Surface Termination Layer Dominating the Moisture Degradation Pathway in CH3NH3PbI3 Perovskite Film.

Theoretical studies have disclosed that the surface terminations such as MAI- or PbI-layers affect greatly on the environmental stability of organic-inorganic perovskite. However, until now, there have been rarely direct evidence trying to experimentally detect the existence of MAI or PbI terminations on MAPbI3 grains, let along disclosing 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 the polycrystalline MAPbI3 films. For the MAI-terminated perovskite film, it followed the moisture degradation process from MAPbI3 to hydrate MAPbI3·H2O and then into PbI2, penetration of water molecules was the main driving force to lead the degradation of MAPbI3 layer by layer. While, for the PbI-terminated perovskite film in humidity, a deprotonation degradation pathway was confirmed, in which it preferred to directly degrade from MAPbI3 into PbI2, while the iodine defects here played a key role in promoting the dissociation of water molecules into hydroxyl OH- and further catalyzing the decomposition of perovskite.