KR20100069427A - Organic light emitting diode display - Google Patents

Abstract

PURPOSE: A device for displaying an organic luminance DIODE recompenses the characteristic deviation of the drive TFT going pixels. The display quality is improved. CONSTITUTION: A pixel(16) is connected between the emitting device and ground voltage source. A driver component controlling the current flowing in the emitting device according to the voltage difference between the source and the gate is included. The data driving circuit(12) supplies data voltage, and the reference current and high potential driving voltage to pixel. The switching circuit makes low the gate voltage of the driver component for the programming term as deviation between the second data voltage and the first data voltage.

Description

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 diode display capable of improving display quality.

Recently, various flat panel displays (FPDs) that can reduce weight and volume, which are disadvantages of cathode ray tubes, have been developed. Such flat panel displays include liquid crystal displays (hereinafter referred to as "LCDs"), field emission displays (FEDs), plasma display panels (hereinafter referred to as "PDPs") and electric fields. Light emitting devices; and the like.

PDP is attracting attention as the most advantageous display device for light and small size and large screen because of its simple structure and manufacturing process. TFT LCDs with thin film transistors (hereinafter referred to as "TFTs") as switching devices are the most widely used flat panel display devices. However, TFT LCDs have a narrow viewing angle and low response speed because they are non-light emitting devices. In contrast, electroluminescent devices are classified into inorganic light emitting diode display devices and organic light emitting diode display devices depending on the material of the light emitting layer. In particular, organic light emitting diode display devices use self-light emitting devices that emit light, and thus, the response speed is high and the light emitting efficiency, There is a great advantage in brightness and viewing angle.

The organic light emitting diode display device has an organic light emitting diode as shown in FIG. 1. The organic light emitting diode includes an organic compound layer (HIL, HTL, EML, ETL, EIL) formed between the anode electrode and the cathode electrode.

The organic compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (Electron Injection layer, EIL).

When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML to form excitons, and as a result, the emission layer EML becomes Visible light is generated.

The organic light emitting diode display device arranges the pixels including the organic light emitting diode in a matrix form and controls the brightness of the pixels selected by the scan pulse according to the gray level of the video data.

Such an organic light emitting diode display is divided into a passive matrix method and an active matrix method using a TFT as a switching element.

Among these, the active matrix method selectively turns on the active TFT, selects a pixel, and maintains light emission of the pixel with a voltage maintained in a storage capacitor.

2 is an equivalent circuit diagram of one pixel in an active matrix organic light emitting diode display.

Referring to FIG. 2, a pixel of an organic light emitting diode display of an active matrix type includes an organic light emitting diode OLED, a data line DL and a gate line GL, a switch TFT SW, and a driving TFT DR that cross each other. ), And a storage capacitor Cst. The switch TFT (SW) and the driving TFT (DR) are implemented with an N-type MOS-FET.

The switch TFT SW is turned on in response to a scan pulse from the gate line GL to conduct a current path between its source electrode and drain electrode. The switch TFT SW applies the data voltage from the data line DL to the gate electrode and the storage capacitor Cst of the driving TFT DR during the on time period.

The driving TFT DR controls the current flowing through the organic light emitting diode OLED according to the difference voltage Vgs between its gate electrode and the source electrode.

The storage capacitor Cst keeps the voltage supplied to the gate electrode of the driving TFT DR constant for one frame period by storing the data voltage applied to one electrode thereof.

The organic light emitting diode OLED is implemented in the structure shown in FIG. 1. The organic light emitting diode OLED is connected between the source electrode of the driving TFT DR and the low potential driving voltage source VSS.

The brightness of the pixel as shown in FIG. 2 is proportional to the current flowing in the organic light emitting diode OLED as shown in Equation 1 below, and the current is the difference voltage between the gate voltage and the source voltage of the driving TFT DR, and the driving TFT DR. Is determined by the threshold voltage.

Here, Ioled is a driving current, k is a constant value determined by mobility and parasitic capacitance of the driving TFT DR, and Vgs is between the gate voltage Vg and the source voltage Vs of the driving TFT DR. The difference voltage and Vth mean threshold voltages of the driving TFT DR, respectively.

As shown in Equation 1, the current Ioled of the organic light emitting diode OLED is greatly influenced by the threshold voltage Vth of the driving TFT DR and the mobility included in 'k'.

In general, non-uniformity of luminance between pixels in the organic light emitting diode display device is caused by variation of electrical characteristics of the driving TFT including the threshold voltage. The cause of variation in the electrical characteristics of the driving TFTs between the pixels varies depending on the backplane of the display panel. In a panel using a low temperature poly silicon (LTPS) backplane, TFT characteristic variation occurs between pixels due to an Excimer Laser Annealing (ELA) process. On the other hand, in the panel using the a-Si (Amorphous Silicon) backplane, the characteristic variation due to the process is hardly generated, but the deterioration degree of the TFT progressed according to the panel driving varies from pixel to pixel, resulting in TFT characteristic variation between pixels. do. The reason why the degree of deterioration of the TFTs differs according to the panel driving is that the threshold voltage fluctuations of the driving TFTs vary from pixel to pixel due to gate-bias stress accumulated on the gate electrodes of the driving TFTs. This is because the degree of deterioration also varies from pixel to pixel depending on the driving time.

Due to the variation of the electrical characteristics of the driving TFT, the current flowing through the organic light emitting diode is changed for each pixel. In order to reduce the driving current variation for each pixel and to improve the display quality, the threshold voltage difference of each driving TFT and the mobility of each driving TFT are increased. It is necessary to make up the difference entirely.

Accordingly, an object of the present invention is to provide an organic light emitting diode display device which improves display quality by compensating for the characteristic variation of the driving TFT between pixels.

In order to achieve the above object, an organic light emitting diode display according to an embodiment of the present invention is a light emitting device that emits light by a high potential driving voltage, and is connected between the light emitting device and the base voltage source and between its gate-source A pixel having a driving element for controlling a current flowing through the light emitting element according to a voltage difference; A data driving circuit configured to supply a data voltage including a first data voltage and a second data voltage having a lower level than the first data voltage, a reference current, and the high potential driving voltage to the pixel; And the gate current of the driving device is set to a sensing voltage including a threshold voltage and a mobility deviation value of the driving device by flowing the reference current through the driving device during the sensing period based on gate signals. And a switch circuit for lowering the gate potential of the driving element by the deviation between the first and second data voltages during the programming period following the sensing period. The supply of the high potential driving voltage is cut off until the programming period, and the supply is allowed in the light emitting period following the programming period.

The data driving circuit may include: a data drive generating the first and second data voltages and applying them to a data voltage supply line; A constant current source generating the reference current and applying it to a reference current supply line; A high potential driving voltage source generating the high potential driving voltage and applying the same to a driving voltage supply line; A first switch for switching the supply of the high potential driving voltage; And a second switch for switching the supply of the reference current.

The gate signals include a sensing signal and a programming signal; The sensing signal is maintained at a turn on level during the sensing period, and is maintained at a turn off level during the programming period and the light emitting period; The programming signal is maintained at a turn on level during the sensing period and the programming period, and is maintained at a turn off level during the light emitting period.

The switch circuit may include: a first switch element for diode-connecting the driving element in response to the sensing signal; A second switch element for switching a current path between a first node connected to a gate electrode of the driving element and a second node connected to the reference current supply line in response to the programming signal; A storage capacitor connected between the first node and a source electrode of the driving device; And a booster capacitor connected between the data voltage supply line and the second node.

The first switch is turned off during the sensing period and the programming period, and turned on during the light emitting period; The second switch is turned on during the sensing period and turned off during the programming period and the light emitting period; The data drive may apply the first data voltage to the data voltage supply line during the sensing period, and the second data voltage to the data voltage supply line during the programming period.

The current (Ioled) flowing through the organic light emitting diode during the light emitting period is characterized by the following formula.

