A new technology for growing nanostructures on optical fibers can be used to make 3-D photovoltaic systems foldable and portable – no longer confined to traditional locations such as rooftops. Researchers at the Georgia Institute of Technology used zinc oxide nanostructures grown on optical fibers and coated with dye-sensitized solar cell materials to develop their photovoltaic (PV) system.Georgia Tech Regents’ Professor Zhong Lin Wang holds a prototype 3-D solar cell that could allow PV systems to be located away from rooftops. (Images courtesy of Georgia Tech University)“Using this technology, we can make photovoltaic generators that are foldable, concealed and mobile,” said Zhong Lin Wang, a Regents’ professor in the Georgia Tech School of Materials Science and Engineering. “Optical fiber could conduct sunlight into a building’s walls, where the nanostructures would convert it to electricity. This is truly a three-dimensional solar cell.” Dye-sensitized solar cells use a photochemical system to generate electricity. They are inexpensive to manufacture, flexible and mechanically robust, but their trade-off for lower cost is conversion efficiency lower than that of more expensive, silicon-based cells. But using nanostructure arrays to increase the surface area available to convert light could help reduce the efficiency disadvantage, while giving architects and designers new options for incorporating PV cells into buildings, vehicles and even military equipment. Fabrication of the new Georgia Tech PV system begins with the type of optical fiber used by the telecommunications industry to transport data. First, the researchers remove the cladding layer, and then they apply a conductive coating to the surface of the fiber before seeding the surface with zinc oxide. Next, they use established solution-based techniques to grow aligned zinc oxide nanowires around the fiber much like the bristles of a bottle brush. The nanowires are coated with the dye-sensitized materials that convert light to electricity. Sunlight entering the optical fiber passes into the nanowires, where it interacts with the dye molecules to produce electrical current. A liquid electrolyte between the nanowires collects the electrical charges. The result is a hybrid nanowire/optical fiber system that can be up to six times as efficient as planar zinc oxide cells with the same surface area. Georgia Tech researchers (l-r) Yaguang Wei, Zhong Lin Wang and Benjamin Weintraub examine a prototype of their 3-D solar cell based on optical fiber.“In each reflection within the fiber, the light has the opportunity to interact with the nanostructures that are coated with the dye molecules,” Wang said. “You have multiple light reflections within the fiber, and multiple reflections within the nanostructures. These interactions increase the likelihood that the light will interact with the dye molecules, and that increases the efficiency.” Wang and his research team have reached an efficiency of 3.3 percent and hope to reach 7 to 8 percent after surface modification. While lower than that of silicon solar cells, this efficiency would be useful for practical energy harvesting. By providing a larger area for gathering light, the technique would maximize the amount of energy produced from strong sunlight, as well as generate respectable power levels even in weak light. The amount of light entering the optical fiber could be increased by using lenses to focus the incoming light, and the fiber-based solar cell has a very high saturation intensity, Wang said. Wang believes this new structure will offer architects and product designers an alternative PV format that can be incorporated into various applications. “This will really provide some new options for photovoltaic systems,” he said. “We could eliminate the aesthetic issues of PV arrays on buildings. We can also envision PV systems for providing energy to parked vehicles, and for charging mobile military equipment where traditional arrays aren’t practical, or you wouldn’t want to use them.” Wang and his research team, which includes Benjamin Weintraub and Yaguang Wei, have produced generators on optical fiber up to 20 cm in length. “The longer the better,” said Wang, “because longer the light can travel along the fiber, the more bounces it will make, and more it will be absorbed.” Close-up showing the brown light-absorbing material for the 3-D solar cell grown on optical fiber by researchers at the Georgia Institute of Technology.Traditional quartz optical fiber has been used so far, but Wang would like to use less expensive polymer fiber to reduce the cost. He is also considering other improvements, such as a better method for collecting the charges, and a titanium oxide surface coating that could further boost efficiency. Though they could be used for large PV systems, Wang doesn’t expect his solar cells to replace silicon devices anytime soon. But he does believe they will broaden the potential applications for photovoltaic energy. “This is a different way to gather power from the sun,” Wang said. “To meet our energy needs, we need all the approaches we can get.” Details of the research were published Oct. 22 in the early view of the journal Angewandte Chemie International. The work was sponsored by DARPA, the KAUST Global Research Partnership and the National Science Foundation. For more information, visit: www.gatech.edu