Tuning bandgap and optical properties of Pb-free perovskites RbGeX3 (X = Cl, Br and I) under pressure: a DFT study

We study structural, electronic and optical properties of inorganic leadfree halide perovskites RbGeX3 (X = Cl, Br and I) under hydrostatic pressure, which could facilitate development of new optoelectronic and solar-cell technologies. ab initio first-principles calculations are employed based on the generalized gradient approximation within the framework of density functional theory. We demonstrate that the bandgap of our perovskites decreases with increasing pressure. At a given pressure, the bandgap becomes narrower when the halogen atom is changed from Cl to I. We also examine the density of states and demonstrate that the energy levels near the Fermi level change significantly under pressure. The optical properties are calculated using the density functional perturbation theory and the Kramers-Kronig relation. The optical parameters such as the real and imaginary parts of the dielectric function, the refractive index and the absorption coefficient are calculated under different pressures.