JP2007206273A - Image display device and driving method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the write efficiency of a pixel circuit in an image display device. <P>SOLUTION: A driving method of an image display device including a light emitting means, a driver means for controlling light emission of the light emitting means, a first capacitive element connected to a control terminal of the driver means, and a second capacitive element which is connected to a pixel signal line for supplying an image signal potential corresponding to light emission luminance of the light emitting means and is connected to the first capacitive element, includes steps of; detecting a threshold voltage of the driver means and causing the first capacitive element to hold a voltage corresponding to the threshold voltage; causing the second capacitive element to hold an image signal voltage by writing a voltage corresponding to an image signal into the second capacitive element through a signal line; and applying an addition voltage of the threshold voltage held in the first capacitive element and the image signal voltage held in the second capacitive element, between the control terminal of the driver means and a first terminal or a second terminal by electrically connecting the first capacitive element and the second capacitive element in series. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image display device such as an organic EL (Electroluminescence) display device and a driving method thereof.

  Conventionally, an image display device using an organic EL element having a function of generating light by recombination of holes and electrons injected into a light emitting layer has been proposed.

  In this type of image display device, for example, a thin film transistor (hereinafter referred to as “TFT”) formed of amorphous silicon, polycrystalline silicon, or the like, or an organic light emitting diode (Organic Light Emitting Diode): (Hereinafter referred to as “OLED”) constitutes each pixel, and the luminance of each pixel is controlled by setting an appropriate current value to each pixel.

  For example, in Non-Patent Document 1 below, a current-driven light-emitting element OLED and an OLED flow for the purpose of improving luminance unevenness caused by variations in threshold voltage among drive transistors provided in each pixel. An image that includes a plurality of pixels that are arranged in series with drive transistors such as TFTs that control the current, for example, and that controls the current that flows through the light-emitting elements of the pixels based on the threshold voltage of the drive transistor that is detected in advance. A display device is disclosed.

  On the other hand, in the image display device that detects the threshold voltage in advance as described above, it is necessary to provide a threshold voltage detection step for detecting the threshold voltage and a step for writing the image signal as the preliminary steps for causing the OLED to emit light. Therefore, it has been pointed out that sometimes it takes a long time to complete these steps, so that sufficient light emission time cannot be secured, and the refresh rate has to be lowered. (For example, Patent Document 1 below). Note that Patent Document 1 discloses various configuration examples of an image display device in which the time until a process of writing an image signal is shortened for the purpose of suppressing a decrease in refresh rate.

JP 2004-341598 A S. Ono, et al. (2003). Pixel Circuit for a-Si AM-OLED. Proceedings of IDW '03, pp. 255-258.

  By the way, the current flowing through the driving TFT is proportional to the square of the difference between the difference between the gate electrode potential with respect to the source electrode potential (hereinafter referred to as “Vgs”) and the threshold voltage specific to the driving TFT (hereinafter referred to as “Vth”). It is known. Therefore, in order to obtain a clear image, it is necessary to increase this Vgs as much as possible.

  On the other hand, an index called “Vgs swing width” (= ΔVgs), which is a difference value of Vgs applied to the driving TFT when the light emission luminance is the highest level and the lowest level, and the “Vgs swing width” and the light emission “Write efficiency” expressed as a ratio of an index (ΔVdata) called “pixel signal line width” that is a difference in potential supplied to the pixel signal line between when the luminance is at the highest level and when the luminance is at the lowest level There is an index called (= ΔVgs / ΔVdata). Between these indexes, since the Vgs amplitude can be increased if the pixel signal line amplitude is increased, the latter is the case from the viewpoint of miniaturizing the drive IC and ensuring the ease of design. Write efficiency is an important indicator. Therefore, in order to ensure the ease of design in the pixel circuit as described above, it is necessary to increase the writing efficiency.

  For example, Patent Document 1 and Non-Patent Document 1 show a pixel circuit of an image display device, but an image display device having higher writing efficiency than such a pixel circuit has been demanded.

  The present invention has been made in view of the above, and an object of the present invention is to provide an image display apparatus and a driving method thereof that effectively improve writing efficiency.

  In order to solve the above-described problems and achieve the object, a driving method of an image display device according to the present invention includes a light emitting means, a control terminal, a first terminal, and a second terminal, and the control terminal, A driver means for controlling light emission of the light emitting means in accordance with a potential difference applied between one end of the first terminal or the second terminal; and directly or indirectly on the control terminal of the driver means. Connected directly or indirectly to the first capacitor element to be connected and a signal line for supplying an image signal potential corresponding to the light emission luminance of the light emitting means, and directly or indirectly connected to the first capacitor element. A method of driving an image display apparatus comprising: a second capacitive element that detects a threshold voltage of the driver means and holds the voltage corresponding to the threshold voltage in the first capacitive element; signal Writing a corresponding voltage to the second capacitive element via the signal line to cause the second capacitive element to hold the image signal voltage; and electrically connecting the first capacitive element and the second capacitive element By connecting them in series, the voltage of the threshold voltage held in the first capacitive element and the voltage of the image signal voltage held in the second capacitive element are added, and the added voltage is added to the driver means of the driver means. Applying between the control terminal and either the first terminal or the second terminal.

  The image display device driving method according to the next invention is characterized in that, in the above invention, the method further comprises the step of discharging the charge accumulated in the light emitting means.

  In the image display apparatus driving method according to the next invention, in the above invention, the discharge of the electric charge accumulated in the light emitting means may be performed after the first capacitive element has held the threshold voltage of the driver means. It is performed before the second capacitance element holds the image signal voltage.

  The image display device driving method according to the next invention is characterized in that, in the above invention, the first capacitor element is in a floating state while the image signal voltage is written to the second capacitor element. To do.

  In the image display device driving method according to the next invention, in the above invention, the first capacitive element and the second capacitive element are electrically in parallel while the threshold voltage of the driver means is detected. It is characterized by being connected to.

  An image display apparatus according to the next invention has a light emitting means, a control terminal, a first terminal, and a second terminal, and the control terminal and either end of the first terminal or the second terminal. By controlling the current flowing between the first terminal and the second terminal according to the potential difference, the driver means for controlling the light emission of the light emitting means and the control terminal of the driver means directly or indirectly A first capacitive element connected and holding a threshold voltage of the driver means; and a second capacitive element to which an image signal voltage corresponding to the light emission luminance of the light emitting means is supplied via a signal line, The two-capacitance element includes a second capacitance element that is directly or indirectly connected to the second electrode of the first capacitance element during a light emission period of the light-emitting means.

  The image display device according to the next invention is characterized in that, in the above invention, the first capacitor element is in a floating state while an image signal voltage is supplied to the second capacitor element.

  The image display device according to the next invention is characterized in that, in the above invention, the first capacitive element is electrically connected to the control electrode of the driver means during the light emission period of the light emitting means. To do.

  Further, in the image display device according to the next invention, in the above invention, the second capacitor element is electrically connected to either the first terminal or the second terminal of the driver means. It is characterized by.

  The image display device according to the next invention is the potential line in which the potential is held substantially constant during the writing period in which the image data is written to the second capacitor element via the signal line. The second capacitor element is electrically connected to the potential line.

  The image display device according to the next invention is characterized in that in the above invention, the image display device further includes a switching element for electrically connecting the pair of electrodes of the second capacitor element for a predetermined period. .

  Further, the image display device according to the next invention further includes a switching element that electrically connects between the pair of electrodes of the second capacitor element for a predetermined period in the above invention, and the threshold voltage detecting means The control terminal and the control terminal of the switching element are electrically connected to a common control line.

  According to the present invention, the second capacitive element different from the first capacitive element that holds the threshold voltage of the driver means is provided, and when the image signal voltage is written to the second capacitive element, the other capacitance including the parasitic capacitance is provided. Since the component is not connected in series to the second capacitor element, an effect that the write efficiency can be effectively improved is obtained.

  Before describing the image display device of the present invention, first, the writing efficiency of the image display devices disclosed in Patent Document 1 and Non-Patent Document 1 will be considered.

In Non-Patent Document 1, the capacitance Coled of the OLED and the auxiliary capacitor Cs into which the image signal is written are electrically connected between the image signal line and the power supply line (or the same potential line) during the writing period. Thus, these capacities greatly affect the write efficiency. When the parasitic capacitance of the driving TFT and other switching transistors and the voltage drop due to the OLED are ignored, the writing efficiency η1 of the image display device (pixel circuit) shown in Non-Patent Document 1 is expressed by the following equation.
η1 = Coled / (Coled + Cs) (1)

Similarly, when the writing efficiency η2 of the image display device (pixel circuit) disclosed in Patent Document 1 is calculated, the following equation is obtained.
η2 = Cs / (Cs + Cs2) (2)
Here, Cs2 in Expression (2) is a capacitance connected (always) between the gate electrode and the source electrode of the driving TFT capacitor as disclosed in Patent Document 1. In Equation (2), Coled that exists in Equation (1) is not included. However, in the pixel circuit of Patent Document 1, the image signal line and the OLED are not connected during the image signal writing period. This is because they are not electrically connected.

  As is clear from the equation (1), in the pixel circuit of Non-Patent Document 1, if the value of the auxiliary capacitor Cs can be reduced, the writing efficiency can be brought close to a value close to 1. However, it is difficult to use an auxiliary capacitor Cs having an extremely small value. This is because if the value of the auxiliary capacitor Cs is reduced, the variation in the manufacturing process increases, and it becomes easy to be affected by the parasitic capacitance of the drive transistor and other switching transistors, and the luminance unevenness between the pixels in each pixel of the pixel circuit. It will be bigger. Therefore, in the pixel circuit of Non-Patent Document 1, it is difficult to increase its write efficiency (for example, a value close to 1) by selecting circuit parameters (for example, selecting a capacitance value).

  This also applies to the pixel circuit of Patent Document 1, and it is difficult to extremely reduce any value of Cs and Cs2 shown in Expression (2). Therefore, even in the pixel circuit of Patent Document 1, it is difficult to increase the writing efficiency by selecting circuit parameters.

