JPH101665A - Electric field light-emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the subject element not requiring a low molecule electron- transporting material, thereby enabling to avoid the deterioration of the element with time due to the crystallization of the low molecule, easy in the manufacture of the element, capable of being made in a large area, and useful for the back lights of liquid crystal displays, etc., by disposing an organic layer containing a specific electron-transpiration polymer between electrodes. SOLUTION: This electric field light-emitting element is made by successively laminating (A) the first transparent electrode, (B) an organic layer containing an organic compound emitting light on the application of an electric voltage as a main component, and the second electrode to a transparent substrate. The component B containing a polymer comprising a polylefin main chain and side chains bound to the main chain and produced from a compound containing (Z) an oxadiazole group, e.g. poly(p- 4-[5-(4-t-butylphenyl)-1,3,4- oxadiazol-2-yl]phenyl}styrene).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an electroluminescent device having an organic layer. Since the electroluminescent element is a planar light emitting body capable of electrically emitting light, it can be used as a display device such as a front display of an automobile or a backlight of a liquid crystal display.

[0002]

2. Description of the Related Art An electroluminescent device has a pair of electrodes attached to a solid organic compound having strong fluorescence, and emits light by applying a voltage. In general, an electroluminescent device includes a transparent electrode (ITO), an organic layer as a light emitting layer made of a solid organic compound having strong fluorescence, and a metal (M) on a transparent glass substrate.
g) electrodes are sequentially stacked. The principle of light emission of this electroluminescent device is as follows. When holes are injected from the anode and electrons are injected from the cathode, the injected holes and electrons move through the solid, collide and recombine, and disappear. The energy generated by the recombination is used to generate the excited state of the light-emitting molecule and emits fluorescence.

[0003] Such an electroluminescent device has no limitation on the viewing angle, can operate at a low voltage and can respond at high speed, and has a higher display performance than other display devices such as a liquid crystal, a plasma display, and an inorganic electroluminescent device. Has excellent characteristics as. However, it has been pointed out that the electroluminescent device in which the light emitting portion is formed of an organic layer has a short life. One of the reasons why the life of the electroluminescent device is short is that there is a problem of deterioration and deterioration due to crystallization of organic substances (hole transporting molecules).
That is, peeling occurs at the junction interface of the element due to heat generation of the element at the time of driving, the crystal structure of the organic substance in the organic layer changes due to heat, and the organic substance itself deteriorates,
The organic layer may be thermally degraded.

In order for an organic electroluminescent device to emit light efficiently, molecules having a hole transporting function, a light emitting function, and an electron transporting function are indispensable. The molecules used for the electron transporting material of the organic electroluminescent device are mainly those found in Alq3 (a kind of aluminoquinol) (Tang et, al., Appl. Phys. Le.
tt., 51, 913 (1987)) Metal complex-based materials and oxadiazole-based materials proposed by Tsutsui et al. (Tsutsui et al., Nikka Kagaku., 11, 1540 (1991)) . These materials are
It is deposited as an electron transporting layer on the hole transporting layer formed on the ITO substrate, and is used as an element constituting an organic electroluminescent device. Since a vapor deposition process is applied to device fabrication, molecules that can be used are necessarily limited to low molecules.

[0005] Among the metal complex-based materials, there are materials having extremely high electron transportability and stable such as Alq3, but the number of metal complex materials that can be used practically is small. On the other hand, various derivatives of oxadiazole-based compounds have been synthesized and their properties have been examined as seen in the examples of Tsutsui et al. It has been clarified that all of these compounds have an electron transporting property. However, oxadiazole-based compounds are easily crystallized. For example, PBD (Chemical Formula 12), which is a kind of oxadiazole derivative, is crystallized immediately after vapor deposition, and the smoothness of the film is impaired. There is a problem that the stability is low for use.

