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. Credit: Nature Photonics

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.

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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.