Patent Publication Number: US-8541790-B2

Title: Lighting device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lighting device using a light emitting element. 
     2. Description of the Related Art 
     A light emitting element is a self light emitting element and is attempted to be used as a lighting device. The light emitting element is a surface-emitting body, and a lighting device capable of emitting near-natural light can be obtained by using the light emitting element for lighting. 
     The light emitting element has an anode, a cathode, and a layer including a light emitting material which provides luminescence (Electroluminescence) by applying an electric field thereto. A hole injected from the anode is combined with an electron injected from the cathode in the layer including a light emitting material, thereby obtaining luminescence. Luminescence obtained from the layer including a light emitting material includes luminescence (fluorescence) that is obtained in returning from a singlet excited state to a ground state and luminescence (phosphorescence) that is obtained in returning from a triplet excited state to a ground state. 
     In a light emitting device using such a light emitting element, an electrode in the direction of light emission needs to be transparent. However, a transparent conductive film which is typically used as a transparent electrode often has relatively high resistivity, and voltage drop is caused in a portion away from a current supply terminal. Specifically, the lighting device is often intended to emit light from the entire surface at the same luminance; therefore, in-plane nonuniformity of luminance becomes further noticeable. 
     When the lighting device has large area, luminance is lowered in a portion through which current is hard to flow. In other words, luminance is nonuniform in a light emitting region of the lighting device. An ITO electrode typically used as the anode has higher electrical resistance than that of metal such as Al used as the cathode. Therefore, voltage drop is caused in a portion away from a current supply terminal, which results in low luminance. In order to solve the problem, Reference 1 reports a structure in which at least a portion of an anode is provided with an auxiliary electrode having lower electrical resistance than that of the anode (Reference 1: Japanese Patent Laid-Open No. 2004-134282). 
     In Reference 1, an auxiliary electrode is provided on a long side or a short side of a light emitting element. When the lighting device has larger area, luminance is lowered in a portion away from the auxiliary electrode (for example, in a central portion of the lighting device). However, the auxiliary electrode can only be partly provided, because light emitted from a light emitting layer cannot be extracted outside when the auxiliary electrode is entirely provided on the anode in the structure of Reference 1. 
     Since the lighting device is often intended to emit light from the entire surface at the same luminance, nonuniformity of luminance becomes further noticeable. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problem, it is an object of the invention to provide a lighting device having favorable luminance uniformity in a light emitting region when the lighting device has large area. 
     One feature of a lighting device of the invention is that a layer including a light emitting material is formed between a first electrode and a second electrode, and a third electrode is formed to connect to the first electrode through an opening formed in the second electrode and the layer including a light emitting material. 
     In other words, the lighting device has a layer including a light emitting material and a second electrode between a first electrode and an auxiliary electrode; the auxiliary electrode is formed opposite to the first electrode with the second electrode therebetween; and the first electrode is electrically connected to the auxiliary electrode through an opening formed in the second electrode and the layer including a light emitting material. Note that the first electrode and the second electrode need to be electrically insulated from each other and the second electrode and the auxiliary electrode need to be electrically insulated from each other. 
     More specifically, another feature of the invention is that a layer including a light emitting material provided with a first opening and a second electrode provided with a second opening are arranged over a first electrode formed of a transparent conductive film so that the second opening overlaps the first opening; and an insulating layer formed over the second electrode, covering the first opening, the second opening, and the side of the second opening, and provided with a third electrode to expose the first electrode and a third electrode formed over the insulating layer to be in contact with the first electrode through the first to third openings are formed. 
     Another feature of the invention according to the above structure is that a plurality of openings is formed in a light emitting region of a lighting device. 
     In the above structure, light emitted from the layer including a light emitting material is emitted from the first electrode side. In other words, the first electrode transmits light and is formed of a transparent conductive film. Specifically, indium tin oxide (hereinafter referred to as ITO), indium tin oxide containing silicon, indium oxide containing zinc oxide (ZnO) of 2% to 20%, or the like can be used. 
     Note that a low-resistivity material is preferably used as the auxiliary electrode. An effect of voltage drop due to relatively high resistivity of the first electrode can be reduced by using a low-resistivity material. 
     In the above structure, the layer including a light emitting material may have a laminated structure of a plurality of layers each including a light emitting material. 
     Further in the above structure, a substrate for supporting the lighting device may be a flexible substrate. 
     In the above structure, no light is lost except in a connection portion of the auxiliary electrode and the first electrode because the auxiliary electrode is not placed in the direction of light emission. Therefore, a material, a thickness, or a formation position of the auxiliary electrode can be freely set. 
     In the case where the area of the connection portion of the auxiliary electrode and the first electrode is sufficiently small, the presence of the auxiliary electrode can be, for the most part, disregarded even when seen from the side of light emission. Therefore, a plurality of openings can be formed in a light emitting region of the lighting device. 
     According to the invention, a lighting device having favorable in-plane uniformity of luminance can be obtained. Since an auxiliary electrode is not placed in the direction of light emission, little light is lost due to the auxiliary electrode, and a material, a thickness, or a formation position of the auxiliary electrode can be freely set. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are a cross-sectional view and a top view of a light emitting region in a lighting device of the present invention, respectively. 
         FIGS. 2A to 2D  show a method for manufacturing a lighting device of the present invention. 
         FIGS. 3A to 3C  show a method for manufacturing a lighting device of the present invention. 
         FIGS. 4A to 4C  show a method for manufacturing a lighting device of the present invention. 
         FIGS. 5A and 5B  are a cross-sectional view and a top view of a light emitting region in a lighting device of the present invention, respectively. 
         FIGS. 6A to 6C  show a method for manufacturing a lighting device of the present invention. 
         FIGS. 7A and 7B  show a method for manufacturing a lighting device of the present invention. 
         FIGS. 8A and 8B  are a top view and a cross-sectional view of a lighting device of the present invention, respectively. 
         FIG. 9  shows an example of a device using a lighting device of the present invention. 
         FIGS. 10A to 10C  show examples of a device using a lighting device of the present invention. 
         FIG. 11  shows an example of a layer including a light emitting material in a lighting device of the present invention. 
         FIG. 12  shows an example of a layer including a light emitting material in a lighting device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment modes of the present invention are described in detail with reference to drawings. However, the present invention is not limited to the following description. As is easily known to a person skilled in the art, the mode and the detail of the invention can be variously changed without departing from the purpose and the scope of the present invention. Thus, the present invention is not interpreted while limiting to the following description of the embodiment modes. 
     Embodiment Mode 1 
     A structure of a lighting device of the invention is described with reference to  FIGS. 1A and 1B . A lighting device shown in  FIGS. 1A and 1B  is a bottom emission lighting device which emits light from a substrate side. Note that  FIG. 1B  is a top view of a light emitting region in a lighting device of the invention and  FIG. 1A  is a cross-sectional view (taken along line A-A′ in  FIG. 1B ) of the vicinity of an opening in the light emitting region. 
     In  FIGS. 1A and 1B , a light transmitting substrate is used as a substrate  101 . Specifically, a light transmitting material such as glass, plastic, polyester resin, or acrylic resin can be used. The substrate  101  may be flexible. 
     A transparent conductive film is formed over the substrate  101  as a first electrode  102 . For example, indium tin oxide (hereinafter referred to as ITO), indium tin oxide containing silicon, indium oxide containing zinc oxide (ZnO) of 2% to 20%, or the like can be used for the transparent conductive film. 
     A layer including a light emitting material  103  is formed over the first electrode  102 . A known material can be used for the layer including a light emitting material  103 , and either a low molecular weight material or a high molecular weight material can be used. A material for forming the layer including a light emitting material may include not only the one formed of only an organic compound material but also the one partly containing an inorganic compound. The layer including a light emitting material is formed by appropriately combining a hole injection layer, a hole transport layer, a hole blocking layer, a light emitting layer, an electron transport layer, an electron injection layer, or the like. The layer including a light emitting material may be a single layer or have a laminated structure of a plurality of layers.  FIG. 11  shows an example of a structure in which the layer including a light emitting material includes a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer. In  FIG. 11 , a first electrode (anode)  1101 , a layer including a light emitting material  1102 , and a second electrode (cathode)  1103  are formed over a substrate  1100 . The layer including a light emitting material  1102  includes a hole injection layer  1111 , a hole transport layer  1112 , a light emitting layer  1113 , an electron transport layer  1114 , and an electron injection layer  1115 . Note that the layer including a light emitting material in a lighting device of the invention is not limited to the structure of  FIG. 11 . Hereinafter, described is a specific material used for the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the electron injection layer. 
     As a hole injection material for forming the hole injection layer, a porphyrin-based compound is effective among other organic compounds, and phthalocyanine (hereinafter referred to as H 2 -Pc), copper phthalocyanine (hereinafter referred to as Cu-Pc), or the like can be used. Further, a chemically doped high molecular weight conductive compound can be used, such as polyethylene dioxythiophene (hereinafter referred to as PEDOT) doped with polystyrene sulfonate (hereinafter referred to as PSS). A benzoxazole derivative and any one or more of TCQn, FeCl 3 , C 60 , and F 4 TCNQ may be included. 
     As a hole transport material for forming the hole transport layer, an aromatic amine-based compound (in other words, the one having a benzene ring-nitrogen bond) is preferably used. For example, N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (hereinafter referred to as TPD) or a derivative thereof such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereinafter referred to as α-NPD) is widely used. Also widely used is a star burst aromatic amine compound such as 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (hereinafter referred to as TCTA), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenyl amine (hereinafter referred to as TDATA), or 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenyl amine (hereinafter referred to as MTDATA). 
     As a light emitting material for forming the light emitting layer, various fluorescent pigments are specifically effective, in addition to a metal complex such as tris(8-quinolinolato)aluminum (hereinafter referred to as Alq 3 ), tris(4-methyl-8-quinolinolato)aluminum (hereinafter referred to as Almq 3 ), bis(10-hydroxybenzo[h]-quinolinato)beryllium (hereinafter referred to as BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (hereinafter referred to as BAlq), bis[2-(2-hydroxyphenyl)-benzoxazolate]zinc (hereinafter referred to as Zn(BOX) 2 ), or bis[2-(2-hydroxyphenyl)-benzothiazolate]zinc (hereinafter referred to as Zn(BTZ) 2 ). 
     In the case of forming the light emitting layer in combination with a guest material, the following material can be used as the guest material: quinacridon, diethyl quinacridon (hereinafter referred to as DEQD), dimethyl quinacridon (hereinafter referred to as DMQD), rubrene, perylene, coumarin, coumarin 545T (hereinafter referred to as C545T), DPT, Co-6, PMDFB, BTX, ABTX, DCM, DCJT, or a triplet light emitting material (phosphorescent material) such as tris(2-phenylpyridine)iridium (hereinafter referred to as Ir(ppy) 3 ) or 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin-platinum (hereinafter referred to as PtOEP). 
     As an electron transport material which can be used for the electron transport layer, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated to PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbreviated to OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviated to TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviated to p-EtTAZ), bathophenanthroline (abbreviated to BPhen), bathocuproin (abbreviated to BCP), or the like can be used, in addition to the above-mentioned metal complex such as Alq 3 , Almq 3 , bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviated to BAlq), BeBq 2 , Zn(BOX) 2 , or Zn(BTZ) 2 , or a metal complex such as tris(8-quinolinolato)gallium (abbreviated to Gaq 3 ) or bis(2-methyl-8-quinolinolato)-4-phenylphenolate-gallium (abbreviated to BGaq) 
     As an electron injection material for forming the electron injection layer, an ultrathin film of an insulating material, for example, alkali metal halide such as LiF or CsF, alkaline earth metal halide such as CaF 2 , alkali metal oxide such as Li 2 O, or the like is often specifically used. In addition, an alkali metal complex such as lithium acetylacetonate (abbreviated to Li(acac)), 8-quinolinolato-lithium (abbreviated to Liq), or the like is also effective. A benzoxazole derivative and any one or more of alkali metal, alkaline earth metal, and transition metal may be included. 
     Note that the layer including a light emitting material  103  may have a laminated structure of a plurality of layers each including a light emitting material.  FIG. 12  shows an example of a laminated structure of a plurality of layers each including a light emitting material.  FIG. 12  shows a structure in which a first electrode  1201 , a first layer including a light emitting material  1202 , a charge generation layer  1203 , a second layer including a light emitting material  1204 , and a second electrode  1205  are laminated over a substrate  1200 . The charge generation layer  1203  needs to be formed of a highly light transmitting material having a function of injecting a carrier. Although  FIG. 12  shows the laminated structure of the two layers each including a light emitting material, the invention is not limited thereto. A laminated structure of three or more layers each including a light emitting material may be used. Further, the first electrode is used as the electrode on the substrate side; however, the second electrode may be used as the electrode on the substrate side. 
     Luminance is improved by using the laminated structure of layers each including a light emitting material. The more layers are laminated, the more the luminance can be improved even with the same amount of current. In particular, the laminated structure of layers each including a light emitting material is suitable for lighting use which requires high luminance. In the case of forming the laminated structure of a plurality of layers each including a light emitting material, the layers each including a light emitting material may be formed of the same material or different materials. 
     For example, layers including a light emitting material formed of materials which emit red (R), green (G), and blue (B) light may be laminated, so that white light emission can be obtained as a whole. The light-emitting layers which emit red (R), green (G), and blue (B) light may each be formed by an evaporation method using an evaporation mask, a droplet discharge method (also referred to as an ink-jet method), or the like. Specifically, CuPC or PEDOT can be used for the hole injection layer; α-NPD, for the hole transport layer; BCP or Alq 3 , for the electron transport layer; and BCP:Li or CaF 2 , for the electron injection layer. The light emitting layer may be formed of, for example, a material doped with a dopant (DCM or the like in the case of R, or DMQD or the like in the case of G) corresponding to each light emission of R, and B. In the case of obtaining white light emission, not only the laminated structure of the light emitting materials of three colors as described above but also a laminated structure of light emitting materials of two colors may be used. For example, white light emission can be obtained by laminating materials which emit blue and yellow light. 
     Note that the structure of the layer including a light emitting material is not limited to the above-described laminated structure. For example, the layer including a light emitting material may be any one of a single layer type, a laminated type, and a mixed type with no interface between layers. A fluorescent material, a phosphorescent material, or a combined material thereof can be used. For example, a phosphorescent material can be used for the material which emits red (R) light, and a fluorescent material can be used for the materials which emit green (G) and blue (B) light. Further, any one of the following may be used: an organic material including a low molecular weight material, a high molecular weight material, and an intermediate molecular weight material, an inorganic material typified by molybdenum oxide which is superior in electron injecting properties, and a composite material of an organic material and an inorganic material. 
     The lighting device of the invention may be formed to provide not only white light but also desired-color light. A color filter, a color conversion layer, a combination of a color filter and a color conversion layer, or the like may be provided separately. 
     A second electrode  104  is formed over the layer including a light emitting material  103 . A known material can be used for the second electrode  104 . In the case of using the second electrode  104  as a cathode, a conductive material having a low work function is preferably used. Rare earth metal such as Yb or Er can be specifically used to form the cathode, in addition to alkali metal such as Li or Cs, alkaline earth metal such as Mg, Ca, or Sr, or an alloy including the metal (Mg:Ag, Al:Li, or the like). In the case of using an electron injection layer of LiF, CsF, CaF 2 , Li 2 O, or the like, a conductive thin film of aluminum or the like can be used. In the case of using the second electrode  104  as an anode, a conductive material having a high work function is preferably used. Specifically, a single-layer film of TiN, ZrN, Ti, W, Ni, Pt, Cr, or the like; a laminated layer of a titanium nitride film and a film containing aluminum as its main component; a three-layer structure of a titanium nitride film, a film containing aluminum as its main component, and a titanium nitride film; or the like can be used. Alternatively, the second electrode may be formed by laminating a transparent conductive film over a reflective electrode of Ti, Al, or the like. 
     An insulating film  105  is formed to cover the second electrode  104  and the layer including a light emitting material  103 . The insulating film  105  electrically insulates the first electrode  102  and the second electrode  104  from the second electrode  104  and an auxiliary electrode  106 , respectively. Note that the insulating film  105  may have a function as a protective film for preventing penetration of a substance, such as moisture and oxygen, to be the cause of promoting deterioration of the layer including a light emitting material. 
     The insulating film  105  has an opening  107 , through which the first electrode  102  is electrically connected to the auxiliary electrode  106 . A low-resistivity material is preferably used for the auxiliary electrode  106 ; specifically, a material such as aluminum, copper, or silver can be used. The diameter of the opening may be 10 μm to 500 μm, preferably, 50 μm to 200 μm. 
     A plurality of the openings  107  is provided in a light emitting region  108 . An effect of voltage drop due to relatively high resistivity of the transparent conductive film can be reduced by electrically connecting the auxiliary electrode  106  to the first electrode  102  through the opening  107 . In other words, the auxiliary electrode  106  practically lowers the resistivity of the first electrode  102  by being electrically connected to the first electrode  102  through the plurality of openings  107  formed in the light emitting region  108 . This can reduce nonuniformity of luminance in which a portion away from a current supply terminal is dark. In the case where the size of the opening is sufficiently small, the presence of the auxiliary electrode can be, for the most part, disregarded when the lighting device is seen from the side of light emission, in other words, from the side of the light transmitting substrate. Since the auxiliary electrode is not placed in the direction of light emission, a material, a thickness, or a formation position of the auxiliary electrode can be freely set. Therefore, the auxiliary electrode can be selectively formed in a position where voltage tends to drop, or can be entirely formed over the light emitting region of the lighting device. 
     Since the lighting device of the invention can reduce the nonuniformity of luminance due to voltage drop caused by relatively high resistivity of the first electrode, a lighting device having favorable in-plane uniformity of luminance can be obtained. Specifically, the lighting device is preferably applied to a large-sized lighting device. 
     Embodiment Mode 2 
     A structure of a lighting device of the invention is described with reference to  FIGS. 5A and 5B . A lighting device shown in  FIGS. 5A and 5B  is a top emission lighting device which emits light from the opposite side of a substrate side. Note that  FIG. 5B  is a top view of a light emitting region of a lighting device and  FIG. 5A  is a cross-sectional view (taken along line A-A′ in  FIG. 5B ) of the vicinity of an opening in the light emitting region. 
     In  FIGS. 5A and 5B , a thin substrate formed of a flexible material is used as a substrate  501 . Specifically, a flexible substrate such as a plastic substrate, more specifically a polyester film or an acrylic resin film, can be used. 
     A second electrode  502  is formed over the substrate  501 . A known material can be used for the second electrode  502 . In the case of using the second electrode  502  as a cathode, a conductive material having a low work function is preferably used. Rare earth metal such as Yb or Er can be used to form the cathode, in addition to alkali metal such as Li or Cs, alkaline earth metal such as Mg, Ca, or Sr, or an alloy including the metal (Mg:Ag, Al:Li, or the like). In the case of using an electron injection layer of LiF, CsF, CaF 2 , or Li 2 O, a typical conductive thin film of aluminum or the like can be used. In the case of using the second electrode  502  as an anode, a conductive material having a high work function is preferably used. Specifically, a single-layer film of TiN, ZrN, Ti, W, Ni, Pt, Cr, or the like; a laminated layer of a titanium nitride film and a film containing aluminum as its main component; a three-layer structure of a titanium nitride film, a film containing aluminum as its main component, and a titanium nitride film can be used. Alternatively, the second electrode may be formed by laminating a transparent conductive film over a reflective electrode of Ti, Al, or the like. 
     A layer including a light emitting material  503  is formed over the second electrode  502 . A known material can be used for the layer including a light emitting material  503 , and either a low molecular weight material or a high molecular weight material can be used. A material for forming, the layer including a light emitting material may include not only the one formed of only an organic compound material but also the one partly containing an inorganic compound. The layer including a light emitting material is formed by appropriately combining a hole injection layer, a hole transport layer, a hole blocking layer, a light emitting layer, an electron transport layer, an electron injection layer, or the like. The layer including a light emitting material may be a single layer or have a laminated structure of a plurality of layers. 
     Note that the layer including a light emitting material  503  may have a laminated structure of a plurality of layers each including a light emitting material. Luminance is improved by using the laminated structure of layers each including a light emitting material. The more layers are laminated, the more the luminance can be improved even with the same amount of current. In particular, the laminated structure of layers each including a light emitting material is suitable for lighting use which requires high luminance. In the case of forming the laminated structure of a plurality of layers each including a light emitting material, the layers each including a light emitting material may be formed of the same material or different materials. 