Here, k is a constant value determined by the mobility and parasitic capacitance of the driving device that is not compensated for the deviation value, Vgs is the voltage difference between the gain-source of the driving device, Vth is the driving device Threshold voltage, Vsen is the sensing voltage, ΔVdata is the voltage difference between the first data voltage and the second data voltage, and k 'is the mobility and parasitic capacitance of the driving element compensated for the deviation value. In the constant value determined by the Vsen ′, the threshold voltage value of the driving element is excluded from the sensing voltage, respectively.

In the organic light emitting diode display according to the present invention, the display quality can be greatly improved by appropriately compensating the characteristic deviation of the driving TFT between the pixels through a hybrid method using a reference current and a data voltage.

Hereinafter, exemplary embodiments of the present invention will be described with reference to FIGS. 3 to 6C.

3 is a block diagram illustrating an organic light emitting diode display according to an exemplary embodiment of the present invention.

Referring to FIG. 3, an organic light emitting diode display according to an exemplary embodiment of the present invention includes a display panel 10 in which pixels 16 are arranged in a matrix form, and a data driving circuit 12 and a gate driving circuit 13. It includes a timing controller 11 for controlling.

In the display panel 10, a plurality of data lines 14a and 14b, a plurality of driving voltage supply lines 14c, and a plurality of gate lines 15a and 15b intersect each other, and the pixel 16 is disposed at an intersection thereof. Are arranged in matrix form. Each of the pixels 16 receives a high potential driving voltage Vdd from the driving voltage supply line 14c and a base voltage Gnd, and includes two data lines 14a and 14b and two gates. Connected to lines 15a and 15b. The two data lines 14a and 14b respectively include a data voltage supply line 14a and a reference current supply line 14b, and the two gate lines 15a and 15b respectively sense a sensing signal supply line 15a. ) And a programming signal supply line 15b. The high potential drive voltage Vdd is generated by the high potential drive voltage source, and the base voltage Gnd is generated by the base voltage source. The pixels 16 will be described later in detail with reference to FIG. 4.

The timing controller 11 rearranges the digital video data RGB input from the outside to the data driving circuit 12 in accordance with the resolution of the display panel 10. In addition, the timing controller 11 may determine the timing of the data driving circuit 12 based on timing signals such as the vertical synchronization signal Vsync, the horizontal synchronization signal Hsync, the dot clock signal DCLK, and the data enable signal DE. The data control signal DDC for controlling the operation timing, the gate control signal GDC for controlling the operation timing of the gate driving circuit 13, and the high potential driving voltage Vdd in the data driving circuit 12; The first switch control signal CS1 for controlling the supply time of the second signal and the second switch control signal CS2 for controlling the supply time of the reference current Iref are generated in the data driving circuit 12.

The data driving circuit 12 includes a data drive 12a connected to the data voltage supply lines 14a, a constant current source IREF connected to the reference current supply lines 14b, and a driving voltage supply line as shown in FIG. High potential drive voltage source VDD connected to 14c), first switch S1 connected between high potential drive voltage source VDD and drive voltage supply line 14c, and constant current source IREF and reference current supply line. And a second switch S2 connected between the 14b.

The data drive 12a generates a data voltage Vdata. In detail, the data drive 12a converts the digital video data RGB into an analog data voltage (hereinafter referred to as a second data voltage) based on the data control signal DDC from the timing controller 11. To the supply lines 14a. In addition, the data drive 12a may transmit a first data voltage having a level higher than the second data voltage, that is, a high potential driving voltage Vdd, to the data voltage supply lines 14a prior to the application of the second data voltage. Is authorized.

The high potential drive voltage source VDD applies a high potential drive voltage Vdd to the drive voltage supply line 14c.

The constant current source IREF is connected to the high potential operating power supply VCC to apply the reference current Iref to the reference current supply lines 14b.

The first switch S1 switches the current path between the high potential drive voltage source VDD and the drive voltage supply line 14c in response to the first switch control signal CS1 from the timing controller 11. The first switch S1 is turned off during the sensing period and the programming period, and turned on during the light emitting period.

The second switch S2 switches the current path between the constant current source IREF and the reference current supply line 14b in response to the second switch control signal CS2 from the timing controller 11. The second switch S1 is turned on during the sensing period and turned off during the programming period and the light emitting period.

