JP6265789B2 - Conductive paste for silicon, method for producing silver electrode, and method for producing solar cell - Google Patents

Description

  The present invention relates to a nickel-coated silver powder that reduces the contact resistance value at the interface between the silver electrode and the semiconductor layer, a method for producing the same, a conductive paste containing the nickel-coated silver powder, and a solar electrode provided with a silver electrode formed by firing this conductive paste. It relates to batteries.

  In recent years, interest in solar cells has also increased with increasing social interest in renewable energy. There are various types of solar cells due to differences in members, and each has its own characteristics. Crystalline silicon solar cells continue to attract attention because of their high power generation efficiency, and further improvements in power generation efficiency are expected. For example, in patent document 1, the silver powder used for the electrically conductive paste for solar cell manufacture is proposed.

JP 2009-231840 A

  In order to improve the power generation efficiency of the solar cell, it is important to reduce the resistance loss at the time of propagation of the current generated by the photoelectric conversion, that is, to reduce the series resistance value of the solar cell element. The surface electrode of the solar cell is a silver electrode obtained by printing and baking a silver paste. The silver paste according to the prior art consists of silver powder, glass frit, resin binder, and solvent. When a silver paste according to the prior art is printed on a constituent member of a solar cell and baked, the glass frit contained in the silver paste dissolves the antireflection layer that is a constituent member of the solar cell (fire-through), and the baked silver Is in contact with an N-type or P-type silicon layer which is a member of a solar cell, and a silver electrode is formed. Contact resistance exists between the silver electrode and the silicon layer. Similarly, not only the surface electrode but also the back contact electrode has a contact resistance between the silver electrode and silicon.

For example, in a crystalline silicon solar cell, the contact resistance value between the silicon semiconductor emitter layer (sun exposure surface) and the silver grid line is about 10 ⁻³ Ω · cm ² . This contact resistance value is a value several orders of magnitude larger than the contact resistance value (for example, about 10 ⁻⁷ Ω · cm ² ) that can be reached in the semiconductor integrated circuit device. The reason why the contact resistance value is large is that the glass frit is non-conductive, the effective contact area between the silicon semiconductor emitter layer and the silver electrode grid line is small, the silicon semiconductor emitter layer and the silver electrode grid It is conceivable that silicon and silver do not form silicide at the effective contact surface between the lines.

  In Patent Document 1, in order to solve this problem, a plurality of contact openings penetrating the passivation layer are formed, and the openings are selectively plated with a contact metal that forms silicon and silicide. The electrode of the solar cell is formed by depositing a metal-containing layer on top of the substrate to form a metal grid line and sintering it. However, the method of Patent Document 1 is not suitable for mass production because the manufacturing process is very complicated and the manufacturing cost increases as compared with the conventional printing method.

  The present invention has been made under the above-mentioned circumstances, and the problem to be solved is a series resistance value suitable for a screen printing method which is the simplest and general method when applying a silver paste. It is providing the electrically conductive paste which can comprise a low silver electrode. Furthermore, the present invention provides a nickel-coated silver powder that is a constituent material of the conductive paste, a method for producing the same, and a solar cell including a silver electrode obtained by firing the conductive paste (hereinafter sometimes simply referred to as “paste”). It is.

  In order to solve the above problems, a first means according to the present invention is nickel-coated silver powder in which nickel is coated with silver powder, and the nickel content in the nickel-coated silver powder is 0.01 mass% or more and 30 mass%. It is as follows.

  A second means according to the present invention is the first means wherein the nickel is metallic nickel deposited by a wet method.

  The 3rd means which concerns on this invention is used for the electrically conductive paste for solar cell manufacture in a 1st or 2nd means. In addition, being used for the electrically conductive paste for solar cell manufacture means using as a material of the electrically conductive paste used for manufacture of the silver electrode which is a structural member of a solar cell.

  According to a fourth means of the present invention, a silver powder and a reducing agent are added to a solution containing a nickel salt and a citrate, and the nickel salt is reduced to precipitate nickel, thereby reducing the mass of the nickel-coated silver powder to 0. The surface of the silver powder is coated with 0.01% by mass or more and 30% by mass or less of the nickel.

  A fifth means according to the present invention includes a first mixed powder of nickel-coated silver powder and silver powder not coated with nickel, glass frit, a resin binder, a solvent, and a dispersant.

