JPH1012381A - Organic electroluminescent device - Google Patents

Abstract

(57) [Problem] To provide an organic electroluminescent element in which the stability of a cathode film is improved and a decrease in emission luminance is suppressed. An organic electroluminescent device having at least an anode, an organic light emitting layer, and a cathode sequentially provided on a substrate, wherein the cathode contains magnesium, lithium, and an alloy containing a metal having a work function of 4.0 eV or more. An organic electroluminescent device comprising: a metal having a lithium content of 0.002 to 2 at% and a work function of 4.0 eV or more of 1 to 30 at%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an organic electroluminescent device (hereinafter abbreviated as an organic EL device). For more information,
The present invention relates to an organic EL device having at least an anode, an organic light emitting layer, and a cathode provided on a substrate, wherein the cathode is made of a specific alloy. The organic EL device of the present invention can be applied to the field of flat panels, displays, surface light emitters, display boards, marker lights, and the like.

[0002]

2. Description of the Related Art Conventionally, as a thin film type electroluminescent (EL) element, Zn, which is a group II-VI compound semiconductor of an inorganic material, is used.
In general, S, CaS, SrS, and the like are doped with Mn or a rare earth element (Eu, Ce, Tb, Sm, or the like) that is a luminescent center. However, EL devices manufactured from the above inorganic materials include: 1) AC drive is required (50-1000Hz), 2) High drive voltage (up to 200V), 3) Full color is difficult (especially blue), 4) Peripheral drive circuit is expensive. ing.

However, in recent years, in order to improve the above problems,
Development of EL devices using organic thin films has been started. In particular, the electrode type was optimized to improve the efficiency of carrier injection from the electrode in order to increase the luminous efficiency, and an organic hole transport layer composed of an aromatic diamine and a luminescent layer composed of an aluminum complex of 8-hydroxyquinoline were used. Of an organic EL device provided with a device (Appl. Phys. Let
t. , Vol. 51, p. 913, 1987), the luminous efficiency has been greatly improved as compared with a conventional EL device using a single crystal such as anthracene.

As the organic light emitting layer, poly (p-phenylenevinylene) (Nature, 347, 539) is used.
Pp. 1990; Appl. Phys. Lett. , 6
1, 2793, 1992), poly [2-methoxy, 5- (2'-ethylhexoxy) -1,4-phenylenevinylene] (Appl. Phys. Lett., 5).
8, 1982, 1991; Thin Solid
Films, 216, 96, 1992; Nat.
ure, 357, 477, 1992), poly (3
-Alkylthiophene) (Jpn. J. Appl. Ph.
ys. , 30, L1938, 1991; Ap
pl. Phys. , Vol. 72, p. 564, 1992) and a device in which a luminescent material and an electron transfer material are mixed with a polymer such as polyvinyl carbazole (Applied Physics, Vol. 61, p. 1044, 1992). Is being developed.

In the organic EL device as described above, the anode is usually made of indium tin oxide (ITO).
Such a transparent electrode is used, but a metal electrode having a low work function is used for a cathode in order to efficiently perform electron injection. For example, an alloy containing a plurality of metals other than alkali metals and having a work function of at least one metal of 4.0 eV or less (JP-A-2-15595) and calcium (Appl. Phys. Lett., Vol. 68, 147)
, 1996) and aluminum alloys obtained by adding a small amount of lithium to aluminum (Japanese Patent Application Laid-Open No. 5-121172). Since these metals have high reactivity, a method of further providing a protective layer on the top thereof (JP-A-4-233194) has been tried in order to prevent deterioration, but it is not enough to suppress the reduction in luminance. At present, it is not possible to obtain such characteristics.

[0006]

In the organic EL devices disclosed so far, EL light emission is caused by recombination of holes injected from an anode and electrons injected from a cathode. Generally, in the case of electrons, carriers must be injected over an injection barrier at the interface between the cathode and the organic light emitting layer. In order to reduce the electron injection barrier and improve the injection efficiency, a metal electrode having a low work function, such as a magnesium alloy or calcium, is used as a cathode. doing. An object of the present invention is to provide an organic EL device in which a decrease in luminance during driving is suppressed.

[0007]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems, and as a result, have found that the use of an alloy system containing magnesium, lithium and a metal having a work function of 4.0 eV or more as a cathode. The inventors have found that luminance degradation is suppressed, and have completed the present invention.

