- New Carbazole Based Materials for Optoelectronic Applications (2006)
- The motivation for this thesis was the synthesis, characterization and the testing of new, environmentally stable materials based on aromatic amines for OFET and OLED applications. The preparation of high quality thin films from solution as well as from the gas phase was an another important issue. The first part of this thesis deals with star-shaped molecular glasses with triphenylamine as core molecule. Substituted fluorene and carbazole units were attached to the core molecule as side arms via a trifold Suzuki cross coupling reaction. The target compounds were highly purified by medium pressure liquid chromatography (MPLC) as purity is an important prerequisite for organic materials to be used for optoelectronic applications. Before the new materials were finally tested in transistor devices, a suitable surface treatment of the OFET substrates was developed. By introducing self-assembled monolayers, prepared from hexamethyldisilazane (HMDS), on top of the SiO2 insulator layer of the FET substrates, the field-effect mobility was increased by at least one order of magnitude. Furthermore it was possible to improve on/off-ratios as well as turn on voltages. In conclusion hole carrier mobilities up to 3 x10-4 cm2/Vs and on/off-ratios of 10000 were achieved from the new star-shaped compounds. The performance of the devices was not affected by a four month storage period in air and daylight. The second part of this thesis describes the synthesis and characterization of a new class of fused heterocycles based on carbazole units. For this issue a series of bisindenocarbazoles is introduced as a new class of fused heterocycles. A synthetical procedure was designed which allows to tailor the thermal properties of the target compounds by introducing different alkyl substituents in the very last step of the synthesis. Yields up to 50 % can be obtained after six synthetical steps including purification of the intermediates. CV measurements showed the electrochemical stability of the novel compounds. Altogether five bisindenocarbazoles with different alkyl substitution patterns have been prepared and characterized. Their morphology varies from highly crystalline materials with short alkyl side chains to amorphous molecular glasses if longer or branched alkyl groups are attached to the core. As the bisindenocarbazoles exhibit a bright blue fluorescence together with high quantum yields, they were tested as blue emitter for OLED applications. In typical setups for blue light emitting LEDs, the blue emitter is doped into a wide band gap host material in order to avoid quenching of the electroluminescence and to adjust the energy levels of the different materials used in the setup. For this issue CBP, mCP and TCTA were tested as matrix materials together with a bisindenocarbazole as emitter. A combinatorial evaporation setup was used for the preparation of the OLED devices in order to dope the different host systems by co-evaporation of the bisindenocarbazole dye. This deposition method also allows the variation of the film thicknesses of the charge transport layers in a single experiment. By using this device architecture a deep-blue emission from the bisindenocarbazole dye at CIE color coordinates of x = 0.19 and y = 0.17 was obtained at a hole blocking layer thickness of 40 nm. Luminance values up to 200 cd/m2 were achieved with this series of devices. The turn on of the light emission was observed at 5 V. These very first results show that the bisindenocarbazole is a promising new blue fluorescent emitter for organic LEDs. Due to the rigid rod-like core of the bisindenocarbazole it was possible to obtain a novel derivative exhibiting a broad nematic mesophase by extending the core with aromatic side groups. By using a suitable alignment method, it is possible to obtain well ordered LC monodomains from which increased charge carrier mobilities can be obtained. The nematic LC phase was characterized with polarizing microscopy (POM) and small angle X-ray scattering (SAXS).