JP2000268957A - Electroluminescence display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To substantially prevent shortage of the life of an EL element caused by accumulation of space charges in the EL element generating by repeating current driving. SOLUTION: In this EL display device having at least a hole transport layer and a luminescent layer between an anode and a cathode and emitting light by supplying a specified bias, a selecting circuit 2 for supplying voltage VBS higher than power source voltage supplying to the anode during driving and either one voltage of earthing voltage and negative voltage Vcd to the cathode is installed, and space charges accumulating in the element are periodically removed by applying reverse bias between the anode and the cathode during a non-display period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electroluminescent display device having at least a hole transport layer and a light emitting layer between an anode and a cathode and emitting light by supplying a predetermined bias.

[0002]

2. Description of the Related Art An organic EL element emits light by itself, so it is not necessary for a backlight necessary for a liquid crystal display device, and is optimal for thinning. In addition, since there is no restriction on a viewing angle, it is practically used as a next-generation display device. It is greatly expected to be used.

As shown in FIG. 7, such an organic EL device has a hole transport layer 5 made of MTDATA between an anode 51 made of a transparent electrode such as ITO and a cathode 55 made of an MgIn alloy.
2, a light emitting layer 53 composed of TPD and Rubrene and an electron transport layer 54 composed of Alq3 are sequentially laminated. And
Light is emitted when the holes injected from the anode 51 and the electrons injected from the cathode 55 are recombined inside the light-emitting layer 53, and light is transmitted from the transparent anode side to the outside as shown by arrows in the drawing. Radiated.

[0004] There are two types of display devices for driving the organic EL, a passive type having a simple matrix structure and an active type using a TFT.
The driving circuit shown in FIG. 6 was used.

In FIG. 6, reference numeral 70 denotes an organic EL element,
The driving circuit for the pixels includes a switching TFT 71 that is turned on / off by the selection signal SCAN when the display signal DATA from the display signal line 75 is applied to the drain and the selection signal SCAN from the selection signal line 76 is applied to the gate. A power supply line connected between the source and a predetermined DC voltage Vsc, charged by a display signal supplied when the TFT 71 is turned on, and holding a charging voltage VG when the TFT 71 is turned off; 77, a source is connected to the anode of the organic EL element 70, and a driving TFT 74 that current-drives the organic EL element 70 by supplying a holding voltage VG from the capacitor 72 to the gate. . Also, usually, organic E
The cathode of the L element is connected to a ground (GND) potential, and the drive power supply voltage Vdd is a positive potential, for example, 10V. The voltage Vsc is, for example, the same potential as Vdd or ground (GN
D) potential.

As shown in FIG. 7, the driving TFT 74 is formed on a glass substrate 60 by forming a polysilicon thin film 65 having a gate electrode 61, a gate insulating film 62, a drain region 63, a channel region and a source region 64, Insulating film 6
6, a planarizing film 67 is sequentially laminated, the drain region 63 is a drain electrode 68 constituting a power supply line 67 (see FIG. 6), and the source region 64 is an organic EL.
It is connected to a transparent electrode 51 which is the anode of the device.

[0007]

As described above, the EL element emits light by current driving. When driven, current flows from the anode to the cathode, and when not driven, no current flows. That is, since current always flows in only one direction, when driving is repeated, space charges accumulate in the EL element such as between the hole transport layer and the light emitting layer, or between the electron transport layer and the light emitting layer, and this is the E
This causes the life of the L element to be shortened. In particular, it is considered that space charges easily accumulate between the hole transport layer and the light emitting layer even within the device. Such a problem is the same even when the driving method is a passive type and an active type.

Accordingly, it is an object of the present invention to drive an EL element with current so as to extend the life as much as possible.

[0009]

According to the present invention, there is provided an electroluminescent display device having at least a hole transport layer and a light emitting layer between an anode and a cathode and emitting light by supplying a predetermined bias. A reverse bias is applied between the anode and the cathode during the period.

Further, according to the present invention, when a pulse signal generated during a non-display period is input, and when the pulse signal is at a first level,
A first potential for supplying the predetermined bias between the anode and the cathode is applied to the cathode or the anode, and when the pulse signal is at a second level, the reverse bias is applied between the anode and the cathode. A selection circuit for applying a second potential to be supplied to the cathode or the anode;

In the present invention, the pulse signal is a blanking pulse signal or a clamp pulse signal generated during a non-display period.

[0012]

FIG. 3 shows a circuit configuration of an EL display panel used in an EL display device according to the present invention, which is basically the same as the conventional one.

