KR100590066B1 - Light emitting display device and display panel thereof - Google Patents

Abstract

The present invention relates to a light emitting display device and a display panel thereof. A display panel according to the present invention is a display panel of a light emitting display device including a plurality of data lines for transmitting a data signal, a plurality of scanning lines for transmitting a selection signal, and a plurality of pixel circuits connected to the data lines and the scanning lines. A capacitor is formed between the first electrode and the second electrode, and is electrically connected to the first transistor, the third electrode of the first transistor for controlling the amount of current output to the third electrode by the voltage charged in the capacitor, A display element for displaying an image corresponding to the amount of input current, a second transistor connected to the first electrode of the first transistor and transferring a first voltage corresponding to the data signal to the first electrode of the first transistor, selection A first switching element for transmitting a data signal to the second transistor in response to the signal, and a first electrode of the first transistor in response to the control signal And a second switching element for initializing the voltage to a negative voltage.

Organic EL, display device, precharge voltage, data voltage, driving transistor

Description

LIGHT EMITTING DISPLAY DEVICE AND DISPLAY PANEL THEREOF}

1 shows a pixel circuit of a conventional voltage writing method.

2 illustrates an organic EL display device according to an embodiment of the present invention.

3 is an equivalent circuit diagram of a pixel circuit according to an exemplary embodiment of the present invention.

4 is a driving waveform diagram for driving the pixel circuit of FIG. 3.

5 is an equivalent circuit diagram of a pixel circuit according to a second exemplary embodiment of the present invention.

FIG. 6 is a driving waveform diagram for driving the pixel circuit of FIG. 5.

The present invention relates to a display device, and more particularly, to an organic electroluminescent (EL) display device and a display panel thereof.

In general, an organic EL display device is a display device for electrically exciting a fluorescent organic compound to emit light, and is capable of representing an image by driving voltage or current driving NxM organic light emitting cells. The organic light emitting cell has a structure of an anode (ITO), an organic thin film, and a cathode layer (metal). The organic thin film has a multilayer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) to improve the light emission efficiency by improving the balance between electrons and holes. It also includes a separate electron injecting layer (EIL) and a hole injecting layer (HIL).

The organic light emitting cell may be driven using a simple matrix method and an active matrix method using a thin film transistor (TFT) or a MOSFET. In the simple matrix method, the anode and the cathode are orthogonal and the line is selected and driven, whereas the active driving method connects the thin film transistor and the capacitor to each indium tin oxide (ITO) pixel electrode to maintain the voltage by the capacitor capacitance. It is a driving method. In this case, the active driving method is divided into a voltage programming method and a current programming method according to the type of signal applied to maintain the voltage on the capacitor.

1 shows a pixel circuit of a conventional voltage writing method.

In the conventional voltage write type pixel circuit, as illustrated in FIG. 1, the transistor M1 is connected to the organic EL element OLED to supply current for emitting light, and the amount of current of the transistor M1 supplies the switching transistor M2. It is to be controlled by the data voltage applied through. At this time, a capacitor C1 for maintaining the applied voltage for a predetermined period is connected between the source and the gate of the transistor M1.

When the switching transistor M2 is turned on, a data voltage is applied to the gate of the transistor M1, and the capacitor C1 is charged with a voltage V _GS applied between the gate and the source, and corresponds to the voltage V _GS . The current I _OLED flows through the transistor M1, and the organic EL element OLED emits light in response to the current I _OLED .

At this time, the current flowing through the organic EL element OLED is represented by Equation 1 below.

Where I _OLED is the current flowing through the organic EL element OLED, V _GS is the voltage between the gate and the source of the transistor M1, V _TH is the threshold voltage of the transistor M1, V _DATA is the data voltage, and β is a constant. Indicates a value.

As shown in Equation 1, a current corresponding to the data voltage is supplied to the organic EL element OECD, and the organic EL element emits light corresponding to the supplied current. At this time, the applied data voltage has a multi-level value in a predetermined range in order to express the gray scale.

However, such a conventional voltage writing pixel circuit has a problem in that it is difficult to obtain a high gradation due to variations in threshold voltage V _TH and electron mobility of the thin film transistor caused by nonuniformity in the manufacturing process. For example, when driving a thin film transistor of a pixel at 3 V, a voltage must be applied to a gate of the thin film transistor at intervals of 12 mV (= 3 V / 256) or less in order to express an 8-bit 256 gray level. When the variation in the threshold voltage of the thin film transistor due to uneven distribution is 100 Hz, it is difficult to express high gray scale. In addition, since the β value in Equation 1 is changed due to the deviation of mobility, it is difficult to express higher gray scales.

An object of the present invention is to provide a pixel circuit that can compensate for a threshold voltage of a driving transistor.

In order to achieve the above object, a display panel of a light emitting display device according to an aspect of the present invention is connected to a plurality of data lines for transmitting a data signal, a plurality of scanning lines for transmitting a selection signal, and the data lines and the scanning lines. A display panel of a light emitting display device including a plurality of pixel circuits, wherein the pixel circuit includes a capacitor formed between a first electrode and a second electrode, and an amount of current output to the third electrode according to a voltage charged in the capacitor. A first transistor for controlling a voltage, a display element for displaying an image corresponding to an amount of an output current of the first transistor, a first voltage connected to the first electrode of the first transistor and corresponding to the data signal; A second transistor which transfers to the first electrode of a first transistor, the data signal in response to the selection signal; In response to the first switching element, and a control signal transmitted to the transistor and the first electrode of the first transistor and a second switching element for resetting to a negative voltage.

In a display panel of a light emitting display device according to an aspect of the present invention, the first voltage is substantially equal to a value obtained by subtracting a threshold voltage of the second transistor from the data voltage.

In the display panel of the light emitting display device according to an aspect of the present invention, the control signal is a selection signal applied before the selection signal is applied.

A light emitting display device according to an aspect of the present invention includes a plurality of data lines for transmitting a data signal, a plurality of scanning lines for transmitting a selection signal, and a plurality of pixel circuits connected to the data lines and the scanning lines. An apparatus, comprising: a first transistor comprising first to third electrodes, the first transistor outputting a current corresponding to a voltage applied between the first and second electrodes to the third electrode, the amount of input current A display element for displaying an image corresponding thereto, a capacitor connected between the first electrode of the transistor and a power supply, and a first switching element which initializes the first electrode of the transistor to a negative voltage in response to a first control signal. A second switching element configured to transfer the data voltage to the second electrode of the first transistor in response to the selection signal; A third switching element for diode-connecting the transistor in response to a signal, a fourth switching element for connecting the second electrode and the power supply of the transistor in response to a second control signal, and in response to the second control signal And a fifth switching element connecting the third electrode of the transistor to the display element.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

In the following description, when a part is connected to another part, it includes not only the case where it is directly connected but also the case where it is electrically connected with another element between them. In the drawings, parts irrelevant to the present invention are omitted for clarity, and like reference numerals designate like parts throughout the specification.

2 illustrates an organic EL display device according to an embodiment of the present invention.

As shown in FIG. 2, an organic EL display device according to an exemplary embodiment of the present invention includes an organic EL display panel 100, a data driver 200, and a scan driver 300.

The organic EL display panel 100 includes a plurality of data lines D1 -Dm extending in the column direction, a plurality of scanning lines S1 -Sn extending in the row direction, and a plurality of pixel circuits 110. The data lines D1 -Dm transmit a data voltage representing an image signal to the pixel circuit 110, and the scan lines S1 -Sn transmit a selection signal for selecting the pixel circuit 110 to the pixel circuit 110. do. The pixel circuit 110 is formed in a pixel area defined by two neighboring data lines D1 -Dm and two neighboring scan lines S1 -Sn.

The data driver 300 applies a data voltage representing an image signal to the data lines D1 -Dm, and the scan driver 200 sequentially applies a selection signal to the scan lines S1 -Sn.

