KR20040025344A - Pixel structure for active matrix organic light emitting diode display - Google Patents

Abstract

PURPOSE: A pixel structure of an active matrix O-LED(Organic Light Emitting Device) is provided to have a uniform light emission characteristics by compensating nonuniform electrical characteristics of a TFT device between pixels. CONSTITUTION: The first and the second switching device select a driving pixel by a scan signal applied from the external, and receive a data current. A capacitor(65) stores charges by a control current applied by the first and the second switching device. The third switching device is selected by the first and the second switching device and writes a data current, and an external power is applied to the third switching device. And a driving device is constituted with a mirror structure with the third switching device, and receives a voltage by the charges stored in the capacitor and applies a current to a corresponding pixel.

Description

PIXEL STRUCTURE FOR ACTIVE MATRIX ORGANIC LIGHT EMITTING DIODE DISPLAY}

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an active matrix organic light emitting diode (O-LED) display pixel structure, and in particular, a current write for self-correcting the nonuniform electrical characteristics of a thin film transistor. The present invention relates to a pixel structure.

Active Matrix LCD (Low Liquid Crystal Display) using Low Temperature Polycrystalline Silicon-Thin Film Transistor (LTPS-TFT) It is expected to be adopted in high resolution LCDs because of its excellent driving ability and density compared to (a-Si TFT: Amorphous Silicon Thin Film Transistor).

1 is a diagram showing a basic pixel structure of a TFT-LCD.

As shown in Fig. 1, the pixel structure of an active matrix LCD using a low temperature polycrystalline silicon thin film transistor is similar to an active driving LCD using an amorphous silicon thin film transistor, and one TFT 11, one liquid crystal cell capacitor 12, and one Two storage capacitors (13) . Since TFTs in active-drive LCDs act as simple switches, and the light transmittance of liquid crystals is determined by the voltage applied to the capacitors through the TFTs, when a sufficient signal voltage is applied to the gates of the TFTs to operate in a linear region, The difference in threshold voltages does not significantly affect image quality.

On the other hand, in the active matrix O-LED, which has been spotlighted as a next-generation display, the uniformity of TFTs, especially the threshold voltage (V _th ) and the field effect mobility (mobility) are controlled because the emission luminance is controlled by controlling the amount of current flowing through the organic thin film device. Ensuring uniformity is important. This is because a uniform pixel current can flow when the threshold voltage of the TFT is corrected. However, in the case of low temperature polycrystalline silicon thin film transistors fabricated at a low temperature of about 450 ° or less, it is difficult to fabricate a uniform threshold voltage and field effect mobility characteristics during the process. Therefore, a circuit approach, that is, a method for solving the non-uniformity problem of the TFT by configuring a compensation circuit in each pixel of the active matrix O-LED panel has been sought.

The basic pixel structure of an active matrix O-LED is divided in two ways. One is a voltage writing method for inputting image data as a voltage, and the other is a current writing method for inputting image data as a current.

2 is a view showing the most basic pixel structure of a conventional voltage writing method, and is composed of two TFTs 21 and 22 and one capacitor 23. In the figure, the T1 TFT 21 acts as a switch like an active matrix LCD, the C _STORAGE 23 stores a data voltage, and the T2 TFT 22 has a current corresponding to the data voltage value stored in the C _STORAGE 23. It serves to flow to the O-LED (24).

However, in the basic pixel structure of the voltage writing method shown in FIG. Flows through the O-LED. Therefore, the structure of FIG. 2 does not correct any variation in TFT threshold voltage between pixels. The current flowing between the source and drain of T2 appears as follows.

I _D = 1/2 xkx (V _GS -V _TH ) ²

= 1/2 xkx (Vdata-V _DD -V _{th_T2} ) ²

(Current-voltage relationship in the saturation region, k = μx Cox x W / L, where μ is the field effect mobility, Cox is the capacitance of the insulating layer, W is the channel width of the TFT, and L is the channel length of the TFT. Each shown)

3 illustrates currents flowing through the O-LEDs by the same data voltage when the threshold voltages of the TFTs in the basic pixel structure of FIG. 2 are -3.8, -4.3, and -4.8 V, respectively. In the active matrix O-LED, since the interval between grays required for high-quality pixel representation is several tens of nA, the current deviation shown in FIG. 3 is considerably large.

