Patent Publication Number: US-2021191468-A1

Title: Display Device

Description:
TECHNICAL FIELD 
     One embodiment of the present invention relates to a display device and particularly relates to a display device including a flexible display. 
     Note that one embodiment of the present invention is not limited to the above technical field. Examples of the technical field of one embodiment of the present invention disclosed in this specification and the like include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, an electronic device, a lighting device, an input device, an input/output device, a driving method thereof, and a fabrication method thereof. A semiconductor device generally means a device that can function by utilizing semiconductor characteristics. 
     BACKGROUND ART 
     Flexible displays whose display surfaces can be curved have been actively developed. Light-emitting elements such as organic EL (electroluminescence) elements, liquid crystal element, and the like are typical display elements used in flexible displays. 
     The basic structure of an organic EL element is a structure in which a layer containing a light-emitting organic compound is provided between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the light-emitting organic compound. A display device using such an organic EL element does not need a light source such as a backlight; thus, a thin, lightweight, high-contrast, and low-power display device can be achieved. 
     For example, Patent Document 1 discloses a flexible light-emitting device using an organic EL element. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2014-197522 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     A problem of flexible displays, which are extremely thinner than conventional displays, is the difficulty in obtaining higher mechanical strength. For example, when a flexible display is made to function as a touch panel, in particular, a strong touch by a finger, a stylus, or the like on a display surface might damage the flexible display. 
     An object of one embodiment of the present invention is to prevent damage to a flexible display. Alternatively, an object is to provide a display device having improved mechanical strength. Alternatively, an object is to provide a display device having high reliability. Alternatively, an object is to provide a display device, or an electronic device having a novel structure. 
     Note that the descriptions of these objects do not preclude the existence of other objects. One embodiment of the present invention does not have to achieve all the objects. Note that objects other than them can be derived from the description of the specification, the drawings, the claims, and the like. 
     Means for Solving the Problems 
     One embodiment of the present invention is a display device including a display panel and a protection cover. The display panel includes a first portion having flexibility. The protection cover has a light-transmitting property and flexibility and is provided to overlap with a display surface side of the display panel. The display device has a function of being reversibly changed in shape to a first mode and a second mode. In the first mode, the display panel and the protection cover are each substantially flat. In the second mode, the first portion of the display panel is curved such that the display surface side becomes a concave surface, and part of the protection cover is curved in the same direction as the first portion. In the second mode, there is a gap between the first portion and the protection cover. 
     In the above, the display panel and the protection cover are preferably provided in contact with each other in the first mode. Alternatively, the display panel and the protection cover are preferably provided apart from each other in the first mode. 
     In the above, the protection cover preferably has a function of a touch panel or a circularly polarizing plate. 
     In the above, a functional layer having flexibility is preferably included between the display panel and the protection cover. At this time, part of the functional layer is preferably curved in the same direction as the first portion in the second mode. Furthermore, the functional layer preferably has a function of a touch panel or a circularly polarizing plate. 
     In the above, preferably, the display panel includes a second portion and a third portion, the first portion is positioned between the second portion and the third portion, the second portion and the third portion are substantially flat in the second mode, and, in the protection cover, a portion overlapping with the second portion and a portion overlapping with the third portion include a substantially flat region. 
     In the above, when an angle formed between a surface of the second portion and a surface of the third portion is an angle θ, the display panel preferably includes, in a range of the angle θ greater than or equal to 90° and less than 180°, an angle range in which the protection cover is changed in shape so that a distance between an end portion of the second portion or an end portion of the third portion of the display panel and an end portion of the protection cover increases continuously when the angle θ is gradually reduced from 180°. 
     In the above, when an angle formed between a surface of the second portion and a surface of the third portion is an angle θ, the display panel preferably includes, in a range of the angle θ greater than or equal to 90° and less than 180°, an angle range in which the curvature radius of the first portion is smaller than the curvature radius of a curved portion of the protection cover; and, in a range of the angle θ greater than or equal to 0° and less than 90°, an angle range in which the curvature radius of the first portion is larger than the curvature radius of the curved portion of the protection cover. 
     In the above, when an angle formed between a surface of the second portion and a surface of the third portion is an angle θ, the display panel preferably includes, in a range of the angle θ greater than or equal to 90° and less than 180°, an angle range in which a distance between the first portion and the protection cover increases continuously when the angle θ is gradually reduced from 180°. 
     In the above, when an angle formed between a surface of the second portion and a surface of the third portion is an angle θ, preferably in the protection cover, tension is applied in a direction perpendicular to a pair of end portions intersecting with a curving direction, in a range of the angle θ greater than or equal to 90° and less than or equal to 180°. 
     In the above, a first support fixed to the second portion and a second support fixed to the third portion are preferably included. In this case, the first portion is preferably not fixed to either the first support or the second support. 
     In the above, preferably, one of the pair of end portions of the protection cover, which intersect with the curving direction, is fixed to the first support, and the other is not fixed to either the first support or the second support. 
     In the above, preferably, the first support has a first rotation axis perpendicular to the curving direction of the second portion and the second support has a second rotation axis parallel to the first rotation axis. In this case, preferably, the first support and the second support are capable of rotating in opposite directions at the same angle upon the first rotation axis and the second rotation axis, respectively, and relative positions of the first rotation axis and the second rotation axis does are constant. 
     In the above, preferably, the first support and the second support each include a retention member, and the protection cover is slidably attached to the retention member. 
     In the above, the protection cover preferably includes one or more of a urethane resin, an acrylic resin, and a silicone resin. 
     Effect of the Invention 
     According to one embodiment of the present invention, damage to a flexible display can be prevented. Alternatively, a display device having improved mechanical strength can be provided. Alternatively, a display device having high reliability can be provided. Alternatively, a display device, an electronic device, or the like having a novel structure can be provided. 
     Note that the descriptions of the effects do not preclude the existence of other effects. Note that one embodiment of the present invention does not need to have all these effects. Note that effects other than them can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  to  FIG. 1E  are views illustrating a structure example of a display device. 
         FIG. 2A  to  FIG. 2F  are views illustrating structure examples of a display device. 
         FIG. 3A  to  FIG. 3F  are views illustrating a structure example of a display device. 
         FIG. 4A  to  FIG. 4D  are views illustrating a structure example of a display device. 
         FIG. 5A  to  FIG. 5F  are views illustrating a structure example of a display device. 
         FIG. 6A  to  FIG. 6F  are views illustrating a structure example of a display device. 
         FIG. 7A  to  FIG. 7D  are views illustrating a structure example of a display device. 
         FIG. 8A  to  FIG. 8C  are views illustrating a structure example of a display device. 
         FIG. 9A  to  FIG. 9D  are views illustrating structure examples of a display device. 
         FIG. 10A  to FIG.  10 F 2  are views illustrating structure examples of a display device. 
         FIG. 11  is a view illustrating a structure example of a display device. 
         FIG. 12A  and  FIG. 12B  are views illustrating a structure example of a display device. 
         FIG. 13  is a view illustrating a structure example of a display panel. 
         FIG. 14  is a view illustrating a structure example of a display panel. 
         FIG. 15  is a view illustrating a structure example of a display panel. 
         FIG. 16A  is a block diagram of a display device.  FIG. 16B  and  FIG. 16C  are circuit diagrams of a pixel. 
         FIG. 17A ,  FIG. 17C , and  FIG. 17D  are circuit diagrams of a display device.  FIG. 17B  is a timing chart. 
         FIG. 18A  to  FIG. 18E  are views illustrating structure examples of a pixel. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments will be described with reference to the drawings. Note that the embodiments can be implemented in many different modes, and it will be readily appreciated by those skilled in the art that modes and details of the embodiments can be changed in various ways without departing from the spirit and scope thereof. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments. 
     Note that in structures of the present invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and a description thereof is not repeated. Furthermore, the same hatch pattern is used for the portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     In each drawing described in this specification, the size, the layer thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, they are not limited to the illustrated scale. 
     Note that in this specification and the like, the ordinal numbers such as “first” and “second” are used in order to avoid confusion among components and do not limit the number. 
     In this specification and the like, a display panel that is one embodiment of a display device has a function of displaying (outputting) an image or the like on (to) a display surface. Thus, the display panel is one embodiment of an output device. 
     In this specification and the like, a substrate of a display panel to which a connector such as an FPC (Flexible Printed Circuit) or a TCP (Tape Carrier Package) is attached, or a substrate on which an IC is mounted by a COG (Chip On Glass) method or the like is referred to as a display panel module, a display module, or simply a display panel or the like in some cases. 
     Note that in this specification and the like, a touch panel that is one embodiment of a display device has a function of displaying an image or the like on a display surface and a function of a touch sensor capable of sensing the contact, press, approach, or the like of a sensing target such as a finger or a stylus with or to the display surface. Thus, the touch panel is one embodiment of an input/output device. 
     A touch panel can be referred to as, for example, a display panel (or a display device) with a touch sensor, or a display panel (or a display device) having a touch sensor function. A touch panel can include a display panel and a touch sensor panel. Alternatively, a touch panel can have a function of a touch sensor in the display panel or on the surface of the display panel. 
     In this specification and the like, a substrate of a touch panel on which a connector and an IC are mounted is referred to as a touch panel module, a display module, or simply a touch panel or the like in some cases. 
     Embodiment 1 
     In this embodiment, a structure example of a display device of one embodiment of the present invention is described. A display device including a flexible display panel is described below. 
     Structure Example 
       FIG. 1A  shows a schematic perspective view of a display device  10 . The display device  10  includes a display panel  11 , a protection cover  12 , a support  21 , and a support  22 . The display panel  11  also includes a display portion  15 . 
     At least part of the display panel  11  has flexibility and can be curved. A plurality of pixels are arranged in a matrix in the display portion  15  of the display panel  11 , whereby an image can be displayed on the display portion  15 . 
     At least one or more display elements are provided in the pixels provided in the display portion  15  of the display panel  11 . As organic EL element can typically be used as the display element. Other than that, a variety of display elements such as light-emitting elements such as an inorganic EL element and an LED element, a liquid crystal element, a microcapsule, an electrophoretic element, an electrowetting element, an electrofluidic element, an electrochromic element, and a MEMS element can be used. 
     The protection cover  12  is positioned on a display surface side of the display panel  11  and has a function of protecting a surface of the display panel  11 . The protection cover  12  has a light-transmitting property, thereby allowing an image displayed on the display portion  15  to be seen by a user through the protection cover  12 . Moreover, at least part of the protection cover  12  has flexibility and can be curved. 
     The protection cover  12  may also function as a touch sensor panel or function as an optical film. When the protection cover  12  functions as a touch sensor panel, the protection cover  12  can include a sensor element such as a capacitive touch sensor, an optical sensor, or a pressure-sensitive touch sensor. As an optical film, a circularly polarizing plate, an anti-reflection film (including an AR (Anti-Reflection) film and an AG (Anti-Glare) film), and the like can be given, for example. 
     For the protection cover  12 , a sheet-like member including at least one or more of a urethane resin, an acrylic resin, a silicone resin, a fluorine resin, an olefin resin, a vinyl resin, a styrene resin, an amide resin, an ester resin, and an epoxy resin is preferably used. In particular, a urethane resin has a relatively high dielectric constant and can increase the sensitivity when a capacitive touch sensor is used. In addition, a urethane resin is preferred because it can give excellent slidability and a self-healing function to a surface of the protection cover  12 . 
     Preferably, an organic resin having a self-healing function is used particularly as the material that is positioned in the outermost surface of the protection cover  12 , in which case surface scattering caused by a scratch or the like can be prevented and display quality can be maintained. When a water-repellent or oil-repellent resin is used as the organic resin or surface treatment is performed to make the organic resin water repellent or oil repellent, the surface of the protection cover  12  can be prevented from being contaminated with fingerprint marks or the like. As a material having a self-healing function, in addition to a urethane resin described above, materials including polyrotaxane, cyclodextrin, polyphenylene ether, or the like can be used. More preferably, in this case, the protection cover  12  has a structure in which the organic resin having a self-healing function is stacked over a sheet made of one or more of a urethane resin, an acrylic resin, and a silicone resin described above. 
     The slidability of the outermost surface of the protection cover  12  is preferably improved by coating, surface treatment, putting a film having excellent slidability, or the like. Furthermore, the slidability not only of the display surface side of the protection cover  12  but also of the surface on the display panel  11  side is preferably improved, in which case the display panel  11  and the protection cover  12  are easy to slide when provided in contact with each other. 
     The support  21  and the support  22  have a function of supporting the display panel  11 . At least the surfaces of the support  21  and the support  22  which support the display panel  11  are preferably flat surfaces or smoothly curved surfaces. Preferably, these surfaces have rigidity to the extent that they are not changed in shape when pressed with a finger, a stylus, or the like by a person. For the surfaces of the support  21  and the support  22  which support the display panel  11 , a material having relatively high rigidity, such as plastic, glass, metal, alloy, ceramics, or wood, for example, is preferably used. 
     The display panel  11  includes a portion fixed to the support  21 , a portion fixed to the support  22 , and, between these two portions, a portion not fixed to either support. Preferably, at least the portion of the display panel  11  which is not fixed to either the support  21  or the support  22  has flexibility. 
       FIG. 1B  shows a schematic perspective view of the display device  10  in a state where the display panel  11  and the protection cover  12  are not curved. 
     This is the state where the display panel  11  is supported by any of the support  21  and the support  22 . In addition, the protection cover  12  is provided on the display surface side of the display panel  11 . In this state, since the whole display panel  11  is supported by the support  21  and the support  22  having rigidity, mechanical strength against pressure from the display surface side is high. Here, the support  21  and the support  22  are preferably in close contact with each other so that, at least in a portion (also referred to as a seam) between their surfaces supporting the display panel  11 , formation of a gap or a step is inhibited as much as possible. 
       FIG. 1C  shows a schematic perspective view of the display device  10  in a state where the display panel  11  and the protection cover  12  are curved. The display panel  11  is curved in such a manner that part of the display surface side becomes a concave surface. The protection cover  12  is also curved in the same direction in such a manner that part of the display surface side becomes a concave surface.  FIG. 1D  and  FIG. 1E  show enlarged views of regions P and Q, respectively, in  FIG. 1C . 
     Here, the direction D of an arrow indicated by the dashed line in  FIG. 1B  and  FIG. 1C  corresponds to the curving direction of the display panel  11 . In this case, the display panel  11  is curved so that the long-side direction and the curving direction of the display panel  11  are aligned. Note that the curving direction is not limited to this and may be aligned with the short-side direction. The curving direction may be a direction non-parallel to the sides of the outline of the display panel  11 . 
     The region P is a region including end portions of the display panel  11  and the protection cover  12  in the curved portion. The region Q is a region including end portions of the display panel  11  and the protection cover  12  in a portion where they are not curved. 
     As illustrated in  FIG. 1C  and  FIG. 1D , the display panel  11  and the protection cover  12  are apart from each other in the portion where the display panel  11  and the protection cover  12  are curved. In other words, there is a gap between the curved portion of the display panel  11  and the protection cover  12 . 
     By curving the display panel  11  and the protection cover  12 , the protection cover  12  is changed in shape as illustrated in  FIG. 1C  and  FIG. 1E , so that an end portion (also referred to as a side) of the protection cover  12 , which intersects with the curving direction, is shifted relatively outward compared to the end portion of the display panel  11  or an end portion of the support  22 . 
     In order that the movement of the above shift be easily understood in  FIG. 1A  to  FIG. 1E , the display device is illustrated such that the end portions of the display panel  11 , the protection cover  12 , the support  21 , and the support  22  are aligned with one another when seen from above in the state where the display panel  11  and the protection cover  12  are not curved (i.e., the state of  FIG. 1B ). 
     When the display panel  11  is curved, the protection cover  12  is changed in shape so as to be shifted relative to the display panel  11 . Thus, without expansion and contraction of both the display panel  11  and the protection cover  12 , a gap can be provided between the display panel  11  and the protection cover  12  in the curved portion. 
     The effect of providing, in the curved portion of the display panel  11 , the gap between the curved portion and the protection cover  12  is described using  FIG. 2A  and  FIG. 2B .  FIG. 2A  is a cross-sectional view along the curving direction in which the display panel  11  is in close contact with the protection cover  12 .  FIG. 2B  is a cross-sectional view in which there is a crescentic gap between these. 
     The curved portion of the display panel  11  is changed in shape as it is lifted up from the support  21  and the support  22 , and consequently the curved portion is not supported by either the support  21  or the support  22 . 
     As illustrated in  FIG. 2A , at the portion where the display panel  11  is not curved and supported by the support  22 , the protection cover  12  can be changed in shape to absorb even the pressure added by a tap of a protruding member (a stylus  29  here) from the protection cover  12  side; thus, the display panel  11  can be prevented from being changed in shape and being damaged. By contrast, in the curved portion of the display panel  11 , since the rear side of the display panel  11  is not supported, the display panel  11  is also changed in shape by following the change in the shape of the protection cover  12 . This might damage the display panel  11  to allow the stylus  29  to penetrate therethrough in the worst case. 
     However, in one embodiment of the present invention, since there is a gap between the protection cover  12  and the display panel  11  in the curved portion of the display panel  11 , even the pressure added by a tap of the stylus  29  is absorbed by a change in the shape of the protection cover  12  to never reach the display panel  11 , as illustrated in  FIG. 2B . Thus, a display device having excellent mechanical strength can be achieved. 
     The case where the curved portion of the display panel  11  is not supported by the support  21  and the support  22  is described so far. However, for example, in the case where the support  21  and the support  22  are replaced by a support that can also support the curved portion of the display panel  11 , at least the surface supporting the display panel  11  needs to be changed in shape or expanded or contracted. Thus, the surface of the support that supports the display panel  11  needs to have flexibility or elasticity, which renders high rigidity difficult to provide. Consequently, when the display surface side of the display panel  11  is tapped, the surface of the support might be changed in shape by the pressure to make the display panel  11  itself have a concave shape, leading to damage. Therefore even in such a case, the structure where a gap between the display panel  11  and the protection cover  12  is provided to prevent a contact therebetween so that pressure can be absorbed by a change in the shape of the protection cover  12  is extremely effective. 
     Here, a stacked-layer structure of the display panel  11  and the protection cover  12  is described.  FIG. 2C  to  FIG. 2F  are enlarged views of a cross section in the region surrounded by the dashed line in  FIG. 2B . 
       FIG. 2C  is an example in which the display panel  11  is provided in contact with the protection cover  12 . 
     In an example shown in  FIG. 2D , the protection cover  12  has a stacked-layer structure in which a functional layer  12   a  and a functional layer  12   b  are stacked. The functional layer  12   b  positioned on the display surface side (the side opposite the display panel  11 ) is a layer including the above-described organic resin having a self-healing function. As the functional layer  12   a  positioned on the display panel  11  side, the above-described sheet-like member including a urethane resin or the like can be used. 
     As illustrated in  FIG. 2E , a protection cover  14  may be provided on the rear surface side (support  21  or support  22  side) of the display panel  11 . Since the curved portion of the display panel  11  is not supported by the support  21  and the support  22 , providing the protection cover  14  on the rear surface side of the display panel  11  can improve the mechanical strength of the display device  10 . For the protection cover  14 , a material similar to that of the protection cover  12  is used. 
     Here, the protection cover  14  may have a stacked-layer structure in which a functional layer  14   a  and a functional layer  14   b  are stacked as illustrated in  FIG. 2F . Materials similar to those of the above functional layer  12   a  and functional layer  12   b  can be used for the functional layer  14   a  and the functional layer  14   b , respectively. 
     Next, a preferred shape when the display panel  11  and the protection cover  12  are curved or the like is described in detail. 
       FIG. 3A  to  FIG. 3F  are schematic cross-sectional views along a curving direction of the display device  10 . In each drawing, a rotation axis  31   a  of the support  21  and a rotation axis  32   a  of the support  22  are each indicated by a circle. 
     The angle shown in each drawing indicates an angle formed between a pair of flat surfaces between which the curved portion of the display panel  11  is interposed. Note that this angle can be rephrased as an angle formed between a pair of surfaces of the support  21  and the support  22  which support the display panel  11  or as an angle obtained by subtracting the sum of the absolute values of the rotation angles of the support  21  and the support  22  (angles of rotations from the state of  FIG. 3A ) from 180°. In the following description, the angle formed between the pair of flat surfaces between which the curved portion of the display panel  11  is interposed may be simply referred to as an “angle”. 
     Note that here, the case where the display panel  11  and the protection cover  12  have an equal length in the cross sectional direction is shown for simplification. The case where the end portions of the display panel  11  and the protection cover  12  are aligned with each other when the display panel  11  is not curved, as illustrated in  FIG. 3A , is shown. 
       FIG. 3A  to  FIG. 3F  show examples in which the end portions of the protection cover  12  and the support  21  are fixed to each other. In other words, the protection cover  12  is changed in shape in such a manner that it slides (shifts) to the support  22  side. 
       FIG. 3B ,  FIG. 3C ,  FIG. 3D ,  FIG. 3E , and  FIG. 3F  show the cases where the angles are 150°, 120°, 90°, 30°, and 0°, respectively. Each drawing clearly shows the amount of shift of the protection cover  12  from the state where the angle is 180° (i.e., the state where the display panel  11  is not curved). Here, D α , denotes the shift amount when the angle is α° and, for example, D 150  denotes the shift amount when the angle is 150°. 
     Preferably, at least in a range of the angle greater than or equal to 90° and less than or equal to 180°, the shift amount of the protection cover  12  gradually increases as the angle decreases. Note that the shift amount is shown to gradually increase even when the angle is less than 90°; in this angle range, however, the shift amount may be unchanged or decrease. 
     Here, when the display panel  11  functions as a touch panel, it is preferably used in a range of the angle greater than or equal to 90° and less than 180° during operation of the display panel  11  in the curved state. Curving the display panel  11  with the angle less than that (i.e., less than) 90° hinders touch operation and input operation with a pen. For this reason, preferably, at least in a range of the angle greater than or equal to 90° and less than 180°, as the angle decreases, the protection cover  12  is changed in shape so that the gap between the display panel  11  and the protection cover  12  increases, i.e., the shift amount of the protection cover  12  increases. 
     Moreover, tension in the curving direction (i.e., force pulling to the outside) is preferably applied to the end portion of the protection cover  12  in the state where the display panel  11  is flat (i.e., with an angle of 180°) or in the state where the display panel  11  is curved with a predetermined angle at least in a range of the angle greater than or equal to 90° and less than 180°. Such a structure can prevent bending of the surface of the protection cover  12  to inhibit surface scattering of external light, whereby the display device can have high visibility. Furthermore, the protection cover  12  always has the same shape at the same angle by being kept pulled tight even after repeated changes in shape between the curved state and the flat state, whereby the display device can have high reliability. 
     A mechanism that applies the tension to the protection cover  12  may be a mechanism that pulls any one of the pair of end portions of the protection cover  12  perpendicular to the curving direction. Alternatively, a mechanism that pulls both of the end portions may be employed. Such a mechanism may be included in any one or both of the support  21  and the support  22  or may be incorporated in a housing of an electronic device or the like separately from the supports. 
       FIG. 4A  to  FIG. 4D  show schematic cross-sectional views of the enlarged curved portion of the display panel  11 .  FIG. 4A ,  FIG. 4B ,  FIG. 4C , and  FIG. 4D  show the states where the angles are 120°, 90°, 30°, and 0°, respectively. 
     Here, the case where the display panel  11  and the protection cover  12  are each curved in an ideal arc shape is described. Although an ideal arc shape is not necessarily formed depending on the structure of the display device, a side surface or a cross section of each curved portion can be approximated by an ideal arc even in such a case. 
       FIG. 4A  to  FIG. 4D  show a center O 1  and a curvature radius r 1  of an arc formed by the curved surface on the display surface side of the display panel  11  and a center O 2  and a curvature radius r 2  of an arc formed by the curved surface on the top surface side (the side opposite the display panel  11 ) of the protection cover  12 . 
     At least in a range of the angle less than 180° and greater than or equal to 90°, the curvature radius r 1  of the display panel  11  and the curvature radius r 2  of the protection cover  12  preferably satisfy r 1 &lt;r 2 . In other words, at least in the above angle range, the protection cover  12  is preferably curved with a larger radius of curvature than that of the display panel  11 . This allows a gap having a crescent shape cross section to be favorably formed between the display panel  11  and the protection cover  12  in the curved portion, as illustrated in  FIG. 4A  and  FIG. 4B . In addition, the portion where the gap is not provided (e.g., the portion where the display panel  11  and the protection cover  12  are in contact with each other) can be a portion where the display panel  11  is always supported by the support  21  or the support  22 . 
     At a predetermined angle less than 90°, the relationship between the curvature radius r 1  and the curvature radius r 2  is reversed to make the curvature radius r 2  smaller than the curvature radius r 1 , as illustrated in  FIG. 4C  and  FIG. 4D , for example. 
     When the center O 1  and the center O 2  are focused on, the center O 1  is preferably always positioned more on the inner side (the display panel  11  side) than the center O 2 . This allows the gap to be formed between the display panel  11  and the protection cover  12  whenever the display panel  11  is curved. 
     Note that the center O 1  and the center O 2  become substantially the same when the display panel  11  and the protection cover  12  that are integrated by adhesion, for example, are curved. 
     If the display panel  11  and the protection cover  12  are integrated by adhesion, for example, the integrated display panel and protection cover is thick in total, and the stress caused by curving the display panel  11  is large accordingly, resulting in a fracture of the display panel  11  in the worst case. By contrast, in the display device  10  where the display panel  11  and the protection cover  12  are curved with different radii of curvature independently of each other, the stress caused by curving the display panel  11  can be reduced, whereby damage can be prevented. 
     Modification Examples 
     Modification examples of the above structure example are described below. 
     Modification Example 1 
     Although the end portions of the protection cover  12  and the support  21  are fixed to each other in the structures exemplified above with reference to  FIG. 3A  and the like, a structure in which the protection cover  12  is not fixed to any support is also possible. 
       FIG. 5A  to  FIG. 5F  illustrate a structure in which the protection cover  12  can be shifted to both the support  21  side and the support  22  side. 
     Here, (L) denotes the amount of the shift of the protection cover  12  to the support  21  side and (R) denotes the amount of the shift to the support  22  side. For example, D 150 (L) and D 150 (R) denote the amount of the shift of the protection cover  12  to the support  21  side and the amount of the shift to the support  22  side, respectively at an angle of 150°. Here, in the case where the shape of the curved portion of the protection cover  12  is similar to the structure illustrated in  FIG. 3B  or the like, for example, the sum of D 150 (L) and D 150 (R) is substantially equal to D 150  of  FIG. 3B . 
     In the above-described structure in which the pair of end portions of the protection cover  12  is thus shifted when the protection cover  12  is curved, the amount of the shift of the protection cover  12  toward the support  22  can be smaller than that in the structure illustrated in  FIG. 3A  or the like. Consequently, an electronic device including the display device can be reduced in size. 
     Note that the amount of the shift of the protection cover  12  to the support  21  side and the amount of the shift to the support  22  side may be equal or may be different from each other. The shift amounts are preferably substantially equal, in which case the amount of the shift of the protection cover  12  toward the support  21  and the amount of the shift toward the support  22  can be minimized. 
     Modification Example 2 
     In the portion where the display panel  11  is not curved, a gap may be provided between the display panel  11  and the protection cover  12 , which are provided in contact with each other in the examples described in the above structure example and Modification Example 1. 
       FIG. 6A  to  FIG. 6F  show an example in which a gap of a distance G is provided between the display panel  11  and the protection cover  12  so that these are not in contact with each other. 
     The portion of the display panel  11  that is not curved is also not in contact with the protection cover  12 , as described above, whereby the mechanical strength can further be improved. 
     In the case where the display panel  11  and the protection cover  12  are not curved as illustrated in  FIG. 6A , the protection cover  12  is preferably supported by the support  21 , the support  22 , a housing of an electronic device, or the like so that the distance (distance G) between the display panel  11  and the protection cover  12  is uniform at least in the display portion  15 . If the distance between the display panel  11  and the protection cover  12  becomes non-uniform by, for example, bending of part of the protection cover  12 , surface reflection of the protection cover  12  might be uneven, resulting in lower visibility. Thus, the distance between the display panel  11  and the protection cover  12  is made uniform, so that a display device with high display quality can be achieved. 
     The support  21 , the support  22 , and the like are provided with a mechanism such as a slit structure for retaining the protection cover  12  to be slidable, for example, outside the display portion  15 . 
     A structure in which air exists (also referred to as a structure in which an air gap is provided) between the display panel  11  and the protection cover  12  can be employed. A fluent material such as a gas, a liquid, a gel, or a sheet-like member having fluidity may be provided between the display panel  11  and the protection cover  12 . As the fluent material here, a material having a refractive index higher than that of air can be used. In particular, the refractive index is preferably close to that of the member positioned on the outermost surface of the display panel  11  or that of a member forming the protection cover  12 , in which case the light extraction efficiency can be increased (e.g., the refractive index differs by 10% or lower, preferably 5% or lower). 
     Modification Example 3 
     One or more sheet-like members may be provided between the display panel  11  and the protection cover  12 . 
       FIG. 7A  to  FIG. 7D  show an example in which a functional layer  13  is provided between the display panel  11  and the protection cover  12 . The functional layer  13  preferably has flexibility like the display panel  11 , the protection cover  12 , and the like. 
     The functional layer  13  may also function as a touch sensor panel or function as an optical film. As a touch sensor panel, the functional layer  13  can include a sensor element such as a capacitive touch sensor, an optical sensor, or a pressure-sensitive touch sensor. As an optical film, a circularly polarizing plate, an anti-reflection film (including an AR film and an AG film), and the like can be given, for example. 
     As illustrated in  FIG. 7B  to  FIG. 7D , when the display panel  11  is curved, part of the functional layer  13  is preferably changed in shape to be shifted relative to the display panel  11  so that there is a gap between the curved portion of the display panel  11  and the functional layer  13 . Here, the functional layer  13  and the protection cover  12  are each preferably changed in shape so that a gap is also provided therebetween. 
     When the display panel  11  is curved in a range greater than or equal to 90° and less than 180° as illustrated in  FIG. 7B  and  FIG. 7C , the functional layer  13  is preferably curved with a curvature radius larger than that of the display panel  11  and smaller than that of the protection cover  12 . When the display panel  11  is folded (when the angle is 0°) as illustrated in  FIG. 7D , the functional layer  13  is preferably curved with a curvature radius smaller than that of the display panel  11  and larger than that of the protection cover  12 . 
     Although  FIG. 7A  and the like show the example in which there are contacts between the display panel  11  and the functional layer  13  and between the functional layer  13  and the protection cover  12  when the display panel  11  is not curved, the structure in which they are not in contact with each other as described above in Modification Example 2 may be employed. 
     Although the example in which the functional layer  13  and the protection cover  12  are shifted only to the support  22  side is shown in  FIG. 7A  and the like, the structure in which they are shifted both to the support  21  side and the support  22  side as described above in Modification Example 1 may be employed. 
     When the functional layer  13  is sufficiently thinner than the display panel  11  or sufficiently more flexible than the display panel  11 , the functional layer  13  may be fixed to the display panel  11  or the protection cover  12 . Particularly when the display panel  11  adheres to the functional layer  13 , the neutral plane of a stacked body where the display panel  11  and the functional layer  13  are stacked is preferably positioned inside the display panel  11 . 
     Structure Example of Support 
     Next, a structure example of the support  21  and the support  22  is described. 
       FIG. 8A  to  FIG. 8C  each show a schematic perspective view of the support  21  and the support  22 . In each drawing, the display panel  11  is indicated by a dashed line.  FIG. 8A  is a state in which the display panel  11  is not curved,  FIG. 8B  is a curved state in which the angle formed between two flat surfaces of the display panel  11  is 120°, and  FIG. 8C  is a curved state in which the two flat surfaces of the display panel  11  are made parallel to each other (i.e., so that the angle formed therebetween is 0°). 
     A pair of gears  31  is attached to each end of the support  21 , and a pair of gears  32  is attached to each end of the support  22 . The gears  31  and the gears  32  are fixed to the support  21  and the support  22 , respectively. The gear  31  and the gear  32  mesh with each other with a gear ratio of 1:1 and can rotate in opposite directions at the same angle. Therefore, the support  21  and the support  22  can rotate in opposite directions at the same angle. This allows the support  21  and the support  22  to be reversibly changed in shape from the mode illustrated in  FIG. 8A  to the mode illustrated in  FIG. 8C  through the mode illustrated in  FIG. 8B . 
