KR20080022695A - Picture element structure of display device and driving method - Google Patents

Abstract

A pixel structure of a display device and a method for driving the display device are provided to suppress a crosstalk phenomenon by maintaining a uniform OLED current during one frame. A pixel structure is controlled by an n-th scan signal and includes a switching transistor for selecting a unit pixel which is defined by data and scan lines. A source/drain current path of a leakage current compensation transistor is connected to a source/drain current path of the switching transistor at a node A. The leakage current compensation transistor delivers a control data signal for compensating for a leakage current of the switching transistor to the node A under the control of an n-1-th scan signal. The pixel structure includes OLEDs(Organic Light Emitting Diodes), a driving transistor(T2), and a capacitor(C1). The driving transistor receives the data voltage through a gate which is connected to the node A, and supplies a corresponding current to the OLED. The capacitor stores the data voltage which is applied to the gate of the driving TFT(Thin Film Transistor).

Description

Pixel structure of display device and driving method thereof {PICTURE ELEMENT STRUCTURE OF DISPLAY DEVICE AND DRIVING METHOD}

1 is a circuit diagram of a unit pixel of a conventional voltage write type active driving type organic light emitting device;

2 is an operation timing diagram of FIG. 1;

3 is a view simulating a change in the amount of current of the OLED according to the driving time in the conventional voltage write type pixel structure shown in FIG.

4 is a view simulating the amount of change in the current of the OLED when there is a change in the amount of leakage current of the switching transistor T1 in the conventional pixel structure shown in FIG.

5 is a circuit diagram of a unit pixel of a voltage write type active driving organic light emitting diode according to a first embodiment of the present invention;

6 is an operation timing diagram of FIG. 5;

FIG. 7 is a view simulating a change in the amount of current of an OLED according to a driving time in the voltage write-in pixel structure of FIG.

FIG. 8 is a diagram showing current-voltage (I _D -V _GS ) characteristic curves when SP _DS modeled V _DS = -2V of the thin film transistors used in FIGS. 3 and 7;

9 is a view simulating the amount of change in the current of the OLED when there is a change in the amount of leakage current of the switching transistor T1 in the pixel structure of FIG.

FIG. 10 is a current-voltage (I _D -V _GS ) characteristic curve when SP _DS modeled V _DS = -2V of the thin film transistors used in FIGS. 4 and 9;

11 is a circuit diagram of a voltage write method pixel structure having a threshold voltage compensation function according to a second embodiment of the present invention;

12 is a circuit diagram of a current write method pixel structure having a threshold voltage compensation function according to a third embodiment of the present invention;

13 is a circuit diagram of a pixel structure of an active driving liquid crystal display device according to a fourth embodiment of the present invention;

14 is a circuit diagram of a voltage write type pixel structure of a hysteresis phenomenon compensation function according to a fifth embodiment of the present invention;

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pixel structure of a display device, and more particularly to a pixel structure of a display device and a driving method thereof for minimizing image quality deterioration due to leakage current of a switching transistor in a pixel.

Recently, development of thinner, lighter and cheaper display devices is actively underway, and one of the things that is attracting attention as such a next-generation display device is an organic light emitting diode (OLED).

The organic light emitting device uses the organic (Lector-Luminescence) phenomenon of a specific organic material, and because it does not have a backlight, the display device using the organic light emitting device can be implemented thinner than the general liquid crystal display device, it is cheaper and easier to manufacture But it has the advantage of wide viewing angle and bright light.

The organic light emitting diode display using the organic light emitting diode includes an organic light emitting diode and a thin film transistor (TFT) for driving the organic light emitting diode. Thin film transistors are classified into polycrystalline silicon thin film transistors and amorphous silicon thin film transistors according to the type of active layer.

In addition, depending on the presence of a switching element provided in the unit pixel of the organic light emitting display panel, it is divided into active driving type and passive driving type.

The organic light emitting display device employing the polycrystalline thin film transistor has various advantages, and thus is widely used. On the other hand, the manufacturing process of the thin film transistor is complicated, and accordingly, the cost increases. Moreover, organic light emitting displays are difficult to obtain large screens.

On the other hand, an organic light emitting display device employing an amorphous silicon thin film transistor is easy to obtain a large screen, and is advantageous in terms of cost because the number of manufacturing processes is relatively smaller than that of an organic light emitting display device employing a polysilicon thin film transistor.

However, as the amorphous silicon thin film transistor continuously supplies a current to the organic light emitting diode, the threshold voltage (V _TH ) of the amorphous silicon thin film transistor itself changes, thereby causing an uneven current to flow through the organic light emitting diode even when the same data voltage is applied. do. As a result, the image quality of the organic light emitting display is deteriorated.

Recently, a thin film transistor (direct desposited nanocrystalline-silicon (nc-Si) thin film transistor) that does not undergo a crystallization process and a solid phase crystallization thin film transistor have been attracting attention.

The two types of thin film transistors have excellent current uniformity compared to conventional polysilicon thin film transistors, and have been reported to have excellent current stability compared to amorphous silicon thin film transistors.

However, since the nc-Si thin film transistor and the solid-state crystallized thin film transistor have a large internal grain density defect, the leakage current of the transistor is severe.

1 is a circuit diagram of a unit pixel of a conventional voltage write type active driving type organic light emitting diode, and FIG. 2 is an operation timing diagram of FIG. 1.

Referring to FIG. 1, a unit pixel of an active driving organic light emitting diode includes a switching transistor T1, a driving transistor T2, a capacitor C, and an organic light emitting diode OLED.

The switching transistor T1 transfers or blocks the data voltage Vdata applied through the image signal line to the driving transistor T2 and the capacitor C by the selection signal Scan applied through the gate signal line. Do it.

The driving transistor T2 serves as a current source device to supply a corresponding current to the organic light emitting diode OLED according to the data voltage Vdata transferred through the switching transistor T1.

The capacitor C stores the data voltage applied to the gate terminal of the driving transistor T2 so that the driving transistor T2 can maintain its operation as a current source even when the switching transistor T1 is cut off.

However, in the basic pixel structure of the voltage write method shown in FIG. 1, when the leakage current of the switching transistor T1 is large due to the continuous change of the data voltage, even if the same data voltage is charged in the capacitor C at the beginning of the frame, the frame As time increases, the data voltage changes. As a result, a current generated by the changed data voltage flows to the organic light emitting diode OLED through the driving transistor T2, and the image of each pixel is distorted. The phenomenon in which the image of each pixel is distorted by the data voltage is called a crosstalk phenomenon.

The current ID flowing between the source and the drain of the driving transistor MDRV is as follows.

ID = 1/2 * k * (V _GS -V _TH ) ²

(Current-voltage relationship in the saturation region k = μ * C _OX * W / L, where μ is the field effect mobility, C _OX is the capacitance of the insulating layer, W is the channel width of the thin film transistor, L is a thin film transistor) Indicates the channel length of

The data voltage stored in the capacitor C through the nth selection signal is changed by the voltage V _DS between the source and the drain of the switching transistor T1 due to the data voltage applied after the nth column.

When the leakage current of the switching transistor T1 is low, the data voltage stored in the capacitor C is not distorted for one frame (generally 16.7 mse at 60 Hz). However, if the leakage current of T1 is high, the data voltage stored in the capacitor C is increased or decreased through the nth selection signal, thereby increasing or decreasing the voltage V _GS between the gate and the source of the driving transistor T2. Let's go.

As a result, an increase or decrease in the voltage V _GS between the gate and the source of the driving transistor T2 in one frame has a problem of causing a crosstalk phenomenon.

FIG. 3 is a SPICE simulation result showing a crosstalk phenomenon caused by a leakage current caused by a change in data voltage in the conventional voltage write-in pixel structure shown in FIG. 1, wherein a data voltage of 7.5V is determined by an nth selection signal. When the data voltage applied to the pixels existing in other horizontal lines after input is changed, the 7.5V data voltage is the amount of current change of the OLED in the input pixel. DC voltage was applied to the data voltages applied to the pixels on the other horizontal lines, and the characteristics were observed by changing the voltage from 0.5V to 6.5V at 0.5V intervals, respectively.

