Abstract:
A light emitting element containing an organic compound has a defect that the light emitting element is easily deteriorated by various factors; therefore, it is the biggest issue of the light emitting element that the light emitting element is formed with high reliability (longer lifetime). An objective of the present invention is to reduce or eliminate generation of the above described various defective modes of the light emitting element containing an organic compound. According to the present invention, current efficiency-luminance characteristics can be improved by orienting organic compound molecules in an applying direction of current. In addition, deterioration can be prevented by using a crystallization inhibitor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic light emitting element including an anode, a cathode, and a layer containing an organic compound (hereinafter, referred to as an electroluminescent layer) that generates light by applying an electric field through itself; and a light emitting device including the light emitting element. Specifically, the present invention relates to an organic light emitting element that exhibits white light emission and a full color light emitting device including the white light emitting element. 
     As used herein, the term “light emitting device” refers to an image display device or a light source (including a lighting system). Further, a module having a light emitting element attached with a connector such as an FPC (Flexible Printed Circuit), a TAB (Tape Automated Bonding), or a TCP (Tape Carrier Package); a module having a TAB or a TCP provided with a printed wiring board at the tip thereof; and a module having a light emitting element directly mounted with an IC (Integrated Circuit) by COG (Chip On Glass) are all included in the light emitting device. 
     2. Related Art 
     An electroluminescent element includes an electroluminescent layer interposed between a pair of electrodes (anode and cathode). The emission mechanism is as follows. Upon applying a voltage between the pair of electrodes, holes injected from the anode and electrons injected from the cathode are recombined with each other at luminescent centers within the electroluminescent layer to lead to formation of molecular excitons, and the molecular excitons return to the ground state while radiating energy to emit photon. 
     An electroluminescent layer in the electroluminescent element may be made of low molecular weight materials or high molecular weight materials by vapor deposition (including vacuum vapor deposition), spin application, ink jetting, dipping, electrolytic polymerization, or the like. 
     These methods are appropriately selected depending on properties of materials or a shape of a film. For example, electrolytic polymerization is used to pattern form a film made of high molecular weight materials. (For example, refer to Japanese Unexamined Patent Publication No. 9-97679.) 
     A light emitting element containing an organic compound has a defect to be easily deteriorated by various factors; therefore, it is the biggest issue to obtain high reliability (longer lifetime) of the light emitting element. 
     The light emitting element containing an organic compound is easily deteriorated, and a defective condition in which a partial decrease in luminance occurs or a non-light-emitting region is generated is observed. When a layer containing an organic compound is crystallized, characteristics (luminance-current characteristics, current efficiency-current characteristics, current-voltage characteristics, or the like) are deteriorated. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to reduce or eliminate generation of the above described various defective modes of the light emitting element containing an organic compound. 
     Inventors of the present invention assume that random arrangement of organic compound molecules in a layer containing an organic compound causes the light emitting element containing an organic compound to be easily deteriorated. According to the present invention, molecules in the layer containing an organic compound are arranged (or oriented) in a certain direction. Specifically, it is preferable to arrange molecules with a structure having a high planarity. 
     One structure of the present invention disclosed in this specification is a light emitting device including a plurality of light emitting elements including: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, wherein molecules in the layer containing an organic compound are oriented in one direction. 
     Another structure of the present invention is a light emitting device comprising a plurality of light emitting elements comprising: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, wherein molecular chains of molecules in the layer containing an organic compound is oriented in the same direction as current flowing from the cathode to the anode. 
     It is preferable to dispose materials for inhibiting crystallization among the arranged molecules in order to suppress crystallization of a material. 
     Another structure of the present invention disclosed in this specification is a light emitting device including a plurality of light emitting elements including: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, wherein molecular chains of molecules in the layer containing an organic compound are continuously oriented in the same direction as current flowing from the cathode to the anode, and a material for inhibiting crystallization of the organic compound is disposed among the arranged molecules. 
     Solution including an organic compound molecule having a group easily reacted and combined with a first electrode material is applied onto the first electrode serving as an anode or a cathode in order to arrange organic compound molecules. For example, thiols (RSH) are reacted with an electrode containing Au, Pt, or Ag to form an Au—S bond, a Pt—S bond, or an Ag—S bond on the surface of the electrode. 
     A structure regarding a method for manufacturing of the present invention is a method for manufacturing a light emitting device including a plurality of light emitting elements comprising: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, including the steps of: forming a cathode containing Au, Pt, or Ag; arranging a long axis of an organic compound molecule having a thiol group (SH group) perpendicular to an electrode surface by reacting the organic compound molecule with a surface of the cathode; and forming an anode. 
     Organic compound molecules may be arranged by evaporating at a slow evaporation rate, after performing surface modification by reacting a group including halogen, for example, an organic compound containing SiCl, COCl, or SO 2 Cl with an electrode made of ITO. 
     The organic compound molecules may be arranged by electrolytic polymerization after performing surface modification for arranging the molecules. The molecules are easily arranged in a direction of current by forming a layer containing an organic compound with current applied in one direction after performing surface modification on an electrode or forming an ultra thin film by application in advance. The molecules may be arranged by intermolecular electrostatic interaction. 
     Another structure regarding a method for manufacturing of the present invention is a method for manufacturing a light emitting device including a plurality of light emitting elements including: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, includes the steps of: forming an anode containing metal oxide; forming a thin film by arranging molecules on an surface of the anode by application; forming a layer containing an organic compound by regularly arranging organic compound molecules along a molecular arrangement in the thin film by vapor deposition; and forming a cathode. 
     After forming a first layer containing an organic compound, regular depressions and projections may be formed by a rubbing treatment. Organic compound molecules may be arranged along the depressions and the projections by forming a second layer containing an organic compound thereover. Liquid crystal molecules having a light emitting substance at the end thereof as the organic compound molecules may be arranged along the depressions and the projections formed by rubbing. In this case, molecular chains are arranged parallel to an electrode plane, thereby forming a p orbit of an aromatic ring in a direction perpendicular to the electrode plane. Accordingly, the organic compound molecules can be arranged so that hopping conduction of carriers occurs in a direction perpendicular to the electrode with electrons moved between the electrodes. 
