JPH06158038A - Organic electroluminescent element - Google Patents

Abstract

PURPOSE:To obtain the subject element which effectively carries out the injection, recombination, and emission of electrons and positive holes and provides a highluminance emission at a low voltage applied by sequentially laminating an anode, specific luminescent layers, and a cathode. CONSTITUTION:This element is obtd. by sequentially laminating an anode, the first luminescent layer which comprises the first org. host substance (e.g. a compd. of formula I) capable of transporting positive holes and the first org. guest substance [e. g. a compd. of formula II (wherein R5 and R6 are each H, alkyl aryl, etc.)] having a smaller energy gap between the first oxidation potential and the first reduction potential (measured by cyclic voltammetry) than the energy gap of the first host substance, the second luminescent layer which comprises the second org. host substance (e.g. a compd. of formula III) capable of transporting electrons and the second guest substance having a smaller energy gap between the first oxidation potential and the first reduction potential (measured as above) than the energy gap of the second host substance, and a cathode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device provided with a light emitting layer in which such a substance is formed into a layer by utilizing the electroluminescence of a substance which emits light when an electric current is injected, and particularly the light emitting layer emits an organic compound. The present invention relates to an organic electroluminescence element configured as a body (hereinafter referred to as an organic EL element).

[0002]

2. Description of the Related Art As an organic EL device, as shown in FIG. 1, an organic light emitting layer 3 is provided between a metal cathode 1 and a transparent anode 2.
And a two-layer structure in which the organic hole transport layer 4 is arranged,
As shown in FIG. 2, a three-layer structure in which an organic electron transport layer 5, an organic light emitting layer 3, and an organic hole transport layer 4 are laminated between a metal cathode 1 and a transparent anode 2 is known ( JP-A-2-216790).

For the light emitting layer 3, an organic material having an electron transporting ability and a light emitting ability (high quantum yield) is used. The hole transport layer 4 has a function of transporting holes from the anode and a function of blocking electrons, and the electron transport layer 5 has a function of transporting electrons from the cathode. These organic EL
In the device, a glass substrate 6 is arranged outside the transparent anode 2, and excitons are generated by recombination of electrons injected from the metal cathode 1 and holes injected from the transparent anode 2 to the light emitting layer 3. The excitons emit light in the process of radiation deactivation, and the light is emitted to the outside through the transparent anode 2 and the glass substrate 6.

In the conventional organic EL device having a two-layer structure shown in FIG. 1, Japanese Patent Laid-Open Nos. 63-264692 and 2-6 are available.
There is a so-called guest-host type EL element in which a guest substance is dispersed only in the light emitting layer 3 as disclosed in No. 6873 and the like. This guest-host type light emitting layer, at least a host material having the ability to transport electrons, and a small amount of organic guest material capable of emitting light in response to recombination,
Composed of.

For the anode 2, a transparent conductive material having a large work function, such as indium tin oxide (hereinafter referred to as ITO) and tin oxide, which emits light emission to the outside is used. The cathode 1 is a single metal of aluminum (Al), magnesium (Mg), indium (In), silver (Ag) or an alloy of these Al-Mg, Ag-Mg, etc., and has a small work function. Is used.

[0006]

However, the light emitting layer in the organic EL device having the two-layer structure and the electron transporting layer in the organic EL device having the three-layer structure cannot be used unless they have high electron transporting ability, and the efficiency is high. In order to emit light at a low voltage well, it is necessary to combine the materials of the functional layers each having a high hole or electron transporting ability, and the selection range thereof is narrow, and a combination that allows sufficient light emission is desired.

Therefore, it is an object of the present invention to provide an organic EL device capable of efficiently injecting electrons and holes and performing recombination emission.

