JP2012023088A - Paste for solar cell electrode, and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a paste for a solar cell electrode that can form an electrode having a high aspect ratio.SOLUTION: In this paste for the solar cell electrode, when an index (μ/μ) obtained by dividing viscosity (μ) measured by rotating a spindle of HB type #7 at a rotational speed of 5 rpm using a Brookfield viscometer at 25°C by viscosity (μ) measured by rotating the spindle at a rotational speed of 200 rpm using the viscometer at 25°C is defined as a thixotropic index (TI), a thixotropic index (TI) obtained when changing the rotational speed of the spindle from 5 rpm to 200 rpm, and a thixotropic index (TI) obtained when changing the rotational speed of the spindle from 200 rpm to 5 rpm satisfy the following formulas (I) and (II): TI<TI(I), and 8≤TI≤16 (II).

Description

  The present invention relates to a solar battery electrode paste and a solar battery cell.

  Solar cells that convert light energy such as sunlight into electrical energy have been actively developed in various structures and configurations as interest in global environmental issues increases. Among them, solar cells using a semiconductor substrate such as silicon are most commonly used due to advantages such as conversion efficiency and manufacturing cost.

As a material for forming such an electrode of a solar cell, a resin-based paste material is known.
For example, Patent Document 1 discloses that “a silver electrode paste comprising at least silver powder, glass frit, a resin and an organic solvent, wherein the glass frit is a residue classified by a sieve having an opening diameter of 24 to 100 μm. Is described.
Patent Document 2 discloses that “a first silver powder having a crystallite diameter of 58 nm or more, a second silver powder having a crystallite diameter different from that of the silver powder, a glass frit, and a resin binder, and a solar cell electrode paste. Is described.
Patent Document 3 describes “a paste for solar cell light-receiving surface electrode containing silver particles having a specific surface area of 0.20 to 0.60 m ² / g, glass frit, a resin binder, and thinner.” Has been.
Patent Document 4 discloses that “a conductive paste containing silver powder, glass frit, and an organic vehicle for forming an electrode on a semiconductor substrate for a solar cell, wherein the silver powder has a BET diameter of 0. Greater than 10 μm and less than or equal to 0.50 μm, average particle diameter (D50) is greater than 0.2 μm and less than 2.0 μm, and average particle diameter (D50) / BET diameter is 10 or less ”Is described.

JP 2004-146154 A JP 2007-194581 A JP 2007-235082 A Japanese Patent No. 3800108

  However, when the surface electrode on the light-receiving surface side is formed using a resin-based paste material as described in Patent Documents 1 to 4, the ratio of the height and width of the cross-section of the electrode (height / width) (hereinafter referred to as the following) It is clear that the aspect ratio of the surface electrode on the light receiving surface side is small, and that the light shielding area on the light receiving surface increases and the current collecting effect is inferior. .

  Then, an object of this invention is to provide the paste for solar cell electrodes which can form an electrode with a high aspect ratio.

As a result of intensive studies to solve the above-mentioned problems, the present inventor has found that a thixoindex measured under a predetermined condition satisfies a specific relationship, and can form an electrode having a high aspect ratio, The present invention has been completed.
That is, the present invention provides the following (1) to (8).

(1) Viscosity (μ ₅ ) measured by rotating a spindle of HB type # 7 at a rotational speed of 5 rpm using a Brookfield viscometer at 25 ° C., and rotating the spindle using the viscometer at 25 ° C. When the index (μ ₅ / μ ₂₀₀ ) obtained by dividing by the viscosity (μ ₂₀₀ ) measured by rotating at 200 rpm is the thixo index (TI), the rotation speed of the spindle was changed from 5 rpm to 200 rpm. When the thixo index (TI ₁ ) and the thixo index (TI ₂ ) when the rotation speed of the spindle is changed from 200 rpm to 5 rpm satisfy the following formulas (I) and (II): paste.
TI ₁ <TI ₂ (I)
8 ≦ TI ₁ ≦ 16 (II)

(2) It contains silver powder (A), organic acid silver salt (B), solvent (C) and / or plasticizer (D), and the silver powder (A) has a specific surface area of 1.5 to 3. 0.0 m ² / g and an average particle diameter of 3 μm or less of silver powder (a), and the content of the organic acid silver salt (B) is 2.5 parts per 100 parts by mass of the silver powder (A). The solar cell electrode paste according to (1), wherein the paste is in mass parts or more.

  (3) The solar cell electrode paste according to (2), wherein the content of the silver powder (a) in the silver powder (A) is 50% by mass or more.

  (4) The organic acid silver salt (B) is a fatty acid silver salt having one or more hydroxyl groups (—OH), a fatty acid silver salt having 18 or less carbon atoms, and a carboxy silver base having no hydroxyl group (—OH). The solar cell electrode paste according to (2) or (3), which is at least one selected from the group consisting of silver polycarboxylates having two or more (—COOAg).

  (5) The solar cell electrode paste according to any one of (2) to (4), wherein the solvent (C) is an organic solvent having a boiling point of 200 ° C. or higher.

  (6) The solar cell electrode paste according to any one of (2) to (5), wherein the plasticizer (D) is a glycol ether plasticizer and / or an ester plasticizer having a boiling point of 250 ° C. or higher. .

  (7) The solar cell electrode paste according to any one of (2) to (6), further comprising glass frit.

  (8) A light-receiving surface side surface electrode, a semiconductor substrate, and a back surface electrode are provided, and at least the surface electrode is formed using the solar cell electrode paste according to any one of (1) to (7). Solar cell.

