JP2013201217A - Solar cell and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a solar cell incorporating a dual-layer electrode structure which restrains electrode separation to exhibit high reliability, and a method for manufacturing the solar cell.SOLUTION: A solar cell of the present invention comprises: a polycrystalline silicon substrate 11 which includes a first conductive type semiconductor region (p-type polycrystalline silicon substrate 11a) and a second conductive type semiconductor region (n-type impurity diffusion layer 11b) and has a pn junction; a light receiving surface side electrode 12 formed on the light receiving side of the polycrystalline silicon substrate 11 so as to constitute a grid; a reverse side electrode 14 formed on the reverse side of the polycrystalline silicon substrate 11. The light receiving surface side electrode 12 has a dual-layer structure consisting of a first layer electrode 12a and a second layer electrode 12b formed so as to cover the top layer of the first layer electrode 12a, the first layer electrode 12a being composed of a dotted pattern.

Description

  The present invention relates to a solar cell and a method for manufacturing the solar cell, and more particularly to an electrode forming method for a solar cell in which an electrode having a thin line width and a large film thickness is formed.

  Conventionally, there is a method using a screen printing method as a typical method used for forming a light receiving surface side electrode of a solar cell. In the method using the screen printing method, a printing mask plate (screen plate) having a transmissive portion of a desired pattern is used, and a printing paste is formed so as to form a grid on the light receiving surface side of the silicon substrate constituting the solar cell. A light-receiving surface side electrode (hereinafter sometimes referred to as a grid electrode) is formed by printing and high-temperature treatment in a baking furnace.

  Specifically, a printing mask is a set of screen plates that are formed by plugging eyes other than the required image lines (patterns of transmission parts) on a printing plate frame on a stainless steel wire knitted in a net shape called a screen mesh. Used as a plate.

  A crystalline silicon substrate on which a solar cell is formed is aligned with this screen plate, a printing paste is placed on it and spread over the pattern of the transmissive part, and a rubber-like plate called a squeegee is pressed against the plate film inside the screen plate. An electrode pattern is formed by moving the electrode pattern. As a result, the printing paste passes through the screen mesh (transmission part pattern) where the plate film is not formed, and is pushed out onto the crystalline silicon substrate disposed under the screen plate to form a desired pattern. . Thereafter, the printing paste is baked after being dried. As a result, a light receiving surface side electrode having a desired pattern is formed. As described above, since the electrode pattern can be easily formed by using the printing plate, this method is most widely used at present.

  On the other hand, a shortage of silicon raw materials is a concern for the rapid spread of silicon solar cells expected in the future. As a countermeasure, by improving the power generation efficiency of solar cells, it is possible to generate large power even with the same amount of raw materials as before, lower the unit price per solar cell power generation amount, and increase the number of production . There is a standard standard for the size of the substrate used for silicon solar cells, and currently 156 mm × 156 mm is generally used, and improving the power generation efficiency per one substrate generates power from the solar cell. It will lead to the improvement of efficiency.

  One method for improving the power generation efficiency is to increase the amount of current obtained from one substrate by ensuring a large area of the substantial light receiving surface that contributes to power generation on the substrate, for example. Generally, in a solar cell, the amount of current generated increases as the light receiving area increases. On the other hand, a solar cell requires an electrode for collecting and flowing the generated current. This electrode needs to be installed on the light receiving surface side unless a special method is used. Therefore, this electrode becomes an obstacle that blocks the light receiving surface. Therefore, even if an electrode is used to pass the current generated on the substrate, the electrode that blocks the light receiving surface is formed with a minimum area, and the area of the light receiving surface that contributes to power generation is maximized to obtain the current obtained. Need to maximize.

  When a grid electrode having a thin line width is formed by a conventional screen printing method, the screen mesh is likely to be clogged by the printing paste, and the print film thickness must be reduced to prevent this clogging. As a result, since the cross-sectional area of the grid electrode is reduced and the electrical resistance of the grid electrode itself is increased, the large current obtained from the substrate cannot be connected to the increase in power generation efficiency, and the output characteristics of the solar cell are improved. I can't.