The data driver 200 and / or the scan driver 300 may be electrically connected to the display panel 100 or may be bonded to the display panel 100 and electrically connected to a tape carrier package (TCP). It may be mounted in the form of a chip. Alternatively, a flexible printed circuit (FPC) or a film may be mounted on the display panel 100 in the form of a chip or the like, and may be mounted on a chip on flexible board, chip on film). Alternatively, the data driver 200 and / or the scan driver 300 may be directly mounted on the glass substrate of the display panel 100 or formed of the same layers as the scan line, the data line, and the thin film transistor on the glass substrate. It may be replaced with a drive circuit or directly mounted.

Hereinafter, the pixel circuit 110 of the organic EL display device according to the first embodiment of the present invention will be described in detail with reference to FIGS. 3 and 4. 3 is an equivalent circuit diagram of a pixel circuit according to a first exemplary embodiment of the present invention, and FIG. 4 is a driving waveform diagram for driving the pixel circuit of FIG. 3.

In FIG. 3, only the pixel circuit connected to the m th data line Dm and the n th scan line Sn is illustrated for convenience of description. When the term “scanning line” is defined, the scanning line to which the current selection signal is to be transmitted is referred to as the “current scanning line”, and the scanning line to which the selection signal is transmitted before the current selection signal is transmitted is referred to as the “previous scanning line”.

As shown in FIG. 3, the pixel circuit 110 according to an exemplary embodiment of the present invention includes an organic EL element OLED, transistors M1-M4, and a capacitor Cst.

In the driving transistor M1, a source is connected to the power supply voltage VDD, and a capacitor Cst is connected between the gate and the source. The capacitor Cst maintains the voltage between the gate and the source of the transistor M1 for a period of time. The compensation transistor M2 is diode connected and the gate is connected to the gate of the driving transistor M1. The switching transistor M3 transfers the data voltage from the data line Dm to the compensation transistor M2 in response to the selection signal from the current scan line Sn. A switching transistor M4 is connected to the drain of the compensation transistor M2, and the switching transistor M4 transfers the precharge voltage Vp to the transistor M2 in response to a selection signal from the previous scan line Sn−1. To pass.

In the organic EL element OLED, a cathode is connected to the reference voltage VSS, and emits light corresponding to an applied current. The reference voltage VSS is a voltage having a level lower than that of the power supply voltage VDD, and a ground voltage or the like may be used.

In FIG. 3, although the transistors M1 -M4 are implemented as transistors having a P type channel, currents flowing to the third electrode by the voltage applied to the first and second electrodes and having the first to third electrodes are provided. Can be implemented using a variety of active devices to control. In addition, the switching transistors M3-M4 may be implemented using various switching elements capable of switching both ends connected in response to a control signal.

Hereinafter, an operation of the pixel circuit according to the first embodiment of the present invention will be described with reference to FIG. 4.

In the period T1, when the selection signal from the previous scan line Sn−1 becomes low, the transistor M4 is turned on, and the precharge voltage Vp is transferred to the gate of the transistor M1. At this time, the precharge voltage Vp applied to the transistor M4 is set to a value lower than the lowest level of the voltage applied to the gate of the transistor, that is, the data voltage applied through the data line Dm, to reach the highest gray level. do. According to an embodiment of the present invention, the precharge voltage Vp is set to a negative value. In this case, when the data voltage is applied through the data line Dm, the data voltage becomes larger than the gate voltage of the transistor M1 so that the transistor M1 is always connected in the forward direction. Therefore, the data voltage is charged in the capacitor Cst, and the switching width of the data voltage can be driven full.

In the period T2, the selection signal from the current scan line Sn becomes low and the transistor M3 is turned on. Then, the data voltage from the data line Dm is transferred to the transistor M2 through the transistor M3. Since the transistor M2 is diode-connected, a voltage corresponding to the difference between the threshold voltage V _TH2 of the transistor M2 and the data voltage is transferred to the gate of the transistor M1. This voltage is charged in the capacitor Cst to be maintained for a certain period of time.

The current I _OLED corresponding to the gate-source voltage V _GS of the transistor M1 is supplied to the organic EL element OLED, and the organic EL element OLED emits light. The current I _OLED flowing through the organic EL element OLED during the light emission period is represented by Equation 2 below.

