KR100683772B1 - Organic light emitting display device - Google Patents

Abstract

The present invention relates to an organic light emitting display device which is advantageous in increasing aperture ratio and high resolution by implementing a pixel circuit that compensates for variations in threshold voltages of a driving transistor while minimizing a leakage path. The organic light emitting diode display includes an organic light emitting diode, first to third transistors, a capacitor, and a fourth transistor. The organic light emitting element emits light correspondingly by the applied current, and the first transistor supplies a driving current to the organic light emitting element. The second transistor is switched in response to the selection signal to control the transfer of the data signal to the first transistor, and the third transistor diode-couples the first transistor while the data signal is transferred to the first transistor. The capacitor stores a voltage corresponding to the driving current of the first transistor, and the fourth transistor controls the voltage of one electrode of the capacitor to be applied to the first electrode of the first transistor during the light emitting period of the organic light emitting element. It is done.

Description

Organic light emitting display device

DETAILED DESCRIPTION OF THE INVENTION A brief description of each drawing is provided to more fully understand the drawings, which are incorporated in the description.

1 is a circuit diagram of a pixel employed in a conventional organic light emitting display device.

2 is a pixel circuit diagram for conventional threshold voltage compensation.

FIG. 3 is a driving waveform diagram for driving the pixel circuit shown in FIG. 2.

4 is a pixel circuit diagram of minimizing a leakage path and compensating a threshold voltage according to the present invention.

FIG. 5 is a first embodiment of a driving waveform diagram for driving the pixel circuit shown in FIG. 4.

FIG. 6 is a second embodiment of a driving waveform diagram for driving the pixel circuit shown in FIG. 4.

The present invention relates to an organic light emitting diode display, and more particularly, to a pixel circuit that compensates for variations in threshold voltages of a driving transistor while minimizing leakage paths, to increase aperture ratio and to achieve high resolution. It relates to a display device.

1 is a circuit diagram of a pixel employed in an organic light emitting diode display.

Referring to FIG. 1, a pixel of an organic light emitting diode display includes an organic light emitting diode OLED, two transistors M1 and M2, and one capacitor Cst, and generally, the first and second transistors ( M1 and M2 are thin film transistors (TFTs).

When the first transistor M1 is turned on, the data signal applied through the data signal line is stored in the capacitor Cst, and then the data stored in the capacitor Cst even when the first transistor M1 is turned off. A current corresponding to the signal is applied to the organic light emitting diode OLED through the second transistor M2. Accordingly, the organic light emitting diode OLED maintains light emission for a predetermined period even when the first transistor M1 is turned off.

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

[Equation 1]

In Equation 1, is a current flowing through the OLED, Vgs is a voltage between the main electrode and the first electrode of the second transistor M2, Vth is the threshold voltage of the second transistor M2, VDD is The power supply voltage, Vdata, represents the data voltage, and the gain factor.

However, in the pixel circuit of the voltage write method as described above, the driving voltage of the second transistor, such that the threshold voltage (Vth) occurs across all the pixels in the manufacturing process, it is difficult to obtain a screen of uniform brightness Occurs.

2 is a diagram illustrating a pixel circuit for compensating a threshold voltage, and includes first to fifth transistors M1 to M5, first and second capacitors Cst and Cvth, and an organic light emitting diode OLED.

3 is a driving waveform diagram for driving the pixel circuit shown in FIG. 2. The operation of the pixel circuit shown in FIG. 2 will be described with reference to FIG. 3 as follows.

In the t1 period, when the low level selection signal S [n-1] is applied through the previous scan line, the second transistor M2 is turned on and the first transistor M1 is in a diode-connected state. Therefore, the voltage between the main electrode and the first electrode of the first transistor M1 is changed until the threshold voltage Vth of the first transistor M1 becomes. At this time, since the power supply voltage VDD is applied to the first electrode of the first transistor M1, the voltage applied to the main electrode of the first transistor M1, that is, the one electrode A of the second capacitor Cvth is do. In addition, the fourth transistor M4 is turned on and the power supply voltage VDD is applied to the other electrode B of the second capacitor Cvth.

Therefore, the voltage between the positive electrodes of the second capacitor Cvth is expressed by Equation 2 below.

[Equation 2]

Here, VCvth denotes a voltage applied between the positive electrodes of the second capacitor Cvth, VCvthA denotes a voltage applied to the one electrode A of the second capacitor Cvth, and VCvthAB denotes the other electrode of the second capacitor Cvth. It means the voltage applied to (B).

In addition, the fifth transistor M5 is blocked in the period t1 to prevent the current flowing through the first transistor M1 from flowing to the organic light emitting diode OLED, and to generate a high level signal on the current scan line S [n]. Is applied, the third transistor M3 is cut off.

