Fabrication and Characterization of Novel Hybrid Nanocomposites with Application in Solar Cells

Doctor of Philosophy Dissertation

Dimitris A. Chalkias Dipl.-Ing Mechanical Engineering & Aeronautics

Supervisor: G.C. Papanicolaou

Dissertation Submitted to the University of Patras for the Award of the Degree of DOCTOR OF PHILOSOPHY in Mechanical Engineering & Aeronautics

Patras, 2019

Examination Committee

1. George Papanicolaou (Dissertation Supervisor) Professor Emeritus, Department of Mechanical Engineering & Aeronautics, University of Patras, Greece

2. Vassilis Kostopoulos (Dissertation Advisor) Professor, Department of Mechanical Engineering & Aeronautics, University of Patras, Greece

3. Theodoros Loutas (Dissertation Advisor) Assistant Professor, Department of Mechanical Engineering & Aeronautics, University of Patras, Greece

4. George Psarras Associate Professor, Department of Materials Science, University of Patras, Greece

5. Dimitris Kondarides Professor, Department of Chemical Engineering, University of Patras, Greece

6. Joannis Kallitsis Professor, Department of Chemistry, University of Patras, Greece

7. Thomas Stergiopoulos Assistant Professor, Department of Chemistry, Aristotle University of Thessaloniki, Greece

Approval of Ph.D. Dissertation of Mr. Dimitris A. Chalkias

Willpower is the key to success.

I dedicate this work to my Mother, who always supported me and believed in me.

Preface

The present Ph.D. dissertation is the outcome of six years of research as a Ph.D. Candidate at the Department of Mechanical Engineering & Aeronautics, University of Patras, Greece, under the supervision of Professor George C. Papanicolaou. Ph.D. Dissertation Outline The present Ph.D. dissertation concerns the application of hybrid nanotechnology in dye-sensitized solar cells (DSSCs), with the main goals of improving their energy conversion efficiency and stability, further reducing their manufacturing cost, and increasing their application range. The dissertation resulted in two published papers in international scientific journals, and seven more already submitted or close to submission. The Ph.D. dissertation is divided into two main sections and consists of six chapters. ➢ Section 1 The first section, which consists of three chapters, describes the current knowledge about the topic of photovoltaics, focusing on DSSCs technology, while the motivation and the research objectives of the Ph.D. dissertation are also presented. In Chapter 1, there is a brief introduction to the recent photovoltaics market trends and solar cell technologies, while the reasons for focusing on DSSCs technology for research, development, and optimization are explained. In Chapter 2, there is an extensive reference to DSSCs technology, presenting a historical background, their design, their operating principle, and their equivalent circuit analysis based on the literature. Chapter 3 provides the state of the art of DSSCs technology, presenting the latest and most important international scientific research efforts to improve the characteristics of DSSCs, from any point of view. In this chapter, the motivation and the research objectives of the Ph.D. dissertation are also presented. ➢ Section 2 The second section of the Ph.D. dissertation, which consists of three chapters, concerns the experimental part. In Chapter 4, the materials, the fabrication and characterization methods, as well as all the experimental setups used in the present research are presented. Chapter 5 concerns the results and discussion part of the dissertation. Chapter 6 lists the most important conclusions and achievements, while future research is also suggested.

Acknowledgements First of all, I would like to express my sincere appreciation for my supervisor, Prof. G.C. Papanicolaou, for giving me the opportunity to pursue my Ph.D. dissertation under his supervision. His guidance always challenged me intellectually and provided the perfect ambience needed to grow as a Ph.D. Candidate, while his constructive criticism and support led me to successfully complete my Ph.D. dissertation. I am also really grateful to him for assigning such an interesting research topic to me, which I would like to follow throughout my next steps of research activity. Additionally, I would like to thank my Ph.D. committee members Prof. V. Kostopoulos, T. Loutas, G.C. Psarras, D.I. Kondarides, J.K. Kallitsis, and T. Stergiopoulos, as well as Prof. E. Stathatos and S. Bebelis. For all the aforementioned Professors, I hold a deep appreciation for their open-handed support all these years. All the aforementioned Professors are very respected by me for their research activity and in many cases are examples to follow. The present Ph.D. dissertation would not have progressed without the help of the laboratory members, Alexis Laios, Dimosthenis Giannopoulos, Dimitris Tasiopoulos, Ioanna Margariti, Dimitris Loizos, and Nektarios Verykokkos, with whom we worked well on the topic of solar cells, serving as co-supervisor of their diploma thesis at the Department of Mechanical Engineering & Aeronautics. Many thanks also to the rest of the lab members for their friendship and support, especially to Lykourgos Kontaxis, with whom we worked side by side all these years of my Ph.D., and Diana Portan for our kind collaboration. I would further like to thank Ms. V. Tsoukala for her assistance in the scanning electron microscopy measurements, Prof. S. Zaoutsos for his assistance in the energy-dispersive X-ray spectroscopy measurements, Ms. P. Lampropoulou for her assistance in the X-ray diffraction measurements, Dr. G. Michanetzis for his assistance in the atomic force microscopy measurements, Prof. P. Koutsoukos for providing technical equipment for the Brunauer–Emmett–Teller analysis measurements, Prof. D.I. Kondarides for providing technical equipment for the ultraviolet-visible spectroscopy measurements, Dr. A. Petala for her assistance in the ultraviolet-visible spectroscopy and diffuse reflectance spectroscopy measurements, Prof. E. Stathatos for providing technical equipment for the photoluminescence spectroscopy measurements, Prof. V. Kostopoulos for providing technical equipment for the differential scanning calorimetry measurements, Ms. E. Kollia for her assistance in the differential scanning calorimetry measurements, Prof. E. Stathatos for providing technical equipment for the Fourier-transform infrared spectroscopy measurements, Dr. A. Soto for her assistance in the Fourier-transform infrared spectroscopy measurements, Prof G.C. Psarras for providing technical equipment for the electrochemical impedance spectroscopy measurements, Prof. J.K. Kallitsis for providing technical equipment for the linear sweep voltammetry measurements, Prof. D.I. Kondarides for providing technical equipment for the cyclic voltammetry measurements, Dr. A. Petala for her assistance in the cyclic voltammetry measurements, Prof. G.C. Psarras for providing technical equipment for the dynamic mechanical analysis measurements, Mr. S. Stavropoulos for his assistance in the dynamic mechanical analysis measurements, and Mr. Dimitris Fakos for providing technical equipment for the solar cells characterization. The difficulties in my Ph.D. dissertation were not exclusively scientific, but often human. After all, the effort was solitary and long-lasting and its side effects in my personal life varied. The value of psychological and moral support in difficult times is not valued and not forgotten. I would not like to offend the people who have shared my worries and my anxiety over these years, mentioning some of them and ignoring others. But at the same time, it is impossible not to mention to my family and special thank them for all their love, continuous support, and encouragement. Without them, executing my Ph.D. dissertation would not be feasible. Additionally, I also feel the need to refer to my girlfriend Theodora and thank her for being next to me, in every sense, in all these years during my Ph.D. II

Last but not least, I would like to acknowledge the contribution of all those who, in one way or another, have questioned me and/or my choices. Without their own doubt, my stubbornness may have been premature. I thank them from my heart for bringing out the best in me.

Dimitris A. Chalkias Ph.D. Candidate Dept. of Mechanical Engineering & Aeronautics University of Patras February 2019

Ph.D. Candidate Qualifications The overall image of the Ph.D. Candidate is shown below: A) Four publications in international scientific journals. B) Twelve publications in international and national conferences. C) In 2015, he gave an invited talk at the symposium held at the University of Patras, entitled “Green University”. D) He is a reviewer of papers in five international scientific journals, namely Electrochimica Acta (I.F. 5.1), Ionics (I.F. 2.3), Journal of Materials Science: Materials in Electronics (I.F. 2.3), Advances in Materials Science and Engineering (I.F. 1.4), and Heliyon (I.F. 1.2). E) He has provided ancillary work in the framework of teaching exercises and applications in the courses “Special Physics for Engineers” (Department of Mechanical Engineering & Aeronautics, University of Patras), "Physics for Chemists" (Department of Chemistry, University of Patras), "Introduction to Composites" (Department of Mechanical Engineering & Aeronautics, University of Patras). F) He was co-supervisor of six diploma theses carried out at the Department of Mechanical Engineering & Aeronautics, in the topic of solar cells. G) He has participated, with a great contribution, in writing both European and National scientific projects on the topic of solar cells. H) Finally, he is a member of the Hellenic Thermal Analysis Company since 2016.

Journal Publications 1. D.A. Chalkias, D.I. Giannopoulos, E. Kollia, A. Petala, V. Kostopoulos, G.C. Papanicolaou, Preparation of polyvinylpyrrolidone-based polymer electrolytes and their application by in-situ gelation in dye-sensitized solar cells, Electrochim. Acta 271 (2018), pp. 632-640. 2. D.A. Chalkias, A.I. Laios, A. Petala, G.C. Papanicolaou, Evaluation of the limiting factors affecting large-sized, flexible, platinum-free dye-sensitized solar cells performance: a combined experimental and equivalent circuit analysis, J. Mater. Sci. Mater. Electron. 29 (2018), pp. 9621-9634. 3. G.C. Papanicolaou, D.A. Chalkias, A.F. Koutsomitopoulou, Thermal shock cycling effect on the mechanical behavior of epoxy matrix-woven flax fabric composites, AIP Conf. Proc. 1932 (2018), 030006. 4. G. Papanicolaou, D. Chalkias, A. Koutsomitopoulou, Low energy impact and post impact behavior of epoxy matrix-woven flax fabric composites, U.P.B. Sci. Bull. Ser. D 78 (2016), pp. 25-36.

III

Conference publications 1. D.A. Chalkias, D.D. Loizos, G.C. Papanicolaou, Environmental factors effect on the degradation of dye-sensitized solar cells performance, 11th National Conference on Low Impact Alternative Energy Sources, 2018, Thessaloniki, Greece. 2. D.A. Chalkias, N.E. Verykokkos, D.I. Tasiopoulos, G.C. Papanicolaou, Mechanical and viscoelastic behavior of a thermoplastic polyblend sandwich structured system for dye-sensitized solar cells application, 7th International Conference on Structural Analysis of Advanced Materials, 2017, Bucharest, Romania. 3. D.A. Chalkias, A.F. Koutsomitopoulou, G.C. Papanicolaou, Thermal shock cycling effect on the mechanical behavior of epoxy matrix-woven flax fabric composites, 7th International Conference on Structural Analysis of Advanced Materials, 2017, Bucharest, Romania. 4. D.A. Chalkias, Ν.Ε. Verykokkos, Ε. Kollia, Α. Petala, G.C. Psarras, D.I. Kondarides, V. Kostopoulos, G.C. Papanicolaou, Preparation of polymer blend electrolytes using mixed iodide compounds and their application in dye-sensitized solar cells. 11th Panhellenic Scientific Conference in Chemical Engineering, 2017, Thessaloniki, Greece. 5. D.A. Chalkias, Ν.Ε. Verykokkos, Ε. Kollia, G.C. Psarras, V. Kostopoulos, G.C. Papanicolaou, Preparation and study of thermal and electrical properties of polymer blend electrolytes with application in dye-sensitized solar cells, 11th Panhellenic Scientific Conference in Chemical Engineering, 2017, Thessaloniki, Greece. 6. D.A. Chalkias, D.I. Giannopoulos, Ε. Kollia, Α. Petala, G.C. Psarras, D.I. Kondarides, V. Kostopoulos, G.C. Papanicolaou, Application of polymer electrolytes solidified by polyvinylpyrrolidone in dye-sensitized solar cells. 11th Panhellenic Scientific Conference in Chemical Engineering, 2017, Thessaloniki, Greece. 7. D.A. Chalkias, D.I. Tasiopoulos, G.C. Papanicolaou, Optimization of dye-sensitized solar cells photo-anode characteristics towards an impressive energy conversion efficiency, 13th International Conference on Nanosciences & Nanotechnologies, 2016, Thessaloniki, Greece. 8. D.A. Chalkias, N.E. Verykokkos, E.K. Kollia, G.C. Papanicolaou, Preparation, thermal analysis, and application of a thermoplastic polyblend electrolyte in dye-sensitized solar cells, 7th Panhellenic Conference on Thermal Analysis and Calorimetry, 2016, Ioannina, Greece. 9. D.A. Chalkias, D.I. Tasiopoulos, G.C. Papanicolaou, Enhanced performances of dye-sensitized solar cells based on hybrid photoanodes, 6th International Conference on Structural Analysis of Advanced Materials, 2015, Porto, Portugal. 10. D.I. Giannopoulos, D.A. Chalkias, G.C. Papanicolaou, Preparation of PVP polymer electrolytes and their application in solid state dye-sensitized solar cells, 6th International Conference on Structural Analysis of Advanced Materials, 2015, Porto, Portugal. 11. D.A. Chalkias, A.F. Koutsomitopoulou, G.C. Papanicolaou, Low-energy impact and post impact behavior of epoxy matrix-woven flax fabric composites, 6th International Conference on Structural Analysis of Advanced Materials, 2015, Porto, Portugal. 12. D.A. Chalkias, G.C. Papanicolaou, Fabrication and characterization of novel dye-sensitized solar cells, 10th National Conference on Low Impact Alternative Energy Sources, 2014, Thessaloniki, Greece.

Invited Talks 1. D.A. Chalkias, Novel technologies of high-strength photovoltaic materials, Green University, 2015, Patras, Greece.

Abstract

Undoubtedly, dye-sensitized solar cells (DSSCs) are one of the most important 3rd generation photovoltaic technologies today, meeting the ever-growing demand of humanity for low-cost, efficient, and clean energy production. DSSCs are hybrid organic-inorganic photovoltaic devices, recently gaining considerable attention in the academic and industrial communities. From a scientific point of view, they are enthralling systems to investigate, develop, and optimize. These devices comprise various materials, while each of the components can be designed, fabricated, and explored individually. Nonetheless, it must be pointed out that the crucial parameter for their efficiency and stability is the interplay between their components. From an industrial point of view, the efficiency and long-term stability of DSSCs have been a subject of concern during the past years of development of this technology. To solve these problems, numerous research efforts have been devoted to the engineering and manufacturing of these devices, in order to meet the standards of the photovoltaics market, for various applications. The present Ph.D. dissertation is merely a small contribution to the gigantic amount of data, models, and theories that have been made over the last years to improve DSSCs technology, but hopefully a valuable research effort to the scientific community. The main research objective of the dissertation is the application of hybrid nanotechnology in DSSCs, with the main goals of improving their energy conversion efficiency and stability, further reducing their manufacturing cost, and increasing their application range. Here, it must be noted that all manufacturing processes used during the present research effort were based on low-cost and simple techniques, capable of reproduction by conventional means. The obtained experimental results were interpreted thoroughly from the physicochemical point of view. The electrical characterization of solar cells, which were all fabricated in the laboratory, comes along with a large number of materials characterization experiments, where the morphology, the crystallinity, the chemical composition and structure, as well as the thermal, optical, electrical, and optoelectrical characteristics of the individual parts of the solar cells were examined. In addition, the one-diode model equivalent circuit analysis of DSSCs contributed to the interpretation of the results obtained from the electrical characterization of the materials as a system (solar cells). The solar cells were characterized both for their performance and their stability. Finally, the mechanical, dynamic mechanical, and viscoelastic behavior of composite materials, which simulate the structure of the solar cells, were investigated. Starting from the first main goal, “enhancement of DSSCs efficiency”, the dissertation deals with the systematic investigation in the direction of replacing the materials and structure of the conventional anode of DSSCs with novel hybrid nanostructures, which exhibit unique and optimized characteristics for the aforementioned application. The modifications concerned interfacial engineering, regulation of the materials porosity, fabrication of composites aiming to improve the electrical and optical characteristics of the anode, enhancement of light scattering, and co- sensitization of the anode for enhanced light-harvesting. The results are very satisfactory since the fabrication of solar cells with an energy conversion efficiency of 13% was achieved, reaching DSSCs efficiency records. Focusing on the second main goal, “enhancement of DSSCs stability”, two separate investigations were conducted. The first one deals with the development of high-efficiency quasi-solid state DSSCs employing novel advanced polymer electrolytes, which were prepared in the laboratory. The aim was the optimization of the characteristics of the polymer electrolytes for achieving a high performance to DSSCs. In all cases, the rest of the materials and structure of the solar cells were identical to the conventional DSSCs. More specifically, the investigation focused on the preparation of iodide-based

electrolytes, using polymer blends as solidification agents, suitable for solar cells application. Further up, the extra use of chemical additives and iodide compound mixtures, which have proven to improve the performance of the corresponding liquid state electrolytes for the aforementioned application, were investigated. The results are very satisfactory since the quasi-solid state DSSCs presented a higher energy conversion efficiency and stability compared to their conventional counterparts, which employed a liquid state factory-available high-stability electrolyte. The second investigation on the DSSCs stability topic deals with the determination of the performance degradation of conventional DSSCs under extreme ageing conditions. The investigation also includes the accurate prediction of the degradation of the solar cells performance after all accelerating ageing conditions, by means of a semi-analytical model (residual property model, RPM) developed by Papanicolaou et al. This achievement is considered important in the direction of the fast and accurate determination of the lifetime and reliability of solar cells for various applications. Concerning the third main goal of the dissertation, “further reduction of DSSCs costs”, a series of platinum-free DSSCs were fabricated, based on novel carbon-based counter electrodes. Even though the study was at the first stages, the platinum-free DSSCs performed equally well compared to the conventional DSSCs employing factory-available platinum-based counter electrodes. The results of this study are considered important in the direction of increasing the competitiveness of DSSCs technology in the photovoltaic market. Finally, concerning the fourth main goal of the dissertation, “DSSCs wide commercialization”, three separate investigations were conducted. The first one deals with the development of high- efficiency back-side illuminated DSSCs, using simple and low-cost techniques, based on highly ordered and mesoporous materials, after their optimization for solar cells application. In this case, the achieved efficiency of the back-side illuminated DSSCs is quite satisfactory since it was higher than the corresponding of the conventional front-side illuminated DSSCs. At this point, it is worth mentioning that the active surface of the back-side illuminated DSSCs was four times larger than the one of the conventional front-side illuminated DSSCs. The results of this study are considered important in the direction of developing high-efficiency, high-stability, and low-cost flexible DSSCs. The second investigation concerns the evaluation of the limiting factors affecting large-sized flexible platinum-free DSSCs performance, which is considered as a topic with a literature gap. This study contributes to the increase of the competitiveness of DSSCs technology in the photovoltaic market for various novel applications. Finally, with the ever-increasing demand for high quality and reliable solar cells, DSSCs were investigated from a mechanical point of view. In this case, the mechanical, the dynamic mechanical, and the viscoelastic behavior of sandwich-like structured composite materials, which simulate the structure of a flexible quasi-solid state DSSC, were studied through three-point blending experiments at different strain rates, dynamic mechanical analysis experiments at different oscillation frequencies, and relaxation experiments at different strain levels. These structures were fabricated using materials already examined for their suitability for DSSCs application. The aforementioned research is a preliminary study in the direction of fabrication of high- mechanical strength solar cells, which is considered as a new hot topic in the field of photovoltaics. All the above-presented concepts and their realizations made a valuable contribution to the DSSCs field. The studies presented hereby are comprehensive and elegant examples of a carefully and logically planned scrutiny, leading to a deeper understanding of the processes taking place in DSSCs. The extensive use of so many different investigation techniques and the successful combination of the obtained results allowed for drawing important conclusions, upon which the base for further optimization of DSSCs and/or other emerging photovoltaic technologies may be built.

Keywords: Photovoltaic; Dye-sensitized solar cell; Hybrid nanocomposite; Physicochemical characterization; Mechanical characterization.

Περίληψη

Αδιαμφισβήτητα, τα φωτοευαίσθητα ηλιακά κύτταρα με χρωστική ουσία (Dye-Sensitized Solar Cells, DSSCs) συγκαταλέγονται στις πιο σημαντικές φωτοβολταϊκές τεχνολογίες 3ης γενιάς, ικανοποιώντας τη διαρκώς αυξανόμενη ανάγκη της ανθρωπότητας για παραγωγή χαμηλού κόστους, αποδοτικής και φιλικής προς το περιβάλλον ενέργειας. Τα DSSCs είναι υβριδικές οργανικές- ανόργανες φωτοβολταϊκές διατάξεις, οι οποίες τα τελευταία χρόνια έχουν προσελκύσει το ενδιαφέρον τόσο της ακαδημαϊκής κοινότητας όσο και της βιομηχανίας. Από επιστημονικής πλευράς, η συγκεκριμένη τεχνολογία παρουσιάζει μεγάλο ενδιαφέρον ως προς την έρευνα, την ανάπτυξη και τη βελτιστοποίησή της. Οι συσκευές αυτές απαρτίζονται από διάφορα υλικά, καθένα εκ των οποίων δύναται να σχεδιαστεί, να κατασκευαστεί και να μελετηθεί μεμονωμένα. Παρόλα αυτά, καθίσταται αναγκαίο να επισημανθεί ότι η κρίσιμη παράμετρος για την απόδοση και τη σταθερότητα των συσκευών αυτών συνιστά η αλληλεπίδρασή μεταξύ των υλικών που τις αποτελούν. Από βιομηχανική άποψη, η απόδοση και η μακροπρόθεσμη σταθερότητα των DSSCs αποτέλεσαν αντικείμενο μελέτης κατά τα τελευταία χρόνια ανάπτυξης αυτής της τεχνολογίας. Για τη βελτίωση των συσκευών αυτών, έχουν διεξαχθεί πολλές ερευνητικές προσπάθειες, οι οποίες επικεντρώθηκαν στη μελέτη και στην τροποποίησή τους, προκειμένου αυτές να ανταποκριθούν στα βιομηχανικά πρότυπα των φωτοβολταϊκών για τις διάφορες εν δυνάμει εφαρμογές τους. Η παρούσα διδακτορική διατριβή αποτελεί μια μικρή συμβολή στο γιγάντιο όγκο δεδομένων, μοντέλων και θεωριών που έχουν αναπτυχθεί τα τελευταία χρόνια ώστε να βελτιωθεί η τεχνολογία των DSSCs, ωστόσο αποβλέπει σε μια αξιοσημείωτη ερευνητική προσπάθεια προς όφελος της επιστημονικής κοινότητας. Το κύριο αντικείμενο έρευνας της διδακτορικής διατριβής συνιστά η εφαρμογή υβριδικής νανοτεχνολογίας στα DSSCs, θέτοντας ως βασικούς στόχους τη βελτίωση της απόδοσης και της σταθερότητάς τους, την περεταίρω μείωση του κόστους κατασκευής τους, καθώς και τη διερεύνηση των πεδίων εφαρμογών τους. Σε αυτό το σημείο, αξίζει να επισημανθεί ότι οι διαδικασίες παρασκευής/κατασκευής των υλικών/ηλιακών κυττάρων, οι οποίες χρησιμοποιήθηκαν κατά τη διάρκεια της παρούσας έρευνας, βασίζονται σε απλές και χαμηλού κόστους τεχνικές, ικανές να αναπαραχθούν με συμβατικά μέσα. Τα αποτελέσματα των πειραμάτων που διεξήχθησαν ερμηνεύονται ενδελεχώς από φυσικοχημική άποψη. Τον ηλεκτρικό χαρακτηρισμό των ηλιακών κυττάρων, τα οποία κατασκευάστηκαν στο εργαστήριο, συνοδεύει ένα μεγάλο πλήθος πειραμάτων χαρακτηρισμού υλικών, κατά τα οποία διευκρινίστηκε η μορφολογία, η κρυσταλλικότητα, η χημική σύσταση και δομή, καθώς και τα θερμικά, οπτικά, ηλεκτρικά και οπτοηλεκτρικά χαρακτηριστικά των υλικών που απαρτίζουν τα υπό μελέτη ηλιακά κύτταρα. Επιπρόσθετα, η ανάλυση των DSSCs μέσω του ισοδύναμου ηλεκτρικού κυκλώματος μονής διόδου συνέβαλε στην ερμηνεία των αποτελεσμάτων, τα οποία συλλέχθηκαν κατά τον ηλεκτρικό χαρακτηρισμό των υλικών ως συστήματος (ηλιακά κύτταρα). Τα ηλιακά κύτταρα χαρακτηρίστηκαν τόσο ως προς την απόδοση ενεργειακής μετατροπής όσο και ως προς τη σταθερότητα που επιδεικνύουν. Τέλος, μελετήθηκε η μηχανική, η δυναμική, και η βισκοελαστική συμπεριφορά συνθέτων υλικών, τα οποία κατασκευάστηκαν με δομή που προσομοιώνει τη δομή των υπό μελέτη ηλιακών κυττάρων. Αρχικά, ως πρώτος βασικός στόχος της διδακτορικής διατριβής τίθεται η βελτίωσης της απόδοσης των DSSCs. Κατά το στόχο αυτόν, διεξάγεται συστηματική έρευνα στην κατεύθυνση της αντικατάστασης των υλικών και της δομής της συμβατικής φωτο-ανόδου των DSSCs με καινοτόμες υβριδικές νανοδομές, οι οποίες παρουσιάζουν μοναδικά και βελτιωμένα χαρακτηριστικά για την προαναφερθείσα εφαρμογή. Συγκεκριμένα, οι μελέτες αφορούν σε διεπιφανειακές τροποποιήσεις στο ηλεκτρόδιο εργασίας, στη ρύθμιση του πορώδους του ημιαγωγού της ανόδου, στην κατασκευή συνθέτων υλικών με στόχο τη βελτίωση των ηλεκτρικών και οπτικών χαρακτηριστικών της ανόδου, στην ενίσχυση του φαινομένου σκέδασης του φωτός από τη φωτο-άνοδο και τέλος, στη συν- VII

ευαισθητοποίηση της ανόδου για την επίτευξη μεγαλύτερης απορρόφησης φωτός από τα ηλιακά κύτταρα. Τα αποτελέσματα είναι πολύ ικανοποιητικά δεδομένου ότι επιτεύχθηκε η κατασκευή DSSCs τα οποία παρουσίασαν απόδοση της τάξεως του 13%, φθάνοντας τις υψηλότερες τιμές απόδοσης που έχουν καταγραφεί ως σήμερα σε αυτή την τεχνολογία φωτοβολταϊκών παγκοσμίως. Ως δεύτερος στόχος της διδακτορικής διατριβής τίθεται η βελτίωση της σταθερότητας των DSSCs, κατά τον οποίο διεξήχθησαν δύο ξεχωριστές μελέτες. Η πρώτη μελέτη αφορά στην ανάπτυξη υψηλής απόδοσης ημι-στερεάς κατάστασης DSSCs, με τη χρήση καινοτόμων προηγμένων πολυμερικών ηλεκτρολυτών, οι οποίοι παρασκευάστηκαν στο εργαστήριο. Σκοπός αποτέλεσε η βελτιστοποίηση των χαρακτηριστικών των πολυμερικών ηλεκτρολυτών για την επίτευξη υψηλής απόδοσης στα DSSCs. Σε όλες τις περιπτώσεις, τα υπόλοιπα υλικά και η δομή των ηλιακών κυττάρων ήταν σύμφωνα με τα αντίστοιχα των συμβατικών DSSCs. Πιο αναλυτικά, διερευνήθηκε η παρασκευή πολυμερικών ηλεκτρολυτών οξειδοαναγωγικού ζεύγους ιωδίου, με τη χρήση πολυμερικών μιγμάτων για τη στερεοποίησή τους, κατάλληλων για αυτή την εφαρμογή. Εν συνεχεία, μελετήθηκε η επιπλέον χρήση χημικών προσθέτων και μιγμάτων ιωδιούχων ενώσεων, τα οποία αποδεδειγμένα βελτιώνουν τη συμπεριφορά των αντίστοιχων υγρών ηλεκτρολυτών για αυτή την εφαρμογή. Τα αποτελέσματα της παρούσας έρευνας είναι πολύ ικανοποιητικά, καθώς τα DSSCs που ενσωμάτωναν τους προαναφερόμενους ηλεκτρολύτες παρουσίασαν υψηλότερη απόδοση και σταθερότητα στο χρόνο σε σύγκριση με αντίστοιχα συμβατικά DSSCs, τα οποία ενσωμάτωναν εργοστασιακά διαθέσιμο υγρό ηλεκτρολύτη υψηλής σταθερότητας. Η δεύτερη μελέτη αφορά στον προσδιορισμό της υποβάθμισης της απόδοσης των συμβατικών DSSCs υπό ακραίες συνθήκες λειτουργίας, μέσω πειραμάτων επιταχυνόμενης γήρανσης. Η έρευνα αυτή περιλαμβάνει και την ακριβή πρόβλεψη της υποβάθμισης της απόδοσης των ηλιακών κυττάρων υπό τις διάφορες συνθήκες επιταχυνόμενης γήρανσής, μέσω του ημι-αναλυτικού μοντέλου Residual Property Model (RPM), το οποίο έχει αναπτυχθεί από τον Καθηγητή Γ.Χ. Παπανικολάου. Το επίτευγμα αυτό θεωρείται σημαντικό στην κατεύθυνση του γρήγορου και ακριβή προσδιορισμού της διάρκειας ζωής και της αξιοπιστίας των ηλιακών κυττάρων για διάφορες εφαρμογές. Ως τρίτος στόχος της διδακτορικής διατριβής τίθεται η περαιτέρω μείωση του κόστους των DSSCs, εστιάζοντας στην ανάπτυξη ηλιακών κυττάρων χωρίς πλατίνα, τα οποία ενσωματώνουν ηλεκτρόδια καθόδου με βάση τον άνθρακα. Μολονότι η συγκεκριμένη έρευνα βρίσκεται σε αρχικά στάδια, τα ηλιακά κύτταρα χωρίς πλατίνα απέδιδαν εξίσου καλά σε σύγκριση με συμβατικά DSSCs, τα οποία ενσωμάτωναν εργοστασιακά διαθέσιμα ηλεκτρόδια καθόδου με βάση την πλατίνα. Τα αποτελέσματα της μελέτης αυτής θεωρούνται σημαντικά στην κατεύθυνση της αύξησης της ανταγωνιστικότητας της τεχνολογίας των DSSCs στη φωτοβολταϊκή αγορά. Τέλος, ως τέταρτος βασικός στόχος της διδακτορικής διατριβής τίθεται η διεύρυνση των πεδίων εφαρμογών των DSSCs, κατά τον οποίο διεξήχθησαν τρεις ξεχωριστές μελέτες. Η πρώτη μελέτη αφορά στην ανάπτυξη υψηλής απόδοσης οπισθο-φωτιζόμενων DSSCs, με τη χρήση απλών και χαμηλού κόστους τεχνικών, βασιζόμενα σε υψηλής οργάνωσης και μεσοπορώδη υλικά, ύστερα από τη βελτιστοποίησή τους για αυτή την εφαρμογή. Κατά τη μελέτη αυτή, η απόδοση που επιτεύχθηκε στα οπισθο-φωτιζόμενα DSSCs αξιολογήθηκε ως αρκετά ικανοποιητική, καθώς ήταν υψηλότερη από την απόδοση αντίστοιχων συμβατικών εμπρόσθια-φωτιζόμενων DSSCs. Σε αυτό το σημείο αξίζει να αναφερθεί ότι η ενεργός επιφάνεια των οπισθο-φωτιζόμενων DSSCs ήταν τετραπλάσια από την αντίστοιχη των συμβατικών εμπρόσθια-φωτιζόμενων DSSCs. Τα αποτελέσματα της μελέτης αυτής θεωρούνται σημαντικά στην κατεύθυνση της ανάπτυξης υψηλής απόδοσης, υψηλής σταθερότητας και χαμηλού κόστους εύκαμπτων DSSCs. Η δεύτερη μελέτη αφορά στην ανάδειξη των παραμέτρων που επηρεάζουν την απόδοση των μεγάλης κλίμακας εύκαμπτων και χωρίς πλατίνα DSSCs, αντικείμενο με διεθνές βιβλιογραφικό κενό. Η μελέτη αυτή συμβάλει στην κατεύθυνση της αύξησης της ανταγωνιστικότητας της τεχνολογίας των DSSCs στη φωτοβολταϊκή αγορά. Τέλος, με γνώμονα τη διαρκώς αυξανόμενη ζήτηση για υψηλής ποιότητας και αξιοπιστίας ηλιακών κυττάρων, τα ηλιακά κύτταρα μελετώνται από μηχανική άποψη. Σε αυτή την περίπτωση, εξετάστηκαν η μηχανική, η δυναμική, καθώς και η βισκοελαστική συμπεριφορά συνθέτων υλικών δομής τύπου sandwich, τα VIII

οποία προσομοιώνουν τη δομή ενός εύκαμπτου ημι-στέρεας κατάστασης DSSC. Η μελέτη αφορούσε σε πειράματα κάμψης τριών σημείων των υλικών σε διαφορετικούς ρυθμούς παραμόρφωσης, σε διαφορετικές συχνότητες ταλάντωσης, καθώς και σε πειράματα χαλάρωσης σε διαφορετικές επιβαλλόμενες παραμορφώσεις. Οι προαναφερθείσες δομές κατασκευάστηκαν χρησιμοποιώντας υλικά τα οποία είχαν ήδη δοκιμαστεί για την καταλληλότητά τους ως προς την εφαρμογή τους στα DSSCs. Η έρευνα αυτή αποτελεί μια προκαταρκτική μελέτη στην κατεύθυνση της κατασκευής ηλιακών κυττάρων υψηλής μηχανικής αντοχής, το οποίο πρόκειται να αποτελέσει ένα δημοφιλές αντικείμενο έρευνας τα επόμενα χρόνια. Οι προαναφερόμενοι στόχοι και οι πραγματώσεις τους συνέβαλαν σημαντικά στο πεδίο έρευνας των DSSCs. Οι μελέτες που παρουσιάζονται στην παρούσα διδακτορική διατριβή αποτελούν υπόδειγμα πλήρους, συστηματικής και καλά οργανωμένης έρευνας, οδηγώντας σε μια βαθύτερη κατανόηση των διαδικασιών που λαμβάνουν χώρα στα DSSCs. Η εκτεταμένη χρήση τόσων διαφορετικών ερευνητικών μεθόδων και ο επιτυχημένος συνδυασμός των αποτελεσμάτων που προέκυψαν, επέτρεψαν να αντληθούν σημαντικά συμπεράσματα με τα οποία μπορεί να δημιουργηθεί η βάση για την περαιτέρω βελτιστοποίηση των DSSC ή/και άλλων αναδυόμενων φωτοβολταϊκών τεχνολογιών.

Λέξεις κλειδιά: Φωτοβολταϊκό, φωτο-ευαίσθητο ηλιακό κύτταρο με χρωστική ουσία, υβριδικό νανοσύνθετο υλικό, φυσικοχημικός χαρακτηρισμός, μηχανικός χαρακτηρισμός.

Acronyms

a-Si:H: hydrogenated amorphous silicon AC: activated carbon AFM: atomic force microscopy ATO: anodic titanium oxide BET: Brunauer-Emmett-Teller BIPV: building-integrated photovoltaic BMII: 1-butyl-3-methylimidazolium iodide BSF: back surface field c-Si: crystalline silicon CB: carbon black CdTe: cadmium telluride CIGS: copper indium gallium (di)selenide CNT: carbon nanotube CPE: constant phase element CSA: camphorsulfonic acid CV: cyclic voltammetry CZTS: copper zinc tin sulphide

− 푫푰ퟑ : diffusion coefficient of triiodides DMA: dynamic mechanical analysis DRS: diffuse reflectance spectroscopy DSC: differential scanning calorimetry DSSC: dye-sensitized solar cell DSSM: dye-sensitized solar module DSSP: dye-sensitized solar panel ECE: energy conversion efficiency EDX: energy-dispersive X-ray spectroscopy EIS: electrochemical impedance spectroscopy FF: fill factor FTIR: Fourier-transform infrared spectroscopy FTO: fluorine-doped tin oxide

G: graphite GaAs: gallium arsenide GNB: graphite nanoball GNF: graphite nanofiber GnNS: graphene nanosheet GnNP: graphene nanoplatelet GNS: graphite nanosheet GnO: graphene oxide GuSCN: guanidine thiocyanate HBC: heterojunction back contact HIT: heterojunction with intrinsic thin-layer HOMO: highest occupied molecular orbital IBC: interdigitated back contact IPCE: incident photon-to-electron conversion efficiency ITO: indium-doped tin oxide

J0: dark saturation current density

JL: light-generated current density

JSC: short-circuit current density LCOE: levelized cost of electricity LSV: linear sweep voltammetry LUMO: lowest unoccupied molecular orbital mc-Si: multi-crystalline silicon MJ: multi-junction MWCNT: multiwall carbon nanotube n: ideality factor n/n0: relative number of free charge carriers OPV: organic photovoltaics P3HT: poly(3-hexylthiophene) PAN: poly(acrylonitrile) PAN-VA: poly(acrylonitrile-co-vinyl acetate) PANI: polyaniline PEDOT: poly(3,4-ethylenedioxythiophene) PEDOT-PSS: poly(3,4-ethylendioxythiophene)-poly(styrene sulfonate) PEG: poly(ethylene glycol) XII

PEO: poly(ethylene oxide) PERC: passivated emitter rear contact PEN: polyethylenenaphtalate PET: polyethyleneterephtalate PL: photoluminescence PMII: 1-methyl-3-propylimidazolium iodide PMMA: poly(methyl methacrylate) PMMA-EA: poly(methyl methacrylate-co-ethyl acrylate) POE-PAI: poly(oxyethylene) amide-imide POEI-IS: poly(oxyethylene)-imide-imidazolium selenocyanate POMA-FGO: poly(o-methoxyaniline) graphene oxide PPC: polypropylene carbonate PProDOT: poly(3,4-propylenedioxythiophene) PPy: polypyrrole PSC: perovskite solar cells PV: photovoltaic PVA: poly(vinyl alcohol) P(VA-co-MMA): polyvinyl acetate-co-methyl methacrylate PVDF: poly(vinylidene fluoride) PVDF-HFP: poly(vinylidenefluoride-co-hexafluoropropylene) PVP: polyvinylpyrrolidone QD: quantum dot QSS-DSSCs: quasi-solid state dye-sensitized solar cells RPM: residual property model rS: area-specific total series parasitic resistance rSH: area-specific shunt parasitic resistance

RSheet: sheet resistance SbGnNPs: antimony-doped graphene nanoplatelets sc-Si: single crystalline silicon SEM: scanning electron microscopy spiro-MeOTAD: 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene SS-DSSCs: solid state dye-sensitized solar cells StS: stainless steel SWCNT: single-walled carbon nanotubes XIII

TBP: 4-tert-butylpyridine TCO: transparent conducting oxide TLM: transmission line model

Tm: melting temperature UV-VIS: ultraviolet–visible

VOC: open-circuit voltage

Xc: crystallinity XRD: X-ray diffraction

ΔHm: experimental melting enthalpy ε: dielectric constant σ: conductivity

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List of Tables

Table 1-1: Comparison of CdTe, CIGs, and a-Si properties...... - 10 - Table 3-1: Current challenges for dye-sensitized solar cells wide commercialization and research objectives of the Ph.D. dissertation...... - 71 - Table 4-1: Working electrodes fabricated under different temperature profiles...... - 75 -

Table 4-2: Configurations of the TiO2 electrodes with the addition of light scatters...... - 78 - Table 4-3: Composition of the prepared polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes with additives and iodide compounds mixture...... - 84 - Table 4-4: Values of the parameters that were used during the DMA experiments...... - 100 - Table 5-1: Electrical characteristics of the dye-sensitized solar cells employing the conventional working electrode sintered under different temperature profiles...... - 102 - Table 5-2: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the conventional working electrode sintered under different temperature profiles...... - 102 - Table 5-3: Electrical characteristics of the dye-sensitized solar cells employing the conventional working electrode and the TiCl4 treaded working electrode...... - 104 - Table 5-4: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the conventional working electrode and the TiCl4 treaded working electrode...... - 104 - Table 5-5: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode and the TiCl4 treated working electrode...... - 106 - Table 5-6: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 working electrode and the corresponding working electrodes fabricated using additionally a blocking layer, of different film thicknesses...... - 108 - Table 5-7: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the conventional TiCl4 working electrode and the corresponding working electrodes fabricated using additionally a blocking layer, of different film thicknesses……...... - 109 - Table 5-8: Parameters obtained by AFM for surface conductive glass and surface conductive glass covered by the blocking layer...... - 109 - Table 5-9: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode...... - 111 - Table 5-10: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode...... - 112 - Table 5-11: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, and the corresponding working electrode fabricated using additionally 1.5 wt% of PVP in the anode...... - 113 - XV

Table 5-12: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally rutile microparticles in different wt% loadings in the anode, in different anode designs...... - 115 - Table 5-13: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally rutile microparticles in different wt% loadings in the anode, in different anode designs...... - 116 - Table 5-14: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, the working electrode employing the optimized scattering layer, and the working electrode employing the optimized scattering layer and the reflecting layer...... - 118 - Table 5-15: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally multi-walled carbon nanotubes in the anode in different wt% loadings...... - 120 - Table 5-16: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally multi-walled carbon nanotubes in the anode in different wt% loadings...... - 120 - Table 5-17: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, and the working electrode fabricated using additionally 0.025 wt% loading of multi-walled carbon nanotubes in the anode...... - 122 - Table 5-18: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally silicon dioxide nanoparticles in different wt% loadings in the anode...... - 124 - Table 5-19: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally silicon dioxide nanoparticles in different wt% loadings in the anode...... - 124 - Table 5-20: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, and the corresponding working electrode fabricated using additionally 0.5 wt% of silicon dioxide nanoparticles in the anode...... - 126 -

Table 5-21: Electrical characteristics of the dye-sensitized solar cells employing the TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes...... - 128 - Table 5-22: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes...... - 128 - Table 5-23: Electrical characteristics of the dye-sensitized solar cells employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes...... - 131 - Table 5-24: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719- RK1 cocktail of dyes...... - 132 - Table 5-25: Electrical characteristics of the dye-sensitized solar cells employing the conventional dye-sensitized working electrode and the optimized dye-sensitized working electrode...... - 132 -

XVI

Table 5-26: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the optimized dye-sensitized working electrode sensitized and the conventional dye-sensitized working electrodes...... - 133 - Table 5-27: Parameters obtained by DSC for the polyvinylpyrrolidone-based polymer electrolytes and the pure polyvinylpyrrolidone...... - 135 - Table 5-28: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone-based polymer electrolytes...... - 137 - Table 5-29: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone-based polymer electrolytes...... - 138 - Table 5-30: Parameters obtained by the one-diode model equivalent circuit analysis for the dye- sensitized solar cells employing the polyvinylpyrrolidone-based polymer electrolytes...... - 138 - Table 5-31: Electrical characteristics of the dye-sensitized solar cells employing the most efficient polyvinylpyrrolidone-based polymer electrolyte, for different TiO2 film thicknesses and dye solution concentrations, as well as after TiCl4 treatment of the optimized TiO2 anode...... - 140 - Table 5-32: Parameters obtained by the one-diode model equivalent circuit analysis for the dye- sensitized solar cells employing the most efficient polyvinylpyrrolidone-based polymer electrolyte, for different TiO2 film thicknesses and dye solution concentrations, as well as after TiCl4 treatment of the optimized TiO2 anode...... - 140 - Table 5-33: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes...... - 148 - Table 5-34: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes...... - 150 - Table 5-35: Parameters obtained by the one-diode model equivalent circuit analysis for the dye- sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes...... - 150 - Table 5-36: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives...... - 154 - Table 5-37: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives...... - 155 - Table 5-38: Parameters obtained by the one-diode model equivalent circuit analysis for the dye- sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives...... - 155 - Table 5-39: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total iodide compounds concentration and for the increasing total iodide compounds concentration...... - 161 - Table 5-40: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture...... - 163 - Table 5-41: Parameters obtained by the one-diode model equivalent circuit analysis for the dye- sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture...... - 163 -

XVII

Table 5-42: Electrical characteristics of the dye-sensitized solar cells employing the factory- available liquid-state high-performance and high-stability electrolytes, as well as the optimized polymer electrolyte...... - 165 - Table 5-43: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the factory-available liquid-state high-performance and high- stability electrolytes, as well as the optimized polymer electrolyte...... - 165 - Table 5-44: Electrical characteristics of the dye-sensitized solar cells...... - 167 - Table 5-45: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells...... - 167 - Table 5-46: Parameters obtained by AFM and CV for the different types of counter electrodes...... …….- 175 - Table 5-47: Electrical characteristics of the dye-sensitized solar cells employing the factory- available Pt-based counter electrode, as well as the carbon-based counter electrodes...... - 176 - Table 5-48: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the factory-available Pt-based counter electrode, as well as the carbon-based counter electrodes...... - 176 - Table 5-49: Parameters obtained by SEM, EDX, and AFM for the anodic titanium oxide films fabricated for different time durations of anodization...... - 180 - Table 5-50: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the highly ordered TiO2 nanotube arrays of different length...... - 181 - Table 5-51: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye-sensitized solar cells employing as anode the highly ordered TiO2 nanotube arrays of different length...... - 181 -

Table 5-52: Parameters obtained by AFM and BET analysis for the as-anodized highly ordered TiO2 nanotube arrays and the TiCl4 treated highly ordered TiO2 nanotube arrays...... - 182 - Table 5-53: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the as-anodized highly ordered TiO2 nanotube arrays of the optimized length and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length...... - 183 - Table 5-54: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye-sensitized solar cells as anode the as-anodized highly ordered TiO2 nanotube arrays of the optimized length and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length...... - 184 -

Table 5-55: Parameters obtained by XRD and DRS for the crystallized highly ordered TiO2 nanotube arrays...... - 186 - Table 5-56: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the TiCl4 treated amorphous and crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays...... - 187 - Table 5-57: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye-sensitized solar cells employing as anode the TiCl4 treated amorphous and crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays...... - 187 -

Table 5-58: Parameters obtained by BET analysis for the highly ordered TiO2 nanotube arrays and the highly ordered TiO2 nanotube arrays – TiO2 nanoparticles hybrids...... - 189 - Table 5-59: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays XVIII

and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids...... - 190 - Table 5-60: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye-sensitized solar cells employing as anode the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids...... - 190 - Table 5-61: Electrical characteristics of the conventional dye-sensitized solar cells and the optimized back-side illuminated dye-sensitized solar cells...... - 191 - Table 5-62: Parameters obtained by the one-diode model equivalent circuit analysis of the conventional dye-sensitized solar cells and the optimized back-side illuminated dye-sensitized solar cells...... - 191 - Table 5-63: Parameters obtained by SEM, AFM, XRD, BET analysis, and DRS for the working electrodes...... - 195 - Table 5-64: Parameters obtained by AFM and CV for the different types of counter electrodes……...... - 197 - Table 5-65: Electrical characteristics of the dye-sensitized solar cells employing the different types of carbon-based counter electrodes...... - 198 - Table 5-66: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells employing the different types of carbon-based counter electrodes...... - 198 - Table 5-67: Electrical characteristics of the different active areas dye-sensitized solar cells. . - 200 - Table 5-68: Parameters obtained by the one-diode model equivalent circuit analysis of the dye- sensitized solar cells for different active areas...... - 200 -

XIX

List of Figures

Figure 1-1: Solar photovoltaics global capacity and annual additions, 2007-2017 [2]...... - 1 - Figure 1-2: Solar photovoltaics global capacity, by country or region, 2007-2017 [2]...... - 2 - Figure 1-3: Photovoltaics global shipment share by technology, 2013-2017 [8]...... - 3 - Figure 1-4: Classification of solar cell technologies based on their primary active material [11]…...... - 4 - Figure 1-5: Solar cells efficiency records [12]...... - 5 - Figure 1-6: Classification of solar cell technologies based on three generations [16]...... - 6 - Figure 1-7: Alternative solar cell technologies classification scheme based on materials complexity [15]...... - 7 - Figure 1-8: The evolution of energy conversion efficiencies of crystalline silicon solar cells in the years 2011-2017 [20]...... - 8 - Figure 1-9: (a) Physical schematic of monolithic triple junction n-on-p solar cell deposited epitaxially upon a substrate, (b) the electrical circuit equivalent diagram showing top, middle, and bottom junction diodes, and interconnecting upper and lower tunnel junctions [30]...... - 11 - Figure 1-10: Top emerging solar cell technologies...... - 12 - Figure 1-11: Some of the applications of dye-sensitized solar cells...... - 13 - Figure 1-12: (a) Perovskite crystal, (b) perovskite solar cell...... - 15 - Figure 2-1: Structure of a conventional dye-sensitized solar cell...... - 18 - Figure 2-2: Dye-sensitized solar cells operating principle...... - 21 - Figure 2-3: Representation of a 1 nm slab sliced from a pore in dye-sensitized solar cells [60].- 23 - Figure 2-4: Transmission line model for dye-sensitized solar cells...... - 25 - Figure 2-5: One-diode model...... - 26 - Figure 3-1: Number of papers published per year, using the searching keyword “dye solar cell” (data source: www.scopus.com)...... - 27 - Figure 3-2: Towards lower solar electricity cost...... - 28 - Figure 3-3: Strategies for improving DSSCs efficiency through photo-anode modifications. .... - 29 - Figure 3-4: Schematic operating principle of (a) n-type dye-sensitized solar cell, (b) p-type dye- sensitized solar cell [120]...... - 33 - Figure 3-5: Main degradation mechanisms of a conventional dye-sensitized solar cell [152]. .. - 37 - Figure 3-6: Replacement of liquid state electrolytes of dye-sensitized solar cells with quasi-solid state electrolytes or solid state hole transport materials...... - 40 - Figure 3-7: Estimated cost analysis for a dye-sensitized solar module, by Solaronix Ltd [230]. - 47 - Figure 3-8: Alternative materials for fabrication of low-cost counter electrodes for dye-sensitized solar cells...... - 53 - Figure 3-9: First years timeline of dye-sensitized solar modules development [230]...... - 60 - XXI

Figure 3-10: Flexible, wearable, transparent, indoor, aesthetically appealing, and mechanically robust solar cells...... - 61 - Figure 3-11: Schematic representation of the four main objectives of the Ph.D. dissertation. ... - 70 -

Figure 4-1: (a) Preparation scheme of the TiO2 paste, (b) chemical model of the TiO2 paste, (c) conventional semi-transparent TiO2 working electrodes [242]...... - 74 - Figure 4-2: Materials that were used additionally during the fabrication of the dye-sensitized working electrodes for improving their characteristics...... - 76 -

Figure 4-3: (a) Preparation scheme of the paste, (b) TiO2 electrodes and dye-sensitized TiO2 electrodes with the addition of the pore-forming agent...... - 77 -

Figure 4-4: (a) Preparation scheme of the paste, (b) TiO2 electrodes and dye-sensitized TiO2 electrodes with the addition of light scatters...... - 78 -

Figure 4-5: (a) Preparation scheme of the paste, (b) TiO2-MWCNTs composite pastes, (c) composite TiO2-MWCNTs electrodes...... - 79 -

Figure 4-6: (a) Preparation scheme of the paste, (b) composite TiO2-SiO2 electrodes...... - 80 -

Figure 4-7: TiO2 electrodes sensitized by N719, cocktail of N719 and RK1, and RK1 dyes...... - 80 - Figure 4-8: (a) Preparation scheme of the pastes, (b) Replacement of the conventional dye-sensitized working electrode of dye-sensitized solar cells by the optimized dye-sensitized working electrode…...... - 81 - Figure 4-9: Materials that were used for the preparation of the polymer electrolytes...... - 83 - Figure 4-10: (a) Polyvinylpyrrolidone-based polymer electrolytes in the sealed vessels, (b) quasi- solid state form of polyvinylpyrrolidone-based polymer electrolytes...... - 83 - Figure 4-11: Replacement of the conventional liquid state factory-available electrolyte of dye- sensitized solar cells by the optimized polymer electrolyte...... - 85 - Figure 4-12: Fabrication scheme of the carbon-based counter electrodes...... - 86 - Figure 4-13: The power supply, the electrochemical cell, the samples before and after anodization, and the detached TiO2 nanotubes membrane...... - 87 - Figure 4-14: Cross-section of dummy cells...... - 88 - Figure 4-15: (a) Solar cell materials characterization methods, (b) characterization methods for each part of the solar cell...... - 89 - Figure 4-16: Equivalent electrical circuit applied to simulate the experimental EIS results...... - 93 - Figure 4-17: Homemade variable load setup...... - 95 - Figure 5-1: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional working electrode sintered under different temperature profiles...... - 101 - Figure 5-2: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional working electrode and the TiCl4 treaded working electrode...... - 104 - Figure 5-3: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the TiCl4 treated working electrode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the TiCl4 treated working electrode...... - 105 -

XXII

Figure 5-4: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 working electrode and the corresponding working electrodes fabricated using additionally a blocking layer, of different film thicknesses...... - 108 - Figure 5-5: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the blocking layer on the surface conductive glass, (c) UV-VIS transmittance spectra of the surface conductive glass and the surface conductive glass coated with blocking layer... - 109 - Figure 5-6: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode...... - 111 - Figure 5-7: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the working electrode fabricated using additionally 1.5 wt% of polyvinylpyrrolidone in the anode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the working electrode fabricated using additionally 1.5 wt% of polyvinylpyrrolidone in the anode...... - 112 - Figure 5-8: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally rutile microparticles in different wt% loadings in the anode, in different anode designs...... - 115 - Figure 5-9: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the optimized scattering layer, (c, d) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the reflecting layer, (e-i) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode, the working electrode employing the optimized scattering layer, and the working electrode employing the optimized scattering layer and reflecting layer...... - 116 - Figure 5-10: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally multi-walled carbon nanotubes in the anode in different wt% loadings...... - 119 - Figure 5-11: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the working electrode fabricated using additionally 0.025 wt% loading of multi-walled carbon nanotubes in the anode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the working electrode fabricated using additionally 0.025 wt% loading of multi-walled carbon nanotubes in the anode...... - 121 - Figure 5-12: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally silicon dioxide nanoparticles in different wt% loadings in the anode...... - 123 - Figure 5-13: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the working electrode fabricated using additionally 0.5 wt% loading of silicon dioxide nanoparticles in the anode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the working electrode fabricated using additionally 0.5 wt% loading of silicon dioxide nanoparticles in the anode...... - 125 - Figure 5-14: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes...... - 127 - XXIII

Figure 5-15: UV-VIS absorbance spectra of (a) dyes ethanolic solutions, (b) dyes anchored on TiO2 working electrodes...... - 128 - Figure 5-16: SEM image showing the cross-section of the optimized multilayered anode...... - 130 - Figure 5-17: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes...... - 131 - Figure 5-18: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional dye-sensitized working electrode and the optimized dye-sensitized working electrode...... - 132 - Figure 5-19: DSC thermograms of the polyvinylpyrrolidone-based polymer electrolytes and the pure polyvinylpyrrolidone...... - 134 - Figure 5-20: (a) FTIR transmittance spectra and (b) XRD patterns of the polyvinylpyrrolidone-based polymer electrolytes and the pure polyvinylpyrrolidone...... - 135 - Figure 5-21: (a) Nyquist plots derived from EIS and (b) linear sweep voltammograms of the polyvinylpyrrolidone-based polymer electrolytes...... - 136 - Figure 5-22: UV-VIS absorption spectra of the polyvinylpyrrolidone-based polymer electrolytes…...... - 137 - Figure 5-23: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone-based polymer electrolytes...... - 138 - Figure 5-24: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the most efficient polyvinylpyrrolidone-based polymer electrolyte, for different (a) TiO2 film thicknesses and (b) dye solution concentrations, as well as (c) with and without TiCl4 treatment of the TiO2 anode...... - 139 - Figure 5-25: SEM image showing the optimal electrode film thickness for DSSCs employing the polyvinylpyrrolidone-based polymer electrolytes...... - 141 - Figure 5-26: DSC thermograms of (a) the pure polyvinylpyrrolidone/polyethylene glycol polymer blends and (b) polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes...... - 142 - Figure 5-27: Crystallinity of (a) polyvinylpyrrolidone, (b) polyethylene glycol, and (c) polyvinylpyrrolidone/ polyethylene glycol blend for the different pure polymer blend weight ratios… ...... - 143 - Figure 5-28: (a, c) FTIR spectra and XRD patterns, respectively, of the pure polyvinylpyrrolidone/polyethylene glycol polymer blends and (b, d) FTIR spectra and XRD patterns, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes……...... - 144 - Figure 5-29: (a) Nyquist plots derived from EIS and (b) linear sweep voltammograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes...... - 147 -

Figure 5-30: AC conductivity (log(σac)) vs frequency (log(f)) for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes...... - 148 - Figure 5-31: UV-VIS absorption spectra of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes...... - 149 - Figure 5-32: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes. .... - 149 -

XXIV

Figure 5-33: DSC thermograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives and of pure polyvinylpyrrolidone/polyethylene glycol polymer blend...... - 151 - Figure 5-34: (a) FTIR transmittance spectra and (b) XRD patterns of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives and of pure polyvinylpyrrolidone/polyethylene glycol polymer blend...... - 152 - Figure 5-35: (a) Nyquist plots derived from EIS and (b) linear sweep voltammograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives...... - 153 - Figure 5-36: UV-VIS absorption spectra of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives...... - 154 - Figure 5-37: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives...... - 155 - Figure 5-38: DSC thermograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes and of pure polyvinylpyrrolidone/polyethylene glycol polymer blend (a) for the fixed total concentration of iodide compounds mixture and (b) for the increasing total concentration of iodide compounds mixture...... - 157 - Figure 5-39: (a, c) FTIR spectra and XRD patterns, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture, (b, d) FTIR spectra and XRD patterns, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the increasing total concentration of iodide compounds mixture...... - 158 - Figure 5-40: (a, c) Nyquist plots derived from EIS and linear sweep voltammograms, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture, (b, d) Nyquist plots derived from EIS and linear sweep voltammograms, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the increasing total concentration of iodide compounds mixture...... - 160 - Figure 5-41: UV-VIS absorption spectra of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes (a) for the fixed total concentration of iodide compounds mixture and (b) for the increasing total concentration of iodide compounds mixture...... - 162 - Figure 5-42: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes(a) for the fixed total concentration of iodide compounds mixture and (b) for the increasing total concentration of iodide compounds mixture...... - 162 - Figure 5-43: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the factory-available liquid-state high-performance and high-stability electrolytes, as well as the optimized polymer electrolyte...... - 164 - Figure 5-44: Degradation of dye-sensitized solar cells efficiency stored under room temperature conditions...... - 166 - Figure 5-45: Representative Current-density–Voltage characteristic curve of the dye-sensitized solar cells...... - 167 - Figure 5-46: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under isothermal ageing at T = 85oC…...... - 168 -

XXV

Figure 5-47: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under isothermal ageing at T = -25oC… ...... - 169 - Figure 5-48: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under thermal shock cycling between Tmin o o = -25 C and Tmax = 85 C...... - 169 - Figure 5-49: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under hydrothermal ageing at T = 65oC and RH = 85%...... - 170 - Figure 5-50: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under reverse biasing at I = 4xISC...... ……..- 171 - Figure 5-51: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under normal ageing at T = 25oC and RH = 50%...... - 172 - Figure 5-52: RPM predictions and corresponding experimental results for the normalized degradation of the dye-sensitized solar cells performance due to their ageing...... - 173 - Figure 5-53: SEM images and three-dimensional AFM images of the surface morphology of (a, d) Pt-based counter electrode, (b, e) MWCNTs-based counter electrode, (c, f) G-based counter electrode, respectively, and (g) cyclic voltammograms of the different types of counter electrodes.… ...... - 174 - Figure 5-54: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the factory-available Pt-based counter electrode, as well as the carbon-based counter electrodes...... - 176 - Figure 5-55: (a, b) SEM images of the top surface of the anodic titanium oxide films before and after ultrasonic cleaning, respectively, (c) SEM image of the bottom surface of the free-standing anodic titanium oxide films, (d) SEM image of the side view of the anodic titanium oxide films, (e) EDX spectrum of the anodic titanium oxide films, (d) three-dimensional AFM image of the top surface of the anodic titanium oxide films...... - 178 - Figure 5-56: Current density–Voltage characteristic curves of the back-side illuminated dye- sensitized solar cells employing as anode the highly ordered TiO2 nanotube arrays of different length...... - 180 -

Figure 5-57: (a) Three-dimensional AFM image and (b) N2 adsorption isotherms of the as-anodized highly ordered TiO2 nanotube arrays and the TiCl4 treated highly ordered TiO2 nanotube arrays…...... - 182 - Figure 5-58: Current density–Voltage characteristic curves of the back-side illuminated dye- sensitized solar cells employing as anode the as-anodized highly ordered TiO2 nanotube arrays of the optimized length and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length...... - 183 - Figure 5-59: (a) XRD patterns, (b) Tauc plots, and (c) PL spectra of the amorphous highly ordered TiO2 nanotube arrays and the crystallized highly ordered TiO2 nanotube arrays...... - 185 - Figure 5-60: Current density–Voltage characteristic curves of the back-side illuminated dye- sensitized solar cells employing as anode the TiCl4 treated amorphous and crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays...... - 186 -

XXVI

Figure 5-61: N2 adsorption isotherms of the highly ordered TiO2 nanotube arrays and the highly ordered TiO2 nanotube arrays – TiO2 nanoparticles hybrids...... - 188 - Figure 5-62: Current density–Voltage characteristic curves of the back-side illuminated dye- sensitized solar cells employing as anode the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids...... - 189 - Figure 5-63: Current density–Voltage characteristic curves of the conventional dye-sensitized solar cells and the optimized back-side illuminated dye-sensitized solar cells...... - 191 - Figure 5-64: (a, b) SEM images of the surface morphology and cross-section, respectively, (c) three- dimensional AFM image of the surface morphology, (d) XRD patterns, (e) N2 adsorption isotherms, (f) Tauc plots, and (g) PL spectra of the un-sensitized TiO2 electrodes, (h) UV-Vis absorbance and transmittance spectra of the dye-sensitized TiO2 electrodes...... - 193 - Figure 5-65: (a, c, e) SEM images, three-dimensional AFM image, and cyclic voltammograms, respectively, for the MWCNTs-based counter electrodes (b, d, f) SEM images, three-dimensional AFM image, and cyclic voltammograms, respectively, for the G-based counter electrodes...... - 196 - Figure 5-66: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing (a) the MWCNTs-based counter electrodes and (b) the G-based counter electrodes……...... - 197 - Figure 5-67: Current density–Voltage characteristic curves of the different active areas dye- sensitized solar cells employing (a) the MWCNTs-based counter electrodes and (b) the G-based counter electrodes...... - 199 - Figure 5-68: Representative stress–strain curve of the dummy cells under three-point bending……...... - 202 - Figure 5-69: Variation of the bending modulus as a function of the strain rate for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material. .. - 203 - Figure 5-70: Variation of the yield stress as a function of the strain rate for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material. .. - 204 - Figure 5-71: Variation of (a) bending modulus and (b) yield stress of the dummy cells under three- point bending as a function of the strain rate and the polymer blend electrolyte composition. . - 204 - Figure 5-72: Variation of (a) relaxation time and (b) viscosity of the dummy cells under three-point bending as a function of the strain rate and the polymer blend electrolyte composition...... - 205 - Figure 5-73: Variation of (a) storage modulus, (b) loss modulus, (c) tan delta, and (d) complex viscosity of the dummy cells under three-point bending as a function of the oscillation frequency...... ….- 206 - Figure 5-74: Variation of (a) storage modulus, (b) loss modulus, (c) tan delta, and (d) complex viscosity of dummy cells under three-point bending as a function of the oscillation frequency and the polymer blend electrolyte composition...... - 207 - Figure 5-75: Stress-time curves for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending at different strain levels...... - 208 -

XXVII

Figure 5-76: Isochronous curves for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending...... - 209 - Figure 5-77: Relaxation modulus of the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending...... - 209 - Figure 5-78: Relaxation modulus of the dummy cells under three-point bending as a function of the time and the polymer blend electrolyte composition...... - 210 - Figure 5-79: Experimental data and RPM predictions for the normalized stress relaxation curves for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending...... - 210 - Figure 5-80: Relaxation time derived from the application of the RPM for the dummy cells under three-point bending as a function of the polymer blend electrolyte composition...... - 211 -

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Contents

Preface ...... I Ph.D. Dissertation Outline ...... I Acknowledgements ...... II Ph.D. Candidate Qualifications ...... III Abstract ...... V Acronyms ...... XI List of Tables ...... XV List of Figures ...... XXI Contents ...... XXIX Chapter 1: Photovoltaics Market and Solar Cell Technologies ...... - 1 - 1.1. Photovoltaics Market Outlook 2018 ...... - 1 - 1.2. A Brief Overview of Solar Cell Technologies ...... - 4 - 1.2.1. Crystalline Silicon Solar Cells ...... - 8 - 1.2.2. Thin-Film Solar Cells ...... - 9 - 1.2.3. Single-Junction Gallium Arsenide and III-V Multi-Junction Solar Cells ...... - 11 - 1.2.4. Emerging Solar Cell Technologies...... - 12 - 1.2.5. Outlook ...... - 16 - Chapter 2: Fundamentals of Dye-Sensitized Solar Cells ...... - 17 - 2.1. Historical background ...... - 17 - 2.2. Dye-Sensitized Solar Cells Design ...... - 18 - 2.3. Dye-Sensitized Solar Cells Operating Principle ...... - 20 - 2.4. Dye-Sensitized Solar Cells Equivalent Circuit Analysis ...... - 24 - 2.4.1. Transmission Line Model ...... - 24 - 2.4.2. One-Diode Model ...... - 25 - Chapter 3: Review of Recent Progress in Dye-Sensitized Solar Cells ...... - 27 - 3.1. Efficiency ...... - 28 - 3.1.1. Dye-Sensitized Working Electrode ...... - 29 - 3.1.2. Electrolyte ...... - 34 - 3.1.3. Counter Electrode ...... - 35 - 3.2. Stability ...... - 36 - 3.2.1. Dye-Sensitized Working Electrode ...... - 37 - 3.2.2. Electrolyte ...... - 39 - 3.2.3. Counter Electrode ...... - 45 - XXIX

3.3. Cost ...... - 46 - 3.3.1. Dye-Sensitized Working Electrode ...... - 48 - 3.3.2. Electrolyte ...... - 50 - 3.3.3. Counter Electrode ...... - 52 - 3.4. Application Range ...... - 59 - 3.4.1. Dye-Sensitized Working Electrode ...... - 60 - 3.4.2. Electrolyte ...... - 66 - 3.4.3. Counter Electrode ...... - 67 - 3.5. Outlook ─ Motivation and Research Objectives ...... - 70 - Chapter 4: Experimental...... - 73 - 4.1. Materials ...... - 73 - 4.2. Fabrication of Conventional Dye-Sensitized Solar Cells ...... - 73 - 4.3. Methodology Towards Higher Efficiency Dye-Sensitized Solar Cells ...... - 76 - 4.4. Methodology Towards Higher Stability Dye-Sensitized Solar Cells ...... - 82 - 4.5. Methodology Towards Lower Cost Dye-Sensitized Solar Cells ...... - 85 - 4.6. Methodology Towards Wider Application Range Dye-Sensitized Solar Cells ...... - 86 - 4.7. Solar Cell Materials Characterization ...... - 89 - 4.8. Solar Cells Characterization ...... - 94 - 4.9. Solar Cells Accelerating Ageing ...... - 96 - 4.10. Mechanical, Dynamic Mechanical, and Viscoelastic Characterization of Dummy Cells Under Three-Point Bending ...... - 98 - Chapter 5: Results and Discussion ...... - 101 - 5.1. Conventional Dye-Sensitized Solar Cells ...... - 101 - 5.2. Towards Higher Efficiency Dye-Sensitized Solar Cells ...... - 107 - 5.2.1. Optimization of Dye-Sensitized Solar Cells Photo-anode Characteristics Towards an Impressive Energy Conversion Efficiency ...... - 107 - 5.3. Towards Higher Stability Dye-Sensitized Solar Cells ...... - 133 - 5.3.1. Development of High-Efficiency Quasi-Solid State Dye-Sensitized Solar Cells ...... - 133 - 5.3.2. Evaluation and Prediction of Dye-Sensitized Solar Cells Stability Under Different Accelerating Ageing Conditions ...... - 166 - 5.4. Towards Lower Cost Dye-Sensitized Solar Cells ...... - 174 - 5.5. Towards Wider Application Range Dye-Sensitized Solar Cells...... - 177 - 5.5.1. Development of High-Efficiency Back-Side Illuminated Dye-Sensitized Solar Cells - 177 - 5.5.2. Evaluation of the Limiting Factors Affecting Large-Sized Flexible Platinum-Free Dye- Sensitized Solar Cells Performance ...... - 192 - 5.5.3. Mechanical, Dynamic Mechanical, and Viscoelastic Behavior of Flexible Quasi-Solid State Dye-Sensitized Solar Cells Under Three-Point Bending ...... - 201 - Chapter 6: Conclusions and Future Work ...... - 213 - References ...... - 217 - XXX

Chapter 1 Photovoltaics Market and Solar Cell Technologies

Chapter 1: Photovoltaics Market and Solar Cell Technologies

In the modern world of technological advancements, energy has become one of the basic needs of life. Energy consumption is rising tremendously year by year, due to the rapid development of the global economy, in combination with the growing world population [1]. It is remarkable that the worldwide power consumption is expected to double in the next three decades. At the moment, fossil fuels are playing a leading role in meeting the energy demands. However, the limited availability of primary exhaustible energy sources and the long-term adverse effects on the environment due to their exploitation reflects the urgency to develop new strategies to effectively utilize other primary energy sources, the renewable ones. In the last 20 years, a lot of work has been done in increasing the amount of energy coming from clean sources, not just in terms of research and development, but also in terms of legislation and politics. The results are great since renewables contributed 18.2% to the humans’ global energy consumption, based on the “Renewable Energy Policy Network for the 21st Century” 2018 report [2]. Amongst them, solar energy is the most abundant source of energy (1.2 x 105 TW reaching the surface of the Earth) and is expected to play a vital role in the future [3]. 1.1. Photovoltaics Market Outlook 2018 Photovoltaics (PVs) are undoubtedly one of the most important sources of sustainable and renewable energy, meeting the ever-growing demand of humanity for clean energy production. PVs market has undergone dramatic development in recent years, while the worldwide growth of PVs is exponential between 1992 to 2018 [4]. Globally, PVs market expansion is mainly due to the increasing competitiveness of solar PVs, combined with the rising demand for electricity in the developing countries, as well as due to the increasing awareness of solar PVs potential to alleviate pollution and reduce CO2 emissions [5].

Figure 1-1: Solar photovoltaics global capacity and annual additions, 2007-2017 [2]. 2017 was a landmark year for solar PVs since the world added more capacity from solar PVs than from any other type of power generating technology. Here, it must be pointed out that, in the last

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Chapter 1 Photovoltaics Market and Solar Cell Technologies

year, more solar PVs capacity was installed than the net capacity additions of fossil fuels and nuclear power combined. During 2017, almost 100 GW of solar PVs capacity was added worldwide (on- and off-grid), increasing the total capacity by nearly one-third, for a cumulative total of approximately 402 GW (see Figure 1-1) [2,6]. On average, the equivalent of more than 40000 solar panels was installed each hour of the year. In August 2017, GTM Research predicted that by 2022, the cumulative installed global PVs capacity is likely to reach 871 GW [7]. By 2050, solar power is anticipated to become the world's largest source of electricity.

Figure 1-2: Solar photovoltaics global capacity, by country or region, 2007-2017 [2]. The significant market growth in 2017 relative to 2016 was primarily due to China, where new installations were up more than 50%. China surpassed all expectations, adding more solar PVs capacity (nearly 53 GW) than was added worldwide in 2015 (51 GW) [2,6]. India’s market also doubled, adding a record 9.1 GW in 2017, more than double the 4 GW installed in 2016. For the fifth year running, Asia eclipsed all other regions, accounting for 75% of global additions. Today, the top solar PVs markets are China, the United States, India, Japan, and Turkey, accounting for about 84% of the newly installed capacity. Concerning the cumulative capacity, the top countries are China, the United States, Japan, Germany, and Italy, with India not far behind (see Figure 1-2) [2,4]. Despite the heavy concentration in a handful of countries, new markets are emerging and countries on all continents have begun to contribute significantly to global growth. By the end of 2017, every continent had installed at least 1 GW and at least 29 countries had 1 GW or more of capacity. At least 22 countries, including China and India, had enough solar PVs capacity to meet 2% or more of their total annual electricity demand. In the year 2017, the leaders of solar PVs capacity per inhabitant were Germany, Japan, Belgium, Italy, and Australia [2]. In European Union countries, the installed solar PVs capacity for 2017 was 6 GW, for a year-end total of nearly 108 GW. Germany confirmed its leading position among the continent and installed 1.8 GW in 2017. The United Kingdom followed with 950 MW, France with 875 MW, and the Netherlands, which continued to progress, installed 853 MW. Some medium-size European markets remained stable, such as Switzerland (260 MW), Austria (153 MW), Hungary (136 MW), and Sweden (93 MW). Other European markets experienced a growth once again, such as Belgium and Spain which installed 284 MW 147 MW, respectively. Poland, Denmark, Finland, and Norway installed 77 MW, 60 MW, 23 MW, and 18 MW. Portugal remained stable and installed 57 MW, respectively [6]. The market is still in transition but is progressing towards reduced dependence on traditional feed-in tariff-based government support.

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Chapter 1 Photovoltaics Market and Solar Cell Technologies

Several countries, including Italy, Greece, and Germany already meet significant portions of their electricity demands with solar PVs. Specifically, solar PVs contributed 8.7%, 7.6%, and 7% to the respective annual domestic consumption in Italy, Greece, and Germany [2]. The year 2017 was also characterized by a number of prominent themes, including record-low auction prices driven by intense competition, thinning margins for producers and developers alike, continual consolidation in the industry among manufacturers and developers, a host of trade-related disputes, and continuing advances in technology. The efforts to advance recycling processes of PVs continued during 2017, although in time there is a relatively small demand for recycling of waste and solar panels (at end-of-life, damaged or defective panels). Besides the environmental benefits due to recycling potential, the process can yield materials either to be sold in global commodity markets or to be used for the production of new solar panels [2]. Innovations and advances continued this year in the manufacturing and performance of solar PVs. They are driven largely by the rapid price reductions as well as the growing customer demands. Throughout 2017, new record solar cell and module efficiencies were achieved, while the advanced module technologies continued to achieve further improvements in efficiency and stability. Module prices continued to fall in 2017, but at a slower rate than in 2016, with the average global prices down an estimated 6% for the year, to US $0.39 per watt. Based on projects completed during this year, the global weighted average levelized cost of energy from large-scale solar PVs plants was US $100 per MWh, down 73% since 2010 [2]. At this level, solar PVs have reached a relatively high level of technological maturity and are competing head-to-head with fossil fuel power sources in many locations and without financial support.

Figure 1-3: Photovoltaics global shipment share by technology, 2013-2017 [8]. Today, the single- and multi-crystalline silicon solar cells dominate the present PVs market and occupy more than 85% (see Figure 1-3), while regard mainly rooftop and ground-mounted applications. Advancements are continuously being made and the efficiency of the most widely used solar cells using crystalline silicon is now higher than 20%. Commercial efficiencies of more than 30% are now routinely achieved using high-efficiency multi-junction solar cells. However, due to cost consideration, these solar cells are primarily being used in space. Concentrated PVs using high- efficiency multi-junction solar cells are designed for terrestrial applications. Thin-film solar cells is a relatively new technology, which now occupies about 10% of PVs market. In addition, strong efforts are underway to develop new solar cell technologies, to improve the performance of existing ones and to develop new applications [9].

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1.2. A Brief Overview of Solar Cell Technologies The PVs field begins in 1953, when Bell Laboratories produced the first silicon solar cell, opening the door to the humankind dream of accessing solar energy for civilian purposes [10]. Since then, there is a widespread diffusion of new solar cell technologies, aiming at a high solar-to-electricity conversion efficiency with low-cost. The classification of the main solar cell technologies based on their primary active material is shown in Figure 1-4. The current record efficiencies for the main solar cell technologies developed are presented in Figure 1-5.

