All-perovskite tandem solar cells show great promise but are hindered by surface defects causing non-radiative recombination in Sn-Pb mixed narrow bandgap perovskite films. This study reports a surface reconstruction strategy using 1,4-butanediamine (BDA) for polishing and ethylenediammonium diiodide (EDAI₂) for passivation. This approach eliminates Sn-related defects and passivates organic cation and halide vacancy defects, leading to high-quality Sn-Pb mixed perovskite films and minimized energy loss. Sn-Pb mixed perovskite solar cells with bandgaps of 1.32 and 1.25 eV achieved power conversion efficiencies of 22.65% and 23.32%, respectively. Two-junction all-perovskite tandem solar cells reached a certified power conversion efficiency of 28.49%.
