First-principles comparative analysis of the physical properties of K2AgMBr6 (M = Sb, Bi, P, In) perovskites: Promising candidates for clean energy applications

A-site doping with alkali ions and metal substitution at the M and M′ sites are key strategies for engineering A 2 MM′X 6 halide double perovskites for energy and device applications. In this work, we employ density functional theory (DFT) to investigate the structural, electronic, optical, and dynamic properties of the lead-free halide double perovskites K 2 AgMBr 6 (M = P, In, Sb, Bi). Structural stability is confirmed by tolerance and octahedral factor criteria, as well as by negative formation energies. Electronic band structures reveal that K 2 AgInBr 6 exhibits a direct band gap, while the other compounds possess indirect gaps. To correct the band gap underestimation inherent to the GGA-PBE functional, we performed additional calculations using the hybrid HSE06 functional. These results yield larger band gaps (1.49–2.48 eV), while preserving the same trends across the series, highlighting K 2 AgSbBr 6 and K 2 AgBiBr 6 as optimal candidates for photovoltaic applications. Optical analysis shows strong absorption in the visible and ultraviolet regions, supported by high dielectric constants and refractive indices, particularly for Sb- and Bi-based compounds. These properties confirm their potential use as absorber layers in optoelectronic and solar energy devices. Phonon dispersion spectra demonstrate dynamic stability for K 2 AgSbBr 6 and K 2 AgBiBr 6 , with no imaginary modes observed. K 2 AgInBr 6 and K 2 AgPBr 6 display shallow soft modes, indicating marginal instability that may be addressed via synthesis or strain engineering. This comprehensive study offers critical insights into the structure–property relationships in halide double perovskites and supports the viability of K 2 AgSbBr 6 and K 2 AgBiBr 6 as efficient, stable, and non-toxic alternatives for next-generation perovskite solar cells.