  Therefore, in order to form an electrode pattern having a large film thickness, it is necessary to perform overprinting a plurality of times, so that a printing paste capable of overprinting a plurality of times is required while improving the printing paste removal property of the screen plate. That is, in order to simultaneously achieve the thinning of the line width of the grid electrode and the thickening of the film thickness by using the screen printing method, an ink paste that can be printed multiple times is necessary, and its flow Therefore, it was necessary to improve the slip-out property from the screen plate while suppressing the property. In addition, this method complicates the printing process, leading to an increase in materials used, an increase in price, and an increase in required time, resulting in a significant increase in manufacturing cost. Even if this method is fully utilized, the cross section of the electrode has a kamaboko shape and a hem-spread shape due to the characteristics of the method of forming a pattern by extruding the ink paste from the image portion provided on the screen plate, and the light receiving surface is The blocking area becomes wider. This leads to a decrease in photoelectric conversion efficiency.

  In addition, in order to form a thin line electrode having a film thickness that satisfies the desired characteristics, the screen printer and its printing conditions, the screen plate and its specifications, and the screen printing printing paste are intricately involved. It is necessary to be familiar with the properties that were present and to build process conditions that are compatible with them. However, the actual situation is that equipment manufacturers and material manufacturers purchase and manufacture products that are sold in the market for solar cell production, and the grids of the desired shape are not accompanied by the process conditions described above. An electrode cannot be formed. That is, there is a limit to the formation of electrodes by the screen printing method.

  In recent years, in order to increase the photoelectric conversion efficiency of a crystalline solar cell, it is particularly important that the electrode on the light receiving surface side has a low resistance and the electrode area is reduced, and the grid size is about 70 μm wide and about 30 μm thick, Thickening is progressing.

  A recently used electrode on the light-receiving surface side is formed by using a printing paste containing 80% or more of silver particles having an average particle diameter of 1 μm or less and performing pattern formation by a screen printing method. After the pattern formation, the film is subsequently dried at 200 ° C. for 1 minute, and then heated in an extremely short time of 2 seconds to 10 seconds to perform sintering under the condition of an ultimate temperature of 800 ° C. As a countermeasure for increasing the thickness of the grid electrode by the screen printing method, a double overlay printing method has been proposed (for example, Patent Document 1). Further, a nozzle coating method is being developed as a measure for thinning the grid electrode (for example, Patent Document 2).

JP 2005-353904 A JP 2005-347628 A

  However, according to the above conventional technique, when the silver electrode exceeds 50 μm thickness or the particle size of the silver particles becomes 0.1 μm or less, the film thickness becomes extremely thick or the particle size becomes small, Stress concentration may occur at the interface between the underlying crystalline silicon substrate and the silver electrode, and the silver electrode may peel from the substrate. This phenomenon becomes prominent especially when the sintering heating time is 3 seconds or less or the sintering temperature reaches 800 ° C. or more. When the particle size of the silver particles is extremely small, such as 0.1 μm or less, the reactivity becomes high, the reaction rate increases, and the stress concentration at the interface between the crystalline silicon substrate and the silver electrode becomes particularly remarkable.

  Thus, in the structure of the conventional solar cell, there is a problem that electrode peeling tends to occur after sintering if the thickness of the grid electrode on the light receiving surface side is increased.

  This invention is made | formed in view of the above, Comprising: It aims at obtaining the solar cell which suppressed electrode peeling, and has a reliable 2 layer electrode structure, and its manufacturing method.

  In order to solve the above-described problems and achieve the object, the present invention includes a semiconductor substrate having a first conductivity type semiconductor region and a second conductivity type semiconductor region, having a pn junction, and light reception by the semiconductor substrate. A solar cell comprising a light receiving surface side electrode provided on the surface side so as to form a grid and a back surface side electrode provided on the back surface side of the semiconductor substrate, wherein the light receiving surface side electrode A first layer electrode, and a second layer electrode formed so as to contact the upper surface and the entire side surface of the first layer electrode and the light receiving surface of the semiconductor substrate so as to cover the upper layer of the first layer electrode It has a two-layer structure, and the first layer electrode has a dot pattern.

  According to the present invention, the light receiving surface side electrode is formed so as to abut on the first layer electrode configured in a dot pattern, the entire upper surface and side surfaces of the first layer electrode, and the light receiving surface of the semiconductor substrate. It has a two-layer structure with the formed second layer electrode. The second layer electrode is formed so as to cover the entire pattern of the first layer electrode. For this reason, there exists an effect that the electrode for current collection which has favorable adhesiveness, is reliable, and has a high aspect ratio can be formed.

FIG. 1 is a cross-sectional view showing a schematic configuration of a solar cell according to Embodiment 1 of the present invention. FIG. 2 is a top view showing a schematic configuration of the solar cell according to the first embodiment of the present invention. FIG. 3 is a bottom view showing a schematic configuration of the solar cell according to the first embodiment of the present invention. FIGS. 4-1 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. FIGS. FIGS. 4-2 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. FIGS. 4-3 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. 4-4 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. FIGS. 4-5 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. FIGS. FIGS. 4-6 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. FIGS. FIGS. 4-7 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 1 of this invention. FIGS. FIG. 5 is a diagram showing a coating apparatus for forming the light receiving surface side electrode according to the first embodiment of the present invention. FIG. 6 is a flowchart showing the method for manufacturing the solar cell according to the first embodiment of the present invention. FIG. 7 is a flowchart showing a method for manufacturing a solar cell according to the second embodiment of the present invention. FIGS. 8-1 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 2 of this invention. FIGS. 8-2 is sectional drawing for demonstrating the manufacturing process of the solar cell concerning Embodiment 2 of this invention.

  EMBODIMENT OF THE INVENTION Below, embodiment of the solar cell concerning this invention, the electrode formation method of a solar cell, and the manufacturing method of a solar cell is described in detail based on drawing. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably. In the drawings shown below, the scale of each member may be different from the actual scale for easy understanding. The same applies between the drawings.

Embodiment 1 FIG.
1-3 is a figure which shows schematic structure of the solar cell 1 concerning this Embodiment, FIG. 1 is sectional drawing of the solar cell 1, FIG. 2 is the upper side figure of the solar cell seen from the light-receiving surface side. FIG. 3 is a bottom view of the solar cell viewed from the side opposite to the light receiving surface. FIG. 1 is a cross-sectional view in the AA ′ direction of FIG. 4-1 to 4-7 are process cross-sectional views illustrating the manufacturing process of the solar cell of the first embodiment. FIG. 5 is an explanatory view showing a coating apparatus used in the manufacturing process of the solar cell of the first embodiment, and FIG. 6 is a flowchart showing the manufacturing process of the solar cell.

  As shown in FIGS. 1 to 3, the solar cell 1 includes a polycrystalline silicon substrate 11 as a semiconductor substrate having a pn junction which is a solar cell substrate having a photoelectric conversion function, and a light receiving surface side of the polycrystalline silicon substrate 11. In addition, a light receiving surface side electrode 12 provided in a grid shape and a back surface side electrode 14 provided on the back surface side of the polycrystalline silicon substrate 11 are provided. The light-receiving surface side electrode 12 is in contact with the first layer electrode 12a and the second layer electrode 12b formed so as to contact the entire upper surface and side surface of the first layer electrode 12a and cover the upper layer of the first layer electrode 12a. The first layer electrode 12a is formed in a dot pattern.

  An antireflection film 13 for preventing the reflection of incident light at the light receiving surface is formed on the surface (front surface) of the polycrystalline silicon substrate 11 on the light receiving surface side. On the light receiving surface side (front surface), the light receiving surface side electrode 12 is formed surrounded by the antireflection film 13. Here, it is not essential for the dot pattern to be scattered all over, and includes a case where there are continuous portions of points, such as forming an aggregate of points.

  A polycrystalline silicon substrate 11 as a semiconductor substrate has a first conductivity type semiconductor region which is a p-type (first conductivity type) polycrystalline silicon substrate 11a and a conductivity type of the surface of the p-type polycrystalline silicon substrate 11a. And a second conductivity type semiconductor region which is an inverted n-type (second conductivity type) impurity diffusion layer 11b, thereby forming a pn junction. As the light-receiving surface side electrode 12, the grid electrode 12G and bus electrode 12B of a photovoltaic cell are included. The grid electrode 12G is locally provided on the light receiving surface in order to collect electricity generated by the polycrystalline silicon substrate 11. The bus electrode 12B is provided substantially orthogonal to the grid electrode 12G in order to take out the electricity collected by the grid electrode 12G. The back side electrode 14 is formed on the entire back side of the polycrystalline silicon substrate 11.

  In the solar cell 1 configured as described above, sunlight is emitted from the light receiving surface side of the solar cell 1 to the pn junction surface of the polycrystalline silicon substrate 11 (the junction surface between the p-type polycrystalline silicon substrate 11a and the n-type impurity diffusion layer 11b). ), Holes and electrons are generated. The generated electrons move toward the n-type impurity diffusion layer 11b by the electric field at the pn junction, and the holes move toward the p-type polycrystalline silicon substrate 11a. As a result, electrons are excessive in the n-type impurity diffusion layer 11b and holes are excessive in the p-type polycrystalline silicon substrate 11a. As a result, photovoltaic power is generated. This photovoltaic power is generated in the direction of biasing the pn junction in the forward direction, the light receiving surface side electrode 12 connected to the n-type impurity diffusion layer 11b becomes a negative electrode, and the back surface side electrode 14 connected to the p-type polycrystalline silicon substrate 11a. Becomes a positive pole, and current flows in an external circuit (not shown).

  In the solar cell 1 according to the first embodiment configured as described above, the light receiving surface side electrode 12 is a thin wire with a grid electrode 12G having a line width of about 70 μm and a film thickness of about 30 μm. It is a thin wire electrode, and the side wall is provided substantially vertically, so that the area blocking the light receiving surface is reduced as much as possible. Thereby, in the solar cell 1 according to the first embodiment, the area of the substantial light receiving surface that contributes to power generation in the polycrystalline silicon substrate 11 is enlarged and secured, and the amount of current obtained from the solar cell 1 is increased. The output characteristics are improved by increasing the output characteristics.

  In addition, the grid electrode 12G is not only thin, but also has a two-layer structure so that the film thickness is large, so that a large cross-sectional area is secured. As a result, it is possible to prevent the electrical resistance of the grid electrode 12G itself from increasing due to the reduction in the line width of the grid electrode 12G, and it is possible to connect the generated current to the increase in power generation efficiency, and to improve the output characteristics. Improvements are being made.

  The first layer electrode 12a is made of silver which is conductive particles. Although the melting point of silver is 962 ° C., the melting temperature can be lowered by reducing the average particle diameter of the silver particles. Silver particles melt and become conductive, penetrate through the antireflection film 13 and melt with the n-type impurity diffusion layer 11b, and when cooled, silver precipitates at the interface of the n-type impurity diffusion layer 11b and forms a grid shape. The light-receiving surface side electrode 12 that is a silver electrode is electrically connected to the n-type impurity diffusion layer 11b. When sintered in a short time, the influence of heavy metal contamination on the surface of the n-type impurity diffusion layer 11b can be reduced, and the cell output tends to be improved. Hereinafter, the particle diameter refers to the average particle diameter.

  If the average particle size of the silver particles is 0.5 μm or less and the film thickness of the light-receiving surface side electrode 12 (grid electrode 12G or bus electrode 12B) is 50 μm or more, cracks may occur in the silver electrode or the underlying n-type The grid electrode 12G and the bus electrode 12B are peeled from the impurity diffusion layer 11b and the antireflection film 13 to cause electrode failure. If the average particle diameter of the silver particles is 1 μm or more, even if the sintering time is as short as 2 seconds, the reaction rate is slow and no significant cracks are likely to occur.

  The average particle diameter of the silver particles of the second layer electrode 12b formed by using the two-time overlay printing method or the nozzle coating method is 0.5 μm or less, and the average particle diameter of the silver particles of the first layer electrode 12a is It is desirable that it is 1 μm or more. Thus, in the present embodiment, silver particles having a large particle size are used in the first layer electrode 12a. If the average particle diameter of the silver particles is 1 μm or more, even if the sintering time is as short as 2 seconds, the reaction rate is slow and no significant cracks are likely to occur. A silver paste having an average particle diameter of 0.5 μm or less is used as the second layer electrode 12b to cover the entire first layer electrode 12a and to come into contact with the underlying silicon substrate surface. It is possible to form a highly accurate electrode pattern with no peeling and uniform pattern edges. On the other hand, in the case of the light receiving surface side electrode of the conventional example, when the thickness is 50 μm or more, the silver electrode is cracked or the light receiving surface side electrode peels off from the underlying n-type impurity diffusion layer 11b or the antireflection film 13. Electrode failure occurs.

