Dye-sensitized Solar Cells

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Researchers in China and Switzerland are reporting the highest efficiency ever for a promising new genre of solar cells, which many scientists think offer the best hope for making the sun a mainstay source of energy in the future. The photovoltaic cells, called dye-sensitized solar cells or Grätzel cells, could expand the use of solar energy for homes, businesses, and other practical applications, the scientists say.


Electricity generation
The research, conducted by Peng Wang and colleagues — who include Michael Grätzel, inventor of the first dye-sensitized solar cell — involves photovoltaic cells composed of titanium dioxide and powerful light-harvesting dyes. Grätzel cells are less expensive than standard silicon-based solar cells and can be made into flexible sheets or coatings.

Although promising, Grätzel cells until now have had serious drawbacks. They have not been efficient enough at converting light into electricity. And their performance dropped after relatively short exposures to sunlight.

In the new study, researchers describe lab tests of solar cells made with a new type of ruthenium-based dye that helps boost the light-harvesting ability. The new cells showed efficiencies as high as 10 percent, a record for this type of solar cell. The new cells also showed greater stability at high temperatures than previous formulas, retaining more than 90 percent of their initial output after 1,000 hours in full sunlight.

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New Efficiency Benchmark For Dye-sensitized Solar Cells
ScienceDaily (July 2, 2008) — Scientists have achieved a record light conversion efficiency of 8.2% in solvent-free dye-sensitized solar cells. This breakthrough in efficiency without the use of volatile organic solvents will make it possible to pursue large scale, outdoor practical application of lightweight, inexpensive, flexible dye-sensitized solar films that are stable over long periods of light and heat exposure.

The new paper published online June 29 in the journal Nature Materials by EPFL professor Michael Graetzel, Shaik Zakeeruddin and colleagues from the Changchun Institute of Applied Chemistry at the Chinese Academy of Sciences.

Dye-sensitized solar cell technology, invented by Michael Grätzel at EPFL in the 1990s, shows great promise as a cheap alternative to expensive silicon solar cells. Dye-sensitized cells imitate the way that plants and certain algae convert sunlight into energy. The cells are made up of a porous film of tiny (nanometer sized) white pigment particles made out of titanium dioxide.

The latter are covered with a layer of dye which is in contact with an electrolyte solution. When solar radiation hits the dye it injects a negative charge in the pigment nanoparticle and a positive charge into the electrolyte resulting in the conversion of sunlight into electrical energy. The cells are inexpensive, easy to produce and can withstand long exposure to light and heat compared with traditional silicon-based solar cells.

Currently, state-of-the-art dye-sensitized cells have an overall light conversion efficiency greater than 11%, still about two times lower than silicon cell technology.

A major drawback to the dye-sensitized cell technology is the electrolyte solution, which is made up of volatile organic solvents and must be carefully sealed. This, along with the fact that the solvents permeate plastics, has precluded large-scale outdoor application and integration into flexible structures.

To overcome these limitations, Grätzel and his colleagues developed a new concept -- a mixture of three solid salts as an alternative to using organic solvents as an electrolyte solution. When the three solid components are mixed together in the right proportion they turn into a melt showing excellent stability and efficiency.

Grätzel is confident that further development of these types of electrolyte mixtures will lead to large-scale practical application of dye-sensitized solar cell technology, reinforcing solar energy's role as a cornerstone of alternative energy production.

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Solar Energy: Popcorn-ball Design Doubles Efficiency Of Dye-sensitized Solar Cells
ScienceDaily (Apr. 15, 2008) — A new approach is able to create a dramatic improvement in cheap solar cells now being developed in laboratories.

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, 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," Cao said. "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 instead 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, 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.

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 percent using only small particles, which is the highest efficiency achieved for this material. With the popcorn-ball design, results presented today at the conference show an efficiency of 6.2 percent, more than double the previous performance.

"The most significant finding is the amount of increase using this unique approach," Cao said.

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 percent 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.