JP2008177560A - Solar cell and string - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell which maintains a good cell behavior and keeps electrodes coated with a lead-free soldering that contains no lead. <P>SOLUTION: The solar cell includes electrodes 5 and 6 which are coated with a lead-free soldering that contains no lead. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to solar cells, in particular solar cells and strings coated with lead-free solder, and materials thereof.

  The structure of the solar cell in the case of performing solder coating is shown in FIG. An n-type diffusion layer 2 is formed on one side which becomes a light-receiving surface of the etched P-type silicon substrate 1, and antireflection for reducing the surface reflectance on the n-type diffusion layer 2 is further provided. A film 3 is formed. On the other hand, a back surface aluminum electrode 4 is formed on the back surface of the P-type silicon substrate 1. Further, silver electrodes 5 and 6 are formed on a part of the back surface of the P-type silicon substrate 1 and the antireflection film 3 on the light receiving surface side. The silver electrodes 5 and 6 are coated with the solder layer 7.

  Such a solar cell has been manufactured by a method as shown in FIG. That is, in the case of crystalline silicon, the P-type silicon substrate 1 is first etched. This is the substrate etching process. Then, an n-type diffusion layer forming step is performed on the etched P-type silicon substrate 1 to form the n-type diffusion layer 2 on one side serving as a light receiving surface, and the surface reflectance is reduced on the n-type diffusion layer forming step An antireflection film forming step for forming the antireflection film 3 is performed.

  Then, on the back surface of the P-type silicon substrate 1, Al paste is printed on almost the entire surface by screen printing, dried, and then fired in an oxidizing atmosphere at a high temperature to form the back surface aluminum electrode 4. This is the back surface aluminum paste printing / drying / firing process.

  Further, silver paste is printed in a pattern on a part of the back surface of the P-type silicon substrate 1 and the antireflection film on the light-receiving surface side by screen printing, and then baked in an oxidizing atmosphere at a high temperature to form silver electrodes 5 and 6. Form. That is, the back surface silver paste printing / drying step is performed after the back surface silver paste printing is performed, and the silver electrode 6 is formed by baking, and the light receiving surface side silver paste is dried after the light receiving surface silver paste printing is performed. -A drying process is performed and baked to form the silver electrode 5. In addition, when baking the silver electrode 5 and the silver electrode 6, it is possible to perform the simultaneous baking process of baking simultaneously. Thereafter, the solar cell element formed as described above is immersed in a flux containing an activator for several tens of seconds at room temperature. This is a flux immersion process. After immersing the solar cell element in the flux, it was dried with warm air and immersed in a 6: 4 eutectic solder bath containing 2% by weight of silver at about 195 ° C. for about 1 minute. Coating with the solder layer 7 is performed. This is the solder coating process.

  After the coating with the solder layer 7 is completed, ultrasonic cleaning in normal temperature water or warm water is repeated several times, and finally pure water rinse is performed, followed by hot air drying. This is the cleaning / drying process. The solar cell is manufactured by the process described above.

  On the other hand, a string in which solar cells are connected by an interconnector is shown in FIG. That is, FIG. 3 is a diagram showing a conventional string. The surface main electrode 11 of the solar cell 10 is coated with 6: 4 solder, and a plurality of solar cells 10 are coated with 6: 4 solder. 12 is connected. Such a string was manufactured by a method as described below. That is, the interconnector 12 in which the copper lead wire is coated with 6: 4 eutectic solder is overlapped on the main electrode 11 of the solar cell 10 and is melted and cooled and solidified by blowing hot air at about 400 ° C. This was repeated for the front and back to produce a string, which was then used for the production of a solar cell module.

  In recent years, from the viewpoint of environmental problems, the harm of lead to the human body is regarded as a problem, and development is progressing in a direction not to use lead from various devices. And also in manufacture of a solar cell, manufacturing in the lead-free state which does not contain lead is strongly desired from the industry.

  However, solar cells using lead-free solder have not been produced so far. For example, when solder coating is performed with an Sn bath in which lead is removed from a conventional 6: 4 eutectic solder, the silver in the silver paste sintered body is taken into the Sn bath, and the electrodes disappear in some places. There was a problem that it could not be converted into solar cells. This is considered to be because the melting point of Sn was 231.9 ° C., which was about 50 ° C. higher than 183 ° C. of 6: 4 eutectic solder.

  The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a solar cell coated with a lead-free solder that maintains good cell characteristics and does not contain lead. Another object of the present invention is to provide an interconnector having a lead wire coated with lead-free solder, and also to provide a reliable string in which solar cells are connected by such an interconnector.

  The solar cell according to the present invention is a solar cell having an electrode coated with lead-free solder not containing lead.

  The solar cell according to the present invention is a solar cell characterized in that the electrode is an electrode formed by firing a paste.

  The solar cell according to the present invention is a solar cell characterized in that the electrode is an electrode formed by any one of a metal vapor deposition method, a sputtering method, and a plating method.

  Moreover, the solar cell according to the present invention is a solar cell characterized in that the lead-free solder is Sn-Bi-Ag solder.

  The solar cell according to the present invention is a solar cell characterized in that the lead-free solder is Sn-Ag solder.

  Moreover, the solar cell according to the present invention is a solar cell characterized in that the paste contains silver powder, glass powder, an organic vehicle, an organic solvent, phosphorus oxide, and a halide.

  Moreover, the solar cell which concerns on this invention is a solar cell which has the electrode coated with the lead-free solder which does not contain lead, after wash | cleaning with the flux which consists of resin, a solvent, and a resin stabilizer.

  The solar cell interconnector according to the present invention is a solar cell interconnector coated with lead-free solder.

  Moreover, the string which concerns on this invention is a string which wired the solar cell which has the electrode coated with the lead-free solder which does not contain lead with the interconnector for solar cells coated with the lead-free solder which does not contain lead.

  In the string according to the present invention, the lead-free solder used for the solar cell and the lead-free solder used for the interconnector have the same composition.

  Moreover, the string which concerns on this invention is a string which contains Bi in at least any one of the lead-free solder used for the said solar cell, or the lead-free solder used for the said interconnector.

  Moreover, the solar cell which concerns on this invention is a solar cell whose content of said Bi is 3-89 weight%.

  Moreover, the string which concerns on this invention is a string whose content of the said Bi is 3-89 weight%.

  Moreover, the string which concerns on this invention is a string which contains Ag in at least any one of the lead-free solder used for the said solar cell, or the lead-free solder used for the said interconnector.

  Moreover, the solar cell which concerns on this invention is a solar cell whose content of the said Ag is 3.5 to 4.5 weight%.

  Moreover, the string which concerns on this invention is a string whose content of said Ag is 3.5 to 4.5 weight%.

  The solar cell according to the present invention is a solar cell coated with lead-free solder that does not contain lead, and is a highly reliable solar cell that maintains good cell characteristics while solving environmental problems. Moreover, the string which connected the solar cell which concerns on this invention with the interconnector for solar cells coated with lead-free solder is a string which has reliability, solving an environmental problem. The solar cell according to the present invention can be manufactured simply by changing the solder material when manufacturing a conventional solar cell. Therefore, the solar cell which concerns on this invention can be easily manufactured by utilizing the manufacturing method of the conventional solar cell.

  The solar cell according to the present invention is a solar cell having an electrode coated with lead-free solder not containing lead. By coating the electrode of the solar cell, it is possible to protect the electrode from mechanical shock and humidity in the atmosphere. In addition, by coating the electrodes of the solar cells, it is possible to facilitate wiring when connecting the solar cells with an interconnector.

  FIG. 4 shows a solar cell according to the present invention, which is the same as the conventional solar cell of FIG. That is, the solder layer 8 is the solder layer 7 that is a 6: 4 eutectic solder in the past, but is a solder layer 8 that is a lead-free solder in this embodiment.

  As the lead-free solder, Sn-Bi-Ag solder or Sn-Ag solder can be used. Sn-Bi-Ag solder or Sn-Ag solder is a solder having a lower melting point than Sn solder. Here, the Sn—Bi—Ag solder is an Sn—Bi—Ag solder containing 0.1% or more of Ag. The Sn—Ag solder is Sn—Ag solder containing 0.1% or more of Ag. In the Sn-Bi-Ag solder, it is preferable that the Bi content is 3 to 89% by weight. Further, it is more preferable that the Bi content is 35 to 60% by weight. The Bi content is set as described above for the following reason. In other words, in order to perform the solder dipping process without any problem, it is desirable to carry out at the current dip temperature of about 195 ° C., and at least 225 ° C. or less, which is a practical limit in terms of characteristics and reliability. is there. In the composition having a melting point of 225 ° C. or lower, Bi is 5 to 88% by weight when 0.1% Ag is contained, and 3 to 89% by weight when 1.3% Ag is contained. On the other hand, in the composition having a melting point of 195 ° C. or lower, Bi is 27 to 79% by weight when 0.1% Ag is contained, and Bi is 35 to 60% by weight when 1.8% Ag is contained. As mentioned above, in Sn-Bi-Ag type solder, if Bi content is 3-89 weight%, it is suitable. Therefore, the Bi content is preferably 3 to 89% by weight, and more preferably 35 to 60% by weight. Similarly, in the Sn-Ag solder, the composition having a melting point of 225 ° C. or lower has an Ag of 3.5 to 4.5% by weight. On the other hand, there is no composition having a melting point of 195 ° C. or lower in this system. As described above, in the Sn-Ag solder, the content of Ag is preferably 3.5 to 4.5% by weight.

  A silver paste used for producing an electrode of a solar cell includes a silver powder, a glass powder, an organic vehicle, and an organic solvent as main components, and contains iridium chloride and phosphorus oxide. Material can be used. Moreover, as a flux used when producing the electrode of a solar cell, the flux material which consists only of polyalkylglycol-type resin and a solvent, and does not contain an activator can be used. That is, a flux composed of a resin, a solvent, and a resin stabilizer can be used. After washing with a flux composed of a resin, a solvent, and a resin stabilizer, the electrode can be coated with lead-free solder.

  The electrode of the solar cell can be formed by firing a paste, and can also be formed by any of a metal vapor deposition method, a sputtering method, or a plating method.

  The interconnector according to the present invention is a solar cell interconnector coated with lead-free solder.

  The string according to the present invention is a string in which a solar cell having an electrode coated with lead-free solder not containing lead is wired with an interconnector having a lead wire coated with lead-free solder not containing lead. . FIG. 5 shows a string using a solar cell according to the present invention. The surface main electrode 21 of the solar cell 10 is coated with lead-free solder, and a plurality of solar cells 10 are coated with lead-free solder. Are connected by an interconnector 22 having a lead wire. Here, if the lead-free solder used for the solar cell and the lead-free solder used for the interconnector have the same composition, there is an advantage that the processing in the manufacturing process is simple and stable. In addition, if the lead-free solder used for solar cells and the lead-free solder used for the interconnector have different compositions, the melting point of each is different, which requires delicate temperature adjustments during the soldering process. It becomes.

  In addition, by containing Ag in at least one of lead-free solder used for solar cells and lead-free solder used for interconnectors, the following effects can be obtained. That is, including Ag in the lead-free solder used in the solar cell has the effect of significantly delaying elution of Ag contained in the Ag paste electrode of the solar cell. On the other hand, even when Ag is not included in the solar cell electrode side solder, by including Ag in the lead-free solder used for the interconnector, the Ag paste electrode slightly dissolves in the interconnector soldering process. However, it can be reduced.

  In addition, when Bi is contained in at least one of lead-free solder used in solar cells and lead-free solder used in interconnectors, the lead wires of the solar cells and interconnectors are It is possible to connect without lowering the melting point of free solder. Here, the Bi content is preferably 3 to 89% by weight, and more preferably 35 to 60% by weight. Similarly, when at least one of the lead-free solder used for the solar cell or the lead-free solder used for the interconnector is Sn-Ag, the content of Ag is preferably 3.5 to 4.5% by weight. It is.