     With such a structure, the support  21  and the support  22  can rotate on their respective rotation axes while the relative positions of their rotation axes are not changed. With such a structure, even when the display panel  11  is fixed to both the support  21  and the support  22 , the display panel  11  can be curved without being expanded and contracted in the curving direction. 
       FIG. 8A  to  FIG. 8C  also illustrate a region  28 , where the display panel  11  is always supported by the support  21  or the support  22 . In other words, the region  28  is where the display panel  11  is always fixed along the surface of the support  21  or the support  22 . In a region between a pair of regions  28 , the display panel  11  can be lifted up from the surface of the support  21  or the support  22  without being fixed to the support  21  and the support  22 . 
     For example, in the region  28 , the display panel  11  can be fixed to the support  21  and the support  22  by adhesion with an adhesive material, an adhesive sheet, or the like. Alternatively, all the regions including the portion where the display panel  11  is to be curved may be bonded to the support  21  and the support  22  with a low-viscosity adhesion sheet which is easy to peel off. In this case, the portion where the display panel  11  is not curved is in a state substantially fixed to the support  21  or the support  22  because the portion does not lift (peel off) from the support  21  or the support  22  even when the support  21  and the support  22  are rotated. 
       FIG. 9A  is a schematic side view of the curved portion where the display panel  11  is curved at 90°, which is seen from the direction perpendicular to the curving direction. In  FIG. 9A , the gear  31  and the gear  32  are indicated by dashed lines. 
       FIG. 9A  shows an example in which the support  21  and the support  22  are each provided with a retention member  23 . The display panel  11  and the protection cover  12  include a region interposed between the support  21  and the retention member  23  and a region interposed between the support  22  and the retention member  23 . The retention member  23  has a function of a guide retaining the display panel  11  or the protection cover  12  to be slidable. 
     The pair of retention members  23  can be provided in a region on the outer side of the display portion of the display panel  11 . For the pair of retention members  23 , a member having a top surface shape of a U-shape surrounding the display portion of the display panel  11  or a square bracket shape (square bracket like shape) can be used. The retention member  23  may be fixed to the support  21  or the support  22  with a screw, an adhesive material, or the like or they may be integrally formed. The retention member  23  is fixed to the support  21  or the support  22  and thus can be construed as being part of the support  21  or the support  22 . 
     The protection cover  12  is retained to be slidable between the display panel  11  and the retention member  23 . 
       FIG. 9B  shows an example in which a spacer  24  is provided between the display panel  11  and the protection cover  12 . With the spacer  24 , the display panel  11  and the protection cover  12  are retained apart from each other by the thickness of the spacer  24 . 
     The protection cover  12  is retained to be slidable between the spacer  24  and the retention member  23 . Thus, it can also be said that the retention member  23  and the spacer  24  form a slit structure for retaining the protection cover  12 . 
     The spacer  24  may have a U-shape top surface like the retention member  23 . Alternatively, the spacer  24  may be provided along both ends of the retention member  23 . The spacer  24  is preferably fixed to the support  21  or the support  22 . The spacer  24  can be construed as being part of the support  21  or the support  22 . 
       FIG. 9C  and  FIG. 9D  show examples in which the shape of an end portion of the retention member  23  is different from the above. 
     In  FIG. 9C  and  FIG. 9D , the end portion of the retention member  23  is processed into a convex surface. For example, the end portion of the retention member  23  is preferably processed into a cross-sectional shape over an arc. 
     In the end portions of the pair of retention members  23 , the protection cover  12  is changed in shape to be curved along the curved surface. Since the end portions of the retention member  23  are processed into a convex surface, the protection cover  12  can be prevented from being curved with a smaller radius than the radius of curvature of the curved surface. That is, the retention member  23  has a function of adjusting the curvature of the protection cover  12 . 
     In the case where such a retention member  23  is provided, the curved portion of the protection cover  12  can be formed of a pair of curved portions with a smaller radius of curvature than that of the display panel  11  and a substantially flat portion between the pair of curved portions. Even with such a structure, the distance between the curved portion of the display panel  11  and the protection cover  12  can be changed depending on the angle at which the display panel  11  is curved. 
     Next, an example of a tensile mechanism for applying tension to the end portion of the protection cover  12  is described. Tension is applied along the curving direction of the protection cover  12  from the end portion side, whereby generation of a crease or a sag in the protection cover  12  can be inhibited. This can prevent misalignment of the protection cover  12  and the display panel  11  even when bending and expanding operations are repeated. 
     FIG.  10 B 1  shows a schematic cross-sectional view of the vicinity of the end portion of the support  22 . The display panel  11  and the protection cover  12  are provided between the support  22  and the retention member  23 . The protection cover  12  is retained to be slidable along the display panel  11 . 
     Furthermore, a spring  41   a  and a movable member  42  are provided in the vicinity of the end portion of the support  22 . In the support  22 , a concave portion where the movable member  42  can move is provided such that the shape of the concave portion adjusts the movable range of the movable member  42 . 
     The movable member  42  is fixed to the protection cover  12  by an adhesive member  43 . Although FIG.  10 B 1  shows an example in which the movable member  42  is fixed to the top surface of the protection cover  12 , the present invention is not limited thereto and the movable member  42  may be fixed to the rear surface of the protection cover. The method of fixing the movable member  42  and the protection cover  12  is not limited to these; for example, the protection cover  12  may be fitted into the movable member  42  so that they are fixed without the adhesive member  43 . 
       FIG. 10A  illustrates the spring  41   a  whose length is an equilibrium length L 0 . As illustrated in FIG.  10 B 1 , the spring  41   a  in a state of being compressed from the equilibrium length L 0  is placed between the support  22  and the movable member  42 . Thus, as indicated by the dashed arrow in FIG.  10 B 1 , outward tension is always applied to the protection cover  12  through the movable member  42 . 
     FIG.  10 B 1  shows a state where the display panel  11  is curved at an angle θ 0  (θ=θ 0 ), and FIG.  10 B 2  shows a state where the display panel  11  is curved at the smaller angle (θ&lt;θ 0 ). Here, θ 0  is 180°, i.e., includes the state where the display panel  11  is not curved. 
     FIG.  10 C 1  and FIG.  10 C 2  show an example of the case where the spacer  24  is provided. In the example shown in FIG.  10 C 1  and FIG.  10 C 2 , the rear surface of the protection cover  12  and the movable member  42  are fixed to each other with the adhesive member  43 . As illustrated in FIG.  10 C 1  and FIG.  10 C 2 , the retention member  23  may be provided to cover the end portion of the protection cover  12 , the spring  41   a , the adhesive member  43 , and the movable member  42 . 
     Although the example where the spring  41   a  is used in the contracted state is described above, the spring may be used in the expanded state. 
       FIG. 10D  illustrates a spring  41   b  whose length is the equilibrium length L 0 . As illustrated in FIG.  10 E 1  and FIG.  10 E 2 , the ends of the spring  41   b , which is in a state of being expanded from the equilibrium length L 0 , are fixed to the movable member  42  and the support  22 . 
     FIG.  10 F 1  and FIG.  10 F 2  show an example of the case where the spacer  24  is provided. 
     The movable range of the movable member  42 , the spring coefficient of the spring, and the like of the tensile mechanism exemplified here are preferably selected such that, in the case where the display panel is curved at an angle from 180° to 0°, tension is applied to the end portion of the protection cover  12  at least in a range of the angle greater than or equal to 90° and less than or equal to 180°. 
     Specific Example of Display Device 
     A specific structure example of the display device is described below.  FIG. 11  shows a schematic perspective view of the display device  10 .  FIG. 12A  shows a schematic perspective view, where the display device  10  illustrated in  FIG. 11  is disassembled into components. 
     As illustrated in  FIG. 11  and  FIG. 12A , the display device  10  includes the support  21 , the support  22 , a retention member  23   a , a retention member  23   b , a spacer  24   a , a spacer  24   b , the gear  31 , the gear  32 , the protection cover  12 , and the display panel  11 . 
     Part of the display panel  11  is interposed between the support  21  and the spacer  24   a , and another part is interposed between the support  22  and the spacer  24   b . The display panel  11  is bonded to the support  21  and the support  22  with a low-viscosity adhesion sheet. 
     Part of the protection cover  12  is interposed between the spacer  24   a  and the retention member  23   a , and another part is interposed between the spacer  24   b  and the retention member  23   a . The protection cover  12  is retained to be slidable between the spacer  24   a  and the retention member  23   a  and between the spacer  24   b  and the retention member  23   a.    
     The gear  31  and the gear  32  are attached to the support  21  and the support  22 , respectively. Furthermore, the gear  31  and the gear  32  are covered with a cover  33 . 
       FIG. 12B  shows an enlarged view of an end portion of the spacer  24   b . In the spacer  24   b , a concave portion for placing the spring  41  is provided. In addition, the spacer  24   b  is provided with the movable member  42 . The spring  41  in a state of being compressed from its equilibrium length is placed in the concave portion. One side of the concave portion on the movable member  42  side is cut such that an end of the spring  41  is provided in contact with the movable member  42 . The protection cover  12  can be attached to the top surface of the movable member  42  with an adhesive material or the like. 
     The spacer  24   a , the spacer  24   b , the retention member  23   a , and the retention member  23   b  overlap a non-display region of the display panel  11 , and have a U-shape top surface so as not to overlap with the display portion. In the region surrounded by the pair of retention members  23   a  and  23   b , an image displayed on the display portion  15  of the display panel  11  can be seen through the protection cover  12  by a user. 
     The above is the description of the structure example of the display device  10 . 
     At least part of the structure examples, the drawings corresponding thereto, and the like exemplified in this embodiment can be implemented in combination with the other structure examples, the other drawings, and the like as appropriate. 
     At least part of this embodiment can be implemented in combination with the other embodiments described in this specification as appropriate. 
     Embodiment 2 
     In this embodiment, a structure example of a display panel which can be applied to the display device of one embodiment of the present invention is described. 
     Structure Example 
       FIG. 13  shows a top view of a display panel  700 . The display panel  700  employs a support substrate  745  having flexibility and can be used as a flexible display. The display panel  700  includes a pixel portion  702  provided over the support substrate  745  having flexibility. Over the support substrate  745 , a source driver circuit portion  704 , a pair of gate driver circuit portions  706 , a wiring  710 , and the like are provided. A plurality of display elements are provided in the pixel portion  702 . 
     Part of the support substrate  745  is provided with an FPC terminal portion  708 , to which an FPC  716  (FPC: Flexible printed circuit) is connected. The pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portions  706  are each supplied with a variety of signals and the like from the FPC  716  through the FPC terminal portion  708  and the wiring  710 . 
     The pair of gate driver circuit portions  706  is provided on opposite sides with the pixel portion  702  interposed therebetween. Note that the gate driver circuit portions  706  and the source driver circuit portion  704  may be formed separately on semiconductor substrates or the like to form packaged IC chips. The IC chip can be mounted on the support substrate  745  by a COF (Chip On Film) technique or the like. 
     Transistors including an oxide semiconductor are preferably used as the transistor included in the pixel portion  702 , the source driver circuit portion  704 , and the gate driver circuit portions  706 . 
