Title: Organic electroluminescent device having backup driving unit
Abstract: An organic electroluminescent device of the present invention includes: a gate line and a data line formed over a substrate, the gate and data lines perpendicularly crossing each other and defining a pixel therebetween; a first driving unit formed in the pixel and comprised of a first switching TFT and a first driving TFT; a power line delivering a current signal to the first driving TFT; an organic electroluminescent diode contacting the first driving TFT and receiving the current signal from the first driving TFT; and a second driving unit formed in the pixel and comprised of a second switching TFT and a second driving TFT, the second driving unit being a backup circuit that can deliver the current signal from the power line to the organic electroluminescent diode when the first driving unit malfunctions.
Patent Number: 6,979,956 Issued on 12/27/2005 to Park, et al.
| Inventors:
|
Park; Jae-Yong (Gyeonggi-do, KR);
Lee; Nam-Yang (Gyeonggi-do, KR);
Yoo; Juhn-Suk (Seoul, KR);
Chung; In-Jae (Gyeonggi-do, KR)
|
| Assignee:
|
LG.Philips LCD Co., Ltd. (KR)
|
| Appl. No.:
|
609025 |
| Filed:
|
June 30, 2003 |
Foreign Application Priority Data
| Jul 16, 2002[KR] | 10-2002-0041612 |
| Current U.S. Class: |
315/169.3; 345/92 |
| Intern'l Class: |
G09G 003/10; G09G 003/36 |
| Field of Search: |
315/1693,169.2,169.1,169.4
345/36,45,76,92
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Dinh; Trinh Vo
Attorney, Agent or Firm: Morgan Lewis & Bockius LLP
Claims
1. An organic electroluminescent device, comprising:
a gate line and a data line formed over a substrate, the gate and data lines
perpendicularly crossing each other and defining a pixel therebetween;
a first driving unit formed in the pixel and comprised of a first switching TFT
and a first driving TFT;
a power line delivering a current signal to the first driving TFT;
an organic electroluminescent diode contacting the first driving TFT and receiving
the current signal from the first driving TFT; and
a second driving unit formed in the pixel and comprised of a second switching
TFT and a second driving TFT, the second driving unit being a backup circuit that
can deliver the current signal from the power line to the organic electroluminescent
diode when the first driving unit malfunctions, wherein the first and second driving
units are connected in parallel.
2. The device of claim 1, wherein the first switching TFT comprises:
a gate connected to the gate line;
a source connected to the data line; and
a drain connected to a gate of the first driving TFT,
wherein the first driving TFT comprises:
the gate connected to the drain of the first switching TFT;
a source connected to the power line; and
a drain connected to the organic electroluminescent diode,
wherein the second switching TFT comprises:
a gate connected to the gate line;
a source connected to the data line; and
a drain connected to a gate of the second driving TFT, and
wherein the second driving TFT comprises:
the gate connected to the drain of the second switching TFT;
a source connected to the power line; and
a drain connected to the organic electroluminescent diode.
3. The device of claim 2, further comprising a first storage capacitor that is
connected in parallel to the first driving TFT and a second storage capacitor that
is connected in parallel to the second driving TFT.
4. The device of claim 3, wherein the first storage capacitor is formed between
the gate of the first driving TFT and the source of the first driving TFT.
5. The device of claim 2, further comprising an open portion between the power
line and the source of the second driving TFT, wherein the open portion electrically
separates the second driving TFT from the power line at the first stage of fabricating
the organic electroluminescent device and then is closed to electrically connect
the power line to the second driving TFT when the first driving unit malfunctions.
6. The device of claim 5, wherein a portion between the power line and the source
of the first driving TFT is cut open when the open portion becomes closed, thereby
disconnecting the first driving unit so that only the second driving unit can operate.
7. The device of claim 3, wherein the second storage capacitor is formed between
the gate of the second driving TFT and the source of the second driving TFT.
8. The device of claim 7, wherein the second storage capacitor includes the gate
of the second driving TFT and the power line.
9. The device of claim 4, wherein the first storage capacitor includes the gate
of the first driving TFT and the power line.
10. The device of claim 1, wherein the organic electroluminescent diode includes
an anode electrode, a cathode electrode and an organic luminous layer between the
anode and cathode electrodes.
11. The device of claim 10, wherein the anode electrode is transparent and the
organic luminous layer includes a hole transporting layer, an emission layer and
an electron transporting layer in sequential order from the anode electrode.
