JP5121124B2 - Organic EL pixel circuit - Google Patents

Description

  The present invention relates to an organic EL pixel circuit that can perform threshold compensation of a driving transistor that controls a driving current supplied to an organic EL element.

  An EL display device using an electroluminescence (hereinafter referred to as EL) element, which is a self-luminous element, as a light-emitting element for each pixel is advantageous in that it is self-luminous and thin and consumes less power. It attracts attention as a display device that replaces a display device such as a device (LCD) or CRT.

  In particular, an active matrix EL display device in which a switching element such as a thin film transistor (TFT) for individually controlling an EL element is provided in each pixel and the EL element is controlled for each pixel enables high-definition display.

  In this active matrix EL display device, a plurality of gate lines extend in a row (horizontal) direction on a substrate, a plurality of data lines and a power supply line extend in a column (vertical) direction, and each pixel is an organic EL. An element, a selection TFT, a driving TFT, and a storage capacitor are provided. The selection TFT is turned on by selecting the gate line, the data voltage (voltage video signal) on the data line is charged to the holding capacitor, and the driving TFT is turned on with this voltage, and the power from the power supply line is supplied to the organic EL element It is flowing.

Special Table 2002-514320

  However, in such a pixel circuit, if the threshold voltage of the driving TFTs of the pixel circuits arranged in a matrix varies, there is a problem that the luminance varies and the display quality deteriorates. It is difficult to make the characteristics of the TFTs constituting the pixel circuit of the entire display panel the same, and it is difficult to prevent the on / off threshold value from varying.

  Therefore, it is desirable to prevent the influence on the display of the variation in threshold value in the driving TFT.

  Here, various proposals have conventionally been made on a circuit for preventing the influence on the fluctuation of the threshold value of the TFT (for example, Patent Document 1).

  However, this proposal requires a circuit for compensating for threshold fluctuation. Therefore, when such a circuit is used, there is a problem that the number of elements of the pixel circuit increases and the aperture ratio becomes small. In addition, when a circuit for compensation is added, there is a problem that a peripheral circuit for driving the pixel circuit needs to be changed.

  The present invention provides a pixel circuit that can effectively compensate for fluctuations in the threshold voltage of a driving transistor.

The present invention includes a drive transistor that causes a drive current corresponding to the potential of the control terminal to flow from a power source to the organic EL element, a drive control transistor that is arranged in series with the drive transistor and the organic EL element, and that turns the drive current on and off. A short-circuit transistor that controls whether the drive transistor is diode-connected, a selection transistor that controls whether a data signal from a data line is supplied to a control terminal of the drive transistor, the selection transistor, and the drive A capacitor inserted between the control ends of the transistor, a potential control transistor for turning on and off the connection between the capacitor and the power supply side of the capacitor, and the drive transistor includes an n-channel transistor. It is a transistor.
The drive control transistor, the short-circuit transistor, the selection transistor, and the potential control transistor are preferably n-channel transistors.
In addition, it is preferable that the drive control transistor is disposed between the drive transistor and a power source.

  As described above, according to the present invention, the control terminal voltage of the drive transistor is set according to the data voltage and the threshold voltage of the drive transistor by turning on the short-circuit transistor while the selection transistor is turned on. can do. Therefore, a driving current corresponding to the data voltage can be supplied to the organic EL element regardless of the fluctuation of the threshold voltage of the driving transistor. In addition, since the driving transistor is an n-channel transistor, the characteristics of the transistor are excellent, and the active layer can be formed of amorphous silicon. Further, even if a capacitor is inserted between the control terminal of the selection transistor and the driving transistor, a data signal having the same polarity as that in the case where the conventional selection transistor is directly connected to the control terminal of the p-channel driving transistor can be used. .

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a configuration of a pixel circuit according to the embodiment. The data line DL extends in the vertical direction and supplies a data signal (data voltage Vsig) about the display luminance of the pixel to the pixel circuit. One data line DL is provided for one column of pixels, and sequentially supplies the data voltage Vsig of the pixel to the pixels in the vertical direction.

