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.
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