American Scientist

We only see about half of our Sun’s light. Most of the other half shines down in the infrared spectrum, warming things up. That radiation makes so-called multijunction solar cells—which can convert the energy from both the visible and infrared spectra into electricity—about twice as efficient as most of the commercially available solar cells that only convert the visible spectrum of light.

“They’re also about 100 times as expensive,” says Chris Giebink, an electrical engineer at Penn State University, “so it’s not realistic to put them on top of your roof.”

Right now, only satellite and space-based industries are willing to pay for the extra engineering and materials required to make multijunction solar cells. That’s because, in addition to being extra efficient, they typically are more robust and so can last longer in space. Of course, many researchers have been trying various strategies to make these cells more economical.

One strategy Giebink and his colleagues are trying uses cheap plastic lenses to concentrate a lot of light onto a very small multijunction solar cell. “So we generate the same amount of power from that light, actually at higher efficiency,” Giebink says, “but we need a much smaller solar cell area to do that.”

Plastic lenses, though, present another problem for efficiency because they reflect some of the light, just as almost all surfaces do. So Giebink and his colleagues have been experimenting with antireflection coatings, too.

“It’s a challenge because most of the antireflection coating formulations that we use for glass are applied at temperatures that would melt plastic,” says Giebink. Plastics such as plexiglass also tend to expand and contract more than the typical antireflection coatings, which therefore flake off over time with exposure to the elements. A further challenge was finding a material that could eliminate reflections over the entire solar spectrum, including parts of the infrared. “We looked at what already existed and there really wasn’t anything,” says Giebink.

Polytetrafluoroethylene, better known by one of its brand-name formulations, Teflon, offered most of the material characteristics the team was seeking. But the remaining question was whether they could deposit it on plastic in a way that would meet all the challenges.

Inspiration came via a study that has been informing optics research for decades: A 1967 electron microscope study showed that nanoscale perturbances on the eyes of nocturnal butterflies and moths both increased light transmission and substantially decreased reflection. Today, researchers refer to such perturbances as subwavelength photonic structures. Those found on the insect eyes measure about 200 nanometers and are repeated about every 200 nanometers, a distance that is about half as short as the shortest wavelength of visible light.

So Giebink and his team experimented to create layers of such subwavelength photonic structures out of Teflon, depositing increasingly porous Teflon onto plexiglass. The first layer, closest to the plexiglass, was just a little bit porous, which also meant light could pass through it overall a little faster than it could through plexiglass. The next layer, more porous, allowed light to pass through even faster. By the fifth, top layer, the Teflon was extremely porous—only slightly denser than air, through which light passes much faster than it does through Teflon or plastic. In this way, each layer of porous Teflon served as a subwavelength photonic structure and, in combination with other layers, prevented the large changes in the speed of light that cause increased reflectivity.

Other research groups working on antireflection coatings—using other materials—have employed a similar nanoporous layering strategy, which Giebink likens to smoothing out a curb that you might drive over: “When you have a big tall bump, you really feel it,” Giebink says. But using Teflon required engineering steps that Giebink says were “a little bit unconventional.” The researchers described those steps and published their results in the January 9 issue of Nano Letters.

The resulting coating is durable, is only a few hundred nanometers thick, and reduces reflectance almost to zero for incidence angles up to 40 degrees and wavelengths between 400 and 2,000 nanometers, a range that covers almost all of the Sun’s power output in both the visible and infrared ranges.

Given how the coating is made, it’s not necessary to coat a material directly: “You could imagine the equivalent of antireflection-coated plastic wrap, and then wrap something else with it to make that object’s reflection disappear,” Giebink says. Other industries are interested, but there’s still a lot more to achieving the goal of making multijunction solar cells economical.

An interview with the researcher, Chris Giebink: