JPH0922782A - Organic electroluminescent element and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescent element which has a cathode layer composed solely of a single metal of a low work function and which degrades little in emission characteristic. SOLUTION: This organic electroluminescent element has at least an anode layer 13, a single or a plurality of organic-film layers 15, and a cathode layer 17 in that order. The cathode layer 17 comprises a first layer 17a composed of Mg to function mainly as an attached layer to the organic film layers 15 and a second layer 17b composed of Mg, provided over the first layer 17a, functioning as a cathode, and being more solid than the first layer 17a. The evaporation speed of an Mg film during formation of the first layer 17a is 1nm/sec at a minimum, while the evaporation speed during formation of the second layer 17b is 0.5nm/sec at a maximum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic EL element utilizing the electroluminescence (EL) phenomenon and a method of manufacturing the same, particularly a method of forming a negative electrode layer.

[0002]

2. Description of the Related Art In an organic EL device having at least a positive electrode layer, one or more organic film layers, and a negative electrode layer in this order, electrons are efficiently injected into the one or more organic film layers functioning as a light emitting layer. From the viewpoint, the negative electrode layer is preferably made of a metal having a low work function (approximately 4.0 eV or less) such as Mg. However, since a simple metal such as Mg is unlikely to adhere to the organic film layer during vapor deposition, and is exposed to the atmosphere after film formation, it is easily corroded and unstable, so that it is actually used as a negative electrode layer. Is difficult. Therefore, it has been attempted to increase the adhesion of the negative electrode layer to the organic film and stabilize it by forming the negative electrode layer with a compound consisting of a low work function metal and a small amount of a second metal. . As a typical example, for example, Japanese Patent Laid-Open No. 2-15
As disclosed in Japanese Unexamined Patent Publication No. 595, in particular, in Example 1, the negative electrode layer is formed of a Mg / Ag alloy (atomic ratio 10: 1, work function about 3.8e).
V). In the case of this example, the adhesiveness of low work function Mg (work function of about 3.6 eV) to the organic film and the stability of the negative electrode itself are determined by Ag (work function of about 4.6 ev).
Is improved by adding.

[0003]

However, since a negative electrode layer containing a low work function metal and a small amount of a second metal, such as the above-mentioned Mg / Ag alloy, is formed by simultaneous binary vapor deposition, : There is a problem that the forming method is complicated ,: It is difficult to control the composition and the like. In addition, since it is difficult to control the composition as described above, the composition of the negative electrode layer formed is likely to be non-uniform, and therefore, it is easy to cause uneven light emission, pinholes, and dark spots in the organic EL element. is there. : Also, a small amount of the second layer contained in the cathode layer
Metals often segregate, and when the negative electrode layer having such a segregated portion comes into contact with the atmosphere, oxidation easily occurs through this segregated portion, so that there is also a problem that corrosion easily proceeds inside the negative electrode layer. .

Further, since the negative electrode layer includes the second metal in addition to the low work function metal, the work function of the negative electrode layer is increased as compared with the case where only the low work function metal is used. Therefore, there is a problem that the efficiency of electron injection into the light emitting layer (organic layer) is reduced accordingly.

There is a demand for a technique for obtaining an organic EL element having a negative electrode layer composed of only a single metal having a low work function and having excellent light emitting characteristics.

[0006]

[Means for Solving the Problems] In order to achieve this object, the inventor of the present application has made intensive studies. As a result, for example, an aluminum quinolinol complex (tris (8-hydroxyquinolinol)) known as an electron transporting light emitting layer is obtained.
(Aluminum) On the organic film, a single metal which is a single metal and which is originally desired to be used as a constituent material of the negative electrode layer, such as Mg.
When a negative electrode layer is obtained by vapor deposition of Mg, the deposition rate of Mg is 1.
It was found that when the film thickness is set to 0 nm / s or more, the film surface becomes rough and unstable to the atmosphere, but the adhesion to the organic film increases, and at the same time, the Mg deposition rate is set to 0.5 nm / s or less. By doing so, the adhesiveness to the organic film disappears,
By forming a negative electrode layer that is dense, has a flat surface, and is relatively stable to the atmosphere, and by forming a Mg film on the organic film at a high vapor deposition rate, it is possible to form an organic film when forming the negative electrode layer. The inventors have found that the damage to the film is reduced, and completed the inventions of this application.

