Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-11-30
2001-08-28
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S506000, C345S045000, C345S076000
Reexamination Certificate
active
06281634
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.
In a color EL display device as described above, however, especially in the case where different emissive materials of the emissive layer are used for each of three RGB colors, an increased number of manufacturing processes is expected such as forming a discrete hole-transport layer
61
, a discrete emissive layer
62
, and a discrete electron-transport layer
63
for each pixel, followed by forming partitions
68
and finally forming the cathode
64
. There has therefore been a demand for a color EL display device with a simpler configuration.
SUMMARY OF THE INVENTION
The present invention is directed to providing an EL display device that can be manufactured through simple processes.
In an EL display device in accordance with the present invention, a charge-transport layer, for example, a hole-transport and/or an electron-transport layer, and a second electrode (e.g. a cathode) are formed over a plurality of pixels, thereby reducing the number of manufacturing processes. Further, the charge-transport layer is necessarily disposed between a first electrode and a second electrode, a short between both electrodes can be prevented without providing any extra partitions or the like.
Further, emissive materials which are different for each of red, green and blue colors are used for the emissive layers, whereby each electroluminescence element directly emits any one of RGB colors. According to the present invention, it is possible to manufacture such a color EL display device having emissive materials which are different for each pixel, with a reduced number of processes.
It is preferable that a first electrode (e.g. an anode) and an emissive layer have substantially the same planar shape. This minimizes the region for forming the emissive layer, prevents mixture of the emissive layers in adjoining pixels, and increases color purity.
It is also preferable that the emissive layer is larger than the anode such that the emissive layer covers the edges of the anode. This allows the emissive layer to be infallibly positioned over the first electrode even when an error is caused in forming the emissive layer. As a result, a leak current generated at the edges of the first electrode and a false field concentration can be prevented.
Further, in the present invention, the above-mentioned switch can be formed as a thin film transistor. The active type EL display transistor using such a thin film transistor allows accurate control of luminescence at each EL element, thereby achieving display of high quality.
In the present invention, the charge-transport layer has higher resistance than the first and the second electrode. By interposing such transport layers having higher resistance between the first and the second electrode, electrical separation of both electrodes can be ensured.
REFERENCES:
patent: 5384517 (1995-01-01), Uno
patent: 5399936 (1995-03-01), Namiki et al.
patent: 5550066 (1996-08-01), Tang et al.
patent: 5670792 (1997-09-01), Utsugi et al.
patent: 5684365 (1997-11-01), Tang e
Cantor & Colburn LLP
Sanyo Electric Co,. Ltd.
Tran Thuy Vinh
Wong Don
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