Halide-Driven Bandgap Engineering and SLME-Based Photovoltaic Performance of Ba₃PX₃ Compounds: A First-Principles Study

In the present work, Ba₃PX₃ (X = F, Cl, Br, I) all-inorganic and lead-free halide compositions have been studied as possible replacements for hybrid perovskites using first-principles calculations. All the considered materials were found to exhibit direct band gaps at the Γ-point, decreasing from 2.37 eV (Ba₃PF₃) to 1.48 eV (Ba₃PI₃). The optical calculations reveal strong absorption in the visible and near-UV regions, with the static dielectric constants ranging from 2.75 to 4.35 in the halide series. All the compounds are mechanically stable and have tuneable ductility and stiffness properties. Lattice stability is confirmed by thermodynamic analysis in broad temperature ranges (0–900 K) and pressure ranges (0–10 GPa). The spectroscopic limit maximum efficiency (SLME), which is a theoretical screening parameter that represents an upper limit, has a value of 39.17% at 300 K for an absorber thickness of 1 μm, comparable to practical thin-film photovoltaic architectures. The findings identify strong trends in the stability of structures, optoelectronic properties, and photovoltaic characteristics within the Ba₃PX₃ family and rank Ba₃PBr₃ and Ba₃PI₃ among the most promising lead-free photovoltaic absorbers.