JP2007294261A - Organic electroluminescence device - Google Patents

Abstract

An organic electroluminescence element capable of suppressing a decrease in efficiency even when the driving voltage is reduced by reducing the thickness of a light emitting layer.
The present invention relates to an organic electroluminescence element formed with a plurality of light emitting layers 3 between an anode 1 and a cathode 2. Two adjacent light emitting layers 3a and 3b formed of a mixture of a plurality of materials and having different main components are provided, and the adjacent light emitting layers 3a and 3b have the electron mobility of the light emitting layer 3a located on the anode 1 side. The value divided by the hole mobility is a combination larger than the value obtained by dividing the electron mobility of the light emitting layer 3b located on the cathode 2 side by the hole mobility.
[Selection] Figure 1

Description

  The present invention relates to an organic electroluminescence element that can be used for a flat panel display, a backlight for a liquid crystal display, an illumination light source, and the like. Specifically, the organic electroluminescence element includes a plurality of light emitting layers and emits light with low voltage and high efficiency. The present invention relates to an electroluminescence element.

  An organic electroluminescence element is formed with at least one organic light emitting layer, and emits light from the organic light emitting layer by injecting current into the element. The efficiency of the organic electroluminescence device is generally discussed in terms of power efficiency (lm / W), which can be divided into two parameters: light emission efficiency (current efficiency; cd / A) and voltage (V). Here, the light emission efficiency (current efficiency) is a value indicating the light emission amount with respect to the energized current amount. Therefore, as a method for obtaining an organic electroluminescence device with improved power efficiency, that is, low power consumption, the light emission efficiency is There are a method for improving and a method for reducing the driving voltage.

  In the organic electroluminescence device, the thickness of the layer made of a material group having a high specific resistance such as a hole transport layer, a light emitting layer, and an electron transport layer is made thin, and the ratio of the hole injection layer and the electron transport layer having a low specific resistance is reduced. It is known that the drive voltage can be reduced by increasing it. However, in this method, as the light emitting layer is made thinner, there is a trade-off in which the driving voltage is reduced but at the same time the light emission efficiency is lowered. This is because the light-emitting region extends beyond the thickness of the light-emitting layer, and the light-emitting region extends to the adjacent electron transport material, hole transport material, and other regions. The cause is that.

  In order to solve this problem, a hole blocking layer is inserted on the cathode side of the light emitting layer and / or an electron blocking layer is inserted on the anode side of the light emitting layer (or an electron transport layer and a hole transport layer having these functions are provided). ) Methods have been proposed (see, for example, Patent Document 1).

For example, a bathophenanthroline derivative or the like is known as a hole block material, and Ir (ppz) ₃ or the like is known as an electron block material. However, the insertion of a hole block layer or the like often adversely affects the lifetime characteristics of the device. This is because holes are accumulated near the interface between the light-emitting layer and the hole-blocking layer due to the insertion of the hole-blocking layer, etc., and the luminous efficiency generally increases, but the accumulated carriers are not only in the light-emitting layer but also in the hole-blocking layer. This is thought to be due to the intrusion and simultaneous oxidation of the hole block material. Excessive oxidation is never preferable for the material constituting the organic electroluminescence element, and as a result, the life characteristics are adversely affected.
JP 2002-260865 A

  The present invention has been made in view of the above points, and provides an organic electroluminescence device capable of suppressing a decrease in luminous efficiency even when the driving voltage is reduced by reducing the thickness of the light emitting layer. It is intended to do.

  The organic electroluminescent device according to claim 1 of the present invention is an organic electroluminescent device formed by providing a plurality of light emitting layers between an anode and a cathode, and is formed of a mixture of a plurality of materials and having different main components. Two adjacent light-emitting layers are provided. The value obtained by dividing the electron mobility of the light-emitting layer located on the anode side by the hole mobility represents the electron mobility of the light-emitting layer located on the cathode side as a hole. It is characterized by comprising a combination larger than the value divided by the mobility.

