Schematic of quasi-random nanowrinkles on a silicon wafer surface.

An optimal material for light management in solar cells has been simultaneously designed and manufactured by an interdisciplinary team of researchers at Northwestern University.

"We have bridged the gap between design and nanomanufacturing," said Wei Chen, professor of mechanical engineering in Northwestern's McCormick School of Engineering, leader of 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."

Streamlined and scalable, the new technique 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, a chemistry professor in Northwestern’s Weinberg College of Arts and Sciences. Odom led the study’s nanofabrication component.

Nanophotonic materials are currently being researched for their ability to absorb light in ultra-thin, flexible solar cells. The ideal nanostructure surface for a solar cell features “quasi-random” structures—structures that appear random, but do in fact have a pattern. Designing these structures to discover the pattern that will absorb the most light involves the simultaneous optimization of thousands of geometric variables.

"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."

Instead, Odom and Chen used wrinkle lithography—a new nanomanufacturing technique that involves applying strain to a substrate. It can rapidly transfer wrinkle patterns into different materials, resulting in a nearly unlimited number of quasi-random nanostructures.

“The complex geometries can be described computationally with only three parameters—instead of thousands typically required by other approaches," added Odom. An iterative search loop determined the optimal nanowrinkle design.

Using their technique, the researchers were able to fabricate material that absorbed 160 percent more light in the 800 to 1,200 nanometer wavelength, a range in which current solar cells are less efficient.

“We did not design for just one frequency," Chen noted. "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."

The team plans next to apply its method to additional materials, such as polymers, metals and oxides, for other photonics applications.

Supported by the National Science Foundation and Office of Naval Research, the research was recently published in the Proceedings of the National Academy of Sciences.