JP2010087251A - Conductive paste for solar battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide conductive paste for a solar battery which can stably obtain high conversion efficiency even if unleaded glass frit is contained in an electrode which is brought into ohmic contact with an n-type semiconductor (an n-type diffusion layer) of a crystal silicon solar battery, a light condensing electrode for the solar battery which is constituted by printing the conductive paste and sintering it, and the solar battery including the light condensing electrode. <P>SOLUTION: The conductive paste for the solar battery includes binder resin, a solvent, glass frit, and conductive powder having silver or a silver compound as a principal component. The conductive paste for the solar battery includes an Si-based organic compound. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

 The present invention relates to a conductive paste for a solar cell, and more specifically, a conductive paste for a solar cell used for forming a condensing electrode for a solar cell, and a solar cell formed by firing the conductive paste for a solar cell. It is related with a solar cell provided with a condensing electrode and the said condensing electrode. Alternatively, in particular, a conductive paste for solar cell suitable for forming an ohmic light-receiving surface electrode (condensing electrode) on an n-type semiconductor layer of a crystalline silicon solar cell, and firing the conductive paste for solar cell are fired. It is related with the condensing electrode of the solar cell which consists of, and the solar cell provided with the said condensing electrode.

In general, as shown in FIG. 1, for example, a crystalline Si solar cell includes a p-type Si substrate 1, a condensing electrode 2 on a light receiving surface, an antireflection film 3, and a back electrode 4 made of aluminum.
As an example of the manufacturing method, first, phosphorus (P) is thermally diffused on one surface of the p-type Si substrate 1b to form the n-type diffusion layer 1a on the entire surface of one surface of the p-type Si substrate 1b. Thus, the p-type Si substrate 1 is produced. The sheet resistance of the n-type diffusion layer 1a is about several tens of Ω / □, and the thickness is about 0.3 to 0.5 μm. Next, etching is performed to clean the surface of the n-type diffusion layer 1a of the p-type Si substrate 1. Next, as an insulating film (antireflection film 3), a thin film such as a silicon nitride film, titanium oxide, or silicon oxide is formed on the n-type diffusion layer 1a in this order by a plasma CVD method or the like.
Next, a silver paste for forming a condensing electrode 2 for forming the condensing electrode 2 on the antireflection film 3 to be the light receiving surface, and a back electrode in a grid shape or a predetermined pattern on the back surface of the p-type Si substrate 1. The aluminum paste to be formed and the silver paste for forming the back electrode are each screen-printed and dried. Thereafter, baking is performed in a near-infrared furnace at 700 ° C. to 900 ° C. for several minutes to several tens of minutes. As a result, on the back side of the p-type Si substrate 1, aluminum diffuses into the p-type Si substrate 1, and a P + layer (Back Surface Field layer) containing high-concentration aluminum impurities is formed. Also, the fired aluminum paste and the back electrode forming silver paste become the back electrode 4 made of aluminum and the back electrode (not shown) made of silver, respectively, and the boundary between these two back electrodes becomes an alloy state and becomes electrically Connected to. Since the back electrode 4 formed of the aluminum paste in this way cannot be soldered, a silver paste is used together with the aluminum paste to form the back electrode for interconnection of solar cell elements by copper foil or the like. .

  On the other hand, the silver paste for the surface electrode (for condensing electrode formation) is printed on the antireflection film 3 (insulating film) in a predetermined pattern, and the silicon nitride film which is an insulating film is melted and penetrated by baking to form an n-type A method (fire through) of forming an ohmic electrode on the diffusion layer 1a is widely used. Usually, as silver paste used for such applications, silver paste, glass frit, binder, solvent and various additives are mixed and dispersed. After printing on the mold Si substrate 1, it is baked at a high speed using a near-infrared furnace to form a thick film collecting electrode 2. In general, a glass frit mixed with a conductive paste such as this silver paste has high conversion efficiency, and thus a Pb-containing glass frit is generally used.

By the way, in recent years, awareness of environmental issues has increased, and solder that joins copper foil to lead terminals has also moved to lead-free solder that does not contain lead, and glass frit is lead-free, even though its content is very small. In fact, it is necessary to switch to glass frit.
In order to obtain good wettability with respect to lead-free solder as a conductive paste for use in a highly safe solar cell that does not affect the global environment, a technique containing Bi ₂ O ₃ in the conductive paste is disclosed. (For example, refer to Patent Document 1).
Also, using a silver paste containing lead-free glass frit, without covering the electrode surface with solder or the like, ensuring an adhesive strength with the substrate for solar cell elements, and having an excellent long-term reliability, A technique for manufacturing with high productivity is disclosed (for example, see Patent Document 2).
The above technique is a technique aiming at the strength and long-term reliability of solder bonding to an electrode formed of a conductive paste containing a lead-free glass frit as the material becomes lead-free. Although these conductive pastes using lead-free glass frit have been achieved for the first time with the goal of lead-free, they are not necessarily aimed at improving the conversion efficiency of solar cells. The same good conversion efficiency as in the case of using a frit was not obtained.

On the other hand, a conductive paste that is screen-printed on an antireflection film formed on an n-type diffusion layer and fired at a high speed and used for forming a condensing electrode has been devised for various paste compositions. The influence of the electrodes on the conversion efficiency and the stability of the battery characteristics of the crystalline silicon solar cell is large, and in particular, the influence of the condensing electrode is very large.
As a measure of electrode performance, there is a solar cell fill factor (FF). When the series resistance of the solar cell is high, FF tends to be small, but one of the factors affecting the series resistance is the contact resistance with the electrodes of the p-type semiconductor and the n-type semiconductor. For this reason, it has been proposed so far to add various additives to the conductive paste for solar cell electrodes in order to reduce the contact resistance and obtain high conversion efficiency and stable characteristics of the solar cell. For example, a technology for containing a Sn compound in a conductive paste (see, for example, Patent Document 3), a metal selected from Y, In and Mo or a compound thereof in the conductive paste, and particles having a size of 0.01 to 0.1 μm A technique (for example, see Patent Document 4) of adding a shaped additive is disclosed.
However, in the conductive pastes described in these prior documents, the glass frit to be added is leaded and the global environment is not considered. As described above, the glass frit does not contain lead and is baked to form an electrode for a solar cell. As a result, the electrode electrode conductivity of the solar cell achieves a low contact resistance of the electrode and a high conversion efficiency of the solar cell. Sex paste has not been developed yet.
Japanese Patent Laid-Open No. 11-329072 JP 2008-109016 A JP 2008-10527 A Japanese Patent No. 3853793

  The present invention has been made in view of such problems of the prior art, and the object thereof is to use lead-free electrodes in ohmic contact with an n-type semiconductor (n-type diffusion layer) of a crystalline silicon solar cell. A conductive paste for solar cell that can stably obtain high conversion efficiency even when glass frit is contained, a condensing electrode for solar cell formed by printing and baking the conductive paste, and the condensing electrode It is in providing the solar cell provided.

 Generally, in a crystalline silicon solar cell, an electrode in contact with an n-type semiconductor (n-type diffusion layer) is made of a group V element such as phosphorus (P), arsenic (As), etc., and a p-type semiconductor (P + layer). It is said that the ohmic contact is improved when a group III element such as boron (B) is present in the contacting electrode. In the former, when an electrode and an n-type semiconductor are bonded, phosphorus (P) and the like in the electrode increase as electrons as donors formed in the n semiconductor. In the latter case, when the electrode and the p-type semiconductor are bonded, boron (B) in the electrode is considered to increase the number of holes formed in the P-type semiconductor. Based on such considerations, it is considered that the effect cannot be obtained even if the Si component, which is the same element as the semiconductor silicon substrate, is mixed in addition to the glass frit. However, as will be understood from the experiment described later, there is little improvement in reducing ohmic contact even when a phosphorus-based organic compound is blended with a conductive paste that becomes an electrode in contact with an n-type semiconductor. On the other hand, the present inventors have found that when a Si-based organic compound having the same element as that of the silicon substrate is blended in the conductive paste, the solar cell has excellent change efficiency, and the present invention is completed. It came.

That is, the present invention is a conductive paste for a solar cell containing a binder resin, a solvent, a glass frit, and a conductive powder mainly composed of silver or a silver compound, characterized by containing a Si-based organic compound. It is the conductive paste for solar cells.
Moreover, this invention relates to the condensing electrode for solar cells formed by printing and baking said electrically conductive paste.
Furthermore, this invention relates to the solar cell provided with said condensing electrode.

 When the Si-based organic compound is mixed with the conductive paste for solar cells, the printing accuracy can be improved because it performs functions such as controlling the thixotropy of the conductive paste for solar cells, preventing bleeding, and preventing bleeding. . In addition, since adhesion to the antireflection film is improved, first of all, troubles in the processing process of the solar cell are reduced. On the other hand, since organic components in such Si-based compounds are decomposed during high-temperature firing, there is no direct negative effect on the performance of the solar cell, and conductive powder (for example, silver particles) is mainly used after firing. It is considered that the condensing electrode, which is a sintered body, and the Si semiconductor are more closely bonded to each other and contact resistance is reduced, so that a solar cell with high conversion efficiency can be obtained, and the performance of the solar cell is improved. It is thought that it can be improved.

The conductive paste for solar cell of the present invention contains a binder resin, a solvent, glass frit, a conductive powder mainly containing silver or a silver compound, and a Si-based organic compound.
Hereinafter, the manufacturing process of the conductive paste for solar cell of the present invention will be described in detail, and various materials used in each process will be described in detail.

As a method for producing the conductive paste for solar cell of the present invention, a binder resin, a solvent, glass frit, conductive powder mainly composed of silver or silver compound, and a mixture containing Si-based organic compound are stirred and dispersed. It can be carried out by a method for producing a dispersion.
Other conductive powder other than the principal conductive powder if necessary, ZnO, such as TiO ₂ fine inorganic particles, surfactants, can be added a dispersing agent or the like.
In addition, an inorganic component such as a conductive powder containing silver or a silver compound as a main component or inorganic fine particles such as ZnO and TiO ₂ may be used after surface treatment with a surfactant, a dispersant or the like in advance.

As the conductive powder mainly composed of silver or a silver compound used in the conductive paste for solar cell of the present invention, silver particles, metal particles surface-coated with silver, or a mixture thereof can be used.
Silver particles having any shape such as a spherical shape, a scale shape, a needle shape, and a dendritic shape can be used.
The method for producing silver particles is not particularly limited, and may be any method such as a mechanical pulverization method, a reduction method, an electrolysis method, or a gas phase method.
The metal particle surface-coated with silver is obtained by forming a silver coating layer on the surface of a particle made of a metal other than silver by a method such as plating.
As the silver particles, spherical silver particles and scaly silver particles made of only silver are preferable from the viewpoint of conductivity and cost.

The volume average particle diameter of the conductive powder containing silver or a silver compound as a main component is preferably 0.05 to 10 μm, more preferably about 0.05 to 5 μm.
As the conductive powder, two or more kinds of conductive powders having different volume average particle diameters are combined to improve the packing density of the conductive powder in the electrode, thereby improving the conductive film constituting the electrode. You may improve electroconductivity.

Any glass frit may be used for the conductive paste for solar cell of the present invention, and a lead-free glass frit can be used.
When the lead-free glass frit is used, the lead-free glass frit is not particularly limited as long as lead is not included. For example, Bi ₂ O ₃ —B ₂ O ₃ —SiO ₂ —ZnO-based, Bi ₂ O ₃ —B ₂ O _{3 -SiO 2 -Al 2 O 3 -ZnO-} BaO _{series, Bi 2 O 3 -B 2 O} 3 -SiO 2 -ZnO-BaO -based, and the like.
The shape and size of the glass frit are not particularly limited, and those known in the art can be used.
Examples of the shape of the glass frit include a spherical shape and an indefinite shape.
The average particle size of the glass frit is preferably 0.01 to 10 μm, more preferably 0.05 to 2 μm, still more preferably 1 to 2 μm from the viewpoint of workability. Here, the average particle size means the average of the longest diameters when the glass frit is irregular.

