A DFT study of structural, electronic and optical properties of Lead-free and Ge based cubic perovskite RbGeX‌3 (X= I, Br and Cl)

The current research uses density functional theory (DFT) approximations in conjunction with the plane wave-pseudopotential method to investigate structural, electronic, and optical properties of Pb-free cubic perovskite RbGeX 3 (X= I, Br and Cl) materials. More specifically, Norm-conserving pseudopotential has been employed to describe the ion and valence electrons interaction, and GGA-PBE flavor is used to represent the exchange-correlation part of the energy of the GGA approximation. Lattice parameters obtained are 5.95, 5.55, and 5.29 Å for RbGeX 3 (where X=I, Br, and Cl), respectively, and they are comparable with available empirical and other values. The direct band gap nature of the three compounds under research here is shown and our values of the band gap energy 𝐸 𝑔 are agree with the other available results. Materials under research show responses to electromagnetic radiation starting from the infrared region to very high energies ( ~ 33 eV). The RbGeI 3 has the lowest 𝐸 𝑔 value at the low region energies and the highest optical response peaks but RbGeCl 3 has the highest optical response peaks at energies located near ~ 20 eV. Our results show that these materials are good candidates for photo electronic applications including solar cells