  Therefore, the inventor of the present application has devised an image display device and a driving method thereof that can make the writing efficiency higher than those of Patent Document 1 and Non-Patent Document 1. Embodiments of an image display apparatus and a driving method thereof according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited by the following embodiment.

(Embodiment 1)
FIG. 1 is a diagram illustrating a configuration of a pixel circuit corresponding to one pixel of the image display device according to the first embodiment of the present invention. The pixel circuit shown in the figure includes an organic EL element OLED that is a light emitting means, a drive transistor Td that is a driver means for driving the organic EL element OLED, and a threshold voltage detection transistor Tth that is mainly used when detecting a threshold voltage. A capacitor Cs that holds a threshold voltage as a first capacitor, a capacitor Cs2 that holds an image signal voltage as a second capacitor, and a switching transistor Ts that controls application of the image signal voltage. Has been. The configuration shown in FIG. 1 shows a circuit configuration of one pixel of a pixel circuit that controls an organic EL element or the like, and the image display device has a configuration in which a plurality of these pixel circuits are arranged in a matrix. is doing.

  In FIG. 1, the drive transistor Td has a function of controlling the amount of current flowing through the organic EL element OLED in accordance with a potential difference applied between a gate electrode that is a control electrode and a source electrode that is a first electrode. . The threshold voltage detection transistor Tth has a function of electrically connecting the gate electrode of the drive transistor Td and the drain electrode as the second electrode when the threshold voltage detection transistor Tth is turned on. It has a function of detecting the threshold voltage Vth of the drive transistor Td by flowing current from the gate electrode of the drive transistor Td to the drain electrode until the potential difference between the gate electrode and the source electrode reaches the threshold voltage Vth of the drive transistor Td. is doing. Note that the functions of the first electrode and the second electrode described above are interchangeable. That is, the drive transistor Td can also control the amount of current flowing through the organic EL element OLED according to the potential difference applied between the gate electrode as the control electrode and the drain electrode as the second electrode.

  The organic EL element OLED is an element having a characteristic of emitting light by causing a current to flow between the anode electrode and the source electrode when a potential difference equal to or higher than the threshold voltage of the organic EL element OLED is generated between the anode electrode and the cathode electrode. . Specifically, the organic EL element OLED includes an anode electrode layer and a cathode electrode layer formed of Al, Cu, ITO (Indium Tin Oxide), and the like, and phthalocyanine between the anode electrode layer and the cathode electrode layer. , And a structure including at least a light emitting layer formed of an organic material such as a trisaluminum complex, benzoquinolinolato, or beryllium complex, and holes and electrons injected into the light emitting layer recombine with each other for light emission. This has the function of generating light.

  The drive transistor Td, the threshold voltage detection transistor Tth, and the switching transistor Ts are, for example, thin film transistors. In each drawing referred to below, the type (n-type or p-type) of the channel of each thin film transistor is not clearly shown, but it is either n-type or p-type. In this embodiment, it is an n-type transistor.

  The power line 10 supplies a predetermined voltage to the drive transistor Td and the switching transistor Ts. The Tth control line 11 supplies a signal for ON / OFF control of the threshold voltage detection transistor Tth. The scanning line 13 supplies a signal for on / off control of the switching transistor Ts. The image signal line 14 supplies an image signal voltage to the capacitor Cs2.

  In FIG. 1, as a configuration for supplying a predetermined voltage to the organic EL element OLED, the organic EL element OLED is arranged between the high-potential power line 10 and the low-potential ground line. Alternatively, the low potential side may be a power source line and the high potential side may be a ground line to be a fixed potential, or both may be driven.

  Next, the operation of the first embodiment will be described with reference to FIGS. Here, FIG. 2 is a sequence diagram for explaining a general operation of the pixel circuit shown in FIG. 1, and FIGS. 3 to 7 show a preparation period (FIG. 3) divided into five periods. It is a figure for demonstrating operation | movement of each area of a threshold voltage detection period (FIG. 4), OLED initialization / Cs2 initialization period (FIG. 5), writing period (FIG. 6), and light emission period (FIG. 7). Each of FIGS. 3 to 7 is shown with the addition of the organic EL element capacitance Coled inherent to the organic EL element OLED. The operations described below are performed under the control of a control unit (not shown).

(Preparation period)
The operation during the preparation period will be described with reference to FIGS. In the preparation period, the power supply line 10 is at a low potential (−Vp), the Tth control line 11 is at a low potential (VgL), the scanning line 13 is at a high potential (VgH), and the image signal line 14 is at a high potential (for example, the maximum potential of the image signal). (VdH: 10 V or 15 V). By this control, as shown in FIG. 3, the switching transistor Ts is turned on, the threshold voltage detection transistor Tth is turned off, and the drive transistor Td is turned on. As a result, a current flows through a path of the ground line → the drive transistor Td → the organic EL element capacitor Coled → the power supply line 10 and electric charges are accumulated in the organic EL element capacitor Coled. The reason for accumulating charges in the organic EL element capacitor Coled during this preparation period is that the current (hereinafter referred to as “Ids”) between the drain electrode and the source electrode of the organic EL element OLED during the threshold voltage detection period described later. Is detected (that is, the potential difference between the gate electrode and the source electrode of the driving transistor Td is equal to the threshold voltage), the organic EL element capacitor Coled is connected to the drain electrode and the source electrode of the driving transistor Td. This is to act as a supply source of a current flowing between the two.

