Patent Publication Number: US-8975647-B2

Title: Lighting device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lighting device including a light-emitting member which exhibits electroluminescence (EL). 
     2. Description of the Related Art 
     Light-emitting elements containing organic compounds as light-emitting bodies have been expected to be applied to next-generation lighting. Light-emitting elements containing organic compounds as light-emitting bodies have features such as drive at a low voltage with low power consumption. 
     An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a layered structure including a hole injection layer, a hole transport layer, an electron transport layer, an electron injection layer, and/or the like, in addition to the light-emitting layer. 
     It is said that, as for a light-emitting mechanism of a light-emitting element, an EL layer is interposed between a pair of electrodes and voltage is applied to the EL layer, so that electrons injected from a cathode and holes injected from an anode are recombined in an emission center of the EL layer to form molecular excitons, and the molecular excitons release energy when returning to a ground state; thus, light is emitted. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states. 
     Further, since the pair of electrodes and the light-emitting layer are formed as films in such a light-emitting element, surface light emission can easily be obtained by forming a large-area light-emitting element. This is a feature which is hard to obtain in light sources such as incandescent lamps and LEDs (point light sources) or in fluorescent lamps (line light sources), so that the above light-emitting element has a high utility value as a light source such as lighting. 
     Patent Documents 1 and 2 disclose light-emitting devices in each of which a plurality of light-emitting elements is connected in series. 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-049853 
         [Patent Document 2] Japanese Published Patent Application No. 2006-108651 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a lighting device which is provided with a relatively large light-emitting region by integration of a plurality of light-emitting elements, and a manufacturing method thereof. 
     Another object of one embodiment of the present invention is to provide a lighting device which is thin and lightweight. 
     Another object of one embodiment of the present invention is to provide a lighting device which can resist an impulse such as a drop impulse. 
     When a lighting device having a relatively large light-emitting region is fabricated, one of a pair of electrode layers of a light-emitting element is formed using a conductive material having a light-transmitting property. A conductive material having a light-transmitting property, such as indium tin oxide (hereinafter referred to as ITO) has a resistance value larger than that of a metal material used for a wiring layer, such as aluminum or titanium. Thus, when a conductive layer, a wiring, or the like is formed using a conductive material having a light-transmitting property, a voltage drop is likely to be caused. Note that a current path in a conductive layer including a material with a large resistance value has resistance in any case, which leads to a voltage drop. 
     For integration of a plurality of light-emitting elements and suppression of a voltage drop, the light-emitting elements are connected in series, a plurality of auxiliary wirings with different widths and different thicknesses is used, and the arrangement of the auxiliary wirings, electrodes, the light-emitting elements, and the like is optimized. 
     One embodiment of the present invention is a lighting device including plural stages of light-emitting element units; a first wiring in contact with the first-stage light-emitting element unit of the plural stages of light-emitting element units; and a second wiring in contact with the last-stage light-emitting element unit of the plural stages of light-emitting element units. The first-stage light-emitting element unit includes at least a first light-emitting element and a second light-emitting element which are connected in parallel. The light-emitting element unit adjacent to the first-stage light-emitting element unit includes at least a third light-emitting element and a fourth light-emitting element which are connected in parallel. The first light-emitting element, the second light-emitting element, the third light-emitting element, and the fourth light-emitting element each include a first electrode layer with a light-transmitting property, an organic compound-containing layer which is in contact with the first electrode layer, a second electrode layer with reflectivity which is in contact with the organic compound-containing layer, and an auxiliary wiring which is in contact with the first electrode layer and has a narrower width than the first wiring and the second wiring. The second electrode layer of the first light-emitting element and the auxiliary wiring of the third light-emitting element are connected to each other, whereby the first light-emitting element and the third light-emitting element are connected in series. The second electrode layer of the second light-emitting element and the auxiliary wiring of the fourth light-emitting element are connected to each other, whereby the second light-emitting element and the fourth light-emitting element are connected in series. The auxiliary wiring of each of the light-emitting elements in the first-stage light-emitting element unit is in contact with the first wiring. The second electrode layer of each of the light-emitting elements in the last-stage light-emitting element unit is in contact with the second wiring. 
     Note that the number of stages of the light-emitting element units is two or more, the light-emitting element unit in contact with the first wiring is referred to as the first-stage light-emitting element unit, and the light-emitting element unit in contact with the second wiring is referred to as the last-stage light-emitting element unit. For example, when the number of stages of the light-emitting element units is four, the first-stage light-emitting element unit is provided so as to be adjacent to the second-stage light-emitting element unit, the second-stage light-emitting element unit is provided so as to be adjacent to the third-stage light-emitting element unit, and the third-stage light-emitting element unit is provided so as to be adjacent to the fourth-stage (last-stage) light-emitting element unit. 
     In one embodiment of the lighting device having the above structure, the thickness of each of the first wiring and the second wiring is greater than or equal to 3 μm and less than or equal to 30 μm, and the thickness of the auxiliary wiring is greater than or equal to 0.1 μm and less than 3 μm. The first wiring, the second wiring, and the auxiliary wiring each include a conductive layer containing copper and have low resistance. The auxiliary wiring is provided in contact with the first electrode layer having a light-transmitting property, whereby a voltage drop is suppressed. 
     With the above structure, the plurality of light-emitting elements can emit light efficiently or the total emission area can be increased. 
     Further, plural kinds of auxiliary wirings each having a smaller thickness and a narrower line width than the first wiring and the second wiring may be provided. One embodiment of the present invention is a lighting device including plural stages of light-emitting element units connected in series; a first wiring in contact with the first-stage light-emitting element unit of the plural stages of light-emitting element units; and a second wiring in contact with the last-stage light-emitting element unit of the plural stages of light-emitting element units. The first-stage light-emitting element unit includes at least a first light-emitting element and a second light-emitting element which are connected in parallel. The light-emitting element unit adjacent to the first-stage light-emitting element unit includes at least a third light-emitting element and a fourth light-emitting element which are connected in parallel. The first light-emitting element, the second light-emitting element, the third light-emitting element, and the fourth light-emitting element each include a first electrode layer with a light-transmitting property, an organic compound-containing layer which is in contact with the first electrode layer, a second electrode layer with reflectivity which is in contact with the organic compound-containing layer, a first auxiliary wiring which is in contact with the first electrode layer and has a narrower width than the first wiring and the second wiring, and a second auxiliary wiring which is in contact with the first electrode layer and has a narrower width than the first auxiliary wiring. The second electrode layer of the first light-emitting element and the first auxiliary wiring of the third light-emitting element are connected to each other, whereby the first light-emitting element and the third light-emitting element are connected in series. The second electrode layer of the second light-emitting element and the first auxiliary wiring of the fourth light-emitting element are connected to each other, whereby the second light-emitting element and the fourth light-emitting element are connected in series. The first auxiliary wiring of each of the light-emitting elements in the first-stage light-emitting element unit is in contact with the first wiring. The second electrode layer of each of the light-emitting elements in the last-stage light-emitting element unit is in contact with the second wiring. 
     In one embodiment of the lighting device having the above structure, the thickness of each of the first wiring and the second wiring is greater than or equal to 3 μm and less than or equal to 30 μm, the thickness of the first auxiliary wiring is greater than or equal to 0.1 μm and less than 3 μm, and the thickness of the second auxiliary wiring is greater than or equal to 3 nm and less than or equal to 30 nm. The first wiring, the second wiring, the first auxiliary wiring, and the second auxiliary wiring each include a conductive layer containing copper and have low resistance. The first auxiliary wiring and the second auxiliary wiring are provided in contact with the first electrode layer having a light-transmitting property, whereby a voltage drop is suppressed. Although the second auxiliary wiring overlaps with a light-emitting region of the light-emitting element, the total area of light emission is less likely to be influenced by the second auxiliary wiring because the second auxiliary wiring has a smaller thickness and a narrower line width than the first auxiliary wiring. 
     In each of the light-emitting elements of one embodiment of the lighting device having the above structure, an insulating layer which is over and in contact with the first electrode layer and has an opening is provided and the organic compound-containing layer is in contact with the first electrode layer in the opening. The insulating layer is called a partition wall or a bank and prevents a short circuit between the adjacent light-emitting elements. The area of the opening formed in the insulating layer, namely, the area of a region where the organic compound-containing layer and the first electrode layer is in contact with each other is equal to or substantially equal to the area of an emission region of one light-emitting element in a plan view. 
     One embodiment of the lighting device having the above structure includes first plural stages of light-emitting element units and second plural stages of light-emitting element units adjacent to the first plural stages of light-emitting element units. The first plural stages of light-emitting element units and the second plural stages of light-emitting element units are arranged axisymmetrically with respect to the length direction of the second wiring in a plan view. The plurality of light-emitting element units is provided axisymmetrically and a plurality of wirings which is extended in the linear direction corresponding to that of the symmetry axis and is given the same potentials may be formed as one wiring, leading to a reduction in total number of wirings. 
     In one embodiment of the lighting device having the above structure, the first light-emitting element and the second light-emitting element are connected in parallel. The second electrode layer of the first light-emitting element is in contact with the first electrode layer of the third light-emitting element, whereby the first light-emitting element and the third light-emitting element are connected in series. The second electrode layer of the second light-emitting element is in contact with the first electrode layer of the fourth light-emitting element, whereby the second light-emitting element and the fourth light-emitting element are connected in series. Serial connection enables application of a higher voltage to the lighting device; thus, the load on a converter can be reduced. 
     In one embodiment of the lighting device having the above structure, the plural stages of light-emitting element units are provided over a substrate having an insulating surface and functioning as a housing. The substrate having an insulating surface has recessed portions in which the first wiring and the second wiring are provided in a plan view. The first wiring and the second wiring are covered with a planarization film. 
     In one embodiment of the lighting device having the above structure, light emitted from the first light-emitting element, the second light-emitting element, the third light-emitting element, and the fourth light-emitting element passes through the substrate having an insulating surface and then is extracted. An optical member such as a microlens array or a diffusion plate may be provided over the substrate having an insulating surface as necessary to provide a large-area lighting device capable of more uniform light emission. 
     An inorganic insulating film (inorganic insulator) covering a top surface of the light-emitting element is preferably provided to improve reliability. Further, an inorganic insulating film may be provided between an emission surface of the light-emitting element and the housing. The inorganic insulating film serves as a sealing film or a protective layer which blocks an external contaminant such as water. The inorganic insulating film can be formed to have either a single-layer structure or a layered structure using any of a nitride film and a nitride oxide film. As the inorganic insulator, thin glass can be used. By providing the inorganic insulating film, deterioration of the light-emitting element can be suppressed and the durability and the lifetime of the lighting device can be improved. 
     The shape of the emission surface of the light-emitting element may be a circular shape or a polygonal shape such as a quadrangle. The shape of the housing covering the light-emitting element may correspond to the shape of the emission surface and can be a rectangular solid, a polygonal cylinder, a cylinder, or the like. 
     Further, the EL layer may have a layered structure including two or more layers provided with an intermediate layer laid therebetween. By stacking a plurality of EL layers with different emission colors, an emission color to be provided can be controlled. By stacking a plurality of EL layers even with the same emission color, an effect of improving power efficiency can be obtained. 
     Further, the EL layer may have a layered structure including, as one of layers, a layer containing a composite material in which an acceptor substance is mixed with an organic compound having a high hole transport property. The layer containing a composite material is in contact with the second electrode layer, whereby a short circuit of the light-emitting element can be suppressed. 
     Note that in this specification, a lighting device refers to a light-emitting device or a light source (including lighting) and includes a light-emitting element provided with at least a light-emitting layer between a pair of electrodes. Further, the lighting device includes the following modules in its category: a module in which a connector such as a flexible printed circuit (FPC), a tape automated bonding (TAB) tape, or a tape carrier package (TCP) is attached to a lighting device; and a module having a TAB tape or a TCP provided with a converter or the like at the end thereof. 
     According to one embodiment of the present invention, a lighting device which is provided with a relatively large light-emitting region by integration of a plurality of light-emitting elements and a manufacturing method thereof can be provided. 
     According to one embodiment of the present invention, a lighting device which is thin and lightweight can be provided. 
     When plastic or a thin metal plate is used as a substrate used for a lighting device, the lighting device can have resistance against an impulse such as a drop impulse. 
     In a lighting device according to one embodiment of the present invention, a plurality of light-emitting elements can emit light efficiently or the total area of light emission can be increased. 
     According to one embodiment of the present invention, a lighting device can be formed by connecting a plurality of light-emitting element units of given stages in series or in series-parallel combination; thus, the lighting device can be increased in size. For example, it is possible to realize a lighting device whose emission region is one surface of a large substrate with a size corresponding to G5.5 (1100 mm×1300 mm) to G11 (3000 mm×3300 mm) of the size of a mother glass substrate of a liquid crystal panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a plan view illustrating a lighting device; 
         FIGS. 2A and 2B  are cross-sectional views illustrating a lighting device; 
         FIG. 3  is a plan view illustrating a lighting device; 
         FIGS. 4A and 4B  are cross-sectional views illustrating a lighting device; 
         FIG. 5  is a plan view illustrating a lighting device; 
         FIGS. 6A and 6B  are cross-sectional views each illustrating a lighting device; 
         FIG. 7  is a cross-sectional view illustrating a lighting device; 
         FIGS. 8A ,  8 B 1 ,  8 B 2 , and  8 C are diagrams each illustrating an example of a light-emitting element applicable to a lighting device; 
         FIG. 9  is a diagram illustrating examples of application of a lighting device; 
         FIG. 10  is a diagram illustrating an example of application of a lighting device; 
         FIGS. 11A and 11B  are cross-sectional views illustrating a lighting device; 
         FIGS. 12A and 12B  are cross-sectional views each illustrating a lighting device; and 
       FIGS.  13 A 1  and  13 A 2  are a cross-sectional view and a plan view illustrating a lighting device, respectively, and FIGS.  13 B 1  and  13 B 2  are a cross-sectional view and a plan view illustrating a lighting device, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that a variety of changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the following descriptions of the embodiments. In the structures to be given below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and descriptions thereof will not be repeated. 
     Embodiment 1 
     In this embodiment, a lighting device according to an embodiment of the present invention will be described with reference to  FIG. 1 ,  FIGS. 2A and 2B ,  FIG. 3 ,  FIGS. 4A and 4B ,  FIG. 5 ,  FIGS. 6A and 6B ,  FIG. 7 ,  FIGS. 11A and 11B ,  FIGS. 12A and 12B , and FIGS.  13 A 1 ,  13 A 2 ,  13 B 1 , and  13 B 2 . Note that common portions are denoted by the same reference numerals in  FIG. 1 ,  FIGS. 2A and 2B ,  FIG. 3 ,  FIGS. 4A and 4B ,  FIG. 5 ,  FIGS. 6A and 6B ,  FIG. 7 ,  FIGS. 11A and 11B ,  FIGS. 12A and 12B , and FIGS.  13 A 1 ,  13 A 2 ,  13 B 1 , and  13 B 2 . 
       FIG. 1  is a plan view illustrating part of a lighting device according to this embodiment.  FIG. 2A  is a cross-sectional view along A 1 -A 2  in  FIG. 1 , and  FIG. 2B  is a cross-sectional view along B 1 -B 2  in  FIG. 1 . 
       FIG. 1  is the plan view of the lighting device in which a plurality of light-emitting elements is arranged between a first wiring  124  and a second wiring  128 . In this embodiment, eight light-emitting elements are used for simplification; however, the number of the light-emitting elements is not particularly limited. 
     The first wiring  124  is a lead wiring which has a great line width and a large thickness and is formed using a low resistance material. The first wiring  124  is electrically connected to a power source (not illustrated here), and current is supplied to each of the light-emitting elements through the first wiring  124 . 
     The second wiring  128  is a lead wiring which has a great line width and a large thickness and is formed using a low resistance material. The second wiring  128  is given a fixed potential (also referred to as a common potential). 
     The first wiring  124  and the second wiring  128  can also be referred to as main wirings and branch at a variety of positions. Note that a line width of a branching portion of the first wiring  124  is substantially equal to a line width of a branching portion of the second wiring  128 . Although  FIG. 1  illustrates an example in which the direction of the portions branching from one first wiring  124  is the same and the first wiring has a comb-like shape, one embodiment of the present invention is not particularly limited thereto. In a layout of the wiring, branching portions thereof may be axisymmetrical with respect to the length direction of the wiring (a layout of a lattice-like wiring), or branching portions thereof may be alternately branched from the wiring. The first wiring  124  and the second wiring  128  are each preferably formed to a thickness in the range of greater than or equal to 3 μm and less than or equal to 30 μm with the use of a low resistance material. For example, the first wiring  124  and the second wiring  128  are each formed to have a single-layer structure or a layered structure using a material selected from aluminum (Al), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), scandium (Sc), nickel (Ni), and copper (Cu) or an alloy material including any of these materials as its main component. 
     To form the first wiring  124  and the second wiring  128 , plating treatment (an electrolytic plating method or an electroless plating method) may be performed. As a metal material to be plated, a low resistance material such as copper, silver, gold, chromium, iron, nickel, platinum, or an alloy thereof can be used. 
     In this embodiment, the first wiring  124  has a two-layer structure of a titanium layer and a copper layer over the titanium layer. A first layer and a second layer of the first wiring  124  are denoted by reference numerals  124   a  and  124   b , respectively, as in  FIGS. 2A and 2B . Further, a first layer and a second layer of the second wiring  128  are denoted by reference numerals  128   a  and  128   b , respectively. The first wiring  124  and the second wiring  128  are provided with the insulating layer  103  laid therebetween to keep a distance between the first wiring  124  and the second wiring  128  so as not to be in contact with each other and not to cause a short circuit, as in  FIG. 2B . 
     The insulating layer  103  can be formed using an inorganic insulating material or an organic insulating material. Note that an organic insulating material having heat resistance, such as an acrylic resin, a polyimide, a benzocyclobutene-based resin, a polyamide, or an epoxy resin, is preferably used as a planarization insulating film. Other than such organic insulating materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. The insulating layer  103  may be formed by stacking a plurality of insulating films formed using any of these materials. 
     There is no particular limitation on the method for forming the insulating layer  103 , and the insulating layer  103  can be formed, depending on a material thereof, by a sputtering method, a spin coating method, a dipping method, a printing method, an ink-jet method, or the like. 
     As illustrated in  FIG. 2A , the first wiring  124  is electrically connected to a first electrode layer  104   a   1  of a first light-emitting element  132   a   1  through an auxiliary wiring  125   a   1 . The first light-emitting element  132   a   1  includes the first electrode layer  104   a   1 , an EL layer  106   a   1 , and a second electrode layer  108   a   1 . Light from the EL layer  106   a   1  is emitted through the first electrode layer  104   a   1  and a housing  100  to the outside; thus, an emission surface is on the first electrode layer  104   a   1  side. Accordingly, the first electrode layer  104   a   1  and a housing  100  transmit at least light from the EL layer  106   a   1 . 
     As a material of the first electrode layer  104   a   1 , indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), ITO, an indium oxide-zinc oxide alloy (In 2 O 3 —ZnO), or any of these metal oxide materials in which silicon oxide is contained can be used. Alternatively, graphene may be used as a material of the first electrode layer  104   a   1 . 
     The first wiring  124  is electrically connected to a first electrode layer  104   a   2  of a second light-emitting element  132   a   2  through an auxiliary wiring  125   a   2 . 
     In this embodiment, eight light-emitting elements are categorized into plural stages of light-emitting element units for simplification. 
     The first light-emitting element  132   a   1  and the second light-emitting element  132   a   2  are collectively referred to as a first-stage light-emitting element unit  133   a   1 . The plurality of light-emitting elements in the first-stage light-emitting element unit  133   a   1  is connected in parallel. 
     Current from the power source is supplied to the EL layer  106   a   1  through the first wiring  124 , the auxiliary wiring  125   a   1 , and the first electrode layer  104   a   1 . The second electrode layer  108   a   1  electrically connected to the EL layer  106   a   1  is electrically connected to the auxiliary wiring  125   a   3 . 
     The second electrode layer  108   a   1  is preferably formed using a metal having a low work function (typically, a metal element which belongs to Group 1 or Group 2 of the periodic table), or an alloy thereof. Specifically, aluminum or an aluminum alloy is used for the second electrode layer  108   a   1 . However, when aluminum or an aluminum alloy is provided in direct contact with ITO or the like, the aluminum or the aluminum alloy might possibly be corroded. Therefore, in this embodiment, the second electrode layer  108   a   1  of the first light-emitting element  132   a   1  is not provided in direct contact with the first electrode layer  104   a   3  of the third light-emitting element  132   a   3  serially connected to the first light-emitting element  132   a   1 , but is connected to the first electrode layer  104   a   3  through the auxiliary wiring  125   a   3 . 
     The auxiliary wiring  125   a   3  can be formed to have a single-layer structure or a layered structure using a material such as copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), scandium (Sc), or nickel (Ni) or an alloy material containing any of these materials as its main component. Aluminum may be used as a material of the auxiliary wiring  125   a   3 . When aluminum is used as a material of the auxiliary wiring  125   a   3 , a layered structure in which aluminum is used as a layer not in contact with ITO or the like may be employed so that the problem of corrosion described above is not caused. The auxiliary wiring  125   a   3  has a narrower line width and a smaller thickness than the first wiring and the second wiring. Specifically, the thickness of the auxiliary wiring  125   a   3  is greater than or equal to 0.1 μm and less than 3 μm. 
     Further, the second-stage light-emitting element unit, the last-stage light-emitting element unit  133   a   2  in this embodiment, is provided so as to be adjacent to the first-stage light-emitting element unit  133   a   1 . 
     The last-stage light-emitting element unit  133   a   2  includes the third light-emitting element  132   a   3  serially connected to the first light-emitting element  132   a   1  in the first-stage light-emitting element unit  133   a   1 . 
     In  FIG. 2A , the plurality of light-emitting elements is connected in series. The auxiliary wiring  125   a   3  is electrically connected to the first electrode layer  104   a   3  of the third light-emitting element  132   a   3 . The first electrode layer  104   a   3  is electrically connected to an EL layer  106   a   3 . The second electrode layer  108   a   3  electrically connected to the EL layer  106   a   3  is electrically connected to the second wiring  128 . 
     Further, as illustrated in  FIG. 2A , in the lighting device, an insulating layer  135  which is over and in contact with the first electrode layers and has openings is provided, and the EL layers are in contact with the first electrode layers in the openings. The insulating layer  135  prevents a short circuit between the adjacent light-emitting elements. As illustrated in  FIG. 2B , the insulating layer  135  is provided between the plurality of light-emitting elements which is adjacent to each other and connected in parallel, and end portions of the EL layers are located over the insulating layer  135 . In addition, as illustrated in  FIG. 2B , end portions of the second electrode layers are located over the insulating layer  135  provided between the plurality of light-emitting elements which is adjacent to each other and connected in parallel. Further, as illustrated in  FIG. 2B , the plurality of light-emitting elements is connected in parallel, and as illustrated in  FIG. 2A , an inorganic insulating film  140  is in contact with the insulating layer  135  between the plurality of light-emitting elements which is adjacent to each other. 
     The insulating layer  135  is formed using an organic insulating material such as a polyimide, acrylic, a polyamide, or epoxy, or an inorganic insulating material. It is particularly preferable that the insulating layer  135  be formed using a photosensitive resin material to have openings over the first electrode layers so that sidewalls of the openings are formed to have tilted surfaces with continuous curvature. 
     Note that the second electrode layers  108   a   2  and  108   a   3  can be formed using a material and a manufacturing process which are similar to those of the second electrode layer  108   a   1 . The auxiliary wirings  125   a   1  and  125   a   2  can be formed using a material and a manufacturing process similar to those of the auxiliary wiring  125   a   3  described above. 
     The last-stage light-emitting element unit  133   a   2  includes a fourth light-emitting element  132   a   4  serially connected to the second light-emitting element  132   a   2  in the first-stage light-emitting element unit  133   a   1 . 
     The light-emitting element unit connected to and in contact with the second wiring  128  is the last-stage light-emitting element unit  133   a   2 . 
     Further, as illustrated in  FIG. 1 , the first wiring  124  branches at two positions to have protruding portions, and the second wiring  128  branches at one position to have a protruding portion. In  FIG. 1 , the protruding portions are provided in comb-like shapes. 
     Furthermore, in  FIG. 1 , the protruding portions branching from the two positions of the first wiring  124  are parallel to or substantially parallel to each other and also parallel to the protruding portion branching from the one position of the second wiring  128 . The first-stage light-emitting element unit  133   a   1  and the last-stage light-emitting element unit  133   a   2 , that is, the first to the fourth light-emitting elements are provided between the protruding portion branching from the first position of the two position of the first wiring  124 , and the protruding portion branching from the one position of the second wiring  128 . 
     It is also a feature of this embodiment that the protruding portion branching from the one position which is a part of the second wiring  128  is straight in the length direction, and a layout is axisymmetric with respect to the straight line as a symmetry axis. In  FIG. 1 , a straight line  170  corresponds to the symmetry axis. 
     A first-stage light-emitting element unit  133   b   1  and a last-stage light-emitting element unit  133   b   2 , that is, the fifth to the eighth light-emitting elements are provided between the protruding portion branching from the second position of the two positions of the first wiring  124 , and the protruding portion branching from the one position of the second wiring  128 . 
     The protruding portion branching from the one position of the second wiring  128  is provided between the last-stage light-emitting element unit  133   a   2  and the last-stage light-emitting element unit  133   b   2 . In this manner, the plurality of light-emitting element units is provided axisymmetrically and a plurality of wirings whose length direction corresponds to that of the symmetry axis and which is given the same potentials is formed as one wiring, leading to a reduction in total number of wirings. 
     Note that the inorganic insulating film  140  covering a top surface of the light-emitting element  132  is preferably provided. In addition, an inorganic insulator  102  may be provided between the emission surface of the light-emitting element  132  and a housing  100 . The inorganic insulating film  140  and the inorganic insulator  102  function as protective layers or sealing films which block an external contaminant such as water. By providing the inorganic insulating film  140  and the inorganic insulator  102 , deterioration of the light-emitting element  132  can be suppressed and thus, the durability and the lifetime of the lighting device can be increased. 
     As each of the inorganic insulating film  140  and the inorganic insulator  102 , a single layer or a stack using a nitride film and a nitride oxide film can be used. Specifically, the inorganic insulating film  140  and the inorganic insulator  102  can be formed using silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, aluminum oxynitride, or the like by a CVD method, a sputtering method, or the like depending on the material. The inorganic insulating film  140  and the inorganic insulator  102  are preferably formed using silicon nitride by a CVD method. The thickness of the inorganic insulating film may be approximately 100 nm to 1 μm. 
     Alternatively, as each of the inorganic insulating film  140  and the inorganic insulator  102 , an aluminum oxide film, a diamond like carbon (DLC) film, a carbon film containing nitrogen, or a film containing zinc sulfide and silicon oxide (e.g., a ZnS.SiO 2  film) may be used. 
     As the inorganic insulator  102 , a thin glass substrate can be used. For example, a glass substrate with a thickness in the range of greater than or equal to 30 μm and less than or equal to 100 μm can be used. 
     When a glass substrate or the like is used for the inorganic insulator  102 , entry of moisture, an impurity, or the like into an organic compound and a metal material which are contained in the light-emitting element from the outside of the lighting device can be suppressed. Consequently, deterioration of the light-emitting element due to moisture, an impurity, or the like can be suppressed, leading to improvement in reliability of the lighting device. In addition, the lighting device can have resistance against bending and breaking, and a reduction in weight of the lighting device can be achieved because the glass substrate has a small thickness, a thickness in the range of greater than or equal to 30 μm and less than or equal to 100 μm. 
     As specific examples of a member used for the housing  100 , plastic (an organic resin), glass, quartz, and the like can be given. As an example of plastic, a member made of polycarbonate, polyarylate, polyethersulfone, or the like can be given. It is preferable to use plastic for the housing  100  because a reduction in weight of the lighting device can be achieved. In addition, the lighting device can have resistance against an impulse such as a drop impulse when plastic is used for the housing  100 . 
       FIG. 1  and  FIGS. 2A and 2B  illustrate an example in which the auxiliary wirings each having a smaller thickness and a narrower line width than the first wiring and the second wiring is provided; however, auxiliary wirings each having a narrower line width and a smaller thickness (specifically, greater than or equal to 3 nm and less than or equal to 30 nm) than the auxiliary wiring may be provided in addition to the auxiliary wirings. 
       FIG. 3  is a plan view where auxiliary wirings each having a small thickness is provided. Note that  FIG. 4A  is a cross-sectional view along A 1 -A 2  in  FIG. 3 , and  FIG. 4B  is a cross-sectional view along B 1 -B 2  in  FIG. 3 . A structure except for the auxiliary wirings each having a small thickness is the same as that of  FIG. 1  and  FIGS. 2A and 2B ; thus, specific description thereof is omitted here. 
       FIG. 3  illustrates an example in which auxiliary wirings  126  whose pattern shapes are mesh-like shapes are provided. As in  FIG. 3 , the auxiliary wirings  126  are provided in contact with the auxiliary wirings  125  and are electrically connected to the auxiliary wirings  125 . As in  FIG. 3  and  FIGS. 4A and 4B , the auxiliary wirings  126  are formed in contact with the first electrode layers  104  and are located to overlap with light-emitting regions. The auxiliary wirings  126  each having a smaller thickness and a narrower line width than the auxiliary wiring  125  is less likely to influence the total area of light emission. Although  FIGS. 4A and 4B  illustrate an example in which the auxiliary wirings  126  are provided over the insulating layer  103 , the auxiliary wirings  126  may be provided over the first electrode layers  104 . 
