Ink composition

Abstract

Provided is a technique capable of contributing to performance improvement of a light-emitting element having a layer composed of metal oxide nanoparticles and disposed between a cathode and an anode. More specifically, the ink composition contains: metal oxide nanoparticles comprising 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 a volume-based cumulative particle size distribution is measured by dynamic light scattering; and a dispersion medium containing a glycol-based solvent.

Description

Ink composition

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

[0002] Layers contained between an anode and a cathode in organic electroluminescent elements and quantum dot light-emitting diodes are sometimes 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.

[0003] Japanese Patent Publication No. 2021-116345 International Publication No. 2022 / 070296

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

[0005] According to one aspect of the present invention, there is provided an ink composition comprising 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 the volume-based cumulative particle size distribution is measured by a dynamic light scattering method, and a dispersion medium containing a glycol-based solvent.

[0006] According to another aspect of the present invention, there is provided an ink composition according to the above aspect, wherein the metal oxide nanoparticles include one or more of a plurality of types of nanoparticles, each of which includes 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, there is provided the ink composition according to any one of the above aspects, wherein the metal oxide nanoparticles have a 95% particle diameter (D95) of 20 nm or more.

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

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

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

[0011] According to yet another aspect of the present invention, there is provided the ink composition according to any one of the above aspects, 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.

[0012] According to yet another aspect of the present invention, there is provided the ink composition according to any one of the above aspects, 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, there is provided an ink composition according to any of the above aspects, wherein the ink composition does not contain any metal or any metal compound other than the metal oxide nanoparticles.

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

[0015] According to yet another aspect of the present invention, there is provided an ink composition comprising 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 50% particle size (D50) of 50 nm or less when the volume-based cumulative particle size distribution is measured by a dynamic light scattering method, and a dispersion medium containing a glycol-based solvent.

[0016] According to yet another aspect of the present invention, there is provided an ink composition according to the above aspect, wherein the metal oxide nanoparticles include one or more of a plurality of types of nanoparticles, each of which includes bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

[0017] According to yet another aspect of the present invention, there is provided the ink composition according to any one of the above aspects, wherein the metal oxide nanoparticles have a 50% particle size (D50) of 10 nm or more.

[0018] According to yet another aspect of the present invention, there is provided the ink composition according to any of the above aspects, wherein the metal oxide nanoparticles have a 95% particle diameter (D95) of 120 nm or less when a volume-based cumulative particle size distribution is measured by a dynamic light scattering method.

[0019] According to yet another aspect of the present invention, there is provided an ink composition according to any one of the above aspects, wherein the metal oxide nanoparticles have a 10% particle diameter (D10) in the range of 3 nm to 30 nm when a volume-based cumulative particle size distribution is measured by a dynamic light scattering method.

[0020] According to yet another aspect of the present invention, there is provided the ink composition according to any one of the above aspects, 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.

[0021] According to yet another aspect of the present invention, there is provided the ink composition according to any one of the above aspects, wherein the glycol-based solvent is one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, triethylene glycol, and hexylene glycol.

[0022] According to yet another aspect of the present invention, there is provided an ink composition according to any of the above aspects, wherein the ink composition does not contain any metal or any metal compound other than the metal oxide nanoparticles.

[0023] According to yet another aspect of the present invention, there is provided an ink composition according to any one of the above aspects, wherein the ink composition consists of only the metal oxide nanoparticles and the dispersion medium.

[0024] According to yet another aspect of the present invention, there is provided a method for forming a printed layer, 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 harden the coating film.

[0025] According to yet another aspect of the present invention, there is provided a printed layer formed by the method according to the above aspect.

[0026] According to yet another aspect of the present invention, there is provided a display device including the print layer according to the above aspect.

[0027] According to the present invention, a technique is provided that can contribute to improving the performance of a light-emitting device having a layer made of metal oxide nanoparticles between a cathode and an anode.

[0028] FIG. 1 is a cross-sectional view of a display device according to one embodiment of the present invention.

[0029] <1> First Embodiment The present inventors have newly discovered that when a layer between an anode and a cathode of a light-emitting element is formed using an ink containing metal oxide nanoparticles, aggregates may form in the coating film (i.e., the layer made of the cured ink), causing the surface of the coating film to bulge, preventing the formation of a coating film with a uniform thickness. These aggregates are large enough to be observed with an optical microscope at 100x magnification, and are not contained in the ink. They are believed to have formed during the formation of the coating film. A coating film with an uneven thickness can lead to a decrease in the performance of the light-emitting element. The present inventors have solved this new problem by using metal oxide nanoparticles exhibiting a specific particle size distribution in the ink, thereby completing the invention according to the first embodiment.

