Realizing High Efficiency over 20% of Low‐Bandgap Pb–Sn‐Alloyed Perovskite Solar Cells by In Situ Reduction of Sn4

Although the theoretical power conversion efficiency (PCE) of low‐bandgap Pb–Sn‐alloyed perovskite solar cells (PSCs) is higher than that of its conventional pure Pb counterpart, its device performance currently has been severely restricted by the large open‐circuit voltage (Voc) loss. Herein, it is discovered that the Sn4+‐induced trap states of the perovskite film can be effectively suppressed by introducing excess Sn powder into the precursor solution (FASnI3) to reduce the Sn4+ content. As a result, the average charge carrier lifetime of the perovskite film increases remarkably from 115 to 701 ns due to the suppressed nonradiative recombination, and the energy levels have up‐shifted by about 0.27 eV, rendering a more favorable energy‐level alignment at the interface. Ultimately, the champion PSCs using a low‐bandgap (FASnI3)0.6(MAPbI3)0.4 perovskite film with Sn4+ reduction show a high Voc of 0.843 V corresponding to a Voc loss as low as 0.397 eV and a high fill factor of 80.34%, leading to an impressive PCE of 20.7%, which is one of the few instances of a PCE over 20% for low‐bandgap mixed Pb–Sn PSCs to date. The in situ reduction of parasitic Sn4+ to Sn2+ by metallic tin powder effectively reduces Sn4+ content and thereby decreases the trap density of the perovskite films, giving rise to a remarkably long charge carrier lifetime and favorable energy‐level alignment at the interfaces. Consequently, a high power conversion efficiency of 20.7% is achieved for low‐bandgap Pb–Sn‐alloyed perovskite solar cells.