A specialized piece of glass called a luminescent solar concentrator can intensify incoming light. The green and orange rings are produced by its fluorescence. Photo: Argonne National Laboratory |
For years,
scientists have dealt with the problem of trying to increase the efficiency and
drive down the cost of solar cells. Now researchers have hit upon a new
idea—trying to give the light collected by solar cells a bit of “amnesia.”
At the
U.S. Department of Energy’s (DOE) Argonne National Laboratory, nanoscientists
Chris Giebink and Gary Wiederrecht, collaborating with Northwestern University
Professor Michael Wasielewski, have investigated the use of fluorescent
plastics called luminescent solar concentrators (LSCs) that can be used to
lower the cost of electricity from solar cells.
“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 says.
Concentrating
sunlight is one strategy to squeeze more power out of existing solar cells,
which ultimately reduces the cost of the energy they produce. The question
occupying researchers today is how best to gather as much sunlight as cheaply
as possible. Although lenses and mirrors are one solution—think burning a piece
of paper with a magnifying glass on a sunny day—they must be continually aimed
at the sun as it moves across the sky each day, which requires an expensive
tracking system.
Luminescent
solar concentrators are an inexpensive, alternative approach that do not
require solar tracking because they capture sunlight and change it to a
different wavelength, according to Giebink. “We’re actively shifting the
frequency of the light by absorbing and re-emitting it,” he says. “LSCs 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
whole process is designed to intensify the light as much as possible.”
The
theoretical potential for this intensification, Giebink says, can exceed the
equivalent of one hundred “suns”—the measurement of solar radiation on one
spot. However, actual implementation has until now failed to produce such high
intensities. “The main problem we’re running into is that light is getting lost
in the slabs either due to reabsorption or to scattering, and that’s the
problem we have to solve,” he says.
Using
instrumentation and equipment at Argonne’s Center for Nanoscale Materials, the
researchers focused on altering the way light is re-emitted and reabsorbed
inside an LSC by taking advantage of optical ‘microcavity’ effects, which occur
when the dimensions of a structure are similar to the wavelength of light. In
this case, the scientists were able to use a series of thin films with
nanometer-scale changes in thickness to produce a ‘resonance-shifting’ effect,
in which light fails to “recognize” the environment that it is emitted from,
drastically reducing reabsorption. “It really is like giving light amnesia—if
light forgets how it came in, it’s less likely to get reabsorbed or scattered
out,” Wiederrecht says.
Although the Argonne experimental research
explored how LSCs work only in one dimension, both Giebink and Wiederrecht
believe that a 2D LSC analysis would show even greater efficacy for
resonance-shifting. “By finding better and better ways to pull this kind of switcheroo,
the higher the efficiency we’re going to get,” Giebink says. “We’ve
demonstrated the general principle, now we just have to find the best pattern
of dye thicknesses or alternatively the best way to vary the thickness of the
glass.”