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This is a continuation of application Ser. No. 08/379,501, filed Mar. 31, 1995 and now abandoned, which is a 371 of PCT/GB93/01574.
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-(4tert-butylphenyl)-l,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.
5 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
1° the light emitting layer.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is
15 provided an electroluminescent device comprising: a first charge carrier injecting layer for injecting positive charge carriers; a first layer of a semiconductive conjugated polymer having a band gap selected such that when it is excited radiation at a first wavelength is emitted; a second layer of
20 a semiconductive conjugated polymer having a band gap selected such that when it is excited radiation at a second wavelength is emitted; a second charge carrier injecting layer for injecting negative charge carriers; and means to enable an electric field to be applied across the said layers
25 wherein at least a part of each of the first and second layers is located in an emission zone of the device, said emission zone extending over a capture region of the device wherein positive and negative charge carriers combine with one another to form excitons and having a width characteristic of
30 the distance over which said excitons migrate before decaying radiatively, such that on application of an electric field to the device both of said first and second polymer layers are caused to emit radiation at their respective wavelengths. As mentioned above, it is not readily apparent from the
35 work which has already been published in relation to conjugated polymers that it is possible to use a plurality of layers to control the colour of emitted radiation.
The inventors have found that it is possible to define an emission zone in which light is emitted through the radiative
40 decay of excitons. This zone has a width resulting from the capture region of the device where excitons are formed and which is also related to the diffusion characteristic of the excitons. The width of the emission zone can be approximately the same as that of the captive region or it can extend
45 beyond it where the excitons diffuse from it before decaying radiatively. Thus, by ensuring that both the first and second layers have parts lying in this characteristic width, excitons will be present in both the layers and cause emission of radiation from the first and second layers. The effect is quite
50 clearly ascertainable from the experiments discussed herein but there are different theories which could be developed to support the practical observations. One theory discussed herein is that the characteristic diffusion length for an exciton determines the critical width of the emission zone
55 but there are other possibilities. The inventors have thus determined that it is possible to control the properties of a multilayer electroluminescent device by selecting the thicknesses of the polymer layers so that at least two layers have parts in the emission zone. In one aspect, the invention
60 involves the use of several polymer layers with different band gaps, with layer thicknesses selected to be smaller than or comparable to, the width of the emission zone. This results in excitons in two or more layers, and thus to light emission from the two or more layers. This then gives light
65 emission with a broader spectral range than can be achieved with one layer. This (together with a colour filter, if necessary) may allow fabrication of a white light source.
Whatever the theory underlying the observed effect, the experimental evidence is to the effect that the width of the emission zone is of the order of 50 nm. Thus, in the preferred embodiment, the first layer has a thickness which is not greater than 50 nm. Of course, more than two such layers 5 can be provided depending on the required colour of emitted radiation. Generally speaking the emission zone will extend for a width not greater than 200 nm, but this depends on the nature of the polymer layers and charge carrier injection layers. 10
The location of the emission zone with respect to the charge carrier injection layers depends on the mobilities of electrons and holes within the polymer layers and on the injection functions and can be determined for each particular case using the models discussed later. 15
In one embodiment there is a third layer of a semiconductive conjugated polymer between said second polymer layer and said second charge carrier injecting layer, the thickness of the third polymer layer being not greater than 50 nm. The emission zone can include part of two layers, part of one layer and a complete other layer or parts of two layers and a complete other layer.
The electroluminescent device can include an additional layer of a conjugated polymer adjacent the second charge 25 carrier injecting layer which is not necessarily electroluminescent but which instead functions as a barrier layer.
Preferably the second charge carrier injecting layer is calcium and the first charge carrier injecting layer is indiumtin oxide coated onto a glass substrate. 30
The term "conjugated polymer" used herein indicates a polymer for which the main chain is either fully conjugated, possessing extended pi molecular orbitals along the length of the chain, or else is substantially conjugated, but with interruptions to conjugation at various positions, either random or regular, along the main chain. It includes within its scope homopolymers and copolymers. The present invention can utilise any conjugated polymer which is capable of forming a thin electroluminescent film.
