JP2010257932A - Paste having conductive particulates dispersed therein - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a paste which has conductive particulates dispersed therein and which is adapted for use in forming rear electrodes of solar cells. <P>SOLUTION: The paste having conductive particulates dispersed therein, the paste being used to form the rear electrodes of the solar cells. The paste includes a (META)-acrylic resin, an organic solvent, and the conductive particulates. The (META)-acrylic resin has polar functional groups at its molecular ends and a weight-average molecular weight of 10,000 to 40,000. The organic solvent is a terpene-based solvent including at least one hydroxyl group in a molecule or a polyol solvent having the number of carbons/the number of hydroxyl groups less than 5. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a conductive fine particle-dispersed paste that can be suitably used for forming a back electrode of a solar cell.

In recent years, photovoltaic power generation has attracted attention as a clean energy alternative to fossil fuel, and the development of solar cells is progressing. As a solar cell, a device in which an electrode is formed on a silicon semiconductor substrate is known. FIG. 1 is a cross-sectional view schematically showing the structure of a general solar cell.

As shown in FIG. 1, the solar cell element has an n-type impurity layer 2 having a thickness of 0.3 to 0.6 μm formed on the light receiving surface side of a p-type silicon semiconductor substrate 1 having a thickness of 200 to 300 μm. Furthermore, an antireflection film 3 and a grid electrode 4 are formed on the n-type impurity layer 2. A back electrode layer 5 is formed on the back side of the p-type silicon semiconductor substrate 1.

The back electrode layer 5 is usually formed by applying a paste containing aluminum powder, glass frit, and organic vehicle by screen printing or the like, drying, and firing at a temperature of 660 ° C. (melting point of aluminum) or higher. . In this firing step, aluminum diffuses into the p-type silicon semiconductor substrate 1, thereby forming an Al—Si alloy layer 6 between the back electrode layer 5 and the p-type silicon semiconductor substrate 1. A p ⁺ layer 7 is formed as an impurity layer by atomic diffusion. The presence of the p ⁺ layer 7 can provide a BSF (Back Surface Field) effect that prevents recombination of electrons and improves the collection efficiency of generated carriers.

In recent years, the demand for cost reduction of solar cells has increased, and as a method for this, it has been studied to reduce the thickness of a silicon semiconductor substrate. However, since the thermal expansion coefficient of silicon and aluminum differ greatly, if the silicon semiconductor substrate becomes thinner, internal stress occurs due to the difference in thermal expansion coefficient, and the back electrode layer is formed after the paste is baked. There is a problem that the silicon semiconductor substrate is deformed so that the side is concave, and warping or cracking occurs.

In order to solve such a problem, in Patent Document 1, the aluminum-containing paste is thinly applied to the entire back surface of the semiconductor substrate, and the aluminum-containing paste is again applied to the portion to be thickened from above, and then fired. A method of forming a back electrode layer composed of two or more layers on the back surface of a semiconductor substrate is disclosed. Patent Document 2 discloses a method of forming backside electrodes in a grid pattern on the backside of a semiconductor substrate.

Patent Document 3 discloses a method using an aluminum powder, a glass frit, an organic vehicle, and an aluminum-containing organic compound, and Patent Document 4 has a smaller coefficient of thermal expansion than aluminum and a melting temperature, A method of using a paste composition containing an inorganic compound, an aluminum powder, and an organic vehicle in which either the softening temperature or the decomposition temperature is higher than the melting point of aluminum is disclosed. Furthermore, in Patent Document 5, warping of the silicon semiconductor substrate is reduced by adding at least one of organic compound particles and carbon particles to the conventional paste composition to suppress shrinkage of the aluminum electrode during firing. A method is disclosed.

However, all the pastes used in these methods use ethyl cellulose as the binder resin. Since ethyl cellulose has poor thermal decomposability, organic substances are likely to remain in a short baking process, and it is difficult to form a dense film. Therefore, it is necessary to apply a thick paste, and thus warp cannot be prevented. There was a problem. Moreover, since it is difficult to form a dense film, there is a problem that the electric resistance is increased.

