Crystallographic orientation and layer impurities in two-dimensional metal halide perovskite thin films

Two-dimensional (2D) metal halide perovskites have achieved similar or even better performance than three-dimensional (3D) perovskites in various optoelectronic device applications. With superior long-term stability, film formation properties, and chemical tunability, 2D perovskites are rising to be a promising alternative to 3D perovskites for research and commercialization. There are unique aspects of 2D perovskites that need to be understood in-depth due to their significant impact on device performance. In particular, understanding the impact of different crystallographic orientations and the presence of different octahedral layer number impurities are critically important. In this review, the authors summarize the formation mechanism of the preferential crystallographic orientation in 2D perovskite thin films and layer impurities, discuss the available strategies to control these aspects for higher device performance in various applications, and propose future research directions that could provide a strategy to rationally improve 2D perovskite-based devices.