JP4956030B2 - Organic EL display device and driving method thereof - Google Patents

Description

  The present invention relates to a novel driving method of an organic EL (electroluminescence) display device and a driving circuit suitable for carrying out this method.

  An organic EL element using electroluminescence (hereinafter abbreviated as EL) of an organic material has an organic compound layer formed by laminating a light emitting layer made of organic molecules and a carrier transport layer between an upper electrode and a lower electrode. Is driven by a current flowing between the electrodes, and its luminance is almost exactly proportional to the flowing current (drive current). An organic EL display device configured by arranging organic EL elements in a matrix is excellent in color reproducibility and excellent in response to an input signal, and is particularly suitable for displaying a color moving image. Furthermore, it can emit light with high brightness and has a wide viewing angle, so it can be used in a wide range of environments. As a material for the organic compound layer, there are a low molecular weight material that can be vacuum-deposited and an oligomer or polymer material that can be applied by a spin coating method or an ink jet method. At present, there are many examples of using low molecular weight materials, but it is expected that there will be more examples of using oligomers and polymer materials suitable for large screen display in the future.

  Further, as a pixel driving method, a simple matrix type in which an organic EL element sandwiched between light is caused to flow directly between a striped lower electrode and an upper electrode extending in directions orthogonal to each other, and each organic EL Pixel circuits composed of thin film transistors (hereinafter abbreviated as TFTs) that drive the elements, capacitances, etc. are arranged in a matrix, send image signals to each pixel, and the pixel circuits hold and hold these signals There is an active matrix type in which an organic EL element emits light based on the signal and displays an image. The active matrix type is particularly suitable for a display device having a large screen, a high definition, and a large number of pixels, with little complication of image signals between pixels.

  The active matrix drive system is roughly classified into a voltage programming system and a current programming system. In the voltage programming method, a potential serving as an image signal is directly applied to the gate of the driving TFT and held. The current flowing through the driving TFT is controlled by the potential of the gate, but the correspondence between the two varies depending on the individual TFT, and often changes over time with the operating time. Therefore, in the voltage programming method, luminance unevenness is easily generated for each pixel and image burn-in is likely to occur. On the other hand, in the current programming method, a current as an image signal is actually supplied to the driving TFT of each pixel immediately before image display and the gate potential at that time is held. Less susceptible to change.

  FIG. 3 shows an example of a current programming drive circuit. This drive circuit includes a pixel circuit 100, a first power supply line 101, a second power supply line 102, a signal line 103, and a signal current source 104 connected to the signal line 103.

  The pixel circuit 100 includes an organic EL element 106 having one electrode connected to the first power supply line 101, a drive TFT 107 having a drain connected to the other electrode of the organic EL element 106, and a gate-source voltage of the drive TFT 107. Holding means 108, a first switch 109 provided between the gate and drain of the driving TFT 107, a second switch 110 provided between the source of the driving TFT 107 and the signal line 103, and a source of the driving TFT 107 And a third switch 111 provided between the second feeder line 102 and the second feeder line 102.

  In the programming period, that is, the signal writing period, the first switch 109 and the second switch 110 are closed, the third switch 111 is opened, and a signal current corresponding to the image signal is supplied from the signal current source 104 to the source of the driving TFT 107. To do. The source-gate voltage at this time is held in voltage holding means (capacitor in FIG. 3) 108 provided between the source and the gate.

  During the image display period, the first switch 109 and the second switch 110 are opened, and the third switch 111 is closed. A current flows through the driving TFT 107 in accordance with the voltage between the source and gate determined and held in the signal writing period, and the organic EL element 106 emits light.

  In FIG. 3, the driving TFT 107 is a p-channel type, and the drain terminal is connected to the anode of the organic EL, and a current flows from the drain toward the organic EL element. When the driving TFT 107 is an n-channel type, the positions of the source and drain may be switched, the drain terminal may be connected to the cathode of the organic EL, and current may flow from the organic EL element toward the drain. An example thereof is proposed in Patent Document 1.

The pixel circuit 100 is made of amorphous silicon or polysilicon on a glass substrate, but a metal oxide such as InGaZnO disclosed in Patent Document 2 can also be used.
US Pat. No. 6,229,506 International Publication No. 05/088726 Pamphlet

  Organic EL display devices are often used in mobile devices such as mobile phones and digital cameras, and it is necessary to reduce power consumption as much as possible. In order to reduce power consumption during the display period, it is effective to keep the power supply voltage (the voltage between the first and second feeder lines in FIG. 3) as low as possible.

