F. Meghdadi

MULTICOLOR MULTILAYER LIGHT-EMITTING DEVICES BASED ON PYRIDINE-CONTAINING CONJUGATED POLYMERS AND PARA-SEXIPHENYL OLIGOMER

There is increased interest in developing multicolor light-emitting devices. We report here the fabrication and study of multilayer color-variable/white light-emitting devices based on pyridine-containing conjugated polymers and para-sexiphenyl (6P) oligomer. Voltage-dependent multicolor emission was observed in both bilayer and trilayer configurations. The emission colors of single devices cover a wide range of visible spectra whose Commission International de I’Eclairage (CIE) color coordinates vary from blue to white to green with increasing voltages. The color coordinate traverses along a straight line in the CIE chromaticity diagram. The role of interfaces in the operation of devices is discussed.

Blue electroluminescence with ladder-type poly(para-phenylene) and para-hexaphenyl

Abstract Conjugated polymers and oligomers are very interesting materials, with a number of possible electronic, optoelectronic and photonic applications. New synthesis techniques allow the preparation of extremely pure soluble ladder-type poly(para-phenylenes) (LPPPs), showing a steepness at the band edge comparable to conventional semiconductors. One consequence of this high purity is an improved photoluminescence quantum yield, around 30% for thin films. Oligomers of poly(poly-phenylene), like para-hexaphenyl (PHP) can be produced with extremely high purity, either as homogeneous disordered layers or well-ordered thin films, which show a high photoluminescence quantum yield in excess of 30%. All these materials are applied as the active layer in light-emitting devices (LEDs), giving bright blue light electroluminescence (EL) emission, and we report on their performance and efficiencies. For the LEd device with LPPP as the active layer, we observe the maximum of the EL emission around 460 nm, whereas for the PHP-based EL devices the emission maximum is located between 400 and 450 nm. The light emission of these EL devices is very high and exceeds a luminance of 2000 cd/m2 for a typical PHP multi-heterolayer EL device. For the overall EL quantum efficiency we obtain values up to 4% in homolayer EL devices. Ordering effects in thin films of conjugated materials, polymers and oligomers, strongly influence their optical properties. This ordering occurs in the active layers of EL devices and determines the threshold field for EL mission. Finally, we demonstrate that the emission colour of our EL devices can be controlled by the driving electric field.

Stable poly(p-phenylene)s and their application in organic light engtting devices

Abstract Organic materials like oligophenyls, poly(para-phenylene) (PPP), and some derivatives (ladder polymers) are used as the active layer of light emitting diodes (LEDs). The electrical characteristics of these devices depend on the band structure of the organic layer as well as on the electrode materials. The optical and electrical LED performance is discussed with regard to the electronic structure of the stratified device arrangement.

Optical absorbance of oriented thin films

We report that the optical absorbance of oriented thin films of para-hexaphenyl (PHP), produced by vacuum deposition, shows strong anisotropy effects. An explanation for the orientation dependence of optical absorption of PHP is proposed, using first principles dielectric tensor calculations. The lack of the π-π* absorption band in the normal UV-Vis absorption spectrum is consistent with theoretical predictions for an orthorhombic thin PHP film with oligomer chains oriented near normal to the substrate surface.

Growth and preferred crystallographic orientation of hexaphenyl thin films

Abstract Vacuum-evaporated hexaphenyl grows in a crystalline state showing strong fibre texture. The fibre axis of this texture is perpendicular to the surface of the substrate. Dependent on the sample preparation conditions, these types of texture (or preferred growth) could be detected: (i) at high substrate temperatures and low deposition rates (DR), the (001) plane of the crystallites develop perpendicular to the fibre axis; (ii) at low substrate temperatures and high DR, the (11-2) and/or (20-3) planes are aligned perpendicular to the fibre axis. Small influences on the preferred growth are observed by different substrate materials like glass, amorphous Indium Tin Oxide (ITO)-coated glass and epiready GaAs. The first hexaphenyl layers on the surface of the substrates are formed by dendritic growth of islands. Molecules perpendicular to the surface of the substrate are observed up to the third monolayer. Strongly structured surfaces and cracks in the films are found for films produced at high substrate temperatures and low DR on epiready GaAs substrates. Smooth surfaces are obtained at low substrate temperatures and high deposition rates on GaAs and ITO substrates. The size of the crystallites is strongly dependent on the substrate temperature, whereas big crystallites are found at high substrate temperatures.