FIG. 4 is another conventional method for solving the problem of the basic pixel structure of the voltage writing method of FIG. After charging the voltage in a capacitor in advance, a method of inputting image data voltage using capacitor couplingRMADawson et al. (Sarnoff Corporation, USA)By Proposed.

However, in the conventional method, TFT threshold voltage correction between pixels is possible, but for this purpose, two signal lines AZ and AZB are additionally required, and two capacitors are required to reduce the light emitting area of the pixel. The light emitting area increases as the number of capacitors decreases. In addition, a separate signal line and an external integrated circuit are required to control two complicated signals, thereby reducing the light emitting area.

5 is a diagram showing a basic pixel structure of a conventional current writing method, and the basic pixel structure of the current writing method is composed of four TFTs 51 to 54 and one Cstg capacitor 55. In this method, a selection signal SEL is applied to apply image data to each pixel, and when a data current flows through the driving TFT P2 52, the gate-source voltage of P2 corresponding to the data current is converted into a Cstg capacitor ( 55 is a driving method for continuously flowing the same current as the data current to the O-LED 56 even after the pixel selection is finished, and thus is not significantly affected by the threshold voltage of the TFT.

In the case of the current write method, various pixel structures have been proposed in addition to the above-described structure. However, there is a problem in that a current source that is difficult to implement compared to a voltage source must be used to apply image data. Most current write-type pixel structures pass the same amount of current as the data current through the O-LED, and thus, there is a difficulty in implementing a current source capable of controlling data currents within several tens of nA for high-quality grayscale expression.

Accordingly, an object of the present invention devised to solve the problems of the prior art as described above is an active matrix O having uniform light emission characteristics by correcting uneven electrical characteristics of TFT elements between pixels using a small number of TFTs and capacitors. To provide a pixel structure of the LED.

Another object of the present invention is to provide a pixel structure of an active matrix O-LED which minimizes the number of signals necessary for threshold voltage correction in addition to the selection signal and data signal necessary for the pixel.

To achieve the above object, a pixel structure of an active matrix O-LED of the present invention includes: first and second switching elements for selecting a driving pixel by a scan signal applied from the outside, and applying a data current; A capacitor for storing a predetermined charge by a control current applied by the first and second switching elements; A third switching element selected by the first and second switching elements, writing a data current, and receiving an external power source; And a driving device configured to receive the voltage by the charge stored in the capacitor and apply a current to the corresponding pixel.

Preferably, the magnitude of the current applied to the corresponding pixel is adjusted according to the channel width W / channel length L of the third switching element.

More preferably, the first and second switching elements are characterized in that the on / off at the same time.

1 is a view showing a basic pixel structure of an active matrix liquid crystal display (TFT-LCD) using a polycrystalline silicon thin film transistor;

2A is a view showing a basic pixel structure of a conventional active matrix O-LED;

2B is an operation timing diagram of FIG. 2A;

3 is a diagram illustrating a current deviation according to a threshold voltage change of a TFT in a basic pixel structure of a conventional active matrix O-LED of FIG. 2A;

4A is a diagram illustrating a pixel structure for voltage write-type threshold voltage correction according to another conventional method;

4B is an operation timing diagram of FIG. 4A;

5 is a view showing a basic pixel structure of a conventional current writing method;

6 is a diagram illustrating a pixel structure of a current write type active matrix O-LED according to a first embodiment of the present invention;

7 is an operation timing diagram of FIG. 6;

8 is a diagram illustrating a current deviation according to a threshold voltage change of a TFT in the pixel structure according to the first embodiment of the present invention;

9 is a view showing a voltage change of node A and node B according to a threshold voltage change of a TFT in the pixel structure according to the first embodiment of the present invention;

