JP2013207054A - Conductive paste for formation of electrode of solar cell element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a conductive paste for formation of an electrode of a solar cell element, with which it is possible to achieve favorable fire through also at the time of electrode formation and realize favorable paste viscosity stability at the time of storage even when a conductive component contains silver oxide.SOLUTION: A conductive paste for formation of an electrode according to the present invention contains conductive particles, a dispersant, a glass frit, an organic binder, a solvent, and a low-melting-point-metal-based component, wherein at least silver oxide is used as a material of the conductive particles and at least one of straight-chained or branched primary alkylamine and (polyoxyalkylene) alkyl phosphoric acid ester, which do not contain double bonds in alkyl group, is used as the dispersant.

Description

  The present invention relates to a conductive paste for forming an electrode of a solar cell element, and particularly relates to a conductive paste for forming an electrode that can be suitably used for forming a surface electrode or the like of a solar cell element.

  A general solar cell element is formed on a semiconductor substrate, a diffusion layer formed on the surface of the semiconductor substrate, an antireflection layer and a surface electrode formed on the diffusion layer, and a back surface of the semiconductor substrate. A back electrode. Of these, the front electrode and the back electrode are generally formed using a conductive paste, and the conductive paste forming these electrodes may be the same or different. A general conductive paste is composed of conductive particles, an organic binder, a solvent, and glass frit, and may contain various additives as necessary.

  Here, the surface electrode is formed not by directly forming the conductive paste on the diffusion layer but by the fire-through (fired through) method. The fire-through method is a method in which a conductive paste is printed (or drawn) so as to have a predetermined electrode pattern (usually a finger-like electrode pattern) on the antireflection layer, and a firing process is performed. In the firing treatment, the glass frit contained in the conductive paste acts on the antireflection layer and dissolves it, so that the electrode pattern penetrates the antireflection layer (fire-through). As a result, the electrode pattern comes into contact with the diffusion layer under the antireflection layer, and as a result, a surface electrode is formed on the diffusion layer.

  The surface electrode formed by the fire-through method is not a Schottky connection (rectifying connection) but an ohmic connection to the diffusion layer. Therefore, the contact resistance between the surface electrode and the diffusion layer is substantially constant regardless of the current and voltage, and is sufficiently smaller than the resistance of the semiconductor substrate.

  However, depending on the composition of the conductive paste or the firing conditions, the fire-through may not be realized satisfactorily, and there is a possibility that a stable ohmic connection cannot be obtained between the surface electrode and the diffusion layer. If the ohmic connection is not stable, the series resistance of the solar cell increases and the value of the fill factor (FF) tends to be small. The performance of a solar cell is represented by an open circuit voltage, a short circuit current, and an FF value, and the conversion efficiency is calculated by multiplying the open circuit voltage, the short circuit current density, and the FF value, so that the conversion efficiency decreases as the FF value decreases. Resulting in. Therefore, in forming the surface electrode, it is important to design a suitable composition of the conductive paste, and it is also important to select firing conditions for the composition.

  By the way, in order to improve the power generation characteristics of the solar cell, it is important to optimize the characteristics of the electrodes themselves regardless of the front electrode and the back electrode. For example, power generation efficiency can be improved if the resistance value of the electrode can be lowered. In view of this, conventionally, various techniques for optimizing the resistance value of the electrode have been proposed in the field of conductive paste for forming a surface electrode of a solar cell.

For example, in the patent document 1, the applicant of the present application described in addition to the conductive particles, the organic binder, the solvent, and the glass frit in order to avoid a decrease in the FF value due to an increase in the contact resistance of the electrode. Alternatively, a conductive paste containing a low melting point metal component such as Se or TeO ₂ has been proposed. In addition, this electrically conductive paste may contain organic acids, such as a stearic acid, as a dispersing agent, and may contain the metal soap of a periodic table 2 group element.

  Further, Patent Document 2 is selected from silver carbonate, silver oxide and silver acetate in addition to silver powder, glass frit and organic vehicle for the purpose of further improving ohmic connection while ensuring high adhesive strength. A conductive paste containing a predetermined amount of at least one silver compound in terms of metallic silver is disclosed.

  Patent Document 3 discloses silver oxide for the purpose of improving the adhesion between the conductive electrode after firing and the silicon substrate without deteriorating the volume resistivity (specific resistance) of the conductive electrode after firing. And an organic solvent having a boiling point of 200 ° C. or higher, and using a conductive composition in which at least 50% by mass or more of the composition is silver oxide, a coating film is formed on the silicon substrate, and 500 to 850 ° C. A solar cell substrate is disclosed which is fired to form a conductive electrode. In Patent Document 3, the conductive composition may further contain silver carboxylate such as aliphatic carboxylate silver or silver 2-methylpropanoate, or may contain silver powder. In addition, it is disclosed that glass frit may be contained.

  Patent Document 4 can satisfy any of the volume resistivity of the conductive electrode after firing, the adhesion between the electrode and the silicon substrate, and the aspect ratio (height / width of electrode cross section). For this purpose, a coating film is formed on a silicon substrate using a conductive composition containing silver powder, silver oxide, and an organic solvent, and containing 50% by mass or more of silver alone and silver compound in silver powder. , A solar cell base material that is fired at 500 to 850 ° C. to form a conductive electrode is disclosed. Patent Document 4 also discloses that the conductive composition may contain silver carboxylate such as aliphatic carboxylate silver.

