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Columnar liquid crystals are useful for the construction of organic light emitting diodes (OLED) due to their high charge carrier mobility and their ability of developing defect-free superstructures. We investigated the electroluminescent properties of various mesogenic derivatives of triphenylene, pyrene, perylene, benzoperylene and hexa-perihexabenzocoronene (HBC) which combine good charge carrying properties and luminescence. The wavelengths of maximum electroluminescence range from about 400 nm to 720 nm, depending on the core and the substitution of the molecules. Nearly white emission can be obtained in samples containing two or more emitting layers. The perylene core is so efficient that even a single organic layer between an ITO anode and an aluminum cathode can give luminance values up to 100 cd/m 2 . In addition to the electroluminescence, we present results on the photovoltaics in hetero junction structures of the compounds. Samples with two organic layers made of a HBC-and a perylene derivative show an open circuit voltage V oc of about 0.8 V, a short circuit current I sc of a few µA/cm 2 , and a filling factor FF of approximately 31 %.

Beyond conventional electrooptic applications, liquid crystals become increasingly important in the topical field of organic electronics. Here, some fundamental findings are reviewed and two examples for the combination of liquid crystals and organic semiconductors are described in more detail. The first part of the paper describes the electroluminescent properties of very thin layers of aligned p-(phenylene vinylene) oligomers, which are embedded in a stack of thin organic semiconductor layers and sandwiched between two electrodes. Both the wavelength of the emitted luminescence (varying from green to red) and the dichroic ratio increase with increasing length of the aromatic backbone of the molecules. High brightness and low threshold voltages could be achieved. The photorefractive system described in the second part of the paper consists of small droplets of a low molar mass liquid crystal, which are dispersed in a photoconducting polymer. Two-beam coupling experiments indicate a high performance at reasonable external voltages. From dynamic diffraction measurements, the amplitude of the internal space charge field can be estimated.