KR20060114537A - An organic light emitting display device - Google Patents

Abstract

An organic electroluminescence display device is provided to prevent the degradation of white balance property due to the luminance nonuniformity of pixels for each color by installing capacitors with different capacities in each pixel according to the colors. An organic electroluminescence display device includes pixel circuits formed in each pixel; a data driving unit electrically connected with each pixel circuit through a data signal line; and a scan driving unit outputting a selection signal(S[n]) and a first control signal(E[n]) to each pixel circuit. The pixel circuit comprises an organic electroluminescent device(OLED) emitting the light correspondingly to the current; a first transistor(M1) feeding driving current to the organic electroluminescent device; a first capacitor(C1) storing voltage corresponding to the driving current; and a second capacitor(C2) varying the potential of a main electrode of the first transistor according to the voltage level variation of the signal applied to one end. The capacities of at least one of the first and second capacitors are different according to each pixel.

Description

Organic electroluminescent display device

1 is a schematic diagram illustrating a conventional organic electroluminescent display.

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

3 is a circuit diagram of a pixel employed in an organic electroluminescent display according to the present invention.

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

5 is a circuit diagram according to another embodiment of a pixel employed in an organic electroluminescent display according to the present invention.

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

7 is a graph illustrating a voltage variation of a main electrode of a driving transistor according to variation of a capacitor capacity.

Explanation of symbols on the main parts of the drawings

M1-M4: first to fourth transistors

C1, C2: first and second capacitors

OLED: organic electroluminescent device

D [m]: data signal S [n]: selection signal

E [n]: first control signal B [n]: boost signal

The present invention relates to an organic electroluminescent display, and more particularly, in a pixel emitting different colors, luminance of pixels is uneven for each color, thereby reducing a problem such as white balance. The present invention relates to an organic electroluminescent display.

1 shows a conventional organic electroluminescent display.

As shown in FIG. 1, the organic electroluminescent display 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 display device, 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 illustrates a circuit of a pixel employed in the organic electroluminescent display of FIG. 1.

Referring to FIG. 2, a pixel of an organic light emitting display device includes an organic light emitting diode OLED, two transistors M1 and M2, and one capacitor Cst, and generally the transistors M1 and M2. ), A thin film transistor (TFT) is used.

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 in response to the selection signal S [n] applied to the main electrode, so that the data signal D [m] is applied into the pixel circuit.

The capacitor Cst is connected between the first electrode and the main electrode of the second transistor M2 to maintain a data voltage for a predetermined period of time. In addition, the second transistor M2 supplies a current corresponding to the voltage applied between both terminals of the capacitor Cst 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 Cst, and then the data stored in the capacitor Cst even when the first transistor M1 is turned off. A current corresponding to the voltage is applied to the organic light emitting diode OLED through the second transistor M2. Accordingly, even when the first transistor M1 is turned off, the organic light emitting diode OLED maintains light emission for a predetermined period of time.

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

= =

= =

는 유기 전계발광소자(OLED)로 공급되는 전류, Vgs는 제2 트랜지스터(M2)의 주전극과 제1전극 사이의 전압, Vth는 제2 트랜지스터(M2)의 문턱 전압, VDD는 전원전압, Vdata는 데이터 전압, β는 이득 계수(gain factor)를 나타낸다.In Equation 1,

Is the current supplied to the organic light emitting diode OLED, Vgs is the 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. Denotes a data voltage and β denotes a gain factor.

However, in the pixel circuit of the voltage write method as described above, a deviation of the threshold voltage Vth occurs in the manufacturing process of the driving transistor like the second transistor, resulting in a problem that it is difficult to obtain a uniform brightness screen.

The current write-type pixel circuit may be applied to the organic light emitting display device so as to improve this and obtain a screen having a uniform brightness despite variation in threshold voltage. In the current write-in pixel circuit, even if the threshold voltage is uneven, if the current applied through the data signal lines is uniform, the pixel has uniform luminance characteristics.

However, in the pixel circuit of the current write method, it takes a long time to charge the parasitic capacitors of the data signal lines. In particular, the finer the driving current applied to the organic light emitting diode OLED, the larger the capacitance of the parasitic capacitor becomes.

In addition, even when the luminance uniformity of the pixel is improved through the aforementioned method, the material constituting the organic light emitting diode OLED has a difference in characteristics.

That is, even when the same data current is applied to all the pixel circuits, the luminance of each color may be uneven due to the characteristic difference of the light emitting material for implementing each color.

SUMMARY OF THE INVENTION The present invention provides an organic electroluminescent display device that can improve a problem of deteriorating characteristics such as white balance due to uneven luminance of pixels in pixels emitting different colors. To provide.

According to an aspect of the present invention, there is provided an organic electroluminescent display device comprising: a pixel circuit formed in each of a plurality of pixels; A data driver electrically connected to each of the pixel circuits through a data signal line; And a scan driver for outputting a selection signal and a first control signal to each pixel circuit, wherein the pixel circuit comprises: an organic electroluminescent device which emits light in response to an applied current; A first transistor supplying a driving current to the organic electroluminescent device; A first capacitor storing a voltage corresponding to the driving current; And a second capacitor configured to change a potential of the main electrode of the first transistor according to a voltage level change of a signal applied to one end, wherein at least one of the first and second capacitors has a capacitance of each pixel. It is characterized by being configured differently.

Each of the pixel circuits may include: a second transistor for diode-connecting the first transistor in response to a selection signal; and a third transistor for controlling application of a driving current to the organic electroluminescent element in response to a first control signal. It may further include;

The first capacitor may be electrically connected between the main electrode and the first electrode of the first transistor, and the second capacitor may be electrically connected between the main electrode of the first transistor and the signal line to which the selection signal is applied. Can be.

Also preferably, the scan driver may further output a second control signal, wherein the first capacitor is electrically connected between the main electrode and the first electrode of the first transistor, and the second capacitor is the main transistor of the first transistor. It may be configured to be electrically connected between the electrode and the signal line to which the second control signal is applied.

DETAILED DESCRIPTION In order to fully understand the present invention, the operational advantages of the present invention, and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings which illustrate preferred embodiments of the present invention and the contents described in the drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals in the drawings denote like elements.

3 is a circuit diagram of a pixel employed in the organic electroluminescent display according to the present invention, and particularly, illustrates a pixel circuit connected to an nth select signal line and an mth data signal line.

As shown in FIG. 3, the pixel circuit applied to the present invention includes first to fourth transistors M1 to M4, first and second capacitors C1 and C2, and an organic light emitting diode OLED. It can be done by.

The first transistor M1 is a driving transistor for supplying a driving current to the organic light emitting diode OLED, and the first electrode is connected to a voltage source for providing a predetermined power supply voltage VDD.

Meanwhile, the second transistor M2 is connected between the main electrode and the second electrode of the first transistor M1 to diode-connect the first transistor M1 in response to the selection signal S [n].

Although not shown, the second transistor M2 may be configured to be electrically connected between the second electrode of the first transistor M1 and the data signal line. As such, the second transistor M2 may be When turned off in response to the selection signal, the voltage applied to the main electrode of the first transistor M1 is affected to prevent the problem of changing the voltage of the first and second capacitors C1 and C2.

In the third transistor M3, a first electrode and a second electrode are respectively connected between the second electrode of the first transistor M1 and the organic electroluminescent element OLED, and the first control signal E [n]. ), The driving current is applied to the organic electroluminescent device (OLED).

In the fourth transistor M4, the first electrode and the second electrode are connected to the data signal line and the main electrode of the first transistor M1, respectively. In addition, the main electrode of the fourth transistor M4 controls the transfer of the data signal D [m] in response to the selection signal S [n].

In addition, the first capacitor C1 is electrically connected between the main electrode and the first electrode of the first transistor M1, and the second capacitor C2 is the main electrode and the selection of the first transistor M1. It is electrically connected between the signal lines to which the signal S [n] is applied.

The pixel circuit having the above configuration is an example of a pixel circuit that can be applied to the present invention, and the present invention is not limited thereto, and may be applied to a pixel circuit in which some modifications are made.