Therefore, according to the first invention of this application, in an organic EL device comprising at least a positive electrode layer, one or more organic film layers, and a negative electrode layer in this order,
The negative electrode layer is a first layer made of a single metal and mainly functions as an adhesion layer to the organic film layer, and the negative electrode layer is provided on the first layer and made of the metal and functions as a negative electrode. It is characterized in that it is composed of a second layer which is denser than the first layer.

According to the second invention of this application, in manufacturing an organic EL device comprising at least a positive electrode layer, a single or plural organic film layers, and a negative electrode layer in this order,
The negative electrode layer is formed by using a single metal as an evaporation source under the first vapor deposition condition under the first vapor deposition condition in which the vapor deposition rate is a range in which the adhesion of the single metal to the organic film can be secured and the film thickness can be controlled. It is characterized by forming a layer and then forming a second layer under a second vapor deposition condition in which the vapor deposition rate is slower than the first vapor deposition condition.

The single metal in the first invention means various metals having suitable physical properties as a constituent material of the negative electrode layer. Further, the simple metal referred to in the second invention refers to various metals having suitable physical properties as a constituent material of the negative electrode layer and capable of vapor deposition. The metal that can be vapor-deposited with a work function having a suitable value for forming the negative electrode layer of the organic EL element is mentioned as a preferable example of the simple metal in the first and second inventions. For example, since magnesium (Mg), indium (In), gallium (Ga), and cadmium (Cd) have a work function of 4 eV or less, they can be cited as preferable examples of the single metal here. Also,
Although aluminum has a work function larger than those listed above (4.2 eV), it can be cited as a preferable example of the simple metal here.

In the first and second inventions, the layer structure of the organic film layer is not particularly limited. Only an organic light emitting layer, a two-layer structure of an organic hole transporting layer and an organic electron transporting light emitting layer (structure described in Examples below), or an organic hole transporting light emitting layer and an organic electron transporting layer. 2 layer structure, or a 3 layer structure in which an organic hole transport layer, an organic light emitting layer, and an organic electron transport layer are laminated in this order, or any other layer structure can be used. The constituent material of the organic film layer is not particularly limited. Various suitable organic compounds and combinations thereof are possible.
For example, the combination of compounds and stacks disclosed in Document I (“Organic EL device development strategy” (published in Science Forum 1992)) can be an example of application of the present invention. In addition to the vacuum vapor deposition method shown in the examples, the organic film layer is formed by a wet method such as a dip coating method, a spin coating method, and a Langmuir-Blodgett (L) method.
It is also possible to use the B) method or the micelle electrolysis method, and as a dry method, an organic molecular beam deposition (OMBD) method, a plasma polymerization method, or the like.

[0011]

According to the structure of the first invention, an organic EL element having a negative electrode layer composed of two functionally separated layers, which is formed of a single metal, is constructed. Here, the first layer ensures the adhesion of the negative electrode layer to the organic film layer. The second element substantially functions as a negative electrode and is dense, and thus contributes to the prevention of the progress of oxidation from the negative electrode layer surface.

Further, according to the constitution of the second invention, each of the first layer and the second layer of the first invention can be formed, so that the organic EL device of the first invention can be easily manufactured.

[0013]

EXAMPLES Examples and comparative examples of each invention of this application will be described below with reference to the drawings. However, the materials used in the following description and the numerical conditions such as the vapor deposition temperature, the degree of vacuum during vapor deposition, the vapor deposition rate, and the film thickness are merely examples within the scope of the present invention. Further, the "electron transport layer" used in the following description means that when an electron injection electrode having a small work function is used, a large amount of electrons can be injected and the injected electrons can move in the film, Holes are a thin film layer that is difficult to inject, or that can inject but does not easily move in the film. Further, the “hole transport layer” means that when a hole injection electrode having a large work function is used, a large amount of holes can be injected, and the injected holes can move in the film, while Is a thin film layer having a property that it is difficult to implant, or even if it can be implanted, it does not easily move in the film.

