Patent Publication Number: US-8968044-B2

Title: Light emitting element, light emitting device, manufacturing method of light emitting device, and sheet-like sealing material

Description:
This application is a divisional of copending U.S. application Ser. No. 11/711,217 filed on Feb. 27, 2007 which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light emitting element and a light emitting device including a light emitting element. In addition, the present invention relates to a method and a material for sealing a light emitting element. 
     2. Description of the Related Art 
     Flat panel displays such as liquid crystal panels have been improved, and attempts have been made on improving quality of picture, reducing power consumption, and improving lifetime. In order to utilize the self-emitting ability of electroluminescent elements for practical application of electroluminescent panels (hereinafter referred to as an EL panels) which employs electroluminescent elements (hereinafter referred to as an EL elements) in the pixels, it is desired to realize vivid and bright displays with reduced power consumption. For this purpose, improvement in power efficiency has been investigated by increasing the current-luminance characteristic of materials used in the EL elements. However, there is a limitation on improvement in the power efficiency by the method described above. 
     The efficiency to extract the light (light extraction efficiency) that is emitted from a light emitting layer of the EL element is only around 20%. The reason of this low light extraction efficiency is that light emitted from the light emitting layer is attenuated since total reflection occurs when the light passes an interface of films having different refractive indexes and that the totally reflected light is absorbed in the EL element. An alternative reason is that the light from the light emitting layer is irradiated through a side surface of the light emitting element, for example, a side surface of a glass substrate. 
     Reference 1 describes an EL element with improved light extraction efficiency, which was achieved by reducing the amount of total reflection. In Reference 1, by providing a film having dispersed particles over a transparent conductive film to scatter the emitted light, the population of the light, which passes the interface between the transparent conductive film and a low refractive index film, with an incidence angle larger than the critical angle. (Reference 1: Japanese Published Patent Application No. 2004-303724). 
     The structure of EL panels are classified into a bottom emission structure (lower surface emission structure) and a top emission structure (upper surface emission structure) depending on the direction to which light is extracted. In the bottom emission structure, light is extracted through a substrate over which an EL element is fabricated. In the top emission structure, light is extracted through the upper side of the EL element. Note that the terms “bottom emission structure” and “top emission structure” are often used to refer to the structure of the organic EL panels. However, in this specification, these words are used to classify the structure of a light emitting element or a light emitting device according to not the kind of the light emitting element but the extracting direction of light. 
     Since the light emission area of the EL element is not strictly limited in the case of the top emission structure compared with the bottom emission structure, the aperture ratio of the active matrix EL panel can be increased by applying the top emission structure. Therefore, in the active matrix EL panels, the top emission structure is advantageous in lowering power consumption and improving quality of the image. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve light extraction efficiency of a light emitting element and to reduce power consumption by decreasing the amount of total reflection of light emitted from a light emitting layer by a means which is different from that described Reference 1. 
     A light emitting element of the present invention includes a first electrode and a second electrode which face each other and at least a light emitting layer between the first electrode and the second electrode. The first electrode, the light emitting layer, and the second electrode are sequentially stacked, and light emitted from the light emitting layer is extracted from the second electrode. 
     The first electrode of the abovementioned light emitting element is an electrode which can reflect light emitted from the light emitting layer. Further, the second electrode is an electrode which can transmit light emitted from the light emitting layer. 
     The light emitting element of the present invention includes at least one light emitting layer between the first electrode and the second electrode. A plurality of light emitting layers may be provided between these electrodes. Further, in the case of fabricating an organic EL element as a light emitting element, for example, in addition to the light emitting layer, a layer such as an electron injecting layer, an electron transporting layer, a hole blocking layer, a hole transporting layer, or a hole injecting layer is appropriately formed. The light emitting element having such a structure is also included in the present invention. In the case of fabricating an inorganic EL element as a light emitting element, an insulating layer can be provided between the light emitting layer and the first electrode and/or between the light emitting layer and the second electrode. 
     One feature of the light emitting element of the present invention is that a plurality of fine particles is provided in contact with a surface of the second electrode on a light extraction side and that the fine particles have a refractive index which is equal to or higher than that of the second electrode. 
     When the second electrode is a single layer film, the refractive index of the second electrode means a refractive index of this single layer film. When the second electrode is a multilayer film, the refractive index of the second electrode means a refractive index of a film which is the closest to the light extraction side, namely, a refractive index of a film having a surface on which the fine particles are located. 
     In the present invention, by providing a plurality of fine particles having a predetermined refractive index, the shape of the surface of the second electrode is changed. That is, the second electrode is an electrode having a plurality of projection portions on its surface. By providing fine particles on the surface, the critical angle of light which passes the surface of the second electrode varies, and light which is totally reflected and cannot be extracted from the conventional EL elements is enabled to pass the second electrode. Accordingly, the amount of total reflection of light which passes through the second electrode decreases, and the light extraction efficiency can be improved. 
     In order to prevent total reflection at an interface between the fine particles and the second electrode, the fine particles have a refractive index which is equal to or higher than that of the second electrode. 
     A protective film formed of a transparent conductive film or an insulating film can be provided in contact with the surface of the second electrode on which the fine particles are provided. In order to prevent total reflection at an interface between the protective film and the second electrode, this protective film has an refractive index which is equal to or higher than that of the second electrode. 
     In another light emitting element of the present invention, a protective film is provided in contact with a surface of a second electrode, and a plurality of fine particles is provided in contact with a surface of the protective film on the light extraction side. Another feature of the light emitting element is that, in order to prevent total reflection at an interface between the protective film and the second electrode, this protective film has a refractive index which is equal to or higher than that of the second electrode, and that the fine particles have a refractive index which is equal to or higher than that of the protective film. 
     Here, when the protective film is a single layer film, the refractive index of the protective film means a refractive index of this single layer film. When the protective film is a multilayer film, the refractive index of the protective film means a refractive index of a film which is the closest to the light extraction side, namely, a refractive index of a film on which the fine particles are provided. 
     In the above-described light emitting element of the present invention, the shape of the surface of the protective film is also changed by providing the fine particles having a predetermined refractive index onto the surface of the protective film on the light extraction side, similarly to the case of providing the fine particles onto the surface of the second electrode. Accordingly, the amount of total reflection of light which passes through the protective film is reduced, and light extraction efficiency of the light emitting element is improved. 
     When light emitted from a light emitting layer is extracted from a second electrode or a protective film, the amount of light which is totally reflected is reduced by the present invention. Accordingly, light extraction efficiency is improved. The improvement of the light extraction efficiency allows reduction of power consumption of a light emitting element and a light emitting device using the light emitting element. Particularly, the effect of the present invention to reduce the power consumption is more remarkably obtained by employing the top emission structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a cross sectional view of a light emitting device (Embodiment Mode 1); 
         FIGS. 2A to 2D  are cross sectional views of a light emitting device (Embodiment Mode 2); 
         FIGS. 3A to 3C  are cross sectional views of a light emitting device (Embodiment Mode 3); 
         FIGS. 4A to 4D  are cross sectional views of a light emitting device (Embodiment Mode 4); 
         FIGS. 5A to 5C  are cross sectional views of a light emitting device (Embodiment Mode 5); 
         FIGS. 6A to 6D  are cross sectional views of a light emitting device (Embodiment Mode 6); 
         FIG. 7  is a cross sectional view of a light emitting device (Embodiment Mode 7); 
         FIGS. 8A to 8C  are cross sectional views of a light emitting device (Embodiment Mode 7); 
         FIG. 9  is a cross sectional view of a light emitting device (Embodiment Mode 8); 
         FIGS. 10A to 10C  are cross sectional views of light emitting devices (Embodiment Mode 8); 
         FIG. 11  is a cross sectional view of a light emitting device (Embodiment Mode 8); 
         FIGS. 12A to 12C  are cross sectional views of light emitting devices (Embodiment Mode 9); 
         FIG. 13  is a top view of a light emitting device (Embodiment Mode 10); 
         FIG. 14  shows circuits of a pixel in a light emitting device (Embodiment Mode 10); 
         FIG. 15  is a cross sectional view of a pixel in a light emitting device (Embodiment Mode 10); 
         FIG. 16  shows a driving method of a light emitting device (Embodiment Mode 10); 
         FIGS. 17A to 17F  show modes of electronic devices to which a light emitting device is applied (Embodiment Mode 11); and 
         FIG. 18  shows a mode of a flat lighting device to which a light emitting device is applied (Embodiment Mode 12). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiment modes of the present invention will be described with reference to the drawings. Note that the present invention can be carried out in many various modes. It is easily understood by those skilled in the art that various changes may be made in forms and details without departing from the concept and the scope of the present invention. Therefore, the present invention should not be limited to the description of the embodiment modes below. 
     In addition, it is possible to combine the embodiment modes appropriately without departing from the concept of the present invention. Since the same reference numerals are commonly given to the same components or components having the same function throughout the embodiment modes, the description thereof may be omitted. 
     Embodiment Mode 1 
       FIG. 1  is a cross sectional view of a light emitting device in which a light emitting element of this embodiment mode is provided. Over a substrate  101 , a support  102  for a light emitting element is provided, and three light emitting elements are provided over the support  102 . 
     In each light emitting element, a first electrode  103 , a light emitting layer  104 , and a second electrode  105  are sequentially stacked over the substrate  101 . A plurality of fine particles  106  is provided on the second electrode  105 , in contact with a surface of the second electrode  105 . Note that the second electrode  105  is commonly provided for the three light emitting elements. An insulating layer  107  is provided for separating the light emitting elements each other, and is often called a partition wall. 
     A sealing substrate  109  is fixed to the substrate  101  with a sealing material  108  which is provided to surround a perimeter of the substrate  101 , thereby sealing the light emitting elements. In this embodiment mode, an airtight space surrounded by the substrate  101 , the sealing material  108 , and the substrate  109  is filled with a gas  110 . An inert gas such as nitrogen or argon is preferable as the gas  110 . 
     The substrate  101  may be anything as long as it can be a support base of the light emitting elements or the support  102 , and a quartz substrate, a semiconductor substrate, a glass substrate, a plastic substrate, a flexible plastic film, or the like can be used. Since a structure where light is extracted from the substrate  101  side is not employed, the substrate  101  is not required to be transparent, and may be colored or opaque. 
     As the sealing substrate  109 , a substrate having a high transmittance to visible light is used in order to extract light from the light emitting elements. For example, a quartz substrate, a glass substrate, a plastic substrate, a flexible plastic film, or the like can be used. A color filter may be provided to the sealing substrate  109  in order to improve color purity of the emitted light or to change an emission color of the light emitting elements. Further, although the substrate  109  having a flat-plate shape is used in this embodiment mode, the shape is not limited to this shape and any shape may be used as long as sealing can be conducted. For example, a substrate having a cap shape like a sealing can is able to be used. 
     There is a case where the support  102  is not needed. In the case of providing an active matrix type pixel in a light emitting device, the support  102  is a circuit including a transistor, a condenser, or the like for controlling luminance or timing of light emission of each light emitting element. 
     The first electrode  103  is formed over the support  102 . The first electrode  103  has a function of reflecting light which is emitted from the light emitting layer and serves as a cathode. The first electrode is formed of a reflective conductive film including a metal or an alloy. For this metal film, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), aluminum (Al), or the like can be used. For the alloy film, an alloy of magnesium and silver, an alloy of aluminum and lithium, or the like can be used. Such a film for forming the first electrode  103  can be fabricated by a sputtering method, a vapor deposition method, or the like. 
     As the first electrode  103 , a multilayer film in which transparent conductive films are stacked on the metal film or the alloy film, or a multilayer film in which the metal film or the alloy film is sandwiched between two transparent conductive films can also be used. Further, as the first electrode  103 , a multilayer film including transparent conductive films having different refractive indices can also be used. Reflectivity can be improved by utilizing multiple interference of light. 
     After forming the first electrode  103 , an insulating layer  107  is formed. The insulating layer  107  is constructed by forming an insulating layer on the surface of the support  102  followed by partly etching the insulating layer to form apertures on which the light emitting layer  104  is fabricated. The insulating layer  107  may be formed by using an organic material including an acrylic resin, a siloxane resin, a polyimide resin, or an epoxy resin; an inorganic material such as a silicon oxide, a silicon oxide containing nitrogen, or a silicon nitride containing oxygen; or a material formed of both inorganic material and the organic material. The organic material film including an acrylic resin or the like is, for example, formed by coating the support  102  with a material solution and baking it. The inorganic material film is formed by a CVD method or a sputtering method. 
     The light emitting layer  104  is formed by a vapor deposition method or the like over the first electrode  103 . The light emitting layer  104  is a layer containing a light emitting substance. A known material can be used for the light emitting layer  104 , and either a low molecular material and a high molecular material can be used. Note that as a material for forming the light emitting layer  104 , not only an organic compound but also an inorganic compound or an organic compound in which an inorganic compound is mixed can be used. To fabricate the light emitting layer  104 , a dry type and/or a wet type film formation methods are selected from, for example, a vapor deposition method using a metal mask, a droplet discharge method without using a metal mask (typically, an inkjet method), a spin coating method, a dip coating method, printing method, and the like, depending on the material of the light emitting layer. 
     The second electrode  105  is formed over the light emitting layer  104 . The second electrode  105  serves as an anode and can transmit the light emitted from the light emitting layer  104 . The light generated in the light emitting layer  104  is extracted from the second electrode  105  either directly or after being reflected by the first electrode  103 . 
     The second electrode  105  is typically a transparent conductive film. In particular, in the case where the light emitting element is an organic EL element, a conductive film formed in the following manner can be used: for adjusting work function, a material having a low transmittance to visible light such as a metal is extremely thinly formed on the first electrode  103  side with a thickness of 1 nm to 50 nm, preferably about 5 nm to 20 nm, and a transparent conductive film is stacked thereon. In this case, for the thin film formed extremely thinly, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or the like can be used. Such a thin film can be formed by, for example, a sputtering method, a droplet discharge method, or the like. 
     The materials of the transparent conductive films used for the second electrode  105  are materials having a high transmittance to light in a visible light range (400 to 800 nm), and typically metal oxides. For example, an oxide of an element selected from zinc (Zn), indium (In), and tin (Sn) or a compound in which a dopant is added to these oxides can be given. As a dopant for zinc oxide, Al, Ga, B, In, Si, or the like or an oxide of these elements is used. Note that zinc oxides containing these dopants are called AZO, GZO, BZO, and IZO respectively. As a dopant of indium oxide, Sn, Ti, or the like is used. Indium oxide doped with Sn is called ITO (Indium Tin Oxide). As a dopant of a tin oxide, Sb, F, or the like is used. Further, for each transparent conductive film, a compound in which two kinds of oxides selected from the above-described zinc oxide, indium oxide, tin oxide, and oxides thereof containing dopants are mixed can be used. 
     Next, the fine particles  106  are sprayed on the surface of the second electrode  105  by a dry method or a wet method in the same manner for spraying spacer of a liquid crystal panel. A dry method is a method in which the fine particles  106  are freely fallen by the action of airflow or static electricity. A wet method is a method in which a mixture of the fine particles  106  and a solvent is sprayed. In the case of spraying the mixture containing fine particles  106  by the wet method, the solvent is evaporated by heating (100° C. or less) in an extent that the light emitting layer  104  is not affected unless a solvent is volatilized before the fine particles  106  reaches the substrate  101  after spraying the mixture containing fine particles  106 . 
     As another method of providing the fine particles  106  on the surface of the second electrode  105 , a method can also be used in which a mixture of the fine particle  106  and a volatile solvent such as alcohol is applied to the surface of the second electrode  105  and then the solvent is volatilized. As the application method, a cast method, a spin coating method, a spray method, an inkjet method, a printing method, a dropping method, or the like can be used. 
     A solvent for a mixture in which fine particles are mixed is selected from water, alcohols such as ethanol or isopropanol (IPA), and the like, depending on the material of the fine particles  106 . 
     Each fine particle  106  is formed of a material having a refractive index which is equal to or higher than that of the second electrode  105 . In this embodiment mode, the refractive index of the second electrode  105  is a refractive index of the transparent conductive film used for the second electrode  105 . 
     In order to seal the light emitting elements, the substrate  109  on the perimeter of which the uncured sealing material  108  is provided is prepared. The uncured sealing material  108  is provided with a predetermined shape on the perimeter of the substrate  109  by a printing method, a dispensing method, or the like. The sealing material  108  can also be provided on the substrate  101  side after spraying the fine particles  106  onto the second electrode  105 . 
     For the sealing material  108 , a resin curable by UV light or the like such as an epoxy resin or an acryl resin, or a heat-curable resin can be used. Since the material of the light emitting layer  104  readily decomposes upon heating, a light-curable resin is optimal for the sealing material  108 . If a heat-curable resin is used, it is preferable that the curing temperature is 100° C. or less. 
     The substrate  109  is provided over the substrate  101  over which the fine particles  106  are sprayed. While pressure is applied to the substrate  101  and the substrate  109 , the uncured sealing material  108  is irradiated with UV light to cure the resign, and the substrate  101  and the substrate  109  are firmly attached. It is obvious that when the heat-curable resin is used as the sealing material  108 , heat treatment is conducted. In addition, it is desirable that an ambient pressure is somewhat reduced from atmospheric pressure in the period after providing the substrate  109  over the substrate  101  and before curing the sealing material  108 . Note that the atmosphere desirably contains as little moisture as possible, and for example, a nitrogen atmosphere can be adopted. 
     By curing the sealing material  108 , the space between the substrate  101  and the substrate  109  is air-sealed and filled with the gas  110 . 
     After sealing the substrate  101  with the substrate  109 , the light emitting device is divided into arbitrary size. 
     One feature of this embodiment mode is that the shape of the surface of the second electrode  105  is changed by providing the plurality of fine particles  106  on the surface of the second electrode  105  on the light extraction side. Due to the plurality of fine particles  106 , the surface of the second electrode  105  has a plurality of projections, and a critical angle of light entering an interface between the second electrode  105  and the gas  110  varies depending on places. In other words, light having an incident angle which normally reflects completely is not totally reflected in the case of the present device, and the light is refracted and scattered by the fine particles  106  so that the light can pass the second electrode  105 . Thus, by providing the fine particles  106  in contact with the surface of the second electrode  105 , the amount of light which is totally reflected at the interface between the second electrode  105  and the gas  110  is reduced. Accordingly, light extraction efficiency is improved. 
     Note that in Reference 1, it is described that light extraction efficiency is improved by providing a particle-containing transparent electrode layer  3 ′, in which fine particles are dispersed, over a transparent electrode layer  3  (see  FIG. 2  and the description thereof). Specifically, Reference 1 describes that extraction efficiency is improved by the change in an angle of light to an angle which does not cause total reflection, which is achieved by scattering the light with the fine particles in the particle-containing transparent electrode layer  3 ′. In Reference 1, it is not described that the conditions of total reflection (critical angle) of light which is extracted from the transparent electrode layer  3  are changed. On the other hand, the invention proposed in this specification is that the total reflection condition of the interface itself between the second electrode  105  and the gas  110  is changed by changing the shape of the interface in order to improve light extraction efficiency. Therefore, the essential principle of the invention proposed in this specification is completely different from that described in Reference 1. 
     As the material of the fine particles  106 , either an organic material or an inorganic material may be used. The oxide or the oxide including a dopant, which are described as the transparent conductive film material of the above-described second electrode  105 , such as tin oxide (SnO 2 ), zinc oxide (ZnO), or ITO; or a metal oxide such as strontium oxide (Sr 3 O 2 ), aluminum oxide (Al 2 O 3 ), titanium oxide (TiO 2 ), yttrium oxide (Y 2 O 3 ), or cerium oxide (CeO 2 , Ce 2 O 3 ) can be given as the material. Further, various ferroelectric materials can also be used. For example, an oxide-based ferroelectric material such as barium titanate (BaTiO 3 ), KNbO 3  or LiNbO 3  is exemplified. Further, an inorganic material such as silicon oxide, silicon nitride, silicon nitride oxide (SiN x O y , 0&lt;x&lt;4/3, 0&lt;y&lt;2, 0&lt;3x+2y≦4), zirconium, DLC (diamond like carbon), or carbon nanotube can be used. 
     The size (particle diameter) of the fine particles  106  is necessarily the size with which the above-described effect can be obtained, and is 2 nm or more and preferably 20 nm or more. Further, it is preferable that the size of the fine particles  106  does not exceed the wavelength of the visible light range, and the upper limit of the size is 800 nm. In consideration of optical design of a light emitting element, the upper limit of the size is preferably 100 nm. 
     The shape of each fine particle  106  is preferably the shape with which light is effectively concentrated or scattered. The shape is, for example, a columnar shape, a polyhedral shape, a polypyramidal shape such as a triangular pyramid, a circular cone shape, a concave lens shape, a convex lens shape, a hog-backed shape, a prism shape, a spherical shape, a semispherical shape, or the like. 
     Many fine particles  106  are provided on the surface of the second electrode  105 . At this time, it is not necessary that all the fine particles  106  have the same material, the same size, and the same shape, and each of them may have different materials, sizes, or shapes. 
     The structure of a light emitting element of the present invention is not limited to the one shown in  FIG. 1  or the like as long as at least one light emitting layer exists between two electrodes. Light emitting elements utilizing electroluminescence are classified depending on whether a light emitting material included in its light emitting layer is an organic compound or an inorganic compound; generally, the former is called an organic EL element, and the latter is called an inorganic EL element. 
     In the case where a light emitting element is an organic EL element, in addition to the light emitting layer, a functional layer such as an electron injecting layer, an electron transporting layer, a hole blocking layer, a hole transporting layer, or a hole injecting layer may be freely combined. In addition, a plurality of light emitting layers may be provided between the electrodes. 
     An inorganic EL element can also be formed as a light emitting element. Inorganic EL elements are classified into a dispersion type inorganic EL element and a thin-film type inorganic EL element depending on its device structure. The former has a light emitting layer in which particles of a light emitting material are dispersed in a binder, whereas the latter has a light emitting layer made of a thin film of a light emitting material. Although they have such a difference therebetween, they have a common feature that electrons accelerated by high electric field are required. Two light-emission mechanisms are accepted. One is the donor-acceptor recombination mechanism, in which a donor level and an acceptor level are utilizes. The other is a localized light emission mechanism which utilizes inner-shell electron transition of metal ions. In general, the dispersion-type inorganic EL element performs the donor-acceptor recombination light emission mechanism, and the thin-film type inorganic EL element performs the localized light emission mechanism. 
     The inorganic EL element emits light by applying voltage between a pair of electrode layers which interpose a light emitting layer therebetween, and can be operated in either DC driving or AC driving. 
     Embodiment Mode 2 
     Embodiment Mode 2 will be described with reference to  FIGS. 2A to 2D . The air-sealed space between the substrate  101  and the substrate  109  is filled with a gas in Embodiment Mode 1. However, in a light emitting device of this embodiment mode, the space is filled with a solid material which is prepared by filling a liquid-phase material and curing it. A sealing structure of a light emitting device in which a solid is provided between substrates is called a solid sealing structure, and this word is frequently used to distinguish it from the structure in which a gas is filled. In this specification, this word will be used to distinguish the structure in which a solid is provided between substrates from the structure in which a gas is filled. 
     By applying the process described in Embodiment Mode 1, a substrate  101  over which fine particles  106  are sprayed on a surface of a second electrode  105  is prepared ( FIG. 2A ). 
     Next, an uncured sealing material  108  is provided with a predetermined shape on a perimeter of the substrate  101  by a printing method, a dispensing method, or the like in a similar manner to Embodiment Mode 1 ( FIG. 2B ). 
     In this embodiment mode, a filler  201  is provided in a space between the substrate  101  and a substrate  109 , which is air-sealed with the sealing material  108 . As a material of the filler  201 , a UV light curable resin such as an epoxy resin or an acryl resin, a visible light curable resin, or a heat-curable resin can be used. When a material of a light emitting layer  104  is an organic material, in consideration of the poor heat resistance ability of the organic material, the UV light curable resin or a visible light curable resin is preferable. In the case of using a heat-curable resin, a resin having a curing temperature of 100° C. or less is selected. After providing the sealing material  108 , the uncured (liquid-phase) filler  201  is dropped into a region surrounded by the sealing material  108  ( FIG. 2C ). 
     Next, the substrate  109  is provided over the substrate  101  over which the uncured sealing material  108  and the filler  201  are prepared. While applying pressure to the substrate  101  and the substrate  109 , the uncured sealing material  108  and the filler  201  are irradiated with light or heated so as to be cured, and the substrate  109  and the substrate  101  are firmly attached. The cured filler  201  is provided in contact with a surface of the second substrate and the surface of the second electrode  105  and fixes the substrate  109  to the substrate  101 . Further, the fine particles  106  are fixed on the surface of the second electrode  105  by the filler  201 . After curing the sealing material  108  and the filler  201 , the device is divided into arbitrary size ( FIG. 2D ). 
     Embodiment Mode 3 
     Embodiment Mode 3 will be described with reference to  FIGS. 3A to 3C . This embodiment mode will also describe a light emitting device having a solid sealing structure similar to Embodiment Mode 2. 
     By the process described in Embodiment Mode 1, a substrate  101  over which light emitting elements each including a first electrode  103 , a light emitting layer  104 , and a second electrode  105  are formed over a support  102  is prepared. In addition, before spraying fine particles, a sealing material  108  is provided on a perimeter of the substrate  101  as described in Embodiment Mode 1 ( FIG. 3A ). 
     An uncured (liquid-phase) filler  302  in which fine particles  106  are dispersed is prepared. As a material of the filler  302 , similar materials to those of the filler  201  of Embodiment Mode 2 can be used. In a region surrounded by the sealing material  108 , the uncured filler  302  in which the fine particles  106  are dispersed is dropwised ( FIG. 3B ). 
     The substrate  109  is provided over the substrate  101 . Then, the substrate  101  is left at rest so that as many fine particles  106  as possible in the filler  302  are in contact with a surface of the second electrode  105 . Then, while applying pressure to the substrate  101  and the substrate  109 , the sealing material  108  and the filler  302  are cured by irradiating UV light or heating to give a light emitting device having a solid sealing structure ( FIG. 3C ). 
     In this embodiment mode, in order to provide the fine particles  106  on the surface of the second electrode  105 , the fine particles  106  are dispersed in the material of the filler  302 , and the filler  302  is dropwised on the surface of the second electrode  105 . In a light emitting device of this embodiment mode, the fine particles  106  are also dispersed in the filler  302 , which distinguishes this embodiment mode from the Embodiment Mode 2. 
     Embodiment Mode 4 
     Embodiment Mode 4 will be described with reference to  FIGS. 4A to 4D . This embodiment mode will describe a light emitting device having a solid sealing structure. In Embodiment Mode 3, the filler in which the fine particles are dispersed is dropwised on the substrate side where the light emitting elements are provided. On the other hand, this embodiment mode will describe an example in which the filler is dropwised onto another substrate for sealing. 
     A sealing material  108  is provided with a predetermined shape on a perimeter of a substrate  109  by a printing method, a dispensing method, or the like ( FIG. 4A ). 
     An uncured (liquid-phase) filler  312  in which fine particles  106  are dispersed is prepared. A material of the filler  312  is similar to that of the filler  201  of Embodiment Mode 2. Into a region surrounded by the sealing material  108 , the uncured filler  312  in which the fine particles  106  are dispersed is dropwised ( FIG. 4B ). 
     By the process described in Embodiment Mode 1, a substrate  101  over which light emitting elements each including a first electrode  103 , a light emitting layer  104 , and a second electrode  105  are formed over a support  102  is prepared. The substrate  101  is provided over the substrate  109  ( FIG. 4C ). 
     After providing the substrate  101  over the substrate  109 , the top and bottom sides are reversed so that the substrate  101  is set below the substrate  109 . Then, the substrate  101  is left at rest so that the fine particles  106  in the filler  312  are precipitated. Then, while applying pressure to the substrate  101  and the substrate  109 , the sealing material  108  and the filler  312  are cured by irradiating UV light or heating to give a light emitting device having a solid sealing structure ( FIG. 4D ). 
     Note that, as described in Embodiment Modes 2-4, in the solid sealing structure in which the sealing material is provided on the perimeter, the cured filler need not necessarily fill the entire space which is surrounded by the sealing material, as long as the cured filler covers at least the region provided with the light emitting elements (the region provided with the light emitting layer  104  or the second electrode  105 ) over the substrate  101 . 
     Embodiment Mode 5 
     Embodiment Mode 5 will be described with reference to  FIGS. 5A to 5C . This embodiment mode will describe a light emitting device having a solid sealing structure. Embodiment Modes 2-4 describe the solid sealing structure in which a solid, prepared by curing a liquid-phase material, is provided. This embodiment mode will describe a solid sealing structure using a solid which is formed by curing a sheet-like (film-like) sealing material provided over a film base. 
     As described in Embodiment Mode 1, a substrate  101  over which fine particles  106  are sprayed on a surface of a second electrode  105  is prepared ( FIG. 5A ). 
     In order to firmly attach a substrate  109  to the substrate  101 , a sheet-like sealing material  501  is prepared. The uncured sheet-like sealing material  501  is a sheet-like sealing material formed of a resin material having an adhesive function. A UV light curable resin, a visible light curable resin, or a heat-curable resin can be used as the resin material. In order to protect adhesive surfaces, each of the surfaces is covered with a film base  502 . The film base  502  on one surface of the sealing material  501  is peeled, and this surface is placed over the surface of the substrate  101  ( FIG. 5B ). 
     The film base on the other surface is next peeled off. Then, the substrate  109  is placed over the substrate  101 . While applying pressure to the substrate  101  and the substrate  109 , the sheet-like sealing material  501  is cured by irradiating UV light or heating, and the substrate  109  is firmly fixed to the substrate  101 . Furthermore, the fine particles  106  are firmly fixed on the second electrode  105  by the cured sealing material  501  ( FIG. 5C ). 
