JP2004304114A - Method for manufacturing solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a method of manufacturing a solar cell with good cell power generation efficiency by suppressing curving due to a heat process to lessen cracking in a stage. <P>SOLUTION: The method includes a stage (step S4) of coating the entire reverse surface of a semiconductor substrate with aluminum paste, a stage (step S6) of burning the semiconductor substrate coated with the aluminum paste and forming a reverse-surface electric field layer on the entire reverse surface of the semiconductor substrate, a stage (step S7) of removing the burnt aluminum paste layer, a stage (step 8) of coating the entire surface of the reverse-surface electric field layer with metal paste for low-temperature burning and then burning it to form a reverse-surface electrode, and a stage (step S9) of soldering a copper wire directly to the reverse-surface electrode. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of manufacturing a solar cell having a small return due to a thermal process and having good cell power generation efficiency.
[0002]
[Prior art]
In a conventional method for manufacturing a solar cell, first, an n-type layer is formed on the surface of a polycrystalline silicon substrate (p-type) to form a pn junction, and an aluminum paste is printed and dried on the back surface of the silicon substrate to form a predetermined opening. A silver paste electrode (or silver aluminum paste electrode) is formed, and then a silver paste (or silver aluminum paste electrode) is printed and dried on the opening of the aluminum paste electrode to form a silver paste electrode (or silver aluminum paste electrode). The silver paste is printed and dried to form a silver paste electrode. Next, using a near-infrared furnace, the silicon substrate is heated at 700 ° C. to 750 ° C. for several tens seconds to several minutes in dry air, and all the electrodes are fired at once. By this firing step, the silver paste electrode (or silver aluminum paste electrode) formed on the back surface side of the silicon substrate and the aluminum paste electrode are electrically connected, and the back surface electric field due to the diffusion of aluminum on the back surface of the silicon substrate. A layer (hereinafter, referred to as a BSF (Back Surface Field) layer) is formed. Then, the silver paste electrode on the front side (light receiving surface) of the plurality of silicon substrates and the silver paste electrode (or silver aluminum paste electrode) on the back side are connected in series / parallel using solder and copper wire. After that, the connected silicon substrate is sealed in tempered glass using a resin such as ethylene, vinyl, acetate, or the like, thereby manufacturing a solar cell module. (For example, see Patent Document 1)
[0003]
[Patent Document 1]
JP-A-10-335267 (FIG. 1)
[0004]
[Problems to be solved by the invention]
In the conventional method of manufacturing a solar cell, the firing temperature when forming a BSF layer by diffusion of aluminum must be equal to or higher than the eutectic temperature of a semiconductor substrate material and aluminum (577 ° C. in the case of silicon and aluminum). As a result, the temperature becomes extremely high. Therefore, when the semiconductor substrate material is cooled to room temperature after firing, a large warpage is caused by a difference in linear expansion coefficient between the semiconductor substrate material and the sintered metal film (aluminum: 23 ppm / K, silicon: 4 ppm / K). Will occur. This warpage causes cracks and cracks during handling and transport in the process, and also during other thermal processes, and lowers the yield in the process. Even a minute crack in the semiconductor substrate leads to a decrease in power generation efficiency.
When a silver-aluminum paste is used on the back surface of the semiconductor substrate, the aluminum diffusion layer formed immediately below the silver-aluminum paste electrode has a low concentration. No aluminum diffusion layer is formed immediately below the aluminum diffusion layer. Therefore, the efficiency of the entire solar battery cell decreases.
[0005]
The present invention has been made in order to solve the above-mentioned problems, and a method for manufacturing a solar cell with high cell power generation efficiency by suppressing the occurrence of warpage due to a thermal process, suppressing the occurrence of cracks in the process, and suppressing the occurrence of cracks in the process. To provide.
[0006]
[Means for Solving the Problems]
A method of manufacturing a solar cell according to the present invention includes a semiconductor substrate having at least a pn junction therein, a light-collecting surface electrode formed on a light-collecting surface of the semiconductor substrate, and a back surface electric field formed on a back surface of the semiconductor substrate. A solar cell including a layer, a back electrode formed on the back surface electric field layer, and a light collecting surface electrode and a copper wire drawn from the back electrode. A step of applying and forming the entire surface; a step of baking the semiconductor substrate on which the aluminum paste is applied and forming; a step of forming the back surface electric field layer on the entire back surface of the semiconductor substrate; and a step of removing the fired aluminum paste layer Applying and forming a metal paste for low-temperature firing on the entire surface of the back surface field layer, and then firing to form the back surface electrode; In which and a step of attaching.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a flowchart illustrating a method for manufacturing a solar cell according to an embodiment of the present invention, and FIG. 2 is a process sectional view illustrating a method for manufacturing a solar cell according to an embodiment of the present invention.
