From Heterostructures to Solid‐Solutions: Structural Tunability in Mixed Halide Perovskites

Abstract The stability, reliability, and performance of halide‐perovskite‐based devices depend upon the structure, composition, and particle size of the device‐enabling materials. Indeed, the degree of ion mixing in multicomponent perovskite crystals, although challenging to control, is a key factor in determining properties. Herein, an emerging method termed evaporation–crystallization polymer pen lithography is used to synthesize and systematically study the degree of ionic mixing of Cs 0.5 FA 0.5 PbX 3 (FA = formamidinium; X = halide anion, ABX 3 ) crystals, as a function of size, temperature, and composition. These experiments have led to the discovery of a heterostructure morphology where the A‐site cations, Cs and FA, are segregated into the core and edge layers, respectively. Simulation and experimental results indicate that the heterostructures form as a consequence of a combination of both differences in solubility of the two ions in solution and the enthalpic preference for Cs–FA ion segregation. This preference for segregation can be overcome to form a solid‐solution by decreasing crystal size (<60 nm) or increasing temperature. Finally, these tools are utilized to identify and synthesize solid‐solution nanocrystals of Cs 0.5 FA 0.5 Pb(Br/I) 3 that significantly suppress photoinduced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials.