JP5323307B2 - Solar cell electrode paste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the characteristics of electrodes to be acquired, and accordingly, improve the characteristics of a solar cell by solving the problem of contraction of an applied film or microcracks to be generated in paste baking, with respect to a paste to be used for manufacturing the electrodes in the solar cell. <P>SOLUTION: A conductive paste for the electrodes for the solar battery includes a first silver powder having a crystal grain size of 58 nm or more, a second silver powder having a crystal grain size different from that of the first silver powder, a glass frit and a resin binder. In the conductive paste, it is preferable (1) that the first silver powder is silver prepared by an atomizing method, (2) that the contraction initiating temperature of the first silver powder is not less than 700&deg;C, (3) that the content of the first silver powder is 10-70 wt.% on the basis of the total weight of the silver, and (4) that the first silver powder has a crystal grain size of 58-90 nm and the second silver powder has a crystal grain size of 30-58 nm. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to a solar cell paste. In more detail, it is related with the paste used for manufacture of the electrode in a solar cell.

  When producing a solar cell electrode, the electrode is formed on the side where the antireflection film is formed. As a method for producing an electrode, a general method is that a conductive powder such as silver powder, a glass frit, a resin binder, and a paste containing an additive as necessary are applied on the antireflection layer and baked.

  In order to enhance the power generation characteristics in the solar cell, the characteristics of the electrodes are important. For example, the power generation efficiency is increased by lowering the resistance value of the electrode. In order to achieve this object, various techniques have been proposed.

  For example, Patent Document 1 (Japanese Patent Laid-Open No. 2005-243500) discloses a technique for manufacturing an electrode having sufficient electrical conductivity. Specifically, a conductive material comprising an organic binder, a solvent, glass frit, conductive powder, and at least one metal selected from Ti, Bi, Zn, Y, In, and Mo or a metal compound thereof. In the conductive paste, a metal or a metal compound thereof has an average particle size of 0.001 μm or more and less than 0.1 μm, and disclosed is a conductive paste that can provide high electrical conductivity with a semiconductor and excellent adhesiveness. Yes.

JP-A-2005-243500

  However, when the paste is fired, there is a problem that contact resistance increases due to shrinkage of the coating film and a problem that micro cracks are generated. These problems can also adversely affect the characteristics of the solar cell. For example, the problem that the in-plane uniformity of a solar cell falls and the problem that the conversion efficiency of a solar cell falls may arise.

  An object of the present invention is to solve these problems that occur during firing of a paste and to improve the characteristics of the resulting electrode, and thus the characteristics of a solar cell.

The present invention relates to a conductive paste for a solar cell electrode, a first silver powder having a crystallite diameter of 58 nm or more, a second silver powder having a crystallite diameter smaller than the first silver powder, a glass frit, and a resin binder only contains the crystallite diameter formula (formula Scherrer)
D = K · λ / βcos θ
(However, D: crystallite diameter
λ: Measurement X-ray wavelength
β: Half width (radian)
θ: Diffraction angle
K: Scherrer constant (0.9). )
It is calculated by .

In the conductive paste, the first silver powder preferably has the following characteristics. (1) The first silver powder is preferably silver produced by an atomizing method. (2) The shrinkage start temperature of the first silver powder is preferably 700 ° C. or higher. (3) It is preferable that content of 1st silver powder is 10-70 wt% based on the total weight of silver. (4) The crystallite diameter of the first silver powder is preferably 58 to 90 nm, and the crystallite diameter of the second silver powder is preferably 30 to 58 nm.
This invention includes the manufacturing method of the solar cell electrode using the said electrically conductive paste for solar cell electrodes. This method
Applying a paste containing a first silver powder having a crystallite diameter of 58 nm or more, a second silver powder having a crystallite diameter smaller than the first silver powder, a glass frit, and a resin binder to a solar cell substrate;
Then, look including the step of firing said paste, said crystallite size is the following formula (Formula Scherrer)
D = K · λ / βcos θ
(However, D: crystallite diameter
λ: Measurement X-ray wavelength
β: Half width (radian)
θ: Diffraction angle
K: Scherrer constant (0.9). )
It is calculated by .

  Increase in contact resistance and generation of microcracks are suppressed. Thereby, the characteristic of the solar cell obtained can be improved.