     Light-emitting elements or the like can be used as the display elements in the pixel portion  702 . Examples of light-emitting elements are self-luminous light-emitting elements such as an LED (Light Emitting Diode), an OLED (Organic LED), a QLED (Quantum-dot LED), and a semiconductor laser. As the display elements, a liquid crystal elements such as transmissive liquid crystal elements, reflective liquid crystal elements, or transflective liquid crystal elements can also be used. MEMS (Micro Electro Mechanical Systems) shutter elements, optical interference type MEMS elements, or display elements using a microcapsule method, an electrophoretic method, an electrowetting method, an Electronic Liquid Powder (registered trademark) method, or the like can also be used, for example. 
       FIG. 13  shows an example where the FPC terminal portion  708  is provided in the portion of the support substrate  745  which has a protrusive shape. In a region P 1  in  FIG. 13 , part of the support substrate  745  that includes the FPC terminal portion  708  can be bent backwards. Bending the part of the support substrate  745  backwards enables the FPC  716  to be placed in a state overlapping with the rear side of the pixel portion  702  when the display panel  700  is mounted on an electronic device or the like, whereby the electronic device or the like can be space-saving or small-sized. 
     An IC  717  is mounted on the FPC  716  connected to the display panel  700 . The IC  717  has a function of a source driver circuit, for example. In this case, a structure can be employed in which the source driver circuit portion  704  in the display device  700  includes at least one of a protection circuit, a buffer circuit, a demultiplexer circuit, and the like. 
     Cross-sectional Structure Example 
     Structures using an organic EL element as the display element are described below with reference to  FIG. 14  and  FIG. 15 .  FIG. 14  and  FIG. 15  are each a schematic cross-sectional view of the display panel  700  illustrated in  FIG. 13  along the dash-dot line S-T. 
     First, portions common to the display panels illustrated in  FIG. 14  and  FIG. 15  are described. 
       FIG. 14  and  FIG. 15  illustrate cross sections including the pixel portion  702 , the gate driver circuit portion  706 , and the FPC terminal portion  708 . The pixel portion  702  includes a transistor  750  and a capacitor  790 . The gate driver circuit portion  706  includes a transistor  752 . 
     The transistor  750  and the transistor  752  are transistors using an oxide semiconductor for a semiconductor layer in which a channel is formed. Note that the transistors are not limited thereto, and a transistor using silicon (amorphous silicon, polycrystalline silicon, or single-crystal silicon) or a transistor using an organic semiconductor for the semiconductor layer can be used. 
     The transistors used in this embodiment includes a highly purified oxide semiconductor film in which formation of oxygen vacancies is suppressed. The off-state current of the transistors can be reduced significantly. Accordingly, in the pixel employing such a transistor, the retention time of an electrical signal such as an image signal can be extended, and the interval between writes of an image signal or the like can also be set longer. Accordingly, the frequency of refresh operations can be reduced, so that power consumption can be reduced. 
     In addition, the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high-speed operation. For example, with such a transistor capable of high-speed operation used for the display panel, a switching transistor in a pixel portion and a driver transistor used in a driver circuit portion can be formed over one substrate. That is, a structure in which a driver circuit formed using a silicon wafer or the like is not used is possible, in which case the number of components of the display device can be reduced. Moreover, the use of the transistor capable of high-speed operation also in the pixel portion can provide a high-quality image. 
     The capacitor  790  includes a lower electrode formed by processing the same film as a film used for the first gate electrode of the transistor  750  and an upper electrode formed by processing the same metal oxide film as a film used for the semiconductor layer. The resistance of the upper electrode is reduced as well as those of a source region and a drain region of the transistor  750 . Part of an insulating film functioning as a first gate insulating layer of the transistor  750  is provided between the lower electrode and the upper electrode. That is, the capacitor  790  has a stacked-layer structure in which an insulating film functioning as a dielectric film is positioned between a pair of electrodes. A wiring obtained by processing the same film as a film used for a source electrode and a drain electrode of the transistor  750  is connected to the upper electrode. 
     A planarization insulating layer  770  which functions as a planarization film is provided over the transistor  750 , the transistor  752 , and the capacitor  790 . 
     The transistor  750  included in the pixel portion  702  and the transistor  752  included in the gate driver circuit portion  706  may have different structures. For example, a top-gate transistor may be used as one of the transistors, and a bottom-gate transistor may be used as the other. Note that t the same applies to the driver circuit portion  704 , as in the gate driver circuit portion  706 . 
     The FPC terminal portion  708  includes a wiring  760  part of which functions as a connection electrode, an anisotropic conductive film  780 , and the FPC  716 . The wiring  760  is electrically connected to a terminal included in the FPC  716  through the anisotropic conductive film  780 . Here, the wiring  760  is formed using the same conductive film as the source electrode and the drain electrode of the transistor  750  and the like. 
     Next, the display panel  700  illustrated in  FIG. 14  is described. 
     The display panel  700  illustrated in  FIG. 14  includes the support substrate  745  and a support substrate  740 . As the support substrate  745  and the support substrate  740 , a glass substrate or a substrate having flexibility such as a plastic substrate can be used, for example. 
     The transistor  750 , the transistor  752 , the capacitor  790 , and the like are provided over the insulating layer  744 . The support substrate  745  and the insulating layer  744  are bonded to each other with the adhesive layer  742 . 
     The display panel  700  includes a light-emitting element  782 , a coloring layer  736 , a light-blocking layer  738 , and the like. 
     The light-emitting element  782  includes a conductive layer  772 , an EL layer  786 , and a conductive layer  788 . The conductive layer  772  is electrically connected to the source electrode or the drain electrode included in the transistor  750 . The conductive layer  772  is provided over the insulating layer  770  and functions as a pixel electrode. An insulating layer  730  is provided to cover an end portion of the conductive layer  772 . Over the insulating layer  730  and the conductive layer  772 , the EL layer  786  and the conductive layer  788  are stacked. 
     For the conductive layer  772 , a material having a property of reflecting visible light can be used. For example, a material including aluminum, silver, or the like can be used. For the conductive layer  788 , a material that transmits visible light can be used. For example, an oxide material including indium, zinc, tin, or the like is preferably used. Thus, the light-emitting element  782  is a top-emission light-emitting element, which emits light to the side opposite the formation surface (the support substrate  740  side). 
     The EL layer  786  includes an organic compound or an inorganic compound such as quantum dots. The EL layer  786  includes a light-emitting material that exhibits white light when current flows. 
     As the light-emitting material, a fluorescent material, a phosphorescent material, a thermally activated delayed fluorescence (TADF) material, an inorganic compound (e.g., a quantum dot material), or the like can be used. Examples of materials that can be used for quantum dots include a colloidal quantum dot material, an alloyed quantum dot material, a core-shell quantum dot material, and a core quantum dot material. 
     The light-blocking layer  738  and the coloring layer  736  are provided on one surface of an insulating layer  746 . The coloring layer  736  is provided in a position overlapping with the light-emitting element  782 . The light-blocking layer  738  is provided in a region not overlapping with the light-emitting element  782  in the pixel portion  702 . The light-blocking layer  738  may also be provided to overlap with the gate driver circuit portion  706  or the like. 
     The support substrate  740  is bonded to the other surface of the insulating layer  746  with an adhesive layer  747 . The support substrate  740  and the support substrate  745  are bonded to each other with a sealing layer  732 . 
     Here, for the EL layer  786  included in the light-emitting element  782 , a light-emitting material that exhibits white light emission is used. White light emission by the light-emitting element  782  is colored by the coloring layer  736  to be emitted to the outside. The EL layer  786  is provided for the whole pixels that exhibit different colors. The pixels provided with the coloring layer  736  transmitting any of red light (R), green light (G), and blue light (B) are arranged in a matrix in the pixel portion  702 , whereby the display device  700  can perform full-color display. 
     A conductive film having a transmissive property and a reflective property may be used for the conductive layer  788 . In this case, a microcavity structure is achieved between the conductive layer  772  and the conductive layer  788  such that light of a specific wavelength can be intensified to be emitted. Also in this case, an optical adjustment layer for adjusting an optical distance may be placed between the conductive layer  772  and the conductive layer  788  such that the thickness of the optical adjustment layer is different for a pixel of each color and accordingly the color purity of light emitted from each pixel can be increased. 
     Note that a structure in which the coloring layer  736  is not provided may be employed when the EL layer  786  is formed into an island shape for each pixel or into a stripe shape for each pixel column, i.e., the above optical adjustment layer is formed by separate coloring. 
     Here, an inorganic insulating film which functions as a barrier film having low permeability is preferably used for each of the insulating layer  744  and the insulating layer  746 . With such a structure in which the light-emitting element  782 , the transistor  750 , and the like are interposed between the insulating layer  744  and the insulating layer  746 , deterioration of them can be inhibited and a highly reliable display panel can be achieved. 
     In a display panel  700 A illustrated in  FIG. 15 , a resin layer  743  is provided between the adhesive layer  742  and the insulating layer  744  illustrated in  FIG. 14 . A protection layer  749  is provided instead of the support substrate  740 . 
     The resin layer  743  is a layer including an organic resin such as polyimide or acrylic. The insulating layer  744  includes an inorganic insulating film of silicon oxide, silicon oxynitride, silicon nitride, or the like. The resin layer  743  and the support substrate  745  are attached to each other with the bonding layer  742 . The resin layer  743  is preferably thinner than the support substrate  745 . 
     The protection layer  749  is attached to the sealing layer  732 . A glass substrate, a resin film, or the like can be used as the protection layer  749 . As the protection layer  749 , an optical member such as a polarizing plate (including a circularly polarizing plate) or a scattering plate, an input device such as a touch sensor panel, or a structure in which two or more of the above are stacked may be employed. 
     The EL layer  786  included in the light-emitting element  782  is provided over the insulating layer  730  and the conductive layer  772  in an island shape. The EL layers  786  are formed separately so that respective subpixels emit light of different colors, whereby color display can be performed without use of the coloring layer  736 . 
     A protection layer  741  is provided to cover the light-emitting element  782 . The protection layer  741  has a function of preventing diffusion of impurities such as water into the light-emitting element  782 . The protection layer  741  has a stacked-layer structure in which an insulating layer  741   a , an insulating layer  741   b , and an insulating layer  741   c  are stacked in this order from the conductive layer  788  side. In that case, it is preferable that inorganic insulating films with a high barrier property against impurities such as water be used as the insulating layer  741   a  and the insulating layer  741   c , and an organic insulating film which functions as a planarization film be used as the insulating layer  741   b . The protection layer  741  is preferably provided to extend also to the gate driver circuit portion  706 . 