Figure 1-4: Classification of solar cell technologies based on their primary active material [11].

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Figure 1-5: Solar cells efficiency records [12]. The most widely used solar cell technologies classification today focuses on two metrics, PV efficiency and cost, that delineate three distinct PVs generations [13,14]. The first PVs generation consists of wafer-based solar cells, and concern solar cell technologies characterized by high- efficiency and high-costs. The second PVs generation consists of thin-film solar cells, and concern solar cell technologies characterized by moderate-efficiency and low-costs. The third PVs generation includes novel thin-film devices, along with a variety of exotic concepts, including spectral-splitting devices, hot-carrier collection, carrier multiplication, and thermophotovoltaics, and concern solar cell technologies developed for achieving high-efficiency with low-costs. The three generations are commonly represented as shaded regions on a plot of efficiency versus area cost. Figure 1-6 shows these regions as originally defined before 2000. All technologies move toward the upper-left corner with time as efficiencies rise and costs fall. Nearly all current first and second PVs generation technologies appear close to the zone designated “II”. The average commercial module prices for both first and second PVs generation tend to cluster along a single $ per W line in any given year, likely due to competitive market dynamics. Furthermore, no third-generation technology has, until now, reached the zone marked “III” [15].

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Figure 1-6: Classification of solar cell technologies based on three generations [16]. An alternative approach to solar cell technologies classification based on materials complexity is shown in Figure 1-7. Material complexity can be defined roughly as the number of atoms in a unit cell, molecule, or another repeating unit. In this framework, all solar cell technologies fall into a spectrum from elemental (lowest) to nanomaterial (highest) complexity. Here, it must be noted that material complexity is not equivalent to processing complexity. On the other hand, higher material complexity is not always “better”. Technological maturity and solar cells efficiency tend to vary inversely with complexity. Increased material complexity, however, does give rise to several novel attributes of value, such as reduced materials usage, improved defect tolerance, versatile form factors, visible transparency, etc [15].

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Figure 1-7: Alternative solar cell technologies classification scheme based on materials complexity [15].

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1.2.1. Crystalline Silicon Solar Cells Crystalline silicon (c-Si) solar cells have been the workhorse of the PVs industry for many years and will continue to be the leader until a more efficient and cost-effective solar cell technology is developed [17]. Modules based on this technology have a long history of rugged reliability, with guarantees lasting 20 to 25 years that are exceptional among manufactured products [5]. However, a key disadvantage is that c-Si is a poor absorber of light, due to its indirect energy bandgap of roughly 1.1 eV at room temperature, encouraging the use of fairly thick, rigid, and brittle wafers to absorb most of the incident light, in absence of advanced light trapping mechanisms. This drawback culminates in a huge capital outlay, low power-to-weight ratios, and limitations in terms of flexibility and design of modules [11,15]. The theoretical energy conversion efficiency limit of these devices is about 30% [18]. The main technological problems of c-Si include the stringent material purity requirements in combination with the high material use, the batch-based solar cell production, the module form factor restrictions, and the module integration processes with relatively low throughput [15]. Current research areas are targeted at manufacturing wafer-based solar cells at lower costs and reduced complexity, increased modular energy conversion efficiencies, reduced quantity of silicon used per watt, and reduced reliance on silver for contact metallization [11]. Novel technologies for the fabrication of high-efficiency c-Si solar cells are introduced, such as passivated emitter rear contact (PERC) cells, interdigitated back contact (IBC) cells, heterojunction with intrinsic thin-layer (HIT) cells, and heterojunction cells with interdigitated back contacts (HBC) (see Figure 1-8). According to the “International Technology Roadmap for Photovoltaic” report for 2018, back surface field (BSF) cells will still dominate the market in the next few years, but PERC, IBC, and HIT cells will gain significant market share over BSF cells, while IBC and HIT cells will become more important [19].

Figure 1-8: The evolution of energy conversion efficiencies of crystalline silicon solar cells in the years 2011-2017 [20].

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Wafer-based c-Si solar cells are produced from slices of either single-crystalline silicon (sc-Si) or multi-crystalline silicon (mc-Si). The sc-Si solar cells are the most established solar cell technology to date. Cell and module fabrication technologies are well developed and reliable [21]. The sc-Si solar cells are typically made using the Czochralski process, the float-zone, or Bridgman techniques, adapted for the production of wafers for integrated circuits. They are endowed with higher crystal quality compared to mc-Si solar cells, which increases charge extraction and power conversion efficiencies, but also increases the need for more expensive wafers, by 20 to 30% compared to mc-Si ones [11]. As sc-Si solar cells are made from single silicon crystals, the processes of making them are complex and costly. Today, the efficiency record of sc-Si solar cells reaches 26.6%, achieved by a back-contact heterojunction design [22]. Additional advantages of sc-Si solar cells compared to mc- Si ones, except for their higher efficiency, are their higher longevity and greater heat resistance [21]. Talking about mc-Si solar cells, they consist of several randomly oriented crystals or grains, which introduce boundaries that hinder charge extraction and electron flow, stimulating them to recombine with holes, leading to a decreased solar cell power output. This makes mc-Si solar cells less efficient under full sun, but at the same time less expensive [11]. Furthermore, their operation in lower light conditions is in many cases better than the sc-Si ones [21]. Today, the efficiency record of these devices reaches 22%, achieved by an mc-Si solar cell with diffused boron front emitter and full-area passivating rear contact [23]. 1.2.2. Thin-Film Solar Cells While c-Si solar cells currently dominate the global PVs market, alternative technologies appear, opening up the way to produce cheaper PV electricity. One of the most promising ways to reduce the cost of PVs is the use of thin-film designs [24]. Combining this fact with a high-efficiency potential makes thin-film solar cells a growing research area. Today, solar cells based on thin semiconducting films occupy about 10% of global PVs module production capacity [19]. Leading commercialized thin-film solar cell technologies include cadmium telluride (CdTe), copper indium gallium (di)selenide (CIGS), and hydrogenated amorphous silicon (a-Si:H) (see Table 1-1). These materials absorb light at a rate that is 10 to 100 times more efficient compared to silicon-based solar cells, allowing their usage in film designs, with thicknesses on the order of a few microns. A key advantage of these technologies is attributed to their low raw material usage with less complex manufacturing procedures. For instance, nowadays factories can fabricate thin-film modules in a greatly streamlined and automated fashion, yielding to modules with low-per-watt costs. Their handling is also easier, more flexible, and less susceptible to damage compared to their c-Si rivals [11]. Their main disadvantage pertains to their low average energy conversion efficiency compared to c-Si solar cells, leading to increased area-dependent balance-of-system costs. Other disadvantages include their sensitivity to moisture and oxygen, which makes encapsulation more expensive to guarantee long- term reliability. Additionally, their reliance on rare elements such as tellurium and indium, and recycling of regulated toxic elements like cadmium limit their potential for large-scale production and application. Current innovation and development opportunities in thin-film technology include module efficiency improvements, materials optimization, and cell architecture improvements. Overall, the reduction of the reliance on rare elements through the development of new materials with similar ease of processing is pertinent [11].

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Table 1-1: Comparison of CdTe, CIGs, and a-Si properties.

CdTe CIGS a-Si Bandgap (eV) ≈ 1.5 (direct) 1.0–1.7 (direct) ≈ 1.7 (direct) Sufficient thickness (μm) 3–5 1–2 1 Temperature coefficient (%/oC) -0.21 -0.36 -0.13 (efficiency, from 25 to 65oC) Toxicity cadmium none none

Among thin-film solar cell technologies, CdTe represents the largest thin-film solar cells production in the world, with a global PVs market share of 3.1% and a thin-film solar cells market share of 52% in 2016 [25]. The main advantage of CdTe for solar cells application is its bandgap, which is in the perfect range for high-efficiency single-junction cells. The bandgap of CdTe is about 1.5 eV for its single crystal form and 1.45 eV for its polycrystalline form. In principle, CdTe could deliver around 1 V for open-circuit voltage and 30 mA/cm2 for short-circuit current density, leading to the fabrication of solar cells with a theoretical maximum energy conversion efficiency over 27% [26]. Efficiencies of 21% and 17.5% for lab cells and module, respectively, have been reported and are among the uppermost for thin-film solar cells. CdTe technology utilizes high-throughput fabrication techniques, requiring a high process temperature, in the region of 600°C. However, they offer the lowest module costs as compared to any solar cell technology that has been commercialized today. Environmental issues such as the toxicity of cadmium and the scarcity of tellurium have prompted the research on alternative material systems, that utilize non-toxic and abundant elements, with similar ease of fabrication [11]. On the other hand, CIGS technology held a global PVs market share of 1.3% and a thin-film solar cells market share of 22% in 2016 [25]. Like CdTe, CIGS films can be deposited by a variety of solution- and vapor-phase techniques on flexible metal- or polyimide-based substrates, favorable for building integration and other un-conventional PVs applications. CIGS solar cells exhibit also high radiation resistance, a necessary property for space applications [15]. Record efficiencies stand at 21.7% for cells and 18.7% for modules [27,28]. Major technological challenges include the difficulty in controlling film stoichiometry and properties, the low open-circuit voltage due to defects in materials, the narrow understanding of the effects of grain boundaries and the processing of higher- bandgap alloys to allow for the fabrication of multi-junction devices [11]. Finally, a-Si:H has been used for decades, doped and as intrinsic absorber layers, in thin-film silicon solar cells. Whereas their efficiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to the better understanding of the device interfaces, allowing for their specific engineering [29]. A 300 nm film of a-Si:H can absorb about 90% of above-bandgap photons in a single pass, enabling the fabrication of lightweight and flexible solar cells [15]. Amorphous silicon solar cell technologies held a global market share of 0.5% and a thin-film solar cells market share of 8.33% in 2016 [25]. An a-Si:H cell can be combined with cells based on nanocrystalline silicon or amorphous silicon–germanium alloys to form multi-junction solar cells, without lattice-matching requirements [15]. Today, the record efficiencies for the aforementioned solar cell technology exceed 12% [27].

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1.2.3. Single-Junction Gallium Arsenide and III-V Multi-Junction Solar Cells Although the cost of silicon-based solar cells has been reduced considerably, stagnation in their energy conversion efficiency prompted scientists and engineers to ponder over alternative materials usage, which would lead to high solar-to-electricity conversion. One of the alternatives for the development of high-efficiency solar cells is gallium arsenide (GaAs). GaAs is the almost perfectly suited material for solar energy conversion, with high optical absorption coefficients, well matched to the solar spectrum bandgap, and very low non-radiative energy loss [15]. These features make GaAs an excellent candidate material for fabrication of solar cells with efficiencies much higher than those of silicon-based solar cells. Today, GaAs solar cells hold the world's record efficiency for single-junction solar cells, reaching 28.8% for lab cells and 24.1% for modules. A key disadvantage is the high cost of the material in terms of producing epitaxial layers or device quality substrates as compared to the crushing commercial edge associated with silicon. This is largely due to factors such as imperfections of its crystals and undesirable impurities which reduce device efficiencies, rendering low-cost deposition routes impossible. In fact, it can cost about €3400 to fabricate a wafer of GaAs. This limits the large-scale application of GaAs solar cells, restricting their use to niche applications (e.g. space communications where higher efficiencies, better radiation resistance, and improved power to weight ratios are required), in which their distinct capabilities justify their exorbitant cost. Cost-effective production processes for GaAs solar cells, which involve the reuse of GaAs wafers, have been reported, but have not been demonstrated in high-volume production [11].

Figure 1-9: (a) Physical schematic of monolithic triple junction n-on-p solar cell deposited epitaxially upon a substrate, (b) the electrical circuit equivalent diagram showing top, middle, and bottom junction diodes, and interconnecting upper and lower tunnel junctions [30]. In order to increase the efficiency of GaAs solar cells, multi-junction (MJ) designs were developed, firstly introduced by the Research Triangle Institute and by Varian Research Center in the late-1970s to mid-1980s [30]. The first MJ solar cells were dual-junction devices, formed from an aluminum gallium arsenide junction stacked or grown on top of a GaAs junction, and interconnected by a semiconductor tunnel junction. Today, MJ solar cells built from III–V semiconductors (concentrator or non-concentrator) are the best-performing solar cells, with a record solar-to-electricity conversion

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efficiency of 46% [27]. Usually, III-V MJ solar cells use a stack of two or more single-junction cells, with different bandgaps to absorb light efficiently across the solar spectrum, by minimizing thermalization losses (see Figure 1-9). Semiconducting compounds of group III elements (aluminum, gallium, indium) and group V elements (nitrogen, phosphorus, arsenic, antimony) can form high- quality crystalline films, with variable bandgaps [15]. The bandgap values chosen for the MJ depend on the number of junctions used. Research has shown that, theoretically, it is possible to appreciably increase the energy conversion efficiency of III-V single-junction devices by increasing the number of junctions, with a range of dissimilar bandgap values matched to the range of photon energies presented in the solar spectrum [11]. Maximum theoretical efficiencies of up to 72% for MJ solar cells are reported [31]. The main disadvantage of III-V MJ solar cells, as with single-junction III-V devices, is their expensive fabrication, due to their complex manufacturing processes and exorbitant material costs. This makes them prohibitively expensive for large-scale terrestrial applications. As such, their use is confined to the demanding environment of space power generation, due to their high radiation resistance, high efficiency, and low-temperature sensitivity, mitigating against the exorbitant cost of materials. Key challenges for developing III-V MJ technologies include improving the long-term reliability and the uniformity in large areas, reduction in materials use, and optimization of cell architecture for different operating conditions [11]. Today, many research efforts are also placed to design concentrator systems using MJ solar cells. These devices promise to deliver electrical power at a lower cost than will be possible with traditional flat-plate systems. To realize this promise, the concentrator PVs are designed to extract maximum performance from the expensive MJ solar cells, while minimizing system costs associated with the concentrating optics, temperature control, and the remaining balance-of-system costs [32]. 1.2.4. Emerging Solar Cell Technologies Moving beyond conventional and niche PVs applications and successfully competing with the incumbent solar cell technologies requires emerging PVs approaches that have a better cost-to- performance ratio, increased stability, and flexible design. Shedding light on the future of PVs, dye- sensitized solar cells (DSSCs), organic photovoltaics (OPVs), copper zinc tin sulphide (CZTS) solar cells, quantum dot (QD) solar cells, and perovskite solar cells (PSCs) are the main emerging solar cell technologies (see Figure 1-10) [11]. These technologies have emerged because of the high level of research and development efforts in materials science and devices engineering. In most of these cases, they utilize nanotechnology to achieve the desired electrical and optical materials characteristics. These technologies are still in the research and development level, and early commercialization stages. They offer promising device-level characteristics, which pave the way for a number of alternative and pioneer PVs applications, such as building integration, indoor applications, automotive, wearable electronics, etc [33].

Figure 1-10: Top emerging solar cell technologies.

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Of all the emerging solar cells options, DSSCs have gained thriving interest and are among the most mature nanomaterial-based solar cell technologies. DSSCs were co-invented by Brian O'Regan and Michael Grätzel in 1988. These devices gained increased interest in 1991, when the light-to- electric conversion efficiency of DSSCs reached 7.9% in simulated solar light and 12% in diffuse daylight [34]. Their satisfactory current density and stability, as well as their low manufacturing cost, made practical applications seem feasible. Today, DSSCs have achieved efficiencies of up to 14% (11.9%, 10.7%, and 8.8% certified for cell, mini-module, and sub-module, respectively) [27,35]. Dozens of companies and industrial research laboratories are now involved in the development, commercialization, and manufacturing of DSSCs technology and products, mostly in Europe, Asia, and Australia. Many products have been demonstrated, as shown in Figure 1-11. In 2009, G24 Innovations was the first to commercialize a DSSC product. Its flexible modules are integrated into items like bags, backpacks, and wireless keyboards for portable recharging of consumer electronics. 3GSolar, Israel, is focused on DSSC modules for off-grid rural applications to provide power for lighting and irrigation pumps. While G24 Innovations and 3GSolar are exclusively working on DSSCs, many large companies also have branches devoted to DSSCs, for both large area panels and indoor electronics. These include Aisin Seiki (in collaboration with Toyota Central R&D Laboratories), Sharp and Sony in Japan [36]. Some of the DSSCs technology benefits are their versatility and low-cost manufacturing, their low-cost, non-toxic, and readily available materials, their high-efficiency in diffuse light conditions, the flexibility of their design, and their appealing aesthetics [37]. Major challenges with DSSCs technology include their moderate energy conversion efficiency under full sun, long-term instability issues, and the rigid and liquid state materials in their conventional structure [38].

Figure 1-11: Some of the applications of dye-sensitized solar cells.

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On the other hand, OPVs have become a subject of attention over the last decades, due to their prospects for low-cost solar-to-electricity conversion, flexible design, and ease of up-scaling to an industrial level [39]. OPVs use organic small molecules or polymers to absorb light. These materials consist mostly of Earth-abundant elements and can be assembled into thin-films by inexpensive fabrication methods, such as roll-to-roll processing and inkjet printing. Due to the possibility of using various absorbers to create colored or transparent organic PV devices, they are appealing for building- integrated applications [11]. Lab efficiencies of 11.2% have been reported for cells, while for mini- modules their efficiency reaches 9.7% [27]. Major setbacks of OPVs technology include ineffective transport of excited electron−hole pairs and charge carriers, poor long-term stability under sunlight, and fairly low-efficiency limits [11]. Today, a lot of research is taking place in the direction of increasing OPVs efficiency by developing novel materials and solar cell designs. Some of the most promising approaches are the development of advanced polymers which combine two units in their structure, one that acts as an acceptor and one that acts as a donor, by forming donor-acceptor copolymers, while organic MJ devices are also under development, which are much easier to fabricate than conventional III-V MJs because of their high defect tolerance and ease of deposition [15,40]. The challenge of finding alternative solar cell materials based on Earth-abundant and non-toxic elements prompted the exploration and development of CZTS solar cells. CZTS is a quaternary compound semiconductor with promising optical absorption properties, direct energy bandgap of roughly 1.5 eV, and large absorption coefficient in the order of 104 cm−1, enabling the absorbance of most of the visible solar spectrum. CZTS film contains neither rare metals nor non-toxic materials, and can be combined with cadmium-free buffer layers to produce solar cells that are completely non- toxic [11]. Several methods for fabrication of CZTS solar cells have been reported in the scientific literature [41]. Every year scanty research papers are published on the CZTS material, but several techniques are introduced to synthesize CZTS thin-films, such as chemical vapor deposition, chemical spray pyrolysis, electrochemical deposition, hydrothermal, spin-coating, hot injection, solution route, sol-gel, sputtering, co-evaporation, pulsed laser deposition, vacuum thermal evaporation, etc. Each technique has a specialty in grain size growth, adhesion, porosity, stoichiometry, thickness sustainability, purity, etc [42]. The current highest energy conversion efficiency for CZTS solar cell technology is 12.6%. A major technological challenge with CZTS solar cells involves the management of cation disorder defects, a phenomenon caused by uncontrolled inter-substitution of zinc and copper cations, resulting into point defects that impede the extraction of charges and decrease the open-circuit voltage [11]. Semiconductor QDs have also drawn considerable interest because of their optoelectronic advantages [43]. A QD is a nanocrystal produced from a semiconductor material which is so small that the laws of quantum mechanics have to be taken into account [11]. Their PV applications, using self-assembled quantum dots and colloidal quantum dots, have the potential to enhance the photo- generation of carriers and subsequently the energy conversion efficiency of solar cells [43]. They are used as absorbing PV materials in solar cells, given their advantage of possessing their bandgap as well as their optical and electrical characteristics that can be tuned simply by altering the size of the nanoparticles. This allows them to be easily fabricated to absorb different parts of the solar spectrum, making room for efficient harvesting of near-infrared photons [11]. Today, the record efficiency of QD solar cells is 16.6% [12]. Major challenges of this technology include the incomplete understanding of QDs surface chemistry, the low open-circuit voltages, and the low charge carrier mobility [15].

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Figure 1-12: (a) Perovskite crystal, (b) perovskite solar cell. Finally, PSCs are the latest type of solar cells and have rapidly become one of the most promising emerging solar cell technologies [15]. Solar-to-electricity conversion efficiencies of devices using perovskite materials have increased from 3.8% in 2009 to 23.3% in late 2018 in single-junction architectures, and in silicon-based tandem cells up 27.3%, exceeding the maximum efficiency achieved in single-junction silicon solar cells. PSCs are therefore the fastest-advancing solar cell technology to date [44]. The remarkable performance of PSCs has thrust these materials to the forefront of cutting-edge optoelectronics research and brought their intriguing underlying properties under deeper scrutiny from materials scientists, chemists, physicists, and engineers [45]. With the potential of achieving even higher efficiencies, combined with their very low production costs, PSCs have become commercially attractive, with start-up companies already promising modules on the market [44]. Their design and operation are largely based on DSSC technology. The term “perovskite” refers to the ABX3 crystal structure, and the most widely investigated perovskite for solar cells is the hybrid organic-inorganic lead halide CH3NH3-Pb(I, Cl, Br)3 (see Figure 1-12). Optical bandgaps can be tuned from 1.25 to 3.0 eV by cation (A, B) or anion (X) substitution. Polycrystalline films can easily be prepared at low temperatures by simple techniques [15]. The main device architectures are based on mesoscopic and planar structures, and can exist in either conventional or inverted configurations [46,47]. In the case of mesoscopic architectures, the perovskite can either be introduced as a thin layer that just adequately covers the oxide scaffold, with the pores in the scaffold infiltrated with the charge transporting material, or the perovskite can form an overlayer on top of the completely infiltrated oxide scaffold. In the case of the planar architecture, there is a lack of mesoporous metal-oxide scaffolds, while the perovskite layer is stacked with an n- type electron selective layer and a p-type hole selective layer. All-perovskite tandems can also be developed, with three main architectures to consider, which are the mechanical stacked, the monolithically integrated, and the spectrally split [48]. Key technological advantages of perovskite materials include low manufacturing cost, long carrier diffusion lengths, low recombination losses, and bandgap tunability. From the early stages of development of PSCs, high open-circuit voltages (>1.1 V) have been achieved, typically being the most difficult PV performance parameter to improve. Key challenges include refined control of film morphology and material properties, unproven cell stability, high sensitivity to moisture, and the use of toxic lead [15]. The potentials for further improvement of PSCs are still great. There is a large library of pure and mixed compositions of organic and inorganic cations, metals, and halides that are still unexplored for solar cell applications. For the next years, it is believed that a major research direction will concentrate on

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understanding the link between atomic composition and material/film/device properties and thus in exploring new perovskite formulations to tailor the materials to the specific PVs applications, and beyond [49]. 1.2.5. Outlook Predicting the future development of any solar cell technology is by nature fraught with uncertainty. While c-Si solar cells dominate the PVs market today, alternative technologies are evolving rapidly. The solar cell of the future may be a refined version of current commercial solar cells or an entirely new technology. Faced with uncertain technological changes and uncertain economic pressures, it is better to avoid betting on any particular solar cell technology. Instead, it is better to view all technologies through the objective lens of application-driven performance metrics. These metrics guide a solar cell designer toward technical themes that mainly include increased efficiency and stability, reduced manufacturing complexity and cost, as well as a wide application range. Taking into account the aforementioned, the author of the present Ph.D. dissertation chose DSSCs technology to investigate, develop, and optimize in 2012. The reasons are both in accordance with the global demand for low-cost, efficient, and clean energy production, and technological. DSSCs technology is one of the most exciting and blooming solar cell technologies today, gaining recently considerable attention both in the academic and industrial communities. The technology, which is already entering the worldwide markets, is being constantly improved and optimized at a laboratory level, by conventional means, while there is still great room for improvement. Of great importance is that the operating principle of DSSCs gives the opportunity to researchers for designing, fabrication, exploitation, and optimization of each component both individually and as a system. DSSCs technology uses also, in most cases, non-toxic and widely available materials. Furthermore, and perhaps the key reason for a Mechanical Engineer, DSSCs design favors the investigation of these systems from a mechanical point of view since their structure is similar to sandwich-structured composite materials, aiming to new and pioneer applications. Finally, the gigantic amount of data, models, and theories that are already available in the literature give the opportunity to new members of the PVs community (as in the case of our group) to join the research and try their new ideas for further development of these devices.

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Chapter 2 Fundamentals of Dye-Sensitized Solar Cells

Chapter 2: Fundamentals of Dye-Sensitized Solar Cells 2.1. Historical background It has been 27 years since the publication of the pioneer paper on DSSCs technology by Brian O’Regan and Michael Grätzel, however, the idea of photo-electrochemical cells is much older and dates back to 19th century [34]. Becquerel laid the foundations of the field of the photo- electrochemistry way back in 1839, when he observed that measurable current passes between two platinum electrodes in presence of sunlight when the electrodes are immersed in an electrolyte containing metal halide salts. Soon after this observation, several attempts have been made to capture the energy of such electron-transfer processes, and this was the beginning of the concept of solar-to- electricity conversion by PV devices [50]. The sensitization of bulk semiconductor electrodes was an extension of the dye sensitization concept introduced by Vogel in 1873, a concept with a pivotal contribution to the progress of photography [51]. The history of photography parallels that of the photo-electric effect. In 1887, the dye sensitization idea was carried over from photography to photo-electric effect by Moser, using the dye erythrosine on silver halide electrodes [52]. Since then, numerous scientists worked in the field of sensitized semiconductors, in order to provide a better understanding of the processes taking place in such systems. Mechanistic details were established through extensive studies of the charge- injection processes under different dye conditions. Photo-sensitization, in general, can occur via transfer of the excitation energy of a sensitizer to a suitable state/energy level of an acceptor or by electron transfer. In the case of dye-sensitized semiconductors, it was found that oxidation of dye takes place through the transfer of an electron from the dye molecule excited energy level to the conduction band of the semiconductor [53]. Tributsch, Genscher, and Calvin pioneered the field, when they examined photo-sensitization of ZnO using chlorophyll derivatives as a model system for the primary process in photosynthesis. As part of his doctoral thesis, Spitler later studied the excited-state charge injection of rose bengal onto a single crystal TiO2 (rutile) electrode. A quantum efficiency of 0.004% was measured for the electron injection from the excited rose bengal dye to the conduction band of the semiconductor. Parkinson extended these studies to oxide surfaces of low-index faces of anatase and rutile forms of TiO2 by covalently attaching dyes to their surfaces [53]. In the following years, a lot of fundamental research was done in this field, but the efficiency of the photo-electrochemical cells was still poor. One of the main problems was that a monolayer of dye molecules onto a flat surface can only absorb up to 1% of the incident light. In general, attempts to harvest more light by using multilayers of dyes were unsuccessful. Increasing the roughness of the semiconductor surface, so that a larger number of dyes could be adsorbed directly to the semiconductor surface and simultaneously be in direct contact with the redox electrolyte, was considered to potentially lead to a dramatical increase in the efficiency of photo-electrochemical cells [37]. The first embodiment of the modern-day DSSCs dates back to late 1980s [54]. However, not until the fundamental work of Brian O’Regan and Michael Grätzel in 1991 was it proven that DSSCs can be a feasible alternative energy source [34]. Today, the highest efficiency reported for DSSCs technology is 14%, making them one of the most exciting PV technologies to investigate, develop, and optimize, while the prospects for further improvements are still considered to be great [35].

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2.2. Dye-Sensitized Solar Cells Design Today’s conventional DSSCs are made in a sandwich configuration, where two pieces of surface conductive glass are used as skin materials, while a hybrid material of 25–50 μm thickness is the core. A schematic representation of a conventional DSSC is shown in Figure 2-1. The main components of the core are the TiO2 nanostructured scaffold sensitized with a ruthenium-based dye and deposited on one of the surface conductive glasses (dye-sensitized working electrode), a liquid state electrolyte with iodide/triiodide redox mediator, and a thin nanostructured Pt film deposited on the other surface conductive glass (counter electrode).

Figure 2-1: Structure of a conventional dye-sensitized solar cell. ➢ Glass substrates As previously mentioned, DSSCs are sandwich structures, involving two transparent conducting oxide (TCO) glass substrates as skins. The main requirements for the TCO substrates are the low sheet resistance (RSheet), which has to be nearly temperature independent to the high temperatures used for the sintering of the TiO2 layer (450–500°C), and high transparency to solar radiation in the visible–infrared region [53,55]. Typical RSheet values of the TCO used in DSSCs is 5–15 Ω/sq. The cost of TCO rises steeply with lower sheet resistance and high light transmittance. The cost of the substrates used in DSSCs accounts for nearly half of the total cost of the solar cell [53]. Both indium- doped tin oxide (In:SnO2, ITO) and fluorine-doped tin oxide (F:SnO2, FTO) have been employed as TCOs. Generally, ITO is the most common TCO used in many photonic and optoelectronic devices. This justifies its mass-production on an industrial scale. Unfortunately, however, it has been found that the thermal stability of ITO glass at high temperatures is not good, with ΙΤΟ layers peeling off the glass and/or formation of defect sites on their surface, which reduces the efficiency of DSSCs. Hence, the currently preferred TCO for DSSCs application is FTO [53,56]. Usage of glass substrates confers good protection against oxygen or water penetration. However, the heavyweight and rigidity of glass render this form of DSSCs non-portable, restricting their use to terrestrial power generation [53,55].

➢ TiO2 nanostructured scaffold

The DSSC revolution actually started from the application of a mesoporous TiO2 layer consisting of 20 nm sized nanoparticles as the anode. Flat substrates made of rather larger crystallites had been used before, and although the PV effect had also been observed therein, the achieved efficiencies

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were not that spectacular. Only the introduction of a highly porous structure with a large surface area sparked intensive research in the field of photo-electrochemical cells. Carefully prepared films of connected regular nanoparticles of narrow size distribution provide the desired large surface area for dye adsorption and maintain the transparency in the visible region [57]. Typically, the increase of surface area by using mesoporous electrodes is by a factor of 1000 [37]. Two distinct improvements are observed when mesoporous electrodes are used. The incident photon-to-electron conversion efficiency (IPCE) increases significantly, due to the more efficient light absorption characteristics of the dye distributed over the larger area within the mesoporous structure. Secondly, the photo-current generation is much more efficient in the low-energy region of the dye absorption, a unique feature of the nanocrystalline mesoporous layers [53]. In DSSCs, the thickness of such films is varied, however, typically used layers are of 10–20 μm thick and about 60–70% porous [57,58]. Many wide-bandgap oxide semiconductors (e.g. TiO2, ZnO, SnO2) have been examined as potential electron acceptors for DSSCs [53]. TiO2 still gives the highest efficiencies in DSSCs. TiO2 is a chemically stable, non-toxic oxide, and readily available in vast quantities. It has a high refractive index (n = 2.4–2.5) and is widely used as a white pigment in paint, toothpaste, sunscreen, self-cleaning materials, and food. TiO2 has many crystalline forms, with anatase, rutile, and brookite being the easily accessible ones. Rutile is the thermodynamically most stable form. Anatase is, however, the preferred structure in DSSCs, because it has a larger bandgap (3.2 eV vs 3.0 eV for rutile) and a higher conduction band edge energy. This leads to a higher Fermi level for the same conduction band electron concentration and better performance in DSSCs [37]. ➢ Ruthenium dye The nanostructured TiO2 absorbs the photons up to about 380 nm. Thus, to harvest more of the solar spectrum reaching the Earth’s surface, dye molecules are chemically bound to its surface, acting as antennas. Each dye molecule, being in intimate contact with the semiconductor, can inject one electron upon the absorption of one quantum of energy. Therefore, in order to maximize the performance, a large surface is needed. Although the choice of dye molecules is almost infinite, there are a few criteria to be fulfilled in order to ensure that sensitization occurs and even more to make it very efficient [37,53,57]. Specifically, the dye molecule has to be soluble in a suitable solvent in order to be transferred to the TiO2 surface (typically sensitization duration is about 4–24 hours), but not to be desorbed by the solvent of the electrolyte. The dye has to be attached to the surface of the semiconductor via a chemical bond. This is typically realized by a carboxylic group. An expected result is a monolayer of the dye grafted neatly on the semiconductor surface. Energetics of the molecule have to be optimized in such a way that the excited state level lies sufficiently higher than the conduction band edge of the semiconductor (for an efficient electron injection) and its ground state redox potential lies lower than the redox potential of the redox couple of the electrolyte (for an efficient dye reduction). The dye absorption spectrum should be broad, preferably extended to the red/infrared (panchromatic), with large optical cross-section throughout the whole absorption spectrum for efficient light-harvesting. Non-aggregation of dye molecules is an important feature because it is believed that non-radiative decay of the excited state to the ground state reduces the electron injection efficiency. The molecule has to be stable, in terms of electrochemistry, decomposition under illumination, and attachment to the semiconductor surface (strong chemical bond preventing the desorption). Historically, after the 1991 breakthrough, the most efficient and extensively studied dyes are Ru complexes. Ru-based dyes are characterized by a broad absorption spectrum, suitable excited and ground state energy levels, relatively long excited-state lifetime, and good electrochemical stability [37]. Amongst them, the most known complexes are the cis- diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II) (N3), cis-diisothiocyanato- bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (N719), cis- diisothiocyanato-(2,2’-bipyridyl-4,4’-dicarboxylic acid)-(2,2’-bipyridyl-4,4’-dinonyl) ruthenium(II)

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(Z907), and triisothiocyanato-(2,2’:6’,6”-terpyridyl-4,4’,4”-tricarboxylato) ruthenium(II) tris(tetrabutylammonium) (N749). ➢ Iodide/triiodide-based liquid state electrolyte The electrolyte plays a very important role in DSSCs, by facilitating the transport of charges between the working and counter electrodes. Among the many redox mediators examined, the most - - - popular redox couple and generally the most efficient remains the I /I3 [53,59]. However, since the I - /I3 redox couple absorbs in the visible-light region, the concentration has to be kept as low as possible. Lil had been used as the iodide source in earlier studies, but now the compositions of liquid state electrolytes for DSSCs include mixtures of alkali salts and ionic liquids [53]. Some of the most commonly used ionic liquids to electrolytes for DSSCs are the 1-methyl-3-propylimidazolium iodide (PMII) and 1-butyl-3-methylimidazolium iodide (BMII). Concerning the electrolyte solvent, the ideal is one that has very low viscosity in order to minimize charge transport problems, melting point below -20oC and boiling point above 100oC in order to minimize thermal instability issues, high dielectric properties so that electrolyte salts are sufficiently soluble and exist in a fully dissociated state, low absorption in the visible spectrum, low toxicity, and low cost [53,59]. Another important demand is to be a good solvent for the redox couple components and various additives, but at the same time not cause significant dissolution of the adsorbed dye or even the semiconducting material of the electrodes. Because many organometallic sensitizing dyes are sensitive toward hydrolysis, water and reactive protic solvents are normally not optimal choices. From an industrial perspective, factors such as robustness (chemical inertness), environmental sustainability, and easy processing are also very important [53]. The most common solvents used in electrolytes for DSSCs are a mixture of acetonitrile:valeronitrile and neat 3-methoxypropionitrile. In many studies, propylene carbonate or ethylene carbonate have also been used. Even though low-viscosity solvents, such as acetonitrile, provide the best solar-to-electricity conversion efficiency, their low boiling points and high vapor pressures limit their usage at elevated temperatures (>80°C). In a closed environment, the local vapor pressure of the solvent rises to very high values when their boiling points are reached, and the ability of the solvents to extract components, even from solids, becomes very high. In DSSCs, this causes slow extraction of the sealing materials to the electrolyte, accompanied by an associated decrease in the performance of the device. Hence, there has been a systematic effort to find alternatives to low- viscosity, high-vapor-pressure solvents [53]. ➢ Platinum-based counter electrode The device is completed with the counter-electrode. Typically, it is an FTO glass piece covered - - with a layer of a catalyst for the redox couple regeneration reaction. Since I /I3 is still the most popular redox mediator, Pt remains a natural choice. Fine particles of Pt are chemically stable and provide sufficient overvoltage for the triiodide reduction [57]. Without Pt, FTO glass is a very poor counter electrode and has a very high charge transfer resistance, more than 106 Ω·cm2, in a standard iodide/triiodide electrolyte. Pt can be deposited using various methods, such as electrodeposition, spray pyrolysis, sputtering, and vapor deposition. Best performance has been achieved using nanoscale Pt clusters prepared by thermal decomposition of Pt chloride compounds. In this case, very low Pt-loadings (5 µgcm-2) are needed, so that the counter electrode remains transparent. Charge transfer resistances of less than 1 Ωcm2 can be achieved [37]. 2.3. Dye-Sensitized Solar Cells Operating Principle It is believed that for 3500 million years, when the first photosynthetic organisms appeared on Earth, Nature has adjusted and optimized the process of photosynthesis along the evolutionary road. Yet, it is a rather complicated mechanism, which could be divided into two main phases; the light-

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dependent reactions and the light-independent or “dark” reactions (Calvin cycle). Having in mind that DSSCs in a way mimic the natural photosynthesis, it could be proudly said that this time mankind did a better job than Mother Nature. The number of steps involved in the photo-current generation cycle in a perfect (lossless) DSSC is 6. In reality, there are some unwanted phenomena occurring, reducing the overall efficiency of the device [37,53,57]. The processes taking place during DSSCs operation are presented in Figure 2-2 and they are briefly described below (the numbers in the brackets correspond to the labels appeared in Figure 2-2).

Figure 2-2: Dye-sensitized solar cells operating principle. ➢ Photon absorption As stated above, the monolayer of the dye anchored on the semiconductor surface helps to absorb most of the solar irradiance reaching the Earth’s surface. A photon of energy (hv) is absorbed (1) by the molecule in the ground state (D) and promotes it to the excited state (D*) (see Equation 2-1). The excess energy could be eliminated by relaxation (7) to the lowest vibrational state (10-12 s) and then emitted (8) (fluorescence or phosphorescence, when the inter-system crossing is involved) (10-8 s) (see Equation 2-2). These processes do not contribute to the injection of electrons to the conduction band of the semiconductor.

퐷 + ℎ푣 → 퐷∗ (2-1)

퐷∗ → 퐷 + ℎ푣′ (2-2) ➢ Electron injection The process of the electron injection (2) from the excited state of the dye molecule to the conduction band of the semiconductor (see Equation 2-3) is one of the fastest known physical phenomena and it occurs on the femtosecond scale. This is a crucial step because this is the moment when the charge separation takes place. Typically, the dyes are designed in such a way that the process is quantitative, as the lowest unoccupied molecular orbital (LUMO) of the dye lies sufficiently high above the conduction band edge of the semiconductor. Nevertheless, the position of the latter can be modified by the local adsorption of some species (e.g. tert-butyl pyridine, causing the upward shift, or intercalation of small cations, like Li+, bringing the conduction band edge down).

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∗ + − 퐷 → 퐷 + 푒푐푏 (2-3) ➢ Electron transport The injected electrons have to travel through the mesoporous network to the external contact (3). The thickness of the film varies from 10–20 μm, however, the actual path that an electron has to travel is longer, due to the tortuosity induced by the mesoporous network of nanocrystallites. The fact that electrons are charged sub-atomic particles would imply that the main mechanism of transport would be the gradient of the field. Nonetheless, the electric field is effectively shielded by the ions in the electrolyte surrounding the particles, so that it is claimed that the electrons coupled with these ions undergo ambipolar diffusion. Additionally, the retention time in the film is prolonged because of the occurrence of the electron traps within the bandgap (the electrons experience multiple trapping/de- trapping events). The timescale at which the transport occurs is in the ms/sub-ms range. ➢ Dye regeneration After the electron injection, the dye molecule becomes oxidized (D+), therefore it has to be consequently reduced to ensure the continuous operation of the device (see Equation 2-4). This is done by the reduced form of the redox mediator presented in the electrolyte (5). The highest occupied molecular orbital (HOMO) of the dye has to be placed lower than the redox potential of the redox couple (it is shown that the difference as small as 150 mV is sufficient). If the process lasts microseconds, then the lifetime of the oxidized state should be in the range of hundreds of seconds.

+ − − 2퐷 + 3퐼 → 2퐷 + 퐼3 (2-4) ➢ Reduction of the redox mediator The oxidized form of the redox mediator needs to be reduced and this takes place at the counter- electrode (6), where the electrons from the external circuit (4) are collected (see Equation 2-5). The triiodide ions diffuse from the working electrode to the counter electrode to become iodide ions. Only in the case of a viscous medium with high concentration of the redox species, e.g. ionic liquid or gel electrolyte, can the transport be enhanced by the occurrence of the hopping Grotthus-like mechanism, involving the formation and cleavage of chemical bonds in the temporarily formed associates. Upon the arrival at the counter electrode, the oxidized form (triiodide) is brought to the reduced state (iodide). The reaction is supposed to be rapid due to the use of a catalyst at the counter electrode surface.

− − − 퐼3 + 2푒푒푥푡 → 3퐼 (2-5) ➢ Electron recombination The main source of losses in the performance of DSSCs can occur via three channels, i.e. recombination of the electrons of the semiconductor with the oxidized redox species in the electrolyte (9), recombination of the electrons from the TCO with the oxidized redox species in the electrolyte (10), and much less common recombination of the electrons of the semiconductor with the oxidized dye molecules (11). It is believed that even in high-efficiency DSSCs, about 10% of electrons readily injected into the conduction band of semiconductor recombines with the electrolyte (see Equation 2- 6). This leads to the lowering of the output voltage because the quasi-Fermi level determined by the population of electrons in the trap states and conduction band is lowered. The recombination with the electrolyte occurs at a timescale comparable to the electron transport, however, in order to obtain a well-performing device, it has to be much slower than the transport. The second recombination channel is when the electrons from TCO recombine with the oxidized redox species of electrolyte (see Equation 2-7). This type of recombination is important under short-circuit conditions, while it is the main reason for lowering the short-circuit current compared to the photo-current. Under short-

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circuit conditions the recombination resistance at the semiconductor/electrolyte interface is high, thus the main recombination losses occur at TCO/electrolyte interface. The third recombination channel is not observed very frequently in DSSCs, as most of the dye molecules are designed in such a way to ensure effective charge separation. The recombination of electrons of semiconductor with the oxidized dye (see Equation 2-8), termed “geminate recombination”, typically competes with the dye regeneration realized via the electrolyte, and that is why the appropriate design of the dye molecule (the spread of the electron cloud) proves to be very important.

− − − 2푒푐푏 + 퐼3 → 3퐼 (2-6)

− − − 2푒푇퐶푂 + 퐼3 → 3퐼 (2-7)

− + 푒푐푏 + 퐷 → 퐷 (2-8) ➢ Idealized description of dye-sensitized solar cells operation It is useful when discussing DSSCs to have a general feel for the materials and the relative concentrations of the various species in the device. Based on the model proposed by O’Regan and Durrant for DSSCs operation under 1 sun illumination (see Figure 2-3), the following numbers for the typical materials and the relative concentration of the different species in the device are derived [60]. A TiO2 particle (18 nm) has about 600 dye molecules on its surface, while there are about 10000 adsorption sites for H+. Each dye molecule absorbs a photon once per second. The flux of electrons -1 injection into a TiO2 particle is about 600 s . Under working conditions, there are about 10 electrons per TiO2 particle, while more than 90% of electrons in TiO2 are in trap states and lower than 10% in its conduction band. Under working conditions, about 1 dye molecule per 150 TiO2 particles is in its oxidized state. The total volume fraction of the solutes in the electrolyte is about 10–20%. In the pore volume around the TiO2 particle, there are about 1000 iodide and 200 triiodide ions. The concentration of iodine is less than 1 µM, which is about one free iodine per 10000 TiO2 particles.

Figure 2-3: Representation of a 1 nm slab sliced from a pore in dye-sensitized solar cells [60]. ➢ Dye-sensitized solar cells operation versus p-n junction solar cells operation By comparing the DSSCs operating principle with the operating principle of conventional p-n junction solar cells, it is perceived that there are fundamental differences in the solar-to-electricity conversion mechanisms of the two devices [61,62]. The main distinguishing characteristics of DSSCs compared to p-n junction solar cells are listed below. At first, in DSSCs, the charge carrier generation and separation are almost simultaneous and occur through exciton dissociation at the TiO2/electrolyte interface, through the absorption of photons from the dye. Then, electrons are transferred to the TiO2

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and holes to the electrolyte, where the charges are moving by diffusion towards the conductive contacts. This is the way it happened since the electrolyte is presented in the whole pore volume of the TiO2 scaffold, disabling the development of powerful electric fields in these devices, which can push the charges out of the cell. The above-mentioned design and operating principle contrast with the spatially and temporally distinct processes of carrier generation in the bulk and subsequent separation in conventional p-n junction solar cells. Furthermore, in the case of p-n junction solar cells, the transportation of the charges in the planar non-porous semiconductors is carried out by drift, due to the electric field appeared in the p-n junction. Due to the fact that electrons and holes in DSSCs are spatially separated, these devices are characterized as majority carrier devices, unlike the conventional p-n junction solar cells, which are primarily characterized as minority carrier devices. Also, taking into account that in DSSCs the most important electronic processes take place at the TiO2/electrolyte interface, these devices are appropriate for development using low to medium purity materials, maintaining the manufacturing costs at low levels. Thus, the cost of DSSCs is lower compared to conventional p-n junction solar cells. Additionally, the fact that during DSSCs operation each material performs its own operation gives the opportunity to DSSCs designers to optimize each material in a single direction. This cannot happen in the conventional p-n junction solar cells since several processes are taking place simultaneously in one material. 2.4. Dye-Sensitized Solar Cells Equivalent Circuit Analysis 2.4.1. Transmission Line Model Electrochemical impedance spectroscopy is a well-established method for the investigation of electronic and ionic processes taking place into DSSCs [63]. The beauty of DSSCs lies in their interfaces formed in their structure, with the main ones to be the TiO2/dye/electrolyte, TCO/electrolyte, electrolyte/counter electrode, and TCO/TiO2 interfaces. Today, electrochemical impedance spectroscopy remains the most versatile technique, giving non-invasive and simultaneous access to almost all processes taking place within DSSCs. The modern electrochemical setups (potentiostats and frequency response analyzers) enable fast yet sensitive experiments. The DSSC is connected to the setup and a steady voltage bias is applied, either in the dark or under illumination. Then, an AC voltage perturbation is applied within a frequency range of 1 mHz to 10 MHz. During the measurements, the voltage amplitude is small (a few mV, typically 10 mV), so that the linearity of the response can be preserved and the resulting impedance be independent of the amplitude of the perturbation. Linearity, causality, and stationarity of the system are necessary requirements to apply Fourier transform to the parameters and move to the frequency domain, as the analysis of so many processes in the time regime would be too complicated. Sets of frequency-varying measurements for many steady-potential bias steps are collected, because only in this way can one follow the changes in the processes occurring into the DSSCs upon changing quasi-Fermi levels. For each bias potential step, a Nyquist plot is usually analyzed and fitted. The main issue of the impedance data treatment is the application of a physically relevant model, where each fitted circuit element describes the process that is suspected to happen in the device under operation. Apparently, the one and only fit to an impedance curve does not exist, therefore a heavy burden of wise choice of fitting parameters and their interpretation lies on the shoulder of the experimentalist. At the beginning of the last decade, two diﬀerent impedance models were developed by Kern et al. [64] and by Bisquert [65]. The former was based on the continuity equations for the conduction

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band and trap states of TiO2, while the latter was mainly concerned with a theoretical point of view for the electron diffusion and recombination in the TiO2. Later, Adachi and co-workers unified these theories, finding a method to determine parameters related to the electron transport in DSSCs [66]. All these works laid the basis for the development of the transmission line model (TLM) for the description of the impedance of DSSCs, which was developed independently by Bisquert's group [67] and Grätzel's group [68]. Figure 2-4 shows the equivalent circuit of DSSCs based on the TLM. In this picture, the representation of the nanostructured TiO2 has been simplified to a columnar model that mimics the mesoporous layer (whose thickness is equal to d), in which the electrolyte solution interpenetrates. The circuit elements and their relative units are the ohmic series resistance of the cell, accounting for the RSheet of the TCO substrates, electrical contacts, and wiring of the solar cell (푅푠, Ω), the substrate contact resistance at the interface between the TCO and the TiO2 film (푅퐶푂, Ω), the substrate contact capacitance at the interface between the TCO and the TiO2 film (퐶퐶푂, F), the substrate charge transfer resistance accounting for the electron recombination from the uncovered layer of the TCO to the electrolyte (푅푡푐표, Ω), the substrate double layer capacitance at the TCO/electrolyte interface (퐶푡푐표, F), the electron transport resistance in the TiO2 (푟푡, Ω⁄m) (the total transport resistance of the film is Rt=rt·d), the recombination charge transfer resistance at the TiO2/electrolyte interface (푟푐푡, Ω·m) (the total recombination resistance of the film is Rct = rct/d), the photo-anode chemical capacitance, which stands for the change of electron density as a function of the Fermi level (푐휇, F⁄m) (the total chemical capacitance is (Cμ=cμ·d)), the electrolyte diffusion impedance, accounting for mass transport of redox species in the electrolyte (훧푑, Ω), the charge transfer resistance at the Pt/electrolyte interface (푅푃푡, Ω), and finally the double layer capacitance at the Pt/electrolyte interface (퐶푃푡, F). Here, it has to be highlighted that the components denoted in lowercase letters in the above presented list (namely rt, rct and cμ), have to be considered as material characteristics that are independent of the photo-anode thickness, i.e. they are distributed in a repetitive arrangement of a transmission line, giving rise to the total photo-anode equivalent impedance. This justifies the TLM name given to this circuit. Furthermore, it has to be pointed out that the ideal capacitors presented in the equivalent circuit are often replaced by constant phase elements (CPEs). This procedure is quite common when analyzing impedance spectra of DSSCs, although a clear justification for the non-perfect capacitive behavior to be related to some specific physical process has not yet been identified. Quite often they are employed with the aim of obtaining a better fitting of the experimental data, but without assigning any physical meaning to all the CPE parameters [63].

Figure 2-4: Transmission line model for dye-sensitized solar cells. Extensive analysis of the electrochemical impedance spectroscopy fundamentals and application in DSSCs can be found in the review paper published by Sacco [63]. 2.4.2. One-Diode Model One-diode model is a steady-state equivalent circuit that can be applied directly to the experimental current−voltage characteristic curve of solar cells [69]. The basic assumption of the model, for its

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application to PVs, is the appearance of a diode-like behavior at one of the interfaces formed between two of their structural parts. In DSSCs, the above-mentioned assumption is satisfied since the resistance element related to the charge transport at the TiO2/dye/electrolyte interfaces displays behavior like that of a diode (Rdiode) [70]. Classical diode models, like the present one, are often applied to solid state solar cells. Concerning DSSCs technology, interpretation of the parameters obtained by these models and their relationship with the physical chemistry fundamental processes governing the performance of these devices is not always provided. Thus, there are only a few scientific articles dealing with their application to DSSCs [71–76]. DSSCs designers mainly use equivalent circuit models for AC conditions. TRL, which was presented at §2.4.1, is the main circuit model and can reveal most of the key electrical circuit elements determining DSSCs energy conversion efficiency. However, when selecting an analytical model for simulating physical and chemical phenomena, it is important to reach an acceptable compromise between analytical complexity, achievable precision, and low cost. One-diode model is considered as a simplified version of the TLM used in electrochemical impedance spectroscopy (see Figure 2-5). One-diode model provides an easy way to understand the critical factors affecting DSSCs performance, without the need for additional characterization equipment.

Figure 2-5: One-diode model.

Equation 2-9 is the basic equation of the one-diode model, where IL represents the photo- generated current, I0 the reverse saturation current of the diode, q the absolute value of the electron charge, RS the total series parasitic resistance, n the diode factor, k the Boltzmann constant, T the environmental temperature, and RSH the shunt parasitic resistance. The Rdiode varies with respect to the photo-voltage according to Equation 2-10 [77].

푞(푉+퐼푅푆) 푉+퐼푅푆 퐼 = 퐼퐿 − 퐼0 [푒푥푝 ( ) − 1] − (2-9) 푛푘푇 푅푆퐻

푉+퐼푅푆 푅푑푖표푑푒 = 푞(푉+퐼푅 ) (2-10) 퐼 [푒푥푝( 푆 )−1] 0 푛푘푇

The n and I0 are the basic parameters of the model describing the diode features and charge recombination kinetics at the TiO2/electrolyte interface. In DSSCs, the reaction order (γ) is commonly used for the description of the non-linear recombination mechanisms at the above-mentioned interface. The parameter γ is the inverse value of n, and is always less than one, as reported in several experimental works [78]. The RS is the contribution of the contact resistance between the TCO and the TiO2, the bulk resistance of TiO2, the bulk resistance of electrolyte, the interfacial resistance at electrolyte/Pt interface, and finally the resistance related to TCO [79]. The RSH is related to the electron−hole recombination occurring at the TCO/electrolyte interface, which becomes more important under short-circuit conditions [71,75,77]. The total back-reaction in DSSCs is determined by the sum of the Rdiode and RSH parallel resistances [77].

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Chapter 3: Review of Recent Progress in Dye-Sensitized Solar Cells

When transferring PV technologies from laboratory-scale fabrication to industrial applications, low-cost, high solar-to-electricity conversion efficiency, long lifetime, large-area, wide application range, and low toxicity are crucial attributes [80]. At the commercialization stage, the performance of PVs is not quantified only by efficiency, but by a figure of merit called levelized cost of electricity (LCOE) [36]. The LCOE is an important standard for comparing different PV technologies. Calculation of the LCOE includes the total cost and the energy output. The cost depends upon raw materials, fabrication, transportation, installation, and maintenance costs, while energy output depends on the efficiency and lifetime of solar modules [80]. Similar to existing PV technologies, the development objectives of DSSCs are mainly two; firstly, lowering the LCOE, which is dictated by the concept of the critical triangle, namely efficiency, stability, and cost, and secondly, increasing their application range. The number of research efforts in the direction of DSSCs technology improvement has been grown greatly in the last few years, as shown in Figure 3-1. The aim is to achieve lower LCOE values than the today corresponding targets of c-Si PV industries, which are in the magnitude of 0.05 €/kWh, using solar cell designs which can lead to DSSCs wide commercialization [81].

Figure 3-1: Number of papers published per year, using the searching keyword “dye solar cell” (data source: www.scopus.com). The present chapter is a comprehensive review of the recent progress in DSSCs technology, in the direction of achieving the “Golden Triangle” requirements for cheap PV electricity (see Figure 3-2), in combination with a wide application range, for increased competitiveness and a high global market share. At the end of this chapter, the current challenges in the direction of DSSCs wide commercialization are demonstrated, while the motivation and the research objectives of the present Ph.D. dissertation are also presented.

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Figure 3-2: Towards lower solar electricity cost. 3.1. Efficiency Increasing the energy conversion efficiency of DSSCs is probably the most important key to their great entry in the huge electricity generation market [82]. The maximum theoretical efficiency of single-junction PVs (Shockley-Queisser limit) is around 32% [83]. For inorganic solar cells, the main deviation from this limit is through the loss-in-potential, which can be defined as the difference between the optical bandgap of the photo-active semiconductor and the open-circuit voltage. For high-efficiency silicon solar cells, the loss-in-potential values range at 400 mV, whereas for high- efficiency GaAs devices, the loss-in-potential values range at 300 mV [84]. In contrast to inorganic solar cells, DSSCs require relatively large over-potentials to drive electron injection into the conduction band of the anode and regenerate the oxidized dye. Typically, a potential difference between the LUMO level of dye and the conduction band of TiO2 is required for fast electron injection. Theoretical calculations show that this value is approximately 150 mV. Moreover, the - - regeneration of a Ru metal complex dye with I /I3 redox couple has a loss of around 600 mV. Therefore, it is estimated that the loss-in-potential for the conventional systems employing Ru metal complex dyes and iodide-based electrolytes is around 750 mV, which limits their maximum energy conversion efficiency to 13.8% [38,84]. Today, there is an increased number of research efforts to reach or even to exceed this limit, by the development and application of novel solar cell materials and designs in DSSCs. Below, there is a brief overview of the recent progress in DSSCs technology, in the direction of achieving high energy conversion efficiency. The research efforts are classified according to the part of the solar cell which is under investigation, namely dye-sensitized working electrode, electrolyte, and counter electrode.

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3.1.1. Dye-Sensitized Working Electrode In the direction of increasing DSSCs efficiency, the dye-sensitized working electrode, which constitutes the photo-anode in a conventional n-type photo-electrochemical cell design, is probably the most important part of the device since there is the heart of the solar cell, which is the dye. The strategies for improving n-type DSSCs efficiency by the modification of the photo-anode are many and include the development of novel nano-architectures, interfacial engineering, increasing light scattering, fabrication of composites, doping, and co-sensitization method (Figure 3-3) [85]. In most of these cases, the TCO substrate used for the fabrication of dye-sensitized working electrode is an FTO glass since it has the best characteristics for the development of high-efficiency DSSCs.

Figure 3-3: Strategies for improving DSSCs efficiency through photo-anode modifications. ➢ Nano-architectures The performance of DSSCs can be enhanced by improving the morphology of the photo-anode. Pore-forming agent usage is a simple way to improve the characteristics of photo-anodes, leading to increased dye-loading, higher incident light illumination for dye excitation, as well as better electron transportation along the conducting channels from nanoparticles. Thus, an increase in short-circuit current and subsequently of the efficiency of solar cells can be achieved. Investigations on this topic have shown that the usage of pore-forming agents can improve the efficiency of DSSCs by 50%, achieving energy conversion efficiencies that exceed even the 9% [86,87]. Self-organized mesoporous networks formed by nanoparticles through a surfactant-assisted or template method are also developed in the direction of improving DSSCs performance. These networks can facilitate electron transport, increase dye adsorption, and enhance porosity [85]. Recently, Ahn et al. fabricated surfactant-templated TiO2 nanoparticles with inter- and intra-particle porosity, leading to even better results by their application to DSSCs as an anode [88]. On the other hand, one-dimensional nanostructures, such as nanotubes, nanowires, and nanorods, have attracted the attention of the PV community, due to their unique physical, chemical, and optical characteristics, enabling extraordinary performance in solar cells [89]. One-dimensional nanostructures are indicated to possess higher electron diffusion coefficient in the photo-anode and better light absorption compared to the non- ordered nanoparticles. In contrast to traditional nanoparticles, well-ordered one-dimensional array nanostructures can offer a direct transfer pathway (known as electron highway) for electron transfer

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from the anode to the conductive substrate, leading to a decrease in recombination rate into the solar cells and to a better performance [85]. Efficiencies of more than 8% have been achieved in DSSCs by using TiO2 nanotubes in DSSCs, in front-side illumination mode [90]. Furthermore, hierarchical architectures have increasingly been used in DSSCs photo-anode in recent years. These architectures consist of primary nanostructures, which form secondary structures of different shapes. Compared with conventional nanoparticles and one-dimensional nanostructures, materials based on hierarchical architectures can lead to enhanced dye loading, higher light-harvesting, and better photo- anode/electrolyte interface, when applied to DSSCs [85]. Xe et al. showed in their recent investigation that when hierarchical rutile TiO2 microspheres are incorporated in a conventional P25 photo-anode, an enhancement in DSSCs performance on the order of 25% is achieved, resulting in efficiencies higher than 9% [91]. Engineering of exposed materials facet is another way to improve the performance of DSSCs. Investigations in this direction have shown that it is possible to increase dye adsorption on the photo-anode, while retardation of the charge recombination kinetics can also be observed. Moreover, rationally exposing materials facet influences the crystal phase and morphology of photo-anode, resulting in facile electron transfer, high light scattering effect, and preferable pore volume distribution [85]. Recently, Zhang et al. [92] achieved efficiencies on the order of 9% using mesocrystalline TiO2 nanosheet arrays with exposed {001} facets in the photo-anode of DSSCs. ➢ Interfacial engineering The performance of DSSCs depends greatly on charge transfer processes taking place at photo- anode/electrolyte, TCO/electrolyte, and TCO/photo-anode interfaces. In high-efficiency DSSCs, the radiationless relaxation of the excited state of the dye and the recombination of the electrons with the oxidized dye have little impact on their operation, while the photo-anode/electrolyte and TCO/electrolyte interfaces being responsible for most of the electron losses. These losses and the mismatches between the energy levels of the different components of the device impose the limits on the maximum achievable photo-potential and photo-current produced by the solar cells. Performance losses also affect the fill factor and are associated with the presence of increased series parasitic resistance in the solar cell. In the direction of improving DSSCs performance, interfacial engineering takes place in these aforementioned interfaces [85]. In particular, the introduction of an ultra-thin dense layer (blocking layer) between the mesoporous photo-anode network and the charge collector in DSSCs has been claimed to improve the performance of solar cells. Two mechanisms have been proposed to explain this behavior, a decrease in the electron−hole recombination at the TCO/electrolyte interface and an enhancement in the electronic contact between the mesoporous photo-anode network and the charge collecting electrode [93]. These layers are usually made by TiO2 and can be prepared on conductive substrates through a large number of techniques, such as sol-gel, spray pyrolysis, DC-magnetron sputtering, electrochemical deposition, and atomic layer deposition [85]. The thickness of this layer plays an important role in the optimal operation of DSSCs since beyond an optimal thickness a decrease in DSSCs energy conversion efficiency is observed [94]. By using blocking layers between the mesoporous photo-anode network and the charge collector, DSSCs efficiency enhancement on the order of 25% can be achieved [93]. On the other hand, concerning the photo-anode/electrolyte interface, a large number of interfacial modifications are also applied [85]. TiCl4 post-treatment of the TiO2 film of DSSC photo-anode is a well-known method for improving the performance of DSSCs [95]. In this method, an extra layer of TiO2 is grown onto the conventional TiO2 anode to form a thin film. This method is not only applied to TiO2 nanoparticle-based photo- anodes, but also to other nanostructured TiO2-based photo-anodes, such as TiO2 nanotube arrays, nanorods, etc. Several hypotheses have been developed to explain the improvement effect of TiCl4 post-treatment on DSSCs. These conjectures include increased specific surface area, improved electron transport, increased light scattering, indicating the multi-functionality and the complicated effect of TiCl4 post-treatment [85,96]. Thus far, the use of TiCl4 treatment to improve the performance

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of DSSCs presents many disputable points. The photo-anode/electrolyte interface can also be improved by the introduction of wide bandgap semiconductors, with a more negative conduction band than that of the main anode semiconductor, or by using a material with an electronic insulating coating to form an energy barrier. These modifications can lead to a decrease in electron−hole recombination at the aforementioned interface, and to an increased open-circuit voltage and efficiency in solar cells. Some of the materials used in this case are Al2O3, ZrO2, Nb2O5, etc [85]. Efficiency improvements that exceed the 30% have been demonstrated by this type of modifications, attaining efficiencies of more than 10% [97]. However, a recent study revealed that the insulating property of these metal oxides alone is not sufficient to suppress the unwanted back-reaction. Other properties such as semiconductor conduction band position and dye oxidation state should be considered as the primary criteria. Meanwhile, the surface isoelectric property of the insulating layer should also be considered with regard to the charge of the electrolyte redox species [98]. These requirements indicate that the interfacial engineering has to be designed and tuned carefully in order to fabricate high- efficiency DSSCs. ➢ Light scattering Addition of light scatters to photo-anode is an efficient and popular approach used to enhance the light absorption and improve the performance of DSSCs. Light scatters are relatively large particles (microparticles) that can scatter and/or reflect the light when incident light passes through the photo- anode film, resulting in a prolonged light path, and enhanced optical absorption and light-harvesting characteristics [85]. Today, scientists have developed many ways to increase light scattering in DSSCs photo-anode [99]. In particular, spherical voids can act as light scatters, developed by polymers that are melted during the sintering procedure of the anode, leaving voids in the film. Carbon spheres are also employed to make pores in the anode. One-dimensional structures can also enhance light scattering, except electron transportation. The light scattering is considerably improved by the incorporation of one-dimensional nanostructures with lengths in the range of hundreds of nanometers to micrometers, providing the same effect as the inclusion of large nanoparticles in the photo-anode. The fabrication of double layered or multilayered films are the main approaches for increasing light scattering in the photo-anode of DSSCs. The idea is to develop a scattering and/or reflective layer placed on the top of the main light absorption layer, using nanoparticles and larger particles in an appropriate proportion. The light scattering depends upon the change in the refractive index between the active and scattering layer. Up until today, various materials of different geometries have been tested as light scatters, such as TiO2 rutile microparticles, ZrO2 microparticles, TiO2 nanotubes, TiO2 nanowires, TiO2 nanospindles, hexagonal TiO2 plates, TiO2 photonic crystals, etc [99]. Additionally, dual function materials, such as nanocrystalline spherical aggregates can provide both higher surface area and light scattering capability. In all the aforementioned cases, the PV parameter that is mainly improved is the short-circuit current, leading to a great energy conversion efficiency enhancement, which can exceed the 25% compared to the corresponding efficiency achieved by a conventional DSSC. By increasing light scattering in photo-anode, energy conversion efficiencies that exceed 9% or even 10% have been reported in DSSCs technology [100–103]. ➢ Composites Forming composites in DSSCs photo-anode can lead to unique features and to a great efficiency enhancement. Materials used to form composites mainly include carbon (such as graphene and carbon nanotubes (CNTs)), noble metals with plasmon effect, and transition metal oxides. These materials can provide different new features to DSSCs, because of their various synergetic and intrinsic properties [85]. Carbon-based materials are commonly used to form composites with TiO2 in the photo-anodes of DSSCs. Graphene and CNTs are the most popular carbon-based materials, because of their excellent electrical properties, high specific surface area, and superior mechanical properties.

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These materials can enhance the electrons transport from the anode to the TCO substrates and reduce the charge recombination kinetics into the solar cells, resulting in an enhancement of energy conversion efficiency [104,105]. The utilization of the plasmon resonance effect of Au and Ag nanomaterials is also a promising pathway to increase the efficiency of DSSCs. The excitation of localized surface plasmon resonance can enhance spectral absorption and light-harvesting. Hence, compositing Au or Ag with TiO2 in the photo-anodes of DSSCs can increase the photo-current density. Furthermore, Au and Ag plasmon can increase the energy levels of TiO2, enhance desirable charge transfer processes in DSSCs, and reduce the electron−hole recombination. Thus, the use of Au and Ag plasmon can considerably improve DSSCs performance [85]. Introducing foreign metal oxide semiconductors into TiO2 or ZnO photo-anodes is another effective approach to enhance the performance of DSSCs. In most cases, these metal oxides possess low-dimensional nanostructures and thus, provide a highway for electron transfer, resulting in reduced recombination with holes, while maintaining high light-harvesting properties and low resistance [85]. Moreover, the fabrication of composite photo-anodes using different TiO2 nanostructured materials have shown to improve the characteristics of DSSCs photo-anode, and thus their energy conversion efficiency [106–109]. Using the aforementioned architectures, a great efficiency enhancement can be achieved in DSSCs, reaching efficiencies on the order of 9%. Finally, the introduction of SiO2 nanoparticles in a conventional TiO2 photo-anode can result in enhanced energy conversion efficiency in DSSCs [110,111]. The fabrication of TiO2-SiO2 composite photo-anodes results in enhanced dye excitation and reduced recombination at the photo-anode/electrolyte interface, attaining energy conversion efficiencies on the order of 9%. ➢ Doping Doping of the conventional TiO2 photo-anode is an effective way to increase DSSCs efficiency [85]. For metal oxides, like the TiO2, doping with a suitable cation/anion modifies their bandgap, which modulates their electrical properties [112]. Introducing cations as a dopant in anode materials (metal oxides) exerts a larger dipole moment that changes the interface energetic for electron transfer. The shift of the conduction band in the negative direction increases the efficiency of electron−hole separation at the photo-anode/electrolyte interface. TiO2 with different cationic dopants and their respective photo-conversion efficiencies are reported by Sengupta et al. [112]. Reading the aforementioned review, it can be stated that, a facile electron injection kinetics involves a strong electronic coupling of the dye LUMO orbital to the metal oxide conduction band states. Due to the doping, using selective cations, electron trapping takes place in the sub-bandgap states of metal oxides, which may lead to faster charge transportation than charge recombination. Besides cation doping, several researchers have also studied doping of anion into TiO2. In anion doped TiO2, the formation of non-oxide phase or new defect level slightly above the valence band generally cause the redshift of the absorption spectrum [112]. Doping of TiO2 photo-anodes can result in a great enhancement of DSSCs efficiency, even on the order of 25%, by introducing different cations or anions as dopants [113,114]. ➢ Co-sensitization Dye-sensitizer plays a key role to ensure effective light-harvesting in DSSCs. It is desirable to have a sensitizer that absorbs all incident light, from the visible to the near-infrared region of the solar spectrum, up to the wavelength of approximately 920 nm [115]. Since the invention of DSSCs, various Ru-based dyes, such as N3, N719, and N749, have been synthesized and served as a paradigm of efficient charge-transfer sensitizers. However, there are several drawbacks of their usage, such as their high cost and the limited amount of noble metals, their sophisticated synthesis and purification steps. On the other hand, metal-free organic dyes have proved to be promising candidates in replacing Ru-based dyes, due to their intrinsic high molar absorptivity and broad absorption by the co-

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sensitization approach. Co-sensitization offers a cost-effective route to the panchromatic sensitization of DSSCs anode, by combining two or more dyes with complementary absorption spectra. In addition to the complimentary spectrums, the criteria for dye co-sensitizer selection include the chemical compatibility between the dye co-sensitizers, and the high molecular extinction coefficient, which is a key factor for co-sensitization, in allowing attachment of multiple dyes, while minimizing their mutual interactions [115]. There are two ways to make a co-sensitization. The first is the cocktail approach, where a mixed dye solution with a certain molar ratio of the two dyes is made, and the stepwise approach, where two or more different dyes are adsorbed at the anode in a consecutive manner [116]. Investigations on co-sensitization of DSSCs anode with two or more dyes have shown that the solar cells performance can be significantly enhanced, attaining efficiencies that exceed 11% [35,117,118]. Kakiage et al. have achieved the efficiency record in DSSCs technology, which is 14% under one sun illumination, using the co-sensitization method [35]. Development in the span of the last two decades has led to a variety of novel DSSC devices originated from Grätzel cell prototype [115]. Amongst them, the development of p-type DSSCs is particularly attractive, due to the following capability of developing high-efficiency tandem pn-type DSSCs [119]. The theoretical limit of tandem DSSCs efficiency could reach as high as 43% under standard test conditions, which is much higher than the 30% of conventional DSSCs with one photo- active dye-sensitized electrode. Different designs and the operation of tandem DSSCs are presented in the review paper of Gong et al. [115].