[0008]
First, in step S1, a single crystal silicon substrate manufactured by, for example, a pulling method or a polycrystalline silicon substrate manufactured by a casting method is washed as the semiconductor substrate 1 (FIG. 2A). In this cleaning step, the surface of the semiconductor substrate 1 is etched by about 10 to 20 μm using an alkaline aqueous solution such as potassium hydroxide or sodium hydroxide, or a mixed solution of hydrofluoric acid and nitric acid. Remove any damage or contamination that occurs. Further, if necessary, the substrate is washed with a mixed solution of hydrochloric acid and hydrogen peroxide to remove heavy metals such as iron attached to the substrate surface.
[0009]
Next, in step S2, if the semiconductor substrate 1 is p-type, the n-type diffusion layer 2 is formed to form a pn junction. In this n-type diffusion layer forming step, an n-type diffusion layer 2 of which conductivity type is inverted by thermally diffusing phosphorus, for example, using phosphorus oxychloride (POCl ₃ ) is formed on the entire surface of the semiconductor substrate 1 (FIG. 2 (b)).
Then, in step S3, a resist is formed so as to cover only the surface (light collecting surface) of the semiconductor substrate 1, the exposed n-type diffusion layer 2 is removed by etching, and the resist is removed using an organic solvent or the like. Then, a semiconductor substrate 1 having an n-type diffusion layer 2 formed on the entire surface is obtained (FIG. 2C).
[0010]
Then, in step S4, using a screen printing method (or a roll coater method), an aluminum paste containing glass is printed and dried over the entire back surface (the surface opposite to the light receiving surface) of the semiconductor substrate 1, and the semiconductor substrate 1 is dried. An aluminum paste layer 3a is formed on the entire back surface (FIG. 2 (d)).
Next, in step S5, a silver paste is printed and dried on the surface of the semiconductor substrate 1 using a screen printing method (or a roll coater method) to form a silver paste layer 4a (FIG. 2E).
[0011]
Then, in step S6, the aluminum paste layer 3a and the silver paste layer 4a formed on the front and back of the semiconductor substrate 1 are baked collectively. By this baking step, aluminum in the aluminum paste layer 3a diffuses into the semiconductor substrate 1 as an impurity, and the p ⁺ -type diffusion layer 5 (back surface electric field layer: BSF layer) containing a high concentration of impurity is formed on the back surface of the semiconductor substrate 1. The silver paste layer 4a formed on the entire surface is fired to become the silver paste electrode 4 (light collecting surface electrode) (FIG. 2 (f)).
In this firing step, in order to form the p ⁺ -type diffusion layer 5, the firing temperature must be at least the eutectic temperature of the semiconductor substrate material and aluminum (577 ° C. in the case of silicon and aluminum). Become. Therefore, when the semiconductor substrate 1 is cooled to room temperature after firing the aluminum paste layer 3a and the silver paste layer 4a, the difference in linear expansion coefficient between the semiconductor substrate 1 and the sintered aluminum paste layer 3 (aluminum: 23 ppm / (K, silicon: 4 ppm / K) causes an internal stress, and the semiconductor substrate 1 is warped. This warping causes cracks and cracks during handling and transport in the process, and also during other thermal processes, and lowers the yield in the process. Furthermore, even if the cracks in the semiconductor substrate 1 are minute, the power generation efficiency is reduced.
[0012]
Next, in step S7, the sintered aluminum paste layer 3 is removed. In this aluminum paste layer removing step, a resist is formed so as to expose only the back surface of the semiconductor substrate 1, and wet etching is performed using an etchant containing hydrofluoric acid as a main component, or dry etching is performed by a reactive ion etching (RIE) method or the like. Is performed, the aluminum paste layer 3 formed on the back surface of the semiconductor substrate 1 is removed.
Since the sintered aluminum paste layer 3 is a residual part of the reaction and includes a silicon-aluminum alloy layer, an aluminum oxide film, an aluminum paste residue film (resin, glass, etc.), the above-described warpage is generated. Factors. Thus, the warpage of the semiconductor substrate 1 from which the aluminum paste layer 3 obtained in step S7 has been removed is reduced (FIG. 2 (g)).
The aluminum paste layer 3 has poor bonding strength with the solder 7 when soldering the copper wire 8 in a later step, and is not suitable for the back electrode 6.
[0013]
Next, in step S8, a metal paste for low-temperature firing is applied and dried over the entire back surface of the semiconductor substrate 1, and then fired to form the back electrode 6 (FIG. 2 (h)).
