Electroluminescent (EL) display is a self-emitting display mode which features excellent visibility (including high brightness, high contrast, very fast response speed and wide viewing angle), an extremely thin profile and very low power consumption. The EL display device itself emits light, as do cathode ray tubes (CRT), fluorescent and plasma displays. Unlike liquid crystal displays (LCDs), there is no need for backlighting. The response speed for EL can be as fast as 1000 times that for LCD, thus making this mode particularly well suited for use with moving images. EL displays may be used in a variety of applications, including aircraft and ship controls, automobile audio equipment, calculators, mobile telephones, portable computers, instrumentation, factory monitors and electronic medical equipment. Another major application for EL displays is as a light source, particularly as backlighting for small LCD panels in order to render them easier to read in low ambient light conditions.
EL displays work by sandwiching a thin film of a phosphorescent or other electroluminescent substance between two plates each of which comprises conductive elements in a predetermined pattern, i.e. electrodes, thereby forming addressable pixels on the display. The electrodes are formed as coatings either on the electroluminescent substance or on a separate support. Where the or each electrode is intended to transmit light, the electrodes are formed as translucent or transparent coatings, for instance using transparent conductive metal oxides. Equally, the or each support may be translucent or transparent, as required. Generally, at least the anode is transparent. The support generally functions both as a base for an electrode and as an insulating layer. The substrate also provides protection against chemical and physical damage in use, storage and transportation. Glass, as well as polymeric film, has been used as the insulating support.
EL display devices have utilised a variety of cathode materials. Early investigations employed alkali metals. Other cathode materials include combinations of metals, such as brass and conductive metal oxides (e.g., indium tin oxide). A variety of single metal cathodes, such as indium, silver, tin, lead, magnesium, manganese, and aluminum, have also been used.
Relatively recent discoveries in EL construction include devices wherein the organic luminescent medium consists of two very thin layers (<1.0 μm in combined thickness) separating the anode and cathode. Representative of OLED devices are those disclosed in, for instance U.S. Pat. No. 4,720,432.
When an electrical current is passed through the conductive elements, the electroluminescent material emits light. EL displays, being an emissive technology, rather than shuttering a light source as per LCD displays, are most useful in applications where high visibility in all light conditions is important.
The development of new, organic electroluminescent materials, which can produce the three primary colours with very high purity, has made possible full-colour displays with uniform levels of brightness and longevity. Polymers having such characteristics can be dissolved in solvents and processed from solution, enabling the printing of electronic devices. Conductive conjugated polymers are of particular interest. As used herein, the term “conjugated conductive polymer” refers to a polymer having pi-electron delocalisation along its backbone. Polymers of this type are reviewed by W. J. Feast in Polymer, Vol. 37 (22), 5017-5047, 1996. In a preferred embodiment, the conjugated conductive polymer is selected from:
(i) hydrocarbon conjugated polymers, such as polyacetylenes, polyphenylenes and poly(p-phenylene vinylenes);
(ii) conjugated heterocyclic polymers with heteroatoms in the main chain, such as polythiophenes, polypyrroles and polyanilines; and
(iii) conjugated oligomers, such as oligothiophenes, oligopyrroles, oligoanilines, oligophenylenes and oligo(phenylene vinylenes), containing at least two, preferably at least three, preferably at least four, preferably at least five, more preferably 6 or more repeating sub-units.
In addition to use in EL devices, such conjugated conductive polymers have been proposed for use in a variety of other electronic and opto-electronic devices, including photovoltaic cells and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally).