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Topic Name: Researchers Innovatively Manipulate Light and More Than Double the Competence of Converting Solar Energy to Electricity Using Popcorn-Ball Design
Category: Solar cells
Research persons: Guozhong Cao
Location: University of Washington, United States
Details
A new approach is able to create 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 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.
Note for Solar Energy
Solar energy is energy from the Sun in the form of heat and light. This energy
drives the climate and weather and supports virtually all life on Earth. Heat
and light from the sun, along with secondary solar resources such as wind and
wave power, hydroelectricity and biomass, account for over 99.9% of the
available flow of renewable energy on earth.
Solar energy technologies harness the sun's heat and light for practical ends
such as heating, lighting and electricity. These technologies date from the time
of the early Greeks, Native Americans and Chinese, who warmed their buildings by
orienting them toward the sun.
Solar power is used synonymously with solar energy or more specifically to refer
to the conversion of sunlight into electricity. This can be done with
photovoltaics, concentrating solar thermal devices and various experimental
technologies.
Earth continuously receives 174 petawatts of incoming solar radiation (insolation)
at the upper atmosphere. When it meets the atmosphere, 6 percent of the
insolation is reflected and 16 percent is absorbed. Average atmospheric
conditions (clouds, dust, pollutants) further reduce insolation traveling
through the atmosphere by 20 percent due to reflection and 3 percent via
absorption. These atmospheric conditions not only reduce the quantity of energy
reaching the earth's surface, but also diffuse approximately 20 percent of the
incoming light and filter portions of its spectrum. After passing through the
atmosphere, approximately half the insolation is in the visible electromagnetic
spectrum with the other half mostly in the infrared spectrum (a small part is
ultraviolet radiation).
The absorption of solar energy by atmospheric convection (sensible heat
transport) and evaporation and condensation of water vapor (latent heat
transport) powers the water cycle and drives the winds. Sunlight absorbed by the
oceans and land masses keeps the surface at an average temperature of 14 °C. The
small portion of solar energy captured by plants and other phototrophs is
converted to chemical energy via photosynthesis. All the food we eat, wood we
build with, and fossil fuels we use are products of photosynthesis. The flows
and stores of solar energy in the environment are vast in comparison to human
energy needs.
The total solar energy available to the earth is approximately 3850 zettajoules
(ZJ) per year.
-
Oceans absorb approximately 2850 ZJ of solar energy per
year.
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Winds can theoretically supply 6 ZJ of energy per year.
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Biomass captures approximately 1.8 ZJ of solar energy per
year.
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Worldwide energy consumption was 0.471 ZJ in 2004.
The upper map on the right shows how solar radiation at the
top of the earth's atmosphere varies with latitude, while the lower map shows
annual average ground-level insolation. For example, in North America, the
average insolation at ground level over an entire year (including nights and
periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day). At
present, photovoltaic panels typically convert about 15 percent of incident
sunlight into electricity; therefore, a solar panel in the contiguous United
States, on average, delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.
The research was funded by the
National Science Foundation, the
Department of Energy,
Washington Technology
Center and the
Air Force Office of Scientific Research. Co-authors are postdoctoral
researcher Qifeng Zhang, research associate Tammy Chou and graduate student
Bryan Russo, all in the UW's department of materials science and engineering and
Samson Jenekhe, a UW professor of chemical engineering.
In figure 1, The thin light-absorbing film is made of zinc
oxide about 10 micrometers (10,000 nanometers) thick. This image was taken using
a scanning electron microscope.
In figure 2, closer image of the film shows that it is composed of tiny balls.
Each ball is about 300 nanometers across.
In figure 3, A close-up of a single ball, taken with a scanning electron
microscope. The 300-nanometer sphere is large enough to scatter light. But its
insides are made of tiny grains just 15 nanometers across.
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