In an advance that could lead to solar cells that more fully utilize sunlight, researchers at Caltech have designed materials that can bend visible light at unusual but precise angles, no matter its polarization. The scientists hope the materials are a step toward perfectly transparent solar-cell coatings that would direct all the sun’s rays into the active area to improve solar power output.
Many groups are working on novel antireflective solar cell coatings in the hopes of getting more light into solar cells. The Caltech group, which includes Harry Atwater, professor of applied physics and materials science, and researcher Stanley Burgos, is addressing the problem by precisely tailoring the structure of materials at the nano and micro scales, creating “metamaterials” that exhibit optical properties that are not found in naturally occurring materials. In the most recent work, Atwater and his coworkers demonstrated a material that precisely controls the path of visible light regardless of the polarization of the light–a first for metamaterials.
The Caltech metamaterial is a metal film several hundred nanometers thick. The films are patterned with circular cavities, each of which surrounds a wirelike column made of the same material. The space between the wire and the cavity wall is filled with a second metal. Depending on the dimensions of the patterns, the material bends, or refracts, light of different colors to a different degree. Atwater says the goal of his project is to make films with a refractive index exactly equal to that of air. Such a material would not bend light at all but would transmit it perfectly, with no reflection. When light moves from one medium to another, it scatters–this is why a straw in a glass of water appears to be broken. There’s a mismatch between the refractive index of water and air. A solar cell coated with a material whose refractive index is identical to that of air would reflect no light at all.
The films that Atwater’s group is making are metallic conductors, and could also serve as the top electrode on a solar cell. Atwater says that while some metamaterial designs have been complex to make and involve multilayered structures, these single-layer films can be made using lithography and etching techniques commonplace in the chip-making industry.
The ability of the material to work with both polarizations of light is exciting, says Nicholas Fang, professor of materials science and engineering at the University of Illinois at Urbana-Champaign. But, he says, one of the major remaining challenges in engineering metamaterials is loss. As these metal structures interact with light, they lose energy to heat. This heat loss is so great in Atwater’s current materials that just 40 percent of incident light passes through them.
For solar applications, Atwater says his goal is a metamaterial film that passes 90 percent of the light. To that end, his group and others in the field are developing ways to amplify light as it passes through metamaterials. Optical amplifiers are used in lasers and in telecommunications; incorporating them with thin films like Atwater’s will enable metamaterials to find their way into practical applications in devices like solar cells.
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