JP3328654B2 - Organic thin-film electroluminescent device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic thin film electroluminescent (hereinafter abbreviated as EL) device. More specifically, the present invention relates to an organic thin-film EL device having high-luminance and high-efficiency light-emitting characteristics and usable as a planar light source and a display device.

[0002]

2. Description of the Related Art An EL device is also called an electroluminescent device, and has been extensively studied for application as a planar light emitting device. This EL element is roughly classified into two types depending on the difference in the light emission excitation mechanism. One is intrinsic (intrinsic) E based on the so-called Destriau effect, which emits light only by applying a strong electric field.
The other is an L element, and the other is a point-contact diode or a charge injection EL based on the so-called Lossev effect, which emits light when a forward current flows through a pn junction.

The former is generally a system in which an element serving as a luminescence center (active element) such as Mn, Cu, or a rare earth element is added to and dispersed in a II-VI compound semiconductor such as ZnS or CdS. Elementary excitation is considered to be the basic emission mechanism. Research on this type of EL device has a long history and actual results, and dispersion type devices have already been put into practical use. Since the 1970s, research on thin film type devices has been actively conducted.

However, the E made of the above-mentioned inorganic material
The L element has many problems, such as the necessity of applying a high electric field of as high as 200 V for driving, the high cost of manufacturing peripheral driving circuits, insufficient durability and luminance, and the difficulty of multicoloring. I have.

[0005] On the other hand, charge injection type EL devices have been studied for a long time, focusing on organic compounds centering on condensed polycyclic aromatic hydrocarbons, but from the practical viewpoint in the late 1970s. Efforts have been made. The most significant feature of the charge injection type EL element is that it can be driven at a lower voltage than the intrinsic EL element.

In the latter half of the 1980s, a charge injection type EL device using an organic thin film as a luminous body was developed to have a high luminance and a high luminance by thinning the material uniformly and forming a multilayered charge injection layer and a light emitting layer by vacuum deposition. It has been demonstrated that it can be an efficient and excellent planar light emitting device (for example, CW Tang et al., Appl.
hys. Lett., 51 , 913 (1987), C. Adachi et al., Jpn.
J. Appl. Phys., 27 , L269 (1988), JP-A-59-194393.
And U.S. Pat. No. 4,720,432). In addition, taking advantage of the diversity of molecular design peculiar to organic materials, it has been shown that various color tone control is possible by introducing substituents, and that it is possible to achieve undesired EL color matching in inorganic EL devices made of inorganic materials. (C. Adachi et al., Appl. Phys. Lett., 56 , 799 (19
90)).

Although these organic thin-film type EL devices can attain a remarkable performance in some device characteristics exceeding practical levels, they have a major drawback of short life and poor durability. This is mainly due to the non-uniformity of a vacuum-deposited film due to the aggregation of molecules over time or by the application of electricity, and the requirement for uniform deposition film-forming ability imposes severe restrictions on material selection. This makes the obstacle to practical application serious.

As an effective means for achieving a longer life while utilizing the features of the organic EL element, it is conceivable to convert the material into a polymer. Recently, EL emission from polyparaphenylene vinylene (PPV) was confirmed (JH Brurr).
oughes et al, Nature, 347,539 (1990), WO-9013148
Attempts have been made to apply conductive polymers to the charge injection layer (S. Adachi et al., Jpn. J. Appl., Phy
s., 25, L773 (1986)). On the other hand, measures such as using a mixed thin film in which a light emitting layer and a charge injection layer are integrated are also taken.
However, none of them have achieved sufficient element characteristics and lifetime, and they are far from practical use.

[0009]

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to solve the above-mentioned problems and to provide an organic thin film type EL device having high luminance, high efficiency, and sufficient durability. It is.

[0010]

Means for Solving the Problems The present inventors have found that in order to achieve the required durability in an organic thin film type electroluminescent device, it is essential that the material constituting the device be made of a polymer. Based on the recognition, we are studying how to achieve the lamination interface control, which is extremely important for the development of device characteristics, with a device made of a polymer material. In addition to increasing the durability of the device by integrating the same, it eliminates the heterojunction interface made of different materials that are likely to be various degradation factors or regulation factors for the development of characteristics, and an integrated homogenous structure in which multiple light emitting layers are made of the same polymer material. With a junction multilayer structure and a charge injection layer by doping a polymer dopant into a part of the polymer light-emitting layer, it has both stable high device characteristics and sufficient durability. Make sure that the EL element is realized,
The present invention has been reached.