That is, this configuration is an active type having a plurality of pixels, and a driving circuit for one pixel for driving the organic EL element 20 is provided with a display signal DATA from a display signal line 25.
Is applied to the drain, the selection signal SCAN from the selection signal line 26 is applied to the gate, and the switching TFT 21 is turned on and off by the selection signal SCAN, and is connected between the source of the TFT 21 and a predetermined DC voltage Vsc. When the TFT 21 is turned off, it is charged by the supplied display signal, the capacitor 22 holding the charging voltage VG, the drain is connected to the power supply line 27 for supplying the drive power supply voltage Vdd, and the source is connected to the anode 201 of the organic EL element 20. The driving TFT 24 drives the organic EL element 20 by supplying a holding voltage VG from the capacitor 22 to the gate.

As in the prior art, the drive power supply voltage Vdd is a positive potential, for example, 10 V, and the voltage Vsc is, for example, Vdd
In this embodiment, the cathode 202 of the organic EL element 20 is not a fixed potential such as a ground (GND) potential, but is a terminal T that supplies a variable potential. It is connected to the.

FIG. 4 shows a plurality of pixels shown in FIG.
It is a sectional view showing the structure of the EL element 20 and the driving TFT 24, 31 is a drain line made of aluminum for supplying a display signal DATA, 32 is a power supply voltage line made of aluminum for supplying a power supply voltage Vdd, and 33 is a selection signal Sc.
a gate line of chromium that supplies an
3 represents the driving TFT 24 shown in FIG. 3, and 37 represents the anode 201 of the EL element 20 made of ITO and constituting the pixel electrode.

The driving TFT 36 is formed as follows. First, a chromium gate electrode 39 is formed on a transparent glass substrate 38, and a gate insulating film 40 is formed thereon. Next, the polysilicon thin film 4 is formed on the gate insulating film 40.
Then, a drain line 31 and a power supply line 32 are formed on the substrate 1 covered with an interlayer insulating film 42. Further, a flattening insulating film 43 is laminated, and an anode 3 made of ITO is formed thereon.
7 is formed. Then, the drain region of the polysilicon thin film 41 is contacted with the power supply line 32, and the source region is contacted with the anode 37. Also, the structure of the switching TFT 21 shown in FIG.
A capacitor 22 connected to 21 is composed of a chrome electrode and a polysilicon thin film with a gate insulating film interposed therebetween.

The anode 37 is formed separately for each pixel on the flattening insulating film 43, and a hole transport layer 44, a light emitting layer 45, an electron transport layer 46, and a cathode 47 are sequentially laminated thereon. As a result, an EL element is formed. Then, the holes injected from the anode 37 and the electrons injected from the cathode 47 are recombined inside the light emitting layer 45 to emit light, and this light is emitted from the transparent anode side to the outside as shown by the arrow. Radiated. The light-emitting layer 45 is formed in the same shape as the anode 37 separately for each pixel.
By using a different light emitting material for each B, each light of RGB is emitted from each EL element.

Here, the hole transport layer 44 and the electron transport layer 4
6, as a material of the cathode 47, for example, MTDATA, Alq3,
An MgIn alloy is used, and as each light emitting layer 45 of R, G, and B, Alq containing DCM as a dopant, Alq containing quinacridone as a dopant, and DPVBi containing distyrylarylene as a dopant are used. ing.

Incidentally, the anode 37 of the EL element is formed independently for each pixel as described above, while the cathode 4 is formed.
7 is formed in common for all pixels as shown in FIG. As is further clear from the plan view shown in FIG.
The cathode 47 is continuously formed on one surface, and the cathode material is stretched as it is to form a connection terminal T with an external circuit. The connection terminal T is a signal board 4 such as TAB or FPC.
8 is connected to a connection terminal 49 made of copper or the like formed on the back surface of the device 8 and connected to an external circuit.

Next, an external circuit connected via the signal board 48 will be described with reference to FIGS.

FIG. 1 is a circuit diagram showing the configuration of an external circuit, which comprises a display controller 1 and a selection circuit 2. The display controller 1 decodes the video input signal to
A decoder 3 for outputting video signals of three primary colors of G and B;
A video buffer 4 for current-amplifying a video signal from the decoder 3, a synchronization separation circuit 5 for separating a synchronization signal from a video input signal, and a blanking pulse BLP and a clamp pulse CLP based on the separated synchronization signal. It comprises a blanking pulse generating circuit 6 and a clamp pulse generating circuit 7, and a timing controller 8 which generates various timing signals used in the organic EL display panel based on the output of the synchronization separating circuit 5.

The selection circuit 2 is configured by connecting n-channel TFTs 9 and 10 in series. One end of the TFT 9 is connected to a reverse bias voltage VBS, and one end of the TFT 10 is connected to a ground potential or a negative potential voltage Vcd. , TFTs 9 and 10 are connected to cathode 202 of EL element 20 shown in FIG.
(47 in FIGS. 4 and 5). The gate of the TFT 9 receives the clamp pulse BLP as it is, and the gate of the TFT 10 receives an inverted signal of the clamp pulse BLP via the inverter 11.
Here, the reverse bias voltage VBS is equal to the power supply voltage Vdd shown in FIG.
It is set to a higher voltage, for example, 20V.

