Spectroscopic Studies of the Charge- Transfer State and Device Performance of Hybrid and Organic Solar Cells Flurin David Eisner A thesis submitted in partial fulfilment of the degree of Doctor of Philosophy Imperial College London Department of Physics April 2019 DECLARATION OF ORIGINALITY This thesis describes the work carried out between October 2015 and December 2018 in the Experimental Solid State Physics group of Imperial College London, under the supervision of Prof. Thomas D. Anthopoulos and Prof. Jenny Nelson. The material presented herein is the product of my own work, unless stated. Where information has been derived from other sources, this has been indicated in the thesis. COPYRIGHT DECLATION The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives License (CC BY-NC-ND). Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the license terms of this work. Flurin Eisner London, April 2019 2 Abstract The last few years have witnessed a remarkable explosion in advances in the field of photovoltaics (PV) based on organic semiconductors as the light absorbing material, both in terms of device efficiency and in terms of the physical understanding of the underlying processes. This thesis aims to contribute to this field by examining two different types of solar cells based on light-absorbing organic semiconductors. It consists of two main parts connected through their focus on the role of charge transfer (CT) states on device performance, and more specifically on a focus on effect of CT states on open-circuit voltage losses. The first two results chapters focus on the fabrication and analysis of novel hybrid heterojunction solar cells based on the molecular inorganic semiconductor copper thiocyanate (CuSCN) in conjunction with light absorbing organic semiconductors, which show surprisingly high charge generation efficiencies. In the first results chapter I show that the formation of a hybrid CT-state formed at the interface between CuSCN and organic semiconductors is key to the charge generation process in these cells. In in the second results chapter I present the discovery of a remarkable nanostructured interface formed between CuSCN and PC70BM via solution processing with which it is possible to fabricate semi-transparent devices with power conversion efficiencies (PCEs) exceeding 5%, which is amongst the highest reported efficiencies for hybrid heterojunction solar cells. The final two results chapters focus on the role of CT-states in affecting the voltage losses of bulk-heterojunction (BHJ) solar cells based on non-fullerene acceptors. In the third results chapter I present the device optimization and characterization of a solar cell based on a novel polymer:non-fullerene acceptor combination, and show that it is possible to fabricate solar cells with PCEs exceeding 13%, amongst the highest reported efficiencies at the time. I extend the analysis of the role of the CT-state in achieving this high efficiency in the last results chapter, where I present a new model that helps to explain the observation of very low non-radiative voltage losses in blends with a low driving energy for charge separation through the processes of hybridization of CT and local excited states. 3 Acknowledgements Of course this thesis would not have been possible without the support of so many people, both at work and my personal life, through the whole three and half years of my PhD. Everyone I’ve had the pleasure of working with has shown me unique ways of ‘doing science’ and I’m eternally grateful to all of you for inspiring and helping me. Thank you all: To my supervisors Thomas Anthopoulos and Jenny Nelson, thank you both for being great supervisors that listen and give ideas in equal measure, and for making my PhD a great experience. Thank you Jenny, for your extreme kindness in adopting me as your PhD student in such unexpected circumstances. Thank you also for your constant guidance since then, and for the sharing of your endless knowledge and ideas with me, and for introducing me to Dervla! Thank you Thomas, for offering me the chance to do a PhD at Imperial, even on the basis of my interview from a hotel room in Nay Pyi Taw, Myanmar. I am grateful for your constant stream of ideas and your continued insistence that with enough persistence experiments will almost always work! A special thank you also to Martin Heeney and Zhuping Fei for some great collaborations and for allowing me to use some of your fantastic materials! Thanks to everyone in the AMD group. From the beginning you showed me that PhDs are not only about trying to get as many papers out as possible, but also about having fun and meeting great people. Thank you Sungho, for being the first person