Conductive paste and solar cell

Abstract

The present invention provides a conductive paste capable of forming an electrode having a relatively low specific resistance even when conductive particles having a relatively low silver content are used.　Provided is a conductive paste containing (A) conductive particles, (B) an epoxy resin, and (C) a curing agent, wherein the (A) conductive particles include silver-coated copper particles, and in the particle size distribution of the silver-coated copper particles obtained by particle size distribution measurement using a laser diffraction scattering method, when the silver-coated copper particle content is set as 100 vol%, the amount of silver-coated copper particles having a particle diameter of less than 5.0 μm is 50 vol% or less.

Description

Conductive paste and solar cells

[0001] The present invention relates to a conductive paste that can be used to form electrodes of semiconductor devices such as solar cells.

[0002] Conductive pastes containing conductive particles such as silver particles are used to form, for example, electrodes and circuit patterns of semiconductor devices and electronic components. Formation of electrodes and circuit patterns using the conductive paste can be achieved by applying the conductive paste in a predetermined pattern onto a substrate or the like by screen printing or the like, and then heating the conductive paste to obtain a conductive film in the predetermined pattern.

[0003] There are two types of conductive paste: high-temperature firing conductive paste and thermosetting conductive paste. High-temperature firing conductive paste is a conductive paste that can form a conductive film by firing at a high temperature of around 550 to 900°C. In the case of high-temperature firing conductive paste, the resin component contained in the conductive paste is burned away during firing. Thermosetting conductive paste is a conductive paste that can form a conductive film by heating at a relatively low temperature of around room temperature (approximately 20°C) to 250°C. In the case of thermosetting conductive paste, the resin component hardens and bonds the silver particles together, forming a conductive film.

[0004] As a thermosetting conductive paste, Patent Document 1 describes a conductive paste characterized by containing silver-coated copper powder in which the surface of copper powder is coated with a silver layer, and an epoxy resin having a naphthalene skeleton.

[0005] Patent Document 2 describes a thermosetting conductive paste composition containing (A) a conductive powder, (B) a thermosetting component, and (C) a curing agent. Patent Document 1 describes that at least one of (A-1) a silver-coated metal powder and (A-2) a powder of copper or an alloy thereof is used as the (A) conductive powder.

[0006] JP 2019-083182 A JP 2019-014786 A

[0007] The conductive particles contained in the conductive paste are silver particles made of silver, which has low resistivity. However, silver is expensive. Therefore, attempts have been made to use conductive particles that are less expensive than silver particles. As a low-cost conductive particle, conductive particles in which the surface of copper particles is coated with silver (silver-coated copper particles) have been proposed.

[0008] However, silver-coated copper particles contain copper particles that have a higher resistivity than silver, and therefore, there is a problem that an electrode formed using a conductive paste containing silver-coated copper particles has a higher resistivity than an electrode formed using a conductive paste containing silver particles.

[0009] Therefore, an object of the present invention is to provide a conductive paste that can form an electrode with relatively low resistivity even when conductive particles with a relatively low silver content are used, and to provide a solar cell having an electrode formed using the conductive paste.

[0010] In order to solve the above problems, the present invention has the following configuration.

[0011] (Configuration 1) Configuration 1 is a conductive paste comprising: (A) conductive particles; (B) an epoxy resin; and (C) a curing agent, wherein the (A) conductive particles comprise silver-coated copper particles; and in a particle size distribution of the silver-coated copper particles obtained by particle size distribution measurement using a laser diffraction scattering method, the content of the silver-coated copper particles having a particle diameter of less than 5.0 μm is 50 vol % or less, where the content of the silver-coated copper particles is 100 vol %.

[0012] (Configuration 2) Configuration 2 is the conductive paste of configuration 1, in which the (A) conductive particles further contain silver particles.

[0013] (Configuration 3) Configuration 3 is the conductive paste of configuration 2, wherein the silver particles have an average particle size (D50) of 0.1 to 4.0 μm.

[0014] (Configuration 4) In Configuration 4, the BET specific surface area of ​​the silver particles is 0.1 to 2.0 m ２ The conductive paste of configuration 2 or 3 is in the range of / g.

[0015] (Configuration 5) Configuration 5 is the conductive paste of any one of Configurations 2 to 4, wherein the silver particles include first silver particles having an average particle diameter (D50) of 0.5 to 2.0 μm and second silver particles having an average particle diameter (D50) of 0.1 to 1.0 μm.

[0016] (Configuration 6) Configuration 6 is the conductive paste of any one of Configurations 1 to 5, wherein the weight ratio (Ag / Cu) of silver (Ag) to copper (Cu) contained in the silver-coated copper particles is in the range of 5 / 95 to 40 / 60.

[0017] (Configuration 7) Configuration 7 is the conductive paste of any one of Configurations 1 to 6, wherein the silver-coated copper particles have an average particle size (D50) in the range of 1.0 to 8.0 μm.

[0018] (Configuration 8) In Configuration 8, the silver-coated copper particles have a BET specific surface area of ​​0.1 to 0.5 m ２ The conductive paste of any one of configurations 1 to 7, wherein the SiO2 content is in the range of / g.

[0019] (Configuration 9) Configuration 9 is the conductive paste of any one of Configurations 1 to 8, wherein the silver-coated copper particles are spherical.

[0020] (Configuration 10) Configuration 10 is the conductive paste of any one of Configurations 1 to 9, wherein the content of the silver-coated copper particles having a particle diameter of less than 3.0 μm is 30 vol % or less, based on 100 vol % of the silver-coated copper particles.

[0021] (Configuration 11) Configuration 11 is the conductive paste of any one of Configurations 1 to 10, wherein the particle diameter (D10) at which the cumulative undersieve value is 10% in a number-based cumulative distribution of particle diameters of the silver-coated copper particles is 1.0 μm or more.

[0022] (Configuration 12) Configuration 12 is the conductive paste of any one of configurations 1 to 11, in which the content of the silver-coated copper particles is 30 to 80 wt % relative to 100 wt % of the conductive particles.

[0023] (Configuration 13) Configuration 13 is the conductive paste of any one of configurations 1 to 12, wherein the content of silver (Ag) is 20 to 60 wt % relative to 100 wt % of the conductive paste.

[0024] (Configuration 14) Configuration 14 is the conductive paste of any one of configurations 1 to 13, in which the content of the (B) epoxy resin is 1 to 10 parts by weight per 100 parts by weight of the (A) conductive particles.

[0025] (Configuration 15) Configuration 15 is the conductive paste of any one of configurations 1 to 14, in which the (C) curing agent includes at least one selected from a cationic curing agent and a boron trifluoride compound.

[0026] (Configuration 16) Configuration 16 is the conductive paste of any one of Configurations 1 to 15, further comprising (D) a solvent.

[0027] (Configuration 17) Configuration 17 is the conductive paste of any one of Configurations 1 to 16, which is a conductive paste for forming a solar cell electrode.

