Tuesday, March 06, 2007

Materials That Reflect No Light

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

By Kevin Bullis

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. Credit: Rensselaer/Fred Schubert

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.

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.