Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1998-03-25
2001-10-23
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000, C345S205000
Reexamination Certificate
active
06307324
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a display apparatus using electroluminescence elements.
Electroluminescence (EL) elements for use in a display apparatus include an inorganic EL element and an organic EL element. The inorganic EL element uses a thin film of an inorganic compound, like zinc selenide or zinc sulfide, as a fluorescent material, and the organic EL element uses an organic compound as a fluorescent material. Preferably, the organic EL element has the following features:
(1) A high external quantum efficiency.
(2) Light is emitted on a low driving voltage.
(3) Multifarious colors (green, red, blue, yellow, etc.) can be generated by selecting a proper fluorescent material.
(4) The display is clear and no back light is needed.
(5) There is no dependency on the viewing angle.
(6) The organic EL element is thin and light.
(7) A soft material like a plastic film can be used for the substrate.
Due to the aforementioned features, a display apparatus using such an organic EL element (hereinafter referred to as “organic EL display apparatus”) is a desirable replacement for a CRT or liquid crystal display.
An organic EL display apparatus employs a dot matrix system which displays an image with dots arranged in a matrix form. The dot matrix system includes a simple matrix system or an active matrix system.
A conventional organic EL display apparatus
101
of the simple matrix system will now be discussed with reference to
FIGS. 1 through 3
.
As shown in
FIG. 1
, a plurality of anodes
103
are arranged, parallel to one another, on an insulator substrate
102
, and a hole transporting layer
104
is provided on the insulator substrate
102
to cover the anodes
103
. A light emitting layer
105
and an electron transporting layer
106
are formed on the hole transporting layer
104
. A plurality of cathodes
107
are arranged, parallel to one another, on the electron transporting layer
106
. The anodes
103
are placed perpendicular to the cathodes
107
. The layers
104
-
106
are formed of an organic compound, and the layers
104
-
106
, the anodes
103
and the cathodes
107
form an organic EL element
108
. The insulator substrate
102
is preferably made of transparent glass, synthetic resin or the like. The anodes
103
are preferably formed of transparent electrodes of ITO (Indium Tin Oxide) or the like. The cathodes
107
are preferably formed of magnesium-indium alloy or the like.
In the organic EL element
108
, holes coming from the anodes
103
are recombined with electrons coming from the cathodes
107
inside the light emitting layer
105
, emitting light. The light is emitted outside via the anodes
103
and the transparent insulator substrate
102
as indicated by the arrow gamma (&ggr;) in FIG.
2
.
The hole transporting layer
104
facilitates the injection of the holes from the anodes
103
, and also blocks the electrons injected from the cathodes
107
. The electron transporting layer
106
facilitates the injection of the electrons from the cathodes
107
. The organic EL element
108
has a high external quantum efficiency, resulting in the display apparatus
101
having an improved luminous intensity.
FIG. 3
is a schematic plan view of the organic EL display apparatus
101
, as viewed from the anodes
103
. In
FIG. 3
, only the anodes
103
and the cathodes
107
are illustrated. In the organic EL element
108
, defined at the individual intersections of anodes
103
a
to
103
c
and cathodes
107
a
to
107
c
are light emitting areas B
1
to B
9
which emit light, as discussed above. That is, the light emitting areas B
1
-B
9
, arranged in a matrix form, form pixels of the organic EL display apparatus
101
.
In a simple matrix system, the positive terminal of a driving power supply is connected to the anodes
103
, and the negative terminal of the driving power supply is connected to the cathodes
107
. In this manner, the anodes
103
and the cathodes
107
are energized.
In order for the light emitting area B
2
at the intersection of the anode
103
b
and the cathode
107
a
to emit light, for example, the positive terminal is connected to the anode
103
b
and the negative terminal is connected to the cathode
107
a
, so that power is supplied through the terminals. As a result, a forward current flows, as indicated by the arrow alpha (&agr;)When energizing the light emitting area B
2
, a leak current may flow, as indicated by the arrow beta (&bgr;). The leak current energizes not only the light emitting area B
2
, but also the light emitting areas B
1
, B
3
and B
5
near the light emitting area B
2
. As a result, the light emitting areas B
1
, B
3
and B
5
emit light. This phenomenon is called optical crosstalk caused by the leak current characteristic of the EL element
108
.
The light produced by the light emitting layer
105
can be scattered. As indicated by the arrow delta (&dgr;) in
FIG. 2
, the light from the light emitting layer
105
is reflected at the electron transporting layer
106
and can be discharged outside from an unnecessary location. Further, the light from the light emitting layer
105
can be discharged outside from an unnecessary location without passing the anodes
103
as indicated by the arrow epsilon (&egr;) in FIG.
2
. As indicated by the arrow eta (&eegr;) in
FIG. 2
, the light from the light emitting layer
105
is optically guided by the optical waveguide effect resulting from the difference in refractive index between the light emitting layer
105
and the hole transporting layer
104
, and can be discharged outside from an unnecessary place. This light scattering causes light emission at a location other than the desired pixel in the organic EL display apparatus
101
. This phenomenon is called the generation of optical crosstalk caused by light scattering that has originated from the structure of the EL element.
The optical crosstalk due to the leak current and the structure of the EL element
108
deteriorates the contrast of images displayed by the organic EL display apparatus
101
, preventing the acquisition of high-definition images. Particularly, a full-color organic EL display apparatus using EL elements causes color “smearing” and does not provide clear images.
The simple matrix system directly drives organic EL elements
108
of a matrix of pixels, arranged on a display panel, in synchronism with a scan signal using an external driving unit. Each pixel on the display panel has only an organic EL element. As the number of scan lines of a display apparatus increases, therefore, the driving time (duty) assigned to each pixel decreases. This reduces the contrast as well as the luminance intensity of the display screen.
Accordingly, it is an object of the present invention to provide a display apparatus, using electroluminescence elements, capable of displaying clearimages.
SUMMARY OF THE INVENTION
To achieve the above objetive, the present invention provides a display apparatus of an active matrix system, comprising: electroluminescence elements; diode type driving elements for driving the electroluminescence elements; and an added capacitor connected in parallel to the electroluminescence elements.
The present invention provides a pixel of a display apparatus in an active matrix system, the pixel comprising: an electroluminescence (EL) element; a diode type driving element connected in series to the EL element for driving the EL element; an added capacitor; and an added resistor connected in series with the added capacitor, wherein the added resistor and the added capacitor are connected in parallel to the EL element.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
REFERENCES:
patent: 3708717 (1973-01-01), Fleming
patent: 6084579 (2000-07-01), Hirano
Hirano Kiichi
Komiya Naoaki
Sanyo Electric Co,. Ltd.
Sheridan & Ross P.C.
Vu David
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