  The data line DL is connected to the drain of an n-channel selection transistor T1, and the source of the selection transistor T1 is connected to one end of a capacitor Cs. The gate of the selection transistor T1 is connected to a gate line GL extending in the horizontal direction. The gate line GL is connected to the gate of the selection transistor T1 of each pixel circuit in the horizontal direction.

  Further, similarly to the gate line GL, a capacitor set line CS extending in the horizontal direction is provided, and the gate of the n-channel potential control transistor T2 is connected to the capacitor set line CS. The source of the potential control transistor T2 is connected to the power supply line PVdd, and the drain is connected to the capacitor Cs and the source of the selection transistor T1. The power supply line PVdd extends in the vertical direction, and supplies the power supply voltage PVdd to each pixel in the vertical direction.

  The other end of the capacitor Cs is connected to the gate of the n-channel drive transistor T4. The drain of the drive transistor T4 is connected to the source of the n-channel drive control transistor T5, and the drain is connected to the anode of the organic EL element EL. The drain of the drive control transistor T5 is connected to the power supply line PVdd, and the gate is connected to the light emission set line ES extending in the horizontal direction. The cathode of the organic EL element EL is connected to a low voltage cathode power source CV.

  Furthermore, the drain of the n-channel short-circuit transistor T3 is connected to the gate of the drive transistor T4, the source of the short-circuit transistor T3 is connected to the drain of the drive transistor T4, and the gate is connected to the gate line GL. .

  Thus, in this embodiment, the data line DL and the power supply line PVdd are arranged in the vertical direction, and the gate line GL and the light emission set line ES are arranged in the horizontal direction.

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

  As shown in FIG. 2, this pixel circuit (including the data line DL) has one horizontal period according to the state (H level, L level) of the gate line GL, the light emission set line ES, and the capacitor set line CS. (I) Data set (GL = H level, ES = L level), (ii) Precharge (GL = H level, ES = H level), (iii) Reset (GL = H level, ES = L level), (Iv) There are four states of potential fixation (GL = L level, ES = L level) and (v) light emission (GL = L level, ES = H level), which are repeated. Note that in this writing period, the capacitor set line CS is at the L level.

  Further, as shown in the drawing, the data in the data line DL is sequentially set in the data line DL of each column of the horizontal line when the writing target line is selected. That is, for each data line DL, data for each pixel is output in a dot sequential manner. Then, after data is set in all the data lines DL, the data (data voltage) is taken into each pixel circuit.

  The various voltages are preferably set as follows. The power supply line PVdd is PVdd, the light emission set line ES is H level = PVdd, L level = VVBB, the gate line GL is H level = VVDD, L level = VVBB, and the capacitor set line CS is H level = VVDD, L level = VVBB, It is preferable to set the cathode power supply CV = CV, PVdd = 8V, VVDD = 10V, VVBB = −2V, and CV = −2V.

  Hereinafter, the write operation will be described.

(I) Data set (GL = H level, ES = L level, CS = L level)
First, the light emission set line ES is set to L level to cut off the current from the power supply line PVdd, and the capacitance set line CS is set to L level to lower the voltage at the connection point between the selection transistor T1 and the capacitor CS. In this state, the gate line GL is set to the H level, and the data voltage of each pixel corresponding to the data line DL is sequentially set. Accordingly, the voltage set in the data line DL is applied to the capacitor CS. Note that data voltages are set to the data lines DL in a dot-sequential manner, but each data line DL is connected to a capacitor, and the once applied data voltage is held.

(Ii) Precharge After the data set to each data line DL is completed, the light emission set line ES is set to H level. As a result, the drain of the drive transistor T4 is connected to the power supply line PVdd, and the short-circuit transistor T3 is turned on, so that the gate of the drive transistor T4 is charged close to the power supply potential PVdd.

