Effects of thermal annealing on carrier recombination in lattice-mismatched InGaAs solar cells on GaAs substrates were investigated. Thermal annealing to the graded buffer layer was effective to increase the minority carrier lifetime in the solar cell layer. Electron beam-induced current (EBIC) measurements revealed that the density of dark line defects decreased after the thermal annealing, but dark spot defects were newly generated. We conclude that dark line defects were primary responsible for the high recombination in the lattice-mismatched InGaAs solar cells. The origin of dark spot defects was discussed and it was found that they were associated with the lattice mismatch between the InGaP back surface field (BSF) layer and the InGaAs cell layer. (C) 2008 Elsevier B.V. All rights reserved.