In Situ Polymerized Redox Additive for Suppressing Sn ²⁺ Oxidation in Sn–Pb Perovskites

Tin–lead (Sn–Pb) narrow-bandgap perovskites have emerged as promising absorbers for single-junction and all-perovskite tandem solar cells. However, Sn2+ oxidation remains a key factor limiting both the performance and long-term stability of Sn–Pb perovskite solar cells (PSCs). Here, we report a multifunctional polymer strategy that integrates chemical passivation with physical sealing at grain boundaries. During thermal annealing, the hydroxyl-functionalized additive 4-(bis(4-vinylphenyl)amino)-2,6-di-tert-butylphenol undergoes in situ polymerization to form a cross-linked network which coordinates Sn2+ ions, stabilizes their oxidation state, and blocks oxygen ingress along grain boundaries. This dual-function strategy effectively suppresses Sn2+ oxidation and enhances operational stability of PSCs. As a result, FA0.7MA0.3Pb0.5Sn0.5I3 PSCs achieve a power conversion efficiency of 23.27% and retain 90% of the initial efficiency after 1060 h of maximum power point tracking at 25 °C under one-sun illumination in nitrogen. This work advances the practical deployment of Sn–Pb perovskite photovoltaics.