JP2008198769A - Organic electroluminescence (el) element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic EL element which can be driven on low voltage and has a high light emission efficiency and a long life. <P>SOLUTION: In the organic EL element, an electric charge generating unit is formed between a positive electrode and a light emission layer. The electric charge generating unit consists of an electron extraction layer and an adjacent layer which is adjacent to the negative electrode surface side of the electron extraction layer and has its electrons to be extracted into the electron extraction layer. The electron extraction layer consists of an aromatic carboxylic acid derivative expressed by a general expression (I), where Ar represents an aromatic group, X represents O or N-R, and k is an integer between 1 and 4. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an organic electroluminescent element (hereinafter sometimes abbreviated as an organic EL element or an element) used for a planar light source or a display element.

  Organic EL elements have been actively developed from the viewpoint of application to displays and lighting. The driving principle of the organic EL element is as follows. That is, holes and electrons are injected from the anode and the cathode, respectively, and these are transported through the organic thin film, recombined in the light emitting layer to generate an excited state, and light emission can be obtained from this excited state. In order to increase the luminous efficiency, it is necessary to inject holes and electrons efficiently and transport the organic thin film. However, since the movement of carriers in the organic EL element is limited by the energy barrier between the electrode and the organic thin film and the low carrier mobility in the organic thin film, there is a limit to improving the light emission efficiency.

  As a method for solving such a problem, there is a method of emitting light efficiently at a low voltage using a charge generation layer. The charge generation layer is formed by a layered or mixed layer of an electron-donating compound and an electron-donating compound, and both compounds efficiently form carriers (holes and electrons) by forming a charge transfer complex by a redox reaction. Can be made.

Patent Document 1 discloses an organic EL element having a multilayer structure in which a charge generation layer having a large specific resistance is provided between a plurality of light emitting units. The charge generation layer functions to inject holes in the cathode direction and inject electrons in the anode direction when a voltage is applied. The charge generation layer is an electrically insulating layer and has an ionization potential of less than 5.7 eV and can form a hole-transporting (electron donating) organic substance and a charge-transfer complex by an oxidation-reduction reaction with the organic substance. It consists of the laminated body or mixture which consists of an inorganic or organic substance. Specifically, a method for generating carriers using a mixed layer of α-NPD and V ₂ O ₅ as a charge generation layer and supplying the carriers to the light emitting layer has been reported. However, since an inorganic oxide is used as the electron-donating compound, the evaporation temperature is very high, and a great amount of radiant heat is generated during thin film deposition. This generated heat causes damage to other organic films and causes re-evaporation of impurities adhering to the inside of the apparatus, resulting in problems such as deterioration of device characteristics and adverse effects on the lifetime. Further, there is an example in which an organic compound is used as the electron-donating compound, but this compound has a problem that the evaporation temperature is low, there are problems in film formability and film stability, and the life is short.

Patent Document 2 relates to the improvement of the invention described in Patent Document 1, but uses MoO ₃ which is an inorganic oxide as an electron-donating compound, and does not solve the above problem.

  Patent Document 3 discloses an organic EL element in which an intermediate unit composed of an electron extraction layer and an electron injection layer is disposed between light emitting units. This intermediate unit draws electrons from the adjacent layer adjacent to the cathode side, supplies holes generated thereby to the first light emitting unit on the cathode side, and draws the extracted electrons on the anode side through the electron injection layer. It serves to supply the second light emitting unit. Here, the electron extraction layer is a layer having a function of extracting electrons from the adjacent layer, the adjacent layer is a layer provided adjacent to the cathode surface side of the electron extraction layer, and the electron injection layer is an anode surface of the electron extraction layer. It is a layer provided adjacent to the side. The difference between the absolute value (A) of the LUMO energy level of the electron extraction layer and the absolute value (B) of the HOMO energy level of the adjacent layer is 1.5 eV or less, and the absolute value of the energy level of the electron injection layer LUMO. (C) or the absolute value (W) of the work function is smaller than the absolute value (A) of the LUMO energy level of the electron extraction layer. And patent document 3 is disclosing the pyrazine derivative as an electron-donating compound used for an electron extraction layer.

  Patent Documents 1 to 3 each disclose an organic EL element including a plurality of light emitting units each including a light emitting layer between a pair of electrodes. When the light emitting units are stacked, the light emission efficiency is lowered. Therefore, a charge generation layer or an intermediate unit is disposed between the light emitting units to prevent the lowering. It is understood that the charge generation layer and the intermediate unit are the same in that holes are injected into the cathode layer adjacent to the charge generation layer and electrons are injected into the anode layer.

  By the way, Patent Document 4 discloses that a certain kind of aromatic bisimide derivative is used as an organic EL element material. Specifically, Patent Document 4 discloses use as a hole transport material or a light emitting material. However, there is nothing that teaches its use as an electron extraction layer material.

  Here, the function required for the electron extraction layer material and the function required for a general hole transport material will be described with reference to FIG. It is important for the electron extraction layer material to extract electrons from the adjacent layer to generate holes, and it is required to make the EA (electron affinity) of the electron extraction layer material as close as possible to the IP (ionization potential) of the adjacent layer material. It is done. Therefore, the IP of the electron extraction layer material is considerably deep, and holes are hardly injected from the anode. On the other hand, it is important for the hole transport material to efficiently transport holes injected from the anode to the light emitting layer, and it is required that the IP of the hole transport material be as close to the work function of the anode as possible. Therefore, even if they are stacked at the same position, the required functions are completely different, and such use is not disclosed in Patent Document 4. Moreover, even if it is a compound similar to patent document 4, by using a compound group having a certain relationship with the adjacent layer material, holes are directly generated in the organic layer without injecting holes from the anode. The driving voltage can be reduced. That is, Patent Document 4 teaches a method of injecting holes from the anode, but nothing teaches that holes are directly generated in the charge generation layer to emit light efficiently at a low voltage.