12. An organic electroluminescent device, comprising:
a gate line and a data line formed over a substrate, the gate and data lines
perpendicularly crossing each other and defining a pixel therebetween;
a first driving unit formed in the pixel and comprised of first and second switching
TFTs and first and second driving TFTs;
a power line delivering a current signal to the first driving TFT;
an organic electroluminescent diode contacting the first driving TFT and receiving
the current signal from the first driving TFT; and
a second driving unit formed in the pixel and comprised of third and fourth switching
TFTs and third and fourth driving TFTs, the second driving unit being a backup
circuit that can deliver the current signal from the power line to the organic
electroluminescent diode when the first driving unit malfunctions.
13. The device of claim 12, wherein the first switching TFT comprises:
a gate connected to the gate line;
a source connected to the data line; and
a drain connected to a source of the second switching TFT,
wherein the second switching TFT comprises:
a gate connected to the gate line;
the source connected to the drain of the first switching TFT; and
a drain connected to gates of the first and second driving TFTs,
wherein the first driving TFT comprises:
the gate connected to the source of the second switching TFT;
a source connected to the power line; and
a drain connected to the organic electroluminescent diode,
wherein the second driving TFT comprises:
the gate connected to the drain of the second switching TFT;
a source connected to the power line; and
a drain connected to both the drain of the first switching TFT and the source
of the second switching TFT,
wherein the third switching TFT comprises:
a gate connected to the gate line;
a source connected to the data line; and
a drain connected to a source of the fourth switching TFT,
wherein the fourth switching TFT comprises:
a gate connected to the gate line;
the source connected to the drain of the third switching TFT; and
a drain connected to gates of the third and fourth driving TFTs,
wherein the third driving TFT comprises:
the gate connected to the drain of the fourth switching TFT;
a source connected to the power line; and
a drain connected to the organic electroluminescent diode, and
wherein the fourth driving TFT comprises:
the gate connected to the drain of the fourth switching TFT;
a source connected to the power line; and
a drain connected to both the drain of the third switching TFT and the source
of the fourth switching TFT.
14. The device of claim 13, further comprising a first storage capacitor that
is connected in parallel to the first and second driving TFTs and a second storage
capacitor that is connected in parallel to the third and fourth driving TFTs.
15. The device of claim 14, wherein the first storage capacitor is formed between
the gates of the first and second driving TFTs and the sources of the first and
second driving TFTs.
16. The device of claim 15, wherein the first storage capacitor includes the
gates of the first and second driving TFTs and the power line.
17. The device of claim 14, wherein the second storage capacitor is formed between
the gates of the third and fourth driving TFTs and the sources of the third and
fourth driving TFTs.
18. The device of claim 17, wherein the second storage capacitor includes the
gates of the third and fourth driving TFTs and the power line.
19. The device of claim 13, wherein the drain of the first switching TFT and
the source of the second switching TFT are formed together as a single piece monolithic structure.
20. The device of claim 13, wherein the gates of the first and second driving
TFTs are formed together as a single piece monolithic structure.
21. The device of claim 13, wherein the drain of the third switching TFT and
the source of the fourth switching TFT are formed together as a single piece monolithic structure.
22. The device of claim 13, wherein the gates of the third and fourth driving
TFTs are formed together as a single piece monolithic structure.
23. The device of claim 13, further comprising a first open portion between the
power line and the source of the third driving TFT and a second open portion between
the data line and the source of the third switching TFT, wherein the first open
portion electrically separates the third driving TFT from the power line at the
first stage of fabricating the organic electroluminescent device and then is closed
to electrically connect the power line to the third driving TFT when the first
driving unit malfunctions, and wherein the second open portion electrically separates
the third switching TFT from the data line at the first stage of fabricating the
organic electroluminescent device and then is closed to electrically connect the
data line to the third switching TFT when the first driving unit malfunctions.
24. The device of claim 23, wherein a portion between the data line and the source
of the first switching TFT is cut open when the first and second open portions
become closed, thereby disconnecting the first driving unit so that only the second
driving unit can operate.
25. The device of claim 12, wherein the organic electroluminescent diode includes
an anode electrode, a cathode electrode and an organic luminous layer between the
anode and cathode electrodes.
26. The device of claim 25, wherein the anode electrode is transparent and the
organic luminous layer includes a hole transporting layer, an emission layer and
an electron transporting layer in sequential order from the anode electrode.
Description
The present application claims the benefit of Korean Patent Application No. 2002-0041612,
filed on Jul. 16, 2002 in Korea, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescent display device,
and more particularly, to a dual plate organic electroluminescent device that includes
a first substrate having a thin film transistor array unit, the thin film transistors
arranged in a way that prevents point defects, such as dark spots, from occurring
in the organic electroluminescent display device.
2. Discussion of the Related Art
An organic electroluminescent display device includes a cathode electrode injecting
electrons, an anode electrode injecting holes, and an organic electroluminescent
layer between the two electrodes. An organic electroluminescent diode has a multi-layer
structure of organic thin films provided between the anode electrode and the cathode
electrode. When a forward current is applied to the organic electroluminescent
diode, electron-hole pairs (often referred to as excitons) combine in the organic
electroluminescent layer as a result of a P-N junction between the anode electrode,
which injection holes, and the cathode electrode, which injects electrons. The
electron-hole pairs have a lower energy when combined together than when they were
separated. The resultant energy gap between the combined and separated electron-hole
pairs is converted into light by an organic electroluminescent element. In other
words, the organic electroluminescent layer emits the energy generated due to the
recombination of electrons and holes in response to an applied current.
As a result of the above-described principles, the organic electroluminescent
device does not need an additional light source as compared with a liquid crystal
display device. Moreover, the electroluminescent device is thin, light weight,
and is very energy efficient. As a result, the organic electroluminescent device
has excellent advantages when displaying images, such as a low power consumption,
a high brightness, and a short response time. Because of these advantageous characteristics,
the organic electroluminescent device is regarded as a promising candidate for
various next-generation consumer electronic appliances, such as mobile communication
devices, PDAs (personal digital assistances), camcorders, and palm PCs. Also, because
the fabricating of such organic electroluminescent devices is a relatively simple
process, it is much cheaper to produce an organic electroluminescent device than
a liquid crystal display device.
The organic electroluminescent display devices may be provided in either a passive
matrix type arrangement or an active matrix type arrangement. The passive matrix
type has a simple structure and fabrication process, but has a high power consumption
when compared with the active matrix type. Further, because the display size of
passive matrix organic electroluminescent display devices is limited by its structure,
the passive matrix type can not easily be adapted to a device that is large in
size. Moreover, the passive matrix type has a decreasing aperture ratio as the
bus lines increase. On the contrary, the active matrix type organic electroluminescent
display devices provide a higher display quality with higher luminosity when compared
with the passive matrix type.
FIG. 1 is a schematic cross-sectional view illustrating an active matrix type
organic electroluminescent display device according to a related art arrangement.
As shown in FIG. 1, an organic electroluminescent display device 10 includes
first and second substrates 12 and 28 which are attached to each
other by a sealant 26. On the first substrate 12, a plurality of
thin film transistors (TFTs) T and array portions 14 are formed. A first
electrode (i.e., an anode electrode) 16, an organic luminous layer 18
and a second electrode (i.e., a cathode electrode) 20 are sequentially formed
on the array portion 14. At this point, the organic luminous layer 18
emits red (R), green (G) or blue (B) color in each pixel P. In particular, to show
color images, organic color luminous patterns are disposed respectively in each
pixel P.
As additionally shown in FIG. 1, the second substrate 28, which is attached
to the first substrate 12 by the sealant 26, includes a moisture
absorbent 22 on the rear surface thereof. The moisture absorbent 22
absorbs the moisture that may exist in the cell gap between the first and second
substrates 12 and 28. When disposing the moisture absorbent 22
in the second substrate 28, a portion of the second substrate 28
is etched to form a dent. Thereafter, the powder-type moisture absorbent 22
is disposed into this dent and then a sealing tape 25 is put on the second
substrate 28 to fix the powder-type moisture absorbent 22 into the dent.
FIG. 2 is an equivalent circuit diagram illustrating a pixel of the organic
electroluminescent display device according to a related art arrangement. As shown
in FIG. 2, a gate line 30 is disposed in a transverse direction and a data
line 32 is disposed in a longitudinal direction substantially perpendicular
to the gate line 30. A switching thin film transistor (switching TFT) T
S
is disposed in a crossing of the gate and data lines 30 and 32 and
a driving thin film transistor (driving TFT) T
D is disposed electrically
connecting with the switching thin film transistor T
S. The driving TFT
T
D is electrically connected with an organic electroluminescent diode
E. A storage capacitor C
ST is constituted between gate 40 and
source 42 of the driving TFT T
D. The organic electroluminescent
diode E is comprised of a first electrode, an organic luminous layer and a second
electrode, as described in FIG. 1. The first electrode of the organic electroluminescent
diode E electrically contacts with a drain 46 of the driving TFT T
D,
the organic luminous layer is disposed on the first electrode, and the second electrode
is disposed on the organic luminous layer.
