Patent Publication Number: US-2007122938-A1

Title: Organic electroluminescent display device

Description:
TECHNICAL FIELD  
      The resent invention relates to an organic electroluminescence display that uses an organic electroluminescence display device.  
     BACKGROUND ART  
      In recent years, organic electroluminescence displays that utilize organic electroluminescence emission (hereinafter, “electroluminescence” is referred to as “EL”) are being developed. Display devices which are used in conventional organic EL displays are constituted so that a light-emitting layer which organically emits light is sandwiched by an anode electrode and a cathode electrode (for example, see patent document  1 ).  
       FIG. 11  illustrates a schematic constitutional diagram of a display device to be used in conventional organic EL displays. The conventional display device  100  has a substrate  101  made of a transparent insulating material such as glass, a transparent electrode as an anode electrode  102  provided onto the substrate  101 , a light-emitting layer  104  for organic light emission, a hole transport layer  103  as a buffer layer which improves joint property of the anode electrode  102  and the light-emitting layer  104 , a cathode electrode  106  made of a material such as aluminum with small work function, an electron transport layer  105  as a buffer layer which improves joint property of the cathode electrode  106  and the light-emitting layer  104 , and the like.  
      Light emitted in the light-emitting layer  104  is emitted outside from a side of the substrate  101  and the cathode electrode  106  whose thickness is very thin, or is reflected by the cathode electrode  106  so as to be emitted from the side of the substrate  101 . At this time, in the display device  100 , since the light emitted in the light-emitting layer  104  transmits through the cathode electrode  106  or the transparent electrode and the substrate  101 , the quantity of the transmitted light becomes smaller than the quantity of the light emitted in the light-emitting layer  104  due to reflection or absorption by the cathode electrode  106  and the substrate  101 . Further, when the ITO is used as the transparent electrode which is the anode electrode  102 , emitting light becomes occasionally reddish because ITO has wavelength selectivity such that transmission is easy to be performed on a long wavelength side of visible light.  
      Therefore, a display device which solves the above problem is developed.  FIG. 12  illustrates a schematic constitutional diagram of the display device which solves the above problem. In the display device  200  shown in  FIG. 12 , an anode electrode  202  and a cathode electrode  206  are arranged in parallel on a substrate  201 , and a light-emitting layer  204  is formed thereon so as to cover the anode electrode  202  and the cathode electrode  206 . Further, a separator  203  is provided in a substrate laminating direction in order to separate to insulate the anode electrode  202  from the cathode electrode  206 . In the display device  200 , since light emitted in the light-emitting layer  204  is not transmitted through a member such as the substrate  201  and can be emitted directly from a side opposite to the substrate  201 , a change in light spectrum such that the light quantity is reduced or emitted light is reddish is not caused.  
      Since, however, the anode electrode  202  and the cathode electrode  206  are arranged in parallel, a moving distance of electrons and holes which move in the light-emitting layer  204  becomes long. When the moving distance of the electrons and the holes becomes long, a driving voltage of the display device  200  should be increased, and thus it is difficult to efficiently emit light in the light-emitting layer  204  with respect to the driving voltage of the display device  200 . Patent document  1 : Japanese Patent Application Laid-Open No. 2001-43980  
     DISCLOSURE OF THE INVENTION  
      It is, therefore, an object of the present invention to provide an organic EL display which has a display device having a constitution such that decrease in the quantity of light emitted in an organic light-emitting element can be suppressed, operates with low driving voltage, and can emit light efficiently.  
      In order to solve the above problem, in an organic EL display of the present invention, an anode electrode and a cathode electrode are arranged on a substrate so as to be adjacent to each other, and an organic light-emitting element is formed so as to cover the anode electrode and the cathode electrode. Further, in order to reduce a driving voltage of the organic EL display, a carbon nanotube is mixed partially in the organic light-emitting element.  
      The carbon nanotube is a substance which is made of carbon where a graphite sheet is formed into a tube shape with diameter of several nm, and it is known that its electrical conductivity is larger than that of metal such as iron and copper. When the carbon nanotube is mixed partially in the organic light-emitting element, mobility of electrons and holes moving in the organic light-emitting element is improved, and resistance of the organic light-emitting element per unit volume can be expected to be reduced.  
      For this reason, a voltage is concentrated on the light-emitting unit as a light emitting area of the organic light-emitting element, and an improvement of luminous efficacy can be expected while the driving voltage is being reduced.  
      Concretely, in the organic EL display of the present invention having a plurality of display devices provided on the substrate, the display devices includes: a first electrode element arranged on the substrate; a second electrode element arranged adjacently to the first electrode element; an organic light-emitting element that emits light by an electric field supplied by the first electrode element and the second electrode element and is formed on the substrate so as to cover both the first electrode element and the second electrode element; and a separator in a substrate laminating direction that is arranged between the first electrode element and the second electrode element and separates to insulate at least the first electrode element from the second electrode element. A carbon nanotube is mixed in the organic light-emitting element.  
      According to the above invention, since the organic EL display has a constitution where the light emitted in the organic light-emitting element can be emitted directly from the organic light-emitting element, decrease in the quantity of the emitted light can be suppressed, and a change in spectrum of the emitted light such that the emitted light becomes reddish is not caused. Further, the driving voltage of the organic EL display can be reduced by the carbon nanotube mixed in the organic light-emitting element.  
      In another organic EL display of the present invention, the carbon nanotube is mixed between a surface crossing the organic light-emitting element in the organic light-emitting element on the side closer to the first electrode than the separator and the first electrode element.  
      When the carbon nanotube is mixed in the above position, mobility of the holes in the carbon nanotube mixed portion is heightened, and electric resistance of the organic light-emitting element between the first electrode element and the second electrode element can be small.  
      Further, in another organic EL display of the present invention, the carbon nanotube is mixed between a surface crossing the organic light-emitting element in the organic light-emitting element on the side closer to the second electrode element than the separator and the second electrode element.  
      When the carbon nanotube is mixed in the above position, the mobility of the electrons in the carbon nanotube mixed portion is heightened. As a result, the carbon nanotube mixed portion becomes an electron source, so that an applied voltage can be concentrated on the light-emitting unit in the organic light-emitting element, and the driving voltage of the organic EL display can be small.  
      Further, in another organic EL display of the present invention, the carbon nanotube is mixed between a surface crossing the organic light-emitting element in the organic light-emitting element on the side closer to the first electrode than the separator and the first electrode element, and between a surface crossing the organic light-emitting element in the organic light-emitting element on the side closer to the second electrode element than the separator and the second electrode element.  
      When the carbon nanotube is mixed in the above position, the mobility of the holes in the carbon nanotube mixed portion is heightened, so that the electric resistance of the organic light-emitting element between the first electrode element and the second electrode element can be small. When the mobility of the electrons in the carbon nanotube mixed portion is heightened, the carbon nanotube mixed portion becomes an electron source, so that the applied voltage can be concentrated on the light-emitting unit in the organic light-emitting element. As a result, the driving voltage of the organic EL display can be small.  
      Further, in another organic EL display of the present invention, the carbon nanotube is mixed between a surface crossing the organic light-emitting element in the organic light-emitting element on the side closer to the first electrode element than the separator and the second electrode element.  
      When the carbon nanotube is mixed in the above position, the mobility of the electrons in the carbon nanotube mixed portion is heightened, so that the carbon nanotube mixed portion becomes an electron source. The applied voltage can be concentrated on the light-emitting unit in the organic light-emitting element, thereby reducing the driving voltage of the organic EL display.  
      Further, in another organic EL display of the present invention, the carbon nanotube is mixed between a surface crossing the organic light-emitting element in the organic light-emitting element on the side closer to the second electrode element than the separator and the first electrode element.  
      When the carbon nanotube is mixed in the above position, the mobility of the holes in the carbon nanotube mixed portion is heightened, so that the electric resistance of the organic light-emitting element can be small between the first electrode element and the second electrode element.  
      In the organic EL display of the present invention, it is desirable that at least one of the first electrode element and the second electrode element is formed transparently.  
      When the transparent material is used for the substrate, the light emitted in the organic light-emitting element can be transmitted through the electrode element formed transparently to be emitted from the side of the substrate. As a result, both the surfaces of the organic EL display can be display surfaces.  
      Further, in the organic EL display, it is desirable that both the first electrode element and the second electrode element are made of a material having resistivity of smaller than  10   −4  Ω·cm.  
      When both the first electrode element and the second electrode element are made of the above material, the joint property of the organic light-emitting element and the first electrode element and the second electrode element is improved, so that the light can be emitted in the organic light-emitting element efficiently with respect to the driving voltage of the organic EL display.  
      Further, in the organic EL display, it is desirable that a plurality of the band-shaped first electrode elements are arranged in a line form, and a plurality of the band-shaped second electrode elements are arranged in a line form via an insulating layer so as to cross the first electrode elements.  
      The organic EL display where the first electrode element and the second electrode element are arranged in a matrix pattern so as to enable image display is proposed.  
      Further, in the organic EL display, it is desirable that a plurality of band-shaped grooves are provided in a line form on the first electrode elements so as to cross the first electrode elements, the second electrode elements are arranged in the grooves, respectively, via the insulating layer, and a height of a boundary between the first electrode elements and the organic light-emitting element from the substrate is approximately equal to a height of a boundary between the second electrode elements and the organic light-emitting element from the substrate.  
      When the surfaces of the first electrode element and the second electrode element are aligned, the moving distance of the electrons moving in the organic light-emitting element becomes shorter than the moving distance of the electrons moving in the organic light-emitting element formed into a thickness which is the same as that of the second electrode element. As a result, the light can be emitted efficiently in the organic light-emitting element with respect to the driving voltage of the organic EL display.  
      Further, in the organic EL display, it is desirable that the first electrode element has an anode side function element which is adjacent to the organic light-emitting element and has at least one of a hole transport function and a hole injecting function, and/or the second electrode element has a cathode side function element which is adjacent to the organic light-emitting element and has at least one of an electron transport function and an electron injecting function, and the first electrode element serves as an anode and the second electrode element serves as a cathode.  
      When a buffer layer intervenes between the first electrode element and the organic light-emitting element and between the second electrode element and the organic light-emitting element, the joint property of the first electrode element and the organic light-emitting element, and the joint property of the second electrode element and the organic light-emitting element are improved. As a result, the light can be emitted efficiently in the organic light-emitting element with respect to the organic EL display.  
      In the organic EL display of the present invention which has the display devices having the constitution such that decrease in the quantity of the light emitted in the organic light-emitting element can be suppressed, the driving voltage of the organic EL display can be reduced. Further, light can be emitted efficiently in the organic light-emitting element with respect to the driving voltage of the organic EL display. 
    
    
     BRIREF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic constitutional diagram illustrating one example of a display device which is a constitutional unit of an organic EL display according to one embodiment of the present invention;  
       FIG. 2  is an enlarged schematic diagram illustrating one example of a form of a carbon nanotube mixed portion of an organic light-emitting layer;  
       FIG. 3  is an enlarged schematic diagram illustrating one example of the form of the carbon nanotube mixed portion of the organic light-emitting layer;  
       FIG. 4  is a schematic constitutional diagram illustrating one example of the organic EL display according to the embodiment of the present invention;  
       FIG. 5  is a schematic constitutional diagram illustrating one example of a constitution of the display device;  
       FIG. 6  is a schematic constitutional diagram illustrating one example of the constitution of the display device;  
       FIG. 7  is a schematic constitutional diagram illustrating one example of the constitution of the display device;  
       FIG. 8  is a schematic constitutional diagram illustrating one example of the constitution of the display device;  
       FIG. 9  is a schematic constitutional diagram illustrating one example of the constitution of the display device;  
       FIG. 10  is a schematic constitutional diagram illustrating one example of the constitution of the display device;  
       FIG. 11  is a schematic constitutional diagram of a display device to be used in a conventional EL display; and  
       FIG. 12  is a schematic constitutional diagram of a display device to be used in a conventional EL display. 
    
    
     DESCRIPTION OF REFERENCE NUMERALS  
     
         
           10 : organic EL display  
           20 : display device  
           30 : substrate  
           31 : anode electrode  
           32 : cathode electrode  
           33 : insulating layer  
           34 : hole transport layer  
           35 : hole injecting layer  
           36 : electron injecting layer  
           37 : electrode transport layer  
           38 : organic light-emitting layer  
           41 : insulating member  
           42 : separator  
           43 : groove  51 ,  52 ,  53 ,  54 ,  55 : organic light-emitting layer including carbon nanotube  
           60 : light-emitting unit  
           100 : display device  
           101 : substrate  
           102 : anode electrode  
           103 : hole transport layer  
           104 : light-emitting layer  
           105 : electron transport layer  
           106 : cathode electrode  
           200 : display device  
           201 : substrate  
           202 : anode electrode  
           203 : separator  
           204 : light-emitting layer  
           206 : cathode electrode  
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Embodiments of the present invention are explained in detail below with reference to the drawings, but the present invention is not limited to the description.  
     FIRST EMBODIMENT  
       FIG. 1  illustrates a schematic constitutional diagram of a display device which is a constitutional unit of an organic EL display of the present invention. The display device  20  shown in  FIG. 1  has an anode electrode  31  as a first electrode element arranged on a substrate  30 , a cathode electrode  32  as a second electrode element arranged adjacently to the anode electrode  31 , an organic light-emitting layer  38  as an organic light-emitting element which emits light due to an electric field supplied by the anode electrode  31  and the cathode electrode  32  and is formed on the substrate  30  so as to cover both the anode electrode  31  and the cathode electrode  32  and in which a carbon nanotube is mixed partially, and a separator  42  which is arranged between the anode electrode  31  and the cathode electrode  32  in a substrate laminating direction and at least separates to insulate the first electrode element side from the second electrode element side. Furthermore, the first electrode element may have a hole injecting layer  35  and a hole transport layer  34  as anode side functional elements that improve joint property with respect to the organic light-emitting layer  38 , and the second electrode element may have an electron injecting layer  36  and an electron transport layer  37  as cathode side functional elements which improve joint property with respect to the organic light-emitting layer  38 . Further, an insulating member  41  which separates to insulate the display devices  20  may be provided.  
      In the display device  20  shown in  FIG. 1 , in order to solve conventional problems such as decrease in emitted light from the display device emitted of the light emitted in the organic light-emitting layer or a change in spectrum or the like, the substrate  30 , the anode electrode  31 , the cathode electrode  32  and the organic light-emitting layer  38  are laminated in this order. Due to such a constitution of the display device  20 , in the case where the laminating direction is a first direction and the opposite direction is a second direction, light generate in the organic light-emitting layer  38  is not transmitted through the substrate  30  and the electrodes, and the light can be led directly to the first direction from the organic light-emitting layer  38 . Since, therefore, reflection loss due to a difference in refractive index and transmission loss due to absorption can be reduced more than the case where the light is transmitted through the electrodes and the substrate so as to be emitted, improvement in emission efficiency of the light to be finally emitted to the outside can be expected. Further, since the light can be emitted without being transmitted through the electrodes or the like, the spectrum of the emitted light is not changed.  
      Since the anode electrode  31  and the cathode electrode  32  are arranged adjacently, however, a moving distance of electrons and holes moving in the organic light-emitting layer  38  becomes long. When the moving distance of the electrons and the holes becomes long, a driving voltage of the display device  20  should be increased, and thus it is difficult to efficiently emit light in the organic light-emitting layer  38  with respect to the driving voltage of the display device  20 . In this embodiment, therefore, a carbon nanotube is mixed partially in the organic light-emitting layer  38 . This will be described later.  
