Molecular ink, method for producing conductive silver traces, substrate and electronic device

Abstract

The invention relates to a molecular ink, a method for producing a conductive silver trace, a substrate and an electronic device. The present invention provides a molecular ink comprising a silver carboxylate, an organic amine, and a polymeric binder comprising a polyester, a polyimide, a polyetherimide, or any mixture thereof, having functional groups that make the polymeric binder compatible with the organic amine. Such inks may have a higher silver loading, a lower viscosity, and a lower processing temperature than existing silver inks. The invention also provides a method for producing a conductive silver trace on a substrate using the molecular ink, and a substrate and an electronic device comprising the conductive silver trace.

Description

[0001] This application is a divisional application of the patent application filed on February 8, 2018, entitled "Silver Molecular Ink with Low Viscosity and Low Processing Temperature" having application number 2018800162233 (international application number PCT / IB2018 / 050791) with the same inventor. TECHNICAL FIELD

[0002] The present application relates to inks, and in particular to low viscosity printable molecular inks with high silver loading and that can be sintered at lower temperatures. BACKGROUND

[0003] Molecular inks based on silver neodecanoate have been formulated for screen printing. While these inks have shown excellent mechanical and electrical properties, along with good print quality, they have at least two limitations.

[0004] First, due to their low solubility in solvents or carriers, the silver salts must be significantly diluted to form a uniform ink. Once printed, the low concentration of silver salts results in thin silver deposits. The resulting inks also have a higher viscosity, greater than that required for many printing techniques (aerosol jet, inkjet printing, flexographic intaglio printing). To print thick traces of the silver neodecanoate inks, multiple layers are printed (inkjet printing or aerosol jet). If the printing technique requires a low viscosity ink, the silver neodecanoate ink must be diluted, reducing the relative silver content of the ink, which in turn results in very thin deposits. Furthermore, printing multiple layers is time consuming and can result in poor print quality.

[0005] Second, these inks require processing temperatures of about 220-240 °C, so when the highest electrical conductivity is sought, thermal sintering is typically only suitable for high cost substrates such as Kapton TM However, only when using longer processing times, can the traces of silver neodecanoate inks be sintered at lower temperatures.

[0006] Accordingly, there is a need for a silver salt ink formulation, and in particular a silver neodecanoate formulation, that allows for a higher loading of silver salts and / or allows for silver to be sintered at lower temperatures. SUMMARY

[0007] Molecular silver inks have been formulated that increase the dispersibility (e.g., solubility) of the silver salts in the ink, thereby increasing the silver loading in the ink. The formulated inks can have a lower viscosity than existing silver inks, without compromising the silver content. The silver inks can also require a lower processing temperature than existing silver inks.

[0008] In one aspect, there is provided a molecular ink comprising: silver carboxylate; an organic amine; and a polymeric binder comprising a polyester, a polyimide, a polyetherimide, or any mixture thereof, having functional groups that render the polymeric binder compatible with the organic amine.

[0009] In another aspect, there is provided a method of producing a conductive silver trace on a substrate, the method comprising depositing a molecular ink on the substrate to form a non-conductive trace of the ink on the substrate, and sintering the non-conductive trace of the ink on the substrate to form a conductive silver trace.

[0010] In another aspect, there is provided a substrate comprising a conductive silver trace produced by the above method.

[0011] In another aspect, there is provided an electronic device comprising a substrate having a conductive silver trace produced by the above method.

[0012] Other features will be described or will become apparent in the course of the following detailed description. It should be understood that each of the features described herein can be used alone or in any combination with one or more other features described herein, and that each feature is not necessarily dependent on the presence of any other feature unless otherwise indicated. BRIEF DESCRIPTION OF DRAWINGS

[0013] For a more complete understanding of the application, reference is now made to the following description taken in connection with the accompanying drawings in which:

[0014] Figure 1A and Figure 1B Plots depicting the viscosity (cP) of various inks as a function of silver neodecanoate (AgND) weight fraction (wt / wt) plotted on a linear scale ( Figure 1A ) and a semi-log scale ( Figure 1B ). Figure 1B The horizontal dashed line at 500 cP indicates that inks II, I2, and I3 have a greater AgND loading at 500 cP than inks CI and C2.