     A transparent conductive film is formed over the layer including a light emitting material  503  as a first electrode  504 . For example, indium tin oxide (hereinafter referred to as ITO), indium tin oxide containing silicon, indium oxide containing zinc oxide (ZnO) of 2% to 20%, or the like can be used for the transparent conductive film. 
     The substrate  501 , the second electrode  502 , the layer including a light emitting material  503 , and the first electrode  504  have an opening  507 . An insulating film  505  is formed to cover the vicinity of the opening of the first electrode  504  and the substrate  501  and a side wall of the opening. The insulating film  505  electrically insulates the first electrode  504  and the second electrode  502  from the second electrode  502  and an auxiliary electrode  506 , respectively. Note that the insulating film  505  may have a function as a protective film for preventing penetration of a substance, such as moisture and oxygen, to be the cause of promoting deterioration of the layer including a light emitting material. 
     The auxiliary electrode  506  is formed over a lower surface of the substrate, in the opening, and in the vicinity of the opening of the first electrode, and is electrically connected to the first electrode in the vicinity of the opening. A low-resistivity material is preferably used for the auxiliary electrode  506 ; specifically, a material such as aluminum, copper, or silver can be used. The diameter of the opening may be 10 μm to 500 μm, preferably, 50 μm to 200 μm. 
     A plurality of the openings  507  is provided in a light emitting region  508 . An effect of voltage drop due to relatively high resistivity of the transparent conductive film can be reduced by electrically connecting the auxiliary electrode  506  to the first electrode  504  through the opening  507 . In other words, the auxiliary electrode  506  practically lowers the resistivity of the first electrode  504  by being electrically connected to the first electrode  504  through the plurality of the openings  507  formed in the light emitting region  508 . This can reduce nonuniformity of luminance in which a portion away from a current supply terminal is dark. In the case where the size of the opening is sufficiently small, the presence of the auxiliary electrode can be, for the most part, disregarded when the lighting device is seen from the side of light emission, in other words, from the side of a light transmitting substrate. Since the auxiliary electrode is not placed in the direction of light emission, a material, a thickness, or a formation position of the auxiliary electrode can be freely set. Therefore, the auxiliary electrode can be selectively formed in a position where voltage tends to drop, or can be entirely formed over the light emitting region of the lighting device. 
     Since the lighting device of the invention can reduce in-plane nonuniformity of luminance which is caused by voltage drop due to relatively high resistivity of the first electrode, a lighting device having favorable in-plane uniformity of luminance can be obtained. Specifically, the lighting device is preferably applied to a large-sized lighting device. 
     Embodiment 1 
     A method for manufacturing the lighting device of the invention shown in  FIGS. 1A and 1B  is described in this embodiment with reference to  FIGS. 2A to 2D . 
     A transparent conductive film serving as a first electrode  202  is formed over a light transmitting substrate  201 . In this embodiment, a glass substrate is used as the light transmitting substrate  201  and an ITO film is formed for forming the first electrode  202 . 
     A layer including a light emitting material  203  is formed over the first electrode  202 . A known material can be used for the layer including a light emitting material  203 . The layer including a light emitting material may have a laminated structure of a plurality of layers each including a light emitting material. 
     A second electrode  204  is formed over the layer including a light emitting material  203  ( FIG. 2A ). The second electrode  204  has an opening. After the second electrode is entirely formed over the layer including a light emitting material, the opening may be formed by pattering the second electrode by a photolithography method. Alternatively, the second electrode  204  having the opening may be formed by using a mask. In this embodiment, an aluminum film is used as the second electrode  204 . After the second electrode is entirely formed over the layer including a light emitting material  203 , it is patterned by a photolithography method. Thereafter, the opening is formed in the layer including a light emitting material, using the patterned second electrode as a mask ( FIG. 2B ). 
     An insulating film  205  is formed to cover the layer including a light emitting material  203  and the second electrode  204  ( FIG. 2C ). The insulating film  205  also has an opening. After the insulating film is entirely formed, the opening may be formed by patterning the insulating film by a photolithography method. Alternatively, the insulating film may be formed by using a mask. In this embodiment, a silicon oxide film is used as the insulating film  205 . 