  Here, the first layer electrode and the second layer electrode are configured in a pattern using a paste containing silver particles, and the average particle diameter of the silver particles of the second layer electrode before sintering is the same as that of the first layer electrode. The average particle diameter of the silver particles is set to half or less. According to this configuration, by increasing the particle size of the silver particles of the first layer electrode, the film thickness of the first layer electrode is increased while suppressing cracks, and this is the particle size of the second layer electrode. By covering with small silver particles, adhesion and pattern accuracy are improved. When formed thick, the first layer electrode having a large particle size and a slow reaction rate is formed in the lower layer of the second layer electrode, which is prone to cracking, and the entire periphery of the first layer electrode is covered with the second layer electrode. The reaction rate between the first layer electrode and the second layer electrode can be adjusted, the generation of cracks can be suppressed, and peeling can be prevented.

  In this silver paste, in addition to silver particles as conductive particles, an inorganic powder component such as frit glass is added. The frit glass has a eutectic temperature with silicon and a melting point of the light-receiving surface side electrode. It is desirable that the main component is a metal oxide higher than the sintering temperature. As a result, the reaction rate at the time of sintering is reduced and the powder is sintered slowly, so that the adhesiveness with the underlying substrate surface is improved.

Further, when the metal oxide contains at least one of zirconia ZrO ₂ , alumina, and vanadium oxide V ₂ O ₅ , adhesion to the underlying substrate surface is improved.

  As described above, in the solar cell 1 according to the first embodiment, the light receiving surface side electrode 12 includes the narrow and thick grid electrode 12G, thereby ensuring a wide light receiving area and excellent photoelectric conversion efficiency. A highly reliable solar cell has been realized.

  Next, an example of a method for manufacturing such a solar cell 1 will be described with reference to FIGS. 4A to 4D are cross-sectional views for explaining the manufacturing process of the solar cell 1 according to the first embodiment.

  First, as a semiconductor substrate, for example, a p-type polycrystalline silicon substrate 11a most frequently used for a consumer solar cell is prepared (FIG. 4-1) (FIG. 6: Step S1001).

  Since the p-type polycrystalline silicon substrate 11a is manufactured by slicing an ingot formed by cooling and solidifying molten silicon with a wire saw, damage at the time of slicing remains on the surface. Therefore, the p-type polycrystalline silicon substrate 11a is first removed by immersing the surface of the p-type polycrystalline silicon substrate 11a in an acid or heated alkaline solution, for example, in an aqueous solution of sodium hydroxide, to etch the silicon substrate. Then, the damaged region existing near the surface of the p-type polycrystalline silicon substrate 11a is removed.

  At the same time as the damage removal or subsequent to the damage removal, fine unevenness may be formed as a texture structure on the light receiving surface side surface of the p-type polycrystalline silicon substrate 11a (not shown). By providing such a texture structure on the light-receiving surface side of the p-type polycrystalline silicon substrate 11a, multiple reflection of light is generated on the surface side of the solar cell 1, and light incident on the solar cell 1 is efficiently transmitted to the semiconductor substrate. 11 can be absorbed, and the reflectance can be effectively reduced to improve the conversion efficiency.

  In addition, since this invention is invention concerning electrode formation, it does not restrict | limit in particular about the formation method and shape (FIG. 6: step S1002) of a texture structure. For example, an alkaline aqueous solution containing isopropyl alcohol or a method using acid etching mainly composed of a mixed solution of hydrofluoric acid and nitric acid, or a mask material partially provided with an opening is formed on the surface of the p-type polycrystalline silicon substrate 11a. Any method such as a method of obtaining a honeycomb structure or an inverted pyramid structure on the surface of the p-type polycrystalline silicon substrate 11a by etching through the mask material, or a method using reactive gas etching (RIE) Can be used.

Next, this p-type polycrystalline silicon substrate 11a is put into a thermal oxidation furnace and heated in an atmosphere of phosphorus (P) which is an n-type impurity. Through this step, phosphorus (P) is diffused on the front and back surfaces of the p-type polycrystalline silicon substrate 11a to form an n-type impurity diffusion layer 11b to form a semiconductor pn junction (FIG. 4-2) (FIG. 6: FIG. 6). (N layer diffusion) Step S1003). In the present embodiment, the n-type impurity diffusion layer 11b is formed by heating the p-type polycrystalline silicon substrate 11a in a phosphorus oxychloride (POCl ₃ ) gas atmosphere at a temperature of 800 ° C. to 850 ° C., for example.

  Here, since the phosphor glass layer mainly composed of glass is formed on the surface immediately after the formation of the n-type impurity diffusion layer 11b, the phosphor glass layer is removed using a hydrofluoric acid solution or the like.