[0015] Figure 2 Plots depicting the sheet resistivity (mW / sq / mil) of a nominal 20 mil line as a function of sintering temperature (°C) for silver neodecanoate (AgND) inks of the present application (inks II, I2, and I3) compared to other AgND inks (inks CI and C2) having different formulations, the inks sintered for 45 minutes.

[0016] Figure 3 Plots depicting the cross-sectional area (mm2) of a nominal 20 mil line as a function of sintering temperature (°C) for silver neodecanoate (AgND) inks of the present application (inks II, I2, and I3) compared to other AgND inks (inks CI and C2) having different formulations, the inks sintered for 45 minutes. 2Graph of AgND weight fraction (wt / wt) as a function of. DETAILED DESCRIPTION

[0017] The silver carboxylate comprises silver ions and an organic group comprising a carboxylic acid moiety. The carboxylate preferably includes 1 to 20 carbon atoms, more preferably 6 to 15 carbon atoms, even more preferably 8 to 12 carbon atoms, for example 10 carbon atoms. The carboxylate is preferably an alkanoate. The silver carboxylate is preferably a silver salt of an alkanoic acid. Some non-limiting examples of preferred silver carboxylates are silver ethylhexanoate, silver neodecanoate, silver benzoate, silver phenylacetate, silver isobutyrylacetate, silver benzoylacetate, silver oxalate, silver neopentanoate, and any mixture thereof. Silver neodecanoate is especially preferred. There can be one or more than one silver carboxylate in the ink. The silver carboxylate is preferably dispersed in the ink. Preferably, the ink is free of flakes or other particles of metallic silver material.

[0018] The silver carboxylate is preferably present in the ink in an amount to provide a silver loading in the ink of about 23 wt% or more, based on the total weight of the ink. More preferably, the silver carboxylate provides a silver loading of about 23.5 wt% or more, or about 25 wt% or more, or about 26.5 wt% or more, or about 27 wt% or more. When the silver carboxylate is silver neodecanoate, the silver neodecanoate can preferably be present in the ink in an amount of about 61 wt% or more, or about 65 wt% or more, or about 68 wt% or more, or about 70 wt% or more, based on the total weight of the ink.

[0019] The organic amine is preferably an alkylamine, a hydroxyalkylamine, or a cyclic amine. The organic amine preferably includes 2 to 12 carbon atoms, more preferably 2 to 8 carbon atoms. Some non-limiting examples of alkylamines are 2-ethyl-1-hexylamine, 1-octylamine, 1-hexylamine, and the like. Some non-limiting examples of hydroxyalkylamines are 1,2-ethanolamine, 1-amino isopropanol (amino-2-propanol), 1,3-propanolamine, 1,4-butanolamine, and the like. Some non-limiting examples of cyclic amines are pyridine, pyrimidine, pyrrole, pyrrolidine, oxazoline, piperidine, isoxazole, morpholine, and the like. There can be one or more than one organic amine in the ink.

[0020] The organic amine can be present in the ink in any suitable amount, preferably in an amount of about 5 wt% or more, more preferably about 8 wt% or more, and preferably in an amount of about 50 wt% or less, more preferably about 25 wt% or less, even more preferably about 20 wt% or less, all weights based on the total weight of the ink. Exemplary preferred ranges are about 5 wt% to about 50 wt%, or about 10 wt% to about 50 wt%, or about 5 wt% to about 25 wt%.

[0021] The polymeric binder can include a polyester, a polyimide, a polyetherimide, or any mixture thereof, having functional groups that render the polymeric binder compatible with the organic amine. Thus, the mixture of organic amines in the polymeric binder does not produce a significant phase separation. The polymeric binder can be dispersible in, for example, soluble in, the organic amine. The functional groups that render the polymeric binder compatible with the organic amine are preferably polar groups capable of forming hydrogen bonds, such as one or more of hydroxyl, carboxyl, amino, and sulfonyl groups. Preferably, the polymeric binder includes terminal hydroxyl and / or carboxyl groups. The polymeric binder preferably includes a polyester having functional groups that render the polyester compatible with the organic amine. More preferably, the polymeric binder includes a hydroxyl and / or carboxyl-terminated polyester.

