Accurate simulation of a-Si:H / c-Si heterojunction (HET) solar cells is mandatory for acquiring a deeper understanding of device physics, better knowledge of material properties, and thus improving solar cells efficiency towards the 26% theoretical limit. The purpose of this paper is to provide relevant guidelines and to highlight key issues for accurate and physically-based HET solar cells simulation. The need for a 2D simulation approach is demonstrated, together with an accurate description of the device optical performance. For the first time, a unified set of models and material parameters is proposed for reproducing experimental IV characteristics under illumination and obscurity conditions, considering state-of-the-art material parameters and localized defects. Finally, the key role of solar cell simulation is demonstrated for further device optimization.