Electronic and optical properties of lead-free K₂AgSbBr₆ double perovskite tuned by doping elements Cu⁺, Bi³⁺, and I⁻.

Lead-free double perovskites have emerged as promising alternatives to conventional lead halide perovskites for optoelectronic applications due to their enhanced stability and reduced toxicity. In this study, we systematically investigate the structural, electronic, and optical properties of K₂AgSbBr₆ double perovskite and its doped variants through density functional theory (DFT) calculations. We examine three strategic doping schemes: Cu⁺ substitution at the Ag⁺ site, Bi³⁺ substitution at the Sb³⁺ site, and I⁻ substitution at the Br⁻ site. Our results reveal that Cu⁺ and I⁻ doping significantly narrow the band gap from 0.554 eV to 0.444 eV and 0.440 eV, respectively, enhancing visible light absorption, while Bi³⁺ doping widens the gap to 1.547 eV, making it suitable for UV applications. Structural analysis shows that Cu⁺ doping leads to lattice contraction with increased mechanical stiffness, while I⁻ substitution causes substantial lattice expansion with reduced bulk modulus, potentially facilitating ion migration. Thermodynamic and mechanical stability analyses were performed and confirmed that all pristine and doped systems are dynamically and mechanically stable, ensuring their viability for practical applications. Optical property analysis reveals enhanced polarizability and absorption coefficients for I⁻-doped systems, while maintaining favorable dielectric properties across all variants. These results explicitly connect the electronic structure modifications to the observed optical behavior, offering a clear design strategy for targeted optoelectronic functionalities. Importantly, the calculated band gaps and optical responses identify Cu⁺- and I⁻-doped K₂AgSbBr₆ as strong candidates for near-infrared and broadband photodetectors, while Bi³⁺-doped K₂AgSbBr₆ is more suitable for tandem solar cells and UV optoelectronic devices. This explicit mapping of material properties to applications provides actionable guidance for experimental synthesis and device prototyping, thereby bridging the gap between computational predictions and practical implementation.