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A flow of methane and hydrogen gas mixture deposits carbon atoms as graphene on a nickel plate. The graphene later is then transferred to a plastic sheet, which is then incorporated into an organic photo voltaic (OPV) cell. Credit: USC Viterbi School of Engineering
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A University of Southern California team has produced flexible
transparent carbon atom films that the researchers say have great
potential for a new breed of solar cells.
"Organic photovoltaic (OPV) cells have been proposed as a means
to achieve low cost energy due to their ease of manufacture, light
weight, and compatibility with flexible substrates," wrote Chongwu
Zhou, a professor of electrical engineering in the USC Viterbi
School of Engineering, in a paper recently published in the journal
ACS Nano.
The technique described in the article describes progress toward
a novel OPV cell design that has significant advantages,
particularly in the area of physical flexibility.
A critical aspect of any OPV photo-electronic device is a
transparent conductive electrode through which light can couple
with active materials to create electricity. The new work indicates
that graphene, a highly conductive and highly transparent form of
carbon made up of atoms-thick sheets of carbon atoms, has high
potential to fill this role.
While graphene's existence has been known for decades, it has
only been studied extensively since 2004 because of the difficulty
of manufacturing it in high quality and in quantity.
The Zhou lab reported the large scale production of graphene
films by chemical vapor deposition three years ago. In this
process, the USC engineering team creates ultra thin graphene
sheets by first depositing carbon atoms in the form of graphene
films on a nickel plate from methane gas.
Then they lay down a protective layer of thermo plastic over the
graphene layer, and then dissolve the nickel underneath in an acid
bath. In the final step they attach the plastic-protected graphene
to a very flexible polymer sheet, which can then be incorporated
into a OPV cell. (see diagram)
The USC team has produced graphene/polymer sheets ranging in
sizes up to 150 square centimeters that in turn can be used to
create dense arrays of flexible OPV cells.
These OPV devices convert solar radiation to electricity, but
not as efficiently as silicon cells. The power provided by sunlight
on a sunny day is about 1000 watts per meter square. "For every
1000 watts of sunlight that hits a one square meter area of the
standard silicon solar cell, 14 watts of electricity will be
generated," says Lewis Gomez De Arco, a doctoral student and a
member of the team that built the graphene OPVs. "Organic solar
cells are less efficient; their conversion rate for that same one
thousand watts of sunlight in the graphene-based solar cell would
be only 1.3 watts."
But what graphene OPVs lack in efficiency, they can potentially
more than make for in lower price and, greater physical
flexibility. Gomez De Arco thinks that it may eventually be
possible to run printing presses laying extensive areas covered
with inexpensive solar cells, much like newspaper presses print
newspapers.
"They could be hung as curtains in homes or even made into
fabric and be worn as power generating clothing. I can imagine
people powering their cellular phone or music/video device while
jogging in the sun," he said.
The USC researchers say graphene OPVs would be major advance in
at least one crucial area over a rival OPV design, one based on
Indium–Tin–Oxide (ITO). In the USC team's tests, ITO
cells failed at a very small angle of bending, while the
graphene-based cells remained operational after repeated bending at
much larger stress angles. This would give the graphene solar cells
a decided advantage in some uses, including the printed-on-fabric
applications proposed by the USC team.
Zhou and the other researchers on the USC team – which
included Yi Zhang, Cody W. Schlenker, Koungmin Ryu, and Mark E.
Thompson in addition to Gomez de Arco - are excited by the
potential for this technology.
Their paper concludes that their approach constitutes a
significant advance toward the production of transparent conductive
electrodes in solar cells. "CVD graphene meets the most important
criteria of abundance, low cost, conductivity, stability,
electrode/organic film compatibility, and flexibility that are
necessary to replace ITO in organic photovoltaics, which may have
important implications for future organic optoelectronic
devices."
SOURCE