Particularly preferred conjugated polymers include poly (p-phenylene vinylene)PPV and copolymers including that polymer. Preferred features of the polymers used with the respective layers are that they should be stable to oxygen, moisture and to exposure to elevated temperatures, they 45 should have good adhesion to an underlying layer, good resistance to thermally-induced and stress-induced cracking, good resistance to shrinkage, swelling, re-crystallisation or other morphological changes. Moreover, the polymer film should be resilient to ion/atomic migration processes, for 50 example by virtue of a high crystallinity and high melting temperature. Particularly preferred polymers are discussed in the literature referred to above, particularly in PCT/W090/ 13148 the contents of which are herein incorporated by reference. A particularly suitable polymer is a poly(2,5- 55 dialkoxyphenylenevinylene). Examples are MEHPPV, poly (2-methoxy-5-(2-methylpentyloxy)-l,4phenylenevinylene), poly(2-methoxy-5-pentyloxy-l,4phenylenevinylene), and poly(2-methoxy-5-dodecyloxy-l, 4-phenylenevinylene), or other poly(2,5- g0 dialkoxyphenylenevinylenes) with at least one of the alkoxy groups being a long solubilising alkoxy group, linear or branched. Other suitable conjugated polymers can also be selected from the poly(alkylthienylene)s. One example is poly(3-dodecylthienylene). 65
The film of conjugated polymer is preferably a film of a poly(p-phenylenevinylene) [PPV] of formula
wherein the phenylene ring may optionally carry one or more substituents each independently selected from alkyl (preferably methyl), alkoxy (preferably methoxy or ethoxy) or any other substituent which maintains electroluminescent properties in the conjugated polymer.
Any poly(arylenevinylene) including substituted derivatives thereof or any poly(arylene) is also suitable. Throughout this specification the term "arylene" is intended to include in its scope all types of arylenes including heteroarylenes as well as arylenes incorporating more than one ring structure including fused ring structures.
Other conjugated polymers derived from poly(pphenylenevinylene) are also suitable for use as the polymer film in the EL devices of the present invention. Typical examples of such derivatives are polymers derived by:
(i) replacing the phenylene ring in formula (I) with a fused ring system, e.g. replacing the phenylene ring with an anthracene or naphthalene ring system to give a structure such as:
discussed in PCT/WO92/03490, the contents of which are herein incorporated by reference. In a preferred embodiment, the copolymer is a conjugated poly (arylenevinylene) copolymer with a proportion of the vinylic groups of the copolymer saturated by the inclusion of a 5 modified group substantially stable to elimination during formation of a film of the copolymer. The proportion of saturated vinylic groups controls the extent of conjugation and thus modulates the semiconductor bandgap of the copolymer. 10
Preferably polymers for use in the present invention are capable of being processed either as precursors which are subsequently converted to a conjugated form or as intrinsically soluble polymers. In this regard reference is made to PCT/WO90/13148, the contents of which are herein incor- 15 porated by reference.
The invention also provides a method of making an electroluminescent device comprising the following steps: providing a first charge carrier injecting layer for injecting positive charge carriers; depositing on said charge carrier 20 injecting layer a first layer of a soluble polymer in a solution of a first solvent and to a first predetermined thickness; depositing a second layer of polymer in the form of a precursor in a solution of a second solvent to a second predetermined thickness; heat treating the device so that the 25 precursor is converted to its polymer which is insoluble; and depositing a second charge carrier injecting layer for injecting negative charge carriers, wherein the first and second predetermined thicknesses are selected so that at least a part of each of the first and second layers is located in an 30 emission zone of the device.
Reference is made to copending Application No. 08/379, 503 filed on 3/31/95 entitled "Manufacture of Electroluminescent Devices" (Page White & Farrer Ref, 74148/VRD), the content of which is incorporated herein by reference. 35
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS, la, lb and lc show the chemical structures of a) PPV, b) a copolymer of PPV and PDMeOPV, c) MEHPPV;
FIG. 2a shows the current density against electric field characteristics of each of four devices on a log-log scale; 45
FIG. 2b shows current density against voltage on a linear scale;
FIGS. 3a to 3d show diagrammaticaily the structures of four electroluminescent devices I to IV; and 5Q
FIG. 4 shows the electroluminescent emission spectra (all normalised to a peak emission of 1 and offset) of (a) sample I, (b) sample II, (c) sample III, (d) sample IV, (e) a unilayer copolymer electroluminescent device, (f) a unilayer PPV electroluminescent device, (g) to (i) the absorption spectra of 55 the polymers a, b, and c themselves. Curve g corresponds to polymer c) MEHPPV; curve h corresponds to polymer a) PPV and curve i corresponds to copolymer b).
