JP5353282B2 - Solar cell - Google Patents

Description

  The present invention relates to an inexpensive and highly efficient solar cell.

  An overview of a solar cell for mass production using a single crystal or polycrystalline silicon substrate is shown in FIG. As the current collecting electrode on the light receiving surface, there are a large number of electrodes of several hundreds to several tens of μm called finger electrodes (101), and the bus bar electrode (102) is used as a current collecting electrode for connecting solar cells. It is common to have several. As a method for forming this electrode, an evaporation method, a sputtering method, or the like may be used. From a cost standpoint, a metal paste in which metal fine particles such as silver are mixed with an organic binder is printed using a screen plate or the like. A method of performing heat treatment at several hundred degrees and bonding to a substrate is widely used.

  In order to increase the photoelectric conversion efficiency of the solar cell, it is necessary to reduce the internal series resistance of the battery as much as possible, and the electrode is required to have high electrical conductivity. For this purpose, it is effective to use copper or silver having high electrical conductivity. Since copper is very cheap compared to silver, it is extremely effective in reducing the cost. However, copper is hardly used for the following reasons.

  Since solar cells have a small amount of power generation per cell, they are usually used by connecting several. For connection, it is common to solder a connection wiring to a bus bar electrode from the viewpoint of manufacturing cost, workability, and the like. Since copper has poor solder wettability compared to silver, there is a risk that reliability during cell connection may be reduced.

  Japanese Laid-Open Patent Publication No. 7-90204 (Patent Document 1) proposes adding a silver-copper alloy, and Japanese Laid-Open Patent Publication No. 2001-15782 (Patent Document 2) proposes adding copper fine particles to a silver paste. This leads to high costs.

  On the other hand, the method disclosed in Japanese Patent Application Laid-Open No. 2008-42095 (Patent Document 3) is a technique in which paste printing is laminated with the same apparatus and the same plate making, and the first electrode layer and the wiring directly bonded to the semiconductor substrate The outermost electrode layer to be joined is formed of silver, but further cost reduction is desired.

JP-A-7-90204 JP 2001-15782 A JP 2008-42095 A

  The present invention has been made in response to the above-described demand, and an object of the present invention is to provide a solar cell that can greatly reduce the manufacturing cost.

As a result of intensive studies to achieve the above object, the present inventors have found that a semiconductor substrate in which a PN junction is formed, a finger electrode formed in a comb shape on at least one surface of the semiconductor substrate, In a solar cell comprising a bus bar electrode connected to a finger electrode, the finger electrode is formed of a conductive paste containing copper fine particles, and in this case, in particular, the first electrode layer directly bonded to the substrate is a copper paste. It is found that if the outermost layer electrode that is formed in the above and is bonded to the connecting wiring is formed of silver paste, a solar cell that can be soldered without degrading the performance can be manufactured, and further cost reduction can be achieved. This has led to the invention.

Accordingly, the present invention is a solar cell comprising a semiconductor substrate on which a PN junction is formed, a finger electrode formed in a comb shape on at least one surface of the semiconductor substrate, and a bus bar electrode connected to the finger electrode. The bus bar electrode is printed and fired with a conductive paste containing only 30 to 90% by mass of copper fine particles as a conductive material, and is directly bonded to the semiconductor substrate, silver, nickel palladium, is formed by a conductive paste containing platinum or gold is fired is printed, have 2 So構 granulation comprising the outermost layer of the electrode layer to be bonded to the wiring, the finger electrode Provided is a solar cell comprising a conductive paste containing the same copper fine particles as the first electrode layer of a bus bar electrode and printed and fired. By using a copper-containing paste as the electrode paste, the cost can be reduced while maintaining the internal resistance as compared with the silver paste.

Further, the higher the content of the copper fine particles in the paste, the lower the cost and the better. However, from the viewpoint of printability, the content is 30 to 90% by mass.

In the case of forming the finger electrodes by multi-layered electrodes, when forming the fingers and the bus bar in either printed at the same time, it improves productivity, leading to cost reduction.

Further, the bus bar electrode, the first electrode layer in contact with the substrate formed with copper paste, to form a top surface of the electrode layer to be bonded to the wiring by using a paste containing copper other than substances such as silver Runode The adhesive strength with the wiring is maintained without impairing the performance of the solar cell.

  According to the present invention, it is possible to significantly reduce the manufacturing cost of a solar cell without impairing the performance of the solar cell and while maintaining the reliability of soldering with the wiring. In addition, since the present invention only changes the paste and does not require a change in equipment, it is extremely easy to implement.

1 is a layout view of electrodes of a general solar cell according to the present invention. It is sectional drawing which shows an example of the finger electrode based on this invention. It is sectional drawing which shows an example of the bus-bar electrode based on this invention. It is sectional drawing which shows the other example of the finger electrode based on this invention. It is sectional drawing which shows another example of the finger electrode based on this invention. It is sectional drawing which shows the other example of the bus-bar electrode based on this invention.

A solar cell according to the present invention includes a semiconductor substrate on which a PN junction is formed, a finger electrode formed in a comb shape on at least one surface of the semiconductor substrate, and a bus bar electrode connected to the finger electrode. In this case, at least the finger electrode is formed by printing and baking a conductive paste containing copper fine particles.

In this case, the bus bar electrode can also be formed of a conductive paste containing fine particles of copper or a copper compound, and at least the bus bar electrode and, in some cases, the finger electrode are formed to have a multilayer structure of two or more layers. At this time, is made of a conductive paste containing at least only the first electrode layer of the first electrode layer及beauty off Inga electrode bus bar electrode to be joined directly to the semiconductor substrate is copper as the conductive material particles, in this case The outermost electrode layer bonded to the wiring is formed using a paste containing a substance such as silver. Further, the finger electrode is formed by using the same paste as the first electrode layer of the bus bar electrode.

Here, as the conductive paste containing copper or copper compound fine particles, the copper or copper compound fine particle content is 30 to 90% by mass in terms of copper , and particularly preferably 60 to 80% by mass. . Examples of the copper compound include alloys composed of one or more of transition metals such as gold, silver, zinc, tin, and nickel. The average particle diameter of the fine particles is 0 by a measuring method using a scanning electron microscope. 15 to 15 μm, particularly 1 to 5 μm is preferable.

  As other components of the conductive paste, known components are used. For example, binders such as ethyl cellulose and terpionol are used in a ratio of 1 to 20% by mass, particularly 2 to 15% by mass, In order to ensure adhesion, glass powder or the like can be used at a ratio of 1 to 20% by mass.

  In addition, as the paste containing a substance such as silver, a conductive material other than copper or a copper compound such as silver, titanium, nickel, palladium, platinum, or gold can be used. It is the same as the sex paste.

  Hereinafter, an example of a method for manufacturing the solar cell of the present invention will be described. However, the present invention is not limited to the solar cell manufactured by this method.

  Slice damage on the surface of an as-cut single crystal {100} p-type silicon substrate doped with a high purity silicon group III element such as boron or gallium and having a specific resistance of 0.1 to 5 Ω · cm, concentration of 5 to 60% by mass Etching is performed using a high concentration alkali such as sodium hydroxide or potassium hydroxide, or a mixed acid of hydrofluoric acid and nitric acid. The single crystal silicon substrate may be manufactured by either the CZ method or the FZ method.

  Subsequently, minute unevenness called texture is formed on the substrate surface. Texture is an effective way to reduce solar cell reflectivity. The texture should be immersed for about 10 to 30 minutes in an alkali solution (concentration 1 to 10% by mass, temperature 60 to 100 ° C.) such as heated sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, and sodium bicarbonate. Easy to make. In many cases, a predetermined amount of 2-propanol is dissolved in the solution to promote the reaction.

  After texture formation, washing is performed in an acidic aqueous solution of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, or the like, or a mixture thereof. From an economic and efficient standpoint, washing in hydrochloric acid is preferred. In order to improve the cleanliness, 0.5 to 5% hydrogen peroxide may be mixed in a hydrochloric acid solution and heated to 60 to 90 ° C. for washing.

