JP2007035432A - Organic el element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a top-emission organic EL element and its manufacturing method with luminescent efficiency improved. <P>SOLUTION: The organic EL element taking out light from a transparent cathode side is made by forming a reflection film consisting of Ag, or Al or their alloy, a transparent anode, an organic EL layer made by containing at least an organic luminescent layer, and a transparent cathode in turn on a substrate. Provided a minimum emission wavelength of the organic luminescent layer is λ, a film thickness of the transparent anode is λ/4 or less, and provided a maximum protrusion height of the transparent anode against the organic layer is h, and a film thickness of the organic EL layer is d, a formula of: h/d<0.5 is satisfied. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an organic EL element used as a light emission source of various display devices used for color televisions, personal computers, pachinko machines, and the like, and a method for manufacturing the same. More specifically, the present invention relates to a top emission type organic EL element with improved luminous efficiency and a method for manufacturing the same.

  As an example of a light-emitting element applied to a display device, an organic electroluminescence element (hereinafter referred to as an “organic EL element”) having a thin film laminated structure of an organic compound is known. Organic EL elements are thin-film self-luminous elements and have excellent characteristics such as low drive voltage, high resolution, and high viewing angle, and thus various studies have been made for their practical application (for example, (Refer nonpatent literature 1).

  As the organic EL element, a so-called “bottom emission” type element that extracts light from the substrate side is widely known. FIG. 4 shows a schematic structure of a general bottom emission type organic EL element. The bottom emission type organic EL element 200 is configured by sequentially forming a transparent anode 207, an organic EL layer 210, and a metal cathode 201 on a substrate 209. The organic EL layer 210 is formed by forming a hole injection layer 206, a hole transport layer 205, an organic light emitting layer 204, an electron transport layer 203, and an electron injection layer 202 on the transparent anode 207.

  In recent years, active matrix drive type displays have been actively developed in the field of organic EL displays using organic EL elements as light emission sources. When the bottom emission type organic EL element configured as described above is applied to an organic EL display of an active matrix driving system, a TFT that occupies the substrate as the number of thin film transistors (TFTs) provided on the substrate as a switching element increases. In some cases, the light extraction area from the substrate side decreases.

  Therefore, in such a situation, the element applied to the organic EL display extracts light from the opposite side (cathode side) of the substrate rather than the bottom emission type element that extracts light from the substrate side shown in FIG. A so-called “top emission” type element is structurally advantageous (see Patent Document 1).

  FIG. 5 shows a schematic structure of a general top emission type element. The top emission type organic EL element is configured by sequentially forming a reflective film 308 functioning as an anode, an organic EL layer 310, and a transparent cathode electrode 301 on a substrate 309. The organic EL layer 310 includes a hole injection layer 306, a hole transport layer 305, an organic light emitting layer 304, an electron transport layer 303, and an electron injection layer 302.

  In order to manufacture such a top emission type organic EL element 300, the reflective film 308 is formed on the substrate 309 and the organic EL layer 310 is formed on the reflective film 308 by a method such as sputtering or vapor deposition. However, the reflective film 308 made of a highly reflective metal has a large surface roughness and has protrusions on the surface. Therefore, in the organic EL layer 310 adjacent to the reflective film 308, the organic EL layer 310 becomes thin at the protrusions, and a leak current occurs between the electrodes or a short circuit occurs.

In order to prevent such a problem, a method of polishing a reflective film made of metal has been reported (see Patent Document 1 and Patent Document 2). However, since the metal used in Patent Document 1 and Patent Document 2 is selected from a material having a high work function so as to have a function as an anode, the reflectivity is not high, and the reflectivity is increased by polishing. It will decline. Furthermore, since Cr, Ni, Cu, etc. have a reduced reflectivity due to polishing and an oxide layer is formed on the surface, the interface resistance rises, resulting in a decrease in luminous efficiency. Therefore, from Cr, Ni, Cu, etc. It is difficult to use the reflection film itself as an electrode.
JP 2001-176660 A Japanese Patent Laid-Open No. 9-7770 C. W. Tang, S.M. A. VanSIyke, Appl. Phys. Lett. , 51, 913 (1987)

  An object of this invention is to provide the top emission type organic EL element which improves luminous efficiency, and its manufacturing method.