The gate driving circuit 13 generates the sensing signal SEN and the programming signal PGM based on the gate control signal GDC from the timing controller 11. The gate driving circuit 13 applies the sensing signal SEN to the sensing signal supply line 15a and the programming signal PGM to the programming signal supply line 15b, respectively.

FIG. 4 shows the data driving circuit 12 and the pixel 16 shown in FIG. 3.

Referring to FIG. 4, a pixel 16 according to an exemplary embodiment of the present invention includes an organic light emitting diode OLED, a driving TFT DR, and a switch circuit 161.

The anode electrode of the organic light emitting diode OLED is connected to the driving voltage supply line 14c, and the cathode electrode is commonly connected to the drain electrode of the driving TFT DR and the switch circuit 161. The organic light emitting diode OLED has the structure as shown in FIG. 1, and the amount of light emitted is controlled by the driving current controlled by the driving TFT DR to realize gray scale.

The gate electrode of the driving TFT DR is connected to the switch circuit 161 through the first node n1, and the drain electrode of the driving TFT DR is the cathode electrode and the switch circuit 161 of the organic light emitting diode OLED. The common electrode of the driving TFT DR is commonly connected to the ground voltage source GND and the switch circuit 161. The driving TFT DR controls the amount of current flowing through the organic light emitting diode OLED according to the difference voltage between its gate electrode and the source electrode. The driving TFT DR may be implemented as an N-type metal oxide semiconductor field effect transistor (MOSFET). The semiconductor layer of the driving TFT DR may be formed of amorphous silicon. It may include.

The switch circuit 161 includes a first switch TFT SW1, a second switch TFT SW2, a storage capacitor Cst, and a booster capacitor Cbst. The switch circuit 161 receives the reference current Iref supplied from the data driving circuit 12 during the sensing period in response to the sensing signal SEN and the programming signal PGM from the gate lines 15a and 15b. Set the gate electrode potential of the driving TFT DR to the sensing voltage including the threshold voltage and the mobility deviation value of the driving TFT DR through the driving TFT DR, and then during the programming period following this sensing period. The driving TFT (DR) gate electrode potential is lowered by a deviation between the first data voltage previously supplied from the data driving circuit 12 and the second data voltage currently supplied.

To this end, the gate electrode of the first switch TFT SW1 is connected to the sensing signal supply line 15a, and the drain electrode of the first switch TFT SW1 is connected to the reference current supply line 14b through the second node n2. ) And the drain electrode of the second switch TFT SW2 and the other electrode of the boost capacitor Cbst. The source electrode of the first switch TFT SW1 is connected to the cathode electrode of the organic light emitting diode OLED and the driving TFT. It is commonly connected to the drain electrode of DR). The first switch TFT SW1 is turned on in response to the sensing signal SEN to diode-connect the driving TFT DR. The sensing signal SEN maintains the high logic level during the sensing period, while maintaining the low logic level during the programming period and the light emission period.

The gate electrode of the second switch TFT SW2 is connected to the programming signal supply line 15b, and the drain electrode of the second switch TFT SW2 is connected to the reference current supply line 14b and the second current through the second node n2. It is commonly connected to the drain electrode of the first switch TFT SW1 and the other electrode of the boost capacitor Cbst, and the source electrode of the second switch TFT SW2 is the gate electrode of the driving TFT DR through the first node n1. And a common connection to one electrode of the storage capacitor. The second switch TFT SW2 switches the current path between the first node n1 and the second node n2 in response to the programming signal PGM. The programming signal PGM maintains the high logic level during the sensing period and the programming period, while maintaining the low logic level during the light emitting period.

The storage capacitor Cst is a voltage applied between the first node n1 and the base voltage source GND by driving one electrode thereof to the first node n1 and the other electrode thereof to the base voltage source GND. The gate-source voltage difference of the TFT DR is kept constant for one frame.

The booster capacitor Cbst has one electrode thereof connected to the data voltage supply line 14a and the other electrode thereof connected to the second node n2 to change the voltage on the data voltage supply line 14a to the second node n2. Reflect on it.

