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Superefficient Solar from Nanotubes

Carbon nanotube photovoltaics can wring twice the charge from light.
September 15, 2009

Today’s solar cells lose much of the energy in light to heat. Now researchers at Cornell University have made a photovoltaic cell out of a single carbon nanotube that can take advantage of more of the energy in light than conventional photovoltaics. The tiny carbon tubes might eventually be used to make more-efficient next-generation solar cells.

Tubular cells: The carbon nanotube at center is connected to several electrodes and acts as a superefficient photovoltaic cell.

“The main limiting factor in a solar cell is that when you absorb a high-energy photon, you lose energy to heat, and there’s no way to recover it,” says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden, CO. Loss of energy to heat limits the efficiency of the best solar cells to about 33 percent. “The material that can convert at a much higher efficiency will be a game-changer,” says Beard.

Researchers led by Paul McEuen, professor of physics at Cornell, began by putting a single nanotube in a circuit and giving it three electrical contacts called gates, one at each end and one underneath. They used the gates to apply a voltage across the nanotube, then illuminated it with light. When a photon hits the nanotube, it transfers some of its energy to an electron, which can then flow through the circuit off the nanotube. This one-photon, one-electron process is what normally happens in a solar cell. What’s unusual about the nanotube cell, says McEuen, is what happens when you put in what he calls “a big photon” – a photon whose energy is twice as big as the energy normally required to get an electron off the cell. In conventional cells, this is the energy that’s lost as heat. In the nanotube device, it kicks a second electron into the circuit. The work was described last week in the journal Science.

There’s evidence that another class of nanomaterials called quantum dots can also convert the energy of one photon into more than one electron. However, making operational quantum-dot cells that can do this has proved a major hurdle, says Beard, whose lab, led by Arthur Nozik, is working on the problem. One of the challenges with quantum-dot solar is that it’s very difficult to get the freed electrons to leave the quantum dot and enter an external circuit. “The system is teasing you; you can’t get those charge carriers out, so what’s the point?” says Ji Ung Lee, professor of nanoscale engineering at the State University of New York in Albany. “McEuen’s group has shown this in a system where you can get the extra carriers out.”

McEuen cautions that his work on carbon nanotube photovoltaics is fundamental. “We’ve made the world’s smallest solar cell, and that’s not necessarily a good thing,” he says. To take advantage of the nanotubes’ superefficiency, researchers will first have to develop methods for making large arrays of the diodes. “We’re not at a point where we can scale up carbon nanotubes, but that should be the ultimate goal,” says Lee, who developed the first nanotube diodes while a researcher at General Electric.

It’s not clear why the nanotube photovoltaic cell offers this two-for-one energy conversion. “It’s mysterious to us,” says McEuen. However, the most likely reason is that while conventional solar materials have only one energy level for electrons to move through, carbon nanotubes have several. And two of them just happen to be very well matched: one of the energy levels, or bandgaps, is twice as high as the other. “We may have gotten lucky, and it has very little to do with the fact that it’s a carbon nanotube,” says McEuen. This means, McEuen hopes, that even if it proves too challenging to make arrays of nanotube solar cells, materials scientists can look for pairs of materials that have these kinds of matched bandgaps, and layer them to make solar cells that do with two materials what the single nanotube cells can do. “Maybe the answer won’t be in nanotubes, but in another pair of materials,” McEuen says.

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