JP2014235961A - Organic electroluminescent element - Google Patents

Abstract

An object of the present invention is to provide an interface configuration of an organic electroluminescence element that improves the light extraction efficiency by re-reflecting light reflected at the interface between a transparent substrate and air to the glass substrate side. SOLUTION: On a transparent substrate 1, at least a scattering layer 8a, a high refractive index resin layer 8b having a refractive index in the range of 1.6 to 2.0, a titanium oxide layer 8c, an anode layer 2, an electroluminescence layer 7, The cathode layer 6 is laminated in this order, and the anode layer 2 can be provided with a metal auxiliary wiring 9 . [Selection drawing] Fig. 1

Description

  The present invention relates to an organic electroluminescence element, and more particularly to a layer structure for improving the extraction efficiency of emitted light.

  An organic electroluminescence element (hereinafter referred to as an organic EL element) used for illumination or display is a self-light-emitting element in which an organic fluorescent material is sandwiched between electrodes facing each other at about 2000 mm or less. Organic fluorescent materials include low molecular weight materials and high molecular weight materials, but the principle of light emission is the same. Holes injected from the anode into the HOMO of the fluorescent material and electrons injected from the cathode into the LOMO Utilizes fluorescence or phosphorescence upon recombination. Expectations are growing as the next generation display devices and flat light sources for liquid crystal displays.

  The configuration of the organic EL element 20 is shown in FIG. 3. On the transparent glass substrate 1, an anode layer 2, a hole transport layer 3, a light emitting layer 4, an electron transport layer 5, a cathode layer 6, a protective film 14, etc. It is made up of. The anode layer 2 is a transparent electrode from which light can be extracted, and usually ITO (indium tin composite oxide) is used. The hole transport layer 3 is an organic layer that facilitates injection of holes from the anode layer 2 into the fluorescent light emitting layer. The cathode 6 is a layer for injecting electrons into the light emitting layer 4 and is made of a metal such as magnesium or an alloy of lithium and silver, and an alkali metal material having a small work function is used.

  In addition, the hole transport layer 3 and the electron transport layer 5 are not only transporting the respective carriers but also being multilayered by adding a functional layer (interlayer layer) for blocking the opposite charge. There are many. In any case, in order to improve the tunneling of holes and electrons, the thickness of each layer is very thin in the range of 100 to 1000 mm.

  For colorization, phosphors with different fluorescent colors are arranged in a predetermined pattern, white light emission is dispersed using a color filter, or a red light emission layer and a green light emission layer are separately deposited on a blue light emission layer. There is a method for color conversion.

  Although the protective film 14 covering the cathode layer 6 is originally unnecessary, the performance of the cathode electrode material and each of the organic layers described above is extremely deteriorated due to the influence of moisture and oxygen. It is installed as a barrier so as not to penetrate into the aforementioned layer. In general, after coating with an epoxy resin or the like, an inorganic film is laminated or covered with a cover glass containing a hygroscopic agent for further completeness.

  In the low molecular system and the high molecular system, the element manufacturing method is different in nature, but in the low molecular system, each layer is sequentially deposited on the glass substrate with a transparent electrode so as to have a predetermined thickness by vapor deposition. In the polymer system, a hole transport material and a conjugated polymer material are separately adjusted to ink in a solvent, and printing or dropping from a fine nozzle is employed. For this reason, low molecular weight systems are difficult to increase in size because it is necessary to mask-deposit RGB phosphors in a predetermined pattern, and are suitable for small and medium-sized displays, while high molecular weight systems are large because printing technology can be applied. It is said to be advantageous for the production of displays. However, there is no clear barrier to the adoption of materials and construction methods according to size, and ultimately it depends on cost.