     An organic insulating film covering the transistor  750 , the transistor  752 , and the like is preferably formed in an island shape inward from the sealing layer  732 . In other words, an end portion of the organic insulating film is preferably inward from the sealing layer  732  or in a region overlapping with an end portion of the sealing layer  732 .  FIG. 15  shows an example in which the insulating layer  770 , the insulating layer  730 , and the insulating layer  741   b  are processed into island shapes. The insulating layer  741   c  and the insulating layer  741   a  are provided in contact with each other in a portion overlapping with the sealing layer  732 , for example. Thus, a surface of the organic insulating film covering the transistor  750  and the transistor  752  is not exposed to the outside of the sealing layer  732 , whereby diffusion of water or hydrogen from the outside to the transistor  750  and the transistor  752  through the organic insulating film can be favorably prevented. This can reduce variations in electrical characteristics of the transistors, so that a display device with extremely high reliability can be achieved. 
     In  FIG. 15 , the region P 1  that can be bent includes a portion where the support substrate  745 , the bonding layer  742 , and the inorganic insulating film such as the insulating layer  744  are not provided. The region P 1  has a structure in which the insulating layer  770  including an organic material covers the wiring  760  not to expose the wiring  760 . When an inorganic insulating film is not provided in the region P 1  that can be bent and only a conductive layer including a metal or an alloy and a layer including an organic material are stacked, generation of cracks at the time of bending can be prevented. When the support substrate  745  is not provided in the region P 1 , part of the display panel  700 A can be bent with an extremely small radius of curvature. 
     In  FIG. 15 , a conductive layer  761  is provided over the protection layer  741 . The conductive layer  761  can be used as a wiring or an electrode. 
     In the case where a touch sensor is provided so as to overlap with the display panel  700 A, the conductive layer  761  can function as an electrostatic shielding film for preventing transmission of electrical noise to the touch sensor during pixel driving. In this case, the structure in which a predetermined constant potential is applied to the conductive layer  761  can be employed. 
     Alternatively, the conductive layer  761  can be used as an electrode of the touch sensor, for example. This enables the display panel  700 A to function as a touch panel. For example, the conductive layer  761  can be used as an electrode or a wiring of a capacitive touch sensor. In this case, the conductive layer  761  can be used as a wiring or an electrode to which a sensor circuit is connected or a wiring or an electrode to which a sensor signal is input. When the touch sensor is formed over the light-emitting element  782  in this manner, the number of components can be reduced, and manufacturing cost of an electronic device or the like can be reduced. 
     The conductive layer  761  is preferably provided in a portion not overlapping with the light-emitting element  782 . The conductive layer  761  can be provided in a position overlapping with the insulating layer  730 , for example. Thus, a transparent conductive film with a relatively low conductivity is not necessarily used for the conductive layer  761 , and a metal or an alloy having high conductivity or the like can be used, so that the sensitivity of the sensor can be increased. 
     As the type of the touch sensor that can be formed of the conductive layer  761 , a variety of types such as a capacitive type, a resistive type, a surface acoustic wave type, an infrared type, an optical type, and a pressure-sensitive type can be used, without limitation to a capacitive type. Alternatively, two or more of these types may be combined and used. 
     [Components] 
     Components such as a transistor that can be used in the display device will be described below. 
     [Transistor] 
     The transistors each include a conductive layer functioning as a gate electrode, a semiconductor layer, a conductive layer functioning as a source electrode, a conductive layer functioning as a drain electrode, and an insulating layer functioning as a gate insulating layer. 
     Note that there is no particular limitation on the structure of the transistor included in the display device of one embodiment of the present invention. For example, a planar transistor, a staggered transistor, or an inverted staggered transistor may be used. A top-gate or bottom-gate transistor structure may be employed. Gate electrodes may be provided above and below a channel. 
     There is no particular limitation on the crystallinity of a semiconductor material used for the transistors, and any of an amorphous semiconductor, a single crystal semiconductor, and a semiconductor having crystallinity other than the single crystal (a microcrystalline semiconductor, a polycrystalline semiconductor, or a semiconductor partly including crystal regions) may be used. It is preferable that a single crystal semiconductor or a semiconductor having crystallinity be used, in which case deterioration of the transistor characteristics can be suppressed. 
     In particular, a transistor that uses a metal oxide film for a semiconductor layer where a channel is formed will be described below. 
     As a semiconductor material used for the transistors, a metal oxide whose energy gap is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, further preferably greater than or equal to 3 eV can be used. A typical example thereof is a metal oxide containing indium, and for example, a CAC-OS described later or the like can be used. 
     A transistor using a metal oxide having a wider band gap and a lower carrier density than silicon has a low off-state current; thus, charges accumulated in a capacitor that is connected in series with the transistor can be held for a long time. 
     The semiconductor layer can be, for example, a film represented by an In-M-Zn-based oxide that contains indium, zinc, and M (M is a metal such as aluminum, titanium, gallium, germanium, yttrium, zirconium, lanthanum, cerium, tin, neodymium, or hafnium). 
     In the case where the metal oxide contained in the semiconductor layer is an In-M-Zn-based oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In-M-Zn oxide satisfy In M and Zn M The atomic ratio of metal elements in such a sputtering target is preferably, for example, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=4:2:3, In:M:Zn=4:2:4.1, In:M:Zn=5:1:6, In:M:Zn=5:1:7, or In:M:Zn=5:1:8. Note that the atomic ratio in the formed semiconductor layer varies from the above atomic ratio of metal elements of the sputtering target in a range of ±40%. 
     A metal oxide film with a low carrier density is used as the semiconductor layer. For example, for the semiconductor layer, a metal oxide whose carrier density is lower than or equal to 1×10 17 /cm 3 , preferably lower than or equal to 1×10 15 /cm 3 , further preferably lower than or equal to 1×10 13 /cm 3 , still further preferably lower than or equal to 1×10 11 /cm 3 , even further preferably lower than 1×10 10 /cm 3 , and higher than or equal to 1×10 −9 /cm 3  can be used. Such a metal oxide is referred to as a highly purified intrinsic or substantially highly purified intrinsic metal oxide. The metal oxide has a low impurity concentration and a low density of defect states and can thus be referred to as a metal oxide having stable characteristics. 
     Note that, without limitation to those described above, an oxide semiconductor with an appropriate composition may be used in accordance with required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of the transistor. To obtain the required semiconductor characteristics of the transistor, it is preferable that the carrier density, the impurity concentration, the density of defect states, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like of the semiconductor layer be set to appropriate values. 
     When silicon or carbon, which is one of Group 14 elements, is contained in the metal oxide contained in the semiconductor layer, oxygen vacancies are increased in the semiconductor layer, and the semiconductor layer becomes n-type. Thus, the concentration of silicon or carbon (concentration obtained by secondary ion mass spectrometry) in the semiconductor layer is set to lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     Alkali metal and alkaline earth metal might generate carriers when bonded to a metal oxide, in which case the off-state current of the transistor might be increased. Thus, the concentration of alkali metal or alkaline earth metal of the semiconductor layer, which is obtained by secondary ion mass spectrometry, is set to lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . 
     When nitrogen is contained in the metal oxide contained in the semiconductor layer, electrons serving as carriers are generated and the carrier density increases, so that the semiconductor layer easily becomes n-type. As a result, a transistor including a metal oxide that contains nitrogen is likely to have normally-on characteristics. Hence, the nitrogen concentration obtained by secondary ion mass spectrometry is preferably set to lower than or equal to 5×10 18  atoms/cm 3 . 
     Oxide semiconductors are classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of the non-single-crystal oxide semiconductor include a CAAC-OS (c-axis-aligned crystalline oxide semiconductor), a polycrystalline oxide semiconductor, an nc-OS (nanocrystalline oxide semiconductor), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor. 
     The aforementioned non-single-crystal oxide semiconductor or CAC-OS can be suitably used for a semiconductor layer of a transistor disclosed in one embodiment of the present invention. As the non-single-crystal oxide semiconductor, the nc-OS or the CAAC-OS can be suitably used. 
     The semiconductor layer may be a mixed film including two or more of a region of a CAAC-OS, a region of a polycrystalline oxide semiconductor, a region of an nc-OS, a region of an a-like OS, and a region of an amorphous oxide semiconductor. The mixed film has, for example, a single-layer structure or a stacked-layer structure including two or more of the above regions in some cases. 
     A CAC-OS (Cloud-Aligned Composite oxide semiconductor) is preferably used for a semiconductor layer of a transistor disclosed in one embodiment of the present invention. The use of the CAC-OS allows the transistor to have high electrical characteristics or high reliability. 
     &lt;Composition of CAC-OS&gt; 
     The composition of a CAC (Cloud-Aligned Composite)-OS that can be used in a transistor disclosed in one embodiment of the present invention will be described below. 
     A CAC-OS refers to one composition of a material in which elements constituting a metal oxide are unevenly distributed with a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size, for example. Note that a state in which one or more metal elements are unevenly distributed and regions including the metal element(s) are mixed with a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size in a metal oxide is hereinafter referred to as a mosaic pattern or a patch-like pattern. 
     Note that the metal oxide preferably contains at least indium. In particular, indium and zinc are preferably contained. In addition, one or more kinds selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like may be contained. 
     For example, a CAC-OS in an In-Ga—Zn oxide (of the CAC-OS, an In-Ga—Zn oxide may be particularly referred to as CAC-IGZO) has a composition (hereinafter, also referred to as cloud-like composition) in which materials are separated into indium oxide (hereinafter, InO X1  (X1 is a real number greater than 0)) or indium zinc oxide (hereinafter, In X2 Zn Y2 O Z2  (X2, Y2, and Z2 are real numbers greater than 0)), and gallium oxide (hereinafter, GaO X3  (X3 is a real number greater than 0)) or gallium zinc oxide (hereinafter, Ga X4 Zn Y4 O Z4  (X4, Y4, and Z4 are real numbers greater than 0)) to form a mosaic pattern, and InO X1  or In X2 Zn Y2 O Z2  forming the mosaic pattern is evenly distributed in the film. 
     That is, the CAC-OS is a composite metal oxide having a composition in which a region including GaO X3  as a main component and a region including In X2 Zn Y2 O Z2  or InO X1  as a main component are mixed. Note that in this specification, for example, when the atomic ratio of In to an element M in a first region is greater than the atomic ratio of In to the element M in a second region, the first region has higher In concentration than the second region. 
     Note that IGZO is a common name, which may specify a compound containing In, Ga, Zn, and O. Atypical example is a crystalline compound represented by InGaO 3 (ZnO) m1  (m1 is a natural number) or In (1+x0) Ga (1−x0) O 3 (ZnO) m0  (−1≤x0≤1; m0 is a given number). 
     The above crystalline compounds have a single crystal structure, a polycrystalline structure, or a CAAC structure. Note that the CAAC structure is a crystal structure in which a plurality of IGZO nanocrystals have c-axis alignment and are connected in the a-b plane direction without alignment. 
     Meanwhile, the CAC-OS relates to the material composition of a metal oxide. The CAC-OS refers to a composition in which, in the material composition containing In, Ga, Zn, and O, some regions that contain Ga as a main component and are observed as nanoparticles and some regions that contain In as a main component and are observed as nanoparticles are randomly dispersed in a mosaic pattern. Therefore, the crystal structure is a secondary element for the CAC-OS. 
     Note that in the CAC-OS, a stacked-layer structure including two or more films with different compositions is not included. For example, a two-layer structure of a film containing In as a main component and a film containing Ga as a main component is not included. 
