Evolution of electronic bandgap by anion variation to explore niobium new halide double perovskites Cs2GeNbX6 (X = Cl, Br, I) for solar cells and thermoelectric applications: first principles analysis

The structural, electronic, optical, and thermoelectric properties of the niobium new halide double perovskites Cs2GeNbX6 (X = Cl, Br, I) were investigated using a density functional theory method. The generalized gradient approximation (GGA) method is used to project the exchange-correlation potential. The tolerance factor and optimizing total energy define the structure's stability. The magnetic moments of our compounds are high, more than 3μB. The compounds have direct narrow band gaps of 0.69, 0.46, and 0.26 eV, respectively, for Cs2GeNbCl6, Cs2GeNbBr6, and Cs2GeNbI6, as determined by band structure calculations. This is ideal for investigating these compounds for use in solar cells. In addition, the investigated compounds were investigated in terms of optical absorption, refractive index, and dielectric constants for energy range 0–12 eV, ensuring absorption in infrared, visible, and ultraviolet regions. This was done in order to study optical characteristics. The investigated compounds are excellent candidates for harvest solar cell applications due to their maximum visible absorption. They are also good candidates for thermoelectric applications due to their Seebeck coefficient, lattice thermal, electric conductivities and figure of merit (ZT) addressed by Boltzmann theory.