  ADVANTAGE OF THE INVENTION According to this invention, the solar cell electrode paste which can form an electrode with a high aspect ratio can be provided. In addition, this makes it possible to thin the surface electrode on the light receiving surface side, so that a solar cell with a high current collecting effect in which the light shielding area on the light receiving surface is reduced can be obtained.

FIG. 1 is a cross-sectional view showing an example of a preferred embodiment of a solar battery cell.

<Solar cell electrode paste>
The solar cell electrode paste of the present invention has a viscosity (μ ₅ ) measured by rotating a spindle of HB type # 7 at 25 ° C. using a Brookfield viscometer at a rotation speed of 5 rpm. in the case of dividing to obtain exponent (μ ₅ / μ ₂₀₀₎ and thixotropy index (TI) in viscosity measured by rotating at a rotation speed 200rpm the spindle (mu ₂₀₀₎ with the rotational speed of the spindle The thixo index (TI ₁ ) when shifting from 5 rpm to 200 rpm and the thixo index (TI ₂ ) when shifting the rotation speed of the spindle from 200 rpm to 5 rpm are represented by the following formulas (I) and (II): It is a solar cell electrode paste that satisfies the above.
TI ₁ <TI ₂ (I)
8 ≦ TI ₁ ≦ 16 (II)

In the present invention, the thixo index (TI ₁ ) and the thixo index (TI ₂ ) are measured continuously.
Specifically, if the spindle is submerged in the solar cell electrode paste to be measured for viscosity, the spindle is first rotated at a rotational speed of 5 rpm for 300 seconds, and then the rotational speed is changed to 200 rpm to 300. Then, the rotation speed is returned again to 5 rpm and the rotation is continued for 300 seconds.
The thixo index (TI = μ ₅ / μ ₂₀₀ ) when the rotation speed of the spindle is changed from 5 rpm to 200 rpm is defined as the thixo index (TI ₁ ), and the rotation speed of the spindle is returned from 200 rpm to 5 rpm. The thixo index (TI = μ ₅ / μ ₂₀₀ ) at this time is used as the thixo index (TI ₂ ).

More specifically, the thixo index (TI ₁ ) is determined by the viscosity (μ ₅ ) at a rotational speed of ₅ rpm for 1 to 3 seconds before the shift point when the rotational speed of the spindle is changed from 5 rpm to 200 rpm, and the speed change. It is determined based on the viscosity (μ ₂₀₀ ) at a rotation speed of 200 rpm in 1 to 3 seconds after the time point.
The same applies to the thixo index (TI ₂ ).

By the way, generally, as a paste to be applied using a coating method such as screen printing, a high thixo index (TI) paste having a large viscosity ratio between a viscosity in a low shear field and a viscosity in a high shear field is used. Is preferred. This is because high fluidity (low viscosity) is required in a high shear field by squeezing, and high shape retention (high viscosity) is required in order to prevent sagging and the like in a low shear field after coating.
However, since such a paste has a tendency to gradually decrease in viscosity due to dissolution of filler aggregation in a shear flow field, the printing state may change over time, which may be a problem. The relationship between the thixo index (TI ₁ ) at the change point from the low shear field to the high shear field and the thixo index (TI ₂ ) at the change point from the high shear field to the low shear field is TI ₁ > TI _2. Therefore, it tends to sag due to the displacement from the high shear field to the low shear field.

  On the other hand, the solar cell electrode paste of the present invention satisfying the above formulas (I) and (II) exhibits high fluidity in a high shear field and also exhibits a property of hardening like dilatancy. Due to this latter property, the viscosity at the time of displacement from a high shear field to a low shear field becomes relatively high, and it is considered that shape retention in the low shear field after displacement is exhibited. Therefore, when the solar cell electrode paste of the present invention satisfying the above formulas (I) and (II) is applied onto a silicon substrate by a coating method such as screen printing and baked to form an electrode, It is thought that the aspect ratio becomes high.

The paste for solar cell electrode of the present invention as described above contains, for example, silver powder (A), organic acid silver salt (B), solvent (C) and / or plasticizer (D), and The silver powder (A) contains a silver powder (a) having a specific surface area of 1.5 to 3.0 m ² / g and an average particle diameter of 3 μm or less, and the content of the organic acid silver salt (B) is Examples include a solar cell electrode paste that is 2.5 parts by mass or more with respect to 100 parts by mass of the silver powder (A).
Hereinafter, each component contained in the said solar cell electrode paste is demonstrated in detail.

[Silver powder (A)]
The silver powder (A) includes a silver powder (a) having a specific surface area of 1.5 to 3.0 m ² / g and an average particle diameter of 3 μm or less.
Here, the specific surface area refers to a value obtained from a nitrogen adsorption isotherm at −196 ° C. based on the BET equation.
Moreover, an average particle diameter means the average value of the particle diameter of silver powder, and means the 50% volume cumulative diameter (D50) measured using the laser diffraction type particle size distribution measuring apparatus. In addition, when the cross-section of the silver powder is an ellipse (including the case where the silver powder is flakes), the particle diameter used as the basis for calculating the average value is the sum of the major and minor diameters. The average value divided by 2 means the diameter of a regular circle.

The specific surface area of the silver powder (a) is preferably 1.5 to 2.5 m ² / g because an electrode having a better aspect ratio can be formed. Moreover, it is preferable that the average particle diameter of the said silver powder (a) is 1.5-3 micrometers from the same reason.