     By using the sheet-like sealing material  501  in this manner, effects such as firmly fixing the substrate  109  to the substrate  101 , forming a light emitting device having a solid sealing structure, and fixing the fine particles  106  can be obtained. 
     In the step shown in  FIG. 5B , the sheet-like sealing material  501  can be provided not over the substrate  101  but on the sealing substrate  109  side as well. In this case, instead of spraying the fine particles  106  on the surface of the second electrode, the fine particles  106  can be sprayed on the surface of the sealing material  501  provided on the substrate  109 . 
     Embodiment Mode 6 
     Embodiment Mode 6 will be described with reference to  FIGS. 6A to 6D . Similarly to Embodiment Mode 5, this embodiment mode will describe a light emitting device having a solid sealing structure which uses a sheet-like sealing material. 
     An uncured sheet-like sealing material  511  is prepared. The uncured sheet-like sealing material  511  is formed of a resin layer having an adhesive function and each of surfaces of the sealing material  511  is covered with a film base  512 . As the resin layer forming the sheet-like sealing material  511 , a UV light curable resin, a visible light curable resin, or a heat-curable resin is used ( FIG. 6A ). 
     The film base  512  on one surface of the sealing material  511  is peeled off, and fine particles  106  are provided on the surface. The fine particles  106  are provided on the one surface of the sealing material  511  by using a dry-type or a wet type spray method as described in Embodiment Mode 1 or a printing method such as a gravure printing method, so that the sheet-like sealing material  511  to which the fine particles  106  are attached is prepared ( FIG. 6B ). 
     As described in Embodiment Mode 1, a substrate  101  over which light emitting elements are formed is prepared. Over a surface of this substrate  101 , the sheet-like sealing material  511  to which the fine particles  106  are attached is placed. At this time, a surface of the sealing material  511  on which the fine particles  106  are provided is made to be in contact with a second electrode  105  ( FIG. 6C ). 
     The other film base  512  is peeled from the sealing material  511 , and a substrate  109  is placed over the surface. While applying pressure to the substrate  101  and the substrate  109 , the sheet-like sealing material  511  is cured by UV light irradiation or heating, and the substrate  109  is firmly fixed to the substrate  101  ( FIG. 6D ). 
     The substrate  109  can be placed over the substrate  101  as well after providing the sealing material  511  having the fine particles  106  over a surface of the substrate  109 . At this time, the surface of the sealing material  511 , to which the fine particles  106  are not sprayed, is put on the substrate  109  side. 
     The sheet-like sealing material  511  having the fine particles  106  shown in  FIG. 6B  has an effect improving light extraction efficiency of light emitting elements, as well as effects such as firmly fixing the substrate  109  to the substrate  101 , forming a light emitting device having a solid sealing structure, and fixing the fine particles  106 . Thus, a sheet-like sealing material having fine particles is very useful as a component of a light emitting device in which light generated from a light emitting element is extracted from the top side of the light emitting element. 
     In the case where a light emitting device having a solid sealing structure is formed using a sheet-like sealing material as shown in Embodiment Modes 5 and 6, the sheet-like sealing material need not necessarily cover the entire surface of the substrate  101  or the substrate  109 . It is acceptable as long as the sheet-like sealing material covers at least a region in which light emitting elements are provided over the substrate  101  (region in which a light emitting layer  104  or the second electrode  105  is provided). 
     Embodiment Mode 7 
     Embodiment Mode 7 will be described with reference to  FIGS. 7 to 8C . This embodiment mode will demonstrates a light emitting device including light emitting elements in which fine particles are interposed between a second electrode and a transparent conductive film. 
     As described in Embodiment Mode 1, a substrate  101  over which fine particles  106  are sprayed on a surface of a second electrode  105  is prepared. 
     The surface of the second electrode  105 , where the fine particles  106  are provided, is formed of a transparent conductive film. Over this transparent conductive film, a protective film  601  is formed. Accordingly, the structure is obtained, where the fine particles  106  are sandwiched between the transparent conductive film which covers the surface of the second electrode  105  and the protective film  601 . Thus, compared with the structure which does not include the protective film  601 , the fine particles  106  are more tightly fixed to the surface of the second electrode ( FIG. 7 ). 
     As a material of the protective film  601 , one can select a material having a refractive index which is equal to or higher than that of the transparent conductive film that covers the surface of the second electrode  105 . This is for suppressing total reflection at an interface between the second electrode  105  and the protective film  601 . Specifically, the material of the protective film  601  can be selected from the materials employed for the transparent conductive film described in Embodiment Mode 1. 
     For example, the transparent conductive film described in Embodiment Mode 1 is formed by the protective film  601 . Such a transparent conductive film can be formed by a sputtering method or a vapor deposition method. 
     Further, for the protective film  601 , as well as the transparent conductive film, silicon oxide (SiO y , 0&lt;y≦2), silicon nitride (SiN x , 0&lt;x≦4/3), silicon nitride oxide (SiN x O y , 0&lt;x&lt;4/3, 0&lt;y&lt;2, 0&lt;3x+2y≦4), DLC, aluminum nitride, or the like can be used. Such a film can be formed by a CVD method, a sputtering method, or a vapor deposition method. In the case of forming silicon oxide, silicon nitride, silicon nitride oxide, or the like by a plasma CVD method for example, adjustment of the refractive index of the protective film  601  can be performed by adjusting the relative permittivity of a stacked film, which is achieved by changing ratio of source gases, kind of source gases, or processing temperature. 
     Optical design of a light emitting device is readily performed by arranging the refractive index of the second electrode  105  to be equal to that of the protective film  601 , which is realized by using the same transparent conductive film as the surface of the second electrode  105  for the protective film  601 . The use of a silicon nitride film having a lower moisture-permeability than that of the transparent conductive film or a silicon nitride oxide film having a lower moisture-permeability than that of the transparent conductive film is advantageous in suppressing deterioration of a light emitting element caused by moisture. Note that the silicon nitride oxide film has a higher proportion of nitrogen than that of oxygen. 
     When the refractive index of the protective film  601  is equal to that of the second electrode, projections and depressions are also made on a surface of the protective film  601  by utilizing the fine particles  106  in order to suppress total reflection of light which passes the protective film  601 . For example, the size of the fine particles  106  is increased to achieve this purpose. In the case where the refractive index of the protective film  601  is higher than that of the second electrode  105 , the projections and depressions made by the fine particles  106  on the surface of the protective film  601  are not necessarily formed prominently. 
     Sealing of the light emitting elements is conducted by fixing the substrate  109  to the substrate  101  as described in Embodiment Modes 1, 2, and 5. Light emitting devices on which the sealing processes of Embodiment Modes 1, 2, and 5 are conducted are shown in  FIGS. 8A to 8C .  FIG. 8A  corresponds to Embodiment Mode 1,  FIG. 8B  corresponds to Embodiment Mode 2, and  FIG. 8C  corresponds to Embodiment Mode 5. 
     Embodiment Mode 8 
     Embodiment Mode 8 will be described with reference to  FIGS. 9 to 11 . This embodiment mode will describe a light emitting device including light emitting elements in which a protective film is provided over a second electrode. 
     By the process described in Embodiment Mode 1, a substrate  101  over which light emitting elements each including a first electrode  103 , a light emitting layer  104 , and a second electrode  105  are formed is prepared. Then, a protective film  611  is formed in contact with a surface of the second electrode  105 . Then, fine particles  106  are provided over the protective film  611 . To provide the fine particles  106 , similarly to Embodiment Mode 1, the fine particles  106  may be sprayed by a dry method or a wet method. 
