Crystallographic Orientation in Metal Halide Perovskite

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.