As for the metal paste for low-temperature firing, the lower the firing temperature, the better. However, since the copper wire 8 is soldered in a later step, it is necessary to select a metal paste that does not soften or melt at the soldering temperature. When using Sn-Pb-based solder (soldering temperature: around 190 ° C.) as the solder 7, for example, model 8368 (thermosetting coating silver, firing temperature: 200 ° C. or more) manufactured by DuPont is used as the metal paste. Can be used.
The back electrode 6 thus manufactured is in electrical contact with any position on the back surface of the semiconductor substrate 1, and by directly connecting the copper wire 8 to the back electrode 6, electric energy of the solar cell can be extracted. .
[0014]
Next, in step S9, the silver paste electrode 4 and the back surface electrode 6 of the plurality of semiconductor substrates 1 are connected in series / parallel using the solder 7 and the copper wire 8 ((i) in FIG. 2). At this time, the type of solder is appropriately selected according to the metal material of the back electrode. For example, when silver is used for the back electrode 6, Sn-Pb-based or Sn-Cu-based solder can be used. When aluminum is used for the back electrode 6, Sn-Zn-based solder can be used. Can be used.
Finally, the solar cell module is manufactured by sealing the plurality of semiconductor substrates 1 connected in series / parallel with tempered glass with a resin such as, for example, ethylene / vinyl / acetate.
[0015]
Here, the results of an experiment using a 15-cm square polycrystalline silicon substrate having a thickness of 300 μm as the semiconductor substrate 1 will be described.
Through steps S1 to S5, silver paste layer 4a and aluminum paste layer 3a were formed on the polycrystalline silicon substrate. Next, using a near-infrared furnace, the polycrystalline silicon substrate was heated at 700 ° C. to 750 ° C. for several tens seconds to several minutes in dry air, and then cooled to room temperature (step S6). Then, when the warpage of the polycrystalline silicon substrate on which the silver paste electrode 4 and the aluminum paste layer 3 were formed was measured, warpage of an average of 1.0 mm occurred.
Next, the polycrystalline silicon substrate was immersed in an etchant containing hydrofluoric acid as a main component, and the sintered aluminum paste layer 3 formed on the back surface of the polycrystalline silicon substrate was removed (step S7). When the warp of the polycrystalline silicon substrate was measured, the warp was reduced to about 0.4 mm.
Then, a silver paste of model 8368 (manufactured by DuPont, thermosetting coating silver, firing temperature: 200 ° C. or more) is applied to the entire back surface of the polycrystalline silicon substrate, and baked at 200 ° C. A back electrode 6 was formed on the entire surface (step 8). When the warpage of the polycrystalline silicon substrate was measured, the increase of the warp was about 0.1 to 0.2 mm.
Thereafter, using a Sn-Pb-based solder and a copper wire, the silver paste electrode 4 and the back surface electrode 6 of a plurality of polycrystalline silicon substrates were connected in series / parallel to produce a solar cell module.
[0016]
Then, when the solar cell module was manufactured as described above, a result was obtained in which the crack rate of the substrate in the process due to the warpage of the substrate could be reduced to about １ ／ of the conventional manufacturing method.
Further, in the solar cell module manufactured in this manner, an increase in power generation efficiency of about 1% on average was confirmed with respect to the solar cell module manufactured by the conventional manufacturing method.
[0017]
Next, the firing temperature of the material of the back electrode 6 will be described.
The functional limit of wafer warpage, that is, the amount of warpage such that one that is cracked in the process and one that does not crack in the process (the cracking rate is 50%) is found to be about 5 mm from the knowledge of computer stress analysis. Here, when the concept of the loss function in the Taguchi method (quality engineering) is introduced, the loss cost is considered to be proportional to the square of the ratio of the quality characteristic value (here, the amount of warpage). Therefore, if T ₀ is the in-process standard value of the warpage, the following equation is established.
(5.0 / T ₀ ) ² = (A ₀ / A)
However, A ₀ is the loss cost when a NG from in-process specification, where is the loss when discarding the solar cell. A is the loss cost when a defect occurs after module assembly, and here is the loss cost when the solar cell module is discarded.
When the value of A ₀ / A is set to 50, T 0, which is the in-process standard value of the warpage, is 0.71 mm from the above equation.
On the other hand, as for the relationship between the firing temperature and the warpage of a wafer having a thickness of 300 μm, the results shown in FIG. 3 are obtained from experiments.
Therefore, the firing temperature of the metal paste as the material of the back electrode 6 is desirably set to a firing temperature corresponding to the in-process standard value of warpage (0.71 mm), that is, about 400 ° C. or less.
