Bi/Sb Alloying-Induced Stability and Optoelectronic Performance in Lead-Free CsSnI₃-Based Perovskite Thin Films

Lead-free halide perovskites have emerged as promising alternatives to lead-based absorbers for environmentally sustainable photovoltaic technologies. In this work, we report the fabrication, characterization, and numerical modeling of a mixed-metal inorganic perovskite, CsSn₀.₅Bi₀.₃Sb₀.₂I₃, processed via a solution-based spin-coating method without the use of SnF₂ additives. Structural analysis using Rietveld refinement confirms the formation of a single-phase cubic perovskite (Pm–3m) with good crystallinity and homogeneous B-site cation distribution. Scanning electron microscopy reveals dense polycrystalline films with closely packed grains and good surface coverage. Temperature-dependent magnetic measurements indicate a low-temperature magnetic transition with FC–ZFC irreversibility, suggesting frustrated or cluster-glass–like magnetic behavior induced by compositional disorder. Optical studies demonstrate strong absorption in the visible–near-infrared region with good thermal stability of the electronic structure. Dielectric measurements reveal a relaxor-type response with non-Debye relaxation behavior arising from mixed-cation disorder and defect-related polarization mechanisms. Density functional theory calculations show a semiconducting electronic structure with p-orbital-dominated band edges governed by I-5p states at the valence band and Sn/Bi/Sb-p states at the conduction band, supporting efficient charge transport. Photovoltaic device simulations based on a planar FTO/TiO₂/CsSn₀.₅Bi₀.₃Sb₀.₂I₃/CuSCN/Pd architecture predict a maximum power conversion efficiency of ~ 11–12% at an optimal absorber thickness of ~ 0.8 µm, with device performance strongly influenced by series and shunt resistances. These results highlight CsSn₀.₅Bi₀.₃Sb₀.₂I₃ as a chemically robust, lead-free perovskite absorber with promising optoelectronic and photovoltaic properties.