Commercially available products can be used as the silver powder (A). Specific examples thereof include AG4-8F (shape: spherical, average particle size: 1.9 μm, specific surface area: 0.43 m ² / g, manufactured by DOWA Electronics Co., Ltd.) ), AG2-1C (shape: spherical, average particle size: 0.8 μm, specific surface area: 0.93 m ² / g, manufactured by DOWA Electronics), AG3-11F (shape: spherical, average particle size: 1.4 μm, Specific surface area: 0.71 m ² / g, manufactured by DOWA Electronics Co., Ltd., AgC-2011 (shape: flake shape, average particle size: 2.6 μm, specific surface area: 2.01 m ² / g, manufactured by Fukuda Metal Foil Powder Industry Co., Ltd. ) And the like.

Further, from the viewpoint of aspect ratio, the content of the silver powder (a) in the silver powder (A) is preferably 50% by mass or more, more preferably 70% by mass or more, and 100% by mass. More preferably.
From the same viewpoint, the content of the silver powder (A) is preferably 50% by mass or more, and more preferably 70% by mass or more of the total mass of the solar cell electrode paste.

[Organic acid silver salt (B)]
The organic acid silver salt (B) is not particularly limited as long as it is a silver salt of an organic acid such as a fatty acid, but a solar cell electrode paste having a high thixo index is obtained, and the volume resistivity after firing becomes low. For this reason, a fatty acid silver salt having one or more hydroxyl groups (—OH), a fatty acid silver salt having 18 or less carbon atoms, and having two or more carboxy silver bases (—COOAg) without hydroxyl groups (—OH). It is preferably at least one selected from the group consisting of silver polycarboxylic acid salts.

  Hereinafter, a fatty acid silver salt having one or more hydroxyl groups, a fatty acid silver salt having 18 or less carbon atoms, and a polycarboxylic acid silver salt having two or more carboxy silver bases without having a hydroxyl group will be described in detail.

(Fatty acid silver salt having one or more hydroxyl groups)
The fatty acid silver salt having one or more hydroxyl groups can be obtained by reacting a fatty acid having one or more hydroxyl groups with silver oxide. Examples of the fatty acid having one or more hydroxyl groups used in this reaction include the following formula: Examples include compounds represented by (1) to (3).

In formula (1), n represents an integer of 0 to 2, R ¹ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and R ² represents an alkylene group having 1 to 6 carbon atoms. When n is 0 or 1, the plurality of R ² may be the same or different from each other. When n is 2, the plurality of R ¹ may be the same or different.
In Formula (2), R ¹ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and a plurality of R ¹ may be the same or different.
In formula (3), R ¹ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and R ³ represents an alkylene group having 1 to 6 carbon atoms. The plurality of R ¹ may be the same or different.

In the above formula (1) to (3), the alkyl group having 1 to 10 carbon atoms of R ^1, a methyl group, an ethyl group, n- propyl group, n- butyl group, n- pentyl group, n- hexyl group , N-heptyl group, n-octyl group, n-nonyl group and n-decyl group. R ¹ is preferably a hydrogen atom, a methyl group, or an ethyl group.
Moreover, in said formula (1), as a C1-C6 alkylene group of R < ² >, a methylene group, ethylene group, propane- 1, 3- diyl group, butane- 1, 4- diyl group, heptane-1 , 5-diyl group and hexane-1,6-diyl group. R ² is preferably a methylene group or an ethylene group. In said formula (1), it is preferable that it is 1 or 2 as an integer of 0-2 of n.
Moreover, in said formula (3), as a C1-C6 alkylene group of R <3>, a methylene group, ethylene group, a propane- 1, ^3- diyl group, butane- 1, 4- diyl group, heptane-1 , 5-diyl group and hexane-1,6-diyl group. R ² is preferably a methylene group or an ethylene group.

  Specific examples of the compound represented by the above formula (1) include 2,2-bis (hydroxymethyl) -n-butyric acid represented by the following formula (1a) and the following formula (1b). 2,2-bis (hydroxymethyl) propionic acid, hydroxypivalic acid represented by the following formula (1c), β-hydroxyisobutyric acid represented by the following formula (1d), and the like. You may use independently and may use 2 or more types together.

  Specific examples of the compound represented by the formula (2) include 2-hydroxy-2-methyl-n-butyric acid represented by the following formula (2a) and the following formula (2b). Examples include 2-hydroxyisobutyric acid, glycolic acid represented by the following formula (2c), DL-2-hydroxybutyric acid represented by the following formula (2d), etc., and these may be used alone. More than one species may be used in combination.

  Specific examples of the compound represented by the above formula (3) include, for example, DL-3-hydroxybutyric acid represented by the following formula (3a) and β-hydroxyvaleric acid represented by the following formula (3b). These may be used, and these may be used alone or in combination of two.

  Among these, the resulting organic acid silver salt (B) is silver 2-hydroxyisobutyrate represented by the following formula (B1), silver glycolate represented by the following formula (B2), and the following formula (B3). The aspect ratio of the electrode formed by using the solar cell electrode paste containing at least one selected from the group consisting of 2,2-bis (hydroxymethyl) -n-silver butyrate represented by For reasons of low resistivity, it is more preferable to use at least one selected from the group consisting of 2-hydroxyisobutyric acid, glycolic acid, and 2,2-bis (hydroxymethyl) -n-butyric acid.

(Fatty acid silver salt having 18 or less carbon atoms)
The fatty acid silver salt having 18 or less carbon atoms can be obtained by reacting a fatty acid having 18 or less carbon atoms with silver oxide. The fatty acid having 18 or less carbon atoms used in this reaction is represented by the following formula (4). The compound which is made is mentioned.

