A model that relates the relatively low Fill Factor (FF) in p(+) /n amorphous/crystalline silicon (a-Si:H/c-Si) hetero-junction solar cells (SHJ) with perfectly ohmic contacts,,to hole reflection at the hetero-interface under illumination is proposed. Such a reflection results from the partial emission and transport of holes imposed by the thermionic emission boundary condition at the hetero-interface. The non-emitted holes scatter back and diffuse towards the substrate. The probability of hole reflection depends on the thermionic emission parameters and also on the diffusion barrier associated with the pn junction adjacent to the hetero-interface. With such hole reflection the cell forward current under illumination is enhanced well above the cell dark current leading to Fill Factor and efficiency degradation. Such effects are found to be particularly significant in cells with deficient active doping in a-Si:H. The model is supported by the results of AFORS-HET simulations at room temperature and at different temperatures. When the model predictions are taken into account, p(+) /n SHJ cells with minimum hole reflection and FF's exceeding 81% could be designed.