(Threshold voltage detection period)
Next, the operation during the threshold voltage detection period will be described with reference to FIGS. In the threshold voltage detection period, the Tth control line 11 is set to the high potential (VgH), and the power supply line 10 is set to the zero potential slightly after the timing when the Tth control line 11 becomes the high potential. The potential (VgL) is maintained. Note that the image signal line 14 is set to zero potential immediately before the threshold voltage detection period starts, and the potential is maintained. By this control, as shown in FIG. 4, the threshold voltage detection transistor Tth is turned on, and the gate electrode and the drain electrode of the drive transistor Td are connected. As a result, the charges accumulated in the organic EL element capacitor Coled and the capacitor Cs are discharged until the potential of the gate electrode with respect to the source electrode of the driving transistor Td reaches the threshold voltage Vth, and current flows through the path of the driving transistor Td → the ground line. . When the potential difference between the gate electrode and the source electrode of the drive transistor Td reaches the threshold voltage Vth of the drive transistor Td, the drive transistor Td is turned off. At this time, the threshold voltage Vth is generated at both ends of the capacitor Cs. If the potential of the image signal line is x (x ≠ 0 V), the capacitor Cs is connected to the gate electrode of the drive transistor Td of the capacitor Cs. The potential on the current side is “Vth−x”.

(OLED / Cs2 initialization period)
Next, the operation in the OLED / Cs2 initialization period will be described with reference to FIGS. In the OLED / Cs2 initialization period, the zero potential of the power supply line 10 and the high potential (VgH) of the scanning line 13 are maintained, while the Tth control line 11 is set to the low potential (VgL). The image signal line 14 is supplied with, for example, the maximum potential (VdH) of the image signal. At this time, as shown in FIG. 5, the drive transistor Td is turned on again, a current flows through the path of the organic EL element OLED → the drive transistor Td → the ground line, and the electric charge remaining in the organic EL element capacitor Coled is discharged. By this operation, the influence on the light emission due to the residual charge of the organic EL element OLED itself is avoided.

  In the OLED / Cs2 initialization period, as shown in FIG. 2, the image signal line 14 is changed from the maximum potential (VdH) to the zero potential, and the scanning line 13 is set after the image signal line 14 is set to the zero potential. After a predetermined time, the high potential (VgH) is changed to the low potential (VgL). This control is for discharging the charge accumulated in the capacitor Cs at the time of initialization of the organic EL element OLED described above, and the influence on the light emission by the remaining charge of the capacitor Cs is avoided.

(Writing period)
Next, the operation in the writing period will be described with reference to FIGS. In the writing period, the zero potential of the power supply line 10 and the low potential (VgL) of the Tth control line 11 are maintained, while the scanning signal by the scanning line 13 and the signal potential by the image signal line 14 (predetermined according to the image signal). Level) is supplied. That is, in the writing process according to the present embodiment, sequential scanning is performed for each scanning line, not for all pixels at once. By this control, as shown in FIG. 6, the switching transistor Ts is turned on, a current flows through the path of the switching transistor Ts → capacitance Cs2 → ground line, and the voltage corresponding to the image signal is held in the capacitor Cs2. The shaded portion in the figure indicates that a predetermined voltage corresponding to the image signal is applied.

(Light emission period)
Finally, the operation during the light emission period will be described with reference to FIGS. In the light emission period, the power supply line 10 is set to the power supply potential (VDD) and the image signal line 14 is set to the zero potential, while the low potential (VgL) of the Tth control line 11 and the low potential (VgL) of the scanning line 13 are maintained. The At this time, the capacitor Cs that holds the threshold voltage of the driving transistor Td and the capacitor Cs2 that holds the image signal voltage corresponding to the image signal are connected in series, and the sum of both voltages is the gate electrode and the source electrode of the driving transistor Td. As shown in FIG. 7, the driving transistor Td is turned on, a current flows through the path of the organic EL element OLED → the driving transistor Td → the ground line, and the organic EL element OLED emits light.

The current (Ids) flowing through the drive transistor Td is a constant β determined from the structure and material of the drive transistor Td, a potential difference Vgs between the gate electrode and the source electrode with reference to the source electrode of the drive transistor Td, and the drive transistor Td. Can be expressed by the following equation.
Ids = (β / 2) ・ (Vgs−Vth) ² (3)

On the other hand, in this light emission period, both the threshold voltage (Vth) held in the capacitor Cs and the image signal voltage (Vdata) held in the capacitor Cs2 are added and applied, so the current Ids in the above equation (3) is applied. Is
Ids = (β / 2) ・ (Vth + Vdata−Vth) ²
= (Β / 2) ・ (Vdata) ² (4)
Theoretically, a current independent of the threshold voltage Vth can be obtained. Further, since the luminous intensity of the organic EL element OLED is proportional to the current flowing through the organic EL element OLED, the luminous intensity that is not affected by the fluctuation of the threshold voltage Vth can be obtained.

