Role of Dipolar Organic Cations on Light-triggered Charge Transfer at TiO /CH NH PbI Interfaces.

The TiO /MAPbI (MA=CH NH ) interfaces have manifested correlation with current-voltage hysteresis in perovskite solar cells (PSCs) under light illumination conditions, but the relations between the photo-induced charge transfer and the collective polarization response of the dipolar MA cations are largely unexplored. In this work, we adopt density functional theory (DFT) and time-dependent DFT approach to study the light-triggered charge transfer across the TiO /MAPbI interfaces with MAI- and PbI-exposed terminations. It is found that regardless of the surface exposure of the MAPbI , the photo-induced charge transfer varies when going from the ground-state geometries to the excited-state configurations. Besides, thanks to the electrostatic interactions between the ends of MA cations and the photogenerated electrons, the photo-induced charge transfer across the interfaces is enhanced (weakened) by the negatively (positively) charged CH (NH ) moieties of the MA species. Resultantly, the positively charged iodine vacancies at the TiO /MAPbI interfaces tend to inhibit the charge transfer induced by light. Combining with the energy level alignment which is significantly modulated by the orientation of the MA species at the interfaces, the dipolar MA cations might be a double-edge sword for the hysteresis in PSCs with the TiO /MAPbI interfaces.