Electron transport layer composition for inkjet printing and method of making same

Abstract

The present invention provides an electron transport layer composition for inkjet printing, which comprises metal oxide nanoparticles and at least three solvents differing in one or more of viscosity, polarity, and vapor pressure, and is inkjet ejectable, a method for producing the same, and a light-emitting device including an electron transport layer formed from the composition.

Description

[Technical Field]

[0001] The present invention relates to an electron transport layer composition for inkjet printing, which comprises metal oxide nanoparticles and at least three solvents that differ in one or more of viscosity, polarity, and vapor pressure, and which is inkjet-dischargeable; a method for producing the same; and a light-emitting device including an electron transport layer formed from the composition. [Background technology]

[0002] Quantum dot light-emitting devices (QLEDs) are attracting attention as next-generation displays due to their high quantum efficiency and color purity. Solution-based spin coating is the most commonly used method for manufacturing QLEDs, but this method is not suitable for large-scale production and commercialization, so much research is being conducted on inkjet printing. The inkjet printing method allows for simple and inexpensive processes, making it more suitable for commercialization and large-scale production.

[0003] In recent years, much research has been done on inkjet-printed quantum dot solutions, but little research has been done on the electron transport layer (ETL) for inkjet printing. In particular, further research is needed on the composition of the inkjet-printed electron transport layer (ETL) to form a uniform thin film of the ETL on a patterned emission layer (EML). Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have noticed that currently widely known ethanol-based electron transport layer compositions cannot be uniformly discharged using an inkjet printing device.

[0005] The present invention has been devised to solve the above-mentioned problems, and a technical object of the present invention is to provide an electron transport layer composition for inkjet printing, which contains a mixture of at least three solvents adjusted to specific physical properties, thereby facilitating uniform ejection by inkjet printing and forming a uniform film from the ejected ink, and a method for producing the same.

[0006] Another technical object of the present invention is to provide a light emitting device including an electron transport layer formed by an ink jet printing method using the above-mentioned electron transport layer composition.

[0007] Other objects and advantages of the present invention will be more clearly set forth in the detailed description of the invention and claims that follow. [Means for solving the problem]

[0008] In order to achieve the above-mentioned technical objectives, the present invention provides an electron transport layer composition for inkjet printing, which comprises metal oxide nanoparticles and at least three solvents differing in one or more of viscosity, polarity, and vapor pressure, and which is inkjet-ejectable.

[0009] In one embodiment of the present invention, the composition may have a viscosity at 20°C of 1.0 to 3.0 cps, a vapor pressure at 20°C of 0.6 to 45 mmHg, a contact angle of 25 to 80°, and a solids content of 5 to 30 wt%.

[0010] In one embodiment of the present invention, the at least three solvents can include dimethyl sulfoxide (DMSO), ethanol, 2-methoxyethanol, and butanol. In one embodiment of the present invention, the mixing ratio of the DMSO, ethanol, 2-methoxyethanol, and butanol may be 3-8:1-3:1-2:0-1 by volume.

[0011] In one embodiment of the present invention, the metal oxide nanoparticles may be Zn-containing metal oxide nanoparticles.

[0012] In one embodiment of the present invention, the Zn-containing metal oxide nanoparticles may be alloyed with a metal that can increase the band gap of ZnO.

[0013] In one embodiment of the present invention, the Zn-containing metal oxide nanoparticles may be ZnMgO. In one embodiment of the present invention, the metal oxide nanoparticles may have organic ligands having hydrophilic moieties attached to a portion or all of their surfaces.

[0014] In one embodiment of the present invention, the organic ligand may include at least two hydrophilic moieties selected from a carboxyl group and a hydroxy group.

[0015] In one embodiment of the present invention, the organic ligand may be contained in an amount ranging from 0.0001 to 10 mol per 1 mol of zinc contained in the metal oxide nanoparticles.

[0016] In one embodiment of the present invention, the metal oxide nanoparticles may include a polymer coating layer formed on a part or all of the surface.

[0017] The present invention also provides a light-emitting element comprising: a first electrode; a second electrode disposed opposite to the first electrode; a light-emitting layer disposed between the first electrode and the second electrode; a hole transport layer disposed between the first electrode and the light-emitting layer; and an electron transport layer disposed between the light-emitting layer and the second electrode and formed from the electron transport layer composition as described above.

[0018] In one embodiment of the present invention, the electron transport layer can be formed by inkjet printing.

[0019] In one embodiment of the present invention, the light emitting device may further include at least one of a hole injection layer and an electron injection layer.

[0020] The present invention further provides a method for producing an electron transport layer composition for inkjet printing as described above, the method comprising: a first step of adding a basic substance to a reaction solution in which a zinc-containing compound and a metal-containing compound capable of increasing the band gap of ZnO are dissolved in a solvent, followed by precipitation to produce a metal oxide nanoparticle dispersion; and a second step of adding at least three solvents differing in one or more of viscosity, polarity, and vapor pressure to the produced metal oxide nanoparticle dispersion and mixing the solvents to produce an electron transport layer composition.

[0021] In one embodiment of the present invention, the metal oxide nanoparticle dispersion liquid produced in the first step further comprises an organic ligand, and the organic ligand may be contained in an amount of 10 wt % or less relative to 100 wt % of the metal oxide nanoparticle dispersion liquid.

[0022] In one embodiment of the present invention, the electron transport layer composition prepared in the second step further comprises at least one of a monomer and a dispersant, and the at least one of the monomer and the dispersant may be included in an amount of 10% by volume or less, based on 100% by volume of the electron transport layer composition. [Effects of the Invention]

[0023] According to one embodiment of the present invention, by mixing at least three solvents having different physical properties such as viscosity, polarity, and vapor pressure, it is possible to provide an electron transport layer composition for inkjet printing, which can be easily ejected uniformly by an inkjet method and can form a uniform film from the ejected ink.

[0024] Furthermore, in the present invention, the uniformity of the film formed can be ensured by applying a specific ligand and monomer (dispersant) during the synthesis of the metal oxide nanoparticles used as the electron transport layer.

