Should color stand in the way of more widespread adoption of solar panels? Theoretically, the answer is, “Of course not.” Generating electricity from sunshine is too important to let aesthetics get in the way, right? Actually, there may be more to it than that. Almost all solar panels manufactured today are black or dark blue. Their color makes them unattractive for uses other than conventional roofs. Some historic preservation guidelines prevent them from being used simply because their appearance disrupts the look of protected buildings. Solar panels could be integrated into walls and other building surfaces if they could be made to blend in better visually.
Researchers at AMOLF in the Netherlands say they have devised a way to do precisely that by imprinting solar panels with silicon nanopatterns that make the panels appear green. The process decreases the efficiency of the panels by about 10%. “Some people say, ‘Why would you make solar cells less efficient?’ But we can make solar cells beautiful without losing too much efficiency,” said Verena Neder, a researcher at AMOLF. “The new method to change the color of the panels is not only easy to apply but also attractive as an architectural design element and has the potential to widen their use.”
The researchers use soft imprint lithography to apply a dense array of silicon nanotubes onto the surface of solar cells. Each nanotube is about 100 nanometers wide, and is carefully shaped to scatter a certain wavelength of light. At this point in the trials, the resulting solar cells appear green to the observer. The color remains constant over a wide range of viewing angles. “The structure we made is not very sensitive to the angle of observation, so even if you look at it from a wide angle, it still appears green,” Neder says.
The color of the solar panels can be adjusted by altering the geometry of the nanotubes. Now that they have green down, the researchers are turning their attention to designing imprints that create red and blue solar cells. Once they master these three primary colors, they can create any color, including white. “You have to combine different nanoparticles, and if they get very close to each other they can interact and that will affect the color,” Polman said. “Going to white is a really big step.”
Colored solar panels are already available for specialized applications, but they reduce efficiency significantly. The AMOLF process reduces efficiency by about 10%, an acceptable trade-off if solar panels could be used on more surfaces of buildings. “In principle, this technique is easily scalable for fabrication technology,” says Albert Polman, a scientific group leader at AMOLF. “You can use a rubber stamp the size of a solar panel that in one step can print the whole panel full of these little, exactly defined nanoparticles.”
One potential application of the technology would make it possible to create tandem solar cells stacked in layers, with each layer fine tuned to absorb certain portions of the electromagnetic spectrum. In theory, that could result in efficiencies of 30% or more. Since current commercially available solar cells are about 20 percent efficient, that would be a major breakthrough on the road toward the widespread use of sustainable energy.