     The use of a material such as copper for formation of the auxiliary wiring  126  can suppress a voltage drop when the first electrode layer  104  has a large area. The material of the second auxiliary wiring is not limited to copper and may be the same as a material of the auxiliary wiring  125 . 
     The pattern shape of the auxiliary wiring  126  is not limited to a mesh-like shape but may be a stripe shape. It is preferable that the pattern shape, the width, and the thickness of the auxiliary wiring  126  and the resistivity of a material of the auxiliary wiring  126  be adjusted as appropriate depending on the shape and the area of the first electrode layer  104  by a practitioner so that a voltage drop is suppressed as much as possible. 
     In the cases of the structures in  FIG. 1 ,  FIGS. 2A and 2B ,  FIG. 3 , and  FIGS. 4A and 4B , the first wiring, the second wiring, the auxiliary wiring (or the second auxiliary wiring), and the first electrode layer can be formed through a photolithography process. The EL layer can be formed using a deposition mask, and a pattern of the second electrode layer can also be formed using a deposition mask. 
     When a glass substrate having a large area, e.g., an area of 720 mm×600 mm or an area of 750 mm×620 mm, is used for the housing  100 , a layout in  FIG. 5  is possible.  FIG. 5  illustrates an example in which a plurality of light-emitting elements is efficiently arranged over a glass substrate having a large area with the use of the structure in  FIG. 3 . 
     A block A includes a first wiring  124 A branching at a plurality of positions, a second wiring  128 A, and plural stages of light-emitting units between the first wiring  124 A and the second wiring  128 A. 
     A block B includes a first wiring  124 B branching at a plurality of positions, a second wiring  128 B, and plural stages of light-emitting units between the first wiring  124 B and the second wiring  128 B. 
     A block C includes a first wiring  124 C branching at a plurality of positions, a second wiring  128 C, and plural stages of light-emitting units between the first wiring  124 C and the second wiring  128 C. 
     It is needless to say that the block A, the block B, and the block C have the same patterns. Further, converters are illustrated in  FIG. 5 ; a first converter  160 A, a second converter  160 B, and a third converter  160 C are provided in the block A, the block B, and the block C, respectively, to form one large lighting device. Note that the structure in  FIG. 5  is only an example; thus, the number of blocks is not limited to that of the structure in  FIG. 5 . For example, the number of blocks may be five. 
     In the cases of the structures in  FIG. 1 ,  FIGS. 2A and 2B ,  FIG. 3 , and  FIGS. 4A and 4B , a photomask is used and exposure is performed in a photolithography process. For example, when a glass substrate with a size of 720 mm×600 mm is used and an exposure apparatus which can expose an area of 310 mm×560 mm to one shot of light, it is preferable to perform exposure of the block A, the block B, and the block C, which are illustrated in  FIG. 5 , to a first shot of light, a second shot of light, and a third shot of light, respectively, for efficient exposure. Note that it is needless to say that the size of an area which can be exposed to one shot of light can be changed by optical adjustment, and the positions to be exposed are adjusted so that a region to be exposed to one shot of light does not overlap with a region to be exposed to another shot of light. 
     In the layout of  FIG. 5 , branching main wirings like costae are provided and the main wirings each have first auxiliary wirings like lateral veins. This state is like a state of a leaf having plural veins with different thicknesses. Further, the first auxiliary wiring has second auxiliary wirings which are extremely thin. It can be said that in the wiring layout in  FIG. 5 , the plural wirings with different thicknesses efficiently supply current from a power source to a plurality of light-emitting elements, like plural veins with different thicknesses of a leaf. 
     Note that in the lighting device described in this embodiment, the light-emitting element  132  may be provided in a housing. In that case, the housing for sealing the light-emitting element  132  may be a combination of a plurality of housings attached to each other. For example, another housing is provided to face the housing  100  so that the light-emitting element  132  is encapsulated, which enables sealing of the light-emitting element  132 . The housing provided opposite to the housing  100  can be formed using a material similar to that of the housing  100 . 
     Further, as illustrated in  FIG. 7 , a top surface of the light-emitting element  132  may be provided with a metal plate  111  opposite to the housing  100 . There is no particular limitation on the thickness of the metal plate  111 ; however, the metal plate with a thickness in the range of greater than or equal to 10 μm and less than or equal to 200 μm is preferably used because a reduction in weight of the lighting device can be achieved. A material of the metal plate  111  is not limited to a particular material, but it is preferable to use a metal such as aluminum, copper, or nickel, an alloy such as an aluminum alloy or stainless steel, or the like. 
     The metal plate  111  and the housing  100  are attached to each other with an adhesive layer  134 , so that the light-emitting element  132  can be sealed in. As the adhesive layer  134 , a visible light curable adhesive, a UV curable adhesive, or a thermosetting adhesive can be used. As examples of materials of such adhesives, an epoxy resin, an acrylic resin, a silicone resin, a phenol resin, and the like can be given. A moisture-absorbing substance serving as a drying agent may be contained in the adhesive layer  134 . 
     Since the metal plate  111  has low permeability, sealing of the light-emitting element  132  with the metal plate  111  and the housing  100  can prevent entry of moisture into the light-emitting element  132 . Thus, by providing the metal plate  111 , a highly reliable lighting device in which deterioration due to moisture is suppressed can be obtained. 
     An inorganic insulating film, a glass substrate, a quartz substrate, or the like may be used instead of the metal plate  111 . 
     A moisture-absorbing substance serving as a drying agent may be provided in a space where the light-emitting element  132  is provided. The moisture-absorbing substance may be placed as a solid such as a powdery substance or may be provided as a film containing a moisture-absorbing substance over the light-emitting element  132  by a deposition method such as a sputtering method. 
     The shape of the emission surface of the light-emitting element  132  may be a circular shape or a polygonal shape such as a quadrangle, and the shape of the housing covering the emission surface may correspond to the shape of the emission surface. 
     Further, a plurality of light-emitting elements having different emission colors is provided and connected to an external power source individually to control a current value and a voltage value, so that light emission color from the lighting device can be adjusted and color rendering properties thereof can be improved. 
       FIGS. 6A and 6B  are examples of cross-sectional views of lighting devices incorporating converters. Note that a converter refers to a constant current power source which converts input power into a constant current suited to specifications of a lighting device and supplies the constant current to the lighting device, or a converter circuit functioning as a constant voltage power source which converts input power into a constant voltage suited to specifications of a lighting device and supplies the constant voltage to the lighting device. 
     The lighting device in  FIG. 6A  is connected to a converter  160  through a terminal electrode  164 . In  FIG. 6A , the terminal electrode  164  has a layered structure of a first layer  164   a  and a second layer  164   b . The terminal electrode  164  can be formed in the same step as the first wiring  124  and the second wiring  128 . In the lighting device in  FIG. 6A , the light-emitting element  132  is sealed with the housing  100 , a housing  168 , and a sealant  162 . For the housing  168  opposite to the housing  100 , a metal plate, a material similar to that of the housing  100 , or the like can be used. 
     The first electrode layer  104  and the second electrode layer  108  which are included in the light-emitting element  132  are electrically connected to the converter  160 , and a current suited to specifications of the lighting device, which is obtained by conversion in the converter  160 , is supplied to the light-emitting element  132 . Since current is supplied from both sides of the light-emitting element to the light-emitting element, luminance unevenness can be reduced and concentration of a load to part of the light-emitting element can be suppressed. 
     Note that it is preferable that a wiring for connecting the converter  160  overlap with a non-light-emitting region of the lighting device in order not to reduce light extraction efficiency of the lighting device. The converter  160  may be provided so as to protrude to be higher than a region sealed with the sealant  162  as in  FIG. 6B . Alternatively, the converter  160  is not provided over the housing  100  and may be provided externally. 
     As illustrated in  FIG. 6B , a region between the inorganic insulating film  140  and the housing  168  may be filled with a resin  151 . As the resin  151 , an epoxy resin, an acrylic resin, a silicone resin, a phenol resin, or the like can be used. Filling of the region between the inorganic insulating film  140  and the housing  168  with the resin  151  allows the lighting device to withstand the weight of an object when the object is placed over the housing  168 . 
     In  FIGS. 6A and 6B , the converter  160  is a DC-DC converter and includes a printed circuit board (not illustrated). When a printed circuit board is used, insulation of a connection surface between the printed circuit board and a terminal electrode is ensured, which facilitates alignment in provision of the printed circuit board over the terminal electrode. As the printed circuit board, a flexible printed circuit board (FPC) or a semi-flexible printed circuit board partly having flexibility may be used. When a flexible printed circuit board is used, a converter can be incorporated in a flexible lighting device or a lighting device having a curved surface. 
     Note that a circuit element provided in the converter  160  may be formed so as to be embedded in the insulating layer  103  in order to prevent the thickness of the lighting device from increasing when the converter  160  is provided over the terminal electrode. 
     As described above, when a converter is incorporated in a lighting device, a malfunction in which overcurrent flows to a light-emitting element can be prevented owing to a function of supplying a stable current suited to an element even if an input voltage varies. Further, a lighting device which is usable even without an external converter can be provided, so that the utilization range of lighting devices is widened. Furthermore, provision of a converter or a connection wiring over a non-light-emitting region enables suppression of a decrease in area of a light-emitting region; thus, a lighting device with high illuminance can be provided. 
     Note that a converter included in the lighting device according to this embodiment is not limited to a DC-DC converter and may be an AC-DC converter which converts AC voltage into DC voltage. When an AC-DC converter is used, an AC power supply can be applied without conversion. Further, the number of light-emitting elements electrically connected to one converter in the lighting device according to this embodiment is not limited to that of the structure described in this embodiment. 
     As illustrated in  FIGS. 11A and 11B , the first wirings  124  ( 124   a  and  124   b ) and the second wirings  128  ( 128   a  and  128   b ) may be provided so as to be embedded in recessed portions of the housing  100 . 
