First-principles discovery of multifunctional K2NaRhX6 (X = F, Cl, Br, I) halide double perovskites for photovoltaic and photocatalytic applications.

Lead-free halide double perovskites have emerged as attractive multifunctional materials for sustainable energy conversion owing to their compositional flexibility and tunable electronic structures. Herein, a comprehensive first-principles investigation combined with SCAPS-1D device simulations is performed to explore the structural stability, electronic structure, optical response, mechanical behavior, photocatalytic activity, and photovoltaic performance of K2NaRhX6 (X = F, Cl, Br, I). All compounds crystallize in the cubic Fm3̄m phase and exhibit negative formation enthalpies relative to their constituent elemental reference states. Halogen substitution systematically modulates the Rh-4d/X-p orbital hybridization, leading to a continuous reduction in the direct band gap from 1.861 to 0.579 eV together with enhanced visible-light absorption and dielectric response. The Br- and I-based compounds exhibit comparatively softer and more ductile mechanical characteristics favorable for thin-film processing, while calculated band-edge positions indicate suitable redox alignment for solar-driven water splitting. DFT-derived electronic parameters were subsequently employed in SCAPS-1D simulations of FTO/SnS2/K2NaRhX6 solar cells. After optimizing the absorber thickness, acceptor concentration, and defect density, K2NaRhCl6 delivered the highest theoretical photovoltaic performance with a power conversion efficiency of 26.64%, together with a V OC of 0.918 V, J SC of 34.89 mA cm-2, and FF of 83.23%. These findings demonstrate that halogen engineering effectively tailors the multifunctional properties of K2NaRhX6 double perovskites and identify K2NaRhCl6 and K2NaRhBr6 as promising lead-free candidates for next-generation photovoltaic and photocatalytic energy-conversion technologies.