     A film having a high transmittance to visible light is used as the protective film  611 . Specifically, silicon oxide (SiO y , 0&lt;y≦2), silicon nitride (SiN x , 0&lt;x≦4/3), silicon nitride oxide (SiN x O y , 0&lt;x&lt;4/3, 0&lt;y&lt;2, 0&lt;3x+2y≦4), DLC, aluminum nitride, or the like can be used. A formation method of the protective film  611  is selected from a vapor deposition method, a sputtering method, a plasma CVD method, a coating method of a material solution which is prepared by dissolving a material in a solvent, and the like, depending on the material of the protective film  611 . 
     In order to prevent the total reflection at an interface between the second electrode  105  and the protective film  611 , a material having a refractive index which is equal to or higher than that of the second electrode  105  is preferably selected as the material of the protective film  611 . In the case of forming silicon oxide, silicon nitride, silicon nitride oxide, or the like by a plasma CVD method for example, adjustment of the refractive index of the protective film  611  can be performed by adjusting the relative permittivity of a stacked film, which is achieved by changing ratio of source gases, kind of source gases, or processing temperature. 
     For the fine particles  106 , a material having a refractive index which is equal to or higher than that of the second electrode  105  is preferably selected in order to prevent total reflection at the interface between the second electrode  105  and the protective film  611 . 
     Next, as described in Embodiment Modes 1, 2, and 5, the substrate  109  is firmly attached to the substrate  101 . Note that sealing can also be performed with a sheet-like sealing material to which the fine particles  106  are attached, as described in Embodiment Mode 6. Light emitting devices on which the sealing processes of Embodiment Modes 1, 2, 5, and 6 are conducted are shown in  FIGS. 10A to 10C .  FIG. 10A  corresponds to Embodiment Mode 1,  FIG. 10B  corresponds to Embodiment Mode 2, and  FIG. 10C  corresponds to Embodiment Modes 5 and 6. 
     Instead of spraying the fine particles  106 , a method of dropwising an uncured filler in which fine particles are dispersed can be employed as shown in Embodiment Modes 3 and 4. A light emitting device fabricated using the method of Embodiment Modes 3 and 4 is illustrated in  FIG. 11 . 
     A light emitting element of this embodiment mode has improved extraction efficiency of light emitted from the light emitting element, which originates from a similar principle to that described in Embodiment Mode 1. In other words, since the shape of the surface of the protective film  611  is changed by providing a plurality of fine particles  106  on a surface of the protective film  611  on the light extraction side, light having an incident angle, which usually leads total reflection of the light at an interface between the second electrode  105  and the fine particles  106 , is not totally reflected, and the light is refracted and scattered by the protective film  611 , allowing the light to pass the fine particles  106 . Thus, by providing the plurality of fine particles in contact with the surface of the protective film  611 , the amount of light which is totally reflected at the interface between the second electrode  105  and the protective film  611  is reduced. Accordingly, light extraction efficiency is improved. 
     Embodiment Mode 9 
     Embodiment Mode 9 will be described with reference to  FIGS. 12A to 12C .  FIG. 1  shows an example where the fine particles  106  are polyhedral and have different shapes and sizes. Effects of lens and a prism become significant depending on the shape of the fine particles. For example, as shown in  FIG. 12A , fine particles  701  are made to be spherical. By passing the spherical fine particles  701 , the light which passes through the second electrode  105  can be concentrated. Note that in the case of solid sealing, the spherical fine particles  701  are fixed in a state that the fine particles  701  are pressed to the surface of the second electrode  105  by the pressure applied when firmly attaching the substrate  101  to the substrate  109 . 
     As shown in  FIG. 12B , the shape of fine particles  702  are allowed to possess a triangular pyramid shape or a triangular pole shape, giving an effect of a prism to the fine particles  702 . Light is scattered by passing the fine particles  702 , and the viewing angle can be increased. Further, by passing the spherical fine particles  701 , the light which passes through the second electrode  105  can be concentrated. 
     As shown in  FIG. 12C , both the spherical fine particles  701  and the fine particles  702  having a triangular pyramid shape or a triangular pole shape may also be employed simultaneously. 
     Although the fine particles  701  and  702  have unequal sizes in  FIGS. 12A to 12C , they may have the same size. Further, the structure shown in Embodiment Mode 2 is employed as an example of a structure of a light emitting device in  FIGS. 12A to 12C , and any structure of other embodiment modes can be employed as well. 
     Embodiment Mode 10 
     Embodiment Mode 10 will be described with reference to  FIGS. 13 to 16 . In this embodiment mode, an example of using an active matrix EL panel having a display function as a light emitting device will be described. 
       FIG. 13  is an exemplary illustrations of an active matrix EL panel when seen from the top. A sealing substrate  801  is firmly fixed to a substrate  800  with a sealing material  802 . A space between the substrate  800  and the sealing substrate  801  is air-sealed. Further, the sealing structure of the EL panel is a solid sealing structure in this embodiment mode, and this space is filled with a filler made of a resin. 
     Over the substrate  800 , a pixel portion  803 , a writing gate signal line driver circuit portion  804 , an erasing gate signal line driver circuit portion  805 , and a source signal line driver circuit portion  806  are provided. The driver circuit portions  804  to  806  are connected, via a wiring group, to an FPC (flexible printed circuit)  807  which is an external input terminal. The source signal line driver circuit portion  806 , the writing gate signal line driver circuit portion  804 , and the erasing gate signal line driver circuit portion  805  receive a video signal, a clock signal, a start signal, a reset signal, and the like from the FPC  807 . In addition, a printed wiring board (PWB)  808  is attached to the FPC  807 . 
     Transistors in the pixel portion  803  and the driver circuit portions  804  to  806  are constructed by thin film transistors (TFTs). Note that the driver circuit portions  804  to  806  need not necessarily be provided over the same substrate  800  as the pixel portion  803 , unlike the example described above. For example, the driver circuit portions  804  to  806  may be provided outside the substrate by utilizing a TCP (tape carrier package) in which an IC chip is mounted on an FPC on which a wiring pattern is formed. A part of the driver circuit portions  804  to  806  may be provided over the substrate  800 , and another part of them may be provided outside the substrate  800 . 
       FIG. 14  is a view of circuits for operating one pixel. A plurality of pixels is planarly arranged in the pixel portion  803 . In one pixel, a first transistor  811 , a second transistor  812 , and a light emitting element  813  are included. Further, a source signal line  814  and a current supply line  815  which extend in columns and a gate signal line  816  which extends in a row are provided. The light emitting element  813  is an EL element having a top emission structure, and light is extracted from the substrate  801  side. 
     Each of the first transistor  811  and the second transistor  812  is a three-terminal element including a gate electrode, a drain region, and a source region, and a channel region is included between the source region and the drain region. Here, since a region serving as the source region and a region serving as the drain region are changed depending on a structure of a transistor, an operational condition, and the like, it is difficult to determine which region is the source region or the drain region. Therefore, in this specification, three terminals of the transistor are referred to as a gate electrode, a first electrode, and a second electrode for being distinguished. 
     In the writing gate signal line driver circuit portion  804 , the gate signal line  816  is electrically connected to the writing gate signal line driver circuit  819  via a switch  818 . By controlling the switch  818 , whether the gate signal line  816  is electrically connected to the writing gate signal line driver circuit  819  or not is selected. 
     In the erasing gate signal line driver circuit portion  805 , the gate signal line  816  is electrically connected to an erasing gate signal line driver circuit  821  via a switch  820 . By controlling the switch  820 , whether the gate signal line  816  is electrically connected to the erasing gate signal line driver circuit  821  or not is selected. 
     In the source signal line driver circuit portion  806 , the source signal line  814  is electrically connected to either a source signal line driver circuit  823  or a power source  824  by a switch  822 . 
