JP5910509B2 - 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 5 to ₂ SiO ₂ by mass%. 15, B ₂ O ₃ and 20 to 40, Al ₂ O ₃ 0-10, 30-45 and ZnO, RO (MgO, CaO, SrO, and at least one total selected from the group consisting of BaO) 5 a 30, is a conductive paste characterized in that it comprises a 0.1 to 6, (at least one of total selected from Li ₂ O, Na ₂ O, and the group consisting of K ₂ O) a R ₂ O .

When using a conductive paste using the glass frit containing lead, the surface resistance of the p ⁺ layer because they exhibit the 20~30Ω / □ extent, the electroconductive paste of the present invention the p ⁺ layer when using 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.

In the present invention, by increasing the amount of R ₂ O contained in the glass frit, the surface resistance of the p ⁺ layer can be made lower than 30 Ω / □, 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.

The glass frit of the present invention is characterized in that the coefficient of thermal expansion at 30 ° C. to 300 ° C. is (55 to 85) × 10 ⁻⁷ / ° C., and the softening point is from 550 ° C. to 650 ° C. In the present invention, the above thermal expansion coefficient means a linear expansion coefficient.

  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 in addition to an organic vehicle containing aluminum powder and a binder such as ethyl cellulose or acrylic resin, and the glass frit contains substantially no lead component and contains SiO ₂ in mass%. 5 to 15, B ₂ O ₃ to 20 to 40, Al ₂ O ₃ to 0 to 10, ZnO to 30 to 45, RO (total of at least one selected from the group consisting of 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.

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, it is possible to form a stable glass, 5 to 15% ( (The same applies to the mass% below). If it exceeds 15%, the softening point of the glass will rise, making it difficult to use as a conductive paste. More preferably, it is 7 to 13% of range.

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 20 to 40% in the glass. If it is less than 20%, the fluidity of the glass becomes insufficient, so that the sinterability is impaired. On the other hand, if it exceeds 40%, the stability of the glass is lowered. More preferably, it is 25 to 35% of range.

Al ₂ O ₃ is a 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.

  ZnO is a component that lowers the softening point of the glass and is contained in the glass at 30 to 45%. If it is less than 30%, the above effect cannot be exhibited, and if it exceeds 45%, the glass becomes unstable and crystals are likely to be formed. Moreover, Preferably it is 35 to 42% of range.

  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. More preferably, it is 10 to 27% of range.

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 2 to 6% of range. R ₂ O preferably contains at least K ₂ O.

In addition, CuO, TiO ₂ , In ₂ O ₃ , Bi 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 (55 to 80) × 10 ⁻⁷ / ° C. and a softening point of 550 to 650 ° C. at 30 to 300 ° C. . When the thermal expansion coefficient is outside (55 to 85) × 10 ⁻⁷ / ° C., problems such as peeling and warping of the substrate occur during electrode formation. Preferably, it is in the range of (60-75) × 10 ⁻⁷ / ° C. Further, when the softening point exceeds 650 ° 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 580 ° C. or higher and 630 ° 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.

  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 and heated and melted in an electric heating furnace at 1000 to 1300 ° C. for 1 to 2 hours. In Examples 1 to 4 in Table 1 , Reference Examples, and Comparative Examples 1 to 4 in Table 2 A glass having the composition shown was obtained. A part of the glass was poured into a mold and used for measurement of thermophysical properties (thermal expansion coefficient, softening point) in a block shape. The remaining glass was made 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 4 μm and a maximum particle size of less than 10 μm.

  In addition, said softening point was measured using thermal analyzer TG-DTA (made 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, paste oil composed of α-terpineol and butyl carbitol acetate is mixed with ethyl cellulose as binder and the above glass powder, and aluminum powder as conductive powder at a predetermined ratio to prepare a conductive paste with a viscosity of about 500 ± 50 poise. did.

  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.

In Tables 1 and 2, “A” indicates that the adhesive strength is good, “B” indicates that the adhesive strength is rather good, and “C” indicates that the adhesive strength is insufficient. Indicates that there was.

As shown in Examples 1 to 4 in Table 1, within the composition range of the present invention, the softening point is 550 to 650 ° C. and a suitable thermal expansion coefficient (55 to 75 ) × 10 ⁻⁷ / ° C. The adhesion with the p-type semiconductor silicon substrate 1 was also good. 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, Comparative Examples 1 to 4 in Table 2 that are out of the composition range of the present invention cannot obtain good adhesion to the p-type semiconductor silicon substrate 1, have a high resistance value of the p ⁺ layer 7, 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 (3)

A conductive paste for solar cells using a semiconductor silicon substrate,
The conductive paste contains aluminum powder, glass frit, and an organic vehicle, and the composition of the glass frit is substantially free of lead components and contains 5 to 15 SiO ₂ by mass%.
B ₂ O ₃ 20-40,
Al ₂ O ₃ from 0 to 10,
35-45 the ZnO,
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)
And a thermal expansion coefficient at 30 to 300 ° C. is (55 to 75) × 10 ⁻⁷ / ° C. The conductive paste according to claim 1, wherein the glass frit has a softening point of 550 ° C. or higher and 650 ° C. or lower. A method for producing a solar cell element , comprising applying the conductive paste according to claim 1 or 2 onto a silicon substrate and firing the paste.

Images