The paper reports on the progress of MOVPE-grown III-V epitaxial structures and tandem solar cell development. A multi-wafer reactor with a capacity of five 4-in wafers is used to grow high-efficiency, large-area tandem solar cells. The benefits of advanced tandem structures for concentrator applications based on Ga0.35In0.65P/Ga0.83In0.17As grown lattice mismatched to GaAs are discussed. In order to achieve super-high efficiencies a mechanical stack of monolithic tandem cells made from Ga0.51In0.49P/GaAs and Al0.25Ga0.75As0.02Sb0.98/GaSb is suggested. (C) 2001 Elsevier Science B.V. All rights reserved.