JP2009244528A - Display device - Google Patents

Abstract

In an organic EL display device, a short dark spot of an organic EL element is made inconspicuous.
A plurality of organic EL elements to which a current obtained by diverting a drive current Ids from the same drive transistor is supplied in one pixel. In each organic EL element, the upper electrode 508 is individually patterned, drawn out on the auxiliary wiring 515 by the drawing pattern 509, and connected by the connection hole 508a. The lead pattern 509 is thin and thin, and functions as a fuse resistance element. When any one of the organic EL elements is short-circuited, almost all of the drive current Ids flows through it, but flows through the extraction pattern 509 and generates heat and burns out, and the damaged organic EL element automatically changes from short-circuit to open. The drive current Ids flows to other normal organic EL elements in the same pixel, and the total current is the same as when the damage is not damaged, and the luminance obtained from one pixel is the same regardless of the presence of a dark spot. .
[Selection] Figure 5

Description

  The present invention relates to a display device having a pixel circuit (also referred to as a pixel) including an electro-optical element (also referred to as a display element or a light emitting element). More specifically, a current-driven electro-optic element whose luminance changes depending on the magnitude of the drive signal is provided as a display element, each pixel circuit has an active element, and display drive is performed on a pixel basis by the active element. The present invention relates to a display device.

  As a display element of a pixel, there is a display device using an electro-optical element whose luminance changes depending on an applied voltage or a flowing current. For example, a liquid crystal display element is a typical example of an electro-optical element whose luminance changes depending on an applied voltage, and an organic electroluminescence (Organic Electro Luminescence, Organic EL, Organic) (Light Emitting Diode, OLED; hereinafter referred to as “organic EL”) A typical example is an element. The organic EL display device using the latter organic EL element is a so-called self-luminous display device using an electro-optic element which is a self-luminous element as a pixel display element.

  An organic EL device has an organic thin film (organic layer) made by laminating an organic hole transport layer and an organic light emitting layer between the lower electrode and the upper electrode, and utilizes the phenomenon that light is emitted when an electric field is applied to the organic thin film. In this electro-optical element, the gradation of color is obtained by controlling the current value flowing through the organic EL element.

  Since the organic EL element can be driven with a relatively low applied voltage (for example, 10 V or less), the power consumption is low. Further, since the organic EL element is a self-luminous element that emits light by itself, an auxiliary illumination member such as a backlight that is required in a liquid crystal display device is not required, and the weight and thickness can be easily reduced. Furthermore, since the response speed of the organic EL element is very high (for example, about several μs), an afterimage at the time of displaying a moving image does not occur. Because of these advantages, development of flat self-luminous display devices using organic EL elements as electro-optical elements has been actively performed in recent years.

  By the way, in a display device using an electro-optic element such as a liquid crystal display device using a liquid crystal display element and an organic EL display device using an organic EL element, a simple (passive) matrix method and an active device are used as the driving method. A matrix method can be adopted. However, a simple matrix display device has problems such as a simple structure and a difficulty in realizing a large and high-definition display device.

  Therefore, in recent years, a pixel signal supplied to a light emitting element in a pixel has been converted into an active element, for example, an insulated gate field effect transistor (generally a thin film transistor (TFT)) as a switching transistor. Active matrix systems that are used and controlled have been actively developed.

  Here, when the electro-optic element in the pixel circuit emits light, the input image signal supplied via the video signal line is supplied to the gate end (control input terminal) of the drive transistor by a switching transistor (referred to as a sampling transistor). The image is taken into a provided storage capacitor (also referred to as a pixel capacitor), and a drive signal corresponding to the input image signal taken in is supplied to the electro-optical element.

  In a liquid crystal display device using a liquid crystal display element as an electro-optical element, the liquid crystal display element is a voltage-driven element, and thus the liquid crystal display element is driven with a voltage signal itself corresponding to an input image signal taken into the storage capacitor. On the other hand, in an organic EL display device using a current-driven element such as an organic EL element as an electro-optical element, a drive signal (voltage signal) corresponding to an input image signal taken into a storage capacitor is supplied to the current signal by a drive transistor. And the drive current is supplied to an organic EL element or the like.

  In a current-driven electro-optical element, typically an organic EL element, the light emission luminance varies depending on the drive current value. Therefore, in order to emit light with stable luminance, it is important to supply a stable drive current to the electro-optical element. For example, driving methods for supplying a driving current to the organic EL element can be broadly classified into a constant current driving method and a constant voltage driving method (this is a well-known technique, and publicly known literature is not presented here).

  Since the voltage-current characteristic of the organic EL element has a large inclination, when constant voltage driving is performed, a slight voltage variation or a variation in element characteristics causes a large current variation, resulting in a large luminance variation. Therefore, generally, constant current driving using a driving transistor in a saturation region is used. Of course, even with constant current driving, if there is a current variation, luminance variations will be caused, but if the current variation is small, only small luminance variations will occur.

  In other words, even in the constant current driving method, the driving signal written and held in the holding capacitor according to the input image signal may be constant because the light emission luminance of the electro-optic element is unchanged. It becomes important. For example, in order that the light emission luminance of the organic EL element remains unchanged, it is important that the drive current corresponding to the input image signal is constant.

  However, the threshold voltage and mobility of an active element (driving transistor) that drives the electro-optical element vary due to process variations. In addition, characteristics of electro-optical elements such as organic EL elements vary with time. If there is such a variation in characteristics of the active element for driving or a characteristic variation of the electro-optical element, even the constant current driving method affects the light emission luminance.

  Therefore, in order to uniformly control the light emission luminance over the entire screen of the display device, a mechanism for correcting the luminance variation caused by the characteristic variation of the driving active element and the electro-optical element described above in each pixel circuit. Various studies have been made.

JP 2006-215213 A

  For example, in the mechanism described in Patent Document 1, as a pixel circuit for an organic EL element, a threshold correction function for making the drive current constant even when the threshold voltage of the drive transistor varies or changes over time, In order to keep the driving current constant even when the mobility-correction function for making the driving current constant even when the mobility of the organic EL element varies or changes with time, or when the current-voltage characteristic of the organic EL element changes with time A bootstrap function has been proposed.

  However, since an electro-optical element such as an organic EL element is an element that is generally formed of a thin film, for example, when the element is damaged due to adhesion of foreign matters such as dust during panel manufacture, This results in a dark spot where the light is not emitted normally (a point where no light is emitted), which causes a pixel defect in the panel and causes a reduction in yield (see, for example, Japanese Patent Publication No. 2003-521094). Such a display defect is an impediment to increasing the non-defective product ratio of the display device, and hinders cost reduction of the display device. In addition, display quality is impaired if the user becomes a dark spot for some reason during use.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a mechanism capable of making a dark spot where light emission is not normally performed inconspicuous.

  One embodiment of a display device according to the present invention includes a pixel circuit including a current-driven electro-optic element that performs display in accordance with a signal amplitude and a drive transistor that drives the electro-optic element. A single pixel circuit is provided with a plurality of electro-optical elements to which a current obtained by diverting a drive current from a common drive transistor in the pixel circuit is supplied. A portion having each electro-optical element is referred to as a divided pixel. In addition, each of the plurality of electro-optic elements individually forms a cathode side electrode, and each cathode side electrode is connected to a predetermined reference potential by an electrode material functioning as a fuse element.

  As an electrode material functioning as the fuse element, a simple formation method is to use a lead pattern drawn from the cathode side electrode.

  When any of a plurality of electro-optic elements in the same pixel circuit is short-circuited, the drive current from the drive transistor is not shunted to the remaining normal electro-optic elements, and this short-circuit phenomenon is exclusively performed. Flows to the electro-optic element causing At this time, each cathode electrode side of the plurality of electro-optical elements is individually patterned, and is further connected to a predetermined reference potential by an electrode material (for example, a lead pattern) functioning as a fuse element, so that a short circuit occurs. The wiring pattern that functions as the fuse element can be blown by the current of time. When the wiring pattern that functions as a fuse element is melted, the short-circuit state up to that point is automatically automatically changed to an open state. In the open state, the drive current from the drive transistor is not shunted to the abnormal electro-optical element on the side that has changed to the open state, but is shunted to the remaining normal electro-optical element, and light emission is performed thereby.

  According to an aspect of the present invention, an electrode material (wiring pattern) that functions as a fuse element on the side of an electro-optic element having a short-circuit abnormality among a plurality of electro-optic elements in one pixel circuit is practically used. The pixel layout structure can be automatically changed to the open state, and the dark spot element and the normal element are electrically separated without special measures. Therefore, even if the electro-optic element of any divided pixel becomes a dark spot due to a short-circuit phenomenon, the dark-point element is electrically connected to the electro-optic elements of the remaining divided pixels that are normal without taking any special measures. Separated. If the display is performed with other normal divided pixel electro-optical elements, it is possible to enjoy the effect that it is not visually recognized as a point defect, and it is possible to prevent one pixel from being completely darkened.

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

<Overview of display device>
FIG. 1 is a block diagram showing an outline of a configuration of an active matrix display device which is an embodiment of a display device according to the present invention. In the configuration example shown here, for example, an organic EL element which is a current-driven element is used as a pixel display element (electro-optical element, light-emitting element), and a polysilicon thin film transistor (TFT) is used as an active element. A case where the present invention is applied to an active matrix type organic EL display (hereinafter referred to as “organic EL display device”) in which an organic EL element is formed on a semiconductor substrate on which a thin film transistor is formed will be described as an example.

  The display device 1 is a mobile terminal device such as a portable music player, a digital camera, a notebook personal computer, or a mobile phone using various electronic devices, for example, a recording medium such as a semiconductor memory, a mini disk (MD), or a cassette tape. In addition, video signals input to electronic devices such as video cameras and video signals generated in the electronic devices can be used in display units of electronic devices in various fields that display still images and moving images (videos).

  In the following description of the overall configuration, an organic EL element is specifically described as an example of a pixel display element. However, this is merely an example, and the target display element is not limited to an organic EL element. In general, all of the embodiments described below (particularly, the dark spot countermeasures) can be similarly applied to all electro-optical elements that emit light by current drive.

  As shown in FIG. 1, the display device 1 has an aspect ratio in which a pixel circuit (also referred to as a pixel) P having organic EL elements (not shown) as a plurality of display elements has a display aspect ratio of X: A display panel unit 100 having a main part of a pixel array unit 102 arranged so as to constitute an effective image area of Y (for example, 9:16), and a panel for generating various pulse signals for driving and controlling the display panel unit 100 A drive signal generation unit (so-called timing generator) 200 that is an example of a control unit and a video signal processing unit 220 are provided. The drive signal generation unit 200 and the video signal processing unit 220 are built in a one-chip IC (Integrated Circuit), and are arranged outside the display panel unit 100 in this example.

  In the case of the configuration shown in FIG. 1, the display panel unit 100 includes a pixel array unit 102 in which pixel circuits P are arranged in a matrix of n rows × m columns on a substrate 101, and the pixel circuits P are arranged vertically. A vertical drive unit 103 that scans in the direction, a horizontal drive unit (also referred to as a horizontal selector or a data line drive unit) 106 that scans the pixel circuit P in the horizontal direction, and a terminal unit for external connection (pad unit) ) 108 is arranged at the end of one side of the display panel unit 100. It should be noted that an interface (IF) unit for interfacing between the driving units 103 and 106 and an external circuit may be mounted as necessary.

  The vertical drive unit 103 includes, for example, a write scan unit (write scanner WS; Write Scan) 104 and a drive scan unit (drive scanner DS; Drive Scan) 105 that functions as a power supply scanner having power supply capability. For example, the pixel array unit 102 is driven by the writing scanning unit 104 and the driving scanning unit 105 from one side or both sides in the horizontal direction shown in the figure, and driven by the horizontal driving unit 106 from one side or both sides in the vertical direction shown in the figure. It has come to be.

  The vertical driving unit 103 (the writing scanning unit 104 and the driving scanning unit 105) and the horizontal driving unit 106 control writing of the signal potential to the holding capacitor, threshold correction operation, mobility correction operation, and bootstrap operation. The control unit 109 is configured to function as a drive circuit that drives the pixel circuit P of the pixel array unit 102.

  As described above, in the mounted state, peripheral drive circuits such as the vertical drive unit 103 and the horizontal drive unit 106 are mounted on the same substrate 101 as the pixel array unit 102.

  In the example shown in FIG. 1, the pulse signal is input from the outside of the display panel unit 100 via the terminal unit 108. However, the drive signal generation unit 200 that generates these various timing pulses is configured by a semiconductor chip. It can also be mounted on the display panel unit 100.

  Various pulse signals are supplied to the terminal unit 108 from the drive signal generation unit 200 arranged outside the display device 1. Similarly, the video signal Vsig is supplied from the video signal processing unit 220. In the case of color display compatibility, video signals Vsig_R, G, and B for each color (in this example, three primary colors of R (red), G (green), and B (blue)) are supplied.

  For example, as a pulse signal for vertical driving, shift start pulses SPDS and SPWS which are examples of vertical write start pulses and vertical scanning clocks CKDS and CKWS (vertical scanning clocks xCKDS and xCKWS whose phases are reversed as necessary) ) And other necessary pulse signals are supplied. In addition, as a pulse signal for horizontal driving, necessary pulse signals such as a horizontal start pulse SPH, which is an example of a horizontal write start pulse, and a horizontal scanning clock CKH (and a horizontal scanning clock xCKH whose phase is inverted as necessary) are supplied. Is done.

  Each terminal of the terminal unit 108 is connected to the vertical driving unit 103 and the horizontal driving unit 106 via a signal line 199. For example, each pulse supplied to the terminal unit 108 is internally adjusted to a voltage level by a level shifter unit (not shown) as necessary, and then supplied to each unit of the vertical driving unit 103 and the horizontal driving unit 106 via a buffer. Supplied.

  Although the pixel array unit 102 is not shown in the drawing (details will be described later), pixel circuits P in which pixel transistors are provided with respect to an organic EL element as a display element are two-dimensionally arranged in a matrix form. On the other hand, scanning lines are wired for each row, and signal lines are wired for each column.

  For example, the pixel array unit 102 is formed with the vertical scanning side scanning lines 104WS and 105DSL and the horizontal scanning side scanning signal lines (data lines) 106HS. An organic EL element (not shown) and a thin film transistor for driving the organic EL element are omitted at the intersection between the vertical scanning lines and the horizontal scanning lines. A pixel circuit P is configured by a combination of an organic EL element and a thin film transistor.

  Specifically, for each pixel circuit P arranged in a matrix, the write scanning lines 104WS_1 to 104WS_n for n rows driven by the write scanning unit 104 with the write drive pulse WS and the drive scanning unit Power supply lines 105DSL_1 to 105DSL_n for n rows driven by the power supply drive pulse DSL by 105 are wired for each pixel row.

  The writing scanning unit 104 and the driving scanning unit 105 are configured by combinations of logic gates (including latches and shift registers), and select each pixel circuit P of the pixel array unit 102 in units of rows, that is, drive signal generation Each pixel circuit P is sequentially selected through the write scanning line 104WS and the power supply line 105DSL based on the vertical drive system pulse signal supplied from the unit 200.

  The horizontal drive unit 106 is configured by a combination of logic gates (including latches and shift registers), and selects each pixel circuit P of the pixel array unit 102 in units of columns, that is, supplied from the drive signal generation unit 200. Based on the pulse signal of the horizontal drive system, a predetermined potential in the video signal Vsig is sampled and written to the storage capacitor via the video signal line 106HS for the selected pixel circuit P.

  The display device 1 of the present embodiment is capable of line-sequential driving or dot-sequential driving, and the writing scanning unit 104 and the driving scanning unit 105 of the vertical driving unit 103 are pixels in line sequential (that is, in units of rows). The array unit 102 is scanned, and in synchronization with this, the horizontal drive unit 106 outputs the image signal for one horizontal line simultaneously (in the case of line sequential) or in units of pixels (in the case of dot sequential). Write to 102.

  As shown in the figure, the product form is provided as a display device 1 in the form of a module (composite part) including all of the display panel unit 100, the drive signal generation unit 200, and the video signal processing unit 220. For example, the display device can be provided only by the display panel unit 100, or the display device can be provided only by the pixel array unit 102.

  For example, the display device 1 includes a module-shaped one having a sealed configuration. For example, the pixel array unit 102 is configured as a display module including only the display panel unit 100 formed by being attached to a facing unit such as transparent glass. The transparent facing portion is provided with a display layer (in this example, an organic layer and electrode layers on both sides thereof), a color filter, a protective film, a light shielding film, and the like. In this case, in addition to the pixel array unit 102, a circuit unit (corresponding to the vertical drive unit 103 and the horizontal drive unit 106) for inputting and outputting the video signal Vsig and various drive pulses to the pixel array unit 102 from the outside. ) Is provided on the edge of the display panel unit 100 as an external connection terminal with an FPC (flexible printed circuit). The other points are basically the same as those of the configuration shown in FIG.

  Note that FIG. 1 shows a configuration in which each element of the vertical drive unit 103 (the write scanning unit 104 and the drive scanning unit 105) is arranged only on one side of the pixel array unit 102. It is also possible to adopt a configuration in which both sides are arranged on both sides. Similarly, FIG. 1 shows a configuration in which the horizontal driving unit 106 is disposed only on one side of the pixel array unit 102, but it is also possible to employ a configuration in which the horizontal driving unit 106 is disposed on both upper and lower sides with the pixel array unit 102 interposed therebetween. It is.

<Pixel circuit>
FIG. 2 is a diagram showing a first comparative example for the pixel circuit P of the present embodiment. In addition, a vertical driving unit 103 and a horizontal driving unit 106 arranged on the periphery of the pixel array unit 102 on the substrate 101 of the display panel unit 100 are also shown. Although details will be described later, a pixel circuit based on the configuration shown in FIG. 2 is adopted as an example of the pixel circuit P of the present embodiment. However, the pixel circuit P applicable to the present embodiment is not limited to the one based on the configuration shown in FIG.

  MOS transistors are used as the transistors including the drive transistor. In this case, for the drive transistor, the gate end is handled as the control input end, and either the source end or the drain end is handled as the input end, and the other is handled as the output end. In particular, regarding a driving transistor that supplies a driving current to the organic EL element 127, one of the source end and the drain end (here, the source end) is handled as an output end, and the other is the power supply end (here, the drain end). ).

