Researchers in North Carolina have developed a way to more than double the performance of organic solar cells by adding a layer of upright optical fibers that act as sunlight traps.

David Carroll, a professor of physics at Wake Forest University, led the development of a prototype solar cell incorporating the fibers. He is the chief scientist at a spinoff company called FiberCell that is developing a reel-to-reel manufacturing process to produce the cells. "We're on the cusp of having working demonstrators that would convince someone to go into production with this," said Carroll.

The best organic solar cells today are nearly 8 percent efficient, although efforts are ongoing to develop organic chemistries that would push the efficiency of such cells above 10 percent. But Carroll says improved chemistries alone won't be enough to catch up to the performance of silicon cells. "The answer doesn't lie in chemistry--it lies in the architecture of the cell itself," he says. Carroll adds that the dollar-per-watt cost of manufacturing fiber-based organic cells should be about the same cost as for flat organic cells. "But they can be produced in a factory costing one-tenth that of a silicon foundry," he says. This would make them much cheaper to produce than silicon cells.

The problem with standard flat cells, whether they're made of an organic or inorganic material, is that some sunlight is lost through reflection. To reduce this effect, cell makers apply antireflective coatings or etch the cell's surface to increase photon absorption. Carroll's team has taken a more dramatic approach by stamping optical fibers onto a polymer substrate that forms the foundation of the cell.

The fibers, which Carroll refers to as "light pipes," protrude from the surface like coarse stubble. They are surrounded by thin organic solar cells applied using a dip-coating process, and a light-absorbing dye or polymer is also sprayed onto the cell. Light can enter the tip of a fiber at any angle. Photons then bounce around inside the fiber until they are absorbed by the surrounding organic cell.

The researchers tested a glass fiber cell in the lab and found that the fiber enhanced light absorption by about half. Carroll says that the cells can also produce twice as many watt-hours over the course of a day compared to flat panels because they can receive light from different angles. "It's the same thing as taking a flat device and pointing it directly at the sun all day long," he says.