[0028] (Configuration 18) Configuration 18 is a solar cell including an electrode, wherein the electrode is a hardened product of the conductive paste of any one of Configurations 1 to 17.

[0029] According to the present invention, it is possible to provide a conductive paste capable of forming an electrode with a relatively low specific resistance even when conductive particles with a relatively low silver content are used, and it is also possible to provide a solar cell having an electrode formed using the conductive paste.

[0030] 1 is a schematic cross-sectional view showing an example of a solar cell having an electrode formed using a conductive paste, and FIG. 2 is a schematic plan view showing a resistivity measurement pattern for an electrode formed using a conductive paste.

[0031] Hereinafter, embodiments of the present invention will be described in detail. Note that the following embodiments are forms for realizing the present invention, and are not intended to limit the scope of the present invention.

[0032] The conductive paste of this embodiment is a thermosetting conductive paste. The conductive paste of this embodiment contains predetermined components, and therefore can be thermoset at low temperatures (for example, 200°C or less) to form an electrode. By using the conductive paste of this embodiment, a conductive film (electrode) with low specific resistance can be formed. The conductive paste of this embodiment can be preferably used as a conductive paste for forming electrodes for solar cells.

[0033] In this specification, the term "conductive film" refers to a thin film pattern formed by printing or the like a conductive paste onto the surface of a predetermined substrate or the like to form a pattern of a predetermined shape, and then curing the printed pattern. The predetermined shape pattern includes any shape, such as a linear, dotted, or planar pattern. The conductive film can be used as an electrode.

[0034] Next, the conductive paste of this embodiment will be described.

[0035] The conductive paste for forming a solar cell electrode of this embodiment contains (A) conductive particles, (B) an epoxy resin, and (C) a curing agent. The conductive paste of this embodiment may further contain (D) a solvent and other additives. Each component contained in the conductive paste of this embodiment will be described below.

[0036] <(A) Conductive Particles> The conductive paste of this embodiment contains (A) conductive particles. The (A) conductive particles include silver-coated copper particles. The (A) conductive particles may further contain silver particles. The conductive paste of this embodiment preferably does not contain any metal other than the (A) conductive particles. The conductive paste of this embodiment preferably contains 85 to 97 wt % of (A) conductive particles, and more preferably 90 to 95 wt %, based on 100 wt % of the conductive paste.

[0037] In this specification, copper particles refer to particles containing copper as a conductive component. The conductive component of the copper particles is preferably copper, and the copper particles preferably contain 50 parts by weight or more of copper per 100 parts by weight of the copper particles, more preferably 90 parts by weight or more of copper, and even more preferably consist of copper alone. The copper particles preferably consist of only a metal containing copper.

[0038] In this specification, silver particles refer to particles containing silver as a conductive component. The conductive component of the silver particles is preferably silver, and the silver particles preferably contain 50 parts by weight or more of silver per 100 parts by weight of the silver particles, more preferably 90 parts by weight or more of silver, and even more preferably consist of silver alone.

[0039] In this specification, the phrase "copper particles consisting only of copper" means that the copper particles are substantially entirely made of copper, excluding unavoidable impurities. In other words, when copper particles consist only of copper, the copper particles may contain unavoidable impurities in addition to copper. The same applies to other particles and components, such as silver particles.

[0040] <<Silver-Coated Copper Particles>> The (A) conductive particles of the conductive paste of this embodiment include silver-coated copper particles. The silver-coated copper particles are metal particles in which the surfaces of copper particles are coated with silver. In this specification, the silver-coated layer on the surface of the silver-coated copper particles may be referred to as a "silver-coating layer."

[0041] The method for producing the copper particles is not particularly limited, and examples of the production method that can be used include a reduction method, a pulverization method, an electrolysis method, an atomization method, a heat treatment method, and a combination thereof.

[0042] The method for forming a silver coating layer on the surface of copper particles is not particularly limited. Examples of the method for forming a silver coating layer on the surface of copper particles include a substitution method and a reduction method. In the substitution method, copper on the surface of the copper particles is replaced with silver, thereby forming a silver coating layer on the surface of the copper particles. In the reduction method, silver or a silver compound is precipitated on the surface of the copper powder, thereby forming a silver coating layer on the surface of the copper particles.

[0043] A silver coating layer can be formed on the surface of copper particles by the following method. First, silver ions are brought into contact with copper particles made of copper in an aqueous solution to perform displacement plating on the surface of the copper particles, thereby depositing silver on the surface of the copper particles. Next, the silver deposited on the surface of the copper particles is brought into contact with silver ions and a reducing agent for the silver ions in an aqueous solution, thereby further depositing silver on the surface of the silver deposited on the surface of the copper particles. In this way, a silver coating layer can be formed on the surface of the copper particles.

[0044] The silver-coated copper particles contained in the conductive paste of this embodiment have a particle size distribution obtained by particle size distribution measurement using a laser diffraction scattering method, and in this particle size distribution of the silver-coated copper particles, the content of silver-coated copper particles having a particle diameter of less than 5.0 μm is preferably 50 vol% or less, more preferably 40 vol% or less, and even more preferably 30 vol% or less, based on 100 vol% of the content of silver-coated copper particles including copper particles and a silver coating layer.

[0045] The particle size of the silver-coated copper particles and other particles contained in the conductive paste of this embodiment can be expressed as a particle size based on the number standard measured by laser diffraction / scattering particle size distribution measurement. Furthermore, the content (volume %) of silver-coated copper particles having a particle size of less than 5.0 μm can be measured by laser diffraction / scattering particle size distribution measurement.

[0046] When the weight ratio of copper to silver contained in silver-coated copper particles is the same, silver-coated copper particles with a small particle size have a large surface area, making it difficult to form a silver coating layer over the entire surface of the copper particles compared to silver-coated copper particles with a large particle size. Therefore, copper particles may be exposed on part of the surface of small-particle-size silver-coated copper particles. The inventors have found that when there are many silver-coated copper particles with exposed copper particles, the resistivity of an electrode obtained using a conductive paste is high. Furthermore, the inventors have found that by setting the content of silver-coated copper particles with a particle size of less than 5.0 μm to 50 vol% or less in a conductive paste, it is possible to avoid the inclusion of many silver-coated copper particles with exposed copper particles, even in cases where the silver content of silver-coated copper particles is relatively low. Based on the above findings, the inventors have found that by using a conductive paste containing silver-coated copper particles with a content of silver-coated copper particles with a particle size of less than 50 vol% or less, an electrode with relatively low resistivity can be formed even when conductive particles with a relatively low silver content are used, and have arrived at the present invention.

[0047] For example, when the silver content in the silver-coated copper particles is 10 wt %, the resistivity of an electrode formed using a conductive paste containing silver-coated copper particles in which the content of silver-coated copper particles having a particle diameter of less than 5.0 μm is 50 vol % or less is lower than the resistivity of an electrode formed using a conductive paste containing silver-coated copper particles in which the content of silver-coated copper particles having a particle diameter of less than 5.0 μm is more than 50 vol %.

