Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1998-07-02
2001-02-13
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C445S046000
Reexamination Certificate
active
06188176
ABSTRACT:
TECHNICAL FIELD
This invention relates to an organic electroluminescent (EL) device using an organic compound and a method for preparing the same. More particularly, it relates to an organic EL device comprising an improved hole injecting electrode for feeding holes to a light emitting layer.
BACKGROUND ART
Recently, active research works have been made on organic EL devices. As a basic configuration, the organic EL device includes a hole injecting electrode, a thin film formed thereon by evaporating a hole transporting material such as triphenyldiamine (TPD), a light emitting layer deposited thereon of a fluorescent material such as an aluminum quinolinol complex (Alq3), and a metal electrode or electron injecting electrode formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from several 100 to several 10,000 cd/m
2
with a drive voltage of approximately 10 volts.
With respect to the material used as the hole injecting electrode of such organic EL devices, a material capable of injecting more holes into the light emitting layer or the hole injecting and transporting layer is believed effective. The hole injecting material is also required to be transparent and electroconductive because in many arrangements, emitted light exits from the substrate side.
Transparent electrodes known to this end include tin-doped indium oxide (ITO), zinc-doped indium oxide (IZO), ZnO, SnO
2
, and In
2
O
3
. Of these, ITO electrodes are widely used as transparent electrodes in liquid crystal displays (LCD), dimmer glass and solar cells because they have both a visible light transmittance of 90% or higher and a sheet resistivity of 10 &OHgr;/or lower. ITO electrodes are considered promising as the hole injecting electrode in organic EL devices.
Organic EL devices tend to deteriorate with the lapse of time. It is a key task to prevent the EL devices from deteriorating. While various factors cause to deteriorate EL devices, the deterioration at a film interface, for example, between an electrode and an organic layer has a significant influence on the lifetime and light emission of EL devices. It is thus important to modify physical properties at the film interface. It is also necessary to prevent an abnormal light emitting phenomenon due to current leakage and a phenomenon of generating and propagating non-luminous regions known as dark spots.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an organic EL device in which the film formation, close contact and physical properties at the interface between a hole injecting electrode and an organic layer are improved so that the device may have a long lifetime, high luminance, high efficacy, and quality display.
Another object of the present invention is to provide an organic EL device which can prevent the occurrence of current leakage and dark spots.
For the purpose of modifying the physical properties at a film interface in organic EL devices, we focused at the orientation of transparent ITO electrodes. We have found that an ITO thin film whose main orientation plane is (222) experiences little deterioration at its interface with an overlying film. We have also found that the occurrence of current leakage and dark spots can be effectively suppressed by restricting the surface roughness of a substrate and an ITO thin film deposited thereon so as to fall within a specific range.
According to the invention, there is provided an organic electroluminescent device comprising a substrate having a surface, a hole injecting electrode disposed on the substrate surface, an electron injecting electrode, and at least one organic layer disposed between the electrodes. The hole injecting electrode comprises an ITO electrode having (111) orientation. Preferably, when analyzed for x-ray diffraction by the rotating crystal method, the ITO electrode exhibits a diffraction intensity ratio of (400)/(222) reflection of up to 0.6; and also preferably, the ITO electrode exhibits an integrated intensity ratio of (400)/(222) reflection of up to 0.5.
In one preferred embodiment, the substrate has a mean surface roughness Ra of up to 10 nm and a maximum surface roughness Rmax of up to 50 nm; and the ITO electrode at its surface remote from the substrate has a mean surface roughness Ra of up to 10 nm and a maximum surface roughness Rmax of up to 50 nm.
Also contemplated herein is a method for preparing an organic electroluminescent device comprising the steps of forming an ITO electrode on a substrate at a substrate temperature of 10° C. to 150° C., and heat treating the electrode at 100° C. to 450° C.
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0083-0855-0 U.S. application No. 09/109,062, filed Jul. 2, 1998, pending.
Codama Mitsufumi
Kagaya Yasunaga
Nakaya Kenji
Onitsuka Osamu
Gerike Matthew J.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patel Vip
TDK Corporation
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