Fluoride-assisted crystallization regulation enables efficient and stable wide-bandgap perovskite photovoltaic

Mixed halogen wide-bandgap perovskite materials are often applied to the top cells of tandem solar cells. Nevertheless, serious halogen vacancy defects in mixed halogen perovskite materials remain one of the main factors restricting the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Fluoride has been proposed to inhibit halogen vacancy defects, but fluoride-induced crystallization behaviour is not thoroughly understood in wide-bandgap mixed halide perovskite systems. Here, we introduce tetrafluoroborate ion (BF4-) into the wide-bandgap perovskite precursor solution to regulate the crystallization dynamics. Due to high electronegativity and strong electron affinity, a strong interaction between BF4- and the [PbX6](4-) octahedron increases the formation energy of perovskite crystal, inhibiting perovskite nucleation. The reduced nucleation site slows down the crystallization rate of the perovskite film. Moreover, we demonstrate that BF4- enters the perovskite lattice and fills the halogen vacancy defect. The reduced trap density and optimized energy level structure guarantee high device performance. As a result, the wide-bandgap PSC with BF4- achieves an excellent PCE of 20.09%. Benefitting from the eliminated halogen vacancies, the device achieves improved operating stability, and it maintains more than 80% of its initial PCE under continuous 1-sun illumination for 1000 h.