Current-voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 Pm before to a value of 4 Pm after irradiation with 5 x 10(12) cm(-2) protons. Good agreement has been obtained between measured and simulated data using DIFFIN,(1) a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation. (C) 2004 Elsevier B.V. All rights reserved.