First-principles investigation of lead-free trigonal CsGeI_3-xBr_x mixed-halide perovskite system for optoelectronic applications: Electronic and optical properties

A stable, non-toxic, environmentally benign substitute to the Pb²⁺ cation is indispensable for large-scale commercialization of the halide perovskite optoelectronic devices in general and the halide perovskite solar cells specifically. In this paper, Density Functional Theory (DFT) with two exchange-correlation functionals PBE and PBEsol, was employed to investigate the structural, electronic, and optical properties of the trigonal inorganic CsGeI_3-xBr_x halide Perovskite system (x = 0, 1, 2 and 3 which representing Br content). The system's properties were compared to the cubic α-CsPbI3 which has been extensively investigated. Our results confirmed that replacing the Pb²⁺ cation with the Ge²⁺ cation caused a distortion in the halide octahedral cage which results in a non-centrosymmetric structure. The changes in the structural properties resulted in changes in the electronic and optical properties. A strong negative correlation was found between the Ge -X bond length and the bandgap. The electronic density of states revealed that only I 5p, Br 4p, and Ge 4s states contributed to the valence band maximum, (VBM), whereas, the conduction band minimum, (CBM), is mainly contributed by Ge 4p states in the CsGeI_3-xBr_x system. Having the mixed halide compounds altered the electronic and optical properties significantly. As the bromide content increases, the bandgap increases, and the optical absorption band widenes. Replacing the Pb²⁺ with Ge²⁺ has red-shifted the absorption peaks and has altered the optical behavior to be anisotropic. This study shed some light on the high-potential of tuning and optimizing the optoelectronic properties of the Ge- based mixed halide perovskite family for the photovoltaic tandem solar cells and other optoelectronic devices.