A Soldering Flux Tackles Complex Defects Chemistry in Sn‐Pb Perovskite Solar Cells

Abstract Developing tin‐lead (Sn‐Pb) narrow‐bandgap perovskites is crucial for the deployment of all‐perovskite tandem solar cells, which can help to exceed the limits of single‐junction photovoltaics. However, the Sn‐Pb perovskite suffers from a large number of bulk traps and interfacial nonradiative recombination centers, with unsatisfactory open‐circuit voltage and the consequent device efficiency. Herein, for the first time, it is shown that abietic acid (AA), a commonly used flux for metal soldering, effectively tackles complex defects chemistry in Sn‐Pb perovskites. The conjugated double bond within AA molecule plays a key role for self‐elimination of Sn 4+ ‐Pb 0 defects pair, via a redox process. In addition, C═O group is able to coordinate with Sn 2+ , leading to the improved antioxidative stability of Sn‐Pb perovskites. Consequently, a ten‐times longer carrier lifetime is observed, and the defects‐associated dual‐peak emission feature at low temperature is significantly inhibited. The resultant device achieves a power conversion efficiency improvement from 22.28% (Ref) to 23.42% with respectable stability under operational and illumination situations.