"Popcorn-ball" design doubles efficiency of dye-sensitized solar cells

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Washington, April 11 : Researchers have used a popcorn-ball design to more than double the efficiency of dye-sensitized solar cells.

Developed by researchers at the University of Washington (UW) in the US, the popcorn-ball design - tiny kernels clumped into much larger porous spheres, when used in solar cells, are able to manipulate light and more than double the efficiency of converting solar energy to electricity.

"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.

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 difficulties 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," he added.

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. 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.

Though the researchers expected some improvement in the performance, what they saw exceeded their hopes.

While the overall efficiency was 2.4 percent using only small particles, with the popcorn-ball design, results showed an efficiency of 6.2 percent, more than double the previous performance.

ANI

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