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

First, when the selection signal S [n] is changed to a low level at t1, the second and fourth transistors M2 and M4 are turned on, so that the first transistor M1 is diode-connected. A current corresponding to the data current Idata value flows through the first transistor M1 to the data signal line.

At the same time, since the first control signal E [n] is changed to a high level, the third transistor M3 is turned off, thereby electrically connecting the first transistor M1 and the organic light emitting diode OLED. Block it with

In this case, the voltage value corresponding to the data current Idata is stored in the first capacitor C1, and the voltage Vg of the main electrode of the first transistor M1 is developed by developing Equation 2 below. Can be obtained as shown in 3.

= Idata = =

= Idata = =

는 제1 트랜지스터의 제2전극으로 흐르는 전류, Vgs는 제1 트랜지스터의 주전극과 제1전극간의 전압, VDD는 제1 트랜지스터의 제1전극으로 인가되는 전원전압, Vth는 제1 트랜지스터(M1)의 문턱 전압, β는 이득 계수(gain factor)를 나타낸다.In Equations 2 and 3

Is the current flowing to the second electrode of the first transistor, Vgs is the voltage between the main electrode and the first electrode of the first transistor, VDD is the power supply voltage applied to the first electrode of the first transistor, and Vth is the first transistor M1. The threshold voltage, β, denotes a gain factor.

Thereafter, when the selection signal S [n] is changed to the high level at t2 and the first control signal E [n] is changed to the low state, the second and fourth transistors M2 and M4 are It is turned off and the third transistor M3 is turned on. At this time, the selection signal S [n] is changed to the high level, so that one end of the second capacitor C2 is increased in voltage by the level rising width of the selection signal S [n].

In this case, the voltage of the main electrode of the first transistor M1 is increased by a predetermined value by the coupling of the first and second capacitors C1 and C2. The level of the selection signal S [n] is increased. When the rising width is ΔVs, the voltage rising width of the main electrode of the first transistor M1 is expressed by Equation 4 below.

As shown in Equation 4, when the voltage of the main electrode of the first transistor M1 increases, the absolute value of the voltage magnitude between the main electrode and the first electrode of the first transistor M1 becomes small. Therefore, in this case, the magnitude of the current flowing to the organic light emitting diode OLED can be reduced.

)의 크기를 감소시키면, 데이터 전류(Idata)를 발광소자로 흐르는 전류(

)보다 큰 값으로 설정할 수 있다. 따라서 데이터 전류(Idata)를 큰 값으로 인가하여 상기 유기 전계발광소자(OLED)로 흐르는 전류(

)의 값을 미세하게 조정할 수 있을 뿐 아니라, 상기 데이터 신호선에 형성되는 기생 커패시터를 충전하는데 소요되는 시간을 감축시킬 수 있게 된다.As described above, the current flowing through the organic electroluminescent device (OLED)

When the size of the? Is reduced, the current (Idata) flowing through the light emitting device (

Can be set to a value greater than). Accordingly, the current flowing to the organic light emitting diode OLED by applying the data current Idata to a large value

Value can be finely adjusted, and the time required for charging the parasitic capacitor formed on the data signal line can be reduced.

By configuring the pixel circuit as shown in FIG. 3, problems caused by variations in threshold voltages of parasitic transistors and parasitic capacitors can be solved, and a uniform driving current can be applied to the plurality of pixels. However, as described above, the luminance difference between colors may occur even when a uniform driving current is applied due to the difference in the properties of the materials constituting the organic light emitting diode OLED of the organic light emitting display device.

)은 제1, 제2 커패시터(C1,C2)의 용량 크기에 따라 가변한다. 즉, 상기 제1, 제2 커패시터(C1,C2) 중 적어도 하나 이상의 커패시터는 화소별로 용량 크기가 상이하게 구성되도록 함으로써, 상기 각 색상에 따른 화소마다 다른 구동전류를 인가하여 휘도 균일도를 향상시킬 수 있다. As shown in Equation 4, the voltage variation of the main electrode of the first transistor M1 (

) Varies depending on the capacitance of the first and second capacitors C1 and C2. That is, at least one or more capacitors among the first and second capacitors C1 and C2 may have different capacitances for each pixel, thereby improving luminance uniformity by applying a different driving current to each pixel according to each color. have.

