KR100731743B1 - Pixel Circuit of Organic Electoluminescent Display Device - Google Patents

Abstract

Disclosed is a pixel circuit of an organic electroluminescent display device for preventing leakage current flowing in a driving transistor while data is applied to the pixel circuit or when representing a black gradation. The pixel circuit of the present invention has a first light emission control transistor connected between the driving transistor and the positive power supply voltage. In addition, a second light emission control transistor connected between the driving transistor and the organic light emitting display device is provided. The first and second light emission control transistors are turned off during the data programming period. Therefore, the leakage current flowing from the positive power supply voltage to the organic light emitting element is cut off, and a clear gray level is expressed in the light emitting period.

Description

Pixel Circuit of Organic Electoluminescent Display Device

1 is a block diagram showing a conventional organic EL display device.

2 is a block diagram illustrating a pixel circuit of an organic EL display device according to an exemplary embodiment of the present invention.

3 is a pixel circuit diagram illustrating a pixel circuit in FIG. 2 in detail.

4 is a timing diagram for describing an operation of a pixel circuit according to the first exemplary embodiment shown in FIG. 3.

FIG. 5 is a pixel circuit diagram illustrating a pixel circuit in FIG. 2 in detail. Referring to FIG.

6 is a timing diagram for describing an operation of a pixel circuit according to the second exemplary embodiment illustrated in FIG. 5.

 BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pixel circuit of an organic electroluminescent display, and more particularly, to an organic electroluminescent display for preventing leakage current flowing in a driving transistor while data is applied to a pixel circuit or when black gradation is expressed. It relates to a pixel circuit of the device.

An organic electroluminescent display (hereinafter referred to as EL ') is a flat panel display that displays a predetermined image by supplying a data signal to an organic EL element that performs self-luminescence operation. It is divided into Matrix type and Active Matrix type.

 The passive type is a method in which an anode and a cathode are arranged in a matrix manner in the display area, and an organic light emitting layer is formed at a portion where the anode and the cathode cross each other.

 In contrast, the active type disposes a thin film transistor (TFT) for each pixel and controls each pixel by using the thin film transistor.

 The biggest difference between the active type and the passive type is in the difference in the light emission time of the organic EL display device. That is, in the passive type, the organic light emitting layer is instantaneously emitted at high luminance, while in the active type, the organic light emitting layer is continuously emitted at low luminance.

 In the passive type, the instantaneous luminous brightness should be increased as the resolution is increased. In addition, since light of high luminance is emitted, it has a great influence on deterioration of the organic EL display device. On the other hand, in the active type, the device is driven using a thin film transistor and continuously emits light from the pixel for one frame, thereby enabling driving with low current. Therefore, the active type has the advantages of less parasitic capacitance and less power consumption than the passive type.

 However, the active type has the disadvantage of uneven brightness. The active type is an active element, and in most cases, an active low temperature polysilicon (LTPS) thin film transistor is used. The LTPS thin film transistor crystallizes amorphous silicon formed in a low temperature state using a laser. At this time, the characteristics of the thin film transistor vary depending on the crystallization. That is, characteristic nonuniformity occurs in which the threshold voltage or the like of the thin film transistor is not constant for each pixel. Therefore, each pixel shows the same brightness for the same screen signal, and when viewed as a whole, the pixels appear to be uneven in brightness. Various attempts have been made to solve the luminance non-uniformity problem.

 The luminance non-uniformity problem is solved by a method of compensating for the characteristics of the driving thin film transistor. Compensation of the characteristics of the driving transistor is largely divided into two methods according to the driving method. That is, there is a method using a voltage writing method and a method using a current writing method.

 The voltage write method is a method of storing the threshold voltage of the driving transistor in a capacitor and compensating the stored threshold voltage of the driving transistor.

In the current write method, an image signal is supplied as a current, and a source-gate voltage difference of the driving transistor corresponding to the image signal current is stored in a capacitor. Thereafter, the driving transistor is connected to a voltage source so that a current equal to the image signal current flows in the driving transistor. That is, regardless of the difference in device characteristics of the driving transistor, the value of the current applied to the organic light emitting layer becomes the value of the current flowing into the image signal. Therefore, the nonuniformity of luminance is improved.

