Solar's Big Future with Small Tech
by David Pescovitz
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Ilan Gur will appear in an episode of Chasing Nature, a new show about engineering and nature that will air on Animal Planet. In 2004, as part of a joint program between the University's Management of Technology (MOT) program and the United Nations Industrial Development Organization (UNIDO), Gur conducted field research in western China to explore solar-powered lighting solutions for off-grid populations.
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UC Berkeley graduate student Ilan Gur and his colleagues have a profound research goal. "In the long run, we'd like to power the world," he says. A PhD candidate in the Department of Materials Science and Engineering, Gur is leveraging advances in nanoscience to develop ultra-thin and cheap solar cells that someday could be batch produced in bulk, perhaps even roll-to-roll like newspapers are printed. Flexible and durable, the cells might eventually wrap the roofs of buildings or transform a cloth automobile cover into a battery charger.
"The reason you don't see solar cells on rooftops everywhere is because they're incredibly expensive to produce," says Gur, who is a student in the research group of nanotechnology pioneer Paul Alivisatos. "Traditional solar cells require high-purity silicon that must be processed in high-temperature, high-vacuum conditions and clean-room environments like those in microchip fabrication facilities."
This scanning electron microscope image depicts a film of nanocrystal solar cells on top of a silicon substrate. (courtesy the researchers)
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Gur spearheaded the development of the first solar cells ever to be made entirely from inorganic nanocrystals, chemically-pure clusters of anywhere from 100 to 100,000 atoms. Unlike the silicon used in today's solar cells, the rod-shaped nanocrystals of cadmium-selenide and cadmium-telluride are deposited in a chemical solution process at fairly low temperatures. Essentially, they're made in a beaker instead of a clean room. The resulting film of photovoltaic material is just 200 nanometers thick, or 500 times thinner than a human hair.
The inspiration for the new approach came from previous breakthroughs in the Alivisatos Group in the creation of prototype solar cells made from a combination of nanocrystals and an organic polymer, or plastic. The researchers showed that the plastic solar cells could be inexpensively manufactured in bulk quantities. Nanosys, a start-up company co-founded by Alivisatos, licensed the technology to further develop it for commercialization.
A close-up image of one of the prototype solar cells. (courtesy the researchers)
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"With that technology moving to the next stage of development, we started thinking about the ultimate limitations of plastic solar cells," says Gur, who is also a researcher in Lawrence Berkeley National Laboratory's Materials Sciences Division. "One problem is that the polymers are organic so they degrade in air. The cells must be encapsulated to protect them, and that's a pretty serious limitation when you're trying to make something that's dirt cheap."
The researchers realized that an all-nanocrystal cell might offer the best of both worlds. The nanocrystals are inorganic, so they aren't sensitive to air. And while they can be processed far more easily than the silicon in traditional photovoltaic cells, the nanocrystals share the semiconductor's desirable sunlight absorption and electron transport characteristics.
"If you can make this technology practical, you could imagine that people would be able to make solar cells where they don't have the tremendous capital needed to build plants, like in developing nations," Gur says.
In October, Gur, Alivisatos, and their collaborators published a paper in Science outlining their novel approach. While the mechanism by which the current flows in nanocrystal cells is similar to the plastic devices, the performance doesn't seem to deteriorate as the material ages. Still, the first prototype cells, layered on a conductive glass substrate, can only convert 3 percent of energy from light into electricity, far less than most of today's commercial solar cells that are more than 10 percent efficient. However, the nanocrystal cells are comparable in efficiency to the state-of-the-art in plastic solar cells and can likely be optimized enough to make the technology marketable.
"If we can produce this for as cheap as we hope, the efficiency doesn't have to be so high because you could just install more of the material," Gur says. "There are certainly places where people would trade space for energy."
The researchers are now working to boost the efficiency by altering the structures of the nanocrystals and tweaking the chemistry of the production process. Meanwhile, they're exploring other kinds of nanocrystals made from elements more common and environmentally-benign than the cadmium-based nanocrystals used in the first experiments.
"The world is facing a massive energy problem," Gur says. "It's not clear that this is the solution, but it has potential. And I know that we have to be working on many potential solutions if we ever hope to solve the problem."
"Sunny Future for Nanocrystal Solar Cells" by Lynn Yarris (Berkeley Lab, October 20, 2005)
Alivisatos Group Homepage
"In Animal Planet reality show, graduate students naturally engineer" by Rachel Shafer (Engineering News, December 5, 2005)
"Nanocrystals, Quantum Dots, and Nature's Own Assembly Line" by David Pescovitz (Lab Notes, November 2002)
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