 In the conductive paste for solar cell of the present invention, in order to ensure sufficient adhesive strength, the glass frit is 0.5 to 10 parts with respect to 100 parts by mass of the conductive powder containing silver or a silver compound as a main component. It is preferable that it is a mass part, More preferably, it is 1-5 mass parts. If the content of the glass frit is within this range, good adhesive strength can be obtained.

In the conductive paste for solar cell of the present invention, a binder resin is used, and a known binder resin can be used as the binder resin.
Examples of the binder resin include polyvinyl alcohol, polyvinyl acetal, butyral resin, phenol resin, and cellulose resins such as ethyl cellulose and nitrocellulose; for example, (meth) acrylic resins such as polymethyl acrylate and polymethyl methacrylate. These resins can be used alone or in admixture of two or more. These binder resins can be appropriately selected in consideration of physical properties that affect coating suitability, such as a solid content ratio, viscosity, and thixotropy, which are suitable for an assumed coating method.
Among these binder resins, ethyl cellulose resin and acrylic resin are preferable. The ethyl cellulose resin and the acrylic resin are decomposed almost completely after decomposing and removing the binder resin in the air and rarely remain in the sintered coating as carbon.
The content of the binder resin in the conductive paste for solar cell of the present invention is preferably in the range of 0.01 to 30 parts by mass with respect to 100 parts by mass of the conductive powder mainly containing silver or a silver compound. The range of 01-15 mass parts is more preferable, and the range of 0.01-5 mass parts is the most preferable.

Solvents used when producing the conductive paste for solar cell of the present invention are alcohols such as methanol, ethanol, n-propanol, benzyl alcohol, 1-phenoxy-2-propanol, and terpineol; acetone, methyl ethyl ketone , Cyclohexanone, isophorone, ketones such as acetylacetone; amides such as N, N-dimethylformamide, N, N-dimethylacetamide; ethers such as tetrahydrofuran, dioxane, methyl cellosolve, diglyme, butyl carbitol; methyl acetate, acetic acid Ethyl, diethyl carbonate, TXIB (1-isopropyl-2,2-dimethyltrimethylene diisobutyrate), carbitol acetate, butyl carbitol acetate, 2,2,4-trimethyl-1,3-penta Esters such as diol monoisobutyrate; Sulfoxides and sulfones such as dimethyl sulfoxide and sulfolane; Aliphatic halogenated hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride, 1,1,2-trichloroethane; benzene, toluene, Aromatics such as o-xylene, p-xylene, m-xylene, monochlorobenzene, dichlorobenzene and the like can be mentioned. These solvents can be appropriately selected in consideration of the applicability of the envisaged application method, the dispersibility and solubility of the binder resin used, the drying properties after application, and the like. These solvents are not limited to those listed here, and can be used alone or in admixture of two or more.
The content of the solvent in the conductive paste for solar cell of the present invention is set to such an extent that the step of applying or printing the conductive paste for solar cell on an appropriate substrate surface can be executed smoothly.

As the Si-based organic compound used in the conductive paste for solar cell of the present invention, a silane coupling agent and various polyorganosiloxanes can be used.
Polyorganosiloxanes include polyalkylsiloxanes such as polydimethylsiloxane and polydiethylsiloxane, polyarylsiloxanes such as polydiphenylsiloxane and polymethylphenylsiloxane, polyoxyalkylene groups, alkoxy groups, and hydroxy groups at side groups and terminal groups. It is also possible to use a modified polysiloxane such as an organopolysiloxane having hydrogen, a carboxyl group, an amino group or a mercapto group. Further, a block copolymer having a siloxane unit or a graft copolymer having a siloxane unit in the side chain can also be used. Among these Si-based organic compounds, one or more members selected from the group consisting of silane coupling agents, polydimethylsiloxane, and polydimethylsiloxane polyether-modified products are preferable.
The silane coupling agent is preferably one represented by the following general formula (I).
_{^{X n R 2 3-n SiR}} 1 -Y (I)
(However, in said general formula (I), X represents a methoxy group or an ethoxy group, Y represents a vinyl group, a mercapto group, an epoxy group, an amino group, a methacryl group, a glycidoxy group, a ureido group, a sulfide group, or a methacryloxy group. R ¹ represents an alkylene group having 2 to 5 carbon atoms or a divalent hydrocarbon group having 2 to 5 atoms in the main chain and containing a nitrogen atom in the main chain, and R ² has 1 to 5 carbon atoms Represents an alkyl group, and n represents an integer of 1 to 3.)

Y is preferably a vinyl group, an amino group, an epoxy group or a mercapto group, more preferably an amino group.
R ¹ is preferably an ethylene group or a propylene group, R ² is preferably a methyl group or an ethyl group, and more preferably a methyl group.

Specifically, as the silane coupling agent, for example, the following groups (a) to (c) are more preferably used.
(A) Those having a glycidoxy group: 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 2- (3,4 epoxycyclohexyl) ethyltrimethoxysilane, etc., (b) amino Having a group: N- (2-aminoethyl) 3-aminopropylmethyldimethoxysilane, N- (aminoethyl) 3-aminopropyltrimethoxysilane, and 3-aminopropyltriethoxysilane, etc. (c) mercapto Those having a group: 3-mercaptopropyltrimethoxysilane, and the like.
In terms of product names, DOW CORNING TORAY SH6020 SILANE (manufactured by Dow Corning Toray) (aminoethylaminopropyltrimethoxysilane (containing 80% by mass or more)), KSE-903, KBM-603 (manufactured by Shin-Etsu Chemical Co., Ltd.) ) Etc.

In terms of product names, polydimethylsiloxane includes KS-66 (dimethylsiloxane units 200 to 350) (Shin-Etsu Chemical Co., Ltd.).
As the polyether-modified product of polydimethylsiloxane, a polyether-modified silicone copolymer represented by the following general formula (1) is preferable.

