Cadmium Telluride A second material which is being extensively studied for thin-film solar cells is cadmium telluride. The bandgap of CdTe is 1.5 eV, which is very well matched to spectrum. It is produced in thin-film form by a wide variety of deposition methods. Best results to date are an AMO efficiency of about 8.6% [32], Like CuInSe2, it is currently not produced on thin substrates. However, unlike CuInSe2, most CdTe deposition methods are ‘superstrate' technologies, where the cell is deposited inverted upon transparent glass, which is used as the front cover. This glass can easily be produced in 50 micron (2 mil) sheets. It is also possible that a transparent plastic could be used. Mixed alloys are also possible in the II-VI system. Ternary alloys of cadmium zinc telluride and cadmium manganese telluride [33] can be made with a higher bandgap than CdTe; ternary alloys of mercury cadmium telluride can be made with lower bandgap. Mercury cadmium telluride (HgCdTe) ternary cells have been made with efficiency as high as 8.5% AMI.5 [34]. HgCdTe with high mercury content (low bandgap) is a material which has been well developed for infrared sensors. Transfer of the technology to solar cells should be straightforward. One advantage to mercury cadmium telluride is that it is easier to make contact to than CdTe, and, in fact, the best CdTe cells utilize HgCdTe in the contacts. Another II-IV compounds which may be useful for thin-film solar cells is cadmium selenide (CdSe) [35]. The bandgap of CdSe is 1.7 eV, slightly high for a single junction cell, but excellent for the top element of a cascade. Amorphous Silicon The material referred to as amorphous silicon is actually a mixed alloy of silicon and hydrogen, where the hydrogen incorporation is necessary for good electronic properties and can range from a few percent to as much as 15%. The material differs from the thin-film materials described above in that the crystal is unstructured. The effective bandgap of amorphous silicon can be varied depending on the deposition parameters within a range of about 1.6-1.7 eV. This is well matched to the solar spectrum. The bandgap can be tailored further by addition of carbon to raise the bandgap, and germanium or tin to reduce it, but so far such amorphous silicon alloy cells have not shown as high performance as pure amorphous silicon. Amorphous silicon solar cells for terrestrial use are the subject of a very large and active research program, currently funded at several million dollars per year. Much of this research will likely be applicable to space applications. The manufacturing technology base on a-Si is very large by space standards. Amorphous silicon solar cell modules are currently in production by a number of companies at the 10 million watts/year level. This yearly production level is considerably larger than the entire amount of conventional solar cells flown in space. The best efficiency of amorphous cells is currently about 9% AMO for single junction cells. Some better efficiencies have been reported, but not independently verified; 5% efficiency is more typical of what we measure. A significant difficulty with amorphous silicon solar cell technology is the light- induced degradation, or Staebler-Wronski effect. First generation a-Si modules experienced about 20% degradation in peak power over two years of exposure to light. The best current a-Si solar cells are more stable, but still experience 10-15% loss of performance. It is believed that future improvements and better understanding of the physics will reduce this degradation still further.
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