JP2009182154A - Solar cell and solar cell fabrication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell having a high conversion efficiency and being low cost by forming an electrode having a high aspect ratio with a more simpler method, and further to provide a method for fabricating such solar cell. <P>SOLUTION: The solar cell in which at least an electrode 10 having a double-layered structure is formed on a substrate W and a finger electrode 2 of the second layer of the electrode 10 is a copper wire and especially the copper wire is plated on the surface with a soldering material, and the method for fabricating such solar cell are provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a solar cell in which an electrode having a high aspect ratio is formed on a substrate and a method for manufacturing the solar cell.

  In a solar cell, a pn junction for receiving light is generally formed on a light receiving surface of a semiconductor substrate such as silicon, and a plurality of electrodes for extracting power are formed on the pn junction. Examples of the electrode include a comb-like finger electrode for directly taking out electric power from a semiconductor substrate, and a bus bar electrode for taking out electric power by connecting to the finger electrode.

In the production process of the solar cell, the electrode is generally formed by a screen printing method which has a great merit in reducing the production cost. For the electrode formation by this screen printing method, a conductive paste containing silver particles (90 wt% or more), glass frit, resin, solvent, etc. is usually used. And after electroconductive paste is screen-printed so that it may have an electrode pattern on the board | substrate for solar cells prepared, and it dries, an electrode is baked by performing high temperature heat processing at 700-900 degreeC.
In particular, finger electrodes for directly extracting power from the substrate are required to have a small area on the substrate and low resistivity so as not to block light, so that the line width is thin and thick, that is, the aspect ratio It is necessary to form a high electrode.

  However, in the screen printing method, an electrode having a wide line width and a low height (a low aspect ratio) such as a finger electrode having a line width of 60 μm and an electrode line height of 20 μm is formed. In the screen printing method, generally, the height is limited to half the line width of the electrode due to the characteristics of the conductive paste material and plate making, and the line height is limited to the line width of the electrode. It is difficult to realize a more ideal aspect ratio.

  Therefore, in Patent Document 1, an appropriate amount of conductive adhesive is attached in advance to a part of a copper wire (diameter 50 μm), which is a metal body, and the metal body to which the conductive adhesive is attached is directly bonded to the electrode forming surface on the substrate. Then, a method has been proposed in which the conductive adhesive is solidified by heating to fix the metal body on the substrate to form finger electrodes. Thus, it is described that the electrode width can be significantly narrower than the line width of the electrode obtained by the screen printing technique at that time, and the area occupied by the electrode on the substrate can be reduced.

  Further, in Patent Document 2, in order to provide a solar cell having a high aspect ratio electrode that improves the photoelectric conversion efficiency by improving the adhesion strength of a metal thin wire to the substrate, the substrate is coated with a conductive material around the thin wire. A method of forming an electrode by disposing the thin wire on top and firing the disposed thin wire is described.

  However, as in Patent Document 1 and Patent Document 2, even if a thin metal wire is used as an electrode, depending on the electrode structure, it is insufficient to obtain high characteristics.

Japanese Patent Laid-Open No. 3-6867 JP 2004-134656 A

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a solar cell with high conversion efficiency and low cost by forming an electrode having a high aspect ratio by a simpler method. Furthermore, it aims at providing the method of manufacturing such a solar cell.

  The present invention for solving the above-described problems is a solar cell in which at least a multilayer electrode is formed on a substrate, wherein the second layer of the electrode is a thin copper wire. A solar cell is provided (claim 1).

Further, the present invention is a method for producing a solar cell having a step of forming an electrode having a multilayer structure on at least a substrate,
In the electrode forming step, a thin copper wire is used as the second layer electrode, and a method for manufacturing a solar cell is provided (claim 4).

  In this way, in the solar cell, the electrode formed on the substrate is a multi-layered electrode, so that the height of the electrode can surely be higher than that of the single-layer electrode. Furthermore, since the second layer electrode is formed of a thin copper wire, the second layer electrode is less likely to protrude from the first layer electrode as compared with the case where the second layer electrode is formed by screen printing. A second layer of electrodes can be stacked. Therefore, an electrode having a higher aspect ratio and a smaller area occupied by wiring on the substrate can be obtained, and the power generation efficiency of the solar cell can be improved.