     Another structure of the present invention disclosed in this specification is a light emitting device includes a plurality of light emitting elements including: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, wherein the anode has regular depressions and projections on its surface, and molecules of the layer containing an organic compound are oriented along the regular depressions and projections. 
     Another structure of the present invention is a light emitting device including a plurality of light emitting elements including: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, wherein the layer containing an organic compound has a laminate structure, a first layer containing an organic compound has regular depressions and projections on its surface, and molecules of a second layer containing an organic compound are arranged along the regular depressions and projections. 
     A method for manufacturing for obtaining the above described structure is a method for manufacturing a light emitting device comprising a plurality of light emitting elements including: a cathode; a layer containing an organic compound in contact with the cathode; and an anode in contact with the layer containing an organic compound, includes the steps of: forming an anode; forming a partition containing an insulating material and covering an edge portion of the anode; forming a first layer containing an organic compound over the anode; forming regular depressions and projections by performing a rubbing treatment on a surface of the first layer containing an organic compound; forming a second layer containing an organic compound oriented along the depressions and the projections; and forming a cathode. 
     In each of the above described structures, the light emitting element emits light of any one of red, green, and blue in the case of displaying in full color. In addition, in each of the above described structures, all of the plurality of light emitting elements emit light of red, green, blue, or white in the case of displaying in monochrome. 
     Note that a light emitting element (EL element) includes a layer containing an organic compound (hereinafter, referred to as an EL layer) which generates luminescence (electro luminescence) by applying an electric field, an anode, and a cathode. Luminescence obtained from organic compounds is divided into luminescence (fluorescence) generated at the time of returning from a singlet excited state to a ground state or luminescence (phosphorescence) at the time of returning from a triplet excited state to a ground state. Both types of the luminescence can be employed in a light emitting device manufactured in accordance with the present invention. 
     A light emitting element (EL element) including an EL layer has a structure in which the EL layer is interposed between a pair of electrodes. Typically, an EL layer has a laminate structure: a hole transporting layer; a light emitting layer; an electron transport layer. The structure provides extremely high light emission efficiency, and is adopted in most of light emitting devices that are currently under development. 
     Further, a structure in which a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer are laminated in this order over an anode or a structure in which a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer are laminated in this order over an anode may be employed. A fluorescent pigment or the like may be doped into the light emitting layer. All of the layers may be made of low molecular weight materials or made of high molecular weight materials. A layer including an inorganic material may also be used. In addition, the term “EL layer” in this specification is a generic term used to refer to all layers interposed between the anode and the cathode. Therefore, the EL layer includes all of the above described hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the electron injection layer. 
     In a light emitting device according to the present invention, a driving method of a screen display is not particularly limited. For example, a dot-sequential driving method, a linear-sequential driving method, a plane-sequential driving method, or the like can be employed. Typically, a linear-sequential driving method is employed, and a time ratio gray scale driving method or an area ratio gray scale driving method is appropriately employed. Video signals inputted to a source line of the light emitting device may be analog signals or digital signals, and driver circuits and the like are designed in accordance with the type of the video signals as appropriate. 
     The present invention can be applied not only to an active matrix light emitting device but also to a passive matrix light emitting device. 
     These and other objects, features and advantages of the present invention will become more apparent upon reading of the following detailed description along with the accompanied drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  shows Embodiment Mode 1; 
         FIGS. 2A and 2B  show Embodiment Mode 2; 
         FIG. 3  shows Embodiment Mode 3; 
         FIGS. 4A and 4B  show Embodiment Mode 4; 
         FIGS. 5A to 5C  show Embodiment Mode 5; 
         FIGS. 6A and 6B  show Embodiment Mode 6; 
         FIGS. 7A and 7B  show a structure of an active matrix EL display device (Embodiment 1); 
         FIG. 8  shows a structure of an active matrix EL display device (Embodiment 2); and 
         FIGS. 9A to 9G  show examples of electronic appliances. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment modes of the present invention are described hereinafter. 
     Embodiment Mode 1 
       FIG. 1  shows a schematic diagram of the present invention. 
     According to the invention, an electroluminescent layer is formed over an electrode (first electrode)  106  that is formed over a substrate (not shown) as shown in  FIG. 1 . As a material for the substrate, glass, quartz, transparent plastics, or the like can be used. 
     In addition, the first electrode  106  may function as either an anode or a cathode. A plurality of the first electrodes  106  may be pattern formed over the substrate. In the case of an active matrix light emitting device, a plurality of TFTs are formed over the substrate. The first electrodes  106  are electrically connected to source electrodes or drain electrodes of the TFTs and are arranged in a matrix configuration. 
     In addition, in the case where the first electrode  106  functions as an anode, metals, alloys, electrically conductive compounds, and mixtures of these materials, which have large work functions (at least 4.0 eV), can preferably be used as anode materials. As a specific example of the anode materials, ITO (indium tin oxide), IZO (indium zinc oxide) composed of indium oxide mixed with zinc oxide (ZnO) of from 2% to 20%, aurum (Au), platinum (Pt), nickel (Ni), tungsten (W), chrome (Cr), molybdenum (Mo), ferrum (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitride of metal materials (for example, TiN), or the like can be used. 
     In the case where the first electrode  106  functions as a cathode, metals, alloys, electrically conductive compounds, and mixtures of these materials, which have small work functions (at most 3.8 eV), can preferably be used as cathode materials. As a specific example of the cathode materials, transition metals containing a rare earth metal can be used, besides elements in the first or second periodic row, that is, alkaline metals such as Li, Cs, and the like, alkaline earth metals such as Mg, Ca, Sr, and the like, alloys of these elements (MgAg, AlLi), or compounds (LiF, CsF, CaF 2 ). Alternatively, the first electrode  106  can be made of transition metals containing a rare earth metal and a laminated layer of the transition metals and metals such as Al, Ag, and ITO (including alloys). 
