Sep
4
A Big Improvement for Carbon Nanotube Solar Cells
September 4, 2014 | Leave a Comment
Northwestern University scientists have now developed a carbon nanotube solar cell (CNT) that is twice as efficient as its predecessors. Early research stalled when CNTs proved to be inefficient, converting far less sunlight into power than other methods. Yet the CNTs have long shown promise for photovoltaics because they’re lighter, more flexible, and cheaper than conventional solar-cell materials.
The Northwestern research team led by Mark Hersam, professor of materials science and engineering and the Bette and Neison Harris Chair of Teaching Excellence at the McCormick School of Engineering created a new type of CNT solar cell that is twice as efficient as its predecessors and is also the first CNT solar cell to have its performance certified by the National Renewable Energy Laboratory.
Hersam said, “The field had been hovering around 1 percent efficiency for about a decade; it had really plateaued. But we’ve been able to increase it to over 3 percent. It’s a significant jump.”
The team reports the secret lies in the CNTs’ chirality, which is a combination of the tubes’ diameter and twist. When a thin sheet of carbon is rolled into a nanotube, several hundred different chiralities are possible. In the past, researchers tended to choose one particular chirality with good semiconducting properties and build an entire solar cell out of that one.
Hersam explained, “The problem is that each nanotube chirality only absorbs a narrow range of optical wavelengths. If you make a solar cell out of a single chirality carbon nanotube, you basically throw away most of the solar light.”
Hersam’s team made a mixture of polychiral, or multiple chirality, semiconducting nanotubes. This maximized the amount of photocurrent produced by absorbing a broader range of solar-spectrum wavelengths. The cells significantly absorbed near-infrared wavelengths, a range that has been inaccessible to many leading thin-film technologies.
Single Walled Carbon Nanotubes Built Up to a Solar Cell. Click image for the largest view. Image Courtesy McCormick School of Engineering, Northwestern University.
While this is major progress for CNT solar cells, the CNT field still lags behind other materials in efficiency. Silicon, for example, can be 15-20 percent efficient, but it is more expensive to manufacture. “If you look at our performance, there’s certainly a big jump,” Hersam said. “But there’s more work to be done. We still have to advance this technology by a factor of three to five.”
Hersam said the next step is to create polychiral CNT solar cells that have multiple layers. Each layer would be optimized for a particular portion of the solar spectrum and, thus, absorb more light. He said they might also incorporate other materials, such as organic or inorganic semiconductors, to complement CNTs.
“What we’d like to do is absorb every photon from the sun and convert it into electricity,” he said. “In other words, we’d like to have a solar cell that has an absorption spectrum perfectly matching solar light. We’re on a path toward that goal.”
This is laudable work with important implications. Even more striking is the idea to, “absorb every photon from the sun and convert it into electricity”. Good work – Keep going folks.
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