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 air. 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 mobile excess electrons were studied by density functional theory calculations. Our results show that molecular O2 only weakly interacts with the perovskite surface. However, a superoxide, which is formed from the reaction between a molecular oxygen and an excess electron, reacts readily with the perovskite surface by forming a Pb–O covalent bond with a surface Pb ion. By further introducing an excess electron, the superoxide is converted into a peroxide and the two O atoms 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 reactions. 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 mobile excess electrons and oxygens in MAPbI3 degradation.