IMPERIAL COLLEGE LONDON Transient Optoelectronic Characterisation and Simulation of Perovskite Solar Cells by Philip Calado A thesis submitted in partial fulfillment for the degree of Doctor of Philosophy in the Faculty of Natural Sciences Department of Physics April 2018 2 Declaration of Originality This thesis is a summary of work undertaken in the Department of Physics at Imperial College London between October 2014 and December 2017 under the supervision of Dr. Piers Barnes and Prof. Jenny Nelson. I declare that the work contained herein is my own except where specific reference is made to the contribution of others. Philip Calado April 2018 The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. 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 licence terms of this work. 3 \The best that most of us can hope to achieve in physics is simply to misunderstand at a deeper level." Wolfgang Pauli 4 Abstract Lead halide perovskites are a class of solution-processable semiconductor materials showing great potential for photovoltaic applications. While perovskite solar cell (PSC) efficiencies have escalated rapidly to beyond 22% in recent years, the materials suffer from a number of chemical instabilities and the processes underlying the optoelectronic response of devices are not well understood. This thesis investigates the device physics of PSCs using novel transient optoelectronic measurements combined with device simulation. A one-dimensional numerical drift-diffusion model capable of solving for electrons, holes and a single ionic charge carrier was developed to simulate perovskite devices. The inclusion of a high density of mobile ionic species in the absorber layer is found to have important consequences on both device performance and the interpretation of established measurements. Transient optoelectronic measurements are presented showing that mobile ions are present in architectures of PSC that do not exhibit current-voltage hysteresis. Simulations of p-i-n structured devices indicate that a combination of mobile ions and field-dependent interfacial recombination rates are critical to reproducing hysteresis. Transient ideality factor measurements are used to identify the dominant recombination mechanisms in PSCs. Changes in the perceived ideality factor are correlated to localised recombination, the charge carrier population overlap, and ion density profiles in simulated devices. Simulations are used to assess the validity of a zero-dimensional model applied to small perturbation transient photovoltage (TPV) measurements on perovskite devices. Two analytical models are proposed to explain the different regimes of behaviour in devices with high rates of interfacial recombination. The TPV decay in perovskites is identified as being predominantly a measure of the transport properties of the absorber layer. Together these findings demonstrate the combined power of experimental measurements and simulation to improve our physical understanding of new semiconductor technologies. 5 6 Acknowledgements For their undying faith in the face of certain failure, I would like to thank the following people: First and foremost my brilliant supervisors Piers and Jenny: thank you for rearing me from an engineering graduate who thought he didn't understand anything to a postgraduate physicist who knows he doesn't understand anything. To Piers `Radio Frequency' Barnes, for being an infinite source of creativity and optimism. For always believing that there was a way to solve a problem no matter what the odds. For having an unquenchable scientific curiosity, even if this did mean a few extended `meetings'. For being a genuine human, an individual thinker, and seeing the potential in everyone. In all seriousness though, I should tell you at this point that I really don't have a clue what you're talking about most of the time. To Jenny Nelson for being a brilliant scientist who also cares. For being such an inspiration to everyone in the group and a strong role model to your staff and students. Also for writing such a brilliant and accessible book, on which much of the work here is based. To Brian O'Regan for being a genuine scientist with real integrity. For being direct yet humble and open with it. For your creativity and for coding the most insanely brilliant transient robot ever. To Andrew Telford who helped me through the early days when I still had no idea what a negative photovoltage transient meant. Thanks for the your great humour and professionalism. To Davide Moia for being so utterly clever and such a nice bloke all round with it. I hope one day I will be able to conceptualise things like you, but somehow I doubt it. You will be Prof. Moia in no time. To Mohammed `Moho' Azzouzi who `in that sense' has an amazing scientific mind! Thank you for such engaging scientific discussions and your great enthusiasm. To Ilario Gelmetti for always having a big smile on your face and for coming to visit us. I feel like you are one of us and you will be missed. Your work in simulating Photo-induced Impedance Spectroscopy will be long-remembered. 