1. Field of the Invention
The invention relates to an organic electroluminescence element (hereinafter, referred to as an organic EL element), a display device including the organic EL element, and a method of manufacturing the organic EL element.
2. Background Art
An organic EL element has a configuration in which an organic luminescent layer showing at least an electroluminescence phenomenon is interposed between an electrode as an anode and an electrode as a cathode. The organic EL element is a spontaneous-luminescent element in which the organic luminescent layer emits light when a voltage is applied between the electrodes, holes and electrons are injected into the organic luminescent layer, and the holes and the electrons are recombined.
To improve the luminescent efficiency, a hole-injection layer, a hole-carrying layer, or an electron-block layer is suitably selected and formed between the anode and the organic luminescent layer, and a hole-block layer, an electron-carrying layer, an electron-injection layer, or the like is suitably selected and formed between the organic luminescent layer and the cathode. Further, the organic luminescent layer is combined with the hole-injection layer, the hole-carrying layer, the electron-block layer, the hole-block layer, the electron-carrying layer, the electron-injection layer, and the like, and the combined layers are referred to as a luminescent medium layer.
Each layer of the luminescent medium layers is formed of an organic material or an inorganic material. The organic material is classified into a low-molecular material and a high-molecular material.
Examples of the low-molecular material include copper phthalocyanine (CuPc) for the hole-injection layer, N,N′-dephenyl-N,N′-bis(3-methyl phenyl)-1,1′-biphenyl-4,4′diamine (TPD) for the hole-carrying layer, tris(8-quinolinol)aluminum (Alq3) for the organic luminescent layer, 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4,-oxazole (PBD) for the electron-carrying layer, and LiF for the electron-injection layer.
Each layer of the luminescent medium layer formed of the low-molecular material is formed so as to have a thickness of about 0.1 nm to about 200 nm mainly by a vacuum deposition method such as a resistance heating method or a vacuum drying method (dry process) such as a sputtering method.
The kinds of low-molecular materials are abundant, and an improvement in the luminescent efficiency, luminescent luminance, lifetime, or the like is expected by combinations thereof.
Examples of the high-molecular material include, for the organic luminescent layer, a material in which a low-molecular luminescent dye is solved in high molecules such as polystyrene, polymethylmethacrylate, or polyvinyl carbazole, a high-molecule fluorescent substance such as a polyphenylene vinylene derivative (hereinafter, referred to as PPV) or a polyalkyl fluorene derivative (hereinafter, referred to as PAF), and a rare-earth metal-based high-molecule phosphor.
The high-molecule material is generally solved or dispersed in a solvent and is film-formed with a thickness of about 1 nm to about 100 nm by a wet method (wet process) such as a coating or printing method.
When the wet method is used, it is possible to obtain the advantages that a film can be formed in the atmosphere, equipment is cheap, an increase in size is easy, and a film can be efficiently formed in a short time, compared to a case where a vacuum drying method such as a vacuum deposition method is used.
An organic thin film formed of the high-molecule material has the advantages that crystallization or agglomeration rarely occurs, defects such as a short-circuit or a dark spot can be prevented from occurring due to coating of a pin hole or a foreign substance of another layer.
On the other hand, examples of the inorganic material include, for the carrier-carrying layer, alkali metal elements such as Li, Na, K, Rb, Ce, and Fr, alkali earth metal elements such as Mg, Ca, Sr, and Ba, lanthanoids such as La, Ce, Pr, Nd, Sm, Eu, Gd, Db, Dy, Ho, Er, Tm, Yb, and Lu, actinoids such as Th, metal elements such as Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Ar, Nb, Mo, Ru, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Al, Ga, In, Sn, Tl, Pb, and Bi, and metalloid elements such as B, Si, Ge, As, Sb, and Te and include inorganic compounds such as alloys, oxides, carbides, nitrides, borides, sulfides, and halides thereof.
