Organic electroluminescent display device and driving method of the same

Abstract

An organic electroluminescent display device includes an organic light-emitting diode, a first transistor outputting a data voltage by an nth scan signal (n is a natural number), a second transistor providing a current to the organic light-emitting by the data voltage, a capacitor storing the data voltage, and a third transistor supplying the second transistor and the capacitor with a pre-charge voltage by an (n−1)th scan signal, the pre-charge voltage having an opposite polarity to the data voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate an embodiment of the present invention and together with the description serve to explain the principles of that invention.

FIG. 1 is an equivalent circuit for a pixel of an OELD device according to the related art.

FIG. 2 is a timing chart of an OELD device.

FIG. 3 is a graph illustrating change of a threshold voltage due to deterioration of a thin film transistor.

FIG. 4 is an equivalent circuit of an OELD device according to a first embodiment of the present invention.

FIG. 5 shows a schematic timing chart for a data voltage and a pre-charge voltage of an OELD device according to embodiments of the present invention.

FIG. 6 is an equivalent circuit of an OELD device according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to an illustrated embodiment of the present invention, examples of which are shown in the accompanying drawings.

FIG. 4 is an equivalent circuit of an organic electroluminescent display (OELD) device according to a first embodiment of the present invention.

In FIG. 4, scan lines S(n−1) and Sn (n is a natural number) and data lines Dm and D(m+1) (m is a natural number) are arranged in a matrix form to define pixels P. Each pixel P includes a first transistor T1, a second transistor T2, a capacitor C, and an organic light-emitting diode OLED are formed. A third transistor T3 is connected at one end of each scan line S(n−1) and Sn.

The first transistor T1 is connected to one of the scan lines S(n−1) and Sn and one of the data lines Dm and D(m+1). That is, a gate electrode of the first transistor T1 is connected to one of the scan lines S(n−1) and Sn, and a source electrode of the first transistor T1 is connected to one of the data lines Dm and D(m+1). A gate electrode of the second transistor T2 is connected to a drain electrode of the first transistor T1, a drain electrode of the third transistor T3, and one electrode of the capacitor C. A source electrode of the second transistor T2 is connected to the other electrode of the capacitor C and a ground GND. A drain electrode of the second transistor T2 is connected to a cathode electrode of the organic light-emitting diode OLED. An anode electrode of the organic light-emitting diode OLED is connected to a power supply line VDD.

A source electrode of the third transistor T3 is connected to a pre-charge line PL. A gate electrode of the third transistor T3, which is connected to the second transistors T2 of pixels P supplied with a signal from the nth scan line Sn, is connected to the (n−1)th scan line S(n−1). A pre-charge voltage Vpre is applied to the third transistor T3 through the pre-charge line PL, and the third transistor T3 connected to one scan line provides the pre-charge voltage Vpre to the second transistors T2 and the capacitors C of the pixels P connected to the next scan line. For convenience of explanation, there shows only the third transistor T3 connected to the (n−1)th scan line S(n−1) in the figure.

The first, second and third transistors T1, T2 and T3 preferably all include amorphous silicon and have the same type channel. The first, second and third transistors T1, T2 and T3 may have an n-type channel, for example. The third transistor T3 and the pre-charge line PL may be disposed in a non-display area where images are not displayed.

The first transistor T1 is switched ON and OFF by a scan signal and outputs a data voltage applied from the data line Dm and D(m+1) into the second transistor T2. The second transistor T2 adjusts currents flowing through its channel according to the data voltage and controls a brightness of light emitted from the organic light-emitting diode OLED. The capacitor C stores the data voltage output from the first transistor T1 and provides the stored data voltage to the second transistor T2 when the first transistor T1 finishes outputting the data voltage, thereby continuing light-emitting time of the organic light-emitting diode OLED.

When a scan signal is applied to the (n−1)th scan line S(n−1) in order to operate the pixels P in an (n−1)th horizontal row in the context of the figure, the third transistor T3 connected to the (n−1)th scan line S(n−1) turns ON and provides a pre-charge voltage Vpre to the second transistors T2 and the capacitors C of the pixels P in an nth horizontal row in the context of the figure. The pre-charge voltage Vpre has an opposite polarity to the data voltage. That is, if the second transistor T2 has an n-type channel, the data voltage may be positive. Accordingly, the pre-charge voltage Vpre may be negative. For example, the pre-charge voltage Vpre may have a value within a range of about −5V to about −10V. If the second transistor T2 has a p-type channel, the data voltage may be negative, and the pre-charge voltage Vpre may be positive.

FIG. 5 shows a schematic timing chart for a data voltage and a pre-charge voltage of an organic electroluminescent display device according to the present invention. The data voltage Vdata and the pre-charge voltage Vpre are supplied to the second transistors T2 in a horizontal row in the context of the figure. Here, the data voltage is positive, and the pre-charge voltage Vpre is negative.

In FIG. 5, the negative pre-charge voltage Vpre is applied to the second transistors T2 and the capacitors C of the pixels P in a horizontal row that will be driven next time through the third transistor T3. After that, the positive data voltage Vdata is applied to the second transistors T2 and the capacitors C of the pixels P in the horizontal row in the context of the figure. Accordingly, the second transistors T2 are prevented from deteriorating and having changed characteristics due to continuous supply of the positive data voltage Vdata. Charges remaining in the capacitors C are discharged due to the negative pre-charge voltage Vpre. Therefore, the next data voltage Vdata is not stored with the previous data voltage Vdata, and more definite images can be displayed.

