Patent Publication Number: US-2022216449-A1

Title: Display panel and display device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 16/637,801, filed Feb. 10, 2020, now allowed, which is a U.S. National Phase Application under 35 U.S.C. § 371 of International Application PCT/IB2018/055989, filed on Aug. 9, 2018, which is incorporated by reference, and which claims the benefit of a foreign priority application filed in Japan on Aug. 25, 2017, as Application No. 2017-162045. 
    
    
     TECHNICAL FIELD 
     One embodiment of the present invention relates to a display panel, a display device, an electronic device, and a manufacturing method thereof. 
     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 include a semiconductor device, a display device, a light-emitting device, an electronic device, a lighting device, an input device (e.g., a touch sensor), an input-output device (e.g., a touch panel), a driving method thereof, and a manufacturing method thereof. 
     BACKGROUND ART 
     In recent years, larger display devices have been required. Examples of uses for a large display device include a television device for home use (also referred to as a TV or a television receiver), digital signage, and a PID (Public Information Display). A larger display region of a display device can provide more information at a time. In addition, a larger display region attracts more attention, so that the effectiveness of the advertisement is expected to be increased, for example. 
     Light-emitting elements (also referred to as EL elements) utilizing electroluminescence (hereinafter referred to as EL) have features such as ease of thinning and lightening, high-speed response to an input signal, and driving with a direct-current low voltage source, and application of the EL elements to display devices has been discussed. For example, Patent Document 1 discloses a flexible light-emitting device including 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 
     An object of one embodiment of the present invention is to increase the size of a display device. An object of one embodiment of the present invention is to provide a display device including a display region with an inconspicuous seam. An object of one embodiment of the present invention is to suppress display unevenness or luminance unevenness of a display device. An object of one embodiment of the present invention is to reduce the thickness or weight of a display device. An object of one embodiment of the present invention is to provide a display device that can display an image along a curved surface. An object of one embodiment of the present invention is to provide a highly browsable display device. An object of one embodiment of the present invention is to provide a novel display device. 
     Note that the description of these objects does not preclude the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects can be derived from the description of the specification, the drawings, and the claims. 
     Means for Solving the Problems 
     A display panel of one embodiment of the present invention includes a display region and a visible-light-transmitting region. The display region is adjacent to the visible-light-transmitting region. The display region includes a first light-emitting element and a second light-emitting element. The first light-emitting element includes a first pixel electrode and a first common electrode. The second light-emitting element includes a second pixel electrode and a second common electrode. The first common electrode includes a first portion overlapping with the first pixel electrode. The second common electrode includes a second portion overlapping with the second pixel electrode. The second common electrode includes a third portion in contact with the first common electrode. The first common electrode has a function of reflecting visible light. The first pixel electrode, the second pixel electrode, and the second common electrode each have a function of transmitting visible light. The second light-emitting element is positioned closer to the visible-light-transmitting region than the first light-emitting element. The second common electrode preferably includes a fourth portion. The fourth portion is a portion overlapping with the second pixel electrode and being in contact with the first common electrode. The second common electrode preferably extends to the visible-light-transmitting region. The display panel preferably includes a protective layer over the first light-emitting element and the second light-emitting element. 
     A display panel of one embodiment of the present invention includes a display region and a visible-light-transmitting region. The display region is adjacent to the visible-light-transmitting region. The display region includes an insulating layer, a partition wall, and a light-emitting element. The light-emitting element includes a pixel electrode and a common electrode. The insulating layer includes an opening. The insulating layer covers an end portion of the pixel electrode. The common electrode overlaps with the pixel electrode with the opening therebetween. The partition wall is positioned over the insulating layer. The partition wall is positioned between the light-emitting element and the visible-light-transmitting region. The partition wall is provided along the visible-light-transmitting region. 
     The level of a top surface of the partition wall is higher than the level of a top surface of a portion of the common electrode that overlaps with the opening. The visible-light-transmitting region and the partition wall are preferably provided along two consecutive sides of the display region. The display panel preferably includes a protective layer over the light-emitting element. 
     A display device of one embodiment of the present invention includes a first display panel and a second display panel. The first display panel includes a first display region and a visible-light-transmitting region. The second display panel includes a second display region. The first display region is adjacent to the visible-light-transmitting region. The first display region includes a first light-emitting element and a second light-emitting element. The first light-emitting element includes a first pixel electrode and a first common electrode. The second light-emitting element includes a second pixel electrode and a second common electrode. The first common electrode includes a first portion overlapping with the first pixel electrode. The second common electrode includes a second portion overlapping with the second pixel electrode. The second common electrode includes a third portion in contact with the first common electrode. The first common electrode has a function of reflecting visible light. The first pixel electrode, the second pixel electrode, and the second common electrode each have a function of transmitting visible light. The second light-emitting element is positioned closer to the visible-light-transmitting region than the first light-emitting element. The second display region includes a portion overlapping with the second light-emitting element and a portion overlapping with the visible-light-transmitting region. The second common electrode preferably includes a fourth portion. The fourth portion is a portion overlapping with the second pixel electrode and being in contact with the first common electrode. The second common electrode preferably extends to the visible-light-transmitting region. The first display panel preferably includes a protective layer over the first light-emitting element and the second light-emitting element. The second display region preferably includes a third light-emitting element and a fourth light-emitting element. The third light-emitting element emits light through the second light-emitting element. The fourth light-emitting element emits light through the visible-light-transmitting region. 
     A display device of one embodiment of the present invention includes a first display panel and a second display panel. The first display panel includes a first display region and a visible-light-transmitting region. The second display panel includes a second display region. The first display region is adjacent to the visible-light-transmitting region. The first display region includes an insulating layer, a partition wall, and a light-emitting element. The light-emitting element includes a pixel electrode and a common electrode. The insulating layer includes an opening. The insulating layer covers an end portion of the pixel electrode. The common electrode overlaps with the pixel electrode with the opening therebetween. The partition wall is positioned over the insulating layer. The partition wall is positioned between the light-emitting element and the visible-light-transmitting region. The partition wall is provided along the visible-light-transmitting region. The level of a top surface of the partition wall is higher than the level of a top surface of a portion of the common electrode that overlaps with the opening. The second display region includes a portion overlapping with the visible-light-transmitting region. The visible-light-transmitting region and the partition wall are preferably provided along two consecutive sides of the first display region. The first display panel preferably includes a protective layer over the light-emitting element. 
     Effect of the Invention 
     One embodiment of the present invention can increase the size of a display device. One embodiment of the present invention can provide a display device including a display region with an inconspicuous seam. One embodiment of the present invention can suppress display unevenness or luminance unevenness of a display device. One embodiment of the present invention can reduce the thickness or weight of a display device. One embodiment of the present invention can provide a display device that can display an image along a curved surface. One embodiment of the present invention can provide a highly browsable display device. One embodiment of the present invention can provide a novel display device. 
     Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects. Other effects can be derived from the description of the specification, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  Top views and a cross-sectional view illustrating examples of display panels. 
         FIGS. 2A and 2B  A top view and a cross-sectional view illustrating a comparison example of a display panel. 
         FIGS. 3A to 3D  Top views and cross-sectional views illustrating examples of display panels. 
         FIGS. 4A to 4C  Cross-sectional views each illustrating an example of a display panel. 
         FIGS. 5A to 5C  Top views and a cross-sectional view illustrating examples of display panels. 
         FIG. 6  A cross-sectional view illustrating an example of a display panel. 
         FIGS. 7A and 7B  Cross-sectional views each illustrating an example of a display panel. 
         FIG. 8  A cross-sectional view illustrating an arrangement example of display panels. 
         FIGS. 9A and 9B  Cross-sectional views each illustrating an example of a display panel. 
         FIGS. 10A and 10B  Cross-sectional views each illustrating an example of a display panel. 
         FIGS. 11A to 11D  Top views and cross-sectional views illustrating examples of display panels. 
         FIGS. 12A and 12B  Cross-sectional views each illustrating an example of a display panel. 
         FIG. 13  A cross-sectional view illustrating an arrangement example of display panels. 
         FIGS. 14A and 14B  Top views each illustrating an arrangement example of display panels. 
         FIG. 15  A diagram illustrating an example of a display system. 
         FIG. 16  (A) A diagram illustrating an example of a display device. (B−1) and (B−2) Diagrams illustrating an example of processing performed in the display system. 
       FIGS.  17 A 1 ,  17 A 2 ,  17 B,  17 C, and  17 D Cross-sectional views illustrating structure examples of transistors. 
         FIGS. 18A to 18D  Diagrams illustrating examples of electronic devices. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments are described in detail with reference to the drawings. Note that the present invention is not limited to the following description and it is readily appreciated by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be construed as being limited to the description in the following embodiments. 
     Note that in the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description of such portions is not repeated. Furthermore, the same hatching pattern is applied to portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     The position, size, range, or the like of each component illustrated in drawings is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like disclosed in the drawings. 
     Note that the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film”. In addition, the term “insulating film” can be changed into the term “insulating layer”. 
     Embodiment 1 
     In this embodiment, display panels and a display device of embodiments of the present invention will be described with reference to  FIG. 1  to  FIG. 17 . 
     When a plurality of display panels are arranged in one or more directions (e.g., in one column or in a matrix), a display device with a large display region can be manufactured. 
     In the case where a large display device is manufactured using a plurality of display panels, each of the display panels is not required to be large. Thus, an apparatus for manufacturing the display panel does not need to be increased in size, whereby space-saving can be achieved. Furthermore, since an apparatus for manufacturing small- and medium-sized display panels can be used and a novel apparatus does not need to be utilized for larger display devices, manufacturing cost can be reduced. In addition, a decrease in yield caused by an increase in the size of a display panel can be suppressed. 
     A display portion including a plurality of display panels has a larger display region than a display portion including one display panel when the display panels have the same size, and has an effect of displaying more information at a time, for example. 
     Here, the case where a display panel includes a non-display region that is provided to surround a display region is considered. In this case, for example, when output images of a plurality of display panels are combined to show one image, the one image is seen by a user of the display device as being divided. 
     Making the non-display regions of the display panels small (using display panels with narrow frames) can inhibit display on the display panels from appearing divided; however, it is difficult to totally remove the non-display regions of the display panels. 
     A small non-display region of the display panel leads to a decrease in the distance between an end portion of the display panel and an element in the display panel, in which case the element easily deteriorates by impurities entering from outside the display panel in some cases. 
     Thus, in one embodiment of the present invention, a plurality of display panels are arranged to partly overlap with one another. In two display panels overlapping with each other, at least the display panel positioned on the display surface side (upper side) includes a visible-light-transmitting region and a display region adjacent to each other. In one embodiment of the present invention, a display region of the display panel positioned on a lower side and the visible-light-transmitting region of the display panel positioned on the upper side overlap with each other. Thus, a non-display region between the display regions of the two overlapping display panels can be reduced and even removed. As a result, a large-sized display device in which a seam between the display panels is hardly seen by the user can be obtained. 
     At least part of a non-display region of the display panel positioned on the upper side transmits visible light, and can overlap with the display region of the display panel positioned on the lower side. Furthermore, at least part of a non-display region of the display panel positioned on the lower side can overlap with the display region or a visible-light-blocking region of the display panel positioned on the upper side. It is not necessary to reduce the areas of these parts because a reduction in the area of the frame of the display device (a reduction in area except a display region) is not affected by these parts. 
     A large non-display region of the display panel leads to an increase in the distance between the end portion of the display panel and an element in the display panel, in which case the deterioration of the element due to impurities entering from outside the display panel can be suppressed. For example, in the case where an organic EL element is used as a display element, impurities such as moisture and oxygen are less likely to enter (or less likely to reach) the organic EL element from outside the display panel as the distance between the end portion of the display panel and the organic EL element increases. Since a sufficient area of the non-display region of the display panel can be secured in the display device of one embodiment of the present invention, a highly reliable large display device can be fabricated even when a display panel including an organic EL element or the like is used. 
     Thus, in the case where the plurality of display panels are provided in the display device, the plurality of display panels are preferably arranged such that the display region is continuous over adjacent display panels. 
     The display panel of this embodiment has a bottom-emission structure. 
     In a top-emission display panel, a common electrode needs to transmit visible light because light emitted from a light-emitting element is extracted to the outside through the common electrode. The use of a visible-light-transmitting conductive material causes a problem of high resistance of the common electrode. When a voltage drop due to the resistance of the common electrode occurs, potential distribution in a display surface becomes non-uniform, variation in luminance of light-emitting elements is caused, and the display quality is degraded. 
     In contrast, the display panel of this embodiment has a bottom-emission structure and the visible-light-transmitting property of the common electrode does not matter because light emitted from a light-emitting element is extracted to the outside through a pixel electrode. The use of a metal, an alloy, or the like having low resistivity can increase the conductivity of the common electrode; thus, a voltage drop due to the resistance of the common electrode can be suppressed and the display quality can be improved. An auxiliary wiring or the like for reducing the resistance of the common electrode does not need to be provided, so that the structure of the display panel can be simplified. 
     Specific Example 1 of Display Panel 
       FIG. 1(A)  shows a top view of a display panel DP 1 . 
     The display panel DP 1  illustrated in  FIG. 1(A)  includes a display region  71 , a visible-light-transmitting region  72 , and a visible-light-blocking region  73 . The visible-light-transmitting region  72  and the visible-light-blocking region  73  are each provided adjacent to the display region  71 .  FIG. 1(A)  shows an example in which the display panel DP 1  is provided with an FPC  74 . 
     The display region  71  includes a plurality of pixels. In the visible-light-transmitting region  72 , a pair of substrates that constitutes the display panel DP 1 , a sealant for sealing a display element interposed between the pair of substrates, and the like may be provided. At this time, for members provided in the visible-light-transmitting region  72 , visible-light-transmitting materials are used. In the visible-light-blocking region  73 , for example, a wiring electrically connected to the pixel included in the display region  71  may be provided. Moreover, one or both of a scan line driver circuit and a signal line driver circuit may be provided in the visible-light-blocking region  73 . Furthermore, a terminal connected to the FPC  74 , a wiring connected to the terminal, or the like may be provided in the visible-light-blocking region  73 . 
     The display panel DPI has a bottom-emission structure.  FIG. 1(A)  illustrates a surface opposite to a display surface of the display panel DP 1 . 
       FIGS. 1(B) and 1(C)  each show an enlarged view of a portion including a boundary between the display region  71  and the visible-light-transmitting region  72  in the display panel DP 1 .  FIG. 1(D)  is a cross-sectional view along dashed-dotted line A 1 -A 2  in  FIG. 1(B) . 
       FIG. 2(A)  shows an enlarged view of a portion including a boundary between the display region  71  and the visible-light-transmitting region  72  in a display panel of a comparative example.  FIG. 2(B)  is a cross-sectional view along dashed-dotted line A 3 -A 4  in  FIG. 2(A) . 
     The display panel of the comparative example in  FIGS. 2(A) and 2(B)  includes a substrate  101 , light-emitting elements  110 , an insulating layer  103 , and a protective layer  105 . 
     The light-emitting element  110  includes a pixel electrode  111 , an EL layer  112 , and a common electrode  113   a . The pixel electrode  111  is provided over the substrate  101 , the EL layer  112  is provided over the pixel electrode  111 , and the common electrode  113   a  is provided over the EL layer  112 . The pixel electrode  111  has a function of transmitting visible light. The common electrode  113   a  has a function of reflecting visible light. The light-emitting element  110  emits light to the substrate  101  side. 
     When a voltage higher than the threshold voltage of the light-emitting element  110  is applied between the pixel electrode  111  and the common electrode  113   a , holes are injected to the EL layer  112  from the anode side and electrons are injected to the EL layer  112  from the cathode side. The injected electrons and holes are recombined in the EL layer  112  and a light-emitting substance contained in the EL layer  112  emits light. 
     Here, when the common electrode is formed, the common electrode is sometimes formed in an area wider than a desired region due to bending of a metal mask. Thus, as illustrated in  FIGS. 2(A) and 2(B) , the common electrode  113   a  is provided not only to be in the display region  71  but also to extend to the visible-light-transmitting region  72  in some cases. When the common electrode  113   a  includes a metal film, an alloy film, or the like, the light-transmitting property of a portion of the visible-light-transmitting region  72  to which the common electrode  113   a  extends decreases. Specifically, in the visible-light-transmitting region  72 , a difference in light-transmitting property arises between a region  124  illustrated in  FIG. 2(B)  and other regions. Thus, when the visible-light-transmitting region  72  overlaps with a display region of another display panel, a difference in light extraction efficiency might be made, leading to display unevenness. In addition, light emitted from a light-emitting element cannot be taken out sufficiently in the region  124  when the region  124  overlaps with a display region of another display panel, and a seam between the two display panels is easily recognized in some cases. 
     In view of this, in one embodiment of the present invention, as illustrated in  FIGS. 1(B) to 1(D) , a common electrode  113   b  transmitting visible light is used for a light-emitting element adjacent to the visible-light-transmitting region  72 . 
       FIG. 1(B)  illustrates an example in which the common electrode  113   b  is used for light-emitting elements in one column on the visible-light-transmitting region  72  side.  FIG. 1(C)  illustrates an example in which the common electrode  113   b  is used for light-emitting elements in three columns on the visible-light-transmitting region  72  side. The common electrode  113   b  can be used for light-emitting elements in one column or a plurality of columns. 
     Furthermore, in the case where the visible-light-transmitting region  72  is provided along two consecutive sides of the display region  71  as illustrated in  FIG. 1(A) , the common electrode  113   b  is preferably used also for light-emitting elements in one row or a plurality of rows on the visible-light-transmitting region  72  side. 
     As illustrated in  FIG. 1(D) , a light-emitting element  110   b  using the common electrode  113   b  is provided between a light-emitting element  110   a  using the common electrode  113   a  and the visible-light-transmitting region  72 . Thus, even when the common electrode  113   a  is formed in an area wider than a desired region due to bending of a metal mask, the common electrode  113   a  can be inhibited from extending to the visible-light-transmitting region  72 . For this reason, the light-transmitting property of the common electrode  113   a  is not limited, and a metal, an alloy, or the like having low resistivity can be used. Accordingly, a voltage drop due to the resistance of the common electrode  113   a  can be suppressed and the display quality can be increased. 
     Even when the common electrode  113   b  extends to the visible-light-transmitting region  72  due to bending of a metal mask, a reduction in light-transmitting property of the visible-light-transmitting region  72  can be inhibited because the common electrode  113   b  has a function of transmitting visible light. Specifically, in the visible-light-transmitting region  72 , a difference in light-transmitting property can be narrowed between a region  123  illustrated in  FIG. 1(D)  and other regions. Thus, when the visible-light-transmitting region  72  overlaps with a display region of another display panel, a difference in light extraction efficiency is not easily made and display unevenness can be inhibited. In addition, light emitted from a light-emitting element can be taken out sufficiently also in the region  123  when the region  123  overlaps with a display region of another display panel, and a seam between the two display panels cannot be easily recognized. Consequently, the display quality of the display device can be increased. 
     The display panel illustrated in  FIG. 1(D)  includes the substrate  101 , the light-emitting element  110   a , the light-emitting element  110   b , the insulating layer  103 , and the protective layer  105 . 
     The light-emitting element  110   a  includes the pixel electrode  111 , the EL layer  112 , and the common electrode  113   a . The pixel electrode  111  is provided over the substrate  101 , the EL layer  112  is provided over the pixel electrode  111 , and the common electrode  113   a  is provided over the EL layer  112 . The pixel electrode  111  has a function of transmitting visible light. The common electrode  113   a  has a function of reflecting visible light. The light-emitting element  110   a  emits light to the substrate  101  side. 
     The light-emitting element  110   b  includes the pixel electrode  111 , the EL layer  112 , and the common electrode  113   b . The pixel electrode  111  is provided over the substrate  101 , the EL layer  112  is provided over the pixel electrode  111 , and the common electrode  113   b  is provided over the EL layer  112 . The pixel electrode  111  and the common electrode  113   b  each have a function of transmitting visible light. The common electrode  113   a  is provided for a portion of the light-emitting element  110   b . The portion of the light-emitting element  110   b  emits light to the substrate  101  side. The other portion of the light-emitting element  110   b  emits light to both the substrate  101  side and the protective layer  105  side (the light-emitting element  110   b  can be said to have a dual-emission structure). 
     The visible-light-transmitting property of the common electrode  113   b  is preferably higher than the visible-light-transmitting property of the common electrode  113   a . For example, the average value of transmittance of light in a wavelength range of greater than or equal to 450 nm to less than or equal to 700 nm of the common electrode  113   b  is preferably higher than the average value of transmittance of light in a wavelength range of greater than or equal to 450 nm to less than or equal to 700 nm of the common electrode  113   a.    
     The common electrode  113   b  includes a portion in contact with the common electrode  113   a . This portion can be provided to overlap with the insulating layer  103 . Furthermore, the portion may be provided to overlap with the pixel electrode  111  included in the light-emitting element  110   b.    
     The pixel electrode  111  is an electrode on a side where light is extracted. It is preferable that the pixel electrode  111  and the common electrode  113   b  each include a visible-light-transmitting conductive film. 
     The visible-light-transmitting conductive film can be formed using, for example, indium oxide, indium tin oxide (ITO), indium zinc oxide, zinc oxide (ZnO), gallium zinc oxide (Ga—Zn oxide), or aluminum zinc oxide (Al—Zn oxide). A metal material such as gold, silver, platinum, magnesium, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or titanium; an alloy containing any of these metal materials; a nitride of any of these metal materials (e.g., titanium nitride); or the like can also be used by being formed to be thin enough to have a light-transmitting property. Alternatively, a stacked film of any of the above materials can be used as the conductive film. For example, a stacked film of ITO and an alloy of silver and magnesium is preferably used, in which case conductivity can be increased. Further alternatively, graphene or the like may be used. 
     The EL layer  112  includes at least a light-emitting layer. The EL layer  112  may include a plurality of light-emitting layers. In addition to the light-emitting layer, the EL layer  112  may further include a layer containing a substance with a high hole-injection property, a substance with a high hole-transport property, a hole-blocking material, a substance with a high electron-transport property, a substance with a high electron-injection property, a substance with a bipolar property (a substance with a high electron-transport property and a high hole-transport property), or the like. The EL layer  112  contains one or more kinds of light-emitting substances. 
     Either a low molecular compound or a high molecular compound can be used for the EL layer  112 , and an inorganic compound may also be contained. The layers that constitute the EL layer  112  can each be formed by a method such as an evaporation method (including a vacuum evaporation method), a transfer method, a printing method, an inkjet method, or a coating method. 
     The light-emitting element may be a single element including one EL layer or a tandem element in which a plurality of EL layers are stacked with a charge generation layer therebetween. 
     In one embodiment of the present invention, a light-emitting element using an inorganic compound such as a quantum dot may be employed. 
     The common electrode  113   a  preferably includes a visible-light-reflecting conductive film. 
     For the visible-light-reflecting conductive film, a metal material such as aluminum, gold, platinum, silver, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy containing any of these metal materials can be used, for example. Lanthanum, neodymium, germanium, or the like may be added to the metal material or the alloy. Moreover, the visible-light-reflecting conductive film can be formed using an alloy containing aluminum (an aluminum alloy) such as an alloy of aluminum and titanium, an alloy of aluminum and nickel, an alloy of aluminum and neodymium, or an alloy of aluminum, nickel, and lanthanum (Al—Ni—La), or an alloy containing silver such as an alloy of silver and copper, an alloy of silver, palladium, and copper (Ag—Pd—Cu, also referred to as APC), or an alloy of silver and magnesium. An alloy containing silver and copper is preferable because of its high heat resistance. When a metal film or a metal oxide film is stacked on an aluminum alloy film, oxidation of the aluminum alloy film can be suppressed. Examples of a material for the metal film or the metal oxide film are titanium and titanium oxide. Alternatively, a visible-light-transmitting conductive film described later and a conductive film containing the metal material or the alloy may be stacked. For example, it is possible to use a stacked film of silver and ITO or a stacked film of an alloy of silver and magnesium and ITO. 
     The pixel electrode  111 , the common electrode  113   a , and the common electrode  113   b  can each be formed by an evaporation method or a sputtering method. Alternatively, a discharging method such as an inkjet method, a printing method such as a screen printing method, or a plating method can be used. 
     The insulating layer  103  is provided over the substrate  101 . The insulating layer  103  has an opening overlapping with the pixel electrode  111 . A light-emitting region of the light-emitting element corresponds to a portion where the opening is formed in the insulating layer  103 . The insulating layer  103  covers an end portion of the pixel electrode  111 . 
     The protective layer  105  is provided to cover a plurality of light-emitting elements. When a film with a high barrier property is used as the protective layer  105 , entry of impurities such as moisture and oxygen into the light-emitting element can be inhibited. Thus, deterioration of the light-emitting elements can be suppressed and the reliability of the display panel can be improved. 
     The protective layer  105  is provided over the common electrode  113   a  and the common electrode  113   b . The protective layer  105  is provided on a side opposite to the side where light is extracted, and the visible-light-transmitting property thereof is not limited. Accordingly, the range of choices for a material that can be used for the protective layer  105  can be widened. For example, an inorganic insulating film having absorption in the visible region, such as a silicon nitride film, can be formed to be thick. Furthermore, an organic insulating film such as a colored polyimide film can be provided. Note that in the case where the protective layer  105  has a low visible-light-transmitting property, the protective layer  105  is preferably provided not to extend to the visible-light-transmitting region  72 . For example, an end portion of the protective layer  105  is preferably positioned in the display region  71 . It is preferable that the protective layer  105  cover an end portion of the EL layer  112  and be in contact with a layer with a high barrier property outside the end portion of the EL layer  112 . Thus, entry of impurities into the light-emitting element can be suppressed and the reliability of the display panel can be improved. 
     The insulating layer  103  and the protective layer  105  each preferably include an inorganic film (or an inorganic insulating film). When the light-emitting element is surrounded by the inorganic films, entry of impurities such as moisture and oxygen from the outside into the light-emitting element can be suppressed. A reaction between impurities and an organic compound or a metal material contained in the light-emitting element might cause deterioration of the light-emitting element. In view of the above, a structure with which impurities are less likely to enter the light-emitting element is employed, whereby deterioration of the light-emitting element can be suppressed and the reliability of the light-emitting element can be improved. 
     In the case where the EL layers  112  of two light-emitting elements are separated from each other as illustrated in  FIG. 1(D) , it is preferable that the common electrode  113   a  or the common electrode  113   b  cover the end portion of the EL layer  112  and that the common electrode  113   a  or the common electrode  113   b  be in contact with the insulating layer  103  and the protective layer  105  outside the end portion of the EL layer  112 . In particular, these three layers (i.e., the common electrode  113   a  or the common electrode  113   b , the insulating layer  103 , and the protective layer  105 ) are preferably inorganic films, in which case impurities can be less likely to enter the EL layer  112 . 
     The inorganic film (or inorganic insulating film) preferably has a high moisture barrier property through which water is less likely to be diffused and transmitted. The inorganic film (or inorganic insulating film) through which one or both of hydrogen and oxygen are less likely to be diffused and transmitted is further preferable. Thus, the inorganic film (or inorganic insulating film) can serve as a barrier film. Diffusion of impurities from the outside into the light-emitting element can be effectively suppressed, so that a highly reliable display panel can be provided. 
     The insulating layer  103  can be formed of one or more insulating films. The protective layer  105  preferably includes one or more insulating films. As each of the insulating layer  103  and the protective layer  105 , an oxide insulating film, a nitride insulating film, an oxynitride insulating film, a nitride oxide insulating film, or the like can be used. Examples of the oxide insulating film include a silicon oxide film, an aluminum oxide film, a gallium oxide film, a germanium oxide film, an yttrium oxide film, a zirconium oxide film, a lanthanum oxide film, a neodymium oxide film, a hafnium oxide film, and a tantalum oxide film. Examples of the nitride insulating film include a silicon nitride film and an aluminum nitride film. Examples of the oxynitride insulating film include a silicon oxynitride film. Examples of the nitride oxide insulating film include a silicon nitride oxide film. 
     Note that in this specification and the like, an oxynitride refers to a material whose oxygen content is higher than the nitrogen content as for the composition, and a nitride oxide refers to a material whose nitrogen content is higher than the oxygen content as for the composition. 
     In particular, a silicon nitride film, a silicon nitride oxide film, and an aluminum oxide film are preferable as the insulating layer  103  and the protective layer  105  because of a high moisture barrier property. 
     An inorganic film containing ITO, Ga—Zn oxide, Al—Zn oxide, In—Ga—Zn oxide, or the like can also be used as the protective layer  105 . The inorganic film preferably has high resistance, specifically, higher resistance than the common electrode  113   b . The inorganic film may further contain nitrogen. 
     The visible-light-transmitting conductive film used as the common electrode  113   b  and the visible-light-transmitting inorganic film used as the protective layer  105  may contain a common metal element, for example. Adhesion between the common electrode  113   b  and the protective layer  105  can be increased when the two films contain a common metal element, whereby film separation and entry of impurities from an interface can be inhibited. 
     A. first ITO film can be used as the common electrode  113   b  and a second ITO film can be used as the protective layer  105 , for example. The second ITO film is preferably a film that has a higher resistivity than the first ITO film. Furthermore, a first Ga—Zn oxide film can be used as the common electrode  113   b  and a second Ga—Zn oxide film can be used as the protective layer  105 , for example. The second Ga—Zn oxide film is preferably a film that has a higher resistivity than the first Ga—Zn oxide film. 
     An inorganic film containing Ga, Zn, and O can be obtained by deposition in an oxygen atmosphere or a mixed atmosphere of argon and oxygen with the use of a Ga—Zn—O-based metal oxide target (a mixed sintered body of Ga 2 O 3  and ZnO), for example. An insulating film containing Al, Zn, and O can be obtained by deposition in a similar atmosphere with the use of an Al—Zn—O-based metal oxide target (a mixed sintered body of Al 2 O 3  and ZnO), for example. An inorganic film containing Ga or A 1  and Zn, O, and N can be obtained by deposition in a mixed atmosphere of argon, oxygen, and nitrogen with the use of a similar target. 
     The specific resistance of each of the insulating layer  103  and the protective layer  105  is preferably higher than or equal to 10 10  Ωcm at 20° C. 
     The insulating layer  103  and the protective layer  105  can each be formed by a chemical vapor deposition (CVD) method (such as a plasma-enhanced chemical vapor deposition (PECVD) method), a sputtering method (such as a DC sputtering method, an RF sputtering method, or an ion beam sputtering method), an atomic layer deposition (ALD) method, or the like. 
     A sputtering method and an ALD method are capable of deposition at a low temperature. The EL layer  112  included in the light-emitting element has a low heat resistance. Therefore, the protective layer  105  formed after the fabrication of the light-emitting element is preferably formed at a comparatively low temperature, typically at lower than or equal to 100° C., and a sputtering method and an ALD method are suitable. 
     The insulating layer  103  formed before the fabrication of the light-emitting element can be deposited at a high temperature. By setting substrate temperature during deposition to a high temperature (e.g., higher than or equal to 100° C. and lower than or equal to 350° C.), a dense film with a high barrier property can be formed. Not only a sputtering method and an ALD method but also a CVD method is suitable for forming the insulating layer  103 . A CVD method has a high deposition rate; thus, it is preferable. 
     As the insulating layer  103  or the protective layer  105 , two or more insulating films formed by different deposition methods may be stacked. 
     It is preferable that a first inorganic film be formed by a sputtering method and a second inorganic film be formed by an ALD method, for example. 
     A film formed by a sputtering method contains fewer impurities and has higher density than a film formed by an ALD method. The film formed by an ALD method has higher step coverage and is less likely to be influenced by the shape of a deposition surface than the film formed by a sputtering method. 
     The first inorganic film contains few impurities and has high density. The second inorganic film is formed so as to cover a portion which is not sufficiently covered with the first inorganic film by the influence of a step of the formation surface. Thus, it is possible to form a protective layer capable of further reducing diffusion of water or the like than a protective layer in which only one of the inorganic films is formed. 
     Specifically, it is preferable that an aluminum oxide film, a zirconium oxide film, an ITO film, a Ga—Zn oxide film, an Al—Zn oxide film, or an In—Ga—Zn oxide film be formed by a sputtering method, and then an aluminum oxide film or a zirconium oxide film be formed by an ALD method. 
     The thickness of the inorganic film formed by a sputtering method is preferably greater than or equal to 50 nm and less than or equal to 1000 nm, further preferably greater than or equal to 100 nm and less than or equal to 300 nm. 
     The thickness of the inorganic film formed by an ALD method is preferably greater than or equal to 1 nm and less than or equal to 100 nm, further preferably greater than or equal to 5 nm and less than or equal to 50 nm. 
     The water vapor transmission rate of each of the insulating layer  103  and the protective layer  105  is lower than 1×10 −2  g/(m 2 ·day), preferably lower than or equal to 5×10 −3  g/(m 2 ·day), further preferably lower than or equal to 1×10 −4  g/(m 2 ·day), still further preferably lower than or equal to 1×10 −5  g (m 2 ·day), yet further preferably lower than or equal to 1×10 −6  g/(m 2 ·day). The lower the water vapor transmission rate is, the more diffusion of water from the outside into the transistor and the light-emitting element can be reduced. 
     The thickness of the insulating layer  103  and the thickness of the protective layer  105  are each greater than or equal to 1 nm and less than or equal to 1000 nm, preferably greater than or equal to 50 nm and less than or equal to 500 nm, further preferably greater than or equal to 100 nm and less than or equal to 300 nm. The thickness of the insulating layer is preferably smaller because the whole display panel can be thinner. The thinner the insulating layer is, the more throughput is improved, so that the productivity of the display panel can be improved. 
     Note that the insulating layer  103  and the protective layer  105  can each have a single-layer structure or a stacked-layer structure including one or both of an inorganic film (an inorganic insulating film) and an organic insulating film. 
     Examples of an organic insulating material that can be used for the organic insulating film include an acrylic resin, an epoxy resin, a polyimide resin, a polyamide resin, a polyimide-amide resin, a polysiloxane resin, a benzocyclobutene-based resin, and a phenol resin. 
     Next, a specific example of the display panel of one embodiment of the present invention, which is different from that in  FIGS. 1(B) to 1(D) , will be described with reference to  FIG. 3  and  FIG. 4 . 
       FIGS. 3(A) and 3(B)  are each an enlarged view of a portion including a boundary between the display region  71  and the visible-light-transmitting region  72 .  FIG. 3(C)  is a cross-sectional view along dashed-dotted line A 5 -A 6  in  FIG. 3(A) .  FIG. 3(D)  is a cross-sectional view along dashed-dotted line A 7 -A 8  in  FIG. 3(B) . 
     The common electrode can be formed in a desired region in some cases such as the case where the degree of bending of a metal mask is small. For example, as illustrated in  FIGS. 3(A) and 3(C) , a structure in which the common electrode  113   b  does not extend to the visible-light-transmitting region  72  and an end portion of the common electrode  113   b  is included in the display region  71  is also one embodiment of the present invention. 
     Furthermore, as illustrated in  FIGS. 3(B) and 3(D) , a structure in which the common electrode  113   a  does not overlap with the light-emitting region of the light-emitting element  110   b  is also one embodiment of the present invention. In this structure, the whole light-emitting region of the light-emitting element  110   b  emits light to both the substrate  101  side and the protective layer  105  side. 
       FIGS. 4(A) to 4(C)  each illustrate a cross-sectional view of a portion including a boundary between the display region  71  and the visible-light-transmitting region  72 . 
     Although an example in which the end portion of the common electrode  113   b  is positioned over the common electrode  113   a  is illustrated in  FIG. 1(D)  and the like, an end portion of the common electrode  113   a  may be positioned over the common electrode  113   b  as illustrated in  FIG. 4(A) . That is, the stacking order of the common electrode  113   a  having a function of reflecting visible light and the common electrode  113   b  having a function of transmitting visible light is not limited. The stacking order can be determined in accordance with the structure of the light-emitting element, or the like. 
     As illustrated in  FIG. 4(B) , an insulating layer  104  having a planarization function may be used instead of the insulating layer  103 . The insulating layer  104  is preferably formed using the above-described organic insulating material. 
     When an inorganic insulating film is used as the insulating layer covering the end portion of the pixel electrode  111 , impurities are less likely to enter the light-emitting element as compared with the case where an organic insulating film is used; therefore, the reliability of the light-emitting element can be improved. When an organic insulating film is used as the insulating layer covering the end portion of the pixel electrode  111 , high step coverage can be obtained as compared with the case where an inorganic insulating film is used; therefore, an influence of the shape of the pixel electrode  111  can be small. Therefore, a short circuit in the light-emitting element can be prevented. 
     Either a side-by-side method or a color filter method may be employed in the display panel. In the case of the color filter method, a combination with a light-emitting element with white light emission is preferable.  FIG. 1(D)  and the like illustrate an example in which the EL layer  112  is provided for each light-emitting element, whereas  FIG. 4(B)  illustrates an example in which the EL layer  112  is provided for a plurality of light-emitting elements. The light emission of each light-emitting element can be extracted through a coloring layer.  FIG. 4(B)  illustrates an example in which light emission of the light-emitting element  110   a  is extracted through a coloring layer  131 A for a first color and light emission of the light-emitting element  110   b  is extracted through a coloring layer  131 B for a second color. Note that a color filter can also be used in the case where the EL layer  112  is provided for each light-emitting element. 
     The coloring layer is a colored layer that transmits light in a specific wavelength range. For example, a color filter that transmits light in a red, green, blue, or yellow wavelength range can be used. As a material that can be used for the coloring layer, a metal material, a resin material, a resin material containing a pigment or dye, and the like can be given. 
     As illustrated in  FIG. 4(C) , the common electrode  113   a  may be provided in the entire light-emitting region of the light-emitting element  110   b . In that case, the entire light-emitting region of the light-emitting element  110   b  emits light to the substrate  101  side (this structure can also be referred to as a bottom-emission structure). 
     Specific Example 2 of Display Panel 
       FIGS. 5(A) and 5(B)  illustrate top views of display panels.  FIG. 5(C)  illustrates a cross-sectional view along dashed-dotted line C 1 -C 2  in  FIG. 5(A) .  FIG. 6  illustrates a cross-sectional view along dashed-dotted line C 3 -C 4  in  FIG. 5(A) . 
     The display panels illustrated in  FIGS. 5(A) and 5(B)  each include the display region  71 , the visible-light-transmitting region  72 , and a driver circuit  78 . The FPC  74  is connected to the display panel.  FIGS. 5(A) and 5(B)  each illustrate an example in which the visible-light-transmitting region  72  is adjacent to the display region  71  and positioned along two sides of the display region  71 . Furthermore, in the examples in  FIGS. 5(A) and 5(B) , the driver circuit  78  is positioned along the other two sides of the display region  71 . 
     The display panel illustrated in  FIG. 5(A)  has a sharp corner and the display panel illustrated in  FIG. 5(B)  has a rounded corner. A display panel using a film substrate can be manufactured to have any of various top surface shapes. For example, a display panel with a corner having a curvature is easily manufactured in some cases because the display panel is less likely to be cracked when divided. 
     As illustrated in  FIG. 5(C)  and  FIG. 6 , a display panel  370 A includes a substrate  361 , a bonding layer  363 , insulating layers  365  and  367 , transistors  301  and  303 , conductive layers  307 ,  355 , and  358 , an insulating layer  314 , the light-emitting element  110   a , the light-emitting element  110   b , the insulating layer  104 , the protective layer  105 , a bonding layer  317 , a substrate  371 , and the like. 
     The display panel  370 A is a bottom-emission display panel employing a side-by-side method. 
     The light-emitting element  110   a  includes the pixel electrode  111 , the EL layer  112 , and the common electrode  113   a . The light-emitting element  110   b  includes the pixel electrode  111 , the EL layer  112 , and the common electrode  113   b . The pixel electrode  111  is electrically connected to a source or a drain of the transistor  303 . These are connected directly or connected through another conductive layer. The EL layer  112  is provided for each light-emitting element. The common electrode  113   a  and the common electrode  113   b  each cover the end portion of the EL layer  112  and are in contact with the insulating layer  104  outside the end portion of the EL layer  112 . The common electrode  113   b  includes a portion in contact with the common electrode  113   a.    
     The pixel electrode  111  has a function of transmitting visible light. The common electrode  113   a  has a function of reflecting visible light. The light-emitting element  110   a  emits light to the substrate  101  side. 
     The common electrode  113   b  has a function of transmitting visible light. The common electrode  113   a  is provided in part (part  110   b   2 ) of the light-emitting element  110   b . The part  110   b   2  of the light-emitting element  110   b  emits light to the substrate  101  side. Another part  110   b   1  of the light-emitting element  110   b  emits light to both the substrate  101  side and the protective layer  105  side. 
     In the vicinity of a connection portion  306  of the display panel  370 A, an opening  308  reaching the insulating layer  313  is provided in the insulating layer  314 , and the insulating layer  313  and the protective layer  105  are in contact with each other in the opening  308 . Even in the case where an organic insulating film exists in an end portion of the display panel as described above, the organic insulating film has an opening and inorganic films (or inorganic insulating films) are in contact with each other in the opening, whereby impurities such as moisture are less likely to enter the display panel from the outside of the display panel; thus, deterioration of the transistor and the light-emitting element can be suppressed. In the display region  71  of the display panel  370 A, the protective layer  105  covers an end portion of the insulating layer  314  and an end portion of the insulating layer  104  in the vicinity of the visible-light-transmitting region  72 . Furthermore, the protective layer  105  is in contact with the insulating layer  313  outside the end portion of the insulating layer  314  and the end portion of the insulating layer  104 . With such a structure, impurities such as moisture are less likely to enter the display panel from the outside of the display panel, whereby deterioration of the transistor and the light-emitting element can be suppressed. 
     As illustrated in  FIG. 6 , the light-emitting element  110   b  using the common electrode  113   b  is provided between the light-emitting element  110   a  using the common electrode  113   a  and the visible-light-transmitting region  72 . Thus, even when the common electrode  113   a  is formed in an area wider than a desired region due to bending of a metal mask, the common electrode  113   a  can be inhibited from extending to the visible-light-transmitting region  72 . For this reason, the light-transmitting property of the common electrode  113   a  is not limited, and a metal, an alloy, or the like having low resistivity can be used. Accordingly, a voltage drop due to the resistance of the common electrode  113   a  can be suppressed and the display quality can be increased. 
     Even when the common electrode  113   b  extends to the visible-light-transmitting region  72  due to bending of a metal mask, a reduction in light-transmitting property of the visible-light-transmitting region  72  can be inhibited because the common electrode  113   b  has a function of transmitting visible light. Specifically, in the visible-light-transmitting region  72 , a difference in light-transmitting property can be narrowed between a region  387  illustrated in  FIG. 6  and other regions. Thus, when the visible-light-transmitting region  72  overlaps with a display region of another display panel, a difference in light extraction efficiency is not easily made and display unevenness can be inhibited. In addition, light emitted from a light-emitting element can be taken out sufficiently also in the region  387  when the region  387  overlaps with a display region of another display panel, and a seam between the two display panels cannot be easily recognized. Consequently, the display quality of the display device can be increased. 
     The common electrode can be formed in a desired region in some cases such as the case where the degree of bending of a metal mask is small. For example, like a display panel  370 B illustrated in  FIG. 7(A) , a structure in which the common electrode  113   b  does not extend to the visible-light-transmitting region  72  and the end portion of the common electrode  113   b  is included in the display region  71  is also one embodiment of the present invention.  FIG. 7(A)  illustrates an example in which the end portion of the common electrode  113   b  is positioned over the insulating layer  104 . 
     Furthermore, like a display panel  370 C illustrated in  FIG. 7(B) , a structure in which the common electrode  113   a  does not overlap with the light-emitting region of the light-emitting element  110   b  is also one embodiment of the present invention. In this structure, the whole light-emitting region of the light-emitting element  110   b  emits light to both the substrate  101  side and the protective layer  105  side.  FIG. 7(B)  illustrates an example in which the end portion of the common electrode  113   a  and the end portion of the common electrode  113   b  are each positioned over the insulating layer  104 . 
     Each of the light-emitting element  110   a  and the light-emitting element  110   b  further includes an optical adjustment layer  114 . When the light-emitting element has a microcavity structure, light with high color purity can be extracted from the display panel. 
     The insulating layer  104  covers the end portion of the pixel electrode  111  and an end portion of the optical adjustment layer  114 . Two adjacent pixel electrodes  111  are electrically insulated from each other by the insulating layer  104 . 
     In the display region  71  of each of the display panels  370 B and  370 C illustrated in  FIGS. 7(A) and 7(B) , in the vicinity of the visible-light-transmitting region  72 , the protective layer  105  covers the end portion of the common electrode  113   a  and the end portion of the common electrode  113   b , and is in contact with the insulating layer  104  outside the end portion of the common electrode  113   a  and the end portion of the common electrode  113   b . Furthermore, the protective layer  105  covers the end portion of the insulating layer  314  and the end portion of the insulating layer  104  and is in contact with the insulating layer  313  outside the end portion of the insulating layer  314  and the end portion of the insulating layer  104 . With such a structure, entry of impurities into the common electrode  113   a  and the common electrode  113   b  can be inhibited. 
     In the display panel of this embodiment, it is preferable that the variety of insulating layers and the protective layer  105  be provided such that an end portion of an inorganic film (or an inorganic insulating film) is positioned outside an end portion of an organic film and inorganic films (or inorganic insulating films) be stacked to be in contact with each other at and in the vicinity of the end portion of the display panel. 
     The substrate  361  and the substrate  371  are attached to each other with the bonding layer  317 . The substrate  361  and the insulating layer  365  are attached to each other with the bonding layer  363 . 
     The display panel  370 A has a structure in which the transistor, the light-emitting element  110   a , the light-emitting element  110   b , and the like formed over a formation substrate are transferred to the substrate  361 . 
     For the substrate  361  and the substrate  371 , a material such as glass, quartz, a resin, a metal, an alloy, or a semiconductor can be used. The substrate on the side where light from the light-emitting element is extracted is formed using a material that transmits the light. A flexible substrate is preferably used as the substrate  361  and the substrate  371 . 
     For the bonding layer, various curable adhesives, e.g., a photocurable adhesive such as an ultraviolet curable adhesive, a reactive curable adhesive, a thermosetting adhesive, and an anaerobic adhesive, can be used. Alternatively, an adhesive sheet or the like may be used. 
     The driver circuit  78  includes the transistor  301 . The display region  71  includes the transistor  303 . 
     Each transistor includes a gate, a gate insulating layer  311 , a semiconductor layer, a back gate, a source, and a drain. The gate (the lower gate) and the semiconductor layer overlap with each other with the gate insulating layer  311  therebetween. The back gate (the upper gate) and the semiconductor layer overlap with each other with an insulating layer  312  and the insulating layer  313  therebetween. It is preferable that the two gates be electrically connected to each other. 
     The structure of the transistor may be different between the driver circuit  78  and the display region  71 . The driver circuit  78  and the display region  71  may each include a plurality of kinds of transistors. 
     A material through which impurities such as water and hydrogen do not easily diffuse is preferably used for at least one of the insulating layer  365 , the insulating layer  367 , the insulating layer  312 , the insulating layer  313 , and the insulating layer  314 . Diffusion of impurities from the outside into the transistors can be effectively inhibited, leading to improved reliability of the display panel. The insulating layer  314  functions as a planarization layer. 
     The display region  71  includes the conductive layer  358 . The conductive layer  358  is an example of wirings in the pixel. The connection portion  306  includes the conductive layer  307 . The conductive layer  307  is electrically connected to an external input terminal through which a signal or a potential from the outside is transmitted to the driver circuit  78 . Here, an example in which the FPC  74  is provided as an external input terminal is shown. The FPC  74  and the conductive layer  307  are electrically connected to each other through a connector  319 . The conductive layer  307  and the conductive layer  358  can be formed using the same material and the same step as those of the source and the drain of the transistor. 
     As the connector  319 , any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), and the like can be used. 
       FIG. 8  illustrates an example in which two display panels  370 A are provided to overlap with each other. 
       FIG. 8  illustrates a display region  71   a  and a visible-light-transmitting region  72   a  of the lower display panel and a display region  71   b  and a driver circuit  78   b  of the upper display panel. 
     In  FIG. 8 , the display panel positioned on the display surface side (lower side) includes the visible-light-transmitting region  72   a  adjacent to the display region  71   a . The display region  71   b  of the upper display panel overlaps with the visible-light-transmitting region  72   a  of the lower display panel. Thus, a non-display region between the display regions of the two overlapping display panels can be reduced and even removed. Accordingly, a large display device in which a seam between display panels is not easily recognized by a user can be obtained. 
     Furthermore, the display region  71   b  also overlaps with the light-emitting region of the light-emitting element  110   b  included in the lower display panel. Since light from the light-emitting element  110   b  is emitted to the protective layer  105  side in some cases, variation in luminance might be caused between display performed with the light-emitting element  110   a  and display performed with the light-emitting element  110   b  in the lower display panel. Thus, it is preferable that the light-emitting element  110   b  included in the display region  71   a  overlap with the light-emitting element  110   a  included in the display region  71   b  and that display is performed with the light-emitting element  110   a . As illustrated in  FIG. 8 , display may be performed with both the light-emitting element  110   b  included in the display region  71   a  and the light-emitting element  110   a  included in the display region  71   b , or the light-emitting element  110   b  may be brought into a non-light-emitting state. Note that when light emitted from the light-emitting element  110   b  is reflected by the common electrode  113   a  included in the display region  71   b , the light can be extracted to the outside of the display panel in some cases. 
     Here, it is preferable that video signals corrected so that discontinuation of an image at the seam between the two display panels can be reduced be input to the display panels. As a result, a natural image having a sense of unity, in which the seam is not easily recognized, can be displayed. 
     In particular, it is preferable to correct the video signals using artificial intelligence (AI). 
     Note that artificial intelligence refers to a computer that imitates the intelligence of human beings. For example, the video signals can be corrected using an artificial neural network (ANN). The artificial neural network refers to a circuit that imitates a neural network composed of neurons and synapses, and the artificial neural network is a kind of artificial intelligence. 
     The display panel of this embodiment can be used for a display portion of a display system ( FIG. 15 ) described below. The display system includes a signal generation portion and the display portion. The signal generation portion can correct image data. The display panels of this embodiment can display a natural image having a sense of unity, in which a seam is not easily recognized, by displaying the image using corrected image data. 
     As illustrated in  FIG. 8 , the two display panels overlap with each other with a light-transmitting layer  318  therebetween. Specifically, the light-transmitting layer  318  that has a higher refractive index than air and transmits visible light is provided between the visible-light-transmitting region  72   a  and the display region  71   b . Thus, air can be inhibited from entering a portion between the visible-light-transmitting region  72   a  and the display region  71   b , so that the interface reflection due to a difference in refractive index can be reduced. In addition, display unevenness or luminance unevenness of the display device can be suppressed. 
     The light-transmitting layer  318  can overlap with part or the whole of a surface of the substrate  371  of the lower display panel. Furthermore, the light-transmitting layer  318  can overlap with part or the whole of a surface of the substrate  361  of the upper display panel. For example, the light-transmitting layer  318  may overlap with only the visible-light-transmitting region  72   a  and the display region  71   b . The light-transmitting layer  318  may overlap with the driver circuit  78   b.    
     Next, specific examples of the display panel of one embodiment of the present invention, which are different from the display panels  370 A,  370 B, and  370 C, will be described with reference to  FIG. 9  and  FIG. 10 . 
       FIG. 9(A)  and  FIG. 10(A)  are cross-sectional views of a display panel  370 D.  FIG. 9(B)  and  FIG. 10(B)  are cross-sectional views of a display panel  370 E.  FIGS. 9(A) and 9(B)  correspond to cross-sectional views along dashed-dotted line C 1 -C 2  in  FIG. 5(A) .  FIGS. 10(A) and 10(B)  correspond to cross-sectional views along dashed-dotted line C 3 -C 4  in  FIG. 5(A) . 
     The display panels  370 D and  370 E are different from the display panel  370 A in that a coloring layer is provided between the insulating layer  315  and the insulating layer  314  covering the transistors and the EL layer  112  is provided for a plurality of pixels. Description of portions in common with the display panel  370 A is omitted. 
     Although the display panel  370 A employing a side-by-side method is described as an example, a color filter method can also be employed as in the display panels  370 D and  370 E. 
     The layers included in the visible-light-transmitting region  72  transmit visible light. The display panel  370 A illustrated in  FIG. 6  illustrates an example in which the visible-light-transmitting region  72  includes the substrate  361 , the bonding layer  363 , the insulating layer  365 , the insulating layer  367 , the gate insulating layer  311 , the insulating layer  312 , the insulating layer  313 , the protective layer  105 , the bonding layer  317 , and the substrate  371 . In this stacked-layer structure, the materials for the layers are preferably selected such that a difference in refractive index at each interface is reduced. By reducing a difference in refractive index between two layers that are in contact with each other, a seam between the two display panels is not easily recognized by a user. 
     As in each of the display panel  370 D illustrated in  FIG. 10(A)  and the display panel  370 E illustrated in  FIG. 10(B) , the number of insulating layers included in the visible-light-transmitting region  72  is preferably smaller than that of insulating layers included in a portion, which is in the vicinity of the visible-light-transmitting region  72 , of the display region  71 . 
     Unlike the display panel  370 A, the display panel  370 D has a structure in which the visible-light-transmitting region  72  does not include the gate insulating layer  311 , the insulating layer  312 , and the insulating layer  313 . 
     Unlike the display panel  370 A, the display panel  370 E has a structure in which the visible-light-transmitting region  72  does not include the insulating layer  367 , the gate insulating layer  311 , the insulating layer  312 , the insulating layer  313 , and the protective layer  105 . 
     The number of insulating layers included in the visible-light-transmitting region  72  is reduced, and thus the number of interfaces at which a difference in refractive index is large can be reduced. In this manner, the reflection of external light in the visible-light-transmitting region  72  can be suppressed. Moreover, the visible light transmittance in the visible-light-transmitting region  72  can be increased. Accordingly, a difference in luminance (brightness) of display on the display panel provided on the lower side between a portion seen through the visible-light-transmitting region  72  and a portion seen not through the region can be made small. 
     Accordingly, display unevenness or luminance unevenness of the display device can be suppressed. 
     The display panel  370 E has a structure in which part of the transistor  303  included in the display region  71  transmits visible light. Specifically, one of the source and the drain (a conductive layer  203   t ), the two gate electrodes, and the semiconductor layer of the transistor  303  transmit visible light. 
     As described above, with the structure in which at least part of the transistor included in the display region  71  transmits visible light, at least the part of the transistor can be provided to overlap with the light-emitting region of the light-emitting element, and the aperture ratio of the display region  71  can be increased. Furthermore, when the capacitor and the like included in the display region  71  also transmit visible light and are provided to overlap with the light-emitting region of the light-emitting element, the aperture ratio of the display region  71  can be further increased. 
     Note that a coloring layer is not provided in a contact portion between the other of the source and the drain (a conductive layer  203   s ) of the transistor  303  and the pixel electrode  111 . Therefore, when the conductive layer  203   s  transmits visible light, light emitted from the light-emitting element  110   a  might leak to the outside of the display panel without passing through the coloring layer. Thus, a conductive film such as a metal film or an alloy film that blocks visible light is preferably used as the conductive layer  203   s.    
     In the display region  71 , a conductive film such as a metal film or an alloy film that blocks visible light is preferably used as a scan line and a signal line. With the use of a metal film, an alloy film, or the like as the scan line and the signal line, the resistance value of the scan line and the signal line can be reduced. A metal film, an alloy film, or the like is also used for the transistor, a wiring, and the like in the driver circuit  78 . The scan line and the signal line are preferably formed in the same process as the transistor  301 , the wiring, and the like in the driver circuit  78 . For example, the conductive layer  358  can function as the scan line or the signal line. For example, a metal film, an alloy film, or the like is preferably used as each of the conductive layer  358 , and a conductive layer  201   a  and the conductive layer  203   s  included in the driver circuit  78 . 
     The gate is preferably formed using a multi-tone mask (a halftone mask, a gray-tone mask, or the like). When a multi-tone mask is used, without increasing the number of masks, a gate that transmits visible light can be formed in the display region  71 , and a low-resistance gate and a low-resistance gate wiring can be formed in the driver circuit  78 . The conductive layer  203   s  and the conductive layer  203   t  are also preferably formed using a multi-tone mask (a halftone mask, a gray-tone mask, or the like). Note that the conductive layer  203   s  may have a stacked-layer structure of the conductive layer  203   t  (a visible-light-transmitting conductive film), and a metal film, an alloy film, or the like. 
     Examples of a material of the visible-light-transmitting semiconductor film include a metal oxide and an oxide semiconductor. The oxide semiconductor preferably contains at least indium. In particular, indium and zinc are preferably contained. Moreover, in addition to these, one kind or a plurality of 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. 
     A material of the visible-light-transmitting conductive film preferably includes one kind or a plurality of kinds selected from indium, zinc, and tin. Specifically, an In oxide, an In—Sn oxide (also referred to as ITO: Indium Tin Oxide), an In—Zn oxide, an In—W oxide, an In—W—Zn oxide, an In—Ti oxide, an In—Sn—Ti oxide, an In—Sn—Si oxide, a Zn oxide, a Ga—Zn oxide, or the like can be used. 
     As the material of the conductive film included in the transistor, an oxide semiconductor that contains an impurity element, for example, and has reduced resistance may be used. The oxide semiconductor with reduced resistance can be regarded as an oxide conductor (OC). 
     For example, regarding an oxide conductor, oxygen vacancies are formed in an oxide semiconductor and then hydrogen is added to the oxygen vacancies, so that a donor level is formed in the vicinity of the conduction band. By the formation of the donor level in the oxide semiconductor, the oxide semiconductor has an increased conductivity and becomes a conductor. 
     An oxide semiconductor has a large energy gap (e.g., an energy gap of 2.5 eV or larger), and thus has a visible-light-transmitting property. An oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band, as described above. Therefore, the influence of absorption due to the donor level is small in the oxide conductor, and the oxide conductor has a visible-light-transmitting property comparable to that of an oxide semiconductor. 
     The oxide conductor preferably includes one or more kinds of metal elements included in the semiconductor film of the transistor. When oxide semiconductors including the same metal element are used for two or more layers of the layers constituting the transistor, the same manufacturing apparatus (e.g., deposition apparatus or processing apparatus) can be used in two or more steps and manufacturing cost can thus be reduced. 
     Specific Example 3 of Display Panel 
       FIG. 11(A)  illustrates a top view of a display panel DP 2 . 
     The display panel DP 2  illustrated in  FIG. 11(A)  includes the display region  71 , the visible-light-transmitting region  72 , and the visible-light-blocking region  73 . The visible-light-transmitting region  72  and the visible-light-blocking region  73  are each provided adjacent to the display region  71 .  FIG. 11(A)  illustrates an example in which the display panel DP 2  is provided with the FPC  74 . 
     A partition wall  118  is provided in the display region  71  of the display panel DP 2 . The partition wall  118  is provided along the visible-light-transmitting region  72 . In  FIG. 11(A) , the visible-light-transmitting region  72  is provided along two consecutive sides of the display region  71 ; thus, the partition wall  118  is also provided along the two sides. 
     The display panel DP 2  has a bottom-emission structure.  FIG. 11(A)  illustrates a surface opposite to a display surface of the display panel DP 2 . 
       FIG. 11(B)  shows an enlarged view of a portion including a boundary between the display region  71  and the visible-light-transmitting region  72  in the display panel DP 2 .  FIGS. 11(C) and 11(D)  are each a cross-sectional view along dashed-dotted line B 1 -B 2  in  FIG. 11(B) . 
     The display panels illustrated in  FIGS. 11(C) and 11(D)  each include the substrate  101 , the light-emitting element  110 , the partition wall  118 , the insulating layer  103 , and the protective layer  105 . 
     The light-emitting element  110  includes the pixel electrode  111 , the EL layer  112 , and the common electrode  113   a . The pixel electrode  111  is provided over the substrate  101 , the EL layer  112  is provided over the pixel electrode  111 , and the common electrode  113   a  is provided over the EL layer  112 . The pixel electrode  111  has a function of transmitting visible light. The common electrode  113   a  has a function of reflecting visible light. The light-emitting element  110  emits light to the substrate  101  side. 
     The partition wall  118  is provided over the insulating layer  103 . A level T 2  of a top surface of the partition wall  118  is higher than a level T 1  of a top surface of the common electrode  113   a  in the light-emitting region of the light-emitting element  110 . A width W 2  of the partition wall  118  is smaller than a width W 1  between the two light-emitting elements  110 . When the width W 2  is larger than the width W 1 , the pixel pitch might deviate when two display panels overlap with each other, leading to a decrease in display quality in some cases. The partition wall  118  is preferably provided such that a pixel pitch does not deviate at a seam between two display panels. 
     When a metal mask is in contact with the partition wall  118  in the formation of the common electrode  113   a , the common electrode  113   a  can be prevented from protruding into the visible-light-transmitting region  72 . Thus, the common electrode  113   a  can be formed such that the end portion of the common electrode  113   a  is positioned inside the partition wall  118  (see  FIGS. 11(B) to 11(D) ). Since the common electrode  113   a  does not extend to the visible-light-transmitting region  72  and can be formed in a desired region, the display quality of the display device can be improved. Note that the common electrode  113   a  is sometimes in contact with the partition wall  118  due to pattern blurring at the deposition or the like, as illustrated in  FIG. 11(D) . 
     It is preferable that the protective layer  105  cover the end portion of the common electrode  113   a  and be in contact with the insulating layer  103  outside the end portion of the common electrode  113   a .  FIG. 11(C)  illustrates an example in which the protective layer  105  extends to the visible-light-transmitting region  72 , and  FIG. 11(D)  illustrates an example in which the protective layer  105  is not provided in the visible-light-transmitting region  72 . 
     Specific Example 4 of Display Panel 
       FIG. 12(A)  is a cross-sectional view of a display panel  370 F.  FIG. 12(B)  is a cross-sectional view of a display panel  370 G.  FIGS. 12(A) and 12(B)  correspond to cross-sectional views along dashed-dotted line C 3 -C 4  in  FIG. 5(A) . Description of portions in common with the display panel  370 A is omitted. 
     As illustrated in  FIGS. 12(A) and 12(B) , the display panels  370 F and  370 G each include the substrate  361 , the bonding layer  363 , the insulating layers  365  and  367 , the conductive layer  358 , the insulating layer  314 , the light-emitting element  110 , the insulating layer  104 , the protective layer  105 , the partition wall  118 , the bonding layer  317 , the substrate  371 , and the like. 
     The light-emitting element  110  includes the pixel electrode  111 , the EL layer  112 , and the common electrode  113   a . The pixel electrode  111  is provided over the insulating layer  314 , the EL layer  112  is provided over the pixel electrode  111 , and the common electrode  113   a  is provided over the EL layer  112 . The pixel electrode  111  has a function of transmitting visible light. The common electrode  113   a  has a function of reflecting visible light. The light-emitting element  110  emits light to the substrate  361  side. 
     The partition wall  118  is provided over the insulating layer  104 . The level of the top surface of the partition wall  118  is higher than the level of the top surface of the common electrode  113   a  in the light-emitting region of the light-emitting element  110 . 
     When a metal mask is in contact with the partition wall  118  in the formation of the common electrode  113   a , the common electrode  113   a  can be prevented from protruding into the visible-light-transmitting region  72 . Thus, the common electrode  113   a  can be formed such that the end portion of the common electrode  113   a  is positioned inside the partition wall  118  (see  FIGS. 12(A) and 12(B) ). Since the common electrode  113   a  does not extend to the visible-light-transmitting region  72  and can be formed in a desired region, the display quality of the display device can be improved. 
     It is preferable that the protective layer  105  cover the end portion of the common electrode  113   a  and be in contact with the insulating layer  104  outside the end portion of the common electrode  113   a .  FIG. 12(A)  illustrates an example in which the protective layer  105  extends to the visible-light-transmitting region  72 , and  FIG. 12(B)  illustrates an example in which the protective layer  105  is not provided in the visible-light-transmitting region  72  and the end portion of the protective layer  105  is positioned over the insulating layer  104 . 
       FIG. 13  illustrates an example in which two display panels  370 F are provided to overlap with each other. 
