Metal—organic interface injection barrier

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

FIGURE 2.4.2 (a) Simple band line-up diagram for a metal-organic semiconductor interface assuming that the Mott-Schottky rule holds and that the vacuum levels for the metal and semiconductor are aligned, (b) Application of a positive bias to the metal can result in hole injection into the semiconductor by thermionic emission over the barrier, (c) Band line-up diagram in the case where an interface dipole is present, causing a shift (A) in the vacuum levels across the junction. 

This offset is in turn a good estimate of the potential barrier to hole injection from the metal to the semiconductor. As shown in Figure 2.4.2(b), applying a positive bias to the metal relative to the semiconductor can result in hole injection (so-called thermionic emission) into the valence band if the holes can surmount the barrier Ep -EyA similar discussion holds for electron injection into the conduction band, but in that case the metal is biased negative and the barrier is determined by Ec - Ep. Thus, the electronic strnctnre of metal-organic semicondnctor interfaces plays a crucial role in determining the charge transport characteristics of the contact. 

In all of these approaches, the injection process is dominated by the charge injection barrier at the interfaces between the active layer and the metal electrodes, which is defined as the energy separation between Fermi-level Ep of the electrode and HOMO (or LUMO) of the organic layer. Thus, it is necessary to consider electronic properties and band alignment of the MO interfaces. 

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