JP 2002-217435 A JP 2002-141533 A JP 2000-90734 A Japanese Patent Laid-Open No. 2003-223813 JP 2004-134775 A

Even if the silicon semiconductor substrate is thinned, the present invention can suppress deformation (warping) of the fired silicon semiconductor substrate without lowering the mechanical strength and adhesion of the electrode, and lower the electrical resistance. An object of the present invention is to provide a conductive fine particle-dispersed paste that can be used.

The present invention is a conductive fine particle-dispersed paste used for forming a back electrode of a solar cell, and contains a (meth) acrylic resin, an organic solvent and conductive fine particles, and the (meth) acrylic resin has molecular terminals. The organic solvent is a terpene-based solvent having at least one hydroxyl group in the molecule, or (carbon number / hydroxyl number) is less than 5 It is a conductive fine particle dispersed paste which is a polyol solvent.
The present invention is described in detail below.

As a result of earnest study, the present inventors have a polar functional group at the molecular end as a (meth) acrylic resin in a conductive fine particle dispersed paste containing a (meth) acrylic resin, an organic solvent and conductive fine particles, And the thing of a weight average molecular weight 10000-40000 was used, and the terpene solvent which has at least one hydroxyl group in a molecule | numerator, or the polyol solvent whose (carbon number / hydroxyl number) is less than 5 was used as an organic solvent. In this case, surprisingly, since a high viscosity is developed as compared with the case where a (meth) acrylic resin having a similar weight average molecular weight is added, the conductive fine particle dispersed paste having such a configuration is screen-printed. It has been found that the viscosity is suitable for screen printing. Furthermore, the film density of the printed film can be increased while fully taking advantage of the high thermal decomposability inherent in the (meth) acrylic resin and the small amount of sintered residue, thereby preventing warping of the substrate after firing. The inventors have found that the electrical resistance value can be reduced, and have completed the present invention.

In the present invention, when the (meth) acrylic resin having the above-described configuration and the organic solvent are used in combination, the reason why a high viscosity is developed as compared with the case where the (meth) acrylic resin having the same weight average molecular weight is added. Although it is not clear, it is considered that this is because the polar functional group present at the molecular end of the (meth) acrylic resin and the hydroxyl group of the organic solvent strongly interact with each other through hydrogen bonding or the like.
In addition, although the reason why the film density of the printed film is greatly improved is not clear, in the present invention, since the resin content can be reduced, the voids in the printed film can be reduced. This is probably because the conductive fine particles can be dispersed and stabilized by the effect of the polar functional group of the (meth) acrylic resin.

The conductive fine particle-dispersed paste of the present invention contains a (meth) acrylic resin, an organic solvent, and conductive fine particles. In this specification, (meth) acrylic resin means acrylic resin or methacrylic resin.

The lower limit of the weight average molecular weight of the (meth) acrylic resin is 10,000, and the upper limit is 40000. When the weight average molecular weight of the (meth) acrylic resin is less than 10,000, even when mixed with an organic solvent described later, the obtained conductive fine particle dispersed paste does not ensure a sufficient viscosity. The ability to form a pattern on a subsequent substrate is reduced. When the weight average molecular weight of the (meth) acrylic resin exceeds 40000, the adhesive strength of the obtained conductive fine particle dispersed paste becomes high, and the screen printability is lowered. A preferred lower limit is 15000 and a preferred upper limit is 35000.
The weight average molecular weight in the present specification is, for example, a polystyrene equivalent value based on the results obtained by gel permeation chromatography (GPC) analysis at room temperature using LF-804 manufactured by SHOKO as a column. Means what is required in

The (meth) acrylic resin preferably has a ratio (Mw / Mn) of 3 or less to the weight average molecular weight (Mw) and the number average molecular weight (Mn). When the above (Mw / Mn) exceeds 3, the proportion of the low molecular weight component and / or the high molecular weight component partially increases, and the resulting conductive fine particle dispersed paste has a reduced viscosity due to the low molecular weight component and / or An increase in adhesive strength due to a high molecular weight component is caused, and as a result, screen printability may be lowered. More preferably, it is 2.5 or less.

