Highly Oriented Large-Grain 2D Cs₃Bi₂X₉ Polycrystalline Films by an Isogenous-Lattice Homoepitaxy Strategy for Photodetection

Outstanding optoelectronic performances, including high carrier mobility and long carrier diffusion length, have only been observed in single-crystalline Cs₃Bi₂X₉, which requires a lengthy fabrication process but not in the easily formed polycrystalline solids. This discrepancy arises from the disordered crystallization and the resultant unsatisfactory film quality. Herein, we propose an isogenous-lattice homoepitaxy strategy to induce the crystallization of highly oriented, large-grain two-dimensional (2D) Cs₃Bi₂X₉ films via the in situ precrystallized, lattice-matched isogenous three-dimensional (3D) Cs₂AgBiBr₆ intermediate. The introduced 3D Cs₂AgBiBr₆ intermediate serves as a primer to initiate and direct the oriented epitaxy of 2D Cs₃Bi₂X₉ while significantly retarding the crystallization process through an additional halogen exchange process, leading to films with grains over 1 μm in size and a highly consistent crystallization orientation. Consequently, the target films exhibit photophysical properties comparable to those of single crystals and superior photodetection performance.