Metal Oxide / Organic Interface Investigations for Photovoltaic Devices Olympia Pachoumi Magdalene College University of Cambridge A thesis submitted for the degree of Doctor of Philosophy July 2014 2 To my parents Acknowledgements This thesis summarises work I have carried out as a PhD student of the Optoelectronics Group at Cavendish Laboratory of the University of Cambridge since October 2010. I am thankful to the Engineering and Physical Sciences Research Council and the A.G. Leventis Foundation for funding my research as well as Magdalene College for financial support towards the attendance of conferences. It has been quite a journey until I was able to complete this manuscript and I would like to take this opportunity to thank all those remarkable individuals who have contributed one way or another towards this work and who have made my journey worthwhile. I would like to thank my supervisor Prof. Henning Sirringhaus for giving me the opportunity to work in his research group and pursue the projects I have, given me being a solar cell enthusiast in his tran- sistor family. I would also like to thank him for all our discussions, his spectrum of knowledge and his insight are really inspiring, and especially for being as supportive as he has been, he appears to have a magic ability to resurrect ones motivation. Special thanks go to Dr Kulbinder Banger who has supported me all the way through my PhD and who I consider as my mentor. I want to thank him for offering me the opportunity to work on one of his projects and sharing his wisdom on experimental methods, showing me the way towards being more efficient and productive. I also want to thank Dr Yana Vaynzof, with whom I have worked very closely, for her support, for discussions and advice regarding my experimental results as well as for running exper- iments together. I thank Dr Cheng Li for sharing his knowledge and experience on metal oxides and for his electroabsorption spectroscopy measurements. It has been an absolute delight collaborating with Cheng. I would like to thank Dr Artem Bakulin, Dr Chaw Keong and Aditya Sadhanala who have contributed towards this work with their expertise. Of course it is always good to have someone around when the spectrophotometer goes crazy in the dark-room or when you are in need of a great working transistor; for that I thank Iyad Nasrallah, who has been a great help with my spectroscopy experiments. I thank the OE and ME Group staff and in specific Dr Radoslav Chakalov, Dr Richard Gymer, Mrs Emily Heavens, Mr. Roger Beadle and Mr Alex Cook for making sure that everything is running smoothly. Furthermore I would like to thank all the OE people for making this group such a pleasant environment to work. Thanks go to my current office mates Carlo, John and Deepak for coping with the tension of having four near-the-end PhD students in the same room and to my former office mates Josie, Chris, Milan, Iyad and Vahe for some scien- tific talk, brainstorming and mostly for the energising breaks. Special thanks to the people who made me feel so welcome in the OE when I was still a fresher; Sebi, Thomas, Aurelie, Riccardo, Mijung, Georgui, Chiara, Michael and Vincenzo and to the 2014 CUTEC Committee for making the past year so much fun. Kyriaki, Marina and Evangelia thank you for all the group therapy sessions along with coffee, tea, or sangria. Thanks to my high-school girlfriends for snapping me out of the hectic PhD life once in a while. Finally, I can't thank my parents, Marina and Michali, enough for their endless support on every step of the way and for always being present, although 2000 miles away. I thank them for all the faith they have in me and for coping with my absence for eight years now. My sisters, Monica and Myrto thank you for always being there when I need you. My grandma, Olympia, thanks for spoiling me. For all the time I spent as a PhD student here in Cambridge, every single day, every single achievement or failure wouldn't have been as fun and unique as they have been and still are if it weren't for Panayiotis. I want to thank him for having to listen to me whine about all things that go wrong, he has been a constant source of motivation for me. I declare that except where specific reference is made to the involve- ment of others, the work presented herein is my own. This dissertation has not been submitted in whole or in part for the award of a degree at this, or any other university and does not exceed 60,000 words in length. Olympia Pachoumi Magdalene College Cambridge, July 2014 Abstract This thesis outlines investigations of metal oxide / organic interfaces in photovoltaic devices. It focuses on