KR100496884B1 - Pixel circuit for organic light-emitting diode - Google Patents

Abstract

A pixel circuit for driving an organic light emitting cell is disclosed. According to the pixel circuit of the present invention, the average current applied to the organic light emitting cell becomes the current of the data signal irrespective of the difference in the threshold voltage between the transistors operating as the current mirror. The image quality of the display device is improved by compensating for the threshold voltage mismatch between the transistors.

Description

Pixel circuit of organic light emitting element {PIXEL CIRCUIT FOR ORGANIC LIGHT-EMITTING DIODE}

The present invention relates to a pixel circuit for driving an organic light emitting device.

In general, an organic light emitting display is a self-luminous display that electrically excites fluorescent organic compounds to emit light, and is capable of representing an image by driving voltage or current by driving NxM organic light emitting cells. As shown in FIG. 1, the organic light emitting cell has a structure of an anode (ITO), an organic thin film, and a cathode layer (Metal), and the organic thin film has a good balance between electrons and holes to improve light emission efficiency. In order to improve, it has a multi-layered structure including an ETL (Electron Transport Layer) and an HTL (Hole Transport Layer) in the emitting layer (EML), sometimes a separate Electron Injecting Layer (EIL) ) And hole injecting layer (HIL).

As such, by placing an organic material including an organic light emitting material between the two electrodes, TFT LCD (Thin Film Transistor Liquid Crystal Display), which is currently the most widely used copyright law of the United States. Compared to All rights reserved., It operates at lower voltages, has a wider viewing angle, and the device's fast response time, making it ideal for high-resolution implementations.

As a method of driving the organic light emitting cell, there is a passive matrix method and an active matrix method using TFTs. In the passive matrix method, the anode and the cathode are orthogonal and the line is selected and driven, whereas the active driving method is a driving method in which a TFT and a capacitor are connected to each ITO pixel electrode to maintain a voltage by the capacitor capacity. The active matrix method has a lot of researches because the driving voltage and current consumption are smaller than the passive matrix method, and it is advantageous for a large screen and a high resolution.

Fig. 2 is a conventional pixel circuit for voltage driving an organic light emitting element using a TFT, which representatively shows one of NxM pixels. Referring to FIG. 2, a PMOS transistor 30 is connected to an organic light emitting diode (OLED) to supply current for emitting light, and an amount of current of the PMOS transistor 30 is applied through the PMOS transistor 10. It is controlled by the voltage which becomes. At this time, a capacitor 20 for maintaining the applied voltage for a predetermined period is connected between the drain and the gate of the PMOS transistor 30, and the n-th select signal SEL [n] is applied to the gate of the PMOS transistor 10. The data signal Data [m] is connected to the drain side.

Referring to FIG. 3, when the PMOS transistor 10 is turned on by the selection signal SEL [n], the data signal DA [m] becomes the PMOS transistor 30. Is applied to the gate of the node A, so that a current flows through the PMOS transistor 30 to the organic light emitting cell 40 to emit light. That is, the pixel to be driven is selected by using the selection signal SEL [n] and then the data signal DA [m] is applied. In this case, the data signal has a multi-level value in a predetermined range to express a gray level. .

However, when the voltage drive is used in the conventional structure as described above, there is a problem in that it is difficult to obtain a high gradation due to the irregularity of the threshold voltage Vth of the TFT. For example, when driving a pixel at 3V, to express an 8-bit 256 gray scale, 3/256 = 12 mV has a 10 mV unit, whereas a change in the threshold voltage of the TFT has a unit of 100 mV. There is a problem that is difficult to express.

Accordingly, an object of the present invention is to provide a pixel circuit of an organic light emitting diode having improved image quality by compensating for a mismatch in threshold voltage between transistors.

