Method for producing ink composition

Abstract

A technology is provided that can contribute to improving the performance of a light-emitting device that has a layer made of metal oxide nanoparticles between a cathode and an anode. A method for producing an ink composition includes preparing a dispersion containing 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 a glycol-based solvent; and h is 15 (MPa) 1 / 2 and filtering the dispersion through a filter made of the above material.

Description

[Technical Field]

[0001] The present invention relates to the preparation of ink compositions. [Background technology]

[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. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-116345 [Patent Document 2] International Publication No. 2022 / 070296 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a technique that can contribute to improving the productivity of ink compositions containing metal oxide nanoparticles and glycol-based solvents. [Means for solving the problem]

[0005] According to one aspect of the present invention, a dispersion liquid containing 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 a dispersion medium containing a glycol-based solvent is prepared, and a dispersion liquid containing metal oxide nanoparticles having a Hansen solubility parameter δh of 15 (MPa) or less is prepared. 1 / 2 and filtering the dispersion through a filter made of the above material.

[0006] According to another aspect of the present invention, there is provided a method for producing an ink composition according to the above aspect, wherein a volume-based cumulative distribution obtained by measuring the particle size distribution of the dispersion by dynamic light scattering has a 50% diameter in the range of 3 nm to 50 nm and a 95% diameter of 100 nm or less.

[0007] According to yet another aspect of the present invention, there is provided a method for producing an ink composition according to any one of the above aspects, wherein the pore size of the filter is in the range of 0.2 μm to 0.5 μm.

[0008] According to yet another aspect of the present invention, there is provided a method for producing an ink composition according to any of the above aspects, wherein the filter contains cellulose acetate.

[0009] According to yet another aspect of the present invention, the dispersion after filtration is subjected to a solubility test using a solution of 100% ethanol with a Hansen solubility parameter of δ h is 15 (MPa) 1 / 2 There is also provided a method for producing an ink composition according to any of the above aspects, further comprising subjecting the ink composition to one or more filtrations, each of which uses a filter made of the material described above.

[0010] According to yet another aspect of the present invention, there is provided an ink composition produced by the production method according to any of the above aspects.

[0011] According to yet another aspect of the present invention, there is provided a method for forming a printing layer, comprising producing an ink composition by the production method according to any of the above aspects, and supplying the ink composition to the surface of a substrate to be printed to form a coating film.

[0012] According to yet another aspect of the present invention, there is provided a printed layer formed by the forming method according to the above aspect.

[0013] According to yet another aspect of the present invention, there is provided a display device including the print layer according to the above aspect.

[0014] According to yet another aspect of the present invention, there is provided a method for manufacturing a display device, which includes forming a print layer by the forming method according to the above aspect. [Effects of the Invention]

[0015] According to the present invention, a technique is provided that can contribute to improving the productivity of ink compositions containing metal oxide nanoparticles and glycol-based solvents. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a cross-sectional view of a display device according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below are more specific embodiments of any of the above aspects. The following items can be incorporated into each of the above aspects, either singly or in combination.

[0018] 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.

[0019] 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.

[0020] <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 is a display device that employs an active matrix driving method and is capable of displaying color images.

[0021] 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, the semiconductor included in the light-emitting element is assumed to be inorganic.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] When the substrate 11 is light-transmitting, the display device 1 may be of a top emission type or a bottom emission type, and when the substrate 11 is light-shielding, the display device 1 is of a top emission type.

[0027] 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.

[0028] 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. Layers made of metals such as elemental metals and alloys can be formed by, for example, vacuum deposition.

[0029] 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 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.

[0030] 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 periphery of each anode 12 is covered with the partition wall layer 13, and the center is exposed to the internal space of the through hole provided in the partition wall layer 13.

[0031] 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 insulator. According to another example, the partition wall layer 13 is made of a cured resin.

[0032] 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.

[0033] The hole injection layer 14 is made of a hole injection material. The hole injection material is, for example, a metal oxide such as nickel oxide (NiO), bismuth oxide (BiO), cobalt oxide (CoO), copper oxide (CuO), molybdenum oxide (MoO), 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.

