JP2010141050A - Lead wire for solar cell and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lead wire for a solar cell hard to generate the warp or damage of a silicon cell during joining the lead wire to the cell even when the solar cell is made into a thin board, and excellent in a bending fatigue property. <P>SOLUTION: The lead wire 13 for a solar cell, that is a conductor formed in a rectangular shape, or the conductor of the surface of which is partially or entirely coated with a solder-plated film, has a 0.2% strength value of 30-90 MPa in a peel test of the conductor. The grain diameter of the conductor is 6-20 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a solar cell lead wire and a manufacturing method thereof, and more particularly to a solar cell lead wire suitable for solder connection with a silicon cell of a solar cell and a manufacturing method thereof.

  In a solar cell 50 in which a silicon crystal is grown on a substrate, as shown in FIG. 3, usually, a connecting lead wire 53 is joined to a predetermined region of a silicon crystal wafer (silicon cell) 51, and power is supplied through this. It is configured to transmit.

  In the connection lead wire 53, a solder plating film for connection to a silicon cell is formed on the surface of a flat conductor. For example, as shown in FIG. 4, pure copper such as tough pitch copper or oxygen-free copper is used as the flat conductor 52, and Sn—Pb eutectic solder is used as the solder plating film 54 formed on the outside (see FIG. 4). For example, see Patent Document 1).

  In recent years, switching to solder containing no Pb (Pb-free solder) as a constituent material of the solder plating film 54 has been studied in consideration of the environment (for example, see Patent Document 2).

  By the way, since the silicon crystal wafer 51 occupies most of the material cost among the members constituting the solar cell 50, the thinning of the silicon crystal wafer 51 is being studied in order to reduce the manufacturing cost. However, when the silicon crystal wafer 51 is thinned, it is flat before the solder connection as shown in FIG. 5A due to the heating process at the time of soldering the connecting lead wire 53 and the temperature change at the time of using the solar cell. As shown in FIG. 5B, the silicon cell 51 and the connecting lead wire 53 that were connected via the solder plating 55 after the solder connection are warped or damaged. There was a risk of it. For this reason, even if the silicon cell 51 is thinned as the connection lead wire 53 to cope with this, the silicon cell 51 is hardly warped or damaged when joining the connection lead wire (low 0.2% proof stress). ) Switching to a solar cell lead wire has been studied (see, for example, Patent Document 3).

  When using a solar cell lead wire with a low 0.2% proof stress, the lead wire is more easily plastically deformed than when using a conventional lead wire, and the thermal stress during soldering can be reduced. The possibility that the connecting lead wire is warped or damaged is reduced.

Japanese Patent Laid-Open No. 11-21660 JP 2002-263880 A JP 2006-54355 A

  However, if the crystal grains are made too large in order to reduce the 0.2% proof stress of the lead wire, cracks are likely to occur due to deformation, and fatigue cracks can easily occur due to the expansion of the cracks and stress concentration. there were.

  That is, even after manufacturing as a solar cell module, mechanical stress is repeatedly applied to the module after installation, so that the portions where the solder plating wires are bent in an L shape between silicon cells, that is, silicon cells are connected to each other There was a case where breakage occurred at the part to be done.

  Accordingly, an object of the present invention is to provide a solar cell lead wire that is less likely to warp or break a silicon cell when the lead wire is joined even when the solar cell is thinned, and has excellent bending fatigue characteristics.

  Another object of the present invention is to provide a solar cell lead wire having good electrical conductivity.

  Furthermore, the other object of this invention is to provide the manufacturing method of the lead wire for solar cells which can suppress the raise of manufacturing cost.

  In order to achieve the above object, the present invention relates to a conductor formed in a rectangular shape or a solar cell lead wire in which a part or all of the surface of the conductor is coated with solder plating. Provided is a solar cell lead wire having a 0.2% proof stress of 30 MPa or more and 90 MPa or less and a crystal grain size of the conductor of 6 μm or more and 20 μm or less.

  Preferably, the volume resistivity of the conductor is 50 μΩ · mm or less. Preferably, the solder plating contains 0.1 wt% or more of Sn-based solder or at least one element selected from Pb, In, Bi, Sb, Ag, Zn, Ni, and Cu as the second component. Sn alloy solder. Preferably, the conductor is one selected from the group consisting of Al, Ag, and Au. Preferably, the conductor is one selected from the group consisting of tough pitch Cu, oxygen-free Cu, phosphorus deoxidized Cu, and high-purity Cu (99.9999% or more).

  In the present invention, the conductor is formed into a rectangular cross section by rolling or slitting, and the conductor formed in the rectangular cross section is subjected to heat treatment so that a 0.2% proof stress value in the tensile test of the conductor is 30 MPa or more. , 90 MPa or less, the crystal grain size of the conductor is 6 μm or more and 20 μm or less, and a method for manufacturing a solar cell lead wire in which a part or all of the surface of the conductor is coated with solder plating is provided.

  Preferably, the conductor has a volume resistivity of 50 μΩ · mm or less. Preferably, the heat treatment conditions are a temperature of 450 to 1000 ° C. and a time of 5 seconds to 60 seconds. Preferably, the conductor is one selected from the group consisting of Cu, Al, Ag, and Au.

  According to the present invention, the following excellent effects are exhibited.

  (1) Even when the solar cell is thinned, it is possible to obtain a solar cell lead wire that is less likely to warp or break the solar cell when the connection lead wire is joined and that has excellent bending fatigue characteristics.

  (2) It can be set as the lead wire for solar cells with favorable electrical conductivity.

  (3) It is possible to provide a method for manufacturing a solar cell lead wire that does not cause adhesion between materials during heat treatment and can suppress an increase in manufacturing cost.

  A preferred embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

  First, the solar cell lead wire of the present invention is one in which a part of or the entire surface of the solar cell conductor is coated with a solder plating film.

  1 and 2 show a solar cell 10 using the solar cell lead wire 13 of the present invention, and in particular, a solar cell 10 in which a plurality of silicon cells 11 are connected by the solar cell lead wire 13 to form a module. 1 is a plan view, and FIG. 2 is a front view.

  1 and 2, the solar cell lead wire 13 for connecting the silicon cell 11 is not shown in detail, but as described in the conventional example of FIG. 4, the surface of the solar cell conductor made of a rectangular conductor. A solder plating film is formed on a part or all of the film.

  This solar cell lead wire 13 is connected to the silicon cell 11 via solder plating (not shown). At this time, in order to connect the silicon cells 11 in series to form a module, one solar cell lead wire 13a is formed to have the same length as the width of the silicon cells 11 and 11 arranged side by side. The solar cell lead wire 13 a is connected to the surface of one silicon cell 11, and the middle is bent in an L shape between the silicon cells 11, 11 and connected to the back surface of the other silicon cell 11. The other solar cell lead wires 13b are formed to be slightly longer than the width of the silicon cell 11, and are connected to the surface facing the one solar cell lead wire 13a.

  In the present invention, the solar cell conductor used for the solar cell lead wire 13 is a conductor formed in a rectangular shape, and has a 0.2% proof stress value in a tensile test of 30 MPa or more and 90 MPa or less, and the conductor. The crystal grain size is 6 μm or more and 20 μm or less, and is formed by covering a part or all of the surface of the conductor with solder plating.

  The solar cell conductor and solder plating film will be described below.

(Solar cell conductor)
The conductor for a solar cell (hereinafter referred to as a conductor) has a soft conductor outer shape having a rectangular cross section so that the solder connection to the silicon cell surface of the solar cell module (silicon crystal wafer) is facilitated. .

(Volume resistivity of conductor)
From the viewpoint of reducing the power generation loss of the solar cell, it is preferable to use a conductor material having a relatively low volume resistivity, for example, a material having a volume resistivity of 50 μΩ · mm or less.

  As shown in Table 1, there are Au, Ag, Al, etc. in addition to Cu as conductive materials having a relatively small volume resistivity. Among these, Ag has the lowest volume resistivity and can maximize power generation efficiency. On the other hand, when priority is given to cost reduction, Cu is good, and when weight reduction is desired, it is desirable to select Al.

  In particular, as the kind of Cu, any of tough pitch Cu, oxygen-free Cu, phosphorus deoxidized Cu, and high purity Cu (purity 99.9999% or more) can be used. In order to minimize the 0.2% yield strength in the conductor tensile test, Cu having a high purity is advantageous, that is, high-purity Cu (purity 99.9999% or more) is selected. On the other hand, when impurities are added and the 0.2% proof stress is increased but cost reduction is desired, tough pitch Cu or phosphorus deoxidized Cu is selected.

(0.2% proof stress value of conductor)
In general, when dissimilar metals having different thermal expansion coefficients are connected at a high temperature, a value obtained by integrating the thermal expansion coefficient and Young's modulus with the temperature change is a force for generating warpage. However, the rigidity of both members to be connected like a solar cell is remarkably different, and when the solder connection temperature is as high as 200 ° C. or higher, the conductor having a smaller cross-sectional area yields, and the thermal expansion coefficient, The force due to the Young's modulus is not a warp generation force.