  Hereinafter, an example of the pixel circuit P in the 2TR configuration will be specifically described. The pixel circuit P of the first comparative example shown in FIG. 2 is characterized in that the drive transistor is basically composed of an n-channel thin film field effect transistor. In addition, a circuit for suppressing fluctuations in the drive current Ids to the organic EL element due to deterioration over time of the organic EL element, that is, driving by correcting a change in current-voltage characteristics of the organic EL element which is an example of an electro-optical element A drive signal stabilization circuit (part 1) for maintaining the current Ids constant is provided.

  It is also characterized by the use of a drive method that maintains a constant drive current Ids by implementing a threshold correction function and mobility correction function that prevent drive current fluctuations due to drive transistor characteristic fluctuations (threshold voltage variations and mobility variations). Have. As a method for suppressing the influence on the drive current Ids due to characteristic variations of the drive transistor 121 (for example, variations and fluctuations in threshold voltage, mobility, etc.), the 2TR configuration drive circuit is directly adopted as the drive signal stabilization circuit (part 1). However, this is dealt with by devising the drive timing of the transistors 121 and 125. Since it has a 2TR drive configuration and the number of elements and wirings is small, high definition can be achieved, and in addition, sampling can be performed without deterioration of the video signal Vsig, so that good image quality can be obtained.

  The pixel circuit P of the first comparative example is characterized by the connection mode of the storage capacitor 120, and is an example of a drive signal stabilization circuit (part 2) as a circuit that prevents drive current fluctuation due to deterioration of the organic EL element 127 over time. This constitutes a bootstrap circuit. A feature is that it has a drive signal stabilization circuit (part 2) that realizes a bootstrap function that makes the drive current constant even when the current-voltage characteristic of the organic EL element changes with time (to prevent fluctuations in the drive current). It has.

  Specifically, as illustrated in FIG. 2, the pixel circuit P of the first comparative example is an n-channel driving transistor 121 and a sampling transistor 125, and is an organic electro-optical element that emits light when current flows. An EL element 127 is included. In general, since the organic EL element 127 has a rectifying property, it is represented by a diode symbol. The organic EL element 127 has a parasitic capacitance Cel. In the figure, this parasitic capacitance Cel is shown in parallel with the organic EL element 127 (diode-like one).

  The storage capacitor 120 is connected between the source end (node ND121) and the gate end (node ND122) of the drive transistor 121, and the source end of the drive transistor 121 is directly connected to the anode end of the organic EL element 127. The storage capacitor 120 functions also as a bootstrap capacitor. The cathode terminal K of the organic EL element 127 is set to a cathode potential Vcath as a reference potential. This cathode potential Vcath is connected to a ground wiring cath (GND) common to all pixels for supplying a reference potential.

  Sampling transistor 125 has a gate end connected to write scan line 104WS from write scan unit 104, a drain end connected to video signal line 106HS, and a source end connected to the gate end (node ND122) of drive transistor 121. Has been. An active H write drive pulse WS is supplied from the write scanning unit 104 to the gate end. The sampling transistor 125 may have a connection mode in which the source end and the drain end are reversed. As the sampling transistor 125, either a depletion type or an enhancement type can be used.

  The drain end of the drive transistor 121 is connected to the power supply line 105DSL from the drive scanning unit 105 that functions as a power scanner. The power supply line 105DSL is characterized in that the power supply line 105DSL itself has a power supply capability to the drive transistor 121. Specifically, the drive scanning unit 105 switches and supplies the first voltage Vcc on the high voltage side and the second voltage Vss on the low voltage side corresponding to the power supply voltage to the drain terminal of the drive transistor 121. A power supply voltage switching circuit is provided. By driving the drain end side of the driving transistor 121 with a power supply driving pulse DSL that takes two values of the first potential Vcc and the second potential Vss, it is possible to perform a preparatory operation prior to threshold correction.

  The second potential Vss is set to a potential sufficiently lower than the offset potential Vofs of the video signal Vsig in the video signal line 106HS. Specifically, the gate-source voltage Vgs of the drive transistor 121 (the difference between the gate potential Vg and the source potential Vs) is larger than the threshold voltage Vth of the drive transistor 121. Two potential Vss is set. The offset potential Vofs is used for an initialization operation prior to the threshold correction operation and also used for precharging the video signal line 106HS in advance.

  In such a pixel circuit P, when driving the organic EL element 127, the first potential Vcc is supplied to the drain end of the drive transistor 121 and the source end is connected to the anode end side of the organic EL element 127. As a whole, a source follower circuit is formed.

  When such a pixel circuit P is employed, a 2TR driving configuration using one switching transistor (sampling transistor 125) for scanning in addition to the driving transistor 121 is adopted, and a power supply driving pulse DSL for controlling each switching transistor is used. In addition, the setting of the on / off timing of the write drive pulse WS has an influence on the drive current Ids due to deterioration with time of the organic EL element 127 and fluctuations in characteristics of the drive transistor 121 (for example, variations and fluctuations in threshold voltage and mobility). prevent.

  In order to drive the pixel circuit P, a writing scanning unit 104, a driving scanning unit 105, and a horizontal driving unit 106 are arranged around the pixel array unit 102. The controller 109 functions as a drive signal stabilization circuit that maintains the drive current Ids flowing through the drive transistor 121 constant by optimizing the drive timing. For this reason, first, the drive scanning unit 105 preferably sets the sampling transistor 125 in a non-conducting state at the time when information corresponding to the signal amplitude Vin is written in the storage capacitor 120, and the video signal to the control input terminal of the drive transistor 121. It is preferable that the supply of Vsig is stopped and a bootstrap operation is performed in which the potential of the control input terminal is interlocked with the potential fluctuation of the output terminal of the driving transistor 121.

  The control unit 109 preferably executes the bootstrap operation even at the beginning of light emission after the end of the sampling operation. That is, the potential difference between the control input terminal and the output terminal of the drive transistor 121 is constant by turning the sampling transistor 125 in a conductive state after the signal potential is supplied to the sampling transistor 125 and then turning the sampling transistor 125 in a non-conductive state. To be maintained.

  In addition, the control unit 109 preferably controls the bootstrap operation so as to realize the temporal variation correction operation of the electro-optic element (organic EL element 127) in the light emission period. For this reason, the control unit 109 continuously keeps the sampling transistor 125 in a non-conductive state during a period in which the drive current Ids based on the information held in the holding capacitor 120 flows to the electro-optical element (organic EL element 127). Thus, it is preferable that the voltage variation at the control input terminal and the output terminal can be kept constant, and the temporal variation correction operation of the electro-optic element is realized. Even if the current-voltage characteristic of the organic EL element 127 varies with time due to the bootstrap operation of the storage capacitor 120 during light emission, the potential difference between the control input terminal and the output terminal of the drive transistor 121 is kept constant by the bootstrap storage capacitor 120. Therefore, a constant light emission luminance is always maintained.

  Preferably, the control unit 109 causes the sampling transistor 125 to conduct in a time zone in which the offset potential Vofs is supplied to the input terminal (the source terminal is a typical example) of the sampling transistor 125, thereby causing the threshold voltage Vth of the driving transistor 121 to be on. Control is performed so as to perform a threshold value correction operation for holding the voltage corresponding to. This threshold value correction operation is repeatedly executed at a plurality of horizontal periods preceding the writing of information corresponding to the signal amplitude Vin to the storage capacitor 120 as necessary, and reliably corresponds to the threshold voltage Vth of the drive transistor 121. It is preferable to hold the voltage in the holding capacitor 120.

  More preferably, prior to the threshold value correcting operation, the control unit 109 conducts the sampling transistor 125 in a time zone in which the offset potential Vofs is supplied to the input terminal of the sampling transistor 125 to perform a threshold value correcting preparatory operation ( Control is performed to execute a discharge operation or an initialization operation. Before the threshold correction operation, the potentials of the control input terminal and the output terminal of the drive transistor 121 are initialized. More specifically, the storage capacitor 120 is connected between the control input terminal and the output terminal, so that the potential difference between both ends of the storage capacitor 120 is set to be equal to or higher than the threshold voltage Vth.

<Characteristic variation and its impact>
FIG. 3 is a diagram for explaining the characteristic variation of pixel constituent elements (organic EL elements and drive transistors) and the influence thereof. Here, FIG. 3A is a diagram for explaining operating points of the organic EL element and the driving transistor. FIG. 3B is a diagram for explaining the influence of variation in characteristics of organic EL elements and drive transistors on the drive current Ids.

<Iel-Vel characteristics of light emitting element>
In general, as shown in FIG. 3A, the drive transistor 121 is driven in a saturation region where the drive current Ids is constant regardless of the drain-source voltage. Therefore, the current flowing between the drain end and the source of the transistor operating in the saturation region is Ids, the mobility is μ, the channel width (gate width) is W, the channel length (gate length) is L, and the gate capacitance (per unit area). When the gate oxide film capacitance) is Cox and the threshold voltage of the transistor is Vth, the driving transistor 121 is a constant current source having a value shown in the following equation (1). “^” Indicates a power. As apparent from the equation (1), in the saturation region, the drain current Ids of the transistor is controlled by the gate-source voltage Vgs and operates as a constant current source.

  However, in general, the IV characteristics of current-driven light-emitting elements such as organic EL elements deteriorate as time passes as shown in FIG. In the current-voltage (Iel-Vel) characteristics of a current-driven light-emitting element typified by the organic EL element shown in FIG. 3 (2), the curve indicated by the solid line indicates the characteristic in the initial state, and the curve indicated by the broken line indicates The characteristic after change with time is shown.

