JP5888493B2 - Conductive paste and solar cell element using the conductive paste - Google Patents

Description

  The present invention relates to a lead-free conductive paste that can be used as an electrode formed in a semiconductor silicon solar cell.

  As an electronic component using a semiconductor silicon substrate, a solar cell element as shown in FIG. 1 is known. As shown in FIG. 1, the solar cell element is formed by forming an n-type semiconductor silicon layer 2 on the light-receiving surface side of a p-type semiconductor silicon substrate 1 having a thickness of about 200 μm, and nitriding to increase the light-receiving efficiency on the light-receiving surface side surface. An antireflection film 3 such as a silicon film, and a surface electrode 4 connected to the semiconductor are formed on the antireflection film 3.

  An aluminum electrode layer 5 is uniformly formed on the back side of the p-type semiconductor silicon substrate 1. The aluminum electrode layer 5 is generally formed by applying an aluminum paste material made of an aluminum powder, glass frit, an organic vehicle containing a binder such as ethyl cellulose or acrylic resin, using screen printing or the like, and having a temperature of about 600 to 900 ° C. It is formed by baking for a short time.

In the baking of the aluminum paste material, aluminum diffuses into the p-type semiconductor silicon substrate 1, so that an Si-Al called BSF (Back Surface Field) layer 6 is formed between the aluminum electrode layer 5 and the p-type semiconductor silicon substrate 1. A eutectic layer is formed, and an impurity layer p ⁺ layer 7 is formed by diffusion of aluminum. The p ⁺ layer 7 has an effect of suppressing loss due to recombination of carriers generated by the photovoltaic effect of the pn junction, and contributes to improvement in conversion efficiency of the solar cell element. As for the BSF effect, as disclosed in, for example, Patent Document 1 and Patent Document 2, it is possible to obtain a high effect by using glass containing lead as a glass frit contained in an aluminum paste material. It is disclosed that.

JP 2007-59380 A JP 2003-165744 A

In general, there is a correlation between the surface resistance of the p ⁺ layer and the BSF effect, and the lower the surface resistance of the p ⁺ layer, the higher the BSF effect and the higher the conversion efficiency as a solar cell element.

  The glass frit containing the lead component described above can be used for a conductive paste such as an aluminum paste material to obtain a high BSF effect, and is an important component for making the conductive paste have a low melting point. However, it has a great negative effect on the human body and the environment. Patent Document 1 and Patent Document 2 described above have a problem that the conductive paste contains a lead component.

  Therefore, an object of the present invention is to obtain a lead-free conductive paste that can be used as an electrode formed in a semiconductor silicon solar cell.

The present invention is an electrically conductive paste for solar cells using a semiconductor silicon substrate, and the composition of the glass frit contained in the electrically conductive paste is substantially free of lead components and contains 1 to ₂ SiO ₂ by mass%. _20, B 2 _{O 3} of 5 to 30, _Al 2 _{O 3} 0-10, 5-35 and ZnO, RO (MgO, CaO, SrO, and at least one total selected from the group consisting of BaO) 5 a 30 to 0.1, R ₂ O (total of at least one selected from the group consisting of Li ₂ O, Na ₂ O, and K ₂ O) 0.1 to 6 and Bi ₂ O ₃ to 10 to 60 The conductive paste is characterized.

When the conductive paste using the lead-containing glass frit is used, the surface resistance of the p ⁺ layer shows about 20 to 30 Ω / □, and therefore the p ⁺ layer of the p ⁺ layer when using the conductive paste of the present invention is used. The surface resistance is preferably 30 Ω / □ or less. When the surface resistance is lower, conversion efficiency is improved when used as a solar cell element.

The glass frit of the present invention is characterized in that the coefficient of thermal expansion at 30 ° C. to 300 ° C. is (70 to 110) × 10 ⁻⁷ / ° C. and the softening point is 450 ° C. or higher and 600 ° C. or lower. In the present invention, the above thermal expansion coefficient means a linear expansion coefficient.

  Moreover, the conductive paste of the present invention is an aluminum paste material having aluminum powder.

The conductive paste of the present invention is characterized in that the composition of the glass frit contains at least K ₂ O as R ₂ O.

  The conductive paste of the present invention is characterized in that the composition of the glass frit contains at least BaO as RO.

  According to the present invention, it is possible to obtain a conductive paste containing glass frit that does not contain lead. By using the conductive paste of the present invention as a solar cell element, a high BSF effect can be obtained. Also, good adhesion to the semiconductor silicon substrate can be obtained. Furthermore, since it does not substantially contain a lead component, there is no harmful effect on the human body and the environment.

