JP2005135623A - Organic el element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve luminous efficiency and reliability of an organic EL element using a cathode having a metal layer and an oxide conductive layer. <P>SOLUTION: The organic EL element 100 is provided with an anode 2, a cathode 5 having translucency, and an organic EL layer 9 formed between the anode 2 and the cathode 5 containing at least a light-emitting layer 4. The cathode 5 has a metal layer 6 containing a first metal and a low work-function metal, and an oxide conductive layer 7 in that order from the side of the organic EL layer 9. A work function of the low work-function metal is smaller than that of the first metal. The metal layer 6 has a first surface at the side of the organic EL layer 9, and a second surface at the side of the oxide conductive layer 7, and the concentration of the low work-function metal on the first surface is larger than that of the one on the second surface. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an organic electroluminescence (EL) device.

  Organic EL elements are attracting attention because they can be driven at a low voltage and can realize light emission with high luminance, and research and development of organic EL elements are actively conducted. In general, an organic EL element has a structure in which a light-emitting layer formed using an organic material is sandwiched between a pair of electrodes each having a light-transmitting property. A hole injecting and transporting layer, an electron injecting and transporting layer, and the like are provided between the pair of electrodes as necessary.

  The organic EL element can be used for a display device, for example. There are a simple matrix method and an active matrix method for driving the display device. In the display device of the simple matrix method, it is necessary to increase the instantaneous luminance of each pixel as the duty ratio increases. When the panel becomes large, there is a problem that power consumption increases. Therefore, particularly when a large display panel is required, an active matrix system is mainly employed.

  An active matrix display device using an organic EL element has a substrate on which a plurality of thin film transistors (hereinafter may be abbreviated as “TFT”) is formed, and an organic EL element formed on the substrate. ing. The TFT is formed for each of a plurality of pixels arranged in a matrix. By turning on / off the TFT of each pixel by the control signal, the light emission state of the organic EL element can be controlled for each pixel, and as a result, an image can be displayed. In the display device having such a configuration, conventionally, a method (bottom emission method) in which light from an organic EL element is extracted from the side of the TFT substrate opposite to the side on which the TFT is formed has been generally employed.

  Organic EL elements are classified into a low molecular type and a high molecular type depending on the type of light emitting material forming the light emitting layer. The low molecular organic EL element has a light emitting layer of a low molecular light emitting material, and the high molecular organic EL element has a light emitting layer of a polymer light emitting material.

  In the bottom emission type low molecular type and high molecular type organic EL elements, ITO (indium tin oxide) or IZO (indium zinc oxide) having a relatively large work function is used as the transparent anode. On the other hand, as a preferable cathode, in a low molecular type organic EL element, for example, an alloy single layer film of Mg (magnesium) and silver (Ag), a laminated film of an alkali metal halide such as LiF and Al, etc. In the polymer type organic EL element used, a laminated film of a low work function metal such as Ca and Ba and a relatively stable protective metal layer such as Al and Ag is used. Thus, the preferred cathode material differs between the low molecular weight organic EL element and the high molecular weight organic EL element. This is because the LUMO level of the polymer light-emitting material is lower than the LUMO level of the low-molecular light-emitting material, so that a polymer material organic EL device uses a cathode material with a lower work function in order to inject electrons efficiently. Is preferable.

  When an active matrix type full color display is manufactured using an organic EL element, it is necessary to form an RGB (red, green, blue) light-emitting layer pattern using mask vapor deposition in the low molecular type organic EL element. For this reason, it is difficult to increase the definition of the organic EL element (for example, 200 PPI), and it is difficult to form the organic EL element on a glass substrate having a large area (for example, 1 m square or more). There is a problem that the production process cannot be adopted. On the other hand, since the polymer type organic EL element can be manufactured by a wet process, it is not necessary to perform mask vapor deposition. Furthermore, according to a technique such as inkjet, a high-definition organic EL element can be formed on a large-area glass substrate. Therefore, in terms of productivity and manufacturing cost, it can be said that the polymer organic EL element is more advantageous than the low molecular organic EL element.

  By the way, when an active matrix display device is configured using organic EL elements, there are the following problems when the bottom emission method is employed.

  A TFT usually has a semiconductor layer made of polysilicon, amorphous silicon or the like, an electrode made of metal, and the like, but such a semiconductor layer or electrode does not have sufficient light transmittance. Therefore, the bottom emission method in which light is extracted through the TFT substrate has a problem that the ratio of the light emission area to the pixel area (aperture ratio) is small. In the organic EL display device, the current driving method is preferably adopted because the variation in display performance of each pixel is suppressed and the change in panel display luminance due to the deterioration of the organic EL material can be further reduced. If adopted, four transistors are required for each pixel. Therefore, the aperture ratio is further reduced as compared with a case where a voltage driving method which is simpler but inferior in display variation or the like for each pixel (two transistors in each pixel) (Non-Patent Document 1).

  In view of this, a configuration of a display device employing a TFT (top emission method) in which light from the organic EL element is extracted from above the TFT and the organic EL element formed on the substrate has been proposed. In the top emission method, it is possible to prevent the problem of the aperture ratio. However, in this method, the upper electrode (cathode) of the organic EL element must first have light transmittance. Second, in order to efficiently inject electrons into the light emitting layer, the upper electrode needs to be formed of a material having a small work function. Third, as will be described in detail below, it is necessary to suppress damage to the lower layer due to the upper electrode fabrication process as much as possible. The upper electrode usually has a laminated structure of an electron injection electrode made of a metal film that is sufficiently thin so as to transmit light and a transparent conductive film formed on the electron injection electrode. The transparent conductive film is provided to protect the electron injection electrode, which is a thin metal film, and to reduce wiring resistance. For the formation of this transparent conductive film, a method of generating relatively high energy particles such as sputtering and ion plating is usually used, so that the electron injection electrode and the light emitting layer in the lower layer are damaged, As a result, device characteristics may be deteriorated.

  Various configurations of such an upper electrode have been conventionally proposed. These conventional configurations basically use cathode materials and electron injection materials that can achieve good characteristics in the bottom emission method as they are in the top emission method as much as possible, and the thickness of the electrode layer formed from these materials. It was devised based on the concept of realizing translucency by reducing the size.

  For example, Patent Document 2 discloses that an alloy of Mg and Ag (such as Patent Document 1), which is one of cathode materials in a bottom emission type organic EL element, is used for a top emission type upper electrode. Yes. The low molecular weight organic EL element of Patent Document 2 includes a metal layer (Mg—Ag layer) having a sufficiently small thickness (10 nm) and a transparent conductive layer (IZO layer) in this order on a low molecular light emitting layer. . Patent Document 3 discloses a top emission type low molecular organic EL element including an upper electrode having a laminated structure of an electron injection layer and an amorphous transparent conductive film.

