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.

"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."