Georgia Tech researchers have developed a system that uses dye-sensitized (Grätzel) solar cells wrapped around optical fibers to increase their overall efficiency, since it’s known Grätzel cells are not very efficient. The fiber solar cells they obtained are 6 times more efficient than zinc-oxide solar cells having the same area.
Inside a fiber optic solar cell the light bounces inside the fiber as it travels along, and gives much more opportunities to the surrounding solar cells to harvest it. “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 (not involved with the research).
So how did the GaTech scientists do it? First, they removed the cladding from conventional optical fibers and grew zinc-oxide nanowires along their surface, so it resembles tiny, nanoscale hair. The fiber thus obtained were treated with dye molecules that were absorbed by the zinc-oxide hair-like structures. Of course, this had a huge advantage over impregnating the dye on a smooth surface: the total area that the wires provide altogether is larger than the same planar surface area. “The more dye molecules there are over a given area of such a cell, the more light it can absorb”, says Zhong Lin Wang, GaTech professor of materials science and engineering. Then, the dye-coated fibers are surrounded by an electrolyte and a metal film carrying the electrons off the device.
Yi Cui, assistant professor of materials at Stanford, says “The question is, can you absorb all the light using a small amount of materials?”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.
Because Wang’s optical fibers only collect light at their tips, relying on its reflection inside the fiber for reaching the solar cells, for trapping more light inside the fibers, there would be need to have smaller fibers bundled together. Those light-capturing smaller fibers could be made of materials that are very efficient at directing light into the “solar” fiber.
This is one of the approaches. The other one would be to make the devices capable of capturing light also on their entire length, rather than only their ends, but it would require the cells being coated with a material that is both electrically conductive and transparent, which is very tricky to obtain.
Fiber-optic solar cells could have unusual uses: they could be embedded into walls on a very large surface, and have the light-capturing opening on the roof, so there would be a much larger area the solar cell could theoretically occupy using very little practical space. Added to that, dye-sensitized solar cells are very cheap, which could make fiber-optic solar cells a very interesting option for everyone. The only price issue that has to be overcome are the fibers themselves which have to be made from inexpensive polymers.