JP 2003-272860 A JP 2006-024791 A JP 2006-066380 A JP 2004-143044 A

  An object of the present invention is to provide an organic EL element including at least one light emitting unit that can be driven at a low voltage, has high luminous efficiency, and has a long lifetime. Another object is to achieve high efficiency and long life of the organic EL element by suppressing the high temperature during production by using a specific organic compound as the electron extraction layer material used for the charge generation layer. The object is to obtain a high-quality organic EL device.

（ここで、Ａｒは１以上の芳香族環を有する芳香族基を示し、-ＣＯ-Ｘ-ＣＯ-はＡｒ
中の芳香族環と縮合してヘテロ環を構成する２価の基を示し、ＸはＯ又はＮ-Ｒを示し、
ＲはＨ又は１価の置換基を示し、ｋは１、２、３又は４を示す。該芳香族基は、縮合芳香族環を有する芳香族基であってもよく、置換基を有してよい。） The present invention relates to an organic electroluminescent device including at least one light emitting layer between an anode and a cathode facing each other, and adjacent to the cathode surface side of the electron extracting layer and the electron extracting layer between the anode and the light emitting layer. And at least one charge generating unit comprising an adjacent layer from which electrons are extracted to the electron extraction layer, and the electron extraction layer is selected from aromatic carboxylic acid derivatives represented by the following general formula (I) It consists of an electron extraction layer material containing at least one electron-accepting aromatic carboxylic acid derivative, and the difference between the ionization potential (IP) of the adjacent layer and the electron affinity (EA) of the electron extraction layer (IP-EA) is 0 to It is an organic electroluminescent device in the range of 2.0 eV.

(Wherein Ar represents an aromatic group having one or more aromatic rings, and —CO—X—CO— represents Ar
A divalent group that forms a heterocycle by condensation with an aromatic ring therein, X represents O or N—R,
R represents H or a monovalent substituent, and k represents 1, 2, 3 or 4. The aromatic group may be an aromatic group having a condensed aromatic ring or may have a substituent. )

  Preferred examples of the aromatic carboxylic acid derivative represented by the general formula (I) include aromatic carboxylic acid derivatives represented by the following formula (1), (2), (3) or (4).

（式（１）、（２）、（３）及び（４）において、Ａｒ₁、Ａｒ₂、Ａｒ₃はそれぞれ含酸素環又は含窒素環と縮合する芳香族環を示し、該芳香族環は１環以上であっても、縮合環であっても、置換基を有してもよい。Ｙは単結合又は２価の結合基を示し、Ｒ₁、Ｒ₂、Ｒ₃は水素原子又は置換基を示し、ｌは１、２又は３を示し、ｍ、ｎは０、１又は２を示すが、ｍ+ｎは１、２、３又は４である。）

(In the formulas (1), (2), (3) and (4), Ar ₁ , Ar ₂ and Ar ₃ each represent an aromatic ring condensed with an oxygen-containing ring or a nitrogen-containing ring, and the aromatic ring is One or more rings, a condensed ring, or a substituent may be present, Y represents a single bond or a divalent linking group, and R ₁ , R ₂ , and R ₃ represent a hydrogen atom or a substituent. And 1 represents 1, 2 or 3; m, n represents 0, 1 or 2; m + n is 1, 2, 3 or 4)

  The charge generation unit may be provided in one layer or in two or more layers. Two or more light emitting units including a light emitting layer are provided, and a charge generation unit is preferably provided between the light emitting units.

  The adjacent layer is preferably a hole transporting material, and more preferably an arylamine-based hole transporting material. Further, the adjacent layer may be formed of a light emitting layer material.

  The electron extraction layer may be formed of a mixed layer of an electron extraction layer material and a hole transporting adjacent layer material.

  In the organic EL device of the present invention, at least one light emitting layer and at least one charge generation unit are provided between an anode and a cathode facing each other. The charge generation unit is provided between the light emitting layer and the anode electrode. Here, the charge generation unit includes an electron extraction layer and an adjacent layer, and the adjacent layer has a function of supplying electrons to the electron extraction layer. The difference (IP-EA) between the ionization potential (IP) of the adjacent layer and the electron affinity (EA) of the electron extraction layer is in the range of 0 to 2.0 eV.

  Also, a plurality of light emitting units can be provided, and a charge generation unit can be disposed between them. In this case, the light emission unit includes one or more light emission layers, and means that the anode, the cathode, the charge generation unit, and the adjacent layers are excluded. Used in. When the light emitting layer or other layer constituting the light emitting unit is an adjacent layer from which electrons are extracted to the electron extracting layer, these layers are understood as layers constituting both the light emitting unit and the adjacent layer. The light emitting unit preferably has a light emitting layer, an electron transport layer and an electron injection layer from the anode side. In Patent Document 3, from the anode side, the electron injection layer / electron extraction layer / adjacent layer are collectively set as an intermediate unit (≈charge generation layer), which is slightly different from the configuration of the charge generation unit of the present invention. It is understood that the adjacent layer of the present invention and the adjacent layer of Document 3 have the same function.

  As the electron extraction layer material, a material containing at least one electron-accepting aromatic carboxylic acid derivative selected from the aromatic carboxylic acid derivatives represented by the above general formula (I) is used. This pull-out layer material may consist only of the electron-accepting aromatic carboxylic acid derivative, or may be mixed with other compounds, preferably the material constituting the adjacent layer. This is selected to keep the above (IP-EA) at a reasonable value.