In addition, if the electrode has a multilayer structure formed only by the screen printing method, the screen printing is repeated a plurality of times, but the electrode forming process is simplified by using a thin metal wire as the second layer electrode. And the deterioration of consumption of the printing plate can be delayed.
Furthermore, the material cost and the manufacturing cost can be greatly reduced when the second layer electrode is a thin copper wire than when the second layer electrode is made of a conductive paste containing Ag.

In this case, it is preferable that the copper fine wire has a surface of the copper fine wire plated with a soldering material (Claim 2), and the surface of the copper fine wire is a solderable material as the copper fine wire. It is preferable to use a material plated with (Claim 5).
As described above, the copper thin wire as the second layer electrode is obtained by plating the surface with a soldering material, so that the solder for attaching the second layer electrode to the first layer electrode is used. The trouble of attaching can be simplified.

Furthermore, the shape of the cross section of the copper fine wire may be a circle or a square (Claim 3), and the copper thin wire having a cross-sectional shape of a circle or a square is used. (Claim 6).
As described above, the shape of the cross section of the copper thin wire as the second layer electrode is circular or square, so that the copper thin wire as the second layer electrode is arranged on the first layer electrode. In this case, since it can be arranged on the first layer electrode without worrying about the arrangement surface of the copper fine wire, the electrode forming process can be further simplified.

  In the present invention, in the electrode forming step, an electrode paste to be a first layer electrode is applied on the substrate in a desired pattern by screen printing, and the first layer electrode is baked, and the first layer electrode is fired. The copper thin wire to be the second layer electrode is placed on the electrode, and an iron at 200 ° C. to 300 ° C. is pressed from the copper thin wire and soldered to the first layer electrode, An electrode having the multilayer structure can be formed (claim 7).

  As described above, in the electrode forming step of the solar cell manufacturing method, first, the first layer electrode is formed by screen printing, so that the adhesion with the substrate as a base is improved and an ohmic electrode can be formed. it can. Subsequently, the second layer electrode is not formed by screen printing, but is placed so as to overlap the first layer electrode using a copper fine wire, and an iron of 200 ° C. to 300 ° C. is placed on the copper thin wire. By pressing and adhering to the first layer electrode by soldering, even if there is wiring scraping due to clogging of the screen printing plate when forming the first layer electrode, it is ensured by the thin copper wire An electrode without disconnection can be obtained. Furthermore, the process is simpler than forming both the first layer and the second layer of the electrode by screen printing.

  In the solar cell and the method for manufacturing the solar cell according to the present invention, the second layer is formed without increasing from the line width of the first layer even when the electrode having the multilayer structure is formed with the aim of improving the aspect ratio. An electrode is formed. In addition, the electrode formation process can be simplified, the cost can be reduced, and the electrode can have a high aspect ratio and a small area occupied by wiring on the substrate. Thereby, the power generation efficiency of the solar cell can be further improved.

  In addition, in a two-layer structure electrode using silver-containing electrode paste by screen printing, the aspect ratio was small, and on average, only an electrode having a width of 150 μm and a height of 50 μm or less could be obtained. In the method of manufacturing a solar cell according to the invention, the first layer electrode is sufficient to have a width of 50 μm and a height of 10 μm or less. There is no reason. In addition, a finger electrode having a height of 50 μm or more can be easily obtained by using a thin copper wire as the second layer electrode.

  Moreover, according to the method of the present invention, the shadow loss on the substrate can be reduced to about 1/3 as compared with a solar cell having an electrode formed by firing two layers of silver paste. That is, if a solar cell having an electrode having a multilayer structure is manufactured by the method of the present invention, the electrode width can be reduced from 150 μm to 50 μm, so that the electrode width can be reduced by about 100 μm. Therefore, for example, when 80 electrodes are formed on a square substrate of 156 mm square, a width of 80 (pieces) × 100 (μm) = 8 (mm) spreads as an effective area on the solar cell substrate, for example, the width is 150 μm. When the power generation efficiency of one solar cell having an electrode with a height of 50 μm is about 18%, the solar cell of the present invention has 8 (mm) / 156 (mm) × 18 (%) = 0.92 (% ) Can also improve power generation efficiency. This is a very big effect.

