JP5630111B2 - Solar cell electrode paste and solar cell - Google Patents

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

In the resin-based paste materials described in Patent Documents 1 to 4, a resin binder and a vehicle play a role of improving printability.
However, when an electrode is formed using such a resin-based paste material, depending on the firing temperature, the resin in the resin binder or vehicle may remain and the volume resistivity (specific resistance) of the electrode itself may increase. Further, the ratio of the height and width of the cross section of the electrode (height / width) (hereinafter referred to as “aspect ratio”) was small, and it became clear that the adhesion between the electrode and the silicon substrate was poor.

  Therefore, the present invention can form an electrode that has a low volume resistivity and a high aspect ratio while maintaining good printability, and also has excellent adhesion to a silicon substrate. It aims at providing the paste for solar cell electrodes, and a photovoltaic cell using the same.

The present inventor can reduce the volume resistivity and increase the aspect ratio of the solar cell electrode paste containing silver powder and a predetermined fatty acid silver salt while maintaining good printability. The inventors have found that an electrode having excellent adhesion to a silicon substrate can be formed, and completed the present invention.
That is, the present invention provides the following (1) to (7).

(1) A solar cell electrode paste that is applied to a silicon substrate and then fired at 350 to 800 ° C., and contains silver powder (A), a fatty acid silver salt (B), and a solvent (C). The fatty acid silver salt (B) is a compound having one carboxy silver base (—COOAg) and one or two hydroxyl groups (—OH), and containing substantially no silver oxide. Solar cell electrode paste.

  (2) The paste for solar cell electrodes according to (1), wherein the fatty acid silver salt (B) is a compound represented by any one of the following formulas (I) to (III).

(In formula (I), n represents 1 or 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. In some cases, the plurality of R ² may be the same or different, and 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. )

  (3) The solar cell electrode according to (1) or (2), wherein the fatty acid silver salt (B) has the hydroxyl group at the α-position and / or β-position carbon atom relative to the carboxy silver base. For paste.

  (4) The solar cell electrode paste according to any one of (1) to (3), further comprising a fatty acid silver salt having 8 or more carbon atoms.

  (5) The solar cell electrode paste according to any one of (1) to (4), further comprising glass frit.

  (6) The sun according to any one of (1) to (5), wherein the content of the fatty acid silver salt (B) is 1 to 100 parts by mass with respect to 100 parts by mass of the silver powder (A). Battery electrode paste.

  (7) A light-receiving surface-side surface electrode, a semiconductor substrate, and a back electrode are provided, and the surface electrode and / or the back electrode is the solar cell electrode paste according to any one of (1) to (6). A solar battery cell formed by using.

  According to the present invention, while maintaining good printability, not only high temperature (about 700 to 800 ° C.) firing but also medium temperature (about 350 to 450 ° C.) firing, the volume resistivity is low, and It is possible to provide a solar battery electrode paste capable of increasing the aspect ratio and forming an electrode excellent in adhesion to a silicon substrate and a solar battery cell using the same.

FIG. 1 is a cross-sectional view showing an example of a preferred embodiment of a solar battery cell. FIG. 2 is a photograph of the silver powder (AgC-103, manufactured by Fukuda Metal Foil Co., Ltd.) used in the examples taken with a scanning electron microscope (SEM). FIG. 3 is a photograph of silver powder (AgC-2011, manufactured by Fukuda Metal Foil Co., Ltd.) taken with a scanning electron microscope (SEM).

The solar cell electrode paste of the present invention contains silver powder (A), a fatty acid silver salt (B), and a solvent (C), and the fatty acid silver salt (B) contains carboxy silver base (—COOAg). A solar cell that has one and has one or two hydroxyl groups (—OH), and the silver oxide content is 10 parts by mass or less with respect to 100 parts by mass of the solvent (C). This is an electrode paste.
Hereinafter, each component contained in the solar cell electrode paste of the present invention will be described.

<Silver powder (A)>
The silver powder (A) used in the solar cell electrode paste of the present invention is not particularly limited, and those blended with conventionally known conductive pastes can be used.
The silver powder (A) is preferably a spherical silver powder having an average particle diameter of 0.5 to 10 μm, because the printability becomes better and an electrode having a smaller volume resistivity can be formed. .
Here, the term “spherical” refers to the shape of a particle having a major axis / minor axis ratio of 2 or less.
Moreover, an average particle diameter means the average value of the particle diameter of spherical 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 spherical silver powder is elliptical, the particle diameter that is the basis for calculating the average value is the average value obtained by dividing the total value of the major axis and the minor axis by 2, and is a regular circle Refers to its diameter.
For example, what is shown in the photograph (FIG. 2) of silver powder (AgC-103, manufactured by Fukuda Metal Foil Co., Ltd.) used in Examples described later corresponds to spherical silver powder, but silver powder (AgC-2011, Fukuda Metal Foil). The photo shown in FIG. 3 (manufactured by Kogyo Co., Ltd.) does not correspond to the spherical silver powder, but corresponds to the flake (scale) -like silver powder.