  The invention of claim 2 is characterized in that, in two adjacent light emitting layers of claim 1, the electron mobility of the light emitting layer located on the anode side is larger than the electron mobility of the light emitting layer located on the cathode side. It is what.

  The invention of claim 3 is characterized in that, in two adjacent light emitting layers of claim 1 or 2, the thickness of the light emitting layer located on the anode side is smaller than the thickness of the light emitting layer located on the cathode side. It is what.

  The invention of claim 4 is characterized in that, in any two adjacent light emitting layers of claims 1 to 3, the thickness of the light emitting layer located on the anode side is 100 mm or less.

  A fifth aspect of the invention is characterized in that in any one of the first to fourth aspects, the light emitting layer has three or more layers.

  The invention of claim 6 is obtained in any one of claims 1 to 5, wherein the plurality of light emitting layers emit light by a combination of two colors having a complementary color relationship, or emit light by a combination of more colors. The emission color is white.

  According to the present invention, the value obtained by dividing the electron mobility of the light emitting layer located on the anode side by the hole mobility is larger than the value obtained by dividing the electron mobility of the light emitting layer located on the cathode side by the hole mobility. By providing the two adjacent light emitting layers, even when the driving voltage is reduced by reducing the thickness of the light emitting layer, it is possible to suppress a decrease in light emission efficiency.

  In addition, by appropriately combining the light emitting layers, it is possible to provide an organic electroluminescence element applicable to either a single color light emitting element or a white light emitting element.

  Hereinafter, the best mode for carrying out the present invention will be described.

  FIG. 1 shows an example of the structure of an organic electroluminescence element formed by laminating an organic light emitting layer 3 between an anode 1 and a cathode 2, and a transparent conductive film is formed on the surface of a translucent substrate 4. The anode 1 is laminated, the hole injection layer 5 and the hole transport layer 6 are laminated on the anode 1, and the light emitting layer 3 is laminated thereon. Furthermore, the electron transport layer 7 and the electron injection layer 8 are laminated on the light emitting layer 3, and the cathode 2 is laminated thereon. Hereinafter, the present structure will be described as an example. However, this structure is merely an example. For example, a hole blocking layer is formed between the light emitting layer 3 and the electron transport layer 7, and the structure of FIG. The structure is not limited.

  In the present invention, the light emitting layer 3 is formed of a plurality of layers. Of the plurality of light emitting layers 3, at least one pair of adjacent light emitting layers 3a and 3b is formed of a mixture of a plurality of materials, and the two light emitting layers 3a and 3b have different main components. is there. For example, when the adjacent light emitting layers 3a and 3b are formed of a so-called host-dopant mixture, the host material of each layer is different, but the dopant material is the same. Further, the plurality of materials constituting each layer may include only two types of host-dopant, or may include a third component that functions in an auxiliary manner. Furthermore, the so-called host-dopant relationship is not limited to the case where the main component host material occupies most of, for example, 80% by mass or more, but the main component host material is more than 50% by mass, and the subcomponent dopant material is 50% by mass. The mixing ratio may be almost the same as a little less than%.

  In the organic electroluminescence device of the present invention, the adjacent light emitting layers 3a and 3b are located on the cathode 2 side, which is obtained by dividing the electron mobility of the light emitting layer 3a located on the anode 1 side by the hole mobility. The light emitting layer 3b is formed in a combination that is larger than the value obtained by dividing the electron mobility by the hole mobility. By forming the adjacent light emitting layers 3a and 3b in such a combination, the hole and the cathode 2 that have moved from the anode 1 side in the vicinity of the interface between the light emitting layer 3a on the anode 1 side and the light emitting layer 3b on the cathode 2 side are formed. The electrons moving from the side can be efficiently recombined, carriers are not easily accumulated in the vicinity of the interface between the light emitting layers 3a and 3b, and the light emitting region is the interface between the light emitting layers 3a and 3b. Since it has the maximum value of the distribution in the vicinity, the recombination energy can be efficiently converted into light in the light emitting layer 3. The value obtained by dividing the electron mobility of the light emitting layer 3a located on the anode 1 side by the hole mobility is 10% of the value obtained by dividing the electron mobility of the light emitting layer 3b located on the cathode 2 side by the hole mobility. It is desirable to be larger than this, and it is more desirable to be larger than 25%.

  Here, the electron mobility and hole mobility of the light emitting layer 3 are values defined as the mobility of a layer made of a mixture of a plurality of materials, rather than the mobility of the main component constituting the light emitting layer 3. For example, it is obtained by time-of-flight measurement, dark injection measurement, calculation from current-voltage characteristics, etc. of a layer made of a mixture. For this reason, even when a material for forming a hole transporting light emitting layer is used in general, the electron mobility of the mixed layer may increase due to the influence of the mixed auxiliary material. Note that the balance of positive and negative carriers in the light emitting layer 3 depends on the relationship between the hole mobility and the electron mobility of the light emitting layer 3, and therefore, the relationship between both of the carrier mobility is larger than the magnitude relationship of either carrier mobility. It is largely determined by the ratio.

  Any element configuration may be used as long as the light emitting layer 3 corresponds to the present invention. As the material that can be used for the light emitting layer 3, any material known as an organic electroluminescence material can be used. For example, anthracene, naphthalene, pyrene, tetracene, coronene, perylene, phthaloperylene, naphthaloperylene, diphenylbutadiene, tetraphenyl Butadiene, coumarin, oxadiazole, bisbenzoxazoline, bisstyryl, cyclopentadiene, quinoline metal complex, tris (8-hydroxyquinolinato) aluminum complex, tris (4-methyl-8-quinolinato) aluminum complex, tris (5 -Phenyl-8-quinolinato) aluminum complex, aminoquinoline metal complex, benzoquinoline metal complex, tri- (p-terphenyl-4-yl) amine, 1-aryl-2,5-di (2-thieni ) Pyrrole derivatives, pyran, quinacridone, rubrene, distyrylbenzene derivatives, distyryl arylene derivatives, distyrylamine derivatives and various fluorescent pigments and the like as an example. Moreover, it is also preferable to mix and use the light emitting material selected from these compounds suitably. Moreover, not only the compound which produces | generates fluorescence emission represented by the said compound but the phosphorescence-emitting material which produces phosphorescence emission, and the compound which has the site | part which consists of them in one part in a molecule | numerator can also be used suitably.

  Further, in the organic electroluminescence device of the present invention, the adjacent light emitting layers 3a and 3b have an electron mobility of the light emitting layer 3a located on the anode 1 side from the electron mobility of the light emitting layer 3b located on the cathode 2 side. Is also preferably large. As described above, since the electron mobility of the light emitting layer 3a on the anode 1 side is large, an element structure that can be positively transported to the anode 1 side can be obtained, and the carrier balance in the light emitting layer 3a on the anode 1 side is further improved. By improving the ratio of light emission in the light emitting layer 3a, it becomes possible to improve the light emission efficiency of the entire organic electroluminescence element. The electron mobility of the light emitting layer 3a located on the anode 1 side is preferably twice or more larger than the electron mobility of the light emitting layer 3b located on the cathode 2 side.

  In the organic electroluminescence device of the present invention, in the adjacent light emitting layers 3a and 3b, the thickness of the light emitting layer 3a located on the anode 1 side is smaller than the film thickness of the light emitting layer 3b located on the cathode 2 side. It is preferable to form as follows. Thus, by reducing the film thickness of the light emitting layer 3a on the anode 1 side, it is possible to suppress an increase in driving voltage due to the light emitting layer 3, and to improve the efficiency by defining the light emitting region in a narrow range in the thickness direction. Improvement is expected. The thickness of the light emitting layer 3a on the anode 1 side is preferably set so that the ratio of the thickness to the total light emitting layer 3 is about 40% or less.

