Porous electrode sheet, method for producing the same, and display device

A porous electrode sheet (1) includes a resin film (2) being transparent and having insulating properties, and a transparent electrode (3) placed on one face (2a) of the resin film (2). The resin film (2) is provided with a plurality of through holes (21) extending linearly from the one face (2a) to the other face (2b). The transparent electrode (3) has openings (31) at positions corresponding respectively to the through holes (21). The through holes (21) of the resin film (2) and the openings (31) of the transparent electrode (3) communicate with each other, and thereby form passages (10) penetrating the porous electrode sheet (1) in the thickness direction.

TECHNICAL FIELD

The present invention relates to a porous electrode sheet including a transparent insulating layer and a transparent electrode placed on the insulating layer, and to a method for producing the porous electrode sheet. The present invention also relates to a display device using the porous electrode sheet.

BACKGROUND ART

Conventionally, transparent electrodes have been used in various devices. A transparent electrode is generally placed on a transparent substrate or insulating layer. For example, Patent Literature 1 discloses an electrochromic multi-color display device100as shown inFIG. 4.

In the display device100, a display substrate11and a counter electrode12are attached to each other via a spacer18, and a cell is thus formed. In the cell, a first display electrode13a, a first electrochromic layer14a, an insulating layer15, a second display electrode13b, and a second electrochromic layer14bare layered on the display substrate11. A counter electrode17and a white reflective layer16are layered on the counter electrode12. In addition, the inside of the cell is filled with an electrolyte19.

The first display electrode13aand the second display electrode13bare transparent electrodes. The first electrochromic layer14ais caused to produce a first color or to become colorless by control of the voltage applied between the first display electrode13aand the counter electrode17. The second electrochromic layer14bis caused to produce a second color different from the first color or to become colorless by control of the voltage applied between the second display electrode13band the counter electrode17.

The first electrochromic layer14aand the second electrochromic layer14bgenerally have a structure in which an electrochromic compound is supported on a metal oxide. The metal oxide used has conducting properties in some cases. The insulating layer15is intended to allow separate control of the potential of the first display electrode13aand the potential of the second display electrode13b. In addition, the insulating layer15is transparent, and is porous in order that the electrolyte19permeates into the insulating layer15so as to allow ion transfer associated with oxidation-reduction reaction.

CITATION LIST

Patent Literature

Patent Literature 1: JP 2010-33016 A

SUMMARY OF INVENTION

Technical Problem

Patent Literature 1 describes sputtering with an inorganic material containing ZnS as a specific method for forming the insulating layer15which is porous. However, in the case where the insulating layer15is formed by sputtering, it is difficult to make the thickness of the insulating layer15uniform over a wide area. If the thickness of the insulating layer15lacks uniformity, the insulating properties are deteriorated at a thin portion, and the transparency is reduced at a thick portion.

In view of such circumstances, the present invention aims to provide a porous electrode sheet that can have an insulating layer whose thickness is uniform along a transparent electrode, and to provide a method for producing the porous electrode sheet. The present invention also aims to provide a display device using the porous electrode sheet.

Solution to Problem

That is, the present invention provides a porous electrode sheet including: a resin film being transparent and having insulating properties, the resin film being provided with a plurality of through holes extending linearly from one face of the resin film to the other face; and a transparent electrode placed on the one face of the resin film and having openings at positions corresponding respectively to the plurality of through holes.

In addition, the present invention provides a display device including the porous electrode sheet.

Furthermore, the present invention provides a method for producing a porous electrode sheet. The method includes the steps of: forming a plurality of modified tracks by irradiating a resin base material that is transparent and imperforate with an ion beam so that the plurality of modified tracks extend in such a way as to penetrate the resin base material in a thickness direction; obtaining a resin film having a plurality of through holes by subjecting the resin base material to chemical etching using the modified tracks as base points; and forming, on one face of the resin film, a transparent electrode having openings at positions corresponding respectively to the plurality of through holes.

Advantageous Effects of Invention

In the above configuration, the transparent electrode is placed on the one face of the resin film. The resin film can easily be fabricated to have a uniform thickness over a wide area. Therefore, the use of the resin film can ensure an insulating layer whose thickness is uniform along the transparent electrode. Furthermore, in the porous electrode sheet of the present invention, passages penetrating the porous electrode sheet in the thickness direction are formed by the through holes of the resin film and the openings of the transparent electrode. Therefore, even in the case where, for example, the insulating layer is disposed in contact with an electrochromic layer, ions can move through the passages.

The conventional insulating layer contacting an electrochromic layer is a porous layer whose pores have no directionality. Therefore, ions cannot move in the insulating layer smoothly. By contrast, in the present invention, since the above-described passages allow ions to move and pass through the porous electrode sheet by the shortest route, the rate of oxidation-reduction reaction can be increased. That is, if the porous electrode sheet is used in an electrochromic display device, the performance of the display device is improved.

DESCRIPTION OF EMBODIMENTS

A porous electrode sheet1according to an embodiment of the present invention is shown inFIGS. 1A and 1B. The porous electrode sheet1includes a resin film2being transparent and having insulating properties, and a transparent electrode3placed on one face2aof the resin film2.

The resin film2can easily be fabricated to have a uniform thickness over a wide area. Accordingly, the porous electrode sheet1using the resin film2can have an insulating layer whose thickness is uniform along the transparent electrode3.

The shape of the porous electrode sheet1is not particularly limited. For example, the shape in plan view (as viewed in the thickness direction) of the porous electrode sheet1may be a rectangle, a circle, or a polygon with rounded corners.

The resin film2is provided with a plurality of through holes21extending linearly from the one face2ato the other face2b. In other words, the through holes21are straight holes having no branches. The resin film2having such through holes21can be fabricated as follows: a resin base material that is transparent and imperforate is irradiated with an ion beam to form a plurality of modified tracks (linear portions formed as a result of modification of the resin) extending in such a way as to penetrate the resin base material in the thickness direction; and the resin base material is then subjected to chemical etching using the modified tracks as base points.

Products fabricated by perforation techniques using the above combination of ion beam irradiation and chemical etching are widely commercially available, and such commercially-available products (e.g., membrane filters sold by Oxyphen AG or Millipore Corporation) can be used as the resin film2.

With an ion beam, a wide area can be irradiated in a short time at high density. In addition, the resin base material can also be irradiated with an ion beam directed at an oblique angle to the resin base material. Heavy ions are preferably used in the ion beam irradiation. This is because the use of heavy ions allows formation of extremely slender through holes21in which the ratio of the length to the hole diameter (a so-called aspect ratio) is 10,000 or more. For example, heavy ions can be emitted toward the resin base material after being accelerated by cyclotron.

Here, assuming that the cross-sectional shape of the through holes21is a circle, the “hole diameter” means the diameter of the circle. In other words, the “hole diameter” means the diameter of a circle having an area equal to the cross-sectional area of the through holes21.

An etching liquid such as an alkali solution is used in the chemical etching for removing the resin of the modified tracks. The hole diameter and the cross-sectional shape of the through holes21can be controlled by the chemical etching.

The cross-sectional shape of the through holes21is not particularly limited, and may be a circle or an indefinite shape. The average hole diameter of the through holes21is preferably 0.05 μm or more and 100 μm or less. However, in the case where the porous electrode sheet1is disposed in contact with an electrochromic layer as described later, the maximum hole diameter of the through holes21is preferably smaller than the average particle diameter of a metal oxide on which an electrochromic compound is supported. The average hole diameter of the through holes21is more preferably 0.1 μm or more and 10 μm or less.

The density of the through holes21preferably falls within specified limits included in a range of 10 to 1×108holes/cm2over the entire faces of the resin film2(e.g., the maximum density is 1.5 times or less the minimum density). The density of the through holes21can be adjusted depending on the number of ions applied at the time of ion beam irradiation. The density of the through holes21is more preferably in a range of 1×103to 1×107holes/cm2.

The aspect ratio of the through holes21is preferably 5 or more.

For example, the thickness of the resin film2is 10 to 100 μm. Specific examples of the resin film2include a polyethylene terephthalate (PET) film, a polyethylene naphthalate (PEN) film, and a polycarbonate (PC) film which are transparent insulating films.

The transparent electrode3has openings31at positions corresponding respectively to the through holes21. That is, the through holes21of the resin film2and the openings31of the transparent electrode3communicate with each other, and thereby form passages10penetrating the porous electrode sheet1in the thickness direction.

In order to form the transparent electrode3having the openings31, a conductive film may be formed directly on the one face2aof the resin film by a sputtering method, an electron beam method, an ion plating method, a vacuum evaporation method, a chemical vapor deposition method (CVD method), or the like.

The transparent electrode3is preferably formed of an indium tin oxide film (ITO film). However, the transparent electrode3may be formed of, for example, a fluorine-doped tin oxide film (FTO film), or a zinc oxide film doped with antimony, indium, or aluminum.

Next, an example of use of the porous electrode sheet1of the present embodiment will be described.FIG. 2shows an electrochromic multi-color display device10A using the porous electrode sheet1. The same components as those described with reference toFIG. 4in BACKGROUND ART are denoted by the same reference characters, and the description thereof is omitted.

In the display device10A, a first transparent electrode4for operation of the first electrochromic layer14ais provided instead of the first display electrode13aofFIG. 4, and the porous insulating sheet1is provided instead of the second display electrode13band the insulating layer15ofFIG. 4. That is, the porous electrode sheet1is disposed in such a way that the resin film2faces the first transparent electrode41across the first electrochromic layer14a, and that the transparent electrode3is in contact with the second electrochromic layer14b. The transparent electrode3of the porous insulating sheet1functions as a second transparent electrode for operation of the second electrochromic layer14b.

Furthermore, address electrodes5are provided instead of the counter electrode17ofFIG. 4. Since the inside of the cell is filled with the electrolyte19, the address electrodes5face the porous electrode sheet1across the space filled with the electrolyte19.

When the porous electrode sheet1is used in the display device10A as describe above, since the passages10(seeFIG. 1B) penetrating the porous electrode sheet1in the thickness direction are formed in the porous electrode sheet1, ions can move through the passages10.

In the conventional display device100shown inFIG. 4, the insulating layer15contacting the first electrochromic layer14ais a porous layer whose pores have no directionality, and therefore, ions cannot move in the insulating layer15smoothly. By contrast, in the display device10A shown inFIG. 2, since the passages10allow ions to move and pass through the porous electrode sheet1by the shortest route, the rate of oxidation-reduction reaction can be increased. That is, the performance of the display device10A can be improved by the porous electrode sheet1.

In addition, the resin film2generally exhibits an increased transparency when immersed in a liquid. Therefore, when a liquid is used as the electrolyte19, the visibility of the display device10A is improved. Furthermore, since such an electrolyte solution is retained in the through holes21, the resin film2can maintain the transparency even when the resin film2undergoes a flexible motion. Therefore, the porous electrode sheet1is particularly suitable for uses that require flexibility such as use in an electronic paper.

Furthermore, since the surface of the resin film2of the porous electrode sheet1is flat, the adhesiveness between the surface and the electrochromic layer (particularly an metal oxide) placed on the surface is high. In addition, the shape of the through holes21of the resin film2can be maintained more stably than the shape of pores formed in a porous insulating layer made of an inorganic material. Therefore, the production yield of the display device can also be improved.

In addition, the porous electrode sheet1of the present embodiment can also be used for an electrochromic monochromatic display device. An example of the configuration in this case is shown inFIG. 3.

In a display device10B shown inFIG. 3, the porous electrode sheet1and the display substrate11are disposed so as to sandwich an electrochromic layer14. More specifically, the transparent electrode3of the porous electrode sheet1is in contact with the electrochromic layer14disposed on the display substrate11, and the resin film2faces the space filled with the electrolyte19.

With this configuration, a transparent electrode need not be formed on the display substrate11which is an outer barrier layer. Therefore, a flexible barrier film such as a plastic film can be used as the display substrate11. In the case of the conventional display device, when a space between the display substrate and the counter electrode is sealed with a spacer at the position of the transparent electrode, a durability problem arises from the adhesiveness between the transparent electrode and the spacer. However, the use of the porous electrode sheet1can eliminate the need of the transparent electrode on the display substrate11, and thus allows adhesion between the same or similar materials, resulting in a good durability.

EXAMPLES

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

A commercially-available product (Oxydisk manufactured by Oxyphen AG) having a thickness of 13 μm and provided with through holes having an average hole diameter of 10 μm was prepared as a transparent insulating resin film. One face of the resin film was sputtered with ITO using a batch-type sputtering device (SMH-2306RE manufactured by ULVAC, Inc.), to form a transparent electrode. The ultimate vacuum in the sputtering was set to 1×10−5Torr, and the sputtering was performed with a voltage of 430 V and a current of 0.88 A for 5 minutes in an atmosphere with a process pressure of 4.3×10−3Torr while Ar and O2were being supplied into the atmosphere at 40 sccm and 3 sccm, respectively. Thus, a porous electrode sheet was obtained.

The surface of the transparent electrode of the obtained porous electrode sheet was observed with an electron microscope at a magnification of 1000. It was found that openings were formed in the transparent electrode at positions corresponding respectively to the through holes of the resin film.

In addition, the surface resistivity of the transparent electrode of the obtained porous electrode sheet was measured using a resistivity meter (Loresta-GP MCP-T610 manufactured by Mitsubishi Chemical Corporation) having an ASP probe. The surface resistivity was 60Ω/□ at a voltage of 10 V.

A similar result was obtained also in the case where the resin film used was a commercially-available product (Oxydisk manufactured by Oxyphen AG) having a thickness of 22 μm and provided with through holes having an average hole diameter of 1.0 μm.