This paper focuses on the metal-organic vapor-phase epitaxy growth of 3-junction (3J) solar cells where the epitaxial Ga0.44In0.56P top and Ga0.92In0.08As middle subcells are grown lattice-mismatched on a Ge substrate. Single-junction metamorphic devices with 8% and 12%-In, GaInAs are grown on 100 mm dia. (001) Ge substrates and evaluated in comparison to approximately lattice-matched GaAs and Ga0.99In0.01As subcells. Layers are observed to be nearly 100% relaxed by high-resolution X-ray diffraction. Threading dislocation densities of similar to2 x 10(5) cm(-2) in the 8%-In layers are observed by electron beam induced current and cathodoluminescence. Single-junction devices show a constant offset between open-circuit voltage and bandgap of similar to 380 mV. Building upon these results, 3J metamorphic Ga0.44In0.56P/ Ga0.92In0.08As/Ge solar cells are fabricated. Very high performances of small area devices are reported with 28.8% efficiency under the AMO spectrum and 31.3% efficiency under the AM1.5G 1-sun terrestrial spectrum. (C) 2003 Elsevier B.V. All rights reserved.