WO2010093088A1

WO2010093088A1 - Pixel circuit for organic light emitting diode (oled) panel, display device having the same, and method of driving oled panel using the same

Info

Publication number: WO2010093088A1
Application number: PCT/KR2009/002570
Authority: WO
Inventors: Dong-Wook Choi
Original assignee: Neoviewkolon Co., Ltd.
Priority date: 2009-02-16
Filing date: 2009-05-14
Publication date: 2010-08-19

Abstract

Disclosed are a pixel circuit (310) of an organic light emitting diode (OLED) panel (300), a display device and a method of driving an OLED panel. The pixel circuit includes a switching element which is driven by a scan signal input through a scan line sequentially selected from among a plurality of scan lines and transfers a driving current corresponding to brightness data input through a data line, and an OLED which is driven by the driving current transferred through the switching element to emit light. According to an exemplary embodiment of the present invention, since a transistor (QN 31 ) which performs a switching functions arranged in a conventional PMOLED panel structure, problems of brightness unbalance and brightness deterioration occurring due to a sequential driving method of a scan line are solved, and a problem of an existing AMOLED panel structure in that a circuit is complicated is solved.

Description

PIXEL CIRCUIT FOR ORGANIC LIGHT EMITTING DIODE (OLED) PANEL, DISPLAY DEVICE HAVING THE SAME, AND METHOD OF DRIVING OLED PANEL USING THE SAME

Example embodiments of the present invention relate to an organic light emitting diode (OLED) panel, and more particularly, to a pixel circuit for an OLED panel, a display device having the same and a method of driving an OLED panel using the same in which an OLED panel has a simple structure and is not affected by reliability characteristic of a transistor used as a driving element.

Generally, an OLED panel is classified into a PMOLED panel and an AMOLED panel. The OLED panel may be manufactured as a transparent OLED panel.

The PMOLED panel is simple in structure due to its characteristic.

However, the PMOLED panel, particularly the transparent PMOLED panel, has a problem in that there is a limitation to manufacturing of a dot matrix type due to high resistance of a transparentcathode and transmittance deterioration of a partition wall. For this reason, the PMOLED panel is usually used as a small size of panel suitable for forming an icon, and has a lot of restrictions to displaying texts or graphics.

On the other hand, the AMOLED panel solves problems of the PMOLED panel such as brightness unbalance and brightness deterioration whichoccur as pixels are sequentially driven according to a scan line. The AMOLED panel has an advantage of being capable of manufacturing a relatively large-sized panel of a dot matrix type unlike the PMOLED panel. However, since the AMOLED panel is driven by adjusting an amount of an electric current of a transistor used as a driving element, a transistorof high reliability is required. The AMOLED panel also has a problem in that brightness unbalance occurs due to characteristic deviation of a transistor used as a driving element. In order to solve the problems, a configuration such as a compensation circuit or a capacitor is additionally required, leading to a relatively complicated structure and high manufacturing cost.

Hereinafter, problems of pixel circuits used in a PMOLED panel and an AMOLED panel will be described in further detail with reference to FIGs. 1 and 2.

FIG. 1 is a circuit diagram illustrating a pixel circuit of a conventional PMOLED panel.

Referring to FIG. 1, a PMOLED panel 100 includes a plurality of pixel circuits 110 which are disposed in the form of a dot matrix. For simplicity, one pixel circuit 110 is exemplarily illustrated in FIG. 1.

Each pixel circuit 110 may be simply expressed as including one OLED₁₁. Terminals of the OLED₁₁ are connected with a scan line and a data line.

The data line is connected to a data driving circuit (not shown), and the scan line is connected to a scan driving circuit (not shown).

The data driving circuit divides brightness by stages and provides each pixel circuit 110 with brightness data corresponding to brightness divided by stages. The scan driving circuit sequentially drives each scan line according to a scan driving method.

Here, when brightness data is provided to the data line, and the scan line is sequentially driven, an electric current flows through each OLED₁₁, so that each OLED₁₁ emits light.

However, when each scan line is connected according to a sequential driving method, brightness unbalance occurs due to resistance deviation of the scan lines. Also, since brightness deterioration occurs due to high resistance of the scan line and the data line, the PMOLED panel 100 has a problem in that a panel size which can be manufactured is limited.

