Andreas Meisel

BLUE POLARIZED ELECTROLUMINESCENCE FROM A LIQUID CRYSTALLINE POLYFLUORENE

demonstrated an organic EL device based on aligned con-jugated polymers which directly emitted polarized light,and realized that such devices would be particularly usefulas backlights for conventional liquid crystal displays(LCDs). However, typical supertwisted nematic LCDs re-quire a contrast ratio of 40 to 50,

Polarized Electroluminescence from Highly Aligned Liquid Crystalline Polymers

Two classes of polymer materials, hexyl-dodecyl copoly(phenylene-ethynylene) and poly(2,7-(9,9-di(ethylhexyl)-fluorene)), respectively, were aligned by two different alignment techniques. For the copolymer, the crystalline and liquid crystalline state were found to differ only slightly and alignment was achieved in the crystalline state, whereas the two phases differ substantially for the polyfluorene and alignment could be obtained in the liquid crystalline state. Light emitting diodes of differing structures were developed using the two materials as emissive layers. In the case of the polyfluorene, a state-of-the-art polarization ratio of 21 was realized.

Liquid crystalline polyfluorenes for blue polarized electroluminescence

A series of 9,9-dialkyl-poly(fluorene-2,7-diyl)s containing linear and branched alkyl substituents with a Mn of up to 200000 g/mol has been synthesized. Moreover, some of the polymers were end capped with a suitable hole transport functionality, such as a triphenylamine derivative, to improve their charge transport properties and to control the molecular weight. The thermal alignment of these novel polymers on a rubbed polyimide layer led to highly anisotropic film formation with dichroic ratios (absorption parallel and perpendicular to the rubbing direction) of up to 26 in absorption and 21 in emission.

Energy and charge transfer in blends of dendronized perylenes with polyfluorene.

Two generations of polyphenylene dendrimers with a perylene diimide core are compared with a nondendronized tetraphenoxyperylene diimide model compound regarding their application in organic light-emitting diodes (OLEDs). Single layer devices with blends of the first and second generation dendrimers in polyfluorene are investigated as active layers in OLEDs, and the effect of dendronization on the emission color and electroluminescence intensity is studied. In photoluminescence, higher degrees of dendronization lead to a reduction in Forster transfer from the polyfluorene host to the perylene, resulting in a larger contribution of the blue host emission in the photoluminescence spectra. In electroluminescence, the dopants appear to act as active traps for electrons, resulting in a predominant generation of excitons on the dye. This gives rise to a remarkably stronger contribution of red emission in electroluminescence than in photoluminescence where energy is exchanged exclusively via Forster transfer. The pronounced color change from red to blue with higher degrees of dendronization and larger driving voltages is explained by the competition of the recombination of free electrons with holes and trapping of electrons by the emitting guest.

Polarized Photoluminescence and Spectral Narrowing in an oriented Polyfluorene Thin Film

The oriented polyfluorene 1 is a promising candidate for a blue polarized emitter in organic optoelectronic devices. By examination of the anisotropic luminescence of thin films of 1, polarization-dependent spectral narrowing was found at high excitation densities, which is of particular interest in reducing the threshold for optically pumped polymer lasers. Femtosecond pump–probe experiments show optical gain at this spectral position.

Anisotropy of the optical and electrical properties of highly-oriented polyfluorenes

Polyfluorenes (PFs) are conjugated polymers with exciting optical and electronic properties and potential applications in optoelectronics. Due to the liquid-crystalline character of most polyfluorene homopolymers, thin layers with a large degree of in-plane alignment can be fabricated with these polymers. Upon appropriate substitution of the polyfluorene backbone, layers with an optical anisotropy in absorption and polarization ratios in emission of more than twenty have recently been fabricated. When included in an organic light-emitting diode structure, highly-polarized blue electroluminescence with an integrated polarization ratio of up to 22 at a brightness of 800 cd/A was realized. These devices represent potential candidates for the illumination of LC displays. Investigations on the anisotropy of the charge carrier mobility revealed that the mobility is highly anisotropic, with only a very small mobility perpendicular to the polymer chains. This is in agreement with expectations based on the results of electron-microscopy studies, which established a structure in which the charge-conducting PF backbone is surrounded by a coaxial, insulating mantle of hydrocarbon side chains. Polyfluorene chains can thus be considered as semiconducting wires, which contain the insulation within their chemical structure.

Polymeric light-emitting diodes with highly polarized blue emission

Polymer light-emitting diodes (LED) with aligned conjugated molecules and polymers as the active layer can emit polarized light as shown recently in several reports. We have synthesized several liquid-crystalline polyfluorenes with different side chain patterns. The thin polymer films were aligned at elevated temperatures on a rubbed polyimide layer. For the most suitable substitution, a dichroic ratio of twelve was observed in absorption. This polymer was used to construct light-emitting devices with ITO and Ca electrodes. Suitable hole transport molecules were added to the polyimide in order to obtain layers with good alignment properties and large hole mobility. Polyimide and poly-fluorene layer thicknesses were varied to optimize the device performance. For those optimized devices, blue light with a dichroic ratio in emission of fifteen was measured and the luminance was approx. 100 Cd/m2 at 18 V. Even though these values are still below the requirements in application, the high dichroic ratio combined with a reasonable brightness represents an important step towards the use of organic LEDs as illumination sources in LCD displays.