Good Charge Transport Properties Research Articles

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.

Studies on UV and thermally radical induced cross-linked polymer networks as charge-transport layers in electrophotographic coating applications

A number of both UV and thermally radically induced cross-linked epoxy acrylates and vinyl polymers/copolymers (cross-linked with divinyl benzene) have been prepared and predoped with different charge-transport media (CTM) and examined as alternative potential durable wear resistant charge-transport layers (CTL) for electrophotographic applications. The CTMs studied were tri- p-tolylamine (TpTA), 2,5-bis(4-diethylaminophenyl-1,3,4-oxadiazole) (OXD), p-diethylaminobenzaldehyde-diphenylhydrazone (DEH) and p-diethylaminophenyl benzaldehyde methylphenyl hydrazone (T191). The polymer binders were based on epoxy acrylates, epoxy resins (amine cured) and divinyl benzene cross-linked styrene/styrene- co-methyl methacrylate, styrene—allyl methacrylate and poly(vinyl cinnamate) and compared with the properties of a conventional polycarbonate CTL. The structure of the cross-linked polymer, nature of the CTM and its concentration controlled the extent of cross-linking and mobility of charge within the coating. UV cured systems were found to fail as CTLs due to the presence of high levels of hydroxy groups on the polymer backbone, residual initiators and photosensitivity and photolysis products of the CTM's trapping and inhibiting charge migration. Polymers containing styrene units, on the other hand, display good charge transport properties compared with polycarbonate due to strong interactions of the pendant phenyl groups stabilising the cation radicals produced on illumination. However, these coatings, although hard were found to be brittle in nature. Copolymerisation with methyl or allyl methacrylate reduced the brittleness but also the mobility of charge due to spacing of the phenyl rings along the polymer network. Thermal cross-linking of a polymer that undergoes a 2 + 2 cycloaddition, poly(allylcinnamate), gave a hard tough coating that exhibited promise as an effective CTL when compared to that of polycarbonate.

The Application of Poly(Phenylene) Type Polymers and Oligomers in Electroluminescent Color Displays

ABSTRACTDue to their high photoluminescence efficiency (>30%), high environmental stability and the good charge transport properties the derivatives of poly(paraphenylene) (PPP), as the laddertype PPP (LPPP) and the oligomer hexaphenyl, are very suitable materials to realise efficient, stable, large area blue organic light-emitting diodes (OLEDs). The emission of blue OLEDs can be efficiently converted into all other emission colors either by an external color conversion technique (ECCT) or an internal color conversion technique (ICCT) and hence are very interesting for a number of display applications:Firstly, we demonstrate the realisation of efficient red-green-blue (RGB) emission colors (representing the RGB-pixels in a multicolor display) by an external CCT. In this case the blue EL device is covered with highly fluorescenct dye/matrix layers, which are excited by the blue emission and emit photoluminescence light in a lower energetic range.Secondly, a new method for producing efficient white light-emitting polymer diodes (which are interesting for e.g. backlight sources in liquid crystal displays) based on a blend of two polymers is presented: a blue light-emitting m-LPPP and a red-orange emitter poly(perylene-co-diethynylbenzene) (PPDB). The red-orange emission is created within the EL device (ICCT) by an excitation energy transfer from m-LPPP into the energetically lower lying states of PPDB. This internal excitation energy transfer is very efficient, so that only a concentration of 0.05 weight % PPDB in the polymer blend is required in order to obtain white light emission.