The (meth) acrylic resin has a polar functional group at the molecular end. It is considered that the presence of the polar functional group at the molecular end enables the (meth) acrylic resin to interact with the organic solvent or the conductive fine particles. As a result, the viscosity of the obtained conductive fine particle dispersed paste Is optimized and the screen printability is improved. Moreover, the polar functional group present at the molecular terminal is highly effective in dispersing and stabilizing the aluminum particles, and as a result, the resulting printed film can be made denser.

The presence of a polar functional group at the molecular end of the (meth) acrylic resin can be confirmed by, for example, ¹³ C-NMR measurement of a polymer extracted from the (meth) acrylic resin. That is, when a bond between carbon and sulfur is confirmed, it can be determined that a polar functional group is introduced at the molecular end.

The polar functional group is not particularly limited as long as it is a functional group having a chemical skeleton having hydrogen bonding properties, and examples thereof include a hydroxyl group, a carboxyl group, an amino group, a phosphate group, and an amide group. Among them, a hydroxyl group and a carboxyl group are preferable because of the small influence of the conductive fine particles in the conductive fine particle-dispersed paste caused by the thermal decomposition compound generated during sintering and the deterioration and damage of the printed substrate.

The production method of the (meth) acrylic resin having a polar functional group at the molecular terminal is not particularly limited. For example, in the presence of a chain transfer agent having a polar functional group, a (meth) acrylic monomer is free radical polymerization method, living (Meth) acrylic monomer in the presence of a polymerization initiator having a polar functional group, a method of polymerization or copolymerization by a conventionally known method such as radical polymerization method, iniferter polymerization method, anion polymerization method, living anion polymerization method, etc. May be polymerized or copolymerized by a conventionally known method such as free radical polymerization, living radical polymerization, iniferter polymerization, anion polymerization, or living anion polymerization. These methods may be used independently and may use 2 or more types together.

Moreover, you may manufacture the (meth) acrylic resin which has a polar functional group in the said molecular terminal using the azo polymerization initiator which has a polar functional group.
The azo polymerization initiator is not particularly limited, and 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis [2- (2-imidazoline). -2-yl) propane] disulfate dihydrate, 2,2′-azobis (2-methylpropionamidine) dihydrochloride, 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropion Amidine] hydrate, 2,2′-azobis {2- [1- (2-droxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, 2,2′-azobis [2- (2-imidazoline- 2-yl) propane], 2,2′-azobis (1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, 2,2′-azobis {2 Methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, 2,2′-azobis [2-methyl-N- (2-hydroxyethyl) propionamide], 4,4 And '-azobis (4-cyanovaleric acid).

The (meth) acrylic monomer is not particularly limited. For example, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, tert-butyl (meth) acrylate, From the group consisting of isobutyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, isobornyl (meth) acrylate, n-stearyl (meth) acrylate, benzyl (meth) acrylate, and (meth) acrylate having a polyoxyalkylene structure Examples include at least one selected. Among these, methyl (meth) acrylate is preferred because it is excellent in low-temperature degreasing property and can obtain a conductive fine particle-dispersed paste having a high viscosity even with a relatively small amount of resin.

The (meth) acrylic resin having a polar functional group at the molecular end may have a polar functional group such as a hydroxyl group, a carboxyl group, or a functional group containing a nitrogen element in the side chain as long as the sinterability is not inhibited. These structures are obtained by copolymerizing a (meth) acrylic acid alkyl ester monomer having a functional group containing a hydroxyl group, a carboxyl group, or a nitrogen element.

The (meth) acrylic acid alkyl ester monomer having a hydroxyl group is not particularly limited. For example, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, cyclohexanedimethanol mono (meth) acrylate, neopentyl glycol mono (meth) acrylate, glycerol mono (meth) acrylate, trimethylolpropane mono (meth) acrylate, pentaerythritol mono (meth) acrylate Etc.

It does not specifically limit as said (meth) acrylic-acid alkylester monomer which has the said carboxyl group, For example, (meth) acrylic acid, 2-carboxyethyl (meth) acrylate, 2-methacryloyloxyethyl succinic acid etc. are mentioned. .

The (meth) acrylic acid alkyl ester monomer having a functional group containing a nitrogen atom is not particularly limited. For example, (meth) acrylamide, (meth) acrylonitrile, N-vinylpyrrolidone, N- (meth) acryloylmorpholine. 2-N, N-dimethylaminoethyl (meth) acrylate, 2-N, N-diethylaminoethyl (meth) acrylate, and the like.

Among the above-mentioned (meth) acrylic acid alkyl ester monomers having each functional group, a residue of decomposition residue is particularly small after degreasing, and therefore a (meth) acrylic acid alkyl ester monomer having a hydroxyl group is suitable. is there.

Although it does not specifically limit as content of the (meth) acrylic-acid alkylester monomer which has each functional group mentioned above in the said (meth) acrylate copolymer, A preferable upper limit in 100 weight part of (meth) acrylate copolymers Is 30 parts by weight. When the amount exceeds 30 parts by weight, it becomes easy to absorb water. Therefore, when a glass powder or the like is dispersed to form a paste, it may be difficult to obtain a composition having a stable viscosity for a long time. A more preferred upper limit is 15 parts by weight. Moreover, although it does not specifically limit as a minimum, Preferably it is 1 weight part.

The chain transfer agent having a polar functional group is not particularly limited. For example, phosphoric acid such as mercaptopropanediol having a hydroxyl group, thioglycerol having a carboxyl group, mercaptosuccinic acid, mercaptoacetic acid, aminoethanethiol having an amino group, etc. And thiophosphoric acid having a group.

The polymerization initiator having the polar functional group is not particularly limited, and examples thereof include P-menthane hydroperoxide (manufactured by NOF Corporation: Permenta H), diisopropylbenzene hydroperoxide (NOF) Company: Parkmill P), 1,2,3,3-tetramethylbutyl hydroperoxide (1,2,3,3-tetramethylbutyloxide) (manufactured by NOF Corporation: Perocta H), cumene hydroperoxide (Cumene hydroperoxide) ( NOF Corporation: Park Mill H-80), t-Butyl hydroperoxide (NOF Corporation: Perbutyl H -69), cyclohexanone peroxide (manufactured by NOF Corporation: Perhexa H), 1,1,3,3-tetramethylbutyl hydroperoxide, t-butyl hydroperoxide, t-amyl hydroperoxide, Examples thereof include Disuccinic acid peroxide (Perroyl SA), etc. Various azo initiators having a nitrogen functional group may be used.

The content of the (meth) acrylic resin in the conductive fine particle dispersed paste of the present invention is not particularly limited, but the preferred lower limit is 0.5% by weight and the preferred upper limit is 10% by weight with respect to the whole conductive fine particle dispersed paste. is there. When the content of the (meth) acrylic resin is less than 0.5% by weight, the viscosity of the obtained conductive fine particle-dispersed paste is too low, and the ability to form a pattern on the substrate after removing the screen may be reduced. is there. When the content of the (meth) acrylic resin exceeds 10% by weight, the adhesive strength of the obtained conductive fine particle dispersed paste is too high, and stringing may occur when the screen is unframed. A more preferred lower limit is 1% by weight, and a preferred upper limit is 5% by weight.

The organic solvent is a terpene solvent having at least one hydroxyl group in the molecule or a polyol solvent having (carbon number / hydroxyl number) of less than 5. By using such an organic solvent in combination with the (meth) acrylic resin, due to the interaction between the polar functional group present at the molecular end of the (meth) acrylic resin and the hydroxyl group in the organic solvent, the above ( Even if the content of the (meth) acrylic resin is reduced, the viscosity of the obtained conductive fine particle dispersed paste can be maintained in a state suitable for the screen printing process. As a result, the conductive fine particle-dispersed paste does not generate stringing or the like when the screen is removed and can form a dense printed film.

In the present invention, a terpene solvent having at least one hydroxyl group in the molecule is used as the organic solvent. When a terpene solvent that does not have a hydroxyl group in the molecule is used, the interaction with the polar functional group present at the molecular end of the (meth) acrylic resin does not occur, so that excellent screen printability cannot be realized, A dense printed film cannot be formed.

Among the organic solvents, the terpene solvent is not particularly limited as long as it has at least one hydroxyl group in the molecule, and examples thereof include α-terpineol and dihydroterpineol. Of these, α-terpineol is preferred. These terpene solvents may be used alone or in combination of two or more.

