Manufacture of electroluminescent devices

Details Manufacture of electroluminescent devices Manufacture of electroluminescent devices

1995-04-03

1998-05-05

Nold, Charles

Stock material or miscellaneous articles

Composite

Of inorganic material

428691, 428917, 313504, 313506, 313509, 427 66, H05B 3312, H05B 3314

Patent

active

057471827

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to electroluminescent devices and to their manufacture and particularly to such devices which have a conjugated polymer as the light emissive layer.


BACKGROUND OF THE INVENTION

Electroluminescent devices of the type with which the present invention is concerned are described for example in PCT/WO90/13148. Reference may also be made to articles by Burroughes et al in Nature (1990) 347,539 and by Braun and Heeger Applied Physics Letters (1991) 58,1982.
These devices offer potential as large-area flat-panel displays since they can be fabricated over large areas using solution-processing techniques. The basic structure of these electroluminescent (EL) devices comprises a polymer film sandwiched between two electrodes, one of which injects electrons, the other of which injects holes.
In the Nature reference the importance of balancing electron and hole injection rates through selection of charge injection electrodes is recognised. For these polymers, it is clear that injection and transport of electrons is less easy to achieve than for holes; this was indicated by the demonstration of improved device efficiencies when low work function metals such as calcium were used as the negative contact layer, as explained in the article in Applied Physics Letters. From photoluminescence studies it has been identified that an important non-radiative decay channel for excitons in these polymers is by exciton diffusion to charged defects which act as quenching sites. Metal injection electrodes can provide many defect states and efficiencies can be raised substantially by introducing an additional layer between the emissive (polymer) layer and the calcium (electrode) layer. For this, a molecular semiconductor, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3, 4-oxadiazole (butyl PBD) in a poly(methyl methacrylate) PMMA matrix has been used. This layer served both to prevent exciton migration to the metal contact and to enhance electron injection. In this context, reference is made to "Light-Emitting Diodes Based on Conjugated Polymers: Control of Colour and Efficiency", P. L. Burn, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley and R. H. Friend, Symposium N, MRS Fall Meeting, Boston December 1991, MRS Symposium Proceedings 247, 647-654 (1992).
As described for example in PCT/WO92/03490, the contents of which are incorporated herein by reference, PPV can be chemically-modified to control its bandgap. For example, poly(2,5-dialkoxyphenylenevinylene) is red-shifted, by some 0.4 eV, with respect to PPV. Copolymers of PPV and poly(2,5-dimethoxy-p-phenylenevinylene), PDMeOPV, allow fine-tuning of the band gap. Furthermore, controlled elimination of precursor leaving-groups allows both red- and blue- shifting of the gap with respect to that for PPV; the latter is achieved by interruption of conjugation along the chain by the presence of non-conjugated groups.
The present inventors have found that particular advantages are to be gained by devices having more than one layer of conjugated polymer between the electrodes. One advantage of this type of structure is that a broader spectrum of emitted radiation can be achieved as discussed in our copending Application No. PCT/GB93/01574 (Page White & Farrer Ref. 74149/VRD), the contents of which are herein incorporated by reference.
One particularly suitable colour emission layer is a soluble conjugated polymer,poly(2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene) MEHPPV. Similarly, poly(2-methoxy-5-(2-methylpentyloxy)-1,4-phenylenevinylene), poly(2-methoxy-5-pentyloxy-1,4-phenylenevinylene), or poly(2-methoxy-5-dodecyloxy-1,4-phenylenevinylene) can be used as colour emission layers. Another suitable layer is poly(3-dodecylthienylene). One object of the present invention is to provide a device in which these and similar layers are protected so that they can function effectively.


SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided an electroluminescent device comprising a first charge carrier inj

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