Ab Initio Study of Interaction of Water, Hydroxyl Radicals, and Hydroxide Ions with CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ Surfaces

Although there have been tremendous breakthroughs in perovskite solar cells over the past few years, degradation of perovskite has been a huge problem. Recently, a number of experimental studies have demonstrated that organic-inorganic halide perovskite materials are sensitive to humid air, and several degradation mechanisms have been proposed. However, the decomposition process of perovskites is only partially known and controversial. In this paper, we theoretically study the structures of the tetragonal CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ (110) surfaces and the degradation mechanism using density functional theory calculations both with and without the van der Waals correction. The computed results indicate that the CH₃NH₃⁺ (MA) cations preferentially orient with the NH₃ group pointing into the surface. This allows the formation of more hydrogen···halide hydrogen bonds between the MA cations and the halides. Moreover, the interactions of water molecules, hydroxyl radicals, and hydroxide ions with the perovskite surfaces are investigated. It has been suggested that the deprotonation of the MA cations followed by the desorption of the CH₃NH₂ molecules is a key step in the degradation mechanism. We found that the hydroxyl radicals and hydroxide ions facilitate this desorption process while water molecules have little effect on it. These present findings are pertinent to revealing the decomposition mechanisms of perovskite materials.