  It has been found that the use of silver having a specific crystallite diameter as the silver contained in the paste as the conductive powder can suppress an increase in contact resistance and the occurrence of microcracks. The present invention is based on such knowledge.

  The present invention relates to a conductive paste for solar cell electrodes, comprising a first silver powder having a crystallite diameter of 58 nm or more, a second silver powder having a crystallite diameter different from that of the silver powder, a glass frit, and a resin binder.

  The electrically conductive paste of this invention is used for formation of the front side electrode of a photovoltaic cell.

  Below, each component of the electrically conductive paste of this invention is demonstrated.

1. Conductive metal Examples of the conductive metal that can be used in the conductive paste of the present invention include silver (Ag) particles, which is preferable.

  The silver particles of the present invention have a crystallite diameter in a predetermined range. Most preferably, two types of silver particles having different crystallite diameters are used as the conductive metal.

In this specification, the crystallite size is calculated by the following formula (Scherrer formula) from the half-value width of the Ag reflection peak (2θ: peak at about 38.1 °) in the X-ray diffraction measurement of Ag powder. Is the value to be It is estimated that the larger the crystallite size, the better the crystallinity.
D = K · λ / βcos θ
Where D: crystallite diameter λ: measured X-ray wavelength β: half width (radian)
θ: Diffraction angle K: Scherrer constant (0.9)

  The crystallite diameter defined in the present invention is a value when Cu = 1.54056 ＝ is used as λ (measurement X-ray wavelength). The crystallite diameter measuring device is not particularly limited, and a commercially available device can be used. In the present application, when a significant difference occurs in the crystallite diameter by the measuring device, a value measured by Mac Science, X-ray diffractometer, MXP18VAHF is adopted.

  As the silver particles, a first silver powder having a crystallite diameter of 58 nm or more and a second silver powder having a crystallite diameter different from that of the first silver powder are used. The crystallite diameter of the first silver powder is preferably 58 to 90 nm, and the crystallite diameter of the second silver powder is 30 to 58 nm, preferably 35 to 50 nm, more preferably 40 to 45 nm. By using a combination of silver powders having crystallite sizes in these ranges, it is possible to suppress an increase in contact resistance of the silver electrode obtained after firing and the occurrence of microcracks.

  The content of the first silver powder is preferably 10 to 70 wt% based on the total weight of silver. More preferably, it is 20-55 weight%, More preferably, it is 25-45 weight%. Silver particles having a large crystallite size such as the first silver powder are considered to have a high crystal suppression effect. On the other hand, in order to obtain high electrical conductivity, it is preferable to obtain a low resistance conductor by increasing sinterability. The above range is considered preferable in order to balance these. That is, if the first silver powder is too small, the sintering may be excessive, and if the first silver powder is too large, the sintering may be insufficient. The total content of the first silver powder and the second silver powder is preferably 60 to 90% by weight based on the weight of the conductive paste.

  The first silver powder preferably has a shrinkage start temperature (also referred to as a sintering start temperature) of 700 ° C. or higher. When sintering starts at a low temperature, excessive residual stress is generated due to shrinkage, which may reduce power generation characteristics.

  The silver particles are preferably formed by an atomizing method. By forming silver particles by the atomization method, silver particles having a crystallite diameter in a specific range of the present invention can be efficiently obtained.

  When used as a general conductive paste, the particle size of the silver particles is not particularly limited from the viewpoint of technical effects, but the particle size affects the sintering characteristics of silver (for example, the particle size Large silver particles are sintered at a slower rate than silver particles having a small particle size.) For the purposes of the present invention, it is preferable to have a specific particle size. Furthermore, the silver particles must have a particle size suitable for a method (for example, screen printing) for applying the conductive paste.

  In order to satisfy the above requirements, the silver particles of the present invention have an average particle size of 0.1 to 14 μm, preferably 1.0 to 8.0 μm. By using silver particles having such a particle size, a paste suitable for application of a conductive paste can be formed. More specifically, the average particle diameter of the first silver powder is preferably 3.5 to 14.0 μm, more preferably 4.0 to 10.0 μm, and still more preferably 4.5 to 8.0 μm. The average particle size of the second silver powder is preferably 0.1 to 3.5 μm, more preferably 0.5 to 3.0 μm, and still more preferably 1.0 to 2.5 μm. The average particle diameter is, for example, a value measured by LA-920 manufactured by Horiba, Ltd., and is calculated as an average particle diameter (50% point).