[0034] Here, "nanoparticles" refers to particles having an average particle diameter in the range of 200 nm to 1 nm as measured by dynamic light scattering. Preferably, metal oxide nanoparticles have an average particle diameter in the range of 3 nm to 50 nm as measured by dynamic light scattering.

[0035] 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.

[0036] 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.

[0037] The hole transport layer 15 is made of a hole transport material. Examples of the hole transport material include metal oxides such as bismuth oxide (BiO), cobalt oxide (CoO), copper oxide (CuO), molybdenum oxide (MoO), and magnesium oxide (MgO). The hole injection material made of these metal oxides may be contained in the hole transport layer 15 in the form of metal oxide nanoparticles. The average particle diameter of these metal oxide nanoparticles, measured by dynamic light scattering, is preferably within the range described above for the hole injection material.

[0038] 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.

[0039] 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 hole transport layer 15 and a larger electron affinity than the hole transport layer 15.

[0040] The light-emitting layer 16 is made of a light-emitting material. The light-emitting layers 16 of the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB contain different light-emitting materials. For example, the light-emitting layer 16 of the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB contain a red light-emitting material, a green light-emitting material, and a blue light-emitting material, respectively.

[0041] 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 nm to about 10 nm.

[0042] 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.

[0043] The shell is a thin layer epitaxially grown on the surface of the core, with 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.

[0044] 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.

[0045] Another example of quantum dots is a ZnSeTe core coated with a first shell of ZnSe, which is then coated with a second shell of ZnS. Such quantum dots emit blue light when their particle size is small.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 hole injection material made of these metal oxides may be contained in the electron transport layer 17 in the form of metal oxide nanoparticles. The average particle size of these metal oxide nanoparticles, measured by dynamic light scattering, is preferably within the range described above for the hole injection material.

[0050] 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.

[0051] The cathode 18 covers the electron transport layer 17 and the exposed portion of the partition layer 13. In this example, the cathode 18 is a common electrode facing the plurality of anodes 12.

[0052] 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. Layers made of metals such as elemental metals and alloys can be formed by, for example, vacuum deposition.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] <2> Display device manufacturing method The display device 1 shown in FIG. 1 can be manufactured, for example, by the following method.

[0058] First, a structure including a substrate 11, an anode 12, and a partition layer 13 is prepared.

[0059] 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. Specifically, the ink composition is first supplied to the surface of the substrate to form a coating film. To supply the ink composition to the surface of the substrate, printing methods such as inkjet printing, spin coating, and slit coating are used. Next, this coating film is dried. For example, the coating film is baked. This results in a layer made of the cured ink composition as a printed layer.

[0060] Next, the light-emitting layer 16 is formed. As described above, the light-emitting layers 16 of the first subpixel PXR, the second subpixel PXG, and the third subpixel PXB are different in the light-emitting material contained therein. 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 using, for example, a lift-off method.

[0061] Subsequently, the electron transport layer 17 is formed. As described above for the hole injection layer 14 and the hole transport layer 15, the electron transport layer 17 can be formed by a printing method using an ink composition containing metal oxide nanoparticles and a dispersion medium, such as inkjet printing, spin coating, or slit coating.

[0062] Next, the cathode 18 is formed. The cathode 18 can be formed by a vacuum deposition method, a sputtering method, or a combination thereof. Thereafter, the light emitting element is sealed as necessary. In this manner, the display device 1 shown in the figure is obtained.

[0063] <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, or slit coating. The ink composition used for this printing, i.e., the ink composition, preferably has excellent printability and dispersion stability of the metal oxide nanoparticles.

[0064] From this perspective, it is preferable to form one or more of the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17 using an ink composition having the composition described below. That is, the ink composition that is preferably used contains metal oxide nanoparticles and a dispersion medium containing a glycol-based solvent.

[0065] Furthermore, if the ink composition contains an additive, and the additive remains in the light-emitting element, the light-emitting element will not achieve the expected light-emitting characteristics. Therefore, it is preferable that the ink composition does not contain any additive that may remain in the light-emitting element. That is, it is preferable that the ink composition contains metal oxide nanoparticles and a dispersion medium containing a glycol-based solvent, and does not contain any other components.

[0066] 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, and can be any of the metal oxide nanoparticles described above for the hole injection layer 14, the hole transport layer 15, and the electron transport layer 17.

[0067] 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, and titanium oxide, for example, any of these nanoparticles. More preferably, the metal oxide nanoparticles include at least one of nanoparticles containing nickel oxide and nanoparticles containing zinc oxide, for example, any one of these nanoparticles.

[0068] The proportion of metal oxide nanoparticles in the ink composition, i.e., the concentration 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.

[0069] The ink composition contains a glycol-based solvent as at least a part of the dispersion medium. The glycol-based solvent can contribute to the dispersion stability of the metal oxide nanoparticles. The glycol-based solvent is, for example, one or more selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, and hexylene glycol. The glycol-based solvent preferably contains ethylene glycol, and more preferably is ethylene glycol.

[0070] The ink composition may further include a non-alcoholic solvent as another component of the dispersion medium. The non-alcoholic solvent may contribute to printability, for example, inkjet printability. The non-alcoholic solvent may be, for example, an aprotic solvent, preferably one or more selected from the group consisting of an amide compound, an aliphatic ester compound, and an aliphatic ether compound. The non-alcoholic solvent may include, for example, one or more of N,N'-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and propylene glycol monomethyl ether acetate (PGMEA or PGMAc).

[0071] When the ink composition contains a glycol-based solvent and a non-alcohol-based solvent, the mass M of the glycol-based solvent A and the mass of the non-alcoholic solvent M B The mass of non-alcoholic solvents in the total of B Percentage of M B / (M A +M B ) is preferably in the range of 0.10 to 0.50, and more preferably in the range of 0.15 to 0.30.

[0072] The proportion of the dispersion medium in the ink composition is preferably in the range of 80% to 99.95% by mass, more preferably in the range of 90% to 99.9% by mass, and even more preferably in the range of 95% to 99.5% by mass.

[0073] The dispersion medium of this ink composition contains a glycol-based solvent, which can maintain the metal oxide nanoparticles in a uniformly dispersed state for a long period of time without causing excessive aggregation of the metal oxide nanoparticles.

[0074] Furthermore, when the ink composition does not contain an additive, the additive or its decomposition product does not remain in the coating film after baking, i.e., in the layer made of the cured product of the ink composition. Therefore, when the ink composition is used, the performance of the light-emitting element does not deteriorate due to the residue.

[0075] In general, ink compositions containing glycol-based solvents have high viscosity and high surface tension. Ink compositions with high viscosity and excessively high surface tension have poor ink ejection properties in inkjet printing and are unsuitable for forming coating films.

[0076] Adding a non-alcoholic solvent to an ink composition containing a glycol-based solvent can adjust the viscosity and surface tension. Therefore, by appropriately adjusting the type and amount of the glycol-based solvent and non-alcoholic solvent, it is easy to simultaneously achieve excellent dispersion stability and excellent printability in inkjet printing. Therefore, by using this ink composition, it is possible to form a layer in which metal oxide nanoparticles are uniformly dispersed with high productivity.

[0077] <4> Method for producing ink composition The above ink composition can be produced by the following method.

[0078] That is, first, a dispersion liquid containing the above-mentioned metal oxide nanoparticles and the above-mentioned dispersion medium is prepared. If the dispersion medium contains multiple types of solvents, a mixture containing all of those solvents may be mixed with the metal oxide nanoparticles, and the metal oxide nanoparticles may be dispersed in this mixture. Alternatively, if the dispersion medium contains multiple types of solvents, first, one of those solvents may be mixed with the metal oxide nanoparticles, and the metal oxide nanoparticles may be dispersed in this mixture, and then the remaining solvent in this mixture may be mixed.

[0079] Next, the dispersion is filtered to obtain an ink composition. This filtration is performed for the purposes of removing coarse particles such as foreign matter from the dispersion and improving the dispersion state of the particles by dissociating excessively aggregated particles from each other. Therefore, to prevent a large amount of metal oxide nanoparticles from being removed from the dispersion by filtration, a filter with a pore size sufficiently large compared to the particle size of the metal oxide nanoparticles and their aggregates is used.

[0080] According to one example, the volume-based cumulative particle size distribution obtained by measuring the particle size distribution of the dispersion by dynamic light scattering has a 50% diameter in the range of 3 to 50 nm and a 95% diameter of 100 nm or less. In this case, the filter used has a pore size in the range of 0.2 to 0.5 μm, preferably 0.2 to 0.45 μm. Here, the "pore size" of the filter refers to the largest pore size in the filter.

[0081] The present inventors have found that even when the pore size of a filter is sufficiently large compared with the particle size of metal oxide nanoparticles or their aggregates, depending on the filter used, the filter may become clogged during filtration, making it impossible to complete the filtration.The present inventors have also found that the above problem can be avoided by using a filter having the characteristics described below.

[0082] That is, in this embodiment, the dispersion is filtered using a method in which the Hansen solubility parameter δ h is 15 (MPa)1 / 2 A filter made of a material where δ is equal to or greater than δ h is the hydrogen bonding strength term of the Hansen solubility parameter.

[0083] δ h If a material with a sufficiently large δ is used for the filter, the clogging described above can be prevented. h is 15 (MPa) 1 / 2 It is preferable that the pressure is 20 (MPa) or more. 1 / 2 It is more preferable that δ is equal to or greater than δ. h For example, 30 (MPa) 1 / 2 or less, and in other examples, 25 (MPa) 1 / 2 The following is the result.

[0084] For clogging of the filter, use δ h has a large effect, while the Hansen solubility parameter δ d and δ p has little effect on filter clogging. d and δ p are the London dispersion term and the dipole-dipole term of the Hansen solubility parameters, respectively.

[0085] δ h Many of the materials in which δ is within the above range d is 15 (MPa) 1 / 2 〜21(MPa) 1 / 2 and δ p is 4 (MPa) 1 / 2 ~20(MPa) 1 / 2 For example, δ d is 15 (MPa) 1 / 2 ~20.5(MPa) 1 / 2 and δ p is 6 (MPa) 1 / 2 ~20(MPa) 1 / 2 is within the range.

[0086] Examples of materials that can be used for the filter and their Hansen solubility parameters are listed in Table 1. Table 1 also lists ethylene glycol and propylene glycol as examples of glycol-based solvents, along with their Hansen solubility parameters.

[0087] [Table 1]

[0088] As shown in Table 1, the filter material may be, for example, cellulose acetate, cellulose, polyacrylic acid, polyvinyl alcohol, polyvinyl sulfonic acid, polyacrylamide, polymethacrylic acid, or a combination of two or more thereof. The filter material is preferably cellulose acetate, cellulose, polyacrylic acid, or a combination of two or more thereof, more preferably cellulose acetate, cellulose, or a combination of two or more thereof, and most preferably cellulose acetate.

[0089] The filter may be a screen filter or a depth filter, and is preferably a screen filter, such as a membrane filter.

[0090] The dispersion may be filtered only once. Alternatively, the dispersion may be filtered multiple times. That is, the filtered dispersion may be subjected to one or more filtrations, each using a filter that satisfies the above-mentioned conditions. When multiple filtrations are performed, the filter used in one filtration and the filter used in the next filtration may have the same or different pore sizes. In the latter case, for example, the pore size of the latter filter is made smaller than the pore size of the former filter.

[0091] In the above-described filtration, the upstream side of the filter can be pressurized higher than the downstream side of the filter. In this case, the pressure difference between the upstream side and the downstream side of the filter is preferably in the range of 0.01 MPa to 0.7 MPa, and more preferably in the range of 0.05 MPa to 0.5 MPa. Increasing this pressure difference allows filtration to be completed in a shorter time. However, if this pressure difference is too large, the filter is more likely to be damaged.

[0092] In this way, by using the above-described filter for filtering the dispersion, clogging of the filter can be prevented, which means that the productivity of the ink composition containing metal oxide nanoparticles and a glycol-based solvent can be improved.

[0093] Furthermore, the ink composition obtained in this manner does not contain coarse particles and has an excellent dispersion state of the metal oxide nanoparticles. Therefore, by using this ink composition, it is possible to keep coating unevenness that occurs during the formation of a printed layer within an acceptable range or to prevent the occurrence of coating unevenness. [Example]

[0094] The following describes tests carried out in connection with the present invention.

[0095] (1) Preparation of ink composition (1.1) Example 1 An ink composition comprising metal oxide nanoparticles and a dispersion medium was prepared by the following method.

[0096] First, a dispersion liquid containing metal oxide nanoparticles and a dispersion medium was prepared. The metal oxide nanoparticles used were nanoparticles made of nickel (II) oxide. The dispersion medium was ethylene glycol (EG), a glycol-based solvent. The amount of metal oxide nanoparticles was adjusted so that the ratio of metal oxide nanoparticles to the dispersion liquid was 2.00 mass%.

[0097] The particle size distribution of this dispersion was measured by dynamic light scattering, and the cumulative volume distribution showed that the 50% diameter was 21 nm and the 95% diameter was 63 nm.

[0098] Next, this dispersion was filtered twice. For each filtration, a syringe filter with a filter diameter of 25 mm was used. For the first filtration, a first filter made of cellulose acetate (CA) with a pore diameter of 0.45 μm was used. For the second filtration, a second filter made of cellulose acetate (CA) with a pore diameter of 0.2 μm was used.

[0099] In this way, an ink composition was obtained. In this example, in each filtration step, there was no difficulty in pushing down the piston due to excessive clogging of the filter.

[0100] (1.2) Example 2 An ink composition was prepared in the same manner as in Example 1, except for the following points: In this example, the first filter used was made of cellulose acetate (CA) and had a pore size of 0.2 μm.

[0101] In this way, an ink composition was obtained. Note that in this example, too, there was no difficulty in pushing down the piston due to excessive clogging of the filter during each filtration.

[0102] (1.3) Example 3 An ink composition was prepared in the same manner as in Example 1, except for the following points: In this example, the second filter used was made of cellulose acetate (CA) and had a pore size of 0.45 μm.

[0103] In this way, an ink composition was obtained. Note that in this example, too, there was no difficulty in pushing down the piston due to excessive clogging of the filter during each filtration.

[0104] (1.4) Example 4 An ink composition was prepared in the same manner as in Example 1, with the following exceptions: In this example, the second filtration was omitted.

[0105] In this way, an ink composition was obtained. Note that in this example, too, there was no difficulty in pushing down the piston during filtration due to excessive clogging of the filter.

[0106] (1.5) Example 5 An ink composition similar to that in Example 1 was prepared, except for the following: In this example, a first filter made of cellulose acetate (CA) with a pore size of 0.2 μm was used, and the second filtration was omitted.

[0107] In this way, an ink composition was obtained. Note that in this example, too, there was no difficulty in pushing down the piston during filtration due to excessive clogging of the filter.

[0108] (1.6) Comparative Example 1 A dispersion similar to that in Example 1 was prepared, and filtration of this dispersion was attempted. A syringe filter with a filter diameter of 25 mm was used for this filtration. The filter (first filter) used was made of polytetrafluoroethylene (PTFE) and had a pore diameter of 0.45 μm. In this example, excessive clogging of the filter occurred during this filtration, making it impossible to press down the piston.

[0109] (1.7) Comparative Example 2 Filtration of the dispersion was attempted in the same manner as in Comparative Example 1, except for the following points. That is, in this example, a filter (first filter) made of polytetrafluoroethylene (PTFE) and having a pore size of 0.2 μm was used. In this example, too, excessive clogging of the filter occurred during the filtration, making it impossible to push down the piston.

[0110] (1.8) Comparative Example 3 Filtration of the dispersion was attempted in the same manner as in Comparative Example 1, except for the following points. That is, in this example, a filter (first filter) made of polypropylene (PP) and having a pore size of 0.2 μm was used. In this example, too, excessive clogging of the filter occurred during the filtration, making it impossible to push down the piston.

[0111] (1.9) Comparative Example 4 Filtration of the dispersion was attempted in the same manner as in Comparative Example 1, except for the following points. That is, in this example, a filter (first filter) made of polyvinylidene fluoride (PVDF) and having a pore size of 0.2 μm was used. In this example, too, excessive clogging of the filter occurred during the filtration, making it impossible to push down the piston.

[0112] (1.10) Comparative Example 5 Filtration of the dispersion was attempted in the same manner as in Comparative Example 1, except for the following points. That is, in this example, a filter (first filter) made of polyethersulfone (PES) and having a pore size of 0.2 μm was used. In this example, too, excessive clogging of the filter occurred during the filtration, making it impossible to push down the piston.

[0113] Table 2 below lists the filter materials used in Examples 1 to 5 and Comparative Examples 1 to 5, and their Hansen solubility parameters.

[0114] [Table 2]

[0115] (2) Evaluation A coating film was formed using each of the ink compositions prepared in Examples 1 to 5, and coating uniformity was evaluated. As a reference example, an ink composition was prepared in the same manner as in Example 1, except that filtration was not performed, i.e., the dispersion of Example 1. Coating film formation using this ink composition and coating uniformity were also evaluated.

[0116] The coating film was formed by the following method. First, a 5 cm square glass substrate was prepared, with an indium tin oxide (ITO) layer on one side of the glass substrate. Next, the ink composition was applied onto the ITO layer by spin coating. The coating film was dried in a room environment for 10 minutes and then baked at 200°C for 10 minutes using a hot plate.

[0117] The coating uniformity was evaluated by the following method. Specifically, each baked coating film was observed under an optical microscope, and the number of coating irregularities occurring within a 4 cm square area 5 mm from the edge of the glass substrate was counted. Here, a coating irregularity is an area where the optical density differs from that of the surrounding area. Ink compositions that produced coatings with three or fewer coating irregularities were rated "AA," ink compositions that produced coatings with four to six coating irregularities were rated "A," and ink compositions that produced coatings with seven or more coating irregularities were rated "B."

[0118] The results of the above evaluations are shown in Tables 3 and 4 below. Tables 3 and 4 also show the evaluation results regarding application properties. Here, ink compositions that were able to be filtered to the end were evaluated as "A," and ink compositions for which it became impossible to press down the piston during filtration were evaluated as "B."

[0119] [Table 3]

[0120] [Table 4]

[0121] As shown in Tables 3 and 4, the ink compositions according to Comparative Examples 1 to 5 were unable to be filtered to completion, whereas the ink compositions according to Examples 1 to 5 were able to be filtered to completion. Furthermore, the ink composition according to the Reference Example was unable to achieve good coating uniformity, whereas the ink compositions according to Examples 1 to 5 were able to achieve good coating uniformity. Furthermore, the ink compositions according to Examples 1 to 3, which were filtered twice, were able to achieve better coating uniformity than the ink compositions according to Examples 4 and 5, which were filtered only once. [Explanation of symbols]

[0122] 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. preparing a dispersion liquid containing 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 a dispersion medium containing a glycol-based solvent; Hansen solubility parameter δ ｈ is 15 (MPa) １／２ filtering the dispersion liquid through a filter made of the above material; A method for producing an ink composition comprising the steps of:

2. 2. The method for producing an ink composition according to claim 1, wherein the volume-based cumulative distribution obtained by measuring the particle size distribution of the dispersion by dynamic light scattering has a 50% diameter in the range of 3 to 50 nm and a 95% diameter of 100 nm or less.

3. 2. The method for producing an ink composition according to claim 1, wherein the pore size of the filter is in the range of 0.2 [mu]m to 0.5 [mu]m.

4. The method for producing an ink composition according to claim 1 , wherein the filter contains cellulose acetate.

5. The dispersion after filtration was subjected to a Hansen solubility parameter δ ｈ is 15 (MPa) １／２ The method for producing an ink composition according to claim 1, further comprising subjecting the ink composition to one or more filtrations, each of which uses a filter made of the above material.

6. An ink composition produced by the method according to any one of claims 1 to 5.

7. Producing an ink composition by the production method according to any one of claims 1 to 5; supplying the ink composition to the surface of a substrate to form a coating film; A method for forming a printing layer comprising:

8. A printing layer formed by the method according to claim 7 .

9. A display device comprising the printed layer according to claim 8.

10. A method for manufacturing a display device, comprising forming a print layer by the forming method according to claim 7.

Images