Multi-quantum well GaAs/In0.19Ga0.81As solar cells have been measured under low concentration levels (1-10 suns) of AM1.5 illumination. An efficiency of 22% has been obtained at a ratio of 4 suns as opposed to 18% under 1 sun AM1.5 conditions. We explain the improvements in conversion efficiency in terms of an enhancement in minority-carrier lifetime under concentration. Even when the concentration ratio is low, the high-injection regime can be achieved since the carrier concentration in the intrinsic layer is very low. The existence of a high concentration of defects at 0.36 eV below the conduction band in the base layer has been observed by the DLTS analysis. Enhancement of the minority-carrier lifetime under concentration is thought to be due to filling of recombination centers by the injection minority carriers. (C) 2000 Elsevier Science B.V. All rights reserved.