JP2010251389A - Aluminum paste composite and solar cell using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum paste composite which is free from swelling or fine protrusions when being applied onto a silicon wafer and baked, and also to provide a solar cell where an aluminum electrode layer is formed with the composite. <P>SOLUTION: The aluminum paste composite for forming the aluminum electrode layer onto the silicon wafer contains aluminum powder, an organic substance binder, and a glass frit. The aluminum powder contains flake shape aluminum powder. The solar cell is the cell where the aluminum electrode layer is formed by applying and baking the aluminum paste composite. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an aluminum paste composition used for forming an aluminum electrode layer of a solar battery cell, and relates to an aluminum paste composition which is applied onto a silicon wafer constituting a crystalline silicon solar battery and fired.

  Conventionally, a solar battery cell in which an aluminum electrode layer is formed on a silicon wafer has a configuration as shown in FIG.

  The solar battery cell 10 is configured with a p-type silicon wafer 1 having a thickness of 180 to 300 μm as a substrate. On the light receiving surface side of the silicon wafer 1, an n-type impurity layer 2 having a thickness of 0.1 to 0.6 μm, and an antireflection film 3 and an Ag surface electrode 4 are formed thereon.

  In FIG. 1, an aluminum electrode layer 5 is formed on the back side of a p-type silicon wafer 1, and a p-type impurity layer 6 and an Ag back electrode 7 are formed on the inside thereof.

The aluminum electrode layer 5 is formed by applying an aluminum paste composition composed of aluminum powder, an organic material binder and glass frit on the back surface of a p-type silicon wafer by screen printing or the like, and drying it at a temperature of 650 ° C. or higher for a short time. The p-type impurity layer 6 is formed by firing. The p-type impurity layer 6 is produced by the diffusion of aluminum atoms into the p-type silicon wafer during firing, and an Al—Si alloy layer is formed between the aluminum electrode layer 5 and the p-type silicon wafer 1. It happens at the same time as it is formed. Due to the presence of the p-type impurity layer 6, recombination of electrons is prevented, and a so-called BSF (Back Surface Field) effect that improves the collection efficiency of generated carriers is obtained.

  In recent years, it has been studied to reduce the thickness of a silicon wafer in order to reduce the cost of solar cells. However, the silicon wafer and the aluminum electrode layer that is applied and baked on almost the entire back surface have a difference in shrinkage rate when it is cooled after firing and stress generated during shrinkage. If it becomes, curvature will generate | occur | produce in a photovoltaic cell after baking. In order to suppress the occurrence of the warp, there is a method of reducing the thickness of the aluminum electrode layer. However, if the aluminum electrode layer is thinned, blisters and minute protrusions are likely to be generated in the aluminum electrode layer during firing. Such blisters and minute protrusions have been a problem of causing cracks in the solar cell itself in the subsequent manufacturing process of the solar cell.

  Various aluminum paste compositions have been proposed as methods for solving these problems.

JP-A-2005-191107 (Patent Document 1) obtains a high-performance back electrode (aluminum electrode layer) that suppresses the formation and swelling of aluminum balls and protrusions in the back electrode (aluminum electrode layer). A method of manufacturing a solar cell element having high productivity with reduced warpage of a semiconductor substrate is disclosed. As the aluminum paste used in this and the average particle diameter D ₅₀ of the cumulative particle size distribution of aluminum powder by volume based 6 to 20 .mu.m, those of less than half the diameter of the average particle diameter D ₅₀ account for all particle size distribution A ratio of 15% or less has been proposed. However, there is a problem that even if aluminum powder satisfying the condition is used, the formation of swelling and protrusions may not be sufficiently suppressed.

  Japanese Patent Application Laid-Open No. 2007-81059 (Patent Document 2) uses an aluminum paste composition capable of suppressing the occurrence of blisters or aluminum balls on the back electrode (aluminum electrode layer) during firing. The solar cell element provided with the electrode (aluminum electrode layer) formed in this way is disclosed. It has been proposed that the aluminum paste composition contains an alkaline earth metal oxide in the glass frit, and the proportion thereof is 5% by weight to 75% by weight or less in the glass frit. However, since glass frit using an alkaline earth metal oxide has a high melting point, it is necessary to maintain a high firing temperature, which increases the production cost and limits the glass frit that can be used. There was a problem that.

JP-A-2005-191107 JP 2007-81059 A

  The problem to be solved by the present invention is to provide an aluminum paste composition that causes very little or no blisters or minute protrusions during firing.

  The invention described in claim 1 is an aluminum paste composition for forming an aluminum electrode layer on a silicon wafer, comprising an aluminum powder, an organic material binder, and a glass frit. The aluminum powder is flaky aluminum. It is an aluminum paste composition characterized by containing powder, and when applied to a silicon wafer and baked, there is very little or no occurrence of blisters or minute protrusions on the aluminum electrode layer.

  The invention according to claim 2 is the aluminum paste composition according to claim 1, wherein the aluminum powder contains flaky aluminum powder in a proportion of 1 wt% to 100 wt%. In addition, when applied to a silicon wafer and baked, the aluminum electrode layer has very little or no blisters or minute protrusions.

  The invention according to claim 3 is a solar cell characterized in that the aluminum paste composition according to claim 1 or claim 2 is applied onto a silicon wafer and then baked to form an aluminum electrode layer. There is very little or no occurrence of blisters and minute protrusions on the aluminum electrode layer, and therefore there is very little or no cracking in the solar cells.

  The aluminum composition of the present invention has the effect that, when applied to a silicon wafer and baked, there are very few or no blisters and minute protrusions in the aluminum electrode layer.

  Further, the solar battery cell of the present invention has very little or no swelling of the aluminum electrode layer and minute protrusions, and therefore, the solar battery cell is hardly or hardly cracked.

It is sectional drawing of a p-type silicon wafer.

  The aluminum paste composition of the present invention includes glass frit in addition to aluminum powder and an organic binder, and the aluminum powder includes flaky aluminum powder.

The glass frit used in the present invention serves as an inorganic binder for bonding aluminum powder after melting, promotes the reaction between aluminum and silicon, and acts as a sintering aid for the aluminum powder itself. As a glass frit having these functions, a PbO—B ₂ O ₃ —SiO ₂ system, a PbO—B ₂ O ₃ —Al ₂ O ₃ system, a PbO—B ₂ O ₃ system, and SiO ₂ are generally used as main components. -B ₂ O ₃ -R ₂ O-based (R: alkali _{metal), B} 2 O 3 -ZnO _{system, Bi 2 O 3 -B 2 O} 3 -SiO 2 based and _{Bi 2 O 3 -B 2 O 3} -ZnO A glass frit containing an oxide such as a system is used, but one containing no lead is preferably used in view of environmental load. The content of the glass frit is 0.5 to 5% by weight, preferably 1 to 3% by weight, based on 100% by weight of the aluminum paste composition. If it is less than 0.5% by weight, the adhesion between the silicon wafer and the fired aluminum electrode layer is poor, and if it exceeds 5% by weight, the resistivity of the aluminum electrode layer is high. If it is less than 1% by weight, the adhesion between the silicon wafer and the aluminum electrode layer after firing tends to decrease, and if it exceeds 3% by weight, the resistivity of the aluminum electrode layer tends to increase.

  The organic material binder used in the present invention is not particularly limited, but it is possible to use a resin in which a resin such as ethyl cellulose, an acrylic resin, or a butyral resin is dissolved in a solvent such as an ester, glycol ether, or terpineol. The content of the resin in the organic material binder is not particularly limited, and depending on the blending ratio of the flaky aluminum powder described later, appropriate application characteristics depending on the viscosity and TI value when the aluminum paste composition is obtained It can be determined after adjustment to obtain. The content of the above resin in the organic binder with respect to 100% by weight of the aluminum paste composition is 1 to 10% by weight, preferably 2 to 6% by weight. When the content is less than 1% by weight, the printability of the aluminum paste composition decreases. If it exceeds 10% by weight, not only will the viscosity of the aluminum paste composition increase, but the organic material will remain in the aluminum electrode layer after firing due to the excess organic material, causing an increase in resistivity. If it is less than 2% by weight, the printability tends to decrease, and if it exceeds 6% by weight, the amount of resin is large, and the resin remains in the electrode and the resistivity tends to increase.