     The above described anode and cathode materials are deposited by vapor deposition or sputtering to form a thin film. The thin film is preferably formed to have a thickness of from 10 nm to 500 nm. 
     In an electroluminescent element according to the invention, in the case where the first electrode  106  serves as an anode, a second electrode that is formed in later process serves as a cathode. 
     An electroluminescent element according to the present invention has a structure that light generated by recombination of carries within the electroluminescent layer is emitted from either the first electrode  106  or the second electrode  115 , or both of the electrodes. When light is emitted from the first electrode  106 , the first electrode  106  is made of a transparent/translucent material. When light is emitted from the second electrode  115 , the second electrode is made of a transparent/translucent material. The case where the first electrode  106  serves as an anode made of transparent/translucent materials and the second electrode serves as a cathode made of materials having light shielding properties is described in this embodiment mode. 
     A first electroluminescent layer  112  is formed over the first electrode  106 , a second electroluminescent layer  113  is formed over the first electroluminescent layer  112 , and a third electroluminescent layer  114  is formed over the second electroluminescent layer  113 . In the case of forming a laminate structure, a hole transport layer, a hole blocking layer, an electron transport layer, or the like as well as a light emitting layer can be used in combination to form the laminate structure by vapor deposition, coating, ink jetting, or the like. 
     The first electroluminescent layer  112  functions as a hole injection layer or a hole transport layer. As a hole injection material, porphyrin compounds are useful, specifically, phthalocyanine (abbreviated to H 2 -Pc), copper phthalocyanine (abbreviated to Cu-Pc), or the like are applicable. Further, chemically doped high molecular weight conductive compounds can be used, such as polyethylene dioxythiophene (abbreviated to PEDOT) doped with polystyrene sulfonate (abbreviated to PSS), polyaniline (abbreviated to PAni), polyvinyl carbazole (abbreviated to PVK), or the like. A thin film of an inorganic semiconductor such as vanadium pentoxide or an ultra thin film of an inorganic insulator such as aluminum oxide can also be used. As hole transport materials, aromatic amine (that is, the one having a benzene ring-nitrogen bond) compounds are preferably used. For example, N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (abbreviated to TPD) or a derivative thereof such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated to α-NPD) is widely used. Also used are star burst aromatic amine compounds, including: 4,4′,4″-tris (N,N-diphenyl-amino)-triphenyl amine (abbreviated to TDATA); 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenyl amine (abbreviated to MTDATA); and the like. 
     The second electroluminescent layer  113  is a light emitting layer. Note that molecules in at least one layer are arranged in one direction in the present invention. Here, in the light emitting layer, a plane of a metal complex molecule  102  is arranged so as to be perpendicular to the first electrode by using a metal complex having a central metal  101  and a planar structure, typically a platinum complex molecule  102  using platinum as a central metal. Current efficiency-luminance characteristics can be improved by adjusting the plane of the metal complex molecule  102  in a flowing direction of current. 
     Specifically, substances represented by following Structural Formulas 1 to 4 may be dispersed in a host material in high concentration, and may appropriately be oriented. A method for orienting the substances is not particularly limited. The light emitting layer is not limited to these metal complexes in the present invention. 
     
       
                 
         
             
             
         
      
     
     Further, a crystallization inhibitor  103  for inhibiting crystallization is preferably disposed among the disposed metal complex molecules to suppress crystallization and to improve reliability. 
     The third electroluminescent layer  114  functions as an electron injection layer or an electron transport layer. As electron transport materials, in specific, metal complexes such as tris(8-quinolinolate) aluminum (abbreviated to Alq 3 ), tris(4-methyl-8-quinolinolate) aluminum (abbreviated to Almq 3 ), bis(10-hydroxybenzo[h]-quinolinato) beryllium (abbreviated to BeBq 2 ), bis(2-methyl-8-quinolinolate)-(4-hydroxy-biphenylyl)-aluminum (abbreviated to BAlq), bis [2-(2-hydroxyphenyl)-benzooxazolate]zinc (abbreviated to Zn(BOX) 2 ), and bis [2-(2-hydroxyphenyl)-benzothiazolatel]zinc (abbreviated to Zn(BTZ) 2 ). Besides, oxadiazole derivatives, such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated to PBD), and 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbreviated to OXD-7); triazole derivatives such as 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviated to TAZ) and 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviated to p-EtTAZ); imidazol derivatives such as 2,2′,2″-(1,3,5-benzenetryil)tris[1-phenyl-1H-benzimidazole] (abbreviated to TPBI); and phenanthroline derivatives such as bathophenanthroline (abbreviated to BPhen) and. bathocuproin (abbreviated to BCP) can be used in addition to metal complexes. 
     As electron injection materials, the above described electron transport materials can be used. Besides, an ultra thin film of an insulator, for example, an alkaline metal halogenated compound such as LiF, CsF, or the like; an alkaline earth halogenated compound such as CaF 2  or the like; or an alkaline metal oxide such as Li 2 O is often used. In addition, an alkaline metal complex such as lithium acetylacetonate (abbreviated to Li(acac)), 8-quinolinolato-lithium (abbreviated to Liq), or the like can also be used. 
     When the light emitting element shown in  FIG. 1  emits light by applying current thereto, current efficiency-luminance characteristics can be improved by orienting organic compound molecules in a flowing direction of current. Further, deterioration can be prevented by using the crystallization inhibitor. 
     Embodiment Mode 2 
     An example of forming a layer containing an organic compound by electrolytic polymerization is described as an example of a method for orienting organic compound molecules. After performing surface modification on an electrode or forming an ultra thin film (not shown) in advance by application, a layer containing an organic compound is formed by electrolytic polymerization. 
     As shown in  FIG. 2A , a reaction tank  201  holds an electrolytic solution  202 , and a substrate  205  on which a first electrode  206  electrically connected to a power source  204  via a wiring  203  is formed, a counter electrode  207 , and a reference electrode  208  are immersed in the electrolytic solution  202 . In addition, the substrate  205  is secured by a support medium  209  that electrically connects the first electrode (anode or cathode, here, an anode)  206  to the wiring  203 . 
