This paper shows that the nature of radiation-induced point defects and dopant interactions can cause a shift in the optimum base doping concentration for terrestrial 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 remain essentially unchanged for GaAs cells compared to their counterpart terrestrial cells. The optimum base doping for Si cells decreases from 8.94 x 10¹⁶ cm ^-3 to ~6.6 x 10¹⁴ cm^-1 after 1-MeV electron irradiation. In the case of GaAs, the optimum base doping concentration remains at ~2 x 10¹⁷ cm ^-3 for both irradiated and unirradiated cells. In contrast to Si and GaAs cells, the InP base doping needs to be increased in the range of 2–6 x 10¹⁷ cm^-3 from 2 x 10¹⁷ cm^-3 for radiation fluences in the range of 10¹⁵ to 10¹⁶ cm^-2 in order to achieve highest efficiency after irradiation. © 1992 IEEE