KR100741076B1 - Organic light emitting display - Google Patents

Abstract

The present invention discloses an organic light emitting display device capable of high resolution by compensating for variation in threshold voltage of a driving transistor to express uniform luminance and minimizing a change in driving voltage caused by a kick back phenomenon of a switching transistor. do. The organic light emitting diode display includes an organic light emitting diode, first to fourth transistors, a first capacitor, and a reservoir. The organic light emitting element emits light correspondingly by the applied current. The first transistor supplies a driving current to the organic light emitting element, and the second transistor is switched in response to the selection signal to control the transfer of the data signal to the first transistor. The third transistor diode-connects the first transistor, and the fourth transistor is interposed between the first transistor and the organic light emitting element, and switches the driving current supplied to the organic light emitting element in response to the second light emission control signal. The first capacitor stores a voltage corresponding to the driving current of the first transistor, and the reservoir is interposed between the first capacitor and the fourth transistor, and stores a part of the leakage current generated when the third transistor is turned on / off.

Description

Organic light emitting display device

BRIEF DESCRIPTION OF THE DRAWINGS In order to better understand the drawings cited in the detailed description of the invention, a brief description of each drawing is provided.

1 is a schematic diagram illustrating a conventional organic light emitting display device.

FIG. 2 is a circuit diagram of a pixel employed in the OLED display of FIG. 1.

3 is a pixel circuit diagram for threshold voltage compensation according to a first embodiment of the present invention.

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

FIG. 5 is a diagram for describing a data voltage variation occurring when a third transistor M3 is turned off in the pixel circuit of FIG. 3.

6 is a pixel circuit diagram to compensate for a kick-back phenomenon according to a second embodiment of the present invention.

FIG. 7 is a driving waveform diagram for driving the pixel circuit shown in FIG. 6.

FIG. 8 is a view for explaining a data voltage variation occurring when the third transistor M3 is turned off in the pixel circuit of FIG. 6.

9 is a pixel circuit diagram to compensate for a kick-back phenomenon according to a third exemplary embodiment of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic light emitting diode display, and more particularly, to compensate for variations in threshold voltages of driving transistors included in pixel circuits, thereby to express uniform luminance, and to be generated by kickback of switching transistors. The present invention relates to an organic light emitting display device capable of realizing high resolution by minimizing a change in driving voltage.

1 illustrates a conventional organic light emitting display device.

As shown in FIG. 1, the organic light emitting diode display 10 includes a data driver 11, a scan driver 12, and a display 13. The display unit 13 includes a plurality of data signal lines extending in the vertical direction and a plurality of selection signal lines extending in the horizontal direction.

In the display unit 13 of the organic light emitting diode display, pixels 14 are defined in a matrix by the data signal lines and the selection signal lines, and pixel circuits are formed in the pixels 14.

The data driver 11 transmits the data signals D [1] to D [m] to the pixel circuit through the data signal lines. The scan driver 12 applies the selection signals S [1] to S [n] through the selection signal lines to select the plurality of pixels 14 constituting the display unit 13 in line units. Information of the data signals D [1] to D [m] is transmitted to the pixels 14 selected by the selection signals S [1] to S [n]. Meanwhile, a predetermined power supply voltage VDD is supplied to all the pixels 14 of the display unit 13 through the bus line.

2 is a circuit diagram of a pixel employed in the OLED display of FIG. 1.

Referring to FIG. 2, a pixel of an organic light emitting diode display includes an organic light emitting diode OLED, two transistors M1 and M2, and one capacitor C _st , and are generally the first and second transistors. Thin film transistors TFT are used for M1 and M2.

In the pixel circuit of FIG. 2, the first electrode of the first transistor M1 is connected to the data signal line. At this time, the first transistor M1 is turned on by the selection signal S [n] applied to the main electrode, so that the data signal D [m] is applied to the pixel circuit.

Meanwhile, the capacitor C _st is connected between the first electrode and the main electrode of the second transistor M2 to maintain the data voltage for a predetermined period. In addition, the second transistor M2 supplies a current corresponding to the voltage applied between both electrodes of the capacitor C _st to the organic light emitting diode OLED.