to show me how to make a solar cell. Thanks Nilushi, Zan, Dimitra, James, Alex, Kornelius, Dongyoon, Yen, Das, Wentao, Gwen for all the fun times in the lab and out of it, and for helping me whenever I needed it. Thanks Hendrik, for being the most helpful person in the world, and sorry for bugging you with things you definitely did not need to be bugged with. Thanks Juliana, for putting up with me, and for managing the labs so brilliantly (almost) entirely by yourself in the last couple of years. Thanks Jon, for some great collaborations in this first half of my PhD, and thank you Yang for allowing me to continue your work on non-fullerene acceptors! Thanks to everyone in Jenny’s group for kindly taking me in! Thanks Sachetan, for being so helpful, Jun and Xingyuan for being the best professors, and Jason for managing to be so upbeat about SCLC. Thank you Alise for putting fun into fluorination. A special thank you Mohammed, for without you my thesis would only have been half as good. Also for being constantly upbeat, fun and inspiring to be around. 4 Thanks Oli, James, Ben, for not pretending to be interested in what I do, and for making London infinitely more fun. Lastly, a big thank you to my whole family. To Nora, for following in my physics footsteps to become a better scientist than me. To Rahel, for your inspiring drive and ambition. To mum and dad, only your unconditional love and continuing encouragement throughout my life has brought me to this point. I do not know how to thank you enough. To Jess, for always believing in me even when I don’t, for keeping me sane by encouraging me to have normal working hours, and for your inspiring passion for making this world a better place for everyone and everything on it. 5 List of Publications 1. Eisner, F+., Azzouzi, M+., Fei, Z., Hou, X., Anthopoulos, TD., Dennis, TJS., Heeney, M., Nelson, J. “Hybridization of Local Exciton and Charge-Transfer States Reduced Nonradiative Voltage Losses in Organic Solar Cells, Journal of American Chemical Society, 141, 6362-6374 (+equal contribution) (2019). 2. Guangjun, S.,Shahid, M., Fei, Z., Xu, S., Eisner, F., Anthopolous, TD., McLachlan, MA., Heeney, M. “Highly-efficient semi-transparent organic solar cells utilising non-fullerene acceptors with optimised multilayer MoO3/Ag/MoO3 electrodes, Materials Chemistry Frontiers 3.3, 450-55 (2019). 3. Karuthedath, S., Gorenflot, J., Firdaus, Y., Sit, W.‐Y., Eisner, F., Seitkhan, A., Ravva, M. K., Anthopoulos, T. D., Laquai, F. :Charge and Triplet Exciton Generation in Neat PC70BM Films and Hybrid CuSCN:PC70BM Solar Cells”, Advanced Energy Materials, 9, 1802476 (2019). 4. Du, T., Burgess, C. H., Lin, C.‐T., Eisner, F., Kim, J., Xu, S., Kang, H., Durrant, J. R., McLachlan, M. A. “Probing and Controlling Intragrain Crystallinity for Improved Low Temperature–Processed Perovskite Solar Cells”, Advanced Functional Materials, 28, 1803943 (2018). 5. Wijeyasinghe, N., Eisner, F., Tsetseris, L., Lin, Y.‐H., Seitkhan, A., Li, J., Yan, F., Solomeshch, O., Tessler, N., Patsalas, P. A., Anthopoulos, T. D. “p‐Doping of Copper (I) Thiocyanate (CuSCN) Hole‐Transport Layers for High‐Performance Transistors and Organic Solar Cells”, Advanced Functional Materials, 28, 1802055 (2018) 6. Firdaus, Y. , Seitkhan, A. , Eisner, F. , Sit, W. , Kan, Z. , Wehbe, N. , Balawi, A. H., Yengel, E. , Karuthedath, S. , Laquai, F. and Anthopoulos, T. D. “Charge Photogeneration and Recombination in Mesostructured CuSCN‐Nanowire/PC70BM Solar Cells”, Solar RRL, 2: 1800095 (2018). 7. Eisner, F. , Seitkhan, A. , Han, Y. , Khim, D. , Yengel, E. , Kirmani, A. R., Xu, J. , García de Arquer, F. P., Sargent, E. H., Amassian, A. , Fei, Z. , Heeney, M. and Anthopoulos, T. D. “Solution‐Processed In2O3/ZnO Heterojunction Electron Transport Layers for Efficient Organic Bulk Heterojunction and Inorganic Colloidal Quantum‐Dot Solar Cells”, Solar RRL, 2: 1800076 (2018). 8. Wijeyasinghe, N., Eisner, F., Tsetseris, L., Lin, Y.‐H., Seitkhan, A., Li, J., Yan, F., Solomeshch, O., Tessler, N., Patsalas, P. A., Anthopoulos, T. D. “p‐Doping of Copper (I) Thiocyanate (CuSCN) Hole‐Transport Layers for High‐Performance Transistors and Organic Solar Cells”, Advanced Functional Materials, 28, 1802055 (2018). 9. Sit, W.‐Y+., Eisner, F+. D., Lin, Y.‐H., Firdaus, Y., Seitkhan, A., Balawi, A. H., Laquai, F., Burgess, C. H., McLachlan, M. A., Volonakis, G., Giustino, F., Anthopoulos, T. D. “High‐ Efficiency Fullerene Solar Cells Enabled by a Spontaneously Formed Mesostructured CuSCN‐ Nanowire Heterointerface” Advanced Science, 5, 1700980 (+equal contribution) (2018). 10. Fei, Z., Eisner, F. D., Jiao, X., Azzouzi, M., Röhr, J. A., Han, Y., Shahid, M., Chesman, A. S. R., Easton, C. D., McNeill, C.