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Upping the Limit on Solar Cell Efficiency

• Kevin Bullis via Technology Review
Although researchers have steadily increased the amount of electricity that solar cells can produce, they face fundamental limits because of the physics involved in converting photons to electrons in semiconductor materials. Now researchers at the University of Wyoming have demonstrated that by using novel nanomaterials called quantum dots, it might be possible to exceed those limits and produce ultraefficient solar cells.

The theoretical limitation of solar cells has to do with the widely varying amounts of energy from photons in sunlight. The amount varies depending on the color of the light. No matter how energetic the incoming photons are, however, solar cells can only convert one photon into one electron with a given amount of energy. Any extra energy is lost as heat. Scientists have hypothesized that quantum dots, because of their unusual electronic properties, could convert some of this extra energy into electrons. They've calculated that this approach could increase the theoretical maximum efficiency of solar cells by about 50 percent.

Initial tests of the idea were encouraging but inconclusive. Researchers couldn't measure the extra electrons directly because the electrons were too short-lived to find their way out of the material and into a circuit. The key advance of the Wyoming researchers was to modify the surface chemistry of the quantum dots and the titanium-dioxide electrode that they're attached to, creating a strong bond that allowed the electrons to escape the quantum dot in just a few trillionths of a second. For the first time, the researchers could directly measure the production of extra electrons in solar cells.

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