  The object of the present invention is solved by using Ag or Al or an alloy thereof as a reflective film, forming a transparent anode thereon to form a reflective electrode, and smoothing the surface. Ag and Al as materials having high reflectivity themselves have a low work function and do not function as an anode, and have a large surface roughness and protrusions on the surface in any film forming method. Therefore, in the reflective film made of a material having a high reflectivity, indium tin oxide (ITO) or indium zinc oxide (IZO) that has a work function capable of hole injection and is not altered by polishing is formed. The present invention is completed by using the anode.

That is, the present invention is an organic EL device that extracts light from the transparent cathode side, and comprises an organic layer comprising a reflective film made of Ag or Al or an alloy thereof, a transparent anode, and at least an organic light emitting layer on a substrate. An EL layer and a transparent cathode are sequentially formed, and when the shortest emission wavelength of the organic light emitting layer is λ, the film thickness of the transparent anode is λ / 4 or less, and the transparent anode with respect to the organic EL layer When the maximum protrusion height is h and the film thickness of the organic EL layer is d, h / d <0.5 is satisfied.
Here, the reflective film made of Ag or Al or an alloy thereof is a reflective film made substantially of only Ag, a reflective film made only of Al, a reflective film made of an alloy of Ag and Al, Ag and A reflective film made of an alloy with a metal selected from Al, Ni, Cr, Ti, Mn, Fe, Co, etc., and a metal selected from Al, Ag, Ni, Cr, Ti, Mn, Fe, Co, etc. A reflective film made of an alloy of The alloy may contain inevitable impurities.

  According to another aspect of the present invention, there is provided a method for manufacturing an organic EL device, comprising: forming a reflective film made of Ag or Al or an alloy thereof on a substrate; and forming a transparent anode on the reflective film. A step of forming, a step of mirror polishing the surface of the transparent anode opposite to the reflective film, and a step of sequentially forming an organic EL layer and a transparent cathode on the mirror-polished transparent anode surface .

  In the mirror polishing step, when the maximum protrusion height of the transparent anode with respect to the organic EL layer is h and the film thickness of the organic EL layer is d, the mirror polishing is performed so as to satisfy h / d <0.5. It is preferable.

  According to the present invention, a transparent anode is formed on a reflective film made of Ag, Al, or an alloy thereof, and a smooth reflective electrode in which the surface of the transparent anode is mirror-polished is formed, so that no reflection or leakage occurs. Efficiency can be improved. As a result, it is possible to efficiently provide a high-quality top emission organic EL element with reduced driving voltage and improved luminous efficiency.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic cross-sectional view showing a top emission type organic EL device according to an embodiment of the present invention. The top emission type organic EL element 100 is configured by sequentially forming a substrate 109, a reflective film 108, a transparent anode 107, an organic EL layer 110, and a transparent cathode 101. In the present specification, a structure in which the reflective film 108 and the transparent anode 107 are laminated may be referred to as a reflective electrode. The organic EL layer 110 includes a hole injection layer 106, a hole transport layer 105, an organic light emitting layer 104, an electron transport layer 103, and an electron injection layer 102.

  As the substrate 109, a normal substrate such as glass can be used. In particular, when the organic EL element 100 is applied to an active matrix driving type organic EL display, a substrate 109 on which a TFT thin film is formed can be preferably used.

  The reflective film 108 is a metal layer provided on the substrate 109, and is preferably Ag or Al having high reflectivity or an alloy thereof. In the case of an alloy, for example, NiAl can be used. In particular, since the reflectivity is as high as 95% and the interface with the adjacent transparent anode 107 is difficult to oxidize, it is preferable to use the reflective film 108 made of Ag. The thickness of the reflective film 108 is preferably 0.02 to 0.3 μm, and more preferably 0.1 to 0.2 μm. This is because the reflectance is ensured and the surface roughness is not deteriorated after the film formation.

