JP2006019768A - Solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a cell from cracking by warping of a substrate generated due to the difference of thermal shrinkage between aluminum and silicon after baking by means of a simple and easy process in a reproducible fashion while the output characteristic of the solar cell is maintained. <P>SOLUTION: The solar cell includes a surface electrode 6 on the front surface of a semiconductor substrate 1 having a semiconductor junction and also includes back electrode including an aluminum electrode 5 on the backside of the semiconductor substrate. The aluminum electrode 5 contains a number of ceramic particles 11. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solar cell element, and more particularly to a solar cell element in which an aluminum electrode is provided on the back side of a semiconductor substrate having a semiconductor junction.

  A conventional back electrode of a solar cell element is formed by applying a metal paste to a semiconductor substrate on which a PN junction is formed by a screen printing method and firing in an oxidizing atmosphere from the viewpoint of cost reduction. In addition, in order to further reduce the cost, after applying silver paste to one area on the back side of the semiconductor substrate and drying, the aluminum paste is applied and dried so as to partially overlap the peripheral edge of the area. In addition, a simultaneous firing method (one-stage firing) in which firing is performed simultaneously has also been used (see Patent Document 1).

  A conventional method for manufacturing a solar cell element will be described with reference to FIGS. On the surface side of the P-type silicon substrate 1, an N-type diffusion layer 2 is formed by a thermal diffusion method using a diffusion source containing P, and an antireflection film 3 made of a silicon nitride film is formed. Further, a groove (not shown) is formed at the end on the back surface side of the semiconductor substrate 1 to physically separate the front surface side and the back surface side of the diffusion layer 2. Next, as shown in FIG. 5, a silver paste to be the silver electrode 4 is applied to the back surface side of the semiconductor substrate 1 by screen printing and dried, and a large portion of the back surface side is overlapped with the peripheral portion thereof. The back electrode 4 and 5 is formed by apply | coating and drying the aluminum paste used as the aluminum electrode 5, and baking simultaneously. At this time, since the silver paste is patterned in a comb shape on the front surface side of the semiconductor substrate 1 as the front surface electrode 6, the front surface electrode 6 can also be formed by simultaneous firing with the back surface electrodes 4 and 5. Finally, solder (not shown) is coated on the silver electrode 4 and the surface electrode 6 to complete a series of cells.

  When the aluminum electrode 5 on the back surface side of the semiconductor substrate 1 is formed, diffusion of aluminum occurs on the back surface side of the semiconductor substrate 1 to form a BSF layer 7 made of a P + layer, and the current-voltage characteristics of the cell are improved. High conversion efficiency is achieved.

  In the solar cell element 10 manufactured as described above, it is general to use a plurality of cells connected in series using a wiring material (not shown) to boost the voltage.

Since solder is required for connection between the cells, the electrodes are solder coated. However, since it is difficult to solder the aluminum electrode 5, the silver electrode 4 having good solder wettability is formed. The wiring material is soldered.

  However, in the conventional solar cell element 10, when the aluminum electrode 5 is baked on substantially the entire back surface of the semiconductor substrate 1, stress is generated due to the difference in thermal shrinkage between the aluminum electrode 5 and the semiconductor substrate 1 made of silicon. There is a problem that 1 is warped and the semiconductor substrate 1 is cracked.

  In order to avoid such a problem, a method of reducing the stress caused by the aluminum electrode 5 by partially etching the surfaces of the back electrodes 4 and 5 in the thickness direction by a blast method or a reactive ion etching method has been proposed. (See Patent Document 2). According to these methods, since the stress on the surface of the aluminum electrode 5 can be reduced, the warpage of the solar cell element 10 can be relaxed and cracking can be suppressed.

  However, according to these methods, if the etching thickness is too small, the stress cannot be sufficiently reduced. On the other hand, if the etching thickness is too thick, the conductive resistance of the aluminum electrode 5 is improved and the function as an electrode cannot be performed. This causes a problem that it is very difficult to control the etching thickness.

