Ab initio study of the role of oxygen and excess electrons in the degradation of CH₃NH₃PbI₃

Excess electrons from photo-excitation, impurities and defects play a significant role in the degradation of CH₃NH₃PbI₃ (MAPbI₃) perovskite in the air [1-4]. However, this role has not fully been understood. Herein, the interactions between the MAI-terminated MAPbI₃ (110) surface and O₂ molecules in the presence of excess electrons were studied by density functional theory calculations. Our results show that molecular O₂ only weakly interact with the perovskite surface. However, a superoxide, which is formed from the reaction between a molecular oxygen and an excess electron, react readily with the perovskite surface by forming a Pb-O covalent bond with a surface Pb. By further introducing an excess electron, the superoxide is converted into a peroxide and the two O form two covalent bonds with the surface Pb in a side-on configuration. With the additional electron, the activation energy of the O-O bond dissociation is significantly reduced compared to that of the superoxide. During these processes, the local Pb-I octahedral structure disintegrates. The formation of the Pb-O covalent bonds can be the precursor of the PbO in the degradation products. An additional O₂ or H₂O molecule was found to only physisorb on the degraded surface with no chemical reaction. However, the physisorbed O₂ can readily abstract an excess electron to form a superoxide and the resulting superoxide spontaneously forms an additional Pb-O bond with the surface Pb. Through this study, we identify a pathway for the formation of the PbO local structure and demonstrate the key roles of excess electrons and oxygens in MAPbI₃ degradation.