[0048] For the reasons described above, it is preferable that the silver-coated copper particles contained in the conductive paste of this embodiment do not contain silver-coated copper particles having a particle diameter of less than 5.0 μm. That is, the content of silver-coated copper particles having a particle diameter of less than 5.0 μm in all coated copper particles is preferably 0 (zero) vol%. However, the coated copper particles may contain silver-coated copper particles having a particle diameter of less than 5.0 μm to the extent that they do not adversely affect the conductive paste of this embodiment. For example, the content of silver-coated copper particles having a particle diameter of less than 5.0 μm in all coated copper particles may be 0.1 vol% or more, 1 vol% or more, or 10 vol% or more.

[0049] In the conductive paste of this embodiment, the content of silver-coated copper particles is preferably 30% by volume or less, more preferably 20% by volume or less, even more preferably 10% by volume or less, and particularly preferably 5% by volume or less, of silver-coated copper particles having a particle diameter of less than 3.0 μm, based on 100% by volume of silver-coated copper particles. Furthermore, the conductive paste of this embodiment may not contain silver-coated copper particles having a particle diameter of less than 3.0 μm. In the case of silver-coated copper particles having a particle diameter of less than 3.0 μm, due to their small particle diameter, it is not easy to form a silver coating layer on the surface of the copper particles so as not to expose the copper particles. Therefore, the fewer silver-coated copper particles having a particle diameter of less than 3.0 μm, the better. By having the content of silver-coated copper particles having a particle diameter of less than 3.0 μm be 30% by volume or less, the adverse effects caused by the presence of silver-coated copper particles having a small particle diameter can be reduced. The conductive paste of this embodiment may contain silver-coated copper particles having a particle diameter of less than 3.0 μm, as long as the effect of the conductive paste is not adversely affected. For example, the silver-coated copper particles having a particle size of less than 3.0 μm can be 0.1% by volume or more, 0.5% by volume or more, or 1% by volume or more.

[0050] In the conductive paste of this embodiment, the weight ratio (Ag / Cu) of silver (Ag) to copper (Cu) contained in the silver-coated copper particles is preferably in the range of 5 / 95 to 40 / 60, more preferably in the range of 7 / 93 to 30 / 70, more preferably in the range of 8 / 92 to 20 / 80, and even more preferably in the range of 9 / 91 to 15 / 85. Silver-coated copper particles are metal particles in which the surfaces of copper particles are coated with silver. Therefore, the silver contained in the silver-coated copper particles exists as a silver coating layer that coats the surfaces of the copper particles. Therefore, in this specification, the content of silver (Ag) contained in 100% by weight of silver-coated copper particles may be referred to as the "silver coating amount" (unit: wt%).

[0051] The greater the amount of silver coating on the silver-coated copper particles, the thicker the silver coating layer of the silver-coated copper particles. Therefore, the resistivity of the final electrode can be reduced. However, if the silver coating layer is thick, the silver content in the silver-coated copper particles increases, resulting in the problem of high costs for the conductive paste. For these reasons, the weight ratio (Ag / Cu) of silver (Ag) to copper (Cu) contained in the silver-coated copper particles is preferably within the above-mentioned range. Since the weight ratio (Ag / Cu) of silver (Ag) to copper (Cu) contained in the silver-coated copper particles is within the above-mentioned range, the use of the conductive paste of this embodiment allows for the formation of an electrode with relatively low resistivity at relatively low cost. That is, the silver-coated copper particles described above can be used as a material for forming an electrode with low resistivity, particularly when the silver content in the silver-coated copper particles is relatively small. Therefore, the use of the conductive paste of this embodiment containing the silver-coated copper particles described above allows for the formation of a relatively low-cost electrode. Furthermore, the conductive paste of this embodiment allows for the formation of an electrode with low resistivity by using silver-coated copper particles with a relatively small silver content. Furthermore, in the conductive paste of this embodiment, it is possible to relatively finely adjust the "content of silver (Ag) in the conductive paste" described below, which allows for a high degree of freedom in designing the composition of the conductive paste.

[0052] As described above, the particle size of particles such as silver-coated copper particles contained in the conductive paste of this embodiment can be expressed by the particle size based on the number standard measured by laser diffraction / scattering particle size distribution measurement. In this specification, the particle size of particles such as silver-coated copper particles may be expressed by the cumulative particle size distribution based on the number standard measured by laser diffraction / scattering particle size distribution measurement. In the cumulative particle size distribution based on the number standard, the cumulative distribution from the smallest particle size is referred to as the "cumulative undersize distribution."

[0053] In the cumulative undersieve distribution of all particles based on a number standard, the particle diameter at which the cumulative value is X% is referred to as "DX". For example, in the cumulative undersieve distribution of all particles based on a number standard, the particle diameter at which the cumulative value is 10% is referred to as "D10". Furthermore, in the cumulative undersieve distribution of all particles based on a number standard, the particle diameter at which the cumulative value is 50% is referred to as "D50". In this specification, D50 may also be referred to as "average particle diameter (D50) based on a number standard" (or simply "average particle diameter (D50)"). Furthermore, in the cumulative undersieve distribution of all particles based on a number standard, the particle diameter at which the cumulative value is 90% is referred to as "D90".

[0054] The average particle diameter (D50) of the silver-coated copper particles contained in the conductive paste of this embodiment is preferably in the range of 1.0 to 10.0 μm, more preferably in the range of 2.0 to 9.0 μm, and even more preferably in the range of 3.5 to 8.0 μm.

[0055] By ensuring that the average particle diameter (D50) of the silver-coated copper particles is within the above-mentioned range, it is possible to easily ensure that the content of silver-coated copper particles having a particle diameter of less than 5.0 μm is within the above-mentioned specified range.

[0056] In the conductive paste of this embodiment, the particle diameter (D10) at which the cumulative undersize distribution based on the number of particle diameters of the silver-coated copper particles reaches 10% is preferably 1.0 μm or more, more preferably 2.0 μm or more, and preferably 3.0 μm or more. In the case of silver-coated copper particles with a particle diameter of less than 1.0 μm, due to the small particle diameter, it is not easy to form a silver coating layer on the surface of the copper particles so as not to expose the copper particles. Therefore, the fewer silver-coated copper particles with a particle diameter of less than 1.0 μm, the better. By having the particle diameter (D10) of the silver-coated copper particles within the above-mentioned range, the adverse effects caused by the presence of silver-coated copper particles with small particle diameters can be reduced. The upper limit of the particle diameter (D10) of the silver-coated copper particles can be, for example, 8.0 μm or less, 6.0 μm or less, or 5.0 μm or less.

