Purification of Tin Iodide through Solid‐State Reaction for Efficient Tin‐Lead Mixed Perovskite Solar Cells

Tin‐lead (Sn‐Pb) mixed perovskite solar cells (PSCs), which serve as the rear subcell, have achieved significant advancements, largely driving the remarkable power conversion efficiency (PCE) seen in recent all‐perovskite tandem solar cells (TSCs). However, the efficiency of Sn‐Pb mixed PSCs is limited by the oxidation susceptibility of Sn 2+ , which induces the formation of Sn vacancies in the perovskite thin films, leading to an increase in defect densities. To address this issue, a novel approach called metal‐assisted solid‐state reduction (MASSR) is introduced that effectively purifies tin iodide (SnI 2 ), a critical component in the fabrication of Sn‐Pb mixed perovskite thin films. The MASSR process controls the oxidation state of the SnI 2 , thereby reducing trap densities and minimizing non‐radiative recombination losses in the perovskite thin films, which improves the overall efficiency of Sn‐Pb mixed PSCs. Consequently, single‐junction Sn‐Pb mixed PSCs utilizing MASSR‐treated SnI 2 achieve a high PCE of 22.86%, and their integration with wide‐bandgap (WBG) PSCs enables the fabrication of all‐perovskite TSCs with a PCE of 26.88%.