Organic electroluminescent devices

Details Organic electroluminescent devices Organic electroluminescent devices

1999-02-25

2002-07-23

Kelly, Cynthia H. (Department: 1774)

Stock material or miscellaneous articles

Composite

Of inorganic material

C313S502000, C313S506000

Reexamination Certificate

active

06423429

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescent device or element (hereinafter, referred also to as an “organic EL device”) which is utilized as a planar light source or utilized in display devices.
2. Description of the Related Art
Attention has been made to an organic EL device in which a luminescent layer, i.e., light emission layer is formed from a specific organic compound, because it ensures a large area display device which can be operated at a low voltage. To obtain a highly efficient EL device, Tang et al., as is reported in Appl. Phys. Lett., 51, 913 (1987), have succeeded in attaining an EL device having a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with a good balance from an anode, and a cathode. In addition, since the thickness of the organic compound layers is not more than about 2,000 Å, the EL device can exhibit a high luminance and efficiency sufficient for practical use; i.e., a luminance of about 1,000 cd/m
2
and an external quantum efficiency of about 1% at an applied voltage of not more than about 10 volts.
In this highly efficient EL device, Tang et al. have used magnesium (Mg) having a low work function in combination with an organic compound which is essentially considered to be an electrically insulating material, in order to reduce an energy barrier which can cause a problem during injection of electrons from a metal-made electrode. However, since the magnesium is liable to be oxidized and is unstable, and also exhibits only a poor adhesion to a surface of the organic layers, magnesium was used after alloying. Alloying is carried out by vapor co-deposition or simultaneous vapor evaporation of magnesium and silver (Ag) which is relatively stable and exhibits good adhesion to a surface of the organic layers.
Further, in the EL device developed by Tang et al., an indium-tin-oxide (ITO) is coated as an anode electrode over a glass substrate. However, the use of the ITO anode electrode device in the Tang et al. to obtain good contact (near to ohmic contact) is considered to be made due to an unexpected and fortunate occurrence; namely, the ITO electrode is frequently used as a transparent anode electrode made of metal oxide in the hole injection of the organic compound to satisfy the requirement for the omission of light in the planar area, and the ITO electrode can exhibit a relatively large work function of a maximum of 5.0 eV.
Furthermore, in their EL device, Tang et al. have inserted a layer of copper phthalocyanine (hereinafter termed as ‘CuPc’) having a thickness of not more than 200 Å between the anode and the hole-transporting organic compound layer to further improve contact efficiency of the anode interface region, thereby achieving the operation of the device at a low voltage.
Similar effects have been also confirmed from the starburst type arylamine compounds, proposed by Shirota et al. of Osaka University, by the researchers of Pioneer Co., Ltd. Both the CuPc compounds and the starburst type arylamine compounds have characteristics that show a work function smaller than that of ITO, and a relatively high mobility of hole charge, and thus improving the stability of the EL devices during the continuous usage thereof, facilitating low-voltage consumption and an improved interfacial contact.
In addition to the above-described devices having the vacuum evaporated layers, there are also known EL devices having the layers formed from a coating solution of a film-forming polymeric material by a coating method such as spin coating. In such EL devices, the coating solution is prepared by previously dispersing an electron-accepting compound in a hole-transporting polymeric material. For example, Partridge, as is reported in POLYMER, Vol.24, June 1983, has confirmed that an ohmic current can be obtained if an antimony pentachloride (hereinafter, SbCl
5
) as an electron-accepting compound is dispersed in dichroromethane solution of polyvinyl carbazole (hereinafter, PVK), whereas such ohmic current could not been realized with the sole use of PVK in the layer formation. In this layer formation, it is understood that SbCl
5
can act as a Lewis acid so that a carbazole pendant group of PVK is oxidized to produce radical cations. SbCl
5
used by Partridge in the layer formation is in a liquid state at a room temperature, and is a Lewis acid compound having a remarkably high reactivity so that fumes can be produced upon reaction with water in atmospheric air. However, contrary to this, if it is reacted with PVK in a glove chamber under an inert atmosphere, SbCl
5
can form a stable complex compound, thereby enabling to form a layer of the complex compound under relatively stable conditions of atmospheric air. Thus, the above layer formation method is considered to be a rational one if it is intended to form a hole injection layer from a side of the ITO electrode. However, in the recent organic EL devices, a highly increased efficiency of the device has been achieved largely relying upon the high purity layer formation process which is based on vacuum evaporation and does not cause cross-contamination. In this production of EL devices, assuming that the above-described method by Partridge is directly applied without any modification, some questions arise because stable driving of the EL devices can be adversely affected by any residues of the solvent used in the coating solution and any impurities in the layer-forming materials.
SUMMARY OF THE INVENTION
The present invention is directed to solve the above-described problems in the EL devices of the prior art. An object of the present invention is to reduce the energy barrier in the hole injection from a transparent ITO anode electrode to a hole-transporting organic layer, and to achieve low-voltage consumption regardless of the work function of the anode material.
To accomplish the above object, the inventors have researched extensively and have now discovered that in the hole injection from an anode electrode to an organic layer adjacent to the anode electrode, an injection barrier (and thus, the voltage) can be reduced if the organic layer is doped with a compound capable of acting as an electron-accepting dopant by a co-deposition or simultaneous evaporation method.
According to the present invention, there is provided an organic electroluminescent (EL) device including at least one light emission layer from an organic compound, the light emission layer being positioned between an anode electrode and a cathode electrode opposed to the anode electrode, in which an organic layer positioned adjacent to the anode electrode is from an organic compound and includes, as an electron-accepting dopant, an electron-accepting compound having a property of oxidizing the organic compound of said organic layer, said electron-accepting compound being doped to said organic layer in vacuum by a simultaneous evaporation method.
In the organic EL devices, the hole injection process from an anode to an organic layer which is basically constituted from an electrically insulating organic compound is intended to carry out oxidation of the organic compound on a surface region of the organic layer, i.e., formation of a radical cation state thereof (cf., Phys. Rev. Lett., 14, 229 (1965)). In the organic EL device, an electron-accepting dopant substance or compound which can act as an oxidizing agent for the organic compound is previously doped in an organic layer in contact with the anode electrode, and thus the energy barrier of the hole injection from the anode electrode can be lowered as a result of such provisional doping of the dopant compound in the organic layer. Since the molecules in the oxidized state (oxidized by the dopant), i.e., in the electron-donated state, are already included in the doped organic layer, a barrier of the hole injection energy is low in the EL device, and therefore a driving voltage of the device can be lowered in compa

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