Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-07-31
2003-01-21
Phan, Tho (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S076000, C345S092000
Reexamination Certificate
active
06509692
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a self-emissive display device of an active matrix type having a self-emissive element which is driven by a selection driving circuit formed of a thin film transistor (TFT) or the like disposed individually for each pixel, and more particularly to an organic electroluminescence (EL) display device of the active matrix type.
2. Description of the Related Art
Because self-emissive display devices are smaller and consume less electric power than CRTs, and exhibit no dependency on the viewing angle unlike LCDs, EL display devices employing EL elements are attracting attentions in recent years as potential replacements for CRTs and LCDs. Further, EL display devices provided with TFTs as switching elements for driving such EL elements, for example, are now being developed.
FIG. 1
is an equivalent circuit diagram of an organic EL display device. In the device, a plurality of gate lines
1
extending in a row direction are disposed, and a plurality of data lines
2
and a plurality of driving lines
3
are disposed in a column direction crossing the gate lines. The driving line
3
is connected to a power source PV, which supplies a positive constant voltage of, for example, 10 V using a ground voltage as a reference voltage. A selection TFT
4
is connected to each intersection between the gate line
1
and the data line
2
. The selection TFT
4
is of a double gate configuration in which two TFTs
4
a
and
4
b
are connected in series. The TFTs
4
a
and
4
b
of the selection TFT
4
each have a gate connected to the gate line
1
, and a drain of the TFT
4
a
is connected to the data line
2
. The selection TFT
4
b
has a source connected to one electrode of a storage capacitor
5
and a gate of a driving TFT
6
, which has a drain connected to the driving line
3
and a source connected to an anode of an organic EL emissive element
7
. The cathode of the organic EL emissive element
7
is connected to ground. A capacitor line
9
extending in the column direction is connected to the other electrode of the storage capacitor
5
.
The gate lines
1
are connected to an unillustrated gate line driver, and sequentially receive a gate signal applied by the gate line driver. The gate signal is a binary signal which assumes either an ON or OFF state. The signal has a positive predetermined voltage in the ON state, and 0V in the OFF state. The gate line driver turns on the gate signal on the predetermined gate line selected among the plurality of gate lines
1
connected thereto. When the gate signal is turned on, TFTs of all the selection transistors
4
connected to that gate line
1
are turned on, and the data line
2
and the gate of a driving transistor
6
are connected through the selection transistor
4
. To the data line
2
, a data signal determined in accordance with a displayed image is applied from a data line driver
8
, and is therefore applied to the gate of the driving transistor
6
and charged in the storage capacitor
5
. The driving transistor
6
connects the driving line
3
and the organic EL emissive element
7
at an electrical conductivity corresponding to the value of the data signal. As a result, a current corresponding to the data signal is supplied from the driving line
3
to the organic EL emissive element
7
through the driving transistor
6
, whereby the organic EL emissive element
7
emits light at a luminance corresponding to the data signal. The storage capacitor
5
forms a static capacitor with another electrode, such as a dedicated capacitor line
9
or the driving line
3
, and is capable of storing the data signal for a predetermined period of time. Even after a particular gate line
1
is deselected to turn off the selection transistor
4
when the gate line driver selects another gate line
1
, the data signal is stored by the storage capacitor
5
for one vertical scanning period, during which the driving transistor
6
maintains the above electrical conductivity. The organic EL emissive element can thereby continue to emit light at the same luminance.
For the active matrix organic EL display device operating in the above-described principles, it should be noted that the term “selection driving circuit” in this specification refers generally to a circuit having the selection transistor
4
and the driving transistor
6
described above and applying a signal simultaneously selecting one or more display elements, such as the gate signal, and the data signal determined by the displayed image to thereby supply a current corresponding to the data signal to a predetermined display element.
FIG. 2
is a cross sectional view of an organic EL display device of the active matrix type. A plurality of driving TFTs
6
are disposed on a glass substrate
11
. In the driving TFT
6
, a gate electrode
6
G is provided facing a source
6
S, a channel
6
C, and a drain
6
D with an interlayer insulating film
12
interposed between them. In the illustrated example, the driving TFT
6
is of a bottom gate type with the gate electrode
6
G disposed below the channel
6
C. An interlayer insulating film
13
is formed on the driving TFT
6
, and the data line
2
and the driving line
3
are disposed on top of the film
13
. The driving line
3
is connected to the drain
6
D of the driving TFT
6
through a contact hole. On such elements a planarization insulating film
14
is formed, on which the organic EL emissive element
7
is disposed for each pixel. The organic EL emissive element
7
is composed of an anode
15
formed of a transparent electrode of, for example, ITO (indium tin oxide), a hole transportation layer
16
, an emissive layer
17
, an electron transportation layer
18
, and a cathode
19
of a metal, such as aluminum, stacked in the above order. Holes injected from the anode
15
to the hole transportation layer
16
and electrons injected from the cathode
19
to the electron transportation layer
18
are recombined in the emissive layer
17
to cause emission of light, which is transmitted through the transparent electrode
15
and the glass substrate
11
to outside, as indicated by the arrow in the figure. The anode
15
and the emissive layer
17
are individually formed for each pixel, while the hole transportation layer
16
, the electron transportation layer
18
, and the cathode
19
are shared by respective pixels.
FIG. 3
shows a correlation curve between a voltage V
EL
applied to the organic EL emissive element
7
and the luminance of light emitted at the corresponding moment. Light is not emitted regardless of the voltage value when the voltage V
EL
is at a predetermined value V
0
or smaller. After light emission begins when the voltage V
EL
exceeds the predetermined value V
0
, the luminance is increased with an increase in the voltage V
EL
. When an organic EL element is used as an emissive element in a display device, images are displayed by controlling the voltage V
EL
applied to the EL emissive element to fall within a range between a minimum voltage V
min
for a current observed when the EL emissive element emits dim light at a predetermined luminance L
min
and a maximum voltage V
MAX
for a current corresponding to a maximum luminance L
MAX
defined as the luminance resulting in a predetermined contrast ratio to the luminance L
min
of, for example, 100:1. Although it is possible to cause stronger light emission and achieve a higher contrast by setting the voltage V
EL
to an even higher value, the life of an organic EL element is shortened by strong light emission, which requires a larger amount of current. Therefore, in view of both the life and current consumption, the maximum luminance and contrast are set at a level appropriate to the circumstances under which the display device is employed.
FIG. 4
is a circuit diagram illustrating only the power source PV, the driving TFT
6
, and the EL emissive element
7
for one pixel, extracted from the circuit diagram of FIG.
1
. As can be seen from the figure, the driving TFT
Cantor & Colburn LLP
Phan Tho
Sanyo Electric Co,. Ltd.
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