Conversion efficiency (AM1.5G) of more than 30% was achieved by adding a small quantity of Indium into a GaAs bottom cell in the conventional tandem solar cell on Ge substrate. It was found that the lattice-mismatch between GaAs and Ge caused misfit-dislocations in thick GaAs layers and reduced an open-circuit voltage (V-oc) of the cell. An In0.49Ga0.51P/In0.01Ga0.99As tandem cell lattice-matched to Ge showed a great improvement in efficiency, which was attributed to an increase in the V-oc of the bottom cell and increases in the photocurrents both in the top and bottom cells due to reductions in band-gap energy. (C) 2001 Elsevier Science B.V. All rights reserved.