1. Example 1 FIG. 1 is a diagram showing a configuration of an organic EL element 10 of Example 1 of the first invention. This organic EL element 10 includes a transparent electrode 13 such as ITO as a positive electrode layer, an organic film layer 15, a first layer 17a and a second layer 17a on a glass substrate (eg, quartz glass substrate) 11 as a substrate. The negative electrode layer 17 according to the present invention composed of the layer 17b is provided at least in this order. Here, the first layer 17a constituting the negative electrode layer 17 is a layer made of a single metal, magnesium in this example, and mainly functions as an adhesion layer to the organic film layer 15. On the other hand, the second layer 17b provided on the first layer 17a is made of the same metal as the constituent metal of the first layer 17a, magnesium in this example, and functions as a negative electrode, but still functions as a negative electrode. It is a more precise layer.
In addition, in Example 1, the second example below, and each comparative example, the organic layer 15 has a two-layer structure of the hole transport layer 15a and the electron transport light emitting layer 15b.

The organic EL device 10 of the first embodiment is manufactured as described below.

After cleaning the quartz glass substrate 11, an ITO film was formed on the quartz glass substrate 11 by the sputtering method. The thickness of this ITO film is m, and the sheet resistance is 1.
It was 0Ω / □ or less. Next, this ITO film is processed by photolithography technology and etching technology to
An m-width positive electrode layer 13 was formed.

Next, this substrate was ultrasonically cleaned by sequentially using acetone and 2-propanol for 10 minutes each and then dried, and then placed in a vacuum deposition apparatus for forming an organic film as follows. The organic film layer 15 was formed.

First, in order to form the hole transport layer 15a, the triphenylamine derivative (N, N'-diphenyl-N, N ') is formed on the substrate 11 on which the positive electrode layer 13 has been formed.
-Bis (3-methylphenyl) -1,1'-biphenyl-4,4'-diamine: hereinafter abbreviated as TPD) with a thickness of 50
Vacuum deposition was performed so as to have a thickness of about nm. Note that TPD is evaporated from a quartz crucible that is resistively heated. At this time, the degree of vacuum is 3.2 × 10 ⁻⁴ Pa, the crucible temperature is 265 ° C.,
The vapor deposition rate was 0.9 nm / s. Subsequently, in order to form the electron transporting light emitting layer 15b, the same vapor deposition apparatus is used.
An aluminum quinolinol complex (tris (8-hydroxyquinolinol) aluminum: hereinafter abbreviated as Alq ₃ ) was vacuum-deposited on the hole transport layer 15a to a thickness of about 50 nm. Alq ₃ was sublimated from a quartz crucible that was resistance-heated. The vacuum degree at this time is 2.9 × 1.
0 ^-6 Pa, crucible temperature 340 ° C, vapor deposition rate 1.0 n
m / s. In this way, the organic film layer 15 was obtained.

Then, the substrate on which the organic film layer 15 is formed as described above is transferred to another vacuum vapor deposition device (vacuum vapor deposition device prepared for a metal electrode), and Mg alone is used as a vapor deposition source on the organic film layer 15. The negative electrode layer 17 is made of a metal as follows.
Was formed. The Mg raw material used was a 10φ × 5 mmt tablet, which was placed in a carbon crucible and indirectly heated and sublimated by a filament. Usually, the Mg raw material in the vacuum vapor deposition method is vaporized by direct resistance heating from a boat made of tungsten, molybdenum, tantalum or the like, but in order to realize a stable low vapor deposition rate with a Mg raw material having a relatively high vapor pressure, An indirect resistance heating method through a crucible was used. Further, in this indirect resistance heating method, since the filament can be covered with the crucible itself, the insulator, etc., radiant heat and thermoelectrons from the raw material boat in the direct heating method, and Mg evaporated particle ions with high energy excited by thermoelectrons. It is also considered to have an effect of suppressing damage given to the organic film due to the above. Although a crucible made of quartz or alumina is usually used, a carbon crucible was adopted in consideration of the reactivity with the Mg raw material.

First, the first layer 17a of the negative electrode layer 17 made of Mg was formed on the organic film layer 15 to a thickness of 30 nm at a raw material part temperature of 630 ° C. and a vapor deposition rate of 1.4 nm / s. Further, on the first layer 17a of the negative electrode layer 17,
170 at a material temperature of 440 ° C. and a deposition rate of 0.2 nm / s
second layer 17b of the negative electrode layer 17 composed of Mg of 20 nm
Was formed.

