Transferable compositions and methods of making and using the same

Abstract

The invention discloses a transferable composition that can be applied to a film, and the film is used to transfer the pattern of the transferable composition to a desired substrate. The transferable composition includes a swellable polymer swollen by a swelling agent and particles dispersed therein. Also provided are methods for preparing the transferable composition, applying it as a pattern to a transfer film, and transferring the pattern to a substrate, as well as articles prepared thereby. The transferred pattern can be used for decorative and / or functional purposes, and the transferable composition can correspondingly contain decorative and / or functional particles. When the functional particles are conductive or can be endowed with conductivity, the pattern formed therewith can be part of a conductive circuit and can be used, for example, in the manufacture of solar cells.

Description

[0001] Related applications

[0002] This application is a divisional application of Chinese patent application No. 2020800733608, entitled “Transferable Compositions and Methods of Making and Using Them,” which entered the Chinese national phase on April 14, 2022. This application claims Paris Convention priority from UK Patent Application No. 1915300.6, filed on October 22, 2019, the contents of which are incorporated herein by reference in their entirety as if fully set forth herein. Technical Field

[0003] The present invention relates to a transferable composition that can be transferred to a substrate for preparing a pattern. In particular, the present invention relates to a transferable composition suitable for preparing a transfer film comprising a transferable pattern formed from the transferable composition. Also provided are methods for preparing and using the transferable composition and the transfer film. Background Art

[0004] In the industrial field, articles with patterns on their surfaces are common. Such patterns can be used for decorative or functional purposes. For example, the material forming the pattern may include a colorant for decorative purposes or conductive particles for functional purposes (e.g., where the pattern functions to form an electrical circuit).

[0005] While some patterns can be applied directly to the intended product (e.g., by painting), some items may require greater precision or consistency, or simply be a challenge to pattern directly. For example, to create a pattern on a glass window already installed in a building, it may be preferable to use a decal.

[0006] When the patterns contain conductive particles, they can be used in a variety of products and applications, such as PCBs (Printed Circuit Boards), radio frequency identification (RFID) tags, flip-chip interconnect boards (sometimes called interposers), printed flexible circuits, touch screens, or solar cells.

[0007] For solar cells, solar cells are widely used as devices that convert light, typically solar energy but also artificial light, into electricity. Solar or photovoltaic cells are typically made of relatively thin silicon wafers of several hundred microns, which are composed of multiple layers, for example, these layers include a phosphorus-doped n-type layer and a boron-doped p-type layer on which photons impinge. The interface between the n-type layer and the p-type layer, regardless of the dopants used to form them, is called a pn junction. A solar cell may include one or more pn junctions. Electrodes are applied on two opposing sides of the wafer to drive and guide the flow of electrons, and the resulting current is connected to an external circuit. The wafer-side electrode used to capture light is usually called the front or front electrode, which is usually formed by a grid-like pattern with relatively narrow conductive lines (also called fingers) and relatively wide lateral strips (also called bus bars), the relatively narrow conductive lines being spaced apart from each other to allow them to be fully exposed to light. The pattern can include various conductive materials, usually metals such as silver. The electrode on the other side of the substrate is called the back or rear electrode, which usually has a less complex pattern and can even be a continuous layer, the area of ​​its coating substantially corresponding to the front electrode. The rear electrode does not need to be made of the same conductive material as the front electrode, for example, it can be made of an aluminum layer. Although the above terms are often used to refer to single-sided solar cells for convenience, they should not be interpreted as restrictive, because there are bifacial solar cells that collect light from any direction by providing a grid pattern on both sides of the wafer to improve efficiency. In addition, although solid wafers of crystalline silicon (c-Si) still dominate (including the recent emitter passivation and back contact (PERC) solar cells), new substrates, such as thin-film solar cells with a thickness of only a few microns, are also being used more and more.

[0008] Various methods are currently available for applying conductive patterns to substrates, and their selection depends primarily on the type of pattern, substrate, and the final use of the patterned substrate. Typically, conductive patterns are formed directly on substrates. In some cases, (for example, in a process similar to printing) this is achieved by the selective deposition of conductive materials, and in other cases, by coating the entire surface of the substrate with conductive materials, and then selectively removing part of the conductive materials (such as by etching or laser ablation) to leave the desired pattern. Although the composition for forming this pattern is generally referred to as conductive compositions, they generally require one or more additional steps to obtain this electrical property and provide the desired function, and the additional steps are for example sintered and fired by applying energy thereto. There is also a composition that does not require any special processing of the pattern after the composition is patterned on the substrate and has sufficient conductivity for the intended pattern or final use. Each known method has its own advantages and disadvantages.

[0009] The inventors have designed and described in WO 2018 / 020479, US2019 / 0172967, WO 2018 / 020481 and US10,645,815 a method for applying a pattern of a composition comprising particles of a conductive material (e.g., made of metal) to a substrate, wherein the conductor pattern formed by the conductor is not produced directly on the substrate, but is formed on a separate transfer film surface or in a pattern of grooves formed therein. The pattern is dried and applied to the intended substrate (e.g., a solar cell wafer or a PCB insulating board) by transfer, if necessary, after which the particles are typically sintered to make the transferred pattern conductive. The inventors further describe in detail in WO 2018 / 020483 and US2019 / 0174635 a device for transferring a pattern of such a transferable composition from the surface of a transfer film (e.g., a flexible web) to the surface of a substrate, which advantageously allows the pattern to be applied synchronously to both sides of the substrate, which in the case of a solar cell may correspond to the front electrode and the back electrode.

[0010] Regardless of the preparation method used (e.g., directly on a substrate or indirectly on a transfer film), it is a common goal for everyone to be able to improve the efficiency of conductive patterns (e.g., solar cells), even if it is only a seemingly insignificant improvement of a few percentage points. This goal is particularly challenging when the conductive composition that can especially contribute to improving such efficiency needs to be transferable in addition to having the properties expected of compositions used in conventional methods. Summary of the Invention

[0011] In a first aspect of the present invention, a transferable composition is provided, which is suitable for applying a pattern on a transfer film and for transferring the pattern from the transfer film to a substrate, the transferable composition comprising:

[0012] i) a swellable polymer;

[0013] ii) a swelling agent; and

[0014] iii) a plurality of particles, said particles providing decorative and / or functional purposes to a pattern comprising said transferable composition;

[0015] wherein a) the swelling agent is capable of swelling the swellable polymer to form a swellable polymer premix; and b) the decorative and / or functional particles (i.e., particles that provide decorative and / or functional purposes for the transferable composition) are dispersed in the polymer premix.

[0016] In a second aspect of the present invention, a transferable composition is provided, which is suitable for applying a pattern on a transfer film and for transferring the pattern from the transfer film to a substrate, the transferable composition comprising:

[0017] i) a swellable polymer;

[0018] ii) a swelling agent; and

[0019] iii) a plurality of particles, said particles providing decorative and / or functional purposes to a pattern comprising said transferable composition;

[0020] wherein a) the swelling agent is capable of swelling the swellable polymer to form a premix of the swellable polymer, the weight of the swelling agent being at least 5 wt.%, at least 10 wt.%, at least 15 wt.%, or at least 20 wt.% of the weight of the swellable polymer; and b) the decorative and / or functional particles (i.e., particles that provide decorative and / or functional purposes to the transferable composition) are dispersed in the polymer premix.

[0021] In a third aspect of the present invention, there is provided a transferable conductive composition suitable for applying a pattern on a transfer film and for transferring the pattern from the transfer film to a substrate, the transferable conductive composition comprising:

[0022] i) a swellable polymer;

[0023] ii) a swelling agent; and

[0024] iii) a plurality of conductive particles;

[0025] wherein a) the swelling agent is capable of swelling the swellable polymer to form a swollen polymer premix; and b) conductive particles are dispersed in the polymer premix.

[0026] In a fourth aspect of the present invention, there is provided a transferable conductive composition suitable for applying a pattern on a transfer film and for transferring the pattern from the transfer film to a substrate, the transferable conductive composition comprising:

[0027] i) a swellable polymer;

[0028] ii) a swelling agent; and

[0029] iii) a plurality of conductive particles;

[0030] wherein a) the swelling agent is capable of swelling the swellable polymer to form a premix of the swellable polymer, the weight of the swelling agent being at least 5 wt.%, at least 10 wt.%, at least 15 wt.% or at least 20 wt.% of the weight of the swellable polymer; and b) conductive particles are dispersed in the polymer premix.

[0031] In one embodiment, the ratio of the weight of the swellable polymer (SP) to the weight of the swelling agent (SA) contained in the transferable (e.g., conductive) composition, SP / SA, satisfies one or more of the following:

[0032] a. SP / SA is at least 1:99, at least 1:49, at least 1:19, at least 1:10, at least 1:6 or at least 1:5;

[0033] b. SP / SA is at most 4:1, at most 2.3:1, at most 1.5:1, at most 1:1 or at most 0.5:1; and

[0034] c. SP / SA ranges between 1:99 and 9:1, between 1:99 and 4:1, between 1:49 and 2.3:1, between 1:49 and 1.5:1, between 1:19 and 1:1, between 1:10 and 0.5:1, between 1:6 and 0.5:1 or between 1:5 and 0.5:1.

[0035] In a specific embodiment, the SP / SA ratio is 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:5.5 or 1:6.

[0036] In another embodiment, the swellable polymer satisfies at least one, at least two, or at least three of the following structural characteristics:

[0037] a) the swellable polymer is selected from amorphous thermoplastic polymers, semi-crystalline thermoplastic polymers and crystalline thermoplastic polymers;

[0038] b) the swellable polymer has a melt flow rate of at least 0.2 g / 10 min, at least 0.3 g / 10 min, at least 0.5 g / 10 min, at least 1 g / 10 min, at least 2 g / 10 min, or at least 5 g / 10 min, measured at 190°C with a load of 2.16 kg;

[0039] c) the swellable polymer has a melt flow rate of at most 500 g / 10 min, at most 250 g / 10 min, at most 200 g / 10 min, at most 150 g / 10 min, at most 100 g / 10 min, at most 50 g / 10 min, or at most 25 g / 10 min, measured at 190°C with a load of 2.16 kg;

[0040] d) the swellable polymer has a melt flow rate in the range of 0.2 g / 10 min to 500 g / 10 min, 0.2 g / 10 min to 250 g / 10 min, 0.2 g / 10 min to 200 g / 10 min, 0.2 g / 10 min to 150 g / 10 min, 0.2 g / 10 min to 100 g / 10 min, 0.5 g / 10 min to 50 g / 10 min, 1 g / 10 min to 100 g / 10 min, 1 g / 10 min to 75 g / 10 min, 1 g / 10 min to 50 g / 10 min, 2 g / 10 min to 50 g / 10 min, or 2 g / 10 min to 25 g / 10 min, measured at 190°C with a load of 2.16 kg;

[0041] e) the swellable polymer has a melting temperature of at least 50°C, at least 60°C, at least 70°C, or at least 80°C;

[0042] f) the swellable polymer has a melting temperature of at most 250°C, at most 200°C, at most 150°C, at most 140°C, at most 130°C, at most 120°C, at most 110°C, or at most 100°C;

[0043] g) the swellable polymer has a melting temperature in the range of 50°C to 250°C, 60°C to 200°C, 65°C to 150°C, 65°C to 140°C, 65°C to 130°C, 70°C to 120°C, or 70°C to 100°C;

[0044] h) the swellable polymer has a Vicat softening temperature of at least 30°C, at least 40°C, at least 50°C, or at least 60°C;

[0045] i) the swellable polymer has a Vicat softening temperature of at most 220°C, at most 180°C, at most 140°C, at most 120°C, at most 100°C, or at most 80°C; and

[0046] j) the swellable polymer has a Vicat softening temperature in the range of 30°C to 140°C, 40°C to 120°C, 50°C to 100°C, or 50°C to 90°C.

[0047] In one embodiment, the swellable polymer satisfies at least the structural property a) listed above. In one embodiment, the swellable polymer satisfies at least the structural property b) listed above. In one embodiment, the swellable polymer satisfies at least the structural property c) listed above. In one embodiment, the swellable polymer satisfies at least the structural property d) listed above. In one embodiment, the swellable polymer satisfies at least the structural property e) listed above. In one embodiment, the swellable polymer satisfies at least the structural property f) listed above. In one embodiment, the swellable polymer satisfies at least the structural property g) listed above. In one embodiment, the swellable polymer satisfies at least the structural property h) listed above. In one embodiment, the swellable polymer satisfies at least the structural property i) listed above. In one embodiment, the swellable polymer satisfies at least the structural property j) listed above.

[0048] In one embodiment, the swellable polymer satisfies at least the structural properties a), b), c), and d) listed above. In one embodiment, the swellable polymer satisfies at least the structural properties a), e), f), and g) listed above. In one embodiment, the swellable polymer satisfies at least the structural properties a), h), i), and j) listed above. In one embodiment, the swellable polymer satisfies at least the structural properties a), b), e), and h) listed above. In one embodiment, the swellable polymer satisfies at least the structural properties a), c), f), and i) listed above. In one embodiment, the swellable polymer satisfies at least the structural properties a), d), g), and j) listed above.

[0049] In one embodiment, in addition to at least one, at least two, or at least three of the structural properties listed in a) to j), the swellable polymer included in the transferable (e.g., conductive) composition of the present invention further satisfies one or more of the following interaction properties with the other components of the composition:

[0050] k) the swellable polymer is immiscible with the swelling agent;

[0051] 1) No more than 10 wt.%, no more than 5 wt.%, no more than 2 wt.%, or no more than 1 wt.% of the swellable polymer is soluble in the swelling agent, based on the weight of the swelling agent.

[0052] In another embodiment, the swellable polymer contained in the transferable (e.g., conductive) composition is a thermoplastic polymer selected from ethylene / vinyl acetate (EVA) copolymers, acid-modified ethylene acrylate resins, ethylene / acrylate / maleic anhydride terpolymers, polyvinyl butyral, polyvinyl alcohol, and polyamides.

[0053] In one embodiment, the swellable polymer is present in the composition at a volume concentration of at least 1 vol.%, at least 2 vol.%, at least 5 vol.%, or at least 10 vol.%, based on the volume of all solids. In another embodiment, the swellable polymer is present in the composition at a volume concentration of at most 25 vol.%, at most 22 vol.%, or at most 20 vol.%, based on the volume of all solids. In yet another embodiment, the swellable polymer is present in the composition at a volume concentration of from 2 vol.% to 25 vol.%, from 5 vol.% to 25 vol.%, or from 10 vol.% to 22 vol.%, based on the volume of all solids in the composition.

[0054] In one embodiment, the swelling agent contained in the transferable (e.g., conductive) composition of the present invention satisfies at least one, at least two, or at least three of the following structural characteristics:

[0055] A) the swelling agent is an aqueous solvent comprising at least 75 wt.%, at least 80 wt.%, at least 85 wt.%, at least 90 wt.%, or at least 95 wt.% water;

[0056] B) the swelling agent is an organic solvent containing at most 25 wt.%, at most 20 wt.%, at most 15 wt.%, at most 10 wt.%, or at most 5 wt.% water;

[0057] C) the swelling agent comprises at most 1.0 wt.%, at most 0.5 wt.%, at most 0.1 wt.%, at most 0.05 wt.%, or at most 0.01 wt.% water;

[0058] D) the swelling agent has a boiling temperature of at least 90°C, at least 100°C, at least 110°C, at least 120°C, at least 130°C, at least 140°C, at least 150°C, or at least 160°C;

[0059] E) the swelling agent has a boiling temperature of at most 450°C, at most 400°C, at most 350°C, at most 300°C, at most 290°C, at most 280°C, at most 270°C, at most 260°C, or at most 250°C;

[0060] F) the swelling agent has a boiling temperature in the range of 90°C to 450°C, 100°C to 450°C, 100°C to 400°C, 100°C to 350°C, 100°C to 300°C, 105°C to 290°C, 110°C to 280°C, 115°C to 270°C, 120°C to 300°C, 120°C to 250°C, 130°C to 400°C, 160°C to 350°C, 150°C to 300°C, or 150°C to 250°C;

[0061] G) The swelling agent has at least 10 -6 kPa, at least 10 -5 kPa, at least 10 -4 kPa, at least 5×10 -4 kPa or at least 10 -3 Vapor pressure in kPa (as measured by standard methods at room temperature, about 20°C to 25°C);

[0062] H) the swelling agent has a maximum of 5 kPa, a maximum of 4 kPa, a maximum of 3 kPa, a maximum of 2 kPa, a maximum of 1 kPa, a maximum of 0.5 kPa, a maximum of 10 -1 kPa, up to 5×10 -2 kPa or up to 10 -2 Vapor pressure in kPa (as measured by standard methods at room temperature, about 20°C to 25°C);

[0063] 1) The swelling agent has a -6 kPa to 5kPa, 10 -5 kPa to 4kPa, 10 -4 kPa to 3kPa, 10 -3 kPa to 2.5kPa, 10 -6 kPa to 0.5kPa, 10 -6 kPa to 10 -1 kPa or 10 -6 kPa to 10 -2 Vapor pressure in the kPa range (measured by standard methods at room temperature, approximately 20°C to 25°C); and

[0064] J) The swelling agent has a viscosity of at least 1 mPa.s, and optionally at most 1,000 mPa.s, at most 500 mPa.s, at most 250 mPa.s or at most 150 mPa.s (measured by standard methods at room temperature between 20°C and 25°C).

[0065] In one embodiment, in addition to at least one, at least two, or at least three of the structural properties listed in A) to J), the swelling agent contained in the transferable (e.g., conductive) composition of the present invention further satisfies one or more of the following interaction characteristics with the other components of the transferable (e.g., conductive) composition:

[0066] K) the swelling agent is chemically inert with respect to the (e.g. conductive) particles;

[0067] L) the swelling agent is immiscible with the swellable polymer;

[0068] M) The swelling agent is capable of dissolving no more than 10 wt.%, no more than 5 wt.%, no more than 2 wt.%, or no more than 1 wt.% of the swellable polymer, based on the weight of the swellable polymer.

[0069] In one embodiment, the swelling agent satisfies at least the structural property A listed above. In one embodiment, the swelling agent satisfies at least the structural property B listed above. In one embodiment, the swelling agent satisfies at least the structural property C listed above. In one embodiment, the swelling agent satisfies at least the structural property D listed above. In one embodiment, the swelling agent satisfies at least the structural property E listed above. In one embodiment, the swelling agent satisfies at least the structural property F listed above. In one embodiment, the swelling agent satisfies at least the structural property G listed above. In one embodiment, the swelling agent satisfies at least the structural property H listed above. In one embodiment, the swelling agent satisfies at least the structural property I listed above. In one embodiment, the swelling agent satisfies at least the structural property J listed above. In one embodiment, the swelling agent satisfies at least the interaction property K listed above. In one embodiment, the swelling agent satisfies at least the interaction property L listed above. In one embodiment, the swelling agent satisfies at least the interaction property M listed above.

[0070] In one embodiment, the swelling agent satisfies at least the structural properties D), I), and J listed above. In one embodiment, the swelling agent satisfies at least the structural properties E), I), and J listed above. In one embodiment, the swelling agent satisfies at least the structural properties F), G), and J listed above. In one embodiment, the swelling agent satisfies at least the structural properties F), H), and J listed above. In one embodiment, the swelling agent satisfies at least the structural properties A), F), I), and J listed above. In one embodiment, the swelling agent satisfies at least the structural properties B), F), I), and J listed above. In one embodiment, the swelling agent satisfies at least the structural properties C), F), I), and J) listed above.

[0071] In one embodiment, the swelling agent satisfies at least the interaction properties K) and L) listed above. In one embodiment, the swelling agent satisfies at least the interaction properties K) and M) listed above. In one embodiment, the swelling agent satisfies at least the interaction properties L) and M) listed above. In some embodiments, in addition to the structural properties or combinations thereof of the swelling agent described above, the swelling agent (or blend thereof) further exhibits the aforementioned interaction properties.

[0072] In another embodiment, the swelling agent is selected from C 6-20 Isoparaffins, C 3-10 Ketone, C 1-12 Alcohol, C 6-10 Aromatic hydrocarbons, glycerin, water and combinations thereof.

[0073] In one embodiment, the particles contained in the transferable composition are or include colorants, the presence of which enables the preparation of decorative patterns and / or facilitates the visualization of functional patterns. The colorant can be selected from natural or synthetic, organic or inorganic pigments and dyes, or from other substances capable of imparting color or any other detectable visual effect (e.g., pearlescent, shimmering, fluorescent, etc.), and combinations thereof.

[0074] Such colorants are well known and do not require detailed description here. For simplicity, organic pigments can be selected from perylene pigments; phthalocyanine pigments; quinacridone pigments; and imidazolinone pigments. Inorganic pigments can include, for example, titanium dioxide, cadmium sulfoselenide, iron oxide, bismuth vanadate, cobalt titanate, sodium sulfoaluminosilicate, mixed Fe-Mg-Ti oxides, manganese ferrite, and metal or alloy pigments.

[0075] In one embodiment, the conductive particles included in the transferable conductive composition include or consist of metals, alloys, organometallics, and their oxides, precursors, hydrates, and / or salts; or include or consist of a conductive polymer. Particles that can provide a functional effect (e.g., conductivity) to the pattern can additionally or alternatively provide a decorative effect and can accordingly be used to prepare transferable compositions other than conductive.

[0076] In one embodiment, the conductive particles can be obtained by having a core made of a different substance and further coating the core with the above-mentioned conductive material. For example, solid or hollow glass, ceramic or plastic microspheres (e.g., made of barium titanate, silicon dioxide or polymethyl methacrylate) can be at least partially coated with a metal (e.g., aluminum, silver, gold, nickel, etc.) to form conductive particles.

[0077] In another embodiment, the conductive particles (whether formed of a conductive material or formed of a different substance at least partially coated with such a conductive material) are present in the composition at a volume concentration of at least 65 vol.%, at least 70 vol.%, or at least 75 vol.%, based on the volume of all solids. In one embodiment, the conductive particles are present in the composition at a volume concentration of at most 98 vol.%, at most 95 vol.%, or at most 90 vol.%, based on the volume of all solids. In yet another embodiment, the conductive particles are present in the composition at a volume concentration of 65 vol.% to 98 vol.%, 70 vol.% to 95 vol.%, or 75 vol.% to 90 vol.%, based on the volume of all solids in the composition.

[0078] In one embodiment, the transferable (e.g., conductive) composition further comprises glass frit. In another embodiment, the glass frit is present in the composition at a volume concentration of 0.1 vol.% to 15 vol.%, 0.2 vol.% to 12.5 vol.%, 0.5 vol.% to 10 vol.%, 0.5 vol.% to 8 vol.%, 0.5 vol.% to 6 vol.%, 0.5 vol.% to 5 vol.%, or 1 vol.% to 5 vol.%, based on the volume of the conductive particles.

[0079] Suitable glass frits for such transferable (e.g., conductive) compositions include silicon dioxide and one or more compounds selected from the group consisting of lead oxide, zinc oxide, boron oxide, bismuth oxide, strontium oxide, aluminum oxide, tellurium oxide, vanadium pentoxide, barium oxide, magnesium oxide, molybdenum oxide, molybdenum trioxide, phosphorus pentoxide, tin oxide, and tungsten oxide.

[0080] In one embodiment, the transferable (e.g., conductive) composition further comprises one or more rheology modifiers. In another embodiment, the one or more rheology modifiers are present in the composition in a total amount of 0.05 wt.% to 20 wt.%, 0.1 wt.% to 15 wt.%, 0.1 wt.% to 10 wt.%, 0.5 wt.% to 5 wt.%, or 1 wt.% to 4 wt.%, based on the weight of all components. In yet another embodiment, the rheology modifier is selected from the group consisting of dibutyl sebacate, butyl stearate, ethylene glycol coconut fatty acid esters, butyl ricinoleate, dibutyl phthalate, castor oil, hydrogenated castor oil, hydrogenated microcrystalline wax, hydrogenated resin, hydrocarbon resin, paraffin wax, beeswax, cetyl alcohol, butyl stearate, diethyl phthalate, diphenyl phthalate, dicyclohexyl phthalate, 2-butoxy-2-oxoethylbutyl phthalate, and dioctyl phthalate.

[0081] In one embodiment, the transferable (e.g., conductive) composition further comprises one or more dispersants. In another embodiment, the one or more dispersants are present in the composition in a total amount of 0.05 wt.% to 20 wt.%, 0.1 wt.% to 15 wt.%, 0.1 wt.% to 10 wt.%, 0.5 wt.% to 5 wt.%, or 1 wt.% to 4 wt.%, based on the weight of all components. In yet another embodiment, the dispersant is selected from the group consisting of ether / ester dispersants, including a composite mixture of a phosphate ester and oleyl alcohol polyglycol ether; a composite mixture of a phosphate ester and polyoxypropylene; a cetyl alcohol polyoxyethylene ether; a composite mixture of a phosphate ester and tridecyl alcohol polyglycol ether; a composite mixture of a phosphate ester and isotridecyl glycol ester; and an anionic dispersant.

[0082] In one embodiment, the transferable (e.g., conductive) composition comprising a swellable polymer, a swelling agent, and particles that provide decorative and / or functional use to a pattern containing the transferable (e.g., conductive) composition satisfies at least one of the following:

[0083] i) the swellable polymer is an EVA copolymer, the weight content of vinyl acetate in the EVA copolymer is at least 10 wt.%, at least 15 wt.%, or at least 20 wt.%, and optionally at most 40 wt.% or at most 30 wt.%;

[0084] ii) the swelling agent is an isoparaffin having a carbon chain length of 10 to 19 carbon atoms; and

[0085] iii) the particles comprise or consist of metal, and the transferable (eg conductive) composition optionally has conductive properties.

[0086] In one embodiment, when applying the transferable composition to the intended surface (e.g., a substrate or a transfer film allowing transfer to a substrate) also involves shear as applied in a screen printing process, the transferable composition preferably has a shear-thinning behavior wherein the transferable composition is thinned within at least 150 seconds. -1 At a shear rate of 1000 nm and a temperature of about 80° C., the viscosity of the transferable composition decreases by at least one order of magnitude.

[0087] In one embodiment, the transferable conductive composition is suitable for applying a pattern to a transfer film and transferring the pattern from the transfer film to a substrate suitable for manufacturing a solar cell. In this case, more simply stated, the transferable conductive composition is suitable for manufacturing a solar cell. In such an embodiment, the "transferability" of the composition relates to its ability to be transferred from the transfer film to the substrate, which presents significant challenges for the composition and is generally not addressed by applying the composition directly to the intended substrate using conventional compositions. In contrast, conventional compositions are generally non-transferable once applied.

[0088] In a preferred embodiment, when the transfer film is required to be separated from the substrate, the transferable composition is substantially completely transferred from the transfer film to the substrate, thereby leaving substantially no residue of the composition on the transfer film (e.g., less than 5 wt%, or less than 4 wt%, less than 3 wt%, less than 2 wt%, less than 1 wt%, less than 0.5 wt%, or less than 0.1 wt% based on the weight of the composition applied to the transfer film). The advantage of this embodiment is that the desired transferable pattern can be formed on the transfer film because the pattern is substantially completely transferred to the substrate.

[0089] In the present invention, the "transferability" of the transferable composition suitable for applying a pattern on a substrate, in particular suitable for applying a pattern on a transfer film and transferring the pattern from the transfer film to the substrate, includes preferential adhesion of the pattern to the substrate before optional separation between the transfer film and the substrate, and after complete transfer to the substrate (after separation from the film and complete release therefrom).

[0090] In another aspect of the present invention, there is provided a method for preparing a transferable composition, the method comprising:

[0091] a) providing a polymer premix comprising a swellable polymer swollen with a swelling agent; said polymer premix being at a temperature above the softening temperature of said swellable polymer;

[0092] The polymer premix of step a) is mixed with decorative or functional particles at a temperature of at least 50°C, at least 60°C, at least 70°C, or at least 80°C and optionally at most 250°C, at most 200°C, at most 150°C or at most 100°C to disperse the particles in the polymer premix to obtain the transferable composition.

[0093] In another aspect of the present invention, there is provided a method for preparing a transferable conductive composition, the method comprising:

[0094] a) providing a polymer premix comprising a swellable polymer swollen with a swelling agent; said polymer premix being at a temperature above the softening temperature of said swellable polymer;

[0095] b) mixing the polymer premix of step a) with conductive particles at a temperature of at least 50° C., at least 60° C., at least 70° C., or at least 80° C., and optionally at most 250° C., at most 200° C., at most 150° C., or at most 100° C., to disperse the conductive particles within the polymer premix, thereby obtaining a transferable conductive composition.

[0096] In one embodiment, the polymer premix of step a) is prepared by combining a swellable polymer and a swelling agent to obtain a mixture, and heating the mixture until the swellable polymer is completely melted. In another embodiment, the heating temperature is at least 20°C, at least 40°C, or at least 60°C higher than the melting point of the polymer, and optionally at least 40°C, at least 60°C, or at least 80°C lower than the boiling point of the swelling agent.

[0097] In one embodiment, swellable polymers, swelling agents, particles, and transferable (e.g., conductive) compositions prepared therefrom may be prepared according to the detailed teachings of the present invention, including previously disclosed embodiments, in which case the method or steps thereof may be adapted accordingly.

[0098] For example, in one embodiment, the method for preparing a transferable composition or a transferable conductive composition further comprises adding glass frit to the polymer premix during step a), or adding glass frit and decorative or functional particles (e.g., conductive particles) to the polymer premix during step b), or both.

[0099] In one embodiment, the transferable conductive composition prepared by the method briefly described above and further detailed herein is a transferable conductive composition suitable for applying a pattern to a transfer film and for transferring the pattern from the transfer film to a substrate suitable for making a solar cell. In this case, more simply stated, the method of preparing the transferable conductive composition is suitable for making a solar cell.

[0100] In another aspect of the present invention, there is provided a method of making a transfer film suitable for applying a decorative or functional pattern to a substrate, the method comprising:

[0101] a) applying a transferable composition comprising (i) a swellable polymer swollen with a swelling agent and (ii) decorative or functional particles to a transfer film using an applicator, the transferable composition being applied at a sufficiently high application temperature at which the transferable composition is sufficiently flowable for the applicator;

[0102] b) optionally, before and / or during application of the transferable composition, heating the transfer film to a temperature that differs by at most 20° C. from the application temperature of the transferable composition;

[0103] c) optionally, removing excess transferable composition from the surface of the transfer film, thereby leaving a pattern thereon;

[0104] d) optionally, removing at least some, but not all, of the swelling agent from the pattern; and

[0105] e) lowering the temperature of the transfer film or allowing the temperature of the transfer film to lower so that the pattern of transferable composition applied thereto is sufficiently adhered to the transfer film.

[0106] In yet another aspect of the present invention, there is provided a method of making a transfer film suitable for applying a pattern (e.g. of an electrical conductor) to a substrate, the method comprising:

[0107] a) applying a transferable conductive composition comprising a swellable polymer swollen with a swelling agent and (e.g. conductive) particles to a transfer film with an applicator, the transferable (e.g. conductive) composition being applied at a sufficiently high application temperature at which the transferable conductive composition is sufficiently flowable for the applicator;

[0108] b) optionally, heating the transfer film before and / or during application of the transferable conductive composition (e.g., to a temperature that differs by at most 20° C. from the application temperature of the transferable conductive composition);

[0109] c) optionally, removing excess transferable composition from the surface of the transfer film, thereby leaving a pattern thereon;

[0110] d) optionally, removing at least some, but not all, of the swelling agent from the pattern; and

[0111] e) lowering the temperature of the transfer film or allowing the temperature of the transfer film to lower so that the pattern of transferable conductive composition applied thereto is sufficiently adhered to the transfer film.

[0112] In some embodiments, the transferable conductive composition applied in step a) for making the transfer film further comprises glass frit.

[0113] In one embodiment, the pattern is formed on the surface of the transfer film. In another embodiment, the pattern is formed in a groove below the surface of the transfer film. In another embodiment, the pattern can be formed partially in the groove and partially on the surface of the transfer film. As an example, a grid-like conductive pattern can form conductor lines (e.g., fingers of a solar cell) in the groove while forming other components (e.g., bus bars of a solar cell or PCB solder bumps) on its surface.

[0114] In a specific embodiment, when the pattern is at least partially formed in a groove below the surface of the transfer film, the transferable composition or transferable conductive composition can be applied to the groove through multiple loading cycles. The groove can be formed in the surface of the transfer film by any suitable method (for example, using a mold or a laser).

[0115] Since similar principles apply to any type of particles in a transferable composition, in the following text, unless the context indicates otherwise, references to transferable (conductive) compositions are transferable compositions comprising decorative particles, functional particles or conductive particles (the conductivity of the particles being inherent or acquired after appropriate treatment (e.g., by sintering) of the pattern made therefrom). Thus, as briefly defined previously and as further detailed below, the term "transferable (conductive) composition" includes both "transferable composition" and "transferable conductive composition." Similarly, the term "(conductive) particles" may be used to refer to decorative particles, functional particles or conductive particles.

[0116] In one embodiment, when transferable composition is applied to the surface of transfer film or groove therein in a plurality of loading cycles, in each cycle, apply and at least partially dry pattern and be all made by identical transferable (conductive) composition.In this case, if the pattern applied after in shape, size and position is in full agreement with the pattern applied subsequently, the pattern applied in each cycle can be regarded as the similar layer of identical final pattern.Or, even when identical transferable (conductive) composition is used for each cycle, the part of pattern can be different because of the cycle, and need not partially or entirely overlap.In such embodiment, the pattern portion applied in each cycle can be regarded as the fragment of identical final pattern.

[0117] In an alternative embodiment, the pattern applied and at least partially dried in each cycle can be made of a transferable (conductive) composition that is different between at least two cycles. As in the above case, the pattern portions can be substantially completely overlapped, in which case at least two cycles can be considered as different layers of patterning, or the pattern portions applied in each cycle can also be different at least in positioning, in which case the cycles can be considered as different fragments of patterning. The differences between the transferable compositions can, for example, relate to the identity of their composition and the relative ratio between the composition.

[0118] In one embodiment, the flexible web contains grooves that at least partially correspond to the desired pattern, and the transferable (conductive) composition is applied by filling the grooves.

[0119] In another embodiment, the transfer film is flexible and optionally composed of a thermoplastic polymer selected from the group consisting of cyclic olefin copolymer (COC), polyvinyl chloride (PVC); polyethylene (PE), polypropylene (PP), thermoplastic polyurethane (TPU), polyethylene terephthalate (PET), and combinations thereof. In another embodiment, the transfer film is pretreated prior to application of the transferable composition, the pretreatment promoting one or more of: i) adhesion of the pattern to the transfer film, and ii) transfer of the pattern to the substrate when the transfer film is in contact with the substrate.

[0120] In another embodiment, the transfer film is a flexible foil made of plastic, metal, paper or a laminated combination thereof, and the flexible foil is impermeable to the transferable conductive composition so that the pattern remains on the surface of the transfer film. In another embodiment, the transfer film is further applied with an anti-permeation agent.

[0121] In another embodiment, the transferable (conductive) composition applied to the transfer film is a transferable (conductive) composition as described in detail herein, including the teachings of the previously briefly described embodiments, and is prepared by a method as described in detail herein, including the teachings of the previously briefly described embodiments.

[0122] In one embodiment, the substrate to which the pattern can be transferred from the transfer film is suitable for manufacturing a solar cell. In this case, when the substrate is the surface of a solar cell, more simply stated, the method of manufacturing a transfer film suitable for applying a pattern of a transferable conductive composition is suitable for manufacturing a solar cell.

[0123] In another aspect of the present invention, there is provided a transfer film suitable for applying a decorative or functional pattern to a substrate, the transfer film having on its surface and / or within its recesses a pattern made of or consisting of a transferable composition comprising a swellable polymer and decorative or functional particles, each swollen with a swelling agent; the transfer film being such that, when the transfer film and the substrate are pressed together, the transferable composition adheres more strongly to the substrate than to the transfer film, and, if desired, the transferable composition remains as a pattern on the substrate when the transfer film is subsequently separated from the substrate.

[0124] In another aspect of the present invention, there is provided a transfer film suitable for applying a pattern of electrical conductors to a substrate, the transfer film having on its surface and / or in its grooves a pattern made of or consisting of a transferable conductive composition, the transferable conductive composition comprising a swellable polymer swollen with a swelling agent and conductive particles, the transferable conductive composition optionally being suitable for becoming conductive when sintered by applying energy thereto; the transfer film being such that when the transfer film and the substrate are pressed together, the transferable conductive composition adheres more strongly to the substrate than to the transfer film, and, if desired, the transferable conductive composition is retained as a pattern on the substrate when the transfer film is subsequently separated from the substrate.

[0125] In one embodiment, the teachings detailed herein include preparing a transfer film suitable for applying a pattern (eg, of an electrical conductor) to a substrate by the method briefly described above.

[0126] In a specific embodiment, the substrate on which the transfer film is transferred to obtain the conductive pattern can be the surface of a solar cell. In this case, it can be simply said that the transfer film is suitable for manufacturing a solar cell.

[0127] In another aspect of the present invention, there is provided a method of applying a decorative or functional pattern to a substrate, the method comprising:

[0128] a) providing a flexible transfer film, wherein a first surface of the transfer film has a transferable pattern of a transferable composition formed thereon and / or within recesses therein, the transferable composition comprising a swellable polymer swollen with a swelling agent and decorative or functional particles, and the transferable pattern at least partially corresponds to a desired pattern to be applied to the substrate;

[0129] b) contacting the transfer film with the substrate, wherein the first surface of the transfer film faces the substrate, the contacting optionally being performed at a transfer temperature greater than 60° C.; and

[0130] d) applying pressure to the transfer film to adhere the transferable pattern of the transferable composition to the substrate, the pressure optionally being at least 0.1 kgf / cm 2 and up to 50kgf / cm 2 .

[0131] In some embodiments, the method of applying a decorative or functional pattern to a substrate further comprises:

[0132] e) separating the transfer film from the substrate to transfer the transferable pattern from the first surface of the transfer film or from the recesses therein to the substrate, the separation optionally being performed at a separation temperature lower than a transfer temperature; and / or

[0133] f) optionally, post-processing the pattern of the composition transferred to the substrate.

[0134] After the transferable pattern of the transferable composition adheres to the substrate by applying pressure, the transfer film need not be separated from the substrate within a short period of time. In some embodiments, the transfer film may remain permanently on the transferable pattern as a protective layer, in which case the transfer film may optionally be adapted to allow post-processing of the transferable pattern, or the transfer film may be separated close to the time when the transferable pattern is to be post-processed or used.

[0135] In yet another aspect of the present invention, there is provided a method of applying a conductive pattern to a substrate, the method comprising:

[0136] a) providing a flexible transfer film, wherein a first surface of the transfer film has a transferable pattern of a transferable conductive composition formed thereon and / or within recesses therein, the transferable conductive composition comprising a swellable polymer swollen with a swelling agent and conductive particles, and the transferable pattern at least partially corresponds to a desired conductive pattern to be applied to the substrate;

[0137] b) contacting the transfer film with the substrate, wherein the first surface of the transfer film faces the substrate, the contacting optionally being performed at a transfer temperature greater than 60° C.; and

[0138] d) applying pressure to the transfer film to adhere the transferable pattern composed of the transferable conductive composition to the substrate, wherein the pressure is optionally at least 0.1 kgf / cm 2 and up to 50kgf / cm 2 .

[0139] In some embodiments, the method of applying a conductive pattern to a substrate further comprises:

[0140] e) separating the transfer film from the substrate to transfer the transferable pattern from the first surface of the transfer film or from the recesses therein to the substrate, the separation optionally being performed at a separation temperature lower than a transfer temperature; and

[0141] f) optionally applying sufficient energy to sinter the conductive particles and render the pattern of composition transferred to the substrate conductive.

[0142] In one embodiment, the method of applying a conductive pattern to a substrate formed by a surface of a solar cell further satisfies at least one of the following:

[0143] i) the swelling agent is present in the transferable conductive composition at a volume concentration of at least 10 vol.% of the transferable composition;

[0144] ii) the swelling agent is present in the transferable conductive composition at a weight concentration of at least 10 wt.% of the transferable composition;

[0145] iii) the swellable polymer has a melt temperature in the range of 50°C to 250°C, 60°C to 200°C, 65°C to 150°C, 65°C to 140°C, 65°C to 130°C, 70°C to 120°C, or 70°C to 100°C;

[0146] iv) the swelling agent has a boiling temperature in the range of 90°C to 450°C, 100°C to 450°C, 100°C to 400°C, 100°C to 350°C, 100°C to 300°C, 105°C to 290°C, 110°C to 280°C, 115°C to 270°C, 120°C to 300°C, 120°C to 250°C, 130°C to 400°C, 160°C to 350°C, 150°C to 300°C, or 150°C to 250°C; and

[0147] v) The vapor pressure of the swelling agent is 10 -6 kPa to 5kPa, 10 -5 kPa to 4kPa, 10 -4 kPa to 3kPa, 10 -3 kPa to 2.5kPa, 10 -6 kPa to 0.5kPa, 10 -6 kPa to 10 -1 kPa or 10 -6 kPa to 10 -2 kPa range.

[0148] vi) heating the transfer film before contacting the substrate;

[0149] vii) heating the substrate before contacting the transfer film;

[0150] viii) contacting at a transfer temperature of at least 60°C, at least 70°C, or at least 80°C;

[0151] ix) contacting at a transfer temperature of at most 250°C, at most 200°C, or at most 180°C; and

[0152] x) if and when separation is performed, the separation is performed at a separation temperature that is at least 5°C, at least 10°C, at least 20°C, or at least 30°C lower than the transfer temperature.

[0153] In a specific embodiment, the substrate to which the flexible transfer film is applied comprises the surface of a solar cell.

[0154] In another aspect of the present invention, there is provided a transferable composition according to the teachings of the present invention for use in manufacturing a pattern on a substrate, the transferable composition particularly comprising a swollen polymer. In one embodiment, the transferable composition comprises decorative particles, and the transfer pattern is decorative. In another embodiment, the transferable composition comprises functional particles and the transfer pattern is functional. In a specific embodiment, the functional particles are conductive particles that can optionally form a conductive pattern after applying energy that is sufficient to sinter the conductive particles contained in the composition, and the conductive pattern is suitable for forming a rear electrode or a front electrode on the surface of a solar cell.

[0155] Other objects, features, and advantages of the present invention will be described in the detailed description that follows, and some of the description will be obvious to those skilled in the art, or will be recognized by practicing the invention as described in the specification and claims. Various features and subcombinations of the embodiments of the present invention may be employed without reference to other features and subcombinations. BRIEF DESCRIPTION OF THE DRAWINGS

[0156] Some embodiments of the present invention will now be further described by way of example with reference to the accompanying drawings, in which like reference numerals or characters indicate corresponding or identical objects. This description, together with the accompanying drawings, makes it clear to those skilled in the art how to practice some embodiments of the present invention. The accompanying drawings are for illustrative purposes only and are not intended to provide a detailed description of the structural details necessary for a further understanding of the present invention. For clarity and ease of presentation, some objects depicted in the drawings are not necessarily shown to scale.

[0157] In FIG1 , a method of preparing a transferable composition according to some embodiments of the present invention is schematically depicted:

[0158] Figure 1A The addition of a swellable polymer to a swelling agent is shown;

[0159] Figure 1B shows the gradual swelling of the swellable polymer;

[0160] Figure 1C shows a polymer premix obtained after swelling the polymer with a swelling agent;

[0161] Figure 1D The addition of functional and / or decorative particles to a polymer premix is ​​shown;

[0162] Figure 1E shows the transferable composition that can be obtained at the end of the process; and

[0163] Figure 1F A magnified view of the gel-like structure of the transferable composition is shown. DETAILED DESCRIPTION

[0164] The present invention relates to transferable compositions. Methods of making and using the transferable compositions are also described.

[0165] The inventors have previously described transferable conductive compositions. These existing compositions (referred to as metal pastes in WO 2018 / 020479 and WO 2018 / 020481) are based, in particular, on polymers commonly referred to as adhesives and are used for transfer after being substantially dried (e.g., containing less than 5 vol.% of liquid). In short, without referring to the details of the aforementioned publications, the previously disclosed polymers, alone or in combination with other ingredients, achieve the following goals in a transferable composition: first, the conductive particles mixed therewith are kept in the shape required to form a pattern on or within a transfer film (e.g., a flexible web), secondly, the pattern is allowed to be substantially completely transferred from the transfer film to the intended substrate (i.e., detached / released from the transfer film and the composition is attached to the substrate (e.g., a solar wafer or an insulating substrate)), and finally, the pattern transferred in a predetermined shape is kept sufficiently adhered to the substrate for subsequent optional steps, such as sintering and firing, or more simply for handling and / or storing the transfer film during or before use.

[0166] To achieve these goals, the binder polymer is typically present in a relatively high amount (e.g., greater than 30 vol.%, or even greater than 40 vol.%, of the solid volume) in the transferred composition, which in turn may result in relatively high levels of burned polymer residues in the sintered and / or fired pattern. Without being bound by any particular theory, these residues may generally affect the electrical efficiency of the pattern. For example, excessive amounts of burned polymer residues may reduce the open circuit voltage (V OC ) thereby reducing the performance of the solar cell, the open circuit voltage (V OC ) is the voltage generated by a solar cell at zero current, which is usually measured in millivolts (mV) under standard test conditions (e.g., at a temperature of 25°C and vertical illumination of 1000 watts / square meter). Those skilled in the art of solar cell manufacturing will readily understand that a relatively high V OC It is only one of the parameters of a solar cell (and a solar module or panel made from it) that can be used satisfactorily. SC ), fill factor (FF) and efficiency (η) are additional parameters commonly used to evaluate the effectiveness of the final product. These parameters are further related to other characteristics of the solar cell, such as relatively high V OC A relatively low reverse saturation current density, J0, which is a measure of recombination in solar cells, may be desired.

[0167] Therefore, using solar cells as an example, reducing the amount of polymer required in the transferable composition would be beneficial in order to prevent or reduce any adverse effects that the presence of the polymer or residues therefrom might have on the subsequent performance of the conductive pattern. The inventors have discovered that using a swellable polymer that swells upon transfer can address this issue, thereby improving the performance of the solar cell while also achieving the functions listed above for non-swellable or non-swellable polymers.

[0168] While the present invention will be described in greater detail within the context of solar cell manufacturing, those skilled in the art will appreciate that the present invention is also applicable to the production of conductive patterns for various non-solar products. Furthermore, the pattern need not be conductive; the present invention can also be used to produce transferable patterns containing particles that provide various functions or simply provide decorative purposes.

[0169] The use of a polymer swollen with a swelling agent allows for the application of a smaller amount of swellable polymer to achieve the same volume of a previously designed transferable paste as compared to a non-swellable or unswellable polymer to achieve the desired pattern. A suitable swellable polymer not only reduces the relative amount of polymer in the composition but also provides rheological properties suitable for the processability of the composition, particularly its application to a transfer film and its transferability from the transfer film to a suitable substrate (typically, at least some, but not all, of the swelling agent has been eliminated, for example, by evaporation). While in a relatively lower amount than a non-swellable polymer, a suitable swellable polymer should still provide the desired mechanical properties and satisfactory transferability to the pattern.

[0170] Before explaining at least one embodiment in detail, it should be understood that the present invention is not necessarily limited in its application to the details of component and / or method construction and arrangement set forth herein. The present invention is capable of other embodiments or can be practiced or implemented in various ways. The words and terms used herein are for descriptive purposes only and should not be considered restrictive.

[0171] It is to be understood that both the foregoing general description and the following detailed description, including materials, methods, and examples, are merely exemplary of the invention and are intended to provide an overview or framework to facilitate understanding the nature and character of the invention as claimed and are not intended to be necessarily limiting.

[0172] Transferable composition

[0173] Transferable composition of the present invention can especially be used for the production of the conductive pattern for the manufacture of solar cells.In short, prepare composition, then it is applied on the transfer film to form pattern, described pattern can be transferred on the solar wafer substrate subsequently, and swellable polymer has swelled to a certain degree (detailed description later) simultaneously.After transfer, if necessary, can sinter and / or burn the pattern that has been attached to the wafer, so that pattern is conductive, or it is processed, thereby the component (for example, swellable polymer and swelling agent) that gives transferability function is reduced or eliminated to the degree that the intended use no longer needs (or even harmful).In short, no matter how the position of transferable composition in transfer film (surface of transfer film or in the groove of transfer film), transferable composition all is applicable to or is applied on the transfer film.Before transfer, the pattern of transferable composition also can be referred to as transferable pattern.

[0174] The aforementioned steps do not need to all be performed at the same location, nor do they need to be performed immediately in chronological order, nor do they need to be performed by the same performing entity. For example, the production of the transfer film (e.g., a flexible web) can be performed by a first performing entity at a first location, the preparation of the transferable composition and its application on or within the transfer film to produce the transferable pattern can be performed by a second performing entity at a second location (e.g., by roll-to-roll production of a disposable film containing the transferable pattern), and if desired, the transfer of the pattern from the transfer film to the intended solar cell substrate can be performed by yet another performing entity at another location.

[0175] The pattern formed by the transferable composition of the present invention can be presented as the spaced arrangement of lines (for example, being presented as a grid comprising fingers and / or busbars), or can be presented as a more continuous pattern forming a composition coating or layer on a transfer film, which is subsequently transferred to the expected substrate. As used in the present invention, the term pattern comprises that the transferable composition and its (for example, conductive) counterpart (and mirror image) after the transfer can be used to manufacture any and all shapes of desired products (for example, solar cells, PCBs, flip-chip interconnect boards, printed flexible circuits, touch screens, RFID tags, etc.). In addition, due to the (functionality or decorative) intended effect of the particles used for the transferable composition being not limited to conductive, the counterpart after the transfer can be a non-conductive decorative pattern, which can generally be a more abundant variety of arbitrary or peculiar shapes.

[0176] In a first aspect of the present invention, a transferable conductive composition is provided. The composition comprises:

[0177] i. a swellable polymer swollen with a swelling agent; and

[0178] ii. A plurality of decorative (eg pigment) or functional (eg conductive) particles.

[0179] Swellable polymers

[0180] The swellable polymers and their swellable counterparts are selected and adapted to perform a variety of functions as detailed herein. For example, they enable the dispersion of particles in the composition formed thereby, they enable the composition to be applied to a transfer film in a desired pattern (e.g., on or within the transfer film), they enable the composition and the pattern formed by the composition to be peeled from the transfer film and transferred to a substrate, they enable the composition and pattern transferred to a substrate to adhere to the substrate, and, in the case where the particles providing the transferable composition function are conductive, the polymers allow the transferred composition and the pattern formed thereby to become conductive upon application of sufficient energy.

[0181] Unless otherwise indicated or clear from the context, the term "swellable polymer" as used in this specification and the appended claims refers to a polymer that increases in volume (and weight) when a swelling agent (e.g., an organic solvent) enters its polymer cross-linked network, rather than dissolving or simply dispersing in the solvent in substantially the same non-swelled form. Upon swelling (e.g., under agitation and heating), the swellable polymer (which may also be referred to as a swollen or swollen polymer) forms a solvated gel-like composition. The gel-like composition is typically opaque.

[0182] While the swellable polymer (or mixture thereof) should preferably be substantially insoluble in the swelling agent (or its blend) to achieve this swollen, gel state, swellable polymers soluble at concentrations up to 10 wt.% of the weight of the swelling agent are contemplated in some embodiments. Advantageously, the amount of swellable polymer dissolved in the swelling agent should be less than 5 wt.%, less than 2 wt.%, less than 1 wt.%, less than 0.5 wt.%, or less than 0.1 wt.%. When this information is not readily available for a particular combination of swellable polymer and swelling agent, it can be estimated by adding a predetermined weight of polymer to the swelling agent, mixing at room temperature, and observing the appearance of the mixture. The polymer concentration at which the mixture changes from clear to turbid (or any other detectable sign of gel formation) can be used as an estimate of the upper limit of solubilization of the polymer (or mixture thereof) in the swelling agent (or its blend).

[0183] During the expansion of the swellable polymer cross-network due to swelling, various forces may be present, including, for example, hydrophilic / hydrophobic and / or ionic interactions. The degree of swelling achieved by the swellable polymer depends, among other things, on the crystallinity of the swellable polymer, the amount of cross-linking, the strength of the attractive and repulsive forces, and the properties of the swelling agent. Some liquids may be more effective than others for a particular swellable polymer. Any given pair of swellable polymer and swelling agent may reach an equilibrium point, above which the swellable polymer can no longer absorb the swelling agent and swell further. Conversely, when the swelling agent is released from the swellable polymer, this is called deswelling, which can be induced, for example, by applying heat to the swollen cross-network.

[0184] Information on the swellability of polymers is often provided by suppliers of these materials, who may even rank suitable swelling agents and their relative effectiveness in swelling the polymer cross-linked network. Swellability can be independently determined using standard methods and conventional measurement methods known to those skilled in the polymer art, such as ASTM D2765, or by any other method, in which, for example, a predetermined weight of polymer is incubated in the presence of an excess of a swelling agent for a sufficient period of time to allow the system to reach equilibrium. At this point, when the weight of the polymer no longer increases, the excess swelling agent is removed, and the increased weight of the polymer is divided by the initial weight of the polymer to yield the degree of swelling, or percentage swelling on a weight / weight basis. This can also be expressed mathematically by the following formula: % Swelling = (Ws - Wd) / Wd × 100, where Ws represents the weight of the swollen polymer and Wd represents the initial weight of the polymer before the addition of the swelling agent.

[0185] The swellable polymer need not be at its maximum swelling (ie, equilibrium point) for the swelling agent to be used therewith to be suitable for use in the present invention. The degree of swelling at such intermediate stages or during the preparation of the transferable composition can be assessed as described above.

[0186] However, as already mentioned, the swellable polymer may need to be maintained in a swollen state to achieve effective transfer. At this stage, the transferable composition containing the swellable polymer also contains (e.g., conductive) particles and any other desired additives. The relative amount of swelling agent can be estimated by routine experimentation according to methods known to those skilled in the art, for example, by drying a known amount of the composition and measuring the weight loss in the process. Drying can be carried out at a temperature corresponding to the elimination of the swelling agent, as determined, for example, by thermogravimetric analysis (TGA). For example, a sample of the composition / pattern / transfer film prepared for transfer can be dried in a vacuum oven at 200°C for 24 hours. Samples that lose less than 5% or less than 10% of their weight may contain an insufficient amount of swelling agent. Further analysis can allow the weight percentage loss to be converted to volume percentage loss.

[0187] In order to achieve adequate transferability, the composition should contain a polymer that retains at least 10 wt.% or 10 vol.% of the swelling agent upon transfer, preferably at least 15 wt.% or 15 vol.%, or at least 20 wt.% or 20 vol.% of the swelling agent. Materials that do not swell the swellable polymer by at least 10%, at least 15%, or at least 20% during composition preparation are not suitable for use in the present invention.

[0188] While the presence of a swelling agent may be required to improve the transferability of a transferable composition comprising a swellable polymer, such compounds are no longer necessary once the pattern has been transferred to its intended substrate. In some embodiments, particularly when the transferred pattern requires subsequent processing to facilitate or achieve its intended use, the swellable polymer and / or swelling agent may be selected to facilitate removal by post-processing. For example, the swellable polymer and / or swelling agent may be selected to have a relatively low melting or boiling temperature (e.g., 250° C. or lower) to facilitate their removal after transfer by heating the transferred pattern to a relatively high temperature (e.g., 300° C. or lower, respectively). It will be appreciated by those skilled in the art that similar considerations may apply to other additives to the transferable composition, such as dispersants, rheology modifiers, and the like.

[0189] In addition to the compatibility between the swellable polymer and its swelling agent (each of which may be used alone or in combination), more broadly, the compatibility between any and all of its other components (e.g., particles, dispersants, glass frits, rheology modifiers, etc.) and the transfer film to which it is to be applied, the compatibility between the transferable composition including the aforementioned components and the transfer film to be applied also needs to be considered. Swellable polymers with relatively low melting points and / or swelling agents with relatively low boiling points allow for the use of a wider variety of films that can withstand such temperatures.

[0190] Transferable compositions comprising relatively high melting point swellable polymers and / or relatively high boiling point swelling agents will require transfer films that can withstand such relatively high temperatures, which may limit the types of transfer films that can be used with the transferable compositions.

[0191] Films having a heat resistance of at least 10°C, at least 20°C, or at least 30°C greater than the highest of the melting point of the swellable polymer and the boiling point of the swelling agent are expected to be suitable for transferable compositions, at least in terms of temperature.

[0192] Transfer films suitable for use with the transferable compositions described herein and methods for their manufacture are described in detail in the following sections.

[0193] In view of the subsequent use of the transfer film of the transferable composition of the present invention, it may be advantageous, but not necessary, to select swellable polymers and swelling agents to allow the pattern comprising them and (e.g., conductive) particles dispersed therein to be quickly transferred to (e.g., solar energy) substrates. In order to promote transfer, substrates and / or the transfer film comprising pattern to be transferred may be heated before or during contact. It is understood that swellable polymers with relatively high melting points require higher temperatures and / or longer heating times to fully soften, thereby achieving satisfactory transfer (e.g., fully transferring, without pattern splitting between transfer film and expected substrate). Extending the transfer process and / or raising the transfer temperature will reduce the efficiency and speed of the process. As described, for the degree of needing to reduce or eliminate swellable polymers after transfer, it may be beneficial to use swellable polymers with relatively low melting points, which helps to reduce or eliminate polymers in the post-transfer processing of the pattern.

[0194] Thus, while the various swellable polymers may have various melting points, for practical reasons of handling and / or storage conditions, the melting point is typically at least 50° C. In one embodiment, the swellable polymer advantageously has a melting point of at least 60° C., at least 70° C., or at least 80° C. In another embodiment, the swellable polymer has a melting point of at most 250° C., at most 200° C., at most 150° C., at most 140° C., at most 130° C., at most 120° C., at most 110° C., or at most 100° C. In yet another embodiment, the swellable polymer has a melting point in the range of 50° C. to 250° C., 60° C. to 200° C., 65° C. to 150° C., 65° C. to 140° C., 65° C. to 130° C., 70° C. to 120° C., or 70° C. to 100° C.

[0195] Likewise, while the various swelling agents may have various boiling points, some even below the boiling point of water, they are generally selected individually or mixed with one another to have individual or combined boiling points of 90°C or higher. In one embodiment, the swelling agent (or mixture thereof) advantageously has a boiling point of at least 100°C, at least 110°C, at least 120°C, at least 130°C, at least 140°C, at least 150°C, or at least 160°C, optionally at most 450°C, at most 400°C, at most 350°C, at most 300°C, at most 290°C, at most 280°C, at most 270°C, at most 260°C, or at most 250°C. In another embodiment, the swelling agent has a boiling point in the range of 90°C to 450°C, 100°C to 450°C, 100°C to 400°C, 100°C to 350°C, 100°C to 300°C, 105°C to 290°C, 110°C to 280°C, 115°C to 270°C, 120°C to 300°C, 120°C to 250°C; 130°C to 400°C, 160°C to 350°C, 150°C to 300°C, or 150°C to 250°C.

[0196] Although it was stated in the previous paragraph that, to facilitate the selection of an appropriate swellable polymer, reference may be made to the melting temperature of the swellable polymer, this term is generally used with polymers that are sufficiently crystalline. These temperatures may be provided by the supplier of the material or may be estimated by routine experimentation according to methods known to those skilled in the art, such as differential scanning calorimetry (DSC), as described in ASTM 3418, ISO 3146, or ISO 11357-3.

[0197] To select a swellable polymer suitable for the present invention, for example, one suitable for application or transfer at temperatures between 60°C and 180°C, the softening temperature of the reference polymer may be selected as an alternative. As used herein, the term "suitable" should not be interpreted as implying that the material needs to be modified before use, but rather that the material is suitable. The term softening temperature is typically used for amorphous or semi-crystalline polymers. The softening temperature may also be provided by the polymer manufacturer or measured through routine experimentation. The Vicat softening temperature can be assessed, for example, according to ASTM D1525 or ISO 306. The Vicat softening temperature is typically about 20°C to 30°C lower than the melting temperature of the polymer. In one embodiment, the Vicat softening temperature of the swellable polymer is at least 30°C, at least 40°C, at least 50°C, or at least 60°C. In one embodiment, the swellable polymer has a softening point of at most 220°C, at most 180°C, at most 160°C, at most 140°C, at most 120°C, at most 100°C, or at most 80°C. In another embodiment, the swellable polymer has a softening point in the range of 30°C to 140°C, 40°C to 120°C, or 50°C to 100°C.

[0198] In the method of the present invention, by mixing a swellable polymer having a melting point or softening point that is relatively higher than that required for the process or use perspective with a plasticizer that can reduce this phase transition temperature, a melting or softening temperature within the scope of the subsequent use that is suitable for the transferable composition or the pattern formed thereby can be optionally achieved. If the presence of the plasticizer has a negative impact on the intended use of the pattern, or when no longer needed for any other reason, the plasticizer can be additionally suitable for at least partial or complete elimination in the subsequent transfer process. As other additives to the transferable composition, the plasticizer should be compatible with the gel-like swellable polymer composition, and according to the present invention, the plasticizer should preferably not dissolve the polymer or at least dissolve the polymer to the extent that it does not affect the degree of swelling by the swelling agent.

[0199] In one embodiment, the swellable polymer is thermoplastic. Thus, it is selected from the group consisting of amorphous thermoplastic polymers, semi-crystalline thermoplastic polymers and crystalline thermoplastic polymers.

[0200] Suitable swellable polymers may have an average molecular weight (MW) of at least 1000 daltons, or at least 10,000 daltons, or at least 50,000 daltons. In one embodiment, the average molecular weight of the swellable polymers suitable for use in the present invention is at most 1,000,000 daltons, at most 500,000 daltons, or at most 250,000 daltons. In another embodiment, the average molecular weight range of the swellable polymers suitable for use in the present invention is 1,000 daltons and 500,000 daltons, 10,000 daltons and 300,000 daltons, or 100,000 daltons and 250,000 daltons. When the swellable polymers used are slightly differentiated molecular populations (e.g., having slightly different numbers of repeating units), the term molecular weight refers to the weight average molecular weight unless otherwise specified by the supplier. The weight average molecular weight can be measured by standard methods, such as by size exclusion chromatography or gel permeation chromatography.

[0201] Melt flow rate (MFR) or melt flow index (MFI) is another parameter that indicates the MW of a swellable polymer, with low MW swellable polymers having conversely higher melt flow rates. The MFR evaluates the ease with which a swellable polymer melt flows and is defined as the mass (in grams) of a preheated swellable polymer sample that flows through a capillary tube of a specified diameter (e.g., 2 mm) and length (e.g., 8 mm) in ten minutes at a predetermined temperature (e.g., 190°C) and a specified load (e.g., 2.16 kg) typically applicable to the chemistry of the swellable polymer. In addition to being an indirect measure of molecular weight, the MFR also indicates a material's ability to flow under pressure and is inversely proportional to the melt viscosity of the melt under plastometer testing conditions.

[0202] In one embodiment, the melt flow rate of the swellable polymer is at least 0.2 g / 10 min, at least 0.3 g / 10 min, at least 0.5 g / 10 min, or at least 1 g / 10 min and up to 100 g / 10 min when measured at a temperature of 190° C. with a test weight of 2.16 kg. The MFR of the swellable polymer may be in the range of 0.2 g / 10 min to 100 g / 10 min, 0.5 g / 10 min to 50 g / 10 min, 1 g / 10 min to 100 g / 10 min, 1 g / 10 min to 75 g / 10 min, 1 g / 10 min to 50 g / 10 min, 2 g / 10 min to 50 g / 10 min, or 2 g / 10 min to 25 g / 10 min. The melt flow rate is typically provided by the polymer manufacturer, but can also be determined independently by methods known to those skilled in the art, such as according to ASTM D1238-13 and ISO 1133.

[0203] Without wishing to be bound by any particular theory, it is generally believed that melt flow rate also indicates the relative hardness that a swellable polymer can exhibit. For example, taking the same amount of two swellable polymers, each swelled with the same amount of the same swelling agent under similar conditions, after swelling, the swellable polymer with the higher MFR (lower MW) will form a softer / looser gel-like structure than the swellable polymer with the lower MFR (higher MW). In other words, the swellable polymer with the lower MFR (higher MW) will be relatively firmer or harder.