When the first transistor M1 is turned on, the data voltage applied through the data signal line is stored in the capacitor C _st , and thereafter, even when the first transistor M1 is turned off, the capacitor C _st is applied to the capacitor C _st . A current corresponding to the stored data voltage 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]

=

=

=

=

는 유기 발광 소자(OLED)에 흐르는 전류, V_gs는 제2 트랜지스터(M2)의 주전극과 제1전극 사이의 전압, V_th는 제2 트랜지스터(M2)의 문턱 전압, V_DD는 전원전압, V_data는 데이터 전압, β는 이득 계수(gain factor)를 나타낸다.In Equation 1,

Is the current flowing through the OLED, V _gs is the voltage between the main electrode and the first electrode of the second transistor M2, V _th is the threshold voltage of the second transistor M2, V _DD is the power supply voltage, V _data represents a data voltage and β represents a gain factor.

값이 낮아지며, 상기 낮은

에 의해 유기 발광 소자(OLED)는 어두운 빛을 발광하게 된다. However, in the pixel circuit of the voltage write method as described above, the driving transistor, like the second transistor, causes variations in the threshold voltage Vth in the manufacturing process, thereby causing a problem that it is difficult to obtain a screen of uniform brightness. That is, when a predetermined driving transistor has an absolute value of a high threshold voltage, as mentioned in Equation 1 above

The lower the value, the lower

As a result, the organic light emitting diode OLED emits dark light.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide an organic light emitting display device capable of expressing uniform luminance by compensating for variations in threshold voltages of driving transistors included in a pixel circuit.

Another object of the present invention is to provide an organic light emitting display device capable of high resolution by minimizing a change in driving voltage caused by a kick back phenomenon.

According to an aspect of the present invention, there is provided an organic light emitting display device including: an organic light emitting device that emits light in response to an applied current; A first transistor supplying a driving current to the organic light emitting element; A second transistor switched in response to a selection signal to control transmission of a data signal to the first transistor; A third transistor for diode-connecting the first transistor; A fourth transistor interposed between the first transistor and the organic light emitting element and switching a driving current supplied to the organic light emitting element in response to a second light emission control signal; And a first capacitor configured to store a voltage corresponding to the driving current of the first transistor.

In accordance with another aspect of the present invention, there is provided an organic light emitting display device including: an organic light emitting device that emits light in response to an applied current; A first transistor supplying a driving current to the organic light emitting element; A second transistor switched in response to a selection signal to control transmission of a data signal to the first transistor; A third transistor for diode-connecting the first transistor; A fourth transistor interposed between the first transistor and the organic light emitting element and switching a driving current supplied to the organic light emitting element in response to a second light emission control signal; A first capacitor storing a voltage corresponding to a driving current of the first transistor; And a reservoir interposed between the first capacitor and the fourth transistor and storing a part of the leakage current generated when the third transistor is turned on / off.

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

As shown in FIG. 3, the pixel circuit for compensating the threshold voltage according to the first exemplary embodiment of the present invention includes an organic light emitting diode (OLED), a first transistor M1 as a driving transistor, and second to fifth transistors as a switching transistor. (M2 to M5) and capacitor C _st .

The second transistor M2 switches the data voltage V _data applied to the data line D [m] in response to the low level of the selection signal from the scan line S [n].

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 has a data voltage V when the second transistor M2 is turned on. _data ), 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 anode of the organic light emitting diode OLED is connected to the first electrode of the fourth transistor M4, and the cathode of the organic light emitting diode OLED is connected to the reference voltage V _SS so that the fourth transistor M4 is turned on. It is turned on to emit light corresponding to the amount of current applied from the first transistor M1. In this case, the reference voltage V _SS is a voltage having a lower level than the power supply voltage V _DD , and a ground voltage and a negative voltage may be used.

The fifth transistor M5 switches the power supply voltage V _DD in response to the low level from the first emission control signal EM1.

