Unravelling the Mechanism of Ionic Fullerene Passivation for Efficient and Stable Methylammonium-Free Perovskite Solar Cells

Despite having attractive stability over the volatile methylammonium (MA) cation, double-cation (Cs, FA) perovskite solar cells are largely overlooked because of their inferior performance compared to MA-based devices. Among all the device engineering strategies, surface passivation represents a promising approach to acquire improved performance. However, effective passivation strategies have not yet been developed for attaining efficiencies of MA-free cells close to their theoretical limit. Herein, fullerene passivators with different binding groups are investigated to establish relationships between molecule structure and perovskite surface properties. It is found that surface treatment with bis-fulleropyrrolidium iodide (bFPI) can have strong interaction with charged defects, leading to effective defect passivation and favorable band-bending at the interface. The resulting bFPI-treated device shows significantly reduced defect density as well as accelerated electron extraction from perovskite into the cathode. Consequently, the MA-free device exhibits an efficiency of 21.1% with long-term environmental stability.