VACUUM DEPOSITION OF ORGANIC MOLECULES FOR PHOTOVOLTAIC APPLICATIONS Peter Kovacik Oriel College A thesis submitted for the degree of DPhil in Materials University of Oxford ABSTRACT The vacuum thermal evaporation of conjugated polymers is presented and their application in organic photovoltaic devices studied. Poly(3-hexylthiophene) (P3HT) and poly(thiophene) (PTh), semiconducting polymers with and without side groups, are deposited by vacuum thermal evaporation. Thermal behaviour of the polymers is studied using DSC and TGA, and the structural changes before and after evaporation investigated by GPC, UV-Vis, NMR, and FT-IR. These studies show that the polymers largely retain their chemical structure, however, their molecular weight decreases. The inferred conjugation length of the evaporated PTh is larger than that of P3HT, as indicated by GPC. Finally, the topography of the polymer thin films is compared using MicroXAM and AFM, and their morphology analysed by TEM and XRD. In contrast to P3HT, evaporated PTh forms microscopically flat films with high molecular order. Functional photovoltaic devices based on vacuum-deposited PTh and P3HT are presented. Different processing and device parameters are examined, and the influence side groups have on the electronic properties of the thin films is studied. Unlike P3HT, the greater crystallinity in PTh films results in significantly improved charge transport properties with -4 2 -1 -1 relatively high hole mobilities (10 cm V s ). PTh/C 60 planar heterojunction devices exhibit an almost 70% increase in efficiency compared to P3HT/C 60 devices, demonstrating enhanced charge extraction in PTh films through improved molecular order. The photovoltaic performance is further related to morphology and optical absorption of the polymer thin films using MicroXAM and UV-Vis. The effect of incident illumination intensity i and post-production thermal annealing is investigated, complementing the detailed characterisation of the devices. Co-deposited bulk heterojunctions with different PTh:C 60 volume ratios are fabricated and their response to post-production thermal annealing examined. Blend morphology is characterised by AFM and XRD and related to the photovoltaic performance. Post- annealing is shown to improve the interpenetrated polymer-fullerene network and enhance efficiency by as much as 80%. Moreover, the development of the PTh:C60 blend morphology is found to be different from standard small-molecule systems. This is illustrated by comparing the morphology of the polymeric PTh:C 60 and oligomeric 6T:C 60 blends. Finally, a device architecture consisting of multiple alternating PTh/C 60 thin films is presented with performance exceeding that of its equivalent co-deposited bulk heterojunction. ii ACKNOWLEDGEMENTS I thank the Engineering and Physical Sciences Research Council and the John Fell Oxford University Press Research Fund for financially supporting my education at Oxford. I am grateful to Oriel College for providing me with a firm social base and repeatedly funding my conference travels. I also thank the Department of Materials for its financial generosity. I thank Dr Andrew Watt for his guidance. I am grateful for our many stimulating debates, and for his enduring patience, his dependability, and particularly his targeted advice on research & life. I did follow most of it. I also believe my research is more punchy and sexy now. I thank Dr Hazel Assender for her supervision. I am grateful for her fast and clever feedback, systematic approach, cooling but sober opinions, support, and encouragement. I want to believe my research is more accurate and truthful now. I thank my officemates, Dr Shawn Willis for letting me have the desk with the only window in our office, for illustrating professionalism in all projects we have undertaken together, and of course for setting up high-productivity standards worth following; and Laura Droessler for taping yellow sunflowers on our walls. I think all our characters blended well on our postcard board. I thank my lovely colleagues, Elva Zou for making the lab environment eccentric, Simon Fairclough friendly, Jenna Holder fairy, and Chris Cattley British. I thank Cheng Cheng for his drive towards endless lab improvements, for being a faithful co-experimentalist, and for cooperatively bouncing and shaping an infinite number of research ideas. I thank Dr Gamal Abbas, Dr Andrew Cook, Dr Jonathan Moghal, Ziqian Ding, and Long Jiang for invaluable discussions, suggestions, and comments. I thank Dr Richard Beal and Dr Alexandros Stavrinadis for demonstrating that even finishing postgraduates can be helpful iii and nice. I thank all part two and summer students, Daniel Camp, Maria Nelson, Assia Kasdi, Miriam Hoener, Tom Walton, Lou Yeoh, Brienne Kugler, Simon Bowcock, Shek