10 is a diagram illustrating I _O-LED current according to the size (W / L) of the data current I _DATA and the T3 TFT in the pixel structure according to the first embodiment of the present invention;

11A illustrates a pixel structure of a current write type active matrix O-LED according to a second embodiment of the present invention;

11B is an operation timing diagram of FIG. 11A;

12A is a diagram showing a pixel structure of a current write type active matrix O-LED according to a third embodiment of the present invention;

12B is an operation timing diagram of FIG. 12A;

FIG. 13 is a diagram showing a gate voltage of a T3 TFT affected by a feedthrough;

14 is a view showing the amount of current flowing in the O-LED,

15A is a diagram showing a pixel structure of a current write type active matrix O-LED according to a fourth embodiment of the present invention;

15B is an operation timing diagram of FIG. 15A;

16A is a diagram showing a pixel structure of a current write type active matrix O-LED according to a fifth embodiment of the present invention;

16B is a timing diagram of FIG. 16A;

17A is a diagram showing a pixel structure of a current write type active matrix O-LED according to a sixth embodiment of the present invention;

17B is an operation timing diagram of FIG. 17A.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; Note that the same components in the drawings are represented by the same reference numerals and symbols as much as possible even though they are shown in different drawings. In addition, in describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

First, FIG. 6 is a diagram illustrating a pixel structure of a current write type active matrix O-LED according to a first embodiment of the present invention.

As shown in Fig. 6, the pixel structure of the current write type active matrix O-LED of the present invention is composed of four P-type TFTs 61 to 64 and one C _STORAGE capacitor 65. Basically, there are no additional signal lines other than the scan signal and the data signal I _DATA which are essential signals to enter the pixel.

The operating principle of the present invention is to store a voltage capable of driving the data current in the C _STORAGE capacitor 65 between the gate (node B) and the source of the T3 TFT by flowing a current through the T3 TFT 63 during pixel selection. will be. After the pixel selection is finished, the T3 TFT 63 operates in the linear region, while the T4 TFT 64 operates in the saturated region, and the current corresponding to the voltage stored in the C _STORAGE capacitor 65 is supplied to the O-LED 66. Shed on. Adjacent polysilicon TFTs simultaneously crystallized with the same laser beam in a low temperature polycrystalline silicon (LTPS) process using an excimer laser are commonly used in current write methods using current-mirror structures because of their almost identical electrical characteristics. . In the present invention, as in the current-mirror structure, the threshold voltage and the field effect mobility of the adjacent T3 TFT 63 and the T4 TFT 64 utilize the same characteristics.

FIG. 7 is a timing diagram of FIG. 6, and thus detailed operations of the pixel structure illustrated in FIG. 6 will be described with reference to FIGS. 6 and 7.

6 and 7, when the first scan signal is high, the voltage of the node B, which is the gate node of the T3 TFT and the T4 TFT, corresponds to the data current of the previous frame. The current value is maintained to flow. At this time, after the scan signal becomes low and the T1 TFT 61 and the T2 TFT 62 are turned on, the data current value I _DATA is applied. When the data current value I _DATA is applied, the data current enters a programming state that flows from V _DD through the T3 TFT 63 to the path through the T1 TFT 61. At this time, the T3 TFT 63 operates in the saturation region with the gate and the drain shorted, and the voltage between the gate and the source of the T4 TFT 64 is shorted, so that the T4 TFT 64 is turned off and the O-LED 66 is turned off. There is no current flowing into. At this time, the voltage V _B of the gate node (node B) of the T3 TFT 63 is a value related only to the data current I _DATA as shown in Equation (1) below.

Formula (1)

At scan ON: I _DATA = k ₁ (V _B -V _DD -V _TH ) ² (k ₁ = μx Cox x W _T3 / L _T3 )

When I _DATA is applied: V _B -V _TH storage

At the end of the programming state, the scan signal is high and the T1 TFT 61 and T2 TFT 62 are turned off, the T3 TFT 63 is operated in the linear region, and the T4 TFT 64 is operated in the saturated region. Since the drain current of the T3 TFT 63 and the drain current of the T4 TFT 64 are the same, the relationship of the following formula (2) is established. Therefore, the O-LED current is 0A during the programming time, and the drain current of the T4 TFT operating in the saturation region flows to the O-LED during the remaining frame time except the programming time.