[0057] In the conductive paste of this embodiment, the particle diameter (D90) at which the cumulative undersieve value is 90% in the cumulative distribution of the particle diameters of the silver-coated copper particles based on the number of particles is preferably 5 to 15 μm, more preferably 6 to 12 μm or more, and even more preferably 8 to 10 μm or more. The particle diameter (D90) of the silver-coated copper particles is a value that is close to the upper limit of the particle diameter of the silver-coated copper particles contained in the conductive paste. By having the particle diameter (D90) of the silver-coated copper particles within the above-mentioned range, it is possible to reduce the adverse effects caused by the presence of silver-coated copper particles with an excessively large particle diameter. Note that if the particle diameter of the silver-coated copper particles is too large, problems may occur when the conductive paste is screen-printed.

[0058] In the conductive paste of this embodiment, the BET specific surface area of ​​the silver-coated copper particles is 0.1 to 0.5 m ２ / g, and 0.2 to 0.45 m ２ It is more preferable that the range is / g.

[0059] When the weight ratio of copper and silver contained in silver-coated copper particles is the same, it is more difficult to form a silver coating layer on the entire surface of silver-coated copper particles with a large BET specific surface area than silver-coated copper particles with a small BET specific surface area. This is presumably because when a silver coating layer is formed on copper particles with a large BET specific surface area, the thickness of the silver coating layer becomes relatively thinner compared to when a silver coating layer is formed on copper particles with a small BET specific surface area. Therefore, when a silver coating layer is formed on copper particles with a large BET specific surface area, the copper particles may be exposed on part of the surface of the silver-coated copper particles, which may increase the specific resistance of the electrode obtained using the conductive paste. By having the BET specific surface area of ​​the silver-coated copper particles within the above-mentioned range, it is possible to avoid the inclusion of a large number of silver-coated copper particles with exposed copper particles, even in the case of silver-coated copper particles with a relatively low silver content. As a result, the specific resistance of the electrode obtained using the conductive paste of this embodiment can be reduced. In addition, if there are exposed copper particles on part of the surface of the silver-coated copper particles, oxidation of copper will proceed from the exposed copper particles, which is thought to increase the resistivity of the silver-coated copper particles.

[0060] The tap density of the silver-coated copper particles is 2.0 to 8.0 g / cm ３ is preferably 2.5 to 7.0 g / cm ３ More preferably, it is 3.0 to 6.0 g / cm ３ When the tap density of the silver particles is in this range, an electrode having sufficient conductivity can be formed.

[0061] The shape of the silver-coated copper particles can be, for example, spherical, flaky, or needle-like. Furthermore, silver-coated copper particles of different shapes can be mixed and used. Flake-shaped silver-coated copper particles can be produced, for example, by crushing spherical copper particles using a ball mill or the like to form flakes, and then forming a silver coating layer on the surface of the flake-shaped copper particles.

[0062] In the conductive paste of this embodiment, the silver-coated copper particles are preferably spherical in shape. The spherical shape of the silver-coated copper particles allows a silver coating layer with a relatively uniform thickness to be formed on the surface of the copper particles. This prevents the presence of exposed copper particles on the surface of the silver-coated copper particles. Furthermore, spherical silver-coated copper particles are easier to control in particle size and contain fewer coarse particles than flake-shaped silver-coated copper particles. Therefore, the use of spherical conductive particles allows for the advantageous formation of fine-wiring electrodes.

[0063] <<Silver Particles>> The (A) conductive particles contained in the conductive paste of this embodiment preferably further contain silver particles.

[0064] As described above, it is preferable that the silver-coated copper particles contain a small amount of silver-coated copper particles with a small particle size. If the conductive particles consist only of silver-coated copper particles with a relatively large particle size, the contact area between the silver-coated copper particles may be small, resulting in an increase in resistivity. By combining silver-coated copper particles with a relatively large particle size and silver particles with a relatively small particle size, it is possible to have silver particles with a relatively small particle size present around the silver-coated copper particles with a large particle size. As a result, the contact area between the silver-coated copper particles and the silver particles is increased, thereby reducing the resistivity of the resulting electrode.

[0065] The average particle size (D50) of the silver particles contained in the conductive paste of this embodiment is preferably 0.1 to 4.0 μm, more preferably 0.2 to 2.0 μm, and even more preferably 0.3 to 1.0 μm. The presence of silver particles having an average particle size (D50) in the above range surrounding silver-coated copper particles with a relatively large particle size can reduce the resistivity of an electrode obtained by heat-treating the conductive paste of this embodiment.

[0066] The conductive paste of this embodiment has a silver particle BET specific surface area of ​​0.1 to 2.0 m ２ The presence of silver particles having a BET specific surface area in the above range around silver-coated copper particles having a relatively large particle size can reduce the resistivity of an electrode obtained by heat treating the conductive paste of this embodiment.

[0067] The silver particles contained in the conductive paste of this embodiment preferably include first silver particles having an average particle diameter (D50) of 0.5 to 2.0 μm and second silver particles having an average particle diameter (D50) of 0.1 to 1.0 μm. By using two types of silver particles with different average particle diameters (D50), it is possible to appropriately arrange the silver particles with relatively small particle diameters around the silver-coated copper particles with relatively large particle diameters. Therefore, by including two types of silver particles with different average particle diameters (D50) in the conductive paste of this embodiment, it is possible to further reduce the resistivity of the electrode obtained by heat treatment.

[0068] The particle sizes of conductive particles such as silver particles generally exhibit a distribution that approximates a normal distribution. When two types of silver particles (first silver particles and second silver particles) with different average particle sizes are present among the silver particles, two types of normal distributions corresponding to the particle size distributions of the first silver particles and the second silver particles will exist. Therefore, when two types of first silver particles and second silver particles are present together, the particle size distribution can be measured using laser diffraction / scattering particle size distribution measurement, and the peaks corresponding to the two types of silver particles can be assumed to be normal distributions, and the average particle sizes (D50) of the two types of first silver particles and the second silver particles can be measured by separating the peaks.

[0069] The shape of the silver particles can be, for example, spherical, flake-like, or needle-like. Silver particles of different shapes can be mixed and used as the silver particles. Flake-like silver particles can be produced, for example, by crushing spherical silver particles using a ball mill or the like.

[0070] Furthermore, in the case of a conductive paste for forming an electrode with fine wiring (e.g., a wiring width of 20 to 60 μm), it is preferable to use spherical silver particles. Compared to flake-shaped silver particles, spherical silver particles are easier to control the particle size of and contain fewer coarse particles. Therefore, the use of spherical silver particles makes it possible to advantageously form an electrode with fine wiring.

[0071] The tap density of the silver particles is 2.0 to 8.0 g / cm ３ is preferably 2.5 to 7.0 g / cm ３ More preferably, it is 3.0 to 6.0 g / cm ３ When the tap density of the silver particles is in this range, an electrode having sufficient conductivity can be formed.

