Patent Publication Number: US-10325556-B2

Title: Display panel and display unit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Priority Patent Application JP 2016-249338 filed on Dec. 22, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The technology relates to a display panel and a display unit. 
     In the technical field of display units that display an image, recently, a display unit utilizing, as a light-emitting device of a pixel, a current-driven optical device such as an organic electroluminescence (EL) device has been developed, and increasingly commercialized. The current-driven optical device has emission luminance which varies depending on a value of a flowing current. The organic EL device is a self-light-emitting device unlike a device such as a liquid crystal device. The display unit utilizing the organic EL device (organic EL display unit) therefore does not need a light source (backlight), thus enabling the organic EL display unit to be more lightweight and thinner, and to have higher luminance than a liquid crystal display unit that needs a light source. Further, the organic EL device has a very high response speed of about several micro seconds, thus preventing the occurrence of an afterimage during display of a motion picture. Hence, the organic EL display unit is expected to be a mainstream next-generation flat panel display. 
     An active-matrix organic EL display unit has a configuration in which each scanning line is sequentially scanned for one horizontal period (1H), and a signal voltage corresponding to an image signal is sampled and is written into a storage capacitor. That is, the line sequential scanning in a 1H cycle allows for the writing operation of the signal voltage. When a threshold voltage and mobility of a driving transistor differ for each pixel, the organic EL device may have irregular emission luminance in the organic EL display unit, resulting in impaired uniformity of a screen. Thus, the active-matrix organic EL display unit performs a correction operation that reduces the irregular emission luminance caused by the irregular threshold voltage and the irregular mobility of the driving transistor, in addition to the line sequential scanning in the 1H cycle. For example, reference is made to Japanese Unexamined Patent Application Publication No. 2013-200541. 
     SUMMARY 
     The disclosure of the above-mentioned Japanese Unexamined Patent Application Publication No. 2013-200541 has an issue in which a correction capability to be obtained by the above-described correction operation is not very high. 
     It is desirable to provide a display panel and a display unit that make it possible to improve a correction capability to be obtained by a correction operation. 
     A display panel with a plurality of pixels, the pixels each including a light-emitting device and a pixel circuit, the pixel circuit including a driving transistor configured to control a current flowing into the light-emitting device, a memory circuit configured to store a signal voltage corresponding to an image signal, and apply the stored signal voltage to a gate of the driving transistor, a writing transistor configured to write the signal voltage into the memory circuit, and a first storage capacitor provided between the gate of the driving transistor and an anode of the light-emitting device, the memory circuit including a second storage capacitor configured to store the signal voltage, a first switching transistor provided between the gate of the driving transistor and the second storage capacitor, and a second switching transistor that is provided between the second storage capacitor and the first switching transistor, or provided on side opposite to the first switching transistor with respect to the second storage capacitor. 
     A display unit including: a display panel with a plurality of pixels, the pixels each including a light-emitting device and a pixel circuit; and a driving circuit configured to drive the plurality of pixels, the pixel circuit including a driving transistor configured to control a current flowing into the light-emitting device, a memory circuit configured to store a signal voltage corresponding to an image signal, and apply the stored signal voltage to a gate of the driving transistor, a writing transistor configured to write the signal voltage into the memory circuit, and a first storage capacitor provided between the gate of the driving transistor and an anode of the light-emitting device, the memory circuit including a second storage capacitor configured to store the signal voltage, a first switching transistor provided between the gate of the driving transistor and the second storage capacitor, and a second switching transistor that is provided between the second storage capacitor and the first switching transistor, or provided on side opposite to the first switching transistor with respect to the second storage capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments and, together with the specification, serve to explain the principles of the technology. 
         FIG. 1  is a schematic configuration diagram of a display unit according to an embodiment of the technology. 
         FIG. 2  illustrates an example of a circuit configuration of each of pixels illustrated in  FIG. 1 . 
         FIG. 3  illustrates an example of a temporal change in each of voltages to be applied to a power supply line, a signal line, and control lines, a gate voltage and a source voltage of a driving transistor, and a voltage at a node A. 
         FIG. 4  illustrates an example of an operation of a pixel. 
         FIG. 5  illustrates an example of an operation of a pixel. 
         FIG. 6  illustrates an example of an operation of a pixel. 
         FIG. 7  illustrates an example of an operation of a pixel. 
         FIG. 8  illustrates an example of a temporal change in the source voltage of the driving transistor. 
         FIG. 9  illustrates an example of an operation of a pixel. 
         FIG. 10  illustrates an example of an operation of a pixel. 
         FIG. 11  illustrates an example of an operation of a pixel. 
         FIG. 12  illustrates an example of an operation of a pixel. 
         FIG. 13  illustrates a modification example of a circuit configuration of each pixel. 
         FIG. 14  is a perspective view of an outer appearance of one application example of a display unit according to any one of the above-mentioned embodiment and the modification example thereof. 
     
    
    
     DETAILED DESCRIPTION 
     Some example embodiments of the technology are described below in detail with reference to the accompanying drawings. 
     It is to be noted that the following description is directed to illustrative examples of the technology and not to be construed as limiting to the technology. Factors including, without limitation, numerical values, shapes, materials, components, positions of the components, and how the components are coupled to each other are illustrative only and not to be construed as limiting to the technology. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the technology are optional and may be provided on an as-needed basis. The drawings are schematic and are not intended to be drawn to scale. It is to be noted that the like elements are denoted with the same reference numerals, and any redundant description thereof will not be described in detail. It is to be noted that the description is given in the following order. 
     1. Example Embodiment (Display Unit) 
     2. Modification Example (Display Unit) 
     3. Application Example (Electronic Apparatus) 
     [1. Example Embodiment] 
     [Configuration] 
       FIG. 1  illustrates a schematic configuration of a display unit  1  according to an embodiment of the technology. The display unit  1  includes a display panel  10 , a controller  20 , and a driver  30 , for example. The controller  20  and the driver  30  correspond to a specific but non-limiting example of a “driving circuit” according to an embodiment of the technology. In the display panel  10 , a plurality of pixels  11  are disposed in matrix. The controller  20  and the driver  30  may drive the plurality of pixels  11  on the basis of an image signal Din and a synchronizing signal Tin which are inputted from the outside. 
     [Display Panel  10 ] 
       FIG. 2  illustrates an example of a circuit configuration of each of the pixels  11  included in the display panel  10 . The controller  20  and the driver  30  may active-matrix-drive each of the pixels  11  to allow the display panel  10  to display an image based on the image signal Din and the synchronizing signal Tin which are inputted from the outside. The display panel  10  may include, for example, a plurality of control lines WSL extending in a row direction, a plurality of control lines CTL 1  and a plurality of control lines CTL 2  both extending in a row direction, and a plurality of signal lines DTL and a plurality of power supply lines DSL both extending in a column direction. It is to be noted that the plurality of power supply lines DSL may extend in a row direction. The display panel  10  further includes the plurality of pixels  11 . Each one of the plurality of pixels  11  may be provided for every location at which corresponding one of the control lines WSL and corresponding one of the signal lines DTL intersect with each other. 
     The control line WSL may select each of the pixels  11 . The signal line DTL may supply to corresponding one of the pixels  11  a signal voltage Vsig in accordance with the image signal Din. The signal line DTL may supply to corresponding one of the pixels  11  a data pulse that includes the signal voltage Vsig. The power supply line DSL may supply power to corresponding one of the pixels  11 . The control line CTL 1  may supply to corresponding one of the pixels  11  a control pulse that performs ON/OFF control of a switching transistor Tr 3  described later. The control line CTL  2  may supply to corresponding one of the pixels  11  a control pulse that performs ON/OFF control of a switching transistor Tr 4  described later. The switching transistor Tr 3  corresponds to a specific but non-limiting example of a “first switching transistor” according to an embodiment of the technology. The switching transistor Tr 4  corresponds to a specific but non-limiting example of a “second switching transistor” according to an embodiment of the technology. 
     Each of the pixels  11  includes a pixel circuit  12  and an organic EL device  13 , for example. The organic EL device  13  corresponds to a specific but non-limiting example of a “light-emitting device” according to an embodiment of the technology. The organic EL device  13  may have a configuration in which an anode electrode, an organic layer, and a cathode electrode are stacked in order, for example. The organic EL device  13  may have a device capacitor, i.e., a device capacitor Ce 1  described later. The pixel circuit  12  may control emission and extinction of the organic EL device  13 . The pixel circuit  12  may serve to store a voltage written into corresponding one of the pixels  11  by means of write scanning described later. The pixel circuit  12  includes a driving transistor Tr 1 , a writing transistor Tr 2 , a storage capacitor Cs, and a memory circuit  12 A, for example. The memory circuit  12 A includes switching transistors Tr 3  and Tr 4  and a storage capacitor Cs 2 , for example. The storage capacitor Cs 1  corresponds to a specific but non-limiting example of a “first storage capacitor” according to an embodiment of the technology. The storage capacitor Cs 2  corresponds to a specific but non-limiting example of a “second storage capacitor” according to an embodiment of the technology. 
     The writing transistor Tr 2  may control writing of the signal voltage Vsig to the memory circuit  12 A. The signal voltage Vsig corresponds to the image signal Din. In a specific but non-limiting example, the writing transistor Tr 2  may sample a voltage of the signal line DTL, and write the voltage obtained by the sampling into the memory circuit  12 A. The driving transistor Tr 1  may be coupled in series to the organic EL device  13 . The driving transistor Tr 1  may drive the organic EL device  13 . The driving transistor Tr 1  may control a current flowing into the organic EL device  13  depending on magnitude of the voltage sampled by the writing transistor Tr 2 . 
     The storage capacitor Cs 1  may store a predetermined voltage between a gate and a source of the driving transistor Tr 1 . The storage capacitor Cs 1  is provided between the gate of the driving transistor Tr 1  and the anode of the organic EL device  13 . 
     The memory circuit  12 A stores the signal voltage Vsig, and applies the stored signal voltage Vsig to the gate of the driving transistor Tr 1 . The storage capacitor Cs 2  stores the signal voltage Vsig. The switching transistor Tr 3  is provided between the gate of the driving transistor Tr 1  and the storage capacitor Cs 2 . Further, the switching transistor Tr 3  may be provided between the gate of the driving transistor Tr 1  and a source or a drain of the writing transistor Tr 2 . The switching transistor Tr 4  is provided on side opposite to the switching transistor Tr 3  with respect to the storage capacitor Cs 2 . In a specific but non-limiting example, the switching transistor Tr 4  may be provided between the storage capacitor Cs 2  and the power supply line DSL. It is to be noted that the pixel circuit  12  may have a circuit configuration in which various capacitors or transistors are added to the above-described circuit including four transistors (Tr) and two capacitors (C), i.e.,  4 Tr 2 C; in an alternative embodiment, the pixel circuit  12  may have a circuit configuration different from that of the above-described  4 Tr 2 C. 
     The driving transistor Tr 1 , the writing transistor Tr 2 , and the switching transistors Tr 3  and Tr 4  may be each formed of n-channel metal oxide semiconductor (MOS) thin film transistor (TFT), for example. It is to be noted that these transistors may be each formed of p-channel MOS TFT. The following description is given on the assumption that these transistors are of enhancement type. However, in one embodiment, these transistors may be of depression type. 
