Role of Surface Features on the Initial Dissolution of CH₃NH₃PbI₃ Perovskite in Liquid Water: An Ab Initio Molecular...

The degradation of CH₃NH₃PbI₃ (MAPbI₃) hybrid organic inorganic perovskite (HOIP) by water has been the major issue hampering its use in commercial perovskites solar cells (PSCs), as MAPbI₃ HOIP has been known to easily degrade in the presence of water. Even though there have been numerous studies investigating this phenomenon, there is still no consensus on the mechanisms of the initial stages of dissolution. Here, we attempt to consolidate differing mechanistic interpretations previously reported in the literature through the use of the first-principles constrained ab initio molecular dynamics (AIMD) to study both the energetics and mechanisms that accompany the degradation of MAPbI₃ HOIP in liquid water. By comparing the dissolution free energy barrier between surface species of different surficial types, we find that the dominant dissolution mechanisms of surface species vary widely based on the specific surface features. The high sensitivity of the dissolution mechanism to surface features has contributed to the many dissolution mechanisms proposed in the literature. In contrast, the dissolution free energy barriers are mainly determined by the dissolving species rather than the type of surfaces, and the type of surfaces the ions are dissolving from is inconsequential toward the dissolution free energy barrier. However, the presence of surface defects such as vacancy sites is found to significantly lower the dissolution free energy barriers. Based on the estimated dissolution free energy barriers, we propose that the dissolution of MAPbI₃ HOIP in liquid water originates from surface defect sites that propagate laterally along the surface layer of the MAPbI₃ HOIP crystal.