  For example, when the light emission current Iel flows through the organic EL element 127 which is an example of the light emitting element, the anode-cathode voltage Vel is uniquely determined. However, as shown in FIG. 3B, during the light emission period, the light emission current Iel determined by the drain-source current Ids (= drive current Ids) of the drive transistor 121 flows through the anode end of the organic EL element 127. As a result, the anode-cathode voltage Vel of the organic EL element 127 increases.

  In the pixel circuit P shown in FIG. 2, it is assumed that the storage capacitor 120 is connected between the gate ground wiring cath of the driving transistor 121 (this circuit is referred to as the pixel circuit P of the second comparative example). As shown in FIG. 3B, the source end is connected to the organic EL element 127 side, and the anode / cathode for the same light emission current Iel due to the Iel-Vel characteristic of the organic EL element 127 that deteriorates with time as shown in FIG. When the voltage Vel changes from Vel1 to Vel2, the operating point of the drive transistor 121 changes, and the source potential Vs of the drive transistor 121 changes even when the same gate potential Vg is applied. As a result, the gate-source voltage Vgs of the drive transistor 121 changes.

  As is clear from the characteristic equation (1), when the gate-source voltage Vgs varies, the drive current Ids varies even if the gate potential Vg is constant, and the current value (light emission current) flowing through the organic EL element 127 at the same time. Iel) changes, and the light emission luminance changes.

  As described above, in the pixel circuit P of the second comparative example, the anode potential variation of the organic EL element 127 due to the temporal variation of the Iel-Vel characteristic of the organic EL element 127 which is an example of the light emitting element is caused between the gate and the source of the driving transistor 121. It appears as a fluctuation in the voltage Vgs and causes a fluctuation in the drain current (drive current Ids). Variations in the drive current Ids due to this cause appear as variations in light emission luminance and temporal variations for each pixel circuit P, resulting in degradation of image quality.

<Vgs-Ids characteristics of drive transistor>
In addition, if the characteristics of the driving transistor 121 are different for each pixel, the influence affects the driving current Ids flowing through the driving transistor 121. As an example, as can be seen from the equation (1), when the mobility μ and the threshold voltage Vth vary from pixel to pixel or change with time, the drive transistor 121 can be used even if the gate-source voltage Vgs is the same. The drive current Ids flowing through the output varies and changes with time, and the light emission luminance of the organic EL element 127 changes for each pixel.

  For example, due to variations in the manufacturing process of the drive transistor 121, there are variations in characteristics such as threshold voltage Vth and mobility μ for each pixel circuit P. Even when the driving transistor 121 is driven in the saturation region, even if the same gate potential is applied to the driving transistor 121 due to this characteristic variation, the drain current (driving current Ids) varies for each pixel circuit P, and the emission luminance is reduced. Appears as variations.

  As described above, the drain current Ids when the driving transistor 121 operates in the saturation region is expressed by the characteristic formula (1). Focusing on the threshold voltage variation of the drive transistor 121, as apparent from the characteristic equation (1), when the threshold voltage Vth varies, the drain current Ids varies even if the gate-source voltage Vgs is constant. In other words, if no countermeasure is taken against the variation of the threshold voltage Vth, the drive current corresponding to Vgs becomes Ids1 when the threshold voltage is Vth1, while the same gate voltage Vgs when the threshold voltage is Vth2. The corresponding drive current Ids2 is different from Ids1.

  When focusing on the mobility variation of the drive transistor 121, as is apparent from the characteristic equation (1), when the mobility μ varies, the drain current Ids varies even if the gate-source voltage Vgs is constant. . In other words, if no measures are taken against the variation in mobility μ, the driving current corresponding to the gate-source voltage Vgs becomes Ids1 when the mobility is μ1, while the mobility is μ2. The drive current Ids2 corresponding to the same gate-source voltage Vgs is different from Ids1.

  If there is a large difference in Vin-Ids characteristics due to the difference in threshold voltage Vth and mobility μ, even if the same signal amplitude Vin is given, the drive current Ids, that is, the emission luminance will be different, and the screen luminance uniformity (uniform) Mitty) is not obtained.

<Concept of threshold correction and mobility correction>
On the other hand, by setting the drive timing (details will be described later) to realize the threshold value correction function and the mobility correction function, the influence of these fluctuations can be suppressed, and the uniformity of the screen luminance (uniformity) can be ensured. .

  In the threshold correction operation and the mobility correction operation employed in this embodiment, when it is assumed that the write gain is 1 (ideal value), the gate-source voltage Vgs at the time of light emission is represented by “Vin + Vth−ΔV”. By doing so, the drain-source current Ids is not dependent on variations and fluctuations in the threshold voltage Vth, and is not dependent on variations and fluctuations in the mobility μ. As a result, even if the threshold voltage Vth and the mobility μ fluctuate due to the manufacturing process and time, the driving current Ids does not fluctuate and the light emission luminance of the organic EL element 127 does not fluctuate. At the time of mobility correction, the mobility correction parameter ΔV1 is increased for a large mobility μ1, while negative feedback is applied so that the mobility correction parameter ΔV2 is also decreased for a small mobility μ2. Become. In this sense, the mobility correction parameter ΔV is also referred to as a negative feedback amount ΔV.

  Although a timing chart showing a detailed example of the threshold correction operation and the mobility correction operation is omitted, before the information corresponding to the signal amplitude Vin is held in the holding capacitor 120, the power supply line 105DSL is set to the first potential Vcc, The video signal line 106HS is set to the offset potential Vofs to turn on the sampling transistor 125, and information on the threshold voltage Vth of the drive transistor 121 is written in the storage capacitor 120 in advance. After that, for example, the sampling transistor 125 is turned on (on) in a state where the gate potential Vg of the drive transistor 121 is at the signal potential (Vofs + Vin). Accordingly, in the write & mobility correction period H, the drive current Ids flows through the drive transistor 121 while the gate end of the drive transistor 121 is fixed to the signal potential (Vofs + Vin). Information on the signal amplitude Vin is held in the holding capacitor 120 in a form that is added to the threshold voltage Vth of the driving transistor 121. As a result, fluctuations in the threshold voltage Vth of the drive transistor 121 are always canceled, and threshold correction is performed. By this threshold correction, the gate-source voltage Vgs held in the holding capacitor 120 becomes “Vin + Vth”.

  Further, after the threshold correction is completed, the sampling transistor 125 is turned on (on) while the gate potential Vg of the drive transistor 121 is at the signal potential (Vofs + Vin). Therefore, the drive current Ids flows through the drive transistor 121 in a state where the gate terminal of the drive transistor 121 is fixed to the signal potential (Vofs + Vin). The gate potential Vg of the drive transistor 121 becomes the signal potential (Vofs + Vin) because the sampling transistor 125 is turned on, but the current flows from the power supply line 105DSL, so that the source potential Vs increases with time.

  When the threshold voltage of the organic EL element 127 is VthEL and the write gain is ideally “1”, the organic EL element 127 is reversed by setting “Vofs−Vth + ΔV <VthEL + Vcath”. Since it is in a bias state and is in a cut-off state (high impedance state), it does not emit light, and exhibits simple capacitance characteristics instead of diode characteristics. If the source potential Vs at this time does not exceed the sum of the threshold voltage VthEL and the cathode potential Vcath of the organic EL element 127, the drain current (drive current Ids) flowing through the drive transistor 121 is equal to the capacitance value Cs of the storage capacitor 120 and the organic EL element. The parasitic capacity (equivalent capacity) 127 of the capacitance 127 of the capacitance value Cel is combined into the capacity “C = Cs + Cel” obtained by combining both, and charging is started. As a result, the source potential Vs of the drive transistor 121 increases. At this time, since the threshold value correcting operation of the driving transistor 121 is completed, the driving current Ids flowing through the driving transistor 121 reflects the mobility μ.

  This increase ΔV, that is, the negative feedback amount ΔV, which is a mobility correction parameter, is subtracted from the gate-source voltage “Vgs = Vin + Vth” held in the holding capacitor 120 by threshold correction, and “Vgs = Vin + Vth−”. Since ΔV ”, negative feedback is applied. At this time, the source potential Vs of the drive transistor 121 becomes a value “Vofs−Vth + ΔV” obtained by subtracting the voltage “Vgs = Vin + Vth−ΔV” held in the storage capacitor from the gate potential Vg (= Vofs + Vin). In consideration of the write gain, the source potential Vs of the drive transistor 121 is obtained by subtracting the voltage “Vgs = (1−g) Vin + Vth−ΔV” held in the storage capacitor from the gate potential Vg (= Vofs + Vin). (1-g) Vofs + g (Vofs + Vin) −Vth + ΔV ”=“ Vofs + gVin−Vth + ΔV ”. In this manner, the sampling of the signal amplitude Vin and the adjustment of the negative feedback amount (mobility correction parameter) ΔV for correcting the variation in the mobility μ are performed. The negative feedback amount can be optimized by adjusting the phase between the active period of the write drive pulse WS and the period of the signal potential (Vofs + Vin).