  Next, on the light receiving surface side of the p-type polycrystalline silicon substrate 11a on which the n-type impurity diffusion layer 11b is formed, for example, a silicon oxyhydroxide film (SiHON) having a film thickness of 70 nm is formed as the antireflection film 13 in order to improve the photoelectric conversion efficiency. Film) is formed (FIG. 4-3) (FIG. 6: Step S1004). For the formation of the antireflection film 13, for example, a plasma CVD method is used, and a silicon oxynitride film is formed using a mixed gas of silane, ammonia and oxygen at a substrate temperature of 500 ° C. The film thickness and refractive index of the antireflection film 13 are set to values that most suppress light reflection.

  As the antireflection film 13, two or more films having different refractive indexes may be laminated. Further, for the formation of the antireflection film 13, a different film forming method such as a sputtering method may be used. Further, a silicon nitride film may be formed as the antireflection film 13. Also in this case, the plasma CVD method can be used, and the substrate temperature can be 500 ° C. and a mixed gas of silane, ammonia, and oxygen can be used.

  Next, the n-type impurity diffusion layer 11b formed on the back surface side of the p-type polycrystalline silicon substrate 11a is removed using the antireflection film 13 as a mask. Thereby, the p-type polycrystalline silicon substrate 11a which is the first conductivity type semiconductor region, and the n-type impurity diffusion which is the second conductivity type semiconductor region formed on the light receiving surface side of the p-type polycrystalline silicon substrate 11a. The semiconductor substrate 11 in which a pn junction is formed by the layer 11b is obtained (FIG. 4-4) (FIG. 6: (rear surface n layer removal) step S1005).

  The removal of the n-type impurity diffusion layer 11b formed on the back surface of the p-type polycrystalline silicon substrate 11a is performed using, for example, a single-sided etching apparatus. Alternatively, a method of using the antireflection film 13 as a mask material and immersing the entire p-type polycrystalline silicon substrate 11a in an etching solution may be used. As the etching solution, an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide heated to room temperature to 95 ° C, preferably 50 ° C to 70 ° C is used. Alternatively, a mixed aqueous solution of nitric acid and hydrofluoric acid may be used as the etching solution.

  Thereafter, a light receiving surface side electrode 12 (before firing), that is, a pattern of silver grid electrodes 12G and bus electrodes 12B (before firing) is formed on the antireflection film 13 (FIG. 4-5) ((light receiving surface electrode Formation) FIG. 6: Step S1006). In the present embodiment, the pattern of the light receiving surface side electrode 12 is received by using two discharge nozzles 100 of the first layer electrode nozzle 100a and the second layer electrode nozzle 100b shown in FIG. The surface side electrode 12 is formed by drawing a pattern.

  Prior to the description of the method of forming the light-receiving surface side electrode 12 having the two-layer electrode structure, an apparatus used for forming the light-receiving surface side electrode 12 having the two-layer electrode structure will be described. FIG. 5 is a view showing a coating apparatus for forming the light-receiving surface side electrode 12 which is the grid electrode of the present embodiment by nozzle coating. In this apparatus, the first layer electrode nozzle 100a and the second layer electrode nozzle 100b are shown. The two-layer electrodes are applied so as to form a pattern at the same time using the discharge nozzle 100 in which the two nozzles are placed close to each other.

  In FIG. 5, 11 is a polycrystalline silicon substrate, 13 is an antireflection film, 100 is a discharge nozzle, 102a is a first layer electrode paste, and 102b is a second layer electrode paste. It is a silver paste. Reference numeral 100a denotes a first layer electrode nozzle, and 100b denotes a second layer electrode nozzle. For the first layer electrode, the n-type impurity diffusion layer 11b is formed on the p-type polycrystalline silicon substrate 11a, and the antireflection film 13 is deposited on the n-type impurity diffusion layer 11b. The nozzle 100a and the second layer electrode nozzle 100b filled with the second layer electrode paste 102b are disposed close to each other. The first layer electrode paste 102a is a silver paste for short-time sintering in which the particle size of silver particles as the main component is large, the reactivity is poor, the reaction rate is slow, and cracks are difficult to occur. The second layer electrode paste 102b is a silver paste having a small particle size and easily cracking after being sintered for a short time.

