Advances in a-Si:H alloys for high efficiency devices [solar cells]

Multibandgap, multijunction devices based on amorphous silicon alloys are the leading candidates for stable, low-cost solar electricity. Progress in understanding the a-SiGe and a-SiC:H alloys has led to improved stabilized device performance. Hydrogen dilution of the feed stocks yields a relatively homogeneous a-SiGe:H alloy with improved minority carrier properties and vastly improved stability. Triple-junction devices, particularly those consisting of relatively thin i-layers, are quite stable and agree well with an ad hoc model of device degradation. Triple-junction devices having a conversion efficiency of 9.6% and retaining 90% of their initial performance after the equivalent of 1 yr outdoors have been prepared. The improvements in cells have been extended to large-area modules. High-efficiency triple-junction modules have been fabricated by combining the improved alloy devices with advances in module design, highly reflecting rear contacts, and laser scribing. The result has been the demonstration of square-foot modules with an aperture area conversion efficiency of 9% measured outdoors