Halide Perovskite Crystallization Processes and Methods in Nanocrystals, Single Crystals, and Thin Films

Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High‐performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (Cmin). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation‐dominated process by rapidly increasing the concentration over Cmin, single crystals are mainly prepared via the growth‐dominated process by keeping the concentration between solubility and Cmin, while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed. The nucleation process is more thermodynamically difficult by involving a higher surface molecular unit ratio than the growth process of solution‐processed halide perovskite (HP) crystals. Nucleation occurs under a higher supersaturation (C > Cmin) while growth can occur when C is over Cs. Typical preparation methods for nucleation‐dominated HP nanocrystals, growth‐dominated HP single crystals, and nucleation‐and‐growth‐balanced HP thin films are reviewed.