Formula (2)

Scan OFF: T3 TFTs operate in linear region, T4 TFTs operate in saturation region

I _O-LED = k ₁ [(V _B -V _DD -V _TH ) (V _DD -V _A )-0.5 (V _DD -V _A ) ² ] (linear current of T3 TFT)

= k ₂ (V _B -V _A -V _TH ) ² (k ₂ = μx Cox x W _T4 / L _T4 ) (Saturation Current of T4 TFT)

At this time, when the voltage V _A of the node A, which is the source portion of the T4 TFT, is solved using the equation (2), the data current is applied only regardless of the non-uniform electrical characteristics of the TFT as shown in the following equation (3). It depends only on the value of V _B -V _TH determined.

Formula (3)

I _O-LED = k ₂ (V _B -V _A -V _TH ) ²

_{= K 2 [V B - f} (V B - V TH) - V TH] 2 _{where, f (V B - V TH} ) is V _B - a function of the V _TH (function) indicates that.

FIG. 8 is a simulation result illustrating a current deviation according to a threshold voltage change in the pixel structure of FIG. 6. As shown in FIG. 3, when the same data current is written when the threshold voltages are -3.8, -4.3, and -4.8 V, respectively, the currents flowing through the O-LEDs are respectively shown.

8 shows almost the same current value in the case of -3.8, -4.3, and -4.8 V, unlike in FIG. 3, and the error is less than about 10 nA when the current flowing through the O-LED is 500 nA. At the current level, the error is as small as 30 nA.

FIG. 9 shows how the voltages of the nodes A and B of FIG. 6 change due to the change of the threshold voltage of the TFT. The voltage of the node A, which is the source node of the T4 TFT, is independent of the non-uniform electrical characteristics of the TFT as described above, and is a voltage determined only by the data current flowing during pixel selection. On the other hand, the voltage of the node B, which is the gate node of the T3 TFT and the T4 TFT, stores a voltage correcting the difference in the threshold voltage of the TFT. That is, since the voltage correcting the non-uniform electrical characteristics of the TFT by the data current flowing during the pixel selection is stored in the gate of the T3 TFT, it is irrelevant to the non-uniform electrical characteristics of the T4 TFT operating in the saturation region after the pixel selection is completed. One drain current flows through the O-LED. In addition, when the W / L (channel width / channel length) of the T3 TFT is increased, since the value of V _B -V _TH stored when writing the same data current becomes small, the current flowing through the O-LED becomes small. By adjusting the W / L value of the T3 TFT, scaling of the current flowing through the O-LED is possible.

10 is a diagram showing I _O-LED current according to the data current I _DATA and the size (W / L) of the T3 TFT, and flows to the O-LED for the programmed data current when the W / L of the T3 TFT is changed. The current can be known.

FIG. 11A is a diagram illustrating a pixel structure of a current write type active matrix O-LED according to a second exemplary embodiment of the present invention, and FIG. 11B is an operation timing diagram of FIG. 11A.

The pixel structure of the current write type active matrix O-LED shown in FIG. 11A is composed of four P-type TFTs 71 to 44 and one C _STORAGE capacitor 75 as in the first embodiment of FIG. . In the structure of FIG. 6, the configuration in which the data current I _DATA is directly applied to the sources of the T1 TFT and the T2 TFT by changing the position of the T2 TFT is different, but as illustrated in the timing diagram of FIG. The basic operation of allowing the saturation region current of the T4 TFT, in which the data current is written and correcting the non-uniform electrical characteristics of the TFT, flows to the O-LED 76 is the same as in the first embodiment, and a detailed description thereof will be omitted.

12A is a diagram showing a pixel structure of a current write type active matrix O-LED according to a third embodiment of the present invention, and FIG. 12B is an operation timing diagram of FIG. 12A.

The pixel structure of the current write type active matrix O-LED shown in FIG. 12A is composed of four P-type TFTs 81 to 84 and one C _STORAGE capacitor 85 as in the first embodiment of FIG. . The position of the T1 TFT is different from the structure of FIG. 6, but as shown in the timing diagram of FIG. 12B, the data current is written to the T3 TFT during programming, and the saturation region of the T4 TFT correcting the nonuniform electrical characteristics of the TFT. The basic operation of causing a current to flow in the O-LED 86 is the same as in FIG.

6 and 12, when the selection signal is applied, the same voltage is stored in the gates of the T3 TFTs 63 and 83. However, when the selection signal is turned off, the degree to which the gate voltage of the T3 TFT is affected by the feedthrough phenomenon is different. That is, in the case of FIG. 6, the feedthrough affecting the gate of the T3 TFT 63 is determined only by the T2 TFT 62, while in the case of FIG. 12, by both the T1 TFT 81 and the T2 TFT 82. Will be affected. Therefore, assuming that conditions such as data current and threshold voltage are the same, the voltages stored in the T3 TFT 63 of FIG. 6 and the T3 TFT 83 of FIG. 12 are different from each other. In the case of FIG. 6, the feedthroughs are eaten less, and in the case of FIG. 12A, the feedthroughs are eaten much, which is larger than in the case of FIG. Therefore, the amount of current flowing through the O-LED 66 in FIG. 6 becomes larger than the amount of current flowing through the O-LED 86 in FIG. 12A. This result can be confirmed through a diagram showing the gate voltage of the T3 TFT affected by the feedthrough shown in FIG. 13 and a diagram showing the amount of current flowing through the O-LED shown in FIG.

The pixel structure according to the present invention can be constructed in a similar manner as the N-type TFT. FIG. 15A is a diagram illustrating a pixel structure of a current write type active matrix O-LED according to a fourth exemplary embodiment of the present invention, and FIG. 15B is an operation timing diagram of FIG. 15A.

Referring to FIGS. 15A and 15B, the P-type structure is opposite to that of the above-described embodiment, and the data signal is also reversed. As shown in Fig. 15A, the pixel structure of the current write type active matrix O-LED of the present invention is composed of four N-type TFTs 91 to 94 and one C _STORAGE capacitor 95.

In the timing diagram of FIG. 15B, when the first scan signal is low, the voltages of the gate nodes of the T3 TFT and the T4 TFT are maintained so that an O-LED current value corresponding to the data current of the previous frame flows. At this time, while the scan signal becomes high, the data current value I _DATA is applied after the T1 TFT and the T2 TFT are turned on. At this time, the data current is supplied through a current source. On the other hand, in the case of the P-type pixel structure, the data current is supplied through a current sink. When the data current value I _DATA is applied, the data current enters a programming state in which a data current flows through the T1 TFT 91 and the T2 TFT 92 through the T3 TFT 93. At this time, the T3 TFT 63 operates in the saturation region with the gate and the drain shorted, and the voltage between the gate and the source of the T4 TFT 94 is also shorted, so that the T4 TFT 94 is turned off and the O-LED 66 is turned off. There is no current flowing into. Therefore, the voltage stored at the gate node of the T3 TFT 93 is similarly applied to the gate node of the T4 TFT 94 in the same manner as the above P-type pixel structure. At the end of the programming state, the scan signal is low, and the T1 TFT 91 and T2 TFT 92 are turned off, the T3 TFT 93 is operated in the linear region, and the T4 TFT 94 is operated in the saturated region. The drain current of the T3 TFT 93 and the drain current of the T4 TFT 94 are the same. Therefore, the O-LED current is 0A during the programming time, and the drain current of the T4 TFT 94 operating in the saturation region flows to the O-LED 96 during the remaining frame time except the programming time.

16A is a diagram illustrating a pixel structure of a current write type active matrix O-LED according to a fifth embodiment of the present invention, and FIG. 16B is an operation timing diagram of FIG. 16A.

