This paper reviews recent work on the development of thin film silicon solar modules and cost-effective production technology. Noting the potential of VHF-PECVD for high rate and high quality deposition, we initiated development of a-Si solar modules. In the first stage, we succeeded in up-scaling a-Si high quality uniform deposition at a high rate of over 1.0 nm/s to a substrate area of 1.1 x 1.4 m(2) to achieve high productivity. Next, the large area a-Si solar modules with stable aperture efficiency of 8% were developed, and the commercial production of a-Si solar modules commenced in October 2002. In the second stage, aiming at stable efficiency of 12%, which could make the PV power generating cost below residential electricity prices in combination with cost-effective production technology, we have been developing a-Si/muc-Si tandem solar modules. Recently, tandem modules of 40 x 50 cm(2) in size with a muc-Si i-layer prepared at a deposition rate of 2.1 nm/s yielded initial conversion efficiencies of 11.1%. As for small sized muc-Si single cells, technologies with a high deposition rate of 2.5 nm/s and efficiency of 8.8% have already been developed. In addition, by improving the up-scaling and light-trapping techniques, we will achieve our current goal of 12% stable efficiency for a-Si/muc-Si tandem modules at a deposition rate of over 2.0 nm/s, leading to cost-effective mass production. (C) 2004 Elsevier Ltd. All rights reserved.