       FIG. 13  illustrates the display region  71   a  and the visible-light-transmitting region  72   a  of the lower display panel and the display region  71   b  and the driver circuit  78   b  of the upper display panel. 
     In  FIG. 13 , the display panel positioned on the display surface side (lower side) includes the visible-light-transmitting region  72   a  adjacent to the display region  71   a . The display region  71   b  of the upper display panel overlaps with the visible-light-transmitting region  72   a  of the lower display panel. Thus, a non-display region between the display regions of the two overlapping display panels can be reduced and even removed. Accordingly, a large display device in which a seam between display panels is not easily recognized by a user can be obtained. 
     Specific Example of Display Device 
     Next, a display device including a plurality of display panels will be described with reference to  FIG. 14 . 
       FIGS. 14(A) and 14(B)  each illustrate an example in which display panels are arranged in a 2×2 matrix (two display panels in each of the longitudinal direction and the lateral direction). 
       FIG. 14(A)  illustrates an example in which the FPC  74  connected to one display panel is provided to extend outside the display panel. 
       FIG. 14(B)  illustrates an example in which the FPC  74  connected to one display panel is provided to overlap with the display region  71  of the display panel. The FPC can be connected to a surface opposite to the display surface of the display panel of this embodiment. Thus, the size of a display device and the size of an electronic device can be reduced. For example, a space for bending an FPC can be sometimes omitted. 
     The four display panels are arranged so as to overlap with each other. Specifically, the display panels are arranged such that the visible-light-transmitting region  72  included in one display panel has a region overlapping with and located over the display region  71  (on the display surface side) of another display panel. The display panels are arranged such that the visible-light-blocking region (a lead wiring  257 , the driver circuit  78 , and the like in  FIGS. 14(A) and 14(B) ) of one display panel does not overlap with and is not located over the display region of another display panel. 
     Thus, a region where the four display regions  71  are arranged with almost no seam can be used as the display region of the display device. 
     Note that to reduce the step between two adjacent display panels, the thicknesses of the display panels are preferably small. For example, the thickness of the display panel is preferably less than or equal to 1 mm, further preferably less than or equal to 300 μm, still further preferably less than or equal to 100 μm. 
     The display panel preferably incorporates both a scan line driver circuit and a signal line driver circuit. In the case where a driver circuit is provided separately from the display panel, a printed circuit board including a driver circuit and a large number of wirings, terminals, and the like are provided on the back side (the side opposite to the display surface side) of the display panel. Thus, the number of components of the whole display device becomes enormous, which leads to an increase in weight of the display device in some cases. When the display panel incorporates both a scan line driver circuit and a signal line driver circuit, the number of components of the display device can be reduced and the weight of the display device can be reduced. This leads to higher portability of the display device. 
     Here, the scan line driver circuit and the signal line driver circuit are required to operate at a high driving frequency in accordance with the frame frequency of an image to be displayed. In particular, the signal line driver circuit is required to operate at a higher driving frequency than the scan line driver circuit. Therefore, some transistors used for the signal line driver circuit require large current supply capability in some cases. Meanwhile, some transistors provided in the display region require adequate withstand voltage for driving a display element in some cases. 
     In view of the above, the transistor of the driver circuit and the transistor of the display region are preferably formed to have different structures. For example, one or a plurality of transistors provided in the display region are transistors with high withstand voltage, and one or a plurality of transistors provided in the driver circuit are transistors with high driving frequency. 
     Specifically, one or a plurality of transistors used for the signal line driver circuit are transistors each including a thinner gate insulating layer than the transistor used for the display region. By forming two kinds of transistors separately as described above, the signal line driver circuit can be formed over the substrate over which the display region is provided. 
     One or a plurality of transistors used for the signal line driver circuit are preferably transistors each having a shorter channel length than the transistor used for the display region. For example, the channel length of the transistor of the signal line driver circuit is shorter than 1.5 μm, preferably shorter than or equal to 1.2 μm, further preferably shorter than or equal to 1.0 μm, still further preferably shorter than or equal to 0.9 μm, yet further preferably shorter than or equal to 0.8 μm, yet still further preferably shorter than or equal to 0.6 μm, and preferably longer than or equal to 0.1 μm. 
     Meanwhile, each transistor provided in the display region preferably has a channel length longer than the shortest channel length among the channel lengths of the transistors of the signal line driver circuit. For example, the channel length of the transistor provided in the display region is longer than or equal to 1 μm, preferably longer than or equal to 1.2 μm, further preferably longer than or equal to 1.4 μm, and shorter than or equal to 20 μm, preferably shorter than or equal to 15 μm, further preferably shorter than or equal to 10 μm. 
     In each transistor used for the scan line driver circuit, the signal line driver circuit, and the display region, a metal oxide is preferably used for a semiconductor in which a channel is formed. Thus, a signal line driver circuit that is hardly achieved in a display panel using amorphous silicon can be mounted on a display panel, for example. In addition, the display panel can be manufactured with a high yield at low cost because variation in characteristics is small and the area of the display panel can be easily increased as compared with the case of using polycrystalline silicon or the like. 
     For example, it is preferable to use a transistor  210   a  described later as the transistor in the display region and to use a transistor  210   b  described later as the transistor in the driver circuit (see  FIGS. 17 (A 1 ) and  17 (A 2 )). 
     Note that in this specification and the like, the channel length direction of a transistor refers to one of directions parallel to the shortest straight line that connects a source and a drain. In other words, the channel length direction corresponds to a direction of current flowing in a semiconductor layer when a transistor is in an on state. The channel width direction refers to a direction orthogonal to the channel length direction. Note that each of the channel length direction and the channel width direction is not fixed to one direction in some cases depending on the structure and the shape of a transistor. 
     In this specification and the like, the channel length of a transistor refers to, for example, the length in the channel length direction of a region where a semiconductor layer and a gate electrode overlap with each other in a top view or a cross-sectional view of the transistor. The channel width of a transistor refers to the length in the channel width direction of the region. 
     Note that each of the channel length and the channel width is not fixed to one value in some cases depending on the structure and the shape of a transistor. Thus, in this specification and the like, each of the channel length and the channel width can be the maximum value, the minimum value, the average value, or a given value between the maximum value and the minimum value. Typically, the minimum channel length and the minimum channel width are used. 
     A transistor may include a pair of gate electrodes (a first gate electrode and a second gate electrode) with a semiconductor layer positioned therebetween depending on its structure. Here, two values corresponding to the gate electrodes can be defined as each of the channel length and the channel width of the transistor. Thus, in this specification and the like, in the case where the term “channel length” is simply used, it represents one or both of a longer channel length and a shorter channel length of the two channel lengths or the average value thereof. Similarly, in this specification and the like, in the case where the term “channel width” is simply used, it represents one or both of a longer channel width and a shorter channel width of the two channel widths or the average value thereof. 
     Configuration Example of Display System 
       FIG. 15  is a block diagram of a display system  10 . 
     The display system  10  has a function of generating image data by using data received from the outside and a function of displaying a picture on the basis of the image data. 
     The display system  10  includes a display portion  20  and a signal generation portion  30 . The display portion  20  includes a plurality of display panels DP. The structure of the display panel described above in this embodiment can be used for the display panel DP. The signal generation portion  30  has a function of generating image data by using data received from the outside. The display panel DP has a function of displaying a picture on the basis of the image data. 
       FIG. 15  illustrates an example in which the display portion  20  includes a plurality of display panels DP arranged in a matrix of x rows and y columns (x and y are each an integer greater than or equal to 1). Note that displays of the display panels DP can be controlled independently of each other. 
     The signal generation portion  30  includes a front end portion  31 , a decoder  32 , a first processing portion  33 , a receiving portion  34 , an interface  35 , a control portion  36 , a second processing portion  40 , and a dividing portion  45 . 
     The front end portion  31  has a function of receiving a signal input from the outside and performing signal processing as appropriate. For example, a broadcast signal encoded and modulated by a predetermined method, or the like is input to the front end portion  31 . The front end portion  31  can have a function of demodulating a received video signal and an analog-digital conversion function, for example. Furthermore, the front end portion  31  may also have a function of correcting an error. Data received and subjected to signal processing in the front end portion  31  is output to the decoder  32 . 
     The decoder  32  has a function of decoding the encoded signal. In the case where image data contained in a broadcast signal input to the front end portion  31  has been compressed, the data is decompressed by the decoder  32 . For example, the decoder  32  can have a function of performing entropy decoding, inverse quantization, inverse orthogonal transform such as inverse discrete cosine transform (IDCT) or inverse discrete sine transform (IDST), intra-frame prediction, and inter-frame prediction. 
     Note that as a coding standard in an 8K broadcast, H.265/MPEG-H High Efficiency Video Coding (hereinafter referred to as HEVC) is employed. In the case where the image data included in the broadcast signal input to the front end portion  31  is encoded according to HEVC, decoding according to HEVC is performed by the decoder  32 . Image data generated by the decoding processing by the decoder  32  is output to the first processing portion  33 . 
     The first processing portion  33  has a function of performing image processing on the image data input from the decoder  32 , generating first image data SD 1 , and outputting it to the second processing portion  40 . 
     Examples of the image processing include noise removal processing, grayscale conversion processing, tone correction processing, and luminance correction processing. The tone correction processing or the luminance correction processing can be performed with the use of gamma correction or the like. Furthermore, the first processing portion  33  may have a function of executing pixel interpolation processing accompanying up-conversion of the resolution, frame interpolation processing accompanying up-conversion of the frame frequency, or the like. 
     Examples of the noise removal processing include removal of various noise such as mosquito noise that appears near the outlines of characters and the like, block noise that appears in high-speed moving images, random noise that causes flicker, and dot noise caused by up-conversion of the resolution. 
     The gray level conversion processing is processing in which the gray level of the first image data SD 1  is converted into gray level corresponding to output characteristics of the display portion  20 . For example, in the case where the number of gray levels is increased, gray levels corresponding to pixels are interpolated to an input image with a small number of gray levels and assigned to the pixels, so that processing for smoothing a histogram can be performed. In addition, high-dynamic range (HDR) processing for increasing a dynamic range is also included in the gray level conversion processing. 
     The tone correction processing is processing in which the tone of a picture is corrected. The luminance correction processing is processing in which the brightness (luminance contrast) of a picture is corrected. The luminance and tone of a picture displayed on the display portion  20  are corrected to be optimal, in accordance with the kind, luminance, or color purity of lighting of a space in which the display portion  20  is provided, for example. 
     The pixel interpolation processing is processing in which data that does not actually exist is interpolated when resolution is up-converted. For example, as newly interpolated data of the colors of a pixel (e.g., the gray level values corresponding to the colors, red (R), green (G), and blue (B)), data is interpolated to be data of the color intermediate between the colors of pixels around the pixel with reference to data of the colors of the pixels around the pixel. 
     In the case where the frame frequency of the displayed picture is increased, the frame interpolation processing is processing in which an image for a frame that does not actually exist (an interpolation frame) is generated. For example, an image for an interpolation frame that is interposed between certain two images is generated from a difference between the two images. Alternatively, images for a plurality of interpolation frames can be generated between the two images. For example, when the frame frequency of image data is 60 Hz, a plurality of interpolation frames are generated, and the frame frequency of a video signal output to the display portion  20  can be increased twofold to 120 Hz, fourfold to 240 Hz, or eightfold to 480 Hz, for example. 
     Note that it is also possible to perform the above image processing by a processing portion which is provided separately from the first processing portion  33 . One or more of the above image processing may be performed by the second processing portion  40 . 
     The receiving portion  34  has a function of receiving data or a control signal input from the outside. The input of the data or the control signal to the receiving portion  34  can be performed with an arithmetic processing device  50 , a remote controller, a portable information terminal (e.g., a smartphone or a tablet), an operation button or a touch panel provided on the display portion  20 , or the like. 
     Note that the arithmetic processing device  50  can supply a weight coefficient used in the second processing portion  40  or the like to the display system  10 . As the arithmetic processing device  50 , a calculator having high arithmetic processing properties, such as a computer, a server, or a cloud, can be used. The arithmetic processing device  50  can supply a weight coefficient obtained by learning to the second processing portion  40  through the receiving portion  34 . 
     The interface  35  has a function of performing, as appropriate, signal processing on the data or the control signal received by the receiving portion  34  and outputting it to the control portion  36 . 
     The control portion  36  has a function of supplying the control signals to the circuits included in the signal generation portion  30 . For example, the control portion  36  has a function of supplying the control signals to the decoder  32 , the first processing portion  33 , and the second processing portion  40 . The control by the control portion  36  can be performed on the basis of the control signal received by the receiving portion  34 , or the like. 
     The second processing portion  40  has a function of correcting the first image data SD 1  input from the first processing portion  33  and generating second image data SD 2 . The second image data SD 2  generated by the second processing portion  40  is output to the signal line driver circuit included in the display portion  20 . 
     For example, the second processing portion  40  has a function of correcting the first image data SD 1  to make display unevenness of the display portion  20  inconspicuous. For example, there might occur display unevenness because of a variation in transistor characteristics or capacitor size in the display panel DP, an effect of the parasitic resistance or parasitic capacitance of signal lines, an in-plane variation in drive capability of the signal line driver circuit, an in-plane variation in display element characteristics, and the like. Even in this case, the second image data SD 2  generated by the second processing portion  40  enables a picture with inconspicuous unevenness to be displayed. 
