JP4915650B2 - Organic electroluminescence device - Google Patents

Description

  The present invention relates to an organic electroluminescence element used for an illumination light source, a backlight, a flat panel display, and the like, and particularly relates to an organic electroluminescence element provided with an improved electron transport layer.

  Light emitters used as illumination light sources, backlights, flat panel displays, etc. realize high-efficiency lighting fixtures, liberalization of lighting fixture shapes, downsizing of electronic devices equipped with liquid crystal displays, longer drive times, flat panel displays In recent years, there has been a strong demand for high efficiency, thinness and light weight in order to reduce the thickness.

  Organic electroluminescence devices have been attracting attention as light emitters that have a possibility of satisfying the above-mentioned requirements and have been actively researched and developed. In particular, in recent years, with the advent of phosphorescent light-emitting materials having a current-light conversion efficiency of 100% in principle, the efficiency of organic electroluminescent elements has increased dramatically, and the practical area of organic electroluminescent elements has expanded greatly. I came. Already, with respect to monochromatic light-emitting elements such as green and red, a high-efficiency light-emitting element considered to correspond to a current-light conversion efficiency of 100% has actually been realized as an actual device. As for blue light-emitting elements, the development of light-emitting materials and peripheral materials suitable for blue light-emitting elements has been slow, and development of organic electroluminescent elements having other light-emitting colors has been delayed. Accordingly, light emitting materials and peripheral materials suitable for blue light emitting elements have been developed, and the efficiency of blue light emitting elements has improved to the same or higher level than other colors. Also, a white light emitting element having a high performance such as 60 lm / W and an external quantum efficiency of 30% has been reported.

As described above, in the organic electroluminescence device, since the efficiency is approaching a so-called theoretical value, recently, research from the viewpoint of extending the lifetime of the device is actively performed. For example, it is considered that the life extension by using a new material is realized by improving the thermal stability and electrical stability of the material. However, when the initial luminance is 1000 cd / m ² , the half-life is 100,000. Values such as time are also reported.

In order to achieve a long lifetime from the viewpoint of the device structure, for example, Patent Document 1 discloses a method of doping a carrier transport layer with a carrier transport dopant, and Patent Document 2 describes a method for doping a carrier transport layer. According to a method of doping a dopant having a specific energy level, Patent Document 3 further discloses that an organic electroluminescence device has an electron transport layer containing a hole trap material having an oxidation potential smaller than that of the electron transport material. It is described that the lifetime is improved.
JP 2000-164362 A Japanese Patent No. 3332491 JP 2005-276665 A

However, for example, when an organic electroluminescence element is applied to illumination, it is necessary to use the current fluorescent lamp at a luminance of several thousand to 10,000 cd / m ² , and the lifetime at that time is shorter than the above-mentioned lifetime. For example, it drops to about several thousand hours. In addition, when an organic electroluminescence element is applied to a display, the occurrence of image sticking, that is, luminance degradation of about 5% is considered to be a lifetime, but in this case, the lifetime is limited to about several thousand hours. .

  Therefore, it is necessary to further study the extension of the lifetime of the organic electroluminescence element from the viewpoint of the device structure as well as the improvement of the material.

  The present invention has been made in view of the above points, and it is an object of the present invention to provide an organic electroluminescence device which can achieve a longer lifetime by improving an electron transport layer.

The organic electroluminescence device according to claim 1 of the present invention includes an organic light-emitting layer 3 and an electron transport layer 5 which are formed between two opposing electrodes 1 and 2 and contain a light-emitting dopant in a host material. In the organic electroluminescence device having the above structure, the main component constituting the electron transport layer 5, the host material of the organic light emitting layer 3, and the electron transport layer 5 are formed on at least a portion of the electron transport layer 5 in contact with the organic light emitting layer 3. And a mixed layer 6 in which an organic semiconductor material different from the host material of the organic light emitting layer 3 is mixed, and the host material of the organic light emitting layer 3 in the mixed layer 6 an electron transport layer 5 mass mixing ratio of the main constituent of the 1: 99-50: a 50, in the mixing layer within 6, the concentration of the organic semiconductor material, the side of the organic light-emitting layer 3 Higher than the side of the cathode 2, the organic semiconductor material mixed in the mixed layer 6 is characterized in that contribution to light emission is not more than 5%.

According to the present invention, by forming the mixed layer 6 as described above in the electron transport layer 5, deterioration due to the presence of a carrier injection barrier between the organic light emitting layer 3 and the electron transport layer 5, and the electron transport layer 5 can reduce the oxidative deterioration of the electron transport material due to the hole penetration into 5, and can improve the lifetime of the organic electroluminescence element. In particular, holes are trapped in the mixed layer 6 by mixing an organic semiconductor material having an ionization potential lower than that of the main component constituting the electron transport layer 5 and different from the host material of the organic light emitting layer 3. In addition, the deterioration of the electron transport layer 5 can be more efficiently suppressed, and the adverse effect on the efficiency characteristics that may occur with the formation of the mixed layer 6 can be avoided. Further, the portion of the mixed layer 6 that is more susceptible to deterioration by holes can be actively protected by the host material of the organic light emitting layer 3 and the organic semiconductor material, and the portion that is not easily damaged by holes is minimized. Thus, a trade-off with other characteristics such as light emission characteristics can be avoided, and an organic electroluminescence device with high efficiency and long life can be obtained.

  The invention of claim 2 is characterized in that, in claim 1, the ionization potential of the organic semiconductor material mixed in the mixed layer 6 is less than or equal to the ionization potential of the host material of the organic light emitting layer 3.

  According to the present invention, the efficiency of hole trapping by the organic semiconductor material in the mixed layer 6 is improved, deterioration of the electron transport layer 5 can be suppressed more favorably, and the life can be further extended. It is possible.

  The invention of claim 3 is characterized in that, in claim 1 or 2, the organic semiconductor material mixed in the mixed layer 6 is the same substance as the luminescent dopant contained in the organic light emitting layer 3. Is.

  According to the present invention, holes that have traveled from the organic light emitting layer 3 to the mixed layer 6 are trapped more efficiently by the organic semiconductor material, and the lifetime of the device can be further increased. In addition, when there are many holes entering the mixed layer 6, it is possible to reduce the influence of the light emission from the organic semiconductor material on the light emission from the organic light emitting layer 3.

  According to the present invention, an organic electroluminescent device comprising an organic light emitting layer 3 and an electron transport layer 5 formed between a pair of opposing electrodes 1 and 2 containing a light emitting dopant in a host material. Thus, an organic electroluminescence device exhibiting a long life and high efficiency can be obtained.

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

  The organic electroluminescence device according to the present invention is formed by providing an organic light emitting layer 3 between two electrodes 1 and 2, that is, an anode (anode) and a cathode (cathode). FIG. 1 shows an example of the structure of such an organic electroluminescence element. An organic light emitting layer 3 formed between an electrode 1 serving as an anode and an electrode 2 serving as a cathode, an organic light emitting layer 3 and an electrode 1. A hole transport layer 4 formed between the organic light-emitting layer 3 and the electrode 2, and an electron transport layer 5 on the side of the organic light-emitting layer 3. A mixed layer 6, which is a characteristic configuration of the present invention, is formed in a part in contact with the organic light emitting layer 3, and these are laminated on the surface of the substrate 7. In the embodiment of FIG. 1, the electrode 1 is formed as a light transmissive electrode on the surface of a transparent substrate 7, and the electrode 2 is formed as a light reflective electrode. Further, a hole injection layer, an electron injection layer, or the like may be provided on the side of the electrodes 1 and 2 of the hole transport layer 4 and the electron transport layer 5, but these are not shown in FIG.

  In the present invention, the mixed layer 6 formed in at least a part of the portion of the electron transport layer 5 in contact with the organic light emitting layer 3 includes a main component constituting the electron transport layer 5, a host material of the organic light emitting layer 3, and An organic semiconductor material having an ionization potential equal to or lower than the ionization potential of the main component constituting the electron transport layer 5 and different from the host material of the organic light emitting layer 3 is mixed. Here, the main component constituting the electron transport layer 5 refers to an electron transport material in which the content ratio in the electron transport layer 5 exceeds 25 mass%, and when there are a plurality of types of materials exceeding 25 mass%. May be any material. The ionization potential is a value calculated by photoelectron spectroscopy measurement.

  The mixing ratio of the above-described components in the mixed layer 6 is not particularly limited, but the main component constituting the electron transport layer 5 and the host material of the organic light emitting layer 3 have a mass of 99: 1 to 1:99. It is preferable to mix in the range of a ratio.

  Here, the mixed concentration of the main component of the electron transport layer 5 and the host material of the organic light emitting layer 3 may be constant in the thickness direction of the mixed layer 6, but the mixed layer 6 is formed of a plurality of layers. Thus, the concentration of the host material of the organic light emitting layer 3 may be high in the layer on the organic light emitting layer 3 side, and the concentration of the host material of the organic light emitting layer 3 may be low in the layer on the electrode 2 side serving as the cathode. Alternatively, the mixed layer 6 is formed as a single layer, the side close to the organic light emitting layer 3 has a high concentration of the host material of the organic light emitting layer 3, and the side close to the electrode 2 has a low concentration of the host material of the organic light emitting layer 3. As such, the mixed concentration may be inclined. Thus, by making the concentration of the host material of the organic light emitting layer 3 in the mixed layer 6 high on the organic light emitting layer 3 side and low on the cathode electrode 2 side, The portion on the side of the organic light emitting layer 3 that is more susceptible to deterioration by holes can be protected by hole trapping by the host material of the organic light emitting layer 3, and the portion that is not easily damaged by holes is the organic light emitting layer. 3 is made low so that the ratio of the host material of the organic light emitting layer 3 in the entire mixed layer 6 does not become too high, and the host material in the mixed layer 6 has the light emission characteristics of the organic electroluminescence device. It is possible to prevent adverse effects.

  Further, in the mixed layer 6, the ionization potential of the host material of the organic light emitting layer 3 and the ionization potential of the main component constituting the electron transport layer are the ionization potential of the host material of the organic light emitting layer 3 ≦ the main component constituting the electron transport layer 5. The ionization potential is preferably set so as to satisfy the following relationship. In particular, when the difference in ionization potential between the two is small, the organic light emitting layer has a mixing mass ratio of the host material of the organic light emitting layer 3 and the main component constituting the electron transport layer 5 of about 1:99 to 20:80. It is preferable to reduce the content of the host material 3.

  Thus, when the ionization potential of the host material of the organic light emitting layer 3 is equal to or lower than the ionization potential of the main component constituting the electron transport layer 5, holes are trapped by the host material of the organic light emitting layer 3 in the mixed layer 6. This improves the efficiency.

  Furthermore, an organic semiconductor material (hereinafter simply referred to as an organic semiconductor material) having an ionization potential equal to or lower than the ionization potential of the main component constituting the electron transport layer 5 mixed with the mixed layer 6 and different from the host material of the organic light emitting layer 3 is used. ) Is preferably a mass ratio of 0.1 / 100 times to 2 times the total amount of the above two components (the main component of the electron transport layer 5 and the host material of the organic light emitting layer 3). Further, as described later, it is preferable to adjust the organic semiconductor material so that the contribution ratio to the light emission is 5% or less, and generally a mass ratio of 1/100 to 1 times is particularly preferable. .

  Here, the mixed concentration of the organic semiconductor material may be constant in the thickness direction of the mixed layer 6 as described above, but the mixed layer 6 is formed of a plurality of layers, and the organic light emitting layer 3 side. This layer may have a high concentration of the organic semiconductor material, and the layer on the electrode 2 side serving as the cathode may have a low concentration of the organic semiconductor material, or the mixed layer 6 may be formed as a single layer to form an organic light emitting layer. The concentration of the organic semiconductor material may be inclined such that the side close to 3 has a high concentration of the organic semiconductor material and the side close to the electrode 2 has a low concentration of the organic semiconductor material. In this way, the concentration of the organic semiconductor material in the mixed layer 6 is higher on the organic light emitting layer 3 side and lower on the cathode electrode 2 side, thereby further deteriorating due to holes in the mixed layer 6. The portion on the side of the organic light-emitting layer 3 that is susceptible to damage can be protected by trapping holes by the organic semiconductor material, and the portion that is not easily damaged by holes is reduced in the concentration of the organic semiconductor material and mixed. The ratio of the organic semiconductor material in the entire layer 6 is prevented from becoming too high, and the organic semiconductor material in the mixed layer 6 can be prevented from adversely affecting the light emission characteristics of the organic electroluminescence element.

  In the present invention, the organic semiconductor material is not particularly limited as long as the above conditions are satisfied, but it is not limited to aromatic hydrocarbon compounds, arylamine derivatives, styrylamine derivatives, anthracene derivatives, perylene derivatives, tetracene derivatives, Any of aluminum-organic complexes, quinacridone derivatives, styrylarylene derivatives, organic-metal complexes, and the like can be used. Examples include perylene having four tertiary butyl groups, rubrene, pentacene having four phenyl groups, α-NPD, and bisnaphthylanthracene, but are not limited thereto. In addition, the organic semiconductor material preferably exhibits reversible oxidation-reduction characteristics. Select a preferred organic semiconductor material from materials that exhibit a reproducible redox profile in multiple oxidation-reduction processes, for example, by cyclic voltammetry measurement, although the redox conditions are not the same as in the device. Can do.

  In the present invention, it is necessary that the contribution ratio of the organic semiconductor material mixed in the mixed layer 6 to light emission is 5% or less. The contribution ratio of the organic semiconductor material to light emission refers to the ratio of the light emission amount of the organic semiconductor material in the mixed layer 6 to the light emission amount of the entire organic electroluminescence element. In the organic electroluminescence element, light emission is mainly performed in the organic light emitting layer 3, but the fact that the organic semiconductor material in the mixed layer 6 emits light means that excess holes are injected from the organic light emitting layer 3 into the mixed layer 6. That is, when the contribution ratio of the light emission by the organic semiconductor material in the mixed layer 6 to the entire light emission of the organic electroluminescent element exceeds 5%, the mixed layer 6 is likely to be deteriorated due to the presence of excess holes. It is. The smaller the contribution ratio of light emission by the organic semiconductor material, the better, and the contribution ratio of 1% or less is more preferable.

  As a method for reducing the contribution ratio of the organic semiconductor material to light emission to 5% or less, a known organic electroluminescence element design method can be arbitrarily used. For example, an electron transport material having a high electron transport property is used. Forming an electron transport layer 5, using an electron injection layer or cathode capable of better electron injection, or increasing the thickness of the organic light emitting layer 3 or hole transport layer 4. It is preferable to use at least one of a method of increasing and a method of reducing the hole transport amount.

  Here, calculation of the contribution ratio of the organic semiconductor material to light emission will be described. When the organic semiconductor material in the mixed layer 6 also emits light, the emission spectrum of the organic electroluminescence element is a combination of the organic light emitting layer 3 and the organic semiconductor material. Then, the emission spectrum of the organic electroluminescence element not containing the organic semiconductor material in the mixed layer 6 and the emission spectrum of the organic semiconductor material are measured in advance, and the two emission spectra and the mixed layer 6 described above are measured. Compared with the emission spectrum of an organic electroluminescence device containing an organic semiconductor material, the percentage of the emission spectrum of this device has two emission spectra, that is, what percentage of the two emission spectra are mixed. It is possible to determine the contribution ratio of the organic semiconductor material to the light emission by calculating whether it is a light source. Moreover, when the organic-semiconductor material in the mixed layer 6 is the same substance as the luminescent dopant contained in the organic light emitting layer 3, the spectrum emitted from the position of the organic light emitting layer 3 of the organic electroluminescence element, Since the spectrum emitted from the misaligned mixed layer 6 is slightly different, an element not containing an organic semiconductor material is prepared in the mixed layer 6 as a model device, and then compared with the emission spectrum of this model device. Then, the contribution rate can be obtained by performing the spectrum analysis.

  By providing the mixed layer 6 having the above composition on at least a part of the electron transport layer 5 on the organic light emitting layer 3 side, an organic electroluminescence device exhibiting a long life and high efficiency can be obtained. . That is, first, the host material of the organic light emitting layer 3 and the main component constituting the electron transport layer 5 are contained as constituents of the mixed layer 6, so that an energy barrier is formed at the interface between the organic light emitting layer 3 and the electron transport layer 5. It is conceivable that the total number of carriers accumulated at the interface can be reduced, and the deterioration of the electron transport layer 5 due to the carriers accumulated at the interface can be reduced. In addition, the host material of the organic light-emitting layer 3 can suppress deterioration due to holes in the electron transport material as one of the reasons for extending the lifetime. Alternatively, it is conceivable as one of the reasons that deterioration of the electron transport layer 5 due to holes can be suppressed by reducing the hole transport property of the mixed layer 6. Further, a local electric field is generated by the holes trapped in the host material of the organic light emitting layer 3, and electrons are injected more efficiently. As a result, the life characteristics can be improved by reducing the driving voltage and disappearing the holes. It is also possible to do it.

  However, if the mixed layer 6 is simply a mixture of the host material of the organic light emitting layer 3 and the main component constituting the electron transport layer 5, the device lifetime may not change or may be shorter depending on circumstances. This is because a clear carrier injection barrier is not formed at the interface between the organic light emitting layer 3 and the electron transport layer 5, so that holes penetrate deeper into the electron transport layer 5 side and the host material of the organic light emitting layer 3 is This is because the deterioration of the electron transport material of the electron transport layer 5 is not reduced or conversely promoted by the inability to sufficiently exhibit such an effect.

  In order to cope with such a problem, in the present invention, by mixing an organic semiconductor material having an ionization potential smaller than the ionization potential of the main component material of the electron transport layer 5 in the mixed layer 6, Is limited to reach the electron transport layer 5 or suppresses the oxidative deterioration of the electron transport material of the electron transport layer 5, thereby extending the life of the organic electroluminescence element. This is because holes that have progressed to the electron transport layer 5 side without recombination in the organic light emitting layer 3 are trapped in the mixed layer 6 by the host material of the organic light emitting layer 3 and also in the organic semiconductor material. By doing so, the progress of holes to the electron transport layer 5 can be strongly restricted, the hole mobility of the mixed layer 6 can be reduced, and the progress of holes to the electron transport layer 5 can be restricted, an organic semiconductor The reason is that holes that have entered the mixed layer 6 due to the presence of the material are converted into light or heat and lost without adversely affecting the mixed layer 6 and the electron transport layer 5, and the result. As a result, it is possible to suppress the deterioration of the electron transport layer 5 due to holes and to extend the lifetime of the organic electroluminescence element.

  Here, it is more preferable that the organic semiconductor material mixed in the mixed layer 6 has a small ionization potential with respect to the ionization potential of the host material of the organic light emitting layer 3. By having an ionization potential smaller than the ionization potential of the host material of the organic light emitting layer 3 as well as the main component constituting the electron transport layer 5, the organic light emitting layer 3 in the mixed layer 6 is made of the host material of the organic light emitting layer 3 by the organic semiconductor material. Deterioration can also be suppressed, or the progress of holes entering the mixed layer 6 to the electron transport layer 5 can be more strongly restricted, and the life of the organic electroluminescence element can be extended. It is something that can be done.

  Furthermore, it is also preferable to use the same substance as the light-emitting dopant contained in the organic light-emitting layer 3 as the organic semiconductor material. In this case, holes that have traveled from the organic light emitting layer 3 to the mixed layer 6 are trapped by the organic semiconductor material, so that the organic electroluminescence element can have a longer lifetime. In addition, when there are many carriers entering the mixed layer 6 and the organic semiconductor material in the mixed layer 6 emits light, the light emission color is the same as the light emission color from the organic light emitting layer 3. It is possible to reduce the influence on the image.

  In the present invention, the entire electron transport layer 5 may be formed of the mixed layer 6, and the mixed layer 6 may be formed only at a part of the electron transport layer 5 on the organic light emitting layer 3 side. May be. When the mixed layer 6 is a part of the electron transport layer 5, the thickness is preferably 0.2 nm or more. If the thickness is equal to or less than this thickness, the effect of forming the mixed layer 6 cannot be sufficiently obtained. Particularly preferred is a thickness in the range of 0.5 nm or more. The thickness of the mixed layer 6 and the fraction of the thickness of the mixed layer 6 in the electron transport layer 5 are appropriately set according to desired element characteristics.

  In the present invention, by providing the mixed layer 6 having the above-described configuration in the electron transport layer 5, it is possible to suppress the injection of excess holes into the electron transport layer 5 during energization. Therefore, it is possible to extend the life of the organic electroluminescent element by preventing the deterioration due to the holes. In particular, since the mixed layer 6 is a layer composed of the main material constituting the host material of the organic light emitting layer 3 and the electron transport layer 5, it is possible to simultaneously improve the characteristics such as reduction of the driving voltage.

  The organic electroluminescence device according to the present invention has a structure in which the organic light emitting layer 3 contains a light emitting dopant in the host material and the electron transport layer 5 is provided on the cathode side of the organic light emitting layer 3. The structure of other parts is not particularly limited. Hereinafter, with respect to the organic electroluminescence device having a structure including the substrate 7 / electrode (anode) 1 / hole transport layer 4 / organic light emitting layer 3 / mixed layer 6 / electron transport layer 5 / electrode (cathode) 2 shown in FIG. An example of the material will be described.

  The material constituting the hole transport layer 4 has the ability to transport holes, has a hole injection effect from the anode, has an excellent hole injection effect with respect to the organic light emitting layer 3, and has an electron The compound which prevents the movement to the hole transport layer 4 and has an excellent thin film forming ability can be mentioned. Examples of such compounds include 4,4′-bis [N- (naphthyl) -N-phenyl-amino] biphenyl (α-NPB or α-NPD), N, N′-bis (3-methylphenyl). )-(1,1′-biphenyl) -4,4′-diamine (TPD), spiro-NPD, spiro-TPD, spiro-TAD, TNB (the phenyl group of α-NPD is a naphthyl group), TPBD (The phenyl group and the naphthyl group of α-NPD are biphenyl groups, and the position of the substitution bond is not limited.) And the like. In particular, in the case of a phosphorescent light emitting device, the energy gap and / or T1 level of the dopant of the organic light emitting layer 3 and the energy gap and / or larger than the energy gap and / or T1 level of the host of the organic light emitting layer 3 A wide energy gap material having a T1 level is preferable, and examples of such a compound include a triarylamine derivative having a tetraphenylsilane skeleton, a site having no conjugated ring such as a cyclohexane ring, and the like. And triarylamine derivatives having a wide energy gap such as a triarylamine derivative, a methyl group-substituted biphenyl skeleton, a quarterphenylene skeleton, and a hexaphenylbenzene skeleton. Alternatively, an amine compound containing a carbazole group, an amine compound containing a fluorene derivative, or the like is also used as appropriate depending on the aforementioned characteristics.

  Further, the material constituting the electron transport layer 5 has the ability to transport electrons, has an electron injection effect from the cathode, and has an excellent electron injection effect with respect to the organic light emitting layer 3. Furthermore, the compound which prevented the movement to the electron carrying layer 5 of a hole, and was excellent in the thin film formation capability can be mentioned. Examples of such compounds include compounds having a heterocyclic ring and derivatives thereof such as bathophenanthroline, bathocuproin, oxazole, oxadiazole, triazole, imidazole, pyridine, furan, and phenanthroline. In particular, in the case of a phosphorescent light emitting device, the energy gap and / or T1 level of the dopant of the organic light emitting layer 3 and the energy gap and / or larger than the energy gap and / or T1 level of the host of the organic light emitting layer 3 A wide energy gap material having a T1 level is preferable. Examples of such compounds include 1,3,5-Tris [3,5-bis (3-pyridinyl) phenyl] benzene, 1,3,5-tri (4-pyrid-3-yl-phenyl) benzene Examples thereof include, but are not limited to, derivatives containing a pyridine ring, pyridine derivatives containing a trimesitylborane skeleton, and the like.

A hole injection layer may be provided between the hole transport layer 4 and the anode (electrode 1), and an electron injection layer may be provided between the electron transport layer 5 and the cathode (electrode 2). These hole injection layer and electron injection layer are formed by using the materials constituting the hole transport layer 4 and the electron transport layer 5 and other materials and materials excellent in carrier injection from the electrodes 1 and 2 alone. Or metals, semiconductors, organic materials, metal oxides, metal carbides, metal borides, metal nitrides, acceptor gases, etc. that form charge transfer complexes with organic materials, Lewis acids, Lewis bases, or Bronsted acids Alternatively, a material that functions as a Bronsted base may be mixed or laminated. For example, the hole injection layer is a phthalocyanine compound, a porphyrin compound, a starburst amine derivative, a triarylamine derivative, a thiophene derivative or other low molecular compound capable of donating electrons, a high molecular compound, etc. For example, it can be formed by mixing or laminating with molybdenum oxide, rhenium oxide, tungsten oxide, vanadium oxide, bromine, iron chloride, titanium chloride, F4TCNQ, DDQ, acid anhydride, or the like. In addition, the electron injection layer may be, for example, any material such as the above-described electron transport layer, phthalocyanines, porphyrins, other low molecular compounds capable of accepting electrons, high molecular compounds, alone, alkali metal, alkaline earth, or the like. It can be formed by using a mixed metal or a stacked layer of organic donors such as metal species, rare earth metals, or [Chemical Formula 1]. Further, a structure in which a mixed or laminated film is formed by releasing a metal component by decomposing or reducing a metal compound at the time of film formation or after film formation may be used. For example, when Cs is mixed with BCP (bathocuproine), when Liq ([Chemical Formula 2]) is laminated on Alq and then Al is deposited to generate Li metal due to the reduction at the interface, Cs ₂ CO _{3 is} added to BCP. An example of this is when layers are laminated or mixed.

  Moreover, as a luminescent dopant used for formation of the organic light emitting layer 3 of an organic electroluminescent element, the arbitrary materials known as a luminescent material for organic electroluminescent elements can be used. For example, anthracene derivatives, pyrene derivatives, tetracene derivatives, fluorene derivatives, perylene derivatives, coumarin derivatives, oxadiazoles, styrylamine derivatives, quinoline metal complexes, tris (8-hydroxyquinolinato) aluminum complexes, tris (4-methyl-8) -Quinolinato) aluminum complex, tris (5-phenyl-8-quinolinato) aluminum complex, quinacridone, rubrene, distyrylbenzene derivatives, distyrylarylene derivatives, and various fluorescent dyes. It is not limited. Moreover, it is also preferable to mix and use the light emitting material selected from these compounds suitably. Further, not only a compound that emits fluorescence, typified by the above-described compound, but also a material system that emits light from a spin multiplet, for example, a phosphorescent material that emits phosphorescence, and a part thereof are included in a part of the molecule. A compound can also be used suitably.

  Moreover, the host material used for formation of the organic light emitting layer 3 of an organic electroluminescent element will not be specifically limited if it is arbitrary materials used as a host material for organic electroluminescent elements. Examples include, but are not limited to, anthracene derivatives, tris (8-hydroxyquinolinate), carbazole derivatives, styrylarylene derivatives, tetracene derivatives, fluorene derivatives, triarylamine derivatives, and the like.

  Further, other members constituting the organic electroluminescence element, such as the substrate 7 holding the stacked elements, the anode (electrode 1), the cathode (electrode 2), etc., are used as they are. can do.

  When the light emitted from the organic light emitting layer 3 is emitted through the substrate 7, the substrate 7 is light transmissive, and may be ground glass even if it is slightly colored in addition to being colorless and transparent. It may be in a shape. For example, transparent glass plates such as soda lime glass and non-alkali glass, plastic films and plastic plates produced by any method from resins such as polyester, polyolefin, polyamide, epoxy, fluororesin, and organic-inorganic hybrid materials Can be used. Furthermore, it is possible to use a material having a light diffusing effect by containing particles, powder, bubbles or the like having a refractive index different from that of the base material of the substrate 7 in the substrate 7. It is also preferable to enhance the light extraction effect by imparting a surface shape. Further, when light is emitted without passing through the substrate 7, the substrate 7 does not necessarily have to be light-transmitting, and any substrate can be used as long as the light emission characteristics, life characteristics, etc. of the element are not impaired. 7 can be used. In particular, it is preferable to use the substrate 7 having high thermal conductivity in order to reduce a temperature rise due to heat generation of the element during energization.

The anode is an electrode for injecting holes into the device, and an electrode material made of a metal, an alloy, an electrically conductive compound, or a mixture thereof having a high work function is preferably used, and the work function is 4 eV or more. It is better to use something. Examples of the material for the anode include metals such as gold, CuI, ITO (indium-tin oxide), SnO ₂ , ZnO, IZO (indium-zinc oxide), PEDOT, polyaniline, and the like. Examples thereof include conductive light-transmitting materials such as conductive polymers doped with molecules and arbitrary acceptors, and carbon nanotubes. The anode can be produced, for example, by forming these electrode materials into a thin film on the surface of the substrate 7 by a method such as vacuum deposition, sputtering, or coating. In order to irradiate the light emitted from the organic light emitting layer 3 through the anode and irradiate the outside, it is preferable that the light transmittance of the anode is 70% or more. Furthermore, the sheet resistance of the anode is preferably several hundred Ω / □ or less, and particularly preferably 100 Ω / □ or less. Here, the film thickness of the anode varies depending on the material in order to control the characteristics such as light transmittance and sheet resistance of the anode as described above, but it is set to 500 nm or less, preferably 10 to 200 nm. Good. The preferred conditions may be appropriately changed depending on the use of the hole injection layer or the auxiliary electrode. That is, by appropriately using the auxiliary electrode, the sheet resistance in combination with the auxiliary electrode can be suppressed to a low value that does not cause a problem in practice, and as a result, the conductive light transmissive material used alone as the anode is so low as a single substance. Even those having a low resistance value can be used.

The cathode 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 low work function, and the work function is It is preferably 5 eV or less. Moreover, you may comprise as a laminated structure etc. combining Al and another electrode material. As a combination of such cathode electrode materials, a laminate of an alkali metal and Al, a laminate of an alkali metal and silver, a laminate of an alkali metal halide and Al, an alkali metal oxide and Al and Laminates of alkaline earth metals or rare earth metals and Al, alloys of these metal species with other metals, and the like. Specific examples include sodium, sodium-potassium alloys, lithium and magnesium. , Etc. and Al, magnesium-silver mixtures, magnesium-indium mixtures, aluminum-lithium alloys, LiF / Al mixtures / laminates, Al / Al ₂ O ₃ mixtures, and the like. Further, as described above, one layer of a material (or an alloy containing them) having an above-described work function of 5 eV or less is used, using an alkali metal oxide, an alkali metal halide, or a metal oxide as a base of the cathode. You may make it laminate | stack above. Further, a transparent electrode typified by ITO, IZO or the like may be used to extract light from the cathode side. Also in this case, it is preferable to use a metal having a work function of 5 eV or less for the base of the transparent electrode.

  The cathode can be produced, for example, by forming these electrode materials into a thin film by a method such as vacuum deposition or sputtering. In order to irradiate the light emitted from the organic light emitting layer 3 to the anode side, the light transmittance of the cathode is preferably 10% or less. On the other hand, when the cathode is formed as a transparent electrode and light is extracted from the cathode side, or when the light is reflected by some means after the transparent electrode is formed and the light is extracted to the anode side, the light transmission of the cathode The rate is preferably 70% or more. The film thickness of the cathode in this case varies depending on the material in order to control the characteristics such as light transmittance of the cathode, but is usually 500 nm or less, preferably 100 to 200 nm. In these cases, as in the case of the anode, suitable conditions may be appropriately changed depending on the use of the electron injection layer or the auxiliary electrode.

  Furthermore, a metal such as Al is sputtered on the cathode, or a fluorine compound, a fluorine polymer, other organic molecules, a polymer, etc. are deposited, sputtered, CVD, plasma polymerization, UV curing after coating, It can be formed as a thin film by thermosetting or other methods so as to have a function as a protective film.

Further, the organic electroluminescence device of the present invention is a so-called multi-photon type, multi-unit type, multi-layer type, in which a plurality of light-emitting layers are stacked via an equipotential surface layer or charge generation layer as an intermediate layer. It may have a tandem structure. Examples of the material of the equipotential surface forming layer or the charge generation layer include metal thin films such as Ag, Au, and Al, metal oxides such as vanadium oxide, molybdenum oxide, rhenium oxide, and tungsten oxide, ITO, IZO, AZO, GZO, Transparent conductive film such as ATO, SnO ₂ , so-called n-type semiconductor and p-type semiconductor laminate, metal thin film or transparent conductive film and n-type semiconductor and / or p-type semiconductor laminate, n-type semiconductor and p-type semiconductor And a mixture of an n-type semiconductor and / or a p-type semiconductor and a metal. The n-type semiconductor or p-type semiconductor may be an inorganic material or an organic material, or a mixture of an organic material and a metal, an organic material and a metal oxide, an organic material and an organic acceptor / It may be obtained by a combination of a donor material, an inorganic acceptor / donor material, etc., and can be selected and used as needed without any particular limitation.

  Next, the present invention will be specifically described with reference to examples. The ionization potential of each material is a value measured by “photoelectron spectrometer AC-3” manufactured by Riken Keiki.

Example 1
A glass substrate having a thickness of 0.7 mm was prepared, in which ITO having a thickness of 110 nm was formed as an anode in the pattern of FIG. The sheet resistance of ITO forming the anode is about 12Ω / □. This was subjected to ultrasonic cleaning for 10 minutes each with detergent, ion-exchanged water, and acetone, then steam-washed with IPA (isopropyl alcohol), dried, and further treated with UV / O ₃ . Next, the buffer material “MCC-PC1020” manufactured by Mitsubishi Chemical Science and Technology Research Center Co., Ltd. is spin-coated to a thickness of 20 nm, baked at 230 ° C. for 30 minutes in a vacuum atmosphere, and cooled to room temperature while maintaining the vacuum to buffer. A layer was formed.

Thereafter, this substrate was set in a vacuum deposition apparatus, and 4,4′-bis [N- (naphthyl) -N-phenyl-amino] biphenyl was used as a hole transport layer under a reduced pressure atmosphere of 1 × 10 ⁻⁴ Pa or less. (Α-NPD) was deposited to a thickness of 20 nm.

  Next, as an organic light emitting layer, TBADN ([Chemical Formula 3]) (ionization potential: 5.55 eV) is doped with 4% by mass of sty-NPD ([Chemical Formula 4]) (ionization potential: 5.35 eV) and has a thickness of 40 nm. Was deposited.

  Next, following the organic light emitting layer, TBADN as the host material of the organic light emitting layer, TmiQPThAZ ([Chemical Formula 5]) (ionization potential: 6.65 eV) as the main component of the electron transport layer, and sty-NPD of the organic semiconductor material Were co-evaporated at a mass ratio of 48: 48: 4 to form a 5 nm thick mixed layer.

  Next, following the mixed layer, as the electron transport layer, TmiQPhTAZ was formed to a thickness of 5 nm, LiF was formed to a thickness of 0.5 nm, aluminum was formed to a thickness of 80 nm, and the Al cathode was patterned as shown in FIG. Thus, an organic electroluminescence element was obtained. The shape of the organic electroluminescence element is as shown in FIG. 2 (in FIG. 2, the organic film is composed of a hole transport layer, an organic light emitting layer, a mixed layer, and an electron transport layer).

(Example 2)
The mixed layer was prepared by mixing TBADN as the host material of the organic light emitting layer, TmiQPTAZ as the main component of the electron transport layer, and rubrene (ionization potential: 5.4 eV) of the organic semiconductor material at a mass ratio of 45:45:10. An organic electroluminescence element was obtained in the same manner as in Example 1 except that it was formed.

(Example 3)
The mixed layer is composed of TBADN as the host material of the organic light emitting layer, TmiQPhTAZ as the main component of the electron transport layer, and TBP ([Chem. 6]) (ionization potential: 5.6 eV) of the organic semiconductor material, 48: 48: 4. An organic electroluminescence device was obtained in the same manner as in Example 1 except that the mixture was formed by mixing at a mass ratio of

Example 4
The mixed layer is composed of TBADN as the host material of the organic light emitting layer, TmiQPHTAZ as the main component of the electron transport layer, and α-NPD (ionization potential: 5.6 eV) of the organic semiconductor material at a mass ratio of 46: 46: 8. An organic electroluminescence element was obtained in the same manner as in Example 1 except that the mixture was formed by mixing.

(Example 5)
Organic light-emitting layer with a thickness of 3 nm, in which the mixed layer is a mixture of TBADN as the host material of the organic light-emitting layer, TmiQPHTAZ as the main component of the electron transport layer, and α-NPD of the organic semiconductor material in a mass ratio of 42:42:16 An organic electroluminescence device is obtained in the same manner as in Example 1 except that the layer is formed of a layer on the layer side and a layer on the cathode side having a thickness of 2 nm mixed at a mass ratio of 35: 57: 8. It was.

(Comparative Example 1)
The electron transport layer was formed of a layer formed by depositing TmiQPhTAZ to a thickness of 10 nm so that a mixed layer was not formed. Except this, it carried out similarly to Example 1, and obtained the organic electroluminescent element.

(Comparative Example 2)
The organic layer was formed in the same manner as in Example 1 except that the mixed layer was formed by mixing TBADN as the host material of the organic light emitting layer and TmiQPTHAZ as the main component of the electron transport layer at a mass ratio of 50:50. An electroluminescence element was obtained.

(Comparative Example 3)
A mixed layer is formed in the same manner as in Example 1 except that the organic light-emitting layer host material TBADN and the organic semiconductor material sty-NPD are mixed at a mass ratio of 96: 4. A luminescence element was obtained.

(Comparative Example 4)
An organic electroluminescent element was obtained in the same manner as in Example 2 except that the organic light emitting layer was formed to a thickness of 30 nm.

(Example 6)
The mixed layer is composed of TBADN as the host material of the organic light emitting layer, Alq (ionization potential: 5.9 eV) as the main component of the electron transport layer, and sty-NPD of the organic semiconductor material at a mass ratio of 5: 91: 4. The electron transport layer was formed of Alq having a thickness of 5 nm. Except this, it carried out similarly to Example 1, and obtained the organic electroluminescent element.

(Example 7)
The mixed layer is composed of TBADN as the host material of the organic light emitting layer, Alq as the main component of the electron transport layer, and a yellow dopant ([Chem. 7]) (ionization potential of 5.6 eV) of 8: 88: 4. An organic electroluminescence device was obtained in the same manner as in Example 6 except that the organic electroluminescence device was formed by mixing at a mass ratio of

(Comparative Example 5)
The electron transport layer was formed of a layer in which Alq was formed to a thickness of 10 nm, and a mixed layer was not formed. Except this, it carried out similarly to Example 6, and obtained the organic electroluminescent element.

(Comparative Example 6)
Except that the mixed layer was formed by mixing TBADN as the host material of the organic light emitting layer and Alq as the main component of the electron transport layer at a mass ratio of 50:50, the same as in Example 6, An organic electroluminescence device was obtained.

(Comparative Example 7)
The organic layer was formed in the same manner as in Example 6 except that the mixed layer was formed by mixing TBADN as the host material of the organic light emitting layer and Alq as the main component of the electron transport layer at a mass ratio of 5:95. A luminescence element was obtained.

(Comparative Example 8)
The mixed layer was prepared by mixing TBADN as the host material of the organic light emitting layer, Alq as the main component of the electron transport layer, and BCP (ionization potential: 6.3 eV) of the organic semiconductor material at a mass ratio of 48: 48: 4. An organic electroluminescent element was obtained in the same manner as in Example 6 except that the organic electroluminescent element was formed.

(Example 8)
The mixed layer is composed of TBADN as the host material of the organic light emitting layer, TpPyPhB ([Chemical Formula 8]) (ionization potential: 6.6 eV) as the main component of the electron transport layer, and sty-NPD of the organic semiconductor material as 48:48. The organic electroluminescence device was obtained in the same manner as in Example 1 except that the mixture was formed at a mass ratio of 4: 4, and the electron transport layer was formed of TpPyPhB having a thickness of 15 nm.

(Comparative Example 9)
An organic electroluminescence device was obtained in the same manner as in Example 8 except that the electron transport layer was formed of a layer formed by depositing TpPyPhB to a thickness of 20 nm and the mixed layer was not formed.

(Comparative Example 10)
The organic electroluminescence was the same as in Example 8 except that the mixed layer was formed with TBADN as the host material of the organic light emitting layer and TpPyPhB as the main component of the electron transport layer at a mass ratio of 50:50. An element was obtained.

As described above, the organic electroluminescence elements obtained in Examples 1 to 8 and Comparative Examples 1 to 10 were energized with a constant current of 20 mA / cm ² and the time required for the luminance to be reduced to 80%, The driving voltage at the initial stage of evaluation was measured. The results are shown in Table 1. Table 1 shows the contribution ratio of the organic semiconductor material to light emission.

  As can be seen from Table 1, the organic electroluminescence device of each example having a mixed layer in which three types of materials, ie, the host material of the organic light emitting layer, the main component of the electron transport layer, and the organic semiconductor material are mixed has a luminance of 80. It took a long time to decrease to a percentage, and the drive voltage could be kept relatively low.

  On the other hand, the device of Comparative Example 1 having no mixed layer has an extremely short lifetime, and Comparative Example 2 in which a mixed layer is formed by mixing two materials of the host material of the organic light emitting layer and the main component of the electron transport layer. Although the lifetime of this device was improved, the lifetime was still shorter than the devices of Examples 1 to 5. In addition, the device of Comparative Example 3 having no electron transport layer as a main component in the mixed layer had a high driving voltage, although its life was extended to some extent. Further, Comparative Example 4 in which the contribution ratio of the organic semiconductor material to light emission was higher than that of Example 2 had a shorter lifetime. In addition, the organic electroluminescent element of the comparative example 4 was white light emission, and was shifting to the thing different from blue originally intended. Therefore, it is meaningless to compare luminance and driving voltage.

  In contrast, the device of Comparative Example 5 in which two kinds of materials, ie, the host material of the organic light emitting layer and the main component of the electron transport layer were mixed, had a short lifetime. Although the device of Comparative Example 6 by reexamining the mixing ratio of the mixed layer materials slightly extended, the device of Comparative Example 8 in which the mixed layer was formed by mixing organic semiconductor materials outside the range intended by the present invention was It had a short life, and was a short life as compared with Examples 6 and 7.

  In addition, Comparative Examples 9 and 10 showed relatively good life, but were inferior to those of Example 8.

It is a schematic sectional drawing which shows an example of the layer structure of the organic electroluminescent element of this invention. It is a schematic plan view which shows the organic electroluminescent element produced in the Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode 2 Electrode 3 Organic light emitting layer 4 Hole transport layer 5 Electron transport layer 6 Mixed layer

Claims (3)

In an organic electroluminescent device comprising an organic light-emitting layer and an electron transport layer, which are formed by containing a light-emitting dopant in a host material between two opposing electrodes, at least the organic light-emitting layer of the electron transport layer The main component constituting the electron transport layer, the host material of the organic light emitting layer, and the host material of the organic light emitting layer having an ionization potential equal to or lower than the ionization potential of the main component constituting the electron transport layer A mixed layer in which different organic semiconductor materials are mixed, and a mixed mass ratio of the host material of the organic light emitting layer to the main component constituting the electron transporting layer is 1:99 to 50:50 in the mixed layer; in the concentration of the organic semiconductor material is higher than the side of the side where the cathode of the organic light-emitting layer, the organic semiconductor material mixed in the mixed layer is contribution to light emission is not more than 5% The organic electroluminescent device characterized.   2. The organic electroluminescence device according to claim 1, wherein an ionization potential of the organic semiconductor material mixed in the mixed layer is equal to or lower than an ionization potential of the host material of the organic light emitting layer.   3. The organic electroluminescence device according to claim 1, wherein the organic semiconductor material mixed in the mixed layer is the same substance as the luminescent dopant contained in the organic light emitting layer.

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