     The first transistor  811  includes the gate electrode electrically connected to the gate signal line  816 , the first electrode electrically connected to the source signal line  814 , and the second electrode electrically connected to the gate electrode of the second transistor  812 . 
     The second transistor  812  includes the gate electrode electrically connected to the second electrode of the first transistor as described above, the first electrode electrically connected to the current supply line  815 , and the second electrode electrically connected to a first electrode of the light emitting element  813 . A second electrode of the light emitting element  813  has a constant potential. 
     The structure of a pixel of this embodiment mode will be described with reference to  FIG. 15 . Since this embodiment mode shows the case of the EL panel having a solid sealing structure, the air-sealed space between the substrate  800  and the sealing substrate  801  is filled with a filler  830  made of a resin. Over the substrate  800 , a support  831  and the light emitting element  813  are formed. As the support  831 , the first transistor  811  and the second transistor  812  shown in  FIG. 14  are formed over a base film  832 . An interlayer insulating film  833  is formed over the first transistor  811  and the second transistor  812 . The light emitting element  813  and an insulating layer  834  serving as a partition wall are formed over the interlayer insulating film  833 . 
     Each of the first transistor  811  and the second transistor  812  is a top-gate thin film transistor in which a gate electrode is provided on the side opposite to the substrate with a semiconductor layer, where a channel formation region is formed, as a center. The structure of the thin film transistors of the first transistor  811  and the second transistor  812  is not particularly limited, and for example, a bottom-gate type may be used. In the case of the bottom-gate type, a protective film may be formed over a semiconductor layer where a channel is formed (channel protective type); alternatively, a part of a semiconductor layer where a channel is formed may have a concave shape (channel etch type). 
     Further, the semiconductor layer where the channel formation region is formed, of the first transistor  811  and the second transistor  812  may be formed of either a crystalline semiconductor or an amorphous semiconductor. 
     As specific examples of the crystalline semiconductor when the semiconductor layer is formed of a crystalline semiconductor, materials which contain single crystalline or polycrystalline silicon, germanium silicon, or the like can be used. These materials may be formed by laser crystallization or a crystallization by a solid-phase growth method using, for example, nickel or the like. 
     In the case where the semiconductor layer is formed of an amorphous semiconductor, for example, amorphous silicon, it is preferable that all thin film transistors forming the pixel portion  803  are n-channel type. In other cases, either or both of an n-channel transistor and a p-channel transistor may be formed in the pixel portion  803 . 
     The same as the first transistor  811  and the second transistor  812  of the pixel portion  803  can be applied to transistors used in the driver circuit portions  804  to  806 . In accordance with the performance of transistors, it is selected whether all the driver circuit portions  804  to  806  are formed of thin film transistors or whether a part of the driver circuit portions is formed of thin film transistors and the other is formed of an IC chip. The transistors of the driver circuit portions  804  to  806  may be either or both of an n-channel type and a p-channel type. 
     In  FIG. 15 , the light emitting element  813  includes a light emitting layer  837  between a first electrode  835  and a second electrode  836 . Over the interlayer insulating film  833 , the first electrode  835 , the light emitting layer  837 , and the second electrode  836  are sequentially stacked. The first electrode  835  is a reflective electrode and serves as a cathode. The second electrode  836  is a light-transmitting electrode and serves as an anode. Light emitted from the light emitting layer  837  is extracted from the second electrode  836 . 
     The first electrode  835  is connected to the second electrode of the transistor  812  by a contact hole provided in the interlayer insulating film  833 . 
     A plurality of fine particles  838  is provided in contact with a surface of the second electrode  836 . By this fine particles, the amount of total reflection of light which enters an interface between the second electrode  836  and the filler  830  is reduced. Accordingly, light extraction efficiency of the light emitting element  813  can be improved. 
     The solid sealing structure described in Embodiment Mode 2 is employed as a sealing structure of the EL panel in this embodiment mode; however, any sealing structure of other embodiment modes can be employed obviously. 
     A driving method of an EL panel of this embodiment mode will be described with reference to  FIG. 16 .  FIG. 16  shows operation of a frame in accordance with the passage of time. In  FIG. 16 , the horizontal direction indicates the passage of time, while the vertical direction indicates the number of scanning stages of a gate signal line. 
     When an image is displayed with an EL panel of this embodiment mode, rewriting operations and displaying operations of a screen are carried out repeatedly in the display period. There is no particular limitation on the number of rewriting operations; however, the rewriting operations are preferably performed about 60 times or more in a second so that a person who watches a displayed image does not sense a flicker in the image. Here, a period of the rewriting and displaying operations for one screen (one frame) is referred to as one frame period. 
     One frame period is time-divided into four sub-frames  841 ,  842 ,  843 , and  844  including address periods  841   a ,  842   a ,  843   a , and  844   a  and sustain periods  841   b ,  842   b ,  843   b , and  844   b , respectively. The light emitting element to which a signal for light emission is applied is in a light emitting state during the sustain periods. The length ratio of the sustain periods of the sub-frames, the first sub-frame  841 : the second sub-frame  842 : the third sub-frame  843 : the fourth sub-frame  844 , satisfies 2 3 :2 2 :2 1 :2 0 =8:4:2:1. This allows the light emitting element to display a 4-bit gray scale. The number of bits and the gray scales are not limited to those shown in this embodiment mode. For example, one frame period may include eight sub-frames so as to display a 8-bit gray scale. 
     The operation of one frame period will be described. First, in the sub-frame  841 , the writing operation is performed sequentially from a first row to a last row. Therefore, the starting time of the writing period varies depending on the row. The sustain period  841   b  sequentially starts in the rows in which the address period  841   a  has been terminated. In the sustain period  841   b , the light emitting element applied with a signal for light emission remains in a light emitting state. The sub-frame  841  is changed to the next sub-frame  842  sequentially in the rows in which the sustain period  841   b  has been terminated. In the sub-frame  842 , a writing operation is performed sequentially from the first row to the last row, in the same manner as in the case of the sub-frame  841 . 
     The above-mentioned operations are carried out repeatedly up to the sustain period  844   b  of the sub-frame  844 , and are then terminated. After terminating the operation of the sub-frame  844 , an operation in the next frame is started. Accordingly, the sum of the light-emitting time in all the sub-frames corresponds to the light emitting time of each light emitting element in one frame period. By varying the light emitting time for each light emitting element and combining the light emitting elements in various ways within one pixel, various display colors with differing brightness and differing chromaticity can be formed. 
     When a sustain period is intended to be forcibly terminated in the row in which the writing operation has already been terminated and the sustain period has started, prior to terminating the writing operation up to the last row as in the sub-frame  844 , an erasing period  844   c  is preferably provided after the sustain period  844   b  so as to stop light emission forcibly. The row where light emission is forcibly stopped does not emit light for a certain period (this period is referred to as a non-light emitting period  844   d ). Right after terminating the address period in the last row, an address period of a next sub-frame (or a next frame) starts sequentially from the first row. This can prevent the address period in the sub-frame  844  from overlapping with the address period in the next sub-frame. 
     Although the sub-frames  841  to  844  are arranged in order from the longest to the shortest length of the sustain period in this embodiment mode, they do not necessarily have to be arranged in this order. For example, the sub-frames may be arranged in order from the shortest length of the sustain period to the longest. Alternatively, the sub-frames may be arranged in random order regardless of the length of the sustain period. In addition, these sub-frames may further be divided into a plurality of frames. In other words, scanning of gate signal lines may be performed a plurality of times during a period of supplying the same video signal. 
     The operations in the address period and the erasing period of the circuit shown in  FIG. 14  will be described. First, the operation in the address period is described. In the address period, the gate signal line  816  in the n-th row (n is a natural number) is electrically connected to the writing gate signal line driver circuit  819  via the switch  818 , and is not connected to the erasing gate signal line driver circuit  821  by the switch  820 . 
