27 June 2011

US researchers create LEDs for flexible arrays

University of Miami professor Jizhou Song together with researchers at Northwestern University have helped a team at University of Illinois at Urbana-Champaign (UIUC) led by professors Ralph Nuzzo and John Rogers to design an LED light that uses an array of LEDs 100 times smaller than conventional LEDs. The new device has flexibility, maintains lower temperature and has an increased life-span over existing LEDs (Kim et al, ‘Unusual Strategies for Using InGaN Grown on Silicon (111) for Solid State Lighting’, Proceedings of the National Academy of Sciences, published online 10 June 2011).

Properties that can now be achieved with blue indium gallium nitride (InGaN) LEDs lead to their potential as replacements for existing infrastructure in general illumination, with important implications for the efficient use of energy. Further advances will benefit from the re-examination of the modes for incorporating this materials technology into lighting modules that manage light conversion, extraction, and distribution, in ways that minimize adverse thermal effects associated with operation, with packages that exploit the unique aspects of the light sources.

In the published work, the researchers focused on improving certain features of LED lights, such as size, flexibility and temperature. Song's role in the project was to analyze the thermal management and establish an analytical model that reduces the temperature of the device.

The researchers have presented ideas in anisotropic etching, microscale device assembly/integration, and module configuration that address these challenges in unconventional ways. “The new model uses a silicon substrate, novel etching strategies, a unique layout and innovative thermal management method,” says Song. “The combination of these manufacturing techniques allows the new design to be much smaller and keep lower temperatures than current LEDs using the same electrical power.”

Various device demonstrations provide examples of the capabilities, including thin, flexible lighting ‘tapes’ based on patterned phosphors and large collections of small light emitters on plastic substrates.

Quantitative modeling and experimental evaluation of heat flow in such structures illustrates one particularly important aspect of their operation: small, distributed LEDs can be passively cooled simply by direct thermal transport through thin-film metallization used for electrical interconnect, providing an enhanced and scalable means to integrate these devices in modules for white light generation.

In the future, the researchers would also like to make the devices stretchable, so that they can be used on any surface, such as deformable display monitors and biomedical devices that adapt to the curvilinear surfaces of the human body.

Tags: LEDs InGaN

Visit: www.pnas.org/content/early/2011/06/06/1102650108.abstract