Structural and electronic properties of hybrid halide perovskites : insights into chemical vapor deposited films
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In the past several years, hybrid perovskite semiconductors that have been incorporated into solar cells have experienced an extremely rapid increase in power conversion efficiency that now rivals traditional silicon based devices. Hybrid organic-inorganic (HOIP) halide perovskites combine inorganic, metallic materials such as lead and tin, with organic compound groups such as methylammonium or formamidinium along with halogen atoms such as iodine, chlorine, or bromine. The combination of ions based off of those elements culminates in thin films with a variety of fortuitous characteristics. HOIP materials exhibit high absorption coefficients, tunable band gaps, extremely high defect tolerance in comparison to silicon, and allow for low-temperature solution processing and vapor deposition growth methods. Unfortunately, the main obstacle preventing mass commercialization of hybrid perovskite solar cells is stability. Moisture, heat, and light induce substantial stress onto perovskite materials and cause irreversible, destructive degradation. As such, research is now focused on developing perovskite thin films that can withstand external stimuli that may cause degradation mechanisms to occur in perovskite based devices. Many methods exist that help improve perovskite stability including modifications to the growth technique to improve the intrinsic sample quality, composition engineering of the perovskite material, encapsulation with polymers or glass, post-treatment annealing or vapor assisted defect passivation, and many more. This work primarily focuses on developing and describing precisely how chemical vapor deposition (CVD) as a growth technique, in contrast to many solution-based methods such as spin coating, greatly impacts the downstream characteristics of the perovskite thin films by enhancing stability, favorable charge transport, and shifting the structural phase diagram of the material. Furthermore, this work demonstrates that many characteristics of perovskite materials that are conventionally thought of as intrinsic are actually greatly dependent, in practice, on the growth environment that ultimately produces the thin films as opposed to idealized models which assume very low defect densities. This work demonstrates air stable perovskite thin films through CVD that exist in the ideal cubic phase at room temperature with increased electronic transport. High hydrostatic pressure was used as a probe to elucidate the phase diagram and optical pressure response of 3D perovskites. Additionally, the CVD method can be augmented to produce chlorine incorporated perovskite thin films that exhibit a single phase structure across a wide temperature range where chlorine acts as a defect passivator. More exotic forms of the material such as 2D Ruddlesden-Popper (RP) were grown with improved columnar alignment in comparison to spin coated films. Finally, a post-deposition electron-beam irradiation treatment is explored and shows how low doses of radiation of CVD grown films can improve optical and electrical performance.
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Ph. D.