WO2018019813A1 - Transparent electroconductive layer and ink for production thereof - Google Patents

Abstract

Described is a composition (ink) comprising metal nanoobjects for preparing a transparent electroconductive layer.

Description

Transparent electroconductive layer and ink for production thereof

The present invention relates to compositions (also referred to as inks) comprising metal nanoobjects, especially metal nanowires, to a process for preparing a layer comprising metal nanoobjects (transparent electroconductive layer) on a surface of a substrate, and to articles comprising a substrate having a surface and, arranged on said surface of said substrate, a transparent electroconductive layer.

Metal nanoobjects, especially metal nanowires like silver nanowires are commonly used for preparing transparent electroconductive layers. The term "transparent electroconductive layer" as used herein refers to a layer which (i) is capable of allowing the flow of an electric current when an appropriate voltage is applied and (ii) has a light transmission of 80 % or more in the visible region (400-700 nm) measured according to ASTM D1003, see e.g. US 8,049,333. Usually, said layer is arranged on the surface of a substrate, wherein said substrate is typically an electrical insulator. Such electroconductive transparent layers are widely used in flat liquid crystal displays, touch panels, electroluminescent devices, organic light emitting diodes, thin film photovoltaic cells, as anti-static layers and as electromagnetic wave shielding layers.

Unfortunately, metal nanoobjects like silver nanowires are sensitive to oxidation, due to the substantially increased specific surface area, compared to their corresponding bulk metals. Oxidation of the metal nanoobjects usually results in a remarkable degradation of the electroconductivity as well as of the optical properties of electroconductive transparent layers comprising such metal nanoobjects.

During use transparent electroconductive layers may be exposed to oxidative stress from the environment, and high current densities as well as electrostatic discharges, which may occur during use of transparent electroconductive layers, may lead to thermal breakup and aggregation and/or oxidation of the metal nanoobjects, due to the released heat.

Accordingly, there is a need to increase the stability of metal nanoobjects in transparent electroconductive layers against thermal and oxidative stress.

In order to solve these problems, US 2014/0020737 A1 proposes a device comprising a substrate and silver nanowires disposed on the substrate, and an oxidation protection layer coated on the silver nanowires, wherein the oxidation protection layer comprises an oxide, and a method, wherein applying of an oxidation protection layer on a surface of the silver nanowires is performed by an atomic layer deposition (ALD) process.

According to US 2014/0020737 A1 , a large part of the surface of each individual nan- owire appears to be coated by the protection layer, so that the number of junctions (mutual contacts) enabling the flow of electrons between adjacent and overlapping electroconductive nanoobjects may be significantly reduced, compared to transparent conductive films wherein the metal nanowires are uncoated. Accordingly the electron conductivity of the transparent conductive layer might be reduced, especially in case the oxides forming the protection layer have a low electronic conductivity. In this regard it is noted that according to US 2014/0020737 A1 the list of suitable oxides for the coating includes oxides having poor electronic conductivity like silica and alumina.

According to the method disclosed in US 2014/0020737 A1 , applying the silver nanowires to the surface of the substrate and applying the protective coating to the silver nanowires is carried out in separate steps, i.e. either the silver nanowires are first deposited on a surface of a substrate and are then coated by the oxidation protection layer, or the oxidation protection layer is first formed on the silver nanowires, and the coated silver nanowires are then deposited on a surface of a substrate. The need to carry out these two distinct steps adds complexity to preparation of transparent electroconductive layers. Related art is also US 201 1/163403 A1 , US 2013/126796 A1 , US 2009/233086 A1 and W0 2016/023887 A1. It is an object of the present invention to provide a composition (ink) for preparing transparent electroconductive layers comprising metal nanoobjects, which are substantially stable against oxidation, thereby avoiding degradation of the electroconductivity as well as of the optical properties of an electroconductive transparent layer.

According to a first aspect of the present invention, there is provided a composition comprising

(A) a carrier liquid having a boiling point of less than 120 °C

(B) metal nanoobjects having two external dimensions in the range of from 1 nm to 100 nm and a third external dimension in the range of from 1 μιη to 100 μιη

(C) metal oxide nanoobjects wherein said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn, Ag, Ce, mixed oxides of In and Sn and mixed oxides of Sb and Sn

(D) one or more metal salts of formula (I)

(M^a+)_z(R^b )_y (I),

wherein

M^a+ represents a metal cation,

R^b" represents the salt anion,

a is 2, 3, 4 or 5,

b is 1 , 2 or 3,

z is the least common multiple of a and b, divided by a

y is the least common multiple of a and b, divided by b.

The above defined composition is suitable for preparing a transparent electroconductive layer on the surface of a substrate. A composition for preparing a transparent electroconductive layer on the surface of a substrate is commonly referred to as an ink.

Surprisingly, it has been found the composition according to the present invention enables obtaining transparent electroconductive layers wherein the metal nanoobjects are substantially stable against oxidation, thereby avoiding degradation of the electroconductivity as well as of the optical properties of electroconductive transparent layer. Furthermore, preparation of transparent electroconductive layers is facilitated by using said ink, because there is no need to apply a protective coating in a separate step, as it is the case in US 2014/0020737 A1 .

The above-defined composition is in the form of a suspension. Preparation of suspensions is known in the art. The term "suspension" denotes a dispersion comprising a continuous phase (in the literature sometimes referred to as an external phase e.p.) that is liquid (herein referred to as the carrier liquid) and at least one dispersed phase (in the literature sometimes referred to as an internal phase i.p.) that is solid and does not dissolve in said continuous phase which is liquid. In the suspension which forms the composition according to the present invention, constituents (B) and (C) form dispersed phases, which are dispersed within carrier liquid (A).

The constituents of the composition and their functions within said composition and within a transparent electroconductive layer obtained from said composition are described in further detail below.

A composition according to the invention comprises a carrier liquid (A). The carrier liquid (A) is merely a vehicle for wet processing and does not remain in the transparent electroconductive layer to be formed from the above-defined composition. Preferably said carrier liquid is selected from the group consisting of water, methanol, ethanol, 1-propanol, 2-propanol, 2-butanol, iso-butanol, acetonitrile, propionitrile and tetrahydrofurane (THF) and mixtures thereof. Preferably, in a composition according to the present invention, the weight fraction of carrier liquid (A) is in the range of from 50 wt.-% to 99.8 wt.-%, preferably 70 wt.-% to 99 wt.-%, most preferably 75 wt.-% to 90 wt.-%, in each case based on the total amount of constituents (A), (B), (C) and (D).

A composition according to the invention comprises metal nanoobjects (B) having two external dimensions in the range of from 1 nm to 100 nm and their third external dimension in the range of from 1 μιη to 100 μιη, in each case determined by transmission electron microscopy.

According to ISO/TS 27687:2008 (as published in 2008), the term "nanoobject" refers to an object having one, two or three external dimensions in the nanoscale, i.e. in the size range from approximately 1 nm to 100 nm. The electroconductive nanoobjects to be used for the present invention are electroconductive nanoobjects having two external dimen- sions in the range of from 1 nm to 100 nm and their third external dimension in the range of from 1 μιη to 100 μιη. Typically, said two external dimensions which are in the range of from 1 nm to 100 nm are similar i.e. they differ in size by less than three times. The third dimension of said electroconductive nanoobjects is significantly larger, i.e. it differs from the other two external dimensions by more than three times.

According to ISO/TS 27687:2008, nanoobjects having two similar external dimensions in the nanoscale, while the third external dimension is significantly larger, are generally referred to as nanofibers. Electrically conductive nanofibers are also referred to as nan- owires. Hollow nanofibers (irrespective of their electrical conductivity) are also referred to as nanotubes. Sometimes nanoobjects having two similar external dimensions in the nanoscale, while the third external dimension is significantly larger, are also referred to as naorods.

Metal nanoobjects (B) as defined above which are to be used for the present invention typically have a cross section close to circular shape. Said cross section extends perpen- dicularly to said external dimension which is in the range of from 1 μιη to 100 μιη. Thus, said two external dimensions which are in the nanoscale are defined by the diameter of said circular cross section. Said third external dimension extending perpendicularly to said diameter is referred to as the length.

Preferably, the metal nanoobjects (B) have a length in the range of from 1 μιη to 100 μιη, preferably of from 3 μιη to 50 μιη, more preferably of from 10 μιη to 50 μιη, and a diameter in the range of from 1 nm to 100 nm, preferably of from 2 nm to 50 nm, more preferably of from 15 nm to 30 nm, length and diameter in each case being determined by transmission electron microscopy.

The term "metal nanoobject" means that the nanoobject comprises or consists of one or more materials selected from the group consisting of metals and alloys of metals. As metals are capable of allowing the flow of electrons, a plurality of such metal nanoobjects deposited on a surface of a substrate may form a conductive network of adjacent and overlapping nanoobjects capable of carrying an electric current, provided that there is sufficient interconnection (mutual contact) between individual metal nanoobjects so as to enable the transport of electrons along the interconnected metal nanoobjects within the network. Preferably, said metal nanoobjects consist of materials selected from the group consisting of silver, copper, gold, platinum, tungsten, aluminum, iron, cobalt and nickel and alloys of two or more metals selected from the group consisting of silver, copper, gold, platinum, tungsten, aluminum, iron, cobalt and nickel. Preferably, said metal nanoobjects are selected from the group consisting of nanowires and nanotubes. Preferred nanowires comprise or consist of one or more metals selected from the group consisting of silver, copper and gold.

Among nanowires and nanotubes, nanowires are preferred.

Preferably, said metal nanoobjects are nanowires consisting of a metal selected from the group consisting of silver, copper, gold, platinum, tungsten, aluminum, iron, cobalt and nickel, or of an alloy of two or more metals selected from silver, copper, gold, platinum, tungsten, aluminum, iron, cobalt and nickel, wherein said nanowires preferably have a length in the range of from 1 μιη to 100 μιη, preferably of from 3 μιη to 50 μιη, more preferably of from 10 μιη to 50 μιη, and a diameter in the range of from 1 nm to 100 nm, preferably 10 nm to 50 nm, most preferably 15 nm to 30 nm, length and diameter in each case being determined by transmission electron microscopy.

Most preferred metal nanoobjects are silver nanowires having the above-mentioned dimensions.

Suitable metal nanoobjects as defined above are known in the art (see e.g. US 7,922,787) and are commercially available.

Silver nanowires (as well as nanowires of other metals) are typically commercially available in the form of an aqueous dispersion wherein polyvinylpyrrolidone is adsorbed onto the surface of the silver nanowires in order to render the dispersion stable. Any matter adsorbed on the surface of the nanowires is not included in the above-defined dimen- sions and composition of the metal nanoobjects.

Preferably, the silver nanowires are obtained by the procedure described by Yugang Sun and Younan Xia in Adv. Mater 2002 14 No. 1 1 , June 5, pages 833-837.

Preferably, in a composition according to the present invention, the weight fraction of metal nanoobjects (B) is in the range of from 0.01 wt.-% to 10 wt.-%, preferably 0.01 wt.-% to 5 wt.-%, most preferably 0.01 wt.-% to 0.5 wt.-%, in each case based on the total amount of constituents (A), (B), (C) and (D).

A composition according to the invention comprises metal oxide nanoobjects (C) wherein said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn, Ag, Ce, mixed oxides of In and Sn and mixed oxides of Sb and Sn. Preferred are oxides of Ti, Si, Sn, Zn and Ce. Especially preferred is cerium (IV) oxide Ce0₂.

Surprisingly it has been found that presence of metal oxide nanoobjects (C) in a transparent electroconductive layer comprising metal nanoobjects (B) as defined above results in an increased stability of the metal nanoobjects against oxidation, even when a high current density flows in said a transparent electroconductive layer.

It is understood that for an ink according to the present invention the kind as well as the amount of metal oxide nanoparticles (C) have to be selected in such manner that the electrical conductivity and the optical properties of a transparent electroconductive layer obtainable from said ink are not compromised. Preferred metal oxide nanoobjects (C) are metal oxide nanoparticles (nanoparticles comprising one or more metal oxides). Herein, the term "nanoobject" has the same meaning as defined above in the context of constituent (B). According to ISO/TS 27687:2008, nanoparticles are nanoobjects having three external dimensions in the nanoscale. Preferred nanoparticles are approximately isometric, i.e. the aspect ratio (longest : shortest direction) of all 3 orthogonal external dimensions is in the range of from 1 to 2.

Particularly preferred metal oxide nanoparticles are primary particles having a primary particle diameter of 1 nm to 100 nm, preferably 3 nm to 50 nm (measured by nitrogen absorption, X-Ray diffraction or transmission electron microscopy). According to DIN 53206-1 : 1972-08, the term "primary particles" refers to entities which are discernible as individuals by means of optical microscopy or transmission electron microscopy.

Advantageously, the metal oxide nanoobjects are nanoparticles which in suspension have a hydrodynamic size D₉₀ of less than 100 nm (measured by dynamic light scattering or centrifugal sedimentation techniques). Preparation of suitable metal oxide nanoobjects comprising one or more metal oxides is known in the art. For instance, the metal oxide nanoobjects are nanoparticles synthesized by a gas phase pyrolysis process, preferably flame spray synthesis. Such nanoparticles are commercially available. Preferably, in a composition according to the present invention, the weight fraction of metal oxide nanoobjects (C) is in the range of from 0.2 wt.-% to 49.9 wt.-%, preferably 1 wt.-% to 20 wt.-%, most preferably 1 wt.-% to 10 wt.-%, in each case based on the total amount of constituents (A), (B), (C) and (D).

The composition according to the present invention further comprises

(D) one or more metal salts of formula (I)

(M^a+)_z(R^b )_y (I),

wherein

M^a+ represents a metal cation,

R^b" represents the salt anion,

a is 2, 3, 4 or 5,

b is 1 , 2 or 3,

z is the least common multiple of a and b, divided by a

y is the least common multiple of a and b, divided by b

wherein at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects (C)

wherein the molar fraction of metal ions M of the metal salts of formula (I) is in the range of from 0.02 to 6 mol%, based on the total amount of metal in the metal ions M of the metal salts of formula (I) and in the metal oxides in the nanoobjects.

The term physisorption, as used herein, defines adsorption in which the forces involved are intermolecular forces (van der Waals or electrostatic forces) and which do not involve a significant change in the electronic orbital patterns of the species involved (see: "International Union of pure and Applied Chemistry" (http://goldbook.iupac.org/P04667.html)). In the context of the present application, "physisorption" denotes the adsorption of a molecule or ion on a surface by either electrostatic or van der Waals attraction. In con- trast to chemisorption, a physisorbed molecule or ion does not alter its chemical proper- ties upon adsorption. Accordingly, by physisorption covalent bonds are neither formed nor broken nor are atoms ionized or ions deionized.

Without being bound to theory, it is believed that the metal salts of formula (I) as defined above act as dispersing aids for the metal oxide nanoobjects (C) and are at least partly physisorbed on the surface of the metal oxide nanoobjects (C), and may be partly dissolved in the liquid phase of the suspension. The term "dispersing aid" as used herein denotes a substance which facilitates the separation of suspended particles and acts to prevent agglomeration or settling of said particles. In the context of the present invention the term "dispersing aid" is used for metal salts of formula (I) as defined herein which stabilize said suspended metal oxide nanoobjects. The dispersing aid is different from the materials forming the liquid external phase (carrier liquid) of suspension.

In the composition according to the present invention, the surfaces of the metal oxide nanoobjects (C) are at least partly coated with physisorbed metal salts of formula (I). The specific fraction of metal salts of formula (I) physisorbed on the surface of the metal oxide nanoobjects (C) and the specific fraction of metal salts of formula (I) dissolved in the carrier liquid (A) are dependent on the specific combination of metal oxide nanoobjects (C)/metal salts of formula (I).

Coating of metal oxide nanoobjects by said one or more metal salts of formula (I) may be achieved by procedures known in the art. For instance, said carrier liquid (A) and said metal oxide nanoobjects (C) are combined, for example by mixing, ultrasonication or ball milling. To the obtained initial suspension, one or more metal salts of formula (I) as defined above are added. Coating of the metal oxide nanoobjects (C) with the one or more metal salts of formula (I) as defined above takes place during mixing at room temperature or upon heating. Alternatively, said carrier liquid (A) and said one or more metal salts of formula (I) are combined, for example by mixing. To the obtained initial solution of one or more metal salts of formula (I) in the carrier liquid (A), the metal oxide nanoobjects (C) are added. Coating of the metal oxide nanoobjects (C) with the one or more metal salts of formula (I) as defined above takes place during mixing at room temperature or upon heating. In formula (I)

(M^a+)_z(R^b )_y (I)

M^a+ represents a metal cation, R^b" represents the salt anion,

a is 2, 3, 4 or 5,

b is 1 , 2 or 3,

z is the least common multiple of a and b, divided by a

y is the least common multiple of a and b, divided by b.

Thus, when a is 2 and b is 1 , z is 1 and y is 2

Thus, when a is 2 and b is 2, z is 1 and y is 1

Thus, when a is 2 and b is 3, z is 3 and y is 2

Thus, when a is 3 and b is 1 , z is 1 and y is 3

Thus, when a is 3 and b is 2, z is 2 and y is 3

Thus, when a is 3 and b is 3, z is 1 and y is 1

Thus, when a is 4 and b is 1 , z is 1 and y is 4

Thus, when a is 4 and b is 2, z is 1 and y is 2

Thus, when a is 4 and b is 3, z is 3 and y is 4

Thus, when a is 5 and b is 1 , z is 1 and y is 5

Thus, when a is 5 and b is 2, z is 2 and y is 5

Thus, when a is 5 and b is 3, z is 3 and y is 5

Preferred are metal salts of formula (I) wherein

M represents one of Zn, Al, Sc, Ga, Y, Pb, Bi, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, La, Mg, Ca, Sr and Ba, most preferably one of Zn, Al and Y

and/or

R^b" represents an organic anion selected from the group consisting of acetate, formiate, citrate, oxalate, or an inorganic anion selected from the group consisting of nitrate, difluorophosphate, hexafluorophosphate and tetrafluroborate. More specifically, preferred are metal salts of formula (I) wherein

M represents one of Zn, Al, Sc, Ga, Y, Pb, Bi, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, La, Mg, Ca, Sr and Ba, most preferably one of Zn, Al and Y

and R^b" represents an organic anion selected from the group consisting of acetate, formiate, citrate, oxalate, or an inorganic anion selected from the group consisting of nitrate, difluorophosphate, hexafluorophosphate and tetrafluroborate.

Especially preferred metal salts of formula (I) are zinc diacteate, aluminium triacetate, yttrium triacetate, zinc dinitrate, aluminium trinitrate, and yttrium trinitrate.

Preferably, in a composition according to the present invention, the metals M of the metal salts (D) of formula (I) differ from the metals of the metal oxides in the metal oxide nanoobjects (C).

Preferred are compositions according to the invention, wherein

(C) said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn and Ce,

and

(D) said metal salts of formula (I) are selected from the group consisting of zinc diacteate, aluminium triacetate, yttrium triacetate, zinc dinitrate, aluminium trini- trate, and yttrium trinitrate. Metal salts of formula (I) as defined above are commercially available.

It is understood that for an ink according to the present invention the kind as well as the amount of metal salts of formula (I) have to be selected in such manner that the electrical conductivity and the optical properties of a transparent electroconductive layer obtain- able from said ink are not compromised.

The molar fraction of metal in the metal ions M of the metal salts of formula (I) is in the range of from 0.1 % to 6 mol%, preferably 0.2 to 4 mol%, most preferably 0.2 to 2 mol%. based on the total amount of metal (i) in the metal ions M of the metal salts of formula (I) and (ii) in the metal oxides in the metal oxide nanoobjects (C). The specific molar fraction of the metal salts of formula (I) may depend on the specific surface exhibited by the metal oxide nanoobjects (C) and may be determined by the skilled person.

Preferably, in a composition according to the present invention, the weight fraction of metal salts (D) is in the range of from 0.005 wt.-% to 10 wt.-%, preferably 0.01 wt.-% to 5 wt.-%, most preferably 0.01 wt.-% to 3 wt.-%, in each case based on the total amount of constituents (A), (B), (C) and (D). Preferably, a composition according to the present invention comprises

(A) said carrier liquid in an amount of from 50 wt.-% to 99.8 wt.-%, preferably 70 wt.-% to 99 wt.-%, most preferably 75 wt.-% to 90 wt.-%

(B) said metal nanoobjects in an amount of from 0.01 wt.-% to 10 wt.-%, preferably 0.01 wt.-% to 5 wt.-%, most preferably 0.01 wt.-% to 0.5 wt.-%

(C) said metal oxide nanoobjects in an amount of from 0.2 wt.-% to 49.9 wt.-%, preferably 1 wt.-% to 20 wt.-%, most preferably 1 wt.-% to 10 wt.-%

(D) said metal salts of formula (I) in an amount of from 0.005 wt.-% to 10 wt.-%, preferably 0.01 wt.-% to 5 wt.-%, most preferably 0.01 wt.-% to 3 wt.-%

in each case based on the total amount of constituents (A), (B), (C) and (D).

It is preferred that in a composition according to the present invention, the ratio between the total weight of (B) said metal nanoobjects

and the total weight of (C) said metal oxide nanoobjects

is in the range of from 0.0002 to 50, preferably 0.001 to 5, most preferably 0.001 to 0.5. At a weight ratio below 0.0002, the fraction of metal nanoobjects (B) in a transparent electroconductive layer obtained from the composition according to the present invention would be quite low, resulting in a rather low electronic conductivity. On the other hand, at a weight ratio above 50, the fraction of metal oxide nanoobjects (C) in a transparent electroconductive layer obtained from the composition according to the present invention would be quite low and accordingly would have no remarkable effect.

Furthermore, it is preferred that in a composition according to the present invention the ratio between

the total weight of said carrier liquid (A) having a boiling point of less than 120 °C, and the total weight of said constituents (B), (C) and (D)

is in the range of from 450 to 1 , preferably 300 to 20, most preferably 100 to 10.

At a weight ratio below 1 , the fraction of solids in the composition is very high, which may impede application of the composition by means of wet processing techniques. On the other hand, at a weight ratio above 450, the fraction of the carrier liquid, which has to be removed in the process of forming a transparent electroconductive layer, is relatively large, and processing may become inefficient. In certain preferred cases, a composition according to the present invention comprises the constituents (A), (B), (C) and (D), and further comprises

(E) one or more organic polymers suspended or dissolved in said carrier liquid (A).

In a transparent electroconductive layer obtained from an ink according to the present invention, said organic polymers (E) form an optically transparent contiguous solid phase (herein referred to as a matrix). Said matrix binds and accommodates the metal nanoobjects (B) and the metal oxide nanoobjects (C) within the transparent electroconductive layer, fills the voids between said nanoobjects, provides mechanical integrity and stability to the transparent electroconductive layer and binds the transparent electroconductive layer to the surface of the substrate. The metal nanoobjects (B) dispersed within said matrix form a conductive network enabling the flow of electrons between adjacent and overlapping electroconductive nanoobjects within the layer. Due to the small dimensions of the metal nanoobjects (B) and the metal oxide nanoobjects (C), their influence on the optical behavior of the layer is minor, thus allowing for the formation of a layer which is optically transparent and electroconductive. Accordingly, said layer is referred to as an "optically transparent conductive layer".

Said matrix forming polymers are selected from the group consisting of polymers dissolved in carrier liquid (A) and polymers suspended in carrier liquid (A). The carrier liquid (A) as defined above and the one or more dissolved polymers (E) as defined above are monophase (i.e. form a single phase). Polymers (E) suspended in mixture (A) substantially do not dissolve in mixture (A) and are present in the ink in the form of dispersed discrete solid particles e.g. fibers or polymer beads.

Preferably, in a composition according to the present invention, the weight fraction of suspended particles of one or more organic polymers is in the range of from of from 1 wt.- % to 1000 wt.-%, preferably 5 wt.-% to 500 wt.-%, most preferably 10 wt.-% to 200 wt.-% in each case based on the total weight of components (B), (C) and (D).

Preferably, in a composition according to the present invention, the weight fraction of dissolved organic polymers is in the range of from of from 1 wt.-% to 1000 wt.-%, preferably 5 wt.-% to 500 wt.-%, most preferably 10 wt.-% to 200 wt.-% in each case based on the total weight of components (B), (C) and (D).

One kind of preferred matrix-forming polymers are styrene/(meth)acrylic copolymers having a number average molecular weight in the range of from 500 g/mol to 22000 g/mol Herein, the term "(meth)acrylic" includes "methacrylic" and "acrylic". In said copolymers each molecule comprises or consists of units derived from monoalkenyl aromatic monomers and units derived from (meth)acrylic monomers, in copolymerized form. Such styrene/(meth)acrylic copolymers are obtainable by copolymerisation of one or more kinds of monoalkenyl aromatic monomers with one or more kinds of (meth)acrylic monomers.

In preferred styrene/(meth)acrylic copolymers, each molecule comprises or consists of units C1 derived from monoalkenyl aromatic monomers MC1

and

units C2 derived from (meth)acrylic monomers MC2

in copolymerized form, wherein

said units C1 (units derived from monoalkenyl aromatic monomers) have the chemical structure

(C1 )

wherein R-i , independently from the R-i of each other unit C1 , is selected from the group consisting of hydrogen and alkyl (including unbranched alkyl, preferably methyl, and branched alkyl, preferably tert-butyl)

and wherein R₂, independently from the R₂ of each other unit C1 , is selected from the group consisting of halogen (preferably chlorine) and alkyl (preferably methyl), and R₂ is situated in a position selected from the group consisting of ortho, meta and para; and said units C2 (units derived from (meth)acrylic monomers) have the chemical structure

(C2)

wherein R₃, independently from the R₃ of each other unit C2, is selected from the group consisting of hydrogen, methyl, halogen (preferably chlorine) and cyano,

and wherein R₄, independently from the R₄ of each other unit C2, is selected from the group consisting of

-COOH,

-COOX wherein X is a cation selected from alkali metal cations, ammonium cations and substituted ammonium cations,

-CN,

-COOR₅ wherein R₅ is selected from the group consisting of branched and unbranched alkyl groups, branched and unbranched alkenyl groups, branched and unbranched alkinyl groups, cycloalkyl groups, aralkyl groups, aralkenyl groups, furfuryl, tetrahydrofurfuryl, isopropylidene glyceryl, glycidyl and tetrahydropyranyl, wherein said branched and unbranched alkyl groups, alkenyl groups and alkinyl groups include branched and unbranched alkyl groups, alkenyl groups and alkinyl groups substituted with one or more groups selected from the group consisting of hydroxy, alkoxy, phenoxy, halogen, sulfo, nitro, oxazolidinyl, monoalkylamino and dialkylamino groups

-CHO,

-NR₆R₇ wherein R₆ and R₇ are independently selected from the group consisting of hydrogen, alkyl and phenyl.