The transparent anode 107 is a film made of a transparent conductive material, which is formed on the surface of the reflective film 108 opposite to the substrate 109. In particular, ITO or IZO is preferably used. The film thickness of the transparent anode 107 is preferably λ / 4 or less when the shortest emission wavelength λ of the organic light emitting layer 104 is set. This is because the film thickness is such that no optical interference occurs. For example, in the case where an aluminum quinolinol complex (Alq ₃ ) is used for the organic light emitting layer 104, λ is 450 nm, so that the film thickness of the transparent anode 107 can be 110 nm or less.

  In the illustrated organic EL element 100, the surface of the transparent anode 107 opposite to the reflective film 108 is polished to reduce the surface roughness. This is to eliminate leakage of the organic EL layer laminated on the transparent anode 107. In particular, the protrusion height of the transparent anode 107 with respect to the organic EL layer is important. Therefore, it is desirable that the maximum protrusion height h of the transparent anode 107 satisfies h / d <0.5, where d is the total thickness of the organic EL layer. This is because when the h / d <0.5 is satisfied, a leak or a short circuit in the organic EL layer can be prevented. Specifically, for example, the maximum protrusion height h of the transparent anode 107 is preferably less than 10 nm, and more preferably 5 nm or less. This is because the thickness of the organic layer may be 20 nm. Here, the maximum protrusion height with respect to the organic EL layer of the transparent anode 107 is extracted from the roughness curve by a reference length in the direction of the average line, and the height Yp from the average line of the extracted portion to the highest peak This is the sum of the depth Yv to the lowest valley. The reference length is suitably about 2 to 100 μm, and can be measured by AFM (Atomic Force Microscope). In addition, although the film thickness of a transparent anode can be measured on the basis of the centerline (average line) of surface roughness, the film thickness d of the organic EL layer which touches a transparent electrode can also be measured similarly.

The material of each layer in the organic EL layer is not particularly limited, and known materials can be used.
The material of the organic light emitting layer 104 can be selected according to a desired color tone. For example, in order to obtain light emission from blue to blue-green, fluorescence such as benzothiazole, benzimidazole, and benzoxazole is used. Brightening agents, metal chelated oxonium compounds, styrylbenzene compounds, aromatic dimethylidin compounds, and the like can be used.

As a material for the electron injection layer 102, an alkali metal such as Li, Na, K, or Cs, an alkaline earth metal such as Ba or Sr, a rare earth metal, or a fluoride thereof can be used. However, the present invention is not limited to these. Further, as a material for the electron transport layer 103, Alq ₃ and benzazur can be used, but the material is not limited to these.

  The transmittance of the electron injection layer 102 and the electron transport layer 103 is preferably 40% or more, more preferably 80% or more, and a material for achieving such transmittance can be appropriately selected. The film thicknesses of the electron injection layer 102 and the electron transport layer 103 can be appropriately selected in consideration of driving voltage, transparency, and the like. Usually, the thickness of the electron injection layer 102 is 10 nm or less, and the thickness of the electron import layer is 40 nm or less. However, it is not limited to these.

  As the hole injection layer 106, copper phthalocyanine can be used, but is not limited thereto. As the hole transport layer 105, triphenyldiamine (TPD) can be used, but is not limited thereto. The transmittance of the hole injection layer 106 and the hole transport layer 105 is preferably 80% or more, more preferably 90% or more, and a material for achieving such transmittance can be appropriately selected. The thickness of the hole injection layer 106 is usually 100 nm or less, and the thickness of the hole transport layer 105 is usually 100 nm or less. However, it is not limited to these.

