Quasiparticle Calculations on Lead-Free Hybrid Germanium Iodide Perovskite CHNHGeI for Photovoltaic Applications.

Lead-free organic-inorganic halide perovskites have gained much attention as nontoxic alternatives to CHNHPbI in next-generation solar cells. In this study, we have examined the geometric and electronic properties of methylammonium germanium iodide CHNHGeI using density functional theory. Identifying a suitable functional to accurately model the germanium halide perovskites is crucial to allow the theoretical investigation for tuning the optoelectronic properties. The performance of various functionals (PBE, PBE+D3, PBEsol, PBEsol+D3, HSE06, and HSE06+D3) has been evaluated for modelling the structure and properties. The calculation of electronic properties was further refined by using the quasiparticle method on the optimized geometries, and that has an excellent agreement with the experiment. We report from our calculations that the characteristic of the density of states for CHNHGeI resembles the density of states for CHNHPbI and the effective masses of the charge carriers of CHNHGeI are comparable to the effective masses of CHNHPbI as well as silicon used in commercially available solar cells.