  Silver is usually preferably of high purity (99 +%). However, a substance with low purity can also be used due to the electrical requirements of the electrode pattern.

2. Glass frit It is preferable that the electrically conductive paste of this invention contains the glass frit as an inorganic binder. The glass binder that can be used in the present invention has a softening point of 450 to 550 ° C. so that the conductive paste is fired at 600 to 800 ° C., appropriately sintered and wetted, and more appropriately bonded to the silicon substrate. A glass frit having If the softening point is lower than 450 ° C., sintering proceeds and the effects of the present invention may not be sufficiently obtained. On the other hand, if the softening point is higher than 550 ° C., sufficient melt flow does not occur at the time of firing, so that sufficient adhesive strength may not be exhibited and liquid phase sintering of silver may not be promoted.

  Here, the “softening point” is a softening point obtained by the fiber elongation method of ASTM C338-57.

  Since the chemical composition of the glass frit is not important in the present invention, any glass frit used in a conductive paste for electronic materials can be used. For example, lead borosilicate glass can be suitably used. Lead silicate glass and lead borosilicate glass are excellent materials in the present invention from the viewpoint of both the softening point range and glass weldability. In addition, lead-free glass such as zinc borosilicate can also be used.

  The content of the glass frit is not particularly limited as long as the object of the present invention can be achieved, but is 0.5 to 10.0% by weight, preferably 1.0 to 3.0% based on the weight of the conductive paste. % By weight.

  If the amount of glass frit is less than 0.5% by weight, the adhesive strength may be insufficient. If the amount of the glass frit exceeds 10.0% by weight, it may hinder soldering, which is a subsequent process, due to glass floating or the like.

3. Resin Binder The conductive paste of the present invention contains a resin binder. In the present specification, the “resin binder” is a concept including a mixture of a polymer and a thinner. Therefore, the resin binder may contain an organic liquid (also called thinner). In the present invention, a resin binder containing an organic liquid is preferable, and when the viscosity is high, the organic liquid can be separately added as a viscosity modifier as necessary.

  In the present invention, any resin binder can be used. In the present invention, a pine oil solution, an ethylene glycol monobutyl ether monoacetate solution, a terpineol solution of ethyl cellulose, or the like of a resin (polymethacrylate or the like) or ethyl cellulose can be used. In the present invention, it is preferable to use a terpineol solution of ethyl cellulose (ethyl cellulose content = 5 wt% to 50 wt%). In the present invention, a polymer-free solvent, such as water or an organic liquid, can be used as a viscosity modifier. Organic liquids that can be used include, for example, alcohols; esters of alcohols (eg acetate or propionate); terpenes (eg pine oil, terpineol etc.).

  The content of the resin binder is preferably 5 to 50% by weight based on the weight of the conductive paste.

4). Additives The conductive paste of the present invention may or may not contain thickeners and / or stabilizers and / or other common additives. When adding an additive, a thickener (thickening agent), a stabilizer, etc. can be added. Or a dispersing agent, a viscosity modifier, etc. can also be added as another general additive. The amount of the additive is determined depending on the properties of the finally obtained conductive paste. The amount of the additive can be appropriately determined by those skilled in the art. A plurality of additives may be added.

  The conductive paste of the present invention preferably has a predetermined range of viscosity as described below. In order to impart an appropriate viscosity to the conductive paste, a thickener (thickening agent) can be added as necessary. As an example of a thickener, what was mentioned above can be mentioned, for example. The amount of the thickener added varies depending on the viscosity of the final conductive paste, but can be appropriately determined by those skilled in the art.

  The electrically conductive paste of this invention is conveniently manufactured by mixing each above-mentioned component with a 3 roll kneader. The conductive paste of the present invention is preferably applied to a desired site on the back surface of the solar cell by screen printing, but when applied by such printing, it preferably has a predetermined range of viscosity. The viscosity of the conductive paste of the present invention is preferably 50 to 300 PaS when measured at 10 rpm and 25 ° C. using a utility cup using a # 14 spindle with a Brookfield HBT viscometer.

