KR20210086110A - Organic light-emitting diode display - Google Patents

Abstract

The present invention relates to an organic light emitting diode display capable of preventing image quality deterioration. According to the present invention, the organic light emitting diode display is configured to divide one frame into a light emitting display time in which image data is displayed and a black display period in which light emitting element does not emit light, vary the black display period according to gradation of image data, and supply an image data voltage generated based on a gamma curve to which an optical compensation coefficient set according to a gray level of the image data is applied to a display panel.

Description

Organic light emitting display device {ORGANIC LIGHT-EMITTING DIODE DISPLAY}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic light emitting diode display, and more particularly, to an organic light emitting display capable of preventing image quality deterioration.

As the information society develops, the demand for a display device for displaying an image is increasing in various forms. For example, it has rapidly developed into a thin, light, and large-area Flat Panel Display Device (FPD) that replaces a bulky cathode ray tube (CRT). As such a flat panel display, a liquid crystal display device (LCD), a plasma display panel (PDP), an organic light-emitting diode display (OLED), and an electrophoretic display device (Electrophoretic) Various flat panel display devices such as Display Device: ED) have been developed and utilized.

Among them, the organic light emitting diode display includes a light emitting element and a pixel driving circuit for controlling a driving current flowing through the light emitting element, and adjusts grayscale (luminance) with the amount of light emitted from the light emitting element proportional to the driving current.

However, in the conventional organic light emitting diode display, display defects occur due to various causes. In particular, a light emitting device driven with a low driving voltage emits light without applying a separate current due to various reasons, resulting in deterioration of image quality.

SUMMARY OF THE INVENTION An object of the present invention is to provide an organic light emitting diode display capable of preventing image quality deterioration.

In order to achieve the above object, an organic light emitting display device according to the present invention divides one frame into a light emitting display time in which image data is displayed and a black display period in which the light emitting element does not emit light, and divides one frame into a gray level of the image data. The black display period is varied, and an image data voltage generated based on a gamma curve to which an optical compensation coefficient set according to a gray level of the image data is applied is supplied to the display panel.

In the present invention, when a low gray level is expressed, a portion of a period of one frame is implemented as black, so that even if the red light emitting device emits bad light, the defective light emission period of the red light emitting device per frame is shortened. Accordingly, according to the present invention, since the red luminance is perceived as low by the user, it is possible to improve the redish defect, which is a color abnormality caused by light emission of different colors.

In addition, in the present invention, when the low gray level is implemented, the target luminance of the sub-pixel displayed with the low gray level can be set to be high in proportion to the shortened light emission display period, so that the luminance fluctuation of the light emitting device implemented with the low gray level can be prevented.

1 is a block diagram illustrating an organic light emitting diode display according to the present invention.
FIG. 2 is a circuit diagram illustrating sub-pixels disposed on the display panel shown in FIG. 1 .
3 is a cross-sectional view illustrating the light emitting device shown in FIG. 2 .
4A and 4B are diagrams illustrating images realized during one frame when the organic light emitting diode display according to the present invention is driven with a low gray scale.
FIG. 5 is a diagram for explaining a black display period and a light emitting display period according to gray levels of the organic light emitting diode display according to the present invention.
6 is a diagram illustrating a blue emission spectrum according to a grayscale change of the organic light emitting diode display according to the present invention.
7 is a diagram illustrating a gamma curve of an organic light emitting diode display according to an exemplary embodiment of the present invention.
8 is a diagram illustrating a gamma curve of an organic light emitting diode display according to a second exemplary embodiment.

Hereinafter, an embodiment according to the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram illustrating an organic light emitting diode display according to the present invention.

The organic light emitting diode display illustrated in FIG. 1 includes a display panel 10 and a panel driver driving the display panel 10 . The panel driver includes a data driver 30 , a gate driver 40 , and a timing controller 20 .

The timing controller 20 includes a control signal generator 22 and a data converter 24 . The control signal generator 22 generates data control signals and gate control signals for controlling driving timings of the data driver 30 and the gate driver 40 , respectively, and supplies them to the data driver 30 and the gate driver 40 . do. The data converter 24 processes the image data and supplies it to the data driver 30 .

The data driver 30 converts the input image data into an image data voltage during the light emitting display period according to the data control signal supplied from the timing controller 20 and supplies it to the data line DL of the display panel 10 . Also, the data driver 30 supplies the black data voltage to the data line DL of the display panel 10 during the black display period according to the data control signal supplied from the timing controller 20 .

