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July 26, 2007 -- Researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), collaborating with Innovalight, Inc., say they have shown that a new and important effect called Multiple Exciton Generation (MEG) occurs efficiently in silicon nanocrystals. MEG results in the formation of more than one electron per absorbed photon.
Silicon is the dominant semiconductor material used in present day solar cells, representing more than 93% of the photovoltaic cell market. Until this discovery, MEG had been reported over the past two years to occur only in nanocrystals (also called quantum dots) of semiconductor materials that are not presently used in commercial solar cells, and which contained environmentally harmful materials (such as lead). The new result opens the door to the potential application of MEG for greatly enhancing the conversion efficiency of solar cells based on silicon because more of the sun's energy is converted to electricity. This is a key step toward making solar energy more cost-competitive with conventional power sources.
In a paper published on July 24 in the initial on-line version of the American Chemical Society's Nano Letters Journal (see), an NREL team reported that silicon nanocrystals, or quantum dots, obtained from Innovalight can produce more than one electron from single photons of sunlight that have wavelengths less than 420 nm. According to the team, when today's photovoltaic solar cells absorb a photon of sunlight, about 50% of the incident energy is lost as heat. MEG provides a way to convert some of this energy lost as heat into additional electricity.
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