     The power source  204  includes a potentiostat which is capable of applying a constant electric potential and a coulombmeter which measures an amount of a flowing electric charge. The counter electrode  207  is made of platinum. Further, the reference electrode  208  is made of Ag/AgCl. 
     The reaction tank  201  is provided over a magnetic stirrer  210 . In the reaction tank  210 , a rotator  211  in the electrolytic solution  202  is controlled by the magnetic stirrer  210  to continuously stir the electrolytic solution  202 . 
     When a predetermined current is applied to the counter electrode  207 , and the first electrode (here, an anode)  206  on the substrate  205  via the support medium  209 , respectively, a monomer or an oligomer in the electrolytic solution  202  is polymerized on the surface of the first electrode  206  by electrolytic polymerization to form a first electroluminescent layer (electrolytic polymerization film)  212  containing a polymer as its main component. According to the invention, an electrolytic polymerization film with surface roughness of at most 6.0 nm, preferably, from 4.0 nm to 5.0 nm can be formed by setting the condition, that is, the first electrode  206  has the size of 0.04 cm 2 , the current is applied from the power source  204  at from 0.016 mA to 0.06 mA, and the current is applied for from 0.8 sec to 3.0 sec. Consequently, decline in luminous efficiency or deterioration of an electroluminescent element due to electric voltage concentration that becomes a problem caused by poor planarity of a film surface can be prevented, and device characteristics and lifetime can be improved. 
     In the present invention, as a supporting electrolyte contained in the electrolytic solution  202 , salts such as natrium perchlorate, lithium perchlorate, tetrabutylammonium perchlorate (hereinafter, TBAP), or tetrabutylammonium tetrafluoroborate; other bases; or other acids can be used. The solvent for the electrolytic solution  202  can be one of water, acetonitrile, benzonitrile, N,N-dimethylformamide, dichloromethane, tetrahydrofuran, propione carbonate; or a mixture of these solvents can be used. 
     As a monomer or an oligomer contained in the electrolytic solution  202 , aniline, phenylene oxide, or the like can be used in addition to thiophene based materials (specifically, thiophene, 3,4-ethylenedioxythiophene, or the like), pyrrol based materials (specifically, pyrrol, indol, or the like), or aromatic hydrocarbon based materials (specifically, benzene, naphthalene, azulene, or the like). 
     Subsequently, an electroluminescent layer (a combined layer of a light emitting layer, a hole transport layer, a hole blocking layer, an electron transport layer, or the like) is appropriately laminated over the electrolytic polymerization film  212 , and lastly, a second electrode  215  serving as a cathode is formed thereover. As cathode materials for the second electrode  215 , materials described above in Embodiment Mode 1 may be used. 
     Accordingly, an electroluminescent element including an electroluminescent layer formed between a pair of electrodes by electrolytic polymerization can be manufactured. Since a layer containing an organic compound is formed with current applied after performing surface modification on an electrode or forming an ultra thin film (not shown) in advance by application, molecules are easily oriented. 
     This embodiment mode can freely be combined with Embodiment mode 1. 
     Embodiment Mode 3 
     Another example of a method for orienting organic compound molecules is described here. 
       FIG. 3  shows a light emitting element in which layers containing an organic compound is used as electroluminescent layers  312  to  314 , a first electrode  306  is used as a cathode, and a second electrode  315  is used as an anode. An organic compound molecule  302  shown in Structural Formula 5 is reacted with a surface of the first electrode containing Au, Pt, or Ag to form an Au—S bond, a Pt—S bond, or an Ag—S bond. 
     [Structural Formula 5]
 
HS—(CH 2 ) n —X—Ar   (5)
 
     Note that n=2 to 6, or 8. Structural Formula 6 shows an example of X in Structural Formula 5, and Structural Formula 7 shows an example of Ar. Ar here is a general abbreviation for an aryl (aromatic) group. 
     [Structural Formula 6]
 
X=nil, —C n H 2n —, —O—, —S—, —N(R)—, —Si(R 2 )— (R═H, C n H 2n , Ar)  (6)
 
[Structural Formula 7]
 
     
       
                 
         
             
             
         
      
     
     Combination of X and Ar may be arbitrary. In addition, Structural Formula 5 may not include X. 
     Solution including these materials is applied or these materials are evaporated to form the first electroluminescent layer  312 . The Au—S bond, the Pt—S bond, or the Ag—S bond is formed on a surface of the first electrode  306 , and the organic compound molecules  302  are arranged in a flowing direction of current as shown in  FIG. 3 . The first electroluminescent layer  312  functions as an electron injection layer or an electron transport layer. 
     A second electroluminescent layer  313  functioning as a light emitting layer is formed over the first electroluminescent layer  312 , and a third electroluminescent layer  314  functioning as a hole injection layer is formed over the second electroluminescent layer  313 . In the case of forming a laminate structure, a hole transport layer, a hole blocking layer, an electron transport layer, or the like as well as a light emitting layer can be used in combination to form the laminate structure by vapor deposition, application, ink-jetting, or the like. 
     Lastly, the second electrode  315  serving as an anode is formed. As anode materials for the second electrode  315 , materials described above in Embodiment Mode 1 may be used. 
     Accordingly, an electroluminescence element including the first electroluminescent layer  312  between a pair of electrodes can be formed. In the first electroluminescent layer  312 , the organic compound molecules  302  are oriented in one direction. Current efficiency-luminance characteristics can be improved by orienting organic compound molecules in a flowing direction of current as shown in  FIG. 3 . 
     This embodiment mode can freely be combined with Embodiment Mode 1 or 2. 
     Embodiment Mode 4 
     Another example of a method for orienting organic compound molecules is described here. 
     Hereinafter, procedures of manufacturing a light emitting element in which a layer containing an organic compound is used as an electroluminescent layer, a first electrode containing metal oxide, typically ITO is used as an anode, and a second electrode is used as a cathode are described. 