DESCRIPTION OF THE PREFERRED
Three different semiconducting poly(arylenevinylene)s are used to demonstrate the invention.
(a) (FIG. la) Poly(p-phenylenevinylene), PPV, was processed from a tetrahydrothiophenium (THT)-leaving 65 precursor polymer which is soluble in methanol, PPV has a it-it* band gap of about 2.5 eV.
(b) (FIG. lb) A copolymer was prepared from a statistical precursor copolymer to PPV and poly(2,5-dimethoxyp-phenylenevinylene), PDMeOPV, which is soluble in methanol. The monomer feed ratio was 9:1. The synthesis of such a copolymer is described for example in a paper entitled "LEDs based on Conjugated Polymers: Control of Colour and Efficiency" by P. Burn, et al given at MRS Boston 1991 and published in Mat. Res. Soc. Symp. 1992 247, 647-654. Under the experimental conditions used here, the band gap of the polymer obtained after thermal conversion is blue-shifted with respect to PPV due to the presence of non-eliminated methoxy groups at the vinylic carbons adjacent to the dialkoxy-substituted phenylenes. The resulting copolymer has a it-it* band gap of about 2.6 eV.
(c) (FIG lc) The third polymer used was poly(2-methoxy5-(2-ethylhexloxy)-1,4-phenylenevinylene), MEHPPV. Due to the long alkyl side-groups this derivative of PPV is soluble in and processed from chloroform. It has a it-it* band gap of about 2.2 eV.
Both the THT-leaving group precursors to PPV and the copolymer are such they can be laid down by spin coating in solution of a solvent, which when dry, forms a stable layer onto which a further layer may be put down. This enables the construction of multilayer structures since, once a layer has dried, subsequent deposition of additional polymer layers will not remove the initial layer. The two THT-leaving group precursor polymers are insoluble in chloroform, but soluble in methanol. MEHPPV is soluble in chloroform, but insoluble in methanol. This difference in solvents allows a layer of precursor to be spin-coated on top of a layer of MEHPPV without removal of the MEHPPV and vice-versa. Hence multilayer structures composed of the three different polymers were fabricated.
The multilayer devices of conjugated polymers were constructed as follows. Indium-tin oxide (ITO)-coated glass substrates were thoroughly cleaned with acetone and subsequently with propan-2-ol, both in an ultrasonic bath. Multilayer structures were formed by spin-coating layers of polymer or precursor, one on top of another as discussed above. All layers were spin-coated within a nitrogen-filled glovebox (02 and H20 content 10 ppm), in which all subsequent processing steps were also performed. Film thicknesses of the polymer layers were set by control of both spin-speed and solution concentration as follows: the copolymer at 20 nm, MEHPPV at 50 nm and PPV at more than 50 nm. The thicknesses of the individual polymer layers and total polymer layer were measured with a Dektak IIA surface profiler. The samples were thermally converted at 200° C. in vacuo (10~6 torr) for 12 hours to convert the precursor polymers. Calcium contacts were vacuum deposited on the samples and the samples were hermetically sealed. Sample areas were 1 mm2. Four multilayer device structures have been studied here; details of construction are summarised in table 1 and illustrated in FIGS. 3a to 3d.
To form device I illustrated in FIG. 3a, an indium-tin oxide coated glass substrate 1 was spin coated firstly with a precursor to the copolymer (b) at a thickness of 20 nm (layer 21). The layer was allowed to dry and then a precursor to PPV (a) in the thickness of 230 nm (layer 22) was laid down by spin coating, and allowed to dry. Finally a layer 23 of MEHPPV (c) in the thickness of 50 nm was laid down by spin coating. Layers 21 and 22 were put down in a solution of methanol and layer 23 was put down in a solution of chloroform. The sample was then heat treated to cause thermal conversion of the precursors to the copolymer in layer 21 and to PPV in layer 22. Finally a calcium contact 2 was vacuum deposited on layer 23.