Figure 3-4: Schematic operating principle of (a) n-type dye-sensitized solar cell, (b) p-type dye-sensitized solar cell [120]. In contrast to conventional dye-sensitized photo-anode (n-type DSSC), dye-sensitized photocathode (p-type DSSC) operates in an inverse mode (see Figure 3-4), where dye-excitation is followed by a rapid electron transfer from the p-type semiconductor to the dye. Upon light absorption, the excited dye injects a hole into the valence band of the p-type semiconductor. The injected holes diffuse through the back TCO, and then in turn pass through the external circuit and reach the counter electrode, where they finally oxidize the redox mediator. The cycle is completed when the oxidized species of electrolyte are reduced by electrons transfer from the dye [115]. In this design, only a few metal oxides exhibit p-type semi-conductivity. Hence, nanostructured NiO is predominantly used in p-type DSSCs. Although fast photo-induced hole transfer from the excited sensitizer to the valence band of NiO is generally evidenced, the relatively short-lived charge-separated state (e.g., hole in NiO and electron on the sensitizer) remains an intrinsic limitation to the performance of p-type DSSCs

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[121]. In addition, poor physical properties of the p-type photo-cathode, as well as the non-optimal energetic configuration at the NiO/dye/electrolyte interface, led to device performance up to 2.5% under standard solar irradiation, far below that of n-type DSSCs [122]. The dominating energy losses in NiO-based p-type DSSCs have been demonstrated in detail by Daeneke et al. [123]. Despite poor reported efficiencies, p-type DSSCs remain highly promising for the fabrication of tandem DSSC devices, while important efforts are made in the international community to develop alternative p- type semiconductors, dyes, and electrolyte systems for p-type DSSCs [120,122,124]. 3.1.2. Electrolyte The improvement in the energy conversion efficiency of DSSCs hinges also on the development and optimization of new electrolytes [38,59,82]. The electrolyte has a great influence on the light-to- electricity conversion efficiency of DSSCs. All the basic PV parameters, named short-circuit current, open-circuit voltage, and fill factor are significantly affected by the electrolyte in DSSCs, and by the interaction of the electrolyte with the electrodes. For instance, short-circuit current can be affected by the transport of the redox couple components in the electrolyte. Open-circuit voltage can be significantly affected by the redox potential of the electrolyte. Fill factor can be affected by the diffusion of charge carriers in the electrolyte and the charge transfer resistance at the electrolyte/counter electrode interface [59]. Up until today, liquid state electrolytes are still the most widely utilized transport medium for DSSCs and have produced the highest efficiencies in DSSCs [35,125]. - - Before 2010, studies on the electrolyte mainly focused on the traditional I /I3 redox couple [126]. Up until today, there are a large number of investigations in the direction of improving iodide-based electrolytes behavior for achieving higher energy conversion efficiency in DSSCs [59,127–130]. These investigations involve the preparation of iodide-based liquid state electrolytes, using different solvents, cations, and chemical compounds. The effect of electrolyte solvent, iodide counterion, iodide concentration, iodide/iodine ration, and additives type and concentration have been extensively studied. The development of new redox mediators falls far behind that of other components of DSSCs. - - However, the field has received renewed attention recently [59,131]. Although I /I3 redox couple has been indicated to show remarkable performance in DSSCs, there are several negative features limiting DSSCs efficiency. Apart from the light absorption in the visible spectrum by triiodide ions and other possible polyiodides, which leads to reduced photo-current production by the solar cells, the main DSSCs performance limitation is the mismatch (about 0.8 V) between the redox potential of a typical - - sensitizer (Eredox ≈ 1.1 V vs. NHE) and that of I /I3 (Eredox ≈ 0.35 V vs. NHE), causing a great loss in open-circuit voltage of DSSCs [59]. According to Marcus theory, a driving force of 0.2–0.3 eV is sufficient to ascertain a fast dye regeneration rate, opening up the opportunity to increase the open- - - circuit voltage of DSSCs. This beneficial effect was observed when the I /I3 redox couple was replaced by new redox mediators and solid state hole conductors [59]. Nowadays, in order to improve the performance of DSSCs, many scientists have been devoting their research to electrolytes. Novel redox mediators have been developed in recent years, in order to increase DSSCs performance. Various types of iodide-free redox couples are tested in DSSCs. Amongst them, one of the most promising seems to be the Cobalt(II/III) redox couple [59,132]. The main advantage of cobalt-based electrolytes is the facile tuning of their redox potential by selecting suitable donor/acceptor substituents on the ligand of cobalt complexes. Thus, the redox potential can be adjusted to match the oxidation potential of the sensitizer, minimizing the energy loss in the dye regeneration step and enabling maximum open-circuit voltage values [132]. By the usage of cobalt complex redox shuttle in DSSCs, open-circuit voltage values higher than 1 V have been reported [133]. Additionally, cobalt-

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based electrolytes have negligible visible-light absorption and do not prevent light absorption from - - the photo-anode [132]. The Br /Br3 redox couple is another alternative in the direction of replacing - - the conventional I /I3 redox couple in DSSCs, to achieve higher energy conversion efficiency [59]. - - Br /Br3 redox couple has a more positive redox potential (about 1.1 V vs. NHE) in comparison to the - - I /I3 redox couple (about 0.35 V vs. NHE), which can lead to higher photo-voltages. The aforementioned conjecture has already been confirmed since the fabrication of DSSCs containing a - - Br /Br3 redox mediator with higher than 1.2 V open-circuit photo-voltage has been reported [134,135]. Investigations on ferrocene-based electrolytes have also demonstrated that they could lead to high-efficiency DSSCs [59]. This non-corrosive mediator exhibits well-defined electrochemistry, - - which is commercially available on a large scale, and has a more desirable redox potential than I /I3 redox couple, which can result in a higher open-circuit voltage. Daeneke et al. have successfully fabricated high-efficiency DSSCs using Fc/Fc+ redox couple electrolytes [136]. These ferrocene- based devices exceed the efficiency achieved by the conventional DSSCs employing iodide-based electrolytes, revealing the great potential of ferrocene-based electrolytes in future DSSCs - - - - applications. Pseudo-halogen redox couples, such as SeCN /(SeCN)3 and SCN /(SCN)3 , have also been studied as redox mediators in electrolytes for the fabrication of high-efficiency DSSCs [59]. The - - redox potentials of these two redox couples are 0.43 V and 0.19 V more positive than that of I /I3 - - redox couple, respectively. The SeCN /(SeCN)3 redox couple was the first alternative redox couple - - that has been identified to rival and even to exceed the performance of the I /I3 couple in DSSCs, - - even at full sunlight [137]. Apart from the aforementioned alternatives to the conventional I /I3 couple for DSSCs, various other redox mediators have been studied in the last few years, with promising results to appear in the direction of DSSCs efficiency enhancement [59]. Deep understanding of the electron transfer kinetics is vital to make a new breakthrough in the development of highly efficient iodine-free DSSCs [138]. Finally, the development of hybrid and tandem electrolyte systems promise even higher energy conversion efficiencies, by combining the beneficial characteristics of different redox couples, eliminating the drawbacks of conventional electrolytes [139,140]. 3.1.3. Counter Electrode The counter electrode in n-type DSSCs has the important task of catalyzing the reduction of the electrolyte by the collected electrons from the external circuit. By improving the counter electrode, the fill factor of the solar cell rises, and subsequently, the energy conversion efficiency rises. As in the cases of dye-sensitized working electrodes, the TCO substrate used for the fabrication of high- performance counter electrodes is FTO glass. Today, Pt is the most preferred material for developing counter electrodes for high-efficiency DSSCs, due to its high electrical conductivity, high electrocatalytic activity towards triiodide reduction, and high reflecting properties [141,142]. DSSCs with energy conversion efficiency over 12% mostly use Pt as cathode material [35,117,143,144]. Many manufacturing techniques have been employed in the fabrication of Pt cathodes for DSSCs, such as sputtering, electrodeposition, thermal vapor deposition, spray pyrolysis, etc [141,142]. Today, Pt 3-D nanostructures with a high surface area have emerged as promising materials for the fabrication of high-performance cathodes for DSSCs. These include structures such as nanotubes, nanowires, nanoflowers, multipods, etc. [141,142]. Jeong et al. [145] synthesized by UV-based nanoimprint lithography periodically aligned Pt nanocups of a controlled diameter (300–600 nm) and pitch size (400–800 nm), for the enhancement of the Pt-based counter electrode performance for DSSCs application. The results were quite satisfactory since a lower charge transfer resistance at counter electrode/electrolyte interface was achieved, leading to an enhancement of solar cell performance, on the order of 25%, compared to the solar cells employing planar Pt-based counter electrodes. Wu group [146] fabricated Pt nanotubes on FTO substrates, using a facile polycarbonate template method. With Pt nanotubes as the cathode, the fabricated DSSC achieved an efficiency of

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9.05%, which was 25% higher compared to that of the DSSC employing a conventional Pt-based counter electrode. In order to further improve the electrocatalytic activity of Pt-based counter electrodes, much attention has been paid to developing hybrids or composite counter electrodes, where except Pt, carbon-based materials or polymers are employed [141,142]. The performance of the hybrids could be improved by taking advantage of the synergetic effects that arise from different components of the hybrids. Currently, hybrids have become amongst the most popular counter electrodes [142]. Guo et al. [147] prepared a nanohybrid cathode, consisting of Pt nanoparticles and CNTs via a sulfur-assisted route. The DSSC employing this cathode produced an energy conversion efficiency of 7.69%, which was 22% higher than the corresponding obtained with the conventional Pt cathode. The PV parameter that showed the greatest enhancement by employing the nanohybrid cathode to DSSC was the fill factor. Miao et al. [148] deposited Pt particles on vertically ordered silicon nanowires and used the hybrid as a cathode for DSSCs. After optimization of the hybrid, the DSSC achieved an efficiency of 8.30%, which was better than the corresponding obtained by the device employing the Pt-based counter electrode. 3.2. Stability In terms of the figure of merit defined by Fonash, the lifetime of solar cells is just as important as their efficiency [149]. Assuming negligible operating costs, a given solar module will produce electricity at approximately half the cost per kWh if its lifetime can be doubled. However, considering the balance of system and solar module manufacturing costs, it is impractical to commercialize solar cells with lab scale efficiencies of less than 10%. Therefore, the majority of past research efforts have been devoted to improving DSSC efficiency, further beyond this threshold. Nonetheless, increasing attention has been paid recently to improving DSSCs stability [36]. For practical applications, DSSCs must be stable at the molecular, cell, and module levels. Each of these scales presents different scientific and engineering challenges. At the molecular level, DSSC components must have complementary kinetics, such that desirable electron transfer reactions are much faster than parasitic degradation pathways. At the cell level, DSSCs must be extremely well sealed to prevent electrolyte leakage and moisture ingress. This challenge is even greater at the module level, where dimensions for sealing are larger [36]. The stability of DSSCs depends on several factors [150]. To systematically study the degradation mechanisms of DSSCs, stability can be categorized into intrinsic and extrinsic stability [151]. To study the intrinsic stability of DSSCs, it is important to ensure excellent sealing of the device, so that factors such as intrusion of moisture and oxygen, as well as leakage and evaporation of liquid state materials do not take place and influence the device performance. If this objective is achieved, the intrinsic stability of DSSCs can be investigated. Intrinsic stability includes the chemical and structural stability of the device over a range of PV operating conditions, in presence of a certain amount of impurities, especially oxygen and water, which are already introduced in the device during manufacturing. By PV operating conditions, both the weather conditions (light, temperature, and humidity exposure) and the electric bias are included. The extrinsic stability primarily deals with the failures of sealing and failures under mechanical loading. Figure 3-5 shows the main degradation mechanisms of a conventional DSSC, during its operation, according to Figgemeier et al [152].

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Figure 3-5: Main degradation mechanisms of a conventional dye-sensitized solar cell [152].

The requirements for DSSCs lifetime depend strongly on the application [36]. Usage of DSSCs as building-integrated photovoltaics (BIPVs) would require lifetimes of more than 25 years, to avoid disruption of the building environment for repair or replacement. Conversely, lifetimes of 5 years may be sufficient for portable electronics chargers integrated into apparel and accessories. Indoor environments are much less harsh in terms of temperature, humidity, and light intensity, so less expensive manufacturing is possible. Currently, there are no standard practices for testing lifetime and stability that are specific to DSSCs. Instead, protocols such as IEC 61646 (United States) and Japan Industrial Standard C-8938 for thin film PVs can be applied [36]. Common treatments, concerning DSSCs technology, include light soaking and high-temperature ageing, with only a few international scientific reports investigating low-temperature ageing, thermal shock cycling, hydrothermal ageing, reverse biasing, and fatigue under mechanical loading [36,150,153–159]. Today, there is an increased number of research efforts to reach the standards of stability for various PV applications, without efficiency limitations, with the development and application of novel solar cell materials in DSSCs. Below, there is a brief overview of the recent progress in DSSCs technology, in the direction of achieving high stability. The research efforts are classified according to the part of the solar cell which is under investigation, namely dye-sensitized working electrode, electrolyte, and counter electrode. 3.2.1. Dye-Sensitized Working Electrode As reported at §3.1.1, the dye-sensitized working electrode is probably the most important part in DSSCs, in view of achieving high energy conversion efficiency. The stability of this component is also of the same importance, in order to maintain the high energy conversion efficiency constant over time. At first, the Fermi level of the anode must not shift relative to the dye and the redox electrolyte.

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The level of the conduction band edge of the anode has a very well-defined energy interval in which the solar cell works most efficiently. If it decreases, open-circuit voltage drops, because of the reduced potential difference between the Fermi level and the energy level of the redox couple. On the other hand, if it increases, the injection of electrons from the excited state of the dye might be less efficient, resulting in a decreased short-circuit current. The development of more surface states, which trigger a drain of charge from the conduction band by electron trapping, must also be avoided. It is known that the electron transfer from the conduction band of the anode to the electrolyte takes place mainly through these states, while investigations have shown that such defects evolve with time. Thus, the development of more surface states over time must be considered and eliminated. Moreover, an increase in the series parasitic resistance with time is a possible factor, due to loss of electrical contact between the particles consisting the anode and/or between the anode with TCO, leading to a decreased energy conversion efficiency [152]. Finally, but of the greatest importance, dye degradation and desorption must be limited since it is regarded as one of the most critical reasons leading to the instability of DSSCs in long-term operation [150]. In the direction of achieving a more stable dye-sensitized working electrode, several scientific efforts have been reported in the worldwide literature in the last few years. Kay and Palomares et al. [160,161] showed that anode surface modification with an insulating layer, like Al2O3, could stabilize the conduction band edge of the anode over time and subsequently DSSCs energy conversion efficiency. Luo et al. [162] fabricated DSSCs employing post-modified TiO2 photo-anodes, using aluminum isopropoxide. This modification led to a depression of dye desorption phenomena over time, and subsequently to a higher DSSCs stability. Guo et al. [163] fabricated DSSCs based on nitrogen-doped TiO2 photo-anode. Nitrogen doping of the conventional TiO2 anode led to a decreased dye degradation over time and to an increased stability in DSSCs. Senevirathna et al. [164] developed SnO2/MgO-based DSSCs, attaining a quite satisfactory efficiency and a better stability compared to the conventional TiO2-based DSSCs. The higher stability achieved by the SnO2/MgO-based DSSCs was, and in this case, attributed to a lesser dye degradation over time. Kim et al. [165] applied atomic layer deposited coatings on the conventional TiO2 anode and demonstrated an improved high- temperature stability in DSSCs. Their infrared spectroscopy experiments, conducted in situ during atomic layer deposition, revealed a transition of carboxylate linker groups from unbound or weakly- bound states, respectively, to more strongly bound bidentate structures. Pathak et al. [166] demonstrated a DSSCs stability enhancement using Al doping of TiO2 photo-anode, while attributed this to the passivation of the electronic trap sites in the bulk and at the surface of the TiO2. Dembele et al. [167] reported an improvement in DSSCs stability by incorporating CNTs in the conventional nanostructured TiO2 photo-anode. Their results showed that by this modification, the short-circuit current of the DSSCs was greatly stabilized over time, leading to a better device stability compared to the conventional counterpart. Murakami et al. [168] developed a new carbazole dye that employed a phosphonic acid anchor for DSSCs application. The stability of DSSCs employing the aforementioned dye was even better than the corresponding of DSSCs employing the famous commercially available Z907 dye. The better stability achieved was attributed to the lesser desorption rate of the novel dye over time compared to the other tested dyes. Novel modifications in dye- sensitized working electrodes have also been reported in the last five years, promising higher stability in DSSCs. Koo et al. [169] presented regenerable DSSCs with a hydrogel-embedded microvascular network, mimicking the regeneration functionality of a plant leaf. A hydrogel medium with embedded channels allowed the rapid and uniform supply of photo-active reagents using a convection-diffusion mechanism, while a washing-activation cycle enabled the reliable replacement of the organic component in the solar cells. Repetitive restoration of PV performance after intensive device degradation was demonstrated. Bella et al. [170] reported that the presence of luminescence coatings at the external surface of the glass working electrode can enhance the performance and long-term

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outdoor stability of DSSCs. Concerning the stability, the better results observed, when the luminescence coatings were applied to solar cells, were ascribed to the action of three combined factors. Firstly, the coating showed easy-cleaning properties, due to its fluorinated nature, keeping the external side of the working electrode clean. Thus, the decrease in the device performance, likely caused by the formation of physical barriers that may prevent incident solar photons from reaching the dye-sensitized working electrode for photon-to-electricity conversion, is avoided. Secondly, the hydrophobic nature of the coating applied to the device contributes to hindering DSSC degradation due to water molecules permeation. Finally, the luminescent species in the coating promote a UV light filtering action, providing protection to the photo-sensitive DSSC components. 3.2.2. Electrolyte The electrolyte is probably the most crucial component in DSSCs in terms of stability [59]. Using liquid state electrolytes as charge carrier transporters, the DSSCs achieved great development in terms of efficiency. However, the stability of these devices is considered insufficient for long-term operation [59]. Experimental studies indicated that the main reasons for the gradual degradation in performance of conventional DSSCs are related to changes in the electrolyte and at the TiO2/electrolyte interface [38]. Thus, the correct choice of solvent, redox couple concentration, as well as electrolyte cations and additives are critical for improving solar cells long-term stability. Mohanty et al. [171] investigated the effect of different electrolyte solvent usage on the photo- electrochemical stability of DSSCs. They demonstrated that, upon DSSCs ageing, phenomena such as dye desorption, conduction band edge sifting, formation of polyiodide species, and sealant dissolution evolve over time, at different rates, depending on the solvent of the electrolyte. Zhang et al. [172] investigated the influence of iodide concentration on the efficiency and stability of DSSCs containing a non-volatile electrolyte. They found that there is a specific iodide compound concentration in the electrolyte where high efficiency and stability in DSSCs can be achieved. Zhang et al. [173] investigated the stability of dyed TiO2/electrolyte interface for DSSCs containing different cations in the electrolyte. They demonstrated that the adsorbed electrolyte cations on the dyed TiO2 electrode could increase the trap state density in the nanostructure TiO2 electrodes with the decrease of cations radius. Tuyet Nguyen et al. [174] investigated the effect of different pyridines and 1- methylbenzimidazole as additives in DSSCs electrolyte on solar cell performance and stability. Concerning the stability, they found a nice correlation of DSSCs efficiency as a function of ageing and dye degradation. The half-life of N719 dye was found to be dependent on the additive used. They showed that none sterically hindered additives protect to some extent the N719 dye against nucleophilic attack from triiodides, most likely by the formation of a pyridine-iodo complex. On the other hand, sterically hindered pyridines cannot prevent dye degradation in DSSCs. Zhang et al. [175] investigated the effect of the addition of guanidinium thiocyanate in DSSCs electrolyte on the performance and stability of solar cells. Concerning DSSCs stability, their investigation indicated that a more stable dyed TiO2/electrolyte interface is achieved when guanidinium thiocyanate is chemisorbed on the TiO2 surface. Sarwar et al. [176] reported an improvement in DSSCs long-term stability by zeolite additive in the electrolyte. The stability enhancement was attributed to the water scavenging effect of the porous zeolite additive at a high temperature. Most often, the degradation of the conventional iodide-based electrolytes is related to the loss of triiodides, which results in changes from a yellow to a colorless liquid, referred to as bleaching [38,177]. The decrease in the triiodides concentration increases the internal series parasitic resistance of the solar cell, due to the increased electrolyte diffusion resistance, leading to a decrease in fill factor. Moreover, the loss of triiodides can reduce the short-circuit current density, because of mass transport limitations, while it also increases the back-reaction kinetics at the TiO2/electrolyte

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interface, leading to a lower open-circuit voltage [38]. Apart from all the above-mentioned, the use of liquid state electrolytes can lead to many practical problems, such as leakage and volatilization of solvent, fast photo-degradation and desorption of the dye, corrosion of counter electrode, and ineffective sealing of the solar cells for long-term applications [59]. In the direction of achieving better stability in DSSCs technology, replacement of liquid state electrolytes by quasi-solid state electrolytes or solid state hole transport materials is considered as the most effective method [59]. Although in most cases the performance of DSSCs employing these materials is still lower than the corresponding of conventional devices, the aforementioned materials are viable alternatives to the liquid state electrolytes, owing to improved stability and better sealing ability [59]. Today, the scientific community, dealing with the enhancement of DSSCs stability, has focused mainly on developing quasi-solid state dye-sensitized solar cells (QSS-DSSCs) or solid state dye-sensitized solar cells (SS-DSSCs) by using quasi-solid state electrolytes or solid state hole transport materials, respectively, without the presence of performance limitations (see Figure 3-6). In this direction, modifications that can improve the anode semiconductor/quasi-solid state electrolyte or anode semiconductor/hole transport material interfaces are also proposed, showing promising results [178,179].

Figure 3-6: Replacement of liquid state electrolytes of dye-sensitized solar cells with quasi-solid state electrolytes or solid state hole transport materials. ➢ Quasi-solid state electrolytes Quasi-solid state or semisolid state is a special state of a substance between solid and liquid states. Quasi-solid state electrolytes are macromolecular or supramolecular nanoaggregate systems characterized by a remarkable ionic conductivity, usually higher than 10−7 S·cm−1, for DSSCs, usually higher than 10−3 S·cm−1 [59]. Quasi-solid state electrolytes possess, simultaneously, both the cohesive property of a solid and the diffusive property of a liquid. Namely, quasi-solid state electrolytes show better long-term stability than liquid state electrolytes and have the merits of liquid state electrolytes, including high ionic conductivity and excellent interfacial contact property [59]. For these reasons, quasi-solid state electrolytes are widely used in DSSCs.

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Generally, there are two methods often used for preparing quasi-solid state electrolytes. The first is when liquid state electrolytes are solidified by organic polymer gelators to form thermoplastic polymer electrolytes or thermosetting polymer electrolytes [59]. Polymer electrolyte is generally defined as polymeric materials complexed with salts [180]. The polar functional group in the main polymeric chain acts as a medium to solvate the ionic species by intermolecular interaction. The ionic transport in polymer electrolytes occurs mainly by two mechanisms [181]. The first mechanism is a result of the segmental motion of the chains surrounding the salt ions, creating a liquid-like environment around the ion. Ions migrate by hopping between adjacent liquid-like environments. This mechanism is dependent on labile interactions between the ion and the polymer chain, and is thus affected by the chemical composition of the polymer and temperature. It is, however, independent of the molecular weight of the chain. The second mechanism of ion conduction is attributed to diffusion of the entire polymer chain with coordinated ions. This mechanism is similar to the diffusion of salts in traditional low molecular weight electrolytes, such as water and alkyl carbonates, wherein ions diffuse with “shells” of coordinated solvent molecules. Since polymer diffusion coefficients decrease rapidly with increasing molecular weight, the second mechanism is only applicable to low molecular weight polymers. The most famous polymer electrolytes used in DSSCs are the thermoplastic. Linear polymers, such as poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(vinylidene fluoride) (PVDF), and poly(methyl methacrylate) (PMMA) are amongst the most often used polymeric gelators of liquid state electrolytes [59]. Secondly, the liquid state electrolytes can also be solidified by inorganic inclusions, to form composite electrolytes [59]. In DSSCs, the main objective of incorporating inorganic nanoparticles into electrolytes aims at the enhancement of long-term stability and ionic conductivity of electrolytes. The inorganic nanoparticles can solidify liquid state electrolyte and convert them to quasi-solid state electrolytes, and thus enhance their long-term stability. Meanwhile, an organic and/or organic- inorganic network is constructed by the incorporation of inorganic nanoparticles in the electrolyte, - - and thus I /I3 ions are able to align and transport on the inorganic particles network, leading to an acceleration of the charge-transport dynamics [59]. Finally, the preparation of polymer composite electrolytes is also possible, by combining polymers and inorganic inclusions [59]. Up until today, many quasi-solid state electrolytes for DSSCs application have been developed. However, as referred above, the performance of these materials is, in most cases, insufficient for DSSCs application since their employment to solar cells leads to a decrease in energy conversion efficiency. Nevertheless, in the last decade, great progress has been made in developing high- performance QSS-DSSCs, by the development and application of novel quasi-solid state electrolytes, that exhibit the appropriate properties for DSSCs application. Some of them are presented below. Starting from the group of polymer electrolytes, a lot of progress has been made recently in the direction of development of novel advanced polymer electrolytes for DSSCs application. In the last years, novel techniques have been applied for polymer electrolytes preparation, in the direction of achieving better properties compared to the conventional ones. Preparation methods of advanced polymer electrolytes include the copolymerization, the polymer blending, the cross-linking, and the plasticization [59,180]. Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) has been used recently as a host in many polymer electrolytes for DSSCs application, leading to promising results. For example, Wang et al. [182] prepared a quasi-solid state electrolyte, using PVDF-HFP (5 wt %) as a polymer host to gel 3-methoxypropionitrile-based liquid electrolyte. The QSS-DSSC employed an amphiphilic ruthenium dye in conjunction with this polymer electrolyte achieved an energy conversion efficiency higher than 6%. Concerning the DSSC stability, the solar cell sustained heating at 80°C for 1000 h, maintaining 94% of its initial performance, while excellent stability was also

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reported under light soaking ageing. Priya et al. [183] developed a QSS-DSSC using electrospun PVDF-HFP membrane electrolyte. The achieved energy conversion efficiency was 7.3%, while the solar cell possessed better stability than the conventional counterpart fabricated with liquid state electrolyte. A highly efficient polymer electrolyte for DSSCs application was also fabricated by Chen et al. [184], using poly(acrylonitrile-co-vinyl acetate) (PAN-VA) as the gelator of a 3- methoxypropionitrile-based liquid state electrolyte. An energy conversion efficiency on the order of 8% was achieved by the application of the aforementioned electrolyte to DSSCs, which was 97% of the value attained by the conventional DSSC employing liquid state electrolyte. Seidalilir et al. [185] fabricated a high-efficiency QSS-DSSCs using poly(methyl methacrylate-co-ethyl acrylate) (PMMA- EA) as gelator of liquid state electrolytes. Energy conversion efficiencies higher than 8% were reported by employing the aforementioned polymer electrolytes in DSSCs, which were very close to the corresponding values achieved by the solar cells employing conventional liquid state electrolytes. Zebardastan et al. [186] prepared PEO/PVDF-HFP blend-based polymer electrolytes using different loadings of NaI. The highest energy conversion efficiency achieved by their application to DSSCs was 5.67%. Tiong et al. [187] prepared poly(ethylene oxide)/poly(vinyl alcohol) (PEO/PVA) blend-based polymer electrolytes of different compositions for application in DSSCs. The DSSC containing the highest conducting polymer electrolyte exhibited the highest energy conversion efficiency, which was 5.36%. Ganesan et al. [188] presented, for the first time, the strategy of blending three different polymers followed by doping of an organic nitrogenous compound, to develop a solvent-free novel doped multi-polymer blend electrolyte system for DSSCs application. The maximum energy conversion efficiency achieved was 9.3% under simulated sunlight radiation of 80 mW/cm2. Shen et al. [189] developed polymer electrolytes, using a cross-linked copolymer synthesized from poly(oxyethylene) amide-imide (POE-PAI) as a polymer host, for DSSCs application. Their investigation concerned the influence of different electrolyte solvents usage on solar cells performance. The highest energy conversion achieved was 8.31%, using propylene carbonate as a solvent, which was higher than the corresponding efficiency achieved by DSSCs employing the liquid state electrolyte. A structurally interconnected block copolymer was facilely prepared by Dong et al. [190], by the oligomerization of poly(oxyethylene)-segmented diamine and 4,4′-oxydiphthalic anhydride, followed by a late-stage curing to generate amide-imide cross-linked gels. This elastomeric copolymer was used as the matrix of polymer electrolytes for DSSCs, showing extremely high PV performance. In particular, a 9.48% energy conversion efficiency was achieved by QSS- DSSC, which was superior to that of DSSC employing the liquid state electrolyte. Wu et al. [191] prepared a high-performance polymer electrolyte for DSSCs application, using poly(ethylene glycol) (PEG) as a polymer host and propylene carbonate as a solvent. In this occasion, the solvent acted as a plasticizer and provided the system with a sol character. The achieved efficiency was 7.22%, which was comparable to the corresponding efficiency attained by the conventional DSSCs employing liquid state electrolytes, while the QSS-DSSC demonstrated a much higher stability over time. Xia et al. [192] fabricated QSS-DSSCs using polymer electrolytes composed of PEO/PVDF-HFP as polymer host and the mixture of hydroxyethyl methylacrylate and ethylene glycol as a plasticizer. The highest energy conversion efficiency achieved by QSS-DSSCs was 6.79%. Farhana et al. [193] prepared polymer electrolytes using polypropylene carbonate (PPC) as a polymer host, NaI as iodide source, and ethylene carbonate and propylene carbonate as plasticizers. The highest efficiency achieved by the QSS-DSSCs was 6.38%. Polymeric ionic liquids belong also to a special kind of advanced polymer electrolytes, presenting unique properties for solar cells application [180]. They are a new class of polymers, that combine both the novel properties of ionic liquids and improved mechanical durability and dimensional control

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resulting from polymerization [194]. Lin et al. [195] synthesized a poly(oxyethylene)-imide- imidazolium selenocyanate (POEI-IS) and used it in preparation of polymer electrolytes for DSSCs application. The QSS-DSSCs demonstrated a high energy conversion efficiency, on the order of 8%, while they also exhibited a high stability over time. Chen et al. [196] prepared bis-imidazolium-based poly(ionic liquid) electrolytes for DSSCs application. Their results were quite satisfactory since an energy conversion efficiency of 5.92% was achieved, which was comparable to the corresponding of conventional devices employing liquid state electrolytes, while the QSS-DSSC exhibited superior long-term stability. Concerning the nanocomposite electrolytes, up until today, many inorganic materials, such as TiO2, SiO2, ZnO, Al2O3, carbon, etc., are introduced as gelators into liquid state electrolytes, to form quasi-solid state electrolytes [59]. Kowsari et al. [197] reported on the preparation of nanocomposite electrolytes using different alkylamin chains modified graphene oxide (GnO) as an additive for DSSCs application. The highest energy conversion efficiency achieved, by the usage of the nanocomposite electrolytes to DSSCs, was 8.33%, which was remarkably enhanced compared to the corresponding of the conventional device using liquid state electrolytes. Senevirathne et al. [198] prepared quasi-solid state electrolytes using fumed silica nanoparticles. Their application to DSSCs gave a light-to-electricity conversion efficiency of 7.46%. Ma et al. [199] reported a novel kind of functional NH2-rich SiO2 nanoparticle as an electrolyte additive, which was employed to assemble high-efficiency QSS-DSSCs. They found that the NH2-rich SiO2 nanoparticles can significantly improve the performance of QSS-DSSCs, especially the open-circuit photovoltage and fill factor, through negatively shifting the TiO2 conduction band edge, effectively facilitating the ions transport, and remarkably inhibiting the charge recombination. Notably, the DSSC fabricated using the aforementioned electrolytes achieved an efficiency of 7.30% under 1 sun illumination. Finally, concerning the polymer composite electrolytes, a number of scientific reports demonstrate that their usage in DSSCs can lead to a high energy conversion efficiency and stability. Chen et al. [200] achieved an impressive 10.58% energy conversion efficiency by QSS-DSSCs using polymer nanocomposite electrolytes, which was higher than the corresponding achieved by the liquid counterpart. The high energy conversion efficiency obtained, was attributed to the in situ gelation property of the electrolyte, the contribution of the PAN-VA to the charge transfer, as well as the enhancement effect of TiO2 fillers on the charge transfer at the counter electrode/electrolyte interface. Accelerating ageing tests, under light-soaking (100 mWcm-2) at 60oC, also showed that the QSS- DSSCs are stable, maintaining their initial performance unchanged. Printable polymer electrolytes based on polyvinyl acetate-co-methyl methacrylate (P(VA-co-MMA)) copolymer and TiO2 nanoparticles as fillers were prepared by Wang et al. [201] for DSSCs application. The achieved efficiencies were higher than 9%, while the QSS-DSSCs demonstrated a high stability, maintaining 96.7% of their initial efficiency after 1000 h exposure to simulative sunlight. Chen et al. [202] prepared polymer composite electrolytes using PAN-VA as matrix and TiO2 as fillers. An 8.65% energy conversion efficiency was achieved using the aforementioned electrolytes to DSSCs. Sakali et al. [203] prepared polymer composite electrolytes using PAN and CNTs for DSSCs application. After regulation of CNTs wt% loading in the polymer electrolyte, a high energy conversion efficiency, on the order of 9%, was achieved. A similar investigation was conducted by Xianhua et al. [204], by preparing polymer electrolytes, using PEO as a solidification agent and CNTs to improve their performance. Their results were quite satisfactory since the introduction of CNTs in the polymer electrolyte led to a great improvement of the electrolyte characteristics, attaining a 7.23% efficiency and a great stability enhancement in DSSCs. Venkatesan et al. [205] prepared a polymer composite electrolyte based on PEO and PVDF polymers and GnO sponge nanofillers. A high energy conversion efficiency of 8.78% was achieved by QSS-DSSCs, which was higher than the liquid state counterpart, while GnO sponge also led to a stability improvement. Zebardastan et al. [206] prepared a polymer

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composite electrolyte by blending PVDF-HFP copolymer and PEO and incorporation of ZnO nanofiller. They observed an enhancement in the energy conversion efficiency of DSSCs in the presence of ZnO into the electrolyte, reaching efficiencies up to 7.33%. The same group [207] had developed earlier a polymer electrolyte composed of the same polymers using them as a matrix, but this time fumed SiO2 nanoparticles as fillers. In this case, the attained energy conversion efficiency was 9.44%. Chang et al. [208] synthesized a novel polymeric ionic liquid, named poly(oxyethylene)- imide-imidazole complex coupled with iodide anions, to prepare polymer electrolytes for DSSCs application. They also used CNTs to enhance the performance of polymer electrolyte. The highest energy conversion efficiency achieved by the QSS-DDSCs was 7.65%, while the solar cells also showed an unfailing durability of greater than 1000 h under 50°C. ➢ Solid state hole transport materials - - According to the operating principle of DSSCs, the mesoporous TiO2 layer and the I /I3 redox electrolyte can be regarded as an electron-transporting layer and a hole-transporting layer, - - respectively [37]. Thus, the I /I3 redox electrolyte can be replaced by a p-type semiconductor material, as a hole-transporting material. According to the definition, the hole transport materials are not electrolytes but semiconductors since the charge carrier transportation is taking place by electrons or holes, not by ions [59]. In hole transport materials, charge carrier transport takes place through hole hopping between neighboring molecules or moieties, which is a typical electronic transport, as opposed to electrolytes, where charge carrier transport is due to movement of ions, a typical ionic transport. However, in many cases, the hole transport materials applied to DSSCs contain, additionally, salts [59]. Thus, they demonstrate electronic and ionic conductivity, which is significant for local charge compensation and high energy conversion efficiency in solar cells. An appropriate hole transport material for fabricating SS-DSSCs must satisfy several requirements [59]. More specifically, it must be able to transfer holes from the dye after the dye injects electrons into the TiO2, and its upper edge of the valence band must be located above the ground state level of the dye. It must be able to be deposited within the mesoporous TiO2 films in an amorphous state because the crystallization of hole transport material will inhibit efficient pore-filling of mesoporous TiO2 film, which is regarded as a major limiting factor for the device performance. The hole mobility of the material should be sufficiently high since low hole mobility is believed to be another limiting factor for the performance of the device. Finally, it should be transparent in the visible range and cannot dissolve or degrade the dye during the depositing process. Known to date, there are only a very limited number of inorganic p-type materials, coincidentally in accordance with the aforementioned requirements and suitable for their application in DSSCs [59]. Familiar inorganic wide-band hole transport materials, such as SiC and GaN, are not suitable to be used in DSSCs since the high-temperature deposition process for these materials will certainly degrade the dye anchored on the working electrode [209]. After extensive experimentation, a type of inorganic p-type semiconductor based on copper compounds, such as CuI, CuBr, or CuSCN, was found suitable. These copper-based materials can be cast from solution or vacuum deposition, to form a complete hole transporting layer, while CuI has good hole conductivity, in excess of 10-2 S·cm-1, which facilitates their hole conducting ability [209]. Up until recently, the energy conversion efficiency of DSSCs employing this group of materials was insufficient for their wide commercialization, due to several drawbacks. However, in 2017, Cao et al. [210] fabricated high- efficiency SS-DSSCs, using copper(II/I) hole transport materials. The energy conversion efficiency of these devices was on the order of 11%, which is the highest achieved so far by a SS-DSSC compared to previously reported devices. Other p-type semiconductors, such as NiO and CuAlO2, are also used as hole transport materials in SS-DSSCs, however, the achieved efficiencies are much lower [59]. Cesium tin iodide, in particular, the orthorhombic perovskite polymorph of this trihalide, is

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gaining rapid interest for DSSCs application in the last years [59]. Amongst the outstanding properties -2 of CsSnI3 are its high p-type metal-like conductivity (∼200 S·cm ), its high hole mobility (∼585 cm2·V-1·s-1), its direct bandgap of ∼1.3 eV, and its strong near-infrared photoluminescent emission at ∼950 nm [59]. Chung et al. [211] used a p-type direct bandgap semiconductor CsSnI3 as hole transport material for fabrication of SS-DSSCs in combination with n-type nanoporous TiO2 and N719 dye. The CsSnI3 is solution processable and can fill into TiO2 mesopores at a molecular level to make intimate contacts with dye molecules and TiO2. With a bandgap of 1.3 eV, CsSnI3 enhances visible light absorption on the red side of the spectrum, even outperforming typical DSSCs in this spectral region. The use of pristine CsSnI3 in SS-DSSCs yielded a conversion efficiency of 3.72%. Doping CsSnI3 with 5% SnF2, the efficiency was increased to 6.81%. By pretreating the TiO2 electrode with fluorine plasma and introducing photonic crystal over the counter electrode, the device yielded an efficiency of 10.2% (8.51% with a mask). Recently, Lee et al. [212] introduced Cs2SnI6, as a hole transporter, and N719 and YD2-o-C8 as a sensitizer, attaining a high energy conversion efficiency, on the order of 8%. Compared with inorganic hole-transport materials, organic hole-transport materials (organic p- type semiconductors) display attractive features, such as plentiful sources, low cost, and easy preparation [59,209]. Most of the organic hole transport materials, either polymers or molecules, are soluble or dispersible in an organic solvent. Simple methods, such as spin-coating, in-situ electrochemical polymerization, or photochemical polymerization methods can be used for fabricating SS-DSSCs with good pore-filling in TiO2 mesoporous films. Moreover, it can be tailored with chemical methods to fit different purposes and thus used widely in solar cells [59]. Organic hole transport materials can be divided into two classes, molecular hole transport materials and polymeric hole transport materials [213]. Some of the organic hole transport materials used in DSSCs are polypyrrole (PPy), polyaniline (PANI), poly(3-hexylthiophene) (P3HT), poly(3,4- ethylenedioxythiophene) (PEDOT), and 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′- spirobifluorene (spiro-MeOTAD) [59]. Amongst them, the spiro-MeOTAD is the most popular hole transporting material for DSSCs, due to the high energy conversion efficiency achieved in solar cells employing this material [59]. Efficiencies on the order of 7% have been achieved by using spiro- MeOTAD as hole transport material in SS-DSSCs [214]. Recently, novel materials have been synthesized that show comparable performance in DSSCs application. In 2016, Xu et al. [215] designed and synthesized, using a two-step synthetic route, a spiro(fluorene-9,9′-xanthene)-based organic hole transport material. SS-DSSCs employing the aforementioned material showed high energy conversion efficiencies, amounting to 7.30%. 3.2.3. Counter Electrode The stability of DSSCs is also affected by the degradation of the counter electrode [150]. Degradation of the counter electrode of DSSCs leads to an increased charge transfer resistance at the counter electrode/electrolyte interface and thus to a decrease in fill factor of the solar cell. Some of the main reasons leading to the degradation of the counter electrode are the poor contact between the catalyst layer and the TCO, or due to the presence of impurities, which also lead to the detachment of the catalyst particles from TCO [150]. The conventional tape tests can be applied to study the adherence of the catalyst layer and can give an idea of the counter electrode stability. The contact between the TCO and the catalyst layer, and between the catalyst particles themselves depends on the preparation method. Other reasons for the performance degradation of counter electrode are chemical reactions, such as corrosion between the electrolyte and the catalyst, or deposition of by-products in the electrolyte onto the counter electrode surface [150]. If the degradation involves a chemical reaction between the catalyst and the iodine in the electrolyte, it would also affect the charge transport in the electrolyte, due to the decreased concentration of triiodides in the electrolyte. The presence of

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corrosion can be confirmed with a chemical analysis of the electrolyte. The deposition of by-products onto the counter electrode could be analyzed with microscopy techniques. Syrrokostas et al. [216] investigated the stability of most common applied Pt-based counter electrodes in DSSCs, which are prepared by electrodeposition and thermal decomposition of hexachloroplatinic acid (H2PtCl6) solutions. To this aim, they stored the electrodes in an iodide-based electrolyte solution or in air, within a closed vessel, for up to 70 days. They found a high degradation of the catalytic activity for both kinds of counter electrodes during storage in air and in the electrolyte, the latter having a more pronounced result. The aforementioned investigation demonstrates that the widely used Pt-based counter electrodes suffer from low robustness, leading to a great effect on DSSCs long-term stability. Up until today, there have been many scientific efforts in the direction of increasing DSSCs stability, by the development of novel counter electrodes with increased performance and robustness. Recently, Li et al. [217] focused on the fabrication of dissolution-resistant counter electrodes for DSSCs application. Their research showed that the use of PtM0.1 alloy counter electrodes to DSSCs can lead to a 20% improvement in solar cell efficiency and stability, compared to the conventional Pt-based DSSCs. The competitive dissolution reactions from guest M atoms and liquid state electrolyte contributed to the better long-term stability of PtM0.1 alloy-based counter electrodes. Kim et al. [218] synthesized edge selectively antimony-doped graphene nanoplatelets (SbGnNPs) by a simple mechano-chemical reaction between graphite (G) and Sb powder. The resultant SbGnNPs were tested as the counter electrode in DSSCs. Notably, the extraordinarily enhanced current-voltage characteristics and long-term stability of the SbGnNPs-based DSSCs outperformed those of the conventional Pt-based DSSCs. Kung et al. [219] fabricated one-dimensional CoS acicular nanorod arrays on FTO substrates, for application as counter electrodes to DSSCs. By the use of the aforementioned counter electrode to DSSCs, a better stability was achieved to the solar cells compared to the corresponding of Pt-based DSSCs, while in both cases the energy conversion efficiency was nearly the same. Gao et al. [220] synthesized polyvinylpyrrolidone/polyaniline (PVP/PANI) nanocomposites and applied them as transparent counter electrodes to DSSCs. The DSSCs fabricated with PVP/PANI-based counter electrodes exhibited improved efficiency and durability compared to the conventional Pt-based DSSCs. Zhang et al. [221] achieved an improvement in DSSCs stability by replacing the conventional Pt-based counter electrodes with novel electrospun FeS nanorods-based counter electrodes, while the energy conversion efficiency of the two devices was comparable. Feng et al. [222] fabricated a nanocomposite graphitic-boxes/NiSe- based counter electrode. Their electrochemical results indicated that the synergetic effect between graphitic boxes and NiSe, along with the large specific surface area can greatly improve the catalytic activity of triiodides reduction to iodides. Furthermore, the presence of graphitic boxes can impressively strengthen the erosion resistance to the electrolyte. Therefore, the DSSCs employing the graphitic-boxes/NiSe counter electrodes yielded a higher efficiency than the corresponding attained by Pt-based counter electrodes, while a quite satisfactory long-term stability was achieved in solar cells, maintaining 95% of their initial performance after 1000 h of light soaking. Wu et al. [223] fabricated a TiC/Pt composite counter electrode for DSSCs application. The solar cells employing the aforementioned counter electrode demonstrated a high energy conversion efficiency, higher than the conventional Pt-based DSSCs, and a high stability, maintaining 86.9% of their initial performance after one year. 3.3. Cost The ultimate goal of any PV technology is to achieve a cost-per-watt ratio that could compete against the conventional fossil fuel technologies. In the past decade, silicon-based solar cells have developed rapidly, and their production cost has decreased remarkably [38]. Today, silicon-based

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solar cells production cost ranges from about 0.40 €/W [224]. According to “Levelized Cost and Levelized Avoided Cost of New Generation Resources in the Annual Energy Outlook 2018”, nowadays PVs have achieved an LCOE on the range of 3.70 €cents/kWh to 9.90 €cents/kWh, which is comparable to the corresponding values of the traditional energy sources [225]. In 2007, Grätzel et al. [226] calculated the manufacturing costs of conventional dye-sensitized solar modulus (DSSMs), based on two analyses. The first was based on a high level of labor and semi-automatic pieces of equipment, whereas the other one was based on a higher level of automation. Their common result was that the DSSMs production costs, projected on a large-volume demand, would be 120–150 €/m2. The cost distributions implied that the production costs of DSSCs technology mainly come from the materials (50–70%), especially from the TCO glass and the dye. In view of costs reduction potential, a second group of analyses was executed. Based on the forecasted development of DSSCs technology, the calculated production costs of DSSMs were 70–90 €/m2. With the efficiencies obtained ranging on the order of 10%, their analysis showed that DSSMs production cost could end up below 2 €/W, while taking the forecasted materials costs reduction into consideration, they calculated the costs at about 1 €/W. In 2009, Kalowekamo et al. [227] estimated the manufacturing costs of DSSMs and compared them with the corresponding of mc-Si solar cell and CdTe solar cell technologies. In their study, the estimated DSSMs cost varied from 0.65 €/W to 0.82 €/W, accounting a manufacturing cost 32–40 €/m2 and an energy conversion efficiency of 5%. According to their analysis, major contributions to the manufacturing cost of DSSMs arise primarily from the conductive substrate. On the other hand, the module cost of mc-Si and CdTe technology was estimated 2.04 €/W and 1.44 €/W, respectively. Based on the aforementioned numbers, DSSCs technology was considered an interesting low-cost alternative solar cell technology to the other well- established solar cell technologies. The next years, DSSMs developers and manufacturers reported analyses of PVs production at large-scales. Solaronix Ltd., one of the world's leading DSSMs developers presented its analysis for a 90 cm x 60 cm solar module [228]. According to its report, two-thirds of the total module cost arise from materials; dyes and electrolytes contributed one third, sealing and interconnections cost more than one third (37%) and substrates cost about 17% of the total materials cost (see Figure 3-7). Following this report, many suggestions took place in the direction of DSSMs production cost reduction. Fujikura Ltd., a DSSM manufacturer, reported that PVs production costs on the order of 0.35 €/W can be achieved, provided that an annual production level of 100 MW is achieved [229]. The last few years of PV market evolution showed that the main issue is not how high the current costs are, but rather how quickly they come down.

Figure 3-7: Estimated cost analysis for a dye-sensitized solar module, by Solaronix Ltd [230]. Today, given the fact that the cost of silicon-based solar cells has continuously decreased during the last years, the aforementioned superiority of conventional DSSCs in cost has almost disappeared. Therefore, the need to further reduce the DSSMs production costs is imperative, aiming to DSSCs high global market share. Scaling up from DSSCs to DSSMs requires better and cost-effective materials, that could lead to low-cost solar cells, without performance and stability limitations.

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Below, there is a brief overview of the recent progress in DSSCs technology, in the direction of achieving lower cost DSSCs. The research efforts are classified according to the part of the solar cell which is under investigation, namely dye-sensitized working electrode, electrolyte, and counter electrode. 3.3.1. Dye-Sensitized Working Electrode

Amongst the most costly components in DSSCs are the TCO substrate, the dye, and the TiO2 [36]. FTO glass is regarded, as shown in many studies and analyses, the most expensive part of DSSCs [55,227]. The cost of glass per module area doubles for sandwich structures compared to monolithic devices. In the case of dye and TiO2, the raw materials themselves are not very costly compared to their processing and synthesis steps. Ru cost amounts only to about 10% of the dye cost. The rest of the dye molecule is inexpensive organics, but the synthesis and attachment of ligands and subsequent purification are quite expensive. TiO2 is abundant and cheap, but the hydrothermal synthesis of TiO2 colloids is relatively expensive. The cost of hydrothermally prepared colloidal TiO2 is usually higher than the targeted production costs. Other preparation methods such as flame pyrolysis are less expensive, but control over particle size and shape is sacrificed and efficiencies are generally lower [36]. The past years, many scientists have been devoted to the fabrication of lower-cost dye-sensitized working electrodes than the conventional counterparts. Starting from the TCO substrate, as suggested by Hashmi et al. [55], if the FTO glass is replaced with a flexible plastic, such as ITO polyethyleneterephtalate (PET) and ITO polyethylenenaphtalate (PEN), a significant cut down on the costs of DSSMs materials is expected. The cost of ITO PET and ITO PEN is much lower than FTO glass, while their usage in the fabrication of dye-sensitized working electrodes opens the way for using low-cost fabrication techniques, such as roll-to-roll production. Nonetheless, it is known that the usage of the aforementioned substrates in DSSCs, in most cases, leads to a lower energy conversion efficiency and stability. From a cost point of view, the scarcity of ITO can be a drawback for DSSCs production on a large scale [55]. Nowadays, alternative TCO coatings on plastics are being sought for. CNTs have been extensively experimented as an alternative to TCO, in numerous electronic and optoelectronic devices [231,232]. CNTs exhibit good electrical conductivity, large surface area, robustness, chemical inertness, while they can be mass produced. However, the difficulty is to get both high transparency and low RSheet at the same time. RSheet of 60–95 Ω/sq with high optical transparency (>80%) on flexible substrates has been obtained. However, these characteristics do not match those of the best TCO layers [55]. Lightweight, highly conductive, and flexible metal wire meshes have also been employed in DSSCs, to achieve a semi-transparent −3 conductive layer, without the use of TCO [233–235]. Ti metal meshes have low RSheet (1×10 Ω/sq) and are able to cope with the high temperature of TiO2 sintering process. However, these materials are comparatively thick in many cases, leading to efficiency limitations, particularly in the case of the usage of high viscosity electrolytes in DSSCs, such as ionic liquid electrolytes. Other factors, such as the space between knitted Ti wires, can also increase the resistance between wires and this can lead to a decrease in the overall performance of the solar cell [55]. Implementation of these meshes in DSSCs has recently yielded to high energy conversion efficiencies, on the order of 8% [233]. An alternative strategy is to use non-corrosive electrolytes, such as polymer electrolytes with commercially available Ag ink printed grids. Such printed metal grids can have 70–80% of the open area and result in lower RSheet (< 1 Ω/sq), which is one order of magnitude lower compared to commonly used TCO layers. With lower RSheet, the solar cell efficiency could be improved and the size of a unit cell in a DSSM enlarged, leading to improved aperture ratio, higher efficiency in a module level, and thus to lower LCOE [55].

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On the other hand, metal foils can also be used to replace the expensive dye-sensitized electrode substrates, in back-side illuminated solar cell designs. For example, when using stainless steel (StS) foil, the costs for one substrate would on average be more than 50% lower compared to ITO PET and 80% compared to FTO glass. Radical reductions (over 99%), in a single substrate cost, compared to TCO glass could be achieved when transferring the technology to very low-cost metals, such as Al [55]. Energy conversion efficiencies on the order of 8% have been reported in DSSCs technology, using metal-based dye-sensitized working electrodes [236]. Park et al. [236] demonstrated a DSSC using a working electrode based on TiO2 deposited on an StS coated with ITO and SiOx layers. The metal-based working electrodes, such as the ones on StS, have shown sufficient characteristics for roll-to-roll production, which can lead to lower manufacturing costs, while crack-free and highly adhesive films on StS were reported after the bending and standard tape tests [237]. However, in the case of StS, the stability is a critical problem since the solar cells can lose even 80–90% of performance in a couple of hours, under 1 sun illumination [238]. An alternative low-cost metal substrate is Al foil, however, the need for the development of non-corrosive electrolytes is, in this case, imperative [239]. Furthermore, novel approaches for the fabrication of lower-cost DSSCs appear, such that of Fuke et al [240]. This group introduced a newly structured DSSC, called back contact dye-sensitized solar cell. In this design, a porous metal layer was used as a back contact electrode instead of the TCO electrode used in the conventional DSSC. By optimization of the novel designed DSSC, an energy conversion efficiency of 8.4% was achieved, while the cost of the solar cell is considered quite reduced. Choosing an optimum fabrication method for the dye-sensitized working electrode, in order to achieve a low-cost high-efficiency DSSM, depends on several factors. Some of them are the production volume, the production flexibility, the capital investment, the energy demand, the human factor etc [241]. There exists a triangular closed loop relationship among materials, properties, and fabrication. The properties of dye-sensitized working electrodes depend upon the manufacturing method selected. First of all, the precursor materials properties are of great importance. Today, more and more researchers are focusing on the fabrication of low-cost TiO2 nanocrystalline porous electrodes with a large surface area, where more dyes can be sufficiently adsorbed and result in a higher light-to-electricity conversion efficiency. In order to fabricate excellent TiO2 porous electrodes, the challenge is to find an optimal method to fabricate the corresponding TiO2 paste with unique quality and characteristics combining with low cost. For the best-performing TiO2 electrodes, o the synthesis of TiO2 paste involves hydrolysis of Ti(OCH(CH3)2)4 in water at 250 C (70 atm) for 12 h, followed by conversion of the water to ethanol by three-times centrifugation. Finally, the ethanol is exchanged with a-terpineol by sonication and evaporation. In other words, this method needs specialized and complicated equipment and techniques which are economically unsuitable for industrial production [242]. In order to simplify the technology, several reports on fabricating TiO2 pastes from commercially-available nanopowders have already been published [243–245]. Some of them are based on low-cost simple chemical techniques, which are even suitable for industrial DSSCs production [242,246]. Concerning the coating method of TiO2 pastes, up until today, a large number of methods for fabrication of DSSCs working electrodes have been developed. These are categorized into physical methods, such as liquid phase precursor (dip coating, spin-coating, doctor-blading, screen-printing, inkjet-printing, electrospray deposition), gas phase precursor (physical vapor deposition), chemical methods, such as liquid phase precursor (electrochemical deposition, solvothermal/hydrothermal method, chemical bath deposition, spray pyrolysis methods, sol-gel coatings, template method), and gas phase precursor (chemical vapor deposition, atomic layer deposition, thermal oxidation) [241]. Amongst them, cost-effective fabrication methods are considered the doctor-blading (low-cost equipment), the inkjet-printing (large production, low materials usage), the electrospray deposition (low materials usage) and the spray pyrolysis (low-cost

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equipment). Finally, in the last decade, alternative methods for fabrication of low-cost dye-sensitized working electrode have been applied. Due to the simplicity of fabrication and high reproducibility, particular attention has been paid to the anodization of metals for the fabrication of low-cost dye- sensitized working electrodes for DSSCs [247–249]. Many scientific efforts are devoted nowadays to the improvement of this type of working electrodes characteristics, towards the fabrication of low- cost and high-efficiency DSSCs [250]. Research on the synthesis of novel dye complexes and development of new dye sensitization methods could also result in lower-cost DSSCs. The development of dyes with higher extinction coefficients can not only improve efficiency but also reduce the cost, by requiring less dye and TiO2 per module area [36]. Several metal-free dyes have been reported as sensitizers in DSSCs, representatives including coumarin-, indoline-, arylamine-, oligoene-, merocyanine-, hemicyanine-, phenothiazine-, carbazole-, and triphenylamine-based compounds [251]. Metal-free organic dyes are considered as a cost-effective solution to the costly and rare Ru-based dyes [252]. Today, the performance of DSSCs using organic dyes is as high as the corresponding employing Ru-based dyes [253–256]. Porphyrins have also a certain advantage over Ru-based dyes since they can be conveniently synthesized by well-established protocols, are inexpensive, stable, durable, and less toxic, while porphyrins derivatives show relatively higher molar absorption coefficients than Ru- based dyes [257]. Recent publications have shown that efficiencies higher than 12% can be achieved using porphyrin-based dyes to DSSCs [125,258]. Furthermore, natural dyes are considered a low-cost alternative to conventional Ru-based dyes [259–261]. Naturally available fruits, flowers, leave, bacteria etc. exhibit various colors and contain several pigments that can be easily extracted and employed in DSSCs. These plant pigments exhibit an electronic structure that interacts with sunlight and alters the wavelengths that are either transmitted or reflected by the plant tissue. This process leads to the occurrence of plant pigmentation and each pigment is described from the wavelength of maximum absorbance and the color perceived by humans. Pigments for natural dyes include chlorophyll, carotenoids, flavonoids, and anthocyanins. Advantages of employing these natural dyes as photo-sensitizers in DSSCs, except cost-effectiveness, are their satisfactory absorption coefficients in the visible region, relative abundance, ease of preparation, and environmental friendliness [260]. However, there are remaining drawbacks towards their wide application in DSSCs, such as their low performance in terms of energy conversion efficiency and low stability compared to synthetic dyes [259]. On the other hand, novel sensitization methods have been developed in the last years, that could lead to reduced manufacturing duration and cost of DSSMs [262–267]. The co-sensitization strategy is a very effective way to obtain high-efficiency DSSCs with lower cost when compared with the synthesis of panchromatic sensitizers [263]. DSSCs fabrication using inkjet-printed dyes also stands out. The unique feature of inkjet printing is that it allows accurate control of dye loading with respect to both the amount and position on the TiO2 film [264]. This offers several advantages over the conventional dye application methods. First, close to full coloration can be achieved without rinsing off any excess dye, which simplifies the DSSC fabrication process and reduces material consumption. Second, the color density of the films can be tuned without changing their thickness, which offers a new degree of freedom in the design of semi-transparent DSSCs, for example, for BIPVs. Third, the high spatial resolution of inkjet printing makes it possible to create color density gradients and patterns of multiple dyes on the same photo-electrode, which suggests a new avenue for creating multi-colored DSSC designs attractive, for example for stylish product integration of DSSCs. 3.3.2. Electrolyte The cost of the electrolyte is not considered as a drawback for DSSCs wide commercialization when only the solvent-based electrolytes are used [268]. In contrast, the advanced quasi-solid state

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electrolytes and solid state hole transport materials are still costly in many cases, and further development is required. Specifically talking, the cost of ionic liquids is still high, while the best- performing hole transport materials are still quite expensive. Recently, many research efforts have been devoted to the development of the corresponding cost-effective materials for DSSCs application. Concerning the ionic liquid electrolytes, alternative cost-effective ionic liquids have been developed in the last years for application in DSSCs. Shi et al. [269] reported the preparation of a cost-effective and low-viscosity ionic liquid, named 1-ethyl-3-methylimidazolium trifluoroacetate, and its application in electrolytes for DSSCs. By the application of the aforementioned low-cost ionic liquid in combination with PMII in DSSCs, they demonstrated a satisfactory solar-to-electricity conversion efficiency. Later, Hao et al. [270] investigated the application of a similar cost-effective binary ionic liquid electrolyte in DSSCs, achieving better results. Their investigation showed an enhancement of the efficiency of the solar cell compared to the device employing a conventional PMII-based electrolyte. The employment of the novel binary ionic liquid electrolyte was able to significantly decrease the diffusion resistance of the triiodide species in the electrolyte and retard the charge recombination between the injected electrons in the anode with triiodide anions in the electrolyte. Furthermore, many novel low-cost quasi-solid state electrolytes have been developed in the last decade for DSSCs application, leading to promising results [271–274]. Preparation and application techniques of quasi-solid state electrolytes, such as photopolymerization, polymer blending, and inkjet-printing, have been on the focus the last few years since they are considered easy, fast, low-cost, and reliable ways for fabrication of QSS-DSSCs [275–277]. Special attention has been given to polymer blend electrolytes since polymer blending is a simple and low-cost method for preparation of advanced polymer electrolytes with a wide variety of properties. A polyblend electrolyte is prepared easily by a simple mixing and its physical properties are devised by changing the compositions of the polyblend [276]. Karthika et al. [278] prepared a novel low-cost solvent-free polymer blend electrolyte comprising of polyethyleneimine, polyethylene glycol, KI, I2 with newly synthesized N,O,S based organic compounds and applied them to DSSCs. Their results were quite satisfactory since an energy conversion efficiency on the order of 9% was achieved under 70 mW/m2. Ganesan et al. [279] reported the preparation of a novel low-cost polymer blend electrolyte comprised of PEO/PVDF/PMMA with KI, I2, and novel and cost-effective organic compounds, such as 1-(2-(2- (2-(2-(benzoate)ethoxy)ethoxy)ethoxy)ethoxy) benzene and 1-(2-(2-(2-(2-(1H-pyrazol-1- yl)ethoxy)ethoxy)ethoxy)ethyl)-1H-pyrazole, for DSSCs application. The DSSCs employing the aforementioned electrolytes demonstrated a high energy conversion efficiency, on the order of 9%. Moreover, it would be an omission not to mention the great research efforts that have been made the last decade for the development of aqueous DSSCs. The use of water as the solvent of electrolytes for DSSCs can lead to cheaper solar cells, and less dangerous in terms of flammability and toxicity, thus to even more environmentally friendly devices [280]. In order to fabricate efficient aqueous DSSCs, several research groups worldwide are involved in an impressive scientific work, proposing new dyes, working electrodes, counter electrodes, redox couples, additives, and sealants [280]. Many reports show that the results obtained so far in terms of efficiency and durability are promising, sometimes outstanding, and fully justify the recent boom in the research and development of aqueous DSSCs. The use of water in solar energy conversion devices was intended to be a big bet by a great part of the scientific community but, based on the present results in the literature, it may very soon become the key for the success of this green technology to finally enter the mass production stage [280]. Concerning the SS-DSSCs, the development of high-performance and cost-effective hole transport materials is still an open issue. Spiro-OMeTAD is still considered as the most successful hole transport material for solar cells application since by its application to PSCs energy conversion efficiencies on the order of 20% have been achieved [281,282]. However, it has been widely

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demonstrated that the complex synthetic route and purification, rendering a high synthetic cost of the 2,2',7,7'-tetrabromo-9,9'-spirobi[fluorene] core, significantly limits Spiro-OMeTAD usage for large- scale application in the future [215]. Although several low-cost alternatives were developed for solar cells application, the synthesis simplification and PV performance are still not competitive in most of these cases with Spiro-OMeTAD [59,283]. Nevertheless, just recently, some scientific reports presented a better performance to cost ratio. In 2017, Lui et al. [284] designed and synthesized two low-cost π-conjugated molecules for application as hole transport materials in DSSCs and PSCs. The synthesis route for the two materials was much more facile and environmentally friendly, compared with Spiro-OMeTAD. The highest energy conversion efficiency achieved using the novel hole transport materials was 6.1% for SS-DSSCs and 16.4% for PSCs, outperforming the Spiro- OMeTAD-based devices. In 2016, Xu et al. [215] designed and synthesized a low-cost spiro[fluorene- 9,9′-xanthene]-based hole transport material and applied it to both DSSCs and PSCs. Their results were quite satisfactory since the devices employing the aforementioned low-cost hole transport material showed high energy conversion efficiencies of up to 7.3% in DSSCs and to 19.84% in PSCs under 100 mW/cm2. These were comparable with the reported efficiencies of 7.7% in DSSCs and 20.8% in PSCs obtained by Spiro-OMeTAD-based devices. The same year, Hua et al. [285] designed and synthesized two novel low-cost fluorene-based hole transport materials, as alternatives to the expensive Spiro-OMeTAD, for the application in SS-DSSCs and PSCs. The two materials were prepared through a facile two-step reaction from cheap starting material and with a total yield higher than 90%. Solar cells based on the aforementioned hole transport materials achieved high energy conversions efficiencies, higher than 6% for SS-DSSCs and 18% for PSCs, which were comparable to the corresponding results attained by Spiro-OMeTAD-based devices. Finally, the last few years, perovskites of the type Cs2SnX6 (X = Cl, Br, I) have been employed in DSSCs as alternative low- cost and nontoxic hole transport materials [286]. The results, as it is reported in §3.2.2, are quite satisfactory and very promising for the development of low-cost, high-efficiency, and high-stability solar cells. 3.3.3. Counter Electrode Today, the main aspects to be considered for counter electrodes of DSSCs are the low-cost and optical transparency [141]. During the last decades, several research efforts have been devoted to the development of cost-effective counter electrodes for DSSCs that could maintain the solar-to- electricity conversion efficiency at high levels. Concerning the Pt-based counter electrodes, the aim is to reduce the amount of the costly Pt, without lowering the catalytic activity of the counter electrode [141]. Alternative methods have been applied for the fabrication of Pt-based counter electrodes, using low Pt quantities. In this direction, Pt nanoparticles can be synthesized by the electrochemical reduction of hexachloroplatinate or by the thermal decomposition of chloroplatinic acid. These alternative methods require only low Pt quantities, around 10–100 mg/cm2, thus the costs can be reduced [141]. Calogero et al. [287] fabricated a low-cost transparent counter electrode for DSSCs, based on Pt nanoparticles, by a bottom-up synthetic approach, in which the Pt nanoparticles have a high surface area and homogeneity compared to Pt-sputtered counter electrodes. The high surface area of the Pt nanoparticles indicates a large number of active sites, which are available for the reduction of triiodides, thus increasing the catalytic activity of the counter electrode. Iefanova et al. [288] fabricated highly transparent Pt-based counter electrodes by spray coating of Pt nanoparticles on hot substrates. This method led to 86% reduction in Pt consumption compared to the conventional Pt-based counter electrodes made by sputter deposition. The simplicity and low cost of this method provided a basis for an up-scalable fabrication process. The achieved results were quite satisfactory since the novel and conventional devices attained a quite similar performance. Furthermore,

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alternative methods for fabrication of cost-effective Pt-based counter electrodes have been reported [141]. On the other hand, with Pt being a costly noble metal, reasonable efforts have been made to find cheaper alternatives. Several varieties of low-cost candidates (see Figure 3-8), such as carbonaceous materials, conductive polymers, and transition metals, including metal sulfides, metal carbides, metal nitrides, and metal oxides, have been widely investigated as novel low-cost counter electrode alternatives for DSSCs and demonstrated outstanding catalytic performance for the triiodides reduction [141,142].

Figure 3-8: Alternative materials for fabrication of low-cost counter electrodes for dye-sensitized solar cells. ➢ Carbon-based counter electrodes Amongst them, carbon-based counter electrodes have attracted great interest, because of their low cost, good catalytic activity, and superior chemical stability, which make them remarkable potential alternatives to expensive Pt-based counter electrodes for large-scale DSSCs application. In particular, carbonaceous materials such as G, carbon black (CB), amorphous porous carbon, activated carbon (AC), CNTs, and graphene have been used with great success as a cathode in DSSCs [289]. G is a promising counter electrode electrocatalyst and conducting layer material, due to the abundant defect sites and good electronic properties. Particularly, the defect sites from edge planes are preferred to those of basal planes, as the former exhibits faster electrons transport and charge transfer. For large G particles, fewer edge planes slow the rate of triiodides reduction, which has a negative influence on the fill factor and efficiency of solar cells [289]. Veerappan et al. [290] tested sub-micrometer-sized colloidal G as a conducting electrode to replace TCO and as a catalytic material to replace Pt for triiodides reduction in DSSCs. Under 1 sun illumination, DSSCs with sub- micrometer-sized G as a catalyst on FTO showed an energy conversion efficiency greater than 6%, which was comparable to the energy conversion efficiency attained by DSSCs employing Pt-based counter electrode. DSSCs with TCO-free G-based counter electrodes showed an energy conversion efficiency greater than 5%, which demonstrated that the G layer could be used both as a conducting layer and catalytic layer, resulting in lower-cost DSSCs. Recently, Li et al. [291] investigated three G materials with different structures, including graphite nanofibers (GNFs), graphite nanosheets (GNSs), and graphite nanoballs (GNBs), as catalytic films of counter electrodes for DSSCs. The GNB-based DSSC showed an energy conversion efficiency of 7.88%, while those of GNF-based and GNS-based DSSCs showed efficiencies of 3.60% and 2.99%, respectively. In their investigation, it

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was observed that the efficiency of the GNB-based DSSC was quite close to that of Pt-based DSSCs. This implies that the GNBs can become a potential candidate to replace the expensive Pt, owing to their low-cost. CB can be a promising and cheap cathode material when taking its electrical conductivity and its catalytic ability for the reduction of triiodides into consideration. Specifically, compared to highly orientated carbons, such as G and CNTs, CB has lower crystallinity and more catalytic sites, which may be helpful for the improvement of charge-transfer ability at counter electrode/electrolyte interface [289]. Grätzel and co-workers explored CB-based counter electrodes as a catalyst for triiodides reduction in DSSCs [292]. By increasing the thickness of CB layer, they observed a great decrease in charge-transfer resistance at the counter electrode/electrolyte interface. Using CB-based counter electrodes in DSSCs, they achieved a high energy conversion efficiency, on the order of 9%, under full sun. A similar result was also confirmed by Rhee et al. [293]. They found that small-sized CB (20 nm) with increased electrode thickness (9 μm) guarantee an excellent catalytic activity, owing to the increased surface area and good conductivity of CB layer. Recently, Wu et al. [294] fabricated high-performance CB-based counter electrodes for DSSCs, achieving efficiencies comparable to the solar cells employing Pt-based counter electrodes. In their investigation, they used PVDF as a binder to regulate the viscosity of the CB paste for the doctor-blading process, while PVDF was removed at the thermal treatment of the counter electrode. Their results showed that after thermal treatment, all CB films showed a mesoporous structure. Another notable recent research effort using CB-based counter electrodes was made by Liu et al [295]. This group tested CB as a cathode in combination with cobalt (iii)/(ii)-based electrolytes. Their investigation definitely verified the feasibility of this cost-effective material for utilization in DSSCs, achieving in some cases even better performance than the corresponding of DSSCs employing Pt-based counter electrode. Amorphous porous carbons with a high specific surface area and unique porous structure are extensively applied as a cathode for DSSCs. Generally, these porous carbons can be classified into three categories according to their pore size, namely microporous (<2 nm), mesoporous (2–50 nm), and macroporous (>50 nm) carbons. The porous carbons with different porous structure exhibit different catalytic behaviors toward triiodides reduction in DSSCs [289]. For example, high surface area microporous carbon with a pore size of ca. 2.12 nm was synthesized by Chen et al. [296]. Owing to the physical and chemical activation process by CO2, H2O, and KOH, the optimum surface area of resultant microporous carbon was 3000 m2g-1, which is favorable for the adsorption of abundant triiodide ions. With the large surface area and different surface defects (dislocations, vacancies, discontinuities, atoms with “free” valences, etc.), the as-prepared microporous carbon showed a good - - catalytic activity for I /I3 redox reactions. The solar cell fabricated with the high surface area microporous carbon-based counter electrode reached the relatively high energy conversion efficiency of 7.36% and a large fill factor of 0.62. Nevertheless, the small pore size resulted in the slow diffusion of the electrolyte ions into numerous catalytic sites and thus restricted the electrocatalytic activity of microporous carbon-based counter electrode to some extent. To solve the above-mentioned problem, Lee and co-workers prepared ordered mesoporous carbon using a simple soft-template approach and studied them as a cathode in DSSCs [297]. Well-connected carbon framework with superior mesoporous structure gives an easy access to the internal surface area of carbon particles, and high surface area (∼1575 m2 g-1) still offers abundant catalytic active sites, thereby leading to a small charge transfer resistance at the counter electrode/electrolyte interface. The achieved energy conversion efficiency, in this case, was 7.46%, with the fill factor to range at 0.68. Zhao’s group [298] also confirmed that high accessible surface area with favorable porosity rendered the ordered mesoporous carbon-based counter electrode a relatively superior catalytic activity compared to that of CB. Zhao and co-workers further evaluated the effect of the pore structure of different carbon materials on the PV performance [299]. In terms of the size of electrolyte ions (triiodides: ca. 1 nm),

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they demonstrated that microporous structure of AC greatly reduces ions-accessible active sites, owing to the difficult diffusion and adsorption of triiodides. In contrast, the mesoporous structure of carbon aerogel facilitates the migration and adsorption of triiodide ions, resulting in the exposure of more active sites for triiodide reduction and achieving much better electrocatalytic activity in comparison with those of the AC. In order to accelerate the electrolyte ions diffusion into the carbon- based counter electrode, three-dimensional ordered macroporous carbon was also fabricated and investigated as the cathode in DSSCs by Chen et al. [300]. The results showed that the DSSCs with three-dimensional ordered macroporous carbon reaches a higher fill factor and larger energy conversion efficiency than those of AC-based devices. However, the macroporous carbon exhibits a relatively low specific surface area compared to those of mesoporous and microporous carbons, which caused its insufficient active sites in catalyzing triiodides to iodides. Combining the structural advantages of macropores and micro-mesopores, the hierarchical porous carbon is of particularly great interest for the design of high-performance counter electrodes for DSSCs. Ko’s group [301] explored the ordered multimodal porous carbon as a cathode in DSSCs and achieved an energy conversion efficiency of 8.67%, which was close to that of DSSCs employing Pt-based counter electrodes. CNTs have also been proposed as a prospective substitute for the conventional Pt, due to their outstanding advantages of the large surface area, high electrical conductivity, and chemical stability [289]. Lee et al. [302] reported the successful application of multiwall carbon nanotubes (MWCNTs) as electrocatalysts for triiodide reduction in DSSCs. Defect-rich edge planes of bamboolike-structure MWCNTs facilitated the electron-transfer kinetics at the counter electrode/electrolyte interface, resulting in low charge-transfer resistance and an improved fill factor. The achieved efficiency using MWCNTs-based counter electrode was comparable to the corresponding obtained by Pt-based counter electrodes. Lou and co-workers investigated the DSSC based on the novel vertically aligned single-walled carbon nanotubes (SWCNTs) [303]. The authors utilized a scalable dry transfer approach to produce the SWCNTs on FTO glass substrates and further studied the effect of lengths of SWCNTs on the electrocatalytic activity. Their experiments showed that the DSSCs employing the aforementioned carbon-based counter electrodes demonstrate a comparable performance to the corresponding of DSSCs employing Pt-based counter electrode. Recently, Chen et al. [304] developed an ultrafast process for synthesizing carbon-nanotube films using atmospheric-pressure plasma jet. The processing time was only 5 s. The synthesized material was used as a counter electrode in DSSCs. With these Pt-free counter electrodes, the assembled DSSCs showed a comparable efficiency with the corresponding of solar cells employing counter electrodes made by a conventional furnace calcination process. The energy consumption was estimated to be 500 J/cm2, which is about one-fifth of the counter electrodes fabrication by conventional furnace process. Their investigation, therefore, demonstrated that this is an eco-friendly, cost- and time-saving process for fabricating energy devices. Graphene is also a nanomaterial that possesses fascinating properties for DSSCs, such as high surface area (2630 m2g-1), high thermal conductivity (∼5000 W/mK), fast charged carrier mobility (∼200,000 cm2V-1s-1), high optical transmittance (97.7%), and chemical inertness [289]. To date, many scientific reports demonstrate the high electrocatalytic activity of graphene in DSSCs. Different techniques are used to produce graphene nanosheets (GnNSs)-based counter electrodes for DSSCs. One of the most efficient methods to produce GnNSs is by the oxidative exfoliation of G, followed by hydrazine reduction. By the use of the aforementioned method, energy conversion efficiencies on the order of 7% have been achieved [305]. Various other techniques are also available to fabricate graphene-based counter electrodes, e.g. by the chemical reduction of GnO colloids under microwave irradiation, electrophoretic deposition followed by annealing, thermal exfoliation from GnO, etc [141]. The DSSCs performance with graphene as cathode depends upon the structure of the graphene.

