Dual-Site Compositional Engineering of Bismuth-Based Halide Perovskites for Stable and Efficient Lead-free Solar Cells

The toxicity of lead in metal halide perovskite solar cells is considered the most critical technical barrier for their commercialization. Among the candidates to replace Pb, trivalent cations (e.g., Bi3+ and Sb3+ are attracting considerable interest owing to their excellent reliability and favorable optical performance. Trivalent cation-based lead-free halide perovskites have two polymorphs: OD phase (dimer, P63/mmc) and 2D phase (layered, P (3) over bar m1). The 2D phase has a lower band gap and is preferred for carrier transport; however, its phase stability at room temperature remains a challenge. This study investigated the effects of dual-site compositional mixing on the stabilization of the 2D phase of A(3)Bi(2)X(9) (A = Cs, methylammonium (MA), formamidinium (FA); X = I, Br, CO and electronic structure modulation. CI mixing was noted to be essential in creating a stable 2D phase. MA mixing can be applied to reduce the band gap widening for the Cl-mixed perovskites. We suggest the optimal criterion as Cs3-chi Bi2I9-gamma Cl gamma, with chi < 0.1 and y > 0.3.