Characterization of Nanoscale Defects in Hybrid Perovskite Thin Films for Photovoltaic Applications

Hybrid halide perovskites have emerged as one of the most promising contenders for next generation, low-cost photovoltaic technologies. Thanks to the remarkable optoelectronic properties of hybrid perovskite absorbers, perovskite solar cells now achieve efficiencies comparable to conventional inorganic solar cells (Si, GaAs), despite being actively researched for only about a decade. The ability to be processed from solution and to be deposited on transparent and flexible substrates, makes them very attractive for various photovoltaic applications. However, before their wide commercialization, hybrid perovskites need to overcome important limitations. In particular, the presence of defects in perovskite thin films has been detrimental to material properties, and has been a critical reason preventing devices from reaching their full potential. To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies. In this thesis, we employed photoemission electron microscopy to directly image nanoscale defects, and uncovered the presence of multiple types of defects in state-of-the-art perovskite thin films. By adding time resolution to our photoemission electron microscopy measurements, we found that depending on their nature, these defects played varied roles in charge carrier trapping – from highly detrimental to relatively benign. Further, we also found them to show varied response to passivation strategies, as seen from our photoemission measurements. With this work, by identifying the origins of various defects occurring in perovskite thin films and highlighting importance of designing meaningful and targeted strategies to overcome them, as well as demonstrating sophisticated yet greatly rewarding tools to detect these very nanoscale defect-rich sites, we hope to contribute to development of more viable and durable perovskite photovoltaics.Okinawa Institute of Science and Technology Graduate Universit