     At first, a first electrode containing metal oxide, typically ITO is formed. As anode materials for the first electrode, materials described above in Embodiment Mode 1 may be used. 
       FIG. 4A  shows a model diagram of a molecular bond at a top surface of metal oxide, and shows a state in which the top surface of the metal oxide includes an OH group. 
     Subsequently, solution including a molecule represented by a structural formula R—Cl in  FIG. 4B  is applied onto the surface of the metal oxide by application, and surface modification is performed by reacting the molecule. A model diagram of a molecular bond on a top surface of the metal oxide after the surface modification is shown in  FIG. 4B . R within the molecule is regularly introduced onto a metal element M by the surface modification. 
     A layer containing an organic compound is laminated by evaporating at a comparatively slowed evaporation rate after the surface modification. Vapor deposition is performed along a functional group R regularly combined with the metal element M. In the case of forming a laminate structure, a hole transport layer, a hole blocking layer, an electron transport layer, or the like as well as a light emitting layer can be used in combination to form the laminate structure by vapor deposition, application, ink-jetting, or the like. 
     Lastly, a second electrode serving as a cathode is formed. As cathode materials for the second electrode, materials described above in Embodiment Mode 1 may be used. 
     Accordingly, an electroluminescence element including an electroluminescent layer between a pair of electrodes can be formed. In the electroluminescent layer, organic compound molecules are oriented in one direction. 
     This embodiment mode can freely be combined with any one of Embodiment Modes 1 to 3. 
     Embodiment Mode 5 
     Here, an example of a method for orienting molecules by intermolecular electrostatic interaction is described with reference to  FIGS. 5A to 5C . 
     At first, an electrode serving as a cathode (or an anode) is formed. Subsequently, an organic compound molecule (organic compound molecule having a comparatively long molecular chain) is introduced into an electrode surface as shown in  FIG. 5A . For example, the method described in Embodiment Mode 4 may be employed as an introducing method. Note that M denotes an arbitrary metal element. 
     Subsequently, a compound shown in  FIG. 5B  is applied or evaporated to regularly arrange molecules as shown in  FIG. 5C . As shown in  FIG. 5C , orientation of molecules is determined by electrostatic interaction, and molecules are arranged regularly. 
     In the case of forming a laminate structure, a light emitting layer, a hole injection layer, a hole transport layer, a hole blocking layer, an electron transport layer, an electron injection layer, or the like can be used in combination to form the laminate structure by vapor deposition, application, ink-jetting, or the like. 
     Lastly, an electrode serving as an anode (or a cathode) is formed. 
     Accordingly, an electroluminescent element including an electroluminescent layer between a pair of electrodes can be formed. In the electroluminescnet layer, organic compound molecules are oriented in one direction. 
     This embodiment mode can freely be combined with any one of Embodiment Modes 1 to 4. 
     Embodiment Mode 6 
     Here, an example of forming regular depressions and projections by means of physical force and orienting organic compound molecules in a certain direction along the regular depressions and the projections is described with reference to  FIGS. 6A and 6B . 
     At first, first electrodes  406  are disposed in a matrix configuration over a substrate  400 , and partitions  401  containing an insulating material and covering edge portions of the first electrodes  406  are formed. Subsequently, a first electroluminescent layer  412  is formed. Here, poly (ethylenedioxy thiophene)/poly (styrenesulfonate) solution (PEDOT/PSS) is applied by spin coating to form a layer functioning as a hole injection layer as the first electroluminescent layer  412 . As another hole injection material, polyaniline/camphor sulfonate solution (PANI/CSA), PTPDES, Et-PTPDEK, PPBA, or the like can be used. 
     Subsequently, a surface of the electroluminescent layer formed over the electrode (the first electrode) disposed over the substrate  400  is rubbed with a roller  420  which is wound with a rubbing fabric (not shown) as shown in  FIG. 6B . The roller  420  rotates, and the surface is rubbed in one direction by moving the substrate  400 . Regular depressions and projections  403  are formed on the surface of the first electroluminescent layer  412  as shown in  FIG. 6A  by rubbing with the roller  420 . 
     Subsequently, a second electroluminescent layer  413  (a layer serving as a light emitting layer) is formed by using a material including a molecule  402  having a comparatively long molecular chain, for example a liquid crystal molecule having a light emitting substance at the end thereof. The molecules  402  having a long molecular chain are oriented along the regular depressions and projections  403  formed by rubbing. In this case, although a molecular chain is arranged parallel to a plane of the electrode  406 , liquid crystal molecules are preferably stacked in a condition that p orbits are formed perpendicular to the plane of the electrode so that electrons move between electrodes due to perpendicular hopping coduction. A direction perpendicular to the plane of the electrode is the same direction as a direction of current flowing through the light emitting element. 
     Subsequently, a third electroluminescent layer  414  functioning as an electron injection layer is formed over the second electroluminescent layer  413 . In the case of forming a laminate structure, a hole transport layer, a hole blocking layer, an electron transport layer, or the like as well as a light emitting layer can be used in combination to form the laminate structure by vapor deposition, application, ink-jetting, or the like. 
     Lastly, a second electrode  415  serving as a cathode or an anode is formed. As cathode materials for the second electrode  415 , materials described above in Embodiment Mode 1 may be used. 
     Accordingly, an electroluminescent element including the second electroluminescent layer  413  between a pair of electrodes can be formed. In the second electroluminescent layer  413 , the molecules  402  having long chains are oriented in one direction. Current efficiency-luminance characteristics can be improved by orienting organic compound molecules in a flowing direction of current as shown in  FIGS. 6A and 6B . 
     Here, an example of orienting molecules of the second electroluminescent layer  413  by performing a rubbing treatment on the first electroluminescent layer  412  is described; however, the present invention is not particularly limited thereto. Molecules of the third electroluminescent layer  414  may be oriented by performing a rubbing treatment on the second electroluminescent layer  413 . In addition, molecules of the first electroluminescent  412  may be oriented by performing a rubbing treatment on the first electrode  406 . 