  However, both of Patent Documents 2 and 3 above relate to the structure of the upper electrode in the case of using a low molecular organic EL element. As described above, the polymer organic EL element and the low molecular organic EL element have different electrode configurations and electrode materials that give good characteristics. Therefore, the configuration of the upper electrode useful for the polymer organic EL element is different. It is necessary to consider.

  Further, the electrode configuration of the above-mentioned patent document has the following problems.

  As the transparent conductive film, an oxide conductive layer containing indium tin oxide (ITO) or indium zinc oxide (IZO) as a main component is used from the viewpoint of translucency and resistivity, but it is made of ITO or IZO. Such an oxide conductive layer is generally formed while oxygen gas is introduced into a film forming apparatus. On the other hand, the upper electrode has a metal layer for injecting electrons into the organic EL layer, and as a material for such a metal layer, a metal having a small work function (hereinafter referred to as the following) in order to improve the electron injection efficiency. For example, an alkaline earth metal such as magnesium (Mg), calcium (Ca), barium (Ba), alkali metal, indium (In), or an alloy containing these metals is used. Many. Therefore, when a metal layer made of a low work function metal is formed and an oxide conductive layer is formed using the metal layer as a base, the low work function metal is easily oxidized by the oxygen gas introduced into the film forming apparatus. There is a problem of deteriorating element characteristics. Even when the oxide conductive layer is formed while introducing only Ar gas into the deposition apparatus without introducing oxygen gas, the low work function metal is oxidized by oxygen contained in the oxide conductive layer. There is a possibility that the device characteristics may deteriorate.

Therefore, particularly when an oxide conductive layer such as ITO is formed on a metal layer made of a low work function metal, oxidation of the metal layer becomes a problem. However, regardless of the order of formation, the metal layer is oxide conductive. When in contact with the layer, there is a problem of deterioration in characteristics or reliability due to oxidation of the metal layer. Such a problem relating to the oxidation of the metal layer is particularly serious in the case of a polymer organic EL element using a metal having a smaller work function as the material of the metal layer.
Japanese Patent No. 2814435 JP 2001-85163 A Japanese Patent Laid-Open No. 10-162959 Shang-Li Chen et al. IDW '01, p.399

  An object of the present invention is to improve the light emission efficiency and reliability of an organic EL element in an organic EL element using a cathode having a metal layer and an oxide conductive layer.

  The organic EL device of the present invention includes an anode, a light-transmitting cathode, and an organic EL layer that is formed between the anode and the cathode and includes at least a light-emitting layer. A metal layer including a metal and a low work function metal, and an oxide conductive layer are provided in this order from the organic EL layer side, and the work function of the low work function metal is smaller than the work function of the first metal. The metal layer has a first surface on the organic EL layer side and a second surface on the oxide conductive layer side, and the concentration of the low work function metal on the first surface is the second surface. Greater than the concentration of the low work function metal.

  The concentration of the low work function metal on the second surface is preferably 30% by mass or less.

  The concentration of the low work function metal on the first surface is preferably 5% by mass or more and 50% by mass or less.

  The organic EL element according to any one of claims 1 to 3, wherein a thickness of the metal layer is 0.5 nm or more and 20 nm or less.

  The first metal may include at least one metal selected from the group consisting of Ni, Os, Pt, Pd, Al, Au, and Rh. The low work function metal may include at least one metal selected from the group consisting of Ca, Ba, Li, and Cs.

  The organic EL element may be a top emission type.

  The light emitting layer may include a polymer light emitting material.

  The oxide conductive layer may be formed by sputtering using a sputtering gas containing oxygen gas.

  A display device of the present invention includes any one of the organic EL elements described above and a thin film transistor electrically connected to the organic EL element.

  According to the present invention, the work function on the surface of the organic EL layer side of the metal layer constituting the cathode can be kept small, and the oxidation of the low work function metal contained in the metal layer can be suppressed, so that the cathode having good electron injection efficiency. Can be formed. As a result, the light emission efficiency of the organic EL element can be improved. Moreover, since the deterioration of the characteristic of an organic EL element can be suppressed, the reliability of an organic EL element can be improved.

  A laminated structure of Ca or Ba and Ag or Al is often used for the upper electrode (that is, the cathode) in the bottom emission type polymer organic EL element. Alternatively, a structure in which LiF and Ca are laminated and Ag or Al is further laminated thereon is used. In accordance with the conventional concept of “converting the configuration of a suitable upper cathode in the bottom emission method to the top emission method” as described above, as the structure of a suitable upper electrode of the top emission type polymer organic EL element, The following structure is conceivable. One example of the structure is a structure in which ITO or IZO is laminated on Ca or Ba instead of Ag or Al. Another structural example is a structure in which ITO or IZO is formed on a laminated structure of Ca or Ba and Ag or Al. The inventors of the present invention formed various electrodes of these structural examples while changing the production conditions of each layer, but a polymer type organic EL device having excellent luminous efficiency could not be obtained.

  Therefore, the present inventors have examined various electrode materials and configurations from various angles without being bound by the conventional common sense that “the preferred top cathode configuration in the bottom emission method is diverted to the top emission method”. It was. As a result, the present inventors have found a very useful cathode configuration that can be applied to a polymer organic EL element.

(Embodiment 1)
Hereinafter, a first embodiment of an organic EL element according to the present invention will be described with reference to the drawings.

  An organic EL element 100 shown in FIG. 1 includes an anode 2, an organic EL layer 9, and a cathode 5 that are sequentially formed on a substrate 1. The organic EL layer 9 only needs to have at least the light emitting layer 4, and may be a single layer or a laminated structure. In the present embodiment, the organic EL layer 9 has the hole injection layer 3 and the light emitting layer 4 in this order from the substrate 1 side. In the organic EL element 100 of the present embodiment, the light from the light emitting layer 4 is extracted from above the cathode 5 (top emission type), so the cathode 5 has translucency. The cathode 5 has a structure in which a metal layer 6 and an oxide conductive layer 7 containing an oxide are stacked in this order on an organic EL layer 9. The metal layer 6 includes a first metal and a low work function metal having a work function smaller than that of the first metal. The concentration of the low work function metal on the surface of the metal layer 6 on the organic EL layer 9 side (hereinafter referred to as “first surface”) is the surface of the metal layer 6 on the oxide conductive layer 7 side. (Hereinafter referred to as the “second surface”) is greater than the concentration of the low work function metal.

  In the organic EL element 100, since the cathode 5 is formed using the metal layer 6 containing a low work function metal, the work function of the cathode 5 can be made sufficiently small. Therefore, electrons are efficiently injected into the organic EL layer 9. it can. In addition, the metal layer 6 is not a metal layer made of only a low work function metal, but contains a first metal in addition to the low work function metal, and the second surface of the metal layer 6 has a low work function. Since the metal concentration is kept low, the oxidation of the low work function metal during the formation of the oxide conductive layer 7 can be effectively suppressed. Therefore, it is possible to suppress a decrease in light emission efficiency of the organic EL element due to oxidation of the low work function metal. In this specification, “light emission efficiency” is the luminance (light flux) of extracted light with respect to the electric power input to the organic EL element.

