Photo credit by ricketyus/flickr
By Kevin Matyi
Some of the greatest scientific discoveries were complete accidents. Antibiotics were first discovered by Alexander Fleming in 1928 after he left a petri dish open and mold began growing alongside a sample of Staphylococcus. Fleming saw that the Staph bacteria could not grow near the mold as easily. While more experiments were needed to determine exactly what was happening, this eventually revolutionized modern medicine, allowing medical practitioners around the world a more effective means of combating infection.
Researchers at Brookhaven National Laboratories stumbled upon a revolutionary design for solar energy: cone-shaped, “moth eyed” nanostructures on the surface of solar panels. The nanostructures get their name from similar structures found in the eyes of moths, which stop light from reflecting outward.
A team of scientists including Stony Brook University’s Matthew Eisaman attempted to apply the same effect to solar panels in a lab. But Eisaman said there is much to be excited about.
“What matters is the dollar per kilowatt hour, how much it costs to do,” Eisaman said. “One of the advantages of this method is that it’s actually potentially very cheap.”
The current industry standard is to coat solar panels with a layer of another material that will allow light through, but then interfere with it leaving the panel if it reflects. However, this only works for a small range of wavelengths, and to expand the range would require multiple layers of interference material, which makes the process more expensive. To combat this, Eisaman said, “usually you tune the thickness so the minimum’s right where the peak of the solar spectrum is, but you get higher reflections in other regions.”
A flat surface using current techniques will have roughly a 35 percent reflectance for a given range of light wavelengths. The method that Eisaman and his team discovered reduces the reflectance to less than 1 percent for the same range of wavelengths. This reduction also continues at a larger variety of angles as well, Eisaman said.
The moth eyes created a gradient between the air and silicon, effectively easing the transition between the refraction indexes of the two materials. Eisaman added the more gradual the transition, the less light would reflect.
At the end of their experiments, the data showed that the taller the moth eyes were, the more effective they would be because “the height is important relative to the wavelength, so if you want longer wavelengths, you have to make it sufficiently tall.” However, there was a curious anomaly from their predicted results.
“Experimentally, the anti-reflection properties were actually better than we would expect,” Eisaman said.
On further testing, the etching chemical used to create the moth eyes, and theoretically allowed the process to be used over large areas, was causing the unexpected increase in anti-reflection. “The optical properties of that coating were between that of air and silicon,” Eisaman said, thus easing the transition between the two other materials more than the cones on their own.
The moth eyes were approximately 200 nanometers tall, or roughly one five-hundredth of the thickness of a piece of standard notebook paper. At such a small scale, the etching chemical’s effects became more apparent due to taking up a relatively larger percentage of the total volume — all of which was a pleasant surprise, Eisaman said.
“[This is] one of those cases where you just try things and you think that it’s going to work a straightforward way,” Eisaman added. “It was one of those nice surprises where it worked a little bit better than you expect naively.”