  As described above, the conductive paste of the present invention is used to form an electrode mainly composed of silver on the light-receiving surface side of the solar cell. That is, the paste of the present invention is printed on the light receiving surface side of the solar battery cell and dried. Separately, a back electrode made of aluminum or silver is also formed on the back surface side of the solar battery cell. These electrodes are preferably fired simultaneously.

  Below, the example which produces a photovoltaic cell using the electrically conductive paste of this invention is demonstrated with reference to FIG.

  First, the Si substrate 102 is prepared. A conductive paste 104 for solder connection is applied to the back side of the substrate by screen printing and dried (FIG. 1A). As this conductive paste, a conventional one, for example, a silver conductive paste containing silver particles, glass particles, and a resin binder can be used. Next, an aluminum paste for a back electrode for a solar cell (which is not particularly limited as long as it is for a solar cell, for example, PV333, PV322 (the present applicant)) is applied by screen printing or the like and dried (FIG. 1 (b The drying temperature of each paste is preferably 180 ° C. or less, and the film thickness of each electrode on the back surface is the film thickness after drying, 20 to 40 μm for aluminum paste, and 15 to 20 for silver conductive paste. The overlap portion of the aluminum paste and the silver conductive paste is preferably about 0.5 mm to about 2.5 mm.

  Next, the conductive paste 108 according to the present invention is applied on the light-receiving side surface (surface) of the Si substrate by screen printing or the like and dried (FIG. 1C). The obtained substrate is co-fired with an aluminum paste and a silver conductive paste for about 2 to 15 minutes at a temperature of, for example, about 600 ° C. to about 900 ° C. in an infrared baking furnace, thereby obtaining a target solar battery cell ( FIG. 1 (d)).

  A solar battery cell obtained using the conductive paste of the present invention is formed from the conductive paste of the present invention on the light-receiving surface (front surface) side of a substrate (for example, Si substrate) 102 as shown in FIG. And an Al electrode (first electrode) 112 mainly composed of Al and a silver electrode (second electrode) 114 mainly composed of Ag on the back side.

1. Preparation of conductive paste (Example 1)
A mixture containing a first silver powder having a crystallite diameter of 59.8 nm, a second silver powder having a crystallite diameter of 43.5 nm, a Si / B / Pb / O-based glass frit, and a sintering auxiliary material was prepared. To this mixture, a terpineol solution containing 20 wt% ethyl cellulose was added as a resin binder. Furthermore, in order to adjust the viscosity, terpineol was added as a thinner. The content of each component is as shown in Table 1. That is, the first silver powder having a crystallite diameter of 58 nm or more is 8.4 wt%, the second silver powder having a crystallite diameter of 43.5 nm is 75.6 wt%, the glass frit is 1.6 wt%, and the resin binder is 10.0 wt%, terpineol added for viscosity adjustment is 0.9 wt%, and the sintering auxiliary material is 3.5 wt%.

  This mixture was premixed with a universal mixer and then kneaded with a three-roll kneader to obtain a solar cell electrode paste. Table 1 shows the particle size, content, and characteristics of the materials used.

(Example 2, Comparative Examples 1-2)
A solar cell electrode paste was obtained in the same manner as in Example 1 except that the type and amount of silver powder used were changed to those shown in Table 1.

2. Production of Solar Cell A solar cell was produced using the four types of pastes obtained. First, a Si substrate was prepared. A conductive paste (silver paste) for solder connection was applied to the back side of the Si substrate by screen printing and dried. Next, an aluminum paste for back electrode (PV333 (by the present applicant)) was applied by screen printing so as to partially overlap with the dried silver paste, and dried. The drying temperature of each paste was 120 ° C. The thickness of each electrode on the back surface was applied after drying so that the aluminum paste was 35 μm and the silver paste was 20 μm.

  Furthermore, the paste of this invention was apply | coated by screen printing on the light-receiving side surface (surface), and it dried. The printing machine was manufactured by Price, and the mask used was an 8 inch x 10 inch frame stainless steel wire 250 mesh. The pattern was a 1.5 inch square evaluation pattern composed of 100 micron wide finger lines and 2 mm wide bus bars, and the film thickness was 13 μm after firing.

  Next, the applied paste was simultaneously fired on the obtained substrate in an infrared firing furnace at a peak temperature of about 730 ° C. and an IN-OUT of about 5 minutes, thereby obtaining a target solar battery cell.