1 is a block diagram showing a conventional organic EL display device.

Referring to FIG. 1, a conventional organic EL display device includes a display panel 100, a scan driver 200, and a data driver 300.

The display panel 100 includes a plurality of data lines D1 -Dm extending in a column direction, a plurality of scan lines S1 -Sn extending in a row direction, a plurality of emission control lines E1 -En and a plurality of light lines. Pixel circuits P11-Pnm. The pixel circuit 110 is formed in a pixel area defined by an area where a plurality of data lines D1 -Dm and a plurality of scan lines S1 -Sn and a plurality of emission control lines E1 -En cross each other. have.

The scan driver 200 is connected to a plurality of scan lines S1 -Sn and a plurality of emission control lines E1 -En to scan signals and emission control for sequentially selecting the plurality of pixel circuits P11 -Pnm. Apply a signal.

The data driver 300 is connected to a plurality of data lines D1 -Dm to apply a data signal for emitting light of the pixel circuit 110 selected by the scan signal of the scan driver 200.

Accordingly, a driving current corresponding to the data signal applied to the pixel circuit 110 is generated, and the driving current is supplied to the organic EL element to emit a predetermined light to display an image.

However, the conventional organic EL display device has a problem in that an undesired leakage current flows during a data programming period in which a data signal is applied to a pixel circuit, resulting in unclear luminance. In addition, even when the black gray is not driven, the leakage current flows through the thin film transistor, so that the black level is deteriorated, thereby increasing power consumption.

An object of the present invention to solve the above problems is to provide a pixel circuit of an organic light emitting display device to block the leakage current generated in the driving transistor while data is being applied or when black gradation is expressed.

In order to achieve the above object, the present invention is connected to a scan line and a data line, is activated by a scan signal transmitted through the scan line, and generates a driving current corresponding to the data signal transmitted through the data line. A pixel driving circuit for; A first light emission control element connected between the pixel driving circuit and a first power line and configured to perform an on / off operation according to a first light emission control signal; And an organic electroluminescent element connected between the pixel driver circuit and the second power line and configured to perform a light emitting operation according to the driving current.

In addition, the object of the present invention is a switching transistor connected to a data line, for transmitting a data signal in response to a scan signal from the scan line; A capacitor connected between the switching transistor and a first power line and configured to store the data signal; A driving transistor having a gate terminal connected to the capacitor and the switching transistor and configured to generate a driving current corresponding to the data signal stored in the capacitor; A first light emission control transistor connected between a first terminal of the driving transistor and a first power line and configured to perform an on / off operation according to a first light emission control signal; A second emission control transistor connected to a second terminal of the driving transistor and configured to perform an on / off operation according to a second emission control signal; And an organic electroluminescent element connected between the second emission control transistor and the second power supply line and including an organic electroluminescent element for performing a light emitting operation according to the driving current. have.

According to the present invention, the leakage current is cut off during the data programming section or the black gradation expression, thereby reducing power consumption and improving the expressive power of the gradation.

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

Example 1

2 is a block diagram showing a pixel circuit of an organic EL display device according to a first embodiment of the present invention.

Referring to FIG. 2, the pixel circuit 110 of the organic EL display device according to the first embodiment of the present invention may include a pixel driving circuit 112, a first light emission control element 114, and a second light emission control element 116. And an organic EL element (OLED).

The pixel driving circuit 112 may be composed of a plurality of transistors and a capacitor. The pixel driving circuit 112 is connected to the scan line Sn and the data line Dm, is activated by a scan signal transmitted through the scan line Sn, and data transferred through the data line Dm. Receive a signal. As such, the time for which the data signal is applied to the pixel driving circuit 112 while the scan signal is applied is referred to as a 'data programming period'. The data signal may be a signal in the form of a voltage or a signal in the form of a current. The data signal is stored for one frame in the form of voltage in a capacitor of the pixel driving circuit 112. Thus, the pixel driving circuit 112 generates a driving current corresponding to the data signal. As such, the period in which the driving current is generated after the data programming period is referred to as the 'light emitting period'.

The first light emission control element 114 is connected between the pixel driving circuit and the first power line, and supplies or cuts off a first power voltage to the pixel driving circuit 112 according to the first light emission control signal. The first power supply voltage may be a positive power supply voltage for generating the driving current, and a negative power supply voltage for absorbing the driving current. Therefore, the first light emission control element 114 is turned off during the data programming period in which the pixel driving circuit 112 receives the data signal, and thus does not apply the first power supply voltage to the pixel driving circuit 112. During the period of time, it is turned on to apply the first power supply voltage. Accordingly, the first light emission control element 114 may prevent the leakage current generated in the pixel driving circuit 112 during the data programming period. The first light emission control element 114 is a switching element that is turned on / off according to the first light emission control signal, and is an N-type or P-type MOSFET (hereinafter, referred to as a metal oxide semiconductor field effect transistor). Can be.

The organic EL element OLED is connected between the pixel driving circuit and the second power supply line, and emits light at a predetermined luminance according to a driving current generated by the pixel driving circuit 112. The second power supply voltage supplied from the second power supply line may correspond to the first power supply voltage and may be a positive power supply source for generating the driving current, and may be negative for absorbing the driving current. It may be a power supply voltage.

In addition, the second emission control element 116 is connected between the pixel driving circuit 112 and the organic EL element OLED, and the driving current is transferred to the organic EL element OLED according to the second emission control signal. Flow or shut off. In addition, the second light emission control element 116 is turned off during the data programming period in which the pixel driving circuit 112 receives a data signal like the first light emission control element 114 and the organic EL element OLED. The driving current is not supplied, and is turned on during the light emitting period to supply the driving current. Therefore, the second light emission control element 116 can prevent the leakage current generated in the pixel driving circuit 112 during the data programming period. The second emission control element 116 is a switching element that is turned on / off according to the second emission control signal, and may be an N-type or P-type MOSFET (hereinafter, referred to as a metal oxide semiconductor field effect transistor). have.

The first light emission control element 114 and the second light emission control element 116 are MOSFETs of the same conductivity type. The first emission control element 114 and the second emission control element 116 are commonly connected to the emission control line En to receive the same emission control signal. That is, the first emission control signal and the second emission control signal are the same control signal transmitted through the emission control line En. Hereinafter, the pixel circuit of the organic EL display device according to the exemplary embodiment of the present invention shown in FIG. 1 will be described in detail with reference to a specific circuit diagram.

FIG. 3 is a pixel circuit diagram according to a first embodiment of the pixel circuit shown in FIG. 2 in detail.

Referring to FIG. 3, the pixel circuit 110 according to the first embodiment includes a pixel driving circuit 112a, a first light emission control element 114a, a second light emission control element 116a, and an organic EL element OLED. It is composed.

The pixel driving circuit 112a includes a switching transistor MS, a capacitor Cst, and a driving transistor MD.

The switching transistor MS is connected to the data line Dm and transmits a data signal supplied from the data line Dm in response to a scan signal applied from the scan line Sn.

The capacitor Cst is connected between the switching transistor MS and the first power line ELVDD and stores a data signal transmitted from the switching transistor MS for one frame.

The driving transistor MD has a gate terminal connected to the capacitor Cst and the switching transistor MS, and generates a driving current corresponding to the data signal stored in the capacitor Cst.

The first light emission control element 114a is connected between the first power supply line ELVDD and the driving transistor MD and is turned on / off according to the first light emission control signal from the light emission control line En connected to the gate terminal. The first light emission control transistor ME1 performs an operation. The first emission control transistor ME1 is turned off during a data programming period in which a data signal is applied to the driving transistor MD, and thus does not apply the first power voltage ELVDD to the driving transistor MD. Therefore, it is possible to prevent the leakage current from occurring in the driving transistor MD during the data programming period. The first power supply voltage ELVDD is a positive power supply voltage and is a power source for supplying power for generating a driving current generated in the driving transistor MD.

The organic EL element OLED is connected between the driving transistor MD and the second power line ELVSS, and emits light at a predetermined luminance according to the driving current generated by the driving transistor MD. The second power supply voltage ELVSS supplied from the second power line is lower than the first power supply voltage ELVDD and may be a negative power supply voltage that receives a driving current passing through the organic EL element OLED. have.

In addition, the second emission control element 116a is connected between the driving transistor MD and the organic EL element OLED and is turned on / off in accordance with the first emission control signal from the emission control line En connected to the gate terminal. The second light emission control transistor ME2 performs an off operation. The second light emission control transistor ME2 is turned off during a data programming period in which a data signal is applied to the driving transistor MD to transfer the driving current generated from the driving transistor MD to the organic EL element OLED. Do not supply Therefore, it is possible to prevent the leakage current from occurring in the driving transistor MD during the data programming period.

Gate terminals of the first and second emission control transistors ME1 and ME2 are commonly connected to the emission control line En. Accordingly, the first light emission control signal and the second light emission control signal are the same waveform signals, and the first and second light emission control transistors ME1 and ME2 are simultaneously turned off during the data programming period, thereby causing leakage current. Can be reduced, and a clear black gradation can be expressed.

As shown in FIG. 3, the pixel circuit of the organic electroluminescent display device according to the first embodiment of the present invention includes all transistors (switching transistor MS, driving transistor MD, first light emission control transistor ME1), and the like. The second light emission control transistor ME2 is formed of a P-type MOSFET.

The operation of the pixel circuit of FIG. 3 will be described in detail with reference to the timing diagram of FIG. 4.

4 is a timing diagram for describing an operation of a pixel circuit according to the first exemplary embodiment shown in FIG. 3.

3 and 4, the timing diagram of FIG. 4 is divided into a data programming section and a light emitting section. First, during a data programming period, when a low level scan signal is applied from the scan line Sn and a high level emission control signal is applied from the light emission control line En, the switching transistor MS is turned on. The first and second light emission control transistors ME1 and ME2 are turned off. Therefore, the data signal is applied to one electrode of the capacitor Cst and the gate terminal of the driving transistor MD through the switching transistor MS. In this case, since the first and second emission control transistors ME1 and ME2 are turned off, the driving transistor MD does not generate a driving current. Therefore, the organic EL element does not emit light in the data programming section.

Next, during a light emitting period, when a high level scan signal is applied from the scan line Sn and a low level light emission control signal is applied from the light emission control line En, the switching transistor MS is turned off. The first and second light emission control transistors ME1 and ME2 are turned on. The first power supply voltage ELVDD is applied to the source terminal of the driving transistor MD through the first light emission control transistor ME1, and the second light emission control transistor ME2 is connected to the drain terminal of the driving transistor MD and the organic light source. The anode terminal of the EL element is connected. Therefore, the driving current having a predetermined value is determined by the data signal Vdata applied to the gate terminal of the driving transistor MD in the data programming period and the first power voltage ELVDD applied to the source terminal in the light emitting period. (OLED) is supplied to emit light at a luminance corresponding to the driving current. The driving current supplied to the organic EL element OLED is defined as in Equation 1 below.

Here, ELVDD is the first power supply voltage, Vdata is the data voltage, Vth is the threshold voltage of the driving transistor MD, and k is a constant.

As described above, the pixel circuit according to the first embodiment of the present invention includes the first and second light emission control transistors ME1 and ME2 so that a leakage current flows through the pixel circuit during the data programming period. This can reliably prevent the organic EL element OLED from emitting light with indefinite luminance. In addition, it is possible to implement accurate black when expressing black gradation.

Example 2

FIG. 5 is a pixel circuit diagram illustrating a pixel circuit in FIG. 2 in detail. Referring to FIG.

Referring to FIG. 5, the pixel circuit according to the second embodiment includes a pixel driving circuit 112b, a first light emission control element 114b, a second light emission control element 116b, and an organic EL element OLED.

The pixel driving circuit 112b includes a switching transistor MS ', a capacitor Cst', and a driving transistor MD '.

The switching transistor MS 'is connected to the data line Dm and transmits a data signal supplied from the data line Dm in response to a scan signal applied from the scan line Sn.

