Dual-Function Halide Exchange Strategy for Simultaneous Sn ⁴⁺ Elimination and Stability Enhancement in Pb–Sn Mixed Perovskite Solar Cells

Pb–Sn mixed perovskites with an optimal bandgap of ∼1.25 eV are essential for high-efficiency all-perovskite tandem solar cells. However, the facile oxidation of Sn 2+ leads to detrimental Sn 4+ defects that cause severe nonradiative recombination and rapid degradation, hindering their commercialization. Here, we demonstrate a halide exchange strategy using inert metal chlorides (MnCl 2 /ZnCl 2 ) to simultaneously reduce the Sn 4+ concentration and form an inorganic protective layer in Pb–Sn perovskite solar cells (PSCs). The metal chlorides react with SnI 4 via ligand substitution, producing volatile SnCl 4, which reduces Sn 4+ concentrations, while forming MnI 2 /ZnI 2 passivation layers at the grain boundaries. In addition, the post-treatment induces partial dissolution-recrystallization, which enlarges the grain size and reduces residual stress. Furthermore, the inorganic passivation layer optimizes the energy-level alignment at the perovskite surface, facilitating carrier separation and extraction. As a result, the champion ZnCl 2 -modified device achieves a high power conversion efficiency (PCE) of 23.06% with an open-circuit voltage of 0.87 V and retains 90% of its initial PCE after 1000 h of continuous illumination in N 2 . This work establishes a novel inert metal chloride post-treatment strategy and elucidates the underlying reaction mechanisms in Pb–Sn mixed perovskites, thereby opening new avenues for developing highly efficient and stable tandem devices.