  The sixth means according to the present invention is a conductive paste containing a mixed powder of nickel-coated silver powder and silver powder not coated with nickel as the first means, and the nickel content in the mixed powder is 0.00. It is a thing of 01 mass% or more and 0.07 mass% or less.

  The seventh means according to the present invention is the fifth or sixth means for manufacturing a solar cell. The term “for solar cell production” means that it is used for the production of a silver electrode that is a constituent member of a solar cell.

  An eighth means according to the present invention comprises a silver electrode obtained by applying a conductive paste to a substrate and firing it in any of the fifth to seventh means.

  A ninth means according to the present invention includes a step of applying a conductive paste, which is any one of the fifth to seventh means, to a substrate and baking it to form a silver electrode.

  A tenth means of the present invention is a silver electrode containing nickel and silver, wherein the nickel content relative to the total mass of the nickel and the silver is 0.01% by mass or more and 0.07% by mass or less. It is to be prepared.

  According to the present invention, since the silver electrode is formed by firing a conductive paste containing nickel-coated silver powder on the semiconductor layer, the nickel adhered to the surface of the silver particles existing at the interface with the semiconductor layer becomes the semiconductor during firing. Since the silicide is formed with the layer, the contact resistance value at the interface between the silver electrode and the semiconductor layer can be reduced. Moreover, according to this invention, since the nickel content rate in an electrically conductive paste is restrained low, the increase in the volume resistance value of a silver electrode can be suppressed. Therefore, according to the solar cell of the present invention, the contact resistance at the interface between the silver electrode and the semiconductor layer is reduced, and the volume resistance value of the silver electrode is kept low, so that a high open-circuit voltage and high conversion efficiency are obtained. Realized.

It is a SEM photograph of nickel covering silver powder concerning the present invention (Example 1). It is a SEM photograph of the nickel covering silver powder concerning this invention (Example 2). It is a figure which shows the qualitative analysis result by FE-AES of the nickel covering silver powder which concerns on this invention. It is a figure which shows the SEM image and element mapping analysis result by FE-AES of the nickel covering silver powder concerning this invention.

  Hereinafter, embodiments of the present invention will be described in detail. In this specification, “silver particles” is an expression when attention is paid to individual silver particles, while “silver powder” is an expression when a large number of silver particles are handled as an aggregate.

  A silver electrode of a solar cell is required to have a low volume resistance value and strong oxidation resistance. In order to realize this characteristic, it is optimal that the silver electrode is composed only of silver powder whose surface is not coated (hereinafter referred to as “uncoated silver powder”). However, a silver electrode made of only uncoated silver powder has a problem that the contact resistance value at the interface with the semiconductor layer is high because silver does not form silicide with silicon. On the other hand, since nickel forms silicide with silicon, the contact resistance value at the interface between the silver electrode and the semiconductor layer can be reduced if nickel is present at the interface between the silver electrode and the semiconductor layer. However, nickel has a higher volume resistance than silver. Therefore, as the nickel content of the silver electrode increases, the volume resistance value of the silver electrode increases, and the effect of reducing the contact resistance value due to nickel silicide formation decreases. In other words, when the nickel content of the silver electrode exceeds a predetermined value, the series resistance value (the sum of the contact resistance value, the volume resistance value, and the specific resistance value) of the solar cell element is composed of a silver electrode made of only uncoated silver powder. It becomes higher than the series resistance value of the solar cell element. Therefore, in order to reduce the contact resistance value at the interface between the silver electrode and the semiconductor layer and reduce the series resistance value of the solar cell element by suppressing the volume resistance value of the silver electrode, Coated silver powder and nickel-coated silver powder are used in combination.

(Nickel coated silver powder)
Nickel-coated silver powder is a coating film formed by metal plating the surface of individual silver particles with nickel by a wet method. Since the core material of the nickel-coated silver powder is silver particles, the volume resistance value of the silver electrode is not significantly increased even when used as a constituent material of the silver electrode. On the other hand, at the interface between the silver electrode and the semiconductor layer, nickel on the surface of the nickel-coated silver powder forms silicide with silicon of the semiconductor layer, so the nickel-coated silver powder has a contact resistance value at the interface between the silver electrode and the semiconductor layer. Can be reduced.