Now, an operation of the organic electroluminescent display device will be explained
briefly with reference to FIG. 2. When a gate signal is applied to a gate 36
of the switching TFT T
S from the gate line 30, a data current
signal flowing via the data line 32 is converted into a voltage signal by
the switching TFT T
S and then this voltage signal is applied to the
gate 40 of the driving TFT T
D. Thereafter, the driving TFT T
D
is operated and then determines current level that flows through the organic
electroluminescent diode E. As a result, the organic electroluminescent diode E
can display a gray scale between black and white.
The voltage signal is also applied to the storage capacitor C
ST such
that a charge is stored in the storage capacitor C
ST. The charge stored
in the storage capacitor C
ST maintains the voltage of the voltage signal
on the gate 40 of the driving TFT T
D. Thus, although the switching
TFT T
S is turned off, the current level flowing to the organic electroluminescent
diode E remains constant until the next voltage signal is applied.
Under different circumstances, the above-mentioned organic electroluminescent
display device can have plural driving and switching TFTs. For example, the organic
electroluminescent display device can have four TFTs, i.e., a four TFT structure.
More particularly, the organic electroluminescent display device can have two parallel-connected
switching TFTs and two parallel-connected driving TFTs within one pixel. When the
organic electroluminescent display device is provided with two switching TFTs,
the stress resulting from the continuously applied DC bias decreases to some extent.
Generally, the driving TFT can deteriorate as a result of persistent stress from
the applied current. As a result, the operating characteristics of the driving
TFT can vary severely. To overcome this problem, two driving TFTs can be provided
within the pixel in a parallel connection to each other. This results in a prolonged
life span of the driving TFT.
FIG. 3 is an equivalent circuit diagram illustrating one pixel of an organic
electroluminescent display device having a four TFT structure. As shown, a gate
line 52 is disposed over a substrate 50 in a transverse direction,
and a data line 54 is disposed in a longitudinal direction substantially
perpendicular to the gate line 52. A first switching TFT T
S1
and a second switching TFT T
S2 are disposed near a crossing of the gate
and data lines 52 and 54. Further, in a pixel defined by the gate
and data lines 52 and 54, there are first and second driving TFTs
T
D1 and T
D2 which are electrically connected with the first
and second switching TFTs T
S1 and T
S2. Gates 58 and
64 of the first and second switching TFTs T
S1 and T
S2 are
both connected to the gate line 52. Source 60 of the first switching
TFT T
S1 is connected to the data line 54. Drain 62 of
the first switching TFT T
S1 is connected with source 66 of the
second switching TFT T
S2. The drain 62 and the source 66
are formed together as a monolithic structure of single piece. Gates 70
and 71 of the first and second driving TFTs T
D1 and T
D2
are also formed together as a monolithic structure of single piece and connected
to the drain 67 of the second switching TFT T
S2. Drain 72
of the second driving TFT T
D2 is connected to both the drain 62
of the first switching TFT T
S1 and the source 66 of the second
switching TFT T
S2. Sources 76 and 74 of the first and
second driving TFTs T
D1 and T
D2 are connected to the power
line 56. A storage capacitor C
ST is formed between the gates
70 and 71 and the sources 76 and 74 of the first and
second driving TFTs T
D1 and T
D2. The drain 78 of the
first driving TFT T
D1 is connected with an organic electroluminescent
diode E.
The above-described organic electroluminescent device having a four TFT structure
operates like the organic electroluminescent device having a two TFT structure.
However, in the organic electroluminescent device of FIG. 3 having a four TFT structure,
the data signal is divided when applied to the parallel-connected TFTs. In particular,
the data current signal of the data line 54 can flow through the first and
second switching TFTs T
S1 and T
S2, dividedly. Then, the first
and second switching TFTs T
S1 and T
S2 drive both the first
and second driving TFTs T
D1 and T
D2. Thereafter, a current
signal flowing in the power line 56 is delivered to the organic electroluminescent
diode E through the first driving TFT T
D1, thereby emitting light.
