Mechanical, optoelectronic and thermoelectric properties of lead-free inorganic Li-based double perovskites Li2GeZ6 (Z = Cl, Br, I) for energy applications

In the present work, we investigate lead-free halide DPs Li(2)GeZ(6) (Z = Cl, Br, I) to explore their optoelectronic device applications. The PBEsol functional and modified Becke-Johnson (mBJ) have been brought to determine the lattice constant and band gap, respectively. Calculated formation energies values are found to be -2.48 eV, -2.04 eV, and -1.88 eV for Li2GeCl6, Li2GeBr6, and Li2GeI6, respectively, and all the studied compounds satisfy Born stability criteria for the cubic phase. Band structures computed using mBJ functional yield band gap values of 3.4 eV, 1.8 eV, and 0.4 eV for Li2GeCl6, Li2GeBr6, and Li2GeI6, accordingly, indicating the suitability of these materials for optical devices. The refractive index, absorption, and reflectivity analyses have been evaluated for optical performance in energy ranges from 0-8 eV. Moreover, thermal and electrical conductivities, power factor, and figure-of-merit have been determined under the effect of temperature with the help of the BoltzTrap code. All these aspects, when taken together, provide a comprehensive picture of optical and thermoelectric characteristics and their applications in energy devices.