Hole injection properties

O. Stephan, F. Tran-Van, and C. Chevrot, New organic materials for light emitting devices based on dihexylfluorene-co-ethylenedioxythiophene copolymers exhibiting improved hole-injecting properties, Synth. Met., 131 31 40, 2002. 

The hole injection properties of PA-based compounds were investigated and cross-compared by V, DISCLC, and AS measurements. We demonstrate that PEDOTPSS can act as an excellent hole injection material for PA compounds... 

Linear PPE-PPV hybrids of the meta and the para varieties (6, 61-64) have been produced by Sonogashira coupling and alkyne metathesis. These polymers show properties that are intermediate between those of the PPVs and the PPEs. Klemm et al. have shown that Horner reactions of suitable substrates lead to linear PPE-PPV hybrids, although the degree of polymerization is not particularly high [98]. The motivation behind these synthetic efforts is to combine the favorable hole injection properties of the PPVs with the blue emission, the stability, and the high fluorescence quantum yields of the PPEs. Several attempts have been made to obtain efficient LED action, but 62, the best one, so far shows a luminance of 147 cd/m at 602 nm, at 14 V, which is not very impressive compared with the numbers that are obtained for PPVs [99-102]. Cross-conjugated PPEs were first synthesized by Bunz et al. (63, 64) and are useful as photodiode-active materials [102]. The dialkylamino substituents in... 

It has been proposed to modify the redox state of PEDOT to tune the hole injection properties of PEDOT in polymeric LEDs and organic photovoltaic cells [69-72]. However, in most cases the effort associated with tuning the oxidation state results in additional processing expense and therefore PEDOT is preferred in the pristine state. 

Fig. 24 Band diagram of a blend of OCiCio-PPV and PCBM sandwiched in between two gold contacts. Due to the blending of both materials an effectively reduced transport gap is obtained leading to more balanced electron and hole injection properties. Adapted with permission from Macmillan Publishers Ltd Nature Materials [35], copyright (2003)...

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

Another approach to molecular assembly involves siloxane chemistry [61]. In this method, the electrically or optically active oligomers are terminated with tii-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular monolayer on the electrode surface, in order to modify its injection properties. 

Table 16-6. Electrical properties and efficiencies of single-layer and double-layer OPVS-LEDs with 1TO hole-injecting contacts in forward-bias operation.

However, a phase separation problem in the above two-component system may affect the device stability. To overcome the problem, Mullen and coworkers [322] introduced triphenylamine groups as side chains at the PF backbone. The triphenylamine substituents simultaneously improve the hole-transport properties of PF (facilitating the injection of holes),... 

The HIL acts as an interface connection layer between the anode and the HTL so as to improve the film forming property of the subsequent organic layer and to facilitate efficient hole injection. Hole injection materials (HIMs) should have good adhesion to the anode and should serve to smooth the anode surface. The most common HIMs are... 

Compared to Alqa, the oxadiazoles have a lower tendency toward reduction and thus a higher barrier for electron injection. Spiro-PBD (140), for example, can accept four electrons, the first electron transfer (merged wave for two electrons) taking place at -2.46 eV vs. Fc/Fc" " (Fig. 3.29) [87]. The oxadiazoles 17a, 18, and 29 exhibit reversible reduction waves at -2.39 and -2.18 eV and an irreversible reduction, respectively [238]. Since the HOMO-LUMO gap is >1 eV larger than for Alqs, the hole blocking properties are better for the oxadiazoles. 

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