The related art organic electroluminescent display device having the two TFT
structure or the four TFT structure frequently results in point defects, such as
dark spots, being displayed. Since the organic electroluminescent display device
is very thin, for example, often having a thickness of 1000 angstroms, the organic
electroluminescent display device may be broken, resulting in these dark spots
appearing in the displayed images. Furthermore, such point defects typically arise
as a result of malfunctions of the thin film transistors within the pixel. For
example, if the switching and driving TFTs improperly operate within the pixel,
the pixel having the malfunctioning TFT can be erroneously displayed as a dark spot.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an organic electroluminescent
display device which substantially obviates one or more of the problems due to
limitations and advantages of the related art.
An object of the present invention is to provide an organic electroluminescent
display device having stable structure elements, resulting in a prolonged operational
life span.
Another object of the present invention is to provide an organic electroluminescent
display device having a stable structure that prevents point defects, thereby providing
high resolution and picture quality.
Additional features and advantages of the invention will be set forth
in the description which follows, and in part will be apparent from the description,
or may be learned by practice of the invention. The objectives and other advantages
of the invention will be realized and attained by the structure particularly pointed
out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the
present invention, as embodied and broadly described, an organic electroluminescent
device includes a gate line and a data line formed over a substrate, the gate and
data lines perpendicularly crossing each other and defining a pixel therebetween;
a first driving unit formed in the pixel and comprised of a first switching TFT
and a first driving TFT; a power line delivering a current signal to the first
driving TFT; an organic electroluminescent diode contacting the first driving TFT
and receiving the current signal from the first driving TFT; and a second driving
unit formed in the pixel and comprised of a second switching TFT and a second driving
TFT, the second driving unit being a backup circuit that can deliver the current
signal from the power line to the organic electroluminescent diode when the first
driving unit malfunctions.
In another aspect, an organic electroluminescent device includes a gate line
and
a data line formed over a substrate, the gate and data lines perpendicularly crossing
each other, and defining a pixel therebetween; a first driving unit formed in the
pixel and comprised of first and second switching TFTs and first and second driving
TFTs; a power line delivering a current signal to the first driving TFT; an organic
electroluminescent diode contacting the first driving TFT and receiving the current
signal from the first driving TFT and a second driving unit formed in the pixel
and comprised of third and fourth switching TFTs and third and fourth driving TFTs,
the second driving unit being a backup circuit that can deliver the current signal
from the power line to the organic electroluminescent diode when the first driving
unit malfunctions.
It is to be understood that both the foregoing general description and the following
detailed description are exemplary and explanatory and are intended to provide
further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further understanding
of the invention and are incorporated in and constitute a part of this specification,
illustrate embodiments of the invention and together with the description serve
to explain the principles of the invention. In the drawings:
FIG. 1 is a schematic cross-sectional view illustrating an active matrix type
organic electroluminescent display device according to a related art arrangement;
FIG. 2 is an equivalent circuit diagram illustrating a pixel of the organic
electroluminescent display device according to the related art arrangement;
FIG. 3 is an equivalent circuit diagram illustrating one pixel of an organic
electroluminescent display device having a four TFT structure according to a related
art arrangement;
FIG. 4 is an equivalent circuit diagram illustrating a pixel of an organic electroluminescent
display device according to a first embodiment of the present invention;
FIG. 5 is an equivalent circuit diagram illustrating a pixel of an organic electroluminescent
display device according to a second embodiment of the present invention; and
FIG. 6 is a schematic cross-sectional view illustrating an active matrix type
organic electroluminescent display device according to the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to the illustrated embodiments of
the present invention, examples of which are illustrated in the accompanying drawings.
FIG. 4 is an equivalent circuit diagram illustrating a pixel of an organic electroluminescent
display device according to a first embodiment of the present invention. Hereinafter
in the present invention, a first switching TFT T
S1 and a first driving
TFT T
D1 are referred to as a first driving unit, and a second switching
TFT T
S2 and a second driving TFT T
D2 are referred to as a
second driving unit. The first driving unit is electrically independent from the
second driving unit.
Referring to FIG. 4, the first driving unit will be explained first. Gate
and data lines
202 and
104 are disposed over a substrate
100.