The pixel structure of the current write type active matrix O-LED shown in FIG. 16A is composed of four N-type TFTs 101 to 104 and one C _STORAGE capacitor 105 as in the fourth embodiment of FIG. . In the structure of FIG. 15A, the configuration in which the data current I _DATA is directly applied to the sources of the T1 TFT and the T2 TFT by changing the position of the T2 TFT is different, but as illustrated in the timing diagram of FIG. Since the basic operation for flowing the saturation region current of the T4 TFT in which the data current is written and correcting the nonuniform electrical characteristics of the TFT to the O-LED 106 is the same as in the fourth embodiment, detailed description thereof will be omitted.

17A is a diagram illustrating a pixel structure of a current write type active matrix O-LED according to a sixth embodiment of the present invention, and FIG. 17B is an operation timing diagram of FIG. 17A.

The pixel structure of the current write type active matrix O-LED shown in FIG. 17A is composed of four P-type TFTs 111 to 114 and one C _STORAGE capacitor 115 as in the first embodiment of FIG. . The difference from the configuration of FIG. 6 is that the T2 TFT 112 is turned off before the T1 TFT 111 by applying two scan signals for stable circuit operation. As such, the concept of controlling switching of the T1 TFT and the T2 TFT by using two scan signals may be equally applicable to the above embodiments.

On the other hand, the TFT constituting the pixel is more advantageous to use a P-type TFT having excellent reliability than an N-type TFT, and using only one P-type TFT uses both an N-type TFT and a P-type TFT. There is a simple advantage in process. In addition, in the detailed description of the present invention has been described with respect to specific embodiments, various modifications are possible without departing from the scope of the invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

As described above, the pixel structure of the current-matrix active matrix O-LED according to the present invention basically does not need any signal other than the scan signal and the I _DATA signal, which are essential signals for the pixel, and thus, the light emitting area of the current matrix type active matrix O-LED is reduced. Since only one P-type TFT having high reliability can be used and is advantageous in terms of process, it is possible to realize a display that is more competitive than other structures.

In addition, most current write pixel structures have a circuit structure in which a current having the same value as the data current flows to the O-LED after the pixel selection is completed. Therefore, considering the material characteristics of the O-LED that should express high-quality gray scales within a maximum of 1-2 dB, a data current driver capable of adjusting several tens of dB is required. On the other hand, according to the pixel structure of the current write type active matrix O-LED of the present invention, since the data current is reduced at a constant rate to drive the O-LED, the current driver can be easily designed.

Claims (4)

First and second switching elements for selecting the driving pixel by a scan signal applied from the outside and receiving a data current; A capacitor for storing a predetermined charge by a control current applied by the first and second switching elements; A third switching element selected by the first and second switching elements, writing a data current, and receiving an external power source; And An active matrix (O-LED) comprising a third switching element and a mirror structure, and including a driving element receiving a voltage by a charge stored in the capacitor and applying a current to a corresponding pixel. Pixel structure. The active matrix O-LED of claim 1, wherein the amount of current applied to the corresponding pixel is adjusted according to the channel width W / channel length L of the third switching element. Pixel structure. The pixel structure of an active matrix O-LED according to claim 1, wherein the first and second switching elements are simultaneously turned on and off. In a pixel structure of an active matrix O-LED (O-LED) having a scan line for selecting a driving pixel, a data line for applying a current to the pixel, and a power supply source for supplying power, A capacitor connected to the power supply and storing a charge determined according to a current applied to a data line; A first switching element formed of a P-type thin film transistor having a gate connected to the scan line and a source / drain current path connected to the data line; A second switching element made of a P-type thin film transistor having a gate connected to the scan line and having a source / drain current path formed between the capacitor and the first switching element; A third switching element made of a P-type thin film transistor having a gate connected to the capacitor and a source / drain current path formed in the power supply source; A driving device formed of a P-type thin film transistor having a gate connected to the gate of the third switching device and having a source / drain current path formed in the power supply source; And And an organic light emitting diode (O-LED) which emits light by a current flowing through the driving element.