JP2003282898A - Solar cell element and method for forming the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that when the output deriving portion of a solar battery element and its each collector portion are so superimposed partially on each other and are so subjected to a screen printing as to bake them and form an electrode, the cell-crack of the solar battery element which starts from their superimposed portion has been generated conventionally. <P>SOLUTION: In a semiconductor substrate 1 of the solar battery element, different conductive regions from each other are so formed on its one principal-surface side and its other principal-surface side as to form respectively a front-surface electrode 4 and rear-surface electrodes 5, 6 on its one and other principal-surface sides. The rear-surface electrodes 5, 6 containing a glass frit comprise a strip-like output deriving portion 5 formed on the other principal-surface side and collector portions 6 formed in the nearly whole region of the other principal-surface side wherefrom the formed region of the output deriving portion 5 is excluded. Further, in this solar battery element, each collector portion 6 is so provided as to superimposed it on the peripheral edge portion of the output deriving portion 5, and the glass-frit content of this superimposed portion 5a of the output deriving portion 5 by each collector portion 6 is made smaller than the one of a region 5b of the output deriving portion wherefrom the superimposed portion 5a is excluded. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell element and a method of forming the same, and more particularly to a solar cell element having a back electrode composed of a strip-shaped output extraction portion and a current collector and a method of forming the same.

[0002]

2. Description of the Related Art A conventional silicon solar cell is shown in FIG.
As shown in FIG. 4, an N-type impurity is diffused to a certain depth to form an N-type diffusion layer 2 on the entire surface near the surface of the P-type semiconductor substrate 1, and an antireflection film is formed on the surface of the diffusion layer 2. 3 is formed. Further, the front surface electrode 4 is formed on the front surface side, and the output extracting portion 5 and the current collecting portion 6 are formed on the rear surface side.
Are formed on the back electrodes 5 and 6.

A method for forming the back electrodes 5 and 6 is described in Japanese Patent Laid-Open No. 5-326990 or Japanese Patent Publication No.
As disclosed in Japanese Patent No. 509910, a silver paste (5) is applied to one region on the back surface of the semiconductor substrate 1 and dried, and then an aluminum paste (6) is formed so as to partially overlap the peripheral portion of the region. ) Is applied, dried and simultaneously fired, that is, a simultaneous firing method (one-step firing) is used. In this conventional solar cell element, the output extracting portion 5 of the back electrode is formed to have a thickness of about 10 μm, and the current collecting portion 6 is formed.
Is formed to a thickness of about 50 μm, and the overlapping portion is 60 μm
It is formed to a total thickness of about.

In the solar cell element manufactured as described above, it is general that a plurality of elements are connected in series by using a wiring material to boost the voltage for use. Since solder is required for connection between these elements, the front surface electrode 4 and the back surface electrodes 5 and 6 are coated with solder. At this time, a material having good solder wettability is used for the output extraction portion 5 of the back surface electrode, and a wiring material (not shown) is soldered thereto.

[0005]

However, in the structure of the back electrodes 5 and 6 as described above, as shown in FIG. 5, when the output extracting portion 5 and the current collecting portion 6 are simultaneously fired, the current collecting portion is The component 6 diffuses into a part of the output extraction portion 5 to form the alloy layer 7. However, since the alloy layer 7 has a large shrinkage rate due to sintering, the interface 8 between the alloy layer 7 and the semiconductor substrate 1 bonded to the alloy layer 7 is large. Tensile stress is generated, and stress concentration occurs in a part of the semiconductor substrate 1.
Therefore, there has been a problem that cell cracking from the overlapping portion of the output extracting portion 5 and the current collecting portion 6 frequently occurs in the process after firing.

In order to avoid the above problems, the thickness of the output extraction portion 5 of the back electrode after firing is set to 2 μm or more and 6 μm or more.
The following has been found to be effective: But,
If the entire thickness of the output extraction portion 5 of the back surface electrodes 5, 6 is reduced, the adhesive strength between the output extraction portion 5 and the semiconductor substrate 1 is reduced, so that the output extraction portion 5a is easily peeled off during solder coating or module fabrication. There was a problem of becoming.

The present invention has been made in view of the above problems. When the output extraction part and the current collecting part are partially screen-printed and baked to form an electrode, the output extraction part and the current collecting part are combined. It is an object of the present invention to provide a solar cell element that solves the conventional problem of cell cracking starting from an overlapping portion.

[0008]

In order to achieve the above object, in a solar cell element according to claim 1, different conductive regions are formed on one main surface side and another main surface side of a semiconductor substrate, A glass frit having a surface electrode formed on the main surface side and a band-shaped output extraction portion on the other main surface side and a current collection portion formed on substantially the entire surface other than the area where the output extraction portion is formed. In a solar cell element provided with a back electrode containing
The current collecting portion was provided so as to overlap the peripheral portion of the output extraction portion, and the glass frit content in the overlapping portion of the output extraction portion was made smaller than the glass frit content in the other area of the output extraction portion. .

In the above solar cell element, it is desirable that the output extracting portion contains silver as a main component and the current collecting portion contains aluminum as a main component.

In the method of forming a solar cell element according to a third aspect, different conductive regions are formed on one main surface side and another main surface side of the semiconductor substrate, and a surface electrode is formed on the one main surface side. At the same time, the sun forming the back electrode containing the glass frit composed of the strip-shaped output extraction portion on the other main surface side and the current collection portion formed on substantially the entire surface other than the area where the output extraction portion is formed In the method of forming a battery element, the current collecting portion is provided so as to overlap the peripheral portion of the output extraction portion,
The overlapping portion of the output extraction portion is formed by using a silver paste containing substantially no glass frit, and the other region of the output extraction portion is formed by using a silver paste containing a glass frit. And

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The basic structure of the solar cell element of the present invention is the same as the structure of the conventional solar cell element shown in FIG. That is,
A diffusion layer 2 of the opposite conductivity type, for example N type, is formed by diffusing an impurity of the opposite conductivity type, for example N type, to a certain depth all over the surface of the semiconductor substrate 1 of one conductivity type, for example P type, and the diffusion layer 2 is formed. The antireflection film 3 is formed on the surface of the. Further, the front surface electrode 4 is formed on the front surface side, and the rear surface electrodes 5, 6 including the output extracting portion 5 and the current collecting portion 6 are formed on the rear surface side.
Are formed.

The semiconductor substrate 1 is composed of a monocrystalline or polycrystalline silicon substrate or the like. This semiconductor substrate 1 is p
Type or n-type may be used. In the case of single crystal silicon, it is formed by a pulling method, and in the case of polycrystalline silicon, it is formed by a casting method. Polycrystalline silicon can be mass-produced and is extremely advantageous over single crystal silicon in terms of manufacturing cost. A silicon block formed by a pulling method or a casting method is cut into a size of about 10 cm × 10 cm or 15 cm × 15 cm to form an ingot, and the semiconductor block 1 is sliced to a thickness of about 300 μm.

A diffusion layer 2 in which semiconductor impurities of opposite conductivity type are diffused is formed on the surface side of the semiconductor substrate 1. The diffusion layer 2 in which the semiconductor impurities of the opposite conductivity type are diffused is provided to form a semiconductor junction in the semiconductor substrate 1. For example, when diffusing n-type impurities, POCl
_{It is formed} by a vapor phase diffusion method using ₃ , a coating diffusion method using P ₂ O ₅ , and an ion implantation method in which P ⁺ ions are directly introduced into the substrate 1 by an electric field. The diffusion layer 2 containing the opposite conductivity type semiconductor impurities is formed to a depth of about 0.3 to 0.5 μm.

An antireflection film 3 is formed on the surface side of the semiconductor substrate 1. The antireflection film 3 is provided to prevent light from being reflected on the surface of the semiconductor substrate 1 and to effectively take in the light into the semiconductor substrate 1. The antireflection film 3 is made of a material having a refractive index of about 2 in consideration of a difference in refractive index with the semiconductor substrate 1, and is made of a silicon nitride film or a silicon oxide (SiO ₂ ) film having a thickness of about 500 to 2000Å. To be done.

On the back surface side of the semiconductor substrate 1, it is desirable to form a BSF layer (not shown) in which one conductivity type semiconductor impurity is diffused at a high concentration. The BSF layer in which the one-conductivity-type semiconductor impurity is diffused at a high concentration forms an internal electric field on the back surface side of the semiconductor substrate 1 in order to prevent a decrease in efficiency due to recombination of carriers near the back surface of the semiconductor substrate 1. is there.

That is, the carriers generated near the back surface of the semiconductor substrate 1 are accelerated by this electric field, and as a result, electric power is effectively taken out, the photosensitivity particularly at long wavelength is increased, and the solar cell characteristics at high temperature are improved. The decrease can be reduced. As described above, the sheet resistance on the back surface side of the semiconductor substrate 1 on which the BSF layer in which the one-conductivity-type semiconductor impurity is diffused at a high concentration is formed is about 15 Ω / □.

On the front surface side and the back surface side of the semiconductor substrate 1,
The front surface electrode 4 and the back surface electrodes 5 and 6 are formed. The front surface electrode 4 and the back surface electrodes 5 and 6 are formed by screen-printing an Ag paste mainly composed of Ag powder, a binder, a glass frit, etc. and firing it to form a solder layer thereon. The surface electrode 4 is composed of, for example, a large number of finger electrodes (not shown) formed with a width of about 200 μm and a pitch of about 3 mm, and two bus bar electrodes that connect the large number of finger electrodes to each other. The surface electrode 4 is formed to have a thickness of about 10 to 30 μm.

The back surface electrode is composed of, for example, a strip-shaped output extracting portion 5 formed to have a width of, for example, about 10 mm over substantially the entire length of the semiconductor substrate 1, and a current collecting portion 6 formed over substantially the entire surface other than the output extracting portion 5. To be done. This output extraction unit 5
Is about 10 μm in thickness after firing, and the current collector 6 is formed in about 50 μm in thickness after firing.

In the solar cell element of the present invention, as shown in FIGS.
As shown in FIG. 5, the strip-shaped output extracting unit 5 and the output extracting unit 5
The back surface electrodes 5 and 6 each including a current collecting portion 6 formed on substantially the entire surface other than the region where the current collecting portion is formed are provided so that the current collecting portion 6 overlaps the peripheral portion of the output extracting portion 5.

In the overlapping portion of the output extracting portion 5 and the current collecting portion 6, the glass frit content of the output extracting portion 5a at the overlapping portion is calculated from the glass frit content of the other area 5b of the output extracting portion 5a. Also reduce. For example, a region 5b other than the overlapping portion of the current collector 6 and the output take-out unit 5
Is coated with a silver paste containing 1% by weight or less of glass frit, and the current collecting portion 6 and the output extracting portion 5 overlap with each other 5a.
Is coated with fritless silver paste. However, since firing causes diffusion, the finished product will also contain some glass frit in this region 5a, but not in a greater amount than in other regions 5b. Therefore,
The overlapping portion 5a between the current collecting portion 6 and the output extracting portion 5 has a smaller glass frit content than the other area 5b.

With this structure, the adhesive force at the interface between the alloy layer 7 formed by firing and the semiconductor substrate 1 is reduced and a part is peeled off, so that the stress concentration generated in the semiconductor substrate 1 is reduced and the back surface is reduced. It is possible to reduce the occurrence rate of cracks in the semiconductor substrate 1 starting from the overlapping portion of the electrodes 5 and 6. Further, since the output take-out portion 5b other than the portion where the output take-out portion 5 and the current collecting portion 6 overlap contains a large amount of glass frit, there is sufficient adhesive strength between the output take-out portion 5b and the semiconductor substrate 1, It is possible to prevent the take-out portion 5 from peeling off.