In a period t2, when a low level scan voltage is applied to the current scan line S [n], the third transistor M3 is turned on to charge the data voltage Vdata to the first capacitor Cst. In addition, since the voltage corresponding to the threshold voltage Vth of the first transistor M1 is charged in the second capacitor Cvth, the main electrode of the first transistor M1 is charged with the data voltage Vdata and the first transistor ( A voltage corresponding to the sum of the threshold voltages Vth of M1 is applied.

That is, the voltage Vgs between the main electrode and the first electrode of the first transistor M1 is represented by Equation 3 below, and a current as shown in Equation 4 is transmitted through the first transistor M1. Is supplied.

[Equation 3]

[Equation 4]

As can be seen from Equation 4, even if the threshold voltage Vth of the first transistor M1 located in each pixel is different from each other, the deviation of the threshold voltage Vth is compensated by the second capacitor Cvth.

However, in the pixel circuit shown in FIG. 2, the second, third and fourth transistors M2 and M3 are turned off while the second transistor M2, the third transistor M3, and the fourth transistor M4 are turned off in the period t2. The charges used to form the respective channels of M4 are leaked to change the voltage of the first capacitor Cst, thereby causing a kick back phenomenon according to a leakage path that changes the data voltage Vdata. It is difficult to implement high resolution.

The present invention has been made in an effort to provide an organic light emitting diode display which is advantageous in increasing aperture ratio and high resolution by implementing a pixel circuit that compensates for variations in threshold voltages of a driving transistor while minimizing a leakage path. .

The organic light emitting display device according to the present invention for solving the technical problem to be achieved by the present invention is one or more scan lines for transmitting a selection signal, one or more data lines for transmitting a data signal, the area intersected by the scan line and the data line And a pixel circuit provided in the pixel defined in the pixel circuit, wherein the pixel circuit is configured to emit light in response to an applied current, a first transistor to supply a driving current to the organic light emitting element, and the selection signal. A second transistor that is switched in response to control the transfer of the data signal to the first transistor, and a third transistor that diode-connects the first transistor while the data signal is transferred to the first transistor; A capacitor for storing a voltage corresponding to a driving current of the first transistor and a fourth transistor for controlling the voltage of one electrode of the capacitor to be applied to the first electrode of the first transistor during the light emitting period of the organic light emitting element; It is preferable to include.

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

As shown in FIG. 4, the pixel circuit which minimizes the leakage path and compensates the threshold voltage according to the present invention includes an organic light emitting diode OLED, a first transistor M1 as a driving transistor, and a second switching transistor. To fifth transistors M2 to M5 and capacitors Cst.

The second transistor M2 switches the data signal D [m] applied through the data line in response to the low level select signal S [n] applied from the scan line.

The first transistor M1 is a transistor for controlling a current flowing in the organic light emitting diode OLED. The first electrode of the first transistor M1 is a data signal when the second transistor M2 is turned on. D [m]), and the second electrode of the first transistor M1 is connected to the first electrode of the fourth transistor M4.

The third transistor M3 diode-connects the first transistor M1 in response to the low level of the selection signal from the scan line S [n].

The fourth transistor M4 switches the driving current for driving the organic light emitting diode OLED in response to the low level from the second emission control signal EM2.

The fourth transistor M4 switches the power supply voltage VDD in response to the low level from the emission control signal EM.

One electrode of the capacitor Cst is connected to the power supply voltage VDD, and the other electrode is connected to the main electrode of the first transistor M1.

The anode of the organic light emitting diode OLED is connected to the first electrode of the fifth transistor M5, and the cathode of the organic light emitting diode OLED is connected to the reference voltage VSS so that the fifth transistor M5 is turned on. And emits light corresponding to the amount of current applied from the first transistor M1. In this case, the reference voltage VSS is a voltage having a lower level than the power supply voltage VDD, and a ground voltage and a negative voltage may be used.

Next, an operation of the pixel circuit which minimizes the leakage path and compensates the threshold voltage shown in FIG. 4 will be described with reference to the first embodiment of the driving waveform diagram shown in FIG. 5. 5 is a driving waveform diagram applied to a pixel circuit when the organic light emitting diode display operates in a normal mode.

The t1 'section is an initialization section, and the data is initialized while the selection signal S [n] immediately before the emission control signal EM changes from a low state to a low state. This data initialization is achieved by discharging the voltage remaining on the capacitor Cst during the previous frame.

