Researchers at Ohio State University
are experimenting with polymer semiconductors that absorb the sun’s energy
and generate electricity. The goal: lighter, cheaper, and more-flexible solar
They have now discovered that adding tiny bits of silver to the plastic boosts
the materials’ electrical current generation.
Paul Berger, professor of electrical and computer engineering and professor
of physics at Ohio State, led the team that reported the results online in the
journal Solar Energy Materials and Solar Cells.
Berger and his team measured the amount of light absorbed and the current density
-- the amount of electrical current generated per square centimeter -- generated
by an experimental solar cell polymer with and without silver nano-particles.
Without silver, the material generated 6.2 milli-amps per square centimeter.
With silver, it generated 7.0 -- an increase of almost 12 percent.
The small silver particles help the polymer capture a wider range of wavelengths
of sunlight than would normally be possible, which in turn increases the current
output, Berger explained.
He added that with further work, this technology could go a long way toward
making polymer solar cells commercially viable.
“The light absorption of polymer solar cells is inadequate today,”
he said. “The top-performing materials have an overall efficiency of about
5 percent. Even with the relatively low production cost of polymers compared
to other solar cell materials, you’d still have to boost that efficiency
to at least 10 percent to turn a profit. One way to do that would be to expand
the range of wavelengths that they absorb. Current polymers only absorb a small
portion of the incident sunlight.”
The new fabrication technique involves encasing each silver particle in an
ultra-thin polymer layer -- a different polymer than the light-absorbing polymer
that makes up the solar cell -- before depositing them below the light-absorbing
polymer; the coating prevents the silver particles from clumping, but also allows
them to self-assemble into a dense and regular mosaic pattern that Berger believes
is key to enhancing the light absorption.
Even though the silver particles allow the material to produce 12 percent more
electrical current, that improvement may not translate directly into a 12 percent
increase in overall solar cell efficiency. Many factors effect efficiency, and
some energy can be lost.
Still, the silver nanoparticles could boost the overall efficiency of virtually
any kind of solar cell -- those made from polymers or other semiconductor materials.
Berger and his colleagues are now studying other nanoparticle formulations that
would increase that number further.
“By changing the organic coating, we could change the spacing of the
particles and alter the size of each particle. By fine-tuning the mosaic pattern,
we could move the enhanced absorption to different wavelengths, and thus get
even more of an improvement. I think we can get several percent more,”
The semiconductor polymer captures more light because the metal nanoparticles
absorb light that would normally be wasted. This extra light energy excites
electrons in the metal particles, creating electron waves called plasmons --
a cross between plasma and photons. The plasmons dance across the surface, depositing
energy inside the solar cell that would otherwise be lost.
Researchers have been looking for a way to generate plasmons in solar cells
without greatly increasing the difficulty and cost of manufacture. Given that
his technique uses simple fabrication equipment at room temperature, and given
that the silver particles self-assemble based only on the chemistry of the coating,
Berger feels that any laboratory could easily make use of this finding.
“Not only do week seek better efficiency, but also lower costs too,”
His co-authors on the paper include student Woo-Jun Yoon, who is conducting
this work for his doctoral degree; Fernando Teixeira, associate professor of
electrical and computer engineering; and Jiwen Liu, Thirumalai Durasisamy, Rao
Revur, and Suvankar Sengupa -- all of MetaMateria Partners, LLC, formerly of
Columbus, Ohio, which coated the silver nano-particles with polymer.
This work was funded by the Wright Center for Photovoltaics Innovation and
Commercialization, and the Institute for Materials Research at Ohio State.