Flexible microLEDs on polyimide film can be bent repeatedly without degradation

University of Texas at Dallas researchers and their colleagues at Los Alamos National Laboratory (Los Alamos, NM) and six different Korean universities and companies have developed a method to create microLEDs that can be folded, twisted, cut, and adhered to different surfaces.¹ The results could help pave the way for the next generation of flexible, wearable technology. MicroLEDs, which can be as small as 2 μm and bundled to be any size, provide higher resolution than other LEDs. Their size makes them a good fit for small devices such as smart watches, but they can be bundled to work in flat-screen TVs and other larger displays. LEDs of all sizes, however, are brittle and typically can only be used on flat surfaces. The researchers’ new microLEDs aim to fill a demand for bendable, wearable electronics.

The flexible LED is fabricated via remote epitaxy, which involves growing a thin layer of LED crystals on the surface of a sapphire crystal wafer. Conventionally, an LED would remain on the wafer; to make it detachable, the researchers added a nonstick film of monolayer graphene to the substrate, enabling the LED to be peeled from the wafer (with the graphene remaining on the wafer), and subsequently adhered to any surface. The South Korean colleagues carried out laboratory tests of the LEDs by adhering them to curved surfaces, as well as to materials that were subsequently twisted, bent, or crumpled. In another demonstration, they adhered an LED to the legs of a Lego minifigure with different leg positions. “You can transfer it onto your clothing or even rubber—that was the main idea,” says Moon Kim, one of the researchers. “It can survive even if you wrinkle it. If you cut it, you can use half of the LED.”

The LEDs have a gallium nitride (GaN)-based microrod p-n junction structure embedded in a film of polyimide in between two metal electrodes. The emission center wavelength was about 500 nm with a full width at half maximum (FWHM) bandwidth of 54 nm. Devices were subjected to 1000 bending cycles with no noticeable degradation of emission properties. The flexible microLEDs have a variety of possible uses, including flexible lighting, clothing, and wearable biomedical devices. From a manufacturing perspective, the fabrication technique offers another advantage: Because the LED can be removed without breaking the underlying wafer substrate, the wafer can be used repeatedly. Reference: J. Jeong et al., Sci. Adv. (2020); doi:10.1126/sciadv.aaz5180.