JP5938363B2 - Organic electroluminescence device - Google Patents

Description

  The present invention relates to an organic electroluminescence (EL) device.

  The organic EL element generally has a structure in which a transparent electrode, a light emitting layer, and a reflective electrode are laminated in this order on a transparent substrate.

  In such an organic EL element, various devices have been made to increase the light extraction efficiency from the transparent substrate.

  For example, Patent Documents 1 to 4 describe organic EL elements in which light extraction efficiency in a wide wavelength region is improved by providing random unevenness at the interface between the transparent substrate and the transparent electrode.

JP 2003-243152 A JP 2006-236748 A JP 2012-178279 A JP 2012-28307 A

  Patent Documents 1 to 4 do not consider how random the spacing between the unevenness provided at the interface between the transparent substrate and the transparent electrode can improve the light extraction efficiency. Further, since the unevenness is formed using a mold or the like, it is difficult to finely control the interval between the unevenness, and the manufacturing cost increases.

  An object of the present invention is to provide an inexpensive organic electroluminescent device capable of improving the light extraction efficiency in a wide wavelength region.

  The organic electroluminescence device of the present invention is formed on a transparent substrate, an optical layer formed on the transparent substrate, having an uneven surface on the surface opposite to the transparent substrate and having a uniform refractive index, and the optical layer. A transparent electrode, a light emitting layer formed on the transparent electrode, and a reflective electrode formed on the light emitting layer. The optical layer includes a binder layer and particles embedded in the binder layer. The particle diameter of the particle is 1000 nm or less, and the interval between the convex portions on the surface of the optical layer varies in the range of 100 nm to 1000 nm, and the appearance of the interval that exists most in the optical layer appears. When the frequency is “1”, the appearance frequency of the above-mentioned interval of 1000 nm is 0.5 or more.

  ADVANTAGE OF THE INVENTION According to this invention, the cheap organic electroluminescent element which can improve the light extraction efficiency in a wide wavelength range can be provided.

The figure which shows the cross-sectional structure of the organic electroluminescent element for describing one Embodiment of this invention The figure when the partial area | region of the optical layer 2 is seen from the light emitting layer 4 side The figure which shows distribution of the convex part space | interval of the organic EL element of an Example The figure which shows distribution of the convex part space | interval of the organic EL element of an Example The figure which shows distribution of the convex part space | interval of the organic EL element of a comparative example

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a diagram showing a cross-sectional configuration of an organic EL element for explaining an embodiment of the present invention.

  The organic EL element 100 includes a transparent substrate 1, an optical layer 2 having unevenness on the surface of the transparent substrate 1 formed on the transparent substrate 1 and a uniform refractive index, and a transparent electrode 3 formed on the optical layer 2. And a light emitting layer 4 including an organic light emitting layer or an inorganic light emitting layer made of an organic or inorganic light emitting material, formed on the transparent electrode 3, and a reflective electrode 5 formed on the light emitting layer 4. There may be auxiliary wiring between the optical layer 2 and the transparent electrode 3. The light emitting layer 4 may include a hole injection layer, a hole transport layer, an electron injection layer, an electron transport layer, and the like as necessary.

  One of the transparent electrode 3 and the reflective electrode 5 is an anode, and the other is a cathode. When a DC voltage is applied between the anode and the cathode, electrons and holes are injected from the cathode and the anode into the organic light emitting layer or the inorganic light emitting layer included in the light emitting layer 4, and excitons are generated by recombination thereof. The light emitting layer 4 emits light by the emission of light when the exciton is deactivated. By taking out the light emitted from the light emitting layer 4 to the transparent substrate 1 side, the organic EL element 100 can be used for illumination, a display element, or the like. In the present embodiment, the light emitting layer 4 that emits white light is used.

  The transparent substrate 1 should just be comprised with the material which can permeate | transmit the light emitted from the light emitting layer 4 enough, for example, a glass substrate, a resin substrate, etc. are used.

  The optical layer 2 is provided to increase the amount of light that can be extracted to the transparent substrate 1 side due to the light diffraction phenomenon.

