Quantum Dot Materials May Improve Efficiency of Silicon Solar Cells

In the race to make clean technologies more efficient and marketable a breakthrough has been made by Researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), collaborating with Innovalight, Inc., showing 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, increasing the amount of light that is converted into usable electricity which is a key step toward making solar energy more cost-competitive with conventional power sources.

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.

Researchers at NREL recently 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.

To date, all experiments showing the production of more than one electron per absorbed photon have been based on various types of optical spectroscopy. In a solar cell device it is necessary to extract the electrons produced in the quantum dots 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 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.

In addition to efficiently extracting the electrons from the quantum dots in solar cells, future research is directed toward producing MEG at wavelengths that have a greater overlap with the solar spectrum, as well as producing a much sharper onset of the MEG processes with decreasing wavelength of the photons.

For additional information visit: www.nrel.gov