However, in the above general formula (1), R ¹ represents a monovalent hydrocarbon group containing no aliphatic unsaturated group, R ^2, R ³ and R ⁴ are monovalent free of aliphatic unsaturation Or a hydrocarbon group of the following formula: —YO (C ₂ H ₄ O) p (C ₃ H ₆ O) q R (wherein Y represents an alkylene group having 2 to 8 carbon atoms not containing an unsaturated group) , R represents a monovalent hydrocarbon group not containing an aliphatic unsaturated group or -H, p represents an integer of 1 to 50, and q represents an integer of 0 to 50). Represents an alkylene group, at least one of R ² , R ³ and R ⁴ represents the polyoxyalkylene group, x represents an integer of 0 or more, y represents an integer of 1 or more, and x + y represents 1 to 300 The siloxane block constitutes 15-95% by weight of the copolymer, and the copolymer is less Also it has an average molecular weight of 300.

 Specific product names include BYK300, BYK301, BYK306, BYK307, BYK335, BYK330, BYK331, BYK341, BYK344, BYK346, BYK302, BYK164 (all manufactured by BWK Chemie Japan).

Of these Si-based organic compounds, silane coupling agents and polydimethylsiloxane are preferred, and silane coupling agents are more preferred.
Si-type organic compounds may be used alone or in combination of two or more.
Moreover, it is preferable that content of Si type organic compound is 0.01-5 mass parts with respect to 100 mass parts of electroconductive powder which has silver or a silver compound as a main component, 0.05-5 mass parts More preferably, it is 0.1-3 mass parts.
When the content of the Si-based organic compound is within this range, the blending effect can be easily obtained.

The Si-based organic compound is usually a liquid and can be blended in the conductive paste for solar cells as it is, but is used by dissolving or dispersing in advance in the solvent used in the method for producing the conductive paste for solar cells. You can also.
The Si-based organic compound is preferably uniformly present in the dried coating film of the solar cell conductive paste of the present invention. Si-based organic compounds are present between various particles constituting the conductive paste, such as conductive powder containing silver or a silver compound as a main component, glass frit, and inorganic fine particles, and are electrically conductive with the Si substrate during sintering. It is considered to function as a preferable inclusion for contact between powders. As a result, it is considered that the contact resistance between the Si substrate and the electrode decreases, leading to an improvement in conversion efficiency.

In addition to the binder resin, solvent, glass frit, conductive powder mainly composed of silver or a silver compound, and the Si-based organic compound, the conductive paste may be applied to the surface of an appropriate substrate. Various additives can be appropriately blended in order to obtain suitable physical properties for printing and to keep the dispersion of the conductive powder and the inorganic fine particles added as necessary.
Examples of suitable additives include dispersants such as phthalate esters, phosphate esters and fatty acid esters, plasticizers such as glycols, additives such as low-boiling alcohols and inorganic fine particles.
In order to improve the dispersibility of the conductive powder in the conductive paste for solar cell, a dispersant may be added to the conductive paste.

As described above, the conductive paste for solar cell of the present invention may be used in combination with inorganic fine particles such as ZnO and TiO ₂ which are effective in obtaining high conversion efficiency.
The shape and average particle size of these inorganic fine particles are not particularly limited. Examples of the shape of a general inorganic fine particle include a spherical shape and an indefinite shape. The average particle size is preferably 0.05 to 1 μm from the viewpoint of dispersibility.
These inorganic fine particles can prevent excessive sintering of the conductive powder during the baking process of the conductive paste. In addition, it suppresses the spread of glass frit liquefied by firing, contributes to creating a field where the conductive powder comes into contact with the n-type semiconductor surface, and is further reduced by CO generated by decomposition of the binder resin. It is believed that the ability to make a semiconductor is also helpful in obtaining good contact between the collector electrode and the semiconductor.

Furthermore, when blending the inorganic fine particles, the content thereof is preferably 0.5 to 15 parts by mass, more preferably 100 parts by mass of the conductive powder containing silver or a silver compound as a main component. Is 2 to 10 parts by weight.
When the content of the inorganic fine particles is within this range, the effects of the above blending can be sufficiently obtained.

It is preferable to treat the surface of the conductive powder mainly composed of silver or a silver compound and the inorganic fine particles added as necessary with the conductive paste for solar cell of the present invention with a surfactant. .
As the surfactant used for the conductive powder and inorganic fine particles added as necessary, it can be selected from many types of surfactants that are usually used, and anionic surfactant Examples thereof include agents, nonionic surfactants, cationic surfactants and amphoteric surfactants.

Examples of the anionic surfactant include alkyl sulfate, polyoxyethylene alkyl sulfate ester salt, alkylbenzene sulfonate, alkyl naphthalene sulfonate, fatty acid salt, salt of naphthalene sulfonate formalin condensate, polycarboxylic acid type Examples thereof include polymeric surfactants, alkenyl succinates, alkane sulfonates, polyoxyalkylene alkyl ether phosphates and salts thereof, polyoxyalkylene alkyl aryl ether phosphates and salts thereof, and the like.
Nonionic surfactants include polyoxyethylene alkyl ether, polyoxyalkylene alkyl ether, polyoxyethylene derivatives, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene sorbitol fatty acid ester, glycerin fatty acid ester, polyoxy Examples include ethylene fatty acid esters, polyoxyethylene hydrogenated castor oil, polyoxyethylene alkylamines, polyoxyalkylene alkylamines, and alkyl alkanolamides.
Examples of the cationic surfactant include alkylamine salts and quaternary ammonium salts.
Examples of amphoteric surfactants include alkyl betaines and alkyl amine oxides.
Among these surfactants, alkylamine surfactants are particularly preferable.

As the alkylamine surfactant, alkylamines and alkylamine salts can be suitably used.
Alkylamine-based nonionic surfactants and alkylamine-salt-based cationic surfactants are effective even when used alone, but especially when used in combination, the dispersibility becomes better and the effect is remarkable. is there.
As the alkylamine surfactant, a polyoxyalkylene alkylamine type surfactant is preferable, and a polyoxyethylene alkylamine type surfactant is more preferable. Among these, those having the following chemical structure (2) are more preferable.

However, in said chemical structure (2), a and b are the integers of 1-20, respectively, R represents a C8-C20 alkyl group or alkylallyl group.
On the other hand, alkylamine salt surfactants are preferably alkylamine acetates, and more preferably those having the following chemical structure (3).

