The applicability of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells for their use in space environment was tested. The cells were subject to proton irradiation at energies between 0.8 and 4 MeV. The optical and electrical parameters were measured. The diffusion lengths, L-D(,) were deduced from the internal quantum efficiency. A plot of 1/L-D(2) vs the applied doses yielded the damage constant. This procedure was repeated for the energy range of 0.8-4 MeV. The damage constants were lower or comparable to those of diffused solar cells. As in the case of conventional solar cells a maximum was found for an energy of 1.7 MeV. The spectral response calculated by means of the AFORSHET simulation program was fitted to the experimental data, using an inhomogeneous defect distribution within the crystalline silicon substrate after irradiation. The defect density used as a fit parameter was proportional to the implanted proton dose. (c) 2006 The Electrochemical Society.