  Further, according to the blast method, when the surface of the aluminum electrode 5 is etched, the silver electrode 4 is also etched at the same time, so that the conductive resistance of the silver electrode 4 is increased and the output characteristics of the solar cell element 10 are decreased. The problem occurs.

  Therefore, although it is necessary to form the silver electrode 4 thicker than necessary in anticipation of the thickness to be etched, silver is originally an expensive material, and if used more than necessary, the cost of the solar cell element 10 is increased. There is a problem.

In order to prevent this problem, there is a method in which only the aluminum electrode 5 is formed and then etched and then the silver electrode 4 is formed. However, in order to form the back electrodes 4 and 5, at least two firing steps are necessary. Therefore, the cost increases due to an increase in the number of processes. Moreover, the output characteristic of the solar cell element 10 may deteriorate due to an increase in the high temperature process.
JP-A-5-326990 JP 2002-353479 A

  On the other hand, if the surface of the aluminum electrode is etched by the reactive ion etching method, the etching amount for each substance can be controlled by selecting the type and conditions of the gas. Although it is possible to control to reduce the etching amount and increase the etching amount on the surface of the aluminum electrode 5 that causes warping, reactive ion etching is expensive and the running cost is high. In addition, since it is a vacuum apparatus, the processing time per substrate is long, and there is a problem that it is unsuitable from the viewpoint of productivity. Further, since the surface is roughened, there is a problem that vacuum adsorption for carrying the solar cell element 10 in the subsequent process cannot be performed, or cracking in the subsequent process occurs.

  The present invention has been made in view of such conventional problems, and is a post-process due to warpage of the substrate that occurs after firing due to the difference in thermal shrinkage between aluminum and silicon while maintaining the output characteristics of the solar cell element. The purpose of this is to suppress cell cracking in a simple manner with good reproducibility.

  In order to achieve the above object, in the solar cell element according to claim 1, the solar cell element has a surface electrode on the front surface side of a semiconductor substrate having a semiconductor junction and a back electrode made of an aluminum electrode on the back surface side. In the method, a large number of ceramic particles are contained in the aluminum electrode.

  In the solar cell element, it is desirable that the ceramic particles have an average particle size of 0.04 to 5 μm.

  Moreover, in the said solar cell element, it is desirable for the said ceramic particle to exist in 3-40 volume% in the said aluminum electrode.

  In the solar cell element, it is preferable that the ceramic particles are an oxide of Al or Si.

  In the solar cell element, the ceramic particles may be hollow particles.

  According to the solar cell element of the first aspect, since a large number of ceramic particles are dispersed in the aluminum electrode on the back surface side of the semiconductor substrate having the semiconductor junction portion, heat shrinkage between aluminum and the semiconductor substrate, which has been a problem in the past, has occurred. Generation of warpage of the semiconductor substrate due to the difference in rate can be suppressed, and cracking of the solar cell element due to the warpage can be prevented.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG. The semiconductor substrate 1 is made of P-type single crystal silicon or P-type polycrystalline silicon. An N-type impurity diffusion layer 2 having a sheet resistance of about 30 to 300 Ω / □ is provided on the surface side of the semiconductor substrate 1.

The diffusion layer 2 is formed by heat-treating the P-type semiconductor substrate 1 in an atmosphere in which N-type impurities are scattered. Further, an antireflection film 3 made of a silicon nitride film or the like is provided on the surface side of the semiconductor substrate 1. This antireflection film 3 is formed by, for example, a plasma CVD method. Further, a surface electrode 6 mainly composed of silver or the like is provided on the front surface side of the semiconductor substrate 1, and an aluminum electrode 5 and a silver electrode 4 for connecting a wiring material (not shown) are provided on the back surface side. Provided.

  In the present invention, a large number of pores 9 are provided in the aluminum electrode 5 as shown in FIG. When a large number of pores 9 are provided in the aluminum electrode 5 in this way, the Young's modulus of the aluminum electrode 5 is reduced, and the warpage of the semiconductor substrate 1 due to the difference in thermal shrinkage between the aluminum and the semiconductor substrate 1, which has been a problem in the past, is caused. Generation | occurrence | production can be suppressed and the crack of the solar cell element 10 resulting from it can be prevented.

  The pores 9 preferably have an average maximum diameter of 0.04 to 5 μm. If the average maximum diameter of the pores 9 is 0.04 μm or less, the added powder is agglomerated, and if it is 5 μm or more, clogging is not desirable when screen printing is performed. The average maximum diameter of the pores 9 can be measured by SEM observation.

  The pores 9 are desirably present in an amount of 3 to 50% by volume in the aluminum electrode. When the pores 9 are present in the aluminum electrode 5 in an amount of 3% by volume or less, the warp cannot be sufficiently suppressed and the object of the present invention cannot be achieved. If it is 50% by volume or more, the resistance in the aluminum electrode 5 becomes high and the function as an electrode is not sufficiently performed, which leads to a decrease in the output characteristics of the solar cell element 10, which is not suitable. The pore portion 9 is preferably 20 to 36% by volume. The volume ratio of the pores 9 in the aluminum electrode 5 can be measured by SEM observation.

  The pores 9 are desirably formed uniformly in the aluminum electrode 5 from the viewpoint of suppressing the warp of the semiconductor substrate 1 and ensuring the output characteristics. However, it is particularly easy to generate a large warp with a large area and a small thickness. When the semiconductor substrate 1 is used, it may be formed densely on the semiconductor substrate 1 side in order to further enhance the warpage suppressing effect. When higher characteristics are required, it is densely formed on the surface side to enhance the BSF effect. May be formed.

  In order to form the pores 9 in the aluminum electrode 5, for example, a pore forming agent such as carbon particles is dispersed in an aluminum paste made of aluminum powder, an organic solvent, a binder and the like, and the temperature is about 700 to 900 ° C. in an oxidizing atmosphere. What is necessary is just to bake at temperature. Thus, when a pore forming agent such as carbon particles is dispersed in the aluminum paste and baked in an oxidizing atmosphere, the carbon particles in the aluminum paste start to become carbon dioxide from a temperature of about 600 ° C. The pores 9 are formed in 5. Therefore, in the temperature range of about 600 to 700 ° C., it is necessary to maintain the temperature for a certain period of time or gradually increase the temperature, and firing at a temperature lower than the melting temperature of aluminum so as not to collapse the cavity. is required.

  In order to form such pores 9 uniformly throughout the aluminum electrode 5, it is necessary to uniformly disperse pore-forming agents such as carbon particles in the aluminum paste. In order to uniformly disperse the pore forming agent in the aluminum paste, it may be sufficiently stirred. In addition, since the pore-forming agent is difficult to be uniformly dispersed when the specific gravity of the pore-forming agent is greatly different from that of aluminum, the pore-forming agent is preferably as close to the specific gravity of aluminum as possible. Also in such a point, carbon can be effectively used as a pore forming agent for the aluminum electrode 5.

  According to the solar cell element described in detail above, since a large number of pores are provided in the aluminum electrode on the back surface side of the semiconductor substrate having a semiconductor junction, the heat of aluminum and the semiconductor substrate, which has been a problem in the past, has been provided. Generation of warpage of the semiconductor substrate due to a difference in shrinkage rate can be suppressed, and cracking of the solar cell element due to the warpage can be prevented.

FIG. 3 is an enlarged view showing an aluminum electrode portion of the solar cell element according to the present invention. This solar cell element is also substantially the same as the above-described solar cell element. In the present invention, Al ₂ O ₃ or SiO ₂ ceramic particles 11 are dispersed in the aluminum electrode. Thus, even when a large number of ceramic particles 11 are dispersed in the aluminum electrode, it becomes an aluminum thermal shrinkage preventing agent, and warpage of the semiconductor substrate 1 due to the difference in thermal shrinkage between the aluminum and the semiconductor substrate 1, which has been a problem in the past, is generated. Can be suppressed, and the solar cell element 10 can be prevented from cracking.

