SQUARAINE DONOR BASED ORGANIC SOLAR CELLS By Guodan Wei A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Materials Science and Engineering) in the University of Michigan 2012 Doctoral Committee: Prof. Stephen R. Forrest, Chair Prof. Pallab Bhattacharya Prof. Rachel S. Goldman Prof. Max Shtein © Copyright by Guodan Wei, 2012 All Rights Reserved DEDICATION To my two sons Anderson Shao, Allen Shao and my husband Jing Shao ii ACKNOWLEDGEMENTS I would like to thank many people for their enormous assistance, contributions and encouragement. With all their support and help, I could continue to work for the completion of this thesis work. Formost, I would like to express my deep appreciation to my advisor Prof. Stephen R. Forrest for his continuous support, instruction and encouragement of my Ph.D course study and research work, for his enormous patience, motivation, enthusism and immense knowledge. His instructive guidance helped me keep the right track of the projects all the time and speed up their completions. Especially, his careful correction and revision have refreshed and deepened each manuscript I have written, where I have gained precious lessons that will benefit my life in the future. Besides my advisor, I would like to thank the rest of my thesis committee: Prof. Rachel S. Goldman, Prof. Max Shtein and Prof. Pallab Bhattacharya, for their encouragement, insightful comments and suggestions. Especially, I would like to thank Prof. Goldman for her kind words and strong support through my ways to find a balance between a graduate student and a working mom. My sincere appreciation also goes to Prof. Mark E. Thompson and Siyi Wang for offering the promising new designed solar materials: squaraines. With more than four years’ close collaboration work, we have carried out systematical and comprehensive studies on these new materials and employed them to improve solar cell performance. Also, I would like thank Dr. Kai Sun for his strong collaboration on the microscopic iii characterization of crystallized squaraines which greatly helps us understand the correlation between crystal structure and device performance. I would like to thank the group members in Optoelectronic Components and Materials (OCM) Group for their friendship and collaboration. In particular, I would like to thank g, Fan YRhonda Bailey-Salzman and Elizabeth Mayo for instructing me how to fabricate organic solar cells. I would also like to thank Kuen-Ting Shiu, Noel Chris Giebink, Stephane Kena-Cohen, Marcelo Davanco and Richard Johnny Lunt for discussions and collaboration on the thermodynamic limit efficiency of quantum solar cells. Their questions, suggestions and advice guided a new direction and enlightened points in this work. The other collaborators and colleagues within OCM were a great help, including Xin Xiao, Jeramy Zimmerman, Brian Lassiter, Yifan Zhang, Ning li, Xiangfei Qi, Kyle Renshaw and Xiaoran Tong. Last but not the least, I would like to thank my mom, my dad in heaven for supporting me spritually throughout my life; and would like to thank my parents-in-law for encouraging me and helping take care of my son, Anderson. Especially, I would like to thank my husband, Jing Shao, for his endless support and patience on my work and family life. Guodan Wei San Jose October, 2011 iv TABLE OF CONTENTS DEDICATION................................................................................................................... ii ACKNOWLEDGEMENTS ............................................................................................ iii List of Figures ................................................................................................................... ix List of Table ................................................................................................................... xvii List of Appendices ........................................................................................................ xviii ABSTRACT .................................................................................................................... xix Chapter 1 Introduction to solution processed organic solar cells ................................ 1 1.1 Principles of organic solar cells .............................................................................. 2 1.2 Device architectures of organic solar cells ............................................................. 3 1.3 Characteristics of organic solar cells ........................................................................ 8 1.4 Recent research progress of solution processed bulk HJ solar cells ..................... 13 1.4.1 Recent research progress of polymer bulk HJ solar cells ................................ 13 1.4.2 Recent research progress of small molecule bulk HJ solar cells ..................... 14 1.5 Thesis overview .................................................................................................... 19 References ..................................................................................................................... 23 Chapter 2 Solution processed squaraine bulk heterojunction photovoltaic cells ..... 27 2.1 Introduction to bulk heterojunction solar cells ....................................................... 27 2.2 Cell series resistance ............................................................................................... 28 2.3 Properties of pure squaraine and mixed squaraine:PC BM thin films .................. 30 70 2.3.1 Crystal structure of squaraine material ............................................................ 30 2.3.2 Optical absorption of squaraine and squaraine:PC BM blend films .............. 31 70 2.3.3 Charge carrier mobilities .................................................................................. 33 2.3.4 Surface morphology ........................ ................................................................. 34 2.4 Squaraine/C control and squaraine:PC BM bulk solar cells ............................... 36 60 70 2.5 Discussion ............................................................................................................... 42 2.6 Summary ................................................................................................................. 46 References ..................................................................................................................... 48 v Chapter 3 Efficient and ordered squaraine/C nanocrystalline heterojunction solar 60 cells ................................................................................................................................... 51 3.1 Introduction ............................................................................................................. 51 3.2 Device fabrication and characterization .................................................................. 52 3.3 Properties of squaraine thin films ........................................................................... 54 3.3.1 Crystallinity evolution through post annealing ................................................ 54 3.3.2 Optical absorption of annealed squaraine films ............................................... 55 3.3.3 Surface morphology and crystallinity .............................................................. 56 3.3.4 Exciton diffusion length ................................................................................... 57 3.4 Device performance of squaraine/C bulk solar cells ............................................ 58 60 3.5 Discussion ............................................................................................................... 62 3.6 Summary ................................................................................................................. 64 References ..................................................................................................................... 