  The nickel content in the nickel-coated silver powder is 0.01% by mass to 30% by mass, preferably 0.5% by mass to 10% by mass, and more preferably 1% by mass to 7% by mass. . Further, the shape of each silver particle in the nickel-coated silver powder may be a spherical shape, an ellipsoidal shape, a flake shape, or an indefinite shape. Incidentally, the shape of the individual silver particles in the uncoated silver powder may be spherical, ellipsoidal, flaky, or indefinite. The average particle diameter D50 of the nickel-coated silver powder is preferably 1 μm or more and 50 μm or less, more preferably 20 μm or less, and more preferably 5 μm or less according to the experiments by the present inventors. The average particle diameter D50 (μm) is the particle size distribution measurement result by Microtrack, the horizontal axis is the particle diameter D (μm), and the volume% (Q%) in which particles having a particle diameter of D μm or less exist is the vertical axis. This is the value of the particle size D (μm) corresponding to Q = 50% when expressed by the cumulative particle size curve with the axis. The nickel coating film preferably covers the surface of the silver particles uniformly and completely, but a part of the surface of the silver particles may be exposed. In addition, by coating with the following wet method, the nickel coating film can cover the surface of the silver particles uniformly and thinly. For example, a very thin coating having an average thickness of 1 to 10 nm, more preferably 1 to 6 nm, can be formed by a wet method. Is less likely to occur and there is little risk of an increase in the volume resistance of the silver electrode.

  FIG. 3 shows the qualitative analysis results of the nickel-coated silver powder by the Auger electron spectroscopy analyzer (FE-AES). FIG. 4 shows the result of elemental mapping analysis of the nickel. 3 and 4 that the silver powder is covered with Ni (nickel). In addition, C (carbon) observed by qualitative analysis by element mapping analysis exists on the surface of silver powder, and P (phosphorus) is observed as a whole as if a part of the silver powder is volatilized. It has been found that Al (aluminum) and O (oxygen) are the background. Carbon is considered to be caused by a dispersing agent and phosphorus is considered to be caused by a reducing agent, and metallic nickel precipitated by reduction is present on the surface of the silver powder. The metallic nickel may contain phosphorus as an impurity. In the vicinity of the interface with the silver particles, nickel may be alloyed with silver.

(Production method of nickel-coated silver powder)
The nickel-coated silver powder can be produced by a wet method such as a chemical reduction method or an electrolytic method. In the wet method, first, silver powder is added to a solution (preferably an aqueous solution) containing a nickel salt and a citrate and stirred to prepare a dispersion. Next, a reducing agent such as hypophosphite as a reducing agent is added to the dispersion containing nickel ions and heated. By adding and heating the reducing agent, nickel ions are reduced to deposit nickel on the surface of the silver particles, thereby obtaining a slurry of nickel-coated silver powder. Next, the nickel-coated silver powder slurry is filtered and the nickel-coated silver powder is collected. Next, the collected nickel-coated silver powder is washed with water and then vacuum-dried. Nickel-coated silver powder is obtained through these series of manufacturing steps.

  As a reducing agent added to the nickel ion-containing dispersion, sodium hypophosphite, hydrazine, or sodium borohydride is preferable. Further, in order to promote the reduction reaction of nickel ions, it is desirable to maintain the temperature of the dispersion containing the reducing agent in the range from room temperature (20 ° C.) to 95 ° C. More preferably, it is 70-90 degreeC. According to the experiments conducted by the present inventors, the liquid temperature when nickel began to precipitate was 70 ° C., and therefore it is considered that the reduction reaction is difficult to proceed below 70 ° C. If exceeded, water evaporation will increase.

  Moreover, it is preferable to keep stirring the slurry at all times so that the silver powder is uniformly dispersed in the nickel ion-containing dispersion and the nickel ion reduction reaction does not stagnate. By continuing this stirring, it is possible to uniformly coat the silver powder with nickel and to prevent the silver powder from precipitating and agglomerating.

  In order to prevent oxygen from being mixed into the nickel ion-containing dispersion from the atmosphere, it is preferable to proceed the nickel ion reduction reaction in an atmosphere purged with an inert gas (nitrogen gas or argon gas).