The gate driver 40 outputs a gate signal including a scan signal and an emission control signal to the gate lines GL under the control of the timing controller 20 . The gate driver 40 sequentially supplies the gate signals to the gate lines GL by shifting the gate signals using a shift register.

A plurality of sub-pixels SP are disposed on the display panel 10 to realize an image. For color realization, red (R), green (G), and blue (B) sub-pixels SP may be disposed, or a white sub-pixel SP may be further disposed.

Each sub-pixel SP includes a light emitting element EL and a pixel driving circuit for driving the light emitting element EL as shown in FIG. 2 . The pixel driving circuit includes first to fifth switch TFTs T1, T2, T3, T4, and T5, a driving TFT DT, and a storage capacitor Cst. Here, the pixel driving circuit is not limited to the structure of FIG. 2 and may be variously changed.

The first to fifth switch TFTs T1, T2, T3, T4, and T5 and the driving TFT DT may be an amorphous TFT, a polycrystalline TFT, an oxide TFT, or an organic TFT, depending on the material of the active layer.

The first switch TFT T1 is controlled by the first scan signal SCAN1 of the first gate line 142 and applies the data voltage Vdata of the data line 178 to the first node during the sampling period after the initialization period. (n1) is supplied. The second switch TFT T2 is controlled by the second scan signal SCAN2 of the second gate line 144 and connects the second node n2 and the third node n3 to the driving TFT ( DT) is connected with a diode. The third switch TFT T3 is controlled by the light emission control signal EM of the third gate line 146 , and during the initialization period, the first node n1 and the storage capacitor Cst through the reference voltage line 152 . is initialized to the reference voltage Vref. The fourth switch TFT T4 is controlled by the light emission control signal EM of the third gate line 146 and applies the driving current supplied from the driving TFT DT during the light emission period after the sampling period to the light emitting element EL. supplied with The fifth switch TFT T5 is controlled by the second scan signal SCAN2 of the second gate line 144 and initializes the fourth node n4 to the reference voltage Vref during the sampling period. The driving TFT DT controls the current flowing through the light emitting element EL according to the gate-source voltage Vgs stored in the storage capacitor Cst.

The storage capacitor Cst is connected between the first node n1 and the second node n2 . The data voltage Vdata compensated by the threshold voltage Vth of the driving TFT DT is charged in the storage capacitor Cst. Accordingly, since the data voltage Vdata is compensated by the threshold voltage Vth of the driving TFT DT, the characteristic deviation of the driving TFT DT between pixels may be compensated.

The light emitting element EL emits light with an amount of current controlled by the driving TFT DT according to the data voltage Vdata. To this end, the light emitting element EL includes an anode electrode 122 connected to the fourth and fifth switch TFTs T4 and T5 through the fourth node n4 as shown in FIGS. 2 and 3 , and a low and a cathode electrode 126 connected to the potential voltage VSS, and a light emitting stack formed between the anode electrode 122 and the cathode electrode 126 . As shown in FIG. 3 , the anode electrode 122 is exposed by the bank 128 and is formed independently for each sub-pixel, and the cathode electrode 126 is formed to be shared by all sub-pixels. The light emitting stack 124 includes an electron blocking layer (EBL), a hole blocking layer (HBL), a hole injection layer (HIL), a hole transport layer (HTL), a light emitting layer. (Emission layer, EML), an electron transport layer (Electron transport layer, ETL), and may include an electron injection layer (Electron Injection layer, EIL), but is not limited thereto.

The light emitting compound layer of at least one of an electron blocking layer, a hole blocking layer, a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer included in the light emitting stack 124 is formed to be shared by all sub-pixels. Among these shared type light emitting compound layers, the light emitting compound layer including the p-type dopant serves as a current passage when driving at a low gray level, so that a horizontal leakage current (L/C) may be generated between the adjacent anode electrodes 122 . In addition, the red light emitting device EL has a lower turn-on voltage than the green and blue light emitting devices EL, so that poor light emission can be achieved at a low gray level without applying a separate current. In particular, when the temperature of the display panel 10 rises, the horizontal leakage current L/C also increases, so that poor emission of the red light emitting device EL increases during low grayscale driving, so that the green and blue sub-pixels become ready ( reddish) is perceived.