     The recessed portions of the housing  100  may be formed by etching or by processing an organic resin with a supporter serving as a mold so that the organic resin has depressions and projections. 
     In the case where the inorganic insulator  102  is provided over the housing  100  having the recessed portions, the inorganic insulator  102  may be formed over the housing  100 , and then the auxiliary wirings may be formed over the inorganic insulator  102  so as to fill the recessed portion of the housing  100 . 
     An optical film may be provided on the emission surface side of the lighting device. For example, the housing  100  may be provided with a diffusion film in a region which covers the emission surface and is opposite to the light-emitting element  132  side. Further, to increase extraction efficiency, a lens array overlapping with the plurality of light-emitting elements  132  may be provided on the emission surface side. 
       FIGS. 12A and 12B  are examples of using housings  241  and  242  each having an uneven shape including a plurality of projections like the shape of a microlens array on the emission surface side (the side opposite to the light-emitting element  132  side). When any of the structures of the housings  241  and  242  each having an uneven shape including a plurality of projections is used as the structure of the housing  100 , total reflection at the interface between the air and the housing  241 / 242  can be suppressed; thus, the efficiency of light extraction to the outside of the housing can be improved. 
     Further, provision of such a housing having an uneven shape allows diffusion of light of a light-emitting element, which is emitted to the outside; thus, even adverse effects caused when a light-blocking auxiliary wiring is provided over the emission surface can be reduced. Accordingly, a lighting device can emit light uniformly and favorably. 
     The housing  241  having an uneven shape can be formed using an organic resin as its material. The organic resin can be processed by heat treatment or light irradiation treatment depending on characteristics of the organic resin. For example, a support having an uneven shape and serving as a mold of the uneven shape of the housing is prepared, a thermoplastic organic resin is used as the material of the housing, the thermoplastic organic resin is pressed in the support while performing heat treatment so that the shape of the thermoplastic organic resin is changed to reflect the shape of the support, and then the thermoplastic organic resin is cured by cooling; thus, the housing  241  having an uneven shape can be formed. 
     As a specific example of a member used for the housing  241 , an organic resin (plastic) is given. As an example of plastic, a member made of polycarbonate, polyarylate, polyethersulfone, or the like is given. 
     FIGS.  13 A 1 ,  13 A 2 ,  13 B 1 , and  13 B 2  illustrate cross-sectional views and plan views of the housing  241  in  FIG. 12A . FIGS.  13 A 2  and  13 B 2  illustrate examples of plan views of housings  241   a  and  241   b  each having an uneven shape. FIG.  13 A 1  is a cross-sectional view along X 1 -Y 1  in FIG.  13 A 2  and FIG.  13 B 1  is a cross-sectional view along X 2 -Y 2  in FIG.  13 B 2 . 
     The housing  241   a  illustrated in FIGS.  13 A 1  and  13 A 2  has an uneven shape in which a projection has a semispherical shape. The housing  241   b  illustrated in FIGS.  13 B 1  and  13 B 2  has an uneven shape in which the base of a projection is a regular hexagon. The pitch or the bottom shape of the plurality of projections included in the housing  241  can be set variously and is not limited to the structures in FIGS.  13 A 1 ,  13 A 2 ,  13 B 1 , and  13 B 2 . For example, the housing  121   a  may have an uneven shape with an apex, such as a circular cone or a pyramid (e.g., a triangular pyramid or a square pyramid). Note that it is preferable to employ a so-called honeycomb structure for an uneven shape in which the base of a projection is a regular hexagon as illustrated in FIGS.  13 B 1  and  13 B 2  because the density of arrangement of the uneven shape can be improved and the extraction efficiency of light to the outside of the housing can be further improved. 
     Further, as illustrated in  FIG. 12B , the housing  242  having an uneven shape including a plurality of projections, like a microlens array, may be used on both the emission surface side (on the side opposite to the light-emitting element  132 ) and the light-emitting element  132  side. 
     The housing  242  illustrated in  FIG. 12B  has uneven shapes each including a plurality of projections on both sides. Further, in  FIG. 12B , the uneven shape including a plurality of projections provided on the light-emitting element  132  side overlap with the light-emitting element  132 . A high refractive index material layer  235  is provided in a region having the uneven shape on the light-emitting element  132  side with respect to the housing  242 , so as to be in contact with the uneven shape. Note that the uneven shape provided on the housing  242  is arranged in a stripe shape, which is effective, but is preferably arranged in a matrix shape. 
     The housing  242  illustrated in  FIG. 12B  has the uneven shape including a plurality of projections on the emission surface side of the housing  242 , whereby total reflection at the interface between the housing  242  and the air can be suppressed. Moreover, the uneven shape including a plurality of projections is provided between the housing  242  and the high refractive index material layer  235 , whereby total reflection at the interface between the high refractive index material layer  235  and the housing  242  can be suppressed and the extraction efficiency of light to the outside of the housing can be further improved. 
     As examples of a material that can be used for the housing  242 , glass, a resin, and the like whose refractive index is greater than 1.0 and less than 1.6 are given. As the resin, a polyester resin, a polyacrylonitrile resin, a polyimide resin, a polymethyl methacrylate resin, a polycarbonate resin, a polyethersulfone resin, a polyamide resin, a cycloolefin resin, a polystyrene resin, a polyamide imide resin, a polyvinylchloride resin, or the like can be used. In particular, a material whose refractive index is greater than or equal to 1.4 and less than 1.6 is preferably used. 
     As a method for forming an uneven shape in the above material, for example, an etching method, a sand blasting method, a microblast processing method, a droplet discharge method, a printing method (screen printing or offset printing by which a pattern is formed), a coating method such as a spin coating method, a dipping method, a dispenser method, an imprint method, a nanoimprint method, or the like can be employed as appropriate. 
     The high refractive index material layer  235  is formed of high refractive index glass, a high refractive index liquid, or a high refractive index resin. The high refractive index material layer  235  has a light-transmitting property and the refractive index is greater than or equal to 1.6, preferably greater than or equal to 1.7 and less than or equal to 2.1. As examples of the high refractive index glass, a resin containing bromine, a resin containing sulfur, and the like are given. For example, a sulfur-containing polyimide resin, an episulfide resin, a thiourethane resin, a brominated aromatic resin, or the like can be used. Alternatively, polyethylene terephthalate (PET), triacetyl cellulose (TAC), or the like may be used. As the high refractive index liquid, a contact liquid (refractive liquid) containing sulfur and methylene iodide, or the like can be used. Any of a variety of methods suitable for the material may be employed for forming the high refractive index material layer  235 . For example, the above resin is selectively formed in contact with the uneven shape including a plurality of projections provided on the light-emitting element  132  side of the housing  242  by a spin coating method and is cured by heat or light, so that the high refractive index material layer  235  can be formed. The material and the deposition method can be selected as appropriate in consideration of the adhesion strength, ease of processing, or the like. Note that the high refractive index material layer  235  also functions as a planarization film for the uneven shape including a plurality of projections provided on the light-emitting element  132  side of the housing  242 . 
     In general, a resin with a high refractive index is expensive, but in the lighting device illustrated in  FIG. 12B , the high refractive index material layer  235  may be selectively formed in a region overlapping with the light-emitting element  132  and in contact with the uneven shape including a plurality of projections, and the thickness of the high refractive index material layer  235  is several tens of micrometers, which is small. Thus, the lighting device with high light extraction efficiency can be manufactured at low cost. 
     The size and the height of the projection of the uneven shape on the side which is in contact with the high refractive index material layer  235  are each preferably approximately 0.1 μm to 100 μm. The size and the height of the projection of the uneven shape on the opposite side are each preferably approximately 0.1 μm to 1000 μm. The size of the projection of the uneven shape on the side which is in contact with the high refractive index material layer  235  affects the amount of material used for the high refractive index material layer  235 ; thus, the allowable range of the size and the height of the projection is narrow. In contrast, the projection included in the uneven shape on the opposite side may have a size or height exceeding 1000 μm. In particular, each of the projections included in the uneven shapes on both sides preferably have a size or height of greater than or equal to 1 μm because an influence of interference of light can be suppressed. 
     Further, in  FIG. 12B , the high refractive index material layer  235  is provided between the light-emitting element  132  and the housing  242 . When a nitride film with a refractive index in the range of greater than or equal to 1.6 is used as the high refractive index material layer  235 , diffusion of impurities to the light-emitting element can be prevented without a decrease in the extraction efficiency of light, which is preferable. 
     According to one embodiment of the present invention, a lighting device can be formed by connecting a plurality of light-emitting element units of given stages in series or in series-parallel combination; thus, the lighting device can be increased in size. For example, it is possible to realize a lighting device whose emission region is one surface of a large substrate with a size corresponding to G5.5 (1100 mm×1300 mm) to G11 (3000 mm×3300 mm) of the size of a mother glass substrate of a liquid crystal panel. 
     Further, the lighting device according to this embodiment can be thin and lightweight. 
     When plastic or a thin metal plate is used for a substrate, a lighting device which can resist an impulse such as a drop impulse can be provided. 
     In the lighting device according to this embodiment, the plurality of light-emitting elements can emit light efficiently or the total emission area can be increased. 
     Embodiment 2 
     In this embodiment, an example of an element structure of a light-emitting element exhibiting organic EL emission, which is used in a lighting device according to one embodiment of the present invention, will be described. The light-emitting element exhibiting organic EL emission generates a smaller amount of heat than an LED. Thus, an organic resin can be used for a housing, so that a reduction in weight of the lighting device is possible, which is preferable. 
     The light-emitting element illustrated in  FIG. 8A  includes the first electrode layer  104 , an EL layer  106  over the first electrode layer  104 , and the second electrode layer  108  over the EL layer  106 . 
     The EL layer  106  includes at least a light-emitting layer containing a light-emitting organic compound. In addition, the EL layer  106  can have a layered structure in which a layer containing a substance having a high electron transport property, a layer containing a substance having a high hole transport property, a layer containing a substance having a high electron injection property, a layer containing a substance having a high hole injection property, a layer containing a bipolar substance (a substance having a high electron transport property and a high hole transport property), and the like are combined as appropriate. In this embodiment, a hole injection layer  701 , a hole transport layer  702 , a light-emitting layer  703 , an electron transport layer  704 , and an electron injection layer  705  are stacked in this order from the first electrode layer  104  side in the EL layer  106 . 