  Moreover, in order to prevent aggregation of the particles of the nickel-coated silver powder, it is preferable to add a dispersant made of an organic polymer (sometimes referred to as an aggregation inhibitor or a surfactant) to the nickel ion-containing dispersion. The addition of this dispersant facilitates the pulverization of the nickel-coated silver powder after vacuum drying, and avoids peeling of the nickel coating coated with the silver powder by the pulverization. Further, if the dispersant is an organic polymer, when the conductive paste is baked in the solar cell manufacturing process, the dispersant vaporizes and disappears. Therefore, by adding the dispersant, the silver electrode of the solar cell according to the present invention is added. There is no degradation in performance. As the organic polymer dispersant, for example, a fatty acid such as stearic acid can be used.

(Conductive paste)
The conductive paste contains a mixed powder of nickel-coated silver powder and non-coated silver powder and a solvent, and further contains a glass frit, a resin binder, and a dispersant as necessary.

  The following are constituent materials of the conductive paste: (1) mixed powder of nickel-coated silver powder and uncoated silver powder, (2) glass frit, (3) resin binder, (4) solvent, (5) dispersant, and (6 ) Other additives will be described.

(1) Mixed powder of nickel-coated silver powder and uncoated silver powder The content of the mixed powder of nickel-coated silver powder and uncoated silver powder in the conductive paste is preferably 65% by mass or more and 95% by mass or less. If the content rate of mixed powder is 65 mass% or more, the silver content rate of the silver electrode after baking will become enough, and it can prevent that the specific resistance value of the silver electrode by the side of a light-receiving surface rises. On the other hand, when the content of the mixed powder is 95% by mass or less, the printability of the paste is ensured and the physical adhesive strength between the silver electrode and the semiconductor layer is ensured.

  The nickel content in the mixed powder is preferably 0.01% by mass to 0.07% by mass, more preferably 0.01% by mass to 0.06% by mass, and preferably 0.01% by mass to 0.04% by mass. Furthermore, 0.02% by mass to 0.027% by mass is optimal according to experiments by the present inventors. If the nickel content in the mixed powder is 0.01% by mass to 0.06% by mass, the effect of reducing the contact resistance value at the interface between the silver electrode and the semiconductor layer due to the formation of nickel silicide is the high volume resistance value of nickel. This exceeds the negative effect of increasing the series resistance value of the solar cell element.

  Mixing of the nickel-coated silver powder and the uncoated silver powder may be performed at any timing before firing. In the following examples, the powders are mixed in a powder state and then made into a paste. However, the present invention is not limited to this, and each of nickel-coated silver powder and non-coated silver powder may be mixed and pasted.

(2) Glass frit The glass frit contained in the conductive paste appropriately corrodes the antireflection layer formed on the semiconductor layer of the solar cell when the conductive paste is baked at 750 ° C. to 950 ° C. The silver electrode, which is a fired product, is bonded to the semiconductor layer. Therefore, the glass frit preferably has a softening point of 300 ° C. or higher and 550 ° C. or lower. If the softening point of the glass frit is 300 ° C. or higher, excessive erosion of the antireflection layer does not occur. On the other hand, if the softening point is 550 ° C. or lower, erosion of the antireflection layer occurs sufficiently and sufficiently.

  The shape of the glass frit is not particularly limited, and may be spherical or indefinite. The content of glass frit in the conductive paste is preferably 0.1% by mass or more and 10% by mass or less. If this content rate is 0.1 mass% or more, the silver electrode can be bonded to the semiconductor layer with sufficient strength. On the other hand, when the content is 10% by mass or less, problems such as glass floating in the conductive paste and poor soldering in a subsequent process do not occur.

(3) Resin binder The resin binder contained in the conductive paste is not particularly limited. For example, cellulose derivatives such as methyl cellulose and ethyl cellulose, acrylic resins, alkyd resins, polypropylene resins, polyurethane resins, and rosin resins. Terpene resin, phenol resin, aliphatic petroleum resin, acrylate ester resin, xylene resin, coumarone indene resin, styrene resin, dicyclopentadiene resin, polybutene resin, polyether resin, A urea resin, a melamine resin, a vinyl acetate resin, a polyisobutyl resin, or the like is preferable.