Japanese Patent No. 4754655 Japanese Patent No. 4556886 JP 2011-35061 A JP 2011-35062 A

  However, compositions containing silver oxide as an essential component as a conductive component, such as the conductive pastes disclosed in Patent Documents 2 to 4, have an increase in viscosity over time that is considered to be caused by decomposition of silver oxide. It becomes remarkable. Therefore, the conductive paste containing silver oxide has a problem that the stability of the viscosity during storage is lacking. In the following description, the stability of the viscosity of the conductive paste during storage is referred to as “paste viscosity stability” for convenience.

On the other hand, since the conductive paste disclosed in Patent Document 1 proposed by the applicant of the present application contains Te, Se, or TeO ₂ , it is possible to suppress sintering of the conductive particles, and thus the obtained electrode. As a result, it is possible to effectively suppress a decrease in the FF value. However, since the conductive paste disclosed in Patent Document 1 may contain silver oxide as the conductive particles, when blending silver oxide, the paste viscosity stability is the same as in Patent Documents 2 to 4. There is room for improvement.

  The present invention has been made to solve such a problem, and can achieve good fire-through even at the time of electrode formation, and can be stored even when the conductive component contains silver oxide. An object of the present invention is to provide a conductive paste for forming an electrode of a solar cell element, which can sometimes achieve good paste viscosity stability.

  As a result of intensive studies in view of the above problems, the present inventors have found that when a conductive component contains silver oxide, by using a specific type of alkylamine or alkyl phosphate ester as a dispersant, a good fire-through and It was found that it was possible to achieve both good paste viscosity stability and that sufficient fire-through could be achieved even when the firing temperature was less than 800 ° C., and the present invention was completed.

  That is, the conductive paste for forming an electrode of a solar cell element according to the present invention includes a conductive particle, a dispersant, a glass frit, an organic binder, a solvent, and a low melting point metal in order to solve the above-described problems. As the material of the conductive particles, at least silver oxide is used, the low melting point metal component is a metal having a melting point of 500 ° C. or lower, or an oxide thereof, and the dispersant is The structure is at least one of a linear or branched primary alkylamine that does not contain a double bond in the alkyl group, and a (polyoxyalkylene) alkyl phosphate ester.

  According to the above configuration, in a composition in which silver oxide is contained in the conductive component, a linear or branched primary alkylamine that does not contain a double bond in the alkyl group as a dispersant, and (polyoxyalkylene) alkylphosphoric acid Since at least one of the esters is used, the obtained conductive paste can achieve good fire-through during firing for electrode formation, and can avoid a decrease in FF value. And since the raise of the viscosity with time of the electrically conductive paste obtained can be suppressed, the improvement of paste viscosity stability can be aimed at. Furthermore, even if the peak temperature when firing the electrode-forming conductive paste is less than 800 ° C., it is possible to sufficiently realize fire-through.

  In the electrode-forming conductive paste having the above structure, the conductive particles are a mixture of silver powder and silver oxide powder, or silver oxide powder, and the weight mixing ratio of these powders is silver powder / silver oxide powder = 97. The structure which exists in the range of / 3-0/100 may be sufficient.

In the electrode-forming conductive paste having the above-described configuration, the low-melting-point metal component may be Te, Se, or TeO ₂ .

  In the conductive paste for electrode formation having the above-described configuration, the content of the dispersant may be in the range of 0.05 to 10% by weight.

  Further, the electrode-forming conductive paste having the above-described configuration may be used for forming a surface electrode of a solar cell element, and may have a baking temperature of less than 800 ° C. when the surface electrode is formed.

  In the present invention, with the above configuration, good fire-through can be achieved even during electrode formation, and good paste viscosity stability can be achieved during storage even when the conductive component contains silver oxide. It is possible to provide a conductive paste for forming an electrode of a solar cell element.

It is typical sectional drawing which shows an example of the general structure of a solar cell element which is the object which forms an electrode using the conductive paste for electrode formation which concerns on this invention.

The conductive paste for forming an electrode of the solar cell element according to the present invention comprises (A) conductive particles, (B) a dispersant, (C) glass frit, (D) an organic binder, and (E) a solvent. (F) a low melting point metal such as Te or Se or a low melting point metal compound such as TeO ₂ , (A) As the conductive particles, a mixture of silver and silver oxide, or silver oxide is used. (B) As a dispersing agent, it becomes the structure which uses the linear or branched primary alkylamine which does not contain a double bond in an alkyl group, or (polyoxyalkylene) alkyl phosphate ester. Hereinafter, preferred embodiments of the present invention will be specifically described. In the following description, the low melting point metal and the low melting point metal compound are collectively referred to as “(F) low melting point metal component” for convenience.

[(A) Conductive particles]
The conductive particles (A) used in the present invention are at least silver oxide powder, silver powder may be used in combination, silver compound such as silver carbonate powder and silver acetate powder, metal powder coated with silver, Other silver-based powders such as silver-containing alloy (silver alloy) powder may be used in combination. Further, a metal powder or a metal compound powder other than the silver-based powder may be used in combination depending on the type or characteristics of the electrode to be formed.