Meanwhile, another embodiment of the pixel circuit applied to the organic electroluminescent display according to the present invention is shown in FIG. 5. In the case of the pixel circuit illustrated in FIG. 3, the capacitance ratio of the first and second capacitors C1 and C2 mentioned in Equation 4 may vary due to a parasitic capacitor component or the like present in the transistors M1 to M4. Can be. That is, since the level rising width of the selection signal S [n] is constant, it is difficult to properly respond to the variation in the capacity ratio.

Since the elements of the pixel circuit of FIG. 5 are configured and operate in the same manner as those of the pixel circuit of FIG. 3 described above, detailed description thereof will be omitted.

In the singularity of the pixel circuit of FIG. 5, the scan driver further outputs boost signals B [n], which are second control signals, through the signal line. In addition, the second capacitor C2 is electrically connected between the main electrode of the first transistor M1 and the signal line to which the boost signal B [n] is applied.

It is assumed that the boost signal B [n] is a pulse signal in which the low voltage level and the high voltage level have an appropriate level rise width according to the variation in the capacitance ratios of the first and second capacitors C1 and C2.

Meanwhile, the pixel circuit as shown in FIGS. 3 and 5 shows an example for implementing the present invention, and the same operation can be performed by slightly modifying the pixel circuit and its configuration and connection state as described above. The present invention is applicable to all pixel circuits present.

FIG. 6 is a waveform diagram for driving the circuit of FIG. 5. As shown in FIG. 6, the selection signal S [n] is changed to a low level at t1 to turn on the second and fourth transistors M2 and M4.

In addition, the first control signal E [n] is changed to a high level to turn off the third transistor M3, and a boost signal B [n] having a low voltage level is applied to the second capacitor C2. It is applied at one end of.

Then, at t2, the selection signal S [n] is changed to the high level and the first control signal E [n] is changed to the low level, whereby the second and fourth transistors M2 and M4 are turned off. The third transistor M3 is turned on.

In addition, since the boost signal B [n] is changed to a high voltage level, the voltage of the main electrode of the first transistor M1 is increased by a predetermined value. At this time, when the difference between the voltage levels of the boost signal B [n] is ΔVb, the voltage rise of the main electrode of the first transistor M1 is expressed by Equation 5 below.

)의 값을 감소시킬 수 있다. 특히, 상기 트랜지스터(M1 내지 M4)에 존재하는 기생 커패시터 성분 등의 원인으로 인하여, 제1, 제2 커패시터(C1, C2)의 용량 비가 변할 수 있는데, 이에 대응하여 상기 부스트 신호(B[n])의 전압레벨간 크기의 차이(ΔVb)를 적절하게 조절함으로써, 유기 전계발광소자(OLED)로 흐르는 구동전류의 정밀도를 증가시킬 수 있다. As described above, since the voltage of the main electrode of the first transistor M1 is increased, the current flowing through the organic electroluminescent element OLED (

Can be reduced. In particular, due to parasitic capacitor components and the like present in the transistors M1 to M4, the capacitance ratios of the first and second capacitors C1 and C2 may vary. In response thereto, the boost signal B [n] By appropriately adjusting the difference ΔVb between the voltage levels, the precision of the driving current flowing to the organic light emitting diode OLED can be increased.

In the pixel circuit of FIG. 5, at least one or more of the first and second capacitors C1 and C2 may have different capacitances for each pixel. Accordingly, a problem of occurrence of luminance deviation due to a difference in characteristics of materials constituting the organic light emitting diode OLED of the organic light emitting display device can be improved.

7 is a graph illustrating an amount of change in voltage of the main electrode of the first transistor according to a change in capacitor capacitance. The graph is related to the case in which the pixel circuit shown in FIG. 3 is employed in the organic electroluminescent display, and the level increase width ΔVs of the selection signal is replaced by the voltage level difference ΔVb of the boost signal. It can be applied to the pixel circuit shown in FIG.

As shown in FIG. 7, as the capacitance of the second capacitor C2 increases, the voltage variation ΔVg of the main electrode of the first transistor M1 increases.

