Transparent conductive film, method for producing it and use

Description

[0001] The invention relates to a transparent conductive film, a method for producing the same, and a use thereof.

[0002] Transparent conductive films, suitable, for example, as heating elements for vehicle windshields, have been described in the prior art; see, for example, EP 2764996 B1, EP 2284134 B1, and WO 2016 / 192858 A1. According to WO 2016 / 192858 A1, a transparent conductive film is produced using a multi-stage process. First, a washable coating is applied to a transparent substrate, which, upon drying, develops numerous cracks in the form of a dense, continuous network. This is followed by vapor deposition with metal, and then the cracked, washable coating is removed in a washing step. The resulting product has a transparent, conductive metallization in the form of a dense, continuous network above the substrate.

[0003] KDM Rao, C. Hunger, R. Gupta, GU Kulkarni, M. Thelakkat: "A cracked polymer templated metal network as a transparent conducting electrode for ITO-free organic solar cells", Phys. Chem. Chem. Phys., 2014, Volume 16, pages 15107-15110; describes a highly transparent, low-resistance Ag-metal network modeled by a cracked polymer thin film and its incorporation into an organic solar cell.

[0004] US 2014 / 218325 A1, a family member of EP 2764996 B1, describes a conductive film comprising a base; and a conductive section formed on at least one principal surface of the base and formed from a plurality of thin metal wires, wherein the conductive section forms a grid pattern in which a plurality of openings of different shapes are arranged in plan view, and a standard deviation of an area of ​​each of the multiple openings is equal to or greater than 0.017 mm² and equal to or less than 0.038 mm².

[0005] In practice, it has been shown that, particularly with copper-based metallizations, the reddish color of the metal on a transparent film is visually striking and therefore distracting to the viewer. With other metals, such as aluminum or silver, a highly reflective metal surface is perceptible when viewed in reflected or transmitted light, primarily as a gloss effect at the angle of reflection.

[0006] The present invention is based on the objective of improving the transparent conductive film known in the prior art. In particular, a transparent conductive film is to be provided which, compared to previously known films, has a neutral color appearance. This objective is achieved by the combinations of features defined in the independent claims. Further developments of the invention are the subject of the dependent claims. Detailed description of the invention

[0007] The present invention is based on the technology for the fine structuring of metallizations known from WO 2016 / 192858 A1, on the basis of which electrical devices, e.g., heating films for use in the windshield of a vehicle, can be provided. The technology includes, inter alia, the use of a crack-forming coating, preferably a dispersion or a solution of a polymer. The crack-forming coating is applied to the transparent substrate, e.g., by printing, so that a thin film is produced which, during drying, forms cracks in the form of a dense, continuous network. Subsequently, the metal is vapor-deposited, followed by the removal of the cracked, washable coating in a washing step.The resulting product is such that it has a transparent, conductive metallization in the form of a tightly woven, continuous network above the transparent substrate.

[0008] The present invention is based on the finding that, particularly in copper-based systems, the reddish color of the metal is strongly perceptible above a transparent substrate. In the case of other metal-based systems, e.g., aluminum or silver, a highly reflective metal surface is perceptible in reflected or backlit light, especially at the glare angle, as a gloss effect. According to the invention, it has been found that a neutral color impression with low reflection can be achieved by designing the metallic network as a multilayer arrangement comprising a first layer of copper and at least one further layer of a material that reduces the visual perceptibility of the first layer. As a result of the neutral color impression and the non-reflective metal surfaces, the transparent conductive film according to the invention achieves higher transparency and lower perceptibility to the viewer.Furthermore, the manufacturing process according to the invention is technically easy to implement because the additional layers can be applied over the entire surface of the cracked, washable coating and then washed away in a single washing step. Either the perceptible color or the perceptible reflection, or both simultaneously, can be controlled by a suitable choice of material that reduces the visual perceptibility of the metallic network.

[0009] In this context, viewing an object in reflected light is understood to mean the situation where both the illumination and the viewing of the object take place from the same side of the object, whereas in the case of viewing the object in transmitted light or backlighting, the illumination source and the viewing location are arranged on opposite sides of the object.

[0010] For achieving the advantageous effect of reducing the visual perceptibility of the metallic copper network, a material is particularly suitable which consists of the group comprising a metal oxide layer, preferably a metal oxide layer based on copper oxide, chromium oxide or a substoichiometric aluminum oxide, an antireflective thin-film structure with in particular the layer sequence metal / dielectric / metal (e.g. a Cu / SiO₂ / Cr structure) or the layer sequence dielectric / metal / dielectric / metal (e.g. a SiO₂ / Cr / SiO₂ / Al structure, a SiO₂ / Cr / SiO₂ / Cu structure or a SiO₂ / Al / SiO₂ / Cu structure), black chrome (i.e. black passivated chrome), black nickel (i.e. black passivated nickel), a metal sulfide layer, an overprint based on a paint or pigment lacquer, a nanostructuring orThe antireflection layer formed by the moth-eye structure is chosen from a combination of two or more of the aforementioned elements.

[0011] Antireflective thin-film structures are known in the prior art, see e.g. Sang-Hwan Cho et al., Journal of the Korean Physical Society, Vol. 55, No. 2, August 2009, 501 - 507.

[0012] Thin-film elements with a multilayer structure and a dark-appearing nanostructured area (so-called moth-eye structure) are known, for example, from EP 2 453 269 A1. An antireflection layer formed by nanostructuring is based, in particular, on a metal, e.g., copper, a metal oxide, a nitride, a polymer, or a dielectric.

[0013] According to a preferred embodiment, the conductive metallization is formed as a dense, continuous mesh such that, starting from the transparent substrate, a first layer of copper and subsequently at least one further layer of a material that reduces the visual perceptibility of the first layer are arranged sequentially. If required, the structure can be additionally provided with a transparent coating that flattens the conductive metallization, e.g., a UV-curing or heat-curing primer. The product thus obtained can then be provided with an adhesive layer, which is, for example, arranged on the side of the transparent substrate opposite the conductive metallization. Alternatively, the adhesive layer can be arranged above the transparent coating that flattens the conductive metallization. A heat-sealable lacquer, for example, is suitable as an adhesive layer.According to a specific variant, the adhesive layer used, e.g. a heat-seal lacquer, can be identical to the transparent coating used for leveling the conductive metallization.

[0014] The transparent substrate is in particular a glass substrate or a plastic film, e.g. a polyethylene terephthalate (PET) film.

[0015] According to a further preferred embodiment, the conductive metallization is formed as a dense, continuous mesh such that it is based on a three-layer arrangement, with a further layer of material that reduces the visual perceptibility of the first layer arranged above and below the first layer of conductive metal. This embodiment has the advantage that the film appears neutral in color to the viewer whether viewed from the front or the back.

[0016] To achieve advantageous conductivity, a copper layer is chosen for the conductive metal layer.

[0017] The present invention further relates to the aspect of producing a transparent, conductive film, comprising the following steps: The provision of a transparent substrate; the application of a crack-forming coating to the transparent substrate and drying of the crack-forming coating, wherein the coating forms numerous cracks in the form of a dense, continuous network upon drying; the application of a conductive metallization based on a multilayer arrangement, comprising a first layer of a conductive metal, namely copper, and at least one further layer of a material reducing the visual perceptibility of the first layer; the removal of the cracked coating together with the multilayer arrangement located above the coating, such that the resulting transparent substrate is such that the conductive metallization forms a dense, continuous network with a multitude of openings of different geometric shapes on its main surface.

[0018] The manufacturing process according to the invention is based on the manufacturing process described in WO 2016 / 192858 A1.

[0019] According to the invention, a dispersion, and more preferably a colloidal dispersion, is preferably used as the crack-inducing coating. Particularly suitable are, for example, dispersions of SiO₂ nanoparticles or of acrylic resin nanoparticles, as described on page 2090 of the document: S. Kiruthika, R. Gupta, KDM Rao, S. Chakraborty, N. Padmavathy, GU Kulkarni: "Large area solution processed transparent conducting electrode based on highly interconnected Cu wire network", J. Mater. Chem. C, 2014, Volume 2, pages 2089-2094. Furthermore, the crack-inducing coating can be based on a polymer in solution. The polymer solution is applied to the substrate, for example by printing, so that a thin polymer film is created. The thin polymer film forms cracks during drying.

[0020] Crack formation depends on the choice of raw materials and substrate, the thickness of the crack-inducing coating, and the drying parameters. The line thicknesses achievable at the end of the manufacturing process, for example in the case of silver, range from 1 µm to 50 µm. The lines are generally so fine that they are only visible under magnification. The human eye cannot resolve the individual lines across the surface, but a difference compared to the untreated or bare foil is noticeable in both reflected and transmitted light. Because the fine lines form an irregular, continuous network, unwanted diffraction effects can be minimized. By varying the island size and crack width, the reflectivity and light transmission can be adjusted accordingly.

[0021] The method for removing the cracked coating is advantageously carried out by dissolving it with a suitable solvent. The choice of solvent is expediently made according to the type of coating. The following solvents can typically be used: methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methoxypropyl acetate, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone, methylene chloride, chloroform, toluene, xylene, methanol, ethanol, and 2-propanol. Acetals or mixtures of the aforementioned solvents can also be used. Alternatively, the cracked coating can be removed by undermining it. In this case, in addition to the solvents mentioned, aqueous solutions, mixtures of solvents and water, possibly with surfactants, defoamers, and other additives, can be used. The removal or...Breaking down the cracked coating can also be supported by spray nozzles or mechanically by brushes, rollers or felts.

[0022] The metallization according to the invention, in the form of a densely meshed, continuous network, exhibits electrical conductivity and optical transmission comparable to a full-surface ITO layer. The fine metallic lines can be used in combination with conventional embossing lacquers, conventional primer compositions, and conventional heat-seal lacquers, thereby acting as reflectors.

[0023] The transparent, conductive film according to the invention is particularly suitable for use in electrical devices, e.g. as a heating film for use in the windshield of a vehicle, in other windows or in building glazing, as well as for current coupling without visible leads, e.g. for use in LED films, in solar cells, in smart glass applications, in OLEDs or in touch panels.

[0024] Further embodiments and advantages of the invention are explained below with reference to the figures, in the representation of which a scale and proportion-accurate reproduction has been omitted in order to increase clarity.

[0025] They show: Figures 1-8 show the individual manufacturing steps for obtaining a transparent, conductive film according to a first embodiment; and Figures 9-13 show the individual manufacturing steps for obtaining a transparent, conductive film according to a second embodiment.

[0026] The Figures 1 to 8 illustrate the individual manufacturing steps for obtaining a transparent, conductive film according to a first embodiment.

[0027] According to the Figures 1 and 2The process begins with the provision of a transparent substrate 1, in this case a PET film, and the application of a crack-inducing coating 2 to the substrate 1. The crack-inducing coating 2 is based, for example, on dispersions of SiO₂ nanoparticles or acrylic resin nanoparticles. The application of the crack-inducing coating 2 is preferably carried out using printing techniques, such as gravure printing, flexographic printing, or inkjet printing. During drying, the crack-inducing coating 2 forms numerous cracks in the form of a dense, continuous network.

[0028] The Figure 1 Figure 2 shows the cracked coating 2 when viewed from above. Figure 2 shows a cross-sectional view along the dashed line A-A'.

[0029] In a further step, a layer 3 based on substoichiometric, black aluminum oxide is applied (see Figure 3The substoichiometric, black aluminum oxide-based layer 3 is deposited both above the cracked coating 2 and within the cracks of the coating 2.

[0030] In a further step, a conductive Cu layer 4 is applied (see Figure 4 The Cu layer 4 is deposited both above the cracked coating 2 and within the cracks of the coating 2.

[0031] In a further step, another layer 5 based on substoichiometric black aluminum oxide is applied (see Figure 5 The substoichiometric, black aluminum oxide-based layer 5 is deposited both above the cracked coating 2 and within the cracks of the coating 2.

[0032] According to the Figure 6The cracked coating 2, including the layers 3, 4, and 5 located above it, is then removed in a washing step. Washing is carried out by dissolving the coating with a suitable solvent, e.g., methyl acetate. A transparent, conductive metallization remains on the transparent substrate 1 in the form of a dense, continuous network. The conductive metallization is based on a multilayer arrangement with three layers, in which two further layers 3 and 5 of substoichiometric black aluminum oxide, which reduces the visual perceptibility of the Cu layer 4, are arranged above and below the conductive Cu layer 4.

[0033] The Figure 8 The transparent, conductive metallization is shown schematically in top view in the form of a tightly woven, continuous network in an enlarged representation.

[0034] According to the Figure 7Optionally, a transparent, leveling lacquer 6 can be applied. Any existing adhesive layer 7, e.g., a heat-seal lacquer layer, serves to apply the transparent, conductive film 8, e.g., to the windshield of a vehicle.

[0035] The Figures 9 to 13 The individual manufacturing steps for obtaining a transparent, conductive film according to a second embodiment are illustrated. The manufacturing process is based on the first embodiment.

[0036] According to the Figure 9 First, a transparent substrate 9, in this case a glass substrate, is provided, and the substrate 9 is coated with a crack-forming coating 10. During drying, the crack-forming coating 10 forms numerous cracks in the form of a dense, continuous network.

[0037] In a further step, a copper oxide layer 11 is applied (see Figure 10 The copper oxide layer 11 is deposited both above the cracked coating 10 and within the cracks of the coating 10.

[0038] In a further step, a conductive layer 12 is applied, in this case an Ag layer (see Figure 11 The Ag layer 12 is deposited both above the cracked coating 10 and within the cracks of the coating 10.

[0039] According to the Figure 12The cracked coating 10, including the layers 11 and 12 located above it, is then removed in a washing step. A transparent, conductive metallization remains on the transparent substrate 9 in the form of a dense, continuous network. The conductive metallization is based on a two-layer arrangement, with layer 11, consisting of black copper oxide that reduces the visual visibility of the Ag layer 12, located below the conductive Ag layer 12.

[0040] According to the Figure 13 Optionally, a transparent, leveling lacquer 13 can be applied. Any existing adhesive layer 14, e.g., a heat-seal lacquer layer, serves to apply the transparent, conductive film 15, e.g., to the windshield of a vehicle.

Claims

1. A transparent, conductive film comprising a transparent substrate on the main surface of which a conductive metallization is formed in the form of a dense, continuous mesh with a plurality of openings of varying geometric shapes, wherein the conductive metallization is based on a multilayer structure comprising a first layer of a conductive metal, namely copper, and at least one additional layer, characterized in that the at least one additional layer consists of a material that reduces the visual perceptibility of the first layer, thereby producing a neutral color impression with low reflectance, wherein the at least one additional layer consists of a material that reduces the visual perceptibility of the first layer, selected from the group consisting of a metal oxide layer, an antireflective thin-film structure with the layer sequence metal / dielectric / metal or the layer sequence dielectric / metal / dielectric / metal, black chrome, black nickel, a metal sulfide layer, an overprint based on a colored lacquer or a pigmented lacquer, an antireflective layer formed by nanostructuring or a moth-eye structure, and a combination of two or more of the aforementioned elements.

2. A transparent, conductive film according to claim 1, wherein the at least one additional layer is selected from a material that reduces the visual perceptibility of the first layer, namely copper oxide, chromium oxide, or a metal oxide layer based on substoichiometric aluminum oxide.

3. A transparent, conductive film according to claim 1 or 2, wherein, starting from the transparent substrate, the first layer of conductive metal and, subsequently, the at least one additional layer of the material that reduces the visual perceptibility of the first layer are arranged in that order.

4. A transparent, conductive film according to claim 1 or 2, wherein the conductive metallization is based on a multilayer structure comprising three layers, such that one additional layer of the material that reduces the visual perceptibility of the first layer is arranged above and below the first layer of a conductive metal, respectively.

5. A transparent, conductive film according to any one of claims 1 through 4, wherein the film additionally comprises a layer that levels the conductive metallization and an adhesive layer.

6. A method for manufacturing a transparent, conductive film according to any one of claims 1 through 5, comprising the following steps: - providing a transparent substrate; - applying a crack-forming coating to the transparent substrate and drying the crack-forming coating, wherein the coating forms numerous cracks in the form of a dense, continuous network during drying; - applying a conductive metallization based on a multilayer assembly, comprising a first layer of a conductive metal and at least one additional layer of a material that reduces the visual perceptibility of the first layer; - removing the cracked coating together with the multilayer assembly located above the coating, so that the resulting transparent substrate is such that the conductive metallization is formed on its main surface in the form of a dense, continuous network with a multitude of openings of varying geometric shapes.

7. Use of the transparent, conductive film according to any one of claims 1 through 5 as a heating film, in particular in a vehicle windshield, in other windows, or in building glazing, as well as for power coupling without visible leads, e.g., for use in LED films, in solar cells, in smart glass applications, in OLEDs, or in touch panels.