1. Field of the Invention
This invention relates to a light emission device, which is utilizable in various fields as a variety of display devices, and more particularly, to an organic electroluminescent device that works at a low drive voltage, is high in luminance and is excellent in stability.
2. Description of the Related Art
Electroluminescent devices are self-luminescent in nature, and thus, are brighter than liquid crystal devices, enabling a clear display. Accordingly, research workers have long studied such devices. Existing electroluminescent devices, which arrive at a level of practical use, are those which make use of inorganic materials such as ZnS. However, such inorganic electroluminescent devices have not been in wide use because their drive voltage for emission is as high as 200 V or over.
In contrast thereto, organic electroluminescent devices, which utilize light-emitting organic materials, were far from the practical level yet. In 1987, C. W. Tang et al of Eastman Kodak Co., (Applied Physics/Letters, Vol. 51, p. 918, 1987) developed a builtup structure device, with a drastic advance in characteristic performance. More particularly, they succeeded in light emission by use of a builtup structure which includes a phosphor or fluorescent body having the stable structure of a deposition film and capable of transporting electrons, and an organic film capable of transporting holes. The carriers from both are injected into the phosphor body. This organic electroluminescent device has a much improved luminous efficiency, and is capable of emission at 1000 cd/m2 or more by application of a voltage of 10V or below.
Furthermore, as set out in Journal of Applied Physics, Vol. 65, p. 3610, 1989), C. W. Tang et al of Eastman Kodak Co., proposed an emission layer using a guest/host system, making it possible to further improve a luminous efficiency and use a diversity of luminescent materials.
Since then, many researchers have extensively made studies and developments, so that light emitting materials and charge transport materials used for the organic electroluminescent devices have been developed, along with improvements in device structure. As a consequence, there has been reported a device that is low in luminance and has a half-life time of luminance of approximately 10,000 hours.
However, for the use of an organic electroluminescent device as an electronic device, its drive life is not satisfactory, thus leaving a problem, from the standpoint of the practical use thereof, on how to drastically improve the life.
Further, in such an organic electroluminescent device as set out hereinabove, the characteristics of the device depend greatly on the types of organic layers including an electron transport layer, an emission layer and a hole transport layer and also the type of electrode metal. Especially, the organic layers have important performances of injection, transport and recombination of charges and also of light emission. In order to realize a device with excellent characteristics, it is important to appropriately select materials suited for the functions of the respective layers. In addition, for obtaining a highly durable device, it is also important to use, as the organic layers, materials, each capable of forming a stable film without causing aggregation.
Needless to say, the charge injection and transport layer is broadly classified into a hole injection and transport layer and an electron injection and transport layer. These layers, respectively, serve to readily inject charges from an electrode and transport the injected charges to an emission region. The charge injection layer and the charge transport layer may be made of one material, or may be made of different materials, respectively. For a hole injection layer material, there is usually used a material whose HOMO (highest occupied molecular orbital) level is small in order to permit easy injection of holes from an anode. Mention is particularly made, as the injection layer material, of copper phthalocyanine (CuPc), tris{4-[(3-methylphenyl)phenylamino]phenyl}amine (M-MTDATA and the like. For a hole transport material, triphenylamine derivatives are ordinarily used. In Japanese Patent Publication No. 2826381, it is stated that conductive oligomers, particularly, thiophene oligomers, are preferred for use as a material for forming an organic semiconductive region as a hole injection layer or a hole injection and transport layer. On the other hand, quinolinol metal complexes, typical of which are oxadiazole derivatives and tris(8-hydroxyquinolinol)aluminium (Alq), have been studied for use as an electron transport material.
However, although these materials exhibit good capability of injection and transport of charges, most of them are not stable when converted to a thin film.
Many studies have been made on an emission layer wherein a number of compounds have been proposed and developed for use as the emission layer. For instance, a small amount of a fluorescent dye is dispersed in a film-forming material to provide a film for use as an emission layer so as to realize a high device efficiency, an elongated life and a proper control of an emission color. This technique is very effective against a fluorescent dye that is likely to undergo concentration quenching. However, such an emission layer is not sufficient to meet characteristic requirements with respect to the emission efficiency and drive durability.
It is accordingly an object of the invention to provide an organic electroluminescent device, which overcomes the problems involved in the prior art.
It is another object of the invention to provide an improved organic electroluminescent device, which has a high luminous efficiency and excellent drive durability.
It is a further object of the invention to provide an improved organic electroluminescent device, which is significantly improved in working or drive life.
The above objects can be achieved, according to one embodiment of the invention, by an organic electroluminescent device, which comprises a pair of electrodes, and a layer structure provided between the paired electrodes and including, at least, an emission layer comprising up to 10 mole % of a thiophene oligomer of the following formula 
wherein R and Rxe2x80x2 independently represent H, an alkyl group having from 1 to 10 carbon atoms, preferably from 1 to 4 carbon atoms, an amino group, an aryl group and n is an integer of 2 to 10.
The electroluminescent unit may consist of the emission layer alone. Preferably, the layer structure includes a hole injection layer, a hole transport layer and the emission layer formed, on one of the electrodes serving as an anode, in this order.
According to another embodiment of the invention, there is also provided an organic electroluminescent device, which comprises a pair of electrodes, and a layer structure provided between the paired electrodes and including an emission layer, a hole injection layer and a hole transport layer wherein at least one of the hole injection layer and the hole transport layer comprises an oligomer selected from the group consisting of a triphenylamine oligomer of the following formula, a thiophene oligomer as defined above and mixtures thereof 
wherein m is an integer of 2 to 6. The triphenylamine oligomer may be substituted or unsubstituted.
According to a further embodiment of the invention, there is provided an organic electroluminescent device, which comprises a pair of electrodes and a layer structure provided between the paired electrodes and including an emission layer and at least one organic layer capable of transporting electrons or holes and in contact with the emission layer wherein the organic layer comprises a fluorescent material having an absorption peak wavelength shorter than a peak wavelength of luminescence emitted from the emission layer.
Further, the at least one organic layer may include two organic sub-layers. In this case, the fluorescent material is present in one of the sub-layers not in contact with the emission layer.
According to a still further embodiment of the invention, there is provided an organic electroluminescent device, which comprises a pair of electrodes, and a layer structure sandwiched between the paired electrodes and including an organic layer capable of transporting electrons or holes and an emission layer wherein the organic layer has a charge transport interference sub-layer therein when the organic layer consists of a hole transport layer made of a hole transport material so that the sub-layer is made of an organic material having an ionization potential greater than the hole transport material of the organic layer, or wherein the organic layer has a charge transport interference sub-layer therein when the organic layer consists of an electron transport layer made of an electron transport material so that the sub-layer is made of an organic material having an electron affinity smaller than the electron transport material of the organic layer.
According to a yet further embodiment of the invention, there is also provide an organic electroluminescent device comprising a pair of electrodes, and a layer structure sandwiched between the paired electrodes and including a charge transport layer and an emission layer wherein the charge transport layer has a charge transport interference sub-layer therein, and the sub-layer is made of a mixture of both a hole transport material and an electron transport material, an inorganic compound or a metal.