     Each of the signal lines DTL may be coupled to an output end of a horizontal selector  31  described later and to the source or the drain of the writing transistor Tr 2 . Each of the control lines WSL may be coupled to one of output ends of a timing generation circuit  22  described later and to a gate of the writing transistor Tr 2 . Each of the power supply lines DSL may be coupled to an output end of a power supply circuit  23  described later and to the source or the drain of the driving transistor Tr 1 . Each of the power supply lines DSL may be further coupled to the output end of the power supply circuit  23  and to a source or a drain of the switching transistor Tr 4 . The power supply lines may be electrically coupled to one another, and thus may have a common potential. In each of the pixels  11 , the source or the drain of the driving transistor Tr 1  may be electrically coupled to one of the plurality of power supply lines DSL that have a mutually common potential. Each of the control lines CTL 1  may be coupled to the other of the output ends of the timing generation circuit  22  described later and to a gate of the switching transistor Tr 3 . Each of the control lines CTL 2  may be coupled to an output end of a control scanner  32  described later and to a gate of the switching transistor Tr 4 . 
     The gate of the writing transistor Tr 2  may be coupled to corresponding one of the control lines WSL. The source or the drain of the writing transistor Tr 2  may be coupled to corresponding one of the signal lines DTL. A terminal, of the source and the drain of the writing transistor Tr 2 , that is not coupled to the corresponding one of the signal lines DTL may be coupled to a source or a drain of the switching transistor Tr 3 . The terminal, of the source and the drain of the writing transistor Tr 2 , that is not coupled to the corresponding one of the signal lines DTL may be coupled further to one end of the storage capacitor Cs 2 . The gate of the switching transistor Tr 3  may be coupled to corresponding one of the control lines CTL 1 . The source or the drain of the switching transistor Tr 3  may be coupled one end of the storage capacitor Cs 2  and to the terminal, of the source and the drain of the writing transistor Tr 2 , that is not coupled to the corresponding one of the signal lines DTL. A terminal, of the source and the drain of the switching transistor Tr 3 , that is coupled neither to the writing transistor Tr 2  nor to the storage capacitor Cs 2  may be coupled to the gate of the driving transistor Tr 1  and to one end of the storage capacitor Cs 1 . The gate of the driving transistor Tr 1  may be coupled to one end of the storage capacitor Cs 1  and to the terminal, of the source and the drain of the switching transistor Tr 3 , that is coupled neither to the storage capacitor Cs 2  nor to the writing transistor Tr 2 . The source or the drain of the driving transistor Tr 1  may be coupled to corresponding one of the power supply lines DSL. A terminal, of the source and the drain of the switching transistor Tr 1 , that is not coupled to the corresponding one of the power supply lines DSL may be coupled to the anode of the organic EL device  13  and to the other end of the storage capacitor Cs 1 . One end of the storage capacitor Cs 1  may be coupled to the gate of the driving transistor Tr 1 . The other end of the storage capacitor Cs 1  may be coupled to the terminal, of the source and the drain of the switching transistor Tr 1 , that is not coupled to the corresponding one of the power supply lines DSL. One end of the storage capacitor Cs 2  may be coupled to the terminal, of the source and the drain of the writing transistor Tr 2 , that is not coupled to the corresponding one of the signal lines DTL. One end of the storage capacitor Cs 2  may be further coupled to a terminal, of the source and the drain of the switching transistor Tr 3 , that is not coupled to the gate of the driving transistor Tr 1 . The other end of the storage capacitor Cs 2  may be coupled to a source or a drain of the switching transistor Tr 4 . The gate of the switching transistor Tr 4  may be coupled to correspond one of the control lines CTL 2 . The source or the drain of the switching transistor Tr 4  may be coupled to the storage capacitor Cs 2 . A terminal, of the source and the drain of the switching transistor Tr 4 , on side opposite to the storage capacitor Cs 2  may be coupled to the corresponding one of the power supply lines DSL. 
     The driver  30  may include the horizontal selector  31  and the control scanner  32 , for example. 
     The horizontal selector  31  may apply to each of the signal lines DTL an analog signal voltage Vsig inputted from an image signal processing circuit  21  in response to (in synchronization with) the input of a control signal, for example. The horizontal selector  31  may be able to output two types of voltages (i.e., Vofs and Vsig), for example. In a specific but non-limiting example, the horizontal selector  31  may supply the two types of voltages (i.e., Vofs and Vsig) to a pixel  11  selected by the timing generation circuit  22  via corresponding one of the signal lines DTL. The signal voltage Vsig may have a voltage value corresponding to the image signal Din. The fixed voltage Vofs may be a constant voltage irrelevant to the image signal Din. The minimum voltage of the signal voltage Vsig may have a voltage value lower than the fixed voltage Vofs. The maximum voltage of the signal voltage Vsig may have a voltage value higher than the fixed voltage Vofs. The horizontal selector  31  may output a data pulse including the signal voltage Vsig to each of the signal lines DTL for one horizontal period. The horizontal selector  31  may output to each of the signal lines DTL a pulse having two values of the signal voltage Vsig and the fixed voltage Vofs, as a data pulse. 
     The control scanner  32  may control ON/OFF operation of the switching transistor Tr 4  of each of the pixels  11  in response to (in synchronization with) the input of the control signal, for example. The control scanner  32  may be able to output two types of voltages (i.e., Von and Voff), for example. In a specific but non-limiting example, the control scanner  32  may supply the two types of voltages (i.e., Von and Voff) to a pixel  11  to be driven via corresponding one of the control lines CTL 2  to perform the ON/OFF control of the switching transistor Tr 4 . The ON-voltage Von may be a value equal to or higher than the ON-voltage of the switching transistor Tr 4 . The OFF-voltage Voff may be a value lower than the ON-voltage of the switching transistor Tr 4  and may be a value lower than the ON-voltage Von. 