     This embodiment mode can freely be combined with any one of Embodiment Modes 1 to 5. 
     The present invention including the above described structures is described in further detail in following embodiments. 
     EMBODIMENT 
     Embodiment 1 
     In this embodiment, a method for manufacturing a light emitting device (dual emission structure) having a light emitting element using an organic compound layer as a light emitting layer, over a substrate having an insulating surface is described with reference to  FIGS. 7A and 7B . 
       FIG. 7A  is a top view of a light emitting device.  FIG. 7B  is a cross-sectional view of  FIG. 7A  taken along a line A-a′. Reference numeral  1101  indicated by a dotted line denotes a source signal line driver circuit;  1102 , a pixel portion;  1103 , a gate signal line driver circuit;  1104 , a transparent sealing substrate;  1105 , a first sealing agent; and  1107 , a second sealing agent. The inside surrounded by the first sealing agent  1105  is filled with the transparent second sealing agent  1107 . In addition, the first sealing agent  1105  contains a gap agent for spacing substrates. 
     Reference numeral  1108  denotes a wiring for transmitting signals inputted to the source signal line driver circuit  1101  and the gate signal line driver circuit  1103 . The wiring receives video signals or clock signals from an FPC (flexible printed circuit)  1109  serving as an external input terminal. Although only FPC is illustrated in the drawing, a PWB (printed wiring board) may be attached to the FPC. In addition, resin  1150  is provided to cover the FPC 1109 . 
     Then, a cross-sectional structure is described with reference to  FIG. 7B . A driver circuit and a pixel portion are formed over a transparent substrate  1110 . In  FIG. 7B , the source signal driver circuit  1101  and the pixel portion  1102  are illustrated as driver circuits. 
     The source signal driver circuit  1101  is provided with a CMOS circuit formed by combining an n-channel TFT  1123  and a p-channel TFT  1124 . A TFT for forming a driver circuit may be formed with a known CMOS, PMOS, or NMOS circuit. In this embodiment, a driver integrated type in which a driver circuit is formed over the substrate is described, but not exclusively, the driver circuit can be formed outside instead of over the substrate. In addition, the structure of a TFT using a polysilicon film or an amorphous silicon film as an active layer is not especially limited. A top gate TFT or a bottom gate TFT can be adopted. 
     The pixel portion  1102  includes a plurality of pixels including a switching TFT  1111 , a current control TFT  1112 , and a first electrode (anode)  1113  electrically connected to a drain of the current control TFT  1112 . The current control TFT  1112  may be either an n-channel TFT or a p-channel TFT. In the case where the current control TFT  1112  is connected to an anode, the TFT is preferably a p-channel TFT. A holding capacitor (not shown) may appropriately be provided. In  FIG. 7B , a cross-sectional structure of only one of thousands of pixels is illustrated to show an example that two TFTs are used for the pixel. However, three or more numbers of pixels can be appropriately used. 
     Since the first electrode  1113  is directly in contact with the drain of a TFT a bottom layer of the first electrode  1113  is preferably made of a material capable of making an ohmic contact with the drain containing silicon, and a top layer, which is in contact with a layer containing an organic compound, is preferably made of a material having a large work function. For example, a transparent conductive film (ITO (indium tin oxide), an indium oxide-zinc oxide alloy (In 2 O 3 —ZnO), zinc oxide (ZnO), or the like), is used. 
     An insulator (also referred to as a bank, a partition, a mound, or the like)  1114  is formed at the both edges of the first electrode (anode)  1113 . The insulator  1114  may be made of an organic resin film or an insulating film containing silicon. In this example, an insulator is made of a positive photosensitive acrylic resin film as the insulator  1114  in the shape as illustrated in  FIG. 7B . 
     In order to make coverage favorable, an upper edge portion or a lower edge portion of the insulator  1114  is formed to have a curved face having a radius of curvature. For example, when a positive photosensitive acrylic resin is used as a material for the insulator  1114 , only upper edge portion of the insulator  1114  preferably has a radius of curvature (from 0.2 μm to 3 μm). As the insulator  1114 , either a negative photosensitive resin that becomes insoluble to etchant by light or a positive photosensitive resin that becomes dissoluble to etchant by light can be used. 
     Further, the insulator  1114  may be covered with a protective film containing an aluminum nitride film, an aluminum nitride oxide film, a thin film containing carbon as its main component, or a silicon nitride film. 
     A layer containing an organic compound  1115  is selectively formed over the first electrode (anode)  1113  by vapor deposition. In this embodiment, the layer containing an organic compound  1115  is formed with a manufacturing device described in Embodiment Mode 2 to obtain uniform film thickness. Moreover, a second electrode (cathode)  1116  is formed over the layer containing an organic compound  1115 . As the cathode, a material having a small work function (Al, Ag, Li, or Ca; or an alloy of these elements such as MgAg, MgIn, AlLi, or CaF 2 ; or CaN) can be used. Here, in order to pass light, the second electrode (cathode)  1116  is made of a laminated layer of a metal thin film (MgAg: 10 nm in thickness) and a transparent conductive film (ITO (indium tin oxide), an indium oxide-zinc oxide alloy (In 2 O 3 —ZnO), zinc oxide (ZnO), or the like) having a film thickness of 110 nm. A light emitting element  1118  including the first electrode (anode)  1113 , the layer containing an organic compound  1115 , and the second electrode (cathode)  1116  is thus formed. In this embodiment, the layer containing an organic compound  1115  is formed by sequentially stacking CuPc (20 nm in thickness), α-NPD (30 nm in thickness), CBP including organometallic complexes (Pt(ppy)acac) using platinum as a central metal (30 nm in thickness), BCP (20 nm in thickness), and BCP: Li (40 nm in thickness) to obtain white emission. The organometallic complex using platinum as a central metal has a planar structure, and a plane thereof is preferably oriented to be perpendicular to a plane of the first electrode. According to the method described in any one of Embodiment Modes 2 to 6, organic compound materials in at least one layer of the layer containing an organic compound  1115  may be oriented by using other organic compound materials. 