  In a conventional current programming type pixel circuit, a driving TFT is diode-connected at the time of current programming, and a gate-source voltage is determined by applying a signal current from the outside. Then, this voltage is maintained even during the image display period, and the same current as the signal current is passed through the organic EL element.

  The gate-source voltage of the driving TFT and the current flowing through the organic EL element are maximized when the organic EL element emits light at the maximum luminance. Therefore, the power supply voltage needs to be a minimum voltage obtained by adding the voltage between the gate and source of the driving TFT at that time to the voltage between both ends of the organic EL element when light is emitted at the maximum luminance, and cannot be lower than that.

  In order to further reduce the power consumption of the organic EL display device, a drive circuit that replaces the conventional current programming method is required.

  Also, the voltage at the output terminal of the signal current source is the same as the above power supply voltage at the maximum luminance, so the power supply voltage for driving the signal current source must be higher. From the viewpoint of power saving, it is required to reduce the power supply voltage of the signal current source.

  The present invention has been made in order to improve the above-described problems, and provides a driving method for an organic EL display device that can display an accurate image with low power consumption and a driving circuit suitable for the driving method. Objective.

The present invention, first,
An organic EL element, a driving TFT, a capacitor provided between the gate and source of the driving TFT, first, second and third feeders for supplying a constant potential, and a signal to which a signal current source is connected An organic EL display device, wherein one electrode of the organic EL element is connected to the first power supply line, and the other electrode of the organic EL element is connected to a drain of the driving TFT Because
(1) connecting the gate of the driving TFT to the third power supply line, connecting the source of the driving TFT to the signal line, and flowing the signal current between the source and drain of the driving TFT;
(2) The step of separating the gate of the driving TFT from the third feeder and maintaining the voltage across the capacitor in the step of flowing the signal current;
(3) a step of separating the source of the driving TFT from the signal line; and (4) connecting the source of the driving TFT to the second power supply line, and connecting the first power supply line and the second power supply line. It has a step of passing a current through the driving TFT and the organic EL element in between.

The second aspect of the present invention is
An organic EL element, a driving TFT, a capacitor provided between the gate and source of the driving TFT, first, second and third feeders for supplying a constant potential, and a signal to which a signal current source is connected An organic EL display device in which one electrode of the organic EL element is connected to the first power supply line and the other electrode of the organic EL element is connected to a drain of the driving TFT. ,
A first switch provided between the gate of the driving TFT and the third power supply line; a second switch provided between a source of the driving TFT and the signal line; and the driving TFT. And a second switch provided between the source and the second feeder, and means for controlling opening and closing of the first to third switches.

  According to the drive circuit of the present invention, when an image signal is written to the drive TFT for controlling the current flowing through the organic EL element, a predetermined potential is applied to the gate from the outside, thereby allowing a signal current source and an image display time to be displayed. Even if the power supply voltage is lowered, the driving TFT can operate in the saturation region. Further, even when a TFT having incomplete saturation characteristics is used, the current flowing through the organic EL can be written with higher accuracy during the image display period. In this way, it operates with less power and realizes high-precision image display. In addition, since the circuit configuration is simple and compatible with various TFTs, it is easy to manufacture and contributes to the realization of a large-area and high-definition organic EL display device.

(Circuit operation of current programming method)
In order to compare the embodiment of the present invention with a conventional driving circuit, the conventional current programming circuit shown in FIG. 3 and its operation will be described first.

  FIG. 4 shows an entire drive circuit of a display device in which the pixel circuits 100 shown in FIG. 3 are arranged in a matrix. The internal structure of each pixel circuit 100 is as shown in FIG. 3, and details are omitted in FIG.

  A first power supply line 101 and a second power supply line 102 are connected to each pixel circuit 100 in common. Further, the pixel circuits 100 in the same column are connected to a common signal line 103. A common scanning line 114 is connected to the pixel circuits 100 in the same row. The first to third switches 109 to 111 are controlled to be opened and closed in accordance with the potential applied to the scanning line 114. A signal current source 104 is individually connected to the signal line 103 in each column.

