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Savings on the Sun

31 May 2005 (All day)
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Sunny delight. Lead-based nanoparticles may help increase the efficiency of solar energy.

Despite decades of improvements to solar cells, the electricity they produce still costs up to 10 times more than that from fossil fuels. But this could change: a new finding shows that nanometer-sized particles can generate electrical charges from sunlight more efficiently than any semiconductor tested before. If the lead-based particles work inside solar cells as well as they do in the lab, they could boost the solar conversion efficiency from a best of about 32% today to perhaps as much as 66%, enough to slash the cost of solar power.

Standard solar cells made from bulk materials have a basic limit. When a photon of sunlight hits a semiconductor, it normally excites one electron. This gives the electron enough energy to escape the grasp of its silicon atom and slip into an electrical circuit to do work. Researchers have known for decades that if incoming photons have enough energy, they can liberate two or even three electrons. But in standard solar cell materials this requires that incoming photons have at least 5 electron volts worth of energy, which corresponds to photons of deep ultraviolet light (UV). Sunlight, unfortunately, doesn't have much deep UV.

In the late 1990s, Arthur Nozik of the National Renewable Energy Laboratory in Golden, Colorado, and the University of Colorado, Boulder, theorized that if the semiconductors were made out of nanoparticles, they could excite multiple electrons with less photon energy, because less of the incoming energy would be sapped by vibrating atoms in the crystalline lattice. Last year, Victor Klimov and Richard Schaller at Los Alamos National Laboratory in New Mexico confirmed the theory. They found that when they shined visible light on nanoparticles made from lead selenide and lead sulfide, they excited just over two electrons on average with every absorbed photon. In their new work, Nozik and his colleagues improved matters by using smaller nanoparticles, showing that they could generate up to three electrons with single photons of visible light. The team reports the finding in the current issue of Nano Letters.

"Scientifically, it's clearly a step forward," says Louis Brus, a chemist and nanoparticle expert at Columbia University in New York City. The trick now, say Nozik and others, will be to show that they can actually make devices that collect these charges and harvest the electricity.

Related sites
Arthur Nozik's page at the University of Colorado
Arthur Nozik's page at the National Renewable Energy Laboratory

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