FIG. 2 is a circuit diagram illustrating a pixel circuit of a conventional AMOLED panel.

Referring to FIG. 2, an AMOLED panel 200 includes a plurality of pixel circuits 210 which are disposed in the form of a dot matrix. For simplicity, one pixel circuit 210 is exemplarily illustrated in FIG. 2.

In each pixel circuit 210, a transistor QP₂₁connected between a data line and a scan line performs a switching function. Predetermined brightness data is provided through each dataline, and a low signal is sequentially applied through each scan line, so that each transistor QP₂₁ is turned on.

A data voltage is input to a gate terminal of a transistor QP₂₂, so that a constant voltage is formed in a capacitor C₂₁. The transistor QP₂₂ determines a supply current of the OLED₂₁ based on the data voltage input to the gate terminal and a threshold voltageof the transistor QP₂₂.

At this time, even though a high signal is applied, so that the transistor QP₂₁ is turned off, a voltage provided to the data line is still maintained in the capacitor C₂₁ "as is".

That is, the AMOLED panel 200 solves a problem of brightness unbalance which occurs because the scan line is sequentially driven.

However, the transistor QP₂₂ of the AMOLED panel 200 has a disadvantage in that characteristic deviation according to a manufacturing process is severe. Therefore, in the AMOLED panel 200 of a dot matrix type structure, brightness unbalance occurs due to characteristic deviation of the transistor QP₂₂ of each unit pixel.

A compensation circuit technique for adding a pluralityof transistors and a capacitor in each pixel is currently used in order to solve a problem of brightness unbalance of the AMOLED panel. However, it is not easy to manufacture the AMOLED panel 200 in which the compensation circuit technique is employed since a manufacturing process is complicated.

Further, the AMOLED panel 200 gets high voltage and current stress since the transistor QP₂₂ directly controls a supply current of the OLED₂₁. Therefore, in order to manufacture the AMOLED panel 200 of a high quality, the transistor QP₂₂ of high reliability is required.

Accordingly, the present invention is provided to substantially obviate one or more problems due to limitations and disadvantages of the related art.

It is a feature of the present invention to provide a pixel circuit for an OLED panel in which an OLED panel is simpler in structure than an AMOLED panel, and a brightness unbalance problem is solved.

It is another feature of the present invention to provide a display device having a pixel circuit in which an OLED panel is simpler in structure than an AMOLED panel, and a brightness unbalance problem is solved.

It is still another feature of the present invention to provide a method of driving an OLED panel using a pixel circuit in which an OLED panel is simpler in structure than an AMOLED panel, and a brightness unbalance problem is solved.

In one example embodiment, a pixel circuit of an organic light emitting diode (OLED) panel, includes: a transistor which is driven by a scan signal input through a scan line sequentially selected from among a plurality of scan lines and transfers a driving current corresponding to brightness data input through a data line; and an OLED which is driven by the driving current transferred through the switching element to emit light.

The transistor may include a gate terminal connected to the scan line, a drain terminal connected to the data line, and a source terminal connected to the OLED.

The OLED may include an anode connected to the source terminal of the transistor and a transparent cathode commonly connected to pixels of the panel.

The transistor may be a metal oxide thin film transistor (MOTFT).

The MOTFT may include an active layer made of one of zinc oxide(ZnO), indium zinc oxide (IZnO), and indium gallium zinc oxide (IGZO).

In another example embodiment, a display device having a pixel circuit of an organic light emitting diode (OLED) panel, includes: a scan driving circuit which applies sequentially scan signals through a plurality of scan lines; a data driving circuit which applies brightness data through a plurality of data lines; and a pixel circuit of an OLED panel which includes a transistor which is formed at a crossing point of the scan line and the data line, is driven by a scan signal input through a scan line sequentially selected from among the scan lines, and transfers a driving current corresponding to brightness data input through a data line, and an OLED which is driven by the driving current transferred through the switching element to emit light.

