DFT study of pure and iodine-doped Cs2GeX6 (X = Cl, Br, I) Halide double perovskites for photovoltaic and photocatalytic water splitting applications

Abstract

The structural, electronic, thermodynamic, mechanical, optical, photocatalytic, and thermoelectric properties of the inorganic cubic halide double perovskites Cs₂GeX₆ (X = Cl, Br, I), along with iodine-doped variants at halide site (X = Cl and Br), have been systematically investigated using first-principles calculations based on Density Functional Theory (DFT). All the investigated structures exhibit p-type semiconducting behavior with a direct band gap. Notably, the substitution of iodine at the chloride or bromide sites results in a significant band gap reduction when spin-orbit coupling (SOC) is considered, from 3.524 eV (y = 0) to 2.478 eV (y = 0.0833) for Cs₂GeCl_6-yI_y, and from 2.183 eV to 1.790 eV for Cs₂GeBr_6-yI_y over the same doping range. This band gap narrowing leads to enhanced optical absorption, particularly in the iodine-doped bromide-based structure, which exhibits an absorption coefficient exceeding 10⁴ cm⁻¹. Conversely, iodine doping does not significantly improve the absorption coefficient of Cs₂GeCl_6-yI_y. Overall, photocatalytic activity assessments highlight Cs₂GeBr₆ as a promising material for photocatalytic applications.