  Each signal current source 104 is simultaneously input with an image signal 112 sent as a time series signal, but at a certain point in time, only the signal current source 104 in a specific column selected by a signal from the horizontal shift register 113 The image signal 112 at that time is captured. Further, the horizontal shift register 113 sequentially selects each signal current source 104, and an image signal is input to the signal current sources 104 in all columns.

  Each signal current source 104 outputs a unique signal current to the corresponding signal line 103. The pixel circuits 100 in the same column are commonly connected to the signal line 103, but at a certain point in time, the signal current on the signal line 103 is specified by the signal output from the vertical shift register 115 to the scanning line 114. It is input only to the pixel circuits 100 in this row. During this period, the pixel circuits 100 belonging to other rows in the same column are electrically disconnected from the signal line 103. Further, the pixel circuits 100 are sequentially selected in the vertical direction by the vertical shift register 114, and a signal current is input to the pixel circuits 100 in all rows.

  FIG. 5 is a chart showing an operation sequence of each switch for explaining the operation of the circuits of FIGS. 3 and 4. Each of 500 and 501 represents one frame period. When displaying 30 frames per second, one frame period is 33 msec. In the period 500 in FIG. 5, display with high luminance is performed, and in period 501, display with low luminance is performed. One frame period includes a signal writing period 502 and an image display period 503.

  Reference numerals 504 to 506 denote operation sequences of the first to third switches. Here, the level of 504 to 506 does not specifically indicate the level of the gate voltage, but merely indicates the distinction between high = open (on) and low = close (off).

  The state of the gate-source voltage of the driving TFT and the change of the driving current at this time are indicated by reference numerals 507 and 508. Here, the dotted line 507 indicates the source potential, and the solid line indicates the gate potential. Since the channel conductivity type of the driving TFT 107 is a p-channel, a driving current 508 flows from the source to the drain when the gate potential is lower than the source potential by a threshold voltage or more.

  6A and 6B show how the operating point of the circuit shown in FIG. 3 is determined. FIG. 6A shows the case of the maximum luminance display and FIG. 6B shows the case of the low luminance display. The horizontal axis 600 indicates voltage, and the vertical axis 601 indicates current. In FIG. 6, a plurality of voltage-current relationships are shown, and the voltage axis 600 represents the source potential and the reference drain potential. However, the direction in which the drain potential increases in the negative direction is taken as the positive direction. It may be considered that the origin is the source potential and the potential decreases as going to the right. The direction in which the current flows into the first feeder is positive.

Hereinafter, the case where the signal current 604 in FIG. 6A is maximum will be described, but FIG. 6B is also described in the same manner.
A curve 602 shows the drain current of the driving TFT 107 when the gate-source voltage is constant. As the drain potential increases in the negative direction, that is, as the source-drain voltage increases, the drain current also increases. However, when the source-drain voltage exceeds a certain voltage 613 (this is called a saturated drain voltage), the drain current is It becomes almost constant. In FIG. 6, it is assumed that the driving TFT 107 has a saturation characteristic in which the drain current is completely constant at a saturation drain voltage or higher.

  A dotted line 603 indicates the relationship between the source-drain voltage and the drain current in a state where the drain and gate of the driving TFT 107 are short-circuited (diode-connected).

  In a saturation region where the drain current is constant, a so-called pinch-off state occurs in which the drain potential is lower than the channel potential at the drain end of the channel and the PN junction is reverse biased. The saturated drain voltage 613 is a pinch-off start voltage, that is, a drain voltage when the channel potential at the drain end is equal to the drain potential. Since the gate potential is lower than the channel potential by the threshold voltage, the source-gate voltage providing the drain current characteristic 602 is higher than the saturation drain voltage 613 by the threshold voltage. Therefore, the drain voltage when the TFT is diode-connected is higher than the saturation drain voltage 612 by the threshold voltage. This is the first operating point 605.

  Reference numeral 611 in FIG. 6 represents the relationship between the voltage and current between the two electrodes of the organic EL element. Since the current becomes zero when the voltage between both electrodes of the organic EL element is zero, the position on the horizontal axis, that is, the drain potential with reference to the source potential is equal to the potential of the first feeder line. As the voltage between the electrodes of the organic EL element increases, the current flowing through the organic EL increases. This corresponds to the direction in which the drain potential increases from the first power supply line potential. In FIG. The organic EL element current increases as it moves.

