Crystallographic Orientation in Metal Halide Perovskite 374 views

Metal halide perovskites are revolutionizing the field of photovoltaics with their phenomenal performance in the past ten years when the efficiency increased from 3.8% to more than 25%. Metal halide perovskites are unique in that they combine low-cost solution processability with superb electronic quality that is comparable to or surpasses the best epitaxial grown semiconductors. A key parameter in perovskite thin film is the crystallographic orientation. However, the crystallization process of the thin film orientation is poorly understood in both two-dimensional and three-dimensional perovskites, as well as its effect on the optoelectronic performance. In the first part of the study, I show the nucleation mechanism for a vertical orientation in two-dimensional perovskite thin films, where nucleation originates from the highly anisotropic liquid-air interface. The initial crystallization at this interface forms a vertically oriented crust, which can template the growth into a vertically oriented thin film. In the thin film, the layers are aligned to facilitate the charge transfer in a vertical direction, which is desirable in devices with vertical charge transport such as solar cells and light emitting diodes. As a consequence of the top-down crystallization mechanism, high-quality 2D metal halide perovskite thin films can be deposited on various substrates ranging from polymers to metal oxides. In the second part, I studied the subsequent growth mechanism following the nucleation at the liquid-air interface to provide a complete picture of the crystallization process. I report a general crystallization mechanism for 2D perovskites, where solvent evaporation and crystal growth compete to influence the level of supersaturation, and a low supersaturation is necessary to crystallize a highly vertically oriented thin film. In the third part of the study I show that in three-dimensional perovskite, the orientation of a thin seed layer at the top can propagate to the thick bottom layer. A simple treatment based on methylammonium chloride was developed to change the preferential orientation of an already-formed perovskite thin film and significantly improve the solar cell performance.

PHD (Doctor of Philosophy)

English

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2019-10-08