In the formula, R ⁴ represents an alkyl group having 1 to 6 carbon atoms, and R ⁵ represents an alkyl group having 1 to 10 carbon atoms.

In the formula (4), the alkyl group having 1 to 6 carbon atoms R ^4, a methyl group, an ethyl group, n- propyl group, n- butyl group, n- pentyl group, and n- hexyl. R ⁴ is preferably a methyl group or an ethyl group.
Further, in the above formula (4), the alkyl group having 1 to 10 carbon atoms R ^5, in addition to an alkyl group having 1 to 6 carbon atoms of the above R ^4, n- heptyl, n- octyl, n- Nonyl group and n-decyl group are mentioned. R ⁵ is preferably a methyl group, an ethyl group, or an n-propyl group.

Specific examples of the carboxylic acid represented by the above formula (4) include 2-methylpropanoic acid (alias: isobutyric acid), 2-methylbutanoic acid (alias: 2-methylbutyric acid), and 2-methylpentane. Acid, 2-methylheptanoic acid, 2-ethylbutanoic acid; and the like.
Among these, the organic acid silver salt (B) obtained contains 2-methylpropanoate silver represented by the following formula (B4) and / or silver 2-methylbutanoate represented by the following formula (B5) 2-methylpropanoic acid and 2-methylbutanoic acid are preferred because the aspect ratio of the electrode formed using the solar cell electrode paste is superior and the volume resistivity is low.

(Silver polycarboxylic acid silver salt having no hydroxyl group and two or more carboxy silver bases)
The polycarboxylic acid silver salt having two or more carboxy silver bases having no hydroxyl group is a silver salt of a polycarboxylic acid having no hydroxyl group and two or more carboxy groups, and the resulting effect is more excellent. And at least one selected from the group consisting of silver malonate, silver glutarate, silver 1,2,3,4-butanetetracarboxylate, and silver 4-cyclohexene-1,2-dicarboxylate .

  A polycarboxylic acid silver salt having no hydroxyl group and two or more carboxy silver bases can be obtained by reacting silver oxide with a polycarboxylic acid having no hydroxyl group and two or more carboxy groups. Examples of the polycarboxylic acid used in this reaction include a compound represented by the following formula (5).

In the above formula (5), n represents an integer of 2 to 6, and R represents an n-valent saturated aliphatic hydrocarbon group having 1 to 24 carbon atoms and an n-valent unsaturated aliphatic group having 2 to 12 carbon atoms. A hydrocarbon group, an n-valent alicyclic hydrocarbon group having 3 to 12 carbon atoms, or an n-valent aromatic hydrocarbon group having 6 to 12 carbon atoms, where R is m, 2m + 2.
In the above formula (5), the carbons of R to which the carbon of the carboxy group is bonded may be the same or different.

  In said formula (5), as an integer of 2-6 which n shows, it is preferable that it is 2-5, and it is more preferable that it is 2-4.

In said formula (5), as carbon number of 1-24 of the saturated aliphatic hydrocarbon group which R shows, it is preferable that it is 1-12, and it is more preferable that it is 1-6.
Examples of the saturated aliphatic hydrocarbon group represented by R when n is 2 include a methylene group, an ethane-1,1-diyl group, an ethane-1,2-diyl group, a propane-1,3-diyl group, Examples include butane-1,4-diyl group, pentane-1,5-diyl group, hexane-1,6-diyl group, heptane-1,7-diyl group, octane-1,8-diyl group and the like.
Examples of the saturated aliphatic hydrocarbon group represented by R when n is 3 include methane-triyl group, ethane-1,1,2-triyl group, propane-1,1,3-triyl group, and propane-1 2,3-triyl group, butane-1,1,3-triyl group, butane-1,1,4-triyl group, butane-1,2,4-triyl group, and the like.
Examples of the saturated aliphatic hydrocarbon group represented by R when n is 4 include ethane-1,2,2,2-tetrayl group, propane-1,2,2,3-tetrayl group, butane-1, Examples include 2,3,4-tetrayl group.

In said formula (5), as 2-12 carbon number of the unsaturated aliphatic hydrocarbon group which R shows, it is preferable that it is 2-10, and it is more preferable that it is 2-6.
Examples of the unsaturated aliphatic hydrocarbon group represented by R when n is 2 include an ethylene-1,2-diyl group and a propene-1,3-diyl group.
Examples of the unsaturated aliphatic hydrocarbon group represented by R when n is 3 include a propene-1,2,3-triyl group and a propene-1,3,3-triyl group.
Examples of the unsaturated aliphatic hydrocarbon group represented by R when n is 4 include a propene-1,1,2,3-tetrayl group.

In said formula (5), as 3-12 carbon number of the alicyclic hydrocarbon group which R shows, it is preferable that it is 3-10, and it is more preferable that it is 3-6.
Examples of the alicyclic hydrocarbon group represented by R when n is 2 include cyclohexane-1,2-diyl group, cyclohexane-1,4-diyl group, cyclohexene-1,2-diyl group, and cyclohexene-4,5. -A diyl group, 1-methylcyclohexene-4,5-diyl group, etc. are mentioned.
Examples of the alicyclic hydrocarbon group represented by R when n is 3 are cyclopropane-1,2,3-triyl group, cyclopentane-1,1,2-triyl group, cyclohexane-1,2,4- A triyl group etc. are mentioned.
Examples of the alicyclic hydrocarbon group represented by R when n is 4 include cyclobutane-1,2,3,4-tetrayl group, cyclopentane-1,2,3,4-tetrayl group, cyclopentane- Examples include 1,2,4,5-tetrayl group, cyclohexane-1,2,4,5-tetrayl group, 3,6-dimethylcyclohexane-1,2,4,5-tetrayl group.