[0030] The invention according to a first embodiment will be described below with reference to the drawings. The embodiment described below is a more specific embodiment of any of the above aspects. The following items can be incorporated into each of the above aspects, either singly or in combination.

[0031] Furthermore, the embodiments shown below are merely examples of 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 components described below. Various modifications can be made to the technical idea of ​​the present invention within the technical scope defined by the claims.

[0032] It should be noted that the drawings are schematic, and 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 differ from the actual situation.

[0033] <1-1> Display Device Fig. 1 is a cross-sectional view of a display device according to one embodiment of the present invention. The display device 1 shown in Fig. 1 employs an active matrix driving method and is capable of displaying color images.

[0034] The display device 1 includes a plurality of pixels arranged in the X and Y directions described below. Each pixel includes a first sub-pixel PXR, a second sub-pixel PXG, and a third sub-pixel PXB. Each of the first sub-pixel PXR, the second sub-pixel PXG, and the third sub-pixel PXB includes a light-emitting element and a pixel circuit. Here, as an example, it is assumed that the semiconductor included in the light-emitting element is inorganic.

[0035] 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 a portion of the cathode 18 facing the anode 12 constitute a light-emitting element.

[0036] 1, the X and Y directions are parallel to the display surface of the display device 1 and intersect with each other. According to one 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.

[0037] According to one example, the substrate 11 includes an insulating substrate such as a glass substrate and an array section provided on one of its main surfaces. According to another example, the substrate 11 includes a semiconductor substrate such as a silicon substrate and an array section provided on one of its surface regions. The array section includes pixel circuits and wiring for supplying signals and power to the pixel circuits. The pixel circuits are arranged in the X and Y directions. Each pixel circuit includes a transistor as a drive element and a switch, a capacitor, and wiring for connecting them to each other. The transistor is, for example, a field effect transistor. Here, as an example, the drive element is a p-channel field effect transistor and the switch is an n-channel field effect transistor.

[0038] Here, the anodes 12 are pixel electrodes arranged in the X and Y directions corresponding to the pixel circuits on the substrate 11. Each anode 12 is connected to the drain of a drive element included in the corresponding pixel circuit.

[0039] When the substrate 11 is light-transmitting, the display device 1 may be of a top emission type or a bottom emission type, whereas when the substrate 11 is light-shielding, the display device 1 is of a top emission type.

[0040] When the display device 1 is a bottom-emission type, the anode 12 is a light-transmitting electrode. Examples of materials that can be used for the light-transmitting electrode include transparent conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide, and fluorine-doped tin oxide (FTO). A layer made of a transparent conductive oxide can be formed by, for example, a sputtering method.

[0041] When the display device 1 is a top-emission type, the anode 12 preferably includes a light-reflecting layer. The light-reflecting layer is made of, for example, an elemental metal such as aluminum or silver, or an alloy containing one or more of these. A layer made of a metal such as an elemental metal or an alloy can be formed by, for example, a vacuum deposition method.

[0042] The anode 12 including the light-reflecting layer may further include a light-transmitting layer on the light-reflecting layer. The light-transmitting layer may be made of, for example, any of the materials exemplified for the light-transmitting electrode. The material constituting the upper surface of the anode 12 preferably has a large work function.

[0043] The partition wall layer 13 is provided on the substrate 11 and the anodes 12. The partition wall layer 13 has through holes at the positions of the anodes 12. Each of these through holes has a shape that tapers from the upper opening to the lower opening. The peripheral edge of each anode 12 is covered with the partition wall layer 13, and the central part is exposed to the internal space of the through hole provided in the partition wall layer 13.

[0044] The partition wall layer 13 is made of an insulating material. According to one example, the partition wall layer 13 is made of an inorganic insulating material. According to another example, the partition wall layer 13 is made of a cured resin.

[0045] The hole injection layer 14 covers the central portion of the anode 12 in the through-hole provided in the partition layer 13. The ionization energy of the hole injection layer 14 is typically larger than the work function of the anode 12.

[0046] The hole injection layer 14 is made of a hole injection material, such as nickel oxide (NiO) or bismuth oxide (BiO). ２ O ３ ), cobalt oxide (CoO), copper oxide (Cu ２ O), molybdenum oxide (MoO ３ The hole injection material may be a metal oxide such as ruthenium (RuO) or magnesium oxide (MgO). The hole injection material made of these metal oxides may be contained in the hole injection layer 14 in the form of metal oxide nanoparticles. The hole injection material may be contained in the hole injection layer 14 without forming agglomerates that would raise the surface of the hole injection layer 14. This allows the hole injection layer 14 to have a uniform thickness.

[0047] 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.

[0048] The hole transport layer 15 covers the hole injection layer 14 in the through-holes provided in the partition layer 13. Typically, the ionization energy of the hole transport layer 15 is greater than the ionization energy of the hole injection layer 14.

