"Light Pipes" Boost Organic Solar Efficiency

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.

Researchers at Georgia Tech are experimenting with similar fiber-based organic solar cells. Zhong Lin Wang, professor of materials science and engineering at the university, says the approach has “major advantages” over conventional flat-cell designs. His lab has developed a hybrid cell consisting of optical fibers and a nanowire fuzz made of zinc oxide that is grown on the outside walls of the fibers. The nanowires, treated with light-absorbing dyes, are meant to provide a greater surface area for capturing sunlight. Wang says this approach enhances efficiency by a factor of six, though his lab has yet to go beyond a single fiber strand. “We are still working on integrating multiple fibers [on a larger surface],” he says.

Carroll started his research back in 2004, giving Wake University a head start on the path to commercialization. “Most of the devices out there now are on individual fibers,” he says. Carroll also says there’s no shortage of roll-to-roll processing techniques on the market that could make substrates covered with optical fibers. “We’ll be borrowing from those; this isn’t difficult to do,” he says. “Sensitivity to film thickness and coatings quality is much less than we had anticipated, meaning that manufacturing routes are much closer than we first anticipated.”

FiberCell is currently talking with investors and aims to produce its fiber-based organic cells for roof tiles and other products that would benefit from the ability to accept light from different angles “If I get this to perform near its maximum, then I have a device that should theoretically be able to surpass 15 percent efficiency, approaching 20 percent,” Carroll says. This would make organic photovoltaic technology competitive with today’s top silicon panels.

John Paul Morgan, an optical engineer and chief technology officer of concentrated photovoltaic solar company Morgan Solar, says the FiberCell approach will have to compete with other techniques designed to increase panel surface area and trap more light. Growing a forest of tightly grouped nanowires on top of a substrate, for example, has been shown to improve the efficiency of organic cells.

“Optical fibers are an interesting approach, but like other approaches it comes down to the challenges of fabrication,” said Morgan. “All new cell technologies face issues with moisture, electrical connections, wear and tear. If they can overcome these, then this could be a very viable idea. I’m excited to see what comes next.”