  Then, the first layer electrode nozzle 100a and the second layer electrode nozzle 100b are moved together so as to trace the position where the desired grid electrode pattern is formed, thereby forming the first layer electrode 12a and the second layer electrode 12b. To do. The first layer electrode nozzle 100a is intermittently coated with the first layer electrode paste 102a so as to be scattered on the antireflection film 13 of the polycrystalline silicon substrate 11. ) 12a is formed. The second layer electrode nozzle 100b applies the second layer electrode paste 102b so as to contact the upper surface and the side surface of the first layer electrode (pattern) 12a having a dot pattern to cover the upper layer and also contact the polycrystalline silicon substrate 11. Form. By using the coating apparatus in which the two nozzles shown in FIG. 5 are integrally arranged, as shown in FIG. 1, a two-layer electrode having a high aspect ratio that is difficult to peel off can be manufactured uniformly and inexpensively. it can.

  As a method for manufacturing the solar cell shown in FIGS. 1 to 4, the nozzle coating device of FIG. 5 was used. However, two electrode pattern masks were prepared, and the first layer, which is the lower layer, was printed twice (screen) printing. A layer electrode and an upper second layer electrode may be sequentially formed. Further, the first layer electrode and the second layer electrode may be sequentially laminated by applying a photoengraving method used in a semiconductor process.

  After forming the light receiving surface side electrode (before firing) in this way, the electrode material of the back surface side electrode 14 such as aluminum (Al), glass, etc. is formed on the entire back surface side of the polycrystalline silicon substrate 11 which is a semiconductor substrate. Is printed using, for example, 100 ° C. to 300 ° C. (FIG. 4-6) (FIG. 6: (Back Electrode Printing) Step S1007).

  And the light-receiving surface side electrode 12 and the back surface side electrode 14 are formed by baking the polycrystalline silicon substrate 11 at 700 to 1000 degreeC, for example. Further, the silver in the light receiving surface side electrode 12 composed of the first layer electrode 12a and the second layer electrode 12b penetrates the antireflection film 13, and the n-type impurity diffusion layer 11b and the light receiving surface side electrode 12 shown in FIG. Are electrically connected to each other (FIG. 4-7) (FIG. 6: Step S1008).

  By performing the steps as described above, the solar cell according to the first embodiment shown in FIGS. 1 to 3 can be manufactured. Note that the order of arrangement of the paste as the electrode material on the polycrystalline silicon substrate 11 may be switched between the light receiving surface side and the back surface side.

  Of the conductive paste used to form the light-receiving surface side electrode 12 in this embodiment, the conductive first layer electrode paste 102a used to form the first layer electrode 12a on the lower layer side is used for the discharge nozzle 100. The first layer electrode nozzle 100a on the upstream side is filled. On the other hand, the second layer electrode paste 102b used to form the second layer electrode 12b on the upper layer side is filled in the second layer electrode nozzle 100b on the downstream side of the discharge nozzle 100.

  These conductive pastes all contain organic components such as a binder resin, a transfer resin, a high volatile solvent, and a low volatile solvent, in addition to inorganic powder components such as a conductive powder and glass frit.

  As the conductive powder used here, silver particles having an average particle diameter of 1.0 μm are used for the first layer electrode paste 102a. On the other hand, silver particles having an average particle diameter of 0.5 μm are used for the second layer electrode paste 102b. Although the melting point of silver is 962 ° C., the melting temperature can be lowered by reducing the average particle diameter of the silver particles. Silver particles melt and become conductive, penetrate the antireflection film 13, melt with the n-type impurity diffusion layer 11b, and when cooled, silver precipitates at the interface with the n-type impurity diffusion layer 11b in the form of islands to form a grid The light-receiving surface side electrode 12 that is a silver electrode is electrically connected to the n-type impurity diffusion layer 11b (FIG. 6: Step S1008). When sintered in a short time, the influence of heavy metal contamination on the surface of the polycrystalline silicon substrate can be reduced, and the cell output is improved.

  In order to melt the antireflection film 13, metal oxide particles such as lead glass that starts melting at a temperature lower than the sintering temperature may be contained in the silver paste. As described above, when the average particle diameter of the silver particles is 0.5 μm or less and the film thickness of the electrode is 50 μm or more, the electrode is cracked or the n-type impurity diffusion layer 11b as the underlayer or the antireflection film 13 is formed. Therefore, the light receiving side electrode 12 is easily peeled off. However, in the present embodiment, the first layer electrode 12a on the lower layer side is formed using a silver paste of silver particles having an average particle diameter of 1.0 μm as a material, and the silver paste material having a low reactivity at the time of sintering is dotted. Patterned. Then, the upper layer is covered with a silver paste using silver particles having a small particle diameter, which is highly reactive during sintering and easily cracked, as the second layer electrode 12b, thereby forming an electrode pattern having a desired shape.