(Configuration)

According to a feature of the present invention for achieving the above object, a pixel circuit for driving an organic light emitting cell includes: a first transistor having a source, a drain and a gate connected with a power supply voltage, a source, a drain connected with the power supply voltage And a second transistor connected to the gate of the first transistor, a capacitor connected between the power supply voltage and the gates of the first and second transistors, a sequential series between the data signal and the gates of the first and second transistors. Third and fourth transistors each having a gate connected to a selection signal, and a common connection node of the third and fourth transistors to either the drain of the first transistor or the drain of the second transistor. A first switch and one end of the organic light emitting cell to drain the second transistor Includes a second switch for connection to any one of the drains of the first transistor. The first and second switches switch the connection state every frame.

In a preferred embodiment, the first to fourth transistors are PMOS transistors.

In a preferred embodiment, in the i th frame, the first switch connects a common connection node of the third and fourth transistors to a drain of the first transistor, and the second switch connects one end of the organic light emitting cell. It is connected to the drain of the second transistor. In an i + 1th frame, the first switch connects a common connection node of the third and fourth transistors to a drain of the second transistor, and the second switch connects one end of the organic light emitting cell to the first transistor. Connect to the drain of.

(Example)

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 4 is a PMOS pixel circuit for driving an organic light emitting element as a current, and shows one of NxM pixels. Referring to FIG. 4, the transistors M1 to M4 are PMOS transistors, respectively.

Transistor M1 has a source, a gate, and a drain connected to a power supply voltage VDD. The transistor M2 has a source connected to the power supply voltage VDD, a drain connected to the organic light emitting cell OLED, and a gate connected to the gate of the transistor M1. The node to which the gates of the transistors M1 and M2 are commonly connected is called a node A. Transistors M3 and M4 are sequentially connected in series between data signal DA [m] and node A, and their gates are connected to select signal SEL [n]. The common connection node of the transistors M3 and M4 is connected with the drain of the transistor M1. The organic light emitting cell OLED is connected between the drain of the transistor M2 and the ground voltage. The capacitor CST is connected between the power supply voltage VDD and the node A. The capacitor CST holds the data signal DA [m] charged to the node A for a predetermined period of time.

FIG. 5 is a timing diagram illustrating an operation of the pixel circuit shown in FIG. 4. 4 and 5, when the selection signal SEL [n] transitions to a low level, the transistors M3 and M4 are turned on. Subsequently, when the data signal DA [m] is applied, the organic light emitting cell OLED emits light with brightness corresponding to the current amount of the data signal DA [m].

In such a pixel circuit, the threshold voltages of the transistors M1 and M2 operating as current mirrors must match completely. If the threshold voltages of the transistors M1 and M2 do not coincide with each other, even though the same data current is applied to the organic light emitting cells, the brightness between the pixels is different. However, in practical circuit implementation, it is not easy to completely match the threshold voltages of the transistors M1 and M2.

FIG. 6 is a PMOS type pixel circuit for driving an organic light emitting diode according to a preferred embodiment of the present invention, and typically shows one of NxM pixels.

Referring to FIG. 6, each of the transistors M1-M4 is a PMOS transistor and is composed of an active device such as a single crystal metal-oxide semiconductor field-effect transistor (MOSFET) or a polysilicon thin film transistor (TFT). do. Transistor M1 has a source, a gate, and a drain connected to a power supply voltage VDD. The transistor M2 has a source connected to the power supply voltage VDD, a drain connected to the organic light emitting cell OLED, and a gate connected to the gate of the transistor M1. The node to which the gates of the transistors M1 and M2 are commonly connected is called a node A. Transistors M3 and M4 are sequentially connected in series between data signal DA [m] and node A, and their gates are connected to select signal SEL [n]. The switch SW1 connects the common connection node of the transistors M3 and M4 to either the drain of the transistor M1 or the drain of the transistor M2. The switch SW1 connects the anode of the organic light emitting cell OLED to either the drain of the transistor M2 or the drain of the transistor M1. The cathode of the organic light emitting cell OLED is connected to the ground voltage. The capacitor CST is connected between the power supply voltage VDD and the node A. The capacitor CST holds the data signal DA [m] charged to the node A for a predetermined period of time.

FIG. 7A shows the connection state of the switches in the pixel circuit shown in FIG. 6 in the i th frame, and FIG. 7B shows the connection state of the switches in the pixel circuit shown in FIG. 6 in the i + 1 th frame.

