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

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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

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  1. 1. j king 5:01 am 06/21/2010

    I would like to reserve credit for this concept and reserve its commercial use by written permission. Use of graphane substrate quantum dot formed by the heated tip of AFM or other means of heating including laser or other beam converting a hexagonal area of the graphane to graphene quantum dot of specific size, here no larger than 6.7 nanometer diameter. A current carrying lead of low resistence can be formed similarly in the same substrate leading from the quantum dot by drawing a heated line with AFM tip, laser or other beam to form an electrode in the hydrogen removed graphene insulated by surrounding graphane remaining on the 2d substrate that began as uniform graphane. Transversely attached electrode of nanotube of metal can be grown from region of this quantum dot as well. J King June 21, 2010

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  2. 2. habanerojo 8:37 pm 06/21/2010

    Dude, I was totally going to say that!

    More seriously, isn’t the problem large scale production more than theoretical feasibility?

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  3. 3. tichead 11:40 am 06/22/2010

    Yeah, I hate getting scooped in a discussion forum. But, habanerojo, J King may not have been published in a journal yet… Honestly, I don’t know quantum dots from quantum holes so I’m glad people are out there using real brains to convert biomass (such as pizza) into new and wonderful technology.

    So, J King, as you have the wire figured out then hook up with the authors of the article in "Science" to get this machine on the market. 66% efficiency is astounding, not just in the solar field, but in almost any energy conversion device. The highest I’ve seen for anything was a concentrated solar parabolic dish with Stirling engines driving an electrical generator with nearly 40% efficiency. I wish my internal combustion vehicle could match that. I understand only about 1-2% of the available energy in the gas actually goes toward moving my brain around town.

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