Further, by using silver having good solderability for the output extraction portion 5 to facilitate the soldering of the wiring material, and by using aluminum for the current collecting portion 6, the P ⁺ layer on the back surface of the semiconductor during firing. To prevent carrier recombination,
The cell characteristics can be improved.

Next, a method of forming the solar cell element according to the present invention will be described. First, as shown in FIG. 3A, one conductivity type, for example, a P-type semiconductor substrate is prepared. Then, as shown in FIG. 3B, the semiconductor substrate 1 is heat-treated in an atmosphere of an opposite conductivity type, for example, an N-type impurity, to diffuse the N-type impurities to a certain depth all over the surface near the surface of the semiconductor substrate 1 to form an N-type impurity. A diffusion layer 2 having a mold is formed. Next, as shown in FIG. 3C, the antireflection film 3 is formed on the surface of the semiconductor substrate 1 by a plasma CVD method or the like.

Next, after separating the diffusion layer 2, the surface electrode 4 is formed.
Print and dry. After that, as shown in FIG. 1A, a silver paste containing glass frit (5
b) is printed and dried, and then a silver paste (5a) substantially free of glass frit is printed and dried around it. Thereafter, an aluminum paste (6) is screen-printed and baked so as to overlap with a part of the silver paste (5a) which does not substantially contain the glass frit, and a solar cell element as shown in FIG. 3 (d) is obtained. be able to. The silver paste containing glass frit (5
The print order of b) and the silver paste (5a) containing no glass frit may be reversed. The portion (5a) not containing the glass frit of the silver electrode 5 due to the firing, the portion (5) containing the glass paste with the aluminum paste (6) or the silver electrode 5
Although the glass frit is slightly diffused from b), the amount thereof is smaller than that in the portion that originally contained the glass frit.

Thereafter, the semiconductor substrate 1 is fired in a firing furnace such as a belt furnace to simultaneously form the front surface electrodes 4 and the back surface electrodes 5 and 6. At this time, aluminum diffuses from the current collector 6 made of aluminum to the output take-out portion 5 made of silver to form the Ag / Al alloy layer 7. Since the Ag / Al alloy layer 7 is formed in the portion 5a having a relatively small glass frit content, the adhesive strength between the Ag / Al alloy layer 7 and the semiconductor substrate 1 is weak and the stress acting on the interface between them is relatively small. Then, the semiconductor substrate 1 on which the electrodes 4, 5 and 6 are formed is dipped in a solder bath to form a solder coating layer (not shown) on the front electrode 4 and the output extraction portion 5 of the back electrode.

The present invention is not limited to the above embodiment, and it is needless to say that many modifications and changes can be made to the above embodiment within the scope of the present invention. For example, a metal paste based on gallium or indium can be used as the metal paste instead of the aluminum paste. It is also possible to use a metal paste based on copper, gold or platinum as a metal paste instead of the silver paste. Further, the back surface electrode pattern of FIG. 1 is an example, and the shape is not limited to this shape.

[0027]

EXAMPLES Next, examples of the present invention will be shown. As shown in FIG. 3A, the semiconductor substrate 1 is 15 cm square and has a thickness of 0.3.
A P-type silicon substrate having a mm resistance and a specific resistance of 1.5 Ω · cm was prepared. Then, as shown in FIG. 3B, a depth of 0.5 is obtained by using phosphorus oxychloride (POCl ₃ ) as a diffusion source by a thermal diffusion method.
An N-type diffusion layer 2 having a thickness of μm was formed.

Next, an antireflection film 3 made of silicon nitride having a thickness of 800 Å was formed on the surface by plasma CVD, and then the diffusion layer 2 was separated.

