Sustainable Energy

Wrapping Solar Cells around an Optical Fiber

Dye-sensitized cells get a double boost from nanowires and optical fiber.

Dye-sensitized solar cells are flexible and cheap to make, but they tend to be inefficient at converting light into electricity. One way to boost the performance of any solar cell is to increase the surface area available to incoming light. So a group of researchers at Georgia Tech has made dye-sensitized solar cells with a much higher effective surface area by wrapping the cells around optical fibers. These fiber solar cells are six times more efficient than a zinc oxide solar cell with the same surface area, and if they can be built using cheap polymer fibers, they shouldn’t be significantly more expensive to make.

Solar on fiber: An optical fiber (left) is covered in dye-coated zinc-oxide nanowires (closeup, right). Both images were made using a scanning electron microscope.

The advantage of a fiber-optic solar-cell system over a planar one is that light bounces around inside an optical fiber as it travels along its length, providing more opportunities to interact with the solar cell on its inner surface and producing more current. “For a given real estate, the total area of the cell is higher, and increased surface area means improved light harvesting and more energy,” says Max Shtein, an assistant professor of materials science and engineering at the University of Michigan who was not involved with the research.

Fiber-optic solar cells could also be used in ways that aren’t possible currently. Zhong Lin Wang, professor of materials science and engineering at Georgia Tech, says fiber solar cells would take up less roof area than planar cells because long lengths of the fibers could be nestled into the walls of a house like electrical wiring.

Dye-sensitized solar cells use dye molecules to absorb light and generate electrons. The Georgia Tech group first removes the cladding from optical fibers and then grows zinc-oxide nanowires along their surface, like bristles on a pipe cleaner. Next, the fibers are treated with dye molecules, which the zinc-oxide structures absorb. The advantage of coating nanowires, rather than a smooth surface, with the dye is that the wires collectively have a very large surface area. The more dye molecules there are over a given area of such a cell, the more light it can absorb, says Wang. The dye-coated fibers are then surrounded by an electrolyte and a metal film that carries electrons off the device. The work is described online in the journal Angewandte Chemie International Edition.

“The question is, can you absorb all the light using a small amount of materials?” says Yi Cui, assistant professor of materials science at Stanford University. Building a nanostructured cell on an optical fiber provides a way to do this by increasing both the surface area covered by the dye and the effective path length of the light, he says. The longer a photon travels through a solar cell, the more opportunities it has to interact and generate an electron.

One potential stumbling block for fiber cells is getting enough light inside them in the first place. Wang’s devices only collect light at their tips, so to get enough light into such a solar cell without having to track the sun, smaller fibers might be bundled together. Cui says the tips of the fibers could be made of materials that are very effective at directing light into the fiber. Another way to overcome this problem is to build fiber cells that can absorb light along their entire length, not just at the tips–which Michigan’s Shtein is working on. This is tricky, because it means the cells’ coatings need to be both electrically conductive and transparent, an unusual combination.

However, Shtein says that fibers that absorb light from the sides offer “an interesting architecture for light capture, because you can distribute the fibers in space in a way that helps you capture more photons more effectively than you can in a planar device.” The shallower the angle at which light hits a planar cell, the more light reflects off its surface. But the light reflecting off the curved surface of a fiber at a shallow angle will hit an adjacent fiber. These cells could be designed so that it’s not necessary to install them with sun-tracking systems, and they would work on cloudy days when the light is diffuse, Shtein says.

Wang says the next step is to try different materials. So far, he has built the cells on quartz optical fibers, which are relatively expensive. Next he plans to try making the cells using cheaper polymer fibers.

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