[0008]

The organic EL device of the present invention comprises an anode, a first organic host material capable of transporting holes and a first oxidation potential and a first reduction potential obtained by cyclic voltammetry measurement. A first light emitting layer comprising a first organic guest material having an energy gap smaller than that of the first organic host material, a second organic host material capable of transporting electrons, and a first obtained in cyclic voltammetry measurement. A second light emitting layer including a second organic guest material having an energy gap between the oxidation potential and the first reduction potential smaller than the energy gap of the second organic host material, and a cathode are sequentially stacked. That is, the organic EL element of the present invention is an organic EL element in which an anode, a hole-transporting layer made of a hole-transporting host material, an electron-transporting layer made of an electron-transporting host material, and a cathode are sequentially stacked. And a guest substance that contributes to light emission is dispersed in each of the hole transporting layer and the electron transporting layer to form a hole transporting light emitting layer and an electron transporting light emitting layer.

[0009]

According to the present invention, by dispersing the guest material throughout the organic functional layer between the electrodes, the chances of injection and recombination emission of electrons and holes are increased.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 3, the organic EL device of this example has an anode 2, a hole transporting light emitting layer 4a made of an organic compound, an electron transporting light emitting layer 3a made of an organic compound, and a metal cathode 1 on a glass substrate 6. Has a structure in which layers are sequentially stacked. Further, as shown in FIG. 4, in addition to the above configuration, an electron transport layer 5 made of an organic compound may be laminated between the light emitting layer 3 and the metal cathode 1 on the glass substrate 6.

The hole transporting compound which can be used as the host material of the hole transporting light emitting layer 4a in the above device structure is
A compound containing a tertiary amine that binds at least one aromatic ring, for example, N, N′-diphenyl-N, N′-bis (3-methylphenyl) -in formula 1
1,1'-biphenyl-4,4'-diamine (hereinafter T
PD) is preferably used, and other chemical formulas 2 and 3 are also used.
The compound known as CTM (Carrier Transporting Materials) shown in 1 can be used alone or as a mixture.

[0012]

[Chemical 1]

[0013]

[Chemical 2]

[0014]

[Chemical 3]

As the host substance, one having a large absolute value of potential on both the oxidation side and the reduction side in CV (cyclic voltammetry) measurement is used. As the guest substance of the hole-transporting light-emitting layer 4a, one having an absolute potential value smaller than that of the host substance on both the oxidation side and the reduction side in CV measurement, for example, a pyrimidopyrimidine derivative represented by Chemical Formula 4 is used.

[0016]

[Chemical 4]

{In the chemical formula 4, R _{5 and} R ₆ are each independently a hydrogen atom, an alkyl group, an aryl group, or R ₅ and R ₆ are taken together to complete a cycloalkyl group or a heterocyclic group. Representing an atom or a group} Specific pyrimidopyrimidine derivatives include depyridamole (Dipyrida) represented by the chemical formula 5 used as a fluorometric reagent.
mole) and this (2,6-bis (diethanolamino) -4,8-di
peridino Pyrimido [5,4-d] Pyrimidine) has an ethanolamino group at the 2,6 position of the pyrimido [5,4-d] pyrimidine skeleton and a piperidino group (pe
ridino).

[0018]

[Chemical 5]

Other pyrimidopyrimidine derivatives used as guest substances include, for example, tri (p-methoxyanilino) pyrimido [5,4-d] pyrimidine {2,4,8-tri (p
-Methoxyanlino) Pyrimido [5,4-d] Pyrimidine}, 2,4,
6,8-Tetramorpholinopyrimido [5,4-d] pyrimidine {2,4,6,8-tetramorpholino Pyrimido [5,4-d] Pyri
midine}, 2,4,6,8-tetraanilinopyrimide [5,4
-D] pyrimidine {2,4,6,8-tetraanilino Pyrimido
[5,4-d] Pyrimidine}, 2,4,6,8-tetra (p-methoxyanilino) pyrimido [5,4-d] pyrimidine {2,4,6,8
− Tetra (p-methoxyanlino) Pyrimido [5,4-d] Pyrimidin
e}, 2,4,6,8-tetrabenzylaminopyrimide [5,4
-D] pyrimidine {2,4,6,8-tetrabenzylamino Pyrimi
do [5,4-d] Pyrimidine}, 2,4,8-trimorpholinoaminopyrimido [5,4-d] pyrimidine {2,4,8-trimorpho
lino Pyrimido [5,4-d] Pyrimidine}, 2,4,8-tripyrrolidinopyrimidine [5,4-d] pyrimidine {2,4,8-t
ripyrrolidino Pyrimido [5,4-d] Pyrimidine}, 2,4,8−
Tetrapiperidino pyrimido [5,4-d] pyrimidine {2,4,6,8-tetrapiperidino Pyrimido [5,4-d] Pyrimidi
ne} and 2,4,8-tetrathiomorpholinopyrimido [5,4-d] pyrimidine {2,4,6,8-tetrathiomorpho
linoPyrimido [5,4-d] Pyrimidine}.