[0072] In the conductive paste of this embodiment, the content of the silver-coated copper particles is preferably 30 to 80 wt %, more preferably 40 to 75 wt %, and even more preferably 50 to 70 wt %, relative to 100 wt % of the conductive particles. By being in the above range, the specific resistance of the electrode obtained by heat treating the conductive paste of this embodiment can be made smaller, and a relatively low-cost electrode can be obtained.

[0073] The conductive particles contained in the conductive paste of this embodiment may include at least silver-coated copper particles and silver particles. When the conductive particles consist of silver-coated copper particles and silver particles, the content of the silver particles is preferably 20 to 70 wt % (the content of silver-coated copper particles is 30 to 80 wt %), more preferably 25 to 60 wt % (the content of silver-coated copper particles is 40 to 75 wt %), and even more preferably 30 to 50 wt % (the content of silver-coated copper particles is 50 to 70 wt %) relative to 100 wt % of the conductive particles, corresponding to the content of the silver-coated copper particles described above. If the content of silver particles is too high, the conductive paste may become expensive. If the content of silver particles is too low, the conductive paste may become expensive. If the content of silver particles is too low, the conductive paste may become expensive. If the content of silver particles is too low, the conductive paste may become expensive. Therefore, if the content of silver-coated copper particles is within the above range, the resistivity of the electrode obtained by heat-treating the conductive paste of this embodiment may be reduced, and a relatively low-cost electrode may be obtained.

[0074] <<Silver (Ag) Content of Conductive Paste>> The conductive paste of this embodiment preferably has a silver (Ag) content of 20 to 60 wt %, more preferably 30 to 55 wt %, and even more preferably 35 to 50 wt %, relative to 100 wt % of the conductive paste. If the silver content in the conductive paste is too high, the conductive paste becomes expensive. On the other hand, if the silver content in the conductive paste is too low, the resistivity becomes high. Therefore, by keeping the silver content in the conductive paste within the above range, the resistivity of the electrode obtained by heat treating the conductive paste of this embodiment can be reduced, and a relatively low-cost electrode can be obtained.

[0075] <(B) Epoxy Resin> The conductive paste of this embodiment contains (B) an epoxy resin.

[0076] In the conductive paste of this embodiment, the weight ratio of the (B) epoxy resin to 100 parts by weight of the (A) conductive particles is preferably 1 to 10 parts by weight, more preferably 2 to 8 parts by weight, and even more preferably 3 to 6 parts by weight. If the weight ratio of the (B) epoxy resin to 100 parts by weight of the (A) conductive particles exceeds 10 parts by weight, the amount of the (B) epoxy resin relative to the (A) conductive particles becomes too large, which may lead to inhibition of fusion between the conductive particles. On the other hand, if the mass ratio of the (B) epoxy resin to 100 parts by weight of the (A) conductive particles is less than 1 part by weight, it may be difficult to maintain the strength of the electrode.

[0077] The type of (B) epoxy resin is not particularly limited, and known epoxy resins can be used. Examples of (B) epoxy resins include bisphenol A type, bisphenol F type, biphenyl type, tetramethylbiphenyl type, cresol novolac type, phenol novolac type, bisphenol A novolac type, dicyclopentadiene phenol condensation type, phenol aralkyl condensation type, and glycidylamine type epoxy resins, as well as brominated epoxy resins, alicyclic epoxy resins, aliphatic epoxy resins, and urethane skeleton-containing epoxy resins. These epoxy resins can be used alone or in combination of two or more.

[0078] An example of the epoxy resin (B) is "AK-601 (trade name)" manufactured by Nippon Kayaku Co., Ltd. This epoxy resin can be used alone or in combination with the other epoxy resins described above to form a mixture of two or more types.

[0079] The epoxy resin (B) is preferably an epoxy resin that is liquid at room temperature. Among epoxy resins that are liquid at room temperature, the epoxy resin (B) preferably contains a compound having the structure of the following formula (4). The compound having the structure of formula (4) is diglycidyl 1,2-cyclohexanedicarboxylate. By containing the compound having the structure of formula (4), it is possible to more reliably form an electrode having low resistivity by heat treatment at a low temperature (for example, 150°C or lower).

[0080] <(C) Curing Agent> The conductive paste of this embodiment further contains a (C) curing agent. By including the (C) curing agent, it is possible to appropriately control the curing of the (B) epoxy resin.

[0081] The curing agent (C) may be a known curing agent. The curing agent (C) preferably includes at least one selected from a phenolic curing agent, a cationic polymerization initiator, an imidazole curing agent, and a boron trifluoride compound. Examples of the boron trifluoride compound include boron trifluoride monoethylamine, boron trifluoride piperidine, and boron trifluoride diethyl ether.

[0082] In the conductive paste of this embodiment, the (C) curing agent preferably includes at least one selected from a cationic curing agent and a boron trifluoride compound.

[0083] The conductive paste of this embodiment preferably contains a compound having a structure of formula (a) as the curing agent (C). The compound having the structure of formula (a) is a cationic polymerization initiator. When the curing agent (C) is a compound having the structure of formula (a), it is possible to more reliably form an electrode having a low specific resistance by heat treatment at a low temperature (for example, 150°C or less). Formula (a)

[0084] In the conductive paste of this embodiment, the ratio of the weight of the (A) conductive particles to the total weight of the (B) epoxy resin and (C) curing agent (weight of the (A) conductive particles:total weight of the (B) epoxy resin and (C) curing agent) is preferably 98.5:1.5 to 90.0:10.0, more preferably 98.0:2.0 to 92.0:8.0, and even more preferably 97.0:3.0 to 94.0:6.0. By appropriately controlling the ratio of the weight of the (A) conductive particles to the total weight of the (B) epoxy resin and (C) curing agent, an electrode with low specific resistance can be formed.

[0085] In the conductive paste of this embodiment, when the total weight of the (A) conductive particles and the (B) epoxy resin is taken as 100 parts by weight, the conductive paste preferably contains 0.1 to 5.0 parts by weight, more preferably 0.15 to 2.0 parts by weight, and even more preferably 0.2 to 1.0 parts by weight of the (C) curing agent. By setting the weight ratio of the (C) curing agent within a predetermined range, the (B) epoxy resin can be appropriately cured, and an electrode of the desired shape can be obtained.

[0086] <(D) Solvent> The conductive paste of the present embodiment preferably further contains a solvent (D). By containing a solvent in the conductive paste, the viscosity of the conductive paste can be set within an appropriate range, and the screen printing performance can be improved.

[0087] Examples of solvents contained in the conductive paste of this embodiment include aromatic hydrocarbons such as toluene, xylene, mesitylene, and tetralin; ethers such as tetrahydrofuran; ketones such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; lactams such as 2-pyrrolidone and 1-methyl-2-pyrrolidone; ether alcohols such as ethyl glycol monophenyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether (butyl carbitol), as well as corresponding propylene glycol derivatives; esters such as corresponding acetates (e.g., diethylene glycol monobutyl ether acetate); and diesters such as methyl and ethyl esters of dicarboxylic acids such as malonic acid and succinic acid. Among these, at least one selected from ethylene glycol monophenyl ether, diethylene glycol monobutyl ether, and butyl carbitol acetate is preferably used.

