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

Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy

With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for next generation, low-cost photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that form during the fabrication process, remains critical to overall device operation, including efficiency and long-term stability. 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. Here, by correlating photoemission and synchrotronbased scanning probe X-ray microscopies, we unveil three different types of defect clusters in state-of-the-art triple cation mixed halide perovskite thin films. Incorporating ultrafast time-resolution into our photoemission measurements, we show that defect clusters originating at grain boundaries are the most detrimental for photocarrier trapping, while lead iodide defect clusters are relatively benign. Hexagonal polytype defect clusters are only mildly detrimental individually, but can have a significant impact overall if abundant in occurrence. We also show that passivating defects with oxygen in the presence of light, a previously used approach to improve efficiency, has a varied impact on the different types of defects. Even with just mild oxygen treatment, the grain boundary defects are completely healed, while the lead iodide defects begin to show signs of chemical alteration. Our findings highlight the need for multi-pronged strategies tailored to selectively address the detrimental impact of the different defect types in hybrid perovskite solar cells

Local nanoscale phase impurities are degradation sites in halide perovskites.

Understanding the nanoscopic chemical and structural changes that drive instabilities in emerging energy materials is essential for mitigating device degradation. The power conversion efficiency of halide perovskite photovoltaic devices has reached 25.7 per cent in single-junction and 29.8 per cent in tandem perovskite/silicon cells1,2, yet retaining such performance under continuous operation has remained elusive3. Here we develop a multimodal microscopy toolkit to reveal that in leading formamidinium-rich perovskite absorbers, nanoscale phase impurities, including hexagonal polytype and lead iodide inclusions, are not only traps for photoexcited carriers, which themselves reduce performance4,5, but also, through the same trapping process, are sites at which photochemical degradation of the absorber layer is seeded. We visualize illumination-induced structural changes at phase impurities associated with trap clusters, revealing that even trace amounts of these phases, otherwise undetected with bulk measurements, compromise device longevity. The type and distribution of these unwanted phase inclusions depends on the film composition and processing, with the presence of polytypes being most detrimental for film photo-stability. Importantly, we reveal that both performance losses and intrinsic degradation processes can be mitigated by modulating these defective phase impurities, and demonstrate that this requires careful tuning of local structural and chemical properties. This multimodal workflow to correlate the nanoscopic landscape of beam-sensitive energy materials will be applicable to a wide range of semiconductors for which a local picture of performance and operational stability has yet to be established

Research Data Supporting Visualising Performance-Limiting Nanoscale Trap Clusters at Grain Junctions in Halide Perovskites

This repository contains data necessary to reproduce figures and results from the associated manuscript. Files included contain data from scanning electron diffraction , photo emission electron microscopy, scanning transmission electron microscopy - energy dispersive X-ray spectroscopy, Kelvin probe force microscopy and photoluminescence measurements