  Furthermore, the thickness of the light emitting layer 3a located on the anode 1 side among the adjacent light emitting layers 3a and 3b is preferably 100 mm or less. When the thickness of the light emitting layer 3a is 100 mm or less, an increase in driving voltage due to the light emitting layer 3 can be suppressed, and an improvement in efficiency can be expected by defining the light emitting region in a narrow range in the thickness direction. It is. The lower limit of the film thickness of the light emitting layer 3a is not particularly limited, but about 10 mm is practically the lower limit of the film thickness.

  By satisfying each of the above conditions, it is possible to suppress a decrease in luminous efficiency even when the thickness of the light emitting layer 3 is reduced.

  The light emitting layer 3 composed of a plurality of layers of the organic electroluminescence device of the present invention may be provided with at least the light emitting layers 3a and 3b having the above-described combination. In addition to the two adjacent light emitting layers 3a and 3b, Other light emitting layers 3c may be laminated.

  That is, as shown in FIGS. 2A and 2B, another light emitting layer 3c may be provided on the anode 1 side or the cathode 2 side of the two adjacent light emitting layers 3a and 3b. As in c), another light emitting layer 3c having the same or different composition may be provided on both the anode 1 side and the cathode 2 side. Specifically, two adjacent light emitting layers 3a and 3b composed of the above combinations are sequentially stacked from the anode 1 side to form a blue light emitting layer, and further an orange light emitting layer 3c is formed on the cathode 2 side. (Structure of FIG. 2A) Or an orange light emitting layer 3c is formed on the anode 1 side, and two adjacent light emitting layers 3a and 3b made of the above combination are stacked on the cathode 2 side to emit blue light An example of forming a layer (configuration in FIG. 2B) can be given. Further, two adjacent light emitting layers 3a and 3b made of the combination of the present invention are laminated in order from the anode 1 side, and then the same light emitting layer 3a located on the anode 1 side is further laminated to form a total of three layers. Forming in the light emitting layer 3 can also be mentioned as an example. These examples are cases where the light emitting layer 3 has a total of three layers, but the number of layers of the light emitting layer 3 may be more than that.

  Moreover, the organic electroluminescent element of this invention combines two colors which are complementary colors as the luminescent color of each layer of the light emitting layer 3, or makes each layer of the light emitting layer 3 light-emit by the combination of the number of colors beyond it. Thus, the emission color obtained from the organic electroluminescence element can be made white.

  In the organic electroluminescence device formed as described above, the material constituting the hole transport layer 6 and the hole injection layer 5 has the ability to transport holes, has the effect of injecting holes from the anode 1, and is organic. Examples thereof include a compound that has an excellent hole injection effect for the light emitting layer 3, prevents movement of electrons to the hole transport layer 6, and has an excellent thin film forming ability. Specific examples of the material constituting the hole transport layer 6 or the hole injection layer 5 are only a part of them. For example, phthalocyanine derivatives, naphthalocyanine derivatives, porphyrin derivatives, N, N′-bis (3-methylphenyl) -Aromatic diamines such as (1,1′-biphenyl) -4,4′-diamine (TPD) and 4,4′-bis [N- (naphthyl) -N-phenyl-amino] biphenyl (α-NPD) Compounds and substances corresponding to those multimers, stilbene derivatives, 4,4 ′, 4 ″ -tris (N- (3-methylphenyl) N-phenylamino) triphenylamine (m-MTDATA) are representative examples. Starburst amine compounds, 4,4′-N, N′-dicarbazole biphenyl (CBP) and polyvinyl carbazole, polysilane, polyester It can be given a polymer material such as range oxide thiophene (PEDOT).

  Moreover, as a material which comprises the electron carrying layer 7 and the electron injection layer 8, it has the capability to convey an electron, and has the electron injection effect from the cathode 2, and the outstanding electron injection effect with respect to the organic light emitting layer 3 In addition, a compound that prevents migration of holes to the electron transport layer 7 and has an excellent thin film forming ability can be given. Specific examples of materials constituting the electron transport layer 7 and the electron injection layer 8 include fluorene, bathophenanthroline, bathocuproine, anthraquinodimethane, diphenoquinone, oxazole, oxadiazole, triazole, imidazole, anthraquinodimethane, 4 , 4′-N, N′-dicarbazole biphenyl (CBP) and the like, and their compounds, derivatives and multimers, metal complex compounds or nitrogen-containing five-membered ring derivatives.

  Specific examples of the metal complex compound include tris (8-hydroxyquinolinato) aluminum, tri (2-methyl-8-hydroxyquinolinato) aluminum, tris (8-hydroxyquinolinato) gallium, bis ( 10-hydroxybenzo [h] quinolinato) beryllium, bis (10-hydroxybenzo [h] quinolinato) zinc, bis (2-methyl-8-quinolinato) (o-cresolate) gallium, bis (2-methyl-8-quinolinato) ) (1-naphtholate) aluminum and the like.

  The nitrogen-containing five-membered ring derivative is preferably an oxazole, thiazole, oxadiazole, thiadiazole or triazole derivative, specifically 2,5-bis (1-phenyl) -1,3,4-oxazole. 2,5-bis (1-phenyl) -1,3,4-thiazole, 2,5-bis (1-phenyl) -1,3,4-oxadiazole, 2- (4′-tert-butyl) Phenyl) -5- (4 ″ -biphenyl) 1,3,4-oxadiazole, 2,5-bis (1-naphthyl) -1,3,4-oxadiazole, 1,4-bis [2- (5-phenylthiadiazolyl)] benzene, 2,5-bis (1-naphthyl) -1,3,4-triazole, 3- (4-biphenylyl) -4-phenyl-5- (4-t- Butylphenyl) -1,2 4- triazole, etc. but not limited thereto.

  Further, the electron transport layer 7 or the electron injection layer 8 may be doped with alkali metal, alkaline earth metal, or rare earth. For example, a material formed by doping cesium with bathophenanthroline at a molar ratio of 1: 1 is exemplified.

  In addition, conventionally used materials can be used as they are for the substrate 4 holding the stacked elements, the anode 1, the cathode 2, and the like, which are members constituting the organic electroluminescence element.

  The substrate 4 is light transmissive when light is emitted through the substrate 4, and may be slightly colored or ground glass in addition to being colorless and transparent. . For example, a transparent glass plate such as soda lime glass or non-alkali glass, a plastic film or a plastic plate produced by an arbitrary method from a resin such as polyester, polyolefin, polyamide, or epoxy, or a fluorine resin can be used. . Further, a material that has a light diffusion effect by containing particles, powder, bubbles, etc. having a refractive index different from that of the base material forming the substrate 4 in the substrate 4 or imparting a shape to the surface can be used. . Further, when light is emitted without passing through the substrate 4, the substrate 4 does not necessarily have to be light transmissive, and may be made of any material as long as the light emission characteristics, life characteristics, etc. of the element are not impaired. Can be formed. In particular, the substrate 4 having high thermal conductivity can be used in order to reduce a temperature rise due to heat generation of the element during energization.

The anode 1 is an electrode for injecting holes into the element, and it is preferable to use an electrode material made of a metal, an alloy, an electrically conductive compound, or a mixture thereof having a large work function, and the work function is 4 eV or more. It is better to use one. As a material for such an anode 1, for example, metal such as gold, CuI, ITO (indium - tin _oxide), SnO 2, ZnO, IZO (indium - zinc oxide) conductive transparent material, and the like . The anode 1 can be produced, for example, by forming these electrode materials into a thin film on the surface of the substrate 4 by a method such as vacuum deposition or sputtering. In order to transmit light emitted from the organic light emitting layer 3 through the anode 1 and irradiate the light to the outside, the light transmittance of the anode 1 is preferably set to 70% or more. Further, the sheet resistance of the anode 1 is preferably several hundred Ω / □ or less, and particularly preferably 100 Ω / □ or less. Here, the film thickness of the anode 1 varies depending on the material in order to control the light transmittance, sheet resistance, and other characteristics of the anode 1 as described above, but is set to a range of 500 nm or less, preferably 10 to 200 nm. It is good.

