“Popcorn-ball” Design Doubles Efficiency of Dye-sensitized Solar Cells

[Karl]

ScienceDaily (Apr. 15, 2008) —

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 were presented in New Orleans at the national meeting of the American Chemical Society April 10. "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 are more flexible, easier to manufacture and cheaper than existing solar technologies. Researchers have tried various rough surfaces to achieve increasingly 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 other devices.

The 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 a greater surface area per volume ratio, 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.

Other researchers have tried mixing larger grains in with the small particles to scatter the light, but have little success in boosting efficiency; however, the UW group 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, creates a much larger surface area 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. With the “popcorn-ball” design, results show an efficiency of 6.2 percent, more than double the previous performance. 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.

Source: Solar Energy: Popcorn-ball Design Doubles Efficiency Of Dye-sensitized Solar Cells