      The substrate  30  has a first surface on which the display device  20  is formed, and a second surface which can be one of emission surfaces when the anode electrode  31  and the substrate  30  are made of a transparent material. Hereinafter, a direction of the light to be transmitted through the second surface of the substrate  30  and emitted is a second direction, and a direction of the light to be emitted to the opposite direction is a first direction. It is desirable that an insulating material is used as the substrate  30 . Its examples are transparent glass, plastic and plastic film. When the substrate  30  is transparent, the light emitted in the organic light-emitting layer  38  can be led to the second direction. Further, since the light emitted in the organic light-emitting layer  38  can be led to the first direction, an opaque material may be used as the substrate  30 . For this reason, a silicon substrate which is opaque but has high thermal diffusivity can be used as the substrate  30 . When the silicon substrate is adopted as the substrate  30 , thermal deterioration of the organic light-emitting layer  38  can be suppressed, and the life of the display device  20  can be long.  
      The anode electrode  31  is formed so as to cover the substrate  30 . At this time, when a transparent electrode such as ITO is used as the anode electrode  31  and the substrate  30  is made of a transparent material, the light emitted in the organic light-emitting layer  38  can be emitted to the second direction. Further, since the light emitted in the organic light-emitting layer  38  can be led to the first direction, materials such as copper (resistivity: 1.67×10 −6  Ω·cm) and aluminum (resistivity: 2.655×10 −6  Ω·cm) which are opaque but has resistivity of smaller than 10 −4  Ω·cm can be used as the anode electrode  31 . For this reason, the light can be emitted in the organic light-emitting layer  38  efficiently by a low driving voltage.  
      The cathode electrode  32  is arranged in a groove  43  provided on the anode electrode  31  via the insulating layer  33 . When the cathode electrode  32  is arranged in the groove  43 , electrode surfaces of the anode electrode  31  and the cathode electrode  32  can be approximately aligned with each other. When a difference in the height of the electrode surfaces is eliminated, the thickness of the organic light-emitting layer  38  to be formed on the anode electrode  31  and the cathode electrode  32  can be reduced, and decrease in mobility of the electrons in such a manner that the mobility is decreased in inverse proportion to third root of the volume of the organic light-emitting layer  38  can be suppressed. In the case where the first electrode element serves as the anode, the second electrode element serves as the cathode. At this time, it is desirable that a material which has small work function or small electron affinity is used as the cathode electrode  32  in order to make reflectance high and facilitate the injection of electrons into the organic light-emitting layer  38 . For example, materials such as magnesium silver alloy and aluminum-lithium alloy can be used. Further, materials with resistivity smaller than 10 −4  Ω·cm such as copper (resistivity: 1.67×10 −6  Ω·cm) and aluminum (resistivity: 2.655×10 −6  Ω·cm) can be used. When the material with resistivity smaller than 10 −4  Ω·cm is used as the cathode electrode  32 , the light can be emitted in the organic light-emitting layer  38  efficiently by a low driving voltage.  
      The separator  42  is provided in the substrate laminating direction so as to separate to insulate the first electrode element from the second electrode element. The provision of the separator  42  ensures the moving of the holes induced from the anode electrode  31  and the electrons induced from the cathode electrode  32 , so that the electrons and the holes can be induced appropriately to the organic light-emitting layer  38 .  
      The anode side functional element has the hole injecting layer  35  which improves injecting efficiency of the holes from the anode electrode  31  as the first electrode element, and the hole transport layer  34  having the function of an electron barrier. Examples of materials of the hole injecting layer  35  are arylamine and phthalocyanine (copper phthalocyanine) . Further, an example of the material of the hole transport layer  34  is arylamine.  
      The cathode side functional element has the electron injecting layer  36  which improves injecting efficiency of electrons from the cathode electrode  32 , and the electron transport layer  37  having the function of a hole barrier. Examples of the material as the electron injecting layer  36  are alkali metal, such as lithium, lithium fluoride, lithium oxide and lithium complex. Further, examples of the material of the electron transport layer  37  are aluminum complex, oxadiazole, triazole and phenanthroline.  
      The organic light-emitting layer  38  has a light-emitting unit  60  which emits light due to an electric field supplied by the anode electrode  31  and the cathode electrode  32 . The light-emitting unit  60  is excited by recoupling of the electrons and the holes which move in the organic light-emitting layer  38  and emits light with high efficiency. A compound having light emitting property such that fluorescence or phosphorescence is strong is used as the organic light-emitting layer  38 . The organic light-emitting layer  38  may include a host material whose light emitting ability is low but deposition property and luminescence are good, and a dopant pigment whose light emitting ability is high but cannot be deposited by itself. An example of the host material is aluminum complex. Examples of the dopant pigment are perylene (red light emitting material), and rubrene (orange light emitting material). At this time, a material of the dopant pigment which satisfies a condition that an excitation energy level of molecules of the host material is higher than an excitation energy level of the dopant pigment molecules is selected. Further, the carbon nanotube is mixed partially in the organic light-emitting layer  38 .  
