Carrier Injection Mechanism

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.