     A method for manufacturing the light-emitting element illustrated in  FIG. 8A  will be described. 
     First, the first electrode layer  104  is formed. The first electrode  104  is provided in the direction in which light is extracted from the EL layer, and thus is formed using a light-transmitting material. 
     As the light-transmitting material, indium oxide, ITO, an indium oxide-zinc oxide alloy (also referred to as IZO (registered trademark)), zinc oxide, zinc oxide to which gallium is added, graphene, or the like can be used. 
     As the first electrode layer  104 , a metal material such as gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or titanium can be used. Alternatively, a nitride of the metal material (such as titanium nitride), or the like may be used. In the case of using the metal material (or the nitride thereof), the first electrode layer  104  may be thinned so as to be able to transmit light. 
     Next, the EL layer  106  is formed over the first electrode layer  104 . In this embodiment, the EL layer  106  includes the hole injection layer  701 , the hole transport layer  702 , the light-emitting layer  703 , the electron transport layer  704 , and the electron injection layer  705 . 
     The hole injection layer  701  is a layer containing a substance having a high hole injection property. As the substance having a high hole injection property, for example, a metal oxide such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, or manganese oxide can be used. A phthalocyanine-based compound such as phthalocyanine (abbreviation: H 2 Pc) or copper(II) phthalocyanine (abbreviation: CuPc) may be used. 
     Further, any of the following aromatic amine compounds which are low molecular organic compounds can be used: 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1). 
     Still alternatively, any of high molecular compounds (e.g., oligomers, dendrimers, or polymers) may be used. Examples of the high-molecular compound include poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD). A high molecular compound to which acid is added, such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS) or polyaniline/poly(styrenesulfonic acid) (PAni/PSS), may be used. 
     In particular, for the hole injection layer  701 , a composite material in which an organic compound having a high hole transport property is mixed with an acceptor substance is preferably used. With the use of the composite material in which an organic compound having a high hole transport property is mixed with an acceptor substance, excellent hole injection from the first electrode layer  104  can be obtained, which results in a reduction in driving voltage of the light-emitting element. Such a composite material can be formed by co-evaporation of a substance having a high hole transport property and an acceptor substance. The hole injection layer  701  is formed using the composite material, whereby hole injection from the first electrode layer  104  to the EL layer  106  is facilitated. 
     As the organic compound for the composite material, any of a variety of compounds such as aromatic amine compounds, carbazole derivatives, aromatic hydrocarbons, and high molecular compounds (e.g., oligomers, dendrimers, and polymers) can be used. The organic compound used for the composite material is preferably an organic compound having a high hole transport property. Specifically, a substance having a hole mobility of 10 −6  cm 2 /Vs or higher is preferably used. Note that any other substance may be used as long as the hole transport property thereof is higher than the electron transport property thereof. Specific examples of the organic compound that can be used for the composite material are given below. 
     Examples of the organic compound that can be used for the composite material include aromatic amine compounds such as TDATA, MTDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), and 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), and carbazole compounds such as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene. 
     Further, any of the following aromatic hydrocarbon compounds may be used: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, and the like. 
     Furthermore, any of the following aromatic hydrocarbon compounds may be used: 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl, 10,10′-bis(2-phenylphenyl)-9,9′-bianthryl, 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA), and the like. 
     Examples of the electron acceptor include organic compounds such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ) and chloranil; and transition metal oxides. Other examples include oxides of metals belonging to Groups 4 to 8 in the periodic table. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because of their high electron accepting property. Among these, molybdenum oxide is particularly preferable because it is stable in the air, has a low hygroscopic property, and is easily handled. 
     The composite material may be formed using the above electron acceptor and the above high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD and used for the hole injection layer  701 . 
     The hole transport layer  702  is a layer containing a substance having a high hole transport property. As the substance having a high hole transport property, any of the following aromatic amine compounds can be used, for example: NPB, TPD, BPAFLP, 4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: DFLDPBi), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB). The substances given here are mainly ones that have a hole mobility of 10 −6  cm 2 /Vs or higher. Note that any other substance may be used as long as the hole transport property thereof is higher than the electron transport property thereof. Note that the layer containing a substance having a high hole transport property is not limited to a single layer and may be formed of a stack of two or more layers containing any of the above substances. 
     For the hole transport layer  702 , a carbazole derivative such as CBP, CzPA, or PCzPA or an anthracene derivative such as t-BuDNA, DNA, or DPAnth may be used. 
     For the hole transport layer  702 , a high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD may be used. 
     The light-emitting layer  703  is a layer containing a light-emitting organic compound. As the light-emitting organic compound, for example, a fluorescent compound which exhibits fluorescence or a phosphorescent compound which exhibits phosphorescence can be used. 
     The fluorescent compounds that can be used for the light-emitting layer  703  will be given. Examples of a material for blue light emission include N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), and 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA). Further, examples of a material for green light emission include N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), and N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA). Examples of a material for yellow light emission include rubrene and 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT). Examples of a material for red light emission include organometallic complexes such as bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C 3′ ]iridium(III) acetylacetonate (abbreviation: Ir(btp) 2 (acac)), bis(1-phenylisoquinolinato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: Ir(piq) 2 (acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq) 2 (acac)), and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP). Examples of a material for red light emission include N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), and 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD). 
     The phosphorescent compounds that can be used for the light-emitting layer  703  will be given. Examples of a material for blue light emission are include bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) picolinate (abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C 2′ }iridium(III) picolinate (abbreviation: Ir(CF 3 ppy) 2 (pic)), and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) acetylacetonate (abbreviation: FIr(acac)). Examples of a material for green light emission include tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: Ir(ppy) 3 ), bis(2-phenylpyridinato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(ppy) 2 (acac)), bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate (abbreviation: Ir(pbi) 2 (acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq) 2 (acac)), and tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq) 3 ). Examples of a material for yellow light emission include bis(2,4-diphenyl-1,3-oxazolato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(dpo) 2 (acac)), bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph) 2 (acac)), bis(2-phenylbenzothiazolato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(bt) 2 (acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)-5-methylpyrazinato]iridium(III) (abbreviation: Ir(Fdppr-Me) 2 (acac)), and (acetylacetonato)bis{2-(4-methoxyphenyl)-3,5-dimethylpyrazinato}iridium(III) (abbreviation: Ir(dmmoppr) 2 (acac)). Examples of a material for orange light emission include tris(2-phenylquinolinato-N,C 2′ )iridium(III) (abbreviation: Ir(pq) 3 ), bis(2-phenylquinolinato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(pq) 2 (acac)), (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me) 2 (acac)), and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr) 2 (acac)). Examples of a material for red light emission include organometallic complexes such as bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C 3′ ]iridium(III)acetylacetonate (abbreviation: Ir(btp) 2 (acac)), bis(1-phenylisoquinolinato-N,C 2′ )iridium(III)acetylacetonate (abbreviation: Ir(piq) 2 (acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq) 2 (acac)), (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr) 2 (acac)), (dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr) 2 (dpm)), and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II) (abbreviation: PtOEP). Further, rare-earth metal complexes, such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac) 3 (Phen)), tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM) 3 (Phen)), and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA) 3 (Phen)), exhibit light emission from rare-earth metal ions (electron transition between different multiplicities), and thus can be used as phosphorescent compounds. 
     Note that the light-emitting layer  703  may have a structure in which the above light-emitting organic compound (a guest material) is dispersed in another substance (a host material). As a host material, a variety of kinds of materials can be used, and it is preferable to use a substance which has a lowest unoccupied molecular orbital level (LUMO level) higher than that of the light-emitting material and has a highest occupied molecular orbital level (HOMO level) lower than that of the light-emitting material. 
     Specific examples of the host material include metal complexes such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), and bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); heterocyclic compounds such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), and bathocuproine (abbreviation: BCP); condensed aromatic compounds such as 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 9,9′-bianthryl (abbreviation: BANT), 9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS), 9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), 3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3), 9,10-diphenylanthracene (abbreviation: DPAnth), and 6,12-dimethoxy-5,11-diphenylchrysene; and aromatic amine compounds such as N,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine (abbreviation: DPhPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine (abbreviation: PCAPBA), N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, and BSPB. 
     As the host material, plural kinds of materials can be used. For example, in order to suppress crystallization, a substance such as rubrene which suppresses crystallization may be further added. In addition, NPB, Alq, or the like may be further added in order to efficiently transfer energy to a guest material. 
     When a structure in which a guest material is dispersed in a host material is employed, crystallization of the light-emitting layer  703  can be suppressed. Further, concentration quenching due to high concentration of the guest material can be suppressed. 
     For the light-emitting layer  703 , a high molecular compound can be used. Specifically, a material for blue light emission, a material for green light emission, and a material for orange to red light emission are given. Examples of a material for blue light emission include poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: PFO), poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)] (abbreviation: PF-DMOP), and poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]} (abbreviation: TAB-PFH). Examples of a material for green light emission include poly(p-phenylenevinylene) (abbreviation: PPV), poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazole-4,7-diyl)](abbreviation: PFBT), and poly[(9,9-dioctyl-2,7-divinylenfluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]. Examples of a material for orange to red light emission include poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation: MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation: R4-PAT), poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenyl amino)-1,4-phenylene]}, and poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]} (abbreviation: CN-PPV-DPD). 
     Note that the light-emitting layer  703  may have a layered structure of two or more layers. When the light-emitting layer  703  has a layered structure of two or more layers and the kinds of light-emitting substances for light-emitting layers vary, a variety of emission colors can be obtained. In addition, a plurality of light-emitting substances of different colors is used as the light-emitting substances, whereby light emission having a broad spectrum or white light emission can also be obtained. In particular, for a lighting device in which high luminance is required, a structure in which light-emitting layers are stacked is preferable. 