      The carbon nanotube is a substance which is made of carbon obtained in such a manner that a graphite sheet is formed into a tube shape with diameter of several nm, and includes a multi-layer one formed by overlapping a plurality of tubes and a single-layer one composed of only one tube. The carbon nanotube can be created by synthesizing methods such as carbon arc discharge, carbon laser evaporation, thermal decomposition of hydrocarbon gas, a plasma CVC (Chemical Vapour Deposition) method and an electron beam irradiation method. The single-layer carbon nanotube is classified into three types: chiral type; arm chair type; and zigzag type according to carbon coupling types. At this time, the arm chair and zigzag type carbon nanotubes have metallic electric conductivity, and the chiral type carbon nanotube has semiconductor type electric conductivity. In this embodiment, it is desirable that the armchair type and zigzag type carbon nanotubes having metallic electric conducting property are used. Since the electric conductivity of the carbon nanotube is extremely high, electric resistance of the organic light-emitting layer  38  in which the carbon nanotube is mixed can be smaller than electric resistance of the organic light-emitting layer in which the carbon nanotube is not mixed, and thus light can be efficiently emitted in the organic light-emitting layer  38  with respect to the driving voltage of the display device  20 .  
      In this embodiment, the carbon nanotube is mixed between a surface crossing the organic light-emitting layer  38  in the organic light emitting layer  38  on the side closer to the anode electrode  31  than the separator  42  and the hole transport layer  34 , so that an organic light-emitting layer  51  including the carbon nanotube is formed. For example as shown in  FIG. 1 , the carbon nanotube is mixed in the organic light-emitting layer  38  so as to cover the hole transport layer  34 .  FIGS. 2 and 3  are enlarged schematic diagrams illustrating another form of the carbon nanotube mixed portion of the inorganic light-emitting layer  38 , but as shown in  FIGS. 2 and 3 , the carbon nanotube may be mixed in the organic light-emitting layer  38 . This will be described later.  
      In the organic light-emitting layer  51  including the carbon nanotube shown in  FIG. 1 , since the carbon nanotube has small work function, it has a function for improving joint property with respect to the hole transport layer  34  and improving the mobility of the holes. When the carbon nanotube is mixed so as to cover the hole transport layer  34 , the joint property can be improved in the entire area of the joint portion between the organic light-emitting layer  51  including the carbon nanotube and the hole transport layer  34 . When the mobility of the hole is compared with the mobility of the electrons in the organic light-emitting layer  38 , since the mobility of the electrons is relatively high, before the carbon nanotube is mixed in the organic light-emitting layer  38 , it is considered that the holes and the electrons are recoupled in the vicinity of the boundary between the hole transport layer  34  and the organic light-emitting layer  38 . When the carbon nanotube is mixed into the organic light-emitting layer  38  so that the mobility of the holes are improved, the holes can be easily retained in the vicinity of the boundary surface of the organic light-emitting layer  51  including the carbon nanotube opposite to the hole transport layer  34 , and a position where the holes and the electrons are recoupled is assumed to be the vicinity of the boundary surface of the organic light-emitting layer  51  including the carbon nanotube opposite to the hole transport layer  34 . When the light emitted in the organic light-emitting layer  38  is emitted to the first direction, the distance of transmission through the organic light-emitting layer  38  becomes short, so that the light emitting efficiency can be improved. When the carbon nanotube approximately covers the hole transport layer  34 , even if a gap is present between the organic light-emitting layers  51   a  and  51   b  including the carbon nanotube as shown in  FIG. 2 , the holes can be easily retained in the vicinity of the boundary surface of the organic light-emitting layers  51   a  and  51   b  including the carbon nanotube opposite to the hole transport layer sufficiently and effectively.  
      Further, the carbon nanotube may be mixed in a position which does not contact with the hole transport layer  34  as shown in  FIG. 3 . At this time, it is desirable that the carbon nanotube is mixed so as to cross the organic light-emitting layer  38 . In the organic light-emitting layer  51  including the carbon nanotube shown in  FIG. 3 , since the carbon nanotube has the function for improving the mobility of the electrons, its electric resistance becomes lower than that of the organic light-emitting layer  38  in which a carbon nanotube is not mixed. For this reason, the driving voltage can be reduced in comparison with the display device  20  having the organic light-emitting layer without the carbon nanotube.  
      When the carbon nanotube is mixed in the organic light-emitting layer  38 , the driving voltage of the display device  20  can be reduced, and the light emitting efficiency of the organic light-emitting layer  38  with respect to the driving voltage of the display device  20  can be improved. Further, it is considered that an emission position of the light emitted in the organic light-emitting layer  38  can be finely adjusted.  
      The organic light-emitting layer  38  can be formed by, for example, an ink-jet method. With the ink-jet method, a solution of an organic material is dropped from a head of the ink jet so that the organic light-emitting layer  38  is formed. At this time, the carbon nanotube may be mixed with the solution of the organic material. Firstly, the organic light-emitting layer  38  in which the carbon nanotube is not mixed is formed, and the solution of the organic material including the carbon nanotube is dropped thereon, so that the dropped portion becomes the organic light-emitting layer  51  including the carbon nanotube shown in  FIG. 1 . In the case where the organic light-emitting layer  38  is formed by the ink-jet method, since the insulating member  41  and the separator  42  shown in  FIG. 4  block out the display devices  20  and inside the display devices, the formation of the organic light-emitting layer  38  is facilitated.  