  If the decrease in light emission efficiency can be suppressed, the power input to the organic EL element can be reduced in order to achieve the required luminance. Therefore, since the current or voltage applied to the organic EL element can be reduced, deterioration of the organic EL element can be suppressed, and as a result, the reliability of the organic EL element can be improved. Further, even after the organic EL element is formed, since the low work function metal contained in the metal layer 6 is small in the vicinity of the oxide conductive layer 7, it is difficult to be oxidized by oxygen contained in the oxide conductive layer 7. Therefore, it is possible to suppress the deterioration of the organic EL element due to the increase in the resistance of the metal layer 6 and the decrease in the electron injection efficiency, and the reliability of the organic EL element can be improved.

  The concentration of the low work function metal on the first surface of the metal layer 6 is preferably 5% by mass or more in order to ensure sufficient electron injection efficiency. On the other hand, if the concentration of the low work function metal on the first surface of the metal layer 6 is 50% by mass or less, it is preferable because the oxidation of the low work function metal can be more reliably suppressed.

  The concentration of the low work function metal on the second surface of the metal layer 6 is preferably 30% by mass or less. Thereby, oxidation of a low work function metal can be suppressed more reliably. The concentration of the low work function metal at the second surface may be substantially zero.

  In this way, by allowing the low work function metal to exist at a higher concentration in the portion close to the organic EL layer 9 in the metal layer 6, it is preferable to use Ca that is preferably used for the light emitting layer 4 having a polymer light emitting material. It is possible to effectively suppress oxidation of electrode metals such as Ba and Ba which are very active and easily oxidized. Therefore, it is possible to provide an organic EL element exhibiting good characteristics, particularly a polymer type organic EL element.

  In order for the metal layer 6 to have sufficient electron injection efficiency, the thickness of the metal layer 6 is preferably 0.5 nm or more. The thickness of the metal layer 6 is preferably 20 nm or less. If the thickness of the metal layer 6 is 20 nm or less, since the light absorption rate from the organic EL layer by the metal layer 6 can be sufficiently reduced, a transparent organic EL panel or an organic EL panel having a top emission structure can be realized. Become.

  The first metal in the present embodiment can be used as the host metal of the metal layer 6. The first metal contained in the metal layer 6 preferably contains at least one metal selected from the group consisting of Ni, Os, Pt, Pd, Al, Au, and Rh. These metals are stable, and films formed from these metals are less likely to have an island-like structure even with a small thickness, have excellent coverage, and prevent oxidation of low work function metals. Since it is excellent, a highly efficient organic EL element can be realized. In this specification, a film that does not have an island-like structure generally includes a wide range of continuous films, and may be, for example, a film having pinholes or a porous film.

  The low work function metal contained in the metal layer 6 preferably contains at least one metal selected from the group consisting of Ca, Ba, Li and Cs. Since the work function of these metals is as small as 4.0 eV or less, when the metal layer 6 containing these metals is formed, electrons can be efficiently injected particularly into the organic EL layer using the polymer light emitting material.

  The cathode 5 only needs to include at least one metal layer 6 and one oxide conductive layer 7, and each layer may have a laminated structure. Further, other layers having different functions from these layers may be further included. However, in order to efficiently inject electrons into the organic EL layer, the metal layer 6 is preferably in contact with the organic EL layer 9.

  In the organic EL element 100, the oxide conductive layer 7 is formed with the second surface of the metal layer 6 as a base, but protection is provided to prevent oxidation of the metal layer 6 between the metal layer 6 and the oxide conductive layer 7. A cap layer may be provided. Preferably, the oxide conductive layer 7 is provided so as to be in contact with the second surface of the metal layer 6 without providing a protective cap layer. In the present embodiment, the low work function metal constitutes an alloy with the metal (Al) as a base material, and the concentration of the low work function metal (Ca) on the second surface of the metal layer is controlled to be low. Even if the cap layer is not provided, that is, even when the low work function metal is exposed on the second surface, the oxidation of the low work function metal can be sufficiently suppressed. Moreover, if the cap layer is not provided, the problem that the brightness of the extracted light decreases due to light absorption in the cap layer, and the manufacturing process becomes complicated due to the addition of the step of forming the cap layer, which increases the manufacturing cost. You can prevent problems.

  By forming the organic EL element of this embodiment on an active matrix substrate, an active matrix display device having excellent display characteristics can be configured.

  The organic EL layer 9 may include an organic layer formed from a solution using a polymer material. Thereby, since the thin film formation method which does not use vacuum, such as a printing method and an inkjet method, can be used, an organic EL element can be produced at lower cost.

  The light emitting layer 4 in the organic EL layer 9 may have a single layer structure or a multilayer structure. The light emitting layer 4 may be a layer in which a base material is doped with a dopant.

As shown in FIG. 1, the organic EL layer 9 in this embodiment has the hole injection layer 3 and the light emitting layer 4 in this order from the anode 2 side. The configuration of the organic EL layer 9 is as shown in FIG. It is not limited to the configuration shown in FIG. For example, the organic EL layer 9 can have the following configuration.
(1) Organic light emitting layer (2) Hole transport layer / organic light emitting layer (3) Organic light emitting layer / electron transport layer (4) Hole transport layer / organic light emitting layer / electron transport layer (5) Hole injection layer / Hole transport layer / organic light emitting layer / electron transport layer The light emitting layer 4 contained in the organic EL layer 9 may be a single layer or may have a multilayer structure.

  The light emitting material contained in the light emitting layer 4 may be a polymer material or a low molecular material, and a known light emitting material for an organic LED element can be used. Known light emitting materials can be classified into low molecular light emitting materials, polymer light emitting materials, precursors of polymer light emitting materials, and the like. Specific compounds of the respective light emitting materials are exemplified below, but the present invention is not limited thereto.

  As low molecular weight light emitting materials, aromatic dimethyldiene compounds such as 4,4′-bis (2,2′-diphenylvinyl) -phenyl ((DPVBi)), 5-methyl-2- [2- [4- (5- Oxadiazole compounds such as methyl-2-benzoxaziryl) phenyl] vinyl] benzoxazole, 3- (4-biphenylyl) -4-phenyl-5-t-butylphenyl-1,2,4-triazole (TZA) ) And other triazole compounds, styryl bezene compounds such as 4-bis (2-methystyryl) benzene, fluorescent organic materials such as thiovirazine dioxide derivatives, benzoquinone derivatives, naphthoquinone derivatives, anthraquinone derivatives, azomethine zinc complexes, (8- Fluorescent organometallic compounds such as hydroxynolinato) aluminum complexes can be used.

Polymer light-emitting materials include poly (2-decyloxy-1,4-phenylene) (DO-PPP), poly [2,5-bis- [2- (N, N, N-triethylammonium) ethoxy] -1 4-phenyl-alt-1,4-phenyllene] dibromide (PPP-NEt ³⁺ ), poly [2- (2′-ethylhexyloxy) -5-methoxy-1,4-phenylenevinylene] (MEH-PPV) ) Etc. can be used.

