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

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%.