  Further, in the present invention, it is only necessary that the silver oxide powder is contained as the conductive particles (A), but the present invention is not limited to this, and it is sufficient that at least silver oxide is contained as the “conductive component”. A) Silver oxide should just be used at least as a material of electroconductive particle. Therefore, for example, a powder made of a mixture of silver oxide and another silver compound may be used as the conductive particles (A), or a silver powder coated with silver oxide may be used.

  The shape of the conductive particles (A) used in the present invention is not particularly limited, and for example, it may be a scale shape, a spherical shape, a flake shape, or an indefinite shape, or a mixture of these shapes may be used.

  The average particle diameter of the conductive particles (A) used in the present invention is not particularly limited, but is preferably in the range of 0.1 to 15 μm. (A) The average particle size of the conductive particles affects the sintering characteristics of the resulting electrode-forming conductive paste. In general, particles having a large particle size are sintered more slowly and slowly than particles having a small particle size. Is done. Therefore, if the average particle size is less than 0.1 μm, the sintering speed is too high, and the physical adhesive strength of the obtained electrode may be insufficient. On the other hand, when the average particle size exceeds 15 μm, the sintering speed is somewhat slow, but the dispersibility in the paste and the printability of the paste tend to deteriorate, and in particular, the electrode pattern is printed with high-definition lines. (Or drawing) may be difficult.

  In addition, the average particle diameter in this invention shall point out the particle diameter of accumulation 50% from the small diameter side in the particle diameter measured by the micro track type particle size distribution measuring method.

The specific surface area of the conductive particles (A) used in the present invention is not particularly limited, but is preferably in the range of 0.05 to ⁵ m ² / g. Within this range, it is possible to print the electrode pattern with high-definition lines, and the paste viscosity can be easily adjusted. On the other hand, if the specific surface area is less than 0.05 m ² / g, the particle size of the conductive particles (A) tends to be too large, and it may be difficult to print the electrode pattern with high-definition lines. is there. In addition, if the specific surface area exceeds 5 m ² / g, a large amount of (E) solvent is required to adjust the paste viscosity, which may reduce the workability of printing and may affect the paste viscosity stability. There is.

  Particularly preferable (A) conductive particles in the present invention are a mixture of silver powder and silver oxide powder, or silver oxide powder. Here, the mixing ratio of the silver powder and the silver oxide powder is preferably in the range of silver powder / silver oxide powder = 97/3 to 0/100 in terms of weight ratio. If the silver oxide powder in the mixture is less than 3% by weight, the effect obtained by using the silver oxide powder cannot be sufficiently obtained, and a decrease in paste viscosity stability derived from silver oxide can be almost ignored.

  The content (blending amount) of (A) conductive particles in the electrode-forming conductive paste according to the present invention is not particularly limited, but is preferably in the range of 65 to 95% by weight with respect to the entire conductive paste. . (A) If content of electroconductive particle is less than 65 weight%, there exists a possibility that the specific resistance value of the electrode obtained may increase by the amount being too small. On the other hand, if the content of (A) the conductive particles exceeds 95% by weight, the printability of the conductive paste may be lowered, and the physical adhesive strength of the obtained electrode may be insufficient. Furthermore, if the content of (A) conductive particles is out of the above range, effects such as paste viscosity stability due to (B) dispersant described later may not be sufficiently obtained.

[(B) Dispersant]
(B) Dispersant used in the present invention includes (B-1) a linear or branched primary alkylamine that does not contain a double bond in the alkyl group, or (B-2) (polyoxyalkylene) alkylphosphorus. It is an acid ester. In the following description, for the sake of convenience, the former may be abbreviated as “(B-1) alkylamine” and the latter “(B-2) alkyl phosphate ester”. The “(polyoxyalkylene) alkyl phosphate ester” in the present invention may be a phosphate ester salt of a linear or branched higher alcohol, or a polyoxyalkylene adduct of a linear or branched higher alcohol. It shall mean that it may be a phosphate ester salt.

  Specifically, as (B-1) alkylamine, for example, hexylamine, heptylamine, octylamine, nonenylamine, decylamine (caprylamine), undecylamine, dodecylamine (laurylamine), tridecylamine, myristyl Examples include amine (tetradecylamine), pentadecylamine, palmitylamine (hexadecylamine), heptadecylamine (margarylamine), stearylamine (octadecylamine), and the like. In any of these primary alkylamines, the alkyl group may be linear or branched.

  Examples of (B-2) alkyl phosphate esters include 2-ethylhexyl phosphate, decyl phosphate, dodecyl phosphate, polyoxyethylene 2-ethylhexyl phosphate, polyoxyethylene branched tridecyl phosphate, and the like. Can be mentioned.

  Only one type of these (B) dispersants may be used, or two or more types may be appropriately selected and used in combination. Therefore, in the present invention, as the (B) dispersant, only one type of (B-1) alkylamine may be used, or two or more types may be used in combination, and (B-2) an alkyl phosphate ester. May be used alone, or two or more may be used in combination. Furthermore, when (B-1) alkylamine and (B-2) alkyl phosphate ester are used in combination, One type may be used, only one type may be used, two or more types may be used, and two or more types may be used.

