Coating and processing of transparent conductive films for stabilization of sparse metal conductive layers

Abstract

To provide a stable transparent conductive film which is manufactured by a cost-effective process, does not crack on a flexible substrate, and has a low sheet resistance.SOLUTION: The transparent conductive film 100 comprising the sparse metal conductive layer 106 is processed after coating with the overcoat 108 or undercoat 104 to reduce the sheet resistance of the film. The sparse metal conductive layer comprises a fused metal nanostructured network. The overcoat or undercoat comprises noble metal ions. The coating further comprises a metal salt stabilizing composition.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to co-pending U.S. Provisional Patent Application No. 62 / 936,681, filed November 18, 2019, to Yang et al., entitled "Post Coating Processing of Transparent Conductive Films Formed With Fused Silver Nanowires," which is incorporated herein by reference. [Background technology]

[0002] Functional films can provide important functions in a variety of situations. For example, conductive films can be important for dissipating static electricity where static electricity may be undesirable or dangerous. Transparent conductive films can be used as electrodes. High-quality displays can include one or more transparent conductive layers.

[0003] Transparent conductors can be used for several optoelectronic applications, including touchscreens, liquid crystal displays (LCDs), flat panel displays, organic light-emitting diodes (OLEDs), solar cells, and smart windows. Historically, indium tin oxide (ITO) has been the material of choice due to its relatively high transparency with high electrical conductivity. However, ITO has several drawbacks. For example, ITO is a brittle ceramic and must be deposited using sputtering, which involves high temperatures and vacuum and is therefore a relatively slow and cost-inefficient manufacturing process. Furthermore, ITO is known to crack easily on flexible substrates. New portable electronic devices tend to be thinner. Summary of the Invention [Means for solving the problem]

[0004] In a first aspect, the present invention relates to a transparent conductive film comprising a transparent polymer substrate, a sparse metal conductive layer supported by the substrate, and a polymer overcoat adjacent to the sparse metal conductive layer, wherein the transparent conductive film has a visible light transmittance of at least about 88% and a sheet resistance of about 120 ohms / sq or less, and the polymer overcoat comprises a polymer and about 0.01 wt % to about 20 wt % of a precious metal ion.

[0005] In a further aspect, the present invention relates to a method for reducing the sheet resistance of a transparent conductive film comprising a substrate, a transparent conductive layer comprising a fused metal nanowire network and a polymer polyol binder, and a polymer overcoat having an average thickness of about 5 nm to about 250 nm, the method comprising heating the transparent conductive sheet to a temperature of at least about 55° C. for at least about 10 minutes to reduce the sheet resistance by at least about 5%. The heating step can be performed in an optically clear, adhesive-free structure. In some embodiments, the fused metal nanostructured network comprises silver, and the film has a sheet resistance of 120 ohms / sq or less and a visible light transmittance of at least about 88%.

[0006] In another aspect, the invention relates to a transparent conductive film comprising a substrate, a transparent conductive layer comprising a fused metal nanostructured network, and a polymer overcoat, wherein the transparent conductive film has a transmittance of at least about 88% and a sheet resistance of about 120 ohms / sq or less, and the transparent conductive film has been modified by processing with heat and optionally humidity for at least about 10 minutes to reduce the sheet resistance by at least about 5%. In some embodiments, the polymer overcoat and / or undercoat comprise metal ions. [Brief explanation of the drawings]

[0007] [Figure 1]FIG. 1 is a partial side view of a film having a sparse metal conductive layer and various additional transparent layers on either side of the sparse metal conductive layer. [Figure 2] FIG. 2 is a schematic side view of a double-sided structure having a sparse metal conductive layer with a thin polymer matrix on both sides. [Figure 3] FIG. 3 is a schematic side view of a roll of transparent conductive film. [Figure 4] Figure 4 is a plot of relative sheet resistance as a function of processing time for a set of silver nanowire structure samples prepared by hand coating silver nanowire ink with various levels of NanoGlue®. The samples were processed at 85°C. [Figure 5] Figure 5 is a plot of relative sheet resistance as a function of processing time for a set of silver nanowire structure samples prepared by hand coating silver nanowire ink with various levels of NanoGlue®. The samples were processed at 85°C and 85% relative humidity. [Figure 6] Figure 6 is a plot of relative sheet resistance as a function of processing time for a set of silver nanowire structure samples prepared by hand coating silver nanowire ink with various levels of NanoGlue®. The samples were processed at 65°C and 90% relative humidity. [Figure 7] Figure 7 shows a plot of relative sheet resistance as a function of processing time for a set of silver nanowire structure samples prepared by roll-to-roll coating of silver nanowire ink with various levels of NanoGlue®. The samples were processed at 65°C and 90% relative humidity. [Figure 8] 8 is a plot of relative sheet resistance as a function of processing time for a combination of laminated optical structure samples prepared with a silver nanowire ink containing NanoGlue® and provided with overcoats having various levels of NanoGlue®. The samples were processed at 85° C. and 85% relative humidity. [Figure 9] 9 is a plot of relative sheet resistance as a function of processing time for two laminated optical structure sample combinations prepared with a silver nanowire ink (containing no NanoGlue®) and an overcoat with or without NanoGlue®. The samples were processed at 85° C. and 85% relative humidity. DETAILED DESCRIPTION OF THE INVENTION

[0008] As demonstrated herein, the controlled application of heat, optionally with the addition of humidity, can be used to reduce the sheet resistance of a fused metal nanostructured network with a protective polymer overcoat. Such processing can be applied to sheets of transparent conductive film with a protective polymer overcoat to further enhance electrical performance. This processing may be effective for fused metal nanostructured networks where the deposited metal is at the junctions between metal nanowires. It has further been discovered that the placement of noble metal ions, particularly silver, in a polymer coating such as an overcoat adjacent to a sparse metal conductive layer such as a fused metal nanostructured network is suitable for further stabilizing the conductive structure. The noble metal in the coating has the advantage that additional processing does not necessarily need to be utilized, as benefits may arise during use. When the sparse metal conductive layer does not contain a fused metal nanostructured network, the results of the present examples suggest that the significant stabilization resulting from the noble metal ions in the overcoat has a profound effect on stability under light and humidity due to fusion, likely due to the metal ions from the overcoat. Results are demonstrated with silver-based conductors. Transparent conductive films can have high transmittance and low haze. Processing times and conditions can be controlled to avoid significant degradation while achieving enhanced performance. Similarly, after processing, the films exhibit significantly enhanced stability under accelerated wear conditions, including high temperature and humidity. This process can be adapted for use with films formed using roll-to-roll processing for large-scale commercial applications.

[0009] Dispersions or inks of silver nanowires can be deposited on surfaces and processed into conductive films. Under appropriate process conditions, the resulting transparent conductive films can be desirable for their mechanical properties, transparency to visible light, flexibility, combinations of these features, or other aspects of conductive films. In particular, the use of nanowires to form transparent conductive films can have significant applications in displays and touch-sensitive devices.

[0010] Metal-based transparent conductive elements, e.g., films, include sparse metal conductive layers. The conductive layer is generally sparser through the conductive structures than around them to provide the desired amount of optical transparency; therefore, the metal coating generally has fine but significant spacing on the conductive element layer. For example, a transparent conductive film can include metal nanowires deposited along the layer, thereby providing sufficient contact to provide an appropriate conduction path by electron percolation. In particularly interesting embodiments, the transparent conductive film can include a fused metal nanostructured network, which has been found to exhibit desirable electrical and optical properties. Unless otherwise indicated, conductivity as referred to herein refers to electrical conductivity.

[0011] As described in more detail below, the fusion process can be controlled to carefully deposit metal at the junctions between metal nanowires. The fusion process can be controlled to deposit a desired amount of silver relative to the junctions. This system can be tailored to provide a thermodynamic drive for fusion to occur at the junctions between adjacent metal nanowires, which are primarily components of a fused metal nanostructured network. After fusion, a unified structure, termed a fused metal nanostructured network, forms, and the original metal nanowires within the conductive structure lose their individual identities. Evidence suggests that the fused metal bonds the original individual nanowires, reducing or eliminating junction resistance. For commercial products, improving the durability of transparent conductive films under a range of real-world conditions is desirable, and the processes described herein demonstrate improved durability under specific accelerated wear tests.

[0012] The benefits of forming a fused metal nanostructured network are fully understood in the context of transparent conductors based on silver nanowires. The fusion process to form a fused metal nanostructured network has been found to result in a highly transparent and highly conductive structure with very low haze. The fused metal nanostructured network has very good stability under accelerated abrasion induction using appropriate stabilizers. The addition of noble metal ions, such as silver, in the overcoat provides an additional level of stabilization, and the results suggest that for the binder systems used herein with hydrophilic binders in the transparent conductive layer, noble metal ions in the fusion and / or overcoat are necessary to obtain reasonable stability under established heat and humidity tests.

[0013] For commercial processing, a thin protective polymer overcoat is typically placed on the transparent conductive layer. The protective coating typically has a thickness of about 250 nm or less. As the results below demonstrate, when a transparent conductive film having a fused metal nanostructured network and a protective polymer overcoat is subjected to heat, optionally with humidity, the sheet resistance is observed to decrease before a final increase due to decomposition. A similar decrease in resistance is not observed in a comparable unfused film. As a result, structures having a fused metal nanostructured network can be found to be more stable than structures previously tested under comparable conditions. Thermal processing can further aid in improving the fusion process. The introduction of humidity during thermal processing can achieve a greater decrease in sheet resistance.

[0014] It has previously been observed that by using humidity during the fusing process, it is possible to use lower temperatures and shorter process times to obtain comparable values ​​of sheet resistance. See U.S. Pat. No. 9,183,968 B to Li et al., entitled "Metal Nanowire Inks for the Formation of Transparent Conductive Films with Fused Networks," incorporated herein by reference. This observation may or may not be related to the present observation. The time frame for this processing is much longer than that used in standard fusing processes.

[0015] If the polymer is hydrophilic, heat and / or humidity can cause the polymer binder to swell around the conductive elements. If the conductive elements are not fused, binder swelling can decrease conductivity and increase sheet resistance due to reduced contact between the conductive elements. The fused elements are held together by a fused metal nanostructured network. Without wishing to be limited by theory, swelling may then promote the migration of metal nanoparticles or any residual metal ions, promoting junction fusion, thereby further reducing sheet resistance. A decrease in sheet resistance is observed in the examples below. Similarly, if the overcoat contains metal ions, these may migrate over time and further contribute to junction fusion. This may mitigate damage and result in a further decrease in sheet resistance.

[0016] The structure after thermal processing, with its protective polymer overcoat, may or may not be different in nature from the previously formed structure. Nevertheless, the observed results are consistent with further migration of silver into the fused metal nanostructured network, and this structure appears to have fewer nanoparticles overall to scatter and absorb light from the fused metal nanostructured network. Following investigations, the new structure achieves improved stability during thermal testing, while optical properties are generally maintained or improved until the new structure is formed.

[0017] The data presented herein provides data related to processes for improving conductivity, as well as data related to abrasion testing of transparent conductive films. Improvements in abrasion testing can be achieved by post-treatment after application of the polymer overcoat and / or by adding metal ions to the overcoat over time. Perhaps if the sheet resistance reaches a lower value and the process is stopped, this material can be used to form products with lower sheet resistance and desirable stability. Generally, post-treated transparent conductive films can have a sheet resistance after fusing that is at least about 5% lower, in some embodiments at least about 7.5% lower, and in other embodiments at least about 10% lower, compared to the initial sheet resistance value.

[0018] Regarding changes in environmental stability, two methods can be considered to evaluate this parameter. In a straightforward sense, initial samples can be examined. Again, these samples contain a thin protective polymer overcoat. These initial samples can be subjected to treatment at 65°C and 90% relative humidity and can maintain a ratio of sheet resistance divided by initial sheet resistance (R / R0) (described as 65 / 90 stability) of less than 1.2 for at least about 200 hours, in some embodiments at least about 300 hours, and in further embodiments at least about 400 hours. If the heat-treated material exhibiting a lower sheet resistance is a new material due to a previously unobtainable structural modification, the thermal stability time observed for the modified material can be compared to that of the initial material.

[0019] Results are shown when heating is carried out at 85°C with ambient humidity, at 85°C and 85% relative humidity, and at 65°C and 90% relative humidity. Generally, processing to form modified materials can be carried out for a time period ranging from about 10 minutes to about 100 hours, depending on process conditions related to temperature and relative humidity. If new materials are formed, processing to form such new materials can likely be optimized.

[0020] Silver provides excellent electrical conductivity. Applicant markets a silver nanowire ink for forming fused metal nanostructured networks under the trade name ActiveGrid®. Other sources of silver nanowires are commercially available, and the basic fusion technology is fully described in the '207 and '807 patents listed below. The majority (>98%) of the silver nanowires in Generation 5 (GEN5) ActiveGrid® products have diameters less than <25 nm, and the majority (>98%) of the silver nanowires in Generation 7 (GEN7) ActiveGrid® products have diameters less than 22 nm. The synthesis of thin silver nanowires is described in U.S. Pat. No. 10,714,230 B2 to Hu et al., entitled "Thin and Uniform Silver Nanowires, Methods of Synthesis and Transparent Conductive Films Formed from the Nanowires," which is incorporated herein by reference.

[0021] Improved properties have been found in fused metal nanostructured networks, with regard to high electrical conductivity and desirable optical properties related to transparency and low haze. The fusion of adjacent metal nanowires can be achieved based on chemical processes under commercially relevant processing conditions.

[0022] In particular, a significant advance in achieving conductive films based on metal nanowires was the discovery of a well-controllable process for forming fused metal networks, in which adjacent sections of metal nanowires fuse into a unified structure without the need for separate nanowires in the conductive network. In particular, it was first discovered that halide ions can fuse metal nanowires to form fused metal nanostructures. Fusion agents containing halide anions have been introduced in various ways to successfully achieve fusion with a corresponding significant reduction in electrical resistance. It should be noted that the halide ions associated with this process should not be confused with the halide ions used in nanowire synthesis reactions. In particular, fusion of metal nanowires with halide anions has been achieved with acid halide vapors and / or solutions, as well as solutions of halide salts. The fusion of metal nanowires using halide sources is described in U.S. Patent No. 10,029,916 to Virkar et al., entitled "Metal Nanowire Networks and Transparent Conductive Material," and U.S. Patent No. 9,920,207 to Virkar et al., entitled "Metal Nanostructured Networks and Transparent Conductive Material" (the '207 patent), both of which are incorporated herein by reference.

[0023] An extension of the process for forming fused metal nanowire networks has been based on reduction / oxidation (redox) reactions that can be applied to produce fused nanowires without destroying the optical properties of the resulting film. The metal for junction deposition can be effectively added as a dissolved metal salt or can be dissolved from the metal nanowire itself. The effective use of redox chemistry to fuse metal nanowires into nanostructured networks is described in U.S. Pat. No. 10,020,807 to Virkar et al. (the '807 patent), entitled "Fused Metal Nanostructured Networks, Fusing Solutions with Reducing Agents and Methods for Forming Metal Networks," which is incorporated herein by reference. The '807 patent also described a single-solution approach for the formation of fused metal nanostructured networks. The single solution approach for the formation of fused metal nanostructured layers is further described in U.S. Pat. No. 9,183,968 B1 to Li et al. (hereinafter the '968 patent), entitled "Metal Nanowire Inks for the Formation of Transparent Conductive Films with Fused Networks," which is incorporated herein by reference, and a single solution or ink process for forming fused metal nanostructured networks is used in the following examples.

[0024] To achieve an effective single-deposition ink that cures into a fused nanostructured metal network, the desired ink contains the desired amount of metal nanowires to achieve the appropriate loading of metal in the resulting film. In the appropriate solution, the ink is stable prior to ink deposition and drying. The ink can contain a reasonable amount of polymer binder that contributes to the formation of a stable conductive film for further processing. To achieve good fusion results with a single ink system, hydrophilic polymers, such as cellulose- or chitosan-based polymers, have been found to be effective as binders. Metal ions, as a source of metal during the fusion process, can be supplied as soluble metal salts.

[0025] A single ink formulation is provided to deposit the desired metal loading as a film on the substrate surface and simultaneously provide components in the ink that induce the coalescence process when the ink is dried under appropriate conditions. These inks can be conveniently referred to as fused metal nanowire inks, with the understanding that coalescence generally does not occur until drying. The ink generally contains an aqueous solvent, and in some embodiments, it can further contain alcohol and / or other organic solvents. The ink can further contain dissolved metal salts as a source of metal for the coalescence process. While not wishing to be limited by theory, it is believed that components of the ink, such as alcohol or other organic compositions, reduce metal ions from solution, inducing the coalescence process. Previous experience with the coalescence process in these systems suggests that metal is preferentially deposited at the junctions between adjacent metal nanowires. A polymer binder can be provided to stabilize the film and influence ink properties. The specific formulation of the ink can be tailored to select ink properties appropriate for a particular deposition approach and to have specific coating properties on the substrate surface. As further described below, drying conditions can be selected to effectively carry out the coalescence process.

[0026] Generally, one or more solutions or inks for forming a metal nanostructured network can include collectively well-dispersed metal nanowires, a fusing agent, and optional additional components, such as a polymer binder, a crosslinker, a wetting agent, such as a surfactant, a thickener, a dispersant, other optional additives, or a combination thereof. The solvent for the metal nanowire ink and / or the fusing solution, if separate from the nanowire ink, can include an aqueous solvent, an organic solvent, or a mixture thereof. Particularly suitable solvents include, for example, water, alcohols, ketones, esters, ethers, such as glycol ethers, aromatic compounds, alkanes, and the like, and mixtures thereof. Specific solvents include, for example, water, ethanol, isopropyl alcohol, isobutyl alcohol, tertiary butyl alcohol, methyl ethyl ketone, glycol ethers, methyl isobutyl ketone, toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate, PGMEA (2-methoxy-1-methylethyl acetate), dimethyl carbonate, or a mixture thereof. The solvent should be selected based on its ability to form a good dispersion of the metal nanowires, but it should also be compatible with other selected additives such that the additives are soluble in the solvent. For embodiments in which the fusing agent is included in a single solution with the metal nanowires, the solvent or a component thereof may or may not be a significant component of the fusing solution, such as an alcohol, and can be selected accordingly, as desired.

[0027] Either the one-ink or two-ink metal nanowire ink can contain about 0.01 to about 1 weight percent metal nanowires, in further embodiments about 0.02 to about 0.75 weight percent metal nanowires, and in additional embodiments about 0.04 to about 0.5 weight percent metal nanowires. For particularly interesting embodiments, the nanowires are silver nanowires, and the metal ion source is a dissolved silver salt. The ink can contain silver ions at a silver ion concentration of about 0.01 mg / mL to about 2.0 mg / mL, in further embodiments about 0.02 mg / mL to about 1.75 mg / mL, and in other embodiments about 0.025 mg / mL to about 1.5 mg / mL. Those skilled in the art will recognize that additional ranges of metal nanowire and metal ion concentrations within the above explicit ranges are contemplated and are within the scope of the present disclosure. The concentration of metal nanowires affects the loading of the metal on the substrate surface as well as the physical properties of the ink.

[0028] For ink formulations, the polymer binder and solvent are generally selected consistently so that the polymer binder is soluble or dispersible in the solvent. In suitable embodiments, the metal nanowire ink generally comprises about 0.02 to about 5 weight percent binder, in further embodiments about 0.05 to about 4 weight percent binder, and in additional embodiments about 0.1 to about 2.5 weight percent binder. In some embodiments, the polymer binder comprises a crosslinkable organic polymer, such as a radiation-crosslinkable organic polymer and / or a thermosetting organic binder. Suitable binders include, for example, polymer polyols, such as polysaccharide-cellulose-based polymers, chitosan-based polymers, and the like. To promote crosslinking of the binder, the metal nanowire ink can comprise, in some embodiments, about 0.0005 wt % to about 1 wt %, in further embodiments about 0.002 wt % to about 0.5 wt %, and in additional embodiments about 0.005 wt % to about 0.25 wt % of a crosslinker. The nanowire ink can optionally include a rheology modifier or a combination thereof. In some embodiments, the ink can include a wetting agent or surfactant to reduce surface tension, and wetting agents can be useful for improving coating properties. A wide range of surfactants, including nonionic, cationic, anionic, and zwitterionic surfactants, are commercially available. Wetting agents are generally soluble in the solvent. In some embodiments, the nanowire ink can include from about 0.001 weight percent to about 1 weight percent, in further embodiments from about 0.002 to about 0.75 weight percent, and in additional embodiments from about 0.003 to about 0.6 weight percent of a wetting agent. A thickener can optionally be used as a rheology modifier to stabilize the dispersion and reduce or eliminate settling. In some embodiments, the nanowire ink can optionally include from about 0.05 to about 5 weight percent, in further embodiments from about 0.075 to about 4 weight percent, and in other embodiments from about 0.1 to about 3 weight percent of a thickener.A person of ordinary skill in the art will recognize that additional ranges of binder, humectant, and thickener concentrations within the explicit ranges above are contemplated and are within the present disclosure.

[0029] The stability of the transparent conductive layer is a significant consideration for the use of these structures in commercial devices such as smartphones, tablets, computer touchscreens, large touchscreens, foldable electronics, wearable electronics, etc. The reliability of touch sensors formed with the transparent conductive layer is significant for widespread commercial use, and the greater the reliability of the material, the greater the range of devices that can be used. The processes and structures described herein offer even greater commercial potential for structures already used in commercial devices.

[0030] Transparent conductive film structure Referring to FIG. 1 , a representative transparent conductive film 100 includes a substrate 102, an optional undercoat layer 104, a sparse metal conductive layer 106, an overcoat layer 108, an optically clear adhesive layer 110, and a protective surface layer 112, although not all embodiments include all layers. Typically, the optically clear adhesive layer 110 and the protective surface layer 112 are added after the completion of significant processing described herein to improve the stability of the transparent conductive layer. A transparent conductive film typically includes a sparse metal conductive layer and at least one layer on both sides of the sparse metal conductive layer. The overall thickness of the transparent conductive film can generally have an average thickness of 5 microns to about 2 millimeters (mm), in further embodiments, about 10 microns to about 1 mm, and in other embodiments, about 12 microns to about 0.5 mm. One of ordinary skill in the art will recognize that additional thickness ranges within the above explicit ranges are contemplated and are within the scope of the present disclosure. In some embodiments, the length and width of the produced film can be selected to be appropriate for a particular application, allowing the film to be directly introduced for further processing into a product. In additional or alternative embodiments, the length of the film can be a length that allows the film to be cut to a desired length for use, while the width of the film can be selected for a particular application. For example, the film can be in the form of a long sheet or a roll. Similarly, in some embodiments, the film can be in a roll or another large standard form, and elements of the film can be cut according to the length and width desired for use.

[0031] The substrate 102 generally comprises a durable support layer formed from a suitable polymer. In some embodiments, the substrate can have an average thickness of about 1 micron to about 1.5 mm, in further embodiments, about 5 microns to about 1 mm, and in additional embodiments, about 10 microns to about 100 microns. In particular, for foldable structures, particularly the double-sided foldable structures described below, the thickness of the substrate is generally about 27 microns or less, and in further embodiments, about 5 microns to about 25 microns. One of ordinary skill in the art will recognize that additional ranges of substrate thickness within the explicit ranges above are contemplated and within the scope of the present disclosure. Any suitable optically clear polymer having very good transparency, low haze, and good protective capabilities can be used for the substrate.

[0032] The amount of nanowires provided on a substrate for the sparse metal conductive layer 106 can involve balancing factors to achieve the desired amount of transparency and conductivity. While nanowire network thickness is typically assessed using scanning electron microscopy, the network can be relatively thin to provide optical transparency, which can make measurement difficult. Generally, sparse metal conductive structures, such as fused metal nanowire networks, will have an average thickness of about 5 microns or less, in further embodiments, 2 microns or less, and in other embodiments, about 10 nm to about 500 nm. However, sparse metal conductive structures are generally relatively open structures with significant surface texture on the submicron scale. Nanowire loading levels can provide useful parameters of the network that can be easily assessed, and loading values ​​provide a surrogate parameter related to thickness. Therefore, as used herein, nanowire loading levels on a substrate are generally expressed as milligrams of nanowires per square meter of substrate. Typically, nanowire networks have average thicknesses of about 0.1 milligrams (mg) / m. 2 ~about 300mg / m 2 and in a further embodiment, about 0.5 mg / m 2 ~about 200mg / m 2and in other embodiments, about 1 mg / m 2 ~about 150mg / m 2 A person of ordinary skill in the art will recognize that additional ranges of thickness and loading within the explicit ranges above are contemplated and are within the present disclosure. If the sparse metal conductive layer is patterned, the discussion of thickness and loading applies only to areas where the metal is not removed or significantly reduced by the patterning process.

[0033] Suitable polymers for the substrate include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyacrylate, poly(methyl methacrylate), polyolefin, polyvinyl chloride, fluoropolymer, polyamide, polyimide, polysulfone, polysiloxane, polyether ether ketone, polynorbornene, polyester, polystyrene, polyurethane, polyvinyl alcohol, polyvinyl acetate, acrylonitrile-butadiene-styrene copolymer, cyclic olefin polymer, cyclic olefin copolymer, polycarbonate, copolymers thereof, or blends thereof, etc. Suitable commercially available polycarbonate substrates include, for example, MAKROFOL SR243 1-1 CG, commercially available from Bayer Material Science; TAP® Plastic, commercially available from TAP Plastics; and LEXAN™ 8010CDE, commercially available from SABIC Innovative Plastics. The protective surface layer 112 can independently have a thickness and composition that includes the same thickness and composition ranges as the substrate described in the paragraph above.

[0034] For structures with transparent conductive layers that can be separately patterned on both sides, a UV-absorbing transparent substrate can be used, as shown in FIG. 2, as described below. Suitable polymers, particularly for these embodiments, but useful for all embodiments, include, for example, polyimides, polysulfides (e.g., Pylux™, Ares Materials), polysulfones (e.g., Udel® from Solvay), or polyethersulfones (e.g., Veradel® from Solvay or Ultrason® from BASF), and polyethylene naphthalates (e.g., Teonex® from DuPont). The following examples are based on transparent polyimides. Conventional aromatic polyimides are colored. However, recently developed polyimides are transparent to visible light. Transparent polyimides absorb ultraviolet light. Transparent polyimides are available from Kolon (Korea), Taimide Tech. (Taiwan), Sumitomo (Japan), SKC Inc. (GA, USA), and NeXolve (AL, USA).

[0035] An optional undercoat 104 and / or overcoat 108 can be disposed below or above the sparse metal conductive layer 106, respectively. The coatings 104, 108 can include a curable polymer, such as a thermally curable or radiation-curable polymer. The coatings 104, 108 can have an average thickness as described in the following section, specifically including the overcoat and undercoat. The thickness and composition of the overcoat can be selected so that sheet resistance measurements through the overcoat do not change significantly compared to measurements without the overcoat. The coatings 104, 108 can include significant additives, further described below, to stabilize the conductive layer. Suitable coating polymers are described below.

[0036] The optional optically clear adhesive layer 110 can have an average thickness of about 10 microns to about 300 microns, in further embodiments, about 15 microns to about 250 microns, and in other embodiments, about 20 microns to about 200 microns. A person of ordinary skill in the art will recognize that additional ranges of optically clear adhesive layer thicknesses within the explicit ranges above are contemplated and within the scope of the present disclosure. Suitable optically clear adhesives can be contact adhesives. Optically clear adhesives include, for example, coatable compositions and adhesive tapes. Optically clear adhesive tapes can be double-sided adhesive tapes having a carrier film between two adhesive layers; see, for example, 3M 8173KCL. UV-curable liquid optically clear adhesives are available based on acrylic or polysiloxane chemistries. Suitable adhesive tapes are commercially available from, for example, Lintec Corporation (MO series); Saint Gobain Performance Plastics (DF713 series); Nitto Americas (Nitto Denko) (LUCIACS CS9621T and LUCIAS CS9622T); LG Hausys OCA (OC9102D, OC9052D); DIC Corporation (DAITAC LT series OCA, DAITAC WS series OCA and DAITAC ZB series); PANAC Plastic Film Company (PANACLEAN series); Minnesota Mining and Manufacturing (3M, Minnesota USA - product numbers 8146, 8171, 8172, 8173, 9894 and similar products) and Adhesive Research (e.g., product 8932).

[0037] Some optically clear adhesive tapes contain a carrier film, such as polyethylene terephthalate (PET), which can be embedded in the tape between two adhesive surfaces. Based on earlier research with organic stabilizers, it was found that the presence of a carrier film in the optically clear adhesive layer, in combination with these stabilizers, is effective in improving the stabilization properties of the optically clear adhesive tape compared to a comparable film made of an optically clear adhesive tape without a carrier film. While not wishing to be limited by theory, it was speculated that the improved stability is potentially due to the reduced water and oxygen permeability of the carrier film. Using the metal-based stabilizers described herein, it has been found that the stabilization properties are not significantly dependent on the specific optically clear adhesive used. This is an advantage of metal-based stabilizers such as V(+5) salts.

[0038] An embodiment of a double-sided structure having two transparent conductive structures is shown in Figure 2. Figure 2 shows the general structure of a double-sided conductive element in a unified form on a core substrate. Referring to Figure 2, a double-sided conductive sheet 150 can include a substrate polymer sheet 152, an optional first hard coat 154, a first sparse metal conductive layer 156, a first overcoat 158, an optional second hard coat 160, a second sparse metal conductive layer 162, a second overcoat 164, an optional first peelable protective film 166, and an optional second peelable protective film 168.

[0039] Either single-sided or double-sided sheets with a transparent conductive layer can be manufactured in roll-to-roll form. Such a roll 190 is shown schematically in FIG. 3. Formation of double-sided flexible structures in roll-to-roll form is described in U.S. Patent Application Publication No. 2020 / 0245457 to Chen et al., entitled "Thin Flexible Structures With Transparent Conductive Films and Processes for Forming the Structures," which is incorporated herein by reference.

[0040] Generally, the silver nanowire ink can be applied using any reasonable approach, such as dip coating, spray coating, knife-edge coating, bar coating, Mayer-rod coating, slot-die coating, gravure printing, spin coating, etc. For commercial roll-to-roll processing, slot-die coating is commonly used. The film can be dried, for example, with a heat gun, oven, thermal lamp, hot air, etc. In some embodiments, the film can be heated to a temperature of about 50°C to about 150°C during drying. To facilitate the fusion process, heat delivery generally continues for about 30 seconds to about 15 minutes. A person of ordinary skill in the art will recognize that additional ranges of temperature and time within the explicit ranges above are contemplated and are within the present disclosure.

[0041] Transparent conductive films can be used in a range of products that incorporate transparent conductive electrodes, such as solar cells. Their excellent optical properties make them particularly beneficial for use in touch sensors integrated into displays. The transparent conductive layer can be patterned using photolithography, laser ablation, or other suitable techniques. The patterned transparent conductive layer can be in a suitable form to form a touch sensor. Such touch sensors find use in portable electronics as well as large-scale displays.

[0042] Polymer coating (overcoat and / or undercoat) The polymer coating can provide the desired functionality and is generally described above with respect to the overall structure. In particular, the polymer overcoat provides several functions. First, it provides mechanical protection for the transparent conductive layer during further processing. Second, it can be provided with a stabilizing composition to improve the abrasion endurance of the transparent conductive layer. The undercoat can provide a desirable surface for the placement of the transparent conductive layer with the fused metal nanostructured network and can provide a stabilizing composition. As described herein, the inclusion of noble metal ions, such as silver ions, in the coating (overcoat and / or undercoat) can lead to increased fusion of the fused metal nanostructured network, reducing sheet resistance and improving thermal stability.

[0043] The sparse metal conductive layer is typically coated with a polymer overcoat to provide mechanical protection for the conductive layer. In some embodiments, it may be possible to select an overcoat that significantly reduces haze after application of the overcoat without significantly degrading other properties. Additionally, the thickness and composition of the overcoat can be selected so that sheet resistance measurements through the overcoat do not significantly change compared to measurements without the overcoat. The incorporation of additional stabilizers into the coating is further described below and in U.S. Patent Application Publication No. 2018 / 0105704 to Yang et al., entitled "Stabilized Sparse Metal Conductive Films and Solutions for Delivery of Stabilizing Compounds" (hereinafter the '704 application), which is incorporated herein by reference.

[0044] In some embodiments, the overcoat layer is generally thin enough that electrical conductivity can occur through the overcoat, even though the overcoat polymer is generally a dielectric. In other words, a surface with the overcoat should generally have approximately the same sheet resistance as a surface without the overcoat. To provide electrical conductivity through the polymer overcoat, it may be desirable for the polymer overcoat to have an average thickness of about 250 nm or less, in some embodiments from about 5 nm to about 200 nm, in other embodiments from about 8 nm to about 125 nm, and in further embodiments from about 10 nm to about 100 nm. A person of ordinary skill in the art will recognize that additional ranges of thickness within the explicit ranges above are contemplated and are within the present disclosure.

[0045] As described above, the conductive structure can include an underlayer between the polymer substrate and the sparse metal conductive layer, typically having a fused metal nanostructured network. The underlayer adjacent to the transparent conductive layer can be referred to as an undercoat. The undercoat can include a polymer hardcoat. As an undercoat, the polymer hardcoat does not provide mechanical protection, but it can provide protection from chemical attack. In some embodiments, the polymer hardcoat undercoat provides significant stability to the fused metal nanostructured layer for some polymer substrates.

[0046] Suitable hard coat polymers are generally highly crosslinked polymers with crosslinked polyacrylates that can be combined with other crosslinked moieties such as polyurethanes, epoxy polymers, polysiloxanes, and / or other crosslinked polymers. Branched acrylate monomers can provide for the formation of highly crosslinked polymers, and crosslinked acrylates can be copolymerized with other moieties such as urethane acrylates to form intermeshed crosslinked polymers. Hard coat polymers are commercially available, for example, coating solutions from POSS® Coatings from Hybrid Plastics, Inc. (Mississippi, USA), silica-filled siloxane coatings from California Hardcoating Company (California, USA), Acier® Hybrid Hard Coating Material from Nidek (Japan), SK1100 Series Hard Coat from Dexerials Corporation (Japan), Lioduras™ from TOYOCHEM (Japan), HC-5619 Hard Coat from Addison Clear Wave (IL, USA), CrystalCoat UV-curable coatings from SDC Technologies, Inc. (California, USA), and Optoclear® from JSR Corporation (Japan). Hardcoat polymers are further described in U.S. Patent Application Publication No. 2016 / 0369104 to Gu et al., entitled "Transparent Polymer Hardcoats and Corresponding Transparent Films," which is incorporated herein by reference. Suitable hardcoat polymers are available that are highly transparent for the relevant thickness so that the hardcoat does not significantly reduce light transmission through the structure.Suitable crosslinkers and additives are described with respect to the binder above, and these apply equally to the coatings (e.g., optional undercoat 104 and overcoat 108) without explicitly repeating the discussion here.

[0047] After initial processing, the structure generally has a crosslinked overcoat that provides a relatively tack-free surface. To protect the structure during transportation, storage, and the like, one or both sides can be protected with a peelable polymer layer. The peelable polymer layer can be formed to a thickness of about 15 microns to about 200 microns, in further embodiments, about 20 microns to about 180 microns, in other embodiments, about 25 microns to about 170 microns, and in additional embodiments, about 40 microns to about 150 microns. Those skilled in the art will recognize that additional thickness ranges within the explicit ranges above are contemplated and within the scope of the present disclosure. A range of suitable non-tacky polymers can be used for the polymer release layer, and optical properties are not relevant because the release layer is removed for further processing. Suitable polymers include, for example, polyethylene, polypropylene, polyester, polyamide, polysiloxane, copolymers thereof, and the like. The particular polymer should be selected to have low tack, so that the peel strength of the release layer must be low enough to avoid damage to the material when peeled from the substrate.

[0048] For further post-coating processing described herein for roll-to-roll applications, a release layer is typically present. The release layer can be selected so that it does not become tacky during the application of heat and humidity selected for further processing. After completion of post-coating processing, the prepared transparent conductive film can be used for patterning and assembly into devices such as touch sensors, which may involve the application of additional layers, often using optically clear adhesives as described above.

[0049] stabilizing composition The overcoat and / or undercoat can include a stabilizing composition that provides improved stability of the fused metal nanostructured network, which can be evaluated, for example, using an accelerated wear test with heat and, optionally, increased humidity. Previous studies have found that vanadium (+5) compounds can be effective in providing the desired stability. Others have found that iron (+2) and other metal salts can be effective stabilizers. See U.S. Patent Application Publication No. 2015 / 0270024 A1 to Allemand, entitled "Light Stability of Nanowire-Based Transparent Conductors," which is incorporated herein by reference. Cobalt (+2) ions complexed with ligands have also been found to provide stabilization within the fused metal nanostructured network layer. The performance of these stabilizing compositions, alone or in combination, can be enhanced by incorporating noble metal ions, particularly silver ions, within the coating (overcoat and / or undercoat) to further enhance stability, likely due to further fusion of the structure through metal ion migration. As with pentavalent vanadium, the benefits of the precious metal ions in the coating can be utilized during the actual use of the structure in a product, but alternatively, or in addition, it can be beneficial to have the precious metal ions in the coating during post-deposition heat / humidity processing prior to assembly into a final product.

[0050] For incorporation into coatings, vanadium compounds with a valence of +5 have been found to provide desirable stabilization during extended wear testing. Suitable compounds include compounds with vanadium as the cation, as well as compounds with vanadium as part of a polyatomic anion, such as metavanadates (VO3 - ) or orthovanadate (VO4 -3). Corresponding salt compounds having an oxometalate pentavalent vanadium anion include, for example, ammonium metavanadate (NH4VO3), potassium metavanadate (KVO3), tetrabutylammonium vanadate (NBu4VO3), sodium metavanadate (NaVO3), sodium orthovanadate (Na3VO4), other metal salts, and the like, or mixtures thereof. Suitable pentavalent vanadium cation compounds include, for example, vanadium oxytrialkoxides (VO(OR)3, where R is an alkyl group, e.g., n-propyl, isopropyl, ethyl, n-butyl, and the like, or combinations thereof), vanadium oxytrihalides (VOX3, where X is Cl, F, Br, or combinations thereof), vanadium complexes such as VO2Z1Z2, where Z1 and Z2 are independently ligands such as those further described below for Co+2 complexes, or combinations thereof. In the coating, pentavalent vanadium can be present, for example, from about 0.01 wt. % to about 9 wt. %, in further embodiments, from about 0.02 wt. % to about 8 wt. %, and in additional embodiments, from about 0.05 wt. % to about 7.5 wt. In a coating solution, the solution generally contains some solvent along with solids, primarily comprising the curable polymer. Generally, a corresponding coating solution can have a pentavalent vanadium compound concentration of about 0.0001 wt. % to about 1 wt. %. One of ordinary skill in the art will recognize that additional ranges of concentrations within the explicit ranges above are contemplated and are within the scope of the present disclosure. In additional or alternative embodiments, iron(+2) or other metal ions can be included in addition to or in place of the pentavalent vanadium ions.

[0051] Furthermore, noble metal ions, particularly silver ions, can be included in the solution for forming the coating. As used herein, noble metal ions refer to ions of silver, gold, platinum, indium, osmium, ruthenium, and rhodium. The noble metal ions can be added as a suitable salt, such as a nitrate, sulfate, perchlorate, hexafluorophosphate, hexafluoroantimonate, or halide. Suitable metal salts for providing the metal ions include, for example, chloroauric acid and palladium chloride. With respect to silver salts, when the coating polymer is deposited by alcohol or other non-aqueous organic solvent, suitable silver salts for sufficient solubility include, for example, silver tetrafluoroborate (AgBF), silver hexafluorophosphate (AgPF), silver perchlorate (AgClO), silver hexafluoroantimonate (AgSbF), silver trifluoroacetate (CFCOO), silver hexafluorobutyrate (AgCHFO), silver methylsulfonate (AgCHSO), silver tolylsulfonate (AgCHCHSO), or mixtures thereof. In the coating, the precious metal ions can be present, for example, from about 0.01 wt. % to about 20 wt. %, in further embodiments from about 0.05 wt. % to about 15 wt. %, in other embodiments from about 0.1 wt. % to about 12 wt. %, in some embodiments from about 0.2 wt. % to about 9 wt. %, and in additional embodiments from about 0.25 wt. % to about 7.5 wt. In the coating solution, the solution generally contains some solvent along with solids, primarily comprising the curable polymer. A person of ordinary skill in the art will recognize that additional ranges of concentrations within the explicit ranges above are contemplated and are within the present disclosure.

[0052] Particularly with regard to the direct use of transparent conductive layers with fused metal nanostructured networks, cobalt, with its +2 valence, has been found to be effective for stabilization without interfering with the fusion process. Suitable cobalt compounds include, for example, nitrite (NO2 -), diethylamine, ethylenediamine, nitrilotriacetic acid, iminobis(methylenephosphonic acid), aminotris(methylenephosphonic acid), ethylenediaminetetraacetic acid (EDTA), 1,3-propylenediaminetetraacetic acid (1,3-PDTA), triethylenetetramine, tri(2-aminoethyl)amine, 1,10-phenanthroline, 1,10-phenanthroline-5,6-dione, 2,2'-bipyridine, 2,2'-bipyridine-4 Cobalt ions have previously been proposed as a suitable ion source for metal fusion at nanowire junctions in the above-cited '833 application. As shown in the '704 application, Co+2 actually destabilizes transparent conductive films unless it is complexed by a ligand. Regarding the use of cobalt+2 stabilizing compounds in layers with fused metal nanostructured networks, the stabilizing compound is added along with a more easily reduced silver salt or other salt of the cation so that the cobalt+2 cation remains in the material after the fusion process. On the other hand, a stoichiometric amount of ligand for Co+2 has been found to interfere with the fusion process to form a fused nanostructured network. In a layer having a fused metal nanostructured network, the concentration of the cobalt+2 stabilizing compound can be from about 0.1 wt. % to about 10 wt. %, in further embodiments, from about 0.02 wt. % to about 8 wt. %, and in additional embodiments, from about 0.025 wt. % to about 7.5 wt. %. For an effective cobalt composition without interfering with the fusion process, the complexing ligand can be present in an amount of from about 0.1 to about 2.6 ligand bond equivalents per mole of cobalt, in further embodiments, from about 0.5 to about 2.5, and in other embodiments, from about 0.75 to about 2.4 ligand bond equivalents per mole of cobalt.With respect to equivalent amounts, the term is intended to indicate that the polydentate ligands have the corresponding molar ratio for the above ranges divided by their coordination numbers. For inks used to deposit metal nanowires, the solution can include a cobalt+2 compound at a concentration of about 0.0001% to about 1% by weight. Further details of nanowire inks are provided below. A person of ordinary skill in the art will recognize that additional ranges of concentrations within the explicit ranges above are contemplated and are within the present disclosure.

[0053] While a wide variety of solutions can be used to form the coating, in some embodiments, the solution is based on an organic solvent along with a crosslinkable hard coating precursor. Generally, the coating solution contains at least about 7% by weight of solvent, and in further embodiments, about 10% to about 70% by weight of solvent, with the remainder being nonvolatile solids. Generally, the solvent can include water, an organic solvent, or a suitable mixture thereof. Suitable solvents generally include, for example, water, alcohols, ketones, esters, ethers, such as glycol ethers, aromatic compounds, alkanes, and the like, and mixtures thereof. Specific solvents include, for example, water, ethanol, isopropyl alcohol, isobutyl alcohol, tert-butyl alcohol, methyl ethyl ketone, methyl isobutyl ketone, cyclic ketones, such as cyclopentanone and cyclohexanone, diacetone alcohol, glycol ethers, toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate, propylene carbonate, dimethyl carbonate, PGMEA (2-methoxy-1-methylethyl acetate), N,N-dimethylformamide, N,N-dimethylacetamide, acetonitrile, formic acid, or mixtures thereof. In some embodiments, non-aqueous solvents may be desirable. The choice of solvent is generally based on the hard coat polymer coating composition. Suitable coating polymers are listed above. Generally, the polymer for the coating, generally a crosslinkable polymer, can be provided as a commercially available coating composition or can be formulated with the selected polymer composition.

[0054] The polymer concentration and, correspondingly, the concentrations of other non-volatile agents can be selected to achieve the desired rheology of the coating solution, such as appropriate viscosity during the selected coating process. Solvent can be added or removed to adjust the total solids concentration. The relative amount of solids can be selected to adjust the composition of the finished coating composition, and the total amount of solids can be adjusted to achieve the desired thickness of the dried coating. Generally, coating solutions can have a polymer concentration of about 0.025% to about 70% by weight, in further embodiments, about 0.05% to about 50% by weight, and in additional embodiments, about 0.075% to about 40% by weight. Those skilled in the art will recognize that additional ranges of polymer concentration within the specific ranges above are contemplated and are within the scope of the present disclosure. Stabilizing salts can be mixed into the polymer coating composition using appropriate mixing equipment.

[0055] Post-coating processing The processing of transparent conductive layers into devices generally involves several steps. After coating the nanowire solution onto a transparent substrate, the coating is dried under conditions that promote the coalescence process. While coalescence is a thermodynamically driven process, some heating has been observed to provide activation of the coalescence process. The amount of coalescence ions can be adjusted to achieve complete coalescence, as indicated by an approximate plateau in sheet resistance, without significant deterioration in optical transmittance or haze, although an increase in color (increase in the magnitude of b*) may be observed with coalescence. An overcoat polymer layer is added onto the transparent conductor layer after coalescence. In a roll-to-roll format, which may be desirable from a commercial manufacturing standpoint, all of this processing is performed on a moving sheet that forms the substrate, and the coated structure is rolled up for further processing. Further processing can then be performed on the roll of coated product, although processing can also be performed in different formats, such as sheets.

[0056] Evidence suggests that further thermal processing involves the formation of additional fusion. Overall observations suggest that these processes have a longer time frame than initial fusion. While not wishing to be bound by theory of the mechanism of operation, overall observations suggest that some residual metal ion sources may be gradually consumed during further processing to form additional metal at the fused joints. Swelling of the hydrophilic binder around the fused metal nanostructured network may facilitate the processing of a fused metal nanostructured network that does not separate due to polymer swelling. Heat and humidity can be used during processing to control processing, both of which promote further fusion and provide a degradation pathway. As demonstrated in the examples, proper control of process conditions can result in lower sheet resistance and slower degradation pathways for transparent conductors based on fused metal nanostructured networks.

[0057] For this post-coating processing, temperatures of at least about 55°C, in further embodiments, from about 60°C to about 100°C, and in additional embodiments, from about 70°C to about 95°C are preferred. While humidity can remain at ambient humidity, improved results are observed by increasing the relative humidity level. Due to increased temperature, the relative humidity in heated air resulting from ambient humidity alone is generally low. In some embodiments, the relative humidity can be at least about 60%, in further embodiments, at least about 65%, and in other embodiments, at least about 70%. The processing time to stabilize the fused metallic nanostructured network can be at least about 10 minutes, in further embodiments, from about 20 minutes to about 50 hours, and in other embodiments, from about 25 minutes to about 40 hours. A person of ordinary skill in the art will recognize that additional ranges of temperature, relative humidity, and processing time within the explicit ranges above are contemplated and are within the present disclosure.

[0058] Processing using heat, and optionally increased humidity, can be carried out in suitable controlled environmental conditions. If humidity is not controlled, the environment can be exposed to the atmosphere, and a suitable structure can be an oven or other suitable heater. Smaller samples can be processed in test chambers designed for the application of controlled temperature and pressure. Commercial test equipment that provides heat and humidity control includes, for example, the ESPEC Model BTL-433 Environmental Chamber (ESPEC Corp. North America, Inc. Hudsonville, MI, USA) or the Thermotron SM-3.5-3800 Benchtop Environmental Chamber (Thermotron Inc., Holland, MI, USA). Larger controlled environmental chambers are commercially available.

[0059] Generally, processing can be performed in coated films without assembling them into a structure with additional layers, such as those used in forming a final device. Accelerated wear testing of layered stacks also includes the placement of additional layers. Heat and optional humidity processing generally cannot be performed on stack structures that include adhesive layers, such as optically clear adhesives. Heat and humidity can destabilize the stack for commercial processing. While some destabilization is acceptable for sample testing, it generally cannot be tolerated for product manufacturing. Therefore, processing for sheet resistance reduction and stabilization is generally performed on samples with an overcoat and potential release layer.

[0060] Heat treatment, with optional high relative humidity, can be performed with the film in roll form loaded onto a roll. As noted above, the overcoat can be coated with a release layer in roll-to-roll form to provide a non-stick surface and facilitate rolling and release; suitable release layers are described above. Because the rolls are typically formed immediately after manufacture and the heat treatment is performed over a period of time, the roll form can be particularly convenient for commercial manufacturing. After heat treatment, the samples can be assembled into final products.

[0061] Electrical and optical properties of transparent films Transparent conductive layers, such as those with fused metal nanostructured networks, can provide low electrical resistance while providing good optical properties. Thus, the films may be useful as transparent conductive electrodes, etc. Transparent conductive electrodes may be suitable for a range of applications, such as electrodes along the light-receiving surface of solar cells. For displays, particularly touchscreens, films can be patterned to provide conductive patterns formed by the film. Substrates with patterned films generally have good optical properties in each portion of the pattern.

[0062] The electrical resistance of thin films can be referred to as sheet resistance, which is reported in units of ohms per square (Ω / □ or ohms / sq) to distinguish it from bulk electrical resistance values, depending on parameters related to the measurement process. Film sheet resistance can typically be measured using a four-point probe measurement or another suitable process. In some embodiments, the fused metal nanowire networks can have a sheet resistance of about 300 ohms / sq or less, in further embodiments about 200 ohms / sq or less, in additional embodiments about 100 ohms / sq or less, and in other embodiments about 60 ohms / sq or less. A person of ordinary skill in the art will recognize that additional ranges of sheet resistance within the explicit ranges above are contemplated and are within the present disclosure. Depending on the particular application, commercial specifications for sheet resistance for use in a device may not necessarily relate to low values ​​of sheet resistance, as additional cost may be involved, and current commercially relevant values ​​may be, for example, target values ​​of 270 ohms / sq, 150 ohms / sq, 100 ohms / sq, 50 ohms / sq, 40 ohms / sq, 30 ohms / sq, 20 ohms / sq or less for touchscreens of various qualities and / or sizes, with each of these values ​​defining a range between the specific values ​​as the end points of the range, such as 270 ohms / sq to 150 ohms / sq, 270 ohms / sq to 100 ohms / sq, 150 ohms / sq to 20 ohms / sq, etc., with 15 specific ranges being defined. Thus, for certain applications, a lower cost film may be appropriate in exchange for a slightly higher sheet resistance value. In general, sheet resistance can be reduced by increasing the nanowire loading, but increasing the loading may not be desirable from other perspectives, and metal loading is only one of many factors for achieving low values ​​of sheet resistance.

[0063] For applications as transparent conductive films, it is desirable for fused metal nanowire networks or other sparse metal conductive layers to maintain good optical transparency. In principle, optical transparency is inversely proportional to loading, with higher loadings leading to decreased transparency, but network processing can also significantly affect transparency. Polymer binders and other additives can also be selected to maintain good optical transparency. Optical transparency can be evaluated with respect to light transmitted through the substrate. For example, the transparency of the conductive films described herein can be measured using a UV-visible spectrophotometer by measuring the total transmission through the conductive film and supporting substrate. Transmittance is a function of incident light intensity (I o The transmittance (T) through the film is the ratio of the transmitted light intensity (I) to the film ) is the measured total transmittance (T) divided by the transmittance through the supporting substrate (T sub ) can be estimated by dividing by (T=I / I o and T / T sub =(I / I o ) / (I sub / I o )=I / I sub =T film). Thus, reported total transmission can be corrected to remove transmission through the substrate and obtain the transmission of the film alone. While having good optical transparency across the visible spectrum is generally desirable, for convenience, optical transmission can be reported for light at a wavelength of 550 nm. Alternatively, or additionally, transmission can be reported as total transmittance for light at wavelengths between 400 nm and 700 nm, and such results are reported in the Examples below. Generally, for fused metal nanowire films, measurements of transmittance at 550 nm and total transmittance between 400 nm and 700 nm (or simply "total transmittance," for convenience) are not qualitatively different. In some embodiments, films formed by the fused network have a total transmittance (TT%) of at least 80%, in further embodiments at least about 85%, in additional embodiments at least about 90%, in other embodiments at least about 94%, and in some embodiments, from about 95% to about 99%. The transparency of films on transparent polymer substrates can be evaluated using standard ASTM D1003 ("Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics"), which is incorporated herein by reference. The TT% throughout the film includes reductions in transmittance due to the substrate and overcoat, which can shift the lower end of the transmittance ranges above from 1% to 10%, and in some embodiments, from 2.5% to 5%. One of ordinary skill in the art will recognize that additional ranges of transmittance within the explicit ranges above are contemplated and within the scope of the present disclosure. When adjusting the measured optical properties of the films in the following examples for the substrate, the films have very good transmission and haze values, which are achieved in addition to the low sheet resistance observed.

[0064] The fused metal network can have low haze along with high visible light transmission while possessing a desirably low sheet resistance. Haze can be measured using a haze meter according to the above-referenced ASTM D1003 standard, and the haze value of the transparent conductive film can be calculated by subtracting the contribution of the substrate haze. In some embodiments, the sintered network film can have a haze value of about 1.2% or less, in further embodiments about 1.1% or less, in additional embodiments about 1.0% or less, and in other embodiments about 0.9% to about 0.2%. As described in the examples, very low haze and sheet resistance values ​​have been achieved simultaneously using appropriately selected silver nanowires. The loading can be adjusted to balance sheet resistance and haze values, so that very low haze values ​​can be obtained while maintaining good sheet resistance values. Haze values ​​of 0.8% or less, particularly about 0.4% to about 0.7%, can be achieved with sheet resistance values ​​of at least about 45 ohms / sq. Also, haze values ​​of 0.7% to about 1.2%, and in some embodiments, about 0.75% to about 1.05%, can be achieved with sheet resistance values ​​of about 30 ohms / sq to about 45 ohms / sq. All of these films maintained good optical clarity. A person of ordinary skill in the art will recognize that additional ranges of haze within the explicit ranges above are contemplated and are within the present disclosure.

[0065] With respect to the comparable properties of the multilayer film, the additional components are generally selected to have only a slight effect on the optical properties, and various coatings and substrates for use with transparent elements are commercially available. Suitable optical coatings, substrates, and related materials are summarized above. Part of the structural material can be electrically insulating, and if a thicker insulating layer is used, the film can be patterned to provide locations where gaps or voids through the insulating layer can provide access and electrical contact to separately embedded conductive elements. Some components of the final device can be covered from view by an opaque or translucent cover to hide visible portions of the structure, such as connections to conductive transparent elements. While the cover can protect the conductive layer from light, it can heat up due to light absorption, and the edges of the covering tape and the transitions between transparent and covered areas can have stability issues, which are addressed in the examples.

[0066] Transparent conductive film stability and stability testing In use, it is desirable for the transparent conductive film to last for a commercially acceptable period of time, such as the lifetime of a corresponding device. The stabilization compositions and structures described herein are intended to achieve this, and the properties of the sparse metal conductive layer, e.g., a fused metal nanostructured network, are fully maintained. Some processing and corresponding evaluations mimic some aspects of accelerated aging tests. However, accelerated aging tests generally use more layered structures that more closely correspond to structures in actual devices, such as displays. While direct quantitative comparisons cannot be established with Applicant's earlier accelerated aging tests, the trends in properties are nonetheless believed to be highly meaningful. [Example]

[0067] Example 1 - Heat and humidity processing to reduce sheet resistance This example demonstrates the performance of silver nanowire structures containing NanoGlue® fusing agent under various environmental conditions by measuring relative sheet resistance. In general, the processing and compositions described in Examples 1 and 2 did not significantly change the optical properties of the structures compared to those reported for earlier versions of these materials.

[0068] Laboratory Coating: Figures 4-6 show plots of relative sheet resistance (R / R) as a function of time for samples in which a transparent polymer substrate was coated with one of four silver nanowire ink compositions in a laboratory set using a hand-slot coating process. The first set of samples (S1 and S2) was prepared using GEN5 ActiveGrid™ ink from Applicant C3Nano, Inc., with silver nanowires less than 25 nm in diameter; one sample (S1) contained NanoGlue® fusing agent, and the other sample (S2) contained no fusing agent. The second set of samples (S3 and S4) was prepared using GEN7 ActiveGrid™ ink with silver nanowires less than 20 nm in diameter; one sample (S3) contained NanoGlue® fusing agent, and the other sample (S4) contained no fusing agent. The silver nanowire ink composition with NanoGlue® and processing to form a transparent conductive film was essentially as described in Example 5 of the above-cited '968 patent, with silver nanowires synthesized as described in U.S. Pat. No. 10,714,230 B2 to Hu et al., entitled "Thin and Uniform Silver Nanowires, Methods of Synthesis and Transparent Conductive Films Formed From the Nanowires," which is incorporated herein by reference.

[0069] After coating, the transparent conductive film structures were air-dried at room temperature and then further baked in an oven at 120°C for up to about 2 minutes to induce coalescence and form a fused metal nanostructured network. The amount of silver deposited was selected to give the film a sheet resistance of about 70 ohms / sq when measured after drying. A commercially available overcoat was hand-slot coated onto each dried transparent conductive film structure. The overcoat was dried and crosslinked using a UV lamp to form an overcoat having a thickness of about 85 nm. The overcoat also included the stabilizing compound described in the above-cited '704 application.

[0070] The effects of heat treatment, or treatment with heat and humidity, were studied using commercially available testing equipment that provides heat and humidity control. Samples were placed in a chamber and tested under various conditions of temperature (65°C or 85°C) and relative humidity (85%, 90%, or uncontrolled) without illumination. In contrast to previous testing of metal nanowire-based films, samples were tested with only the polymer overcoat. This contrasts with previous testing in which transparent conductive films are tested in stacks with an additional, thicker protective polymer layer.

[0071] The relative sheet resistance (R / R) of the samples was measured as a function of time. Figure 4 shows data for the 85°C condition without humidity control, Figure 5 shows data for the 85°C and 85% relative humidity conditions, and Figure 6 shows data for the 65°C and 90% relative humidity conditions. The transparent conductive film formed from thinner silver nanowires and NanoGlue® fusing agent, sample S3, exhibited the lowest relative sheet resistance at the longest test time for the taught conditions, demonstrating the best stability performance. Comparing Figure 4 with Figures 5 and 6 reveals that dry heating at 85°C did not significantly increase sheet resistance, while high humidity caused a large increase in sheet resistance. Comparing Figures 4 and 5 with Figure 6, the superior performance of the samples with NanoGlue® fusing agent (S1 and S3) at high relative humidity (90%) was evident, especially for times longer than 50 hours. The stabilizing effect of the NanoGlue® fusing agent was particularly pronounced for smaller diameter nanowire structures at high relative humidity, as can be seen by the dramatically different results for S3 and S4 in Figures 5 and 6. Thinner silver nanowires have a larger surface-to-volume ratio, and the observed effect of NanoGlue® in the S3 and S4 samples can be attributed to an increased number of contacts and a decreased contact area for the thinner wires.

[0072] Processing at 85°C and 85% relative humidity (Figure 5) resulted in the greatest initial decrease in relative sheet resistance. At each environmental condition, sheet resistance decreased (by about 10% or more) during the initial time of testing for samples with NanoGlue® in the coating (S1 and S3). With the exception of S1 at 85°C without a humidity-controlled environment after about 75 hours (Figure 4), samples with NanoGlue® fusing agent exhibited lower relative sheet resistance than samples without fusing agent at a given time of testing. Samples without NanoGlue® fusing agent (S2 and S4) did not show any decrease in relative sheet resistance with processing. For these samples, relative sheet resistance only increased over time.

[0073] Roll-to-roll coating: Figure 7 shows the results of samples in which substrates were coated with one of three silver nanowire ink compositions using slot-die coating in a roll-to-roll configuration. The three silver nanowire ink compositions were prepared using GEN7 ActiveGrid™ ink and different amounts of NanoGlue® fusing agent: 1.5x NanoGlue®, 3x NanoGlue®, or no NanoGlue® (as a control). Note that the amount of NanoGlue® is referenced relative to the standard amount in commercially available inks. After coating, the structures were air-dried at room temperature and then further dried in an oven at 120°C for up to approximately 10 minutes to dry the ink. The resistance of the nanowire ink-coated samples as a function of time was tested in an air atmosphere at 65°C and 90% relative humidity. The two samples coated with the NanoGlue®-containing ink showed an initial decrease in relative sheet resistance of approximately 10% over approximately 80 hours. After an initial drop, the relative sheet resistance of the NanoGlue®-coated sample increased but remained below the initial resistance value until t = approximately 300 hours. The final relative sheet resistance (R / R) for the two samples with NanoGlue® fusing agent was 1.4. The 1.5x NanoGlue® sample generally exhibited lower sheet resistance than the 3x sample. The sample without fusing agent exhibited an initial increase in sheet resistance of approximately 19%, and then the sheet resistance increased throughout the test period. The final relative sheet resistance was approximately 2.1. The target limit of 1.2 R / R for sheet resistance increase is indicated by the horizontal line.

[0074] This experiment showed that nanowire inks with 1.5x or 3x NanoGlue® fusing agent reduced the resistance of nanowire-coated structures by approximately 33%. Samples without NanoGlue® crossed the 1.2 target limit after approximately 125 hours, while samples with NanoGlue® crossed the 1.2 target limit after approximately 650 hours.

[0075] Laboratory and roll-to-roll results demonstrated that the environmental durability of conductive silver nanowire films was improved with the use of NanoGlue® fusing agent. The results suggest that the initial resistance drop observed in the NanoGlue® samples is an indication of providing an additional metal source to the transparent conductive film. The improved stability observed at high temperature / high humidity conditions with NanoGlue® is consistent with fusing reducing / eliminating the junction resistance, which can increase more rapidly at high temperature / high humidity conditions without fusing. These results indicate that initial heat and humidity treatment of coated products can be used to reduce sheet resistance and potentially stabilize transparent conductive films before assembly into devices.

[0076] Example 2 - Stabilization of fused metal nanostructured networks with silver ions in the overcoat This example demonstrates the stabilizing effect of NanoGlue® fusing agent in the overcoat layer adjacent to the fused metal nanostructured network.

[0077] Figure 8 shows a plot of relative sheet resistance (R / R) as a function of time for laminated optical structure samples provided with various overcoats. The structures included a transparent polymer substrate with a sparse metal conductive layer that was either unfused or fused to form a sparse metal nanostructured network. The formation of this structure is described in Example 1. The layers of the laminated optical structure samples are illustrated in Figure 8. Each sample had a PET substrate. The substrate was coated with GEN5 silver nanowire ink with NanoGlue® fusing agent as described in Example 1 and then dried. Next, an overcoat consisting of a UV-curable polymer resin containing a selected amount of silver salt (as NanoGlue®) was coated onto each structure. Finally, each structure was laminated onto type A glass. Five overcoat compositions were prepared using different amounts of NanoGlue® fusing agent: levels 1x, 1.5x, 2x, 3x, or no NanoGlue® (as a control). The reference amount of NanoGlue® (1x) is about 3.0 wt. % to about 3.5 wt. % silver ions. Samples were tested at high temperature / high humidity conditions: 85°C and 85% relative humidity. Each sample with an overcoat containing NanoGlue® showed an initial decrease in relative sheet resistance, and the decrease became more pronounced as the level of NanoGlue® increased. NanoGlue® in the overcoat showed a clear effect on the sheet resistance of the silver nanowire structure upon further high temperature / high humidity processing of the optical stack.

[0078] This example shows that the decrease in sheet resistance during high temperature / high humidity treatment is more significant in structures where both the overcoat and the silver nanowire layer contain NanoGlue®. For example, comparing S1 in Figure 5 at approximately 95 hours with the 3x NanoGlue® level in Figure 8, the samples have relative sheet resistances of 1.07 and 0.85, respectively. This decrease in sheet resistance is consistent with the chemical reduction of silver ions in the overcoat to the metallic silver conductor at the junction location.

[0079] Figure 9 shows the effect of tripling the level of NanoGlue® in the overcoat on a GEN5 silver nanowire film that did not contain NanoGlue®. The laminate was tested with only the polymer overcoat, with no additional OCA or other layers. A sample with NanoGlue® in the overcoat and another sample without NanoGlue® in the overcoat are shown for high temperature / high humidity conditions of 85°C and 85% relative humidity. The sample with NanoGlue® in the overcoat showed an initial decrease in relative sheet resistance, followed by a continuous decrease. In contrast, the sample without NanoGlue® in the overcoat showed a significant increase. At approximately 45 hours, the two samples were 0.87 vs. 1.36.

[0080] This example shows that NanoGlue® in the overcoat, with or without NanoGlue® in the silver nanowire layer, can significantly reduce sheet resistance and provides significant stabilization in accelerated wear tests.

[0081] The above-described embodiments are intended to be illustrative and not limiting. Additional embodiments are within the scope of the claims. In addition, while the present invention has been described with reference to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the present invention. Any incorporation by reference of the above documents is limited so as not to incorporate subject matter to the contrary as explicitly disclosed herein. To the extent that specific structures, compositions, and / or processes are described in terms of components, elements, materials, or other moieties, the disclosure herein is understood to include specific embodiments, embodiments that include specific components, elements, materials, or other moieties, or combinations thereof, unless otherwise indicated, as well as embodiments that consist essentially of such specific components, materials, or other moieties, or combinations thereof, which may include additional features that do not alter the basic nature of the subject matter presented in this disclosure. The use of the term "about" herein refers to the expected uncertainty in the associated value that would be understood in a particular situation by a person of ordinary skill in the art.

Claims

1. 1. A transparent conductive film comprising a transparent polymer substrate, a sparse metal conductive layer supported by the substrate, and a polymer overcoat adjacent to the sparse metal conductive layer, wherein the transparent conductive film has a visible light transmittance of at least about 88% and a sheet resistance of about 120 ohms / sq or less, and the polymer overcoat comprises a polymer and about 0.01% to about 20% by weight of a precious metal ion.

2. 10. The transparent conductive film of claim 1, wherein the overcoat has an average thickness of about 5 nm to about 250 nm.

3. 3. The transparent conductive film according to claim 1, wherein the polymer of the overcoat comprises polysiloxane, polysilsesquioxane, polyurethane, acrylic resin, acrylic copolymer, cellulose ether and / or ester, nitrocellulose, other water-insoluble structural polysaccharides, polyether, polyester, polystyrene, polyimide, fluoropolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, acrylonitrile butadiene styrene copolymer, polysulfide, epoxy-containing polymer, copolymer thereof, and mixture thereof.

4. 4. The transparent conductive film of claim 1, wherein the polymer overcoat comprises from about 0.1% to about 9% by weight of the vanadium(+5) stabilizing composition.

5. The polymer overcoat has an average thickness of about 10 nm to about 125 nm, the polymer of the overcoat comprises a polyacrylate, and the polymer overcoat comprises ammonium metavanadate (NH ４ VO ３ ), tetrabutylammonium vanadate (NBu ４ VO ３ ), potassium metavanadate (KVO ３ ), sodium metavanadate (NaVO ３ ), sodium orthovanadate (Na ３ VO ４ 5. The transparent conductive film of claim 1, comprising about 0.5 wt. % to about 5 wt. % of a vanadium(+5) stabilizing composition comprising vanadium(+5) oxytripropoxide, vanadium oxytriethoxide, vanadium oxytriisopropoxide, vanadium oxytributoxide, or a mixture thereof.

6. 6. The transparent conductive film of claim 1, wherein the sparse metal conductive structure comprises a fused metal nanostructured network comprising silver, and the noble metal ions comprise silver ions.

7. 7. The transparent conductive film according to claim 1, wherein the transparent conductive film has a transmittance of at least about 90% and a sheet resistance of about 90 ohms / sq or less.

8. The polymer overcoat is made of silver tetrafluoroborate (AgBF ４ ), silver hexafluorophosphate (AgPF ６ ), silver perchlorate (AgClO ４ ), silver hexafluoroantimonate (AgSbF ６ ), silver trifluoroacetate (CF ３ COO), silver hexafluorobutyrate (AgC ４ HF ６ O ２ ), silver methylsulfonate (AgCH ３ SO ３ ), silver tolylsulfonate (AgCH ３ C ６ H ４ SO ３ 8. The transparent conductive film of claim 1, comprising about 0.25% to about 12% by weight of silver ions provided as a silver ion complex, such as ammonium nitrate, ...

9. The transparent conductive film according to any one of claims 1 to 8, wherein the sparse metal conductive layer is patterned.

10. 1. A method for reducing the sheet resistance of a transparent conductive film comprising a substrate, a transparent conductive layer comprising a fused metal nanowire network and a polymer polyol binder, and a polymer coating having an average thickness of about 5 nm to about 250 nm, comprising:

10. A method comprising heating the transparent conductive sheet to a temperature of at least about 55° C. for at least about 10 minutes to reduce sheet resistance by at least about 5%.

11. 11. The method of claim 10, wherein during the heating step, the transparent conductive sheet is on a roll, the polymer coating is an overcoat, and the overcoat is covered by a release layer.

12. 12. The method of claim 10 or 11, wherein the heating step is carried out with a relative humidity adjusted to at least about 60%.

13. 12. The method of claim 10 or 11, wherein the heating is carried out at a temperature of about 60°C to about 100°C for about 20 minutes to about 50 hours at a relative humidity of at least about 60%, and the transparent conductive film is free of an optically transparent adhesive.

14. 14. The method of any one of claims 10-13, wherein the fused metallic nanostructured network comprises silver and the film has a sheet resistance of 120 ohms / sq or less and a visible light transmittance of at least about 88%.

15. 15. The method of any one of claims 10-14, further comprising heating the substrate with the coating of solvent, metal nanowires, and metal ions to a temperature of about 45°C to about 130°C for at least about 2 minutes prior to application of the polymer overcoat to dry the coating and form the fused metal nanostructured network.

16. 16. The method of claim 15, wherein the coating comprises a vanadium(+5) stabilizing composition and silver ions.

17. 17. The method of any one of claims 10 to 16, wherein the polymeric overcoat has an average thickness of from about 20 nm to about 125 nm, the polymer of the polymeric overcoat comprises a polyacrylate, and the polymeric overcoat comprises from about 0.5 wt % to about 5 wt % of a vanadium(+5) stabilizing composition.

18. 18. The method of claim 17, wherein the polymer overcoat comprises from about 0.1% to about 20% by weight of precious metal ions.

19. 19. The method of any one of claims 10 to 18, wherein the fused metallic nanostructured network comprises silver and the polymer overcoat comprises a polymer and about 0.25% to about 15% by weight of silver ions.

20. 1. A transparent conductive film comprising a substrate, a transparent conductive layer comprising a fused metal nanostructured network, and a polymer overcoat, wherein the transparent conductive film has a transmittance of at least about 88% and a sheet resistance of about 120 ohms / sq or less, and wherein the transparent conductive film has been modified by processing with heat and optionally humidity for at least about 10 minutes to reduce the sheet resistance by at least about 5%.

21. 21. The transparent conductive film of claim 20, wherein the polymer overcoat and / or undercoat comprises metal ions.

22. 22. The transparent conductive film of claim 21, wherein the metal ions comprise silver ions at a concentration of about 0.01% to about 20% by weight.

23. 23. The transparent conductive film according to claim 21 or 22, wherein the metal ions comprise vanadium (+5) at a concentration of about 0.5% to about 5% by weight.

24. 21. The transparent conductive film of claim 20, wherein the fused metallic nanostructured network comprises silver and the processing comprises heating at a temperature of about 60°C to about 100°C at a relative humidity of at least about 60%, but the transparent conductive film does not comprise an optically clear adhesive.

25. 25. The transparent conductive film of claim 24, wherein the overcoat has an average thickness of about 5 nm to about 250 nm and comprises a crosslinked polyacrylate and vanadium (+5) ions at a concentration of about 0.5 wt % to about 5 wt %.

Images