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Organic electroluminescent device and display

The invention relates to an organic electroluminescent device and display.

In order to make an organic electroluminescence (EL) device have a longer durability and higher efficiency, there is a technique of stacking a plurality of units, a unit being cathode/organic EL device/anode (for example, JP-A-H6-176870, JP-A-H11-312584 and JP-A-H11-312585).

Such a device requires a lower current density than a device having one unit to obtain the same luminance. The device with plural units thus has a longer durability. However the technique has the following problem. An electrode must be extracted from an injection electrode arranged between adjacent organic emitting layers, resulting in complicated wiring and substantial loss in electric power due to the wiring resistance.

As a technique for avoiding the problem, there is used an intermediate electrode with a hole-injection surface and the electron-injection surface opposed thereto, thereby eliminating the need of an intermediate extraction electrode (for example, JP-A-H11-329748 and JP-A-2003-45676).

However the technique has a problem that the color of emitted light observed from the font of a device is different from that observed obliquely, i.e., a high dependency on viewing angle. It also has a problem of an insufficient luminous efficiency.

In view of the above problems, an object of the invention is to provide an organic EL device and a display with good viewing-angle properties and high efficiency.

For the structure of an intermediate conductive layer, JP-A-H11-329748 discloses laminates of a metal thin film and transparent electrode, and JP-A-2003-45676 discloses laminates of metal, and conductive inorganic compound and/or dielectric material. The inventors have studied these structures in detail and found the following. For example, when using a conductive inorganic compound, the refractive index thereof is larger than that of an organic emitting layer, thereby degrading the viewing-angle properties. When laminating a metal film, the luminous efficiency is reduced due to the large absorption coefficient thereof.

Further in order to solve the subject of enhancing the viewing-angle properties and luminous efficiency, the inventors have found the following. A difference in refractive index between intermediate conductive layer and an organic emitting layer as small as 0.2 or less can suppress a change in chromaticity to a degree that a user cannot feel the change even when observing the emission device obliquely.

According to a first embodiment of the invention, there is provided an organic EL device comprising: a transparent electrode, a counter electrode arranged opposite to the transparent electrode, one or more intermediate conductive layers and one or more organic emitting layers arranged between the transparent electrode and the counter electrode, wherein the difference between na and nb is 0.2 or less when na is the refractive index of an intermediate conductive layer and nb is the refractive index of an organic emitting layer.

According to a second embodiment of the invention, there is provided an organic EL device comprising: a transparent electrode, a counter electrode arranged opposite to the transparent electrode, one or more intermediate conductive layers and a plurality of organic emitting layers sandwiching an intermediate conductive layer therebetween, the intermediate conductive layers and the organic emitting layers arranged between the transparent electrode and the counter electrode, wherein the difference between na, and nb and/or nc is 0.2 or less when na is the refractive index of an intermediate conductive layer, nb is the refractive index of a first organic emitting layer and nc is the refractive index of a second organic emitting layer, the intermediate conductive layer sandwiched between the first and second organic emitting layers.

In these organic EL devices, the difference between na, and nb and/or nc is preferably 0.15 or less. nb and nc may be the same as each other.

The intermediate conductive layer can be a laminate of a layer which has a refractive index larger than nb and/or nc, and a layer which has a refractive index smaller than nb and/or nc. The laminate may be made of 2 or 3 or more layers.

Alternatively, the intermediate conductive layer can be a layer comprising a mixture of a material which has a refractive index larger than nb and/or nc, and a material which has a refractive index smaller than nb and/or nc.

According to a third embodiment of the invention, there is provided a display comprising the above mentioned organic EL device of the first or second embodiments.

FIG. 1 is a view showing an embodiment of the organic EL device according to the invention;

FIG. 2 is a partially enlarged view of FIG. 1;