Researchers from Taiwan's National Chiao Tung University have developed highly flexible, versatile white LEDs with potential applications in wearable displays and non-flat surfaces.
The new LED is based on already available technologies so as to enable others to easily modify or build up 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. In addition, all of the technologies associated with this design are currently available," said Chien-Chung Lin, associate professor at College of Photonics, National Chiao Tung University, Taiwan.
The findings of the research carried out by Lin, Professor Hao-Chung Kuo and their team were published in the Optics Express journal from The Optical Society (OSA). This invention is the team's first flexible LED device. Their previous work focused on traditional gallium-nitride LEDs.
The flexibility of the off-the-shelf LED device was achieved through two primary materials — polydimethylsiloxane and polyimide. The new LED was fabricated by covering a polyimide substrate using a copper foil shielding tape. The researchers then mounted 81 blue LED chips, measuring 1.125 x 1.125 mm, to the foil in a bottom up position through a flip-chip bonding process. This process minimizes thermal resistance, resulting in higher heat dissipation than conventional wire bonding.
Following this, the researchers added a layer of yellow phosphor film, mixed and spin-coated in popular silicone-based organic polymer - polydimethylsiloxane or PDMS, to offer a warm white-yellow light. It was selected due to its flexibility, stability and high transparency degree. The measure of the final film is 5 x 5 cm. However, the size of the film is not limited to any specific value.
The device was then made to run for a standard time of 1000 hours to check durability, with the results showing a 5% emission decay. It was also subjected to bending tests to demonstrate its capabilities for application in wearable devices. The power output was held when the device is bent to a 1.5 cm radius curvature. The light efficiency of the device was found to be 120 lumens per watt.
"Because the components are all available by current technology, the combined reliability can be very good," Lin said. "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."
The research team's future work will involve minimizing the thickness of the LED device and improving its service life and energy efficiency.