Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1997-10-15
2001-01-30
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S503000, C313S506000, C313S509000, C313S112000
Reexamination Certificate
active
06181062
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the structure of an organic electroluminescent device.
BACKGROUND TECHNOLOGY
An electroluminescent device includes an inorganic electroluminescent device and an organic electroluminescent device. The inorganic electroluminescent device includes a thin film type and a dispersion type both of which generally require a high AC voltage having several tens of V or more for emitting light. On the other hand, the organic electroluminescent device has an effect that it can emit light with high luminance at a DC voltage of 10 V or less.
The organic electroluminescent device is generally formed of a luminescent body (organic electroluminescent body) comprising a transparent electrode, an organic luminescent layer, and a metal electrode which are laminated in that order on a transparent substrate.
The organic luminescent layer is formed of a laminated body of various organic thin films, for example, there is known a body having various combinations of layers such as a laminated layer formed of a hole injection layer made of triphenylamine derivative and a luminescent layer made of fluorescent organic solid such as anthracene, a laminated layer formed of such a luminescent layer and an electron injection layer made of perylene derivative, and a laminated layer formed of a hole injection layer, the luminescent layer and the electron injection layer set forth above.
The organic electroluminescent device emits light based on the following principle. That is, when the voltage is applied to the transparent electrode and metal electrode, holes and electrons are injected into the organic luminescent layer, wherein the holes and electrons are recombined to generate energy, which energy excites the fluorescent material, and the organic electroluminescent device emits light when the excited fluorescent material returns to the ground state.
The mechanism of recombination of the holes and electrons in the course of emitting light is similar to that of ordinary light emitting diodes, so that the current and also the luminescent intensity exhibit non-linear characteristics with respect to the applied voltage. This results in rectification characteristics of the organic electroluminescent device.
In the organic electroluminescent device, at least one of the electrodes must be transparent for permitting light to travel outside, so that the transparent electrode formed of a transparent conductor made of indium tin oxide (ITO) is generally employed as an anode.
To enhance luminous efficiency by facilitating the injection of electrons, it is important to employ material having a lower work function as a cathode, and hence a metal electrode made of Mg—Ag, or Al—Li is normally employed.
In such an organic electroluminescent device having the arrangement set forth above, the organic luminescent layer is formed of a very thin film having a thickness of about 100 nm. Accordingly, the organic luminescent layer as well as the transparent electrode is substantially transparent. Consequently, when the organic electroluminescent device is turned off, environmental light transmits through the transparent substrate, the transparent electrode and the organic luminescent layer, then it is reflected by the metal electrode, and successively is ejected from the front surface of the transparent substrate. As a result, the display area of the organic electroluminescent device looks like a mirror.
In case that the organic electroluminescent device is employed as a light emitting element or a display element, it is very unnatural for the display area to look like a mirror when the device is turned off, and hence the use of this device is remarkably limited. This problem becomes particularly serious under conditions of a bright environment.
DISCLOSURE OF THE INVENTION
The present invention has been made to solve the problems of the prior art organic electroluminescent device, and it is a main object to prevent a display area from looking like a mirror, which is caused by the light reflection by the metal electrode when the device is turned off.
To achieve the above object, a first aspect of the invention comprises an organic electroluminescent device comprising an organic electroluminescent body (hereinafter occasionally abbreviated as an organic EL body) which emits light when a voltage is applied thereto, including a transparent electrode formed on the front surface of an organic luminescent layer and a metal electrode formed on the back surface of the organic luminescent layer, wherein the transparent electrode of the organic electroluminescent body is formed on the back surface of a transparent substrate, and a diffusing plate having light diffusion characteristics is formed on the front surface of the transparent substrate.
In the specification and claims, the “front surface” of each component is a surface of the viewing side of the display area, while the “back surface” is a surface which is opposite to the front surface (surface close to the metal electrode).
As the diffusing plate, a thin ceramic plate can be employed. As mentioned above, according to the present invention having the diffusing plate formed on the front surface of the transparent substrate, and when the driving voltage is not applied to the organic EL body (when light is not emitted), a part of external incident light is reflected diffusedly by the front side of or within the diffusing plate and the rest of the light travels through the diffusing plate. The light traveling through the diffusing plate is reflected by the metal electrode, and reenters the diffusing plate, then it is diffused again. As a result, there is an advantage that a mirror originated in the metal electrode is not visually perceived from outside.
The first aspect of the invention can provide a color filter which transmits light having a given distribution of transmittancy to the wavelength of light or emitting fluorescence having a given distribution of intensity to the wavelength of light, and which is formed on the front side of the diffusing plate or between the diffusing plate and the transparent substrate.
When the organic EL body does not emit light, the mirror of the metal electrode is screened by the diffusing plate, and, since the color filter is provided, the entire display area appears in the color of the color filter. This is caused by the fact that the diffusing plate is milk white and does not have wavelength sensitive characteristics, and it functions as a kind of reflecting panel when the organic EL body does not emit light.
When the organic EL body emits light, it is possible to visually perceive a color of the light, in which the spectral curve is obtained by overlapping a spectral curve of the light emitted from the organic EL body and a spectral curve of the transmittancy of the color filter, thereby realizing a variety of colors on the display area.
It is also possible to realize a variety of colors on a display area by not providing the color filter but by coloring the diffusing plate with an arbitrary color.
In the first aspect of the invention, the transparent electrode may be divided into plural segments to form plural transparent electrodes, and a screening layer for screening light may be formed between the transparent substrate and the diffusing plate. The portions of the screening layer formed at least the area except at least that confronts the plural transparent electrodes.
The plural transparent electrodes are arranged in an arbitrary pattern representing characters, numerals, symbols, etc., wherein each transparent electrode arranged in respective arbitrary patterns can emit light independently and display certain patterns when selected electrodes are energized.
The light emitted from the organic EL body radiates radially while traveling through the transparent substrate. However, since a screening layer is formed on the front surface of the transparent substrate only the light which passes through the gaps of the screening layer enters the diffusing plate and then is ejected from the front side thereof
Hiraishi Hisato
Kazama Ayako
Armstrong, Westerman Hattori, McLeland & Naughton
Citizen Watch Co. Ltd.
DelGizzi Ronald E.
O'Shea Sandra
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