That is, the present invention relates to an organic electroluminescent device in which at least an anode, an organic light emitting layer and a cathode are sequentially provided on a substrate, wherein the cathode has magnesium, lithium and a work function of 4.0 eV or more. A metal-containing alloy having a lithium content of 0.002 to 2 atomic% and a metal having a work function of 4.0 eV or more;
0 atomic%.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an organic EL device of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows the organic EL of the present invention.
FIG. 2 is a cross-sectional view schematically illustrating a structural example of an element, where 1 is a substrate,
2 represents an anode layer, 3 represents an organic light emitting layer, and 4 represents a cathode layer. The substrate 1 serves as a support for the organic EL element of the present invention, and may be a quartz or glass plate, a metal plate or metal foil, a plastic film or sheet, or the like.
Transparent synthetic resin substrates such as polyester, polymethacrylate, polycarbonate, and polysulfone are preferred.

An anode layer 2 is provided on a substrate 1. The anode layer 2 is usually made of a metal such as aluminum, gold, silver, nickel, palladium, tellurium, indium and / or indium.
Or a metal oxide such as a tin oxide, copper iodide, carbon black, or a conductive polymer such as poly (3-methylthiophene). The formation of the conductive layer is usually
Sputtering, often performed by vacuum deposition, etc., in the case of fine metal particles such as silver or copper iodide, carbon black, conductive metal oxide fine particles, conductive polymer fine powder, etc. Dispersed in binder resin solution,
It can also be formed by coating on a substrate. Further, in the case of a conductive polymer, a thin film can be formed directly on a substrate by electric field polymerization, or can be formed by coating on a substrate (Appl. Phys. Lett., Vol. 60, No. 27).
11, p. 1992). The above conductive layers can be stacked with different materials.

The thickness of the anode layer 2 varies depending on the required transparency. When transparency is required, the transmittance of visible light is preferably at least 60%, more preferably at least 80%. In this case, the thickness is usually 5 to 1000 n
m, preferably about 10 to 500 nm. If opaque, the anode layer 2 may be the same as the substrate 1. Further, it is also possible to laminate the above-mentioned anode layer with a different material.

An organic light-emitting layer 3 is provided on the anode layer 2. The organic light-emitting layer 3 can efficiently transport holes injected from the anode and electrons injected from the cathode between electrodes to which an electric field is applied. It is formed from materials that recombine well and emit light. Normally, the organic light emitting layer 3 is formed as shown in FIG.
As shown in (1), a function separation type is divided into a hole transport layer 3a and an electron transport layer 3b (Appl. Ph.
ys. Lett. 51, 913, 1987).
Further, as shown in FIG. 3, the hole transporting layer 3a is further separated into a hole injecting layer 3a1 mainly responsible for hole injection and a hole transporting layer 3a2 mainly responsible for hole transport. (Appl. Phys. Lett., 65,
807, 1994).

In the function-separated type device shown in FIG. 3, it is desired that the hole injection layer 3a1 has a high hole injection efficiency from the anode layer 2 as a material. For that purpose, a small ionization potential is required. Further, it is desired that the adhesion to the anode layer 2 is high and stable. Such materials include copper phthalocyanine, starburst amine (Mol. Cryst. Liq. Cryst., 211).
Volume, 431, 1992, Appl. Phys. Le
tt. 65, p. 807 (1994)). These are formed by a spin coating method, a vacuum evaporation method, or the like. The thickness of the hole injection layer is usually 1 to 10
0 nm, preferably 5 to 50 nm. In order to uniformly form this thin film, a vacuum deposition method is often used. The hole transport layer 3a2 is stacked on the hole injection layer 3a1. In order for the holes injected by the hole injection layer to be injected into the hole transport layer, it is desired that the ionization potential of the hole transport layer is smaller than or similar to the ionization potential of the hole injection layer. Further, it is required that the hole mobility is large and impurities serving as traps are hardly generated during manufacturing or use.

Examples of such a hole transport compound include N, N 'described in JP-A-59-194393 and US Pat. No. 4,175,960, columns 13 to 14. -Diphenyl-N, N '-(3-methylphenyl) -1,1'-biphenyl-4,4'-diamine: 1,1'-bis (4-di-p-tolylaminophenyl) cyclohexane: 4, Aromatic amine compounds such as 4'-bis (diphenylamino) quadrophenyl, hydrazone compounds disclosed in JP-A-2-311591, and silazane compounds disclosed in U.S. Pat. No. 4,950,950. Can be These compounds may be used alone or, if necessary, may be used as a mixture. In addition to the above compounds, polyvinyl carbazole and polysilane (Appl. Phys. Lett., 59
Vol., Page 2760, 1991).