In still another embodiment, a methodof driving an organic light emitting diode (OLED) panel using a pixel circuit which includes a transistor for selectively driving a pixel, includes: driving the transistor by a scan signal input through a scan line sequentially selected from among a plurality of scan lines; transferring a drivingcurrent to an OLED through the transistor, the driving current corresponding to brightness data input through a data line; and emitting light through the OLED driven by the driving current.

Indriving the transistor, the scan signal may be applied to a gate terminal of the transistor.

In transferring the driving current, the driving current may be input through a drain terminal of the transistor, and the transistor may transfer the driving current to the OLED through a source terminal.

in emitting light through the OLED, the OLED may include an anode connected to a source terminal of the transistor and a cathode connected to a transparent common electrode, and the drivingcurrent transferred through the anode may be transferred to the transparent common electrode connected to the cathode.

The transistor may be an n- or p-type metal oxide thin film transistor (MOTFT).

TheMOTFT may include an active layer made of one of zinc oxide(ZnO), indium zinc oxide (IZnO), and indium gallium zinc oxide (IGZO).

As described above, according to an exemplary embodiment of the present invention, by employing a transistor which performs a switching function in a conventional PMOLED panel structure, a brightness unbalance problem occurring due to a sequential driving method of a scan line is solved, and a problem ofan existing AMOLED panel structure in that a circuit is complicated is solved.

Example embodiments of the present invention will become more apparent by describing in detail example embodiments of the present invention with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram illustrating a pixel circuit of a conventional PMOLED panel;

FIG. 2 is a circuit diagram illustrating a pixel circuit of a conventional AMOLED panel;

FIG. 3 is a circuit diagram illustrating a pixel circuit for an OLED panel according to an exemplary embodiment of the present invention;

FIG. 4 is a block diagram illustrating a display device having a pixel circuit according to an exemplary embodiment of the present invention; and

FIG. 5 is a flowchart illustrating a method of driving an OLED panel according to an exemplary embodiment of the present invention.

* Description of Major Symbol in the above Figures

110: Pixel circuit 210: Pixel circuit

310: Pixel Circuit 410: Data driving circuit

420: Scan driving circuit

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail.

However, it should be understood that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

Hereinafter, one example embodiment of the present invention will be described in more detail with reference to the accompanying drawings. In the following description, elements having the same functions as those of the elements which have been previously described will be indicated with the same reference numerals, and a detailed description thereof will be omitted.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

FIG. 3 is a circuit diagram illustrating a pixel circuit of an OLED panel according to an exemplary embodiment of the present invention.

Referring to FIG. 3, an OLED panel 300 according to an exemplary embodiment of the present invention may include a plurality of pixels which are disposed in the form of a dot matrix. The OLED panel 300 of the dot matrix type is suitable for a large-sized display device. For simplicity, FIG. 3 exemplarily illustrates one of a plurality of pixels disposed in the OLED panel 300. A pixel circuit 310 included in each pixel may include a transistor QN₃₁ and an OLED₃₁. A structure and operation of the OLED panel 300 will be described below.

First, a structure of the pixel circuit 310 of the OLED panel 300 will be described.

The transistor QN31 has a gate terminal connected to a scan line X_N and a drain terminal connected to a data line Y_N. A scan signal is applied through the scan line, and brightness data representing brightness is applied through the data line. A source terminal of the transistor QN₃₁is connected to the OLED₃₁.

The scan signal is applied from a scan driving circuit (see 420 in FIG. 4), and brightness data as a data signal is applied from a data driving circuit (see 410 in FIG. 4). The scan signal is input through a scan line which is sequentially selected from among a plurality of scan lines by the scan driving circuit 420.

The data driving circuit 410 and the scan driving circuit 420 may be identical to the scan driving circuit and the data driving circuit which are used in the PMOLED panel of FIG. 1.

The OLED panel 300 may be of a transparent type, and in this case, the transistor QN₃₁ is preferably a metal oxide thin film transistor (MOTFT). It is because the MOTFT may be manufactured as a transparent type. Of course, the MOTFT may be made of various materials. For example, an active layer may be made of zinc oxide (ZnO), indium zinc oxide (IZnO) or indium gallium oxide (IGZO).