  Moreover, a poly (P-phenylene vinylene) precursor (Pre-PPV), a poly (P-naphthalene vinylene) precursor (Pre-PNV), etc. can be used as a precursor of the polymer light emitting material.

  The light emitting layer 4 can be formed by a known method. For example, the light emitting layer can be formed by depositing an organic light emitting material using a dry process such as a direct vacuum evaporation method, an EB method, or an MBE method. Instead, a solution for forming an organic light emitting layer containing an organic light emitting material is applied to a spin coating method, a doctor blade method, a discharge coating method, a spray coating method, an ink jet method, a relief printing method, an intaglio printing method, a screen printing method, a micro gravure coating. The light emitting layer may be formed by applying using a wet process such as a method.

  In the case of using a wet process, the organic light emitting layer forming solution may be a solution containing at least one kind of light emitting material, and may contain two or more kinds of light emitting materials. Further, in addition to the light emitting material, a leveling agent, a light emission assisting agent, an additive (donor, acceptor, etc.), a charge transport agent, a light emitting dopant, and the like may be included. Further, the solvent of the organic light emitting layer forming solution may be any solvent that can dissolve or disperse the light emitting material, such as pure water, methanol, ethanol, THF (tetrahydrofuran), chloroform, toluene, xylene, trimethylbenzene and the like. May be.

  Each of the hole transport layer and the electron transport layer (collectively referred to as “charge transport layer”) may have a single layer structure or a multilayer structure. The charge transport layer can be formed, for example, by a known method (dry process, wet process) as described above as a method for forming the light emitting layer. When the charge transport layer is formed by a wet process, the solvent of the charge transport layer forming solution may be any solvent that can dissolve or disperse the charge transport material, and the solvent exemplified above as the solvent of the light emitting layer forming solution. It may be.

  As the charge transport material contained in the charge transport layer, a known material can be used. Although these specific compounds are shown below, this invention is not limited to this.

  Examples of hole transport materials contained in the hole transport layer include porphyrin compounds, N, N′-bis- (3-methylphenyl) -N, N′-bis- (phenyl) -benzidine (TPD), N, Aromatic tertiary amine compounds such as N′-di (naphthalen-1-yl) -N, N′-diphenyl-benzidine (NPD), low molecular materials such as hydrazone compounds, quinacridone compounds and stilamine compounds, polyaniline, 3 , 4-polyethylenedioxythiophene / polystyrene sulfonate (PEDT / PSS), poly (triphenylamine derivative), polyvinylcarbazole (PVCz) and other polymer materials, poly (P-phenylene vinylene) precursor, poly (P A polymer material precursor such as a (naphthalene vinylene) precursor can be used.

  As an electron transport material included in the electron transport layer, for example, a low molecular material such as an oxadiazole derivative, a triazole derivative, a benzoquinone derivative, a naphthoquinone derivative, or a fluorenone derivative, or a polymer material such as poly [oxadiazole] is used. it can.

  The anode and the cathodes 2 and 5 can be formed from electrode materials as exemplified below.

The anode (anode) 2 may be a layer made of an electrode material such as a metal material having a large work function such as Au, Ni or Pt or a conductive metal oxide such as ITO, IZO or SnO _2. . Moreover, you may have a multilayer structure containing the layer which consists of the said electrode material. For example, the anode 2 has a structure in which an SiO ₂ layer having a thickness (for example, about 1 nm) is provided on the organic EL layer 9 side of the layer made of the above electrode material so as not to greatly hinder the conductivity of this layer. Also good. Since the surface of the SiO ₂ layer is excellent in affinity (wetting property) with the organic light emitting layer forming solution and the charge transport layer forming solution, the addition of the SiO ₂ layer causes the anode 2 and the organic EL layer 9 to be in contact with each other. Adhesion can be improved.

  As the low work function metal contained in the metal layer 6 of the cathode 5, Ca, Ce, Yb, Cs, Rb, Sr, Ba, Al, an alloy of Mg and Ag, an alloy of Al and Li, or the like can be used. . Among these, when Ca, Ce, Cs, Rb, Sr, Ba, Mg, and Li having a work function of 4.0 eV or less are used, high electron injection efficiency can be obtained. When forming the light emitting layer 4 of the polymer light emitting material, Ca and Ba are preferably used as the low work function metal. The metal layer 6 can be formed by doping the low work function metal into the host metal (first metal). The first metal is preferably formed using a chemically relatively stable metal such as Ni, Os, Pt, Pd, Al, Au, and Rh. Thereby, not only the oxidation of the low work function metal contained in the metal layer 6 is suppressed, but also the deterioration of the cathode 5 can be suppressed because the metal layer 6 itself is not oxidized. Moreover, it is preferable that the metal layer 6 has a transmittance of about 10% or more over the entire visible light region. For example, a Ni film (thickness: 35 nm) formed on a synthetic quartz substrate by an electron beam evaporation method exhibits a transmittance of about 10% over the entire visible light region, and islands are observed in surface morphology observation by scanning electron microscopy. Since it does not show a shape, it was confirmed that the covering property was also excellent. It was confirmed that the films made of the other materials described above also have good transmittance and covering properties, although there are differences in degree. Note that in order to form a film that does not have an island structure, it is preferable to select a metal material that does not easily form an island structure as a host metal, depending on the formation method and film thickness. For example, when Ni, Al, or the like is used as the host metal, it has been confirmed in the above-described experiment that a film that does not have an island-like structure can be formed even if the thickness is small. In addition, since materials (Os, Pd, etc.) generally used for thin film coating are also materials that do not easily form an island structure, they can be used as the host metal of the metal layer 6.

  The organic EL element 100 of FIG. 1 is a top emission type, but by using a transparent substrate such as a glass substrate as the substrate 1, light from the light emitting layer 4 can be extracted from the lower side of the substrate 1 (bottom emission). . Even in this case, by controlling the concentration of the low work function metal to be high on the surface on the organic EL layer side and low on the surface on the oxide conductive layer side, oxidation of the metal layer 6 can be prevented. The effect is obtained.

  The configuration of the organic EL device of the present invention is particularly advantageous when applied to a polymer organic EL device. As described above, in order to inject electrons into the light emitting layer 4 containing the polymer light emitting material, a metal having a lower work function is required, and oxidation of such a metal can be prevented.

  It is further advantageous to apply the present invention to a polymer type organic EL device having a PEDOT / PSS layer as a hole injection layer. The reason will be described in detail below.

