Conductive film, dispersion, manufacturing methods for these, and device including conductive film

Abstract

To provide a dispersion suitable for the production of a conductive film containing semiconductor nanoparticles as conductive elements.SOLUTION: A dispersion comprises: a dispersion medium; semiconductor nanoparticles dispersed in the dispersion medium; and a first compound, wherein the first compound is present as an adhered compound and a free compound, the adhered compound adhering to the semiconductor nanoparticles, the free compound being free from the semiconductor nanoparticles, and the dispersion satisfies at least one selected from i) to iv): i) a ratio of an amount of the first compound present as the free compound to an amount of the first compound present as the adhered compound is 1 or more; ii) a proportion of an amount of the first compound to a sum of amounts of the semiconductor nanoparticles and the first compound is 10% or more in terms of mass; iii) the semiconductor nanoparticles include at least one selected from an oxide, a sulfide, a selenide, and a telluride; and iv) the semiconductor nanoparticles are each a columnar body and / or a polyhedron.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a conductive film, particularly a conductive film having optical transparency. The present invention also relates to a method for producing the conductive film, a dispersion for producing the conductive film, and a method for producing the dispersion, and further to a device including the conductive film. [Background technology]

[0002] Conductive films, particularly transparent conductive films with high optical transparency, are used in various devices as electrodes, antistatic films, etc. Industrial mass production of conductive films is often carried out by vacuum film formation methods, typified by sputtering.

[0003] Vacuum film formation requires a reduced pressure atmosphere, which requires large-scale equipment and limits the substrates that can be used. In light of this situation, conductive films containing silver nanowires (e.g., Patent Document 1 and Non-Patent Document 1) and conductive films containing carbon nanotubes (e.g., Patent Document 2) have been proposed. These conductive films can be formed by printing methods, coating methods, and other methods that do not require a reduced pressure atmosphere. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-92036 [Patent Document 2] International Publication No. 2015 / 115102 [Non-patent literature]

[0005] [Non-Patent Document 1] Eri Nakazawa et al., "Development of Silver Nanowire Ink for Transparent Conductive Films," Journal of the Society of Smart Processing, Society of Smart Processing, 2015, Vol. 4, No. 4, pp. 202-206 Summary of the Invention [Problem to be solved by the invention]

[0006] However, it is not easy to reduce the production costs of precious metals and special carbon materials such as silver nanowires and carbon nanotubes. The present invention aims to provide a conductive film that can be formed without using a vacuum film formation method and that contains a material other than a precious metal or a special carbon material as a conductive element for exhibiting conductivity. Another object of the present invention is to provide a method and dispersion for producing such a conductive film, a method for producing the dispersion, and a device including such a conductive film. [Means for solving the problem]

[0007] Although the presence of conductive paths formed by mutual contact of conductive elements in a film has been considered essential for the development of practical conductivity, the inventors' investigations have shown that unexpectedly high conductivity can also be developed from films containing semiconductor nanoparticles that are spaced apart from each other.

[0008] That is, the present invention is including an array of semiconductor nanoparticles; When a cross section including the array portion is observed, the semiconductor nanoparticles are arranged in a line in the array portion while being spaced apart from each other, A conductive film having a conductivity C1 of 7 S / cm or more measured along at least one direction is provided.

[0009] The conductivity required for a conductive film may be sufficient for practical use if it is achieved in a specific direction. From this perspective, a conductive film does not necessarily need to exhibit high conductivity in all directions. Based on this concept, it is possible to obtain a highly useful conductive film that contains semiconductor nanoparticles as conductive elements.

[0010] That is, from another aspect, the present invention provides: Contains semiconductor nanoparticles as conductive elements, The conductive element The electrical conductivity C1 measured along the first direction is 7 S / cm or more, Anisotropic conductivity is exhibited, in which the conductivity C2 measured along the second direction is less than 10% of the conductivity C1; The conductive film is provided such that one of the first direction and the second direction is a film surface direction and the other is a film thickness direction.

[0011] Furthermore, the present invention provides a dispersion suitable for producing the above-mentioned conductive film.

[0012] That is, from another aspect, the present invention provides: a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound; the first compound exists as an attached compound attached to the semiconductor nanoparticles and as a free compound liberated from the semiconductor nanoparticles, A dispersion that satisfies at least one of the following conditions: i) to iv). i) The ratio of the amount of the first compound present as the free compound to the amount of the first compound present as the attached compound is 1 or greater. ii) The ratio of the amount of the first compound to the total amount of the semiconductor nanoparticles and the first compound is 10% or more by mass. iii) The semiconductor nanoparticles contain at least one selected from an oxide, a sulfide, a selenide, and a telluride. iv) The semiconductor nanoparticles are columnar and / or polyhedral.

[0013] The present invention further provides a method for producing a dispersion according to the present invention, the method comprising the steps of: mixing a stock solution containing a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, a first compound, and impurities with a solvent to obtain a mixed solution; Centrifuging the mixture at a rotation speed of less than 5000 rpm; removing the solvent together with at least a portion of the impurities from the mixture after the centrifugation; A manufacturing method comprising:

[0014] Further, from another aspect, the present invention provides a first ink comprising a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound, the first compound being present as an attached compound attached to the semiconductor nanoparticles and a free compound liberated from the semiconductor nanoparticles; and a second ink containing a second compound that has a smaller molecular weight than the first compound and is capable of substituting the first compound and attaching to the semiconductor nanoparticles.

[0015] Further, from another aspect, the present invention provides A conductive film according to the present invention is provided, The present invention provides a device comprising the conductive film as at least one functional film selected from an electrode film, an antistatic film, a heat generating film, and an electromagnetic wave shielding film.

[0016] From another aspect, the present invention provides: A conductive film containing semiconductor nanoparticles and at least one electrode in contact with the conductive film, The conductive film provides a device that satisfies the following a) and / or b): a) comprising an arrangement portion of the semiconductor nanoparticles, When a cross section including the array portion is observed, the semiconductor nanoparticles are arranged in a line in the array portion while being spaced apart from each other, The electrical conductivity C1 measured along at least one direction is 7 S / cm or more. b) containing the semiconductor nanoparticles as conductive elements; The conductive element The electrical conductivity C1 measured along the first direction is 7 S / cm or more, Anisotropic conductivity is exhibited, in which the conductivity C2 measured along the second direction is less than 10% of the conductivity C1; One of the first direction and the second direction is the film surface direction, and the other is the film thickness direction.

[0017] In another aspect, the present invention provides a method for producing a conductive film according to the present invention, the method comprising the steps of: a first step of applying a first ink to a surface of a substrate to form a coating film, the first ink including a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound, the first compound being a dispersion that exists as an attached compound attached to the semiconductor nanoparticles and a free compound liberated from the semiconductor nanoparticles; a second step of contacting the coating film with a second ink containing a second compound that has a smaller molecular weight than the first compound and is capable of substituting the first compound and adhering to the semiconductor nanoparticles, thereby substituting at least a portion of the first compound in the coating film with the second compound. [Effects of the Invention]