     The control scanner  32  may scan a plurality of pixels  11  for each predetermined unit during memory writing described later. In a specific but non-limiting example, the control scanner  32  may sequentially output a control pulse to each of the control lines CTL 2  in one frame period. The control scanner  32  may select the plurality of control lines CTL 2  through a predetermined sequence in response to (in synchronization with) the input of the control pulse, for example, to thereby cause the memory writing to be executed in a desired order. As used herein, the term “memory writing” refers to writing the signal voltage Vsig into the memory circuit  12 A (i.e., storage capacitor Cs 2 ). 
     [Controller  20 ] 
     Description is given next of the controller  20 . The controller  20  may include the image signal processing circuit  21 , the timing generation circuit  22 , and the power supply circuit  23 , for example. 
     The image signal processing circuit  21  may perform a predetermined correction to a digital image signal Din inputted from the outside, for example, and may generate the signal voltage Vsig on the basis of the image signal obtained by the predetermined correction. The image signal processing circuit  21  may output the generated signal voltage Vsig to the horizontal selector  31 , for example. Non-limiting examples of the predetermined correction may include gamma correction and overdrive correction. 
     The timing generation circuit  22  may control circuits in the driver  30  to operate in conjunction with one another. The timing generation circuit  22  may output a control signal to each of the circuits in the driver  30  in response to (in synchronization with) a synchronizing signal Tin inputted from the outside, for example. The timing generation circuit  22  may further output a predetermined control signal to each of the control lines CTL 1  and each of the control lines WSL in the display panel  10 . The timing generation circuit  22  may be able to output the two types of voltages (i.e., Von and Voff), for example. In a specific but non-limiting example, the timing generation circuit  22  may supply the two types of voltages (i.e., Von and Voff) to a pixel  11  to be driven via corresponding one of the control lines CTL 1  and corresponding one of the control lines WSL to perform the ON/OFF control of each of the writing transistor Tr 2  and the switching transistor Tr 3 . The ON-voltage Von may be a value equal to or higher than an ON-voltage of each of the writing transistor Tr 2  and the switching transistor Tr 3 . The OFF-voltage Voff may be a value lower than the ON-voltage of each of the writing transistor Tr 2  and the switching transistor Tr 3 , and may be a value lower than the ON-voltage Von. 
     The power supply circuit  23  may generate various fixed voltages necessary for various circuits, and may supply the generated various fixed voltages. Non-limiting examples of the various circuits may include the horizontal selector  31 , the control scanner  32 , the image signal processing circuit  21 , and the timing generation circuit  22 . The power supply circuit  23  may further generate various fixed voltages necessary for each of the power supply lines DSL in the display panel  10 , and may supply the generated various fixed voltages. 
     [Operation] 
     Description is given next of operations (from extinction operation to emission operation) of the display unit  1  according to the present example embodiment. The present example embodiment may incorporate a compensation operation for variation in I-V characteristics of the organic EL device  13 , in order to keep emission luminance of the organic EL device  13  constant without being affected by possible temporal change in the I-V characteristics of the organic EL device  13 . Further, the present example embodiment may incorporate a compensation operation for variation in a threshold voltage and mobility of the driving transistor Tr 1 , in order to keep the emission luminance of the organic EL device  13  constant without being affected by possible temporal change in the threshold voltage and the mobility of the driving transistor Tr 1 . 
     In  FIG. 3 , the term “threshold correction preparation” refers to initializing a gate voltage of the driving transistor Tr 1  (refers to changing the gate voltage to Vofs, in a specific but non-limiting example), and initializing a source voltage of the driving transistor Tr 1  (refers to changing the source voltage to Vss, in a specific but non-limiting example). The term “threshold correction” refers to a correction operation in which the gate-source voltage Vgs of the driving transistor Tr 1  is made closer to a threshold voltage Vth of the driving transistor Tr 1 . The term “mobility correction” refers to a correction operation in which a voltage stored between the gate and the source of the driving transistor Tr 1  (gate-source voltage Vgs) is corrected depending on magnitude of mobility of the driving transistor Tr 1 . Signal voltage transfer and the mobility correction may be performed at different timings in some cases. According to the present example embodiment, the signal writing and the mobility correction may be performed together (or continuously without an interval). The signal voltage transfer refers to an operation in which the signal voltage Vsig written into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) is transferred to the gate of the driving transistor Tr 1 . 
       FIG. 3  illustrates an example of a temporal change in each of voltages to be applied to the control line WSL, the power supply line DSL, the signal line DTL, and the control lines CTL 1  and CTL  2 , a gate voltage Vg and a source voltage Vs of the driving transistor Tr 1 , and a voltage Va at a node A.  FIGS. 4 to 7 and 9 to 11  each illustrate an example of an operation of the pixel  11 .  FIG. 8  illustrates an example of a temporal change in the source voltage Vs of the driving transistor Tr 1 . 
     It is to be noted that a voltage of the power supply line DSL is a voltage that is applied simultaneously to all of the power supply lines DSL in the display panel  10 . A voltage of the control line CTL 1  may also be a voltage that is applied simultaneously to all of the control lines CTL 1  in the display panel  10 . An ON-voltage in the control line CTL 2  during extinction may be a voltage that is applied simultaneously to all of the control lines CTL 2  in the display panel  10 . An ON-voltage in the control line CTL 2  during emission may be a voltage that is applied line-sequentially to each of the control lines CTL 2  in the display panel  10 . 