Furthermore, the solar cell of the present invention can further reduce the specific resistance of the electrode.
The silver paste has independent silver particles of micron order or larger flaky silver and glass frit in a solvent, and when these silver particles are fused at the time of firing, it becomes an electric conductor. Therefore, the specific resistance is about 1.5 times higher than that of Muku's silver wire. Although the specific resistance of silver is 1.62 μΩ · cm and copper is 1.72 μΩ · cm, which is about 6% higher, the volume effect of making a copper wire is large.

  As described above, the light-receiving surface electrode material of the solar cell is generally an electrode paste containing silver particles, glass frit, resin, solvent, etc. To do. Electrode formation by the screen printing method is usually performed in one layer, and an electrode having a wide line width and a thin width (a low aspect ratio, eg, a width of 60 μm and a height of 20 μm) is formed.

In addition to the formation of electrodes by screen printing, there is a method using fine metal wires.
However, when the fine metal wire is directly bonded to the substrate with solder or the like, there is a problem that the adhesion to the substrate is not so good and it is difficult to obtain an ohmic contact.

  Therefore, in order to form an electrode that has good adhesion to the underlying substrate and can aim at a high aspect ratio, the present inventor repeats the screen printing method a plurality of times using the same screen printing plate. The idea was to form an electrode having a structure, and an electrode having a multilayer structure was formed.

  However, when an electrode having a multilayer structure was actually manufactured only by screen printing, management of electrode paste containing Ag (storage temperature, humidity, solvent evaporation, firing temperature and time, etc.) and printing conditions (scratch) Plate deformation and wear due to pressure and speed of the plate), the thickness of the line does not become constant, and scumming occurs, making it difficult to obtain stable characteristics. In addition, for the electrode of the second layer electrode It was very difficult to completely overlay the paste on the fired first layer electrode. Furthermore, even if the second layer paste overlaps the first layer electrode, the second layer paste flows over the first layer electrode until the second layer electrode is fired. However, there has been a problem that it protrudes from the desired electrode pattern.

  As a result of further diligent research conducted by the inventor to solve such problems, an electrode having a high aspect ratio can be formed by a simpler method, and conversion efficiency can be increased and manufacturing cost can be reduced. In order to manufacture a solar cell, if the electrode of the second layer is not formed by screen printing using an Ag-containing electrode paste, but a copper multilayer wire is used to form a multilayer electrode. We came up with a good idea and completed the present invention.

Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited thereto.
First, the solar cell of this invention is demonstrated, referring FIG.1 and FIG.2.
FIG. 1 is a diagram for explaining a multilayer structure of an electrode of a solar cell according to the present invention, and FIG. 2 is an overall schematic diagram of the solar cell according to the present invention.

The solar cell 20 of the present invention is such that a finger electrode 10 having a multilayer structure is formed on a gallium-doped p-type single crystal silicon substrate W. In this finger electrode 10, as shown in FIG. 1, the first-layer finger electrode 1 is formed by screen printing with an Ag-containing electrode paste, and the second-layer finger electrode 2 is made of copper. The thin line is used.
The first layer electrode 1 and the second layer electrode 2 are bonded together by solder 2a.

  The substrate W has a two-stage emitter formed of a high-concentration diffusion layer 12b and a low-concentration diffusion layer 12a. A nitride film 11 serving as a surface protection film is deposited on the emitter layer. The film thickness is preferably 70 nm to 100 nm because it also serves as an antireflection film. Other antireflection films include oxide films, titanium dioxide films, zinc oxide films, tin oxide films, and the like, which can be substituted. On the back surface of the substrate W, there is a back electrode 13.

As described above, in the solar cell, the electrode formed on the substrate is an electrode having a multilayer structure, so that the height of the electrode is surely higher than that of the electrode having a single layer structure. Further, since the second layer electrode is formed of a thin copper wire, the aspect ratio is higher than that of the second layer electrode formed by screen printing, and the area occupied by the wiring on the substrate Therefore, the solar cell having such a multi-layered electrode has improved power generation efficiency compared to the conventional one.
In addition, if the second layer electrode is a thin copper wire, the material cost and the manufacturing cost are greatly reduced compared to the case where the second layer electrode is formed from an Ag-containing electrode paste. .

  The copper thin wire 2 used as the second layer electrode of the finger electrode 10 is preferably one in which a soldering material 2a is plated on the surface of the thin wire. As described above, the soldering material 2a is plated on the surface of the copper thin wire 2 in advance, so that the soldering process can be simplified when bonding to the electrode 1 of the first layer. The solar cell is reduced.

