Ink composition

Abstract

This invention provides a technology that can contribute to improving the performance of a light-emitting element having a layer made of metal oxide nanoparticles between the cathode and anode. [Solution] The ink composition comprises metal oxide nanoparticles selected from the group consisting of hole injection materials, hole transport materials, and electron transport materials, wherein the 95% particle size (D95) when the volume-based cumulative particle size distribution is measured by dynamic light scattering is 60 nm or less, and a dispersion medium containing a glycol-based solvent.

Description

Technical Field

[0001] The present invention relates to an ink composition.

Background Art

[0002] Layers included between an anode and a cathode in an organic electroluminescence element or a quantum dot light-emitting diode may be formed by a printing method using an ink composition. For example, Patent Document 1 describes forming an electron transport layer by an inkjet printing method. Patent Document 2 describes forming a hole injection layer, a hole transport layer, and an electron transport layer by an inkjet printing method.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a technique that can contribute to improving the performance of a light-emitting element having a layer made of metal oxide nanoparticles between a cathode and an anode.

Means for Solving the Problems

[0005] According to one aspect of the present invention, there is provided an ink composition including metal oxide nanoparticles made of a material selected from the group consisting of a hole injection material, a hole transport material, and an electron transport material, and having a 95% particle diameter (D95) of 60 nm or less when measuring a volume-based cumulative particle size distribution by a dynamic light scattering method, and a dispersion medium including a glycol-based solvent.

[0006] According to another aspect of the present invention, the ink composition according to the above aspect is provided, which comprises one or more types of nanoparticles, each containing bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

[0007] According to yet another aspect of the present invention, an ink composition relating to any of the above aspects is provided, wherein the metal oxide nanoparticles have a 95% particle size (D95) of 20 nm or more.

[0008] According to yet another aspect of the present invention, an ink composition relating to any of the above aspects is provided in which the metal oxide nanoparticles have a 50% particle size (D50) in the range of 3 nm to 50 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

[0009] According to yet another aspect of the present invention, an ink composition relating to any of the above aspects is provided, wherein the metal oxide nanoparticles have a cumulative 50% particle size (D50) in the range of 3 nm to 30 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

[0010] According to yet another aspect of the present invention, an ink composition relating to any of the above aspects is provided in which the metal oxide nanoparticles have a 10% particle size (D10) in the range of 3 nm to 30 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

[0011] According to yet another aspect of the present invention, an ink composition is provided in which the proportion of the metal oxide nanoparticles in the ink composition is in the range of 0.05% by mass to 20% by mass.

[0012] According to yet another aspect of the present invention, an ink composition according to any of the above aspects is provided, wherein the glycol-based solvent is one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, and hexylene glycol.

[0013] According to yet another aspect of the present invention, the ink composition provided is an ink composition relating to any of the above aspects that does not contain metal and metal compounds other than the metal oxide nanoparticles.

[0014] According to yet another aspect of the present invention, the ink composition provided is an ink composition according to any of the above aspects, comprising only the metal oxide nanoparticles and the dispersion medium.

[0015] In yet another aspect of the present invention, a method for forming a printed layer is provided, comprising applying an ink composition according to any of the above aspects to a substrate to form a coating film, and drying the coating film to cure the coating film.

[0016] According to yet another aspect of the present invention, a printed layer formed by the method relating to the above aspect is provided.

[0017] According to yet another aspect of the present invention, a display device is provided that has a printed layer according to the above aspect. [Effects of the Invention]

[0018] The present invention provides a technology that can contribute to improving the performance of a light-emitting element having a layer made of metal oxide nanoparticles between the cathode and the anode. [Brief explanation of the drawing]

[0019] [Figure 1] Figure 1 is a cross-sectional view of a display device according to one embodiment of the present invention. [Modes for carrying out the invention]

[0020] The inventors have newly found that when a layer included between an anode and a cathode of a light-emitting element is formed using an ink containing metal oxide nanoparticles, aggregates may occur in the coating film (i.e., a layer composed of a cured product of the ink), causing the surface of the coating film to bulge, and thus it may be impossible to form a coating film having a uniform thickness. These aggregates have a size that can be observed with an optical microscope at a magnification of 100 times, are not contained in the ink, and are considered to have occurred during the formation of the coating film. A coating film with non-uniform thickness may lead to a deterioration in the performance of the light-emitting element. The inventors have solved this new problem by using metal oxide nanoparticles having a specific particle size distribution in the ink and have thus completed the present invention.

[0021] Embodiments of the present invention will be described below with reference to the drawings. The embodiments described below are more specific forms of any of the above aspects. The matters described below can be incorporated into each of the above aspects alone or in combination.

[0022] In addition, the embodiments shown below illustrate configurations for embodying the technical idea of the present invention, and the technical idea of the present invention is not limited by the materials, shapes, structures, etc. of the following constituent members. Various changes can be made to the technical idea of the present invention within the technical scope defined by the claims described in the claims.

[0023] Note that the drawings are schematic, and relationships such as the relationship between dimensions in one direction and dimensions in another direction, and the relationship between the dimensions of one member and the dimensions of another member, etc. may be different from actual ones.

[0024] I<1> Display device FIG. 1 is a cross-sectional view of a display device according to an embodiment of the present invention. The display device 1 shown in FIG. 1 employs an active matrix driving method and is a display device capable of displaying a color image.

[0025] The display device 1 includes a plurality of pixels arranged in the X and Y directions, as described later. Each pixel includes a first subpixel PXR, a second subpixel PXG, and a third subpixel PXB. Each of the first subpixel PXR, second subpixel PXG, and third subpixel PXB includes a light-emitting element and a pixel circuit. Here, as an example, the semiconductor included in the light-emitting element is assumed to be an inorganic material.

[0026] The display device 1 includes a substrate 11, an anode 12, a partition layer 13, a hole injection layer 14, a hole transport layer 15, a light-emitting layer 16, an electron transport layer 17, and a cathode 18. The anode 12, the hole injection layer 14, the hole transport layer 15, the light-emitting layer 16, the electron transport layer 17, and the portion of the cathode 18 facing the anode 12 constitute a light-emitting element.

[0027] In Figure 1, the X and Y directions are parallel to the display surface of the display device 1 and intersect each other. For example, the X and Y directions are orthogonal to each other. The Z direction is perpendicular to the X and Y directions, i.e., the thickness direction of the display device 1.

[0028] In one example, the substrate 11 includes an insulating substrate such as a glass substrate and an array portion provided on one of its main surfaces. In another example, the substrate 11 includes a semiconductor substrate such as a silicon substrate and an array portion provided on one of its surface regions. The array portion includes pixel circuits and wiring that supplies signals and power to the pixel circuits. The pixel circuits are arranged in the X and Y directions. Each pixel circuit includes transistors and capacitors as driving elements and switches, and wiring that connects them to each other. The transistors are, for example, field-effect transistors. Here, as an example, the driving element is a p-channel field-effect transistor and the switch is an n-channel field-effect transistor.

[0029] The anode 12 in this case is a pixel electrode arranged on the substrate 11 in the X and Y directions, corresponding to the pixel circuit. Each anode 12 is connected to the drain of the driving element included in the corresponding pixel circuit.

[0030] If the substrate 11 is light-transmitting, the display device 1 may be a top-emission type or a bottom-emission type. If the substrate 11 is light-shielding, the display device 1 shall be a top-emission type.

[0031] When the display device 1 is of the bottom emission type, the anode 12 is a light-transmitting electrode. For example, transparent conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide, and fluorine-doped tin oxide (FTO) can be used as the material for the light-transmitting electrode. A layer made of the transparent conductive oxide can be formed, for example, by sputtering.

[0032] When the display device 1 is of the top emission type, it is preferable that the anode 12 includes a light-reflecting layer. The light-reflecting layer is made of, for example, elemental metals such as aluminum and silver, or an alloy containing one or more of them. Layers made of elemental metals and alloys can be formed, for example, by vacuum deposition.

[0033] The anode 12, which includes a light-reflecting layer, may further include a light-transmitting layer on top of the light-reflecting layer. As the material for the light-transmitting layer, for example, those exemplified as materials for light-transmitting electrodes can be used. It is preferable that the material constituting the upper surface of the anode 12 has a large work function.

[0034] The partition layer 13 is provided on the substrate 11 and the anode 12. The partition layer 13 has through holes at the positions of the anode 12. Each of these through holes has a shape that tapers from the upper opening to the lower opening. Each of the anodes 12 has its peripheral edge covered by the partition layer 13, and its central part is exposed in the internal space of the through holes provided in the partition layer 13.

[0035] The partition layer 13 is made of an insulator. In one example, the partition layer 13 is made of an inorganic insulator. In another example, the partition layer 13 is made of a cured resin.

[0036] The hole injection layer 14 covers the central portion of the anode 12 within through-holes provided in the partition layer 13. The ionization energy of the hole injection layer 14 is typically greater than the work function of the anode 12.

[0037] The hole injection layer 14 consists of a hole injection material. The hole injection material is, for example, a metal oxide such as nickel oxide (NiO), bismuth oxide (Bi2O3), cobalt oxide (CoO), copper oxide (Cu2O), molybdenum oxide (MoO3), and magnesium oxide (MgO). These metal oxides can be incorporated into the hole injection layer 14 in the form of metal oxide nanoparticles. The hole injection material can be incorporated into the hole injection layer 14 without forming aggregates and causing the surface of the hole injection layer 14 to bulge. This allows the hole injection layer 14 to have a uniform thickness.

[0038] The thickness of the hole injection layer 14 is preferably in the range of 1 nm to 200 nm, and more preferably in the range of 5 nm to 50 nm.

[0039] The hole transport layer 15 covers the hole injection layer 14 within through-holes provided in the partition layer 13. Typically, the ionization energy of the hole transport layer 15 is greater than that of the hole injection layer 14.

[0040] The hole transport layer 15 is composed of a hole transport material. The hole transport material is, for example, a metal oxide such as bismuth oxide (Bi2O3), cobalt oxide (CoO), copper oxide (Cu2O), molybdenum oxide (MoO3), and magnesium oxide (MgO). These metal oxide hole transport materials can be contained in the hole transport layer 15 in the form of metal oxide nanoparticles. The hole transport material can be contained in the hole transport layer 15 without forming aggregates and causing the surface of the hole transport layer 15 to bulge. This allows the hole transport layer 15 to have a uniform thickness.

[0041] The thickness of the hole transport layer 15 is preferably in the range of 1 nm to 200 nm, and more preferably in the range of 10 nm to 50 nm.

[0042] The light-emitting layer 16 covers the hole transport layer 15 within through-holes provided in the partition layer 13. Typically, the light-emitting layer 16 has a higher ionization energy and electron affinity compared to the hole transport layer 15.

[0043] The light-emitting layer 16 is made of a light-emitting material. The first subpixel PXR, the second subpixel PXG, and the third subpixel PXB have different light-emitting materials in their light-emitting layers 16. For example, the light-emitting layer 16 of the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB uses a red-emitting material, a green-emitting material, and a blue-emitting material, respectively.

[0044] One example of a light-emitting material is a quantum dot. A quantum dot is, for example, a semiconductor particle having a core-shell structure and a particle size in the range of a few nanometers to about 10 nanometers.

[0045] The core consists of a semiconductor responsible for light emission. The emission spectrum of a quantum dot changes by changing the type of semiconductor that makes up the core and the particle size of the core.

[0046] The shell is a thin layer epitaxially grown on the surface of the core, having a thickness of 1 to 4 atoms. The shell contributes to improving and stabilizing the luminescence efficiency. The shell may have a single-layer structure or a multi-layer structure.

[0047] One example of a quantum dot is a structure in which a core made of InP is covered with a first shell made of ZnSe, and this is then covered with a second shell made of ZnS. Such quantum dots emit red light when the particle size is large, and green light when the particle size is small.

[0048] Another example of a quantum dot has a structure in which a core made of ZnSeTe is covered with a first shell made of ZnSe, and this is then covered with a second shell made of ZnS. Such quantum dots emit blue light when the particle size is small.

[0049] Quantum dots may have ligands on the surface of their core-shell particles. The ligands are hydrocarbons with functional groups that contribute to improved resistance and prevention of aggregation in dispersions. Note that the ligands may be at least partially absent in the display device 1.

[0050] The thickness of the light-emitting layer 16 is preferably in the range of 1 nm to 200 nm, and more preferably in the range of 10 nm to 50 nm.

[0051] The electron transport layer 17 covers the light-emitting layer 16 within through-holes provided in the partition layer 13. Typically, the electron transport layer 17 has a higher ionization energy and electron affinity compared to the light-emitting layer 16.

[0052] The electron transport layer 17 is made of an electron transport material. The electron transport material is, for example, a metal oxide such as magnesium-doped zinc oxide (MgZnO), zinc oxide (ZnO), and titanium oxide (TiO). These metal oxide electron transport materials can be contained in the electron transport layer 17 in the form of metal oxide nanoparticles. The electron transport material can be contained in the electron transport layer 17 without forming aggregates and causing the surface of the electron transport layer 17 to bulge. This allows the electron transport layer 17 to have a uniform thickness.

[0053] The thickness of the electron transport layer 17 is preferably in the range of 1 nm to 200 nm, and more preferably in the range of 10 nm to 50 nm.

[0054] The cathode 18 covers the electron transport layer 17 and the exposed portion of the partition layer 13. Here, the cathode 18 is a common electrode facing multiple anodes 12.

[0055] When the display device 1 is of the bottom emission type, it is preferable that the cathode 18 includes a light-reflecting layer. The light-reflecting layer is made of, for example, elemental metals such as aluminum and silver, or an alloy containing one or more of them. Layers made of elemental metals and alloys can be formed, for example, by vacuum deposition.

[0056] When the display device 1 is of the top emission type, the cathode 18 is a light-transmitting electrode. As the material for the light-transmitting electrode, for example, transparent conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide, and fluorine-doped tin oxide (FTO) can be used. A layer made of transparent conductive oxide can be formed, for example, by sputtering.

[0057] The work function of the cathode 18 is typically smaller than that of the anode 12 and larger than that of the electron affinity of the electron transport layer 17. The cathode 18 may include a layer made of a material with a low work function, such as a MgAg alloy or an AlLi alloy, between the layer made of a metal or transparent conductive oxide and the electron transport layer 17.

[0058] The display device 1 may further include one or more other elements. For example, the display device 1 may further include an electron injection layer, such as a LiF layer, between the electron transport layer 17 and the cathode 18. The display device 1 may also further include a sealing film or sealing substrate that seals the light-emitting element.

[0059] Furthermore, the display device 1 employs a forward structure for the light-emitting element. The light-emitting element may also employ an inverse structure in which the stacking order of the layers it contains is reversed.

[0060] <2> Method for manufacturing a display device The display device 1 shown in Figure 1 can be manufactured, for example, by the following method.

[0061] First, a structure (substrate) including a substrate 11, an anode 12, and a partition layer 13 is prepared.

[0062] Next, the hole injection layer 14 and the hole transport layer 15 are formed sequentially. Each of the hole injection layer 14 and the hole transport layer 15 can be formed, for example, by a printing method using an ink composition containing metal oxide nanoparticles and a dispersion medium. The hole injection layer 14 can be formed as a printed layer by applying the ink composition to the above structure (substrate) to form a coating film, and then drying the coating film to harden it. The hole transport layer 15 can be formed as a printed layer by applying the ink composition to the hole injection layer 14 (substrate) to form a coating film, and then drying the coating film to harden it. In forming the hole injection layer 14 and the hole transport layer 15, the application of the ink composition to the substrate can be carried out using printing methods such as inkjet printing, spin coating, and slit coating.

[0063] Next, the light-emitting layer 16 is formed. As described above, the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB have different light-emitting materials in their light-emitting layer 16. Therefore, the light-emitting layer 16 of the first subpixel PXR, the light-emitting layer 16 of the second subpixel PXG, and the light-emitting layer 16 of the third subpixel PXB are formed separately. Each of these light-emitting layers 16 can be formed, for example, using the lift-off method.

[0064] Next, the electron transport layer 17 is formed. The electron transport layer 17 can be formed, for example, by a printing method using an ink composition containing metal oxide nanoparticles and a dispersion medium. The electron transport layer 17 can be formed as a printed layer by applying the ink composition to the light-emitting layer 16 (substrate) to form a coating film, and then drying the coating film to harden it. In forming the electron transport layer 17, the application of the ink composition to the substrate can be carried out using printing methods such as inkjet printing, spin coating, and slit coating.

[0065] Next, a cathode 18 is formed. The cathode 18 can be formed by vacuum deposition, sputtering, or a combination thereof. Afterward, the light-emitting element is sealed as needed. In this way, the display device 1 shown in the figure is obtained.

[0066] <3> Ink composition As described above, the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17 can be formed by printing methods such as inkjet printing, spin coating, and slit coating. The ink composition used for this printing is preferably such that it can form the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17 as layers of uniform thickness without causing the surfaces of these layers to bulge due to the generation of aggregates.

[0067] From this perspective, it is preferable that one or more of the hole injection layer 14, hole transport layer 15, and electron transport layer 17 be formed using an ink composition having the composition described below.

[0068] In other words, a preferred ink composition comprises metal oxide nanoparticles and a dispersion medium containing a glycol-based solvent.

[0069] The metal oxide nanoparticles consist of materials selected from the group consisting of hole injection materials, hole transport materials, and electron transport materials. As for the metal oxide nanoparticles, those described above can be used for the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17.

[0070] Preferably, the metal oxide nanoparticles include one or more of several types of nanoparticles, each containing bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide, for example, any of these nanoparticles. More preferably, the metal oxide nanoparticles include at least one of the following: nanoparticles containing nickel oxide, nanoparticles containing zinc oxide, and nanoparticles containing magnesium-doped zinc oxide, for example, any one of these nanoparticles.

[0071] Metal oxide nanoparticles have a 95% particle size (D95) (hereinafter also referred to as "D95") of 60 nm or less, when the volume-based cumulative particle size distribution is measured by dynamic light scattering. Here, "D95" refers to the particle size at which the volume-based cumulative distribution corresponds to 95% when the volume-based cumulative distribution is measured by dynamic light scattering and the particle size is plotted on the x-axis and the volume-based cumulative (%) is plotted on the y-axis. D95 is, for example, in the range of 10 nm to 60 nm, preferably 10 nm to 50 nm, and more preferably 10 nm to 30 nm.

[0072] D95 is preferably 20 nm or greater. Specifically, D95 is preferably in the range of 20 nm to 60 nm, more preferably 20 nm to 50 nm, and even more preferably 20 nm to 30 nm.

[0073] The metal oxide nanoparticles have a 50% particle size (D50) (hereinafter also referred to as "D50"), measured by dynamic light scattering, where the volume-based cumulative particle size distribution is preferably in the range of 3 nm to 50 nm, more preferably 3 nm to 30 nm, and even more preferably 10 nm to 25 nm. Here, "D50" refers to the particle size at which the volume-based cumulative distribution corresponds to 50% when the volume-based cumulative distribution is measured by dynamic light scattering and the particle size is plotted on the x-axis and the volume-based cumulative (%) is plotted on the y-axis.

[0074] The metal oxide nanoparticles have a 10% particle size (D10) (hereinafter also referred to as "D10"), which is preferably in the range of 3 nm to 30 nm, more preferably 3 nm to 20 nm, when the volume-based cumulative particle size distribution is measured by dynamic light scattering. Here, "D10" refers to the particle size when the volume-based cumulative distribution corresponds to 10% when the volume-based cumulative distribution is measured by dynamic light scattering and the particle size is plotted on the x-axis and the volume-based cumulative (%) is plotted on the y-axis.

[0075] The proportion of metal oxide nanoparticles in the ink composition is preferably in the range of 0.05% to 20% by mass, more preferably in the range of 0.1% to 10% by mass, and even more preferably in the range of 0.5% to 5% by mass.

[0076] The dispersion medium contains a glycol-based solvent. The glycol-based solvent is one or more selected from the group consisting of, for example, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, and hexylene glycol.

[0077] The dispersion medium may further contain other solvents. The proportion of glycol-based solvent in the dispersion medium is preferably in the range of 50% to 100% by mass, and more preferably in the range of 80% to 100% by mass.

[0078] The other solvents mentioned above are preferably one or more aprotic solvents such as amide compounds, aliphatic ester compounds, and aliphatic ether compounds. The one or more aprotic solvents may include, for example, one or more N,N'-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and propylene glycol monomethyl ether acetate (PGMEA). The proportion of the aprotic solvent in the other solvents is preferably in the range of 80% to 100% by mass, and more preferably 100% by mass.

[0079] The proportion of the dispersion medium in the ink composition is preferably in the range of 80% to 99% by mass, and more preferably in the range of 85% to 99% by mass.

[0080] The above ink composition comprises the above metal oxide nanoparticles and the above dispersion medium, but preferably does not contain metal or metal compounds other than the above metal oxide nanoparticles. It is more preferable that the above ink composition consists only of the above metal oxide nanoparticles and the above dispersion medium. When the above ink composition consists only of the above metal oxide nanoparticles and the above dispersion medium, the proportion of metal oxide nanoparticles in the ink composition is preferably in the range of 0.05% to 20% by mass, more preferably 0.1% to 10% by mass, and even more preferably 0.5% to 5% by mass, and the proportion of dispersion medium in the ink composition is preferably in the range of 80% to 99.95% by mass, more preferably 90% to 99.9% by mass, and even more preferably 95% to 99.5% by mass.

[0081] As described above, the ink composition contains metal oxide nanoparticles exhibiting a specific particle size distribution. This allows the ink composition to form a layer of metal oxide nanoparticles as a uniformly thick layer without causing surface elevation due to the formation of aggregates. As a result, the ink composition can contribute to improving the performance of a light-emitting device having a layer of metal oxide nanoparticles between the cathode and anode. [Examples]

[0082] The tests conducted in connection with the present invention are described below.

[0083] <1> Preparation of ink composition <1.1>Example 1 An ink composition containing metal oxide nanoparticles and a dispersion medium was prepared.

[0084] As the metal oxide nanoparticles used, "nanoparticles consisting of nickel(II) oxide with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm" were used. The amount of metal oxide nanoparticles was adjusted so that the proportion of metal oxide nanoparticles in the ink composition was 1% by mass.

[0085] Ethylene glycol (EG) and propylene glycol monomethyl ether acetate (PGMEA) were used as dispersion media. The amount of ethylene glycol added relative to the total amount of the ink composition was 85% by mass. The amount of propylene glycol monomethyl ether acetate added relative to the total amount of the ink composition was 14% by mass.

[0086] <1.2>Example 2 An ink composition similar to that in Example 1 was prepared, except for the following: In this example, instead of "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm," "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 28 nm, a D50 of 12 nm, and a D10 of 8 nm" were used as the metal oxide nanoparticles.

[0087] <1.3> Example 3 An ink composition similar to that in Example 1 was prepared, except for the following: In this example, instead of "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm," "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 43 nm, a D50 of 20 nm, and a D10 of 13 nm" were used as the metal oxide nanoparticles.

[0088] <1.4>Example 4 An ink composition similar to that in Example 1 was prepared, except for the following: In this example, instead of "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm," "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 57 nm, a D50 of 21 nm, and a D10 of 14 nm" were used as the metal oxide nanoparticles.

[0089] <1.5> Comparative Example 1 An ink composition similar to that in Example 1 was prepared, except for the following: In this example, instead of "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm," "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 66 nm, a D50 of 29 nm, and a D10 of 19 nm" were used as the metal oxide nanoparticles.

[0090] <1.6> Comparative Example 2 An ink composition similar to that in Example 1 was prepared, except for the following: In this example, instead of "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm," "nanoplastic nanoparticles made of nickel(II) oxide with a D95 of 65 nm, a D50 of 23 nm, and a D10 of 15 nm" were used as the metal oxide nanoparticles.

[0091] <2> evaluation <2.1> Particle size of metal oxide nanoparticles For each of the metal oxide nanoparticles used in Examples 1 to 4 and Comparative Examples 1 to 2, D95, D50, and D10 were determined from the volume-based cumulative particle size distribution. The cumulative particle size distribution was measured by dynamic light scattering. A Nanotrac® UPA-EX150 particle size analyzer manufactured by Nikkiso Co., Ltd. was used to measure the cumulative particle size distribution.

[0092] <2.2> Uniformity of the coating film A coating film was prepared using each of the ink compositions described in Examples 1 to 4 and Comparative Examples 1 to 2, and the uniformity of the coating film was evaluated. The evaluation was carried out as follows: First, the ink composition was applied to a 5 cm square glass substrate by spin coating, air-dried for 5 minutes, and then baked on a hot plate at 200°C for 5 minutes. The resulting coating film (i.e., the layer consisting of the cured ink composition) was observed with an optical microscope at 100x magnification. The observation area with the optical microscope was the area excluding the portion from the outer edge of the 5 cm square glass substrate to 0.5 mm inward (i.e., the 4 cm square area in the center of the glass substrate). If no aggregates were observed, it was evaluated as "A," and if aggregates were observed, it was evaluated as "B."

[0093] <2.3> Summary of Evaluations The results of the above evaluation are shown in Table 1 below.

[0094] [Table 1]

[0095] As shown in Table 1, the ink compositions of Comparative Examples 1 and 2 showed aggregates in the coating film, and the coating film did not have a uniform thickness. These aggregates were large enough to be observed at 100x magnification with an optical microscope (on the order of microns). These aggregates were not present in the ink composition and are thought to have formed during the formation of the coating film. In contrast, the ink compositions of Examples 1 to 4 showed no aggregates in the coating film and the coating film had a uniform thickness. [Explanation of symbols]

[0096] 1...Display device, 11...Substrate, 12...Anode, 13...Blocking layer, 14...Hole injection layer, 15...Hole transport layer, 16...Light-emitting layer, 17...Electron transport layer, 18...Cathode, PXB...Third subpixel, PXG...Second subpixel, PXR...First subpixel.

Claims

1. A metal oxide nanoparticle comprising a material selected from the group consisting of hole injection materials, hole transport materials, and electron transport materials, wherein the 95% particle size (D95) when the volume-based cumulative particle size distribution is measured by dynamic light scattering is 60 nm or less, A dispersion medium containing a glycol-based solvent An ink composition containing the following:

2. The ink composition according to claim 1, wherein the metal oxide nanoparticles comprise one or more of a plurality of nanoparticles, each containing bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

3. The ink composition according to claim 1, wherein the metal oxide nanoparticles have a 95% particle size (D95) of 20 nm or more.

4. The ink composition according to claim 1, wherein the metal oxide nanoparticles have a 50% particle size (D50) in the range of 3 nm to 50 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

5. The ink composition according to claim 1, wherein the metal oxide nanoparticles have a cumulative 50% particle size (D50) in the range of 3 nm to 30 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

6. The ink composition according to claim 1, wherein the metal oxide nanoparticles have a 10% particle size (D10) in the range of 3 nm to 30 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

7. The ink composition according to claim 1, wherein the proportion of the metal oxide nanoparticles in the ink composition is in the range of 0.05% by mass to 20% by mass.

8. The ink composition according to claim 1, wherein the glycol-based solvent is one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, and hexylene glycol.

9. The ink composition according to claim 1, which does not contain a metal or a metal compound other than the metal oxide nanoparticles.

10. The ink composition according to claim 1, comprising only the metal oxide nanoparticles and the dispersion medium.

11. The method involves applying the ink composition according to any one of claims 1 to 10 to a substrate to form a coating film, The coating film is dried and hardened. A method for forming a printed layer containing [a specific material].

12. A printed layer formed by the method described in claim 11.

13. A display device comprising the printed layer according to claim 12.

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