In the above general formula (I), X represents O or N—R, and R represents H or a monovalent substituent, preferably an alkyl group having 1 to 6 carbon atoms or an aryl group having 1 to 8 carbon atoms. It is. -CO-X-CO- represents a divalent group constituting a heterocycle by condensation with an aromatic ring in Ar, Ar represents an aromatic group having one or more aromatic rings, and contains X An aromatic ring fused with the ring,
The aromatic ring may be one or more aromatic rings or a condensed aromatic ring, and may have a substituent. The total number of aromatic rings and heterocycles is preferably 3-8. k represents 1, 2, 3 or 4, and is preferably 2 or 3.

The aromatic carboxylic acid derivative represented by the general formula (I) is preferably an aromatic carboxylic acid derivative represented by the following formula (1), (2), (3) or (4). In the above formulas (1), (2), (3) and (4), Ar ₁ , Ar ₂ and Ar ₃ each represent an aromatic ring condensed with an oxygen-containing or nitrogen-containing ring, and the aromatic ring is One or more rings, a condensed ring, or a substituent may be present. Y represents a single bond or a divalent linking group, R ₁ , R ₂ and R ₃ represent a hydrogen atom or a substituent, l represents 1, 2 or 3, m, n represents 0, 1 or 2 As shown, m + n is 1, 2, 3 or 4.

In the formulas (1) to (4), Ar ₁ , Ar ₂ , and Ar ₃ each represent an aromatic ring, and preferred aromatic rings are a benzene ring, a naphthalene ring, an anthracene ring, a phenanthrene ring, a tetracene ring, and a chrysene ring. Perylene ring, pyrene ring and the like. The number of rings constituting the aromatic ring is 1 to 8, preferably 1 to 6.

In the formulas (2) and (4), Y represents a single bond or a divalent linking group, and is preferably a single bond, O, S, CO, or CH ₂ . In the formulas (1) and (3), l represents 1, 2 or 3, preferably 2 or 3. In the formulas (2) and (4), m and n represent 0, 1 or 2, but preferably both m and n are 1.

  The aromatic ring may have a substituent. Preferred substituents include alkyl groups (for example, methyl group, ethyl group, propyl group, isopropyl group, tertiary butyl group, pentyl group, hexyl group, octyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, etc.) A cycloalkyl group (eg, cyclopentyl group, cyclohexyl group, etc.), an alkenyl group (eg, vinyl group, allyl group, etc.), an alkynyl group (eg, ethynyl group, propargyl group, etc.), an aryl group (eg, phenyl group, naphthyl group, etc.) Group), aromatic heterocyclic group (for example, furyl group, thienyl group, pyridyl group, pyridazinyl group, pyrimidinyl group, pyrazinyl group, triazinyl group, imidazolyl group, pyrazolyl group, thiazolyl group, quinazolinyl group, phthalazinyl group, etc.), Heterocyclic groups (eg, pyrrolidyl groups, imidazoles Group, morpholyl group, oxazolidyl group, etc.), alkoxyl group (for example, methoxy group, ethoxy group, propyloxy group, pentyloxy group, hexyloxy group, octyloxy group, dodecyloxy group, etc.), cycloalkoxyl group (for example, Cyclopentyloxy group, cyclohexyloxy group, etc.), aryloxy group (eg, phenoxy group, naphthyloxy group, etc.), alkylthio group (eg, methylthio group, ethylthio group, propylthio group, pentylthio group, hexylthio group, octylthio group, dodecylthio group) Etc.), a cycloalkylthio group (eg, cyclopentylthio group, cyclohexylthio group, etc.), an arylthio group (eg, phenylthio group, naphthylthio group, etc.), an alkoxycarbonyl group (eg, methyloxycarbonyl group, ethyl) Oxycarbonyl group, butyloxycarbonyl group, octyloxycarbonyl group, dodecyloxycarbonyl group, etc.), aryloxycarbonyl group (eg, phenyloxycarbonyl group, naphthyloxycarbonyl group, etc.), sulfamoyl group (eg, aminosulfonyl group, methyl) Aminosulfonyl group, dimethylaminosulfonyl group, butylaminosulfonyl group, hexylaminosulfonyl group, cyclohexylaminosulfonyl group, octylaminosulfonyl group, dodecylaminosulfonyl group, phenylaminosulfonyl group, naphthylaminosulfonyl group, 2-pyridylaminosulfonyl group, etc. ), Acyl group (for example, acetyl group, ethylcarbonyl group, propylcarbonyl group, pentylcarbonyl group, cyclohexylcarbonyl group, octyl group) Rucarbonyl group, 2-ethylhexylcarbonyl group, dodecylcarbonyl group, phenylcarbonyl group, naphthylcarbonyl group, pyridylcarbonyl group, etc.), acyloxy group (for example, acetyloxy group, ethylcarbonyloxy group, butylcarbonyloxy group, octylcarbonyloxy group) Group, dodecylcarbonyloxy group, phenylcarbonyloxy group, etc.), amide group (for example, methylcarbonylamino group, ethylcarbonylamino group, dimethylcarbonylamino group, propylcarbonylamino group, pentylcarbonylamino group, cyclohexylcarbonylamino group, 2 -Ethylhexylcarbonylamino group, octylcarbonylamino group, dodecylcarbonylamino group, phenylcarbonylamino group, naphthylcarbonylamino group, etc.), Vamoyl group (for example, aminocarbonyl group, methylaminocarbonyl group, dimethylaminocarbonyl group, propylaminocarbonyl group, pentylaminocarbonyl group, cyclohexylaminocarbonyl group, octylaminocarbonyl group, 2-ethylhexylaminocarbonyl group, dodecylaminocarbonyl group) , Phenylaminocarbonyl group, naphthylaminocarbonyl group, 2-pyridylaminocarbonyl group, etc.), ureido group (for example, methylureido group, ethylureido group, pentylureido group, cyclohexylureido group, octylureido group, dodecylureido group, phenylureido) Group naphthylureido group, 2-pyridylaminoureido group, etc.), sulfinyl group (for example, methylsulfinyl group, ethylsulfinyl group, butylsulfuric group) Nyl group, cyclohexylsulfinyl group, 2-ethylhexylsulfinyl group, dodecylsulfinyl group, phenylsulfinyl group, naphthylsulfinyl group, 2-pyridylsulfinyl group, etc.), alkylsulfonyl group (for example, methylsulfonyl group, ethylsulfonyl group, butylsulfonyl group) Cyclohexylsulfonyl group, 2-ethylhexylsulfonyl group, dodecylsulfonyl group, etc., arylsulfonyl group (phenylsulfonyl group, naphthylsulfonyl group, 2-pyridylsulfonyl group etc.), amino group (for example, amino group, ethylamino group, dimethyl) Amino group, butylamino group, cyclopentylamino group, 2-ethylhexylamino group, dodecylamino group, anilino group, naphthylamino group, 2-pyridylamino group, etc.), halogen atom ( For example, fluorine atom, chlorine atom, bromine atom, etc.), fluorinated hydrocarbon group (eg, fluoromethyl group, trifluoromethyl group, pentafluoroethyl group, pentafluorophenyl group, etc.), cyano group, nitro group, hydroxy group , Mercapto group, silyl group (for example, trimethylsilyl group, triisopropylsilyl group, triphenylsilyl group, phenyldiethylsilyl group, etc.).

  These substituents may be further substituted with the above substituents. In addition, a plurality of these substituents may be bonded to each other to form a ring.

In the formulas (3) and (4), R ₁ , R ₂ and R ₃ each represent hydrogen or a substituent bonded to a nitrogen atom, and preferred substituents are exemplified below.

  An alkyl group (for example, methyl group, ethyl group, propyl group, isopropyl group, tertiary butyl group, pentyl group, hexyl group, octyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, etc.), cycloalkyl group (for example, , Cyclopentyl group, cyclohexyl group, etc.), alkenyl group (eg, vinyl group, allyl group, etc.), alkynyl group (eg, ethynyl group, propargyl group, etc.), aryl group (eg, phenyl group, naphthyl group, etc.), aromatic A heterocyclic group (for example, furyl group, thienyl group, pyridyl group, pyridazinyl group, pyrimidinyl group, pyrazinyl group, triazinyl group, imidazolyl group, pyrazolyl group, thiazolyl group, quinazolinyl group, phthalazinyl group, etc.), heterocyclic group (for example, Pyrrolidyl, imidazolidyl, morpholyl, o Sazolidyl group, etc.), alkoxyl group (eg, methoxy group, ethoxy group, propyloxy group, pentyloxy group, hexyloxy group, octyloxy group, dodecyloxy group, etc.), cycloalkoxyl group (eg, cyclopentyloxy group, cyclohexyloxy) Group), aryloxy group (for example, phenoxy group, naphthyloxy group, etc.), alkylthio group (for example, methylthio group, ethylthio group, propylthio group, pentylthio group, hexylthio group, octylthio group, dodecylthio group, etc.), cycloalkylthio group (For example, cyclopentylthio group, cyclohexylthio group, etc.), arylthio group (for example, phenylthio group, naphthylthio group, etc.), seleno group (for example, methylseleno group, phenylseleno group, etc.), acyl group (for example, acetyl group, ethyl group, etc.) Carbonyl group, propylcarbonyl group, pentylcarbonyl group, cyclohexylcarbonyl group, octylcarbonyl group, 2-ethylhexylcarbonyl group, dodecylcarbonyl group, phenylcarbonyl group, naphthylcarbonyl group, pyridylcarbonyl group, etc.), acyloxy group (for example, acetyl) Oxy group, ethylcarbonyloxy group, butylcarbonyloxy group, octylcarbonyloxy group, dodecylcarbonyloxy group, phenylcarbonyloxy group, etc.), sulfinyl group (for example, methylsulfinyl group, ethylsulfinyl group, butylsulfinyl group, cyclohexylsulfinyl group) 2-ethylhexylsulfinyl group, dodecylsulfinyl group, phenylsulfinyl group, naphthylsulfinyl group, 2-pyridylsulfinyl group Group), alkylsulfonyl group (for example, methylsulfonyl group, ethylsulfonyl group, butylsulfonyl group, cyclohexylsulfonyl group, 2-ethylhexylsulfonyl group, dodecylsulfonyl group, etc.), arylsulfonyl group (phenylsulfonyl group, naphthylsulfonyl group, 2-pyridylsulfonyl group, etc.), amino group (for example, amino group, ethylamino group, dimethylamino group, butylamino group, cyclopentylamino group, 2-ethylhexylamino group, dodecylamino group, anilino group, naphthylamino group, 2 -Pyridylamino group, etc.), fluorinated hydrocarbon group (eg, fluoromethyl group, trifluoromethyl group, pentafluoroethyl group, pentafluorophenyl group, etc.), cyano group, nitro group, hydroxy group, mercapto group, silyl group ( example , Trimethylsilyl group, triisopropylsilyl group, triphenylsilyl group, a phenyl diethyl silyl group and the like) and the like.

  These substituents may be further substituted with the above substituents. In addition, a plurality of these substituents may be bonded to each other to form a ring.

  Although the specific example of the aromatic carboxylic acid derivative represented by general formula (I) or formula (1)-(4) below is shown, it is not limited to this.