     A boundary between the region containing GaO X3  as a main component and the region containing In X2 Zn Y2 O Z2  or InO X1  as a main component is not clearly observed in some cases. 
     In the case where one or more kinds selected from aluminum, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like are contained instead of gallium, nanoparticle regions containing the selected metal element(s) as a main component are observed in part of a CAC-OS and nanoparticle regions containing In as a main component are observed in part of the CAC-OS, and these nanoparticle regions are randomly dispersed to form a mosaic pattern. 
     The CAC-OS can be formed by a sputtering method under a condition where a substrate is intentionally not heated, for example. Moreover, in the case of forming the CAC-OS by a sputtering method, any one or more selected from an inert gas (typically, argon), an oxygen gas, and a nitrogen gas are used as a deposition gas. The flow rate of the oxygen gas to the total flow rate of the deposition gas in deposition is preferably as low as possible, and for example, the flow rate of the oxygen gas is higher than or equal to 0% and lower than 30%, preferably higher than or equal to 0% and lower than or equal to 10%. 
     The CAC-OS is characterized in that a clear peak is not observed when measurement is conducted using a θ/2θ scan by an Out-of-plane method, which is an X-ray diffraction (XRD) measurement method. That is, it is found from X-ray diffraction measurement that no alignment in the a-b plane direction and the c-axis direction is observed in a measured region. 
     In an electron diffraction pattern of the CAC-OS which is obtained by irradiation with an electron beam with a probe diameter of 1 nm (also referred to as a nanometer-sized electron beam), a ring-like region with high luminance and a plurality of bright spots in the ring-like region are observed. Therefore, the electron diffraction pattern indicates that the crystal structure of the CAC-OS includes an nc (nano-crystal) structure with no alignment in the plan-view direction and the cross-sectional direction. 
     Moreover, for example, it can be checked by EDX mapping obtained using energy dispersive X-ray spectroscopy (EDX) that the CAC-OS in the In-Ga—Zn oxide has a composition in which regions including GaO X3  as a main component and regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are unevenly distributed and mixed. 
     The CAC-OS has a composition different from that of an IGZO compound in which metal elements are evenly distributed, and has characteristics different from those of the IGZO compound. That is, in the CAC-OS, regions including GaO X3  or the like as a main component and regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are phase-separated from each other to form a mosaic pattern. 
     The conductivity of a region including In X2 Zn Y2 O Z2  or InO X1  as a main component is higher than that of a region including GaO X3  or the like as a main component. In other words, when carriers flow through the regions including In X2 Zn Y2 O Z2  or InO X1  as a main component, the conductivity of a metal oxide is exhibited. Accordingly, when the regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are distributed in a metal oxide like a cloud, high field-effect mobility (μ) can be achieved. 
     By contrast, the insulating property of a region including GaO X3  or the like as a main component is higher than that of a region including In X2 Zn Y2 O Z2  or InO X1  as a main component. 
     In other words, when regions including GaO X3  or the like as a main component are distributed in a metal oxide, leakage current can be suppressed and favorable switching operation can be achieved. 
     Accordingly, when a CAC-OS is used for a semiconductor element, the insulating property derived from GaO X3  or the like and the conductivity derived from In X2 Zn Y2 O Z2  or InO X1  complement each other, whereby high on-state current (I on ) and high field-effect mobility (μ) can be achieved. 
     A semiconductor element using a CAC-OS has high reliability. Thus, the CAC-OS is suitably used in a variety of semiconductor devices typified by a display. 
     Since a transistor including a CAC-OS in a semiconductor layer has high field-effect mobility and high driving capability, the use of the transistor in a driver circuit, typically a scan line driver circuit that generates a gate signal, enables a display device with a narrow frame width (also referred to as a narrow bezel) to be provided. Furthermore, the use of the transistor in a signal line driver circuit included in the display device (particularly in a demultiplexer connected to an output terminal of a shift register included in a signal line driver circuit) can reduce the number of wirings connected to the display device. 
     Furthermore, unlike a transistor including low-temperature polysilicon, the transistor including a CAC-OS in the semiconductor layer does not need a laser crystallization step. Thus, the manufacturing cost of a display device can be reduced, even when the display device is formed using a large substrate. In addition, the transistor including a CAC-OS in the semiconductor layer is preferably used for a driver circuit and a display unit in a large display device having high resolution such as ultra-high definition (“4K resolution”, “4K2K”, and “4K”) or super high definition (“8K resolution”, “8K4K”, and “8K”), in which case writing can be performed in a short time and display defects can be reduced. 
     Alternatively, silicon may be used for a semiconductor in which a channel of a transistor is formed. As silicon, amorphous silicon may be used but silicon having crystallinity is particularly preferably used. For example, microcrystalline silicon, polycrystalline silicon, or single crystal silicon are preferably used. In particular, polycrystalline silicon can be formed at a temperature lower than that for single crystal silicon and has higher field-effect mobility and higher reliability than amorphous silicon. 
     [Conductive Layer] 
     Examples of materials that can be used for conductive layers of a variety of wirings and electrodes and the like included in the display device in addition to a gate, a source, and a drain of a transistor include metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten and an alloy containing such a metal as its main component. A single-layer structure or stacked-layer structure including a film containing any of these materials can be used. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which an aluminum film or a copper film is stacked over a titanium film or a titanium nitride film and a titanium film or a titanium nitride film is formed thereover, a three-layer structure in which an aluminum film or a copper film is stacked over a molybdenum film or a molybdenum nitride film and a molybdenum film or a molybdenum nitride film is formed thereover, and the like can be given. Note that an oxide such as indium oxide, tin oxide, or zinc oxide may be used. Copper containing manganese is preferably used because it increases controllability of a shape by etching. 
     [Insulating Layer] 
     Examples of an insulating material that can be used for the insulating layers include, in addition to a resin such as acrylic or epoxy and a resin having a siloxane bond, an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, or aluminum oxide. 
     The light-emitting element is preferably provided between a pair of insulating films with low water permeability. In that case, impurities such as water can be inhibited from entering the light-emitting element, and thus a decrease in the reliability of the display device can be inhibited. 
     Examples of the insulating film with low water permeability include a film containing nitrogen and silicon, such as a silicon nitride film and a silicon nitride oxide film, and a film containing nitrogen and aluminum, such as an aluminum nitride film. Alternatively, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or the like may be used. 
     For example, the moisture vapor transmission rate of the insulating film with low water permeability is lower than or equal to 1×10 −5  [g/(m 2 ·day)], preferably lower than or equal to 1×10 −6  [g/(m 2 ·day)], further preferably lower than or equal to 1×10 −7  [g/(m 2 ·day)], still further preferably lower than or equal to 1×10 −8  [g/(m 2 ·day)]. 
     The above is the description of the components. 
     At least part of the structure examples, the drawings corresponding thereto, and the like exemplified in this embodiment can be implemented in combination with the other structure examples, the other drawings, and the like as appropriate. 
     At least part of this embodiment can be implemented in combination with the other embodiments described in this specification as appropriate. 
     Embodiment 3 
     In this embodiment, configuration examples of a display device will be described with reference to  FIG. 16A  to  FIG. 16C . 
     The display device illustrated in  FIG. 16A  includes a pixel portion  502 , a driver circuit portion  504 , protection circuits  506 , and a terminal portion  507 . Note that a configuration in which the protection circuits  506  are not provided may be employed. 
     The pixel portion  502  includes a plurality of pixel circuits  501  that drive a plurality of display elements arranged in X rows and Y columns (X and Y each independently represent a natural number of 2 or more). 
     The driver circuit portion  504  includes driver circuits such as a gate driver  504   a  that outputs a scanning signal to gate lines GL_ 1  to GL_X and a source driver  504   b  that supplies a data signal to data lines DL_ 1  to DL_Y. The gate driver  504   a  includes at least a shift register. The source driver  504   b  is formed using a plurality of analog switches, for example. Alternatively, the source driver  504   b  may be formed using a shift register or the like. 
     The terminal portion  507  refers to a portion provided with terminals for inputting power, control signals, image signals, and the like to the display device from external circuits. 
     The protection circuit  506  is a circuit that, when a potential out of a certain range is applied to a wiring to which the protection circuit  506  is connected, establishes continuity between the wiring and another wiring. The protection circuit  506  illustrated in  FIG. 16A  is connected to a variety of wirings such as the gate lines GL that are wirings between the gate driver  504   a  and the pixel circuits  501  and the data lines DL that are wirings between the source driver  504   b  and the pixel circuits  501 , for example. Note that the protection circuits  506  are hatched in  FIG. 16A  to distinguish the protection circuits  506  from the pixel circuits  501 . 
     The gate driver  504   a  and the source driver  504   b  may be provided over a substrate over which the pixel portion  502  is provided, or a substrate where a gate driver circuit or a source driver circuit is separately formed (e.g., a driver circuit board formed using a single crystal semiconductor or a polycrystalline semiconductor) may be mounted on the substrate over which the pixel portion  502  is provided by COG or TAB (Tape Automated Bonding). 
     The plurality of pixel circuits  501  illustrated in  FIG. 16A  can have a configuration illustrated in  FIG. 16B  or  FIG. 16C , for example. 
     The pixel circuit  501  illustrated in  FIG. 16B  includes a liquid crystal element  570 , a transistor  550 , and a capacitor  560 . The data line DL_n, the gate line GL_m, a potential supply line VL, and the like are connected to the pixel circuit  501 . 
     The potential of one of a pair of electrodes of the liquid crystal element  570  is set appropriately in accordance with the specifications of the pixel circuit  501 . The alignment state of the liquid crystal element  570  is set depending on written data. Note that a common potential may be supplied to one of the pair of electrodes of the liquid crystal element  570  included in each of the plurality of pixel circuits  501 . Moreover, a different potential may be supplied to one of the pair of electrodes of the liquid crystal element  570  of the pixel circuit  501  in each row. 
     The pixel circuit  501  illustrated in  FIG. 16C  includes a transistor  552 , a transistor  554 , a capacitor  562 , and a light-emitting element  572 . The data line DL_n, the gate line GL_m, a potential supply line VL_a, a potential supply line VL_b, and the like are connected to the pixel circuit  501 . 
     Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other. Current flowing through the light-emitting element  572  is controlled in accordance with a potential applied to a gate of the transistor  554 , whereby the luminance of light emitted from the light-emitting element  572  is controlled. 
     At least part of the configuration examples, the drawings corresponding thereto, and the like exemplified in this embodiment can be implemented in combination with the other configuration examples, the other drawings, and the like as appropriate. 
     At least part of this embodiment can be implemented in combination with the other embodiments described in this specification as appropriate. 
     Embodiment 4 
     A pixel circuit including a memory for correcting gray levels of pixels and a display device including the pixel circuit will be described below. 
     [Circuit Configuration] 
       FIG. 17A  shows a circuit diagram of a pixel circuit  400 . The pixel circuit  400  includes a transistor M 1 , a transistor M 2 , a capacitor C 1 , and a circuit  401 . A wiring S 1 , a wiring S 2 , a wiring G 1 , and a wiring G 2  are connected to the pixel circuit  400 . 