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Extremely pure graphene has an excellent conductivity but possesses a limited number of active sites for triiodides reduction [141]. Kavan et al. [306] proved that the electrocatalytic activity of GnNSs is mainly associated with the defects and oxygen-containing functionalized groups present in them. The GnNSs prepared from G by the oxidation-reduction approach contains lattice surface defects and these lattice defects are considered as the electrocatalytic active sites. Roy-Mayhew et al. [307] found that the electrocatalytic activity towards the triiodides reduction can be increased by increasing the number of oxygen-containing functional groups in the graphene sheet. On the other hand, Xu et al. [308] reported that reduced GnO functionalized with the –NHCO– group exhibited a higher catalytic activity than the original, indicating that –NHCO– groups can increase the catalytic activity. In 2018, Kim et al. [309] prepared a series of edge-functionalized graphene nanoplatelets (GnNPs) through a simple mechano-chemical ball-milling process, to create edge-halogenated GnNPs and metalloid- doped GnNPs. Their aim was to apply them as a cathode for DSSCs, using different dyes and electrolytes, for a comparative study. Their results were quite satisfactory since a better performance was achieved using GnNPs as a cathode for DSSCs compared to the Pt, reaching efficiencies that even exceed the 12% under full sun. Today, achieving a comparable or even a better catalytic activity to that of Pt becomes the focus of research for carbon-based counter electrodes. Many efforts have been devoted to optimizing the physical and chemical properties of carbon from different perspectives, such as their pore structure, electrical conductivity, and catalytic active sites [289]. Carbon composites are studied intensively nowadays in the direction of fabrication of high-efficiency Pt-free DSSCs. Mehmood et al. [310] investigated the application of different carbon/carbon nanocomposites, such as AC/MWCNTs, MWCNTs/graphene, graphene/AC, and AC/MWCNTs/graphene, as counter electrodes for DSSCs. Their investigation demonstrated that the AC/MWCNTs/graphene-based counter electrode lead to the highest energy conversion efficiency when employed to DSSCs, which was on the order of 11%. The aforementioned efficiency was comparable to the corresponding obtained by Pt-based devices. Zhang et al. [311] synthesized a highly interconnected 3D nitrogen-doped graphene/reduce CNT-OH composite aerogel (NG/CNT-OH) with unique hierarchical porosity. This composite was used as a catalyst for fabrication of TCO-free counter electrodes for DSSCs, exhibiting high electrocatalytic activity, excellent carrier (electron/ion) transport ability, and exceptional mechanical flexibility, simultaneously. DSSCs employing NG/CNT-OH-based counter electrodes demonstrated a better efficiency compared to Pt-based DSSCs, revealing that the aforementioned 3D structure presents a great potential way to fabricate low-cost, metal-free, flexible, and TCO-free counter electrodes for DSSCs, with excellent performance. In 2018, Sun et al. [312] fabricated graphite nanoplatelet/AC composites of different wt% AC fillers for their intended use as counter electrodes in DSSCs. The high defect-rich morphology and good bonding strength of the composite led to the fabrication of high-efficiency DSSCs, higher than 8% and comparable to the corresponding of Pt-based devices. ➢ Polymer-based counter electrodes Conducting polymers are spectacular materials that can be used as a cathode in DSSCs. A conducting polymer has the potential to replace Pt because of its high electrochemical activity and lower cost [141]. The most preferred conducting polymer is PEDOT, due to its high stability, electrochemical activity, and transparency. Other polymers such as PPy and PANI have also been used for the same reasons [141]. A cost-effective conducting material exhibiting a high catalytic activity should be used simultaneously to act as a catalyst for triiodides reduction and charge transport. Lee et al. [313] synthesized a PEDOT-based counter electrode for DSSCs, without a TCO, by the in situ polymerization method. The PEDOT had a thickness of 60 nm and an 88% transmittance. The cyclic voltammetry curve of PEDOT coated on a glass substrate showed a higher peak compared to Pt, indicating that PEDOT-based counter electrodes are electrochemically active

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and possess a higher transfer rate, even though the PEDOT-based counter electrode has no TCO substrate. Ahmad et al. [314] reported the preparation of poly(3,4-propylenedioxythiophene) (PProDOT) as a low-cost alternative to Pt for triiodides reduction in DSSCs. In their investigation, they fabricated nanoporous PProDOT-based counter electrodes, by electrical field assisted growth in hydrophobic ionic solvents as the medium. A promising PV performance of 9.20% under standard test conditions was observed for PProDOT-based solar cells, which was comparable to the corresponding of Pt-based devices. During the last few years, there have been many research efforts for the fabrication of low-cost and high-performance polymer composite counter electrodes for DSSCs. In 2017, Li et al. [315] fabricated a novel transparent honeycomb-like PEDOT/MWCNT-based counter electrode for DSSCs. Cyclic voltammetry and electrochemical impedance spectroscopy measurements indicated that the honeycomb-like PEDOT/MWCNT-based counter electrode has a higher electrocatalytic activity for the triiodides reduction and a smaller charge transfer resistance than those of the flat PEDOT-based counter electrode. Ultraviolet-visible spectroscopy measurements indicated that the PEDOT/MWCNT-based counter electrode with the honeycomb-like nanostructure demonstrates high transparency for the back-side illumination. The bifacial DSSC based on this honeycomb-like PEDOT/MWCNT-based counter electrode showed energy conversion efficiencies of 9.07% and 5.62% from the front and rear side illumination, respectively, which were higher than those of the bifacial DSSC based on the flat PEDOT-based counter electrode (7.51% and 3.49%, respectively). The same year, Maiaugree et al. [316] fabricated NiS nanoparticle-poly(3,4-ethylendioxythiophene)- poly(styrene sulfonate) (PEDOT-PSS) composites on FTO substrates for their intended use as counter electrodes for DSSCs. By using the aforementioned composite counter electrodes, the highest solar cell efficiency was 8.18%, which was very close to the corresponding of the DSSCs employing the Pt-based counter electrode. Yu et al. [317] prepared a series of covalent bond–grafted soluble poly(o- methoxyaniline) graphene oxide (POMA-FGO) composite materials doped with 1S-(+)- camphorsulfonic acid (CSA) for application as a cathode in DSSCs. DSSCs with the POMA-FGO and POMA-FGO-CSA counter electrodes exhibited substantially low interfacial impedance, which may be attributed to the dual improvement from the well-dispersed FGO and the doping effect of CSA. Therefore, the efficiency levels of DSSCs employing the POMA-FGO counter electrodes were higher than those of the DSSCs employing the POMA counter electrodes. When CSA was additionally used as a dopant, the efficiency of DSSCs was further enhanced. In particular, the efficiency of DSSCs based on the POMA-FGO counter electrodes doped with CSA reached 8.81%, which was higher than the corresponding of DSSCs employing the conventional Pt-based counter electrode. Another remarkable scientific effort was made by Shih et al. [318]. This group successfully fabricated through electrochemical method PANI/GnNP/MWCNT composite films deposited on FTO substrates to provide a lower-cost counter electrode for DSSCs. By varying the contents of GnNP and MWCNT in the PANI composite counter electrode, the short-circuit current density of - - DSSC was found to linearly relate to the reduction current density of I /I3 redox couple measured by cyclic voltammetry. The open-circuit voltage was also highly dependent on the reduction potential of redox couple, while the charge transfer resistance at the electrolyte/counter electrode interface measured by electrochemical impedance spectroscopy had an approximately linear relationship with the RSheet of PANI composite-based counter electrode. Notably, the DSSC with the counter electrode fabricated by the optimized weight ratio of PANI/GnNP/MWCNT yielded a high energy conversion efficiency of 7.67%, which was comparable with the corresponding obtained by the conventional Pt- based device. In 2018, Ma et al. [319] reported a facile, low-cost, and easy-controllable method to fabricate PEDOT/reduced GnO composites by electrochemical deposition onto FTO substrates, for their application as counter electrodes for DSSCs. The electro-deposition process was accomplished by electropolymerization of the composites onto FTO substrates followed by electrochemical

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- - reduction of the GnO component. Their electrochemical measurements showed that the I /I3 catalytic activity of the as-prepared PEDOT/reduced GnO counter electrode was improved compared with that of the pure PEDOT and PEDOT/GO-based counter electrodes. The solar-to-electricity conversion efficiency of the DSSCs assembled with PEDOT/reduced GnO electrode reached 7.79%, close to 8.33% of the cell with the Pt-based counter electrode and increased by 13.2% compared with the device employing the PEDOT-based counter electrode. ➢ Transition metal-based counter electrodes As early as 1973, Levy and Boudart first proved that WC showed a Pt-like catalytic behavior, due to its specific electronic structure [320]. Other transition metal compounds, such as sulfides, nitrides, oxides, not limited to the carbides, were also proven consecutively to own Pt-like catalytic behavior [321]. In DSSCs, application of transition metal compound catalysts to replace the expensive Pt-based counter electrode kicked off with CoS and TiN in 2009 [322,323]. Since then, there are many transition metal-based counter electrodes that have been applied in DSSCs, with great success. In 2010, Lee’s group [324] first introduced WC in DSSCs as the counter electrode catalyst. After optimization, the DSSCs employing the WC-based counter electrode yielded a high energy conversion efficiency of 7.01%, which was, however, lower than the corresponding of DSSCs employing the Pt-based counter electrode (8.23%). Several research efforts followed containing carbides as a cathode for triiodide reduction, including WC, TiC, VC, Cr3C2, ZrC, NbC, Mo2C, etc, demonstrating that the carbides show great potential to replace the expensive Pt in DSSCs [321]. However, there are remaining disadvantages, such as the high energy consumption and the series particle aggregation in their synthesis process. Nitrides are also used as a cathode in DSSCs with great success [321]. TiN is one of the first used metal compounds at counter electrodes in DSSCs. Gao’s group [323] fabricated TiN nanotube arrays on Ti sheet (TiN/Ti) through anodization and nitridation steps. This kind of TiN/Ti can be used directly as a counter electrode for DSSCs, where the TiN worked as the catalyst and Ti as the electron collector. The corresponding DSSCs attained an energy conversion efficiency of 7.73%, slightly higher than the performance of the counterpart employing the Pt-based counter electrode (7.45%). Other nitrides, such as MoN, WN, and Fe2N, have also been used in DSSCs, showing promising results for replacing the expensive Pt [321]. Mesoporous structures of nitrides are on the focus today for overcoming the transport limitation at the counter electrode/electrolyte interface and achieving a high energy conversion efficiency. Several reports demonstrate the superiority of mesoporous nitrides-based counter electrodes usage in DSSCs compared to the compact one. Song et al. [325] reported that mesoporous MoN-based counter electrodes show a much better performance compared to compact MoN-based counter electrodes for DSSCs. Wang’s group [326] reported a unique mesoporous counter electrode using TiN microspheres on a Ti substrate. The DSSCs employing the mesoporous TiN-based counter electrode gave an energy conversion efficiency of 6.8%, much higher than the DSSCs employing the flat TiN-based counter electrodes (2.4%). The same as the carbides and nitrides, the oxides have also been used as catalysts to replace Pt in DSSCs [321]. According to Zhou et al. [327] investigation, WO2.72 have shown high catalytic activity for triiodides reduction and a comparable performance to the expensive Pt when applied to DSSCs. Ma’s group [328] synthesized and applied W18O49 nanofibers and hierarchical spheres in DSSCs, reporting quite satisfactory results. By using W18O49 nanofibers as a cathode, DSSCs showed a high efficiency of 8.58%, which was close to the conventional device employing a Pt-based counter electrode. Yang’s group [329] prepared RuO2 nanocrystals via the hydrothermal method and the prepared RuO2 presented high catalytic activity as a counter electrode catalyst in DSSCs. The achieved energy conversion efficiency was 7.22%, exceeding the corresponding of Pt-based DSSCs (7.17%).

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Concerning the sulfides, in 2009 CoS-based counter electrode was reported as a promising alternative to the expensive Pt. Grätzel’s group [322] fabricated a DSSC employing a CoS-based counter electrode that demonstrated higher performance than the corresponding of the Pt-based device, while durability tests confirmed that CoS was stable in severe conditions. Recently, Qian’s group [330] demonstrated the potential application of Ag2S as a cathode in DSSCs for catalyzing triiodides reduction. In their investigation, they prepared Ag2S nanocrystals ink, for fabrication of Ag2S-based counter electrodes for DSSCs. The solar cells consisting of the aforementioned counter electrodes displayed a high energy conversion efficiency of 8.40%, which was superior to the corresponding obtained with Pt-based counter electrodes (8.11%). The last decade, many other sulfides have also been used in DSSCs with great success, demonstrating even better performance than the conventional Pt-based counterpart, such as WS2, CuS, Sb2S3, etc [321]. Today, transition metal-based counter electrodes, including phosphide, selenide, boride, silicide, and telluride compounds, except carbides, nitrides, oxides, and sulfides, are considered promising low-cost alternatives to the conventional Pt-based counter electrodes of DSSCs [321]. However, the main problem with this type of catalysts is the large energy consumption and the poisonous gas released in their synthesis procedure. Thus, the exploration of new synthesis routes with green technology and lower energy consumption for this kind of counter electrode catalysts is necessary. 3.4. Application Range The rapid growth of PV industry over the past few years has also been associated with the expanded range of PV system applications in the municipal and industrial sectors [331]. Solar cells of today provide not only efficient power-generation, high stability, and low-cost, but also unique properties that open the way for mass production for many novel applications such as building integration, wearable electronics, automotive, indoor use, military, and so on [33]. For about 25 years, DSSCs technology has been the subject of research and development of a great part of PV community, receiving constant funding through International, European, and National research programs [332]. The aim is the achievement of the “Golden Triangle” requirements for cheap PV electricity. Constant further development and integration of new concepts of materials and devices are foreseen in the future. Introduction to the market has been accompanied by a strong increase in patent applications in the field, which is a good indication of further commercialization activity. Materials and solar cell concepts have been developed to such an extent that easy uptake by industrial manufacturers is possible. The critical phase for broad market acceptance has therefore been reached, which implies that the standardization-related research topics are on the focus [332]. During the last decade, the first commercial DSSC products were successfully launched. The first years timeline of DSSMs development is shown in Figure 3-9. Due to the high performance of DSSCs under low-light conditions (>18% energy conversion efficiency has been shown under indoor-light conditions), the first attractive market for DSSC products is consumer electronics, which goes together with the development of suitable electronics. Flexible DSSCs have already been introduced into the global lifestyle product market. The success in scaling up DSSMs on glass, plastics, and foils also demonstrated the suitability of DSSCs for the very large BIPVs market [332]. One of the first and most important milestones for the commercial deployment of DSSCs in large scale is the dye- sensitized solar panels (DSSPs) developed by Solaronix, which were applied in April 2014 at the École Polytechnique Fédérale de Lausanne (EFPL) campus, in Switzerland. These DSSPs are estimated to be able to generate about 2000 kWh of annual solar electricity [230]. The unique characteristics of DSSCs, due to their various degrees of transparency and color (determined by the dye), also made them a suitable choice to be considered as a photo-selective covering for greenhouses [333]. Nowadays, many companies are developing DSSCs for greenhouses, with Brite Solar being

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one of the leading-edge PV technology companies already established in Greece. Additionally, DSSMs can also be used as electrochromic windows. Electrochromic devices that simultaneously harvest and store solar energy by integrating two electrochemical devices (DSSCs and supercapacitors) have already been developed [230]. Many other novel applications are also demonstrated year by year, establishing DSSCs technology as one of the most promising PV technologies for solar-to-electricity conversion. In parallel, the number of scientific publications on DSSCs technology is growing further (>15000 since 2018) and the range of new or renewed fundamental topics is broadening. Today, widening of DSSCs application range is even more imperative since, as it is already stated in §3.3, the superiority of conventional DSSCs in cost in comparison to silicon-based solar cells has almost disappeared.

Figure 3-9: First years timeline of dye-sensitized solar modules development [230]. Below, there is a brief overview of the recent progress in DSSCs technology, in the direction of achieving a wider application range. The research efforts are classified according to the part of the solar cell which is under investigation, namely dye-sensitized working electrode, electrolyte, and counter electrode. 3.4.1. Dye-Sensitized Working Electrode Flexibility, low-cost, lightweight, non-toxicity, transparency, high performance under low light conditions, appealing aesthetics, and mechanical robustness are some important aspects that if they are combined with a high energy conversion efficiency can lead to a wider commercialization of solar cells (see Figure 3-10). Today, the efforts of the PV community in designing high-efficiency DSSCs that also combine all or some of these unique characteristics are growing year by year, in order to achieve their usage in many pioneer applications. Many novel modifications of the conventional dye-

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sensitized working electrode have been reported in this direction in the last few years, showing promising results.

Figure 3-10: Flexible, wearable, transparent, indoor, aesthetically appealing, and mechanically robust solar cells.

For any PV technology, flexible solar panels offer some advantages over the classic rigid ones, such as a wider application range, and in most cases a lower cost and weight [334]. Glass substrates used in conventional DSSCs have a series of drawbacks, namely rigidity, high weight, and frangibility, which limit the potential integration of solar cells in many novel applications. Based upon these considerations, alternative, flexible, low-cost, and lightweight substrates, such as plastic films and thin metal foils, have been investigated intensively for fabrication of flexible electrodes for DSSCs the last decade [335]. The development of flexible DSSCs not only addresses the need for applications where flexibility, conformability, portability, low weight, and reduced dimensions are important, but is also conducive to industrial roll-to-roll fabrication, which, as it is already stated in §3.3.1, enables the implementation of high throughput production lines and its associated potential reduction in manufacturing costs. However, the commercialization of flexible DSSCs has not yet fully taken off due to a few persisting gaps in the optimization of employed materials and processes. Concerning the all-plastic flexible DSSCs, ITO PET and ITO PEN are the most widely used substrates for fabrication of flexible plastic working electrodes [55,334,335]. The main research efforts today deal with the challenge of fabrication of high-performance flexible plastic working electrodes, for achieving a high energy conversion efficiency by the solar cells. The main drawback here is the restriction of working electrode manufacturing temperature below 150oC, which leads to unsuitable anode characteristics for dye sensitization. Normally, the TiO2 layer is sintered in high temperature (400–500oC) to get good electrical contact between the particles. Additionally, the typical titania paste contains an organic binder and viscous solvents that have to be removed in a high-temperature sintering process [55]. As a result, novel techniques for fabrication of low- temperature plastic dye-sensitized working electrodes have to be developed to attain the desirable anode characteristics. Other drawbacks of plastic-based dye-sensitized working electrodes application in DSSCs are the easier penetration of water and other contaminants into the solar cells, and the high degradation of plastics under UV irradiation [55]. Preparation of suitable pastes with appropriate rheology to produce high quality and mechanically stable films, without organic binders, is the first major challenge facing when fabricating DSSCs on plastic substrates [334]. Up until today, there have been many scientific efforts in the direction of developing the appropriate pastes for fabrication of low-temperature processed working electrodes. Nanocrystalline films of TiO2 are the most popular and still the best-performing dye-sensitized working electrodes structures [335]. The most-used approach consists of preparing a binder-free paste (made of nanoparticles, solvents such as alcohols and/or water, and additives with low boiling points), coating it on an ITO substrate and heating it up to 150oC to remove solvents and additives, and promote slight sintering [335]. Nanoparticles can be synthesized in the laboratory or commercially sourced. For the latter kind, P25 Degussa powder is widely utilized. The maximum published energy conversion efficiency obtained with this powder in an all-plastic flexible DSSC (sensitized by a single dye) is 6.3%, whereas efficiencies on the order of 8% have been obtained using hydrothermally custom-synthesized TiO2 nanoparticles [336,337]. The most efficient all-plastic flexible DSSCs

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amongst these have been realized by mixing together different particles with diameters of 20 and 100 nm, in order to obtain high specific surface area, efficient transport through the mesoporous structure, and better light scattering [335]. Furthermore, the concept of adding materials smaller than 5 nm, to promote necking between the nanoparticles at low or even room temperature, sometimes referred to as nanoglues, has been an effective route for the fabrication of high-performance plastic-based working electrodes for DSSCs [338]. Alternative approaches, such that of preparing much larger submicron- to micron-sized “building blocks” (such as pre-fabricated mesoporous TiO2 beads), by using solvothermal synthesis or by sintering nanoparticles into clusters (even at high temperatures before their deposition) have also been used. In this way, it was possible to obtain strongly interconnected TiO2 networks, while maintaining the advantage of solution-processing of the TiO2 layer, delivering devices with energy conversion efficiencies higher than 7% [335,339]. Composites are also used to improve the transport and/or flexing characteristics of the TiO2 layer, such as TiO2- CNTs, TiO2-polymer, etc. [335]. By using composites, efficiency enhancements on the order of 50% compared to the pristine TiO2-based working electrodes have been reported [340]. The last years, surface plasmons are also used with great success for the enhancement of flexible DSSCs efficiency [341].

In contrast to the TiO2 pastes used for high-temperature sintering on glass or metal foils, which are commonly screen-printed, most of the low-temperature processed pastes described above are deposited by the doctor-blading technique, because the absence of binders and additives in the TiO2 paste makes screen-printing difficult [335]. Although optimization studies on the rheological properties of these pastes have been carried out (by adding an increasing quantity of ammonia or hydrochloric acid in water for example), the appropriateness of the doctor-blading technique for a large area (e.g. roll-to-roll) fabrication needs to be validated [335]. Other techniques, such as spin- coating, gravure-printing, and inkjet-printing have also been used, although lower efficiencies have been reported [342–344]. Alternative techniques to printing include the electrophoretic deposition. One of the best-performing flexible solar cells fabricated with electrophoretic deposition showed an energy conversion efficiency of 6.6% [345]. Since room or low-temperature (120–150oC) treatments do not typically guarantee sufficient necking between nanoparticles required to reach those maximum efficiencies that are achievable at high temperatures, apart from the introduction of necking agents (or nanoglues) in the paste formulations as outlined previously, other strategies consist of subjecting the working electrodes to further processing such as compression, UV irradiation, and chemical baths (compatible with plastic substrates), in order to improve device performance. A different route that has already been used to fabricate strongly interconnected mesoporous TiO2 layers consisted of transferring on ITO plastic substrates pre-sintered (at high temperature) mesoporous TiO2, previously made on glass or ceramic substrates [335]. Adhesion of the transferred film on the plastic substrate can be improved by depositing a thin adhesion layer prior to the transfer process. By using double layered TiO2 structures, formed on plastic substrates by transferring high temperature-processed N719/TiO2 over an organic dye-sensitized TiO2 film by a typical compression process at room temperature, energy conversion efficiencies higher than 6% have been obtained [346]. Completely TCO-free and flexible DSSCs fabricated on a plastic substrate using a unique transfer method and back-contact architecture has also been reported, reaching efficiencies higher than 7% [347]. The 1D nanostructures, such as nanotubes, nanowires, and nanorods can also be fabricated by various techniques and transferred afterward to the plastic substrates, or they can be grown directly on plastics substrates via alternative relatively low-temperature processing methods, to form high-performance flexible dye-sensitized working electrodes for DSSCs [348–350]. Such 1D structures not only hold fast electron diffusion channels but also provide better properties in bending. Although the efficiency of all-plastic flexible DSSCs has been improved gradually in the last years, it is still lower than that of glass-based DSSCs. Metal-based dye-sensitized working electrodes have

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an edge over plastics due to their compatibility with high-temperature sintering process, to get high quality and highly adhesive TiO2 films, with good inter-particle connections, as well as non- permeability [55]. Additionally, metals present a much better surface in terms of conductivity and recombination inhibition [335]. However, the optical losses, due to back-side illumination, where absorption of light by the catalyst and the electrolyte layer occurs, result in a decreased photo-current production, and hence in a decreased overall solar cell performance. Nevertheless, the solar cell configuration can be optimized, by controlling the thickness and the composition of the catalyst layer and electrolyte, to achieve good results [55]. Grätzel’s group was one of the first that demonstrated that high-efficiency metal-based flexible back-side illuminated DSSCs can be developed, reporting efficiencies higher than 7% [351]. After this report, several research efforts followed, showing promising results [352]. Most of them concern the fabrication of TiO2 nanotubes on Ti foils, for their intended use as working electrodes for flexible DSSCs. Alternative 1D nanostructures have also been reported. Lin et al. [353] reported the fabrication of conical-shaped anodic TiO2 nanotube arrays with a length of a few tens of micrometer and rough tube walls for their intended use in flexible backilluminated DSSCs. Their investigation showed that these working electrodes lead to an enhanced light-harvesting, rapid electron-transport rate, prolonged electron lifetime, reduced dark current, and thus to an increased energy conversion efficiency compared to the TiO2 nanoparticles/Ti-based DSSCs. Today, the energy conversion efficiency record of the flexible back-side illuminated DSSCs exceeds 8% [354]. The last decade, many alternative flexible DSSCs have also been developed, using novel types of dye-sensitized working electrodes, showing promising results. Sheehan et al. [355] demonstrated for the first time in 2015 that flexible DSSCs can be fabricated using flexible glass as substrates. These substrates combine high transparency, high heat resistance needed for high-temperature processing, flexibility, and low-cost manufacturing. Using flexible glass as substrates, a quite satisfactory energy conversion efficiency was achieved by DSSCs, comparable to the corresponding employing commercially available FTO and ITO glass substrates, attaining additionally flexibility in solar cells for a wider application range. In 2018, Zhao et al. [356] used for the first time heat-resistant borosilicate glass papers as substrates to fabricate flexible DSSCs. These substrates show extraordinary flexibility, while they can also be treated at high temperatures (ca. 500°C). Their work provided a new perspective for fabricating flexible DSSCs in the direction of developing paper-based electronics. Vomiero et al. [348] reported the fabrication of TiO2 nanotube arrays on Kapton HN substrate, for their intended use as working electrodes for DSSCs. In their approach, they used anodization of a titanium thick film for obtaining nanotubes directly on Kapton HN substrate. The use of the aforementioned substrate led to better results compared to the PET one, due to its higher heat resistance, enabling the mandatory post-anodization annealing process of TiO2 nanotubes. Yun et al. [357] worked for the development of textile-based solar cells. In their study, a new deposition method, called floating printing method, was developed to obtain a uniform and controllable deposition of electrode materials on textile or wire type substrates. TCO-free flexible DSSCs were fabricated utilizing a metal textile substrate, a paper spacer, and Pt-coated carbon counter electrodes, obtaining a satisfactory energy conversion efficiency. Liang et al. [358] developed novel flexible fiber-type DSSCs with multi-working electrodes. In each solar cell, all the components were assembled into a flexible plastic capillary tube. A Pt microwire along the axis of the tube was used as the sole counter electrode and a number of Ti microwires surrounding it, which were all covered with highly ordered TiO2 nanotube arrays, were jointly used as the working electrodes. This new configuration brings about good flexibility, the capability of harvesting light from all directions, and a conversion efficiency competitive with those of the conventional DSSCs. Using the aforementioned configuration, Liang et al. reported impressive energy conversion efficiencies, reaching even 9%. Lv et al. [359] developed a highly efficient completely flexible fiber-shaped solar cell based on TiO2

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nanotube arrays, showing an energy conversion efficiency of 6.72%. They reported that these solar cells can be woven into three-dimensional solar modules with different shapes, suitable for producing power supplies for portable devices or power suits. Moreover, they can be woven into curtains, tents, and bags. Such energy cloths can become a power supply for woven electronic devices made up of electroluminescent devices, sensors, and field-effect transistors. The three-dimensional light collection capability of these fiber-shaped solar cell modules is particularly suitable for usage in a PV ‘‘house’’, such as gauze windows or curtains, which can make use of both direct outdoor sunlight and scattered indoor reflection. Hence, it has great potential for extensive applications. Fan et al. [360] also reported the fabrication of weaveable wire-shaped flexible DSSCs, formed by two fiber- like electrodes twisted together, demonstrating unique properties. An interesting result of their investigation was that the output of these solar cells was not influenced by the angle of the incident light. The transparency of DSSCs is also of great importance for their wider application range. Today, many research efforts focus on the development of high transparent working electrodes for DSSCs, using novel nanopastes, printing methods, and dyes, that can lead to a high energy conversion efficiency [246,361–363]. Recently, photonic crystals have also been used in DSSCs photo-anode to increase the performance of high transparent solar cells. Photonic crystals are materials that possess a macroscopic crystal structure with a one-, two-, or three-dimensional periodic lattice, where the lattice distance is comparable to the wavelength of visible light [364]. These materials can selectively prevent light propagation in certain directions with specified frequencies (i.e. a certain range of wavelengths or “colors” of light) [365]. Thus, their application in DSSCs can allow the transmission of some range of visible light and reflect the light of other wavelengths, increasing light-harvesting and therefore the solar cells performance, maintaining although their transparency in visible light. In 2017, Baek et al. [364] demonstrated that photonic crystals nanofillers can selectively and strongly reflect the light, resulting in an enhanced photo-current production, maintaining the transparency of working electrode at high levels. In 2018, Guo et al. [366] demonstrated the coupling of plasmonic nanoparticles with TiO2 nanotube photonic crystals for enhanced DSSCs performance. Their investigation showed that the aforementioned design can lead to a great efficiency enhancement of 44.7%, which exceeds the sum enhancement produced by employing individual photonic crystals or plasmon resonance effect. Novel modifications have also been applied in the last few years to enhance the efficiency of transparent DSSCs. In 2018, Selvaraj et al. [367] reported a 67% energy conversion efficiency enhancement using an optical concentrator in DSSCs under 1000 W/m2 compared to the conventional devices. A maximum short-circuit current of 25.55 mA/cm2 was achieved at 40oC for the concentrated coupled device compared with the short-circuit current of 13.06 mA/cm2 for the bare solar cell at the same temperature. As it is already stated at the beginning of §3.4, the first attractive market for DSSC products is consumer electronics, which in most cases operate under indoor low light conditions [332]. The development of high-efficiency DSSCs that are intended to operate indoors, even by the artificial light, is attracting more and more the interest of the scientific community in the last few years. Modifications in dye-sensitized working electrode seem to bear the desired results. In 2017, Tingare et al. [368] reported the achievement of a pivotal improvement in DSSCs performance for indoor applications, through stepwise modifications on the anthracene-based organic sensitizers. Under low light illumination, the devices employing the new sensitizers displayed outstanding efficiencies of 20.72% and 28.56% under 6000 lux of LED and T5 fluorescent light source, respectively. The 28.56% efficiency under T5 irradiance is the highest among any dye reported till date. Such extraordinary performance is due to the solar cells high open-circuit voltage, which was over 0.6 V even under 300 lux dim light, and matching of solar cells absorption spectrum to that of the T5 light source. The same year, Freitag et al. [369] reported the fabrication of one of the highest performance

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DSSCs for indoor applications, using a new PV design that combined two judiciously selected sensitizers and a copper electrolyte. The co-sensitization method enabled light-harvesting over the whole visible range. Strikingly, the energy conversion efficiency of this new DSSC design under 1000 lux indoor illumination was 28.9%, which resulted in a power output of 88.5 µW/cm2, outperforming even the GaAs solar cells in terms of efficiency and cost under ambient light conditions. In standard AM 1.5G sunlight, these solar cells reached an energy conversion efficiency of 11.3%, which was a new record for a system based on a copper electrolyte. Enhanced performance in thin and semi-transparent DSSCs under low photon flux conditions using TiO2 nanotube photonic crystals was reported by Xie et al. [370]. Compared with the single-layer TiO2 nanoparticles-based DSSCs, the solar cells that employed the photonic crystals yielded enhancements up to 99.1% and 130% under 1 and 0.5 sun conditions, respectively. Furthermore, the compensation effect of photonic crystals usage to reduce the output power drop of these cells under tilted incident light was also demonstrated. The electricity generation by DSSCs in the dark has also been discussed very recently. Tang et al. [371] proposed the incorporation of long persistence phosphors into the mesoscopic TiO2 photo-anodes for fabrication of all-weather DSSCs that can generate electricity in the daytime and in the dark. Under full sun illumination, these solar cells yielded a maximum energy conversion efficiency of 10.08%. The unabsorbed red and infrared light across dye-sensitized working electrode was stored in the phosphors, and was subsequently converted into monochromatic fluorescence for persistent dye illumination under dark conditions, yielding energy conversion efficiencies up to 26.69%, lasting for several hours. The aesthetics of DSSCs are also of great importance when a wider solar cells application range is desirable. DSSCs are provided for the development of aesthetically appealing solar cells, due to the various colors that can be obtained using different dyes as sensitizers [53]. Additionally, using the inkjet-printing technique, it is possible to create highly complex, multi-layered patterns on electrodes at a short time, with no masking between the materials or layers to be required, only by software control [372]. Using inkjet-printed dyes, it is also possible to produce patterns of multiple dyes on the same electrode and to create color density gradients, which constitutes a new avenue for creating multi-colored DSSCs [264]. Mechanical robustness in solar cells is also a critical aspect for the widening of their application range. Metal-based, textile-based, and fiber-based flexible DSSCs are indicated for the development of flexible and mechanical robust solar cells, which can undergo complex mechanical loadings and deformations, without degradation of their energy conversion efficiency. Rui et al. [373] reported the fabrication of robust DSSCs in bending, using three-dimensional titania network on a Ti foil as the working electrode. These solar cells showed a good mechanical stability, exhibiting 97.3% retention of the initial efficiency after twenty consecutive bending cycles. Yun et al. [374] presented high flexible and efficient DSSCs, fabricated by sewing textile-structured electrodes onto casual fabrics such as cotton, silk, and felt, or paper. These textile-based devices showed high flexibility and high performance under 4 mm radius of curvature over thousands of deformation cycles. Considering the vast number of textile types, these textile-based solar cells offer a huge range of applications, including stretchable and wearable devices. Liang et al. [375] developed fiber-based flexible DSSCs using highly ordered hierarchical TiO2 nanotube arrays as the anode. Except for the high energy conversion efficiency of 8.6%, these solar cells demonstrated a high stability under extreme mechanical deformations such as 180o bending. The fabrication of flexible and mechanical robust all- plastic flexible DSSCs is also possible when using a one-dimensional nanostructured anode. Fu et al. [376] reported the fabrication of all-plastic flexible DSSCs using well-designed TiO2 nanotube array membrane as the anode. These plastic-based DSSCs showed a high efficiency of 6.25% and at the same time maintain 90% of their initial performance after hundreds of bending cycles. Jiang et al. [377] fabricated a high-bendability flexible DSSC based on a ZnO-nanowire photo-electrode, which

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was fabricated on ITO PET by a low-temperature hydrothermal growth. Their investigation showed that the ZnO-nanowire film can be bent to an extreme radius of 2 mm with no crack to be observed, while flexible DSSCs based on this kind of photo-electrodes also showed a good bending stability. 3.4.2. Electrolyte The composition of the electrolyte is directly connected with the intended application of DSSCs. Solar cells that are about to operate outdoors in a long-term have to integrate high thermal, electrical, chemical, and optical stability electrolytes [59]. Up to date, iodide-based electrolytes are the most commonly used material to transfer the charges from the cathode to anode and regenerate the dye. - - The redox mediator I /I3 is the preferred one from the beginning of DSSCs development since it yields the most stable and efficient photo-electrochemical cells [126]. The introduction of the optimal iodine concentration, appropriate additives, and solvents are crucial for obtaining the desirable properties. For assembling long-lasting devices for outdoor operation, high boiling point and viscosity solvents are fundamental to achieve a satisfactory stability, and avoid electrolyte leakage and evaporation. Many reports demonstrate that even with the integration of liquid state electrolytes, lifetimes that allow DSSCs commercialization for normal solar cells outdoor operation conditions are achievable [59]. The last decade, the development of advanced quasi-solid state electrolytes and solid state hole transport materials, as it is already stated in §3.2.2, led to the fabrication of higher stability DSSCs, without performance limitations [59]. QSS-DSSCs and SS-DSSCs demonstrate an increased lifetime compared to the liquid state counterparts, while their integration in many novel applications is now possible. The requirements are still higher in the case of flexible DSSCs, while the mechanical stability of the devices is also of great importance [335]. To achieve high-stability flexible DSSCs, the implementation of quasi-solid state electrolytes or solid state hole transport materials is even more imperative since the degradation of solar cells employing the conventional flexible plastic-based electrodes is pronounced [55]. One of the first realizations and studies on flexible SS-DSSCs was made in 2002, by Longo et al [378]. However, the energy conversion efficiency of these devices was quite low, on the order of 0.1%. Two years later, Kumar et al. [379] reported the fabrication of flexible QSS-DSSCs employing biocatalytically synthesized polymer electrolytes, achieving energy conversion efficiencies higher than 4%. Over the last five years, the development of high-efficiency and high-stability flexible QSS-DSSCs has been on the focus, with promising results to be reported. In 2013, Lee et al. [380] fabricated efficient, stable, and flexible DSSCs based on nanocomposite gel electrolytes, achieving efficiencies that exceeded 6%. These devices also exhibited a great durability, maintaining higher than 97% of their original efficiency after 500 h of ageing tests under continuous light irradiation at 60oC. The same year, Chae et al. [381] developed all-solid, flexible solar textiles based on DSSCs with ZnO nanorod arrays on stainless steel wires. The solar textile with 10 × 10 wires exhibited an energy conversion efficiency higher than 2.5%, with a short-circuit current density of 20.2 mA/cm2 at 100 mW/cm2 illumination. The reported photo-current is one of the greatest values reported in flexible DSSCs. In 2015, Han et al. [382] demonstrated a satisfactory efficiency and stability in flexible DSSCs using ionic liquid electrolytes. These flexible DSSCs showed less than 10% drop in their performance after 1000 h of ageing at 60oC. More recently, Liu et al. [383] fabricated flexible QSS-DSSCs using poly(acrylic acid-co-carbon nanotubes) gel electrolytes and CoS/Ag/Ti-based counter electrodes for increased stability and efficiency. Their results are quite satisfactory since high energy conversion efficiencies that exceed 7% were achieved, which were comparable to the corresponding devices employing liquid state electrolytes and Pt-based counter electrodes. Another interesting investigation was made by Bella et al. [384]. This group fabricated the first DSSC based on two paper-based components, i.e. the photo-anode and the electrolyte. In this way, cost-effective, lightweight, eco-friendly, mechanically deformable and comfortable wearing PV

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devices are a step closer to being developed. Notably, the newly developed paper-based solar cells displayed an energy conversion efficiency of 3.55% under 1 sun illumination, which remained remarkably stable even after accelerated ageing at 50oC of 1000 h. On the contrary, for indoor applications, high-performance electrolytes that will ensure a high energy conversion efficiency under low and diffuse light conditions are preferred since the solar cells are not intended to operate under harsh environment. Recently, De Rossi et al. [385] designed and studied customized flexible DSSCs for indoor operation under artificial light. One of their main modifications concerned the electrolyte. Particularly, they focused on the development of transparent electrolytes by decreasing the iodine concentration, which is the main reason for optical losses. Thus, they observed considerable improvement in the photo-generated current under low light conditions. Flexible DSSCs with the lowest iodine concentration in the electrolyte reached a mean efficiency of 12.4% under 200 lux illumination, which was almost 2.7 times higher than the corresponding value obtained by conventional flexible DSSCs under the same conditions. Noteworthy is the fact that these modified DSSCs delivered a maximum power density of 8 μW/cm2, which was 35% higher than the corresponding obtained by the most efficient a-Si device measured under the same conditions. Transparency in electrolytes is also considered an important aspect for many other novel applications, such as building integration. The scientific efforts in the development of transparent electrolytes for DSSCs have been increased greatly in the last few years. The usage of cobalt-based electrolytes in DSSCs can not only lead to high energy conversion efficiency but also to high transparency [132]. Iodine-free electrolytes have also attracted the interest of the scientific community towards the fabrication of high-performance and transparent photo-electrochemical cells. One of the first reports of high-performance polymer iodine-free electrolytes was made by Wang et al. [386]. In their investigation, they reported that amongst the prepared electrolytes employing different wt% iodine loadings, the highest efficiency in DSSCs was achieved by the iodine-free electrolyte, which was 5.29%. These solar cells also exhibited a great stability, maintaining about 85% after 1000 h without sealing. Moreover, iodine-free electrolytes are indicated for the development of transparent bifacial DSSCs, which have the advantage of higher light-harvesting efficiency, because of their capabilities of utilizing the incident light from both sides. In 2015, Rong et al. [387] fabricated transparent bifacial QSS-DSSCs using an iodine-free polymer electrolyte. The assembled solar cells showed an energy conversion efficiency of 6.35% under front-side irradiation, and 4.98% under rear-side irradiation with the intensity of 100 mW/cm2, which approached almost 80% of that of the front-side irradiation. Furthermore, the usage of iodine-free electrolytes is found beneficial for back-side illuminated devices since in this design, the light has to pass through the counter electrode and electrolyte before its absorption from the dye molecules anchored to anode semiconductor. Thus, the need for the development of high transparency counter electrodes and electrolytes is imperative. Recently, Ri et al. [388] fabricated high-efficiency back-side illuminated QSS-DSSCs using iodine-free polymer blend electrolytes. The maximum energy conversion achieved by these solar cells was 6.44%. In their investigation, they demonstrated that the prepared electrolytes not only overcome the visible light absorption and the leakage problems but also reduce charge recombination in solar cells, delivering high energy conversion efficiencies. 3.4.3. Counter Electrode In terms of widening DSSCs application range, flexibility, low-cost, lightweight, non-toxicity, transparency, and mechanical robustness are some important aspects to consider in the development of counter electrodes, as in the case of working electrodes. So far, a large number of materials and processes have been utilized for the fabrication of novel types of counter electrodes, which

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demonstrate both high performance and some of the aforementioned characteristics, opening up the way for many pioneer solar cells applications. Today, the development of flexible counter electrodes for DSSCs is as important as lowering their manufacturing cost. In this direction, thin plastics and metal foils are the most commonly used alternative substrates to the conventional glass [55,335]. Their implementation in solar cells can lead, except for the development of flexible devices, to lower cost and weight compared to the conventional glass-based devices. Concerning the plastic-based counter electrodes, the progress in their development in the last decade has been great, with Pt being the most commonly utilized catalyst material, due to its impressive catalytic properties. In 2010, Yamaguchi et al. [337] demonstrated one of the most efficient all-plastic flexible DSSCs, using sputtered Pt on ITO PEN as the counter electrode, achieving an energy conversion efficiency higher than 8%. Four years later, Konarka/G24 Power Ltd., a DSSC manufacturer, reported an NREL-certified lab solar cell efficiency of 7.17%, using the same type of counter electrode [335]. Since then, many important research efforts have been reported on the fabrication of plastic-based counter electrodes for DSSCs, using Pt or alternative cathode materials, such as carbon in various forms, conductive polymers, and transition metals [389– 392]. The fabrication of composites and hybrid cathodes on plastic substrates has also been of increasing interest in the last few years, in view of achieving unique properties that are not found in conventional single-component materials. In 2017, Zhu et al. [393] reported the fabrication of flexible Pt/carbon spheres composite counter electrodes for DSSCs, using ITO PEN as a substrate, achieving energy conversion efficiencies even higher than 9%. By using composites, the development of high- performance flexible Pt-free counter electrodes is also feasible, even with the use of TCO-free substrates. Zhang et al. [311] reported the fabrication of flexible TCO-free carbon composite counter electrodes and their application in DSSCs, achieving even higher performance than the corresponding of the conventional Pt-based device. Concerning the metal-based counter electrodes, their development has also been of increasing interest in the last decade. Adopting metal foils as substrates, flexibility in counter electrodes is achievable, while constraints on low-temperature processing and low stability faced by the usage of plastic-based substrates are lifted [335]. The metals also possess lower sheet resistance compared to ITO substrates, while their opacity and in many cases high reflective properties can lead to an improved energy conversion efficiency. Up until today, Ti foils are the most commonly used substrates for the fabrication of metal-based counter electrodes for DSSCs, while Pt is the most preferred catalyst [335]. Yamaguchi et al. [337] reported one of the highest certified energy conversion efficiencies for flexible DSSCs using sputtered Pt on Ti foil as the counter electrode. Nevertheless, alternative metal-based substrates (foils and meshes) and cathode materials have also been used for counter electrodes fabrication, leading to satisfactory energy conversion efficiencies when applied to DSSCs [394–397]. Novel nanostructures have also been employed in the last few years towards the development of high-performance metal-based counter electrodes. As an example, highly ordered TiN nanotube arrays, prepared by anodization of a Ti foil and subsequent nitridation in an ammonia atmosphere, demonstrated high catalytic activity, leading to energy conversion efficiencies close to 8% when applied to DSSCs (with an FTO glass photoelectrode) [323]. Furthermore, many alternative substrates have already been tested for fabrication of flexible counter electrodes for DSSCs, leading to promising results. Meng’s group [398] was one of the first who fabricated flexible pure carbon-based counter electrodes for DSSCs, using an industrial flexible G sheet as the substrate and AC as the catalytic layer. The main advantage of the pure carbon- based counter electrode over the conventional TCO substrate-based counter electrode lies on its low sheet resistance, leading to improved fill factor and efficiency in solar cells. This characteristic was found to be even more important for large-scale devices. A year later, the same group [399] demonstrated the fabrication of a flexible PANI/G composite counter electrode for DSSCs, using flexible G as conducting substrate, achieving energy conversion efficiencies that exceed 7%. Textiles

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have also been used as flexible, cost-effective, lightweight, and non-toxic substrates for fabrication of counter electrodes for DSSCs, opening up the way for wearable electronics application. Li et al. [400] reported the fabrication of flexible TCO-free conducting polymer/carbon cloth-based counter electrodes for DSSCs, achieving energy conversion efficiencies higher than 8%, which were even better than the corresponding of the device employing the conventional Pt/FTO-based counter electrode. Sahito et al. [401] also fabricated high-performance flexible textile-based counter electrodes for DSSCs, made from cotton fabric and GnNSs, achieving energy conversion efficiencies on the order of 7%. In their investigation, they reported that the novel counter electrodes demonstrated a low sheet resistance of 7 Ω/sq, which was unchangeable even if the substrate was subjected to various bending positions. On the other hand, the development of transparent counter electrodes is also a very important topic in the direction of widening DSSCs application range (i.e. for building integration), while it can also lead to an increased light-to-electricity conversion efficiency by the solar cells, when taking advantage of light-harvesting both from the front and rear side illumination. One of the first reports on the development of high transparent (>80% at visible wavelengths) counter electrodes for DSSCs was made by Hong et al. [402]. In their investigation, they used graphene/PEDOT-PSS composite films as the cathode material, achieving an energy conversion efficiency of 4.5%. Today, a large number of highly transparent and efficient counter electrodes have been reported. Many of them utilize Pt as the catalyst, however, many novel high-performance and transparent Pt-free counter electrodes have also been demonstrated recently [287,288,389,403,404]. Ku et al. [405] fabricated highly transparent (~ 90% at visible wavelengths) NiS-based counter electrodes for DSSCs with a facile electrodeposition technique, achieving a maximum energy conversion efficiency of 6.25%. Jia et al. [406] reported the fabrication of transparent (> 75% at visible wavelengths) Ni0.85Se-based counter electrodes for DSSCs, using a facile solvothermal reaction. The solar cells employing these novel counter electrodes displayed efficiencies on the order of 9%, higher than the corresponding value obtained by the conventional Pt-based device. Additionally, this group demonstrated that energy conversion efficiencies higher than 10% are achievable by using Ni0.85Se/mirror-based electrodes in DSSCs. The development of bifacial DSSCs, for the exploitation of light-harvesting both from the front and rear side illumination, has also been investigated extensively the last few years. By using highly transparent counter electrodes in DSSCs, energy conversion efficiencies that exceed 10% from bifacial irradiation have been reported [407]. It is also important to mention that the percentage ratio of the rear to the front illumination efficiency for bifacial DSSCs (rigid, flexible, and quasi-solid state) has been improved greatly in the last few years, with reported values reaching or even exceeding the 80% [220,404,408–410]. Finally, the development of mechanically robust counter electrodes for DSSCs has been of increasing interest in the last few years, in order to improve solar cells stability under mechanical loadings. Bending is the main type of mechanical loadings to consider for solar cells, especially for flexible devices. Most of the investigations concern the fabrication of novel TCO-free polymer- and textile-based counter electrodes, in view of replacing the conventional TCO substrate-based ones, which are found to suffer from severe degradation after bending. In 2015, Pan’s group [389] reported the fabrication of highly flexible and mechanical robust DSSCs using Pt networks on TCO-free polymer substrates as counter electrodes. The novel fabricated counter electrodes exhibited a remarkable stability in bending and twisting, leading to the development of solar cells that can maintain higher than 90% of their initial performance after 200 bending cycles. A year later, the same group [392] reported the fabrication of mechanical robust flexible TCO-free and Pt-free DSSCs, employing CuS nanosheet on plastic substrates as counter electrodes. In their investigation, they reported that the aforementioned counter electrodes not only exhibited high conductivity and transparency but also excellent electrochemical and mechanical stability. The novel solar cells

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displayed 14% higher performance compared to the conventional Pt-based devices, while bending tests showed only a ~10% decrease in their performance after 100 cycles of bending and relaxing. Grätzel’s group [390] has also demonstrated the fabrication of high durable ITO-free and Pt-free plastic-based counter electrodes for DSSCs, using SWCNTs as the cathode and PET as the substrate. The novel counter electrodes exhibited a remarkable durability when exposed to severe mechanical stability tests (bending tests and tape adhesion tests), while with their implementation in DSSCs an energy conversion efficiency on the order of 7% was achieved. Hashmi et al. [411] demonstrated the fabrication of mechanical robust counter electrodes for DSSCs, using fiber-based substrates and carbon nanotubes as the catalyst. In their investigation, they reported that these novel counter electrodes show a remarkable durability when subjected to very severe bending and tape adhesion tests, while with their implementation in solar cells an energy conversion efficiency on the order of 6% was achieved. Arbab et al. [412] fabricated flexible textile-based counter electrodes for DSSCs, using MWCNTs as the catalyst and polyester fabric as the substrate. The novel counter electrodes demonstrated a satisfactory sheet resistance of 15 Ω/sq, a great stability under bending, while the polyester fabric offers a good sealing capacity due to its hydrophobic nature. 3.5. Outlook ─ Motivation and Research Objectives Successful wide commercialization of any PV technology requires a combination of high- efficiency, long-term stability, and low-cost, utilizing solar cell materials and designs that allow a wide range of PVs applications. DSSCs have shown great potential as alternatives to conventional silicon-based solar cells. Their development in the last decades has been great, as discussed in the present Chapter. The first commercialized devices have already been demonstrated for a significant number of applications. At the same time, the interest of the scientific community in their further development has been increased in the last few years, with many novel solar cell materials and designs being reported. However, DSSCs wide commercialization is still hampered by a number of issues, concerning their efficiency, stability, cost, and application range, which have not fully been addressed during the years past. The present Ph.D. dissertation is devoted to the improvement of DSSCs technology from all the aforementioned point of views, as shown in Figure 3-11. The main current remaining challenges of DSSCs technology towards its wide commercialization and the research objectives of the present Ph.D. dissertation are tabulated in detail in Table 3-1. Figure 3-11: Schematic representation of the four main objectives of the Ph.D. dissertation.

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Table 3-1: Current challenges for dye-sensitized solar cells wide commercialization and research objectives of the Ph.D. dissertation.

Challenges Research objectives

Efficiency Efficiency

➢ The efficiency of DSSCs is still low ➢ Optimization of DSSCs photo-anode characteristics (The certified efficiency is lower than 12%) towards a high energy conversion efficiency

Stability Stability

➢ The long-term stability of DSSCs is still an open ➢ Development of high-efficiency QSS-DSSCs using issue novel advanced polymer electrolytes (The efficiency of QSS-DSSCs and SS-DSSCs is less ➢ Evaluation and prediction of DSSCs stability under than the corresponding of conventional DSSCs) different accelerating ageing conditions

Cost Cost

➢ The cost of DSSCs has to be further reduced ➢ Development of high-efficiency Pt-free DSSCs

Application range Application range

➢ Flexible DSSCs are characterized by low ➢ Development of high-efficiency back-side illuminated efficiency and stability DSSCs ➢ Scaling up of DSSCs technology needs further ➢ Evaluation of the limiting factors affecting large-sized improvement flexible Pt-free DSSCs performance ➢ Novel solar cell materials and designs should be ➢ A preliminary study of the mechanical, dynamic developed for widening DSSCs application mechanical, and viscoelastic characteristics of flexible range QSS-DSSCs

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Chapter 4: Experimental 4.1. Materials Fluorine doped tin oxide glasses (8 Ω/sq sheet resistance, 2.2 and 3.2 mm thickness, Dyesol), indium tin oxide PET (60 Ω/sq sheet resistance, 0.127 mm thickness, Sigma Aldrich), titanium foil (99.7% trace metals basis, 0.25 mm thickness, Sigma Aldrich), PET (amorphous, 0.62 mm thickness, NGP Plastic), platinum counter electrode (Dyesol), titania paste for blocking layer (BL-1 Blocking Layer, Dyesol), titanium dioxide nanopowder (P25, Sigma Aldrich), titanium dioxide micro-powder (Rutile 2902, Vellis Chemicals), silicon dioxide nanopowder (particles size 10-20 nm, Sigma Aldrich), titanium (IV) chloride (purity ≥ 99%, Sigma Aldrich), acetic acid (assay 99%, Penta), hydrochloric acid (ACS reagent 37%, Sigma Aldrich), water (analytical grade, Carlo Erba), N719 dye (Dyesol), RK1 dye (Solaronix), ethanol (purity ≥ 99.8%, Acros Organics), high-performance liquid state electrolyte (EL-HPE, Dyesol), high-stability liquid state electrolyte (EL-HSE, Dyesol), polyvinylpyrrolidone (average molecular weight 40000, Sigma Aldrich), polyethylene glycol (average molecular weight 20000, Sigma Aldrich), potassium iodide (ACS reagent ≥ 99%, Sigma Aldrich), 1-butyl-3-methylimidazolium iodide (assay ≥ 99.8%, Sigma Aldrich), iodine (ACS reagent ≥ 99.8%, Sigma Aldrich), 4-tert-butylpyridine (purity 96%, Sigma Aldrich), guanidine thiocyanate (purity 99%, Fisher Scientific), methanol (analytical reagent grade, Fisher Chemical), platinum paste (PT1-Platinum paste, Dyesol), carbon nanotubes (MWCNTs, NTX1, Nanothinx), graphite (spray, Ν- 77), low temperature thermoplastic sealant (50 μm thickness, Dyesol), ammonium fluoride (purity ≥ 98%, Acros Organics), ethylene glycol (assay 99.97%, Lach-Ner), lithium perchlorate (99.99% trace metals basis, Sigma Aldrich), acetonitrile (ultragradient grade, Carlo Erba reagents) have been used. 4.2. Fabrication of Conventional Dye-Sensitized Solar Cells DSSCs of 0.25 cm2 active area, all in a square shape, were fabricated according to the following procedure. A simple chemical technique for preparing TiO2 paste from commercially-available P25 nanopowder was used as reported elsewhere [242]. The preparation scheme and the chemical model of the TiO2 paste are given in Figure 4-1. More specifically, in order to make acetic acid adsorb on the TiO2 surface sufficiently by chemisorption, the TiO2 powder and acetic acid are mixed with ethanol, and then the mixture is kept at 80oC for 12 h. After that, the mixture is dried in a drying oven o − at 60 C for 6 h, and the CH3COO /TiO2 powder is obtained. The coordination of CH3COOH with hydroxides (−OH) on the TiO2 surface takes place in the heating process, by an ester bond. The ethanol facilitates the coordination in the heating process and avoids the aggregation during the − dryness. Due to the hydrophilic group, the CH3COO /TiO2 powder is easy to form water-based TiO2 − colloid by sonication assistance. When the CH3COO /TiO2 powder is dispersed in water, the groups − of CH3COO ionize and form a negative anion layer on particles, which is called “Stern layer”. Other positive ions, such as protons (H+), are then attracted to counterbalance the negative layer, forming the “Diffusion layer”. The positive “Diffusion layer” results in repelling the particles from each other. − In order to form the negative anion layer on the TiO2 surface, the powder (CH3COO /TiO2) must be − mixed firstly with water. If not, the ionization process of CH3COO will not happen, and the repulsive force will not exist. Then it is hard to prevent aggregation. At last, a little hydrochloric acid is necessary to change the TiO2 colloid into a viscous TiO2 paste, by the flocculating reaction. The hydrochloric acid releases numerous ions by ionization, destroying the positive “Diffusion layer”. Without the electrostatic force, the TiO2 particles would gather together by Van der Waals forces and

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turn into a viscous paste. The temperature control during the preparation of the TiO2 paste regulates its viscosity for doctor-blading or spin-coating usage. Due to the abundance of ethanol existing in the paste, numerous pores are formed in the quick evaporating process during sintering, while acetic acid and hydrochloric acid are volatilized, resulting in the fabrication of pure porous material.

Figure 4-1: (a) Preparation scheme of the TiO2 paste, (b) chemical model of the TiO2 paste, (c) conventional semi- transparent TiO2 working electrodes [242].

The working electrodes were fabricated either by doctor-blading or spin-coating of the TiO2 paste on the surface conducting substrates (FTO glass), while the sintering procedure took place at 500oC for 90 min. The thickness of the anode semiconductor was fixed at about 10 μm and 15 μm using the doctor-blading and spin-coating technique, respectively. In both cases, the resulted TiO2 working electrodes were characterized by a semi-transparent nature (see Figure 4-1). The sensitization of the TiO2 working electrodes was carried out by their immersion into a standard ruthenium dye (N719) ethanolic solution at room temperature for 24 h; 0.2 mM and 0.3 mM dye solution concentrations were tested. Pt nanoclusters coated on FTO glasses were used as the counter electrodes to DSSCs. The counter electrodes were fabricated by doctor-blading of the PT1 paste on the surface conductive glasses and firing of the system at 500oC for 30 min. The manufacturing process of DSSCs was completed when the sensitized working electrodes were sandwiched with counter electrodes, separated by a 50 μm spacer/sealant. The intervening space was filled with a drop of the liquid state factory-available electrolyte EL-HPE, through a hole already drilled in counter electrodes, using

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vacuum and filling process. In all cases, DSSCs active area was equal to the illuminated area, corresponding to the aperture area of a proper shading mask that covers the rest of the electrode, preventing the light incidence from the edges [413].

➢ Fabrication of conventional dye-sensitized TiO2 electrodes using different temperature profiles during sintering In this group of experiments, the fabrication of conventional TiO2 electrodes using different temperature profiles during sintering was investigated (see Table 4-1). All electrodes were fabricated by the spin-coating technique (film thickness ≈ 15 μm) and sensitized using a standard 0.3 mM N719 dye ethanolic solution, at room temperature for 24 h. In the cases where a gradual increase and/or decrease in the temperature was applied, the temperature change was kept lower than 10oC/min, to avoid a thermal shock effect. According to literature, the sintering/annealing temperature of the photo-anode influences its characteristics at high rates, such as its microstructure, specific surface area, electrical characteristics, and dye-adsorption, and subsequently the energy conversion efficiency of DSSCs [414]. Table 4-1: Working electrodes fabricated under different temperature profiles. Working electrode Temperature Gradual increase Temperature of the Gradual decrease name profile in temperature sintering procedure in temperature (oC) RE 1 1 x 450 x RE 2 2 x 500 x RE 3 3 x 550 x RE 4 4 x 600 x RE 5 5 ✓ 550 x RE 6 6 x 550 ✓ RE 7 7 ✓ 550 ✓

➢ Fabrication of conventional dye-sensitized TiO2 electrodes using additionally TiCl4 treatment

In order to further improve the characteristics of the conventional TiO2 electrodes (E RE 6) and enhance the performance of DSSCs, the working electrodes were treated with TiCl4 before their sensitization. TiCl4 treatment of TiO2 electrodes aims mainly to an increase in the specific surface area of TiO2, thus increasing the dye loading onto TiO2 [96]. The TiCl4 treatment was carried out, after their sintering (using temperature profile 6), by their immersion in a 40 mM TiCl4 aqueous o solution at 70 C for 30 min. The 40 mM TiCl4 aqueous solution was derived from further dilution of o a stock 2 M TiCl4 aqueous solution, which was previously prepared and stored at 0 C. The dense solution was prepared carefully in a special laboratory bench equipped with a hood since during the preparation process a lot of vapors were formed. The TiCl4 treatment was completed after sintering o the TiO2 electrodes at 550 C for 30 min once again. The concentration of the TiCl4 aqueous solution was determined according to the literature to bring the desired effect [96]. The name assigned to the conventional working electrode treated with TiCl4 was RE 6 T.

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4.3. Methodology Towards Higher Efficiency Dye- Sensitized Solar Cells In order to achieve a high energy conversion efficiency by DSSCs, modifications on the conventional dye-sensitized working electrodes treated with TiCl4 (RE 6 T) were made. The aim was the fabrication of a novel hybrid multilayered photo-anode, with optimized morphological, crystallographic, electrical, and optical characteristics. The modifications concerned interfacial engineering, regulation of the materials porosity, fabrication of composites aiming to improve the electrical and optical properties of the anode, enhancement of light scattering, and co-sensitization of the anode for enhanced light-harvesting (see Figure 4-2). In all cases, the sensitization of the working electrodes was carried out using a standard 0.3 mM ruthenium dye (N719) ethanolic solution at room temperature for 24 h unless otherwise stated. The remaining parts, the structure, and the manufacturing procedure of the solar cells were identical to the conventional devices, for comparison purposes. In all cases, DSSCs active area was equal to the illuminated area, corresponding to the aperture area of a proper shading mask that covers the rest of the electrode, preventing the light incidence from the edges [413].

Figure 4-2: Materials that were used additionally during the fabrication of the dye-sensitized working electrodes for improving their characteristics.

➢ Fabrication of dye-sensitized TiO2 electrodes with the addition of a blocking layer In order to improve the contact between the anode semiconductor and the conductive substrate and to reduce the charge recombination at the TCO/electrolyte interface, an ultra-thin (on the order of nm) blocking layer was introduced between the FTO glass and the TiO2 film [93]. The blocking layer was fabricated by spin-coating the commercially-available TiO2 paste (BL-1 Blocking Layer, Dyesol) on FTO glass for 1 min. Different film thicknesses were obtained by regulating the revolutions per minute of the coater; 500, 1000, 1500, and 1800 rpm were tested. Subsequently, the system was fired o at 500 C for 1 h. The TiO2 film was fabricated on the top of the blocking layer, according to the procedures that have already been discussed. The active area of the blocking layer was equal to the active area of the solar cell (0.25 cm2). The names assigned to the working electrodes were BL 500 T, BL 1000 T, BL 1500 T, and BL 1800 T, respectively.

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➢ Fabrication of dye-sensitized TiO2 electrodes with the addition of a pore- forming agent In order to regulate the porosity of the TiO2 film, PVP was introduced during the synthesis of the TiO2 paste, according to the preparation scheme given in Figure 4-3. PVP acts as a pore-forming agent by its sublimation during sintering of the TiO2 electrodes [86]. The polymer loading in the pastes was fixed at 1, 1.5, 2, and 3 wt% in regard to TiO2. In all cases, the novel working electrodes were fabricated according to the fabrication procedure of the conventional working electrodes. The names assigned to the novel working electrodes were 1.0 wt% PVP T, 1.5 wt% PVP T, 2.0 wt% PVP T, and 3.0 wt% PVP T, respectively. As shown in Figure 4-3, the working electrodes possessed different transparency and dye loading with the usage of different wt% loadings of PVP in the TiO2 paste.

Figure 4-3: (a) Preparation scheme of the paste, (b) TiO2 electrodes and dye-sensitized TiO2 electrodes with the addition of the pore-forming agent.

➢ Fabrication of dye-sensitized TiO2 electrodes with the addition of light scatters With the aim of improving light-harvesting by the dye-sensitized working electrode, light scatters were introduced into the conventional TiO2 film. In the present investigation, Rutile 2902 micropowder was used to promote light scattering in dye-sensitized working electrodes since these TiO2 microparticles are characterized by the appropriate size and refractive index to scatter the light effectively and to increase the energy conversion efficiency of DSSCs [101]. In this direction, TiO2 nanoparticles were mixed with TiO2 microparticles, in different weight loadings, to form TiO2 composite pastes, according to the preparation scheme given in Figure 4-4. The aforementioned pastes were used to fabricate scattering and reflecting layers on the top of the main active layer, creating multilayered films.

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Figure 4-4: (a) Preparation scheme of the paste, (b) TiO2 electrodes and dye-sensitized TiO2 electrodes with the addition of light scatters. The different configurations of novel working electrodes that were investigated, as well as the revolutions per minute of the coater that were used to fabricate the corresponding layers are listed in Table 4-2. As it is observed, in all cases, the first layer consists of TiO2 nanoparticles only, acting as the main active layer of the dye-sensitized working electrode. Concerning the second layer, different wt% ratios of TiO2 nano:micro particles were investigated towards the fabrication of an effective scattering layer. The third layer, which is of reduced thickness and mainly consisting of microparticles, constitutes the reflecting layer. The high loading of microparticles aims at the fabrication of a fully opaque layer at the visible spectrum, while the introduction of the low content of nanoparticles aims to achieve a good adhesion of the reflecting layer with the other layers of the anode. Finally, the thin nanoparticles layer (fourth layer) was added on the top of the reflecting layer to improve the TiO2/electrolyte interface in DSSCs, according to the literature [415]. In all the above- mentioned cases, the total thickness of the anode semiconductor film was fixed at about 15 μm, while the novel working electrodes were fabricated according to the fabrication procedure of the conventional working electrodes. The difference in the transparency and the dye loading using different wt% ratio of TiO2 nano:micro particles in the scattering layer in a double layered design of working electrode is shown in Figure 4-4.

Table 4-2: Configurations of the TiO2 electrodes with the addition of light scatters. Working 1st layer 2nd layer 3rd layer 4th layer electrode wt% of nano:micro TiO2 particles name (500rpm) (500rpm) (1800rpm) (1800rpm) RE 6 T 100:0 100:0 x x N:M 70:30 T 100:0 70:30 x x N:M 50:50 T 100:0 50:50 x x N:M 30:70 T 100:0 30:70 x x N:M 10:90 T 100:0 10:90 x x N/N/R T 100:0 100:0 10:90 x N/S/R T 100:0 70:30 10:90 x N/S/R/N T 100:0 70:30 10:90 100:0

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➢ Fabrication of composite dye-sensitized TiO2-MWCNTs electrodes In the direction of improving the electrical characteristics of the conventional dye-sensitized working electrodes and subsequently the performance of DSSCs, composites using additionally high conductive materials in the conventional anode semiconductor were fabricated [105]. In the present investigation, MWCNTs in 0.025, 0.05, and 0.1 wt% loadings in regard to TiO2 were introduced in conventional TiO2 pastes, in order to fabricate composite dye-sensitized TiO2-MWCNTs electrodes. The preparation scheme and the resulted composite pastes, as well as the fabricated working electrodes with the different wt% of MWCNTs are shown in Figure 4-5. In all cases, the novel working electrodes were fabricated according to the fabrication procedure of the conventional working electrodes. The names assigned to the working electrodes were 0.025 wt% MWCNTs T, 0.05 wt% MWCNTs T, and 0.1 wt% MWCNTs T, respectively. The difference in the transparency of the composite working electrodes using different wt% loadings of MWCNTs was obvious even visually.

Figure 4-5: (a) Preparation scheme of the paste, (b) TiO2-MWCNTs composite pastes, (c) composite TiO2-MWCNTs electrodes.

➢ Fabrication of composite dye-sensitized TiO2-SiO2 electrodes In the direction of improving the optical characteristics of the conventional dye-sensitized working electrodes, composites using additionally SiO2 nanoparticles, of smaller size compared to the TiO2 nanoparticles, were fabricated. The smaller-sized SiO2 nanoparticles increase the transparency of the anode semiconductor film, leading to an increased light-harvesting by the dye, and thus to an increased energy conversion efficiency by DSSCs [110]. In the present investigation, SiO2 nanoparticles in 0.5, 1, 1.5, and 2 wt% loadings in regard to TiO2 were introduced in the conventional TiO2 pastes, in order to fabricate composite dye-sensitized TiO2-SiO2 electrodes. The preparation scheme of the composite paste and the fabricated working electrodes with the different wt% of SiO2 are shown in Figure 4-6. In all cases, the novel working electrodes were fabricated according to the fabrication procedure of the conventional working electrodes. The names assigned to the novel working electrodes were 0.5 wt% SiO2 T, 1 wt% SiO2 T, 1.5 wt% SiO2 T, and 2 wt% SiO2 T, respectively. The difference in the transparency of the composite working electrodes using different wt% loadings of SiO2 was obvious even visually.

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Chapter 4 Experimental

Figure 4-6: (a) Preparation scheme of the paste, (b) composite TiO2-SiO2 electrodes.

➢ Fabrication of co-sensitized dye-sensitized TiO2 electrodes In order to improve the optical characteristics of the conventional dye-sensitized working electrodes, the co-sensitization method in a cocktail approach was used. Two commercially-available high-performance dyes of complementary absorption spectra, namely N719 and RK1, were mixed in concentrations 0.2 mM and 0.1 mM in ethanol, respectively, to prepare a cocktail dye ethanolic solution [253]. The aim was the fabrication of a dye-sensitized TiO2 electrode with a more intense and broad absorption spectrum. The fabrication of TiO2 electrodes sensitized by 0.3 mM N719 and 0.3 mM RK1 dye ethanolic solutions was also conducted for comparison purposes (see Figure 4-7). In all cases, the novel working electrodes were fabricated according to the fabrication procedure of the conventional working electrodes. The names assigned to the novel dye-sensitized working electrodes were RE 6 T N719, RE 6 T RK1, and RE 6 T cocktail, respectively.

Figure 4-7: TiO2 electrodes sensitized by N719, cocktail of N719 and RK1, and RK1 dyes. ➢ Fabrication of the optimized dye-sensitized working electrodes In view of achieving a high energy conversion efficiency by DSSCs, novel hybrid multilayered photo-anodes were fabricated, by applying all the aforementioned modifications, in the direction of optimization of dye-sensitized working electrodes characteristics. The fabrication method of the novel working electrodes was based on the results of the previous investigations, which determined the appropriate design of the photo-anode, in view of achieving a high performance by the solar cells. The preparation scheme of the composite pastes, as well as the optimized design of the dye-sensitized

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working electrode is shown in Figure 4-8. The names assigned to the novel working electrodes sensitized with N719, RK1, or cocktail of N719 & RK1 dyes are Multi T N719, Multi T RK1, and Multi T cocktail, respectively.

Figure 4-8: (a) Preparation scheme of the pastes, (b) Replacement of the conventional dye-sensitized working electrode of dye-sensitized solar cells by the optimized dye-sensitized working electrode.

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As it is observed, the optimized dye-sensitized working electrode consists of five layers, namely the blocking layer, the main active layer, the scattering layer, the reflecting layer, and the cover layer. The blocking layer (first layer) is a compact TiO2 layer that was fabricated using the commercially available TiO2 paste (BL-1 Blocking Layer, Dyesol). The main active layer (second layer) was fabricated by TiO2 nanoparticles, MWCNTs, and SiO2 nanoparticles, while its porosity was regulated with the addition of PVP. The scattering layer (third layer) was fabricated by TiO2 nanoparticles and TiO2 microparticles, while its porosity was also regulated with the addition of PVP. The reflecting layer (fourth layer) is of reduced thickness and consisted mainly of TiO2 microparticles with a few amounts of TiO2 nanoparticles. Finally, the cover layer (fifth layer) is also of reduced thickness and was fabricated only by TiO2 nanoparticles. Before the sensitization procedure, the multilayered anode was TiCl4 treated, according to the procedure already described, while the cocktail N719 and RK1 dyes ethanolic solution was applied as a sensitizer. The wt% loading of each material was in accordance with the investigation already presented. Τhe impact of the simultaneous usage of the additional materials on the optimum design of the photo-anode was considered negligible since their amounts were too low. 4.4. Methodology Towards Higher Stability Dye- Sensitized Solar Cells In order to improve the stability of DSSCs, novel polymer electrolytes were prepared and implemented in the conventional structure of solar cells. The aim was the fabrication of high- efficiency QSS-DSSCs, after the optimization of the polymer electrolytes. The usage of the appropriate polymers as solidification agents of liquid state electrolytes, creating polymer blend electrolytes, with the extra use of the appropriate additives and iodide compounds mixtures, which have proven to improve the performance of the electrolytes for the aforementioned application, were investigated (see Figure 4-9) [276,416–421]. In all cases, the polymer electrolytes were applied to DSSCs by in-situ gelation, by their casting onto the dye-sensitized working electrodes, while in a low viscosity solution form, for their complete penetration through the voids of the mesoporous network [422]. The gelation of polymer electrolytes took place at room temperature for 2 h. Finally, a drop of the polymer electrolytic solution was added before the electrodes were sandwiched with counterelectrodes. A hot-melt spacer of 50 μm thickness was used to assemble the solar cells. The thickness of the working electrodes as well as the concentration of the dye (N719) ethanolic solution, which was used as the sensitizer, were optimized to achieve high energy conversion efficiency by the QSS- DSSCs. The remaining parts, the structure, and the manufacturing procedure of the solar cells were identical to the conventional devices. Finally, conventional DSSCs employing the liquid state factory- available electrolyte EL-HSE were fabricated for comparison purposes. In this case, the thickness of the anode semiconductor was fixed at 15 μm, while its sensitization was carried out by the usage of a 0.3 mM dye (N719) ethanolic solution. Before the sensitization procedure, the working electrodes were treated with TiC4. In all cases, DSSCs active area was equal to the illuminated area, corresponding to the aperture area of a proper shading mask that covers the rest of the electrode, preventing the light incidence from the edges [413].

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Chapter 4 Experimental

Figure 4-9: Materials that were used for the preparation of the polymer electrolytes. ➢ Preparation of polyvinylpyrrolidone-based polymer electrolytes PVP-based polymer electrolytes were prepared by dissolving 500 mg of PVP and different wt% loadings of KI and I2 in 4 mL of methanol, under continuous stirring in sealed glass vessels, for their complete dissolution. The x wt% KI contents (where x = 20, 25, 30, 35 and 40) of PVP:KI mixtures were studied, while the I2 amount was in all cases fixed at 10 wt% of KI. The electrolytes were stored in sealed vessels until their in-situ gelation onto the DSSCs photo-anode (see Figure 4-10). The photo-anode thickness was fixed at about 10 μm, using a one-layered film fabricated by the doctor- blade technique, while a 0.2 mM dye (N719) ethanolic solution was used for its sensitization. Next, DSSCs employing the best-performing electrolyte (PVP:KI wt% ratio was fixed at 70:30) were fabricated, and an investigation on optimization of the dye-sensitized working electrode thickness and afterward of the dye loading of the anode semiconductor was conducted. Different film thicknesses were obtained by the spin-coating technique, either by configuring the revolutions per minute of the coater or by using more layers. The investigated dye ethanolic solution concentrations concerned the 0.2, 0.3, and 0.5 mM. Finally, DSSCs employing the best-performing PVP-based polymer electrolyte, and the optimized (thickness and dye loading) and TiCl4 treated working electrode were fabricated.

Figure 4-10: (a) Polyvinylpyrrolidone-based polymer electrolytes in the sealed vessels, (b) quasi-solid state form of polyvinylpyrrolidone-based polymer electrolytes. ➢ Preparation of polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes Based on the synthesis of the best-performing electrolyte of the previous group of experiments, PVP/PEG polymer blend electrolytes were prepared by dissolving PVP and PEG (total amount 500 mg), 214 mg of KI, and 21.4 mg of I2 in 4 ml of methanol, under continuous stirring in sealed glass vessels, for their complete dissolution. The wt% ratio of PVP:PEG was varied (100:0, 80:20, 60:40, 40:60, 20:80, and 0:100) in the direction of improving the polymer electrolyte performance for

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DSSCs application. The electrolytes were stored in sealed vessels until their in-situ gelation onto the DSSCs photo-anode. The anode semiconductor thickness and the dye ethanolic solution concentration, which was used for its sensitization, was fixed at about 15 μm and 0.3 mM, respectively, according to the previous group of experiments. In all cases, before the dye sensitization, the anode semiconductor was treated with TiCl4. Furthermore, PVP/PEG polymer blends of the aforementioned wt% ratios of PVP:PEG, without the addition of KI and I2, were prepared for materials characterization purposes. ➢ Preparation of polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes with additives Based on the synthesis of the best-performing polymer electrolyte of the previous group of experiments, two new PVP/PEG polymer blend electrolytes were prepared using additionally 4-tert- butylpyridine (TBP) and guanidine thiocyanate (GuSCN) in their composition as chemical additives. More specifically, the preparation of both polymer electrolytes included the dissolution of 200 mg of PVP, 300 mg of PEG, 214 mg of KI, and 21.4 mg of I2 in 4ml of methanol, under continuous stirring in sealed glass vessels. Subsequently, in order to obtain the first electrolyte, 0.5 M of TBP was added in the polymer electrolytic solution, while the second one included 0.5 M of TBP and 0.1 M of GuSCN in its composition. In both cases, the concentration of the TBP and GuSCN was determined according to the literature. The electrolytes were stored in the sealed vessels until their in-situ gelation onto the DSSCs photo-anode [419]. The anode semiconductor thickness and the dye ethanolic solution concentration, which was used for its sensitization, was fixed at about 15 μm and 0.3 mM, respectively, according to the previous group of experiments. In all cases, before the dye sensitization, the anode semiconductor was treated with TiCl4. ➢ Preparation of polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes with additives and iodide compounds mixture Based on the synthesis of the best-performing polymer electrolyte of the previous group of experiments, two new groups of PVP/PEG polymer blend electrolytes were prepared, this time using an iodide compound mixture in their composition, i.e. KI and BMII. The first group concerned the preparation of PVP/PEG polymer blend electrolytes with a total fixed at 0.287 M concentration of the iodide compound mixture in the polymer electrolytic solution, while the concentration percentage ratio of KI:BMII was varied. The second group concerned the preparation of PVP/PEG polymer blend electrolytes with an increasing total concentration of iodide compound mixture, i.e. 0.4 M, 0.5 M, and 0.6 M, by adding 0.113 M, 0.213 M, and 0.313 M of BMII in the reference electrolytic solution, respectively. The compositions of the prepared polymer electrolytic solutions are shown in more detail in Table 4-3. The optimized composition of the prepared polymer electrolyte in view of the fabrication of high-efficiency QSS-DSSCs is shown in Figure 4-11. Table 4-3: Composition of the prepared polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes with additives and iodide compounds mixture.

Polymer electrolytic solution Methanol PVP PEG KI BMII I2 TBP GuSCN (ml) (mg) (mg) (M) (M) (M) (M) (M) KI: BMII 100:0 4 200 300 0.287 0 0.188 0.5 0.1 KI: BMII 80:20 4 200 300 0.230 0.057 0.188 0.5 0.1 KI: BMII 60:40 4 200 300 0.172 0.115 0.188 0.5 0.1 KI: BMII 40:60 4 200 300 0.115 0.172 0.188 0.5 0.1 KI: BMII 20:80 4 200 300 0.057 0.230 0.188 0.5 0.1 KI: BMII 0:100 4 200 300 0 0.287 0.188 0.5 0.1 0.287 M KI + 0.113 M BMII 4 200 300 0.287 0.113 0.262 0.5 0.1 0.287 M KI + 0.213 M BMII 4 200 300 0.287 0.213 0.328 0.5 0.1 0.287 M KI + 0.313 M BMII 4 200 300 0.287 0.313 0.393 0.5 0.1

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Chapter 4 Experimental

Figure 4-11: Replacement of the conventional liquid state factory-available electrolyte of dye-sensitized solar cells by the optimized polymer electrolyte. 4.5. Methodology Towards Lower Cost Dye-Sensitized Solar Cells In order to reduce the manufacturing costs of DSSCs, novel carbon-based counter electrodes were fabricated and implemented in the conventional structure of DSSCs. The aim was the replacement of the high-cost Pt-based counter electrode, with novel counter electrodes that are characterized by lower-cost and simple manufacturing, without lowering the energy conversion efficiency of DSSCs. In this direction, carbon-based counter electrodes were fabricated, by depositing MWCNTs or G on the conductive glass substrates (see Figure 4-12). To ensure the uniform deposition of MWCNTs on the surface conductive substrates, a homogeneous ethanolic mixture containing 0.5 wt% MWCNTs, without additional treatment, was prepared by sonication assistance. The mixture deposition carried out dropwise using a graduated pipette and spin-coating at 1000 rpm. The G counter electrodes were fabricated by the spray deposition technique, using a commercially available G spray. The G amount and the spray distance was in all cases approximately the same. The fabrication of carbon-based counter electrodes was completed by their firing at 500oC for 30 min. Factory-available counter electrodes, fabricated by Pt nanoclusters, were also used to DSSCs for comparison. In all cases, the dye-sensitized working electrode was fabricated by doctor-blading of the homemade TiO2 paste (one- layered film). The thickness of the anode semiconductor was fixed at about 10 μm, while its sensitization was carried out by the usage of a 0.2 mM dye (N719) ethanolic solution. The employed electrolyte in all solar cells was the liquid state factory-available electrolyte EL-HPE. In all cases, the DSSCs active area was equal to the illuminated area, corresponding to the aperture area of a proper shading mask that covers the rest of the electrode, preventing the light incidence from the edges [413].

