Potassium tetrafluoroborate-induced defect tolerance enables efficient wide-bandgap perovskite solar cells

Wide-bandgap (WBG) perovskites play a crucial role for top cells in tandem solar cells (TSCs), which provides a promising avenue to boost the performance of widely used commercial solar cells. However, such WBG perovskite solar cells (PSCs) show poor performance compared to that of ~1.6 eV bandgap PSCs due to high defects density and photo-instability, resulting in relatively large open-circuit voltage loss (Vloss). Herein, we introduce alkali pseudo-halide KBF4 into the perovskite precursor solution for preparing less-defect WBG perovskite film. It is showed that the interstitial occupancy of K+ in the perovskite lattice and the suppression of recombination by BF4-, thereby inhibiting the ion migration and reducing the trap density. As a result, the champion WBG PSC (Energy gap (Eg), Eg = 1.74 eV) delivers a high open-circuit voltage (VOC) of 1.21 V and a power conversion efficiency (PCE) of 17.49%. This work provides new insight into the defect tolerance upon metal pseudo-halides doping in the WBG perovskite.