Revealing the Structural, Electronic, Optical, and Thermoelectric Aspects of the Gold-Based Double Perovskites X2Au+Au3+Br6 (X = Cs, Rb) Using a First-Principles Approach

Lead-free double perovskites are now assumed to be suitable candidates for green energy harvesting in particular as active materials for solar cells and thermoelectric generators, which can meet future generation energy needs. Therefore, we explore the Au-based halide double perovskites X₂Au⁺Au³⁺Br₆ (X = Cs, Rb) from the first principles approach. Density functional theory (DFT) is utilized to explore the electronic structure with DFT code Quantum ESPRESSO. The mechanical, thermodynamic, and structural stability is ensured from Burn-Haun criterion, formation energies, and Goldschmidt factors, respectively. The examined materials have stable structures with direct band gaps i.e. 1.54 and 1.72 eV. The existence of band gaps in the visible region motivates us to explore the optical properties, which give fascinating outcomes. The absorption coefficients and optical conductivity peaks are found to be significant in the visible region i.e., ≈10⁴cm⁻¹ and ≈10¹⁵s⁻¹, respectively. Additionally, the thermoelectric properties are also investigated using Boltzmann transport theory. There are several good gestures for the usage in the thermoelectric generators since the values of Seebeck coefficients (446.5, and 225.2 μV K⁻¹), power factors (1.75 × 10¹¹W mk⁻² s, and 1.24 × 10¹¹W mk⁻² s), and figure of merits (0.92 and 0.73) are noteworthy for Cs₂AuAuBr₆ and Rb₂AuAuBr₆, respectively, at room temperature T = 300 K.