In the present invention, a polyol solvent having (carbon number / hydroxyl number) of less than 5 is used as the organic solvent. When the polyol solvent has a carbon number / hydroxyl group number of 5 or more, the interaction with the polar functional group present at the molecular end of the (meth) acrylic resin is weakened, thus realizing excellent screen printability. It is not possible to form a dense printed film.

Among the above organic solvents, the polyol solvent is not particularly limited as long as (the number of carbon atoms / the number of hydroxyl groups) is less than 5. For example, 1,3-butanediol, 1,5-pentanediol, 3-methyl-1 , 5-pentanediol, 2,4-diethyl-1,5-pentanediol, 2-ethyl-1,3-hexanediol, glycerin and the like. Among these, 2,4-diethyl-1,5-pentanediol and 2-ethyl-1,3-hexanediol are preferable. These polyol solvents may be used independently and may use 2 or more types together.

The organic solvent has a preferred lower limit of viscosity at room temperature of 50 cps. When the viscosity is less than 50 cps, the conductive fine particle dispersed paste obtained does not have a viscosity suitable for screen printing, and a dense printed film may not be formed after firing.
In addition, the said viscosity means the viscosity when a viscosity is measured using a B-type viscometer at normal temperature.

The conductive fine particle dispersed paste of the present invention contains conductive fine particles.
Examples of the conductive fine particles include metal powders such as aluminum powder, gold powder, silver powder, copper powder, nickel powder, platinum powder and palladium powder, and metal oxide powders such as alumina, zinc oxide and ferrite. Of these, aluminum powder is preferred.

The aluminum powder preferably has an average particle diameter of 0.5 to 20 μm and a substantially spherical shape. When the average particle size is less than 0.5 μm, the specific surface area of the aluminum powder is increased, and when it exceeds 20 μm, the dispersion stability may be lowered when a conductive fine particle-dispersed paste is obtained. When applied to a silicon wafer, the contact between the silicon wafer and the aluminum powder decreases, and a uniform aluminum-silicon alloy layer may not be obtained after firing. More preferably, it is 1-10 micrometers. The substantially spherical shape includes particles having a shape close to a sphere in addition to a true sphere shape.

Moreover, as said aluminum powder, it is preferable to use what combined the aluminum particle with an average particle diameter of 0.5-5 micrometers and the aluminum particle with an average particle diameter of 5-20 micrometers in arbitrary ratios. Thereby, the packing of the aluminum particles in the film obtained after sintering becomes denser.

The content of the conductive fine particles in the conductive fine particle-dispersed paste of the present invention is not particularly limited, but a preferable lower limit with respect to the entire conductive fine particle-dispersed paste is 60% by weight, and a preferable upper limit is 85% by weight. If the content of the conductive fine particles is less than 60% by weight, the resulting conductive fine particle-dispersed paste is too low in viscosity to be suitable for screen printing, or the sintered aluminum film Resistance may increase, leading to a decrease in energy conversion efficiency of the solar cell. On the other hand, when the content of the conductive fine particles exceeds 85% by weight, the applicability of the conductive fine particle-dispersed paste during screen printing may deteriorate. A more preferred lower limit is 65% by weight, and a more preferred upper limit is 80% by weight.

The conductive fine particle dispersion paste of the present invention preferably further contains an anionic dispersant having at least one polar functional group. Such an anionic dispersant is a stabilizing aid for the microphase separation structure of the mixture phase of the (meth) acrylic resin and the organic solvent, and a dispersion stabilizing aid for the aluminum powder and the (meth) acrylic resin. It is thought to work as an agent. Such a stabilizing effect cannot be expected from a nonionic dispersant having no polar functional group.

Although it does not specifically limit as said anionic dispersing agent, For example, polycarboxylic acid (aliphatic polyhydric carboxylic acid) or its salt, salt of polyaminoamide and high molecular acid polyester, phosphate ester, polyamino amide phosphoric acid or The salt etc. are mentioned, Among these, the dispersing agent which has polycarboxylic acid as a main component is especially preferable. In addition, when the anionic dispersant is added in a large amount, the thermal decomposability of the obtained conductive fine particle-dispersed paste is lowered, the degreasing property is impaired, or the conductive fine particle-dispersed paste adheres to the substrate. May decrease. Therefore, the upper limit with preferable content is 5 weight% with respect to the whole electroconductive fine particle dispersion paste.