<Embodiment of the Invention>
(Embodiment 1)
As shown in FIG. 4, an n-type diffusion layer 2 is formed by thermal diffusion of P on one surface of a texture-etched P-type silicon substrate 1, and silicon nitride is formed thereon as an antireflection film 3 by plasma CVD. A film was formed. A commercially available aluminum paste was printed on the back surface by a screen printing method, dried at about 150 ° C., and then fired in air to form a back surface aluminum electrode 4.

  A silver paste firing, vapor deposition method, sputtering method, or plating method is applied to a P-type silicon substrate 1 provided with an n-type diffusion layer 2 and an antireflection film 3 on one surface and further provided with a back surface aluminum electrode 4 on the back surface. Thus, electrode formation can be performed.

  When forming an electrode by firing silver paste, it can be performed by the following procedure. That is, a silver paste having the composition shown in Table 1 is printed at a predetermined position on the back surface of the P-type silicon substrate 1 by a paste method to a thickness of about 30 microns and dried at 150 ° C. for about 4 minutes. Next, a silver paste is printed in a pattern on the light-receiving surface side of the P-type silicon substrate 1 and dried, and then baked in an oxidizing atmosphere for 2 minutes at a temperature of 600 ° C. to thereby form the silver electrodes 5 and 6 on the front and back surfaces. Can be formed.

  Moreover, when performing electrode formation by a vapor deposition method, it can carry out in the procedure shown below. That is, a predetermined pattern is extracted on the surface of the antireflection film by a resist method, the antireflection film is removed by etching with HF, dried, and then Ti, Pd, Ag in the order of about 70 ° C. Vapor deposition is performed at a thickness of 0.1, 0.1, and 1 μm. Thereafter, after removing the resist, a silver electrode can be formed by performing heat treatment in nitrogen at 350 ° C. In addition, also when forming a silver electrode by sputtering method, the procedure of electrode formation can be performed similarly to the procedure by a vapor deposition method.

  Moreover, when performing electrode formation by the plating method, it can carry out in the procedure shown below. That is, a predetermined pattern is extracted on the surface of the antireflection film by a resist method, and the antireflection film is etched away with HF. Then, after pre-plating treatment, Ni and Ag electroless plating are formed with thicknesses of 0.5 and 2.5 μm, respectively. Thereafter, the resist is peeled off, and a silver electrode can be formed by heat treatment in nitrogen at 150 ° C.

  The solar battery cell on which the silver electrode is formed can be immersed in a flux having the composition shown in Table 2, dried with hot air, and then immersed in a solder bath having the composition shown in Table 3 to form the solder layer 8. The solder can contain trace amounts of phosphorus, antimony, and gallium to improve wettability. Then, after rinsing with pure water or warm pure water, it was dried to complete a solar cell. In Table 3, Sn-Bi-Ag solder and Sn-Ag solder are described as lead-free solders, but any of them can cover the electrode.

  This solar cell was inspected for characteristics, and those having an FF value of 0.69 or less were determined to be defective. When the phenomenon that the silver paste sintered body is eaten by the solder bath, that is, the phenomenon that the silver in the silver paste sintered body is taken into the Sn bath and the electrode disappears in some places and cannot be made into a solar cell occurs. Since the FF value is greatly reduced, a defect can be easily determined. Table 4 shows the results of comparison with the conventional Sn-only bath. The electrodes were formed using paste forming methods, vapor deposition methods, sputtering methods, and plating methods. The number of tests was 5. In the plating composition according to the present invention, the electrode was coated with both Sn-Bi-Ag solder or Sn-Ag solder.

  When the solder composition was Sn-Bi-Ag type or Sn-Ag type, none was judged to be defective in the FF value regardless of the electrode forming method. On the other hand, when the solder composition was only Sn, the FF value was judged to be defective in all the test numbers 5 regardless of the electrode formation method.

  Therefore, by using Sn-Bi-Ag solder or Sn-Ag solder, which is a low temperature solder compared to Sn solder, any electrode formation among paste firing method, vapor deposition method, sputtering method and plating method can be performed. It can be understood that even with this method, a good solder coating can be applied to the metal electrode.