      In this embodiment, the first electrode element is composed of the anode electrode  31 , the hole transport layer  34  and the hole injecting layer  35 , and the second electrode element is composed of the electron injecting layer  36 , the electron transport layer  37  and the cathode electrode  32 . Depending on the relationship with the organic light-emitting layer  38 , however, the element may have the two-layer structure composed of the electron transport layer  37  and the organic light-emitting layer  38 , or the hole transport layer  34  and the organic light-emitting layer  38 , or may have the three-layer structure composed of the electron transport layer  37 , the hole transport layer  34  and the organic light-emitting layer  38 . In this embodiment, the first electrode element serves as the anode, and the second electrode element serves as the cathode, but “first” and “second” are only reference symbols for convenience.  
      The light emitting steps of the display device  20  in this embodiment are explained with reference to  FIG. 1 . The display device  20  is driven by, for example, a driver IC (not shown) which outputs a pulse voltage. When the a voltage of not less than a threshold value is applied to the anode electrode  31  and the cathode electrode  32  of the display device  20 , the holes are injected from the anode electrode  31  into the hole injecting layer  35 , and the electrons are injected from the cathode electrode  32  into the electrode injecting layer  36 . The holes are transported into the organic light-emitting layer  38  via the hole transport layer  34 , and the electrons are transferred into the organic light-emitting layer  38  via the electron transport layer  37 . In the organic light-emitting layer  51  including the carbon nanotube, since the mobility of the holes and the electrons is improved, the holes and the electrons are recoupled in the vicinity of the light-emitting unit  60  so that an exciter is generated, and the exciter moves in the organic light-emitting layer  38 . When the exciter discharges corresponding energy between bands of the dopant pigment so that the dopant pigment emits light.  
      The organic EL display using the above display device according to the present invention is explained below with reference to  FIG. 4 .  FIG. 4  illustrates a schematic constitutional diagram of the organic EL display according to the embodiment.  FIG. 4  partially includes a sectional view for easy understanding of the constitution of the organic EL display. The organic EL display  10  shown in  FIG. 4  has the band-shaped anode electrodes  31  as the first electrode elements which are arranged in a line form, the band-shaped grooves  43  which are provided in a line form so as to cross the anode electrodes  31 , the band-shaped cathode electrodes  32  as the second electrode elements which are arranged in a line form in the grooves  43  via the insulating layer  33 , and the organic light-emitting layer  38  as the organic light-emitting element which is formed so as to cover the anode electrodes  31  and the cathode electrodes  32  and in which the carbon nanotube is mixed partially. The organic light-emitting layer  38  is separated to be insulated by the insulating member  41  in the substrate laminating direction, and respective insulated and separated elements emit light independently as the display devices  20 .  
      In this embodiment, the anode electrode  31  covers the substrate  30 . As a result, when a transparent electrode such as ITO is used as the anode electrode  31 , a transparent insulating material such as glass is used as the substrate  30 , so that the light emitted in the organic light-emitting layer  38  can be emitted from the substrate  30  side. Different two surfaces of the organic EL display  10  can be, therefore, used as the display surfaces.  
      When a plurality of the anode electrodes  31  and the cathode electrodes  32  are formed in a line form, the display devices  20  can be driven by passive driving in a line sequential direction. The passive driving is a driving method of applying a voltage to one anode electrode  31  and one cathode electrode  32  simultaneously so as to emit light at a portion in the organic light-emitting layer  38  where the anode electrode  31  and the cathode electrode  32  cross each other. At this time, the anode electrode  31  and the cathode electrode  32  are conductively connected to a drive IC, not shown, for example. A signal voltage according to a display image is input from the driver IC to the plural anode electrodes  31  in synchronization with a clockpulse, and a scanning voltage is applied sequentially to the plural cathode electrodes  32 .  
      In the case where the organic EL display  10  is constituted for color display, red light, blue light and yellow light may be emitted in the three adjacent display devices  20  sequentially, for example. At this time, light emitting substances may be mixed to the organic light-emitting layer  38  in order to emit color light, or the display devices  20  may be covered with color filters corresponding to the respective colors.  
      The electrodes and the grooves  43  can be formed in the following manner, for example. After photolithography, the anode electrodes  31  are deposited by vacuum evaporation or sputtering. Thereafter, the anode electrodes  31  are formed into a band shape by etching or sandblast. The grooves  43  are formed into a band shape by etching or sandblast. Thereafter, the cathode electrodes  32  are formed via the insulating layers  33  by vacuum evaporation or sputtering.  
     SECOND EMBODIMENT  
       FIG. 5  illustrates a schematic constitutional diagram of the display device  20  according to the embodiment. The display device  20  according to this embodiment is a display device where the position where the carbon nanotube is mixed in the organic light-emitting layer  38  is different from the mixing position explained in the first embodiment. Since all the components such as the first electrode element other than the mixing position of the carbon nanotube in the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described. Further, in the organic EL display  10  shown in  FIG. 4  using the display device  20  according to this embodiment, since all the components such as the first electrode element other than the organic light-emitting layer  38  of the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described.  
      In the display device  20  shown in  FIG. 5 , the carbon nanotube is mixed between the surface crossing the organic light-emitting layer  38  in the organic light-emitting layer  38  on the side closer to the cathode electrode  32  than the separator  42  and the hole transport layer  34 , so that the organic light-emitting layer  54  including the carbon nanotube is formed. At this time, for example, the carbon nanotube covers the hole transport layer  34  and the cross surface and is mixed so that the organic light-emitting layer  54  including the carbon nanotube continues from the hole transport layer  34  to the cross surface.  