Such styrene/(meth)acrylic copolymers are obtainable by copolymerisation of one or more kinds of monoalkenyl aromatic monomers MC1 having the formula

(MC1 )

wherein R-i , independently from the R-i of each other monomer MC1 , is selected from the group consisting of hydrogen and alkyi (including unbranched alkyi, preferably methyl, and branched alkyi, preferably tert-butyl)

and wherein R₂, independently from the R₂ of each other monomer MC1 , is selected from the group consisting of halogen (preferably chlorine) and alkyi (preferably methyl) and R₂ is situated in a position selected from the group consisting of ortho, meta and para, with one or more kinds of (meth)acrylic monomers MC2 having the formula

(MC2)

wherein R₃, independently from the R₃ of each other monomer MC2, is selected from the group consisting of hydrogen, methyl, halogen (preferably chlorine) and cyano, and wherein R₄, independently from the R₄ of each other unit MC2 is selected from the group consisting of

-COOH,

-COOX wherein X is a cation selected from alkali metal cations, ammonium cations and substituted ammonium cations, -CN,

-COOR₅ wherein R₅ is selected from the group consisting of branched and unbranched alkyl groups, branched and unbranched alkenyl groups, branched and unbranched alkinyl groups, cycloalkyl groups, aralkyl groups, aralkenyl groups, furfuryl, tetrahydrofurfuryl, isopropylidene glyceryl, glycidyl and tetrahydropyranyl, wherein said branched and unbranched alkyl groups, alkenyl groups and alkinyl groups include branched and unbranched alkyl groups, alkenyl groups and alkinyl groups substituted with one or more groups selected from the group consisting of hydroxy, alkoxy, phenoxy, halogen, sulfo, nitro, oxazolidinyl, monoalkylamino and dialkylamino groups

-CHO,

-NR₆R₇ wherein R₆ and R₇ are independently selected from the group consisting of hydrogen, alkyl and phenyl.

The term "(meth)acrylic monomer" MC2 as employed herein includes acrylic acid and salts, esters and amides of acrylic acid, acrylonitrile and acrolein, as well as methacrylic acid and salts, esters and amides of methacrylic acid, methacrylonitrile, and methacrolein.

(Meth)acrylic monomers, wherein R₃ is hydrogen or methyl, resp., and R₄ is -COOH, are acrylic acid or methacrylic acid, resp.

(Meth)acrylic monomers wherein R₃ is hydrogen or methyl, resp., and R₄ is -COOR₅ as defined above, are esters of acrylic acid or esters of methacrylic acid, resp.

(Meth)acrylic monomers wherein R₃ is hydrogen or methyl, resp., and R₄ is -COOX as defined above, are salts of acrylic acid or salts of methacrylic acid, resp.

(Meth)acrylic monomers wherein R₃ is hydrogen or methyl, resp., and R₄ is -CN, are acrylonitrile or methacrylonitrile, resp. Styrene/(meth)acrylic copolymers obtainable by copolymerization of one or more kinds of monoalkenyl aromatic monomers MC1 and one or more (meth)acrylic monomers from the group consisting of acrylonitrile and methacrylonitrile and no other (meth)acrylic monomers MC2 are not preferred. In this regard it is preferred that for the preparation of said copolymer (meth)acrylic monomers selected from the group consisting of acrylonitrile and methacrylonitrile are used in combination with other (meth)acrylic monomers MC2 as defined herein.

(Meth)acrylic monomers wherein R₃ is hydrogen or methyl, resp., and R₄ is -NR₆R₇ as defined above, are amides of acrylic acid or amides of methacrylic acid, resp.

(Meth)acrylic monomers wherein R₃ is hydrogen or methyl, resp., and R₄ is -CHO are acrolein or methacrolein, resp.

Preferable monoalkenyl aromatic monomers MC1 are selected from the group consisting of alpha-methyl styrene, styrene, vinyl toluene, tertiary butyl styrene and ortho- chlorostyrene.

Examples of suitable (meth)acrylic monomers include the following methacrylate esters (methacrylic acid esters): methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, isopropyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, n-hexyl methacrylate, isoamyl methacrylate, 2-hydroxyethyl methacrylate,

2- hydroxy propyl methacrylate, Ν,Ν-dimethylaminoethyl methacrylate, Ν,Ν-diethylaminoethyl methacrylate, t-butylaminoethyl methacrylate, 2-sulfoethyl methacrylate, trifluoroethyl methacrylate, glycidyl methacrylate, benzyl methacrylate, allyl methacrylate, 2-n-butoxyethylmethacrylate, 2-chloroethyl methacrylate, secbutyl-methacrylate, tert-butyl methacrylate, 2-ethylbutyl methacrylate, cinnamyl methacrylate, crotyl methacrylate, cyclohexyl methacrylate, cyclopentyl methacrylate, 2-ethoxyethyl methacrylate, furfuryl methacrylate, hexafluoroisopropyl methacrylate, methallyl methacrylate,

3- methoxybutyl methacrylate, 2-methoxybutyl methacrylate, 2-nitro-2-methylpropyl methacrylate, n-octylmethacrylate, 2-ethylhexyl methacrylate, 2-phenoxyethyl methacrylate, 2-phenylethyl methacrylate, phenyl methacrylate, prop-2-inyl methacrylate, tetrahydrofurfurylmethacrylate and tetrahydropyranylmethacrylate. Typical acrylate esters employed include: methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate and n-decyl acrylate, methyl alpha-chloroacrylate, methyl 2-cyanoacrylate. Other suitable (meth)acrylic monomers include methacrylonitrile, methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, N,N-diethylmethacryl- amide, Ν,Ν-dimethylmethacrylamide, N-phenylmethacrylamide and methacrolein, acrylo- nitrile, acrylamide, N-ethylacrylamide, Ν,Ν-diethylacrylamide and acrolein. Esters of methacrylic acid or acrylic acid containing a suitable condensable cross linkable functional group may be used as the monomer. Among such esters are t-butylaminoethyl methacrylate, isopropylidene glyceryl methacrylate and oxazolidinylethyl methacrylate.

Typical preferred cross-linkable acrylates and methacrylates include hydroxy alkyl acry- lates, hydroxyl alkyl methacrylates and hydroxyesters of glycidyl acrylates or methacrylates. Examples of preferred hydroxy functional monomers include 2-hydroxyethyl acry- late, 3-chloro-2-hydroxypropyl acrylate, 2-hydroxy-butyl acrylate, 6-hydroxyhexyl acrylate, 2-hydroxy methyl methacrylate, 2-hydroxypropyl methacrylate, 6-hydroxyhexyl methacrylate, 5,6-dihydroxyhexyl methacrylate and the like. The term "styrene/(meth)acrylic copolymer" as employed herein includes copolymers obtainable from mixtures consisting of two or more (meth)acrylic monomers and one or more monoalkenyl aromatic monomers, as well as copolymers obtainable from mixtures of at least one (meth)acrylic monomer and at least one non-acrylic ethylenic monomer and one or more monoalkenyl aromatic monomers. Suitable ethylenic monomers include: vinyl pyridine, vinyl pyrrolidone, sodium crotonate, methyl crotonate, crotonic acid and maleic anhydride.

For further details regarding the above-defined styrene/(meth)acrylic copolymers, reference is made to US 2008/0182090, US 4,414,370, US 4,529,787, US 4,546, 160, US 5,508,366 and the prior art cited therein. The number average molecular weight of said styrene/(meth)acrylic copolymers is in the range of from 500 g/mol to 22000 g/mol, preferably of from 1700 g/mol to 15500 g/mol, further preferably of from 5000 g/mol to 10000 g/mol.

Typically, said styrene/(meth)acrylic copolymers are amphiphilic, because their molecules contain non-polar hydrophobic regions derived from the monoalkenyl aromatic monomers and polar hydrophilic regions derived from the (meth)acrylic monomers. Thus, the desired amphiphilic behavior is obtainable by appropriate selection of the hydrophobic monoalkenyl aromatic monomers and the hydrophilic (meth)acrylic monomers and appropriate adjustment of the ratio between monoalkenyl aromatic monomers and (meth)acrylic monomers so that a styrene/(meth)acrylic copolymer is obtained which has an appropriate ratio between hydrophobic units derived from monoalkenyl aromatic monomers and hydrophilic units derived from (meth)acrylic monomers to allow for amphiphilic behavior of the copolymer. In aqueous solution said water-soluble styrene/(meth)acrylic copolymers behave like surfactants (tensides), i.e. they are capable of forming micelles. A micelle is an aggregate formed by association of dissolved amphiphilic molecules. Preferably said micelles have a diameter of up to 5 nm. Typical water-soluble styrene/(meth)acrylic copolymers are known in the art and commercially available. Typically such copolymers are commercially available in the form of aqueous solutions.

A second kind of preferred matrix-forming polymers is crystalline cellulose. Preferably said crystalline cellulose is in the form of fibers having a length in the range of from 80 nm to 300 nm and a diameter in the range of from 5 nm to 30 nm. Preferably, said fibers of crystalline cellulose have a length in the range of from 80 nm to 150 nm and a diameter in the range of from 5 nm to 10 nm. Said fibers are also referred to as nanocrystalline cellulose or cellulose nanofibers or cellulose II (see WO 2010/127451 ). They are obtainable by disrupting the amorphous domains of natural cellulose fibers and disintegration of the micrometer-sized cellulose fibers into rod-like rigid crystallites. The obtained crystallites typically have the above-mentioned dimensions.

More specifically, crystalline cellulose fibers having the above-mentioned dimensions are obtainable by chemical treatment or by enzymatic treatment or by mechanical treatment of natural cellulose fibers or by combinations of different types of treatment, e.g. chemical treatment (e.g. with sulfuric acid or sodium chlorite) or enzymatic treatment followed by high-pressure homogenization, or by milling of natural cellulose fibers and subsequent hydrolysis to remove amorphous regions.

When a dispersion of fibers of crystalline cellulose in a liquid is dried (i.e. the liquid is removed from the dispersion), the cellulose fibers become packed together by capillary action during the evaporation of the water. Accordingly, said cellulose fibers are capable of forming a matrix and binding metal nanoobjects so as to form a coating as defined above. Furthermore, due to their outstanding mechanical stability, said fibers impart mechanical reinforcement to the obtained coating.

Due to their external dimensions, said fibers of crystalline cellulose are nanoobjects in the sense of to ISO/TS 27687:2008 (as published in 2008), see below, and do not scatter visible light. However, said fibers of crystalline cellulose do not comprise any materials capable of allowing the flow of electrons. Preferably, said fibers of crystalline cellulose are fibers of sulfated crystalline cellulose. They are obtainable by treatment of cellulose with sulfuric acid. Fibers of this kind of crystalline cellulose contain sulfur in the form of sulfate groups. Especially preferred are fibers of sulfated crystalline cellulose II obtainable by be the process described in WO 2010/127451. Said sulfated crystalline cellulose II has a degree of polymerization of 60 or below. For further details, reference is made to WO 2010/127451.

Preferably, the composition according to the present invention does not comprise fibers of crystalline cellulose obtained by means of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of cellulose. This kind of crystalline cellulose fibers exhibits a high density of carboxylate groups on their surfaces. Said carboxylate groups are formed by oxidation of primary hydroxyl groups of cellulose.

Suitable fibers of crystalline cellulose are commercially available, e.g. from Celluforce.

A third kind of preferred matrix-forming polymers are polymers having a number average molecular weight of 25000 g/mol or higher in the form of dispersed particles. Preferably, the number average molecular weight of said polymer is not higher than 200000 g/mol. Said polymer is either a homopolymer or a copolymer. Said particles are either particles of one kind of polymer or a mixture of particles of different polymers.

Such particles are also referred to as polymer beads. Typically, a polymer bead consists of several entangled polymer chains. Said polymer beads have an average particle diameter in the range from 10 nm to 1000 nm, in particular in the range from 50 nm to 600 nm determined by dynamic light scattering on an aqueous polymer dispersion (from 0.005 to 0.01 percent by weight) at 23 °C by means of an Autosizer IIC from of Malvern Instruments, England.

Preferred polymers beads comprise, in copolymerized form, from 50 % to 99.9 % by weight of

esters of acrylic acid and/or methacrylic acid with alkanole having from 1 to 12 carbon atoms, or

styrene, or

styrene and butadiene, or

- vinyl chloride and/or vinylidene chloride,

or from 40 % to 99.9 % by weight of vinyl acetate, vinyl propionate and/or ethylene. Particularly preferred are polyacrylates, polymethacrylates, copolymers of acrylates and methacrylates, and copolymers of styrene and (meth)acrylates. Herein, the term "(meth)acrylate" includes "methacrylate" and "acrylate"."

The polyacrylates are either homo- or copolymers. In the case of homopolymers each molecule consists of units each derived from one kind of acrylate monomer. In the case of copolymers each molecule comprises or consists of units derived from different kinds of acrylate monomers in copolymerized form.

The polymethacrylates are either homo- or copolymers. In the case of homopolymers each molecule consists of units each derived from one kind of methacrylate monomer. In the case of copolymers each molecule comprises or consists of units derived from different kinds of methacrylate monomers in copolymerized form.

In said copolymers of acrylates and methacrylates each molecule comprises or consists of units derived from acrylate monomers and units derived from methacrylate monomers in copolymerized form In said copolymers of styrene and (meth)acrylates, each molecule comprises or consists of units derived from monoalkenyl aromatic monomers and units derived from (meth)acrylic monomers in copolymerized form.

Such polymer beads are known in the art and are commercially available in the form of aqueous dispersions of said polymer beads (aqueous polymer dispersions). Typically, the dispersed polymers are present in colloidal dispersion. Such aqueous polymer dispersions are obtainable by polymerization of suitable monomers in an aqueous liquid phase, e.g. by means of suspension polymerization or emulsion polymerization. Preferred aqueous polymer dispersions are obtainable by free-radically initiated aqueous emulsion polymerization of ethylenically unsaturated monomers. The free-radically initiated aque- ous emulsion polymerization is effected typically in such a way that at least one ethylenically unsaturated monomer, frequently in the presence of dispersing assistants, is distributed in a disperse manner in an aqueous medium and polymerized by means of at least one free-radical polymerization initiator. For further details, reference is made to US 7,999,045 B2 and the prior art cited therein. An aqueous colloidal dispersion of polymer particles is also referred to as a latex. Colloidal stability of a latex is achieved by a balancing of electrostatic repulsion, van der Waals attraction and steric attraction or repulsion. The above-defined polymer dispersions typically comprise dispersing assistants which serve to ensure the stability of the aqueous polymer dispersions. Suitable dispersing agents are selected from the group consisting of protective colloids and surfactants. Preferred surfactants are sodium dodecylsulfate (SDS), and water soluble amphiphilic styrene/(meth)acrylic copolymers having a number average molecular weight in the range of from 500 g/mol to 22000 g/mol as defined above. Said water soluble amphiphilic styrene/(meth)acrylic copolymers are preferred surfactants because they are capable of co-acting with the polymer beads in forming a matrix and binding the above-defined metal nanoobjects. When a latex is deposited on a substrate and evaporation of the liquid phase is allowed to proceed, a continuous, a homogeneous film (coating) is formed under appropriate conditions. This process is called film formation. The mechanism of film formation from latex is described e g. in Materials Science and Engineering, 21 (1997) 101-170 and in Advances in Colloid and Interface Science 86 (2000) 195-267. Generally, the formation of a latex film arises from the 'coalescence' i.e. compaction, deformation, cohesion and polymer chain interdiffusion of the individual latex particles (polymer beads) which in the aqueous dispersion are held apart by stabilizing forces (electrostatic and/or steric) resulting e g. from charged polymer chain endgroups or adsorbed surfactants. These and other forces resisting particle deformation are overcome upon evaporation of the continuous phase of the latex (water).

Further preferred matrix-forming polymers are selected from the group consisting of cellulose alkyl ethers, cellulose hydroxyalkyl ethers (e.g. hydroxypropyl methyl cellulose), cellulose esters (e.g. carboxymethyl cellulose), polyacrylamides, polyvinylalcohol, polyvinylpyrrolidone, polystyrenesulfonic acid, and dextranes. Other suitable matrix-forming polymers are polyolefin copolymer resins comprising an olefin monomer and an acrylic acid comonomer or (meth)acrylic acid comonomer as described in EP 2 960 310 A1 .

In certain other preferred cases, a composition according to the present invention comprises the constituents (A), (B), (C) and (D), and further comprises

(F) one or more polymerizable monomers,

and (G) one or more initiators for initiating radical polymerization of said polymerizable monomers.

Said polymerizable monomers (F) are precursors of a polymer matrix. In the process of preparing a transparent electroconductive layer from such an ink, said polymerizable monomers form a polymer matrix (as defined above) by polymerization on a surface of a substrate to which the above-defined composition has been applied.

The term "polymerizable monomer" includes co-polymerizable monomers (i.e. monomers capable of forming a copolymer by copolymerization with another kind of monomers). Preferred polymerizable monomers are copolymerizable monomers selected from the group consisting of alkyl acrylates and alkyl methacrylates and copolymerizable monomers selected from the group of hydroxyalkyl acrylates and hydroxyalkyl methacrylates. Preferably, monomers selected from the group consisting of alkyl (meth)acrylates are copolymerized with monomers selected from the group of hydroxyalkyl (meth)acrylates. As used herein, the term (meth)acrylates in each case includes acrylates and methacry- lates.

Preferred initiators (G) for initiating radical polymerization decompose into radicals when exposed to irradiation, especially irradiation having a wavelength in the range of from 360 nm to 420 nm. Suitable initiators (G) are known in the art and commercially available.

Preferably, in a composition according to the present invention, the weight fraction of polymerizable monomers is in the range of from of from 1 wt.-% to 1000 wt.-%, preferably 5 wt.-% to 500 wt.-%, most preferably 10 wt.-% to 200 wt.-% in each case based on the total weight of components (B), (C) and (D).

Particularly preferred compositions according to the present invention are those wherein two or more of the above-defined preferred features are combined. In a second aspect, the present invention relates to a process for preparing a layer comprising metal nanoobjects (i.e. a transparent electroconductive layer as defined above) on a surface of a substrate, said process comprising the steps of

forming on a surface of said substrate a wet film by applying a composition according to the invention to said surface of said substrate

- removing said carrier liquid having a boiling point below 120 °C from the wet film formed on said surface of said substrate. Said substrate is preferably in a form selected from the group consisting of foils, films, webs, panes and plates. Preferably, said substrate has a thickness in the range of from 10 μιη to 200 μιη, preferably from 50 μιη to 100 μιη.

Preferably, said substrate comprises an optically transparent material selected from the group of glasses and organic polymers, which is electronically insulating.

Preferred types of glass are e.g. float glass, low iron float glass, heat strengthened glass and chemically strengthened glass. Optionally, the glass has a low-emissivity (low-e) coating, sun-protection coating or any other coating on the surface facing away from the above-described transparent electroconductive layer. Preferred organic polymers are selected from the group consisting of polymethylmethacrylate (PMMA, commercially available e.g. as Plexiglas™), polycarbonate (PC), polyethylene (PE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene (PP), low density polypropylene (LDPP), polyethylene therephthalate (PET), glycol modified polyethylene therephthalate, polyethylene napthalate (PEN), cellulose acetate butyrate, polylactide (PL), polystyrene (PS), polyvinyl chloride (PVC), polyimides (PI), polypropyleneoxide (PPO) and mixtures thereof. PET and PEN are particularly preferred.

Preferably, said substrate has a light transmission of 80 % or more measured according to ASTM D1003 (Procedure A) as published in November 2013. Preferably, the composition applied to said substrate is selected from the above-defined preferred compositions.

Preferably, the composition according to the present invention is applied to the surface of said substrate by coating or printing, preferably by a coating technique selected from the group consisting of roll-to-roll-, slot-die-, spray-, ultrasonic spray-, dip- and blade coating, or by a printing technique selected from the group consisting of ink-jet-, pad-, offset-, gravure-, screen-, intaglio- and sheet-to-sheet- printing.

Preferably, the wet film formed by applying the composition according to the present invention to said surface of said substrate has a thickness in a range of from 1 μιη to 100 μιη, preferably of from 2 μιη to 50 μιη. Said thickness is also referred to as "wet thick- ness". In the above-defined process according to the present invention, the carrier liquid is removed from said wet film on said surface of said substrate by exposing said wet film to a temperature in the range of from 20 °C to 120 °C, preferably 40 °C to 120 °C, most preferably 80 °C to 120 °C, thereby forming on said surface of said substrate a transparent electroconductive layer.

In certain cases, preparing said transparent electroconductive layer further comprises the step of annealing the layer formed on the surface of the substrate after removal of the carrier liquid, preferably at a temperature in the range of from 70 °C to 200 °C, more preferably at a temperature in the range of from 100 °C to 150 °C.

In certain cases, after removal of the carrier liquid from the surface of said substrate is completed, a sequence comprising the steps of

forming a wet film by applying the above-defined composition according to the invention to the surface of the layer formed the surface of the substrate after removal of the carrier liquid,

removing from said wet film said carrier liquid having a boiling point below 120 °C, optionally annealing the layer formed the surface of the substrate after removal of the carrier liquid, preferably annealing at a temperature in the range of from 70 °C to 200 °C, more preferably at a temperature in the range of from 100 °C to 150 °C is carried out and optionally repeated at least once.

In certain cases, after annealing the layer formed on the surface of the substrate is completed, a sequence comprising the steps of

forming a wet film by applying the above-defined composition according to the invention to the surface of the layer formed the surface of the substrate after removal of the carrier liquid,

removing from said wet film said carrier liquid having a boiling point below 120 °C, optionally annealing the layer formed the surface of the substrate after removal of the carrier liquid, preferably annealing at a temperature in the range of from 70 °C to 200 °C, more preferably at a temperature in the range of from 100 °C to 150 °C is carried out and optionally repeated at least once. In an especially preferred process according to the present invention said composition applied to said surface of said substrate comprises (F) one or more polymerizable monomers, and said process further comprises the step of

polymerizing the polymerizable monomers on said surface of said substrate. Herein, the step of polymerizing the polymerizable monomers on said surface of said substrate is performed after the step of removing said carrier liquid having a boiling point below 120 °C from the wet film formed on said surface of said substrate.

Herein, particularly preferably, the composition applied to said surface of said substrate comprises

(F) one or more polymerizable monomers,

and

(G) one or more initiators for initiating radical polymerization of said polymerizable monomers.

Polymerization of the polymerizable monomers is preferably initiated by irradiation, espe- cially irradiation having a wavelength in the range of from 360 nm to 420 nm, in the presence of an initiator which decomposes into radicals when exposed to said irradiation. Suitable initiators are known in the art and are commercially available.

In certain cases, preparing said transparent electroconductive layer further comprises the step of annealing the layer formed on the surface of the substrate after polymerizing the polymerizable monomers, preferably annealing at a temperature in the range of from 70 °C to 200 °C, more preferably a temperature in the range of from 100 °C to 150 °C.

In certain cases, after said polymerization of said polymerizable monomers on the surface of said substrate is completed, a sequence comprising the steps of

forming a wet film by applying the above-defined composition according to the invention on the surface of the layer wherein the polymerizable monomers have been polymerized,

removing from said wet film said carrier liquid having a boiling point below 120 °C, polymerizing said polymerizable monomers in the layer,

optionally annealing the layer after polymerizing the polymerizable monomers, preferably annealing at a temperature in the range of from 70 °C to 200 °C, more preferably at a temperature in the range of from 100 °C to 150 °C

is carried out and optionally repeated at least once.

In certain cases, after annealing the layer formed on the surface of the substrate is completed, a sequence comprising the steps of

forming a wet film by applying the above-defined composition according to the invention on the surface of the layer wherein the polymerizable monomers have been polymerized,

removing from said wet film said carrier liquid having a boiling point below 120 °C, polymerizing said polymerizable monomers in the layer,

optionally annealing the layer after polymerizing the polymerizable monomers preferably annealing at a temperature in the range of from 70 °C to 200 °C, more preferably at a temperature in the range of from 100 °C to 150 °C

is carried out and optionally repeated at least once.

In a third aspect, the present invention relates to the use of a composition as defined above for preparing a layer comprising metal nanoobjects (i.e. a transparent conductive layer as defined above) on a surface of a substrate. Regarding specific and preferred features of the composition to be used and the substrate, reference is made to the disclosure provided above.

In a fourth aspect, the present invention relates to an article comprising or consisting of a substrate having a surface

and, arranged on said surface of said substrate, a layer comprising

(B) metal nanoobjects, said metal nanoobjects having two external dimensions in the range of from 1 nm to 100 nm and a third external dimension in the range of from 1 μιη to 100 μιη,

(C) metal oxide nanoobjects wherein said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn, Ag, Ce, mixed oxides of In and Sn and mixed oxides of Sb and Sn, (D) one or more metal salts of formula (I) as defined above wherein at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects (C).

Preferably said substrate is optically transparent, i.e. exhibits a light transmission of 80 % or more measured according to DIN EN 410. Preferably, said substrate comprises one or more materials selected from the group consisting of glasses and organic polymers. Said substrate is preferably in a form selected from the group consisting of foils, films, webs, panes and plates. Regarding further specific and preferred features of the substrate, reference is made to the disclosure provided above.

Regarding specific and preferred metal nanoobjects (B), metal oxide nanoobjects (C) and metal salts (D) of formula (I) and combinations thereof, reference is made to the disclosure provided above in the context of the first aspect of the present invention.

In said transparent electroconductive layer arranged on said surface of said substrate, the metal nanoobjects (B) form a conductive network enabling the flow of electrons between adjacent and overlapping electroconductive nanoobjects within the layer, and metal oxide nanoobjects (C) are dispersed between the metal nanoobjects (B). Surprisingly it has been found that the presence of metal oxide nanoobjects (C) in a transparent electroconductive layer comprising metal nanoobjects (B) as defined above results in an increased stability of the metal nanoobjects (B) against oxidation, even when a high current density flows in said transparent electroconductive layer. Said transparent electroconductive layer further comprises one or more metal salts of formula (I) wherein at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects.

Preferably, in an article according to the invention, the mass of metal nanoobjects (B) per unit area is in the range of from 1 mg/m² to 1000 mg/m², preferably 5 mg/m² to 300 mg/m², and the mass of metal oxide nanoobjects (C) per unit area is in the range of from 20 mg/m² to 5000 mg/m², preferably 100 mg/m² to 1500 mg/m².

In particularly preferred articles according to the present invention, said transparent electroconductive said layer arranged on said surface of said substrate further comprises a matrix formed of one or more organic polymers, wherein said metal nanoobjects (B) and said metal oxide nanoobjects (C) and said one or more metal salts of formula (I) are dispersed within said matrix, and at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects.

Said matrix is formed from the above-defined polymers (E) or from the polymerization products of the above-defined polymerizable monomers (F). Regarding specific and preferred polymers (E) and polymerizable monomers (F), reference is made to the disclosure provided above.

Said matrix binds and accommodates the metal nanoobjects (B) and metal oxide nanoobjects (C) within the transparent electroconductive layer, fills voids between said nanoobjects, provides mechanical integrity and stability to the transparent electroconductive layer and binds the transparent electroconductive layer to the surface of the substrate.

Accordingly, a preferred article according to the present invention comprises or consists of

a substrate having a surface

and, arranged on said surface of said substrate, a layer comprising

a matrix formed of one or more organic polymers

and dispersed within said matrix

(B) metal nanoobjects, said metal nanoobjects having two external dimensions in the range of from 1 nm to 100 nm and a third external dimension in the range of from 1 μιη to 100 μιη,

(C) metal oxide nanoobjects wherein said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn, Ag, Ce, mixed oxides of In and Sn and mixed oxides of Sb and Sn,

(D) one or more metal salts of formula (I) as defined above wherein at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects (C).

Preferably said transparent electroconductive layer has a thickness in the range of from 10 nm to 10 μιη. Said thickness may be determined by profilometry, atomic force, microscopy or electron microscopy. The above-defined transparent electroconductive layer is arranged on a surface of said substrate in such manner that it partially or completely covers a surface of said substrate. In specific cases the layer forms a pattern on said surface of said substrate. The pattern may be selected from any random and non-random structures, like grids, stripes, waves, dots and circles.

In a preferred article according to the present invention said layer arranged on said surface of said substrate has

a haze of 3 % or less as measured according to ASTM D1003 (procedure A), a sheet resistance of 100 Ohm/square or less as measured by the four point probe, - a light transmission of 80 % or more as measured according to ASTM D1003 (procedure A).

The measurement of haze and light transmission (in ASTM D1003 as published in November 2013 referred to as luminous transmittance which is the ratio of the luminous flux transmitted by a body to the flux incident upon it) by means of a hazemeter is defined in ASTM-D1003 as published in November 2013 as "Procedure A -Hazemeter". The values of haze and light transmission (corresponding to the luminous transmittance as defined in ASTM D1003 as published in November 2013) given in the context of the present invention refer to this procedure.

The parameter "light transmission" refers to the percentage of an incident light which is transmitted through a medium. Preferably the light transmission of the transparent electroconductive layer is 85 % or more, more preferably 90 % or more, further preferably 95 % or more, in each case measured according to ASTM D1003 (Procedure A) as published in November 2013.

Generally, the parameter haze is an index of the light diffusion. It refers to the percentage of the quantity of light which is separated from the incident light and scattered during transmission. Unlike light transmission, which is largely a property of the medium, haze is often a production concern and is typically caused by surface roughness, and by embedded particles or compositional heterogeneities in the medium.

According to ASTM D1003 as published in November 2013, in transmission, haze is the scattering of light by a specimen responsible for the reduction in contrast of objects viewed through said specimen, i.e. the percent of transmitted light that is scattered so that its direction deviates more than a specified angle (2.5 °) from the direction of the incident beam.

Preferably the haze of the transparent electroconductive layer is 1.8 % or less, more preferably 1.5 % or less, further preferably 1 % or less, in each case measured according to ASTM D1003 (Procedure A) as published in November 2013.

The sheet resistance is a measure of resistance of a thin body (sheet) namely uniform in thickness. The term "sheet resistance" implies that the current flow is along the plane of the sheet, not perpendicular to it. For a sheet having a thickness t, a length L and a width W, the resistance R is

wherein R_sh is the sheet resistance. Accordingly the sheet resistance R_sh is

W

R sh ⁼ R *—

L

In the formula given above the bulk resistance R is multiplied with a dimensionless quantity (W/L) to obtain the sheet resistance R_sh, thus the unit of sheet resistance is Ohms. For the sake of avoiding confusion with the bulk resistance R, the value of the sheet resistance is commonly indicated as "Ohms per Square" (Ohms/square) because in the specific case of a square sheet W = L and R = R_sh. The sheet resistance is measured by means of a four point-probe.

Preferably the sheet resistance of the transparent electroconductive layer is 100 Ohms/square or less, more preferably 80 Ohms/square or less, further preferably 40 Ohms/square or less, in each case measured by the four point probe.

Further preferably, said transparent electroconductive layer (as defined above) exhibits one or more of the following features:

a haze of 1 % or less as measured according to ASTM D1003 (procedure A) as published in November 2013,

a sheet resistance of 50 Ohms/square or less as measured by the four point probe, a light transmission of 90 % or more as measured according to ASTM D1003 (procedure A) as published in November 2013. Particularly preferably, said transparent electroconductive layer (as defined above) exhibits the following features:

a haze of 1 % or less as measured according to ASTM D1003 (Procedure A) as published in November 2013, and

- a sheet resistance of 50 Ohms/square or less as measured by the four point probe, and

a light transmission of 90 % or more as measured according to ASTM D1003 (Procedure A) as published in November 2013.

Examples The oxidation resistance of metal nanoobjects (in the form of silver nanowires) in transparent electroconductive layers (according to the invention and comparison) was studied by means of electrochemical methods (cyclic voltammetry and chronoamperometry). The electrochemical measurements were accompanied by visual inspection of the transparent conductive layers and UV/VIS spectroscopy. The substrates for the transparent electroconductive layers were glass plates (2 cm x 5 cm) coated with indium-tin-oxide (ITO) in order to facilitate application of a voltage to the transparent electroconductive layer. The transparent electroconductive conductive layers were disposed on said indium-tin-oxide coatings.

Transparent electroconductive layers were prepared as follows: A first comparative transparent electroconductive layer was prepared by forming on the ITO-coated surface of a glass substrate (as defined above) a wet film by applying an ink comprising 0.5 wt.-% of silver nanowires suspended in iso-propanol. The ink did not contain metal oxide nanoparticles, nor metal salts of formula (I) as defined above. The ink was applied by means of spin coating. After coating, the iso-propanol was allowed to evaporate. The sequence of spin-coating and evaporation of the iso-propanol was repeated twice (total number of three spin coating steps). Thereafter the coated substrate was annealed at 130 °C for 10 minutes in an oven.

A second comparative transparent electroconductive layer was prepared by forming on the ITO-coated surface of a glass substrate (as defined above) a wet film by applying an ink obtained by combining in a volume ratio of 1 : 1 a suspension comprising 0.5 wt.-% of silver nanowires suspended in iso-propanol and a solution of 1 wt.-% of a polymerizable binder system in iso-propanol. The ink did not contain metal oxide nanoparticles, nor metal salts of formula (I) as defined above. The ink was applied by means of spin coating. After coating, the iso-propanol was allowed to evaporate. Thereafter, polymerization of the polymerizable monomers in the binder system is initiated by means of irradiation from a UV lamp for 4 seconds in dry ice atmosphere. The distance between the UV lamp and the layer comprising the polymerizable monomers was 4 cm. The sequence of spin- coating, evaporation of the iso-propanol and polymerization of the polymerizable monomers in the binder system was repeated twice (total number of three spin coating steps). Thereafter the coated substrate was annealed at 130 °C for 10 minutes in an oven.

A transparent electroconductive layer according to the invention was prepared by forming on the ITO-coated surface of a glass substrate (as defined above) a wet film by applying an ink according to the present invention. The ink according to the invention was obtained by combining in a volume ratio of 85:10:5

a suspension comprising 0.5 wt.-% of silver nanowires suspended in iso-propanol, a suspension comprising 5 wt.-% nanoparticles of Ce0₂ suspended in methanol with Y(N0₃)3^*6(H₂0) added in an amount of 2 wt.-% relative to the Ce0₂ content and

a solution of 5 wt.-% of a polymerizable binder system in iso-propanol.

The ink was applied by means of spin coating. After coating, the iso-propanol was allowing to evaporate. Thereafter, polymerization the polymerizable monomers in the binder system is initiated by means of irradiation from a UV lamp for 4 seconds in dry ice atmosphere. The distance between the UV lamp and the layer comprising the polymerizable monomers was 4 cm. The sequence of spin-coating, evaporation of the iso-propanol and polymerization of the polymerizable monomers in the binder system was repeated twice (total number of three spin coating steps). Thereafter the coated substrate was annealed at 130 °C for 10 minutes in an oven.

The silver nanowires dispersion was obtained from Seashell Technology LLC (product name: AgNW-25). The average diameter of the nanowires was 25 nm, and length of the nanowires was 22 μιη.

The cerium oxide (Ce0₂) nanoparticles were obtained from Nanograde AG. They had an average crystallite size of 1 1 nm, based on powder XRD data. Y(N0₃)₃ ^*6(H₂0) was obtained from Aldrich.

The polymerizable binder system consists of 586.5 parts by weight n-butyl acrylate, 127.5 parts by weight 1 ,4-butanediol monoacrylate, 5.1 parts by weight Irgacure 184, 2.6 parts by weight Irgacure TPO and 0.9 parts by weight Tinuvin 123. Irgacure 184 and Irgacure TPO are commercially available UV initiators, Tinuvin 123 is a commercially available stabilisator.

Cyclic voltammograms (scan rate 20 mV/s) and chronoamperometric curves were recorded using a SP300 potentiostat (supplier: Biologic) in a cuvette containing propylene carbonate with 2 wt.-% lithium trifluoromethanesulfonate. Ag/Ag+ was used as the refer- ence electrode and a platinum sheet as the counter electrode. In the figures, all voltages are vs. the Ag/Ag+ reference.

A cyclic voltammogramm of the pristine ITO-coated glass substrate (i.e. without any further layer deposited on the ITO coating) was recorded for comparison in order to determine the current response of ITO as function of the applied potential and distinguish it from the contribution of the tested transparent electroconductive conductive layers.

The cyclic voltammogramm of the pristine ITO-coated glass (figure 1 ) exhibits practically no current response in the voltage regime in a range between ca. -0.3V and +0.8V and an apparently Ohmic dependence of the current on the applied potential at lower and higher potentials. The cyclic voltammograms of the first and second comparative transparent electroconductive layer are shown in figure 2. In both cases, a remarkable anodic current was observed in a potential range between 0 and 0.5 V, accompanied by the appearance of a brownish color (visible to the naked eye) and loss of conductivity. In the next cycle (not shown), the anodic current is significantly reduced, and almost disappeared in the next cycles. Obviously, the silver nanowires have been oxidized in the first cycle and have not been completely reduced when the potential scan was reversed.

The cyclic voltammogram of the transparent electroconductive layer according to the invention is shown in figure 3. No anodic current attributable to oxidation of the silver nanowires was observed, despite the anodic potential limit (1.5 V) of the cyclic voltammogramm was significantly larger than in the cyclic voltammograms of the first and second comparative transparent electroconductive layer. The shape of the cyclic voltammogramm remained practically unchanged over 5 cycles. The visual appearance of the transparent electroconductive layer did not change.

UV-Vis-spectra of the transparent electroconductive layer according to the invention recorded before and after recording the cyclic voltammogramm were identical within the limits of error (see figures 4A and B), thus confirming that virtually no oxidation of the silver nanowires occurred.

In chronoamperometric studies of the transparent electroconductive layer according to the invention, a potential of +2.0 V was applied for 15 minutes followed by switching the potential to -1 .0 V which was applied for one minute. During this sequence the current was recorded. The sequence was repeated 5 times. The results are shown in figure 5. Like in the cyclic voltammetry studies, no anodic current attributable to oxidation of the silver nanowires could be observed.

Claims

Claims:

1. Composition comprising

(A) a carrier liquid having a boiling point of less than 120 °C

(B) metal nanoobjects having two external dimensions in the range of from 1 nm to 100 nm and a third external dimension in the range of from 1 μιη to 100 μιη

(C) metal oxide nanoobjects wherein said nanoobjects have one, two or three external dimensions in the range of from 1 nm to 100 nm, and said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn, Ag, Ce, mixed oxides of In and Sn and mixed oxides of Sb and Sn

(D) one or more metal salts of formula (I)

(M^a+)_z(R^b )_y (I),

wherein

M^a+ represents a metal cation,

R^b" represents the salt anion,

a is 2, 3, 4 or 5,

b is 1 , 2 or 3,

z is the least common multiple of a and b, divided by a

y is the least common multiple of a and b, divided by b.

2. Composition according to claim 1 wherein

(A) said carrier liquid is selected from the group consisting of water, methanol, ethanol, 1-propanol, 2-propanol, 2-butanol, iso-butanol, acetonitrile, propionitrile and tetrahydrofuran (THF) and mixtures thereof.

3. Composition according to any preceding claim wherein

(B) said metal nanoobjects are nanowires consisting of a metal selected from the group consisting of silver, copper, gold, platinum, tungsten and nickel, or of an alloy of two or more metals selected from the group consisting of silver, copper, gold, platinum, tungsten, aluminum, iron, cobalt and nickel, wherein said nanowires preferably have a length in the range of from 1 μιη to 100 μιη, and a diameter in the range of from 1 nm to 100 nm, preferably 10 nm to 50 nm, most preferably 15 nm to 30 nm, length and diameter in each case determined by transmission electron microscopy.

Composition according to any preceding claim wherein

(C) said metal oxide nanoobjects are nanoparticles having a primary particle diameter of 1 nm to 100 nm measured by nitrogen absorption, X-ray diffraction or transmission electron microscopy,

wherein said metal oxide is preferably selected from the group consisting of oxides of Ti, Si, Sn, Zn, and Ce, most preferably Ce0₂.

Composition according to any preceding claim wherein

(D) in said metal salts of formula (I)

M represents a metal selected from the group consisting of Zn, Al, Sc, Ga, Y, Pb, Bi, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, La, Mg, Ca, Sr and Ba, most preferably one of Zn, Al and Y and/or

R^b" represents an organic anion selected from the group consisting of acetate, formiate, citrate, oxalate, or an inorganic anion selected from the group consisting of nitrate, difluorophosphate, hexafluorophosphate and tetrafluroborate,.

wherein said metal salts of formula (I) are preferably selected from the group consisting of zinc diacteate, aluminium triacetate, yttrium triacetate, zinc dinitrate, aluminium trinitrate, and yttrium trinitrate, most preferably yttrium trinitrate.

Composition according to any preceding claim wherein

(C) said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, and Ce

and

(D) said metal salts of formula (I) are selected from the group consisting of zinc diacteate, aluminium triacetate, yttrium triacetate, zinc dinitrate, aluminium trinitrate, and yttrium trinitrate. Composition according to any preceding claim, further comprising

(E) one or more organic polymers suspended or dissolved in said carrier liquid (A).

Composition according to any of claims 1 to 6, further comprising

(F) one or more polymerizable monomers,

and

(G) one or more initiators for initiating radical polymerization of said polymerizable monomers.

Process for preparing a layer comprising metal nanoobjects on a surface of a substrate,

said process comprising the steps of

forming on a surface of said substrate a wet film by applying a composition according to any of claims 1 to 8 to said surface of said substrate removing said carrier liquid having a boiling point below 120 °C from the wet film formed on said surface of said substrate.

Process according to claim 9, wherein

said composition is applied to said surface of said substrate by coating or printing,

and/or

said carrier liquid having a boiling point of less than 120 °C is removed by exposing the wet film formed on said surface of said substrate to air having a temperature in the range of from 20 °C to 120 °C.

Process according to claim 9 or 10, wherein

said composition applied to said surface of said substrate comprises (F) one or more polymerizable monomers, and said process further comprises the step of polymerizing the polymerizable monomers on said surface of said substrate.

12. Use of a composition according to any of claims 1 to 8 for preparing a layer comprising metal nanoobjects on a surface of a substrate. Article comprising

a substrate having a surface

and, arranged on said surface of said substrate, a layer comprising

(B) metal nanoobjects, said metal nanoobjects having two external dimensions in the range of from 1 nm to 100 nm and a third external dimension in the range of from 1 μιη to 100 μιη,

(C) metal oxide nanoobjects wherein said nanoobjects have one, two or three external dimensions in the range of from 1 nm to 100 nm, and said metal oxide is selected from the group consisting of oxides of Ti, Si, Sn, Zn, Ag, Ce, mixed oxides of In and Sn and mixed oxides of Sb and Sn,

(D) one or more metal salts of formula (I) as defined in any of claims 1 , 5 and 6, wherein at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects (C).

Article according to claim 13,

wherein said layer arranged on said surface of said substrate further comprises a matrix formed of one or more organic polymers,

wherein said metal nanoobjects (B) and said metal oxide nanoobjects (C) and said one or more metal salts of formula (I) are dispersed within said matrix, and at least a portion of said metal salts of formula (I) is physisorbed on the surfaces of said metal oxide nanoobjects (C).

Article according to claim 13 or 14, wherein

said substrate comprises one or more materials selected from the group consisting of glasses and organic polymers and has a light transmission of 80 % or more measured according to ASTM D1003 (Procedure A) as published in November 2013.

Article according to any of claims 13 to 15, wherein said layer has

a haze of 3 % or less as measured according to ASTM D1003 (procedure A),

a sheet resistance of 100 Ohm/square or less as measured by the four point probe, a light transmission of 80 % or more as measured according to ASTM D1003 (procedure A).