That is, the present invention relates to an organic thin film type electroluminescent device comprising an organic thin film light emitting layer sandwiched between a pair of electrodes at least one of which is transparent.
The light emitting layer is formed by laminating at least two layers of the same polymer light emitting body, and at least one of the light emitting layers is a charge injection layer doped with a polymer dopant. A luminescent element is provided. Hereinafter, the present invention will be described in more detail.

FIG. 1 is a sectional view showing a schematic structure of an example of the organic thin film electroluminescent device of the present invention.
On a support substrate 1, a transparent electrode (anode) 2, a light-emitting layer 3 also serving as a charge (hole) injection layer, a light-emitting layer 4, and an electrode (cathode) 5
Are laminated.

The structure of the organic thin-film electroluminescent device according to the present invention is such that a transparent electrode (anode), a first charge (hole) injection layer, a light-emitting layer, a second A laminate of a charge (electron) injection layer and an electrode (cathode), and a light-emitting layer and an electrode (cathode) also serving as a transparent electrode (anode), a light-emitting layer, and a charge (electron) injection layer on a support substrate. And the like.

The supporting substrate 1 is preferably transparent, and generally, a transparent resin material such as glass, quartz, or polyethylene terephthalate is used. The electrodes (anode 2 and cathode 5 ) include gold, aluminum,
Metals such as indium, alloys, mixtures thereof, or indium tin oxide (ITO), SnO ₂ , Zn
It is preferable to use a material such as O. At least one of these electrodes is made of a transparent material in order to increase the transmittance of light emission.

In the present invention, as the light emitting layer, a layer obtained by laminating a plurality of layers of the same polymer light emitting body is used. One of the laminates is a light emitting layer composed of only a polymer light emitting body, and the other layer is a light emitting layer doped with a polymer dopant.

The light-emitting material used in the present invention is not particularly limited as long as it is a polymer light-emitting material. From the viewpoint of realizing a modulation dope structure, the polymer light-emitting material itself or its precursor is
Desirably, it is soluble in water or organic solvent.
As a specific example of such a polymer light emitting body, particularly, the structural formula [1]

[0017]

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A polymer compound having a polyparaphenylenevinylene (PPV) skeleton represented by the following formula and a structural formula [2]:

[0019]

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A poly-1,4-naphthenylene represented by
A polymer compound having a vinylene skeleton is preferable. The polymer light-emitting materials of the structural formulas [1] and [2] are polyarylene.
-1 ', 2'-ethylene-1'-sulfonium salt or polyarylene-1'-alkoxy-1', 2'-
It is synthesized via a precursor such as ethylene . That is, the polymer luminous body of the structural formula [1] is represented by the following structural formula [3] or [4]

[0021]

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[0022] (In the formula, ^{R 1} is a lower alkyl group, pairs Io
Emissions X ^- represents a halogen ion. Water-soluble poly-1,4-phenylene-1 ', 2'-ethylene-
1'-sulfonium salt or organic solvent-soluble poly-
1,4-phenylene-1'-alkoxy-1 ', 2'-
Port of obtained by thermal decomposition of the ethylene and the structural formula [2]
The polymer compound having 1,4-naphthenylenevinylene skeleton is represented by the following structural formula [5] or [6]

[0023]

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[0024] (symbols in the formula are as defined above.) Water-soluble poly-1,4-represented by Nafuteniren
-1 ', 2'-ethylene-1'-sulfonium salt or organic solvent-soluble poly-1,4-naphtenylene-1'
-Alkoxy-1 ', 2'-Ethylene is obtained by thermal decomposition. Further, the structural formula [7]

[0025]

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(In the formula, R ² and R ³ each represent an alkyl group.) A polymer compound having a structure represented by the following formula is also a preferable polymer light-emitting material. This compound also has the structural formula [8] or [9] as described above.

[0027]

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[0028] (symbols in the formula are as defined above.) With a water-soluble poly-2,5-dialkoxy shown
-1,4-phenylene-1 ', 2'-ethylene-1'-
Poly-2,5 soluble in sulfonium salt or organic solvent
-Dialkoxy-1,4-phenylene-1'-alkoxy
It is obtained by thermal decomposition with -1 ', 2'-ethylene .