It is a schematic sectional drawing of a common semiconductor silicon photovoltaic cell.

The conductive paste of the present invention contains glass frit (1 to 5% by mass) in addition to an organic vehicle containing aluminum powder and a binder such as ethyl cellulose or acrylic resin, and the glass frit is substantially free of a lead component. And selected from the group consisting of SiO ₂ in mass% 1-20, B ₂ O ₃ 5-30, Al ₂ O ₃ 0-10, ZnO 5-35, RO (MgO, CaO, SrO, and BaO). 5 to 30, and R ₂ O (total of at least one selected from the group consisting of Li ₂ O, Na ₂ O, and K ₂ O) 0.1 to 6, Bi ₂ O ₃ is a conductive paste characterized by containing 10 to 60.

In the glass frit of the present invention, SiO ₂ is a glass forming component, and by coexisting with B ₂ O ₃ which is another glass forming component, a stable glass can be formed, and 1 to 20% ( (The same applies to the mass% below). If it exceeds 20%, the softening point of the glass will rise, making it difficult to use as a conductive paste. More preferably, it is 5 to 17%, and further preferably 8 to 15%.

B ₂ O ₃ is a glass forming component, facilitates glass melting, suppresses an excessive increase in the thermal expansion coefficient of glass, imparts fluidity to glass during firing, and lowers the dielectric constant of glass. And 5 to 30% in the glass. If it is less than 5, the fluidity of the glass becomes insufficient and the sinterability is impaired. On the other hand, if it exceeds 30%, the stability of the glass decreases. Moreover, More preferably, it is 10 to 25%, More preferably, it is 15 to 25% of range.

Al ₂ O ₃ is an optional component that suppresses crystallization of glass. Although it is made to contain 0 to 10% in glass, when it exceeds 10%, the softening point of glass will rise and it will become difficult to use as an electrically conductive paste. More preferably, it may be 0 to 5%.

  ZnO is a component that lowers the softening point of glass and is contained in the glass in an amount of 5 to 35%. If it is less than 5%, the above-mentioned action cannot be exhibited. If it exceeds 35%, the glass becomes unstable and crystals are likely to be formed. Further, it is more preferably 8 to 30%, and further preferably 10 to 20%.

  RO (total of at least one selected from the group consisting of MgO, CaO, SrO and BaO) lowers the softening point of the glass and is contained in the glass in an amount of 5 to 30%. If it is less than 5%, the softening point of the glass is not sufficiently lowered, and the sinterability is impaired. On the other hand, if it exceeds 30%, the thermal expansion coefficient of the glass may become too high. Preferably it is 10 to 30%, More preferably, it is 10 to 20% of range. In addition, RO may be a single component or a mixture of a plurality of components, but more preferably contains BaO.

R ₂ O (total of at least one selected from the group consisting of Li ₂ O, Na ₂ O, and K ₂ O) lowers the softening point of the glass and adjusts the thermal expansion coefficient to an appropriate range. It is made to contain in 1 to 6% of range. If it is less than 0.1%, the softening point of the glass is not sufficiently lowered and the sinterability is impaired. On the other hand, if it exceeds 6%, the thermal expansion coefficient may be excessively increased. More preferably, it is 1-6%, More preferably, it is 1-3% of range.

In the present invention, it is possible to make the surface resistance of the p ⁺ layer lower than 30 Ω / □ by increasing the amount of R ₂ O contained in the glass frit, but the R ₂ O exceeds 6% by mass. If containing Te, the order by R _{2 O} alkali component increases sometimes exhibit deliquescence, in the present invention to 6% by mass or less the R _{2 O.}

R ₂ O may be a single component or a mixture of a plurality of components. In particular, when the amount of K ₂ O in the R ₂ O component is the main component or only the K ₂ O component is used, the appearance And the adhesion to the substrate is favorable. Here, the "main component" described above may be any K _{2 O} mass to the total of the weight of R 2 _O component is more than 50 wt%, preferably may be more than 70 wt%.

Bi ₂ O ₃ lowers the softening point of the glass and adjusts the thermal expansion coefficient, and is contained in the range of 10 to 60%. If it is less than 10%, the softening point of the glass is not sufficiently lowered, and the sinterability is impaired. On the other hand, if it exceeds 60%, the thermal expansion coefficient is excessively increased. More preferably, it is 15 to 55% of range.