 However, in said chemical structure (3), R represents a C8-C20 alkyl group or alkylallyl group.

When using alkylamine surfactants and alkylamine salt-based cationic surfactants alone or in combination, the total amount of surfactant in the conductive powder based on silver or silver compounds Can be appropriately adjusted depending on the type of conductive powder. For example, when silver particles are used as the conductive powder, the compounding amount of the surfactant with respect to the silver particles needs to be adjusted slightly depending on the type of silver particles, but is 0.01% with respect to 100 parts by mass of the silver particles. -3.00 mass part is preferable, and 0.05-1.50 mass part is further more preferable.
When the total amount of the surfactant is less than 0.01 parts by mass, sufficient dispersibility tends to be hardly obtained. On the other hand, when the total amount of the surfactant exceeds 3.00 parts by mass, the surface of the silver particles is thick and coated with the organic component of the surfactant, and it becomes difficult to obtain contact between the silver particles after firing. Tend to decrease.
When an alkylamine surfactant and an alkylamine salt cationic surfactant are used in combination, the mixing ratio of the alkylamine surfactant to the alkylamine salt cationic surfactant is A range of 1:20 to 1: 5 is preferred.

 In the conductive paste for solar cell of the present invention, when a surfactant is used, it is preferable that the dispersion is in an acidic condition or an alkaline condition. Thereby, an interfacial electric double layer is generated on the surface of the conductive powder containing silver or a silver compound as a main component via the surfactant, and dispersion stability is obtained. It is preferable to use the conductive powder and a repulsive force between the particles according to the sign of the surface charge of the inorganic fine particles added as necessary.

 The conductive paste for solar cell of the present invention is basically a mixture prepared by blending the basic components of the above-mentioned conductive paste for solar cell in the above-mentioned blending amount range, and further various additives as necessary. The mixture in which is added as an optional component can be agitated, dispersed, or kneaded as a pretreatment if necessary to produce a paste.

 In producing the conductive paste for solar cell of the present invention, the kneading and dispersing means that can be used to paste the mixture of the constituents of the conductive paste include, for example, two rolls, three rolls, ball mill , Sand mill, pebble mill, tron mill, sand grinder, seg barrier triter, high speed impeller disperser, high speed stone mill, high speed impact mill, planetary mixer, Henschel mill, kneader, homogenizer, ultrasonic disperser, etc. Can be used for kneading and dispersing.

 As described above, the components constituting the conductive paste for solar cell of the present invention may be mixed at a time to prepare a mixture, and kneading and dispersing may be performed. For some, pretreatment is performed in advance to improve dispersibility, or a dispersion is prepared by kneading and dispersing these materials in advance, and then the dispersion is used as the remaining other components. May be mixed with the conductive paste for solar cell of the present invention. In particular, among the conductive powders used in the present invention, silver particles have a high specific gravity and are difficult to disperse. Therefore, it is preferable to use a technique for increasing the dispersibility.

 Among the materials used for the conductive paste for solar cell of the present invention, when conducting the above-mentioned pretreatment on conductive powder mainly composed of silver or a silver compound or inorganic fine particles added as necessary, the above-mentioned pretreatment is performed. A method of treating the surface of conductive powder, inorganic fine particles, glass frit or the like with the above-described surfactant or dispersant is preferred. In particular, since the silver particles have a high specific gravity, they are likely to settle, and it is preferable that the surface of the silver particles is previously subjected to a surface treatment in order to enhance dispersibility. It is preferable to improve the dispersibility of these conductive powder and inorganic fine particles added as necessary using a surfactant by adsorbing the surfactant on the surface of the conductive powder in advance. These pretreatments may be performed separately on the conductive powder and the inorganic fine particles, but some of them may be performed simultaneously.

A pretreatment for the conductive powder or inorganic fine particles containing silver or a silver compound as a main component can be performed by a known method, but in a dispersion step of dispersing the conductive powder and the surfactant in a pretreatment solvent. It is preferable to use a method of preparing a dispersion of conductive powder, drying the dispersion and volatilizing the solvent in the drying step, and particularly preferably using a vacuum freeze-drying method in the drying step.
When the above method is used, particularly when the conductive powder and inorganic fine particles added as necessary are produced in a liquid phase, these highly active conductive powders can be effectively used. Surfactant can be added in the liquid phase when these conductive powders are manufactured, and surface treatment can be performed in the presence thereof, so that the processing is easy and the original characteristics of these conductive powders etc. Can be sufficiently exhibited.

Hereinafter, the details of a preferable pretreatment method for conductive powder or the like mainly composed of silver or a silver compound will be described.
(1) Dispersing Step In the production of the conductive paste for solar cell of the present invention, when conducting pretreatment of conductive powder mainly composed of silver or a silver compound, the conductive powder and surfactant, or as necessary. Inorganic fine particles and the like are added to the solvent, and the mixture is mixed with the conductive powder and the surfactant through a stirrer or a disperser. At this time, stirring or dispersion may be performed under the condition that the conductive powder and the inorganic fine particles can be simultaneously crushed.

 More specifically, the dispersion treatment with the surfactant for the conductive powder mainly containing silver or the silver compound used in the present invention or the inorganic fine particles added as necessary is carried out by adding the surfactant to the solvent. It is preferable to blend the conductive powder after sufficiently dissolving or dispersing. After mixing, when dispersed for 0.5 to 4.0 hours, the conductive powder and the like are disintegrated into primary particles, and the surfactant, the conductive powder, and inorganic fine particles added as necessary are in an adsorption equilibrium. Reach.

Thus, for example, a conductive powder mainly composed of silver or a silver compound, a surfactant, or inorganic fine particles added as necessary, and a solvent are mixed in a desired ratio and dispersed by a dispersing means. A dispersion liquid of the conductive powder or the like can be obtained.
The concentration range of the conductive powder and the like in the dispersion can be set as appropriate according to the apparatus used for stirring and dispersion. However, when lyophilization is performed in the next step, the solid content in the dispersion of the conductive powder. The concentration range is preferably 0.5 to 80% by mass, particularly 1 to 50% by mass.