[0088] The conductive paste of this embodiment preferably contains ethylene glycol monophenyl ether, diethylene glycol monobutyl ether (butyl carbitol) and / or diethylene glycol monobutyl ether acetate (butyl carbitol acetate) as the solvent (D).

[0089] (D) When the solvent contains ethylene glycol monophenyl ether, diethylene glycol monobutyl ether (butyl carbitol), and / or diethylene glycol monobutyl ether acetate (butyl carbitol acetate), it is possible to more reliably obtain a conductive film (e.g., a solar cell electrode) with low specific resistance. Furthermore, by using a specific solvent, the viscosity of the conductive paste can be more appropriately adjusted, thereby improving the printing characteristics of the printed conductive film. Specifically, when the conductive film pattern has a fine line shape, the line width can be made thin and uniform, the film thickness can be made uniform, and a shape with a high aspect ratio can be obtained.

[0090] When the conductive paste of this embodiment is applied to the surface of a transparent conductive film or the like by screen printing, the apparent viscosity of the conductive paste at room temperature is preferably 100 to 1000 Pa·s, more preferably 200 to 900 Pa·s, and even more preferably 300 to 800 Pa·s. The viscosity is measured using a Brookfield viscometer: HBD type (manufactured by Brookfield) at a rotation speed of 5 rpm (shear rate: 2 sec －１ ), and a value measured at a temperature of 25° C. can be used. By adjusting the blending amount of the (D) solvent in the conductive paste, the viscosity of the conductive paste can be adjusted to a predetermined range.

[0091] <Other Components> The conductive paste of the present embodiment may contain the following components in addition to the above-described components (A), (B), (C), and (D).

[0092] The conductive paste of this embodiment may contain a thermosetting resin other than an epoxy resin. Examples of such thermosetting resins include amino resins such as urea resins, melamine resins, and guanamine resins; oxetane resins; phenolic resins such as resol-type phenolic resins, alkyl resol-type phenolic resins, novolac-type phenolic resins, alkyl novolac-type phenolic resins, and aralkyl novolac-type phenolic resins; silicone-modified resins such as silicone epoxy and silicone polyester; bismaleimide and polyimide resins.

[0093] The conductive paste of this embodiment may contain (B) a thermoplastic resin other than an epoxy resin. Examples of (B) a thermoplastic resin other than an epoxy resin include novolac phenolic resin, allylphenol resin, phenoxy resin, butyral resin, cellulose resin, acrylic resin, methacrylic resin, polyester resin, polyurethane resin, polyamide resin, thermoplastic xylene resin, hydroxystyrene polymer, cellulose derivative, and a mixture of two or more of these.

[0094] The conductive paste of the present embodiment may further contain a phenoxy resin. By further containing a phenoxy resin, the conductive paste of the present embodiment can improve the adhesive strength to the substrate while maintaining a low resistivity.

[0095] The conductive paste of this embodiment may further contain a coupling agent. A silane coupling agent can be used as the coupling agent. By further containing a coupling agent in the conductive paste, the adhesiveness between the inorganic component such as the conductive particles and the thermosetting resin can be improved.

[0096] The conductive paste of the present embodiment may further contain at least one selected from the group consisting of an inorganic pigment, an organic pigment, a leveling agent, a thixotropic agent, and an antifoaming agent.

[0097] <Conductive Paste> The conductive paste of this embodiment can be suitably used in the field of thermosetting conductive pastes. In particular, it can be suitably used as a thermosetting conductive paste for forming a collector electrode of a solar cell, a conductive adhesive, or an internal or external electrode of a chip-type electronic component. The conductive paste of this embodiment can be suitably used in particular as a conductive paste for forming an electrode of a solar cell.

[0098] The method for producing the conductive paste of this embodiment is not particularly limited. The conductive paste of this embodiment can be produced by adding the components of the conductive paste of this embodiment in a predetermined ratio to a mixer such as a mortar and pestle mixer, a propeller mixer, a kneader, a three-roll mill, or a pot mill, and mixing them.

[0099] The conductive paste of this embodiment can be applied to the surface of a transparent electrode, etc., by a known method such as screen printing. After the conductive paste is applied to the surface of a transparent electrode, etc., the conductive paste is heat-treated at a predetermined temperature to harden, thereby forming a conductive film (electrode).

[0100] In this specification, the conductive paste of this embodiment, or a conductive film, electrode, or the like obtained by heating (heat treating) the conductive paste of this embodiment to a predetermined temperature and curing it, is referred to as a "cured body." The cured body of this embodiment is the above-mentioned conductive paste, or a cured body of the above-mentioned conductive paste. The cured body of this embodiment can be preferably used as an electrode for a solar cell.

[0101] The cured product of this embodiment is preferably a cured product obtained by applying a conductive paste to the surface of a transparent conductive film and curing it at 200° C. for 30 minutes. Furthermore, the specific resistance of the solar cell electrode is preferably less than 7 μΩ cm, more preferably 6.9 μΩ cm or less, and even more preferably 6.8 μΩ cm or less. Because the electrode that is the cured product of this embodiment has such properties, it can be preferably used as an electrode for amorphous silicon solar cells, heterojunction solar cells, perovskite solar cells, and the like.

[0102] The heat treatment temperature for thermally curing the conductive paste (heat treatment temperature when forming electrodes for solar cells) is preferably 220°C or less, and more preferably 200°C or less. Specifically, the heat treatment temperature for thermally curing the conductive paste is preferably 180 to 220°C, and more preferably 200 to 210°C. The heat treatment time is preferably 20 to 60 minutes, and more preferably 25 to 45 minutes. A specific example of the heat treatment conditions is 30 minutes at 200°C.

[0103] The thickness of the conductive paste applied to the surface of the transparent electrode or the like is preferably 5 to 40 μm, more preferably 10 to 30 μm, and even more preferably 15 to 20 μm.

[0104] The conductive paste of this embodiment can be preferably used to form solar cell electrodes with a wiring width of 20 to 60 μm. Therefore, solar cell electrodes heat-treated with the conductive paste of this embodiment can include electrodes with a wiring width of 20 to 60 μm. The width (wiring width) of the conductive paste applied to the surface of a transparent electrode or the like is preferably 20 to 60 μm, more preferably 20 to 50 μm, and even more preferably 20 to 40 μm.

[0105] The conductive film obtained by heat-treating the conductive paste of this embodiment has the characteristic of low resistivity (high conductivity), and therefore, by using the thermosetting conductive paste of this embodiment, it is possible to form a good electrode for a semiconductor device or the like.

[0106] The conductive paste of this embodiment can be used to form electrodes and circuit patterns for semiconductor devices, electronic components, etc. The conductive paste of this embodiment can be used to form electrodes and / or circuit patterns not only on the surfaces of inorganic materials such as semiconductors, oxides, and ceramics, but also on substrates with low heat resistance such as PET (polyethylene terephthalate) and PEN (polyethylene naphthalate).

