Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), collaborating with Innovalight, Inc., have shown that a new and important effect called Multiple Exciton Generation (MEG) occurs efficiently in tiny silicon crystals. MEG results in a larger percentage of solar energy converted to electrical energy.

Silicon is the main semi-conductor material used in present day solar cells, which covers more than 93% of the photovoltaic cell market.  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 on-line version of the American Chemical Society’s Nano Letters Journal (visit: http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/nl071486l), an NREL team reported that tiny silicaon-based crystals, which were obtained from Innovalight can produce more electrical energy from the same amount of solar energy (sunlight) than the commercially manufactured solar cells produce. When today’s photovoltaic solar cells absorb a photon of sunlight, about 50% of the energy is lost as heat. MEG provides a way to convert some of this energy lost as heat into additional electricity.

The silicon-based semi-conductor crystals produced by Innovalight, Inc., which is a thin-film solar cell developer based in Santa Clara, California, were studied by NREL while working with Innovalight scientists. The NREL team consisted of Matthew C. Beard, Kelly P. Knutsen, Joseph M. Luther, Qing Song, Wyatt Metzger, Randy J. Ellingson and Arthur J. Nozik.

The findings represent an important extension of the pioneering work by Nozik, who in 1997 predicted that tiny semi-conductor crystals could increase the efficiency of solar cells.

To date, all experiments that propose to demonstrate the improvement in electrical generation have been based upon various types of optical analyses of a light source. In a solar cell device it is necessary to extract the electrons produced in the tiny crystals and pass them through an external circuit to generate electrical power. Such experiments are currently underway at NREL, Innovalight and other laboratories to demonstrate that MEG can indeed lead to enhanced solar cell efficiencies. Calculations at NREL by Mark Hanna and Nozik have shown that the maximum theoretical efficiency of these tiny partical solar cells exhibiting optimal MEG is about 44% with normal sunlight and 68% with sunlight  that has been highly concentrated. Today’s conventional solar cells have maximum efficiencies of 33% and 40%, respectively, under the same solar conditions.

In addition to efficiently extracting the electrons from the tiny crystals in solar cells, future research is directed toward producing MEG more within a sunlight range.

About the National Renewable Energy Laboratory:

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. The NREL research was funded by DOE’s Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. NREL is operated for DOE by Midwest Research Institute and Battelle. NR20-07

For more and more detailed information visit their websites: http://www.nrel.gov and http://www.innovalight.com.

Source for this post: U.S. DOE National Renewable Energy Laboratory; Innovalight, Inc.

George Douglas, of U.S. Department of Energy’s National Renewable Energy Laboratory, +1-303-275-4096, george_douglas@nrel.gov; or Jennifer Watts, of Innovalight, Inc., +1-408-987-9400, ext 300, jwatts@innovalight.com

Websites: http://innovalight.com
http://www.nrel.gov

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