The gate line
102 is located in a transverse direction, and the data line
104 is located in a longitudinal direction substantially perpendicular to
the gate line
102. A gate
108 of the first switching TFT T
S1
is connected to the gate line
102, and a source
107 of the first
switching TFT T
S1 is connected to the data line
104. Further,
a drain
110 of the first switching TFT T
S1 is connected to a
gate
112 of the first driving TFT T
D1. A source
114 of
the first driving TFT T
D1 is connected to a power line
106, and
a drain
116 of the first driving TFT T
D1 is connected to an organic
electroluminescent diode E. A first storage capacitor C
ST1 is formed
between the gate
112 and the source
114 of the first driving TFT
T
D1. For example, the first storage capacitor C
ST1 substantially
comprises the gate
112 and the power line
106.
When the first switching TFT T
S1 is turned on, the first driving
TFT T
D1 is also turned on and then a current signal flowing the power
line
106 is delivered to the organic electroluminescent diode E through
the first driving TFT T
D1. As a result, the organic electroluminescent
diode E emits light.
The second driving unit will now be explained with reference to FIG. 4. A gate
118 of the second switching TFT T
S2 is connected to the gate
line
102, and a source
120 of the second switching TFT T
S2 is
connected to the data line
104. Further, a drain
122 of the second
switching TFT T
S2 is connected to a gate
124 of the second driving
TFT T
D2. A source
126 of the second driving TFT T
D2 is
connected to a power line
106, and a drain
128 of the second driving
TFT T
D2 is connected to an organic electroluminescent diode E. A second
storage capacitor C
ST2 is formed between the gate
124 and the
source
126 of the second driving TFT T
D2. For example, the second
storage capacitor C
ST2 substantially comprises the gate
124 and
the power line
106.
The second driving unit comprised of the second switching and driving TFTs T
S2
and T
D2 is actually a backup circuit that does not operate when the
organic electroluminescent diode E emits light by the proper operation of the first
driving unit (first switching and driving TFTs T
S1 and T
D1).
Therefore, an open portion A exists between the power line
106 and the source
126 of the second driving TFT T
D2. This open portion A is opened
at the first stage of fabricating the organic electroluminescent display device.
Therefore, although the second switching TFT T
S2 applies the signal
to the second driving TFT T
D2, the power line signal flowing in the
power line
106 does not influence the organic electroluminescent diode E
because of the open portion A.
On the contrary, when the first driving unit has some defects and malfunctions,
the open portion A is closed by welding or the like to connect the power line
106
to the second driving TFT T
D2 and then the second driving unit operates
instead of the first driving unit. At this time, a portion B between the power
line
106 and the first driving TFT T
D1 is opened to surely disconnect
the first driving TFT T
D1 from the power line
106, and thus the
organic electroluminescent diode E is not influenced by the first driving unit.
When the second driving unit operates instead of the first driving unit, the
second switching TFT T
S2 is turned on and the second driving TFT T
D2
is also turned on, thereby delivering a current signal flowing the power
line
106 to the organic electroluminescent diode E through the second driving
TFT T
D2. Therefore, the organic electroluminescent diode E emits light.
The reason for opening the portion B between the power line
106 and the
first driving TFT T
D1 is to stably supply the power line signal from
the power line
106 to the organic electroluminescent diode E. If the portion
B is not cut, the power line signal may flow into the organic electroluminescent
diode E through the first driving TFT T
D1 and then deteriorate the image
quality of the organic electroluminescent display device. Accordingly, when the
second driving unit replaces the first driving unit with the open portion A closed
by welding or the like, the portion B between the power line
106 and the
first driving TFT T
D1 is opened.
FIG. 5 is an equivalent circuit diagram illustrating a pixel of an organic electroluminescent
display device according to a second embodiment of the present invention. As shown,
gate and data lines
202 and
204 are formed over a substrate
200.
The gate line
202 is disposed in a transverse direction, and the data line
204 is disposed in a longitudinal direction substantially perpendicular
to the gate line
202.
As shown in FIG. 5, the pixel according to the second embodiment of the present
invention includes a first driving unit that is disposed in a crossing of the gate
and data lines
202 and
204. The first driving unit is comprised of
parallel-connected first and second switching TFTs T
S1 and T
S2
and parallel-connected first and second driving TFTs T
D1 and T
D2.
The first and second switching TFTs T
S1 and T
S2 are electrically
connected to the first and second driving TFTs T
D1 and T
D2.
