Light wave filtering technique boosts photovoltaics efficiency potential
The work is described in a paper appearing in the journal Science, written by MIT graduate student Yichen Shen, professor of physics Marin Soljacic, and four others.
The new structure consists of a stack of ultrathin layers of two alternating materials where the thickness of each layer is precisely controlled. "When you have two materials, then generally at the interface between them you will have some reflections," explained Soljacic. But at these interfaces, "there is this magical angle called the Brewster angle, and when you come in at exactly that angle and the appropriate polarization, there is no reflection at all."
While the amount of light reflected at each of these interfaces is small, by combining many layers with the same properties, most of the light can be reflected away – except for that coming in at precisely the right angle and polarization.
Using a stack of about 80 alternating layers of precise thickness, Shen explained: "We are able to reflect light at most of the angles, over a very broad band [of colors]: the entire visible range of frequencies."
Previous work had demonstrated ways of selectively reflecting light except for one precise angle, but those approaches were limited to a narrow range of colors of light. The new system’s breadth could open up many potential applications, the team says.
"This could have great applications in energy, and especially in solar thermophotovoltaics," said Shen. Solar energy is harnessed by using it to heat a
material, which in turn radiates light of a particular color. That light emission can then be harnessed using a photovoltaic cell tuned to make maximum use of that color of light. But for this approach to work, it is essential to limit the heat and light lost to reflections, and re-emission, so the ability to selectively control those reflections could improve efficiency.
In principle, the angular selectivity can be made narrower simply by adding more layers to the stack, the researchers say. For the experiments performed so far, the angle of selectivity was about 10 degrees; roughly 90 percent of the light coming in within that angle was allowed to pass through.
While these experiments were done using layers of glass and tantalum oxide, Shen says that in principle any two materials with different refractive indices could be used.
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