JP2007242912A - Conductive paste and solar cell element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conductive paste with excellent adhesive strength and electric characteristics that is used for a surface electrode of a solar cell element. <P>SOLUTION: The conductive paste is used for forming the surface electrode of the solar cell element in which a region of an inverse conductivity is formed on the light receiving surface side of a semiconductor substrate of a conductivity, an antireflective film and the surface electrode are provided on the region, and an underside electrode is provided on the opposite side surface thereof. The conductive paste contains 100 w/t parts of silver powder, 0.1 to 10 w/t parts of glass frit, 0.05 to 3.0 w/t parts, a total amount in terms of metal silver, of at least one kind of silver compound selected from a silver carbonate, a silver oxide and silver acetate, and an organic vehicle. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fired conductive paste that contains silver powder and glass frit and is used for forming an electrode of a solar cell element, and a solar cell element formed using the same.

  As shown in FIG. 1, a general solar cell element includes at least a semiconductor substrate 1, a diffusion layer 2, an antireflection film 3, a back electrode 4, and a surface electrode 5.

Such a solar cell element is manufactured as follows, for example. First, an impurity diffusion layer 2 and an insulating antireflection film 3 made of silicon nitride, silicon oxide, titanium oxide, or the like are sequentially formed on the light receiving surface side (surface side) of the semiconductor substrate 1 made of silicon. . Here, the semiconductor substrate 1 contains, for example, a semiconductor impurity such as boron at about 1 × 10 ¹⁶ to 10 ¹⁸ atoms / cm ³ so as to have one conductivity type (for example, p-type) having a specific resistance of about 1.5 Ωcm. It is intended to be presented. In the case of monocrystalline silicon, it is formed by a pulling method or the like, and in the case of 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 semiconductor substrate 1 is obtained by slicing an ingot formed by a pulling method or a casting method to a thickness of about 300 μm.

The diffusion layer 2 is a region exhibiting an opposite conductivity type (for example, n-type) of the semiconductor substrate 1 formed by diffusing impurities such as phosphorus on the light receiving surface of the semiconductor substrate 1. The diffusion layer 2 is formed, for example, by placing the semiconductor substrate 1 in a diffusion furnace and heating it in phosphorus oxychloride (POCl ₃ ) or the like.

The antireflection film 3 is formed on the light receiving surface side of the diffusion layer 2 in order to protect the solar cell element together with the antireflection function. When the antireflection film 3 is a silicon nitride film, it is formed by, for example, a plasma CVD method in which a mixed gas of silane (SiH ₄ ) and ammonia (NH ₄ ) is plasmatized by glow discharge decomposition and deposited. The antireflection film 3 is formed so as to have a refractive index of about 1.8 to 2.3 in consideration of a refractive index difference with the semiconductor substrate 1 and is formed to a thickness of about 500 to 1000A.

  Further, a high-concentration p-type BSF layer (not shown) in which, for example, aluminum is diffused is formed on the surface opposite to the light receiving surface (rear surface side) of the semiconductor substrate 1. The electrode 5 is formed on the back surface by applying and baking a conductive paste on the back surface 4.

  Here, the surface electrode 5 is formed by printing a conductive paste containing silver powder, glass frit and an organic vehicle on the surface of the antireflection film 3 in a predetermined pattern and baking it at a high temperature of 500 to 900 ° C. . Usually, during baking, glass frit contained in the conductive paste acts on the antireflection film 3 to dissolve the film, and as a result, the surface electrode 5 penetrates the antireflection film 3 and the diffusion layer 2 and An electrical contact method is employed (fire-through method).

  In this method, when the surface electrode 5 is fired, a stable ohmic contact cannot be obtained between the surface electrode 5 and the diffusion layer 2 of the semiconductor substrate 1 due to variations in the action of the glass frit, and the adhesive strength also varies. There is a problem. When the ohmic contact is insufficient, a loss occurs when the output is taken out, and the conversion efficiency of the solar cell is lowered or the current-voltage characteristics are deteriorated.

