Solar crystals get 2-for-1
Technology Research News
Researchers from Los Alamos National Laboratory
have tapped the efficiencies of nanotechnology to increase solar cells'
potential energy production by as much as 37 percent.
Solar cells generate electricity by absorbing photons and directing
the resulting energy to move an electron from the low-energy valence band
in a material to a higher-energy conduction band where it is free to flow.
Researchers working to squeeze more energy from sunlight are generally
aiming for solar cells that can absorb and use a higher percentage of
the wavelengths of light in the sun's spectrum. Today's commercial solar
cells can use anywhere from 10 percent to 35 percent.
The Los Alamos researchers have found that it is possible to increase
a cell's energy production by making each photon move two electrons. "Carrier-multiplication-enhanced
solar cells can, in principle, produce twice as large a current as conventional
solar cells," said Victor Klimov, a team leader at Los Alamos National
The method could increase what has been thought of as the maximum
power conversion of solar cells by as much as 37 percent, depending on
the materials used, resulting in a solar cell with a potential efficiency
of over 60 percent. The method could also be used to increase the efficiency
of other optical components, including amplifiers, lasers, switches and
light absorbers, according to Klimov.
Key to the method is the use of lead selenium nanocrystals. The
nanocrystals measure about ten nanometers in diameter, which is the span
of 100 hydrogen atoms, or about 7,500 times narrower than human hair.
In today's solar cells a photon moves one electron and produces
some waste heat. Carrier multiplication, a phenomenon discovered in the
1950s, happens when a photon moves more than one electron at a time.
This happens via impact ionization. "In this effect, the conduction-band
electron [excess] energy is transferred to the valence-band electron and
excites it across the energy gap," said Klimov. "As a result, instead
of one conduction-band electron we have two electrons that can contribute
to electrical current," he said. "Normally, without impact ionization,
the... energy is lost as heat."
In traditional semiconductor materials, carrier multiplication
can be used to increase energy production by about 1 percent. In nanocrystals,
however, carrier multiplication occurs much more efficiently. "Carrier
multiplication occurs with extremely high-efficiency -- up to 100 percent
-- at photonenergies that are relevant to solar power generation," said
The breakthrough that enabled the discovery was a method for detecting
impact ionization, said Klimov. Detecting impact ionization involves measuring
the time difference between a single electron and double electron reaction.
The single electron reactions happen more slowly than the double electron
reactions -- in under one microsecond, or millionth, of a second versus
less than 100 picoseconds, or trillionths of a second.
The researchers measured the difference in lead selenium nanocrystals
by hitting the crystals with pulses of light that were only 50 femtoseconds,
or million billions of a second, long. The researchers were surprised
to find that the ionization effect, "which is almost nonexistent in bulk
semiconductors, turned out to be 100 percent efficient and semiconductor
nanocrystals," said Klimov.
Solar cells that use the researchers method could become practical
in two to three years, said Klimov. Klimov's research colleague was R.
D. Schaller. The work is scheduled to appear in Physical Review Letters.
The research was funded by the U.S. Department of Energy.
Timeline: 2-3 years
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "High-efficiency Carrier
Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion,"
Physical Review Letters, accepted for publication
May 19/26, 2004
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