Bridging first-principles calculations and device simulations of A₃GaI₆ (A = K, Rb, Cs) double perovskites for...

Halide perovskites have garnered significant interest as advanced materials for optoelectronic applications and energy harvesting devices, owing to their adjustable bandgaps, elevated absorption coefficients, and exceptional charge transport characteristics. Among them, double perovskites of the less explored formula, specifically A₃BX₆-type halide double perovskites, remain relatively underrepresented in the literature. In this work, the structural, mechanical, thermodynamic, electronic, optical, and thermoelectrical properties of the lead-free halide perovskites A₃GaI₆ (A = Cs, K, Rb) were systematically investigated using first-principles calculations within the WIEN2k framework employing GGA-PBE, TB-mBJ, and TB-mBJ+SOC functionals. Structural stability was confirmed through Goldschmidt tolerance factors, negative formation energies, convex-hull analysis, and elastic constants. The calculated direct band gaps based on both functionals (TB-mBJ/TB-mBJ+SOC) of 2.06/1.88 eV (Cs₃GaI₆), 1.83/1.65 eV (K₃GaI₆), and 1.94/1.76 eV (Rb₃GaI₆) indicate strong optical absorption in the visible to near-infrared region. Carrier-density and Bader-charge analyses reveal that the Ga-I framework governs electronic transport, while the A-site cations tune the charge distribution, with K₃GaI₆ and Rb₃GaI₆ exhibiting higher carrier densities and stronger charge transfer than Cs₃GaI₆. Among the studied compounds, K₃GaI₆ possesses the most suitable band gap (∼1.65 eV), lower carrier effective masses, higher carrier mobilities, and a larger static dielectric constant, indicating efficient charge separation and transport, and thus superior photovoltaic potential. Based on DFT-derived parameters, SCAPS-1D simulations of sixteen n-i-p device architectures based on K₃GaI₆ yield power conversion efficiencies ranging from 19.48% to 22.48%, with the AZO/STO/K₃GaI₆/Zn₂P₂ configuration showing the best performance due to favorable band alignment and transport-layer properties. After optimization, the efficiency reaches 27.19%, highlighting K₃GaI₆ as a highly promising lead-free absorber for high-performance perovskite solar cells. This work establishes a direct link between material properties and device performance and provides a solid theoretical foundation for the experimental realization of A₃GaI₆-based optoelectronic and energy-harvesting applications.