Patent Publication Number: US-8110983-B2

Title: Self light emitting device

Description:
This application is a continuation of U.S. application Ser. No. 11/021,070, filed on Dec. 22, 2004 now U.S. Pat. No. 7,317,282 which is a continuation of U.S. application Ser. No. 09/696,619, filed on Oct. 25, 2000 (now U.S. Pat. No. 6,847,163 issued Jan. 25, 2005). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a self light-emitting device including an EL element, which is formed on a substrate, and comprises an opaque electrode (cathode), a transparent electrode (anode), and a luminescing organic material (hereinafter referred to as an organic EL material) sandwiched therebetween. Specifically, the present invention relates to an improvement of efficiency for extracting light from the EL element. 
     2. Description of the Related Art 
     In recent years, developments of EL display devices employing as an element an organic EL material are advancing. This is a current driving type self light-emitting device utilizing light emitting, which is caused by re-coupling of an electron and a hole injected into an organic thin film from the electrodes of both surfaces by applying a voltage. The light emitted is extracted as a sheet luminescent. However, the efficiency of extracting light, generated in a solid-state thin film having a large refractive index, to outside of the light-emitting element as a sheet luminescent is extremely low, normally 20% or less. 
     As shown in  FIG. 2 , among the lights outputted from a light source “A” in an “a” layer  202  having a large refractive index (n=n 1 ) to “b” layers  201  and  203  having small refractive indexes (n=n 2 ), the light entering at angles (θ 1  and θ 2 ) greater than the radiation angle of θ 0  (provided that θ 0 =sin −1 (n 2 /n 1 )) is entirely reflected and is waveguided in the “a” layer with a large refractive index. Thus, light that is waveguided is referred as a guided light. A part of the components of the guided light light is absorbed and disappears while the rest is propagated in the “a” layer  202  and escapes to the edge surface. Therefore, only a portion of the outgoing light can be extracted as a sheet luminescent. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above stated problem, and therefore it is an object of the present invention to improve the efficiency of extracting light in a luminescent element, particularly, an EL element. Further, another object of the present invention is to provide a self light-emitting device having a high luminescing efficiency. 
     In the structure of the self-light emitting device of the present invention, as shown in  FIG. 1 , each of an EL layer  102  and a transparent electrode  103  contained in an EL element is formed so as to have a thickness in which a guided light does not occur, and an inert gas is filled between the transparent electrode and a cover material  105  (a region denoted by the reference numeral  104 ). Note that the region denoted by the reference numeral  104  is referred to as a “gas space” throughout the present specification. 
     Throughout the present specification, an EL element is an element whose structure is composed of a cathode made of an opaque electrode, an anode made of a transparent electrode, and an EL layer sandwiched therebetween. In the present invention, the cathode is the opaque electrode (light shielding electrode) and the anode is the transparent electrode. Further, it is to be noted that a layer where the injection, transportation, and re-coupling of carriers are performed is referred to as the EL layer throughout the present specification. 
     Furthermore, the film thickness in which there is no occurrence of a guided light is referred to as a film thickness (d) derived from d≦λ/(4 n), where the refractive index of the film is n and the wavelength of the light generated in the EL element is λ. For instance, if the wavelength of the light generated in the EL element is 560 nm and the refractive index of the film is n x , then d≦(140/n x ). Therefore, there is no existence of a guided light in a film having a film thickness that is thinner than the film thickness (d). Note that it is appropriate to set the total film thickness to (140/n x ) or less when the refractive indexes of both the film for constituting the EL layer and the transparent electrode are n x . 
     Next, an explanation will be made regarding the structure in which an inert gas is filled into the space between the transparent electrode and the cover material. It is generally known that when light travels through a gas space, a solid-state layer, and a gas space again in that order, the light can be extracted efficiently. Therefore, the structure is formed such that light generated in the EL element travels through the gas space, the solid-state layer, and the gas space in that order after it has transmitted the transparent electrode, thereby making it possible to improve the efficiency of extracting light. In the present invention, the structure of the EL element is a structure in which the inert gas is sandwiched between the transparent electrode and the cover material, and thus by forming the above stated structure, light can be extracted efficiently. 