The above organic hole transporting material is formed by a coating method or a vacuum evaporation method. In the case of coating, a coating solution is prepared by adding and dissolving one or more kinds of organic hole transporting compounds and additives such as a binder resin that does not trap holes as required and a coating property improving agent such as a leveling agent. Then, it is coated on the anode layer or the hole injection layer by a method such as spin coating, and dried to form an organic hole transport layer. Examples of the binder resin include polycarbonate, polyarylate, and polyester. Since a large amount of the binder resin decreases the hole mobility, a small amount is desirable, and the amount is preferably 50% by weight or less.

In the case of the vacuum evaporation method, the organic hole transporting material is put into a crucible provided in a vacuum vessel, and the inside of the vacuum vessel is evacuated to 10 ^-6 Torr by a suitable vacuum pump.
The crucible is heated to evaporate the hole transport material and form a layer on the substrate placed opposite the crucible. The thickness of the hole transport layer is usually 10 to 300 nm, preferably 30 to 300 nm.
100 nm. In order to uniformly form such a thin film, a vacuum deposition method is often used.

As a material for the hole transport layer, an inorganic material can be used instead of an organic compound. The conditions required for the inorganic material are the same as those for the organic hole transport compound. Examples of the inorganic material used for the hole transport layer include p-type hydrogenated amorphous silicon, p-type hydrogenated amorphous silicon carbide,
Examples include p-type hydrogenated microcrystalline silicon carbide, p-type zinc sulfide, and p-type zinc selenide. These inorganic hole transport layers are formed by CVD, plasma CVD, vacuum deposition,
It is formed by a sputtering method or the like. The film thickness of the inorganic hole transport layer is also usually 10 to 300 nm, like the organic hole transport layer,
Preferably it is 30 to 100 nm.

An electron transporting layer 3b is provided on the hole transporting layer. The electron transporting layer 3b efficiently transports electrons from the cathode between the electrodes to which an electric field is applied in the direction of the hole transporting layer. Formed from a compound that As an organic electron transport compound, the electron injection efficiency from the cathode layer 4 is high,
In addition, the compound must be able to transport the injected electrons efficiently. For that purpose, it is required that the compound has a high electron affinity, a high electron mobility, and is excellent in stability, and hardly generates impurities serving as traps during production or use.

Materials satisfying such conditions include aromatic compounds such as tetraphenylbutadiene (Japanese Patent Laid-Open No.
No. 51781) and metal complexes such as aluminum complexes of 8-hydroxyquinoline (JP-A-59-194393).
), Cyclopentadiene derivatives (JP-A-2-28)
9675), perinone derivatives (JP-A-2-289)
676), oxadiazole derivatives (Japanese Unexamined Patent Publication No.
No. 216791), bisstyrylbenzene derivatives (Japanese Patent Laid-Open Nos. 1-245087 and 2-222484).
), Perylene derivatives (JP-A-2-189890 and JP-A-3-7991), coumarin compounds (JP-A-2-191694 and JP-A-3-792), rare earth complexes (JP-A-1-256584). ), Distyrylpyrazine derivatives (JP-A-2-252793), p-phenylene compounds (JP-A-3-33183), thiadiazolopyridine derivatives (JP-A-3-37292), and pyrrolopyridine derivatives (JP-A-3-37292). JP-A-3-37293), naphthyridine derivatives (JP-A-3-203982)
Publication).

The organic electron transporting layer using these compounds has a role of transporting electrons and a role of providing light emission upon recombination of holes and electrons, and also serves as a light emitting layer. When the organic hole transporting compound has a light emitting function, the organic electron transporting layer only plays a role of transporting electrons.

For the purpose of improving the luminous efficiency of the device and changing the luminescent color, for example, doping a fluorescent dye for laser such as coumarin using an 8-hydroxyquinoline aluminum complex as a host material (J. Appl. Phys.
s. 65, 3610, 1989). Also in the present invention, the light emission characteristics of the device can be further improved by doping various fluorescent dyes with the organic electron transporting material as a host material at 10 ⁻³ to 10 mol%. The thickness of the electron transporting layer 3b is usually 10 to 20.
0 nm, preferably 30-100 nm.

The organic electron transporting layer can be formed in the same manner as the organic hole transporting layer, but usually a vacuum evaporation method is used. As a method for further improving the luminous efficiency of the organic EL element, another electron transport layer 3 is further formed on the electron transport layer 3b.
It is conceivable to stack c (see FIG. 4). The compound used for the electron transport layer 3c is required to be capable of easily injecting electrons from the cathode and having a higher electron transport ability. As such an electron transport material,

[0023]

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[0024]

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Oxadiazole derivatives (Appl.
Phys. Lett. 55, 1489, 1989.
Years) or a system in which they are dispersed in a resin such as PMMA (App
l. Phys. Lett. 61, 2793, 19
1992) or n-type hydrogenated amorphous silicon carbide, n-type zinc sulfide, n-type zinc selenide, and the like. The thickness of the electron transporting layer 3c is usually 5 to 200 nm, preferably 1 to 200 nm.
0-100 nm.