In said formula (5), as 6-12 carbon number of the aromatic hydrocarbon group which R shows, it is preferable that it is 6-10, and it is more preferable that it is 6-8.
As the aromatic hydrocarbon group represented by R when n is 2, benzene-1,2-diyl group, benzene-1,3-diyl group, benzene-1,4-diyl group, naphthalene-1,4- A diyl group, a naphthalene-2,3-diyl group, a naphthalene-2,6-diyl group, etc. are mentioned.
Examples of the aromatic hydrocarbon group represented by R when n is 3 include a benzene-1,2,4-triyl group and a benzene-1,3,5-triyl group.
Examples of the aromatic hydrocarbon group represented by R when n is 4 include a benzene-1,2,4,5-tetrayl group and a biphenyl-3,3 ′, 4,4′-tetrayl group.

Examples of the compound represented by the above formula (5) include aliphatic polycarboxylic acids, alicyclic polycarboxylic acids, and aromatic polycarboxylic acids.
Specific examples of the aliphatic polycarboxylic acid include saturated dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, and sebacic acid; unsaturated dicarboxylic acids such as fumaric acid, maleic acid, and itaconic acid; And saturated tricarboxylic acids such as tricarballylic acid; unsaturated tricarboxylic acids such as aconitic acid; saturated tetracarboxylic acids such as 1,2,3,4-butanetetracarboxylic acid; and the like.
Specific examples of the alicyclic polycarboxylic acid include unsaturated alicyclic dicarboxylic acids such as 4-cyclohexene-1,2-dicarboxylic acid and 4-methyl-4-cyclohexene-1,2-dicarboxylic acid. Saturated alicyclic tetracarboxylic acids such as 1,2,3,4-cyclopentanetetracarboxylic acid; and the like.
Specific examples of the aromatic polycarboxylic acid include benzenedicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid; tricarboxylic acids such as trimellitic acid and trimesic acid; pyromellitic acid, 3,3 ′, 4 , 4′-biphenyltetracarboxylic acid and the like; 1,4-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid and 2,3-naphthalenedicarboxylic acid and the like; and the like.
These may be used alone or in combination of two or more.

  Of these, aliphatic polycarboxylic acid and alicyclic polycarboxylic acid are preferred because the effect of containing the obtained polycarboxylic acid silver salt is better, and saturated dicarboxylic acid, saturated tetracarboxylic acid, More preferably, it is a saturated alicyclic dicarboxylic acid, selected from the group consisting of malonic acid, glutaric acid, 1,2,3,4-butanetetracarboxylic acid, and 4-cyclohexene-1,2-dicarboxylic acid. More preferably, it is at least one.

On the other hand, silver oxide used in the above reaction is Ag ₂ O.

The organic acid silver salt (B) is selected from the group consisting of the above-mentioned fatty acid having one or more hydroxyl groups, fatty acid having 18 or less carbon atoms, and polycarboxylic acid having no hydroxyl group and two or more carboxy groups. A compound represented by the following formulas (i) to (iii) in a reaction formula shown below, a compound represented by the following formula (iv), and It is preferably at least one selected from the group consisting of compounds represented by the following formula (v).
For example, when the compounds represented by the above formulas (1) to (5) are used, this reaction is not particularly limited as long as the reaction represented by the following reaction formula proceeds, but the above silver oxide A method of proceeding while pulverizing and a method of reacting the fatty acid after pulverizing the silver oxide are preferred. Specifically, as the former method, the above silver oxide and a solution obtained by dissolving the above fatty acid with a solvent are kneaded with a ball mill or the like, and the above solid silver oxide is pulverized at room temperature, The reaction is preferably performed for about 24 hours.

In formula (i), n represents an integer of 0 to 2, R ¹ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and R ² represents an alkylene group having 1 to 6 carbon atoms. When n is 0 or 1, the plurality of R ² may be the same or different from each other. When n is 2, the plurality of R ¹ may be the same or different.
In formula (ii), R ¹ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and a plurality of R ¹ may be the same or different.
In formula (iii), R ¹ represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and R ³ represents an alkylene group having 1 to 6 carbon atoms. The plurality of R ¹ may be the same or different.

In formula (iv), R ⁴ represents an alkyl group having 1 to 6 carbon atoms, and R ⁵ represents an alkyl group having 1 to 10 carbon atoms.

In the formula (v), n represents an integer of 2 to 6, and R represents an n-valent saturated aliphatic hydrocarbon group having 1 to 24 carbon atoms and an n-valent unsaturated aliphatic group having 2 to 12 carbon atoms. A hydrocarbon group, an n-valent alicyclic hydrocarbon group having 3 to 12 carbon atoms, or an n-valent aromatic hydrocarbon group having 6 to 12 carbon atoms, where R is m, 2m + 2.
In the above formula (v), the carbons of R to which the carbon of the carboxy silver base is bonded may be the same or different.

  Specific examples of the solvent for dissolving the fatty acid and the polycarboxylic acid include butyl carbitol, methyl ethyl ketone, isophorone, α-terpineol, and the like. May be used in combination.

  Content of the said organic acid silver salt (B) is 2.5 mass with respect to 100 mass parts of said silver powder (A) from the reason that a high thixo index is obtained and an electrode with low volume resistivity can be formed. It is preferably at least 5 parts by weight, and more preferably 5-10 parts by weight.