      When the carbon nanotube is mixed so as to cover the hole transport layer  34 , the joint property can be satisfactorily in entire area of the joint portion between the organic light-emitting layer  54  including the carbon nanotube and the hole transport layer  34 . Further, when the carbon nanotube is mixed so as to cover the cross surface and so that the organic light-emitting layer  54  including the carbon nanotube continues from the hole transport layer  34  to the cross surface, the electrons or the holes are allowed to securely pass through the organic light-emitting layer  54  including the carbon nanotube from the cross surface to the hole transport layer  34 , so that the mixing of the carbon nanotube into the organic light-emitting layer  38  can be made to be effective. Since the work function of the carbon nanotube is small, the mixing of the carbon nanotube into the above position improves the joint property with respect to the hole transport layer  34 . Further, the mixing of the carbon nanotube into the organic light-emitting layer  38  can improve the mobility of the holes in the organic light-emitting layer  38 . For this reason, the electric resistance of the organic light-emitting layer  38  between the anode electrode  31  and the cathode electrode  32  can be small, so that the driving voltage can be reduced more than that of the display device having the organic light-emitting layer without the carbon nanotube.  
      Since the mobility of the holes in the organic light-emitting layer  38  is improved, it can be assumed that the holes are easily retained in the vicinity of the boundary surface of the organic light-emitting layer  54  including the carbon nanotube on the side of the cathode electrode  32 , and the position where the holes and the electrons are recoupled can be moved to the vicinity of the boundary surface of the organic light-emitting layer  54  including the carbon nanotube on the side of the cathode electrode  32 . For this reason, when the position of the boundary surface of the organic light-emitting layer  54  including the carbon nanotube on the side of the cathode electrode  32  is adjusted, the position of the light-emitting unit  60  can be finely adjusted. When the light emitted in the light-emitting layer  38  is emitted to the first direction, the transmitting distance of the light in the organic light-emitting layer  38  can be shortened, and the light emitting efficiency can be improved.  
     EMBODIMENT 3  
       FIG. 6  illustrates a schematic constitutional diagram of the display device  20  according to this embodiment. The display device  20  according to this embodiment is a display device where the position where the carbon nanotube is mixed in the organic light-emitting layer  38  is different from the mixing position explained in the first embodiment. Since all the components such as the first electrode element other than the mixing position of the carbon nanotube in the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described. Further, in the organic EL display  10  using the display device  20  according to this embodiment, all the components such as the first electrode element other than the organic light-emitting layer  38  of the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described.  
      In the display device  20  shown in  FIG. 6 , the carbon nanotube is mixed between the surface crossing the organic light-emitting layer  38  in the organic light-emitting layer  38  on the side closer to the anode electrode  31  than the separator  42  and the electron transport layer  37 , so that the organic light-emitting layer  53  including the carbon nanotube is formed. At this time, for example, the carbon nanotube covers the electron transport layer  37  and the cross surface and is mixed so that the organic light-emitting layer  53  including the carbon nanotube continues from the electron transport layer  37  to the cross surface.  
      When the carbon nanotube is mixed so as to cover the electron transport layer  37 , the electrons easily move from the electron transport layer  37  to the organic light-emitting layer  53  including the carbon nanotube. Further, when the carbon nanotube is mixed so as to cover the cross surface and so that the organic light-emitting layer  53  including the carbon nanotube continues from the electron transport layer  37  to the cross surface, the electrons or the holes are allowed to securely pass through the organic light-emitting layer  53  including the carbon nanotube from the cross surface to the hole transport layer  34 , thereby making the mixing of the carbon nanotube in the organic light-emitting layer  38  effective. When the carbon nanotube is mixed in the above position, the mobility of the electrons can be improved, and the organic light-emitting layer  53  including the carbon nanotube can serve as an electron discharge source. At this time, an applied voltage can be concentrated onto the light-emitting unit  60  in the organic light-emitting layer 38 . For this reason, the electric resistance of the organic light-emitting layer  38  between the anode electrode  31  and the cathode electrode  32  can be small, so that the driving voltage can be lower than that of the display device having the organic light-emitting layer  38  without the carbon nanotube.  
      When the mobility of the holes and the electrons in the organic light-emitting layer  38  is considered, it is assumed that the position where the electrons and the holes are recoupled is the vicinity of the boundary surface of the organic light-emitting layer  53  including the carbon nanotube on the side of the anode electrode  31 . For this reason, when the position of the surface of the anode electrode  31  side among the boundary surfaces of the organic light-emitting layer  53  including the carbon nanotube is adjusted, the position of the light-emitting unit  60  can be finely adjusted. When the light emitted in the organic light-emitting layer  38  is emitted to the first direction, the transmission distance in the organic light-emitting layer  38  is shortened, so that the light emitting efficiency can be improved.  
      As shown in  FIG. 7 , the carbon nanotube may be mixed so as to cross the organic light-emitting layer  38  of the anode electrode  31  side from the position which does not contact with the electron transport layer  37 . In the organic light-emitting layer  53  including the carbon nanotube shown in  FIG. 7 , since the electric resistance becomes lower than that of the organic light-emitting layer  38  without the carbon nanotube, the driving voltage can be lower than that of the display device having the organic light-emitting layer without carbon nanotube effectively.  
