A theoretical design analysis using numerical two dimensional computer aided design tool (i.e., TCAD) is presented for a-Si/c-Si based heterojunction (HJ) solar cells. A set of optical beam propagation models, complex refractive index models and defect models for a-Si material implemented (in-built) in the simulation software are first evaluated for single (SHJ) and double heterojunction (DHJ) devices. Assessment is further carried out by varying physical parameters of the layer structures such as doping, thickness of the c-Si and a-Si layers, defect density in the a-Si layer and bandgap discontinuity parameter. With varying bandgap discontinuity and using standard transport model in numerical device simulation, HJ solar cell performance is undervalued (η = 19.5%). This is the result of poor photogenerated carrier collection due to the presence of heterojunction at the respective n and p-contacts of the device. Implementing thermionic field emission tunneling model at the heterojunction, we obtained improved performance (η = 24 %) over large range of bandgap discontinuities. Keeping improved efficiency of HJ cell, implementing a step graded a-Si layer, further helps to widen the range of bandgap discontinuity parameter.