[0107] In this specification, the term "semiconductor device" refers to a device using a semiconductor chip, for example, semiconductor devices such as transistors and integrated circuits, flat panel displays such as liquid crystal displays (LCDs) and plasma display panels (PDPs), and devices using semiconductors such as solar cells. A semiconductor device is a device that utilizes the properties of electrons and / or holes in a semiconductor, and has electrodes for direct or indirect electrical connection to the semiconductor.

[0108] Depending on the type of semiconductor material of the semiconductor device, the type of material of the electronic component, and the type of non-semiconductor materials constituting the semiconductor device and electronic component, etc., high temperatures, for example, temperatures exceeding 150°C, during the electrode formation process may cause deterioration of the semiconductor chip and / or other materials. By using the conductive paste of this embodiment, it is possible to form an electrode with low resistivity at low temperatures (for example, 150°C or lower). By using the conductive paste of this embodiment, it is possible to form an electrode with a predetermined low resistance without causing deterioration of the semiconductor device due to high temperatures. Therefore, the conductive paste of this embodiment is particularly suitable as a conductive paste for forming electrodes for amorphous silicon solar cells and heterojunction solar cells, which use amorphous silicon as a material, as well as perovskite solar cells.

[0109] <Solar Cell> This embodiment is a solar cell including the above-described conductive paste or a cured product of the conductive paste. The cured product can be an electrode of the solar cell. By using the conductive paste or the conductive paste of this embodiment, a solar cell electrode with low resistivity can be formed at low cost.

[0110] The conductive paste of this embodiment can be preferably used to form an electrode on the surface of a transparent conductive film of a solar cell. Depending on the type of solar cell, a material that is adversely affected by a high-temperature heating process may be used. The conductive paste of this embodiment can be particularly preferably used to form an electrode on the surface of a transparent conductive film of such solar cells with low heat resistance. Examples of materials that are adversely affected by a high-temperature heating process include amorphous silicon and perovskite compounds. Examples of solar cells using amorphous silicon as a material include amorphous silicon solar cells and heterojunction solar cells using amorphous silicon and crystalline silicon (hereinafter simply referred to as "heterojunction solar cells"). Examples of solar cells using perovskite compounds include perovskite solar cells. The conductive paste of this embodiment can be preferably used to form an electrode on the surface of a transparent conductive film of an amorphous silicon solar cell, a heterojunction solar cell, or a perovskite solar cell. Furthermore, the conductive paste of this embodiment can be used to form solar cell electrodes with low resistivity and low cost.

[0111] A heterojunction solar cell, which is a preferred application of the conductive paste of this embodiment, will be described with reference to FIG.

[0112] FIG. 1 shows a cross-sectional schematic diagram of an example of a heterojunction solar cell. The heterojunction solar cell shown in FIG. 1 has an n-type crystalline silicon substrate 10 (e.g., a single-crystal silicon substrate or a polycrystalline silicon substrate) on its light-incident surface, on which an i-type amorphous silicon layer 12 (approximately 10 nm thick) made of intrinsic amorphous silicon and a p-type amorphous silicon layer 14a (approximately 10 nm thick) made of p-type amorphous silicon are stacked in this order. A transparent conductive film, e.g., a transparent conductive film 16 (approximately 70 nm thick) made of indium tin oxide (ITO), is disposed on the p-type amorphous silicon layer 14a. A grid-shaped light-incident electrode 18a is formed on the surface of the transparent conductive film 16 to obtain a slit-shaped light-incident surface.

[0113] 1, an i-type amorphous silicon layer 12 (thickness: approximately 10 nm) made of intrinsic amorphous silicon and an n-type amorphous silicon layer 14b (thickness: approximately 10 nm) made of highly doped n-type amorphous silicon are stacked in this order on the back surface of the crystalline silicon substrate 10. A transparent conductive film 16 and a lattice-shaped back electrode 18b are formed on the n-type amorphous silicon layer 14b, similar to the light-incident surface.

[0114] In the case of the heterojunction solar cell shown in FIG. 1 , the layers other than the crystalline silicon substrate 10 can all be formed at temperatures of approximately 200° C. or less using methods such as plasma CVD, sputtering, vapor deposition, or screen printing. Furthermore, because amorphous silicon is adversely affected by high-temperature heating processes, it is preferable to form the light-incident-side electrode 18 a and the back electrode 18 b on the surface of the transparent conductive film 16 at low temperatures. By using the conductive paste of this embodiment, the light-incident-side electrode 18 a and the back electrode 18 b can be formed at low temperatures (e.g., 250° C. or less, preferably 220° C. or less) and at low cost, with low resistivity.

[0115] Examples and comparative examples of this embodiment will be described below, but this embodiment is not limited to the following examples and comparative examples.

[0116] [Preparation of Conductive Paste] As examples and comparative examples, conductive pastes containing (A) conductive particles, (B) epoxy resin, (C) curing agent, and (D) solvent were produced. Tables 1 and 2 show the blending ratios of materials in Examples 1 to 7 and Comparative Examples 1 to 5. The blending ratios shown in Tables 1 and 2 are shown in parts by weight when the weight of the (A) conductive particles is taken as 100 parts by weight. The "silver content in the conductive paste" is shown as % by weight when the weight of the conductive paste is taken as 100% by weight.

[0117] (A) Silver-coated copper particles 1 to 8 shown in Table 3 and silver particles 1 and 2 shown in Table 4 were used as the conductive particles. Tables 1 and 2 show the blending amounts of silver-coated copper particles 1 to 8 and silver particles 1 and 2.

[0118] In Table 3, the "Less than 3 μm (vol %)" and "Less than 5 μm (vol %)" columns show the proportions of silver-coated copper particles having particle diameters of less than 3 μm and less than 5 μm in units of "vol %" with the content of silver-coated copper particles taken as 100 vol %. The "Silver coating amount (wt %)" column in Table 3 shows the weight ratio of silver contained in the silver coating layer of the silver-coated copper particles relative to the weight of the silver-coated copper particles. In Table 3, the "BET (m ２ The "TAP (g / cm)" column in Table 3 shows the BET specific surface area of ​​the silver-coated copper particles. ３ The "D10 (μm)" column, the "D50 (μm)" column, and the "D90 (μm)" column in Table 3 show the particle sizes (D10, D50 (average particle size), and D90) at which the cumulative values ​​are 10%, 50%, and 90% in the cumulative undersieve distribution based on the number of particle sizes of the silver-coated copper particles measured by laser diffraction / scattering particle size distribution measurement. The "Shape" column in Table 3 shows the particle shape of the silver-coated copper particles.

