Dual-Function Halide Exchange Strategy for Simultaneous Sn⁴⁺ 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 Sn²⁺ leads to detrimental Sn⁴⁺ defects that cause severe nonradiative recombination and rapid degradation, hindering their commercialization. Here, we demonstrate a halide exchange strategy using inert metal chlorides (MnCl₂/ZnCl₂) to simultaneously reduce the Sn⁴⁺ concentration and form an inorganic protective layer in Pb–Sn perovskite solar cells (PSCs). The metal chlorides react with SnI₄ via ligand substitution, producing volatile SnCl₄, which reduces Sn⁴⁺ concentrations, while forming MnI₂/ZnI₂ 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 ZnCl₂-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 N₂. 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.