device instabilities originating from the metal oxide layer surface sensitivity and it presents suggested mechanisms behind these instabilities. A simple sol-gel solution deposition technique for the fabrication of stable and highly performing transparent conducting mixed metal ox- ides (ZnMO) is presented. It is demonstrated that the use of amor- phous, mixed metal oxides allows improving the performance and sta- bility of interfacial charge extraction layers for organic solar cells. Two novel ternary metal oxides, zinc-strontrium-oxide (ZnSrO) and zinc-barium-oxide (ZnBaO), were fabricated and their use as electron extraction layers in inverted organic photovoltaics is investigated. We show that using these ternary oxides can lead to superior devices by: preventing a dipole forming between the oxide and the active organic layer in a model ZnMO / P3HT:PCBM OPV as well as lead to im- proved surface coverage by a self assembled monolayer and promote a significantly improved charge separation efficiency in a ZnMO / P3HT hybrid device. Additionally a spectroscopic technique allowing a versatility of char- acterisation for long-term stability investigations of organic solar cells is reported. A device instability under broadband light exposure in vacuum conditions for an inverted ZnSrO/PTB7:PC70BM OPV is ob- served. Direct spectroscopic evidence and electrical characterisation indicate the formation of the PC70BM radical anion associated with a loss in device performance. A charge transfer mechanism between a heavily doped oxide layer and the organic layers is suggested and discussed. vi Contents 1 Introduction 1 2 Background 5 2.1 Oxide Semiconductors: zinc oxide . .6 2.1.1 Electronic Structure . .6 2.1.2 Electronic Properties . 11 2.1.3 Surface Oxygen . 14 2.1.4 Sol-gel Processing . 17 2.2 Organic Semiconductors . 18 2.2.1 Origin of Semiconducting Property . 18 2.2.2 Optical Processes . 19 2.2.3 Neutral and Charged Excitations . 21 2.3 Organic Photovoltaic Devices . 23 2.3.1 Charge Photogeneration . 23 2.3.2 Built-in Potential . 26 2.3.3 Open-circuit Voltage . 28 2.3.4 Stability and Degradation . 29 2.4 Organic Field Effect Transistors . 32 References . 33 3 Experimental Methods 41 3.1 Materials . 41 3.1.1 P-type Polymers . 41 3.1.2 Fullerene Derivatives . 44 3.1.3 ZnO, ZnSrO and ZnBaO . 45 vii 3.1.4 Tungsten Trioxide . 46 3.2 Thin Film Fabrication . 46 3.2.1 Organic Layer . 47 3.2.2 Mixed Metal Oxides . 48 3.2.3 Electron Blocking Layer . 49 3.2.4 Electrode Deposition . 49 3.3 Material Characterisation . 51 3.3.1 Profilometry . 51 3.3.2 Morphology Probes (AFM, SEM, XRD).......... 51 3.3.3 UV-Vis Spectroscopy . 52 3.3.4 Photoemission Spectroscopy . 53 3.3.5 Photothermal Deflection Spectroscopy . 54 3.3.6 Electrical Conductivity . 55 3.4 Device Characterisation . 56 3.4.1 Photovoltaic Device Performance . 56 3.4.2 Electroabsorption Spectroscopy . 58 3.4.3 Pump-Push Photocurrent Spectroscopy . 62 References . 63 4 ZnSrO and ZnBaO Electron Extraction Layers for Inverted Or- ganic Photovoltaics 67 4.1 Introduction . 68 4.2 Metal Oxide Characterisation . 71 4.2.1 Chemical Composition . 72 4.2.2 Surface and Bulk Microstructure . 75 4.2.3 Optical Transmission . 77 4.2.4 Surface Potentials . 78 4.2.5 Electrical Conductivity . 81 4.3 Device Performance . 83 4.3.1 Doping Level . 84 4.3.2 EQE Preliminary Measurements . 86 4.3.3 UV Light Exposure and Nitrogen Annealing . 87 4.4 Improvement Mechanism . 91 viii CONTENTS 4.4.1 Interfacial Band Alignment . 92 4.4.2 Chemical Composition Stability . 94 4.4.3 Built-in Potential . 97 4.4.4 Electrical Hysteresis . 99 4.5 Conclusions . 101 References . 101 5 Interfacial Modification for Oxide/Polymer Hybrid Photovoltaics107 5.1 Introduction . 108 5.2 ZnSrO/P3HT Hybrid PV Devices . 109 5.2.1 Doping Level and Device Performance . 109 5.2.2 Open-circuit Voltage Improvement Discussion . 111 5.2.3 Interfacial Charge Separation . 113 5.3 Interfacial Modification Using PCBA . 114 5.3.1 Device Performance . 115 5.3.2 Charge Separation Efficiency . 117 5.4 Conclusions . 119 References . 120 6 Charge Accumulation Spectroscopy for Organic Photovoltaics 123 6.1 Introduction . 124 6.2 Optical Transmittance . 126 6.3 High Sensitivity Optical Spectroscopy Setup . 128 6.4 External Light Source . 130 6.5 Device Architecture Optimisation . 133 6.5.1 Device Active Area . 135 6.5.2 Transparent Top Electrode . 137 6.6 Charge Induced Absorption in PTB7 Devices . 138 6.6.1 PTB7 Field Effect Transistor . 138 6.6.2 PTB7:PCBM Solar Cell . 142 6.7 Conclusions . 147 References . 148 ix 7 Inverted Organic Photovoltaic Device Stability Investigations 151 7.1 Introduction . 152 7.2 Operational Stability in Vacuum .