A cathode may be formed as a transparent electrode. In the OLED panel 300 according to an exemplary embodiment of the present invention, unlike the existing PMOLED panel 100, the cathode is used as a common electrode, and thus resistance or transmittance does not deteriorate.

As described above, the OLED panel 300 according to an exemplary embodiment of the present invention is a PMOLED panel but uses a common cathode electrode. Since an electric current flows from the data line to the common cathode, a brightness deterioration problem resulting from high resistance of the scan line does not occur.

In the OLED panel 300 according to an exemplary embodiment of the present invention, the scan driving circuit of a sequential driving method and the data driving circuit which are used in the conventional PMOLED panel may be used "as is".

Next, operation of the pixel circuit 310 will be described.

First, brightness data is applied to the transistor QN₃₁ through the data line. Brightness data may have different electric currents according to stages.

The scan signal is applied to the gate terminal of the transistor QN₃₁from the scan line. The scan signal is sequentially applied through each scan line.

In a state in which constant brightness data is applied to the data line, the scan signal is applied to the gate terminal of the transistor QN₃₁ of each pixel circuit 310 through the scan line, so that the transistor QN₃₁ is turned on.

A brightness data current is transferred through the transistor QN₃₁ which is turned on. The current flows through the OLED₃₁, so that the OLED₃₁ emits light.

As described above, the transistor QN₃₁ operates as a switching element for the OLED₃₁. At this time, when the transistor QN₃₁is turned on, a transistor saturation voltage is exceeded, so that it operates as a current path.

In the conventional AMOLED panel, an OLED is driven by an electric current generated by a driving transistor, and thus there is a limitation in performance, depending on characteristic deviation, such as reliability for a threshold voltage of a transistor. However, in the OLED panel 300 according to an exemplary embodiment of the present invention, since the transistor QN₃₁ merely operates as a switching element, a problem such as performance deviation or performance deterioration does not occur. Therefore, a complicated circuit configuration such as a compensation circuit technique employed in the AMOLED panel is not required.

Meanwhile, in the conventional PMOLED panel, since an electric current is applied to the data line or the scan line, there may be a limitation to driving due to resistance, but in the OLED panel 300 according to an exemplary embodiment of the present invention, a scan line current path of the PMOLED panel disappear due to the transistor QN₃₁, and the cathode is used as a common grounding electrode, whereby a limitation resulting from resistance can be overcome.

FIG. 4 is a block diagram illustrating a display device having a pixel circuit according to an exemplary embodiment of the present invention.

Referring to FIG. 4, a display device 400 according to an exemplary embodiment of the present invention may include an OLED panel 300, a data driving circuit 410, and a scan driving circuit 420.

The data driving circuit 410 provides brightness data through a plurality of data lines. That is, different currents having different brightness according to stages are provided through the data lines.

The scan driving circuit 420 sequentially outputs scan signals through a plurality of scan lines. The scan driving circuit 420 may use a scan line driving method used in the conventional PMOLED panel "as is".

Here, an inverter circuit may be additionally connected to an output of the conventional scan driving circuit. The inverter circuit functions to invert a scan signal output from the scan driving circuit 420. Since when the transistor QN₃₁ used in the OLED panel 300 is of an n-type, a scan line of the conventional PMOLED driving method forms a p-type negative voltage, an additional configuration such as an inverter circuit is necessary. From a point of view of manufacturing an integrated circuit (IC) chip, the inverter may be manufactured to be installed inside a circuit and be selectively used according to whether of the transistor QN₃₁ is of an n-type or a p-type.

The OLED panel 300 includes a plurality of pixels which are disposed in the form of a dot matrix, and each pixel includes the pixel circuit 310 according to an exemplary embodiment of the present invention.

Each pixel circuit 310 may be formed at a crossing point of the scan line and the data line and include the transistor QN₃₁ and the OLED₃₁.

The transistor QN₃₁of the pixel circuit 310 is driven by a scan signal input trough a scan lien sequentially selected from among the scan lines and outputs a driving current according to brightness data input through the data line.

The OLED₃₁ of the pixel circuit 310 is driven by a driving current transferred from the transistor QN₃₁ to emit light.