     An auxiliary electrode  206  is formed ( FIG. 2D ). The auxiliary electrode  206  preferably has low resistivity, and aluminum is used in this embodiment. The auxiliary electrode  206  is electrically connected to the first electrode  202  through an opening  207  and insulated from the second electrode  204 . 
     Thus, the ITO film serving as the first electrode  202  is connected to the auxiliary electrode  206  through the opening  207 , and an effect of voltage drop due to relatively high resistivity of the first electrode can be reduced. Therefore, in-plane nonuniformity of luminance can be reduced when the lighting device is applied to a large-sized lighting device. In the case where the opening  207  is sufficiently small, the presence of the auxiliary electrode can be, for the most part, disregarded when the lighting device is seen from the side of light emission, in other words, from the side of the light transmitting substrate. 
     In this embodiment, the layer including a light emitting material is entirely formed and then patterned using aluminum as a mask. However, the layer including a light emitting material may be formed using a mask to have an opening. 
       FIGS. 3A to 3C  show a method for forming each of a layer including a light emitting material  303  and a second electrode  304  using a mask. In other words, each of the layer including a light emitting material  303  and the second electrode  304  is formed using a mask after a first electrode  302  is entirely formed. An insulating film  305 , an auxiliary electrode  306 , and an opening  307  may be formed by a similar method to the above-described method. At this time, the first electrode is more certainly insulated from the second electrode by making the opening of the second electrode  304  larger than that of the layer including a light emitting material  303 . 
     Embodiment 2 
     A method for manufacturing the lighting device of the invention shown in  FIGS. 1A and 1B , which is different from that of Embodiment 1, is described in this embodiment with reference to  FIGS. 4A to 4C . 
     A transparent conductive film serving as a first electrode  402  is formed over a light transmitting substrate  401 . In this embodiment, a glass substrate is used as the light transmitting substrate  401  and an ITO film is formed as the first electrode  402 . 
     A layer including a light emitting material  403  and a second electrode  404  are sequentially formed ( FIG. 4A ). In this embodiment, an aluminum film is formed for the second electrode  404 . 
     Then, laser light is emitted to the light transmitting substrate  401  side to form an opening ( FIG. 4B ). As the laser light, used is laser light with a wavelength enough to be transmitted through the glass substrate and ITO and absorbed by the layer including a light emitting material  403  and the second electrode  404 . In this embodiment, laser light with a wavelength of 532 nm is used. The laser light with a wavelength of 532 nm can be obtained by converting a fundamental wave (wavelength: 1064 nm) of a YAG laser, YVO 4 , or the like into a second harmonic by a nonlinear optical element. The layer including a light emitting material and the second electrode after absorbing the laser light are heated and sublimed, thereby forming the opening. After the opening is formed, an insulating film  405  and an auxiliary electrode  406  are formed as in Embodiment 1 so that the first electrode  402  is electrically connected to the auxiliary electrode  406  through the opening  407  ( FIG. 4C ). 
     Embodiment 3 
     A method for manufacturing the lighting device of the invention shown in  FIGS. 5A and 5B  is described in this embodiment with reference to  FIGS. 6A to 6C  and  FIGS. 7A and 7B . 
     A second electrode  602 , a layer including a light emitting material  603 , and a first electrode  604  are formed over a thin substrate  601  formed of a flexible material. In this embodiment, an aluminum film is formed as the second electrode  602  and an ITO film is formed as the first electrode  604  over a polyester film ( FIG. 6A ). 
     The substrate  601 , the second electrode  602 , the layer including a light emitting material  603 , and the first electrode  604  are provided with an opening ( FIG. 6B ). Since the substrate  601  is formed of a flexible material, the opening can be easily formed by applying physical force. 
     A silicon oxide film is formed as an insulating film  605 . The silicon oxide film is formed using a mask by a sputtering method or an evaporation method ( FIG. 6C ). The silicon oxide film is also formed to wrap around to the opposite surface of an intended surface through the opening by using a sputtering method or an evaporation method. According to this, the second electrode  602  is more certainly insulated from an auxiliary electrode  606 . 