  As for the content rate of the resin binder in an electrically conductive paste, 0.1 to 10 mass% is preferable. If this content is 0.1% by mass or more, the silver electrode can be bonded to the semiconductor layer with sufficient strength. On the other hand, if the content is 10% by mass or less, an increase in the viscosity of the conductive paste is avoided, so that the printability of the conductive paste is ensured.

(4) Solvent The solvent contained in the conductive paste is not particularly limited. For example, hexane, toluene, ethyl cellosolve, cyclohexanone, butyl cellosolve, butyl cellosolve acetate, butyl carbitol, butyl carbitol acetate, diethylene glycol diethyl ether, Diacetone alcohol, terpineol, methyl ethyl ketone, benzyl alcohol or the like is preferable. The content of the solvent in the conductive paste is preferably 1% by mass or more and 40% by mass or less. If this content rate is 1 mass% or more and 40 mass% or less, the printability of an electrically conductive paste is ensured.

(5) Dispersant The dispersant contained in the conductive paste is preferably stearic acid, palmitic acid, myristic acid, oleic acid, lauric acid, or the like. In addition, if a dispersing agent is a general thing, it will not be limited to an organic acid. The content of the dispersant in the conductive paste is preferably 0.05% by mass or more and 10% by mass or less. If this content rate is 0.05 mass% or more, the dispersibility of the mixed powder and glass frit in an electrically conductive paste is ensured. On the other hand, if the content is 10% by mass or less, an increase in the specific resistance value of the light receiving surface side silver electrode in the solar cell can be avoided.

(6) Other additives Various additives such as stabilizers, antioxidants, ultraviolet absorbers, silane coupling agents, antifoaming agents, and viscosity modifiers are added to the conductive paste as long as the effects of the present invention are not hindered. An agent may be added as appropriate. However, these additives are preferably selected such that the sodium content in the total mass of the conductive paste is less than 100 ppm.

(Solar cell)
The solar cell is provided with silver electrodes obtained by firing a conductive paste that connects adjacent photoelectric conversion elements in series on both sides facing each other across a semiconductor layer that is a photoelectric conversion element. Either side of the silver electrode facing the semiconductor layer can be formed by firing the conductive paste. The conductive paste according to the present invention can also be used for an Si-based back contact electrode.

  As a method for applying the conductive paste for forming the silver electrode, a screen printing method, an offset printing method, a jet printing method, or the like is preferable. In particular, the screen printing method which is the simplest and general method is suitable.

  The nickel-coated silver powder has the effect of reducing the contact resistance value at the interface by forming nickel silicide at the interface between the silver electrode and the semiconductor layer. However, as the nickel content in the silver electrode increases, the negative effect that the volume resistance value of the silver electrode increases increases. Therefore, one ideal configuration of the silver electrode is a configuration in which the content of nickel-coated silver powder is high near the interface with the semiconductor layer, while the content of uncoated silver powder is high at a position away from the semiconductor layer. In order to realize this ideal configuration, first, a conductive paste containing only nickel-coated silver powder is applied to the semiconductor layer by screen printing or the like, and then a conductive paste containing only uncoated silver powder is applied again on the screen printing surface. Can be considered. In addition, it is possible to apply each paste of nickel-coated silver powder and uncoated silver powder while changing the mixing ratio.

  If the series resistance value of the solar cell element is reduced, the fill factor (FF) of the solar cell is increased, and the conversion efficiency is improved.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
<Manufacture of nickel-coated silver powder>
16 g of nickel sulfate and 17 g of ammonium citrate were dissolved in 125 mL of water, and 31.5 g of uncoated silver powder was added while stirring at a liquid temperature of 30 ° C., followed by an aqueous sodium hypophosphite solution as a reducing agent. This aqueous sodium hypophosphite solution is obtained by dissolving 14 g of sodium hypophosphite in 50 mL of water. While continuing the stirring, the liquid temperature was heated to 90 ° C., and nickel was deposited on the surface of the uncoated silver powder by a reduction reaction to obtain a nickel-coated silver powder-containing slurry. The slurry was separated by filtration, the collected material was washed with water, and then vacuum-dried at 75 ° C. to obtain nickel-coated silver powder A according to Example 1. A 5000 times SEM photograph of nickel-coated silver powder A is shown in FIG.