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Figure 4-12: Fabrication scheme of the carbon-based counter electrodes. 4.6. Methodology Towards Wider Application Range Dye-Sensitized Solar Cells ➢ Fabrication of back-side illuminated dye-sensitized solar cells In view of increasing DSSCs application range, novel back-side illuminated DSSCs were fabricated according to the following procedure. Highly ordered TiO2 nanotubes were developed directly on Ti foils using the electrochemical anodization method, to apply them as an anode in DSSCs. At first, Ti foils were cut into specimens of 1 x 2 cm2 and polished separately by ultrasonication in acetone, isopropanol, and ethanol, for a total duration of 9 min. Then, they were risen with deionized water and dried in air at 60oC. The anodization of the Ti foils took place in an electrochemical cell, employing the Ti foil (active area ≈ 1 cm2) as the working electrode, a Pt nanoclusters-coated FTO glass as the counter electrode, and an ethylene glycol solution containing 0.25 wt% NH4F and 0.75 wt% deionized water as the electrolyte. According to the author’s knowledge, it is the first time that a Pt nanoclusters-coated FTO glass is employed as a counter electrode for the anodization of Ti foils. The aim was to reduce the cost of the anodization procedure, by replacing the high-cost Pt foil/mesh with a much lower-cost but high-performance counter electrode. The Ti foils were anodized at 50 V for 2, 4, 6, and 8 h at room temperature to develop TiO2 nanotubes of various lengths. Then, the anodized samples were cleaned for 10 min in an ultrasonication bath containing deionized water, to remove the surface debris from the anodic titanium oxide (ATO) films introduced during anodization. Finally, the samples were fired at 150oC to burn the remaining organics on their surface. The names assigned to the samples fabricated by Ti foil anodization for 2, 4, 6, and 8 h were TiO2 NTs L15, TiO2 NTs L22, TiO2 NTs L25, and TiO2 NTs L28, respectively. The detachment of the TiO2 nanotubes from the Ti foil in the form of a membrane was also possible, by the immersion of the anodized samples in deionized water and their ultrasonic vibration, using an ultrasonic probe sonicator. The power supply, the electrochemical cell, the samples before and after anodization, as well as the detached TiO2 nanotubes membrane is shown in Figure 4-13.

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Figure 4-13: The power supply, the electrochemical cell, the samples before and after anodization, and the detached TiO2 nanotubes membrane.

A second group of the same type of specimens was also fabricated by the additional TiCl4 treatment of the TiO2 nanotubes, to increase their specific surface area. The TiCl4 treatment of the TiO2 nanotubes was carried out by the immersion of the anodized samples in a 40 mM TiCl4 aqueous solution at 70oC for 30 min, following their firing at 550oC for 30 min. The names assigned to the TiCl4 treated samples fabricated by Ti foil anodization for 2, 4, 6, and 8 h were TiO2 NTs L15 T, TiO2 NTs L22 T, TiO2 NTs L25 T, and TiO2 NTs L28 T, respectively. Moreover, a group of crystallized TiO2 nanotubes were fabricated. In this case, the specimens were anodized at 50 V for 6 h, leading to the development of TiO2 nanotubes of a specific length, which was optimized for DSSCs application, according to the previous group of experiments. The anodized samples were afterward crystallized by their annealing at 450, 500, 550, or 600oC for 2 h, aiming at the development of different crystal structured TiO2 nanotubes. All the specimens were afterwards treated with TiCl4, according to the procedure already presented. The names assigned to the TiCl4 treated samples fabricated by Ti foil anodization for 6 h and crystallization by their annealing at 450, 500, 550, and o 600 C were TiO2 NTs L25 A450 T, TiO2 NTs L25 A500 T, TiO2 NTs L25 A550 T, and TiO2 NTs L25 A600 T, respectively. Finally, double-layered oxide samples were fabricated, consisting of one layer of TiO2 nanotubes and one layer of TiO2 nanoparticles. The thickness of the TiO2 nanotubes layer was regulated by the anodization of the Ti foils for 2, 4, 6, and 8 h. On the top of this layer, a TiO2 nanoparticles layer was fabricated by spin-coating the TiO2 homemade paste, presented in §4.2, o at 500 rpm. The system was fired at 450 C for 2 h, to obtain crystallized TiO2 nanotubes and sintered TiO2 nanoparticles. Finally, the system was treated with TiCl4, according to the procedure already presented. The names assigned to the double-layered oxide samples fabricated by Ti foil anodization

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for 2, 4, 6, and 8 h were Hybrid TiO2 NTs L15 A450 T, Hybrid TiO2 NTs L22 A450 T, Hybrid TiO2 NTs L25 A450 T, and Hybrid TiO2 NTs L28 A450 T, respectively. In all the aforementioned cases, the specimens were sensitized by their immersion in a 0.3 mM dye (N719) ethanolic solution to serve as photo-anode to DSSCs. The employed electrolyte in all solar cells was the liquid state factory-available electrolyte EL-HPE, while as a counter electrode was employed the Pt nanoclusters-coated FTO glass, which was fabricated by doctor-blading the PT1 paste on surface conductive glasses and firing the system at 500oC for 30 min. In all cases, the solar cells active area was equal to the anodized area of the Ti foil (1 cm2). ➢ Fabrication of large-sized flexible platinum-free dye-sensitized solar cells DSSCs of 0.25, 0.49, and 1 cm2 active area, all in a square shape, were fabricated according to the following procedure. Semi-transparent TiO2 working electrodes were fabricated by doctor-blading the homemade TiO2 paste (the preparation scheme is shown in §4.2) on surface conducting glass substrates (FTO glass), or polymer substrates (ITO PET). The glass working electrodes were heated o o at 500 C and the polymer electrodes at 80 C, for 90 min. The sensitization of the TiO2 semiconductor was carried out by immersing the electrodes into a 0.2 mM N719 ethanolic solution for 24 h, at room temperature. Carbon-based counter electrodes were fabricated, by depositing MWCNTs or G on conductive glass or polymer substrates. To ensure the uniform deposition of MWCNTs on the surface conductive substrates, a homogeneous ethanolic mixture containing 0.5 wt% MWCNTs, without additional treatment, was prepared by sonication assistance. The mixture deposition carried out dropwise using a graduated pipette and spin-coating at 1000 rpm. The G counter electrodes were fabricated by the spray deposition technique, using a commercially available G spray. The G amount and the spray distance was in all cases approximately the same. The glass counter electrodes were heated at 500oC and the polymer counter electrodes at 80oC for 30 min. The manufacturing process of DSSCs was completed when the sensitized working electrodes were sandwiched with counter electrodes, separated by a 50 μm spacer/sealant. The intervening space was filled with a drop of the liquid state factory-available electrolyte EL-HPE. The DSSCs active area was in all cases equal to the illuminated area, corresponding to the aperture area of a proper shading mask that covers the rest of the electrode, preventing the light incidence from the edges [413]. ➢ Fabrication of dummy cells Sandwich like-structured composite materials, simulating the structure of a flexible QSS-DSSC (dummy cells), were fabricated for the mechanical, dynamic mechanical, and viscoelastic characterization of the solar cells. In all cases, PET films of 0.62 mm thickness were used as the skins, while PVP/PEG blend-based polymer electrolytes of 60 μm thickness were used as the core. The cross-section of the dummy cells is shown in Figure 4-14. The preparation of the polymer electrolytes was according to the procedure presented in §4.3. Different compositions of polymer electrolytes were prepared by varying the PVP:PEG wt% ratio and applied as core to prepare the sandwich like- structured composite materials. The dimensions of the specimens were determined according to the ASTM D790 for the mechanical and viscoelastic characterization, while specimens of 60 mm length and 12.8 mm width were fabricated for the dynamic mechanical characterization.

Figure 4-14: Cross-section of dummy cells.

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4.7. Solar Cell Materials Characterization The characterization of solar cells, which were all fabricated in the laboratory, comes along with a large number of materials characterization experiments, where the morphology, the crystallinity, the chemical composition and structure, as well as the thermal, optical, electrical, and optoelectrical characteristics of the individual parts of the solar cells were examined (see Figure 4-15).

Figure 4-15: (a) Solar cell materials characterization methods, (b) characterization methods for each part of the solar cell.

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➢ Scanning electron microscopy Scanning electron microscopy (SEM) was used to investigate the surface morphology of the different anodes and cathodes and to measure the thickness of the anodes. During this method, the scanning electron microscope (here GEOL JSM 6610 LV) produces images of a sample by scanning its surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography of the sample. The electron beam scans the surface in a raster scan pattern, and the position of the beam is combined with the detected signal to produce the image [423]. The modern scanning electron microscopes are capable of imaging details on the order of tens of Angstroms (i.e. sub-nanometer), subject to the limits of electron–specimen interactions. ➢ Atomic force microscopy Atomic force microscopy (AFM) was used to investigate the surface morphology of the different anodes and cathodes, in terms of surface roughness. The atomic force microscope (here Nanoscope Veeco Santa Barbara) consists of a cantilever with a sharp nanometer-sized tip (probe), which is used to scan the specimen surface in a raster scan pattern, with resolutions on the order of fractions of nanometers. When the tip is brought into proximity of the sample surface, forces between the tip and the sample lead to the deflection of the cantilever, while an electronic feedback loop is employed to keep the probe-sample force constant during scanning. There are three modes of atomic force microscope operation according to the nature of tip motion, namely contact mode, tapping mode, and non-contact mode [424]. In the present investigation, contact mode operation was applied. ➢ Energy-dispersive X-ray spectroscopy Energy-dispersive X-ray spectroscopy (EDX) was used to investigate the chemical composition of the different anodes. EDX is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on the interaction of some source of X-ray excitation and the sample. Its characterization capabilities are in large part due to the fundamental principle that each element has a unique atomic structure, allowing a unique set of peaks on its electromagnetic emission spectrum [425]. ➢ X-ray diffraction X-ray diffraction (XRD) was used to investigate the crystallinity of the different anodes and polymer electrolytes. XRD is a technique used for determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their crystallographic disorder, and various other information [426]. In the present work, a Bruker D8 Advance diffractometer equipped with a monochromatic Cu-Ka radiation source (λ = 1.5496 Å) was used to investigate the crystallinity of the different anodes and polymer electrolytes, in a 2θ range of 2° to 80° and at a scan rate of 5°/min. The weight percentage of anatase (X wt%) in the samples was estimated using Spurr-Myers equation (see Equation 4-1), where IA represents the intensity of the anatase peak at about 2θ = 25.3° and IR is that of the rutile peak at about 2θ = 27.5° [427]. The mean primary size of anatase and rutile TiO2 crystallites was determined from the full width at half maximum (FWHM) of the corresponding XRD peaks by Scherrer’s formula (see Equation 4-2), where K is usually taken as 0.9, λ is the wavelength of the X-ray radiation, and β is the line width at half-maximum height [428]. The crystal lattice distortion was evaluated from Equation 4-3 [428].

푋(%) = 100⁄(1 + 1.265 퐼푅⁄퐼퐴) (4-1)

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Chapter 4 Experimental

퐿 = 훫휆⁄(훽 푐표푠 휃) (4-2)

훥푑⁄푑 = 훽⁄(4 푡푎푛 휃) (4-3) ➢ Brunauer-Emmett-Teller analysis Brunauer-Emmett-Teller (BET) analysis was used to investigate the porosity of the different anodes. BET theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials. Nitrogen is the most commonly employed gaseous adsorbate used for surface probing by BET method [429]. In the present work, the analyzer Micromeritics Gemini III 2375 employing nitrogen physisorption was used to investigate the porosity of the different anodes, at the temperature of liquid nitrogen (−195oC). ➢ Photoluminescence spectroscopy Photoluminescence (PL) spectroscopy was used to investigate the optoelectronic characteristics of the different anodes, in terms of material imperfections and impurities, and recombination mechanisms. PL spectroscopy delivers information about the optoelectronic characteristics of materials and helps to unveil the performance of charge carrier trapping and separation, as well as to make out the fate of excitons. PL includes the excitation, migration, and transfer of photo-induced charge carriers. PL emission is the result of the recombination of the excited state electrons with valance holes, either directly (band–band) or indirectly (via a bandgap state). A low PL intensity may be due to the low recombination rate of electrons and holes, and high separation efficiency under light irradiation, which is desirable during DSSCs operation. PL spectroscopy uses a laser beam to capture light generated from a substance as it falls from the excited state to ground state when irradiated by the laser beam. By measuring the luminescence spectrum, the aforementioned important information about the materials optoelectronic characteristics is obtained [430]. In the present work, the PL spectra of the different un-sensitized working electrodes were taken using an F-2500 fluorescence spectrophotometer, at the excitation wavelength of 325 nm. ➢ Ultraviolet–visible spectroscopy Ultraviolet-visible (UV–VIS) spectroscopy was used to investigate the absorption spectra of the different sensitizers, the transparency of the different working electrodes and dye-sensitized working electrodes, as well as the chemical composition and optical characteristics of the different polymer electrolytes. UV–VIS spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes. The UV–Vis spectrophotometer (here Varian Cary 3) measures the intensity of light passing through a sample and compares it to the intensity of light before it passes through the sample, to calculate the absorbance spectrum of the materials [430]. In the present work, the absorbance/transmittance spectra of the materials were obtained at the range from 200 nm to 800 nm and at a scan rate of 2400 nm/min. ➢ Diffuse reflectance spectroscopy Diffuse reflectance spectroscopy (DRS) was used to measure the absorption spectra of the different dye-sensitized working electrodes. DRS is very closely related to UV–VIS spectroscopy. The difference in these techniques is that UV–VIS spectroscopy measures the relative change of transmittance of light as it passes through the material, whereas in DRS, the relative change in the amount of reflected light of a surface is measured. Thus, it is possible to measure the optical properties even of opaque materials [430]. In the present work, the investigated spectra of the different anodes were obtained using a Hitachi U-2001 UV–VIS spectrophotometer equipped with an integrating sphere, using BaSO4 as a reference, at the range from 200 nm to 800 nm and at a scan rate of 2400 nm/min. The optical bandgap of the samples was estimated according to the Tauc’s formula (see

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Equation 4-4), where a is the absorption coefficient, hv is the photon energy, A is a proportionality constant, and Eg is the bandgap, while m determines the characteristics of transition in the semiconductor [431]. A value m=1/2 is assigned for direct bandgap semiconductors and m=2 is assigned for indirect bandgap semiconductors. Here, the values of the bandgap of the samples were obtained by extrapolating the linear portion of the plot hv versus (ahv)1/2 to its intersection with the abscissa, assuming an indirect transition. 푚 (푎ℎ푣) = 퐴(ℎ푣 − 퐸푔) (4-4) ➢ Differential scanning calorimetry Differential scanning calorimetry (DSC) was used to measure the crystallinity (Xc) and estimate the melting temperature (Tm) of the different polymer electrolytes and pure polymers. DSC is a thermoanalytical technique where the difference in the amount of heat required to increase the temperature of a sample and a reference is measured as a function of temperature. The basic principle underlying this technique is that when the sample undergoes a physical transformation, such as a phase transition, more or less heat is needed to flow into it than the reference to maintain both at the same temperature [432]. In the present work, a Dupont Model 910 differential scanning calorimeter was used to obtain the DSC thermograms of the different polymer electrolytes and pure polymers, at the range from 25oC to 200oC and at a heating rate of 10oC/min. The crystallinity of the different samples was estimated according to Equation 4-5, where ΔHm is the measured melting enthalpy, 0 0 0 ΔHm the melting enthalpy of the 100% crystallized polymer (훥퐻푚푃푉푃 = 619.4 J/g, 훥퐻푚푃퐸퐺 = 213.7 J/g) and f is the mass fraction of the polymer in the sample [433–435]. Concerning the polymer blend samples, their crystallinity was estimated according to Equation 4-6. DSC curves, which were obtained from first run heating, were studied for all the samples, simulating the gelation process of polymer electrolytes onto the TiO2 electrodes. 0 푋푐(%) = [훥퐻푚⁄(훥퐻푚 푓)] · 100 (4-5)

푋푐푏푙푒푛푑 = 푋푐푃푉푃 푓푃푉푃 + 푋푐푃퐸퐺 푓푃퐸퐺 (4-6) ➢ Fourier-transform infrared spectroscopy Fourier-transform infrared spectroscopy (FTIR) was used to measure the chemical structure of the different polymer electrolytes. FTIR is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid, or gas, providing qualitative compound identification. The term “Fourier-transform” originates from the fact that a Fourier transform (a mathematical process) is required to convert the raw data into the actual spectrum [436]. In the present work, Jasco FT/IR-400 and Bruker FRA 106/S were used to record the FTIR spectra of the different polymer electrolytes, over the wavenumbers region between 4000 cm-1 to 500 cm-1 and at a resolution of 1 cm-1. ➢ Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) was used to extract electrical characteristics of the different polymer electrolytes, i.e. their conductivity (σ), dielectric constant (ε), relative number of − free charge carriers (n/n0), and the diffusion coefficient of triiodides (퐷퐼3 ). EIS is a steady-state method, measuring the current response to the application of an ac voltage as a function of frequency. A constant dc voltage may be superimposed to the ac signal; the amplitude of the latter should be as low as possible (small signal approximation) in order to consider the system under study as pseudo- linear. The frequency may span over a wide range, usually from some MHz down to a few mHz. It is usually carried out through the usage of a potentiostat and a frequency response analyzer, and it can be performed in two- or three-electrodes configuration, depending on the absence (two-electrodes) or the presence (three-electrodes) of a reference electrode, whose potential is known and fixed. The main

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advantages of EIS over other techniques are the multitude of information that it provides and that it is a non-destructive testing method [437]. In the present work, EIS spectra were recorded on symmetrical Pt/electrolyte/Pt cells, at 0 V vs Pt, with an amplitude of 10 mV, over a frequency range from 1 MHz to 100 mHz, at room temperature, using Alpha-N Frequency Response Analyzer. The conductivity was calculated using Equation 4-7, where Rb is the electrolyte resistance, l is the electrolyte thickness (equal to the spacer/sealant known thickness), and A is the cell active area. Rb was estimated from the difference between the ohmic resistance obtained by the Nyquist plot at the point where the curve intercepts with the real axis (RS) minus the electrodes resistance (Rel), assuming that the different RS values that were measured for the different samples were attributed to the different electrolytes since the electrodes were identical [273]. The electrodes resistance was estimated by a calibration, which was carried out in a corresponding system, employing an electrolytic solution with known conductivity. The dielectric constant of each sample was calculated at 1 MHz, provided by the frequency response analyzer. The relative number of free charge carriers was calculated, according to Barker's theory, by the Equation 4-8, where U is the dissociation energy of the iodide compound (푈퐾퐼 = 6.726 eV and 푈퐵푀퐼퐼 = 3.430 eV), ε is the dielectric constant of the sample, k is the Boltzmann constant, and T is the temperature of the sample [438]. An electrical equivalent circuit of the type RS(CDL[RCTO]) (see Figure 4-16) was applied on the experimental − results, using ZSimpWin software, in order to estimate the 퐷퐼3 of each polymer electrolyte, where RS is the sum of the electrolyte resistance plus the electrodes resistance (푅푆 = 푅푏 + 푅푒푙), CDL and RCT are the double layer capacitance and the charge transfer resistance at the platinum/electrolyte interfaces, respectively, and O is an element defining the diffusion complex impedance (ZDif), 1⁄2 1⁄2 expressed by the equation 푍퐷푖푓(휔) = 푅퐷푖푓{[푐표푡ℎ(푗휔휏) ]/(푗휔휏) }, with 푅퐷푖푓 = 퐵⁄푌푂 (RDif being the diffusion resistance, B the reaction beta coefficient, and YO a component of the constant 2 − phase element) and 휏 = 퐵 (τ being the electron lifetime) [273,439]. 퐷퐼3 of each polymer electrolyte was estimated using the Equation 4-9, where B was obtained after fitting the experimental EIS spectra.

휎 = 푙⁄(푅푏퐴) (4-7)

푛 = 푛0푒푥푝[−푈⁄(2휀푘푇)] (4-8)

2 − [ ] 퐷퐼3 = 푙⁄(2퐵) (4-9)

Figure 4-16: Equivalent electrical circuit applied to simulate the experimental EIS results. ➢ Linear sweep voltammetry − Linear sweep voltammetry (LSV) was used to measure the 퐷퐼3 of the different polymer electrolytes, under dc conditions. LSV is a voltammetric method where the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time. The experimental setup for LSV utilizes a potentiostat and a two- or three- electrode setup to deliver a potential to a solution and monitor its change in current. The three- electrode setup consists of a working electrode, a counter electrode, and a reference electrode [440]. In the present work, linear sweep voltammograms were recorded on symmetrical Pt/electrolyte/Pt cells, at the range from -0.7 V to +0.7 V vs Pt and at a scan rate of 10 mV/s, at room temperature,

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− using the potentiostat/galvanostat Autolab PGSTAT 302 N. 퐷퐼3 of the different polymer electrolytes was estimated from the cathodic steady state currents of the linear sweep voltammograms, according to Equation 4-10, where jlim is the limiting current, l is the electrolyte thickness (equal to the spacer/sealant known thickness), F is the Faraday constant, C is the bulk concentration of the − − − diffusion limiting species, and n is the number of electrons transferred in the 퐼3 + 2푒 → 3퐼 redox reaction [441]. The concentration of the triiodides in the different polymer electrolytes was estimated assuming that iodine molecules have fully reacted with iodide anions presented in the solution and by taking into account the solvent evaporation during gelation of the polymer electrolytes.

− 퐷퐼3 = 푗푙푖푚푙⁄(2푛퐹퐶) (4-10) ➢ Cyclic voltammetry Cyclic voltammetry (CV) was used to investigate the catalytic activity of the different counter electrodes. CV is a dc electrochemical technique, where the response in the current is recorded while a potential scan is applied to the working electrode at a constant scan rate in the forward and reverse directions, once or several times [440]. In the present work, the catalytic activity of the different counter electrodes was investigated by CV measurements, which were performed in a three-electrode system using PGSTAT 128N potentiostat/galvanostat instrument, at the range from -0.4 V to +1.2 V (vs Ag/AgCl) and at a scan rate of 50 mV/s. The three-electrode system used an Ag/AgCl electrode as the reference electrode, a Pt foil as the counter electrode, the as-prepared counter electrodes as the working electrodes, and an acetonitrile solution containing 0.1 M LiClO4, 10 mM KI, and 1 mM I2 as the electrolyte. 4.8. Solar Cells Characterization ➢ Current-density ─ Voltage characteristic curves The current−voltage (I−V) characteristic curves of DSSCs were obtained using a homemade variable load setup (see Figure 4-17), under real test conditions, at 1000 ± 10W/m2 incident solar irradiance, 48 ± 2o zenith angle, and 25−30oC environmental temperature, sweeping from zero applied voltage to open-circuit voltage. The light intensity was measured using a HT304N multicrystalline silicon pyranometer. A polynomial fitting followed, using Origin Pro Graphing and Analysis software, so as to extract a continuous and without deviation to experimental results, I−V characteristic curve. PVs characterization was performed on a batch of at least three samples for each type of DSSC, in order to extract a mean value and a standard deviation for each PV parameter.

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Figure 4-17: Homemade variable load setup.

The energy conversion efficiency (ECE) of DSSCs was calculated according to Equation 4-11, where IMPP is the current at maximum power point, VPPM is the voltage at maximum power point, Pin is the solar light intensity, A is the solar cell active area, ISC is the short-circuit current, VOC is the open-circuit voltage, FF is the fill factor, and JSC is the short-circuit current density. 퐼 푉 퐼 푉 퐹퐹 퐽 푉 퐹퐹 퐸퐶퐸 = 푀푃푃 푀푃푃 = 푆퐶 푂퐶 = 푆퐶 푂퐶 (4-11) 푃푖푛퐴 푃푖푛퐴 푃푖푛 ➢ One-diode model equivalent circuit analysis of dye-sensitized solar cells The PVs characterization was followed by a theoretical analysis using the one-diode equivalent circuit model, which is already presented in §2.4.2. The application of the model was based on the De Blas et al. [442] approach. The validation of the method has already been investigated by many scientists, for various PV technologies [443–445]. The aim of the method proposed by Blas et al. was to develop a simple procedure, able to extract the five parameters of the one-diode model, with a satisfactory degree of accuracy. The input parameters of the model are the experimental values of ISC, VOC, IMPP, VMPP, the reciprocal of the slope of the I–V characteristic curve of the solar cell for V = VOC (RSO), and the reciprocal of the slope of the I–V characteristic curve of the solar cell for V = 0 (RSHO).

Initially, a stating RS value is hypothesized. Then, n and RSH values are calculated from Equation 4-12 and Equation 4-13, respectively. The RS value is recalculated from Equation 4-14. An iterative procedure verifies the convergence of the RS value. When RS converges, the correct values of RSH and n are recalculated. Finally, the values of I0 and IL are calculated using Equation 4-15 and Equation 4-16, respectively. The above-mentioned equations are extracted using a series of simplifications, based on the analytical positions of Equation 4-17 and Equation 4-18, which are usually widely satisfied [446]. 푉 +퐼 푅 −푉 푞 푛 = 푀푃푃 푀푃푃 푆 푂퐶 푅푆 푉푀푃푃 (4-12) (퐼 −퐼푚푝푝)(1+ )− 푘푇 푆퐶 푅푆퐻 푅푆퐻 푙푛[ 푅 푉 ] 퐼 (1+ 푆 )− 푂퐶 푆퐶 푅푆퐻 푅푆퐻

푅푆퐻 = 푅푆퐻0 − 푅푆 (4-13)

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푉푂퐶푞 퐼푆퐶푅푆0푞 푅푆0( −1)+푅푆퐻0(1− ) 푅 = 푛푘푇 푛푘푇 (4-14) 푆 (푉푂퐶−퐼푆퐶푅푆퐻0)푞 푛푘푇

푅푆 푉푂퐶 푉푂퐶푞 퐼0 = [퐼푆퐶 (1 + ) − ] 푒푥푝 (− ) (4-15) 푅푆퐻 푅푆퐻 푛푘푇

푉푂퐶푞 푉푂퐶 퐼퐿 = 퐼0 [푒푥푝 ( ) − 1] + (4-16) 푛푘푇 푅푆퐻

푞퐼푆퐶푅푆 푞푉푂퐶 푒 푛푘푇 ≪ 푒 푛푘푇 (4-17)

푞퐼푆퐶푅푆 푞퐼0 1 푒 푛푘푇 ≪ (4-18) 푛푘푇 푅푆퐻 In the present work, Origin Pro Graphing and Analysis software facilitated the model application. Specifically, as it has already been stated, all experimental I–V characteristic curves of DSSCs were firstly fitted using a high order polynomial function. In all cases, the polynomial fitting deviated from the experimental I–V points less than 1%. Subsequently, the derivative of the polynomial fitting gave the slope of the curve with respect to V, leading to RS0 and RSH0 determination with high accuracy. For each type of solar cell, using one-diode model equivalent circuit analysis for DSSCs, the light- generated current density (JL), the ideality factor (n), the dark saturation current density (J0), the area- specific total series parasitic resistance (rS), and the area-specific shunt parasitic resistance (rSH) were extracted. 4.9. Solar Cells Accelerating Ageing Concerning DSSCs stability topic, a large series of solar cells accelerating ageing experiments were conducted, for the determination of the factors leading to the performance degradation of conventional DSSCs subjected to extreme ageing conditions. The accelerating ageing tests involved isothermal ageing at high or low temperatures, thermal shock cycling, hydrothermal ageing, and reverse biasing of DSSCs, simulating partial shading of PV modules. ➢ Isothermal ageing at T = 85oC The solar cells were thermally aged at 85oC, using a laboratory oven, while the temperature increment and decrease were regulated to a maximum rate of 1oC/min, from room temperature to 85oC, and vice versa. The controlled low-temperature increment and decrease prevented any solar cells degradation due to thermal shock. The total experimental duration was 1000 h, with measurement intervals being set at 100 h. ➢ Isothermal ageing at T = -25oC The solar cells were thermally aged at -25oC. The experiment was carried out using a freezer. The solar cells were exposed from room temperature to -25oC, and vice versa, with a maximum temperature variation rate of 1oC/min. The airtight sealing of solar cells, in thin plastic pouches, limited any moisture effect. The total experimental duration and the measurement intervals were set at 1000 h and 100 h, respectively. o o ➢ Thermal shock cycling between Tmin = -25 C and Tmax = 85 C The solar cells were subjected to thermal shock cycling tests. Each cycle had a total duration of 30 min, 15 min at -25oC and 15 min at 85oC. To prevent any moisture effect, the solar cells were airtight sealed in thin plastic pouches. The measurements intervals were set at 10, 20, 40, 60, 80, 100, 130, 160, and 200 cycles.

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➢ Hydrothermal ageing at T = 65oC and RH = 85% The hydrothermal ageing of the solar cells was performed in a special chamber, which provided the desired environmental conditions for their ageing. The solar cells were exposed at 65oC and 85% RH. The experiments lasted 100 h, with measurement intervals being set at 20 h. The experimental duration was short since the solar cells demonstrated serious instability issues during their ageing in the aforementioned environmental conditions.

➢ Reverse biasing at I = 4xISC In order to study the effect of partial shading of PV modules, reverse biasing experiments were carried out on the solar cells. The solar cells were connected to a dc power supply, while the current density was constant, four times higher than the short-circuit current of the solar cells. The experiment took place in the dark, so that the solar cells would not generate electricity. The effect of reverse biasing on the degradation of the solar cells performance was calculated after 10, 20, 30, 40, 60, 80, 100, and 150 hours. ➢ Normal ageing at T = 25oC and RH = 50% Finally, the degradation of solar cells performance was evaluated by storing them in room temperature conditions (T = 25oC and RH = 50%). Solar cells were placed under shade conditions to prevent their degradation due to their operation. ➢ Prediction of dye-sensitized solar cells stability under different accelerating ageing conditions The residual property model (RPM) is developed by Professor G.C. Papanicolaou and can be applied for the description of the properties degradation of a material after damage, irrespective of the type of the material and the cause of the damage [447–453]. Among usual damage sources are mechanical, thermal or hydrothermal fatigue, single or repeated impact, etc. The final expression of the RPM used for the prediction of the residual property value after damage is given by Equation 4- 19, where Pr is the current value of the property of the material considered, P0 and P∞ correspond to the value of the same property for the virgin and the damage saturated material, respectively, t is the time/thermal shock cycle, and p is a parameter depended on the material and the damage. 푃 푃 푡 푟 = 푠 + (1 − 푠)푒−푠푀 where 푠 = ∞, 푀 = (4-19) 푃0 푃0 푝 For the application of the RPM, one needs to know only two experimental points, with the condition that the property of the virgin material is known. The first point needed is on the initial stages of the experiment (t1, P1) and the second point is on the final stages of the experiment, where the damage saturation usually appears (t∞, P∞). The parameter p can be calculated using Equation 4- 20, which is derived from Equation 4-19 for t=t1. 푠 푝 = 푃1 (4-20) 1 푃 −푠 − 푙푛 0 푡1 1−푠 In the present study, P represents the energy conversion efficiency of the solar cells as a function of time of accelerating ageing or thermal shock cycles.

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4.10. Mechanical, Dynamic Mechanical, and Viscoelastic Characterization of Dummy Cells Under Three-Point Bending The mechanical, dynamic mechanical, and viscoelastic characterization of dummy cells, which simulate the structure of flexible QSS-DSSCs, was conducted through three-point bending experiments at different strain rates, oscillation frequencies, and strain levels, respectively. Bending is one of the main mechanical loadings that solar cells (especially flexible solar cells) are subjected to during their lifetime. Typical examples are their daily applications where flexibility is required (e.g. integration into portable electronic devices, clothing, etc.), wind, and impact loads. Furthermore, the viscoelastic characterization of solar cells in bending is of great importance. Solar cells are often subjected to constant mechanical loads or deformations, leading to creep or relaxation phenomena (e.g. constant mechanical loads due to snow accretion, constant deformations due to their application to curved surfaces). On the other hand, there are many advantages of materials characterization in bending, such as the simple geometry of the specimens, the simple experimental procedure, while information concerning the behavior of the material in tension and compression by a single bending experiment are derived. However, a major disadvantage of bending experiments is the development of complex stress distributions in the material. For reliable experimental results in bending, the span-to-depth ratio must be large enough to minimize the influence of shear stresses [454,455]. ➢ Mechanical characterization The mechanical characterization of dummy cells concerned three-point bending experiments at different strain rates. The term “strain rate” refers to the rate of change in strain of a material with respect to time and was established for the first time by the American metallurgist Jade Lecocq in 1867. The strain rate influences significantly the mechanical behavior of a material and must be seriously considered when designing materials for integration to structures [456]. Polymers, which are often used in solar cells, are characterized as viscoelastic materials, even at ambient temperature, with their behavior being intermediate to the high viscosity fluids and elastic solids [457]. The yield point of polymers, which separates the rubbery state from the glassy state, depends strongly on the strain rate. At high strain rates, the polymers behave as brittle materials, where their yielding and break take place in low strains, while the modulus of elasticity and yield stress are relatively high. At low strain rates, the polymers behave more elastically, by developing larger strains for the same level of stresses. Conversely analogous is the behavior of the polymers observed with the temperature variation. The aforementioned correlation is described by the time-temperature superposition principle [457]. In the present work, the mechanical behavior of the dummy cells was specified by the calculation of the bending modulus, the yield stress, and the maximum stress in bending, for different strain rates. The bending experiments were according to ASTM D790. The calculation of the aforementioned sizes was conducted easily using Origin Pro Data Analysis and Graphing Software, which was used to obtain the stress–strain curves from the force-displacement curves measured by the universal testing machine Instron 4301. In this group of experiments, Maxwell model (under a constant strain rate loading) was also applied for the description of the experimental results, and the determination of the relaxation time and the viscosity of the materials under investigation [457]. According to this model, if at 푡 = 0 a

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tensile load is applied, strains ε1 and ε2 will develop on the spring and the dashpot, respectively. The total strain of the model is described by Equation 4-21.

휀 = 휀1 + 휀2 (4-21) The derivative to time of Equation 4-21 leads to Equation 4-22, where the total strain rate of the model is described.

휀̇ = 휀1̇ + 휀2̇ (4-22) Because the spring and the dashpot are connected in series, the stress in each element is equal to σ. For the spring and the dashpot, Equation 4-23 and Equation 4-24 are derived, respectively, where E is the modulus of elasticity and η is the viscosity of the material.

휎̇ = 퐸휀1̇ (4-23)

휎 = 휂휀2̇ (4-24) From Equation 4-22, Equation 4-23, and Equation 4-24, the constitute equation of the Maxwell model (see Equation 4-25) is derived. 휎̇ 휎 휂 휀̇ = 휀1̇ + 휀2̇ = + ⇒ 휎 + 휎̇ = 휀̇휂 (4-25) 퐸 휂 훦

푑휀 = 휇 = 푐표푛푠푡푎푛푡 If a specific strain rate is applied to the Maxwell model, i.e. 푑푡 , then from Equation 4-25 derives Equation 4-26. From Equation 4-26, assuming that for 푡 = 0, 휎 = 0, Equation 4-27 is derived, where 휏 = 휂⁄퐸 is the relaxation time of the material. 휎 1 푑휎 + = 휇 (4-26) 휂 훦 푑푡

푡 휎(푡) = 휂휇 [1 − 푒푥푝 (− )] (4-27) 휏 In the present work, the relaxation time and the viscosity of the materials were determined by fitting Equation 4-27 to the experimental stress–strain curves. ➢ Dynamic mechanical characterization The dynamic mechanical characterization of dummy cells was conducted through dynamic mechanical analysis (DMA) experiments at different oscillation frequencies in three-point bending. DMA is a technique used to measure the changes in the dynamic response of a material over time, temperature, frequency, or amplitude of oscillation. The response of the material depends on the stresses and strains that develop in the material due to the dynamic excitation. Basic characteristics of the material, such as its inertia and molecular kinetics, are directly linked to its dynamic response. The time-varying stresses σ(t) or strains ε(t) developed in the material determine its dynamic behavior. Consequently, each material has a different behavior depending on the dynamic mechanical loading to which it is subjected [458]. The dynamic response of a material is determined by the oscillation of the material under specific conditions (temperature, frequency, deformation, and stress). Through the harmonic stresses (or strains) which the material under investigation is subjected to, important mechanical sizes are determined [458]. The parameters that were used to conduct the DMA experiments of the present work are tabulated in Table 4-4. In each case of the investigated material, the storage modulus, the loss modulus, the tan delta, and the complex viscosity were determined, using Q800 TA Instruments dynamic mechanical analyzer.

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Table 4-4: Values of the parameters that were used during the DMA experiments.

Parameter Value Temperature 30oC Oscillation frequencies 0.1 ─ 80 Hz Dimensions of the specimens 60 mm x 12.8 mm x 1.3 mm Maximum amplitude 150 μm

➢ Viscoelastic characterization The viscoelastic characterization of dummy cells was conducted through stress relaxation experiments at different strain levels. The term “viscoelasticity” refers to the fact that the behavior of the materials is intermediate to that of the fluids and that of the completely elastic bodies, resulting in their time dependence behavior on excitation. Just like the elastic behavior, the viscoelastic behavior is distinguished in linear and non-linear [457]. In the linear elastic behavior, regardless of the moment of the observation, the stress–strain curve is unique and linear. In linear viscoelastic behavior, there is a different stress–strain curve for each time and it is also linear. Finally, in non- linear viscoelastic behavior, for each time corresponds a different stress–strain curve, which is non- linear. Relaxation is called the phenomenon in which the developed stresses in a material, which is in a constant strain, temperature, and humidity, decrease over time [457]. In the present work, the viscoelastic behavior of dummy cells was investigated by stress relaxation experiments in three-point bending. The experiments were conducted at the universal testing machine Instron 4301, according to ASTM D790. The displacement rate of the grabs was set at 100 mm/min. The imposed strains of the dummy cells were quite low, to investigate their linear viscoelastic behavior, according to their mechanical characterization already conducted. In order to describe the stress relaxation behavior of the dummy cells in three-point bending and calculate the relaxation time of the materials in each imposed strain, the residual property model (RPM) was applied. As it has already been stated, the RPM is developed by Professor G.C. Papanicolaou and is used to describe and predict the behavior of any material after any kind of damage [447–453]. According to this model, it turns out that the degradation of the mechanical properties of a material, due to any damage, follows an exponential decrease which is described by Equation 4- 28. In this equation, Pr denotes the value of the residual property (e.g. modulus of elasticity) after the damage, P0 denotes the corresponding material property before the damage, P∞ is the value of the 푃0+ residual property of the material when it is under damage saturation conditions, s0 is the ratio , s 푃0 푃∞ is the ratio , and M is a degradation parameter, which depends on the nature of the damage. 푃0

푃푟 −푠푀 = 푠 + (푠0 − 푠)푒 (4-28) 푃0

푡 In the case of stress relaxation, M parameter equals to the ratio 푠휏, where t is the current time of the experiment and τ is the relaxation time. The parameter Pr is the stress relaxation and P0 is the initial zero-time stress developed in the material after the strain is applied. When M tends to zero, the + ration tends to s0, whereas when M tends to infinity, the ratio tends to s. RPM is applied from 0 time point.

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Chapter 5: Results and Discussion 5.1. Conventional Dye-Sensitized Solar Cells ➢ Effect of the application of different temperature profiles during sintering of the conventional anode on dye-sensitized solar cell characteristics The temperature profile during thermal processing of working electrodes of DSSCs has a great effect on the electrical characteristics of the solar cells and has to be studied thoroughly in view of achieving a high energy conversion efficiency. The influence of thermal processing of anode on its microstructure, specific surface area, electrical characteristics, and dye-adsorption, and subsequently on DSSCs electrical characteristics has been investigated by many scientists since the invention of the modern-day DSSCs [414,459–461]. Many results demonstrate the importance of this procedure. The appropriate temperature profile depends on many factors, including the type of electrode substrate and the precursor material used for the fabrication of the anode (e.g. concerning amorphous structures, thermal processing also refers to crystallization procedure). The present study is a preliminary investigation on the effect of the application of different temperature profiles during sintering of the commercially-available TiO2 nanoparticles (P25 Degussa) composing the anode on DSSCs electrical characteristics. Figure 5-1 shows the J–V characteristic curves of the DSSCs employing the conventional working electrode sintered under different temperature profiles. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-1 and Table 5-2, respectively.

Figure 5-1: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional working electrode sintered under different temperature profiles. The maximum ECE attained by the DSSCs in this group of experiments was 5.83% and was achieved using the working electrode sintered according to temperature profile 6. In general, the results showed an increase in JSC and a decrease in VOC of the solar cells by increasing the sintering temperature of the working electrodes from 450oC to 550oC. However, regarding the 600oC, a decrease in JSC, VOC, and FF of the solar cells was observed. Talking about the temperature profiles 5, 6, and 7, the gradual increase and/or decrease in the temperature before and/or after the sintering process of the anode led to an enhanced ECE.

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Table 5-1: Electrical characteristics of the dye-sensitized solar cells employing the conventional working electrode sintered under different temperature profiles.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 1 12.38±0.11 740±3 0.60±0.01 5.53±0.01 RE 2 12.72±0.03 736±2 0.60±0.00 5.62±0.01 RE 3 12.99±0.08 732±2 0.59±0.01 5.64±0.01 RE 4 12.69±0.22 724±4 0.58±0.01 5.33±0.03 RE 5 13.22±0.17 726±2 0.59±0.01 5.70±0.01 RE 6 13.16±0.03 739±1 0.60±0.00 5.83±0.01 RE 7 13.28±0.12 728±1 0.59±0.01 5.73±0.01

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of DSSCs contributed to a better understanding of the above-mentioned results. More specifically, the increase in n and J0 of the solar cells, as well as the slight decrease in rSH by increasing the sintering temperature of the working electrode justifies the corresponding decrease in VOC. On the other hand, the rS of the solar cells was not altered by increasing the sintering temperature of the working electrode up to 550oC. Concerning the DSSCs employing the working electrodes sintered at o 600 C, a high increase in the values of n, J0, and rS was observed, while the rSH was decreased. The aforementioned variation of the parameters justifies the decrease in FF of these solar cells. Regarding the temperature profiles 5–7, when a gradual increase in temperature up to the sintering temperature o of the working electrode (550 C) was applied, an increase in n, J0, and rSH of the solar cells was observed, while rS showed a slight decrease. On the other hand, the gradual decrease in the o temperature after sintering the working electrode at 550 C led to a decrease in n, J0, and rS. Regarding the JL, in all the aforementioned cases, its value was at a constant rate higher than JSC. Table 5-2: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the conventional working electrode sintered under different temperature profiles.

Working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 1 12.42±0.11 3.15±0.03 1455±72 4.42±0.14 1460±42 RE 2 12.76±0.03 3.17±0.01 1701±69 4.50±0.00 1449±20 RE 3 13.03±0.08 3.20±0.01 1971±27 4.42±0.14 1447±25 RE 4 12.73±0.23 3.36±0.02 3200±341 5.00±0.50 1326±42 RE 5 13.25±0.17 3.26±0.01 2502±113 4.17±0.14 1635±23 RE 6 13.20±0.03 3.13±0.01 1502±65 4.25±0.00 1456±23 RE 7 13.31±0.12 3.24±0.01 2346±67 4.17±0.14 1658±24

The increase in JSC and the decrease in VOC of the solar cells by increasing the sintering temperature of the working electrode was attributed to morphological and crystallographic changes in the anode, as well as to the increased electron−hole recombination rate at TiO2/electrolyte interface. When P25 powder is subjected to heat treatment at high temperatures (higher than 500oC), an increase in its specific surface area and rutile content is reported [462]. By increasing the specific surface area of the anode, a larger amount of dye molecules is anchored on it, leading to an increased photo-current production [86]. On the other hand, it has been reported that an increase of rutile phase percentage in TiO2 anode could lead to an increased electron−hole recombination rate inside the solar cells [463]. This phenomenon was also observed in the present study, where the electron−hole recombination rate at TiO2/electrolyte increased by increasing the sintering temperature of the working electrode. This was probably the main reason for the corresponding decrease in VOC of the solar cells. Another factor that could contribute to the decrease in VOC of the solar cells is the positive shift of the quasi-Fermi

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level of the anode due to the increase of the rutile phase percentage. Rutile has a lower conduction band edge energy than anatase, which generally leads to a lower Fermi level and VOC in DSSCs [37]. The decrease in JSC, VOC, and FF that was observed for the solar cells employing the working electrodes sintered at 600oC was mainly attributed to the evident deterioration of the FTO glass, resulting in a decrease in its surface conductivity and optical transparency. Thus, a high increase in electron−hole recombination rate and in the internal series parasitic resistance of the solar cells, as well as a decrease in JSC and VOC of the solar cells were observed. Concerning the solar cells employing the working electrodes sintered according to the temperature profiles 5, 6, and 7, the possible mechanisms that affected their electrical characteristics are described below. By applying a gradual increase in the temperature up to the final sintering temperature, the organic materials included in the TiO2 paste evaporate at a lower rate. Thus, the developed cracks in the anode film could be less than in the corresponding cases where an abrupt evaporation of the organics takes place. Furthermore, the sintering of TiO2 nanoparticles could be more effective since there is more time for particles re-arrangement and sintering necks formation. Thus, a more compact structure is developed [464]. These are possibly the reasons why the solar cells employing RE 5 or RE 7 working electrodes showed a decreased rS and an increased JSC. In the same cases, the decrease in VOC and the increase in n, J0, and rSH that were observed is possibly a result of the development of smaller-sized pores in the anode film due to the more effective sintering, as it has already been described. The development of small-sized pores in the anode leads to an increased difficulty of electrolyte penetration through the entire thickness of anode, while the diffusion of triiodides into the anode could also be retarded. Thus, dye molecules anchored on TiO2 are ineffectively regenerated and the lifetime of electrons in the anode is decreased [465]. On the other hand, the development of smaller-sized pores in the anode leads to the decreased electron−hole recombination rate at FTO/electrolyte interface since a lower amount of electrolyte comes in contact with the conductive substrate. Regarding the cases where a gradual decrease in temperature after sintering of the working electrodes was applied, the improvement of the electrical characteristics of the solar cells is possibly attributed to the fact that the negative effects caused by the thermal shock were avoided.

➢ Effect of TiCl4 post-treatment of the anode on dye-sensitized solar cell characteristics A known method to improve the performance of conventional DSSCs is the post-treatment of the TiO2 anode with a TiCl4 aqueous solution. Different explanations of the working principle of the developed coating have been reported [95,96]. These hypotheses concern increased specific surface area, improved electron transport, improved light scattering towards higher light absorption, etc. In the present investigation, the conventional working electrode (E RE 6) was post-treated with TiCl4, before its sensitization, in view of improving its characteristics for DSSCs application and further enhancing the energy conversion efficiency. Figure 5-2 shows the J–V characteristic curves of the DSSCs employing the conventional working electrode and the TiCl4 treaded working electrode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5- 3 and Table 5-4, respectively.

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Figure 5-2: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional working electrode and the TiCl4 treaded working electrode. The post-treatment of the conventional working electrode led to an increase of the ECE of DSSCs by 18%, reaching the 6.88%. The DSSCs performance enhancement was due to JSC, VOC, and FF increase. Table 5-3: Electrical characteristics of the dye-sensitized solar cells employing the conventional working electrode and the TiCl4 treaded working electrode.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 13.16±0.03 739±1 0.60±0.00 5.83±0.01 RE 6 T 14.50±0.09 761±2 0.62±0.01 6.88±0.01

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of DSSCs contributed to a better understanding of the above-mentioned results. The treatment of the conventional working electrode with TiCl4 resulted in a significant decrease in n and J0, while an increase was also observed in rSH, justifying the corresponding increase in VOC. Furthermore, the solar cells employing the TiCl4 treated working electrode showed an increased JL, justifying the increase in JSC, while a slight increase was also observed in rS. Table 5-4: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the conventional working electrode and the TiCl4 treaded working electrode.

Working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 6 13.20±0.03 3.13±0.01 1502±65 4.25±0.00 1456±23 RE 6 T 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34

In order to further examine the TiCl4 post-treatment of the conventional working electrode and interpret the PV and one-diode equivalent circuit model results, the characteristics of the conventional working electrode and TiCl4 treated working electrode were compared by means of SEM, AFM, XRD, BET, DRS, PL, and UV-VIS (see Figure 5-3). The parameters obtained by AFM, XRD, BET, and DRS are listed in Table 5-5.

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Figure 5-3: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the TiCl4 treated working electrode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the TiCl4 treated working electrode. Surface analysis by means of SEM revealed that when the working electrode was treated with TiCl4, its morphology changed. The surface of the TiCl4 treated anode was found less uniform and rougher compared to the conventional one, possibly attributed to the formation of small TiO2 clusters on the top of the conventional TiO2 film during the TiCl4 post-treatment [96]. AFM gave additional evidence for the surface morphology of the two compared anodes. The surface root-mean-square roughness, the surface area, and the Z-range were found increased in the case of the TiCl4 treated anode compared to the conventional one. The XRD patterns of both fabricated working electrodes showed sharp and intense diffraction peaks that can be indexed to anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276), arising from the TiO2 anode, as well as to cassiterite (JCPDS 21-1250), arising from FTO glass. The XRD analysis revealed an increase of wt% content of anatase in the anode, a slight decrease in anatase mean crystallite size, and an increase of anatase crystal lattice distortion, after the TiCl4 treatment of the anode. On the other hand, the rutile mean crystallite size was found slightly increased, while there was no change in rutile crystal lattice distortion. Concerning the BET analysis and according to IUPAC classification, the isotherms of both the conventional anode and TiCl4 treated anode showed a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that in both cases the nanoparticles were aggregated to plate-like particles, creating slit-shaped mesopores [429,466]. However, the TiCl4 treated anode showed an increase in its specific surface area, a decrease in the average pore diameter, and a decrease

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in the total pore volume. Similar results are found in the literature [96]. From Tauc plots derived from DRS, it was found that the optical bandgap of the anode after TiCl4 treatment was slightly increased, possibly attributed to the increased wt% content of anatase in the anode compared to the conventional one, according to XRD results. Furthermore, by comparing the PL spectra of the working electrodes, a decrease in the intensity of the emission peak at about 398 nm and a decrease in the intensity of the emission peaks ranging from 440 to 500 nm in the case of the TiCl4 treated working electrode was observed. The aforementioned observations are possibly attributed to the small increase of wt% content of anatase and decrease in the defect-states, respectively, in the anode after the TiCl4 treatment [467]. The decrease in the surface defects of TiO2 after TiCl4 post-treatment has also been reported by many scientists in the past years [96,468]. Finally, the transmittance spectra of the TiCl4 treated working electrode was found slightly decreased compared to the conventional one. According to the literature, the TiCl4 post-treatment of TiO2 working electrodes can lead to an improvement of light scattering towards higher light absorption in the long wavelength region of the visible spectrum [96]. Table 5-5: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode and the TiCl4 treated working electrode.

Parameter Sample reference with TiCl4 Root-mean-square roughness (nm) a 20 22 Surface area (μm2) a 31.8 32.2 Z-range (nm) a 171 178

b Anatase 78 82 TiO2 phase composition (wt%) Rutile 22 18 Anatase 25 24 Mean primary crystallite size (nm) b Rutile 42 44 Anatase 0.006 0.007 Crystal lattice distortion b Rutile 0.003 0.003 Specific surface area (m2g-1) c 49.9 53.9 Average pore diameter (nm) c 18.7 16.4 Total pore volume (cm3g-1) c 0.260 0.220 Bandgap (eV) d 3 3.02 determined by a AFM, b XRD, c BET, d DRS

The increase in JL and subsequently in JSC for the solar cells employing the TiCl4 treated working electrode is possibly attributed to the increased amount of dye adsorbed on the anode, arising from the increase in its specific surface area and roughness, as well as from the increase of anatase crystal lattice distortion. By increasing the specific surface area and roughness of the anode, more dye molecules can anchor on its surface [96,244]. On the other hand, the crystal lattice distortion affects semiconductor sensitization. The adsorption ability of dyes like N719 is dependent on the adsorption of carboxylate groups of dye either on defect sites or on the hydroxyl groups on the surface of TiO2 [469]. The increase in VOC of the solar cells employing the TiCl4 treated working electrode was mainly attributed to the reduced electron−hole recombination rate at TiO2/electrolyte and FTO/electrolyte interfaces, as well as to the increased wt% content of anatase in the anode. As it has already been shown, there is a decrease in n and J0 for the solar cells employing the TiCl4 treated working electrode, which can be attributed to the decreased defect-states in TiO2 after TiCl4 treatment, according to PL analysis. On the other hand, it is known that anatase is characterized by a greater bandgap than rutile and lead to a lower recombination rate [463]. The increased wt% content of anatase in the anode also increases its quasi-Fermi level and thus the VOC of DSSCs, which is determined as the difference between the quasi-Femi level of the anode and the electrochemical potential of the electrolyte [37]. The increase in rSH is possibly attributed to the less amount of electrolyte that comes in contact with the conductive substrate after TiCl4 treatment of the anode since the decrease in the average pore

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diameter increases the difficulty of electrolyte penetration through its entire thickness [465]. The increase in FF of the solar cells employing the TiCl4 treated working electrode was attributed to the reduced recombination rate into the solar cells. The increased rS could be ascribed to a higher resistance in electrons transport in the anode. The narrowed pore size in the case of the TiCl4 treated anode could lead to diffusion-limited kinetics in the electrolyte, thus to a reduced ambipolar diffusion of electrons in the anode [96]. ➢ Conclusions In the present investigation, a satisfactory energy conversion efficiency, on the order of 6.88%, was achieved by the conventional DSSCs, after optimization of the sintering temperature profile and TiCl4 treatment of the conventional TiO2 working electrode. The investigation demonstrated the importance of choosing the appropriate temperature profile during sintering of the TiO2 nanostructured working electrode of DSSCs as well as the effect of its TiCl4 post-treatment on solar cells electrical characteristics. The results are quite satisfactory by considering the low-cost and simplicity of the manufacturing procedure of the conventional working electrodes, while the performance of DSSCs is considered satisfactory according to the literature, setting them as reference solar cells towards their further development. 5.2. Towards Higher Efficiency Dye-Sensitized Solar Cells 5.2.1. Optimization of Dye-Sensitized Solar Cells Photo-anode Characteristics Towards an Impressive Energy Conversion Efficiency In view of achieving a high energy conversion efficiency by DSSCs, modifications were made on the conventional dye-sensitized working electrodes treated with TiCl4 (E RE 6 T). The aim was the fabrication of a novel hybrid multilayered photo-anode, with optimized morphological, crystallographic, electrical, and optical characteristics. These modifications concerned interfacial engineering, regulation of the materials porosity, fabrication of composites aiming to improve the electrical and optical characteristics of the anode, enhancement of light scattering, and co- sensitization of the anode for enhanced light-harvesting. ➢ Effect of the addition of a blocking layer on dye-sensitized solar cell characteristics The performance of DSSCs depends highly on interfacial phenomena and processes taking place during their operation. Interfacial engineering is frequently used by many scientists to improve DSSCs performance, with many novel techniques to have appeared in the last decades [85]. Amongst them, the introduction of a blocking layer between the conductive substrate and the anode is a promising modification of the conventional working electrode in view of decreasing the electron−hole recombination rate at TCO/electrolyte interface and improving the contact between the anode and the conductive substrate, thus subsequently enhancing DSSCs performance [93]. In the present investigation, the effect of the introduction of a blocking layer between the FTO glass and the TiO2 anode on DSSCs electrical characteristics was studied, in view of achieving a higher energy conversion efficiency by the solar cells (different blocking layer thicknesses were tested).

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Figure 5-4 shows the J–V characteristic curves of the DSSCs employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally a blocking layer of different film thicknesses between the FTO glass and the anode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-6 and Table 5-7, respectively.

Table 5-6: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 working electrode and the corresponding working electrodes fabricated using additionally a blocking layer, of different film thicknesses.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 T 14.50±0.09 761±2 0.62±0.01 6.88±0.01 BL 500 T 14.54±0.12 771±2 0.64±0.01 7.14±0.01 BL 1000 T 15.80±0.21 775±2 0.64±0.01 7.80±0.02 BL 1500 T 16.53±0.09 790±3 0.65±0.01 8.44±0.02 BL 1800 T 16.71±0.16 782±3 0.64±0.01 8.41±0.02

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of DSSCs contributed to a better understanding of the above-mentioned results. The introduction of a blocking layer between FTO glass and the anode resulted in a high increase in JL and rSH, while the values of n and J0 slightly decreased, justifying the corresponding increase in JSC and VOC of the solar cells. On the other hand, rS showed a slight increase with the introduction of the blocking layer of the increased thickness. However, as the thickness of the blocking layer was decreasing, rS was decreasing, justifying the corresponding increase in FF. Regarding the difference between JL and JSC, it was found to be lower in the case of DSSCs employing the blocking layer.

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Table 5-7: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the conventional TiCl4 working electrode and the corresponding working electrodes fabricated using additionally a blocking layer, of different film thicknesses.

Working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 6 T 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 BL 500 T 14.56±0.12 2.73±0.01 285±10 4.75±0.25 4313±29 BL 1000 T 15.82±0.21 2.72±0.01 286±23 4.33±0.14 4296±36 BL 1500 T 16.55±0.09 2.67±0.01 197±12 4.08±0.14 4272±51 BL 1800 T 16.73±0.16 2.66±0.01 216±20 4.08±0.14 4249±48

In order to further examine the modification of the conventional working electrode of DSSCs with the addition of a blocking layer, the characteristics of the FTO glass and the blocking layer on FTO glass, in the optimized thickness (spin-coating at 1500 rpm) according to solar cells characterization results, were compared by means of SEM, AFM and UV-VIS (see Figure 5-5). The parameters obtained by AFM are listed in Table 5-8.

Figure 5-5: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the blocking layer on the surface conductive glass, (c) UV-VIS transmittance spectra of the surface conductive glass and the surface conductive glass coated with blocking layer.

Surface analysis by means of SEM revealed that the surface morphology of the FTO glass covered or not with the blocking layer is quite similar. Blocking layer thickness was on the order of nm scale. Thus, its surface morphology was highly dependent on the morphology of the underlayer material (FTO glass). AFM quantified the differences in the surface morphology between the bare FTO glass and the FTO glass covered with the blocking layer. From the analysis, it was found that the surface root-mean-square roughness, the surface area, and the Z-range decreased in the case of FTO glass covered with the blocking layer. Furthermore, UV-VIS revealed a decrease in the transparency of FTO glass when it was covered by the blocking layer. Table 5-8: Parameters obtained by AFM for surface conductive glass and surface conductive glass covered by the blocking layer.

Parameter Sample FTO glass BL 1500 Root-mean-square roughness (nm) a 18 16 Surface area (μm2) a 25.8 25.5 Z-range (nm) a 118 102 determined by a AFM

The increase in JSC with the introduction of a blocking layer between the conductive substrate and the anode is commonly reported in the literature. In most cases, this observation was attributed to the decrease in electron−hole recombination rate at TCO/electrolyte interface and to the higher collection

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efficiency of the electrons to the external circuit [93,470,471]. Some other reports show that the aforementioned observation is a result of an increased transparency of the conductive substrate with the introduction of the blocking layer [472]. However, in the present investigation, according to one- diode model equivalent circuit analysis and materials characterization results, the corresponding increase in JSC could be not attributed to the reasons described above. In this case, two possible explanations are listed below. At first, it can be supposed that the dye excitation in the front-side of the anode is increased since the blocking layer hinders the electrolyte penetration through the entire thickness of the anode, thus the light absorption by the triiodides of the electrolyte is lower. This can be supported, after taking into account that there is only a small difference between the bare FTO glass and the FTO glass covered with the blocking layer, in the transparency in the visible spectrum, according to UV-VIS results. Another explanation that can be used additionally to support the present results, is that reported by Yoo et al [473]. This group found that even there is a little difference in the transparency of the conductive substrate before and after the introduction of the thin blocking layer, the transmittance spectrum in 300 to 600 nm region of the system increases when it comes in contact with the electrolyte. Thus, the increased light intensity in the aforementioned wavelength region reaching the photo-anode could lead to an increased JL production. Regarding the decrease in JL and subsequently in JSC by increasing the thickness of the blocking layer, this could be a result of a decrease in the transparency of FTO glass/blocking layer system [93]. Thus, more light is absorbed by the blocking layer, leading to a decreased dye excitation. On the other hand, the increase in VOC for the solar cells employing additionally the blocking layer was attributed to the reduced electron−hole recombination rate at the FTO/electrolyte and TiO2/electrolyte interfaces, which is indicated by the high increase in rSH and the decrease in n and J0. The increase in FF of the solar cells employing additionally the blocking layer was attributed to the lower resistance in electrons transfer in the working electrode as well as to the high decrease in electron−hole recombination rate at the FTO/electrolyte interface. These are shown by the decrease in rS and the high increase in rSH, respectively. The decreased rS could possibly be ascribed to the development of a better contact between TiO2 and conductive substrate since the roughness and the Z-range of the conductive substrate with the addition of the blocking layer decreases, according to AFM results. Thus, the contact between the conductive substrate and the photo-anode could be improved [474]. The great decrease in rSH is a result of the reduced amount of electrolyte coming in contact with the FTO glass after the introduction of the blocking layer. Thus, the electrons in FTO glass recombine at a lower rate with holes existing in the electrolyte. On the other hand, the increase in rS by increasing the thickness of the blocking layer is ascribed to the fact, that the blocking layer of increased thickness can act as a barrier in the electrons transfer from the anode to the FTO glass. Similar results are reported in the literature [93]. ➢ Effect of the addition of a pore-forming agent into the anode on dye- sensitized solar cells characteristics The characteristics of the mesoporous structure of the nanocrystalline TiO2 anode play a key role in the optimal operation of DSSCs, while the optimal regulation of its porosity can lead to an enhanced energy conversion efficiency [85]. Many scientists introduce pore forming agents into the anode during the fabrication of working electrodes for DSSCs, to improve their characteristics and subsequently enhance solar cells performance [86,87,475]. Some of the benefits of the pore forming agents usage are the increased dye adsorption on the anode, the higher electrons transmission efficiency in the anode, the reduced electron−hole recombination at TiO2/electrolyte interface, etc. In the present investigation, PVP in different wt% loadings was used as a pore-forming agent in the anode, during the fabrication of the working electrodes for DSSCs, in view of enhancing their higher energy conversion efficiency.

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Chapter 5 Results and Discussion

Figure 5-6 shows the J–V characteristic curves of the DSSCs employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-9 and Table 5-10, respectively.

Figure 5-6: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode. The maximum ECE attained by the DSSCs in this group of experiments was 9.14% and was achieved with the addition of 1.5 wt% of PVP into the anode. This value is almost 33% higher compared to the corresponding value attained by the solar cells without the addition of PVP into the anode. The improvement in DSSCs performance was due to JSC, VOC, and FF increase. In general, the results showed an increase in JSC and VOC of solar cells with the addition of PVP as a pore forming agent into the anode, while in some cases, an increase in FF was also observed. However, after an optimal wt% loading of PVP into the anode, the JSC, VOC, and FF of the solar cells showed a decrease.

Table 5-9: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 T 14.50±0.09 761±2 0.62±0.01 6.88±0.01 1.0 wt% PVP T 18.69±0.16 768±2 0.62±0.01 8.95±0.01 1.5 wt% PVP T 18.62±0.11 775±2 0.63±0.01 9.14±0.01 2.0 wt% PVP T 17.68±0.15 772±2 0.63±0.01 8.65±0.02 3.0 wt% PVP T 17.06±0.09 764±3 0.62±0.01 8.13±0.02

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of DSSCs contributed to a better understanding of the above-mentioned results. The introduction of PVP into the anode resulted in an increased JL. However, as the wt% loading of PVP was increasing, the JL was decreasing. On the other hand, the n and J0 showed a different variation, with their values decreasing until the 1.5 wt% loading of PVP in the anode, while for higher wt% loadings of PVP in the anode, their values showed an increase. A similar variation to n and J0 was observed for the rS, while in this case, its minimum value was attained for 1 wt% loading of PVP in the anode. Finally, the rSH showed a continuous decrease with the increase of wt% loading of PVP in the anode.

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Table 5-10: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally PVP in different wt% loadings in the anode.

Working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) E RE 6 T 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 1.0 wt% PVP T 18.74±0.16 2.69±0.02 320±16 4.00±0.00 1613±37 1.5 wt% PVP T 18.67±0.11 2.55±0.02 158±9 4.08±0.14 1601±42 2.0 wt% PVP T 17.73±0.15 2.60±0.03 196±17 4.08±0.14 1593±47 3.0 wt% PVP T 17.11±0.10 2.70±0.04 321±34 4.17±0.14 1537±53

In order to further examine the modification of the conventional working electrode of DSSCs with the addition of PVP in the anode, the characteristics of the conventional working electrode and the corresponding working electrode fabricated using additionally PVP into the anode, in the optimized wt% loading (1.5 wt% loading of PVP) according to solar cells characterization results, were compared by means of SEM, AFM, XRD, BET, DRS, PL, and UV-VIS (see Figure 5-7). The parameters obtained by AFM, XRD, BET, and DRS are listed in Table 5-11.

Figure 5-7: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the working electrode fabricated using additionally 1.5 wt% of polyvinylpyrrolidone in the anode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the working electrode fabricated using additionally 1.5 wt% of polyvinylpyrrolidone in the anode.

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Surface analysis by means of SEM revealed that when PVP was introduced as a pore-forming agent during the fabrication of the working electrode, large pores were created, while the uniformity of the surface of the anode was maintained as in the case of the conventional working electrode. AFM gave additional evidence for the surface morphology of the two compared anodes. The surface root- mean-square roughness, the surface area, and the Z-range of the conventional working electrode and corresponding working electrode fabricated using additionally PVP in the anode were found almost the same. In both cases, the XRD patterns of the fabricated working electrodes showed sharp and intense diffraction peaks that can be indexed to anatase (JCPDS 21-1272) and rutile (JCPDS 21- 1276), arising from the TiO2 anode, as well as to cassiterite (JCPDS 21-1250), arising from FTO glass. The XRD analysis showed that there is no change in the TiO2 phase composition of the anode. However, the mean crystallite size and the crystal lattice distortion of both anatase and rutile decreased and increased, respectively, in the case of the working electrode fabricated using additionally PVP in the anode. Concerning the BET analysis and according to IUPAC classification, the isotherms of the conventional anode and the anode fabricated using additionally PVP showed in both cases a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that in both cases the nanopowder was aggregated to plate-like particles, creating slit-shaped mesopores [429,466]. However, by comparing the two anodes, the anode fabricated using additionally PVP showed a decrease in its specific surface area, as well as a high increase of the average pore diameter and the pore volume. From Tauc plots derived from DRS, it was found that there was no significant change in the optical bandgap of the two compared working electrodes. Furthermore, by comparing the PL spectra of the conventional working electrode and the working electrode fabricated using additionally PVP in the anode, there was no alteration in the intensity of the emission peak at 398 nm, while an increase in the intensity of the emission peaks ranging from 440 to 500 nm in the latter was observed. The increase in the intensity of the emission peaks ranging from 440 to 500 nm shows an increase of the defect-states of TiO2 when PVP was added as a pore forming agent into the anode [467]. The small shift to lower intensities of the curve corresponding to the working electrode fabricated using additionally PVP as a pore forming agent in the anode could be attributed to its increased transmittance compared to the conventional working electrode, according to UV-VIS results. As it is shown in the UV-VIS transmittance spectra of the two compared working electrodes, the transmittance of the working electrode fabricated using additionally PVP in anode was increased. The aforementioned results are in most cases in agreement with the results reported by Hu et al [86]. Table 5-11: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, and the corresponding working electrode fabricated using additionally 1.5 wt% of PVP in the anode.

Parameter Sample conventional 1.5 wt% PVP Root-mean-square roughness (nm) a 20 20 2 a Surface area (μm ) 31.8 31.7 Z-range (nm) a 171 173 b Anatase 78 78 TiO2 phase composition (wt%) Rutile 22 22 Anatase 25 23 Mean primary crystallite size (nm) b Rutile 42 37 Anatase 0.006 0.007 Crystal lattice distortion b Rutile 0.003 0.004 Specific surface area (m2g-1) c 49.9 44.2 Average pore diameter (nm) c 18.7 37 Total pore volume (cm3g-1) c 0.260 0.512 Bandgap (eV) d 3 2.99 determined by a AFM, b XRD, c BET, d DRS

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The increase in JL and subsequently in JSC with the addition of low wt% amount of PVP into the anode was attributed to the increase in the amount of dye adsorbed on the surface of TiO2, due to the increase of crystal lattice distortion and average pore diameter, according to XRD and BET analysis, respectively. The increase of crystal lattice distortion favors the dye adsorption onto the surface of TiO2, while the development of large pores leads to reduced dye diffusion resistance into the anode during the sensitization procedure [86,469]. Furthermore, according to UV-VIS results, the transparency of the working electrode increased with the addition of PVP into the anode. Thus, light- harvesting from the entire thickness of the photo-anode could be more efficient [86]. The increase in VOC of the solar cells employing the working electrodes fabricated using additionally low wt% loading of PVP in the anode could be attributed to an increase in the quasi-Fermi level of TiO2, due to the increased JL, as well as to the reduced electron−hole recombination rate at TiO2/electrolyte interface, according to one diode model equivalent circuit analysis. On the other hand, the corresponding increase in FF of the solar cells was attributed to a decreased electron transport resistance in the anode, which is indicated from the corresponding decrease in rS. The introduction of a pore forming agent in the anode could lead to the development of connected conducting channels from TiO2 nanoparticles, resulting in a rapid electrons travel along the anode and enhanced electron lifetime [86]. This could also be the reason for the reduced electron−hole recombination rate at TiO2/electrolyte interface observed in these cases. Concerning the solar cells employing the working electrodes fabricated using additionally a high wt% loading of PVP in the anode, there was a decrease in JSC, VOC, and FF. The decrease in JSC was a result of the decrease in JL, possibly due to a further decrease in the specific surface area of the anode [87]. Thus, a lower amount of dye is adsorbed on it. The decrease in VOC and FF is possibly attributed to the non-uniform dispersion of the pore forming agent into the anode, leading to the development of a non-ideal porosity and dead-end conductive channels from TiO2 nanoparticles. As a result, the electrons transport in the photo-anode becomes less sufficient, which is indicated by the increase in rS, while the electron−hole recombination rate at TiO2/electrolyte is increased, which is indicated by the increase in n and J0. Furthermore, due to the non-uniform dispersion of the pore forming agent into the anode, larger pore could be developed, leading to a reduced resistance in electrolyte penetration through the entire thickness of the anode. Thus, a larger amount of electrolyte comes in contact with the conductive substrate, resulting in an increase in the electron−hole recombination rate at FTO/electrolyte interface, which is indicated by the decrease in rSH. ➢ Effect of the addition of light scatters into the anode on dye-sensitized solar cells characteristics In recent years, the light scattering effect generated by large particles in the sub-micrometer scale has been widely used in DSSCs as an efficient strategy to make better use of the incident solar light [99]. Microparticles are introduced in the photo-anode to extend the optical path length of incident light within the photo-anode, so as to enhance the light-harvesting by DSSCs. In the present investigation, rutile microparticles were introduced in different wt% loadings in the anode, to create a scattering layer and/or reflecting layer, in a bilayer or multilayer structure, in view of achieving a higher energy conversion efficiency.

Figure 5-8 shows the J–V characteristic curves of the DSSCs employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally rutile microparticles in different wt% loadings in the anode as light scatters, in different anode designs. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-12 and Table 5-13, respectively.

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Figure 5-8: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally rutile microparticles in different wt% loadings in the anode, in different anode designs. The maximum ECE attained by the DSSCs in this group of experiments was 9.11% and was achieved using the working electrode design “E N/S/R/N T”. This value is almost 32% higher compared to the corresponding value attained by the solar cells without the usage of rutile microparticles in the anode. The improvement in DSSCs performance was due mainly to the increase in JSC, while a small increase was also observed in VOC. Concerning the double layer anode designs, in general, an increase in JSC of the solar cells was observed with the addition of rutile microparticles in the anode, while there was no change in the values of VOC and FF. However, as the wt% loading of rutile microparticles in the scattering layer was increased, a decrease in JSC was observed. Concerning the solar cells employing the multilayered anodes, the usage of both scattering and reflecting layer in the anode led to a further increase in JSC, while the introduction of a thin nanoparticles layer on the top of the multilayer was found to increase the VOC and FF.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 T 14.50±0.09 761±2 0.62±0.01 6.88±0.01 N:M 70:30 T 18.28±0.19 765±2 0.62±0.01 8.62±0.01 N:M 50:50 T 16.51±0.22 765±2 0.62±0.01 7.79±0.02 N:M 30:70 T 14.93±0.26 765±3 0.62±0.01 7.08±0.02 N:M 10:90 T 13.36±0.15 765±2 0.62±0.01 6.30±0.03 N/N/R T 16.80±0.16 766±2 0.62±0.01 7.94±0.02 N/S/R T 19.16±0.42 766±3 0.61±0.01 8.95±0.02 N/S/R/N T 19.12±0.30 768±2 0.62±0.01 9.11±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of DSSCs contributed to a better understanding of the above-mentioned results. In general, the introduction of light scatters in the anode resulted in an increased JL. Concerning the double layer anode designs, as the wt% loading of rutile microparticles in the scattering layer increased, a decrease in JL and rSH was observed, while n, J0, and rS showed an increase. The introduction of a reflecting layer on the top of the main active layer or on the top of the optimized scattering layer resulted in a further increase in the values of JL, n, J0, and rSH. Finally, when an extra thin nanoparticles layer was used on the top of the reflecting layer, n and J0 showed a decrease and rSH an increase.

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Chapter 5 Results and Discussion

Table 5-13: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally rutile microparticles in different wt% loadings in the anode, in different anode designs.

Working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 6 T 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 N:M 70:30 T 18.33±0.19 2.78±0.02 482±43 4.33±0.14 1615±56 N:M 50:50 T 16.55±0.23 2.84±0.02 540±52 4.33±0.14 1599±70 N:M 30:70 T 14.97±0.27 2.86±0.03 526±85 4.50±0.25 1593±105 N:M 10:90 T 13.40±0.16 2.93±0.03 597±45 4.75±0.25 1573±121 N/N/R T 16.85±0.17 2.82±0.02 463±51 4.58±0.14 1649±78 N/S/R T 19.21±0.43 2.81±0.05 552±122 4.33±0.14 1672±157 N/S/R/N T 19.17±0.30 2.73±0.09 402±148 4.33±0.14 1741±200

In order to further examine the modification of the conventional working electrode of DSSCs with the introduction of rutile microparticles in the anode, the characteristics of the conventional working electrode, the working electrode employing the optimized scattering layer, according to the solar cells characterization results, and the working electrode employing the optimized scattering layer and reflecting layer were compared by means of SEM, AFM, XRD, BET, DRS, PL, and UV-VIS (see Figure 5-9). The parameters obtained by AFM, XRD, BET, and DRS are listed in Table 5-14.

