Suns-Voc measurements are frequently used to study the intrinsic potential of a solar cell technology, by excluding the parasitic series resistance effects. However, when applied to a-Si/c-Si heterojunction (SHJ) solar cells, the results show a peculiar turn-around at illumination intensities that has been attributed to an extrinsic Schottky contact. In this paper, we demonstrate that this voltage turn-around rather may arise from the heterojunction (HJ), inherent to SHJ solar cell, without having to invoke the Schottky contact assumption. We use numerical simulations to explore the full J-V under different illumination and temperature conditions. Using these J-Vs, we establish the bias, intensity, temperature conditions necessary to observe the voltage turn-around in these cells. We validate the HJ hypothesis using an extensive set of experiments on a high- efficiency SHJ solar cell. Our work establishes Suns-Voc as a powerful characterization tool for extracting the cell parameters that limit efficiency in heterojunction devices.