Lifetime and Reliability of Polymer Solar Cells A
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LIFETIME AND RELIABILITY OF POLYMER SOLAR CELLS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Craig Homer Peters November 2011 © 2011 by Craig H Peters. All Rights Reserved. Re-distributed by Stanford University under license with the author. This dissertation is online at: http://purl.stanford.edu/fs540ky3123 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Michael McGehee, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Reinhold Dauskardt I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Alan Sellinger Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract The power conversion efficiency of organic photovoltaic (OPV) cells has increased from 4-5% in 2005 to 8.3% in 2010. The goal of a 10% single junction OPV device seems attainable making the commercialization of OPV more realistic. With advances made on the efficiency front, the lifetime and reliability of OPV devices has come into focus. To date there has been considerable work done in understanding and quantifying the lifetime and degradation of bulk heterojunction solar cells (BHJs) based on poly-(para- phenylene-vinylene) (PPV) and poly(3-hexylthiophene) (P3HT) polymers. A comparison of OPV lifetime experimental results across different research groups has posed challenges due to the lack of standardized testing and reporting procedures; however, great strides have been made in this regard during the most recent International Summit on OPV Stability (ISOS-3). Modules based on P3HT/fullerene BHJs have shown lifetimes of 5,000 hours when state-of-the-art encapsulation with a glass-on-glass architecture is used. Assuming negligible degradation in the dark and 5.5 hours of one- sun intensity per day, 365 days per year, this translates into an operating lifetime approaching three years. More recently P3HT/PCBM devices utilizing an inverted architecture have been shown to retain more than 50% of their initial efficiency after 4,700 hours of continuous exposure to one-sun intensity at elevated temperatures and have exhibited a long shelf life when stored in the dark in ambient conditions. However, results to date have yet to show polymer based OPV lifetimes greater than 3-4 years. ii In my dissertation I present a detailed operating lifetime study of encapsulated solar cells comprised of poly[N-9'-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'- benzothiadiazole) (PCDTBT) in BHJ composites with the fullerene derivative [6,6]- phenyl C70-butyric acid methyl ester (PC70BM). PCDTBT/PC70BM solar cells achieved an efficiency greater than 6%, making this one of a small number of polymers able to achieve this level of performance. I describe an experimental set-up that is capable of testing large numbers of solar cells simultaneously, holding each device at its maximum power point while controlling and monitoring the temperature and light intensity. Using this set-up we were able to compare the PCDTBT/PC70BM system with the well-studied P3HT/PCBM system and demonstrate a lifetime for PCDTBT devices that approaches 7 years, which is the longest reported operating lifetime for a polymer based solar cell. I will further present a systematic study of the burn-in degradation mechanism behind PCDTBT:PC70BM solar cells. I will show that a photochemical reaction in the photoactive layer creates states in the bandgap of PCDTBT. These sub-bandgap states increase the energetic disorder in the system, which reduces the FF, Voc and to a lesser extent Jsc. The photochemical reactions are shown to progress rapidly when first exposed to light but subsequently decrease in occurrence, which results in the stabilization of the Voc and FF. iii Acknowledgements First and foremost I would like to thank Professor Michael McGehee. Mike looked at an older graduate student and had the foresight and guts to take me into his group. He provided an exceptional environment for me to develop my scientific skills and pushed me to think about my work in a bigger picture manner. Mike taught me how to give compelling scientific talks and how to write exceptional scientific publications. Finally, Mike became a close friend through the process. As for the research itself, the saying that, “it takes a village to raise a child,” should now read, “it takes a village to perform a lifetime and reliability experiment.” I have to thank the incredibly hard work that was put in by Toby Sachs-Quintana. Toby worked countless hours and helped in all aspects of the work presented. Jack Kastrop also worked for hours on end soldering leads, building reflectors and testing solar cells. There is no doubt that without the two of them the first lifetime study would not have been launched as early or as well. I also have to thank Billy Mateker and Thomas Heumueller for their hard work in getting the second paper to print. Both worked tirelessly on building testing equipment and taking measurements into the weekends. I would like to thank my labmates who offered countless hours of discussion and brainstorming. Zach Beiley and Eric Hoke have been co-author on one of my papers and have both helped me understand energetic disorder and models associated with these concepts as well as charge-transfer states and the implications on device parameters. Eric has built many of the characterization tools that were used to take the data presented in this thesis. George Burkhard has been a go-to iv person for me when trying to tackle complex physics problems. George was also a consultant for most of what was constructed to perform the reliability experiments. I-Kang Ding was my neighbor and we had numerous discussions about solar cells and the greatness of badminton even though I wasn’t great at badminton. The rest of the McGehee group, including G-5, Nicky, Roman, Mark, Sam, Jon and all of the newest members, made the experience one of the most memorable in my life. I also have to thank Shawn Scully for developing my understanding of the physics of polymer solar cells. He was my mentor for the first 1.5 years of my PhD and provided me with incredibly valuable insight into organic PV. Numerous discussions with Michael Rowell were also critical to my understanding of OPV and I want to thank him for keeping me healthy by going off to the gym or volleyball. Brian Hardin was instrumental in keeping the pressure on the lab to push itself to new levels. His enthusiasm and creativity were valuable to everyone around him. He also initiated numerous “Ab-offs” which helped me continue to get in shape. He is now a trusted partner in our new business and will continue to be a life-long friend. I need to thank my amazing wife, Kathryn, for being so incredibly supportive in my final year of graduate school. She pushed me to be exceptional in every way and gave me the time and love that only comes from a true life partner. My family has been there for me at every step of this process. From going back to undergraduate studies at the age of 31 to completing a PhD at the age of 41 they have lifted me up the whole way through. I want to especially thank my mother for being my greatest cheerleader and Harrison Griswold, one of my fathers, who’s love v of science and engineering inspired me to enter this field. He has promoted my endeavor down this path from the beginning and been there for me through the good and tough times. Finally, Eric and Mara, my oldest brother and his wife, deserve a special thanks. They inspire others to follow their dreams and not the money. They have never let up their support of me and continue to push me to achieve greater things with my life. vi Dedication This work is dedicated to my father, Harrison Griswold, who instilled in me a love of science and to anyone else who wants to pursue their dreams regardless of their age. vii Table of Contents Abstract .............................................................................................................................. ii Acknowledgements .......................................................................................................... iv Dedication ........................................................................................................................ vii List of Figures ................................................................................................................... xi 1. Background and Motivation ..................................................................................... 1 1.1 The reliability of organic solar cells comes into focus ...................................... 1 1.2 The similarity of OPV and organic light emitting diodes (OLEDs) .................. 2 1.3 A brief look at the lifetime of state-of-the-art OLEDs ...................................... 3 1.4 State-of-the-art lifetime studies