  Furthermore, in the present invention, since the thixotropy of the solar cell electrode paste becomes better and an electrode having a higher aspect ratio can be formed, the content of silver oxide is 100 mass of the solvent (C) described later. The amount is preferably 5 parts by mass or less, more preferably 1 part by mass or less, and most preferably an embodiment containing substantially no silver oxide.

[Solvent (C)]
The solvent (C) is not particularly limited as long as it can disperse the silver powder (A) and can dissolve or disperse the organic acid silver salt (B), but it is an organic solvent having a boiling point of 200 ° C. or higher. Preferably there is.
Specific examples of the organic solvent having a boiling point of 200 ° C. or higher include, for example, butyl carbitol, isophorone, α-terpineol, triethylene glycol and the like, and these may be used alone or in combination of two or more. May be.

[Plasticizer (D)]
The plasticizer (D) is not particularly limited, but is excellent in dispersibility of the silver powder (A) and the organic acid silver salt (B), and also serves as a drying measure during coating (printing). A glycol ether plasticizer and / or an ester plasticizer having a boiling point of 250 ° C. or higher is preferable.
Examples of glycol ether plasticizers having a boiling point of 250 ° C. or higher include dibutyl diglycol (DBDG), butyl triglycol (BTG), hexyl diglycol (HeDG), 2 ethylhexyl diglycol (EHDG), and phenyl diglycol (PhDG). , Benzyl glycol (BzG), benzyl diglycol (BzDG) and the like.
Examples of the ester plasticizer include phthalic acid esters, adipic acid esters, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, and the like.

[Glass frit]
The solar cell electrode paste preferably contains glass frit because the adhesion between the electrode to be formed and the silicon substrate becomes better.
As the glass frit, it is preferable to use a glass frit having a softening temperature of 300 ° C. or higher and a baking temperature (heat treatment temperature) or lower. Specific examples of such a glass frit include a borosilicate glass frit having a softening temperature of 300 to 800 ° C.
The shape of the glass frit is not particularly limited, and may be spherical or crushed powder. The average particle diameter (D50) of the spherical glass frit is preferably 0.1 to 20 μm, and more preferably 1 to 3 μm. Furthermore, it is preferable to use a glass frit having a sharp particle size distribution from which particles of 10 μm or more are removed.
The content of the glass frit is preferably 0.1 to 10 parts by mass, and more preferably 1 to 5 parts by mass with respect to 100 parts by mass of the silver powder (A).

[Other additives]
The said solar cell electrode paste may contain additives, such as metal powder other than the said silver powder (A), a reducing agent, as needed.
Specific examples of the metal powder include copper and aluminum. Among them, copper is preferable. Moreover, it is preferable that it is a metal powder with a particle size of 0.01-10 micrometers.
Specific examples of the reducing agent include ethylene glycols.

<Manufacturing method>
The manufacturing method of the said solar cell electrode paste is not specifically limited, The said silver powder (A), the said organic acid silver salt (B), the said solvent (C), and / or the said plasticizer (D) are contained depending on necessity. Examples thereof include a method of mixing glass frit and additives which may be mixed with a roll, a kneader, an extruder, a universal agitator or the like.

<Solar cell>
The solar cell of the present invention is a solar cell comprising a light-receiving surface side surface electrode, a semiconductor substrate and a back surface electrode, and at least the surface electrode is formed using the solar cell electrode paste of the present invention. is there.
Below, the structure of the photovoltaic cell of this invention is demonstrated using FIG.

As shown in FIG. 1, the solar cell 1 includes a surface electrode 4 on the light receiving surface side, a pn junction silicon substrate 7 in which a p layer 5 and an n layer 2 are joined, and a back electrode 6.
Further, as shown in FIG. 1, the solar battery cell 1 is preferably provided with an antireflection film 3 by, for example, etching the wafer surface to form a pyramidal texture in order to reduce the reflectance.

<Front electrode / Back electrode>
If the surface electrode or the back electrode included in the solar battery cell of the present invention is formed using the surface electrode or both of the paste for solar battery electrode of the present invention, the electrode arrangement (pitch), shape, height, The width and the like are not particularly limited.
Here, as shown in FIG. 1, the front surface electrode and the back surface electrode usually have a plurality, but in the present invention, for example, only a part of the plurality of front surface electrodes is for the solar cell electrode of the present invention. It may be formed of a paste, or part of the plurality of front surface electrodes and part of the plurality of back surface electrodes may be formed of the solar cell electrode paste of the present invention.

<Antireflection film>
The antireflection film that the solar battery cell of the present invention may have is a film (film thickness: about 0.05 to 0.1 μm) formed on a portion of the light receiving surface where the surface electrode is not formed. For example, a silicon oxide film, a silicon nitride film, a titanium oxide film, or a laminated film thereof.

<Silicon substrate>
The silicon substrate included in the solar battery cell of the present invention is not particularly limited, and a known silicon substrate (plate thickness: about 100 to 450 μm) for forming a solar battery can be used, and a single crystal or polycrystal Any silicon substrate may be used.

The silicon substrate has a pn junction, which means that a second conductivity type light-receiving surface impurity diffusion region is formed on the surface side of the first conductivity type semiconductor substrate. When the first conductivity type is n-type, the second conductivity type is p-type. When the first conductivity type is p-type, the second conductivity type is n-type.
Here, examples of the impurity imparting p-type include boron and aluminum, and examples of the impurity imparting n-type include phosphorus and arsenic.

