Stock material or miscellaneous articles – Composite – Of inorganic material
Patent
1997-09-18
2000-03-21
Nold, Charles
Stock material or miscellaneous articles
Composite
Of inorganic material
428 69, 428917, 313504, 345 36, 345 45, 345 76, H05B 3314
Patent
active
060400692
DESCRIPTION:
BRIEF SUMMARY
There is a great industrial need for large-area solid-state light sources for a range of applications, predominantly in the field of display elements, VDU technology and light engineering. The demands made of these light sources can at present not be met completely satisfactorily by any of the existing technologies.
As an alternative to conventional display and/or lighting elements such as incandescent lamps, gas discharge lamps and non-self-illuminating liquid crystal display elements, use has for some time been made of electroluminescence (EL) materials and devices such as light-emitting diodes (LEDs).
Apart from inorganic electroluminescence materials and devices, low molecular weight, organic electroluminescence materials and devices have been known for about 30 years (see, for example, U.S. Pat. No. 3,172,862). However, until recently such devices have been greatly restricted in their practical usability.
WO 90/13148 and EP-A 0 443 861 describe electroluminescence devices comprising a film of a conjugated polymer as light-emitting layer (semiconductor layer). Such devices offer numerous advantages such as the opportunity to produce large-area, flexible displays simply and inexpensively. Such displays are self-illuminating and therefore require no additional backward lighting source. Unstructured electroluminescence devices can also be used as background lighting source in liquid crystal displays. In contrast to conventional devices having a lighting tube and a light scattering grating, such devices offer advantages in respect of thickness and weight. However, owing to the random distribution of the main polymer chains, the light emitted from electroluminescence devices is not polarized.
However, particularly in the field of display elements, emission of polarized light is of particular importance. The background lighting of conventional LCD displays, e.g. in VDUs for computers, has to be polarized. If the emitted light is itself polarized, a polarizing film with its associated intensity losses can be omitted.
A typical electroluminescence device as described in WO 90/13148 comprises a light-emitting layer in the form of a thin, coherent polymer film (semiconductor layer) comprising at least one conjugated polymer. A first contact layer is in contact with a first surface of the semiconductor layer and a second contact layer is in contact with a further surface of the semiconductor layer. The polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers for charge carriers to be introduced into the semiconductor layer on application of an electric field between the two contact layers, so that one contact layer becomes positive compared with the other and the semiconductor layer emits radiation.
The polymers used in such devices are generally conjugated. For the purposes of the present invention, a conjugated polymer is a polymer which has a delocalized electron system along the main chain. The delocalized electron system gives the polymer semiconductor properties and enables it to transport positive and/or negative charge carriers with high mobility.
Polarized electroluminescence on polymer films has been observed on stretched polythiophene (P. Dyreklev, M. Berggren, O. Inganas, M. R. Andersson, O. Wennerstrom, T. Hjertberg, Adv. Mat. 1995, 7, 43). The ratio of light polarized in parallel and perpendicular directions was 2.4. However, in this method it is not possible to set an exact film thickness and homogeneity and cracks can occur in the polymer film as a result of the mechanical stretching process.
The building up of the light-emitting layers by means of conventional techniques such as spin coating (see, for example H. Frey, G. Kienel, Dunnschichttechnologie, VDI Verlag, Dusseldorf, 1987) results in a virtually isotropic orientation of the conjugated polymers in relation to the substrate surface. The emitted radiation can therefore also have no preferential polarization.
Up to now, it has been possible, by means of the Langmuir-Blodgett (LB) technique, to
REFERENCES:
Advanced Materials, vol. 8, No. 2, 1996, pp. 146-149.
Synthetis Metals, vol. 71, No. 1-3, pp. 2121-2124.
Advanced Materials, vol. 7, No. 11, 1995, pp. 923-925.
Abstract of JP-A-03/043991.
Lupo Donald
Neher Dieter
Remmers Marcus
Hoechst Aktiengesellschaft
Nold Charles
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