Converting sunlight to electricity might no longer mean large panels of photovoltaic
cells atop flat surfaces like roofs.
Using zinc oxide nanostructures grown on optical fibers and coated with dye-sensitized
solar cell materials, researchers at the Georgia
Institute of Technology have developed a new type of three-dimensional photovoltaic
system. The approach could allow PV systems to be hidden from view and located
away from traditional locations such as rooftops.
“Using this technology, we can make photovoltaic generators that are
foldable, concealed and mobile,” said Zhong Lin Wang, a Regents professor
in the Georgia Tech School of Materials Science and Engineering. “Optical
fiber could conduct sunlight into a building’s walls where the nanostructures
would convert it to electricity. This is truly a three dimensional solar cell.”
Details of the research were published in the early view of the journal Angewandte
Chemie International on October 22. The work was sponsored by the Defense Advanced
Research Projects Agency (DARPA), the KAUST Global Research Partnership and
the National Science Foundation (NSF).
Dye-sensitized solar cells use a photochemical system to generate electricity.
They are inexpensive to manufacture, flexible and mechanically robust, but their
tradeoff for lower cost is conversion efficiency lower than that of silicon-based
cells. But using nanostructure arrays to increase the surface area available
to convert light could help reduce the efficiency disadvantage, while giving
architects and designers new options for incorporating PV into buildings, vehicles
and even military equipment.
Fabrication of the new Georgia Tech PV system begins with optical fiber of
the type used by the telecommunications industry to transport data. First, the
researchers remove the cladding layer, then apply a conductive coating to the
surface of the fiber before seeding the surface with zinc oxide. Next, they
use established solution-based techniques to grow aligned zinc oxide nanowires
around the fiber much like the bristles of a bottle brush. The nanowires are
then coated with the dye-sensitized materials that convert light to electricity.
Sunlight entering the optical fiber passes into the nanowires, where it interacts
with the dye molecules to produce electrical current. A liquid electrolyte between
the nanowires collects the electrical charges. The result is a hybrid nanowire/optical
fiber system that can be up to six times as efficient as planar zinc oxide cells
with the same surface area.
“In each reflection within the fiber, the light has the opportunity to
interact with the nanostructures that are coated with the dye molecules,”
Wang explained. “You have multiple light reflections within the fiber,
and multiple reflections within the nanostructures. These interactions increase
the likelihood that the light will interact with the dye molecules, and that
increases the efficiency.”
Wang and his research team have reached an efficiency of 3.3 percent and hope
to reach 7 to 8 percent after surface modification. While lower than silicon
solar cells, this efficiency would be useful for practical energy harvesting.
If they can do that, the potentially lower cost of their approach could make
it attractive for many applications.
By providing a larger area for gathering light, the technique would maximize
the amount of energy produced from strong sunlight, as well as generate respectable
power levels even in weak light. The amount of light entering the optical fiber
could be increased by using lenses to focus the incoming light, and the fiber-based
solar cell has a very high saturation intensity, Wang said.
Wang believes this new structure will offer architects and product designers
an alternative PV format for incorporating into other applications.
“This will really provide some new options for photovoltaic systems,”
Wang said. “We could eliminate the aesthetic issues of PV arrays on building.
We can also envision PV systems for providing energy to parked vehicles, and
for charging mobile military equipment where traditional arrays aren’t
practical or you wouldn’t want to use them.”
Wang and his research team, which includes Benjamin Weintraub and Yaguang Wei,
have produced generators on optical fiber up to 20 centimeters in length. “The
longer the better,” said Wang, “because longer the light can travel
along the fiber, the more bounces it will make and more it will be absorbed.”
Traditional quartz optical fiber has been used so far, but Wang would like
to use less expensive polymer fiber to reduce the cost. He is also considering
other improvements, such as a better method for collecting the charges and a
titanium oxide surface coating that could further boost efficiency.
Though it could be used for large PV systems, Wang doesn’t expect his
solar cells to replace silicon devices any time soon. But he does believe they
will broaden the potential applications for photovoltaic energy.
“This is a different way to gather power from the sun,” Wang said.
“To meet our energy needs, we need all the approaches we can get.”