The aluminum powder other than flaky aluminum powder used in the present invention, an average particle diameter D ₅₀ of 1~20μm is used, 3 to 10 [mu] m is more preferable. When the average particle diameter is less than 1 μm, the aluminum powder is melted quickly during firing, and the aluminum electrode layer is swollen and minute protrusions are formed. On the other hand, when the average particle diameter exceeds 20 μm, the film thickness of the aluminum electrode layer becomes large, and a large warp is caused in the silicon wafer due to shrinkage after firing. When the thickness is less than 3 μm, the electrical resistivity of the aluminum electrode layer tends to increase, and when it exceeds 10 μm, the unevenness of the aluminum electrode layer tends to increase.

  Content of aluminum powder is 20 to 90 weight% with respect to 100 weight% of aluminum paste compositions, Preferably it is 30 to 85 weight%. If it is less than 20% by weight, the resistivity of the aluminum electrode layer is high, and if it exceeds 90% by weight, the aluminum paste composition has a high viscosity and the smoothness after printing becomes poor. If it is less than 30% by weight, the resistivity of the aluminum electrode layer tends to increase. If it exceeds 85% by weight, the viscosity of the aluminum paste composition increases and the leveling property decreases, and the smoothness after printing on a silicon wafer is poor. Tend to be.

The flaky aluminum powder used in the present invention is not particularly limited, but the average particle diameter D ₅₀ is 55 μm or less, the thickness is 2 μm or less, the specific surface area is 50000 cm ² / g or less, and the average particle diameter D ₅₀ And those having a thickness to thickness ratio of 5 to 100 can be used. Preferably, the average particle diameter of 30μm or less, there is the 0.5μm or less in thickness, the ratio of the average particle diameter _{D 50} thickness of 20 to 80. When the average particle diameter D ₅₀ is 55μm greater, there is a fear that clogging of the aluminum paste composition between the yarn and the yarn of a screen printing mesh when attached coating (250 mesh or equivalent). In the present invention, the amount of flaky aluminum powder blended suppresses excessive melting of aluminum during firing, and the occurrence of blisters and minute protrusions in the aluminum electrode layer is extremely reduced or not generated at all. Can be. The blending amount of the flaky aluminum powder is determined by the cost of the aluminum paste composition and the yield of solar cells.

  The content of the flaky aluminum powder used in the present invention is preferably 1% by weight or more in the proportion in the aluminum powder, and further 5% by weight or more is caused by blisters and minute fines generated during firing. The number of protrusions is greatly reduced. If the amount is less than 1% by weight, the effect of suppressing predetermined blisters and minute protrusions tends not to be obtained. If the amount is less than 5%, blisters and minute protrusions are included as compared with the case where no flaky aluminum powder is contained. The number of occurrences is greatly reduced, but a small number of blisters and minute protrusions tend to occur.

  The method for producing the aluminum powder and flaky aluminum powder of the present invention is not particularly limited.

  The aluminum paste composition of the present invention should be prepared by blending a dispersant, a surfactant, a plasticizer, a coupling agent, an antifoaming agent, an anti-settling agent, a leveling agent, etc., depending on the application characteristics. Can do. Various types of mixing, kneading, and dispersing machines can be used for blending preparation. Examples thereof include a two-roll mill, a three-roll mill, a planetary mixer, a high-speed mixer, and a self-revolving stirrer.