<Evaluation of nickel-coated silver powder>
The measurement results of nickel content, particle size distribution, and specific surface area (BET value) of the nickel-coated silver powder A are shown in “Table 1”. Since the uncoated silver powder does not contain nickel, the nickel content 2.0% by mass of the nickel-coated silver powder A can be regarded as the nickel coating amount as it is. The nickel content was calculated by composition analysis using a weight method (mass method) and emission spectroscopic analysis (ICP). Specifically, after the nickel-coated silver powder was dissolved with nitric acid, hydrochloric acid was added to precipitate silver chloride and filtered, and the mass of silver chloride was measured to calculate the silver content. Also, ammonia water was added to the filtrate in which nickel was dissolved to precipitate nickel hydroxide, and the nickel hydroxide was precipitated and filtered. The nickel hydroxide was calcined to form nickel oxide, and the mass was measured to calculate the nickel content. . Furthermore, the amount of impurities was measured by using ICP for a solution obtained by dissolving nickel-coated silver powder with nitric acid.

  The particle size distribution of nickel-coated silver powder A is measured using Microtrack 9320-X100 (Honeywell-Nikkiso Co., Ltd.) after adding 0.3 g of nickel-coated silver powder A to 50 mL of isopropyl alcohol and dispersing for 5 minutes with a 50 W ultrasonic cleaner. did. Table 1 shows values of D10 (cumulative 10 mass% particle diameter), D50 (cumulative 50 mass% particle diameter), D90 (cumulative 90 mass% particle diameter), and Dmax (maximum particle diameter).

The specific surface area of the nickel-coated silver powder A was 0.76 m ² / g as a BET value. Since the nickel content in the nickel-coated silver powder A is 2.0% by mass, the average thickness of the nickel-coated film calculated from the specific surface area of the nickel-coated silver powder is 3 nm.

<Manufacture of conductive paste>
0.86 parts by mass of nickel-coated silver powder A, 85.14 parts by mass of uncoated silver powder having an average particle size of 3 μm, 1 part by mass of a Ba-based glass frit having a softening point of 530 ° C., 1 part by mass of ethyl cellulose (resin binder), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate 11 parts by weight (organic solvent), stearic acid 0.5 part by weight (dispersing agent), magnesium stearate 1 part by weight, tellurium dioxide Two parts by mass were weighed and blended. This blend was made into a paste by mixing with a three-roll mill. Furthermore, an organic solvent was appropriately added to this paste so that the paste viscosity at the time of screen printing described later was about 400 Pa · s, and the conductive paste according to Example 1 was obtained.

<Printing of conductive paste>
A p-type single crystal silicon wafer having an outer shape of 156 mm × 156 mm, an n-type diffusion layer formed on the surface, and an SiNx antireflection layer formed on the n-type diffusion layer was prepared. An aluminum paste for forming the back electrode was applied to the entire back surface of the silicon wafer by screen printing, dried at 200 ° C. for 20 minutes, and then cooled to room temperature by natural cooling. And the electrically conductive paste which concerns on Example 1 was apply | coated to the surface side of the silicon wafer by screen printing, and it dried at 200 degreeC for 20 minutes, Then, it cooled to room temperature by natural cooling.

<Manufacture of solar cell elements>
The silicon wafer on which the conductive paste was screen-printed was inserted into a high-speed baking furnace and baked at a maximum temperature of 780 ° C. for 1 minute to obtain a solar cell element according to Example 1.

<Characteristic evaluation of solar cell element>
The solar cell element was loaded into a solar simulator, and the characteristics of the solar cell element were measured. The measurement results are shown in “Table 1”. The values of the series resistance value, the fill factor (FF), and the conversion efficiency (η _n ) in Table 1 are based on the characteristic values of the solar cell element according to Comparative Example 1 using a paste that does not contain nickel-coated silver powder described later. It is a value normalized as a value. As is apparent from Table 1, the solar cell according to Example 1 had a lower series resistance value and an improved curve factor and conversion efficiency (nominal conversion efficiency) as compared with the solar cell element according to Comparative Example 1.

  Here, the fill factor (FF) is expressed by the following formula (1).

The conversion efficiency input energy due to the incident light by the measured normalized by 100 mW / cm ² (or 1000W / ^{m 2)} (nominal conversion efficiency: eta _n) is denoted by the following formula (2).