[0018]
And since the metal paste used as the material of the back surface electrode 6 reduces the internal stress which causes the warpage of the semiconductor substrate 1, it is desirable to lower the sintering temperature. However, in the subsequent soldering step (step 9), it is necessary to prevent the back electrode 6 from softening and melting. Therefore, the sintering temperature of the metal paste needs to be higher than the soldering temperature in the subsequent process. For example, when using Sn—Pb-based solder (soldering temperature: around 190 ° C.) as the solder 7, it is desirable that the firing temperature of the metal paste used as the material of the back electrode 6 be 200 ° C. or higher.
Therefore, it is desirable to use a metal paste for firing at a low temperature of about 200 ° C. to about 400 ° C. as the material of the back electrode 6.
[0019]
As described above, the BSF layer is formed on the entire back surface of the semiconductor substrate 1 since the aluminum paste containing glass is formed on the entire back surface of the semiconductor substrate 1 and then baked to form the BSF layer on the back surface of the semiconductor substrate 1. As a result, power generation efficiency is improved.
Further, since the sintered aluminum paste layer 3 is removed after the BSF layer forming step, the aluminum paste layer 3 which causes internal stress causing warpage, particularly a semiconductor-aluminum alloy generated by high-temperature firing The layer is removed, and the warpage of the semiconductor substrate 1 can be reduced. Then, since the low-temperature firing metal paste is formed on the BSF layer and then fired to form the back electrode 6, the warpage generated by forming the back electrode 6 can be minimized. Therefore, the warpage generated in the semiconductor substrate 1 is reduced, and the occurrence of cracks and cracks during handling and transport in the process and also during other thermal processes is suppressed, and the yield in the process can be increased. Further, the generation and development of minute cracks in the semiconductor substrate 1 leading to a decrease in power generation efficiency can be suppressed.
[0020]
Further, since the silver paste layer 4a and the aluminum paste layer 3a formed on the front surface and the back surface of the semiconductor substrate 1 are baked at a time, the process can be simplified.
Further, since the copper wire 8 is directly soldered to the back surface electrode 6, it is not necessary to provide a new electrode on the back surface side of the semiconductor substrate unlike the related art. Therefore, there is no decrease in power generation efficiency due to no diffusion layer being formed in a region immediately below the back electrode portion formed of silver or the like, which is generated in the conventional technique, and power generation efficiency can be improved.
[0021]
Note that, in the above embodiment, the light-collecting surface electrode forming process and the BSF layer forming process are described as being performed in the same baking process, but the light-collecting surface electrode forming process and the BSF layer forming process are performed. May be performed in different firing steps. In this case, it is desirable to carry out both steps in the descending order of the firing temperature.
[0022]
【The invention's effect】
As described above, the present invention provides a step of applying and forming an aluminum paste on the entire back surface of a semiconductor substrate, firing the semiconductor substrate on which the aluminum paste is applied and forming, and forming a back surface field layer on the entire back surface of the semiconductor substrate. Forming, removing the fired aluminum paste layer, applying a low-temperature firing metal paste over the entire surface of the back surface field layer, forming the back surface electrode by firing, and forming a copper wire. And a step of directly soldering to the back electrode, so that a method for producing a solar cell with high cell power generation efficiency can be obtained by suppressing the occurrence of warpage due to a thermal process, suppressing the occurrence of cracks in the process. Can be
[Brief description of the drawings]
FIG. 1 is a flowchart illustrating a method for manufacturing a solar cell according to an embodiment of the present invention.
FIG. 2 is a process sectional view illustrating the method for manufacturing the solar cell according to the embodiment of the present invention.
FIG. 3 is a diagram showing the relationship between firing temperature and warpage.
[Explanation of symbols]
Reference Signs List 1 semiconductor substrate, 2 n-type diffusion layer, 3 sintered aluminum paste layer, 3a aluminum paste layer, 4 silver paste electrode (light collecting surface electrode), 4a silver paste layer, 5 p ⁺ type diffusion layer (BSF layer) , 6 back electrode, 7 solder, 8 copper wire.

Claims (1)

A semiconductor substrate having at least a pn junction therein, a light-collecting surface electrode formed on the light-collecting surface of the semiconductor substrate, a back surface electric field layer formed on the back surface of the semiconductor substrate, and formed on the back surface electric layer. Back electrode, and a method of manufacturing a solar cell comprising a light-collecting surface electrode and a copper wire drawn from the back electrode,
A step of applying and forming an aluminum paste on the entire back surface of the semiconductor substrate,
Baking the semiconductor substrate coated with the aluminum paste and forming the back surface electric field layer on the entire back surface of the semiconductor substrate;
Removing the fired aluminum paste layer;
A step of applying and forming a low-temperature firing metal paste on the entire back surface field layer, and then firing to form the back electrode;
Soldering the copper wire directly to the back electrode.

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