Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-04-02
2002-10-22
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S084000, C345S092000
Reexamination Certificate
active
06469450
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a display device. particularly a thin film transistor driven organic electro-luminescent display device (hereafter referred to as TFT-OELD) which is driven by a thin film transistor (hereafter referred to as TFT) and provided with an organic electro-luminescent element (hereafter referred to as OELD) of a high polymer system formed in a liquid phase process.
2. Description of Related Art
TFT-OELD is promising because it is a display device which realizes light-weightness, thinness, smallness, higher accuracy, wider view angle, lower electric consumption, and the like.
FIG. 1
shows a conventional TFT-OELD.
FIG. 2
shows a cros-ssectional view of the conventional TFT-OELD. Here, there is only one pixel
11
depicted, but there are actually many pixels
11
in plural rows and lines. Here, OELD
18
is a high polymer system, formed by a liquid phase process, such as spin coating, blade coating, ink jet, or the like.
In order to implement gradation, in the case of the structure shown in
FIG. 1
, a gate voltage of a driving TFT
17
is made to change and conductance is changed, so electric current which flows in the OELD
18
needs to be controlled. However, gradation according to this method, particularly in half tone, result in irregularity of transistor characteristics of the driving TFT
17
and appears as brightness irregularity of the OELD
18
, and there is a problem such that the screen becomes non-uniform.
Therefore, as shown in
FIG. 3
, a method is considered which implements gradation by changing a light emitting area of the OELD
18
(Japanese Patent Application
9-233107
).
FIG. 4
shows a driving method of this method. A scanning electric potential
31
is applied to a scanning line
12
, and a signal line
13
is formed of a signal line (lower bit)
131
and a signal line (upper bit)
132
. A signal electric potential (lower bit)
321
and signal electric potential (upper bit)
322
are respectively applied as a signal electric potential
32
. A driving TFT
17
is formed of a driving TFT (lower bit)
171
and a driving TFT (upper bit)
172
, and the OELD
18
is formed of an OELD (lower bit)
181
and an OELD (upper bit)
182
. In this example, 2-bit
4
gradation is considered, so an area ratio between OELD (lower bit)
181
and OELD (upper bit)
182
is 1:2.
In this method, the driving TFT
17
takes either a substantially completely on state or a substantially completely off state. In the on state, the resistance of the driving TFT
17
is small enough to be ignored, compared to the resistance of OELD
18
, and the electric current amount which flows in the driving TFT
17
and OELD
18
is substantially determined by only the resistance of the OELD
17
.
Therefore, irregularity of transistor characteristics of the driving TFT
17
does not appear as brightness irregularity of the OELD
18
. Furthermore, in the off state, voltage applied to the OELD
18
becomes less than a threshold voltage, so the OELD
18
does not emit light at all, and, needless to say, irregularity of transistor characteristics of the driving TFT
17
does not appear as brightness irregularity of the OELD
18
.
FIG. 5
is a cross-sectional view of TFT-OELD which implements gradation display by changing a light emitting area of the OELD
18
shown in
FIGS. 3 and 4
. FIG.
5
(
a
) is a cross-sectional view of the OELD (lower bit)
181
, and FIG.
5
(
b
) is a cross-sectional view of the OELD (upper bit)
182
. The ratio between the light emitting part
25
of the OELD (lower bit)
181
and the light emitting part
25
of the OELD (upper bit)
182
is preferably 1:2.
A light emitting layer
22
is an OELD of a high polymer system and formed in a liquid phase process. A surface of a bank
24
is lyophobic and the light emitting layer
22
does not remain. Therefore, the area of the OELD
18
is determined by patterning. With respect to a side surface of the bank
24
, the materials and processing determine whether the side surface of the bank
24
becomes lyophobic or lyophilic.
FIG. 5
shows the case of a lyophilic side surface of the bank
24
. As a phenomenon that is characteristic of a liquid phase process, the light emitting layer
22
has a cross-sectional shape which is pulled toward the side surface of the bank
24
. In this case, electric current flows into a thinner part of the light emitting layer
22
, and this part becomes a light emitting part
25
. The cross-sectional shape of the light emitting layer
22
described here is sensitive to liquid amount, liquid material, an initial position of the liquid, and a state, temperature, atmosphere, or the like of a substrate, and which are difficult to control. That is, it is difficult to obtain an absolute value of a desired light emitting area. Because of this, it is difficult to obtain an accurate ratio of 1:2, between the light emitting part
25
of the OELD (lower bit)
181
and the light emitting part
25
of the OELD (upper bit)
182
, and ultimately, it is difficult to obtain accurate gradation.
FIG. 6
is a cross-sectional view of OELD (lower bit)
181
(FIG.
6
(
a
)) and a cross-sectional view of OELD (upper bit)
182
(FIG.
6
(
b
)) in the same manner as in FIG.
5
. In
FIG. 6
, the side surface of the bank
24
is lyophobic. As a phenomenon that is characteristic of a liquid phase process, the light emitting layer
22
has a cross-sectional shape which is distant from the side surface of the bank
24
. In this case as well, electric current flows into the thinner part of the light emitting layer
22
, and this part becomes the light emitting part
25
. In this case as well, in the same manner as in the case of
FIG. 5
, it is difficult to obtain an accurate ratio of 1:2 between the light emitting part
25
of the OELD (lower bit)
181
and the light emitting part
25
of the OELD (upper bit)
182
, so it is difficult to obtain accurate gradation.
SUMMARY OF THE INVENTION
Therefore, one aspect of this invention is to obtain an accurate ratio of the light emitting parts, and accurate gradation. Therefore, the invention may provide a display device in which gradation is implemented by forming a plurality of TFTs and a plurality of OELDs in each pixel, directly connecting the TFTs and OELDs, switching an on and off state of the TFTs, and controlling an area of the OELDs, that emits light, wherein the plurality of OELDs have the same shape, and gradation is implemented by controlling the number of OELDs that are created to emit light and by controlling an appropriate on/off state of the TFT connected to each OELD.
According to this structure, as a characteristic phenomenon of a liquid phase process, even if an OELD becomes a cross-sectional shape which is pulled in to a side surface of a bank or is distant from the side surface of the bank, the light emitting part of each OELD is the same area, and accurate gradation can be obtained. In this structure as well, it is difficult to obtain an absolute value of a desired light emitting area, but the light emitting area of a plurality of OELDs becomes equal, so the ratio of the light emitting areas can be accurate by controlling the number OELDs.
The display device may also include a plurality of OELDs that have a round shape. According to this structure, the light emitting part of each OELD can reliably be the same area, and accurate gradation can be obtained. The reasons are as follows. When an OELD has a shape with a rectangular vertex (or vertices), there is a possibility that a phenomenon may occur, for example, that the vertex becomes pulled in or the vertex cannot be filled. This phenomenon prevents a user from obtaining accurate gradation for the same reason as in the problems of a cross-sectional shape as described above. This phenomenon is more sensitive to the liquid amount, liquid material, initial position of liquid, and the state, temperature, and atmosphere of a substrate, more so than the problems in a cross-sectional shape described above, and it is difficult to control this phenomenon
Philogene Haissa
Seiko Epson Corporation
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