By using a popcorn-ball design — tiny kernels clumped into much larger porous spheres — researchers at the University of Washington have been able to manipulate light and more than double the efficiency of converting solar energy to electricity. It turns out that dye-sensitized solar cells are more flexible, easier to manufacture and cheaper than existing solar technologies. But so far, current lab prototypes of this popcorn-ball version 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.

So the researchers at the U of W went about raising the efficiency. They realized that smaller grains have a larger surface area per volume and thus can absorb more rays. But larger clumps, closer to the wavelength of visible light rays, cause light to ricochet from one thin light-absorbing surface to another, so the ray has a greater likelihood of being absorbed.

The U of W group 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, thereby create a surface area of about 1000 square feet for each gram of material. This internal surface is coated with a dye that captures the light.

The overall efficiency was 2.4 percent using only small particles, which is the highest efficiency achieved for this material. But with the popcorn-ball design, show an efficiency rose to 6.2 percent, more than double the previous performance.

The experiments were performed using zinc oxide, which is less stable chemically than the more commonly used titanium oxide but easier to work with.

Titanium oxide based dye-sensitized solar cells are now at 11 percent maximum efficiency. The hope is to push dye-sensitized solar cell efficiency even higher.

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