Recent progress in inverted OLEDs: Key materials, air stability, and application to flexible display (Conference Presentation)

Abstract

Although flexible optoelectronic devices can be easily fabricated by integrating OLEDs on flexible substrates, this technique suffers from the rapid growth of the non-emitting area due to the oxygen and moisture degradation of reactive electron injection layer materials such as alkali metals. Flexible substrates that can completely block oxygen and moisture are essential for extending the lifetime of flexible devices, but such flexible substrates cannot be easily fabricated. In recent years, inverted OLEDs (iOLEDs) with a bottom cathode have been intensively studied as an ideal structure for realizing air-stable OLEDs. As an alternative to the alkali metals that are commonly used in conventional OLEDs, metal oxides and organic interlayers such as polyethylene imine are employed in most reported iOLEDs. Despite the recent advances in the iOLED technology, the development of interlayers that can prevent the decrease in brightness caused by iOLED operation is lacking. Here, we report the design strategy of an interlayer for the fabrication of efficient and stable iOLEDs. The efficiency and the operational lifetime of the optimized iOLED were comparable to that of the conventional OLED that used the same emitter. Two flexible displays were fabricated to ascertain the feasibility of the application of the interlayer to real devices and the air stability of the iOLED-based devices: one using iOLEDs and the other using conventional OLEDs. The iOLED-based flexible display emits light over 1 year under the simplified encapsulation though the conventional OLED-based flexible display shows almost no luminosity only after 21 days under the same encapsulation.