In this embodiment, the organic EL layer 110 includes a hole injection layer 106, a hole transport layer 105, an organic light emitting layer 104, an electron transport layer 103, and an electron injection layer 102. However, the organic EL layer of the present invention is not limited to the illustrated form, and includes at least an organic light emitting layer that emits light by recombination of holes and electrons generated by applying a voltage to the anode and the cathode. If it is. Specifically, the organic EL layer has a structure as shown below, for example.
(1) Organic light emitting layer (2) Hole injection layer / organic light emitting layer (3) Organic light emitting layer / electron transport layer (4) Hole injection layer / organic light emitting layer / electron transport layer (5) Hole injection layer / Hole transport layer / organic light emitting layer / electron transport layer (6) hole injection layer / hole transport layer / organic light emitting layer / electron transport layer / electron injection layer

  In the organic EL layer having the above-described structures (1) and (3), a transparent anode is connected to the organic light emitting layer, and in the organic EL layers having the structures (2) and (4) to (6), A transparent anode is connected to the hole injection layer. On the other hand, in the organic EL layers having the structures (1) and (2), a transparent cathode is connected to the organic light emitting layer, and in the organic EL layers having the structures (3) to (5) A transparent cathode is connected, and in the organic EL layer having the structure (6), the transparent cathode is connected to the electron injection layer. The connection between the organic light emitting layer, the electron transport layer or the electron injection layer and the transparent cathode may be a connection through a protective film described later.

  In addition, in the organic EL element of this invention, it is preferable to provide an electron carrying layer and an electron injection layer from the point of the improvement of electron injection efficiency. Further improvement in electron injection efficiency can be realized by appropriately selecting materials for the electron transport layer and the electron injection layer.

  A protective film (not shown) can be provided on the surface of the organic EL layer 110 opposite to the transparent anode 107. The protective film can alleviate damage to the organic EL layer 110 when the transparent cathode 101 is produced. In particular, when an electron injection layer is provided in the organic EL layer, the electron injection layer and the transparent cathode are separated by a protective film, so that the electron injection layer and the transparent cathode form an interface to form the electron injection layer. It is possible to prevent the surface of the metal from being oxidized and deteriorated. The protective film may be a thin film consisting essentially of carbon or a thin film containing a metal element other than carbon, and specific examples include a diamond-like carbon film and an amorphous carbon film. Moreover, the film thickness of a protective film can be normally determined in the range below 50 nm so that a desired transmittance | permeability may be achieved.

  The transparent cathode 101 is formed directly on the surface of the organic EL layer 110 opposite to the transparent anode 107 or via the protective film. As the transparent cathode 101, it is preferable to use a transparent conductive material such as IZO or ITO. In particular, the transparent cathode 101 desirably has a transmittance of 80% or more at a wavelength of 380 to 780 nm which is a visible light region. This is because light is extracted from the transparent cathode 101 side. The film thickness of the transparent cathode 101 can usually be 100 to 300 nm, but is not limited to a specific film thickness.

  The organic EL element 100 having the above structure has a high reflectance at the reflective film 108 and a small protrusion height of the transparent anode 107 with respect to the organic EL layer 110. For this reason, light emission in the organic light emitting layer 104 can be extracted from the transparent cathode 101 side with high efficiency.

  Next, the organic EL element 100 concerning this invention is demonstrated from the aspect of a manufacturing method. The organic EL device manufacturing method according to the present invention includes a step of forming a reflective film 108 on a substrate 109, a step of forming a transparent anode 107 on the reflective film 108, and the reflective film 108 of the transparent anode 107. A step of mirror-polishing the opposite surface, and a step of sequentially forming the organic EL layer 110 and the transparent cathode on the mirror-polished transparent anode 107 surface.

  In the step of forming the reflective film 108 on the substrate 109, Ag or Al, or an alloy thereof can be formed on the cleaned substrate by vapor deposition, sputtering, or other known methods. A step of mirror polishing the surface of the reflective film 108 after forming the reflective film 108 on the substrate 109 and before forming the transparent anode 107 on the reflective film 108 may be included. This is because the surface roughness after film formation is reduced and particles are removed. The polishing process can be performed by polishing using free abrasive grains such as diamond grains having a particle diameter of 1 μm or less, or tape processing using a resin tape (fixed abrasive grains) in which the abrasive grains are embedded on the surface. Further, the maximum protrusion height can be 10 nm or less by polishing.