Here, the manufacturing conditions (vapor deposition conditions) of the first layer 17a and the second layer 17b of the negative electrode layer 17 made of Mg will be described. The vapor deposition rate and the vapor deposition film thickness were monitored by a crystal oscillator type film thickness meter installed in the vapor deposition machine near the substrate. The deposition rate of the electrode film used in the description is a monitor value by this film thickness meter. Further, the actual film thickness of the deposited film was measured by a stylus type step gauge.

When the first layer 17a of the negative electrode layer 17 is formed on the Alq ₃ organic film, the deposition rate of Mg is 1.0.
The film thickness of the film formed at the time of nm / s or less measured by the step gauge is thinner than that displayed by the crystal oscillator type film thickness meter,
The difference increases as the deposition rate decreases. This is because when the deposition rate is slow, the negative electrode component does not easily adhere to the organic film at the early stage of film formation of the Mg negative electrode on the organic film, and the actual film thickness of the unformed film at the initial stage of film formation This is because it has become thinner. Therefore, in order to secure the adhesion of Mg on the Alq ₃ organic film, a vapor deposition rate of 1.0 nm / s or more is required. The range of 1.0 to 2.0 nm / s is preferably selected. This is because if the vapor deposition rate is too fast, the film will have severe surface irregularities. Also, it is difficult to control a desired film thickness. Also, Alq ₃
In the case of vapor deposition on an organic film other than the above, the vapor deposition rate at which the negative electrode layer 17 begins to sufficiently adhere on the organic film varies depending on the organic film, so it is necessary to select the vapor deposition rate suitable for the organic film. There is.

Next, regarding the vapor deposition conditions for the second layer 17b of the Mg negative electrode layer, prior to the actual device production, the vapor deposition rate was changed and the Mg film was vapor deposited on the quartz glass to investigate. As a result, the surface of the Mg film formed at a vapor deposition rate of 0.5 nm / s or less is flat, but as the vapor deposition rate is increased to 0.5 nm / s or more, irregularities on the film surface gradually increase. I understand. As a concrete experimental result, the surface of the Mg film formed at a deposition rate of 0.2 nm / s in FIG. 2 was observed by an atomic force microscope (AFM).
The results of observation are shown below. In addition, the deposition rate is 1.4 in FIG.
The results of observing the surface of the Mg film formed at nm / s with an atomic force microscope (AFM) are shown. Further, when the X-ray diffraction measurement was performed on the Mg films respectively formed with the vapor deposition rates of 0.2 nm / s and 1.4 nm / s, the intensity of the diffraction peak from Mg and the half-value width were the vapor deposition rates of 0.2 nm / s. The Mg film formed in 1) was about twice as excellent, indicating that the Mg film deposited at a slow deposition rate had good compactness. Based on these results, the Mg negative electrode layer 17
It is considered that the vapor deposition rate of the second layer 17b is preferably 0.5 nm / s or less. However, if the vapor deposition rate is too slow, it is feared that impurities such as oxygen present in the vacuum vapor deposition machine may be mixed into the negative electrode film. Is 0.1 to 0.
It is considered that 5 nm / s is preferable.

Regarding the film thickness of each of the layers 17a and 17b, in the case of the first layer 17a of the Mg negative electrode layer 17, it is 5
A range of -40 nm is suitable. When the film thickness is larger than this, the voltage at which the device starts to emit light tends to increase. Therefore, it is preferable that the first layer 17a be as thin as possible due to the nature of the film. However, the lower limit of this film thickness range is 5 nm
That is, it is considered that the organic film 15 can be covered to some extent even if the Mg film having a high deposition rate and a large number of irregularities as shown in FIG. 3 has a film thickness of at least about 5 nm. Because it will be done. On the other hand, in the case of the second layer 17b of the Mg negative electrode layer 17, the first layer 17a having surface irregularities
On top, a second layer 17 of the negative electrode layer 17 having a flat surface
A film thickness of 150 nm or more is required to form b.