(Iii) Reset Thereafter, the light emission set line ES is returned to the L level, and the drive transistor T4 is disconnected from the power source PVdd. As a result, as shown in FIG. 3, the gate potential of the driving transistor T4 is lowered from the source potential to a potential offset by the threshold voltage Vtn. On the other hand, since the drain potential of the transistor T4 becomes the threshold voltage Ve of the organic EL element EL, the gate voltage Vg of the drive transistor T4 becomes Ve + Vtn. At this time, the data line DL side of the capacitor Cs is at the data voltage Vsig of the data line DL.

(Iv) Potential Fixation Next, the gate line GL is set to L level, and the selection transistor T1 and the short-circuit transistor T3 are turned off. As a result, the gate voltage Vg of the driving transistor T4 is fixed to Ve + Vtn. At this time, the voltage on the opposite side of the capacitor Cs is Vsig, and the capacitor Cs is charged with a voltage of Vsig−Vg = Vsig− (Ve + Vtn).

(V) Light emission After the potential is fixed, the light emission set line ES and the capacitance set line CS are set to the H level. As a result, as shown in FIG. 4, the voltage on the selection transistor T1 side of the capacitor Cs becomes PVdd, and therefore the gate voltage Vg of the drive transistor T4 = PVdd−Vsig + Ve + Vtn. Since the drive control transistor T5 is also turned on, the drive transistor T4 passes a current corresponding to the gate-source voltage Vgs, which is supplied to the organic EL element EL. Here, the source potential of the driving transistor T4 is Vs = Ve + I · R. Here, I is a current value flowing through the organic EL element EL, and R is an on-resistance of the organic EL element EL. Therefore, the gate-source voltage of the driving transistor T4 is Vgs = Vg−Vs = PVdd−Vsig + Vtn−I · R.

The on-resistance R of the organic EL element EL can be considerably reduced by increasing the area of the organic EL element and making the organic layer of the organic EL element thin. Since the drain current I in the driving transistor T4 is determined by I = (1/2) β (Vgs−Vtn) ² , the drain current I corresponds to the data voltage Vsig regardless of the threshold voltage of the driving transistor T4. A current can be passed through the drive transistor T4. Here, β is the drive transistor T4 amplification factor, which is expressed by β = μεGW / GL, μ is the carrier mobility, ε is the dielectric constant, GW is the gate width, and GL is the gate length.

  In particular, the gate-source voltage Vgs of the drive transistor T4 is determined based on a voltage obtained by subtracting the data voltage Vsig from PVdd. Therefore, the data voltage Vsig can be the same as the data voltage Vsig directly supplied to the gate of the p-channel driving transistor. Therefore, the circuit for driving the data line DL can be configured similarly to the conventional one.

  In the above description, basically only the operation for one pixel has been described. Actually, the display panel has pixels arranged in a matrix, and for each of them, the data voltage Vsig corresponding to the corresponding luminance signal is supplied to cause each organic EL element to emit light. That is, as shown in FIG. 5, the display panel is provided with a horizontal switch circuit HSR and a vertical switch VSR, and the data line DL, gate line GL, other light emission set line ES, and capacitor set line are output by these outputs. The state such as CS is controlled. In particular, one gate line GL is associated with each pixel in the horizontal direction, and the gate lines GL are sequentially activated one by one by the vertical switch VSR. Next, in the period until one half of the horizontal period in which one gate line GL is activated, the data voltage is supplied to all the data lines DL in a dot-sequential manner by the horizontal switch HSR and is set to all the data lines DL. The Then, data is simultaneously written into each pixel circuit for one horizontal line corresponding to the data voltage of each data line DL (the data voltage taken into each pixel is determined). Each pixel circuit emits light according to the data voltage written until after one vertical period.

  Next, a data writing procedure for each pixel in one horizontal line will be described with reference to FIG.

  First, the dot sequential data voltage Vsig is written to all the data lines DL after the L level of the enable signal ENB indicating the start of one horizontal period. That is, a capacitor or the like is connected to the data line DL, and the data voltage Vsig is held in the data line DL by setting a voltage signal. Therefore, the data voltage Vsig for the pixels in each column is sequentially set to the corresponding data line DL, thereby setting the data voltage Vsig to all the data lines DL.