First, referring to FIG. 7A, in the i th frame, the switch SW1 connects the common connection node of the transistors M3 and M4 with the drain of the transistor M1. The switch SW2 connects the anode of the organic light emitting cell OLED to the drain of the transistor M2. At this time, the voltage of the node A and the drain current of the transistor M2 applied to the organic light emitting cell OLED are represented by Equation 1 below.

If the difference in the threshold voltage between the transistors M1 and M2 in Equation 1 is Δv, it can be summarized as in Equation 2.

In equation (2), it is assumed that Δv <

7B shows the connection state of the switches SW1 and SW2 in the i + 1th frame. The switch SW1 connects the common connection node of the transistors M3 and M4 with the drain of the transistor M2. The switch SW3 connects the anode of the organic light emitting cell OLED to the drain of the transistor M1. At this time, the drain current of the transistor M1 applied to the organic light emitting cell OLED is expressed by Equation 3 below.

From equations (2) and (3), the average current applied to the organic light emitting cell OLED in the i-th frame and the i + 1th frame is expressed by the following equation (4).

As can be seen from Equation 4, according to the pixel circuit of the present invention, the average current applied to the organic light emitting cell OLED is the data signal DA [regardless of the difference in the threshold voltage between the transistors M1 and M2. m]) is the current Idata.

While the invention has been described using exemplary preferred embodiments, it will be understood that the scope of the invention is not limited to the disclosed embodiments. Rather, the scope of the present invention is intended to include all of the various modifications and similar configurations. That is, the structure of the pixel circuit of the present invention can be equally applied to the voltage driven pixel circuit and other kinds of pixel structures. Accordingly, the claims should be construed as broadly as possible to cover all such modifications and similar constructions.

According to the present invention, the image quality of the display device is improved by compensating for the mismatch of the threshold voltage between the transistors.

1 is a view showing the structure of an organic light emitting cell;

FIG. 2 is a conventional pixel circuit for voltage driving an organic light emitting element using a TFT, which representatively shows one of NxM pixels; FIG.

3 is a timing diagram showing the operation of the pixel circuit shown in FIG. 2;

FIG. 4 is a pixel circuit for driving an organic light emitting element, representatively showing one of N × M pixels; FIG.

5 is a timing diagram showing the operation of the pixel circuit shown in FIG. 4;

6 illustrates a pixel circuit for driving an organic light emitting diode according to a preferred embodiment of the present invention;

FIG. 7A shows a connection state of switches in the pixel circuit shown in FIG. 6 in an i th frame; FIG. And

FIG. 7B shows the connection state of the switches in the pixel circuit shown in FIG. 6 in the i + 1th frame.

Claims (6)

In a pixel circuit for driving an organic light emitting cell: A first transistor having a source, a drain, and a gate connected to the power supply voltage; A second transistor connected to a source, a drain connected to the power supply voltage, and a gate of the first transistor; A capacitor coupled between the power supply voltage and the gates of the first and second transistors; Third and fourth transistors sequentially connected in series between a data signal and gates of the first and second transistors, each having a gate connected to a selection signal; A first switch connecting the common connection node of the third and fourth transistors to either the drain of the first transistor or the drain of the second transistor; A second switch connecting one end of the organic light emitting cell to either the drain of the second transistor or the drain of the first transistor; And the first and second switches switch the connection state every predetermined period. The method of claim 1, And the first to fourth transistors are PMOS transistors. The method of claim 1, And the first and second switches switch the connection state every frame. The method of claim 3, wherein In an i th frame, the first switch connects a common connection node of the third and fourth transistors to the drain of the first transistor, and the second switch connects one end of the organic light emitting cell to the drain of the second transistor. And a pixel circuit. The method of claim 4, wherein In an i + 1th frame, the first switch connects a common connection node of the third and fourth transistors to a drain of the second transistor, and the second switch connects one end of the organic light emitting cell to the first transistor. And a drain circuit of the pixel circuit. The method of claim 1, And the first and second switches switch the connection state every predetermined frames.