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A First for NREL Scientists: Multiple Exciton Generation Found in Silicon Nanocrystals

August 08, 2007

Scientists at the National Renewable Energy Laboratory (NREL), collaborating with Innovalight, Inc., have shown that a quantum effect called multiple exciton generation (MEG) occurs efficiently in silicon nanocrystals. MEG results in the formation of more than one electron per absorbed photon and therefore has potential to greatly enhance the conversion efficiency of solar cells because more of the sun’s energy can be converted to electricity. 

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. Silicon is the dominant semiconductor material used in present day solar cells, representing more than 93% of the photovoltaic cell market.

In a paper published on July 24, 2007, in the initial online version of the American Chemical Society’s Nano Letters Journal, 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.  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.

The silicon nanocrystals produced by Innovalight, Inc., a thin-film solar cell developer based in Santa Clara, California, were studied at NREL as part of a collaboration between NREL and Innovalight scientists. The NREL team consisted of Matthew Beard, Kelly Knutsen, Joseph Luther, Qing Song, Wyatt Metzger, Randy Ellingson, and Arthur Nozik.

The findings represent an important extension of the range of semiconductor materials that exhibit MEG and are a further confirmation of pioneering work by Nozik, who in 1997 predicted that semiconductor quantum dots could exhibit efficient electron multiplication and hence increase the efficiency of solar cells.

Calculations at NREL by Mark Hanna and Nozik have shown that the maximum theoretical efficiency of quantum dot solar cells exhibiting optimal MEG is about 44% with normal unconcentrated sunlight and 68% with sunlight concentrated by a factor of 500 with special lenses or mirrors. Today’s conventional solar cells that produce one electron per photon have maximum efficiencies of 33% and 40%, respectively, under the same solar conditions.