On this substrate, an emitter layer is formed by vapor phase diffusion using phosphorus oxychloride. Common silicon solar cell, it is necessary to form only on the light receiving surface of the PN junction, or diffused in a state superimposed two substrates to each other in order to achieve this, SiO ₂ film Ya on the back surface before spreading An SiN _x film or the like is formed as a diffusion mask, and it is necessary to devise such that a PN junction cannot be formed on the back surface. After diffusion, the glass formed on the surface is removed with hydrofluoric acid or the like. The silicon nitride film (SiN _x film) has a composition of Si ₃ N ₄ in terms of the stoichiometric ratio, but usually has a stoichiometric ratio so as to improve the masking property and become effective as an antireflection film as will be described later. More Si is used in many cases, and in the present invention, it is described as SiN _x for convenience.

Next, an antireflection film is formed on the light receiving surface. A SiN _x film is formed to a thickness of about 100 nm using a plasma CVD apparatus or the like. As the reaction gas, monosilane (SiH ₄ ) and ammonia (NH ₃ ) are often mixed and used, but nitrogen can be used instead of NH ₃ , and the process pressure can be adjusted and the reaction gas diluted. Furthermore, when polycrystalline silicon is used for the substrate, hydrogen may be mixed into the reaction gas in order to promote the bulk passivation effect of the substrate. As a method of exciting the reactive gas in CVD, thermal CVD, photo CVD, or the like may be used in addition to the above-described plasma.

  Next, the back electrode and the light receiving electrode are formed by screen printing. A paste obtained by mixing Al powder with an organic binder is screen-printed on the back surface of the substrate. After printing, the back electrode is formed by firing at a temperature of 600 to 800 ° C. for 3 to 30 minutes. The back electrode is preferably formed by a printing method from the viewpoint of manufacturing cost, but can also be formed by a vapor deposition method, a sputtering method, or the like.

  The light receiving surface electrode also uses screen printing. As the printing plate making, it is preferable to use a plate making that can be printed simultaneously by the busbar fingers because the productivity is improved. For the first electrode layer, a paste containing copper is used to reduce the cost. A paste containing silver is printed on the second electrode layer using the same plate making as the first electrode layer. As a result, the cross-section of the finger electrode has the structure shown in FIG. 2, and the electrical resistance is greatly reduced as compared with the case of only one layer. In FIG. 2, 200 is a substrate, 203 is a finger first electrode layer formed of a conductive paste containing copper or copper compound fine particles (hereinafter referred to as copper paste), and 204 is a conductive material containing silver. It is the finger 2nd electrode layer formed with the paste (henceforth silver paste). Further, the cross section of the bus bar electrode has the structure shown in FIG. 3, and since copper is not exposed on the outermost layer, the reliability of the adhesive strength with the wiring by soldering is increased. 3, 300 is a substrate, 303 is a bus bar first electrode layer formed of copper paste, and 304 is a bus bar second electrode layer formed of silver paste. If the patterns of the plate making for printing of the first electrode layer and the second electrode layer are made different from each other, the finger electrode can have the structure shown in FIG. 4 or FIG. Since the wiring is connected on the bus bar electrode, it is not necessary to reduce the wiring resistance of the bus bar. Therefore, the bus bar may have a structure shown in FIG.

  4, 400 is a substrate, 403 is a finger first electrode layer formed of copper paste, 500 in FIG. 5 is a substrate, 504 is a finger first electrode layer formed of silver paste, 503 Is a finger second electrode layer formed of a copper paste, in FIG. 6, 600 is a substrate, and 604 is a bus bar first electrode layer formed of a silver paste.

After these printings, silver powder is passed through the SiN _x film by heat treatment (fire through), and the electrode and silicon are made conductive. The back electrode and the light receiving surface electrode can be baked at a time.

  The unit price of the metal paste is almost determined by the unit price of the material metal. Since the unit price of copper is about 1/50 that of silver, the unit price of paste is also about 1/50. The ratio of the silver paste to the total material cost in manufacturing the solar battery cell by the conventional manufacturing method is about 12%, but according to the present invention, the ratio of the electrode paste on the light receiving surface can be halved to about 6%. . According to the present invention, it is not necessary to change the plate making, printing machine, etc. from the existing equipment, so the capital investment is not added to the production cost, and the production cost of the solar cell is reduced by the reduction of the material cost. It is valid.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not restrict | limited to the following Example.

