Microscopic investigations on the surface-state dependent moisture stability of a hybrid perovskite

Hybrid organic-inorganic perovskite (HOIP) materials have caught significant attention in photovoltaics and photoelectronics for their outstanding photovoltaic properties. However, their instability to various environment, such as illumination, temperature, moisture and oxygen, hinders their way to commercialization. To figure out the interaction mechanism between H₂O and CH₃NH₃PbI₃ (MAPbI₃), extensive theoretical studies have been carried out; however, the experimental results are insufficient and inconsistent. Here, we systematically investigate and compare the influence of H₂O on MAPbI₃ perovskite films with or without DMF) post-annealing in dark or light condition. The interaction between H₂O and the surface of pristine MAPbI₃ leads to the fusion of grain boundaries thus grain growth into micron level in short-time moisture exposure. While the penetration of H₂O into MAPbI₃ results in swelled crystalline whisker, cracking into smaller grains in long-time exposure upon the release of H₂O. However, no degradation occurs in dark condition. As the DMF post-annealing treatment changes the surface states of MAPbI₃, the interactions between the external H₂O and internal MAPbI₃ significantly varies from the pristine MAPbI₃. Three different surface states with different topographies have influence on the interaction process and mechanism with H₂O, leading to different decomposition rates, the striped surface that is the most rough among the three and experiencing the minimum change in surface potential with exposure to 80% humidity decomposes into PbI₂ fastest. However, the addition of light will once again affect the aforementioned process. It is found that even ambient light could severely speed up the moisture-induced decomposition of MAPbI₃, while the N,N-dimethylformamide (DMF) post-annealing treatment significantly improves the stability of MAPbI₃ films upon exposure to humidity and illumination, benefiting from the MAI-deficient thus H₂O resistant surface.