  In the signal writing period 502, the first switch 109 and the second switch 110 in the circuit of FIG. 3 are closed and the third switch 111 is open, so that the characteristics of the driving TFT 107 are represented by a dotted line 603. . At this time, a signal current 604 represented by a horizontal line in FIG. 6 enters the source of the driving TFT 107 from the signal current source and flows as a drain current. Therefore, a characteristic curve (dotted line 603) when the diode is connected in FIG. The drain potential is determined at the intersection. This point 605 is referred to as a first operating point. Further, since the same drive current flows through the organic EL element 106 connected in series with the drive TFT 107, the organic EL characteristic at this time passes through the first operating point 605 as shown by a curve 607. The interelectrode voltage of the organic EL element 106 is represented by 608.

  The gate potential of the driving TFT 107 is determined by the source-gate voltage of the drain current characteristic curve 602 that passes through the first operating point 605. This source-gate voltage determines the charge of the capacitor 108 provided between the source and the gate.

  Further, since the voltage drop from the output terminal of the signal current source 104 to the first power supply line in the signal writing period 502 is the sum of the source-drain voltage 606 of the driving TFT and the organic EL both-ends voltage 608, the arrow in FIG. The size is represented by 609. This is also the output terminal voltage of the signal current source 104 with respect to the first feeder line potential.

  In the image display period 503, the first switch 109 and the second switch 110 are opened, and the third switch 111 is closed. The source potential of the driving TFT 107 becomes the potential of the second power supply line 102. Since the first switch is opened, the capacitor 108 holds the charge determined in the signal writing period, and the source-gate voltage of the driving TFT 107 is also held.

  Therefore, the drain current characteristic of the driving TFT 107 represented by the curve 602 in FIG. 6 does not change between the signal writing period 502 and the image display period 503 and is represented by the same curve.

  Since the diode connection of the driving TFT 107 is eliminated, the drain voltage is not determined by the characteristics 603 when the diode is connected. Instead, since the source potential is fixed at the second feeder line potential, the position of the current zero in the current-voltage characteristic 611 of the organic EL is the voltage between the first feeder line and the second feeder line (in FIG. 6). The operating point is determined by the intersection (hereinafter referred to as the second operating point 612) of the characteristic 611 of the fixed organic EL element and the drain current characteristic 602 of the driving TFT. .

  When the driving TFT 107 has a complete saturation characteristic, the operating point is changed from 605 to 612 if the voltage between the first and second feeder lines (hereinafter referred to as the power supply voltage) is set to the signal source voltage 609 or higher. Even if it moves, the drive current does not change.

  The case where the signal current 604 in FIG. 6A is maximum has been described above. Even if the signal current is lowered to the level shown in FIG. 6B, the basic operation is the same as in FIG.

  In FIG. 6 (b), the numbers in the figure are basically matched with (a), and when the values and shapes are different from (a), dashes are added to the numbers. In (b), since the signal current 604 ′ is small, the first operating point 605 ′ is on the low potential side and the second operating point 611 ′ is on the high potential side, the movement of the operating point 605 ′ → 612 ′ is ( Although larger than a), the drive current does not change because it is within the saturation characteristic region.

  In the above description, it is assumed that the driving TFT has perfect saturation characteristics as shown in FIG.

  In the future, with the increase in area of display devices, the use of semiconductors that are easy to manufacture even in large areas such as amorphous silicon, ZnO, metal oxides such as InGaZnO disclosed in Patent Document 2, and organic semiconductors such as polythiophene and pentacene will be used. It is expected to increase. These semiconductor TFTs often have incomplete saturation characteristics. Even in the case of polysilicon TFTs that have been widely used in the past, saturation characteristics tend to be incomplete if the channel length is shortened as the definition becomes higher.

  A case where the saturation characteristic of the driving TFT 107 is incomplete will be described with reference to FIG. As in FIG. 6, FIG. 7A shows the case of display with the highest luminance, and FIG. 7B shows the case of display with low luminance. Since the saturation characteristics are incomplete, the drain currents 702 and 702 ′ are not constant even when the drain-source voltage is increased to the saturation voltage or higher, and the currents at the first operating point 705 and the second operating point 712 are different. This means that the current at the time of programming is different from the current at the time of display, so that accurate display cannot be performed. In the case of FIG. 7B, the movement of the operating point increases from 705 'to 712', which is particularly remarkable.