In patent document 1, ohmic contact property and adhesive strength are achieved by including at least one of Ti, Bi, Co, Zn, Zr, Fe, Cr, or a compound thereof in a silver-based conductive paste for a surface electrode. It is improving. Patent Document 2 discloses a silver-based conductive paste mainly composed of silver powder, silver oxide, silver carbonate, silver acetate, and the like, and Ti, Bi, Zn, Y, In, Mo, or a compound thereof is extremely fine. It is described that a more stable ohmic contact is obtained by including a powder. Patent Document 3 describes that ohmic contact is improved by adding a small amount of phosphorus or a phosphorus compound to a silver paste for a surface electrode.
JP 2001-313400 A JP-A-2005-243500 Japanese Patent Publication No. 3-46985

  However, the effects of the additives described in these documents are not so great, and further improvements are desired. The present invention has been made in view of the above points, and further improves the ohmic contact while ensuring high adhesive strength, and reduces the loss at the time of taking out the output, thereby improving the conversion efficiency of the solar cell. An object is to provide a conductive paste for a surface electrode of a solar cell element, which can be further increased.

In order to solve the above problems, the invention according to claim 1 is a conductive paste,
A solar cell in which a region exhibiting a reverse conductivity type is formed on a light receiving surface side of a semiconductor substrate exhibiting one conductivity type, an antireflection film and a surface electrode are provided thereon, and a back electrode is provided on a surface opposite to the light receiving surface A conductive paste used for forming a surface electrode of an element, comprising 100 parts by weight of silver powder, 0.1 to 10 parts by weight of glass frit, at least one silver compound selected from silver carbonate, silver oxide and silver acetate In an amount of 0.05 to 3.0 parts by weight in terms of metallic silver, and an organic vehicle.

The invention according to claim 2 is the conductive paste according to claim 1,
The silver compound is in the form of particles having an average particle size of 5 μm or less.

The invention described in claim 3
A solar cell in which a region exhibiting a reverse conductivity type is formed on a light receiving surface side of a semiconductor substrate exhibiting one conductivity type, an antireflection film and a surface electrode are provided thereon, and a back electrode is provided on a surface opposite to the light receiving surface In the device, the surface electrode is formed by baking the conductive paste according to claim 1 or 2 at 500 to 900 ° C.

  By baking the conductive paste on the anti-reflection film of the solar cell element, the fire-through can be performed satisfactorily, and there is no variation in the surface electrode exhibiting high adhesive strength and excellent ohmic contact with the semiconductor substrate. Can be formed. As a result, a solar cell element having high power generation efficiency and good current-voltage characteristics can be obtained.

  Below, one Embodiment of the electrically conductive paste and solar cell element which concern on this invention is described. However, the scope of the invention is not limited to the following.

First, the conductive paste according to the present invention will be described.
In the conductive paste of the present invention, 0.1 to 10 parts by weight of glass frit with respect to 100 parts by weight of silver powder, and at least one silver compound of silver carbonate, silver oxide and silver acetate is converted into metal silver The total amount is 0.05 to 3.0 parts by weight and dispersed in an organic vehicle.

Hereinafter, each component will be described.
There is no restriction | limiting in particular as silver powder, What is used conventionally, such as spherical silver powder, flaky silver powder, dendritic silver powder, is used. In addition to pure silver powder, a silver-coated composite powder having a silver layer at least on the surface, an alloy containing silver as a main component, or the like may be used. The silver powder preferably has an average particle size of 0.1 to 10 μm.

  The glass frit is not limited as long as it can be softened at a low temperature during firing, penetrated by dissolving the antireflection film, and can firmly adhere the surface electrode to the semiconductor substrate through the diffusion layer. Commonly used lead borosilicate glass, bismuth borosilicate glass, zinc borosilicate glass, etc. having a softening point of about 300 to 600 ° C. are used. If the blending amount of the glass frit is less than 0.1 parts by weight with respect to 100 parts by weight of the silver powder, the adhesiveness and the electrode strength become extremely weak. On the other hand, if it exceeds 10 parts by weight, it is not desirable because glass floats on the electrode surface or solderability is hindered.

  By adding a specific amount of any one or a plurality of silver compounds of silver carbonate, silver oxide, and silver acetate as an additive to a conductive paste containing silver powder and glass frit, adhesion and reflection to a semiconductor substrate The ohmic contact between the surface electrode and the semiconductor substrate is improved without impairing the solubility of the prevention film. For this reason, the loss at the time of taking out an output can be reduced, and current-voltage characteristics (curve factor) and power generation efficiency can be improved.

  Although the mechanism of action of these silver compounds has not yet been elucidated, these silver compounds decompose during firing to produce active nano-sized silver ultrafine particles, which react directly with the semiconductor substrate to form silicon or the like. It is alloyed with the semiconductor element of the above, or dissolved in the molten glass, and when cooled, the nano-sized silver ultrafine particles that are active are precipitated and react with the semiconductor substrate to form an alloy. It is thought that ohmic contact is improved.