Figure 5-9: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the optimized scattering layer, (c, d) SEM image and three-dimensional AFM image, respectively, of the surface

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Chapter 5 Results and Discussion

morphology of the reflecting layer, (e-i) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode, the working electrode employing the optimized scattering layer, and the working electrode employing the optimized scattering layer and reflecting layer. Surface analysis by means of SEM revealed that the working electrode employing the scattering layer possesses a quite uniform surface, similar to the conventional working electrode. The TiO2 nanoparticles and microparticles were in most cases dispersed homogeneously over a large surface area, without the development of irregular aggregations. On the other hand, the surface of the working electrode employing the scattering layer and the reflecting layer is non-uniform. In this case, large and irregular pores and aggregates appeared on its surface. The inset of Fig. 5-7c is a tilted-view SEM image of the surface of the reflecting layer, showing more clearly the non-uniformity of this surface. AFM gave additional evidence for the surface morphology of the compared anodes. The surface root- mean-square roughness and the surface area were found lower in the case where the scattering layer was employed in the working electrode, compared to the conventional one, and even lower in the case of the working electrode employing the scattering layer and reflecting layer. On the other hand, an inverse variation was observed for the Z-range. The working electrode employing the scattering layer showed a higher Z-range compared to the conventional one, while an even higher Z-range was observed for the working electrode employing the scattering layer and reflecting layer. The XRD patterns of the fabricated working electrodes showed sharp and intense diffraction peaks that can be indexed to anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276), arising from the TiO2 anode, as well as to cassiterite (JCPDS 21-1250), arising from FTO glass. The XRD analysis revealed an increase of the rutile phase in the case of the working electrode employing the scattering layer, compared to the conventional one, while an even higher increase was recorded in the case of the working electrode employing the scattering layer and reflecting layer. This is in accordance with the increased amount of rutile microparticles that were used for the fabrication of the scattering layer and reflecting layer. The mean primary crystallite size and crystal lattice distortion of anatase were found to be the same in all working electrodes. However, in the case of rutile, the corresponding values were found increased and decreased, respectively, in the cases of the working electrode employing the scattering layer and the working electrode employing the scattering layer and reflecting layer. Concerning the BET analysis and according to IUPAC classification, in all cases, the isotherms showed a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that in all cases the mixtures of nanopowder and micropowder were aggregated to plate-like particles, creating slit-shaped mesopores [429,466]. However, by comparing the anodes, the specific surface area was found decreased, and the average pore diameter and total pore volume increased in the cases of the anode employing the scattering layer and the anode employing the scattering layer and reflecting layer. From Tauc plots derived from DRS, it was found that there was a small decrease in the optical bandgap of the working electrode employing the scattering layer and a further decrease of its value for the working electrode employing the scattering layer and reflecting layer. Furthermore, by comparing the PL spectra of the working electrodes, there was a shift and an increase in the intensity of the peak corresponding to the band to band emission in the cases where the anode employed the scattering layer, which were even more intense in the case of the working electrode employing the scattering layer and reflecting layer. These can be attributed to the increased amount of rutile when the scattering layer or the scattering layer and reflecting layer was employed in the working electrode. On the other hand, a corresponding increase was also found in the intensity of emission peaks ranging from 440 to 500 nm, showing an increase of defect-states in TiO2 [467]. The shift to higher intensities of the curves corresponding to the working electrode employing the scattering layer and the working electrode employing the scattering layer and reflecting layer could be ascribed to the decrease in the transparency of the corresponding working electrodes, according to UV-VIS results. As it is shown in the UV-VIS transmittance spectra, the transmittance of the working electrode employing the scattering layer in the visible spectrum was quite reduced, while the working

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Chapter 5 Results and Discussion

electrode employing the scattering layer and reflecting layer was characterized totally opaque in the light spectrum from 200 to 800 nm. Table 5-14: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, the working electrode employing the optimized scattering layer, and the working electrode employing the optimized scattering layer and the reflecting layer.

Parameter Sample conventional N:M 70:30 N/S/R Root-mean-square roughness (nm) a 20 18 13 2 a Surface area (μm ) 31.8 29.2 28.3 Z-range (nm) a 171 217 261 b Anatase 78 46 20 TiO2 phase composition (wt%) Rutile 22 54 80 Anatase 25 25 25 Mean primary crystallite size (nm) b Rutile 42 64 119 Anatase 0.006 0.006 0.006 Crystal lattice distortion b Rutile 0.003 0.002 0.001 Specific surface area (m2g-1) c 49.9 43.4 40.5 Average pore diameter (nm) c 18.7 22.9 24.4 Total pore volume (cm3g-1) c 0.260 0.283 0.288 Bandgap (eV) d 3 2.97 2.95 determined by a AFM, b XRD, c BET, d DRS

The increase in JSC and VOC with the introduction of a scattering layer in the anode was attributed to the better light-harvesting capability of the anode [100]. The rutile microparticles scatter the light effectively, promoting the interaction of incident photons with the dye molecules, thus increasing the JL and subsequently the JSC. According to the literature, rutile microparticles of this size show a high scattering efficiency [101]. The results were even better in the case of solar cells employing the multilayered working electrodes. When the anode employed the optimized scattering layer and reflecting layer, a total opacity was achieved, according to the UV-VIS results. In this way, the light- harvesting capability of the anode was further improved, thus the ECE of solar cells was further enhanced. The greatest enhancement in the ECE of solar cells was achieved when a thin layer of nanoparticles was added on the top of the reflecting layer of the anode. This was attributed to the decreased electron−hole recombination rate at TiO2/electrolyte interface, according to one-diode equivalent circuit analysis. Similar results were found in the literature, demonstrating the advantages of the application of multilayered photo-anodes in view of enhancing DSSCs performance [102,415]. Concerning the solar cells employing the double-layered photo-anode, i.e. only a scattering layer, for increased wt% loading of rutile microparticles in the scattering layer, a decrease in JL and subsequently in JSC was observed. This was attributed to the decreased surface roughness, rutile crystal lattice distortion, and specific surface area of the anode, according to AFM, XRD, and BET analysis. Thus, a lesser amount of dye is adsorbed on the anode, leading to a reduction of its light- harvesting capability [86,244,469]. Similar results were found in the literature [100]. The increase in n and J0 with the increase of wt% loading of rutile microparticles in the scattering layer was attributed to the development of large and irregular pores and aggregates in the anode, according to SEM, AFM, and BET analysis. Thus, the electrons diffusion in the anode becomes ineffective due to the development of more tortuous and extended pathways for electrons transfer, with an increased fraction of terminating particles (dead-ends) [476]. Besides these, the increase of the rutile phase in the anode results in more defect states in TiO2 and a higher recombination rate, according to PL analysis and literature [463]. The variation of rS relates to the electrons diffusion resistance in the anode. When large particles, like microparticles, are used in the anode, there is a decrease in the particles boundaries that the electrons have to pass before being collected in the conductive substrate,

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which are considered as points where electron lose much of their energy [465]. However, when the wt% loading of large particles in the anode becomes high, the path that the electrons have to pass until being collected becomes non-optimal, due to the decreased coordination number (number of ways electrons can pass through a given particle) [465]. Thus, the electrons diffusion resistance in the anode and subsequently the rS of the solar cell is increased. On the other hand, the variation of rSH is highly dependent on the developed porosity of the anode. The creation of large pore leads to an easier penetration of electrolyte through the entire thickness of the anode. Thus, a larger amount of electrolyte comes in contact with the conductive substrate, increasing the electron−hole recombination rate at FTO/electrolyte interface. In the present investigation, the increase of the average pore diameter with the introduction of the scattering layer in the anode was demonstrated by BET analysis, while the increase in electron−hole recombination rate at FTO/electrolyte interface by increasing the wt% loading of rutile microparticles in the scattering layer was demonstrated by one- diode model equivalent circuit analysis. ➢ Effect of the addition of MWCNTs into the anode on dye-sensitized solar cells characteristics The electrons collection efficiency is an important parameter that has to be investigated in view of developing high-efficiency DSSCs. For an ideal and hassle-free transportation of electrons in the photo-anode, there is a need for additional electron transport channels. In this direction, the introduction of high conductive nanomaterials into the conventional anode is considered a promising strategy to enhance the energy conversion efficiency of the solar cells [85]. In the resent investigation, different wt% loadings of MWCNTs were introduced into the conventional anode, creating composite anodes, in view of improving its electrical characteristics and achieving a higher energy conversion efficiency.

Figure 5-10 shows the J–V characteristic curves of the DSSCs employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally MWCNTs in the anode in different wt% loadings. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5- 15 and Table 5-16, respectively.

Figure 5-10: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally multi-walled carbon nanotubes in the anode in different wt% loadings.

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The maximum ECE attained by the DSSCs in this group of experiments was 8.38% and was achieved with the addition of 0.025 wt% of MWCNTs into the anode. This value is almost 22% higher compared to the corresponding value attained by the solar cells without the addition of MWCNTs into the anode. The improvement in DSSCs performance was due to JSC, VOC, and FF increase. In general, the results showed an increase in JSC, VOC, and FF with the addition of MWCNTs into the anode, while after an optimal wt% loading of MWCNTs in the anode, all the PV parameters showed a decrease.

Table 5-15: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally multi-walled carbon nanotubes in the anode in different wt% loadings.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 T 14.50±0.09 761±2 0.62±0.01 6.88±0.01 0.025 wt% MWCNTs T 16.83±0.10 770±2 0.65±0.01 8.38±0.02 0.050 wt% MWCNTs T 16.38±0.23 768±3 0.65±0.01 8.14±0.02 0.100 wt% MWCNTs T 15.64±0.16 764±4 0.64±0.01 7.69±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The introduction of MWCNTs into the anode resulted in an increased JL. However, as the wt% loading of MWCNTs in the anode was increasing, the JL of the solar cells was decreasing. On the other hand, n and J0 showed a different variation, with their values being highly decreased in the case of 0.025 wt% loading of MWCNTs in the anode, while for higher wt% loadings of MWCNTs in the anode, their values increased. Regarding the rS, its value showed a decrease by increasing the wt% loading of MWCNTs in the anode, while the value of rSH did not show any significant variation. Table 5-16: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode, and the corresponding working electrodes fabricated using additionally multi-walled carbon nanotubes in the anode in different wt% loadings.

Working electrode JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 6 T 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 0.025 wt% MWCNTs T 16.87±0.10 2.44±0.03 93±12 3.83±0.14 1618±31 0.050 wt% MWCNTs T 16.42±0.23 2.49±0.03 117±23 3.83±0.14 1614±41 0.100 wt% MWCNTs T 15.68±0.16 2.62±0.05 221±59 3.75±0.00 1613±47

In order to further examine the modification of the conventional working electrode of DSSCs with the addition of MWCNTs into the anode, the characteristics of the conventional working electrode and the corresponding working electrode fabricated using additionally MWCNTs in the anode, in the optimized wt% loading (0.025 wt% loading of MWCNTs) according to solar cells characterization results, were compared by means of SEM, AFM, XRD, BET, DRS, PL, and UV-VIS (see Figure 5- 11). The parameters obtained by AFM, XRD, BET, and DRS are listed in Table 5-17.

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Figure 5-11: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the working electrode fabricated using additionally 0.025 wt% loading of multi-walled carbon nanotubes in the anode, (c- g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the working electrode fabricated using additionally 0.025 wt% loading of multi-walled carbon nanotubes in the anode. Surface analysis by means of SEM revealed that the working electrode fabricated using additionally MWCNTs in the anode possessed a quite uniform surface, similar to the conventional working electrode. The TiO2 nanoparticles were in most cases dispersed homogeneously over a large surface area, without the development of irregular aggregations. AFM gave additional evidence for the surface morphology of the two compared working electrodes. The surface root-mean-square roughness and the Z-range were found slightly increased in the case of TiO2-MWCNTs composite working electrode, while its surface area was found slightly decreased. The XRD patterns of the fabricated working electrodes showed in both cases sharp and intense diffraction peaks that can be indexed to the anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276), arising from the TiO2 anode, as well as to cassiterite (JCPDS 21-1250), arising from FTO glass. Concerning the TiO2-MWCNTs composite working electrode, no diffraction peaks were observed corresponding to the MWCNTs. The XRD analysis revealed quite similar characteristics in the TiO2 crystal structure of the two compared working electrodes, with no great differences to have appeared in the TiO2 phase composition, the mean primary anatase and rutile crystallite size, and the anatase crystal lattice distortion. However, a slight increase was observed in the rutile crystal lattice distortion of the TiO2-

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MWCNTs composite working electrode. Concerning the BET analysis and according to IUPAC classification, in both cases, the isotherms showed a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that in both cases the nanomaterials were aggregated to plate-like particles, creating slit-shaped mesopores [429,466]. However, by comparing the two anodes, the anode fabricated using additionally MWCNTs showed a decreased specific surface area, as well as an increase of the average pore diameter and the total pore volume. From Tauc plots derived from DRS, it was found that there was a small decrease in the optical bandgap of the TiO2-MWCNTs composite working electrode compared to the conventional one. A similar observation was reported by Mehmood et al. [477]. Furthermore, by comparing the PL spectra of the conventional working electrode and the working electrode fabricated using additionally MWCNTs in the anode, there was no obvious alteration in the intensity of the emission peak at 398 nm, while a slight decrease in the intensity of the emission peaks ranging from 400 to 500 nm in the latter was observed. The decrease in the intensity of the emission peaks ranging from 440 to 500 nm shows a decrease in the defect-states of TiO2 when MWCNTs were added into the anode [467]. Finally, as it is shown in the UV-VIS transmittance spectra, the transmittance of the TiO2-MWCNTs composite working electrode was reduced in the visible spectrum compared to the conventional working electrode. Similar results were reported by Sawatsuk et al. [105]. Table 5-17: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, and the working electrode fabricated using additionally 0.025 wt% loading of multi-walled carbon nanotubes in the anode.

Parameter Sample conventional 0.025 wt% MWCNTs Root-mean-square roughness (nm) a 20 22 Surface area (μm2) a 31.8 30.6 a Z-range (nm) 171 180 b Anatase 78 79 TiO2 phase composition (wt%) Rutile 22 21 Anatase 25 25 Mean primary crystallite size (nm) b Rutile 42 41 Anatase 0.006 0.006 Crystal lattice distortion b Rutile 0.003 0.004 Specific surface area (m2g-1) c 49.9 39.8 Average pore diameter (nm) c 18.7 25.0 3 -1 c Total pore volume (cm g ) 0.260 0.308 Bandgap (eV) d 3 2.97 determined by a AFM, b XRD, c BET, d DRS

The increase in JL and subsequently in JSC for the DSSCs employing the TiO2-MWCNTs composite working electrodes is possibly attributed to an increase in the amount of dye adsorbed on the TiO2 surface, due to the increased surface roughness, crystal lattice distortion, and average pore diameter in the anode, according to AFM, XRD, and BET analysis. The increase of the surface roughness and crystal lattice distortion of the anode favors the dye adsorption onto the surface of TiO2, while the increase of the average pore diameter can lead to a reduced dye diffusion resistance into the anode during the sensitization procedure [244,469,478]. Furthermore, the reduced bandgap of the TiO2-MWCNTs composite working electrodes compared to the conventional one, according to DRS results, could increase the electrons injection from the dye molecules to TiO2, thus increase the JL and subsequently the JSC of the solar cells [477]. The increase in VOC was attributed to the reduced electron−hole recombination rate at the TiO2/electrolyte interface and the decrease in the defect states of TiO2, according to one-diode model equivalent circuit analysis and PL analysis results, respectively. The reduced recombination rate inside the solar cells could be mainly ascribed to the increase of electrons lifetime in the anode due to the presence of MWCNTs [479]. Finally, the increase in FF of the solar cells employing the TiO2-MWCNTs composite working electrodes is the

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result of the high decrease in the rS, possibly due to a decrease in electrons transport resistance in the anode [480]. On the other hand, the reduced performance of DSSCs employing the TiO2-MWCNTs composite working electrodes with high wt% loadings of MWCNTs was attributed to the increased electron−hole recombination rate at TiO2/electrolyte interface and reduced transmittance of the anode, according to one-diode model equivalent circuit analysis and UV-VIS results, respectively. Similar reports are found in literature, where for high wt% loadings of carbon materials in TiO2 anode, there is an electrons leakage inside the solar cell and a solar-to-electricity conversion loss due to the light absorption from the carbon materials [105,481].

➢ Effect of the addition of SiO2 nanoparticles into the anode on dye-sensitized solar cells characteristics The last few years of development of DSSCs have shown that the fabrication of composite photo- anodes is an efficient strategy in the direction of enhancing their energy conversion efficiency. By creating composites, novel materials emerge, showing unique characteristics, which are different from their individual components. The research in this direction has demonstrated many new combinations of materials that lead to an improvement in DSSCs performance [85]. Amongst them, the fabrication of TiO2-SiO2 composite photo-anodes is a promising way to increase the transparency and to reduce the charge recombination in DSSCs [110,111]. In the present investigation, different wt% loadings of SiO2 nanoparticles were introduced into the conventional anode, during the fabrication of the working electrodes, creating composite anodes, in view of enhancing the energy conversion efficiency of solar cells.

Figure 5-12 shows the J–V characteristic curves of the DSSCs employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally SiO2 nanoparticles in different wt% loadings in the anode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-18 and Table 5-19, respectively.

Figure 5-12: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally silicon dioxide nanoparticles in different wt% loadings in the anode. The maximum ECE attained by the DSSCs in this group of experiments was 8.30% and was achieved with the addition of 0.5 wt% loading of SiO2 nanoparticles into the anode. This value is almost 21% higher compared to the corresponding value attained by the solar cells without the usage of SiO2 nanoparticles in the photo-anode. The improvement in DSSCs performance was due to the increase in JSC and VOC. In general, the results showed an increase in JSC and VOC of the solar cells with the addition of SiO2 nanoparticles into the anode, while after an optimal wt% loading, these PV parameters showed a decrease.

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Table 5-18: Electrical characteristics of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally silicon dioxide nanoparticles in different wt% loadings in the anode.

Working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 T 14.50±0.09 761±2 0.62±0.01 6.88±0.01 0.5 wt% SiO2 T 17.43±0.11 764±2 0.62±0.01 8.30±0.01 1.0 wt% SiO2 T 16.49±0.24 767±3 0.62±0.01 7.89±0.02 1.5 wt% SiO2 T 16.11±0.24 765±3 0.62±0.01 7.60±0.02 2.0 wt% SiO2 T 15.48±0.17 764±4 0.62±0.01 7.29±0.02

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The introduction of SiO2 nanoparticles in the anode resulted in an increased JL. However, when wt% loading of SiO2 nanoparticles in the anode increased, the JL was decreasing. On the other hand, n and J0 showed a decrease up to the 1.5 wt% loading of SiO2 nanoparticles in the anode, where an increase of their values was observed once again. Regarding the rS and rSH, their values showed an increase and decrease, respectively, by increasing the wt% loading of SiO2 nanoparticles in the anode. Table 5-19: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode and the corresponding working electrodes fabricated using additionally silicon dioxide nanoparticles in different wt% loadings in the anode.

Working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 6 T 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 0.5 wt% SiO2 T 17.48±0.11 2.65±0.05 273±49 4.58±0.14 1634±55 1.0 wt% SiO2 T 16.54±0.24 2.61±0.04 212±42 4.67±0.14 1603±24 1.5 wt% SiO2 T 16.16±0.24 2.67±0.03 274±45 4.92±0.14 1596±20 2.0 wt% SiO2 T 15.53±0.17 2.73±0.03 342±55 5.00±0.00 1586±15

In order to further examine the modification of the conventional working electrode of DSSCs with the addition of SiO2 nanoparticles, the characteristics of the conventional working electrode and the corresponding working electrode fabricated using additionally SiO2 nanoparticles in the anode, in the optimized wt% loading (0.5 wt% loading of SiO2 nanoparticles) according to solar cells characterization results, were compared by means of SEM, AFM, XRD, BET, DRS, PL, and UV-VIS (see Figure 5-13). The parameters obtained by AFM, XRD, BET, and DRS are listed in Table 5-20.

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Figure 5-13: (a, b) SEM image and three-dimensional AFM image, respectively, of the surface morphology of the working electrode fabricated using additionally 0.5 wt% loading of silicon dioxide nanoparticles in the anode, (c-g) XRD patterns, N2 adsorption isotherms, Tauc plots, PL spectra, and UV-VIS transmittance spectra, respectively, of the conventional working electrode and the working electrode fabricated using additionally 0.5 wt% loading of silicon dioxide nanoparticles in the anode. Surface analysis by means of SEM revealed that the working electrode fabricated using additionally SiO2 nanoparticles in the anode possessed a quite uniform structure, similar to the conventional working electrode. The TiO2 nanoparticles were dispersed homogeneously over a large surface area, without the development of irregular aggregations. AFM gave additional evidence for the surface morphology of the two compared working electrodes. The surface root-mean-square roughness, the surface area, and the Z-range of the compared anodes were found quite similar, with a slight increase to appear in the aforementioned parameters in the case of TiO2-SiO2 composite working electrode. In both cases, the XRD patterns of the fabricated working electrodes showed sharp and intense diffraction peaks that can be indexed to the anatase (JCPDS 21-1272) and rutile (JCPDS 21-1276), arising from the TiO2 anode, as well as to cassiterite (JCPDS 21-1250), arising from FTO o glass, while an extra diffraction peak at 2θ = 28.4 was observed in the case of TiO2-SiO2 composite working electrode that can be indexed to silicon (JCPDS 5-565). The XRD analysis revealed quite similar characteristics in the TiO2 crystal structure of the two compared anodes, with no differences in the TiO2 phase composition, mean primary anatase crystallite size, and anatase crystal lattice distortion. However, a slight decrease and increase were observed in the mean primary crystallite size

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of rutile and rutile crystal lattice distortion, respectively, for the TiO2-SiO2 composite working electrode. Concerning the BET analysis and according to IUPAC classification, the isotherms of the conventional anode and anode fabricated using additionally SiO2 nanoparticles showed in both cases a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that in both cases the nanomaterials were aggregated to plate-like particles, creating slit- shaped mesopores [429,466]. However, by comparing the two anodes, the anode fabricated using additionally SiO2 nanoparticles showed a decrease in its specific surface area, while the average pore diameter and the total pore volume were quite increased. From Tauc plots derived from DRS, it was found that there was an increase of the optical bandgap of the TiO2-SiO2 composite working electrode compared to the conventional one. Furthermore, by comparing the PL spectra of the conventional working electrode and the working electrode fabricated using additionally SiO2 nanoparticles in the anode, there was no obvious difference between the corresponding intensities of the emission peaks. The shift to lower intensities of the curve corresponding to the TiO2-SiO2 composite working electrode, compared to the conventional one, could be attributed to its increased transparency, according to UV-VIS results. As it is shown in the UV-VIS transmittance spectra, the transmittance of the TiO2-SiO2 working electrode increased in the visible spectrum compared to the conventional working electrode. Similar results were reported by Xu et al [110]. Table 5-20: Parameters obtained by AFM, XRD, BET analysis, and DRS for the conventional working electrode, and the corresponding working electrode fabricated using additionally 0.5 wt% of silicon dioxide nanoparticles in the anode.

Parameter Sample conventional 0.5 wt% SiO2 Root-mean-square roughness (nm) a 20 21 2 a Surface area (μm ) 31.8 31.9 Z-range (nm) a 171 174 b Anatase 78 78 TiO2 phase composition (wt%) Rutile 22 22 Anatase 25 25 Mean primary crystallite size (nm) b Rutile 42 40 Anatase 0.006 0.006 Crystal lattice distortion b Rutile 0.003 0.004 Specific surface area (m2g-1) c 49.9 42.9 Average pore diameter (nm) c 18.7 32.9 Total pore volume (cm3g-1) c 0.260 0.432 Bandgap (eV) d 3 3.05 determined by a AFM, b XRD, c BET, d DRS

The increase in JL and subsequently in JSC with the addition of SiO2 nanoparticles into the anode is possibly attributed to an increase in the amount of dye adsorbed onto the surface of TiO2 due to the increase of the surface roughness, crystal lattice distortion, and average pore diameter of the anode, according to AFM, XRD, and BET analysis, respectively, as well as to the increased transparency of the TiO2-SiO2 composite anode, according to UV-VIS results. The increase of the surface roughness and crystal lattice distortion favors the dye adsorption onto the surface of TiO2, while the development of larger pores leads to a reduced dye diffusion resistance into the anode during the sensitization procedure [244,469,478]. Furthermore, the increase in the transparency of the anode could enhance dye excitation. By increasing the transparency of the anode, the gradient descent of incident light intensity within the dye-sensitized TiO2 film is decreased, because of reflection and absorption from the TiO2 nanoparticles. Thus, the mean light intensity across the TiO2 film thickness is increased and subsequently, so is the dye excitation [110,111]. The increase in VOC of the solar cells employing the working electrodes fabricated using additionally SiO2 nanoparticles in the anode could be attributed to the reduced electron−hole recombination rate at the TiO2/electrolyte interface, as well as to the

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increased optical bandgap of the TiO2-SiO2 composite anode, according to one diode model equivalent circuit analysis and DRS results, respectively. According to the literature, the coverage of TiO2 surface by insulating SiO2 nanoparticles can significantly decrease the recombination of electrons traveling in the anode with triiodides of electrolyte [110,111]. The control of charge recombination dynamics in DSSCs by the use of conformally deposited metal oxides (e.g. SiO2, Al2O3, ZrO2, etc.) as coatings of TiO2 anodes is a well-known strategy in the direction of improving the performance of these solar cells [161]. On the other hand, the reduced performance of DSSCs employing the working electrodes fabricated using high wt% loadings of SiO2 nanoparticles could be attributed to the reduced amount of dye adsorbed onto the surface of TiO2 due to a high decrease in its specific surface area, according to BET analysis, and the higher coverage of TiO2 by SiO2 nanoparticles [110,111]. This could be indicated by the decrease in JL, which results in a decrease in JSC. ➢ Effect of co-sensitization of the anode on dye-sensitized solar cells characteristics Co-sensitization approach is an effective strategy in view of enhancing DSSCs performance, through a selective combination of two or more dyes anchored on the anode semiconductor surface together. Thus, an improvement of the light-harvesting capability of the working electrode and subsequently of the solar cell is achieved, leading to an increased photo-current production. Up until now, a large number of dye molecules combinations have been investigated, either in a cocktail approach or in a stepwise approach, in the direction of enhancing DSSCs performance, with promising results [35,116,482,483]. In the present investigation, the co-sensitization of the conventional working electrodes was investigated using N719 and RK1 dyes in a cocktail approach, in view of enhancing the energy conversion efficiency of the solar cells.

Figure 5-14 shows the J–V characteristic curves of the DSSCs employing the conventional TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-21 and Table 5-22, respectively.

Figure 5-14: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes. The maximum ECE attained by the DSSCs in this group of experiments was 8.60% and was achieved with the application of N719-RK1 cocktail of dyes as the sensitizer of the working electrode. This value is almost 25% higher compared to the corresponding value attained by the solar cells employing the working electrodes sensitized with N719 dye or RK1 dye. The improvement in DSSCs performance was due to the increase in JSC VOC, and FF. On the other hand, the solar cells employing the working electrodes sensitized with N719 dye or the working electrodes sensitized with RK1 dye

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showed an equal performance, with the latter to attain a higher JSC but a lower VOC compared to the first.

Table 5-21: Electrical characteristics of the dye-sensitized solar cells employing the TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes.

Dye-sensitized working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) RE 6 T N719 14.50±0.09 761±2 0.62±0.01 6.88±0.01 RE 6 T RK1 14.83±0.08 752±2 0.62±0.01 6.88±0.02 RE 6 T cocktail 17.83±0.03 766±1 0.63±0.00 8.60±0.01

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The co- sensitization of the conventional working electrode with N719-RK1 cocktail of dyes resulted in an increase in JL, as well as to a decrease in n and J0, while the values of rS and rSH did not show any significant variation compared to the corresponding values of the solar cells employing the working electrodes sensitized with N719 dye or RK1 dye. When comparing the solar cells employing the working electrodes sensitized with N719 dye or RK1 dye, an increase in the JL, n, J0, and rS was observed in the latter, while the value of rSH did not show any significant variation. Table 5-22: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the conventional TiCl4 treated working electrode sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes.

Dye-sensitized working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) RE 6 T N719 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 RE 6 T RK1 14.87±0.08 2.78±0.03 462±32 4.67±0.14 1621±35 RE 6 T cocktail 17.88±0.03 2.56±0.01 184±5 4.50±0.00 1644±21

In order to further examine the modification of the conventional working electrode of DSSCs with the application of the co-sensitization method, the characteristics of the N719, RK1, and N719-RK1 cocktail of dyes ethanolic solutions as well as of the corresponding dye-sensitized working electrodes were compared by means of UV-VIS and DRS, respectively (see Figure 5-15).

Figure 5-15: UV-VIS absorbance spectra of (a) dyes ethanolic solutions, (b) dyes anchored on TiO2 working electrodes. The UV-VIS absorbance spectra of N719 dye, RK1 dye, and N719-RK1 cocktail of dyes in ethanol are shown in Figure 5-15a. The N719 dye ethanolic solution shows, as expected, two broad visible bands at 535 nm and 395 nm, as well as one narrow absorption peak at 312 nm (not shown in the figure). The former peaks can be attributed to metal-to-ligand charge transfer transitions and the latter

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can be allotted to intra ligand (π-π*) charge transfer transitions [484]. On the other hand, the RK1 dye ethanolic solution shows, as expected, two absorption bands between 300 nm and 600 nm. The first one which was located in the UV region at 366 nm was assigned to a π-π* transition of the aromatic rings, whereas the second absorption band in the visible region at 470 nm was attributed to the internal charge transfer transition that occurs between the electron-withdrawing and electron- donating segments of the molecule [253]. At this point, it is worth mentioning that the absorbance of the visible band of RK1 dye ethanolic solution is almost double than the corresponding visible bands of N719. This observation is in agreement with the literature since it is referred that the RK1 molar extinction coefficient at the band in the visible spectrum (470 nm, ε=26600 M-1cm-1) is almost two times higher than the corresponding values of N719 (λ=395 nm, ε=14300 M-1cm-1 and λ=535 nm, ε=14700 M-1cm-1) [253,484]. Finally, the N719-RK1 cocktail of dyes ethanolic solution showed a broad and intense absorption spectrum, owing to the synergistic effect of the two dyes.

Absorption spectra of individually sensitized TiO2 films with N719 dye or RK1 dye, as well as co- sensitized TiO2 films with N719-RK1 cocktail of dyes are shown in Figure 5-15b. In all cases, the absorption spectra of the dye-sensitized TiO2 films differed from the corresponding ones of dye ethanolic solutions. In general, the former ones were quite broader and shifted compared to the latter ones, possibly due to the strong interaction of dye molecules with the TiO2 and the development of dye aggregates onto the TiO2 surface [485]. By comparing the absorption peaks in the visible spectrum of the dye-sensitized TiO2 films with the corresponding absorption peaks of the dye ethanolic solutions, some extra observations were made. Concerning the N719 dye, the position of the band corresponding to the low-energy metal-to-ligand charge transfer transition of the complex anchored on TiO2 and of the ethanolic solution were found quite similar, in agreement with the literature [486]. On the other hand, the maximum absorption peak of RK1 anchored on TiO2 films is red-shifted to 485 nm with compared to that of the solution, possibly due to a bathochromic- aggregation (J-aggregation) and interaction of the dye molecules with the TiO2 [482]. The absorption peak of the N719-RK1 cocktail of dyes was also redshift to 510 nm compared to that of the solution (485 nm). By comparing the absorption spectra of the dye-sensitized TiO2 films, the TiO2 film sensitized with RK1 dye showed a much more intense absorbance up to 575 nm compared to the corresponding of N719, while in larger wavelengths, the absorbance of the latter was higher. Finally, the absorption spectrum of the co-sensitized TiO2 film with N719-RK1 cocktail of dyes was more intense compared to the corresponding of N719 in the whole visible spectrum, as well as more intense compared to the corresponding of RK1 in the spectrum between 550 nm and 800 nm. As it is already shown, the DSSCs characteristics employing the N719 dye and RK1 dye are quite similar. The slightly higher JL and subsequently JSC achieved by the solar cells employing the RK1 dye was attributed to the higher light absorption capability of RK1 dye in the visible spectrum compared to the corresponding of N719, according to the UV-VIS and DRS results. The lower difference in the photo-current production compared to the corresponding difference observed when comparing the absorption spectrum of dyes ethanolic solutions or dye-sensitized working electrodes was attributed to the fact that the electrons injection from the dye to the anode is dependent on several factors and not only on the optical characteristics of the working electrode. Some extra factors are the type of the bond between the dye and the anode, the presence of dye aggregations and so on [487]. Regarding the difference in the VOC of the solar cells employing the N719 dye or RK1 dye as a sensitizer, the lower VOC attained with the latter is possibly a result of two reasons. The first concerns the higher recombination rate at TiO2/electrolyte interface, according to one-diode model equivalent circuit analysis. According to previous studies, electrons lifetime for RK1 devices is shorter than the corresponding of N719 devices [253]. This is routinely observed in DSSCs employing organic sensitizers. The second reason is that the quasi-Fermi level of TiO2 sensitized with RK1 dye is more positive than the corresponding of N719 dye [253]. Therefore, the combined effect of shorter electron

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lifetime and the lower lying TiO2 conduction band explains the general trend of lower VOC of DSSCs employing the RK1 dye. Finally, the higher performance of DSSCs employing the cocktail of N719- RK1 dyes as a sensitizer is possibly the result of the broader and more intense absorption spectrum of the working electrode achieved, leading to the increased JL and subsequently JSC, as well as to the reduced electron−hole recombination rate inside the solar cells, leading to the increased VOC and FF. The decrease in the electron−hole recombination rate at the TiO2/electrolyte of the co-sensitized DSSCs could be attributed to the formation of a more compact and ordered monolayer of N719 and RK1 dye molecules on the photo-anode surface and to a lower dye aggregation [483]. At this point, it should be mentioned that during the final stages of preparation of the present dissertation a paper was published by Mehmood et al. [483], demonstrating the superiority of the N719-RK1 cocktail of dyes as a sensitizer for DSSCs application, presenting similar results with the corresponding ones of the present investigation. ➢ Dye-sensitized solar cells employing the optimized photo-anode In view of achieving a high energy conversion efficiency by DSSCs, a novel hybrid multilayered photo-anode was fabricated in the direction of optimizing the characteristics of dye-sensitized working electrodes. The design of the novel dye-sensitized working electrode was based on the results presented in the previous paragraphs, which determined the appropriate wt% loadings of the additive materials. Figure 5-16 is an SEM image showing the cross-section of the optimized multilayered anode. From this image, three of the five layers of the multilayer are distinct. Starting from the bottom, the blocking layer is not discernible in the present image since its thickness is on the order of nm scale, thus it seems one with the conductive substrate. On the blocking layer-conductive substrate system lies the main active layer which consists only of nanomaterials. On the top of the main active layer, the scattering layer is distinct, which is approximately of the same thickness as the main active layer and consists of nanoparticles and microparticles. Finally, on the top of the scattering layer lies the reflecting layer and the nanostructured cover layer. These two layers seem one in the present image since their thickness is small, while the thin nanostructured cover layer covers the large pores developed in the reflecting layer due to the presence of a large number of microparticles.

Figure 5-16: SEM image showing the cross-section of the optimized multilayered anode.

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Figure 5-17 shows the J–V characteristic curves of the DSSCs employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-23 and Table 5-24, respectively.

Figure 5-17: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes. The maximum ECE attained by the DSSCs in this group of experiments was 13.08% and was achieved with the application of the novel multilayered working electrode sensitized with N719-RK1 cocktail of dyes. This value is almost 15% higher compared to the corresponding value attained by the solar cells employing the novel multilayered working electrodes sensitized with N719 dye or RK1 dye. The improved ECE of DSSCs employing the N719-RK1 cocktail of dyes as sensitizer was mainly due to the increase in JSC, while a slight increase was also observed in VOC, and in FF. On the other hand, the solar cells employing the novel multilayered working electrodes sensitized with N719 dye or RK1 dye showed quite similar characteristics. Table 5-23: Electrical characteristics of the dye-sensitized solar cells employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes.

Dye-sensitized working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) Multi T N719 22.37±0.31 795±3 0.64±0.01 11.38±0.03 Multi T RK1 22.39±0.33 789±2 0.64±0.01 11.31±0.03 Multi T cocktail 25.30±0.27 799±4 0.65±0.01 13.08±0.04

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The co- sensitization of the optimized working electrode with N719-RK1 cocktail of dyes resulted in an increase in JL, as well as in a decrease in n and J0, while the values of rS and rSH did not show any significant variation compared to the corresponding values of the solar cells employing the working electrodes sensitized with N719 dye or RK1 dye. On the other hand, by comparing the solar cells employing the optimized working electrodes sensitized with N719 dye or RK1 dye, it is observed that the values of the parameters obtained by the one-diode model equivalent circuit analysis for the two types of solar cells were almost the same.

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Table 5-24: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the optimized working electrodes sensitized with N719 dye, RK1 dye, or N719-RK1 cocktail of dyes.

Dye-sensitized working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) Multi T N719 22.39±0.31 2.50±0.04 113±19 3.92±0.29 4287±61 Multi T RK1 22.41±0.33 2.51±0.06 133±41 3.83±0.14 4299±101 Multi T cocktail 25.32±0.27 2.30±0.02 42±7 3.83±0.14 4298±24

Figure 5-18 shows the J–V characteristic curves of the DSSCs employing the conventional dye- sensitized working electrode (RE 6 T N719) and the optimized dye-sensitized working electrode (Multi T cocktail). The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-25 and Table 5-26, respectively.

Figure 5-18: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the conventional dye-sensitized working electrode and the optimized dye-sensitized working electrode. By comparing the characteristics of the DSSCs, a 90% enhancement of the ECE of the solar cells is demonstrated with the application of the optimized dye-sensitized working electrode compared to the conventional one. The enhancement of DSSCs performance was due to the increase in JSC, VOC, and FF, with the greatest increase appear in the value of JSC, which was increased by almost a factor 1.75. The ECE of the solar cells employing the optimized photo-anode is one of the highest reported in the literature concerning DSSCs technology. The PV parameter which led to the achievement of such a high ECE was the JSC, which is amongst the highest achieved in DSSCs technology, with only a few reports demonstrating similar values [106,483,488,489]. The VOC is also high according to the literature since DSSCs employing iodide-based electrolytes usually show a VOC lower than 800 mV [59]. Table 5-25: Electrical characteristics of the dye-sensitized solar cells employing the conventional dye-sensitized working electrode and the optimized dye-sensitized working electrode.

Dye-sensitized working JSC VOC FF ECE electrode (mA/cm2) (mV) (–) (%) conventional 14.50±0.09 761±2 0.62±0.01 6.88±0.01 N719 + RK1 multi 25.30±0.27 799±4 0.65±0.01 13.08±0.04

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The application of the optimized dye-sensitized working electrode in DSSCs resulted in a high increase in JL and rSH, a high decrease in n and J0, as well as in a decrease in rS, compared to the conventional devices. The aforementioned parameters variation implies that the novel solar cells produce a much

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higher photo-current compared to the conventional ones, while the electron−hole recombination inside the solar cells is highly retarded. On the other hand, the decrease in rS could be a result of a better electrons transport in the anode. Table 5-26: Parameters obtained by the one-diode model equivalent circuit analysis of dye-sensitized solar cells employing the optimized dye-sensitized working electrode sensitized and the conventional dye-sensitized working electrodes.

Dye-sensitized working JL n J0 rS rSH electrode (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) conventional 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 N719 + RK1 multi 25.32±0.27 2.30±0.02 42±7 3.83±0.14 4298±24

➢ Conclusions In conclusion, an impressive energy conversion efficiency, on the order of 13%, was achieved by the solar cells employing the novel multilayered photo-anode, reaching DSSCs efficiency records [12,35]. For the first time, a morphological, optical, and electrical optimized photo-anode was fabricated, by combining with great success materials that compensate for the disadvantages of the conventional nanostructured TiO2 anode. The prepared composite pastes were based on low-cost and simple chemical techniques, possibly even suitable for industrial production of DSSCs. On the other hand, the fabricated dye-sensitized working electrodes were based on low-cost and simple manufacturing techniques, capable of reproduction by conventional means. Finally, it has to be mentioned that the novel anode design is also a very promising design for its application in PSCs, in view of achieving an even higher performance than current efficiency records. 5.3. Towards Higher Stability Dye-Sensitized Solar Cells 5.3.1. Development of High-Efficiency Quasi-Solid State Dye- Sensitized Solar Cells In view of developing high-stability DSSCs without performance limitations, novel polymer electrolytes were prepared and implemented in the conventional structure of the solar cells. The aim was the fabrication of high-efficiency QSS-DSSCs, after the optimization of the applied polymer electrolytes. The usage of the appropriate polymers as solidification agents of the liquid state electrolytes, creating polymer blend electrolytes, with the extra use of the appropriate additives and iodide compounds mixtures, which have proven to improve the performance of the electrolytes for the aforementioned application, were investigated. Finally, the electrical characteristics of the novel QSS-DSSCs were compared with the corresponding ones obtained by the DSSCs employing factory- available liquid state high-performance and high-stability electrolytes. ➢ Polyvinylpyrrolidone-based polymer electrolytes PVP, as a nitrogen-containing heterocyclic polymer, is a promising candidate for preparation of high-performance iodide-based polymer electrolytes for DSSCs [276,417,490,491]. Nitrogen- - - containing heterocyclic compounds in I /I3 redox electrolytic solutions have proven, by their application to DSSCs, to lead to an increased photo-voltage production [492]. Furthermore, PVP shows high stability under normal environmental conditions, easy processability, good solubility in polar solvents, and excellent wetting properties, while its highly amorphous nature facilitates the movement of ions [276,417]. In the present investigation, PVP-based polymer electrolytes were prepared, thoroughly characterized, and applied by in-situ gelation to DSSCs. The redox couple

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concentration of the polymer electrolyte varied, to achieve high ionic conductivity, combined with low absorbance in the visible light, and low back-reaction when the electrolyte is applied to DSSCs. The determination of the optimum working electrode film thickness and dye loading, as well as the TiCl4 treatment of the working electrode was carried out, in order to further improve the performance of solar cells.

DSC was used to study the influence of KI–I2 wt% loading on the crystallinity of PVP-based polymer electrolytes. The crystallinity of polymer electrolytes is an important characteristic to investigate since it has a direct effect on the ionic conductivity of electrolytes and subsequently on the electrical characteristics of solar cells. The increase in a polymer electrolyte crystallinity leads to a decrease in the movement of ions through the polymer network [418]. Figure 5-19 shows the DSC thermograms of the PVP-based polymer electrolytes, as well as of the pure PVP, i.e. without the addition of KI and I2. The melting temperature, the melting enthalpy, and the crystallinity of the samples are tabulated in Table 5-27.

Figure 5-19: DSC thermograms of the polyvinylpyrrolidone-based polymer electrolytes and the pure polyvinylpyrrolidone. As it is observed, the melting temperature of the PVP-based polymer electrolyte crystals decreased by increasing the KI–I2 wt% loading, indicating the formation of weaker PVP crystals. This phenomenon was attributed to the reaction of PVP, as a nitrogen-containing heterocyclic polymer, - with the triiodides which were formed in the electrolyte, according to the relation Heterocycles+I3 - ↔Heterocycles-I2+I [276,493]. The melting temperature decrease was also attributed to the interpolation of free potassium cations between the polymer chains of PVP, acting as defects. According to previous studies, there is no chemical interaction between PVP and KI, and the system shows a composite nature, while each material retains its identity [417]. The increase in KI–I2 wt% loading in the polymer electrolyte also led to a decrease in electrolyte crystallinity. However, for high KI–I2 wt% loadings, the increase in polymer electrolyte crystallinity was probably due to KI aggregates formation and sedimentation. This can also be supported by the fact that in the thermograms of the polymer electrolytes with high KI–I2 wt% loadings, abnormal peaks at the range of PVP melting temperature appeared, showing a melt heterogeneity. The relatively high crystallinity measured in the pure PVP sample, in contrast to PVP characterization as high amorphous polymer, was attributed to the preparation process of the samples. During the samples gelation, the solvent evaporation rate was relatively low. Based on the literature, solution-processed polymers show higher crystallinity compared to those processed above their melting temperature [494].

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Table 5-27: Parameters obtained by DSC for the polyvinylpyrrolidone-based polymer electrolytes and the pure polyvinylpyrrolidone.

Sample Tm ΔΗm Xc o ( C) (J/g) (%) PVP 139 264 43 PVP:KI 80:20 127 108 22 PVP:KI 75:25 121 70 15 PVP:KI 70:30 116 44 10 PVP:KI 65:35 111 51 13 PVP:KI 60:40 110 80 22

In order to examine the influence of KI–I2 wt% loading on the structural characteristics of the PVP-based polymer electrolytes, the interactions among atoms and ions in the polymer electrolytes as well as the crystallinity of the polymer electrolytes were investigated by means of FTIR and XRD.

Figure 5-20: (a) FTIR transmittance spectra and (b) XRD patterns of the polyvinylpyrrolidone-based polymer electrolytes and the pure polyvinylpyrrolidone. The FTIR transmittance spectra of the PVP-based polymer electrolytes as well as of the pure PVP are shown in Figure 5-20a. The band observed at about 2900 cm-1 corresponds to symmetric C-H stretching mode of PVP. The absorption peaks at about 1660 cm-1 and 1440 cm-1 are assigned to symmetric and asymmetric C=O stretching and CH2 wagging modes of PVP, respectively, while the -1 one at about 1280 cm is attributed to asymmetric CH2 twisting mode of PVP [495,496]. Finally, the band at about 3450 cm-1 is ascribed to the presence of water in the samples, resulting possibly from the strong hygroscopic nature of PVP [497]. A close look and comparison of the IR spectra of PVP- based polymer electrolytes and pure PVP lead to the conclusion that there is no significant change in the intensities and shifting of the vibrational bands of PVP to indicate the formation of a complex between PVP and KI, in agreement with the literature [417]. However, the small broadening of the -1 vibrational band of PVP at about 1660 cm , which appeared as the KI–I2 wt% loading in the polymer electrolyte increases, could possibly be due to PVP-I2 complexation, taking into account that KI:I2 wt% ratio was fixed at 10:1 [498].

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The XRD patterns of the PVP-based polymer electrolytes as well as of the pure PVP are shown in Figure 5-20b. Pure PVP exhibits two broad peaks at 2θ regions of 8o to 16o and 17o to 29o, which are attributed to the semi-crystalline nature of the polymer [499]. However, as it is observed in the XRD profiles of all PVP-based polymer electrolytes, these peaks were seemingly found to have disappeared, indicating the decrease in polymer crystallinity, in agreement with DSC results. The well-defined sharp peaks that appeared in the XRD patterns of the polymer electrolytes are attributed to KI (JCPDS No 1-555), while there were no peaks attributed to I2 (JCPDS No 44-63). The relative intensity of KI peaks increased as the amount of the salt in the sample increases. The afore-mentioned observation confirms that the polymer electrolytes show a composite nature, while there is no complex between PVP and KI. The absence of XRD peaks attributed to KI in the polymer electrolyte composition PVP:KI 80:20 may be due to the good dispersion and dissolution of the iodide compound in the polymer, resulting in small size crystallites formation, possibly undetectable by XRD. The electrical characterization of the PVP-based polymer electrolytes was carried out under ac and dc conditions, by means of EIS and LSV, respectively. Figure 5-21 shows the Nyquist plots derived from EIS and the linear sweep voltammograms of the PVP-based polymer electrolytes, while the parameters obtained by EIS and LSV are tabulated in Table 5-28.

Figure 5-21: (a) Nyquist plots derived from EIS and (b) linear sweep voltammograms of the polyvinylpyrrolidone- based polymer electrolytes. Ionic conductivity, dielectric constant, and diffusion coefficient of triiodides presented a similar variation as the KI–I2 wt% loading in the polymer electrolyte increases. The afore-mentioned correlation is usually observed in this type of electrolytic systems [273,417,500]. The results obtained by EIS and LSV were compatible. However, the triiodide diffusion coefficients obtained by LSV were higher compared to EIS, which was attributed to the great dependence of LSV results on the scan rate [501]. The distinctive reduction peak at about 0.1 V presented in the linear sweep voltammograms was attributed to the high viscosity of electrolytes, since PVP is in its glassy state at the experiment temperature, which hinders the mobility of ions, retarding momentarily the reduction process [273,434]. The increase of this peak as the KI–I2 wt% loading in the polymer electrolyte increases is due to the development of a more intense local “overload” of ions at the beginning of the reduction process [273]. Regarding the relative number of free charge carriers, its value did not show any noticeable variation.

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Table 5-28: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone-based polymer electrolytes.

a b Sample σdc ε n/n0 DI3- DI3- (mS/cm) (–) (–) (x10-7 cm2/s) (x10-7 cm2/s) PVP:KI 80:20 0.77 763 0.85 0.28 0.39 PVP:KI 75:25 0.99 826 0.86 0.33 0.58 PVP:KI 70:30 1.18 881 0.86 0.34 0.69 PVP:KI 65:35 1.10 848 0.86 0.26 0.49 PVP:KI 60:40 0.88 774 0.85 0.15 0.26 determined by a EIS, b LSV

The increase in the PVP-based polymer electrolyte conductivity when increasing the KI–I2 wt% loading is due to the increase in the electric charge carriers and their diffusion coefficient. The decrease in the polymer electrolyte conductivity and triiodide diffusion coefficient for PVP:KI higher to 70:30 was attributed to the increased crystallinity and heterogeneity of the sample, as well as to KI aggregates formation and sedimentation. At high redox couple concentrations in polymer electrolytes, a decrease in mobility of polymer chains has also been reported since ions that have chemically bonded to the polymer matrix act as crosslink agents, limiting the movement of macromolecules [418].

Figure 5-22: UV-VIS absorption spectra of the polyvinylpyrrolidone-based polymer electrolytes.

Figure 5-22 shows the absorption spectra of PVP-based polymer electrolytes obtained by UV- VIS. The peaks at 290 nm and 360 nm indicate the formation of triiodides in the polymer electrolytes. The characteristic absorption peak of iodine in the visible spectrum did not appear, indicating that all iodine molecules have reacted with iodide anions [502]. The intensity and the width of triiodide absorption peaks increased by increasing the KI–I2 wt% loading in the polymer electrolyte, changing significantly the electrolyte optical properties in the spectrum of light that primarily contributes to the photoelectric effect. Increasing the absorption capacity of electrolyte in the visible spectrum is undesirable for its application to DSSCs, while absorption of light from the dye is prevented [503]. Subsequently, the prepared PVP-based polymer electrolytes were incorporated in DSSCs by in- situ gelation onto the photo-anode. In all cases, the TiO2 electrode was fabricated using the doctor- blading technique. The TiO2 film possessed a uniform compact microstructure, with numerous nanopores, while its thickness was found at about 10 μm. Figure 5-23 shows the J–V characteristic curves of the DSSCs employing the PVP-based polymer electrolytes, while their electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-29 and Table 5-30, respectively.

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Figure 5-23: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone-based polymer electrolytes. The maximum ECE attained by the DSSCs in this group of experiments was 2.00% and was achieved with the application of the PVP-based polymer electrolyte PVP:KI 70:30. The results showed an increase in JSC, FF, and ECE of the solar cells by increasing the KI–I2 wt% loading in the polymer electrolyte up to PVP:KI wt% ratio 70:30, while for higher wt% loadings all these parameters showed a decrease. On the other hand, the VOC showed a continuous decrease by increasing the KI– I2 wt% loading in the polymer electrolyte. Table 5-29: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone-based polymer electrolytes.

Electrolyte JSC VOC FF ECE (mA/cm2) (mV) (–) (%) PVP:KI 80:20 5.27±0.08 613±1 0.53±0.01 1.70±0.01 PVP:KI 75:25 5.53±0.03 583±1 0.54±0.01 1.75±0.02 PVP:KI 70:30 6.38±0.03 578±1 0.54±0.01 2.00±0.02 PVP:KI 65:35 5.79±0.03 529±1 0.52±0.01 1.59±0.03 PVP:KI 60:40 5.17±0.14 494±2 0.49±0.01 1.24±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The JL showed an increase by increasing the KI–I2 wt% loading in the polymer electrolyte up to PVP:KI wt% ratio 70:30, while for higher wt% loadings its value showed a decrease. On the other hand, n and rS showed the opposite variation, with their values decreasing up to PVP:KI wt% ratio 70:30, while for higher wt% loadings their values showed an increase. Finally, the values of J0 and rSH showed a continuous increase and decrease, respectively, by increasing the KI–I2 wt% loading in the polymer electrolyte. Table 5-30: Parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells employing the polyvinylpyrrolidone-based polymer electrolytes.

Electrolyte JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) PVP:KI 80:20 5.29±0.08 3.16±0.02 2678±125 16.00±0.00 3346±38 PVP:KI 75:25 5.55±0.03 3.12±0.01 3723±93 12.67±0.14 3144±48 PVP:KI 70:30 6.40±0.03 3.10±0.01 4360±162 10.67±0.14 2611±53 PVP:KI 65:35 5.81±0.03 3.13±0.02 7707±322 11.25±0.25 2455±74 PVP:KI 60:40 5.20±0.14 3.15±0.01 10909±428 14.17±0.14 2016±57

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The increase in JL and subsequently in JSC, as well as the decrease in rS by increasing the KI–I2 wt% loading in the polymer electrolyte up to PVP:KI wt% ratio 70:30 was attributed to the increase in electrolyte conductivity, according to the EIS results. The increase in electrolyte conductivity results in a more efficient regeneration of dye molecules by the electrolyte, thus in a higher photo- current production [37]. However, for higher KI–I2 wt% loadings in the polymer electrolyte, its conductivity decreased, leading to a decrease in JL and subsequently in JSC, as well as to an increase in rS. The decrease in JL and subsequently in JSC in these cases was also attributed to the increased absorption capacity of electrolyte in the visible spectrum, according to the UV-VIS results. The increase in J0 and the decrease in rSH by increasing the KI–I2 wt% loading in the polymer electrolyte show a higher electron−hole recombination rate inside the solar cells, justifying the corresponding decrease in VOC. By increasing the concentration of the redox couple in the electrolyte, more triiodides are presented in the electrolyte, which take part in the main charge recombination processes in DSSCs [59]. Figure 5-24 shows the J-V characteristic curves of the DSSCs employing the most efficient PVP electrolyte (PVP:KI 70:30), for different TiO2 film thicknesses and dye loadings of the working electrode, as well as with and without the TiCl4 treatment of the TiO2 anode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-31 and Table 5-32, respectively.

Figure 5-24: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the most efficient polyvinylpyrrolidone-based polymer electrolyte, for different (a) TiO2 film thicknesses and (b) dye solution concentrations, as well as (c) with and without TiCl4 treatment of the TiO2 anode.

The maximum ECE attained by the DSSCs in this group of experiments was 3.74% and was achieved after optimization of the TiO2 film thickness and dye solution concentration, as well as TiCl4 treatment of the TiO2 anode. The results showed an increase in JSC, VOC, and ECE of the solar cells by increasing the TiO2 film thickness up to 14.2 μm, while for the solar cells employing the working electrodes with TiO2 film thickness at 19.7 μm, all these parameters showed a decrease. On the other

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hand, the FF showed a continuous decrease by increasing the TiO2 film thickness. The increase in dye solution concentration applied as a sensitizer of the TiO2 anode from 0.2 mM to 0.3 mM led to an increase in JSC, VOC, and ECE of the solar cells. Concerning the solar cells employing the working electrodes sensitized by the 0.5 mM dye solution, their JSC further increased, however, VOC and FF decreased, resulting in a decreased ECE. Finally, the TiCl4 treatment of the optimized TiO2 anode resulted in an increase of all the PV parameters, attaining the best results in this group of experiments. Table 5-31: Electrical characteristics of the dye-sensitized solar cells employing the most efficient polyvinylpyrrolidone-based polymer electrolyte, for different TiO2 film thicknesses and dye solution concentrations, as well as after TiCl4 treatment of the optimized TiO2 anode.

Dye-sensitized working electrode JSC VOC FF η 2 TiO2 thickness Dye concentration TiCl4 (mA/cm ) (mV) (–) (%) (μm) (mM) treatment 4.1 0.2 x 5.94±0.07 561±1 0.57±0.00 1.90±0.03 4.8 0.2 x 6.33±0.06 565±1 0.57±0.00 2.04±0.03 6.3 0.2 x 7.17±0.05 570±1 0.57±0.01 2.32±0.02 8.5 0.2 x 7.83±0.04 577±1 0.56±0.01 2.52±0.02 14.2 0.2 x 10.45±0.07 583±2 0.53±0.01 3.21±0.01 19.7 0.2 x 9.63±0.17 574±3 0.50±0.02 2.78±0.02 14.2 0.3 x 10.81±0.09 585±1 0.53±0.01 3.33±0.01 14.2 0.5 x 11.06±0.07 579±1 0.52±0.01 3.31±0.01 14.2 0.3 ✓ 11.64±0.04 592±2 0.54±0.01 3.74±0.02

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The JL showed an increase by increasing the TiO2 film thickness of the working electrode up to 14.2 μm, while for the solar cells employing the working electrodes with TiO2 film thickness at 19.7 μm, its value showed a decrease. On the other hand, n, J0, rS, and rSH showed a continuous increase by increasing the TiO2 film thickness of the working electrode. By increasing the concentration of the dye solution from 0.2 mM to 0.3 mM, the JL showed an increase, while the n, J0, and rSH decreased. Concerning the solar cells employing the working electrodes sensitized with 0.5 mM dye solution, a further increase was observed in JL, but also the values of n and J0 increased and rSH decreased. Finally, the solar cells employing the optimized and TiCl4 treated working electrodes showed an increase in JL, rS, and rSH, as well as a decrease in n and J0, compared to the solar cells employing the corresponding working electrodes that were not treated with TiCl4. Table 5-32: Parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells employing the most efficient polyvinylpyrrolidone-based polymer electrolyte, for different TiO2 film thicknesses and dye solution concentrations, as well as after TiCl4 treatment of the optimized TiO2 anode.

Dye-sensitized working electrode JL n J0 rS rSH 2 2 2 2 TiO2 Dye TiCl4 (mA/cm ) (–) (nA/cm ) (Ω·cm ) (Ω·cm ) thickness concentration treatment (μm) (mM) 4.1 0.2 x 5.96±0.07 2.70±0.00 2015±5 8.25±0.00 2755±76 4.8 0.2 x 6.35±0.06 2.71±0.00 2094±9 8.25±0.00 2888±65 6.3 0.2 x 7.19±0.05 2.73±0.01 2381±85 8.58±0.14 2977±31 8.5 0.2 x 7.85±0.04 2.75±0.01 2512±89 8.58±0.14 3462±35 14.2 0.2 x 10.47±0.07 2.82±0.01 3729±82 9.08±0.14 4382±66 19.7 0.2 x 9.64±0.17 3.25±0.03 10980±444 9.92±0.38 5137±109 14.2 0.3 x 10.83±0.09 2.79±0.01 3531±79 9.08±0.14 4291±44 14.2 0.5 x 11.08±0.07 2.84±0.01 4407±123 9.08±0.14 4182±87 14.2 0.3 ✓ 11.66±0.04 2.50±0.02 1313±57 9.33±0.29 4605±47

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The variation of DSSCs performance by altering the TiO2 thickness was described and interpreted in detail by many scientists in the last decade [504,505]. Concerning DSSCs employing liquid state electrolytes, the TiO2 film thickness is usually determined between 10 μm and 20 μm [242,256,505]. In the present investigation, the optimum TiO2 film thickness was determined at 14.2 μm (see Figure 5-25), much higher than the corresponding usually attained in SS-DSSCs [506]. This was attributed to the fabrication method applied, where the PVP-based polymer electrolytes were in-situ gellated onto the photo-anode. One of the main problems in QSS-DSSCs fabrication is the difficulty of polymer electrolyte penetration into the mesoporous network of the photo-anode [179]. In this direction, in-situ gelation of polymers or organic monomers polymerization pre-penetrated into the pores of the DSSCs working electrode is considered the most appropriate technique for fabrication of high-efficiency QSS-DSSCs and SS-DSSCs [422,507–509]. Concerning the anode dye loading effect on DSSC performance, the appropriate dye solution concentration in the present investigation was found at 0.3 mM. For 0.5 mM, the DSSC performance was slightly reduced, probably due to the formation of a higher amount of dye molecules aggregates, resulting in a higher charge recombination rate inside the solar cells [510]. Finally, the TiCl4 treatment of the TiO2 anode led to an improvement in DSSCs performance, due to reasons already discussed in §5.1.

Figure 5-25: SEM image showing the optimal electrode film thickness for DSSCs employing the polyvinylpyrrolidone- based polymer electrolytes. ➢ Polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes Polymer blending is one of the most useful techniques to produce new polymeric materials with a wide variety of properties with low cost. In DSSCs technology, polymer blending has been recently used to develop novel advanced polymer electrolytes, that show unique characteristics for the aforementioned application [276,279,435,511]. Even though there have been only few studies, the results seem very promising. In the present investigation, PVP/PEG blend-based polymer electrolytes, with fixed iodide compound to total polymer blend weight ratio, were prepared, thoroughly characterized, and applied by in-situ gelation to DSSCs. The unique properties of PVP, as a nitrogen-containing heterocyclic polymer, in combination with the ability of PEG to dissolve high quantities of ionic compounds were the main reasons for developing PVP/PEG blend-based polymer electrolytes for DSSCs application [276,416,418]. The influence of the PVP/PEG blend weight ratio on the electrolyte and solar cell characteristics was studied for the entire blend weight ratio range, and the results were discussed in detail.

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DSC was used to study the crystallinity of the polymer blend electrolytes for the different PVP/PEG blend weight ratios, as well as of the pure PVP/PEG polymer blends, i.e. without the addition of KI and I2. The crystallinity of polymer electrolytes is an important characteristic to investigate since it has a direct effect on the ionic conductivity of electrolytes and subsequently on the electrical characteristics of solar cells. The increase in a polymer electrolyte crystallinity leads to a decrease in the movement of ions through the polymer network [418]. The DSC thermograms of the pure PVP/PEG polymer blends as well as of the PVP/PEG blend-based polymer electrolytes are shown in Figure 5-26.

Figure 5-26: DSC thermograms of (a) the pure polyvinylpyrrolidone/polyethylene glycol polymer blends and (b) polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes.

From the DSC thermograms of the pure PVP/PEG polymer blends shown in Figure 5-26a, a change is observed in the heat flow in the temperature range of 55-75oC (endothermic process due to melting of PEG crystals) in most of the samples, which was continuously increasing and shifting to higher temperatures by decreasing the PVP content in the polymer blend [418]. This shift suggests the formation of stronger PEG crystals, which is possibly attributed to the lower amount of PVP in the polymer blend, which intercalates between PEG macromolecules, possibly creating hydrogen bonding between PVP carboxyl groups and PEG hydroxyls [512]. The increase in PEG melting enthalpy with the decrease of PVP in the polymer blend was almost linear. On the other hand, the sharp change in the heat flow around 140°C observed in the sample PVP:PEG 100:0 concerns the melting of PVP crystals [416]. Concerning the melting of PVP crystals, the pure polymer blend samples showed small change in the heat flow across a wide range of temperatures. The crystallinity of PVP was strongly influenced by the presence of PEG in the mixture, which acts as a plasticizer of PVP, reducing its crystallinity (see Figure 5-27a) [513]. Concerning the crystallinity of PEG, in most cases, it was not affected by the presence of PVP in the polymer blend and remained constant at high values, with the exception of pure PVP:PEG 80:20 sample, where its crystallinity was found slightly reduced (see Figure 5-27b). The high crystallinity of PEG obtained in all cases was attributed to the high flexibility that characterizes the PEG chains, which favors the crystals growth [494]. Finally, with regard to the crystallinity of the whole pure polymer blend, it showed an increase by increasing the PEG amount in the polymer blend, with the exception of the pure PVP:PEG 80:20 sample, where its crystallinity was found minimum (see Figure 5-27c).

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Figure 5-27: Crystallinity of (a) polyvinylpyrrolidone, (b) polyethylene glycol, and (c) polyvinylpyrrolidone/ polyethylene glycol blend for the different pure polymer blend weight ratios. Concerning the PVP/PEG blend-based polymer electrolytes, in the DSC thermograms, no change appeared in the heat flow corresponding to the melting of PEG crystals, while melting of PVP crystals appeared only in the sample PVP:PEG 100:0 (see Figure 5-26b). PEG did not develop crystallinity due to the Lewis base-acid interaction between the oxygens of its macromolecules and the potassium cations, which intercalated and separated its polymeric chains effectively [418]. Based on the literature, potassium cations suppress the PEO/PEG crystallinity effectively due to their appropriate size [514]. The change in the heat flow that appeared in most of the polymer blend electrolyte samples between 140oC and 150oC is due to the development of PEG-potassium cations crystalline complex, a phenomenon observed in PEO/PEG-based electrolytes with a high concentration of cations [503,515]. In the present study, there was an increase in the melting enthalpy of PEG-potassium cations crystalline complex in the midrange compositions of the polymer blend, with the maximum appearing in the case of the sample PVP:PEG 40:60. This is possibly due to the fact that as the amount of PVP in the sample increases, the cations:PEG weight ratio increases. However, after a threshold (here PVP:PEG 40:60), the amount of PEG is low to support the development of a high amount of PEG-potassium cations crystalline complex in the sample. In order to examine the structural characteristics of the PVP/PEG blend-based polymer electrolytes as well as of the pure PVP/PEG polymer blends, the interactions among atoms in the polymers and atoms with ions in the polymer blend electrolytic systems, as well as the crystallinity of the polymer blend electrolytes and the pure polymer blends were investigated by means of FTIR and XRD (see Figure 5-28).

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Figure 5-28: (a, c) FTIR spectra and XRD patterns, respectively, of the pure polyvinylpyrrolidone/polyethylene glycol polymer blends and (b, d) FTIR spectra and XRD patterns, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes. The FTIR transmittance spectra of the pure PVP/PEG polymer blends and PVP/PEG blend-based polymer electrolytes are shown in Figure 5-28a and Figure 5-28b, respectively. Concerning the pure PVP/PEG polymer blends and specifically the spectrum of the pure PVP:PEG 100:0 sample (pure PVP backbone), the band observed at about 2900 cm-1 corresponds to symmetric C-H stretching mode of PVP, the absorption peaks at about 1660 cm-1 and 1440 cm-1 are assigned to symmetric and asymmetric C=O stretching and CH2 wagging modes of PVP, respectively, while the one at about -1 1280 cm is attributed to asymmetric CH2 twisting mode of PVP [494,495]. Finally, the band at about 3450 cm-1 is ascribed to the presence of water in the sample, possibly resulting from the strong hygroscopic nature of PVP [496]. On the other hand, concerning the spectrum of the pure PVP:PEG 0:100 sample (pure PEG backbone), the high-intensity band observed at about 2886 cm-1 and the low-

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-1 intensity peak at about 2690 cm are ascribed to asymmetric C-H stretching mode of CH2 of PEG, -1 the absorption peak at about 1467 cm corresponds to CH2 scissoring mode of PEG, while the ones -1 -1 at about 1280 cm and 1234 cm are attributed to asymmetric CH2 twisting and symmetric CH2 twisting modes of PEG, respectively [496]. The intense triplet centered at about 1116 cm-1 and shoulder peaks at about 1149 cm-1 and 1061 cm-1 concern the C-O-C stretching mode of PEG. In this mode, the peak at 1116 cm-1 represents the amorphous phase region of PEG, whereas the peaks at 1149 cm-1 and 1061 cm-1 represent the characteristic crystalline phase of PEG. In addition to these vibration mode peaks, the characteristic CH2 wagging and CH2 bending modes of PEG, exhibiting sharp doublet at about 1361 cm-1 and 1343 cm-1, respectively, also denote the crystalline region of -1 -1 PEG [516]. Finally, the two peaks near 947 cm and 841 cm are assigned to CH2 rocking vibrations of methylene groups and are related to the helical structure of PEG. Apart from this, the mode -1 responsible for the peak at 947 cm is primarily due to C-O stretching motion with some CH2 rocking -1 motion, while the peak at 841 cm originates primarily in CH2 rocking motion with a little contribution from C-O stretching motion of PEG [517]. Concerning the spectra of the samples pure PVP:PEG 80:20, pure PVP:PEG 60:40, pure PVP:PEG 40:60, and pure PVP:PEG 20:80, a decrease in the intensity of the peaks corresponding to PVP backbone and an increase in the intensity of the peaks corresponding to PEG backbone were observed as the amount of PEG in the polymer blend was increasing. Another interesting observation is that the band corresponding to the presence of water in the samples decreases as the amount of the hygroscopic PVP in the sample decreases. However, with a closer look at the spectrum of the Pure PVP:PEG 20:80 sample, several extra spectrum changes appeared compared to the spectra of the rest polymer blend samples. At first, the shoulder peaks at 1149 cm-1 and 1061 cm-1 corresponding to the characteristic crystalline phase of PEG did not appear, while there is also a downshift of the central peak at 1116 cm-1 of the pure PEG -1 -1 to 1107 cm . Furthermore, the peak at 841 cm corresponding to CH2 rocking of PEG was found quite reduced. The aforementioned observations show that the crystallinity of PEG is reduced, in agreement with the DSC results. This is possibly attributed to the achievement of a better miscibility in the polymer blend, with the PVP chains to intercalate effectively between PEG chains and prevent the PEG crystals growth. PVP chains can interact chemically with PEG chains since the carbonyl group of the former can develop hydrogen bonding with the hydroxyl group of the later. The H- bonding interactions between PVP and PEG could be shown by a change in the intensity and/or shift of the band corresponding to C=O stretching mode of PVP at 1660 cm-1. However, this interaction was not clear in the present case due to the strong hygroscopic nature of PVP. The presence of water molecules in the sample complicate elucidation of H-bonding interactions between PVP and PEG [518]. The range of hydroxyl stretching bands (3700-3200 cm-1) of PEG was not considered in these cases, because of the relatively weak absorbance in this region and the overlap of the spectral bands of PEG with the bands corresponding to the presence of water in the samples [519]. With regard to the spectra of PVP/PEG blend-based polymer electrolytes, several changes were observed when comparing them with the corresponding spectra of the pure PVP/PEG polymer blends. These changes concerned mostly the bands of PEG backbone, while no significant changes in the intensities and shifting appeared in the vibrational bands of PVP. This demonstrates that the KI additive, which is in high amount in the polymer blend electrolyte, interacts mainly with PEG, while there is no obvious interaction of it with PVP, in agreement with the literature [417,418]. The small broadening of the vibrational band of PVP at 1660 cm-1 that appeared in the PVP/PEG blend-based polymer electrolyte samples compared to the pure PVP/PEG polymer blend samples could be due to PVP-I2 complexation, taking into account that KI:I2 wt% ratio was fixed at 10:1 [498]. Another noteworthy observation in all spectra of the PVP/PEG blend-based polymer electrolyte samples is that the triplet band corresponding to C-O-C stretching mode of PEG has significantly changed compared to the pure PVP/PEG polymer blend samples. More specifically, concerning this band, there is a downshift of the central peak, while the shoulder peaks have disappeared. This demonstrates that the

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crystallinity of PEG is quite reduced in all compositions of the PVP/PEG blend-based polymer electrolytes, in agreement with the DSC results. Finally, the band at about 3450 cm-1, corresponding to the presence of water in the sample, appeared in all spectra of the PVP/PEG blend-based polymer electrolytes, in contrast to the spectra of the pure PVP/PEG polymer blends, where the intensity of this band was dependent on the amount of the hygroscopic PVP in the sample. The presence of water in the PVP:PEG 0:100 polymer blend electrolyte sample was also the reason of the appearance of the additional absorption peak at about 1660 cm-1, which is ascribed to the bending mode of molecularly absorbed water in the sample [520]. The aforementioned observations are possibly attributed to the presence of a high amount of the hydrophilic KI in the polymer blend electrolyte samples. The XRD patterns of the pure PVP/PEG polymer blends and PVP/PEG blend-based polymer electrolytes are shown in Figure 5-28c and Figure 5-28d, respectively. Concerning the pure PVP/PEG polymer blends and specifically the pattern of the pure PVP:PEG 100:0 sample (pure PVP), two low-intensity and broad peaks at 2θ regions of 8o to 16o and 17o to 29o appeared, which are attributed to the semi-crystalline nature of the polymer [499]. On the other hand, the pure PVP:PEG 0:100 sample (pure PEG) showed two intense peaks at 19.4o and 23.5o and a few minor peaks before and after this 2θ region, which are attributed to the high-crystalline nature of the polymer [499]. Concerning the patterns of the samples pure PVP:PEG 80:20, pure PVP:PEG 60:40, pure PVP:PEG 40:60, and pure PVP:PEG 20:80, an increase in the intensity of the peaks corresponding to PEG was observed as the amount of PEG in the polymer blend was increasing, also showing an increase in the crystallinity of the samples, in agreement with the DSC results. With regard to the patterns of PVP/PEG blend-based polymer electrolytes, several changes were observed when comparing them with the corresponding patterns of the pure PVP/PEG polymer blends. More specifically, the intense peaks corresponding to the PEG crystals did not appear in any of the samples, showing that the Lewis base-acid interaction between the oxygens of PEG macromolecules and the potassium cations effectively suppress the PEG crystals growth, in agreement with the DSC and FTIR results. An interesting observation in the PVP/PEG blend-based polymer electrolytes patterns is the appearance of the sharp peaks at about 13o and 26o and some other low-intensity peaks at the samples PVP:PEG 80:20 and PVP:PEG 60:40. These peaks are attributed to the formation of PEG-potassium cations crystalline complex, in agreement with the DSC results. Similar observations are reported in the literature [521,522]. The absence of these peaks in the samples PVP:PEG 40:60, PVP:PEG 20:80, and PVP:PEG 0:100 may be due to the formation of smaller size crystallites, possibly undetectable by XRD. Finally, the well-defined sharp peak that appeared in the XRD pattern of PVP:PEG 100:0 is attributed to KI (JCPDS No 1-555), while there were no peaks attributed to I2 (JCPDS No 44-63) in any of the polymer blend electrolyte samples. The electrical characterization of PVP/PEG blend-based polymer electrolytes was carried out under ac and dc conditions, by means of EIS and LSV, respectively. Figure 5-29 shows the Nyquist plots derived from EIS and the linear sweep voltammograms of the PVP/PEG blend-based polymer electrolytes, while the parameters obtained by EIS and LSV are tabulated in Table 5-33.

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Figure 5-29: (a) Nyquist plots derived from EIS and (b) linear sweep voltammograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes.

As shown in Table 5-33, the variation in the electrolyte conductivity does not follow the variation in the diffusion coefficient of triiodides obtained by EIS, as it is usually observed [273,500]. The increase in the diffusion coefficient of triiodides with the increase of the amount of PEG in the polymer blend electrolyte was attributed to the higher flexibility that characterizes the PEG chains compared to the PVP ones. PEG, which is in its rubbery state at the experiment temperature, decreases the impedance in the movement of ions through the polymer chains compared to the PVP, which is in its glassy state at the experiment temperature and its chains are characterized by a much higher stiffness [59]. The decrease in the conductivity of the polymer blend electrolytes with a high amount of PEG is in contrast to the increase in the diffusion coefficient of triiodides calculated by EIS in corresponding cases. This reveals the co-existence of other charge transport mechanisms in the polymer blend electrolyte, which are found enhanced in the midrange compositions of the polymer blend. In the present case, the increase in the polymer blend electrolyte conductivity in the midrange compositions was attributed to the formation of PEG-potassium cations crystalline complexes, where according to thermal analysis, the melting enthalpy of these crystalline complexes is higher. These crystalline complexes are usually observed in polymer electrolytes with high salt concentration and are attributed to the development of ionic clusters on the polymer chains [503,523]. These ionic clusters are considered to act as transient bridges when connected with each other, forming segments of infinite percolation pathways where there is an exchange of charges (“polymer-in-salt” conduction behavior). Here, the increase of the crystalline complex is achieved by decreasing the amount of PEG in the sample, which is the only polymer that complexes with potassium cations, according to FTIR results. However, after a threshold, the amount of PEG is low to support the development of these percolation pathways for charge exchange, so the contribution of this conduction mechanism to the conductivity of the polymer blend electrolyte decreases. The aforementioned hypothesis is also in agreement with the variation of the ac conductivity calculated from the dielectric losses by the relation ′′ −12 -1 휎푎푐 = 휀0휔휀 , where 휀0 = 8.85 10 Fm is the permittivity of the free space and 휔 = 2휋푓 is the angular frequency. More specifically, at midrange compositions of the polymer blend electrolyte, there is a greater increase in the conductivity at high frequencies, showing an enhancement of hopping conduction mechanism (see Figure 5-30) [524]. Generally, the increase in electrolytes conductivity at high frequencies regime is attributed to the fact that the charges transport in shorter distances inside the polymer blend electrolyte, favoring hopping events and giving a lower impedance compared to the low frequencies regime, where the charge transport distance is longer and the impedance higher. Finally, concerning the dielectric constant, its values presented a much lower variation compared to the other EIS parameters that have already been discussed, while the relative number of free charge carriers was almost constant for all polymer blend electrolyte compositions.

