Tiny hills and valleys on the surface of leaves have guided a team of Princeton University scientists in the creation of a new kind of solar cell. By wrinkling up a layer of adhesive, the team designed solar cells that can capture more light, even in the infrared region.
“If you look at leaves very closely, they are not smooth,” said Yueh-Lin Loo, Princeton’s principle investigator on the project. “We’d like to mimic this geometric effect in synthetic, man-made light-harvesting systems.”
The research team worked in the Mechanical and Aerospace Engineering Department’s laboratory to design a unique surface by curing a layer of liquid photographic adhesive with ultraviolet light. They produced ripples on the surface by introducing stress to the material and controlling how fast it was cured. Solar cells are then constructed with inexpensive plastic placed over the “folded” material. The best results came when a mixture of shallow ripples (wrinkles) and deep ripples (folds) occupied the surface. It makes sense-if you increase the surface area, you increase the number of places where light can be collected.
The new cells produced 47 percent more electricity than their flat-surfaced counterparts along with a significant increase in the notorious infrared region of the spectrum. Conventional solar cells struggle to capture the long waves of light in the red region, but with the wrinkles and folds, absorption in the region was upped by a whopping 600 percent.
“On a flat surface, light is either absorbed or it bounces back,” said Loo. “By adding these curves, we create a kind of wave guide, and that leads to a greater chance of the light being absorbed.” Loo noted that the design is “flexible, bendable, light weight, and low cost.” Panels made with this technique will be durable and unaffected by mechanical stress from bending, unlike standard plastic panels, which lose some ability to produce energy upon bending.
Most solar panels are made from silicon, a brittle and expensive material. Plastic panels have not caught on because the solar conversion efficiency has been too low. The new folding technique holds promise for an assortment of surfaces, including window inserts, exterior wall overlays, or backpacks. “This is a very simple process that you can use with any material,” said Loo. “We have tested it with other polymers and it works well.”
Other researchers have focused on increasing the efficiency of the plastic photovoltaic material itself, and UCLA scientists did reach improvements, but with this new technique, efficiency could soar even higher. Princeton’s Jong Bok Kim, the lead author on the paper describing the technique (published in Nature Photonics), said, “I expected that it would increase the photocurrent because the folded surface is quite similar to the morphology of leaves, a natural system with high light-harvesting efficiency. However, when I actually constructed solar cells on top of the folded surface, its effect was better than my expectation.”