Fig. 5 shows the operation timing of signals for driving the pixel, the operation timing of the switches, and the gate electrode potential of the driving TFT at each timing. 6A to 6C show a connection state of pixels in a sensing period, a programming period, and a light emission period, respectively.

5 and 6A, the sensing signal SEN is generated as a high logic voltage during the sensing period Ts to turn on the first switch TFT SW1, and the programming signal PGM is generated as a high logic voltage. To turn on the second switch TFT SW2. During this sensing period Ts, the first switch S1 is turned off to cut off the supply of the high potential driving voltage Vdd, and the second switch S2 is turned on to permit the supply of the reference current Iref. . During the sensing period Ts, the data drive applies the first data voltage Vd1 at the high potential driving voltage Vdd level to the data voltage supply line.

Accordingly, the driving TFT DR is diode-connected, and the reference current Iref flows to the base voltage source GND through the driving TFT DR to which the diode is connected, and thus the potential Vn1 of the first node n1. Is set to the sensing voltage (Vsen). The sensing voltage Vsen includes a threshold voltage Vth value and a mobility deviation value of the driving TFT DR. Here, the reference current Iref is preferably set to a large value in order to reduce the time for sensing.

5 and 6B, during the programming period Tp, the sensing signal SEN is inverted to a low logic voltage to turn off the first switch TFT SW1, and the programming signal PGM is maintained at a high logic voltage. The second switch TFT SW2 is continuously turned on. During this programming period Tp, the first switch S1 remains turned off to continuously cut off the supply of the high potential drive voltage Vdd, and the second switch S2 is inverted to the turn off state to thereby the reference current. Shut off the supply of (Iref). During the programming period Tp, the data drive applies a second data voltage Vd2 at a level lower than the first data voltage Vd1 to the data voltage supply line.

Accordingly, the potential on the data voltage supply line is lowered by the data voltage deviation ΔVdata, and according to the coupling effect of the booster capacitor Cbst, the potential Vn1 of the first node n1 is also reduced by the data voltage deviation ( Boost-Down by ΔVdata). The sensing voltage Vsen boosted down by the data voltage deviation ΔVdata is stored in the storage capacitor Cst including the threshold voltage and the mobility value of the driving TFT DR. Here, since the data voltage deviation ΔVdata is a difference value between the first data voltage Vd1 and the second data voltage Vd2, its magnitude is determined according to the level of the second data voltage Vd2.

5 and 6C, the sensing signal SEN is maintained at a low logic voltage during the light emission period Te to continuously turn off the first switch TFT SW1, and the programming signal PGM is low logic. The voltage is inverted to turn off the second switch TFT SW2. During this light emission period Te, the first switch S1 is inverted to a turn-on state to permit the supply of the high potential driving voltage Vdd, and the second switch S2 is kept in the turn-off state so that the reference current Iref ) Continue to shut off the supply.

Accordingly, the current Ioled flowing in the organic light emitting diode OLED is represented by Equation 2 below.

Here, k 'is a constant value determined by the mobility and parasitic capacitance of the driving TFT DR whose deviation is compensated for, and Vsen' is a threshold voltage Vth value of the driving TFT DR from the sensing voltage Vsen. This means the remaining voltage values after excluding them.

As can be seen from Equation 2, since Equation 2 does not include the threshold voltage Vth factor of the driving TFT DR, the driving current Ioled flowing through the organic light emitting diode OLED is the driving TFT DR. The threshold voltage Vth is not affected by the change. In addition, since the mobility deviation value of the driving TFT DR is compensated for in Equation 2, the driving current Ioled flowing through the organic light emitting diode OLED is free from the influence of the mobility deviation value of the driving TFT DR. Lowered. Accordingly, the luminance unevenness caused by the difference in the threshold voltage Vth and the mobility in the driving TFT DR between the pixels is minimized.

