Researchers Combine the Advantages of Organic and Inorganic LEDs

September 8, 2009

3 Min Read
Researchers Combine the Advantages of Organic and Inorganic LEDs

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Stretchable LED display consists of an interconnected mesh of printed micro LEDs bonded to a rubber substrate. Photo by D. Stevenson and C. Conway, Beckman Institute, University of Illinois.

Organic light-emitting diodes (LEDs) can be formed on flexible substrates in dense, interconnected arrays--an advantage they enjoy over standard inorganic LEDs. But inorganic LEDs are brighter, more robust, and longer-living than their organic cousins. Now, scientists at the University of Illinois (Urbana-Champaign), Northwestern University, the Institute of High Performance Computing (Singapore), and Tsinghua University (Beijing) have devised a method for making inorganic LEDs that combine the features of both types of technologies.To overcome requirements on device size and thickness associated with conventional wafer dicing, packaging, and wire bonding, the researchers developed epitaxial growth techniques for creating LEDs with sizes up to 100 times smaller than usual. They also developed printing processes for assembling these devices into arrays on stiff, flexible, and stretchable substrates.As part of the growth process, a sacrificial layer of material is embedded beneath the LEDs. When fabrication is complete, a wet chemical etchent removes this layer, leaving the LEDs undercut from the wafer but still tethered at anchor points.To create an array, a rubber stamp contacts the wafer surface at selected points, lifts off the LEDs at those points, and transfers them to the desired substrate. "The stamping process provides a much faster alternative to the standard robotic 'pick and place' process that manipulates inorganic LEDs one at a time," explains John Rogers, professor of materials science and engineering at the University of Illinois. "The new approach can lift large numbers of small, thin LEDs from the wafer in one step and then print them onto a substrate in another step." By shifting position and repeating the stamping process, LEDs can be transferred to other locations on the same substrate. In this fashion, large light panels and displays can be crafted from small LEDs made in dense arrays on a single, comparatively small wafer.The LED arrays are interconnected using thin-film processing, Rogers remarks. Because the LEDs can be placed far apart and still provide sufficient light output, the panels and displays can be nearly transparent. The thin device geometries allow the use of thin-film processing methods for the interconnects, rather than wire bonding.The new process for creating ultrathin miniature inorganic LEDs and assembling them into large arrays offers the possibility of creating new classes of lighting and display systems with properties that cannot be achieved using existing technologies, such as see-through construction and mechanical flexibility. Applications for the arrays, which can be printed onto flat or flexible substrates such as glass, plastic, and rubber, include wearable health monitors and biomedical imaging devices."Wrapping a stretchable sheet of tiny LEDs around the human body offers interesting opportunities in biomedicine and biotechnology, including applications in health monitoring, diagnostics, and imaging," Rogers comments.

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