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Tech Review;
Researchers at University of Notre Dame, in Indiana, have
demonstrated a way to significantly improve the efficiency
of solar cells made using low-cost, readily available materials,
including a chemical commonly used in paints.
The researchers added single-walled carbon nanotubes to a
film made of titanium-dioxide nanoparticles, doubling the
efficiency of converting ultraviolet light into electrons when
compared with the performance of the nanoparticles alone.
The solar cells could be used to make hydrogen for fuel cells
directly from water or for producing electricity. Titanium
oxide is a main ingredient in white paint.
Such cells are appealing because nanoparticles have a great potential
for absorbing light and generating electrons. But so far, the efficiency
of actual devices made of such nanoparticles has been considerably
lower than that of conventional silicon solar cells. That's largely because
it has proved difficult to harness the electrons that are generated to
create a current.
Indeed, without the carbon nanotubes, electrons generated when light
is absorbed by titanium-oxide particles have to jump from particle to
particle to reach an electrode. Many never make it out to generate an
electrical current. The carbon nanotubes "collect" the electrons and
provide a more direct route to the electrode, improving the efficiency
of the solar cells.
The future of solar cells and solar panels... Nano Solar Cells?
As they wrote online in the journal Nano Letters, the Notre Dame
researchers form a mat of carbon nanotubes on an electrode. The
nanotubes serve as a scaffold on which the titanium-oxide particles
are deposited. "This is a very simple approach for bringing order
into a disordered structure," Kamat says.
The new carbon-nanotube and nanoparticle system is not yet a
practical solar cell. That's because titanium oxide only absorbs
ultraviolet light; most of the visible spectrum of light is reflected
rather than absorbed. But researchers have already demonstrated
ways to modify the nanoparticles to absorb the visible spectrum.
In one strategy, a one-molecule-thick layer of light-absorbing dye
is applied to the titanium-dioxide nanoparticles. Another approach,
which has been demonstrated experimentally by Kamat, is to coat
the nanoparticles with quantum dots--tiny semiconductor crystals.
Unlike conventional materials in which one photon generates just
one electron, quantum dots have the potential to convert high-energy
photons into multiple electrons.
Several other groups are exploring approaches to improve the
collection of electrons within a cell, including forming titanium-oxide
nanotubes or complex branching structures made of various
semiconductors. But experts say that Kamat's work could be a
significant step in creating cheaper, more-efficient solar cells.
"This is very important work," says Gerald Meyer, professor of
chemistry at Johns Hopkins University. "Using carbon nanotubes
as a conduit for electrons from titanium oxide is a novel idea,
and this is a beautiful proof-of-principle experiment."
Nano Solar Cells; what's in the future of solar cell production?
