Clustered Films Double Efficiency of Dye-Sensitized Solar Cells
Aaron Hand, Executive Editor, Electronic Media -- Semiconductor International, 4/17/2008 10:09:00 AM
Various groups around the world are looking at ways to improve solar cells’ ability to capture more of the sun’s energy. Efficiencies of another kind are also being heavily explored — namely, to improve the cost and efficiency of manufacturing the cells. Researchers at the University of Washington (Seattle) have developed a method to boost conversion efficiency in dye-sensitized solar cells, which are more flexible, easier to manufacture and cheaper than existing silicon-based solar cells.
By using a popcorn-ball design — tiny kernels clumped into much larger porous spheres — the UW researchers have been able to more than double the efficiency of converting solar energy to electricity. The findings were presented this week in New Orleans at the national meeting of the American Chemical Society. “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.
Although dye-sensitized solar cells offer hope in terms of cost and manufacturability, they have so far been able to convert only about half of the sun’s energy that has been achieved with silicon-based solar cells. Nonetheless, researchers have been trying a variety of methods to drive dye-sensitized efficiencies higher, looking in particular for rough surfaces to help the absorption rate.
Other researchers have attempted to satisfy both demands by mixing larger and smaller grains together, but with little success. What the UW group did instead was to use only very tiny grains (~15 nm across), then clump those grains into larger clusters ~300 nm 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 ~1000 square feet for each gram of material. This internal surface is then coated with a dye that captures the light.
The researchers expected some improvement in the performance, but the new method actually more than doubled previous efficiency results. The overall efficiency was 2.4% using only small particles. With the popcorn-ball design, efficiencies have reached 6.2%, more than double the previous performance.
There is still work to be done to improve the efficiency rate further, the first step being to use a different material with the same approach. The initial experiments were done 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.
Swiss and Japanese researchers reported last week on a dye-sensitized solar cell that has achieved an energy conversion yield of 7.2%. The maximum efficiency reported for dye-sensitized solar cells based on titanium oxide is 11%, according to Cao, who hopes his strategy could push dye-sensitized solar cells’ efficiency significantly over that threshold.
The UW 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 UW’s department of materials science and engineering, and Samson Jenekhe, a UW professor of chemical engineering.