  Next, the writing efficiency of the pixel circuit shown in FIG. 1 will be described in comparison with the writing efficiency in Non-Patent Document 1 and Patent Document 1 described above.

First, the writing efficiency in Non-Patent Document 1 and Patent Document 1 described above will be described again.
Write efficiency in Non-Patent Document 1 η1 = Coled / (Coled + Cs) (1)
Write efficiency in Patent Document 1 η2 = Cs / (Cs + Cs2) (2)

  Here, in the pixel circuit of Non-Patent Document 1, in the image signal writing period, the capacitor Cs and the organic EL element capacitor Coled are connected in series, and a writing voltage is applied to both ends of both capacitors while emitting light. In the period, since only the voltage written in the capacitor Cs is applied to the driving transistor, the writing efficiency shown in the above equation (1) is the upper limit.

  In the pixel circuit of Patent Document 1, the capacitor Cs and the other capacitor Cs2 are connected in series in the image signal writing period, and a writing voltage is applied to both ends of both capacitors. Similar to Non-Patent Document 1, since only the voltage written in the capacitor Cs is applied to the drive transistor, the write efficiency shown in the above equation (2) is the upper limit.

  On the other hand, in the pixel circuit shown in FIG. 1, in the image signal writing period, the image signal supplied from the image signal line 14 is applied and held only to the capacitor Cs2. In addition, in the light emission period, an addition voltage of the threshold voltage held in the capacitor Cs and the image signal voltage held in the capacitor Cs2 is applied to the driving transistor, so that the writing efficiency η3 is theoretically “1”. . In the pixel circuit shown in FIG. 1, the switching transistor Ts is interposed between the image signal line 14 and the capacitor Cs2. However, the switching transistor Ts is on while the image signal is applied, and during this time, the switching transistor Ts is switched. Since the potential difference between both ends of the transistor Ts is substantially zero (substantially the same potential), the parasitic capacitance of the switching transistor Ts does not become a problem.

  As described above, the image display device (pixel circuit) according to the first embodiment includes the second capacitor element different from the first capacitor element that holds the threshold voltage (Vth) of the drive transistor Td, and this second capacitor element. In addition, when the image signal voltage (Vdata) is written to the second capacitor element, other capacitance components including parasitic capacitance are not connected in series to the second capacitor element. Thus, the writing efficiency can be effectively improved.

In the driving method of the image display device (pixel circuit) according to the first embodiment,
(A) Step of holding the threshold voltage (Vth) of the driving transistor Td in the capacitor Cs (b) Step of holding the image signal voltage (Vdata) in the capacitor Cs2 (c) Threshold voltage (Vth) held in the capacitor Cs Since the step of adding both of the image signal voltage (Vdata) held in the capacitor Cs2 and applying between the gate electrode and the source electrode of the driving transistor Td is included, effectively improving the writing efficiency, It is possible to obtain luminous intensity (brightness) that is not affected by fluctuations in threshold voltage.

  In the pixel circuit shown in FIG. 1, one end of the capacitor Cs2 is always connected to the source electrode (first electrode) of the drive transistor Td, but one end of the capacitor Cs2 is at least in the light emission period. In this case, it may be connected to the source electrode (first electrode) or the drain electrode (second electrode) of the driving transistor Td. In particular, even during the writing period of the image signal, it is not necessary to be connected to the source electrode or the drain electrode of the drive transistor Td, and a substantially constant potential is maintained during this writing period. It may be configured to be electrically connected to a constant potential line such as the Tth control line 11 or a ground line.

  The constant potential mentioned above does not have to be a constant potential in all of the preparation period, the threshold voltage detection period, the writing period, and the light emission period, and the constant potential is maintained at least for a predetermined period. That's fine.

  Further, the meaning of the constant potential does not need to be a constant potential in a strict sense, and a predetermined potential fluctuation can be allowed within the scope of the purpose of obtaining an effect of increasing the writing efficiency by the additional capacitor Cs2. .

  In the pixel circuit shown in FIG. 1, the other end of the capacitor Cs2 is always connected to the gate electrode (control electrode) of the drive transistor Td via the capacitor Cs. May be configured to be electrically connected to the gate electrode (control electrode) of the drive transistor Td at least during the light emission period.

  In the pixel circuit shown in FIG. 1, one end of the capacitor Cs is connected to a connection point between the gate electrode (control electrode) of the driving transistor Td and the threshold voltage detection transistor, and the other end is connected to one end of the capacitor Cs2 and the switching transistor. Although it is configured so that it is always connected to the connection point with Ts, at least in the writing period, one end or the other end of the capacitor Cs is in a floating state (a state in which neither end is electrically connected and is floating). And one end or the other end of the light emitting period is connected to the gate electrode (control electrode), the source electrode (first electrode), or the drain electrode (second electrode) of the driving transistor Td. It only has to be.

