No matter how efficient photovoltaic cells are, the amount of electricity they can produce is limited by the number of photons they absorb. And when sunlight hits the surface of the cells at a steep enough angle, photons are more likely to reflect than to penetrate.
An advance by researchers at Rensselaer Polytechnic Institute in Troy, N.Y., may change that equation. A research team there has demonstrated an anti-reflective coating that absorbs almost all the light falling on it, no matter the angle. Applied to a solar panel, the coating promises not only to make photovoltaic cells more powerful, but could eliminate the need for cells to track the sun throughout the day.
Non-reflective coatings are commonly found on high-end optics. Lenses on some eyeglasses are coated to absorb and transmit light entering perpendicular to the surface while reflecting away light from angles that could produce glare. Such coatings work by bending the incoming light. Bending the light toward the bottom boundary of the coating enables it to pass through; bending the light away from the bottom boundary causes increased reflection.
While most optical coatings involve one thin layer of material, the antireflective coating created by the Rensselaer team led by Shawn-Yu Lin, stacks seven layers, one atop another. Each layer is an array of nanoscale rods made of silicon dioxide and titanium dioxide aligned at a specific angle with the neighboring layers. As light enters this forest of nanorods, almost all of it winds up being bent toward the layers below. The layers also act to prevent the escape of light that might be reflected upward.
In tests of the coating, the material absorbed more than 96 percent of the light that shone on it. That compares favorably to untreated solar cells that absorb only 67 percent of incident light. What’s more, the coating captured light coming in at oblique angles that might otherwise be reflected away.
The hope is that such a coating could enable solar cells to produce as much as 50 percent more electricity than they do at present merely by capturing sunlight more efficiently. What’s more, since coated panels would be less subject to power variation due to the changing angle of the sun, they could be mounted more simply and cheaply.
It was uncertain when photovoltaic cells using this technology will enter commercial production.
— Jeffrey Winters