Tuesday, October 24, 2017

From Butterfly Wings to Solar Cells

You may associate butterfly wings more closely with pop culture and chaos theory than with cutting-edge materials science, but the delicate wings have a lot more to offer than the plot of sci-fi movies. In new research published last week in the journal Science Advances, a team of scientists from Germany and the United States reveal how a technique inspired by black-winged butterflies could lead to more efficient solar cells.

The beautiful Pachliopta aristolochiae, also known as the “common rose” butterfly, is native to South and Southwest Asia. The insect has predominantly black wings with some red and white spots, and more vivid red can be seen on the underside of its wings and body. The scales that protect the butterfly’s wings are covered in tiny structures that absorb sunlight, turning it into heat. These structures enable the butterfly to harvest solar energy over a wide variety of angles and wavelengths, helping it to maintain a high body temperature and regulate its appearance.

Black butterfly sketch.
Image Credit: Radwanul Hasan Siddique, KIT/Caltech.
The ability to harvest sunlight over a wide range of angles and wavelengths is not only useful for butterflies, it’s also exactly what we need in photovoltaics, systems that convert light into electricity. Photovoltaic systems made of second-generation solar cells, called thin-film solar cells, are more flexible and less expensive than traditional solar panels, but they fall behind in efficiency. The scientists behind this new project, which was led by Radwanul Siddique from the California Institute of Technology along with Gomard Guillaume and Hendrik Hölscher from Karlsruhe Institute of Technology (KIT) in Germany, hope that biology can inspire new ways of overcoming this limitation.

Their work focuses on understanding the tiny structures that cover the scales of Pachliopta aristolochiae wings. These structures range in size from the nanoscale, close to the wavelength of light, to the microscale, a thousand times larger. Scientists used to think that these structures were periodic—that they followed an organized pattern—but lately they’ve realized that systems with some disorder do better over a wider range of angles and wavelengths. In light of this discovery, the collaborators on the butterfly project focused not only on the structures, but on the disorder within the structures.

As a first step, the scientists carefully studied the scales of black-winged butterflies under a microscope. Up close, they could see periodic ridges running along the length of the scales that were connected by cross-ribs. Tiny holes, varying randomly in diameter, were suspended from the ridges and cross-ribs.

Scanning electron microscope image of black butterfly nanoholes.
Image Credit: Radwanul Hasan Siddique, KIT/Caltech.
Some areas of the butterfly wings are darker than others, and the researchers noticed that this correlated, at least in part, to a difference in the density of holes. This implies that the density of holes is related to the region’s ability to absorb light. In addition, areas with lower hole density appear to play a larger role in mechanical stability. It seems that hole density reflects a balancing act between efficiently absorbing light and supporting the butterfly’s wing structure.

Next, the researchers created a 3-dimenstional model of the nanoholes in order to investigate their impact on light absorption. They considered two slabs of the same volume of the same material, one smooth and unpatterned and the other with a structure inspired by the black butterfly. Their results showed that when light hit the slabs, the patterned slab absorbed significantly more light over a wider range of wavelengths. Exactly how the light interacts with the pattern depends on the wavelength of light and its size relative to the structures. However, the bottom line is the nanoholes make a big difference, and the ridges contribute too.

As the authors report in the paper, “The outstanding absorption properties originate from a multilevel system.” First, light is collected and partly absorbed by the nanoholes and the ridges. Then any light that makes it through those structures has a second chance of being absorbed, this time by a layer of material at the bottom of the structures.

In order to take what they learned from the butterfly and study how it might apply to a thin-film absorber, like a solar cell, the scientists simulated four nanohole arrangements, described below, in a material commonly used to make photovoltaic absorbers.

• An unpatterned slab (for reference)
• A periodic, ordered arrangement where all of the holes were evenly spaced and had the same diameter
• A periodic, perturbed arrangement where all of the holes were evenly spaced but they had different diameters
• A “correlated” combination of position and diameter size disorder inspired by the black butterfly scales

Next, they simulated how well each arrangement harvested light over a range wavelengths and incoming angles. While all of the patterned slabs performed significantly better than the unpatterned slab, the butterfly-inspired design performed the best over the largest range of wavelengths and angles.

Scientists from KIT and Caltech utilize the disordered nanoholes of the black butterfly to improve solar cell performance.
Image Credit: Radwanul Hasan Siddique, KIT/Caltech.
As a final step in this work, the team designed and fabricated a thin photovoltaic absorber from silicon that mimicked the butterfly structure. They used a simple, scalable technique and again saw major improvement in its optical performance. The “remarkably improved performance” of absorbers that incorporate this butterfly-inspired pattern, along with the ease of the patterning process makes this a promising route for future photovoltaic applications, say the authors.

Technological advances inspired by nature are nothing new. Many of the creatures that populate our world have evolved to survive and thrive in conditions that seem harsh and unnerving to humanity. The more we study their creative adaptations, the more likely we are to find solutions to our own challenges. And, of course, the more likely we are to find inspiration for the next generation of science fiction movies.

Kendra Redmond

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