As shown in FIG. 2A, the video input signal input to the display controller 1 has a display period and a non-display period which are clearly separated from each other, and the blanking pulse BLP has a non-display period as shown in FIG. 2B. Is output to Further, the clamp pulse CLP is output as shown in FIG. 2C and is also output during the non-display period. FIG. 2D shows the horizontal synchronization signal Hsync separated from the synchronization.

As shown in FIG. 2B, the clamp pulse BLP is
It becomes L level during the display period, and this L level signal
9 is input to the gate of the TFT 10, and an H level signal obtained by inverting the L level signal is input to the gate of the TFT 10.
9 turns off and the TFT 10 turns on. Therefore, the selection circuit 2
In this case, the voltage Vcd of the ground potential or the negative potential is output to the connection terminal T during the display period, and this voltage Vcd is supplied to the cathodes 202 of all the EL elements 20 through the terminal T. All EL elements 20
Since the anode 201 is connected to the positive power supply voltage Vdd via the driving TFT 24 as described above, the EL element is biased in the forward direction, and current driving similar to the conventional one is realized.

On the other hand, the clamp pulse BLP goes to the H level during the non-display period, and the H level signal is input to the gate of the TFT 9 and the L level signal obtained by inverting the H level signal is input to the gate of the TFT 10. The TFT 9 turns on and the TFT 10 turns off. Therefore, in the selection circuit 2, the reverse bias voltage VBS is output to the connection terminal T during the non-display period, and this voltage VBS is supplied to the cathodes 202 of all the EL elements 20 through the terminal T. Since the voltage VBS is set to a voltage higher than the power supply voltage Vdd as described above,
A voltage higher than that of the anode 201 is applied to the cathode 202 of the L element 20, and a reverse bias is applied to the EL element 20.

In the EL element 20, when current driving is repeated during the display period, space charges accumulate between the hole transport layer 44 and the light emitting layer 45 and between the electron transport layer 46 and the light emitting layer 45, which shortens the life. Cause you to However, in the present embodiment, since a reverse bias is applied to the EL element 20 during the non-display period, space charges accumulated between the hole transport layer 44 and the light emitting layer 45 and between the electron transport layer 46 and the light emitting layer 45 are reduced. It is discharged. In particular, since the blanking pulse BLP is periodically output every one horizontal period in the non-display period, electric charges are frequently discharged, and accumulation of electric charges can be prevented as much as possible. Therefore, the life of the EL element 20 can be extended.

In the present embodiment, the blanking pulse BLP from the display controller 1 is input to the selection circuit 2. Instead, the clamp pulse CLP or another pulse output only during the non-display period is used instead. You may make it input.

Further, in this embodiment, the voltage supplied to the cathode is changed by the selection circuit while the anode is fixed, but the voltage supplied to the anode is changed by the selection circuit by setting the cathode to the fixed potential. You may do
Further, the voltage supplied to both the anode and the cathode may be changed by a selection circuit.

[0029]

According to the present invention, the space charge accumulated in the EL element is discharged in the non-display period by repeating the current driving, so that the EL in the display period is not affected at all. The life of the element can be extended.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an external circuit configuration according to an embodiment of the present invention.

FIG. 2 is a timing chart for explaining the operation of the circuit shown in FIG.

FIG. 3 is a circuit diagram illustrating a configuration of an EL display panel according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a structure of an EL display panel according to the embodiment of the present invention.

FIG. 5 is a plan view showing a structure of an EL display panel according to the embodiment of the present invention.

FIG. 6 is a circuit diagram showing a configuration of a conventional EL display device.

FIG. 7 is a cross-sectional view illustrating a structure of a conventional EL display device.

[Explanation of symbols]

 Reference Signs List 1 display controller 2 selection circuit 6 blanking pulse generation circuit 7 clamp pulse generation circuit 20 EL element 21 switching TFT 24 driving TFT 201, 37 anode 202, 47 cathode 44 hole transport layer 45 light emitting layer 46 electron transport layer

Claims (3)

[Claims] 1. An electroluminescent display device having at least a hole transport layer and a light emitting layer between an anode and a cathode, and emitting light by supplying a predetermined bias, wherein a light is applied between the anode and the cathode during a non-display period. An electroluminescent display device, wherein a reverse bias is applied to the display. 2. A pulse signal generated during a non-display period is input, and when the pulse signal is at a first level, a first potential for supplying the predetermined bias between the anode and the cathode is applied to the cathode. Or a selection circuit for applying a second electric potential for supplying the reverse bias between the anode and the cathode when the pulse signal is applied to the anode and the pulse signal is at a second level. The electroluminescent display device according to claim 1, wherein 3. The electroluminescent display device according to claim 1, wherein the pulse signal is a blanking pulse signal or a clamp pulse signal generated during a non-display period.