Water in hybrid perovskites: Bulk MAPbI3 degradation via super-hydrous state

Abstract

Here, first-principles density functional theory calculations are presented which reveal how water incorporation in hybrid halide perovskite [CH3NH3]PbI3 (MAPbI3) catalyzes the phase transition to the ([CH3NH3]PbI3.H2O edge-sharing) monohydrate (colorless) phase, eliminating its favorable photovoltaic properties. First, fundamental chemical and electrostatic interactions between water and each component of MAPbI3 are analyzed, demonstrating their dependence on water concentration. Second, the energetics of incorporated water is explored, leading to the discovery of spontaneous phase segregation into dry regions and regions with more than one water per formula unit - termed the “super-hydrous state.” Third, the properties of the super-hydrous state are analyzed, including the acceleration of octahedron breaking and rearrangement by the high water density. This reveals the phase transformation to be a bulk process, initiated at the super-hydrous regions. This paper concludes with a discussion of how this super-hydrous model explains disparate recent experimental observations concerning the water-induced transition from (black) perovskite to edge-sharing PbI2 (yellow) phase.