Heterojunction silicon solar cells, also known as heterojunction having intrinsic thin layer (HIT) of the type TCO/n-a-Si:H/i-a-Si:H/p-c-Si/p+-a-Si:H/BSF solar cells (where TCO is transparent conducting oxide involving Al-doped ZnO; a-Si:H is hydrogenated amorphous silicon; c-Si:H is hydrogenated crystalline silicon; BSF is back surface field, n- and p- refer to n-type and p-type respectively; n-a-Si is the emitter layer, i-a-Si is the passivation layer of intrinsic type semiconductor) attract special interest due to their suitability and high efficiencies. Thickness and work function for the TCO layer, together with thickness and doping density of the emitter layer, are all optimized here. With optimal parameters, TCO/n-a-Si:H/i-a-Si:H/p-c-Si/p+-a-Si:H/BSF solar cell may exhibit high simulation characteristics in terms of conversion efficiency (25.62%), open circuit potential (VOC, 744 mV), short circuit current density (JSC, 42.43 mA/cm2) and fill factor (FF, 83.7%). The Automat for Simulation of Heterostructures (AFORS-HET) program is used. The energy band diagram, current density, quantum efficiency, and charge-carrier generation/recombination behaviours are investigated to find out how hetero-junction cell performance enhancement occurs.