  According to the organic EL element of the present invention, by using an organic compound having an appropriate vapor pressure as a material used for the charge generation unit, thermal damage due to heat generation and re-evaporation of impurities attached to the apparatus wall surface are suppressed, As compared with the conventional technology, an element having high luminous efficiency and greatly improved driving stability can be obtained. As a result, excellent performance can be exhibited in application to full-color or multi-color panels. The organic EL device of the present invention can emit light with high efficiency at a low voltage, and can further provide a device with little deterioration during high-temperature storage. Therefore, the organic electroluminescent device according to the present invention is a flat panel display (for example, for OA computers and wall-mounted televisions), an in-vehicle display device, a light source utilizing characteristics of a mobile phone display or a surface light emitter (for example, a light source of a copying machine, It can be applied to liquid crystal displays and back light sources for instruments, display panels, and indicator lights, and its technical value is great.

  Hereinafter, the present invention will be described with reference to the drawings. 1 to 3 are schematic cross-sectional views showing an example of the organic EL element of the present invention.

  FIG. 1 shows an organic EL element in which a plurality of light emitting units and charge generating units are stacked. The anode 2, the charge generation unit 31, and the light emitting unit 41 are stacked on the substrate 1 to constitute a first unit. On top of that, the charge generation unit 32 and the light emitting unit 42 are stacked to constitute a second unit. If necessary, a third unit and an Nth unit are formed, and a cathode 7 is laminated thereon. In the organic EL device of the present invention, the substrate, the anode, the charge generation unit, the light emitting layer or the light emitting unit, and the cathode are included as essential layers, but other layers may be provided as necessary. Here, the Nth unit such as the first unit is configured by laminating the charge generation unit 31 and the light emitting unit 41 in this order from the anode side.

  Here, the light emitting unit 41 includes at least one light emitting layer, and a plurality of light emitting layers may be stacked as necessary. The light emitting unit includes a light emitting layer as an essential layer, but usually comprises a plurality of layers. The light emitting layer 4a, the electron transport layer 4b, and the electron injection layer 4c are preferable from the anode side. The light emitting unit 41 may be a single layer or two or more layers.

  The charge generation unit 31 has a structure in which an electron extraction layer 3a and an adjacent layer 3b are stacked in this order from the anode side. The charge generation unit 31 may be a charge generation unit provided in contact with the anode, or may be a charge generation unit provided between the light emitting units. When the light emitting unit 41 is composed of one layer, the charge generating unit 31 is provided in contact with the anode. When the light emitting unit 41 is composed of two or more layers, the charge generation unit 31 can be a single layer charge generation unit provided in contact with the anode, or a charge generation unit provided in part or all of the light emission units. However, it is desirable to have a charge generation unit provided between the charge generation unit provided in contact with the anode and the light emitting unit.

  In the present invention, it is important for the electron extraction layer to efficiently extract electrons from the adjacent layer and generate holes in the adjacent layer. For this purpose, as shown in FIG. 5, the electron extraction layer material is adjacent to EA. The difference in the IP of the layer material is within 2.0 eV, preferably in the range of 0 to 1.0 eV. If the difference is larger than this, it becomes difficult for the electron extraction layer material to extract electrons from the adjacent layer material, which causes a high voltage for driving the element.

  The adjacent layer is preferably formed from a hole transporting material, particularly preferably an arylamine-based hole transporting material. In the case where the hole transporting material also functions as a light emitting layer, this light emitting layer is also an adjacent layer.

  FIG. 2 shows an organic EL element in which a light emitting unit and a charge generation unit are stacked in one layer. An anode 2, a charge generation unit 3, a light emitting unit 4, and a cathode 7 are stacked on the substrate. Here, preferred configurations of the charge generation unit 3 and the light emitting unit 4 are the same as those in FIG.

  FIG. 3 shows an organic EL element in which two layers each of a light emitting unit and a charge generating unit are laminated. A charge generation unit 31 and a light emitting unit 41 are stacked on the anode 2 to constitute a first unit. On top of that, the charge generation unit 32 and the light emitting unit 42 are stacked to constitute a second unit. Here, preferred configurations of the charge generation unit and the light emitting unit are the same as those in FIG.

Although the preferable structural example of the organic EL element of this invention is shown below, it is not limited to this.
1) Anode / charge generating unit / light emitting unit / cathode
2) Anode / charge generating unit / light emitting unit / charge generating unit / light emitting unit / cathode
3) Anode / charge generating unit / light emitting unit / charge generating unit / light emitting unit / charge generating unit / light emitting unit / cathode

  As described above, the element configuration of the present invention may be a single light emitting unit or a plurality of units. In the case of a plurality of units, by providing a charge generation unit between the plurality of light emitting units, holes generated by the extraction of electrons from the adjacent layer by the electron extraction layer are supplied to the cathode side light emitting unit, and the extracted electrons are supplied to the anode. The light emitting units can be supplied to the side light emitting units, and each individual light emitting unit can emit light in the same manner as in the case of a single unit.

  The electron extraction layer constituting the charge generation unit may be formed from a single electron extraction layer material or a mixed layer with a hole transporting adjacent layer material. It should be noted that the charge generation unit also has an adjacent layer even when it is a mixed layer with the adjacent layer material.

  The substrate 1 serves as a support for the organic electroluminescent element, and a quartz or glass plate, a metal plate or a metal foil, a plastic film, a sheet, or the like is used. In particular, a glass plate or a transparent synthetic resin plate such as polyester, polymethacrylate, polycarbonate, or polysulfone is preferable. When using a synthetic resin substrate, it is necessary to pay attention to gas barrier properties. If the gas barrier property of the substrate is too small, the organic electroluminescent element may be deteriorated by the outside air that has passed through the substrate, which is not preferable. For this reason, a method of providing a gas barrier property by providing a dense silicon oxide film or the like on at least one surface of the synthetic resin substrate is also a preferable method.