[0022] Other classes of polymeric binders can be used. These other classes of polymeric binders can be homopolymers or copolymers. These other classes of polymeric binders can be thermoplastics or elastomers. Some examples of other polymeric binders include, for example, thermoplastic polyurethanes and silicone elastomers.

[0023] The polymeric binder can be present in the ink in any suitable amount, preferably ranging from about 0.1 wt% to about 10 wt% based on the total weight of the ink. More preferably, the amount ranges from about 0.5 wt% to about 10 wt%, or from about 0.1 wt% to about 5 wt%, or from about 0.5 wt% to about 3 wt%, or from about 1 wt% to about 2 wt%.

[0024] In a particularly preferred embodiment, the molecular ink includes: about 61 wt% or more of silver neodecanoate; about 5 wt% to about 50 wt% of amino-2-propanol, 2-ethyl- 1-hexylamine, or 2-ethyl-2-oxazoline; and about 0.5 wt% to about 3 wt% of a hydroxyl and / or carboxyl-terminated polyester, all weights based on the total weight of the ink.

[0025] The molecular ink can optionally include a solvent. The solvent is preferably compatible with one or both of the organic amine or the polymeric binder. The solvent is preferably compatible with both of the organic amine and the polymeric binder. The organic amine and / or the polymeric binder is preferably dispersible in, for example, soluble in, the solvent. The solvent is preferably an organic solvent, more preferably a non-aromatic organic solvent. Non-aromatic organic solvents include, for example, glycol ethers (e.g., dipropylene glycol methyl ether), alcohols (e.g., methylcyclohexanol, octanol, heptanol), carbitols (e.g., 2-(2-ethoxyethoxy)ethanol), or any mixture thereof. Alcohols are preferred, preferably C1-C 10alkanols, more preferably C6-C8 alkanols, such as octanol. When used, the solvent can be present in the ink in any suitable amount, preferably ranging from about 1 wt% to about 50 wt% based on the total weight of the ink. More preferably, the amount ranges from about 2 wt% to about 50 wt%, or from about 5 wt% to about 50 wt%, or from about 5 wt% to about 40 wt% or from about 5 wt% to about 20 wt%. When present, the solvent generally makes up the balance of the ink.

[0026] The ink can have a viscosity ranging from about 1 cP to about 50,000 cP. For example, the viscosity of the ink can range from about 1 cP to about 15,000 cP.

[0027] The relative amounts of each component of the ink can play an important role in regulating the dispersibility (e.g., solubility) of the silver carboxylate in the ink, while providing a lower viscosity and lower processing temperature than existing silver neodecanoate inks. Preferred embodiments of the amounts of each component in the ink enable the ink to have particularly improved processing temperatures at lower viscosities and higher silver loadings.

[0028] The ink can be deposited on a substrate in any suitable method to form a non- conductive trace of the ink on the substrate. The ink is particularly suitable for printing, such as screen printing, inkjet printing, flexographic printing (e.g., stamps), gravure printing, offset printing, airbrushing, aerosol printing, letterpress, stencil printing, or any other method. The ink can be uniquely optimized for various different printing techniques.

[0029] After deposition on the substrate, the silver carboxylate in the non-conductive trace is dried and decomposed to form a conductive trace. Drying and decomposition can be accomplished by any suitable technique, with the technique and conditions depending on the type of substrate on which the trace is deposited and the type of silver carboxylate in the ink. For example, drying the ink and decomposing the silver carboxylate can be accomplished by heating and / or photonic sintering.

[0030] In one technique, the substrate is heated to dry and sinter the trace to form a conductive trace. Sintering decomposes the silver carboxylate to form conductive particles (e.g., nanoparticles) of silver. Advantageously, the heating can be performed at a relatively low temperature range of less than about 185 °C, particularly about 150-185 °C, or about 150-175 °C, while producing a relatively high conductive silver trace. Although the ability to sinter at lower temperatures is an advantage of the ink, if desired, the heating can be performed at higher temperatures, such as at a temperature of about 185 °C or higher or up to about 250 °C.