Efficient full-colour electroluminescence and stimulated emission with polyphenylenes

Abstract Soluble and stable polyparaphenylene-type ladder polymers (LPPPs) with an extraordinarily high degree of intrachain order and exceptionally low concentration of defects belong to the class of best-defined conjugated low-dimensional organic semiconductors currently available. Parahexaphenyl, the highly stable oligomer of polyparaphenylene, can be synthesized in the form of single crystals, disordered thin films and highly ordered epitaxially grown thin films. We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light-emitting devices with these electroactive materials. These RGB devices are fabricated based on a new technique, which allows the realization of full-colour flat-panel displays. Using this new technique, we are able to produce devices having any desired emission colour in the visible and near-infrared spectrum. An efficient white-light emission is generated by an internal excitation energy (Forster-type) transfer from the blue LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present an optically pumped methyl-substituted LPPP waveguide, laser structure, which shows a spectrally very narrow and highly directional blue-green light output.

Electroluminescence with Conjugated Polymers and Oligomers

Abstract We report on the optical properties of polymers and oligomers used in light emitting diodes (LEDs) prepared by solution processing or vacuum deposition. Special emphasis is put on the comparison of PPP and its oligomers to planarized forms of PPP.

Blue electroluminescence devices based on parahexaphenyl

Abstract Homogeneous thin films of the organic oligomer parahexaphenyl (PHP) allow the application of this material as a stable emitting layer in blue electroluminescence (EL) devices. We used oligoazomethine (OAM) and diaminoctofluorobiphenyl (DOB) as charge transport layers, which allowed us to achieve excellent EL performance with enhanced EL quantum efficiencies above 1%, improved brightness up to 2000 cd/m2 Luminance and increased rectification ratio in excess of 103 compared to single layer devices.

Self-assembled aromatic oligoazomethines on polar surfaces

The reaction between aromatic dialdehydes and diamines is studied by differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA) measurements. It can be shown that the condensation reaction starts at temperatures below 100 °C. Thus, the high rate and the high conversions of this reaction lead to a new technique of thin-film growth on polar substrates, by successive deposition of monomer layers under high vacuum. Gravimetric measurements show that the first step is a bonding between the organic molecules and the surface. This leads to an oriented prime layer of deposited monomers. In the following steps reactive partners are deposited successively on the surface, layer by layer. Thin films grown by this technique are studied by UV-Vis spectroscopy. These well-ordered films can be applied as interfaces between organic and inorganic layers in solid-state optoelectronic devices.

Multicolour electroluminescence diodes using oligophenylene and oligophenylenevinylene multilayers

Abstract We present electroluminescence (EL) characteristics for single-layer and multilayer thin films with oligophenylenevinylenes and oligo-phenylenes. Light-emitting diodes (LEDs) were fabricated by vacuum evaporation techniques of both the oligomer films and the negative electrodes. Depending on the substrate temperature, highly ordered or polycrystalline layers were obtained. Our LEDs emit blue light in the range of 425–460 nm (oligophenylenes) and in a broad visible range of 400–600 nm (oligophenylene/oligophenylenevinylene multilayers). The current-voltage ( I–V ) characteristics of our devices show rectifying behaviour with high rectification ratios. We report I–V characteristics and the respective threshold voltages, quantum efficiencies, photoluminescence (PL), EL and optical absorbance of the thin films and the devices.