Led by KAIST Prof. Yoo Seung-Hyup (School of Electrical Engineering) and Postech Prof. Lee Tae Woo (Department of Materials Science and Engineering), the joint research team used graphene, titanium dioxide, and a conductive polymer as a composite electrode to develop OLED technology characterized by outstanding flexibility.
Flexible OLEDs are currently used for smartphones with edgy displays and curved OLED televisions, but these flexible OLED displays can be applied only by bending them into a curved shape.
It is important for a flexible OLED to maintain its components unbroken and undamaged when it is repeatedly bent.
Graphene features excellent flexibility, electric properties, and optical transparency based on its thinness, and such outstanding properties make it suitable to address the breakage of the oxide transparent electrode commonly used for OLEDs.
In general, the luminous efficiency of OLEDs can be improved through a resonance phenomenon, and a transparent electrode with a minimum level of light reflection is required to induce the resonance phenomenon. The use of graphene as the only component for a transparent electrode results in less reflection and low luminous efficiency.
To address the flexibility and efficiency issues, the research team developed a composite electrode layer that combines the existing graphene with titanium dioxide and a conductive polymer. Each electrode layer mutually compensates the other layer’s drawbacks, thereby maximizing the resonance effect.
Composite electrode layers use both the high refractive index of titanium dioxide and the low refractive index of the conductive polymer. The effective reflexibility from the electrodes can be used for the resonance phenomenon, and the low refractive index of the conductive polymer restricts the efficiency reduction due to the loss of surface plasmons.
The research team’s OLED realized 40.5% external quantum efficiency, which is 1.5 times higher than the existing efficiency of 27.4%.
The team also observed that titanium dioxide’s original capability to prevent breakage when layers are bent helps the composite material bear four times greater transformation than the existing oxide transparent electrodes.
The recently developed flexible OLED satisfies high performance and flexibility simultaneously as the luminosity property remains the same despite 1,000 times bending at a 2.3mm radius of curvature.
“If not for the past convergent research, our study would have not been possible,” explains Prof. Yoo at KAIST. “The recent research outcomes will lay a significant foundation for the success of flexible, wearable displays or flexible light sources of sensors to be worn on the human body.”
The research outcomes were published in the online edition of Nature Communications on June 2.