Tuning the Physical Properties of Lead-Free Halide Double Perovskite Cs₂MInCl₆ (M = Na, K) via Cu Doping for Optoelectronic Applications: ADFT Investigation

Lead-free halide double perovskites (DPs) have gained remarkable attention as potential materials for optoelectronic applications due to their outshining electronic and optical properties, stability, and nontoxicity. First-principles calculations on the structural, electronic, optical, and mechanical properties of Cs2Na1-xCuxInCl6 and Cs2K1-xCuxInCl6 (x = 0.00, 0.25, 0.50, 0.75, and 1.00) double perovskites are investigated employing density functional theory. The formability factors and decomposition enthalpies confirm the structural and thermodynamic stability of the compounds. Only the Cs2CuInCl6 compound shows instability in terms of the octahedral factor, tolerance factor, and decomposition enthalpy. The band structures are obtained by using both generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) and the HSE06 functional. The band gap of materials was reduced with increasing Cu content in Cs2MInCl6 (M = Na, K), which made the materials a better candidate for optoelectronic devices. The density of states and optical properties are also computed by the HSE06 method to correspond with the band structure. The different parameters of the optical properties support the electronic properties of the materials. Increasing the copper doping concentration in the pure Cs2MInCl6 compound produces better optical activity in the visible region, implicating the materials in photovoltaics technology. Based on the components of the stiffness tensor, Cs2MInCl6, Cs2M0.75Cu0.25InCl6, and Cs2M0.50Cu0.50InCl6 DPs ascertain the mechanical stability. On the other hand, Cs2M0.25Cu0.75InCl6 and Cs2CuInCl6 compounds are mechanically unstable. The Cs2NaInCl6 double perovskite (DP) is brittle in nature, but the Cu-doped Cs2NaInCl6 crystal systems follow ductility. In the case of K-based materials, all of the Cs2K1-xCuxInCl6 (x = 0.00, 0.25, 0.75, and 1.00) DPs are ductile in nature, except the Cs2K0.50Cu0.50InCl6 material, which displays brittleness.