     EMBODIMENT 4  
       FIG. 8  illustrates a schematic constitutional diagram of the display device  20  according to this embodiment. The display device  20  in this embodiment is a display device where the mixing position of the carbon nanotube in the organic light-emitting layer  38  is different from the mixing position explained in the first embodiment. Since all the components such as the first electrode element other than the mixing position of the carbon nanotube in the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described. Further, in the organic EL display  10  using the display device  20  in this embodiment, since all the components such as the first electrode element other than the organic light-emitting layer  38  of the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described.  
      The carbon nanotube is mixed between the surface crossing the organic light-emitting layer  38  in the organic light-emitting layer  38  on the side closer to the cathode electrode  32  than the separator  42  and the electron transport layer  37 , so that the organic light-emitting layer  52  including the carbon nanotube is formed in the display device  20  shown in  FIG. 8 . At this time, as shown in  FIG. 8 , for example, the carbon nanotube is mixed in the organic light-emitting layer  38  so as to cover the electron transport layer  37 .  
      When the carbon nanotube is mixed so as to cover the electron transport layer  37 , the electrons easily move from the electron transport layer  37  to the organic light-emitting layer  52  including the carbon nanotube. When the carbon nanotube is mixed in the above position, the mobility of the electrons can be improved, and the organic light-emitting layer  52  including the carbon nanotube can serve as an electron discharge source. At this time, an applied voltage can be concentrated on the light-emitting unit  60  in the organic light-emitting layer  38 . For this reason, the electric resistance of the organic light-emitting layer  38  between the anode electrode  31  and the cathode electrode  32  can be small, so that the driving voltage can be lower than that of the display device having the organic light-emitting layer  38  without carbon nanotube.  
      Further, as shown in  FIG. 9 , the carbon nanotube may be mixed in a position which does not contact with the electron transport layer  37 . At this time, the carbon nanotube is mixed so as to cross the organic light-emitting layer  38  on the side closer to the cathode electrode  32  than the separator  42 . In the organic light-emitting layer  52  including the carbon nanotube shown in  FIG. 9 , since the carbon nanotube has the function for improving the mobility of the electrons, its electric resistance becomes lower than that of the organic light-emitting layer  38  without carbon nanotube. For this reason, the driving voltage can be lower than that of the display device  20  having the organic light-emitting layer without carbon nanotube.  
     EMBODIMENT 5  
       FIG. 10  illustrates a schematic constitutional diagram of the display device  20  in this embodiment. The display device  20  in this embodiment is a display device where the mixing position of the carbon nanotube in the organic light-emitting layer  38  is a combination of the mixing positions of the carbon nanotube explained in the first ( FIG. 1 ) and fourth ( FIG. 8 ) embodiments. Since all the components such as the first electrode element other than the mixing position of the carbon nanotube in the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described. In the organic EL display  10  using the display device  20  in this embodiment, since all the components such as the first electrode element other than the organic light-emitting layer  38  of the display device  20  in this embodiment are similar to those explained in the first embodiment, the explanation thereof is not described.  
      In the display device  20  shown in  FIG. 10 , the carbon nanotube is mixed between the surface crossing the organic light-emitting layer  38  in the organic light-emitting layer  38  on the side closer to the cathode electrode  32  than the separator  42  and the electron transport layer  37 , so that the organic light-emitting layer  55   a  including the carbon nanotube is formed. The carbon nanotube is mixed between the surface crossing the organic light-emitting layer  38  in the organic light-emitting layer  38  on the side closer to the anode electrode  31  than the separator  42  and the hole transport layer  34 , so that the organic light-emitting layer  55   b  including the carbon nanotube is formed. At this time, as shown in  FIG. 10 , for example, the carbon nanotube is mixed in the organic light-emitting layer  38  so as to cover the electron transport layer  37  on the side of the cathode electrode  32 , and the carbon nanotube is mixed in the organic light-emitting layer  38  so as to cover the hole transport layer  34  on the side of the anode electrode  31 .  
      When the carbon nanotube is mixed so as to cover the electron transport layer  37 , as explained in the fourth embodiment, the electrons easily move from the electron transport layer  37  to the organic light-emitting layer  55   a  including the carbon nanotube, so that the mobility of the electrons is improved and the organic light-emitting layer  55 a including the carbon nanotube can serve as the electron discharge source. As a result, the applied voltage can be concentrated on the light-emitting unit  60  in the organic light-emitting layer  38 . As a result, the electric resistance of the organic light-emitting layer  38  between the anode electrode  31  and the cathode electrode  32  can be small, so that the driving voltage can be lower than that of the display device having the organic light-emitting layer  38  without the carbon nanotube.  
      When the carbon nanotube is mixed so as to cover the hole transport layer  34 , as explained in the first embodiment, the joint property can be improved in the entire area of the joint portion between the organic light-emitting layer  55   b  including the carbon nanotube and the hole transport layer  34 . Further, the holes can be easily retained in the vicinity of the boundary surface of the organic light-emitting layer  55   b  including the carbon nanotube on the side opposite to the hole transport layer  34 . For this reason, when the light emitted in the light-emitting unit  60  is emitted to the first direction, the transmission distance of the light to be transmitted through the organic light-emitting layer  38  becomes short, so that the emitting efficiency can be improved.