First, when the data voltage input from the next line in the pixel to which the data voltage 7.5V is applied by the nth selection signal is DC 6.5V, the data voltage of 7.5V is 6.5V due to the leakage current of the switching transistor of FIG. This is applied to the applied data line. When the data voltage of 7.5V stored in the capacitor C is discharged and decreased through the leakage current of the switching transistor T1, the voltage V _GS between the gate and the source of the driving transistor T2, which is a current supply source of the OLED, becomes As a result, the OLED current increases.

Accordingly, when the leakage current of the switch transferring the data voltage is large when the active driving display is driven using the pixel structure of FIG. 1, the image is distorted by random data voltages input from the next line even when the same data voltage is applied. There is this.

4 is a diagram simulating the amount of change in the current of the OLED when there is a change in the amount of leakage current of the switching transistor T1 in the conventional pixel structure shown in FIG.

Referring to FIG. 4, when the leakage current amount of the switching transistor T1 increases, the current change amount of the OLED increases in the pixel structure of FIG. 1 due to the voltage change applied to the data line. The simulation condition of FIG. 4 assumes that a data voltage of 7.5 V is applied by the n-th selection signal and then DC 8.5 V is applied from the next line.

If the leakage current (V _GS = 2V) of the switching transistor T1 of FIG. 1 increases from 2pA to 67pA, the data voltage of 7.5V stored in the capacitor is faster from 8.5V as the leakage current increases. Therefore, the V _GS of the driving transistor T2 decreases in a shorter time as the leakage current of the switching transistor T1 increases, so that the OLED current decreases.

Accordingly, an object of the present invention is to solve the above-described problems of the related art, and an object of the present invention is to provide a pixel structure of a display device and a driving method thereof for minimizing a change in data voltage due to leakage current of a switching transistor in a pixel. In providing.

In order to achieve the above object, a pixel structure of a display device according to an exemplary embodiment of the present invention is controlled by an nth scan signal and includes at least a switching transistor for selecting a unit pixel defined by a data line and a scan line. In the structure, the source / drain current path of the switching transistor is connected to the source / drain current path of the switching transistor at node A, and is controlled by an n−1 th scan signal to control the leakage current of the switching transistor. And a leakage current compensation transistor for transmitting a signal to the node A.

Preferably, the pixel structure and the organic light emitting element; A driving transistor for receiving the data voltage transferred through the switching transistor to a gate connected to the node A and supplying a corresponding current to the organic light emitting element; And a capacitor for storing a data voltage applied to the gate of the driving transistor.

Preferably, the pixel structure includes a first transistor having a source / drain current path connected between the node A and a gate of the driving transistor, and each gate connected to a gate of the node A and the driving transistor. And a voltage write method pixel structure having a threshold voltage compensation function including a second transistor.

Preferably, the pixel structure includes a storage capacitor connected between the node A and a power supply voltage terminal to store a corresponding charge in accordance with a pixel voltage applied from the data line; The pixel structure of the liquid crystal display further comprises a liquid crystal whose transmittance is controlled by the voltage stored in the storage capacitor.

More preferably, the separate control data signal for compensating for the leakage current is an inverted signal of the digital bit of the original image data signal.

In order to achieve the above object, a voltage write method driving method according to an embodiment of the present invention is controlled by an nth scan signal and includes at least a switching transistor for selecting a unit pixel defined by a data line and a scan line. In the pixel structure, a separate control data signal for compensating for the leakage current of the switching transistor is provided to the source / drain current path of the switching transistor.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; Note that the same components in the drawings are denoted by the same reference numerals and symbols as much as possible even if shown on different drawings. In addition, in describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

5 is a circuit diagram of a unit pixel of a voltage write type active driving type organic light emitting diode according to an embodiment of the present invention, and FIG. 6 is an operation timing diagram of FIG. 5.

Referring to FIG. 5, a unit pixel according to an exemplary embodiment may include a switching transistor T1, a driving transistor T2, a leakage current compensating transistor T3, a capacitor C1, and an organic light emitting diode. An element OLED is provided.