(Embodiment 2)
FIG. 8 is a diagram illustrating a configuration of a pixel circuit corresponding to one pixel of the image display device according to the second embodiment of the present invention. The pixel circuit shown in the figure is configured to include the switching transistor Tm connected to both ends of the capacitor Cs2 and controlled to be turned on / off by a Tth control line in the pixel circuit shown in FIG. Yes. Other configurations are the same as or equivalent to those of the pixel circuit according to the first embodiment shown in FIG. 1, and the same or equivalent parts are denoted by the same reference numerals.

  In FIG. 8, as a configuration for supplying a predetermined voltage to the organic EL element OLED, the organic EL element OLED is arranged between the high-potential power line 10 and the low-potential ground line. Alternatively, the low potential side may be a power source line and the high potential side may be a ground line to be a fixed potential, or both may be driven.

  In the configuration shown in FIG. 8, the Tth control line 11 is configured to control both the threshold voltage detection transistor and the switching transistor Tm, but is connected to another control line that controls the switching transistor Tm. May be.

  Next, the operation of the second embodiment will be described with reference to FIGS. Here, FIG. 9 is a sequence diagram for explaining a general operation of the pixel circuit shown in FIG. 8, and FIGS. 10 to 14 show a preparation period (FIG. 10) divided into five periods. It is a figure for demonstrating the operation | movement of each area | region of Cs / Cs2 initialization period (FIG. 11), threshold voltage detection period (FIG. 12), writing / OLED initialization period (FIG. 13), and light emission period (FIG. 14). . In addition, in each drawing of FIGS. 10-14, what added the organic EL element capacity | capacitance Coled which the organic EL element OLED has inherently is added and shown. The operations described below are performed under the control of a control unit (not shown).

(Preparation period)
The operation during the preparation period will be described with reference to FIGS. In the preparation period, the power supply line 10 is set to a low potential (−Vp), the Tth control line 11 is set to a low potential (VgL), and the scanning line 13 is set to a high potential (VgH). The potential of the image signal line 14 is arbitrary. This arbitrary meaning means that the image signal line 14 may have any potential, and the degree of freedom of design or control increases. By this control, as shown in FIG. 10, the threshold voltage detection transistor Tth is turned off and the drive transistor Td is turned on. As a result, a current flows through a path of the ground line → the drive transistor Td → the organic EL element capacitor Coled → the power supply line 10 and electric charges are accumulated in the organic EL element capacitor Coled. In this preparation period, the reason why charges are accumulated in the organic EL element capacitor Coled is that the current between the drain electrode and the source electrode of the organic EL element OLED during the threshold voltage detection period described later, as in the first embodiment. When detecting a state in which (Ids) stops flowing (that is, a state in which the potential difference between the gate electrode and the source electrode of the driving transistor Td is equal to the threshold voltage), the organic EL element capacitance Coled is connected to the drain electrode of the driving transistor Td. This is to act as a supply source of a current flowing between the source electrode.

(Cs / Cs2 initialization period)
Next, the operation during the Cs / Cs2 initialization period will be described with reference to FIGS. In the Cs / Cs2 initialization period, the zero potential of the power supply line 10 and the high potential (VgH) of the scanning line 13 are maintained, while the Tth control line 11 is set to the high potential (VgL). Further, the potential of the image signal line 14 is arbitrary. By this control, as shown in FIG. 11, the driving transistor Td is kept on, the gate electrode and the drain electrode of the driving transistor Td are connected, and a part of the charge of the capacitor Cs is discharged. Further, when the switching transistor Tm is turned on, the charge remaining in the capacitor Cs2 is also discharged. Even if the driving transistor Td is kept on, the electric power stored in the organic EL element capacitor Coled is retained because the potential of the power supply line 10 is kept at a low potential (−Vp). Is done.

(Threshold voltage detection period)
Next, the operation during the threshold voltage detection period will be described with reference to FIGS. In the threshold voltage detection period, the power supply line 10 is set to zero potential, while the high potential (VgH) of the Tth control line 11 is maintained. Note that the potential of the image signal line 14 is arbitrary as in the preparation period and the Cs / Cs2 initialization period. By this control, as shown in FIG. 12, the threshold voltage detection transistor Tth is kept on, and the organic EL element until the potential of the gate electrode with respect to the source electrode of the drive transistor Td reaches the threshold voltage Vth of the drive transistor Td. The electric charge accumulated in the capacitor Coled is discharged, and a current flows through the path of the drive transistor Td → the ground line. Then, when the potential difference between the gate electrode and the source electrode of the drive transistor Td reaches the threshold voltage Vth of the drive transistor Td, the drive transistor Td is turned off. Note that, since the switching Tm continues to be on, no charge is accumulated in the capacitor Cs2.

