Critical role of post-treatment induced surface reconstruction for high performance inorganic tin-lead perovskite solar cells

•Combined 1-4FP additive and post-treatment strategy was adopted in tin–lead PSCs.•High-quality perovskite film was obtained via significant surface reconstruction.•Crosslinking neighboring perovskite grains with N-H bonds prevented tin oxidation.•Target PSC delivered a PCE of 17.19 % with a record small Voc deficit of 0.426 V.•17.19% is the highest efficiency among all-inorganic Pb-Sn PSCs reported to date. Inorganic CsPb0.7Sn0.3I3 perovskites with low-bandgap (1.2 to 1.4 eV) are desired absorber materials for solar cells owing to their ideal bandgap and compositional stability. However, the performances of inorganic Pb-Sn perovskite solar cells are much lower than their Pb-based and hybrid Pb-Sn (e.g. CsPbX3 and FAMAPbSnX3) PSCs, which might be ascribed to its poor film-forming morphology and easy oxidation of Sn2+ at the surface. Here, for the first time, a combined 1-(4-fluorophenyl) piperazine (1-4FP) additive and 1-4FP post-treatment strategy was adopted to obtain high-quality pinhole-free films through a significant surface reconstruction. Specifically, the interaction introduced by 1-4FP post-treatment can effectively prevent the inevitable Sn2+ oxidation on perovskite surface, resulting from crosslinking neighboring perovskite grains with hydrogen bonds of amine group binding to the perovskite surface. As a result, the device treated by this combined strategy achieved a PCE of 17.19 %, which is the highest efficiency of Pb-Sn alloyed inorganic PSCs (sub-1.4 eV) reported to date. In addition, the unsealed 1-4FP-treated devices maintained their initial efficiencies of approximately 100 % after storage in a nitrogen atmosphere for 4000 h. Our findings open a new avenue to obtain high-quality inorganic Pb-Sn perovskite films and associated solar cells.