Highly efficient organic light-emitting diodes (OLEDs) have been fabricated as ultra-thin fibers by a team led by Kyung Cheol Choi of the School of Electrical Engineering at KAIST (Korea Advanced Institute of Science and Technology) in Daejeon, South Korea Korea. The OLED fibers might be woven into the very fabric of future wearable technologies for a wide range of applications.
We usually think of OLEDs as being flat, yet putatively flexible, devices with an emissive electroluminescent layer based on film of organic materials with at least one of the two electrodes being transparent. From early work in the 1950s it was not until 1987 that American physical chemist Ching Tang and Steven Van Slyke at Eastman Kodak demonstrated the first practical OLED device. Researchers at Cambridge University took up the challenge of developing efficient, inexpensive, flexible OLEDs in the 1990s. Ultimately, the science led to the familiar technology we all use in the displays for our smart devices.
With the emergence of a viable concept of wearable technology, the focus on OLEDs has turned from the thin film to the fiber-based device. However, fiber OLEDs have proven to be of much lower performance compared to their planar counterparts. This poor performance has until now limited their potential somewhat.
"Existing fiber-based wearable displays had limitations for applicability due to their low performance. However, this technology can fabricate OLEDs with high performance on fibers. This simple, low-cost process opens a way to commercialize fiber-based wearable displays," Choi explains
The KAIST team of Seonil Kwon, Hyuncheol Kim, Seungyeop Choi, Eun Gyo Jeong, Dohong Kim, Somin Lee, Ho Seung Lee, Young Cheol Seo, and Kyung Cheol Choi used a low-temperature, dip-coating method with a polyethylene terephthalate (PET) fiber substrate. The device has a bottom-emission inverted fluorescent structure, in which emitted light passes through the substrate. The conducting polymer is poly(3,4-ethylenedioxythiophene) with polystyrene sulfonate (PEDOT:PSS) and indium tin oxide (ITO) cathodes. The team added zinc oxide nanoparticles and polyethyleneimine, Super Yellow, molybdenum oxide and aluminum as the electron injection layer, emitting layer, hole injection layer, and anode, respectively.
With this approach, the team has developed efficient OLED fibers, which they say can be hand woven into fabrics and last a lifetime and have performance on a par with planar substrates. Indeed, the fiber OLEDs have luminance and current efficiency values of over 10,000 candela per /square meter and 11 candela/ampere. The fibers can withstand a tensile strain of up to 4.3% while retaining more than 90% current efficiency, the team reports. Fibers of diameter as low as 90 micrometers were possible, but they can be made up to 300 micrometers thick. [Choi et al., Nano Lett, online; DOI: 10.1021/acs.nanolett.7b04204]
"The next step will be to fabricate a real clothing display," Choi told Materials Today. "We hope to combine TFT sensors and communication modules with displays for medical applications. Choi told us that the team is now working with Seoul National University Hospital at Bundang in the Republic of Korea on using OLED fibers in a dressing fabric to promote would healing through photobiomodulation.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".