Synergistic Treatments of Bulk and Surface in Tin‐Lead Mixed Perovskite for Efficient All‐Perovskite Tandem Solar Cells

Abstract Narrow‐bandgap (NBG) perovskites have attracted significant attention for their promising potential as light‐absorbing materials in tandem solar cells. However, tin‐lead mixed perovskites face a major limitation of the facile oxidation of Sn²⁺, leading to Sn vacancies in the perovskite films. In this study, sodium triacetoxyborohydride (STAB) is introduced as an antioxidant, while potassium (5‐bromo‐2‐fluoropyridin‐3‐yl)trifluoroborate (PBFT) is employed as a surface modifier for mixed tin‐lead perovskites. This combined approach not only suppresses Sn²⁺ oxidation but also promotes the formation of high‐quality perovskite films. Furthermore, the induced energy band bending at the perovskite/C₆₀ interface substantially enhances charge transfer. Consequently, the optimized device achieves a power conversion efficiency (PCE) of 23.41% with excellent illumination stability. When paired with a wide‐bandgap (WBG) semi‐transparent perovskite solar cell (PSC), the resulting four‐terminal (4‐T) all‐perovskite tandem solar cell attains a remarkable efficiency of 27.37%.