Organic electroluminescence display

Details Organic electroluminescence display Organic electroluminescence display

1999-09-16

2002-04-30

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S500000

Reexamination Certificate

active

06380673

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an organic EL (electroluminescence) display having an organic EL device comprising an organic light emitting layer.
BACKGROUND OF THE INVENTION
In an organic EL display comprising numeral pixels each constituted by an organic electric field light emitting device (hereinafter referred to as an organic EL device), an electron and a hole are injected to an organic light emitting layer from a cathode and an anode, respectively, by applying a voltage to the organic EL device, and recombination of the electron and the hole occurs in the organic light emitting layer to cause emission of light.
Examples of the organic EL device equipped in an organic EL display include a single hetero type organic EL device shown in FIG.
1
. The organic EL device comprises a transparent substrate
1
, such as a glass substrate, having thereon an anode
2
comprising a transparent conductive film, such as ITO (indium tin oxide), and having further thereon an organic layer
5
comprising a hole transfer layer
3
and a light emitting layer
4
, and a cathode
6
comprising aluminum, in this order.
In the organic EL device having such a constitution, a positive voltage is applied to the anode
2
, and a negative voltage is applied to the cathode
6
. A hole injected from the cathode
2
reaches the light emitting layer
4
through the hole transfer layer
3
, and an electron injected from the cathode
6
reaches the light emitting layer
4
, to cause recombination of the electron and the hole to occur in the light emitting layer
4
. As a result, light having a prescribed wavelength is generated and emitted from the transparent substrate
1
as shown by the arrow in FIG.
1
.
Accordingly, an organic EL display can be formed by arranging a large number of the organic EL devices in a matrix form, as described in the foregoing.
An example of the conventional organic EL display is shown in FIG.
2
. The organic EL display shown in
FIG. 2
comprises a transparent substrate
7
having thereon transparent electrodes
8
in a stripe form as an anode, and organic layers
11
a,
11
b,
11
c
et al. in a stripe form are formed further thereon to cross the transparent electrodes
8
at right angles, and cathodes
12
having the substantially same dimension as the organic layer
11
a
(
11
b
and
11
c
) are formed on the organic layers
11
a,
11
b,
11
c
et al. The organic layers
11
a
,
11
b
and
11
c
have light emission characteristics corresponding to one of red (R), green (G) and blue (B), respectively, and thus the organic EL display becomes a full-color or multi-color display.
The mode of displaying an image in the color organic EL display shown in
FIG. 2
will be described. In the color organic EL display, as shown in
FIG. 3
, a scanning circuit
13
is connected to the transparent electrodes
8
, and a brightness signal circuit
14
is connected to the cathodes
12
. A signal voltage varying with time is applied to the organic layers
11
a
to
11
c
at the point of intersection between the transparent electrodes
8
and the cathodes
12
by the scanning circuit
13
and the brightness signal circuit
14
, and thus the respective organic layers
11
a
to
11
c
emit light. By using such a mode of control, the organic EL display can also function as an image reproduction apparatus.
However, the organic EL display involves the following problems.
In the case where the organic EL device having several hundreds scanning lines is driven in a simple matrix mode, an electric current of about 1 A/cm
2
is necessary to ensure sufficient brightness. In this case, while depending on the size of the display, an electric current of about from 0.5 to 1 A momentarily flows through the transparent electrodes
8
on the side of the scanning circuit
13
.
ITO generally used as the transparent electrodes
8
has a resistance larger than a metal, such as aluminum, and its alloy by about 100 times, and therefore, when a large electric current of about from 0.5 to 1 A flows therein, a large voltage drop occurs in the transparent electrodes
8
. When such a large voltage drop occurs in the transparent electrodes
8
, the voltage applied to the respective organic EL devices in the organic EL display becomes uneven to considerably deteriorate the display performance of the organic EL display.
In other words, in the case where the display is driven in the single matrix mode, while depending on the size of the display, the electric current flowing the electrode of the scanning side becomes larger than the electric current flowing in the electrode of the brightness signal side by 100 to 1,000 times due to the driving principles thereof. However, in the organic EL display, because a large electric current flows in the transparent electrodes
8
having a large resistance, a large voltage drop occurs in the transparent conductive film constituting the transparent electrodes
8
, and the voltage applied to the organic layers
11
a
,
11
b
,
11
c
, et al. constituting the respective pixels becomes uneven, so as to deteriorate the display performance and to increase the consuming electric power in the transparent electrodes
8
.
In the color organic EL display shown in
FIG. 2
, because the organic layers
11
a
,
11
b
,
11
c
, et al. are formed along the lengthwise direction of the cathodes
12
in a stripe form under the whole surface thereof, a brightness signal necessary for each colors, R, G and B, should be applied from the cathodes
12
. Therefore, it is necessary that the brightness signal circuit
14
is connected to the cathodes
12
, and the scanning circuit
13
is connected to the transparent electrodes
8
.
Because the electric power consumed in the transparent electrodes
8
becomes large as described above, the property of low power consumption of the whole organic EL display is impaired. Therefore, in order to obtain an organic EL display of a low electric power consumption, it is necessary to lower the resistance of the electrode of the scanning side to decrease the voltage drop.
As measures of lowering the resistance of the electrode of the scanning side, a technique is described in JP-A-5-307997 in that a metallic wiring is attached to transparent electrodes. According to this technique, a metal with low resistance is provided at a part between the transparent electrodes and the organic layer to lower the resistance of the scanning electrode.
However, in order to sufficiently lower the resistance by this technique, the area of the metallic wiring attached to the transparent electrodes is necessarily made as large as possible. When the area of the metallic wiring is large, it covers the organic layer
11
a
(
11
b
and
11
c
) to be a light emission part, and as a result, the light emission area of the organic EL device becomes small to deteriorate the light emission efficiency.
While it is also considered to lower the resistance by increasing the thickness of the metallic film, such measures may cause a short circuit between the anode and the cathode and unevenness in thickness of the organic layer.
SUMMARY OF THE INVENTION
The invention has been developed in view of the circumstances described above.
An object of the invention is to provide an organic EL display that realizes low electric power consumption and display with high brightness by decreasing the resistance of the electrode on the scanning side.
The invention relates to a display device comprising a substrate,
first electrodes in a form of a stripe comprising a transparent conductive material formed on the substrate,
an insulating film having prescribed openings formed on the first electrodes,
an organic layer comprising an organic light emitting material formed on the openings and the insulating film, and
second electrodes in a form of a stripe formed on the organic layer,
wherein the first electrodes comprised of a material having a larger resistance than a resistance of the second electrodes,
the first electrodes and the second electrodes are arranged to cross each other,
the organic layer i

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