Scientists at the U.S. Dept. of Energy’s Brookhaven National Laboratory have found a novel way of maximizing solar cell efficiency – by inscribing a nanoscale texture on the silicon itself, creating an antireflective surface that enhances the cell’s production of solar energy.
The new method, published recently in Nature Communications, could dramatically reduce the cost, and streamline the production of, solar cells. Additionally, it signals an advance in anti-reflective technology that could have huge implications not only for solar cells, but also for windows, radar camouflage for military equipment, and the efficacy of light-emitting diodes.
“For antireflection applications, the idea is to prevent light or radio waves from bouncing at interfaces between materials,” Charles Black, lead physicist of the research from Brookhaven Lab’s Center for Functional Nanomaterials (CFN), said in a recent BNL press release.
In order to prevent reflections, the scientists must handle the dramatic change that happens when two materials with very different refractive indices meet, which in the case of solar cells, means silicon and air. Because of this, adding a coating with an in-between refractive index smoothes the transition and reduces reflection. However, Black explains that these coatings are not perfect.
“The issue with using such coatings for solar cells is that we’d prefer to fully capture every color of the light spectrum within the device, and we’d like to capture the light irrespective of the direction it comes from,” Black explained. “But each color of light couples best with a different antireflection coating, and each coating is optimized for light coming from a particular direction. So you deal with these issues by using multiple antireflection layers. We were interested in looking for a better way.”
This “better way” was inspired by the compound eyes of moths, which are comprised of textured patterns made of miniscule “posts” that are each smaller than the wavelengths of light. “We set out to recreate moth eye patterns in silicon at even smaller sizes using methods of nanotechnology,” Atikur Rahman, researcher at CFN and first author of the study, said.
In order to construct their nanotextured silicon, the researchers began by coating the top of a silicon solar cell with a material called “block copolymer,” which self-assembles into an ordered surface pattern at dimensions as small as fractions of a nanonmeter. The pattern created by the block copolymer becomes a template for creating nanoposts using a plasma of reactive gases, a method more often used in the production of semiconductor electronic circuits. The resulting nanotexture works to gradually modify the refractive index of the surface material, drastically reducing the reflections produced by numerous wavelengths of light simultaneously, no matter the direction they propagate from.
This method not only reduces cost by a significant amount – it also increases efficiency by up to 20% compared to single-surface-coated solar cells, and performs at the same high level as multi-coated solar cells. “We are working to understand whether there are economic advantages to assembling silicon solar cells using our method, compared to other, established processes in the industry,” Black said.
Part of the success of Black’s method is due to a thin layer of silicon oxide that yields nanoposts up to half the height predicted by the scientists’ mathematical model, but with increased efficiency.
“On a flat surface, this layer is so thin that its effect is minimal,” Matt Eisaman of Brookhaven’s Sustainable Energy Technologies Department, said. “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.”
“This ‘hidden’ layer was the key to the extra boost in performance,” Black said.
Moving forward, the BNL scientists involved in the research are interested in transposing their technology onto other materials apart from silicon, such as glass and plastic, in order to create antireflective surfaces on windows and other objects.