As the single-layer type organic light emitting layer 3 which does not perform the function separation, the poly (p-phenylenevinylene) (Nature, 347, 539, 1990) described above is used.
Year; Appl. Phys. Lett. , 61, 279
3, p. 1992), poly [2-methoxy, 5- (2 '
-Ethylhexoxy) -1,4-phenylenevinylene]
(Appl. Phys. Lett., Vol. 58, 1982).
1991, Thin Solid Films,
216, 96, 1992; Nature, 357
Volume, 477, 1992), poly (3-alkylthiophene) (Jpn. J. Appl. Phys., 30).
Volume, L1938, 1991; Appl. Phys
s. , Vol. 72, p. 564, 1992), or a system in which a light emitting material and an electron transfer material are mixed with a polymer such as polyvinyl carbazole (Applied Physics, Vol. 61, 1044).
P. 1992).

On this, the cathode layer 4 is formed. The cathode layer 4 plays a role of injecting electrons into the organic light emitting layer 3b. In addition, it is required that the film quality is stable in order to operate stably for a long period of time. As a material satisfying such conditions,
The present inventor has found that an alloy containing magnesium, lithium and a metal having a work function of 4.0 eV or more is suitable.

To facilitate electron injection, lithium is added, but its content is 0.0
It is 02 to 2 atomic%, preferably 0.01 to 1.5 atomic%. A metal having a work function of 4.0 eV or more is added mainly for the purpose of increasing the stability of the film. Examples of such a metal include gold, silver, aluminum, indium, chromium, manganese, nickel, cobalt, tin, and copper. Of these, silver and indium are preferred. The metal content having a work function of 4.0 eV or more is as follows:
1 to 30 atom%, preferably 5 to 20 atom% based on the whole alloy
Atomic%.

The thickness of the cathode layer is usually 1 nm or more and 10 nm or more.
000 nm or less, preferably 10 nm or more and 100
0 nm or less. Further, another metal may be formed thereon as a protective layer. Examples of a method for forming these films include a method of forming a film by a simultaneous evaporation method using each metal as an independent evaporation source, a method of forming an alloy in advance and forming a film from a single evaporation source, and the like. Can be. By mixing a metal having a work function of 4.0 eV or more, the stability of the cathode film was increased, and as a result, electron injection was performed stably, and as a result, the deterioration of luminance was moderated. It is.

[0030]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the description of the following examples unless it exceeds the gist.

Example 1 An organic EL device having the structure shown in FIG. 2 was manufactured by the following method. Indium tin oxide (IT
O) A transparent conductive film deposited to a thickness of 120 nm is subjected to ultrasonic cleaning with acetone, water cleaning with pure water, ultrasonic cleaning with isopropyl alcohol, drying with dry nitrogen, and UV / ozone cleaning, and then is placed in a vacuum evaporation apparatus. And the degree of vacuum in the device is 3 ×
Evacuation was performed using an oil diffusion pump until the pressure became 10 ^-6 Torr or less.

As an organic hole injection layer material, a copper phthalocyanine compound represented by the following structural formula (I-1) was placed in a molybdenum moat, and heated to perform vapor deposition. At this time, the degree of vacuum was 2 × 10 ⁻⁶ Torr, the deposition rate was 2 nm / sec, and an organic hole injecting and transporting layer 3a1 having a thickness of 20 nm was obtained in a deposition time of 1 minute and 45 seconds.

[0033]

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Next, as a material for the organic hole transporting layer, an aromatic amine compound represented by the following structural formula (H-1) was placed in a ceramic crucible, and heated by a tantalum wire heater around the crucible to perform vapor deposition. The temperature of the crucible at this time is 2
Control was performed in the range of 20 to 260 ° C. The degree of vacuum during evaporation is 3
× 10 ⁻⁶ Torr, deposition time of 2 minutes and 00 seconds, and film thickness of 60 n
m of the organic hole transport layer 3a2 was obtained.

