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Novel Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes
(2011)
- Organic light-emitting diodes (OLEDs) have been commercially used in full-colour active matrix (AMOLED) displays for a couple of years. Only recently, a new application of OLEDs in the field of lighting has been opened up. For white emission monochrome systems of the three primary colours red, green and blue need to be combined. The major issue from the materials’ point of view is still the lack of stable host-emitter systems for blue emission. This thesis deals with the development of new host materials for blue phosphorescent emitters. The host material has to meet a complex profile of requirements. As most crucial feature the triplet energy of the host material has to exceed the triplet energy of the emitter. An increase of triplet energy of the host material is achieved by reducing the conjugated π-system in the host molecule. This thesis describes three synthetic approaches to high triplet energies by confining the π-conjugation: by introducing torsion in the molecular structure, by choosing a meta-linkage and by a non-conjugated linkage. The first and second approach was applied to carbazole-based host materials, whereas the third was demonstrated on phosphazene-based host materials. In the first approach, the molecular structure of a well-known carbazole-based host material, 4,4’-bis(carbazol-9-yl)-2,2’-biphenyl (CBP), was optimised by introducing torsion via methyl or trifluoromethyl substituents in the 2- and 2’-positions of the central biphenyl moiety to yield twisted CBP-derivatives. By confining the conjugated system in combination with selective methyl substitution a series of host materials with superior thermal and photophysical properties was obtained. Compared with the triplet energy of 2.58 eV for CBP, high triplet energies of 2.95 eV could be realised for the twisted CBP-derivatives. In addition, appropriate substitution of the crystalline CBP results in amorphous materials with high glass transition temperatures of up to 120°C. In cyclic voltammetry the electrochemical properties were studied. Here, it was found that the systematic variation of the substitution patterns enables fine-tuning of the energetic positions of the HOMO and LUMO. This helps to avoid injection barriers at materials’ interfaces in the OLED device. By blocking the activated sites in the host molecules a stability of the electrochemically oxidised species against dimerisation could be demonstrated. In the second approach, the conjugation in the same parent carbazole-based compound CBP was reduced by choosing a meta-type of linkage instead of the common para-linkage of the carbazole substituents to the central biphenyl unit. As a result of the meta-linkage, triplet energies of more than 2.90 eV were achieved. No further increase in triplet energy was observed by introducing additional torsion in the molecular structure as described in the first approach. Moreover, the thermal properties were optimised by selective methyl substitution to yield host materials with glass forming properties and high glass transition temperatures of up to 120°C. All host materials were tested in a comparative OLED device study in combination with a phosphorescent emitter with saturated blue emission. For the best host material of this series an external quantum efficiency of 9.7 % and a high brightness of 10 800 cd/m2 were achieved. Both series of carbazole based host materials – the twisted and the meta-linked CBP-derivatives – were synthesised by Ullmann reaction of a dihalogenated biphenyl unit with two (substituted) carbazole units under classic conditions. Noteworthy is the intermediate 5,5’-diiodo-2,2’-dimethyl-biphenyl – a simple and versatile building block in the synthesis of materials with confined conjugation. The synthesis by direct iodination of 2,2’-dimethylbiphenyl, to the best of our knowledge, has not been described in literature before. In the third approach, the class of low molecular weight phosphazenes, which is less described in the context of OLED-materials, was chosen as hosts for blue phosphorescent emitters. As a common characteristic all host materials consist of a six-membered ring of alternating phosphorus and nitrogen atoms. Each phosphorus atom bears two aromatic substituents attached via a non-conjugated linkage. Depending on the type of linkage to the central phosphazene core two sets of host materials can be distinguished: phenoxy substituted phosphazenes with phosphorus-oxygen bonds and phenyl substituted phosphazenes with phosphorus-carbon bonds. The phenoxy substituted derivatives were synthesized by nucleophilic substitution of the chlorine atoms in hexachlorocyclotriphosphazene with phenolates as nucleophils whereas the phenyl substituted derivatives were prepared by cyclocondensation of three equivalents of phosphinic amides. Due to their superior thermal properties compared to the phenoxy substituted series the phenyl substituted phosphazenes are better suited for the use in OLED devices. They exhibit particularly high triplet energies of up to 3.4 eV. Thus, they can be combined with deep blue phosphorescent emitters. Another specialty of the phenyl substituted phosphazenes is a balanced charge carrier transport characteristic. To conclude, each of the three presented approaches yields host materials with triplet energies high enough for a combination with blue phosphorescent emitters. Regarding the morphological stability the extensively studied carbazole based host materials exceed the novel phosphazene based host materials.