The detailed operation of the transistor QN₃₁ and the OLED₃₁ are already described above with reference to FIG. 3.

The OLED panel 300 and the display device 400 having the OLED panel 300 may be manufactured according to a typical circuit manufacturing process, and a transistor employed in the OLED panel 300 and the display device 400 may have a bottom gate TFT structure or a top gate TFT structure.

Referring to FIG. 5, in the pixel circuit 310 having the transistor QN₃₁for selectively driving a pixel, the transistor QN₃₁ is driven by a scan signal input through a scan line sequentially selected from among a plurality of scan lines (S100). The scan signal may be applied to the gate terminal of the transistor QN₃₁.

The transistor QN₃₁ transfers a driving current corresponding to brightness data input through the data line to the OLED₃₁ (S200). The driving current may be applied to the drain terminal of the transistor QN₃₁ and be transferred to the OLED₃₁ through the source terminal of the transistor QN₃₁.

The OLED₃₁ is driven by the driving current transferred through the source terminal of the transistor QN₃₁to emit light (S300). At this time, an anode of the OLED₃₁may be connected to the source terminal of the transistor QN₃₁, and a cathode thereof may be connected to a transparent common electrode.

Preferably, the transistor QN₃₁ is a MOTFT of an n- or p-type. The MOTFT may include an active layer which is made of any one of ZnO, IZnO, and IGZO.

While the present invention has been shown and described with reference to certain example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

The present invention has industrial applicabilty to dispaly device having a pixel circuit for an OLED panel in which an OLED panel is simpler in structure than an AMOLED panel.

Claims

A pixel circuit of an organic light emitting diode (OLED) panel, comprising:

a switching element which is driven by a scan signal input through a scan line sequentially selected from among a plurality of scan lines and transfers a driving current corresponding to brightness data input through a data line; and

an OLED which is driven by the driving current transferred through the switching element to emit light.
The pixel circuit of claim 1, wherein the switching element includes a transistor which has a gate terminalconnected to the scan line, a drain terminal connected to the data line, and a source terminal connected to the OLED.
The pixel circuit of claim 2, wherein the OLED has an anode connected to the source terminal of the transistor and a cathode connected to a transparent common electrode.
The pixel circuit of claim 2, wherein the transistor is an n- or p-type metal oxide thin film transistor (MOTFT).
The pixel circuit of claim 4, wherein the MOTFT includes an active layer made of one of zinc oxide(ZnO), indium zinc oxide (IZnO), and indium gallium zinc oxide (IGZO).
A display device havinga pixel circuit of an organic light emitting diode (OLED) panel, comprising:

a scan driving circuit which applies sequentially scan signals through a plurality of scan lines;

a data driving circuit which applies brightness data through a plurality of data lines; and

a pixel circuit of an OLED panel which includes a switching element which is formed at a crossing point of the scan line and the data line, is driven by a scan signal input through a scan line sequentially selected from among the scan lines, and transfers a driving current corresponding to brightness data input through a data line, and an OLED whichis driven by the driving current transferred through the switching element to emit light.
A method of driving an organic light emitting diode (OLED) panel using a pixel circuit which includes a transistor for selectively driving a pixel, comprising:

driving the transistor by a scan signal input through a scan line sequentially selected from among a plurality of scan lines;

transferring a driving current to an OLED through the transistor, the driving current corresponding to brightness data input through a data line; and

emitting light through the OLED driven by the driving current.
The method of claim 7, wherein in driving the transistor, the scan signal is applied to a gate terminal of the transistor.
The method of claim 8, wherein in transferring the driving current, the driving current is input through a drain terminalof the transistor, and the transistor transfers the driving current to the OLED through a source terminal.
The method of claim 9, wherein in emitting light through the OLED, the OLED has an anode connected to a source terminal of the transistor and a cathode connected to a transparent common electrode, and the driving current transferred through the anode is transferred to the transparent common electrode connected to the cathode.
The method of claim 10, wherein the transistor is an n- or p-type metal oxide thin film transistor (MOTFT).
The method of claim 11, wherein the MOTFT includes an active layer made of one of zinc oxide(ZnO), indium zinc oxide (IZnO), and indium gallium zinc oxide (IGZO).