Mechanism and performance effects of water and oxygen molecules on the surface stability of FAPbI₃

The degradation of metal halide perovskite surfaces induced by H₂O and O₂ molecules severely limits the performance and practical application of perovskite solar cells. In this work, density functional theory (DFT) calculations are performed to systematically investigate the degradation mechanisms occurring on the FAPbI₃ (001) surface upon exposure to H₂O and O₂, and the resulting changes in electronic and optical properties. We find that the outermost Pb-terminated surface is particularly susceptible to degradation by H₂O and O₂, mainly due to the disruption of Pb-I bonds by these small molecules, which induces severe lattice distortion and further promotes molecular adsorption. H₂O and O₂ together adsorb more readily to the surface of the perovskite than when the two are present alone. Furthermore, a capping graphene layer can effectively prevent the permeation of H₂O and O₂ molecules, significantly improving the environmental stability of the perovskite. Graphene coating enhances both infrared and visible-light absorption, achieving a 16% increase in the visible range compared to the pristine surface. Graphene oxide provides superior blocking capability while further enhancing infrared absorption.