Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-05-18
2002-05-14
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C428S704000, C428S212000, C313S504000, C313S506000
Reexamination Certificate
active
06387546
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an organic electroluminescent device, and more particularly, to an organic electroluminescent device characterized in that light having a high luminance can be stably emitted, and light having a predetermined color can be emitted.
2. Description of the Related Art
In recent years, the needs of flat panel display devices, the consumption of electric power and the size of which are smaller than those of a CRT (Cathode Ray Tube) which has been heretofore generally employed, have been increased as information equipments are diversified, for example. An electroluminescent device has been paid attention to as one of the flat panel display devices.
The electroluminescent device is roughly divided depending on a used material, that is, roughly divided into an inorganic electroluminescent device using an inorganic material and an organic electroluminescent device using an organic material.
The organic electroluminescent device is so adapted that holes injected from a hole injection electrode and electrons injected from an electron injection electrode are recombined with each other in the interface between an emitting layer and a carrier transport layer and in the emitting layer, and has the advantage that it can be driven at a lower voltage, as compared with the inorganic electroluminescent device.
In the case of the organic electroluminescent device, a light emitting device emitting light in a suitable color can be obtained by selecting a luminescent material. It is expected that the organic electroluminescent device can be also utilized as a multi-color or full-color display device, for example.
In recent years, various studies have been conducted on such an organic electroluminescent device.
In such an organic electroluminescent device , an emitting layer and a carrier transport layer which is constituted by a hole transport layer for transporting holes to the emitting layer and an electron transport layer for transporting electrons thereto are generally provided between the hole injection electrode and the electron injection electrode.
Generally used as the structure of the organic electroluminescent device are a three-layer structure referred to as a DH structure obtained by laminating a hole transport layer, an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode, a two-layer structure referred to as an SH-A structure obtained by laminating a hole transport layer and an emitting layer between a hole injection electrode and an electron injection electrode, and a two-layer structure referred to as an SH-B structure obtained by laminating an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode.
An electrode material having a large work function, for example, gold or an indium-tin oxide is used as the hole injection electrode, and an electrode material having a small work function, for example, Mg is used as the electron injection electrode.
An organic material having the property of a p-type semiconductor is used as a hole transporting material in the hole transport layer, and an organic material having the property of an n-type semiconductor is used as an electron transporting material composing the electron transport layer. The property of a material composing the emitting layer is determined depending on the device structure of the organic electroluminescent device. For example, the material having the property of the n-type semiconductor, the material having the property of the p-type semiconductor, and the material having the property close to neutrality are respectively used in the SH-A structure, the SH-B structure, and the DH structure.
Conventionally, a chelate metal complex such as tris(8-quinolinolato)aluminum (hereinafter referred to as Alq
3
) indicated by the following structural formula 1 has been generally used as the electron transporting material composing the electron transport layer.
The above-mentioned chelate metal complex is superior in film stability, but does not have sufficient electron transporting properties. In order to transport electrons in sufficient quantity to emit light having a high luminance, a driving voltage to be applied to the organic electroluminescent device must be increased.
Conventionally used as the hole transporting material composing the hole transport layer is a material having a phenylamine structure such as N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (hereinafter abbreviated as TPD) indicated by the following structural formula (2).
Although the above-mentioned material having a phenylamine structure is superior in hole transporting properties to some extent, the hole transporting properties are not necessarily sufficient.
Although the electron transport layer has the properties of transporting electrons, and the hole transport layer has the properties of transporting holes, they are very poor in the properties of transporting carriers.
Therefore, holes and electrons are respectively stored in the interface between the electron transport layer and the emitting layer and the interface between the hole transport layer and the emitting layer. Consequently, the luminescent properties, the life, and the like of the organic electroluminescent device are reduced.
The material composing the emitting layer is generally low in the properties of transporting electrons and holes. Therefore, electrons and holes are not sufficiently injected into the emitting layer, so that sufficient luminance cannot be obtained.
Conventionally used as the organic electroluminescent device is one obtained by doping a dopant having high luminescent properties into a host material in an emitting layer to obtain sufficient luminance.
Excitation energy is not satisfactorily moved to the dopant from the host material depending on the types of the host material and the dopant which are used for the emitting layer. Therefore, sufficient luminance may not, in some cases, be obtained.
Furthermore, the host material also emits light in addition to the dopant depending on the types of the host material and the dopant which are used for the emitting layer, so that light having a suitable color cannot be emitted. Particularly, it is very difficult to emit red light having a high luminance and having a high color purity.
SUMMARY OF THE INVENTION
An object of the present invention is to increase hole transporting properties in a hole transport layer and electron transporting properties in an electron transport layer while maintaining film stability in the hole transport layer and the electron transport layer, to make it possible to stably emit light having a high luminance.
Another object of the present invention is to make it easy to inject electrons and holes into an emitting layer, to make it possible to emit light having a high luminance.
Still another object of the present invention is to satisfactorily move excitation energy from a host material in an emitting layer to a dopant having luminescent properties, to emit light having a high luminance.
A further object of the present invention is to prevent a host material from emitting light in addition to a dopant in an emitting layer, to emit light in a suitable color.
In an organic electroluminescent device having a carrier transport layer and an emitting layer which are composed of an organic material provided between a hole injection electrode and an electron injection electrode, a first organic electroluminescent device according to the present invention is characterized in that a dopant for transporting carriers is doped into the carrier transport layer.
When the carrier transport layer is an electron transport layer, it is possible to dope as the dopant for transporting carriers a dopant having electron transporting properties, a dopant having hole transporting properties, and a dopant having both electron transporting properties and hole transpor
Hamada Yuji
Kanno Hiroshi
Tsujioka Tsuyoshi
Usuki Tatsuro
Armstrong Westerman & Hattori, LLP
Kelly Cynthia H.
Sanyo Electric Co,. Ltd.
Xu Ling
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