As described above, the organic light emitting diode display according to the present invention can greatly improve the display quality by appropriately compensating the characteristic variation of the driving TFT between the pixels through a hybrid method using a reference current and a data voltage.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. For example, in the exemplary embodiment of the present invention, only the case where the TFT is implemented as the N type MOSFET is described. In addition, in the exemplary embodiment of the present invention, only the case in which the display panel is implemented as the a-Si backplane has been described. However, the technical concept of the present invention is not limited thereto and may be applied to the LTPS backplane. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

1 is a diagram illustrating a light emission principle of a general organic light emitting diode display.

Fig. 2 is a circuit diagram equivalently showing one pixel in a conventional organic light emitting diode display having a 2T1C structure.

3 is a block diagram illustrating an organic light emitting diode display device according to an exemplary embodiment of the present invention.

4 is a circuit diagram illustrating a data driving circuit and a pixel illustrated in FIG. 3.

Fig. 5 shows operation timings of signals for driving pixels, operation timings of switches, and gate electrode potentials of the driving TFTs at each timing.

6A is an equivalent circuit diagram of a pixel during the sensing period of FIG. 5.

6B is an equivalent circuit diagram of a pixel during the programming period of FIG.

6C is an equivalent circuit diagram of a pixel during the light emission period of FIG.

<Description of Symbols for Main Parts of Drawings>

10: display panel 11: timing controller

12: data driving circuit 13: gate driving circuit

16: pixel 161: switch circuit

Claims (6)

A pixel having a light emitting element emitting light by an applied high potential driving voltage, and a driving element connected between the light emitting element and a ground voltage source and controlling a current flowing through the light emitting element according to a gate-source voltage difference thereof; A data driving circuit configured to supply a data voltage including a first data voltage and a second data voltage having a lower level than the first data voltage, a reference current, and the high potential driving voltage to the pixel; And The gate current of the driving device is set to a sensing voltage including the threshold voltage and the mobility deviation value of the driving device by flowing the reference current through the driving device during the sensing period based on gate signals. And a switch circuit for lowering the gate potential of the driving device by the deviation between the first and second data voltages during the programming period following the sensing period. And the supply of the high potential driving voltage is cut off until the programming period, and the supply of the high potential driving voltage is allowed in the light emitting period subsequent to the programming period. The method of claim 1, The data driving circuit, A data drive generating the first and second data voltages and applying them to a data voltage supply line; A constant current source generating the reference current and applying it to a reference current supply line; A high potential driving voltage source generating the high potential driving voltage and applying the same to a driving voltage supply line; A first switch for switching the supply of the high potential driving voltage; And And a second switch for switching the supply of the reference current. The method of claim 1, The gate signals include a sensing signal and a programming signal; The sensing signal is maintained at a turn on level during the sensing period, and is maintained at a turn off level during the programming period and the light emitting period; And the programming signal is maintained at a turn on level during the sensing period and the programming period, and is maintained at a turn off level during the light emitting period. The method according to claim 2 or 3, The switch circuit, A first switch element for diode-connecting the driving element in response to the sensing signal; A second switch element for switching a current path between a first node connected to a gate electrode of the driving element and a second node connected to the reference current supply line in response to the programming signal; A storage capacitor connected between the first node and a source electrode of the driving device; And And a booster capacitor connected between the data voltage supply line and the second node. The method of claim 2, The first switch is turned off during the sensing period and the programming period, and turned on during the light emitting period; The second switch is turned on during the sensing period and turned off during the programming period and the light emitting period; And the data drive applies the first data voltage to the data voltage supply line during the sensing period and the second data voltage to the data voltage supply line during the programming period. . The method of claim 1, An organic light emitting diode display device wherein the current (Ioled) flowing through the organic light emitting diode during the light emitting period is as shown in the following equation.

Here, k is a constant value determined by the mobility and parasitic capacitance of the driving device that is not compensated for the deviation value, Vgs is the voltage difference between the gain-source of the driving device, Vth is the driving device Threshold voltage, Vsen is the sensing voltage, ΔVdata is the voltage difference between the first data voltage and the second data voltage, and k 'is the mobility and parasitic capacitance of the driving element compensated for the deviation value. In the constant value determined by the Vsen ′, the threshold voltage value of the driving element is excluded from the sensing voltage, respectively.

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