     Since the light emitting element  1118  is given as an example of exhibiting white emission in this embodiment, a color filter comprising a coloring layer  1131  and a light shielding layer (BM)  1132  is provided (for simplification, an over coat layer is not illustrated). 
     Further, optical films  1140  and  1141  are provided for such dual emission display devices so as not to be transparent to see a background therethrough and so as not to reflect outside light. For the optical films  1140  and  1141 , a polarizing film (a highly transmissive polarizing plate, a thin type polarizing plate, a paper white polarizing plate, a high-performance dye type polarizing plate, an AR polarizing plate, or the like), a retardation film (a broadband quarter-wave plate, a temperature compensating retardation film, a twisted-nematic retarder film, a wide viewing angle polarizing film, a biaxial oriented retardation film, or the like), a brightness enhancement film, and the like may appropriately be used in combination. For example, effect of preventing the device from being transparent to see a background therethrough and from reflecting light can be obtained by using polarizing films as the optical films  1140  and  1141  and arranging the polarizing films so that polarizing directions of light are perpendicular to each other. In this case, a portion except a light emitting portion for performing a display becomes black not to be transparent to see a background, even if the display is watched from either side. Since light emitted from a light emitting panel passes through only one polarizing plate, an image is displayed as it is. 
     Note that similar effect can be obtained without making two polarizing films perpendicular to each other when polarizing directions of light are within ±45°, preferably ±20°. 
     The optical films  1140  and  1141  can prevent a display from being hard to be recognized due to transparency to see a background when watched from one side. 
     Further, one more optical film may be added. For example, although either of the two polarizing films absorbs an S wave (or a P wave), a brightness enhancement film that reflects an S wave (or a P wave) to a light emitting element side and reuses the S wave may be provided between a polarizing plate and a light emitting panel. Consequently, more P wave (or S wave) passes through the polarizing plate, and a total amount of light increases. In a dual emission panel, since structures of layers through which light passes from a light emitting element are different, conditions of light (luminance, color purity, or the like) are also different. The optical film is useful for adjusting balance of light emission on both sides. In addition, since degrees of reflection of outside light are different in a dual emission panel, a brightness enhancement film is preferably disposed between a polarizing plate and a light emitting panel on a more reflective side. 
     In order to seal the light emitting element  1118 , a transparent protective laminated layer  1117  is formed. The transparent protective laminated layer  1117  includes a first inorganic insulating film, a stress relaxation film, and a second inorganic insulating film. As the first inorganic insulating film and the second inorganic insulating film, a silicon nitride film, a silicon oxide film, a silicon oxynitride film (a SiNO film (composition ratio: N&gt;O) or a SiON film (composition ratio: N&lt;O)), or a thin film containing carbon as its main component (for example, a DLC film or a CN film) can be used. These inorganic insulating films have high blocking properties against moisture. However, when the film thickness is increased, film stress is also increased; consequently, film peeling easily occurs. By interposing the stress relaxation film between the first inorganic insulating film and the second inorganic insulating film, moisture can be absorbed and stress can be relaxed. Even when fine holes (such as pin holes) are formed on the first inorganic insulating film at film formation for any reason, the stress relaxation film can fill in the fine holes. The second inorganic insulating film formed over the stress relaxation film gives the transparent protective laminated film excellent blocking properties against moisture or oxygen. The stress relaxation film is preferably made of a material having smaller stress than that of an inorganic insulating film and hygroscopic properties. In addition, a material that is transparent to light is preferable. As the stress relaxation film, a material film containing an organic compound such as α-NPD (4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl), BCP (bathocuproin), MTDATA (4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenyl amine), Alq 3  (tris-8-quinolinolate aluminum complex), or the like can be used. These films have hygroscopic properties and are almost transparent in case of having thin film thickness. Further, MgO, SrO 2 , or SrO can be used as the stress relaxation film since they have hygroscopic properties and light transparency/translucency, and can be formed into a thin film by vapor deposition. In this embodiment, a silicon nitride film having high blocking properties against impurities such as moisture or alkaline metals is formed by vapor deposition using a silicon target in the atmosphere containing nitrogen and argon as the first inorganic insulating film or the second inorganic insulating film. A thin film made of Alq 3  by vapor deposition is used as the stress relaxation film. In order to pass light through the transparent protective laminated layer, the total film thickness of the transparent protective laminated layer is preferably formed to be as thin as possible. 
     In order to seal the light emitting element  1118 , the sealing substrate  1104  is pasted with the use of the first sealing agent  1105  and the second sealing agent  1107  in an inert gas atmosphere. An epoxy resin is preferably used for the first sealing agent  1105 . There is no particular limitation of a material for the second sealing agent  1107  as long as the material has light transparency/translucency. Typically, an ultraviolet curable or heat curable epoxy resin is preferably used. A highly heat resistant UV epoxy resin (product name: 2500 Clear, manufactured by Electrolite Cooperation) having an index of refraction equal to 1.50, a viscosity equal to 500 cps, a Shore D hardness equal to 90, a tensile strength equal to 3,000 psi, a Tg point of 150° C., a volumetric resistivity equal to 1×10 15  Ω·cm, and a withstand voltage of 450 V/mil is used here. Total transmittance can be improved by filling a space between a pair of substrates with the second sealing agent  1107 , compared to a case where the space between the pair of the substrates is an open space (innert gas). It is preferable that the first sealing agent  1105  and the second sealing agent  1107  are materials that shields as much moisture or oxygen as possible. 
     In this embodiment, as a material for the sealing substrate  1104 , a plastic substrate made of FRP (Fiberglass-Reinforced Plastics), PVF (polyvinyl fluoride), Myler, polyester, acrylic, or the like can be used besides a glass substrate or a quartz substrate. After pasting the sealing substrate  1104  with the first sealing agent  1105  and the second sealing agent  1107 , a third sealing agent can be provided to seal the side face (exposed face). 