  An anode 2 is provided on the substrate 1. This anode is usually a metal such as aluminum, gold, silver, nickel, palladium, platinum or the like, an oxide of indium and / or tin, an oxide of zinc and / or tin, an oxide of tungsten and / or tin. It is composed of an oxide, a metal halide such as copper iodide, carbon black, or a conductive polymer such as poly (3-methylthiophene), polypyrrole, or polyaniline. In general, the anode is often formed by a sputtering method, a vacuum deposition method, or the like. Also, in the case of fine metal particles such as silver, fine particles such as copper iodide, carbon black, conductive metal oxide fine particles, conductive polymer fine powder, etc., it is dispersed in an appropriate binder resin solution and placed on the substrate. An anode can also be formed by coating. Further, in the case of a conductive polymer, a thin film can be directly formed on the substrate by electrolytic polymerization, or an anode can be formed by applying a conductive polymer on the substrate (Appl. Phys. Lett., Vol. 60). 2711, 1992). The anode can be formed by stacking different materials. The thickness of the anode 2 varies depending on the required transparency. When transparency is required, the visible light transmittance is usually 60% or more, preferably 80% or more. Although there is no restriction | limiting in particular about the film thickness of the anode 2, Usually, 1-1000 nm, Preferably it is 10-500 nm. In addition, when it may be opaque, an anode may be the same as a board | substrate. Furthermore, it is also possible to laminate different conductive materials on the anode.

  A charge generation unit 3 is provided on the anode 2. The charge generation unit includes an electron extraction layer and an adjacent layer, and the adjacent layer has a function of supplying electrons to the electron extraction layer.

  The electron extraction layer may be formed of only the electron extraction layer material represented by the general formula (I) or the formulas (1) to (4), or a mixed layer may be formed at an arbitrary ratio with the adjacent layer. However, in order to generate electric charges efficiently, it is sufficient that the difference between the EA of the electron extraction layer and the IP of the adjacent layer is 2.0 eV or less, and preferably a combination of 1.0 eV or less. Thus, electrons can be extracted from the adjacent layer and flowed to the anode side, holes can be generated in the adjacent layer, and holes can flow to the cathode side. Thereby, even if there are a plurality of light emitting units, all the light emitting units can be made to emit light efficiently.

  The electron extraction layer can be formed by thinning the electron extraction layer material by a known method such as a vacuum deposition method, a spin coating method, a casting method, a printing method including an ink jet method, or an LB method. .

  Although there is no restriction | limiting in particular about the film thickness of an electron extraction layer, Usually, 1-300 nm, Preferably it is 5-100 nm. The electron extraction layer may have a single layer structure composed of one or more of the above materials.

  The adjacent layer is preferably formed of a hole transporting compound, and generally known hole transport layer materials can be used. For example, triazole derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives and pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, amino-substituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives, stilbenes Derivatives, silazane derivatives, aniline copolymers, conductive polymer oligomers, particularly thiophene oligomers, and the like can be given.

  As the adjacent layer material, the above-mentioned materials can be used, and among them, an arylamine-based hole transporting material is particularly preferable.

  Representative examples of arylamine hole transport materials include N, N, N ′, N′-tetraphenyl-4,4′-diaminophenyl; N, N′-diphenyl-N, N′-bis (3- Methylphenyl)-[1,1′-biphenyl] -4,4′-diamine (TPD); 2,2-bis (4-di-p-tolylaminophenyl) propane; 1,1-bis (4-di) -P-tolylaminophenyl) cyclohexane; N, N, N ', N'-tetra-p-tolyl-4,4'-diaminobiphenyl; 1,1-bis (4-di-p-tolylaminophenyl)- 4-phenylcyclohexane; bis (4-dimethylamino-2-methylphenyl) phenylmethane; bis (4-di-p-tolylaminophenyl) phenylmethane; N, N′-diphenyl-N, N′-di (4 -Methoxy N), 4'-diaminobiphenyl; N, N, N ', N'-tetraphenyl-4,4'-diaminodiphenyl ether; 4,4'-bis (diphenylamino) quadriphenyl; N, N, N-tri (p-tolyl) amine; 4- (di-p-tolylamino) -4 '-[4- (di-p-tolylamino) styryl] stilbene; 4-N, N-diphenylamino- (2-diphenyl) Vinyl) benzene; 3-methoxy-4'-N, N-diphenylaminostilbenzene; N-phenylcarbazole and two condensed aromatics described in US Pat. No. 5,061,569 Having a ring in the molecule, for example, 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl (α-NPD), described in JP-A-4-308688 4,4 ', 4 "-tris [N- (3-methylphenyl) -N-phenylamino] triphenylamine (MTDATA) in which three triphenylamine units are linked in a starburst type Can be mentioned.

  Although there is no restriction | limiting in particular about the film thickness of an adjacent layer, Usually, it is 1-300 nm, Preferably it is 5-100 nm, and a thin film is formed on an electron extraction layer by the method similar to an electron extraction layer. This adjacent layer may have a single layer structure composed of one or more of the above materials.

  A light emitting unit 4 is provided on the charge generation unit 3. The light emitting unit includes a light emitting layer and an electron transport layer, and an electron injection layer may be provided as necessary. The light emitting unit has a function of recombining holes and electrons to emit light.

  The light emitting layer in the light emitting unit may be formed from a single light emitting layer, or may be configured by laminating a plurality of light emitting layers so as to be in direct contact with each other. The light emitting layer is composed of a host material and a fluorescent light emitting material or a phosphorescent light emitting material, and any material conventionally used for forming these layers can be used. In the case where the host material of the light emitting layer is a hole transporting material suitable as the adjacent layer, the light emitting layer may also be used as the adjacent layer.

