JP2005310473A - Manufacturing method of organic el element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve positive hole-injection characteristics by carrying out oxidation treatment of a metal reflecting electrode in a manufacturing method of an organic EL element wherein the reflecting electrode is provided on a plate board, and light is taken out from the opposed transparent electrode side. <P>SOLUTION: In the manufacturing method of the organic EL element wherein a pair of the electrodes composed of the opposing anode and cathode, and a light-emitting layer composed of at least an organic compound in the pair of the electrodes are installed, and after the anode composed of the metal reflecting electrode has been formed on the plate board, this is reverse sputtering-treated, and succeedingly after the oxidation treatment of the surface is carried out, the light-emitting layer and the cathode are formed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing an organic EL element having a reflective metal electrode on a substrate and extracting light from the opposite transparent cathode side.

  The organic EL device, which is currently under development, is basically composed of an anode / light emitting layer / cathode laminate, and a basic structure is adopted in which a transparent anode is formed on a substrate using a glass plate, and light is emitted from the substrate side. It is said. On the other hand, in recent years, a panel of a method (active matrix method) in which a driving transistor is provided for each light emitting pixel has been studied. However, when light is extracted from the substrate side, these drive circuits and wiring portions block light, so that there is a problem that the aperture ratio of the pixel (the area ratio of the portion that actually emits light in the element) becomes small.

  Therefore, as disclosed in Patent Documents 1 and 2, it has been attempted to extract light from the cathode side by forming the cathode with a transparent electron injection metal layer and an amorphous transparent conductive film. In this element configuration, a metal reflective electrode (anode) as a pixel electrode is formed on a TFT drive circuit substrate, and an organic EL layer and a transparent cathode are further provided. Since light is extracted from the cathode, the problem of lowering the aperture ratio is solved. However, sufficient improvement has not yet been obtained in terms of hole injection from the pixel electrode (anode), device process stability, and the like.

  As an anode material that is a metal reflective electrode, Patent Document 1 mentions a metal material having a work function of 4.8 eV or more, such as Au, Pt, Ni, and Pd. However, these materials are difficult to form a pattern and have a problem in adhesion to an organic film.

  Patent Document 2 proposes to use a periodic table 5 or group 6 metal material containing Cr, which is easy to form a pattern, as the anode material. However, since the work function is 4.5 eV or less, hole injection from the electrode is performed. There is a problem that the driving voltage is increased by about 3 to 10 V.

In a conventional organic EL device in which a transparent anode is formed on a substrate and light emission is extracted from the substrate side, an indium oxide compound represented by ITO (indium-tin oxide) is mainly used as a transparent anode material. Used. In order to improve the hole injection property of the transparent anode and to remove organic contamination caused by process steps such as patterning, electrode pretreatments such as O ₂ plasma treatment and UV ozone treatment are generally used. Yes. The principle of improving the hole injection property is said as follows. Taking ITO as an example, there is an oxygen defect in the very surface portion of the ITO electrode formed in the first place, and this oxygen defect lowers the work function of the surface and inhibits the hole injection property. Therefore, by performing oxidation treatment such as O ₂ plasma treatment and UV ozone treatment, oxygen defects on the surface are eliminated and hole injection properties are improved.

Patent Document 3 also discloses a technique in which a portion having oxygen defects on the surface is etched by reverse sputtering to expose a portion having no internal oxygen defects to form an organic EL layer.
Japanese Patent Laid-Open No. 10-162959 Japanese Patent Laid-Open No. 2001-043980 JP 2002-170666 A

By the way, in an organic EL device having a structure in which a metal reflective electrode on a substrate as an anode is used as in the present invention and light is extracted from the opposite transparent electrode side, holes are injected from a single metal or an alloy, but the object is to eliminate organic contamination. Although the O ₂ plasma treatment and UV ozone treatment are effective as in the case of the transparent electrode material, the oxidation treatment for the purpose of improving the hole injection property may not always be effective.

  Therefore, the present invention provides a method for producing an organic EL device in which a pair of electrodes composed of an anode and a cathode opposed to each other and a light emitting layer composed of at least an organic compound is provided between the pair of electrodes. A method of manufacturing an organic EL device is provided, in which a light emitting layer and a cathode are formed after reverse sputtering treatment is performed, followed by surface oxidation treatment.

In a method for manufacturing an organic EL element in which a metal reflective electrode is arranged on a substrate and light is extracted from the opposite transparent electrode side, reverse sputtering treatment to the metal reflective electrode of the present invention, O ₂ plasma, UV ozone treatment, etc. By using the electrode oxidation treatment in combination, it was possible to obtain an organic EL element having a low driving voltage and stable emission without light emission unevenness.

  As a result of detailed studies by the present inventors, the reason why the oxidation treatment is not effective for the metal reflective electrode is that an oxide film is formed on the metal surface by the treatment, and the work function is increased, but the electrical function is increased. It has been found that the resistance is in the increasing direction, and as a result, the hole injection property is hindered. In addition, when the oxidation was promoted, the light reflectance of the electrode was lowered, and in some cases, the target luminance could not be obtained. The condition that the oxidation treatment is effective, that is, the work function of the surface is increased but the resistance component does not hinder the injection property is a very narrow range of conditions.

