This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells in Sanyo. Much effort has been devoted to increasing both the stabilized efficiency and the process throughput. A key issue in increasing the stabilized efficiency is thinner i-layer structure with an improved optical confinement effect. High-rate deposition of the i-layers has been investigated using rf (13.56MHz) plasma-CVD method while keeping the substrate temperature below 200 °C. A high photosensitivity of 106 of a-Si:H films maintain up to the deposition rate (Rd) of 15 Å/s by optimizing hydrogen dilution and other deposition conditions. It is of great importance to utilize the effect of hydrogen dilution which can reduce the incorporation of excess hydrogen in the films. The world's highest conversion efficiency of 11.2% has been achieved for a large-area (8252cm2) a-Si/a-SiGe tandem by combining the optimized hydrogen dilution and other solar cell related technologies.