In the polymer organic EL element, PEDOT / PSS is suitably used as the material for the hole injection layer. However, as is well known, the conventional organic EL element has a problem that the life characteristics are deteriorated due to diffusion of sulfur (S) contained in PEDOT / PSS. A trace amount of moisture (H ₂ O) remains in the PEDOT / PSS layer. A sulfur compound is generated by the desulfurization reaction of H ₂ O and PSS, and the sulfur compound diffuses into the light emitting layer. The diffused sulfur compound causes an oxidation-reduction reaction with the cathode (low work function metal) at the interface between the organic EL layer and the cathode. This oxidation-reduction reaction is further promoted by applying a voltage to the organic EL element. As a result of the oxidation-reduction reaction, a new metal sulfur compound is formed at the interface between the organic EL layer and the cathode, and this new metal sulfur compound becomes a factor that degrades the life characteristics of the organic EL element. In particular, Ca and Ba that are suitably used as the cathode material of the polymer organic EL element have high reactivity with S, and form sulfides (CaS and BaS), respectively, and deteriorate the life characteristics. On the other hand, according to the present invention, the low work function metal (for example, Ca) contained in the cathode constitutes an alloy layer with another metal. Thus, since the low work function metal contained in the alloy is considered to be difficult to react with S (not easily sulfided), it is possible to suppress the formation of a metal sulfur compound at the interface between the cathode and the organic EL layer. Life characteristics can be improved.

<Example 1>
The organic EL element (FIG. 1) of Example 1 is produced by the following method.

  A striped Pt electrode (width: 2 mm, length: 5 cm, thickness: about 150 nm) 2 is formed as an anode 2 on an insulating substrate 1 of 5 cm square by an electron beam evaporation apparatus. Next, a hole injection layer 3 is formed by applying a mixed aqueous solution of polyethylene dioxythiophene (PEDOT) and polystyrene sulfonic acid (PSS) on the anode 2 by spin coating and drying at 150 ° C. for 20 minutes. To do. At this time, the thickness of the hole injection layer 3 is set to about 60 nm by controlling the concentration of the solution, the number of rotations during spin coating, and the like. Next, similarly to the hole injection layer 3, the light emitting layer 4 is formed by applying and drying a solution of a polyfluorene derivative by a spin coating method.

Subsequently, the cathode 5 is formed on the light emitting layer 4. First, the metal layer 6 (thickness: 10 nm) is formed by resistance heating vapor deposition using Al containing 5 mass% of Ca as a vapor deposition source (starting material). The shape of the metal layer 6 is a stripe shape (width: 2 mm, length: 5 cm) along the direction orthogonal to the anode 2. Thereafter, an IZO layer (thickness: 100 nm) is formed as the oxide conductive layer 7 by a DC sputtering method. The IZO layer is formed using an IZO sintered target as a target and a mixed gas of Ar and O ₂ as a sputtering gas. The IZO layer is formed to have the same stripe pattern as the metal layer 6. Thereby, the organic EL display device of Example 1 is obtained. In addition, when the metal layer 6 and the oxide conductive (IZO) layer 7 are formed, the process of heating the substrate is not performed.

  When a direct current voltage was applied to the obtained organic EL device of Example 1 so that the Pt electrode 2 was positive and the IZO layer 7 was negative, and light emission from the upper part of the IZO layer 7 was observed, Green emission is observed under fluorescent light.

  Next, in order to compare with the organic EL element of Example 1, as shown in FIGS. 7A and 7B, an organic EL element of a comparative example is fabricated. As shown in FIG. 7A, the organic EL element 102 of Comparative Example 1 is the same as that of Example 1 except that it has a Ca layer (thickness: 10 nm) 8 made of only Ca as the metal layer of the cathode 5. It has the same configuration as the organic EL element and is manufactured by the same method. The Ca layer 8 is formed by resistance heating vapor deposition. As shown in FIG. 7B, the organic EL element 103 of Comparative Example 2 includes a Ca layer (thickness: 5 nm) 20 made of only Ca and an Al layer (thickness: 5 nm) made of only Al as metal layers. Except having a laminated structure with No. 21, it has the same structure as the organic EL element of Example 1, and is produced by the same method. The Ca layer 20 and the Al layer 21 are each formed by resistance heating vapor deposition.

  When a direct-current voltage was applied to each of the organic EL elements 102 and 103 of Comparative Examples 1 and 2 so that the Pt electrode 2 was positive and the IZO layer 7 was negative, and light emission from the top of the IZO layer 7 was observed, Only light emission (low brightness) that can be finally confirmed at a certain place can be obtained. Therefore, the luminance of light from the organic EL elements 102 and 103 is significantly lower than the luminance of light from the organic EL element of Example 1. In other words, the light emission efficiency of the organic EL elements 102 and 103 is significantly lower than the light emission efficiency of Example 1. Comparing the current efficiencies, the current efficiency of the organic EL element 102 is about two orders of magnitude smaller than the current efficiency of the organic EL element of Example 1, and the current efficiency of the organic EL element 103 is the current of the organic EL element of Example 1. About 1.5 orders of magnitude less than efficiency.

  The cause is considered as follows. In the organic EL element 102 of Comparative Example 1, the Ca layer 8 is at least partially oxidized to an oxide (insulator) by oxygen in the sputtering gas used when forming the IZO layer 7, and as a result, the cathode 5 The electron injection efficiency is reduced. Therefore, sufficient light emission cannot be obtained in the light emitting layer 4. Further, in the organic EL element 103 of Comparative Example 2, by providing the Al layer 21 between the Ca layer 20 and the IZO layer 7, oxidation of the Ca layer 20 during the formation of the IZO layer 7 can be slightly suppressed. The effect is extremely small compared to the effect in the case of forming an alloy layer of Ca and Al. Therefore, it cannot be sufficiently suppressed that the electron injection efficiency of the cathode 5 is lowered due to the oxidation of the Ca layer 20.

  On the other hand, in the organic EL element of Example 1, Ca is present only at a low concentration in the vicinity of the IZO layer 7, so that it is not easily oxidized by oxygen in the sputtering gas used when forming the IZO layer 7. The metal layer 6 is not a Ca layer made of only Ca that is easily oxidized, but an alloy layer of Al and Ca. Thus, Ca contained in the alloy is further difficult to be oxidized. Therefore, since oxygen in the sputtering gas does not damage the cathode 5, the cathode 5 can inject electrons into the organic EL layer 9 with high efficiency.

<Composition analysis of metal layer>
Hereinafter, in order to examine the configuration of the metal layer 6 in the organic EL element of Example 1 in detail, only a metal layer (thickness: 10 nm) is formed on a glass substrate (referred to as “metal layer sample”), and this metal layer The composition analysis in the depth direction was performed, and the results will be described. The metal layer is formed by the same method as the method for forming the metal layer 6 in the organic EL element of Example 1 described above. The composition analysis of the metal layer is performed using an Auger electron spectrometer.

  The result of the composition analysis of the metal layer is shown in FIG. FIG. 2 shows that the concentration of Ca on the surface of the metal layer on the glass substrate 1 side is about 35% by mass. It can also be seen that the concentration of Ca deposited decreases as the deposition proceeds, and the Ca concentration is about 15% by mass on the upper surface of the metal layer. This is considered to be because when vapor deposition is performed using an alloy of Ca and Al as a vapor deposition source, Ca is vapor deposited earlier due to the difference in vapor pressure between Ca and Al. In FIG. 2, the total content of Ca and Al is smaller than 100% by mass, because the metal layer contains impurities mainly composed of oxygen (not shown).

