PNNL

Abstract

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 have been known to easily degrade in the presence of water molecules. However, even though there have been numerous studies investigating this phenomenon, there is still no consensus on the mechanisms of 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 the mechanisms, kinetics, and thermodynamics that accompany the degradation of MAPbI3 HOIP in liquid water. We consider not only the dissolution of the species found on the pristine MAPbI3 HOIP surfaces, but also the dissolution from defect sites to imitate the successive degradation steps, and propose a sequence of events in the initial phase of MAPbI3 HOIP dissolution. By comparing the dissolution free energy barrier between surface species of different surficial types, we find that the dominant dissolution mechanisms of surface species varies widely based on the specific surface features adjacent to the dissolving ion. The high sensitivity of 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 are inconsequential toward the dissolution free energy barrier. However, the presence of surface defects such as vacancy sites are found to significantly lower the dissolution free energy barriers. Based on the estimated dissolution free energy barriers from different types of surfaces that we investigated in this study, we proposed 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.