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U of M researchers have cleared a major hurdle in the drive to build solar cells with potential efficiencies up to twice as high as current levels.
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A team of Univ. of
Minnesota-led researchers has cleared a major hurdle in the drive to build
solar cells with potential efficiencies up to twice as high as current levels,
which rarely exceed 30%.
By showing how energy
that is now being lost from semiconductors in solar cells can be captured and
transferred to electric circuits, the team has opened a new avenue for solar
cell researchers seeking to build cheaper, more efficient solar energy devices.
The work is published in Science.
A system built on the
research could also slash the cost of manufacturing solar cells by removing the
need to process them at very high temperatures.
The achievement crowns
six years of work begun at the university Institute of Technology (College of
Science and Engineering) chemical engineering and materials science professors
Eray Aydil and David Norris and chemistry professor Xiaoyang Zhu (now at the
university of Texas-Austin) and spearheaded by U of M graduate student William
Tisdale.
In most solar cells now
in use, rays from the sun strike the uppermost layer of the cells, which is
made of a crystalline semiconductor substance—usually silicon. The problem is
that many electrons in the silicon absorb excess amounts of solar energy and
radiate that energy away as heat before it can be harnessed.
An early step in
harnessing that energy is to transfer these “hot” electrons out of the
semiconductor and into a wire, or electric circuit, before they can cool off.
But efforts to extract hot electrons from traditional silicon semiconductors
have not succeeded.
However, when
semiconductors are constructed in small pieces only a few nanometers wide—“quantum
dots”—their properties change.
“Theory says that quantum
dots should slow the loss of energy as heat,” said Tisdale. “And a 2008 paper
from the Univ. of
Chicago showed this to be
true. The big question for us was whether we could also speed up the extraction
and transfer of hot electrons enough to grab them before they cooled. ”
In the current work, Tisdale
and his colleagues demonstrated that quantum dots—made not of silicon but of
another semiconductor called lead selenide—could indeed be made to surrender
their “hot” electrons before they cooled. The electrons were pulled away by
titanium dioxide, another common inexpensive and abundant semiconductor
material that behaves like a wire.
“This is a very promising
result,” said Tisdale. “We’ve shown that you can pull hot electrons out very
quickly – before they lose their energy. This is exciting fundamental science.”
The work shows that the
potential for building solar cells with efficiencies approaching 66% exists,
according to Aydil.
“This work is a necessary
but not sufficient step for building very high-efficiency solar cells,” he
said. “It provides a motivation for researchers to work on quantum dots and
solar cells based on quantum dots.”
The next step is to
construct solar cells with quantum dots and study them. But one big problem
still remains: “Hot” electrons also lose their energy in titanium dioxide. New
solar cell designs will be needed to eliminate this loss, the researchers said.
Still, “I’m comfortable
saying that electricity from solar cells is going to be a large fraction of our
energy supply in the future,” Aydil noted.
The research was funded
primarily by the U.S. Department of Energy and partially by the National
Science Foundation. Other authors of the paper were Brooke Timp from the
University of Minnesota
and Kenrick Williams from UT-Austin.
SOURCE
