Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2002-03-26
2003-05-27
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S103000
Reexamination Certificate
active
06570180
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-089412, filed Mar. 27, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode material of an organic light-emitting device and its structure, and more specifically, to an erbium electrode having characteristics of such a low work function that can enhance the efficiency of electron injection to an organic thin film, and a structure which can lower its contact resistance.
2. Description of the Related Art
As a man-machine interface of the recent information society, light-weight and compact flat display panels have become the focus of attention. The most popular device now used is the liquid crystal display, and its market share has now grown greater than that of the conventional CRT (cathode ray tube). However, the liquid crystal display now used is of a non-emissive type, and it cannot be monitored without external light. Under these circumstances, there is a great demand for development of a new type of display which is of an emissive type and whose total power consumption of the system is lower than that of a backlit liquid crystal display.
At present, much attention is being paid to the organic electro-luminescent device, that is, organic light-emitting device, as a device which can overcome the above-described drawbacks in the non-emissive type of the conventional liquid crystal display, which has superior characteristics to those of conventional liquid crystal displays.
In connection with the above, there have been reports regarding the organic light-emitting device. The first example is a report on an organic light-emitting device having the characteristics of high brightness, low driving voltage, small size, high efficiency, etc., published by C. W. Tang and S. A. VanSlyke in a magazine “Applied Physics Letters” in 1987 (C. W. Tang and S. A. VanSlyke: Applied Physics Letters, 51(12), PP. 913-915 (1987)). This publication was an epoch-making report on a device which improved the efficiency of the device by more than ten times that of the conventional organic light-emitting device, and such characteristics were achieved with a film formed of an organic dye, which can easily give rise to an amorphous film, by vacuum deposition, followed by making it extremely thin. According to this report, an external quantum efficiency of 1%, a visible light-emission efficiency of 1.51 m/W and a brightness of 1000 cd/m
2
were realized at a drive voltage of 10V or less. Further, a magnesium/silver alloy, which has a relatively small work function, was used as the cathode, in order to lower the voltage. At present, more than 10 years have passed since the publication of this report, and along with time, the efficiency of the organic light-emitting device has been improved further, and the lifetime of the element has been improved. In addition, the matrix panels are presently commercially available on the market.
The second example is a report published by C. W. Tang, S. A. VanSlyke and C. H. Chen, on an improvement of the light-emitting efficiency, which was achieved by mixing a dye having a high fluorescent quantum efficiency, such as a coumarin dye or a pyran derivative, into a light-emitting layer (C. W. Tang, S. A. VanSlyke and C. H. Chen: Journal of Applied Physics, 65(9), PP. 3610-3616 (1989)).
The third example is a report published by H. Nakada and T. Tohma on the formation of a high-efficiency organic electric field light-emitting device. In this publication, it is reported that a light-emission efficiency of 121 m/W and a brightness of 100,000 cd/m
2
could be achieved when a Quinacridone derivative having a high fluorescent quantum efficiency was mixed into the light-emitting layer and an aluminum alloy containing lithium having a small work function was used as the cathode (H. Nakada and T. Tohma: Inorganic and Organic Electroluminescence (EL96 Berlin), (Edited by R. H. Mauch and H. -E. Gumlich) PP. 385-390 (1996)).
Here, it should be noted that the organic light-emitting device is of a type in which an organic film is impregnated with electrons and holes so as to create excitons, each of which is a pair of an electron and a hole, and light is emitted by recombination of an electron-hole pair. Therefore, the light-emission intensity of the organic light-emitting device is proportional to the amount of impregnated electrons and holes. Due to this characteristic, in order to realize a high-efficiency organic light-emitting device, it is necessary to apply a large electrical current at a low voltage into the device. Therefore, the use of a metal which can easily release electrons even at a low voltage, that is, having a low work function, is very effective.
However, low-work function metals, for example, an aluminum alloy containing lithium, are, in general, atmospherically unstable, as they are easily oxidized. For this reason, when such a metal is used for the organic light-emitting device, the device deteriorates quickly, which is undesirable. Under these circumstances, there has been a demand for development of a more stable cathode, together with an improved structure thereof.
The fourth example is a report published by T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada and M. Tsuchida on a solution to the above-described problem, that is, manufacture of a stable organic light-emitting device. This was achieved by forming an extremely thin film of 1 nm or less, made of an oxide of a low work function metal such as lithium or cesium, as the cathode, on an organic layer, and then evaporating an aluminum electrode on the thin film (T. Wakimoto, Y. Fukuda, K. Nagayama, A. Yokoi, H. Nakada and M. Tsuchida: IEEEE Transaction on Electron Devices, 44(8), PP. 1245/1248 (1997)). It is stated in the fourth report that, with the above-described structure, it is possible to manufacture an organic light-emitting device exhibiting an excellent reproducibility and having a high efficiency.
There is further a report published by L. S. Hung, C. W. Tang and M. G. Mason, on a high-efficiency organic light-emitting device. This has a structure in which a fluoride layer of an extremely thin lithium film having a thickness of 1 nm or less is formed between an organic layer and an aluminum cathode layer (L. S. Hung, C. W. Tang and M. G. Mason: Applied Physics Letters, 70(2), PP. 153-154 (1997).
However, in the device discussed in the above report, insulator layers such as an oxide layer and a fluoride layer are formed extremely thin, and therefore due to a slight variation in thickness of these insulator layers, the drive voltage is increased or the non-light emitting portion is enlarged, which is undesirable. Thus, due to the variation in thickness of each insulator layer, the characteristics of the organic light-emitting device dramatically change. For this reason, especially in the case where an organic light-emitting device is manufactured on a large substrate, it is extremely important to maintain a uniform thickness of each insulator layer. In order to achieve this, it is further necessary to solve the drawbacks entailed in the manufacturing steps of the conventional technique.
BRIEF SUMMARY OF THE INVENTION
The present invention has been achieved as a solution to the above-described drawbacks entailed in the conventionally used cathode materials, and the object thereof is to provide an electrode structure for an organic light-emitting device which can display a high-quality image at a low voltage on a uniform light-emitting surface without using insulator intermediate layers such as extremely thin oxide layer and fluoride layer.
According to an aspect of the present invention, there is provided an organic light-emitting device including a substrate having a light transmitting property in a visible range, such as glass, quartz or resin; a first electrode having a light transmitting property in a visible range, formed
Naka Shigeki
Okada Hiroyuki
Onnagawa Hiroyoshi
Tabatake Shigeru
Meier Stephen D.
President of Toyama University
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