The capacitor Cst 'is connected between the switching transistor MS' and the first power line ELVSS, and stores a data signal transmitted from the switching transistor MS 'for one frame.

The driving transistor MD 'is connected to a gate terminal of the capacitor Cst' and the switching transistor MS ', and generates a driving current corresponding to the data signal stored in the capacitor Cst'.

The first emission control element 114b is connected between the first power supply line ELVSS and the driving transistor MD and is turned on / off according to the first emission control signal from the emission control line En connected to the gate terminal. The first light emission control transistor ME1 ′ performs an operation. The first light emission control transistor ME1 ′ is turned off during a data programming period in which a data signal is applied to the driving transistor MD ′ so as not to apply the first power voltage ELVSS to the driving transistor MD ′. Do not. Therefore, it is possible to prevent the leakage current from occurring in the driving transistor MD during the data programming period. The first power supply voltage ELVSS is a voltage lower than the second power supply voltage ELVDD, which will be described later. The first power supply voltage ELVSS may be a negative power supply voltage that receives a driving current passing through the organic EL element OLED.

The organic EL element OLED is connected between the driving transistor MD 'and the second power supply line ELVDD, and emits light at a predetermined luminance according to a driving current generated by the driving transistor MD'. The second power supply voltage ELVDD supplied from the second power supply line is a positive power supply voltage, and is a power source for generating a driving current of the driving transistor MD '.

In addition, the second light emission control element 116b is connected between the driving transistor MD 'and the organic EL element OLED and according to the first light emission control signal from the light emission control line En connected to the gate terminal. The second light emission control transistor ME2 ′ performs an on / off operation. The second emission control transistor ME2 ′ is turned off during a data programming period in which a data signal is applied to the driving transistor MD ′ so that a driving current is not generated in the driving transistor MD ′. Therefore, it is possible to prevent the leakage current from occurring in the driving transistor MD 'during the data programming period.

Gate terminals of the first and second emission control transistors ME1 ′ and ME2 ′ are commonly connected to the emission control line En. Accordingly, the first emission control signal and the second emission control signal are the same waveform signals, and the first and second emission control transistors ME1 'and ME2' are simultaneously turned off during the data programming period to leak the leakage current. Current can be reduced, and a clear black gradation can be expressed.

As shown in FIG. 5, the pixel circuit of the organic electroluminescent display device according to the second embodiment of the present invention includes all transistors (switching transistor MS ', driving transistor MD', and first emission control transistor ME1). ') And the second light emission control transistor ME2' are constituted by an N-type MOSFET.

The operation of the pixel circuit of FIG. 5 will be described in detail with reference to the timing diagram of FIG. 6.

6 is a timing diagram for describing an operation of a pixel circuit according to the second exemplary embodiment illustrated in FIG. 5.

5 and 6, the timing diagram of FIG. 6 is divided into a data programming section and a light emitting section. First, during a data programming period, when a high level scan signal is applied from the scan line Sn and a low level emission control signal is applied from the light emission control line En, the switching transistor MS 'is turned on. The first and second light emission control transistors ME1 'and ME2' are turned off. Therefore, the data signal Vdata is applied to one electrode of the capacitor Cst and the gate terminal of the driving transistor MD 'through the switching transistor MS. At this time, since the first and second emission control transistors ME1 'and ME2' are turned off, the driving transistor MD 'does not generate a driving current. Therefore, the organic EL element OLED does not emit light in the data programming section.

Next, during the light emitting period, when the low level scan signal is applied from the scan line Sn and the high level light emission control signal is applied from the light emission control line En, the switching transistor MS 'is turned off. The first and second light emission control transistors ME1 'and ME2' are turned on. The first power supply voltage ELVSS is applied to the source terminal of the driving transistor MD 'through the first light emitting control transistor ME1', and the second light emitting control transistor ME2 'is the drain terminal of the driving transistor MD. And the cathode terminal of the organic EL element are connected. Therefore, a driving current corresponding to the data signal Vdata applied to the gate terminal of the driving transistor MD 'in the data programming period and the first power voltage ELVSS applied to the source terminal in the light emitting period is generated. The driving current supplied from the second power supply voltage ELVDD is supplied to the organic EL element OLED so that the organic EL element OLED emits light with luminance corresponding to the driving current. The driving current supplied to the organic EL element OLED is defined as shown in [Equation 1] of FIG. 4 described above.

