In this review we describe some of our recent research in Jerusalem concerning electroluminescence phenomena from conjugated polymers with emphasis on poly(arylenevinylene) polymers and copolymers as the active emitting polymers. Electroluminescence (EL) is described in both simple spin-cast thin films and multilayers produced by a layer-by-layer deposition technique. Newly synthesized copolymers and polymer heterostructures especially designed and useful for organic light-emitting diodes (OLEDs) were prepared by a two-step polymerization of α,α-bis-sulfonium salt of a p-xylene derivative. Copolymerization of different monomers allows one to control the HOMO-LUMO energy gap of the copolymer, and thus to tune the color of the emitted light. This notion of tuning the color of light emitted in EL by performing relatively simple chemical modifications has been termed "chemical tuning" and is an important part of the research carried out in the field of electroluminescence from conjugated polymers. Although most commonly thin film OLED devices are prepared by spin-casting, we have also developed a novel method of layer-by-layer (self-)assembly for device fabrication by which heterostructures containing active luminescent layers are formed by successive deposition of charged layers of an insulating polymer and conjugated polymer by a simple dipping procedure. We have shown that heterostructures prepared in this manner are structurally modulated and are "ordered" perpendicular to the substrate. The polymer/polymer interface roughness at the active layer/spacer layer interface is of the order of 10 Å. Heterostructures prepared by the layer-by-layer technique showed increased stability in OLED devices. Furthermore, the luminescence in these multilayers is blue-shifted relative to spin-cast films provided that the thickness of the spacer layer is large in comparison to the thickness of the emitting conjugated polymer layer. We attribute this phenomenon to confinement effects. An additional emphasis in this review is given to recent studies on transient EL from poly(phenylenevinylene) in the presence of strong electrical pulses. We have demonstrated that at high fields an additional UV-violet EL emission, in addition to the typical green-yellow EL emission, is observed. We have proposed that the UV-violet emission is due to the formation of "hot" carriers in the strong electric field.
|Number of pages||10|
|Publication status||Published - 1998|
ASJC Scopus subject areas
- Chemical Engineering(all)
- Polymers and Plastics