The switching transistor T1 has a drain connected to the data line Vdata, a source connected to the gate of the driving transistor T2, and a data voltage according to the nth selection signal Scan (n) applied through the gate. Serves to transfer Vdata to the gate of the driving transistor T2.

The driving transistor T2 has a source connected to the supply power supply line V _DD , a drain connected to the anode of the organic light emitting diode OLED, and a corresponding current supplied to the organic light emitting diode according to the data voltage Vdata transferred to the gate. (OLED) serves to supply.

The leakage current compensating transistor T3 has a drain connected to a separate control data signal line, a source connected to a gate of the driving transistor T2, and an n-1th selection signal Scan (n) applied through the gate. -1)) to transfer a separate control data signal to the gate of the driving transistor T2. The separate control data signal is a signal obtained by inverting the digital bit of the original image data signal and analogizing it using a digital to analog converter. It becomes the data voltage (/ Vdata).

The capacitor C stores the data voltage applied to the gate terminal of the driving transistor T2 so that the driving transistor T2 can maintain its operation as a current source even when the switching transistor T1 is cut off.

The operation of the unit pixel of the voltage write method active driving organic light emitting device of the present invention having the above configuration is as follows. For reference, the pixel structure of FIG. 5 described in the present embodiment will be described based on the assumption that the data voltage is stored by the n-th scan signal Scan (n) to control the light emission of the OLED. will be.

Referring to FIGS. 5 and 6, in (1) of FIG. 6, the n-1 th scan signal Scan (n-1) is low and the n th scan signal Scan (n) is high ( In the period of high, the leakage current compensating transistor T3 is turned on to apply the n−1th data voltage to the gate node A of the driving transistor T2.

FIG. 6 (2) shows a section in which the n-1 th scan signal Scan (n-1) is high and the n th scan signal Scan (n) is low. T1 is turned on and a data voltage to be applied to the unit pixel is applied to the gate node A of the driving transistor T2.

FIG. 6 (3) is a section in which both the n-1 th scan signal Scan (n-1) and the n th scan signal Scan (n) are high, and the switching transistor T1 and the leakage current are shown in FIG. A leakage current is generated in the compensation transistor T3. Section (3) is a section for compensating for leakage current generated in the switching transistor T1. When a leakage current occurs in the switching transistor T1, it is discharged through the leakage current compensation transistor T3, which is a switch for leakage current circulation. By doing so, distortion of the data voltage stored in the gate node A of the driving transistor T2 can be minimized.

It is possible to form the leakage current discharge path in the leakage current compensation transistor T3 by applying a separate control data signal. The separate control data signal is a signal obtained by inverting the digital bits of the original image data signal and then analogizing them using a digital / analog converter.

For example, when a PMOS type transistor is used as a current driving transistor as in the driving transistor of FIG. 5, the leakage compensation voltage is 6.85V when the original image data voltage is 8.5V, and the original image. When the data voltage is 6.5V, 8.8V is applied to the leakage current compensation voltage, thereby preventing distortion of the data voltage due to the leakage current of the switching transistor T1 using the leakage current of the leakage current compensation transistor T3.

Table 1 below shows examples of leakage current compensation voltages corresponding to data voltages.

Table 1.

Data voltage (Vdata) Leakage current compensation voltage 8.5 V 6.85 V 8V 7.2 V 7.5V 7.5V 7 V 8.2V 6.5V 8.8V

In the pixel structure according to the present invention, the smaller the change of the leakage current due to the change of the voltage V _GS between the gate and the source of the driving transistor T2, the smaller the influence of the leakage current can be minimized. Minimizing the change of leakage current by the voltage V _GS between the gate and the source can be easily secured by applying a lightly doped drain (LDD) process technology.

In addition, as the leakage current of the switching transistor T1 and the leakage current compensating transistor T3 is similar, the ability to minimize the influence due to the leakage current is increased. Since the switching transistor T1 and the leakage current compensating transistor T3 are adjacent to each other in the unit pixel, the characteristics can be ensured so that the amount of leakage current is similar.

