Thermal and illumination effects on a PbI2 nanoplate and its transformation to CH3NH3PbI3 perovskite

Vapor reaction is one of the major methods used for large-scale fabrication of high-quality organic–inorganic halide perovskite solar cells, but its detailed process is rarely investigated. In this study, the thermal and illumination effects on PbI2 and its transformation to CH3NH3PbI3 (MAPbI3) under CH3NH3I (MAI) vapor were investigated. Atomic shifting of the PbI2 layer was observed, which caused the generation of defects and distortion in PbI2 without breaking the phase structure. The reaction with MAI started with decoupling of PbI2 layers and formation of nanoislands and then, the transformation of PbI2 to MAPbI3 proceeded randomly on the surface from top to bottom. Additional illumination of the reaction suppressed the transformation, causing the generation of PbI2 nano-walls along [100] and [010] directions, leading to decrease in power conversion efficiency.