  If the amount of silver compound added is less than 0.05 parts by weight in terms of metallic silver with respect to 100 parts by weight of silver powder, the effect cannot be confirmed. Moreover, when it is contained in a large amount, the fill factor and conversion efficiency tend to decrease, and the adhesive strength also decreases. Therefore, the total amount in terms of metallic silver needs to be up to 3.0 parts by weight. Among these silver compounds, silver carbonate is particularly effective. In particular, it is preferable to add 0.1 to 2.0 parts by weight of silver carbonate in terms of metallic silver to 100 parts by weight of silver powder because extremely excellent conversion characteristics can be obtained without reducing the adhesive strength.

  There is no restriction | limiting in the addition form of a silver compound, Usually, a particulate thing is used. In particular, it is preferable that the silver compound is in the form of fine particles having an average particle diameter of 5 μm or less because a better ohmic contact improvement effect can be obtained. The lower limit of the average particle diameter of the silver compound is not particularly limited, but 100 nm or more is practical from the viewpoint of cost and handling.

  The silver powder, glass frit and silver compound are mixed with an organic vehicle to form a rheological paste, paint or ink suitable for screen printing and other printing methods. The vehicle is not particularly limited, and an organic binder, a solvent and the like that are usually used as a vehicle for silver paste are appropriately selected and blended. For example, as organic binders, celluloses, acrylic resins, phenol resins, alkyd resins, rosin esters, etc., and as solvents, alcohol-based, ether-based, ester-based, hydrocarbon-based organic solvents, water, and mixed solvents thereof Is mentioned. In addition, a plasticizer, a viscosity modifier, a surfactant, an oxidizing agent, a metal organic compound, and the like can be appropriately blended as necessary. The blending amount of the vehicle is not particularly limited, and is an appropriate amount capable of retaining the inorganic component in the paste, and is appropriately adjusted according to the coating method and the like, but is usually 10 to 100 parts by weight of the silver powder. About 40 parts by weight.

  The conductive paste of the present invention may be blended with metal oxides, metal organic compounds, and the like that are usually added as additives, as long as the effects of the present invention are not impaired.

  The solar cell element according to the present invention is manufactured, for example, as follows. The semiconductor substrate is preferably made of single crystal silicon or polycrystalline silicon, and has a conductivity type (for example, p-type) by containing a semiconductor impurity such as boron. A region having a reverse conductivity type (for example, n-type) is formed by diffusing impurities such as phosphorus atoms to form a diffusion layer on the light receiving surface side surface of the semiconductor substrate. Further, an antireflection film such as silicon nitride is provided thereon, and a back electrode is formed by baking aluminum paste and silver paste, or silver-aluminum paste on the substrate surface opposite to the light receiving surface, and at the same time a high concentration p-type The BSF layer is formed. Thus, after forming a general semiconductor substrate, a diffusion layer, an antireflection film, and a back electrode, a surface electrode is formed using the conductive paste according to the present invention. Specifically, the conductive paste according to the present invention is applied onto the antireflection film by a normal method such as a screen printing method, dried, and then baked at a high temperature of 500 to 900 ° C. for about 1 to 30 minutes, thereby forming an organic material. The vehicle component is decomposed and volatilized to form a surface electrode. The front electrode may be fired simultaneously with the back electrode, or the back electrode may be formed after firing the front electrode.

  As described above, by producing a solar cell element using the conductive paste according to the present invention, the surface electrode can be fired through satisfactorily. Therefore, since the adhesion strength and ohmic contact between the surface electrode and the semiconductor substrate are improved, the output of the semiconductor element can be taken out efficiently, and the conversion efficiency and current-voltage characteristics of the solar cell element can be further enhanced. .

  In the following, production of spherical silver powder and evaluation thereof will be shown as examples, but the present invention is not limited thereto.

(Production of samples 1 to 9)
5 parts by weight of lead borosilicate glass frit with respect to 100 parts by weight of silver powder, and 0.05 to 4 parts by weight of silver carbonate powder having an average particle size of about 3 μm as shown in Table 1 in terms of metallic silver, Conductive pastes (samples 1 to 8) were prepared by dispersing in 12 parts by weight of an organic vehicle. As the silver powder, a mixed powder of a spherical powder having an average particle diameter of about 2 μm and a flaky powder having an average particle diameter of about 3 μm was used. As the organic vehicle, a butyl carbitol solution of ethyl cellulose was used. Further, a conductive paste (Sample 9) was prepared in the same manner except that no silver carbonate powder was blended. Samples 8 and 9 are outside the scope of the present invention.