  Therefore, in the organic EL element of Example 1, Ca, which is a low work function metal, is present in the vicinity of the surface of the metal layer 6 in contact with the light emitting layer 4 at a concentration of about 35% by mass. It can be confirmed that it exists in the vicinity of the surface in contact with the conductive layer 7 at a concentration of about 15% by mass.

  In this way, when an alloy is deposited as a starting material, a single deposition source is doped with a low work function metal only near one surface, and the doping concentration of the low work function metal decreases with increasing distance from the surface. A layer can be formed. In addition, by controlling the Ca content in the vapor deposition source, the formation rate of the metal layer (film formation rate), and the timing of opening the shutter during vapor deposition, the Ca content in the metal layer 6 and the Ca in the metal layer 6 are controlled. Concentration gradient can be adjusted. Furthermore, the content of impurities mainly composed of oxygen in the metal layer 6 can be reduced by keeping the pressure before and during the formation of the metal layer 6 low. In the organic EL element of Example 1, the content of impurities in the metal layer 6 is the same as or higher than the content of Ca. Despite the inclusion of such a relatively large amount of impurities, the organic EL device of Example 1 exhibits good characteristics as described above.

<Example 2>
The organic EL element of Example 2 has the same configuration as the organic EL element of Example 1 and is manufactured by the same method, but differs in the following points. In the organic EL element of Example 2, the Ca concentration on the surface (second surface) of the metal layer 6 on the side of the oxide conductive layer 7 is substantially zero. Such a metal layer 6 can be formed by controlling the deposition conditions and thickness of the metal layer 6. Here, Al containing 5% by mass of Ca is used as a vapor deposition source, and the metal layer (thickness: 15 nm) 6 in which the Ca concentration on the second surface is substantially zero is formed. When the composition analysis is performed on the metal layer 6 by the same method as described above, the result shown in FIG. 8 is obtained.

  Further, when a direct current voltage was applied to the organic EL element of Example 2 so that the Pt electrode 2 was positive and the IZO layer 7 was negative, and light emission from the upper part of the IZO layer 7 was observed, the green color from the light emitting layer 4 was observed. Luminescence is observed under fluorescent light.

<Examination of Ca concentration in metal layer 6>
Next, since the suitable Ca density | concentration of the metal layer 6 was examined, it demonstrates below. Here, a plurality of sample elements are obtained by changing the Ca concentration on the surface (first surface) of the metal layer 6 on the organic EL layer 9 side and the Ca concentration on the surface (second surface) of the oxide conductive layer 7 side. And a suitable range of the Ca concentration is obtained by examining the relationship between the Ca concentration on each surface and the current efficiency of the sample element.

  First, a sample element is manufactured by a method similar to the method for manufacturing the organic EL element of Example 1 described above. However, in order to change the Ca concentration in the metal layer 6 for each sample element, the metal layer formation conditions such as the Ca concentration in the vapor deposition source, the formation speed of the metal layer, and the timing of opening the shutter are appropriately selected. The thickness of the metal layer 6 is 10 nm.

  Subsequently, using these sample elements, the relationship between the Ca concentration on the first and second surfaces of the metal layer 6 and the current efficiency (external current efficiency) of the sample elements is examined. The results are shown in Table 1.

The current efficiency (cd / A) of the sample element is the luminance (unit: cd / m ² ) of the element sample measured with a luminance meter, the current value (unit: A) flowing through the element sample at that time, and the light emission of the element It can be determined from the area (unit: m ² ).
The current efficiency shown in Table 1 is the sample element No. showing the highest current efficiency. This is a normalized value with the current efficiency of 3 as 1.

  Further, the Ca concentration shown in Table 1 can be obtained for each sample element by the same method as the method using the metal layer sample described in Example 1. That is, a plurality of metal layer samples are produced by forming a metal layer similar to that formed by each sample element on a glass substrate. About each of this metal layer sample, the Ca concentration of the surface which is in contact with the glass substrate in a metal layer is calculated | required using the Auger electron spectrometer, and it is set as Ca concentration of the 1st surface of each sample element.

  In any of the sample elements, green light emission from the light emitting layer 4 is observed under a fluorescent lamp and shows good current efficiency. As is clear from Table 1, the Ca concentration on the first surface is 5 mass% or more and 50 mass%. If it is below, especially favorable current efficiency is obtained (sample element No.2-No.4). The reason is considered as follows. If the Ca concentration on the first surface is, for example, 50% by mass or less, it is difficult to be affected by oxygen or moisture in the atmosphere when forming the oxide conductive layer 7. That is, Ca becomes difficult to be oxidized, and as a result, it is possible to more reliably prevent a decrease in the electron injection efficiency of the metal layer 6. On the other hand, when the concentration of Ca on the first surface is, for example, 5% by mass or more, the work function on the first surface of the metal layer can be further reduced, so that the electron injection efficiency of the metal layer 6 can be improved.

<Examination of life characteristics>
Then, since the lifetime characteristic of the organic EL element of this embodiment was investigated, the result is demonstrated.

  Here, the organic EL element of Example 3 is used. The organic EL element of Example 3 has the same configuration as the organic EL element of Example 1 except that an ITO layer is used as the oxide conductive layer 7 instead of the IZO layer, and is manufactured by the same method. The For comparison, an organic EL element of Comparative Example 3 is also produced. The organic EL element of Comparative Example 3 is the same as the organic EL element of Comparative Example 2 described with reference to FIG. 7B except that an ITO layer is used as the oxide conductive layer 7 instead of the IZO layer. It has a structure and is manufactured by a similar method.

  The results of examining the lifetime characteristics of the organic EL device of Example 3 and the organic EL device of Comparative Example 2 are shown in FIG.

  As can be seen from FIG. 6, the luminance half-life of the organic EL element of Example 3 is about 80 hours, whereas the luminance half-life of the organic EL element of Comparative Example 3 is about 25 hours. From this, it can be confirmed that in the polymer organic EL element having the PEDOD / PSS layer 3, when an alloy layer containing Ca is provided as the metal layer 6, the life of the organic EL element is extended about three times. Although not shown, in the case of a low molecular organic EL element not having a PEDOT / PSS layer, the life characteristics of an element having a Ca-doped Al layer (alloy layer) as the metal layer 6 and the metal layer 6 There is no significant difference as shown in FIG. 6 between the lifetime characteristics of the element having the laminated structure of the Ca layer and the Al layer.