One of the means for producing a thin film which has reduced crystallinity and is stable enough to be used as an electron transporting layer of an organic electroluminescent device is a method of polymerizing a target molecule. Such examples are often found in polymerized hole transporting molecules. For example, a polymer dispersed electroluminescent device having a matrix of polyvinyl carbazole (Applied Physics 61 (10), 1044 (1992)), or a polymer containing triphenylamine or TPD in the side chain (Polymer Transactions, 52,
(4) 216 (1995)) and a material in which a hole transport molecule is introduced into the main chain of polycarbonate (JP-A-5-247458).
No.) is known.

[0007] As described above, by polymerizing the hole transport molecule, the molecular motion and the arrangement state of the molecules are regulated, the amorphous state becomes a thermodynamically stable state, and the crystal as seen in a small molecule is obtained. The problem of conversion can be avoided. In the case of the polymer-dispersed electroluminescent device, the polymer in the matrix is inactive and must not hinder the movement of carriers (electrons and holes). For this reason, it is better to eliminate polar groups such as carbonyl groups, halogens, esters, and amides that may cause carrier trap formation. However, in many cases, a polar group has to be introduced due to synthesis. The aforementioned Kido and Idemitsu Kosan polymers contain such polarity.

When a molecule having a carrier transport function is polymerized in a side chain form, the main chain portion is irrelevant to carrier transport. Therefore, the density of the electron transport functional molecules (hopping sites) is increased, and the carrier transport is increased. In order to improve the efficiency, the main chain preferably has a simple structure. In the above-mentioned polymers of Kido and Idemitsu Kosan, the molecular weight of the portion of the polymer that is not involved in carrier transfer is larger than the polymer having a main chain of polyolefin.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to easily obtain an organic electroluminescent device having a thermally stable thin film structure and excellent characteristics.

[0010]

According to the electroluminescent device of the present invention, a transparent first electrode, an organic layer mainly composed of an organic compound which emits light by applying a voltage, and a second electrode are formed on a transparent substrate. In the electroluminescent device formed by sequentially stacking the organic layers, the organic layer includes a polymer including a main chain of a polyolefin and a side chain of a compound including an oxadiazole group bonded to the main chain.

The side chain is 2 through an oxadiazole group.
Desirably, two aromatic compounds are bonded and one aromatic compound is bonded to the main chain. The aromatic compound is a phenyl group, a naphthyl group, an anthranyl group, an aromatic hydrocarbon such as a biphenyl group, a nitrogen-containing aromatic compound such as triphenylamine, an oxygen-containing aromatic compound such as diphenyl ether, pyridine, bipyridine, indole, Heterocyclic compounds such as quinoline, thiophene and furan can be mentioned.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The electroluminescent device of the present invention comprises a transparent first substrate, a transparent first electrode, an organic layer, and a second electrode, which are laminated in this order. is there. This organic layer is formed including a polymer comprising a polyolefin main chain and a side chain containing a compound containing an oxadiazole group bonded to the main chain.

This polymer has the general formulas 1 and 2
It is expressed by an equation. That is, the main chain of polyolefin and
And a side chain containing an xadiazole group. The main chain is poly
Olefin polymer, polyethylene, polypropylene
Etc. are available. Alkyl substitution in the main chain as shown in Chemical formula 2.
A substituent may be bonded. This side chain can be compared to the main chain
For example, Ar represented by the following general formula₁(Formula 1), sp-A
r₁Bonding to an oxadiazole group through
I have. The oxadiazole group has another substituent Ar_TwoIs combined
And Ar₁And Ar_TwoThat it is sandwiched between
preferable. In addition, sp in the chemical formula 2 is a main chain as a spacer.
And Ar₁And, for example, (CH_Two)_n, (CO_Two-
(CH_Two)_n), (CONH (CH_Two) _n), O (CH
_Two)_nIt has a bond represented by n ≧ 0.

[0014]

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[0015]

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Examples of Ar ₁ include aromatic compound groups such as phenyl, naphthyl, anthranyl and biphenyl, oxygen-containing aromatic compound groups such as diphenyl ether, pyridine, bipyridine, indole, quinoline, thiophene and furan. Heterocyclic compounds can be mentioned. Further, compounds represented by Chemical Formulas 3 and 4 can also be used.

[0017]

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[0018]

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As Ar ₂ , the compounds mentioned for Ar ₁ can be used, and compounds of the following formulas 5 to 13 having the following hole transport function and light emitting function can be used. Note that Ar ₂ is A
It may be bonded to the oxadiazole group in the same or different combination as r ₁ .

[0020]

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[0021]

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[0022]

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[0023]

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[0024]

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[0025]

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[0026]

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The above side chain forming compounds (Ar ₁ , Ar ₂ )
May have a substituent introduced therein, for example, an alkyl group, an allyl group, an aryl group, an amino group, an alkoxy group, an allyloxy group, an aryloxy group, a thioalkyl group, a thioallyl group, a sulfone group, a phosphoryl group, a carboxyl group And substituents such as carbonyl group, thiocarbonyl group, imino group, hydroxy group, amide group, alkoxysilyl group and cyano group.

Further, such a polymer may contain, as a unit, a compound having a hole transport function and a light emitting function (formulas 3 to 11). This polymer can be prepared by polymerization using a side chain-forming compound containing the oxadiazole having a vinyl group as a monomer by a radical polymerization method, a cation polymerization method, or an anion polymerization method.

This side chain compound is a hole transport function molecule,
Alternatively, it may be a copolymer with a light emitting functional molecule. Here, the copolymer is prepared by polymerizing a monomer having a vinyl group by a radical polymerization method, a cation polymerization method, or an anion polymerization method. At this time, different monomers may be added. Alternatively, a mixture obtained by dispersing a molecule having a hole transporting function, an electron transporting function, and a light emitting function in the above polymer may be included. Here, the dispersed molecules are
Not only low molecular weight but also high molecular weight may be used.

The organic layer constituting the organic layer of the electroluminescent device may be a single layer, but may be a laminated film of a thin film formed by a coating method or a vapor deposition method and a thin film containing the above polymer. It may be. Further, the organic layer containing the polymer may be formed by a coating method such as a spin coating method or a dip coating method, or may be formed by a vacuum evaporation method.

When a molecule having a carrier transport function is polymerized in a side chain type, the main chain portion is irrelevant to carrier transport, so that the density of electron transport functional molecules (hopping sites) is increased and carrier transport is increased. In order to improve the efficiency, the main chain preferably has a simple structure. The hydrocarbon-based polymer main chain (polyolefin) used in this study had a molecular weight of 27.0, and 86.1 of methacryl or 66.1 of carbonate.
It is smaller than 0.0 or 86.1 of amide. A polymer having a structure in which the aromatic ring of an electron transport molecule (which is an aromatic compound) is bonded to the side chain of a polymer having a polyolefin as a main chain is considered to be a polymer having a high electron transport density. Can be

Since this polymer is bonded to the side chain via a short ethylene, steric repulsion between the side chains increases. For this reason, the above-mentioned polymer having a polyolefin as a main chain forms a stable amorphous phase because the overlapping of side chains causing crystallization is inhibited. That is, a uniform thin film can be formed.

[0033]

Embodiments will be described below with reference to embodiments. (Example) (Acetylation of PBD by Friedel-Crafts reaction) 56.7 g of aluminum chloride was added to carbon disulfide 6 under nitrogen atmosphere.
Dissolved in 00 ml and cooled to 0 ° C. Acetyl chloride 9m
1 over 1 minute, and then maintain PB at -3 ° C.
A solution of 20.15 g of D (formula 12) in 200 ml of carbon disulfide was added dropwise over 45 minutes. After the reaction solution was returned to room temperature and stirred for 15 minutes, carbon disulfide was removed by decantation and poured into crushed ice (600 ml). After stirring the reaction solution for 15 minutes, it was extracted with 300 ml of chloroform. The extracted chloroform solution was washed twice with 200 ml of water and dried over potassium carbonate. Filter potassium carbonate and concentrate the filtrate.
Vacuum dried. The product was purified by silica gel column chromatography to give 14.3 g of acetyl PBD.