(Production of samples 10 to 12)
A conductive paste (samples 10 to 12) was prepared in the same manner except that the average particle diameter of the silver carbonate powder was as shown in Table 1, and the amount of silver carbonate was 0.3 parts by weight in terms of metallic silver. .

(Preparation of Samples 13 to 22)
As shown in Table 1, 0.005 to 4 parts by weight of silver oxide (Ag ₂ O) powder having an average particle diameter of about 2 μm and silver acetate powder having an average particle diameter of about 2 μm are used instead of silver carbonate powder. A conductive paste (samples 13 to 22) was produced in the same manner except that it was blended. Sample 17 and sample 22 are outside the scope of the present invention.

(Preparation of samples 23 to 27)
Instead of silver carbonate powder, as shown in Table 1, titanium oxide (TiO ₂ ) powder with an average particle diameter of about 1 μm, bismuth oxide (Bi ₂ O ₃ ) powder with an average particle diameter of about 2 μm, oxidation with an average particle diameter of about 1 μm In the same manner, except that 0.5 parts by weight of cobalt (Co ₂ O ₃ ) powder, zinc oxide powder having an average particle diameter of about 1 μm, and phosphorus oxide (P ₂ O ₅ ) powder having an average particle diameter of about 3 μm are blended, Conductive paste (samples 23 to 27) was produced.

(Production and evaluation of solar cell elements)
Using the conductive pastes of Samples 1 to 27, solar cell elements were manufactured as follows.
Using a 15 cm × 15 cm square p-type polycrystalline silicon substrate as a semiconductor substrate, an n-type region (diffusion layer) in which phosphorus is diffused is formed on one main surface side, and silicon nitride is formed thereon as an antireflection film. A film was formed. In addition, an aluminum paste and then a silver paste were baked by a conventional method on the opposite surface of one main surface to form a back electrode. The conductive paste was screen-printed in a comb pattern on the antireflection film of the silicon substrate and baked at 700 ° C. for 2 minutes to form a surface electrode.

The obtained solar cell elements were measured for electrical characteristics (curve factor and conversion efficiency) and surface electrode adhesive strength (tensile strength), and are shown in Table 1. Tensile strength is whether or not the copper foil peels off when a solder-plated copper foil is attached to the electrode surface using a bond tester (SD-30WD manufactured by Nishishin Shoji Co., Ltd.) and the copper foil is pulled up in a direction perpendicular to the electrode surface. Or the load (Kg) when the solar cell element is destroyed.

  As is clear from Table 1, when silver carbonate, silver oxide, and silver acetate were contained within the scope of the present invention, improvement in the fill factor and conversion efficiency was observed as compared with those not containing. Also, the adhesive strength was good. In particular, the effect of silver carbonate was extremely excellent in the range of 0.1 to 2.0 parts by weight. Further, when the average particle size of silver carbonate was smaller than 5 μm, the improvement effect was extremely large.

It is sectional drawing which shows schematic structure of a general solar cell element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Diffusion layer 3 Antireflection film 4 Back surface electrode 5 Front surface electrode

Claims (3)

  A solar cell in which a region exhibiting a reverse conductivity type is formed on a light receiving surface side of a semiconductor substrate exhibiting one conductivity type, an antireflection film and a surface electrode are provided thereon, and a back electrode is provided on a surface opposite to the light receiving surface A conductive paste used for forming a surface electrode of an element, comprising 100 parts by weight of silver powder, 0.1 to 10 parts by weight of glass frit, at least one silver compound selected from silver carbonate, silver oxide and silver acetate Containing 0.05 to 3.0 parts by weight of the total amount in terms of metallic silver, and an organic vehicle.   2. The conductive paste according to claim 1, wherein the silver compound is in the form of particles having an average particle diameter of 5 μm or less.   A solar cell in which a region exhibiting a reverse conductivity type is formed on a light receiving surface side of a semiconductor substrate exhibiting one conductivity type, an antireflection film and a surface electrode are provided thereon, and a back electrode is provided on a surface opposite to the light receiving surface In the device, the surface electrode is formed by baking the conductive paste according to claim 1 or 2 at 500 to 900 ° C.

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