Here, V _TH1 is the threshold voltage of the transistor M1, V _DATA is the data voltage from the data line Dm, and β represents a constant value.

At this time, if the threshold voltages V _TH1 and V _TH2 of the transistors M1 and M2 are the same, Equation 2 is expressed by Equation 3 below.

Therefore, a current corresponding to the data voltage applied through the data line Dm flows through the organic EL element OLED regardless of the threshold voltage V _TH1 of the transistor M1.

According to an embodiment of the present invention, the data voltage applied through the data line Dm is always higher than the gate voltage of the transistor M1 by setting the precharge voltage Vp applied to the transistor M4 to a negative voltage. Increase

That is, when the driving transistor M1 is set as a P-type transistor, the data voltage range may be set to, for example, about 0V to about 5V to drive the organic EL element OLED to express a desired gray scale. However, when a constant voltage is charged to the gate of the transistor M1 due to the parasitic capacitance component, the transistor M2 is forward biased only when the data voltage is greater than the voltage charged to the gate of the transistor M1 to obtain a desired gray scale. I can express it.

Therefore, when the transistor M4 is turned on before the data voltage is written, and the gate of the transistor M1 is initialized to the precharge voltage which is a negative voltage, even when a data voltage having a low voltage is written, a current corresponding to a desired gray scale is obtained. It is possible to flow to the organic EL element OLED, so that the swing width of the data voltage can be operated in full.

FIG. 5 is an equivalent circuit diagram of a pixel circuit of the organic EL display device according to the second embodiment of the present invention, and FIG. 6 is a drive waveform diagram for driving the pixel circuit of FIG.

As shown in Fig. 5, the pixel circuit according to the second embodiment of the present invention includes transistors M1-M6, a capacitor Cst, and an organic EL element OLED.

The transistor M1 is a driving transistor for driving the organic EL element OLED, and is connected to the organic EL element OLED through the transistor M6. The cathode of the organic EL element OLED is connected to the power supply VSS.

Transistor M2 diode-connects transistor M1 in response to a selection signal from current scan line Sn, and transistor M3 is applied to data line Dm in response to a selection signal from current scan line Sn. The data voltage is transferred to the transistor M1. The transistor M4 initializes the gate of the transistor M1 in response to the selection signal from the immediately preceding scan line Sn-1.

The transistor M5 applies the power supply voltage VDD to the source of the transistor M1 in response to the light emission signal from the light emission scan line En. The transistor M6 transfers a current flowing through the transistor M1 to the organic EL element OLED in response to a light emission signal from the light emission scan line En.

Hereinafter, an operation of the pixel circuit according to the second exemplary embodiment of the present invention will be described with reference to FIG. 6.

When the low level select signal is applied to the previous scan line Sn-1 in the period T1, the transistor M4 is turned on, and the precharge voltage Vp is applied to the transistor M1 so that the gate of the transistor M1 is applied. Initialize

In the section T2, the selection signal of the immediately preceding scan line Sn-1 becomes high level, and the selection signal of the current scanning line Sn-1 becomes low level. Then, the transistor M3 is turned on so that the data voltage from the data line Dm is applied to the source of the transistor M1.

In addition, since the transistor M2 is turned on and the transistor M1 is in a diode-connected state, a voltage equal to the gate voltage of the transistor M1 minus the threshold voltage of the transistor M1 is applied to the gate of the transistor M1, thereby providing a capacitor C1. Are stored in.

In the section T3, a high level signal is applied to all of the scan lines Sn-1, Sn, and En. By providing a short period in which the immediately preceding selection signal, the current selection signal, and the light emission signal are all at a high level, the parasitic currents generated while the data voltage is written in the section T2 prevent the current flowing into the organic EL element OLED. You can prevent it. Therefore, the organic EL display device can display an image more stably.

Next, a high level selection signal is applied to the previous scan line Sn-1 and the current scan line Sn in the section T4, and a low level light emission signal is applied to the light emission scan line En. Then, the transistors M5 and M6 are turned on, and a current corresponding to the voltage stored in the capacitor Cst flows to the organic EL element OLED, thereby displaying an image of a desired gray scale.