In addition to the above, CuO, TiO ₂ , In ₂ O ₃ , SnO ₂ , TeO ₂ or the like represented by a general oxide may be added.

  By substantially not containing lead (hereinafter also referred to as PbO), it is possible to eliminate the influence on the human body and the environment. Here, “substantially free of PbO” means an amount of PbO mixed as an impurity in the glass raw material. For example, if it is in the range of 0.3% or less in the low-melting glass, there is almost no influence on the adverse effects described above, that is, the influence on the human body and the environment, the insulation characteristics, etc. Become.

By using the glass frit, it is possible to obtain a conductive paste having a thermal expansion coefficient of (70 to 110) × 10 ⁻⁷ / ° C. and a softening point of 450 to 600 ° C. at 30 to 300 ° C. . When the coefficient of thermal expansion deviates from (70 to 110) × 10 ⁻⁷ / ° C., problems such as peeling and warping of the substrate occur during electrode formation. Preferably, it is in the range of (75-100) × 10 ⁻⁷ / ° C. Further, when the softening point exceeds 600 ° C., it does not flow sufficiently at the time of firing, so that problems such as poor adhesion to the semiconductor silicon substrate occur. The softening point is preferably 480 ° C or higher and 580 ° C or lower.

  The conductive paste of the present invention can be used for a solar cell element as described above. Furthermore, since the conductive paste can be baked at a low temperature, it can be used as a substrate for electronic materials such as a wiring pattern forming material using silver or aluminum or various electrodes.

  One preferred embodiment of the conductive paste of the present invention is a conductive paste containing glass frit, aluminum powder, and an organic vehicle, and the viscosity of the conductive paste is preferably 200 Pa · s or less. The conductive paste is applied and baked on a semiconductor silicon substrate to form an aluminum electrode layer. If the viscosity is out of the above range, the moldability and workability may be deteriorated.

  The glass frit contained in the conductor paste preferably has an average particle size of 1 to 10 μm and a maximum particle size of 30 μm or less. The particle size of the glass frit was measured using a laser diffraction / scattering particle size / particle size distribution measuring device (manufactured by Nikkiso Co., Ltd.). If the average particle size of the glass frit exceeds 10 μm and the maximum particle size exceeds 30 μm, the adhesion between the semiconductor silicon substrate and the aluminum electrode layer may be reduced when the aluminum electrode layer is formed on the semiconductor silicon substrate. .

  In addition, the aluminum powder has conductivity, and it is preferable to have 50 to 80% by mass with respect to the conductive paste in order to exhibit conductivity that can be used as the aluminum electrode layer.

  The organic vehicle is composed of an organic solvent and a binder, and volatilizes when fired to form the aluminum electrode layer. The viscosity of the organic solvent and the binder may be adjusted as appropriate so that the viscosity is in the above-described range and volatilizes during the firing process. For example, the organic solvent is 10 to 40 mass with respect to the conductive paste. %, And the binder may be contained in an amount of 1 to 10% by mass.

  Examples of the organic solvent include N, N′-dimethylformamide (DMF), α-terpineol, higher alcohol, γ-butyllactone (γ-BL), tetralin, butyl carbitol acetate, ethyl acetate, isoamyl acetate, diethylene glycol monoethyl. Ether, diethylene glycol monoethyl ether acetate, benzyl alcohol, toluene, 3-methoxy-3-methylbutanol, triethylene glycol monomethyl ether, triethylene glycol dimethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether , Tripropylene glycol monobutyl ether, propylene carbonate, dimethyl sulfoxide (DMSO), - methyl-2-pyrrolidone or the like can be used. In particular, α-terpineol is preferable because it is highly viscous and has good solubility in resins and the like.

  As the binder, for example, acrylic acid ester (acrylic resin), ethyl cellulose, polyethylene glycol derivative, nitrocellulose, polymethylstyrene, polyethylene carbonate, methacrylic acid ester and the like can be used. In particular, acrylic acid ester, nitrocellulose, and ethylcellulose are preferable because of their good thermal decomposability.

  Hereinafter, a description will be given based on examples.

(Conductive paste)
First, various inorganic raw materials were weighed and mixed so that the glass powder had a predetermined composition described in the examples to prepare a raw material batch. This raw material batch was put into a platinum crucible, heated and melted at 1000 to 1300 ° C. for 1 to 2 hours in an electric heating furnace, and the compositions shown in Examples 1 to 6 in Table 1 and Comparative Examples 1 to 5 in Table 2 were used. Glass was obtained. A part of the glass was poured into a mold and made into a block shape for use in measuring the thermal expansion coefficient. The remaining glass was formed into flakes with a rapid cooling twin roll molding machine, and sized with a pulverizer into a powder having an average particle size of 1 to 10 μm and a maximum particle size of less than 30 μm.