  In the solar battery cell of the present invention, at least the surface electrode is formed using the solar battery electrode paste of the present invention, and therefore the aspect ratio of the electrode is increased. As a result, the surface electrode can be thinned, so that a solar cell with a high current collecting effect in which the light shielding area of the light receiving surface is reduced can be obtained.

Although the manufacturing method of the photovoltaic cell of this invention is not specifically limited, The wiring formation process which apply | coats the solar cell electrode paste of this invention on a silicon substrate, and forms wiring, heat-treats the obtained wiring, and an electrode ( And an electrode forming step of forming a front electrode and / or a back electrode).
In addition, when the photovoltaic cell of this invention comprises an antireflection layer, an antireflection film can be formed by well-known methods, such as a plasma CVD method.
Below, a wiring formation process and a heat treatment process are explained in full detail.

<Wiring formation process>
The said wiring formation process is a process of apply | coating the paste for solar cell electrodes of this invention on a silicon base material, and forming wiring.
Here, specific examples of the coating method include inkjet, screen printing, gravure printing, offset printing, letterpress printing, and the like.

<Heat treatment process>
The heat treatment step is a step of obtaining a conductive wiring (electrode) by heat-treating the coating film obtained in the wiring forming step.
By heat-treating the wiring, when the silver decomposed from the organic acid silver salt (B) melts, the silver powder (A) is connected to form an electrode (silver film).

  In the present invention, the heat treatment is not particularly limited, but is preferably a treatment of heating (firing) at a temperature of 400 to 800 ° C. for several seconds to several tens of minutes. When the temperature and time are within this range, even when an antireflection film is formed on the silicon substrate, the electrode can be easily formed by the fire-through method.

  In the present invention, since the wiring obtained in the wiring formation step can form electrodes even by irradiation with ultraviolet rays or infrared rays, the heat treatment step may be performed by irradiation with ultraviolet rays or infrared rays. .

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

(Examples 1-5, Comparative Examples 1-3)
The silver powder shown in the following Table 1 was added to the ball mill so as to have the composition ratio shown in the following Table 1, and these were mixed to prepare a solar cell electrode paste.
The prepared solar cell electrode paste is dipped in an alkaline etching solution and the surface oxide film is removed, on a silicon substrate (single crystal silicon wafer, LS-25TVA, 156 mm × 156 mm × 200 μm, manufactured by Shin-Etsu Chemical Co., Ltd.) by screen printing. The wiring was formed by coating.
Then, the sample of the photovoltaic cell which baked at 800 degreeC for 5 second (s) in oven, and formed the conductive wiring (electrode) was produced.
Further, the prepared paste for solar cell electrode was applied on the silicon substrate by screen printing to form a 25 mm × 25 mm test pattern, and then dried in an oven at 100 ° C. for 2 minutes, and at 800 ° C. for 5 minutes. The sample for volume resistivity measurement and adhesive evaluation was produced by baking for 2 seconds.

<Thixo Index (TI ₁ ), Thixo Index (TI ₂ )>
The viscosity (μ ₅ , μ ₂₀₀ ) of the prepared solar cell electrode paste was measured to determine the thixo index (TI ₁ , TI ₂ ).
As an instrument for measuring the viscosity, a Brookfield viscometer (HBDV-II + Pro, manufactured by BrookField) and a cylindrical spindle (HB type # 7) were used.
First, 65 g of the prepared solar cell electrode paste was filled in an M-70 bottle (manufactured by Yoyo Glass Co., Ltd.), capped, and placed at 25 ° C. for 1 hour to evacuate the air. Then, the spindle set in the viscometer was submerged in the center position of the solar cell electrode paste filled in the M-70 bottle and rotated to measure the viscosity.
At this time, the spindle was rotated for 900 seconds while changing the rotation speed. More specifically, first, 1 to 300 seconds were rotated at a rotation speed of 5 rpm, 301 to 600 seconds were rotated at a rotation speed of 200 rpm, and 601 to 900 seconds were returned again to the rotation speed of 5 rpm and rotated.
Then, the viscosity at ₅ rpm measured at 300 seconds (μ ₅ ) was divided by the viscosity at 200 rpm measured at 301 seconds (μ ₂₀₀ ) to obtain a thixo index (TI ₁ ). Further, the viscosity (μ ₅ ) measured at 601 seconds and the rotational speed of ₅ rpm was divided by the viscosity (μ ₂₀₀ ) measured at 600 seconds and the thixo index (TI ₂ ) was obtained. The results are shown in Table 1 below.

<Aspect ratio>
About the sample of each produced photovoltaic cell, the electrode was observed with the laser microscope, height and width were measured, and the aspect ratio (height / width) was calculated | required. The results are shown in Table 1 below.

<Adhesiveness>
Whether the cellophane adhesive tape (width 18 mm) is completely attached to the surface of the prepared sample for evaluation of adhesiveness, and then immediately hold one end of the tape at a right angle and pull it off instantaneously to determine whether the electrode (print pattern) is peeled off. I investigated.
If the electrode was confirmed to be peeled off, it was evaluated as “△” as having a practically inferior but slightly inferior adhesive property. ". The results are shown in Table 1 below.

<Volume resistivity>
About the produced sample for volume resistivity measurement, the volume resistivity of the electrode was measured by a 4-terminal 4-probe method using a resistivity meter (Lorestar GP, manufactured by Mitsubishi Chemical Corporation). The results are shown in Table 1 below.

