A detailed study of the antireflecting properties of Indium Tin Oxide (ITO) thin films used as front electrodes in a-Si/c-Si heterojunction (HJ) solar cells is presented. Radiofrequency magnetron sputtering deposition conditions of ITO layers were optimised for HJ solar cells applications. An XPS analysis was performed on the deposited films and a correlation between the film composition and the experimental parameters used in the sputtering process was established. The ITO layer thickness was optimised considering the a-Si layer thickness, its optical characteristics and the HJ solar cell spectral response. In our devices, the optimal ITO film thickness was established to be in the range 80 - 90 nm depending on the solar cell spectral response. A thickness tolerance of ± 10 nm was found to be suitable by accepting a minor degradation of the device performance. Finally, device simulation results obtained by the AMPS code are reported.