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New Solar Cell Tech Generates 2 Electrons from 1 Photon

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A material that splits particles of light into multiple packets of energy could ultimately lead to more efficient solar cells, scientists say.
The material, known as pentacene, when made into a thin coating on a prototype solar cell, can generate two excitons, or "packets of energy," for every photon of light hitting it. Traditional solar cells can produce only one electron per photon.
The trick uses wavelengths of light that would normally be wasted, said Marc Baldo, professor of electrical engineering at MIT. "All solar cells work best at one wavelength of light," Baldo said. All other wavelengths of light get wasted as heat.
Instead of wasting the high-energy photons, Baldo and his team had the idea of turning them into two excitons, each with half the energy of the original. [Read also: Trees Inspire Recyclable Solar Cells]
"Instead of trying to take those [high-energy] blue and green photons and convert them into one electron with a larger voltage, we're trying to convert them into two electrons with a smaller voltage." The pentacene is "very good at taking the photons and splitting them into two packets of energy," he said.
Pentacene's properties — including its exciton-generating capabilities — have been known since the 1960s, but practical uses for the material have thus far been hard to come by.
In 2006, however, researchers at the National Renewable Energy Lab published research showing that exploiting the double-exciton phenomenon with a related technique could increase the efficiency of solar panels by more than 43 percent above their theoretical limit. Those solar cells would convert a full 46 percent of the light hitting them into electricity.
Baldo's proof of concept is simpler, he said, but "it's not useful yet. This is a proof of principle that this mechanism … does exist and [that] it can be efficient."
If pentacene is implemented into current silicon cells — likely as a coating or a film —solar panels could see efficiencies of 30 or 31 percent.
The best solar cells reach about 25 percent efficiency. Solar cell makers "are fighting to get every tenth of a percent," he said.

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