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

Abstract

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.