Here, the solvent used in the dispersion step is not particularly limited. For example, water, a water-soluble solvent, or a mixture (aqueous solution) of water and a water-soluble solvent is used.
Examples of the water-soluble solvent include lower alcohols such as ethanol and isopropyl alcohol; ethylene oxide adducts of alkyl alcohols such as ethylene glycol hexyl ether and diethylene glycol butyl ether, and propylene oxide adducts of alkyl alcohols such as propylene glycol propyl ether. .

In the pretreatment of conductive powder mainly composed of silver or silver compound, vacuum is used as a drying method for obtaining a pretreated conductive powder by drying a dispersion of a conductive powder or the like and a surfactant and a solvent. When using freeze-drying, it is preferable to select a solvent that can be easily freeze-dried in vacuum from among the above-mentioned solvents for the reasons described later about the drying process, and it is preferable to use it as a solvent in the dispersion process. It is preferable that it is -40 degreeC or more.
Solvents used in the dispersion step are not limited to those listed here, and may be used singly or may be used by appropriately selecting and mixing two or more solvents. .
The stirrer or the disperser that can be used in the dispersion step is not particularly limited, and can be appropriately selected from the well-known stirrers or dispersers described below.

(2) Drying step Through the above dispersion step, the dispersion containing the conductive powder mainly composed of silver or silver compound and the inorganic fine particles, the surfactant and the solvent added as necessary is a solvent by the drying step. The conductive powder surface-treated with the surfactant is obtained.
In the drying step, any known method can be applied as long as the surfactant is not subject to thermal change or chemical change. In particular, the drying method by the vacuum freeze-drying method is a solvent without increasing the dispersion temperature. Therefore, the conductive powder and the inorganic fine particles are less likely to aggregate and the surfactant is less likely to be unevenly distributed.

Hereinafter, the drying process using the vacuum freeze-drying method will be described in more detail.
In the drying process using the vacuum freeze-drying method, basically only the solvent is sublimated and removed from the dispersion frozen at a low temperature. Since there is no surfactant lost by elution in the solvent, almost all of the surfactant remains on the surface of the treated conductive powder and, if necessary, the inorganic fine particles. In the dispersion, the surfactant is localized near the surface of the conductive powder and the inorganic fine particles added as necessary, and when performing the freeze-drying in which only the solvent is removed, the surfactant is There is a high possibility that the conductive powder and the like can be taken out in a state of being uniformly adsorbed on the surface of the conductive powder and inorganic fine particles added as necessary, and the solvent is removed by a normal method other than vacuum freeze-drying. Thus, the conductive powder or the like does not agglomerate as in the case of performing the process, which can be said to be an extremely efficient treatment method. Since all the surfactants used in this manner remain on the surface of the conductive powder and the like, and the surface-treated conductive powder and the like are efficiently formed, the effect of the surfactant on the conductive powder and the like It is easy to grasp the relationship of the amount used, and it is easy to optimize the amount of the surfactant used with respect to the amount of the conductive powder used.

 It is preferable that the molecule of the surfactant has a hydrophilic group side and a hydrophobic group side in the molecule, and the molecule has such a configuration, so that at the end of the hydrophilic group side included in the molecule. Since it adsorbs on the surface of the conductive powder or the like, the end of the hydrophobic group side contained in the molecule faces outward with respect to the surface of the conductive powder. Thereby, when the conductive paste for solar cells of the present invention is produced using the pretreated conductive powder, etc., the affinity between the conductive powder contained in the conductive paste and the binder resin is improved. Then, the binder resin is adsorbed on the surface of the conductive powder or the like through the surfactant to coat the surface of the conductive powder or the like. As a result, dispersibility of the conductive powder and the like is improved. Further, because of the steric hindrance effect due to the binder resin adsorbed on the surface of the conductive powder or the like, aggregation of the conductive powder or the like is suppressed, and the state dispersed in the primary particles can be maintained.

When using the freeze vacuum drying method in the drying step, for example, in the case of a dispersion containing conductive powder, inorganic fine particles added as necessary, water, and a surfactant, the dispersion is at atmospheric pressure. The degree of vacuum may be controlled so that it is pre-frozen to 0 ° C. or lower, and theoretically the vapor pressure of water at 0 ° C. does not exceed 4.5 mmHg (= 600 Pa). Considering the ease of controlling the drying speed, it is preferable to raise the temperature to the melting point (freezing point) at the vapor pressure by setting the degree of vacuum to 1 mmHg (= 133.32 Pa) or less.
As described above, in the drying method by vacuum freeze-drying, the solvent contained in the dispersion liquid is sublimated and evaporated in vacuum and dried, so that the shrinkage due to drying is slight, and aggregation of the conductive powder or the like is difficult to occur. In addition, drying is not performed while moving a liquid component such as water in the sample at a high temperature as in hot air drying, but is directly dried at a low temperature by sublimation from a solid frozen state. It is preferable because there is almost no partial component concentration such as drying and almost no partial component change.

In order to produce the conductive paste for solar cell of the present invention using the conductive powder or the like having the surfactant adsorbed on the surface by the dispersing step and the drying step (pretreated conductive powder or the like) Conductive powder or the like having a surfactant adsorbed on the surface, Si-based organic compound, glass frit, binder resin, and solvent are mixed and the conductive paste for solar cell of the present invention is mixed using an appropriate disperser. Can be produced.
Since the surfactant is adsorbed on the surface of the conductive powder as described above, there is no aggregation of the conductive powder or the like. Therefore, the constituent components of the conductive paste for solar cell of the present invention are mixed. The conductive paste for solar cell of the present invention can be obtained only by performing a simple stirring operation.

 That is, for example, when the condensing electrode is formed on the surface of the n-type diffusion layer formed on the p-type Si substrate constituting the solar cell using the conductive paste for solar cell of the present invention, the solar of the present invention Immediately before printing the conductive paste for a battery, the necessary constituents of the conductive paste are added and a simple stirring operation is performed, so that a conductive paste for a solar cell with good dispersibility can be obtained. The equipment for adjusting the paint attached to can be simple. However, in order to perform the dispersion more reliably, the kneading process or the dispersion process may be performed using the kneading apparatus or the dispersing apparatus described above.