  By blending such (B-1) alkylamine and / or (B-2) alkyl phosphate ester into the electrode-forming conductive paste, it only acts in the same manner as a general (B) dispersant. Even if the conductive paste contains silver oxide as a “conductive component”, the paste viscosity stability can be improved, and even when the electrode is formed using the conductive paste. , Can contribute to the realization of good fire-through.

  In the conductive paste for electrode formation according to the present invention, the content of (B-1) the alkylamine and / or (B-2) the alkyl phosphate ester as the dispersant (B) is not particularly limited. It is preferable to be within the range of 0.05 to 10% by weight with respect to the whole. If the content of these (B) dispersants is less than 0.05% by weight, various effects due to the addition of these (B) dispersants may not be stably obtained. On the other hand, if the content of the (B) dispersant exceeds 10% by weight, various effects at a level commensurate with the added amount of the (B) dispersant may not be obtained, and the specific resistance of the obtained electrode is low There is also a risk of rising.

[(C) Glass frit]
(C) Glass frit used by this invention is not specifically limited, In the field | area of the electroconductive paste for electrode formation, it is preferable to use well-known (C) glass frit used in order to implement | achieve a fire through. it can.

  As a preferred example of the (C) glass frit that can be used in the present invention, for example, those having a softening point in the range of 300 to 550 ° C. are preferable. If the softening point is lower than 300 ° C., depending on the specific composition or firing conditions of the conductive paste, good fire-through will not occur in the process of firing the electrode pattern (coating film) formed from the conductive paste. There is a case.

  On the other hand, if the softening point of (C) the glass frit is higher than 550 ° C., there is a possibility that sufficient melt flow does not occur during firing, and the obtained electrode may not be able to realize sufficient adhesive strength. In particular, in the present invention, since the firing temperature can be less than 800 ° C., (C) the glass frit preferably has a softening point of 550 ° C. or less, and more preferably 530 ° C. or less.

Specific examples of the (C) glass frit having a softening point in the range of 300 to 550 ° C. include, for example, Bi glass, Bi ₂ O ₃ —B ₂ O ₃ —ZnO glass, and Bi ₂ O ₃ —. Examples thereof include glass powders made of B ₂ O ₃ glass, Bi ₂ O ₃ —B ₂ O ₃ —SiO ₂ glass, Ba glass, BaO—B ₂ O ₃ —ZnO glass, and the like. The specific shape of these (C) glass frit is not specifically limited, A spherical shape, an indefinite shape may be sufficient, and another shape may be sufficient.

  The content of (C) glass frit in the electrode-forming conductive paste according to the present invention is not particularly limited, but is preferably in the range of 0.1 to 10% by weight with respect to the entire conductive paste. (C) If the glass frit content is less than 0.1% by weight, the resulting electrode may have insufficient adhesive strength. Further, if the content of (C) glass frit exceeds 10% by weight, the resulting electrode may float on the glass or may be defective in soldering performed on the electrode.

[(D) Organic binder and (E) solvent]
The (D) organic binder and (E) solvent used in the present invention are not limited to the electrode forming application of the solar cell element, and are known organic resins and solvents, or organic resins and solvents in the field of conductive paste. An organic vehicle containing can be preferably used.

  Specific organic binders include, for example, cellulose derivatives such as methyl cellulose and ethyl cellulose; polyolefin resins such as polypropylene resins, polyvinyl chloride resins, styrene resins, vinyl acetate resins, polybutene resins, and vinyl resins; alkyds Polyester resins such as resins; Natural resins such as rosin resins and terpene resins; Acrylic resins such as acrylic resins and acrylate resins; Amino resins such as urea resins and melamine resins; Xylene resins , Dicyclopentadiene resins, phenolic resins, and the like, and cross-linked thermosetting resins having a cyclic structure; coumarone indene resins; polyether resins; polyurethane resins; polyisobutylene resins; . These organic resins may be used alone or in appropriate combination of two or more.

  The content of the organic binder (D) in the electrode-forming conductive paste according to the present invention is not particularly limited, but is preferably in the range of 0.1 to 30% by weight with respect to the entire conductive paste. (D) If content of an organic binder is less than 0.1 weight%, there exists a possibility that sufficient adhesive strength cannot be implement | achieved in the coating film (electrode pattern) printed with an electrically conductive paste. On the other hand, when the content of the organic binder (D) exceeds 30% by weight, the paste viscosity is excessively increased and the printability of the conductive paste may be deteriorated.

  Specific examples of the solvent (E) include saturated hydrocarbons such as hexane; aromatic hydrocarbons such as toluene; glycol ethers (cellosolve) such as ethyl cellosolve, butyl cellosolve, and butyl cellosolve acetate; diethylene glycol diethyl Glycol ethers such as ether and butyl carbitol (diethylene glycol monobutyl ether); Acetic esters of glycol ethers such as butyl carbitol acetate; Alcohols such as diacetone alcohol, terpineol and benzyl alcohol; Ketones such as cyclohexanone and methyl ethyl ketone; DBE, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, etc. And the like can be given; ester compounds. These solvents may be used alone or in combination of two or more.