[0034]

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(1-hydroxyethyl PBD by reduction
Production of 3.17 g of acetyl PBD in 125 ml of ethanol
Then, 0.66 g of sodium borohydride was added while stirring under a nitrogen stream. After 18 hours, the reaction solution was concentrated, and 20 ml of water and 20 ml of chloroform were added.
The aqueous layer was extracted twice with 10 ml of chloroform, and the combined chloroform layers were washed with 20 ml of water. After drying over sodium sulfate, the mixture was concentrated. The concentrated product was purified by silica gel column chromatography to obtain 2.76 g of 1-hydroxyethyl PBD.

(Production of Vinyl PBD by Dehydration Reaction)
2.76 g of 1-hydroxyethyl PBD (0.51 g) and p-toluenesulfonic acid (0.19 g) were mixed with benzene (200 m).
and heated and refluxed under a nitrogen atmosphere in a reaction vessel equipped with a product water separator. After 3 hours, the reaction solution was concentrated to 40 ml, and the concentrated solution was washed with 30 ml of a saturated aqueous sodium hydrogen carbonate solution, and then washed twice with 20 ml of water. Dried over sodium sulfate,
The concentrated residual product was purified by silica gel column chromatography to obtain 300 mg of vinyl PBD.

(Production of PPBD by Polymerization Reaction) 3.44 g of vinyl PBD and 8 mg of AIBN were dissolved in 50 g of distilled benzene and injected into a reaction vessel. After degassing under vacuum, the reaction vessel was sealed and reacted at 60 ° C. for 5 days. The reaction solution was poured into 1 liter of methanol, and
The reprecipitation purification with methanol was repeated 5 times and freeze-dried from benzene to obtain 2.95 g of PPBD (formula 13).

[0038]

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As a result of DSC measurement of the obtained PPBD, P
The Tg of the PBD was 213 ° C. Melting points were not observed below 250 ° C. PPBD was found to be an amorphous polymer by observation with a polarizing microscope under crossed Nichols. The PPBD cast film became a colorless and transparent film. The maximum absorption of PPBD film is 320n
m, the fluorescence maximum was 395.5 nm.

Comparative Example The melting point of PBD was 136-138.
° C. PBD was crystalline below the melting point and liquefied above the melting point. PBD is deposited on ITO by evaporation method.
0 nm was deposited and thinned. Immediately after film formation, the film was a transparent amorphous film, but after about 30 minutes, crystallized and became a white cloudy film.

(Preparation of Dispersion Type Element) An organic layer was formed on an ITO electrode by spin coating. The spin coat solution is prepared by mixing PPBD and TPD at a predetermined weight ratio (the composition ratio with PPBD is from 50 mol% to 100% of PBD (50,
67, 80, 89, 95, 100%) The weight of the mixture is 2
The mixture was mixed so as to be 5 mg, coumarin 6 was added at 1% by weight, and dissolved in 1.5 g of 1,2-dichloroethane. This was filtered through a 0.2 μm filter and spin-coated on an ITO substrate at 3000 rpm. At this time, the film thickness was almost 100 nm. Metal electrodes are Mg and Ag
Was manufactured by a binary co-evaporation method. Metal composition is Mg: A
g = 10: 1, 150 n / min at 150 ° / min
m was deposited.

The device was evaluated in a nitrogen atmosphere. The initial characteristics were obtained by increasing the voltage of the device by 1 V every about 5 seconds and measuring the luminance and current density at each voltage. Luminance was measured using Minolta nt-1 °. The structure of the fabricated device is ITO / PPBD, TP
D, Coumarin 6 (1% by weight) / Mg: Ag (organic layer thickness about 100 nm).

The device emitted green light which was emitted from coumarin 6. FIG. 2 shows current density-luminance characteristics of each PPBD / TPD ratio. The composition ratio of PPBD and TPD is 89:
At 11, the luminance (green) of 150 cd / m ² was obtained by applying 28V. (Preparation of Two-Layer Element) PPV was formed on ITO by spin coating. After spin-coating the precursor aqueous solution of PPV, it was heat-treated under vacuum at 200 ° C for 1 hour. After that, PPB
A solution prepared by dissolving 25 mg of D and 0.3 mg of coumarin 6 in 1.5 ml of dichloroethane was spin-coated at 3000 rpm. At this time, the film thickness was almost 100 nm.
The metal electrode was produced by a binary co-evaporation method of Mg and Ag. The metal composition is Mg: Ag = 10: 1, and the deposition rate is 15
0 ° / min. Then, 150 nm was deposited. This element
At 26 V and 41 mA / cm ² , light was emitted at a luminance of 62 cd / m ² .

