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Materials That Reflect No Light

Solar cells, camera lenses, and LEDs could benefit from new antireflection coatings.
March 6, 2007

Unwanted reflections limit the performance of light-based technologies, such as solar cells, camera lenses, and light-emitting diodes (LEDs). In solar cells, for example, reflections mean less light that can be converted into electricity. Now researchers at Rensselaer Polytechnic Institute (RPI), in Troy, NY, and semiconductor maker Crystal IS, in Green Island, NY, have developed a new type of nanostructured coating that can virtually eliminate reflections, potentially leading to dramatic improvements in optical devices. The work is published in the current issue of Nature Photonics.

Seeing no reflection: Two pieces of aluminum nitride, a semiconducting material that can be used in light-emitting devices, reflect different amounts of light. The piece at the top reflects 12 percent of the light. A new antireflective coating on the lower piece reduces reflection to about 0.1 percent. The bluish tinge is because the coating allows more blue light to reflect. Such coatings could improve many optical devices.

The researchers showed that they can prevent almost all reflection of a wide range of wavelengths of light by “growing” nanoscale rods projected at specific angles from a surface. In contrast, conventional antireflective coatings work best only for specific colors, which is why, for example, eyeglasses with such coatings still show faint red or green reflections. Fred Schubert, professor of physics and electrical, computer, and systems engineering at RPI and one of the authors of the study, says that the material stops reflections from nearly all the colors of the visible spectrum, as well as some infrared light, and it also reduces reflections from light coming from more directions than conventional coatings do. As a result, he says, the total reflection is 10 times less than it is with current coatings.

Applied to a solar cell, the new coating would increase the amount of light absorbed by a few percentage points and convert it into electricity, Schubert says. A more remarkable 40 percent improvement could be seen in LEDs, he says, in which a large amount of light generated by a semiconductor is typically trapped inside the device by reflections. The work is part of a growing effort among researchers to alter the properties of materials, such as their optical properties, by controlling nanoscale structures.

To make less-reflective surfaces, the RPI engineers created a multilayered, porous coating that eases the transition as light moves from air into a solid material or as light is emitted from a semiconductor in an LED. Reflectivity is related to the difference between the amount that two substances, such as air and glass, refract or bend light. Reducing the difference reduces reflection where two materials meet. In the new coating, each successive layer bends light more as light moves from air into a substrate. Likewise, as in the example of an LED, light emerging from a semiconductor is bent less in each successive layer until it reaches the air.

The theory behind this has been known for decades, says Steven Johnson, a professor of applied mathematics at MIT, but the challenge has been fabricating a structure that is both porous enough and small enough to work with the short wavelengths of visible light.

The RPI researchers made such a porous structure by depositing materials on a surface to create nanoscale rods. Tilting the surface makes it possible to grow the nanorods at an angle. The researchers found that by changing the angle of the nanorods, they can control the way the nanorods bend light–the index of refraction. Air has an index of refraction of very nearly one. The researchers were able to make a top layer of nanorods with what Schubert says is an unprecedented index of 1.05. (For comparison, glass has an index of refraction of 1.45, and a light-emitting semiconductor, aluminum nitride, has an index of about 2.05.) Each successive layer has a higher index of refraction until the last layer nearly matches the substrate. The top two layers incorporate glass nanorods. The bottom three are made of titania. The researchers tested the coating on aluminum nitride, but it should work on a variety of substrates, Schubert says.

“We have developed a new class of materials that has a refractive index that is lower than anything else–any other viable optical thin-film material that has been available in the past,” Schubert says. Since “everything in optics depends upon the refractive index,” he says it could have applications other than antireflective coatings. Indeed, the nanorods could be used to do the opposite, creating very highly reflective mirrors by pairing layers of nanorods that bend light very differently, rather than by creating a gradual transition.

Schubert is working with a spinoff company to commercialize the technology, and he anticipates that products could be available in three to five years. The technology will face competition with inexpensive conventional coatings as well as with other new nanostructured materials. “This is very elegant, beautiful work,” says Michael Rubner, a professor of materials science and engineering at MIT. “They’ve been able to get some exceptionally low refractive-index values for a coating. The key question is always going to be cost versus performance.”

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