[0049] The hole transport layer 15 is made of a hole transport material. The hole transport material is, for example, bismuth oxide (Bi ２ O ３ ), cobalt oxide (CoO), copper oxide (Cu ２ O), molybdenum oxide (MoO ３ ), and metal oxides such as magnesium oxide (MgO). Hole transport materials made of these metal oxides 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 agglomerates that would raise the surface of the hole transport layer 15. This allows the hole transport layer 15 to have a uniform thickness.

[0050] 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.

[0051] The light-emitting layer 16 covers the hole transport layer 15 in the through-holes provided in the partition layer 13. The light-emitting layer 16 typically has a larger ionization energy than the ionization energy of the hole transport layer 15 and a larger electron affinity than the electron affinity of the hole transport layer 15.

[0052] The light-emitting layer 16 is made of a light-emitting material. The light-emitting layers 16 of the first sub-pixel PXR, the second sub-pixel PXG, and the third sub-pixel PXB contain different light-emitting materials. For example, the light-emitting layer 16 of the first sub-pixel PXR, the second sub-pixel PXG, and the third sub-pixel PXB use a light-emitting material that emits red light, a light-emitting material that emits green light, and a light-emitting material that emits blue light, respectively.

[0053] According to one example, the light-emitting material is a quantum dot, which is a semiconductor particle having, for example, a core-shell structure and a particle size ranging from a few nanometers to about 10 nanometers.

[0054] The core is made of a semiconductor that is responsible for emitting light. The emission spectrum of the quantum dot can be changed by changing the type of semiconductor that makes up the core and the particle diameter of the core.

[0055] 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 luminous efficiency. The shell may have a single-layer structure or a multi-layer structure.

[0056] An example of a quantum dot has a core made of InP, covered with a first shell made of ZnSe, and 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.

[0057] Another example of quantum dots is a quantum dot with a core made of ZnSeTe, covered with a first shell made of ZnSe, and covered with a second shell made of ZnS. Such quantum dots emit blue light when their particle size is small.

[0058] The quantum dots may have ligands on the surface of the core-shell particles. The ligands are hydrocarbons with functional groups that contribute to improving durability and preventing aggregation in the dispersion. Note that the ligands may be at least partially lost in the display device 1.

[0059] 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.

[0060] The electron transport layer 17 covers the light-emitting layer 16 in the through-holes provided in the partition layer 13. The electron transport layer 17 typically has a larger ionization energy than that of the light-emitting layer 16 and a larger electron affinity than that of the light-emitting layer 16.

[0061] The electron transport layer 17 is made of an electron transport material. Examples of the electron transport material include metal oxides such as magnesium-doped zinc oxide (MgZnO), zinc oxide (ZnO), and titanium oxide (TiO). The electron transport material made of these metal oxides may be contained in the electron transport layer 17 in the form of metal oxide nanoparticles. The electron transport material may be contained in the electron transport layer 17 without forming agglomerates that would raise the surface of the electron transport layer 17. This allows the electron transport layer 17 to have a uniform thickness.

[0062] 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.

[0063] The cathode 18 covers the electron transport layer 17 and the exposed portion of the partition wall layer 13. In this example, the cathode 18 is a common electrode facing the plurality of anodes 12.

[0064] When the display device 1 is a bottom-emission type, the cathode 18 preferably includes a light-reflecting layer. The light-reflecting layer is made of, for example, an elemental metal such as aluminum or silver, or an alloy containing one or more of these. A layer made of a metal such as an elemental metal or an alloy can be formed by, for example, a vacuum deposition method.

[0065] When the display device 1 is a top-emission type, the cathode 18 is a light-transmitting electrode. Examples of materials that can be used for the light-transmitting electrode include transparent conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), tin oxide, and fluorine-doped tin oxide (FTO). A layer made of a transparent conductive oxide can be formed by, for example, a sputtering method.

[0066] The work function of the cathode 18 is typically smaller than the work function of the anode 12 and larger than 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 an MgAg alloy or an AlLi alloy, between the layer made of a metal or a transparent conductive oxide and the electron transport layer 17.

[0067] 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 further include a sealing film or a sealing substrate that seals the light-emitting element.

[0068] Furthermore, the display device 1 employs a forward structure for the light-emitting element. The light-emitting element may employ an inverted structure in which the stacking order of the layers included therein is reversed.

[0069] <1-2> Manufacturing Method of Display Device The display device 1 shown in FIG. 1 can be manufactured, for example, by the following method.

[0070] First, a structure (base material) including the substrate 11, the anode 12, and the partition wall layer 13 is prepared.

