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Thin film a-Si(n+)/c-Si1-xGex(p)/c-Si(p+) heterojunction solar cells are fabricated with Ge content up to 56 atomic percent. Solar cells with junction layers consisting of Si, Si0.75Ge0.25, Si0.59Ge0.41, and Si0.44Ge0.56 are compared to study the effect of increasing Ge concentration. The measured short-circuit current (Jsc) increases from ~14 mA/cm2 for Si cells to 21 mA/cm2 for the Si0.44Ge0.56 cells, for one light pass and a 2 μm-thick SiGe layer. The results show an open-circuit voltage (Voc) of 0.61 V for Si cells, dropping to 0.32 V for Si0.44Ge0.56, consistent with the reduction in band-gap. Quantum efficiency measurements highlight the improved spectral response for higher Ge percentages. Physics based TCAD simulations combined with the experimental results are used to extract lifetime and interface velocity.