Formamidinium-based Pb–Sn mixed-halide perovskite solar cells with near-optimal bandgap: FAPb _{1− <i>x</i>} Sn _<i>x</i> (I _0.8 Br _0.2 ) ₃

The development of lead-tin (Pb-Sn) binary perovskites with ideal bandgaps (1.3-1.4 eV) is crucial for single-junction perovskite solar cells (PSCs) to approach the Shockley-Queisser efficiency limit. Compositional engineering, the most effective strategy for improving PSC efficiency, offers a promising route to tune the perovskite bandgap into the ideal range. In this study, we developed FA-based Pb-Sn mixed-halide perovskites, FAPb_1-<i>x</i>Sn_<i>x</i>(I_0.8Br_0.2)₃, <i>via</i> compositional engineering to target near-optimal bandgaps (∼1.4 eV), improve crystallinity and enhance absorption coefficient. Structural and morphological analyses reveal that Sn incorporation stabilizes the photoactive phase of pure FA-based perovskites, whereas Pb incorporation promotes the formation of dense films. This synergistic effect enables Pb-Sn binary PSCs to outperform their Pb-only and Sn-only counterparts. Furthermore, it is found that a Sn content of <i>x</i> = 0.4 yields optimal photovoltaic performance under ambient conditions and the lowest photovoltage deficit. Our analysis of the charge recombination processes indicates that structural disorder and band alignment act as competing factors governing the photovoltage deficit. This study elucidates how compositional engineering fine-tunes the optoelectronic properties of perovskites and provides insights into developing novel Pb-Sn binary PSCs with near-optimal bandgaps.