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Research Team Uses Nanostructure Surface Textures to Improve Solar Cells
The method, described in Nature Communications, is inspired by structure of moths’ eyes which prevents the reflection of light
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Stony Brook University’s Matthew Eisaman, PhD

Stony Brook, NY, January 23, 2015 – Light that bounces off of solar panels is lost energy and presents a ubiquitous problem for efficiently converting the sun’s rays into electricity. Now a team of scientists from the U.S. Department of Energy’s Brookhaven National Laboratory and Stony Brook University have developed a new anti-reflective design for solar panels based on nanostructured surface textures and inspired by the structures of the eyes of moths, which are designed to keep light from reflecting back out. Their findings are published in Nature Communications .

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This closeup shows how the nanotextured square of silicon completely blocks reflection compared with the surrounding silicon water.

The team, which includes Matthew Eisaman, PhD, Assistant Professor in the Electrical & Computer Engineering Department at Stony Brook University, showed that etching a nanoscale texture onto the silicon material itself creates an antireflective surface that works as well as state-of-the-art thin-film multilayer coatings. Their method, submitted for patent protection, has potential for streamlining silicon solar cell production and reducing manufacturing costs. The approach may find additional applications in reducing glare from windows, providing radar camouflage for military equipment, and increasing the brightness of light-emitting diodes.

"For antireflection applications, the idea is to prevent light or radio waves from bouncing at interfaces between materials," said physicist Charles Black, PhD, who led the research at Brookhaven Lab's Center for Functional Nanomaterials (CFN), a Department of Energy (DOE) Office of Science User Facility. 

Preventing reflections requires controlling an abrupt change in "refractive index," a property that affects how waves such as light propagate through a material. This occurs at the interface where two materials with very different refractive indices meet, for example at the interface between air and silicon. Adding a coating with an intermediate refractive index at the interface eases the transition between materials and reduces the reflection, Dr. Black explained.

To design a surface without the need for such coatings, the scientists turned to a well-known example of an antireflective surface in nature, the eyes of common moths. The surfaces of their compound eyes have textured patterns made of many tiny "posts," each smaller than the wavelengths of light. This textured surface improves moths' nighttime vision, and also prevents the "deer in the headlights" reflecting glow that might allow predators to detect them.

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Details of the nanotextured antireflective surface as revealed by a scanning electron microsope. The tiny posts, each smaller than the wavelengths of light, are reminiscent of the structure of moths’ eyes. 

The scientists started by coating the top surface of a silicon solar cell with a polymer material called a "block copolymer," which can be made to self-organize into an ordered surface pattern with dimensions measuring only tens of nanometers. The self-assembled pattern served as a template for forming posts similar to those in the moth eye (but much smaller) on the solar cell surface using a plasma of reactive gases—a technique commonly used in the manufacture of semiconductor electronic circuits.  

The resulting surface nanotexture served to gradually change the refractive index to drastically cut down on reflection of many wavelengths of light simultaneously, regardless of the direction of light impinging on the solar cell. 

Additionally, by using a combination of computational modeling, electron microscopy, and surface science, the team deduced that a thin layer of silicon oxide similar to what typically forms when silicon is exposed to air seemed to be having an outsized effect. 

"On a flat surface, this layer is so thin that its effect is minimal," explained Dr. Eisaman. "But on the nanopatterned surface, with the thin oxide layer surrounding all sides of the nanotexture, the oxide can have a larger effect because it makes up a significant portion of the nanotextured material."  

Said Dr. Black, "This 'hidden' layer was the key to the extra boost in performance." 

The scientists are now interested in developing their self-assembly-based method of nanotexture patterning for other materials, including glass and plastic, for antiglare windows, and coatings for solar panels.

This research was supported by the DOE Office of Science.