This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10(6) for a-Si:H films has been maintained up to the deposition rate of 15 Angstrom/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm(2) a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies. (C) 2002 Elsevier Science B.V. All rights reserved.