     The auxiliary electrode  606  is formed. First, silver is deposited on the first electrode  604  side by a printing method. With the use of a printing method at this time, the opening  607  is filled with silver ( FIG. 7A ). Then, silver is entirely deposited on the substrate  601  side ( FIG. 7B ). According to this, the first electrode  604  is electrically connected to the auxiliary electrode  606 . Thus, an effect of voltage drop due to relatively high resistivity of the first electrode can be reduced. Therefore, a lighting device having favorable in-plane uniformity of luminance can be obtained when the lighting device is applied to a large-sized lighting device. 
     In the structure of this embodiment, an ITO film serving as the first electrode is connected to the auxiliary electrode formed of silver, on a light emitting side. However, in the case where the opening  607  is sufficiently small, the presence of the auxiliary electrode can be, for the most part, disregarded when the lighting device is seen from the side of light emission, in other words, from the side of the first electrode. 
     Embodiment 4 
     An example of an overall structure of a lighting device of the invention is described in this embodiment with reference to  FIGS. 8A and 8B . 
       FIGS. 8A and 8B  are a top view and a cross-sectional view of a lighting device of the invention, respectively. The lighting device of the invention includes a substrate  801 , a first electrode  802 , a layer including a light emitting material  803 , a second electrode  804 , an insulating film  805 , and an auxiliary electrode  806 . The first electrode  802  is electrically connected to the auxiliary electrode  806  through an opening  807 . A plurality of the openings  807  is provided in a light emitting region. A second insulating film  808  is at the end of the light emitting region, which serves to prevent the first electrode  802  and the second electrode  804  from shorting. Current supply terminals  809  are each connected to the second electrode  804  and the auxiliary electrode  806 . The light emitting region is sealed with a sealant  810 . The sealant is preferably a material which let penetrate as little moisture and oxygen as possible to prevent deterioration of the layer including a light emitting material. A space  811  surrounded by the sealant is filled with a filler. The space  811  surrounded by the sealant may be filled with an inert gas (nitrogen, argon, or the like) in place of the filler. Alternatively, the space may be filled with a sealant. 
       FIGS. 8A and 8B  show the structure described in Embodiment 1 as an example; however, the structure described in Embodiment 2 or 3 can also be used as a lighting device by sealing similarly. 
     Embodiment 5 
     Examples of a device using a lighting device of the invention are described in this embodiment with reference to  FIG. 9  and  FIGS. 10A to 10C . 
       FIG. 9  shows an example of a liquid crystal display device using a lighting device of the invention as a backlight. The liquid crystal display device shown in  FIG. 9  includes a chassis  901 , a liquid crystal layer  902 , a backlight  903 , and a chassis  904 . The liquid crystal layer  902  is connected to a driver IC  905 . The lighting device of the invention is used as the backlight  903  and current is supplied through a terminal  906 . 
     A backlight having favorable in-plane uniformity of luminance can be obtained by using the lighting device of the invention as a backlight of a liquid crystal display device; thus, high quality of a display device is obtained. Since the backlight can have large area, the liquid crystal display device can also have large area. Further, the light emitting element is thin and consumes low power; therefore, the display device can also be thinned and made to consume low power. 
       FIG. 10A  shows the lighting device of the invention used as indoor lighting. The lighting device of the invention is a surface-emitting lighting device and has favorable in-plane uniformity of luminance even when it has large area. Therefore, an entire ceiling, for example, can be provided with the lighting device of the invention. Not only the ceiling but also a wall, a floor, or a column can be provided with the lighting device of the invention. Further, the lighting device of the invention can be provided on a curved surface because the lighting device is flexible. Moreover, the lighting device can be used not only indoors but also outdoors and can be provided on a building wall or the like as an outdoor light. 
       FIG. 10B  shows the lighting device of the invention used as lighting in a tunnel. The lighting device of the invention can be formed along a curved inner wall of a tunnel because the lighting device is flexible. 
       FIG. 10C  shows an example of the lighting device of the invention used as interior lighting. Since the lighting device of the invention is thin and flexible and is a surface-emitting type, it can be processed into a desired shape as shown in  FIG. 10B . 
     The lighting device of the invention can also be used for lighting in taking a picture. In the case of taking a picture, a picture similar to one taken with a subject illuminated by natural light can be taken when a subject is illuminated by large-sized light with uniform luminance.