  The average particle diameter of the silver powder (A) is preferably 0.7 to 5 μm because the printability is further improved, and is 1 to 3 μm because the sintering speed is appropriate and the workability is excellent. Is more preferable.

  Content of the said silver powder (A) becomes 300-700 mass with respect to 100 mass parts of said solvent (C) mentioned later from the reason that printing property becomes more favorable and an electrode with a smaller specific resistance can be formed. Part is preferable, and 400 to 600 parts by mass is more preferable.

  Commercially available products can be used as the silver powder (A), and specific examples thereof include AgC-103 (shape: spherical, average particle size: 1.5 μm, manufactured by Fukuda Metal Foil Co., Ltd.), AG4-8F (shape) : Spherical, average particle diameter: 2.2 μm, manufactured by DOWA Electronics), AG2-1C (shape: spherical, average particle diameter: 1.0 μm, manufactured by DOWA Electronics), AG3-11F (shape: spherical, average particle diameter) : 1.4 μm, manufactured by DOWA Electronics), EHD (shape: spherical, average particle size: 0.5 μm, manufactured by Mitsui Kinzoku Co., Ltd.), AgC-2011 (shape: flake shape, average particle size: 2-10 μm, Fukuda Metal) Foil Co., Ltd.).

  Furthermore, in this invention, spherical silver powder and flaky silver powder can be used together as said silver powder (A). The content when flaky silver powder is used in combination is preferably 50% by mass or less of the total mass of the silver powder (A).

<Fatty acid silver salt (B)>
The fatty acid silver salt (B) contained in the solar cell electrode paste of the present invention is a compound having one carboxy silver base (—COOAg) and one or two hydroxyl groups (—OH). .
The fatty acid silver salt (B) has the hydroxyl group at the α-position and / or β-position carbon atom with respect to the carboxy silver base because the silver salt is readily reduced and silver particles are easily generated. It is preferably a fatty acid silver salt, and since the temperature at which the reduction starts is low and the time at which the reduction ends is short, the silver 2-hydroxyisobutyrate represented by the following formula (B1) And at least one selected from the group consisting of silver glycolate represented by the following formula (B2) and silver 2,2-bis (hydroxymethyl) -n-butyrate represented by the following formula (B3) Is more preferable.

  The fatty acid silver salt (B) can be obtained by reacting a silver oxide with a fatty acid having one carboxy group and one or two hydroxyl groups. The fatty acid used in the reaction is not particularly limited as long as it has one carboxy group and one or two hydroxyl groups, and is represented by, for example, the following formulas (1) to (3). Compounds.

In formula (1), n represents 1 or 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 1, the plurality of R ² may be the same or different. 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.
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, as the fatty acid silver salt (B), a fatty acid silver salt having a hydroxyl group at a carbon atom at the α-position and / or β-position with respect to the carboxy silver base can be obtained. It is preferable to use a fatty acid having a hydroxyl group at a carbon atom at the β-position.

  In particular, the fatty acid silver salt (B) is represented by the silver 2-hydroxyisobutyrate represented by the above formula (B1), the silver glycolate represented by the above formula (B2), and the above formula (B3). Because at least one selected from the group consisting of 2,2-bis (hydroxymethyl) -n-silver butyrate can be obtained, 2-hydroxyisobutyric acid, glycolic acid, and 2,2-bis (hydroxymethyl)- It is more preferable to use at least one selected from the group consisting of n-butyric acid.

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

The fatty acid silver salt (B) is obtained by reacting a fatty acid having one or more hydroxyl groups described above with silver oxide, and is represented by the following formulas (I) to (III) in the reaction formulas shown below. Is preferred.
For example, when the compounds represented by the above formulas (1) to (3) are used, this reaction is not particularly limited as long as the reaction represented by the following reaction formula proceeds. 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 1 or 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 1, the plurality of R ² may be the same or different. 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.

  Specific examples of the solvent for dissolving the fatty acid include butyl carbitol, methyl ethyl ketone, isophorone, α-terpineol, and the like. These may be used alone or in combination of two or more. Good.

  The content of the fatty acid silver salt (B) is preferably 1 to 100 parts by mass and 2 to 80 parts by mass with respect to 100 parts by mass of the silver powder (A) because the volume resistivity can be further reduced. More preferably, it is a part.

In the present invention, for the solar cell electrode, the silver powder (A) and the fatty acid silver salt (B) are used, and the content of silver oxide is 10 parts by mass or less with respect to 100 parts by mass of the solvent (C) described later. By using paste, while maintaining good printability, not only high temperature (about 700 to 800 ° C.) firing but also medium temperature (about 350 to 450 ° C.) firing, the volume resistivity is low, and An aspect ratio can be increased, and an electrode having excellent adhesion to a silicon substrate can be formed.
This is because when the silver decomposed from the fatty acid silver salt (B) is melted by heat treatment, the silver powder (A) is connected to express high conductivity, and the fatty acid silver salt (B) has a hydroxyl group. Due to the presence, decomposition (reduction) into silver by heat treatment is greatly promoted, so that it is considered that the volume resistivity can be lowered not only at high temperature baking but also at medium temperature baking.
Moreover, since the said fatty acid silver salt (B) provides moderate thixotropy to the paste for solar cell electrodes, and can suppress the spread of the coating surface while ensuring fluidity during printing, the printability is improved, and the aspect ratio It is thought that can be raised.
And since the silver decomposed | disassembled from the said fatty acid silver salt (B) by heat processing melt | dissolves moderately to a silicon substrate, it is thought that adhesiveness with a silicon substrate improves.

  Furthermore, in the present invention, since the thixotropy of the solar cell electrode paste becomes better and the aspect ratio can be further increased, the content of silver oxide is based on 100 parts by mass of the solvent (C) described later. It 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.

<Fatty acid silver salt with 8 or more carbon atoms>
The solar cell electrode paste of the present invention preferably further contains a fatty acid silver salt having 8 or more carbon atoms because the aspect ratio can be further increased.
The fatty acid silver salt having 8 or more carbon atoms is represented by the following formula (4), and may be linear or branched and may have an unsaturated bond.
The number of carbon atoms of the fatty acid silver salt having 8 or more carbon atoms is preferably 8 to 22 and more preferably 10 to 18 from the viewpoint of superior thixotropy.

R- (COOAg) _n (4)
In the above formula (4), R is an aliphatic hydrocarbon group having 2 or more carbon atoms, n is an integer of 1 or more, and the sum of the carbon number of R and n is 8 or more. n is preferably an integer of 1 to 4.
The aliphatic hydrocarbon group as R has 2 or more carbon atoms, preferably 4 or more, more preferably 4 to 21, and more preferably 6 to 21.

  Examples of the fatty acid silver salt having 8 or more carbon atoms include monocarboxylic acid-containing saturated aliphatic hydrocarbon compounds such as silver caprylate, silver caprate, silver laurate, silver myristate, silver palmitate, and silver stearate. Silver salt; Silver salt of polyvalent carboxylic acid-containing saturated aliphatic hydrocarbon compound such as silver salt of compound represented by the following formula (5); silver oleate, silver linoleate, silver linolenate, silver arachidonic acid, Unsaturated fatty acid silver salts such as silver icosapentaenoate and silver docosahexaenoate, etc. may be mentioned, and these may be used alone or in combination of two or more.

  Of these, silver salts of monocarboxylic acid-containing saturated aliphatic hydrocarbon compounds and silver salts of polyvalent carboxylic acid-containing saturated aliphatic hydrocarbon compounds are preferred from the viewpoint of superior thixotropy. Silver caprate, silver laurate, myristine Acid silver, silver palmitate, silver stearate, and the silver salt of the compound represented by the above formula (5) are more preferred.

The method for producing the fatty acid silver salt having 8 or more carbon atoms is not particularly limited, and the same method as that for the fatty acid silver salt (B) can be used.
The content of the fatty acid silver salt having 8 or more carbon atoms is preferably 1 to 50 parts by mass or less with respect to 100 parts by mass of the silver powder (A) because the aspect ratio can be further increased. More preferably, it is 25 parts by mass.

<Solvent (C)>
The said solvent (C) will not be specifically limited if the paste for solar cell electrodes of this invention can be apply | coated on a board | substrate.
Specific examples of the solvent (C) include butyl carbitol, methyl ethyl ketone, isophorone, α-terpineol, and the like. These may be used alone or in combination of two or more.

