Ab-initio investigation of novel lead-free halide based Rb2CsXI6 (X = Ga, In) double perovskites: Mechanical, structural, thermoelectric, and optoelectronic potential for photovoltaics and green energy applications

In this study, the investigation of Rb₂CsXI₆ (X = Ga, In) double perovskites (DPs) utilized the full potential linearized augmented plane wave (FP-LAPW) approximation and semi-classical Boltzmann transport theory. The study aimed at exploring the electronic, optical, and transport properties of these compounds. We also confirmed the mechanical stability of these compounds by checking the Born’s stability criteria. Pugh’s ratio above 1.75 and Poisson ratio ($\sigma$) larger than 0.26 suggested the ductile nature of Rb₂CsXI₆ (X = Ga, In). The bandgap values were computed using PBE-GGA, TB-mBJ and TB-mBJ + SOC approximations, show that both systems maintain direct bandgaps 0.339 eV and 0.337 eV using PBE-GGA, 1.133 and 0.973 eV using GGA + mBJ, 1.011 and 0.836 eV using mBJ + SOC for Rb₂CsGaI₆ and Rb₂CsInI₆ respectively. The dielectric function, reflectivity, and absorption coefficient of the studied DPs were evaluated for the optical characteristics. Using the BoltzTraP code, the thermoelectric characteristics were calculated, with thermal (κ/τ) and electrical (σ/τ) conductivities, power factor (PF), and figure of merit (ZT) against temperature (T) and chemical potentials (μ). The high value of ZT values and highest absorption in visible region of both DPs could show promise use for a thermal and optoelectronics applications.