Predicting the Morphology of Perovskite Thin Films Produced by Sequential Deposition Method: A Crystal Growth Dynamics Study

We performed a kinetic analysis of the sequential deposition method (SDM) to investigate how to form perovskite (CH₃NH₃PbI₃) phases, and the effects of processing conditions on the final perovskite morphology. The reaction was found to consist of two periods with distinct kinetics. During the first period, perovskite crystals nucleated on the lead iodide (PbI₂) surface, and the reaction proceeded until the surface was completely converted to perovskites. The reaction during this period determined the surface morphology of the perovskites. We were able to extract the value of the rate of the phase transformation during the first period by applying the Johnson-Mehl-Avrami-Kolmogorov model, in which the rate r is related to the average grain size R by R ∝ r^-1/3. In this way, r was used to predict the surface morphology of the perovskite under certain processing conditions. During the second period, the remaining lead iodide under the top perovskite layer was converted. Methylammonium iodide (CH₃NH₃I, MAI) molecules apparently diffused into the buried PbI₂ through intergrain gaps of the top perovskite layers. Added MAI molecules reacted with PbI₂ but also generated single-crystal perovskite nanorods, nanoplates, and nanocubes. The current study has furthered the understanding of detailed features of the SDM, enabled a reliable prediction of the final perovskite morphology resulting from specified processing conditions, and contributed to a reproducible fabrication of high-quality perovskite films.