In general, in order to allow a significant drain current (Ids) to flow through a TFT, the gate-source voltage (Vgs) needs to be equal to or higher than the threshold voltage (Vth) of the driving TFT. In the case of a low-temperature polysilicon TFT, Vth is often about 1 to 3V. FIG. 8 shows the drain-source voltage (Vds) dependence of the Ids of a TFT with perfect saturation characteristics under the condition that Vgs is constant (where Vgs> Vth).

In the non-saturated region 815 (Vds ≦ Vgs−Vth), Ids is generally compared to Vds. Ids = k {2 (Vgs−Vth) −Vds} · Vds (Equation 1)
It is known to increase according to. Here, k is a constant determined by the structure of the TFT and the characteristics of the semiconductor used. Expression 1 is a quadratic curve having a maximum at a saturated drain voltage 813 (= Vgs−Vth), and the maximum value of Ids (saturated drain current) is Ids = k (Vgs−Vth) ² (Expression 2)
It becomes. Reference numeral 817 denotes a locus of the saturation drain voltage 813 when Vgs is changed. The saturated drain voltage is about 5 to 10 V under the condition of performing high luminance display. In the saturation region 816 (Vds> Vgs−Vth), Ids is a constant value given by Equation 2, and depends on Vgs but does not depend on Vds.

  Therefore, in order to make the Ids of the image display period coincide with the predetermined signal current 604, the operating point may be set so as to come to the saturation region 816. The minimum Vds necessary for that purpose may be lower than Vgs by Vth according to the definition of the saturation drain voltage 813. However, in the case of diode connection, the first switch 109 is closed and Vds = Vgs. Therefore, 805 (first operating point) that is larger than the saturated drain voltage 813 by Vth must be added as Vds. Therefore, the voltage-current characteristic of the diode-connected driving TFT is a curve 803 obtained by shifting the saturation drain voltage locus 817 to the high voltage side by Vth. On the other hand, if Vds is lowered to the saturation drain voltage 813 with the diode connected, Vgs is also lowered and Ids becomes smaller than the signal current 814, so that the signal cannot be written accurately.

(Improved drive circuit operation)
If Vgs can be set independently of Vds, Vds can be lowered to the saturation drain voltage 813 while maintaining Vgs. In FIG. 6, in order for the signal current source 104 to flow the signal current 604, the signal current source 104 has a voltage equal to (the voltage drop 608 between the electrodes of the driving TFT Vds 606 and the electrode of the organic EL element 106) when the signal current flows. Although it is necessary to output, if Vds can be lowered, the maximum output voltage required for the signal current source 104 can be lowered.

  A circuit suitable for carrying out the present invention based on the above concept is shown in FIG. The difference between this circuit and the circuit of FIG. 3 is that a third power supply line 105 is provided to supply a potential independent of the drain to the gate of the driving TFT 107. The first switch 109 is provided between the gate of the driving TFT 107 and the third power supply line 105. Further, the circuit of FIG. 1 is assumed to operate according to the sequence shown in FIG. Although the driving TFT 107 in FIG. 1 is depicted as a p-channel, the gist of the present invention can be similarly achieved even when the driving TFT has an n-channel as shown in Example 2 (FIG. 12) described below.

  An overall view corresponding to the drive circuit of FIG. 1 is shown in FIG. FIG. 2 differs from the overall view of FIG. 4 in that a third feeder 105 is provided. In FIG. 2, only one type of scanning line 114 is drawn for each row. However, in some cases, multiple types of scanning lines are provided for each row, and a plurality of pixel circuits belonging to the same row are provided. It may be connected to a kind of scanning line. Specific examples will be described in detail in Example 3 (FIG. 13) and Example 4 (FIG. 15).

  FIG. 9 shows the operation of the circuit of FIG. 1 for (a) high luminance display and (b) low luminance display. First, description will be made in the case of (a). In the circuit of FIG. 1 also, in the signal writing period 502, the first switch 109 and the second switch 110 are closed, and the third switch 111 is open. Since the potential of the third power supply line 105 is applied to the gate of the driving TFT 107, the difference between this and the voltage output from the signal current source 104 becomes Vgs. Since the Ids of the TFT is determined by Vgs in the saturation region, the signal current source 104 outputs a voltage such that Ids is equal to the signal current 904. At this time, Vgs is written in the voltage holding means 108. After the first switch 109 is opened, the written potential changes in the potential of the first to third power supply lines, or causes a potential drop in the vicinity of the pixel circuit due to wiring resistance in the display device. Even if it is not affected.