(Write / OLED initialization period)
Next, the operation in the write / OLED initialization period will be described with reference to FIGS. In the writing / OLED initialization period, the zero potential of the power supply line 10 is maintained, while the Tth control line 11 is set to a low potential (VgL). Further, a scanning signal from the scanning line 13 and a signal potential (a predetermined level corresponding to the image signal) from the image signal line 14 are supplied. By this control, as shown in FIG. 13, the switching transistor Ts is turned on, a current flows through the path of the switching transistor Ts → the capacitor Cs2 → the ground line, and the voltage corresponding to the image signal is held in the capacitor Cs2. Further, when the signal voltage corresponding to the image signal is written in the capacitor Cs2, the charge accumulated in the organic EL element capacitor Coled is discharged. That is, during this period, the OLED initialization process is performed together with the image signal voltage writing process.

(Light emission period)
Finally, the operation during the light emission period will be described with reference to FIGS. In the light emission period, the power supply line 10 is set to the power supply potential (VDD), while the low potential (VgL) of the Tth control line 11 and the low potential (VgL) of the scanning line 13 are maintained. Further, the potential of the image signal line 14 is arbitrary as in the preparation period, the Cs / Cs2 initialization period, and the threshold voltage detection period. At this time, the capacitor Cs that holds the threshold voltage of the driving transistor Td and the capacitor Cs2 that holds the image signal voltage corresponding to the image signal are connected in series, and the sum of both voltages is the gate electrode and the source electrode of the driving transistor Td. As shown in FIG. 14, the driving transistor Td is turned on, a current flows through the path of the organic EL element OLED → the driving transistor Td → the ground line, and the organic EL element OLED emits light.

  Therefore, in the same way as in the first embodiment, a current that does not depend on the threshold voltage Vth is obtained in the drive transistor Td, and a luminous intensity that suppresses the influence of fluctuations in the threshold voltage Vth is obtained.

  The pixel circuit shown in FIG. 8 has the same basic configuration as the pixel circuit of the first embodiment shown in FIG. 1, and the writing efficiency η4 of this pixel circuit is theoretically “1”.

  As described above, the image display device (pixel circuit) according to the second embodiment includes the second capacitor element different from the first capacitor element that holds the threshold voltage (Vth) of the drive transistor Td, and this second capacitor element. In addition, when the image signal voltage (Vdata) is written to the second capacitor element, other capacitance components including parasitic capacitance are not connected in series to the second capacitor element. Thus, the writing efficiency can be effectively improved.

  Note that the image display apparatus according to the second embodiment is configured to include the switching transistor Tm that is connected to both ends of the capacitor Cs2 and is on / off controlled by the Tth control line. In this sequence, it is not necessary to provide a special OLED initialization period, and it can be performed simultaneously with the writing period, so that a sufficient light emission period can be secured, and the ease of design is secured.

In the driving method of the image display device (pixel circuit) according to the second embodiment,
(A) Step of holding the threshold voltage (Vth) of the driving transistor Td in the capacitor Cs (b) Step of holding the image signal voltage (Vdata) in the capacitor Cs2 (c) Threshold voltage (Vth) held in the capacitor Cs Since the step of adding both of the image signal voltage (Vdata) held in the capacitor Cs2 and applying between the gate electrode and the source electrode of the driving transistor Td is included, effectively improving the writing efficiency, It is possible to obtain luminous intensity (brightness) that is not affected by fluctuations in threshold voltage.

  In the pixel circuit shown in FIG. 8, one end of the capacitor Cs2 is always connected to the source electrode (first electrode) of the drive transistor Td. However, one end of the capacitor Cs2 is at least in the light emission period. In this case, it may be connected to the source electrode (first electrode) or the drain electrode (second electrode) of the driving transistor Td. In particular, even during the writing period of the image signal, it is not necessary to be connected to the source electrode or the drain electrode of the drive transistor Td, and a substantially constant potential is maintained during this writing period. It may be configured to be electrically connected to a constant potential line such as the Tth control line 11 or a ground line.

  The constant potential mentioned above does not have to be a constant potential in all of the preparation period, the threshold voltage detection period, the writing period, and the light emission period, and the constant potential is maintained at least for a predetermined period. That's fine.

  Further, the meaning of the constant potential does not need to be a constant potential in a strict sense, and a predetermined potential fluctuation can be allowed within the scope of the purpose of obtaining an effect of increasing the writing efficiency by the additional capacitor Cs2.

  In the pixel circuit shown in FIG. 8, the other end of the capacitor Cs2 is always connected to the gate electrode (control electrode) of the drive transistor Td via the capacitor Cs. May be configured to be electrically connected to the gate electrode (control electrode) of the drive transistor Td at least during the light emission period.

  In the pixel circuit shown in FIG. 8, one end of the capacitor Cs is connected to the connection point between the gate electrode (control electrode) of the drive transistor Td and the threshold voltage detection transistor, and the other end is connected to the switching transistors Ts and Tm and the capacitor. Although it is configured such that it is always connected to each connection point of Cs2, at least one end or the other end of the capacitor Cs is floated (a state where it is floating without being electrically connected to either end) at least during the writing period. In the light emission period, either one end or the other end is connected to the gate electrode (control electrode), the source electrode (first electrode), or the drain electrode (second electrode) of the drive transistor Td. It only has to be.

