electronics

Bend it like bright LED screens

NORTHWESTERN (US)—A research team has developed a new way of printing inorganic LEDs onto plastic. The technique creates screens that are small, thin, flexible, and less expensive to produce—all the while retaining their high quality.

Inorganic light-emitting diode screens, or LED, made of silicon are bright, efficient, and last a long time, but have typically been expensive to manufacture, heavy, and difficult to make in small sizes. In contrast, organic LED screens are cheap and flexible, but they produce a lower-quality image and have a short lifespan.

“This technology could have many applications in health-monitoring devices and screen technology,” says Yonggang Huang, Joseph Cummings Professor of Civil and Environmental Engineering and Mechanical Engineering at the McCormick School of Engineering and Applied Science at Northwestern University.

Huang worked on the research with professors from the University of Illinois-Urbana Champaign and others from across the globe. Their results are published in the Aug. 21 issue of the journal Science.

Huang and his colleagues knew that the way to reduce the cost of expensive inorganic LED screens was to reduce the material. To do this, they created microscale inorganic LEDs and developed a method to assemble them onto plastic substrates.

This method allows them to print tiny islands of LEDs onto a surface (rather than cover the entire surface with the materials) while still keeping the image quality. The space between the islands is small enough that the human eye can’t tell the difference between it and a fully covered screen. The process uses just 1 percent of the material that standard inorganic LED screen processes use.

“You save a tremendous amount in the material cost, but you still have the same image quality,” Huang explains.

This method of printing LEDs also allows for much smaller screens—even as small as a fingernail. Such a small screen could have uses in medical health-monitoring devices. But such devices, if they are to be placed on the body, need to be flexible.

To connect islands of LEDs, Huang and his colleagues use the same “pop-up” technology they developed to connect tiny islands of circuits used in stretchable electronics. The tiny wires enable electronic transfer while also allowing the device to bend and stretch.

The technology allows for transparent electronics, like “head-up” displays, which superimpose images onto the inside of a windshield to enable pilots and drivers to view information without looking down.

For his part, Huang’s research group created the mechanics models for the design. His colleagues contributed through their specialties, including materials science, electrical engineering, and manufacturing.

“To work on something like this you need people from many different backgrounds,” says Huang. “We really worked together.”

The Ford Motor Company, the National Science Foundation, and the U.S. Department of Energy supported the research.

Northwestern University news: www.northwestern.edu/newscenter

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