First-principles Investigations of Structural, Thermodynamic, Optoelectronic and Thermoelectric Properties of Rb₂CuMF₆ (M = As³⁺, Bi³⁺) Eco-friendly Halide Double Perovskites: Materials for Green Energy Applications

Lead-free halide double perovskites (HDPs) have emerged as aspirant members of the optoelectronic and thermoelectric (TE) materials family due to their non-toxicity, tunable and noticeable performances. Herein, the structural, mechanical, thermodynamic, optoelectronic, and TE properties of novel Rb₂CuMF₆ (M = As³⁺, Bi³⁺) HDPs are comprehensively investigated. This study has been performed using accurate first principle calculations with the Boltzmann transport theory. The evaluated elastic constants ensure their structural and thermal stability in cubic phase and anisotropic with ductile behavior. Using Tran-Balaha modified Becke-Johnson (TB-mBJ-GGA) potential, indirect semiconducting band gaps of 1.43 eV and 1.27 eV are recorded for Rb₂CuAsF₆, and Rb₂CuBiF₆, respectively. Correlated analyses of the wavelength-dependent optical properties are conducted. The studied HDPs exhibited different optical absorption abilities in both UV and VIS working regions. Besides, the entire TE properties are addressed for n- and p-type doping in a wide operating temperature range. The p-type doping is found to be effective in enhancing the TE performances of both HDPs. The highest figure of merit (ZT)_max = 0.994 is recorded at 300 K for intrinsic Rb₂CuMF₆ (M = As³⁺, Bi³⁺). Accordingly, the favorable combination of the present outcomes makes Rb₂CuMF₆ (M = As³⁺, Bi³⁺) an interesting candidate for widespread optoelectronic and TE applications in a wide working temperature range.