Design and Screening of Inorganic Mixed-Valent Halide Perovskites A ₄ B ^III B ^V X ₁₂ for Optoelectronic Applications

Hybrid halide perovskites have emerged as highly efficient and low cost solar absorbers due to their unique physical properties and record power conversion efficiencies. However, their practical deployment remains hindered by Pb toxicity and poor structural stability. Mixed valent double perovskites (DPs) offer a promising alternative due to their non-toxic composition, ambient-phase stability, and tunable optoelectronic properties enabled by the synergistic interaction between two distinct metal cations. Here, we design and explore a series of mixed valent DPs A4BIIIBVX12 (with A = Cs and Rb; BIII = Bi, Ga, La, Rh, Tl, In; BV = Sb and X = Cl, Br, I) using first-principles calculations. Phase stability is assessed through decomposition enthalpy, phonon spectra, and the Born mechanical stability criterion. Through a targeted screening for photovoltaic applications, we identify seven promising compounds that exhibit clear thermodynamic stability, desired band gaps (0.63–2.21 eV), high dielectric constants, large transition dipole moments, and maximum absorption coefficients, all of which are desirable for solar energy harvesting. Remarkably, Cs4BiSbCl12 and Rb4BiSbCl12 show near-ideal band gaps (1.25 and 1.27 eV) and predicted efficiencies exceeding 30%, comparable to the performance of the benchmark MAPbI3 perovskite. These findings highlight the strong potential of mixed valent DPs in optoelectronics and present a viable strategy for developing high-performance, lead-free halide perovskite solar absorbers.