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Figure 5-30: AC conductivity (log(σac)) vs frequency (log(f)) for the polyvinylpyrrolidone/polyethylene glycol blend- based polymer electrolytes. Concerning the LSV results, in this case, the variation of the diffusion coefficient of triiodides is in agreement with the conductivity calculated by EIS. This is possibly due to the co-existence of two transport mechanisms as it has already been described, which cannot be distinguished under dc conditions. Thus, a different variation of the calculated diffusion coefficient of triiodides was observed, which were calculated under the assumption that the conductivity of the electrolyte is determined only by triiodides diffusion. Another noteworthy observation in the LSV results is the appearance of a distinct reduction peak at about 0.1 V at some of the samples. This reduction peak is due to the development of a local “overload” of ions at the beginning of the reduction process and it was attributed to the high viscosity of the polymer blend electrolytes [273]. However, as the amount of PEG in the polymer blend electrolyte increases, this overload decreases. This is possibly due to a decrease in the viscosity of the polymer blend electrolyte by increasing the amount of PEG in the sample since PEG is in its rubbery state at the experiment temperature, while PVP is in its glassy state at the experiment temperature [59]. Table 5-33: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes.

a b Sample σdc ε n/n0 DI3- DI3- (mS/cm) (–) (–) (x10-7 cm2/s) (x10-7 cm2/s) PVP:PEG 100:0 1.18 881 0.86 0.34 0.69 PVP:PEG 80:20 1.28 902 0.87 0.54 0.83 PVP:PEG 60:40 1.41 933 0.87 0.35 0.95 PVP:PEG 40:60 1.44 937 0.87 0.40 0.98 PVP:PEG 20:80 1.40 936 0.87 0.95 0.93 PVP:PEG 0:100 1.34 936 0.87 2.62 0.87 determined by a EIS, b LSV

Figure 5-31 shows the absorption spectra of PVP/PEG blend-based polymer electrolytes obtained by UV-VIS. In all spectra, two absorption peaks appeared, one at 290 nm and one at 360 nm, while the characteristic absorption peak of iodine in the visible spectrum did not appear. This indicates that all iodine molecules have reacted with iodide anions, creating triiodides [502]. On the other hand, as it is observed, there is a slight increase in the intensity and the width of triiodide absorption peaks by increasing the amount of PEG in the electrolyte. This is possibly attributed to the decrease of the amount of PVP in the sample, which interacts with triiodides according to the relation - - Heterocycles+I3 ↔Heterocycles-I2+I , binding iodine molecules [276]. According to the literature,

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the characteristic absorption peak of iodine molecules bonded on heterocycles, like PVP, does not appear because of spectral interference and usual low-intensity peak of iodine [525].

Figure 5-31: UV-VIS absorption spectra of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes.

Figure 5-32 shows the J–V characteristic curves of the DSSCs employing the PVP/PEG blend- based polymer electrolytes, while their electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-34 and Table 5-35, respectively.

Figure 5-32: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes. The maximum ECE attained by the DSSCs in this group of experiments was 4.66% and was achieved with the application of the PVP/PEG blend-based polymer electrolyte PVP:PEG 40:60. The results showed an increase in JSC, VOC, FF, and ECE of the solar cells with the application of the polymer blend electrolytes compared to the corresponding solar cells employing the conventional PVP-based and PEG-based polymer electrolytes, while the best results were attained at the midrange compositions of the polymer blend electrolytes.

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Table 5-34: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes.

Electrolyte JSC VOC FF ECE (mA/cm2) (mV) (–) (%) PVP:PEG 100:0 11.64±0.04 592±2 0.54±0.01 3.74±0.02 PVP:PEG 80:20 12.38±0.28 568±3 0.54±0.01 3.80±0.03 PVP:PEG 60:40 12.78±0.14 616±2 0.57±0.01 4.46±0.02 PVP:PEG 40:60 13.50±0.07 620±2 0.56±0.01 4.66±0.02 PVP:PEG 20:80 12.37±0.21 603±3 0.55±0.01 4.11±0.03 PVP:PEG 0:100 11.25±0.09 586±3 0.52±0.01 3.45±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The JL showed an increase, while the n, J0, and rS showed a decrease for the solar cells employing the polymer blend electrolytes, with the best results to be attained by the application of the polymer blend electrolytes in the midrange compositions. Regarding the rSH, in general, its value showed a decrease by decreasing the amount of PVP in the polymer blend electrolyte. Table 5-35: Parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes.

Electrolyte JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) PVP:PEG 100:0 11.66±0.04 2.50±0.02 1313±57 9.33±0.29 4605±47 PVP:PEG 80:20 12.44±0.28 2.32±0.03 756±64 8.92±0.38 2106±66 PVP:PEG 60:40 12.81±0.14 2.40±0.02 481±33 8.33±0.14 4002±71 PVP:PEG 40:60 13.53±0.08 2.48±0.02 669±33 8.00±0.25 3104±72 PVP:PEG 20:80 12.43±0.22 2.55±0.04 1041±197 8.50±0.25 1789±110 PVP:PEG 0:100 11.42±0.10 2.65±0.03 1617±196 8.67±0.14 588±49

The increase in JSC, VOC, and FF of the solar cells employing the PVP/PEG blend-based polymer electrolytes was attributed to the unique properties achieved by the polymer blend, by combining the beneficial for DSSCs application characteristics of PVP and PEG [276,416,418]. As shown from the variation of the parameters obtained by one-diode model equivalent circuit analysis, by the combination of PVP and PEG as matrix for the iodide-based liquid state electrolyte gelation, it is possible to achieve a higher charge transport from the cathode to the photo-anode that can lead to a quicker dye regeneration, while at the same time, a decreased electron−hole recombination rate inside the solar cells can be achieved. In this way, a higher photo-current and photo-voltage production by the solar cells is attained. The rS variation was similar to the electrolyte conductivity obtained by EIS, showing that except triiodides diffusion, charge transport through the PEG-potassium cations crystalline complex contributes effectively to the photoelectric effect. On the other hand, the decrease in rSH with the decrease of the amount of PVP in the polymer blend electrolyte, which shows an increase in electron−hole recombination rate at FTO/electrolyte interface, was an expected variation when considering the properties of PVP [416].

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➢ Polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes with additives Over the past years, a series of chemical compounds have been tested as additives in electrolytes for DSSCs in the direction of improving their performance [59]. Usually, such additives act favorably with respect to some of the PV parameters and negatively to others. The combined action of them often yields better results. Amongst the most famous additives for DSSCs electrolytes are TBP and GuSCN, sometimes being used singly and others as co-additives. Concerning their application to liquid state electrolytes, several investigations demonstrate their effect on the behavior of DSSCs [175,419,526]. In the present investigation, TBP and GuSCN were used in a specific concentration, according to the literature, to the PVP/PEG blend-based polymer electrolytes, in the direction of improving the DSSCs electrical characteristics and achieving a higher energy conversion efficiency [419]. DSC was used to study the crystallinity of the PVP/PEG blend-based polymer electrolytes employing the TBP or TBP and GuSCN co-additives. The crystallinity of polymer electrolytes is an important characteristic to investigate since it has a direct effect on the ionic conductivity of electrolytes and subsequently on the electrical characteristics of solar cells. The increase in a polymer electrolyte crystallinity leads to a decrease in the movement of ions through the polymer network [418]. The DSC thermograms of the PVP/PEG blend-based polymer electrolytes as well as of the pure PVP/PEG polymer blend are shown in Figure 5-33.

From the DSC thermograms of all PVP/PEG blend-based polymer electrolytes, a change in heat flow was observed at about 140oC, while there was no change in the heat flow corresponding to the melting of PEG and PVP crystals. The change in the heat flow at about 140oC was attributed to the melting of PEG-potassium cations crystalline complex [515]. The addition of TBP in the polymer blend electrolyte did not lead to any particular change in its thermal characteristics, with the melting temperature and melting enthalpy of the PEG-potassium cations crystalline complex not showing any particular change. For samples containing additionally GuSCN in their composition, a different behavior was observed, regarding the melting of the crystalline complex. In this case, there is a small shift of the melting temperature of the crystalline complex to a slightly higher temperature, while the melting peak became more intense. This phenomenon suggests the creation of a slightly stronger crystalline complex, while the kinetics of the endothermic reaction also change. In order to examine the influence of TBP and GuSCN addition on the structural characteristics of the PVP/PEG blend-based polymer electrolytes, the interactions among atoms and ions in the polymer

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blend electrolytic systems as well as the crystallinity of the polymer blend electrolytes were investigated by means of FTIR and XRD.

Figure 5-34: (a) FTIR transmittance spectra and (b) XRD patterns of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives and of pure polyvinylpyrrolidone/polyethylene glycol polymer blend. The FTIR transmittance spectra of the PVP/PEG blend-based polymer electrolytes with and without the addition of TBP and GuSCN and of pure PVP/PEG polymer blend (pure PVP:PEG 40:60) are shown in Figure 5-34a. Concerning the pure PVP/PEG polymer blend (pure PVP:PEG 40:60), the high-intensity band observed at about 2886 cm-1 and the low-intensity peak at about 2690 cm-1 are ascribed to asymmetric C-H stretching mode of CH2 of PEG with a small contribution of symmetric C-H stretching mode of PVP, the absorption peak at about 1660 cm-1 is assigned to -1 asymmetric C=O stretching mode of PVP, while that at about 1467 cm corresponds to CH2 -1 scissoring mode of PEG and that at about 1440 cm corresponds to CH2 wagging mode of PVP. The -1 -1 sharp doublet at about 1361 cm and 1343 cm is attributed to CH2 wagging and CH2 bending modes of PEG, respectively, while the peaks at about 1280 cm-1 and 1234 cm-1 are attributed to asymmetric CH2 twisting and symmetric CH2 twisting modes of PEG, respectively, with a contribution of CH2 twisting mode of PVP [495,496]. The intense triplet centered at about 1116 cm-1 and shoulder peaks at about 1149 cm-1 and 1061 cm-1 concern the C-O-C stretching mode of PEG. In this mode, the peak at 1116 cm-1 represents the amorphous phase region of PEG, whereas the peaks at 1149 cm-1 and 1061 cm-1 represent the characteristic crystalline phase of PEG [516]. Finally, the two peaks near 947 -1 -1 cm and 841 cm are assigned to CH2 rocking vibrations of methylene groups and are related to the helical structure of PEG. Apart from this, the mode responsible for the peak at 947 cm-1 is primarily -1 due to C-O stretching motion with some CH2 rocking motion, while the peak at 841 cm originates primarily in CH2 rocking motion with a little contribution from C-O stretching motion of PEG [517]. With regard to the spectra of PVP/PEG blend-based polymer electrolytes, several changes were observed when comparing them with the corresponding spectrum of the pure PVP/PEG polymer blend. Amongst them, noteworthy is that the triplet band corresponding to C-O-C stretching mode of PEG has significantly changed compared to the pure PVP/PEG polymer blend. More specifically, concerning this band, there is a downshift of the central peak, while the shoulder peaks have disappeared in the spectra of the polymer blend electrolytes. This demonstrates that the crystallinity of PEG is quite reduced in all compositions of the PVP/PEG blend-based polymer electrolytes, in agreement with the DSC results. Moreover, there is a small broadening of the vibrational band of PVP at 1660 cm-1 that appeared in the PVP/PEG blend-based polymer electrolytes compared to the pure PVP/PEG polymer blend which could be due to PVP-I2 complexation, taking into account that -1 KI:I2 wt% ratio was fixed at 10:1 [498]. Furthermore, the band at about 3450 cm , corresponding to the presence of water in the sample, appeared more intense in all spectra of the PVP/PEG blend-based polymer electrolytes. The aforementioned observation is possibly attributed to the presence of the

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high amount of the hydrophilic KI in the polymer blend electrolyte samples. Finally, in the spectrum of the PVP/PEG blend-based polymer electrolyte employing TBP and GuSCN additives, a low- intensity peak at 2057 cm-1 appeared, which is attributed to the C-N stretching of SCN anion of GuSCN [527]. The XRD patterns of the PVP/PEG blend-based polymer electrolytes with and without the addition of TBP and GuSCN and of the pure PVP/PEG polymer blend (pure PVP:PEG 40:60) are shown in Figure 5-34b. The pure polymer blend sample showed two intense peaks at 19.6o and 23.7o and a few minor peaks before and after this 2θ region, which are attributed to the crystalline nature of the PEG [499]. With regard to the patterns of PVP/PEG blend-based polymer electrolytes with and without the additives, a different pattern was observed when comparing them with the corresponding pattern of the pure PVP/PEG polymer blend. More specifically, the intense peaks corresponding to the PEG crystals did not appear in any of the samples, showing that the Lewis base-acid interaction between the oxygens of PEG macromolecules and the potassium cations effectively suppress the PEG crystals growth, in agreement with the DSC and FTIR results. The electrical characterization of PVP/PEG blend-based polymer electrolytes with and without the addition of TBP and GuSCN was carried out under ac and dc conditions, by means of EIS and LSV, respectively. Figure 5-35 shows the Nyquist plots derived from EIS and the linear sweep voltammograms of the PVP/PEG blend-based polymer electrolytes, while the parameters obtained by EIS and LSV are tabulated in Table 5-36.

Figure 5-35: (a) Nyquist plots derived from EIS and (b) linear sweep voltammograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives. From the EIS results, it is shown that the addition of TBP in the sample did not affect the conductivity of the electrolyte. The slight increase that was observed in the diffusion coefficient of triiodides with the addition of TBP in the sample is possibly due to the decrease in the viscosity of the polymer blend electrolyte by trapping an amount of TBP in the polymer blend. Regarding the dielectric constant and the relative number of free charge carriers, their values did not show any significant variation. With regard to the electrolyte employing additionally GuSCN in its composition, an increase in the order of 25% in its conductivity, an increase in its dielectric constant, and a high increase in the diffusion of triiodides was observed, while the relative number of free charge carriers did not show any significant variation, when comparing to the electrolyte employing only TBP as additive in its composition. The increase in the conductivity of the polymer blend electrolyte employing TBP and GuSCN as co-additives in its composition was mainly attributed to the high increase in the diffusion coefficient of triiodides. Furthermore, an enhancement of charge transport mechanism through the PEG-cations crystalline complex has possibly taken place since an increase in the melting enthalpy of the PEG-cations crystalline complex was observed. The increase in the diffusion coefficient of triiodides is possibly due to a reduction in the viscosity of the polymer

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blend electrolyte since the presence of GuSCN in the sample was found to reduce solvent evaporation. Also, this phenomenon possibly justifies the large increase in the charge transfer resistance at the platinum/electrolyte interfaces, which can be derived from the diameter of the semicircles shown in the Nyquist plots [77]. The increase of the amount of solvent in the sample reduces the final concentration of the redox couple in the electrolyte after its gelation and consequently increases the charge transfer resistance at the platinum/electrode interfaces. Concerning the LSV results, the variation of the diffusion coefficient of triiodides was compatible with the corresponding obtained by EIS. The higher values of the triiodide diffusion coefficients obtained by LSV compared to the corresponding ones obtained by EIS were attributed to the great dependence of LSV results on the scan rate [501]. Finally, it has to be mentioned that the reduction peak at about 0.1 V, which was observed in the previous group of experiments, was not observed in any of the samples, showing that the viscosity of the electrolytes is decreased. Table 5-36: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives.

a b Sample σdc ε n/n0 DI3- DI3- (mS/cm) (–) (–) (x10-7 cm2/s) (x10-7 cm2/s) PVP:PEG 40:60 1.44 937 0.87 0.40 0.98 PVP:PEG 40:60 + TBP 1.44 935 0.87 0.51 1.08 PVP:PEG 40:60 + TBP + GuSCN 1.81 981 0.88 1.55 2.91 determined by a EIS, b LSV

Figure 5-36 shows the absorption spectra of PVP/PEG blend-based polymer electrolytes with and without the addition of TBP and GuSCN, obtained by UV-VIS. In all spectra, two absorption peaks appeared, one at 290 nm and one at 360 nm, while the characteristic absorption peak of iodine in the visible spectrum did not appear. This indicates that all iodine molecules have reacted with iodide anions, creating triiodides [502]. However, as it is observed, there is an alteration of the absorption spectra of the electrolytes with the addition of TBP in their composition. This is possibly attributed to the complexation of TBP with iodine molecules. Changes in the absorption spectrum of iodine after its complexation with additives employed in the electrolyte have been reported in the literature [528]. On the other hand, there was no significant change in the absorption spectrum of the electrolyte with the addition of GuSCN.

Figure 5-36: UV-VIS absorption spectra of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives.

Figure 5-37 shows the J–V characteristic curves of the DSSCs employing the PVP/PEG blend- based polymer electrolytes with and without the addition of TBP and GuSCN, while their electrical

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characteristics and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-37 and Table 5-38, respectively.

Figure 5-37: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives. The maximum ECE attained by the DSSCs in this group of experiments was 5.77% and was achieved with the application of the PVP/PEG blend-based polymer electrolyte PVP:PEG 40:60 + TBP + GuSCN. The results showed a decrease in JSC and an increase in VOC, FF, and ECE of the solar cells with the addition of TBP in the electrolyte of DSSCs, while a further increase in all PV parameters was observed with the addition of TBP and GuSCN in the polymer blend electrolyte of DSSCs. Table 5-37: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives.

Electrolyte JSC VOC FF ECE (mA/cm2) (mV) (–) (%) PVP:PEG 40:60 13.50±0.07 620±2 0.56±0.01 4.66±0.02 PVP:PEG 40:60 + TBP 12.87±0.22 682±3 0.57±0.01 5.00±0.03 PVP:PEG 40:60 + TBP + GuSCN 13.05±0.21 737±3 0.60±0.01 5.77±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. With the addition of TBP in the polymer blend electrolytes, a decrease in JL, rS, and rSH, and an increase in n and J0 was observed. On the other hand, with the addition of TBP and GuSCN in the polymer blend electrolytes, a slight increase in JL and rSH, as well as a decrease in n, J0, and rS, were observed compared to the solar cells employing the polymer blend electrolyte with only TBP as an additive in its composition. Table 5-38: Parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes with and without the additives.

Electrolyte JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) PVP:PEG 40:60 13.53±0.08 2.48±0.02 669±33 8.00±0.25 3104±72 PVP:PEG 40:60 + TBP 12.91±0.23 2.80±0.03 1117±157 7.67±0.38 2516±56 PVP:PEG 40:60 + TBP + GuSCN 13.09±0.21 2.71±0.03 385±64 6.83±0.38 2643±55

The increase in VOC of the solar cells with the addition of TBP in the electrolyte is commonly reported in the literature. Huang et al. [529] showed that the use of nitrogen-containing heterocyclic

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additives, such as TBP, significantly reduce the electron−hole recombination rate inside the solar cells, leading to a significant increase in the VOC. On the other hand, Boschloo et al. [526] interpreted the corresponding increase in terms of a shift of the TiO2 Fermi level to more negative values. Using this consideration also justified the decrease in JSC since in this way the electrons injection rate from the excited state of dye decreases. In the present investigation, the introduction of TBP in the polymer blend electrolyte resulted in a decrease in JSC and in an increase in VOC and FF. One-diode model equivalent circuit analysis showed an increase in recombination rate inside the solar cells in the corresponding case. Thus, the aforementioned PV parameters variation was attributed to a negative shift of TiO2 Fermi level, in agreement with the Boschloo report. Talking about the solar cells employing additionally GuSCN in the polymer blend electrolyte, a further enhancement of their performance was achieved compared to the solar cells employing only TBP as an additive in the polymer blend electrolyte. Similar results were found in the literature [419]. The enhancement of DSSCs performance, in this case, was mainly attributed to the lower electron−hole recombination rate and the better charge transport inside the solar cells, according to the one-diode model equivalent circuit analysis, EIS, and LSV results. ➢ Polyvinylpyrrolidone/polyethylene glycol bend-based polymer electrolytes with additives and iodide compounds mixture The influence of cations in liquid state redox electrolytes on the performance and stability of DSSCs has been widely discussed in the literature since the invention of DSSCs [129,530–532]. Cations have been found to affect many parameters that influence the performance of the solar cells, such as the conduction band edge of the photo-anode, the electron−hole recombination rate at the photo-anode/electrolyte interface, the charge transport in the electrolyte, etc. The last decade, the main focus has been on the cation size effect on the performance of QSS-DSSCs employing polymer electrolytes, in view of achieving high-stability solar cells, without performance limitations [514,533]. Moreover, the multiple cation effect on the performance of QSS-DSSCs has attracted the attention of DSSCs developers very recently [420,421,534,535]. By combining small and large cations in one polymeric membrane, unique characteristics in polymer electrolytes and in solar cells can be achieved. In the present investigation, the mixed cation effect on the electrolyte and solar cells characteristics was studied thoroughly, in a binary alkali salt/ionic liquid-based polymer electrolytic system. The combination of KI and BMII in the novel PVP/PEG blend-based polymer electrolytes was studied for a fixed and an increasing total concentration of iodide compounds mixture. In the first group of experiments, the KI:BMII concentration ratio in the polymer blend electrolyte was varied keeping the total concentration of iodide compounds mixture fixed. In the second group of experiments, the concentration of BMII in the polymer blend electrolyte increased, without altering the concentration of KI, leading to an increased total concentration of iodide compounds mixture. DSC was used to study the crystallinity of the PVP/PEG blend-based polymer electrolytes employing the TBP and GuSCN co-additives and the iodide compounds mixture in the different compositions. The crystallinity of polymer electrolytes is an important characteristic to investigate since it has a direct effect on the ionic conductivity of electrolytes and subsequently on the electrical characteristics of solar cells. The increase in a polymer electrolyte crystallinity leads to a decrease in the movement of ions through the polymer network [418]. The DSC thermograms of the PVP/PEG blend-based polymer electrolytes as well as of the pure PVP/PEG polymer blend are shown in Figure 5-38.

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Figure 5-38: DSC thermograms of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes and of pure polyvinylpyrrolidone/polyethylene glycol polymer blend (a) for the fixed total concentration of iodide compounds mixture and (b) for the increasing total concentration of iodide compounds mixture. From the DSC thermograms of the most PVP/PEG blend-based polymer electrolytes, a change in heat flow was observed at the range of 135-150oC, while there was no change in the heat flow corresponding to the melting of PEG and PVP crystals. The change in the heat flow at the range of 135-150oC was attributed to the melting of PEG-potassium cations crystalline complex [503,515]. More specifically, with a closer look at this region of the DSC thermograms, the lower melting temperature peak was assigned to the melting of PEG-potassium cation crystalline complex, while the higher melting temperature peak was assigned to the melting of PEG-imidazolium cation crystalline complex. According to the literature, the accurate position of the melting temperature of the PEG-cation crystalline complex cannot be provided since it depends on many factors, including the cation size, the PEO/PEG molecular weight, the choice of the solvent, the thermal history, etc [536]. On the other hand, a decrease in the melting enthalpy of the crystalline complex was observed as the concentration of ionic liquid in the polymer blend electrolyte increases, in both groups of experiments. This was due to the development of weaker Lewis base-acid interaction between imidazolium cations with the oxygen of PEG macromolecules compared to the corresponding of potassium cations, due to the greater size and lower charge density of the former [537]. This was also the reason of the development of a low-intensity and broad endothermic peak at the range of PEG crystals melting at the sample KI:BMII 0:100, showing the development of PEG crystallinity. In order to examine the mixed cation effect on the structural characteristics of the PVP/PEG blend- based polymer electrolytes, the interactions among atoms and ions in the polymer blend electrolytic systems, as well as the crystallinity of the polymer blend electrolytes, were investigated by means of FTIR and XRD.

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Figure 5-39: (a, c) FTIR spectra and XRD patterns, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend- based polymer electrolytes for the fixed total concentration of iodide compounds mixture, (b, d) FTIR spectra and XRD patterns, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the increasing total concentration of iodide compounds mixture.

The FTIR transmittance spectra of the PVP/PEG blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture, as well as of pure PVP/PEG polymer blend (pure PVP:PEG 40:60) are shown in Figure 5-39. Concerning the pure PVP/PEG polymer blend, the high-intensity band observed at about 2886 cm-1 and the low-intensity peak at about 2690 cm-1 are ascribed to asymmetric C-H stretching mode of CH2 of PEG with a small contribution of symmetric C-H stretching mode of PVP, the absorption peak at about 1660 cm-1 is assigned to asymmetric C=O stretching mode of PVP, while -1 -1 that at about 1467 cm corresponds to CH2 scissoring mode of PEG and that at about 1440 cm

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-1 -1 corresponds to CH2 wagging mode of PVP. The sharp doublet at about 1361 cm and 1343 cm is attributed to CH2 wagging and CH2 bending modes of PEG, respectively, while the peaks at about -1 -1 1280 cm and 1234 cm are attributed to asymmetric CH2 twisting and symmetric CH2 twisting modes of PEG, respectively, with a contribution of CH2 twisting mode of PVP [495,496]. The intense triplet centered at about 1116 cm-1 and shoulder peaks at about 1149 cm-1 and 1061 cm-1 concern the C-O-C stretching mode of PEG. In this mode, the peak at 1116 cm-1 represents the amorphous phase region of PEG, whereas the peaks at 1149 cm-1 and 1061 cm-1 represent the characteristic crystalline -1 -1 phase of PEG [516]. Finally, the two peaks near 947 cm and 841 cm are assigned to CH2 rocking vibrations of methylene groups and are related to the helical structure of PEG. Apart from this, the -1 mode responsible for the peak at 947 cm is primarily due to C-O stretching motion with some CH2 -1 rocking motion, while the peak at 841 cm originates primarily in CH2 rocking motion with a little contribution from C-O stretching motion of PEG [517]. Talking about the spectra of PVP/PEG blend- based polymer electrolytes, several changes were observed when comparing them with the corresponding spectrum of the pure PVP/PEG polymer blend. Amongst them, noteworthy is that the triplet band corresponding to C-O-C stretching mode of PEG has significantly changed compared to the pure polymer blend. More specifically, concerning this band, there is a downshift of the central peak, while the shoulder peaks have disappeared in the spectra of the polymer blend electrolytes. This demonstrates that the crystallinity of PEG is quite reduced in the polymer blend electrolytes, in agreement with DSC results. Moreover, there is a small broadening of the vibrational band of PVP at 1660 cm-1 that appeared in the PVP/PEG blend-based polymer electrolytes compared to the pure PVP/PEG polymer blend which could be due to PVP-I2 complexation, taking into account that KI:I2 wt% ratio was fixed at 10:1 [497]. The absorption peak at about 2057 cm-1 that appeared in all polymer blend electrolyte spectra is attributed to the C-N stretching of SCN anion of GuSCN [527]. By increasing the amount of BMII in the polymer blend electrolytes, some extra peaks and differences in their spectra appeared. The band observed at about 3120 cm-1 is attributed to C-H stretching of the imidazole ring of BMII, while the differences that appeared at the band at 2886 cm-1 are due to the contribution of aliphatic C-H stretching of the methyl group of BMII [538]. The high increase in the intensity and the small downshift of the absorption peak at 2057 cm-1 could be the result of some interaction between BMI cations of BMII and SCN anions of GuSCN. The increase in the intensity and the small broadening of the absorption peak at 1660 cm-1 is due to the contribution of O-H bending of BMII, while the peak at about 1567 cm-1 represents ring stretching of BMII. The absorption peak at about 1168 cm-1 is attributed to H-C-C and H-C-N bending mode of the imidazole ring of BMII, while these at about 750 cm-1 and 623 cm-1 are attributed to out of plane C-H bending and C2-N1-C5 bending of BMII, respectively. Furthermore, the increase in the intensity of the absorption peak at about 841 cm-1 is due to the contribution of in-plane bending of the imidazole ring of BMII [538]. Finally, the increase in the intensity of the band at about 3450 cm-1 as the concentration of ionic liquid in the polymer blend electrolyte increases is attributed to an increase of the amount of water presented in the sample, due to the high hydrophilicity of BMII [538]. The XRD patterns of the PVP/PEG blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture, as well as of pure PVP/PEG polymer blend (pure PVP:PEG 40:60) are also shown in Figure 5-39. The pure polymer blend sample showed two intense peaks at 19.6o and 23.7o and a few minor peaks before and after this 2θ region, which are attributed to the crystalline nature of the PEG [499]. With regard to the patterns of PVP/PEG blend-based polymer electrolytes, a different pattern was observed when comparing them with the corresponding pattern of the pure polymer blend. More specifically, the intense peaks corresponding to the PEG crystals did not appear in most of the samples, showing that the Lewis base-acid interaction between the oxygens of PEG macromolecules and the potassium-imidazolium cations mixture effectively suppress the PEG

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crystals growth, in agreement with the DSC results. However, concerning the polymer blend electrolyte employing only BMII (KI:BMII 0:100), a peak at about 19.6o and a low-intensity peak at 23.7o appeared, showing the development of PEG crystallinity, in agreement with the DSC results. On the other hand, by increasing the amount of BMII in the polymer blend electrolyte, peaks at 13o, 26o, and 52o, as well as some other low-intensity peaks appeared, which are attributed to the development of PEG-potassium/imidazolium cations crystalline complex formation, ones again in agreement with the DSC results. Similar observations are reported in the literature [521,522]. The electrical characterization of PVP/PEG blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture was carried out under ac and dc conditions, by means of EIS and LSV, respectively. Figure 5-40 shows the Nyquist plots derived from EIS and the linear sweep voltammograms of the PVP/PEG blend-based polymer electrolytes, while the parameters obtained by EIS and LSV are tabulated in Table 5-39.

Figure 5-40: (a, c) Nyquist plots derived from EIS and linear sweep voltammograms, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture, (b, d) Nyquist plots derived from EIS and linear sweep voltammograms, respectively, of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the increasing total concentration of iodide compounds mixture. From the EIS results, it is observed that the variation in the electrolyte conductivity follows in most of the cases the variation in the diffusion coefficient of triiodides, as it is usually observed [273,500]. Regarding the first group of experiments, the increase in the diffusion coefficient of triiodides and subsequently in electrolyte conductivity with the increase of BMII concentration in the polymer blend electrolyte is possibly attributed to a decrease in the viscosity of the polymer blend electrolyte by trapping the non-volatile BMII in the polymer blend. Thus, the polymer segmental mobility and subsequently the ionic transportation in the polymer blend electrolyte increases [421].

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However, this increase is not continuous. The decrease in the conductivity of the polymer electrolytes employing alkali salt and ionic liquid mixtures in some concentrations could be interpreted by an increase in the viscosity of the liquid mixture. Nevertheless, the fluid nature of the ionic liquid materials still makes them more conductive than their solid counterparts [539]. The decrease in the electrolyte conductivity with the simultaneous increase in the diffusion coefficient of triiodides in the polymer blend electrolyte KI:BMII 0:100 may be due to a decrease of the fast charge transport mechanism taking place through the PEG-cations crystalline complex [503,523]. In this electrolyte composition, the melting enthalpy of the PEG-cations crystalline complex was found quite reduced, according to the DSC results. In this group of experiments, the dielectric constant showed a similar variation to that of the polymer blend electrolyte conductivity. The relative number of free charge carriers showed a slight increase with the increase of the amount of BMII in the polymer blend electrolyte. This can be attributed to the larger size of imidazolium cations compared to potassium cation, which leads to an increased ionic dissociation [537]. Regarding the second group of experiments, a decrease in diffusion coefficient of triiodides and subsequently in electrolyte conductivity appeared for total concentrations of iodide compounds mixture higher than 0.4 M. The appearance of a critical redox couple concentration in electrolyte, above which the charge transport in electrolyte decreases, is commonly reported in the literature [417,421]. Noteworthy is also the decrease in the charge transfer resistance at the platinum/electrolyte interfaces, which can be derived from the diameter of the semicircles shown in the Nyquist plots, as the total concentration of iodide compounds mixture increases, with similar results reported in the literature [441]. In this group of experiments, the dielectric constant showed a similar variation to these of conductivity and triiodides diffusion coefficient, while the relative number of free charge carriers did not show any variation. Concerning the LSV results, the variation of the diffusion coefficient of triiodides was similar to the variation of the electrolyte conductivity obtained by EIS. This justifies the conjecture that the fast charge transport through the PEG-cations crystalline complex in the polymer blend electrolyte KI:BMII 0:100 decreases. In LSV, the co-existence of the two charge transport mechanisms cannot be distinguished. Thus, in this case, the variation of the diffusion coefficient of triiodides calculated by EIS and LSV is incompatible since the latter is calculated under the assumption that the conductivity of the electrolyte is determined only by triiodides diffusion. The higher values of the triiodide diffusion coefficients obtained by LSV compared to the corresponding ones obtained by EIS was attributed to the great dependence of LSV results on the scan rate [501]. An extra noteworthy observation in the LSV results is the appearance of a reduction peak at about 0.1 V at the linear sweep voltammogram of the polymer blend electrolyte KI:BMII 0:100, which did not appear in the other polymer blend electrolyte compositions. This reduction peak is possibly attributed to an increase in the electrolyte viscosity due to PEG crystallization, according to the DSC and XRD results [273]. Table 5-39: Parameters obtained by EIS and LSV for the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total iodide compounds concentration and for the increasing total iodide compounds concentration.

a b Sample σdc ε n/n0 DI3- DI3- (mS/cm) (–) (–) (x10-7 cm2/s) (x10-7 cm2/s) KI: BMII 100:0 1.81 981 0.88 1.55 2.91 KI: BMII 80:20 3.27 1065 0.90 2.45 5.03 KI: BMII 60:40 1.83 982 0.90 1.55 3.22 KI: BMII 40:60 2.15 1000 0.91 3.04 6.24 KI: BMII 20:80 3.73 1083 0.93 3.15 7.01 KI: BMII 0:100 3.37 1073 0.94 3.21 6.83 0.287 M KI + 0.113 M BMII 2.64 1039 0.91 2.20 4.68 0.287 M KI + 0.213 M BMII 2.24 999 0.91 2.08 4.40 0.287 M KI + 0.313 M BMII 1.29 905 0.91 1.79 3.69 determined by a EIS, b LSV

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Figure 5-41 shows the absorption spectra of PVP/PEG blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture, obtained by UV-VIS. In all spectra, two absorption peaks appeared, one at 290 nm and one at 360 nm, while the characteristic absorption peak of iodine in the visible spectrum did not appear. This indicates that all iodine molecules have reacted with iodide anions, creating triiodides [502]. Concerning the polymer blend electrolytes with the fixed total concentration of iodide compounds mixture, their absorption spectra were found quite the same in all cases. On the other hand, the absorption capacity of the polymer blend electrolytes was found increased by increasing the total concentration of iodide compounds mixture. Iodine and its derivatives are the ones most responsible for the absorption in the visible spectrum of light in this type of electrolytic systems. Increasing the absorption capacity of electrolyte in the visible spectrum is undesirable for its application to DSSCs, while absorption of light from the dye is prevented [503].

Figure 5-41: UV-VIS absorption spectra of the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes (a) for the fixed total concentration of iodide compounds mixture and (b) for the increasing total concentration of iodide compounds mixture.

Figure 5-42 shows the J–V characteristic curves of the DSSCs employing the PVP/PEG blend- based polymer electrolytes with additives and iodide compounds mixture, while their electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-40 and Table 5-41, respectively.

Figure 5-42: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes(a) for the fixed total concentration of iodide compounds mixture and (b) for the increasing total concentration of iodide compounds mixture. The maximum ECE attained by the DSSCs in this group of experiments was 6.33% and was achieved with the application of the PVP/PEG blend-based polymer electrolyte 0.287 M KI + 0.113 M BMII. Concerning the solar cells with the fixed total concentration of iodide compounds mixture,

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a decrease in JSC and an increase in VOC was observed in general by increasing the amount of BMII in the polymer blend electrolyte, while the FF was found increased or decreased, depending on electrolyte composition. With regard to the solar cells employing the polymer blend electrolytes with increasing total concentration of iodide compounds mixture, there was an optimal concentration of total concentration of iodide compounds mixture (here 0.287 M KI + 0.113 M BMII), where all PV parameters were found increased, while for higher concentrations, all PV parameters showed a decrease. Table 5-40: Electrical characteristics of the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture.

Electrolyte JSC VOC FF ECE (mA/cm2) (mV) (–) (%) KI: BMII 100:0 13.05±0.21 737±3 0.60±0.01 5.77±0.03 KI: BMII 80:20 12.03±0.31 763±3 0.62±0.01 5.69±0.03 KI: BMII 60:40 11.82±0.19 777±2 0.60±0.01 5.54±0.03 KI: BMII 40:60 10.26±0.07 796±2 0.62±0.01 5.03±0.03 KI: BMII 20:80 11.50±0.34 747±3 0.53±0.01 4.55±0.03 KI: BMII 0:100 13.52±0.13 762±2 0.55±0.01 5.70±0.02 0.287 M KI + 0.113 M BMII 13.68±0.24 739±3 0.63±0.01 6.33±0.03 0.287 M KI + 0.213 M BMII 12.23±0.32 736±3 0.61±0.01 5.49±0.03 0.287 M KI + 0.313 M BMII 11.70±0.29 737±2 0.60±0.01 5.17±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. Concerning the solar cells employing the polymer blend electrolytes with the fixed total concentration of iodide compounds mixture, a decrease in JL, rS, and rSH, as well as an increase in n and J0 was observed in general with the increase of the amount of BMII in the polymer blend electrolyte. With regard to the solar cells employing the polymer blend electrolytes with increasing total concentration of iodide compounds mixture, at the 0.4 M total concentration of iodide compounds mixture, they showed the maximum JL and rSH, as well as the minimum n, J0, and rS compared to the solar cells employing the polymer blend electrolyte of a different total concentration of iodide compounds mixture. Above the aforementioned concentration, the solar cells showed a decrease in JL and rSH, as well as an increase in n, J0, and rS. Table 5-41: Parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells employing the polyvinylpyrrolidone/polyethylene glycol blend-based polymer electrolytes for the fixed total concentration of iodide compounds mixture and for the increasing total concentration of iodide compounds mixture.

Electrolyte JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) KI: BMII 100:0 13.09±0.21 2.71±0.03 385±64 6.83±0.38 2643±55 KI: BMII 80:20 12.06±0.31 2.88±0.04 466±96 5.33±0.38 2387±55 KI: BMII 60:40 11.87±0.19 3.02±0.03 594±73 6.33±0.29 1564±39 KI: BMII 40:60 10.30±0.08 2.95±0.03 322±38 6.08±0.14 1445±42 KI: BMII 20:80 11.61±0.36 3.90±0.05 6707±932 5.25±0.25 518±39 KI: BMII 0:100 13.63±0.14 3.57±0.02 3504±252 5.17±0.14 631±33 0.287 M KI + 0.113 M BMII 13.70±0.24 2.47±0.03 147±27 5.92±0.14 3617±40 0.287 M KI + 0.213 M BMII 12.27±0.32 2.64±0.04 276±58 7.00±0.50 2480±85 0.287 M KI + 0.313 M BMII 11.75±0.29 2.83±0.04 528±92 7.00±0.50 1936±69

The alteration of PVP/PEG blend-based polymer electrolytes composition when changing the concentration ratio of KI:BMII in the electrolyte as well as with the increase of the total concentration

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of iodide compounds mixture with the addition of BMII in the electrolyte affected significantly the solar cells electrical characteristics and subsequently their ECE. Concerning the solar cells employing the electrolytes with the fixed total concentration of iodide compounds mixture, the decrease in JSC and the increase in VOC by increasing the amount of BMII in the polymer blend electrolyte could be interpreted with the theory of intercalation and adsorption of cations on the surface of the photo- anode [537,540]. Imidazolium cations are much larger than potassium cation, thus the photo-anode band edge movement to positive values is lower. In this way, the electrons injection from the dye is lower (lower JSC), while the difference between the quasi-Fermi level of the photo-anode and the electrochemical potential of the electrolyte is higher (higher VOC). At the same time, by increasing the amount of BMII in the polymer blend electrolyte, the impedance of charges transfer in the electrolyte decreases, according to EIS, LSV, and one-diode model equivalent circuit analysis, possibly due to a decrease in viscosity of electrolytes when the low-volatile ionic liquid compound is employed in the polymer blend electrolyte. However, the usage of imidazolium cations in the polymer blend electrolytes increased the recombination rate inside the solar cells, according to one-diode model equivalent circuit analysis. According to the literature, the higher recombination rate is one of the limiting factors of DSSCs employing ionic liquids [77]. With regard to the solar cells employing the polymer blend electrolytes with the increasing total concentration of iodide compounds mixture, the enhancement of the ECE of the solar cells employing the polymer blend electrolyte 0.287 M KI + 0.113 M BMII was attributed to the unique characteristics of the polymer blend electrolyte achieved with the application of the iodide compounds mixture, according to DSC, FTIR, XRD, EIS, LSV, and UV-VIS analysis. However, for higher concentrations of total iodide compounds mixture, the decrease in their ECE was mainly attributed to the decreased conductivity of the polymer blend electrolyte and to its increased absorption capacity, which reduces dye excitation and subsequently photo-current production, as well as to the increased recombination rate inside the solar cells. ➢ Dye-sensitized solar cells employing the optimized polymer electrolyte Figure 5-43 shows the J–V characteristic curves of the DSSCs employing the factory-available liquid-state high-performance and high-stability electrolytes, as well as the corresponding solar cells employing the optimized polymer electrolyte. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-42 and Table 5-43, respectively.

Figure 5-43: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the factory- available liquid-state high-performance and high-stability electrolytes, as well as the optimized polymer electrolyte. By comparing the characteristics of the DSSCs, it is demonstrated that the ECE of the QSS-DSSCs employing the optimized polymer electrolyte is quite satisfactory, almost equal to the corresponding of the solar cells employing factory-available liquid-state high-performance and high-stability

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electrolytes. More specifically, the QSS-DSSCs showed a lower JSC, a lower VOC, and a higher FF compared to the solar cells employing the factory-available liquid-state high-performance electrolyte, and a higher JSC, a lower VOC, and a higher FF compared to the solar cells employing the factory- available liquid-state high-stability electrolyte. Table 5-42: Electrical characteristics of the dye-sensitized solar cells employing the factory-available liquid-state high- performance and high-stability electrolytes, as well as the optimized polymer electrolyte.

Electrolyte JSC VOC FF ECE (mA/cm2) (mV) (–) (%) EL-HPE, Dyesol 14.50±0.09 761±2 0.62±0.01 6.88±0.01 EL-HSE, Dyesol 12.87±0.10 771±3 0.61±0.01 6.02±0.01 Optimized polymer electrolyte 13.68±0.24 739±3 0.63±0.01 6.33±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The JL of the QSS-DSSCs was found lower compared to the corresponding of DSSCs employing the factory- available liquid-state high-performance electrolyte and higher than the corresponding of DSSCs employing the factory-available liquid-state high-stability electrolyte. QSS-DSSCs also showed a lower n and J0, and a higher rS and rSH compared to the solar cells employing the factory-available liquid-state high-performance and high-stability electrolytes. The aforementioned results show that the photo-current production of QSS-DSSCs is almost equal to the corresponding of the conventional DSSCs, while the electron−hole recombination rate inside QSS-DSSCs is lower and charge transfer resistance inside QSS-DSSCs higher. Table 5-43: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the factory-available liquid-state high-performance and high-stability electrolytes, as well as the optimized polymer electrolyte.

Electrolyte JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) EL-HPE, Dyesol 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 EL-HSE, Dyesol 12.93±0.10 2.88±0.02 436±12 5.08±0.14 1075±6 Optimized polymer electrolyte 13.70±0.24 2.47±0.03 147±27 5.92±0.14 3617±40

The stability of the QSS-DSSCs employing the optimized polymer electrolyte was also evaluated by measuring the ECE of the solar cells that were stored at room temperature conditions (T=25oC and RH=50%) for a period of about 2300 h. The results were compared to the corresponding of the DSSCs employing the factory-available liquid-state high-stability electrolyte. As shown in Figure 5-44, the QSS-DSSCs demonstrated a higher stability compared to the conventional DSSCs, even at room temperature conditions, where the liquid state electrolytes present the minimum instability issues.

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Figure 5-44: Degradation of dye-sensitized solar cells efficiency stored under room temperature conditions. ➢ Conclusions In conclusion, the fabrication of high-efficiency QSS-DSSCs was achieved, by the replacement of the conventional liquid state electrolytes with a novel polymer electrolyte. The QSS-DSSCs demonstrated a higher efficiency and stability compared to DSSCs employing the factory-available liquid-state high-stability electrolyte. Moreover, the efficiency of QSS-DSSCs was very close to the corresponding value obtained by DSSCs employing the factory-available liquid-state high- performance electrolyte. Finally, it is worth mentioning that the preparation method of the novel polymer electrolyte is considered simple and of low cost, having great prospects for further optimization for DSSCs application. 5.3.2. Evaluation and Prediction of Dye-Sensitized Solar Cells Stability Under Different Accelerating Ageing Conditions The stability of DSSCs, as of all emerging PV technologies, is one of the most important criteria for their wide commercialization. Nevertheless, there are only few studies dealing with DSSCs stability to date, in terms of in-depth analysis of their performance degradation due to their ageing. Determining solar cells stability is not easy since the degradation of their performance during their lifetime depends on many factors, which may act simultaneously [150]. It is even more difficult to describe their degradation due to their ageing through simple mathematical equations, in order to predict their reliability for various applications. The present work constitutes a systematic investigation on DSSCs stability under different accelerating ageing conditions, fabricated in the laboratory using mainly commercially available materials, prepared for DSSCs application. The accelerating ageing tests involved isothermal fatigue at high or low temperature, thermal shock cycling, hydrothermal fatigue, and reverse biasing of DSSCs, simulating partial shading of PV modules. The variation of DSSCs electrical characteristics is discussed for each type of accelerating ageing test. In all cases, the one-diode equivalent circuit model was applied to the experimental J-V characteristic curves of the solar cells, in order to demonstrate the key factors leading to the degradation of DSSCs performance. Finally, a semi-analytical predictive model, developed previously by Professor G.C. Papanicolaou, was applied to predict the DSSCs performance degradation in each case of accelerating ageing, and the predictions were compared with the corresponding experimental results. Before the accelerating ageing experiments, the ECE of DSSCs was, in all cases, determined at about 6%. A representative J-V characteristic curve of the solar cells is shown in Figure 5-45. The

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electrical characteristics of the DSSCs and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-44 and Table 5-45, respectively.

Figure 5-45: Representative Current-density–Voltage characteristic curve of the dye-sensitized solar cells.

Table 5-44: Electrical characteristics of the dye-sensitized solar cells.

JSC VOC FF ECE (mA/cm2) (mV) (–) (%) DSSCs 12.87±0.10 771±3 0.61±0.01 6.02±0.01

Table 5-45: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells.

JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) DSSCs 12.93±0.10 2.88±0.02 436±12 5.08±0.14 1075±6

The results of the accelerating ageing tests discussed in the following paragraphs are presented in terms of normalized values, in order to easily determine DSSCs performance degradation in each type of experiment. In each measurement, a mean value and a standard deviation are presented, obtained by at least three identical solar cells for each type of experiment. ➢ Isothermal ageing at T = 85oC Figure 5-46a and Figure 5-46b show the electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis, respectively, for the DSSCs subjected under isothermal ageing at T = 85oC. As it is demonstrated, the degradation of the ECE of the solar cells was abrupt at the initial experimental stages, while for 1000 h of experiment, their ECE presented a mean total decrease of about 35%. The main reason for their ECE degradation was the decrease in JSC, while VOC and FF did not present any significant variation. Regarding the variation of the parameters obtained by the one-diode model equivalent circuit analysis, JL showed a great decrease at the initial experimental stages, which reached the 35% after 1000 h of experiment, justifying the corresponding decrease in JSC. On the other hand, n showed a small, almost linear increase, while J0 decreased until the 200 h of experiment and afterwards increased. Finally, rS showed an increase, which finally reached 15%, while rSH showed a decrease, which was of a higher rate at the initial experimental stages.

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Figure 5-46: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under isothermal ageing at T = 85oC.

The degradation of DSSCs performance due to their isothermal ageing at T = 85oC was mainly attributed to the degradation and desorption of the dye molecules from the TiO2 film [150,541]. This is evidenced by the high decrease in JL and subsequently in JSC. Degradation and desorption phenomena of the conventional N719 dye is one of the main reasons for DSSCs degradation under accelerating ageing at constant high temperatures above 85oC [68,541]. This was also the main reason for the decrease in J0 that appeared at the initial experimental stages. The increase in J0 after 200 h of experiment was attributed to the electrolyte degradation, which was also confirmed by the increase in rS. According to the literature, electrolyte degradation is due to the decrease in triiodide concentration and irreversible reactions of the redox couple with impurities existing in the electrolyte [150,542]. The side products in the electrolyte in combination with dye desorption and the detachment of catalyst particles from counter electrode increase the recombination rate inside the solar cells, which is also confirmed in the present study by the increase in J0 and the decrease in rSH [150]. ➢ Isothermal ageing at T = -25oC Figure 5-47a and Figure 5-47b show the electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis, respectively, for the DSSCs subjected under isothermal ageing at T = -25oC. As it is demonstrated, in this case, the degradation of the solar cells electrical characteristics was lower compared to the corresponding at T = 85oC. In the present case, the solar cells maintained almost 80% of their initial ECE after 1000 h of experiment. The decrease in JSC was determinant of the ECE degradation of the solar cells once again, while VOC and FF did not present any significant variation. Regarding the variation of the parameters obtained by the one- diode model equivalent circuit analysis, JL showed a notable decrease, reaching almost the 20% after 1000 h of experiment, justifying the corresponding decrease in JSC. A decrease was also observed in J0 and rSH. Finally, n and rS did not present any significant variation.

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Figure 5-47: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under isothermal ageing at T = -25oC. The degradation of DSSCs performance due to their isothermal ageing at T = -25oC was lower compared to the corresponding at T = 85oC. Although the degradation mechanisms for solar cells under isothermal ageing at low temperatures has been investigated for other PV technologies, the studies that have been already carried out for DSSCs are only few, with similar results to be reported [543,544]. In the present investigation, the degradation of DSSCs performance was mainly attributed o to the degradation and desorption of dye molecules from the TiO2. Isothermal ageing at T = -25 C caused an evident discoloration of the photo-anode. Thus, a high decrease in JL and subsequently in JSC was observed, also leading to a decrease in J0. The effect of solar cells isothermal ageing at T = - o 25 C on electrolyte characteristics was possibly not great since n and rS did not present any significant variation. Furthermore, the electrolyte did not present any thermal instability phenomena during the experiment. o o ➢ Thermal shock cycling between Tmin = -25 C and Tmax = 85 C Figure 5-48a and Figure 5-48b show the electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis, respectively, for the DSSCs subjected under thermal o o shock cycling between Tmin = -25 C and Tmax = 85 C. As it is demonstrated, the solar cells maintained higher than 85% of their initial performance after 200 thermal shock cycles. The degradation of the ECE of the solar cells was due to a decrease in JSC, while VOC and FF did not present any variation. Regarding the variation of the parameters obtained by the one-diode model equivalent circuit analysis, JL, J0, and rSH presented a decrease, while n and rS did not vary intensively.

Figure 5-48: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit o o analysis for the dye-sensitized solar cells under thermal shock cycling between Tmin = -25 C and Tmax = 85 C.

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DSSCs showed high stability under thermal shock cycling. The decrease in JL was attributed to degradation and desorption phenomena of dye molecules from TiO2, a phenomenon that was observed more intensively in isothermal ageing of the solar cells at high or low temperature. Concerning the rest of the component materials of the solar cells, thermal shock cycling did not strongly affect their properties, which is evidenced by the small variation of n, rS, and rSH. In literature, there is a small number of studies on DSSCs stability under thermal shock cycling, without further evidence of the degradation mechanisms [154,545,546]. ➢ Hydrothermal ageing at T = 65oC and RH = 85% Figure 5-49a and Figure 5-49b show the electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis, respectively, for the DSSCs subjected under hydrothermal ageing at T = 65oC and RH = 85%. As it is demonstrated, the solar cells presented low stability under hydrothermal ageing since most of them presented an almost 20% degradation of their ECE after only 100 h of experiment. The ECE decrease was due to JSC decrease, while VOC and FF did not present any significant variation. Regarding the variation of the parameters obtained by the one-diode model equivalent circuit analysis, JL, J0, and rSH showed a continuous decrease, while n and rS remained almost constant at their initial values.

Figure 5-49: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under hydrothermal ageing at T = 65oC and RH = 85%. DSSCs showed quite low stability under conditions of increased temperature and humidity, with some of the devices being completely destroyed after 60 h of experiment. The main reason for their degradation was the penetration of the moisture through the solar cells sealants, leading to the great degradation of their component materials. In particular, the solar cells performance degradation was mainly attributed to the degradation and desorption of the dye molecules from the TiO2, which was the reason for the high decrease in JL and subsequently in JSC. Obvious was also the degradation of the electrolyte by its discoloration. According to the literature, water molecules interfere within TiO2, which exhibits hydrophilic properties. This results in the desorption and degradation of the dye from the TiO2, especially when the dye has hydrophilic properties, like N719 [150,547]. In this way, the electron−hole recombination rate inside DSSCs is also reduced, as water molecules inhibit the reduction of triiodides of electrolyte from the electrons of the semiconductor [548,549]. The aforementioned phenomenon explains the high decrease in J0. Furthermore, iodide/triiodide-based redox electrolytes show high degradation in presence of water molecules since irreversible chemical reactions occur between triiodides and the later. The aforementioned reaction is commonly perceived even visually, by electrolyte bleaching since triiodides are reduced and undesirable derivatives of iodine are generated [150].

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➢ Reverse biasing at I = 4xISC Figure 5-50a and Figure 5-50b show the electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis, respectively, for the DSSCs subjected under reverse biasing at I = 4xISC. As it is demonstrated, the solar cells presented high stability under reverse biasing, maintaining about 90% of their initial performance after 150 h of experiment. In the present case, VOC and FF showed a small decrease, which ranged below 5%, while JSC remained stable at its initial value. Regarding the variation of the parameters obtained by the one-diode model equivalent circuit analysis, JL and n remained almost unchanged during the experiment, while J0 showed a sharp increase, which finally reached the 80%. On the other hand, rS showed a small increase and rSH a great decrease, which finally reached 10% and 40%, respectively.

Figure 5-50: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under reverse biasing at I = 4xISC. Reverse biasing did not affect significantly DSSCs performance. Gas bubbles inside solar cell, sealant failure, and photo-diode breakdown were not observed in the present study, which are common observations in corresponding accelerating ageing experiments on DSSCs [158]. According to the literature and the one-diode model equivalent circuit analysis, the degradation of DSSCs performance due to reverse biasing is attributed to electrolyte degradation. The aforementioned phenomenon is also justified by the electrolyte bleaching, which was also observed in the present study. Electrolyte bleaching is due to the decrease in triiodide concentration, which leads to a decrease in electrolyte conductivity [158,550]. Visual alterations were also observed on the counter electrode, which was possibly due to its degradation. Similar reports were found in the literature [550]. The high increase in J0 and the high decrease in rSH, which indicate a high increase in the electron−hole recombination rate inside the solar cells, are attributed to a potential change in the Helmholtz thickness bilayer and the development of side products in the electrolyte, respectively, which act as charge recombination centers [551]. ➢ Normal ageing at T = 25oC and RH = 50% Figure 5-51a and Figure 5-51b show the electrical characteristics and the parameters obtained by the one-diode model equivalent circuit analysis, respectively, for the DSSCs subjected under normal ageing at T = 25oC and RH = 50%. As it is demonstrated, the solar cells showed a high decrease in their ECE, even under normal ageing, which was on the order of 25% after 2300 h of experiment. The decrease in the ECE of the solar cells was due to the JSC decrease, while VOC and FF did not present any significant variation. Regarding the variation of the parameters obtained by the one-diode model equivalent circuit analysis, JL showed a high decrease over time, leading to a corresponding decrease in J0. On the other hand, n, rS, and rSH presented a quite lower variation.

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Figure 5-51: (a) Electrical characteristics and (b) parameters obtained by the one-diode model equivalent circuit analysis for the dye-sensitized solar cells under normal ageing at T = 25oC and RH = 50%. The results of the present study showed a high decrease in DSSCs performance even under normal ageing. The degradation of the ECE of the solar cells was attributed and in this case to dye desorption phenomena from the TiO2. The high decrease in JL and subsequently in JSC, even at room temperature conditions, shows a possible existence of some extra mechanisms, except for the ones already discussed, which contribute to the desorption of dye from TiO2. The presence of multilayer dye aggregations onto TiO2 is a possible reason that could accelerate the phenomenon [485]. During DSSCs fabrication, the dye-sensitized working electrodes were not rinsed in view of reducing the multilayer dye aggregation and achieving a monomolecular dye sensitization of the TiO2. Dye aggregation dictates structural and optoelectronic properties of the dye-sensitized working electrodes in DSSCs and can take place either prior to dye adsorption onto the semiconductor surface during the solar cells fabrication process, or thereafter [485]. The rinsing of dye-sensitized working electrodes during DSSCs fabrication contributes to a decrease of multilayer dye aggregation [552]. The dye aggregates are easily desorbed from TiO2 and penetrate in the electrolyte over time, reaching even the counter electrode. The dye molecules that come in direct contact with TiO2 are the only ones that create strong chemical bonds and contribute satisfactorily to the photo-current generation [553,554]. In contrast, dye molecules that are not chemically bonded on the surface of the semiconductor penetrate to the electrolyte over time. The speed of the phenomenon depends highly on the viscosity of the electrolyte [555]. Furthermore, the electrolyte composition, i.e. solvent and cations, plays an important role in the long-term stability of the TiO2/electrolyte interface and thus to DSSCs performance [171,173]. Research has also shown a decrease in DSSCs performance in presence of dye molecules in the electrolyte, reporting a similar variation of their electrical characteristics to that of the present investigation [556]. ➢ Prediction of dye-sensitized solar cells stability under different accelerating ageing conditions Figure 5-52 shows the RPM predictions and the corresponding experimental results for the normalized degradation of DSSCs performance due to their ageing. The dashed black line presented in all diagrams is the RPM prediction corresponding to solar cells ageing under room temperature conditions. As it is demonstrated, the predicted values derived from the RPM were very close to the corresponding experimental results, for all cases of ageing. Here, it is noteworthy mentioning that the only input of the predictive model is two experimental points, one at the initial stages of the experiment and one at the final stages of the experiment, where the saturation of the solar cells performance degradation takes place. These results show that RPM could be a powerful tool in the direction of prediction of solar cells stability, irrespective of the type of the solar cell and the type of ageing.

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Figure 5-52: RPM predictions and corresponding experimental results for the normalized degradation of the dye- sensitized solar cells performance due to their ageing. ➢ Conclusions The present study is a systematic investigation of DSSCs stability under various types of ageing. The results showed that the main reason leading to the degradation of the solar cells performance is the degradation and desorption of the dye from the TiO2. The moisture in combination with the high temperature was found to be the harshest condition leading to the degradation of DSSCs performance. The moisture was able to penetrate easily through the sealants inside the solar cells at 65oC and destroy them completely. Concerning the accelerating ageing experiments at constantly high or low temperature, the degradation of the DSSCs performance was mainly attributed to the dye degradation and desorption from the TiO2, while the electrolyte did not show any thermal instability issues or significant alteration of its characteristics. The DSSCs showed high stability under thermal shock cycling. Their degradation, in this case, was attributed once again to the degradation and desorption of the dye from the TiO2, while the characteristics of the rest component materials of the solar cells did not present any significant alteration. Reverse biasing of the DSSCs did not lead to any significant degradation of their electrical characteristics, while there were no phenomena indicating the photo- diode breakdown. Finally, the DSSCs presented a significant degradation even when storing them under room temperature conditions. This phenomenon was mainly attributed to the desorption of the multilayered aggregates of dye molecules from the TiO2. Finally, it is noteworthy to mention that the degradation of DSSCs performance due to their ageing was in all cases accurately predicted by applying a semi-analytical predictive model developed previously by Professor G.C. Papanicolaou. The results showed that this model could be a powerful tool in the direction of prediction of solar cells stability, irrespective of the type of the solar cell and the type of ageing.

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5.4. Towards Lower Cost Dye-Sensitized Solar Cells Pt is up to now the main cathode material in DSSCs, due to its high catalytic activity and high stability when it comes in contact with the electrolyte [141]. However, it is costly to apply it in commercially available PVs. For this reason, carbon in various forms, such as CNTs, G, and CB, has been suggested as a suitable candidate material for fabrication of low-cost cathodes for DSSCs [290,294,557]. Although, in most cases, it is not as effective as Pt, its low cost is a great advantage for its usage to DSSCs at an industrial level. In the present investigation, carbon-based counter electrodes were fabricated, by depositing MWCNTs or G on conductive glass substrates, for their intended use as low-cost, Pt-free counter electrodes for DSSCs. The results were compared to the corresponding ones obtained by the solar cells whose counter electrodes were factory-made using Pt nanoclusters. Figure 5-53 displays the SEM images, the three-dimensional AFM images, and the cyclic voltammograms of the Pt-based, MWCNTs-based, and G-based counter electrodes, while the parameters obtained by AFM and CV are listed in Table 5-46.

Figure 5-53: SEM images and three-dimensional AFM images of the surface morphology of (a, d) Pt-based counter electrode, (b, e) MWCNTs-based counter electrode, (c, f) G-based counter electrode, respectively, and (g) cyclic voltammograms of the different types of counter electrodes. According to SEM images, the form and the size of the structural units, and therefore the surface morphology of each cathode differed dramatically. Pt film consisted of imprecise boundary nanogranules, the MWCNTs length far exceeded 500 nm, while the structural unit size of G was

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defined at the microscale. By comparing the AFM results of the different cathodes, it was observed that the Pt-based counter electrode showed by far the lowest surface roughness, resulting in the lowest active surface area. However, the Pt-based counter electrode demonstrated a high surface smoothness, with Z-range to be defined at the nanoscale. The roughness of the carbon-based counter electrodes was found much higher compared to the Pt-based counter electrode, while at least one of the dimensions of the structural units of the former materials was defined at the microscale, leading the Z-range to high values. The highest active surface area was found at MWCNTs-based counter electrode, which was almost 80% higher compared to the corresponding of the factory-available Pt- based counter electrode. Generally, by increasing the active surface area of the cathode, an increase of triiodide reduction kinetics on the counter electrode is observed, minimizing the energy loss [289]. Concerning the cyclic voltammograms related to the counter electrodes, two pairs of redox peaks - - - - - appeared, which were assigned to the redox reactions I3 +2e ↔3I and 3I2+2e ↔2I3 [558]. These peaks were observed on the cyclic voltammograms of all the types of counter electrodes. In the anodic sweep, iodide is oxidized sequentially to triiodide (peak I) and then to iodine (peak II), while when the potential scan is reversed, iodine is reduced first to triiodide (peak II′) and then to iodide (peak I′). From the CV profiles of counter electrodes, their electrocatalytic activity was compared in terms of cathodic peak current density (JCP), cathodic peak potential (ECP), and peak-to-peak separation (EPP) of the potential difference between the anodic and cathodic peaks of iodide/triiodide couple. JCP and EPP portray the electrode reaction kinetics, and their values are proportional and inversely proportional, respectively, to the electrocatalytic activity of the electrode [559]. By comparing the characteristics of the different counter electrodes, MWCNTs-based counter electrode demonstrated the highest electrocatalytic activity, even higher than the corresponding of the factory-available Pt- based counter electrode, while the electrocatalytic activity of G-based counter electrode was found quite reduced. Table 5-46: Parameters obtained by AFM and CV for the different types of counter electrodes.

Parameter Counter electrode Pt-based MWCNTs-based G-based Root-mean-square roughness (nm) a 32 421 347 Surface area (μm2) a 25.4 45.2 34.6 Z-range (nm) a 156 1937 1786 Anodic peak current density (mA/cm2) b 2.06 2.29 1.87 Cathodic peak current density (mA/cm2) b -1.95 -2.05 -1.14 Anodic peak potential (V) b 0.489 0.484 0.589 Cathodic peak potential (V) b -0.170 -0.134 -0.153 Peak-to-peak separation (V) b 0.659 0.618 0.742 determined by a AFM, b CV

Figure 5-54 shows the J–V characteristic curves of the DSSCs employing the factory-available Pt-based counter electrode, as well as the carbon-based counter electrodes. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-47 and Table 5-48, respectively.

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Figure 5-54: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing the factory- available Pt-based counter electrode, as well as the carbon-based counter electrodes. By comparing the characteristics of the DSSCs, it is demonstrated that the ECE of the Pt-free DSSCs is quite satisfactory, almost equal to the corresponding of the solar cells employing the factory-available Pt-based counter electrode. More specifically, the MWCNTs-based DSSCs and G- based DSSCs showed an equal and higher JSC, respectively, compared to the solar cells employing the factory-available Pt-based counter electrode, while in both cases, their VOC and FF were found slightly reduced. Table 5-47: Electrical characteristics of the dye-sensitized solar cells employing the factory-available Pt-based counter electrode, as well as the carbon-based counter electrodes.

Counter electrode JSC VOC FF ECE (mA/cm2) (mV) (–) (%) Pt-based 7.15±0.03 732±2 0.58±0.01 3.05±0.02 MWCNTs-based 7.14±0.13 725±3 0.57±0.01 2.95±0.02 G-based 7.29±0.07 721±3 0.57±0.01 2.98±0.02

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The JL of the MWCNTs-based DSSCs and G-based DSSCs was found equal and higher, respectively, compared to the corresponding of DSSCs employing the factory-available Pt-based counter electrode. However, the Pt-free DSSCs showed a higher n and J0, and a lower rSH compared to the solar cells employing the factory-available Pt-based counter electrode. Finally, rS showed an almost equal value for the compared solar cells. Table 5-48: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the factory-available Pt-based counter electrode, as well as the carbon-based counter electrodes.

Counter electrode JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) Pt-based 7.19±0.03 3.25±0.01 1166±27 5.17±0.14 1021±41 MWCNTs-based 7.18±0.13 3.31±0.04 1421±206 5.25±0.25 815±54 G-based 7.34±0.07 3.31±0.03 1504±171 5.17±0.14 739±34

The MWCNTs-based DSSCs and G-based DSSCs demonstrated an almost equal performance to the corresponding of the Pt-based DSSCs. The increased active surface area of the carbon cathodes compared to the Pt one was capable to maintain the electrocatalytic activity for triiodide reduction high and thus all the PV parameters at high values; experimental results have shown that carbon has

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lower catalytic activity for triiodide reduction compared to Pt, leading to higher charge transfer resistance at counter electrode/electrolyte interface, and thus to lower solar cell performance [560]. On the other hand, according to the one-diode model equivalent circuit analysis, the Pt-free DSSCs showed an increased electron−hole recombination rate at TiO2/electrolyte and conductive substrate/electrolyte interfaces compared to the conventional devices. ➢ Conclusions In conclusion, the fabrication of Pt-free DSSCs was achieved, by the replacement of the conventional Pt-based counter electrode with novel MWCNTs-based or G-based counter electrodes. The Pt-free DSSCs demonstrated an equal performance to the corresponding attained by the solar cells employing a factory-made Pt nanoclusters-based counter electrode. The fabrication method of the novel carbon-based counter electrodes is considered simple and of low cost, having great prospects for further optimization for DSSCs application. The present study demonstrates that the development of lower-cost DSSCs without performance limitations is achievable. 5.5. Towards Wider Application Range Dye-Sensitized Solar Cells 5.5.1. Development of High-Efficiency Back-Side Illuminated Dye- Sensitized Solar Cells The rapid development of modern electronics gives rise to higher demands of flexible and wearable energy resources. Back-side illuminated configuration in DSSCs technology is considered a promising way to develop high-efficiency, high-stability, and low-cost flexible solar cells by applying metal foils as working electrode substrates [561]. Metal-based working electrodes have an edge over the plastic-based ones due to their compatibility with the high-temperature sintering process to get high quality and high adhesive anode, lower sheet resistance facilitating the movement of electrons, higher mechanical robustness, non-permeability of moisture shielding the internal structure of the solar cells from degradation, low cost since there is no need to apply a TCO, etc. Nevertheless, one of the main drawbacks of the back-side illuminated configuration to DSSCs is the increased optical losses due to the absorption of light by the catalyst and the electrolyte layer, resulting in a decrease in the photo-current production and hence, in the overall solar cell performance [55]. Up until today, many different metal foils have been used as working electrode substrates in back-side illuminated DSSCs, with the most famous being the Ti foil [237,562–564]. Nowadays, the energy conversion efficiency of the back-side illuminated DSSCs employing a Ti foil as working electrode substrate has reached a satisfactory level compared to the corresponding ones usually attained by the conventional front-side illuminated devices [351,565,566]. However, in most cases, the performance of the former is still lower than the corresponding ones of the latter. The present work deals with the development of novel high-efficiency back-side illuminated DSSCs employing highly ordered and mesoporous TiO2 nanostructures, fabricated on Ti foils using low-cost and simple chemical techniques, after their optimization for DSSCs application. More specifically, the investigation includes the development of highly ordered TiO2 nanotube arrays of different length directly on the Ti foils using the electrochemical anodization method, their subsequent treatment with TiCl4 aqueous solution and their crystallization at different annealing temperatures for the improvement of their morphological and electrical characteristics. In all cases, the anodized Ti foils were applied as working electrodes in DSSCs and the solar cells characteristics were compared. Furthermore, hybrid double-layered photo-anodes were fabricated on Ti foils, using TiO2 nanotubes and nanoparticles,

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and subsequently applied as working electrodes in back-side illuminated DSSCs. Finally, the characteristics of the optimized back-side illuminated DSSCs were compared to the corresponding ones of conventional front-side illuminated DSSCs.

➢ Highly ordered TiO2 nanotube arrays of different length The application of one-dimensional nanostructured photo-anodes in DSSCs is considered a promising way to improve the efficiency of charge collection, by promoting both more rapid electron transport and slower charge recombination inside the solar cells [85]. Amongst the most famous one- dimensional nanostructures for the aforementioned application are the TiO2 nanotubes, which can be synthesized by several methods, with the most known to be the electrochemical anodization [250,567]. The present work deals with the development of highly ordered TiO2 nanotube arrays using the electrochemical anodization method. In the present investigation, the characteristics of the ATO films fabricated directly on the Ti foils by electrochemical anodization for different time durations were studied using SEM, EDX, and AFM (see Figure 5-55). The parameters obtained by SEM, EDX, and AFM are tabulated in Table 5-49.

Figure 5-55: (a, b) SEM images of the top surface of the anodic titanium oxide films before and after ultrasonic cleaning, respectively, (c) SEM image of the bottom surface of the free-standing anodic titanium oxide films, (d) SEM image of the side view of the anodic titanium oxide films, (e) EDX spectrum of the anodic titanium oxide films, (d) three-dimensional AFM image of the top surface of the anodic titanium oxide films.

Surface analysis by means of SEM revealed the fabrication of highly ordered nanotube arrays on the Ti foil after its anodization, for all the examined time durations of anodization. However, as it is observed in Figure 5-55a, the as-anodized nanotubular structures were in most cases covered by a bundle layer introduced during the anodization procedure. Similar reports were found in the literature [565,568]. The phenomenon is attributed to the high concentration of acids presented in the electrolyte and the prolonged duration of anodization of the Ti. Because of the robust structure of the nanotube arrays and the loose structure of the surface debris, the unwanted deposits on the surface of the ATO films introduced during the anodization procedure can be effectively removed by ultrasonication of the anodized samples in deionized water. After the ultrasonic cleaning, the disordered clumps were completely removed from ATO films as shown in Figure 5-55b. In this case, the surface of the ATO films was quite uniform, without the presence of any particular imperfections

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and cracks in the whole structure, which is considered to be one of the key factors leading to the development of high-efficiency DSSCs [568]. The diameter of the nanotubes was found at about 100 nm, while their tubular shape appears to be well-formed almost in all cases. The detachment of the ATO films from the Ti foils in the form of membranes was also possible, by the immersion of the anodized samples in deionized water and their ultrasonic vibration, using an ultrasonic probe sonicator. Figure 5-55c shows the bottom view of the free-standing ATO films. As it is observed, the tubular shape is maintained throughout the whole ATO film thickness, while at the bottom side, the nanotubes were closed-ended. The wall thickness of the nanotubes appeared to be slightly larger near the bottom side of the ATO films compared to the corresponding at the upper side. The opposite happens with their internal diameter. This means that the nanotubes showed an asymmetric morphology, probably due to the higher rate of etching of the ATO films in its front-side area, where the electrolyte is in better contact with them. By the detachment of the ATO film from the Ti foil, the fabrication of front-side illuminated DSSCs based on highly ordered tubular nanostructured anode is possible [250]. Figure 5-55d shows an SEM image of the side view of the ATO films, where the tubular nanostructures are also clearly identified. From the side view, it was also possible to measure the length of the developed nanotubes (ATO film thickness), which is an important characteristic since the ATO films are about to be applied as an anode in DSSCs. The length of the highly ordered nanotube arrays greatly affects the performance of DSSCs when they are used as an anode, with many studies being done on this topic in the last few years [90,565]. In this group of experiments, the influence of the time duration of anodization of the Ti foils on the length of the developed highly ordered nanotube arrays was studied, maintaining the other parameters (electrolyte, cathode, voltage, temperature) constant. With the increase of the anodization duration, an increase of the length of the highly ordered nanotube arrays was found. However, the rate of the increase of their length with the increase of the anodization time duration was not constant. This phenomenon was more pronounced for long anodization durations. According to the literature, from one point onwards, the growth rate of the length of the highly ordered nanotube arrays is equal to the rate of their dissolution. At this point, the maximum length of the highly ordered nanotube arrays is achieved [569]. In the present investigation, the values of the obtained lengths of the highly ordered nanotube arrays for the different anodization time durations of the Ti foil were quite similar to the corresponding ones reported by Chen et al. [565]. EDX was used for the elemental analysis of the anodized samples. In all cases, only Ti and O peaks were observed in their spectra, while no other peaks appeared corresponding to residues from the anodizing process, proving the formation of pure titanium oxide on Ti foil after its anodization and cleaning process. The low-intensity Si peak appeared in the EDX spectrum of some ATO films was considered an artifact peak generated from the silicon detector crystal [570]. From the EDX spectra of all samples, it was evident that the ratio of the Ti peak height to the O peak height slightly changed from one measure to another, indicating that the composition of the nanotubes was not fixed as in the case of TiO2. Similar reports were found in the literature [571]. For this reason, the term TiOx is often used in the literature when reference is made to nanotubes which have been fabricated by anodization of Ti. However, in the present investigation, because the deviation in the results regarding the elemental analysis of the different samples was quite low (< 2%) and the calculated elemental composition of the ATO films was found at 62 wt% of Ti and 38 wt% of O, which is very close to the wt% content of Ti and O elements in TiO2 (approximately 60 wt% of Ti and 40 wt% of O), it has been considered appropriate to use the term TiO2 nanotubes from now on. AFM gave additional evidence for the surface morphology of the compared anodized samples. The surface morphology of the ATO films was compared in terms of root-mean-square roughness, surface area, and the Z-range. The roughness of the ATO films is an essential parameter to be considered since they are about to be applied as an anode in DSSCs. Generally, the higher the surface

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roughness, the greater the real surface extension and chemisorption of the dye-complex molecules onto the semiconductor surface, and thus, the higher the energy conversion efficiency attained by the photo-electrochemical cell [244]. As it is observed, the uniformity of the surface of the ATO films changed by altering the time duration of anodization. For short anodization time durations, the surface of the ATO films was characterized rougher and uneven, with the values of the surface root-mean- square roughness and the Z-range to decrease with the increase of the anodization time duration. This is possibly attributed to a non-uniform erosion of the Ti foil at the early stages of anodization, leading to the development of large peaks and valleys on its surface, which are smoothed with the increase of anodization time. At this point, it has to be mentioned that the cost of anodization was quite lower than the corresponding ones taking place in conventional electrochemical cells employing a Pt foil/mesh as a counter electrode. To the author’s knowledge, it is the first time that a Pt nanoclusters-coated FTO glass is employed as a counter electrode for the anodization of Ti foils, demonstrating similar results with those reported in the literature. Table 5-49: Parameters obtained by SEM, EDX, and AFM for the anodic titanium oxide films fabricated for different time durations of anodization.

Parameter Sample Duration of anodization (h) 2 4 6 8 Film thickness (μm) a 15 22 25 28 Titanium ≈ 62 Elemental composition (wt%) b Oxygen ≈ 38 Other elements < 0.2 Rout-mean-square roughness (nm) c 661 646 634 626 Surface area (μm2) c 2747 2732 2722 2717 Z-range (nm) c 2.3 2.0 1.8 1.7 determined by a SEM, b EDX, c AFM

Figure 5-56 shows the J–V characteristic curves of the back-side illuminated DSSCs employing the highly ordered TiO2 nanotube arrays of different length as an anode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-50 and Table 5-51, respectively.