  Although there is no restriction | limiting in particular about the film thickness of a light emitting layer, Usually, it is 1-300 nm, Preferably it is 5-100 nm, and a thin film is formed by the method similar to an electron extraction layer on an adjacent layer.

  The electron transport layer is formed of a compound capable of efficiently transporting electrons injected from the cathode between the electrodes to which an electric field is applied in the direction of the light emitting layer. The electron transporting compound used in the electron transporting layer needs to be a compound that has high electron injection efficiency from the cathode and that has high electron mobility and can efficiently transport injected electrons. is there.

  As an electron transport material satisfying such conditions, a metal complex such as Alq3 (Japanese Patent Laid-Open No. 59-194393), a metal complex of 10-hydroxybenzo [h] quinoline, an oxadiazole derivative, a distyrylbiphenyl derivative, Silole derivatives, 3- or 5-hydroxyflavone metal complexes, benzoxazole metal complexes, benzothiazole metal complexes, trisbenzimidazolylbenzene (US Pat. No. 5,645,948), quinoxaline compounds (JP-A-6-207169) Phenanthroline derivatives (Japanese Patent Laid-Open No. 5-33159), 2-t-butyl-9,10-N, N′-dicyanoanthraquinonediimine, n-type hydrogenated amorphous silicon carbide, n-type zinc sulfide, n-type Examples include zinc selenide.

  Although there is no restriction | limiting in particular about the film thickness of an electron carrying layer, Usually, it is 1-300 nm, Preferably it is 5-100 nm, and a thin film is formed on a light emitting layer by the method similar to an electron extraction layer. This electron transport layer may have a single layer structure composed of one or more of the above materials.

  The cathode serves to inject electrons into the light emitting unit. The material used as the cathode can be the material used for the anode, but a metal having a low work function is preferable for efficient electron injection, such as tin, magnesium, indium, calcium, aluminum, A suitable metal such as silver or an alloy thereof is used. Specific examples include low work function alloy electrodes such as magnesium-silver alloy, magnesium-indium alloy, and aluminum-lithium alloy.

  Further, for the purpose of protecting a cathode made of a low work function metal, further laminating a metal layer having a high work function and stable to the atmosphere on the cathode increases the stability of the device. For this purpose, metals such as aluminum, silver, copper, nickel, chromium, gold, platinum are used.

  Although there is no restriction | limiting in particular about the film thickness of a cathode, Usually, it is 1-1000 nm, Preferably it is 10-500 nm, and a thin film is formed on a light emitting unit by the method similar to an electron extraction layer. This cathode may have a single layer structure composed of one or more of the above materials.

Further, the cathode side light-emitting unit includes an alkali metal salt such as LiF, MgF ₂ , Li ₂ O, an alkaline earth metal salt, an alkali metal oxide, an alkaline earth metal salt, an alkali metal complex such as Liq, and Li, Cs. It is also an effective method for improving the efficiency of the device to provide alkali metal such as Ca, alkaline earth metal as the electron injection layer 4c.

  Moreover, after producing the said metal by the film thickness of 1 nm-20 nm to a cathode, the transparent or semi-transparent cathode can be produced by producing the electroconductive transparent material quoted by description of the anode on it, By applying this, it is possible to produce a device in which both the anode and the cathode are transparent.

  Note that the structure opposite to that shown in FIG. 1, that is, a cathode, a light emitting unit, a charge generation unit, and an anode can be laminated on the substrate in this order. It is also possible to provide the organic EL element of the present invention between the substrates. Also in this case, layers can be added or omitted as necessary. Further, in the present invention, the organic EL element can be applied to any of a single element, an element having a structure arranged in an array, and a structure in which an anode and a cathode are arranged in an XY matrix. .

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to these.

Example 1
In FIG. 2, each thin film was laminated at a vacuum degree of 1.0 × 10 ⁻⁵ Pa by a vacuum deposition method on a glass substrate on which an anode electrode made of ITO having a thickness of 110 nm was formed. First, NTCDA (Exemplary Compound 2) was formed on ITO as an electron extraction layer to a thickness of 10 nm at a deposition rate of 0.1 nm / s. Next, NPB (4,4′-bis (N- (1-naphthyl) -N-phenylamino) biphenyl) was formed as an adjacent layer to a thickness of 25 nm at a deposition rate of 0.1 nm / s. Next, on the adjacent layer, DNA (9,10-di (2-naphthyl) anthracene) and TBP (2,5,8,11-tetratertiary butylperylene) are used as the light emitting layer from different vapor deposition sources, and TBP. Was co-evaporated so as to have a thickness of 30 nm. Next, Alq3 (tris (8-quinolinolato) aluminum complex) was formed as an electron transport layer to a thickness of 30 nm at a deposition rate of 0.1 nm / s. Further, Alq3 and Liq ((8-quinolinolato) lithium complex) were co-deposited on the electron transport layer from different deposition sources so that Liq would be 25% by weight to form a thickness of 10 nm. . Finally, aluminum (Al) was formed as a cathode electrode on the electron injection layer to a thickness of 100 nm at a vapor deposition rate of 0.5 nm / s to produce one unit of an organic EL device. Here, the IP difference between EA and NPB of NTCDA is 0.7 eV.

Example 2
An organic EL device was produced in the same manner as in Example 1 except that the electron extraction layer was a mixed layer of NTCDA and NPB (weight ratio 50:50) 10 nm, and the adjacent layer was NPB 25 nm.

Comparative Example 1
An organic EL device was produced in the same manner as in Example 1 except that the electron extraction layer was omitted and the adjacent layer NPB was 35 nm.

When an external power source was connected to the obtained organic EL element and a DC voltage was applied, it was confirmed that the organic EL element had the light emission characteristics as shown in Table 1. In Table 1, luminance, voltage, and luminous efficiency are values at 100 A / m ² , and half-life is 250 A / m ² . The maximum wavelength of the device emission spectrum was 460 nm, indicating that light emission from TBP was obtained.