(Copolymer of hole transport function molecule (polyvinyl carbazole (PVK)) and PPBD (50: 5
Synthesis of 0 mol ratio) 660 mg of vinyl PBD, 340 mg of 9-vinylcarbazole and 3 mg of AIBN were dissolved in 15 g of benzene, degassed and sealed, and then heated at 60 ° C. for 40 hours. After allowing to cool, reprecipitation was performed with benzene-methanol, followed by freeze-drying from benzene and PVK-PPBD.
A copolymer was obtained.

(Production and Evaluation of Element) PVK-PPB
Dissolve 250 mg of D copolymer and 2.5 mg of coumarin 6 in 7.6 g of dichloroethane and put 100 n on the ITO electrode.
m was applied by dip coating. The metal electrode was produced by a binary co-evaporation method of Mg and Ag. Metal composition is Mg: Ag
= 10: 1, deposition rate 150 ° / min. And 150
nm. This element is 31 V, 45 mA / cm ²
In this case, light was emitted at a luminance of 19 cd / m ² .

[0046]

According to the present invention, the crystallization of a low-molecular compound, which is a problem when using a low-molecular compound (PBD) having an electron-transporting function in the organic layer of the electroluminescent device, is suppressed by the polymerization and the steric hindrance between side chains. We were able to. That is, PPBD is Tg
= 210 ° C., and can maintain the film form without softening to Tg. On the other hand, a PBD thin film of a low-molecular-weight electron transport function material cannot maintain its film form due to melting of the material at 135 ° C. In addition, PPBD, which is a polymer electron transport function material, was able to stably maintain an amorphous state until Tg.

By using a polymer having an electron transporting function in an electroluminescent element, it is not necessary to use a low-molecular-weight electron-transporting material, and the problem of aging due to crystallization, which is a problem with a low-molecular-weight electron-transporting material. Can be avoided. A thin film of PBD, a low molecular weight electron transport material, crystallizes in a few days and cannot retain the film form, but no crystallization of a high molecular weight PPBD is observed after several months.

Since the organic layer containing the electron transporting material can be formed only by the spin coating method and does not use the vapor deposition method, the preparation is simple and the area can be easily increased. By changing the copolymer ratio of the copolymer of the hole transporting molecule and the electron transporting molecule, it is easy to control the balance between the hole transporting property and the electron transporting property, and an element having excellent characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between current density and luminance characteristics of a device manufactured by changing the composition of a PPBD in this example.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H05B 33/14 H05B 33/14 (72) Inventor Shizutoshi Tokuji 41st Nagakute-cho, Yakumichi, Nagakute-cho, Aichi-gun, Aichi (1) Toyota Central Research Laboratory Co., Ltd. (72) Inventor Yasunori Taga 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture

Claims (4)

[Claims] 1. An electroluminescent device comprising a transparent substrate and a transparent first electrode, an organic layer mainly composed of an organic compound emitting light by application of a voltage, and a second electrode laminated in this order on the transparent substrate. An electroluminescent device, wherein the layer comprises a polymer comprising a polyolefin main chain and a side chain of a compound containing an oxadiazole group bonded to the main chain. 2. The side chain according to claim 1, wherein two aromatic compounds are bonded via an oxadiazole group, and one of the aromatic compounds is bonded to the main chain. An electroluminescent device according to claim 1. 3. The electroluminescent device according to claim 2, wherein the aromatic compound is bonded to the main chain of the side chain via a functional group. 4. The aromatic compound is selected from phenyl, naphthyl, anthranyl, biphenyl, triphenylamine, diphenyl ether, pyridine, bipyridine, indole, quinoline, thiophene, and furan. 4. The electroluminescent device according to 2 or 3.