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An easily fabricated, flexible and wearable white-light LED

21 August 2015

By taking advantage of novel design patterns, researchers in Taiwan have developed a flexible white light LED using cheap, readily-accessible components.

A large area warm white light source with bending capability (photo: Chin-Wei Sher, Chien-chung Lin, Hao-Chung Kuo/National Chiao Tung University)

Researchers from National Chiao Tung University, Taiwan have created highly flexible, efficient white LEDs with potential use in wearable displays and non-flat surfaces, such as curved and flexible television screens. While the design itself is new, the LED was completely fabricated from pre-existing technologies, allowing others to easily replicate and build on the platform.

"Compared to organic light-emitting diodes, this design of flexible LEDs can be very attractive, due to the low cost, prolonged lifetime and high efficiency," says Chien-Chung Lin, associate professor, College of Photonics, National Chiao Tung University, Taiwan. "In addition, all of the technologies associated with this design are currently available." 

A paper by Lin, Professor Hao-Chung Kuo, and their research team appears this week in the journal, Optics Express.

This off-the-shelf LED device gets its flexibility from two primary materials, polyimide and polydimethylsiloxane. To construct it, Lin and his colleagues first covered a polyimide substrate with copper foil shielding tape. In a process known as flip-chip bonding, which reduces thermal resistance and results in higher heat dissipation than traditional wire bonding, they mounted 81 blue LED chips, measuring 1.125mm x 1.125mm, to the foil in an upside down position.

To provide a warm white-yellow light, the researchers then added another layer consisting of a yellow phosphor film that had been mixed and spin-coated in polydimethylsiloxane (PDMS) a widely used silicone-based organic polymer. It was chosen for its high degree of transparency, stability, and flexibility. The final film measured 5cm by 5cm, but there is no reasonable limitation to the size of the film.

The researchers ran the device for a standard 1,000 hours, to test its durability, finding that its emission decayed by only 5 percent. Its potential for use in wearables was demonstrated when subjected to bending tests. It held its power output when bent to a curvature with a 1.5cm radius. It also exhibited a light efficiency of 120 lumens per Watt.

"Because the components are all available by current technology, the combined reliability can be very good," Lin adds. "Most of the novel processes or materials require a lengthy procedure to verify their reliability, but our design uses only available parts to avoid this issue."

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