Tuning optoelectronic and thermoelectric attributes of Na2GeX6 (X = Br, Cl) halide double perovskites for high-efficiency solar cells applications

In this study, by implementing Full Potential augmented plane −wave (FP-LAPW) method employed in density functional theory (DFT), we computed the structural, electronic, optical, thermoelectric and elastic properties of two novel double perovskites halides Na₂GeX₆(X = Cl, Br). We confirmed the ground state energy and stability of these compounds by computing the cohesive and formation energies. Using the Tran Blaha modified Becke Johnson approximation and generalized gradient approximations (TB-mBJ-GGA and PBE-GGA), the electronic band structure and density of states plots of these compounds reveal their semiconducting nature. The Perdew-Burke-Ernzerhof generalized gradient approximation reveals a direct band-gap of (0.943) for Na₂GeCl₆ and a metallic character for Na₂GeBr₆ respectively. However, for Na₂GeCl₆ band-gap value of (3.351) with TB-mBJ, direct-bandgap value of (1.423 eV) for Na₂GeBr₆. The optical properties are also computed for both the compounds. The compounds show large absorption coefficient values in visible and UV regions, showing promising optoelectronic properties. Moreover, the transport properties were calculated using BoltzTrap code, which discussed about the thermoelectric behavior of the compounds.