Accordingly, in the present invention, at least a portion of one frame is driven with a luminance higher than the corresponding gray level as shown in FIG. 4A and the remaining period is implemented as black as shown in FIG. 4B when a low gray level is expressed.

As described above, according to the present invention, even when the red light emitting device EL emits bad light, the bad light emission period of the red light emitting device EL is shortened per frame by implementing a black period in a portion of one frame when the low gray level is expressed. Accordingly, according to the present invention, red luminance is perceived as low by the user, and thus redish failure can be improved.

To this end, as shown in FIG. 5 , the control signal generator 22 according to the present invention sets the ratio of the black display period in one frame differently for each gray level of the input image data.

One frame is divided into an active period and a blank period. The active period is a period in which input image data is written into the sub-pixels, and the blank period is a period in which input image data is not written into the sub-pixels. That is, the blank period refers to a period in which the data enable signal (Data Enable, DE) input to the timing controller 20 is maintained at a low logic level.

The active period is divided into a light emitting display period and a black display period. In the active period, the light emission control TFT T4, which is the fourth switch TFT, is turned on in a line sequential manner. Since a driving current is supplied to the light emitting element EL by the turned-on fourth switch TFT T4 , the light emitting element EL emits light. In the black display period Tb, the fourth switch TFT T4 is turned off in a line sequential manner. Since the driving current applied to the light emitting element EL is blocked by the turned-off fourth switch TFT T4 , the light emitting element EL does not emit light.

The control signal generator 22 determines the input image data as image data of the first low grayscale, the second low grayscale, and the high grayscale. The control signal generator 22 determines the gray level of each image data supplied to each sub-pixel, or applies the control signal to a plurality of sub-pixels connected to the same third gate line 146 to which the emission control signal EM is supplied. The gray level of the image data is determined based on the average value of the supplied input image data.

The first low gradation is a gradation having a relatively high frequency of occurrence of redish defects, and means a gradation capable of implementing the first low luminance including the lowest luminance. The second low gradation is a gradation in which the frequency of occurrence of redish failure is relatively low, and means a gradation capable of realizing a second low luminance between the first low luminance and the high luminance. The high gradation does not cause redish failure, is a gradation higher than the first and second low gradations, and refers to a gradation capable of realizing high luminance including the highest luminance. In the present invention, a case in which the first low gradation is 0 to 15 gradations, the second low gradation is 16 to 31 gradations, and the high gradation is 32 to 255 gradations will be described as an example.

In addition, the control signal generator 22 adjusts the black display period by controlling the timing of the light emission control signal EM supplied to the fourth switch TFT T4 according to the determined gray level of the input image data. That is, the control signal generator 22 sets the duty ratio of the light emission control signal EM to be lower as the gray level of the input image data is lower, so that the black display period per frame becomes longer.

When the input image data is determined to be high grayscale input image data, by setting the PWM (Pluse Width Modulation) duty ratio of the light emission control signal EM to 100%, the active period of one frame becomes the light emission display period without the black display period. only done

When it is determined that the input image data is the input image data of the second low grayscale, the second duty ratio DR2 of the light emission control signal EM is set lower than that of the input image data of the high grayscale (for example, 50%) Accordingly, the ratio of the black display period per frame increases compared to the high gradation display, and the ratio of the light emitting display period decreases compared to the high gradation display.

When it is determined that the input image data is the input image data of the first low grayscale, the black display period per frame is occupied by setting the first duty ratio DR1 of the light emission control signal EM to a low level (eg, 25%). The ratio increases compared to the second low grayscale expression, and the ratio occupied by the light emitting display period decreases compared to the second low grayscale expression.

As described above, even when the red light emitting device EL emits bad light due to the horizontal leakage current L/C when expressing the low grayscale of the input image data, the display period of the bad light emission of the red light emitting device EL is shortened per frame. It is possible to improve the redish phenomenon of grayscale.

In particular, as shown in FIG. 6 , the intensity of red light (a dotted line region in FIG. 6 ) emitted by the horizontal leakage current is constant even if the gray level of the blue light emitting device adjacent to the red light emitting device EL increases. In this case, regardless of the gray level of the blue light emitting device, the red luminance perceived by the user's eyes decreases in proportion to the light emission sustain period, so that the redish phenomenon of the low gray level can be improved.