     In the transistor M 1 , a gate is connected to the wiring G 1 , one of a source and a drain is connected to the wiring S 1 , and the other is connected to one electrode of the capacitor C 1 . In the transistor M 2 , a gate is connected to the wiring G 2 , one of a source and a drain is connected to the wiring S 2 , and the other is connected to the other electrode of the capacitor C 1  and the circuit  401 . 
     The circuit  401  is a circuit including at least one display element. Any of a variety of elements can be used as the display element, and typically, a light-emitting element such as an organic EL element or an LED element, a liquid crystal element, a MEMS (Micro Electro Mechanical Systems) element, or the like can be used. 
     A node connecting the transistor M 1  and the capacitor C 1  is denoted as a node N 1 , and a node connecting the transistor M 2  and the circuit  401  is denoted as a node N 2 . 
     In the pixel circuit  400 , the potential of the node N 1  can be retained when the transistor M 1  is turned off. The potential of the node N 2  can be retained when the transistor M 2  is turned off. When a predetermined potential is written to the node N 1  through the transistor M 1  with the transistor M 2  being in an off state, the potential of the node N 2  can be changed in accordance with a change in the potential of the node N 1  owing to capacitive coupling through the capacitor C 1 . 
     Here, the transistor using an oxide semiconductor can be used as one or both of the transistor M 1  and the transistor M 2 . Accordingly, owing to an extremely low off-state current, the potential of the node N 1  or the node N 2  can be retained for a long time. Note that in the case where the period in which the potential of each node is retained is short (specifically, the case where the frame frequency is higher than or equal to 30 Hz, for example), a transistor using a semiconductor such as silicon may be used. 
     Driving Method Example 
     Next, an example of a method of operating the pixel circuit  400  is described with reference to  FIG. 17B .  FIG. 17B  is a timing chart of the operation of the pixel circuit  400 . Note that for simplification of description, the influence of various kinds of resistance such as wiring resistance, parasitic capacitance of a transistor, a wiring, or the like, the threshold voltage of the transistor, and the like is not taken into account here. 
     In the operation shown in  FIG. 17B , one frame period is divided into a period T 1  and a period T 2 . The period T 1  is a period in which a potential is written to the node N 2 , and the period T 2  is a period in which a potential is written to the node N 1 . 
     [Period T 1 ] 
     In the period T 1 , a potential for turning on the transistor is supplied to both the wiring G 1  and the wiring G 2 . In addition, a potential V ref  that is a fixed potential is supplied to the wiring S 1 , and a first data potential V w  is supplied to the wiring S 2 . 
     The potential V ref  is supplied from the wiring S 1  to the node N 1  through the transistor M 1 . The first data potential V w  is supplied from the wiring S 2  to the node N 2  through the transistor M 2 . Accordingly, a potential difference V w −V ref  is retained in the capacitor C 1 . 
     [Period T 2 ] 
     Next, in the period T 2 , a potential for turning on the transistor M 1  is supplied to the wiring G 1 , and a potential for turning off the transistor M 2  is supplied to the wiring G 2 . A second data potential Vaasa is supplied to the wiring S 1 . The wiring S 2  may be supplied with a predetermined constant potential or brought into a floating state. 
     The second data potential V data  is supplied from the wiring S 1  to the node N 1  through the transistor M 1 . At this time, capacitive coupling due to the capacitor C 1  changes the potential of the node N 2  in accordance with the second data potential V data  by a potential dV. That is, a potential that is the sum of the first data potential V w  and the potential dV is input to the circuit  401 . Note that although dV is shown as a positive value in  FIG. 17B , dV may be a negative value. That is, the second data potential V data  may be lower than the potential V ref . 
     Here, the potential dV is roughly determined by the capacitance of the capacitor C 1  and the capacitance of the circuit  401 . When the capacitance of the capacitor C 1  is sufficiently larger than the capacitance of the circuit  401 , the potential dV is a potential close to the second data potential V data . 
     In the above manner, the pixel circuit  400  can generate a potential to be supplied to the circuit  401  including the display element, by combining two kinds of data signals; hence, a gray level can be corrected in the pixel circuit  400 . 
     The pixel circuit  400  can also generate a potential exceeding the maximum potential that can be supplied by a source driver connected to the wiring S 1  and the wiring S 2 . For example, in the case where a light-emitting element is used, high-dynamic range (HDR) display or the like can be performed. In the case where a liquid crystal element is used, overdriving or the like can be achieved. 
     Application Examples 
     Example Using Liquid Crystal Element 
     A pixel circuit  400 LC illustrated in  FIG. 17C  includes a circuit  401 LC. The circuit  401 LC includes a liquid crystal element LC and a capacitor C 2 . 
     In the liquid crystal element LC, one electrode is connected to the node N 2  and one electrode of the capacitor C 2 , and the other electrode is connected to a wiring supplied with a potential V com2 . The other electrode of the capacitor C 2  is connected to a wiring supplied with a potential V com1 . 
     The capacitor C 2  functions as a storage capacitor. Note that the capacitor C 2  can be omitted when not needed. 
     In the pixel circuit  400 LC, a high voltage can be supplied to the liquid crystal element LC; thus, high-speed display can be performed by overdriving or a liquid crystal material with a high driving voltage can be employed, for example. Moreover, by supply of a correction signal to the wiring S 1  or the wiring S 2 , a gray level can be corrected in accordance with the operating temperature, the deterioration state of the liquid crystal element LC, or the like. 
     Example Using Light-Emitting Element 
     A pixel circuit  400 EL illustrated in  FIG. 17D  includes a circuit  401 EL. The circuit  401 EL includes a light-emitting element EL, a transistor M 3 , and the capacitor C 2 . 
     In the transistor M 3 , a gate is connected to the node N 2  and one electrode of the capacitor C 2 , one of a source and a drain is connected to a wiring supplied with a potential V H , and the other is connected to one electrode of the light-emitting element EL. The other electrode of the capacitor C 2  is connected to a wiring supplied with a potential V com . The other electrode of the light-emitting element EL is connected to a wiring supplied with a potential V L . 
     The transistor M 3  has a function of controlling a current to be supplied to the light-emitting element EL. The capacitor C 2  functions as a storage capacitor. The capacitor C 2  can be omitted when not needed. 
     Note that although the configuration in which the anode side of the light-emitting element EL is connected to the transistor M 3  is described here, the transistor M 3  may be connected to the cathode side. In that case, the values of the potential V H  and the potential V L  can be appropriately changed. 
     In the pixel circuit  400 EL, a large amount of current can flow through the light-emitting element EL when a high potential is applied to the gate of the transistor M 3 , which enables HDR display, for example. Moreover, a variation in the electrical characteristics of the transistor M 3  and the light-emitting element EL can be corrected by supply of a correction signal to the wiring S 1  or the wiring S 2 . 
     Note that the configuration is not limited to the circuits illustrated in  FIG. 17C  and  FIG. 17D , and a configuration to which a transistor, a capacitor, or the like is further added may be employed. 
     At least part of this embodiment can be implemented in combination with the other embodiments described in this specification as appropriate. 
     Embodiment 5 
     In this embodiment, structure examples of the pixel of the display device of one embodiment of the present invention are described below. 
     Structure examples of a pixel  300  are shown in  FIG. 18A  to  FIG. 18E . 
     The pixel  300  includes a plurality of pixels  301 . The plurality of pixels  301  each function as a subpixel. One pixel  300  is formed of the plurality of pixels  301  exhibiting different colors, and thus full-color display can be achieved in a display unit. 
     The pixels  300  illustrated in  FIG. 18A  and  FIG. 18B  each include three subpixels. The combination of colors exhibited by the pixels  301  included in the pixel  300  illustrated in  FIG. 18A  is red (R), green (G), and blue (B). The combination of colors exhibited by the pixels  301  included in the pixel  300  illustrated in  FIG. 18B  is cyan (C), magenta (M), and yellow (Y). 
     The pixels  300  illustrated in  FIG. 18C  to  FIG. 18E  each include four subpixels. The combination of colors exhibited by the pixels  301  included in the pixel  300  illustrated in  FIG. 18C  is red (R), green (G), blue (B), and white (W). The use of the subpixel that exhibits white can increase the luminance of the display unit. The combination of colors exhibited by the pixels  301  included in the pixel  300  illustrated in  FIG. 18D  is red (R), green (G), blue (B), and yellow (Y). The combination of colors exhibited by the pixels  301  included in the pixel  300  illustrated in  FIG. 18E  is cyan (C), magenta (M), yellow (Y), and white (W). 
     When subpixels that exhibit red, green, blue, cyan, magenta, yellow, and the like are combined as appropriate with more subpixels functioning as one pixel, the reproducibility of halftones can be increased. Thus, the display quality can be improved. 
     The display device of one embodiment of the present invention can reproduce the color gamut of various standards. For example, the display device of one embodiment of the present invention can reproduce the color gamut of the following standards: the PAL (Phase Alternating Line) or NTSC (National Television System Committee) standard used for TV broadcasting; the sRGB (standard RGB) or Adobe RGB standard used widely for display devices in electronic devices such as personal computers, digital cameras, and printers; the ITU-RBT.709 (International Telecommunication Union Radiocommunication Sector Broadcasting Service (Television) 709) standard used for HDTV (High Definition Televisions, also referred to Hi-Vision); the DCI-P3 (Digital Cinema Initiatives P3) standard used for digital cinema projection; and the ITU-R BT.2020 (REC.2020 (Recommendation 2020)) standard used for UHDTV (Ultra High Definition Television, also referred to as Super Hi-Vision); and the like. 
     Using the pixels  300  arranged in a matrix of 1920×1080, a display device that can achieve full color display with a resolution of what is called full high definition (also referred to as “2K resolution”, “2K1K”, “2K”, or the like) can be obtained. For example, using the pixels  300  arranged in a matrix of 3840×2160, a display device that can achieve full color display with a resolution of what is called ultra high definition (also referred to as “4K resolution”, “4K2K”, “4K”, or the like) can be obtained. For example, using the pixels  300  arranged in a matrix of 7680×4320, a display device that can achieve full color display with a resolution of what is called super high definition (also referred to as “8K resolution”, “8K4K”, “8K”, or the like) can be obtained. By increasing the number of pixels  300 , a display device that can achieve full color display with 16K or 32K resolution can be achieved. 
     At least part of this embodiment can be implemented in combination with the other embodiments described in this specification as appropriate. 
     REFERENCE NUMERALS 
       10 : display device,  11 : display panel,  12 : protection cover,  12   a ,  12   b ,  13 : functional layer,  14 : protection cover,  14   a ,  14   b : functional layer,  15 : display portion,  21 ,  22 : support,  23 ,  23   a ,  23   b : retention member,  24 ,  24   a ,  24   b : spacer,  28 : region,  29 : stylus,  31 ,  32 : gear,  31   a ,  32   a : rotation axis,  33 : cover,  41 ,  41   a ,  41   b : spring,  42 : movable member,  43 : adhesive member