Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-08-30
2002-03-12
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C257S040000, C313S504000, C345S205000
Reexamination Certificate
active
06356032
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of Japanese Patent Applications No. 11-254493 filed on Sep. 8, 1999, and No. 2000-225774 filed on Jul. 26, 2000, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an organic electroluminescent (EL) element that is transparent or translucent when the element does not emit light.
2. Description of the Related Art
A luminous type transparent (or translucent) display panel is conventionally applied to a vehicle meter by being disposed at the front (driver side) of the meter. In this case, a light emitting part of the transparent display panel provides a message to a driver, which can be visually observed by the driver. When the panel does not emit light, i.e., at a non-emitting state, the meter disposed behind the panel can be visually observed because the panel is transparent.
An inorganic EL element is used as such a transparent display panel. The inorganic EL element is typically composed of a luminescent layer made of inorganic material such as ZnS:Mn and two electrode films made of a transparent conductive material such ITO (indium tin oxide) and interposing the luminescent layer therebetween, thereby providing a transparent structure.
In addition to the inorganic EL element, there is known an organic EL element including a luminescent layer containing an organic compound that is provided between a cathode and an anode. While the inorganic EL device necessitates several hundreds volts as a drive voltage, the organic EL element can be driven at a low voltage of several volts to several tens volts. Therefore, the organic EL device can be provided together with a drive circuit readily with a lightened weight at low cost. Incidentally, the organic EL device is driven with direct current.
A widely known material for the anode of the organic EL element is transparent conductive ITO. Light is generally emitted from this ITO electrode side. The cathode is generally composed of a metal layer made of Mg—Ag (alloy of magnesium and silver) or Al/LiF (lamination structure composed of an aluminum layer and a lithium fluoride layer).
When the organic EL element is applied to a transparent display panel, the cathode should be made transparent. However, the cathode is required to have a capability (work function) for supplying electrons into the luminescent layer. This makes it difficult to simply replace the metallic film with a transparent conductive film such as an ITO film.
In this connection, it is conceivable that the metallic film for the cathode is thinned to have a thickness (for instance, approximately 10 nm) sufficient for transmitting light. However, thinning the metallic film increases wiring resistance of the cathode, and reduces an amount of current supplied into the luminescent layer. The decreased amount of current might be insufficient for making the luminescent layer emit light. In such a case, current supplied from the drive circuit needs to be increased to compensate the decreased amount of current, thereby increasing the cost at the drive circuit side.
Especially when the organic EL element is constructed for a matrix type panel, a wiring length of the cathode is increased as a distance from a terminal portion positioned at the periphery of the panel is increased. Therefore, the increase in wiring resistance is prominent as the distance from the terminal portion is increased, and produces variation in amount of current supplied to pixels in accordance with the wiring length. This results in malfunctions of the panel such as nonuniformity of brightness.
As another countermeasure for making the organic EL element transparent, there is known a method for making the organic EL device translucent by thinning the cathode material to a thickness capable of transmitting light, and simultaneously for decreasing wiring resistance of the cathode by disposing a transparent conductive film such as ITO on the cathode (JP-A-8-185984, JP-A-10-294182, and JP 270013). In this method, however, organic layers are readily damaged by plasma charged particles, rise in temperature of a substrate and the like during the formation of the ITO to impair element characteristics.
To the contrary, JP-A-10-223377 proposes a method for reducing damage to the organic layers by forming the transparent conductive film not from ITO but from ZnSe or ZnS that can be formed by vapor deposition. In this method, however, an ITO film is further formed on the transparent conductive film, which serves as a damage preventive barrier for the organic layers, to further reduce resistance. Therefore, the element structure is complicated, and the damage to the organic layers is not prevented completely. Another structure is proposed using this method, in which an Al layer is disposed on the transparent conductive film made of ZnSe or the like. However, the transparency of this structure is insufficient.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems. An object of the present invention is to provide an organic EL element with sufficient transparency and low wiring resistance of a cathode.
According to an organic EL element of the present invention, a cathode has a thin film region having a first thickness capable of transmitting light, and a thick film region having a second thickness thicker than the first thickness. The thin film region makes the EL element transparent, and the thick film region suppresses an increase in wiring resistance of the cathode.
Preferably, the first thickness is in a range of approximately 5 nm to approximately 50 nm. Preferably, a ratio in area of the thick film region with respect to a light emitting region of the luminescent layer is less than approximately 70%. A transparent electrode can be further provided on a surface of the cathode at an opposite side of the anode.
REFERENCES:
patent: 5739545 (1998-04-01), Guha et al.
patent: 5747930 (1998-05-01), Utsugi
patent: 5834894 (1998-11-01), Shirasaki et al.
patent: 6037718 (2000-03-01), Nagami
patent: 6046543 (2000-04-01), Bulovic et al.
patent: 6246179 (2001-06-01), Yamada
patent: 6307324 (2001-10-01), Hirano et al.
patent: 845924 (1998-06-01), None
patent: 8-185984 (1996-07-01), None
patent: B2-2770013 (1998-04-01), None
Nagakubo Masao
Suzuki Harumi
Denso Corporation
Law Offices of David G. Posz
Vu Jimmy T
Wong Don
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