[0035]

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Next, as a material for the organic electron transporting layer and the light emitting layer, an aluminum complex of 8-hydroxyquinoline represented by the following structural formula (E-1), Al (C ₉ H ₆ N
O) ₃ was used, and rubrene represented by the following structural formula (D-1) was used as a dopant material, and vapor deposition was similarly performed on the organic hole transport layer 3a2. At this time, the temperature of the crucible containing the host material is 310 to 320 ° C.
The temperature of the crucible containing the dopant ranges from 130 to
The temperature was controlled in the range of 140 ° C. The degree of vacuum during evaporation is 3 × 10
^-6 Torr, the deposition time was 3 minutes and 30 seconds, and the film thickness was 75 nm. At this time, the concentration of the dopant was 1.1%.

[0037]

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[0038]

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Finally, as a cathode, an alloy electrode of magnesium, silver and lithium was formed to a thickness of 50 nm by a ternary simultaneous vapor deposition method.
m. The degree of vacuum at this time is 3 × 10 ^-6 Torr,
The deposition time was 4 minutes and 30 seconds, and a glossy film was obtained. The contents of magnesium, silver and lithium are respectively 91.
They were 8 atomic%, 7.8 atomic% and 0.4 atomic%. When this device was driven at a constant current of a current density of 90 mA / cm ² , the time required for the brightness to decrease to 80% of the initial brightness was 10 hours.

Example 2 A device was produced in the same manner as in Example 1 except that the composition of the cathode was changed to an alloy electrode of magnesium, indium and lithium. At this time, the contents of magnesium, indium and lithium were 90.1 at%, 8.5 at% and 1.4 at%, respectively. The current density of this device was 90 mA /
When driven at a constant current of cm ² , the time required for the luminance to fall to 80% of the initial luminance was 10 hours.

Comparative Example 1 A device was manufactured in the same manner as in Example 1 except that the composition of the cathode was changed to an alloy electrode of magnesium and silver. At this time, the contents of magnesium and silver were 88.5 atomic% and 11.5 atomic%.
Met. When this device was driven at a constant current of a current density of 90 mA / cm ² , the time required for the luminance to decrease to 80% of the initial luminance was 0.5 hour.

Comparative Example 2 An element was produced in the same manner as in Example 1 except that the composition of the cathode was changed to an alloy electrode of magnesium and silver. At this time, the contents of magnesium, silver and lithium were 88.2 atomic%, 8.6 atomic% and 3.2 atomic%, respectively. When this device was driven at a constant current of a current density of 90 mA / cm ² ,
The time during which the luminance decreased to 80% of the initial luminance was 3.5 hours.

Comparative Example 3 An element was produced in the same manner as in Example 1 except that the composition of the cathode was changed to an alloy electrode of magnesium and silver. At this time, the contents of magnesium, silver and lithium were 55.1 at%, 44.6 at% and 0.3 at%, respectively. When this device was driven at a constant current of 90 mA / cm ² at a current density of 3.5 hours, the luminance decreased to 80% of the initial luminance.

[0044]

According to the organic electroluminescent device of the present invention, at least an anode (anode), an organic light emitting layer, and a cathode (cathode) are sequentially provided on a substrate. Uses an alloy containing a metal of 4.0 eV or more, so that the stability of the cathode film is improved, and as a result, a decrease in emission luminance can be suppressed. Accordingly, the EL element of the present invention is a light source (for example, a light source of a copying machine, a backlight of a liquid crystal display or an instrument) utilizing the characteristics of a flat panel display (for example, for an OA computer or a wall-mounted television) and a surface light emitting body. It can be applied to light sources, display boards, and marker lights, and its technical value is great.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of the organic EL device of the present invention.

FIG. 2 is a schematic sectional view showing another example of the organic EL device of the present invention.

FIG. 3 is a schematic sectional view showing another example of the organic EL device of the present invention.

FIG. 4 is a schematic sectional view showing another example of the organic EL device of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Substrate 2 Anode layer 3 Organic light emitting layer 3a Hole transport layer 3a1 Hole injection layer 3a2 Hole transport layer 3b Organic electron transport layer 3c Organic electron transport layer composed of a compound different from 3b 4 Cathode layer

Claims (2)

[Claims] 1. An organic electroluminescent device comprising at least an anode, an organic light emitting layer and a cathode provided in this order on a substrate, wherein the cathode contains magnesium, lithium and a metal having a work function of 4.0 eV or more. An organic electroluminescent device comprising an alloy, wherein the content of lithium is 0.002 to 2 atomic% and the content of a metal having a work function of 4.0 eV or more is 1 to 30 atomic%. 2. A metal having a work function of 4.0 eV or more is gold,
The organic electroluminescent device according to claim 1, wherein the device is at least one selected from the group consisting of silver, aluminum, indium, chromium, manganese, nickel, cobalt, tin, and copper.