The pixel of the second embodiment also includes a second driving unit that is
comprised of parallel-connected third and fourth switching TFTs T
S3 and
T
S4 and parallel-connected third and fourth driving TFTs T
D3 and
T
D4. The third and fourth switching TFTs T
S3 and T
S4
are electrically connected to the third and fourth driving TFTs T
D3
and T
D4. As mentioned before, the second driving unit is electrically
independent from the first driving unit.
In the first driving unit of FIG. 5, a gate
207 of the first switching
TFT T
S1 and a gate
214 of the second switching TFT T
S2 are
both connected to the gate line
202. A source
210 of the first switching
TFT T
S1 is connected to the data line
204 and a drain
212
of the first switching TFT T
S1 is connected to a source
216 of
the second switching TFT T
S2. The drain
212 and the source
216
are formed together as a monolithic structure of single piece. A gate
220
of the first driving TFT T
D1 and a gate
221 of the second driving
TFT T
D2 are connected to the drain
218 of the second switching
TFT T
S2 and also are formed together as a monolithic structure of single
piece. A drain
224 of the second driving TFT T
D2 is connected
to both the drain
212 of the first switching TFT T
S1 and the
source
216 of the second switching TFT T
S2. A source
222
of the first driving TFT T
D1 and a source
226 of the second driving
TFT T
D2 are both connected to a power line
206. Further more,
a drain
228 of the first driving TFT T
D1 is connected to an organic
electroluminescent diode E. A first storage capacitor C
ST1 is formed
between the two gates
220 and
221 and the two sources
222
and
226.
In the above-mentioned driving unit, when the first and second switching TFTs
T
S1 and T
S2 are turned on, the first and second driving TFTs
T
D1 and T
D2 are also turned on and then a current signal
flowing in the power line
206 is delivered to the organic electroluminescent
diode E through the first driving TFT T
D1. As a result, the organic
elecroluminescent diode E emits light.
The second driving unit will now be explained with reference to FIG. 5. In FIG.
5 of the present invention, the second driving unit is illustrated by dotted lines.
As mentioned before, the second driving unit includes the parallel-connected third
and fourth switching TFTs T
S3 and T
S4 and the parallel-connected
third and fourth driving TFTs T
D3 and T
D4. Gates
230
and
242 of the third and fourth switching TFTs T
S3 and T
S4
are connected to the gate line
202, respectively. A source
232
of the third switching TFT T
S3 is connected to the data line
204
and a drain
234 of the third switching TFT T
S3 is connected to
a source
244 of the fourth switching TFT T
S4. The drain
234
and the source
244 are formed together as a monolithic structure of single
piece. A gate
236 of the third driving TFT T
D3 and a gate
237
of the fourth driving TFT T
D4 are connected to the drain
246
of the fourth switching TFT T
S4 and also formed together as a monolithic
structure of single piece. A drain
240 of the fourth driving TFT T
D4
is connected to both the drain
234 of the third switching TFT T
S3
and the source
244 of the fourth switching TFT T
S4. A source
238 of the third driving TFT T
D3 and a source
248 of the
fourth driving TFT T
D4 are both connected to a power line
206.
Furthermore, a drain
250 of the third driving TFT T
D3 is connected
to an organic electroluminescent diode E. A second storage capacitor C
ST2
is formed between the two gates
236 and
237 and the two sources
238 and
248.
The second driving unit comprised of the third and fourth switching TFTs T
S3
and T
S4 and the third and fourth driving TFTs T
D3 and T
D4
is actually a backup circuit that does not operate when the organic electroluminescent
diode E emits light by the proper operation of the first driving unit (i.e., the
first and second switching TFTs T
S1 and T
S2 and the first
and second driving TFTs T
D1 and T
D2). Therefore, a first
open portion C between the power line
206 and the source
238 of the
third driving TFT T
D3 is opened, and a second open portion D between
the data line
204 and the source
232 of the third switching TFT T
S3
is also opened. The first and second open portions C and D are designed to
be opened together at the first stage of fabricating the organic electroluminescent
display device.
Therefore, the second driving unit does not properly operate at the first
time by cutting off the TFTs from communication with the source lines (i.e., the
data and power lines). However, when the first driving unit has some defects and
malfunctions, the open portions C and D are closed by welding or the like to connect
the power line
206 to the third driving TFT T
D3 and the data
line
204 to the third switching TFT T
S3. And thus the second
driving unit operates instead of the first driving unit. At this time, a portion
F between the data line
204 and the source of the first switching TFT T
S1
is opened to surely disconnect the first driving unit from the data line
204. As a result of the disconnection of the portion F, the organic electroluminescent
diode E is not influenced by the first driving unit.
