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

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 (EC-PPL) 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, we utilized these tools to identify and synthesize solid-solution nanocrystals of Cs 0.5 FA 0.5 Pb(Br/I) 3 that significantly suppress photo-induced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials. This article is protected by copyright. All rights reserved.