     The source signal line  814  is electrically connected to the source signal line driver circuit  823  via the switch  822 . In this case, a signal is input to the gate of the first transistor  811  connected to the gate signal line  816  in the n-th row (n is a natural number), thereby turning the first transistor  811  on. At this time, video signals are simultaneously input to the source signal lines  814  in the first to the last columns. Further, the video signals input from each source signal line  814  are independent in columns from one another. 
     The video signal input from the source signal line  814  is input to the gate electrode of the second transistor  812  via the first transistor  811  connected to each source signal line  814 . At this time, it is determined whether the light emitting element  813  emits light or not depending on the current value of the signal that is input to the second transistor  812 . For instance, when the second transistor  812  is a p-channel type, the light emitting element  813  emits light by inputting a low level signal to the gate electrode of the second transistor  812 . On the other hand, when the second transistor  812  is an n-channel type, the light emitting element  813  emits light by inputting a high level signal to the gate electrode of the second transistor  812 . 
     Next, the operation in the erasing period will be described. In the erasing period, the gate signal line  816  in the n-th row (n is a natural number) is electrically connected to the erasing gate signal line driver circuit  821  via the switch  820 , and is not connected to the writing gate signal line driver circuit  821  by the switch  818 . The source signal line  814  is electrically connected to the power source  824  via the switch  822 . In this case, by inputting a signal to the gate of the first transistor  811  connected to the gate signal line  816  in the n-th row, the first transistor  811  is turned on. At this time, erasing signals are simultaneously input to the source signal lines  814  in the first to the last columns. 
     The erasing signal input from the source signal line  814  is input to the gate electrode of the second transistor  812  via the first transistor  811  connected to the source signal line  814 . Then, the supply of a current flowing from the power supply line  815  to the light emitting element  813  is stopped by the signal input to the second transistor  812 . This forcibly makes the light emitting element  813  emit no light. For example, when the second transistor  812  is a p-channel type, the light emitting element  813  emits no light by inputting a high level signal to the gate electrode of the second transistor  812 . On the other hand, when the second transistor  812  is an n-channel type, the light emitting element  813  emits no light by inputting a low level signal to the gate electrode of the second transistor  812 . 
     In the erasing period, a signal for erasing is input to the n-th row (n is a natural number) by the above-mentioned operation. However, as mentioned above, the n-th row sometimes remains in the erasing period while another row (referred to as an m-th row (m is a natural number)) is in the writing period. In this case, since a signal for erasing is necessary to be input to the n-th row and a signal for writing is necessary to be input to the m-th row by utilizing the source signal line  814  in the same column, the operation mentioned below is preferably carried out. 
     Right after the light emitting element  813  in the n-th row stops emitting light by the above-described operation in the erasing period, the gate signal line  816  and the erasing gate signal line driver circuit  821  are disconnected from each other, while the source signal line  814  is connected to the source signal line driver circuit  823  by switching the switch  822 . Then, the gate signal line  816  and the writing gate signal line driver circuit  819  are connected to each other by the switch  818 . Then, a signal is selectively input to the gate signal line  816  in the m-th row from the writing gate signal line driver circuit  819 , and the first transistor  811  is turned on. Meanwhile, signals for writing are input to the source signal lines  814  in the first to the last columns from the source signal line driver circuit  823 . The light emitting element in the m-th row emits light or no light depending on the signal. 
     After terminating the address period in the m-th row as mentioned above, the erasing period immediately starts in the (n+1)-th row. Therefore, the gate signal line  816  and the writing gate signal line driver circuit  819  are disconnected from each other by the switch  818 , and the gate signal line  816  is connected to the erasing gate signal line driver circuit  821  by switching the switch  820 . In addition, the source signal line  814  is connected to the power source  824  by switching the switch  822 . Then, a signal is input to the gate signal line  816  in the (n+1)-th row from the erasing gate signal line driver circuit  821  to turn on the first transistor  811 , while an erasing signal is input from the power source  824 . Similarly, an erasing period and an address period are repeated alternately up to the erasing period of the last row. 
     Embodiment Mode 11 
     Reduction of the power consumption of the light emitting devices described in Embodiment Modes 1 to 8 can be realized by improving light extraction efficiency of the light emitting element. Accordingly, by mounting these light emitting devices as a display portion, vivid and bright display with low power consumption can be performed. 
     Therefore, the light emitting devices of Embodiment Modes 1 to 9 can be favorably used for a display portion of a battery-powered electronic device, a display device with a large-sized screen, or a display portion of an electronic device. The following can be given as examples: a television device (a TV or a television receiver), a camera such as a digital camera or a digital video camera, a cellular phone device (a cellular phone handset), a portable information terminal such as PDA, a portable game machine, a monitor, a computer, an audio reproducing device such as a car audio, an image reproducing device provided with a recording medium such as a home game machine, and the like. Specific examples thereof will be described with reference to  FIGS. 17A to 17F . A light emitting device used in a display portion may have either an active matrix type or a passive type. 
     A light emitting device is used in a display portion  911  of a portable information terminal device shown in  FIG. 17A . 
     A light emitting device is used in a finder  914  and a display portion  913  for displaying a taken image in a digital video camera shown in  FIG. 17B . 
     A light emitting device can be applied to a display portion  915  of a cellular phone handset shown in  FIG. 17C . 
     The light emitting device of the above-described embodiment mode is used in a display portion  916  of a portable television device shown in  FIG. 17D . 
     The light emitting device of the above-described embodiment mode can be applied to a display portion  917  of a notebook or laptop computer shown in  FIG. 17E . 
     The light emitting device of the present invention can be applied to a display portion  918  of a television device shown in  FIG. 17F . Note that the light emitting device of the above-described embodiment mode can be applied to display portions of television devices with various screen sizes including a small television device mounted on a portable terminal such as the cellular phone handset shown in  FIG. 17D , a medium television device which is portable, and a large (for example, 40-inch or larger) television device. 
     Embodiment Mode 12 
     Embodiment Mode 12 will describe a mode where a light emitting device is applied to a planar lighting device. The light emitting devices of Embodiment Modes 1 to 9 can be used in a planar lighting device as well as in a display portion. For example, in the case of using a liquid crystal panel in a display portion of an electronic device exemplified in the above-described embodiment mode, the light emitting device of the above-described embodiment mode can be mounted as a backlight of the liquid crystal panel. In the case of using the light emitting device as a lighting device, a passive light emitting device is preferably used. 
       FIG. 18  shows an example of a liquid crystal display device using the light emitting device as a backlight. The liquid crystal display device shown in  FIG. 18  includes a housing  921 , a liquid crystal layer  922 , a backlight  923 , and a housing  924 , and the liquid crystal layer  922  is connected to a driver IC  925 . The light emitting device of the present invention is used for the backlight  923 , and current is supplied through a terminal  926 . 
     A liquid crystal display device including the backlight of this embodiment mode can be used for display portions of various electronic devices as described in Embodiment Mode 11. 
     By using the light emitting device to which the present invention is applied as the backlight of the liquid crystal display device, a backlight with brightness and reduced power consumption can be obtained. The light emitting device to which the present invention is applied is a lighting device with plane emission, and can have a large area. Therefore, the backlight can have a large area, and the liquid crystal display device can have a large area, too. Furthermore, the light emitting device has a thin shape and consumes low power; therefore, a thinner shape and lower power consumption of a display device can also be achieved. 
     This application is based on Japanese Patent Application serial no. 2006-057154 filed in Japan Patent Office on Mar. 3, 2006, the entire contents of which are hereby incorporated by reference.