Ultrasmooth metal halide perovskite thin films via sol–gel processing

Abstract

We demonstrate that lead halide perovskite thin film formation displays the characteristics of a sol–gel process. By performing a solvent exchange at different times, the stages of the sol–gel process are elucidated and their sensitivity to processing conditions are examined. For CH₃NH₃PbI₃, the reaction and aging kinetics are found to be extremely rapid, complete within 10 s, and a competing formation of a highly crystalline PbI₂:N,N-dimethylformamide (DMF) complex introduces additional complications relative to a well-behaved sol–gel process. Perovskite formation from strongly polar solvents can be described, in most cases, by the sol–gel processing of PbI₂ regarding other solution components as additives. Understanding the details of additive and environmental influences on the sol–gel properties allow us to exploit fundamental concepts of sol–gel engineering to direct solvent and surfactant choices to control particle size and demonstrate multiple widely employed lead halide perovskite films (CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CsPbBr₃) with unprecedented surface roughness of less than 2 nm.