The following components were used as the components in Table 1 (units are parts by mass).
Silver powder 1: AgC-2011 (shape: flake shape, average particle size: 2.6 μm, specific surface area: 2.01 m ² / g, manufactured by Fukuda Metal Foil Powder Industry Co., Ltd.)
Silver powder 2: Ag4-8F (shape: spherical, average particle size: 1.9 μm, specific surface area: 0.43 m ² / g, manufactured by DOWA Electronics)
Silver powder 3: FA-5-1 (shape: spherical, average particle size: 5.0 μm, specific surface area: 0.73 m ² / g, manufactured by DOWA Electronics)

Organic acid silver salt 1: First, 50 g of silver oxide (manufactured by Toyo Chemical Co., Ltd., the same shall apply hereinafter), 38 g of silver 2-methylpropanoate (manufactured by Kanto Chemical Co., Ltd.), and 300 g of methyl ethyl ketone (hereinafter referred to as “MEK”), It was made to react by throwing into a ball mill and stirring for 24 hours at room temperature. Subsequently, MEK was removed by suction filtration, and the obtained powder was dried to obtain white silver 2-methylpropanoate (silver isobutyrate).
Organic acid silver salt 2: First, 50 g of silver oxide, 45 g of 2-hydroxyisobutyric acid (manufactured by Tokyo Chemical Industry Co., Ltd.) and 300 g of MEK were put into a ball mill and reacted by stirring at room temperature for 24 hours. Subsequently, MEK was removed by suction filtration, and the obtained powder was dried to obtain white silver 2-hydroxyisobutyrate.
Organic acid silver salt 3: First, 50 g of silver oxide, 57 g of glutaric acid (manufactured by Tokyo Chemical Industry Co., Ltd.) and 300 g of MEK were put into a ball mill and reacted by stirring at room temperature for 24 hours. Subsequently, MEK was removed by suction filtration, and the obtained powder was dried to obtain white silver glutarate.

Solvent: α-terpineol Plasticizer 1: 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (manufactured by Chisso Corporation)
-Plasticizer 2: DBDG (manufactured by Nippon Emulsifier Co., Ltd.)
・ Glass frit: lead borosilicate glass powder

From the results shown in Table 1, it was found that Examples 1 to 5 satisfying “TI ₁ <TI ₂ ” and “8 ≦ TI ₁ ≦ 16” have high aspect ratios.
Looking at Examples 1 to 5 in more detail, it was found that Example 1 using only silver powder 1 had a higher aspect ratio than Example 2 using silver powder 1 and silver powder 2 in combination. .
In addition, Example 3 using the organic acid silver salt 2 and Example 4 using the organic acid silver salt 3 may have a lower volume resistivity than Example 1 using the organic acid silver salt 1. I understood.
Moreover, it turned out that Examples 1-4 using a glass frit are excellent by the adhesiveness with respect to a silicon substrate compared with Example 5 which does not contain a glass frit.

On the other hand, it was found that all of Comparative Examples 1 to 3 had a low aspect ratio. In particular, it was found that the comparative example 3 has a lower aspect ratio than the comparative examples 1 and ₂ that do not satisfy “TI ₁ <TI ₂ ”.

DESCRIPTION OF SYMBOLS 1 Solar cell 2 N layer 3 Antireflection film 4 Surface electrode 5 P layer 6 Back electrode 7 Silicon substrate

Claims (8)

Viscosity (μ ₅ ) measured by rotating HB type # 7 spindle at 5 rpm using a Brookfield viscometer at 25 ° C., and rotating the spindle at 200 rpm using the viscometer at 25 ° C. in the case of dividing to obtain exponent (μ ₅ / μ ₂₀₀₎ and thixotropy index (TI) in viscosity as measured by (mu _200),
A thixo index (TI ₁ ) when the rotational speed of the spindle is changed from 5 rpm to 200 rpm;
The solar cell electrode paste, wherein the thixo index (TI ₂ ) when the rotation speed of the spindle is changed from 200 rpm to 5 rpm satisfies the following formulas (I) and (II).
TI ₁ <TI ₂ (I)
8 ≦ TI ₁ ≦ 16 (II) Silver powder (A), organic acid silver salt (B), solvent (C) and / or plasticizer (D),
The silver powder (A) includes a silver powder (a) having a specific surface area of 1.5 to 3.0 m ² / g and an average particle diameter of 3 μm or less,
The solar cell electrode paste according to claim 1, wherein the content of the organic acid silver salt (B) is 2.5 parts by mass or more with respect to 100 parts by mass of the silver powder (A).   The paste for solar cell electrodes according to claim 2, wherein the content of the silver powder (a) in the silver powder (A) is 50% by mass or more.   The organic acid silver salt (B) is a fatty acid silver salt having one or more hydroxyl groups (—OH), a fatty acid silver salt having 18 or less carbon atoms, and a carboxy silver base (—COOAg having no hydroxyl group (—OH). The solar cell electrode paste according to claim 2, which is at least one selected from the group consisting of silver carboxylates having two or more).   The solar cell electrode paste according to any one of claims 2 to 4, wherein the solvent (C) is an organic solvent having a boiling point of 200 ° C or higher.   The paste for solar cell electrodes according to any one of claims 2 to 5, wherein the plasticizer (D) is a glycol ether plasticizer and / or an ester plasticizer having a boiling point of 250 ° C or higher.   Furthermore, the paste for solar cell electrodes in any one of Claims 2-6 containing a glass frit. It comprises a surface electrode on the light receiving surface side, a semiconductor substrate and a back electrode
The solar cell in which the said surface electrode is formed using the paste for solar cell electrodes in any one of Claims 1-7 at least.