Examining Spectral Converters for Emerging Photovoltaic Technologies Barry McKenna A thesis submitted to the University of Dublin, Trinity College for the degree of Doctor of Philosophy Under the supervision of Dr Rachel C. Evans School of Chemistry February 2018 Trinity College Dublin Declaration I declare that this thesis has not been submitted as an exercise for a degree at this or any other university and it is entirely my own work, except where otherwise cited, referenced, acknowledged or accredited. I agree to deposit this thesis in the University’s open access institutional repository or allow the Library to do so on my behalf, subject to Irish Copyright Legislation and Trinity College Library conditions of use and acknowledgement. .............................................................. Barry McKenna M.Sc. i Acknowledgements This part is far more nerve wracking than I expected but I’ll start first and foremost with the most important, my supervisor Dr Rachel Evans. Rachels dedication to getting the best out of her students and (oft tested) patience are the only reason I have gotten this far. Her guidance with work has allowed me to mature professionally, and her commitment to fostering a friendly atmosphere within the group has meant I am not only leaving with experience but with friends for life. I can’t imagine many other supervisors who would join their new student in a stirring rendition of a Seal song (Kiss From A Rose) on their first group night out, it made me feel right at home. I would like to thank Rachel for allowing me to be part of her group, the last four (and a bit) years have been an amazing experience that without you would not have been possible. I had the pleasure during this PhD of working with amazing collaborators in Swansea University. I would like to thank Dr Trystan Watson for supporting me throughout my four visits to Swansea, he is the nicest man in science and made me feel welcome every time. Without the help of Dr Joel Troughton and Ben Smith my perovskite cells would still be less than 1% efficient, their support has been essential to this body of work. I would like to thank Dr Francesca DeRossi for her help with all the stability measurements and Emily Speller for putting a roof over my head making sure I experienced the Swansea nightlife. This work could not have been done without the support of the staff here in the School of Chemistry. I would like to thank, Dr Noelle Scully for her guidance on outreach, Dr Brendan Twamley for all his help with the XRD, Dr Manuel Ruether and Dr John O’Brien for their support and training with the equipment, and Dr Clive Downing for his help and guidance with electron microscopy. I would like to thank the school administrative alumni Tess and Helen for still being great to gossip with. Special thanks to Patsy Greene whose events are the highlight of the social calendar and for supporting me through my four years demonstrating in the physchem labs. I think we all dream of a team of him. To the past and present members of the Evans group (gang, posse, cult also appropriate) I would like to thank you all for making the last four years so enjoyable. Judith thank you for being insanely supportive, while also just being a bit insane, and thank you for showing me that penguins can do handstands and are not just upside down. Adam, when you’ve helped to put a nappy on a grown man I don’t think any words are needed. Niamh who had me indoctrinated into the group ways within days of arrival, thank you for the vast repository of awful photos of me you now hold at ransom. Ilaria who started at the same time as me and made me look bad by comparison for four years, thank you for all the dating gossip, it makes my issues seem a lot less significant. Camille, the male model of the group, thank you for allowing me to expand my French vocabulary considerably. Elaine, thank you for being the only other group member in four years to make the tea occasionally. Steve, thank you for your frequent deadpan put-downs of myself and other group members (Judith in particular), you keep us on our toes. Special shout-out to Andy who put up with my moaning every night after work for the two and half years we lived together. Actually, on consideration, it was the other way round. Last and by no means least I would like to thank my parents Paula and Martin, whose support over the last four years both emotionally and financially is the only reason I was able to do this in the first place. They have encouraged me throughout my life to do what I want without question of if it is possible or feasible and they have made me the person I am today. ii Abstract Spectral converters are photoluminescent layers which can selectively absorb solar photons of wavelengths which are not efficiently converted to electricity by a photovoltaic (PV) cell, and re- emit these photons at wavelengths more suitable for use. These materials are attractive for complementary application with PV devices due to the versatility afforded to them by the choice of luminophore which can be tuned to the PV cell in question. The ability of these materials to harvest ultraviolet (UV) radiation and emit it at longer, less harmful wavelengths means they can potentially improve the photostability of emerging PV technologies, in particular, perovskite solar cells (PSCs). PSCs are limited currently by their instability to moisture, which when present in conjunction with light and heat can cause irreversible degradation. In order to address the current limitations of PSCs, the first part of this work will investigate a two-fold approach of both polymer encapsulation to improve the resistance of the device to moisture-induced degradation (Chapter 3) and the addition of luminescent down-shifting (LDS) layers to act as UV-filters (Chapter 4). To investigate the requirements of an encapsulant for PSCs, bare perovskite layers were thermally degraded with and without polymer encapsulation. This degradation was monitored by X-ray diffraction, UV/Vis absorption spectroscopy and epi- fluorescence microscopy. It was found that the water vapour transport rate within the polymer had the greatest impact on its ability to act as an encapsulant for PSCs. Poly(methyl methacrylate) (PMMA) performed the best, with stabilities of over 400 h achieved for elevated temperatures (60 °C) and thus, was used as the host material for the LDS layers coated on the PSCs in Chapter 4. LDS coatings applied to CH3NH3PbI3 PSCs using the luminophores Lumogen F Violet 570, polyfluorene and 2-(2-hydroxyphenyl)quinazolin-4(3H)-one exhibited enhanced power conversion efficiencies (PCE) and external quantum efficiencies (EQE). The device photostability, however, was not measurably enhanced upon coating due to device degradation caused by alternative degradation pathways (thermal, moisture) occurring on a faster timescale. The use of nanocrystals for spectral converters was then investigated. The tunable emission properties of CH3NH3PbBr3-xIx mixed-halide perovskite nanocrystals (PNCs) upon dilution was examined in Chapter 5. It was determined by photoluminescence spectroscopy and transmission electron microscopy that a red-shift in PNC emission occurred due to aggregation and uncontrolled growth of the PNCs caused by decreasing capping ligand concentration in solution. This growth led to particles with dimensions ca. 1 µm and the uptake of the excess iodide bound within the capping ligands. This red-shift occurs independently of the makeup of the precursor solution and is directly related to the iodide content and anti-solvent utilised. Finally, due to the issues with the solution phase instability of PNCs, in Chapter 6 CdSe@ZnS/ZnS QDs were investigated as luminophores for large-area luminescent solar concentrators (LSCs). Due to the inhomogeneity of the edge emission observed for these LSCs, a detailed study into the optical characterisation protocols for large planar LSCs was undertaken. It was found that determining the mean edge emission from multiple overlapping intervals is the most accurate characterisation method. The LSCs were coupled with a thin strip DSSC and a photocurrent was obtained. These examples illustrate the versatility of spectral converters and highlight the range of applications they can be used for with emerging PV technologies. The stability and efficiency of PV cells can be improved, the materials required can be minimised and the optical properties of our conversion techniques can be carefully tuned for the application and device architecture required. iii Table of Contents Declaration ............................................................................................................................. i Acknowledgements ............................................................................................................... ii Abstract ................................................................................................................................ iii Abbreviations ....................................................................................................................... ix Symbols ............................................................................................................................... xii Chapter One ‒ Introduction................................................................................................... 1 1.1 Introduction .............................................................................................................................. 2 1.2 Solar