Identification and optimization of mixed light emitting layers in phosphorescent multilayer OLEDs
Phosphorescent OLEDs (PHOLEDs) based on organic small molecules offer a great potential for a variety of future applications. In terms of efficiency PHOLEDs can already compete with conventional light sources. However, PHOLEDs typically suffer from a steady decrease in efficiency at high current densities, known as efficiency roll-off. One of the reasons for this is the high probability for triplet-triplet quenching processes. On the one hand these processes happen due to the long lifetime of the triplet phosphors and on the other hand due to the high triplet density in the rather narrow recombination zone in common OLED heterostructures. One approach to minimize this roll-off is to introduce a mixed host structure in the light emitting layer. This is done by mixing one mostly hole-transporting material and one mostly electron-transporting material with each other. In the first part of this thesis, the characteristics of PHOLEDs with a conventional heterostructure, a uniformly mixed host structure and a graded mixed host structure were studied in detail. PHOLEDs with a graded mixed structure showed an increased quantum efficiency, a reduced efficiency roll-off and a longer lifetime compared to them with conventional heterostructure. These effects are attributed to the broadening of the recombination zone and a better charge balance inside the light emitting layer. Existing techniques for the detection of intermixing are very expensive and time consuming. In the second part of this thesis, it was tested whether impedance spectroscopy can be used as a suitable and cheap tool to measure the degree of intermixing between two layers. Measurements of the single materials were made to extract their characteristic parameters. Two fit functions were formulated and applied to measurements of devices with different degrees of intermixing. One of the fit functions was able to identify intermixing for all tested devices very well.
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