“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.