Further, even when the conditions for obtaining a preferable hole injection property were found, the obtained light emitting state was often uneven and unstable. The reason for this is that many of the metal materials that can be used for the metal reflective electrode have a natural oxide film formed on the surface after the process steps after electrode formation, or organic contamination remains as described above. It can be considered that this is caused by the fact that a uniform and appropriate oxide film is not formed on the surface by the oxidation treatment such as O ₂ plasma treatment or UV ozone treatment.

  Therefore, the present invention proposes a manufacturing method in which reverse sputtering treatment is performed on the metal reflective electrode, and then surface oxidation treatment is performed. Thus, by cleaning the surface and creating a uniform and appropriate oxidation state, an organic EL element having excellent hole injection properties and a low driving voltage can be obtained.

  If reverse sputtering is simply performed on the metal reflective electrode, for example, if a Cr electrode is taken as an example, the surface has a work function of 4.5 eV or less as described above, and the hole injection property is not improved.

  For the reverse sputtering treatment, a conventionally known technique may be used, and magnetron sputtering, radio frequency (RF) sputtering, or the like can be suitably used. As the sputtering gas, an inert gas, particularly Ar can be preferably used. Conditions such as the back pressure, sputtering pressure, and electric output vary depending on the electrode material and its film thickness, and should be appropriately adjusted from the element characteristics.

For the oxidation treatment, a method such as thermal oxidation treatment can be used in addition to the above-described O ₂ plasma treatment and UV ozone treatment. These may be used in combination, and in particular, an excellent effect may be obtained by performing UV ozone treatment and O ₂ plasma treatment successively. Further, it is more effective not to expose to the atmosphere during the process, such as performing O ₂ plasma treatment in the same chamber after the reverse sputtering treatment.

  The organic EL device of the present invention has a structure in which a metal reflective electrode (anode) is formed on a substrate, an organic EL layer and a transparent cathode are provided, and light is extracted from the cathode.

  The cathode is a transparent conductive layer and desirably has a sheet resistance of 40Ω / □ or less so that uniform light emission can be obtained as a display element. For example, a transparent conductive film including an In—Sn—O-based oxide or an In—Zn—O-based oxide is preferable.

  The electron injection layer is a layer that can inject electrons from the cathode into the organic EL layer including the light emitting layer.

  For example, a thin metal or alloy thin film containing at least a metal having a low work function (3.8 eV or less), for example, Mg, Ca, Ba, Li, Sc, or the like is used.

  As another preferred embodiment, a thin film of an alkaline earth metal such as BaO, SrO, or CuO is used.

  As still another preferred example, an electron injecting metal or a mixture of an alkaline earth metal oxide and an electron transporting compound may be used.

The most preferable electron injection layer is a co-evaporated film of a salt typified by cesium carbonate (Cs ₂ CO ₃ ) and an electron transporting compound typified by Alq ₃ . A preferable mixing ratio is about 5 to 30% by volume of cesium carbonate (desirably about 10%, corresponding to about 50% in molar ratio).

The present invention shows a method for manufacturing an element that can stably inject holes from an anode metal electrode from a low voltage, and has good characteristics such as a co-deposited film of an electron transporting compound such as cesium carbonate and Alq _3. It should be used in combination with the electron injection layer shown.

  The organic EL layer may be a layer consisting only of the light emitting layer interposed between the anode and the cathode, or may have a multilayer structure in which an electron transport layer, a hole transport layer and the like are laminated.

  The organic EL element has a function of emitting light by recombination of electrons and holes by injecting holes from the anode and electrons from the cathode when an electric field is applied. As these, known materials in conventional organic EL elements can be used.

  FIG. 1 is a cross-sectional view showing a basic configuration of an organic EL element according to the present invention.

  Each pixel includes an anode A, a cathode K, and an organic EL layer 10 held between the two. The organic layer 10 includes a light emitting layer 102 that emits light by recombination of holes supplied from the anode A and electrons supplied from the cathode K. A hole transport layer 101 and an electron injection transport layer 103 are included.

Adjacent pixels are separated by an element isolation film 104. For the element isolation film 104, a known resin material, an inorganic material, or the like can be used. Specifically, an acrylic resin, polyimide, SiO ₂ , SiN, or the like can be used. In order to avoid damage to the element isolation film due to reverse sputtering, it is also effective to provide a protective film with an inorganic material such as SiO ₂ after forming the element isolation film with a resin material.

  For the anode A, a metal material that is easy to form a pattern is selected as an element electrode. Al, Cr, Ta, W, Ni, Pd, Ag or an alloy containing them can be used.

  Examples of the present invention will be described below.

Cr electrode A Cr film was formed as an anode A by sputtering on a glass substrate by a magnetron DC sputtering method using a Cr target. The film formation was performed with a back pressure of 3 × 10E (−4) Pa, an Ar gas pressure of 0.4 Pa, and an electrical output of 300 W. At this time, a film formation mask was used to form a stripe shape of 3 mm and a thickness of 150 nm. .