At this time, the current I _OLED flowing through the organic EL element _OLED is represented by Equation 4 below.

Where I _OLED is the current flowing through the organic EL element OLED, V _GS is the voltage between the gate and the source of the transistor M1, Vth is the threshold voltage of the transistor M1, V _DATA is the data voltage, and β is a constant value. Respectively.

As can be seen from Equation 4, the current I _OLED is applied to the anode of the organic EL element _OLED corresponding to the data voltage V _DATA applied to the data line irrespective of the threshold voltage of the driving transistor M1. .

In the second embodiment of the present invention, since the deviation of the threshold voltage of the driving transistor M1 is detected through the transistor M2 and compensates for itself, the current flowing through the organic EL element can be finely controlled. Therefore, a high gradation organic EL display device can be provided.

In addition, the pixel circuit is driven in units of frames to which n + 1 selection signals from the immediately preceding selection signal S0 to the selection signal Sn of the selection signal S1 are applied to display an image. In this case, when the data voltage written to the pixel for the next frame is lower than the data voltage written to the pixel for the previous frame, the desired data voltage is applied to the gate electrode of the transistor M1 by the diode connection characteristic of the transistor M1. Will not be.

Therefore, in the second embodiment of the present invention, the gate electrode of the transistor M1 is initialized to the negative precharge voltage Vp before data writing by using the switching transistor M4 so that the data voltage can be stably applied. can do.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

For example, in the above description, the pixel illustrated in FIGS. 3 and 5 is exemplarily described. However, the scope of the present invention is not limited thereto, and the pixel circuit of the driving transistor may be initialized by various types of pixel circuits. The concept can be applied as it is.

According to the present invention, a light emitting display device and a display panel including a pixel circuit capable of compensating threshold voltages of a driving transistor can be provided.

Further, a light emitting display device and a display panel thereof capable of driving the swing width of the data voltage to the maximum can be provided.

Claims (6)

A display panel of a light emitting display device comprising: a plurality of data lines for transmitting a data signal, a plurality of scanning lines for transmitting a selection signal, and a plurality of pixel circuits connected to the data lines and the scanning lines. The pixel circuit, A first transistor is formed between the first electrode and the second electrode, and the first transistor for controlling the amount of current output to the third electrode by the voltage charged in the capacitor, A display element electrically connected to the third electrode of the first transistor and configured to display an image corresponding to an input current amount; A second transistor connected to the first electrode of the first transistor and transferring a first voltage corresponding to the data signal to the first electrode of the first transistor, A first switching element transferring the data signal to the second transistor in response to the selection signal; A second switching element for initializing the first electrode of the first transistor to a negative voltage in response to a control signal A display panel of the light emitting display device comprising a. The method of claim 1, And the first voltage is substantially equal to the data voltage minus the threshold voltage of the second transistor. The method of claim 1, And the control signal is a selection signal applied before the selection signal is applied. A light emitting display device comprising a plurality of data lines for transmitting a data signal, a plurality of scanning lines for transmitting a selection signal, and a plurality of pixel circuits connected to the data lines and the scanning lines. The pixel circuit, A first transistor including first to third electrodes and outputting a current corresponding to a voltage applied between the first and second electrodes to the third electrode, A display element for displaying an image corresponding to the amount of current input; A capacitor connected between the power supply and the first electrode of the transistor, A first switching element for initializing the first electrode of the transistor to a negative voltage in response to a first control signal, A second switching element transferring the data voltage to the second electrode of the first transistor in response to the selection signal; A third switching device for diode-connecting the transistor in response to the selection signal; A fourth switching element connecting the second electrode and the power supply of the transistor in response to a second control signal, and A fifth switching element connecting the third electrode of the transistor and the display element in response to the second control signal A light emitting display device comprising a. The method of claim 4, wherein And the first control signal is a selection signal applied before the selection signal is applied. The method according to claim 4 or 5, And the second control signal is applied at a timing after the selection signal and the first control signal are applied.

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