<i>In Situ</i> Observation of Surface Decomposition and Recrystallization of Post Synthesis Metal Halide Perovskite Thin Films

We present an in situ atomic force microscopy (AFM) study of surface dynamics in solution-processed MAPbI3 thin films under ambient conditions. Time-resolved AFM imaging reveals location-dependent decomposition in certain exposed regions alongside simultaneous recrystallization in adjacent areas, with crystal growth primarily occurring via edges from surface screw dislocations or pre-existing ones. These dynamic processes are significantly suppressed on the surface of MA0.4FA0.6PbI3 films. We propose a surface decomposition-diffusion-integration mechanism to explain these phenomena. The decomposition products (PbI2 and MAI) can stay and diffuse across surface steps and combine at low-energy edges, as supported by first-principles calculations. Our work provides direct experimental evidence for rich surface dynamics in the post synthesis MAPbI3 and MA0.4FA0.6PbI3 thin films under ambient conditions and reveals decomposition and surface recrystallization as key processes affecting film surface properties. These findings offer new insights into the metal halide perovskite film growth, surface stability, and microscopic degradation mechanisms.