  In the step of forming the transparent anode 107 on the reflective film 108, the transparent anode 107 is formed by sputtering. The transparent anode 107 is preferably ITO or IZO. The film thickness of the transparent anode 107 at that time is preferably adjusted to be λ / 4 or less, where λ is the shortest emission wavelength of the organic light emitting layer 104.

  Next, a process of mirror polishing the surface of the transparent anode 107 is performed. The mirror polishing can be performed by polishing using loose abrasive grains, CMP (Chemical Mechanical Polishing) with an etching action, tape polishing using fixed abrasive grains, or the like. The mirror polishing is desirably performed so that the protrusion height h with respect to the organic film thickness on the surface of the transparent anode 107 is h / d <0.5 or less with respect to the total film thickness d of the organic EL layer 110. This is because leakage of the organic EL layer 110 can be reduced when h / d <0.5. With this process, the surface roughness of the surface of the transparent anode 107 that contacts the organic EL layer 110 can be reduced.

  After the surface of the transparent anode 107 is mirror-polished, a reflective electrode formed by laminating the reflective film 108 and the transparent anode 107 on the substrate 109 can be obtained by appropriately patterning using a conventional technique.

  Next, a step of sequentially forming the organic EL layer 110 and the transparent cathode on the surface of the mirror-polished transparent anode 107 is performed. For example, when the organic EL layer 110 having the configuration shown in FIG. 1 is formed, the hole injection layer 106, the hole transport layer 105, the organic light emitting layer 104, the electron injection layer 102, and the electron transport layer 103 are disposed on the transparent anode 107 surface. Laminate sequentially. The hole injection layer 106, the hole transport layer 105, the organic light emitting layer 104, the electron transport layer 103, and the electron injection layer 102 can be usually formed by a vacuum deposition method, but may be formed by a coating method.

  Next, the transparent cathode 101 is laminated on the organic EL layer 110. The transparent cathode 101 can be formed by a sputtering method or the like. Moreover, you may include the process of forming a protective film between the organic EL layer 110 and the transparent cathode 101 as needed. The protective film can be formed by a chemical vapor deposition method using a mixed gas containing a hydrocarbon-based gas as a main component.

  According to the method for producing an organic EL element according to the present embodiment, an organic EL element having high luminous efficiency can be produced by a step of mirror polishing the surface adjacent to the organic EL layer of the transparent anode.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, they do not limit the present invention, and needless to say, various modifications can be made without departing from the gist of the present invention.

Example 1
Based on the present invention, an organic EL device having a light emission region of 2 mm × 2 mm (main light emission wavelength 510 nm, shortest wavelength λ450 nm) was produced by the following procedure. First, an Ag reflective film having a thickness of 50 nm was formed on a glass substrate by a vacuum deposition apparatus. On the reflective film, 100 nm thick IZO was formed by a DC sputtering method (target: IZO (In ₂ O ₃ -10% ZnO), sputtering gas: Ar, power 250 W) to form an electrode.

Next, the surface of IZO was polished for 2 minutes at a load of 50 gf / cm ² using colloidal silica as a slurry so as to satisfy h / d <0.5. A reflective electrode having a maximum protrusion height of 4.5 nm was obtained. After sufficiently washing the reflective electrode, it was irradiated in the atmosphere for 5 minutes under a 50 mW output low pressure mercury lamp having emission wavelengths of 184.9 nm and 253.7 nm (UV irradiation treatment).

Thereafter, in the organic vapor deposition apparatus, a 40 nm thick hole injection layer made of αNPD, a 60 nm thick organic light emitting layer made of Alq _3, and a 1 nm thick electron injection made of LiNP as an organic EL layer on the reflective electrode. The layers were laminated in order. The film formation conditions for each layer were a pressure of 1 × 10 ⁻³ Pa or less and a film formation rate of 0.5 nm / s. A protective film made of diamond-like carbon was formed on the organic EL layer to a thickness of 5 nm by DC sputtering.

Next, 100 nm thick IZO was deposited on the protective film by facing DC sputtering to form a transparent cathode. Ar gas was used as the sputtering gas, and IZO (In ₂ O ₃ -10% ZnO) was used as the target.

