Synergistic Effects of Water and Oxygen Molecule Co-adsorption on (001) Surfaces of Tetragonal CH₃NH₃PbI₃: A First-Principles Study

The poor environmental stability of organometallic halide perovskite solar cells presents a big challenge for its commercialization, which is mainly due to the degradation of perovskite materials in humid air. The role played by water molecules has been extensively studied in the degradation processes, where strong interactions between water molecules and perovskite surfaces are found. Using first-principles simulations, we find that oxygen molecules also have strong interactions with (001) surfaces of tetragonal CH₃NH₃PbI₃ through the formation of a chemical Pb–O bond on the PbI₂-terminated surface and a hydrogen bond on the CH₃NH₃I-terminated surface. The adsorbed oxygen molecules introduce empty states near the Fermi level of the surfaces, which can facilitate charge transfer between the surface and oxygen molecules. Furthermore, when an oxygen molecule is located atop a Pb atom on PbI₂-terminated surface, the calculated adsorption energies indicate that the surface is more attractive to water molecules, making the surface even more sensitive to humidity. These findings reveal that oxygen molecules also play an important role in the initial stage of the degradation of perovskite materials