  From these results, the following can be understood. In the organic EL element of Comparative Example 3, as described above, sulfur (S) contained in the PEDOT / PSS layer 3 as the hole injection layer reacts with Ca in the Ca layer 20 to form a metal sulfur compound (CaS or the like). As a result, the life characteristics are degraded. On the other hand, in the organic EL element of Example 3, the use of the alloy layer containing Ca as the cathode can suppress the generation of the metal sulfur compound as described above, and thus can suppress the deterioration of the life characteristics.

(Embodiment 2)
Hereinafter, a second embodiment of the organic EL element according to the present invention will be described.

  The organic EL element of this embodiment has the same configuration as that of the organic EL element 100 described with reference to FIG. However, the organic EL element of this embodiment and the organic EL element 100 differ in the point of the formation method of the metal layer 6. FIG. In the organic EL element 100, the metal layer 6 is formed using a single vapor deposition source. However, in this embodiment, a multi-source (a plurality of vapor deposition sources) of a base metal and a low work function metal is used. The metal layer 6 is formed by the co-evaporation used. First, a vapor deposition boat containing a metal (for example, Al) as a base material and a vapor deposition boat containing a low work function metal (for example, Ca) are prepared. Next, co-evaporation is performed while controlling the current applied to each deposition boat so that the deposition rate of the base metal and the low work function metal is a desired ratio. Thereby, the metal layer 6 (thickness: for example, 10 nm) in which the concentration of the low work function metal is different between the first surface and the second surface can be formed.

  In the organic EL element of this embodiment, since the metal layer 6 is formed by co-evaporation, the concentration of the low work function metal in the metal layer 6 can be controlled in a wider range and more strictly. Therefore, the metal layer 6 having excellent electron injection efficiency can be more reliably formed by suppressing the oxidation of the metal layer 6 while keeping the work function of the metal layer 6 small.

<Examination of Ca concentration in metal layer 6>
In the present embodiment, a plurality of sample elements having different Ca concentrations in the metal layer 6 were prepared, and the relationship between the Ca concentration and the current efficiency of the sample elements was examined in more detail. The results will be described.

  Here, a plurality of sample elements are manufactured by the following method. First, an anode (Pt) 2, a hole injection layer 3, and a light emitting layer 4 are sequentially formed on the substrate 1 by using the same material as that of the organic EL element of Example 1 and the same method. Thereafter, a Ca-doped Al layer is formed as the metal layer 6 by co-evaporation. At this time, first, the Ca concentration on the second surface of the metal layer 6 is fixed to 10 mass%, and the Ca concentration on the first surface of the metal layer 6 is changed for each sample element. Similarly, when the Ca concentration on the second surface is fixed to 30% by mass and 40% by mass, the Ca concentration on the first surface is changed for each sample element. The Ca concentration of these sample elements is controlled by the formation conditions of the metal layer 6 (for example, the magnitude of current applied to the evaporation boat). The formation conditions of the metal layer 6 are determined in advance from the results obtained by forming the Ca-doped Al layer on the glass substrate under various conditions and obtaining the Ca concentration in each Ca-doped Al layer with an Auger electron spectrometer. deep. Finally, the oxide conductive layer 7 is formed on the metal layer 6 by the same method using the same material as the organic EL element of Example 1.

  The current efficiency is measured for each of the obtained sample elements. The results are shown in FIG. The current efficiency in FIG. 3 is a value normalized by setting the current efficiency of the sample element showing the highest current efficiency among the sample elements manufactured by the above method to 1.

  As is apparent from FIG. 3, sample elements having Ca concentrations of 10 mass% and 30 mass% on the second surface of the metal layer 6 exhibit particularly good current efficiency. This is because the amount of Ca existing on the second surface serving as the base for forming the oxide conductive layer 7 is suppressed to 30% by mass or less, whereby Ca due to oxygen or moisture in the atmosphere when the oxide conductive layer 7 is formed. This is thought to be because the oxidation of can be suppressed more effectively.

  Moreover, it turns out that the sample element whose Ca concentration in the 2nd surface is 10 mass% and 30 mass%, and Ca concentration in the 1st surface is 5 mass% or more and 50 mass% or less shows still higher current efficiency. This is considered because if the Ca concentration on the first surface is 5% by mass or more, the metal layer 6 has a sufficient amount of Ca to efficiently inject electrons into the organic EL layer. On the other hand, if the Ca concentration on the first surface is 50% by mass or less, the oxidation of Ca due to oxygen and moisture in the atmosphere when forming the oxide conductive layer 7 is more reliably suppressed, so that the metal layer 6 deteriorates. It is thought that it does not.

<Examination of the thickness of the metal layer 6>
Furthermore, since the suitable thickness of the metal layer 6 was examined, it demonstrates below. Here, a plurality of sample elements having different thicknesses of the metal layer (Ca-doped Al layer) 6 are produced, and by examining the relationship between the thickness of the metal layer 6 and the current efficiency (external current efficiency) of the sample element, A suitable thickness range of the metal layer 6 is obtained.

  The metal layer 6 of these sample elements is formed by the co-evaporation method as described above. When the metal layer 6 is formed, the deposition rate is adjusted so that the Ca concentration on the first surface of the metal layer 6 of the sample element is 30% by mass and the Ca concentration on the second surface is 10% by mass. Further, the thickness of the metal layer 6 in each sample element is controlled by appropriately adjusting the vapor deposition rate and the timing for closing the vapor deposition boat shutter. Except for the above points, the sample element has the same configuration as the organic EL element of Example 1, and is manufactured by the same method.

  Next, the relationship between the thickness of the metal layer 6 and the current efficiency of the sample element is examined using these sample elements. The results are shown in FIG. The current efficiency shown in FIG. 4 is a value normalized with the current efficiency of the sample element showing the highest current efficiency as 1.

  As is apparent from FIG. 4, the current efficiency of the organic EL element increases rapidly when the thickness of the metal layer 6 increases slightly from zero, and gradually decreases when the thickness exceeds 20 nm. As described above, the reason why the current efficiency gradually decreases as the thickness of the metal layer 6 increases is that the absorption and reflection of light from the light emitting layer 4 by the metal layer 6 increases as the thickness of the metal layer 6 increases. This is because the amount of light transmitted through the metal layer 6 is reduced. Moreover, FIG. 4 shows that when the thickness of the metal layer 6 is 0.5 nm or more and 20 nm or less, an organic EL element having excellent current efficiency can be obtained.

(Embodiment 3)
Hereinafter, with reference to FIG. 5, the structure of a display device using the organic EL element will be described.