     [Memory Writing] 
     First, the controller  20  and the driver  30  may write the signal voltage Vsig into the memory circuit  12 A (i.e., storage capacitor Cs 2 ). In a specific but non-limiting example, the controller  20  and the driver  30  may write the signal voltage Vsig into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) on a pixel row basis when each organic EL device  13  emits light. In other words, the controller  20  and the driver  30  may write, on a pixel row basis, the signal voltage Vsig into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) by turning, on a pixel row basis, each switching transistor Tr 4  ON, with each writing transistor Tr 2  being turned ON. 
     In a specific but non-limiting example, suppose that the voltage of the signal line DTL is Vsig; the writing transistor Tr 2  is turned ON; and the switching transistor Tr 3  is turned OFF, when each organic EL device  13  emits light. In this situation, the timing generation circuit  22  may change the voltage of the control line CTL 2  from Voff to Von on a pixel row basis to turn the switching transistor Tr 4  ON. As a result, when each organic EL device  13  emits light, the signal voltage Vsig may be written into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) on a pixel row basis to cause a voltage (Vcc−Vsig) to be applied across the storage capacitor Cs 2  (at T 13 ,  FIG. 4 ). 
     After elapse of a certain period of time (e.g., after elapse of one horizontal period), the timing generation circuit  22  may change the voltage of the control line CTL 2  from Von to Voff on a pixel row basis to turn the switching transistor Tr 4  OFF (at T 14 ). In this situation, the writing transistor Tr 2  may remain ON, and thus the voltage of the signal line DTL may be inputted as it is to the node A between the switching transistor Tr 3  and the writing transistor Tr 2 . Meanwhile, the switching transistor Tr 4  may be OFF, and thus the voltage stored by the storage capacitor Cs 2  may remain as the voltage (Vcc−Vsig) without any change. In this situation, the switching transistor Tr 3  may remain OFF, and therefore the gate-source voltage Vgs of the driving transistor Tr 1  may not be changed, thus causing a current Ids to flow into the organic EL device  13 . 
     [Extinction] 
     Next, the controller  20  and the driver  30  may extinguish the organic EL device  13 . In a specific but non-limiting example, the controller  20  and the driver  30  may extinguish the organic EL device  13  in all of the pixels  11  altogether when each organic EL device  13  emits light. 
     In a specific but non-limiting example, when each organic EL device  13  emits light, the power supply circuit  23  may decrease the voltage of the power supply line DSL from Vcc to Vss in all of the pixels  11  altogether (at T 1 ). In this situation, the fixed voltage Vss may be smaller than a sum of a threshold voltage Vthe 1  and a cathode voltage Vcat of the organic EL device  13  (Vthe 1 +Vcat). Thus, when the source voltage Vs is decreased to Vss, the organic EL device  13  is extinguished, and a terminal, of the driving transistor Tr 1 , on side of the power supply line DSL may be a source. In this situation, the gate voltage Vg may also be decreased by coupling via the storage capacitor Cs 1 , thus causing the anode of the organic EL device  13  to be charged to Vss. 
     [Threshold Correction Preparation] 
     Next, the controller  20  and the driver  30  may prepare threshold correction, in which the gate-source voltage Vgs of the driving transistor Tr 1  may be made closer to the threshold voltage Vth of the driving transistor Tr 1 . In a specific but non-limiting example, during a period of time when each organic EL device  13  is extinguished (V-blanking period), the controller  20  and the driver  30  may prepare the threshold correction in all of the pixels  11  altogether. 
     In a specific but non-limiting example, the horizontal selector  31  may first change the voltage of the signal line DTL from Vsig to Vofs in all of the pixels  11  altogether (at T 2 ,  FIG. 5 ). Accordingly, the voltage Va of the node A may be changed from Vsig to Vofs. Thereafter, the timing generation circuit  22  may change the voltage of the control line CTL 1  from Voff to Von in all of the pixels  11  altogether to turn the switching transistor Tr 3  ON (at T 3 ,  FIG. 6 ). This brings the gate voltage Vg of the driving transistor Tr 1  to Vofs. In this situation, the gate-source voltage Vgs of the driving transistor Tr 1  may be a voltage (Vofs−Vss). The voltage (Vofs−Vss) may be greater than the threshold voltage Vth of the driving transistor Tr 1 . In other words, the fixed voltages Vofs and Vss may be set to allow the voltage (Vofs−Vss) to have a value greater than the threshold voltage Vth of the driving transistor Tr 1 . 
     [Threshold Correction] 
     Next, the controller  20  and the driver  30  may perform threshold correction. In a specific but non-limiting example, the controller  20  and the driver  30  may perform the threshold correction in all of the pixels  11  altogether when each organic EL device  13  is extinguished, i.e., during the V-blanking period. 
     In a specific but non-limiting example, the power supply circuit  23  may increase the voltage of the power supply line DSL from Vss to Vcc in all of the pixels  11  altogether (at T 4 ,  FIG. 7 ). Accordingly, a current may flow between the drain and the source of the driving transistor Tr 1  to cause the source voltage to be increased. Here, an equivalent circuit of the organic EL device  13  is represented by a diode and a device capacitor Ce 1 , as illustrated in  FIG. 7 . Thus, insofar as the source voltage Vs is smaller than the sum of the threshold voltage Vthe 1  and the cathode voltage Vcat of the organic EL device  13  (Vthe 1 +Vcat), the current flowing between the drain and the source of the driving transistor Tr 1  may charge the storage capacitor Cs 1  and the device capacitor Ce 1 . The source voltage Vs may be the anode voltage Ve 1  of the organic EL device  13 . As illustrated in  FIG. 8 , the source voltage Vs, i.e., the anode voltage Ve 1  may be increased with elapse of time, for example. As a result, the storage capacitor Cs 1  may be charged to cause the gate-source voltage Vgs to come closer to Vth. 