Li, and Kayla Nguyen, for making many bits of my DPhil time joyful. I thank Dr John Topping for introducing me to the world of vacuum and for many, many inspiring and educational discussions. I thank Richard Turner for all his help with measurements and technical issues, Gabriella Chapman and Sverre Myhra for training me on microscopes, and Laurie Walton for help with fabricating the equipment. I thank Marion Beckett, Adrian Taylor, Lyn Richmond, Barry Fellows, Ian Sutton, and Paul Warren for various forms of assistance. I thank Prof Harry Anderson, Dr Giuseppe Sforazzini, and Dr Jonathan Matichak, great organic chemists, for fruitful inter-departmental collaborations. I thank my closest ones who turned my “outlab” life at Oxford into 51% of my overall growth. Finally, I thank my family for their support and love. This work, which none of you will probably read, is unsurprisingly dedicated to you. iv DECLARATION OF ORIGINALITY This thesis is an account of work carried out by the author in the Materials Department, University of Oxford under the supervision of Dr Hazel Assender and Dr Andrew Watt. Where the work of others has been drawn upon this is duly acknowledged in the text, and a list of references is presented at the end of each chapter. No part of this thesis has been submitted towards the completion of another degree at the University of Oxford or elsewhere. Parts of this thesis have been submitted to or published in the following scientific journals or conference presentations: Journal articles Kovacik, P.; Sforazzini, G.; Cook, A. G.; Willis, S. M.; Grant, P. S.; Assender, H. E.; Watt, A. A. R., Vacuum-Deposited Planar Heterojunction Polymer Solar Cells. ACS Applied Materials & Interfaces 2011 , 3, (1), 11-15. Kovacik, P.; Willis, S. M.; Matichak, J. D.; Assender, H. E.; Watt, A. A. R., Effect of Side Groups on the Vacuum Thermal Evaporation of Polythiophenes for Organic Electronics. Organic Electronics 2012 , 13, (4), 687-696. Kovacik, P.; Assender, H. E.; Watt, A. A. R., Morphology Control in Co-evaporated Bulk Heterojunction Solar Cells. Journal of Physical Chemistry C 2012 , submitted. Kovacik, P.; Assender, H. E.; Watt, A. A. R., Importance of Oligothiophene Conjugation Length in Phase-Separated Vacuum-Deposited Heterojunction Blends. 2012 , in preparation. Kovacik, P.; Assender, H. E.; Watt, A. A. R., Enhancement of Charge Extraction in Polymer Bulk Heterojunctions through Alternating Thermal Deposition. 2012 , in preparation. Conference proceedings Kovacik, P.; Willis, S. M.; Sforazzini, G.; Assender, H. E.; Watt, A. A. R. In Vacuum Deposition of Conjugated Polymers for Solar Cells , 7th Photovoltaic Science Application and v Technology (PVSAT-7) Conference and Exhibition, Edinburgh, United Kingdom, 2011; Edinburgh, United Kingdom, 2011. Kovacik, P.; Assender, H. E.; Watt, A. A. R. In Vacuum Deposition of Conjugated Polymers for Organic Photovoltaics , 55th Annual SVC Technical Conference, Santa Clara, USA, 2012; Society of Vacuum Coaters Santa Clara, USA, 2012. Conference papers Kovacik, P.; Sforazzini, G.; Assender, H. E.; Watt, A. A. R., Thermal Deposition of Large Orga- nic Molecules for Photovoltaic Applications. In 2009 MRS Fall Meeting , Boston, USA, 2009. Kovacik, P.; Sforazzini, G.; Willis, S. M.; Assender, H. E.; Watt, A. A. R., Thermal Deposition of Large Organic Molecules for Photovoltaic Applications. In UK Semiconductors 2010 , Sheffield, United Kingdom, 2010. Kovacik, P.; Sforazzini, G.; Willis, S. M.; Assender, H. E.; Watt, A. A. R., Vacuum-Deposited Polythiophene Organic Solar Cells. In ElecMol 2010, 5th International Meeting on Molecular Electronics , Grenoble, France, 2010. Kovacik, P.; Willis, S. M.; Matichak, J. D.; Sforazzini, G.; Assender, H. E.; Watt, A. A. R., Vacuum Deposition of Conjugated Polymers for Solar Cells. In 7th Photovoltaic Science Application and Technology (PVSAT-7) Conference and Exhibition , Edinburgh, United Kingdom, 2011. Kovacik, P.; Willis, S. M.; Assender, H. E.; Watt, A. A. R., Vacuum-Deposited Polythiophene Organic Solar Cells. In IOP Advances in Photovoltaics , London, United Kingdom, 2011. Kovacik, P.; Matichak, J. D.; Willis, S. M.; Assender, H. E.; Watt, A. A. R., Vacuum Deposition of Conjugated Polymers for Organic Photovoltaics In IOP Physical Aspects of Polymer Science , Guildford, United Kingdom, 2011. Kovacik, P.; Assender, H. E.; Watt, A. A. R., Vacuum-Deposited Polythiophene Organic Solar Cells. In 2012 MRS Spring Meeting , San Francisco, USA, 2012. Kovacik, P.; Assender, H. E.; Watt, A. A. R., Vacuum Deposition of Conjugated Polymers for Organic Photovoltaics. In 55th Annual SVC Technical Conference , Santa Clara, USA, 2012. vi TABLE OF CONTENTS Abstract .........................................................................................................................