  The ceramic particles 11 preferably have an average particle size of 0.04 to 5 μm. If the average particle size of the ceramic particles 11 is 0.04 μm or less, aggregation is caused, and if the average particle size is 5 μm or more, clogging is undesirable when screen printing is performed.

The ceramic particles 11 are preferably dispersed in the aluminum electrode 5 in an amount of 3 to 50% by volume. When the ceramic particles 10 are present only in an amount of 3% by volume or less in the aluminum electrode, the warp cannot be sufficiently suppressed and the object of the present invention cannot be achieved. If it is 50% by volume or more, the resistance in the aluminum electrode 5 becomes high and the function as an electrode is not sufficiently performed, which leads to a decrease in the output characteristics of the solar cell element 10, which is not suitable. For SiO _2, it is preferably 10 to 30 vol%, if the _Al 2 _{O 3,} is optimal from 5 to 15% by volume. The volume ratio of the ceramic particles 11 in the aluminum electrode 5 can be measured by SEM observation.

  The ceramic particles 11 are desirably formed uniformly in the aluminum electrode 5 from the viewpoint of suppressing warpage of the semiconductor substrate 1 and securing output characteristics. However, the ceramic particles 11 are particularly likely to generate large warpage with a large area and a small thickness. When the semiconductor substrate 1 is used, it may be formed densely on the semiconductor substrate 1 side in order to further enhance the warpage suppressing effect. When higher characteristics are required, it is densely formed on the surface side in order to enhance the BSF effect. May be formed.

In order to disperse the ceramic particles 11 in the aluminum electrode 5, ceramic particles 11 such as Al ₂ O ₃ and SiO ₂ are dispersed in an aluminum paste made of, for example, an aluminum powder, an organic solvent, a binder, etc., and about 700 to 900 ° C. It may be fired at a temperature of.

In order for such ceramic particles 11 such as Al ₂ O ₃ and SiO ₂ to be uniformly present throughout the aluminum electrode 5, it is necessary to uniformly disperse the ceramic particles 11 in the aluminum paste. In order to uniformly disperse the ceramic particles 11 in the aluminum paste, it may be sufficiently stirred. Further, if the specific gravity of the ceramic particles 11 is significantly different from that of aluminum, it is difficult to uniformly disperse the ceramic particles 11. Also in this respect, Al ₂ O ₃ , SiO _{2 and the} like can be effectively used as the ceramic particles 11 dispersed in the aluminum electrode 5.

  The ceramic particles 11 may be hollow ceramic particles. In the case of hollow ceramic particles, the volume can be increased even if the added weight is small.

  As described above, according to the solar cell element described in detail, since a large number of ceramic particles are dispersed in the aluminum electrode on the back surface side of the semiconductor substrate having the semiconductor junction portion, the aluminum and the semiconductor substrate, which have been problems in the past, are used. It is possible to suppress the warpage of the semiconductor substrate due to the difference in thermal shrinkage between the solar cell elements and to prevent the solar cell element from cracking.

As an electrode material of an aluminum electrode, an area of 14.5 cm × 14.5 cm of an aluminum paste in which an aluminum paste made of aluminum powder, a binder, an organic solvent, and a glass frit is contained at a carbon powder content of 14% with respect to the total weight ratio of the aluminum paste 15cm × 15cm × 0.35mm solar cell element coated with 1600mg and fired at 760 ° C to form a pore portion (burned pore portion is 28% by volume), output characteristics (Pm), aluminum paste 1600 mg each of the aluminum paste according to the present invention containing 14% of SiO ₂ and Al ₂ O ₃ with respect to the total weight ratio of the aluminum paste, respectively, in an area of 14.5 cm × 14.5 cm, and firing at 760 ° C. 15cm x 15cm x 0.35 obtained by The warpage and output characteristics (Pm) of the solar cell element of mm are shown in Table 1 (20% by volume of burned SiO _{2 and} 10% by volume of Al ₂ O ₃ ). Table 1 also shows warpage and output characteristics (Pm) of a 14.5 cm × 14.5 cm solar cell element having a conventional aluminum electrode that does not form pores as described above and does not disperse ceramic particles.