  A voltage drop 908 corresponding to the signal current 904 occurs between the electrodes of the organic EL element 106. When displaying the maximum brightness, the voltage drop 908 is typically about 3 to 5V. Therefore, (the output voltage 909 of the signal current source 104 -the voltage drop 908 of the organic EL element) becomes Vds of the driving TFT 107. However, in order to ensure the operation in the saturation region, Vds needs to be larger than the saturation drain voltage 913, and FIG. 9 shows a case where Vds is matched with the saturation drain voltage 913. This is a typical first operating point 905 of the present invention. In the circuit of FIG. 1, unlike the circuit of FIG. 3, Vds is independent of Vgs, so that Vgs can be adjusted by the potential of the third power supply line 105. In order to realize the state of FIG. 9, the potential of the third power supply line is changed from the potential of the first power supply line 101 by (the voltage drop of the organic EL element−the threshold voltage of the driving TFT) of the second power supply line. The value is close to the potential.

  Since the corresponding saturation drain voltage increases as the signal current 904 increases, the maximum saturation drain voltage to be assumed is a value corresponding to the maximum signal current to be assumed. Assuming that 904 in FIG. 9A is the maximum signal current to be assumed, 909 which is (driving TFT saturation drain voltage + voltage drop in the organic EL element) is the maximum to be assumed as the output terminal voltage of the signal current source 104. Value. Since this value is lower than the first operating point 605 of the conventional current programming circuit shown in FIG. 3 by the threshold voltage Vth of the driving TFT 107, according to the present invention, the maximum voltage at the output terminal of the signal current source 104 is made higher than that of the conventional driving TFT. The threshold voltage Vth can be lowered.

  For example, when the saturation drain voltage of the driving TFT = 6V, the threshold voltage = 2V, and the voltage drop at the organic EL element = 4V, the signal current source conventionally needs to output 12 V at the maximum, but in the typical example of the present invention, 10V is good. However, even if the first operating point is set between the saturated drain voltage 905 and the first operating point 605 in FIG. 6, there is an improvement effect, and this is included for the purpose of absorbing the characteristic variation of the organic EL element. It may be set to an appropriate point. That is, the maximum value of the output terminal voltage is not less than (driving TFT saturated drain voltage + voltage drop at the organic EL element) when the maximum signal current flows and (driving TFT saturated drain voltage + voltage drop at the organic EL element + It may be less than the threshold voltage. Correspondingly, the potential of the third feeder line is not less than (voltage drop in the organic EL element−threshold voltage of the driving TFT) and less than (voltage drop of the organic EL element) when the maximum signal current flows. What should I do?

  Further, when the power supply voltage 910 is matched with 909 during image display, power consumption during image display can be reduced. In this case, in the case of the maximum luminance display (a), the second operating point 912 of the image display period coincides with 905, and the operating point does not move. In the case of the low luminance display (b), the movement of the operating point from 905 'to 912' remains, but the movement amount becomes smaller than the conventional movement from 605 'to 612' shown in FIG. 6 (b). Therefore, even when the saturation characteristics of the driving TFT are incomplete as shown in FIG. 10, the operating point 1005 and 1012 coincide with each other in the display with the highest luminance (a), and no movement occurs, and the driving current coincides with the signal current 1004. Even in the low-brightness display (b), the movement of the operating point from 1005 ′ to 1012 ′ is less than the movement from 705 ′ to 712 ′ in FIG. 7 of the conventional current programming method, and the drive current error is reduced and saturated. The accuracy of image display is improved when a driving TFT having incomplete characteristics is used.

  In this way, the gate voltage at the time of programming is set to a fixed potential regardless of the signal current, and the gate-source voltage at the time of programming is held by disconnecting the gate from the fixed voltage power source in this state. At the time of display, by switching the source terminal from the signal current source to the fixed power supply voltage of the second feeder line, a current corresponding to the gate-source voltage flows from the source to the drain terminal, and the organic EL element can be driven. According to this method, the fixed potential of the second feeder line can be set as a voltage obtained by adding the voltage across the organic EL to the saturation drain voltage of the TFT at that time with respect to the maximum signal current in the range in which the signal current changes. This power supply voltage is lowered by a threshold voltage value as compared with the conventional driving method in which the TFT is diode-connected during current programming, and as a result, power consumption is reduced.

