Unique Quantum Effect Found in Silicon
Quantum Dot Materials May Improve
Efficiency of Silicon Solar Cells
July 24, 2007 - Press Release - National Renewable
Energy Laboratory (NREL)
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 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 percent 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
In a paper published on July 24 in the
initial on-line 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 percent 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 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 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.
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 by Mark Hanna and Nozik have
shown that the maximum theoretical efficiency of quantum
dot solar cells exhibiting optimal MEG is about 44
percent with normal unconcentrated sunlight and 68
percent 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 percent and 40 percent,
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
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.
For further information contact NREL
Public Relations at (303) 275-4090.