  As described above, the image display device and the driving method thereof according to the present invention are useful as an invention that can greatly contribute to the improvement of the writing efficiency in the pixel circuit.

It is a figure which shows the structure of the pixel circuit corresponding to 1 pixel of the image display apparatus concerning Embodiment 1 of this invention. FIG. 2 is a sequence diagram for explaining a general operation of the pixel circuit shown in FIG. 1. It is a figure explaining operation | movement of the preparation period shown in FIG. It is a figure explaining the operation | movement of the threshold voltage detection period shown in FIG. FIG. 3 is a diagram for explaining an operation in an OLED initialization / Cs2 initialization period shown in FIG. 2. FIG. 3 is a diagram illustrating an operation during a writing period illustrated in FIG. 2. It is a figure explaining the operation | movement of the light emission period shown in FIG. It is a figure which shows the structure of the pixel circuit corresponding to 1 pixel of the image display apparatus concerning Embodiment 2 of this invention. FIG. 9 is a sequence diagram for explaining a general operation of the pixel circuit shown in FIG. 8. It is a figure explaining operation | movement of the preparation period shown in FIG. It is a figure explaining the operation | movement of the Cs / Cs2 initialization period shown in FIG. It is a figure explaining the operation | movement of the threshold voltage detection period shown in FIG. FIG. 10 is a diagram for explaining the operation in the write / OLED initialization period shown in FIG. 9. It is a figure explaining the operation | movement of the light emission period shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Power supply line 11 Tth control line 12 Merge line 13 Scan line 14 Image signal line OLED Organic EL element Td Drive transistor Tth Threshold voltage detection transistor Ts, Tm Switching transistor Cs, Cs2 Capacitance

Claims (12)

Light emitting means;
A control terminal; a first terminal; and a second terminal, wherein the light emitting means emits light according to a potential difference applied between the control terminal and one of the first terminal and the second terminal. Driver means to control;
A first capacitive element connected directly or indirectly to the control terminal of the driver means;
A second capacitive element connected directly or indirectly to a signal line for supplying an image signal potential corresponding to the light emission luminance of the light emitting means, and directly or indirectly connected to the first capacitive element;
In a method for driving an image display device comprising:
Detecting a threshold voltage of the driver means and holding the voltage corresponding to the threshold voltage in the first capacitor element;
Writing the voltage corresponding to the image signal to the second capacitive element via the signal line, thereby causing the second capacitive element to hold the image signal voltage;
By electrically connecting the first capacitive element and the second capacitive element in series, the threshold voltage held in the first capacitive element and the image signal voltage held in the second capacitive element Adding a voltage and applying the added voltage between the control terminal of the driver means and either the first terminal or the second terminal;
A method for driving an image display device, comprising:   The method for driving an image display device according to claim 1, further comprising the step of discharging the charge accumulated in the light emitting means.   The discharge of the electric charge accumulated in the light emitting means is performed after the first capacitive element holds the threshold voltage of the driver means and before the second capacitive element holds the image signal voltage. The method for driving an image display device according to claim 2.   4. The driving method of the image display device according to claim 1, wherein the first capacitor element is in a floating state while the image signal voltage is written to the second capacitor element. 5. .   The first capacitor element and the second capacitor element are electrically connected in parallel while the threshold voltage of the driver means is detected. A driving method of the image display device according to the above. Light emitting means;
A control terminal, a first terminal, and a second terminal are provided, and between the first terminal and the second terminal according to a potential difference between the control terminal and one end of the first terminal or the second terminal. Driver means for controlling the light emission of the light emitting means by controlling the current flowing through
A first capacitive element connected directly or indirectly to a control terminal of the driver means and holding a threshold voltage of the driver means;
A second capacitive element to which an image signal voltage corresponding to the light emission luminance of the light emitting means is supplied via a signal line,
The second capacitive element includes a second capacitive element connected directly or indirectly to the second electrode of the first capacitive element during a light emission period of the light emitting means;
An image display device comprising:   The image display apparatus according to claim 6, wherein the first capacitor element is in a floating state while an image signal voltage is supplied to the second capacitor element.   8. The image display device according to claim 6, wherein the first capacitive element is electrically connected to the control electrode of the driver unit during a light emission period of the light emitting unit. 9.   9. The device according to claim 6, wherein the second capacitor element is electrically connected to one of the first terminal and the second terminal of the driver unit. 10. Image display device. A potential line that holds the potential substantially constant during a writing period in which the image data is written to the second capacitor element via the signal line;
The image display apparatus according to claim 6, wherein the second capacitor element is electrically connected to the potential line.   The image display device according to claim 6, further comprising a switching element that electrically connects the pair of electrodes of the second capacitor element during a predetermined period. A switching element that electrically connects between the pair of electrodes of the second capacitor element for a predetermined period;
The image display apparatus according to claim 7, wherein a control terminal of the threshold voltage detection unit and a control terminal of the switching element are electrically connected to a common control line.

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