(Example 2)
<Manufacture of nickel-coated silver powder>
32 g of nickel sulfate, 34 g of ammonium citrate and 0.5 g of sodium hydroxide were dissolved in 750 mL of water, and 80 g of uncoated silver powder and then an aqueous sodium hypophosphite solution were added while stirring at a liquid temperature of 30 ° C. This aqueous sodium hypophosphite solution is obtained by dissolving 30 g of sodium hypophosphite in 50 mL of water. The liquid temperature was heated to 80 to 95 ° C. while stirring was continued, and stearic acid was added thereto. In the presence of stearic acid, nickel was precipitated on the surface of the silver particles by a reduction reaction to obtain a nickel-coated silver powder-containing slurry. The slurry was filtered and the collected product was washed with water, followed by vacuum drying at 75 ° C. to obtain nickel-coated silver powder B according to Example 2. An SEM photograph of the nickel-coated silver powder B at 5000 times is shown in FIG.

<Evaluation of nickel-coated silver powder>
The nickel content, particle size distribution, and specific surface area (BET value) of the nickel-coated silver powder B were measured by the same method as in Example 1. The measurement results are shown in “Table 1”. Since the uncoated silver powder does not contain nickel, the nickel content of 2.7% by mass of the nickel-coated silver powder B can be regarded as the nickel coating amount as it is. Since the specific surface area of the nickel-coated silver powder is 0.75 m ² / g, the calculated average thickness of the nickel-coated film is 4 nm.

<Manufacture of conductive paste>
A conductive paste according to Example 2 was obtained by the same manufacturing method as Example 1 except that nickel-coated silver powder B was used instead of nickel-coated silver powder A.

<Printing of conductive paste>
The conductive paste according to Example 2 was applied to the surface side of the silicon wafer by screen printing by the same paste printing method as in Example 1, dried, and then cooled to room temperature by natural cooling.

<Manufacture of solar cell elements>
The silicon wafer on which the conductive paste according to Example 2 was screen-printed was baked in the same manner as in Example 1 to obtain a solar cell element according to Example 2.

<Characteristic evaluation of solar cell element>
The solar cell element according to Example 2 was loaded into a solar simulator, and the characteristics of the solar cell element were measured in the same manner as in Example 1. The measurement results are shown in “Table 1”.

  As is clear from the results shown in Table 1, the solar cell element according to Example 2 has a series resistance value lower than that of the solar cell element according to Comparative Example 1, and has a fill factor and conversion efficiency (nominal conversion efficiency). Improved.

(Comparative Example 1)
<Manufacture of conductive paste>
A conductive paste was obtained by the same production method as in Example 1 except that 86 parts by weight of uncoated silver powder having an average particle size of 3 μm was used without using nickel-coated silver powder.

<Printing of conductive paste>
The conductive paste according to Comparative Example 1 was applied to the surface side of the silicon wafer by screen printing in the same manner as in Example 1, dried, and then cooled to room temperature by natural cooling.

<Manufacture of solar cell elements>
The silicon wafer on which the conductive paste according to Comparative Example 1 was screen-printed was baked by the same method as in Example 1 to obtain a solar cell element according to Comparative Example 1.

<Characteristic evaluation of solar cell element>
The solar cell element according to Comparative Example 1 was loaded into a solar simulator, and the characteristics of the solar cell element were measured by the same method as in Example 1. The measurement results are shown in “Table 1”.

  As is clear from Table 1, the solar cell according to Comparative Example 1 has a higher series resistance value and lower curve factor and conversion efficiency (nominal conversion efficiency) than the solar cell elements according to Examples 1 and 2. Met.

(Example 3)
<Manufacture of nickel-coated silver powder>
27 g of nickel sulfate hexahydrate and 27 g of diammonium citrate were dissolved in 400 mL of water, and an aqueous sodium hypophosphite solution was added thereto. This aqueous sodium hypophosphite solution is obtained by dissolving 29.5 g of sodium hypophosphite in 50 mL of water. While stirring at room temperature, 20 g of uncoated silver powder (average particle size 3 μm, spherical shape) was added together with 50 mL of water. While continuing the stirring, the liquid temperature was heated to 94 ° C., and nickel was precipitated on the surface of the uncoated silver powder by a reduction reaction to obtain a nickel-coated silver powder-containing slurry. To this slurry, 0.07 g of BTA-Na was diluted and added as a dispersant. The slurry was then filtered off and the collected material was washed with water, followed by vacuum drying at 70 ° C. for 8 hours. Next, the nickel-coated silver powder C according to Example 3 was obtained by crushing the aggregate of the nickel-coated silver powder that had been vacuum-dried.

