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September 18th, 2009

Efficient solar from a carbon nanotube

Published on September 18th, 2009 | By: pwray@ceramics.org
Tubular cells: The carbon nanotube at center is connected to several electrodes and acts as a superefficient photovoltaic cell.

Tubular cells: The carbon nanotube at center is connected to several electrodes and acts as a superefficient photovoltaic cell. (Credit: AAAS/Science)

Technology Review reported that researchers at Cornell University have made a photovoltaic cell out of a single-walled carbon nanotube. They claim it is more energy efficient than conventional photovoltaics. The group hopes their work will eventually lead the use of SWNTs to make ultra-efficient solar cells and highly sensitive photon detectors.

“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 began by putting a single nanotube in a circuit and giving it three electrical contacts, one at each end and one underneath (i.e., a split-gate field-effect geometry). They used these gates to apply a voltage across the nanotube, then illuminated it.

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 basic one-photon, one-electron process is also what normally happens in a typical PV cell. What’s unusual about the nanotube “cell,” says Cornell physic professor Paul McEuen, is what happens when it is struck by what he calls “a big photon” – a photon whose energy is twice the amount normally required to kick out an electron. In conventional PVs, this energy is lost as heat. But, in the nanotube device, it kicks a second electron into the circuit.

The work was also described last week in Science.

In the TR review story, McEuen confesses researchers aren’t sure if they exactly understand what is occurring, or if this property is only found in nanotubes:

“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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