     Furthermore, the second processing portion  40  has a function of correcting the first image data SD 1  to compensate for the picture discontinuity at the boundary between the two display panels DP. The second image data SD 2  generated by the second processing portion  40  enables a picture with an inconspicuous seam to be displayed. 
     The dividing portion  45  has a function of dividing the second image data SD 2  input from the second processing portion  40 . The second image data SD 2  is divided into pieces of data that are the same in number as the display panels DP provided in the display portion  20 . In  FIG. 15 , the second image data SD 2  is divided into x×y pieces of data (second image data SD 2 [ 1 ,  1 ] to SD 2 [x, y]) and output to the display portion  20 . Second image data SD 2 [p, q] (p is an integer greater than or equal to 1 and less than or equal to x, and q is an integer greater than or equal to 1 and less than or equal toy) is image data corresponding to an image displayed on a display panel DP[p, q]. A control signal is supplied from the control portion  36  to the dividing portion  45 . 
     A video signal supplied from the signal generation portion  30  is input to the display panel DP. 
     In the display portion  20 , there is a region where the display panels DP are adjacent to each other, that is, a seam region between the display panels DP (a region S in the diagram), as illustrated in  FIG. 16(A) . When a picture is displayed using the plurality of display panels DP, picture continuity in the region S is preferably ensured. 
     However, there can be variations in the transistor characteristics or capacitor size in the pixels, the parasitic resistance or parasitic capacitance of the signal lines, the drive capability of the signal line driver circuit, and the like among the display panels DP. This can make an error in a picture displayed on each display panel DP when the video signals are supplied to the display panels DP, which might result in picture discontinuity in the seam region. Furthermore, in the case where the display region  71  of one display panel DP includes a region overlapping with the visible-light-transmitting region  72  of another display panel DP, in the seam region, the picture displayed in the display region  71  is viewed through the visible-light-transmitting region  72  and a gray level error can occur. Thus, if pieces of data (first image data SD 1 [ 1 ,  1 ] to SD 1 [x, y]) obtained by directly dividing the first image data SD 1  generated by the first processing portion  33  are supplied to the display panels DP, a picture that is discontinuous in the region S can be viewed as illustrated in  FIG. 16 (B−1). 
     Here, the display system of one embodiment of the present invention includes the second processing portion  40  having a function of correcting a video signal by utilizing artificial intelligence. Specifically, the second processing portion  40  can correct the video signal to relieve the picture discontinuity at a seam between two display panels DP. In this manner, in the case where the display portion  20  is formed using the plurality of display panels DP, the picture distortion at the seam between the display panels DP can be made inconspicuous, improving the quality of the picture. 
     The second processing portion  40  illustrated in  FIG. 15  has a function of correcting the video signal input from the first processing portion  33 . Specifically, the second processing portion  40  has a function of correcting the first image data SD 1  so that a picture which is continuous at a boundary between two display panels DP is displayed, that is, the picture discontinuity at the seam is compensated for. 
     The correction of the first image data SD 1  is performed by the second processing portion  40 . In the second processing portion  40 , learning is performed to appropriately correct a video signal so that picture discontinuity in a seam region is relieved. Then, when the first image data SD 1  is supplied to the second processing portion  40 , the second processing portion  40  performs inference and outputs the second image data SD 2 . Then, when the second image data SD 2  generated by the second processing portion  40  is divided into x×y pieces of data by the dividing portion  45  and the second image data SD 2 [p, q] is supplied to the display panel DP[p, q], a picture with an inconspicuous seam is displayed as illustrated in  FIG. 16 (B−2). 
     Specifically, processing for making the seam region brighter than other regions can be performed. As a result, a natural picture having a sense of unity, in which the seam is not easily recognized, can be displayed over the plurality of display panels DP. Furthermore, since display unevenness can be corrected at the same time, display quality of the display portion can be further improved. 
     Structure Example of Transistor 
     Next, transistors that can be used in the display panel or the display device will be described. 
     The structures of the transistors in the display panel or the display device are not particularly limited. For example, a planar transistor may be used, a staggered transistor may be used, or an inverted staggered transistor may be used. The transistor structure may be either a top-gate structure or a bottom-gate structure. Alternatively, gate electrodes may be provided above and below a channel. 
       FIG. 17  illustrates structure examples of transistors. Each transistor is provided between an insulating layer  141  and an insulating layer  208 . The insulating layer  141  preferably has a function of a base film. The insulating layer  208  preferably has a function of a planarization film. 
     The transistors  210   a  and  210   b  illustrated in  FIGS. 17 (A 1 ) and  17 (A 2 ) are each a top-gate transistor including a metal oxide in a semiconductor layer. The metal oxide can function as an oxide semiconductor. 
     An oxide semiconductor is preferably used as the semiconductor of the transistor. Using a semiconductor material having a wider band gap and a lower carrier density than silicon is preferable because the off-state current of a transistor can be reduced. 
     The transistors  210   a  and  210   b  each include a conductive layer  201 , an insulating layer  202 , a conductive layer  203   a , a conductive layer  203   b , a semiconductor layer, a conductive layer  205 , and an insulating layer  207 . The transistor  210   a  further includes an insulating layer  206   a , and the transistor  210   b  further includes an insulating layer  206   b . The conductive layer  201  functions as a gate. The conductive layer  205  functions as a back gate. The insulating layer  202 , the insulating layer  206   a , and the insulating layer  206   b  function as gate insulating layers. The semiconductor layer includes a channel formation region  204   a  and a pair of low-resistance regions  204   b . The channel formation region  204   a  overlaps with the conductive layer  205  with the insulating layer  206   a  or the insulating layer  206   b  therebetween. The channel formation region  204   a  overlaps with the conductive layer  201  with the insulating layer  202  therebetween. The conductive layer  203   a  is electrically connected to one of the pair of low-resistance regions  204   b  through an opening provided in the insulating layer  207 . In a similar manner, the conductive layer  203   b  is electrically connected to the other of the pair of low-resistance regions  204   b . Any of a variety of inorganic insulating films can be used as the insulating layer  202 , the insulating layer  206   a , the insulating layer  206   b , and the insulating layer  207 . In particular, an oxide insulating film is suitable for the insulating films which are included in the insulating layer  202 , the insulating layer  206   a , and the insulating layer  206   b  and are in contact with the channel formation region  204   a , and a nitride insulating film is suitable for the insulating layer  207 . 
     The thickness of the insulating layer  206   b  functioning as a gate insulating layer of the transistor  210   b  is smaller than the thickness of the insulating layer  206   a  functioning as a gate insulating layer of the transistor  210   a . A channel length Lb of the transistor  210   b  is shorter than a channel length La of the transistor  210   a . Therefore, the driving frequency of the transistor  210   b  can be higher than that of the transistor  210   a , and the withstand voltage of the transistor  210   a  can be higher than that of the transistor  210   b . Accordingly, it is preferable that, in the display panel, the transistor  210   a  be used as a transistor in the display region and the transistor  210   b  be used as a transistor in the driver circuit. 
     Note that the display panel may be manufactured using only one of the transistors  210   a  and  210   b . Alternatively, the display panel may be manufactured by combining one of the transistors  210   a  and  210   b  and another transistor. 
     The structure in which the semiconductor layer where a channel is formed is sandwiched between the two gates is used for the transistors  210   a  and  210   b . The two gates are preferably connected to each other and supplied with the same signal so that the transistor operates. Such a transistor can have a higher field-effect mobility and thus have a higher on-state current than other transistors. Consequently, a circuit capable of high-speed operation can be manufactured. Furthermore, the area occupied by a circuit portion can be reduced. The use of the transistor having a high on-state current can reduce signal delay in wirings and can reduce display unevenness even in a display panel or a display device in which the number of wirings is increased because of increase in size or definition. Alternatively, by supplying a potential for controlling the threshold voltage to one of the two gates and a potential for driving to the other, the threshold voltage of the transistor can be controlled. 
     A metal oxide film that functions as a semiconductor layer can be deposited using one or both of an inert gas and an oxygen gas. Note that there is no particular limitation on the flow rate ratio of oxygen (the partial pressure of oxygen) at the time of depositing the metal oxide film. However, to obtain a transistor having a high field-effect mobility, the flow rate ratio of oxygen (the partial pressure of oxygen) at the time of depositing the metal oxide film is preferably higher than or equal to 0% and lower than or equal to 30%, further preferably higher than or equal to 5% and lower than or equal to 30%, still further preferably higher than or equal to 7% and lower than or equal to 15%. 
     The metal oxide preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. The metal oxide will be described in detail in Embodiment 3. 
     The energy gap of the metal oxide is preferably 2 eV or more, further preferably 2.5 eV or more, still further preferably 3 eV or more. With the use of a metal oxide having such a wide energy gap, the off-state current of the transistor can be reduced. 
     The metal oxide film can be formed by a sputtering method. Alternatively, a PLD method, a PECVD method, a thermal CVD method, an ALD method, a vacuum evaporation method, or the like may be used. 
     A transistor  220  illustrated in  FIG. 17(B)  is a bottom-gate transistor including a metal oxide in a semiconductor layer  204 . 
     The transistor  220  includes the conductive layer  201 , the insulating layer  202 , the conductive layer  203   a , the conductive layer  203   b , and the semiconductor layer  204 . The conductive layer  201  functions as a gate. The insulating layer  202  functions as a gate insulating layer. The semiconductor layer  204  overlaps with the conductive layer  201  with the insulating layer  202  therebetween. The conductive layer  203   a  and the conductive layer  203   b  are each electrically connected to the semiconductor layer  204 . The transistor  220  is preferably covered with an insulating layer  211  and an insulating layer  212 . Any of a variety of inorganic insulating films can be used as the insulating layer  211  and the insulating layer  212 . In particular, an oxide insulating film is suitable for the insulating layer  211 , and a nitride insulating film is suitable for the insulating layer  212 . 
     A transistor  230  illustrated in  FIG. 17(C)  is a top-gate transistor including low-temperature polysilicon (LTPS) in a semiconductor layer. 
     The transistor  230  includes the conductive layer  201 , the insulating layer  202 , the conductive layer  203   a , the conductive layer  203   b , a semiconductor layer, and an insulating layer  213 . The conductive layer  201  functions as a gate. The insulating layer  202  functions as a gate insulating layer. The semiconductor layer includes a channel formation region  214   a  and a pair of low-resistance regions  214   b . The semiconductor layer may further include an LDD (Lightly Doped Drain) region.  FIG. 17(C)  shows an example in which an LDD region  214   c  is provided between the channel formation region  214   a  and the low-resistance region  214   b . The channel formation region  214   a  overlaps with the conductive layer  201  with the insulating layer  202  therebetween. The conductive layer  203   a  is electrically connected to one of the pair of low-resistance regions  214   b  through an opening provided in the insulating layer  202  and the insulating layer  213 . In a similar manner, the conductive layer  203   b  is electrically connected to the other of the pair of low-resistance regions  214   b . Any of a variety of inorganic insulating films can be used as the insulating layer  213 . In particular, a nitride insulating film is suitable for the insulating layer  213 . 
     A transistor  240  illustrated in  FIG. 17(D)  is a bottom-gate transistor including hydrogenated amorphous silicon in a semiconductor layer  224 . 
     The transistor  240  includes the conductive layer  201 , the insulating layer  202 , the conductive layer  203   a , the conductive layer  203   b , an impurity semiconductor layer  225 , and the semiconductor layer  224 . The conductive layer  201  functions as a gate. The insulating layer  202  functions as a gate insulating layer. The semiconductor layer  224  overlaps with the conductive layer  201  with the insulating layer  202  therebetween. The conductive layer  203   a  and the conductive layer  203   b  are electrically connected to the semiconductor layer  224  through the impurity semiconductor layer  225 . The transistor  240  is preferably covered with an insulating layer  226 . Any of a variety of inorganic insulating films can be used as the insulating layer  226 . In particular, a nitride insulating film is suitable for the insulating layer  226 . 
     As described above, the display panel of this embodiment has a structure in which formation of a conductive film with low visible-light-transmitting property in a visible-light-transmitting region, which might be caused by bending of a metal mask, is not easily caused at the time of forming a common electrode. Therefore, with the use of the display panels of this embodiment, a display device with an inconspicuous seam between two display panels and high display quality can be provided. 
     This embodiment can be combined with the other embodiments as appropriate. In this specification, in the case where a plurality of structure examples are shown in one embodiment, the structure examples can be combined as appropriate. 
     Embodiment 2 
     Described in this embodiment is a metal oxide applicable to a transistor disclosed in one embodiment of the present invention. In particular, details about a metal oxide and a CAC (Cloud-Aligned Composite)-OS are described below. 