Furthermore, the shape of the cross section of the thin copper wire 2 can be circular or square.
In the present invention, the cross-sectional shape of the copper fine wire is not particularly limited. However, since it is circular or square, the copper thin wire is stably bonded onto the first layer electrode.

Next, the method for producing the solar cell of the present invention described above will be described below.
3 and 4 are diagrams for explaining a method for producing a solar cell having a multilayer electrode according to the present invention.

  First, a gallium-doped p-type single crystal silicon substrate W is prepared. This silicon single crystal substrate may be produced by any one of the Czochralski (CZ) method and the float zone (FZ) method. The specific resistance of the substrate is preferably, for example, 0.1 to 20 Ω · cm, and in particular, 0.5 to 2.0 Ω · cm is suitable for producing a high-performance solar cell.

  Next, the prepared substrate 1 is immersed in an aqueous sodium hydroxide solution, and the damaged layer is removed by etching. For removing damage from the substrate, a strong alkaline aqueous solution such as potassium hydroxide may be used. The same object can be achieved with an aqueous acid solution such as hydrofluoric acid.

A random texture is formed on the substrate W subjected to damage etching.
In general, a solar cell preferably has an uneven shape on the surface. The reason is that in order to reduce the reflectance in the visible light region, it is necessary to cause the light receiving surface to perform reflection at least twice as much as possible. The size of each of these peaks may be about 1 to 20 μm. Typical surface uneven structures include V-grooves and U-grooves. These can be formed using a grinding machine. In order to create a random concavo-convex structure, it is possible to use wet etching by dipping in an aqueous solution of sodium hydroxide and isopropyl alcohol, or to use acid etching or reactive ion etching. . In the figure, the texture structure formed on both sides is omitted because it is fine.

  Next, a two-stage emitter composed of the high concentration diffusion layer 12b and the low concentration diffusion layer 12a is formed. The high-concentration diffusion layer is printed and baked by a screen printer using a phosphorus paste for diffusion. The low-concentration diffusion layer can be formed by spin-coating a diffusing agent for spin coating containing diphosphorus pentoxide and silicon alkoxide and performing diffusion heat treatment. According to such a manufacturing method, the photoelectric conversion efficiency can be improved by suppressing the surface recombination of the light receiving surface other than the electrode and the recombination in the emitter while obtaining an ohmic contact.

  Next, junction separation is performed using a plasma etcher. In this process, the sample is stacked so that plasma and radicals do not enter the light-receiving surface and back surface, and in this state, the end surface is cut by several microns.

  Subsequently, after phosphorous glass formed on the surface is etched with hydrofluoric acid, a nitride film 11 as a surface protective film is deposited on the emitter layer using a direct plasma CVD apparatus. The film thickness is preferably 70 nm to 100 nm because it also serves as an antireflection film. Other antireflection films include oxide films, titanium dioxide films, zinc oxide films, tin oxide films, and the like, which can be substituted. In addition to the above, the formation method includes a remote plasma CVD method, a coating method, a vacuum deposition method, and the like. From the economical viewpoint, it is preferable to form the nitride film by the plasma CVD method.

  Furthermore, if a film having a refractive index between 1 and 2, such as a magnesium difluoride film, is formed on the antireflection film so that the total reflectance is minimized, the reflectance is further reduced. The current density is increased.

  Next, using a screen printing apparatus, a paste made of, for example, aluminum is applied to the back surface and dried. The application of the aluminum paste on the back surface is not limited to the screen printing method, and may be applied by other methods.

Subsequently, the electrode paste for the first layer 1 of the finger electrode 10 on the surface side is printed on the substrate W (see the upper diagram of FIG. 4) and dried.
The printing of the first layer 1 is performed by using an Ag-containing electrode paste in a screen printing apparatus having a printing plate having openings of a desired comb-shaped electrode pattern. At this time, a valley may be formed at the center of the first-layer electrode paste printed as shown in FIG. 5 in anticipation of placing the copper thin wire on the second layer. By forming a valley in the first layer electrode 1 in this way, when a copper thin wire having a circular cross-sectional shape is used, the adhesiveness with the first layer 1 is improved. It is possible to suppress peeling of the joint portion of the multilayer structure.
The electrode paste used for forming the first layer of the finger electrode can be managed in the same manner as when the finger electrode is formed of one layer.