     Thereafter, the control scanner  32  may decrease the voltage of the control line WSL from Von to Voff in all of the pixels  11  altogether. Further, the timing generation circuit  22  may decrease the voltage of the control line CTL 1  from Von to Voff in all of the pixels  11  altogether. This causes the writing transistor Tr 2  and the switching transistor Tr 3  to be turned OFF (at T 5  and T 6 ,  FIG. 9 ). Accordingly, the gate of the driving transistor Tr 1  may be brought into a floating state. 
     Next, the control scanner  32  may increase the voltage of the control line CTL 2  from Voff to Von in all of the pixels  11  altogether, thereby turning the switching transistor Tr 4  ON (at T 7 ,  FIG. 10 ). This causes respective switching transistors Tr 4  of all of the pixels  11  to be turned ON all at once. As a result, the node A may have the signal voltage Vsig. 
     [Signal Voltage Transfer and Mobility Correction] 
     Next, the controller  20  and the driver  30  may perform the signal voltage transfer and the mobility correction. In a specific but non-limiting example, the controller  20  and the driver  30  may perform the signal voltage transfer and the mobility correction in each of the pixels  11  altogether when each organic EL device  13  is extinguished, i.e., during the V-blanking period. The controller  20  and the driver  30  may transfer the signal voltage Vsig written into the storage capacitor Cs 2  to the gate of the driving transistor Tr 1  in each of the pixels  11  altogether. The controller  20  and the driver  30  may turn each of the switching transistors Tr 3  and Tr 4  ON, with each writing transistor Tr 2  being turned OFF, thereby transferring the signal voltage Vsig written into the storage capacitor Cs 2  to the gate of the driving transistor Tr 1  in each of the pixels  11  altogether. 
     In a specific but non-limiting example, the timing generation circuit  22  may increase the voltage of the control line CTL 1  from Voff to Von in all of the pixels  11  altogether, thereby turning the switching transistor Tr 3  ON (at T 8 ,  FIG. 11 ). This causes charge partitioning to occur via the switching transistor Tr 3 , thus allowing the gate voltage Vg of the driving transistor Tr 1  to have a voltage value Vsig 1  corresponding to a gradation. As a result, the driving transistor Tr 1  may cause a current that is based on the gate voltage Vg of the driving transistor Tr 1 , from the power supply line DSL, thus allowing the source voltage Vs of the driving transistor Tr 1  to be increased with elapse of time. In this situation, in a case where the source voltage Vs of the driving transistor Tr 1  does not exceed the sum of the threshold voltage Vthe 1  and the cathode voltage Vcat of the organic EL device  13  (Vthe 1 +Vcat), the current of the driving transistor Tr 1  may charge the storage capacitor Cs 1  and the device capacitor Ce 1 . In this situation, the threshold correction of the driving transistor Tr 1  has been completed. Thus, the current flowing between the drain and the source of the driving transistor Tr 1  reflects the mobility of the driving transistor Tr 1 . 
     [Emission] 
     Next, the controller  20  and the driver  30  may cause the organic EL device  13  to emit light. In a specific but non-limiting example, the controller  20  and the driver  30  may cause the organic EL device  13  to emit light in all of the pixels  11  altogether when each organic EL device  13  is extinguished. 
     In a specific but non-limiting example, the timing generation circuit  22  may decrease the voltage of the control line CTL 1  from Von to Voff in all of the pixels  11  altogether, thereby turning the switching transistor Tr 3  OFF (at T 9 ,  FIG. 12 ). Accordingly, the gate of the driving transistor Tr 1  may be brought into the floating state, thus causing the current Ids to flow between the drain and the source of the driving transistor Tr 1 , which leads to increase in the source voltage Vs. In association with the increase in the source voltage Vs, the gate voltage Vg may also be increased. The source voltage Vs may be increased to a voltage Vx at which a current Ids&#39; flows into the organic EL device  13 , thus causing the organic EL device  13  to emit light at desired luminance. 
     [Effects] 
     Description is given next of effects of the display unit  1  according to the present example embodiment. 
     An organic EL display unit typically performs a correction operation that reduces irregular emission luminance caused by an irregular threshold voltage and irregular mobility of the driving transistor. When the correction operation is performed, Vcc and Vss have been applied line-sequentially to a plurality of power supply lines provided for respective pixel rows. Vcc is a voltage that causes the organic EL device to emit light, thus making it necessary to design a circuit, that drives the power supply lines line-sequentially, to have a large current capability. This has resulted in large width of the circuit in accordance with the large current capability, thus making it difficult to have a narrow bezel. 
     Therefore, it is conceivable, for example, that the respective power supply lines may be driven altogether instead of driving them line-sequentially. For example, it is conceivable that the threshold correction may be performed for all of the pixels altogether during the blanking period and thereafter the mobility correction and the signal writing may be performed on a pixel row basis. In such a case, it is possible to eliminate the circuit that drives the power supply lines line-sequentially, thus making it possible to achieve the narrow bezel by the size of such an eliminated circuit that drives the power supply lines line-sequentially. In this method, however, time from the threshold correction to the signal writing (standby time) varies for each pixel row. Thus, an amount of a leak current generated during the standby time also varies for each pixel row. This results in occurrence of shading. In addition, it is difficult, in this method, to have longer emission time, thus causing flickering to occur. 
