Role of Dipolar Organic Cations on Light-triggered Charge Transfer at TiO₂ /CH₃ NH₃ PbI₃ Interfaces

The TiO₂ /MAPbI₃ (MA=CH₃ NH₃ ) 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 TiO₂ /MAPbI₃ interfaces with MAI- and PbI-exposed terminations. It is found that regardless of the surface exposure of the MAPbI₃ , 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 CH₃ (NH₃ ) moieties of the MA species. Resultantly, the positively charged iodine vacancies at the TiO₂ /MAPbI₃ 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 TiO₂ /MAPbI₃ interfaces.