A schematic of quasi-random nanowrinkles on the surface of a silicon wafer. Image: Northwestern University.An interdisciplinary team of researchers at Northwestern University has used mathematics and machine learning to design an optimal material for light management in solar cells, then simultaneously fabricated the nanostructured surfaces with a new nanomanufacturing technique.
"We have bridged the gap between design and nanomanufacturing," said Wei Chen, professor in engineering design and professor of mechanical engineering at Northwestern's McCormick School of Engineering, who led the study's design component. "Instead of designing a structure element by element, we are now designing and optimizing it with a simple mathematic function and fabricating it at the same time."
The fast, highly scalable, streamlined method could replace cumbersome trial-and-error nanomanufacturing and design methods, which often take vast resources to complete.
"The concurrent design and processing of nanostructures paves the way to avoid trial-and-error manufacturing, increasing the cost effectiveness to prototype nanophotonic devices," said Teri Odom, professor of chemistry in Northwestern's Weinberg College of Arts and Sciences and leader of the study's nanofabrication component.
Researchers are currently interested in nanophotonic materials for light absorption in ultra-thin, flexible solar cells. The same principle could also be applied to implement color into clothing without dyes and to create anti-wet surfaces. For solar cells, the ideal nanostructure surface features quasi-random structures – meaning the structures appear random but do have a pattern. Designing these patterns can be difficult and time consuming, since there are thousands of geometric variables that must be optimized simultaneously to discover the optimal surface pattern able to absorb the most light.
"It is a very tedious job to fabricate the optimal design," Chen said. "You could use nano-lithography, which is similar to 3D printing, but it takes days and thousands of dollars just to print a little square. That's not practical."
To bypass the issues of nano-lithography, Odom and Chen manufactured the quasi-random structures with wrinkle lithography, a new nanomanufacturing technique that can rapidly transfer wrinkle patterns into different materials to realize a nearly unlimited number of quasi-random nanostructures. Formed by applying strain to a substrate, wrinkling is a simple method for the scalable fabrication of nanoscale surface structures.
"Importantly, the complex geometries can be described computationally with only three parameters – instead of thousands typically required by other approaches," Odom said. "We then used the digital designs in an iterative search loop to determine the optimal nanowrinkles for a desired outcome."
Supported by the US National Science Foundation and the US Office of Naval Research, the research is published in a paper in the Proceedings of the National Academy of Sciences. Won-Kyu Lee, a PhD student in Odom's laboratory, served as the paper's first author. Shuangcheng Yu, a PhD student who recently graduated from Chen's Integrated Design Automation Laboratory (IDEAL), served as the paper's second author.
The team demonstrated this concurrent design and manufacturing method by using it to fabricate 3D photonic nanostructures on a silicon wafer for potential use as a solar cell. The resulting material absorbed 160% more light at 800–1200nm wavelengths – a range in which current solar cells are less efficient – than other designs.
"Light wavelengths have different frequencies, and we did not design for just one frequency," Chen said. "We designed for the whole spectrum of sunlight frequencies, so the solar cell can absorb light over broadband wavelengths and over a wide collection of angles."
Next, the team plans to apply its method to other materials, such as polymers, metals and oxides, for other photonics applications.
This story is adapted from material from Northwestern University's McCormick School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.