[0018] The conductive film of the present invention can be formed by methods other than vacuum deposition, specifically, for example, coating methods, and uses semiconductor nanoparticles as conductive elements for exhibiting conductivity. Therefore, it can be formed on a wide range of substrates, and is advantageous in terms of reducing manufacturing costs over films using a noble metal or special carbon material as a conductive element. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a cross-sectional view illustrating an example of a conductive film. [Figure 2] 1A and 1B are diagrams showing an example of the shape and arrangement of semiconductor nanoparticles contained in a conductive film. [Figure 3] 10A and 10B are diagrams showing another example of the shape and arrangement of semiconductor nanoparticles contained in a conductive film. [Figure 4] 10A and 10B are diagrams showing still another example of the shape and arrangement of semiconductor nanoparticles contained in a conductive film. [Figure 5] FIG. 1 is a cross-sectional view showing an example of a device including a conductive film. [Figure 6] FIG. 10 is a cross-sectional view showing another example of a device including a conductive film. [Figure 7] FIG. 10 is a diagram showing an example of light transmittance. [Figure 8] 1 is an example of an image obtained by observing a conductive film with a transmission electron microscope (TEM). [Figure 9] 10 is another example of an image obtained by observing a conductive film with a TEM. [Figure 10] FIG. 10 is a diagram for explaining a bending test. [Figure 11] FIG. 10 is a diagram showing an example of a change in resistivity of a conductive film due to a bending test. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, but the following description is not intended to limit the present invention to specific embodiments. In this specification, the term "semiconductor" refers to not only ordinary semiconductors but also semimetals. Semimetals are materials that have a band structure in which the lower part of the conduction band and the upper part of the valence band slightly overlap across the Fermi level due to distortion of the crystal structure or interactions between crystal layers. The term "nanoparticle" refers to particles with a minimum diameter of less than 1 μm, for example, in the range of 0.1 nm to less than 1 μm. Nanoparticles typically refer to particles with a maximum dimension of 5 μm or less, preferably in the range of 3 nm to 2 μm. The "minimum diameter" is determined by the smallest dimension passing through the center of gravity of the particle, and the "maximum dimension" is determined by the longest line segment that can be drawn within the particle. The term "substantially parallel" is used to mean that the difference in direction between the particles is limited to 10° or less, particularly 5° or less. The term "autonomously align" or "autonomous alignment" is used to mean that semiconductor nanoparticles align themselves by self-organization using interactions between particles as a driving force. The term "functional group" is also used to include halogen atoms.

[0021] <Conductive film> The conductive film of this embodiment includes semiconductor nanoparticles as conductive elements. The conductive film includes an array portion of semiconductor nanoparticles. When a cross section including the array portion is observed, the semiconductor nanoparticles are arranged in a spaced-apart relationship in the array portion. The semiconductor nanoparticles may be arranged in a line in the array portion. The conductive film may include a plurality of array portions that extend substantially parallel to one another, or may include a plurality of array portions that intersect with one another or are connected to one another at their ends. At least one cross section of the conductive film may include an array portion in which the semiconductor nanoparticles are arranged in a line in a spaced-apart relationship.

[0022] The conductivity C1 measured along at least one direction of the conductive film is 7 S / cm or more. When the direction in which the conductivity C1 is measured is defined as a first direction, the conductivity C2 measured along a second direction different from the first direction may be, for example, less than 10%, particularly less than 1% of the conductivity C1 (i.e., C2 / C1<0.1, particularly C2 / C1<0.01). The first direction and the second direction may be perpendicular to each other. One of the features of the conductive film of this embodiment is the anisotropy of the conductivity. Here, the first direction and the second direction are, for example, one parallel to the film surface, i.e., the film surface direction, and the other perpendicular to the film surface, i.e., the film thickness direction. The lower limit of the conductivity C2 is not particularly limited, but may be, for example, 2×10 -5 S / cm or more.

[0023] In a cross section where the semiconductor nanoparticle arrangement portion is observed, the average spacing between adjacent semiconductor nanoparticles is preferably 10 nm or less. The "average spacing" of the semiconductor nanoparticles is determined by individually measuring the shortest distance between two adjacent particles that are spaced apart in the semiconductor nanoparticle arrangement portion that appears in the film cross section and taking the arithmetic average of the measurements. As an example, the average distance can be determined by measuring the shortest distance between two adjacent particles that are spaced apart at 10 points along the portion where the two particles face each other and taking the arithmetic average (so-called 10-point average). Adjacent semiconductor nanoparticles in the arrangement portion in the cross section are preferably spaced apart so that their opposing sides are substantially parallel. In the conductive film, the first direction in which a conductivity C1 of 7 S / cm or more is obtained may be parallel to the cross section where the arrangement portion appears. The arrangement portion may be formed by the semiconductor nanoparticles autonomously arranging themselves during the formation of the conductive film.

[0024] The conductive film may contain a compound attached to the semiconductor nanoparticles, i.e., an attachment compound. The attachment compound is a compound different from the semiconductor constituting the semiconductor nanoparticles, typically an organic or inorganic compound that does not correspond to a semiconductor, and is preferably a compound different from the attachment agent used to protect the semiconductor nanoparticles from aggregation contained in the conductive ink, which is a dispersion of semiconductor nanoparticles. The molecular weight of the attachment compound is preferably 300 or less, 200 or less, 150 or less, or even less than 110, and particularly less than 60. The attachment compound may have a functional group suitable for bonding with the semiconductor nanoparticles, such as a ligand capable of bonding to a metal atom through a coordinate bond. This functional group may be included as an ion constituting a salt. However, in this case, the molecular weight of the compound is calculated excluding the counterion of the salt. Thus, for example, the molecular weight of KSCN, which contains SCN and can be attached to semiconductor nanoparticles, is calculated excluding K and is 58.1.

[0025] The three-dimensional shape of the semiconductor nanoparticles may be a columnar and / or polyhedron. The base of the columnar body is not particularly limited and may be, for example, a circle, an ellipse, or a polygon. A columnar body is typically a columnar body whose two bases are identical and parallel to each other, but is not limited thereto. For example, a columnar body may be a solid body whose longitudinal cross section cut along a plane passing through both bases is a rectangle or can be regarded as a rectangle. The polyhedron may be a regular polyhedron such as a regular hexahedron, but is not limited thereto. One preferred three-dimensional shape is a rod-shaped columnar body having a height that is at least two times, preferably at least three times, and particularly at least four times the diameter of a circle with the same area as the base (hereinafter, such a shape will be simply referred to as a "rod-shaped" body). However, the columnar body is not limited thereto and may also be a disk-shaped columnar body having a height that is at most 1 / 2 times, preferably at most 1 / 3 times, and particularly at most 1 / 4 times the diameter of a circle with the same area as the base (hereinafter, such a shape will be simply referred to as a "disk-shaped" body). The semiconductor nanoparticles may have a rod-like and / or disk-like shape.

[0026] The semiconductor nanoparticles may contain at least one selected from oxides, sulfides, selenides, and tellurides, or may contain at least one selected from sulfides, selenides, and tellurides, or may be sulfides and / or selenides. However, for applications requiring durability such as heat resistance, semiconductor nanoparticles containing oxides are generally more suitable.

[0027] FIG. 1 shows an example of a conductive film. The conductive film 1 is formed on the surface 7a of a substrate 7. FIG. 1 shows a cross section of the conductive film 1 cut along a direction perpendicular to the surface 7a. FIG. 2 shows semiconductor nanoparticles 11 and 12 contained in the conductive film 1. The shape and arrangement of the semiconductor nanoparticles 11 and 12 shown in FIG. 2 can be observed using, for example, a TEM. The minimum diameter of the semiconductor nanoparticles 11 and 12 is less than 1 μm, for example, 500 nm or less, and preferably 100 nm or less. In this embodiment, an insulating material is present in the matrix 40 where the semiconductor nanoparticles 11 and 12 are not present.

[0028] Semiconductor nanoparticles 11 and 12 appear in the cross section of the conductive film 1 shown in FIG. 2. The semiconductor nanoparticles 11 in the cross section are substantially polygonal with five or more vertices, specifically a pentagon or hexagon. The semiconductor nanoparticles 12 in the cross section are substantially quadrangular. The semiconductor nanoparticles 12 in the cross section are more specifically rectangular, and more specifically rectangular with the long side corresponding to the base being at least twice the length of the short side corresponding to the height. The semiconductor nanoparticles 11 and 12 observed in this manner are, for example, disk-shaped. However, the shape of the semiconductor nanoparticles in the cross section is not limited to the above and may be circular, elliptical, triangular, or any other quadrangle than a rectangle, such as a square.

[0029] When the semiconductor nanoparticles 12 appearing in the film cross section are rectangular, the length of the long side of the rectangle is, for example, 3 to 2000 nm, further 5 to 900 nm, or even 10 to 50 nm. The length of the short side of the semiconductor nanoparticles 12 is, for example, 2 to 100 nm, or further 3 to 20 nm. The ratio of the long side to the short side of the semiconductor nanoparticles 12 may be, for example, 2 or more.