     When employing the EL element having the structure shown in  FIG. 1 , it is desirable to provide a buffer layer between the EL layer  102  comprising only a light-emitting layer and the transparent electrode  103 , or between the EL layer  102  made of only a light-emitting layer and the opaque electrode. It is to be noted that the buffer layer indicates a layer for promoting the injection and transportation of carriers (electrons or holes). In other words, an electron injection layer, a hole injection layer, an electron transporting layer, or a hole transporting layer may be utilized as the buffer layer. In the case of providing a buffer layer, the buffer layer is contained in the EL layer throughout the present specification. 
     The provision of the buffer layer improves the state of interface between the light-emitting layer and the electrode, and therefore the efficiency of extracting light generated in the light-emitting layer is improved. In the case of providing a buffer layer in the EL element with the structure of  FIG. 1  of the present invention, the luminescing efficiency is further enhanced. Moreover, a problem of damaging the light-emitting layer during forming the transparent electrode thereon can be resolved by forming the buffer layer sandwiched between the light-emitting layer and the transparent electrode. 
     Thus, in the present invention, the film thicknesses of the respective layers forming the EL layer  102  and the film thickness of the transparent electrode, which are included in the EL element, are film thicknesses in which there is no occurrence of a guided light, and there is inert gas in the space between the transparent electrode and the cover material  105 , thereby making it possible to largely increase the efficiency of extracting light. In addition, by providing the buffer layers between the electrodes that sandwich the light-emitting layer and the light-emitting layer within the EL element to form the EL layer made of the light-emitting layer and the buffer layers, the efficiency of extracting light can further be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a diagram showing a structure of a self light-emitting device of the present invention; 
         FIG. 2  is a diagram showing the state of generating a guided light; 
         FIG. 3  is a diagram showing a structure of a self light-emitting device of the present invention; 
         FIG. 4  is a diagram showing a structure of a self light-emitting device of the present invention; and 
         FIG. 5  is a diagram showing a structure of a self light-emitting device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the present invention, as shown in  FIG. 1 , an EL element has a structure wherein an EL layer  102  and a transparent electrode  103  contained therein is formed so as to have a film thickness where there is no occurrence of a guided light, respectively, and a gas space  104  is formed sandwiched between the transparent electrode  103  and a cover material  105 , and spacers  106  are utilized in the gas space  104  to secure it. To give an actual example, the film thickness of the EL layer  102  is set to 30 nm and the film thickness of the transparent electrode  103  is set to 100 nm. 
     A polymer-based organic EL material or a monomer-based organic EL material is used for the EL layer  102 . The polymer-based organic EL material can be dissolved in a solvent in the polymer state and then applied, or it can be dissolved in a solvent in the monomer state and then polymerized after it has been applied. 
     Note that the gas space  104  refers to the space filled with the inert gas (typically argon, nitrogen, neon, and krypton). The cover material  105  indicates a transparent member, and specifically, glass, quartz, plastic, etc. can be used. 
     Further, in order to improve the interface state in the transparent electrode and the EL layer comprising only a light-emitting layer, as shown in  FIG. 3 , the structure has buffer layers  302  and  304  sandwiched between a light-emitting layer  303  and a transparent electrode  306 , and between the light-emitting layer  303  and an opaque electrode  301 , thereby forming an EL layer  305  comprising the light-emitting layer  303  and the buffer layers  302  and  304 . Note that a gas space  307 , spacers  308 , and a cover material  309  are similar to the gas space  104 , the spacers  106 , and the cover material  105  in  FIG. 1 , respectively. 
     Embodiment 1 
       FIG. 4  is a diagram showing the cross sectional structure of an active matrix type self light-emitting device of the present invention. In  FIG. 4 , reference numeral  401  denotes a substrate and  402  denotes a TFT. It is to be noted that a known TFT is used as the TFT  402 . Furthermore, reference numeral  403  denotes an electrode having aluminum (AL) as its principal constituent, and a polyparaphenylene vinylene (PPV) is used for an EL layer  404 . Reference numeral  405  denotes a transparent electrode made of ITO, and argon is filled in a gas space denoted by  406 . Also, glass is used as a cover material  407  and spacers  408  are utilized in the gas space  406  to secure it. 