The polymer dopant for forming the charge injection system is selected from water-soluble or organic solvent-soluble ones, similarly to the polymer compound of the light emitting layer. If the conductive polymer light emitting polymer is synthesized via such precursor polyarylene vinylene will become possible to perform structural conversion treatment after the addition of polymer dopant in the precursor solution, high As a molecular dopant , stability of the doped state under processing conditions is also required. Therefore, the polymer dopant not only exerts a doping effect, but also has heat resistance to withstand heat generated during driving of the device or heating during the precursor treatment process, and the applied electric field gradient and thermal motion during driving of the device make it possible to use the polymer dopant in the system. It is a requirement to have a stability without moving. Examples of polymer dopants that satisfy this requirement include polymer cations that hold sulfonic acid groups, such as polystyrene sulfonic acid and polynaphthyl sulfonic acid, for p-type doping, and ammonium groups for n-type doping. The preferred polymeric anions are:

It is preferable that both the p-type and the n-type polymer dopant have a molecular weight of 50,000 or less. More preferably,
This is a system that functions as a polymer dopant by being mixed as a monomer in a precursor solution and simultaneously thermally polymerized during heat treatment. Such materials include parastyrene sulfonic acid, vinyl sulfonic acid, and the like.

In constructing the device, the polymer light emitting layer and the charge injection layer are formed by a coating method. In this case, the application methods include spinner application, roll application,
In addition to known methods such as spray coating, dip coating, and gravure coating, a casting method can be used.

In order to realize the modulation doping structure, a doped layer and an undoped layer may be sequentially formed according to the element structure. For example, when the light-emitting layer of the device has a three-layer structure of a hole injection layer, a light-emitting layer, and an electron injection layer, first, a light-emitting polymer containing a polymer anion is disposed on a transparent electrode (ITO or the like) substrate as an anode. The solution is coated to form a hole injection layer, and then a light-emitting layer containing no dopant (polymer ion) and an electron injection layer containing a polymer cation are sequentially formed to form a desired homomultilayer structure. obtain.

At the time of laminating coating, it is not unequivocally determined by the type of the luminescent polymer and the type of the solvent, but it is basically preferable to continue to the next coating step in a semi-dry state. The optimum thickness of each layer is determined in each case depending on the light emitting material, the kind of the dopant, the kind of the solvent, and the like, but is preferably in the range of 500 to 3,000 angstroms in view of the element characteristics.

[0034]

Hereinafter, the present invention will be described with reference to examples.
The present invention is not limited only to the following examples.

Example 1 1.5% by weight of PPV precursor (R in the above formula [3]
1.5% by weight of polystyrenesulfonic acid (molecular weight: 8000) is dissolved in an aqueous solution of a high molecular compound ( ¹ is CH ₃ and X is Cl), and this is used as a PPV precursor solution for a charge (hole) injection layer. This solution was applied on an ITO glass substrate by a spin coating method to form a film having a thickness of about 1000 Å,
Air dried. Next, a 3 wt% PPV precursor aqueous solution was formed thereon by spin coating similarly to form a light emitting layer having a thickness of about 8000 Å. During this time, the work was carried out in a clean booth to minimize contamination of the interface. After the light-emitting layer was air-dried, a heat treatment was performed at 200 ° C. for 2 hours in an argon atmosphere to convert the PPV precursor into a PPV structure. After slow cooling, an aluminum electrode (cathode) was formed to a thickness of 3000 Å by vacuum evaporation. Lead wires were attached using silver paste to complete the device.

FIG. 2 shows the relationship between the voltage and light emission luminance when a positive DC voltage was applied to the ITO electrode side and a negative DC voltage was applied to the aluminum electrode side for the EL device shown in FIG. 1 prepared as described above. FIG. 3 shows the relationship with the light emission luminance, and FIG. 4 shows the change with time in the light emission luminance.

COMPARATIVE EXAMPLE 1 When a DC voltage was applied to an EL device having a single layer structure in which a PPV light emitting layer (about 8000 angstroms) and an aluminum electrode (cathode, 3000 angstroms) were provided on an ITO electrode in the same manner as in Example 1. FIG. 2 shows the relationship between the voltage and the light emission luminance, FIG. 3 shows the relation between the current density and the light emission luminance, and FIG. 4 shows the change over time in the light emission luminance.

