Abstract

Abstract Research on mixed Sn‐Pb perovskite solar cells (PSCs) is gaining significant attention due to their potential for high efficiency in all‐perovskite tandem solar cells. However, Sn 2+ in Sn‐Pb perovskite is susceptible to oxidation, leading to a high defect density. The oxidation primarily occurs through two pathways: one involving a reaction with oxygen, and the other related to iodine defects, which generate I 2 and further accelerate the oxidation of Sn 2 ⁺, greatly reducing stability. First, to tackle the photo‐stability issues caused by iodine defects, amber acid (AA) is screened as the additive. The Carboxyl group on AA can strongly coordinate with Sn 2+ , reinforcing the Sn─I bond and electrostatically interacting with negatively charged defects. This interaction inhibits the photoinduced formation of I 2 and the subsequent oxidation of Sn 2+ , thereby enhancing the stability of Sn─Pb PSCs under continuous illumination. Building on the foundation of AA, a reductive sulfhydryl group is introduced to synthesize thiomalic acid (TA). It inhibits the formation of Sn 4+ in both the perovskite precursor and the perovskite film, thereby improving air stability while maintaining strong photostability. Consequently, single PSCs achieved a champion efficiency of 22.7%. The best‐performing two‐terminal all‐perovskite tandem solar cell achieved a power conversion efficiency of 28.6% with improved operational stability.