WO2012168941A1

WO2012168941A1 - Flexible transparent conductive coatings by direct room temperature evaporative lithography

Info

Publication number: WO2012168941A1
Application number: PCT/IL2012/050200
Authority: WO
Inventors: Shlomo Magdassi; Michael Layani
Original assignee: Yissum Research Development Company Of The Hebrew University Of Jerusalem Ltd.
Priority date: 2011-06-09
Filing date: 2012-06-07
Publication date: 2012-12-13
Also published as: KR20150028759A; WO2012168941A8

Abstract

The invention provides a process for forming highly ordered, conductive and transparent patterns on flexible heat-sensitive surfaces. The invention relates to sintering nanoparticles at room temperature. Patterning is done by a patterning device and the self-assembly of the nanoparticles.

Description

FLEXIBLE TRANSPARENT CONDUCTIVE COATINGS BY DIRECT ROOM TEMPERATURE EVAPORATIVE LITHOGRAPHY

TECHNOLOGICAL FIELD

This invention relates to conductive patterns formed on flexible substrates.

BACKGROUND

The current industrial demand for transparent and conductive coatings for printed electronics has led to various approaches for fabrication of such coatings [1, 2]. These approaches are mainly based on patterning methods and self-assembly of nanoparticles (NPs).

Kang et al. [3] used nano-imprinting lithography for fabrication of gold transparent grid, by which the metal line thickness and aperture ratio were varied, and demonstrated the use of the grid as the anode in an OLED device. Printing of arrays of micrometer-sized rings composed of silver nanoparticles has also been demonstrated to lead to the formation of a transparent and conductive grid [4]. The rings were formed due to the coffee stain effect [5], where a droplet containing particles was pinned to the substrate and during evaporation of the liquid the particles traveled to the periphery of the droplet. Once the droplet was dried, a ring composed of the closely packed nanoparticles was formed, enabling use of the ring pattern as a transparent conductive electrode.

The self-assembly of nanoparticles was also studied by Vakarelski et al. [6] as a method for patterning of gold nanoparticles. On top of a first layer composed of latex particles, a droplet was placed which contained gold NPs. The flux caused by evaporation forced the gold particles to assemble around the large latex particles. Once the droplet was fully evaporated the latex layer was removed, and a small area, 0.5 mm², of patterned gold NPs was obtained. This approach of evaporative lithography [7, 8] was suggested by Vakarelski et al. [6] as a method for fabrication of transparent and conductive coatings, which should require an additional step of sintering the nanoparticles.

Higashitani et al. [9] have published an indirect fabrication method for obtaining a large area of transparent conductive films on plastic substrates (PET) using an evaporative lithography method. As formation of a conductive pattern on the plastic substrate was not possible due to the nature of the substrate (being heat sensitive and soft, flexible), the fabrication was based on a three-step process, which included: placing a stainless-steel mesh on top of a plasma-treated glass substrate; placing a droplet containing gold NPs on top of the mesh, permitting instantaneous spreading of the droplet over the mesh and the glass substrate and flowing of the droplet towards the wires of the mesh, leaving an empty area of glass in each square of the mesh; heating the substrate to 425°C for 20 min; and transferring the grid pattern formed on the glass onto a poly(ethylene terephthalate) film, with the aid of UV curable monomers, thus affording the pattern on the PET film.

The process of Higashitani et al. [9] does not permit direct application onto PET, being a heat-sensitive flexible substrate, as the sintering conditions applied on such a substrate would damage the substrate to an extent that sustainable pattering would not be possible.

The ability to sinter nanoparticles chemically at room temperature [10] was demonstrated on a printed pattern made of silver nanoparticles. The chemical sintering was conducted at room temperature.

REFRENCES

[1] Hecht, D. S.; Hu, L. B. ; Irvin, G., Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures. Advanced Materials 2011, 23, (13), 1482-1513.

[2] Kumar, A.; Zhou, C. W., The Race To Replace Tin-Doped Indium Oxide: Which Material Will Win? Acs Nano 2010, 4, (1), 11-14.

[3] Kang, M. G.; Guo, L. J., Nanoimprinted semitransparent metal electrodes and their application in organic light-emitting diodes. Advanced Materials 2007, 19, (10), 1391.

[4] Layani, M.; Gruchko, M.; Milo, O.; Balberg, I.; Azulay, D.; Magdassi, S., Transparent Conductive Coatings by Printing coffee Ring Arrays Obtained at Room Temperature. Acs Nano 2009, 3, (11), 3537-3542.

[5] Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A., Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, (6653), 827-829. [6] Vakarelski, I. U.; Chan, D. Y. C; Nonoguchi, T. ; Shinto, H.; Higashitani, K., Assembly of Gold Nanoparticles into Microwire Networks Induced by Drying Liquid Bridges. Physical Review Letters 2009, 102, (5), 058303.

[7] Harris, D. J.; Lewis, J. A., Marangoni effects on evaporative lithographic patterning of colloidal films. Langmuir 2008, 24, (8), 3681-3685.

[8] Harris, D. J.; Hu, H.; Conrad, J. C; Lewis, J. A., Patterning colloidal films via evaporative lithography. Physical Review Letters 2007, 98, (14), 148301.

[9] Higashitani, K.; McNamee, C. E.; Nakayama, M., Formation of Large-Scale Flexible Transparent Conductive Films Using Evaporative Migration Characteristics of Au Nanoparticles. Langmuir 2011, 27 (6), 2080-2083.

[10] Grouchko, M.; Kamyshny, A.; Mihailescu, C. F.; Anghel, D. F.; Magdassi, S., Conductive Inks with a "Built-in" Mechanism That Enables Sintering at Room Temperature. ACS Nano 2011, 5 (4), 3354-3359.

[11] Magdassi, S.; Grouchko, M. ; Berezin, O.; Kamyshny, A., Triggering the Sintering of Silver Nanoparticles at Room Temperature. ACS Nano 2010, 4, (4), 1943- 1948.

[12] Tsunomura, Y.; Yoshimine, Y.; Taguchi, M.; Baba, T.; Kinoshita, T.; Kann, H.; Sakata, H.; Maruyama, E.; Tanaka, M., Twenty-two percent efficiency HIT solar cell. Solar Energy Materials and Solar Cells 2009, 93, (6-7), 670-673.

GENERAL DESCRIPTION

Transparent conductive patterns of pre-defined and controllable characteristics are essential for fabrication of a variety of electronic devices, such as optoelectronic devices, in particular in flat and touch panel displays. As the traditional evaporative lithography methods involve sintering at high temperatures, these methods do not permit pattern forming on heat-sensitive surfaces, nor do they permit control of the resulting pattern characteristics, thus limiting the selection of substrates which can be used and the end applications. These evaporative lithography methods, being based on material transfer from a less sensitive surface (e.g., glass) to a more sensitive surface (e.g., highly heat sensitive such as those used in plastic electronics), are plagued with material loss and disintegration and therefore do not guarantee transfer/formation of highly ordered, highly controlled and industrially feasible pattern forming on sensitive substrates. Thus, in order to perform evaporative lithography directly on heat- sensitive substrates, while maintaining high quality pattern features, the development of a direct process for directly forming a pattern on such heat-sensitive substrates was necessitated.

Herein, the inventors of the present invention disclose a room-temperature or low temperature fast process for obtaining transparent conductive patterns on heat- sensitive and flexible substrates, processes which may be carried out directly on heat sensitive substrates. This direct process is both industrially feasible and industrially more preferable.

Thus, in one aspect there is provided a process for producing a pattern on a heat- sensitive material substrate, the process comprising:

providing a substrate of a heat-sensitive material, the substrate having on at least one region thereof a patterning device defined by a plurality of intersecting wire-like elements, said elements being of another material and defining confined regions enclosed by walls of said intersecting wire-like elements (a mesh);

placing a dispersion of nanoparticles or a solution of nanoparticles' precursors in at least one confined region on said substrate of a heat- sensitive material;

allowing migration of the nanoparticles to the walls of said confined region to form a pattern having substantially the contour of said confined region; and

sintering said pattern of nanoparticles.

In some embodiments, the heat- sensitive substrate is flexible.

In some embodiments, the sintering of the pattern of nanoparticles on the heat- sensitive surface is carried out under conditions which substantially do not affect any one or more of substrate integrity (e.g., the substrate substantially remains whole and uncracked), substrate deformation (e.g., the substrate does not undergo any change in size or form), substrate decomposition (e.g., the substrate does not chemically degrade or chemically decompose) and substrate degradation (e.g., the substrate does not undergo physical or chemical cracking, breaking or etching).

In some embodiments, the "patterning device defined by a plurality of intersecting wire-like elements" is a grid-like patterning device (screen, lattice mash) covering at least a region of the heat- sensitive substrate. The grid-like patterning device may be of any shape and size. The intersecting wire-like elements defining the grid-like patterning device have a height above the top surface of the heat-sensitive material, thus forming the confined regions (voids confined by the walls of the wire-like elements and the heat-sensitive surface). The wire elements may be straight and parallel to each other, or may be randomly oriented, or may be of any shape and at any angle to a neighboring element or to the edge of the substrate, such that their intersecting forms confined regions in the shape of circles, squares, triangles, pentagons, hexagons, and others, or any combination thereof. In some embodiments, the shape of the regions is geometrically undefined.

The pattern/shape of the confined regions may be random or repetitive.

The pattern/shape of the resulting nanoparticle pattern formed by the process of the invention on the heat-sensitive substrate is thus at least similar in pattern/shape to at least part of the pattern/shape of the patterning device.

In some embodiments, the substrate is a clean plastic substrate with no material deposited on it. In some embodiments, the substrate is pre-coated with a conductive material similar to the dispersion material. In further embodiment, the substrate is coated with a different conductive material such as carbon nanotubes, graphite etc. In further embodiment, the substrate is coated with a non-conductive material.

In some embodiments, the average dimension (longest axis) or diameter of an average confined enclosure defined by walls of the wire-like elements is greater than about 0.01 μιη. In some embodiments, the average dimension is in the range of about 0.01 μιη to about 1000 μιη. In some embodiments, the average dimension is in the range of about 1 μιη to about 500 μιη. In some embodiments, the average dimension is in the range of about 1 μιη and about 300 μιη. In other embodiments, the average dimension is in the range of about 10 μιη to about 100 μιη. In other embodiments, the average dimension is below about 50 μιη. In other embodiments, the average dimension is about 50 μκι.

As stated above, the "wire-like elements" have any width, length or height, depending on the nature of the pattern to be formed on the substrate. In some embodiments, the width of each such wire-like element is greater than about 0.1 μιη. In some embodiments, the width is between about 0.1 μιη to about 300 μιη. In some embodiments, the width is between about 1 μιη to about 300 μιη. In other embodiments, the width is between about 1 to about 10 μηι. In further embodiments, the width is between about 0.1 to about 1 μιη.

In additional embodiments, the average width of a wire-like element is between about 5 to about 10 μιη.

In some embodiments, the height of a wire-like element is greater than 30 nm. In other embodiments, the height is greater than 0.1 μιη. In some embodiments, the height is between about 30 nm to about 100 μκι. In some embodiments, the height is between about 100 nm to about ΙΟμκι.

In further embodiments, the height is between about 800 nm to about 1300 nm.

The wire-like elements constituting the patterning device (grid, screen) may be comprised of any material. In some embodiments, the material from which the device is made is different from the heat-sensitive material. In further embodiments, the material is selected to decompose under etching conditions which do not affect the integrity and composition of the heat-sensitive material. In further embodiments, the material may be washed off the substrate.

In some embodiments, the material of the patterning device (e.g., grid) comprises at least one of the following materials: metallic material, insulator material, polymeric material, carbon-based material. In some embodiments, the material of the patterning device is of a metallic material or comprises a metallic material, said metallic material being selected from iron, copper, titanium, platinum, stainless steel, carbon, aluminum and nickel.

In some embodiments, the material of the patterning device is stainless steel.

In some embodiments, the patterning device is formed on the heat- sensitive material by a process of material deposition forming a pre-defined blueprint.

In other embodiments, the patterning device is a self-standing (removable, detachable, preformed) device such as a screen or grid or mesh (defined by a plurality of material voids, holes) which is placed on the heat-sensitive substrate and which may be removed therefrom prior to or subsequent to sintering.

Thus, the process comprising:

providing a substrate of a heat-sensitive material;

placing on a region of said substrate a patterning device (grid, screen, mesh); placing a dispersion of nanoparticles or a solution of nanoparticles' precursor on said substrate of a heat-sensitive material; allowing migration of the nanoparticles to the walls of said voids to form a nanoparticle pattern having substantially the contour of said voids; and sintering said pattern of nanoparticles.

In some embodiments, the patterning device is lifted from the heat- sensitive surface prior to sintering to permit further migration of the nanoparticle.

In further embodiments, prior to sintering, the process further comprises the step of permitting evaporation of the dispersion medium in which nanoparticles are contained. In some embodiments, the dispersion medium is permitted to completely dry.

In some embodiments, the patterning device is lifted or removed after the sintering period.

In some embodiments, the substrate and/or the patterning device, being a self- standing device (screen or grid) are subjected to a pre-treatment procedure, prior to placing the patterning device on a substrate. The pre-treatment may include solvent or chemical washing or physical washing (e.g., by a non-liquid medium such as a gas), etching, heating, plasma treatment (e.g., oxygen plasma), UV-ozone treatment, corona discharge, laser or microwave irradiation, flash lamp (Xenon) electroless plating, coating by a protective layer, or any combination thereof.

In some embodiments, pre-treatment is applied onto the patterning device and substrate, prior to their use. In some embodiments, the pre-treatment comprises plasma treatment. In further embodiments, the plasma treatment involves oxygen plasma.

As used herein, the self-standing patterning device may be pre-made in any shape desired, and therefore provides a manner to control and pre-determine, with a high accuracy, the final shape of the nanoparticle pattern to be formed on the heat- sensitive substrate.

The "heat-sensitive substrate" may be of a flexible or rigid material. The substrate can be of any smoothness. In most general terms, the substrate may be of a solid material such as glass, paper, a semiconductor inorganic or organic, a polymeric material or a ceramic material, or any combination thereof, which is heat sensitive, namely which at a temperature of above 100°C, 120°C, 140°C, 160°C or 180°C it undergoes at least one of decomposition, structural deformation, structural disintegration, chemical decomposition, or any other chemical or physical (structural) change. In some embodiments, the heat-sensitive substrate is stable at a temperature below 180°C. In some embodiments, the substrate is an inorganic semiconductor material. In other embodiments, the substrate is a polymeric material, such as polyamide, polyester, polyacrylate, polyolefin, polyimide, polycarbonate, polymethyl methacrylate, and polyethylene terephthalate.

In some embodiments, the substrate is polyethylene terephthalate.

Thus, in some embodiments, the process of the invention comprises:

providing a polymeric substrate of a heat-sensitive polymeric material; placing on a region of said polymeric substrate a patterning device (grid, screen);

placing a dispersion of nanoparticles or a solution of nanoparticles' precursor on said polymeric substrate in one or more of the material voids in said patterning device;

allowing migration of the nanoparticles to the walls of said voids to form a pattern having substantially the contour of said voids; and

sintering said pattern of nanoparticles.

In some embodiments, the patterning device is lifted (removed) prior to sintering of the pattern of nanoparticles. In other embodiments, the patterning device is lifted (removed) after the sintering period.

The "dispersion of nanoparticles" refers to a dispersion comprising a plurality of nanoparticles of at least one type, in a liquid medium, the medium being any liquid in which the nanoparticles may be dispersed. The medium may be aqueous or non-aqueous (organic). In some embodiments, the medium is water or a water-containing liquid mixture. In some embodiments, the medium may be an organic solvent, such as, ethanol, chloroform, hexane or any combination thereof.

Where the process involves in-situ forming of nanoparticles, the patterning device is employed for patterning nanoparticles precursors on the substrate. In such embodiments, the process of the invention requires the placing of a solution (or a medium) of nanoparticles' precursors, namely a salt solution on the heat-sensitive substrate in one or more of the material voids in said patterning device. In some embodiments, the precursor solution (interchangeable referred to also as a salt solution) may contain metallic salts, such as silver lactate, silver acetate, silver format, gold tetra chloride HAuCl₄, AgN0₃, (NH₄)₂PdCl₆, H₂PtCl₆, NiCl₂, Cu(N0₃)₂ and other metal precursors. The sintering of a salt pattern on the substrate may proceed at a low temperature, by chemical means, e.g., by exposing the salt pattern to one or more reducing agents, e.g., formic acid vapors and sodium borohydrate, or by physical means, e.g., by irradiating the salt pattern (UV, LASER, Xenon). In some embodiment, the salt pattern is reduced by chemical means.

In some embodiments, the nanoparticle dispersion or the precursor solution further comprises at least one additive selected from a stabilizing agent(s), a rheological agent, a wetting agent, a buffer, pH adjusting agent, a biocide, a sequestering agent, a chelating agent, a corrosion inhibitor, a humectant, a co-solvent, a fixative, a penetrant and a surfactant.

In some embodiments, the at least one additive is a stabilizing agent being selected from polyacrylic acid salt, polycarboxylate ethers, metal cations or any other stabilizer that assist in the sintering of the nanoparticles.

The nanoparticles employed in accordance with the present invention are of a material, e.g., a conductive material, in the form of nanoparticles, characterized by at least one dimension being in the nano-scale (i.e., between 1 and 1,000 nm or between 1 and 100 nm).

Nanoparticles include spherical nanoparticles and nanoparticles of any polyhedral or non-spherical shapes, as well as, nanotubes, nanowires, nanorods and nanoribbons. In some embodiments, the nanoparticles are selected amongst spherical nanoparticles having an averaged diameter in the nano-scale (between 1 and 100 nm).

The nanoparticle material may be selected from a metal, a transition metal, a semiconductor, an alloy, an intermetallic material, a metal oxide, a conducting polymer, a carbon based material such as carbon black, carbon nanotubes (CNT), graphite, graphene, fullerenes, carbon allotropes, indium tin oxide or any combination thereof.

In some embodiments, the material is or comprises an element of Groups IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of block d of the Periodic Table of the Elements.

In other embodiments, the material is or comprises a transition metal selected from Groups IIIB, IVB, VB, VIB, VIIB, VIIIB, IB and IIB of block d the Periodic Table. In some embodiments, the transition metal is a metal selected from Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt, Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir and Hg. In other embodiments, the material is selected amongst semiconductor materials. The semiconductor material may be selected from elements of Group II- VI, Group III- V, Group IV- VI, Group III- VI, Group IV semiconductors and combinations thereof.

In some embodiments, the semiconductor material is being selected from Si, Ge, Sn, Pb, CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, HgS, HgSe, HgTe, CdZnSe, InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb, A1P, A1N, AlAs, AlSb, CdSeTe, ZnCdSe, PbSe, PbTe, PbS, PbSnTe, Tl₂SnTes and any combination thereof.

In additional embodiments, the material is a carbon based material such as carbon black, carbon nanotubes (CNT), graphite, graphene, fullerenes, or other carbon allotropes.

In additional embodiments, the material may alternatively be a dissolved conductive polymer such as poly(3,4-dioctyloxythiophene) (PDOT), poly(3,4- ethylenedioxythiophene):poly (styrenesulfonate) (PDOT:PSS), polyaniline and polypyrrole.

In some embodiments, the nanoparticle material is or comprises Ag or Au.

In some embodiments, the nanoparticle material is or comprises Ag.

In accordance with the present invention, the dispersion of the nanoparticles or the salt solution containing nanoparticles precursors is placed on at least a region of the heat-sensitive substrate within the confined regions (material voids, holes) formed by the grid walls. The dispersion may be placed at the region by any method known in the art, including one physical or chemical methods such as immersing the substrate in a dispersion comprising said nanoparticles, placing droplets of said dispersion of nanoparticles at the desired region(s), printing, jet printing, differential roll printing, contact printing, coating, spin coating, or any combination thereof.

In some embodiments, the dispersion or solution may be placed several times in different areas to connect between different patterns or on the same area in which a conductive pattern was already formed, in order to increase surface coverage by the conductive lines.

Once the dispersion is placed on the heat-sensitive surface, it is permitted to flow outwards in the direction of the grid walls to substantially follow the contour of the confined regions. Without wishing to be bound by theory, the migration of the nanoparticles occurs by a self-assembly mechanism in the absence of any one of heat and pressure. In some embodiments, the medium is dried either fully or partially. In other embodiments, the dispersion medium is volatile, permitting efficient evaporation under ambient conditions (at room temperature and under atmospheric pressure or under a reduced pressure).

In further embodiments, the migration of the nanoparticles may be assisted by employing an external driving force such as vacuum, heating, and humidity.

In order to achieve a continuous (or fused) or quasi-continuous pattern, which enables, e.g., conductivity and improves other properties of the pattern, the pattern of nanoparticles obtained subsequence to nanoparticle migration is subjected to a sintering process which does not substantially affect substrate integrity (e.g., the substrate substantially remains whole and uncracked), substrate deformation (e.g., the substrate does not undergo any change in size or form), substrate decomposition (e.g., the substrate does not chemically degrade or chemically decompose) and substrate degradation (e.g., the substrate does not undergo physical or chemical cracking, breaking or etching). In some embodiments, the sintering can be a chemical sintering and is performed directly after the nanoparticles have fully migrated to the walls of the confined regions.

In some embodiment, the sintering process involves irradiation by e.g., UV, LASER, Xenon, plasma etc.

In some embodiments, the chemical sintering is performed directly after the nanoparticles have fully migrated to the walls of the confined regions.

In accordance with the present invention, the chemical sintering involves exposure of the nanoparticles pattern (before or after removal of the structured element: grid, screen) to at least one chemical sintering agent. In some embodiments, the chemical sintering agent is an acid.

In other embodiments, the chemical sintering agent is selected amongst materials capable of causing the nanoparticles to come into close contact and sinter together. The sintering agent is selected to cause at least one of: (i) irreversible coalescence of the closely located NPs due to neutralization of the charges at the NPs surface, (ii) screening charges at the NPs surface, (iii) desorption of the dispersing agent, or (iv) any other mechanism which enables coagulation and coalescence (sintering) of the nanoparticles that enables formation of conductive lines

In some embodiments, the sintering agents are selected amongst salts, e.g., agents containing chlorides such as KC1, NaCl, MgCl₂, AICI₃, LiCl, CaC^; agents containing bromide or iodide such as NaBr, HBr, Nal; charged polymers; polycations, e.g., poly(diallyldimethylammonium chloride) (PDAC); polypyrroles; polyanions; polyacrylic acid (PAA), polyethyleneimine, carboxymethyl cellulose (CMC), polynaphthalene sulfonate/formaldehyde poly(y-glutamic acid); acids, e.g., HC1, H₂SO₄, HNO₃, H₃PO₄, acetic acid, acrylic acid, formic acid ; and bases, e.g., ammonia, e.g. aminomethyl propanol (AMP), NaOH and KOH.

In some embodiments, the molar concentration of the sintering agent is between about 0.1 to 500 mM of the formulation.

In some embodiments, the chemical sintering agent is applied to the pattern in the form of vapors.

In some embodiments, the chemical sintering agent is applied to the pattern in the form of a solution, such as sodium chloride solution, or a solution of poly dialyldimethyl ammonium chloride.

In some embodiments, sintering is carried out at room temperature (23-37°C). In other embodiments, sintering is carried out at a temperature above room temperature. In further embodiments, sintering is carried out at a temperature higher than about 50°C, higher than about 75°C, or higher than about 100°C. In some embodiments, sintering is carried out at a temperature lower than about 150°C.

In some embodiments, sintering is carried out at a temperature between room temperature and about 150°C. In further embodiments, sintering is carried out at a temperature between about 100°C and about 150°C.

Following sintering, the nanoparticle pattern may be peeled off the substrate, thus affording a grid, or mesh.

In some embodiments, the invention provides a process for producing a nanoparticle pattern with controllable height and width on a plastic substrate, the process comprising:

providing a plastic (e.g., PET) substrate;

placing on the substrate a patterning device covering at least a region thereof, said patterning device having square-like voids being between about 10 and about 100 μιη in size;

placing a dispersion of metal nanoparticles or a solution of nanoparticles' precursor within said voids; allowing migration of the metal nanoparticles or precursor solution to form a pattern having substantially the counter of said voids;

chemically sintering said nanoparticle pattern by exposing said nanoparticles to HC1 vapors or said precursor solution to a reducing agent, at room temperature.

The resulting sintered nanoparticle patterns obtained on a substrate according with the present invention have a high ratio of width-to-height, and are in particular transparent and conductive. The resulting sintered nanoparticle patterns of the invention are useful in devices that require transmission of visible, NIR, IR, and/or UV regions of the electromagnetic spectrum.

In some embodiments, the resulting sintered nanoparticle patterns (grid) of the invention are composed of wires having an averaged width of between about 0.1 and about 30 μιη and the height being in the range of between about 50 nm to about 30 μιη. The dimensions are controlled by the metal dispersion properties such as surface tension, viscosity, fraction of dispersed nanoparticles and evaporation rate.

The invention also contemplates a flexible conductive pattern, said pattern being composed of a plurality of wire-like elements of sintered nanoparticles, said elements having one or more of the following:

(1) averaged width of between about 0.1 μιη to about 300 μιη;

(2) averaged height of between about 50 nm to about 30 μιη.

In some embodiments, the elements have an averaged width of between about 5 to about 10 μκι. In other embodiments, the elements have an average height between about 100 nm to about 10 μιη.

As stated herein, the pattern may be on a substrate, e.g., a heat-sensitive substrate.

In some embodiments, the pattern is bendable to a bending degree above 20° (external angle relative to the plane surface). In further embodiments, the bending degree (external angle relative to the plane surface) is between 20° and 160°.

The invention also provides a conductive transparent pattern obtainable (or obtained) by a process according to the invention.

Also provided is a conductive pattern obtainable by the process of the invention or which is composed of a plurality of wire-like elements of sintered nanoparticles, said elements having one or more of the following: (1) averaged width of between 0.1 μηι to 300 μηι;

(2) averaged height of between 50 nm to 30 μηι.

In some embodiments, the elements have an averaged width of between about 5 to 10 μκι. In further embodiments, the elements have an averaged height between 100 nm to 10 μιη.

Light transparency of patterns of the invention is of at least 50%, in some embodiments at least 70%, in other embodiments at least 80%, in other embodiments at least 90% and in further embodiments of about 95% and more light transparency. The transmission is of visible light, being measured in the wavelength range of 400nm to 700nm. In some embodiments, the transmission is of the NIR and/or IR light.

In some embodiments, the patterns of the invention have a high conductivity and low sheet resistance. In some embodiments, the sheet resistance may be below 10 Ohm/square. In some embodiments, the sheet resistance may be below 1 Ohm/square. In some embodiments, the sheet resistance may be below 100 Ohm/square. In some embodiments, the sheet resistance may be within the range of 0.1 Ohm/square to 100 Ohm/square.

In some embodiments, the pattern according to the invention is characterized by a transparency above 70% and a sheet resistance of between 0.1 Ohm/square to 50 Ohm/square.

In some embodiments, the pattern according to the invention is characterized by a transparency of about 75% and a sheet resistance of about 10 Ohm/square.

In another aspect the invention also provides a device implementing a pattern of the invention.

In the some embodiments, the substrate on which the pattern is provided may be an integral part of the device or may be a substrate which is implemented in the device.

The device may be an electronic device or an optoelectronic device.

The patterns of the invention may be integrated in devices, that requires transmittance of visible, UV, IR, and/or NIR regions of the electromagnetic spectrum, including for example, photoconductors, photodiodes; solar cells; light emitting diodes (LEDs), including organic light emitting diodes and lasers; light sensors, as well as specialized transistors, including organic transistors, inorganic transistors, or hybrid transistors. Other applications for utilization of such coatings are related to the following categories: printed electronics, touch screens, display backplanes and large or small area flexible applications. Flexible applications further include large area arrays, flexible displays, and e-paper (electronic books, journals, newspapers).

In some embodiments, the pattern of the invention obtained by the process of the invention is used as the transparent component in the electronic/optoelectronic device, such as, solar cells, electromagnetic shielding (EMS).

In some embodiments, transparent patterns of the invention or produced according to the processes of the present invention may be integrated as in silicon, CdTe, CIGS (cupper Indium Galium Sellenide), T1O₂, organic solar cells, dye- sensitized solar cells (DSSC), and other types of solar cells.

In some embodiments, the pattern of the invention or obtained by the process of the invention is a transparent and conductive electrode.

The transparent conductive electrode composed of the patterns of the present invention or obtained by the process of the invention comprises wires which have thin lines width concomitant with a broad height which enables achievement of high transmittance of light with low resistivity. Lower resistivity can lead to lower power losses, for example, in conversion of light to electrical power in solar cells.

Further, transparent electrodes comprising the pattern of the invention or obtained by the process of the invention tend to have geometries which improve conductance, owing to the fact that the lines can be close to each other as desired, limited only by the pre-made patterning device. The dense thin lines are a great advantage, for example, in the case of solar cells, due to less distantly spaced grid lines, photo generated charge carriers should travel shorter distances through lower resistivity regions (and with less chance of carrier recombination) leading to less power loss.

Furthermore, the geometry of the pattern of the invention is predetermined and precisely controlled, which enables a high accuracy in engineering and design of the device required. This may lead to a new emerged devices as well as more reliable devices. As this method of the invention easily controls the pattern dimension, versatile end products with different designs and functions can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the disclosure and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Figs. 1A-D depict HR-SEM images of the transparent grid composed of silver NP (formed using 8% wt silver nanoparticle dispersion). Fig. 1A- 2x2 mm section of the sintered silver grid; Figs. IB, 1C and ID show one line of the grid at various magnifications.

Figs. 2A-D depict HR-SEM images of the grid lines before and after sintering, at different magnifications.

Figs. 3A-B are a grid profile of Ag sintered grid lines. Fig. 3A shows the height of a series of sintered grid lines; Fig. 3B shows the height of a single sintered line.

Figs. 4A-B are Fig. 4A- a plot of average height of lines as a function of metal concentration; Fig. 4B- a plot of lines average width as a function of metal concentration.

Figs. 5A-D demonstrate a device according to the present invention. Figs. 5A- C- pictures of an electroluminescent device with a bias of 110V at different magnifications; Fig. 5D- demonstration of the flexibility and transparency of the conductive silver grid.

DETAILED DESCRIPTION OF EMBODIMENTS

Experimental section

The synthesis of the silver NP dispersion (42% wt) was performed as described by Magdassi et al. [11], yielding nanoparticles which were stabilized by polyacrylic acid sodium salt (MW 8kD), having an average size of 14+3 nm and zeta potential of -42 mv. Dispersions with various concentrations of the nanoparticles were made by diluting the concentrated dispersion with triple distilled water (TDW, 0.55 μ8Λ;ιη). The following properties of the dispersion were controlled to achieve optimal performance: liquid composition, metal load, surface tension, viscosity, etc. The surface tension may be in the range of 17-72 dyne/cm, and the viscosity up to 500 cPs. The metal dispersion may be prepared by a variety of methods.

Fabrication of the transparent grid was performed as described by Higashitani et al. [9] on glass, but the whole process was conducted directly on the plastic substrate, without any transfer and heat sintering processes and without using any UV crosslinkable monomers on the plastic substrate. Briefly, a stainless steel mesh was placed on a PET substrate, followed by addition of a 35 microliter droplet of a silver dispersion with various silver contents. The liquid immediately wets the mesh, and after 5 minutes the stainless steel grid was removed, leaving a grid pattern on the plastic substrate. The PET substrate (Jolybar, Israel) and the stainless steel 420 mesh (Emmtech, Israel) were treated by plasma prior to their use (in 20% oxygen for 2 minutes, PICO system, deiner electronics). The diameter of the mesh wires was 25 micrometers. Chemical sintering of the grids was performed at room temperature by exposing the PET with the Ag grid pattern to HC1 vapors for 30 seconds. This was achieved by putting it in a 10 mL glass Petri dish, which was placed in a larger Petri dish (100 mL) that contained 10 mL of cone. HC1 (37% wt, Aldrich). The sintering could also be achieved by contact with a solution of sintering agent such as NaCl.

To demonstrate the utilization of the plastic flexible conductive transparent grid as a replacement of ITO transparent electrode, a four-layer (PET: grid pattern: ZnS: BaTiOs) electroluminescent device was fabricated as follows: Ag grid patterns were fabricated on PET and was followed by chemical sintering, as described above. On top of the conductive grid, a layer of ZnS and colorant particles (MOBIChem Scientific Engineering, Israel) was coated by Drawdown with K-coater; after drying at 60°C, this was coated with a double layer of BaTitante (MOBIChem Scientific Engineering, Israel). On top of the BaTitanat, the non-transparent pattern of the device was achieved by inkjet printing of the silver dispersion (20% wt Ag) containing 10% wt PG and 0.05% of BYK 348 as wetting agent. The printing was performed by an Omnijet printer (Unijet, Korea) with 30 pL (Samsung) printheads, at 1000 Hz. The printed pattern was then sintered under HC1 vapors, as described above.

The cross-sectional profiles of the lines were measured using a Veeco® Dektak 150+ Surface Profiler. The surface-tension measurements were carried out by a pendant drop tensiometer (First-Ten- Angstrom 32).

The grid patterns were imaged using an optical microscope and an HR-SEM microscope (Philips, Sirion HR-SEM).

The transparency and electric resistivity of the networks were measured at different points using a spectrometer (UV) and a four-pin probe surface resistivity meter, respectively VARIAN carry 100 bio and Cascade Microtech Inc.

Results and Discussion

Fig. 1 shows HR-SEM images of the chemically sintered grid and line formed by using 8% wt silver dispersion. It can be seen that the grid is composed of closely packed silver nanoparticles, which assemble in the form of a cone in each line of the grid. This structure is probably due to the capillary forces that cause the upward movement of the dispersion at the gap between the metal wires and the plastic substrate. The effect of the exposure to HC1 vapors is presented in Fig. 2; the cone form is not affected by the exposure to the vapors, but there is a significant increase in silver particle size and formation of percolation paths due to the sintering of the nanoparticles. The sintering process led to the formation of a conductive array having a low sheet resistance of 9±0.8O/cm².

Fig. 3 shows a typical height profile of the sintered grid, in which the average height was 1123+163 nm, and the average width at the base of the cone is 5.7+1.3 μιη. These very narrow lines led to high transparency, 77+5%, as demonstrated in Fig. 5D.

It should be noted that the sheet -resistance and transparency did not change after storage of the sintered grid for at least 2 months at room temperature. Also, the transparency values did not change before and after exposure to the vapors.

Interestingly, the average height of the silver lines in the system of the invention was about 10 times higher than that obtained by Higashitani et al. [9] by using gold nanoparticles of a similar size on a glass substrate. The main differences between the two systems (besides having gold and silver) are the different substrates (glass and PET) and the much larger concentration of nanoparticles in the system of the invention (8% wt compared to 2% wt). Since the PET substrate was treated by plasma and the contact angle of water was similar to that of the glass (below 5°), the dominant parameter was the concentration of the particles in the dispersion. Indeed, while evaluating the dependence of line height on the concentration of the silver in the dispersion (Fig. 4A), it was discovered that the larger the concentration of silver the greater the line height, so by the present invention the height of the conductive pattern can be controlled.

This finding is important, since achieving high aspect ratio of the conductive lines is advantageous in a variety of applications, such as solar cells [12] and OLED devices. However, as shown in Fig. 4A, above 10% wt silver this dependency levels off. In addition, the line width also increases with the increase in metal load of the dispersion; at 20% wt silver the line thickness is about 25 microns, which causes the transparency of the grid to decrease to about 40%. By comparing Fig. 4A and Fig. 4B, it seems that the height is strongly dependent on metal load at the low concentrations, while the width is strongly dependent on metal load at the high concentrations. It should be mentioned that at low metal load (0.5% wt) the silver NP formed grids; however they were composed of separated lines rather than a continuous grid. This problem of disconnected lines occurred up to 5% wt metal load, so conductive grids can be obtained by this method only at higher metal load. From an application point of view, these different effects of the metal load in the dispersion would require a trade off between conductivity and transparency through the control of dispersion properties.

The applicability of the transparent conductive grid is demonstrated in a plastic EL device (Fig. 5), for which all transparent electrode preparation steps were conducted at room temperature. Fig. 5A shows the HU symbol, observed by applying 110 volts between the transparent grid and the inkjet printed pattern. It should be noted that the whole fabrication process took less than 30 minutes, largely due to use of the rapid sintering technique.

It should be noted that qualitative bending experiments show that the electroluminescence remained functional even after bending the substrate to angles below about 20° (namely to nearly 160° measured from the plane), showing that the arrays may be suitable for applications in which flexibility is required.

Adhesion of the grid was tested by a tape test and by immersing the grid in water. It was found that the tape test caused failure of some parts of the grid, and if the samples were immersed in water for a prolonged time the patterns start to dissolve. However, if the grids are heated at 130°C for ten minutes, the durability of the patterns improved significantly in both tape and immersion tests. As known in plastic electronics, such heat post-treatment is acceptable for many plastic substrates.

In conclusion, a simple process is demonstrated herein for fabricating conductive and transparent electrodes on plastic substrates. The process is based on direct patterning of silver nanoparticles on a plastic substrate, while the sintering process is conducted at room temperature. The sintering is achieved rapidly at room temperature by using nanoparticles stabilized by polyacrylic acid salt, upon contact with HC1 vapors. This makes the process suitable for a variety of conductive nanomaterials that can undergo sintering at low temperatures and for a variety of heat sensitive plastic substrates and open new applications based on low cost substrates. In particular, since the process is very suitable for flexible substrates, it can be easily performed in large scale production processes such as roll-to-roll processes.

Claims

CLAIMS:

1. A process for producing a conductive transparent pattern on a heat-sensitive material substrate, the process comprising:

providing a substrate of a heat-sensitive material, the substrate having on at least one region thereof a patterning device defined by a plurality of intersecting wire-like elements, said elements being of another material and defining confined regions enclosed by walls of said intersecting wire-like elements;

placing a dispersion of nanoparticles or a solution of nanoparticles' precursor in a confined region on said substrate of a heat-sensitive material; allowing migration of the nanoparticles to the walls of said confined region to form a pattern having substantially the contour of said confined region; and

sintering said pattern of nanoparticles under conditions maintaining at least one of substrate integrity, substrate deformation, substrate decomposition and substrate degradation.

2. The process according to claim 1, wherein the heat-sensitive substrate is flexible.

3. The process according to claim 1, wherein the patterning device is a grid-like patterning device.

4. The process according to claim 1, wherein the wire-like elements are straight and parallel to each other, or randomly oriented.

5. The process according to claim 1, wherein the confined regions enclosed by walls of said intersecting wire-like elements have a shape selected from circles, squares, triangles, pentagons, hexagons, or any combination thereof, or an undefined shape.

6. The process according to claim 1, wherein the confined regions are random or repetitive.

7. The process according to claim 1, wherein the substrate is pre-coated with one or more material selected from a conductive material similar to the dispersion material, a different conductive material, a non-conductive material.

8. The process according to claim 1, wherein the confined regions have average dimension in the range of 0.01 μιη to 1000 μιη.

9. The process according to claim 8, wherein the confined regions have average dimension in the range of 1 μιη to 500 μιη.

10. The process according to claim 8, wherein the confined regions have average dimension in the range of 1 μιη and 300 μιη.

11. The process according to claim 8, wherein the confined regions have average dimension in the range of 10 μιη to about 100 μιη.

12. The process according to claim 8, wherein the confined regions have average dimension is 50 μιη.

13. The process according to claim 1, wherein the wire-like elements have a width between 0.1 μκι to 300 μιη.

14. The process according to claim 13, wherein the wire-like elements have a width between 1 μιη to 300 μιη. In other embodiments.

15. The process according to claim 13, wherein the wire-like elements have a width between 1 to 10 μκι.

16. The process according to claim 13, wherein the wire-like elements have a width between 0.1 to 1 μιη.

17. The process according to claim 13, wherein the wire-like elements have an average width between 5 to 10 μιη.

18. The process according to claim 1, wherein the wire-like elements height is between 800 nm to 1300 nm.

19. The process according to claim 1, wherein the wire-like elements height is between 30 nm to ΙΟΟμκι.

20. The process according to claim 1, wherein the wire-like elements height is between 100 nm to 10 μιη.

21. The process according to claim 1 , wherein the patterning device is of a material different from the heat-sensitive material.

22. The process according to claim 1, wherein the patterning device is of at least one material selected from a metal, an insulator, a polymeric material and a carbon-based material.

23. The process according to claim 22, wherein the material is a metal or comprises a metal.

24. The process according to claim 23, wherein the material is a metal selected from iron, copper, titanium, platinum, stainless steel, carbon, aluminum and nickel.

25. The process according to claim 24, wherein the material is stainless steel.

26. The process according to claim 1, wherein the patterning device is formed on the heat-sensitive material by material deposition.

27. The process according to claim 1, wherein the patterning device is a self- standing device which is placed on the heat-sensitive substrate.

28. The process according to claim 1, comprising:

providing a substrate of a heat-sensitive material;

placing on a region of said substrate a patterning device; placing a dispersion of nanoparticles or a solution of nanoparticles' precursor on said substrate of a heat-sensitive material in one or more of the material voids in said patterning device;

allowing migration of the nanoparticles to the walls of said voids to form a nanoparticle pattern having substantially the contour of said voids; and

sintering said pattern of nanoparticles.

29. The process according to claim 28, wherein the patterning device is lifted from the heat-sensitive surface prior to sintering.

30. The process according to claim 29, wherein prior to sintering, the process further comprises the step of permitting evaporation of the dispersion medium in which nanoparticles are contained.

31. The process according to claim 30, wherein dispersion medium is permitted to completely dry.

32. The process according to claim 31, wherein the substrate and/or the patterning device, being a self-standing device, are subjected to a pre-treatment procedure, prior to placing the patterning device on a substrate.

33. The process according to claim 32, wherein pre-treatment including solvent or chemical washing or physical washing, etching, heating, plasma treatment, UV-ozone treatment, corona discharge, laser or microwave irradiation, flash lamp (Xenon) electroless plating, coating by a protective layer, or any combination thereof.

34. The process according to claim 33, wherein pre-treatment is applied onto the patterning device and substrate, prior to use.

35. The process according to claim 33, wherein pre-treatment comprises plasma treatment.

36. The process according to claim 35, wherein plasma treatment involves oxygen plasma.

37. The process according to claim 1, wherein the substrate is of a flexible or rigid material.

38. The process according to claim 1, wherein the substrate is smooth or rough.

39. The process according to claim 1, wherein the substrate is a solid material.

40. The process according to claim 1, wherein the material is selected from paper, a semiconductor inorganic or organic, a polymeric material or a ceramic material, or any combination thereof, which is heat sensitive substrate.

41. The process according to claim 40, wherein the substrate is an inorganic semiconductor material.

42. The process according to claim 40, wherein the substrate is selected from a polymeric material.

43. The process according to claim 42, wherein said polymeric material is selected from polyamide, polyester, polyacrylate, polyolefin, polyimide, polycarbonate, polymethyl methacrylate, and polyethylene terephthalate.

44. The process according to claim 43, wherein the substrate is polyethylene terephthalate.

45. The process according to claim 1, comprising:

providing a polymeric substrate of a heat- sensitive polymeric material; placing on a region of said polymeric substrate a patterning device; placing a dispersion of nanoparticles or a solution of nanoparticles' precursor on said polymeric substrate in one or more of the material voids in said patterning device;

sintering said pattern of nanoparticles.

46. The process according to claim 45, wherein the patterning device is lifted (removed) prior to sintering of the pattern of nanoparticles.

47. The process according to claim 45, wherein the patterning device is lifted (removed) after the sintering period.

48. The process according to claim 1, wherein the dispersion of nanoparticles or a solution of nanoparticles' precursors being aqueous or non-aqueous.

49. The process according to claim 48, wherein the medium is water or a water- containing liquid mixture.

50. The process according to claim 48, wherein the medium is an organic solvent, selected from ethanol, chloroform, hexane, or any combination thereof.

51. The process according to claim 1, wherein the solution of nanoparticles' precursors contains metallic salts.

52. The process according to claim 51, wherein the metallic salts are selected from silver lactate, silver acetate, silver formate, gold tetra chloride HAuCU, AgN0_3, (NH₄)₂PdCl₆, H₂PtCl₆, NiCl₂, Cu(N0₃)₂, or any combination thereof.

53. The process according to claim 52, wherein the solution of nanoparticles' precursors is sintered by chemical means or by physical means.

54. The process according to claim 53, wherein the chemical means involving exposing the salt pattern to one or more reducing agents.

55. The process according to claim 54, wherein the reducing agent is formic acid vapors or sodium borohydrate.

56. The process according to claim 53, wherein the physical means involving irradiating the salt pattern.

57. A flexible transparent conductive pattern, said pattern being composed of a plurality of wire-like elements of sintered nanoparticles, said elements having one or more of the following:

(3) averaged width of between 0.1 μιη to 300 μιη;

(4) averaged height of between 50 nm to 30 μιη.

58. The pattern according to claim 57, wherein the elements having an averaged width of between about 5 to 10 μιη.

59. The pattern according to claim 57, wherein the elements having an average height between 100 nm to 10 μιη.

60. The pattern according to claim 57, being on a substrate.

61. The pattern according to claim 57, wherein said substrate is heat-sensitive.

62. The pattern according to claim 60, being bendable to a bending degree above 20°.

63. A conductive pattern obtainable by the process of any one of claims 1 to 56.

64. A conductive pattern obtainable by the process of any one of claims 1 to 56, said pattern being composed of a plurality of wire-like elements of sintered nanoparticles, said elements having one or more of the following:

(1) averaged width of between 0.1 μιη to 300 μιη;

(2) averaged height of between 50 nm to 30 μιη.

65. The pattern according to claim 64, wherein the elements having an averaged width of between about 5 to 10 μιη.

66. The pattern according to claim 64, wherein the elements having an averaged between 100 nm to 10 μιη.

67. A device comprising at least one patterned region, said pattern being obtainable by the process of any one of claims 1 to 56 or being a pattern of any one of claims 57- 62.

68. A process according to claim 1, wherein sintering of the nanoparticles is achievable by radiation.

69. The process accordion to any one of claims 1 to 56 or the pattern according to any one of claims 57 to 62, wherein the sintered pattern on said heat-sensitive substrate having a light transparency of 50%.

70. The process or the pattern according to claim 69, wherein the sintered pattern on said heat-sensitive substrate having a light transparency of 70%.

71. The process or the pattern according to claim 69, wherein the sintered pattern on said heat-sensitive substrate having a light transparency of 80%.

72. The process or the pattern according to claim 69, wherein the sintered pattern on said heat-sensitive substrate having a light transparency of 90%.

73. The process or the pattern according to claim 69, wherein the sintered pattern on said heat-sensitive substrate having a light transparency of 95%.

74. The process accordion to any one of claims 1 to 56 or the pattern according to any one of claims 57 to 62, wherein the sintered pattern on said heat-sensitive substrate having a sheet resistance of below 10 Ohm/square.

75. The process or the pattern according to claim 74, wherein the sheet resistance being below 1 Ohm/square.

76. The process or the pattern according to claim 74, wherein the sheet resistance being below 100 Ohm/square.

77. The process or the pattern according to claim 74, wherein the sheet resistance being within the range of 0.1 Ohm/square to 100 Ohm/square.

78. The process accordion to any one of claims 1 to 56 or the pattern according to any one of claims 57 to 62, wherein the pattern is characterized by a transparency above 70% and a sheet resistance of between 0.1 Ohm/square to 50 Ohm/square.

79. The process accordion to any one of claims 1 to 56 or the pattern according to any one of claims 57 to 62, wherein the pattern according to the invention is characterized by a transparency of about 75% and a sheet resistance of about 10 Ohm/square.