The organic EL element 10 of the first embodiment manufactured in this manner is applied with a DC voltage of 3 V or more,
It emits yellow-green light and has 13,620 cd when 15 V is applied.
The brightness was / m ² . Current density at this time is 375m
It was A / cm ² . When an organic EL device similar to this was driven at a constant current of 50 mA / cm ^{2 in} the atmosphere, the half life of the initial luminance was about 55 hours. In addition, an AFM image of the surface of the negative electrode layer 17 of this organic EL element is shown in FIG.
It was a flat surface similar to.

2. Example 2 Similar to Example 1, the positive electrode layer 13, the hole transport layer 15a and the electron transport light emitting layer 15 were formed on the quartz glass substrate 11.
b, an organic film layer 15, a first layer 17 and a second layer 1
After forming the negative electrode layer 17 composed of 7b and 7b, the whole element was further sealed in the air with a protective film using an ultraviolet curable resin. That is, the negative electrode layer 17 made of Mg should be transferred from the vacuum vapor deposition apparatus in which it is originally formed to the protective film forming means without breaking the vacuum by any means to cover it with the protective film. Although it is preferable from the viewpoint of preventing oxidation, in the second embodiment, the negative electrode layer 17 was intentionally covered with a protective film after being left in the atmosphere once. The organic EL device manufactured in this manner showed substantially the same light emission characteristics as in Example 1 even after being left in the atmosphere for about 1 month. This is the negative electrode layer 1 produced according to the present invention.
7 shows that it is stable even if it is left in the air to some extent.

3. Comparative Example 1 Similarly to Example 1, the positive electrode layer 1 was formed on the quartz glass substrate 11.
3 and the organic film layer 15 are formed, and a layer 200n made of Mg / Ag alloy (atomic ratio 10: 1) is used as a negative electrode layer.
m was formed (not shown). Mg was sublimated and evaporated from the carbon crucible described in Example 1 and Ag from a tungsten boat so that the deposition rates were 1.0 nm / s and 0.1 nm / s, respectively.

Organic E of Comparative Example 1 produced in this way
The L element emits light when a DC voltage of 4 V or more is applied,
When applying 6 V (current density: 336 mA / cm ² ),
It showed a luminance of 450 cd / m ² . Since the negative electrode layer made of the Mg / Ag alloy has a higher work function than the negative electrode layer of Mg alone, it is considered that the driving voltage was slightly higher and the emission brightness was lower than in Example 1. When the same organic EL device as in Comparative Example 1 was driven in the atmosphere at a constant current of 50 mA / cm ² , the half life of initial luminance was about 30 hours, which was about half that of Example 1. Further, the AFM image on the surface of the negative electrode of this organic EL element was dotted with many island-shaped portions having a diameter of several hundred nm. It is considered that this island-shaped portion was formed with Ag as a nucleus.

4. Comparative Example 2 As in Example 1, the positive electrode layer 1 was formed on the quartz glass substrate 11.
3 and the organic film layer 15 are formed, a Mg negative electrode layer having a film thickness of 200 nm is formed on the organic film layer 15 under the condition of Mg deposition rate of 1.5 nm / s. A device was obtained (not shown).

Organic E of Comparative Example 2 produced in this way
The L element emits light when a DC voltage of 8 V or more is applied, and 2
When 4 V was applied, it showed a luminance of 9,490 cd / m ² . The current density at this time was 337 mA / cm ² . When an organic EL device similar to that of Comparative Example 2 was driven in the atmosphere at a constant current of 100 mA / cm ² , the half life of the initial luminance was about 1 hour.

The AFM image of the surface of the negative electrode layer of the organic EL device of Comparative Example 2 has almost the same shape as that of FIG. 3, and the surface is occupied by large grains having a diameter of several hundred nm. It was very rough.

5. Comparative Example 3 The positive electrode layer 1 was formed on the quartz glass substrate 11 in the same manner as in Example 1.
3 and the organic film layer 15 are formed, a Mg negative electrode layer having a film thickness of 200 nm is formed on the organic film layer 15 under the condition of Mg deposition rate of 0.1 nm / s. A device was obtained (not shown). The vapor deposition at this time is
It is performed in consideration of the undeposited film at the initial stage of vapor deposition on the organic film. The above 200 nm is the film thickness actually obtained (film thickness confirmed by a stylus type step meter).

