Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-03-17
2001-03-20
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C315S224000, C345S076000, C345S077000, C345S204000, C345S214000
Reexamination Certificate
active
06204610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active type electroluminescence (EL) display device in which organic EL elements are driven using thin film transistors (TFT).
2. Description of the Related Art
Organic EL elements are suited for liquid crystal displays with reduced thickness because organic EL elements are self-emissive and therefore do not require a backlight. Furthermore, organic EL elements do not restrict the viewing angle of display devices in which they are employed. For these reasons, it is widely expected that organic EL displays will be as the primary display devices of the next generation.
Organic EL display devices are commonly classified as being either passive matrix type, having a simple matrix structure, or an active matrix type employing Thin Film Transistors (TFTs). In a conventional active matrix device, a drive circuit as shown in
FIG. 1
is employed.
In
FIG. 1
, numeral
70
denotes an organic EL element. A drive circuit for one pixel comprises a switching TFT
71
which turns on and off according to a selection signal SCAN. In the TFT
71
, a display signal DATA from a display signal line
75
is applied to the drain, while the selection signal SCAN from a selection signal line
76
is applied to the gate. The drive circuit also comprises a capacitor
72
connected between the source of the TFT
71
and a predetermined direct current voltage Vsc. When the TFT
71
is turned on, the capacitor
72
is charged with the display signal supplied from the display signal line
75
. The capacitor
72
retains the charge voltage VG when the TFT
71
is turned off. The drive circuit further includes a driving TFT
74
. In the TFT
74
, the drain is connected to a power source line
77
that supplies a power source voltage Vdd, while the source is connected to the anode of the organic EL element
70
. The retained voltage VG from the capacitor
72
is supplied to the gate of the TFT
74
, which allows the TFT
74
to drive the organic EL element
70
by a current. The cathode of the organic EL element is typically connected to a ground (GND) potential. The power source voltage Vdd is a positive potential of, for example, 10V. The voltage Vsc may be the same potential as Vdd, or alternatively, a ground (GND) potential.
As shown in
FIG. 2
, the organic EL element
70
comprises an anode
51
constituted by a transparent electrode made of ITO (indium tin oxide) or a similar material, and a cathode
55
composed of a magnesium-indium alloy. Laminated between the anode
51
and the cathode
55
are, in order, a hole-transport layer
52
composed of MTDATA(4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), an emissive layer
53
composed of TPD (N,N′-diphenyl-N, N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine) and rubrene, and an electron transport layer
54
made of Alq3 (8-hydroxyquinoline aluminium). Light is emitted when a hole injected from the anode
51
and an electron injected from the cathode
55
recombine within the emissive layer
53
. The light radiates outside through the side of the transparent anode
51
, as indicated by an arrow in the figure.
The driving TFT
74
is configured by forming on a glass substrate
60
, in order, a gate electrode
61
; a gate insulating film
62
; a poly-silicon thin film
65
including a drain region
63
, a channel region, and a source region
64
; an interlayer insulating film
66
; and a planarization film
67
. The drain region
63
is connected to a drain electrode
68
constituting the power source line
77
(see FIG.
1
). The source region
64
is connected to the transparent electrode
51
serving as the anode of the organic EL element.
In a conventional arrangement, the cathode of the EL element is connected to the ground potential. The anode is connected to the TFT
74
for driving the EL element by current, and this TFT
74
is supplied with a fixed positive power source voltage Vdd. With such an arrangement, the maximum current flowing in one EL element is fixed, and the emissive luminance of the pixel is therefore also fixed.
When displaying an image in which light-emitting pixels dominate a large area in the overall display screen, if the luminance of the light-emitting pixels is too high, the displayed image may become glaring or bright, and unpleasant to the viewer's eyes. The above-mentioned power source voltage may therefore be lowered to set a lower maximum current value, such that the pixels emit light at a slightly reduced luminance. Under such a setting, the emissive luminance becomes similarly reduced when displaying an image in which light-emitting pixels cover only a small area of the overall display screen, producing a display image having a low contrast. However, if the power source voltage is set at a high level to allow the pixels to emit light at an increased luminance suitable for an image having a small area covered with light-emitting pixels, the display screen again becomes glaringly bright in the viewer's eyes when displaying an image having a large area dominated by light-emitting pixels. Furthermore, power consumption will be undesirably increased.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a device for displaying images having suitable contrast according to the area covered by light-emitting pixels, namely, the number of light-emitting pixels.
To accomplish the above object, the present invention provides an electroluminescence display device having a plurality of pixels. Each pixel comprises an electroluminescence element including at least an emissive layer between an anode separately provided for each pixel and a cathode commonly provided for the plurality of pixels. Each pixel further comprises at least a switch element for controlling a current supply from a power source commonly provided for the plurality of pixels to the anodes of the electroluminescence elements. In this display device, a current flowing from the common cathode provided for the plurality of pixels is detected, and emissive luminance of the electroluminescence elements is controlled according to the detected current.
According to another aspect of the present invention, the display device comprises a current detector circuit for detecting current flowing from the common cathode provided for the plurality of pixels, and a control circuit for controlling the emissive luminance of the electroluminescence elements according to the detected current.
According to a further aspect of the present invention, there is provided a drive circuit for an electroluminescence display device. In the device, each of a plurality of pixels comprises an electroluminescence element including at least an emissive layer between an anode separately provided for each pixel and a cathode commonly provided for the plurality of pixels. Each pixel further comprises at least a switch element for controlling a current supply from a power source commonly provided for the plurality of pixels to the anodes of the electroluminescence element. The drive circuit includes a current detector circuit for detecting current flowing from the common cathode provided for the plurality of pixels, and a control circuit for controlling the emissive luminance of the electroluminescence elements according to the detected current.
In a still further aspect of the present invention, the current detector circuit generates a voltage corresponding to the detected current. Furthermore, the control circuit includes a voltage inverting amplifier circuit for inverting and amplifying a voltage output from the current detector circuit, and a current amplifier circuit for performing current amplification of an output from the voltage inverting amplifier circuit.
According to another aspect of the present invention, there is provided a method for driving an electroluminescence display device having a plurality of pixels. Each pixel comprises an electroluminescence element including at least an emissive layer between an anode separately p
Cantor & Colburn LLP
Philogene Haissa
Sanyo Electric Co,. Ltd.
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