Next, the back surface electrode 13 and the first layer 1 of the finger electrode are simultaneously fired with a predetermined thermal profile.
Subsequently, a thin copper wire 2 serving as a second layer electrode is disposed on the fired first layer electrode 1 (see the lower diagram of FIG. 4). As shown in the lower diagram of FIG. 4, one tip of the copper thin wire 2 is aligned with the position A so that the plurality of copper thin wires 2 overlap the first layer electrode 1. Then, the first-layer electrode 1 and the second-layer electrode 2 are bonded by pressing an iron of 200 ° C. to 300 ° C. from the copper thin wire 2.

  Solder wire may be inserted between the first layer electrode 1 and the copper thin wire 2 when the copper thin wire 2 is disposed, but the solder is more accurately placed on the first layer electrode 1. In order to adhere the second-layer electrode 2, it is preferable to use a material in which a soldering material 2 a is plated on the surface of the copper thin wire 2. Thereby, the labor of soldering can be simplified. Examples of the solder material 2a include tin (Sn) and silver (Ag).

  The copper thin wire 2 has a cross-sectional area having the same resistance value as that of the electrode paste, and in particular, a cross-sectional shape having a circular or square shape can be used. As a result, when the copper thin wire 2 is disposed on the first-layer electrode 1, it can be disposed on the first-layer electrode without worrying about the copper thin-wire arrangement surface, thereby simplifying the electrode formation process. Can be

  Next, a plurality of thin copper wires 2 soldered to the first layer 1 are cut at a position B as shown in the lower diagram of FIG. Thereby, the solar cell 20 as shown in FIG. 2 can be manufactured.

  In this way, by forming the first layer 1 of the finger electrode on the surface by screen printing using an electrode paste, the adhesion with the substrate W as a base is improved and an ohmic electrode can be formed. . In particular, in the case where a random texture is formed on the substrate W, the first layer formed with the electrode paste is more closely adhered to the substrate W than the metal fine wire is directly soldered.

  Further, the second layer 2 of the finger electrode on the surface is not formed by screen printing in the same manner as the first layer, but is formed by using the thin copper wire 2, so that the first layer electrode Even when wiring 1 is clogged due to clogging of the screen printing plate mesh, the second layer of copper fine wires can reliably provide an electrode without disconnection.

  Furthermore, if both the first layer and the second layer of the electrode are formed by screen printing using an Ag-containing electrode paste, the process is simple and the manufacturing cost is reduced to about 1/10 of the present invention. Can be reduced. In addition, in the two-layered finger electrode, the material cost can be reduced to about 1/10 by using a copper thin wire plated with a solder material for the second layer, rather than using an Ag-containing electrode paste for both layers. .

  In addition, by forming the second layer as a copper thin wire, it is easy to overlap the first layer electrode, and the second layer electrode is less likely to protrude from the desired finger electrode pattern. Therefore, in the solar cell manufacturing method of the present invention described above, an electrode having a high aspect ratio is formed by a simple method by using a finger electrode as a multilayer structure and using a thin copper wire as the second layer. It is possible to provide a solar cell with high conversion efficiency at low cost.

  Further, in the two-layer structure electrode formed of the silver-containing electrode paste, the aspect ratio was small, and on average, only an electrode having a width of 150 μm and a height of 50 μm or less could be obtained. In the battery manufacturing method, the first layer electrode is sufficient to have a width of 50 μm and a height of 10 μm or less, so even if the first layer electrode is formed by screen printing, there is no difficulty in the printing process. . In addition, an electrode having a height of 50 μm or more can be easily obtained by using a copper fine wire as the second layer electrode.

  In addition, according to the method of the present invention, the shadow loss on the substrate can be reduced to about 1/3 compared to a solar cell having electrodes formed of silver paste in both layers. That is, if a solar cell having an electrode having a multilayer structure is manufactured by the method of the present invention, the electrode width can be reduced from 150 μm to 50 μm, so that the electrode width can be reduced by about 100 μm. Therefore, for example, when 80 electrodes are formed on a square substrate of 156 mm square, a width of 80 (pieces) × 100 (μm) = 8 (mm) spreads as an effective area on the solar cell substrate, for example, the width is 150 μm. When the power generation efficiency of one solar cell having an electrode with a height of 50 μm is about 18%, the solar cell of the present invention has 8 (mm) / 156 (mm) × 18 (%) = 0.92 (% ) Can also improve power generation efficiency. This is a very big effect.