[0071] 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-mentioned structure (substrate) to form a coating film, and then drying the coating film to harden the coating. 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 the coating. In forming the hole injection layer 14 and the hole transport layer 15, the ink composition can be applied to the substrate using a printing method such as inkjet printing, spin coating, or slit coating.

[0072] Next, the light-emitting layer 16 is formed. As described above, the light-emitting layers 16 of the first sub-pixel PXR, the second sub-pixel PXG, and the third sub-pixel PXB are made of different light-emitting materials. Therefore, the light-emitting layer 16 of the first sub-pixel PXR, the second sub-pixel PXG, and the third sub-pixel PXB are formed separately. Each of these light-emitting layers 16 can be formed by using, for example, a lift-off method.

[0073] 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 ink composition can be applied to the substrate using a printing method such as inkjet printing, spin coating, or slit coating.

[0074] Next, the cathode 18 is formed. The cathode 18 can be formed by vacuum deposition, sputtering, or a combination thereof. Thereafter, the light-emitting element is sealed, if necessary. In this manner, the display device 1 shown in the figure is obtained.

[0075] <1-3> 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 a printing method such as inkjet printing, spin coating, slit coating, etc. The ink composition used for this printing is preferably one that 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 protrude due to the generation of aggregates.

[0076] From this viewpoint, it is preferable that one or more of the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17 are formed using an ink composition having the composition described below.

[0077] That is, a suitably used ink composition contains metal oxide nanoparticles and a dispersion medium containing a glycol-based solvent.

[0078] The metal oxide nanoparticles are made of a material selected from the group consisting of a hole injection material, a hole transport material, and an electron transport material. The metal oxide nanoparticles may be those described above for the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17.

[0079] Preferably, the metal oxide nanoparticles include one or more of a plurality of 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 nanoparticles containing nickel oxide, nanoparticles containing zinc oxide, and nanoparticles containing magnesium-doped zinc oxide, for example, any one of these nanoparticles.

[0080] The metal oxide nanoparticles have a 95% particle diameter (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 diameter at which the volume-based cumulative distribution corresponds to 95% when the volume-based cumulative distribution is measured by dynamic light scattering, with particle diameter on the horizontal axis and volume-based cumulative distribution (%) on the vertical 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.

[0081] D95 is preferably 20 nm or more. 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.

[0082] The metal oxide nanoparticles preferably have a 50% particle diameter (D50) (hereinafter also referred to as "D50") in the range of 3 to 50 nm, more preferably 3 to 30 nm, and even more preferably 10 to 25 nm, when the volume-based cumulative particle size distribution is measured by dynamic light scattering. Here, "D50" refers to the particle diameter at which the volume-based cumulative distribution corresponds to 50% when the volume-based cumulative distribution is measured by dynamic light scattering, with particle diameter on the horizontal axis and volume-based cumulative distribution (%) on the vertical axis.

[0083] The metal oxide nanoparticles preferably have a 10% particle diameter (D10) (hereinafter also referred to as "D10") in the range of 3 to 30 nm, more preferably 3 to 20 nm, when the volume-based cumulative particle size distribution is measured by dynamic light scattering. Here, "D10" refers to the particle diameter at which the volume-based cumulative distribution corresponds to 10% when the volume-based cumulative distribution is measured by dynamic light scattering, with particle diameter on the horizontal axis and volume-based cumulative distribution (%) on the vertical axis.

[0084] The proportion of metal oxide nanoparticles in the ink composition is preferably in the range of 0.05% by mass to 20% by mass, more preferably in the range of 0.1% by mass to 10% by mass, and even more preferably in the range of 0.5% by mass to 5% by mass.

[0085] The dispersion medium contains a glycol-based solvent, for example, one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, and hexylene glycol.

[0086] The dispersion medium may further contain other solvents. The proportion of the glycol solvent in the dispersion medium is preferably in the range of 50% by mass to 100% by mass, and more preferably in the range of 80% by mass to 100% by mass.

[0087] The other solvent is preferably one or more aprotic solvents such as an amide compound, an aliphatic ester compound, and an aliphatic ether compound. The one or more aprotic solvents may include, for example, one or more of N,N'-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and propylene glycol monomethyl ether acetate (PGMEA). The proportion of the aprotic solvent in the other solvent is preferably in the range of 80% by mass to 100% by mass, and more preferably 100% by mass.

[0088] 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.

[0089] The ink composition contains the metal oxide nanoparticles and the dispersion medium, but preferably does not contain metal or metal compounds other than the metal oxide nanoparticles. The ink composition more preferably consists of only the metal oxide nanoparticles and the dispersion medium. When the ink composition consists of only the metal oxide nanoparticles and the dispersion medium, the proportion of the metal oxide nanoparticles in the ink composition is preferably 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 the dispersion medium in the ink composition is preferably 80% to 99.95% by mass, more preferably 90% to 99.9% by mass, and even more preferably 95% to 99.5% by mass.

[0090] As described above, the ink composition contains metal oxide nanoparticles that exhibit a specific particle size distribution. This allows the ink composition to form a layer of metal oxide nanoparticles with a uniform thickness without causing the surface of the layer to rise due to the generation of aggregates. As a result, the ink composition can contribute to improving the performance of a light-emitting device that has a layer of metal oxide nanoparticles between a cathode and an anode.

[0091] <2> Second Embodiment The invention according to the second embodiment is the same as the invention according to the first embodiment, except that the ink composition described below is used as the ink composition. Therefore, in the following description, explanations that overlap with those of the first embodiment, i.e., explanations regarding the "display device" and the "manufacturing method for the display device", will be omitted.

[0092] The present inventors have newly discovered that inks containing metal oxide nanoparticles may experience a concentration gradient of metal oxide nanoparticles in the depth direction during storage due to the sedimentation of the metal oxide nanoparticles. When such a concentration gradient occurs, even when coating films are formed under the same conditions, the content of metal oxide nanoparticles may vary between coating films, potentially resulting in variations in the thickness of the printed layer formed by drying the coating film. Printed layers that do not maintain consistent quality may lead to reduced performance of light-emitting devices. The present inventors have solved this new problem by using metal oxide nanoparticles exhibiting a specific particle size distribution in the ink, thereby completing the invention according to the second embodiment.

[0093] The invention according to a second embodiment will be described below. The embodiment described below is a more specific embodiment of any of the above aspects. The following items can be incorporated into each of the above aspects, either singly or in combination.

[0094] Furthermore, the embodiments shown below are merely examples of 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 components described below. Various modifications can be made to the technical idea of ​​the present invention within the technical scope defined by the claims.

[0095] As described above, the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17 can be formed by a printing method such as inkjet printing, spin coating, or slit coating. The ink composition used for this printing preferably has excellent dispersion stability and does not cause a concentration gradient of the metal oxide nanoparticles in the depth direction due to settling of the metal oxide nanoparticles during storage.

[0096] From this viewpoint, it is preferable that one or more of the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17 are formed using an ink composition having the composition described below.

[0097] That is, a suitably used ink composition contains metal oxide nanoparticles and a dispersion medium containing a glycol-based solvent.

[0098] The metal oxide nanoparticles are made of a material selected from the group consisting of a hole injection material, a hole transport material, and an electron transport material. The metal oxide nanoparticles may be those described above for the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17.

[0099] Preferably, the metal oxide nanoparticles include one or more of a plurality of 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 nanoparticles containing nickel oxide, nanoparticles containing zinc oxide, and nanoparticles containing magnesium-doped zinc oxide, for example, any one of these nanoparticles.

[0100] The metal oxide nanoparticles have a 50% particle diameter (D50) (hereinafter also referred to as "D50") of 50 nm or less when the volume-based cumulative particle size distribution is measured by dynamic light scattering. Here, "D50" refers to the particle diameter at which the volume-based cumulative distribution corresponds to 50% when the volume-based cumulative distribution is measured by dynamic light scattering, with particle diameter on the horizontal axis and volume-based cumulative distribution (%) on the vertical axis. D50 is, for example, in the range of 3 nm to 50 nm, preferably 3 nm to 30 nm, and more preferably 3 nm to 20 nm.

[0101] D50 is preferably 10 nm or more. Specifically, D50 is preferably in the range of 10 nm to 50 nm, more preferably 10 nm to 30 nm, and even more preferably 10 nm to 20 nm.

[0102] The metal oxide nanoparticles have a 95% particle diameter (D95) (hereinafter also referred to as "D95") of, for example, 120 nm or less when the volume-based cumulative particle size distribution is measured by dynamic light scattering. D95 is preferably in the range of 10 nm to 120 nm, more preferably 10 nm to 60 nm. Here, "D95" refers to the particle diameter at which the volume-based cumulative distribution corresponds to 95% when the volume-based cumulative distribution is measured by dynamic light scattering, with particle diameter on the horizontal axis and volume-based cumulative distribution (%) on the vertical axis.

[0103] D95 is preferably 20 nm or more. Specifically, D95 is preferably in the range of 20 to 120 nm, more preferably 20 to 110 nm.

[0104] The metal oxide nanoparticles preferably have a 10% particle diameter (D10) (hereinafter also referred to as "D10") in the range of 3 to 30 nm, more preferably 3 to 20 nm, when the volume-based cumulative particle size distribution is measured by dynamic light scattering. Here, "D10" refers to the particle diameter at which the volume-based cumulative distribution corresponds to 10% when the volume-based cumulative distribution is measured by dynamic light scattering, with particle diameter on the horizontal axis and volume-based cumulative distribution (%) on the vertical axis.

[0105] The proportion of metal oxide nanoparticles in the ink composition is preferably in the range of 0.05% by mass to 20% by mass, more preferably in the range of 0.1% by mass to 10% by mass, and even more preferably in the range of 0.5% by mass to 5% by mass.

[0106] The dispersion medium contains a glycol-based solvent, which may be, for example, one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, triethylene glycol, and hexylene glycol.

[0107] The dispersion medium may further contain other solvents. The proportion of the glycol solvent in the dispersion medium is preferably in the range of 50% by mass to 100% by mass, and more preferably in the range of 80% by mass to 100% by mass.

[0108] The other solvent is preferably one or more aprotic solvents such as an amide compound, an aliphatic ester compound, and an aliphatic ether compound. The one or more aprotic solvents may include, for example, one or more of N,N'-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and propylene glycol monomethyl ether acetate (PGMEA). The proportion of the aprotic solvent in the other solvent is preferably in the range of 80% by mass to 100% by mass, and more preferably 100% by mass.

[0109] 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.

[0110] The ink composition contains the metal oxide nanoparticles and the dispersion medium, but preferably does not contain metal or metal compounds other than the metal oxide nanoparticles. The ink composition more preferably consists of only the metal oxide nanoparticles and the dispersion medium. When the ink composition consists of only the metal oxide nanoparticles and the dispersion medium, the proportion of the metal oxide nanoparticles in the ink composition is preferably 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 the dispersion medium in the ink composition is preferably 80% to 99.95% by mass, more preferably 90% to 99.9% by mass, and even more preferably 95% to 99.5% by mass.

[0111] As described above, the ink composition contains metal oxide nanoparticles that exhibit a specific particle size distribution. As a result, the ink composition does not experience concentration gradients of the metal oxide nanoparticles in the depth direction due to settling of the metal oxide nanoparticles during storage, and has excellent dispersion stability.

[0112] The ink composition has excellent dispersion stability, and therefore, when applied to a substrate to form a coating film, the metal oxide nanoparticles can be uniformly distributed in the coating film. For example, when a coating film is formed by inkjet printing, the ink composition has excellent dispersion stability, and therefore the amount of metal oxide nanoparticles contained in the ink droplets ejected from the inkjet head can be kept constant, thereby allowing the metal oxide nanoparticles to be uniformly distributed in the coating film. In other words, the ink composition can reduce variation in the content of metal oxide nanoparticles between formed coating films. This reduces variation in the thickness of the printed layer formed by drying the coating film, allowing the formation of a printed layer that maintains consistent quality. 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 a cathode and an anode.

[0113] On the other hand, if a concentration gradient of metal oxide nanoparticles occurs in the depth direction of the ink composition due to settling of the metal oxide nanoparticles during storage, the amount of metal oxide nanoparticles contained in a given amount of ink applied to a substrate will differ between, for example, when a sufficient amount of ink is contained in the tank and when the ink in the tank is about to be used up. For example, when a coating film is formed by inkjet printing, if a concentration gradient of metal oxide nanoparticles occurs in the depth direction of the ink composition due to settling of the metal oxide nanoparticles during storage, the amount of metal oxide nanoparticles contained in the ink droplets ejected from the inkjet head will differ between, for example, when a sufficient amount of ink is contained in the tank and when the ink in the tank is about to be used up. Such a difference leads to variations in the thickness of the printed layer, making it impossible to form a printed layer with consistent quality.

[0114] Example 1 Tests carried out in relation to the first embodiment of the invention will be described below.

[0115] <1> Preparation of Ink Composition <1.1> Example 1A An ink composition containing metal oxide nanoparticles and a dispersion medium was prepared.

[0116] The metal oxide nanoparticles used were "nanoparticles made of nickel (II) oxide, with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm." The amount of the metal oxide nanoparticles was adjusted so that the proportion of the metal oxide nanoparticles in the ink composition was 1% by mass.

[0117] Ethylene glycol (EG) and propylene glycol monomethyl ether acetate (PGMEA) were used as dispersion media. The amount of ethylene glycol added based on the total amount of the ink composition was 85 mass %. The amount of propylene glycol monomethyl ether acetate added based on the total amount of the ink composition was 14 mass %.

[0118] <1.2> Example 2A An ink composition similar to that of Example 1A was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 28 nm, a D50 of 12 nm, and a D10 of 8 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0119] <1.3> Example 3A An ink composition similar to that of Example 1A was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 43 nm, a D50 of 20 nm, and a D10 of 13 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0120] <1.4> Example 4A An ink composition similar to that of Example 1A was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 57 nm, a D50 of 21 nm, and a D10 of 14 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0121] <1.5> Comparative Example 1A An ink composition similar to that of Example 1A was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 66 nm, a D50 of 29 nm, and a D10 of 19 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0122] <1.6> Comparative Example 2A An ink composition similar to that of Example 1A was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 65 nm, a D50 of 23 nm, and a D10 of 15 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0123] <2> Evaluation <2.1> Particle size of metal oxide nanoparticles For each of the metal oxide nanoparticles used in Examples 1A to 4A and Comparative Examples 1A to 2A, 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 (registered trademark) UPA-EX150 particle size distribution analyzer manufactured by Nikkiso Co., Ltd. was used to measure the cumulative particle size distribution.

[0124] <2.2> Uniformity of Coating Film: Coating films were prepared using each of the ink compositions of Examples 1A to 4A and Comparative Examples 1A to 2A, and the uniformity of the coating films was evaluated. The evaluation was performed as follows. First, the ink composition was applied to a 5 cm square glass substrate by spin coating, allowed to dry naturally for 5 minutes, and then baked on a hot plate at 200°C for 5 minutes. The resulting coating film (i.e., a layer consisting of the cured ink composition) was observed with an optical microscope at 100x magnification. The optical microscope observation was performed on an area excluding the area extending 0.5 mm inward from the outer edge of the 5 cm square glass substrate (i.e., a 4 cm square area in the center of the glass substrate). Cases where no aggregates were observed were evaluated as "A," and cases where aggregates were observed were evaluated as "B."

[0125] <2.3> Summary of Evaluation The results of the above evaluation are shown in Table 1 below.

[0126]

[0127] As shown in Table 1, for the ink compositions of Comparative Examples 1A and 2A, aggregates were observed in the coating film, and the coating film did not have a uniform thickness. The aggregates were small enough to be observed with an optical microscope at 100x magnification (on the order of microns). The aggregates were not contained in the ink composition, and are thought to have been generated during the formation of the coating film. In contrast, for the ink compositions of Examples 1A to 4A, no aggregates were observed in the coating film, and the coating film had a uniform thickness.

[0128] Example 2 Tests carried out in relation to the second embodiment of the invention will be described below.

[0129] <1> Preparation of Ink Composition <1.1> Example 1B An ink composition containing metal oxide nanoparticles and a dispersion medium was prepared.

[0130] The metal oxide nanoparticles used were "nanoparticles made of nickel (II) oxide, with a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm." The amount of the metal oxide nanoparticles was adjusted so that the proportion of the metal oxide nanoparticles in the ink composition was 1% by mass.

[0131] Ethylene glycol (EG) was used as the dispersion medium, and the amount of ethylene glycol added was 99% by mass based on the total amount of the ink composition.

[0132] <1.2> Example 2B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 28 nm, a D50 of 12 nm, and a D10 of 8 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0133] <1.3> Example 3B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 43 nm, a D50 of 20 nm, and a D10 of 13 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0134] <1.4> Example 4B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 57 nm, a D50 of 21 nm, and a D10 of 14 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0135] <1.5> Example 5B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 66 nm, a D50 of 29 nm, and a D10 of 19 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0136] <1.6> Example 6B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 65 nm, a D50 of 23 nm, and a D10 of 15 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0137] <1.7> Example 7B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 103 nm, a D50 of 43 nm, and a D10 of 25 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0138] <1.8> Comparative Example 1B An ink composition similar to that of Example 1B was prepared, except for the following: In this example, as the metal oxide nanoparticles, "nanoparticles made of nickel(II) oxide having a D95 of 137 nm, a D50 of 52 nm, and a D10 of 32 nm" were used instead of "nanoparticles made of nickel(II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm."

[0139] <1.9> Comparative Example 2B An ink composition similar to that of Example 1B was prepared, except for the following points. That is, in this example, as the metal oxide nanoparticles, "nanoparticles made of nickel (II) oxide having a D95 of 119 nm, a D50 of 53 nm, and a D10 of 29 nm" were used instead of "nanoparticles made of nickel (II) oxide having a D95 of 21 nm, a D50 of 11 nm, and a D10 of 7 nm." Also, in this example, 1,3-propanediol (1,3-PG) was used as the dispersion medium instead of ethylene glycol (EG).

[0140] <2> Evaluation <2.1> Particle size of metal oxide nanoparticles For each of the metal oxide nanoparticles used in Examples 1B to 7B and Comparative Examples 1B to 2B, 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 (registered trademark) UPA-EX150 particle size distribution analyzer manufactured by Nikkiso Co., Ltd. was used to measure the cumulative particle size distribution.

[0141] <2.2> Ink Storage Stability The ink storage stability was evaluated for each of the ink compositions according to Examples 1B to 7B and Comparative Examples 1B to 2B. The evaluation was performed as follows. The prepared ink compositions were stored at room temperature (25°C) for 14 days. After storage, the ink compositions were visually observed to confirm whether phase separation had occurred (i.e., the metal oxide nanoparticles had settled, and the ink composition had separated into a phase with a low concentration of metal oxide nanoparticles and a phase with a high concentration of metal oxide nanoparticles). Cases where no phase separation was observed were evaluated as "A," and cases where phase separation was observed were evaluated as "B."

[0142] <2.3> Summary of Evaluation The results of the above evaluation are shown in Table 2 below.

[0143]

[0144] As shown in Table 2, phase separation was observed in the ink compositions of Comparative Examples 1B to 2B after storage. Furthermore, when the particle size distribution of the ink compositions of Comparative Examples 1B to 2B was measured immediately after preparation and after storage, the particle size distribution of the ink compositions after storage was the same as the particle size distribution of the ink compositions immediately after preparation, and no difference was observed in the particle size distribution data. These results indicate that the phase separation was not due to the formation of aggregates of the metal oxide nanoparticles, but rather due to sedimentation.

[0145] In contrast, in the ink compositions according to Examples 1B to 7B, no phase separation was observed after storage, and the metal oxide nanoparticles remained uniformly dispersed even after storage.

[0146] 1...display device, 11...substrate, 12...anode, 13...partition layer, 14...hole injection layer, 15...hole transport layer, 16...light-emitting layer, 17...electron transport layer, 18...cathode, PXB...third sub-pixel, PXG...second sub-pixel, PXR...first sub-pixel.

Claims

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

2. The ink composition according to claim 1, wherein the metal oxide nanoparticles comprise one or more of a plurality of types of nanoparticles, each of which contains 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 or 2, wherein the metal oxide nanoparticles have a 95% particle diameter (D95) of 20 nm or more.

4. An ink composition according to any one of claims 1 to 3, wherein the metal oxide nanoparticles have a 50% particle diameter (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. An ink composition according to any one of claims 1 to 4, wherein the metal oxide nanoparticles have a cumulative 50% particle diameter (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. An ink composition according to any one of claims 1 to 5, wherein the metal oxide nanoparticles have a 10% particle diameter (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 any one of claims 1 to 6, wherein the proportion of said metal oxide nanoparticles in said ink composition is in the range of 0.05% by mass to 20% by mass.

8. The ink composition according to any one of claims 1 to 7, 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 any one of claims 1 to 8, wherein the ink composition does not contain any metal or metal compound other than the metal oxide nanoparticles.

10. The ink composition according to any one of claims 1 to 9, wherein the ink composition consists only of the metal oxide nanoparticles and the dispersion medium.

11. An ink composition comprising metal oxide nanoparticles made of a material selected from the group consisting of hole injection materials, hole transport materials, and electron transport materials, and having a 50% particle size (D50) of 50 nm or less when the volume-based cumulative particle size distribution is measured by dynamic light scattering, and a dispersion medium containing a glycol-based solvent.

12. The ink composition according to claim 11, wherein the metal oxide nanoparticles comprise one or more of a plurality of types of nanoparticles, each of which contains bismuth oxide, cobalt oxide, copper oxide, magnesium oxide, nickel oxide, zinc oxide, magnesium-doped zinc oxide, and titanium oxide.

13. The ink composition according to claim 11 or 12, wherein the metal oxide nanoparticles have a 50% particle diameter (D50) of 10 nm or more.

14. An ink composition according to any one of claims 11 to 13, wherein the metal oxide nanoparticles have a 95% particle diameter (D95) of 120 nm or less when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

15. An ink composition according to any one of claims 11 to 14, wherein the metal oxide nanoparticles have a 10% particle diameter (D10) in the range of 3 nm to 30 nm when the volume-based cumulative particle size distribution is measured by dynamic light scattering.

16. The ink composition according to any one of claims 11 to 15, 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.

17. The ink composition according to any one of claims 11 to 16, wherein the glycol-based solvent is one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, triethylene glycol, and hexylene glycol.

18. The ink composition according to any one of claims 11 to 17, which does not contain any metal or metal compound other than the metal oxide nanoparticles.

19. The ink composition according to any one of claims 11 to 18, wherein the ink composition consists only of the metal oxide nanoparticles and the dispersion medium.

20. A method for forming a printed layer, comprising: applying the ink composition according to any one of claims 1 to 19 to a substrate to form a coating film; and drying the coating film to harden the coating film.

21. A printed layer formed by the method of claim 20.

22. A display device comprising the printed layer according to claim 21.

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