One electrode of the capacitor C _st is connected to the power supply voltage V _DD , and the other electrode is connected to the main electrode of the first transistor M1.

Next, an operation of the pixel circuit according to the first embodiment of the present invention for threshold voltage compensation will be described with reference to FIGS. 4 and 5.

The t1 'section is an initialization section in which the first emission control signal EM1 is high, the second emission control signal EM2 is low, and the selection signal S [n] is low. In the t1 'period, the second to fourth transistors M2, M3, and M4 except for the fifth transistor M5 are turned on. Since the third transistor M3 is turned on, the first transistor M1 is in a diode connection state. At this time, the charge stored in the capacitor C _st is cleared.

The t2 'section is a data writing section in which the first emission control signal EM1 and the second emission control signal EM2 are high and the selection signal S [n] 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 third transistor M3 maintains a turn-on state, the first transistor M1 maintains a diode connection state. In the t2 'period, the voltage across the capacitor C _{st is} shown in Equation 2.

[Equation 2]

The t3 'section is a light emission delay section where the first emission control signal EM1, the second emission control signal EM2, and the selection signal S [n] are high. In the t3 'period, all of the second to fifth transistors M2 to M5 are turned off, and the voltage across the capacitor C _st is represented by Equation 2 above. In the t3 'section, the selection signal S [n] section preferably satisfies the range below the first and second emission control signals EM1 and EM2 sections. This makes it possible to stably turn off the fourth and fifth transistors M4 and M5 while the select signal S [n]] turns on the second and third transistors M2 and M3 to write data. For sake. If the time point at which the second and third transistors M2 and M3 are turned on and the time point at which the fourth and fifth transistors M4 and M5 are turned on are the same, they are connected to the selection signal S [n]]. Due to the difference in load, the second and third transistors M2 and M3 may be turned on when the fourth and fifth transistors M4 and M5 are turned on. When data is written while the fourth and fifth transistors M4 and M5 are turned on, a problem arises in that a voltage corresponding to the data cannot be correctly written to the capacitor C _st . Therefore, in the t3 'section, the selection signal S [n] section is set within a range below the first and second emission control signals EM1 and EM2 sections so that the voltage corresponding to the data is correctly written to the capacitor C _st . do.

The t4 'section is a display section in which the first and second emission control signals EM1 and EM2 are low and the selection signal S [n] is high. In the period t4 ', the second and third transistors M2 and M3 are turned off, and the fourth and fifth transistors M4 and M5 are turned on, so that the organic light emitting diode OLED has a capacitor C _st . A current corresponding to the stored voltage flows to emit light. The current I _OLED flowing in the organic light emitting diode OLED may be expressed by Equation 3 below.

[Equation 3]

As a result, uniform luminance can be expressed by compensating for variation in threshold voltage.

변동이 발생한다. However, as the third transistor M3 is turned off in the t3 'period, a kick-back phenomenon occurs in which charges used to form the channel of the third transistor M3 leak to the capacitor C _st . . This leakage current is changed, as can affect the voltage across the capacitor (C _st), the capacitor (C _st) shown in Figure 5 the data voltage across the. Due to the kickback current, the voltage across the capacitor C _st is equal to that of Equation 4

Fluctuations occur.

[Equation 4]

In Equation 3, Q _ch represents the amount of charge leaking into the capacitor C _st as the third transistor M3 is turned off, and C _st represents the capacitance value of the capacitor C _st .

Subsequently, a current corresponding to the voltage stored in the capacitor C _st flows to the organic light emitting diode OLED in the period t4 ′, thereby emitting light. As shown in Equation 4, the voltage across the capacitor C _st is changed. As a result, the current I _OLED flowing in the organic light emitting diode OLED may be expressed by Equation 5.

