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Designing novel host materials for blue phosphorescent organic light-emitting diodes
(2009)
- The overall efficiency of an organic light-emitting diode (OLED) is always limited to the efficiency of its individual components. The most important component is the emission layer, where excitons are formed and light is generated. This thesis deals with the improvement of one class of OLED component, namely host materials for blue phosphorescent emitters. Three generations of 1,3,5-triazine-based materials with varying donor-substituents are presented in this work. In the first generation carbazole units are bound to the triazine core. The second generation consists of triazines with diarylamino substituents. The third generation compounds are a combination of disubtituted triazines from the first two generations and a phenoxy-carbazole unit. While the first and second generation comprise substituents that are directly bound to the triazine core, in the third generation triazines the phenylcarbazole-donor is attached via a nonconjugated ether bond. Within each generation various properties are tailored to fulfill the complex profile of requirements for host materials. Known nucleophilic substitution reactions were further improved to enable the efficient synthesis of novel host materials in very high purity and high yields. The sequential replacement of the chlorines of cyanuric chloride is dependent on temperature, actual ring substitution and the nature of the nucleophile. Effective methods were developed to yield asymmetrically substituted triazines in a controlled manner. The thermal properties, including the thermal stability to enable the processing by vapor deposition and the glass forming properties to result in a morphological stability of prepared thin emission layers, were controlled by systematic investigation of different substitution patterns. Thus, glass transition temperatures up to 170 °C are presented. Studies of the long term stability of amorphous host films, carried out for several materials, revealed its importance for long term efficient devices. The electrochemical properties of the novel compounds were investigated by cyclic voltammetry to study the energetic position of the HOMO and the LUMO as well as the stability of the material upon oxidation and reduction. Using this method the injection properties of the materials were determined. The blocking of activated positions resulted in reversible redox behavior. Furthermore the ionization potential was decreased for the third generation triazines to yield an improved hole injection into these materials. Additionally computational calculations were carried out to understand and further improve the energy levels by substituent exchange. This led directly to the development of bipolar host materials with separated hole and electron transport units within one molecule. Furthermore single carrier devices were fabricated to demonstrate the benefits of the transport bipolar characteristics. For the efficient operation of a device the triplet energy of the host material has to be higher compared to the emitter. First generation triazines exhibit triplet energies up to 2.96 eV and therefore enable the use of light and middle blue phosphorescent emitters. Second generation triazines comprise exceptionally high triplet energies up to 3.24 eV. These are amongst the highest values reported in the literature and facilitate the use of deep blue phosphorescent emitters. For hosts of the third generation the triplet energy depends on the choice of the triazine moiety. They are therefore suited for light and deep blue emitter. Extensive photo physical characterizations of all materials have been carried out in solutions, neat films and doped films. Energy transfer experiments with several emitters additionally gained valuable information about the compatibility of host and guest molecules All generations of triazines are tested as host material in OLEDs. The optimization of the device configurations was carried out by combinatorial evaporation. The sequential adaption of layer thickness and composition helped to improve the device performance. The stepwise optimization of the host material properties resulted in an enduring progression concerning the luminance and efficiency. For the third generation triazines 11.5 % external quantum efficiency and a high brightness of 33000 cd/m2 were achieved.