<Glass frit>
The solar cell electrode paste of the present invention 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 solar cell electrode paste of the present invention may contain additives such as a metal powder other than the above-described silver powder (A) and a reducing agent, if necessary.
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 method for producing the solar cell electrode paste of the present invention is not particularly limited, and the silver powder (A), the fatty acid silver salt (B), the solvent (C), and a glass frit and an additive that may be optionally contained. Can be mixed by a roll, a kneader, an extruder, a universal agitator or the like.

<Solar cell>
The solar cell of the present invention comprises a light-receiving surface-side surface electrode, a semiconductor substrate, and a back electrode, and the surface electrode and / or the back electrode is formed using the solar cell electrode paste of the present invention described above. It is a solar battery cell.
Here, since the solar cell electrode of the present invention described above can be applied to the formation of the back electrode of the all back electrode type (so-called back contact type) solar cell, The present invention can also be applied to an electrode type solar cell.
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 either one or both of the front electrode and the back electrode provided in the solar battery cell of the present invention are formed using the solar cell electrode paste of the present invention, the arrangement (pitch), shape, and height of the electrodes The width and the like are not particularly limited. In addition, although the height of an electrode is normally designed by several to several dozen micrometer, the aspect ratio of the electrode formed using the solar cell electrode paste of this invention will be 0.4 or more.
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 cell of the present invention, since the front electrode and / or the back electrode is formed using the solar cell electrode paste of the present invention, the electrode has an aspect ratio of 0.4 or more, and the electromotive force generated by light reception Can be efficiently extracted as a current.

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 fatty 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 350 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 addition, in the present invention, since the solar cell electrode paste of the present invention is used, not only high temperature (about 700 to 800 ° C.) but also medium temperature (about 350 to 450 ° C.), good heat treatment ( Firing).

  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. .

  Hereinafter, the present invention will be described in detail with reference to examples. However, the present invention is not limited to these.

(Examples 1-8, 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 was applied 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 to form a wiring.
Then, the sample of the photovoltaic cell which baked in 700 degreeC for 10 minutes or 450 degreeC for 10 minutes, and formed the conductive wiring (electrode) was produced.

<Volume resistivity (specific resistance)>
About the sample of each produced photovoltaic cell, 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.

<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.

<Adhesion>
On the surface of each solar cell sample produced, 100 1 mm bases (10 × 10) are made, and a cellophane adhesive tape (width 18 mm) is completely attached on the bases, and one end of the tape is immediately sampled. The number of electrodes (printed patterns) remaining without being completely peeled off from the silicon substrate was checked while being kept at a right angle to the surface. The results are shown in Table 1.

<Photoelectric conversion efficiency>
For each of the solar cell samples produced, a solar simulator was used as a light source, and AM1.5 simulated sunlight was irradiated from the photoelectrode side with a light intensity of 100 mW / cm ² , and a current-voltage measuring device (Digital by Keithley Instruments Inc.) Conversion efficiency was determined using a source meter 2400). The results are shown in Table 1 below.

<Printability>
The wiring in the wet state (before firing) after screen printing was observed with an optical microscope for the presence or absence of disconnection, the presence or absence of bleeding, the presence or absence of meandering of the lines, and the presence or absence of screen mesh marks. Samples that did not show any disconnection, blemishes, line meandering, or mesh marks were evaluated as “Good” as having good printability, and any of disconnection, blemishes, line meandering, or mesh marks were confirmed. The sample was evaluated as “x” as having poor printability. The results are shown in Table 1 below.

The following components were used for each component in Table 1 (units are parts by mass).
Silver powder 1: AgC-103 (shape: spherical, average particle size: 1.5 μm, manufactured by Fukuda Metal Foil Co., Ltd.)
Silver powder 2: Ag-4-8F (shape: spherical, average particle size: 2.2 μm, manufactured by DOWA Electronics)
Silver powder 3: Ag-2-1C (shape: spherical, average particle size: 1.0 μm, manufactured by DOWA Electronics)
Silver powder 4: AgC-2011 (shape: flake shape, average particle size: 2 to 10 μm, manufactured by Fukuda Metal Foil Co., Ltd.)

  Silver 2-hydroxyisobutyrate: First, 50 g of silver oxide (manufactured by Toyo Chemical Co., Ltd., the same shall apply hereinafter), 45 g of 2-hydroxyisobutyric acid (manufactured by Tokyo Chemical Industry Co., Ltd.), and 300 g of methyl ethyl ketone (hereinafter referred to as “MEK”) The mixture was 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.

  Silver glycolate: First, 50 g of silver oxide, 32.8 g of glycolic acid (manufactured by Tokyo Chemical Industry Co., Ltd.), and 300 g of MEK were charged 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 glycolate.

