Solar cell absorbs invisible light
21 June 2012

Image credit: MIT / Rishabh Jain

Analysis by Jesse Emspak
Thu Jun 21, 2012 03:55 PM ET

A new kind of solar cell made from carbon harvests sunlight that other cells can't see. The technology could greatly boost the efficiency of solar panels and help bring the down the price of solar panels.

The invisible sunlight being captured is the near-infrared part of the spectrum, which makes up a whopping 40 percent of the wavelengths of light beaming down from the sun. Most solar cells, made from silicon or special plastics, harvest visible light, the wavelengths of light we see in a rainbow. But so far, that's not very efficient.

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Looking for a way to improve the efficiency of solar cells, a research team at the Massachusetts Institute of Technology turned to carbon nanotubes and C60, otherwise known as buckyballs. Carbon solar cells absorb energy from the near-infrared wavelengths of without heating up, the way that silicon cell do.

Previous attempts at making photovoltaic cells of carbon have been tried before, but the resulting cells required a layer of polymer to hold the carbon nanotubes in place. The MIT team's cell is stable when exposed to air. Layers of carbon nanotubes and buckyballs are also transparent. That means one can put a layer of the carbon on top of an ordinary solar cell and boost the energy output.

There are still some kinks to work out. One is mass-producing the carbon cells, as it's not always easy to get the kind of pure carbon nanotubes necessary. Michael Strano, a professor of chemical engineering at MIT and senior author of the paper, told Discovery News that his lab has licensed a method to a private company to address that. Another is the energy efficiency. At 0.1 percent it's only a hundredth of a typical cell.

"The morphology is not ideal," Strano said. He noted that the carbon nanotubes and buckyballs are distributed randomly on the cell surface, with the bockyballs filling the gaps between tubes and sometimes sitting on top of them. "In future work, we have to better organize these two materials."

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But even a low efficiency could make a big difference. The sun typically sends about 4-6 kilowatt-hours per square meter per day onto the United States. About 43 percent of that energy is in the near-infrared and totally wasted. If carbon solar cells were added to the mix, about half of what is usually just lost could be harvested, Strano said. Even a fraction of that added to a typical solar cell would be an improvement. RIshabh Jain, the lead author of the paper, said the team hopes to reach the 20 percent efficiency mark of commercial cells.

The work was published in the journal Advanced Materials.

Image: An atomic-force microscope image of a layer of single-walled carbon nanotubes deposited on a silicon surface. Individual nanotubes can be seen in the image. Credit: MIT / Rishabh Jain


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