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 RbGeX3 (X= I, Br and Cl) materials. More specifically, Norm-conserving pseudopotential has been employed to describe the ion and valence electrons interaction, and Perdew-Burke-Ernzerhof (PBE) flavor is used to represent the exchange-correlation part of the energy of the GGA approximation. Our calculated lattice constants are 5.95, 5.55, and 5.29 Å for RbGeX3 (where X=I, Br, and Cl), respectively, and they are of are in good agreement with available empirical and other values. The band structure shows the direct band gap nature of the three compounds under research here and our values of the band gap energy E_g are in good agreement with the other available results. Materials under research show response to the electromagnetic radiation starting from the infrared region to the very high energies (~33 eV). The RbGeI3 has the lowest E_g value at the low region energies and the highest optical response peaks but RbGeCl3 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.