Example 1 (ratio of 2% by weight of flaky aluminum powder in aluminum powder)
STD45 (trade name, manufactured by Dow Chemical Co., Ltd.), which is ethyl cellulose, is prepared in advance. A 8.4 wt% organic material binder A dissolved in a mixed solvent of 1 is prepared. The organic substance binder A was used in all the following examples and comparative examples.
This organic substance binder A 44.2 parts by weight, aluminum powder H-3 (made by VALIME, trade name, average particle diameter D ₅₀ : 4.6 μm, particle shape: spherical) 51.9 parts by weight, flaky aluminum No. which is powder. 800 parts (manufactured by Daiwa Metal Co., Ltd., average particle diameter D ₅₀ is 28 μm, thickness is 0.44 μm, ratio of average particle diameter D ₅₀ to thickness is 64, specific surface area is 18000 cm ² / g), 1.1 parts by weight, glass AFB3211 which is a frit (Central Glass Co., Ltd., zinc bismuth borate system, average particle diameter D ₅₀ 0.6 μm) 1.5 parts by weight, Floren G-900 (Kyoeisha Chemical Co., Ltd., carboxyl group-containing) which is a dispersant Polymer modified product) 0.5 part by weight and 0.8 part by weight of DOP (manufactured by J Plus Co., Ltd., bis (2-ethylhexyl) phthalate) as a plasticizer were uniformly mixed using a three-roll mill. The viscosity was 45 Pa · s / 23 ° C., and the TI value was 1.6 / 23 ° C.

Example 2 (Ratio of flaky aluminum powder in aluminum powder 5% by weight)
Organic substance binder A 44.2 weight part, H-3 which is aluminum powder 50.3 weight part, No. which is flaky aluminum powder. 800 2.7 parts by weight, 1.5 parts by weight of AFB3211 as a glass frit, 0.5 parts by weight of Floren G-900 as a dispersant, and 0.8 parts by weight of DOP as a plasticizer are uniformly using a three-roll mill. Mixed. The viscosity was 49 Pa · s / 23 ° C., and the TI value was 1.6 / 23 ° C.

Example 3 (ratio of flaky aluminum powder in aluminum powder 20% by weight)
Organic substance binder A 44.2 weight part, H-3 which is aluminum powder 42.4 weight part, No. which is flaky aluminum powder 800 10.6 parts by weight, 1.5 parts by weight of AFB3211 as a glass frit, 0.5 parts by weight of Floren G-900 as a dispersant, and 0.8 parts by weight of DOP as a plasticizer using a three-roll mill. Mixed. The viscosity was 55 Pa · s / 23 ° C., and the TI value was 1.7 / 23 ° C.

Example 4 (ratio of flaky aluminum powder in aluminum powder 100%)
Organic substance binder A 64 weight part, No. which is flaky aluminum powder. 800 30 parts by weight, 1.5 parts by weight of AFB3211 as a glass frit, 1.5 parts by weight of Floren G-900 as a dispersant, and 3.0 parts by weight of DOP as a plasticizer are uniformly mixed using a three-roll mill. did. The viscosity was 25 Pa · s / 23 ° C., and the TI value was 2.1 / 23 ° C.

Example 5 (ratio of flaky aluminum powder in aluminum powder 20%)
Organic substance binder A 44.2 weight part, H-3 which is aluminum powder 42.4 weight part, No. which is flaky aluminum powder 18000 (manufactured by Daiwa Metal Co., Ltd., trade name, average particle diameter D ₅₀ is 15 μm, thickness is 0.22 μm, ratio of average particle diameter D ₅₀ to thickness is 68, specific surface area is 36000 cm ^{2 /} g) 10.6 Part by weight, 1.5 parts by weight of AFB3211 as a glass frit, 0.5 parts by weight of Floren G-900 as a dispersant, and 0.8 parts by weight of DOP as a plasticizer were uniformly mixed using a three-roll mill. The viscosity was 59 Pa · s / 23 ° C., and the TI value was 1.8 / 23 ° C.