  As shown in FIG. 1, a solar battery cell obtained using the conductive paste of the present invention has an Ag electrode 110 on the light-receiving surface (front surface) side of a substrate (for example, Si substrate) 102 and Al on the back surface side. An Al electrode (first electrode) 112 mainly containing Ag and a silver electrode (second electrode) 114 mainly containing Ag.

3. Evaluation of Cell The electric characteristics (IV characteristics) of the obtained solar cell substrate were evaluated by a cell tester. The cell tester used was a device (NCT-M-150AA) manufactured by NPC.
The obtained characteristic values are: Eff: conversion efficiency (%), FF: fill factor (%), Voc: open circuit voltage (mV), Jsc: short circuit current (mA · cm 2), Rs: series resistance (Ω · cm ² ) , Rsh: Shunt resistance (Ω · cm ² ). The higher the characteristic value other than Rs, the better the power generation performance as a solar cell. The results are shown in Table 2. The numerical value of each electrical characteristic shown in Table 2 is an average of the measured values of the five solar cell substrate samples, and is a relative value when each numerical value of Comparative Example 1 is 100.0.

  As described above, the characteristics of the obtained solar cell are improved by using two kinds of silver powder having a predetermined crystallite diameter defined in the present invention in combination.

(Example 3)
A test for estimating an appropriate content of the first silver powder in the silver powder was conducted. As the first silver powder (A), a crystallite diameter of 59.8 nm (shrinkage start temperature 740 ° C.) was used, and as the second silver powder (B), a crystallite diameter of 43.5 nm (shrinkage start temperature 670 ° C.) was used. The composition ratio of the first silver powder and the second silver powder was changed as shown in Table 3 below, and a conductive paste and a solar cell substrate were prepared in the same procedure as in Examples 1 and 2. The conversion efficiency (Eff) of the obtained solar cell was measured. The results are shown in Table 3. Moreover, the result of Table 3 was represented with the graph in FIG.

  From the above results, it was possible to estimate that the proper content of the first silver powder was 10 to 70% by weight based on the total weight of silver.

  The present invention is applicable to solar cells.

(A)-(d) is a figure for demonstrating the manufacturing process at the time of manufacturing a photovoltaic cell using the electrically conductive paste of this invention. In order to estimate the optimal content of 1st silver powder and 2nd silver powder, it is a graph when the electrical property of the solar cell at the time of changing these content is measured.

Explanation of symbols

102 Si substrate 102 Back surface Ag electrode conductive paste composition 106 Back surface Al electrode paste composition 108 Light receiving surface side electrode conductive paste composition 110 Light receiving surface side Ag electrode 112 Back surface Al electrode 114 Back surface Ag electrode

Claims (5)

A first silver powder having a crystallite diameter of 58 to 90 nm,
A second silver powder having a smaller crystallite diameter than the first silver powder , wherein the second silver powder has a crystallite diameter of 30 to 58 nm ,
Including glass frit, and resin binder,
The silver powder has an average particle size of 0.1 to 14 μm,
The crystallite diameter is the following formula (Scherrer formula)
D = K · λ / βcos θ
(However, D: Crystallite diameter λ: Measurement X-ray wavelength β: Half width (radian)
θ: diffraction angle K: Scherrer constant (0.9). )
A paste for solar cell electrode, characterized by being calculated by:   The paste according to claim 1, wherein the first silver powder is silver produced by an atomizing method.   The paste according to claim 1, wherein the shrinkage start temperature of the first silver powder is 700 ° C or higher.   The paste according to claim 1, wherein the content of the first silver powder is 10 to 70 wt% based on the total weight of silver. A method for producing a solar cell electrode, comprising:
The first silver powder crystallite size of 58 to 90 nm, a second silver powder having a small crystallite diameter than the first silver powder, crystallite size of the second silver powder with 30~58nm Applying a paste containing a second silver powder , a glass frit, and a resin binder to a solar cell substrate;
Then, the step of firing the paste,
The silver powder has an average particle size of 0.1 to 14 μm,
The crystallite diameter is the following formula (Scherrer formula)
D = K · λ / βcos θ
(However, D: Crystallite diameter λ: Measurement X-ray wavelength β: Half width (radian)
θ: diffraction angle K: Scherrer constant (0.9). )
A method characterized by being calculated by:

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