Amorphous-crystalline silicon (a-Si:H/c-Si) heterojunctions have recently drawn much attention owing to their low-temperature fabrication and high-efficiency photovoltaics. a-Si:H/c-Si heterojunctions were studied for the first time using the constant photocurrent method (CPM). The doping concentration in the p-type a-Si:H was varied. CPM derived absorption for energies greater than 1.4 eV is observed to increase with decreasing dopant concentration in the p- layer. This is attributed to a decrease in the density of defect states in the amorphous layer and the interface. A model is proposed wherein the amorphous layer and the interface constitute one absorbing layer while the crystalline substrate forms the other absorbing layer. A combined defect density in the amorphous layer and interface of 2.8x1018 cm-3 eV-1 at 0.4 eV from the valence band edge was measured for our best device. By comparing the combined defect density with that of a single amorphous layer the defect density at the interface is inferred to be 5x1012 cm -2.