Figure 5-56: Current density–Voltage characteristic curves of the back-side illuminated dye-sensitized solar cells employing as anode the highly ordered TiO2 nanotube arrays of different length. The maximum ECE attained by the DSSCs in this group of experiments was 3.59% and was achieved with the application of highly ordered TiO2 nanotube arrays of length 25 μm as anode. The

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results showed an increase in JSC, and a slight decrease in VOC with the increase of the length of the highly ordered TiO2 nanotube arrays, while FF did not present any significant variation. Table 5-50: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the highly ordered TiO2 nanotube arrays of different length.

Working electrode JSC VOC FF ECE (mA/cm2) (mV) (–) (%) TiO2 NTs L15 6.23±0.06 732±2 0.65±0.01 2.95±0.01 TiO2 NTs L22 7.34±0.04 731±2 0.64±0.01 3.45±0.01 TiO2 NTs L25 7.69±0.03 730±2 0.64±0.00 3.59±0.01 TiO2 NTs L28 7.70±0.06 728±3 0.64±0.01 3.57±0.01

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. In the present case, all the parameters calculated by the one-diode equivalent circuit model, namely JL, n, J0, rS, and rSH showed an increase with the increase of the length of the highly ordered TiO2 nanotube arrays that were used as an anode in DSSCs. Table 5-51: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye- sensitized solar cells employing as anode the highly ordered TiO2 nanotube arrays of different length.

Working electrode JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) TiO2 NTs L15 6.25±0.06 2.43±0.03 58±6 5.83±0.14 1595±26 TiO2 NTs L22 7.36±0.04 2.46±0.02 79±6 5.92±0.14 1651±36 TiO2 NTs L25 7.72±0.03 2.48±0.02 95±9 6.00±0.00 1658±27 TiO2 NTs L28 7.73±0.06 2.50±0.02 107±5 6.33±0.14 1676±13

The increase in JL and subsequently in JSC of DSSCs with the increase of the length of the highly ordered TiO2 nanotube arrays was attributed to the increased optical capacity of the anode. By increasing the length of the highly ordered TiO2 nanotube arrays (anode thickness), an increased number of dye molecules are presented in the anode, while the light scattering efficiency of the anode also increases. TiO2 nanotubes scatter the light effectively, resulting in better exploitation of solar irradiation. Thus, many researchers use them as light scatters in DSSCs, mostly in a double-layered anode design [572,573]. In this way, an increased number of excited electrons is obtained and hence an increased JSC is attained by DSSCs. However, there is a critical length above which the JSC of the solar cells shows a decrease and the electron-hole recombination rate inside the solar cells shows a high increase. The photo-generated electrons have a specific lifetime before collected to the external circuit, which is greatly dependent on the design of the anode. If their diffusion in the anode exceeds this time limit, they are recombined with the holes [37]. The increase in the rate of electron-hole recombination inside the solar cells is generally found to be great for thicknesses higher than the aforementioned optimal thickness. In the present investigation, the increase in the electron-hole recombination rate inside the solar cells with the increase of the length of the highly ordered TiO2 nanotube arrays is shown by the increase in n and J0 and the subsequent decrease in VOC. In the present case, the optimal length of the highly ordered TiO2 nanotube arrays for their application as an anode in DSSCs was found at 25 μm. Similar results corresponding to DSSCs employing highly ordered TiO2 nanotube arrays were found in the literature [90,565]. In the case of DSSCs employing anodes composed of TiO2 nanoparticles, the optimal thickness is much lower [256,504]. This shows that the highly ordered TiO2 nanotube arrays, with their ideal shape and orientation, facilitate the diffusion of electrons in the anode and reduce their recombination with holes, thus the optimal thickness of the anode varies at higher values.

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➢ TiCl4 treated highly ordered TiO2 nanotube arrays of different length

In order to increase the specific surface area of the highly ordered TiO2 nanotube arrays, to achieve an increased dye adsorption and subsequently an enhanced energy conversion efficiency to DSSCs, the anodized samples were treated with TiCl4 [565]. The characteristics of the TiCl4 treated highly ordered TiO2 nanotube arrays were compared to the corresponding ones of the as-anodized samples by means of AFM and BET analysis (see Figure 5-57). The parameters obtained by AFM and BET analysis are tabulated in Table 5-52.

Figure 5-57: (a) Three-dimensional AFM image and (b) N2 adsorption isotherms of the as-anodized highly ordered TiO2 nanotube arrays and the TiCl4 treated highly ordered TiO2 nanotube arrays.

The surface morphology of the as-anodized and the TiCl4 treated highly ordered TiO2 nanotube arrays were compared in terms of root-mean-square roughness, surface area, and Z-range. The roughness of the highly ordered TiO2 nanotube arrays is an essential parameter to consider since they are about to be applied as an anode in DSSCs. Generally, the higher the surface roughness, the greater the real surface extension and chemisorption of the dye-complex molecules onto the semiconductor surface, and thus, the higher the energy conversion efficiency attained by the photo-electrochemical cell [244]. As it is observed, the surface root-mean-square roughness of the highly ordered TiO2 nanotube arrays was highly increased after their TiCl4 treatment, also leading to an increase of their surface area, while the value of Z-range did not present any variation.

The N2 adsorption/desorption isothermal tests were carried out with the aim of determining the differences in the specific surface area and mesoporosity between the as-anodized and the TiCl4 treated highly ordered TiO2 nanotube arrays. According to IUPAC classification, in both cases, the isotherms showed a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that the highly ordered TiO2 nanotube arrays presented slit-shaped mesopores in both cases [429,466]. In more detail, the TiCl4 treated samples present a higher specific surface area compared to the as-anodized samples, while the average pore diameter and the total pore volume of the former were found decreased compared to the latter. The macroporosity of the materials was not investigated since the TiCl4 treatment of the samples is not expected to create a significant difference in their porosity at this level.

Table 5-52: Parameters obtained by AFM and BET analysis for the as-anodized highly ordered TiO2 nanotube arrays and the TiCl4 treated highly ordered TiO2 nanotube arrays.

Parameter Sample TiO2 NTs L25 TiO2 NTs L25 T Rout-mean-square roughness (nm) a 634 983 Surface area (μm2) a 2722 2726 Z-range (nm) a 1.8 1.8 Specific surface area (m2g-1) b 38.5 43.0 Average pore diameter (nm) b 14.3 7.3 Total pore volume (cm3g-1) b 0.172 0.054 determined by a AFM, b BET

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Figure 5-58 shows the J–V characteristic curves of the back-side illuminated DSSCs employing the as-anodized highly ordered TiO2 nanotube arrays of the optimized length as an anode, according to the previous group of experiments, and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-53 and Table 5-54, respectively.

Figure 5-58: Current density–Voltage characteristic curves of the back-side illuminated dye-sensitized solar cells employing as anode the as-anodized highly ordered TiO2 nanotube arrays of the optimized length and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length.

As it is observed, the performance of DSSCs greatly increased with the TiCl4 treatment of the highly ordered TiO2 nanotube arrays. The maximum ECE attained by the DSSCs in this group of experiments was 5.65% and was achieved with the application of TiCl4 treated highly ordered TiO2 nanotube arrays of length 25 μm as an anode. The aforementioned ECE was almost 57% higher compared to the corresponding attained by the DSSCs employing the as-anodized highly ordered TiO2 nanotube arrays as an anode. In the present group of experiments, the results showed an increase in JSC with the increase of the length of the TiCl4 treated highly ordered TiO2 nanotube arrays up to 25 μm, while in these cases, VOC and FF did not present any significant variation. The further increase of the length of the TiCl4 treated highly ordered TiO2 nanotube arrays led to a slight decrease in JSC, VOC, FF, and subsequently in the ECE of the solar cells. Table 5-53: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the as- anodized highly ordered TiO2 nanotube arrays of the optimized length and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length.

Working electrode JSC VOC FF ECE (mA/cm2) (mV) (–) (%) TiO2 NTs L25 7.69±0.03 730±2 0.64±0.00 3.59±0.01 TiO2 NTs L15 T 10.61±0.11 751±3 0.65±0.01 5.15±0.02 TiO2 NTs L22 T 11.30±0.10 750±3 0.64±0.01 5.45±0.02 TiO2 NTs L25 T 11.83±0.14 750±3 0.64±0.01 5.65±0.02 TiO2 NTs L28 T 11.66±0.09 746±3 0.63±0.01 5.51±0.01

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. As it is observed, the DSSCs employing the TiCl4 treated highly ordered TiO2 nanotube arrays as anode presented in general a higher JL and rSH, and a lower n, J0, and rS compared to the corresponding devices employing the as-anodized highly ordered TiO2 nanotube arrays as an anode. In the present group of experiments, the results showed an increase in the values of all the parameters calculated by

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the one-diode equivalent circuit model, namely JL, n, J0, rS, and rSH with the increase of the length of the TiCl4 treated highly ordered TiO2 nanotube arrays that were used as anode in DSSCs. An exception was the decrease in JL of the solar cells employing the TiCl4 treated highly ordered TiO2 nanotube arrays of higher length than 25 μm. Table 5-54: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye- sensitized solar cells as anode the as-anodized highly ordered TiO2 nanotube arrays of the optimized length and the TiCl4 treated highly ordered TiO2 nanotube arrays of different length.

Working electrode JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) TiO2 NTs L25 7.72±0.03 2.48±0.02 95±9 6.00±0.00 1658±27 TiO2 NTs L15 T 10.64±0.11 2.39±0.03 60±8 5.58±0.14 1869±57 TiO2 NTs L22 T 11.33±0.10 2.40±0.03 68±10 5.75±0.25 1929±56 TiO2 NTs L25 T 11.86±0.15 2.42±0.02 80±10 5.83±0.14 1944±30 TiO2 NTs L28 T 11.70±0.09 2.43±0.02 90±7 6.08±0.14 1959±30

As it is demonstrated, the DSSCs employing the TiCl4 treated highly ordered TiO2 nanotube arrays as anode showed a higher performance compared to the corresponding solar cells employing the as- anodized highly ordered TiO2 nanotube arrays. The enhancement of the ECE of DSSCs after the TiCl4 treatment of the anode was attributed to the increased photocurrent production and the decreased electron-hole recombination rate inside the solar cells, according to the one-diode model equivalent circuit analysis. The TiCl4 treatment of the highly ordered TiO2 nanotube arrays resulted in a high increase in the roughness of the anode and its specific surface area, according to AFM and BET analysis results. Thus, a higher amount of dye molecules can be adsorbed on the anode surface, which is considered the main factor responsible for the high increase in JL and subsequently in JSC. The corresponding increase in VOC was attributed to the decreased electron-hole recombination rate inside the solar cells in the case where the anode was treated with TiCl4. This observation is already reported and discussed thoroughly by many scientists developing DSSCs based on anodes composed of highly ordered TiO2 nanotube arrays or TiO2 nanoparticles [96,565]. The TiCl4 treatment is a methodology developed for over ten years to improve the performance of DSSCs and has been widely used as a surface modification for TiO2 anodes. On the other hand, the variation of DSSCs characteristics as a function of the length of the TiCl4 treated highly ordered TiO2 nanotube arrays that were applied as an anode was in agreement with the corresponding variation for DSSCs employing the as-anodized highly ordered TiO2 nanotube arrays, discussed in the previous group of experiments.

➢ TiCl4 treated highly ordered TiO2 nanotube arrays crystallized under different annealing temperatures Crystallization by thermal annealing at a high temperature in an oxidative atmosphere is an effective way to enhance the electron mobility and to decrease the density of both bulk and surface trap states in highly ordered TiO2 nanotube arrays [461]. Thus, in the present investigation, the anodized samples were crystallized under different annealing temperatures before their TiCl4 treatment and their characteristics were compared by means of XRD, DRS, and PL spectroscopy (see Figure 5-59). The parameters obtained by XRD and DRS are tabulated in Table 5-55.

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Figure 5-59: (a) XRD patterns, (b) Tauc plots, and (c) PL spectra of the amorphous highly ordered TiO2 nanotube arrays and the crystallized highly ordered TiO2 nanotube arrays.

The crystal form of the TiO2 electrode influences DSSC performance. Park et al. [574] demonstrated that DSSCs employing TiO2 anode in anatase form leads to a higher energy conversion efficiency compared to the corresponding of rutile. In the present investigation, XRD patterns confirmed the amorphous nature of the as-anodized highly ordered TiO2 nanotube arrays as well as their crystallization after their annealing. The crystallinity of the highly ordered TiO2 nanotube arrays was found to be highly dependent on the annealing temperature. The samples annealed at 450oC presented a crystal structure of anatase phase, which was evidenced by the characteristic anatase (101) diffraction peak at 2θ = 25.3°. On the other hand, the samples crystallized under higher annealing temperatures presented both anatase and rutile crystallites, which was evidenced by the extra formation of the characteristic rutile (110) diffraction peak at 2θ = 27.5°. The weight percentage of anatase to rutile ratio was found to decrease as the annealing temperature was increased. The phenomenon of crystals transformation from anatase phase to rutile phase in TiO2 structures under high-temperature annealing/calcination is commonly reported in the literature [461,575,576]. In the present case, where the highly ordered TiO2 nanotube arrays were supported by the Ti foil, the transformation was greatly considered a consequence of the “substrate effect” [461,577]. According to this effect, as the metallic Ti substrate is directly oxidized to rutile under high-temperature annealing, a thin compact rutile layer is formed at the interface region between the Ti foil and the highly ordered TiO2 nanotube arrays, which gradually becomes thicker. This initiates a geometry change (compactization) and phase transformation (from anatase to rutile) of the highly ordered TiO2 nanotube arrays, from the bottom side (substrate side) to the top side. For this reason, the evidence of the phenomenon depends greatly on the length of the highly ordered TiO2 nanotube arrays. Concerning the mean primary crystallite size, the size of anatase crystals was found to decrease by increasing the annealing temperature, while an inverse variation was observed for the rutile crystals. Generally, smaller crystallites result in more defects in crystal structure compared to larger ones [578]. The crystal lattice distortion affects semiconductor sensitization. The adsorption ability of dyes like N719 is dependent on the adsorption of carboxylate groups of dye either on defect sites or on the hydroxyl groups on the surface of TiO2 [469]. In the present investigation, the crystal lattice distortion of anatase crystals was found to increase with the increase of the annealing temperature, while an inverse variation was observed for the rutile crystals. The electronic band structure of the anode semiconductor of DSSCs also has to be considered since the exact position of the band edges plays a decisive role in the energy conversion efficiency of the solar cells [579]. In the present investigation, the energy bandgap of highly ordered TiO2 nanotube arrays crystallized under different annealing temperatures was estimated from their DRS spectra, according to the Tauc’s formula. The bandgap of the crystallized highly ordered TiO2 nanotube arrays was found to slightly decrease with the increase of the annealing temperature. This is consistent with the phase transformation of TiO2 nanostructures from anatase to rutile as their annealing temperature

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increased, demonstrated by XRD. The blue-shift in the values of the bandgap of the crystallized highly ordered TiO2 nanotube arrays calculated in the present work compared to the values reported in the literature (anatase Eg ≈ 3.2 eV, rutile Eg ≈ 3.0 eV) is possibly attributed to a quantization effect in the nanotubular films, while the dependence of the calculated bandgaps on the semiconductor film thickness has also to be taken into account [569,580]. PL spectroscopy was used to deliver information about the optoelectronic characteristics of the materials. In the present investigation, the emission peak at about 400 nm corresponds to the direct band emission of TiO2, whereas the emission peaks at longer wavelengths are associated with the indirect recombination through defect and surface-mediated recombination centers [467]. When the PL spectra of the crystallized highly ordered TiO2 nanotube arrays were compared, an increase in the intensity of the emission peak at about 400 nm and the emission peaks ranging from 430 to 500 nm was observed as the annealing temperature was increasing. The aforementioned observation is possibly attributed to the increase of the rutile content and defect-states as the annealing temperature increases. Similar reports were found in the literature [577].

Table 5-55: Parameters obtained by XRD and DRS for the crystallized highly ordered TiO2 nanotube arrays.

Parameter Sample TiO2 NTs L25 A450 A500 A550 A600

a Anatase 100 96 93 91 TiO2 phase composition (wt%) Rutile 0 4 7 9 Anatase 55 49 41 38 Mean primary crystallite size (nm) a Rutile - 52 57 72 Anatase 0.003 0.003 0.004 0.004 Crystal lattice distortion a Rutile - 0.003 0.003 0.002 Bandgap (eV) b 3.29 3.27 3.26 3.25 determined by a XRD, b DRS

Figure 5-60 shows the J–V characteristic curves of the back-side illuminated DSSCs employing the TiCl4 treated amorphous or crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays of the optimized length as an anode, according to the previous group of experiments. The electrical characteristics of the solar cells and the parameters obtained by the one- diode model equivalent circuit analysis are tabulated in Table 5-56 and Table 5-57, respectively.

Figure 5-60: Current density–Voltage characteristic curves of the back-side illuminated dye-sensitized solar cells employing as anode the TiCl4 treated amorphous and crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays.

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As it is observed, the performance of DSSCs significantly increased with the application of the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays as an anode. The maximum ECE attained by the DSSCs in this group of experiments was 6.53% and was achieved with the application o of TiCl4 treated highly ordered TiO2 nanotube arrays that were crystallized at 450 C. The aforementioned ECE was almost 16% higher compared to the corresponding attained by the DSSCs employing the TiCl4 treated amorphous highly ordered TiO2 nanotube arrays as an anode. In the present group of experiments, the results showed a decrease in JSC, VOC, FF, and subsequently in the ECE of the solar cells with the increase of the annealing temperature of the highly ordered TiO2 nanotube arrays. Table 5-56: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the TiCl4 treated amorphous and crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays.

Working electrode JSC VOC FF ECE (mA/cm2) (mV) (–) (%) TiO2 NTs L25 T 11.83±0.14 750±3 0.64±0.01 5.65±0.02 TiO2 NTs L25 A450 T 12.94±0.13 768±3 0.66±0.01 6.53±0.02 TiO2 NTs L25 A500 T 12.51±0.19 762±3 0.66±0.01 6.26±0.02 TiO2 NTs L25 A550 T 12.13±0.18 744±3 0.65±0.01 5.90±0.02 TiO2 NTs L25 A600 T 12.11±0.13 736±3 0.65±0.01 5.76±0.02

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. As it is observed, in general, the DSSCs employing the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays as anode presented a higher JL and rSH, and a lower n, J0, and rS compared to the corresponding devices employing the TiCl4 treated amorphous highly ordered TiO2 nanotube arrays as an anode. In the present group of experiments, the results showed a decrease in JL and rSH, and an increase in n, J0, and rS with the increase of the annealing temperature of the highly ordered TiO2 nanotube arrays. Table 5-57: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye- sensitized solar cells employing as anode the TiCl4 treated amorphous and crystallized under different annealing temperatures highly ordered TiO2 nanotube arrays.

Working electrode JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) TiO2 NTs L25 T 11.86±0.15 2.42±0.02 80±10 5.83±0.14 1944±30 TiO2 NTs L25 A450 T 12.96±0.13 2.27±0.02 30±5 5.58±0.14 3008±23 TiO2 NTs L25 A500 T 12.53±0.19 2.28±0.02 35±5 5.58±0.14 2967±24 TiO2 NTs L25 A550 T 12.15±0.18 2.30±0.03 50±10 5.67±0.14 2918±35 TiO2 NTs L25 A600 T 12.14±0.14 2.33±0.03 66±11 5.83±0.14 2863±49

As it is demonstrated, the DSSCs employing the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays as anode showed in all cases a higher performance compared to the corresponding solar cells employing the amorphous nanotubular structures as an anode. The enhancement of the ECE of the DSSCs after the crystallization of the highly ordered TiO2 nanotube arrays was attributed to the increased photo-current production and the decreased electron-hole recombination rate inside the solar cells, according to the one-diode model equivalent circuit analysis. The crystallization of highly ordered TiO2 nanotube arrays is an effective way to enhance the electron mobility in the nanotubular structures, while a decrease in the density of both bulk and surface trap states is achieved, by reducing the number of defects and oxygen vacancies/Ti3+ existing in the bandgap induced by the amorphous domains, grain boundaries, and impurities embedded during anodization [461]. The

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structural changes with the annealing procedure also influence the light-harvesting and charge- injection efficiency in DSSCs, which are determinants of the photo-current production [577]. On the other hand, the decrease in the ECE of the DSSCs with the increase of the annealing temperature of the highly ordered TiO2 nanotube arrays is attributed to several reasons. At first, the decreased photo- current production could be due to the compactization of the material, arising from the “substrate effect”, which is more pronounced at high annealing temperatures [461]. In this way, a lower amount of dye molecules is adsorbed on the surface of the material, thus, a decreased number of excited electrons is obtained. At the same time, a strong increase in the resistance and structural destruction of the materials takes place, which hinder the electron transport to the external circuit and increase the electron-hole recombination rate inside the solar cells [461]. The aforementioned explanations could well be applied in the present investigation, according to XRD, PL spectroscopy, and one-diode model equivalent circuit analysis results.

➢ TiCl4 treated crystallized highly ordered TiO2 nanotube arrays – TiO2 nanoparticles hybrid

Although highly ordered TiO2 nanotube arrays demonstrate better electron transport characteristics than TiO2 nanoparticles, the application of the former material to DSSCs leads generally to a lower energy conversion efficiency compared to the corresponding ones attained by the devices employing the latter material. According to the literature, the main reason responsible for this observation is considered the lower specific surface area of the nanotubes compared to nanoparticles, which leads to the adsorption of an insufficient amount of dye molecules on the anode and subsequently to a low photo-current production by the solar cells [106,581]. In the present investigation, in view of achieving a high energy conversion efficiency to back-side illuminated DSSCs, novel hybrid double- layered materials composed of TiCl4 treated crystallized highly ordered TiO2 nanotube arrays – TiO2 nanoparticles were fabricated on Ti foils, using low-cost and simple chemical techniques, and subsequently applied as an anode to the solar cells. The length of the highly ordered TiO2 nanotube arrays varied in order to achieve the highest performance to DSSCs.

In the present investigation, the characteristics of the highly ordered TiO2 nanotube arrays and the hybrid materials were compared by means of BET analysis (see Figure 5-61). The parameters obtained by BET analysis are tabulated in Table 5-58.

Figure 5-61: N2 adsorption isotherms of the highly ordered TiO2 nanotube arrays and the highly ordered TiO2 nanotube arrays – TiO2 nanoparticles hybrids.

The N2 adsorption/desorption isothermal tests were carried out with the aim of determining the differences in the specific surface area and the porosity between the highly ordered TiO2 nanotube arrays and the highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids.

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According to IUPAC classification, in all cases, the isotherms showed a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that the materials presented slit-shaped mesopores in all cases [429,466]. More specifically, in general, the highly ordered TiO2 nanotube arrays – TiO2 nanoparticles hybrids presented a higher specific surface area, while the average pore diameter and the total pore volume were found lower compared to the highly ordered TiO2 nanotube arrays. On the other hand, concerning the former materials, these parameters showed a decrease with the increase of the length of the highly ordered TiO2 nanotube arrays.

Table 5-58: Parameters obtained by BET analysis for the highly ordered TiO2 nanotube arrays and the highly ordered TiO2 nanotube arrays – TiO2 nanoparticles hybrids.

Parameter Sample TiO2 NTs Hybrid TiO2 NTs Lx 15 22 25 28 Specific surface area (m2g-1) a 38.5 45.8 44.7 44.3 44 Average pore diameter (nm) a 14.3 10.0 9.5 9.3 9.2 Total pore volume (cm3g-1) a 0.172 0.123 0.115 0.112 0.110 determined by a BET

Figure 5-62 shows the J–V characteristic curves of the back-side illuminated DSSCs employing the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids as an anode. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-59 and Table 5-60, respectively.

Figure 5-62: Current density–Voltage characteristic curves of the back-side illuminated dye-sensitized solar cells employing as anode the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids.

As it is observed, in some cases, the application of the hybrid materials as an anode to DSSCs led to an improvement in the solar cells performance compared to the devices employing only the highly ordered TiO2 nanotube arrays as an anode. The maximum ECE attained by the DSSCs in this group of experiments was 7.05% and was achieved with the application of TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of 15 μm length – TiO2 nanoparticles hybrid. The aforementioned ECE was almost 8% higher compared to the corresponding attained by the DSSCs employing the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays as an anode. In the present group of experiments, the results showed a decrease in JSC, VOC, and subsequently in the ECE of the solar cells with the increase of the length of the highly ordered TiO2 nanotube arrays in the hybrid anode, while the FF did not present any significant variation.

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Table 5-59: Electrical characteristics of the back-side illuminated dye-sensitized solar cells employing as anode the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids.

Working electrode JSC VOC FF ECE (mA/cm2) (mV) (–) (%) TiO2 NTs L25 A450 T 12.94±0.13 768±3 0.66±0.01 6.53±0.02 Hybrid TiO2 NTs L15 A450 T 14.38±0.34 766±4 0.64±0.01 7.05±0.03 Hybrid TiO2 NTs L22 A450 T 13.45±0.22 758±3 0.64±0.01 6.52±0.04 Hybrid TiO2 NTs L25 A450 T 12.90±0.12 754±3 0.64±0.01 6.19±0.06 Hybrid TiO2 NTs L28 A450 T 12.33±0.29 750±3 0.63±0.01 5.82±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. As it is observed, the DSSCs employing the hybrid material as anode presented in general a higher n, J0, and rSH compared to the corresponding devices employing the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays. In the present group of experiments, the results showed a decrease in JL and an increase in n, J0 rS, and rSH with the increase of the length of the highly ordered TiO2 nanotube arrays in the hybrid anode. Table 5-60: Parameters obtained by the one-diode model equivalent circuit analysis of the back-side illuminated dye- sensitized solar cells employing as anode the optimized TiCl4 treated crystallized highly ordered TiO2 nanotube arrays and the TiCl4 treated crystallized highly ordered TiO2 nanotube arrays of different length – TiO2 nanoparticles hybrids.

Working electrode JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) TiO2 NTs L25 A450 T 12.96±0.13 2.27±0.02 30±5 5.58±0.14 3008±23 Hybrid TiO2 NTs L15 A450 T 14.40±0.34 2.42±0.04 79±22 5.33±0.14 3250±57 Hybrid TiO2 NTs L22 A450 T 13.47±0.22 2.47±0.04 108±25 5.42±0.14 3627±49 Hybrid TiO2 NTs L25 A450 T 12.92±0.12 2.50±0.04 121±25 5.67±0.14 3765±40 Hybrid TiO2 NTs L28 A450 T 12.35±0.29 2.54±0.04 154±34 6.00±0.25 3893±34

As it is demonstrated, the DSSCs employing the hybrid anode showed in some cases a higher performance compared to the corresponding solar cells employing the optimized crystallized highly ordered nanotube arrays as an anode. In these cases, the enhancement of the ECE of the DSSCs was mainly attributed to the increased photo-current production. This is possibly a result of the achievement of a higher specific surface area anode by the usage of the hybrid materials, according to BET analysis, which can lead to an increased amount of dye molecules anchored on it [106,581]. However, for DSSCs employing the hybrid anode, the electron-hole recombination rate was found increased compared to the corresponding solar cells employing only the highly ordered TiO2 nanotube arrays as an anode. This was considered the main reason responsible for the decreased VOC attained by the former solar cells compared to the latter solar cells. Furthermore, according to one diode model equivalent circuit analysis of DSSCs, it was found that with the increase of the length of the highly ordered nanotube arrays of the hybrid anode, a decrease in photo-current production and an increase in electron-hole recombination rate inside the solar cells takes place. This is possibly attributed to the increased thickness of the anode, which is estimated to be about the length of the highly ordered TiO2 nanotubes arrays plus 8 μm from the top nanoparticles layer, according to previous studies, possibly exceeding the optimal anode thickness for DSSCs application [416]. Thus, there is an inefficient dye regeneration in the anode and an increased electron-hole recombination rate at the anode/electrolyte interface.

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➢ Back-side illuminated dye-sensitized solar cells employing the optimized working electrode Figure 5-63 shows the J–V characteristic curves of the conventional DSSCs and the optimized back-side illuminated DSSCs. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-61 and Table 5- 62, respectively.

Figure 5-63: Current density–Voltage characteristic curves of the conventional dye-sensitized solar cells and the optimized back-side illuminated dye-sensitized solar cells.

By comparing the characteristics of the solar cells, it is demonstrated that the novel optimized back-side illuminated DSSCs attain a slightly higher ECE compared to the corresponding of the conventional DSSCs, even though the active area of the former was four times larger than the active area of the latter. This was mainly attributed to the increased FF of the novel optimized back-side illuminated DSSCs compared to the conventional DSSCs, while JSC and VOC of the two types of solar cells presented smaller differences. Table 5-61: Electrical characteristics of the conventional dye-sensitized solar cells and the optimized back-side illuminated dye-sensitized solar cells.

Solar cell JSC VOC FF ECE (mA/cm2) (mV) (–) (%) Conventional 14.50±0.09 761±2 0.62±0.01 6.88±0.01 Optimized back-side illuminated 14.38±0.34 766±4 0.64±0.01 7.05±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. The optimized back-side illuminated DSSCs presented a lower n, and J0, and a higher rS and rSH compared to the conventional DSSCs, while the JL of the two compared solar cells was of the same values. The aforementioned show that the photo-current production of both solar cells was almost the same, while the electron transport resistance and the electron-hole recombination rate inside the solar cells increased and decreased, respectively, for the optimized back-side illuminated DSSCs compared to the conventional DSSCs. Table 5-62: Parameters obtained by the one-diode model equivalent circuit analysis of the conventional dye-sensitized solar cells and the optimized back-side illuminated dye-sensitized solar cells.

Solar cell JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) Conventional 14.54±0.09 2.75±0.01 356±9 4.50±0.00 1637±34 Optimized back-side illuminated 14.40±0.34 2.42±0.04 79±22 5.33±0.14 3250±57

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➢ Conclusions In conclusion, high-efficiency back-side illuminated DSSCs were developed using highly ordered and mesoporous materials as an anode, fabricated by simple and low-cost chemical techniques. The novel back-side illuminated DSSCs demonstrated a higher performance than the corresponding of the conventional DSSCs, even though the active area of the former was four times larger than the active area of the latter. At this point, it has to be mentioned that the energy conversion efficiency of the fabricated solar cells was at a quite satisfactory level regarding DSSCs technology. These results are considered important since in most cases the performance of the back-side illuminated DSSCs is lower than the corresponding of the front-side illuminated DSSCs [90,582,583]. The aforementioned achievement can be used for the development of high-efficiency, high-stability, and low-cost large- sized flexible solar cells based on metal substrates. 5.5.2. Evaluation of the Limiting Factors Affecting Large-Sized Flexible Platinum-Free Dye-Sensitized Solar Cells Performance Reducing further the cost, increasing the application range, and scaling up of DSSCs are important issues for their wide commercialization. Nowadays, the fabrication of high-efficiency Pt-free DSSCs is one of the main targets of DSSCs designers, in an attempt to reduce the manufacturing costs of these devices without limiting their performance, thus, making them more competitive [584]. On the other hand, based on the industrial trend noticed so far with DSSCs technology, the rigidity, the high cost and weight of glass substrates make the conventional DSSCs less likely to be mass produced and widely used for large scale exterior applications [355]. Flexible DSSCs based on thin polymer substrates have attracted wide attention in recent years, due to their light-weight, high flexibility, high impact resistance, and low cost [55]. Finally, one of the main remaining aspects for the widespread application of DSSCs is their scaling up [585]. Investigation on the scaling up of DSSCs technology is still in progress. Most of the scientific reports relate to small laboratory solar cells, with only few studies investigating the parameters affecting the large-sized cells and modules energy conversion efficiency [586–588]. The present work constitutes an extensive investigation of the limiting factors affecting large-sized flexible Pt-free DSSCs performance, using a combined experimental and equivalent circuit analysis. The investigated solar cells employed either MWCNTs-based or G-based counter electrodes as low-cost alternatives to the conventional Pt-based counter electrode. The glass conductive substrates of the dye-sensitized working electrodes and/or counter electrodes were replaced by thin flexible polymer substrates, and an analysis of the parameters limiting the flexible DSSCs performance was conducted. The scaling up effect on DSSCs electrical characteristics was also experimentally justified and quantitated, using glass and polymer electrodes. ➢ Towards all-plastic flexible dye-sensitized solar cells The photo-anode is probably the most important part of an n-type DSSC. The dye, which is the heart of the solar-to-electricity conversion processes in such a device, is chemisorbed on the anode semiconductor. Thus, in-depth analysis and optimal engineering of the semiconductor morphology and of the semiconductor electrical and optical characteristics are mandatory when designing and fabricating novel n-type photo-electrochemical cells.

In the present investigation, the un-sensitized TiO2 films characteristics, both for the glass and PET working electrodes, were studied using SEM, AFM, XRD, BET analysis, DRS, and PL spectroscopy. The optical characteristics of the dye-sensitized TiO2 electrodes were determined by UV–Vis and DRS (see Figure 5-64). The parameters obtained by SEM, AFM, XRD, BET analysis and DRS are tabulated in Table 5-63.

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Figure 5-64: (a, b) SEM images of the surface morphology and cross-section, respectively, (c) three-dimensional AFM image of the surface morphology, (d) XRD patterns, (e) N2 adsorption isotherms, (f) Tauc plots, and (g) PL spectra of the un-sensitized TiO2 electrodes, (h) UV-Vis absorbance and transmittance spectra of the dye-sensitized TiO2 electrodes.

Surface analysis by means of SEM revealed that the TiO2 films, both for the glass and PET working electrodes, possessed a uniform and compact microstructure, with numerous nanopores. The TiO2 nanoparticles were dispersed in all cases homogeneously over a large area, without the development of irregular aggregations. The fabrication of TiO2 electrodes composed of uniform particles and aggregates with regular shape is one of the key factors leading to the development of high-efficiency DSSCs [589]. The TiO2 film thickness was determined at about 10 μm for the glass working electrode and at about 11 μm for the PET working electrode.

AFM gave additional evidence for the surface morphology of the two compared TiO2 electrodes. The surface root-mean-square roughness and the surface area of the glass working electrode were found lower compared to the corresponding ones of the PET working electrode. This is probably attributed to the increased compactness of the glass working electrode that arose from the sintering of the TiO2 nanoparticles. The glass working electrode also displayed a more uniform surface compared to the PET one; lower Z-range was determined for the glass working electrode compared to the PET working electrode. The roughness of the anode semiconductor is an essential parameter to be considered. Generally, the higher the surface roughness, the greater the real surface extension and chemisorption of the dye-complex molecules onto the semiconductor surface, and thus, the higher the energy conversion efficiency attained by the photo-electrochemical cell [244].

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The N2 adsorption/desorption isothermal tests were carried out with the aim of determining the differences in the specific surface area and porosity between the two types of working electrodes. According to IUPAC classification, the isotherms showed in both types of working electrodes a typical shape of type IV curve and their narrow hysteresis loops exhibited a typical H3 pattern. This indicates that the powder was aggregated to plate-like particles, creating slit-shaped mesopores [429,466]. The size of dye molecules and triiodide ions is approximately 1–2 nm, so mesoscopic semiconductors, like the present one, are considered to be the appropriate materials that satisfy high dye adsorption and increased triiodide diffusion kinetics. In more detail, both working electrodes demonstrated the same specific surface area, while the glass working electrode possessed a more compact structure, attributed to the formation of a strong chemical bonding between TiO2 nanoparticles, by dehydration of hydroxides on their surface during sintering [242]. The electronic band structure of the anode semiconductor of DSSCs also has to be considered since the exact position of the band edges plays a decisive role for the energy conversion efficiency of the solar cells [579]. In the present investigation, the energy bandgap of the glass and PET TiO2 electrodes was estimated from their DRS spectra, according to the Tauc’s formula. The values of the bandgap of both TiO2 electrodes were quite similar and in agreement with the reported values found in the literature [590]. The small difference that was observed in the bandgap of the two TiO2 electrodes is probably attributed to the small increase of the rutile content in the glass TiO2 electrode compared to the PET one, according to XRD results, while the dependence of the calculated bandgaps on the semiconductor film thickness also has to be taken into account [580]. PL spectroscopy was used to deliver information about the optoelectronic characteristics of the two compared working electrodes. In the present investigation, the emission peak at 398 nm corresponds to the direct band emission of TiO2, whereas the emission peaks at longer wavelengths associate with the indirect recombination through defect and surface-mediated recombination centers [467]. When the PL spectra of the glass TiO2 electrode is compared to the PET one, an increase of the emission peak at 398 nm and a decrease in the intensity of the emission peaks ranging from 430 to 500 nm are observed. The aforementioned observations are possibly attributed to the small increase of the rutile content and a decrease of the defect-states, respectively, in the glass TiO2 electrode compared to the PET one, due to the high-temperature annealing procedure of TiO2 taking place during the fabrication of the glass working electrode. The variation of the trap density and the surface states in the TiO2 films due to their annealing at different temperatures is discussed by many scientists working with DSSCs [459,460].

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Finally, the absorbance and transmittance spectra of the glass and PET dye-sensitized working electrodes were determined. In both cases, the TiO2 film thickness was set at about 2 μm. The two working electrodes presented small differences in their optical characteristics, which concerned the spectra of 450–600 nm. The glass dye-sensitized working electrode possessed slightly higher absorbance and lower transmittance compared to the PET one in the aforementioned spectral range. This was attributed to the higher light absorption from dye molecules since one of the broad visible bands of N719 dye is at 535 nm [591]. Table 5-63: Parameters obtained by SEM, AFM, XRD, BET analysis, and DRS for the working electrodes.

Parameter Working electrode glass PET Film thickness (μm) a 10 11 Root-mean-square roughness (nm) b 20 33 Surface area (μm2) b 31.8 38.8 Z-range (nm) b 171 297 c Anatase 77 82 TiO2 phase composition (wt%) Rutile 23 18 Anatase 25 22 Mean primary crystallite size (nm) c Rutile 41 37 Anatase 0.006 0.007 Crystal lattice distortion c Rutile 0.004 0.004 Specific surface area (m2g-1) d 49.9 49.9 Average pore diameter (nm) d 18.7 21.3 Total pore volume (cm3g-1) d 0.260 0.314 Bandgap (eV) e 2.99 3.02 determined by a SEM, b AFM, c XRD, d BET, e DRS

High catalytic activity of counter electrodes is an important prerequisite for achieving high energy conversion efficiency to DSSCs. A robust counter electrode must be catalytically active to accelerate − − − I3 + 2e → 3I reaction and efficiently reduce the overpotential. Reduction of triiodides at counter electrode depends significantly on the cathode material type and its surface morphology [141]. In the present investigation, the surface morphology of the MWCNTs and G films, both for the glass and PET counter electrodes, was determined using SEM and AFM, while the catalytic activity of the counter electrodes was investigated by CV measurements. Figure 5-65 displays the SEM images, the three-dimensional AFM images, and the cyclic voltammograms of the MWCNTs-based and G-based counter electrodes, while the parameters obtained by AFM and CV are tabulated in Table 5-64.

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Figure 5-65: (a, c, e) SEM images, three-dimensional AFM image, and cyclic voltammograms, respectively, for the MWCNTs-based counter electrodes (b, d, f) SEM images, three-dimensional AFM image, and cyclic voltammograms, respectively, for the G-based counter electrodes. Generally, by increasing the active surface area of the cathode, an increase in triiodide reduction kinetics on the counter electrode is observed, minimizing the energy loss. In the present investigation, by comparing the SEM images of the corresponding glass and PET counter electrodes (SEM images of the PET counter electrodes are not shown), no obvious differences were observed. However, from AFM analysis, it was found that the surface roughness and the active surface area of the glass carbon- based counter electrodes were slightly increased compared to the corresponding ones of the PET carbon-based counter electrodes. On the other hand, the Z-range of the former was found slightly reduced compared to the corresponding values of the latter. Based on the literature, by increasing the annealing temperature during the fabrication of carbon-based counter electrodes, an increase in surface roughness is observed, attributed to the increase of the sp2 domains and the ordered sp2 clusters content at the film surface [592]. Annealing of carbon-based counter electrodes contributes to a structural change from amorphous to nanocrystalline clusters. Concerning the cyclic voltammograms, related to the counter electrodes, the two pairs of redox − − − − − peaks that were observed are assigned to the redox reactions I3 + 2e ⇄ 3I and 3I2 + 2e ⇄ 2I3 [558]. These peaks were observed on all the types of counter electrodes, except for the PET G-based counter electrode, indicating its low catalytic activity. In the anodic sweep, iodide is oxidized sequentially to triiodide (peak I) and then to iodine (peak II), while when the potential scan is reversed, iodine is reduced first to triiodide (peak II′) and then to iodide (peak I′). From the CV profiles of the counter electrodes, their electrocatalytic activity was compared in terms of cathodic peak current density (JCP), cathodic peak potential (ECP), and peak-to-peak separation (EPP) of the potential difference between the anodic and cathodic peaks of the iodide-triiodide couple. JCP and EPP portray the electrode reaction kinetics, and their values are proportional and inversely proportional, respectively, to the electrocatalytic activity of the electrode [559]. A comparison of the electrocatalytic activity of the corresponding glass and PET counter electrodes shows that the latter demonstrate much lower performance, which was attributed to their higher surface resistivity and their obligatory low-temperature manufacturing process [334].

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Table 5-64: Parameters obtained by AFM and CV for the different types of counter electrodes.

Parameter MWCNTs-based G-based counter electrode counter electrode glass PET glass PET Root-mean-square roughness (nm) a 421 392 347 318 Surface area (μm2) a 45.2 42.5 34.6 31.9 Z-range (nm) a 1937 1950 1786 1793 Anodic peak current density (mA/cm2) b 2.29 1.90 1.87 - Cathodic peak current density (mA/cm2) b -2.05 -1.77 -1.14 - Anodic peak potential (V) b 0.484 0.638 0.589 - Cathodic peak potential (V) b -0.134 -0.310 -0.153 - Peak-to-peak separation (V) b 0.618 0.948 0.742 - determined by a AFM, b CV

Figure 5-66 shows the J–V characteristic curves of the DSSCs employing the different types of carbon-based counter electrodes. The electrical characteristics of the solar cells and the parameters obtained by the one-diode model equivalent circuit analysis are tabulated in Table 5-65 and Table 5- 66, respectively. The names assigned indicate the transparent conductive substrate used to the dye- sensitized working electrode and the counter electrode; for the counter electrode, the cathode material is also listed.

Figure 5-66: Current density–Voltage characteristic curves of the dye-sensitized solar cells employing (a) the MWCNTs-based counter electrodes and (b) the G-based counter electrodes.

By comparing the characteristics of the DSSCs, it is demonstrated that the FTO-coated glass-based DSSCs exhibited higher performance than the corresponding of the ITO-coated PET-based DSSCs in all cases. More specifically, the replacement of the FTO glass substrate of the working electrodes with ITO PET led to an average decrease in JSC, VOC, FF, and ECE by almost 26%, 1%, 10%, and 35%, respectively, for both MWCNTs-based and G-based DSSCs. On the other hand, the respective substrate alteration in the counter electrode caused less decrease in solar cells performance. JSC and VOC decreased in average by lower than 2%, while FF and ECE decreased by almost 4% and 6%, respectively. Namely, the all-plastic flexible DSSCs showed an average decrease in JSC by 27%, in VOC by 2%, in FF by 14%, and subsequently in ECE by almost 40% compared to the corresponding values of the conventional rigid DSSCs.

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Table 5-65: Electrical characteristics of the dye-sensitized solar cells employing the different types of carbon-based counter electrodes.

Solar cell JSC VOC FF ECE (mA/cm2) (mV) (–) (%) Glass/Glass MWCNTs 7.14±0.13 725±3 0.57±0.01 2.95±0.02 Glass/PET MWCNTs 6.97±0.11 723±5 0.55±0.01 2.77±0.03 PET/Glass MWCNTs 5.22±0.14 715±4 0.52±0.01 1.93±0.03 PET/PET MWCNTs 5.19±0.09 712±6 0.50±0.01 1.84±0.04 Glass/Glass G 7.29±0.07 721±3 0.57±0.01 2.98±0.02 Glass/PET G 7.24±0.06 717±4 0.54±0.01 2.78±0.02 PET/Glass G 5.37±0.04 712±3 0.51±0.01 1.94±0.03 PET/PET G 5.30±0.04 708±3 0.48±0.01 1.80±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. By comparing the results corresponding to FTO-coated glass-based DSSCs with those of ITO-coated PET-based DSSCs, several differences were observed. More specifically, the replacement of the FTO glass substrate of the working electrodes with ITO PET, both for the MWCNTs-based and G-based DSSCs, led to an average decrease in JL and rSH by 26% and 27%, respectively, and to an average increase in n, J0, and rS by 6%, 17%, and 52%, respectively. On the other hand, the respective substrate alteration in counter electrode caused an average decrease in JL and rSH by 1% and 15%, respectively, and to an average increase in n, J0, and rS by 1%, 11%, and 46%, respectively. Namely, the all-plastic flexible DSSCs showed on average 26% lower JL, 7% higher n, 27% higher J0, 104% higher rS, and 37% lower rSH compared to the corresponding values of the conventional rigid DSSCs. Table 5-66: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells employing the different types of carbon-based counter electrodes.

Solar cell JL n J0 rS rSH (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) Glass/Glass MWCNTs 7.18±0.13 3.31±0.04 1421±206 5.25±0.25 815±54 Glass/PET MWCNTs 7.04±0.12 3.33±0.06 1499±345 7.50±0.25 736±42 PET/Glass MWCNTs 5.29±0.15 3.50±0.02 1631±155 7.83±0.14 609±46 PET/PET MWCNTs 5.29±0.09 3.52±0.03 1728±88 10.50±0.25 538±22 Glass/Glass G 7.34±0.07 3.31±0.03 1504±171 5.17±0.14 739±34 Glass/PET G 7.33±0.06 3.37±0.03 1758±79 7.75±0.25 596±25 PET/Glass G 5.45±0.04 3.53±0.03 1797±3 8.00±0.25 527±20 PET/PET G 5.42±0.06 3.59±0.04 2005±177 10.75±0.25 445±33

From the above-presented analysis, it is demonstrated that in an effort to fabricate all-plastic flexible DSSCs, the replacement of the glass dye-sensitized working electrode substrate by the thin polymer one causes the maximum reduction in the solar cell performance. This observation was not surprising, owing to the high surface resistivity of the polymer substrates compared to the glass ones, as well as to the obligatory low-temperature manufacturing process of the polymer working electrodes, which led to unsuitable characteristics of the anode semiconductor. According to the literature, electron diffusion length is much shorter for the low-temperature processed TiO2 films compared to the high-temperature ones [459,593]. This is due to the decreased compactness of the TiO2 semiconductor, different TiO2 crystal structure, and increased charge recombination rate at TiO2/electrolyte interface of the former compared to the latter, which were also observed in the present investigation. The decreased JL that was attained by the DSSCs employing the polymer working electrodes compared to the corresponding attained by the DSSCs employing the glass

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working electrodes was attributed to the lower light absorption by the PET dye-sensitized working electrode compared to the corresponding glass one. On the other hand, the respective substrate alteration in the counter electrode caused less effect on solar cells performance. In this case, the decrease in the performance of the solar cells employing the polymer counter electrodes was attributed to the higher surface resistivity of polymer counter electrodes compared to the glass counter electrodes, as well as to the decreased active surface area and catalytic activity for the triiodides reduction of the former compared to the latter. The effect of annealing temperature during the fabrication of carbon-based counter electrodes for DSSCs on solar cells electrical characteristics is investigated in detail by Park et al. [592]. ➢ Scaling up effect on dye-sensitized solar cells performance Figure 5-67 shows the J–V characteristic curves of the different active areas DSSCs employing FTO glass or ITO PET as working electrode and counter electrode substrates, and MWCNTs or G as a cathode. The electrical characteristics of the solar cells and the parameters obtained by the one- diode model equivalent circuit analysis are tabulated in Table 5-67 and Table 5-68, respectively. The names assigned indicate the transparent conductive substrate used to the dye-sensitized working electrode and the counter electrode; for the counter electrode, the cathode material is also listed.

Figure 5-67: Current density–Voltage characteristic curves of the different active areas dye-sensitized solar cells employing (a) the MWCNTs-based counter electrodes and (b) the G-based counter electrodes. By comparing the characteristics of the DSSCs, it is demonstrated that the increase in the active area of the solar cells leads to a decrease in their JSC, VOC, FF, and subsequently in their ECE, irrespective of the applied electrode substrate and cathode material type. However, the rate of the aforementioned decrease in the values of the PV parameters was found different when the conventional rigid DSSCs and the all-plastic flexible DSSCs were compared. More specifically, the increase in active area from 0.25 cm2 to 1 cm2 of the conventional rigid DSSCs led to an average decrease in their JSC, VOC, and FF by lower than 2%, and subsequently in their ECE by lower than 4%. On the other hand, the corresponding increase in the active area of the all-plastic flexible DSSCs caused a much higher decrease in their performance. In this case, their JSC, VOC, FF, and subsequently their ECE decreased on average by 16%, 1%, 41%, and 51%, respectively.

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Table 5-67: Electrical characteristics of the different active areas dye-sensitized solar cells.

Solar cell Active area JSC VOC FF ECE (cm2) (mA/cm2) (mV) (–) (%) Glass/Glass MWCNTs 0.25 7.14±0.13 725±3 0.57±0.01 2.95±0.02 Glass/Glass MWCNTs 0.49 7.06±0.07 723±3 0.57±0.01 2.89±0.03 Glass/Glass MWCNTs 1.00 6.99±0.12 723±2 0.56±0.01 2.83±0.01 PET/PET MWCNTs 0.25 5.19±0.09 712±6 0.50±0.01 1.84±0.04 PET/PET MWCNTs 0.49 4.91±0.12 708±5 0.42±0.01 1.45±0.03 PET/PET MWCNTs 1.00 4.42±0.07 704±4 0.29±0.01 0.89±0.03 Glass/Glass G 0.25 7.29±0.07 721±3 0.57±0.01 2.98±0.02 Glass/Glass G 0.49 7.27±0.07 721±3 0.56±0.01 2.92±0.02 Glass/Glass G 1.00 7.26±0.08 721±3 0.55±0.01 2.86±0.01 PET/PET G 0.25 5.30±0.04 708±3 0.48±0.01 1.80±0.03 PET/PET G 0.49 4.99±0.04 704±2 0.40±0.01 1.40±0.03 PET/PET G 1.00 4.38±0.05 700±3 0.29±0.01 0.88±0.03

The application of the one-diode equivalent circuit model on the experimental J–V characteristic curves of the DSSCs contributed to a better understanding of the above-mentioned results. Concerning the conventional rigid DSSCs, the increase in their active area from 0.25 cm2 to 1 cm2 led to on average decrease in JL and rSH by 1% and 5%, respectively, while n, J0, and rS increased in average by 1%, 9%, and 25%, respectively. On the other hand, the respective increase in the active area of the all-plastic flexible DSSCs caused an average increase in JL, n, and J0 by 2%, 1%, and 18%, respectively, while rS increased by a factor 10 and rSH decreased by 8%. Table 5-68: Parameters obtained by the one-diode model equivalent circuit analysis of the dye-sensitized solar cells for different active areas.

Solar cell Active area JL n J0 rS rSH (cm2) (mA/cm2) (–) (nA/cm2) (Ω·cm2) (Ω·cm2) Glass/Glass MWCNTs 0.25 7.18±0.13 3.31±0.04 1421±206 5.25±0.25 815±54 Glass/Glass MWCNTs 0.49 7.10±0.07 3.32±0.04 1503±173 5.50±0.25 843±49 Glass/Glass MWCNTs 1.00 7.04±0.13 3.34±0.07 1550±231 5.75±0.25 784±78 PET/PET MWCNTs 0.25 5.29±0.09 3.52±0.03 1728±88 10.50±0.25 538±22 PET/PET MWCNTs 0.49 5.30±0.14 3.55±0.01 1880±37 40.50±0.25 512±12 PET/PET MWCNTs 1.00 5.37±0.11 3.57±0.05 2066±296 105.50±0.50 496±34 Glass/Glass G 0.25 7.34±0.07 3.31±0.03 1504±171 5.17±0.14 739±34 Glass/Glass G 0.49 7.33±0.07 3.32±0.04 1550±192 5.75±0.25 712±31 Glass/Glass G 1.00 7.33±0.08 3.35±0.04 1649±160 7.17±0.29 699±38 PET/PET G 0.25 5.42±0.06 3.59±0.04 2005±177 10.75±0.25 445±33 PET/PET G 0.49 5.48±0.05 3.61±0.04 2180±203 41.08±0.38 420±25 PET/PET G 1.00 5.52±0.08 3.62±0.05 2325±276 105.50±0.50 405±25

The increase in the active area of the solar cells led, as expected, to a decrease in their energy conversion efficiency. This was attributed to the increased path length of electrons into the solar cells before collection, which are eventually recombined with the hole-conducting species of the electrolyte [594]. The aforementioned is shown by the increase in n, J0, and rS, and the decrease in rSH of the solar cells. The scaling up effect was much more pronounced in the all-plastic flexible DSSCs, where the rS increased at a much higher rate, finally reaching values that were comparable to the corresponding ones of rSH. This was the main reason for the high decrease in FF, and subsequently in ECE of the all-plastic flexible DSSCs by their scaling up. Similar results were found in the literature, with the PV parameters of DSSCs to be highly dependent on rS during their scaling up [595].

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➢ Conclusions The key limiting factors affecting the large-sized flexible Pt-free DSSCs performance were investigated in detail, using a combined experimental and equivalent circuit analysis. The investigation showed that in the effort to fabricate all-plastic flexible DSSCs, the replacement of the glass working electrode substrate by the polymer one causes a much higher decrease in solar cells performance (ECE decreased by 35%) than the corresponding alteration in counter electrode substrate (ECE decreased by 6%). In the present investigation, the all-plastic flexible DSSCs showed on average a 40% lower performance than the corresponding of the conventional rigid DSSCs. This was mainly attributed to the obligatory low-temperature manufacturing process of the polymer working electrodes, which led to unsuitable characteristics of the anode semiconductor. Concerning the scaling up of DSSCs from the active area of 0.25 cm2 to 1 cm2, the investigation showed that the scaling up effect was much more pronounced in the flexible devices (ECE decreased by 50%) compared to the corresponding in the rigid devices (ECE decreased by 4%). In the former case, the high decrease in the solar cells performance was attributed to the high surface resistivity of the polymer substrates. 5.5.3. Mechanical, Dynamic Mechanical, and Viscoelastic Behavior of Flexible Quasi-Solid State Dye-Sensitized Solar Cells Under Three- Point Bending DSSCs are not only required to be efficient, but durable too, exhibiting chemical, thermal, and mechanical stability [150]. As a low-cost PV technology, they may also be applied to various engineering constructions in the future, presupposing high mechanical strength to different external loads [36,230,596]. Thus, their structural integrity is crucial for their reliability for various applications and their mechanical, dynamic mechanical, and viscoelastic behavior have to be studied thoroughly. In the present investigation sandwich-like structured symmetrical dummy cells, simulating the structure of a flexible QSS-DSSC, were fabricated using PET flexible plates as skins and a thermoplastic polymer blend electrolyte as the core material. The structure of PET was amorphous, leading to high transparency, needed for solar cells application. The polymer blend electrolyte was prepared in the laboratory, while its behavior has already been investigated in previous studies for DSSCs application, showing the appropriate thermal, structural, electrical, and optical characteristics. The present work deals with three-point bending at different strain rates, oscillation frequencies, and relaxation experiments on the sandwich-like structured symmetrical dummy cells employing the polymer blend electrolyte; different wt% ratios of the polymers used in the polymer blend as gelation agent of the liquid state electrolyte were tested. Finally, analytical modeling was applied to the experimental results to determine the rheological characteristics of the systems. ➢ Mechanical behavior of the dummy cells Figure 5-68 shows a representative stress–strain curve of the sandwich-like structured symmetrical dummy cells under three-point bending. All experiments were stopped at strain 0.05, according to ASTM D0790. In general, at low strains, there was a linear increase in stress with strain, followed by a region of material yielding. The strain in material yielding was high and almost the same for all the dummy cells and the investigated strain rates, demonstrating the ability of the systems to undergo large elastic deformations, needed for the development of flexible solar cells.

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Figure 5-68: Representative stress–strain curve of the dummy cells under three-point bending. The shape of stress–strain curve of a polymeric material strongly depends on strain rate [597–599]. Depending on the strain rate, the relaxation time that the material has to respond to the external loads alters. Looking at the entire range of the strain rate, a typical sigmoidal stress–strain rate curve is obtained. At a low strain rate, the relaxation time is large, thus the polymer chains have the time to change their position, a process in which the stresses in the material remain low. At a higher strain rate, the stresses developed in the material are influenced greatly by the strain rate, with their values increasing with strain rate. For high strain rates, the relaxation time that the material has to respond to the external loads is quite low, the movement of polymer chains is limited, and thus high stresses are developed in the material. The strain rate strongly affects the stiffness of a material, which is strongly affected by inertial phenomena. On the other hand, the maximum strength of a material is mainly dependent on internal discontinuities and imperfections in the material. Figure 5-69 and Figure 5-70 show the variation of the bending modulus and the yield stress, respectively, of the sandwich-like structured symmetrical dummy cells employing the different polymer blend electrolytes as a function of the strain rate. As it is demonstrated, the bending modulus of the dummy cells employing the electrolyte PVP:PEG 100:0 strongly depended on the strain rate, with its value increasing with strain rate, while the variation of the yield stress with strain rate was almost negligible. PVP is a thermoplastic polymer, which was on its glassy state at the experiment o temperature (Tg ≈ 110 C) [59]. The presence of the side groups on PVP chains greatly hampers the movement of chains due to the limitations of rotation of the bonds formed therein, making them stiff [494]. Generally, the greater the size of the side groups of the polymer chains, the more rigid the polymer chains are. Thus, inertia phenomena are more pronounced for these polymers. On the other hand, the mechanical behavior of the dummy cells employing the electrolyte PVP:PEG 0:100 was different. As it is demonstrated, in this case, the bending modulus and the yield stress did not present any significant variation with strain rate. PEG was on its rubbery state at the experiment temperature o (Tg ≈ -63 C) [59]. PEG chains are characterized by high flexibility, due to the nature of the chemical bonds that form them, which allow their easy rotation [494]. The flexibility of PEG chains was found high even for the high strain rates, leading to a constant low bending modulus for the entire range of the investigated strain rates. The dummy cells employing the polymer blend electrolytes showed an intermediate behavior to the dummy cells employing the polymer electrolytes PVP:PEG 100:0 and PVP:PEG 0:100 regarding the variation of their bending modulus with strain rate. The effect of strain rate on the mechanical behavior of the dummy cells was less pronounced as the PEG content in the polymer blend increased. At high strain rates, the bending modulus of the dummy cells employing the electrolyte PVP:PEG 0:100 was found almost half compared to the corresponding of the dummy cells employing the electrolyte PVP:PEG 100:0. Concerning the yield stress, its value did not present

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any significant variation with strain rate for all the dummy cells. Only the yield stress of the dummy cells employing the polymer blend electrolytes was found slightly decreased for low strain rates compared to the corresponding ones of the dummy cells employing the polymer electrolytes PVP:PEG 100:0 and PVP:PEG 0:100. This is possibly happening due to the immiscibility of the polymer blend, according to previous studies. Immiscible polymer blends usually show poor mechanical characteristics due to the poor interfacial adhesion of the polymeric components, arising from the lack of physical and chemical interaction across the phase boundaries [600,601]. Thus, the load transfer between the polymeric components decreases, while premature failure of the material under stress can take place as a result of cracks initiation and propagation at stress concentration points. In immiscible polymer blends, it is considered that there is an increased number of stress concentration points caused by the development of additional imperfections and discontinuities in these material compared to single-component materials [600,601]. Generally, the achievement of a miscible polymer blend is difficult, especially in its intermediate compositions, and is directly dependent on the compatibility of the polymers, their molecular weight, and the manufacturing procedure [602]. Another factor that could have led to the decreased bending modulus and yield stress for the dummy cells employing the polymer blend electrolytes is the development of the PEG- potassium crystalline complex, according to previous studies. Due to Lewis Base-Acid interaction, potassium cation clusters were developed on PEG chains, which could be considered as imperfections in the material and a reason for an increase in the stress concentration factor. This phenomenon was more pronounced in the intermediate compositions of the polymer blend. For high strain rates, all the dummy cells presented almost the same yield stress.

Figure 5-69: Variation of the bending modulus as a function of the strain rate for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material.

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Figure 5-70: Variation of the yield stress as a function of the strain rate for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material.

Figure 5-71 shows the variation of the bending modulus and the yield stress of the dummy cells under three-point bending as a function of the strain rate and the polymer blend electrolyte composition.

Figure 5-71: Variation of (a) bending modulus and (b) yield stress of the dummy cells under three-point bending as a function of the strain rate and the polymer blend electrolyte composition.

Figure 5-72 shows the variation of the relaxation time and the viscosity of the dummy cells under three-point bending as a function of the strain rate and the polymer blend electrolyte composition, derived from the application of the Maxwell model. As it was expected, the relaxation time presented an intense decrease with the increase of strain rate for all the dummy cells. A similar variation was observed for the viscosity of all the dummy cells as a function of the strain rate. According to the Maxwell model, the relaxation time and the viscosity of a material vary proportionately [457].

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Figure 5-72: Variation of (a) relaxation time and (b) viscosity of the dummy cells under three-point bending as a function of the strain rate and the polymer blend electrolyte composition. ➢ Dynamic mechanical behavior of the dummy cells The variation of storage modulus, loss modulus, and tan delta of a material as a function of its oscillation frequency is attributed to the different time provided to the material to respond to the externally applied deformation [458]. For polymeric materials at low oscillation frequencies, the polymer chains have enough time to respond to the externally applied deformation. Thus, relaxation to a lower energy state takes place through the movement of the entangled polymer chains. As a result, the storage modulus of the polymer attains low values. For higher oscillation frequencies, the time provided to the polymer to relax is less, thus, higher stresses are developed in the material and the storage modulus increases. At high oscillation frequencies, the time provided for movement of the polymer chains is very low and the polymer chains do not present any particular movement, resulting in a high storage modulus. According to the literature, when looking at the entire range of oscillation frequencies, a typical sigmoidal-shaped storage modulus-oscillation frequency curve is obtained [457]. On the other hand, the loss modulus of a polymeric material presents a bell-shaped curve as a function of its oscillation frequency [457]. At low oscillation frequencies, where the material has the time to respond to the externally applied deformation, the dissipation energy increases with the increase in oscillation frequency since the energy provided to the material and the movement of polymer chains increase. Thus, the loss modulus of the polymer increases. The increase in loss modulus of the polymer continues up to its glassy regime. After this point, the loss modulus decreases since the time that the material has to respond to the externally applied deformation is quite low, causing it to behave more elastically. The tan delta-oscillation frequency curve is also bell- shaped since it is determined as the ratio of loss modulus/storage modulus. Concerning the viscosity of a polymer, there is a decrease of its values as a function of its oscillation frequency. This is happening because with the increase of its oscillation frequency a decrease in the movement of the polymer chains and thus in the ratio of shear stresses/strain rate take place. Figure 5-73 shows the variation of the storage modulus, the loss modulus, the tan delta, and the complex viscosity of the sandwich-like structured symmetrical dummy cells under three-point bending as a function of their oscillation frequency.

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Figure 5-73: Variation of (a) storage modulus, (b) loss modulus, (c) tan delta, and (d) complex viscosity of the dummy cells under three-point bending as a function of the oscillation frequency. DMA showed that the dynamic mechanical behavior of the dummy cells is greatly dependent on the polymer blend electrolyte composition. In particular, the usage of PVP as a gelation agent of the liquid state electrolyte resulted in the highest values of storage modulus. In this case, the storage modulus was greatly dependent on the oscillation frequency. Its value showed an increase even for low oscillation frequencies of the material. The corresponding variation of the loss modulus and tan delta was small for the same material. PVP is a thermoplastic polymer, which was on its glassy state o at the experiment temperature (Tg ≈ 110 C) [59]. The presence of the side groups on PVP chains greatly hampers the movement of chains due to the limitations of rotation of the bonds formed therein, making them stiff [494]. Thus, inertia phenomena were pronounced for the dummy cells employing the polymer electrolyte PVP:PEG 100:0. Conversely, the usage of PEG as gelation agent of the liquid state electrolyte resulted in much lower values of the storage modulus. In this case, the variation of storage modulus was quite small and related mainly to the high oscillation frequency region. The corresponding variation of loss modulus and tan delta was great for the same material. PEG is a o thermoplastic polymer, which was on its rubbery state at the experiment temperature (Tg ≈ -63 C) [59]. PEG chains are characterized by high flexibility, due to the nature of the chemical bonds that form them, which allow their easy rotation [494]. Thus, inertia phenomena for the dummy cells employing the polymer electrolyte PVP:PEG 0:100 were much less pronounced compared to the corresponding ones employing the polymer electrolyte PVP:PEG 100:0. Concerning the dummy cells employing the polymer blend electrolytes, their dynamic mechanical behavior was intermediate to these of the dummy cells employing the polymer electrolyte PVP:PEG 100:0 and PVP:PEG 0:100. The lower values observed for the storage modulus of the dummy cells employing the polymer blend electrolytes at low oscillation frequency region is possibly attributed to the immiscibility of the polymer blend, according to previous studies. Similar observations were made at the mechanical behavior of the dummy cells under three-point bending at low strain rates. Finally, it is worth mentioning that the tan delta varied in all cases at lower values than unity. This demonstrates the

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quasi-solid state nature of the systems employing the polymer blend electrolytes. Concerning the viscosity of the dummy cells, its variation was in all cases a linear decrease with the increase of the oscillation frequency. Figure 5-74 shows the variation of the storage modulus, the loss modulus, the tan delta, and the complex viscosity of the dummy cells under three-point bending as a function of the oscillation frequency and the polymer blend electrolyte composition.

Figure 5-74: Variation of (a) storage modulus, (b) loss modulus, (c) tan delta, and (d) complex viscosity of dummy cells under three-point bending as a function of the oscillation frequency and the polymer blend electrolyte composition. ➢ Viscoelastic behavior of the dummy cells Figure 5-75 shows the stress-time curves for the sandwich-like structured symmetrical dummy cells under three-point bending for different strain levels. The strains were quite low, according to the quasi-static characterization of the dummy cells, in order to investigate the linear viscoelastic behavior of the materials.

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Figure 5-75: Stress-time curves for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending at different strain levels. As expected, the stresses developed in the materials increased with the increase of strain. For each applied strain, the stresses decreased exponentially over time, reaching a saturation point where there was no further change of their values. The slope of the stress-time curves tended to 0 after just 6 min. Another observation is that for the same strain, the stresses developed in the materials decreased as the amount of PEG in the polymer blend electrolyte increased. PEG chains exhibit much higher flexibility than the corresponding ones of PVP, thus the dummy cells employing a high amount of PEG in the electrolyte showed low resistance to the applied strain. Figure 5-76 shows the isochronous curves for five representative time points for the dummy cells employing the different polymer blend electrolytes. As it is observed, the stresses exhibit in all cases a linear increase with the increase of the applied strain, indicating that for the investigated strain levels, the material exhibits a linear viscoelastic behavior.

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Figure 5-76: Isochronous curves for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending. From the slope of the isochronous curves, the relaxation modulus was calculated for each time point for the different dummy cells (see Figure 5-77). The relaxation modulus presented in all cases an exponential decrease over time. Moreover, as it is observed, the relaxation modulus for time zero decreased as the amount of PEG in the polymer blend electrolyte increased. This is happening since the PEG chains present a much lower stiffness than the corresponding ones of PVP. Finally, it is worth mentioning that the values of relaxation modulus for time zero were in absolute agreement with the values of the bending modulus calculated from the mechanical characterization of the dummy cells at the corresponding strain rate.

Figure 5-77: Relaxation modulus of the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending.

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Chapter 5 Results and Discussion

Figure 5-78 shows the variation of the relaxation modulus of the dummy cells under three-point bending as a function of the time and the polymer blend electrolyte composition.

Figure 5-78: Relaxation modulus of the dummy cells under three-point bending as a function of the time and the polymer blend electrolyte composition. For the description and prediction of the viscoelastic behavior of the dummy cells employing the different polymer blend electrolytes under stress relaxation, the RPM model was applied, developed by Professor G.C. Papanicolaou. Figure 5-79 shows the normalized to time zero stress relaxation curves for the different dummy cells, showing the convergence of the experimental with theoretical data derived from the RPM. As it is shown, the deviation of the theoretical from the experimental data was lower than 1.5%.

Figure 5-79: Experimental data and RPM predictions for the normalized stress relaxation curves for the dummy cells employing the polymer blend electrolyte (a) PVP:PEG 100:0, (b) PVP:PEG 80:20, (c) PVP:PEG 60:40, (d) PVP:PEG 40:60, (e) PVP:PEG 20:80, and (f) PVP:PEG 0:100 as core material under three-point bending.

Figure 5-80 shows the relaxation time for the different dummy cells derived from the RPM. As it is demonstrated, the relaxation time increases almost linearly as the amount of PEG in the polymer blend electrolyte increases. This was an expected variation since PEG exhibits a more viscous behavior than the corresponding of PVP. Finally, it is worth mentioning that the relaxation time was lower than the minute for all the dummy cells, showing that all the materials behave high elastically.

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Figure 5-80: Relaxation time derived from the application of the RPM for the dummy cells under three-point bending as a function of the polymer blend electrolyte composition. ➢ Conclusions In conclusion, the mechanical, the dynamic mechanical, and the viscoelastic behavior under three- point bending of sandwich-like structured symmetrical dummy cells, which simulate the structure of flexible QSS-DSSCs, employing different polymer blend electrolytes were investigated. All the dummy cells showed a high elastic behavior needed for the development of flexible solar cells. In all cases, their mechanical, dynamic mechanical, and viscoelastic behavior was greatly dependent on the composition of the polymer blend electrolyte. The present study is a preliminary investigation that could contribute to the novel research topic of fabrication of high mechanical robustness DSSCs.

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Chapter 6 Conclusions and Future Work

Chapter 6: Conclusions and Future Work

The work presented within the scope of the present Ph.D. dissertation revolves around one of the most exciting and blooming 3rd generation PV systems, DSSCs. This PV technology, which now enters the worldwide markets, is being constantly improved and optimized at the laboratory level. The research in the DSSCs field is going parallelly to many different tracks, including the development of novel nanostructures, semiconductors, light absorbing materials, electrolytes/hole transport materials, substrates, and last but not least new theories and methods for DSSCs investigation to shed brighter light on the physical phenomena occurring in these solar cells. However, the wide commercialization of DSSCs is still hampered by a number of issues, concerning their efficiency, stability, cost, and application range, which have not fully been addressed during the past years. The present Ph.D. dissertation was devoted to the improvement of DSSCs technology from all the aforementioned point of views. In the next paragraphs, the most important achievements of the present Ph.D. dissertation are presented, while future work is also suggested. ➢ General achievements ✓ Great enhancement of the energy conversion efficiency of dye-sensitized solar cells An increase of 90% in the ECE of DSSCs was achieved compared to the corresponding of conventional DSSCs. The aforementioned increase is one of the greatest reported in DSSCs technology, while in absolute terms, the achieved ECE is one of the highest reported in DSSCs technology of today. This increase was achieved by the replacement of the reference nanostructured photo-anode with a novel multilayered hybrid nanostructured photo-anode, which presents optimized morphological, electrical, and optical characteristics, without changing the manufacturing processes and cost. ✓ Higher stability in dye-sensitized solar cells, without performance limitations Fabrication of high-efficiency QSS-DSSCs, using novel advanced polymer electrolytes. The performance and stability of the QSS-DSSCs exceed the corresponding ones of conventional DSSCs employing a commercially available high-stability liquid state electrolyte. In this way, the long-term instability issues of solar cells are reduced, keeping the performance at high levels, the cost at low levels, while the prospects for further performance and stability improvements are considered great. ➢ Technological achievements Novel multilayered hybrid nanostructured anode A morphological, electrical, and optical optimized anode was fabricated for the first time, by combining with great success materials that have proven to compensate for disadvantages of a conventional nanostructured titanium dioxide anode of DSSCs. The aforementioned hybrid material is also very promising for application in PSCs, leading to even higher performances than today records. Furthermore, it has to be mentioned that the prepared composite pastes were based on low- cost and simple chemical techniques, possibly even suitable for industrial production of DSSCs. Novel sensitizer A novel sensitizer that demonstrates unique optical characteristics was prepared by combining N719 and RK1 high-performance dyes for the first time, characterized by complementary absorption spectra, achieving a more intense and boarder absorption spectrum and a great enhancement in the performance of DSSCs.

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High-efficiency back-side illuminated dye-sensitized solar cells High-performance back-side illuminated DSSCs were fabricated, using simple and low-cost manufacturing processes, based on highly ordered and mesoporous materials, after their optimization for DSSCs application. The proposed solar cells pave the way for the development of high-efficiency, high-stability, and low-cost flexible DSSCs. Novel advanced polymer electrolyte A novel advanced thermoplastic polymer blend electrolyte based on iodide compounds mixture was prepared and applied to DSSCs. The QSS-DSSCs demonstrate a higher performance and stability to the corresponding ones attained by DSSCs employing a commercially available high-stability liquid state electrolyte. The present electrolyte paves the way for the fabrication of high-efficiency QSS- DSSCs, achieving efficiencies even higher than the records of conventional liquid state DSSCs, when combined with solid organic or inorganic fillers. Studies on the stability of dye-sensitized solar cells A systematic investigation of the stability of conventional DSSCs under extreme ageing conditions was conducted. The present study includes isothermal ageing at 85oC and -25oC, thermal shock cycling between -25oC and 85oC, hydrothermal ageing at 65oC & 85% RH, and reverse biasing at 4xISC, revealing in each case the key factors leading to the degradation of the solar cells. The investigation also includes the accurate prediction of the degradation of solar cells performance after all the accelerating ageing conditions, by the application of the RPM model developed by Papanicolaou et. al. This achievement is considered important in the direction of the fast and accurate determination of the lifetime and reliability of solar cells for various applications. Towards large-sized flexible and lower-cost DSSCs The limiting factors affecting large-sized flexible platinum-free DSSCs performance were demonstrated in an extensive investigation, using a combined experimental and equivalent circuit analysis. This study contributes to the increase of the competitiveness of DSSCs technology in the photovoltaic market for various novel applications. Studies on the mechanical, dynamic mechanical, and viscoelastic behavior of flexible quasi-solid state dye-sensitized solar cells The mechanical, the dynamic mechanical, and the viscoelastic behavior of sandwich-like structured composite materials, which simulate the structure of a flexible quasi-solid state DSSC, were studied through three-point blending experiments at different strain rates, dynamic mechanical analysis experiments at different oscillation frequencies, and relaxation experiments at different strains. These structures were fabricated using materials already examined for their suitability for DSSCs application. The aforementioned research is a preliminary study in the direction of fabrication of high- mechanical strength solar cells, which is considered as a new hot topic in the field of photovoltaics. ➢ Future work ▪ Further optimization of solar cells performance, stability, and cost using “Design of Experiment” method and/or novel materials and structures (perovskites, tandem designs, etc.). ▪ Replacement of the conventional skin materials (glass, PET, metal foils) with advanced high strength composite materials towards the fabrication of high mechanical robustness solar cells. ▪ Transition from R&D to pilot scale and scale-up of the present solar cell technology. ▪ Development of hybrid energy cells based on multifunctional materials.

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After 6 years of study, gaining experience, and research in the DSSCs field, I stay strong in my opinion that the DSSCs technology still has a lot of potential and should be explored further and deeper. During my Ph.D. stay, I witnessed one of the most important shifts in solar cells research: hybrid organic-inorganic light absorber - perovskites - which established a novel low-cost and efficient way for solar-to-electricity conversion. I believe that the last word has not been said yet.

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