Advances in amorphous silicon based alloys and device structure have led to the demonstration of 10.5% initial efficiency and 9.1% stabilized efficiency on 1 ft/sup 2/ a-Si/a-SiGe tandem junction module. Scale-up to 4 ft/sup 2/ in pilot production has resulted in the best initial efficiency of 9.75% as measured by NREL, which should result in a stabilized efficiency over 8%. The 4 ft/sup 2/ modules have an average initial efficiency of 8.75% and a stabilized average efficiency of about 7.5%, as measured by Solarex.