Investigating the Role of Various Substitution Strategies in Cs ₂ AgBiI ₆ Double Perovskite Toward the Enhancement of Its Optoelectronic Properties

Halide double perovskites (HDPs) have emerged as promising non-toxic alternatives to lead-based perovskites for optoelectronic applications. Their electronic and optical properties, particularly the bandgap, can be tuned through elemental doping. In this study, first-principles calculations based on density functional theory (DFT) are employed using periodic boundary conditions to investigate the effects of Cu and Na doping on the structural, electronic, and optical properties of the Cs₂AgBiI₆ HDP. All calculations are performed using the PBE exchange-correlation functional. To improve the accuracy of the band structure evaluations, additional calculations are conducted using the HSE06 hybrid functional with spin-orbit coupling included (HSE06+SOC). It is found that a low level of doping typically decreases the structural stability, but the structures become more stable at higher doping levels. The pristine structure of Cs₂AgBiI₆ exhibits a band gap of 1.21 eV at the HSE06+SOC level. Cu doping introduces new states near the Fermi level and reduces the bandgap. The 50% Cu-doped structure has the smallest band gap of 0.49 eV (HSE06+SOC). Na doping does not introduce new states near the Fermi level, but it changes the valence band and affects band clustering. Due to a narrow bandgap, the 50% Cu doped structure is expected to have the highest electrical conductivity. The pristine Cs₂AgBiI₆ HDP absorbs strongly in the UV-visible region with absorption maxima near 91, 185, 412, 485, 526, and 654 nm. The corresponding absorption coefficients are of the order of 10⁶ cm^-1. Notably, its absorption coefficient in the near-infrared region (800 nm) is comparable to that of typical infrared absorbers (∼10⁵ cm^-1). Doping modifies the absorption profile significantly: for instance, Cu doping at 75% red-shifts the 505 nm peak by 93 nm and enhances the absorption coefficient by a factor of 1.5-2.0. Na doping enhances absorption only at intermediate concentrations (25-50%) and alters the position of the absorption maxima. The ability to tune the absorption peaks and improve light absorption by doping makes these materials attractive for optoelectronic applications.