Next, in some of the cells, a silver paste (5) having a structure shown in FIG.
a, 5b) were printed and fired. Further, in the remaining cells, a silver paste (5a) containing no glass frit, a silver paste containing glass frit (5b), and an aluminum paste (6) having the structure shown in FIG. Further, electrodes 4, 5, 6 were formed by screen-printing silver paste (4) on the surface and baking it. At this time, as shown in FIG. 1B, the output extracting portion 5 (5a, 5b) made of silver and the current collecting portion 6 made of aluminum.
The alloy layer 7 was formed in a region of about 0.5 mm around the overlapping portion with and.

Thereafter, the substrate 1 was dipped in a solder bath at 200 ° C. and pulled up to cover the output extraction portions 5 of the front surface electrode 4 and the rear surface electrode with a solder to prepare a solar cell element. Moreover, the solar cell module was created using the solar cell element. Table 1 shows the rates of occurrence of cracking and peeling starting from the overlapping portion of the output extracting portion 5 and the current collecting portion 6 and the photoelectric conversion efficiency in the post-firing process of this solar cell element, as compared with the conventional method.

[0031]

[Table 1]

As shown in Table 1, when the method of the present invention is used, cracks originating from the overlapping portion of the output extracting portion 5 and the current collecting portion 6 occurring after firing can be reduced as compared with the conventional method. did it. When a paste containing no glass frit is used only in the overlapping portion between the output extracting portion 5 and the current collecting portion 6,
The cracks were reduced and the output take-out portion 5 was not peeled off. In addition, the photoelectric conversion efficiency of these solar cell elements was almost the same as when the conventional method was used.

[0033]

As described above, according to the solar cell element of the present invention, the current collecting portion of the back electrode is provided so as to overlap the peripheral portion of the output extracting portion, and the glass frit in the overlapping portion of the output extracting portion is provided. Since the content was made smaller than the content of the glass frit in the region other than that of the output extraction part, it is possible to prevent cracking of the solar cell element starting from this overlapping part, and between the back electrode and the semiconductor substrate. Sufficient adhesive strength can be strengthened to prevent peeling of the back electrode. Further, the photoelectric conversion efficiency of the solar cell element is not impaired.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a back electrode structure before and after firing of a solar cell element according to the present invention.

FIG. 2 is a diagram for explaining a back electrode pattern of the solar cell element according to the present invention.

FIG. 3 is a diagram for explaining the steps of the method for forming a solar cell element according to the present invention.

FIG. 4 is a diagram showing a conventional solar cell element.

FIG. 5 is an enlarged view showing a back electrode portion of a conventional solar cell element.

[Explanation of symbols]

1 ... Semiconductor substrate, 2 ... N-type diffusion layer, 2a ...
Junction separating part, 3 ... Antireflection film, 4 ... Surface electrode,
5 ... Output extraction part, 6 ... Current collection part, 5a ... Overlap part of output extraction part with current collection part

Claims (3)

[Claims] 1. A semiconductor substrate having one main surface side and another main surface side having different conductive regions to form surface electrodes on the one main surface side, and a strip-shaped output extraction portion on the other main surface side. In a solar cell element provided with a back electrode containing a glass frit, which is composed of a current collecting portion formed on substantially the entire surface other than the region where the output extracting portion is formed, the current collecting portion is the output extracting portion. And a glass frit content in the overlapping portion of the output extraction portion is set to be smaller than the glass frit content in other areas of the output extraction portion. 2. The solar cell element according to claim 1, wherein the output extraction part contains silver as a main component, and the current collection part contains aluminum as a main component. 3. A semiconductor substrate having one main surface side and another main surface side having different conductive regions to form surface electrodes on the one main surface side, and a strip-shaped output extracting portion on the other main surface side. In the method of forming a solar cell element for forming a back electrode containing a glass frit, which comprises a current collector formed on substantially the entire surface other than the region where the output extraction portion is formed, the current collector is It is provided so as to overlap the peripheral portion of the output take-out portion, and the overlapping portion of the output take-out portion is formed by using a silver paste containing substantially no glass frit, and the other area of the output take-out portion is made into a glass frit. A method for forming a solar cell element, which is characterized by forming using a silver paste containing.