In this study, a novel 10 mu m thick interdigitated back contact silicon-germanium heterojunction IBC-Si(1-x)Ge(x)HJ) solar cell device has been designed and simulated for standalone and four-terminal mechanically stacked tandem applications. Optimization of i-a-SiGe: H thickness, the width of n-a-SiGe: H region, p-aSiGe: H region and gap along with composition fraction (x) lead to 15.5% power conversion efficiency (PCE) in a stand-alone configuration. Whereas in combination with perovskite top subcell we further demonstrate 25.7% PCE in four-terminal tandem configuration. In mechanical stacking, top and bottom subcells are fabricated individually and then assembled in a module, which avoids the need for current matching between subcells, thereby giving greater process and design flexibility. The proposed IBCSiGeHJ solar cell is similar to(25-30) times thinner than conventional Si solar cells which are used as bottom subcell in perovskite/silicon tandem solar cell. The results reveal that the proposed 4-terminal mechanically stacked perovskite/IBC-SiGeHJ tandem device may open new doors for the energy efficient applications. All the simulations have been done using Silvaco technology computer aided design (TCAD) simulator. (C) 2017 Elsevier Ltd. All rights reserved.