  In the case of a conductor, if the yield stress is small, it is plastically deformed with a small force, and no further deformation resistance is obtained. That is, the lower the strength and the lower the proof stress, the less the load on the silicon crystal wafer during bonding. For this reason, the 0.2% yield strength value in the tensile test is used as an index of plastic deformation, and the 0.2% yield strength of the conductor is set to 90 MPa or less. By selecting such a soft conductor having a low 0.2% proof stress value, it is possible to reduce the thermal stress (force to warp the silicon cell) when the conductor is bonded to the silicon crystal wafer. However, since a conductor with a 0.2% yield strength that is too low is inferior in fatigue characteristics, the 0.2% yield strength of the conductor is preferably 30 MPa or more.

(Crystal grain size of conductor)
The 0.2% proof stress is known to have a large correlation with the crystal grain size of the conductor. On the other hand, the smaller the grain boundary, that is, the larger the crystal grain size, the smaller the deformation resistance. On the other hand, if the particle size becomes too large, the fatigue properties of the material will be reduced, resulting in a brittle material. Therefore, the crystal grain size and the 0.2% proof stress of the conductor need to be within a certain range of conditions. The crystal grain size here is an average of the crystal grain size of the conductor.

  When the crystal grain size of the conductor is 20 μm or more, the conductor becomes brittle and crack resistance is lowered. For this reason, if the conductor has a crystal grain size of 20 μm or more, after being installed in the solar cell module, cracks may occur at the portions where the lead wires for solar cells, particularly the flat wires between cells, are bent into an L shape, and long-term reliability is not good. This may cause problems such as becoming sufficient. If the crystal grain size is 6 μm or less, crack resistance is not a problem, but the softness of the conductor is lost, and the warpage of the silicon cell increases. Accordingly, the crystal grain size of the conductor is preferably in the range of 6 to 20 μm.

(0.2% proof stress value of suitable conductor and crystal grain size)
From the above, the 0.2% proof stress of the conductor is preferably 30 MPa or more and 90 MPa or less, and the crystal grain size of the conductor is preferably 6 to 20 μm. If it is this range, since the curvature of a silicon cell can be reduced significantly compared with the conventional system, and also the brittleness by crystal grain coarsening can be prevented, a crack and the fall of long-term reliability can be prevented.

(Method for producing flat conductor for solar cell)
Either a rolling process or a slit process can be applied as the conductor processing method. The method of rolling from a round wire and flattening can produce a long and uniform product. The slit method has the merit that it can cope with materials of various widths.

  For example, a suitable flat conductor for a solar cell is formed by forming a conductor into a rectangular shape by die drawing or roll rolling, or a composite process thereof, and then heat-treating with a current-carrying type or batch type equipment or a continuous type equipment. It can be obtained by reducing the 2% proof stress. Here, as a heat treatment method for reducing the 0.2% proof stress, a batch type heating method or a continuous type heating method using a heater capable of giving sufficient heat energy is preferable to the current heating method. Alternatively, from the viewpoint of preventing oxidation, a furnace having an inert gas atmosphere such as nitrogen or a hydrogen reducing atmosphere may be used.

  In the case of heat treatment, it is necessary to define time and temperature in order to obtain a desired crystal grain size.

  At high temperatures, the heat treatment is completed in a relatively short time. Although it takes a long time at low temperatures, an inexpensive equipment can be used.

  When the conductor is Cu, the heating time is most appropriate when the temperature is 450 to 1000 ° C., and the crystal grain size of 6 to 20 μm can be obtained.

  The heat treatment conditions are not limited to the above case, and the crystal grain size can be controlled by a combination of temperature and time. For example, when the temperature is 200 to 450 ° C., the heating time is appropriately about 1 to 60 minutes, and a crystal grain size of 6 to 20 μm can be obtained.

  For Ag, Au, and Al other than Cu as the conductor, the crystal grain size can be controlled under the heat treatment conditions as in the case of Cu.

(Solar cell lead wire)
The lead wire for solar cell of the present invention is obtained by applying a solder plating film to the entire surface of a conductor. From the environmental point of view, the solder plating is preferably a lead-free product, and is performed on part or all of the outer periphery of the conductor surface.

  As for the solder composition, what used Cu as the conductor so far has been required to be capable of low-temperature connection in consideration of thermal expansion matching with the silicon cell, but by using the conductor of the present invention, Since the warpage of the silicon cell is small, it is possible to use a solder having a Sn—Ag—Cu composition having a high connection temperature.

