Instilling Amnesia in 'Photon' Beams

US Department of Energy's (DOE) Argonne National Laboratory (ANL) physicists argue that they may have just developed a new approach to increasing the efficiency of solar cells that convert light into electricity. The team here managed to develop a way to give light “amnesia.”

At this point, conversion efficiency levels are very low, not exceeding 18 percent. The theoretical limit imposed on current materials is 31 percent. The ANL team believes that its approach could bring that limit within reach over a very short period of time.

Together with Northwestern University professor Michael Wasielewski, ANL nanoscientists Chris Giebink and Gary Wiederrecht studied fluorescent plastics called luminescent solar concentrators (LSC) in great detail. For a while, the devices were touted as means of reducing electricity costs.

Due to the high technologies involved in producing solar arrays, the cost of electricity produced from this renewable source is currently significant larger than that associated with energy produced by burning oil, coal or natural gases.

“In order to make solar power competitive in energy markets, we either need to get more energy from the cells we've already developed or find ways to make cheaper cells that give us the same amount of energy,” Giebink explains.

All team members are part of the staff at the Argonne-Northwestern Solar Energy Research Center. The organization is one of the 46 Energy Frontier Research Centers the DOE Office of Science established in 2009, and continued financing since.

“We're actively shifting the frequency of the light by absorbing and re-emitting it. LSC act kind of like flat funnels – we try to absorb a lot of light over the face of a plastic slab filled with dye, and then re-direct it all back out the edges,” the expert says.

“The whole process is designed to intensify the light as much as possible,” he explains, adding that this type of light intensification can lead to a theoretical potential of up to 100 suns, a unit of measure determining the amount of solar radiation that falls on a single spot.

In order to improve LSC performances, the team made use of the optical microcavity effect, which basically alters the way in which photons are re-emitted and reabsorbed inside the devices. Previously, the effect only took place in one dimension.

By creating a series of thin films with nanometer-scale changes in thickness to produce a ‘resonance-shifting’ effect, the investigators were in effect able to make photons fail in recognizing the environments from which they were emitted, drastically improving solar cell efficiency.

This approach “really is like giving light amnesia – if light forgets how it came in, it's less likely to get reabsorbed or scattered out,” Wiederrecht concludes.