  The optical layer 2 includes a binder layer 2A and a large number of particles 2B embedded in the binder layer 2A. Due to the large number of particles 2 </ b> B, irregularities are formed on the surface of the optical layer 2 opposite to the transparent substrate 1. Although the particles 2B are spherical in FIG. 1, particles having various shapes such as a triangular prism and a quadrangular prism are used.

  The interval between the convex and concave portions on the surface of the optical layer 2 includes a wavelength range of visible light (generally 380 nm to 780 nm) and varies in a wider wavelength range than this wavelength range. In this way, the spacing between the convex portions corresponding to the grating spacing of the diffraction grating varies, so that not only light of a specific wavelength in the visible light wavelength range but also light of many wavelengths in this wavelength range. Thus, the light extraction efficiency can be improved.

  The wavelength range is preferably set to a range of 100 nm to 1000 nm in order to increase the light extraction efficiency at various wavelengths of visible light.

  The interval between the convex portions refers to the interval between all the convex portions formed on the surface of the optical layer 2 and other convex portions adjacent to each of the convex portions. Here, the adjacent two convex portions means that no other convex portions other than the two convex portions are arranged on a straight line connecting the top portions of the two convex portions.

  FIG. 2 is a diagram when a partial region of the optical layer 2 is viewed from the light emitting layer 4 side, and is a diagram for explaining an interval between the convex portions. In the partial area shown in FIG. 2, there are six intervals indicated by reference numerals 20, 21, 22, 23, 24, and 25 as the intervals between the convex portions according to the above definition.

  The inventor has a particle size of the particle 2B of the optical layer 2 of 1000 nm or less, and furthermore, among the intervals between the convex portions formed on the surface of the optical layer 2, the most existing first length interval. When the appearance frequency is “1”, the appearance frequency of the interval of the second length (= 1000 nm) that is the same as the wavelength of the end on the long wavelength side in the wavelength range is 0.5 or more. It has been found that the light extraction efficiency can be improved as compared with the case where the interval between the convex portions is simply random.

  The particle diameter of the particle 2B is a diameter of a circle having a size corresponding to the area of the particle in plan view. The particle diameter of the particles 2B in the optical layer 2 is preferably in the range of 200 nm to 800 nm, and more preferably in the range of 300 nm to 600 nm.

  Furthermore, when the inventor plots the appearance frequency for each of the intervals of all the lengths included in the optical layer 2, the appearance frequency is between the first length and the second length. It has been found that the light extraction efficiency can be further improved by the non-monotonic decrease.

  The above-mentioned intervals of all lengths included in the optical layer 2 can be measured by performing a fast Fourier transform (FFT) process on a microscope image obtained by imaging the surface of the optical layer 2 where the irregularities are formed with a laser microscope. It is.

  So far, the conditions for the spacing between the convex portions of the optical layer 2 have been described. However, if the average height of the irregularities formed on the surface of the optical layer 2 is too large, the transparent electrode 3 formed on the optical layer 2. And a reflection electrode 5 leak. For this reason, it is preferable that the average height of the unevenness formed on the surface of the optical layer 2 is 200 nm or less. Of course, if measures against leakage are taken by other means (such as flattening the light emitting layer 4), the average height of the irregularities may be larger than 200 nm. The lower limit value of the average height of the irregularities may be about 20 nm as long as the irregularities to the extent that a diffraction phenomenon can be expected are formed.

  The average height of the unevenness means the average height from the bottom of the uneven concave portion to the top of the convex portion, and can be determined as follows, for example.

  The uneven pattern is observed with an atomic force microscope or the like, and a cross-sectional view of the surface fine uneven body is obtained from the observation. The depth to the bottom of one recess is 1/2 of the sum of the distances from the tops of the two adjacent protrusions to the bottom of the recess. Therefore, for each of 10 or more randomly extracted recesses, the sum of the distances from the top of the two adjacent protrusions to the bottom of the recess was obtained, and 1/2 of each was further obtained. The average value is the average height.

  The optical layer 2 having the above configuration can be formed by, for example, applying a binder in which resin particles such as acrylic are dispersed to the transparent substrate 1.

  As a material of the binder layer used for the optical layer 2, a transparent appropriate resin is used.

  As this resin, for example, acrylic resin, polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate (PMMA), polystyrene, polyethersulfone, polyarylate, polycarbonate resin, polyurethane, polyacrylonitrile, polyvinyl acetal, polyamide, polyimide, diester Examples thereof include acrylic phthalate resins, cellulose resins, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, other thermoplastic resins, and copolymers of two or more monomers constituting these resins.

  In the optical layer 2, the relationship between the refractive index of the binder layer 2A and the refractive index of the particles 2B is preferably determined so that light incident from the transparent electrode 3 is not scattered inside.

  By making the difference between the refractive index of the binder layer 2A and the refractive index of the particles 2B 0.1 or less, preferably 0.05 or less, the refractive index of the optical layer 2 can be made substantially uniform. 2 can effectively prevent light scattering inside. As a result, the amount of light incident on the optical layer 2 returning to the light emitting layer 4 side by scattering can be reduced, and the light extraction efficiency of the organic EL element 100 can be increased.

  In order to prevent light scattering at the interface between the optical layer 2 and the transparent substrate 1, it is preferable that the binder layer 2B and the transparent substrate 1 have substantially the same refractive index.

  The transparent electrode 3 is an electrode made of a conductive material that can sufficiently transmit the light emitted from the light emitting layer 4 and has a sufficiently higher conductivity than the material contained in the light emitting layer 4. For the material of the transparent electrode 3, for example, a conductive polymer such as ITO or PEDOT-PSS is used.

  Examples of the conductive polymer used for the transparent electrode 3 include, but are not limited to, conductive polymers such as polythiophene, polyaniline, polypyrrole, polyphenylene, polyphenylene vinylene, polyacetylene, polycarbazole, polyacetylene, and polyethylenedioxythiophene. It is not something. These may be used alone or in combination.

  As shown in FIG. 1, the transparent electrode 3 has a shape reflecting the shape of the unevenness on the surface of the optical layer 2, and the surface on the light emitting layer 4 side of the transparent electrode 3 is the surface on the optical layer 2 side. Asperity is formed.

  In FIG. 1, unevenness having a shape reflecting the unevenness formed on the surface of the transparent electrode 3 on the light emitting layer 4 side is formed on the surface of the light emitting layer 4 on the reflective electrode 5 side. However, the surface of the light emitting layer 4 on the reflective electrode 5 side may be flat.

  The reflective electrode 5 is made of a conductive material that can reflect light emitted from the light emitting layer 4 and has sufficiently higher conductivity than the material used for the light emitting layer 4. The reflective electrode 5 is made of, for example, aluminum or silver.

  Next, an example of a method for manufacturing the organic EL element 100 will be described.

(Glass substrate surface treatment)
A glass substrate (Corning, Eagle XG, refractive index 1.51) is placed in a washing container, ultrasonically washed in a neutral detergent, then ultrasonically washed in pure water, and heated and dried at 120 ° C. for 120 minutes. went. The glass substrate was subjected to silane coupling treatment.

(Optical layer forming process)
TMPT (made by Shin-Nakamura Chemical Co., Ltd.) 2500 mg and EA-200 (made by Osaka Gas Chemical Co., Ltd.) 650 mg as binders, PGME (propylene glycol monomethyl ether) 12000 mg and MEK (methyl ethyl ketone) 4500 mg as rollers, stirrer The mixture is sufficiently mixed to form a binder layer material liquid.

  Next, a total of 4500 mg of crosslinked acrylic particles (Techpolymer (manufactured by Sekisui Chemical Co., Ltd., registered trademark)) and 30 mg of a polymerization initiator (BASF Japan) are added to the binder layer material liquid, and a stirrer is added. To form a material liquid for the optical layer 2.

  The optical layer 2 is formed by applying the material liquid of the optical layer 2 to the glass substrate using a wire bar, and then curing by UV irradiation (365 nm) for 10 minutes.

  The number of the particles 2B included in the optical layer 2 thus formed is N, and the particles 2B when the particles 2B are arranged in a close-packed manner so that the particles 2B do not overlap in a plane region on the transparent substrate 1 on which the optical layer 2 is formed. When the number is M, when N = α × M (α is a natural number of 1 or more), the particle density of the optical layer 2 is said to be α.

  The said conditions regarding the space | interval of the convex parts of the optical layer 2 are realizable by adjusting the particle size of the particle | grains contained in a material liquid, and the value of said (alpha). The particle density α is preferably 1.5 or more, and more preferably 2 or more.

(Transparent electrode formation process)
On the optical layer 2, Ag is formed as an auxiliary wiring with a thickness of 100 nm through a metal mask, and then ITO is sputter-deposited by a vacuum deposition apparatus to form the transparent electrode 3.

(Light emitting layer forming step)
On the transparent electrode 3, HATCN (hexaazatriphenylene hexacarbonitrile) is vacuum-deposited to a thickness of 10 nm to form a hole injection layer.

  On the hole injection layer, α-NPD (Bis [N- (1-naphthyl) -N-phenyl] benzidine) is vacuum-deposited to a thickness of 500 nm to form a first hole transport layer.

  On the 1st positive hole transport layer, the organic material A represented by the following structural formula is vacuum-deposited so that it may become thickness 5nm, and a 2nd positive hole transport layer is formed.

  Next, MCP (meta-dicarbazo-9-rylbenzene) is used as a host material on the second hole transport layer, and the following structural formula, which is a phosphorescent material of 40% by mass with respect to the host material, is represented by the following structural formula. A material doped with the light emitting material A is vacuum-deposited to a thickness of 30 nm to form an organic light emitting layer.

  On the organic light emitting layer, BAlq (Bis- (2-methyl-8-quinolinolato) -4- (phenyl-phenolate) -aluminum (III)) represented by the following structural formula was vacuum-deposited to a thickness of 39 nm. Thus, the first electron transport layer is formed.

  On the first electron transport layer, BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) represented by the following structural formula was vacuum-deposited so as to have a thickness of 1 nm. The electron transport layer is formed.

  LiF is deposited on the second electron transport layer so as to have a thickness of 1 nm to form an electron injection layer. Thereby, the formation of the light emitting layer 4 is completed.

(Reflective electrode formation process)
Aluminum is deposited on the electron injection layer to a thickness of 200 nm to form the reflective electrode 5.

(Sealing process)
A constituent other than the transparent substrate 1 is sealed with a sealing glass can in which a desiccant is attached in a nitrogen gas atmosphere and a sealing material is applied to the installation surface with the transparent substrate.

  According to the organic EL element 100, the optical layer can be formed by applying the material liquid as described above. Moreover, the space | interval of the convex parts of an optical layer can be easily adjusted with the particle size and density of particle | grains. Therefore, the light extraction efficiency can be improved without increasing the manufacturing cost.

  Next, the result of verifying the light extraction efficiency in the organic EL element 100 having the above configuration will be described below.

<Example 1>
The organic EL element was manufactured by the said manufacturing method by making the particle size of the particle | grains which comprise an optical layer into 300 nm, and setting the density (alpha) = 2 of the particle | grains in an optical layer. In the element of Example 1, the refractive index difference between the particles in the optical layer and the binder layer was set to 0.01 by adjusting the mixing ratio of the two binders contained in the binder layer material. In the element of Example 1, the distribution of the convex portions of the optical layer was as shown in the graph of FIG. In the graph shown in FIG. 3, the horizontal axis indicates the interval between the convex portions, and the vertical axis indicates the appearance frequency for each interval between the convex portions as the relative frequency when the appearance frequency of the most existing interval is 1. ing.

<Example 2>
An organic EL element was produced in the same manner as in Example 1 except that the particle size of the particles constituting the optical layer was 500 nm. In the element of Example 2, the distribution of the convex portions of the optical layer was as shown in FIG.

<Example 3>
An organic EL device was produced in the same manner as in Example 1 except that the particle size of the particles constituting the optical layer was 800 nm. In the element of Example 3, the distribution of the convex portions of the optical layer was as shown in FIG.

<Example 4>
An organic EL element was produced in the same manner as in Example 1 except that the particle size of the particles constituting the optical layer was 500 nm and α was set to 3. In the element of Example 4, the distribution of the convex portions of the optical layer was as shown in FIG. The horizontal axis and horizontal axis of the graph shown in FIG. 4 are the same as those in FIG.

<Example 5>
An organic EL element was produced in the same manner as in Example 1 except that the particle diameter of the particles constituting the optical layer was 500 nm and α was set to 4. In the element of Example 5, the distribution of the convex portions of the optical layer was as shown in FIG.

<Example 6>
An organic EL device was produced in the same manner as in Example 1 except that the particle diameter of the particles constituting the optical layer was 500 nm and the material constituting the transparent electrode was changed to PEDOT-PSS. In the element of Example 6, the distribution of the convex portions of the optical layer was almost the same as that of the element of Example 2.

<Example 7>
The organic EL element was fabricated in the same manner as in Example 1 except that the particle diameter of the particles constituting the optical layer was 500 nm and the refractive index difference between the particles in the optical layer and the binder layer was adjusted to 0.05. Produced. In the element of Example 7, the distribution of the convex portions of the optical layer was almost the same as that of the element of Example 2.

<Example 8>
An organic EL element was produced in the same manner as in Example 1 except that the particle size of the particles constituting the optical layer was 500 nm and α was set to 1.5. In the element of Example 8, the distribution of the convex portions of the optical layer was as shown in FIG.

<Comparative Example 1>
An organic EL element was produced in the same manner as in Example 1 except that the optical layer was omitted.

<Comparative Example 2>
An organic EL element was produced in the same manner as in Example 1 except that the particle size of the particles constituting the optical layer was 500 nm and α = 1. In the element of Comparative Example 2, the distribution of the convex portions of the optical layer was as shown in FIG. The horizontal axis and the horizontal axis of the graph shown in FIG. 5 are the same as those in FIG.

<Comparative Example 3>
An organic EL device was produced in the same manner as in Example 1 except that the particle size of the particles constituting the optical layer was 1500 nm. In the element of Comparative Example 3, the distribution of the convex portions of the optical layer was as shown in FIG.

<Comparative example 4>
The organic EL element was fabricated in the same manner as in Example 1 except that the particle diameter of the particles constituting the optical layer was 500 nm and the refractive index difference between the particles in the optical layer and the binder layer was adjusted to 0.15. Produced. In the element of Comparative Example 4, the distribution of convex portions of the optical layer was as shown in FIG.

The external quantum efficiency and the driving voltage were measured for each device thus fabricated. As the external quantum efficiency and driving voltage, “C9920-12” manufactured by Hamamatsu Photonics was used, and the external quantum efficiency and the voltage value applied to the device when a current of ^2.5 mA / cm ² was passed through the device were read. Tables 1 and 2 below summarize the measurement results and device configuration.

  From comparison between Examples 1 to 8 and Comparative Example 2, it can be seen that the quantum efficiency increases when the relative frequency at the interval = 1000 nm is 0.5 or more.

  Comparative Example 3 satisfies the requirement that the relative frequency at the interval = 1000 nm is 1 and the relative frequency at the interval = 1000 nm is 0.5 or more, but the quantum efficiency is as low as 14.4%. This is presumably because the quantum efficiency decreases because the particle size of the particles is as large as 1500 nm, and the interval having a high relative frequency is biased toward the longer wavelength side. As in Examples 1 to 8, by setting the particle size to 1000 nm or less, intervals with high relative frequency are dispersed in a wavelength range of 100 nn to 1000 nm, and quantum efficiency can be increased.

  In Comparative Example 4, the relative frequency at the interval = 1000 nm is 0.9, but the quantum efficiency is as low as 10.1%. This is presumably because the refractive index difference between the particles and the binder layer is large in the optical layer, and light is scattered in the optical layer. From the results of Examples 1 to 8 and Comparative Example 4, it can be seen that the difference in refractive index between the particles and the binder layer in the optical layer is 0.1 or less, thereby preventing the quantum efficiency from being lowered.

  Also, Examples 1, 2, 6, and 7 have higher quantum efficiencies than Examples 3 and 8. What is common to Examples 1, 2, 6, and 7 is that the relative frequency decreases non-monotonically from the interval at which the relative frequency becomes “1” to the interval of 1000 nm. Non-monotonic decrease means that the value does not continue to decrease, but increases or disappears along the way.

  That the relative frequency monotonously decreases from the peak means that the interval having a high relative frequency is difficult to disperse in the wavelength range of 100 nn to 1000 nm. Therefore, by providing the optical layer with a distribution in which the relative frequency decreases non-monotonically, the light extraction efficiency can be further improved as can be seen from the results of Examples 1, 2, 6, and 7.

  In addition, the results of Examples 1, 2, 3, 6, and 7 show that the efficiency can be maximized when the particle diameter of the optical layer is in the range of 300 nm to 600 nm. This is because the organic EL material used for the organic EL element has a refractive index as high as about 1.8, so the wavelength range of visible light (380 nm to 780 nm) is 200 nm to 450 nm in this organic EL material. This is because if the optical layer has a particle size, the extraction efficiency in the entire visible light can be increased.

  As described above, the following items are disclosed in this specification.

  The disclosed organic electroluminescent device is formed on a transparent substrate, an optical layer formed on the transparent substrate, having an uneven surface on the surface opposite to the transparent substrate and having a uniform refractive index, and the optical layer. A transparent electrode, a light emitting layer formed on the transparent electrode, and a reflective electrode formed on the light emitting layer. The optical layer includes a binder layer and particles embedded in the binder layer. The particle diameter of the particle is 1000 nm or less, and the interval between the convex portions on the surface of the optical layer varies in the range of 100 nm to 1000 nm, and the appearance of the interval that exists most in the optical layer appears. When the frequency is “1”, the appearance frequency of the above-mentioned interval of 1000 nm is 0.5 or more.

  The disclosed organic electroluminescence device has a frequency range from the most existing interval in the optical layer to the interval of 1000 nm when the appearance frequency is plotted for each interval of all the lengths included in the optical layer. In which the appearance frequency is non-monotonically decreasing.

  In the disclosed organic electroluminescent element, the number of the particles contained in the optical layer when the particles are arranged in a close-packed manner so that the particles do not overlap in a plane region where the optical layer is formed. Including those that are 1.5 times or more.

  The disclosed organic electroluminescent element includes those in which the particle diameter is in the range of 300 nm to 600 nm.

  The disclosed organic electroluminescent element includes those in which the difference in refractive index between the particles and the binder layer is 0.1 or less.

  The disclosed organic electroluminescent elements include those in which the average height of the irregularities is in the range of 20 nm to 200 nm.

DESCRIPTION OF SYMBOLS 100 Organic EL element 1 Transparent substrate 2 Optical layer 2A Binder layer 2B Particle 3 Transparent electrode 4 Light emitting layer 5 Reflective electrode

Claims (6)

A transparent substrate;
An optical layer formed on the transparent substrate and having a uniform refractive index having irregularities on the surface opposite to the transparent substrate;
A transparent electrode formed on the optical layer;
A light emitting layer formed on the transparent electrode;
A reflective electrode formed on the light emitting layer,
The optical layer is composed of a binder layer and particles embedded in the binder layer,
The particle diameter is 1000 nm or less,
The spacing between the convex portions on the surface of the optical layer varies in the range of 100 nm to 1000 nm,
An organic electroluminescent element in which the appearance frequency of the interval of 1000 nm is 0.5 or more, when the appearance frequency of the interval existing most in the optical layer is “1”. The organic electroluminescent device according to claim 1,
When the appearance frequency is plotted for each of the intervals included in the optical layer, the appearance frequency decreases non-monotonically between the interval that exists most in the optical layer and the interval of 1000 nm. Organic electroluminescent device. The organic electroluminescent element according to claim 1 or 2,
The number of the particles contained in the optical layer is 1.5 times or more of the number of the particles when the particles are arranged in a close-packed manner so that the particles do not overlap in a plane region where the optical layer is formed. Organic electroluminescent device. It is an organic electroluminescent element of any one of Claims 1-3, Comprising:
The organic electroluminescent element having a particle size of 300 nm to 600 nm. It is an organic electroluminescent element of any one of Claims 1-4, Comprising:
The organic electroluminescent element whose refractive index difference of the said particle | grain and the said binder layer is 0.1 or less. It is an organic electroluminescent element of any one of Claims 1-5, Comprising:
The organic electroluminescent device having an average height of the irregularities in a range of 20 nm to 200 nm.

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