A compositionally graded silicon germanium (Si1-xGex) buffer layer is a leverage technology that grows III-V materials on silicon (Si) substrates with low threading dislocation density. This study determined average optical absorption coefficients of Si1-xGex graded buffer layers at wavelengths beyond the band gap of Si by measuring the transmittances of Si1-xGex on Si wafers. The results show that assuming linear changes of absorption coefficients of Si1-xGex with the change of germanium (Ge) composition will significantly overestimates the absorption in the Si1-xGex graded buffer layers. Using determined average absorption coefficients of graded buffer layers, an accurate optical model was developed for a fabricated planar Si(0.15)Geo(0.85) on Si solar cell. This model shows that significant amounts of light at the band edge of Si are absorbed by the Si substrate. However, the majority of the light absorbed by the Si substrate cannot contribute to the short-circuit current of the solar cell, as the generated electron-hole carriers are too far away from the p-n junction and thus have a very low probability of being collected by the p-n junction. By thinning the Si substrate from 675 gm to 200 gm, a 0.5 mA/cm(2) short-circuit current density (J(sc)) improvement under AM1.5 G spectrum was observed on the Si(0.15)Geo(0.85) solar cell. Further thinning the Si substrate to 50 gm should lead to a total J(sc) gain of 1.6 mA/cm(2). This study provides a practical path for achieving a highly efficient two-terminal current matched GaAsP/SiGe tandem solar cell on Si substrate. (C) 2016 Elsevier B.V. All rights reserved.