Alexander Karbach

PEDT/PSS for efficient hole-injection in hybrid organic light-emitting diodes

Light-emitting diodes have been prepared by depositing three organic layers successively by spin-coat and evaporation techniques. The first layer of PEDT/PSS smoothens the ITO surface, reduces the probability of electrical shorts and is beneficial for a high overall yield of the operating devices. Above all, this layer promotes hole-injection as an important parameter for higher efficiencies and prolonged operation life. The second layer of spin-coated dendritic phenylamines (TDAPB) with high glass transition temperature modulates the injection of holes into the emitting layer, formed by evaporated Alq. By comparing characteristics and operation lifetime data of devices with and without PEDT/PSS it is shown that the combination of polymeric and monomeric organic layers leads to highly efficient devices, opening new ways to modify device architectures.

Surface roughness effects and their influence on the degradation of organic light emitting devices

Organic light emitting devices typically consist of one or several organic layers which are sandwiched between two electrodes, one of which has to be transparent. In most cases indium tin oxide (ITO) is employed as the transparent, hole-injecting anode material. Usually, the functional organic layers possess a thickness of about 100 nm. For such thin films the homogeneity and the surface roughness are especially important factors for the device performance. Therefore, the surface roughness of all those layers which are the basis for subsequent deposition processes were systematically studied by atomic force microscopy (AFM). For these investigations both the ITO substrate and the layers consisting of different organic materials deposited onto the ITO substrate were analyzed. In addition, the two different basic deposition methods for the organic materials, namely the deposition from solution by spin coating and the deposition by thermal evaporation, were compared to one another with respect to their resulting surface roughness. It was found that the large surface roughness of the ITO substrate induces layer inhomogeneities, especially for the vapor deposited organic layers. They can be reduced by the incorporation of a polymeric smoothing layer.

Bound rubber morphology and loss tangent properties of carbon-black-filled rubber compounds

The bound rubber phenomenon of carbon-black-filled rubber compounds, which is still an intensively discussed subject, is visualized in this research as a stable nanoscale interphase. Using the novel amplitude and phase-modified atomic force microscope technique, a viscoelastic mapping mode, it becomes possible to quantify mechanical loss tangent properties that are defined as the ratio of loss modulus G″ to storage modulus G′. Imaging loss tangent enables the observation of separated energy dissipation of single constituents within a blend system as well as bound rubber dimensions. Determined with the conventional quantification of insoluble rubber, the amount of bound rubber is correlated with values from the analytical evaluation of loss tangent images. Comparing the loss tangent images and histograms to dynamic mechanical analyses allows the characterization of each single component. On the base of the time-temperature superposition principle, bound rubber dimensions and mechanical properties of filled compounds can be optimized.

Annealing effects on the electrical conductivity of polymerized liquid crystalline 3,4-ethylenedioxythiophene derivatives

Abstract 3,4-Ethylenedioxythiophene derivatives with aromatic, in most cases mesogenic, side groups were synthesized and their liquid crystal behaviour was characterized. These monomers were polymerized oxidatively to charged, electrically conductive polythiophenes. X-ray and atomic force microscopy studies were performed. Films of theses polythiophenes achieved via in situ polymerization were prone to a significant increase of the conductivity by annealing.