[0025] As a result, the electron transport layer composition of the present invention is useful for producing light-emitting devices, specifically, self-luminous displays, by an inkjet printing process, and by applying the inkjet process simply and inexpensively, advantageous effects can be obtained for commercialization and large-scale production.

[0026] The effects of the present invention are not limited to the above-mentioned contents, and various effects are included in this specification. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a graph showing the absorption spectrum of ZnMgO nanoparticles produced in Example 1. [Figure 2] 1 is a graph showing the absorption spectrum of ZnMgO nanoparticles produced in Comparative Example 1. [Figure 3] 1 shows an image and a graph of a surface profile analysis of the ejection profile of the ink composition of the electron transport layer prepared in Example 1. [Figure 4] FIG. 10 is an image diagram of ejected ink using the ink composition for the electron transport layer produced in Example 3. [Figure 5] FIG. 10 is an image showing one drop and a 1*5 pattern of ZnMgO in the ink composition of the electron transport layer produced in Example 2. [Figure 6] FIG. 10 is an image showing one drop and a 1*5 pattern of ZnMgO in the ink composition for the electron transport layer produced in Example 3. [Figure 7] 1 is a conceptual diagram showing one drop and a 1*5 pattern formed on a substrate using the ink composition for an electron transport layer prepared in Example 2. FIG. [Figure 8] FIG. 10 is an image showing the shape of one drop of ZnMgO in the ink composition for the electron transport layer produced in Example 4. [Figure 9] FIG. 10 is an image showing the shape of one drop of ZnMgO in the ink composition for the electron transport layer produced in Example 5. [Figure 10]FIG. 10 is an image showing the shape of one drop of ZnMgO in the ink composition for the electron transport layer produced in Example 6. [Figure 11] FIG. 10 is an image showing the shape of one drop of ZnMgO in the ink composition for the electron transport layer produced in Example 7. [Figure 12] FIG. 1 is an image diagram showing the ejected ink produced in Comparative Example 1. [Figure 13] FIG. 1 is a conceptual diagram showing a one-dot pattern of Comparative Example 1. [Figure 14] FIG. 10 is a conceptual diagram showing a one-dot pattern of Comparative Example 2. [Figure 15] 1 is a graph showing the CRF (Coffee Ring Factor) results of inks deposited on a substrate using electron transport layer compositions in Examples 1-3 and 6-7. [Figure 16] 1 is a graph showing the luminous efficiency of light-emitting devices fabricated using electron transport layer compositions in Examples 1-3 and 6-7. [Figure 17] 1 is a graph showing quantum efficiency of light-emitting devices fabricated using electron transport layer compositions in Examples 1-3 and 6-7. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will be described in detail below. Unless otherwise specified, all terms used herein (including technical and scientific terms) have the meanings that are commonly understood by those of ordinary skill in the art to which this disclosure belongs. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless otherwise defined.

[0029] Furthermore, throughout this specification, when a part "comprises" a certain component, unless otherwise specified, it should be understood as an open-ended term that does not exclude other components but includes the possibility of further including other components. Furthermore, throughout this specification, "on" or "above" means not only being located above or below the target part, but also including cases where another part exists between them, and does not necessarily mean being located above with respect to the direction of gravity.

[0030] In addition, in this specification, "(meth)acrylate" means acrylate and methacrylate, "(meth)acrylic" means acrylic and methacrylic, and "(meth)acryloyl" means acryloyl and methacryloyl.

[0031] In this specification, the terms "monomer" and "monomer" have the same meaning. In the present invention, a monomer is distinguished from an oligomer and a polymer and refers to a compound having a weight-average molecular weight of 1,000 or less.

[0032] The present invention provides an electron transport layer composition that can be ejected by inkjet printing and that can realize the desired device characteristics.

[0033] For this reason, in the present invention, at least three or more solvents that can be ejected are selected in consideration of the appropriate viscosity and vapor pressure in an inkjet device, and these are mixed in a predetermined ratio and used as solvents for the ink composition.

[0034] In addition, by adding a specific ligand, monomer, or dispersant to prevent aggregation of fine particles to the surface of metal oxide nanoparticles, the coffee ring effect (CRF) can be improved during inkjet ejection, ensuring film uniformity.

[0035] As a result, the present invention can provide an electron transport layer produced by an inkjet printing process and a self-luminous display having the same.

[0036] <Electron transport layer composition for inkjet printing> The electron transport layer composition according to one embodiment of the present invention is an ink composition that can be ejected by a general inkjet method to form an electron transport layer (ETL).

[0037] In one embodiment, the composition comprises metal oxide nanoparticles and at least three solvents differing in any one or more of viscosity, polarity, and vapor pressure, in a predetermined ratio, and may optionally contain at least one or more conventional additives known in the art.

[0038] The composition of the electron transport layer will be described in detail below.

[0039] Metal Oxide Nanoparticles The electron transport layer composition according to the present invention comprises metal oxide nanoparticles.

[0040] The metal oxide nanoparticles can be any nanoparticles used in the art for electron transport layers without limitation. For example, conventional metal oxide nanoparticles used as dopant materials can be used, including, but not limited to, In2S3, Cu2S, Ag2S, ZnSe, ZnS, ZnO, ZnTe, ZnSe, TiO2, SnO2, ZnS, or nanoparticles containing at least one of the above elements.

[0041] Specifically, the metal oxide nanoparticles are Zn-containing metal oxide nanoparticles, and more specifically, they can be alloyed with a metal (M) that can increase the band gap of ZnO [ZnMO (M = Ca, Mg)]. The metal (M) that can increase the band gap of ZnO is Ca or Mg. These metals have an ionic radius similar to that of Zn, so they can be incorporated into the ZnO lattice without causing stress, thereby reducing the size of ZnO and increasing the band gap of ZnO. ZnMgO is preferred.

[0042] Increasing the band gap of ZnO nanoparticles by alloying them and applying them to the electron transport layer results in an upward shift in the conduction band minimum (CBM) level, bringing the energy closer between the CBM of the quantum dot light-emitting layer and the electron transport layer. This lowers the electron energy barrier and ultimately promotes electron injection into the quantum dot region. As a result, light-emitting devices with electron transport layers containing alloyed ZnO nanoparticles have improved luminance and efficiency compared to devices with electron transport layers containing ZnO nanoparticles, enabling them to achieve higher luminous efficiency even at lower drive voltages. In other words, applying such an electron transport layer reduces the electron injection barrier, resulting in lower drive voltages, improved efficiency, and even reduced power consumption for QLEDs. Lowering the drive voltage of devices reduces heat generation, which is expected to extend device life.

[0043] In one embodiment, the metal oxide nanoparticles may have organic ligands with hydrophilic moieties attached to some or all of their surfaces.

[0044] As the organic ligand, any known organic ligand in the art can be used without limitation, for example, C5 to C 20Examples of suitable organic ligands include alkyl carboxylic acids, alkenyl carboxylic acids, or alkynyl carboxylic acids, pyridine, mercaptoalcohol, thiol, phosphine, phosphine oxide, primary amines, secondary amines, and combinations thereof. Specifically, the organic ligand may contain at least two hydrophilic moieties selected from a carboxyl group and a hydroxyl group. More specifically, a ligand substance having the hydrophilic moieties of a carboxyl group and / or a hydroxyl group at both ends of the molecular structure is preferred. Examples of organic ligands that can be used include, but are not limited to, mono-(2-acryloyloxyethyl) succinate (MAES), mono-2-(methacryloyloxy)ethyl succinate (MMES), 2-(2-methoxyethoxy)acetic acid (MEAA), or mixtures thereof.

[0045] Such organic ligands adhere to the surface of metal oxide nanoparticles, thereby improving the coffee ring effect (CRF) during inkjet ejection, thereby ensuring film uniformity. The coffee ring effect (CRF) is a phenomenon in which colloidal particles migrate to the periphery due to the hydrodynamic effect of the evaporation process, resulting in uneven particle density distribution. Smaller particles tend to migrate to the periphery, resulting in smaller particles accumulating at the periphery of the droplet, while larger particles are distributed in the center of the droplet. In the present invention, the coffee ring effect can be minimized and film uniformity can be maximized by incorporating a specific organic ligand and / or a monomer (described below) into the electron transport layer composition.

[0046] The content of the organic ligand is not particularly limited and can be appropriately adjusted within a range known in the art. In consideration of dispersibility and film uniformity, the content may be in the range of 0.0001 to 10 mol, specifically 0.001 to 5 mol, per mol of the metal contained in the metal oxide nanoparticles, for example, zinc (Zn).

[0047] In one embodiment, the metal oxide nanoparticles may include a polymer coating layer formed on a part or all of the surface.

[0048] The components of the polymer coating layer are not particularly limited, and conventional polymers known in the art may be used. For example, the layer may be formed using any of acrylic or methacrylic polymers or polyethylene glycols known in the art. Specifically, it is preferable to use a (meth)acrylic polymer containing a predetermined range of polar functional groups in the molecule and / or polyethylene glycol having a hydrophilic portion. Such functional groups can react with certain chemicals, and this reaction may be used to induce a polymer coating on the surface of the metal oxide nanoparticles.

[0049] The monomer (monomer) forming the polymer coating layer may be, but is not limited to, a conventional (meth)acrylate monomer known in the art. Usable monomers include, for example, monofunctional (meth)acrylate monomers such as caprolactone acrylate, orthophenylphenol ethoxy acrylate, lauryl acrylate, isodecyl acrylate, tetrahydrofurfuryl acrylate, 2-hydroxyethyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, ethoxyethoxyethyl acrylate, isobornyl (meth)acrylate, alkoxylated tetrahydrofurfuryl (meth)acrylate, and 2-ethylhexyl acrylate; ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyolefin glycol di(meth)acrylate, and ethoxylated polypropylene glycol di(meth). ) acrylate, 2-hydroxy-3-acryloyloxypropyl methacrylate, 2-hydroxy-1,3-dimethacryloxypropane, dioxane glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, glycerin di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 2-methyl-1,8-octanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, butyl ethyl propanediol di(meth)acrylate, 3-methyl-1,Di(meth)acrylates having an aromatic ring such as 5-pentanediol di(meth)acrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate, triethylene glycol diacrylate, and trichlorodecane dimethanol diacrylate, trifunctional (meth)acrylate monomers such as ethoxylated glycerin tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane (ethylene oxide)-modified triacrylate, ethoxylated trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, trimethylolpropane (propylene oxide)-modified triacrylate, and tris(2-hydroxyethyl)isocyanurate triacrylate; Examples of the monomer include, but are not limited to, tetrafunctional (meth)acrylate monomers such as dipentaerythritol pentaacrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate (tetrafunctional monomer), and mixtures thereof; pentafunctional (meth)acrylate monomers such as dipentaerythritol pentaacrylate, propionic acid-modified dipentaerythritol pentaacrylate, dipentaerythritol pentamethacrylate, propionic acid-modified dipentaerythritol pentamethacrylate, and mixtures thereof; and hexafunctional (meth)acrylate monomers such as dipentaerythritol hexaacrylate, caprolactone-modified dipentaerythritol hexaacrylate, dipentaerythritol hexamethacrylate, caprolactone-modified dipentaerythritol hexamethacrylate, and mixtures thereof.

[0050] By forming the polymer coating layer on the surface of the metal oxide nanoparticles, an improvement effect of the coffee ring effect (CRF) is achieved during inkjet ejection, and the uniformity of the film can be ensured.

[0051] In the present invention, metal oxide nanoparticles are mainly described as the material constituting the electron transport layer. However, the present invention is not limited to the above, and the application of organic substances or organic-inorganic composites that can be used as electron transport layer materials in the relevant field also falls within the scope of the present invention.

[0052] In the present invention, the content of the metal oxide nanoparticles is not particularly limited and can be appropriately adjusted within a range known in the art. For example, it may be 5 to 30 parts by weight, specifically 10 to 20 parts by weight, relative to the total weight (e.g., 100 parts by weight) of the electron transport layer composition.

[0053] solvent The electron transport layer composition according to the present invention contains at least three solvents having different physical properties such as viscosity, polarity, and vapor pressure.

[0054] Conventionally, metal oxide nanoparticles, such as ZnMgO, have been dispersed in alcohol-based solvents due to the characteristics of the material. However, when ethanol is used alone, the device characteristics are realized, but there is a problem that inkjet ejection is not possible.

[0055] The ejection conditions for an inkjet device are broadly classified into viscosity and vapor pressure. If the viscosity is too high or too low, a uniform film cannot be obtained, and the degree of ejection is determined by the vapor pressure. The present invention is characterized by considering the viscosity and vapor pressure appropriate for inkjet ejection, selecting at least three solvents that satisfy these physical properties, and controlling the mixing ratio of these solvents within a predetermined range to form a solvent for the ink composition of the electron transport layer.

[0056] In one embodiment, the electron transport layer composition of the present invention containing at least three solvents may have a viscosity of 1.0 to 3.0 cps at 20°C, a vapor pressure of 0.6 to 45 mmHg at 20°C, a contact angle of 25 to 80°, and a solids content of 5 to 30 wt%. More specifically, the composition may have a viscosity of 1.2 to 2.0 cps, a vapor pressure of 1.0 to 30 mmHg, a contact angle of 30 to 50°, and a solids content of 5 to 25 wt%.

[0057] When the ink has the physical properties of viscosity, vapor pressure, and contact angle as described above, inkjet ejection is easily performed, and the ejected ink has uniformity, so that the characteristics of the element can be realized.

[0058] The electron transport layer composition according to the present invention is not particularly limited in terms of the specific components and / or the contents of the at least three or more solvents constituting the composition, as long as the composition satisfies the above-mentioned viscosity, vapor pressure, and contact angle properties.

[0059] In one embodiment, the at least three solvents include dimethyl sulfoxide (DMSO), ethanol, 2-methoxyethanol, and butanol.

[0060] Specifically, the present invention uses dimethyl sulfoxide (DMSO), which has good dispersibility and viscosity, as the primary solvent. However, when DMSO is used alone for inkjet ejection, wettability may be reduced. On the other hand, when DMSO is mixed with ethanol, which has excellent wettability, the viscosity and contact angle required for inkjet ejection are not met. Therefore, the present invention uses DMSO and ethanol, and further includes 2-methoxyethanol and butanol, which are alcohol-based solvents with appropriate viscosities, to control the final composition so that it simultaneously satisfies the viscosity, vapor pressure, and contact angle characteristics described above. For example, the viscosity of each solvent may be DMSO (1.98 cps), ethanol (1.07 cps), 2-methoxyethanol (1.70 cps), and butanol (2.57 cps). The vapor pressure of each solvent may be DMSO (0.6 mmHg), ethanol (44.62 mmHg), 2-methoxyethanol (6 mmHg), or butanol (5.47 mmHg). By mixing the above three or more solvents in a predetermined ratio, an optimal electron transport layer composition can be obtained, with a viscosity of 1.0 to 3.0 cps, a contact angle of 25 to 80°, and a vapor pressure of 0.6 to 45 mmHg.

[0061] In one embodiment of the present invention, the at least three solvents contain dimethyl sulfoxide (DMSO) as a primary solvent, and the DMSO may be present in an amount of 30% by volume or more relative to 100% by volume of the total solvents. The ethanol content may be 10% by volume or more relative to 100% by volume of the total solvents. The 2-methoxyethanol content may be 10% by volume or more relative to 100% by volume of the total solvents, and the butanol content may be in the range of 0 to 50% by volume relative to 100% by volume of the total solvents. The term "total solvents" may refer to the total volume of the at least three or more solvents. It may also refer to the at least three solvents further including a dispersion solvent contained in the metal oxide nanoparticle dispersion liquid described below.

[0062] In one embodiment, the mixing ratio of dimethyl sulfoxide (DMSO), ethanol, 2-methoxyethanol, and butanol among the at least three solvents is 3-8:1-3:1-2:0-1 by volume, preferably 5:3:1:1 by volume. If the DMSO ratio is less than 3, inkjet ejection becomes unstable, and the alcohol-based solvent ratio becomes relatively high, resulting in only favorable device characteristics. Furthermore, 2-methoxyethanol and butanol are added to ensure the desired viscosity and contact angle of the metal oxide nanoparticles (ZnMgO). If the ratio of these solvents exceeds 1, the viscosity becomes too high, adversely affecting inkjet ejection and device realization.

[0063] In the present invention, the content of the at least three solvents is not particularly limited and can be appropriately adjusted within a range known in the art, for example, the remainder in 100 parts by weight of the electron transport layer composition, specifically, 70 to 95 parts by weight.

[0064] additives In addition to the above-mentioned components, the electron transport layer composition of the present invention may contain at least one additive known in the art without limitation, as long as the effect of the present invention is not impaired.

[0065] Examples of additives that can be used include light stabilizers, thermal stabilizers, photoinitiator accelerators, thermal initiation accelerators, leveling agents, reinforcing agents, thickening agents, colorants, reactive diluents, coupling agents, dispersants, and solvents. These may be used alone or in combination of two or more. The content of the additives can be adjusted appropriately within a range known in the art and is not particularly limited. For example, the content of the at least one additive may be 0.01 to 5 parts by weight, specifically 0.01 to 2 parts by weight, based on the total weight of the electron transport layer composition.

[0066] The electron transport layer composition for inkjet printing according to the present invention can be prepared by mixing and stirring the above-described metal oxide nanoparticles, at least three solvents, and optionally, a ligand, a monomer, and other additives according to a conventional method well known in the art.

[0067] One embodiment of the method for producing the electron transport layer composition includes, for example, (i) a first step of adding a basic substance to a reaction solution in which a zinc-containing compound and a metal-containing compound capable of increasing the band gap of ZnO are dissolved in a solvent, followed by precipitation to produce a dispersion of metal oxide nanoparticles; and (ii) a second step of adding at least three solvents, each having a different viscosity, polarity, or vapor pressure, to the dispersion of metal oxide nanoparticles produced above, followed by mixing the solvents to produce the electron transport layer composition.

[0068] In the first step, the zinc-containing compound and the metal-containing compound are not particularly limited, and any compounds known in the art can be used without limitation, such as zinc acetate dihydrate, zinc chloride, zinc nitrate, zinc sulfate, magnesium acetate tetrahydrate, etc.

[0069] The basic substance is not particularly limited, and any basic substance known in the art can be used without limitation. For example, at least one selected from the group consisting of tetramethyl ammonium hydroxide (TMAH), potassium hydroxide (KOH), sodium hydroxide (NaOH), and amines can be used.

[0070] In the first step, the dispersion of metal oxide nanoparticles may further contain an organic ligand, the content of which may be in the range of 0 to 10 wt % relative to the total weight (e.g., 100 wt %) of the dispersion of metal oxide nanoparticles, and more specifically, may be greater than 0 to 5 wt % or less.

[0071] In the first step, the dispersion of metal oxide nanoparticles may further contain a polymer. Such a polymer may contain a conventional hydrophilic moiety known in the art, for example, at least two hydrophilic moieties selected from a carboxyl group and a hydroxyl group. Usable polymers include, for example, polyethylene glycol (PEG) having a carboxyl group. The polymer may be present in an amount of 0 to 10 wt % relative to the total weight (e.g., 100 wt %) of the dispersion of metal oxide nanoparticles, but is not limited thereto.

[0072] After adding the basic substance, the mixture is heated to 55 to 65°C, and then stirred and reacted at a uniform speed for about 0.5 to 2 hours to precipitate nanoparticles. The metal oxide nanoparticles may then be separated using the properties of the solvent and non-solvent, producing a dispersion in which the metal oxide nanoparticles are uniformly dispersed.

[0073] Examples of solvents that can be used include, but are not limited to, hexane, benzene, xylene, toluene, octane, chloroform, chlorobenzene, tetrahydrofuran (THF), methylene chloride, 1,4-dioxane, diethyl ether, cyclohexane, and dichlorobenzene. These may be used alone or in combination of two or more. Examples of non-solvents that can be used include, but are not limited to, acetone, ethanol, methanol, butanol, propanol, isopropyl alcohol, tetrahydrofuran, dimethyl sulfoxide, and dimethylformamide. These may be used alone or in combination of two or more.

[0074] In the second step, the mixing ratio of the metal oxide nanoparticles to the at least three solvents may be a weight ratio of 5-30:95-70, but is not limited thereto and can be appropriately adjusted within a range known in the art.

[0075] The mixing method is not particularly limited, and can be carried out using, for example, a mixer known in the art, such as a homodisper, homomixer, universal mixer, planetary mixer, kneader, or three-roll mixer.

[0076] The electron transport layer composition prepared in the second step may further include at least one of a monomer and a dispersant, and the content of the monomer and / or dispersant may be in the range of 0 to 10% by volume, specifically, more than 0 to 5% by volume, based on 100% by volume of the electron transport layer composition.

[0077] The electron transport layer composition of the present invention configured as described above has optimized viscosity and vapor pressure properties, thereby achieving excellent workability and processability. In particular, the composition ensures uniformity and stability in terms of inkjet ejection, the shape of the ejected ink, and the shape of the ink deposited on the substrate, making it useful for inkjet printing methods.

[0078] <Light-emitting element> The light emitting device according to one embodiment of the present invention is distinguished from conventional light emitting devices in that it includes an electron transport layer formed from the above-described electron transport layer composition.

[0079] In one embodiment, the light-emitting element includes a first electrode, a second electrode facing the first electrode, a light-emitting layer between the first electrode and the second electrode, a hole transport layer between the first electrode and the light-emitting layer, and an electron transport layer between the light-emitting layer and the second electrode and formed by inkjet printing the electron transport layer composition. Optionally, the light-emitting element may include at least one of a hole injection layer and an electron injection layer.

[0080] Hereinafter, the present invention will be described using a quantum dot light emitting device as an example, but the present invention is not limited thereto, and the light emitting device can be applied to various light emitting devices such as an organic light emitting device.

[0081] The first electrode is disposed on a substrate. Such a substrate may be a transparent, flat glass substrate or a transparent plastic substrate. To remove contaminants, the substrate may be ultrasonically cleaned in a solvent such as isopropyl alcohol, acetone, or methanol, followed by UV ozone treatment.

[0082] The first electrode serves as an anode. For example, the anode may be made of a metal, a metal oxide that meets the respective transparent / opaque conditions, or other non-oxide inorganic material. For bottom emission, the first electrode may be made of a transparent conductive metal such as transparent ITO, IZO, ITZO, or AZO.

[0083] The hole injection layer and hole transport layer are located on the first electrode. These hole injection layer and hole transport layer facilitate hole injection from the first electrode and transport holes to the light-emitting layer. The hole transport layer can be organic or inorganic. Organic examples include CBP (4,4'-N,N'-dicarbazole-biphenyl), -NPD (N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4"-diamine), TCTA (4,4',4"-tris(N-carbazolyl)-triphenylamine), TFB, and DNTPD (N,N'-di(4-(N,N'-diphenyl-amino)phenyl)-N,N'-diphenylbenzidine). Inorganic examples include NiO or MoO3 oxides. As an example, the hole injection layer may be poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), and the hole transport layer may be TFB or poly(9-vinylcarbazole) (PVK), etc.

[0084] The light-emitting layer is disposed on the hole transport layer, and the quantum dots are provided in the light-emitting layer. For example, the light-emitting layer can be formed by dispersing the quantum dots in a solvent, coating the hole transport layer with the dispersion using a solution process, and then volatilizing the solvent. Examples of the coating method include drop casting, spin coating, dip coating, spray coating, flow coating, screen printing, and inkjet printing, which can be used alone or in combination.

[0085] The quantum dots (QDs) that make up the light-emitting layer can also refer to nano-sized semiconductor materials. Atoms form molecules, and molecules form clusters, which are molecular aggregates of a small number of molecules, to form nanoparticles. When such nanoparticles have semiconductor properties, they are called quantum dots. When the quantum dots receive external energy and become floating, they themselves emit energy according to the corresponding energy band gap.

[0086] The quantum dots may have a homogeneous single layer structure, a multilayer structure such as a core-shell type or a gradient structure, or a mixture of these structures.

[0087] The components of the single-layer quantum dot (QD) or the multi-layer shell (excluding the core and / or surface (outermost shell)) can be independently selected from the group II-VI compounds, group III-V compounds, group IV-VI compounds, group IV elements, group IV compounds, and combinations thereof, as described below. In this case, when the shell has multiple layers, each layer may contain a different component, for example, a (quasi)metal oxide, and can be freely composed of the components exemplified below.

[0088] In one example, the II-VI compound may be a binary compound selected from the group consisting of CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, MgSe, MgS, and mixtures thereof, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdH and ternary compounds selected from the group consisting of CdZnSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, and mixtures thereof, and quaternary compounds selected from the group consisting of CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, and mixtures thereof.

[0089] In another example, the III-V compound may be selected from the group consisting of binary compounds selected from the group consisting of GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, and mixtures thereof; ternary compounds selected from the group consisting of GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, and mixtures thereof; and quaternary compounds selected from the group consisting of GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, and mixtures thereof.

[0090] In another example, the IV-VI compound may be selected from the group consisting of binary compounds selected from the group consisting of SnS, SnSe, SnTe, PbS, PbSe, PbTe, and mixtures thereof; ternary compounds selected from the group consisting of SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, and mixtures thereof; and quaternary compounds selected from the group consisting of SnPbSSe, SnPbSeTe, SnPbSTe, and mixtures thereof.

[0091] In another example, the group IV element may be selected from the group consisting of Si, Ge, and mixtures thereof, and the group IV compound may be a binary compound selected from the group consisting of SiC, SiGe, and mixtures thereof.

[0092] The two-element compound, three-element compound, or four-element compound described above may be present in a particle at a uniform concentration, or may be present in the same particle with partially different concentrations. Also, quantum dots may have a core-shell structure in which one quantum dot envelops another. The interface between the core and shell may have a concentration gradient in which the concentration of the element present in the shell decreases toward the center.

[0093] The shape of the quantum dots is not particularly limited as long as it is a shape commonly used in the art, and may be, for example, spherical, rod-shaped, pyramidal, disc-shaped, multi-armed, or cubic nanoparticles, nanotubes, nanowires, nanofibers, or plate-shaped nanoparticles.

[0094] The size of the quantum dots is not particularly limited and can be adjusted appropriately within a normal range known in the art. 50) is 1 to 20 nm, and specifically, may be 2 to 15 nm. When the particle size of the quantum dots is controlled in this manner within the range of approximately 1 to 20 nm, light of a desired color can be emitted. For example, blue-emitting quantum dots may be used in the present invention. Specifically, the blue-emitting quantum dots (QDs) may be Cd-based II-VI group QDs (e.g., CdZnS, CdZnSSe, CdZnSe, CdS, CdSe), non-Cd-based II-VI group QDs (e.g., ZnSe, ZnTe, ZnS, HgS), or non-Cd-based III-V group QDs (e.g., InP, InGaP, InZnP, GaN, GaAs, GaP).

[0095] The electron transport layer facilitates electron injection from the second electrode and transports electrons to the light-emitting layer. Such an electron transport layer contains Zn-containing metal oxide nanoparticles alloyed with a metal that can increase the band gap of ZnO. For example, the electron transport layer can be formed by inkjet printing the electron transport layer composition onto the light-emitting layer and then volatilizing the solvent. The electron transport layer of the present invention can have a single-layer structure that also functions as an electron injection layer, or it can be formed as a laminate structure with a separate electron injection layer.

[0096] The second electrode may be positioned on the electron injection / transport layer and serve as a cathode. The second electrode may be made of a metal, a metal oxide, or other non-oxide inorganic material that meets the requirements for transparency or opacity. In particular, the second electrode may be made of a metal having a low work function and excellent internal reflectivity to facilitate electron injection at the LUMO level of the emissive layer. Specifically, metals with a low work function, such as I, Ca, Ba, Ca / Al, LiF / Ca, LiF / Al, BaF / Al, BaF / Ca / Al, Al, Mg, and Ag:Mg alloys, may be used to facilitate electron injection.

[0097] The light-emitting device according to this embodiment has been described as a quantum dot light-emitting device. However, the light-emitting device may be various other light-emitting devices. For example, the light-emitting device may be an organic light-emitting device. Furthermore, although the electron injection / transport layer is described as being made of a single material in this embodiment, the electron injection layer and the electron transport layer may be provided separately.

[0098] The present invention will be described in detail below with reference to examples. However, the examples described below are merely illustrative of the present invention, and the present invention is not limited to these examples. [Example]

[0099] Example 1: Preparation of an electron transport layer composition for inkjet printing Zinc acetate dehydrate (Zn(OAc)2) and magnesium acetate tetrahydrate (magnesium acetate tetrahydrate) were dissolved in dimethyl sulfoxide (DMSO), followed by the addition of the basic substance tetramethyl ammonium hydroxide (TMAH). The mixture was heated to 60°C and allowed to react for approximately 1 hour. After the reaction was complete, hexane and acetone were added to the ZnMgO solution, and the particles were centrifuged. The ZnMgO solution was then dispersed in a 5:3:1:1 volumetric mixture of DMSO, ethanol, 2-methoxyethanol, and butanol. As described above, an electron transport layer composition (ZnMgO nanoparticle dispersion) suitable for inkjet printing was prepared for Example 1.

[0100] [Example 2] Preparation of an electron transport layer composition having a ligand for inkjet printing (1) Zinc acetate dehydrate (Zn(OAc)2) and magnesium acetate tetrahydrate (magnesium acetate tetrahydrate) were dissolved in dimethyl sulfoxide (DMSO), followed by the addition of a basic substance, tetramethyl ammonium hydroxide (TMAH), and the mixture was heated to 60°C. 0.2 ml of mono(2-acryloyloxyethyl)succinic acid (MAES) diluted to 5 wt% in ethanol was added to the reaction solution, followed by approximately 1 hour of reaction. After the reaction was complete, hexane and acetone were added to the ZnMgO solution, and the quantum dot particles were centrifuged. Then, a mixture of DMSO, ethanol, 2-methoxyethanol, and butanol (volume ratio: 5:3:1:1) was added and dispersed. As described above, an electron transport layer composition suitable for inkjet printing (Example 2) was prepared.

[0101] [Example 3] Preparation of an electron transport layer composition having a ligand for inkjet printing (2) Zinc acetate dehydrate (Zn(OAc)2) and magnesium acetate tetrahydrate (magnesium acetate tetrahydrate) were dissolved in dimethyl sulfoxide (DMSO), followed by the addition of a basic substance, tetramethyl ammonium hydroxide (TMAH), and the mixture was heated to 60°C. 0.2 ml of mono(2-acryloyloxyethyl)succinic acid (MAES) was added to the reaction solution, and the mixture was allowed to react for approximately 1 hour. After the reaction was complete, hexane and acetone were added to the ZnMgO solution, and the particles were centrifuged. Then, the mixture was dispersed in a 5:3:1:1 volumetric ratio of DMSO, ethanol, 2-methoxyethanol, and butanol. As described above, an inkjet-printable electron transport layer composition for Example 3 was prepared.

[0102] [Example 4] Preparation of an electron transport layer composition for inkjet printing containing a monomer (1) An electron transport layer composition for inkjet printing of Example 4 was prepared in the same manner as in Example 1, except that 1,6-hexanediol diacrylate monomer was mixed at a ratio of 1 volume % relative to 100 volume % of the total composition for the electron transport layer (ZnMgO nanoparticle dispersion).

[0103] [Example 5] Preparation of an electron transport layer composition for inkjet printing containing a monomer (2) An electron transport layer composition for inkjet printing of Example 5 was prepared in the same manner as in Example 1, except that 2% by volume of 1,6-hexanediol diacrylate monomer was mixed with 100% by volume of the total composition for the electron transport layer.

[0104] [Example 6] Preparation of an electron transport layer composition for inkjet printing containing a monomer (3) An electron transport layer composition for inkjet printing of Example 6 was prepared in the same manner as in Example 1, except that 3% by volume of 1,6-hexanediol diacrylate monomer was mixed with 100% by volume of the total composition for the electron transport layer.

[0105] Example 7: Preparation of a polymer-coated electron transport layer composition for inkjet printing 0.2 ml of carboxyl-containing polyethylene glycol (PEG) was added to the reaction solution of Example 1 and allowed to react for approximately 1 hour. After the reaction was complete, the ZnMgO:PEG solution was centrifuged and dispersed in a 5:3:1:1 volumetric mixture of DMSO, ethanol, 2-methoxyethanol, and butanol. In this manner, an inkjet-printable electron transport layer composition of Example 7 was prepared.

[0106] Comparative Example 1: Preparation of electron transport layer composition for spin coating Zinc acetate dehydrate and magnesium acetate tetrahydrate were dissolved in dimethyl sulfoxide (DMSO), and then the basic substance tetramethylammonium hydroxide (TMAH) was added and the mixture was heated to 60°C and reacted for approximately 1 hour. After the reaction was completed, the ZnMgO solution was mixed with hexane and acetone in a volume ratio of 1:2:6. The solvent was separated using a centrifuge, and the mixture was dispersed in ethanol to prepare the electron transport layer composition of Comparative Example 1.

[0107] Comparative Example 2: Preparation of an electron transport layer composition for spin coating containing a ligand Similarly to the above, Zn(OAc)2 and TMAH were dissolved, and during the reaction of the two substances, 0.2 ml of mono(2-acryloyloxyethyl)succinic acid (MAES) was added to 60 ml of Zn(OAc)2:TMAH, heated to 60°C, and reacted for approximately 1 hour. After the reaction was completed, hexane and acetone were added to the ZnMgO:MAES solution, and the solvent was separated using a centrifuge. Then, ethanol was added and the mixture was dispersed to produce the electron transport layer composition of Comparative Example 2.

[0108] [Experimental Example 1] Spectroscopic evaluation The electron transport layer compositions prepared in Example 1 and Comparative Example 1 were used to measure absorption spectra.

[0109] FIG. 1 shows the UV-visible absorption spectrum of the ZnMgO nanoparticles synthesized in Example 1, and FIG. 2 shows the UV-visible absorption spectrum of the ZnMgO nanoparticles synthesized in Comparative Example 1.

[0110] As shown in Figures 1 and 2, the maximum absorption wavelength of the produced ZnMgO is about 330 nm, and it has a wide absorption band in the ultraviolet region. This confirms that the same spectral characteristics are exhibited when using a spin coating composition and an inkjet composition.

[0111] [Experimental Example 2] Evaluation of inkjet ejection and shape In Examples 1 to 3, 6 to 7, and Comparative Examples 1 and 2, the electron transport layer compositions were used and the ejection and shape thereof were analyzed by inkjet printing.

[0112] Specifically, each of the prepared ink compositions was ejected using an inkjet printing device (OMNIJET200), and at this time, the ink compositions were ejected in the form of one drop and in the form of a 1*5 pattern formed by five drops.

[0113] The ink that had landed on the substrate was analyzed using a three-dimensional surface profilometer (NV9000, manufactured by Zygo). The following equation 1 was used to quantify the degree of the coffee ring effect, and the results are shown in Table 1 and Figures 3 to 15.

[0114] [Formula 1] JPEG0007719941000001.jpg30166 (in the formula, H max is the thickest thickness of the pattern, and H min is the thinnest thickness of the pattern, and the CRF value indicates the degree of the coffee ring effect. That is, CRF=1 indicates that the coffee ring is completely removed.

[0115] [Table 1] JPEG0007719941000002.jpg68166

[0116] As shown in Table 1 above, ejection and pattern formation by the inkjet method were impossible in Comparative Examples 1 and 2. In contrast, the electron transport layer composition of the present invention, which contains a mixture of at least three solvents with different physical properties, was easily ejected from a general inkjet printing device, and exhibited the characteristics that both the ejected ink and the ink that landed on the substrate had a uniform shape, confirming its usefulness in the inkjet printing method (see FIGS. 3 to 15).

[0117] [Experimental Example 3] Fabrication of light-emitting device and evaluation of its physical properties In Examples 1 to 3, 6 to 7, and Comparative Examples 1 and 2, light-emitting devices were produced using the electron transport layer compositions, and their physical properties were evaluated.

[0118] Specifically, an indium tin oxide (ITO) substrate was washed with isopropyl alcohol and acetone for 15 minutes each, and then dried in an oven at 60°C for 30 minutes. After drying, the substrate was treated with UV ozone for 20 minutes, and then spin-coated with PEDOT:PSS to form a hole injection layer (HIL). The spin-coating conditions were 4,500 rpm / 60 seconds and heat treatment conditions were 150°C / 20 minutes.

[0119] Next, a TFB material dissolved in chlorobenzene at 6 mg / ml was deposited under a nitrogen gas (N2) atmosphere at 4,500 rpm for 30 seconds, and then heat-treated at 15°C for 30 minutes to form a hole transport layer (HTL).

[0120] Then, to form the light-emitting layer (EML), a dispersion of indium phosphide (InP)-based quantum dots dispersed in 5 ml of octane was used to form a thin film by spin coating at 3,500 rpm for 20 seconds.

[0121] Next, the electron transport layer compositions prepared in Examples 1 to 3, 6 to 7, and Comparative Examples 1 to 2 were coated by an inkjet method to prepare an electron transport layer (ETL), and then electrodes were formed by a vacuum thermal deposition method to prepare light-emitting devices.

[0122] The efficiency of the light-emitting device manufactured by the method described above was evaluated using an IVL measurement device, and the results are shown in Table 2 below and in FIGS.

[0123] [Table 2] JPEG0007719941000003.jpg67166

[0124] As shown in Table 2 above, it was found that the light-emitting devices of Comparative Examples 1 and 2 could not be driven. In contrast, it was confirmed that the light-emitting devices of Examples 1 to 3 and 6 to 7, which had electron transport layers formed using the electron transport layer compositions according to the present invention, simultaneously exhibited low driving voltages, excellent luminous efficiency, and excellent external quantum efficiency (EQE) (see FIGS. 16 and 17).

Claims

1. Metal oxide nanoparticles, and at least three solvents that differ in one or more of viscosity, polarity, and vapor pressure; Including, 1. An inkjet-dischargeable electron transport layer composition for inkjet printing, comprising: the at least three solvents include dimethyl sulfoxide (DMSO), ethanol, 2-methoxyethanol, and butanol; The electron transport layer composition for inkjet printing, wherein the mixing ratio of the dimethyl sulfoxide (DMSO), the ethanol, the 2-methoxyethanol, and the butanol is 3-8:1-3:1-2:0-1 by volume.

2. The composition comprises: The viscosity at 20°C is 1.0 to 3.0 cps, The vapor pressure at 20°C is 0.6 to 45 mmHg, The contact angle is 25 to 80°, 2. The electron transport layer composition for ink jet printing of claim 1, wherein the solids content is from 5 to 30 weight percent.

3. 10. The electron transport layer composition for inkjet printing of claim 1, wherein the metal oxide nanoparticles are Zn-containing metal oxide nanoparticles.

4. 4. The electron transport layer composition for inkjet printing of claim 3, wherein the Zn-containing metal oxide nanoparticles are alloyed with a metal that can increase the band gap of ZnO.

5. 4. The electron transport layer composition for inkjet printing of claim 3, wherein the Zn-containing metal oxide nanoparticles are ZnMgO.

6. 2. The electron transport layer composition for inkjet printing of claim 1, wherein the metal oxide nanoparticles have organic ligands having hydrophilic moieties attached to a portion or all of their surfaces.

7. 7. The electron transport layer composition for ink jet printing of claim 6, wherein the organic ligand comprises at least two hydrophilic moieties selected from the group consisting of carboxyl groups and hydroxyl groups.

8. 7. The electron transport layer composition for inkjet printing according to claim 6, wherein the organic ligand is contained in an amount ranging from 0.0001 to 10 mol per 1 mol of the metal contained in the metal oxide nanoparticles.

9. 10. The electron transport layer composition for inkjet printing according to claim 1, wherein the metal oxide nanoparticles have a polymer coating layer formed on a part or all of their surfaces.

10. a first electrode; a second electrode disposed opposite the first electrode; a light-emitting layer disposed between the first electrode and the second electrode; a hole transport layer disposed between the first electrode and the light-emitting layer; and an electron transport layer disposed between the light-emitting layer and the second electrode, the electron transport layer being formed from the electron transport layer composition according to any one of claims 1 to 9; A light-emitting element comprising:

11. The light-emitting device according to claim 10, wherein the electron transport layer is formed by inkjet printing.

12. The light-emitting device of claim 10 , further comprising at least one of a hole injection layer and an electron injection layer.

13. a first step of adding a basic substance to a reaction solution in which a zinc-containing compound and a metal-containing compound capable of increasing the band gap of ZnO are dissolved in a solvent, followed by precipitation to produce a metal oxide nanoparticle dispersion; and a second step of adding at least three solvents having different viscosities, polarities, and vapor pressures to the prepared metal oxide nanoparticle dispersion and mixing them to prepare an electron transport layer composition; A method for making the electron transport layer composition for ink jet printing of claim 1 , comprising:

14. the metal oxide nanoparticle dispersion in the first step further comprises an organic ligand; The method according to claim 13 , wherein the organic ligand is contained in an amount of 10% by weight or less relative to 100% by weight of the metal oxide nanoparticle dispersion.

15. the electron transport layer composition prepared in the second step further comprises a monomer; The method according to claim 13 , wherein the monomer is contained in an amount of 10% by volume or less relative to 100% by volume of the electron transport layer composition.

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