The conductive fine particle-dispersed paste of the present invention preferably further contains glass frit. For example, when the glass frit is applied to a silicon substrate, it is considered that the glass frit functions to further strengthen the bond between the aluminum electrode layer and the silicon substrate. The glass frit content is preferably 5% by weight or less. If the content of the glass frit exceeds 5% by weight, the glass may be segregated. The glass frit is most preferably non-lead glass that does not adversely affect the environment, but lead-containing glass may also be used. The average particle size of the glass frit particles is not particularly limited, but is preferably 20 μm or less.

The conductive fine particle-dispersed paste of the present invention may contain other conventionally known additives as necessary within the range not impairing the effects of the present invention.

The method for producing the conductive fine particle-dispersed paste of the present invention is not particularly limited. For example, the (meth) acrylic resin, the organic solvent, the conductive fine particles, and an additive to be added as necessary are conventionally used, such as a three roll. It can manufacture by the method of mixing for a predetermined time using a well-known stirrer.
The method of mixing is not particularly limited, and examples thereof include a method of mixing using a homodisper, a universal mixer, a Banbury mixer, a kneader and the like.

Since the conductive fine particle-dispersed paste of the present invention has an appropriate viscosity characteristic, it is excellent in screen printability and can be suitably used for the production of a back electrode of a solar cell. Moreover, since the film density is increased and a dense film is formed after firing, the electric resistance of the obtained electrode can be kept low. Furthermore, by using the conductive fine particle dispersion paste of the present invention, it is not necessary to apply the conductive fine particle dispersion paste thickly, and it is possible to prevent the electrode from warping.

According to the present invention, even when the silicon semiconductor substrate is thinned, the mechanical strength and adhesion of the electrode are not lowered, deformation (warping) of the fired silicon semiconductor substrate can be suppressed, and electrical resistance can be reduced. A conductive fine particle dispersed paste that can be lowered can be provided.

It is sectional drawing which shows the structure of a common solar cell typically.

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

Example 1
In a 2 L separable flask equipped with a stirrer, cooler, thermometer, hot water bath, and nitrogen gas inlet, 25 parts by weight of methyl methacrylate (MMA), 75 parts by weight of n-butyl methacrylate (BMA), as a chain transfer agent 2 parts by weight of mercaptosuccinic acid and 100 parts by weight of terpineol as an organic solvent were added to obtain a monomer mixture.

The obtained monomer mixture was bubbled with nitrogen gas for 20 minutes to remove dissolved oxygen, and then the temperature inside the separable flask system was replaced with nitrogen gas and heated until the hot water bath boiled with stirring. A solution obtained by diluting the polymerization initiator with ethyl acetate was added. During the polymerization, an ethyl acetate solution containing a polymerization initiator was added several times. Seven hours after the start of polymerization, the polymerization was terminated by cooling to room temperature. Thereby, a terpineol solution of a copolymer of methyl methacrylate and butyl methacrylate having a carboxyl group at the terminal was obtained.

To the terpineol solution of the copolymer of methyl methacrylate and butyl methacrylate obtained, terpineol was further added so as to have the composition ratio shown in Table 1, and dispersed with a high-speed disperser. Furthermore, BykP104 (manufactured by Big Chemie) as a nonionic dispersant, aluminum fine particles (average particle size of 5 μm) and glass frit (average particle size of 1 μm) as conductive fine particles were added so as to have a composition ratio described in Table 1, After sufficiently kneading using a high-speed stirrer, treatment with a three-roll mill was performed while paying attention not to flatten the aluminum fine particles, thereby preparing a conductive fine particle-dispersed paste.

(Examples 2 to 13, Comparative Examples 1 to 11)
A conductive fine particle-dispersed paste was prepared in the same manner as in Example 1 except that each raw material was adjusted so that the composition ratio described in Table 1 was obtained. As the surfactant, Disparon 2150 (manufactured by Enomoto Kasei Co., Ltd.) was used in addition to BykP104.

