“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.