  Silver 2,2-bis (hydroxymethyl) -n-butyrate: First, 50 g of silver oxide, 57 g of 2,2-bis (hydroxymethyl) -n-butyric acid (manufactured by Tokyo Chemical Industry Co., Ltd.), and 300 g of MEK were ball milled. 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 2,2-bis (hydroxymethyl) -n-butyrate silver.

  Silver pivalate: First, 50 g of silver oxide, 44.1 g of pivalic acid (manufactured by Kanto Chemical Co., Inc.) 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 pivalate (represented by the following formula (X)).

  Silver stearate: First, 50 g of silver oxide, 122.7 g of stearic acid (manufactured by Kanto Chemical 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 stearate.

Solvent: α-Terpinel Glass frit: Borosilicate lead glass powder Silver paste: Resin silver paste (DWP-025, manufactured by Toyobo Co., Ltd.)

From the results shown in Table 1, Comparative Example 1 prepared with a conventionally known resin-based silver paste has high volume resistivity, low aspect ratio, and poor adhesion in both high temperature baking and medium temperature baking. I understood that.
Moreover, it was found that Comparative Example 2 using silver pivalate has a high volume resistivity in both high temperature baking and medium temperature baking.
Moreover, it turned out that the comparative example 3 which used silver oxide over 10 mass parts with respect to 100 mass parts of solvent becomes low in aspect ratio in both high temperature baking and medium temperature baking.

On the other hand, it turned out that Examples 1-8 are excellent in printability and have low volume resistivity, high aspect ratio, and excellent adhesion not only in high temperature baking but also in medium temperature baking.
Moreover, it turned out that Examples 1-8 show sufficient photoelectric conversion efficiency for a solar cell electrode use.

  Furthermore, by comparing Example 2 and Comparative Example 2, high-temperature baking is performed by using a fatty acid silver salt having one carboxy silver base and one or two hydroxyl groups as the fatty acid silver salt (B). It was found that the volume resistivity was low in both the medium temperature firing and the medium temperature firing.

In addition, when Examples 2, 4 and 5 are compared, if a spherical silver powder having an average particle diameter of 0.5 to 10 μm is used as the silver powder (A), the printability is excellent regardless of the average particle diameter. I understood.
Moreover, when Examples 1-5 are compared with Examples 6 and 7, if a spherical silver powder having an average particle diameter of 0.5 to 10 μm is used as the silver powder (A), silver powders having different average particle diameters ( It was found that even when A) was used in combination, the printability was excellent.
Moreover, when comparing Examples 1 to 3 in which only the content of silver 2-hydroxyisobutyrate is different, Example 3 has a lower volume resistivity in high-temperature baking than in Examples 1 and 2, and high-temperature baking and medium temperature. It was found that the aspect ratio was higher than those of Examples 1 and 2 in both firings.
Further, when Example 6 and Example 7 are compared, Example 6 containing silver stearate, which is a fatty acid silver salt having 8 or more carbon atoms, has a higher aspect ratio than Example 7 not containing this. I understood that.
Further, when Example 2 and Example 8 are compared, Example 2 using only silver powder 1 has a higher aspect ratio than Example 8 using silver powder 1 and silver powder 4 in combination. I understood that.

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 (7)

A solar cell electrode paste that is fired at 350 to 800 ° C. after being applied on a silicon substrate,
Silver powder (A), fatty acid silver salt (B), and solvent (C),
The fatty acid silver salt (B) is a compound having one carboxy silver base (—COOAg) and one or two hydroxyl groups (—OH),
A paste for solar cell electrode , substantially free of silver oxide . The paste for solar cell electrodes according to claim 1, wherein the fatty acid silver salt (B) is a compound represented by any one of the following formulas (I) to (III).

(In formula (I), n represents 1 or 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. In some cases, the plurality of R ² may be the same or different, and 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. )   The paste for solar cell electrodes according to claim 1 or 2, wherein the fatty acid silver salt (B) has the hydroxyl group at the α-position and / or β-position carbon atom with respect to the carboxy silver base.   Furthermore, the solar cell electrode paste in any one of Claims 1-3 containing a C8 or more fatty acid silver salt.   Furthermore, the solar cell electrode paste in any one of Claims 1-4 containing a glass frit.   The paste for solar cell electrodes according to any one of claims 1 to 5, wherein a content of the fatty acid silver salt (B) is 1 to 100 parts by mass with respect to 100 parts by mass of the silver powder (A). 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 and / or the said back surface electrode are formed using the paste for solar cell electrodes in any one of Claims 1-6.