     The electron transport layer  704  is a layer containing a substance having a high electron transport property. As the substance having a high electron transport property, any of the following can be used, for example: a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq 2 ), or bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq). Alternatively, a metal complex or the like including an oxazole-based or thiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX) 2 ) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ) 2 ), can be used. Other than the metal complexes, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or the like may be used. The substances mentioned here are mainly ones that have an electron mobility of 10 −6  cm 2 /V·s or higher. The electron transport layer is not necessarily a single layer and may be formed of a stack including two or more layers made of the aforementioned substance. 
     The electron injection layer  705  is a layer containing a substance having a high electron injection property. For the electron injection layer  705 , an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium, cesium, calcium, lithium fluoride, cesium fluoride, calcium fluoride, or lithium oxide, can be used. Further, a rare earth metal compound such as erbium fluoride may be used. A substance for forming the electron transport layer  704  may be used. 
     Note that the hole injection layer  701 , the hole transport layer  702 , the light-emitting layer  703 , the electron transport layer  704 , and the electron injection layer  705  which are described above can each be formed by an evaporation method (e.g., a vacuum evaporation method), an ink-jet method, a coating method, or the like. 
     Note that a plurality of EL layers may be stacked between the first electrode layer  104  and the second electrode layer  108  as illustrated in FIGS.  8 B 1  and  8 B 2 .  FIG. 8B   1  illustrates an example including two EL layers, in which a first EL layer  800  and a second EL layer  801  are provided between the first electrode layer  104  and the second electrode layer  108 , with a charge generation layer  803  laid between the first EL layer  800  and the second EL layer  801 . FIG.  8 B 2  illustrates an example including three EL layers, in which the first EL layer  800 , the second EL layer  801 , and a third EL layer  802  are provided between the first electrode layer  104  and the second electrode layer  108 , with charge generation layers  803   a  and  803   b  laid between the first EL layer  800  and the second EL layer  801  and between the second EL layer  801  and the third EL layer  802 , respectively. 
     When the EL layers are stacked, the electron generation layer (the electron generation layer  803 ,  803   a , or  803   b ) is preferably provided between the stacked EL layers (between the first EL layer  800  and the second EL layer  801  or between the second EL layer  801  and the third EL layer  802 ). The charge generation layers  803 ,  803   a , and  803   b  can each be formed using the above composite material. Further, the charge generation layers  803 ,  803   a , and  803   b  may each have a layered structure including a layer containing the composite material and a layer containing another material. In that case, as the layer containing another material, a layer containing an electron donating substance and a substance having a high electron transport property, a layer formed of a transparent conductive film, or the like can be used. As for a light-emitting element having such a structure, problems such as energy transfer and quenching are less likely to occur, and thus a light-emitting element which has both high luminous efficiency and long lifetime can be easily obtained due to expansion in the choice of materials. Moreover, a light-emitting element which provides phosphorescence from one of the EL layers and fluorescence from the other can be readily obtained. Note that this structure can be combined with any of the above structures of the EL layer. 
     When the charge generation layer is provided between the stacked EL layers as illustrated in FIGS.  8 B 1  and  8 B 2 , the element can have high luminance and a long lifetime while the current density is kept low. In addition, a voltage drop due to resistance of an electrode material can be reduced, whereby uniform light emission in a large area is possible. 
     In the case of a stack-type element having a structure in which two EL layers are stacked, white light emission can be extracted outside by allowing a first EL layer and a second EL layer to emit light of complementary colors. White light emission can also be obtained with a structure including a plurality of light-emitting layers in which light emission color of a first EL layer and light emission color of a second EL layer are colors complementary to each other. As complementary relations blue and yellow, blue-green and red, and the like can be given. A substance which emits blue light, yellow light, blue-green light, or red light may be selected as appropriate from, for example, the light-emitting substances given above. 
     An example of a light-emitting element having a structure in which a plurality of EL layers is stacked will be described below. First, an example of a structure in which each of the first EL layer and the second EL layer includes a plurality of light-emitting layers which emit light of complementary colors will be described. With this structure, white light emission can be obtained. 
     For example, the first EL layer includes a first light-emitting layer which has an emission spectrum with a peak in the wavelength range of blue to blue-green, and a second light-emitting layer which has an emission spectrum with a peak in the wavelength range of yellow to orange. The second EL layer includes a third light-emitting layer which has an emission spectrum with a peak in the wavelength range of blue-green to green, and a fourth light-emitting layer which has an emission spectrum with a peak in the wavelength range of orange to red. 
     In this case, light emission from the first EL layer is a combination of light emission from both the first light-emitting layer and the second light-emitting layer and thus exhibits an emission spectrum having both a peak in the wavelength range of blue to blue-green and a peak in the wavelength range of yellow to orange. That is, the first EL layer emits light of two-wavelength type white or a two-wavelength type color close to white. 
     In addition, light emission from the second EL layer is a combination of light emission from both the third light-emitting layer and the fourth light-emitting layer and thus exhibits an emission spectrum having both a peak in the wavelength range of blue-green to green and a peak in the wavelength range of orange to red. That is, the second EL layer emits light of two-wavelength type white color or a two-wavelength type color close to white, which is different from that of the first EL layer. 
     Accordingly, by combining the light emission from the first EL layer and the light emission from the second EL layer, white light emission which covers the wavelength range of blue to blue-green, the wavelength range of blue-green to green, the wavelength range of yellow to orange, and the wavelength range of orange to red can be obtained. 
     Further, the wavelength range of yellow to orange (greater than or equal to 560 nm and less than 580 nm) is a wavelength range of high spectral luminous efficacy; thus, application of an EL layer which includes a light-emitting layer having an emission spectrum peak in the wavelength range of yellow to orange is useful. For example, a structure can be used in which a first EL layer which includes a light-emitting layer having an emission spectrum peak in a blue wavelength range, a second EL layer which includes a light-emitting layer having an emission spectrum peak in an yellow wavelength range, and a third EL layer which includes a light-emitting layer having an emission spectrum peak in a red wavelength range are stacked. 
     Further, two or more EL layers exhibiting yellow to orange color may be stacked. The power efficiency can be further improved by stacking two or more EL layers exhibiting yellow to orange color. 
     For example, in the case of a light-emitting element in which three EL layers are stacked as in FIG.  8 B 1 , a second EL layer and a third EL layer each of which includes a light-emitting layer having an emission spectrum peak in yellow to orange wavelength range may be stacked over a first EL layer which includes a light-emitting layer having an emission spectrum peak in a blue wavelength range (greater than or equal to 400 nm and less than 480 nm). Note that the wavelengths of the peaks of the spectra of light emitted from the second EL layer and the third EL layer may be the same or different from each other. 
     When the number of EL layers which are stacked is increased, the power efficiency of a light-emitting element can be improved; however, there occurs a problem that the manufacturing process becomes complicated. Thus, the structure in which three EL layers are stacked as in FIG.  8 B 2  is preferable because the power efficiency is high as compared to the case of a structure of two EL layers and the manufacturing process is simple as compared to the case of a structure of four or more EL layers. 
     The use of the EL layer which has an emission spectrum peak in the yellow to orange wavelength range makes it possible to utilize the wavelength range of high spectral luminous efficacy, allowing improvement in power efficiency. Accordingly, the power efficiency of the whole light-emitting element can be improved. Such a structure is advantageous in terms of spectral luminous efficacy and thus enables improvement in power efficiency as compared to the case where, for example, an EL layer which emits green light and an EL layer which emits red light are stacked to obtain a light-emitting element which emits yellow to orange light. Further, the emission intensity of light of the blue wavelength range of low spectral luminous efficacy is relatively low as compared to the case of using one EL layer which has an emission spectrum peak in the yellow to orange wavelength range of high spectral luminous efficacy; thus, the color of emitted light is close to incandescent color (or warm white), and the power efficiency is improved. 
     In other words, in the above light-emitting element, the color of light which is obtained by combining light whose emission spectrum peak is in the yellow to orange wavelength range and whose wavelength of the peak is greater than or equal to 560 nm and less than 580 nm and light whose emission spectrum peak is in the blue wavelength range (i.e., the color of light emitted from the light-emitting element) can be natural color like warm white or incandescent color. In particular, incandescent color can be easily achieved. 
     As a light-emitting substance which emits light having a peak in the yellow to orange wavelength range, for example, an organometallic complex in which a pyrazine derivative functions as a ligand can be used. Alternatively, the light-emitting layer may be formed by dispersing a light-emitting substance (a guest material) in another substance (a host material). A phosphorescent compound can be used as the light-emitting substance which emits light having a peak in the yellow to orange wavelength range. The power efficiency in the case of using a phosphorescent compound is three to four times as high as that in the case of using a fluorescent compound. The above organometallic complex in which a pyrazine derivative functions as a ligand is a phosphorescent compound, has high emission efficiency, and easily emits light in the yellow to orange wavelength range, and thus is favorable. 
     As a light-emitting substance which emits light having a peak in the blue wavelength range, for example, a pyrene diamine derivative can be used. A fluorescent compound can be used as the light-emitting substance which emits light having a peak in the blue wavelength range. The use of a fluorescent compound makes it possible to obtain a light-emitting element which has a longer lifetime than a light-emitting element in which a phosphorescent compound is used. The above pyrene diamine derivative is a fluorescent compound, can obtain an extremely high quantum yield, and has a long lifetime; thus, the above pyrene diamine derivative is favorable. 
     As illustrated in  FIG. 8C , the EL layer may include the hole injection layer  701 , the hole transport layer  702 , the light-emitting layer  703 , the electron transport layer  704 , an electron injection buffer layer  706 , an electron relay layer  707 , and a composite material layer  708  which is in contact with the second electrode layer  108 , between the first electrode layer  104  and the second electrode layer  108 . 
     It is preferable to provide the composite material layer  708  which is in contact with the second electrode layer  108  because damage caused to the EL layer  106  particularly when the second electrode layer  108  is formed by a sputtering method can be reduced. The composite material layer  708  can be formed using the above composite material in which an organic compound having a high hole transport property is mixed with an acceptor substance. 
