© 2021 Elsevier LtdFinding a highly stable and appropriate perovskite solar cell with optimum design parameters is crucial for being included as the top cell in tandem structure, with Si as the bottom cell, to realize the power conversion efficiencies (PCEs) over the limits imposed by Shockley-Queisser theory. Here, we quantize the performance of methylammonium (MA) and lead (Pb)-free all-inorganic cesium tin-germanium triiodide (CsSnxG1-xI3) by combining CsSnI3 (Eg = 1.31 eV) and CsGeI3 (Eg = 1.63 eV) perovskites for standalone and integrated multijunction structure in tandem with Silicon solar cell. The maximum PCE of the standalone cell is shown to be about 16.5% and 13.6% for n-i-p and p-i-n structures, respectively, almost twice the present PCE value obtained in the experiment. Optimum perovskite absorber bandgap of 1.38 eV and electron affinity of 4eV with composition ratio of X = 0.25 (CsSn1-xGexI3) in both n-i-p and p-i-n structures are obtained. The optimum perovskite absorber thickness of 600 nm leads to the highest PCEs. The effect of doping concentration and defect density on absorber was also explored. Finally, four-terminal and six-terminal tandem solar cells are designed and optimized. With CsSn1-xGexI3 perovskite thickness of 385 nm and bandgap of 1.56 eV in the four-terminal, it demonstrates the high PCE of 26.9%. Interestingly, with FASnI2Br (Eg = 1.68 eV and thickness = 200 nm) and CsSn0.75Ge0.25I3 (Eg = 1.38 eV and thickness = 200 nm) as the absorber layer of the top cell and middle cell in the six-terminal PSC, respectively, the superior PCE of 27.8% is realized for MA and Pb-free six-terminal perovskite/Si tandem solar cell. This work provides a route toward the development of highly stable all-inorganic perovskite/Si tandem solar cells.