The t2 'section is a data writing section in which the emission control signal EM is high and the selection signal S [n] input through the scan line is low. In the t2 'period, the second and third transistors M2 and M3 except for the fourth and fifth transistors M4 and M5 are turned on. Since the second transistor M2 is turned on, the data signal D [m] is transmitted, and the first electrode of the first transistor M1 has a data voltage Vdata corresponding to the data signal D [m]. Is approved. Since the third transistor M3 maintains a turn-on state, the first transistor M1 maintains a diode connection state. In the t2 'period, one electrode of the capacitor Cst is the power supply voltage VDD, and the other electrode of the capacitor Cst, which is connected to the main electrode of the first transistor M1, becomes a voltage across (Vdata-Vth) to form a capacitor ( The voltage across Cst) is as shown in Equation 5.

[Equation 5]

Subsequently, the t3 'section is a display section in which the emission control signal EM is changed from high to low and the selection signal S [n] input through the scan line is high. In the t3 'period, the second and third transistors M2 and M3 are turned off and the voltage stored in the capacitor Cst corresponds to the voltage between the main electrode and the first electrode of the first transistor M1. In the OLED, a current corresponding to the voltage stored in the capacitor Cst flows to emit light. In addition, since the fourth transistor M4 is turned on, the driving current corresponding to Equation 4 is applied to the organic light emitting diode OLED.

The pixel circuit shown in FIG. 4 has a smaller capacitor element than the conventional pixel circuit shown in FIG. 2, so that the pixel circuit can be made smaller. In addition, the kick-back phenomenon according to the leakage paths of the second, third, and fourth transistors M2, M3, and M4 generated in the pixel circuit of FIG. 2 is leaked from the second transistor M2 in the pixel circuit of FIG. 4. By reducing the kickback along the path, it is possible to minimize the kickback back along the leakage path. As a result, it is advantageous to increase the aperture ratio of the organic light emitting diode display and to implement high resolution.

Next, an operation of the pixel circuit which minimizes the leakage path and compensates the threshold voltage shown in FIG. 4 will be described with reference to the second embodiment of the driving waveform diagram shown in FIG. 6. 6 is a driving waveform diagram applied to a pixel circuit when the organic light emitting diode display is operated in a demux mode. In FIG. 6, descriptions of sections t1 ″ and t4 ″ which are the same sections as t1 ′ and t3 ′ of FIG. 5, respectively, will be omitted.

 After the t1 " section (initialization section), the light emission control signal EM is changed to a high state in the t2 " section, which is the data writing section, and the fourth transistor M4 and the fifth transistor M5 are turned off. The R, G, and B data voltages Vdata are stored in an external capacitor (not shown). In this case, in order to prevent the previous data voltage Vdata from being stored in the capacitor Cst, the second transistor M2 and the third transistor are changed by changing the selection signal input through the scan line S [n] to a high state. Turn off (M3).

When the data voltage Vdata is stored in the external capacitor, the selection signal input through the scan line S [n] in the t3 " section is changed from the high state to the low state. Then, the second transistor M2 and the third transistor M3 are turned on, and the data voltage Vdata stored in the external capacitor is stored in the capacitor Cst. When the storage of the data voltage Vdata to the capacitor Cst is completed in the t3 " section, the selection signal S [n] is changed from the low state to the high state and enters the display period t4 ".

As described above, the optimum embodiment has been disclosed in the drawings and the specification. Although specific terms have been used herein, they are used only for the purpose of describing the present invention and are not intended to limit the scope of the invention as defined in the claims or the claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

As described above, the organic light emitting diode display according to the present invention can implement the pixel circuit smaller because the capacitor element is smaller than the conventional pixel circuit, and minimizes the kick-back phenomenon due to the leakage path to increase the aperture ratio and high resolution. It is advantageous to the implementation.

Claims (3)

At least one scan line for transmitting a selection signal, at least one data line for transmitting a data signal, and a pixel circuit provided in a pixel defined in an area intersected by the scan line and the data line, The pixel circuit An organic light emitting device that emits light correspondingly by an applied current; A first transistor supplying a driving current to the organic light emitting element; A second transistor switched in response to the selection signal to control transmission of a data signal to the first transistor; A third transistor for diode-connecting the first transistor while a data signal is transmitted to the first transistor; A capacitor storing a voltage corresponding to a driving current of the first transistor; And A fourth transistor configured to control the voltage of one electrode of the capacitor to be applied to the first electrode of the first transistor during the light emitting period of the organic light emitting element; And And a fifth transistor interposed between the second electrode of the first transistor and one electrode of the organic light emitting element to control the application of current to the organic light emitting element in response to a light emission control signal. delete The method of claim 1, And the fourth transistor controls the voltage transfer of the capacitor one electrode in response to the light emission control signal.

Images