  FIG. 11 shows an example in which the circuit of the present invention is realized by a low-temperature polysilicon CMOS. In the case of a display device in which a drive circuit is provided on the substrate side and an organic EL element 106 is stacked thereon, the drain of the drive TFT 107 is contacted with the anode of the organic EL element 106 as a pixel electrode. Manufacturing is easy if metal, a transparent conductive film, or the like is formed on the entire surface. Further, when formed in this order, it is known that the organic EL element 106 exhibits good carrier injection characteristics, and the voltage drop in the organic EL element is lowered. Therefore, the maximum output voltage and power supply voltage of the signal current source are further increased. Can be lowered.

  The driving TFT is a p-channel TFT, the first switch 109 and the second switch 110 are p-channel TFTs, and the third switch 111 is an n-channel TFT. The gates of the TFTs are connected to a common scanning line 114. When a low level signal is applied to the scanning line 114 from the vertical shift register 115, the first switch 109 and the second switch 110 are closed, and the third switch 111 is opened. Further, when a high level signal is applied, the operation of all the switches is inverted, so that the sequence shown in FIG. 5 can be realized with only one scanning line 114.

  FIG. 12 shows another example in which the circuit of the present invention is realized by a low-temperature polysilicon CMOS. Here, the drain of the driving TFT 107 is used as a pixel electrode to be in contact with the cathode of the organic EL element 106, and the first power supply line 101 can be easily manufactured by forming a metal, a transparent conductive film or the like on the entire surface.

  As the driving TFT, the first switch 109 and the second switch 110 are n-channel TFTs, and the third switch 111 is a p-channel TFT. The gates of the TFTs are connected to a common scanning line 114. When a signal having a level higher than that of the vertical shift register 115 is applied to the scanning line 114, the first switch 109 and the second switch 110 are closed, and the third switch 111 is opened. Further, when a low level signal is applied, the operation of all the switches is inverted, so that the sequence shown in FIG. 5 can be realized with only one scanning line 114.

  In the circuits of the first and second embodiments, the number of circuit elements such as TFTs is increased, although the number of the third feeder lines 105 is increased as compared with the conventional current programming circuit disclosed in US Pat. No. 6,229,506. It is easy to manufacture and can be said to be a practical circuit.

  In the circuit of FIG. 1, capacitance is widely used as the voltage holding means 108. In the sequence of FIG. 5, since the first switch 109 is closed during the signal writing period 502, a current flows into the voltage holding means 108, and a gate for properly flowing the signal current between the drain and source of the driving TFT. The source-to-source voltage is written. The written potential needs to be held reliably in the image display period 503.

  In the image display period 503, since the first switch 109 is opened, the written potential does not normally change. However, before the first switch 109 is opened, the third switch 111 is closed and the source of the driving TFT 107 is the second power supply line. If it is connected to 102, a current flows into the voltage holding means 108 during that time, and the potential written correctly may fluctuate. The first switch 109 and the third switch 111 often have different channel conductivity type specifications as shown in FIG. In addition, since the second power supply line 102 and the third power supply line 105 have different wiring capacities, it may be assumed that the switch is not ideally switched.

  FIG. 13 shows an example of a circuit for completely eliminating such fears and surely switching from the signal writing period 502 to the image display period 503. FIG. 13 is based on the circuit of FIG. 11 and the circuit elements such as TFTs are the same as those of FIG. 11, but the scanning line 117 of the first switch 109 is replaced with the scanning line of the second switch 110 and the third switch 111. 116 and independent. Therefore, as shown in FIG. 14, the switching 504 ′ of the first switch 109 from the signal writing period 502 to the image display period 503 is changed from the switching 505 of the second switch 110 and the switching 506 of the third switch 111. It can be preceded by a significant period Δt. This prevents an erroneous current from flowing into the voltage holding means 108 and ensures that a correct drive current flows during the image display period 503.

  If CMOS is used like the circuits shown in FIGS. 11 to 13, there is a merit that a plurality of switches can be driven in reverse phases by a signal from one scanning line, but in the case of polysilicon, the manufacturing process is To be complicated. Furthermore, when only n-channel TFTs can provide good characteristics, such as metal oxide semiconductors such as amorphous silicon, ZnO, and InGaZnO, and when only p-channel TFTs can provide good characteristics, such as organic semiconductors Can not respond.