The solar cell of the present invention can further reduce the specific resistance of the electrode.
The silver paste has independent silver particles of micron order or larger flaky silver and glass frit in a solvent, and when these silver particles are fused at the time of firing, it becomes an electric conductor. Therefore, the specific resistance is about 1.5 times higher than that of Muku's silver wire. Although the specific resistance of silver is 1.62 μΩ · cm and copper is 1.72 μΩ · cm, which is about 6% higher, the volume effect of making a copper wire is large.

  In addition, the bus bar electrode (not shown) which takes out electric power by connecting with this finger electrode further to the finger electrode of the solar cell manufactured as mentioned above can be formed. An interconnector (not shown) can be further soldered onto the bus bar electrodes of a plurality of solar cells in order to modularize the solar cell on which the bus bar electrodes are formed.

  In addition, as an embodiment of the present invention, a method of forming a finger electrode having a multilayer structure on the light receiving surface side (front surface) of the substrate has been described, but the method of forming a multilayer structure electrode of the present invention is applied to an electrode on the back surface. It is also possible to do.

Examples of the present invention will be specifically described below, but the present invention is not limited thereto.
(Example)
<Production of solar cell substrate>
A solar cell is manufactured by the method shown in FIG. First, a boron-doped p-type as-cut silicon single crystal substrate grown by the CZ method with a side of about 156 mm and a thickness of 300 μm, a specific resistance of 0.5 Ω · cm, and an orientation of {100} is prepared. The damage layer was removed by etching with an aqueous potassium oxide solution. Then, it was immersed in a potassium hydroxide / 2-propanol mixed solution, textured on the surface of the silicon single crystal substrate, and then washed with a hydrochloric acid / hydrogen peroxide mixed solution.

  Next, this p-type silicon single crystal substrate was heat-treated in a phosphorus oxychloride atmosphere at 850 ° C. to form an emitter layer having a thickness of 0.4 μm. After forming the emitter layer, the phosphorous glass formed by diffusion was immersed in a hydrofluoric acid aqueous solution to remove it.

  Thereafter, a silicon nitride film was formed as an antireflection film using a remote plasma CVD method.

<Formation of electrode>
Electrodes are formed on the solar cell substrate W thus obtained. First, an aluminum paste was applied to the entire surface (back surface) opposite to the light receiving surface of the substrate W using a screen printing method and dried.
Next, an electrode paste was applied by screen printing with a linear electrode pattern as shown in the upper diagram of FIG. 4 and dried. As the first layer electrode paste, a mixture of silver particles, glass frit, resin and solvent was used.
Thereafter, the substrate W was heat-treated with a desired thermal profile. As a result, the back electrode made of aluminum having a thickness of 20 μm is baked, and at the same time, the first layer of the finger electrode having a height of about 15 μm in a pattern with a line width of about 50 μm and a distance between each line of about 2.5 mm Baked.

Next, about 80 to 100 thin copper wires 2 serving as the second layer electrodes were prepared.
The copper thin wire 2 has a thin film of tin (Sn) coated on its surface so that the thin wire itself has solderability. The cross-sectional shape of the copper thin wire is circular, and its diameter is about It is about 50 μm.

  Then, a copper thin wire is placed on the first layer electrode 1 baked by the screen printing method, and an iron of about 250 ° C. is pressed from the copper thin wire to the first layer electrode and the solder. The second layer electrode was fixed. Subsequently, the thin copper wire 2 was cut at a position B, and a solar cell was manufactured.

<Characteristics of manufactured solar cell>
The conversion efficiency etc. of the solar cell manufactured in this way were measured. The measurement results are shown in Table 1 below. As shown in Table 1, the aspect ratio (height / width) of the finger electrode having a two-layer structure manufactured according to the example was 65/50. That is, if a copper wire having a diameter of about 50 μm is soldered onto the first layer of the finger electrode having a width of 50 μm, the second layer finger electrode does not protrude from the first layer electrode, and thus has a two-layer structure. This finger electrode can maintain the width fired by screen printing in the first layer. Therefore, the area occupied by the finger electrodes per unit area can be about 2%, and the conversion efficiency is about 17.2%. Therefore, it can be seen that the shadow loss due to the electrodes can be reduced.

