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Structure of plastic solar cells impedes their efficiency Share:
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News EurekAlert | Thursday, October 7, 2010 Double-click any
word to search Highlight any phrase & click HotSearch A team of
researchers from North Carolina State University and the U.K. has found that
the low rate of energy conversion in all-polymer solar-cell technology is
caused by the structure of the solar cells themselves. They hope that their
findings will lead to the creation of more efficient solar cells. Polymeric
solar cells are made of thin layers of interpenetrating structures from two
different conducting plastics and are increasingly popular because they are
both potentially cheaper to make than those currently in use and can be
"painted" or printed onto a variety of surfaces, including flexible films
made from the same material as most soda bottles. However, these solar cells
aren't yet cost-effective to make because they only have a power conversion
rate of about three percent, as opposed to the 15 to 20 percent rate in
existing solar technology. "Solar cells have to be simultaneously thick
enough to absorb photons from the sun, but have structures small enough for
that captured energy — known as an exciton — to be able to travel to the
site of charge separation and conversion into the electricity that we use,"
says Dr. Harald Ade, professor of physics and one of the authors of a paper
describing the research. "The solar cells capture the photons, but the
exciton has too far to travel, the interface between the two different
plastics used is too rough for efficient charge separation, and its energy
gets lost." The researchers' results appear online in Advanced Functional
Materials and Nano Letters. In order for the solar cell to be most
efficient, Ade says, the layer that absorbs the photons should be around
150-200 nanometers thick. (A nanometer is thousands of times smaller than
the width of a human hair.) The resulting exciton, however, should only have
to travel a distance of 10 nanometers before charge separation. The way that
polymeric solar cells are currently structured impedes this process. Ade
continues, "In the all-polymer system investigated, the minimum distance
that the exciton must travel is 80 nanometers, the size of the structures
formed inside the thin film. Additionally, the way devices are currently
manufactured, the interface between the structures isn't sharply defined,
which means that the excitons, or charges, get trapped. New fabrication
methods that provide smaller structures and sharper interfaces need to be
found." Ade and his team plan to look at different types of polymer-based
solar cells to see if their low efficiencies are due to this same structural
problem. They hope that their data will lead chemists and manufacturers to
explore different ways of putting these cells together to increase
efficiency. "Now that we know why the existing technology doesn't work as
well as it could, our next steps will be in looking at physical and chemical
processes that will correct for those problems. Once we get a baseline of
efficiency, we can redirect research and manufacturing efforts." SOURCE
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