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Pushing Plastic Solar Cells

Researchers make cells with near-perfect internal efficiency.

Plastic solar cells are lightweight, flexible, and, most important, cheap to make. But so far, these devices have been too inefficient to compete with silicon solar cells for most applications. Now researchers from a few institutions claim to have made polymer solar cells with record-breaking efficiencies. These cells still aren’t good enough to compete with silicon, but polymer efficiencies have been increasing at a rate of about 1 percent a year. If they can keep this up, say researchers, plastic solar cells will be competing with silicon within a few years.

Powerful polymers: This illustration shows the different layers that make up a new plastic solar cell with nearly perfect internal efficiency. From bottom to top, the layers are glass, a transparent electrode, two polymer layers, a titanium oxide layer that redistributes light, and an aluminum electrode.

This week, in the online edition of Nature Photonics, researchers reported on polymer solar cells that convert about 6.1 percent of the energy in sunlight into electricity–inching a bit closer to the 10 percent that they say will be needed to gain a significant foothold in the market. (Conventional silicon cells are about 15 percent efficient.) The new efficiency numbers “show that we’re in the game,” says Alan Heeger, a professor of physics at the University of California, Santa Barbara, who led the research. Heeger shared the Nobel Prize in Chemistry in 2000 for his role in the development of the first conducting polymers, and he’s cofounder and chief scientist at Konarka, a plastic solar cell company headquartered in Lowell, MA.

The California researchers’ results compare very favorably with previous published descriptions of polymer solar cells, whose efficiency has hovered around 5 percent. Konarka says that the company’s cells, which use different materials than the cells made in Heeger’s university lab, have recently been rated at about 6.4 percent. And a competitor in San Mateo, CA, called Solarmer Energy has made plastic cells with similar efficiencies, according to an affiliated researcher.

Plastic solar cells, no matter how well designed, have intrinsic limits dictated by the polymers that make up their active layer. The polymers made so far can only absorb relatively narrow bands of light. It’s possible to boost their power-conversion efficiency by stacking films of polymers designed to pick up different bands of light; Heeger’s group has, in fact, had some success with this in the past. But this approach has a major disadvantage. “Layering is self-defeating because you increase the fabrication costs,” says Luping Yu, a professor of organic chemistry at the University of Chicago, who is also working on solar cells.

One way to improve these cells’ performance is by making sure that every single photon that does get absorbed by the polymers is converted into an electron that can be collected. Heeger’s group boosted its cells’ overall efficiency by improving this internal efficiency. How well electrons can move within these films depends on the quality of the interface between the two components that make up the film: in the University of California cell, these are a conductive polymer and a version of a soccer-ball-shaped carbon compound called a fullerene. Heeger’s group tested films made of different ratios of these two components, as well as different solvents for processing them.

The result is a cell with nearly perfect internal efficiency. “All the light that’s been absorbed has been converted into charges,” says Zhenan Bao, an associate professor of chemical engineering at Stanford University, who was not involved in the research. “This group has done very good engineering on the cells.”

“I’m excited about the progress,” says Yang Yang, a professor of materials science and engineering at the University of California, Los Angeles. “You push the record incrementally.” Heeger agrees: “The size of the market depends on the cost in dollars per watt, so every increase in efficiency is important.”

While Yang, Bao, and Heeger say that the figures achieved by Heeger’s group are an important demonstration of the potential of polymer solar cells, all acknowledge that existing materials will not be the ones that will push the industry forward. “Ten percent [efficiency] would be a breakthrough,” says Heeger. “We get there by synthesizing new materials that respond to more of the energy spectrum.”

“Organic materials are still limited to visible light,” says Yang, but much of the sun’s energy is in the neighboring part of the spectrum–the infrared–so polymer scientists are working on solar-cell materials that can also absorb this band. The University of Chicago’s Yu, who is collaborating with Solarmer Energy, says that the company has used his polymers, which absorb shorter-wavelength light, to make cells that should achieve more than 7 percent efficiency, but he cannot disclose the details because the results have not yet been published.

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