Since many inorganic materials have higher adhesion or thermal stability than organic materials, suppression of an erroneous luminescent phenomenon caused by a leakage current, reduction in the generation of non-luminescent regions called dark spots, and an improvement in luminescent characteristics or lifetime are expected for the inorganic materials. Further, since the inorganic materials can be applied to large-sized displays and mass-produced products at relatively lower costs compared to the organic materials, the inorganic materials are more advantageous in terms of reduction in costs.
There are known configurations (see JP-A-11-307259, JP-A-5-41285, JP-A-2000-68065, JP-A-2000-215985, JP-A-2006-114521, and JP-A-2006-155978) in which an inorganic hole-injection layer formed of an inorganic material is formed between an organic luminescent layer and an anode serving as a hole-injection electrode.
Further, there are known configurations (see JP-A-2002-367784, JP-A-5-41285, JP-A-2000-68065, and JP-A-2006-155978) in which an inorganic electron-injection layer formed of an inorganic material is formed between the organic luminescent layer and a cathode serving as an electron-injection electrode.
When a process subsequent to a process of forming the inorganic material layer includes a manufacturing process of forming a film in the atmosphere by a wet method, a problem may arise in that the light luminescent efficiency, the luminescent luminance, the lifetime, or the like may deteriorate since an undesirable phenomenon such as a change in the physical properties of the inorganic material occurs due to a liquid used in the process or the oxygen or moisture of the atmosphere.
For example, molybdenum oxide is known as a useful material which is an example of a hole-carrying material or an electron-injection material, since film formation is easy, a hole-injection function from a hole-injection electrode is high, a function of reliably carrying holes is excellent, and stability is high.
The molybdenum oxide is classified broadly into molybdenum trioxide (MoO3) and molybdenum dioxide (MoO2). Since the transmittance of molybdenum trioxide is high and molybdenum dioxide is low at the time of forming a film, molybdenum trioxide is generally used.
However, molybdenum trioxide is slightly soluble in water. Therefore, since molybdenum trioxide is formed and then reacts to moisture, the physical properties of molybdenum trioxide may easily vary. On the contrary, since molybdenum dioxide or most of the other inorganic compounds is insoluble in water, the physical properties of molybdenum dioxide or most of the other inorganic compounds scarcely vary.
In particular, when a process of forming a luminescent medium layer adjacent to molybdenum oxide includes a manufacturing process, such as transportation or film formation, performed in the atmosphere, the film may deteriorate due to a deterioration causing factor such as moisture. For this reason, a problem may arise in that display characteristics such as luminescent efficiency, luminescent luminance, or lifetime may deteriorate in some cases.
That is, when the luminescent medium layer is laminated only by a vacuum drying method, the amount and influence of deterioration causing factor adsorbed on the surface of molybdenum trioxide is small. However, when the manufacturing process of forming the film in the atmosphere is included, the display characteristics may considerably deteriorate.
Further, when a water-based solvent, an alcohol-based solvent, a ketone-based solvent, a carboxylic acid-based solvent, a nitryl-based solvent, or an ester-based solvent is used in the manufacturing process of forming the film in the atmosphere as in a wet method, a problem may arise in that particularly the luminescent efficiency and the luminescent luminance may deteriorate since molybdenum trioxide is solved and the physical properties and the film thickness are changed.
As a method of preventing the influence of the deterioration causing factor, there is a known method of preventing the deterioration causing factor by containing another metal compound having a strong environmental tolerance or a strong solvent tolerance in the metal compound used to form one layer of the luminescent medium layer and using the metal compound as a mixture film.    Patent document 1: JP-A-H11-307259    Patent document 2: JP-A-2002-367784    Patent document 3: JP-A-H05-41285    Patent document 4: JP-A-2000-68065    Patent document 5: JP-A-2000-215985    Patent document 6: JP-A-2006-114521    Patent document 7: JP-A-2006-155978