The larger the overlap between the excitation wavelength spectrum distribution of the guest substance and the fluorescence wavelength spectrum distribution of the host substance, the more efficiently the light emission occurs. The guest substance is selected from fluorescent dyes having a high fluorescence quantum yield, and 0.01 wt. % To 10 wt. It is preferably contained at a concentration of%. This is because light emission with high brightness can be obtained with a low applied voltage.

In the above device structure, the electron transporting light emitting layer 3
The compound that can be used as the host substance of a is a HOMO (highest occupied molecule) compound than the above hole transporting compound.
ar orbital: highest occupied molecular orbital or LUMO (lo
west unoccupied molecular orbital: lowest unoccupied molecular orbital)
Is a fluorescent substance having a large energy of, for example, Alq ₃ {tris (8-hydroxyquinolin
e) aluminium} and TPB {1,1,4,4-tetraphenyl-1,3-b
utasiene} or t-Bu-PBD {2- (4'-tert-butylp
henyl) -5- (4 ″ -biphenyl) -1,3,4-oxadiazole} and the like. These can also be used as the organic electron transport layer 5 in the case of a device having a three-layer structure.

[0022]

[Chemical 6]

[0023]

[Chemical 7]

[0024]

[Chemical 8]

The guest substance used as the light-emitting body of the electron-transporting light-emitting layer 3a emits extremely strong fluorescence, and the energy gap between the first oxidation potential and the first reduction potential obtained by CV measurement is that of the electron-transporting light-emitting layer 3a. It is a compound smaller than the energy gap of the host material. For example, the above pyrimidopyrimidine derivative is used. Specifically,
Each thin film was formed by continuous vapor deposition (resistance heating method) in a vacuum on an ITO substrate that had been subjected to a predetermined cleaning process, to produce an organic EL device. First, TPD is used as the first organic host material having a capability of transporting holes, which is the main component of the first light emitting layer, that is, the hole transporting light emitting layer 4a, and the first oxidation obtained as a second organic guest material in CV measurement. The film was formed by a co-evaporation method using depyridamole in which the energy gap between the potential and the first reduction potential was smaller than the energy gap of TPD.

Next, t-Bu-PBD is used as the first organic host material capable of transporting electrons, which is the main constituent of the second light emitting layer, that is, the electron transporting light emitting layer 3a, and CV measurement is performed as the second organic guest material. The energy gap between the first oxidation potential and the first reduction potential obtained in step is t-Bu-
A film was formed by a co-evaporation method using depyridamole having a smaller energy gap than PBD. Depyridamole was commonly used as the guest material, and Mg and A1 were prepared as the cathode by a co-evaporation method. The film thickness of each organic functional layer was 500 angstroms, and the cathode was 2000 angstroms.

As Comparative Example 1, except that a hole-transporting light-emitting layer in which depyridamole was added as a guest substance to the hole-transporting substance TPD and an electron-transporting layer consisting only of the electron-transporting substance t-Bu-PBD were formed. An electron transport / guest host type light emitting layer type EL device formed in the same manner as in (1) was produced. Also, as Comparative Example 2, except that a hole transport layer composed only of the hole transport material TPD and an electron transport light emitting layer in which depyridamole was added as a guest substance to the electron transport material t-Bu-PBD were formed. A guest-host type light emitting layer / hole transport layer type EL device formed in the same manner was produced.

