Formamidinium-based Pb–Sn mixed-halide perovskite solar cells with near-optimal bandgap: FAPb 1− x Sn x (I 0.8 Br 0.2 ) 3

FA-based Pb–Sn binary perovskites with I–Br blend were developed via compositional engineering, demonstrating bandgaps close to the ideal range for single-junction solar cells.