We fabricated silicon-germanium (Si1-xGex) based HIT solar cells with x = 0, 0.25, 0.41 and 0.56 in order to quantify the effect of germanium fraction on key solar cell performance parameters. The p-type absorber layer consists of 2 and 4 mu m Si1-xGex layer grown on p+ silicon substrate using a graded buffer layer to reduce the threading dislocation density. The emitter is n+ amorphous-Si. A thin strained-Si layer is grown on the c-Si1-xGex layer prior to a-Si deposition and is believed to improve a-Si-H/c-Si1-xGex interface quality. The short-circuit current (J(sc)) increases, from similar to 14 mA/cm(2) for Si cells to 21 mA/cm(2) for Si0.44Ge0.56 cells with 2 mu m-thick active layers, while open-circuit voltage decreases. The spectral response of the Si1-xGex solar cells improves due to a reduction in absorption depth and smaller band-gap associated with the higher germanium fractions. (C) 2014 Elsevier Ltd. All rights reserved.