CN1312650C - Method and device for image uniformity of active organic light emitting diode display - Google Patents

Abstract

A method and device for making image uniform on active organic LED display is prepared as setting a driving unit on each pixel unit to drive organic LED to light up, connecting cathode of said organic LED to a positive power supply to raise electric potential of said organic LED by voltage supplied by said positive power supply, reducing Vsd (source-drain voltage difference) and keeping Vsg (source-gate voltage difference) of each driving unit when each driving unit is operated, reducing difference of output current when each driving unit is conducted when each driving unit has different initial voltage due to characteristic change.

Description

Active organic light emitting diode display method and device for image uniformity

technical field

The present invention relates to a method and device for uniform image of an active organic light emitting diode display, in particular to a method for reducing the Vsd (source-drain voltage difference) of a driving thin-film transistor (Driving TFT) and maintaining its Vsg (source-drain voltage difference). Gate voltage difference), so that the output current of each driving thin film transistor (Driving TFT) will not change too much.

Background technique

The known thin film transistor liquid crystal display (TFT LCD) technology is currently divided into two types: amorphous silicon thin film transistor (α-Si TFT) and polysilicon thin film transistor (Poly-Si TFT), generally called thin film transistor liquid crystal display (TFT LCD) ) refers to the amorphous silicon thin film transistor (α-Si TFT), which is currently a mature technology and is the mainstream product of liquid crystal display (LCD). The biggest difference between low temperature polysilicon thin film transistor (LTPS TFT) and amorphous silicon thin film transistor (α-Si TFT) is that the low temperature polysilicon (LTPS) transistor needs to be further processed by laser tempering, and the amorphous silicon thin film transistor (α-Si TFT) -SiTFT) film is transformed into a polysilicon film layer, which makes the low temperature polysilicon (LTPS) more orderly in the silicon crystal structure than the amorphous silicon thin film transistor (α-Si TFT), so it can increase the electron conduction rate to 200cm ² / V-sec. Low-temperature polysilicon (LTPS) technology can make components smaller, reducing the overall TFT component area by more than 50%; and improving the aperture ratio (aperture ratio), compared with α-Si TFT LCD, it can manufacture higher resolution, and reduce power consumption. In addition, low temperature polysilicon thin film transistor (LTPS TFT) also has the advantages of power saving, high brightness, fine picture, light and thin, and fewer contacts (less than 200 contacts, which increases the pass rate, and α-Si TFT needs more than 3842 contacts).

However, since the thin film transistors produced in the low temperature polysilicon (LTPS) process must undergo a laser tempering process, the threshold voltage (ThresholdVoltage) and drift rate (Mobility) of the thin film transistors will often change, so that each thin film The characteristics of transistor (TFT) components will be different. Therefore, when the driving system uses analog modulation to express grayscale, it is often because the thin film transistor (TFT) has different characteristics after undergoing the manufacturing step of laser tempering. , even if the same voltage signal is written, the OLEDs of different pixels generate different currents and emit different luminances. This phenomenon will cause the OLED panel to display images with wrong gray scales, which seriously damages the image uniformity (Image Uniformity).

In addition, US Patent No. 5,684,365 "Thin Film Display of Organic Electroluminescent Elements" proposes a pixel circuit composed of two thin film transistors and a capacitor. When the pixel device is scanning image data, the switch unit is turned on. At this time, the image data enters the switch unit from the data line, and is stored in the storage unit after being scanned by the scanning line. The Vsg (source-gate voltage difference) makes the drive unit output current to the organic electroluminescent element, and the brightness emitted by the organic electroluminescent element is proportional to the current flowing through the element. However, if the element characteristics of the driving unit of this kind of pixel device vary due to the limitation of the manufacturing process, it will cause uneven light emission of the organic electroluminescent element, and damage the uniformity of the image.

Therefore, in order to improve the image uniformity of the display, some operators have developed a digital driving architecture, and the grayscale is expressed through Time Ratio modulation. The operating principle is to control the thin film transistor (TFT) Turn on (ON) and cut off (OFF) to control the lighting and non-lighting of the organic light-emitting diode (OLED), and determine the gray scale of the image through the ratio of the lighting time of the OLED to the frame time (Frame Time).

However, to digitally drive organic light-emitting diodes, there are the following key technologies that need to be broken through:

(1) When each thin film transistor on the display panel is in a conducting state, the difference in output current must be small enough.

(2) When each thin film transistor on the display panel is in the cut-off state, it must be ensured that all TFTs can be completely cut off without any output.

