Investigation of structural, electronic, elastic and optical properties of Ge-halide perovskites NaGeX3 (X = Cl, Br and I): A first-principles DFT study

Lead-free perovskites are among materials that are currently most investigated for their potential application in photovoltaic and optoelectronic applications due to their non-toxic effect on the environment. In this study, a first-principle Density Functional Theory (DFT) calculation to study structural, elastic, electronic, and optical properties of Ge-halide perovskites namely cubic NaGeX3 (X = Cl, Br, and I) is presented and reported for the first time. The Quantum Espresso code with GGA-PBE functional was used for conducting the DFT calculation on these lead-free perovskite materials. The structural investigation revealed that the optimized lattice constants for NaGeCl3, NaGeBr3, and NaGeI3 are 5.24 Å, 5.50 Å, and 5.90 Å, respectively. Meanwhile, all compounds are shown to possess semiconducting behaviour and have direct R→R energy bandgap. Consistent with other isoelectronic compounds, it is observed that the energy gaps are tuned to slightly lower values as Cl is replaced by Br and I, with energy gaps 0.76 eV, 0.45 eV, and 0.44 eV for the respective compounds. From their Bulk modulus and other elastic parameters calculated, the materials are stable and ductile, making them easily incorporated into thin films in solar cells and other applications. The materials also exhibit excellent optical properties including high absorption coefficient and therefore they have the potential to be used in developing lead-free perovskite solar cells and other optoelectronic applications.