     The method disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2013-200541 is conceivable in order to have longer emission time while driving respective power supply lines altogether. In the method disclosed in Japanese Unexamined Patent Application Publication No. 2013-200541, the signal voltage is written into the memory circuit in the pixel circuit during the emission period, and the signal voltage having been written into the memory circuit is written all at once into respective driving transistors of all of pixels during the blanking period, thus causing all of the pixels to emit light all at once. In the pixel circuit disclosed in Japanese Unexamined Patent Application Publication No. 2013-200541, however, parasitic capacitance of two transistors in the memory circuit is added to the gate of the driving transistor, thus causing a bootstrap gain to be small. This results in lowered capability in threshold correction and in mobility correction. In order to avoid the lowered capability, it is conceivable to have a larger area of the storage capacitor between the gate and the source of the driving transistor, for example. In such a case, however, a distance between wiring lines in the pixel circuit becomes smaller by such a larger area of the storage capacitor, thus making it difficult to obtain high yield. In addition, a charge amount stored by the storage capacitor between the gate and the source of the driving transistor is more likely to vary, thus making unevenness and roughness more likely to occur on a displayed screen. 
     In contrast, according to the present example embodiment, only parasitic capacitance of one transistor, i.e., the switching transistor Tr 3  is added to the gate of the driving transistor, in the memory circuit  12 A provided in each pixel circuit  12 . This enables the bootstrap gain to be larger than the bootstrap gain obtained by the method disclosed in Japanese Unexamined Patent Application Publication No. 2013-200541, thus making it possible to suppress a loss of the bootstrap gain caused by the parasitic capacitance of the memory circuit  12 A. As a result, it becomes possible to improve the capability of each of the threshold correction and the mobility correction, as compared with the method disclosed in Japanese Unexamined Patent Application Publication No. 2013-200541, thus making it possible to achieve an image of high display quality. In addition, in the present example embodiment, it is unnecessary to make the distance between the wiring lines in the pixel circuit  12  smaller to such a degree that the yield may be lowered. In addition, neither unevenness nor roughness is likely to occur on a displayed screen. 
     Further, in the present example embodiment, the source or the drain of the driving transistor Tr 1  may be electrically coupled to one of the plurality of power supply lines DSL having a mutually common potential in each of the pixels  11 . This makes it possible to omit the circuit that drives the plurality of power supply lines DSL line-sequentially, thus enabling the display panel  10  to have a narrower bezel. 
     Furthermore, in the present example embodiment, a terminal, of the source and the drain of the switching transistor Tr 4 , on side opposite to the storage capacitor Cs 2  may be electrically coupled to the power supply line DSL. This makes it possible to allow a wiring line coupled to the switching transistor Tr 4  and a wiring line coupled to the driving transistor Tr 1  to be in common, thus enabling the display panel  10  to have a narrower bezel. 
     Moreover, in the present example embodiment, the signal voltage Vsig is written into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) on a pixel row basis. Further, the signal voltage Vsig written into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) is transferred to the gate of the driving transistor Tr 1  in each of the pixels  11  altogether. This enables the signal voltage Vsig to be written into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) when each organic EL device  13  emits light. Thus, it becomes possible to transfer the signal voltage Vsig written into the memory circuit  12 A (i.e., storage capacitor Cs 2 ) to the gate of the driving transistor Tr 1 , in each of the pixels  11  altogether, when each organic EL device  13  is extinguished. As a result, it becomes possible to increase an emission period of each of the pixels  11 , as compared with a case where the signal voltage Vsig is written when each organic EL device  13  is extinguished. 
     [2. Modification Example] 
     Description is given below of a modification example of the display unit  1  according to the foregoing example embodiment. It is to be noted that the same reference numerals are assigned to components common to those of the display unit  1  of the foregoing example embodiment. Further, descriptions of the components common to those of the display unit  1  of the foregoing example embodiment are omitted where appropriate. 
     In the foregoing example embodiment, positions of the storage capacitor Cs 2  and the switching transistor Tr 4  may be reversed. For example, as illustrated in  FIG. 13 , the switching transistor Tr 4  may be provided between the storage capacitor Cs 2  and the switching transistor Tr 3 . In this situation, for example, the source or the drain of the switching transistor Tr 4  may be coupled to one end of the storage capacitor Cs 2 . A terminal, of the source and the drain of the switching transistor Tr 4 , that is not coupled to the storage capacitor Cs 2  may be coupled to the node A. Further, in this situation, the other end of the storage capacitor Cs 2  may be electrically coupled to a wiring line that provides a fixed voltage (a wiring line to which the cathode voltage Vcat is applied, in this example). 
     Even in a case where the memory circuit  12 A has such a configuration, it is possible to drive each of the pixels  11  using the voltage signals illustrated in (A) to (E) of  FIG. 3 , for example. Hence, also in the present modification example, effects similar to those of the foregoing example embodiment are achieved. 
     [3. Application Example] 
     Description is given below of an application example of the display unit  1  described in any one of the foregoing example embodiment and modification example thereof (hereinafter, referred to as “the foregoing example embodiment, etc.”). The display unit  1  of the foregoing example embodiment is applicable to a display unit of an electronic apparatus in various fields, which may display an image signal supplied from the outside or an image signal generated inside, as an image or as a picture. Non-limiting examples of the electronic apparatus with such a display unit may include a television, a digital camera, a laptop personal computer, a portable terminal unit such as a mobile phone, and a video camera. 
       FIG. 14  illustrates a schematic configuration example of an electronic apparatus  2  according to the present application example. The electronic apparatus  2  may be a laptop foldable personal computer including a display surface  2 A on a main surface of one of two plate-shaped casings, for example. The electronic apparatus  2  may include the display unit  1  according to the foregoing example embodiment, etc., as well as the display panel  10  at a location of the display surface  2 A, for example. The display unit  1  is provided in the present application example, thus making it possible to achieve an image of high display quality. 