Comparative Example 2 The following structural formula which is a typical hole transport system on an ITO electrode

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A charge injection layer (about 1000 angstroms) of an aromatic diamine (TAD) is provided by a vacuum evaporation method, and a PPV light emitting layer (about 8000 angstroms) is formed thereon by applying an organic solvent-soluble precursor and heating. And then aluminum electrode by vacuum evaporation
An EL element was formed with a thickness of 3000 angstroms. FIG. 2 shows the relationship between the voltage and the emission luminance when a DC voltage was applied to this EL element in the same manner as in Example 1.
FIG. 3 shows the relationship between the current density and the light emission luminance, and FIG. 4 shows the change over time in the light emission luminance. It is clear from the light emission behavior of the examples and the comparative examples that the organic thin-film electroluminescent device according to the present invention has improved luminance and significantly extended light emission life.

Example 2 Example 1 was repeated except that 1.5% by weight of p-styrenesulfonic acid was dissolved in an aqueous solution of 1.5% by weight of PPV precursor (same as in Example 1), and this was used as an aqueous solution of PPV precursor for a charge injection layer. An EL element was manufactured in exactly the same manner as in Example 1. In this example, p-styrenesulfonic acid is thermally polymerized during heat treatment at 200 ° C. to become a polymer dopant. When the light emission behavior of this device was measured, a remarkable improvement in luminance and extension of light emission life were realized as in Example 1.

[0041]

According to the present invention, there is provided an organic thin-film electroluminescent device in which an organic thin-film light-emitting layer is sandwiched between a pair of electrodes at least one of which is transparent. Laminated,
The organic thin-film electroluminescent device of the present invention, in which at least one of the light-emitting layers is a charge injection layer doped with a polymer dopant, can be constructed by a very simple method using a coating film forming method, This is an element having a long life, high efficiency, and excellent durability in which the problem of the short life of the conventional organic EL element has been solved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a schematic configuration of an example of an organic thin film electroluminescent device of the present invention.

FIG. 2 is a graph showing a relationship between voltage and light emission luminance of the EL element of Example 1 and two kinds of EL elements of Comparative Examples .

FIG. 3 is a graph showing a relationship between current density and light emission luminance of the EL element of Example 1 and two kinds of EL elements of Comparative Examples .

FIG. 4 is a graph showing the change over time in the emission luminance of the EL element of Example 1 and the EL elements of two types of comparative examples .

[Explanation of symbols]

 Reference Signs List 1 support substrate 2 transparent electrode (anode) 3 charge injection layer 4 light emitting layer 5 electrode (cathode)

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-202355 (JP, A) JP-A-3-273087 (JP, A) JP-A-3-244630 (JP, A) JP-A-6-206 1972 (JP, A) JP 5-320634 (JP, A) JP 5-263307 (JP, A) JP 5-247460 (JP, A) JP 4-145192 (JP, A) JP-A-64-79217 (JP, A) JP-A-61-55185 (JP, A) JP-T-Hei 6-508477 (JP, A) JP-T-Hei 4-500582 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H05B 33/14 C09K 11/06

Claims (6)

(57) [Claims] 1. An organic thin-film electroluminescent device having an organic thin-film light-emitting layer sandwiched between a pair of electrodes, at least one of which is transparent, wherein the light-emitting layer comprises at least two layers of the same polymer light-emitting material. Laminated,
An organic thin-film electroluminescent device, wherein at least one of the light-emitting layers is a charge injection layer doped with a polymer dopant. 2. The light-emitting body has a structural formula [1] The organic electroluminescent device according to claim 1, which is a polymer compound having a polyparaphenylene vinylene skeleton represented by the following formula: 3. The light-emitting body has a structural formula [2]: The organic electroluminescent device according to claim 1, which is a polymer compound having a poly-1,4-naphtenylenevinylene skeleton represented by: 4. The organic electroluminescent device according to claim 1, wherein the light emitting layer is a layer formed by a coating method. 5. A method for manufacturing an organic thin-film electroluminescent device in which an organic thin-film light-emitting layer is sandwiched between a pair of electrodes at least one of which is transparent, wherein the light-emitting layer comprises the same polymer light-emitting body. At least two layers are stacked, and at least one of the luminous body layers includes a step of applying a solution in which a polymer dopant is added to an aqueous solution or an organic solution of the luminous body or a precursor thereof. Of manufacturing an organic thin film type electroluminescent device. 6. The precursor of the luminescent material is polyarylene-
6. The method for producing an organic thin film type electroluminescent device according to claim 5, which is 1 ', 2'-ethylene-1'-sulfonium salt or polyarylene-1'-alkoxy-1', 2'-ethylene. Method.