The solar cell conductive paste of the present invention, which has been dispersed by the above-described method, is generally printed on a semiconductor substrate by a known and commonly used coating method or printing method as a solar cell conductive paste. A condensing electrode can be formed by heating and baking.
As a coating method, it can form as a coated material by various coating methods. For example, dip coating or known roll coating methods such as air doctor coating, blade coating, rod coating, extrusion coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating A coating can be formed on an electronic device or a substrate by gravure coating, kiss coating, cast coating, spray coating, or the like.
Various printing methods can also be applied. There are also printing methods in which the optimum viscosity region is in a relatively low viscosity region, such as intaglio printing, and in the high viscosity region, such as screen printing. Specifically, the coating material can be printed in a predetermined size on the substrate using a stencil printing method, an intaglio printing method, a lithographic printing method, or the like.

 Since the conductive paste for solar cell of the present invention can be used for manufacturing a concentrating electrode of a solar cell and can obtain good contact with an n-type semiconductor, an n-type semiconductor (for example, n-type diffusion) It is useful for forming a collecting electrode on the layer).

In order to manufacture a concentrating electrode of a solar cell and a solar cell using the conductive paste for solar cell of the present invention, the following method can be used.
Hereinafter, an example of the manufacturing method of the condensing electrode of a solar cell and the manufacturing method of a solar cell is demonstrated using FIG.
First, a texture is optionally formed on one surface of the p-type Si substrate 1b, and then P (phosphorus) or the like is thermally diffused at about 900 ° C. to form an n-type diffusion layer 1a. Get one.
Next, an antireflection film 3 made of a silicon nitride thin film, titanium oxide or the like is formed on the n-type diffusion layer 1a with a film thickness of 50 to 100 nm by a plasma CVD method or the like.
Next, in order to form a light-incident-side condensing electrode, the solar cell conductive paste of the present invention is screen-printed on the antireflection film 3 with a film thickness of about 20 μm and dried.
Next, in order to form an electrode on the back surface side, an aluminum paste for forming a back surface electrode is printed on the back surface of the p-type Si substrate 1 by screen printing with a film thickness of 30 to 50 μm and dried.
Next, the p-type Si substrate 1 having electrodes printed on both the front and back surfaces of the p-type Si substrate in this manner is baked at a maximum baking temperature of 750 to 850 ° C., and is made of the light-collecting electrode 2 and aluminum. A solar battery cell provided with the back electrode 4 is obtained.
When forming the electrode on the back surface side, the back surface electrode forming aluminum paste is printed on the back surface of the p-type Si substrate 1 and dried, and if necessary, the back surface electrode forming silver paste is further printed thereon. Then, it can be dried and fired to form a back electrode 4 made of aluminum and a back electrode (not shown) made of silver.

 EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further more concretely, this invention is not limited to a following example.

(Preparation of surface treated product of silver particles (b1))
First, prior to the production of the conductive paste for solar cell of the present invention, pretreatment of conductive powder with a surfactant is performed by the method described below.
<Dispersing process>
50 g of silver particles having a D diameter of 1.4 to 2.5 μm as a conductive powder (trade name: SPN10JS, manufactured by Mitsui Kinzoku Co., Ltd.), 10% by mass of coconut amine acetate as a cationic surfactant for alkylamine salt 4 g of aqueous solution, 0.4 g of 10% by mass aqueous solution of polyoxyethylene coconut alkylamine ether as surfactant for alkylamine, 50 g of water as solvent, and 400 g of 2 mm zirconia beads are placed in a 250 ml plastic bottle. The mixture was mixed and kneaded for 4 hours using a rotating machine (ball mill) to obtain a dispersion (a1) of silver particles.

<Drying process>
100 g of this silver particle dispersion (a1) was transferred to a flat tray having a bottom dimension of 200 mmL × 150 mmW, pre-lyophilized, and then freeze-dried.
As the freeze vacuum dryer, “DFM-05AS” manufactured by Nippon Vacuum Co., Ltd. was used.
The pre-frozen silver particle dispersion (a1) is placed on a shelf that has been cooled to about −40 ° C. in advance, and after freeze-drying for 20 hours at a degree of vacuum of 7 to 10 Pa, 50 g of surface-treated product (b1) was obtained. This corresponds to a conductive powder pretreated with a surfactant.

(Preparation of surface treated product of silver particles (b2))
Silver particles, except that Prisurf A215C (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) is mixed as a phosphate anionic surfactant instead of the cationic surfactant in an amount of 1% by mass with respect to the silver particles. In the same manner as in the surface-treated product (b1), 50 g of a surface-treated product (b2) of silver particles was obtained. This silver particle surface-treated product (b2) also corresponds to the conductive powder pretreated with the surfactant.

"Example 1"
(Manufacture of conductive paste for solar cells)
A conductive paste for solar cell was prepared by kneading a mixture having the following composition with three rolls.
The viscosity of the prepared conductive paste for solar cell was measured with a TVE-20H type rotational viscometer (manufactured by Toki Sangyo Co., Ltd.).
Surface treatment of the silver particles (b1) 100 parts of ethyl cellulose resin 2.5 parts Pb-free glass frit 5.0 parts _{(Bi 2 O 3 -B 2 O} 3 -SiO 2 -Al 2 O 3 -ZnO-BaO system)
ZnO powder (spherical D50 diameter 700 nm) 5.0 parts Kao wax 85P (manufactured by Kao Corporation) 1.0 part silane coupling agent (amino type) 1.0 part (DOW CORNING TORAY SH6020 SILANE manufactured by Toray Dow Corning)
Dinormal butyl phthalate (DBP) 2.5 parts Texanol / 1-phenoxy-2-propanol (mixed solvent) 15.05 parts

"Examples 2-5, Comparative Examples 1-4"
As shown in Table 1, using pretreated silver powder (surface treated products of silver particles (b1) and (b2)) or untreated silver powder, binder resin, lead-free glass frit, solvent, Si-based organic A mixture containing the compound and various additives as required was kneaded with three rolls to prepare a conductive paste for solar cell.
As the polydimethylsiloxane of Example 2, KS-66 (dimethylsiloxane units 200 to 350, manufactured by Shin-Etsu Chemical Co., Ltd.) is used, and as the polyether-modified polydimethylsiloxane of Example 3, BYK330 (B & K Chemie Japan, Inc.) is used. Made).
As the silane coupling agent of Example 4 and Example 5, the same silane coupling agent as used in Example 1 was used.