     A CAC-OS or a CAC-metal oxide has a conducting function in a part of the material and has an insulating function in a part of the material; as a whole, the CAC-OS or the CAC-metal oxide has a function of a semiconductor. Note that in the case where the CAC-OS or the CAC-metal oxide is used in a channel formation region of a transistor, the conducting function is to allow electrons (or holes) serving as carriers to flow, and the insulating function is to not allow electrons serving as carriers to flow. By the complementary action of the conducting function and the insulating function, the CAC-OS or the CAC-metal oxide can have a switching function (On/Off function). In the CAC-OS or the CAC-metal oxide, separation of the functions can maximize each function. 
     Furthermore, the CAC-OS or the CAC-metal oxide includes conductive regions and insulating regions. The conductive regions have the above-described conducting function, and the insulating regions have the above-described insulating function. Furthermore, in some cases, the conductive regions and the insulating regions in the material are separated at the nanoparticle level. Furthermore, in some cases, the conductive regions and the insulating regions are unevenly distributed in the material. Furthermore, the conductive regions are observed to be coupled in a cloud-like manner with their boundaries blurred, in some cases. 
     Furthermore, in the CAC-OS or the CAC-metal oxide, the conductive regions and the insulating regions each have a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 0.5 nm and less than or equal to 3 nm, and are dispersed in the material, in some cases. 
     Furthermore, the CAC-OS or the CAC-metal oxide includes components having different bandgaps. For example, the CAC-OS or the CAC-metal oxide includes a component having a wide gap due to the insulating region and a component having a narrow gap due to the conductive region. When carriers flow in this composition, carriers mainly flow in the component having a narrow gap. Furthermore, the component having a narrow gap complements the component having a wide gap, and carriers also flow in the component having a wide gap in conjunction with the component having a narrow gap. Therefore, in the case where the above-described CAC-OS or CAC-metal oxide is used in a channel formation region of a transistor, the transistor in an on state can achieve high current driving capability, that is, a high on-state current and a high field-effect mobility. 
     In other words, the CAC-OS or the CAC-metal oxide can also be called a matrix composite or a metal matrix composite. 
     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 a metal oxide preferably contains at least indium. It is particularly preferable that a metal oxide contain indium and zinc. Moreover, in addition to these, one kind or a plurality of 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 instance, a CAC-OS in an In—Ga—Zn oxide (an In—Ga—Zn oxide in the CAC-OS may be particularly referred to as CAC-IGZO) has a 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)), for example, so that a mosaic pattern is formed, and mosaic-like InO X1  or In X2 Zn Y2 O Z2  is evenly distributed in the film (which is hereinafter also referred to as cloud-like). 
     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 Min a first region is larger than the atomic ratio of In to the element Min a second region, the first region is regarded as having a higher In concentration than the second region. 
     Note that IGZO is a commonly known name and sometimes refers to one compound formed of In, Ga, Zn, and O. A typical example is a crystalline compound represented by InGaO 3 (ZnO) m1  (m1 is a natural number) or In (i+x0) Ga (1−x0) O 3 (ZnO) m0  (−1≤x0≤1; m0 is a given number). 
     The above crystalline compound has a single crystal structure, a polycrystalline structure, or a CAAC (c-axis aligned crystal) 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. 
     On the other hand, 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 include Ga as a main component and are observed as nanoparticles and some regions that include 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 the CAC-OS is regarded as not including a stacked-layer structure of two or more kinds of films with different compositions. For example, a two-layer structure of a film including In as a main component and a film including Ga as a main component is not included. 
     Note that a clear boundary cannot sometimes be observed between the region including GaO X3  as a main component and the region including In X2 Zn Y2 O Z2  or InO X1  as a main component. 
     Note that in the case where one kind or a plurality of 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, the CAC-OS refers to a composition in which some regions that include the metal element(s) as a main component and are observed as nanoparticles and some regions that include In as a main component and are observed as nanoparticles are randomly dispersed in a mosaic pattern. 
     The CAC-OS can be formed by a sputtering method under a condition where a substrate is not heated intentionally, 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. Furthermore, the ratio of the flow rate of an oxygen gas to the total flow rate of the deposition gas at the time of deposition is preferably as low as possible, and for example, the flow rate ratio of the oxygen gas is preferably higher than or equal to 0% and lower than 30%, further preferably higher than or equal to 0% and lower than or equal to 10%. 
     The CAC-OS is characterized in that no clear peak is observed in measurement using θ/2θ scan by an Out-of-plane method, which is one of X-ray diffraction (XRD) measurement methods. That is, it is found from the X-ray diffraction that no alignment in the a-b plane direction and the c-axis direction is observed in a measured region. 
     In addition, 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 nanobeam electron beam), a ring-like high-luminance region and a plurality of bright spots in the ring region are observed. It is therefore found from the electron diffraction pattern 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 confirmed 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 the metal elements are evenly distributed, and has characteristics different from those of the IGZO compound. That is, the CAC-OS has a composition in which 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 and form a mosaic pattern. 
     Here, a region including In X2 Zn Y2 O Z2  or InO X1  as a main component is a region whose conductivity 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 an oxide semiconductor is exhibited. Accordingly, when the regions including In X2 Zn Y2 O Z2  or InO X1  as a main component are distributed in an oxide semiconductor like a cloud, a high field-effect mobility (μ) can be achieved. 
     In contrast, a region including GaO X3  or the like as a main component is a region whose insulating property 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 an oxide semiconductor, leakage current can be suppressed and favorable switching operation can be achieved. 
     Accordingly, when the 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 a high on-state current (Ion) and a high field-effect mobility (μ) can be achieved. 
     Moreover, a semiconductor element using the CAC-OS has high reliability. Thus, the CAC-OS is the most suitable for a variety of semiconductor devices such as displays. 
     This embodiment can be combined with the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, electronic devices of embodiments of the present invention will be described with reference to  FIG. 18 . 
     An electronic device in this embodiment is provided with the display device of one embodiment of the present invention. Thus, a display portion of the electronic device can display a high-quality picture. 
     The display portion of the electronic device in this embodiment can display a picture with a resolution of, for example, full high definition, 2K, 4K, 8K, 16K, or higher. In addition, as a screen size of the display portion, the diagonal size can be greater than or equal to 20 inches, greater than or equal to 30 inches, greater than or equal to 50 inches, greater than or equal to 60 inches, or greater than or equal to 70 inches. 
     Examples of the electronic device include a digital camera, a digital video camera, a digital photo frame, a mobile phone, a portable game console, a portable information terminal, and an audio reproducing device in addition to electronic devices provided with a comparatively large screen, such as a television device, a desktop or laptop personal computer, a monitor for a computer and the like, digital signage, and a large game machine such as a pachinko machine. 
     The electronic device of one embodiment of the present invention may include an antenna. When a signal is received by the antenna, the electronic device can display a picture, data, or the like on the display portion. When the electronic device includes the antenna and a secondary battery, the antenna may be used for contactless power transmission. 
     The electronic device of one embodiment of the present invention may include a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, a chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radioactive rays, flow rate, humidity, gradient, oscillation, a smell, or infrared rays). 
     The electronic device of one embodiment of the present invention can have a variety of functions. For example, the electronic device can have a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of executing a variety of software (programs), a wireless communication function, and a function of reading out a program or data stored in a recording medium. 
       FIG. 18(A)  illustrates an example of a television device. In a television device  7100 , a display portion  7000  is incorporated in a housing  7101 . Here, a structure in which the housing  7101  is supported by a stand  7103  is illustrated. 
     The display device of one embodiment of the present invention can be used for the display portion  7000 . 
     Operation of the television device  7100  illustrated in  FIG. 18(A)  can be performed with an operation switch provided in the housing  7101  or a separate remote controller  7111 . Alternatively, the display portion  7000  may include a touch sensor, and the television device  7100  may be operated by touch on the display portion  7000  with a finger or the like. The remote controller  7111  may be provided with a display portion for displaying data output from the remote controller  7111 . With operation keys or a touch panel provided in the remote controller  7111 , channels and volume can be operated and pictures displayed on the display portion  7000  can be operated. 
     Note that the television device  7100  has a structure in which a receiver, a modem, and the like are provided. A general television broadcast can be received with the receiver. When the television device is connected to a communication network with or without wires via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers, for example) data communication can be performed. 
       FIG. 18(B)  illustrates an example of a laptop personal computer. A laptop personal computer  7200  includes a housing  7211 , a keyboard  7212 , a pointing device  7213 , an external connection port  7214 , and the like. In the housing  7211 , the display portion  7000  is incorporated. 
     The display device of one embodiment of the present invention can be used for the display portion  7000 . 
       FIGS. 18(C) and 18(D)  illustrate examples of digital signage. 
     Digital signage  7300  illustrated in  FIG. 18(C)  includes a housing  7301 , the display portion  7000 , a speaker  7303 , and the like. Furthermore, the digital signage can include an LED lamp, operation keys (including a power switch or an operation switch), a connection terminal, a variety of sensors, a microphone, and the like. 
       FIG. 18(D)  is digital signage  7400  attached to a cylindrical pillar  7401 . The digital signage  7400  includes the display portion  7000  provided along a curved surface of the pillar  7401 . 
     The display device of one embodiment of the present invention can be used for the display portion  7000  in  FIGS. 18(C) and 18(D) . 
     A larger area of the display portion  7000  can provide more data at a time. The larger display portion  7000  attracts more attention, so that the effectiveness of the advertisement can be increased, for example. 
     The use of a touch panel in the display portion  7000  is preferable because in addition to display of a still image or a moving image on the display portion  7000 , intuitive operation by a user is possible. Moreover, for an application for providing information such as route information or traffic information, usability can be enhanced by intuitive operation. 
     Furthermore, as illustrated in  FIGS. 18(C) and 18(D) , it is preferable that the digital signage  7300  or the digital signage  7400  work with an information terminal  7311  or an information terminal  7411  such as a user&#39;s smartphone through wireless communication. For example, information of an advertisement displayed on the display portion  7000  can be displayed on a screen of the information terminal  7311  or the information terminal  7411 . By operation of the information terminal  7311  or the information terminal  7411 , display on the display portion  7000  can be switched. 
     It is possible to make the digital signage  7300  or the digital signage  7400  execute a game with the use of the screen of the information terminal  7311  or the information terminal  7411  as an operation means (controller). Thus, an unspecified number of users can join in and enjoy the game concurrently. 
     This embodiment can be combined with the other embodiments as appropriate. 
     REFERENCE NUMERALS 
     DP 1 : display panel, DP 2 : display panel,  10 : display system,  20 : display portion,  30 : signal generation portion,  31 : front end portion,  32 : decoder,  33 : first processing portion,  34 : receiving portion,  35 : interface,  36 : control portion,  40 : second processing portion,  45 : dividing portion,  50 : arithmetic processing device,  71 : display region,  71   a : display region,  71   b : display region,  72 : visible-light-transmitting region,  72   a : visible-light-transmitting region,  73 : visible-light-blocking region,  74 : FPC,  78 : driver circuit,  78   b : driver circuit,  101 : substrate,  103 : insulating layer,  104 : insulating layer,  105 : protective layer,  110 : light-emitting element,  110   a : light-emitting element,  110   b : light-emitting element,  110   b   1 : portion,  110   b   2 : portion,  111 : pixel electrode,  112 : EL layer,  113   a : common electrode,  113   b : common electrode,  114 : optical adjustment layer,  118 : partition wall,  123 : region,  124 : region,  131 A: coloring layer,  131 B: coloring layer,  141 : insulating layer,  201 : conductive layer,  201   a : conductive layer,  202 : insulating layer,  203   a : conductive layer,  203   b : conductive layer,  203   s : conductive layer,  203   t : conductive layer,  204 : semiconductor layer,  204   a : channel formation region,  204   b : low-resistance region,  205 : conductive layer,  206   a : insulating layer,  206   b : insulating layer,  207 : insulating layer,  208 : insulating layer,  210   a : transistor,  210   b : transistor,  211 : insulating layer,  212 : insulating layer,  213 : insulating layer,  214   a : channel formation region,  214   b : low-resistance region,  214   c : LDD region,  220 : transistor,  224 : semiconductor layer,  225 : impurity semiconductor layer,  226 : insulating layer,  230 : transistor,  240 : transistor,  257 : wiring,  301 : transistor,  303 : transistor,  306 : connection portion,  307 : conductive layer,  308 : opening,  311 : gate insulating layer,  312 : insulating layer,  313 : insulating layer,  314 : insulating layer,  315 : insulating layer,  317 : bonding layer,  318 : light-transmitting layer,  319 : connector,  355 : conductive layer,  358 : conductive layer,  361 : substrate,  363 : bonding layer,  365 : insulating layer,  367 : insulating layer,  370 A: display panel,  370 B: display panel,  370 C: display panel,  370 D: display panel,  370 E: display panel,  370 F: display panel,  370 G: display panel,  371 : substrate,  387 : region,  7000 : display portion,  7100 : television device,  7101 : housing,  7103 : stand,  7111 : remote controller,  7200 : laptop personal computer,  7211 : housing,  7212 : keyboard,  7213 : pointing device,  7214 : external connection port,  7300 : digital signage,  7301 : housing,  7303 : speaker,  7311 : information terminal,  7400 : digital signage,  7401 : pillar,  7411 : information terminal.