Inhibition of Sn (II) Oxidation Via Triacetoxyboro‐Hydride for Stable Tin‐Lead Perovskite Solar Cells

Tin‐lead (Sn‐Pb) perovskites with narrow bandgaps can be used as bottom cells for tandem cells to boost the efficiency of perovskite solar cells (PSCs). However, the uncontrollable crystallization of Sn–Pb perovskites and the straightforward oxidation of Sn 2+ to Sn 4+ are two key factors that restrict their efficiency and stability. Compared to the anti‐solvent method, vacuum flash assisted solution processing (VASP) offers the advantages of environmental protection and excellent controllability. In this study, we combined VASP with the post‐processing of sodium triacetoxyboro‐hydride (STAB) as a reducing agent and passivator. The experimental and theoretical results demonstrat that interactions occurred between specific functional groups (−CH 3 , B–H, and CO) and the perovskite. Therefore, the Sn 2+ oxidation was successfully inhibited and the film quality was improved. The efficiency of the optimized PSCs with the STAB increased from 18.2% to 21.64%, which is the highest power conversion efficiency among the Sn–Pb PSCs fabricated using the VASP method. In addition, the device retained 81% of its initial efficiency after being stored in N 2 under −0.1 MPa for 1400 h.