Electroluminescent devices

Details Electroluminescent devices Electroluminescent devices

1997-04-30

1998-09-15

Yamnitzky, Marie

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Including components having same physical characteristic in...

428690, 428917, 4284111, 428457, 313504, 313506, 257 40, 257103, 427 66, 4273855, 427402, 4274071, H05B 3314, B05D 138

Patent

active

058076271

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

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


BACKGROUND TO 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 Dec. 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.
To date therefore it has been possible to have a limited amount of control over the colour of light emitted from an electroluminescent device using conjugated polymers. The present invention seeks to provide an electroluminescent device having a broader range of colour emission.
This has not been achieved in an EL device using conjugated polymer layers and is not a simple matter since the inventors have found that it requires that at least two conjugated polymer layers be put down and be simultaneously excited to emit radiation without one having a detrimental effect on the other.
Reference is made to EP-A-0443861 to Sumitomo which discloses electroluminescent devices made with two layers of conjugated polymers. In this device, only one layer is excited to emit radiation and the other layer is used as a charge transport layer to enhance the transfer of charges into the light emitting layer.


SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided an electroluminescent device comprising: a first charge carrier injecting layer for injecting positive charge carriers; a first layer of a semiconductive conjugated polymer h

REFERENCES:
patent: 5283132 (1994-02-01), Ogura et al.

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