The market for devices based on organic semiconductors such as organic light emitting diodes (OLEDs) is continuously growing. Therefore, it is increasingly important to get detailed insight into the processes that take place in these materials. In Particular triplet states play an important role for OLEDs. The thesis presented here focuses on one hand on the properties of the transfer of triplet excitons and on the other hand on energetic processes of triplet excitations in organic semiconductors. The phosphorescence resulting from the relaxation of the triplet state was examined, in particular its time and temperature dependent change in intensity as well as the temperature dependent shift of its spectrum. By investigating temperature dependent diffusion rates, I could show that there exist two temperature ranges for diffusion. At low temperatures, diffusion is scarcely temperature activated whereas it is strongly activated at higher temperatures. The effective activation energy consists of a contribution from the energetic disorder due to the random distribution of the energy levels of an ensemble of chromophores and a second contribution which is related to the reorganization energy. The latter is dominating in the Marcus model, above a transition temperature, and describes the energy that must be applied due to changes in the electron density in order to allow subsequent changes in the positions of the nuclei. At low temperatures, thermal activation energy is hardly available and tunneling processes occur. Here, the contribution of the energetic disorder dominates. This second mechanism can be described by the so called Miller-Abrahams rate. In order to elucidate the influence of energetic disorder and reorganization energy on the transfer of triplet excitons, a material system on the basis of poly(p-phenylene) was analyzed in which both parameters were systematically varied. Compared to a model developed by theoreticians, a deviation of the experimentally determined parameters occurs at low temperatures. Measurements of the spectral diffusion of triplet excitons revealed that the assumption of a thermal equilibrium does not always hold, and this can sometimes lead to frustration of the spectral relaxation. To check the experimental results, additional Monte Carlo simulations were performed. It could be shown that first the occurrence of frustration could be reproduced. Second the experimentally estimated triplet diffusion rates can be described by Miller-Abrahams rates at low temperatures and Marcus rates in the high temperature range and thus applied to triplet transfer. In the second part of the thesis the importance of triplet states in materials used for electronic devices was studied. Triplet states play a significant role for the host and guest materials used in OLEDs. CBP (4,4 '-bis (N-carbazolyl) -2,2'-biphenyl) derivatives were investigated as host materials. All non-radiative decay processes in the host material lead to a poor efficiency of the OLED. Therefore, processes taking place in these materials have to be elucidated. Triplet levels in CBP are normally high enough to prevent energy transfer from taking place in the host. In thin films, however, a special feature could be observed. Depending on the substituents of CBP derivatives, the formation of a triplet sandwich excimer, in which the carbazole units of two molecules overlap, takes place. Because of the good wave function overlap of the carbazole units the excimer stabilization energy is very high. This can reduce the efficiency of the device by unwanted energy transfer from guest to host material. Blue emitting Iridium complexes were analyzed as an example for host materials. Because of the high triplet energies required, it is a challenge to synthesize efficient blue triplet emitters. In a series of such complexes it was possible to identify an intramolecular energy transfer to the ancillary ligand in a blue emitting but inefficient complex using spectroscopy and quantum chemical calculations. Possible approaches to overcome these efficiency reducing processes could be shown. To summarize, it can be said that the work presented elucidates important and fundamental aspects that enhance the understanding of the transfer of triplet excitons and can generally serve as a model for charge transfer. Furthermore, it was shown how important it is to understand efficiency limiting processes and states in host as well as in guest systems in order to develop strategies to prevent these processes.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
|Keywords:||Organic light emitting diodes , Triplet states, Transfer of triplet excitons|
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