On the other hand, in order to prevent the luminance of the green and blue light emitting devices EL from being reduced due to the shortening of the light emission display period of the green and blue light emitting devices EL during low gradation, in the present invention, in the first and second low gradations, The target luminance is set higher than before.

In order to set the target luminance of the low gray scale high, the data converter 24 of the present invention receives the input data based on the gamma curve shown in FIG. 7 to which the optical compensation coefficient that is the reciprocal to the duty ratio of the emission control signal EM is applied. Correct the image data. The gamma-corrected image data by the data converter 24 is supplied to the data driver 30 .

In the gamma curve shown in FIG. 7 , the first low grayscale, the second low grayscale, and the high grayscale use a gamma curve set to the same gamma value. For example, in the present invention, a gamma curve of 1.5 to 2.5 is used. Such a gamma cover is determined by Equations 1 to 3. The target luminance of the high gradation, the target luminance of the first low gradation, and the target luminance of the second low gradation are proportional to the optical compensation coefficients a1, a2, a3. In Equation 1, a3 is an optical compensation coefficient applied to the high grayscale region of the gamma curve, and is set to 1, which is the inverse of the duty ratio set for realizing the high grayscale. In Equation 2, a2 is an optical compensation coefficient applied to the second low grayscale region of the gamma curve, is a positive integer greater than 1, and is set as the reciprocal of the second duty ratio DR2 set for realizing the second low grayscale. do. For example, when the second duty ratio DR2 set for realizing the second low gray is 50% (=1/2), the optical compensation coefficient a2 of the second low gray is set to 2. In Equation 3, a1 is an optical compensation coefficient applied to the first low grayscale region of the gamma curve, is a positive integer greater than a2, and is set as the reciprocal of the first duty ratio DR1 set for realizing the first low grayscale . For example, when the first duty ratio DR1 set when the first low gray is implemented is 25% (= 1/4), the optical compensation coefficient a2 of the first low gray is set to 4.

Accordingly, from the minimum target luminance to the maximum target luminance of each of the first low gradation, the second low gradation, and the high gradation, as shown in Tables 1 and 7, a positive slope gradually increases.

gradation target luminance High gradation (32-255 gradations) LHmin~LHmax (where LHmax is the maximum luminance) 2nd low gradation (16-31 gradations) LLmin2 to LLmax2 (where LLmax2>LHmin) 1st low gradation (0 to 15 gradations) LLmin1 to LLmax1 (where LLmin1 = 0; LLmax1 > LLmin2)

Also, the gamma curve has a negative slope from the maximum target luminance LLmax1 of the first low grayscale to the minimum target luminance LLmin2 of the second low grayscale. That is, since the maximum target luminance LLmax1 of the first low gray is set to be higher than the minimum target luminance LLmin2 of the second low gray, the maximum data voltage for realizing the first low gray is the minimum data for realizing the second low gray. set higher than the voltage. In addition, from the maximum target luminance LLmax2 of the second low gradation to the minimum target luminance LHmin of the high gradation, there is a negative slope. That is, since the maximum target luminance LLmax2 of the second low gray is set higher than the minimum target luminance LHmin of the high gray, the maximum data voltage for realizing the second low gray is set higher than the minimum data voltage for realizing the high gray do.

As described above, when the first low gray level and the second low gray level are implemented, the target luminance of the sub-pixels displayed with the first low gray level and the second low gray level can be set to be high in proportion to the shortened light emission display period. And it is possible to prevent the luminance fluctuation of the light emitting device implemented with the second low grayscale.

Meanwhile, in the present invention, a structure for gamma correction of image data using the gamma curve shown in FIG. 7 having two negative slopes has been described as an example, but in addition, the gamma curve shown in FIG. 8 having one negative slope has been described. can be used to gamma-correct image data.

The gamma curve shown in FIG. 7 has two negative slopes as the low gradation is subdivided into the first low gradation and the second low gradation according to the frequency of occurrence of redish defects, whereas the gamma curve shown in FIG. By limiting all grayscales in which defects occur to low grayscales, one negative gradient is obtained. Accordingly, the gamma curve shown in FIG. 8 has a negative slope from the maximum target luminance LLmax2 of the low grayscale (0 to 31 grayscale) to the minimum target luminance LHmin of the high grayscale. In addition, the optical compensation coefficient (for example, 2) applied to the low grayscale of the gamma curve shown in FIG. 8 is the inverse of the duty ratio of the emission control signal set for realizing the low grayscale, and is applied to the high grayscale of the gamma curve. The optical compensation coefficient (eg, 1) is a reciprocal of a duty ratio set for realizing a high grayscale.