  The content of the solvent (E) in the electrode-forming conductive paste according to the present invention is not particularly limited, but is preferably in the range of 1 to 40% by weight with respect to the entire conductive paste. (E) If the content of the solvent is less than 1% by weight, the absolute amount of the solvent may be too small to make a paste. Also, if the content of the solvent (E) exceeds 40% by weight, depending on the type and composition of other components, fluidity suitable for printing cannot be obtained, and the printability of the conductive paste decreases. There is a risk.

[(F) Low melting point metal component]
The (F) low melting point metal component (low melting point metal compound) used in the present invention acts as a sintering inhibitor, and includes a metal having a melting point of 500 ° C. or lower or a compound thereof.

  Specific examples of the low melting point metal having a melting point of 500 ° C. or lower include, for example, zinc (melting point 419.6 ° C.), lead (melting point 327.5 ° C.), tin (melting point 231.97 ° C.), bismuth (melting point 271). .3 ° C.), tellurium (melting point: 449.5 ° C.), selenium (melting point: 217 ° C.), and the like. Among these, tellurium (Te) and selenium (Se) are particularly preferable.

These low melting point metal compounds are not specifically limited. For example, tellurium compounds include tellurium chloride, tellurium dioxide, tellurite compounds, zinc telluride (ZnTe), tellurium tetrabromide, aluminum telluride. , Cadmium telluride, hydrogen telluride, potassium telluride, sodium telluride, gallium telluride, silver telluride, chromium telluride, germanium telluride, cobalt telluride, mercury telluride, tin telluride, tungsten telluride, tellurium Titanium fluoride, copper telluride, lead telluride, bismuth telluride, arsenic telluride, manganese telluride, molybdenum telluride, telluric acid, ammonium metatellate, potassium metatellate, rubidium metatellate, sodium metatellate, lead metatellurate, Tellurium iodide, tellurium sulfide, diphe Rujiterurido, mention may be made of tellurium octylate. Among these, tellurium dioxide (TeO ₂ ) and zinc telluride (ZnTe) are preferable, and TeO ₂ is particularly preferable.

  Examples of selenium compounds include ferroselen, selenized alloys, selenium dioxide, selenite, selenate, selenium disulfide, and selenium organometallic compounds.

The content of the (F) low-melting-point metal component in the electrode-forming conductive paste according to the present invention is not particularly limited, but within a range of 0.1 to 5% by weight if it is a low-melting-point metal such as Te or Se. If it is a compound such as TeO ₂ , it is preferably in the range of 0.01 to 10% by weight, more preferably in the range of 0.1 to 8% by weight, More preferably, it is within the range of 4% by weight. If the content of these (F) low melting point metal-based components is less than the lower limit of the above range, (A) the effect of suppressing the sintering of the conductive particles may not be sufficiently obtained, and the upper limit of the above range is exceeded. When it exceeds, there exists a possibility that the resistance value of the electrode obtained may increase and FF value may become small.

[Manufacture and use of conductive paste]
The manufacturing method of the electroconductive paste for electrode formation which concerns on this invention is not specifically limited, A well-known method can be used suitably in the field | area of an electroconductive paste. As a typical example, there is a method in which the above-described components are blended at a predetermined blending ratio and made into a paste using a known kneading apparatus. Examples of the kneading apparatus include a three roll mill.

  In the conductive paste for electrode formation according to the present invention, the above-mentioned (A) conductive particles, (B) dispersant, (C) glass frit, (D) organic binder, (E) solvent, and (F ) In addition to the low melting point metal component, various additives known in the field of conductive paste may be contained. Examples of such additives include, but are not limited to, stabilizers, antioxidants, ultraviolet absorbers, silane coupling agents, antifoaming agents, viscosity modifiers, and the like. Also, the additive amount (content) of the additive is not particularly limited, and it should be added within a range that does not affect the physical properties required of the electrode forming conductive paste or interfere with the effects obtained by the present invention. Can do.

  The conductive paste for forming an electrode according to the present invention is preferably used for forming an electrode of a solar cell element, and particularly preferably used for forming a surface electrode which is an electrode formed by a fire-through method. Can do. A general configuration of the solar cell element and a manufacturing method thereof will be specifically described.

  As shown in FIG. 1, a general solar cell element includes a surface electrode 1, an antireflection layer 2, an N-type diffusion layer 3, a P-type silicon substrate 4, and a back electrode 5. The N-type diffusion layer 3 is formed on the surface of the P-type silicon substrate 4, and the antireflection layer 2 and the surface electrode 1 are formed on the N-type diffusion layer 3. Further, the back electrode 5 is formed on the back surface of the P-type silicon substrate 4.

An example of a method for manufacturing this solar cell element will be described. First, an N-type diffusion layer 3 of impurities, silicon nitride, silicon oxide, or the like is formed on the light-receiving surface side (surface side) of a P-type silicon substrate 4 made of silicon. An insulating antireflection layer 2 made of titanium oxide or the like is sequentially formed. Here, in the present embodiment, the P-type silicon substrate 4 contains, for example, a semiconductor impurity such as boron of about 1 × 10 ¹⁶ to 10 ¹⁸ atoms / cm ^3, so that the P-type silicon substrate 4 has a specific resistance of about 1.5 Ωcm. It is comprised so that the conductivity type of this may be exhibited.

