A DFT analysis of structural and electronic modulation of Cs2AgBiX6 (X = Cl, Br) via A-site NH4+ substitution for photovoltaic applications

Abstract

To address environmental pollution and sustainable energy challenges, lead-free Ag–Bi double perovskites Cs₂AgBiX₆ (X = Cl, Br) and their ammonium-substituted variants CsNH₄AgBiX₆ and (NH₄)₂AgBiX₆ are investigated using first-principles FP-LAPW calculations within density functional theory. Ammonium incorporation slightly reduces lattice size while enhancing structural flexibility. Band-structure analysis (GGA, SOC, hybrid-PBE) shows decreasing band gaps with NH₄ doping from 2.52 eV to 2.09 eV, with the CBM dominated by Bi states and the VBM by halide p states. Effective mass calculations indicate high carrier mobility due to the low effective mass of (NH₄)₂AgBiX₆ (X = Cl, Br) compared to Cs-based double perovskites, which results in values that are between 0.524 and 0.939 eV, and values that are between 1.2 and 1.645 eV. The stability of these compounds is confirmed through mechanical (C_ij), formation of enthalpy ΔH_f and Goldschmidt tolerance factor (τ_G) analyses. The elastic constants confirm the mechanically stable and ductile nature of these materials. Furthermore, ab initio molecular dynamics simulations and phonon band-structure calculations have been performed and confirm the stability of the materials. Optical properties reveal stronger light absorption (∼45 × 10⁴ cm⁻¹ in the visible region) and an enhanced dielectric response after NH₄⁺ substitution. Band-edge alignment analysis supports the potential for photocatalytic water splitting, while SLME analysis identifies (NH₄)₂AgBiBr₆ (η_max = 6.47%) as the most promising photovoltaic absorber. Overall, A-site ammonium engineering effectively tunes the structural, electronic, optical, and photocatalytic properties of Ag–Bi double perovskites for energy applications.