Examples and Comparative Examples 1 and 2 are 500n
An electroluminescence (EL) spectrum having a peak around m was obtained. The emission spectrum peak of t-Bu-PBD is 3
62 nm, TPD emission spectrum peak is 396 nm
And the emission spectrum peak of depyridamole is 50.
From 0 nm and the shape of the emission spectrum, it was confirmed that the electroluminescence spectra of Example and Comparative Examples 1 and 2 were those of depyridamole.

Further, as shown in Table 1, a structure in which a guest material is dispersed in a hole transport layer which is a light emitting layer (Comparative Example 1)
The luminous efficiency at a constant current value is higher than that of the structure in which the guest material is dispersed in the electron transport layer (Comparative Example 2), but the structure in which the guest material is dispersed in both layers (Example ) Also showed a higher value. Electrons are injected from the cathode side, and the electrons move in the electron transport layer according to the electric field direction. Further, holes are injected from the anode side, and the holes move in the hole transport layer according to the electric field direction. It is considered that recombination of electrons and holes occurs near the interface between the hole transport layer and the electron transport layer, and as a result, excitons are generated, which is considered to be a state having no charge. It is considered that the excitons diffuse in all directions because the movement has little directivity. In the conventional structure as in Comparative Examples 1 and 2, the guest substance is dispersed only in the light emitting layer, and thus the excitons that have moved into the charge transport layer are extinguished as some kind of current or heat through some non-radiative process. It is thought that However, by using it as the host layer of both carrier transport layers as in the example, the above-mentioned energy, which has been wasted in the past, is effectively used because the opportunities for recombination and emission of both carriers are increased. Conceivable.

[0030]

[Table 1] * Current: I = 1 (mA / 4mm ² ) ** Current: I = 3.6 × 10 ⁻⁴ (A / 4mm ² ), Voltage:
V = 14.75 (V) *** Current: I = 1.07 × 10 ⁻⁴ (A / 4 mm ² ), Voltage: V = 14 (V) *** Current: I = 1.32 × 10 ^-4 (A / 4mm ² ),
Voltage: V = 15 (V)

[0031]

As described above, according to the present invention, the anode,
An organic EL device comprising a hole transport layer made of a host substance having a hole transporting property, an electron transporting layer made of a host substance having an electron transporting property, and a cathode, which are sequentially stacked. The layer and the electron-transporting layer are dispersed in the respective layers to form the hole-transporting light-emitting layer and the electron-transporting light-emitting layer, so that high-luminance light emission can be performed at a low applied voltage. According to this structure, even if the combination of the functional layers is constant, the organic EL
The device can emit light with higher brightness.

[Brief description of drawings]

FIG. 1 is a structural diagram showing an organic EL device having a two-layer structure.

FIG. 2 is a structural diagram showing an organic EL device having a three-layer structure.

FIG. 3 is a structural diagram showing an organic EL device having a two-layer structure according to an example of the present invention.

FIG. 4 is a structural diagram showing an organic EL device having a three-layer structure according to an example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cathode 2 Anode 3a Electron transporting light emitting layer 4a Hole transporting light emitting layer 5 Electron transporting layer 6 Glass substrate

Claims (2)

[Claims] 1. An anode, a first organic host material capable of transporting holes, and an energy gap of a first oxidation potential and a first reduction potential obtained by cyclic voltammetry measurement, the energy gap of the first organic host material. An energy gap between a first light emitting layer composed of a first organic guest material smaller than the gap, a second organic host material capable of transporting electrons, and a first oxidation potential and a first reduction potential obtained in cyclic voltammetry measurement. An organic electroluminescent device comprising: a second light emitting layer, which is composed of a second organic guest material having a smaller energy gap than the second organic host material, and a cathode, which are sequentially stacked. 2. The organic electroluminescence device according to claim 1, wherein an electron transport layer made of an organic compound is provided between the second light emitting layer and the cathode.