[Examples and Comparative Examples]
In order to confirm the effectiveness of the present invention, the characteristics were compared under the condition A (Example) when a copper paste was used for the first electrode layer and the condition B (Comparative Example) when a silver paste was used. The second electrode layer was a silver paste for both conditions.
After removing the damaged layer with a hot concentrated potassium hydroxide aqueous solution on 60 boron-doped {100} p-type ascut silicon substrates of 100 × 100 mm ² , thickness 250 μm, specific resistance 1 Ω · cm, potassium hydroxide / 2 -Dipping in an aqueous propanol solution to form a texture, followed by washing in a hydrochloric acid / hydrogen peroxide mixed solution. Next, it heat-processed in the phosphorus oxychloride atmosphere in the state which accumulated the back surfaces at 870 degreeC, and formed the emitter layer. After diffusion, the glass was removed with hydrofluoric acid, washed and dried.
After the above treatment, a SiN _x film was formed as a light-receiving surface antireflection film on all samples using a plasma CVD apparatus.

Next, an Al paste as a back electrode was screen-printed on the entire back surface and dried on a hot plate at 180 ° C.
Next, a copper paste was printed as the first electrode layer on the light receiving surface on 30 sheets of the above, and dried (Condition A). The viscosity of the copper paste was adjusted to about 180 Pa · s with an organic solvent, and almost no disconnection or bleeding occurred. On the remaining 30 sheets, a silver paste was printed and dried as a comparative example (Condition B).

  Subsequently, using the same plate making as the first electrode layer, a silver paste was printed as the second electrode layer for all samples. At this time, the stage was appropriately finely adjusted so that the second electrode layer was directly above the first electrode layer. After printing, it was dried and finally baked in an air atmosphere at 780 ° C.

The composition of the copper paste was as follows. About 76% by mass of copper, about 8% by mass of ethyl cellulose, about 4% by mass of terpionol, about 5% by mass of glass powder, and about 7% by mass of organic solvent were added to adjust the viscosity.
The composition of the silver paste was as follows. About 86% by mass of silver, about 5% by mass of ethyl cellulose, about 2% by mass of terpionol, about 3% by mass of glass powder and about 4% by mass of organic solvent were added to adjust the viscosity.

The solar cell characteristics of the fabricated cell were measured under the conditions of AM 1.5 spectrum, irradiation intensity 100 mW / cm ² and 25 ° C. using a solar simulator manufactured by Yamashita Denso Co., Ltd. The average value of the obtained results is shown in Table 1.

In condition A, the form factor slightly decreased, but the short circuit current was improved and the conversion efficiency was equivalent to that of the comparative example.
Ten arbitrary sheets were extracted from each condition, and the connection wiring was soldered and a tensile strength test was performed. None of the cells showed any separation of wiring.

100, 200, 300, 400, 500, 600 Substrate 101 Finger electrode 102 Busbar electrodes 203, 303, 403, 503 Copper paste 204, 304, 504, 604 Silver paste

Claims (3)

A solar cell comprising: a semiconductor substrate on which a PN junction is formed; a finger electrode formed in a comb shape on at least one surface of the semiconductor substrate; and a bus bar electrode connected to the finger electrode, wherein the bus bar electrode However, a conductive paste containing only 30 to 90% by mass of copper fine particles as a conductive material is printed and fired, and a first electrode layer directly bonded to the semiconductor substrate, and silver, nickel, palladium, platinum or gold is formed by a conductive paste containing is fired is printed, have 2 So構 granulation comprising the outermost layer of the electrode layer to be bonded to the wiring, the first electrode of the finger electrodes is a bus bar electrode A solar cell comprising a conductive paste containing the same copper fine particles as the layer and printed and fired. Are those wherein the finger electrodes have a 2 So構 granulation, claims, characterized in that the first electrode layer of the finger electrodes directly joined to the semiconductor substrate is formed of a conductive paste containing fine particles of the copper 1. The solar cell according to 1. The solar cell according to claim 2, wherein the outermost electrode layer of the finger electrode is formed by printing and baking a conductive paste containing silver, nickel, palladium, platinum, or gold.

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