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Charge of the light brigade: How quantum dots may improve solar cells


Photovoltaic cells remain woefully inefficient at converting sunlight into electricity. Although layered cells composed of various elements can convert more than 40 percent of (lens-concentrated) sunlight into electricity, more simple semiconducting materials such as silicon hover around 20 percent when mass-produced. And, at best, such cells could convert only a third of incoming sunlight due to physical limits.

But one of those physical limits may have just been stretched: heat loss. Nanosize crystals of semiconducting material, in this case a mixture of lead and selenium, move electrons fast enough to channel some of them faster than they can be lost as heat, according to new work from researchers at the universities of Minnesota and Texas.

Solar cells employ semiconducting material because when a photon of sunlight of the right wavelength strikes that material, it knocks loose an electron, which can then be harvested as electrical current. But many of those loosened electrons dissipate as heat rather than being funneled out of the photovoltaic cell. Previous work in 2008 had shown that nanocrystals of semiconducting material can, in effect, slow down such "hot" electrons. As a result, these nanocrystals, also known as quantum dots, might be able to boost the efficiency of a solar cell.

The new research published in Science June 18 shows that is indeed the case: Not only can quantum dots capture some of the "hot" electrons but they can also channel them to a typical electron-accepting material—the same titanium dioxide used in conventional solar cells.

In fact, that transfer takes place in less than 50 femtoseconds (a femtosecond is one quadrillionth of a second, or really, really, really fast). Because that transfer is so fast, fewer of the excited electrons are lost as heat, thus boosting the theoretical efficiency to as high as 66 percent.

Unfortunately, that's not all that's required to build such a highly efficient solar cell. The next step would be to show that the captured electrons and transferred current can be carried away on a wire, as in a conventional solar cell. The challenge will be making a wire small enough to connect to a solar cell incorporating a quantum dot no bigger than 6.7 nanometers in diameter—and one that won't lose much of the current as heat. And it would be years if not decades before such quantum dot-based solar cells might be manufactured. But these chemists have lit at least one path to a more efficient solar future.

Image: Courtesy of Cornell University and John Silcox

The views expressed are those of the author and are not necessarily those of Scientific American.

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