Contents of the invention

Therefore, the main purpose of the present invention is to solve the above-mentioned traditional defects. In order to avoid the existence of this defect, the method and device for uniform image of the active organic light emitting diode display of the present invention are connected to a positive power supply through the cathode of each organic light emitting diode , so as to increase the potential of the organic light-emitting diode, so that the Vsd (source-drain voltage difference) driving the TFT is reduced, while the Vsg (source-gate voltage difference) remains unchanged, so that each thin film transistor is in the on-state When , the difference in the magnitude of the output current is reduced.

In order to achieve the above-mentioned purpose, the present invention adopts a method and device for uniform image of an active organic light emitting diode display. The cathode of the organic light-emitting diode is connected to a positive power supply, so that the voltage provided by the positive power supply increases the potential of the organic light-emitting diode, and then the Vsd (source-drain voltage difference) of the driving unit is reduced when the driving unit is in operation, and Vsg ( The voltage difference between the source and the gate) remains unchanged, and when each drive unit has a different initial voltage (Threshold Voltage) due to characteristic variation, the difference in output current of each drive unit when it is turned on can be reduced.

Description of drawings

Fig. 1 is a schematic circuit diagram of the present invention.

Fig. 2 is a schematic diagram of the current-voltage of the driving unit.

Fig. 3 is a current-voltage schematic diagram (2) of the driving unit.

Detailed ways

Now to detailed content and technical description of the present invention, be described as follows with reference to accompanying drawing:

Please refer to FIG. 1 , which is a schematic circuit diagram of the present invention. As shown in the figure: the method and device for uniform image of an active organic light emitting diode display of the present invention, the display is composed of a plurality of pixel devices 10, each pixel device 10 has a driving unit 2 to drive organic light emitting diodes 4 to emit light Bright, the cathode of the organic light emitting diode 4 is connected to a positive power supply 5, thereby providing a voltage through the positive power supply 5 to increase the potential of the organic light emitting diode 4, and then make the Vsd (source, drain) of the driving unit 2 operate. Voltage difference) decreases, while Vsg (source, gate voltage difference) remains unchanged, then when each drive unit 2 has a different initial voltage (Threshold Voltage) due to characteristic variation, each drive unit 2 can be The difference in output current narrows during turn-on.

In order to achieve the above-mentioned method, the pixel (pixel) device 10 adopted in the present invention includes: a switch unit 1, a drive unit 2, a storage unit 3 and an organic light emitting diode 4; wherein,

The above-mentioned switch unit 1 is a thin film transistor (TFT), and the two input terminals 11 and 12 of the switch unit 1 are respectively connected to a scan line 60 (Scan Line) and a data line 61 (Data Line);

The drive unit 2 is a thin film transistor (TFT), the input terminal 21 of the drive unit 2 is connected to a power line 62 (Supply Line), and the other input terminal 22 is connected to the output terminal 13 of the switch unit 1;

The storage unit 3 is composed of a capacitor, one end is connected to a power line 62 (Supply Line), and the other end is connected to the output terminal 13 of the switch unit 1;

The anode of the organic light emitting diode 4 is connected to the output terminal 23 of the driving unit 2, and its cathode is connected to a positive power supply 5;

The voltage provided by the positive power supply 5 increases the cathode and anode potentials of the organic light emitting diode 4, and jointly increases the potential of the output terminal 23 of the drive unit 2, so that the Vsd (source, drain) of the drive unit 2 is activated. Pole voltage difference) decreases, while Vsg (source, gate voltage difference) remains unchanged, then when each drive unit 2 has a different initial voltage (Threshold Voltage; Vth) due to characteristic changes, each The difference of the output current of the driving unit 2 is reduced when it is turned on.

In addition, in order to fully understand the effects achieved in this case, please refer to Figure 2, which is a schematic diagram of the current-voltage of the drive unit. As shown in the figure: when the input voltage Vdd of the power line 62 is 13V and the input voltage signal of the data line 61 is 0V, under the known pixel device structure, the driving unit 2 has a known load curve 71, so the known driving unit The Vsd operating point (Operating Point) is located in the known load curve 71 and the characteristic curve 72 of the drive unit 2 (when the input voltage (Vdd) of the power line is 13V and the input voltage signal (Vdata) of the data line is 0V ) at the point of intersection.

Assuming that the threshold voltage (Threshold Voltage; Vth) of the TFT element has a change of ±1.5V due to the limitation of the manufacturing process, it can be seen from the figure that when the Vth changes by -1.5V, the output current of the existing drive unit will be 23.3 %The change.