Further, the cathode 2 is an electrode for injecting electrons into the organic light emitting layer 3, and it is preferable to use an electrode material made of a metal, an alloy, an electrically conductive compound and a mixture thereof having a small work function. Is preferably 5 eV or less. Examples of the electrode material for the cathode 2 include alkali metals, alkali metal halides, alkali metal oxides, alkaline earth metals and the like, and alloys thereof with other metals such as sodium, sodium-potassium alloys, Examples include lithium, magnesium, a magnesium-silver mixture, a magnesium-indium mixture, an aluminum-lithium alloy, and an Al / LiF mixture. Aluminum, Al / Al ₂ O ₃ mixture, etc. can also be used. Alternatively, an alkali metal oxide, an alkali metal halide, or a metal oxide may be used as the base of the cathode 2, and one or more conductive materials such as metals may be stacked and used. For example, an alkali metal / Al laminate, an alkali metal halide / alkaline earth metal / Al laminate, an alkali metal oxide / Al laminate, and the like can be given as examples. Further, a transparent electrode typified by ITO, IZO or the like may be used to extract light from the cathode 2 side. Furthermore, the organic layer at the interface of the cathode 1 may be doped with an alkali metal such as lithium, sodium, cesium, or calcium, or an alkaline earth metal.

  For example, the cathode 2 can be produced by forming these electrode materials into a thin film by a method such as a vacuum deposition method or a sputtering method. In order to irradiate the light emitted from the organic light emitting layer 3 to the anode 1 side, the light transmittance of the cathode 2 is preferably 10% or less. Conversely, when light emission is extracted from the cathode 2 side using the cathode 2 as a transparent electrode, the light transmittance of the cathode 2 is preferably 70% or more. In this case, the film thickness of the cathode 2 varies depending on the material in order to control the characteristics such as the light transmittance of the cathode 2 in this way, but it is usually 500 nm or less, preferably 100 to 200 nm.

  In addition, in an organic electroluminescent element, it is possible to use together each member and structure in the range which does not impair the meaning of this invention.

  Next, the present invention will be specifically described with reference to examples.

Example 1
As the substrate 4, a glass substrate having a thickness of 0.7 mm in which an anode 1 was formed by forming an ITO film having a thickness of 1100 mm was used. The sheet resistance of ITO is about 12Ω / □. This was subjected to ultrasonic cleaning with a detergent, ion-exchanged water and acetone for 10 minutes each, then steam cleaned with IPA (isopropyl alcohol), and then dried.

Next, this substrate 4 was set in a vacuum deposition apparatus, and 4,4′-bis [N- (naphthyl) -N-phenyl-amino was deposited on the anode 1 under a reduced pressure atmosphere of 1 × 10 ⁻⁴ Pa or less. The hole injection layer 5 was formed by vapor-depositing a biphenyl (α-NPD) and vanadium oxide co-evaporated body (molar ratio 1: 1) in a thickness of 600 mm. Next, α-NPD was deposited thereon to a thickness of 200 mm to form a hole transport layer 6.

  Next, a layer in which 4% of sty-NPD (formula 2) is co-deposited on DPYFL (formula 1) is formed as a light emitting layer 3a on the anode 1 side on the hole transport layer 6 with a thickness of 25 mm, and further on this As the light-emitting layer 3b on the cathode 2 side, a layer in which 4% of sty-NPD was co-evaporated on TBADN (formula 3) was formed to a thickness of 175 mm to form a light-emitting layer 3 composed of two layers.

  Next, bathophenanthroline (BCP: manufactured by Dojindo Laboratories Co., Ltd.) is deposited on the light emitting layer 3 to a thickness of 50 mm to form the electron transport layer 7, and BCP and Cs (cesium) are further added at a molar ratio of 1: The electron injection layer 8 was formed by co-evaporation at a rate of 1 to a thickness of 200 mm.

  Subsequently, aluminum was vapor-deposited on the electron injection layer 8 at a vapor deposition rate of 4 mm / s to a thickness of 800 mm to form the cathode 2, thereby obtaining an organic electroluminescence device having the configuration shown in FIG. 1.

The hole mobility and electron mobility were calculated by a time-of-flight method or calculation from current-voltage characteristics. As a result, DPYFL: sty-NPD forming the light emitting layer 3a on the anode 1 side has a hole mobility of 2 ×. 10 ⁻⁴ cm ² / Vs, the electron mobility was 1 × 10 ⁻³ cm ² / Vs, and the value obtained by dividing the electron mobility of the light emitting layer 3 a by the hole mobility was 5. TBADN: sty-NPD forming the light emitting layer 3b on the cathode 2 side has a hole mobility of 8 × 10 ⁻⁵ cm ² / Vs and an electron mobility of 1 × 10 ⁻⁴ cm ² / Vs. The value obtained by dividing the electron mobility of the layer 3b by the hole mobility was 1.25.

(Example 2)
Except that the light-emitting layer 3a on the anode 1 side is formed with a thickness of 175 mm, the light-emitting layer 3b on the cathode 2 side is formed with a thickness of 25 mm, and the light-emitting layer 3 composed of two layers is formed. Similarly, an organic electroluminescence element having the configuration of FIG. 1 was obtained.

(Comparative Example 1)
An organic electroluminescence device was obtained in the same manner as in Example 1 except that the light emitting layer 3 was formed as a single layer having a thickness of 200 mm in which 4% of sty-NPD was co-deposited on TBADN.

(Comparative Example 2)
The light emitting layer 3a on the anode 1 side is formed as a layer having a thickness of 175 mm by co-depositing 4% sty-NPD on TBADN, and the light emitting layer 3b on the cathode 2 side is formed by co-depositing 4% sty-NPD on DPYFL with a thickness of 25 mm. An organic electroluminescent device was obtained in the same manner as in Example 1 except that the above layer was formed.

(Comparative Example 3)
An organic electroluminescence device was obtained in the same manner as in Example 1 except that the light emitting layer 3 was formed as one layer having a thickness of 500 mm in which 4% of sty-NPD was co-deposited on TBADN.

(Example 3)
On the hole transport layer 6, as a yellow light emitting layer 3c, a layer in which 0.5% of rubrene was doped into TBADN was formed to a thickness of 50 mm. On the yellow light emitting layer 3c, a layer of 4% co-deposited sty-NPD on DPYFL is formed as a light emitting layer 3a on the anode 1 side with a thickness of 25 mm. Further, a light emitting layer 3b on the cathode 2 side is formed on TBADN with a sty- A layer in which 4% of NPD was co-evaporated was formed to a thickness of 175 mm, and the light emitting layer 3 consisting of a total of three layers was formed. Except these, it carried out similarly to Example 1, and obtained the organic electroluminescent element of the structure of FIG.2 (b).

(Comparative Example 4)
On the hole transport layer 6, as a yellow light emitting layer 3c, a layer in which 0.5% of rubrene was doped into TBADN was formed to a thickness of 50 mm. Then, 4% of sty-NPD was co-evaporated on TBADN as a light emitting layer 3b on this yellow light emitting layer 3c to form a 200 mm thick light emitting layer 3b, and a light emitting layer 3 consisting of two layers was formed. Except these, it carried out similarly to Example 1, and obtained the organic electroluminescent element.

Example 4
The light emitting layer 3a on the anode 1 side is formed as a 50 mm thick layer obtained by co-evaporating 4% of TBP (Formula 4) on TBADN, and the light emitting layer 3b on the cathode 2 side is formed of 4% TBP on CBP (Formula 5). An organic electroluminescent device having the structure shown in FIG. 1 was obtained in the same manner as in Example 1 except that the light-emitting layer 3 formed of a two-layered laminate was formed with a deposited layer having a thickness of 150 mm.

The hole mobility and the electron mobility were calculated by a time-of-flight method or calculation from current-voltage characteristics. As a result, TBP has a hole mobility of 9 × 10 ⁻ in TBADN forming the light emitting layer 3a on the anode 1 side. ^{The value was 5} cm ² / Vs, the electron mobility was 1 × 10 ⁻⁴ cm ² / Vs, and the value obtained by dividing the electron mobility of the light emitting layer 3a by the hole mobility was 1.1. CBP: TBP forming the light emitting layer 3b on the cathode 2 side has a hole mobility of 2 × 10 ⁻³ cm ² / Vs and an electron mobility of 3 × 10 ⁻⁴ cm ² / Vs, and this light emitting layer 3b The value obtained by dividing the electron mobility by the hole mobility was 0.15.

(Comparative Example 5)
The light emitting layer 3a on the anode 1 side is formed as a 50 mm thick layer of 4% TBP co-deposited on CBP, and the light emitting layer 3b on the cathode 2 side is formed as a 150 mm thick layer co-deposited with 4% TBP on TBAND. An organic electroluminescence device was obtained in the same manner as in Example 1 except that this was done.

As described above, the organic electroluminescence elements obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were connected to a power source (KEYTHLEY 2400), and the luminance and voltage values were measured when driven at a constant current of 10 mA / cm ² . . For luminance evaluation, “BM-9” manufactured by Topcon Corporation was used. The results are shown in Table 1.

  As can be seen from Table 1, the elements of each example had high light emission luminance, that is, current efficiency, and at the same time low driving voltage. On the other hand, the device of the comparative example resulted in low voltage but low efficiency (particularly comparative example 5 in which the light emitting layer was thin), and high efficiency but also high voltage (particularly in comparison with the thick light emitting layer). Example 3), which is a bad characteristic with respect to the characteristics of the device of the example.

It is the schematic which shows an example of the layer structure of the organic electroluminescent element of this invention. (A) (b) (c) is the schematic which shows the other example of a layer structure same as the above.

Explanation of symbols

1 Anode 2 Cathode 3 Light emitting layer

Claims (6)

  In an organic electroluminescence device formed with a plurality of light emitting layers between an anode and a cathode, the light emitting device has two adjacent light emitting layers formed of a mixture of a plurality of materials and having different main components. The layer has a combination in which the value obtained by dividing the electron mobility of the light emitting layer located on the anode side by the hole mobility is larger than the value obtained by dividing the electron mobility of the light emitting layer located on the cathode side by the hole mobility. An organic electroluminescence device characterized.   2. The organic electroluminescence according to claim 1, wherein, in the two adjacent light emitting layers, the electron mobility of the light emitting layer located on the anode side is larger than the electron mobility of the light emitting layer located on the cathode side. element.   3. The organic electroluminescence according to claim 1, wherein in the two adjacent light emitting layers, the thickness of the light emitting layer located on the anode side is smaller than the thickness of the light emitting layer located on the cathode side. element.   4. The organic electroluminescent element according to claim 1, wherein in the two adjacent light emitting layers, the thickness of the light emitting layer located on the anode side is 100 mm or less. 5.   The organic electroluminescent element according to claim 1, wherein the light emitting layer has three or more layers.   6. The light emitting layer according to claim 1, wherein the plurality of light emitting layers emit light in a combination of two colors having a complementary color relationship, or emit light in a combination of more colors, and the obtained emission color is white. An organic electroluminescence device according to any one of the above.

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