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 (V-oc) 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 (FASnI(3)) to reduce the Sn4+ content. As a result, the average charge carrier lifetime of the perovskite film increases remarkably from 115 to 701ns due to the suppressed nonradiative recombination, and the energy levels have up-shifted by about 0.27eV, rendering a more favorable energy-level alignment at the interface. Ultimately, the champion PSCs using a low-bandgap (FASnI(3))(0.6)(MAPbI(3))(0.4) perovskite film with Sn4+ reduction show a high V-oc of 0.843V corresponding to a V-oc loss as low as 0.397eV 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.