FIG. 7 is a SPICE simulation result diagram illustrating a change in the amount of current of an OLED according to a driving time in the voltage write type pixel structure according to the exemplary embodiment of FIG. 5. As in FIG. 3, when the data voltage of 7.5V is input by the nth selection signal and the data voltage applied to the pixels existing in other horizontal lines is changed, the current of the OLED in the pixel in which the 7.5V data voltage is input. The amount of change. DC voltage was applied to the data voltages applied to the pixels on the other horizontal lines, and the characteristics were observed by changing the voltage from 0.5V to 6.5V at 0.5V intervals, respectively.

First, when the data voltage input from the next line in the pixel to which the data voltage 7.5V is applied by the nth selection signal is DC 6.5V (in this case, the separate control data voltage for compensating for leakage current is 8.8V), 7.5V data The voltage is about to discharge to the data line to which the 6.5V voltage is applied due to the leakage current of the switching transistor T1 of FIG. 5, and the separate voltage with the 8.8V voltage due to the leakage current of the leakage current compensation transistor T3. There is a phenomenon of charging to the control data line.

Therefore, the data voltage of 7.5V stored in the capacitor C is discharged through the leakage current of the switching transistor T1 and charged through the leakage current compensating transistor T3 so as to be maintained as long as possible for one frame. As such, the V _GS of the driving transistor T2, which is the current source of the OLED, is kept constant, thereby keeping the OLED current constant.

If the data voltage input from the next line in the pixel to which the data voltage 7.5V is applied by the nth selection signal is DC 8.5V (the separate control data voltage for leakage current compensation is 6.85V), While the data voltage is trying to charge from the data line with the 8.5V voltage due to the leakage current of the switching transistor T1, the separate control data with the 6.85V voltage due to the leakage current of the leakage current compensating transistor T3. There is a phenomenon of discharging to a line.

Accordingly, the data voltage of 7.5 V stored in the capacitor is maintained to the maximum for one frame by charging the phenomenon that the discharge current is caused by the leakage current of the switching transistor T1 through the leakage current of the leakage current compensating transistor T3. As such, V _GS of the driving transistor, which is the current source of the OLED, does not change, and the current of the OLED is maintained.

FIG. 8 is a current-voltage (I _D -V _GS ) characteristic curve when SP _DS modeled V _DS = -2V of the thin film transistors used in FIGS. 3 and 7.

9 is a result of simulating the amount of change in the current of the OLED when there is a change in the amount of leakage current of the switching transistor (T1) in the pixel structure according to an embodiment of the present invention shown in FIG.

Referring to FIG. 9, even though the leakage current amount of the switching transistor T1 increases, the current change amount of the OLED is maintained regardless of the voltage change applied to the data line in the pixel structure of FIG. 5. The simulation condition of FIG. 9 assumes that the data voltage 7.5V is applied by the nth selection signal and then the data voltage DC 8.5V is applied from the next line.

Even if the leakage current (V _GS = 2V) of the switching transistor T1 of Fig. 5 increases from 2pA to 67pA, the data voltage of 7.5V stored in the capacitor is maintained in one frame regardless of the leakage current increase. The V _GS of the transistor T2 can confirm that the current change of the OLED is controlled within an error of 2.5%, which is difficult for the human eye, even when the leakage current of the switching transistor T1 increases.

FIG. 10 is a current-voltage (I _D -V _GS ) characteristic curve when the SPICE modeled V _DS = -2V of the thin film transistors used in FIGS. 4 and 9.

FIG. 11 is a circuit diagram of a voltage write type pixel structure having a threshold voltage compensation function according to a second embodiment of the present invention, in which a switch T5 for leakage current compensation is performed using four conventional transistors T1, T2, T3, and T4. It shows an example applied to a voltage write type pixel circuit having a threshold voltage compensation function.

12 is a circuit diagram of a current write type pixel structure having a threshold voltage compensation function according to a third embodiment of the present invention, in which a switch T5 for leakage current compensation is performed using four conventional transistors T1, T2, T3, and T4. An example of application to a current write method pixel circuit having a threshold voltage compensation function is shown.

As shown in FIGS. 11 and 12, a leakage current compensation switch and a separate control data signal can be added to an existing threshold voltage compensation circuit to suppress an influence caused by a leakage current of a transistor in an active driving display device. have.