     By encapsulating the light emitting element  1118  in the first sealing agent  1105  and the second sealing agent  1107 , the light emitting element  1118  can be shielded completely from outside to prevent moisture or oxygen that brings deterioration of the organic compound layer from penetrating into the light emitting element  1118 . Therefore, a highly reliable light emitting device can be obtained. 
     When a top emission light emitting device is manufactured, an anode is preferably a metal film having reflectivity (chromium, titanium nitride, or the like). When a bottom emission light emitting device is manufactured, a cathode is preferably a metal film (from 50 nm to 200 nm in thickness) containing Al, Ag, Li, or Ca or an alloy of these elements MgAg, Mgln, or AlLi. 
     This embodiment can be freely combined with any one of Embodiment modes 1 to 6. 
     Embodiment 2 
     In this embodiment, another example of a different sealing method from that in Embodiment 1 is described with reference to  FIG. 8 . An example of white emission is described in Embodiment 1; however, an example of a light emitting device which can display in full color by providing three types (R, G, and B) of light emitting elements is described in this embodiment. 
     As shown in  FIG. 8 , an inorganic insulating layer  620   b  may be formed by sputtering over a sealing layer  621   a  after forming the sealing layer  621   a  by application and solidifying the sealing layer in order to further firmly seal a light emitting element covered with an inorganic insulating layer  620   a.  In addition, a sealing layer  621   b  may again be formed thereon by application and be solidified. Moisture or an impurity particularly from a side of a panel is shielded with a laminated layer of the sealing layer and the inorganic insulating film. 
     In  FIG. 8 , reference numeral  600  denotes a substrate;  601 , a transparent electrode;  603 , a polarizing plate;  606 , a cover;  607 , a sealing agent (containing a gap agent);  620   a  to  620   c,  inorganic insulating layers (a silicon nitride film (SiN), a silicon oxynitride film (SiNO), an aluminum nitride film (AlN), an aluminum nitride oxide film (AlNO), or the like);  621   a  to  621   c,  sealing layers;  622 , a transparent electrode;  623 , a partition (also referred to as a bank). Further, reference numeral  624   b  denotes an EL layer which exhibits blue emission as a light emitting element;  624   g,  an EL layer which exhibits green emission as a light emitting element;  624   r,  an EL layer which exhibits red emission as a light emitting element. Accordingly, a full color display is realized. The transparent electrode  601  is an anode (or a cathode) of a light emitting element connected to a source electrode or a drain electrode of a TFT. 
     This embodiment can be freely combined with any one of Embodiment Modes 1 to 6 and Embodiment 1. 
     Embodiment 3 
     In this embodiment, examples of electronic appliances having two or more display devices are described with reference to  FIGS. 9A to 9G . Electronic appliances comprising an EL module can be completed by implementing the present invention. Such electric appliances are as follows: a video camera; a digital camera; a goggle type display (head mounted display); a navigation system; audio reproducing devices (a car audio, an audio component, and the like); a laptop computer; a game machine; personal digital assistants (a mobile computer, a cellular phone, a portable game machine, an electronic book, and the like); and an image reproducing device including a recording medium (specifically, a device capable of processing data in a recording medium such as a Digital Versatile Disk (DVD) and having a display that can display the image of the data). 
       FIG. 9A  is a perspective view of a laptop computer, and  FIG. 9B  is a perspective view showing a folded state of the laptop computer. The laptop computer comprises a main body  2201 , a casing  2202 , display portions  2203   a  and  2203   b,  a keyboard  2204 , an external connection port  2205 , a pointing mouse  2206 , and the like. 
     The laptop computer shown in  FIGS. 9A and 9B  comprises a high-resolution display portion  2203   a  that mainly displays an image in full color and a display portion  2203   b  that mainly displays characters and symbols in monochrome. 
       FIG. 9C  is a perspective view of a mobile computer, and  FIG. 9D  is a perspective view showing a back side. The mobile computer comprises a main body  2301 , display portions  2302   a  and  2302   b,  a switch  2303 , operation keys  2304 , an infrared port  2305 , and the like. The mobile computer comprises a high-resolution display portion  2302   a  that mainly displays an image in full color and a display portion  2302   b  that mainly displays characters and symbols in monochrome. 
       FIG. 9E  shows a video camera, which comprises a main body  2601 , a display portion  2602 , a casing  2603 , an external connection port  2604 , a remote control receiving unit  2605 , an image receiving unit  2606 , a battery  2607 , an audio input section  2608 , operation keys  2609 , and the like. The display portion  2602  is a dual emission panel, which can mainly display a high-quality image in full color on one side and can mainly display characters and symbols in monochrome on the other side. Note that the display portion  2602  can be turned at an attaching portion. The present invention can be applied to the display portion  2602 . 
       FIG. 9F  is a perspective view of a cellular phone, and  FIG. 9G  is a perspective view showing a folded state of the cellular phone. The cellular phone comprises a main body  2701 , a casing  2702 , display portions  2703   a  and  2703   b,  an audio input section  2704 , an audio output section  2705 , operation keys  2706 , an external connection port  2707 , an antenna  2708 , and the like. 
     The cellular phone shown in  FIGS. 9F and 9G  comprises a high-resolution display portion  2703   a  that mainly displays an image in full color and an area color display portion  2703   b  that mainly displays characters and symbols. In this case, a color filter is used for the display portion  2703   a,  and an optical film for a display in area color is used for the display portion  2703   b.    
     This embodiment can freely be combined with any one of Embodiment Modes 1 to 6 and Embodiments 1 and 2. 
     According to the present invention, current efficiency-luminance characteristics can be improved by orienting organic compound molecules in an applying direction of current. In addition, deterioration can be prevented by using a crystallization inhibitor. 
     This application is based on Japanese Patent Application serial no. 2003-133950 filed in Japanese Patent Office on May 13 in 2003, the contents of which are hereby incorporated by reference. 
     Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.