[0031] The heating is preferably performed for a time of about 3 hours or less, more preferably about 2 hours or less, for example a time ranging from about 1-180 minutes or about 2-120 minutes. The heating is performed with sufficient balance between temperature and time to sinter the traces on the substrate to form electrically conductive traces. The type of heating device also affects the temperature and time required for sintering. The substrate can be sintered under an oxidizing atmosphere (e.g., air) or an inert atmosphere (e.g., nitrogen and / or argon).

[0032] In another technique, the photonic sintering system can feature a high intensity lamp (e.g., a pulsed xenon lamp) that delivers a broad band spectrum. The lamp can deliver about 5-20 J / cm 2 of energy to the traces. The pulse width is preferably in the range of about 0.58-1.5 ms. The driving voltage is preferably in the range of about 1.6-2.8 kV. The photonic sintering can be performed under ambient conditions (e.g., in air). Photonic sintering is particularly suitable for polyethylene terephthalate and polyimide substrates. The use of low energy (e.g., less than 10 J / cm 2 ) increases the compatibility of the ink on low temperature substrates such as polyethylene terephthalate and polyethylene naphthalate, where the use of a silver neodecanoate based ink containing an amine as described herein significantly reduces the occurrence of substrate damage (e.g., melting and / or warping).

[0033] The electrically conductive traces formed by drying and sintering the ink on the substrate can have any desired thickness and width. Advantageously, the ink can be dried and sintered to form relatively thin and / or narrow electrically conductive traces while maintaining a relatively high electrical conductivity (i.e., a relatively low resistivity). Furthermore, the sintered traces formed from the ink can be flexible, capable of passing the ASTM F1683-02 Bend & Crease Test without any open circuit break (i.e., without open circuit failure). In manufacturing applications, the lowest possible variation in electrical resistance is necessary. Open circuit break is defined as a complete loss of electrical conductivity (i.e., infinite resistivity).

[0034] The substrate (print receiving material) can be any suitable surface, particularly a printable surface. The printable surface can include, for example, polyethylene terephthalate (PET) (e.g., Melinex TM ), polyethylene naphthalate (PEN), polyolefin (e.g., silica-filled polyolefin (Teslin TM )), polydimethylsiloxane (PDMS), polystyrene, acrylonitrile / butadiene / styrene, polycarbonate, polyimide (e.g., Kapton TM ), thermoplastic polyurethane (TPU), silicone film, wool, silk, cotton, flax, jute, modal, bamboo, nylon, polyester, acrylic, aramid, spandex, polylactide, paper, glass, coated glass (e.g., ITO coated glass), metal, dielectric coating, and the like.

[0035] Conductive traces deposited on substrates can be incorporated into electronic devices such as circuits, conductive busbars (e.g., for photovoltaic cells), sensors (e.g., touch sensors, wearable sensors), antennas (e.g., RFID antennas), thin-film transistors, diodes, smart packaging (e.g., smart pharmaceutical packaging), conformal inserts in devices and / or vehicles, and multilayer circuits and MIM devices, including low-pass filters, frequency selective surfaces, transistors, and antennas on conformal surfaces that can withstand high temperatures. Inks enable the miniaturization of such electronic devices.

[0036] Example:

[0037] Example 1: Neodecanoate Silver Ink

[0038] A series of silver neodecanoate (AgND)-based inks were formulated as described in Table 1. Inks I1, I2, and I3 were formulated according to the present invention, while inks C1 and C2 serve as comparative examples of other AgND-based ink formulations. The inks contain various types of carriers, which can affect the decomposition temperature of the inks. Each ink has a different component that contributes to the largest weight fraction of the carrier: octanol (Ink I1 and Ink C2), an alkylamine (Ink I2), an oxazoline (Ink I3), and a terpene alcohol (Ink C1).

