Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-11-30
2002-08-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S504000
Reexamination Certificate
active
06429599
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active type color EL (electroluminescence) display device in which an electroluminescence (EL) element is driven using a thin film transistor(TFT).
2. Description of Related Art
Practical use of organic EL elements in next generation display devices is greatly expected, because such displays can eliminate need for a back light as required in a liquid crystal display device for self-emission, can be optimally made thin, and can have an unlimited viewing angle.
Three methods have commonly been proposed for achieving color display in a display device comprising such an organic EL element.
In the first method, different emissive materials for each of the primary RGB colors are used in corresponding emissive layers to individually form discrete color pixels directly emitting respective RGB light rays. In another method, an emissive layer generates white luminescence which is then converted into three primary colors using color filters. A third method is based on conversion of light from a blue emissive layer into three primary colors using color conversion mediums (CCM). As light energy is lost in the second and third methods above due to the use of color filters or color conversion mediums, the first method is the most effective of these in this respect because a desired light ray is directly emitted.
Meanwhile, to drive an organic EL display device, two types of driving methods, a passive type using a passive matrix and an active type employing TFTS, are available. The circuit configuration shown in
FIG. 1
may be used in an active display.
FIG. 1
illustrates a circuit configuration for a single pixel in such a display pixel. Each pixel comprises an organic EL element
20
, a first TFT
21
for switching, in which a display signal DATA is applied to a drain and a scan signal SCAN is applied to a gate to switch the TFT on and off, a capacitor
22
which is charged by a display signal DATA applied when the TFT
21
is on and which holds a charge voltage Vh when the TFT
21
is off, a second TFT
23
in which a drain is connected to a drive source of a voltage V
COM
, a source is connected to an anode of the organic EL element
20
and a hold voltage Vh is applied to a gate from the capacitor
22
to drive the organic EL element
20
.
A scan signal SCAN rises to an H level during one horizontal scanning period (1H). When the TFT
21
is switched on, a display signal DATA is applied to one end of the capacitor
22
, which is then charged by a voltage Vh corresponding to the display signal DATA. This voltage Vh remains held in the capacitor
22
for one vertical scanning period (1V) even after the signal SCAN becomes a low level to switch the TFT
21
off. Because the voltage Vh is supplied to the gate of the TFT
23
, the EL element is controlled so as to emit light with a luminance in accordance with the voltage Vh.
The conventional configuration of such an active type EL display device for achieving color display by means of the above-mentioned first method will be now described.
FIG. 2
depicts a conceptual plan view showing a configuration of a related art device, and
FIG. 3
is a cross section taken along line C—C in FIG.
2
. Each of the drawings depicts three pixels.
In
FIGS. 2 and 3
, numeral
50
represents a drain line for supplying a display signal DATA, numeral
51
represents a drive source line for supplying a supply voltage V
COM
, and numeral
52
represents a gate line for supplying a scan signal SCAN. Further, numerals
53
,
54
, and
55
designate features corresponding the first TFT
21
, the capacitor
22
, and the second TFT
23
in
FIG. 1
, respectively, and numeral
56
designates an anode of the EL element
20
which constitutes a pixel electrode. As shown, discrete anodes
56
are separately formed for each pixel on a planarization insulating film
60
. A hole-transport layer
61
, an emissive layer
62
, an electron-transport layer
63
, and a cathode
64
are sequentially laminated on the discrete anode
56
, thereby forming an EL element. Holes injected from the anodes
56
and electrons injected from the cathodes
64
are recombined inside the emissive layer
62
, which emits light in the direction of the transparent anodes toward outside, as shown by arrows in FIG.
3
. Here, discrete hole-transport layers
61
, discrete emissive layers
62
and discrete electron-transport layers
63
having substantially the same shape as the discrete anodes
56
are provided for respective pixels. Emissive materials which are different for each RGB are used in the corresponding emissive layers
62
, and therefore light rays having respective RGB colors are emitted from respective EL elements. The cathode
64
, which applies a common voltage to each pixel, extends over the pixels. Partitions
68
are interposed between adjoining emissive layers
62
. Further, numerals
65
,
66
, and
67
designate a transparent glass substrate, a gate insulating film, and an interlayer insulating film, respectively.
However, the arrangement of the first TFT
53
, the capacitor
54
, the second TFT
55
, and the anode
56
of the related examples do not take sufficient consideration of integration efficiency and therefore a more highly-integrated configuration is in demand.
Further, the color display device generally adopts a stripe arrangement as shown in
FIG. 4A
or a delta arrangement as shown in
FIG. 4C
as an arrangement for three primary colors of RGB. At the same time, it is necessary to use different luminescent materials for each of RGB such that discrete EL elements can directly emit light rays of respective RGB colors. Therefore, if the stripe arrangement shown in
FIG. 4A
is adopted, for example, a metal mask
70
shown in
FIG. 4B
may be used to form the luminescent layers as follows. First, a luminescent layer for R is formed by evaporating only an R color luminescent material onto the hole transport layer. Then, the metal mask
70
is displaced by a distance corresponding to one pixel in the horizontal direction to form a luminescent layer for G by evaporating only a G color luminescent materials on the hole transport layer. Finally, the metal mask
70
is further displaced by one pixel in the horizontal direction to form a luminescent layer for B by evaporating only a B color luminescent material. In the case of the delta arrangement shown in
FIG. 4C
, the luminescent layers can be similarly formed using the metal mask shown in FIG.
4
D.
However, during the process for forming the luminescent layers by evaporating the luminescent materials, a so-called “diffusion” phenomenon is caused in which luminescent materials are deposited onto regions other than the regions directly under the openings in the metal masks
70
and
71
. Because of such diffusion phenomenon or because of imperfect construction of the metal mask itself, colors in adjoining pixels are adversely mixed causing color purity to deteriorate. Particularly in delta arrangements, wherein adjoining pixels in the column and row directions differ from one another, this disadvantage is further pronounced.
SUMMARY OF THE INVENTION
The present invention provides a color display device suitable for a highly integrated configuration.
In accordance with one aspect of the present invention, a first thin film transistor is disposed in a region between a gate line and a capacitor, and a second thin film transistor is disposed in a region between the capacitor and an EL element. This configuration allows the capacitor, the first thin film transistor, and the second thin film transistors to be densely arranged, thereby facilitating formation of a highly integrated configuration.
In accordance with another aspect of the present invention, said first thin film transistor is connected to one end of one electrode of said capacitor while a gate of said second thin film transistor is connected to the other end which is opposed to said one end of the capacitor. Thus, the first thin film transistor and the gate of the second thin
Cantor & Colburn LLP
Dinh Trinh Vo
Sanyo Electric Co,. Ltd.
Wong Don
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