Example 6 (ratio of flaky aluminum powder in aluminum powder 5%)
Organic substance binder A 44.2 weight part, H-3 which is aluminum powder 50.3 weight part, No. which is flaky aluminum powder. E (manufactured by Daiwa Metal Co., Ltd., average particle diameter D ₅₀ is 50 μm, thickness is 1.54 μm, ratio of average particle diameter D ₅₀ to thickness is 32, specific surface area is 5200 cm ² / g), 2.7 parts by weight, glass 1.5 parts by weight of AFB3211 as a frit, 0.5 parts by weight of Floren G-900 as a dispersant, and 0.8 parts by weight of DOP as a plasticizer were uniformly mixed using a three-roll mill. The viscosity was 39 Pa · s / 23 ° C., and the TI value was 1.6 / 23 ° C.

Comparative Example 1 (when not containing flaky aluminum powder)
Organic material binder A 44.2 parts by weight, aluminum powder H-3 53 parts by weight, glass frit AFB3211 1.5 parts by weight, fluorene G-900 0.5 parts by weight, plasticizer 0.8 parts by weight of DOP was uniformly mixed using a three roll mill. The viscosity was 43 Pa · s / 23 ° C., and the TI value was 1.7 / 23 ° C.

  The viscosity of the various aluminum paste compositions was measured using a BH rotary viscometer (Toki Sangyo Co., Ltd.), the rotor was No. 7, and the rotation was measured at 20 rpm. The TI value was 2 rpm / 20 rpm. When the aluminum paste composition is applied by screen printing, the viscosity is preferably 20 to 60 Pa · s / 23 ° C., and the TI value is preferably 1.5 to 3.0. In this case, the coating film after printing is The thickness is kept uniform and smooth, and the coating workability is excellent.

Evaluation method The above-mentioned various aluminum paste compositions were centered on a polycrystalline p-type silicon wafer having a thickness of 200 μm and a size of 156 mm × 156 mm using a 250-mesh screen printing plate so as to have an area of 154 mm × 154 mm. And dried at 150 ° C. for 30 minutes using a hot air dryer. For firing, a 4-zone wire belt furnace was used. Zone 1 temperature is set to 400 ° C., Zone 2 temperature is set to 600 ° C., Zone 3 temperature is set to 800 ° C., Zone 5 temperature is set to 600 ° C., the stop time in Zone 1 is 25 seconds, and the remaining Zone 2 to 2 The stop time of 4 was 7 seconds, and the moving speed between the zones was 20 m / min. The film thickness of the aluminum electrode layer after firing was 35 μm on average. In the aluminum electrode layer formed on the silicon wafer, the number of blisters and minute protrusions on the entire surface of the aluminum electrode layer (154 mm × 154 mm) was visually counted. The total value is shown in Table 1.

Summary of evaluation results
In Table 1, Examples 1 to 6 have a smaller amount of blisters, aluminum blisters and minute protrusions than Comparative Example 1. This has been found by the inventor of the present invention, and it is presumed that flake-like aluminum powder mitigates a local excessive temperature rise that occurs during firing on a silicon wafer. It is thought that the cause of the swelling of aluminum and minute protrusions is due to the local rise in temperature at the time of firing. For this reason, the aluminum in the part melts and liquefies, and the liquefied aluminum is still in the surrounding area. It is thought that it is caused by a slight overflow and solidification for some reason from the gaps between the non-liquefied aluminum particles. The flaky aluminum powder has a greater ability to conduct heat to others than spherical aluminum, and therefore, local increase in heat is suppressed or prevented. For this reason, it is presumed that as a result, the local generation number of aluminum that is completely melted and liquefied is reduced or zero.

1 p-type silicon wafer 2 n-type impurity layer 3 antireflection film 4 Ag surface electrode 5 aluminum electrode layer 6 p-type impurity layer 7 Ag back electrode 10 solar cell

Claims (3)

  An aluminum paste composition for forming an aluminum electrode layer on a silicon wafer, comprising an aluminum powder, an organic material binder, and a glass frit, wherein the aluminum powder contains flaky aluminum powder. Aluminum paste composition. 2. The aluminum paste composition according to claim 1, wherein the aluminum powder contains flaky aluminum powder in a proportion of 1 wt% to 100 wt%. A solar cell, wherein the aluminum paste composition according to claim 1 or 2 is applied onto a silicon wafer and then baked to form an aluminum electrode layer.

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