深川 弘彦

We have developed a universal method to inject electrons into organic semiconductors without using unstable conventional n-dopants. The reduction of the injection barrier is induced by the formation of hydrogen bonds between a base, which is widely used as a catalyst in organic syntheses, and an organic semiconductor.

Both hole and electron injection layers are commonly used in recent OLEDs to reduce the injection barrier between electrodes and organic layers. This injection barrier originates from the energy difference between the work function (WF) of the electrode and the energy level of the organic layer. For instance, the hole injection barrier is defined as the energy difference between the Fermi level of the anode and the highest occupied molecular orbital (HOMO) level of the organic layer on the anode, as shown in Fig. 4.1a. Thus, an ideal hole injection material is the material that can make the surface WF of the anode larger (Fig. 4.1b). On the other hand, an ideal electron injection material is the material that can make the surface WF of the cathode smaller (Fig. 4.1c). Since the relationship between the WF of the electrode and the energy level of the organic layer is important in determining the carrier injection barrier, both material-dependent OLED characteristics and energy diagrams between the electrode and the injection materials have been extensively studied using a Kelvin probe (KP) and by ultraviolet photoelectron spectroscopy (UPS). In particular, hole injection materials have been intensively studied since the anode and the cathode of a conventional OLED are the bottom electrode and the top electrode, respectively. Alkali metals such as Li, Cs and Ba have been used as EILs in conventional OLEDs [1, 2] however, it was difficult to investigate the electronic structures at the organic layer/EIL/cathode interfaces since alkali metals can react with both the organic layer and the cathode [3]. On the other hand, the electronic structures at the anode/HIL interface in conventional OLEDs, which can be easily investigated by KP and UPS, have been intensively studied [4]. Similarly, the electronic structures at the cathode/EIL interface in inverted OLEDs have recently been studied. In addition, the electronic structures at the organic layer/HIL/anode interfaces in inverted OLEDs, which are as complicated as the electronic structures at organic layer/EIL/cathode interfaces, have also become the subject of investigation. In this chapter, we first explain the basic carrier injection mechanism and related electronic structures using examples of anode/HIL interfaces in conventional OLEDs that have been widely studied. Then, we introduce the electron/hole injection mechanism related to inverted OLEDs.

As discussed in Chap. 2, it is difficult to inject electrons from the electrode to the organic layer by employing only metal oxides in HOILEDs. To reduce the driving voltage of inverted OLEDs, the EIL in an inverted OLED should be prepared by incorporating metal oxides and an interlayer, as illustrated in Fig. 3.1. Accordingly, the work function (WF) of the EIL surface is reduced, which facilitates the electron injection (the relationship between surface WF and electron injection efficiency will be described in Chap. 4). From this viewpoint, various materials have been studied as interlayers, and improvements of inverted OLED characteristics using such interlayers have been demonstrated.

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.

We describe the development of an air-stable OLED and its application to a flexible display. The air-stable OLED is realized by employing a suitable electron injection layer. Dark spot formation is not observed after 250 days in the OLED encapsulated by a barrier film with a WVTR of 10-4 g/m 2/day. © 2014 Society for Information Display.

An oxide-TFT-driven flexible AMOLED display using inverted OLEDs was fabricated. The fabricated TFT backplane exhibited good electrical characteristics, and we confirmed the display could show color moving images.

We describe a highly efficient inverted OLED with high stability in air. The EQE of the inverted OLED is improved to above 15% by employing a phosphorescent material and an air-stable electron injection layer. No dark spot formation is observed after over 100 days for the optimized inverted OLED, which is encapsulated by a barrier film with a water vapor transmission rate of 10-4 g/m2/day. © 2013 Society for Information Display.