Organic E of Comparative Example 3 produced in this way
The L element emits light when a DC voltage of 14 V or more is applied, but the light emission is non-uniform even with the naked eye, and the brightness thereof is less than 1 cd / m ² even when the applied voltage is increased. This is M
When the vapor deposition rate of g is slow, it is difficult for Mg to adhere to the organic film at the early stage of vapor deposition, and the surface of the organic film is not covered with Mg for a considerable time.
Probably because it was exposed to evaporated particles and damaged.

[0034]

As is apparent from the above description, according to the first invention of this application, in an organic EL device having at least a positive electrode layer, a single or plural organic film layers, and a negative electrode layer in this order, The negative electrode layer is composed of a predetermined first layer and a predetermined second layer. Therefore, it is possible to obtain an organic EL element which is suitable as a constituent material of the negative electrode layer and whose negative electrode layer is composed of a single metal such as magnesium. For this reason, an organic EL element having a lower drive voltage than the conventional one, easy to maintain the initial drive voltage even during long-term drive, easy to maintain uniformity of light emission, and less deteriorated in light emission characteristics can be obtained.

Further, according to the method for manufacturing an organic EL element of the second invention, when forming the negative electrode layer of the organic EL element, a single metal suitable as a constituent material of the negative electrode layer is used as a vapor deposition source, In addition, the negative electrode layer is formed by changing the vapor deposition rate in stages such as the first condition and the second condition and performing vapor deposition. As a result, a negative electrode layer can be formed by laminating layers having different properties such as a first layer which is the same material but has adhesiveness to an organic film and a dense and relatively stable second layer. . As a result, a negative electrode layer that is relatively stable, has a flat surface, and has good electron injection efficiency into the light emitting layer is obtained. An organic EL element having good characteristics can be obtained. It is considered that this negative electrode manufacturing method can be used for manufacturing any organic EL element using an organic thin film.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the first embodiment and is a cross-sectional view of an organic EL device of the first embodiment.

FIG. 2 is an explanatory diagram of an example, where a vapor deposition rate is 0.2 nm /
It is the figure which showed the AFM image of the Mg film formed by s.

FIG. 3 is an explanatory diagram of an example, where a vapor deposition rate is 1.4 nm /
It is the figure which showed the AFM image of the Mg film formed by s.

[Explanation of symbols]

 10: Organic EL Element of Examples 11: Glass Substrate (Quartz Glass Substrate) 13: Positive Electrode Layer (for example, ITO Film) 15: Organic Film Layer 15a: Hole Transport Layer 15b: Electron Transport Light Emitting Layer 17: Cathode Electrode Layer 17a: first layer 17b: second layer

Claims (6)

[Claims] 1. An organic EL device comprising at least a positive electrode layer, one or more organic film layers, and a negative electrode layer in this order, wherein the negative electrode layer is made of a single metal as an adhesion layer to the organic film layer. A first layer which mainly functions and a second layer which is provided on the first layer and which is made of the metal and functions as a negative electrode and which is denser than the first layer. Characteristic organic EL device. 2. The organic EL element according to claim 1, wherein each of the first layer and the second layer is formed of a magnesium layer. 3. When manufacturing an organic EL device comprising at least a positive electrode layer, one or more organic film layers and a negative electrode layer in this order, the negative electrode layer is formed by vapor deposition using a single metal as an evaporation source. The first layer is formed under the first vapor deposition condition in which the speed is such that the adhesion of the elemental metal to the organic film can be secured and the film thickness can be controlled, and then,
A method for manufacturing an organic EL element, which comprises performing the second layer under a second vapor deposition condition in which the vapor deposition rate is slower than the first vapor deposition condition. 4. The method for manufacturing an organic EL device according to claim 3, wherein the vapor deposition rate under the second vapor deposition condition is 0.5 at a maximum.
nm / sec. 5. The method for manufacturing an organic EL device according to claim 3, wherein the metal is magnesium, and the vapor deposition rate under the first vapor deposition condition is at least 1 nm.
/ Sec, and the vapor deposition rate under the second vapor deposition condition is 0.5 at the maximum.
nm / sec. 6. The method of manufacturing an organic EL element according to claim 3, wherein the vapor deposition is performed by an indirect resistance heating method using a carbon crucible. Production method.