Inexpensive thin-film solar cells aren't as efficient as conventional solar cells, but a new coating that incorporates nanoscale metallic particles could help close the gap. Broadband Solar, a startup spun out of Stanford University late last year, is developing coatings that increase the amount of light these solar cells absorb.
Based on computer models and initial experiments, an amorphous silicon cell could jump from converting about 8 percent of the energy in light into electricity to converting around 12 percent. That would make such cells competitive with the leading thin-film solar cells produced today, such as those made by First Solar, headquartered in Tempe, AZ, says Cyrus Wadia, codirector of the Cleantech to Market Program in the Haas School of Business at the University of California, Berkeley. Amorphous silicon has the advantage of being much more abundant than the materials used by First Solar. The coatings could also be applied to other types of thin-film solar cells, including First Solar's, to increase their efficiency.
Solar antenna: The square at the center is an array of test solar cells being used to evaluate a coating that contains metallic nanoantennas tuned to the solar spectrum.
Broadband believes its coatings won't increase the cost of these solar cells because they perform the same function as the transparent conductors used on all thin-film cells and could be deposited using the same equipment.
Broadband's nanoscale metallic particles take incoming light and redirect it along the plane of the solar cell, says Mark Brongersma, professor of materials science and engineering at Stanford and scientific advisor to the company. As a result, each photon takes a longer path through the material, increasing its chances of dislodging an electron before it can reflect back out of the cell. The nanoparticles also increase light absorption by creating strong local electric fields.
The particles, which are essentially nanoscale antennas, are very similar to radio antennas, says Brongersma. They're much smaller because the wavelengths they interact with are much shorter than radio waves. Just as conventional antennas can convert incoming radio waves into an electrical signal and transmit electrical signals as radio waves, these nanoantennas rely on electrical interactions to receive and transmit light in the optical spectrum.
Their interaction with light is so strong because incoming photons actually couple to the surface of metal nanoparticles in the form of surface waves called plasmons. These so-called plasmonic effects occur in nanostructures made from highly conductive metals such as copper, silver, and gold. Researchers are taking advantage of plasmonic effects to miniaturize optical computers, and to create higher-resolution light microscopes and lithography. Broadband is one of the first companies working to commercialize plasmonic solar cells.
In his lab at Stanford, Brongersma has experimented with different sizes and shapes of metallic nanostructures, using electron-beam lithography to carve them out one at a time. Different sizes and shapes of metal particles interact strongly with different colors of light, and will direct them at varying angles. The ideal solar-cell coating would contain nanoantennas varying in size and shape over just the right range to take advantage of all the wavelengths in the solar spectrum and send them through the cell at wide angles. However, this carving process is too laborious to be commercialized.
Through his work with Broadband, Brongersma is developing a much simpler method for making the tiny antennas over large areas. This involves a technique called "sputter deposition" that's commonly used in industry to make thin metal films (including those that line some potato-chip bags). Sputtering works by bombarding a substrate with ionized metal. Under the right conditions, he says, "due to surface tension, the metal balls up into particles like water droplets on a waxed car." The resulting nanoparticles vary in shape and size, which means they'll interact with different wavelengths of light. "We rely on this randomness" to make the films responsive to the broad spectrum found in sunlight, he says.
Broadband is currently developing sputtering techniques for incorporating metal nanoantennas into transparent conductive oxide films over large areas. Being able to match the large scale of thin-film solar manufacturing will be key to commercializing these coatings.
The company has been using money from angel investors to test its plasmonic coatings on small prototype cells. So far, says Brongersma, enhanced current from the cells matches simulations. Broadband is currently seeking venture funding to scale up its processes, says CEO Anthony Defries.
By Katherine Bourzac
From Technology Review