In addition, although the structure in which the control signal generator 22 and the data converter 24 are built in the timing controller 20 has been described as an example in the present invention, in addition, the control signal generator 22 and the data converter 24 may be formed as a separate driving chip from the timing controller 20 . have.

In addition, although a structure in which the turn-on voltage of the red light-emitting device is lower than that of the green and blue light-emitting devices has been described as an example in the present invention, the present invention is not limited thereto, and the turn-on voltage of the light-emitting device implementing a color other than red may be low. Accordingly, perceived luminance may be lowered by driving a light emitting device having a low turn-on voltage among a plurality of light emitting devices implementing different colors through a black data voltage and an image data voltage for one frame.

The above description is merely illustrative of the present invention, and various modifications may be made by those of ordinary skill in the art to which the present invention pertains without departing from the technical spirit of the present invention. Therefore, the embodiments disclosed in the specification of the present invention do not limit the present invention. The scope of the present invention should be construed by the following claims, and all technologies within the scope equivalent thereto should be construed as being included in the scope of the present invention.

10: display panel 20: timing controller
22: control signal generation unit 24: data conversion unit
30: data driver 40: gate driver
122: anode electrode 124: light-emitting stack
126: cathode electrode

Claims (11)

a display panel provided with a plurality of light emitting devices;
a timing controller that divides one frame into a light emitting display time during which image data is displayed and a black display period in which the light emitting element does not emit light, and varies the black display period according to a gradation of the image data;
and a data driver configured to supply an image data voltage generated based on a gamma curve to which an optical compensation coefficient set according to a gray level of the image data is applied to the display panel. The method of claim 1,
An organic light emitting diode display, wherein the light emitting display period of the light emitting device is increased when the input image data is high grayscale image data than when the input image data is low grayscale image data. The method of claim 1,
An organic light emitting diode display for reducing a duty ratio of a light emission control signal applied to gate lines of the display panel when the input image data is low grayscale image data rather than high grayscale image data. 4. The method of claim 3,
The optical compensation coefficient applied to the low grayscale region of the gamma curve is
The organic light emitting diode display is larger than the optical compensation coefficient applied to the high grayscale region of the gamma curve. 5. The method of claim 4,
The optical compensation coefficient is a reciprocal of the duty ratio. The method of claim 1,
The lowest target luminance of the high gradation region of the gamma curve is lower than the highest target luminance of the low gradation region. The method of claim 1,
The input image data is divided into first and second low grayscale image data and high grayscale image data,
The first low grayscale image data is image data implementing the first low luminance including the lowest target luminance,
The high grayscale image data is image data implementing high luminance including the highest target luminance,
The second low grayscale image data is image data implementing a second low luminance excluding the first low luminance and high luminance,
An organic light emitting diode display, wherein the light emitting display period of the light emitting device is increased when the input image data is high grayscale image data than when the input image data is the first and second low grayscale image data. 8. The method of claim 7,
When the input image data is second low grayscale image data than when the input image data is high grayscale image data, a second duty ratio of the emission control signal applied to the gate lines of the display panel is reduced;
When the input image data is the first low grayscale image data than when the input image data is the second low grayscale image data, a first duty ratio of the emission control signal applied to the gate lines of the display panel is lower than the second duty ratio organic light emitting display device. 9. The method of claim 8,
The first optical compensation coefficient applied to the first low grayscale region of the gamma curve is
greater than a second optical compensation coefficient applied to a second low grayscale region of the gamma curve;
A second optical compensation coefficient applied to the second low grayscale region of the gamma curve is
The organic light emitting diode display is larger than a third optical compensation coefficient applied to the high grayscale region of the gamma curve. 10. The method of claim 9,
The first and second optical compensation coefficients are inverses of the first and second duty ratios. 10. The method of claim 9,
The lowest target luminance of the high gradation region of the gamma curve is lower than the highest target luminance of the second low gradation region;
The lowest target luminance of the second grayscale region of the gamma curve is lower than the highest target luminance of the first low grayscale region.

Images