SiO ₂ element isolation film Next, SiO ₂ was deposited to a thickness of 250 nm as an insulating layer to be an element isolation film by RF sputtering. At this time, the film formation mask was replaced so that Cr / CN was exposed as a pixel electrode through a 2 mm square opening on the Cr stripe. The measurement was performed under the electrical output conditions of a back pressure of 3 × 10E (−4) Pa, an Ar gas pressure of 0.4 Pa, 13.56 MHz, and 300 W.

Reverse Sputtering Treatment The substrate on which the Cr electrode and the SiO ₂ element separation film were formed was subjected to reverse sputtering treatment in the same sputtering apparatus. On the SiO ₂ target, the treatment was performed for 10 minutes under the electrical output conditions of a back pressure of 3 × 10E (−4) Pa, an Ar gas pressure of 0.4 Pa, 13.56 MHz, and 200 W.

Electrode oxidation treatment Subsequently, the substrate was transferred to an O ₂ plasma treatment apparatus without being exposed to the atmosphere. In the O ₂ plasma treatment, RF power of 50 W was applied to a ring electrode provided in the vicinity of the substrate, the oxygen pressure was 0.6 Pa, and the treatment time was 40 seconds.

Organic EL Layer Formation After the vacuum chamber was evacuated to 1 × 10E (−4) Pa, an organic EL layer was deposited on a predetermined portion by using a deposition mask. The predetermined portion is a portion where the Cr is exposed on the substrate (pixel electrode).

First, TPD (N, N′-diphenyl-N, N′-bis (3methylphenyl) -1,1′-biphenyl-4,4′-diamine) is 50 nm as the hole transport layer, and Alq ₃ (as the light emitting layer). aluminum - Toriki Norino rate) of 30 nm, electron injection transport layer as Alq ₃ 9: providing the Cs ₂ CO ₃ 1 (by volume) composed of co-deposited film to a film thickness of 50nm.

  The material set in each vapor deposition boat is evaporated by resistance heating method, the organic layer is ~ 0.5nm / sec., And the co-deposition layer is also adjusted to each boat current value. Film formation was performed at a deposition rate of.

  Subsequently, an ITO (In—Sn—O) cathode film was formed by DC sputtering so as to cover the Cr pixel electrode by mask deposition and cross the Cr stripe.

  The ITO target was sputtered using Ar gas at a pressure of 0.4 Pa and a power output condition of 300 W. The film thickness was 100 nm, and the specific resistance value of the single film under the same conditions was 4 × 10E−6 Ωcm.

Element Evaluation When a voltage-current-luminance characteristic was measured using an ITO transparent conductive film as a cathode and a Cr electrode as an anode, the voltage at which a luminance of 300 Cd / m ² was obtained was 6.5V. Further, light emission within a 2 mm square pixel was uniform.

  Table 1 shows the reverse sputtering treatment conditions, oxidation treatment conditions, and element characteristic evaluation results of the metal reflective electrode in this example.

The organic EL device was prepared and evaluated in exactly the same manner as in Example 1 except that UV ozone treatment was performed as the electrode oxidation treatment in Example 1. In UV ozone treatment, the substrate after reverse sputtering treatment is transported to a glove box with N ₂ : O ₂ of 8: 2 and dew point adjusted to -50 ° C. or less without being exposed to the atmosphere. UV irradiation was performed using a mercury lamp for 20 minutes.

The voltage at which a luminance of 300 Cd / m ² was obtained was 6.7V. Further, light emission within a 2 mm square pixel was uniform.

  The organic EL device was fabricated and evaluated in exactly the same manner as in Example 1 except that the reverse sputtering treatment in Example 1 was changed to a power output of 100 W.

The voltage at which a luminance of 300 Cd / m ² was obtained was 6.7V. Further, light emission within a 2 mm square pixel was uniform.

(Comparative Example 1)
The organic EL element was produced and evaluated without performing the reverse sputtering process in Example 1.

The voltage was 7.9 V to obtain a luminance of 300 Cd / m ² . Further, uneven light emission within the pixel was conspicuous, and the light emission state was unstable.

(Comparative Example 2)
The organic EL device was produced and evaluated without performing any electrode oxidation treatment in Example 1.

  The voltage was 8.5 V to obtain the luminance of 300 Cd / m 2, and the driving voltage was significantly increased.

It is a schematic diagram which shows the laminated structure example of the organic EL element in this invention.

Claims (3)

  In a method of manufacturing an organic EL element in which a pair of electrodes composed of an anode and a cathode facing each other and a light emitting layer composed of at least an organic compound is provided between the pair of electrodes, the anode is formed on a substrate. A method for producing an organic EL element, comprising performing a reverse sputtering process and subsequently performing a surface oxidation process, followed by forming a light emitting layer and a cathode. The method for producing an organic EL element according to claim 1, wherein the oxidation treatment is at least one of UV ozone treatment and O ₂ plasma treatment.   3. The method for producing an organic EL element according to claim 1, wherein the organic EL element performs light extraction from the cathode side.

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