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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,” said Cao. “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 made only very tiny grains, about 15 nanometers across, and then they clumped these grains into larger agglomerations of 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 creates a surface area of about 1000 square feet for each gram of material. The 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% using only small particles, which is the highest efficiency achieved for this material. With the popcorn-ball design, results show an efficiency of 6.2%, more than double the previous performance. “The most significant finding is the amount of increase using this unique approach,” said Cao.
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% maximum efficiency. Cao hopes his strategy could push dye-sensitized solar cells’ efficiency significantly over that threshold.
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.
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