[0039] Table 1

[0040]

[0041] Inks containing 1-amino-2-propanol (hydroxylamine), 2-ethyl-1-hexylamine (alkylamine), and 2-ethyl-2-oxazoline (oxazoline) were prepared by first preparing a concentrated suspension of silver neodecanoate in octanol. 1-amino-2-propanol, 2-ethyl-1-hexylamine, or 2-ethyl-2-oxazoline was then slowly incorporated into the suspension using a mortar and pestle and mixed until a clear, homogeneous, viscous solution was formed. Other inks were prepared by combining all components and mixing in a mixer until the solution was homogeneous. One of two polymer binders, Rokrapol, was added to the ink. TM 7075 (polyester) or ethyl cellulose 46 cP.

[0042] The viscosity of the inks was determined as a function of AgND loading. For these experiments, the silver neodecanoate component of the inks was diluted with the corresponding vehicle and amine in the same proportions as the formulation in Table 1. The viscosity of these inks was measured at 20.5°C using a Brookfield RV-DV-III Ultrarheometer and a UL adapter.

[0043] The four inks were screen printed onto Kapton paper using a 400 mesh screen (stainless steel screen with a wire diameter of 19 μm and a mesh opening of 45 μm). TMon 8.5 x 11" sheets of HPP-ST. The screen included lines that were 10 cm long and 2-20 mil wide. The printed traces were heat sintered in air at different reflow temperatures (T) from 151 °C to 229 °C using the heating program described in Table 2 (the temperatures in Table 2 correspond to the target temperature of the oven). The resistivity data in Table 3 and Figure 2 TM The temperature for obtaining the resistivity data in Table 3 and

[0044] Table 2

[0045] Zone Before After Time, seconds Preheat 1 100℃ 100℃ 300 Preheat 2 150℃ 150℃ 300 Soak 160℃ 160℃ 300 Reflux 160-260℃ 160-260℃ 2700 Cooling 60℃ 60℃ 300

[0046] The electrical properties of the traces were characterized by measuring the resistance across the 10 cm long traces with an ohmmeter. An optical surface profilometer (Cyber Technologies 3D Surface Profiler) was used to measure the width and thickness of the sintered traces. The trace width can be used to determine the number of squares per 10 cm long trace and subsequently to calculate the sheet resistance. The cross-sectional area of the traces was calculated by multiplying the measured width and thickness of the traces. Using the thickness measurement, the sheet resistance value of the traces was calculated. The electrical properties of the sintered traces are provided in Table 3. Figure 2

[0047] As evident from Table 1, the neodecanoic acid silver loading of the inks I1, I2, I3, I4, and I5 of the present application can be significantly greater than the neodecanoic acid silver loading in other AgND-based inks. The higher neodecanoic acid silver loading results in higher silver deposition, as shown by the cross-sectional area of the traces in Table 3. Table 3 provides a comparison of the resistance, line width, line thickness, cross-sectional area, and sheet resistance of silver traces 10 cm in length made from ink I1, ink I2, ink I3, ink CI, and ink C2 after heating to 200 °C for 45 minutes. Figure 3 The relationship between the neodecanoic acid silver loading in the inks and the cross-sectional area of the silver traces after sintering at 200 °C for 45 minutes is shown.

[0048] Based on Figure 2 It is also evident that inks can be formulated by adding additional amines and octanols to produce inks with low viscosity (suitable for flexographic printing and plotter-based printing) and maintain high neodecanoic acid silver loading (from about 48% to about 67%, depending on the amine). It is also evident from Figure 2 Table 3 that inks I1, I2, and I3 can be sintered at much lower temperatures than inks CI and C2 while providing silver traces with good electrical conductivity. In addition, the photonic curing of these inks printed on low temperature substrates can be performed with lower energy, which results in significantly less damage to the underlying substrate.

[0049] Table 3​​

[0050]

[0051] Three other experiments were performed with ink I3, varying the sintering conditions and substrate.

[0052] In a first experiment, ink I3 was screen printed on HPP-ST and sintered using the method described above except that sintering was performed at a temperature of 200°C for 30 minutes instead of 45 minutes. The results are shown in Table 4A and Table 4B.

[0053] Table 4A

[0054]

[0055] Table 4B

[0056]

[0057] In a second experiment, ink I3 was screen printed on and sintered using the method described above except that sintering was performed at a temperature of 180°C for 30 minutes instead of 45 minutes instead of 200°C. The results are shown in Table 5A and Table 5B. Table 5A and Table 5B provide the mechanical properties (resiliency according to ASTM F1683-02 Flex & Crease Test).

[0058] Table 5A

[0059]

[0060]

[0061] Table 5B

[0062]

[0063] In a third experiment, ink I3 was screen printed on using a stainless steel screen with a mesh count of 360 threads / inch (SS360), sintered using photonic sintering (340V / 1500μsec) and then thermally sintered at a temperature of 160°C for 30 minutes. The results are shown in Table 6A and Table 6B.

[0064] Table 6A

[0065]

[0066] Table 6B

[0067]

[0068] The data in the table associated with ink I3 indicates that the combination of 1-octanol and 2-ethyl-oxazoline is able to produce highly conductive traces (volume resistivity values of ~10 μΩ-cm for most line widths). This performance can be due to the ability to convert the traces to conductive silver using less energy than, for example, ink CI. The mechanical properties of the traces derived from ink I3 are also excellent, with no more than a 3% increase in resistance after bending and creasing tests (ASTM 1683-02) (Tables 5A and 5B).

[0069] Ink I4 was screen printed onto a glass substrate having an RMS roughness of 0.06 μm using a stainless steel screen having a mesh count of 360 threads / inch (SS360) and a thickness of the emulsion of about 7-10 μm, and sintered at 200 °C for 30 minutes. The cross-hatch of the screen printed pattern was 4B. The results are shown in Table 7.

[0070] Table 7

[0071]

[0072] The data indicates that the combination of the polymeric binder and ink components results in silver traces that adhere well to the glass substrate (4B) while remaining very smooth (RMS surface roughness of ~0.06 μm).

[0073] Ink I5 was screen printed onto a glass substrate having an RMS roughness of 0.06 μm by high resolution screen printing The high resolution screen printing included woven tungsten wire having a wire diameter of 13 μm, and the screen had a mesh count of 430 threads / inch. The screen printed ink was dried for 5 minutes and then heat sintered at 165 °C for 30 minutes. The results are shown in Table 8. As can be seen from Table 8, traces having measured line widths of 50 μm or less, and volume resistivity values between 10-20 μΩ-cm can be readily produced. This indicates that the ink can be used for transparent conductive electrode (TCE) applications.

[0074] Table 8

[0075]

[0076] The novel features will become apparent to those skilled in the art upon consideration of the specification together with the drawings. However, it should be understood that the scope of the claims is not to be limited to the embodiments but is to be accorded the broadest interpretation of the language employed in the claims and the specification as a whole.

Claims

1. A molecular ink comprising: Silver carboxylate; an organic amine; and a polymer binder comprising polyester, polyimide, polyetherimide, or any mixture thereof, having functional groups that make the polymer binder compatible with the organic amine.

2. The molecular ink according to claim 1, wherein The silver carboxylate includes silver neodecanoate.

3. The molecular ink according to claim 2, wherein: The silver neodecanoate is present in an amount of about 23 wt % or more based on the total weight of the molecular ink.

4. The molecular ink according to any one of claims 1 to 3, wherein The organic amine includes alkylamine, hydroxyalkylamine or cyclic amine.

5. The molecular ink according to any one of claims 1 to 4, wherein The organic amine includes amino-2-propanol, 2-ethyl-1-hexylamine or 2-ethyl-2-oxazoline.

6. The molecular ink according to any one of claims 1 to 5, wherein The organic amine is present in an amount of about 5 wt % to about 50 wt % based on the total weight of the molecular ink.

7. A method for producing conductive silver traces on a substrate, the method comprising depositing the molecular ink as defined in any one of claims 1 to 6 on a substrate to form non-conductive traces of the molecular ink on the substrate, and sintering the non-conductive traces of the molecular ink on the substrate to form conductive silver traces.

8. The method according to claim 7, wherein: The sintering is performed at a temperature ranging from about 150° C. to about 185° C. for a time ranging from about 1 minute to 120 minutes.

9. A substrate comprising conductive silver traces produced from a molecular ink as defined in any one of claims 1 to 6.

10. An electronic device comprising the substrate as defined in claim 9.

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