Candidate: 00513629 The synthesis and analysis of polymers and small molecules for use in organic electronics Samuel Jarvis Cryer Submitted in partial fulfilment of the requirements for the degree of: Doctor of Philosophy in Physics Imperial College London July 2017 I Declaration of Originality All work presented within this thesis was performed by myself (unless stated otherwise) within the department of Chemistry and Physics at Imperial College London between October 2013 and June 2017. This was done under the supervision of Professor Iain McCulloch. S. J Cryer June 2017 ii II Copyright Declaration 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. iii III Contributions and Acknowledgements The fabrication of devices discussed in this thesis was undertaken by Andrew Wadsworth at Imperial College London (OPV) and Mark Nikolka at Cambridge University (OFET) and I am very thankful for their time and effort in these endeavours. The journey through this PhD has been a tough one with a personal breakdown in the middle being a monumental mountain to overcome; which I could not have done without the considerable help from my family, friends and supervisor. In particular, I will never be able to thank enough my parents Binny and Mike Cryer, Sophie, Richard and Michael O’Rourke, and Jenny, Robert, Grace and George Ripley who carried me through my darkest days with their unending love and support. I know this journey has been tough for them too! I would also like to thank my supervisor Iain McCulloch who has supported me both academically and personally through this PhD and always been there for me when I’ve needed his help – his guidance and management has taught me so much and has made me a better scientist and academic. This PhD has therefore not just taught me a vast amount academically, but also personally about myself and although painful at times, as a result has given me the skills to be truly happy in my life. And for that I could not be more appreciative. Organic chemistry is a highly practical field and much like cooking, everyone has their own style. In particular, I must thank Bob Schroeder, Maud Jenart and Hugo Bronstein for their continuous patience in teaching me the art of chemistry through all of the tips, tricks and advice they gave me – “if you’re doing it like in the text book, you’re probably doing it wrong”. I would also like to thank Karl Thorley for his extended help in using the HPC and performing complex molecular modelling calculations and for always answering all of my questions. I have been so lucky to be surrounded by such great colleagues, many of whom have become extremely close friends. In particular, I have to thank Abby, Adam, Alex, iv Alexandros, Andy, Andreea, Bob, Cameron, Christian, Craig, Dan, Derya, Fei, George, Hugo, Iain, Jenny, Jess, Joe, Josh, Karl, Konstantinos, Laure, Lisa, Mark, Matt, Maud, Mike, Mindaugas, Notina, Pierre, Petruta, Sarah, Shahid, Sophie, Yang and all of the many other members in the McCulloch and Heeney group who have made working their great fun on the good days and worth getting out of bed for on the bad ones! Their interesting discussions on my and their own work and the many tips and tricks has been invaluable advice and made me the chemist I am today. I must also give a special mention to Josh Green for the great deal of help he gave me in putting together all the technical aspects of this thesis. Finally, I would like to thank the Centre for Plastic Electronics for awarding me the funding that has allowed me to pursue the last 4 years of research and all of the wonderful staff and students who I’ve met along the way. v IV Abstract Organic Semiconducting materials (OSCs) are integral to the next generation of devices such as flexible screens and solar panels, thanks to their flexible, solution processable and synthetically cheap nature. Although the field of OSCs has moved forward rapidly in the last decade, further research must be conducted to improve performance and efficiencies so as to be competitive with current silicon technologies whilst opening up new markets. One method of improving the current technology is through the chemical modification of these materials to tune their parameters whether this is optical, energetic or morphological. In this thesis, we report the complex synthesis of new thiazole based polymers for use in organic field effect transistors (OFETs) and organic photovoltaics (OPV) with exceptional planarity through a ‘conformational lock’ between the thiazole and thiophene units. Although these polymers performed poorly in OPV and OFET, their interesting properties give new insight into thiazole based polymers, whilst also showing a novel ring closure reaction previously unpublished. This thesis also reports on the synthesis of new diketopyrrolopyrrole polymers with electron rich flanking units used to form strong push-pull hybridisation across the backbone to create low band gap, near-IR absorbing materials. The high molecular weight pDTP-DPP-TT behaveds in a rod like manner due to a ‘hydrogen-bond’ bond-like’ interaction between the DPP core and the DTP flanking unit. This results in exceptionally high extinction coefficients – a prerequisite for high currents in OPV devices. Finally, the thesis reports on the extension of the XBR family with the addition of two new non-fullerene acceptor materials CBR and CBI. The use of non-fullerene acceptors in OPV should not only heavily bring down the materials cost of devices, but also open up new absorption pathways to allow greater device efficiencies. This work looked into how vi chemical modifications of the central core and flanking units can be used to tune the electronic energy levels, as well as the optoelectronic properties of the materials both individually and in devices. vii V Table of Contents I Declaration of Originality ....................................................................................................... ii II Copyright Declaration .......................................................................................................... iii III Contributions and Acknowledgements ................................................................................ iv IV Abstract ................................................................................................................................ vi V Table of Contents ............................................................................................................... viii VI List of Publications .............................................................................................................. xi VII Abbreviations ................................................................................................................... xii Chapter 1 Introduction ............................................................................................................ 1 1.1 Background .......................................................................................................................... 2 1.1.1 From wooden sticks to silicon chips – humanity’s progress ..................................... 2 1.1.2 A plastic fantastic future ............................................................................................ 3 1.1.3 Plastic electronics ...................................................................................................... 4 1.2 Polymer chemistry ............................................................................................................... 6 1.2.1 Polymer definitions ................................................................................................... 6 1.2.2 Polymer synthesis – chain and step growth ............................................................... 7 1.2.3 Predictable polymer properties ................................................................................ 10 1.3 Conducting polymer fundamentals .................................................................................... 11 1.3.1 The pz band .............................................................................................................. 11 1.3.2 Defining charge ....................................................................................................... 14 1.3.3 Charge transport ...................................................................................................... 17 1.4 OPV devices ....................................................................................................................... 19 1.4.1 General operating principles ................................................................................... 19 1.4.2 Device architecture .................................................................................................. 21 1.4.3 Device physics – judging performance ................................................................... 22 1.4.4 Non-fullerene acceptors .......................................................................................... 25 1.5 OFET devices ....................................................................................................................