深川 弘彦

We introduce the importance of air stability on the basis of the principles and the history of organic light-emitting diodes (OLEDs), and the way of realising air-stable OLEDs finally. OLEDs are current-driven self-emitting devices that, in principle, have the features of lightness and thinness. Therefore OLEDs are expected to have unprecedented flexibility. However, it is difficult to achieve air stability in OLEDs, which is the key property for the realisation of flexible devices, because it has been essential to use air-active materials in consideration of the operational mechanism. The trend toward lighter products, thinner devices, and greater toughness is inevitable. Lightweight products conserve resources, leading to an environmentally friendly society. Products based on thin devices are attractive to consumers, leading to a creative society. Tough devices increase the reliability of products, ensuring a safe society. In the field of electronic devices, substrates have shifted from metals to glass, and now to polymers, and we are now in a transition period between the use of glass and polymer substrates. Organic electronics have been developed during this period and their future development in line with the above trend is expected. Although organic light-emitting diodes (OLEDs) are one of the most promising types of device, there are major challenges to be overcome in their fabrication. The stability of the performance of OLEDs in air has been an important and major challenge since the principles of existing organic EL devices have been published. OLEDs are current-driven self-emitting devices and also serve as practical devices, in which carriers move through the organic/organic interface and through the organic/inorganic interface[1–4]

To achieve air stability in organic light-emitting diodes (OLEDs), a metal oxide layer was introduced on the cathode side. In these novel OLEDs, referred to as hybrid organic-inorganic light-emitting diodes (HOILEDs), an inverted structure was better than the conventional one from the viewpoint of the fabrication process. These HOILEDs have insufficient performance for practical use, even though the electron-injection barrier was reduced by the insertion of the metal oxide layer. In this chapter, by introducing the history of development of HOILED, we will explain the purpose of inverted OLED and the essential problem as the cornerstone of current inverted OLED (iOLED).

It was found that stable bases widely used in organic syntheses as catalysts can lower the electron injection barrier in organic light-emitting diodes. In contrast to conventional n-doping, the reduction of the injection barrier caused by adding bases is induced by the formation of hydrogen bonds between hosts and bases.