As described above, an organic EL element having a reflective electrode was obtained. About the obtained element, it was made to light with the brightness | luminance of 100 cd / m < ² >, and the current efficiency was calculated | required by measuring the electric current in that case. Moreover, it measured about the light emission start voltage which starts light emission. The results are shown in Table 1.

(Comparative Example 1)
A reflective film made of Pt having a thickness of 100 nm was formed as a reflective electrode on a glass substrate by a DC sputtering method (target: Pt), sputtering gas: Ar, power 250 W). Then, the mirror polishing of the reflective film was performed. As a result, the maximum protrusion height on the reflective film surface was 6.5 nm. In the same manner as in Example 1, an organic EL layer and a transparent cathode made of IZO were formed to produce an organic EL element. About the obtained organic EL element, it carried out similarly to Example 1, and measured the current efficiency and the light emission start voltage. The results are shown in Table 1.

  As is clear from Table 1, it was found that the organic EL element of Example 1 showed extremely high luminous efficiency as compared with the organic EL element of Comparative Example 1. This is considered to be because Ag having a high reflectance can be used, and the protrusion height of the transparent anode is reduced to reduce the leakage current.

(Reference Example 1)
A reflective electrode is formed on a glass substrate by vapor-depositing Ag so as to have a film thickness of 100 nm using a vacuum vapor deposition apparatus, and then forming a reflective electrode by depositing IZO as a transparent anode so as to have a thickness of 100 nm. did. A photograph of the surface of the deposited IZO measured with AFM is shown in FIG. At this time, an AFM image was observed in a 2 μm square region. As a result, the maximum protrusion height R _max on the surface of IZO was 72 nm. FIG. 3 shows a photograph of the surface of this IZO film that has been polished for 2 minutes at a load of 50 gf / cm ² using colloidal silica as a slurry using a polishing machine. The maximum protrusion height R _max on the IZO surface after polishing was 4.2 nm. From this, it was found that the height of the protrusion on the surface was reduced to about 1/17 by the step of mirror polishing the surface of the transparent anode.

  Using the organic EL element of the present invention, it is possible to configure a display device such as a display for information equipment. In particular, since the organic EL element of the present invention is a top emission type, it is effective as an element constituting a display device that requires a large screen.

It is typical sectional drawing which shows the structure of the organic EL element based on this invention. It is a figure which shows the AFM photograph in the state which formed IZO into the reflective film which consists of Ag formed on the board | substrate. It is a figure which shows the AFM photograph after forming IZO into the reflecting film which consists of Ag formed on the board | substrate, and also grind | polishing. It is typical sectional drawing which shows the structure of a general bottom emission type organic EL element. It is a typical sectional view showing the structure of a general top emission type organic EL device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Top emission type organic EL element 101 Transparent cathode 102 Electron injection layer 103 Electron transport layer 104 Organic light emitting layer 105 Hole transport layer 106 Hole injection layer 107 Transparent anode 108 Reflective film 109 Substrate 110 Organic EL layer

Claims (3)

On the substrate, a reflective film made of Ag or Al or an alloy thereof, a transparent anode, an organic EL layer including at least an organic light emitting layer, and a transparent cathode are sequentially formed. An organic EL element to be extracted,
When the shortest emission wavelength of the organic light emitting layer is λ, the film thickness of the transparent anode is λ / 4 or less,
An organic EL element that satisfies h / d <0.5, where h is the maximum protrusion height of the transparent anode with respect to the organic EL layer, and d is the thickness of the organic EL layer. Forming a reflective film made of Ag or Al or an alloy thereof on the substrate;
Forming a transparent anode on the reflective film;
Mirror-polishing the surface of the transparent anode opposite to the reflective film;
The method for producing an organic EL element according to claim 1, comprising a step of sequentially forming an organic EL layer and a transparent cathode on the mirror-polished transparent anode surface.   In the mirror polishing step, when the maximum protrusion height of the transparent anode with respect to the organic EL layer is h and the film thickness of the organic EL layer is d, the mirror polishing is performed so as to satisfy h / d <0.5. The method of claim 2.

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