Hole injection efficiency from metals

Figure 6. Hole injection efficiency figure of merit for substrate contacts of varying work function vs. energy step across the contact polymer interface estimated from published work function data and electrochemical redox potential data. The height of each bar reflects the variability in injection efficiency due primarily to variation in substrate surface pretreatment and for the particular case of Au, diffusion to the interface of metal atoms from underlying binder layers.

Judging from the present OLED status, the most important research was Partridge s report on the EL utilized poly(vinyl carbazole) (PVCz) thin films in 1982.26-29 He used the 500-nm-thick PVCz thin films doped with fluorescent molecules as an emissive center, equipped with the efficient hole-injection electrode (SbCls/PVCz) and the electron-injection electrode (cesium) as a low-workfunction metal. Although no quantitative measurement of luminance was described, surprisingly the injection current density reached 1 mA/cm2. Nowadays, we can fabricate very similar OLED devices with superior EL performance. Thus, Partridge s device contributed to establishing the prototypes of present OLED devices. 

An important issue for the performance of an organic electronic device like an OFET is the injection of charge carriers, electrons or holes, from the electrode into the organic material. In case of the commonly used metal electrodes an efficient electron injection is possible only if the Fermi level of the metal and the energy of the lowest unoccupied molecular orbital (LUMO) of the organic material differs by a small amount only. A similar statement applies for hole injection, in this case the position of the highest occupied molecular orbital (HOMO) has to match with the position of the Fermi level. When noble metals, in particular Au, are being used for an electrode one may naively assume... 

As for a6T, Mg led to superior rectifying behaviour for ECnT thin films, whereas the electroluminescence yields are not enhanced for lower workfunction electrodes [313], The latter unexpected effect could be due to different indiflusion of the metals or chemical reactions at the interface, as discussed in Section 6.1. From asymmetric cells comprising two different ECnTs it can be concluded that electroluminescence always takes place in the layer at the electron-injecting electrode. Thus the hole-injection and transport seems to be more efficient if compared with the electron transfer. The majority charge carriers are therefore holes injected at the ITO electrode. 

Even when the energetic requirements for electron transfer are satisfied, some oxidants with sufficiently positive electrochemical potentials, particularly H2O2, can exhibit poor hole injection kinetics into bare Si. Hole injection can be efficiently catalyzed by the presence of a metal, which can affect reactivity in two ways. The first is simply that since it is a metal, it provides state density from E f all the way down to the valence band maximum. In effect, the metal can short-circuit the connection between the oxidant and the valence band. Second, the metal can act electrocatalytically to enhance either electrochemical or chemical steps in the redox reactions required to achieve etching. Thus, metals such as Au and Pt both can act as electron/hole conduits, but R is particularly effective at electrocatalytically enhancing reactions of H, H2O, H2O2, and O2. 

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