Upping the Limit on Solar Cell Efficiency

Nanomaterials could help solar cells convert more sunlight into electricity than once thought possible.

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.

Solar dots: A micrograph shows lead sulfide quantum dots, each about five nanometers across, coating an electrode of titanium dioxide.

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.

The advance is important for two reasons. First, it demonstrates that it’s possible to use the extra electrons to help generate an electrical current, which is essential if they’re to be of any use in a solar cell. Second, the measurements indicate that the quantum dots are more effective at generating extra electrons than some researchers thought–about three times better for some wavelengths of light, if the results are accurate, says Eran Rabani, a professor of chemistry at Tel Aviv University. The performance, however, is still not good enough to make ultraefficient solar cells, he says. Bruce Parkson, a professor of chemistry at the University of Wyoming who led the work, agrees. “It isn’t optimal. This is only a first step,” he says.

Two major hurdles remain before the trick can be used to make ultraefficient solar cells. Parkinson used lead sulfide quantum dots with an electrode of crystalline titanium dioxide. Researchers need to try other combinations of quantum dots and electrode materials to find ones that can convert more photons into multiple electrons. Parkinson says his new methods for making quantum dot solar cells will help them directly test these other combinations.

Researchers also need to increase the total amount of light that the quantum dot solar cells can absorb. In the experimental cells, the layer of quantum dots is so thin that most light passes through without being absorbed. Parkinson says a possible next step is to bind the quantum dots to a porous material with high surface area, which would give them more opportunity to absorb light, while still allowing the electrons to quickly escape.

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