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I OPTIMIZATION of ORGANIC SOLAR CELLS a DISSERTATION OPTIMIZATION OF ORGANIC SOLAR CELLS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Seung Bum Rim March 2010 i © 2010 by Seung Bum Rim. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/yx656fs6181 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. Peter Peumans, 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. Michael McGehee 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. Philip Wong 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 Organic solar cell is a promising technology because the versatility of organic materials in terms of the tunability of their electrical and optical properties and because of their relative insensitivity to film imperfections which potentially allows for very low-cost high-throughput roll-to-roll processing. However, the power conversion efficiency of organic solar cell is still limited and needs to be improved to be competitive with grid parity. In this thesis, I'll discuss major factors to limit efficiencies of bilayer organic solar cells such as light absorption, exciton diffusion and open circuit voltage. Light trapping enhances light absorption and increases efficiencies with thinner devices structure. The technique is particularly important in organic solar cells because internal quantum efficiency of organic solar cells is low with thick films while absorption is weak with thin films. V-trap configuration is a simple and effective light trapping scheme for organic solar cells since there is no need to modify active layers, thinner films achieve high efficiencies and no tracking system is necessary. The effects of total internal reflection in shaped substrates and the comparison with shapes other than V-shape will be also provided in Chapter 2. Exciton diffusion is a main bottleneck in bilayer organic solar cells and thus the exciton diffusion length (L D) is an important parameter that determines efficiency. However, different groups report different L Ds because there are many factors that affect the diffusion length or because there is a systematic error in the measurement iv Abstract method. The photocurrent spectroscopy method to estimate L D in Chapter 3 and the effect of molecular packing on L D will be discussed in Chapter 4. Even when light absorption and exciton diffusion are optimized, the efficiency of a single junction organic solar cell is too low for commercial applications. Multi- junction cells are a way to achieve the efficiencies needed. I'll discuss the practical efficiencies of tandem organic solar cells in the case of a series-connected tandem cell and an unconstrained (multi-terminal) tandem cell. In practical cases, unconstrained tandem cells result in higher efficiencies because of the increased freedom in choosing materials and device structures without requiring current matching. Semitransparent solid state dye sensitized cells are demonstrated as a route to realize three terminal tandem cells in Chapter 5. Curved focal plane arrays on stretchable silicon mesh networks can lead to realize high performance optical system with simple design. In Chapter 6, I show that curved focal plane arrays have optical advantages such as small number of elements, bright and accurate imaging for off-axis locations. Fabrication method is briefly introduced. v Acknowledgement I would like to gratefully thank to my advisor, Professor Peter Peumans, for his encouragement and guidance. I appreciate all his contributions of time, ideas and funding to make my Ph.D. program motivated and productive. It has been really my pleasure to learn from him to solve challenging problems with deep understanding and creativity. His guidance with deep knowledge on broad spectrum of science and bright intuition keeps me motivated and going forward. I am also thankful to my reading committees; Professor Michael D. McGehee and Professor Philip Wong. It would not be possible to complete my projects without Prof. McGehee’s and his students’ help. I have shared ideas and have done many experiments with his students in his lab. I also appreciate Prof. Wong for his great teaching about nanoelectronics and advanced silicon devices. I appreciate BASF, Samsung scholarship foundation and center for advanced molecular photonics and KAUST for sponsoring my Ph.D. program. I also thank my co-workers; Peter Erk, Jan Schoneboom, Felix Eickemeyer in BASF for perylene project, Shanbin Zhao and Shawn R. Scully for V-trap project, Rostam Dinyari and Kevin Huang for curved focal plane array project, Brian E. Hardin for multi-junction dye sensitized cell project and Jung-Yong Lee and Whitney Gaynor for multi-terminal multi-junction cell project. I thank Junbo Wu, Albert Liu, Nicholas Sergeant and all members in Peumans’ group for fruitful discussions on various topics. I acknowledge Taeksoo Kim, Sungwoo Kim, Daeho Lee, Sangwook Lee and Intaik Park for their advices and consulting throughout Ph.D. program. vi Table of Contents I greatly appreciate my wife, Hye Jung Lee, for endless support and my kids, Aiden and Katie, for their being. Seung Rim vii Table of Contents Abstract ............................................................................................... iv Acknowledgement ............................................................................... vi List of Tables ........................................................................................ xi List of Figures ...................................................................................... xii List of Equations ............................................................................... xviii List of Symbols ................................................................................... xix List of Abbreviations ........................................................................... xx List of Chemicals ................................................................................. xxi List of Publications, Conference Contributions .................................. xxii Chapter 1 Introduction ................................................................... 25 1.1 Thin film photovoltaic cells......................................................................... 25 1.2 Cost analysis of organic solar cells ............................................................. 26 1.2.1 Introduction ................................................................................. 26 1.2.2 Levelized cost of energy .............................................................. 27 1.2.3 Efficiency goal for organic solar cells........................................... 30 1.3 Current status of organic solar cells ........................................................... 31 1.4 Physics of organic solar cells ...................................................................... 32 1.4.1 Introduction ................................................................................. 33 1.4.2 Light absorption ........................................................................... 36 1.4.3 Exciton diffusion .......................................................................... 40 1.4.4 Charge transfer and separation ................................................... 46 1.4.5 Charge collection ......................................................................... 49 1.5 Dye sensitized solar cells ............................................................................ 49 1.6 Multi-junction cells ..................................................................................... 50 1.7 Conclusion and outlook .............................................................................. 51 Bibliography .......................................................................................................... 52 viii Table of Contents Chapter 2 V-shaped light trapping in organic solar cells .................. 63 2.1 Introduction ................................................................................................ 63 2.2 Light trapping in thin film solar cells .......................................................... 64 2.3 Principles of V-shaped light trap ................................................................ 66 2.3.1 Structure ...................................................................................... 66 2.3.2 Optical pathlength enhancement ............................................... 67 2.4 Modeling methods ..................................................................................... 69 2.5 V-shaped light trap ....................................................................................
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