Exploration of Lead-Free Rb₂InGeX₆ Double Perovskites: Correlating Halide Substitution with Electronic, Optical, and Thermoelectric Performance

Density functional theory (DFT) was used to investigate the structural, electronic, optical, and thermodynamic properties of the double perovskite Rb₂InGeX₆ (X = Cl, Br, I). The performed calculations reveal that the three investigated structures have semi-metallic behavior defined by valence band crossings at the Fermi level, with band gaps ranging from Rb₂InGeCl₆ > Rb₂InGeBr₆ > Rb₂InGeI₆. The Fermi level's Ge-P, In-P, and halide-P orbitals significantly influence the conductivity of the materials. The density of states (DOS) investigations emphasize the role of individual atomic orbitals in shaping the electronic structure. The analyzed optical properties of Rb₂InGeX₆ (X = Cl, Br, I) show different light-matter interaction properties: Rb₂InGeBr₆ shows notable transparency in the infrared-visible spectrum (IR), while Rb₂InGeCl₆ shows strong absorption in the visible-UV range. Whereas Rb₂InGeCl₆ is appropriate for UV photodetectors and light-harvesting technologies, the computed refractive index and reflectivity verify the fit of Rb₂InGeBr₆ for IR photodetectors and optoelectronic devices. Furthermore, the thermodynamic calculations over a wide range of temperatures show that halide substitution significantly affects the thermoelectric properties. Rb₂InGeBr₆ exhibits n-type conduction with an enhanced absolute Seebeck coefficient and better power factor; Rb₂InGeCl₆ and Rb₂InGeI₆ present p-type behavior with more modest thermoelectric performance. The obtained results offer a complete understanding and a new perspective on the tunability of Rb₂InGeX₆ perovskites for next-generation optoelectronic, photovoltaic, and energy harvesting uses.