Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-01-31
2003-05-27
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S704000, C428S917000, C313S504000, C313S506000, C252S301160, C252S301260, C257S040000, C257S103000
Reexamination Certificate
active
06569544
ABSTRACT:
The present invention relates to an electroluminescent device.
Electroluminescent devices can in particular be used as electroluminescent diodes forming part of the design of display devices or flat screens for computers or television sets.
In such multilayer structure devices, a semiconducting electroluminescent solid organic substance is arranged in a layer between two electrodes at least one of which consists of a transparent, or at least translucent material.
When an electrical voltage is applied between the two electrodes, this organic substance has the property of emitting light. The current flow is then accompanied by a transfer of charges between, on the one hand, each of the electrodes and, on the other hand, the semiconducting and electroluminescent organic substance. The electrode connected to the negative terminal injects electrons into the organic substance. As for the electrode connected to the positive terminal, it injects holes into the organic substance, in other words it captures the electrons emitted by this substance.
Electroluminescent devices of this kind are already known, in which the semiconducting and electroluminescent organic substance is chosen from fluorescent, monomer or polymer organic molecules. It can also consist of a mixture of these fluorescent molecules. As an example of fluorescent molecules, we may cite molecules belonging to the family of naphthalene, anthracene, coronene, perylene, as well as acridine, carbazole, phthalocyanines, metal complexes of 8-hydroxyquinoline possibly doped with coumarin or polymer molecules with a conjugated electronic system such as poly(para-phenylene-vinylene) or poly(para-phenylene).
In general, an emission band corresponding to a particular colour is associated with a type of fluorescent organic molecule. The wide choice of molecules now enables almost the whole colour spectrum to be covered. The choice of the materials of the electrodes is important in producing such an electroluminescent device.
In addition, of course, to these materials having to possess good electrical conducting properties, they must be chosen so that at least one of the two electrodes is transparent to light, or at least translucent, in order to allow the radiation emitted by the electroluminescent organic substance to pass through. Moreover, these materials must be chosen so that each of the electrodes can inject charges, holes for one, electrons for the other, into the semiconducting and electroluminescent organic substance. This transfer of either one of the charges is heavily dependent upon the height of the energy barrier that may exist between on the one hand the work function of the electrode, in other words its capacity to extract or capture electrons, and on the other hand the oxidation or reduction potential of the organic substance.
According to the previous state of technology relating to devices of this kind, only the material of the hole-injecting electrode can be transparent to light. The material of the electron-injecting electrode is generally opaque to light. The result of this is that it is impossible to imagine that the available devices to date could emit the same quality of light through either of their faces equally, and even simultaneously through both faces.
The materials used for making transparent hole-injecting electrodes are generally chosen from metal oxides. We may cite the example of the mixed oxide of indium and tin.
These metal oxide-based materials are capable of injecting holes directly into numerous organic emitters such as, for example, poly(para-phenylene-vinylene). It has been shown by I. D. Parker that the work function of the electrode is then very close to the oxidation potential of the semiconductor and electroluminescent organic substance (J. Appl. Phys., 1994, 75, 1656).
However, it happens that electrodes consisting of metal oxide-based materials cannot adequately inject holes directly into certain organic emitters. This is particularly the case when the electroluminescent substance is tris(8-hydroxyquinoline) of aluminium. This substance possesses remarkable emission properties, but its oxidation potential is unfortunately too high, much greater than the electrode's work function. Therefore, organic substances, that may be termed electrocatalysts, are used, whose role is to facilitate the crossing of the energy barrier and consequently the injection of holes into the electroluminescent substance. They also enable the transport of holes from the electrode into the electroluminescent substance whilst impeding the reverse transfer of electrons to the electrode.
Thus, C. W. Tang et al. have described an electroluminescent device in which a layer consisting of an electrocatalyst derived from triphenylamine is interposed between the metal oxide-based electrode and the layer consisting of the electroluminescent substance (Appl. Phys. Lett., 1987, 51, 913).
U.S. Pat. No. 5,231,329 discloses a similar device, in which an electrocatalyst, with a polymer structure derived from aniline, is arranged between the mixed indium and tin metal oxide-based electrode and the organic emitter with a polymer structure derived from 8-hydroxyquinoline.
Recently, Q. Pei et al. have described an electroluminescent device in which the transported charges are not just electrons and holes, but also ions (Science 1995, 269, 1086). In this type of device, the semiconductor and electroluminescent substance, in this case a polymer or a mixture of polymers, also acts as a solid electrolyte for ion transport. According to the authors, the optimization of such a substance that transports both ions, electrons and holes, still poses many problems, especially in contact with the electrodes.
The materials generally used for making electron-injecting electrodes are chosen from metals or electrically conductive metal alloys. We may cite the examples of aluminium, magnesium, titanium, molybdenum or an alloy of magnesium and silver.
Evidently such metal materials cannot be transparent to light when they are arranged in a layer within such electroluminescent devices.
These metal materials also display other drawbacks, unlike the metal oxides, of being sensitive to corrosion and of not being fashioned according to the techniques of photolithography or silkscreen printing.
It also happens that these metal electrodes cannot adequately inject electrons directly into certain organic emitters. This occurs when this substance possesses a too low reduction potential in relation to the work function of the electrode. It is possible, in this case also, to interpose a layer consisting of electrocatalysts, such as substances derived from oxadiazole for example.
Since the introduction of layers consisting of organic electrocatalysts into electroluminescent devices has enabled the constraints due to crossing energy barriers to be overcome, it is now possible to envisage using a great variety of electroluminescent substances and electrode pairs and making numerous combinations of them.
The fact remains that the charge transfers between the electrodes and the various kinds of organic substances, emitters or electrocatalysts, take place at the interface of heterogeneous materials, inorganic on the one hand, organic on the other. According to the previous examples, this can be a metal oxide-organic substance interface in the case of hole injection, or a metal-organic substance interface in the case of electron injection.
Strong electrical fields are created at these interfaces, in particular when charge transfer is not very effective. One of the consequences of this is the appearance of detachments of the organic layer applied onto the electrode. This phenomenon could be the cause of part of the deterioration of the materials observed in these devices, which precludes them from consideration for an industrial application.
Recently, F. Nuesch et al. became interested in the phenomena that could occur at the interfaces of heterogeneous materials (Adv. Mater., 1997, 9, 222). An electroluminescent device was constructed comprising an electrode con
Alain Nüesch Frank
François Rotzinger
Libero Zuppiroli
Lynda Si-Ahmed
Browning Clifford W.
Ecole Polytechnique Federale de Lausanne
Jones Deborah
Woodard, Emhardt, Naughton Moriarty & McNett LLP
Xu Ling
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