[0119] (B) As the epoxy resin, the following resins 1 and 2 were used: Resin 1: Urethane skeleton-containing epoxy resin "EPU-7N (trade name)" manufactured by ADEKA Corporation Resin 2: Epoxy resin "AK-601 (trade name)" manufactured by Nippon Kayaku Co., Ltd. (diglycidyl 1,2-cyclohexanedicarboxylate, see formula (4))

[0120] Resin 2, diglycidyl 1,2-cyclohexanedicarboxylate, is a compound having the structure of the following formula (4).

[0121] (C) As the curing agents, the following curing agent 1 and curing agent 2 were used. Note that curing agent 2 is a compound having the structure of formula (a). Curing agent 1: boron trifluoride piperidine (manufactured by Stella Chemical Co., Ltd.) Curing agent 2: cationic polymerization initiator of formula (a), product name "CXC1614" manufactured by King Industries, Inc. Formula (a)

[0122] (D) As the solvent, the following solvent 1 was used: Solvent 1: Butyl carbitol acetate (diethylene glycol monobutyl ether acetate)

[0123] [Measurement of Resistivity] The conductive pastes of Examples 1 to 7 and Comparative Examples 1 to 5 were heat-treated to obtain conductive films, and the resistivity of the conductive films was measured.

[0124] The resistivity of the examples and comparative examples was measured by the following procedure. A silicon substrate measuring 15 mm in width, 15 mm in length, and 180 μm in thickness was prepared. A pattern made of conductive paste as shown in FIG. 2 was printed on the substrate using a 325-mesh stainless steel screen.

[0125] Next, the patterns made of the conductive pastes of the examples and comparative examples applied onto the substrate were heated at 200° C. for 30 minutes to obtain samples for measuring resistivity.

[0126] The resistivity of the conductive film patterns of the resistivity measurement samples obtained by heat-treating the conductive pastes of the Examples and Comparative Examples was measured. First, the resistance value was measured using a four-terminal method using a Toyo Corporation Model 2001 multimeter. The cross-sectional area of ​​the pattern was measured using a Lasertec Corporation OPTELICS H1200 confocal microscope and a Lasertec Corporation 1500SD2 surface roughness and shape measuring instrument. Measurements were taken at 50 locations over a 1.6 mm range, and the average value was calculated. The resistivity was calculated using the cross-sectional area and the measured resistance value.

[0127] Four samples for measuring resistivity were prepared under the same conditions, and the measured value was calculated as the average of the four values. The measurement results are shown in Tables 1 and 2.

[0128] As is clear from Tables 1 and 2, the resistivity of the conductive films obtained using the conductive pastes of Examples 1 to 7 of this embodiment was 6.6 μΩ cm (Example 6) or less. Generally, it can be said that a conductive film with a resistivity of less than 7 μΩ cm can be suitably used as an electrode. In contrast, the resistivity of the conductive films obtained using the conductive pastes of Comparative Examples 1 to 5 ranged from 7.0 μΩ cm (Comparative Examples 2 and 5) to 8.9 μΩ cm (Comparative Example 1). Therefore, it was revealed that electrodes with lower resistivity can be obtained by forming conductive films (electrodes) using the conductive pastes of Examples 1 to 7 of this embodiment.

[0129] Furthermore, as shown in Tables 1 and 2, the silver content in the conductive pastes of Examples 1 to 7 was 37.4 to 46.7 wt %, which was less than half the silver content of silver particles consisting only of silver. Therefore, it is clear that by using the conductive paste of this embodiment, electrodes with low resistivity can be formed at low cost.

[0130]

[0131]

[0132]

[0133]

[0134] REFERENCE SIGNS LIST 10 Crystalline silicon substrate 12 i-type amorphous silicon layer 14a p-type amorphous silicon layer 14b n-type amorphous silicon layer 16 Transparent conductive film 18a Light incident side electrode 18b Back electrode

Claims

1. A conductive paste comprising: (A) conductive particles; (B) an epoxy resin; and (C) a curing agent, wherein the (A) conductive particles comprise silver-coated copper particles; and in a particle size distribution of the silver-coated copper particles obtained by particle size distribution measurement using a laser diffraction scattering method, the content of the silver-coated copper particles having a particle diameter of less than 5.0 μm is 50 vol % or less, with the content of the silver-coated copper particles being 100 vol %.

2. The conductive paste according to claim 1, wherein the conductive particles (A) further comprise silver particles.

3. The conductive paste according to claim 2, wherein the average particle diameter (D50) of the silver particles is 0.1 to 4.0 μm.

4. The BET specific surface area of ​​the silver particles is 0.1 to 2.0 m ２ The conductive paste according to claim 2 or 3, wherein the SiO 2 content is in the range of / g.

5. The conductive paste according to any one of claims 2 to 4, wherein the silver particles comprise: first silver particles having an average particle diameter (D50) of 0.5 to 2.0 μm; and second silver particles having an average particle diameter (D50) of 0.1 to 1.0 μm.

6. The conductive paste according to any one of claims 1 to 5, wherein the weight ratio (Ag / Cu) of silver (Ag) to copper (Cu) contained in the silver-coated copper particles is in the range of 5 / 95 to 40 / 60.

7. The conductive paste according to any one of claims 1 to 6, wherein the average particle diameter (D50) of the silver-coated copper particles is in the range of 1.0 to 8.0 µm.

8. The BET specific surface area of ​​the silver-coated copper particles is 0.1 to 0.5 m ２ The conductive paste according to any one of claims 1 to 7, wherein the range is / g.

9. The conductive paste according to any one of claims 1 to 8, wherein the silver-coated copper particles are spherical in shape.

10. A conductive paste according to any one of claims 1 to 9, wherein the content of the silver-coated copper particles having a particle diameter of less than 3.0 μm is 30% by volume or less, based on 100% by volume of the silver-coated copper particles.

11. The conductive paste according to any one of claims 1 to 10, wherein the particle diameter (D10) at which the cumulative under-sieve distribution based on the number of particle diameters of the silver-coated copper particles reaches 10% is 1.0 μm or more.

12. The conductive paste according to any one of claims 1 to 11, wherein the content of the silver-coated copper particles is 30 to 80% by weight relative to 100% by weight of the conductive particles.

13. The conductive paste according to any one of claims 1 to 12, wherein the content of silver (Ag) is 20 to 60 wt % relative to 100 wt % of the conductive paste.

14. The conductive paste according to any one of claims 1 to 13, wherein the content of the epoxy resin (B) is 1 to 10 parts by weight per 100 parts by weight of the conductive particles (A).

15. The conductive paste according to any one of claims 1 to 14, wherein the (C) curing agent includes at least one selected from the group consisting of cationic curing agents and boron trifluoride compounds.

16. The conductive paste according to any one of claims 1 to 15, further comprising (D) a solvent.

17. The conductive paste according to any one of claims 1 to 16, which is a conductive paste for forming solar cell electrodes.

18. A solar cell including an electrode, wherein the electrode is a hardened product of the conductive paste according to any one of claims 1 to 17.

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