High Efficiency Triple-Junction Amorphous Silicon Alloy Photovoltaic Technology: Final Technical Report, 6 March 1998 - 15 October 2001

This report describes the research program intended to expand, enhance, and accelerate knowledge and capabilities for developing high-performance, two-terminal multijunction amorphous silicon (a-Si) alloy cells, and modules with low manufacturing cost and high reliability. United Solar uses a spectrum-splitting, triple-junction cell structure. The top cell uses an amorphous silicon alloy of~1.8-eV bandgap to absorb blue photons. The middle cell uses an amorphous silicon germanium alloy (~20% germanium) of~1.6-eV bandgap to capture green photons. The bottom cell has~40% germanium to reduce the bandgap to~1.4-eV to capture red photons. The cells are deposited on a stainless-steel substrate with a predeposited silver/zinc oxide back reflector to facilitate light-trapping. A thinlayer of antireflection coating is applied to the top of the cell to reduce reflection loss. The major research activities conducted under this program were: 1) Fundamental studies to improve our understanding of materials and devices; the work included developing and analyzing a-Si alloy and a-SiGe alloy materials prepared near the threshold of amorphous-to-microcrystalline transition andstudying solar cells fabricated using these materials. 2) Deposition of small-area cells using a radio-frequency technique to obtain higher deposition rates. 3) Deposition of small-area cells using a modified very high frequency technique to obtain higher deposition rates. 4) Large-area cell research to obtain the highest module efficiency. 5) Optimization of solar cells and modules fabricatedusing production parameters in a large-area reactor.