  Then, at the stage when this data setting is completed, the light emission set line ES is precharged to the H level, and then the light emission set line ES is returned to the L level to perform the reset. Then, by returning the gate line GL to the L level, the charging voltage of the capacitor Cs in the pixel circuit is fixed, and thereafter, by setting the capacitance set line CS to the H level, the gate of the driving transistor T4 is shifted, and the horizontal Light is emitted in all pixels of the line.

  In this way, a normal video signal (data voltage Vsig) can be sequentially written into the data line DL, and this can be set in the pixel circuit to emit light.

"Effect of the embodiment"
As shown in FIG. 1, it is preferable that all transistors (thin film transistors: TFTs) used in the pixel circuit are n-channel transistors. The characteristics of the n-channel transistor are superior to those of the p-channel transistor. For this reason, even if the active layer of the transistor is made of amorphous silicon, sufficient operation is possible. Therefore, the yield of the active layer can be improved by eliminating the need for polysilicon processing.
Even if the capacitor Cs is inserted between the gates of the selection transistor T1 and the driving transistor T4, a data signal having the same polarity as that in the case where the conventional selection transistor is directly connected to the control terminal of the p-channel driving transistor should be used. Can do.

"Modification"
FIG. 7 shows a configuration of a pixel circuit of a modified example. In this example, one end (drain) of the potential control transistor T2 is connected to the light emission set line ES instead of the power supply line PVdd. With this configuration, the same operation as in the example of FIG. 1 can be obtained. Further, although the power supply is connected to the same PVdd, the light emission set line ES is a line different from the power supply line PVdd, and there is no voltage fluctuation compared to the power supply line PVdd that drives and supplies the organic EL element EL. Stable operation can be obtained. That is, when setting the voltage Vn by the potential control transistor T2, it is not affected by the voltage drop of the power supply line PVdd.

It is a figure which shows the structure of the pixel circuit which concerns on embodiment. It is a chart figure explaining operation. It is a figure explaining writing of data. It is a figure explaining the time of light emission. It is a figure which shows the whole structure of a panel. It is a figure which shows the example of timing of a data set. It is a figure which shows the structure of the pixel circuit of a modification.

Explanation of symbols

  Cs capacitor, CS capacitance set line, CV cathode power supply, DL data line, EL organic EL element, ENB enable signal, ES light emission set line, GL gate line, HSR horizontal switch, PVdd power supply voltage, T1 selection transistor, T2 potential control transistor , T3 short-circuit transistor, T4 drive transistor, T5 drive control transistor, VSR vertical switch, Vg drive transistor gate voltage, Vsig data voltage.

Claims (2)

A driving transistor connected in series between the power source and the organic EL element, and causing a driving current corresponding to the potential of the control terminal to flow from the power source to the organic EL element;
A drive control transistor arranged in series between the drive transistor and the power source to turn on and off the drive current;
One end connected between the drive transistor and the drive control transistor, the other end connected to the control end of the drive transistor, and a short-circuit transistor for controlling whether the drive transistor is diode-connected,
A selection transistor for controlling whether to supply a data signal from a data line to the control terminal of the driving transistor;
A capacitor inserted between the selection transistor and the control terminal of the drive transistor;
A potential control transistor for turning on and off a connection between the selection transistor side of the capacitor and the power source;
The driving transistor is an n-channel transistor;
By turning on the selection transistor and the short-circuit transistor, the data signal is supplied to the control terminal of the drive transistor,
While the short-circuit transistor is turned on, the drive control transistor is turned on, then the drive control transistor is turned off, and then the short-circuit transistor and the selection transistor are turned off, and then the potential control transistor and An organic EL pixel circuit, wherein the drive control transistor is turned on. The circuit of claim 1, wherein
An organic EL pixel circuit, wherein the drive control transistor, the short-circuit transistor, the selection transistor, and the potential control transistor are n-channel transistors.

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