(Production of solar cells and measurement of conversion efficiency)
A texture was formed by wet etching on the surface of a P-type polycrystalline silicon substrate (size 150 mm × 150 mm, substrate thickness 200 μm) doped with B (boron).
Thereafter, P (phosphorus) was thermally diffused to form an n-type diffusion layer (thickness 0.3 μm).
Next, an antireflection film made of a silicon nitride thin film (thickness: about 60 nm) was formed on the n-type diffusion layer by plasma CVD from silane gas and ammonia gas.
Using the conductive paste for solar cell prepared in Examples 1 to 5 and Comparative Examples 1 to 4, the film thickness was 10 on the antireflection film by screen printing (325 mesh, wire diameter 28 μm, emulsion thickness 10 μm). It printed so that it might become ~ 20micrometer, and it dried and formed the electrode pattern which consists of a bus electrode which consists of this electrically conductive paste, and a finger electrode. Although the dry film thicknesses of the bus electrode and the finger electrode differ depending on the configuration of the conductive paste for solar cell, it was possible to print a good electrode pattern without bleeding or repelling.
Next, the back surface electrode forming aluminum paste (trade name: 08-6429, manufactured by Toyo Aluminum Co., Ltd.) is printed on the back surface of the P-type polycrystalline silicon substrate by screen printing so that the film thickness becomes 30 to 50 μm. , Dried.
Thereafter, the substrate on which various electrode pastes formed on both sides of the P-type polycrystalline silicon substrate are printed and dried is baked in an infrared furnace at a maximum baking temperature of 750 to 850 ° C. to collect light on the light incident side. A solar cell provided with an electrode and a back electrode made of aluminum was obtained.
In producing solar cells using the conductive paste for solar cells of Examples 1 to 5, the formed condensing electrodes had sufficient adhesive strength, and none of them caused trouble in the processing process. .
Moreover, the thickness of the finger electrode was measured with a laser microscope (trade name: VK-9500, manufactured by Keyence Corporation).

 The current-voltage characteristics of the solar cells were measured under a solar simulator (trade name: WXS-155S-10, manufactured by Wacom Denso Co., Ltd.), and the conversion efficiency was measured.

The composition of the conductive paste for solar cell used in Examples 1 to 5 or Comparative Examples 1 to 4 is shown in Table 1 below. Table 1 shows conversion efficiency values.
Examples 1 to 5 using the conductive paste for solar cell of the present invention are all better than Comparative Examples 1 to 4 using the conventional conductive paste for solar cell not containing Si organic compound. The conversion efficiency was shown.

As can be seen from Table 1, the solar cells using the conductive paste for solar cell of the present invention shown in Examples 1 to 5 containing Si-based organic compounds are Comparative Examples 1 to 4 not using this. It showed higher conversion efficiency than the solar cell using the conventional conductive paste for solar cell shown. In particular, as shown in Examples 1 and 2, the surface treated product (b1) of silver particles surface-treated with an alkylamine-based surfactant is used, and a silane coupling agent or polydimethylsiloxane is contained as a Si-based organic compound. High conversion efficiency can be obtained when using the conductive paste for solar cell, and the conversion efficiency when using the conductive paste for solar cell containing the silane coupling agent of Example 1 is the most excellent. It was.
Moreover, from the comparison of the finger electrode thickness in Examples 2 and 3 and Example 5 and Comparative Example 1 and Comparative Examples 2 to 4, the surface treated product of silver particles (b1) surface-treated with an alkylamine surfactant. ) Is used compared to the case of using a surface treated product (b2) of silver particles surface-treated with a phosphate ester-based surfactant or the case of using silver powder that has not been surface-treated at all. Since it tends to be easy to improve, and it tends to pass through a screen printing mesh well when forming an electrode pattern to form a thick film electrode, these conversions contribute to high conversion efficiency. As can be seen, the conductive paste using the Si-based organic compound further improved the conversion efficiency. In particular, those using the solar cell conductive paste containing the silane coupling agent of Example 1 and Example 2 showed high conversion efficiency despite the reduction in the thickness of the finger electrode. It was found that there is a high possibility that the contact resistance could be reduced.

It is a schematic sectional drawing which shows the basic structure of a solar cell.

Explanation of symbols

1 p-type Si substrate 1a n-type diffusion layer 1b p-type Si substrate 2 condensing electrode 3 antireflection film 4 back electrode

Claims (7)

 A conductive paste for a solar cell containing a binder resin, a solvent, a glass frit, and a conductive powder containing silver or a silver compound as a main component, and containing a Si-based organic compound. Conductive paste.  The electrically conductive paste for solar cells of Claim 1 which contains 0.01-5 mass parts of said Si type organic compounds with respect to 100 mass parts of conductive powder which has the silver or silver compound as a main component.  3. The conductive paste for solar cell according to claim 2, wherein the Si-based organic compound is one or more selected from the group consisting of a silane coupling agent, polydimethylsiloxane, and a polydimethylsiloxane polyether-modified product.  The conductive paste for solar cell according to any one of claims 1 to 3, wherein the glass frit does not contain lead.  The conductive powder containing silver or silver compound as a main component is pre-treated by vacuum freeze-drying a mixed solution containing a solvent, the conductive powder mainly containing silver or silver compound, and a dispersant. The conductive paste for solar cell according to any one of claims 1 to 4.  By printing and baking the conductive paste for solar cell according to any one of claims 1 to 5 on an antireflection film formed on a single crystal or polycrystalline Si substrate constituting the solar cell. Condensing electrode of the formed solar cell. A single crystal or polycrystalline Si substrate, an antireflection film formed on one surface of the Si substrate, a condensing electrode formed on the antireflection film, and formed on the other surface of the Si substrate. The solar cell comprised from the back electrode by which the said condensing electrode was formed with the electrically conductive paste for solar cells of any one of Claims 1-5.

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