[Equation 5]

이 발생하게 되면, 고해상도 구현에 문제가 발생한다. 고해상도 구현을 위해서는 캐패시터(C_st)의 크기를 최소로 구현해야 하는데, 이렇게 되면 캐패시터(C_st)의 크기 감소에 따른 킥-백 현상에 의한 전압 변화량이 커진다. 따라서 킥-백 현상에 의한 전압 변화를 줄이는 방법은 캐패시터(C_st)의 크기를 크게 하는 방법 밖에는 없고, 이 또한 고해상도 구현에 적합하지 않은 구조가 된다.As shown in Equation 5, the voltage across the capacitor C _st is changed due to the kickback phenomenon.

If this occurs, problems with the high resolution implementation occur. In order to implement a high resolution to implement the size of the capacitor (C _st) to a minimum, so when the kick according to reduce the size of the capacitor (C _st) - the greater the voltage variation due to the back phenomenon. Therefore, the only way to reduce the voltage change caused by the kick-back phenomenon is to increase the size of the capacitor (C _st ), which is also unsuitable for high resolution.

Accordingly, to solve this problem, FIGS. 6 and 9 illustrate first and second embodiments of a pixel circuit diagram for compensating a kickback phenomenon.

First, a pixel circuit for compensating a kickback phenomenon according to the first exemplary embodiment will be described with reference to FIGS. 6 to 8.

In the pixel circuit shown in FIG. 6, a sixth electrode in which the first and second electrodes are connected to the other electrode of the capacitor C _st and the main electrode is connected to the second emission control signal EM2 in the pixel circuit shown in FIG. 3. The transistor M6 is further provided. The sixth transistor M6 is turned on / off by the second emission control signal EM2 and serves as a storage element for storing a part of the variable voltage generated by the kick-back phenomenon.

FIG. 7 is a driving waveform diagram for driving the pixel circuit shown in FIG. 6, and the description is omitted since sections t1 to t3 are the same as sections t1 'to t3' of FIG.

The t4 section is a display section in which the first and second emission control signals EM1 and EM2 are low and the selection signal S [n] is high. In the period t4, the second and third transistors M2 and M3 are turned off, and the fourth, fifth and sixth transistors M4, M5, and M6 are turned on so that the capacitors are disposed in the organic light emitting diode OLED. A current corresponding to the voltage stored in C _st ) flows to emit light.

가 발생하며, 이는 도 8에 도시되어 있고, 수학식6과 같이 표현할 수 있다.The sixth transistor M6 is turned on at the moment when the second emission control signal EM2 is turned low in the t4 period. In this case, the sixth transistor M6 serves as a capacitor. Hundred the charge from leaking from the second transistor (M2) by the developer is distributed to the capacitor (C _st), and the sixth transistor (M6) the capacitor (C _st) - a sixth transistor (M6) is turned on, kick Change in voltage across

Is generated, which is shown in FIG. 8 and can be expressed as Equation 6.

[Equation 6]

In Equation 6, Q _ch1 represents the first charge amount leaked into the capacitor C _st as the third transistor M3 is turned off, and Q _ch2 represents the sixth transistor (M3) as the third transistor M3 is turned off. Each of the second electric charges leaked into M6) is shown, and C _st represents the capacitance value of the capacitor C _st .

Therefore, it can be seen that the amount of fluctuation in the voltage across the capacitor C _st is significantly reduced than before. Finally, the current I _OLED flowing in the organic light emitting diode OLED in the t4 section may be expressed by Equation 7.

[Equation 7]

A sixth transistor serving as a reservoir is further included in the pixel circuit of FIG. 3, which compensates for the threshold voltage, thereby expressing uniform brightness and minimizing a change in driving voltage caused by a kickback phenomenon. Implementation is possible.

