In CH NH PbI Perovskite Film, the Surface Termination Layer Dominates the Moisture Degradation Pathway.

Theoretical studies have shown that surface terminations, such as MAI or PbI layers, greatly affect the environmental stability of organic-inorganic perovskite. However, until now, there has been little effort to experimentally detect the existence of MAI or PbI terminations on MAPbI grains, let alone disclose their effects on the humidity degradation pathway of perovskite solar cell. Here, we successfully modified and detected the surface terminations of MAI and PbI species on polycrystalline MAPbI films. MAI-terminated perovskite film followed the moisture degradation process from MAPbI to hydrate MAPbI ⋅H O and then into PbI , with penetration of water molecules being the main driving force leading to the degradation of MAPbI layer by layer. In contrast, for the PbI-terminated perovskite film in a humid atmosphere, a deprotonation degradation pathway was confirmed, in which the film preferentially degraded directly from MAPbI into PbI , here the iodine defects played a key role in promoting the dissociation of water molecules into OH and further catalyzing the decomposition of perovskite.