Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2003-03-06
2004-06-01
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C428S704000, C313S503000, C313S504000, C313S505000, C313S506000, C257S040000, C257S103000, C252S301350
Reexamination Certificate
active
06743526
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to an organic electroluminescent device constructed by providing at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode, and more particularly, to an organic electroluminescent device capable of performing stable luminance for a long time by using an organic material composed of a stable chelate compound as the organic material in the emitting layer and the carrier transport layer to prevent the organic material from being crystallized in the emitting layer and the carrier transport layer.
BACKGROUND 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 into an inorganic electroluminescent device and an organic electroluminescent device depending on a used material.
The inorganic electroluminescent device is so adapted that a high electric field is generally applied on a luminescent portion, and electrons are accelerated within the high electric field to collide with a luminescence center, whereby the luminescence center is excited to emit light. On the other hand, the organic electroluminescent device is so adapted that electrons and holes are respectively injected into a luminescent portion from an electron injection electrode and a hole injection electrode, the electrons and the holes thus injected are recombined with each other in a luminescence center to bring an organic molecule into its excited state, and the organic molecule emits fluorescence when it is returned from the excited state to its ground state.
In the case of the inorganic electroluminescent device, a high voltage of 100 to 200 volts is required as its driving voltage because the high electric field is exerted as described above. On the other hand, the organic electroluminescent derive can be driven at a low voltage of approximately 5 to 20 volts.
In the case of the organic electroluminescent device, a luminescent device emitting light in a suitable color can be obtained by selecting a fluorescent material that is 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. Further, it is considered that the organic electroluminescent device is utilized as a back light of a liquid crystal display device or the like because it can emit light at a low voltage.
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 a hole injection electrode and an electron injection electrode. Specifically, used as the structure thereof 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 abundant in electron transporting properties 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 abundant in hole transporting properties and an electron transport layer between a hole injection electrode and an electron injection electrode.
In forming the carrier transport layer and the emitting layer in the organic electroluminescent device, various types of organic materials have been conventionally used, to form uniform layers composed of the organic material generally by vacuum evaporation or the like.
However, the organic material generally used for the emitting layer and the carrier transport layer has not conventionally showed sufficient stability. The organic material is gradually crystallized with the elapse of time, so that crystals are deposited on the emitting layer and the carrier transport layer. Consequently, there are some problems. For example, the organic electroluminescent device is short-circuited, for example, so that the overall organic electroluminescent device cannot uniformly emit light.
Conventionally, a chelate compound composed of a plurality of ligands coordinated to one metal ion, for example, tris(8-quinolinol)aluminum (hereinafter referred to as Alq
3
) indicated by the following chemical formula 1 has been widely utilized as the organic material in the emitting layer and the carrier transport layer.
However, the chelate compound heretofore widely utilized does not necessarily have sufficient properties. In the case of the above-mentioned Alq
3
, for example, its luminescent peak wavelength is as large as 520 nm to 530 nm, and its excitation energy is small. When the Alq
3
is used as a host material in the emitting layer, there are some problems. For example, a dopant such as perylene having large excitation energy cannot be caused to emit light.
A conventional organic electroluminescent device is so constructed that a hole injection electrode composed of an indium-tin oxide (hereinafter referred to as ITO) having a large work function is provided on a transparent glass substrate, to take out light emitted in the emitting layer outward through the hole injection electrode and the glass substrate.
Since the refractive indices of the emitting layer and the ITO are generally 1.5 to 1.7, and the refractive index of the glass substrate is also generally about 1.5, while the refractive index of air is 1.0, however, a part of the light emitted in the emitting layer is reflected on the interface of the glass substrate and the air, to be confined in the organic electroluminescent device. Therefore, the luminance of the light irradiated outward is reduced.
Furthermore, in the conventional organic electroluminescent device, the electron injection electrode is generally formed of a metal having a small work function in order that electrons are efficiently injected into the organic electroluminescent device from the electron injection electrode.
When the electron injection electrode is formed of a metal having a small work function, however, the electron injection electrode is easily oxidized upon reacting with oxygen and water in the air. When the electron injection electrode is thus oxidized, properties in the electron injection electrode are degraded, so that there are some problems. For example, the luminance is reduced, and dark spots (non-luminescent portions) grow.
An object of the present invention is to solve the above-mentioned various problems in an organic electroluminescent device having at least an emitting layer and a carrier transport layer using an organic material between a hole injection electrode and an electron injection electrode.
That is, an object of the present invention is to prevent, in the above-mentioned organic electroluminescent device, an organic material used for an emitting layer and a carrier transport layer from being gradually crystallized with the elapse of time as in the conventional example, to prevent short, for example, of the organic electroluminescent device.
An object in the organic electroluminescent device according to the present invention is to make it possible to perform stable luminance for a long time by using an organic material composed of a stable chelate compound for the emitting layer and the carrier transport layer.
An object in the organic electroluminescent device according to the present invention is t
Hamada Yuji
Matsushita Yasuhiko
Shono Masayuki
Jones Deborah
Sanyo Electric Co,. Ltd.
Westerman Hattori Daniels & Adrian LLP
Xu Ling X.
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