Stephan A. Benning

Color-Tuned Electroluminescence from Columnar Liquid Crystalline Alkyl Arenecarboxylates

Simple ester derivatives of polycyclic arenes offer access to light-emitting diodes of nearly any visible color by making use of the good charge-transport properties of the columnar liquid crystals of these derivatives. The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester (structure shown, R=Et). Through use of multiple layers of different esters light-emitting diodes with almost white luminescence can be obtained.

Fluorescent columnar liquid crystalline 3,4,9,10-tetra-(n-alkoxycarbonyl)-perylenes

The title compounds were recently demonstrated to be useful as luminescent electron transport materials in organic light emitting diodes. Here, we present studies of the fluorescence of the homologues with chain lengths between 2 and 10 carbon atoms. Dilute solutions of the investigated compounds show absorption in the range between 410 and 490 nm and fluorescence between 475 and 555 nm. However, the columnar phase of the pure compounds exhibits fluorescence at considerably larger wavelength (550-650 nm) and gives larger fluorescence lifetimes. This behaviour can be explained by the formation of excimers in the mesophase. The phase transitions to the liquid crystal state are associated with a distinct change of the fluorescence intensity.

Absorption and luminescence spectra of electroluminescent liquid crystals with triphenylene, pyrene and perylene units

New derivatives of triphenylene, perylene, and pyrene are described, some of which form columnar mesophases. The absorption spectra are investigated both experimentally and theoretically. The spectra calculated using the density functional tight binding (DFTB) theory are in good agreement with experimental results. The investigated compounds show photoluminescence of violet-blue (triphenylene), yellow-green (pyrene), and orange-red (perylene) colours. In addition, electroluminescence is observed in thin films of these compounds between a positively charged ITO electrode and a negatively charged aluminium electrode. The brightness of the electroluminescence decreases in the order perylene > pyrene > triphenylene. Threshold voltages below 20 V and a luminance up to 100 cd m-2 were observed.

Polarized electroluminescence of a discotic mesogenic compound

Polarized electroluminescence is observed for a uniaxially aligned sample of a discotic compound in its crystalline state. The alignment was achieved by coating the ITO anode with an aligned film of poly(tetrafluoroethylene) (PTFE) deposited using the friction method. Organic light emitting devices with the layer sequence glass/ITO/PTFE/tetraethyl perylene-3,4,9,10-tetracarboxylate (55 nm)/aluminium (100 nm) show linearly polarized emission of light with a dichroic ratio up to 3.2.

ORGANIC HETEROJUNCTION PHOTOVOLTAIC CELLS MADE OF DISCOTIC, MESOGENIC MATERIALS

We made photovoltaic cells containing two organic layers. The hole-conducting layer consists of a hexa-peri-hexabenzocoronene (HBC) derivative. The electron-conducting component is an alkylester of perylene tetracarboxylic acid. Both compounds form columnar mesophases which can provide 1D pathways for transport. The current/voltage-characteristics of the two-layer devices indicate a photovoltaic effect with an open circuit voltage of Voc ≈ 0.8 V, a short circuit current of Isc ≈ 3.9 μA/cm2, and a filling factor of FF ≤ 31%. The spectral distribution of the photocurrent of different samples indicates that excitations of the perylene derivative or of the HBC derivative both can lead to a photovoltaic effect.

Electroconvection of liquid crystals : Tool for fabricating modulated polymer surfaces

Electrically induced dissipative structures in liquid crystals can be stored by the photocuring of reactive mesogens. Thus, spontaneous pattern formation can be used to fabricate phase gratings or polymer films with a periodic surface modulation from an initially uniform sample. Different patterns, both in the conductive and in the dielectric regimes, have been stored. Photopolymerization and subsequent investigation by atomic force microscopy provide a useful tool in analyzing dissipative structures, even if the lattice constants are smaller than 1 μm. This method indicates surface modulations with amplitudes of up to 12 nm and periodicities between 800 nm and several tens of micrometers. The influences of composition and curing conditions on the topography have been studied.

Electroluminescence and photovoltaic effects using columnar mesogenic compounds

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-peri-hexabenzocoronene (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/m2. 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 Voc of about 0.8 V, a short circuit current Isc of a few μA/cm2, and a filling factor FF of about 31 %.

Bold relief fabrication by means of electroconvection: basic properties of a suitable mixture

Electroconvection of liquid crystals does not only lead to a large variety of spontaneously formed patterns, but these patterns can also be used as a template for the fabrication of polymer surfaces exhibiting a periodic bold relief. Here, a useful room-temperature nematic mixture consisting of two photostable liquid crystals and a photosensitive cross-linker is presented. Its stability diagram in the frequency-voltage plane, the influence of monomer concentration and the topography of the resulting polymer surfaces are described.

Optoelectronic and photonic properties of liquid crystals: electroluminescence and photorefractivity

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.