Ab initio study of the dynamics of electron trapping and detrapping processes in the CH3NH3PbI3 perovskite

Charge trapping and detrapping are responsible for a number of unusual properties in the CH3NH3PbI3 (MAPbI(3)) perovskite such as photoinduced ion migration, photodegradation, and current density-voltage hysteresis. However, little is known about the dynamic processes of charge trapping and detrapping at the trap sites. Herein, the behavior of excess electron trapping and its interplay with iodine vacancies in the MAI-terminated and PbI2-terminated MAPbI(3) surfaces were studied using density functional theory (DFT) calculations. The results show that the positively charged iodine vacancy (V-I(+)) is energetically more favorable to stay at the top-surface layer. By trapping excess electrons, the iodine vacancy migrates from the top-surface layer to the subsurface layer due to the formation of deep-level trap states. On the other hand, the adsorption of OH radicals and O-2 molecules induces electron detrapping from the deep-level trap states and iodine vacancies migration from the subsurface back to the top-surface layer. Furthermore, we studied the electron trapping and detrapping processes in bulk MAPbI(3). We found that in bulk MAPbI(3), the excess electrons facilitate the clustering of the iodine vacancies together and form deep trap states via the iodine migration process. However, electron detrapping was found to induce the hopping of two adjacent iodine vacancies away from each other. Our results show that while excess electrons trapped in both the shallow-level and deep-level states enhance the photodegradation of MAPbI(3) under humid air conditions, the electrons in the deep-level traps play a significantly more important role in facilitating photoinduced ion migration and the hysteresis behavior.