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.