(Example 14)
In a 2 L separate flask equipped with a stirrer, cooler, thermometer, hot water bath and nitrogen gas inlet, 30 parts by weight of methyl methacrylate (MMA), 40 parts by weight of butyl methacrylate (BMA), 2-hydroxyethyl methacrylate (HEMA) ) 30 parts by weight and 100 parts by weight of terpineol as an organic solvent were mixed to obtain a monomer mixture.

The obtained monomer mixture was bubbled with nitrogen gas for 20 minutes to remove dissolved oxygen, and then the temperature inside the separable flask system was replaced with nitrogen gas and heated up until the oil bath reached 130 ° C. while stirring. . As a polymerization initiator, a solution obtained by diluting 4,4'-azobis (4-cyanovaleric acid) with terpineol was added. Further, a terpineol solution containing a polymerization initiator was added several times during the polymerization, and 3 parts by weight of the polymerization initiator was added to 100 parts by weight of the monomer in total.

Seven hours after the start of polymerization, the polymerization was terminated by cooling to room temperature. As a result, a terpineol solution of a (meth) acrylic resin having a cyano group and a carbonyl group at the molecular end was obtained. The obtained polymer was analyzed by gel permeation chromatography using column LF-804 (manufactured by SHOKO) as a column, and the weight average molecular weight in terms of polystyrene was as shown in Table 1.
To the terpineol solution of the methacryl binder modified with the terminal polar group, terpineol was further added so as to have the composition ratio shown in Table 1, and dispersed with a high-speed disperser. Furthermore, Disparon 2150 (manufactured by Enomoto Kasei Co., Ltd.) as a nonionic dispersant, aluminum fine particles (average particle size of 5 μm), and glass frit (average particle size of 1 μm) as conductive fine particles are added so as to have the composition ratio shown in Table 1. Then, after sufficiently kneading using a high-speed stirrer, treatment was performed with a three-roll mill while paying attention not to flatten the aluminum fine particles, thereby preparing a conductive fine particle dispersed paste.

(Example 15)
A conductive fine particle dispersion paste was prepared in the same manner as in Example 14 except that 2-ethyl-1,3-hexanediol was used as a solvent and BykP104 (manufactured by Big Chemie) was used as a surfactant.

(Evaluation)
The (meth) acrylic resin and conductive fine particle dispersed paste obtained in the examples and comparative examples were evaluated by the following methods. The results are shown in Tables 1 and 2.

(1) As a measurement column for the weight average molecular weight of the (meth) acrylic resin, a column LF-804 manufactured by SHOKO was used, and analysis by gel permeation chromatography was performed. The weight average molecular weight was calculated in terms of polystyrene.

(2) Evaluation of thermal decomposition behavior of (meth) acrylic resin Thermogravimetric measuring device (trade name: simulated SDT 2960, manufactured by TA Instruments Co., Ltd.) was used and the temperature was increased from 23 ° C. to 800 ° C. at a temperature rising rate of 200 ° C./min. And the thermal decomposition behavior of the resin was measured. As a result, what decomposed | disassembled resin within 2 minutes was evaluated as (circle), what took 2 minutes or more, or the thing which the resin did not decompose | disassemble but remained and a sintering residue remained.

(3) Screen printing property The obtained conductive fine particle dispersed paste was applied to a screen printing machine ("MT-320TV" manufactured by Microtech), screen plate making ("ST200" emulsion 12 [mu] manufactured by Tokyo Process Service Co., Ltd.), screen frame: 320 mm x 320 mm) and a printed glass substrate (soda glass: 150 mm × 150 mm, thickness: 1.5 mm), printing was performed in an environment of a temperature of 23 ° C. and a humidity of 50%.
○ If the screen printing plate does not leave a trace on the glass substrate and a clean printing surface is obtained, the glass substrate sticks to the screen plate making and a clean printing surface cannot be obtained, or conductive fine particle dispersed paste on the screen plate making Was left in large quantities and could not be printed.
As a result of the evaluation, although Examples and Comparative Examples 10 and 11 using ethylcellulose could be printed satisfactorily, Comparative Examples 1 to 9 could not be printed.

