We reported numerical simulations of device performances made of methylammonium germanium halide (CH3NH3GeI3)-based perovskite solar cells. The main goal here is to seek for an efficient method to improve the device efficiency of alternative lead-free perovskite based on germanium solar cells by using various organic and inorganic hole transport materials. For that aspiration, the effect of several parameters on the solar cell performance were investigated such as thicknesses of perovskite, HTM, defect density, hole mobility, and metal electrode work function on the charge collection. The device simulation revealed that the optimum thickness of CH3NH3GeI3 absorber is found around 600 Tim. Furthermore, Ge-based perovskite solar cells with Cu2O and D-PBTTT-14 as HTM exhibited a remarkable overall power conversion efficiency reaching 21%. The defect density reduction is a critical factor to improve the solar cell performance and should be controlled under the order of similar to 10(15) cm(3). Further simulations were performed to study the effect of operating temperature on the performance. Our simulation results advocate for a viable route to design hole-transporting materials for highly efficient and stable perovskite solar cells with low cost.