  Here, “optimizing the negative feedback amount” means that the mobility correction can be appropriately performed at any level in the range from the black level to the white level of the video signal potential. The amount of negative feedback applied to the gate-source voltage Vgs depends on the extraction time of the drain current Ids, and the longer this period, the larger the negative feedback amount. The negative feedback amount ΔV is ΔV = Ids · Cel / t.

  As is apparent from this equation, the negative feedback amount ΔV increases as the drive current Ids, which is the drain-source current of the drive transistor 121, increases. Conversely, when the drive current Ids of the drive transistor 121 is small, the negative feedback amount ΔV is small. Thus, the negative feedback amount ΔV is determined according to the drive current Ids.

  Further, as the signal amplitude Vin increases, the drive current Ids increases and the absolute value of the negative feedback amount ΔV also increases. Therefore, mobility correction according to the light emission luminance level can be realized. At this time, the writing & mobility correction period H is not necessarily constant, and conversely, it may be preferable to adjust it according to the drive current Ids. For example, when the drive current Ids is large, the mobility correction period t should be set short, and conversely, when the drive current Ids becomes small, the write & mobility correction period H should be set long.

  Further, the negative feedback amount ΔV is Ids · Cel / t, and even if the drive current Ids varies due to variations in the mobility μ for each pixel circuit P, the negative feedback amount ΔV corresponds to each. Variations in mobility μ for each pixel circuit P can be corrected. That is, when the signal amplitude Vin is constant, the absolute value of the negative feedback amount ΔV increases as the mobility μ of the drive transistor 121 increases. In other words, since the negative feedback amount ΔV increases as the mobility μ increases, the variation in mobility μ for each pixel circuit P can be removed.

  The control unit 109 also has a bootstrap function. That is, during the light emission period, the write scanning unit 104 cancels the application of the write drive pulse WS to the write scan line 104WS when the information of the signal amplitude Vin is held in the holding capacitor 120 (ie, in Active L (low)), the sampling transistor 125 is turned off, and the gate terminal of the drive transistor 121 is electrically disconnected from the video signal line 106HS. In the light emission period, the horizontal driving unit 106 returns the potential of the video signal line 106HS to the offset potential Vofs at an appropriate time point thereafter. Thereafter, the process proceeds to the next frame (or field), and the threshold correction preparation operation, the threshold correction operation, the mobility correction operation, and the light emission operation are repeated again.

  In the light emission period, the gate terminal of the drive transistor 121 is disconnected from the video signal line 106HS. Since the application of the signal potential (Vofs + Vin) to the gate terminal of the driving transistor 121 is released, the gate potential Vg of the driving transistor 121 can be increased. A storage capacitor 120 is connected between the gate terminal and the source terminal of the driving transistor 121, and a bootstrap operation is performed by the effect of the storage capacitor 120. Assuming that the bootstrap gain is 1 (ideal value), the gate potential Vg is interlocked with the variation of the source potential Vs of the drive transistor 121, and the gate-source voltage Vgs can be kept constant. .

  At this time, the drive current Ids flowing through the drive transistor 121 flows through the organic EL element 127, and the anode potential of the organic EL element 127 rises according to the drive current Ids. Let this increase be Vel. Eventually, as the source potential Vs rises, the reverse bias state of the organic EL element 127 is canceled, so that the organic EL element 127 actually starts to emit light by the inflow of the drive current Ids. The rise (Vel) of the anode potential of the organic EL element 127 at this time is nothing but the rise of the source potential Vs of the drive transistor 121, and the source potential Vs of the drive transistor 121 rises by Vel.

  The relationship between the drive current Ids and the gate voltage Vgs can be expressed as in Expression (2-1) by substituting “Vin−ΔV + Vth” into Vgs in Expression (1) representing the previous transistor characteristics. Incidentally, when the write gain is taken into consideration, it can be expressed as equation (2-2) by substituting “(1−g) Vin−ΔV + Vth” into Vgs of equation (1). In Expression (2-1) and Expression (2-2) (collectively referred to as Expression (2)), k = (1/2) (W / L) Cox.

  From this equation (2), it can be seen that the term of the threshold voltage Vth is canceled and the drive current Ids supplied to the organic EL element 127 does not depend on the threshold voltage Vth of the drive transistor 121. Basically, the drive current Ids is determined by the signal amplitude Vin (specifically, the sampling voltage held in the holding capacitor 120 corresponding to the signal amplitude Vin = Vgs). In other words, the organic EL element 127 emits light with a luminance corresponding to the signal amplitude Vin.

  At this time, the information held in the holding capacitor 120 is corrected by the feedback amount ΔV. The correction amount ΔV works to cancel out the effect of the mobility μ located in the coefficient part of the equation (2). Therefore, the drive current Ids substantially depends only on the signal amplitude Vin. Since the drive current Ids does not depend on the threshold voltage Vth, even if the threshold voltage Vth varies depending on the manufacturing process, the drain-source drive current Ids does not vary, and the light emission luminance of the organic EL element 127 does not vary.

  Further, even when the n-channel type driving transistor 121 is used, a bootstrap function is realized in which the gate end potential Vg is interlocked with the fluctuation of the source end potential Vs of the driving transistor 121 as shown in FIG. By adopting the circuit configuration and the drive timing, even if there is an anode potential fluctuation (that is, a source potential fluctuation of the driving transistor 121) of the organic EL element 127 due to a change in characteristics of the organic EL element 127 with time, the fluctuation is canceled out. The gate potential Vg is changed. Thereby, the uniformity (uniformity) of screen luminance can be secured. With the bootstrap function, it is possible to improve the temporal variation correction capability of a current-driven light-emitting element typified by an organic EL element. Of course, in the bootstrap function, the light emission current Iel begins to flow through the organic EL element 127 at the start of light emission, and as a result, the anode-cathode voltage Vel rises until it becomes stable. It also functions when the source potential Vs of the drive transistor 121 varies with the variation of the voltage Vel.

  For example, a storage capacitor 120 is connected between the gate terminal and the source terminal of the drive transistor 121, and due to the effect of the storage capacitor 120, a bootstrap operation is performed at the beginning of the light emission period. The gate potential Vg and the source potential Vs of the drive transistor 121 rise while the source voltage Vgs is kept constant, and the gate potential Vg becomes “Vofs + Vin + Vel”. At this time, since the gate-source voltage Vgs of the drive transistor 121 is constant, the drive transistor 121 passes a constant current (drive current Ids) to the organic EL element 127. As a result, the potential at the anode end A of the organic EL element 127 (= potential at the node ND121) rises to a voltage at which a current called a drive current Ids in a saturated state can flow through the organic EL element 127.

  Here, the organic EL element 127 has its IV characteristic changed as the light emission time becomes longer. Therefore, the potential of the node ND121 also changes with time. However, even if the anode potential fluctuates due to such deterioration of the organic EL element 127 with time, the gate-source voltage Vgs held in the holding capacitor 120 is always kept constant.

  Since the drive transistor 121 operates as a constant current source, the IV characteristic of the organic EL element 127 changes with time, and even if the source potential Vs of the drive transistor 121 changes accordingly, the drive transistor 121 drives the drive transistor 121. Since the gate-source potential Vgs 121 is kept constant (≈Vin−ΔV + Vth or ≈ (1−g) Vin−ΔV + Vth), the current flowing through the organic EL element 127 does not change. The light emission brightness is also kept constant.

  Regardless of the characteristic variation of the organic EL element 127, an operation for correction (operation based on the effect of the storage capacitor 120) for maintaining the gate-source voltage of the driving transistor 121 constant and maintaining the luminance constant is performed. This is called a bootstrap operation. By this bootstrap operation, even if the IV characteristic of the organic EL element 127 changes with time, it is possible to display an image without luminance deterioration associated therewith.

  Thus, according to the drive timing by the pixel circuit P of the first comparative example shown in FIG. 2 (in fact, the pixel circuit P of the present embodiment described later) and the control unit 109 for driving the pixel circuit P, the drive transistor Even if there is a characteristic variation (variation or variation with time) of 121 or the organic EL element 127, by correcting these variations, the influence does not appear on the display screen, and a high-quality image without luminance deterioration. Display is possible.

  However, if each organic EL element 127 in the pixel array unit 102 has a defect, the part is visually recognized as a point defect that does not emit light, and the display quality is deteriorated, which causes a problem. Hereinafter, this problem and its improvement method will be described in detail.

<< About pixel defects >>
4 to 4E are diagrams for explaining point defects in the pixel circuit P of the pixel array unit 102. FIG. Here, FIG. 4 is a diagram for explaining an equivalent circuit of the organic EL element 127 when a dark spot occurs. FIG. 4A is a diagram for explaining the arrangement relationship of the organic EL elements 127 on the semiconductor substrate. Specifically, FIG. 4A is a diagram showing an outline of a layer structure for one pixel in a general organic EL display device, and FIG. 4A (1) is a schematic plan view (focusing on electrodes) for one pixel. 4A (2) is a schematic cross-sectional view. FIG. 4B is a diagram illustrating a layout example of the lower electrode and the auxiliary wiring of the organic EL element 127.

  FIG. 4C is a diagram illustrating a pixel circuit P of a third comparative example having a dark spot element countermeasure function. FIG. 4D is a diagram showing an outline of the layer structure for one pixel of the third comparative example, and FIG. 4D (1) is a schematic diagram of a plane for one pixel (a plan perspective view focusing on the electrodes). FIG. 4D (2) is a schematic cross-sectional view. FIG. 4E is a diagram illustrating a problem in the third comparative example illustrated in FIG. 4C.