(4) Dry film strength After the substrate on which the conductive fine particle dispersed paste was printed on the silicon wafer (thickness: 300 μm) by screen printing under the same printing conditions as “(3) Screen printability” was dried at 150 ° C. for 10 minutes, The film strength was evaluated using a pencil hardness meter (manufactured by Yasuda Seiki Seisakusho, Model 553-M).
As a result of the evaluation, the hardness of HB was confirmed in all dry films for the examples, but only a strength of about 6B was obtained for Comparative Examples 9 and 10 using ethyl cellulose. If the dry film strength is low, the wire mark may remain in the wire furnace where the printed surface directly touches the wire, which may cause a problem.

(5) The board | substrate created by sinterability "(4) dry film intensity | strength" was baked by hold | maintaining for 2 minutes in oven of 730-750 degreeC. The case where the silicon wafer or the conductive fine particle dispersed paste was distorted, warped or cracked, or the strength of the sintered film was weak and peeled off was evaluated as x.
As a result of the evaluation, the examples could be resolved without problems.

(6) Surface roughness (Ry)
The surface roughness of the sintered film prepared in “(5) Sinterability” was measured using a stylus meter (using a stylus meter manufactured by KLA, Scan Length: 15 mm, Scan Speed: 0.2 mm / s, Sampling Rate: 100 Hz). The maximum height Ry was measured.
As a result of the evaluation, it was confirmed that the examples had less surface irregularities than the comparative example using ethyl cellulose. That is, in the Example of this invention, it turned out that the dispersibility of the aluminum powder in a paste is favorable compared with the paste using ethyl cellulose. This difference in dispersibility is considered to affect the shape after printing.

(7) Electrical resistance value The left and right terminals of a digital tester (custom, CDM-6000) were applied to the sintered film formed on the silicon wafer after firing, and the electrical resistance value was measured.
As a result of the evaluation, in the examples, a low resistance value was confirmed as compared with the comparative example using ethyl cellulose. A small amount of residue after sintering is considered to have an effect on electrical characteristics.

(8) Wafer-Al interface surface resistance value The aluminum film sintered on the silicon wafer prepared in “(5) Sinterability” is peeled off by treating with hydrochloric acid, and the wafer has a metallic luster-Al interface. Was taken out. The surface resistance value of the appeared interface was measured using Loresta GP (Mitsubishi Chemical Corporation). The surface resistance value of the wafer-Al interface correlates with the BSF effect, and the smaller the value, the better. In particular, it is preferably 11Ω / □ or less.
As a result of the evaluation, it can be seen that the example of the present invention forms the BSF layer without any problem.

ADVANTAGE OF THE INVENTION According to this invention, the electroconductive fine particle dispersion | distribution paste which can be used suitably for the back surface electrode formation of a solar cell can be provided.

1 p-type silicon semiconductor substrate 2 n-type impurity layer 3 antireflection film 4 grid electrode 5 back electrode layer 6 Al—Si alloy layer 7 p ⁺ layer

Claims (6)

A conductive fine particle dispersion paste used to form a back electrode of a solar cell,
Containing (meth) acrylic resin, organic solvent and conductive fine particles,
The (meth) acrylic resin has a polar functional group at the molecular end, has a weight average molecular weight of 10,000 to 40,000, and
The conductive fine particle dispersion paste, wherein the organic solvent is a terpene solvent having at least one hydroxyl group in the molecule, or a polyol solvent having (carbon number / hydroxyl number) of less than 5. The conductive fine particle-dispersed paste according to claim 1, wherein the organic solvent has a viscosity at room temperature of 50 cps or more. The conductive fine particle-dispersed paste according to claim 1 or 2, wherein the conductive fine particles are aluminum powder having an average particle diameter of 0.5 to 20 µm and a substantially spherical shape. 4. The conductive fine particle dispersed paste according to claim 3, wherein the content of the aluminum powder is 60 to 85% by weight. 5. The conductive fine particle dispersion paste according to claim 1, further comprising an anionic dispersant having at least one polar functional group. Furthermore, glass frit is contained, The electroconductive fine particle dispersion paste of Claim 1, 2, 3, 4 or 5 characterized by the above-mentioned.

Images