65 Chapter 4 Solvent annealed nanocrystalline squaraine: PC BM (1:6) solar cells ... 67 70 4.1 Introduction ............................................................................................................. 67 4.2 Effect of solvent annealing to squaraine:PC BM (1:6) films ................................ 68 70 4.2.1 Crystallinity of solvent annealed squaraine:PC BM blend films ................... 68 70 4.2.2 Optical properties of solvent annealed squaraine:PC BM blend films .......... 69 70 4.2.3 Surface morphology of squaraine:PC BM blend films .................................. 71 70 4.3 Device performance of squaraine:PC70BM cells ................................................... 73 4.4 Summary ................................................................................................................. 76 References ..................................................................................................................... 77 Chapter 5 Arylamine-Based Squaraine Donors for Use in Organic Solar Cells ...... 80 5.1 Introduction ............................................................................................................. 80 5.2 1-NPSQ/C device fabrication ............................................................................... 82 60 5.3 Absorption spectra of SQ and 1-NPSQ thin films and device characteristics ........ 84 5.3 SQ/C and 1-NPSQ/C device characteristics ...................................................... 86 60 60 5.4 The thermal annealing of 1-NPSQ thin films ......................................................... 88 5.5 The 1-NPSQ/C cells with low series resistance (R ) ............................................ 90 60 s 5.6 Conclusion .............................................................................................................. 91 References ..................................................................................................................... 93 Chapter 6 Functionalized Squaraine Donors for Nanocrystalline Organic vi Photovoltaics .................................................................................................................... 95 6.1 Introduction ............................................................................................................. 95 6.2 Experiment of six squaraine/C devices ................................................................ 97 60 6.3 Physical properties of six functionalized squaraines .............................................. 99 6.3.1 HOMO and LUMO levels ............................................................................... 99 6.3.2 Optical properties ........................................................................................... 101 6.3.3 Molecular stacking of the single crystal squaraine (SQ) and arylamine-based Squaraine (DPSQ) ................................................................................................... 101 6.3.4. Exciton diffusion length of fSQs .................................................................. 104 6.3.5. Surface morphology of the fSQ thin films .................................................... 106 6.4 Squaraines/C NcHJ solar cells ........................................................................... 107 60 6.5 Discussion ............................................................................................................. 112 6.6 Conclusion ............................................................................................................ 118 References ................................................................................................................... 119 Chapter 7 ....................................................................................................................... 121 Conclusions and outlooks ............................................................................................. 121 7.1 Conclusions ........................................................................................................... 121 7.2 Outlook ................................................................................................................. 125 7.2.1 The design and synthesis of new squaraine donors ....................................... 126 7.2.2. Synthesis of single crystal squaraine nanowires ........................................... 128 7.2.3 Nanocomposite squaraine:PC BM solar cells .............................................. 131 70 7.2.4 Two squaraine donor blending solar cells ..................................................... 133 7.2.5 Solvent annealing of functionalized squaraines (fSQ)/C solar cells ........... 135 60 7.3 Afterword .............................................................................................................. 136 References ................................................................................................................... 137 Appendix 1 ..................................................................................................................... 138 Thermodynamic limits of quantum photovoltaic cell efficiency based on one photon absorption .................................................................................................................... 138 Appendix 2 ...................................................... ............................................................... 150 Thermodynamic limits of quantum photovoltaic cell efficiency based on two photon absorption .................................................................................................................... 150 Appendix 3 ..................................................................................................................... 153 vii List of Publications, Conference Presentations and Patents ..................................... 153 viii List of Figures Figure 1.1: Schematic illustration of the four consecutive steps in the generation of photocurrent from incident light to an organic solar cell [8]. ............................................. 2 Figure 1.2: Schematic illustration of two organic solar cell architectures: (a) planar and (b) bulk heterojunction ........................................................................................................ 4 Figure 1.3: (a) the structure of a CuPc/PTCBI bilayer solar cell;(b) the current density (J)-Voltage (V) is measured in the dark and under 1 sun illumination [23]. ...................... 9 Figure 1.4: Equivalent circuit of a photovoltaic cell [24]. ................................................ 10 Figure 1.5: (a) increasing series and (b) reducing parallel resistance. In each case the effect of the resistances is to reduce the area of the maximum power rectangle compared to J × V [25].................................................................................................................. 12 sc oc Figure 1.6: Efficiency improvement over years for solution processed small molecule bulk HJ solar cells ............................................................................................................. 15 Figure 1.7: Chemical structure of merocyanine dye used in fabricating Al/merocyanine/Ag organic solar cells [43]. .................................................................... 17 Figure 1.8: New squaraine donors 1 (R=2-ethylhexyl) and 2 (R=n-dodecyl); schematic diagram of squaraine:PCBM bulk solar cells [39]. ........................................................... 18 Figure 1.9: Two new squaraine dyes substituted at the pyrrolic rings with n-hexyl (squaraine 1) or n-hexenyl (squaraine 2) chains. The presence of the terminal double bond results in a much more compact solid-state structure, dramatically affecting charge transport in the thin films [5]. ........................................................................................... 18 ix