When the second driving unit operates instead of the first driving unit, the
third and fourth switching TFTs T
S3 and T
S4 are turned on,
and then the third and fourth driving TFTs T
D3 and T
D4 are
also turned on, thereby delivering a current signal flowing in the power line
206
to the organic electroluminescent diode E through the third driving TFT T
D3.
Therefore, the organic electroluminescent diode E emits light.
As mentioned before, opening the portion F between the data line
204 and
the first switching TFT T
S1 results in the ability to stably supply
the power line signal from the power line
206 to the organic electroluminescent
diode E via only the second driving unit without any interruption by the first
driving unit. If the portion F is not cut open, the power line signal may flow
into the organic electroluminescent diode E through the first driving unit as well,
which would deteriorate the image quality of the organic electroluminescent display
device. Accordingly, when the second driving unit is set to replace the first driving
unit by welding shut the open portions C and D, the portion F between the data
line
204 and the first switching TFT T
S1 is opened.
In the above-mentioned organic electroluminescent display device, the switching
and/or driving TFTs can be p-type or n-type. If p-type TFTs are used for the driving
TFT, the electrode contacting the power line
106 or
206 can be a
drain and the electrode contacting the organic electroluminescent diode E can be
a source.
In the present invention, it is distinguishable that the backup circuit is included
in the pixel of the organic electroluminescent display device. Furthermore, the
inventive structure including the backup circuits can be adopted, for example,
to the active matrix type organic electroluminescent display device and particularly
to the top emission type organic electroluminescent display device. The top emission
type organic electroluminescent display device emits light in a direction away
from the TFTs and arrays so that it can be easily designed to have a high aperture
ratio. As an example of the top emission type organic electroluminescent display
device, the organic electroluminescent display device having a dual plate structure
(i.e., Dual Plate LED:DPLDE) is introduced.
FIG. 6 is a schematic cross-sectional view illustrating an active matrix type
organic electroluminescent display device having a dual plate structure according
to the present invention. As shown in FIG. 6, an organic electroluminescent display
device
300 of the present invention includes a first substrate
400
and a second substrate
500 which are attached to each other by a sealant
600. On a front surface of the first substrate
400, a plurality of
pixels P are defined and disposed. Each pixel P includes TFT portion T that has
switching and driving thin film transistors, and array portion
410 that
has a plurality of lines and other array elements.
On a rear surface of the second substrate
500, a first electrode
502
is disposed. The first electrode
502 is made of a transparent material,
e.g., indium tin oxide, and injects the holes to an organic luminous layer
508.
A second electrode
510 is formed on the organic luminous layer
508
so that the organic luminous layer
508 is disposed between the first and
second electrodes
502 and
510. The organic luminous layer
508
can be comprised of monolayer or multilayer. When the organic luminous layer
508
has the multilayer structure, the organic luminous layer
508 includes a
hole transporting layer
508a, an emission layer
508b and
an electron transporting layer
508c in sequential order from the
first electrode
502. The second electrode
510 electrically communicates
with the TFT portion T through a electrical connector
430. The first substrate
400 is usually fabricated to have the electrical connector
430 and
then the second substrate
500, including the organic luminous layer
508
and the first and second electrodes
502 and
510, is attached to the
first substrate
400. Thus, the electrical connector
430 connects
the switching and driving TFTs T to the second electrode
510 that injects
the electrons to the organic luminous layer
508.
According to the present invention, the inventive dummy circuits shown
in FIGS. 4 and 5 can be adopted to the TFT portions T of FIG. 6. In the present
invention, since the organic electroluminescent display device includes backup
driving circuits, if the switching and/or driving TFTs have defects or malfunctions,
the backup driving circuits can be substituted to avoid poor image quality that
would otherwise result in being displayed from such defects or malfunctions. Therefore,
a point defect in the pixel, for example, a dark spot, can be prevented from being
displayed in accordance with the present invention. As a result, the organic electroluminescent
display device of the instant invention provides a superior picture quality.
It will be apparent to those skilled in the art that various modifications and
variations can be made in the organic electroluminescent device of the present
invention without departing from the spirit or scope of the invention. Thus, it
is intended that the present invention cover the modifications and variations of
this invention provided they come within the scope of the appended claims and their equivalents.
*
