Insight into the reaction mechanism of water, oxygen and nitrogen molecules on a tin iodine perovskite surface

Abstract

In this article, in order to study the lead-free perovskite CH₃NH₃SnI₃ (MASnI₃) reaction mechanism between perovskite surface molecules and gas molecules, we fabricated models for three different gas molecules adsorbing on an MASnI₃ surface. First, the surface energies of seven low-index surfaces ((100), (010), (001), (110), (101), (011) and (111)) of orthorhombic MASnI₃ were calculated by density functional theory (DFT). The results showed the (011) surface was the most stable among the seven low index surfaces by the PW91 method. Secondly, three stable configurations were sorted out from these adsorption models of different molecules (H₂O, O₂, N₂) at six positions on the (011) surface. For H₂O, O₂ and N₂ adsorption, the smallest adsorption energies were −1.931(934) eV, −0.134(67) eV and −0.049(523) eV, respectively. The stabilities of configurations for H₂O adsorption were affected by the strong interaction of the H–I bond. However, for O₂ adsorption, the Sn–O bond and charge transfer between the Sn atom and O atom mostly affected the stability of the configuration. We found that the MASnI₃ surface under a N₂ atmosphere maintained a higher stability than in the other two cases. In addition, we further confirmed that the mechanism of H₂O adsorption was not the same as O₂ and N₂ molecular adsorption from the Mulliken population, energy band and electrical density of state calculations. These results indicated that the H–I bond and Sn–O bond mainly affect the Sn–I bond, leading to MASnI₃ stability being worse in H₂O and O₂ environments, respectively, and performance in a N₂ environment is more stable than in O₂ and H₂O environments. This investigation enhanced understanding of the degradation of perovskite and provided some suggestions for developing more stable lead-free perovskite solar cells and devices.