JP2016046248A - Transparent conductive film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transparent conductive film excellent in transparency and conductivity and suitably used for electronic device applications.SOLUTION: There is provided a transparent conductive film 1 having a substrate 2 and a conductive layer 3 consisting of indium tin oxide nanowire coating a surface of the substrate 2 with coating percentage of 20 to 70% and having total light transmittance of 90% or more and surface resistance of 250 Ω/sq or less. There is provided a transparent conductive film 1 having average length of the indium tin oxide nanowire of 200 nm or more, a shape having a ball with diameter of 1 to 500 nm on a taper like tip as a shape of the indium tin oxide nanowire and SnOcontent of the indium tin oxide nanowire of 3.0 to 50 wt.%, preferably 7.0 to 35 wt.%.SELECTED DRAWING: Figure 1

Description

The present invention relates to a transparent conductive film that is excellent in transparency and conductivity, and is suitably used for electronic equipment.

A transparent conductive film is a thin film in which a conductive layer is provided on the surface of a substrate made of a polymer film or the like, and is used for electronic devices such as an organic EL display, a liquid crystal display, a transparent touch panel, and a solar battery.
Conventionally, silver has been used as a material for the conductive layer. However, silver is easily oxidized, and there is a problem in that the surface resistance of the conductive layer increases due to the oxidation and the conductivity is impaired. In some cases, a protective layer is provided on the surface of the conductive layer to prevent oxidation, but in this case, the number of manufacturing steps increases and the operation becomes complicated.

As an alternative to silver, metal oxides such as zinc oxide, indium oxide, tin oxide, and indium tin oxide (ITO), which are more stable than silver, are known. Indium tin oxide (ITO) is frequently used from the viewpoint of conductivity and the like.
The conductive layer made of indium tin oxide (ITO) is formed by a dry method such as a vacuum deposition method, a sputtering method, or an ion plating method (for example, Patent Document 1).
In addition, since a conductive layer formed by a dry method is weak against a physical force such as bending, and a crack may occur when a physical force is applied, a wet method including a coating process is also being studied. For example, Patent Document 2 describes a transparent conductive film formed using an ink composition in which ITO fine particles are dispersed. Patent Document 3 describes a substrate with a transparent conductive film provided with an ITO thin film obtained by firing an indium compound and a tin compound on a glass substrate.

However, it is difficult to achieve both transparency and conductivity in a transparent conductive film. For example, the transparent conductive film described in Patent Document 1 does not have a sufficiently high total light transmittance, and improvement in transparency is required. It is done. Moreover, although the transparent conductive film described in Patent Documents 2 and 3 has high total light transmittance, the surface resistance is insufficiently low, and improvement in conductivity is required.

JP 2007-149546 A JP 2001-279137 A Japanese Patent Laid-Open No. 10-22615

An object of this invention is to provide the transparent conductive film which is excellent in transparency and electroconductivity, and is used suitably for an electronic device use.

The present invention has a substrate and a conductive layer made of indium tin oxide nanowires that covers the surface of the substrate at a coverage of 20 to 70%, has a total light transmittance of 90% or more, and a surface resistance of 250 Ω / sq. It is the transparent conductive film which is the following.
The present invention is described in detail below.

The inventor has a specific range of total light transmittance and surface resistance by forming on the substrate a conductive layer made of indium tin oxide nanowires covering the surface of the substrate with a specific range of coverage. And it discovered that the transparent conductive film excellent in transparency and electroconductivity was obtained, and came to complete this invention.
In such a transparent conductive film, the conductive layer is formed not in the entire surface of the substrate but in a specific range of coverage, so that a sufficiently high total light transmittance can be obtained, and a single crystal with high conductivity can be obtained. The surface resistance is sufficiently low even if the conductive layer is not formed on the entire surface of the substrate by forming a network of nanowires with indium tin oxide nanowires having a very large crystallite and high crystallinity. Is obtained.
In addition, since such a transparent conductive film is excellent in transparency and conductivity, it is suitably used for electronic devices such as organic EL displays, liquid crystal displays, transparent touch panels, solar cells, etc. It is also possible to handle large screens.

The transparent conductive film of the present invention has a substrate and a conductive layer made of indium tin oxide nanowires (also referred to as ITO nanowires in this specification) that covers the surface of the substrate with a coverage of 20 to 70%.

Although the said board | substrate is not specifically limited, It is preferable that it is transparent, for example, inorganic substrates, such as a glass substrate and a metal substrate, a plastic film, etc. are mentioned. As the plastic film, a PET film, a polyethylene naphthoate film, a polycarbonate film, a polyimide film, a cycloolefin resin film and the like are preferable.
The thickness of the substrate is not particularly limited, but the thickness of the inorganic substrate such as the glass substrate is preferably 20 μm to 10 mm, and the thickness of the plastic film is preferably 8 to 200 μm.

Since the ITO nanowire is more stable than silver, when the conductive layer is made of the ITO nanowire, the problem that the surface resistance of the conductive layer is increased by oxidation and the conductivity is impaired is suppressed. Further, it is not necessary to provide a protective layer on the surface of the conductive layer for preventing oxidation. However, the transparent conductive film of the present invention may have other layers such as a protective layer on the surface of the conductive layer.
The composition of the ITO nanowire is not particularly limited as long as it contains indium tin oxide, that is, oxygen, indium and tin, and may contain a small amount or a trace amount of other elements.

The ITO nanowire preferably has an average length of 200 nm or more. When the average length is less than 200 nm, the network of the ITO nanowires is not sufficiently formed, and the conductivity of the transparent conductive film may be lowered. The average length is more preferably 500 nm or more, and still more preferably 1000 nm or more.
The upper limit of the average length of the ITO nanowire is not particularly limited, and it is preferable that the ITO nanowire is longer because the network of the ITO nanowires is good. However, the ITO nanowire may be easily broken by a physical force. The preferred upper limit is 10 μm.

The average diameter (average thickness) of the ITO nanowire is not particularly limited, but a preferable lower limit is 10 nm and a preferable upper limit is 300 nm. If the average diameter is out of the above range, the network of the ITO nanowires may not be sufficiently formed or the transparency as the nanowire may be greatly reduced, and the transparency or conductivity of the transparent conductive film is reduced. There are things to do. A more preferable lower limit of the average diameter is 30 nm, and a more preferable upper limit is 100 nm.

The aspect ratio of the ITO nanowire, that is, the ratio of the average length to the average diameter (average length / average diameter) is not particularly limited, but the preferable lower limit is 1 and the preferable upper limit is 1000. If the aspect ratio is out of the above range, the ITO nanowire network may not be sufficiently formed, or it may be difficult to adjust the coverage of the conductive layer, and the transparency or conductivity of the transparent conductive film may be reduced. May decrease. A more preferred lower limit of the aspect ratio is 10, and a more preferred upper limit is 500.

The shape of the ITO nanowire is not particularly limited as long as it is a wire shape or a linear shape, but it is preferably a taper shape, that is, a shape gradually narrowing toward the tip.
The tapered shape is preferably a shape having a sphere having a diameter of preferably 1 to 500 nm, more preferably 30 to 250 nm at the tapered tip. By having such a shape having a sphere at the tapered tip, the ITO nanowires are less likely to agglomerate or form a bundle, and a network of the ITO nanowires can be uniformly formed with high dispersibility. .

The average length, average diameter (average thickness), aspect ratio, and shape of the ITO nanowire can be obtained from an electron micrograph (SEM image) obtained by observing the cross section or surface of the transparent conductive film. The average length, average diameter (average thickness), and aspect ratio mean an average value of 30 or more ITO nanowires arbitrarily selected.

The conductive layer covers the surface of the substrate with a coverage of 20 to 70%.
In the transparent conductive film of the present invention, the conductive layer is formed not with the entire surface of the substrate but with a coverage in the above range, so that a sufficiently high total light transmittance can be obtained, and the ITO nanowire can be used to form a nanowire. By forming a network between them, a sufficiently low surface resistance can be obtained even if the conductive layer is not formed on the entire surface of the substrate.
When the coverage is less than 20%, the surface resistance of the transparent conductive film increases and the conductivity decreases. When the coverage exceeds 70%, the total light transmittance of the transparent conductive film is lowered and the transparency is lowered. The preferable lower limit of the coverage is 30% and the preferable upper limit is 68%, the more preferable lower limit is 40%, and the more preferable upper limit is 66%.

In addition, the coverage of a conductive layer can be calculated | required from the electron micrograph (SEM image) which observed the cross section or the surface of the transparent conductive film. The coverage of the conductive layer means a coverage calculated for a region having an area of 10 μm × 10 μm arbitrarily selected.

The transparent conductive film of the present invention has a total light transmittance of 90% or more. If the total light transmittance is less than 90%, the transparency of the transparent conductive film is lowered, so that it is not suitable for applications requiring high transparency, or the photoelectric conversion efficiency is lowered when used in solar cells. . The total light transmittance is preferably 91% or more, and more preferably 93% or more.
The total light transmittance is measured by using a haze meter (for example, HM-150 manufactured by Murakami Color Research Laboratory Co., Ltd.) and the light transmittance in the visible light region (370 to 780 nm) according to JIS K 7361-1. Can be obtained.

The transparent conductive film of the present invention has a surface resistance of 250 Ω / sq or less. If the surface resistance exceeds 250 Ω / sq, the conductivity of the transparent conductive film decreases, making it unsuitable for applications that require high conductivity, making it difficult to save power, increase the display screen, and the like. The surface resistance is preferably 150Ω / sq or less, and more preferably 100Ω / sq or less.
The surface resistance can be measured by a four-probe method using a resistivity meter (for example, Loresta AX MCP-T370 manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

1 and 2 are diagrams schematically showing an example of the transparent conductive film of the present invention. A transparent conductive film 1 of the present invention shown in FIGS. 1 and 2 includes a substrate 2 and a conductive layer 3 made of ITO nanowires that covers the surface of the substrate 2 with a coverage of 20 to 70%. The ITO nanowire has a shape having a sphere 4 at a tapered tip.

Although the manufacturing method of the transparent conductive film of this invention is not specifically limited, On the said board | substrate, the dispersion liquid (ITO nanowire dispersion liquid) containing the said ITO nanowire is apply | coated, and the electroconductivity which consists of the said ITO nanowire by drying a solvent etc. A method of forming a layer is preferred. Note that when the baking is performed at a temperature lower than the heat resistant temperature of the substrate after drying, the adhesive force of the conductive layer to the substrate can be improved.
As a method for producing individual ITO nanowires contained in the ITO nanowire dispersion liquid, after forming an underlayer and an ITO nanowire extending from the underlayer on the sputtering substrate by sputtering, a polishing apparatus or super A method of scraping the ITO nanowire using a sonic cutter or the like, or a method of obtaining the ITO nanowire by removing the underlayer by chemical etching is preferable.

The sputtering apparatus used for the sputtering is not particularly limited, and may be a batch type sputtering apparatus or a roll-to-roll type sputtering apparatus.

By adjusting the sputtering conditions (for example, the gas atmosphere, the temperature (film formation temperature) and time for heating the sputtering substrate, the SnO ₂ content in the sputtering target, etc.), the above-mentioned sputtering substrate is used. ITO nanowires can be formed.
Among these, it is preferable to perform sputtering without introducing oxygen gas, and it is more preferable to perform sputtering in an argon gas atmosphere. Note that sputtering without introducing oxygen gas may include at least a step of performing sputtering without introducing oxygen gas, and even if sputtering is performed by introducing oxygen gas before that step. Good. In the case of introducing oxygen gas, it is preferable to perform sputtering in a mixed gas atmosphere of argon gas and oxygen gas, and then perform sputtering in an argon gas atmosphere not containing oxygen gas.

In the sputtering, the temperature at which the sputtering substrate is heated (film formation temperature) is not particularly limited, but among the elements constituting the sputtering target, the temperature is equal to or higher than the melting point of the metal element that is solid at normal temperature and normal pressure. More preferably, the preferable lower limit is 130 ° C., and the preferable upper limit is 500 ° C. By setting it as such film forming temperature, ITO nanowire can fully be formed.
Especially, when the said sputtering base material is inorganic base materials, such as a glass base material, the minimum with preferable film forming temperature is 150 degreeC, and a more preferable minimum is 175 degreeC. By setting the film forming temperature to 175 ° C. or higher, it becomes easy to form an ITO nanowire having a target shape.
In addition, since the plastic film is generally thin and has high thermal conductivity, when the sputtering substrate is a plastic film, the preferable lower limit of the film forming temperature is 120 ° C.

Sputtering target used in the sputtering (ITO target) is not particularly limited, the lower limit is 3.0% by weight of the content of SnO ₂ in the ITO target, it is preferable upper limit is 50 wt%. By the content of SnO ₂ in such a range, the ITO nanowire shape formed can as appropriate, can be excellent in balance of more transmittance and resistance. Note that as the SnO ₂ content increases, the ITO nanowires become longer and the density increases, and the diameter of the sphere present at the tapered tip tends to decrease. A more preferred lower limit of the SnO ₂ content is 5.0% by weight, a more preferred upper limit is 45% by weight, a still more preferred lower limit is 7.0% by weight, and a still more preferred upper limit is 35% by weight.

The sputtering method is not particularly limited, and may be DC sputtering or RF sputtering, or may be superimposed sputtering of DC sputtering and RF sputtering. The sputtering pressure, input power, and the like are not particularly limited. For example, a pressure of 0.666 Pa, input power of 300 W, or the like can be used.

In the sputtering, it is preferable to perform the sputtering in a state where the sputtering base material and the sputtering target are preferably separated by 60 mm or more, more preferably 100 mm or more, and further preferably 150 mm or more. Thereby, ITO nanowire can be stably formed in a wide area | region.
The upper limit of the distance between the sputtering base and the sputtering target is not particularly limited, but the preferable upper limit is 500 mm from the viewpoint of manufacturing feasibility and film forming efficiency.
The “state where the sputtering substrate and the sputtering target are separated by 60 mm or more” means that the distance (shortest distance) from the center of the surface of the sputtering substrate to the center of the surface of the sputtering target facing the sputtering substrate is 60 mm or more. It means a separated state.

In the sputtering, it is preferable to perform sputtering in a state where the sputtering target is inclined and opposed to the sputtering base material.
In this case, the preferable lower limit of the angle θ formed by the vertical axis of the sputtering base and the vertical axis of the sputtering target is 5 °, the preferable upper limit is 60 °, the more preferable lower limit is 10 °, and the more preferable upper limit is 55 °. A more preferred lower limit is 15 °, and a more preferred upper limit is 45 °. By forming the film at an angle in such a preferable range, the density of the ITO nanowire can be increased.

In the sputtering, heating, plasma treatment, or the like may be performed on the obtained ITO nanowires in order to increase crystallinity.

The method for removing the underlayer by the chemical etching is not particularly limited. For example, by immersing ITO nanowires obtained by sputtering together with the sputtering base material in an etching solution, the polycrystalline underlayer is dissolved, and a single crystal is obtained. And a method for obtaining a dispersion containing ITO nanowires.
The etching solution is not particularly limited, and a commercially available etching solution (for example, H-1000A manufactured by Sanhayato Co., Ltd.) mainly containing an acid such as hydrochloric acid, nitric acid, sulfuric acid, or aqua regia, iron chloride, or the like can be used. Although the immersion time is not particularly limited, for example, when 10 wt% hydrochloric acid is used as the etching solution, about 30 minutes is preferable.
By removing the underlayer in this way, individual ITO nanowires can be obtained. Individual ITO nanowires obtained are separated by a filter or centrifugation, washed with water, and then dried to obtain ITO nanowires with few impurities.

The ITO nanowire dispersion liquid preferably contains water or an organic solvent in addition to the ITO nanowire. The organic solvent is not particularly limited, and examples thereof include methanol, ethanol, acetone, methyl ethyl ketone, and ethyl acetate. Of these, ethanol and acetone are preferable.

The ITO nanowire dispersion liquid may further contain a binder resin. The binder resin is not particularly limited, and examples include conductive polymers such as polyacetylene, polythiophene, and polyaniline. Examples of the polythiophene include poly (3,4-ethylenedioxythiophene) (PEDOT) and poly (3,4-ethylenedioxythiophene) -poly (styrenesulfonic acid) (PEDOT / PSS). Of these, PEDOT and PEDOT / PSS are preferable.

The content of each component in the ITO nanowire dispersion is not particularly limited, but from the viewpoint of adjusting the coverage of the conductive layer to the above range, the preferred lower limit of the content of the ITO nanowire is 0.1% by weight, and the preferred upper limit. Is 10% by weight. If the content of the ITO nanowire is out of the above range, the ITO nanowire network may not be sufficiently formed, or it may be difficult to adjust the coverage of the conductive layer. Or, the conductivity may decrease. A more preferable lower limit of the content of the ITO nanowire is 0.5% by weight, and a more preferable upper limit is 5% by weight.

The method for applying the ITO nanowire dispersion on the substrate is not particularly limited. For example, coating using a bar coater, die coater, dip coater, spin coater, spray, etc., gravure printing, screen printing, etc. Printing that is performed while producing a specific pattern using the above-mentioned method can be used.
Moreover, the method of drying and / or baking the applied ITO nanowire dispersion liquid is not particularly limited, and examples thereof include oven heating and infrared heating.

Since the transparent conductive film of the present invention is excellent in transparency and conductivity, it is suitably used for electronic equipment applications such as organic EL displays, liquid crystal displays, transparent touch panels, solar cells, etc. It is possible to cope with the conversion.

ADVANTAGE OF THE INVENTION According to this invention, it is excellent in transparency and electroconductivity, and can provide the transparent conductive film used suitably for an electronic device use.

It is the figure which showed typically an example of the transparent conductive film of this invention. It is the figure which showed typically an example of the transparent conductive film of this invention.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

Example 1
(1) Mounting an ITO target of _SnO 2 7.0 wt% as a sputtering target to the target electrode of ITO nanowire fabrication DC magnetron sputtering device, sputtering the substrate (Corning glass # 1737 substrate in an argon gas atmosphere, the thickness 0.7mm And the sputtering substrate was sputtered to form an underlayer and ITO nanowires extending from the underlayer on the sputtering substrate.
At this time, the distance from the center of the sputtering substrate surface to the center of the surface of the sputtering target facing the sputtering substrate side (distance between the sputtering substrate and the target) was 110 mm. Further, the angle θ formed by the vertical axis of the sputtering substrate and the vertical axis of the sputtering target was 5 °. The argon pressure was 0.666 Pa, the film forming temperature was 300 ° C., the input power was 300 W, and the film forming time was 10 minutes.
Next, the ITO nanowire obtained by sputtering was immersed in 10 wt% hydrochloric acid together with the sputtering substrate for 30 to remove the underlayer. Thereafter, the obtained individual ITO nanowires were filtered with a membrane filter, washed with water, and then dried to obtain individual ITO nanowires.

(2) Production of transparent conductive film 1 part by weight of the obtained ITO nanowire, 50 parts by weight of water, and 0.1 part by weight of PEDOT / PSS as a binder resin were mixed to prepare an ITO nanowire dispersion. The obtained ITO nanowire dispersion liquid was applied onto a substrate (Corning glass # 1737 substrate, thickness 0.7 mm) using a bar coater, and heated in an oven at 150 ° C. for 30 minutes to form a conductive layer made of ITO nanowires. A transparent conductive film was obtained.

(Examples 2-8, Comparative Examples 1-4)
The average length of the ITO nanowire was changed as shown in Table 1 by extending the film formation time in sputtering, and / or the SnO ₂ content in the ITO target, the content of the ITO nanowire in the ITO nanowire dispersion liquid A transparent conductive film was obtained in the same manner as in Example 1 except that the coverage ratio of the conductive layer was changed as shown in Table 1 by changing.

<Evaluation>
The following evaluation was performed about the transparent conductive film obtained by the Example and the comparative example. The results are shown in Table 1.

(1) Average length of ITO nanowire The average length of nanowire was calculated | required as an average value of 30 or more ITO nanowires arbitrarily selected using the electron micrograph (SEM image) which observed the surface of the transparent conductive film. .

(2) Coverage ratio of conductive layer to substrate surface Using an electron micrograph (SEM image) obtained by observing the surface of the transparent conductive film, a coverage ratio was calculated for a region having an area of 10 μm × 10 μm arbitrarily selected. In addition, the area | region where the board | substrate is not substantially visible with the ITO nanowire in a SEM image was judged as the area | region where the surface of the board | substrate was coat | covered with the conductive layer.

(3) Evaluation of transparency (measurement of total light transmittance)
About the obtained transparent conductive film, the total light transmittance (%) was measured using the haze meter (Murakami Color Research Laboratory make, HM-150). In addition, the measurement was performed using the test piece which cut out the transparent conductive film into the size of 50 mm length and 50 mm width according to JISK7361-1. Three test pieces were used, and the average thickness of the three test pieces was 0.7 μm. The total light transmittance was determined from the average of the three measurements.

(4) Conductivity evaluation (measurement of surface resistance)
About the obtained transparent conductive film, the surface resistance (Ω / sq) was measured by a four-probe method using a resistivity meter (Loresta AX MCP-T370, manufactured by Mitsubishi Chemical Analytech).

ADVANTAGE OF THE INVENTION According to this invention, it is excellent in transparency and electroconductivity, and can provide the transparent conductive film used suitably for an electronic device use.

DESCRIPTION OF SYMBOLS 1 Transparent conductive film 2 of this invention Substrate 3 Conductive layer 4 made of ITO nanowire Sphere existing at tapered tip

Claims (5)

A substrate and a conductive layer made of indium tin oxide nanowires covering the surface of the substrate with a coverage of 20 to 70%,
A transparent conductive film having a total light transmittance of 90% or more and a surface resistance of 250 Ω / sq or less. 2. The transparent conductive film according to claim 1, wherein the average length of the indium tin oxide nanowire is 200 nm or more. The transparent conductive film according to claim 1 or 2, wherein the shape of the indium tin oxide nanowire is a shape having a sphere having a diameter of 1 to 500 nm at a tapered tip. The transparent conductive film according to claim 1, wherein the SnO ₂ content of the indium tin oxide nanowire is 3.0 to 50% by weight. The transparent conductive film according to claim 1, wherein the SnO ₂ content of the indium tin oxide nanowire is 7.0 to 35% by weight.

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