An Indium Tin Oxide (ITO) layer is frequently used as front contact for a-Si/c-Si heterojunction (HJ) solar cells. It must have appropriate characteristics from the electrical point of view (i.e., a good conductivity) as well as from the optical point of view (low absorption coefficient and good antireflection properties). However, higher conductivities imply more absorbent materials and then less transparency, thus a compromise between these effects should define the optimised device. In this paper, the optimisation of the ITO layer thickness was performed taking into account the influence of the a-Si layer thickness, the optical refraction indexes of both materials and the spectral response of the device. The code D-AMPS-1D was used for device simulation. Calculated spectral response and I-V curve are presented and compared with experimental results. The optimisation of the front contact grid is also carried out taking into account the ITO sheet resistance. Optimal ITO thickness between 80 and 90 nm were obtained depending on the particular spectral response. A tolerance of ± 10nm was found acceptable with a minimal degradation on the device performance. Finally, the experimental conditions for the deposition of suitable ITO layers by means of a radiofrequency (RF) magnetron sputtering system were determined.