  As shown in Table 1, the devices of Examples 1 and 2 can achieve lower voltage driving, higher efficiency, and longer life as compared with Comparative Example 1.

Example 3
In FIG. 3, each thin film was laminated at a vacuum degree of 1.0 × 10 ⁻⁵ Pa by a vacuum deposition method on a glass substrate on which an anode electrode made of ITO having a thickness of 110 nm was formed. First, NTCDA was formed on ITO as an electron extraction layer to a thickness of 10 nm at a deposition rate of 0.1 nm / s. Next, NPB was formed as an adjacent layer to a thickness of 25 nm at a deposition rate of 0.1 nm / s. Next, on the adjacent layer, DNA and TBP were co-deposited as a light emitting layer from different vapor deposition sources so that the TBP was 1.0% by weight to form a thickness of 30 nm. Next, Alq3 was formed as an electron transport layer to a thickness of 30 nm at a deposition rate of 0.1 nm / s. Further, on the electron transport layer, Alq3 and Liq were co-deposited from different vapor deposition sources so that Liq was 25% by weight to form a thickness of 10 nm, and subsequently Al was added at 0.05 nm / s. An electron injection layer was formed by vapor deposition of 2 nm. Next, NTCDA, which is an electron extraction layer, was again formed to a thickness of 50 nm at the same rate as described above. Subsequently, the layers from the adjacent layer to the electron injection layer were formed in the same manner as described above. Finally, aluminum (Al) was formed as a cathode electrode on the electron injection layer to a thickness of 100 nm at a deposition rate of 0.5 nm / s, thereby producing a 2-unit organic EL device.

Comparative Example 2
An organic EL device was prepared in the same manner as in Example 2 except that the second unit electron extraction layer was omitted and the adjacent layer NPB was changed to 70 nm.

When an external power source was connected to the obtained organic EL element and a DC voltage was applied, it was confirmed that the organic EL element had the light emission characteristics as shown in Table 2. In Table 2, luminance, voltage, and luminous efficiency are values at 100 A / m ² and half-life is 250 A / m ² . The maximum wavelength of the device emission spectrum was 460 nm, indicating that light emission from TBP was obtained.

  As shown in Table 2, by forming a charge generation unit between a plurality of light emitting units, light can be emitted efficiently even with the same current, and a long life can be achieved.

The schematic cross section which showed an example of the organic EL element. Schematic cross-sectional view of an organic EL device having a light emitting unit and a charge generation unit each in one layer Schematic sectional view of an organic EL device having two layers each of a light emitting unit and a charge generating unit Diagram showing the function of electron extraction layer material and hole transport material Diagram showing the energy diagram of the charge generation unit

Explanation of symbols

1 substrate, 2 anode, 3 charge generation unit, 3a electron extraction layer, 3b adjacent layer, 4 light emitting unit, 4a light emitting layer, 4b electron transport layer, 4c electron injection layer, 7 cathode

Claims (9)

In an organic electroluminescent device including at least one light-emitting layer between an anode and a cathode facing each other, the electron-extracting layer and the electron-extracting layer are adjacent to the cathode side of the electron-extracting layer between the anode and the light-emitting layer. At least one charge generation unit comprising an adjacent layer from which electrons are extracted to the layer is provided, and the electron extraction layer is at least one selected from aromatic carboxylic acid derivatives represented by the following general formula (I) It consists of an electron extraction layer material containing an electron-accepting aromatic carboxylic acid derivative, and the difference between the ionization potential (IP) of the adjacent layer and the electron affinity (EA) of the electron extraction layer (IP-EA) is 0 to 2.0 eV. An organic electroluminescent device having a range.

(Wherein Ar represents an aromatic group having one or more aromatic rings, and —CO—X—CO— represents Ar
A divalent group that forms a heterocycle by condensation with an aromatic ring therein, X represents O or N—R,
R represents H or a monovalent substituent, and k represents 1, 2, 3 or 4. The aromatic group may be an aromatic group having a condensed aromatic ring or may have a substituent. ) The organic compound according to claim 1, wherein the aromatic carboxylic acid derivative represented by the general formula (I) is an aromatic carboxylic acid derivative represented by the following formula (1), (2), (3) or (4). Electroluminescent element.

(In the formulas (1), (2), (3) and (4), Ar ₁ , Ar ₂ and Ar ₃ each represent an aromatic ring condensed with an oxygen-containing ring or a nitrogen-containing ring, and the aromatic ring is One or more rings, a condensed ring, or a substituent may be present, Y represents a single bond or a divalent linking group, and R ₁ , R ₂ , and R ₃ represent a hydrogen atom or a substituent. And 1 represents 1, 2 or 3; m, n represents 0, 1 or 2; m + n is 1, 2, 3 or 4)   The organic electroluminescent device according to claim 1 or 2, wherein one layer of charge generation units is provided.   The organic electroluminescent element according to claim 1, wherein two or more light emitting units including a light emitting layer are provided, and a charge generation unit is provided between the light emitting units.   The organic electroluminescent element according to any one of claims 1 to 4, wherein the electron extraction layer is formed from an aromatic carboxylic acid derivative represented by the formula (1) or (2).   The organic electroluminescent element according to claim 1, wherein the adjacent layer is formed from a hole transporting material.   The organic electroluminescent device according to claim 6, wherein the hole transporting material is an arylamine-based hole transporting material.   The organic electroluminescent element according to claim 1, wherein the adjacent layer is formed from a light emitting layer material. The organic electroluminescent element according to claim 1, wherein the electron extraction layer is formed of a mixed layer of an electron extraction layer material and a hole transporting adjacent layer material.

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