  In the pixel circuit P of the first comparative example shown in FIG. 2, consider a case where the organic EL element 127 becomes a dark spot (a pixel that does not emit light) due to a defect such as dust. In the case where the anode and cathode of the organic EL element 127 become open (open) and become a dark spot, the equivalent circuit of the organic EL element 127 is omitted from the illustration, but an ultrahigh resistance is connected in series with the normal organic EL element 127. It can be considered that there is a resistance element having a value, and in fact, it can be considered that the drive current Ids from the drive transistor 121 does not flow to the organic EL element 127 and the organic EL element 127 does not emit light.

  On the other hand, when the anode and cathode of the organic EL element 127 are short-circuited and become a dark spot, the equivalent circuit of the organic EL element 127 is parallel to the normal organic EL element 127 as shown in FIG. It may be considered that the resistance element 127R exists in The resistance element 127R may be considered to have a low resistance value, and the driving current Ids from the driving transistor 121 flows more to the resistance element 127R side than the organic EL element 127, so that the organic EL element 127 does not emit light. Good.

  4A (1), a lower electrode (for example, an anode electrode) 504 is disposed on the substrate 101, and an opening portion of the organic EL element 127 (on the lower electrode 504 (as shown in FIG. 4A (1)). 127a) (hereinafter referred to as EL opening). The lower electrode 504 is provided with a connection hole (for example, TFT-anode contact) 504a, and is connected to the input / output terminal (source electrode in this example) of the drive transistor 121 disposed below the lower electrode 504 through the connection hole 504a. An electrode 504 is connected.

  The periphery of the lower electrode 504 is covered with an opening defining insulating film 505 that is an insulating film pattern, and an upper electrode 508 (not shown) is provided on the lower electrode 504 so as to cover almost the entire surface of the pixel array unit 102. In addition, only the portion where the lower electrode 504 constituting the organic EL element 127 and the organic layer 506 and the upper electrode 508 (not shown) are laminated is an EL opening 127a that is widely exposed so as to be a light emission effective region 127b.

  FIG. 4A (2) shows a schematic diagram of a cross section. As shown in FIG. 4A (2), although not shown at positions corresponding to the pixel circuits P on the substrate 101, the thin film transistors Q and the storage capacitors 120 such as the drive transistors 121 and the sampling transistors 125 that constitute the pixel circuits. Circuit elements such as (capacitance value Cs) are arranged by internal wiring, and an interlayer insulating film (oxide film) is provided above the layer (first wiring layer). A source electrode line and a drain electrode line connected to the thin film transistor Q are provided further above the interlayer insulating film. In addition, other wirings constituting the pixel circuit P are formed by the conductive layer constituting each element (thin film transistor Q, storage capacitor 120) and the conductive layer (second wiring layer) constituting the source electrode line and the drain electrode line. Is done. The first wiring layer and the second wiring layer are collectively referred to as a TFT layer L_TFT.

  Then, an interlayer insulating film functioning as an upper leveling film is provided as an insulating flat film 503 in a state of covering layers (second wiring layers) such as source electrode lines and drain electrode lines, and on the insulating flat film 503. In addition, an organic EL element 127 is formed. The organic EL element 127 includes a lower electrode (for example, an anode electrode) 504, an organic layer 506, and an upper electrode (for example, a cathode electrode) 508 that are sequentially stacked from the lower layer side. Since the organic EL element 127 has a structure in which an organic layer 506 that is a dielectric is sandwiched between the lower electrode 504 and the upper electrode 508, the organic EL element 127 has a capacitance component (parasitic capacitance Cel).

  Specifically, the organic layer 506 has a multilayer structure made of, for example, a low molecular material. For example, a hole injection layer, a hole transport layer, and a light emitting layer are sequentially arranged from the lower electrode 504 side to the upper electrode 508 side. Layer, and an electron transport layer (also serving as an electron injection layer). And in the case of a color display correspondence, the thing suitable for a display color is used as an organic material of a light emitting layer.

  The lower electrode 504 is patterned as a pixel electrode, and is connected to the source electrode of the drive transistor 121 through a connection hole 504a (a connection contact between the anode metal and the drive transistor 121) formed in the interlayer insulating film. In addition, the upper electrode 508 facing the lower electrode 504 is formed as a solid film covering all the pixel circuits P. Although not shown, a light shielding metal layer is provided on the surface of the substrate 101 opposite to the side on which the transistor Q and the organic EL element 127 are disposed for light leakage and temperature diffusion.

  In the organic EL display device 1 having such a layer structure, the organic EL element 101 can be configured as a so-called top emission method in which the emitted light L1 is extracted from the side opposite to the substrate 101 on which the organic EL elements 127 are arranged. Effective in securing the aperture ratio. Further, in such a top emission method, the aperture ratio of the organic EL element 127 does not depend on the layout of the thin film transistor Q constituting the pixel circuit P. For this reason, a pixel circuit P using a plurality of thin film transistors Q and storage capacitors 120 can be arranged corresponding to each pixel.

  Since the display device 1 is a top emission type in which emitted light is extracted from the side opposite to the substrate 101, the lower electrode 504 is made of a material having high light shielding properties and high reflectance. On the other hand, the upper electrode 508 is configured using a material having high light transmittance. Therefore, the wiring resistance of the upper electrode 508 is increased. Even if the upper electrode 508 is a solid wiring, there is a limit in reducing the resistance value. The auxiliary wiring 515 contributes to reducing the resistance value of the entire cathode wiring by wiring in parallel with the high resistance upper electrode 508 in electrical circuit. In other words, since the upper electrode 508 (cathode metal) has a high resistance and is insufficient as a cathode as a panel, the auxiliary wiring 515 has a cathode potential using the same metal material as the lower electrode 504 (anode metal). In order to reduce the resistance of the cathode wiring.

  For example, FIG. 4B shows a layout example of the lower electrode of the organic EL element 127 and the auxiliary wiring. As shown in this figure, the lower electrode 504 is arranged in a two-dimensional matrix corresponding to the arrangement of the pixel circuits P arranged in a matrix. Between the lower electrodes 504, auxiliary wirings 515 configured in the same layer as the lower electrodes 504 are arranged in a lattice shape so as to surround the lower electrodes 504 (that is, the pixel circuits P), and further on the outer periphery of the pixel array section. The configuration is wired so as to surround the whole 102. As described above, the auxiliary wiring 515 of the anode layer L3 in which the lower electrode 504 is formed has the cathode contact KC at an appropriate location (in the example shown in the figure, at the center corresponding to each pixel and the center corresponding to the outer peripheral pixel). Thus, the upper electrode 508 of the upper layer is connected.

  Since there is one EL opening 127a in such a pixel circuit P per pixel, if the organic EL element 127 becomes a dark spot due to dust or the like, the pixel becomes a point defect, resulting in a decrease in yield. Cause. As a countermeasure, it is conceivable to take a mechanism to alleviate the problem that the pixel becomes a point defect when the organic EL element 127 itself becomes a dark spot due to dust or the like. As a basic mechanism, for example, it is conceivable to divide a conventional pixel into a plurality of divided pixel areas and to provide an organic EL element 127 for each divided pixel. In the case of color display compatibility, each subpixel for each color is divided into a plurality of divided pixel areas.

  For example, it is conceivable to provide a plurality of organic EL elements 127 in one pixel and drive them in common by one drive transistor 121. As in the pixel circuit P of the third comparative example shown in FIG. 4C, one conventional pixel is divided into two regions, a divided pixel P_1 and a divided pixel P_2, and each divided pixel P_1, P_2 has one organic first. An EL element 127 is provided. The 2TR driving circuit that drives each of the organic EL elements 127_1 and 127_2 employs a configuration in which, for example, the same configuration as the pixel circuit P of the second comparative example described above is provided in common to each of the divided pixels P_1 and P_2. . Thereby, the organic EL element 127_1 of the divided pixel P_1 and the organic EL element 127_2 of the divided pixel P_2 are driven by a common drive circuit (specifically, the drive transistor 121).

  Assuming that the characteristics of the two organic EL elements 127_1 and 127_2 are the same, the drive current Ids from the drive transistor 121 is evenly distributed to the two organic EL elements 127_1 and 127_2 in a normal time when no dark spot occurs. The drive current Ids flowing from one drive transistor 121 is divided and emitted. That is, one pixel emits light with luminance corresponding to the drive current Ids by the two organic EL elements 127_1 and 127_2.

  As a planar configuration, as shown in FIG. 4D (1), the lower electrode 504 serving as an anode electrode is individually patterned for each of the plurality of organic EL elements 127_1 and 127_2 to form the lower electrodes 504_1 and 504_2. Even if the upper electrode 508 serving as a cathode electrode is shared, a plurality of organic EL elements 127 are arranged in one pixel. The lower electrodes 504_1 and 504_2 (that is, the anode electrodes) of the organic EL elements 127_1 and 127_2 are connected to the source of the driving transistor 121 through the connection holes 504a_1 and 504a_2, respectively, which have contact functions. Thus, one EL pixel has two EL openings 127a_1 and 127a_2 corresponding to the divided pixel P_1 and the divided pixel P_2 divided into two regions. If the two organic EL elements 127_1 and 127_2 are not dark spots, both the EL openings 127a_1 and 127a_2 serve as light emitting parts. Therefore, the total area of the EL openings 127a_1 and 127a_2 is set to the area of the EL opening 127a before the division. In other words, the aperture ratio of the display device is not substantially reduced.

