Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1515 Discovering Hidden Traps in Nickel Oxide Nanoparticles for Dye-Sensitised Photocathodes LUCA D'AMARIO ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-554-9911-2 UPPSALA urn:nbn:se:uu:diva-320187 2017 Dissertation presented at Uppsala University to be publicly examined in Häggsalen, Ångströmlab, Uppsala, Wednesday, 7 June 2017 at 21:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor James Durrant (Faculty of Natural Sciences, Department of Chemistry, Imperial College London). Abstract D'Amario, L. 2017. Discovering Hidden Traps. in Nickel Oxide Nanoparticles for Dye- Sensitised Photocathodes. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1515. 95 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9911-2. The finite nature of fossil fuels and their effect on the global climate, raised the need to find an alternative source of energy. This source should be environment compatible, cheap and abundant. The light coming from the Sun is a promising alternative. To be fruitful, the solar energy needs to be transformed in storable and transportable energy forms like electricityor fuels. Amongst the most studied techniques dye sensitised devices offer the possibility to be designed for both the scopes: solar-to-electricity and solar-to-fuel conversions. In these applications a photocathode and a photoanode, constructed by mesoporous semisconductor films sensitised with dyes, are placed in series with one another.It follows that the photocurrent generated by one electrode should be sustained by the photocurrent produced by the other electrode. At the moment there is a substantial difference between the conversion efficiencies and the photocurrent produced by photoanodes and photocathodes. In this thesis the reasons for this discrepancy are investigated. The main responsible of the bad performance is identified in the semiconductor normally used in photocathodes, Nickel Oxide (NiO). Electrochemical impedance spectroscopy was used to elucidate the electrical properties of mesoporous NiO films. The study revealed that NiO films are able to carry a large enough current to establish that conductivity is not a limiting factor. The recombination reactions were then accused as the cause of the power losses. A time resolved spectroscopic study revealed that NiO can host two kinds of holes. One of these holes is responsible for a fast dye-NiO recombination (100 ns) and the other one for a slow recombination (10 ms). A cell featuring only the slow dye-NiO recombination would possibly reach high efficiency. The characterisation of the species associated with these two holes was performed by density-of-state assisted spectroelectrochemistry. The holes were found to be trapped by Ni2+ and Ni3+ sites located on the NiO surface forming respectively Ni3+ and Ni4+ states. A study by fs and ns transient absorption spectroscopy revealed that Ni3+ sites can trap a hole in subpicosecond time scale and this hole relaxes into a Ni2+ trap in ns timescale. The control of the Ni2+/Ni3+ratio on the NiO surface was found to be crucial for a high cell photovoltage. In the thesis these results are discussed and used to propose an explanation and some solutions to the poor performance of NiO-based dye sensitised cells. Luca D'Amario, Department of Chemistry - Ångström, Physical Chemistry, Box 523, Uppsala University, SE-75120 Uppsala, Sweden. © Luca D'Amario 2017 ISSN 1651-6214 ISBN 978-91-554-9911-2 urn:nbn:se:uu:diva-320187 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-320187) Alla mia famiglia. List of papers This thesis is based on the following papers, which are referred to in the text by their roman numerals. I Tuning of Conductivity and Density of States of NiO Meso- porous Films Used in p-Type DSSCs Luca D’Amario, Gerrit Boschloo, Anders Hagfeldt, and Leif Ham- marström J. Phys. Chem. C, 2014, 118 (34), 19556-19564 II Kinetic Evidence of Two Pathways for Charge Recombina- tion in NiO-Based Dye-Sensitized Solar Cells Luca D’Amario, Liisa J. Antila, Belinda Pettersson Rimgard, Gerrit Boschloo, and Leif Hammarström J. Phys. Chem. Lett., 2015, 6 (5), 779-783 III Chemical and Physical Reduction of High Valence Ni States in Mesoporous NiO Film for Solar Cell Application Luca D’Amario, Roger Jiang, Ute Cappel, Elizabeth A. Gibson, Ger- rit Boschloo, Håkan Rensmo, Licheng Sun, Leif Hammarström and Haining Tian ACS Appl. Mater. Interfaces, accepted. IV Unveiling Hole Trapping and Surface Dynamics of NiO Na- noparticles Luca D’Amario, Jens Föhlinger, Gerrit Boschloo and Leif Hammarström Manuscript ready for submission Reprints were made with permission from the publishers. Papers not in the thesis