A tin fluoride-free, efficient and durable tin-lead perovskite solar cell

The photo-thermal stability of tin-lead perovskite solar cells remains a major challenge. SnF₂ is commonly used to inhibit Sn²⁺ oxidation and reduce hole density, however, the stability of devices remains poor. Here, we found that the poor stability partially results from an adverse effect of SnF₂, which reacts with formamidine iodide during photo-thermal treatments. This reaction leads to degradation of perovskite and release of hydrofluoric acid, which corrodes electrodes. To address this issue, we develop a strategy that combines lead powder in precursor with PbF₂ post-treatment, replacing the role of SnF₂ as in film formation and surface defect passivation, respectively. The d-electron polarization in Pb²⁺ strengthens its bond with F^⁻, making it react inert to perovskite. In this work, the efficiency of SnF₂-free devices increases from 16.43% to 24.07%. The cells retain 60% of their initial efficiency after 550 hours operating at 85 °C under maximum power point.