     Further, by providing the electron injection buffer layer  706 , an injection barrier between the composite material layer  708  and the electron transport layer  704  can be reduced; thus, electrons generated in the composite material layer  708  can be easily injected to the electron transport layer  704 . 
     Any of the following substances having high electron injection properties can be used for the electron injection buffer layer  706 : an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metal (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and carbonate), a rare earth metal compound (e.g., an oxide, a halide, and carbonate), and the like. 
     Further, in the case where the electron injection buffer layer  706  contains a substance having a high electron transport property and a donor substance, the donor substance is preferably added so that the mass ratio of the donor substance to the substance having a high electron transport property is from 0.001:1 to 0.1:1. Note that as the donor substance, an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene may be used as well as an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metal (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and carbonate), and a rare earth metal compound (e.g., an oxide, a halide, and carbonate). Note that as the substance having a high electron transport property, a material similar to the material for the electron transport layer  704  described above can be used. 
     Furthermore, the electron relay layer  707  is preferably formed between the electron injection buffer layer  706  and the composite material layer  708 . The electron relay layer  707  is not necessarily provided; however, by providing the electron relay layer  707  having a high electron transport property, electrons can be rapidly transported to the electron injection buffer layer  706 . 
     The structure in which the electron relay layer  707  is sandwiched between the composite material layer  708  and the electron injection buffer layer  706  is a structure in which the acceptor substance contained in the composite material layer  708  and the donor substance contained in the electron injection buffer layer  706  are less likely to interact with each other, and thus their functions hardly interfere with each other. Accordingly, an increase in driving voltage can be prevented. 
     The electron relay layer  707  contains a substance having a high electron transport property and is formed so that the LUMO level of the substance having a high electron transport property is located between the LUMO level of the acceptor substance contained in the composite material layer  708  and the LUMO level of the substance having a high electron transport property contained in the electron transport layer  704 . In the case where the electron relay layer  707  contains a donor substance, the donor level of the donor substance is controlled so as to be located between the LUMO level of the acceptor substance in the composite material layer  708  and the LUMO level of the substance having a high electron transport property contained in the electron transport layer  704 . As a specific value of the energy level, the LUMO level of the substance having a high electron transport property contained in the electron relay layer  707  is preferably greater than or equal to −5.0 eV, more preferably greater than or equal to −5.0 eV and less than or equal to −3.0 eV. 
     As the substance having a high electron transport property contained in the electron relay layer  707 , a phthalocyanine-based material or a metal complex having a metal-oxygen bond and an aromatic ligand is preferably used. 
     As the phthalocyanine-based material contained in the electron relay layer  707 , specifically, any of the following is preferably used: CuPc, a phthalocyanine tin(II) complex (SnPc), a phthalocyanine zinc complex (ZnPc), cobalt(II) phthalocyanine, β-form (CoPc), phthalocyanine iron (FePc), and vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (PhO-VOPc). 
     As the metal complex having a metal-oxygen bond and an aromatic ligand, which is contained in the electron relay layer  707 , a metal complex having a metal-oxygen double bond is preferably used. The metal-oxygen double bond has an acceptor property (a property of easily accepting electrons); thus, electrons can be transferred (donated and accepted) more easily. Further, the metal complex which has a metal-oxygen double bond is considered stable. Thus, the use of the metal complex having a metal-oxygen double bond makes it possible to drive the light-emitting element at low voltage more stably. 
     As the metal complex having a metal-oxygen bond and an aromatic ligand, a phthalocyanine-based material is preferable. Specifically, any of vanadyl phthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex (SnOPc), and a phthalocyanine titanium oxide complex (TiOPc) is preferable because a metal-oxygen double bond is more likely to act on another molecular in terms of a molecular structure and an acceptor property is high. 
     Note that as the phthalocyanine-based material described above, a phthalocyanine-based material having a phenoxy group is preferable. Specifically, a phthalocyanine derivative having a phenoxy group, such as PhO-VOPc, is preferable. A phthalocyanine derivative having a phenoxy group is soluble in a solvent; thus, the phthalocyanine derivative has an advantage of being easily handled during formation of a light-emitting element and an advantage of facilitating maintenance of an apparatus used for film formation. 
     The electron relay layer  707  may further contain a donor substance. Examples of the donor substance include organic compounds such as tetrathianaphthacene (abbreviation: TTN), nickelocene, and decamethylnickelocene, in addition to an alkali metal, an alkaline earth metal, a rare earth metal, and compounds of the above metals (e.g., alkali metal compounds (including an oxide such as lithium oxide, a halide, and carbonates such as lithium carbonate and cesium carbonate), alkaline earth metal compounds (including an oxide, a halide, and a carbonate), and rare earth metal compounds (including an oxide, a halide, and a carbonate)). When such a donor substance is contained in the electron relay layer  707 , electrons can be transferred easily and the light-emitting element can be driven at lower voltage. 
     In the case where a donor substance is contained in the electron relay layer  707 , other than the materials described above as the substance having a high electron transport property, a substance having a LUMO level greater than the acceptor level of the acceptor substance contained in the composite material layer  708  may be used. As a specific energy level of the substance having a LUMO level, a LUMO level is greater than or equal to −5.0 eV, preferably greater than or equal to −5.0 eV and less than or equal to −3.0 eV. As examples of such a substance, a perylene derivative and a nitrogen-containing condensed aromatic compound can be given. Note that a nitrogen-containing condensed aromatic compound is preferably used for the electron relay layer  707  because of its stability. 
     As specific examples of the perylene derivative, the following can be given: 3,4,9,10-perylenetetracarboxylicdianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (abbreviation: PTCBI), N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), and N,N′-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (Hex PTC). 
     As specific examples of the nitrogen-containing condensed aromatic compound, the following can be given: pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT(CN) 6 ), 2,3-diphenylpyrido[2,3-b]pyrazine (2PYPR), and 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (F2PYPR). 
     Other examples are 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation: NTCDA), perfluoropentacene, copper hexadecafluoro phthalocyanine (abbreviation: F 16 CuPc), N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NTCDI-C8F), 3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″terthiophen (abbreviation: DCMT), and methanofullerene (e.g., [6,6]-phenyl C 61  butyric acid methyl ester). 
     Note that in the case where a donor substance is contained in the electron relay layer  707 , the electron relay layer  707  may be formed by a method such as co-evaporation of the substance having a high electron transport property and the donor substance. 
     The hole injection layer  701 , the hole transport layer  702 , the light-emitting layer  703 , and the electron transport layer  704  may each be formed using any of the above materials. 
     Then, the second electrode layer  108  is formed over the EL layer  106 . 
     The second electrode layer  108  is provided on the side opposite to the side from which light is extracted and is formed using a reflective material. As the reflective material, a metal material such as aluminum, gold, platinum, silver, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium can be used. Alternatively, any of the following may be used: alloys containing aluminum (aluminum alloys) such as an alloy of aluminum and titanium, an alloy of aluminum and nickel, and an alloy of aluminum and neodymium; and an alloy containing silver, such as an alloy of silver and copper. An alloy of silver and copper is preferable because of its high heat resistance. Further, a metal film or a metal oxide film is stacked in contact with an aluminum alloy film, whereby oxidation of the aluminum alloy film can be suppressed. As examples of a material of the metal film or the metal oxide film, titanium and titanium oxide are given. The above materials are preferable because they are present in large amounts in the Earth&#39;s crust and inexpensive and thus a reduction in manufacturing cost of a light-emitting element can be achieved. 
     Note that this embodiment can be freely combined with any of the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, application examples of a lighting device will be described. 
       FIG. 9  illustrates an example in which the lighting device according to one embodiment of the present invention is used as an indoor lighting device. The lighting device according to one embodiment of the present invention can be used not only as a ceiling-mounted lighting device  8202  but also as a wall-mounted lighting device  8204  and a floor-mounted lighting device  8205 . When the lighting device according to one embodiment of the present invention is used as the floor-mounted lighting device  8205 , a second housing of the floor-mounted lighting device  8205  is loaded with the people and objects; therefore, a substrate and a sealing structure which can sustain the weight of the people and the objects which are loaded on the second housing of the floor-mounted lighting device  8205 . In the case where a fragile glass substrate is used for the second housing of the floor-mounted lighting device  8205 , the glass substrate may be protected by providing over the glass substrate a thick plastic plate which has a light-transmitting property and can sustain the weight of people and objects which are loaded thereon. Further, since the lighting device according to one embodiment of the present invention can be reduced in thickness and increased in size, it is also possible that a wall surface itself serves as a light source like lighting devices  8203  so that a room is illuminated. 
     The lighting device according to one embodiment of the present invention, which has a surface light source, is preferably used as an indoor lighting device because it has advantages such as a reduction in number of components such as a light-reflecting plate as compared with the case of using a point light source, or less heat generation as compared with a filament bulb. Further, the lighting device as a whole is thin and lightweight; thus, the lighting device can be mounted on a variety of places. 
     Next, an example in which the lighting device according to one embodiment of the present invention is applied to an emergency exit light is illustrated in  FIG. 10 . 
       FIG. 10  illustrates an example of the appearance of an emergency exit light. An emergency exit light  8232  can be formed by combination of the lighting device and a fluorescent plate provided with a fluorescent portion. Alternatively, the emergency exit light  8232  may be formed by combination of a lighting device which emits light of a specific color and a light-blocking plate provided with a transmissive portion with a shape illustrated in the drawing. The lighting device according to one embodiment of the present invention can emit light with a constant luminance, and thus is preferably used as an emergency exit light that needs to be on at all times. 
     The lighting device according to one embodiment of the present invention can be thin and lightweight and have a large area and high reliability. 
     Note that what is described in this embodiment with reference to each drawing can be freely combined with or replaced with what is described in any of the other embodiments as appropriate. 
     This application is based on Japanese Patent Application serial no. 2010-288677 filed with the Japan Patent Office on Dec. 24, 2010, the entire contents of which are hereby incorporated by reference.