During the Ph.D. studies the author contributed in other scientific works that are not reported in this thesis. They are listed in the following: A comprehensive comparison of dye-sensitized NiO photocathodes for solar energy conversion, Wood C. J., Summers G. H., Clark C. A., Kaeffer N., Braeutigam M., Carbone L. R., D’Amario L., Fan K., Farre Y., Narbey S., Oswald F., Stevens L. A., Parmenter C. D. J., Fay M. W.,La Torre A., Snap C. E., Dietzek B., Dini D., Hammarström L., Pellegrin Y., Odobel F., Sun L., Artero V., and Gibson, E. A.Phys. Chem. Chem. Phys., 2016, 18, 10727-10738; Supramolecular hemicage Cobalt mediators for dye-sensitized solar cells, M. Freitag, W. Yang, L. A. Fredin, L. D’Amario, K. M. Karlsson, A. Hagfeldt, G. Boschloo, ChemPhysChem 2016, 17, 3845; Ultra long-lived electron-hole separation within water-soluble colloidal ZnO nanocrystals: Prospective applications for solar energy produc- tion, Cieslak A.M., Pavliuk M.V., D’Amario L., Abdellah M., Sokolowski K., Rybinska U., Fernandes D.L.A., Leszczynski M.K., Mamedov F., El-Zhory A.M., Fohlinger J., Budinska A., Wolska-Pietkiewicz M., Hammarstrom L., Lewinski J., Nano Energy, 2016, 30, 187-192. Contribution to papers Paper I: Main responsible for the design of the project, performed all the measurements, the analysis of the data and the interpretation. Wrote the first draft of the manuscript. Paper II: Participated in the design of the project and co-supervised the student that performed the measurements. Analysis of the major part of the data and main responsible for the interpretation. Wrote the first draft of the manuscript. Paper III: Participated in the design of the project, prepared the sam- ple and performed all the spectroscopic measurements and their analysis. Main responsible for the interpretation of the results and wrote the first draft of the manuscript. Paper IV: Main responsible for the design of the project, performed the ns-transient absorption measurements and steady state measure- ments and their analysis. Main responsible for the interpretation of the results. Wrote the first draft of the manuscript. List of abbreviations BG band gap C343 coumarin c343 dye ued in Paper I CB conduction band CE counter electrode DOS Density of States (cm−3eV−1) DSC dye-sensitised solar cell DSFC dye-sensitised solar fuel cell FTO Fluorine-doped Tin Oxide HEC Hydrogen Evolving Catalyst HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital NP nanoparticle OEC Oxygen Evolving Catalyst P1 dye used in Paper III RE reference electrode Ru-NMI ([Ru(dcb)2(NMI-phen)](PF6)2)dye TA transient absorption TAS transient absorption spectroscopy VB valence band WE working electrode C capacitance (in F) E electrochemical potential (in J/mol) E energy (in J or eV) EF Fermi level (in eV) q EF quasi-Fermi level (in eV) e− electron FF fill-factor fkww stretched exponential function h+ hole, electron vacancy 2 Jsc short circuit current density(in A/cm ) R resistance (in Ω) Voc open circuit voltage (in V) β stretching parameter of the fkww τ time constant of the fkww Contents 1 Introduction ............................................................................. 11 1.1 Motivations ..................................................................... 11 1.2 Solar energy conversion challenges ..................................... 13 2 Fundamentals ........................................................................... 23 2.1 Fermi level and Density of states (DOS) ............................. 23 2.2 quasi-Fermi level .............................................................. 25 3 Materials and methods .............................................................. 26 3.1 Materials ......................................................................... 26 3.1.1 Semiconductor: Nickel Oxide ................................. 26 3.1.2 Sensitisers ............................................................ 27 3.2 Techniques ...................................................................... 29 3.2.1 Electrochemical Impedance Spectroscopy (EIS) ....... 29 3.2.2 ns-Transient Absorption Spectroscopy (ns-TAS) ...... 34 4 Main findings and Discussion ..................................................... 38 4.1 DSC performance: n-type vs. p-type .................................. 38 4.2 NiO electrical conductivity ................................................ 40 4.3 The lithium effect on the DOS and conductivity ................. 43 4.4 The recombination issue ................................................... 45 4.5 NiO trap characterisation ................................................. 52 4.6 NiO surface trap dynamics ...............................................