[0030] The conductive film 1 includes arrangement sections 21, 22, 23, and 24 in which semiconductor nanoparticles 11 and 12 are arranged. In the arrangement sections 21 to 24, the semiconductor nanoparticles are arranged in a row without contacting adjacent particles. In the arrangement sections 21 and 22, substantially pentagonal or hexagonal semiconductor nanoparticles 11 are arranged, and in the arrangement sections 23 and 24, substantially rectangular semiconductor nanoparticles 12 are arranged. There are no particular restrictions on the number of semiconductor nanoparticles arranged in an arrangement section, and the number of semiconductor nanoparticles arranged may be 3 or more, 7 or more, or in some cases 10 or more, or even 20 or more.

[0031] In the array sections 21 to 24, a straight line can be drawn that passes through the semiconductor nanoparticles contained in each array section. In other words, each array section is arranged along a straight array direction. However, in FIG. 2, only the array directions 33 and 34 of the array sections 23 and 24 are shown, and other array directions are not shown. Note that in the film cross section of FIG. 2, there are array sections of semiconductor nanoparticles other than those shown, but these are not shown for simplification.

[0032] In the array sections 23 and 24, adjacent semiconductor nanoparticles 12 are spaced apart so that their opposing sides are substantially parallel. In such array sections, the average spacing between adjacent semiconductor nanoparticles 12 is likely to be maintained small. Because the gaps between semiconductor nanoparticles are usually insulating, an array with small spacing between semiconductor nanoparticles is advantageous for achieving high conductivity. Furthermore, in the array sections 23 and 24, the opposing sides of adjacent semiconductor nanoparticles 12 form the long sides of a rectangle. In an array structure with relatively long opposing sides, even if there is partial disorder in the arrangement of the semiconductor nanoparticles 12, the portions where the semiconductor nanoparticles 12 are close to each other are likely to be maintained, and the arrangement is likely to be maintained over a long distance. This characteristic is also advantageous for achieving high conductivity.

[0033] The average spacing between the semiconductor nanoparticles is, for example, 10 nm or less, further 7 nm or less, and in some cases 5 nm or less, preferably 3 nm or less, further preferably 2 nm or less, and particularly preferably 1.8 nm or less. The average spacing may be, for example, 0.3 nm or more, further preferably 0.5 nm or more.

[0034] 3 and 4 show cross sections of other examples of conductive films. In the cross section of conductive film 2, only substantially pentagonal or hexagonal semiconductor nanoparticles 13 appear. In the cross section of conductive film 3, only rectangular semiconductor nanoparticles 14 appear. In FIG. 4, rectangular semiconductor nanoparticles 14 are arranged in array sections 25, 26, 27, 28, and 29 extending along array directions 35, 36, 37, 38, and 39 that are substantially parallel to each other. Furthermore, substantially rectangular semiconductor nanoparticles 14 are also arranged in array sections 61, 62, 63, and 64 extending along array directions 71, 72, 73, and 74 that are not substantially parallel to the array directions 35 to 39 but are substantially parallel to each other. The cross section of FIG. 4 shows many array sections in which substantially rectangular semiconductor nanoparticles are arranged. In these array sections, the semiconductor nanoparticles 14 are arranged in a line over a long distance, spaced apart but adjacent to each other so that opposing sides are substantially parallel to each other. Such a film cross section is advantageous in achieving a sufficiently high conductivity in the in-plane direction.

[0035] However, it should be noted that even in a conductive film 2 in which a film cross section (FIG. 3) is observed in which no arrangement portion in which rectangular semiconductor nanoparticles are arranged is observable, an arrangement portion in which rectangular semiconductor nanoparticles are arranged may be observed in a different film cross section, for example, in a direction perpendicular to the paper surface of FIG. 3. For example, a film in which the semiconductor nanoparticles 13 have a disk-like three-dimensional shape and adjacent particles are arranged so that they are spaced apart in the height direction and their bases face each other may have a film cross section such as that shown in FIG. 4, depending on the cutting direction.

[0036] 2 to 4 show an embodiment in which all semiconductor nanoparticles 11 to 14 are spaced apart, but in this embodiment, as long as an arrangement portion in which semiconductor nanoparticles are spaced apart is included, some of the semiconductor nanoparticles may be in contact with each other in other portions. Note that even if they are actually spaced apart, the semiconductor nanoparticles may be observed to be in contact or overlapping depending on the observation direction, the resolution of the equipment used for observation, and other factors.

[0037] The arrangement of semiconductor nanoparticles in a wider area than those shown in Figures 2 to 4 is illustrated in Figures 8 and 9. As shown in these figures, the arrangement of semiconductor nanoparticles does not have to extend in the same direction. The path formed by multiple arrangements may be curved, bent, cross each other, or partially overlap. The path formed by multiple arrangements forms a conductive path.

[0038] The conductive films 1 to 3 have a matrix 40 in which the semiconductor nanoparticles 11 to 14 are not present. The matrix 40 contains a binder and other materials, which will be described later. The binder and other materials are also present between adjacent semiconductor nanoparticles in the arrangement section. The material constituting the matrix 40 may be entirely insulating, or at least a portion of it may be conductive.

[0039] The matrix 40 constitutes an element that allows the semiconductor nanoparticles 11-14 to be autonomously arranged. The appropriate arrangement of the semiconductor nanoparticles 11-14 generates anisotropy in the conductivity of the conductive films 1-3, and can raise the conductivity to a practical level at least in a specific direction. The matrix 40 can also be an element that contributes to improving the light transmittance of the conductive films 1-3. The matrix 40 can also be an element that suppresses a decrease in the conductivity of the conductive films 1-3 due to bending. In a cross section where an arrangement portion of the semiconductor nanoparticles is observed, the matrix 40 where no semiconductor nanoparticles are present may account for 20% or more, 30% or more, or even 40% or more of the entire area, or may be 90% or less, or even 80% or less.

[0040] The conductive films 1 to 3 preferably contain a compound attached to the semiconductor nanoparticles 11 to 14. This compound may also be present between adjacent semiconductor nanoparticles in the array portion. In order to maintain a narrow gap between the semiconductor nanoparticles, it is preferable that the molecular weight of the compound is small. However, the compound attached to the semiconductor nanoparticles can be removed or reduced after film formation, for example, by heating. In particular, compounds with a small molecular weight, such as hydrazine, tend to disappear from the conductive film over time by volatilization, even without intentional heating.

[0041] The electrical conductivity C1 of the conductive film is 7 S / cm or more, or even 10 S / cm or more, preferably 20 S / cm or more, more preferably 50 S / cm or more, even more preferably 100 S / cm or more, particularly preferably 150 S / cm or more, and in some cases 180 S / cm or more. The electrical conductivity C1 may be obtained by measurement along at least one direction. This direction may be the film surface direction parallel to the film surface, or the film thickness direction perpendicular to the film surface. In practice, high electrical conductivity of a conductive film is often required in a specific direction. For example, in the case of a very thin conductive film, the length in the film surface direction is significantly longer than the film thickness, so improving the electrical conductivity in the film surface direction can often substantially solve practical problems.

[0042] As can be understood from the above, the conductivity of the conductive films 1 to 3 may have anisotropy due to the arrangement state of the semiconductor nanoparticles. The conductive film according to this embodiment may be a film in which a conductivity C1 is measured along a first direction and a conductivity C2 lower than the conductivity C1 is measured along a second direction. When the first direction is the film surface direction, the second direction may be the film thickness direction. When the first direction is the film thickness direction, the second direction may be the film surface direction.

[0043] The conductivity C2 may be, for example, less than 10%, less than 1%, or even less than 0.1%, particularly less than 0.001%, or even less than 0.0001% of the conductivity C1. However, by properly arranging the semiconductor nanoparticles or narrowing the spacing between the semiconductor nanoparticles, it is possible to improve not only the conductivity C1 but also the conductivity C2. The conductivity C2 may be, for example, 2×10 -5 S / cm or more, and even 1×10 -4 S / cm or more, especially 1×10 -3 S / cm or more, and in some cases 1×10 -2 S / cm or more, and even 1×10 -1 The electrical conductivity may be 1 S / cm or more, particularly 1 S / cm or more, but the conductive film of this embodiment does not necessarily need to have a significant anisotropy in electrical conductivity.

[0044] Disordered arrangement of semiconductor nanoparticles prevents the achievement of the above-mentioned high level of conductivity. To prevent this disorder, it is desirable to include a sufficient amount of compounds that can adhere to semiconductor nanoparticles in the dispersion used as the conductive ink for forming the conductive film. In a film in which the arrangement of semiconductor nanoparticles is disordered, not only the conductivity C1 but also the conductivity C2 tends to decrease.

[0045] The directional dependence of the conductivity of a conductive film is affected by at least the affinity of the semiconductor nanoparticles for the surface of the substrate and the three-dimensional shape of the semiconductor nanoparticles. For example, semiconductor nanoparticles having a disk-like three-dimensional shape are arranged so that the disk surface is approximately perpendicular to the surface of the substrate on the surface of a substrate with low affinity for the semiconductor nanoparticles. These semiconductor nanoparticles are arranged so that the disk surface is approximately parallel to the surface of the substrate on the surface of a substrate with high affinity for the semiconductor nanoparticles. Furthermore, for example, semiconductor nanoparticles having a rod-like three-dimensional shape are arranged so that the axial direction (long axis) of the rod is approximately perpendicular to the surface of the substrate on the surface of a substrate with low affinity for the semiconductor nanoparticles, in other words, the rod stands upright relative to the surface. These semiconductor nanoparticles are arranged so that the axial direction of the rod is approximately parallel to the surface of the substrate on the surface of a substrate with high affinity for the semiconductor nanoparticles, in other words, the rod lies down.

[0046] To control the directional dependence of the conductivity of a conductive film through the arrangement of semiconductor nanoparticles, it is useful to select an appropriate substrate or adjust the affinity of the substrate surface. For example, to arrange rod-shaped semiconductor nanoparticles on the surface of a substrate with low affinity for semiconductor nanoparticles so that the longitudinal axis of the rods is approximately parallel to the surface, it is useful to pre-coat the surface of the substrate with a material that has high affinity for semiconductor nanoparticles. This arrangement is suitable for forming a conductive film with a relatively high conductivity C1 in the film thickness direction.

[0047] The light transmittance of the conductive film at a wavelength of 650 nm is, for example, 80% or more, more preferably 85% or more, preferably 90% or more, and even more preferably 92% or more. A conductive film having such a high light transmittance is sometimes called a transparent conductive film. However, the light transmittance of the conductive film at the same wavelength may be about 50 to 80%, and depending on the application, 60% or more may be sufficient. Furthermore, the light transmittance of the conductive film in the visible light region (wavelength region of 400 to 800 nm) is preferably, for example, 50% or more throughout. This results in a nearly transparent conductive film.

[0048] Similarly, the light transmittance of the conductive film at a wavelength of 550 nm is, for example, 80% or more, further 85% or more, preferably 90% or more, and more preferably 92% or more. The light transmittance of the conductive film at the same wavelength may be about 50 to 80%, and depending on the application, 60% or more may be sufficient.

[0049] The thickness of the conductive film is not particularly limited and may be appropriately set depending on the application, but is, for example, 5 to 5000 nm, 5 to 2000 nm, further 10 to 1000 nm, and particularly 100 to 800 nm.

[0050] The conductive film of this embodiment can have excellent folding resistance. Conventional conductive films, in which conductive elements contact each other to form conductive paths, tend to exhibit a decrease in conductivity due to bending. For example, as shown in FIG. 7 of Non-Patent Document 1, the conductivity of a conductive film containing silver nanowires in contact with each other as conductive elements often decreases significantly after approximately 300 to 400 bendings. In contrast, the conductive film of this embodiment, in which the conductive elements, semiconductor nanoparticles, form conductive paths while spaced apart, is less likely to exhibit a decrease in conductivity even with repeated bending. As will be described later, an example of a conductive film of this embodiment showed that its conductivity did not decrease significantly even after being repeatedly bent hundreds of times, even 1,000 times (see FIG. 11). To prevent such a decrease in the conductivity of a conductive film containing silver nanowires, the silver nanowires must be melted and integrated by photonic curing. As shown in FIG. 7 of Non-Patent Document 1, even after photo-curing, the conductivity of the silver nanowires rapidly decreases after being bent approximately 900 times or more. The conductive film of this embodiment has excellent folding resistance and is therefore suitable for use in flexible devices.

[0051] When the conductive film of this embodiment is subjected to the following bending test, the resistance change rate calculated from the resistivity R1 before the test and the resistivity R2 after the test by (|R2-R1| / R1)×100 can be 30% or less, further 20% or less, and particularly 10% or less. The bending test involves bending a test piece on which the conductive film is formed at a bending angle of 180° so that the radius of curvature of the bent portion is 5 cm or less, and then bending it in the same way in the opposite direction, counting this as one bending. The test is repeated 500 times, or if necessary, 800 times, or even 1000 times. The radius of curvature is usually set to 5 cm. However, if a smaller bending is possible, a more stringent bending test can be performed with a radius of curvature of less than 5 cm, for example, 1 cm, or even 5 mm. The resistivity R1 used in the measurement is preferably 7 S / cm or more when converted into conductivity.

[0052] As shown in Figure 10, for each bending test, the test specimen 100 is bent once on each side. The radius of curvature r of the bent portion 101 is determined at the deepest portion 102 of the bent portion 101. In the bent state, the ends 103 of the test specimen 100 are held parallel to each other so that the bending angle is 180°.

[0053] The conductive film of this embodiment can have excellent weather resistance. For example, a conductive film containing silver nanowires as conductive elements will undergo oxidation and migration in a hot and humid environment, causing a rapid decrease in conductivity, unless the conductive film is covered with an overcoat. In contrast, the conductive film of this embodiment uses semiconductor nanoparticles as conductive elements, so its conductivity is less affected even when the film surface is exposed to a hot and humid atmosphere.

[0054] <Conductive film components> (semiconductor nanoparticles) The semiconductor nanoparticles preferably contain at least one compound semiconductor selected from oxides, sulfides, selenides, and tellurides. The semiconductor nanoparticles may be composed of a semiconductor equivalent to a metal compound. Furthermore, trace components, particularly components called dopants, for improving conductivity may be added to the semiconductor nanoparticles.

[0055] Examples of preferred oxides include indium oxide, zinc oxide, tungsten oxide, molybdenum oxide, cadmium oxide, copper oxide, vanadium oxide, and copper gallium oxide. Examples of preferred sulfides include copper sulfide and copper indium sulfide. Examples of preferred selenides include copper selenide. Examples of preferred tellurides include copper telluride.

[0056] Semiconductor nanoparticles may be crystalline or amorphous. However, if they contain crystals, the crystalline structure may affect their conductivity. The type of crystal may also affect the three-dimensional shape of the semiconductor nanoparticles. The preferred crystalline structure varies depending on the type of semiconductor nanoparticle. For example, copper sulfide preferably contains at least one selected from the group consisting of covellite, anilite, roxybite, digenite, and djuleite, particularly covellite and / or roxybite. Nanoparticles containing copper sulfide may contain only at least one crystalline structure selected from the above.

[0057] The semiconductor nanoparticles preferably have a three-dimensional shape that results in a locally ordered structure due to autonomous alignment. Specifically, this three-dimensional shape is a shape having a cross section with substantially parallel opposite sides, and is typically a columnar body and / or a polyhedron. An example of the three-dimensional shape is a columnar body whose base is a circle, an ellipse, or a polygon, or can approximate any of these. This shape may be the rod-like shape described above, or a disk-like shape.

[0058] The content of semiconductor nanoparticles in the conductive film is not particularly limited, but is, expressed in mass %, for example, 5 to 98%, further 10 to 50%, and preferably 20 to 40%.

[0059] (binder) The conductive film may contain a binder together with the semiconductor nanoparticles. The binder is a preferred component that can impart flexibility to the conductive film by being interposed between the semiconductor nanoparticles. The binder can also contribute to improving the film-forming properties of the conductive film and ensuring proper alignment of the nanoparticles.

[0060] The binder preferably contains an attachment compound attached to the semiconductor nanoparticles. The attachment compound may contain at least one functional group capable of bonding to the semiconductor nanoparticles, such as fluoride (F), chloride (Cl), bromide (Br), iodide (I), cyanide (CN), thiocyanato (SCN), isothiocyanato (NCS), hydroxide (OH), mercapto (SH), carbonyl (CO), amino (NR), nitrosyl (NO), nitrite (NO), phosphane (PR), carbene (RC), and pyridine (NCH). Here, each R is independently an organic residue or a hydrogen atom. As can be understood from the above examples, the functional group capable of bonding to the semiconductor nanoparticles may also be other functional groups capable of functioning as ligands for metal atoms or anions.

[0061] The attachment compound may be an inorganic compound or an organic compound. The attachment compound may also be a salt composed of an ion containing or consisting of one of the functional groups exemplified above and its counterion. The attachment compound may also be a compound having multiple of the above functional groups, such as hydrazine (H2NNH2), ethylenediamine (H2NCH2CH2NH2), ethylenedithiol (HSCH2CH2SH), mercaptopropionic acid (HSCH2CH2COOH), acetylacetonate (HCCOCHCOCH3), or aminobenzonitrile (NH2C6H4CN).

[0062] The molecular weight of the attachment compound is, for example, 300 or less, preferably 200 or less, more preferably 100 or less, even more preferably 80 or less, and in some cases less than 60. There is no particular limitation on the lower limit of the molecular weight, but it is, for example, 20 or more, or even 30 or more. The use of an attachment compound whose molecular weight is not too large is suitable for controlling the spacing between semiconductor nanoparticles to be narrow.

[0063] The content of the attached compound in the conductive film may be adjusted appropriately depending on the type of the attached compound, and may be, expressed as the mass ratio of the attached compound to the total amount of the semiconductor nanoparticles and the attached compound, for example, 1% or more, further 2% or more, particularly 3% or more, and in some cases 5% or more, preferably 8% or more. The upper limit of this content is not particularly limited, but is 30% or less, further 20% or less.

[0064] Materials other than the above-mentioned adhesive compounds may be used as the binder. Examples of such compounds include various resins, specifically polyvinyl alcohol, polyvinyl acetal, polyvinyl pyrrolidone, carboxymethyl cellulose, acrylic resin, polyvinyl acetate, polyethylene terephthalate, polystyrene, polyethylene, etc. In addition, pH adjusters, colorants, thickeners, surfactants, etc. may also be used depending on the needs for forming the conductive film and the intended use.

[0065] <device> The conductive film according to the present invention is suitable for use in various devices, particularly devices that use a light-transmitting conductive film. Examples of such devices include photovoltaic devices such as solar cells, image display devices such as liquid crystal displays and organic electroluminescence (EL) displays, heat-generating devices such as heated windshields, and electromagnetic wave shielding devices such as electromagnetic wave shielding windows and heat-shielding windows. In these devices, the conductive film is used as an electrode film, an antistatic film, a heat-generating film, an electromagnetic wave shielding film, etc. The use of a highly transparent conductive film for the above-mentioned applications offers advantages such as not interfering with the design of the product, not interfering with information communication, and achieving invisibility.

[0066] In the above-described device, at least one electrode may be disposed in contact with the conductive film for the purpose of passing current or discharging charges. Examples of electrode arrangements are shown in FIGS. 5 and 6. In the example shown in FIG. 5, a pair of electrodes 51 and 52 are disposed spaced apart in the direction of the surface of the conductive film 4. By applying a potential difference between the electrodes 51 and 52, a current flows in the direction of the surface of the conductive film 4. This conductive film may function, for example, as a heat-generating film. The electrodes 51 and 52 do not need to be formed on the surface 7a of the substrate 7, but may be formed on the surface 4a of the conductive film 4. Note that, for discharging charges from the conductive film for purposes such as preventing static electricity rather than generating heat, only one electrode is sufficient.

[0067] In the example shown in FIG. 6, the conductive film 4 in contact with the electrode 53 is used as a light-transmitting electrode. For example, when light 9 transmitted through the conductive film 4 is incident on the functional film 8, a potential difference is generated between the electrodes 53 and 54 due to the photoelectric conversion function of the functional film 8. In another device, when a potential difference is applied between the conductive film 4 and the electrode 54, a voltage is applied to the functional film 8 along its film thickness direction (the z direction in FIG. 6), causing the functional film 8 to emit light in the opposite direction to the incident light 9. In the example shown in FIG. 6, a transparent substrate such as a glass plate or a transparent resin plate can be used as the base material 7, and a metal film such as aluminum can be used as the electrode 54. However, examples of the base material 7 and the electrode 54 are not limited thereto. For example, the base material 7 may be a substrate having a thin film on its surface.

[0068] 6, the electrode 53 extends in a direction perpendicular to the paper surface (the y direction in FIG. 6). In this case, in order to reduce the difference in potential within the conductive film 4 depending on the distance from the electrode 53, it is preferable that the conductive film 4 have a high conductivity C1 at least in the film surface direction perpendicular to the direction in which the electrode extends (the x direction).

[0069] 6, when observed from the z direction perpendicular to the film surface of the conductive film 4, the area S1 of the region where the electrode 53 and the conductive film 4 overlap is less than half, i.e., less than 50%, of the area S2 of the conductive film 4. The ratio of the area S1 to the area S2 may be, for example, 30% or less, or even 20% or less, or in some cases 10% or less.

[0070] However, the shape and arrangement of the electrodes in the device are not limited to the examples shown in FIGS.

[0071] To take advantage of the excellent folding resistance of the conductive film, the present embodiment also provides a flexible device. A flexible device, for example, can be bent without breakage at a bending angle of 180° with a bending radius of 5 cm or less, e.g., 1 cm, or even 5 mm. Flexible substrates such as woven fabric, nonwoven fabric, paper, and film are suitable for such devices. The device of the present embodiment may have a resistance change rate of 30% or less, preferably 20% or less, and particularly 10% or less, of the conductive film measured by the above-described bending test. However, the device of the present embodiment may also be a rigid device lacking flexibility.

[0072] As described above, the conductive film in this embodiment can have excellent durability. To take advantage of this characteristic, at least a portion of the conductive film in the device of this embodiment may be exposed.

[0073] <Dispersion> Dispersions used to form conductive films are sometimes called conductive inks. Following this convention, hereinafter in this specification, dispersions may also be referred to as conductive inks. As can be understood from the above, conductive inks do not necessarily have to be conductive themselves. A dispersion that is a conductive ink contains a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound, and the first compound exists as an attached compound attached to the semiconductor nanoparticles and as a free compound free from the semiconductor nanoparticles, and satisfies at least one selected from the following i) to iv). It is desirable that the dispersion satisfies at least i). The dispersion may also satisfy all of i) to iv). i) The ratio of the amount of the first compound present as a free compound to the amount of the first compound present as an attached compound is 1 or greater. ii) The ratio of the amount of the first compound to the total amount of the semiconductor nanoparticles and the first compound is 10% or more by mass. iii) The semiconductor nanoparticles contain at least one selected from an oxide, a sulfide, a selenide, and a telluride. iv) The semiconductor nanoparticles are columnar and / or polyhedral.

[0074] The conductive ink contains a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound. A portion of the first compound is contained in the conductive ink as an attached compound attached to the semiconductor nanoparticles, while the remainder of the first compound is contained in the conductive ink as a free compound liberated from the semiconductor nanoparticles. The first compound may be the same as the second compound to be attached to the semiconductor nanoparticles in the conductive film, but it is preferable that they are different compounds. This is because a compound desirable for improving the conductivity of a conductive film does not necessarily coincide with a compound desirable for forming a conductive film in which semiconductor nanoparticles are properly aligned from the conductive ink. The first compound can be a compound suitable for stably dispersing the semiconductor nanoparticles in the ink and for autonomously aligning the semiconductor nanoparticles during the formation of the conductive film. The second compound can be introduced into the conductive film by replacing the first compound after the film is formed.

[0075] The autonomous alignment of semiconductor nanoparticles can be achieved by interactions between the semiconductor nanoparticles, specifically, by using electrostatic force, van der Waals force, dipole force, dispersion force, etc. As the dispersion medium is removed from the applied ink, the semiconductor nanoparticles can be aligned in a self-regulating manner with the first compound interposed between them.

[0076] As with the second compound already described as the attachment compound, the first compound may be an inorganic or organic compound, particularly an organic compound, containing at least one, and in some cases, a plurality of, of the functional groups exemplified above. However, it is preferable that the molecular weight of the first compound is larger than that of the second compound. The molecular weight of the first compound is preferably 2000 or less, more preferably 1000 or less, and even more preferably 500 or less, for example, 60 to 300. When the first compound is a salt, the molecular weight of the first compound is also calculated based on the method described above.

[0077] The content of the first compound in the conductive ink may be adjusted appropriately depending on the type of compound. Expressed as the ratio of the mass of the first compound to the total mass of the semiconductor nanoparticles and the first compound, it is preferably 10% or more, 15% or more, 16% or more, 17% or more, particularly 18% or more, and in some cases may be 20% or more, 25% or more, 28% or more, particularly 30% or more. The upper limit of this ratio is not particularly limited, but may be, for example, 90% or less, 86% or less, or in some cases 75% or less. To autonomously and appropriately align the semiconductor nanoparticles in the film after film formation, it is preferable to add more of the first compound than when the sole purpose is to stably disperse the semiconductor nanoparticles in the ink. Conductive films formed from conductive inks lacking the first compound often have insufficient conductivity.

[0078] To properly align the semiconductor nanoparticles, it is desirable for the conductive ink to contain an appropriate amount of the first compound as a free compound. If the amount of the free compound is below the appropriate range, the semiconductor nanoparticles may partially aggregate, even if they are properly dispersed in the conductive ink. This aggregation can hinder the proper alignment of the semiconductor nanoparticles in the conductive film. Because the state of the free compound attached to the semiconductor nanoparticles is generally relatively stable compared to the free state, the appropriate range of the amount of the free compound relative to the attached compound can be expressed as a mass ratio or molar ratio. The ratio of the amount of the first compound contained as a free compound to the amount of the first compound contained as an attached compound is preferably 1 or more, 2 or more, 3 or more, 4 or more, 5 or more, 6 or more, 7 or more, particularly 8 or more, and in some cases may be 10 or more. The upper limit of this ratio is not particularly limited, but may be, for example, 100 or less, 50 or less, or even 30 or less, or in some cases, 28 or less, or even 27 or less.

[0079] A dispersion of semiconductor nanoparticles may contain impurities derived from raw materials for synthesizing the semiconductor nanoparticles, additives added for the synthesis, and the like. The impurities are typically unreacted raw materials. To remove these impurities, a solvent may be added and the impurities may be removed together with the solvent. As such a solvent, a polar solvent, particularly a polar organic solvent, may be used. As described below, a nonpolar organic solvent or a polar organic solvent with low polarity is usually used as the dispersion medium. The removal of impurities can be carried out, for example, by a method including the following steps.

[0080] Adding solvent: A step of mixing a stock solution containing a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, the first compound, and impurities with a solvent to obtain a mixed solution. Centrifugation: A process of centrifuging the mixture at a predetermined rotation speed. Removal of solvent: after centrifugation, removing the solvent together with at least a portion of the impurities from the mixture.

[0081] If the rotation speed of the centrifugation is too high, a large amount of the first compound will be removed together with the solvent. Therefore, the rotation speed of the centrifugation is preferably less than 5000 rpm, particularly in the range of 500 to 4000 rpm. For the same reason, the centrifugation is preferably carried out at a temperature of the mixture near room temperature, specifically in the range of 10 to 30°C. A suitable polar solvent is a poor solvent for the semiconductor nanoparticles, and when the dispersion medium is a polar solvent, a polar solvent with a higher polarity than the dispersion medium is suitable. Specific examples of polar solvents include low-boiling point solvents, such as water, or polar organic solvents such as alcohols with 1 to 4 carbon atoms and acetone. There are no particular limitations on the number of times the centrifugation is performed, but approximately 2 to 3 times is preferred.

[0082] In addition to the conductive ink described above (which may be referred to as the "first ink" when combined with the second ink described below), it is desirable to prepare an ink containing a second compound to replace the first compound (hereinafter, this may be referred to as the "second ink"). An ink set including the first ink and the second ink is convenient for forming a conductive film. The ink set may include, for example, the first ink, a first container containing the first ink, a second ink, and a second container containing the second ink. As described above, the first compound and the second compound are a combination of compounds in which the molecular weight of the latter is smaller than the molecular weight of the former.

[0083] The conductive ink (first ink) and the second ink may contain a pH adjuster, a co-solvent, and other components. The other components may be the above-mentioned components that the conductive film may contain, or may be volatile components that do not remain in the conductive film.

[0084] Preferred aspects of the shape, crystallinity, etc. of the semiconductor nanoparticles are as described above. The content of the semiconductor nanoparticles in the conductive ink (first ink) is not particularly limited, but is, expressed in mass %, for example, 1 to 70%, further 5 to 50%, and preferably 10 to 20%. The dispersion medium in the conductive ink (first ink) is not particularly limited as long as it can disperse the semiconductor nanoparticles, and suitable examples include organic solvents such as toluene, hexane, octane, and chloroform. The solvent for the second ink is also not particularly limited as long as it can dissolve the second compound, and it is recommended to use a solvent appropriate for the type of second compound.

[0085] <Method of manufacturing conductive film> The method for manufacturing a conductive film according to this embodiment includes the following first and second steps in this order. After performing the second step, the following third and fourth steps may be performed. If the desired film thickness is not achieved, the third and fourth steps may be repeated two or more times. When using the ink set described above, the first ink is used as the conductive ink, and the second ink is used as the solution containing the second compound.

[0086] (1st step) In the first step, a conductive ink containing semiconductor nanoparticles and a first compound is applied to the surface of a substrate to form a coating film. (Second step) In the second step, the coating film formed in the first step is brought into contact with a solution containing a second compound, thereby replacing at least a portion of the first compound with the second compound, thereby obtaining a conductive film. (Third Step) In the third step, a conductive ink is applied to the surface of the formed conductive film to form an additional coating film. (Step 4) In the fourth step, the formed additional coating film is brought into contact with a solution containing a second compound, and at least a portion of the first compound contained in the additional coating film formed in the third step is replaced with the second compound, thereby obtaining a thickened conductive film.

[0087] The application of the conductive ink or solution in each step can be carried out by a known coating method such as spin coating, roll coating, flow coating, or dip coating.

[0088] The present invention will be further described below with reference to examples, but the following description is not intended to limit the present invention to specific examples.

[0089] (Example A1) -Preparation of conductive ink 1.891 g of copper acetate, 1.13 g of 1,3-dibutyl-2-thiourea, and 10 ml of oleylamine were placed in a three-necked flask, and the atmosphere was replaced with nitrogen while stirring. Next, the liquid temperature was raised to 80°C using a mantle heater and maintained at that temperature for 1 hour. After that, when the liquid temperature had dropped to 40°C, 40 ml of chloroform was gradually added to the three-necked flask to dissolve the solids.

[0090] The contents of the three-way flask were transferred to a centrifuge tube. After confirming that the solids were fully dissolved, 40 ml of ethanol was added. The tube was then centrifuged at 2000 rpm (revolutions per minute) for 10 minutes, and the supernatant was immediately discarded. The precipitate was then dissolved in 5 ml of octane, after which 30 ml of ethanol was added and the tube was again centrifuged at 2000 rpm for 5 minutes to collect the precipitate. The mass of the precipitate was measured, and based on this, octane was added to make the concentration 200 mg / ml. Copper sulfide nanoparticles were dispersed in the tube, resulting in a conductive ink.

[0091] The conductive ink contains copper sulfide nanoparticles and oleylamine, a compound that can coordinate with the copper sulfide nanoparticles. Thermogravimetric analysis (TGA) of the precipitate revealed that the mass ratio of oleylamine to the total mass of copper sulfide nanoparticles and oleylamine was 10%. This ratio was maintained throughout the following examples.

[0092] Conductive film formation A spin coater was used to apply 50 μl of conductive ink (first ink) onto a glass plate on which a comb-shaped electrode had been formed, yielding a coating film. The concentration of the conductive ink was adjusted to 50 mg / ml before application. The glass plate on which the comb-shaped electrode was formed was a commercially available product (G-IDEU5, manufactured by Drop Sens) with an electrode width and electrode spacing of 5 μm or 10 μm, a number of pairs of 250 (for a 5 μm electrode width) or 125 (for a 10 μm electrode width), and a beam length of 6760 μm.

[0093] A 200 μl solution (second ink) containing KSCN, which acts as an adhesive compound, was applied onto the coating film using a spin coater to obtain a conductive film. The solvent for this solution was octane, and the concentration of KSCN was 10% by mass. Upon contact with the second ink, at least a portion of the compound coordinated to the copper sulfide nanoparticles was replaced from oleylamine (first compound) to KSCN (second compound).

[0094] An additional coating film was formed on the conductive film in the same manner as above using the first ink, and then the compound was replaced in the same manner as above using the second ink, thereby obtaining a conductive film with an increased thickness.

[0095] (Examples A2 to A4) A conductive film was prepared in the same manner as in Example A1, except that the second compound in the second ink, which was the adhesion compound, was a compound shown in Table 1. At this time, the number of times n (n = 2 in Example A1) of repeated application to thicken the film was also set to the number shown in Table 1.

[0096] (Examples A5 to A8) -Preparation of conductive ink 0.246 g of copper acetate and 20 ml of oleylamine were placed in three-way flask A, and the flask was evacuated while stirring. Next, flask A was heated using an oil bath to raise the liquid temperature to 160°C, and this state was maintained for 1 hour. The temperature increase rate was 8°C / min. Meanwhile, 0.096 g of sulfur and 30 ml of 1-octadecene were placed in another three-way flask B, and the flask was repeatedly evacuated and replaced with nitrogen gas while stirring. Then, nitrogen gas was introduced to maintain a nitrogen gas atmosphere. Next, flask B was heated using an oil bath to raise the liquid temperature to 160°C, dissolving the sulfur. The temperature increase rate was 5°C / min. After leaving flask B for 1 hour, nitrogen gas was introduced to maintain a nitrogen gas atmosphere.

[0097] The contents of Flask A were transferred to a centrifuge tube, and the contents of Flask B were added to the centrifuge tube using a syringe and held for 10 minutes. The heater was then turned off, and after the liquid temperature had dropped to 40°C, approximately 30 ml of hexane was added to the centrifuge tube. After visually confirming that the solids had dissolved, 30 ml of ethanol was added and the tube was centrifuged at 2000 rpm for 5 minutes to collect the precipitate. The precipitate was then dissolved in 5 ml of octane, and 30 ml of ethanol was added again. The tube was centrifuged again at 2000 rpm for 5 minutes to collect the precipitate. The mass of the precipitate was measured, and based on this, octane was added to a concentration of 200 mg / ml, and copper sulfide nanoparticles were dispersed to obtain a conductive ink.

[0098] Conductive film formation A conductive film was prepared in the same manner as in Example A1, except that the second compound in the second ink, which was the adhesion compound, was a compound shown in Table 1. At this time, the number of times n of repeated application to thicken the film was also the number shown in Table 1.

[0099] (Example A9) 50.5 mg of copper chloride (CuCl2), 271.8 mg of copper nitrate (Cu(NO3)2 / 3H2O), 2.46 ml of oleylamine, 15 ml of 1-octadecene, and 6 ml of dodecanethiol were added to a three-arm flask, and the flask was repeatedly evacuated and replaced with nitrogen gas while stirring, and finally evacuated. The three-arm flask was then heated using a mantle heater to raise the liquid temperature to 100°C and maintained at that temperature for 30 minutes. While the liquid temperature was being raised to 180°C, 10 ml of t-dodecanethiol was injected via syringe, and the temperature was maintained at 180°C for 5 minutes, after which the flask was rapidly cooled using tap water.

[0100] The contents of the three-way flask were transferred to a centrifuge tube, and approximately 30 ml of hexane was added. After visually confirming that the solids had dissolved, 15 ml of isopropanol and 15 ml of acetone were added, and the tube was centrifuged at 2000 rpm for 5 minutes to collect the precipitate. The precipitate was then dissolved in 5 ml of octane, after which 30 ml of ethanol was added and the tube was centrifuged at 2000 rpm for 5 minutes to collect the precipitate. The mass of the precipitate was measured, and based on this, octane was added to a concentration of 200 mg / ml to disperse the copper sulfide nanoparticles, resulting in a conductive ink. After the liquid temperature had cooled to 40°C, approximately 30 ml of hexane was added to the centrifuge tube. After visually confirming that the solids had dissolved, 30 ml of ethanol was added and the tube was centrifuged at 2000 rpm for 5 minutes to collect the precipitate. The precipitate was then dissolved in 5 ml of octane, 30 ml of ethanol was added, and the mixture was centrifuged again at 2000 rpm for 5 minutes to collect the precipitate. The mass of the precipitate was measured, and based on this, octane was added to make the concentration 200 mg / ml, and copper sulfide nanoparticles were dispersed to obtain a conductive ink.

[0101] Conductive film formation A conductive film was prepared in the same manner as in Example A1, except that the second compound in the second ink, which was the adhesion compound, was a compound shown in Table 1. At this time, the number of times n of repeated application to thicken the film was also the number shown in Table 1.

[0102] The conductivity and film thickness of the obtained conductive film were measured, and the crystal system of the copper sulfide contained in the copper sulfide nanoparticles was also measured by X-ray diffraction. Furthermore, the light transmittance was also measured for Examples A1 to A3. The results are shown in Table 1 and FIG. 7. The light transmittance in Table 1 is the value at a wavelength of 650 nm.

[0103] [Table 1]

[0104] A conductive film was formed on the surface of an ITO-coated glass plate in the same manner as in Example A5, and a gold electrode was partially vapor-deposited on the surface of the conductive film. The conductivity of this conductive film in the thickness direction was measured and found to be 9×10 -5 S / cm. It was confirmed that the conductivity in the film thickness direction of the conductive films obtained in the other examples was also lower than the conductivity in the film surface direction measured using an interdigitated electrode, being significantly less than 1%. In the other examples, too, in view of the arrangement of the nanoparticles, the conductivity in the film thickness direction was considerably smaller than the conductivity in the film surface direction, and is thought not to exceed 10%.

[0105] In addition, when observed using a TEM, it was confirmed that the nanoparticles produced in Example A9 had a rod-like shape and were arranged so that their long axis direction was perpendicular to the surface of the substrate. On the other hand, it was confirmed that the nanoparticles produced in Examples A1 to A8 had a disk-like shape and were arranged so that the disk surface was perpendicular to the surface of the substrate. These crystal particles had a substantially rectangular cross section and were arranged close to each other so that the opposing long sides were substantially parallel to each other. It was also confirmed that the arrangement direction of this arrangement extended in the film surface direction. Furthermore, in all of the Examples, the average spacing of the copper sulfide nanoparticles was 10 nm or less. For example, when a conductive film was produced on the surface of a glass plate on which an interdigitated electrode had been formed in the same manner as in Example A5, the average spacing of the copper sulfide nanoparticles after substitution with EDT was 1.1 nm.

[0106] The conductive film formed from the conductive ink obtained in the same manner as above was observed using a TEM, and the results are shown in Figures 8 and 9. Figures 8 and 9 are images of the film surface of the conductive film.

[0107] As is clear from Table 1, roxyvite is a preferred crystal that can stably provide relatively high electrical conductivity. To the best of the inventor's knowledge, this point was previously unknown. Furthermore, covellite is a crystal that can provide significantly improved electrical conductivity, especially when combined with a compound having a small molecular weight. In other words, the electrical conductivity obtained from covellite is easily affected by the attached compound.

[0108] (Example A10) Conductive film formation A conductive film was fabricated on a borosilicate glass substrate in the same manner as in Example A1, except that the second compound in the second ink (the adhesion compound) was hydrazine. The coating process was repeated 15 times to thicken the film. The film was then cut into a 10 mm x 10 mm size to prepare a measurement sample. Subsequently, Au electrodes with thicknesses of 100 to 200 nm were formed on each of the four corners of the surface of the measurement sample by vapor deposition. Four terminals were then contacted to the Au electrodes formed on the four corners, and the sheet resistance of the measurement sample was measured using the Van Der Pauw method using a ResiTest 8400 sheet resistance measurement device (manufactured by Toyo Corporation). The measured sheet resistance, which is the resistance in the film surface direction, was 100 Ω / □. In other words, for a conductive film of a size close to practical use, performance almost equivalent to that of ITO (indium tin oxide) was obtained. Unlike ITO, which must be formed by vapor deposition, this conductive film can be formed by coating, which makes manufacturing easier and confirms its advantages.

[0109] Example B1 A conductive ink was obtained in the same manner as in Example A5, except that octadecene was used instead of octane as the organic solvent added last to obtain the conductive ink. In this conductive ink, oleylamine is contained as an attached compound attached to the copper sulfide nanoparticles and as a free compound liberated from the copper sulfide nanoparticles. The ratio of the free compound to the attached compound in this conductive ink was calculated as follows.

[0110] First, 5 ml of conductive ink and then 45 ml of ethanol were added to a centrifuge tube and centrifuged at 2000 rpm for 10 minutes. The resulting precipitate was then evacuated to remove residual solvent. Thermogravimetric analysis (TGA) was performed on the resulting dried product to measure the amounts of copper sulfide nanoparticles and oleylamine (attached compound) adhering to the precipitate. The ratio of the amount of oleylamine (attached compound) to the total amount of copper sulfide nanoparticles and oleylamine (attached compound) was 10% by mass. Using an evaporator, the supernatant obtained by centrifugation was then removed. The amount of oleylamine (free compound) liberated from the copper sulfide nanoparticles was calculated based on the amount of liquid after ethanol removal and the ratio of the amount of oleylamine to octadecene added. Based on the above, the ratio of the amount of free compound to the amount of attached compound was calculated to be 21.4.

[0111] A conductive film was obtained in the same manner as in Example A5, except that the conductive ink of Example B1 was used. However, the film was formed to a thickness of 135 nm. The measured conductivity is shown in Table 2.

[0112] (Example B2) The conductive ink prepared in Example A1 underwent a centrifugation process to remove impurities twice. However, from the perspective of removing impurities such as unreacted compounds, it is desirable to perform the centrifugation process repeatedly. Taking this into consideration, in Example B2, centrifugation was performed twice under the same conditions as in Example B1, followed by a third centrifugation under the same conditions as the second centrifugation. Using the conductive ink obtained in the same manner as in Example B1, the ratio of free compounds to attached compounds was determined, and a conductive film was formed and its conductivity was measured in the same manner as in Example B1. The results are shown in Table 2.

[0113] (Comparative Example B3) The conductive ink prepared in Example A1 was centrifuged to remove impurities at a rotation speed of 2000 rpm. However, from the perspective of impurity removal, it is desirable to use a centrifugation method with a higher rotation speed. Taking this into consideration, in Comparative Example B3, the centrifugation conditions were changed to 8000 rpm for 2 minutes, and a total of three centrifugations were performed. The second and third washings were performed using a mixture of chloroform and ethanol in a 1:20 ratio. Using the conductive ink prepared in the same manner as in Example B1, the ratio of free compounds to attached compounds was determined, a conductive film was formed, and its conductivity was measured in the same manner as in Example B1. The results are shown in Table 2.

[0114] [Table 2]

[0115] In each of the conductive films obtained in Examples B1 and B2 and Comparative Example B3, the conductivity in the film surface direction was higher than the conductivity in the film thickness direction. Furthermore, in Comparative Example B3, the semiconductor nanoparticles in the conductive ink were partially aggregated, which is thought to have resulted in the semiconductor nanoparticles being locally misaligned or misaligned across the entire conductive film. In such a film, sufficient conductivity cannot be obtained even in the direction where the conductivity is relatively high (in the film surface direction in the above example).

[0116] Example C1 A conductive film was obtained in the same manner as in Example A6, except that a PET plate with a comb-shaped electrode formed thereon was used instead of the glass plate with a comb-shaped electrode formed thereon and the number of coatings was five. The PET plate with a comb-shaped electrode formed thereon was a commercially available product (DRIP-P-IDEAU100 manufactured by Drop Sens). The PET plate with a conductive film formed thereon was folded 1,000 times so that the radius of curvature of the folded portion was approximately 5 mm, as shown in Figure 10 . The number of foldings was counted as one set, consisting of folding with the conductive film facing inward and folding with the conductive film facing outward. The resistivity of the conductive film was measured using a comb-shaped electrode every 50 foldings. Figure 11 shows the change in resistivity of the conductive film due to folding. The rate of change in resistivity was less than 10%.

[0117] Example D1 A conductive film was obtained in the same manner as in Example A6, except that anisidine (molecular weight 123.2) was used as the second compound in the second ink, which served as the adhesion compound. The resistance of this conductive film was measured in an exposed state immediately after preparation and after more than one and a half years (13,140 hours) had elapsed since preparation. No increase in the resistivity of the conductive film was observed.

[0118] (Example D2) The resistance of the conductive film of Example A6 was measured in an exposed state at the time of production and one and a half years after production. As a result, the resistance value was 44 Ω at the time of production and increased to 78 Ω after one and a half years. Although the resistance value increased, it is believed that the increase in resistance value was clearly suppressed compared to silver nanowires without an overcoat.

Claims

1. a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound; the first compound exists as an attached compound attached to the semiconductor nanoparticles and as a free compound liberated from the semiconductor nanoparticles; A dispersion satisfying at least one of the following conditions: i) to iv). i) the ratio of the amount of the first compound present as the free compound to the amount of the first compound present as the attached compound is 1 or greater; ii) The ratio of the amount of the first compound to the total amount of the semiconductor nanoparticles and the first compound is 10% or more by mass. iii) The semiconductor nanoparticles contain at least one selected from an oxide, a sulfide, a selenide, and a telluride. iv) The semiconductor nanoparticles are columnar and / or polyhedral.

2. The dispersion of claim 1 , which satisfies at least i).

3. The dispersion according to claim 2, which satisfies all of i) to iv).

4. The dispersion according to any one of claims 1 to 3, wherein, when the dispersion is applied to a substrate to form a coating film, the semiconductor nanoparticles autonomously arrange themselves, thereby forming an arrangement portion in which the semiconductor nanoparticles are arranged at a distance from one another on a cross section of the coating film.

5. The dispersion of any one of claims 1 to 4, wherein the semiconductor nanoparticles comprise copper sulfide.

6. 6. The dispersion of claim 5, wherein the copper sulfide comprises covellite and / or roxyvite.

7. A method for producing the dispersion according to any one of claims 1 to 6, comprising the steps of: mixing a stock solution containing a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, a first compound, and impurities with a solvent to obtain a mixed solution; Centrifuging the mixture at a rotation speed of less than 5000 rpm; removing the solvent together with at least a portion of the impurities from the mixture after the centrifugation; A method for producing a dispersion, comprising:

8. a first ink comprising a dispersion medium, semiconductor nanoparticles dispersed in the dispersion medium, and a first compound, wherein the first compound exists as an attached compound attached to the semiconductor nanoparticles and a free compound liberated from the semiconductor nanoparticles; a second ink containing a second compound that has a smaller molecular weight than the first compound and is capable of substituting the first compound to adhere to the semiconductor nanoparticles.

9. The ink set according to claim 8 , wherein the second compound has a molecular weight of less than 60.

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