     Although the technology has been described hereinabove by way of example with reference to the example embodiment, the modification example, and the application example, the technology is not limited thereto but may be modified in a wide variety of ways. Moreover, the effects described hereinabove are mere examples. The effects according to an embodiment of the technology are not limited to those described hereinabove. The technology may further include other effects in addition to the effects described hereinabove. 
     For example, the technology may also have the following configurations. 
     (1) 
     A display panel with a plurality of pixels, the pixels each including a light-emitting device and a pixel circuit, 
     the pixel circuit including 
     a driving transistor configured to control a current flowing into the light-emitting device, 
     a memory circuit configured to store a signal voltage corresponding to an image signal, and apply the stored signal voltage to a gate of the driving transistor, 
     a writing transistor configured to write the signal voltage into the memory circuit, and 
     a first storage capacitor provided between the gate of the driving transistor and an anode of the light-emitting device, 
     the memory circuit including
         a second storage capacitor configured to store the signal voltage,   a first switching transistor provided between the gate of the driving transistor and the second storage capacitor, and   a second switching transistor that is provided between the second storage capacitor and the first switching transistor, or provided on side opposite to the first switching transistor with respect to the second storage capacitor.
 
(2)
       

     The display panel according to (1), in which, in each of the pixels, a source or a drain of the driving transistor is electrically coupled to one of a plurality of power supply lines that have a mutually common potential. 
     (3) 
     The display panel according to (1) or (2), in which a terminal, of a source and a drain of the second switching transistor, on side opposite to the second storage capacitor is electrically coupled to the one of the power supply lines. 
     (4) 
     The display panel according to (1) or (2), in which a terminal, of a source and a drain of the second switching transistor, on side opposite to the second storage capacitor is electrically coupled to a node between the first switching transistor and the writing transistor. 
     (5) 
     A display unit including: 
     a display panel with a plurality of pixels, the pixels each including a light-emitting device and a pixel circuit; and 
     a driving circuit configured to drive the plurality of pixels, 
     the pixel circuit including 
     a driving transistor configured to control a current flowing into the light-emitting device, 
     a memory circuit configured to store a signal voltage corresponding to an image signal, and apply the stored signal voltage to a gate of the driving transistor, 
     a writing transistor configured to write the signal voltage into the memory circuit, and 
     a first storage capacitor provided between the gate of the driving transistor and an anode of the light-emitting device, 
     the memory circuit including
         a second storage capacitor configured to store the signal voltage,   a first switching transistor provided between the gate of the driving transistor and the second storage capacitor, and   a second switching transistor that is provided between the second storage capacitor and the first switching transistor, or provided on side opposite to the first switching transistor with respect to the second storage capacitor.
 
(6)
       

     The display unit according to (5), in which 
     in each of the pixels, a source or a drain of the driving transistor is electrically coupled to one of a plurality of power supply lines that have a mutually common potential, 
     the driving circuit writes, on a pixel row basis, the signal voltage into the second storage capacitor, and 
     the driving circuit transfers the signal voltage that is written into the second storage capacitor to the gate of the driving transistor in each of the pixels altogether. 
     (7) 
     The display unit according to (6), in which 
     the driving circuit writes, on the pixel row basis, the signal voltage into the second storage capacitor by turning, on the pixel row basis, the second switching transistor ON, with the writing transistor being turned ON, and 
     the driving circuit transfers the signal voltage that is written into the second storage capacitor to the gate of the driving transistor in each of the pixels altogether by turning, in each of the pixels, both of the first switching circuit and the second switching transistor ON, with the writing transistor being turned OFF. 
     (8) 
     The display unit according to (6), in which the driving circuit writes the signal voltage into the second storage capacitor when each of the light-emitting devices emits light. 
     (9) 
     The display unit according to (6), in which the driving circuit transfers the signal voltage that is written into the second storage capacitor to the gate of the driving transistor in each of the pixels altogether when each of the light-emitting devices is extinguished. 
     In the display panel and the display unit according to the respective embodiments of the technology, the pixel circuits each include the memory circuit. In the memory circuit, the first switching transistor is provided between the second storage capacitor and the gate of the driving transistor. The second storage capacitor stores the signal voltage. The memory circuit further includes the second switching transistor. The second switching transistor is provided between the second storage capacitor and the first switching transistor. In an alternative embodiment, the second switching transistor is provided on side opposite to the first switching transistor with respect to the second storage capacitor. This makes it possible to prevent parasitic capacitance of the memory circuit from being added to the gate of the driving transistor, thus allowing for suppression of loss of a bootstrap gain caused by the parasitic capacitance of the memory circuit. 
     According to the display panel and the display unit of the respective embodiments of the technology, it is possible to suppress the loss of the bootstrap gain caused by the parasitic capacitance of the memory circuit, thus allowing for improvement in the correction capability by the above-described correction operation. It is to be noted that the effects described herein are not necessarily limitative, and may be any effects described in the disclosure. 
     Although the technology has been described in terms of exemplary embodiments, it is not limited thereto. It should be appreciated that variations may be made in the described embodiments by persons skilled in the art without departing from the scope of the technology as defined by the following claims. The limitations in the claims are to be interpreted broadly based on the language employed in the claims and not limited to examples described in this specification or during the prosecution of the application, and the examples are to be construed as non-exclusive. For example, in this disclosure, the term “preferably” or the like is non-exclusive and means “preferably”, but not limited to. The use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. The term “substantially” and its variations are defined as being largely but not necessarily wholly what is specified as understood by one of ordinary skill in the art. The term “about” as used herein can allow for a degree of variability in a value or range. Moreover, no element or component in this disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.