가 발생하며, 이는 도 8에 도시되어 있고, 상기 수학식6과 같이 표현할 수 있다. 수학식 6에서 Q_ch1은 제3 트랜지스터(M3)가 턴-오프 되면서 캐패시터(C_st)로 누설되는 제1 전하량을 나타내고, Q_ch2은 제3 트랜지스터(M3)가 턴-오프 되면서 임의의 캐패시터(C_c)로 누설되는 제2 전하량을 각각 나타내고, C_st는 캐패시터(C_st)의 캐패시턴스 값을 나타낸다. 도 6과 마찬가지로 캐패시터(C_st)의 양단 전압에 변동 양이 종래 보다 현저하게 줄어듦을 알 수 있으며, 캐패시터(C_c)의 크기를 잘 조절하면 분자항을 "0"으로 만들 수 있다. t4 구간에서 유기 발광 소자(OLED)에 흐르는 전류(I_OLED)는 상기 수학식7과 같다. Figure 9 is a kick in accordance with a third embodiment of the present invention as a pixel circuit for compensating for the back current, and the other of the pixel circuit is a capacitor (C _st) the first electrode in the pixel circuit shown in FIG. 3 shown in FIG. 9 An optional capacitor C _c connected to the electrode and the other electrode connected to the second emission control signal EM2 is further provided. Any capacitor C _c serves as a storage element for storing a part of the fluctuation voltage generated by the kick-back phenomenon. Kick-back by the developing electric charge leaking from the third transistor (M3) is distributed to the capacitor (C _st) and a capacitor (C _c) changing the voltage across the capacitor (C _st)

Is generated, which can be expressed as shown in Equation 6 above. In Equation 6, Q _ch1 represents the first charge amount leaked into the capacitor C _st as the third transistor M3 is turned off, and Q _ch2 represents an arbitrary capacitor as the third transistor M3 is turned off. Each of the second electric charges leaked into C _c ) is represented, and C _st represents the capacitance value of the capacitor C _st . As shown in FIG. 6, it can be seen that the amount of fluctuation in the voltage across the capacitor C _st is significantly reduced compared to the prior art. When the size of the capacitor C _c is well controlled, the molecular term can be made “0”. The current I _OLED flowing through the organic light emitting diode OLED in the t4 section is expressed by Equation 7 above.

As described above, since the sixth transistor M6 and the capacitor C _c are provided in the pixel circuit of the organic light emitting device, the size of the capacitor C _st can be minimized. That is, while implementing a smaller capacitor (C _st ) than Figure 3, it is possible to obtain a better driving characteristics. This has the disadvantage of adding one more element to the pixel circuit to reduce the voltage fluctuation, but the larger gain can be obtained because the area of the capacitor C _st that occupies the largest area in the pixel circuit can be reduced.

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 present 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 achieve high resolution by compensating the threshold voltage and expressing uniform luminance and minimizing a change in driving voltage caused by a kick back phenomenon.

Claims (8)

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 a selection signal to control transmission of a data signal to the first transistor; A third transistor for diode-connecting the first transistor; A fourth transistor interposed between the first transistor and the organic light emitting element and switching a driving current supplied to the organic light emitting element in response to a second light emission control signal; A first capacitor storing a voltage corresponding to a driving current of the first transistor; And And a fifth transistor configured to control the voltage stored in the first capacitor to be applied to one electrode of the first transistor in response to a first emission control signal. delete 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 a selection signal to control transmission of a data signal to the first transistor; A third transistor for diode-connecting the first transistor; A fourth transistor interposed between the first transistor and the organic light emitting element and switching a driving current supplied to the organic light emitting element in response to a second light emission control signal; A first capacitor storing a voltage corresponding to a driving current of the first transistor; A fifth transistor configured to control the voltage stored in the first capacitor to be applied to one electrode of the first transistor in response to a first light emission control signal; And And a reservoir interposed between the first capacitor and the fourth transistor and storing a part of the leakage current generated when the third transistor is turned on or off. delete The organic light emitting display device of claim 3, wherein the reservoir is a sixth transistor. The method of claim 5, wherein the sixth transistor is And a first electrode and a second electrode are electrically connected to the main electrode of the first transistor, and the main electrode is electrically connected to the main electrode of the fourth transistor. The organic light emitting display device of claim 3, wherein the reservoir is a second capacitor. The method of claim 7, wherein the second capacitor And one electrode is electrically connected to the main electrode of the first transistor, and the other electrode is electrically connected to the main electrode of the fourth transistor.

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