High efficiency Si heterojunction (HJ) solar cells must exhibit low interface recombination, as it limits the cell open circuit voltage (VOC). The study of the interface recombination of various a-Si:H/c-Si lifetime test samples gives insight into the recombination mechanisms, which are found compatible with an amphoteric recombination model . We find that there is a trade-off between reduced interface defect density, yielding high-VOC cells (713mV), and increased field effect passivation resulting in higher efficient cells (19.1%) on flat wafers. Predicted VOCs of 725mV are reached for optimally textured n- and p-type c- Si wafers passivated by intrinsic a-Si:H, but the VOCs of the cells are lower. The injection-level dependence of the surface recombination identifies the efficiency limiting factors of HJ solar cells. Such measurements are thus a powerful indicator to achieve highly efficient devices.