  As shown in Table 1, the warpage of the solar cell element obtained by the conventional method was 1.02 mm, whereas the warpage of the solar cell element obtained by the present invention was reduced to 0.69 to 0.73 mm. did. The output characteristics of the solar cell element were 3.3369 W for the conventional solar cell, whereas the solar cell element according to the present invention was almost equivalent to 3.372 to 3.385 W.

It is a figure which shows the solar cell element which concerns on this invention. It is a figure which shows the example which formed the pore part in the aluminum electrode in the solar cell element which concerns on this invention. It is a figure which shows the example which disperse | distributed the ceramic particle to the aluminum electrode in the solar cell element which concerns on this invention. It is a figure which shows the conventional solar cell element. It is a figure which shows the back surface electrode of the conventional solar cell element.

Explanation of symbols

  1: Semiconductor substrate, 2: Diffusion layer, 3: Antireflection film, 4: Back side silver electrode, 5: Aluminum electrode, 6: Front side electrode, 7: Back side solder layer, 8: Front side solder layer, 10: Sun Battery element

Claims (15)

In a solar cell element having a surface electrode on the front surface side of a semiconductor substrate having a semiconductor junction and a back electrode made of an aluminum electrode on the back surface side,
A solar cell element characterized in that a large number of ceramic particles are contained in the aluminum electrode.   2. The solar cell element according to claim 1, wherein the ceramic particles have an average particle size of 0.04 to 5 μm.   3. The solar cell element according to claim 1, wherein the ceramic particles are present in an amount of 3 to 40% by volume in the aluminum electrode.   The solar cell element according to any one of claims 1 to 3, wherein the ceramic particles are an oxide of Al or Si.   The solar cell element according to claim 1, wherein the ceramic particles are hollow particles.   6. The solar cell element according to claim 1, wherein the specific gravity of the ceramic particles approximates the specific gravity of aluminum. The solar cell element according to any one of claims 1 to 6, wherein the ceramic particles are present dispersed in the aluminum electrode.   The solar cell element according to any one of claims 1 to 6, wherein the ceramic particles are present more on the semiconductor substrate side than on the surface side of the aluminum electrode.   7. The solar cell element according to claim 1, wherein the ceramic particles are present more on the aluminum electrode surface side than on the semiconductor substrate side. In the method for manufacturing a solar cell element having a surface electrode on the front surface side of a semiconductor substrate having a semiconductor junction, and having a back electrode made of an aluminum electrode on the back surface side,
Applying an aluminum paste containing a large number of ceramic particles on the back side of the semiconductor substrate;
And baking the applied aluminum paste. A method for manufacturing a solar cell element.   An aluminum paste for forming solar cell element electrodes, comprising aluminum powder, a binder, an organic solvent, glass frit and ceramic particles.   The aluminum paste for forming a solar cell element electrode according to claim 11, wherein the ceramic particles have an average particle size of 0.04 to 5 µm.   The said ceramic particle is an oxide of Al or Si, The aluminum paste for solar cell element electrode formation of Claim 12 or 13 characterized by the above-mentioned.   14. The aluminum paste for forming a solar cell element electrode according to claim 11, wherein the ceramic particles are hollow particles.   15. The aluminum paste for forming a solar cell element electrode according to claim 11, wherein the specific gravity of the ceramic particles approximates the specific gravity of aluminum.

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