  The softening point was measured using a thermal analyzer TG-DTA (manufactured by Rigaku Corporation). Moreover, the said thermal expansion coefficient calculated | required the linear expansion coefficient from the amount of elongation at 30-300 degreeC when it heated up at 5 degree-C / min using the thermal dilatometer.

  Next, 39% by mass of paste oil composed of a mixture of α-terpineol and butyl carbitol acetate was mixed with 1% by mass of ethyl cellulose as a binder and 3% by mass of the above glass powder, and 57% by mass of aluminum powder as a conductive powder. A conductive paste of about 100 ± 50 Pa · s was prepared.

  Next, the p-type semiconductor silicon substrate 1 was prepared, and the conductive paste produced above was screen-printed thereon. These test pieces were dried in an oven at 140 ° C. for 10 minutes and then baked in an electric furnace at 800 ° C. for 1 minute to form an aluminum electrode layer 5 and a BSF layer 6 on the p-type semiconductor silicon substrate 1. A structure was obtained.

  Next, in order to examine the adhesiveness of the aluminum electrode layer 5 to the p-type semiconductor silicon substrate 1, a peeling tape of the aluminum electrode layer 5 when a mending tape (manufactured by Nichiban) is applied to the aluminum electrode layer 5 and peeled off is applied. Was visually evaluated.

Thereafter, a p-type semiconductor silicon substrate 1 formed with the aluminum electrode layer 5 was immersed in an aqueous solution of sodium hydroxide, the p ⁺ layer 7 by an aluminum electrode layer 5 and the BSF layer 6 is etched to expose the surface, p ⁺ The surface resistance of the layer 7 was measured with a four-probe type surface resistance measuring instrument.

(result)
The lead-free low melting point glass composition and various test results are shown in the table.

As shown in Examples 1 to 6 in Table 1, within the composition range of the present invention, the softening point is 450 ° C. to 600 ° C., and a suitable thermal expansion coefficient (70 to 110) × 10 ⁻⁷ / ° C. And had good adhesion to the p-type semiconductor silicon substrate 1. Furthermore, the resistance value of the p ⁺ layer 7 related to the conversion efficiency of the solar cell element is also 26Ω / □ or less, and can be used as a conductive paste for semiconductor silicon solar cells.

On the other hand, in Comparative Examples 1 to 5 in Table 2 outside the composition range of the present invention, good adhesion to the p-type semiconductor silicon substrate 1 is not obtained, the resistance value of the p ⁺ layer 7 is high, or glass after melting Since it exhibits deliquescence, it cannot be applied as a conductive paste for semiconductor silicon solar cells.

1 p-type semiconductor silicon substrate 2 n-type semiconductor silicon layer 3 antireflection film 4 surface electrode 5 aluminum electrode layer 6 BSF layer 7 P ⁺ layer

Claims (6)

A conductive paste for a solar cell using a semiconductor silicon substrate, wherein the conductive paste contains glass frit, and the composition of the glass frit is substantially free of a lead component, and SiO ₂ is contained in 1% by mass. 20,
B ₂ O ₃ 5 to 30,
Al ₂ O ₃ from 0 to 10,
ZnO 8 to 35,
RO (total of at least one selected from the group consisting of MgO, CaO, SrO, and BaO) is 5 to 30,
0.1 to 6, R ₂ O (total of at least one selected from the group consisting of Li ₂ O, Na ₂ O, and K ₂ O),
Bi ₂ O ₃ 10-60,
A conductive paste comprising: 2. The conductive glass according to claim 1, wherein the glass frit has a coefficient of thermal expansion at 30 ° C. to 300 ° C. of (70 to 110) × 10 ⁻⁷ / ° C. and a softening point of 450 ° C. or more and 600 ° C. or less. Sex paste. The conductive paste according to claim 1, wherein the conductive paste includes aluminum powder. Wherein the composition of the glass frit, R 2 _O, characterized in that it comprises at least K _{2 O} as claims 1 to 3 of any one in the conductive paste according. The conductive paste according to any one of claims 1 to 4, wherein the composition of the glass frit includes at least BaO as RO. The claims 3 to conductive paste according to any one of claims 5, applied to the p-type silicon substrate, the manufacturing method of the aluminum electrode layer and firing.

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