PORTLAND, Ore. — Solar cells are no longer confined to flat panels thanks to a new technique that turns optical fibers into long, thin solar cells.

By assembling the components of a dye-sensitized solar cell along the outside of an optical cable, Georgia Tech researchers said they were able to demonstrate thin, flexible solar cells six times more efficient than flat panels using the same materials.

"We face the fiber's end directly at the sun," said Georgia Tech professor Zhong Lin Wang. "There is an electrode located next to the fiber that collects the generated charge, which is connected to the external load." Wang performed the work with Georgia Tech researchers Benjamin Weintraub and Yaguang Wei.

Georgia Tech professor Zhong Lin Wang shows solar cells assembled as a thin-film coating on fiber optical cables.

Dye-sensitized solar cells were invented in the 1990s as a thin-film version of photovoltaics. Unlike silicon solar cells, which use an expensive semiconductor materials to generate a charge, the researchers created dye-sensitized solar cells by substituting an inexpensive thin film of dye molecules atop zinc oxide nanowires in an electrolyte surrounded by a metal film. When light shines on the dye cells, they shed their electrons into the metal film, which transports them to the external electrode.

One problem with conventional solar panels is that incident photons have little chance of energizing electrons in the thin panels before degenerating into heat. By contrast, optical fibers provide photons many more chances to interact with the active layer.

Wang's prototype used fibers 20-cm long to ensure that most photons were absorbed somewhere along its length.

The technique only collects sunlight from the tips of optical fibers, which is then channeled down the length of the fiber. Future versions may use a transparent outer metallic sheath, thereby allowing light to enter from both inside and outside. The researchers also want to create bundled subsystems consisting of hundreds of parallel fiber optical cables.

Efficiency also must be increased from 3.3 percent to as much as 8 percent in order to commercialize the technology. Wang said he also plans to experiment with cheaper optical fiber materials to reduce costs while substituting titanium oxide for zinc oxide to boost efficiency.

For commercial systems, designers might create non-traditional shapes beyond flat panels. For military applications, the fibers could be built into equipment to produce power.

Funding for the solar research was provided by the Defense Advanced Research Projects Agency, the KAUST Global Research Partnership and the National Science Foundation.