  In the display device 200 of FIG. 5, the organic EL element 100 is formed on the active matrix substrate 101. The active matrix substrate 101 includes a substrate 1, a plurality of TFTs formed for each pixel on the substrate 1, and a planarization film 14 that covers these TFTs. Each TFT is provided so as to cover the gate electrode 12, an island-like semiconductor layer (not shown) formed on the gate electrode 12 via the gate insulating film 13, and both ends of the island-like semiconductor layer. TFT electrodes (source and drain electrodes) 10 (bottom gate structure). Each TFT is connected to the source line 11 ′ and the gate line 11. A through hole 15 reaching the drain electrode 10 of each TFT is provided in the planarizing film 14. An organic EL element 100 is formed on the planarization film 14. The anode 2 of the organic EL element 100 is formed for each pixel by patterning a conductive layer deposited on the planarizing film 14 and inside the through hole 15. Each anode 2 is connected to the corresponding drain electrode 10 of the TFT via a through hole 15. These anodes 2 are insulated from each other by an insulating film 16 formed so as to cover the respective edge portions and through holes 15 of each anode 2. On the anode 2 and the insulating film 16, a hole injection layer 3, a light emitting layer 4, a metal layer (Ca-doped Al layer) 6, and an oxide conductive layer (IZO layer) 7 are formed in this order.

  Since the display device 200 in FIG. 5 has the above-described configuration, the display device 200 has high light emission efficiency by suppressing a decrease in electron injection efficiency due to oxidation of a low work function metal. Therefore, high-definition and bright display can be realized. In addition, the variation in luminance between pixels (that is, for each organic EL element 100) is small, and a high-quality display can be obtained. Furthermore, since the display device 200 can suppress deterioration of the metal layer 6 in the organic EL element 100, the display device 200 has high reliability.

  The configuration of the display device according to the present invention is not limited to the above. For example, the TFT may have a top gate structure. The configuration of the organic EL element 100 is not limited to the above, and various configurations as described with reference to FIG. 1 can be applied. In addition, a simple matrix display device can be formed using the organic EL element 100. Further, the display device 200 may adopt a voltage driving method that requires two TFTs for each pixel, or may adopt a current driving method that requires four TFTs for each pixel. .

  The organic EL display device (FIG. 5) of Example 4 is manufactured by the following method.

A TFT using polysilicon as a semiconductor layer is formed on the insulating substrate 1, and then covered with a planarizing film 14 in order to eliminate unevenness on the surface of the substrate 1. Next, the Ni layer 2 is formed on the planarizing film 14 and inside the through hole 15 provided in the planarizing film 14. The Ni layer is connected to the drain electrode 10 of the TFT through the through hole 15. The thickness of the Ni layer formed on the planarizing film 14 is 150 nm. After forming the Ni layer, a NiO film (thickness: 1 nm) is formed on the surface of the Ni layer by plasma oxidation (not shown). Thereby, the anode 2 is formed. Next, an SiO ₂ film is formed so as to cover the through hole 15 and the edge portion of the Ni electrode 2. Subsequently, the hole injection layer (thickness: about 60 nm) 3 and the light emitting layer 4 are formed in this order on the substrate by the same method as described with reference to FIG. On the light emitting layer 4, the translucent metal layer 6 is formed by co-evaporation of Al and Ca. Al is evaporated by electron beam evaporation, and Ca is evaporated by resistance heating evaporation. First, the ratio of the deposition rate of Al and Ca is controlled to 80:20 so that the Ca concentration on the first surface is about 30% by mass, and the deposition of Al and Ca is started simultaneously. Thereafter, the deposition rate of Ca is gradually lowered, and at the end of deposition, the ratio of the deposition rate of Al and Ca is controlled to 90:10 so that the Ca concentration on the second surface is about 10% by mass. Thereby, a Ca-doped Al layer (thickness: 10 nm) 6 doped with Ca can be formed as the metal layer 6. Finally, an IZO layer (thickness: 100 nm) 7 is formed by a method similar to the method described with reference to FIG.

  When a control signal is applied to the TFT of the display device obtained in Example 4, green light emission from the light emitting layer 4 is observed from above the IZO layer 7. Therefore, it can be confirmed that the oxidation in the metal layer 6 of the organic EL element 100 is sufficiently suppressed.

  According to the present invention, since oxidation of a low work function metal at the cathode can be suppressed, a highly reliable and highly efficient organic EL element can be provided.

  The present invention can be suitably applied to a polymer organic EL element. In the polymer type organic EL element, since the cathode is formed using a metal having a smaller work function, oxidation of such a metal can be effectively suppressed.

  Further, the present invention can be suitably applied to a top emission type organic EL element. Further, when such an organic EL element is used in an active matrix display device, a remarkable effect is sometimes obtained.

It is sectional drawing which shows typically the structure of the organic EL element of Embodiment 1 by this invention. 3 is a graph showing the composition of a metal layer in the organic EL element of Example 1. FIG. It is a graph which shows the relationship between Ca density | concentration of a metal layer, and the external current efficiency of an organic EL element. It is a graph which shows the relationship between the thickness of a metal layer, and the external current efficiency of an organic EL element. It is sectional drawing which shows typically the structure of the organic electroluminescence display of Embodiment 3 by this invention. It is a graph which shows the lifetime characteristic of the organic EL element of Example 3 and Comparative Example 3, respectively. (A) And (b) is sectional drawing which shows typically the structure of the organic EL element of a comparative example. 6 is a graph showing the composition of a metal layer in the organic EL element of Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Anode 3 Hole injection layer 4 Light emitting layer 5 Cathode 6 Metal layer 7 Oxide conductive layer 8 Ca layer 9 Organic EL layer 10 Source, drain electrode 11, 11 ′ wiring 12 Gate metal 13 Gate insulating film 14 Planarization film 15 Through hole 16 Insulating film 20 Ca layer 21 Al layer 100 Organic EL element 101 Active matrix substrate 200 Organic EL display device

Claims (10)

The anode,
A light-transmitting cathode;
An organic EL layer formed between the anode and the cathode and including at least a light emitting layer;
The cathode has a metal layer including a first metal and a low work function metal, and an oxide conductive layer in this order from the organic EL layer side, and the work function of the low work function metal is the first work function. Smaller than the work function of metal,
The metal layer has a first surface on the organic EL layer side and a second surface on the oxide conductive layer side, and the concentration of the low work function metal on the first surface is low on the second surface. Greater than the concentration of the work function metal,
Organic EL element.   The organic EL element according to claim 1, wherein the concentration of the low work function metal on the second surface is 30% by mass or less.   3. The organic EL element according to claim 1, wherein the concentration of the low work function metal on the first surface is 5% by mass or more and 50% by mass or less.   The organic EL element according to any one of claims 1 to 3, wherein a thickness of the metal layer is 0.5 nm or more and 20 nm or less.   5. The organic EL element according to claim 1, wherein the first metal includes at least one metal selected from the group consisting of Ni, Os, Pt, Pd, Al, Au, and Rh.   The organic EL device according to claim 1, wherein the low work function metal includes at least one metal selected from the group consisting of Ca, Ba, Li, and Cs.   The organic EL element according to claim 1, wherein the organic EL element is a top emission type.   The organic EL device according to claim 1, wherein the light emitting layer includes a polymer light emitting material.   The organic EL element according to claim 1, wherein the oxide conductive layer is formed by sputtering using a sputtering gas containing oxygen gas. An organic EL device according to any one of claims 1 to 19,
A display device comprising a thin film transistor electrically connected to the organic EL element.

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