  FIG. 15 shows an example in which the driving TFT 107 and the first switch 109 to the third switch 111 are all configured by p-channel TFTs. Here, the gates of the first switch 109 and the second switch 110 are connected to the first scanning line 118. On the other hand, the gate of the third switch 111 is separately connected to the second scanning line 119. Therefore, the sequence shown in FIG. 5 can be realized by adding signals having opposite phases to the scanning lines 118 and 119.

  Further, as shown in FIG. 16, the signal of the first scanning line 118 is switched prior to the switching of the second scanning line 119 by a significant time Δt, whereby the circuit of the third embodiment shown in FIG. In the same manner as described above, the effect of accurately holding the signal written in the voltage holding means 108 can be obtained.

FIG. 3 is a diagram illustrating a driving circuit of an organic EL display device according to the present invention, focusing on a pixel circuit. BRIEF DESCRIPTION OF THE DRAWINGS The figure explaining the drive circuit of the organic electroluminescent display apparatus of this invention entirely. The figure explaining the drive circuit of the conventional organic electroluminescence display centering on a pixel circuit. The figure explaining the drive circuit of the conventional organic EL display apparatus entirely. The figure explaining the drive sequence in the drive circuit of this invention and the conventional organic electroluminescent display apparatus. The figure explaining the operation | movement of the drive circuit in the conventional organic electroluminescence display which uses the drive TFT with a perfect saturation characteristic. The figure explaining operation | movement of the drive circuit in the conventional organic electroluminescence display which uses the drive TFT with incomplete saturation characteristics. The figure explaining the operating point in diode connection. The figure explaining operation | movement of the drive circuit in the organic electroluminescent display device of this invention using TFT with perfect saturation characteristics. The figure explaining operation | movement of the drive circuit in the organic electroluminescent display device of this invention using TFT with an incomplete saturation characteristic. 1 is a drive circuit of an organic EL display device according to a first embodiment of the present invention. 4 is a drive circuit of an organic EL display device according to a second embodiment of the present invention. 4 is a drive circuit of an organic EL display device according to a third embodiment of the present invention. The drive timing chart of the organic electroluminescence display in the 3rd Example of this invention. 9 is a drive circuit of an organic EL display device according to a fourth embodiment of the present invention. The drive timing chart of the organic electroluminescence display in the 4th Example of this invention.

Explanation of symbols

100 pixel circuit 101, 102, 105 feeder line 103 signal line 104 signal current source 106 organic EL element 107 driving TFT
108 Voltage holding means 109, 110, 111 Switch 114 Scan line

Claims (3)

An organic EL element; a p-channel type driving TFT; a capacitor provided between the gate and source of the driving TFT; first, second and third feeders each supplying a constant potential; and a signal current source. An organic EL display having a connected signal line, one electrode of the organic EL element connected to the first power supply line, and the other electrode of the organic EL element connected to a drain of the driving TFT A method for driving an apparatus, comprising:
(1) connecting the gate of the driving TFT to the third power supply line, connecting the source of the driving TFT to the signal line, and causing a signal current to flow between the source and drain of the driving TFT;
(2) The step of separating the gate of the driving TFT from the third feeder and maintaining the voltage across the capacitor in the step of flowing the signal current;
(3) a step of separating the source of the driving TFT from the signal line; and (4) connecting the source of the driving TFT to the second power supply line, and connecting the first power supply line and the second power supply line. a step of passing a current through said drive TFT and the organic EL element possess between,
The voltage between the first and third feeder lines is determined when the signal current source supplies a signal current that maximizes the luminance of the organic EL display device to the signal line (both electrodes of the organic EL element). The voltage between the electrodes + the threshold voltage of the driving TFT))) or higher (the voltage between both electrodes of the organic EL element) is less than (the voltage between the two electrodes of the organic EL element) . When the signal current source supplies a signal current that maximizes the luminance of the organic EL display device to the signal line, the voltage between the first and second feeder lines (in the step (1)) The voltage between the source and drain of the driving TFT + the voltage between both electrodes of the organic EL element in the same step) and (the voltage between the source and drain of the driving TFT in the step (1) + in the same step) 2. The driving method of the organic EL display device according to claim 1, wherein the driving voltage is less than a voltage between both electrodes of the organic EL element + a threshold voltage of the driving TFT. When the signal current source supplies a signal current that maximizes the luminance of the organic EL display device to the signal line, the potential of the signal line in the step (1) is the potential of the second feeder line. The driving method of the organic EL display device according to claim 2, wherein

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