FIG. 13 is a circuit diagram of a pixel structure of an active driving liquid crystal display device according to a fourth embodiment of the present invention, in which a leakage current compensation switch T2 and a separate control data signal are applied to a pixel structure of a conventional liquid crystal display device. An example is shown.

FIG. 14 is a circuit diagram of a voltage write type pixel structure having a current error compensation function due to a hysteresis phenomenon of a transistor, according to a fifth embodiment of the present invention. An example of application to a voltage write type pixel circuit having a current error compensation function due to a hysteresis phenomenon composed of T1, T2, and T3) is shown.

On the other hand, in the description of the present invention has been described with respect to specific embodiments, various modifications are possible without departing from the scope of the invention. For example, a circuit configuration consisting of only the P-type thin film transistor (TFT) proposed in the present invention may be configured as an N-type thin film transistor.

In addition, it can be useful for driving AMOLED displays using thin film transistors such as nanocrystalline-silicon thin film transistors, amorphous silicon thin film transistors, oxide thin film transistors (oxide-TFT), organic thin film transistors (organic-TFT), and the like.

Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by those equivalent to the scope of the claims.

As described above, according to the present invention, the OLED current can be uniformly maintained for one frame regardless of the magnitude of the leakage current of the thin film transistor in the pixel, thereby successfully suppressing the crosstalk phenomenon.

Claims (9)

A pixel structure controlled by an nth scan signal and including at least a switching transistor for selecting a unit pixel defined by a data line and a scan line, At node A, the source / drain current path of the switching transistor is connected to the source / drain current path of the switching transistor, and is controlled by an n−1 th scan signal to control the signal to control the leakage current of the switching transistor. A pixel structure of a display device, further comprising a transistor for compensating for leakage current transmitted to A. The method of claim 1, wherein the pixel structure An organic light emitting device; A driving transistor for receiving the data voltage transferred through the switching transistor to a gate connected to the node A and supplying a corresponding current to the organic light emitting element; And a voltage write type pixel structure further comprising a capacitor for storing a data voltage applied to the gate of the driving transistor. The method of claim 2, wherein the pixel structure And a first transistor and a second transistor, wherein a source / drain current path is connected between the node A and a gate of the driving transistor, and each gate is connected to the node A and a gate of the driving transistor. A pixel structure of a display device, characterized in that the voltage write method pixel structure of the threshold voltage compensation function. The method of claim 1, wherein the pixel structure A storage capacitor connected between the node A and a power supply voltage terminal to store a corresponding charge according to a pixel voltage applied from the data line; And a pixel structure of the liquid crystal display device further comprising a liquid crystal whose transmittance is controlled by a voltage stored in the storage capacitor. The method according to any one of claims 1 to 4, wherein the separate control data signal for compensating for the leakage current is A pixel structure of a display device, characterized in that the inverted signal of the digital bit of the original image data signal. The transistor of claim 1, wherein the leakage current compensating transistor And the leakage current amount is similar to the leakage current amount of the switching transistor. First and second switching transistors configured to select a driving pixel based on an n-th scan signal applied from the outside and to which a data current is applied; A storage capacitor for storing a corresponding charge according to a control current applied by the first and second switching transistors; A third switching transistor selected by the first and second switching transistors, writing a data current, and receiving an external power source; A fourth driving transistor having a mirror structure with the third switching transistor, the fourth driving transistor receiving a voltage according to the charge stored in the storage capacitor and providing a current to the corresponding pixel; and a leakage current compensation transistor controlled by an n−1 th scan signal and transferring a control data signal for compensating for leakage currents of the first and second switching transistors to a gate of the third switching transistor. A pixel structure of a display device. A pixel structure controlled by an nth scan signal and including at least a switching transistor for selecting a unit pixel defined by a data line and a scan line, And a separate control data signal for compensating for the leakage current of the switching transistor to a source / drain current path of the switching transistor. The method of claim 8, wherein the separate control data signal for compensating for the leakage current A method for driving a voltage write-in pixel structure, characterized in that it is generated as an inverted signal of a digital bit of an original image data signal.

Images