Virginia Tech researchers' ability to create films in one-nanometer-thick layers is bringing flexible solar cells closer to reality, and has resulted in a thin film that can be changed from transparent to deep violet and back as rapidly as 20 times per second. The work will be presented at the 223nd national meeting of the American Chemical Society, April 7-11 in Orlando.
A nanometer is about 10 atoms thick. Creating material layers a few atoms thick is not hands-on work. Researchers select materials that will self-assemble. Positively and negatively charged molecules are elercctrically attracted to one another. Building materials based on this attraction is called ionic self-assembled multilayers (ISAM).
Virginia Tech researchers are creating flexible photovoltaic devices, or solar cells, by building up nanometer-thick layers of materials selected for their ability to self-assemble and to convert light to electricity.
The researchers are using polymers and molecules called fullerenes. The advantages of these carbon-based (organic) materials over silicon are flexibility and light weight. "You can fabricate a large area all at once, limited only by the size of your vat of solution from which you grow the films," says James R. Heflin, associate professor of physics at Virginia Tech. "Organic solar cells can be flexible, so you could have deployable sails on a space craft, or fold your solar cell into your briefcase or backpack."
So far, the efficiency of organic solar cells is only about 20 percent of silicon. But the Virginia Tech researchers are using ultra-thin layers of fullerenes that act as electron acceptors, which they have demonstrated increases the efficiency of the organic solar cells. "Starting with a conducting polymer, which is a light emitter, we can apply a fullerene layer and produce electrical current from incident light," says Heflin.
The problem being solved by nanotechnology is the distance between the materials that are electron donors and acceptors. The fullerene has to be within 10 nanometers of where the light is absorbed for current to be created. "We believe we can improve the efficiency by factors of five or 10 through nanoscale control of the composition and thickness," Heflin says. "We expect organic solar cells will be at least as efficient as silicon within five years."
The second ISAM application, electrochromic films, is also existing technology being improved with nanotechnology. Films are presently being produced that will change from transparent to dark by applying a small voltage, and changed back by reversing the voltage. The electric field drives ions from one layer of the film to another layer to activate or deactivate optically-absorbing molecules. Applications already include rear view mirrors in automobiles that darken automatically and coated windows that can be darkened with the push of a button. But current materials require several seconds to change color.
Researchers from Virginia Tech and Luna Innovations Inc., funded by the SBIR program, are working on the application of such materials for flat panel displays. Current LCD flat panel displays must be viewed head-on "An electrochromic display will allow you to view the screen of your lap top computer from an angle," explains Heflin.
The problem to be overcome is to increase the speed for the color of the film to change. The refresh rate on a computer screen is 60 to 80 times per second.
Electrochromic films presently being produced consist of two materials, each 100 nanometers thick. The Virginia Tech researchers are using self-assembly to create alternating one-nanometer thick layers of the ion conductor and an electrochromic polymer -- reducing the distance the ions must travel by a significant amount and increasing the response time.
"We have now shown switching times faster than 20 times per second, which is getting close to what is needed for a computer screen," says Heflin.
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