  When the P-type silicon substrate 4 is monocrystalline silicon, it is formed by a pulling method or the like, and when it is polycrystalline silicon, it is formed by a casting method or the like. Polycrystalline silicon can be mass-produced and is more advantageous than single crystal silicon in terms of manufacturing cost. The P-type silicon substrate 4 is obtained, for example, by slicing an ingot formed by a pulling method or a casting method into a thickness of about 100 to 300 μm.

The N-type diffusion layer 3 is an N-type layer that is formed by diffusing impurities such as phosphorus on the light-receiving surface of the P-type silicon substrate 4 and has the opposite conductivity type of the P-type silicon substrate 4. The N-type diffusion layer 3 is formed, for example, by placing a P-type silicon substrate 4 in a furnace and heating it in phosphorus oxychloride (POCl ₃ ) or the like. If the conductivity type of the silicon substrate 4 is N-type, the diffusion layer 3 may be P-type.

The antireflection layer 2 is a layer formed on the light receiving surface side of the N-type diffusion layer 3 for protecting the solar cell element together with the antireflection function. When the antireflection layer 2 is a silicon nitride film, for example, it is formed by a plasma CVD method or the like in which a mixed gas of silane (SiH ₄ ) and ammonia (NH ₃ ) is converted into plasma by glow discharge decomposition and deposited. For example, the antireflection layer 2 has a refractive index of about 1.8 to 2.3 in consideration of a refractive index difference with the P-type silicon substrate 4 and has a thickness of about 0.05 μm to 1.0 μm. Formed.

  A front electrode 1 is formed on the surface of the P-type silicon substrate 4, and a back electrode 5 is formed on the back surface. In addition, in order to improve the output of a solar cell element, you may form a BSF (Back Surface Field) layer in the back surface as needed. The front electrode 1 is generally composed of a bus bar electrode and a finger electrode, and the back electrode 5 is generally composed of a bus bar electrode and a current collecting electrode. The conductive paste composition for forming an electrode according to the present invention can be used for forming both the back electrode 5 and the front electrode 1, but can be particularly suitably used for forming the front electrode 1.

  Specifically, a coating film having a predetermined pattern of the conductive paste for electrode formation according to the present invention is formed on the upper surface of the antireflection layer 2 by screen printing or the like (pattern formation step), and the coating film is peaked within a predetermined range. Firing is performed for several tens of seconds to several tens of minutes so that the temperature is reached (firing step). During firing, the binder is removed from the organic binder to sinter the conductive particles, and fire through occurs due to the action of the glass frit contained in the conductive paste for electrode formation. Therefore, the surface electrode 1 that penetrates the antireflection layer 2 and is in ohmic contact with the N-type diffusion layer 3 is formed.

  In addition, although the conductive paste for electrode formation which concerns on this invention may be used for formation of the back surface electrode 5, the conductive paste of a different composition can be used. For example, in the case of bus bar electrode formation, (C) glass frit, (D) organic binder, similarly to the electrode forming conductive paste according to the present invention, except that silver powder and aluminum powder are used as the conductive component. (E) What contains a solvent, (B) a dispersing agent, etc. is used. Moreover, if it is for current collection electrode formation, except using only aluminum powder as an electroconductive component, (C) glass frit, (D) organic binder, (E) solvent similarly to the electroconductive paste which concerns on this invention The thing containing is used. Then, a coating film having a predetermined pattern may be formed on the substantially front surface by screen printing using these conductive pastes and fired.

Thus, in the conductive paste for electrode formation according to the present invention, (A) conductive particles containing silver oxide are used as the “conductive component”, and (B) (B-1) as the dispersant. Alkylamines and / or (B-2) alkyl phosphates are used, and (F) low melting point metal components such as Te, Se or TeO ₂ are used as sintering inhibitors. As a result, good fire-through can be realized, a decrease in the FF value can be avoided, and an increase in the viscosity of the obtained conductive paste over time can be suppressed. Improvements can be made.

  In particular, regarding paste viscosity stability, the viscosity change in one month can be suppressed to ± 50 Pa · s or less, and depending on the blending ratio of each component, it can be suppressed to less than ± 10 Pa · s. Therefore, even when the viscosity of the conductive paste is adjusted (manufactured) to a value suitable for printing and then stored for a long period of time, the printability is not impaired. Therefore, according to the present invention, it is possible to obtain a conductive paste having a small change in viscosity over time (excellent paste viscosity stability).

  Furthermore, in the present invention, the peak temperature for realizing sufficient fire-through can be suppressed to less than 800 ° C. when firing the electrode-forming conductive paste.

  Specifically, since the conductive paste for electrode formation according to the present invention contains (C) glass frit and (F) low melting point metal component, Patent Document 1 is a prior patent by the present applicant. It has the same composition as the conductive paste disclosed in (1). In the conductive paste disclosed in Patent Document 1, excellent fire-through can be achieved by setting the peak temperature during firing to 800 ° C. or higher, and effectively reducing the FF value can be effectively avoided. When the firing temperature is 800 ° C. or higher, although depending on various conditions, there is a case where the shrinkage of the back electrode 5 becomes so large that the warpage of the P-type silicon substrate 4 cannot be ignored. If the warpage of the P-type silicon substrate 4 becomes a significant degree, there is a possibility that the P-type silicon substrate 4 may be damaged during modularization or the characteristics of the solar cell may be deteriorated.

