Microcrystalline silicon-germanium (muc-SiGe) solar cells were investigated for a unit cell of multi-junction structures. Despite having a thinner muc-SiGe layer of 0.5 mum, a conversion efficiency of 5.6% (the highest value ever reported for muc-SiGe solar cells) was achieved by using a muc-SiGe film with 20% Ge concentration. A short-circuit current (I-sc) of 28 mA/cm(2) was obtained in the case of 27% Ge concentration. This is higher than the I-sc of the best muc-Si solar cells with 2 mum thickness. The collection efficiency spectrum shows more than 20% at 1000 nm, and sensitivity is observed in wavelengths up to 1200 nm. The results suggest that a reduction in the thickness of microcrystalline solar cells may achieve higher productivity, and the utilization of longer wavelength light allows us to obtain higher efficiency. (C) 2002 Elsevier Science B.V. All rights reserved.