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The numerical simulation of lead-free perovskite cesium tin germanium halide (CsSnGeI 3) and methyl ammonium germanium halide (CH 3 NH 3 GeI 3) solar cells have been performed. It has been elucidated from the numerical simulation that as compare to the CH 3 NH 3 GeI 3 based structure, CsSnGeI 3 based structure depicts better photovoltaic performance under constant illumination condition. To enhance the device photovoltaic performance, the effects of different hole transport layer (HTL), defect density of the absorber layer, metal work function and the effect of temperature has been studied. In the present investigation, the effect of hole transport layers are analyzed by correlating the built-in voltage (V bi) with the open circuit voltage (V OC). It has been obtained from the simulation results, that V bi has a significant impact on V OC as the higher V bi corresponds to the highest V OC. Moreover, the effect of defect density and metal work function is also studied. It is obtained from the simulation results, that the optimum defect density is found to be 1 × 1014 cm−3 and metal work function should be greater than or equal to 5 eV for both lead-free CH 3 NH 3 GeI 3 and CsSnGeI 3 based perovskite layer. Furthermore, to suggest possible lead-free alternatives for all-perovskite two-terminal multi-junction solar cell, an attempt is made to match the current of bottom perovskite layer i.e., CsSnGeI 3 based perovskite layer to the top perovskite layer i.e., CH 3 NH 3 GeI 3 based perovskite layer. To attain the current matching for bottom perovskite layer (CsSnGeI 3 based perovskite layer) to top perovskite layer (CH 3 NH 3 GeI 3 based perovskite layer), the bottom perovskite sub cell is fed with the filtered spectrum which is obtained by transmission from top electrode. After, the realization of current matching the short circuit current density of CsSnGeI 3 based perovskite layer drops from 25.75 mA/cm2 to 15.32 mA/cm2 which is similar to CH 3 NH 3 GeI 3 based perovskite absorber layer. In addition, it has been obtained from literature, that both the perovskite layers can be fabricated using solution–processing low-temperature technology. Hence, the numerical study suggests the possible alternatives for wide and narrow bandgap perovskite layers for the application to obtain highly efficient lead-free all-perovskite multi-junction solar cells. • The numerical simulation of lead-free wide and narrow bandgap CH 3 NH 3 GeI 3 and CsSnGeI 3 perovskite layers is performed. • The effect of different hole transport layers is realized by correlating V bi with the V OC. • Cu 2 O as an HTL shows highest V bi and hence shows better photovoltaic performance. • The study of defect density, metal work function and temperature are explored for lead-free perovskite layers. • The lead-free perovskite layers proves to be the promising candidate for all-perovskite multi-junction solar cell. [ABSTRACT FROM AUTHOR]