As described above, the pixel circuit according to the second embodiment of the present invention includes the first and second emission control transistors ME1 ′ and ME2 ′, so that a leakage current is applied to the pixel circuit during the data programming period. Can be surely prevented from flowing, and the organic EL element OLED can be prevented from emitting light with indefinite luminance. In addition, it is possible to implement accurate black when expressing black gradation.

Although described above with reference to the preferred embodiments of the present invention, those skilled in the art various modifications and changes to the present invention without departing from the spirit and scope of the invention described in the claims below I can understand that you can.

The organic electroluminescent display device of the present invention having the above-described configuration blocks the leakage current by using the first and second light emitting control transistors as a source of driving current during the data programming period. Therefore, the luminance is accurately expressed in the light emitting section through the blocking of the leakage current.

In addition, since no leakage current is generated when expressing black gradations, a clear black state can be expressed.

In addition, since unnecessary leakage current is not generated, power consumption is reduced.

Claims (14)

A pixel driving circuit connected to a scan line and a data line, activated by a scan signal transmitted through the scan line, and configured to generate a driving current corresponding to the data signal transmitted through the data line; A first light emission control element connected between the pixel driving circuit and a first power line and configured to perform an on / off operation according to a first light emission control signal; And And an organic electroluminescent element connected between the pixel driver circuit and a second power supply line and configured to perform a light emitting operation according to the driving current. The method of claim 1, The first light emission control element, And the data signal is turned off while the data signal is applied to the pixel driving circuit. The method of claim 2, The pixel circuit, And a second light emission control element connected between the pixel driving circuit and the organic electroluminescent element and configured to perform an on / off operation according to a second light emission control signal. . The method of claim 3, wherein The second light emission control element, And the data signal is turned off while the data signal is applied to the pixel driving circuit. The method of claim 4, wherein And the first light emission control element and the second light emission control element are connected to a light emission control line in common. The method of claim 5, The pixel driving circuit, A switching transistor connected to the data line and configured to transfer the data signal in response to the scan signal; A capacitor connected between the switching transistor and a first power line and configured to store the data signal; And And a driving transistor connected between the first light emitting control element and the second light emitting control element and configured to generate the driving current corresponding to the data signal stored in the capacitor. Pixel circuit. The method of claim 6, And the switching transistor, the driving transistor, the first light emitting control element and the second light emitting element are P-type MOSFETs. The method of claim 7, wherein And wherein the first power line supplies a positive power supply voltage and the second power supply line supplies a negative power supply voltage. The method of claim 6, And the switching transistor, the driving transistor, the first light emitting control element and the second light emitting element are N-type MOSFETs. The method of claim 9, And wherein the first power supply line supplies a negative power supply voltage and the second power supply line supplies a positive power supply voltage. A switching transistor coupled to the data line and configured to transfer a data signal in response to a scan signal from the scan line; A capacitor connected between the switching transistor and a first power line and configured to store the data signal; A driving transistor having a gate terminal connected to the capacitor and the switching transistor and configured to generate a driving current corresponding to the data signal stored in the capacitor; A first light emission control transistor connected between a first terminal of the driving transistor and a first power line and configured to perform an on / off operation according to a first light emission control signal; A second emission control transistor connected to a second terminal of the driving transistor and configured to perform an on / off operation according to a second emission control signal; And And an organic electroluminescent element connected between the second emission control transistor and a second power supply line and configured to perform a light emitting operation according to the driving current. The method of claim 11, And a gate terminal of the first light emission control transistor and a gate terminal of the second light emission control transistor are commonly connected to a light emission control line. The method of claim 12, And the first light emission control transistor and the second light emission control transistor are turned off while the data signal is applied to the driving transistor. The method of claim 13, And the switching transistor, the driving transistor, the first light emission control transistor and the second light emission control transistor are MOSFETs of the same conduction type.

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