<Evaluation of nickel-coated silver powder>
The nickel content, particle size distribution, and specific surface area (BET value) of the nickel-coated silver powder C were measured in the same manner as in Example 1. The measurement results are shown in “Table 1”. Since the uncoated silver powder does not contain nickel, the nickel content of 6.0% by mass of the nickel-coated silver powder C can be regarded as the nickel coating amount as it is.

Moreover, the specific surface area (BET value) of the nickel-coated silver powder C was 1.1 m ² / g. Since the nickel content in the nickel-coated silver powder C is 6.0% by mass, the average thickness of the nickel-coated film calculated from the specific surface area of the nickel-coated silver powder C is 6 nm. The silver content was 91.8% by mass. The total amount of impurities other than silver and nickel is 2.2% by mass, which are considered to be components such as a dispersant and a reducing agent that could not be removed.

<Manufacture of conductive paste>
A conductive paste according to Example 3 was obtained by the same manufacturing method as Example 1 except that nickel-coated silver powder C was used instead of nickel-coated silver powder A.

<Printing of conductive paste>
The conductive paste according to Example 3 was applied to the surface side of the silicon wafer by screen printing in the same manner as in Example 1, dried, and then cooled to room temperature by natural cooling.

<Manufacture of solar cell elements>
The silicon wafer on which the conductive paste according to Example 3 was screen-printed was baked in the same manner as in Example 1 to obtain a solar cell element according to Example 3.

<Characteristic evaluation of solar cell element>
The solar cell element according to Example 3 was loaded into a solar simulator, and the characteristics of the solar cell element were measured by the same method as in Example 1. The measurement results are shown in “Table 1”.

Example 4
In the production of the conductive paste of Example 3, the same method as Example 3 except that the blending amount of nickel-coated silver powder C was 0.43 parts by mass and the blending amount of uncoated silver powder was 85.57 parts by mass. A solar cell element was manufactured and evaluated for its characteristics. The configuration and characteristic evaluation results of the solar cell element according to Example 4 are described in “Table 1”.

(Reference Example 1)
In the production of the conductive paste of Example 2, the same amount as in Example 2 except that the amount of nickel-coated silver powder B was tripled to 2.58 parts by mass and the amount of uncoated silver powder was 83.42 parts by mass. The solar cell element was manufactured by the method and the characteristic was evaluated. The configuration and characteristic evaluation results of the solar cell element according to Reference Example 1 are described in “Table 1”.

(Comparison study of characteristics of solar cell elements)
If the characteristics of the solar cell elements manufactured in Examples 1 to 4, Comparative Example 1 and Reference Example 1 are compared, if the content of the nickel-coated silver powder in the conductive paste is too large, the series resistance value of the solar cell element increases. And it turns out that the fill factor and conversion efficiency deteriorate. Although it is considered to be caused by the composition of the conductive paste, it is strongly inferred that there is an appropriate amount region in the nickel content in the mixed powder. Specifically, when the nickel content rate in the mixed powder is 0.01% by mass or more and 0.07% by mass or less, the conversion efficiency of the solar cell element is surely improved. In the range of 020 mass% or more and 0.027 mass% or less, the conversion efficiency of the solar cell element reaches the maximum range.

  Since the contact resistance value at the interface between the silver electrode and the semiconductor layer can be reduced by using the nickel-coated silver powder according to the present invention, the present invention is applied to a device using a semiconductor element, such as a diode, a semiconductor memory, or an integrated circuit. It can also be applied to.

Claims (3)

A conductive paste containing a mixed powder of nickel-coated silver powder having a nickel content of 0.01% by mass to 30% by mass and silver powder not coated with nickel,
The conductive paste for silicon, wherein the nickel content in the mixed powder is 0.01% by mass or more and 0.07% by mass or less. A method for producing a silver electrode , wherein the conductive paste for silicon according to claim 1 is applied to silicon and baked.   The manufacturing method of the solar cell including the process of apply | coating and baking the electrically conductive paste for silicon | silicones of Claim 1 to silicon | silicone, and making it a silver electrode.