Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1999-12-13
2002-05-28
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S506000, C313S503000, C428S917000
Reexamination Certificate
active
06396209
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 can be utilized as a planar light source or utilized in display devices, for example.
2. Description of the Related Art
Recently, attention has been drawn to organic EL devices in which a luminescent layer, i.e., light emission layer, is formed from a specific organic compound. The reason for the recent attention is that such organic EL devices can achieve 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 good balance via an anode and a cathode, respectively. In addition, since the thickness of the organic compound layers is less than or equal to 2,000 Å, the EL device can exhibit a high luminance and efficiency sufficient for practical use; i.e., a luminance of about 1,0000 cd/m
2
and an external quantum efficiency of about 1% at an applied voltage of no 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 electrode. However, since the magnesium is unstable and is liable to oxidization, 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 evaporation of magnesium and silver (Ag) which is relatively stable and exhibits good adhesion to the surface of the organic layers.
The researchers of Toppan Printing Co. (cf, 51st periodical meeting, Society of Applied Physics, Preprint 28a-PB-4, p.1040) and researchers of Pioneer Co. (cf, 54th periodical meeting, Society of Applied Physics, Preprint 29p-ZC-15, p.1127) have had developments in the usage of lithium (Li), which has an even lower work function than that of Mg, and alloying Li with an aluminum (Al) to obtain a stabilized cathode, thereby embodying a lower driving voltage and a higher emitting luminance in comparison with those of the EL device using Mg alloy. In addition, as is reported in IEEE Trans. Electron Devices., 40, 1342 (1993), the inventors of the present application have found that a two-layered cathode produced by depositing lithium (Li) alone with a very small thickness of about 10 Å on an organic compound layer, followed by laminating a silver (Ag) to the deposited Li layer is effective to attain a low driving voltage in an EL device.
Recently, Pei et al. of Uniax Co. have proceeded to reduce the driving voltage of an EL device by doping a polymeric luminescent layer with a Li salt (cf. Science, 269, 1086 (1995)). This doping method is intended to dissociate the Li salt dispersed in the polymeric luminescent layer to distribute Li ions and counter ions near the cathode and near the anode, respectively, thus ensuring an in-situ doping of the polymer molecules positioned near the electrodes. According to this method, since the polymers near the cathode are reduced with Li as a donor dopant, i.e., electron-donating dopant, and thus the reduced polymers are contained in the state of radical anions, a barrier of the electron injection from the cathode can be considerably reduced, contrary to the similar method including no Li doping.
More recently, the inventors of the present application have found that the driving voltage of an EL device can be reduced by doping an alkali metal such as lithium and the like, an alkali earth metal such as strontium and the like, or a rare earth metal such as samarium and the like, to an organic layer adjacent to the cathode electrode (cf. SID 97, Digest, P.775). It was believed that such reduction of the driving voltage could be obtained because a barrier in the electron injection from the cathode electrode can be notably reduced due to a radical anion state in the organic layer adjacent to the electrode produced by metal doping therein.
However, due to oxidation of the electrodes and other reasons, deterioration of the device can result in the above-described EL devices using an alloy of Mg or Li as the electrode material. In addition, the use of such alloy electrodes has the disadvantage of limited selection of a material suitable for an electrode, because the electrode material to be used has to simultaneously satisfy the requirement for the function as a wiring material. Furthermore, the above described two-layered cathode developed by the present inventors is unable to act as a cathode when the thickness of the Li layer is more than about 20 Å (cf. IEEE Trans. Electron Devices., 40, 1342 (1993), and also has the disadvantage of low reproducibility, because there is difficulty in the control of the layer thickness when the Li layer is deposited at a considerably reduced thickness in the order of about 10 Å. Furthermore, in the in-situ doping method developed by Pei et al. in which the Li salt is added to the luminescent layer to cause their dissociation in the electric field, there is a problem with the transfer time of the dissociated ions to the close vicinity of the electrodes having a controlled velocity, thereby causing a considerable retardation of the response speed of the devices.
Moreover, in the method which includes doping the metal as a dopant in the organic layer, it is necessary to precisely control the concentration of the dopant during formation of the organic layer, because the doping concentration may affect the properties of the resulting devices.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above-described problems of the prior art EL devices, and accordingly, one object of the present invention is to reduce the energy barrier in the electron injection from a cathode electrode to an organic compound layer in accordance with a simple and reliable method to thereby ensure a low driving voltage of the EL devices regardless of the work function of the cathode material.
Another object of the present invention is to provide a device (organic EL device) capable of ensuring satisfactory characteristics which are similar to, or better than, those obtained using the above-described alloy as the electrode material, even if aluminum or other low-cost stable metals which are conventionally used as the wiring material in the prior art are used solely as the cathode material.
In order to achieve the above mentioned objects, an organic electroluminescent device is provided which includes at least one luminescent layer, constituted from an organic compound, between a cathode electrode and an anode electrode opposed to the cathode electrode. The electroluminescent device further includes an organic layer adjacent to the cathode electrode, the organic layer being a mixed layer of an electron-transporting organic compound and an organic metal complex compound containing at least one member selected from the group including an alkali metal ion, an alkali earth metal ion and a rare earth metal ion. The cathode electrode includes a metal capable of reducing the metal ion(s) in the organic metal complex compound of the mixed layer, in a vacuum, to the corresponding metal.
Preferably, the mixed layer is a layer formed upon co-deposition of the organic metal complex compound and the electron-transporting organic compound.
Preferably, the metal used in the formation of the cathode electrode is any one of aluminum, zirconium, titanium, yttrium, scandium and silicon.
Preferably, the metal used in the formation of the cathode electrode is an alloy containing at least one of aluminum, zirconium, titanium, yttrium, sc
Endoh Jun
Kido Junji
Mizukami Tokio
Mori Kohichi
International Manufacturing and Engineering Services Co., Ltd.
McCormick Paulding & Huber LLP
Patel Ashok
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