  As a result, as shown in FIG. 4E (1), one of the plurality of organic EL elements 127_1 and 127_2 (in the example shown, the organic EL element 127_1) is damaged by a foreign object or the like, and becomes an open dark spot. In this case, since the drive current Ids from the drive transistor 121 flows to another organic EL element (in the example of the figure, the organic EL element 127_2) in the same pixel, the total current damages the organic EL element when viewed as a whole pixel. The drive current Ids similar to that in the case where it is not performed flows.

  Since the organic EL element is a current-emitting element, luminance can be obtained in proportion to the current. For this reason, even when one organic EL element is damaged and becomes a dark spot, it is possible to separate the dark spot and emit light only by another normal organic EL element existing in the same pixel. Since the total currents flowing through the organic EL elements are equal, the luminance obtained from one pixel can be equivalent regardless of the presence of a dark spot.

  When a conventional pixel is divided into a plurality of areas, each of which is provided with an organic EL element and driven by a common drive transistor 121, one of the divided pixels becomes an open dark spot Even if it displays with the organic EL element of another normal division | segmentation pixel, it can enjoy the effect that it is not visually recognized as a point defect apparently.

  However, as shown in FIG. 4E (2), one of the plurality of organic EL elements 127_1 and 127_2 (the organic EL element 127_1 in the example in the figure) is damaged by a foreign substance or the like, or is not connected to the anode for some reason during use. In the case of a dark spot where the cathode is short-circuited, almost all of the drive current Ids from the drive transistor 121 flows through the short-circuited organic EL element (in the example shown, the organic EL element 127_1). In addition, current hardly flows into other organic EL elements (organic EL element 127_2 in the example shown in the figure) in the same pixel. Therefore, in the case of damage to the organic EL element in which the anode and the cathode are short-circuited, even if a plurality of organic EL elements are produced in one pixel, luminance can hardly be obtained from the pixel, which becomes a dark spot. There is a problem.

  Therefore, in the present embodiment, when a plurality of organic EL elements 127 are arranged in one pixel and a countermeasure against dark spots is taken, even if the organic EL element 127 becomes a dark spot due to a short circuit, the countermeasure effect can be enjoyed. To. The basic idea is that the cathode electrode is individually patterned for each of the plurality of organic EL elements 127, and the organic EL element 127 is damaged in part on the current path passing through each organic EL element 127. When the cathode is short-circuited, an electrode material (wiring portion) that functions as a fuse resistance element that is disconnected (fused) by the drive current Ids from the drive transistor 121 flowing through the damaged (short-circuited) organic EL element 127 It is to arrange (form). As an electrode material functioning as a fuse element, it is easy to use a lead pattern drawn from the cathode side electrode.

  In the conventional configuration, the upper electrode 508 of the organic EL element 127 is a solid film so as to cover the entire surface of the pixel array unit 102, but first, the cathode electrodes are individually patterned for each of the plurality of organic EL elements 127. As a result, even if the lower electrode 504 serving as the anode electrode is connected to the source of the driving transistor 121 in common, a plurality of organic EL elements 127 are arranged in one pixel.

  On the other hand, various methods are conceivable as to how to arrange (form) an electrode material that functions as a fuse resistance element in a part of the current path passing through the organic EL element 127. In particular, a technique is adopted in which each cathode side of the plurality of organic EL elements 127 in one pixel is electrically connected to the cathode potential Vcath as a reference potential in a thin and thin pattern. For example, an auxiliary wiring 515 is used as a portion for applying the cathode potential Vcath serving as a reference potential. That is, paying attention to the lower electrode 504 serving as an anode electrode and the upper electrode 508 among the upper electrode 508 serving as a cathode electrode provided across the organic layers 506 of the plurality of organic EL elements 127 in one pixel, The electrode 508 and the auxiliary wiring 515 are connected in a thin pattern with a thin film thickness.

  By connecting the upper electrode 508 and the auxiliary wiring 515 in a thin pattern, the pattern becomes high resistance and thin and thin, that is, the cross-sectional area is small, so that it can function as a fuse resistance element. If a large current flows in a thin thin film pattern when the organic EL element 127 is short-circuited, the pattern is fused. The wiring on the cathode side of the organic EL element 127 is opened by fusing, and no current flows through the shorted organic EL element 127, and light is emitted from the remaining normal organic EL element 127 in one pixel. It becomes like this. This will be specifically described below.

<< Pixel circuit for dark spot element countermeasures >>
FIG. 5 is a diagram for explaining an example of the dark spot element countermeasure of the present embodiment. Here, FIG. 5A is a schematic diagram of a plane for one pixel (a plan perspective diagram focusing on the electrodes), and FIG. 5B is a schematic diagram of a cross section.

  As shown in FIG. 5, the pixel circuit P of the present embodiment divides a conventional pixel into two regions, a divided pixel P_1 and a divided pixel P_2, and each divided pixel P_1, P_2 first has one organic pixel. It is similar to the third comparative example shown in FIG. 4C in that an EL element 127 is provided. The difference is that in the divided pixels P_1 and P_2 divided into two regions, not only the anode side but also the cathode side of each organic EL element 127_1 and 127_2 is individually patterned to form the lower electrodes 504_1 and 504_2 and the upper electrodes 508_1 and 508_2. Form. The anode side does not necessarily need to be individually patterned.

  Further, the upper electrodes 508_1 and 508_2 (that is, the respective cathode electrodes) of the organic EL elements 127_1 and 127_2 are formed of thin lead patterns 509_1 and 509_2 from a part of the edges of the upper electrodes 508_1 and 508_2. Metal wiring) is extended to the auxiliary wiring 515 side, and is connected to the auxiliary wiring 515 through connection holes 508a_1 and 508a_2 each having a contact function. The auxiliary wiring 515 functions as a ground wiring cath. By forming the lead patterns 509_1 and 509_2 functioning as fuse elements with the same material as the upper electrode 508, it is only necessary to draw out a part of the upper electrode 508 and pattern it, and the lead patterns 509_1 and 509_2 can be easily formed by patterning. Can be formed.

  As for how to arrange (form) an electrode material that functions as a fuse resistance element in a part of the current path that passes through the organic EL element 127, various methods can be considered. In particular, with respect to each cathode side of the plurality of organic EL elements 127 in one pixel, thin lead patterns 509_1 and 509_2 are thin and thin at a portion where the taper angle of the opening defining insulating film 505 is a forward direction (referred to as a forward taper portion). Is forming. The thin lead patterns 509_1 and 509_2 having the forward taper portion are electrode material portions that function as fuse resistance elements. Roughly, the upper electrode 508 seems to cover almost the entire surface of the pixel array portion 102, but minutely, due to the presence of the extraction patterns 509_1 and 509_2, most of the forward tapered portions are opened, and the extraction patterns 509_1 and 509_2 It will be in the state connected only by a part by.

  By connecting the upper electrodes 508_1 and 508_2 to the auxiliary wiring 515 with thin lead patterns 509_1 and 509_2, the lead patterns 509_1 and 509_2 have high resistance and are thin and thin. Function. If the leading taper portion drawing pattern 509 is formed thin and thin, the forward taper portion (drawing pattern 509) burns out due to overcurrent and changes from a short to an open state and is automatically cut.

  As a result, when one of a plurality of organic EL elements 127_1 and 127_2 existing in one pixel is short-circuited due to damage caused by a foreign substance mixed at the time of manufacturing the panel, the short-circuited organic EL element is the same as the conventional one. Almost all of the drive current Ids flows from the drive transistor 121, but this drive current Ids flows through the thin lead pattern 509 (thin film thickness is thin and high resistance) shown in FIG. Load is applied. For this reason, the portion of the drawing pattern 509 generates heat, which burns out and breaks. Therefore, the anode and cathode of the short-circuited organic EL element are opened.

  Therefore, in effect, the damaged organic EL element 127 automatically changes from the shorted dark spot state to the open dark spot state. As a result, as in FIG. 4E (1), the drive current Ids from the drive transistor 121 flows to another normal organic EL element in the same pixel. Therefore, when viewed as one pixel as a whole, the total current is the organic EL element. The drive current Ids is the same as in the case where there is no damage, and the luminance obtained from one pixel can be equivalent to the luminance regardless of the presence of a dark spot.

  Even if one of the divided pixels becomes a short-circuited dark spot, a drive current Ids at the time of short-circuit is supplied to the lead-out pattern 509, and the lead-out pattern 509 is blown to change to an open dark spot. If the display is performed with an organic EL element having a large number of divided pixels, it is possible to enjoy the effect that it does not appear as a point defect. Thereby, it is possible to prevent one pixel from completely becoming a dark spot, and it is possible to avoid a decrease in yield due to a point defect.

  Even if there is a dark spot defect due to a short circuit of the current-driven electro-optic element, it is possible to prevent a complete dark spot defect, thereby reducing a decrease in luminance due to the dark spot. In addition, the number of dark spots can be reduced, a high yield can be realized, and a display device with good image quality can be obtained. In addition, by reducing the short circuit dark spots, the current consumed by the short dark spots can be reduced, and the power consumption can be reduced.

