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

Abstract

Excess electrons from photo-excitation, impurities and defects play a significant role in the degradation of CH 3 NH 3 Pbl 3 (MAPbI 3 ) perovskite in the air [1-4]. However, this role has not fully been understood. Herein, the interactions between the MAI-terminated MAPbI 3 (110) surface and O 2 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.