Abstract 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 μm 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 (Jsc) increases, from ∼14 mA/cm2 for Si cells to 21 mA/cm2 for Si0.44Ge0.56 cells with 2 μm-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.