[0204] Another parameter that can be considered when selecting a swellable polymer suitable for use in the transferable composition of the present invention is the "pot life" that it can impart to the transferable composition. The term "pot life" as used herein does not refer to the polymer's participation in any reaction that occurs or does not occur in the transferable composition, but rather simply refers to the duration of time over which the transferable composition can be used or stored after its preparation. In other words, how long will the composition exhibit a sufficiently stable viscosity to allow it to be applied to the transfer film by any suitable means selected for the application process? As will be readily understood by those skilled in the art of polymer processability, beyond a viscosity level that depends, inter alia, on the selected application device, the desired pattern, and its location on or within the transfer film, the composition may no longer be applied to the film in a satisfactory manner.

[0205] When selecting the swellable polymer that is applicable to the transferable composition of the present invention, another parameter that can be considered is the " open time " of polymer. Although relevant with the pot life of composition before it is applied to required flexible film, the term open time is usually more specifically relevant with the performance of swellable polymer in subsequent transfer process. Open time generally refers to the time period that polymer has enough mobility to present desired shape, and this value depends on the condition (for example, temperature or pressure) that polymer is subjected to. The open time of swellable polymer in turn affects the ductility of the transferable composition that comprises it. The open time of polymer can depend on the temperature that it is subjected to, therefore selecting satisfactory open time especially depends on the condition that is expected to be used for the use of transferable composition. Such swellable polymer is considered to be suitable: have sufficiently long open time to allow after the pattern containing transferable composition is heated to the transfer temperature that rises, it is suitably transferred to substrate, and open time is short enough to keep desired shape after transfer and adhering to substrate simultaneously. For swellable polymer that is applicable to the present invention, open time less than several seconds, less than 1 second, less than hundreds of milliseconds or tens of milliseconds is considered to be satisfactory.

[0206] Swellable polymers having a low ash content are believed to be advantageous because such polymers can promote sintering of the conductive particles when the conductive particles are made of metal or alloy.

[0207] In one embodiment, the swellable polymer is selected from: ethylene / vinyl acetate (EVA) copolymers, such as commercially available 3101, 3861 (vinyl acetate content (% VA) is 25%), Elvax TM 350 (25% VA), Elvax TM 410 (18% VA), Elvax TM 420 (18% VA), Elvax TM 470 (18% VA) and Elvax TM 670 (12% VA) (DuPont, USA); acid-modified ethylene acrylate resin, such as 2022 (DuPont, USA); ethylene / acrylate / maleic anhydride terpolymers, such as 4700 (Arkema, France); polyvinyl butyral (PVB), such as B-72, B-74, B-76, B-79, B-90 or B-98 (Eastman, USA or Kuraray, Japan); polyvinyl alcohol (PVA), such as PovalTM Lm-30 (Kuraray, Japan); and polyamide. The polyamide may be an ester-terminated polyamide, such as CrystaSensean, CrystaSense TM LP1 (Croda, UK), polyaminoamide or polyesteramide.

[0208] In one embodiment, the swellable polymer is an EVA copolymer, which may be optionally grafted (e.g., anhydride grafted), and the copolymer has a vinyl acetate content (% VA) of at least 5 wt.%, at least 10 wt.%, at least 15 wt.%, or at least 20 wt.%, based on the weight of the polymer. In some embodiments, the swellable polymer is an EVA copolymer having a vinyl acetate content of up to 55 wt.%, up to 40 wt.%, or up to 30 wt.%. In further embodiments, the EVA copolymer has a vinyl acetate content of 5 wt.% to 50 wt.%, 10 wt.% to 40 wt.%, 10 wt.% to 30 wt.%, or 15 wt.% to 30 wt.%. The vinyl acetate content of the EVA copolymer is typically provided by the manufacturer, but if not provided, it can be independently determined by routine experimentation according to methods known to those skilled in the art, such as Fourier transform infrared (FT-IR) spectroscopy, as described in ASTM D5594.

[0209] Typically, the amount of swellable polymer is measured as a volume percent (vol.%) of the volume of all solids in the transferable composition, such as the swellable polymer, (conductive) particles, and glass frit (if present). The volume of each component can be calculated based on its density and solid weight.

[0210] In one embodiment, the swellable polymer is present in the transferable composition at a concentration of at least 1 vol.%, at least 2 vol.%, at least 5 vol.%, at least 10 vol.%, or at least 15 vol.%, based on the volume of all solids in the composition. In one embodiment, the swellable polymer is present in the transferable composition at a concentration of at most 80 vol.%, at most 60 vol.%, at most 40 vol.%, or at most 30 vol.%, based on the volume of all solids in the composition. For example, the swellable polymer can be present in the composition at a concentration of 1 vol.% to 60 vol.%, 1 vol.% to 40 vol.%, 5 vol.% to 30 vol.%, 2 vol.% to 25 vol.%, 5 vol.% to 25 vol.%, 10 vol.% to 22 vol.%, or 15 vol.% to 22 vol.%, based on the volume of all solids in the composition.

[0211] Swelling agent

[0212] Any suitable swelling agent can be used, as long as it swells the swellable polymer, rather than dissolving it or dispersing it in a non-swelled form. In other words, the swelling agent should not be miscible with the swellable polymer, and vice versa. While a swelling agent that cannot dissolve a particular swellable polymer may be preferred, lesser miscibility is also acceptable. When the swelling agent is present in an amount exceeding 100-fold relative to the polymer, a swelling agent that cannot dissolve more than 10 wt.% of the swellable polymer at room temperature and advantageously dissolves less than 5 wt.%, less than 2 wt.%, or less than 1 wt.% of the polymer may be suitable. In the present invention, the ability of a liquid used as a swelling agent to dissolve a particular polymer can be determined from the literature or provided by the manufacturer. This ability can be assessed by adding a predetermined amount of swellable polymer to an excess of the swelling agent, allowing it to swell under appropriate stirring and heating conditions, and then removing the swollen polymer, for example, by filtration. The swelling agent is in excess when the volume of the filtrate remaining after separation of the swollen polymer is approximately 10% of the volume of the swollen polymer. In other words, if adding a swellable polymer to a swelling agent results in a gel-like structure in a volume of approximately 100 ml, the amount of swelling agent used should be greater than 10 ml. The presence of polymer and its content can then be determined by analyzing the volume of swelling agent that may contain dissolved polymer. Alternatively, the swelling agent in the filtrate can be removed by evaporation at a temperature slightly above its boiling point (e.g., 10 to 20° C. above) for at least 12 hours. The residue remaining after evaporation of the swelling agent can be assumed to represent the maximum weight of the swellable polymer dissolved in the swelling agent that was not swollen thereby, and the potential degree of solubility of the swellable polymer can be calculated accordingly.

[0213] As described above, information regarding the ability of a particular solvent to swell a particular polymer is typically provided by the manufacturer of the swellable polymer, but can also be determined experimentally as described above. The particular solvent can be an aqueous solvent as well as an organic solvent, wherein the aqueous solvent consists of water or contains water and a water-miscible compound. Both types of liquid solvents need not be pure substances (e.g., at least 95 wt.%, at least 97.5 wt.%, or at least 99 wt.%), and can each be a mixture comprising an aqueous solution or a mixture of organic solvents. In addition, in some embodiments, a predominantly aqueous solvent can contain some (e.g., at most 25 wt.%, at most 20 wt.%, at most 15 wt.%, at most 10 wt.%, or at most 5 wt.%) inorganic or organic components or solvents, and / or conversely, in some embodiments, a predominantly organic solvent can contain some (e.g., at most 25 wt.%, at most 20 wt.%, at most 15 wt.%, at most 10 wt.%, or at most 5 wt.%) aqueous components. Thus, in contrast, the aqueous swelling agent or aqueous solvent may be defined by a water content of at least 75 wt.%, at least 80 wt.%, at least 85 wt.%, at least 90 wt.%, or at least 95 wt.%.

[0214] While it is expected that swelling agents that form a single phase are preferred for more uniform swelling of the swellable polymer, it is generally believed that this is not necessary and that swelling agents that form a two-phase system (e.g., an emulsion) can also be used to swell the swellable polymer. Although aqueous solvents are generally preferred for environmental reasons, organic solvents may be advantageous from a processing perspective, as their relatively high boiling points allow for delayed deswelling.

[0215] The swelling agent is preferably a liquid with a relatively low viscosity so that, when mixed with the swellable polymer, (conductive) particles, and any other additives, the resulting transferable composition has a viscosity suitable for application to the transfer film at the application temperature. In one embodiment, the swelling agent is a liquid having a relatively low viscosity of no more than 1,000 mPa·s at room temperature between 20°C and 25°C, typically no more than 500 mPa·s, no more than 250 mPa·s, no more than 200 mPa·s, or no more than 150 mPa·s. The viscosity of the swelling agent can be provided by the manufacturer or measured by conventional methods, such as ASTM D445. Although these values ​​are measured at 20-25°C, temperatures typically provided by manufacturers, it will be understood that the swelling agent generally exhibits a lower viscosity at the temperatures used for preparation, application, or transfer in the present invention.

[0216] The swelling agent may have a vapor pressure that is high enough to be eliminated quickly after transfer, but low enough so that when mixed with the swellable polymer, (conductive) particles and any other additives, a sufficient amount of the swelling agent is retained within the swellable polymer in the resulting transferable composition so that upon transfer, the amount of swelling agent per swellable polymer is not less than 10 wt.%, advantageously at least 15 wt.%, or at least 20 wt.%. In the present invention, the vapor pressure of the swelling agent may be at least 10 -6 kPa, at least 10 -5 kPa, at least 10 -4 kPa, at least 5×10 -4 kPa, or at least 10 -3 In some embodiments, the swelling agent has a maximum of 5 kPa, a maximum of 4 kPa, a maximum of 3 kPa, a maximum of 2 kPa, or a maximum of 1 kPa, a maximum of 0.5 kPa, a maximum of 10 -1 kPa, up to 5×10 -2 kPa or up to 10 -2 In other embodiments, the vapor pressure of the swelling agent is between 10 -6 kPa to 5kPa, 10 -5 kPa to 5kPa, 10 -4 kPa to 5kPa, 10 -3 kPa to 2.5kPa, 10 -6 kPa to 0.5kPa, 10 -6 kPa to 10 -1 kPa or 10 -6 kPa to 10 -2 The vapor pressure of the swelling agent (or blend thereof) can be determined at room temperature between approximately 20°C and 25°C by standard methods, such as described in ASTM E1194 and E1782.

[0217] Without wishing to be bound by any particular theory, it is generally believed that the swollen state of the polymer facilitates the transfer of the transferable composition containing it. Thus, while a portion of the swelling agent may be eliminated, intentionally or unintentionally, during any step prior to transfer, a non-negligible residual amount (e.g., 10 vol. % or more) is necessary to satisfactorily transfer the pattern from the transfer film to the substrate. The swelling agent preferably evaporates relatively slowly at a temperature below the transfer temperature. A swelling agent with a relatively low flash point or boiling point has a higher evaporation rate than another swelling agent with a relatively high flash point or boiling point. As mentioned above, the boiling point of the swelling agent may be in the range of 90 to 450°C.

[0218] When the (conductive) particles added to the swellable polymer and swelling agent are water-reactive metals, the presence of water in the swelling agent above a certain amount can be harmful. For example, aluminum can react dangerously with water to form hydrogen. Therefore, while organic-based swelling agents can contain up to 25 wt.% water or similar aqueous components, in some embodiments, the swelling agent is an organic solvent that contains up to 1.0 wt.%, up to 0.5 wt.%, up to 0.1 wt.%, up to 0.05 wt.%, or up to 0.01 wt.% water.

[0219] Alternatively or additionally, water-sensitive (conductive) particles may be protected by suitable agents to reduce or prevent their reaction with aqueous environments, thereby optionally allowing the use of swelling agents with water contents above 1 wt.%. Such protection may be achieved by suitable surfactants or wetting agents that can encapsulate the (conductive) particles by adsorption to their outer surface. For example, anionic phosphate esters, such as the commercially available 3D33 (Solvay, Belgium) or 1306 (Clariant, Switzerland), and alcohol alkoxylates, such as BYK-3410 (BYK Chemie, Germany), can provide effective protection for water-sensitive particles in appropriate amounts. Such water protectants (e.g., phosphate esters), when present on or together with the (conductive) particles, allow the use of water or aqueous swelling agents and swellable polymers that can be swelled thereby to prepare transferable compositions. For example, the protected (conductive) particles can be added to a transferable composition wherein the premix includes a PVA polymer swelled with water or an alcohol solution.

[0220] For transferable compositions in which the (conductive) particles are not readily reactive with water (eg noble metals such as silver, non-metallic conductive materials or non-conductive particles such as pigments), the swelling agent may be or contain water.

[0221] Those skilled in the art of swellable polymers can readily understand which swelling agent is suitable for which type of polymer. For example, when the swellable polymer is an EVA copolymer, a suitable swelling agent may be C 6-20 Isoparaffins, such as the commercially available Isopar TM M,Isopar TM C,Isopar TM E,Isopar TM G,Isopar TM H,Isopar TM L, or Isopar TM V (ExxonMobil Corporation, USA) or polyethylene oxide (PEO; also known as high molecular weight polyethylene glycol (PEG)); For PVB polymer, suitable swelling agents can be selected from: C3-10 Ketones, such as acetone, methyl ethyl ketone or diisobutyl ketone; C 1-12 Alcohols, such as methanol or butanol; and C 6-10 Aromatic hydrocarbons, such as xylene or toluene. Glycerol, as well as the aforementioned alcohols, water, and combinations thereof, can be used as swelling agents for PVA polymers. If desired, the swelling agents can be mixed with one another. For example, the swelling agent can be a mixture of a first agent, wherein the first agent has a relatively high boiling point and a relatively high viscosity that may prolong its absorption, and a second agent, which has a relatively low viscosity for "diluting" the first agent, and the second agent itself has a boiling point temperature that may not be suitable when used alone in the present invention.

[0222] In preparing the transferable (conductive) composition, the weight / weight ratio of the swellable polymer to the swelling agent, SP / SA, is at least 1:99, at least 1:49, at least 1:19, at least 1:10, or at least 1:5. Although the latter is in significant excess relative to the swellable polymer and the swelling agent (e.g., resulting in an SP / SA ratio of up to 1:100), in some embodiments, the transferable (conductive) composition is prepared with an SP / SA of at most 9:1, at most 6:1, at most 4:1, at most 3:1, at most 2.3:1, at most 1.5:1, at most 1:1, or at most 0.5:1. Typically, the transferable (conductive) composition has an SP / SA ratio of 1:99 to 9:1, 1:49 to 6:1, 1:49 to 4:1, 1:49 to 1.5:1, 1:19 to 3:1, 1:19 to 1:1, 1:10 to 0.5:1, or 1:5 to 0.5:1 when prepared and applied to the transfer film. In a specific embodiment, the SP / SA ratio is 1:2.5, 1:3, 1:3.5, or 1:4. Although the weight ratio of the two materials is expressed as SP / SA in the above description, the inverse ratio of SA / SP can also be easily obtained, that is, an SP / SA ratio of at least X is equal to an SA / SP ratio of at most X, and an SP / SA ratio of at most Y is equal to an SA / SP ratio of at least Y.

[0223] Although some of the swelling agent may be removed spontaneously or intentionally during the preparation of the transferable composition, application, or subsequent storage after application to the transfer film, at least a portion of the swelling agent must remain in the transferable pattern upon transfer to the substrate. Therefore, in the transferable (conductive) composition applied to the transfer film, the weight / weight ratio SA / SP of the swelling agent to the swellable polymer in the swellable polymer is at least 1:9, at least 1:7, at least 1:5, at least 1:4, at least 1:3, or at least 1:2, and this ratio is generally not more than 10:1.

[0224] Considering the weight or volume of the entire transferable composition, the swelling agent should remain at least 10%, at least 15%, or at least 20% of the weight or volume of the transferable composition, respectively, upon transfer.

[0225] Conductive particles

[0226] As used herein, the term "conductive particles" encompasses particles comprising any conductive material or consisting of any conductive material, including metals, metal oxides, metal salts, organometallics, alloys and conductive polymers, as well as any electrochemically compatible combination of the foregoing materials (e.g., a mixture of two metals, aluminum and silver). The term includes particles made of the above-mentioned conductive materials, or particles that are at least partially coated with the above-mentioned conductive materials to an extent sufficient to provide sufficient conductivity for the final product. Electrochemical compatibility means that when the conductive material is applied to the transfer film (on its surface and / or in the grooves formed therein) by repeated coating or filling steps, any conductive material of any layer is chemically inert relative to any other conductive material of the same layer or other layers. In particular, none of these materials will have a negative impact on the intended effect, more specifically, these materials will not affect the conductivity and / or conductivity of the final conductive pattern obtained by the present invention, nor will they affect its ability to be transferred and attached to the substrate.

[0227] A particle is said to be made of a particular material if it comprises, consists essentially of (e.g., comprises 50.1-75 wt.%), consists primarily of (e.g., comprises 75.1-95 wt.%), or consists essentially of (e.g., comprises 95.1-100 wt.%) a particular material, and any other components of the particle other than the particular material are detrimental to its function.

[0228] The metal may be selected from aluminum, copper, gold, silver, tin, nickel, platinum, zinc; the alloy may be selected from bronze, brass and palladium / silver. The organometallic may be selected from copper (II) formate (C2H2CuO4), copper (II) hexanoate (C 12 H 22 CuO4), mesitylene copper (I) (C9H 11 Cu), vinyl-trimethylsilane-copper (I) hexafluoroacetylacetonate, silver neodecanoate (C 10 H 19 AgO2), its precursor, its hydrate and / or its salt.

[0229] When metal particles are needed for preparing conductive patterns (for example, for solar cells), it is possible to select metal particles having a thickness of at least 1×10 6 Siemens / meter (S / m, measured at 20 ° C) of the metal. In some embodiments, the conductivity is at least 3 × 10 6 S / m, at least 5×10 6 S / m or at least 7×10 6S / m. Alternatively, this property can be expressed as the resistivity of the material from which the particles are made. In some embodiments, the resistivity is at most 1×10 -6 Ω·m (measured at 20°C). When other conductive particles are desired, such as organic metals, they can be made of materials with lower conductivity (or higher resistivity) than the aforementioned, as will be understood by those skilled in the art. When the conductive particles are used for decorative purposes, their conductivity does not need to meet any specific threshold.

[0230] The particles may have a core made of a first material, while the particles are at least partially coated with a second material to provide the particles with their intended effect (e.g., decorative or functional). For conductive particles, the core can be a spherical, flake, or rod-shaped element made of glass (e.g., borosilicate), ceramic (e.g., silicon dioxide), natural polymers (e.g., latex), synthetic plastics (e.g., polyethylene), or carbon, and such a core is coated with a metal such as listed above. In addition, some coated particles may have a core made of a first metal species (e.g., copper) and an outer layer coated with a different metal species than the first metal species (e.g., silver).

[0231] In one embodiment, when the transferable composition is used to prepare a pattern on a transfer film, which is then contacted with a solar wafer and the pattern is transferred thereto to form a rear electrode on the back side of the substrate, the conductive particles are made of aluminum.

[0232] Suitable volume concentrations of the conductive particles are at least 20 vol.%, at least 30 vol.%, at least 40 vol.%, at least 65 vol.%, at least 70 vol.%, or at least 75 vol.% based on the volume of all solids in the composition (e.g., conductive particles, swellable polymer, and glass frit, if present). In some embodiments, the volume concentration of the conductive particles is at most 98 vol.%, at most 95 vol.%, at most 90 vol.%, at most 85 vol.%, at most 80 vol.%, or at most 75 vol.% based on the volume of all solids; and optionally, from 20 vol.% to 98 vol.%, from 30 vol.% to 95 vol.%, from 40 vol.% to 90 vol.%, from 65 vol.% to 98 vol.%, from 70 vol.% to 95 vol.%, or from 75 vol.% to 90 vol.% of the volume of all solids.

[0233] Although this section describes conductive functional particles in detail, it is worth noting that this should not be construed as limiting the scope of the present invention. In another embodiment, the particles do not need to be conductive, nor do they need to be functional in any other way. For example, when the transferable pattern is used for decorative purposes, the particles can be organic or inorganic pigments or dyes, or any such chromophores that can provide the desired color or coloring for the pattern, either alone or in combination. The amount of such particles in the transferable composition does not need to be particularly limited, and can be adapted to meet the desired visual effect.

[0234] It will be appreciated that the distinction between "functional particles" that provide a functional effect (e.g., allowing the formation of an electrical circuit) and "decorative particles" that provide a visual effect is intended to clarify the primary effect sought. It will be understood that functional conductive patterns also provide a visual effect, however in this case the particles are considered functional. Similarly, the distinction between functional and decorative particles does not necessarily mean that the particles are different. For example, taking metal particles as an example, they can be functional (e.g., particles for PCBs or RFID that are used from the outset without further sintering or firing, or particles that are subsequently "functionalized" such as for solar cells) or decorative (e.g., forming a metallic appearance pattern).

[0235] glass frit

[0236] In addition to swellable polymers, swelling agents and (e.g., conductive) particles, the transferable composition may also optionally include glass frit, which is particularly etchable on the surface (e.g., front) of the solar wafer and improves the adhesion between the particles (e.g., metal) and the wafer of the optional sintering after the temperature of the transferred pattern is applied to the increased temperature (if necessary). Before transfer and optional sintering, glass frit can be used as an adhesive and / or rheology modifier for the transferable composition. In addition, glass frit can be used to protect the conductive particles, e.g., from moisture in the air. Glass frit is compatible with other components (e.g., swellable polymers, swelling agents or conductive particles) and process conditions of the transferable conductive composition, e.g., glass frit provides the composition on the transfer film or in the groove and / or removed from the transfer film surface. Glass frit has the non-brittleness to maintain pattern integrity, or if conductive particles need to be sintered, glass frit has enough "heat resistance" to maintain a suitable shape by a certain amount.

[0237] Glass frits suitable for c-Si solar cells are typically made from the following: i) lead oxide (PbO)-based glass, typically lead borosilicate glass: PbO-SiO2-B2O3; ii) bismuth oxide (Bi2O3)-based glass, typically bismuth borosilicate glass: Bi2O3-SiO2-B2O3 and strontium bismuth borosilicate glass: SiO2-SrO-Bi2O3-B2O3; iii) tellurium oxide (Te2O)-based glass; and iv) vanadium pentoxide (V2O5)-based glass. In some embodiments, combinations of the above glass frits are used.

[0238] Glasses based on lead oxide, bismuth oxide, and tellurium oxide as a supplement to silicon dioxide (SiO2) are generally suitable for standard c-Si cells, while less abrasive or corrosive glass frits are preferred for PERC cells. Therefore, glasses based on bismuth oxide, vanadium pentoxide, lead oxide, and zinc oxide (ZnO), preferably also containing boron oxide (B2O3) and mixtures thereof, are more advantageous for PERC cells, as glass frits made from such materials do not harm the passivation layer on the back side of this particular type of solar cell.

[0239] In addition to silicon dioxide, each of the above glasses may also contain one or more of the following: aluminum oxide (Al2O3), barium oxide (BaO), magnesium oxide (MgO), molybdenum oxide (MoO2), molybdenum trioxide (MoO3), phosphorus pentoxide (P2O5), strontium oxide (SrO), tin oxide (SnO2) and tungsten oxide (WO3).

[0240] Exemplary glass frits include, for example, commercially available glass frit 2083 (3M, USA), which contains strontium oxide, bismuth oxide, boron oxide, and aluminum oxide in addition to silicon dioxide, and has a CTE of about 86.7×10 -7 ℃ -1 , and ASF-1100B (AGC, Japan), which contains bismuth oxide and boron oxide in addition to silicon dioxide and has a CTE of about 107×10 -7 ℃ -1 . Glass frits can be specially designed and prepared by a chemist proficient in the field of glass materials by combining and processing appropriate starting materials in the required amounts and proportions. Therefore, various glass frits suitable for use in the transferable composition of the present invention can be prepared, for example containing lead oxide, zinc oxide, boron oxide and silicon dioxide. For example, an exemplary glass frit can be prepared to contain: silicon dioxide in an amount between 0.5 wt.% and 4 wt.%, lead oxide in an amount between 70 wt.% and 85 wt.%, boron oxide in an amount between 5 wt.% and 15 wt.% and zinc oxide in an amount between 1 wt.% and 10 wt.%.

[0241] The amount of glass frit is typically in the range of 0.1 vol.% to 15 vol.%, preferably in the range of 0.2 vol.% to 12.5 vol.%, 0.5 vol.% to 10 vol.%, 0.5 vol.% to 8 vol.%, 0.5 vol.% to 6 vol.%, 0.5 vol.% to 5 vol.% or 1 vol.% to 5 vol.% of the volume of the (conductive) particles.

[0242] When the transferable composition is used to create solar cell patterns or to apply decorative patterns to glass substrates, the inclusion of glass frit is recommended. In this case, glass frit further helps adhere the transferred pattern to the substrate, and combined with functional particles, it can enhance the efficacy of the resulting pattern. While glass frit can also act as a filler, it is typically not present in transferable compositions used to form conductive patterns in electronic products other than solar cells.

[0243] granularity

[0244] Particles (whether (conductive) particles or glass frit) can have any shape, for example, form regular or irregular spherical beads / pieces / rods etc., preferably but not necessarily, if the composition needs the step of sintering or firing, single particles do not have the cavity that can hinder subsequent appropriate sintering or firing. When the transferable composition is applied in the groove formed in the transfer film, the maximum size of the particle on any axis / dimension should be less than the minimum size of the groove. (For example, the particle is on the order of a few microns, usually no more than 10-20 μm, or less). If the transferable composition that the particle is used for is applied on the outer surface of the transfer film, the restriction to the maximum size can be relaxed so, and particles of tens of microns (for example, the maximum size is at most 100 μm, at most 75 μm or at most 50 μm) can be used. Whether applied on the surface of the film or in the groove, depending on their shape, too large particles may not be satisfactorily filled in the pattern of the transferable composition. This filling is insufficient and may reduce or prevent the formation of a pattern that is fully conductive (or other functions), and / or weaken the resulting visual effect. Although smaller particles are more likely to pack together, in some cases, particles that are too small may not be advantageous, for example, they may undergo uncontrolled sintering at temperatures relatively lower than the sintering temperature of larger particles (for example, at a transfer temperature of about 135°C). This premature sintering of particles that are too small may subsequently affect the transfer of the composition (impeding flow) and the interfacial contact of the composition with the substrate. Secondly, a large number of small particles will increase the surface area of ​​the particles, which may require more swellable polymer, which in turn may affect the rheology of the composition and the processability of the paste. The increase in surface area may also require an increase in the concentration of certain additives in the transferable composition, for example, if a protective agent for water-sensitive particles is included, its amount may need to be increased. In addition, the increase in swellable polymer may also interfere with sintering (affecting the desired conductivity).

[0245] Therefore, depending on the application mode of the transfer film surface, particles with an average (D50) or maximum size of at least 200 nanometers (nm) and optionally up to 30 μm are preferred. In some embodiments, the (conductive) particles have an average or maximum size in the range of 0.5 to 15 μm, 1 to 10 μm, 3 to 5 μm, 3 to 20 μm or 3 to 30 μm. A group of particles of uneven size, although certain shapes are not required, can improve the packing of particles, with smaller particles inserted between larger particles to achieve a packing with fewer inter-particle gaps. This filling can promote sintering and / or firing (if necessary), which can improve the conductivity of the final sintered / fired pattern when these steps are required to make the pattern conductive or have other functions.

[0246] Information on particle size is usually provided by the supplier or can be determined by routine experimentation, for example using dynamic light scattering (DLS) techniques, where particles are approximated as spheres with equivalent scattering responses and the size is expressed as the hydrodynamic diameter, the value observed for 50% of the population by volume or by number, referred to as D, respectively. V 50 and D N 50. These values, often also referred to as D50, are often referred to as the average particle size. The size of the particles can also be estimated by microscopic methods and analysis of images captured by scanning electron microscopy (SEM), transmission electron microscopy (TEM), focused ion beam (FIB) and / or confocal laser scanning microscopy techniques. These methods are well known to those skilled in the art and do not need to be described in further detail. Since particles generally have different dimensions in different directions (unless they are perfect spheres), for simplicity the longest dimension in the largest plane of projection of the particles is considered. When the particles are spherical or nearly spherical, the "longest dimension" is approximately their diameter, which can be estimated by the DLS method. In this case, the hydrodynamic diameter of 90% of the particle population, but more typically 50% of the particle population, can be used to evaluate the size of the particles. In other words, depending on the particle shape, the longest length L of the particle, the thickness and the diameter in D V 90.D V 50. D N 90 or D N When estimated by DLS, preferably volume analysis, in one embodiment, the average particle size of the particles can be D V 50.

[0247] additive

[0248] Transferable conductive compositions may include various additional materials, commonly referred to as additives. When water-sensitive particles are incorporated into the transferable composition (the swellable polymer is swollen with a swelling agent comprising water), one such additive has been previously described as a water protectant, although other types of additives may also be included instead or in addition. For example, at least one rheology modifier (e.g., a plasticizer) may optionally be added to the blend comprising the swellable polymer and the swelling agent, such addition typically, but not necessarily, being performed before the (conductive) particles are added to the blend. Further optionally, at least one dispersant may be added to promote and / or stabilize the dispersion of the particles within the swollen polymer, such addition typically, but not necessarily, being performed before or after the conductive or other functional or decorative particles are added to the swollen polymer blend, which optionally contains the rheology modifier or any other desired premixed additives.

[0249] It will be understood that the name of any particular additive is not intended to limit its function to that name, but rather to indicate its primary role. For example, an additive specifically adapted to modify the viscosity of a transferable composition may also in turn facilitate the dispersion of particles and thus also function in some manner as a dispersant. As a converse example, an additive formally employed as a dispersant may also modify the rheological properties of the composition or protect the particles from water.

[0250] Plasticizers can not only improve the processability of transferable (e.g., conductive) compositions for their preparation, application to transfer films, and / or transfer to substrates, but can also enhance the mechanical properties of the resulting pattern. For example, the presence of a plasticizer in the composition can make the pattern more flexible and stretchable compared to the fragility of the pattern in the absence of the plasticizer. Such rheology modifiers can be selected from phthalates, phosphates, glycerides, and esters of higher fatty acids and amides. For example, when necessary, such additives may be one or more of the group consisting of dibutyl sebacate, butyl stearate, coconut oil fatty acid glycol ester, butyl ricinoleate, dibutyl phthalate, castor oil, hydrogenated castor oil, hydrogenated microcrystalline wax, hydrogenated resin, hydrocarbon resin, paraffin, beeswax, cetyl alcohol, butyl stearate, diethyl phthalate, diphenyl phthalate, dicyclohexyl phthalate, 2-butoxy-2-oxoethylbutyl phthalate, and dioctyl phthalate.

[0251] A dispersant may be optionally added to improve the dispersion of the conductive, other functional or decorative particles in the swollen polymer and / or the dispersion of the glass frit (if present), and may be selected from: (a) ether / ester dispersants, including but not limited to complex mixtures of phosphate esters and oleyl alcohol polyglycol ethers, such as the commercially available Crodafos TM O3A-LQ-(RB) or Crodafos TM O10A; a complex mixture of phosphate esters and polyoxypropylene, cetyl alcohol polyoxyethylene ether, such as Crodafos TM SG; a complex mixture of phosphate esters and tridecyl glycol ether, such as Crodafos TM T6A (all the aforementioned exemplary dispersants are provided by Croda, UK) or a complex mixture of phosphate ester and isotridecyl glycol ester, such as 1306 (Clariant, Switzerland); and (b) anionic dispersants, such as Hypermer TM KD4 (Croda, UK), or VPTZ100 (Elementis, UK).

[0252] If present, the total amount of these additives (e.g., rheology modifiers, dispersants, water protectants, etc.) is at least 0.05 wt.%, at least 0.1 wt.%, or at least 0.5 wt.%, relative to the weight of the total components. In some embodiments, the total amount of one or more additives is at most 30 wt.%, at most 25 wt.%, at most 20 wt.%, at most 15 wt.%, or at most 10 wt.%, relative to the weight of the total components.

[0253] Whether added individually or as a result of the addition of a dispersant, the particles can be dispersed or well dispersed in the transferable composition when they form a relatively uniform distribution in the cross-network of the swollen polymer. The relative uniformity of the particle dispersion can be visually assessed, for example, by microscopic examination. Although not all particles need to be individually dispersed in the composition to be considered sufficiently uniform, small clusters of a small number of particles may be tolerated, but the absence of coarse aggregates is preferred. Major aggregates can be detected by the naked eye of a skilled formulator. The presence of such aggregates or the upper size limit of the cluster can also be determined microscopically, but more easily can be determined using a fineness of grind meter, also known as a Hegman fineness meter.

[0254] thermal expansion

[0255] In some cases, it may be necessary to heat transferable composition, pattern, transfer film and / or substrate. For example, heating can be performed to reduce the viscosity of the composition, promote the formation of the pattern, increase the flexibility of the transfer film, strengthen the transfer to the substrate, and / or provide desired functions by thermal activation. The linear thermal expansion coefficient (CTE) of the material indicates that it changes the trend of its shape, area and volume in response to temperature changes. When two different materials in contact with each other are subjected to identical temperature changes (for example heating or cooling), they should preferably have similar CTE, because if their CTE differences are too large, at least one of them may produce harmful deformation, thereby may cause the loss of integrity, or any other adverse impact on their intended use.

[0256] The difference in the CTE of the materials that can be tolerated depends in particular on the temperature to which the materials are subjected, the duration of heating / cooling, and the intended use of the final product. In some cases, slight differences may even be beneficial. For example, if the transferable composition comprises glass frit and conductive particles, as in the case of a solar cell, a glass frit having a CTE slightly higher than that of the conductive particles can help to bring the conductive particles closer to each other after the composition is heated and cooled, thereby improving electrical conductivity. On the other hand, in order to prevent or reduce the stress that may be generated on relatively fragile silicon wafers, the transferable composition is preferably one having a CTE similar to or slightly lower than that of the substrate. In the manufacturing process of solar cells, in the firing step of generating electrical contact on the silicon wafer by making the conductive particles reach a high temperature, the mismatch of the thermal expansion coefficients of the materials and different mechanical properties may all cause at least local stress. Therefore, if the mechanical stress caused by the CTE difference is too high, the silicon wafer may bend to form a convex or concave body when cooling, and may even cause it to break.

[0257] The CTE of the glass frit is usually provided by its manufacturer or can be measured by routine experiments, for example, according to ASTM E228-17. In some embodiments, the CTE of the glass frit used in the transferable composition of the present invention is 50×10 -7 Up to 120×10 -7 ℃ -1 .

[0258] Solar cell efficiency

[0259] The transferable composition of the present invention can be used to form conductive patterns in various electronic products. Considering the specific situation of solar cells, the transferable pattern can generally form conductive grid lines for the front electrode of a typical single-sided solar cell, or can form a conductive layer that serves as the rear electrode on the back side of such a solar cell wafer.

[0260] While transferable conductive compositions according to the present invention are beneficial for both types of patterns and both sides of a solar cell, it is generally recognized that more continuous conductive patterns, such as those required for the back electrode of a solar cell, may be more important and / or easier to assess in terms of their contribution to the performance and efficiency of the solar cell.

[0261] One of the key factors in solar cell efficiency is low surface recombination, which is usually achieved by providing a passivation layer on the front side and forming a satisfactory back surface field (BSF) on the back side. Simply put, the back surface field is a higher doped region. The electric field formed at the interface of the high and low doped regions reduces dark current and reflection of minority carriers. The BSF has the net effect of passivating the back side, which in turn can increase the open circuit voltage, V OCA standard c-Si cell in which the back side is not passivated and comprises a BSF layer on most of its surface is also referred to as a BSF cell.

[0262] Another type of c-Si solar cell, known as a passivated emitter and rear contact (PERC) solar cell, is widely used due to its relatively high efficacy, which is attributed to the dielectric passivation layers on its front and back sides. For more conventional BSF cells, the front passivation layer may be an anti-reflective coating, whereas their rear passivation layer is perforated by chemical or laser. This enables contact between a conductive electrode on the back side of the wafer and the silicon layer underneath this perforated passivation layer, thereby creating a localized BSF under favorable conditions (e.g., after firing). The perforated passivation layer on the back side of the PERC cell has an additional contribution, allowing PERC cells to have higher cell efficiency and / or other advantages (e.g., heat resistance) compared to more conventional cells.

[0263] The open circuit voltage of the solar cell (V OC ) is an indicator of the degree of recombination, with reduced recombination levels translating into increased V OC , indicating that the efficiency of the battery is higher. Although in a laboratory environment and standard conditions (for example, at a temperature of 25°C and a vertical illumination of 1000 watts / square meter), a V of 700mV or even greater can be obtained OC , but generally speaking, in commercial sizes, for solar cells, especially conventional BSF cells, a V of at least 580 mV, at least 590 mV, at least 600 mV, at least 610 mV, at least 620 mV, at least 630 mV, or at least 640 mV OC For PERC cells, a V of at least 660mV or at least 670mV is likely sufficient to be considered effective. OC When the battery produces no current under no load, measure the battery's V by connecting a voltmeter across the positive and negative electrodes. OC .

[0264] While it is believed that swellable polymers can improve the efficiency of solar cells provided by transferable conductive compositions, it is primarily to reduce the amount of swellable polymer required for processability of the transferable composition, thereby reducing the amount of harmful residues remaining in the final product, which is one of the advantages of the transferable compositions of the present invention. The reduction of harmful residues that may hinder the efficiency of patterns or solar cells formed by polymers containing them is expected to reduce the possibility of recombination, thereby increasing the V OCWithout wishing to be bound by any particular theory, the present invention further proposes that the use of a relatively low amount of a swellable polymer (compared to a relatively high amount of a conventional non-swellable binder) may provide, among other things, at least one of the following advantages: i) increased fluidity of the transferable composition; ii) facilitated application thereof to a transfer film; iii) enhanced release thereof from the transfer film; iv) ease of transfer thereof to a substrate; v) enhanced contact with said substrate; and vi) increased effectiveness of the conductive particles dispersed therein relative to each other (e.g., enabling more uniform sintering if desired) and / or relative to the substrate surface with which they will interact (e.g., facilitating etching of the frit if present in the pattern due to formation of the front electrode, or facilitating formation of a better BSF by increasing the relative doping of the back side of the solar cell).

[0265] In order to maintain a satisfactory solar cell efficiency, the thickness of the back electrode layer and V OC Without being bound by theory, it is generally believed that there is a minimum required thickness of the conductive layer on the back side of the solar wafer to allow the wafer to have a satisfactory V OC The formed electrode has sufficient sheet resistance. Increasing the thickness of the conductive layer to or beyond a certain level will not further increase V OC , so any further increase in thickness is unnecessary. The thickness of the conductive layer is described in further detail below.

[0266] Preparation of transferable compositions

[0267] In another aspect of the present invention, a method for preparing a transferable (e.g., conductive) composition comprising a swellable polymer, a swelling agent, and decorative and / or functional (e.g., conductive) particles is provided. Figures 1A-1E As shown, Figure 1F A magnified image of the resulting transferable (eg, conductive) composition is depicted.

[0268] In the first step, a polymer premix containing a swellable polymer is prepared. The swelling of the polymer is carried out by mixing the swellable polymer 1 with the swelling agent 2 to obtain a mixture, such as Figure 1A The polymer-containing mixture 3 that gradually begins to swell is then heated under suitable mixing conditions (e.g., stirring) until the polymer is completely melted, and the mixture then forms a uniform gel-like preparation 4 (e.g., Figure 1C). The polymer premix can be prepared by heating the mixture to a temperature above the melting temperature of the swellable polymer, typically at least 20°C, at least 40°C, or at least 60°C above the melting point of the polymer. Although further increasing the heating temperature can accelerate the melting of the swellable polymer and the preparation of the polymer premix, this step should preferably be performed under sufficiently mild conditions to prevent premature elimination of the swelling agent. Therefore, the temperature for preparing the polymer premix is ​​typically at least 40°C, at least 60°C, or at least 80°C below the boiling point of the swelling agent. In addition, the mixing container can be sealed and / or, if necessary, the swelling agent can be added during the preparation of the polymer premix to compensate for undesirable evaporation, or it can be provided in excess from the beginning. In some embodiments, the polymer premix comprising the swellable polymer and its swelling agent is heated to a temperature of at least 100°C, at least 120°C, or at least 140°C. In some embodiments, the polymer premix is ​​heated to a temperature of at most 200°C, at most 180°C, or at most 160°C. In particular embodiments, the premix is ​​heated to a temperature in the range of about 100° C. to 200° C., 120° C. to 180° C., or 140° C. to 160° C. Heating may be performed gradually until the premix reaches the desired temperature.

[0269] As previously mentioned, additives (such as rheology modifiers) may optionally be added at this stage as part of the polymer premix (not depicted in the figure).

[0270] The polymer premix thus obtained can be cooled or allowed to cool for subsequent use, in which case a reheating step may be required to fully soften the swollen polymer before the (conductive) particles are added to the polymer premix. Alternatively, the polymer premix can be used directly after its preparation, with the swollen polymer remaining in a softened state. In the context of the present invention, the term "directly after" does not necessarily mean immediately after obtaining a homogeneous polymer premix, as this can occur at relatively high temperatures. Such high temperatures may not be suitable for some (conductive) particles because they may trigger undesirable reactions. Therefore, in some embodiments, the temperature of the polymer premix is ​​allowed to passively cool or actively lowered before the (conductive) particles are added to the mix. The temperature at which the (conductive) particles are added to the mix can be considered the softening temperature of the polymer premix. At this temperature, the polymer premix and the transferable composition prepared therefrom are fully ductile / flowable for the ongoing steps (e.g., mixing of the ingredients, application or deposition onto a film, loading into a groove, etc.). The fluidity of the transferable composition also depends on the equipment used. Although the viscosity of the transferable composition need not be particularly limited except in the case of a specific equipment performing a predetermined step, in some embodiments, it is generally believed that the viscosity does not need to exceed 50,000 mPa·s at temperatures relevant to evaluating fluidity (e.g., within the temperature range of 50-100° C.). Under relevant operating conditions, the viscosity can be no more than 40,000 mPa·s, no more than 20,000 mPa·s, or no more than 10,000 mPa·s.

[0271] When the added particles do not require special temperature control during mixing with the polymer premix, the mixing temperature can be selected from a wide range having a lower limit of the softening temperature of the polymer premix (or 10-30°C higher) and an upper limit of the lesser of the lowest boiling point of the swelling agent and the maximum processing temperature of the swellable polymer minus at least 40°C, 50°C, 60°C or 80°C. For example, if the softening temperature of the polymer premix is ​​about 50° C., the temperature at which the (e.g., conductive) particles are dispersed and mixed within the polymer premix to obtain the transferable conductive composition can be at least 50° C., at least 60° C., at least 70° C., or at least 80° C. If the polymer premix consists of a swelling agent having a boiling point of about 320° C. and a swellable polymer having a maximum processing temperature of 290° C., then for heat-insensitive particles, the mixing temperature can be at most 250° C. (290-40° C.), at most 230° C. (290-60° C.), at most 210° C. (290-80° C.), at most 150° C., or at most 100° C. Advantageously, once swollen, the polymer premix can be used at a temperature in the range of 60° C. to 120° C., or 80° C. to 100° C.

[0272] Whether or not performed "directly" after the first step, in a second step, the polymer premix 4 containing the swollen polymer is combined with particles 5 (e.g., functional conductive or decorative particles) at a temperature that allows it to soften sufficiently (e.g., at about 80-90° C.), such as Figure 1D and mixing until a uniform dispersion of particles is obtained in the gel-like swollen polymer cross-network (eg, transferable composition 6), as shown in FIG. Figure 1E As shown in .

[0273] Figure 1F Schematically shows an enlarged view of a transferable composition 6, which can be Figures 1A to 1E Without being bound by theory, it is believed that the swollen polymer forms a cross-linked network of elongated chains 7 which form the boundaries of an internal compartment 8 in which the particles and any other additives are dispersed within the swelling agent.

[0274] If desired, can add glass frit and / or at least a dispersion agent (not shown) in this stage, the interpolation of these additives (or any other additive) is carried out under the softening temperature of the polymer premix, so that promote in the swollen polymer gel-like structure, prepare uniform dispersion. So dispersed particles (and any additive added in any step) form transferable composition of the present invention together with the swollen polymer.If particle is conductive, then its dispersion in the swollen polymer can be called transferable conductive composition.

[0275] (Conductive) particles, glass frit and other potential additives (e.g. rheology modifiers, dispersants) can be incorporated into the transferable composition at various stages. For example, prior to adding the (conductive) particles, glass frit can be added to the polymer premix to form a glass frit premix. Alternatively, the glass frit can be combined with particles (e.g. aluminum) to form a glass frit / particle premix, typically in the presence of a dispersant, which is then further combined with the polymer premix. In another alternative, the (conductive) particles and / or glass frit can be combined separately with a dispersant to prepare a premix, which is then combined with the polymer premix.

[0276] While the processing of the polymer premix and / or transferable composition can be accomplished by hand mixing, various equipment can also be used to provide a more uniform swollen polymer mixture. Such equipment includes, but is not limited to, vortex mixers, overhead stirrers, magnetic stirrers, ultrasonic dispersers, high shear homogenizers, planetary centrifuges or planetary mills, attritor media grinders, bead mills, ball mills, three-roll mills, extruders, and extrusion kneaders. In one embodiment, the premix or transferable composition is prepared by high shear mixing.

[0277] Regardless of the steps used to combine the various components that may form a particular transferable composition, it is noteworthy that the gel-like structure provided by the swollen polymer should be substantially retained at the end of the preparation. In other words, the transferable composition of the present invention forms a cross-structure of swollen polymer molecules that interact through strong intermolecular forces and covalent bonds that may be facilitated by certain additives. The spaces within the cross-structure between the interlocking polymer molecules contain the swelling agent, the dispersed particles, and any other optional additives, such as Figure 1F As shown in the enlarged view of . With this satisfactory dispersion mode, particles are embedded in the internal liquid phase of the gel, and subsequently they are applied on the transfer element with the state of part immobilization, and are applied on the substrate thus, so as to allow to still keep the required particle dispersion state after the transfer in case of transfer. Under the situation that is not bound by a specific theory, it is generally believed that the gel-like structure of the transferable composition of the present invention is better than other more conventional compositions comprising similar particles, and contrary to the present invention, the similar particles of conventional compositions can fully migrate in a continuous, outside or independent liquid phase to flocculate or agglomerate in a random manner. For example, by realizing or improving the suitable dispersion of particles in the relatively stable cross structure of swelling polymers, transferable composition can reduce the risk of forming particles and have discontinuous patterns. Taking conductive pattern as example, the pattern portion of insufficiently loaded particles, for example, prevents the gap of sintering, and may cause the disconnection of conductive pattern or the reduction of its efficacy.

[0278] The rheology of the transferable (conductive) composition thus obtained generally depends on two main parameters: i) the temperature, an increase of which leads to a decrease in viscosity; and ii) the shear-thinning behavior of the transferable (conductive) composition, if any.

[0279] The shear thinning or pseudoplastic flow behavior of a composition, characterized by a decrease in viscosity with increasing shear rate, is generally attributed to the structural reorganization of the polymer molecules within the composition when shear is applied. Without being bound by any particular theory, it is generally believed that when subjected to shear, the polymer chains disentangle and align in a manner that causes a decrease in viscosity. After the shear stops, the composition returns to its previous state, with an increase in viscosity, and the composition may even regain its initial higher viscosity. This behavior may be desirable when the application of the transferable (conductive) composition to the transfer film involves shearing by the applicator or the process employed for this purpose.

[0280] For example, if the transferable composition is applied by screen printing or doctor blade, it is advantageous to initially have a relatively high initial viscosity of the transferable composition (e.g., to prevent unintentional flowing prior to the intended selective application), which may then exhibit a relatively low viscosity during shear application (e.g., to allow the composition to flow through the mesh of the screen print), and then thicken again once applied to the transfer film (e.g., to allow the pattern to maintain its intended shape).

[0281] In these embodiments, the transferable compositions of the present invention exhibit a -1 The viscosity of the transferable composition decreases by at least one order of magnitude at a shear rate of 0.1 s. The viscosity can be assessed using any suitable rheometer, such as a ThermoScientific MARS III, using a 60 mm diameter disk at 0-1000 s. -1 The principal axis geometry of the 60 / 1° Til is measured by operating at a shear rate and shear stress (rotational ramp). The measurements should be carried out at a temperature related to the application temperature of the transferable composition, the current observations of the dilution behavior were carried out at 80°C.

[0282] Application of transferable composition

[0283] Then, the transferable (conductive) composition obtained by the method for above-mentioned non-limiting examples can be applied to the transfer film by any suitable method known in the art.For example, the composition can be deposited on the transfer film and spread, for example, by smearing the composition of deposition with a scraper, scraper or air knife or any device suitable for leveling the composition when in contact with or without contact with the composition, to produce a coating of desired shape and thickness on the surface of the transfer film. Alternatively, the pattern (for example, the grid shape of the spacer lines of conductive particles) with a specific design can be applied by screen printing or any other method allowing to selectively deposit the composition with a predetermined pattern. Pattern can also be formed in the groove below the transfer film surface, similar to gravure printing in principle. For example, groove can be formed by laser, by applying a predetermined heat engraving plastic film to form the pattern of expectation. Also can be formed groove by mechanical engraving or making film contact with the mold (optionally with bottom mold) of suitable shape, described mold is suitable for forming at least a portion of desired pattern. For example, the mold of suitable shape can be a pad with parallel projections, and the profile that described parallel projections have and spacing correspond to the profile of the groove formed accordingly with a distance similar to each other. The gasket can be mounted on a plate or a cylinder and pressure is applied to form a groove. The composition can then be deposited on the transfer film and applied to fill the groove, and this loading can be performed in one or more cycles until the composition is substantially flush with the surface of the transfer film. This method is described by the applicant in WO 2018 / 020479 and WO 2018 / 020481. The transferable pattern can constitute at least a portion of the intended pattern on the substrate, because such an intended pattern can be complex and / or applied separately, optionally by different methods. For example, taking the front electrode of a solar cell as an example, the transferable pattern can form conductive fingers in the shape of a guide grid, and the lateral busbars are applied separately to the substrate.

[0284] Transferable composition is applied to the transfer film and can be carried out at one or more temperatures high enough relative to ambient temperature, to promote deposition and patterning of transferable composition by the device selected for this purpose.The temperature of selection is such, makes it high enough to allow composition to have enough fluidity so that it is applied to the transfer film, makes it low enough to prevent fluidity from extending beyond the required forming expected pattern simultaneously.As those skilled in the art will readily appreciate, such temperature can depend on applying device and / or applying method, depend on the pattern to be formed and the persistent period of formation thereof and depend on composition to be applied to the transfer film thereon.Also may depend on the composition of transferable composition, its softening temperature, be loaded in the groove or be applied on factors such as demand on the transfer film surface.

[0285] In some embodiments, the application temperature is in the range of 50°C to 150°C, 60°C to 120°C, or 70°C to 100°C. For example, the application temperature may be at least 50°C, at least 60°C, at least 70°C, or at least 80°C. In some embodiments, the temperature at which the transferable conductive composition is applied to the transfer film is at most 150°C, at most 120°C, at most 100°C, at most 95°C, or at most 90°C. This temperature may be related to the temperature of the composition when it is fed to the applicator or when it is deposited on the transfer film, and is subsequently reduced as the pattern is formed. In some embodiments, the application temperature is maintained substantially constant or reduced during application, for example, by applying heat to the transfer film to which the composition is applied. Depending on the method selected for applying the composition to the transfer film, the temperature applied or experienced, and the time the composition may be subjected to the temperature, at least a portion of the swelling agent may be removed during the application process. If desired, additional swelling agent removal may be achieved after the pattern is formed on the film (e.g., by a stream of hot air). However, it is not necessary to completely eliminate the presence of the swelling agent, as long as the pattern formed on the film is sufficiently "dry" to maintain its desired shape and / or be non-sticky for subsequent steps. On the contrary, it is generally believed that the presence of at least some swelling agent (e.g., 10 vol.% or more based on the volume of the composition on the film) is desirable for the polymer to remain swollen, as this can facilitate subsequent transfer.

[0286] If desired, a diluent may be added to the transferable composition to facilitate its application to the transfer film. The diluent may provide a viscosity suitable for the application method or apparatus, any suitable apparatus also being referred to as an applicator. Since the effect of the diluent is short-lived, it is desirable that it be rapidly eliminated. For example, the diluent may be a short-chain alcohol having a relatively low boiling point, such as, for example, isopropyl alcohol, as a non-limiting example.

[0287] The pattern applied can have any desired thickness, and this depends on the intended purpose.If after transferable composition is applied, before any significant change occurs in its content (for example, before the evaporation of relative volatile components), thickness is measured, then this thickness value is referred to as the " wet " thickness of pattern.After applying on transfer film, transferable pattern can have the wet thickness of at least 3 μm, at least 5 μm or at least 10 μm.In some embodiments, transferable pattern can have the wet thickness of at most 250 μm, at most 200 μm, at most 150 μm, at most 100 μm or at most 80 μm.For solar cell back electrode, the wet thickness of the pattern of transferable composition can be in the range between 25 μm and 50 μm.

[0288] If the measurement is made after at least a portion of the swelling agent has been removed, and typically after the pattern has been intentionally dried, these values ​​refer to the "dry" thickness of the pattern. The transferable pattern can have a dry thickness of at least 2 μm, at least 3 μm, or at least 5 μm. In some embodiments, the transferable pattern can have a dry thickness of at most 200 μm, at most 170 μm, at most 130 μm, at most 90 μm, or at most 70 μm.

[0289] transfer film

[0290] Once applied, the transferable composition of the present invention should preferably have relatively low film adhesion to the transfer film (e.g., regardless of the remaining amount of swelling agent, only sufficient to allow the composition to remain attached to the surface and / or within the grooves of the transfer film during manufacturing, handling, or storage and when applied to a substrate, but to allow the transferable composition to be released from the transfer film if peeled from the substrate after lamination to the substrate). On the other hand, the composition should preferably have relatively high adhesion to the receiving substrate (e.g., to allow transfer of a pattern including, for example, conductive particles to the substrate).

[0291] The transfer film can be appropriately selected to facilitate this relatively low adhesion / relatively high releasability of the transferable composition.

[0292] While the transfer film can be rigid (e.g., the outer surface of an impression cylinder), a flexible film is believed to facilitate subsequent transfer of the pattern formed thereon, for example, using an apparatus such as that described by the inventors in WO 2018 / 020483. Advantageously, but not necessarily, the flexible film can be disposable if peeled from the substrate after transfer. In one embodiment, the flexible film is in web form and can be used in a roll-to-roll process.

[0293] Flexible films with low surface energy can be used in the method disclosed herein. When the pattern of the transferable conductive composition is formed in a groove located below the surface of the transfer film, a film comprising a thermoplastic polymer is particularly suitable, wherein the thermoplastic polymer is selected from cyclic olefin copolymer (COC), polyvinyl chloride (PVC), polypropylene (PP), polyethylene (PE), thermoplastic polyurethane (TPU) and polyethylene terephthalate (PET).

[0294] When the pattern of transferable conductive composition is formed on the surface of transfer film, a foil made of this thermoplastic polymer can be used, in this case, other plastic polymers can also be used, and it is not necessary to be thermoplastic. Owing to not wishing to carve on the support of this transferable pattern, other materials can also be used for these transfer films, for example can be a metal foil or paper foil, usually formed by laminating with a layer of plastic polymer layer. The transfer film that is applicable to transferable pattern remains on its outer surface is basically impermeable for the transferable composition applied thereon, to prevent the improper penetration that may have a negative impact on the shape of the expected pattern or its subsequent transfer ability. The material constituting this transfer film or its outer layer can be itself impermeable, or the transfer film can be pre-treated with an impermeability agent.

[0295] The flexible film made of this polymer preferably has enough inelasticity, to remain on it or in the shape of the pattern formed in the groove and the profile of the composition applied thereto.On the other hand, film is preferably enough pliable to fit the surface of substrate (for example, solar wafer), thereby allows enough close contact to be used for the transfer of the pattern formed by transferable composition. For similar reasons (for example, filling groove with a scraper, using substrate etc.) for improving interface in the process, it may be necessary to have a film with a relatively smooth surface. Advantageously, owing to contact with substrate, the average roughness Rz of the flexible film surface is 1 μ m or less, 500nm or less, 250nm or less or 100nm or less. It is understandable that transfer film also needs to meet other mechanical properties, so that transfer film and the method (for example, tensile strength, stress resistance, heat-resisting, radiation resistance etc.) and the transferable composition (for example, chemical resistance, chemical inertness etc.) used therein are compatible.

[0296] While in some embodiments, the flexible film can be pre-formed and provided as is (e.g., in roll or sheet form), the flexible film can alternatively be cast from a suitable material (e.g., a thermoplastic polymer that solidifies upon cooling, or a photopolymer that solidifies upon exposure to radiation, which may be referred to herein as a plastic polymer) as part of the production process. Formable plastic polymers that can be embossed or cast are well known to those skilled in the art.

[0297] Pattern transfer

[0298] The pattern formed on the transfer film of the transferable composition of the present invention can be transferred to a suitable substrate or its side, depending on the pattern. Advantageously, the pattern transfer from the flexible film to the substrate is usually performed under applied pressure (e.g., at about 0.1 kgf / cm 2 to about 50kgf / cm 2 The transfer temperature can be adjusted to a temperature within a range of 100°C (-100°F) and optionally at an elevated temperature. The basic principles for transferring at temperatures above ambient temperature are similar to those detailed above regarding applying the transferable composition to the transfer film. Simply put, because higher temperatures reduce the viscosity of the pattern-forming composition and increase its ductility, improved wetting of the receiving substrate can be achieved, which in turn can facilitate transfer and adhesion to the substrate. This transfer temperature can facilitate release of the pattern from the transfer film and / or facilitate adhesion to the substrate. This transfer temperature can depend on the transferable composition, its softening temperature, the transfer film bearing the pattern, and the substrate in contact therewith. The transfer temperature can further depend on the pressure applied at the contact line or contact area between the transfer film and the substrate. For example, the transfer temperature can be at least 80°C, at least 90°C, or at least 100°C. In some embodiments, the transfer temperature can be at most 150°C or at most 140°C. In some embodiments, the transfer temperature is in the range of 80°C to 150°C, 100°C to 150°C, or 130°C to 140°C.

[0299] In some embodiments, the transfer temperature is achieved by heating the substrate and the transfer film together, for example, by bringing them into contact between two rollers, at least one of which is heated to a temperature that allows them to quickly reach the desired transfer temperature. In other embodiments, the transfer temperature is achieved by heating the substrate alone, while the film is kept or maintained at ambient temperature. The substrate can be heated by any suitable method, such as by conduction (e.g., by a hot plate), convection (e.g., using a stream of hot air), radiation (e.g., using an IR lamp), or a combination of these heating means.

[0300] In some embodiments, the temperature is reduced after pressure is applied to achieve appropriate contact and before the transfer film is peeled off to achieve transfer of the pattern to the substrate if necessary. In this intermediate stage, the film is said to be "attached to the substrate" or the substrate is "attached to the film", even if the attachment is temporary and will not be separated until needed. Cooling can be actively achieved by conduction (e.g., passing the substrate with the film attached over a cold surface), convection (e.g., blowing cooled air onto the substrate with the film attached), or a combination of these cooling means. If separation is not required, or separation is not required immediately, cooling can be passive.

[0301] substrate

[0302] The substrate is selected to be suitable for the intended use to which the pattern is transferred, and this selection is known to those skilled in the art. When used for the production of solar cells, the substrate can be a semiconductor wafer. The substrate can be rigid or flexible, consisting of one or more layers of material, for transferring on one or more sides thereof, can be doped or undoped, and the like. As a non-limiting example, the substrate suitable for preparing a solar cell can be a rigid wafer or a flexible film. The substrate can be the face of a solar cell made of an inorganic material or an organic material, the inorganic material being for example single crystal silicon or polycrystalline silicon (single crystal c-Si or polycrystalline c-Si), amorphous silicon (a-Si), gallium arsenide (GaAs), polycrystalline silicon (p-Si) and any other similar substrate for solar cells, the organic material being for example a flexible polymer. Such a wafer can be of various sizes (e.g., approximately 156×156 mm or 125×125 mm) and thickness (e.g., in the range of 150-300 μm or 100-250 μm).

[0303] The semiconductor solar substrate may have a patterned surface forming, for example, an array of pyramid-like protrusions.

[0304] When used for non-solar technologies, other substrates may be used, which may generally be referred to as insulating substrates. These insulating substrates may be rigid or flexible, consisting of one or more layers of material. As non-limiting examples, substrates suitable for preparing circuit boards may be rigid wafers or flexible films. The insulating substrate may be made of phenolic resin, ceramic, epoxy resin, glass, plastic material or any other non-conductive material, or may have only a surface made of phenolic resin, ceramic, epoxy resin, glass, plastic material or any other non-conductive material. As an example, the substrate of a PCB may be a glass fiber rigid board (e.g., FR4) or a flexible high temperature plastic (e.g., available from DuPont). Polyimide film), also known as flexible board.

[0305] In some embodiments, the insulating substrate can be a substrate of any material having an outer insulating surface formed by a thin layer of electrically insulating material. Such an insulating layer can have a thickness of tens of nanometers to hundreds of micrometers (e.g., in the range of 50 nm to 100 μm) and can be applied to the substrate body by any suitable method known to those skilled in the art, such as spin coating, dip coating, or vapor deposition. As a non-limiting example, the thin film forming the insulating surface of the electrically insulating substrate can be made of (a) a photopolymer derived from B-stage bisbenzocyclobutene (BCB) (e.g., Cyclotene commercially available from Dow Chemical Company) TM4000 series), (b) SU-8 epoxy negative photoresist, or (c) poly(p-xylylene) polymer (also known as parylene). Such a thin insulating layer can be applied, for example, on a substrate whose underlying layer is made of metal or ceramic.

[0306] sintering

[0307] When used to make solar cells or other conductive circuits, the transferable composition, the pattern formed on the transfer film, and the pattern transferred thereto to the substrate may include conductive particles, optionally requiring sintering to make the pattern conductive. Depending on the material of the particles, various sintering methods may be considered. Typically, for metals, thermal sintering may be selected. In addition, the sintering temperature selected may substantially remove all previous organic components (e.g., swellable polymers, swelling agents, etc.) in the composition, while the conductive circuit produced by the sintering process is primarily formed by sintered conductive particles.

[0308] When the transferable composition further comprises glass frit, and the transferable composition within the transferred pattern is subsequently sintered (if necessary), the substrate and the pattern transferred thereto can be fired to fuse the conductive pattern to the substrate. The molten glass can etch the dielectric layer on the front side of the semiconductor wafer while allowing metal ions (e.g., silver) to migrate through it to reach the substrate (e.g., a silicon wafer or thin film), thereby creating a conductive path between the surface metal pattern and the dielectric layer or passivation layer (if present) underneath the wafer.

[0309] When used in a transferable composition applied to the backside of a solar cell wafer (BSF or PERC), the molten glass frit can coat the conductive particles during sintering, promoting their migration to the silicon layer, thereby enabling contact between the conductive particles and the silicon and thereby forming the BSF (or localized BSF for PERC cells). Sintering can also increase the sheet resistance, as evidenced by the cell conductivity. Without being bound by theory, it is generally believed that when the molten glass frit cools after sintering, the sintered composition undergoes some shrinkage, which in turn may bring the conductive particles closer together, thereby increasing conductivity.

[0310] Firing can be performed separately, or simply as an extension of the step used to sinter the conductive particles, if desired. However, the temperature and time required to fuse the conductive pattern to the semiconductor substrate are different from those required for simple thermal sintering. For example, sintering the conductive particles can be performed at a sintering temperature ranging from about 100-150°C to about 800°C, while firing the pattern of the composition to form electrical contact with the substrate requires a firing temperature ranging from about 500°C to about 900°C, which is greater than the sintering temperature.

[0311] As is known to those skilled in the art of solar cell manufacturing, once applied to the substrate, this post-processing of the pattern is typically performed according to a temperature ramp profile. A first stage (e.g., at 200°C-250°C) allows any volatile components to substantially dry out at the selected first stage temperature, a second stage (e.g., at 500°C-550°C) ensures the elimination of the polymer by combustion while the conductive particles are at least partially sintered, and a third stage (e.g., at 680°C, 790°C, 860°C, and / or 920°C) ensures the conductive particles are gradually fired into the solar cell substrate.

[0312] Depending on the pattern and the side of the solar substrate to which it will be applied, the transferable conductive composition of the present invention can be used to ultimately produce a conductive pattern on the front surface of a solar cell (i.e., front electrode) or a conductive coating on the back side of a solar cell (i.e., back electrode).

[0313] Thus, in another aspect of the present invention, a transferable composition and pattern formed therefrom is provided which can be applied to a transfer film and subsequently released from the transfer film onto a substrate. In a particular aspect of the present invention, the transferable composition produces a conductive pattern that can be used to manufacture solar cells.

[0314] Furthermore, in another aspect, a solar cell is provided, which is manufactured from the transferable composition and / or the transfer film of the present invention, the transfer film carrying a pattern prepared from the transferable composition.

[0315] After transfer and any desired post-transfer processing (e.g., sintering, firing, etc.), the thickness of the conductive pattern may be at least 2 μm, at least 3 μm, or at least 5 μm. In some embodiments, the thickness of the pattern may be at most 200 μm, at most 170 μm, or at most 130 μm. The thickness of the pattern or its conductive pattern can be measured by FIB microscopy or by a thickness gauge by comparing the thickness of the transferred film in the area with and without the pattern.

[0316] Alternatively, the transferred pattern may not require sintering and / or firing to become conductive, the transferable composition used to form it comprising inherently conductive particles. Furthermore, the particles themselves may be functional and / or decorative, without requiring any special treatment after transfer. In this case, a ready-to-use final product may be obtained directly after transferring a pattern comprising such "ready-to-use" particles.

[0317] Example

[0318] Material

[0319] The materials used in the following examples are listed in Table 1 below. The reported properties were taken from the product data sheets provided by the respective suppliers or estimated by standard methods. Unless otherwise stated, all materials were purchased at the highest purity level available. N / A indicates that specific information is not available.

[0320] Table 1

[0321]

[0322]

[0323]

[0324] equipment

[0325] Ball mill: YKQ-2-5L, purchased from Yonglekang Equipment Company, Changsha, China.

[0326] Oven: purchased from China Bo Ke.

[0327] Flat screen printing machine: AT-60P, purchased from ATMA, Taiwan, China.

[0328] 4-terminal resistance meter: DT-5302, purchased from CEM, China.

[0329] Heating mantle: purchased from Huanghua Xinshi Instrument Co., Ltd., China.

[0330] High shear homogenizer: HOG-500-2, purchased from Dalong Xingchuang Experimental Instrument Co., Ltd., China.

[0331] The IV tester consisted of a Kepco BOP 20-20 bipolar power supply, a Keithley 2700 multimeter capable of reading current and voltage, a temperature-controlled chuck, and an EYE SolarLux 150R400nm-1100nm as a solar simulator.

[0332] IV tester: Suns V OC , purchased from Sinton Instruments, USA.

[0333] IR heater: Infrared hanging oven bath 3 degrees 1500W, purchased from Electro Hanan, Israel.

[0334] Laminating machine: LM-380, purchased from Epoch International, USA.

[0335] A rubber roller with a diameter of 3 cm, a total length of 15 cm, an outer surface hardness of 80 Shore A, and a total weight of 102 g

[0336] Solar belt furnace: CDF-SL7210, purchased from Despated Industries, USA.

[0337] Vacuum automatic film applicator: BDG 218, purchased from China Biaogeda Precision Instrument (Guangzhou) Co., Ltd.

[0338] Example 1: Preparation of Transferable Compositions - Testing of Various Swellable Polymers

[0339] 1.1 Preparation of swellable polymer premix

[0340] 400g Isopar used as swelling agent TM The swellable polymer (100 g) listed in Table 2 and Castor wax HCO 70 (used as a rheology modifier) ​​were added to the swellable agent in a weight / weight ratio of 3:1 (75 g and 25 g, respectively) using a heating mantle. The resulting mixture was heated to approximately 150-160° C. using a standard overhead stirrer at approximately 800-2000 rpm until a clear solution was obtained. The solution was transferred to a sealable glass container and maintained in a sealed container at room temperature (approximately 23° C.) for at least 2 hours. The swellable polymer premix was allowed to cool, thereby obtaining a whitish, dense, rubbery composition containing 15 wt% of the swellable polymer based on the weight of the polymer premix.

[0341] The swellable polymers used to prepare the polymer premix according to the above method are listed in Table 2, including their melt flow rates (provided by the supplier, tested under a 2.16 kg load and a temperature of 190°C). Their respective melting temperatures, softening temperatures and densities are also provided.

[0342] Table 2

[0343]

[0344] 1.2. Preparation of Conductive Composition

[0345] Place 59.2 g of the polymer premix prepared in the previous step (8.88 g of which is swellable polymer) in a 250 ml glass container. Heat the gel-like swellable polymer composition to approximately 80-90° C. using a heating mantle until it reaches a sufficiently homogeneous flowable state for the addition of solid particles. While stirring at approximately 800-2300 rpm using an overhead stirrer, gradually add 3.4 g of glass frit (V2083, density 5.6 g / ml) to the softened polymer premix. Continue stirring for an additional 5 minutes until a uniform dispersion is achieved.

[0346] Conductive particles were prepared by mixing two types of aluminum powder (density 2.7 g / ml) in a weight / weight ratio of 2:1, with the larger particles predominating in this example. Specifically, 88.4 g of aluminum particles with an average diameter of 5-6 μm and 44.2 g of aluminum particles with an average diameter of 1-2 μm were gradually added to the polymer premix to which the glass frit had been added, with continuous stirring. The resulting mixture was stirred for a further 5 minutes until a uniform dispersion was obtained. Since the proportion of the swelling agent decreased with the gradual addition of solid particles, 4.8 g of Isopar was added during this process. TM M to improve the fluidity of the mixture and the dispersion of the particles therein. The dispersions of the conductive particles in the respective swelling polymers obtained by this method were kept in a closed glass container at room temperature for 2-3 hours, allowed to cool, and then formed into a gray paste.

[0347] The volume of each component was calculated by dividing its weight by its density, and then the vol.% of each component was calculated by dividing its volume by the volume of all solids (swellable polymer, glass frit, and aluminum) and multiplying the result by 100. Table 3 provides 3861 Example calculation results for swellable polymers.

[0348] Table 3

[0349]

[0350] (*) indicates that the volume percentage of glass frit is calculated relative to the volume of aluminum.

[0351] Example 2: Preparation of Transferable Compositions - Testing of Various Premixes and Formulations of the Same Swellable Polymer

[0352] 2.1 Preparation 3861 premix

[0353] Prepare a mixture containing 18 wt% by weight according to the preparation method of Example 1.1. 3861、Isopar TM The premix of M and rheology modifier is shown in Table 4.

[0354] Table 4

[0355]

[0356] 2.2. Preparation of Conductive Compositions Containing Aluminum Powder

[0357] Prepare the mixture containing The transferable composition of the 3861 polymer premix, the details of the different components and contents are shown in Table 5. The solid volume % (vol.%) of each component was calculated according to the calculation method specified in Example 1.2 (calculation process not shown).

[0358] Table 5

[0359]

[0360]

[0361] (*) Glass frit concentration is given as volume percentage of aluminum powder or a mixture of aluminum powder and aluminum-silicon alloy powder.

[0362] Example 3: Alternative Method of Preparing Transferable Compositions

[0363] 3.1. Provide swelling 3861 premix

[0364] The polymer premix was prepared according to Example 1.1, wherein in the absence of castor wax HCO 70, the premix was prepared by Isopar TM M swells the polymer to produce a premix having a polymer content of 26 wt.%.

[0365] Will The 3861 polymer premix was placed in a glass container and heated to approximately 80-90°C using a heating mantle until it reached a sufficiently homogeneous flowable state to allow the solid particles to be added. The heated premix was maintained at this temperature for further use.

[0366] Similarly, a polymer content of 25 wt.% was prepared. 3861 polymer premix.

[0367] 3.2. Preparation of Transferable Compositions Using Aluminum / Glass Frit Premix

[0368] In a separate 300ml glass container, place 10.8g of Isopar TM M and 0.5g Crodafos TM O10A was added and mixed with an overhead stirrer, followed by the gradual addition of 2.2 g of glass frit (first 1.1 g of V2083, then 1.1 g of ASF-1100B). Aluminum powder was then gradually added to the above blend with continuous stirring, starting with 28.8 g of aluminum powder with an average particle size of 1-2 μm, followed by 57.7 g of aluminum powder with an average particle size of 5-6 μm. Once the aluminum powder and the blend containing the glass frit were thoroughly mixed, the resulting combination was further mixed at 20,000 rpm for 1-2 minutes using a high shear homogenizer to obtain aluminum / glass frit premix I.

[0369] In a similar manner, aluminum / glass frit premixes II and III were prepared according to Table 6 (where various sizes of aluminum powder were used as indicated in the table). Data are provided in wt. %.

[0370] Table 6

[0371]

[0372] Then 22.5 g of heated 3861 polymer premix (26 wt% polymer content) was placed in another 300 ml glass container, and 77.5 g of aluminum / glass frit premix I was added. The container was placed in a heating mantle at about 80-90° C., and its contents were mixed with an overhead stirrer at a speed of about 800-2300 rpm until fully homogenized and flowable, and then further mixed with a high shear homogenizer at a speed of 20,000 rpm for 1-2 minutes to obtain a transferable conductive composition I.

[0373] Similarly, the aluminum / glass frit premix II was prepared by mixing it with 21.4 wt.% of Transferable Conductive Composition II was prepared by combining 3861 polymer premix (25 wt. % polymer content) and stirring hot without further mixing by high shear homogenization.

[0374] Similarly, the aluminum / glass frit premix III was prepared by mixing it with 20.6 wt.% of Transferable Conductive Composition III was prepared by combining 3861 polymer premix (25 wt.% polymer content) with stirring under heating but without further mixing by high shear homogenization.

[0375] 3.3. Preparation of transferable compositions using aluminum premix

[0376] In a separate 300ml glass container, place 13.4g of Isopar TM M and 0.6g Crodafos TM O3A-LQ-(RB) and mixed with an overhead stirrer. Aluminum powder was then gradually added with continuous stirring, first adding 28.7 g of aluminum powder with an average particle size of 1-2 μm, then adding 57.3 g of aluminum powder with an average particle size of 5-6 μm, followed by mixing at 20,000 rpm for 1-2 minutes using a high shear homogenizer to obtain an aluminum premix.

[0377] Then the heated 3861 polymer premix (polymer content 26 wt%) was placed in another 300 ml glass container, and 1.8 g of glass frit was gradually added (first 0.9 g of V2083, then 0.9 g of ASF-1100B). The container was placed in a heating mantle at about 80-90° C., and its contents were mixed with an overhead stirrer at about 800-2300 rpm for about 5 minutes until fully homogeneous and flowable.

[0378] 75.7 g of the aluminum premix was added to the blend of the polymer premix and glass frit and the combination was stirred for an additional 5 minutes followed by further mixing using a high shear homogenizer at 20,000 rpm for 1-2 minutes to obtain a transferable conductive composition.

[0379] 3.4. Preparation of Transferable Compositions Using Alternative Aluminum Premixes

[0380] Referring to the preparation method of Example 3.3, use 1306 as a dispersant instead of Crodafos TM O3A-LQ-(RB), prepare an aluminum premix according to the following amounts: Isopar TM M: 12.1g; 1306: 0.6g; aluminum powder (1-2μm): 29.1g; aluminum powder (5-6μm): 58.2g.

[0381] Add 75g of aluminum premix to the glass frit and the heated In a blend of 3861 polymer premix, a blend was prepared according to the following amounts: 3861 polymer premix: 23 g; glass frit (V2083): 1 g; glass frit (ASF-1100B): 1 g.

[0382] 3.5. Preparation of Transferable Compositions Using a Frit Premix

[0383] In a separate 200ml glass container, add 12.5g of Isopar TM M and 0.5g Crodafos TM O3A-LQ-(RB), mixed by overhead stirrer, then gradually added 87g of glass frit (first added 43.5g V2083, then added 43.5g ASF-1100B) and mixed using a high shear homogenizer at 20000 rpm for 1-2 minutes to obtain a glass frit premix.

[0384] Then 31.5 g of heated The 3861 polymer premix was placed in another 300 ml glass container and 2 g of the glass frit premix was added. The container was placed in a heating mantle at about 80-90°C and its contents were mixed with an overhead stirrer at about 800-2300 rpm for about 5 minutes until fully homogeneous and flowable.

[0385] Then, 66.5 g of aluminum powder was gradually added to the blend of the swollen polymer premix and the glass frit (first adding 22 g of aluminum powder with an average particle size of 1-2 μm, and then adding 44.5 g of aluminum powder with an average particle size of 5-6 μm), stirred for another 5 minutes, and then further mixed for 1-2 minutes using a high shear homogenizer at 20,000 rpm to obtain a transferable conductive composition.

[0386] Transferable compositions prepared according to the alternative method described above are summarized in Table 7. Their compositions are expressed as vol.% of the components based on the total solid volume, calculated as described in Example 1.2 and Table 3. For the glass frits, the volume concentrations are calculated based on the cumulative volume of the aluminum particles.

[0387] Table 7

[0388]

[0389] (*) indicates the volume percentage of glass frit relative to the total volume of aluminum powder.

[0390] Without being bound by any particular theory, it is generally believed that preparing the transferable composition of the present invention by mixing premixes containing multiple main ingredients separately facilitates the preparation of premixes with better compatibility between the components, which in turn facilitates the optimization of the components in the premix.

[0391] Example 4: Application of the transferable composition by coating

[0392] In this example, a thin polyethylene flexible sheet is used to illustrate a transfer film onto which can be applied a transferable composition such as prepared according to Examples 1 to 3. The flexible film is pre-coated with film-forming ethylcellulose to facilitate subsequent release / transfer of the pattern.

[0393] 4.1 Coating PET sheet with ethyl cellulose

[0394] 190 g of 1-pentanol was placed in a 250 ml glass container and 10 g of Ethocel as a release agent was gradually added while stirring with a magnetic stirrer. TM 100 until a clear solution is obtained. Ethocel was applied manually using a 6 μm wire wound LabRod. TM100 was transferred to a soft, transparent polyethylene (PET, 36 μm thick, purchased from Johlybar, Israel). The thus-coated PET sheet was kept at room temperature overnight to allow the organic solvent to evaporate and the ethylcellulose coating to dry, forming a film approximately 1 μm thick. For simplicity, this transfer film will be referred to as pretreated PET sheet hereinafter.

[0395] 4.2. Coating of pretreated PET with transferable composition

[0396] The pretreated PET sheet was placed on a vacuum auto-film applicator equipped with a 60 μm wire rod and preheated to 80° C. The vacuum auto-film applicator was post-heated to 80° C. and top-heated to 60-80° C. by an IR heater.

[0397] The transferable compositions prepared in Example 1 were preheated to approximately 100°C in a drying oven to restore fluidity. Each transferable composition was applied to a preheated, pretreated PET sheet in a sufficient amount to cover a substantial portion of the PET surface upon spreading, and then cast using a wire rod at a rate of 15 mm / second. Each coated sheet was then held in a vacuum automatic film applicator for at least 20 seconds to allow for partial evaporation of the solvent and enhance the coating's anchorage to the sheet. Typically, after partial removal of the swelling agent, a coating approximately 35-40 μm thick and having a rectangular shape of 200 x 300 mm was formed.

[0398] Example 5: Application of the transferable composition by screen printing

[0399] The Ethocel-coated TM A pretreated sheet of 100% flexible transparent polyethylene was placed on a vacuum automatic film applicator, and the top and back were heated to approximately 80° C. A mesh screen (Polyester α Series EX 43-080 / 110PW, purchased from NBC, China) coated with a polymer mesh emulsion (Fotecoat 1090, purchased from Foteco, Italy) was placed on the pretreated PET sheet to form a mask that allowed the designed pattern to pass through in a series of experiments.

[0400] Referring to Example 4.2, the transferable conductive compositions prepared in Example 1 were preheated to approximately 100° C. in a drying oven. Once flowable, each composition was applied to a mesh screen and then manually screen-printed using a squeegee on a polyurethane screen printer to form a designed line pattern or layer with a dry thickness of approximately 35-40 μm.

[0401] In addition, the transferable compositions prepared in Examples 1 to 3 were respectively printed on a pretreated PET sheet using an industrial flat screen printing machine heated to 80° C. by an IR heater, thereby forming a coating layer of about 35-40 μm in thickness.

[0402] Example 6: Pattern transfer from film to BSF solar cell wafer

[0403] A flexible PET sheet comprising an aluminum pattern prepared from various transferable compositions comprising a swellable polymer (see Examples 1 to 3) and coated by various coating methods (see Examples 4 and 5) is brought into contact with the surface of a suitable substrate. In this example, the surface on which the various transfer film samples are placed is the back side of a textured silicon wafer having a phosphorus-doped emitter front side and a boron-doped silicon back side. The silicon wafers used in this example (purchased from YangliSolar, China) are standard BSF c-Si cells, which are squares with a side length of approximately 156 mm and a thickness of approximately 175 μm. In some experiments, the silicon wafer already included a standard 5BB grid electrode on the front side before the transfer film was applied to the back side of the silicon wafer. The transfer films patterned with the different transferable compositions were subjected to a 6 kg / cm 2 The wafer is then pressed against the backside of the wafer under pressure of 100°C and passed over a pair of standard rubber rollers at a speed of 1.6 m / min, with the pressure rollers heated to approximately 100-130°C. The silicon wafer is then allowed to cool to ambient temperature, with each transfer film still attached to the backside surface of each substrate and the respective aluminum pattern (e.g., a continuous coating) attached thereto. The PET sheet is then peeled from the wafer, while the transferred pattern remains on the wafer after removal of the PET sheet. The transferred aluminum coating is then sintered and fired on the wafer in a solar belt furnace operating at a speed of 550 cm / min, with a temperature profile comprising a drying phase at approximately 250°C for approximately 30 seconds, a burn phase between approximately 450°C and 550°C for approximately 20 seconds, and a peak temperature of approximately 860°C held for approximately 15 seconds to produce the conductive pattern on the substrate. The above sintering profile for BSF c-Si cells is exemplary. Anyone skilled in the solar cell field can design temperature profiles suitable for various cell types and / or compositions to be sintered.

[0404] Example 7: Alternative Transferable Compositions and Substrate Surfaces

[0405] Although Examples 1-6 describe in detail the preparation of transferable compositions containing aluminum particles, their application to a transfer film, and their transfer to the back side of a silicon wafer suitable as a solar cell substrate, the present invention is not limited to this particular type of conductive particles, nor to a particular pattern or a particular side of the substrate, as will be demonstrated in this Example.

[0406] 7.1 Preparation of swellable polymer premix

[0407] Will 3861 and 80g Isopar TMThe swellable polymer premix was placed in a 100 ml sealed glass container equipped with a condenser to prevent or reduce solvent evaporation. The resulting mixture was heated to approximately 150-160°C using a heating mantle while stirring at approximately 800-2000 rpm with an overhead stirrer until a clear solution of the swellable polymer was obtained. The solution was maintained in the sealed container at room temperature for at least 2 hours. The swellable polymer premix was allowed to cool, at which point a whitish, dense, rubbery composition was obtained.

[0408] 7.2 Preparation of Transferable Conductive Composition Containing Silver Powder

[0409] Place 19.5 g of the polymer premix obtained in Example 7.1 in a 300 ml glass container. Heat the swollen polymer to about 80-90°C using a heating mantle until it is sufficiently softened. While stirring at about 800-2300 rpm using an overhead stirrer, gradually add 2.3 g of dispersant. VPTZ 100 was added and the resulting mixture was stirred for 1-2 minutes until a uniform dispersion of the dispersant was achieved. 128.3 g of silver powder was gradually added with stirring, and the resulting mixture was stirred for 5 minutes until a uniform dispersion of the silver particles in the swollen polymer was achieved. The resulting dispersion was held in a sealed glass container at room temperature for 2-3 hours, allowed to cool, and then formed into a gray, rubbery silver paste. No glass frit was added to the formulation.

[0410] With reference to Example 5, the silver-containing transferable composition was reheated and manually screen-printed onto a flexible, transparent sheet of siliconized PET (Cat. no. 942018, 30 μm, purchased from Nirotek, Israel). In this example, the manufacturer pre-treated the PET sheet with siliconization, a coating that facilitates the release of the subsequent pattern. Next, with reference to Example 6, a printed pattern, presented as a series of spaced lines, was applied to the front of a solar cell wafer by transfer. The transferability of the transferable composition was subsequently confirmed by peeling off the PET sheet, while the pattern was not sintered on the wafer.

[0411] Example 8: Electrical performance of standard BSF solar cells with transferred patterns

[0412] The electrical conductivity of the pattern sintered onto the solar substrate as described in Example 6 was measured using an IV tester having a device capable of extracting the V of the tested solar cell from the obtained data. OC 、J SC , FF and efficiency software.

[0413] The front side of the BSF solar cell has a standard 5BB grid electrode applied directly by the manufacturer (Yingli Solar, China), and the back side of the BSF solar cell is coated with a pattern of transferable composition transferred from a film prepared and used as described above. The solar cell with sintered electrodes on both sides to be tested is isolated from devices that can cause electrical interference and placed in a light box with its back side facing the support. Its front side that collects solar energy is exposed to a light source that provides 1000W / m 2 The incident radiation is essentially perpendicular to the front surface at a distance from the solar cell. A probe with an array of contacts aligned with the ends of the solar cell's front conductors is applied to the front electrode, enabling it to make contact with a similar array of contacts located below the back electrode. The measurement begins when both sides of the solar cell are properly contacted by the relevant probe, which allows closing the circuit between the commercial front electrode and the back electrode. The voltage between the two electrodes is measured in the absence of current while the solar cell is maintained at approximately 25°C, and V is recorded in at least three repeated experiments. OC The average of all values ​​for the patterns / BSF cells formed from each transferable composition is given in Table 8. For comparison, the V OC The cell has a similar front electrode and a conventional back electrode (purchased from Yingli Solar, China), and its V OC is 625mV.

[0414] Table 8

[0415]

[0416]

[0417] Example 9: Electrical performance of PERC solar cells with transferred patterns

[0418] The patterns of transferable compositions transferred to and sintered on PERC cells were tested for electrical activity.

[0419] A P-type PERC cell (supplied by Trina Solar, China, with a standard 5BB grid electrode applied directly by the manufacturer on its front side, metallized on the front side, non-metallized on the back side and laser-perforated to produce 1 mm × 30 μm openings) was coated with a pattern of the transferable composition on its back side and further sintered according to the method in Example 6.

[0420] A commercial standard p-type PERC cell was used as a reference, with similar front and back electrodes (pre-applied by the manufacturer (Trina Solar, China)).

[0421] The transferable conductive compositions of the analyzed patterns / PERC cells are as follows:

[0422] 9.1. The transferable conductive composition prepared in Example 2.2.2;

[0423] 9.2. The transferable conductive composition prepared in Example 2.2.4;

[0424] 9.3. The transferable conductive composition prepared in Example 2.2.5;

[0425] 9.4. Transferable conductive composition III prepared in Example 3.2;

[0426] 9.5. The transferable conductive composition prepared in Example 3.4;

[0427] 9.6. Commercially available aluminum paste composition (BL02A, purchased from Leed-Ink, China);

[0428] 9.7. Modified commercial pastes in which a swellable polymer replaces the original resin.

[0429] An alternative approach is as follows: Commercially available BL02A aluminum paste is mixed with excess isopropyl alcohol to wash away the original resin. The mixture is stirred at room temperature for approximately 20 minutes and then centrifuged to separate the solids of the commercial paste from the wash solvent. The washing step is repeated two to four more times, and the solids from the final wash step are then dried in an oven at approximately 80°C for at least three hours, so that the primary glass frit remains in the paste.

[0430] 87 g of the washed and dried paste was then dispersed in 12.7 g of Isopar by ball milling at 100-250 rpm for 12 hours. TM M and 0.3g Crodafos TM A uniform dispersion was obtained in O10A;

[0431] The above 71.2 g dispersion was placed in a separate container and 28 g of the mixture containing 3861 polymer premix. Add 0.8g Isopar TM M, and the resulting mixture was mixed using an overhead stirrer at 2000 rpm for five minutes.

[0432] Compositions 9.1-9.5 and 9.7 were screen printed onto a plate made of Ethocel® with the reference to Example 5. TM 100 pre-treated flexible transparent polyethylene sheet, and then transferred to the back of the PERC cell, referring to the relevant content for the BSF cell in Example 6.

[0433] Composition 9.6 was screen printed directly onto a PERC cell wafer using the same screen as the composition of the present invention, resulting in a similar pattern.

[0434] Using Suns-V OC IV tester measures the V OC As a result, the sheet resistance of the transferred or applied pattern was measured using a 4-terminal resistance meter. The results are summarized in Table 9.

[0435] Table 9

[0436]

[0437]

[0438] V OC The results show that the efficiency of PERC cells produced using the transferable compositions of the present invention (Examples 9.1-9.5) is comparable to the V OC The sheet resistance of the pattern formed by the transferable composition of the present invention is also substantially equivalent to, or even more advantageously lower than, that of commercial batteries, indicating that the transferable composition of the present invention and the pattern formed and transferred therefrom can provide higher efficiency.

[0439] The commercial composition (Example 9.7) whose polymer content was replaced by the swelling polymer of the present invention was able to be transferred and patterned on a PERC cell with a measured V OC The results are comparable to the original commercial paste (Example 9.6) which was directly screen printed.The sheet resistance provided by the two conductive patterns is also similar.

[0440] Unless otherwise defined or understood from the disclosure of the present invention, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0441] It should be understood that certain features of the present subject matter described in the context of different embodiments for the sake of clarity may also be provided in combination in the same embodiment. Conversely, various features of the present subject matter described in the context of the same embodiment for the sake of brevity may also be provided separately or in any suitable subcombination or in any other described embodiment of the invention as appropriate. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperable without those features.

[0442] Although various specific embodiments of the present invention have been described, these specific disclosed embodiments are for illustrative purposes only, and should not be considered limiting. Based on the applicant's disclosure herein, it will be apparent to those skilled in the art that numerous other alternatives, modifications, variations, substitutions, and variations of these embodiments and their features may be implemented. Therefore, the present invention is intended to encompass all such alternatives, modifications, and variations, as well as any changes within the meaning and scope of their equivalents.

[0443] Unless otherwise stated, use of the expression "and / or" between the last two members of a list of options for selection means that one or more of the listed options are applicable and can be selected.

[0444] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments and / or to exclude the incorporation of features from other embodiments. Furthermore, features described as preferred or advantageous in some embodiments may not necessarily be preferred or advantageous in other embodiments.

[0445] In the present specification and claims, each of the verbs "comprise", "include" and "have" and their conjugations are used to indicate that one or more objects of the verb are not necessarily a complete list, and the list includes a complete list of features, components, steps, parts, elements or parts of the subject or subjects of the verb.

[0446] As used herein, the singular forms "a," "an," and "the" include plural referents and mean "at least one" or "one or more" unless the context clearly dictates otherwise. At least one of A and B is intended to mean A or B, and in some embodiments can mean A and B.

[0447] Positional or motion terms, such as "upper," "lower," "right," "left," "bottom," "down," "lower," "low," "top," "above," "elevated," "high," "rear," "front," "side," "vertical," "horizontal," "backward," "forward," "upstream," and "downstream," and grammatical variations thereof, may be used herein for exemplary purposes only to illustrate the relative positioning, placement, or displacement of certain components, and / or to indicate first and second components in the accompanying drawings. These terms do not necessarily indicate that, for example, the "bottom" component is below the "top" component, as these orientations and / or components may be flipped, rotated, shifted in space, placed in a diagonal orientation or position, positioned horizontally or vertically, or similarly modified.

[0448] Unless otherwise stated, when outer limits of a range for a feature of an embodiment are indicated in the present disclosure, it should be understood that in the embodiment, possible values ​​for the feature can include the outer limits and values ​​between the outer limits.

[0449] As used herein, unless otherwise indicated, adjectives such as "substantially," "approximately," and "about" that modify one or more conditions or relational features of embodiments of the present subject matter should be understood to mean that the condition or feature is defined within a tolerance acceptable for the operation of the embodiment to which it is intended to be applied, or within variations that are expected from the measurements being performed and / or from the measuring instrument being used. When the terms "about" and "approximately" precede a numerical value, they are intended to mean + / - 15%, or + / - 10%, or even just + / - 5%, or any other suitable + / - deviation within such ranges, and in some cases, the exact value. Furthermore, unless otherwise indicated, terms (e.g., numbers) used in embodiments of the present subject matter, even without such adjectives, should be interpreted as having a tolerance that may deviate from the exact meaning of the relevant term, but that will enable the embodiment, or relevant portions thereof, to operate and function as described (as understood by those skilled in the art).

[0450] To the extent necessary to understand or complete the present disclosure, all publications, patents, and patent applications mentioned herein, including applicant's applications, are expressly incorporated by reference in their entirety. Citation or identification of any reference herein should not be construed as an admission that such reference is available as prior art to the present invention.

[0451] Certain trademarks referenced herein may be common law or registered trademarks of third parties. The use of these marks is by way of example only and should not be construed as descriptive or to limit the scope of the invention to only the materials associated with these marks.

Claims

1. A transferable conductive composition suitable for applying a pattern on a transfer film and for transferring the pattern from the transfer film to a substrate, the transferable conductive composition having a gel-like structure and comprising: i) a swellable polymer; ii) a swelling agent; and iii) a plurality of conductive particles; wherein a) the swelling agent is capable of swelling the swellable polymer to form a premix of the swellable polymer, and the weight of the swelling agent is at least 20 wt % of the weight of the swellable polymer; and b) the conductive particles are dispersed in the premix.

2. The transferable conductive composition of claim 1 , wherein the weight / weight ratio SP / SA of the swellable polymer to the swelling agent satisfies one or more of the following: a. SP / SA is at least 1:99, at least 1:49 or at least 1:19; b. SP / SA is at most 4:1, at most 2.3:1, at most 1.5:1 or at most 1:1; and c. SP / SA ranges between 1:99 and 9:1, between 1:99 and 4:1, between 1:49 and 2.3:1, between 1:49 and 1.5:1, or between 1:19 and 1:

1.

3. The transferable conductive composition of claim 2, wherein the swellable polymer satisfies one or more of the following structural properties: a. The swellable polymer is selected from the group consisting of an amorphous thermoplastic polymer, a semi-crystalline thermoplastic polymer and a crystalline thermoplastic polymer; b. The swellable polymer has a melt flow rate of at least 0.2 g / 10 min, at least 0.3 g / 10 min, at least 0.5 g / 10 min, at least 1 g / 10 min, at least 2 g / 10 min, or at least 5 g / 10 min measured at 190 ° C with a load of 2.16 kg; c. the swellable polymer has a melt flow rate of at most 100 g / 10 min, at most 50 g / 10 min, or at most 25 g / 10 min, measured at 190 ° C with a load of 2.16 kg; d. the swellable polymer has a melt flow rate in the range of 0.2 g / 10 min to 100 g / 10 min, 0.5 g / 10 min to 50 g / 10 min, 1 g / 10 min to 100 g / 10 min, 1 g / 10 min to 75 g / 10 min, 1 g / 10 min to 50 g / 10 min, 2 g / 10 min to 50 g / 10 min, or 2 g / 10 min to 25 g / 10 min, measured at 190°C with a load of 2.16 kg; e. The swellable polymer has a melting temperature of at least 50°C, at least 60°C, at least 70°C or at least 80°C; f. The swellable polymer has a melting temperature of at most 250°C, at most 200°C, at most 150°C, at most 120°C or at most 100°C; g. The swellable polymer has a melting temperature in the range of 50°C to 250°C, 60°C to 200°C, 65°C to 150°C, 70°C to 120°C or 70°C to 100°C; h. The swellable polymer has a Vicat softening temperature of at least 30°C, at least 40°C, at least 50°C or at least 60°C; i. the swellable polymer has a Vicat softening temperature of at most 220°C, at most 180°C, at most 140°C, at most 120°C, at most 100°C or at most 80°C; and j. The swellable polymer has a Vicat softening temperature in the range of 30°C to 140°C, 40°C to 120°C, or 50°C to 100°C.

4. The transferable conductive composition according to any one of claims 1 to 3, wherein The swellable polymer is a thermoplastic polymer selected from the group consisting of ethylene / vinyl acetate (EVA) copolymers, acid-modified ethylene acrylate resins, ethylene / acrylate / maleic anhydride terpolymers, polyvinyl butyral, polyvinyl alcohol, and polyamides.

5. The transferable conductive composition according to any one of claims 1 to 3, wherein The swellable polymer is present in the composition at a concentration by volume of: (I) at least 1 vol.%, at least 2 vol.%, at least 5 vol.%, or at least 10 vol.%, based on the volume of all solids; and / or (II) at most 25 vol.%, at most 22 vol.%, or at most 20 vol.%; and / or (III) in the range of 2 vol.% to 25 vol.%, 5 vol.% to 25 vol.%, or 10 vol.% to 22 vol.%.

6. The transferable conductive composition of any one of claims 1 to 3, wherein the swelling agent satisfies one or more of the following properties: a. The swelling agent is an aqueous solvent comprising at least 75 wt.%, at least 80 wt.%, at least 85 wt.%, at least 90 wt.% or at least 95 wt.% of water; b. The swelling agent is an organic solvent comprising at most 25 wt.%, at most 20 wt.%, at most 15 wt.%, at most 10 wt.% or at most 5 wt.% of water; c. The swelling agent comprises at most 1.0 wt.%, at most 0.5 wt.%, at most 0.1 wt.%, at most 0.05 wt.%, or at most 0.01 wt.% of water; d. The swelling agent has a boiling temperature of at least 100°C, at least 130°C or at least 160°C; e. The swelling agent has a boiling temperature of at most 450°C, at most 400°C or at most 350°C; f. the swelling agent having a viscosity of at least 1 mPa·s and optionally at most 1,000 mPa·s, at most 500 mPa·s, at most 250 mPa·s or at most 150 mPa·s; and g. The swelling agent is chemically inert relative to the conductive particles.

7. The transferable conductive composition according to any one of claims 1 to 3, wherein the swelling agent is selected from the group consisting of C 6-20 Isoparaffins, C 3-10 Ketone, C 1-12 Alcohol, C 6-10 The group consisting of aromatic hydrocarbons, glycerol and combinations thereof.

8. The transferable conductive composition according to any one of claims 1 to 3, wherein The conductive particles include or consist of: metals, alloys, organic metals, and their precursors, hydrates and / or salts; or conductive polymers thereof.

9. The transferable conductive composition according to any one of claims 1 to 3, wherein The conductive particles are present in the composition at a concentration by volume of all solids of: (I) at least 65 vol.%, at least 70 vol.%, or at least 75 vol.%; and / or (II) at most 98 vol.%, at most 95 vol.%, or at most 90 vol.%; and / or (III) in the range of 65 vol.% to 98 vol.%, 70 vol.% to 95 vol.%, or 75 vol.% to 90 vol.%.

10. The transferable conductive composition of any one of claims 1-3, further comprising glass frit, wherein the amount of the glass frit is in a range of 0.1 vol.% to 15 vol.%, 0.2 vol.% to 12.5 vol.%, 0.2 vol.% to 12.5 vol.%, 0.5 vol.% to 10 vol.%, 0.5 vol.% to 8 vol.%, 0.5 vol.% to 6 vol.%, 0.5 vol.% to 5 vol.%, or 1 vol.% to 5 vol.%, based on the volume of the conductive particles.

11. The transferable conductive composition of any one of claims 1-3, further comprising one or more rheology modifiers present in a total amount of 0.05% wt.% to 20 wt.% based on the weight of the total composition, the rheology modifier further selected from the group consisting of dibutyl sebacate, butyl stearate, ethylene glycol coconut fatty acid esters, butyl ricinoleate, dibutyl phthalate, castor oil, paraffin, beeswax, cetyl alcohol, butyl stearate, diphenyl phthalate, dicyclohexyl phthalate, and dioctyl phthalate.

12. The transferable conductive composition of any one of claims 1-3, further comprising one or more dispersants present in a total amount of 0.05% wt.% to 20 wt.% based on the weight of the total composition, the dispersant further selected from the group consisting of: ether / ester dispersants, including complex mixtures of phosphate esters and polyethylene glycol ethers of oleyl alcohol; complex mixtures of phosphate esters and polyoxypropylene; hexadecanol polyoxyethylene ether; complex mixtures of phosphate esters and hexatriacontol polyethylene glycol ether; and anionic dispersants.

13. The transferable conductive composition of any one of claims 1 to 3, wherein: i) the swellable polymer is an EVA copolymer, the polymer having a vinyl acetate content of at least 10 wt.%, at least 15 wt.%, or at least 20 wt.%, by weight, the vinyl acetate content being at most 40 wt.%, or at most 30 wt.%; ii) the swelling agent is an isoparaffin having a carbon chain length of 10 to 19 carbon atoms; and iii) the conductive particles comprise or consist of a metal.

14. A method for preparing a transferable conductive composition suitable for applying a pattern on a transfer film and for transferring the pattern from the transfer film to a substrate, the composition having a gel-like structure and comprising: a) providing a premix comprising a swellable polymer swollen with a swelling agent; the premix being at a temperature above the softening temperature of the swellable polymer; as well as b) mixing the premix of step a) with conductive particles at a temperature of at least 50° C., at least 60° C., at least 70° C., or at least 80° C. and at most 250° C., at most 200° C., at most 150° C., or at most 100° C. to disperse the conductive particles in the premix to obtain a transferable conductive composition.

15. The method according to claim 14, wherein The premix of step a) is prepared by combining the swellable polymer and the swelling agent to obtain a mixture, and heating the mixture until the swellable polymer is completely melted, wherein the heating temperature is at least 20°C, at least 40°C, or at least 60°C higher than the melting point of the polymer and at least 40°C, at least 60°C, or at least 80°C lower than the boiling point of the swelling agent.

16. The method according to claim 14 or 15, wherein The swellable polymer, the swelling agent, the conductive particles and the transferable conductive composition prepared therefrom are according to any one of claims 1-13.

17. A method of making a transfer film suitable for applying a pattern of electrical conductors to a substrate, the method comprising: a) applying a transferable conductive composition to a transfer film with an applicator, the transferable conductive composition comprising a swellable polymer swollen with a swelling agent and conductive particles, the transferable conductive composition having a gel-like structure and being applied at a sufficiently high application temperature so that the transferable conductive composition has sufficient fluidity for the applicator; b) heating the transfer film before and / or during application of the transferable conductive composition to a temperature that is no more than 20° C. higher than the application temperature of the transferable composition; as well as c) removing excess transferable composition from the surface of the transfer film to leave a pattern thereon.

18. The method of claim 17, further comprising eliminating at least some, but not all, of the swelling agent from the pattern; The temperature of the transfer film is lowered so that the pattern of the transferable conductive composition applied thereon is sufficiently adhered to the film.

19. The method according to claim 17 or 18, wherein the pattern is formed on the surface of the transfer film.

20. The method of claim 17 or 18, wherein the pattern is formed in recesses beneath the surface of the transfer film, the transferable conductive composition being loaded into the recesses in one or more loading cycles.

21. The method of claim 17 or 18, wherein the transfer film is flexible and is composed of a thermoplastic polymer selected from the group consisting of cyclic olefin copolymer (COC), polyvinyl chloride (PVC); polyethylene (PE), polypropylene (PP), thermoplastic polyurethane (TPU) and combinations thereof.

22. The method of claim 17 or 18, wherein the transfer film is pretreated prior to applying the transferable composition, the pretreatment promoting one or more of: i) adhesion of the pattern to the film, and ii) transfer of the pattern to the substrate when the transfer film is contacted with the substrate.

23. The method of claim 17 or 18, wherein the transfer film is a flexible foil made of plastic, metal, paper or a laminated combination thereof, and the flexible foil is impermeable to the transferable conductive composition so that the pattern remains on the surface of the transfer film.

24. The method of claim 17 or 18, wherein the transferable conductive composition applied to the transfer film is a transferable conductive composition according to any one of claims 1 to 13; the transferable composition is prepared according to claim 14 or claim 15.

25. A transfer film suitable for applying a pattern of electrical conductors to a substrate, the transfer film having on its surface or in its recesses a pattern consisting of a transferable conductive composition, the composition having a gel-like structure and comprising a swellable polymer swollen with a swelling agent and conductive particles, the transferable conductive composition being suitable for becoming conductive when sintered by applying energy thereto; the transfer film being such that when the film and the substrate are pressed together, the transferable conductive composition adheres more strongly to the substrate than to the transfer film, and such that subsequent separation of the transfer film from the substrate results in the transferable conductive composition remaining as a pattern on the substrate.

26. A transfer film suitable for applying a pattern of electrical conductors to a substrate, the transfer film being prepared by a method according to any one of claims 17 to 24.

27. A method of applying a pattern of electrical conductors to a substrate, the method comprising: a) providing a flexible transfer film, wherein a first surface of the transfer film has a transferable pattern of a transferable conductive composition formed thereon or within recesses therein, the transferable conductive composition having a gel-like structure and comprising conductive particles and a swellable polymer swollen by a swelling agent, and the transferable pattern at least partially corresponds to a desired pattern of the electrical conductor to be applied to the substrate; b) contacting the transfer film with the substrate, wherein the first surface of the transfer film faces the substrate, and the contacting is performed at a transfer temperature greater than 60° C.; d) applying pressure to the transfer film to adhere the transferable pattern of the transferable conductive composition to the substrate, the pressure being at least 0.1 kgF / cm 2 , up to 50kgF / cm 2 ; e) separating the transfer film from the substrate to transfer the transferable pattern from the first surface of the film or from the recesses therein to the substrate, the separation being performed at a separation temperature lower than a transfer temperature; and f) applying sufficient energy to sinter the conductive particles and render the pattern of composition transferred to the substrate conductive.

28. The method according to claim 27, further comprising: i) the swelling agent is present in the transferable conductive composition at a volume concentration of at least 10 vol.% of the volume of the transferable conductive composition; ii) the swelling agent is present in the transferable conductive composition at a weight concentration of at least 10 wt.% based on the weight of the transferable conductive composition; iii) the transferable conductive composition is according to any one of claims 1 to 13; iv) the transferable conductive composition is prepared by the method according to any one of claims 14 to 16; v) the transfer film is one according to claim 25 or 26; vi) the transfer film is manufactured according to the method of any one of claims 17-24; vii) heating the transfer film before contacting the substrate; viii) heating the substrate before contacting the transfer film; ix) contacting at a transfer temperature of at least 60°C, at least 70°C, or at least 80°C; x) contacting at a transfer temperature of at most 250°C, at most 200°C, or at most 180°C; and xi) performing the separation at a separation temperature that is at least 5°C, at least 10°C, at least 20°C, or at least 30°C lower than the transfer temperature.

29. Use of a transferable conductive composition according to any one of claims 1 to 13, prepared by the method according to any one of claims 14 to 16, for producing a conductive pattern after applying energy sufficient to sinter the conductive particles contained in the composition, wherein the conductive pattern is suitable for forming a rear electrode or a front electrode on the surface of a solar cell.

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