Vacancy-ordered CsRbGeCl6 and CsRbGeBr6 perovskites as new promising non-toxic materials for photovoltaic applications: A DFT investigation

Abstract

In this study, we have proposed novel vacancy-ordered double perovskites, CsRbGeCl6 and CsRbGeBr6, and conducted computational analyses to investigate their mechanical, optoelectronic and thermoelectric properties, for the first time. All calculations are performed using the Density Functional Theory (DFT) as implemented in the Quantum Espresso package. The two materials exhibit considerable chemical and mechanical stability, with CsRbGeCl6 additionally exhibiting thermodynamic stability. Using the PBE (SCAN) functional, the calculated band gap energies for CsRbGeCl6 and CsRbGeBr6 perovskites are 2.17 eV (2.57 eV) and 0.88 eV (1.23 eV), respectively. Consequently, the perovskites, particularly CsRbGeBr6, possess the optimal band gap energy for photovoltaic applications. Their excellent electronic properties are further supported by their remarkable optical properties spanning from visible to ultraviolet region, characterized by high absorption coefficients (up to 106 cm−1) and minimal reflectivity (below 23 %). Moreover, the analysis of their thermoelectric properties revealed that they exhibit the required characteristics as thermoelectric materials.