  On the other hand, the conductive paste for electrode formation according to the present invention can realize not only paste viscosity stability but also good fire-through even when the peak temperature during firing is less than 800 ° C. Thus, it is possible to effectively suppress the decrease in the FF value and increase the conversion efficiency of the obtained solar cell element. Therefore, according to the present invention, a highly efficient solar cell can be stably manufactured.

  The present invention will be described more specifically based on examples and comparative examples, but the present invention is not limited to this. Those skilled in the art can make various changes, modifications, and alterations without departing from the scope of the present invention. In addition, preparation of the solar cell element sample in the following Examples and Comparative Examples, preparation of various conductive pastes, and evaluation of characteristics of the solar cell element sample were performed as follows.

(Preparation of solar cell element samples and preparation of various conductive pastes)
[Preparation of semiconductor substrate]
As a semiconductor substrate of the solar cell element sample used in Examples and Comparative Examples, a semiconductor in which an n-type diffusion layer and an antireflection layer made of SiN _x are formed in this order on the surface of a polycrystalline silicon p-type semiconductor substrate. A wafer was prepared. The semiconductor wafer has a thickness of 200 μm and an outer shape of 156 mm × 156 mm.

[Preparation of various conductive pastes]
In preparation of the solar cell element sample, three types of conductive pastes for electrode formation shown in Table 1 were prepared.

  The conductive paste (1) was used to form the BSF layer on the back surface of the semiconductor substrate and the current collecting electrode on the back surface, and the conductive paste (2) was used to form the bus bar electrode on the back surface. The conductive paste (3) corresponds to the electrode forming conductive paste according to the present invention and was used to form a surface electrode on the surface of the semiconductor substrate.

  Each of these conductive pastes (1) to (3) is prepared by blending the components shown in Table 1 and Table 2 so as to have the composition shown in Table 1 and mixing them with a three roll mill to form a paste. Prepared. As for the conductive paste (3), as described later, at least one of (A) conductive particles, (B) a dispersant, (C) glass frit, and (F) a low-melting-point metal component is used. A plurality of types appropriately changed were prepared and used in Examples and Comparative Examples. Moreover, the solvent shown in Table 1 was added suitably and the paste viscosity was adjusted to about 300 Pa.s so that it might become a paste viscosity suitable for electroconductive paste (3) screen printing.

[Preparation of solar cell element sample]
A film to be a bus bar electrode is formed on the back surface of the semiconductor wafer by screen printing using the conductive paste (2), and then the BSF layer and the collector are also formed on the back surface using the conductive paste (1). A coating film to be an electric electrode is formed by screen printing, and further, a coating film to be a solar cell electrode (finger electrode and bus bar electrode) is formed on the surface of the semiconductor wafer by screen printing using the conductive paste (3). did.

  In addition, after forming each coating film and before forming the next coating film, the drying process for 5 minutes at 150 degreeC, and the natural cooling process after that to room temperature were performed. Here, the screen printing conditions were # 325 screen mesh, and the emulsion thickness (that is, the thickness of the conductive paste) was 25 μm. Moreover, the width of the coating film to be the finger electrode was 80 μm, and the width of the coating film to be the bus bar electrode was 2 mm.

  Thereafter, the semiconductor wafer having the coating film formed on both the front and back surfaces was baked in a high-speed baking furnace (BTU international Inc., trade name: PV309). The temperature data of the high-speed baking furnace can be confirmed with a temperature logger manufactured by Datapaq Ltd., and the maximum value of the actual temperature of the semiconductor wafer was evaluated as the baking peak temperature.

(Characteristic evaluation of solar cell element sample)
[Evaluation of electrical characteristics]
The electrical characteristics of the finger electrode formed using the conductive paste (3) of Example or Comparative Example are as follows. Tester (trade name: KST-15Ce-1s) manufactured by Kyoshin Electric Co., Ltd. and Solar manufactured by Wacom Denso Co., Ltd. A simulator (trade name: WXS-156S-10, AM1.5G) was used. Data acquisition was performed by a self-made program. The FF value and the conversion efficiency were calculated from the obtained IV curve and used as representative characteristics of electrical characteristics.

[Evaluation of paste viscosity stability]
The conductive paste (3) of Example or Comparative Example was stored in a constant temperature bath at 40 ° C. for 1 month, and the paste viscosity of each conductive paste before and after storage was measured. For measuring the paste viscosity, a viscometer manufactured by Brookfield (trade name: HBDV-III Ultra) was used. If the viscosity change after 1 month of storage is less than ± 10 Pa · s before storage, “◎”, if it is within the range of ± 10-50 Pa · s, “◯”, within ± 50-100 Pa · s. In this case, the paste viscosity stability of each conductive paste was evaluated as “X” if it exceeded “±” and ± 100 Pa · s.

