TECHNICAL FIELD

This invention relates to preparing transparent, electrically conductive films.

BACKGROUND

Transparent electrically conductive films can be formed by depositing electrically conductive metals onto a transparent substrate. In some cases, the metals can be deposited onto the substrate in the form of a network such that transparent cells are defined by the network of metal traces. In other cases, the metals can be deposited in the form of intersecting metal fibers or wires. The reflection of visible light from the metallic surfaces of the electrically conductive elements may visually detract from the image being viewed through the film, e.g., when the electrically conductive network is disposed on the viewing side of a display screen.

Blackening processes have been described to lower the reflectivity of electrically conductive elements by depositing a dark colored layer onto the surface of the metal, e.g. a sulfide layer on a silver network. However the blackening processes may undesirably increase the sheet resistance of the metal network. Some blackening processes also introduce complexity in manufacturing processes for the film. It would be beneficial to have a blackening process that is economical and minimizes any increase in sheet resistance.

SUMMARY

There is described a process for blackening an electrically conductive metal network that includes: (a) providing a transparent electrically conductive film

comprising a metal network; and (b) treating the metal network with a blackening composition comprising (i) an organic solvent, (ii) a chalcogen selected from the group consisting of sulfur, tellurium, selenium, and combination thereof, and (iii) an amine.

The term "blackening" generally refers to the process of treating a metallic surface with a chemical composition to add a thin layer of color, typically a colored compound of the metal itself, thus darkening the overall appearance and reducing the amount of light reflected from the surface. The actual color of the coating on the metal may be black, or may be other colors that simply appear dark as compared with the color of the untreated metal. When the transparent conductive film is used in an application where the film is disposed on the viewing side of an electronic display such as a computer monitor or video screen, the visual effect of blackening is to reduce a gray appearance caused by light reflecting from the metal surface, and restore the appearance to what would be seen if the conductive film were not present.

In certain embodiments, the amine may be an aliphatic amine. For example, the aliphatic amine may be a hydroxy aliphatic amine. The amount of the amine in the blackening composition may range from 0.01 to 5 wt.%, 0.1 to 1.0 wt.%, or 0.1 to 0.5 wt.%, where all weight percentages are based upon the weight of the blackening composition. The chalcogen may be sulfur, The sulfur may be in the form of colloidal sulfur. The amount of chalcogen in the blackening composition may range from 0.01 to 0.5 wt.%, 0.02 to 0.2 wt.%, or 0.025 to 0.1 wt.%, where all weight percentages are based upon the weight of the blackening composition.

The blackening composition may be in the form of a solution or a colloidal dispersion. The metal network may include a pattern of electrically conductive traces formed of at least partially -joined metal nanoparticles defining cells that are transparent to visible light.

The blackening process described herein is beneficial in several ways. The process is effective at blackening all surfaces of the electrically conductive metal network, i.e. both the surface facing the film substrate and the surface opposite the film substrate. The chemicals used are relatively safe as compared to processes using more toxic blackening compositions. The process is conducted under relatively mild conditions, e.g., room temperature for the coating step, and no subsequent washing step is required to remove residual blackening agents. Importantly, the process minimizes increases in electrical sheet resistance of the electrically conductive network. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

DETAILED DESCRIPTION

The blackening process blackens electrically conductive metal networks while minimizing an increase in sheet resistance. The process includes treating the network with a blackening composition comprising (i) an organic solvent, (ii) a chalcogen selected from the group consisting of sulfur, tellurium, selenium, and combinations thereof, and (iii) an amine. By including an amine in the composition, an increase in sheet resistance is less than would be found using a comparable composition that lacked the amine.

Transparent conductive films

Transparent conductive films include films having an electrically conductive metal network disposed on a transparent substrate or free standing electrically conductive metal networks having no supporting substrate. Substrates may include polymeric films or sheets such as polyesters, polyamides, polyimides, polycarbonates, polyolefins, poly(meth)acrylates, copolymers, and multilayer films. Glass can also be a useful substrate for transparent electrically conductive networks. The substrates may be rigid or flexible for roll to roll processing, and preferably are selected to be compatible with the solvent used in the blackening composition.

The electrically conductive metal network may be a transparent, electrically conductive network of metal traces. The network of metal traces can be continuously conductive to electricity and defines cells that are transparent to visible light, i.e. visible radiation. Shapes, e.g., patterns, of the network and cells defined by the network may be regular, irregular, or random. Useful networks can be formed from electrically conductive metal (e.g., silver) nanoparticles that self-assemble into a transparent, conductive network of traces and cells, as described in U.S. Patent 7,601,406, which is hereby incorporated by reference. Such networks comprise traces formed of at least partially -joined metal nanoparticles to provide electrical conductivity. Useful networks include those sold by Cima NanoTech, St. Paul, MN, under the trade name "SANTE."

Other useful electrically conductive networks include networks deposited from conductive inks using printing processes, networks formed by patterned exposure of silver halide emulsions to radiation and subsequent development, and networks formed from the deposition of conductive particles into preformed patterns of grooves in the surface of a substrate. Other electrically conductive metal networks include metal networks formed from intersecting metal fibers or wires. In some cases, transparent electrically conductive networks can have enhanced electrical conductivity provided by electroplating or electroless plating of a base electrically conductive metal network.

Blackening composition

The blackening composition can include sulfur, an aliphatic amine, and a solvent. Sulfur may be used in the form of colloidal sulfur, i.e. elemental sulfur having a colloidal particle size. Useful sulfur concentrations in the composition may be 0.01 to 0.5 weight percent, or 0.02 to 0.2 weight percent, or 0.025 to 0.1 weight percent. As shown in the examples, higher concentrations of sulfur can provide more blackening. Sulfur is a member of the chalcogen family, which also includes selenium and tellurium, either of which can included in the blackening composition. Combinations of sulfur, selenium, and/or tellurium may be used as well.

The amine may be an aliphatic amine such as octylamine or a hydroxy amine. Examples of hydroxy amines include ethanolamine and amino butanol. Combinations of two or more amines can be used as well. The amine concentration in the composition can be 0.01 to 5.0 weight percent, or 0.1 to 1.0 weight percent, or 0.1 to 0.5 weight percent.

The solvent may be an organic solvent, or blend of organic solvents, and is selected to be compatible with the substrate used for the transparent conductive film, compatible with the process, and for the ability to disperse or dissolve the other components in the blackening composition. Examples of solvents include methyl ethyl ketone, ethanol, and toluene. Dispersions, e.g., colloidal suspensions of small particles in a solvent, may be preferred for slower and more controlled blackening, particularly if the metal conductive network is porous. Solutions may be preferred for faster blackening. Depending on the solvent, the blackening compositions may contain both dispersed and dissolved chalcogen (e.g., sulfur).

The degree of blackening can be controlled by one or more of the quantity of the composition applied, the concentration of chalcogen in the composition, and the solvent chosen.

Process

The transparent electrically conductive film can be treated with the blackening composition using several known coating techniques, including dip coating, flood coating, spray coating, die coating, and bar coating.

After the composition is applied, the film can be dried and heated to elevated temperatures, e.g., 80-200 deg. C for times in the range of 10 sec. to 10 min.

Temperatures and times chosen should be compatible with any substrates used to support the electrically conductive network.

Batch treatment processes may be used, or continuous processes, e.g. roll-to-roll, may be used.

Examples

Test methods

Sheet resistance (Rs) was measured using a Loresta-GP MCP T610 4 point probe (Mitsubishi Chemical, Chesapeake, VA).

Color was characterized visually using the non-blackened film as a reference. A scale of 0-5 was used with 0 corresponding to no blackening, 1 to light blackening, and 5 to very black color. "Front side color" was the color viewed from the surface of the film having the electrically conductive network. "Back side color" was the color viewed from the surface of the film opposite the electrically conductive network, i.e. viewed through the substrate.

The components shown in Table 1 were mixed with methyl ethyl ketone (MEK) until uniform using an ultrasonic homogenizer to form a dispersion. The components were obtained from Sigma Aldrich, St. Louis, MO. The uniform dispersion was coated onto a transparent, conductive film having a silver nanoparticle network of traces and cells on a PET substrate (SANTE® FS200 Touch Film, available from Cima NanoTech, St. Paul, MN) using a Mayer rod to give a wet thickness of 40 microns. The dispersion was applied onto the surface of the film having the electrically conductive network. The coated films were dried for 30 seconds at 150 deg. C. The films thus formed were tested 5 and the results are shown in Table 1.

TABLE 1 ("C" indicates a comparative example)

A number of embodiments of the invention have been described. Nevertheless, it o will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.