A new approach creates a dramatic improvement in cheap solar
cells now being developed in laboratories.
By using a popcorn-ball design – tiny kernels
clumped into much larger porous spheres – researchers at the University
of Washington are able to manipulate light and more than
double the efficiency of converting solar energy to electricity. The
findings will be presented today in New Orleans at the national meeting
of the American Chemical Society.
"We think this can lead to a significant breakthrough in
dye-sensitized solar cells," said lead author Guozhong Cao, a UW
professor of materials science and engineering.
Dye-sensitized solar cells, first popularized in a scientific
article in 1991, are more flexible, easier to manufacture and cheaper
than existing solar technologies. Researchers have tried various rough
surfaces and achieved higher and higher efficiencies. Current lab
prototypes can convert just over one tenth of the incoming sun's energy
into electricity. This is about half as efficient as the commercial,
silicon-based cells used in rooftop panels and calculators.
The UW researchers did not attempt to maximize the overall
efficiency of a dye-sensitized solar cell to match or beat these
previous records. Instead, they focused on developing new approaches
and compared the performance of a homogeneous rough surface with a
clumping design. One of the main quandaries in making an efficient
solar cell is the size of the grains. Smaller grains have bigger
surface area per volume, and thus absorb more rays. But bigger clumps,
closer to the wavelength of visible light, cause light to ricochet
within the thin light-absorbing surface so it has a higher chance of
being absorbed.
"You want to have a larger surface area by making the grains
smaller," Cao said. "But if you let the light bounce back and forth
several times, then you have more chances of capturing the energy."
Other researchers have tried mixing larger grains in with the
small particles to scatter the light, but have little success in
boosting efficiency. The UW group instead made only very tiny grains,
about 15 nanometers across. (Lining up 3,500 grains end to end would
equal the width of a human hair.) Then they clumped these into larger
agglomerations, about 300 nanometers across. The larger balls scatter
incoming rays and force the light to travel a longer distance within
the solar cell. The balls' complex internal structure, meanwhile,
creates a surface area of about 1,000 square feet for each gram of
material. This internal surface is coated with a dye that captures the
light.
The researchers expected some improvement in the performance
but what they saw exceeded their hopes.
"We did not expect the doubling," Cao said. "It was a happy
surprise."
The overall efficiency was 2.4 percent using only small
particles, which is the highest efficiency achieved for this material.
With the popcorn-ball design, results presented today at the conference
show an efficiency of 6.2 percent, more than double the previous
performance.
"The most significant finding is the amount of increase using
this unique approach," Cao said.
The experiments were performed using zinc oxide, which is less
stable chemically than the more commonly used titanium oxide but easier
to work with.
"We first wanted to prove the concept in an easier material.
Now we are working on transferring this concept to titanium oxide," Cao
said. Titanium oxide based dye-sensitized solar cells are now at 11
percent maximum efficiency. Cao hopes his strategy could push
dye-sensitized solar cells' efficiency significantly over that
threshold.
Posted 11th April 2008