  The solder used for covering the conductor is Sn-based solder or Sn-based containing 0.1 wt% or more of at least one element selected from Pb, In, Bi, Sb, Ag, Zn, Ni and Cu as the second component. Although it is alloy solder, you may use what contains 1000 ppm or less of trace elements as a 3rd component.

  The solar cell assembly is obtained by connecting the solar cell lead wire to a predetermined contact region (for example, an Ag electrode region) on the silicon cell surface of the silicon crystal wafer (solar cell module).

(Effect of solar cell lead wire)
As described above, even if the solar cell lead wire of the present invention is solder-connected to the silicon cell of the solar cell, there is less warpage of the silicon cell than the conventional method, and embrittlement due to coarsening of the crystal grains can be prevented, It also has excellent electrical characteristics.

  That is, since the 0.2% proof stress value of the conductor is 30 MPa or more and 90 MPa or less, the thermal stress generated in the silicon cell can be reduced by the thermal contraction of the conductor after the solder connection. For this reason, the curvature of a silicon cell can be reduced in the case of the thermal contraction after a solder connection with a silicon cell.

  Moreover, since the maximum value of the crystal grain size is regulated by using a conductor whose crystal grain size falls within the range of 6 to 20 μm, the conductor can be prevented from being embrittled, and the minimum value of the crystal grain size can be prevented. Therefore, warpage of the silicon cell can be prevented from the viewpoint of crystal grain size.

  Further, if the crystal grain size of Cu is 20 μm or more, it may cause problems such as cracks in the flat wire or insufficient long-term reliability during the molding process to be incorporated into a solar cell panel or the like. Since the crystal grain size of Cu is 6 to 20 μm, it is possible to sufficiently obtain suitable fatigue characteristics of the solar cell solder-plated wire, and it is excellent in molding processing and long-term reliability.

  Moreover, since the lead wire for solar cells of the present invention uses a highly conductive conductor having a volume resistivity of 50 μΩ · mm or less, the power generation efficiency as a solar cell can be favorably maintained.

  Furthermore, according to the manufacturing method of the present invention, the 0.2% proof stress value of the conductor is set to a predetermined value or less depending on the heat treatment conditions, and the crystal grain size of the conductor is within a predetermined range. The solar cell lead can be provided by a simple method without cost.

(Examples 1-14 and Comparative Examples 1-4)
A Cu material (OFC: oxygen-free copper, TPC: tough pitch copper) having a width of 1.5 mm and a thickness of 0.15 mm is rolled into a rectangular wire to form a conductor, and heat treatment conditions are 1010 ° C./30 seconds (Comparative Example 1). 1000 ° C./62 seconds (Comparative Example 2), 1000 ° C./60 seconds (Example 1),... 450 ° C./5 seconds (Example 14), and 450 ° C./4 seconds (Comparative Example 3), 500 The heat treatment was carried out at a temperature changed from ℃ / 30 seconds (Comparative Example 4).

  After the heat treatment, the periphery of the conductor is covered with Sn-3% Ag-0.5% Cu-based lead-free solder to provide a solder plating film, and a solder-coated Cu rectangular wire is formed. The 0.2% proof stress of Cu and solder coating Cu was examined. The results are shown in Table 2.

  The crystal grain size here is the average of the crystal grain diameters in a 120 μm square section of the solder-plated rectangular wire. Specifically, based on the cutting method shown in JIS H0501, the number of crystal grains completely cut by six 120 μm-long line segments (vertical lines × 3, horizontal lines × 3) on the micrograph is counted. The average value of the cutting length is obtained.

  Further, the 0.2% proof stress σ of the solder coated product Cu is obtained by dividing the cross-sectional area A of the conductor excluding the solder by the load (external force) F in a tensile test that gives 0.2% strain to the conductor. . This can be expressed by the following formula.

σ = F / A
When the heat treatment conditions were changed to 1000 ° C./60 seconds (Example 1),... 450 ° C./5 seconds (Example 14), the crystal grain size of the rectangular Cu was 20 to 6 μm. Further, the 0.2% proof stress of flat Cu was 30 to 90 MPa, and all were 90 MPa or less.

  In contrast, the heat treatment conditions were 1010 ° C./30 seconds (Comparative Example 1), 1000 ° C./62 seconds (Comparative Example 2; conductor is TPC), 450 ° C./4 seconds (Comparative Example 3), 500 ° C./30 seconds. (Comparative Example 4; conductor is TPC) When the treatment was performed, the crystal grain size of the rectangular Cu was 21.8 μm, 21.3 μm, 5.8 μm, and 5.6 μm, respectively. The 0.2% yield strength of Cu was 28 MPa, 29 MPa, 91 MPa, and 92 MPa, respectively.

