Indium phosphide solar cells are more radiation resistant than gallium arsenide and silicon solar cells and their growth by heteroepitaxy offers additional advantages leading to the development of lighter, mechanically strong and cost‐effective cells. Changes in heteroepitaxial InP cell efficiency under 0.5 and 3 MeV proton irradiations have been explained by the variation in the minority‐carrier diffusion length. The base diffusion length versus proton fluence has been calculated by simulating the cell performance. The diffusion length damage coefficientKLhas also been plotted as a function of proton fluence.