Nanostructured Electrodes and Their Use in Organic Solar Cells Master Thesis Reza Abolhassani Mads Clausen Institute University of Southern Denmark Supervisors: Associate Professor Morten Madsen Associate Professor Jost Adam November 2015 Master Thesis- Reza Abolhassani MCI, University of Southern Denmark Acknowledgements I would like to express my sincere gratitude to my supervisors, Morten Madsen and Jost Adam, for giving me the chance to work on this project, and for their continuous support and guidance throughout my work. I have learned so much from them and it is my honor to be their student. I would also like to thank PhD students: Mina Mirsafaei, André Luis Fernandez Cauduro, Arkadiusz Jaroslaw Goszczak, and Mehrad Ahmadpour. They were not only good mentors that gave me their valuable experiences and provided me information, but also good friends that motivated me to finish the project as good as possible for me. Last but not least, I wish to thank my family for their borderless love, supports, and encouragement. They will be in my heart forever. 1 Master Thesis- Reza Abolhassani MCI, University of Southern Denmark Abstract Energy demands have increased dramatically in last decades and predicted to increase by three times in 2050. Fossil fuels are the main reasons for environmental degradation, acid rain, ozone depletion, forest destruction, and global climate change. Therefore, renewable sources of energy have attracted attention from industries and scientists. One of these renewable energy sources is solar power. Solar cells convert sun light directly to the electricity. However organic solar cells power conversion efficiency is rather low in comparison with conventional solar cells, it is an interesting topic for researchers due to their flexibility and lightweight, low cost materials and manufacturing, and simple fabrication process. In this research, light-trapping structures in OSC electrodes, and their effect on light absorption profile is studied. The project divided into two main parts: modeling and experiments. Modeling is a good tool to study devices optical properties which experimentally are time and cost consuming. OSCs were simulated using finite element method by COMSOL Multiphysics 5. Different device configurations were studied and the effects of pitch and height dimensions on light absorption profile were investigated. In the experimental part, wrinkled PDMS substrates used as stamps for imprinting and transferring structures onto the electrodes. Then optical properties of fabricated devices were characterized and light absorption enhancement in respect to planar device was studied. Finally, three different configurations were characterized for OSC parameters under 1 sun illumination by solar simulator. 2 Master Thesis- Reza Abolhassani MCI, University of Southern Denmark Table of Contents Acknowledgements ....................................................................................................................................... 0 Abstract ......................................................................................................................................................... 2 List of Figures ................................................................................................................................................ 5 List of Tables ................................................................................................................................................. 9 List of Acronyms .......................................................................................................................................... 10 1. Introduction ............................................................................................................................................ 11 1.1. Project Background ..................................................................................................................... 11 1.2. Project Objectives ....................................................................................................................... 12 1.3. Outline......................................................................................................................................... 13 2. Theory ..................................................................................................................................................... 15 2.1. Introduction ................................................................................................................................ 15 2.2. Light Absorption in Thin-Films .................................................................................................... 16 2.3. Photovoltaic Effect ...................................................................................................................... 19 2.4. Basics of Solar Cell Operation ..................................................................................................... 20 2.5. Organic Solar Cells....................................................................................................................... 22 2.5.1. Organic Semiconductors ..................................................................................................... 22 2.5.2. Principles of Organic Solar Cells Operation .............................................................................. 23 2.5.3. Organic Solar Cells Structure............................................................................................... 24 2.5.4. Organic Solar Cell Characterization ........................................................................................... 27 2.5.5. Prospects and Challenges ......................................................................................................... 30 3. Simulation ............................................................................................................................................... 31 3.1. Introduction ..................................................................................................................................... 31 3.2. Simulation Methods ......................................................................................................................... 31 3.3. Comsol Multiphysics ........................................................................................................................ 33 3.3.1. Wave Optics Module; Theories and Principles ......................................................................... 33 3.4. Simulation Setup .............................................................................................................................. 35 4. Experimental ........................................................................................................................................... 38 4.1. Introduction ..................................................................................................................................... 38 4.2. Stamp Fabrication for Nano-imprinting ........................................................................................... 38 3 Master Thesis- Reza Abolhassani MCI, University of Southern Denmark 4.3. Fabrication Methods and Equipment .............................................................................................. 43 4.3.1. Sample Preparation................................................................................................................... 43 4.3.2. Nano-Imprinting ........................................................................................................................ 43 4.3.3. Metal Deposition ...................................................................................................................... 46 4.3.4. Organic Material Deposition ..................................................................................................... 48 4.4. Measurements and Characterization .............................................................................................. 50 4.4.1. Surface Characterization; Atomic Force Microscope ................................................................ 50 4.4.2. Absorption Measurement; Optical Microscope ....................................................................... 51 5. Results and Discussions........................................................................................................................... 52 5.1. PDMS Stamps ................................................................................................................................... 52 5.2. Simulation Results ............................................................................................................................ 56 5.3. Light Absorption in Organic Solar Cells ............................................................................................ 62 5.4. Organic Solar Cells Parameters ........................................................................................................ 67 6. Conclusion ............................................................................................................................................... 71 7. Outlook.................................................................................................................................................... 73 8. Bibliography ............................................................................................................................................ 75 9. Appendices