7 To Dr Xingyuan Shi for making me feel better about being incompetent by falsely claiming to be incompetent yourself. I really appreciate it. To the best Plastic Electronic CDT cohort ever, Cohort 5: To Alex for always wearing a smile, Andika for being strong in adversity, Das for not taking life too seriously, Iain A. for saying it just like it is, Gwen for being a great hippie scientist, Heavy Metal Iain for being hilarious, Jam for being so open, Jason for keeping it `Rohr', Madeleine for sorting us all out, Matt for being a great ski buddy, Nathan for laughing at my bad sense humour, Tony for falling asleep, and Yiren for being a TRPL wizard. To all the members of the Nelson and Barnes groups past and present: Anna, Aurelien, Beth, Drew, Eli, Jarv, Jizhong, Sachetan, Sam, Scot and Will. Thanks for sharing your scientific observations and great humour. To Prof James Durrant for directing me towards Jenny and agreeing to examine this work. To Li, Dan, Pabitra, Seb and everyone else in the Durrant group for chemical and lab coat assistance. To everyone who I shared office H1101 with: Alise, Elysia, Jun, Mike, Michelle, Nicola, and Viktoria. Thanks for creating such a nice working environment. To Martin Neukom for fighting for a better planet in every way imaginable - from solar research, to being an MP for the Green Party, to shutting down nuclear power stations with direct action. You put the rest of us to shame. Also for the great conversations about how to fix the world. I hope we can do that one day. To Matt Carney for your amazing humour and humility. We still owe you a publication and I haven't forgotten. To Trystan Watson, Joel Troughton and all the staff and students at SPECIFIC. To Pablo Docampo and Hongi Hu for your incredible efficiency and professionalism. To Shaun Armstrong and Duncan McLellan for teaching me the fundamentals of mathematics and physics with infinite patience. And to my sponsors, the UK Engineering and Physical Sciences Research Council, for providing me with this unique opportunity. 8 My personal thanks for their unconditional love and support: Most importantly to Abigail Mortimer for being my guiding light. For being my ethical compass and my companion in life. And for putting up with an increasingly scruffy mad-scientist-Bubba throughout the thesis writing process. Also for the heroic prof readinfg. To my soul brother Matt Fairclough for walking side-by-side and sharing so many important experiences with me. It is a great honour to call you my friend. To George Richardson for seeing me through those earlier lab experiments when I was still just a noob. Big shout out to the Imperial College Hackers Ultimate Distruption (ICCHUD) crew- you know who you are! To Oliver Levers for making me laugh so much that it hurt my ribs almost everyday during the MRes year and for all the good times since with the General Farmer's Alliance and ICCHUD. To General Prospero Taroni Junior Waste for being such a stylish and singular human. Only a mind as creative as yours could've invented PEDOT-Spandex my friend. To Squ Ldr Ian `Flash' Mortimer and Jane for being such supportive and inspirational `older people'. The pens were really useful in the end Jane, so thanks. To Steve Baker for your services to the environment and humanity, and your endless positivity! To Jamie Innes for showing me that all the answers are in the plants. To Paul Allen and everyone at the Centre for Alternative Technology for reminding me that the world could be so different. And finally to my family: To Maria Bridges for all your support through my various stages of education and life. I promise to stop being a student now mum. To Paulo, Sophie and the Shrimpmonks for giving me hope in the children of tomorrow. And finally to my Big-bro Carlos for your strength in the face of adversity. Keep on fighting the good fight and never give up! 9 10 Contents Declaration of Originality3 Abstract 5 Acknowledgements7 List of Figures 16 List of Tables 19 Physical Constants 20 Abbreviations 21 Symbols 24 1 Introduction 29 1.1 Abstract..................................... 29 1.2 Motivation................................... 29 1.2.1 The climate crisis............................ 29 1.2.2 The potential for solar energy..................... 30 1.3 Lead halide perovskite solar cells......................