Nanostructures Catch the Light
Razor-thin solar cells could be cheap but need a little help holding light in
Efficiency may be the first parameter you think of when you hear the word photovoltaics. However, a less-talked-about factor can have a big impact, too—how thin a solar cell is. Researchers have recently come up with new ways of slimming down cells using structures smaller than the wavelengths of visible light.
"The main aim is to use as little material as possible to absorb sunlight," says Shanhui Fan, associate professor of electrical engineering at Stanford. High-efficiency materials, such as III-V semiconductors and crystalline silicon, are expensive. With other materials such as amorphous silicon, cost is less of an issue, but the electrons and holes that carry charge travel only short distances before being lost as heat. "The thinner the cell gets, the easier it is to get the carrier out," Fan explains. However, the thinner a solar cell is, the more likely that photons will pass right through it before they can be absorbed.
Commercial crystalline silicon cells can be 180 micrometers thick, Fan says. But some companies are pushing to get down to 50 μm, while his lab and other researchers are aiming for designs that are only a micrometer or two thick. In theory, techniques such as adding random nanoscale texturing to the surface of cells could enhance light absorption by as much as 50-fold, Fan says, by changing the angles at which photons travel through the cells, but nanophotonics can improve that by another factor of 10.
One approach is called plasmonics. Photons striking small, metallic structures can create plasmons, which are oscillations of electron density in the metal. The effect can increase the scattering of light within the solar cell, giving it more of an opportunity to absorb the photons. Vivian Ferry, a postdoctoral researcher at Caltech, says her team is creating plasmons using hemispheric bumps on the contacts of a 90-µm-thick solar cell made of hydrogenated amorphous silicon. Ferry says the nanostructured device produces 15 percent more current than a commercially produced, randomly textured solar cell.
Another nanophotonics trick in the works is using photonic crystals to construct reflectors. Photonic crystals are periodic structures with features smaller than the wavelength of the light they are designed to deal with. Miro Zeman, who heads the photonics materials and devices group at Delft University of Technology, in the Netherlands, says his lab has built photonic crystal reflectors at both the back of the cell and in the middle. The reflectors force light to bounce around inside the silicon, increasing the chances that it will be turned into electricity.
Another photonic crystal scheme would use the structures in a 1-µm-thick layer of crystalline silicon. According to Ounsi El Daif, a researcher at Imec in Leuven, Belgium, the photonic crystal layer can then be joined to an amorphous silicon layer. Because the film is so thin, "traditional texturing techniques cannot really work in this case," El Daif says. Theoretically, he says, such a photonic crystal could increase photon absorption by 37 percent.
These technologies are still years from being commercial products, Fan says. But they might be worth the wait.