Water-Assisted Diffusional Phase Transitions in Inorganic Metal Halide Perovskites

In experiments, water is observed to accelerate the black-yellow phase transition in inorganic metal halide perovskites (MHPs). However, the underlying microscopic mechanisms for this phenomenon remain unclear. In this study, we employ classical molecular dynamics simulations to examine the role of water molecules in the yellow-black phase transition in the typical inorganic MHP CsPbI3. Our simulation results demonstrate that the black-yellow phase transitions in CsPbI3 follow a crystal-amorphous-crystal two-step mechanism and that water molecules in the air can enter the amorphous interface between the black and yellow regions. The rate of the yellow-black phase transition markedly increases with the influx of interfacial water molecules, which enhance ion diffusivity by reducing the diffusion barrier, thereby expediting the yellow-black phase transition in CsPbI3. We discuss implications for the reverse black-yellow phase transition, which is known to degrade the photovoltaic properties of perovskites, and we present a general mechanism through which solvent molecules can greatly facilitate phase transitions that otherwise have prohibitively high transition energies.