FIG. 6 is an equivalent circuit of an OELD device according to a second embodiment of the present invention. The OELD has a plurality of pixels, and a structure and an operation of the pixel are the same as that of the first embodiment of FIG. 4. The explanation for the structure and the operation of the pixel will be abbreviated.

In FIG. 6, a third transistor T3 is connected at one end of each scan line S(n−1) and Sn. A gate electrode of the third transistor T3, which is connected to second transistors T2 of pixels P supplied with a signal from the nth scan line Sn, is connected to the (n−1)th scan line S(n−1). A source electrode of the third transistor T3 is connected to the nth scan line Sn, and a drain electrode of the third transistor T3 is connected to the second transistors T2 and capacitors C of the pixels P supplied with a signal from the nth scan line Sn. For convenience of explanation, only the third transistor T3 connected to the (n−1)th scan line S(n−1) are shown in the figure.

When a scan signal, that is, a high level voltage, is applied to the (n−1)th scan line S(n−1), the third transistor T3 provides the second transistors T2 and the capacitors C of the pixels P connected with the nth scan line Sn with a low level voltage of the nth scan line Sn as a pre-charge voltage Vpre.

The first, second and third transistors T1, T2 and T3 all include amorphous silicon and have the same type channel. The first, second and third transistors T1, T2 and T3 may have an n-type channel, for example. The third transistor T3 may be disposed in a non-display area where images are not displayed.

The first transistor Ti is switched ON and OFF by a scan signal and outputs a data voltage applied from the data line Dm and D(m+1) into the second transistor T2. The second transistor T2 adjusts currents flowing through its channel according to the data voltage and controls a brightness of light emitted from the organic light-emitting diode OLED. The capacitor C stores the data voltage output from the first transistor T1 and provides the stored data voltage to the second transistor T2 when the first transistor T1 finishes outputting the data voltage, thereby continuing light-emitting time of the organic light-emitting diode OLED.

With reference to FIG. 5, when a scan signal, that is, a high level voltage, is applied to the (n−1)th scan line S(n−1) in order to operate the pixels P in an (n−1)th horizontal row in the context of the figure, the third transistor T3 connected to the (n−1)th scan line S(n−1) turns ON and provides a low level voltage of the nth scan line Sn, that is, a pre-charge voltage Vpre, to the second transistors T2 and the capacitors C of the pixels P in an nth horizontal row in the context of the figure. For example, the pre-charge voltage Vpre may have a value within a range of about −5V to about −10V.

In the same way as the first embodiment, the negative pre-charge voltage Vpre is applied to the second transistors T2 and the capacitors C of the pixels P in a horizontal row that will be driven next time through the third transistor T3. After that, a positive data voltage is applied to the second transistors T2 and the capacitors C of the pixels P in the horizontal row in the context of the figure. Accordingly, the second transistors T2 are prevented from deteriorating and having changed characteristics due to continuous supply of the positive data voltage. Charges remaining in the capacitors C are discharged due to the negative pre-charge voltage Vpre. Therefore, the next data voltage Vdata is not stored with the previous data voltage Vdata, and more definite images can be displayed.

It will be apparent to those skilled in the art that various modifications and variation can be made in an organic electroluminescent display device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. An organic electroluminescent display device, comprising: an organic light-emitting diode;a first transistor outputting a data voltage by an nth scan signal (n is a natural number);a second transistor providing a current to the organic light-emitting by the data voltage;a capacitor storing the data voltage; anda third transistor supplying the second transistor and the capacitor with a pre-charge voltage by an (n−1)th scan signal, the pre-charge voltage having an opposite polarity to the data voltage.

2. The device of claim 1, wherein the first, second and third transistors include amorphous silicon.

3. The device of claim 1, wherein the first, second and third transistors have an n-type channel.

4. The device of claim 1, wherein the capacitor is disposed between a gate electrode of the second transistor and a ground.

5. The device of claim 1, wherein the data voltage is positive and the pre-charge voltage is negative.

6. The device of claim 1, wherein the pre-charge voltage has a value within a range of about −5V to about −10V.

7. The device of claim 1, further comprising a pre-charge line providing the pre-charge voltage to the third transistor.

8. The device of claim 1, wherein the third transistor is connected to an nth scan line providing the nth scan signal to the first transistor.

9. The device of claim 1, wherein n is larger than two.

10. A driving method for an organic electroluminescent display device including pixels, n scan lines (n is a natural number) and m data lines (m is a natural number), the method comprising: sequentially applying scan signals to the scan lines;applying data voltages to the data lines according to the scan signals; andsupplying a pre-charge voltage to driving transistors and capacitors of the pixels connected to an nth scan line by an (n−1)th scan signal.

11. The method of claim 10, wherein the pre-charge voltage has an opposite polarity to the data voltages.

12. The method of claim 10, wherein the pre-charge voltage is provided from the (n+1)th scan line.

13. An organic electroluminescent display device, comprising: an organic light-emitting diode;a first transistor outputting a data voltage by an nth scan signal (n is a natural number);a second transistor providing a current to the organic light-emitting by the data voltage;means for storing the data voltage; andmeans for supplying the second transistor and the means for storing the data voltage with a pre-charge voltage by an (n−1)th scan signal, the pre-charge voltage having an opposite polarity to the data voltage.