  In addition, in any electrode forming method among paste baking, vapor deposition, sputtering, and plating, copper or the like can be used as the upper metal instead of silver as the upper metal. In addition, in the case of a plating method or a vapor deposition method, the upper layer metal is generally a metal in the outermost layer at that time, although two to three kinds of metals are generally formed in order. Moreover, in the plating method, electrodes can be formed not only by electroless plating but also by electrolytic plating. Moreover, the solar cell according to the present invention only changes the solder material, has no complexity, and can apply all conventional solar cell forming steps as they are.

(Embodiment 2)
The silver paste material was compared with the lead-free solder material. As the silver paste, a silver paste containing silver powder, glass powder, an organic vehicle, an organic solvent, phosphorus oxide, and a halide was used. Specifically, the silver paste described in Table 1. On the other hand, as a comparative example, a commercially available silver paste (DuPont 8050S) was used. As the cell formation procedure, the same cell formation procedure as described in the first embodiment was used. As the electrode formation method, the same method as in the first embodiment was used. The evaluation method for the FF value is also as described in the first embodiment. The results are shown in Table 5. The number of tests was 5.

  As a solder material, when any of Sn-Bi-Ag solder or Sn-Ag solder is used and an electrode is formed using the silver paste shown in Table 1, the FF value is poor. There were no FF defects. On the other hand, when Sn solder was used, even if the electrodes were formed using the silver paste shown in Table 1, all of the test numbers 5 were FF failures. In addition, even when using any of Sn-Bi-Ag solder or Sn-Ag solder, when the electrode was formed using a commercially available silver paste, FF failure was observed in all of the five tests. It was. Moreover, when using Sn solder and forming an electrode using a commercially available silver paste, it was FF failure in all of the test number 5. From the results in Table 5, it can be seen that when an electrode is formed using a commercially available silver paste, the solder coating described in Table 3 is insufficient for solder coating.

(Embodiment 3)
In Embodiment 3, the flux materials were compared. The silver paste used for electrode formation was the one described in Table 1. The flux was compared using the flux described in Table 2 and a commercially available flux (Sanwa Chemical's SF-60). The commercially available flux (Sanwa Chemical's SF-60) contains a halogen compound.

  As the cell formation procedure, the same cell formation procedure as described in the first embodiment was used. As the electrode formation method, the same method as in the first embodiment was used. The evaluation method for the FF value is also as described in the first embodiment. The number of tests was 5. The number of FF defects when the FF value was defective was determined.

  On the other hand, an appearance inspection after a moisture resistance test (85 ° C., 85% RH, 500 h) was added as a simple reliability evaluation. This moisture resistance test is performed on a cell that has been solder coated and washed and dried by the above method. If the cleaning of corrosive substances such as halogen is insufficient, the solder or silver may be discolored, or It has been found that corrosion powder is generated and defects can be easily identified. As a result of the moisture resistance test, those determined to be defective were determined to be poor reliability. Table 6 shows the result of the number of FF defects and the result of the number of reliability defects.

  From the results shown in Table 6, it can be understood that when the flux shown in Table 2 is used, if the lead-free solder composition is used, there is no number of defects in electrode coating and there is no number of defects in reliability. On the other hand, when a flux containing a commercially available activator is used, it can be seen that the lead-free solder composition can be used for electrode coating, but most of the reliability is poor and impractical.

(Embodiment 4)
In Embodiment 4, the solderability and the reliability in the string state were evaluated by combining the solder composition covering the electrodes of the solar cell and the solder composition covering the lead wires on the interconnector side. Flux is not used for soldering. The results are shown in Table 7.

  As a result, it can be seen that, in any combination, wiring connection is possible when forming a string, and the reliability in the string state is also achieved.

  A solar cell according to the present invention as shown in FIG. 4 was produced by the following method. That is, an n-type diffusion layer 2 having a surface resistance of about 50Ω / □ is formed on one surface of a 125 mm square P-type silicon substrate 1 having a thickness of 330 μm by texture etching by P thermal diffusion at 900 ° C. A silicon nitride film having a thickness of about 60 nm was formed thereon as the antireflection film 3 by plasma CVD. A commercially available aluminum paste 4 was printed on the back surface by a screen printing method, dried at about 150 ° C., and then fired at 700 ° C. in air to form a back aluminum electrode 4.