<Contrast with similar configuration example>
As a mechanism similar to the configuration of the present embodiment described above, for example, Japanese Patent Publication No. 2003-521094 discloses a plurality of sub-pixels in a branch arranged electrically in parallel with respect to one electroluminescent element. Arrangements have been proposed in which, in each branch, a fuse element is provided between one of the connections and part of the layer of electroluminescent material. In the embodiment (see paragraphs 12 to 15), a mechanism for forming a fuse element by pattern formation on the anode side of the sub-pixel is specifically shown, and focusing on the cathode side of the sub-pixel, It has been suggested that a fuse element is formed by pattern formation between elements (for example, by a cathode material). Further, paragraph 16 suggests that the same mechanism can be applied not only to the anode side but also to the cathode side.

  On the other hand, in the structure of the present embodiment, thin leading patterns 509_1 and 509_2 are formed on the forward taper portion of the opening defining insulating film 505, so that most of the forward taper portion is opened, and the lead patterns 509_1 and 509_1 By being in a state in which only a part is connected by 509_2, the lead-out patterns 509_1 and 509_2 having thin forward tapered portions function as a fuse resistance element. When an abnormality occurs, the forward taper portion (drawer pattern 509) burns out due to overcurrent, changes from short to open, and automatically cuts, so there is an advantage that it is extremely simple and patterning is easy. Such a mechanism of the present embodiment is not disclosed in Japanese Patent Publication No. 2003-521094.

  As mentioned above, although this invention was demonstrated using embodiment, the technical scope of this invention is not limited to the range as described in the said embodiment. Various changes or improvements can be added to the above-described embodiment without departing from the gist of the invention, and embodiments to which such changes or improvements are added are also included in the technical scope of the present invention.

  Further, the above embodiments do not limit the invention according to the claims (claims), and all combinations of features described in the embodiments are not necessarily essential to the solution means of the invention. Absent. The embodiments described above include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. Even if some constituent requirements are deleted from all the constituent requirements shown in the embodiment, as long as an effect is obtained, a configuration from which these some constituent requirements are deleted can be extracted as an invention.

<Modification of Pixel Circuit>
On the pixel circuit side, as a configuration example of a bootstrap circuit and a threshold & mobility correction circuit which are examples of a drive signal stabilization circuit that maintains a drive current constant, a 2TR configuration using an n-channel type as the drive transistor 121 is adopted. Although an example in which the drive timing is devised is shown, this is merely an example of a drive signal stabilization circuit and a drive timing for maintaining a drive signal for driving the organic EL element 127 constant. As the drive signal stabilizing circuit for preventing the influence on the drive current Ids due to deterioration or characteristic variation of the n-channel type drive transistor 121 (for example, variation or fluctuation of threshold voltage or mobility), various other circuits are used. Can be applied.

  For example, since “dual theory” holds in circuit theory, the pixel circuit P can be modified from this point of view. In this case, although not shown in the figure, the pixel circuit P having the 2TR configuration shown in FIG. 2 is configured using the n-channel driving transistor 121, whereas the p-channel driving transistor (hereinafter referred to as p) is used. The pixel circuit P is configured using a type driving transistor 121p. In accordance with this, a change is made in accordance with the dual reason, such as reversing the polarity of the signal amplitude Vin of the video signal Vsig and the magnitude relation of the power supply voltage.

  The modification described here is a modification of the 2TR configuration shown in FIG. 2 in accordance with the “dual theory”, but the method of circuit modification is not limited to this. For example, as shown in FIG. 6, it is possible to adopt a configuration in which the polarities are different, such as an n-channel sampling transistor 125 and a p-channel driving transistor 121. In FIG. 6, the storage capacitor 120 is connected between the gate and the source of the driving transistor 121, the source is fixed at the first potential Vcc, and a plurality of organic EL elements 127_1 and 127_2 are connected to the drain side. Yes.

  In such a system, the increase in the anode-cathode voltage Vel of the organic EL element 127 appears on the drain end side of the drive transistor 121. However, since the drive transistor 121 operates in a saturation region, The constant current Ids continues to flow through the organic EL element 127, and even if the Iel-Vel characteristic of the organic EL element 127 deteriorates, the emission luminance does not deteriorate with time. A driving transistor 121, a light emission control transistor 122, a storage capacitor 120, and a sampling transistor 125, which corrects a change in current-voltage characteristics of an organic EL element 127, which is an example of an electro-optical element, and maintains the driving current constant. A signal stabilization circuit is configured. That is, when the pixel circuit P is driven with the pixel signal Vsig, the source end of the p-channel type driving transistor 121 is connected to the first potential Vcc and is designed to always operate in the saturation region. The constant current source has the value shown in (1).

  Further, the voltage at the drain end of the drive transistor 121 changes as the Iel-Vel characteristic of the organic EL element 127 changes with time (FIG. 3B), but the drive transistor 121 includes the gate and source of the drive transistor 121. Since the gate-source voltage Vgs is held constant in principle by the bootstrap function of the holding capacitor 120 connected therebetween, the drive transistor 121 operates as a constant current source. As a result, the organic EL element 127 , A constant amount of current flows, and the organic EL element 127 can emit light with constant luminance, and the light emission luminance does not change. The potential at the source end of the driving transistor 121 (source potential Vs) is determined by the operating point between the driving transistor 121 and the organic EL element 127. Since the driving transistor 121 is driven in the saturation region, it corresponds to the source voltage at the operating point. With respect to the gate-source voltage Vgs, the drive current Ids having the current value defined in the above equation (1) continues to flow, and the light emission luminance does not change.

  Although not shown, other than the 2TR configuration in which other switching transistors for controlling the driving current to be constant are provided in addition to the sampling transistor (an example of a switching transistor) and the driving transistor. However, in order to realize a small display device that requires high-definition display, it is optimal to realize a drive signal stabilization function with a 2TR configuration.

  Here, also in various modified examples, one conventional pixel is divided into a plurality of regions, each having an organic EL element, and the cathode electrode of each organic EL element in one pixel is individually patterned. Further, it is possible to adopt a mechanism in which each cathode electrode is connected to the ground wiring cath by a fine line pattern that functions as a fuse resistance element. Even if one of the divided pixels has a shorted dark spot, the fine line pattern that functions as a fuse resistance element is blown by a short current to electrically isolate the dark spot and cause the other divided pixels to emit light. The dark spot location of the divided pixel is made inconspicuous, and the yield reduction due to the point defect can be avoided.

1 is a block diagram showing an outline of a configuration of an active matrix display device which is an embodiment of a display device according to the present invention. It is a figure which shows the 1st comparative example with respect to the pixel circuit P of this embodiment. It is a figure explaining the characteristic variation of a pixel component, and its influence. It is a figure explaining the equivalent circuit of the organic EL element at the time of a dark spot generation | occurrence | production. It is the figure which showed the outline of the layer structure for 1 pixel in a general organic electroluminescent display apparatus. It is the figure which showed the example of a layout of the lower electrode of an organic EL element, and auxiliary wiring. It is a figure which shows the pixel circuit of the 3rd comparative example provided with the dark spot element countermeasure function. It is the figure which showed the outline of the layer structure for 1 pixel of a 3rd comparative example. It is a figure explaining the problem in the 3rd comparative example shown in Drawing 4C. It is a figure explaining the dark spot element countermeasure of this embodiment. It is a figure which shows the modification of the pixel circuit provided with the dark spot element countermeasure function.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Display apparatus, 100 ... Display panel part, 101 ... Board | substrate, 102 ... Pixel array part, 103 ... Vertical drive part, 104 ... Write scanning part, 104WS ... Write scanning line, 105 ... Drive scanning part, 105DSL ... Power supply Supply line 106 ... Horizontal drive unit 106HS ... Video signal line 109 ... Control unit 120 ... Retention capacitor 121 ... Drive transistor 125 ... Sampling transistor 127 ... Organic EL element 127a ... EL opening 200 ... Drive Signal generation unit 220 ... Video signal processing unit 504 ... Lower electrode (anode electrode), 504a ... Connection hole, 505 ... Opening defining insulating film, 506 ... Organic layer, 508 ... Upper electrode (cathode electrode), 515 ... Auxiliary wiring , Cel ... parasitic capacitance, P ... pixel circuit, KC ... cathode contact

Claims (4)

A pixel circuit including a current-driven electro-optic element that performs display according to the signal amplitude and a drive transistor that drives the electro-optic element;
One pixel circuit has a plurality of the electro-optic elements to which a current obtained by diverting a drive current from the common drive transistor in the pixel circuit is supplied,
Each of the plurality of electro-optic elements has a cathode-side electrode individually patterned, and each cathode-side electrode is connected to a predetermined reference potential by an electrode material that functions as a fuse element. Characteristic display device. The display device according to claim 1, wherein the electrode material functioning as the fuse element is a drawing pattern drawn from the cathode-side electrode. An auxiliary wiring connected to the reference potential is formed in the wiring layer on which the anode electrode of the electro-optic element is formed,
3. The display device according to claim 2, wherein an electrode on each cathode side of the plurality of electro-optic elements is connected to the auxiliary wiring by the lead pattern functioning as the fuse element. The display device according to claim 2, wherein the lead-out pattern that functions as the fuse element is formed of the same material as the cathode-side electrode of the plurality of electro-optic elements.

Images