OPEN HOUSE 2018 NHK Appealed Atmospheric Stability of Inverted Structure Type OLED

May 24 - 27, OPEN HOUSE 2018 was held in NHK Science & Technology Research Laboratories. As regards display, sheet type OLEDs inclusive of 88-inch 8K display were exhibited, however, new technology was not found in this display. In this here, inverted structure type OLED is picked up.

Picture 2. Flexible inverted structure OLED

Picture 1. 88-inch 8K OLED

NHK's inverted structure type OLED is composed of ITO cathode/electron injection layer (EIL)/electron transporting layer (ETL)/emitting layer/hole transporting layer (HTL)/hole injection layer (HIL)/Al anode. In short, ITO and Al are used as cathode and anode respectively unlike of the conventional structure OLED.

ZnO inorganic material (by sputtering deposition) and Nippon Shokubai's secret organic material are stacked as EIL. Atmospheric stability of this device is superior compared to the conventional structure device because of non-use of alkali EIL. Picture 3 shows emission state of the inverted device and the conventional device. In both devices, OLED was manufactured on the plastic film substrate, and then, it was simply encapsulated by commercial seal material and the plastic film. As you can see, in the conventional device, a lot of non-emitting areas are observed after 21 days by occurrence of dark spot, on the other hand, in the inverted device, it is not almost observed even after 156 days. And also, atmospheric stability is superior in high-temperature and humidity environment, too.

Picture 3. Comparison of the conventional device (left) and the inverted device (right)

As concerns carrier injection property, practical hole injection property is gained because of low carrier injection barrier essentially. On the other hand, work function of as-deposition ITO is relatively high same as 4.7 eV, however, work function of EIL is 3.7 eV by stacking of ZnO. For this reason, electron injection property of the inverted device is almost same as that of the conventional device. In fact, EQE (external quantum efficiency) of red and green is approximate 20 % by use of phosphorescence, that of blue is 5 % because of use of fluorescence.