(Comparative example)
<Production of solar cell substrate>
From the preparation of the silicon single crystal substrate to the formation of the silicon nitride film, the substrate W was manufactured by the same method as in the example.

<Formation of electrode>
Electrodes are formed on the solar cell substrate W thus obtained. First, an aluminum paste was applied to the entire surface (back surface) opposite to the light receiving surface of the substrate W using a screen printing method and dried.
Next, an electrode paste was applied by screen printing with the same electrode pattern as in the example and dried. As the first layer electrode paste, a mixture of silver particles, glass frit, resin and solvent was used.
Thereafter, the substrate W was heat-treated with a desired thermal profile. As a result, an aluminum back electrode having a thickness of 20 μm is fired, and at the same time, a first electrode of a finger electrode having a line width of about 50 μm and a distance of each line of about 1.8 ± 0.2 mm and a height of about 20 μm. The first layer was fired.

Next, a second layer electrode was formed on the first layer electrode by screen printing.
The second layer electrode paste used here was the same as the first layer electrode paste.
Thereafter, the substrate W was dried, and the substrate W was heat-treated with a desired thermal profile. Thereby, the 2nd layer of the finger electrode was baked.

<Characteristics of manufactured solar cell>
The conversion efficiency etc. of the solar cell manufactured in this way were measured. The measurement results are shown in Table 1 below. As shown in Table 1, the conversion efficiency of the finger electrode having a two-layer structure manufactured according to the comparative example was about 16.5%. The aspect ratio (height / width) was 50/150. That is, if both layers are formed of electrode paste, the first layer finger electrode can be formed with a width of 50 μm. However, when the second layer electrode paste is printed, printing misalignment or paste It can be seen that the paste protrudes considerably from the first layer electrode due to the flow. Furthermore, the occupied area of the finger electrode per unit area was about 6%.

  From the results of Examples and Comparative Examples, as in the present invention, by using a copper fine wire as the second layer electrode on the first layer electrode, the multi-layer structure electrode protrudes from the desired pattern. It can be seen that an electrode having a high aspect ratio and a low resistivity can be obtained by a simpler method and a solar cell with high conversion efficiency can be manufactured.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration substantially the same as the technical idea described in the claims of the present invention and exhibits the same function and effect. It is included in the technical scope.

It is an enlarged view of the electrode part of the solar cell which concerns on this invention. It is a figure explaining embodiment of the solar cell which concerns on this invention. It is a figure explaining the method of forming the solar cell concerning this invention. It is a figure explaining the method of forming the electrode of the multilayer structure of the solar cell which concerns on this invention. It is an enlarged view of the electrode part of the solar cell which concerns on this invention.

Explanation of symbols

1 ... first layer finger electrode, 2 ... second layer finger electrode,
2a ... soldering material, 10 ... finger electrode of multi-layer structure,
11: Passivation film and antireflection film,
12a ... Low concentration diffusion layer, 12b ... High concentration diffusion layer, 13 ... Back electrode,
20 ... solar cell, A, B ... position, W ... (for solar cell) substrate.

Claims (7)

At least a solar cell in which an electrode having a multilayer structure is formed on a substrate,
The solar cell, wherein the second layer of the electrode is a thin copper wire.   2. The solar cell according to claim 1, wherein the copper thin wire is obtained by plating a surface of the copper thin wire with a soldering material.   The solar cell according to claim 1 or 2, wherein a shape of a cross section of the copper fine wire is a circle or a square. At least a method for manufacturing a solar cell having a step of forming an electrode having a multilayer structure on a substrate,
In the electrode forming step, a thin copper wire is used as the second layer electrode.   5. The method for manufacturing a solar cell according to claim 4, wherein the copper fine wire is a surface of the copper fine wire plated with a soldering material.   The method for manufacturing a solar cell according to claim 4 or 5, wherein the copper thin wire has a cross-sectional shape of a circle or a square.   In the electrode forming step, an electrode paste to be a first layer electrode is applied on the substrate in a desired pattern by screen printing, and the first layer electrode is baked, and the first layer electrode is baked on the first layer electrode. The copper thin wire to be the second layer electrode is disposed, and an iron of 200 ° C. to 300 ° C. is pressed from the copper thin wire to be soldered to the first layer electrode, and the multilayer structure is formed. An electrode is formed, The manufacturing method of the solar cell of any one of Claims 4 thru | or 6 characterized by the above-mentioned.

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