Conversely, when using the pixel device 10 of the present invention, assuming that the cathode potential of the organic light emitting diode (OLED) has a size of 5V due to the voltage provided by the positive power supply, the driving unit 2 has a load curve 73 of the present invention, so the Vsd of the driving unit 2 The operating point (OperatingPoint) is located on the intersection point of the load curve 73 of the present invention and the characteristic curve 172 of the drive unit 2, then when the starting voltage (Vth) of the TFT element also has a change of -1.5V due to the limitation of the manufacturing process , it is confirmed by experiments that the output current of the driving unit 2 only changes by 13.6%.

Please refer to FIG. 3 again, which is the current-voltage schematic diagram (2) of the driving unit. As shown in the figure: when the characteristic curve 3 83 (this characteristic curve is when the input voltage (Vdd) of the power line 62 is 8V, and the input voltage signal (Vdata) of the data line 61 is 0V), under the structure of the traditional pixel device , the Vsg of the traditional drive unit is reduced to 8V, and the Vsd operating point (Operating Point) of the traditional drive unit is located at the intersection of the load curve 2 81 and the characteristic curve 3 83 of the drive unit 2 (Vsg=8V). Then, when the initial voltage Vth (Threshold Voltage) of the TFT element varies by ±1.5V due to the limitation of the manufacturing process, the output current of the traditional driving unit will vary by 39.6%.

However, when using the driving device of the present invention, because the common cathode potential of the organic light emitting diode (OLED) panel is equal to 5V, the input voltage (Vdd) is 13V, the input voltage signal (Vdata) of the data line 61 is 0V, and the drive unit 2 The Vsd operating point (Operating Point) is located at the intersection of the load curve 2 81 and the traditional drive unit characteristic curve 2 82 (Vsg=13V). Then, when the initial voltage Vth (Threshold Voltage) of the TFT element varies by ±1.5V due to the limitation of the manufacturing process, the output current of the driving unit 2 only varies by 13.6%.

Therefore, when the power supply voltage (Vdd) of the traditional pixel device becomes smaller, although the Vsd of the traditional driving unit can be reduced, Vsg is also reduced at the same time, and the output current of each traditional driving unit is greatly affected by the characteristic change. However, it is impossible to reduce the Vsd of the driving unit 2 while maintaining its Vsg at a certain value as in the present invention, so that when the TFT is turned on (ON), the output current is not affected by the characteristic change of the TFT.

The foregoing are only preferred embodiments of the present invention, and are not intended to limit the implementation scope of the present invention. That is, all equivalent changes and modifications made according to the patent scope of the present invention are covered by the patent scope of the present invention.

Claims (5)

1. An apparatus for homogenizing an image of an active matrix organic light emitting diode display, said display comprising a plurality of pixel devices (10), wherein each pixel device (10) comprises:

a switch unit (1) having two input terminals (11), (12) and an output terminal (13), wherein the two input terminals (11), (12) are respectively connected to the scan line (60) and the data line (61);

a storage unit (3) having one end connected to a power line (62) and the other end connected to the output terminal (13) of the switch unit (1);

a driving unit (2) having two input terminals (21), (22) and an output terminal (23), wherein one input terminal (21) is connected to a power line (62), and the other input terminal (22) is connected to the output terminal (13) of the switching unit (1);

an organic light emitting diode (4) having an anode connected to the output terminal (23) of the driving unit (2) and a cathode connected to a positive power supply (5);

the positive power supply (5) provides voltage to increase the potentials of the cathode and the anode of the organic light emitting diode (4) and increase the potential of the output end (23) of the driving unit (2), so that the voltage difference between the source and the drain of the driving unit (2) is reduced and the voltage difference between the source and the gate is kept unchanged during action.

2. The device for image uniformity of an active organic light emitting diode display according to claim 1, wherein said switch unit (1) is a thin film transistor.

3. The device for image uniformity of an active organic light emitting diode display according to claim 1, wherein said driving unit (2) is a thin film transistor.

4. The device for image uniformity of an active organic light emitting diode display according to claim 1, wherein said storage unit (3) is formed by a capacitor.

5. A method for making image of active organic LED display uniform, the said display is mainly made up of multiple pixel devices, characterized by that, every said pixel device (10) has a driving unit (2) in order to drive the said organic LED (4) to shine, the negative pole of the said organic LED (4) is connected to a positive power supply (5), provide the voltage and improve the electric potential of the organic LED (4) through the said positive power supply (5), and then make the said driving unit (2) reduce its source, drain-source voltage difference when acting, and the source, grid voltage difference keeps unchanged, when the said each said driving unit (2) has different initial voltages because of characteristic variation, can make the difference of its output current of each said driving unit (2) reduce when turning on.

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