It is shown that the nature of radiation-induced point defects and dopant interactions can cause a shift in the optimum base doping concentration for terrestial and space solar cells. The base doping concentration has been optimized for high-efficiency Si, GaAs, and InP solar cells before and after electron irradiation. A combination of detailed carrier lifetime calculations and cell modeling is used to show that the optimum doping concentration for irradiated cells increases for InP cells, decreases for Si cells, and remains essentially unchanged for GaAs cells compared to their counterpart terrestial cells. The optimum base doping for Si cells decreases from 8.94 × 1016 cm-3 to approximately 6.6 × 1014 cm-1 after 1-MeV electron irradiation. In the case of GaAs, the optimum base doping concentration remains at approximately 2 × 1017 cm-3 for both irradiated and unirradiated cells. The InP base doping needs to be increased in the range of (2-6) × 1017 cm-3 from 2 × 1017 cm-3 for radiation fluences in the range of 1015 to 1016 cm-2 in order to achieve the highest efficiency after irradiation.