  The above-described solder-coated Cu rectangular wire was solder-connected to a silicon cell having a length of 155 mm × width of 155 mm and a thickness of 180 μm, and the crack resistance and warpage of the silicon cell were examined. For crack resistance, a small module is made with strings in which 10 silicon cells are connected by a flat wire, both ends of the small module are fixed, and the center is mechanically bent up and down 2cm from the horizontal position. Evaluation was made based on the number of cycles (number of bendings) until the wire broke. The results are also shown in Table 2.

  In Table 2, the number of bendings shows an average value of n = 5. In addition, the evaluation marks O, Δ, and X in the warp column of the silicon cell mean good (<3 mm), slightly good (3 mm), and poor (> 3 mm), respectively.

  In Examples 14 to 1, the 0.2% proof stress of Cu decreases as the crystal grain size increases. When the crystal grain size at which the 0.2% proof stress of Cu is 90 MPa is 6.1 μm (Example 14), the warpage of the silicon cell tends to decrease, so the Cu 0.2, which was slightly good (Δ), was 0.2. When the crystal grain size at which the% proof stress is 89 MPa is 6.2 μm (Example 13), the warpage of the silicon cell can be reduced and it is good (◯). When the crystal grain size of Cu was 20 μm (Example 1), the crack resistance, which was good up to 19.8 μm (Example 2), was slightly reduced.

  When the crystal grain size of Cu was 21.3 μm (Comparative Example 2), the crack resistance was poor. Further, when the crystal grain size in which the 0.2% proof stress of Cu was 91 MPa was 5.8 μm (Comparative Example 3), the warpage of the silicon cell was large and it was defective.

  From the above, the 0.2% proof stress of the conductor is preferably 30 MPa or more and 90 MPa or less, and the crystal grain size of the conductor is preferably in the range of 6 to 20 μm.

It is a top view of the solar cell made into the module using the lead wire for solar cells of this invention. It is a front view of FIG. It is a perspective view which shows the connection state of the lead wire for a connection to a general photovoltaic cell. It is sectional drawing which shows the solder plating wire for solar cells of a prior art example. It is explanatory drawing which shows the curvature which generate | occur | produces when soldering the silicon cell and connecting lead wire of a prior art example.

Explanation of symbols

10 solar cell 11 silicon cell 13 solar cell lead wire

Claims (10)

  In a solar cell lead wire in which a conductor formed in a rectangular shape or a part or all of the surface of the conductor is coated with solder plating, a 0.2% proof stress value in a tensile test of the conductor is 30 MPa or more and 90 MPa or less. And the conductor has a crystal grain size of not less than 6 μm and not more than 20 μm.   The lead wire for a solar cell according to claim 1, wherein the volume resistivity of the conductor is 50 μΩ · mm or less.   The solder plating is Sn-based solder or Sn-based alloy solder containing 0.1 mass% or more of at least one element selected from Pb, In, Bi, Sb, Ag, Zn, Ni, and Cu as the second component. The solar cell lead wire according to claim 1 or 2.   The solar cell lead according to any one of claims 1 to 3, wherein the conductor is one type selected from the group consisting of Al, Ag, and Au.   The sun according to any one of claims 1 to 3, wherein the conductor is one selected from the group consisting of tough pitch Cu, oxygen-free Cu, phosphorus deoxidized Cu, and high-purity Cu (99.9999% or more). Battery lead wire.   The lead wire for solar cell according to claim 5, wherein the conductor has an oxygen content of 25 ppm to 450 ppm. The conductor is formed into a flat cross section by rolling or slitting,
Heat treatment is performed on the conductor formed in the rectangular cross section so that the 0.2% proof stress value in the tensile test of the conductor is 30 MPa or more and 90 MPa or less, and the crystal grain size of the conductor is 6 μm or more and 20 μm or less. Furthermore, a method for producing a lead wire for a solar cell, wherein a part or all of the surface of the conductor is coated with solder plating.   The method for manufacturing a lead wire for a solar cell according to claim 7, wherein the conductor has a volume resistivity of 50 μΩ · mm or less.   The method for manufacturing a solar cell lead according to claim 7 or 8, wherein the heat treatment conditions are a temperature of 450 to 1000 ° C and a time of 5 seconds to 60 seconds.   The method for producing a solar cell lead according to any one of claims 7 to 9, wherein the conductor is one selected from the group consisting of Cu, Al, Ag, and Au.

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