Therefore, in the pixel in which the second capacitor C2 of the large capacitance is employed, the absolute value of the voltage between the main electrode and the first electrode of the first transistor M1 increases in magnitude. Therefore, as shown in Equation 2, the magnitude of the current flowing to the organic light emitting diode OLED is relatively small.

On the contrary, as the capacitance of the second capacitor C2 decreases, the voltage variation ΔVg of the main electrode of the first transistor M1 decreases.

Therefore, in the pixel in which the second capacitor C2 of small capacity is employed, the absolute value of the voltage between the main electrode and the first electrode of the first transistor M1 has a small amount of decrease in magnitude. Therefore, the magnitude of the current flowing to the organic light emitting diode OLED is relatively large.

Accordingly, in the case where the luminance deviation occurs due to the difference in characteristics of the organic light emitting diode (OLED) material provided in the pixels emitting different colors, the luminance uniformity may be improved by taking the following configuration.

First, a pixel having an organic electroluminescent device (OLED) having a relatively low luminance characteristic is increased so that its brightness is increased, so that the second capacitor C2 included in the pixel is made of a capacitor having a smaller capacity than other pixels. do.

On the contrary, in order to increase the luminance uniformity for each color, a pixel having an organic electroluminescent element (OLED) having a relatively high luminance characteristic needs to reduce its luminance. The capacitor C2 is formed of a capacitor having a larger capacity than other pixels.

On the other hand, by changing the capacitance of the first capacitor (C1) according to the principle described above, it is possible to adjust the brightness of the pixel provided with the first capacitor (C1).

That is, since the voltage variation ΔVg of the main electrode is inversely proportional to the capacitance of the first capacitor C1, the pixel having the organic light emitting diode OLED having a relatively low luminance characteristic increases the luminance. The first capacitor C1 included in the pixel is formed of a capacitor having a larger capacitance than other pixels.

On the contrary, when a pixel having an organic light emitting diode OLED having a relatively high luminance characteristic needs to be reduced in brightness, the first capacitor C1 included in the pixel is smaller than other pixels. It should consist of a capacitor of capacity.

As described above, optimal embodiments have 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 electroluminescent display device according to the present invention includes a capacitor having a different capacitance size for each pixel according to each color, so that the luminance of the pixel is uneven for each color, such as white balance. This deterioration problem can be improved.

Claims (6)

A pixel circuit formed in each of the plurality of pixels; A data driver electrically connected to each of the pixel circuits through a data signal line; And a scan driver for outputting a selection signal and a first control signal to the pixel circuits, the organic light emitting display device comprising: The pixel circuit may include an organic electroluminescent device that emits light in response to an applied current; A first transistor supplying a driving current to the organic electroluminescent device; A first capacitor storing a voltage corresponding to the driving current; And a second capacitor configured to change a potential of the main electrode of the first transistor according to a change in the voltage level of the signal applied to one end. And at least one of the first and second capacitors has different capacitances for each pixel. The method of claim 1, Each of the pixel circuits may include: a second transistor for diode-connecting the first transistor in response to a selection signal; and a third transistor for controlling application of a driving current to the organic electroluminescent device in response to a first control signal. An organic electroluminescent display further comprising. The method of claim 2, The first capacitor is electrically connected between the main electrode and the first electrode of the first transistor, the second capacitor is electrically connected between the main electrode of the first transistor and the signal line to which the selection signal is applied. Organic electroluminescent display. The method of claim 2, The scan driver further outputs a second control signal, The first capacitor is electrically connected between the main electrode and the first electrode of the first transistor, and the second capacitor is electrically connected between the main electrode of the first transistor and the signal line to which the second control signal is applied. Organic electroluminescent display. The method according to claim 3 or 4, The organic light emitting display device of claim 1, wherein the first capacitor of the pixel has a larger capacitance than that of the first capacitor of the other pixel so that the luminance of the pixel emitting a predetermined color is increased. The method according to claim 3 or 4, And the second capacitor provided in the pixel has a smaller capacitance than the second capacitor provided in the other pixel so that the luminance of the pixel emitting a predetermined color is increased.

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