(Examples 1-20)
As shown in Table 3, (A) As the conductive particles, silver powder and silver oxide powder are used by changing the mixing ratio (Examples 1 to 18), or only silver oxide powder is used (Examples 19 and 20). , (B) As a dispersant, any one of eight types of (B-1) alkylamines shown in Table 2 (Examples 1-4, 6, 7, 9, 10, 12-14, 16, 17, 19, 20), or any one of five types of (B-2) alkyl phosphate esters (Examples 5, 8, 11, 15, 18), and (C) a Ba system shown in Table 2 as a glass frit (Examples) 1-10) or Bi type (Examples 11 to 20), and (F) TeO ₂ (Examples 1, 3, 6, 9, 11, 12, 14, 16) or Te ( Examples 2, 4, 7, 10, 13, 15, 17, 19) or Se (Examples 5, 8, 18) 20) was used to prepare a conductive paste (3) as described above.

Similarly, after preparing other conductive pastes (1) and (2), solar cell element samples were prepared as described above, and the characteristics of the samples were evaluated. Table 3 shows the evaluation results and the firing peak temperature at the time of sample preparation.
(Comparative Examples 1-6)
As shown in Table 3, (A) use only silver powder as conductive particles (Comparative Examples 1 to 3), or mix silver powder and silver oxide powder at a mixing ratio of silver powder / silver oxide powder = 70/30 Used (Comparative Examples 4 to 6), (B) Three types of comparative dispersants shown in Table 2 were used as the dispersant (Comparative Examples 1 to 4: Comparative Dispersant 1, Comparative Example 5: Comparative Dispersant 2, Comparative Example 6: Comparative dispersant 3), (C) Bi type shown in Table 2 was used as the glass frit, and (F) TeO ₂ (Comparative examples 1, 2, 5) or Te (Comparative example) was used as the low melting point metal component. 3, 6) or Se (Example 4) was used to prepare a comparative conductive paste (3) as described above.

Similarly, after preparing other conductive pastes (1) and (2), solar cell element samples were prepared as described above, and the characteristics of the samples were evaluated. Table 3 shows the evaluation results and the firing peak temperature at the time of sample preparation.
As is clear from the evaluation results of Examples 1 to 20, the FF value of the finger electrode (surface electrode) using the electrode forming conductive paste according to the present invention is 0.780 or more, which is good when the finger electrode is formed. It can be seen that true fire-through has been realized. In addition, the paste viscosity stability is “」 ”or“ ◯ ”, indicating that the change in paste viscosity over time is small and excellent stability is exhibited. Furthermore, it can be seen that the firing peak temperature is less than 800 ° C. in any of the examples, and therefore fire-through can be realized even when firing at a temperature lower than 800 ° C.

  On the other hand, in Comparative Example 1, although the FF value is 0.780 or more and the paste viscosity stability is “◯”, the firing peak temperature is 820 ° C., and although the fire-through can be realized, the firing peak temperature is lowered. Not done. In Comparative Examples 2 and 3, although the firing peak temperature is less than 800 ° C. and the paste viscosity stability is “◯”, the FF value is less than 0.780, so that fire-through is not sufficiently realized. Further, in Comparative Examples 4, 5, and 6, although the firing peak temperature is less than 800 ° C., the FF value is less than 0.780 except for Comparative Example 6, and thus fire-through is not sufficiently realized. Also the paste viscosity stability is getting worse.

In Examples 1 to 20, three types of TeO ₂ , Te and Se are used as particularly preferable (F) low melting point metal components, and these exhibit substantially the same behavior as a sintering inhibitor ( For example, refer to Examples and Comparative Examples in Patent Document 1.) Further, other low-melting-point metal components other than those described above exhibit substantially the same behavior as a sintering inhibitor like these preferable elements or compounds.

  Further, the present invention is not limited to the description of the above-described embodiment or examples, and various modifications are possible within the scope shown in the claims, and different examples and a plurality of modified examples are respectively provided. Embodiments obtained by appropriately combining the disclosed technical means are also included in the technical scope of the present invention.

  The present invention can be suitably used for forming an electrode of a solar cell element, particularly a surface electrode. Therefore, the present invention can be used widely in a field related to solar cells.

DESCRIPTION OF SYMBOLS 1 Front electrode 2 Antireflection layer 3 N type diffused layer 4 P type silicon substrate 5 Back electrode

Claims (5)

Contains conductive particles, a dispersant, a glass frit, an organic binder, a solvent, and a low melting point metal component,
As the material of the conductive particles, at least silver oxide is used,
The low melting point metal component is a metal having a melting point of 500 ° C. or lower or an oxide thereof;
The dispersant is at least one of a linear or branched primary alkylamine that does not contain a double bond in the alkyl group, and a (polyoxyalkylene) alkyl phosphate ester,
A conductive paste for forming an electrode of a solar cell element. The conductive particles are a mixture of silver powder and silver oxide powder, or silver oxide powder;
The weight mixing ratio of these powders is characterized by being in the range of silver powder / silver oxide powder = 97/3 to 0/100,
The conductive paste for electrode formation of the solar cell element according to claim 1. The low-melting-point metal component is Te, Se, or TeO ₂ ,
The electrically conductive paste for electrode formation of the solar electronic element of Claim 1 or 2. The content of the dispersant is in the range of 0.05 to 10% by weight,
The electrically conductive paste for electrode formation of the solar cell element of any one of Claim 1 thru | or 3. It is used for forming the surface electrode of the solar cell element, and the firing temperature when forming the surface electrode is less than 800 ° C.,
The electrically conductive paste for electrode formation of the solar cell element of any one of Claim 1 thru | or 4.

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