By the way, in the organic EL element, as can be seen from FIG. 3, the light 11 emitted from the phosphor has the hole transport layer 3, the anode layer 2, and the transparent layer that support the light emitting layer 4 with the metal electrode 6 behind. It comes out through a multilayer medium called the substrate 1. In this situation, it is not possible to take out all of the phosphor emission in the front direction. One cause is that the singlet excited state of the phosphor does not contribute to light emission due to non-radiative deactivation when interacting with the adjacent cathode layer (metal electrode) 6 (Non-patent Document 1).

  On the other hand, the light 11 passing through the multilayer medium is modulated by the mutual refractive index relationship and the thickness. There is a problem that the transmission wavelength varies due to interference, or cannot be extracted as light guided in the layer due to total reflection. In particular, when light enters from a layer having a high refractive index to a layer having a low refractive index, total reflection may occur depending on the incident angle, and the total reflected light may be guided through the medium and emitted from the edge. Disappear. There is a problem that the amount of light that can be extracted forward is only about 20% of the light emitted by the phosphor.

  The refractive index of the electron transport layer 5 / the light emitting layer 4 / the hole transport layer 3 is approximately 1.7 to 1.8 when the refractive index difference between the layers is ignored, and the refractive index of the anode layer 2 (ITO) is 2 Since the glass substrate 1 is about 1.45 and the air is 1, the light extraction by total reflection at the interface between the anode layer 2 and the glass substrate 1 and at the interface between the glass substrate 1 and the air. It can be said that the decrease in efficiency is severe.

  In order to avoid a sudden change in the refractive index, a high refractive index layer 8b or scattering layer 8a having a refractive index between the glass substrate 1 and the anode layer 2 between the anode layer 2 and the glass substrate 1 as shown in FIG. A configuration is possible in which these are arranged. However, the light that is totally reflected by the front and back surfaces of the glass substrate 1 has a problem that the light extraction efficiency is not sufficient because components that cannot be extracted to the outside after being guided inside the device remain.

Japanese Patent Laid-Open No. 9-272863 JP-A-5-159881

A. N. Safonov et al., Synthetic Metals, 116, 145 (2001).

  Therefore, the present invention has a problem that the amount of light transmitted through the transparent substrate is reduced due to total reflection when entering from the high refractive index anode electrode to the low refractive index transparent substrate interface, and the total reflection at the glass substrate / air interface. Therefore, it was an object to improve the problem that the light extraction efficiency decreases. In particular, an object of the present invention is to provide an interface configuration of an organic electroluminescence element that improves light extraction efficiency by re-reflecting light reflected at the interface between the transparent substrate and air toward the glass substrate.

  In order to achieve the above object, the invention described in claim 1 includes at least a scattering layer, a high refractive index resin layer having a refractive index in the range of 1.6 to 2.0, a titanium oxide layer, and an anode on a transparent substrate. An organic electroluminescent device is characterized in that a layer, an organic light emitting layer, and a cathode layer are laminated in this order.

  The invention according to claim 2 is the organic electroluminescence device according to claim 1, wherein the scattering layer, the high refractive index resin layer, the titanium oxide layer, and the anode layer are patterned. .

The invention according to claim 3 is the organic electroluminescence element according to claim 1 or 2, wherein the anode layer includes a metal auxiliary wiring.

  In an organic electroluminescence element, light emitted from a relatively high refractive index transparent substrate to the low refractive index air side and light incident from the anode layer to the transparent substrate have total reflection components. In addition, a part of the refractive index is extracted to the outside by the stepwise change of the refractive index by the scattering layer and the high refractive index resin layer, but the present invention has a higher refractive index than the resin layer on the high refractive index resin layer. It has a layer structure in which a titanium oxide layer is disposed. By introducing the titanium oxide layer, a component incident on the titanium oxide layer at a critical angle or more out of the light reflected at the glass substrate / air interface is re-reflected, so that the effect of increasing the light component that can be extracted to the outside can be expected.

  According to the second aspect of the present invention, the scattering layer, the high refractive index resin layer, the titanium oxide layer, and the anode layer can be shielded from the outside by sealing, and moisture and oxygen can be prevented from entering the device. I can do it.

  According to the third aspect of the present invention, it is possible to reduce the resistance of the anode layer by the metal auxiliary line, and the effect that the light extraction efficiency to the outside increases as a result of the metal serving as the reflection layer can be expected.

It is a figure of the cross-sectional view explaining an example of the organic EL element structure which becomes this invention. It is a figure of the cross-sectional view explaining another example of the organic EL element structure which becomes this invention. It is a figure of the cross-sectional view explaining the structure of the conventional organic EL element. It is a figure of the cross-sectional view explaining the conventional structure of the organic EL element which improved the light extraction efficiency.

  As shown in FIG. 3, the organic EL element basically includes a transparent substrate 1, an anode layer 2 (generally ITO), an organic light emitting layer 7 (a hole transport layer 3, a light emitting layer 4, an electron transport layer 5, etc. ) And the cathode layer 6. The organic light emitting layer 7 exemplifies a layer structure in which the hole transport layer 3, the light emitting layer 4, and the electron transport layer 5 are laminated in this order, but the case where the light emitting layer 4 also serves as the hole transport layer 3. As described above, other than the light emitting layer may be omitted, or an additional functional layer may be included although not shown.

The light 11 emitted from the organic light emitting layer 7 is dissipated for various reasons, but the main light loss is from the anode layer 2 (refractive index: n ₁ ≧ 2.0) having a high refractive index to the transparent substrate having a low refractive index. 1 (generally a glass substrate having a refractive index n ₂ = 1.45) and a total reflection component at the interface between the transparent substrate 1 and the air layer.

In order to reduce the total reflection component when entering the transparent substrate 1 having a low refractive index from the anode layer 2 having the high refractive index, the difference in refractive index between the anode layer 2 and the transparent substrate 1 is not discontinuous and is a step. A structure in which a high refractive index resin layer 8b having a refractive index n satisfying the relationship of n ₁ <n <n ₂ is inserted between the anode layer 2 and the transparent substrate 1 so as to change in a shape is employed. Further, between the high refractive index resin layer 8b and the transparent substrate 1, as shown in FIG. 4, inorganic fine particles 12 such as titanium oxide, zirconia, or silicon oxide having a diameter of about 0.4 to 1.5 μm. There is a configuration in which a scattering layer 8a made of is arranged. The scattering layer 8a is not limited to being formed only from inorganic fine particles, but may be formed by dispersing the fine particles 12 in the high refractive index resin 8b.

In the present invention, in order to further improve the light extraction efficiency, as shown in FIG. 1, between the anode layer 2 (ITO electrode having a refractive index n _{2 of} about 2.0 or more) and the high refractive index resin layer 8b. The titanium oxide layer 8c having a higher refractive index than the refractive index of the high refractive index resin layer 8b and the anode layer 2 is arranged.

The refractive index n of titanium oxide is about 2.5 to 2.7 (rutile type and anatase type), and the light emitted from the light emitting layer 7 is a titanium oxide layer because the refractive index is higher than that of ITO. In 8c, total reflection does not occur and the light is transmitted in the forward direction. Of the transmitted light, the light reflected from the front and back of the transparent substrate 1, particularly the transparent substrate / air interface, returns to the light emitting layer 7 side, but the component incident on the titanium oxide layer 8 c at a critical angle or more is re-reflected and reflected on the transparent substrate 1 side. Change the direction. This can be expected to improve the front light extraction efficiency.
The configuration will be described below.

  The inorganic fine particle scattering layer 8a or the dispersion layer can be obtained by dissolving fine particles having a diameter of about 0.4 to 1.5 μm made of an inorganic substance such as titanium oxide or zirconium oxide in a solvent and then applying and drying. Alternatively, it can be formed by dispersing a predetermined amount in the high refractive index resin layer 8b and then applying it. Since the refractive index of the titanium oxide layer 8c is larger than the refractive index of the anode layer 2, the light extraction efficiency in the front direction increases for the reason described above.

  The refractive index n of the high refractive index resin layer 8b is preferably in the range of 1.6 to 2.0, which is an intermediate value between the refractive indexes of the transparent substrate 1 and the anode layer 2. The refractive index of the titanium oxide high refractive index reflective layer 9 is in the range of 2.5 to 2.7. When the refractive index of the binder resin layer is 1.4 to 1.6 (typical acrylic resin), the high refractive index resin layer 8b is an inorganic material of titanium oxide, zirconia oxide, or silicon oxide having a diameter of about 50 nm. It is obtained by dispersing fine particles of the system. When the refractive index of the binder resin is as high as 1.8 to 1.9, it can be used as it is.

  On the anode layer 2, the metal auxiliary wiring 9 can be laid in stripes as shown in FIG. Metal wiring can reduce the resistance of the anode in combination with ITO. Moreover, it is desirable to lay it periodically, and in that case, a uniform light emission state can be obtained. As the metal wiring, a single layer type of copper or aluminum, or a laminated type of molybdenum / aluminum / molybdenum can be adopted. It is laid to have a thickness of 0.05 to 0.3 μm and a line width of 10 to 100 μm. The metal auxiliary wiring 9 has a function of re-reflecting light totally reflected at the interface between the transparent substrate 1 and air and at the interface between the anode layer 2 and the transparent substrate 1 in various directions. Alternatively, the light emitted from the light emitting layer side is also reflected. In any case, the front light extraction efficiency is increased.

  The present invention can be applied regardless of whether the light emitting layer 7 is a low molecular weight light emitting layer 7 or a light emitting layer 7 based on a conjugated polymer. The refractive index value for the material is described. As a hole transport material, TPD (n = 1.8 to 1.9) is used for a low molecular weight system, PEDOT-PSS (n = 1.5) is used for a high molecular weight system, and aluminum quinoline ( n = 1.8 to 1.9), and polymer systems include PPV (n = 2.0). Although both have chromatic dispersion, the wavelength λ is a value for light of approximately 0.5 μm.

  As the transparent substrate 1 used in the present invention, a glass substrate, a quartz substrate, a plastic substrate or the like can be used, but a glass substrate excellent in transparency, strength, heat resistance, chemical resistance, and workability is most preferable. The refractive indexes of the glass substrate and the quartz substrate are both about 1.45 if slight wavelength dispersion is ignored. The difference in refractive index from the air layer is about 0.4, which is a cause of a decrease in extraction efficiency due to total reflection.

  First, silicon oxide, which is inorganic fine particles having a diameter of about 0.4 to 1.5 μm, is dissolved in a predetermined solvent, and then coated on the glass substrate 1 and dried to form the scattering layer 8a. .

Next, a transparent high refractive index resin layer 8b that is transparent in the visible region having a refractive index in the range of 1.6 to 2.0 is formed. A resin having a refractive index intermediate between the refractive index of the glass substrate 1 and the refractive index of the anode layer (ITO transparent electrode) (preferably a combination having a refractive index difference of 0.3 or less) is desirable. Urethane (~ 1.8), episulfide (~ 1.7), acrylic (~ 1.6), polyimide (~ 1.75), triazine, silsesquio, etc. it can. The thickness of the resin layer 8b is set within a range of 1 to 3 μm that is not colored. The high refractive index resin layer 8b can be manufactured by dissolving in an appropriate solvent and then coating and drying.

  Moreover, the uneven | corrugated shape can be formed in the surface of the glass substrate 1 by the sandblasting method etc., and the light-scattering effect can be provided. The unevenness preferably has a pitch of about the wavelength of light, and is preferably about 0.5 to 1 μm. If there is unevenness, the total reflection effect can be reduced by changing the angle of light incident from the resin side. Unevenness can also be formed on the surface of the glass substrate by wet etching. The scattering layer 8a made of fine particles may be formed in advance under the high refractive index resin layer 8b. The fine particles 12 made of an inorganic material having a diameter of about 0.4 to 1.5 μm may be dispersed in the high refractive index resin layer 8b.

  Next, a titanium oxide layer 8c is provided on the high refractive index resin layer 8b. The titanium oxide layer 8c is set in the range of 0.01 to 0.03 μm by sputtering. Titanium oxide has a rutile or anatase crystal structure and a refractive index of about 2.5 to 2.7. The titanium oxide layer 8 c may be patterned so as to be the light emission pattern of the organic light emitting layer 7.

Next, the anode layer 2 made of ITO (n ₂ = about 2.0 to 2.1) is provided on the titanium oxide layer 8c. As an electrode material, in addition to ITO, transparent electrode materials such as IZO (indium zinc composite oxide), tin oxide, zinc oxide, indium oxide, and aluminum oxide composite oxide can be used. ITO is preferable because it has high transparency and high transparency. ITO is sputtered on a substrate with a thickness of 0.1 to 0.3 μm and then annealed at 230 ° C. to lower the resistance.

  Next, the metal auxiliary wiring 9 is periodically formed on the anode layer 2. The metal auxiliary wiring 9 is patterned by a regular photolithography method after a metal thin film is formed by a sputtering method.

  After the anode layer 2 is formed as described above, the hole transport layer 3 is formed. The hole transport layer 3 is provided to lower the injection barrier so that holes are easily injected from the anode layer 2 to the light emitting layer 4.

Examples of the low-molecular hole transport material constituting the hole transport layer 3 include TPD which is a triphenylamine dimer and starburst amine obtained by binding these in a starburst form. In any case, it is desirable to exhibit an amorphous state and to exhibit a high glass temperature so as not to crystallize. It is formed by vacuum deposition at a low pressure of 10 ⁻⁴ Pa or less.

  Moreover, as a hole transport material composing the hole transport layer 3, in addition to the organic compounds, inorganic oxides such as vanadium oxide (VOx), molybdenum oxide (MoOx), nickel oxide (NiOx), etc. Is mentioned. These materials can be formed by various deposition methods such as sputtering and resistance heating.

  As the polymer-based hole transport material, a polyaniline derivative, a polythiophene derivative, a polyvinylcarbazole (PVK) derivative, poly (3,4-ethylenedioxythiophene) (PEDOT-PSS), or the like may be used. These materials are dissolved or dispersed in a solvent and formed as a hole transport material ink by various coating methods such as letterpress printing, gravure printing, reverse offset printing, spin coater, bar coater, roll coater, die coater, gravure coater, etc. can do.

  An interlayer layer (not shown) can be formed on the hole transport layer 3. Interlayer functions as an electron blocking layer that blocks electrons moving from the organic light-emitting layer to the anode side and increases the charge coupling probability in the organic light-emitting layer, by contact between the excited organic light-emitting material and the hole transport layer It is installed for the purpose of preventing quenching and preventing a decrease in luminous efficiency due to a chemical reaction between an organic light emitting material and a hole transport material.

  The material used for the interlayer can be a polymer material such as polyarylene, polyarylene vinylene, or polyfluorene, or a mixture of these polymer materials with low molecules such as aromatic amines. In particular, poly (2,7- (9,9-di-n-octylfluorene) -alt- (1,4-phenylene-((4-sec-butylphenyl) amino) -1,4-phenylene) ( In order to avoid color mixing with the organic light-emitting layer, the interlayer is usually subjected to a heat treatment at about 200 ° C. for 10 minutes or more before forming the organic light-emitting layer. Insolubilized.

Next, the light emitting layer 4 is formed.
Examples of the low molecular weight system include electron-transporting metal complexes such as aluminum quinoline, benzoxazole Zn complex, and benzoquinolinol Be complex, and dye-based materials used for adjusting the emission color by doping them. Examples of the dye system include distyrylarylene derivatives, pyrazoquinoline derivatives, and oxadiazole derivatives. These also preferably have a high glass transition temperature.

  In the case where a separate coating is required for a polymer system, a relief printing method, a gravure printing method, a printing method such as a reverse offset printing method, an ink jet method or the like can be used. Examples of organic light emitting materials include coumarin, perylene, pyran, anthrone, porphyrene, quinacridone, N, N′-dialkyl substituted quinacridone, naphthalimide, N, N′-diaryl substituted pyrrolopyrrole, iridium. Examples include complex-based luminescent dyes dispersed in polymers such as polystyrene, polymethyl methacrylate, and polyvinyl carbazole, and polyarylene-based, polyarylene vinylene-based, and polyfluorene-based polymer materials. However, it is not limited to these.

  These organic light emitting materials are dissolved or stably dispersed in a solvent to form an organic light emitting ink. Examples of the solvent for dissolving or dispersing the organic light emitting material include toluene, xylene, acetone, anisole, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, or a mixed solvent thereof. Among them, aromatic organic solvents such as toluene, xylene, and anisole are preferable from the viewpoint of the solubility of the organic light emitting material. Moreover, surfactant, antioxidant, a viscosity modifier, a ultraviolet absorber, etc. may be added to organic luminescent ink as needed.

  Finally, the cathode layer 6 is formed of a stripe or solid electrode. As the material of the cathode layer 6, those according to the light emission characteristics of the organic light emitting layer 7 can be used. For example, simple metals such as lithium, magnesium, calcium, ytterbium and aluminum, and stable metals such as gold and silver can be used. And alloys thereof. Alternatively, a conductive oxide such as indium, zinc, or tin can be used.

  Although not shown in FIG. 1, an electron transport layer 5 such as an alkali metal fluoride (LiF) or an oxide can be formed thinly between the cathode 6 and the light emitting layer 4. Electron injection efficiency is improved and light emission efficiency is also improved. Examples of a method for forming these cathodes and related layers include a method for forming by vacuum evaporation using a mask.

The organic EL element can have a laminated structure in which layers such as a hole blocking layer, an electron transport layer, and an electron injection layer are selected as necessary in addition to the hole transport layer, the interlayer layer, and the organic light emitting layer.
The laying of the high refractive index layer according to the present invention is applied regardless of the detailed laminated structure of the organic light emitting layer 7 described above.

  In addition, since the refractive index of the glass substrate is larger than the refractive index of air, there is also a mode in which the glass substrate is guided and dissipates from the edge of the glass substrate. In order to suppress this, a film having a lower refractive index than the glass substrate may be formed on the glass substrate, or a light scattering film may be laid.

1. Transparent substrate (glass substrate)
2. Anode layer (ITO transparent electrode)
3, hole transport layer 4, light emitting layer 5, electron transport layer 6, cathode layer (metal electrode)
7, organic light emitting layer 8, high refractive index layer 8a, scattering layer 8b, high refractive index resin layer 8c, titanium oxide layer 9, metal auxiliary wirings 10, 20, organic EL element,
11, outgoing light 12, fine particles 13, reflection component (outgoing light) (substrate / air interface)
14. Protective film

Claims (3)

  On the transparent substrate, at least a scattering layer, a high refractive index resin layer having a refractive index in the range of 1.6 to 2.0, a titanium oxide layer, an anode layer, an organic light emitting layer, and a cathode layer are laminated in this order. An organic electroluminescence device characterized.   The organic electroluminescence device according to claim 1, wherein the scattering layer, the high refractive index resin layer, the titanium oxide layer, and the anode layer are patterned.   The organic electroluminescence element according to claim 1, wherein the anode layer includes a metal auxiliary wiring.

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