  Subsequently, an electrode was formed by a silver paste firing method. That is, the silver paste 6 having the composition shown in Table 1 was printed at a predetermined position on the back surface with a thickness of about 30 microns by the same method and dried at 150 ° C. for about 4 minutes. Next, the silver paste 5 was printed on the light-receiving surface side in a pattern and dried, and then fired in an oxidizing atmosphere at 600 ° C. for 2 minutes to form silver electrodes on the front and back surfaces.

  The solar cell thus produced was immersed in a flux having the composition shown in Table 2 for 1 minute at room temperature, dried with hot air at 100 ° C. for 1 minute, and then immersed in a solder bath having the composition shown in Table 3 for 1 minute. Layer 8 was formed. The solder contains trace amounts of phosphorus, antimony and gallium to improve wettability. This was rinsed with pure water and warm pure water for a total of 5 minutes and then dried to complete the solar cell.

  In addition, the interconnector according to the present invention can be manufactured by a method in which a copper wire having a width of 2 mm and a thickness of 0.2 mm is immersed in a solder bath having a desired composition, and wound at a constant speed. is there.

  The string according to the present invention is manufactured as follows. That is, as shown in FIG. 3, the interconnector cut to a desired length is set so as to be in contact with the light receiving surface electrode of the solar cell, hot air of about 400 ° C. is blown together with the interconnector, and the solder is once melted. Then, the interconnector and the solar cell are integrated by cooling and solidifying. Thereafter, the string according to the present invention can be manufactured by performing the same process on the back surface side electrode of the solar cell by inverting the solar cell.

  It should be understood that the embodiments and examples disclosed this time are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is sectional drawing of the conventional solar cell. It is a figure explaining the manufacturing process of a solar cell. It is a figure explaining the conventional string. It is sectional drawing of the solar cell which concerns on this invention. It is a figure explaining the string which concerns on this invention.

Explanation of symbols

  1 P-type silicon substrate, 2 n-type diffusion layer, 3 antireflection film, 4 back aluminum electrode, 5,6 silver electrode, 7 solder layer, 8 solder layer, 10 solar cell, 1 16: 4 main coated with solder Electrode, 126: 4 solder coated interconnector, 21 Lead-free solder coated main electrode, 22 Lead-free solder coated interconnector.

Claims (16)

  A solar cell having an electrode coated with lead-free solder that does not contain lead.   The solar cell according to claim 1, wherein the electrode is an electrode formed by firing a paste.   The solar cell according to claim 1, wherein the electrode is an electrode formed by any one of a metal vapor deposition method, a sputtering method, and a plating method.   The solar cell according to claim 1, wherein the lead-free solder is Sn—Bi—Ag solder.   The solar cell according to claim 1, wherein the lead-free solder is Sn—Ag solder.   The solar cell according to claim 2, wherein the paste contains silver powder, glass powder, an organic vehicle, an organic solvent, phosphorus oxide, and a halide.   A solar cell having an electrode coated with a lead-free solder containing no lead after being washed with a flux comprising a resin, a solvent, and a resin stabilizer.   Solar cell interconnector coated with lead-free solder.   A string in which a solar cell having electrodes coated with lead-free solder that does not contain lead is wired with a solar cell interconnector that is coated with lead-free solder that does not contain lead.   The lead-free solder used for the solar cell and the lead-free solder used for the interconnector have the same composition.   The string according to claim 9, wherein Bi is contained in at least one of lead-free solder used for the solar cell and lead-free solder used for the interconnector.   The solar cell according to claim 4, wherein the Bi content is 3 to 89% by weight.   The string according to claim 11, wherein the Bi content is 3 to 89% by weight.   The string of Claim 9 which contains Ag in at least any one of the lead-free solder used for the said solar cell, or the lead-free solder used for the said interconnector.   The solar cell according to claim 5, wherein the content of Ag is 3.5 to 4.5% by weight.   The string according to claim 14, wherein the content of Ag is 3.5 to 4.5% by weight.

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