Trapping Tetravalent Tin and Protecting Stannous in Tin‐Lead Mixed Perovskites for Efficient All‐Perovskite Tandem Solar Cells

Abstract Tin‐lead (Sn‐Pb) mixed perovskite solar cells (PSCs), which serve as bottom subcells in all‐perovskite tandem solar cells, are pivotal for developing highly efficient solar cells. However, the vulnerability of stannous (Sn²⁺) to spontaneous oxidation to harmful tetravalent tin (Sn⁴⁺) presents significant challenges. Here, a “mouse glue trap” strategy is proposed to mitigate this issue by introducing a multifunctional additive, oxamidic acid potassium salt (OAPS). This approach effectively traps undesired Sn⁴⁺ impurities through strong interactions between the oxaminic acid groups of OAPS and Sn⁴⁺ impurities. Additionally, OAPS, with its unique functional groups, can inhibit Sn²⁺ oxidation, passivate defects, alleviate stress, and improve crystalline quality in Sn‐Pb mixed perovskite films. As a result, the enhanced Sn‐Pb mixed narrow‐bandgap PSCs incorporating OAPS achieve a power conversion efficiency of 22.04% and exhibit improved storage stability, with 91% retention after 3072 h of storage in an N₂‐filled glovebox. Moreover, all‐perovskite tandem cells using OAPS‐incorporated Sn‐Pb narrow‐bandgap PSCs as subcells demonstrate efficiencies of 27.17% and 28.31% for two‐terminal and four‐terminal tandems, respectively, presenting a straightforward approach to optimizing performance in Sn‐Pb mixed PSCs and their tandems.