     Embodiment 2 
       FIG. 5  is a diagram showing a cross sectional structure of a passive matrix type self light-emitting device of the present invention. In  FIG. 5 , reference numeral  501  denotes a substrate and  502  denotes an EL layer. PPV is used for the EL layer. Reference numeral  503  denotes a plurality of opaque electrodes (cathodes) arranged in stripe shapes and a plurality of transparent electrodes (anodes)  504  are provided in stripe shapes so as to be orthogonal to the plurality of opaque electrodes  503 . 
     In addition, the EL layer  502  is formed sandwiched between the plurality of opaque electrodes  503  and the plurality of transparent electrodes  504 . At this point, a cover material  506  made of glass is provided on the plurality of transparent electrodes  504 , sandwiching spacers  507 . A gas space  505  is thus formed between the cover material  506  and the plurality of electrodes  504 . Nitrogen is filled in the gas space  505  in Embodiment 2. Note that the constitution of Embodiment 2 may be implemented by freely combining it with the constitution of Embodiment 1. 
     Embodiment 3 
     As shown in  FIG. 3 , the EL layer  305  comprises the light-emitting layer  303  and, as buffer layers, the electron injection layer  302  and the hole injection layer  304 . A polymer-based material is used for the layers constituting the El layer  305 . For example, polyparaphenylene vinylene can be used for the light-emitting layer  303 , copper pthalocyanine or PEDOT for the buffer layer (hole injection layer)  304 , and lithium fluoride or lithium for the buffer layer (electron injection layer)  302 . 
     Note that in forming the EL layer  305 , it is desirable that the treatment atmosphere for conducting the formation of this layer is a dry atmosphere having as little moisture as possible and in inert gas. The inert gas here is one such as nitrogen or argon. Because the EL layer easily deteriorates due to the presence of moisture or oxygen, it is therefore necessary to eliminate these factors as much as possible when forming this layer. In addition, these layers which constitute the EL layer  305  may be provided commonly for all the pixels, and therefore they may be formed by employing the spin coating method or the printing method. 
     The electron injection layer  302 , which is the buffer layer, has a role to inject electrons from the cathode  301  to the light-emitting layer  303 , and the hole injection layer  304  has a role to inject holes from the transparent electrode (anode)  306  to the light-emitting layer  303 . In addition, by providing the hole injection layer  304 , the prevention of damages to the light-emitting layer  303  during the formation of the electrodes can be expected. Note that the constitution of Embodiment 3 may be implemented by freely combining it with the constitution of Embodiments 1 or 2. 
     Embodiment 4 
     In manufacturing the EL element, a tris (8-quinolinolate) aluminum complex (Alq 3 ) is used for the light-emitting layer, and when using magnesium and silver (MgAg) for the cathode, the evaporated complexes of both the Alq 3  and the acetylacet sodium complexes can be used as the buffer layer sandwiched between the light-emitting layer and the cathode. Note that the constitution of Embodiment 4 may be implemented by freely combining it with any of the constitutions of Embodiments 1 to 3. 
     Embodiment 5 
     A self light-emitting device having a structure in which an organic EL material is used as a light-emitting layer sandwiched between a cathode and an anode has excellent brightness as well as low power consumption, and hence may be utilized as a back light of a liquid crystal display device, etc. Regarding the cathode, the anode, and the light-emitting layer, each can be formed on the entire surface of the substrate provided that the cathode and the anode are formed in a range that they do not directly contact each other. Furthermore, materials such as PPV and PVK (polyvinyl carabazole) may be used as the organic EL material. The present invention may be employed as the back light of a liquid crystal display utilized in the display portion of a mobile phone, the monitor of a personal computer (PC), and the like. Note that the constitution of Embodiment 5 may be implemented by freely combining it with any of the constitutions of Embodiments 1 to 4.