Mapping and manipulating optoelectronic processes in emerging photovoltaic materials By Sibel Yontz Leblebici A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering – Materials Science and Engineering in the Graduate Division of the University of California, Berkeley Committee in charge: Doctor Alexander Weber-Bargioni, Co-Chair Professor Ting Xu, Co-Chair Professor Junqiao Wu Professor Ana Arias Spring 2016 Mapping and manipulating optoelectronic processes in emerging photovoltaic materials Copyright 2016 By Sibel Yontz Leblebici Abstract Mapping and manipulating optoelectronic processes in emerging photovoltaic materials by Sibel Yontz Leblebici Doctor of Philosophy in Engineering - Materials Science and Engineering University of California, Berkeley Doctor Alexander Weber-Bargioni, Co-Chair Professor Ting Xu, Co-Chair The goal of the work in this dissertation is to understand and overcome the limiting optoelectronic processes in emerging second generation photovoltaic devices. There is an urgent need to mitigate global climate change by reducing greenhouse gas emissions. Renewable energy from photovoltaics has great potential to reduce emissions if the energy to manufacture the solar cell is much lower than the energy the solar cell generates. Two emerging thin film solar cell materials, organic semiconductors and hybrid organic-inorganic perovskites, meet this requirement because the active layers are processed at low temperatures, e.g. 150 °C. Other advantages of these two classes of materials include solution processability, composted of abundant materials, strongly light absorbing, highly tunable bandgaps, and low cost. Organic solar cells have evolved significantly from 1% efficient devices in 1989 to 11% efficient devices today. Although organic semiconductors are highly tunable and inexpensive, the main challenges to overcome are the large exciton binding energies and poor understanding of exciton dynamics. In my thesis, I optimized solar cells based on three new solution processable azadipyrromethene-based small molecules. I used the highest performing molecule to study the effect of increasing the permittivity of the material by incorporating a high permittivity small molecule into the active layer. The studies on two model systems, small donor molecules and a polymer-fullerene bulk heterojunction, show that Frenkel and charge transfer exciton binding energies can be manipulated by controlling permittivity, which impacts the solar cell efficiency. Hybrid organic-inorganic perovskite materials have similar advantages to organic semiconductors, but they are not excitonic, which is an added advantage for these materials. Although photovoltaics based on hybrid halide perovskite materials have exceeded 20% efficiency in only a few years of optimization, the loss mechanisms are not understood but critical to systematically improve the efficiency towards the theoretical limit. Thus, I correlated morphology to local power conversion efficiency determining properties by mapping local short circuit current, open circuit voltage, and dark drift current in state-of-the-art methylammonium lead iodide solar cells with sub-30 nm spatial resolution using photoconductive atomic force microscopy. I found, within individual grains, spatially-correlated heterogeneity in short circuit current and open circuit voltage. This heterogeneity is attributed to a facet-dependent density of trap states. These results imply that controlling crystal grain and facet orientation will enable a systematic optimization of hybrid halide perovskite photovoltaics. 1 Table of Contents List of Acronyms and Symbols ..................................................................................................... iii Acknowledgements ........................................................................................................................ vi Introduction ................................................................................................................................... 1 1 Motivation .................................................................................................................................... 2 2 Photovoltaic fundamentals ........................................................................................................... 6 2.1 Solar cell efficiency .............................................................................................................. 7 2.2 Shockely-Queisser limit: detailed balance ........................................................................... 8 3 Organic solar cells...................................................................................................................... 10 3.1 Photocurrent generation in organic solar cells ................................................................... 11 3.2 History of organic solar cells: materials and device architectures ..................................... 13 3.3 Technical challenges in organic solar cells ........................................................................ 16 4 Hybrid organic-inorganic perovskite solar cells ........................................................................ 27 4.1 Hybrid organic-inorganic perovskite material properties .................................................. 29 4.2 Short history of perovskite solar cells ................................................................................ 31 4.3 Perovskite solar cell operation ........................................................................................... 33 4.4 Advantages of hybrid perovskite solar cells ....................................................................... 34 4.5 Challenges of hybrid perovskite solar cells ........................................................................ 37 5 Solar cell characterization techniques ........................................................................................ 41 5.1 Power conversion efficiency .............................................................................................. 42 5.2 Solar cell quantum efficiency ............................................................................................. 42 5.3 Scanning probe microscopy ............................................................................................... 43 Exciton manipulation in organic solar cells.............................................................................. 51 6 Solution processable azadipyrromethene electron donors for planar heterojunction solar cells 52 6.1 Introduction ........................................................................................................................ 53 6.2 Materials and instrumentation ............................................................................................ 53 6.3 Device fabrication .............................................................................................................. 54 6.4 Material properties ............................................................................................................. 55 6.5 Organic solar cells .............................................................................................................. 59 6.6 Conclusions ........................................................................................................................ 61 7 Increasing organic solar cell efficiency by increasing permittivity to reduce exciton binding energy ............................................................................................................................................ 62 i 7.1 Introduction ........................................................................................................................ 63 7.2 Materials and methods ........................................................................................................ 63 7.3 Results and discussion ........................................................................................................ 65 7.4 Conclusion .......................................................................................................................... 74 8 Controlling charge transfer exciton binding energy in organic bulk heterojunction photovoltaics ................................................................................................................................. 75 8.1 Introduction ........................................................................................................................ 76 8.2 Film and device formation ................................................................................................. 77 8.3 Film characterization .......................................................................................................... 78 8.4 Results and discussion ........................................................................................................ 78 8.5 Conclusion .......................................................................................................................... 85 9 Exciton mobility and charge transfer in organic solar cell bulk heterojunctions from femtosecond stimulated Raman spectroscopy .............................................................................. 87 9.1 Introduction ........................................................................................................................ 88 9.2 Experimental methods .......................................................................................................