The glass frit is a component added to fire the conductive powder after firing to maintain the electrode shape and to secure the adhesion between the electrode and the printed material (polycrystalline silicon substrate 11). Such a glass frit desirably has a property that it does not dissolve at the thermal decomposition temperature of the organic component, can reduce the amount of warping of the polycrystalline silicon substrate 11 after firing, and can be fired at as low a temperature as possible. This is because it is economically advantageous if it can be fired at a low temperature. Therefore, it is preferable to appropriately change the softening point of the glass frit depending on the type of organic component contained in the conductive paste. However, in the conductive paste used as the material of the first layer electrode 12a of the present embodiment, silica It is desirable to use glass or a metal oxide having a melting point higher than 800 ° C. or a eutectic temperature with silicon as the glass frit. For example, zirconia ZrO ₂ , alumina, vanadium oxide V ₂ O ₅ or the like is desirable. This is because it has the property that it does not dissolve at the thermal decomposition temperature of the organic component, and the amount of warpage of the polycrystalline silicon substrate 11 after firing can be reduced.

  Moreover, as a glass frit, you may use a softening point within the range of 350-600 degreeC. As such a glass frit, specifically, one or two selected from glasses containing metal oxides such as lead oxide, borosilicate glass, boron oxide, aluminum oxide, silicon oxide, zinc oxide, and bismuth oxide. More than a seed glass powder is desirable. The particle size of the glass frit is not particularly limited, but for the same reason as the conductive powder, it is preferable that the particle size is the same. Moreover, it is preferable that the particle size of a glass frit is 1/3 or less with respect to the particle size of electroconductive powder. This is because if the particle size of the glass frit is larger than 1/3 with respect to the particle size of the conductive powder, the glass frit cannot be efficiently sintered and the conductivity of the electrode increases.

  Alternatively, the glass frit is used in combination with a metal oxide having a softening point in the range of 350 to 600 ° C. and a melting point higher than the sintering temperature of 800 ° C. and having a eutectic temperature with silicon, The sintering speed and the melting point may be adjusted.

  The binding resin is a component added to maintain the shape of the conductive paste on the polycrystalline silicon substrate 11 after printing. For this reason, a resin is selected in which the conductive paste after printing has flexibility but is strong. In addition, a resin that is completely removed without leaving a resin component or a residue thereof when the binder resin is thermally decomposed and removed by baking is preferable. Examples of such a resin include a phenol resin, an epoxy resin, a polyester resin, ethyl cellulose, and the like. From these, the resin powder is appropriately selected including dispersibility and viscosity adjustment of the conductive powder.

  According to the method for forming the solar cell 1 according to the first embodiment described above, it is possible to form an electrode having an electrode size on the order of microns to several tens of microns and a dimensional accuracy of the order of microns. According to the method of forming the solar cell 1 according to the first embodiment, for example, the line width is 10 μm to 90 μm, the electrode thickness is 10 μm to 90 μm, and the aspect ratio (electrode thickness / electrode width) is 1/9. ~ 9 grid electrodes can be formed. More preferably, the aspect ratio is 0.35 to 1. That is, as compared with a line width of 100 μm to 200 μm and a film thickness of about 10 μm to 20 μm, which are electrode dimensions formed by a method using a conventional screen printing method, it is possible to greatly reduce the thickness and thickness. The light-receiving surface side electrode 12 which is a grid electrode can be formed. The appropriate values of the electrode thickness and aspect ratio (electrode thickness / electrode width) vary depending on the line width of the electrode to be designed.

  Therefore, the aspect ratio of the electrode (electrode thickness / electrode width) is preferably in the range of 0.35 to 1 from the viewpoint of function. If the aspect ratio of the electrode (electrode thickness / electrode width) is within this range, the electrode will not break, the resistance value of the electrode itself will be stable at the original value of the material, and the formed electrode will collapse. In addition, the probability of occurrence of a decrease in adhesion strength between the electrode and the substrate does not increase.

  In the above embodiment, the light receiving surface side electrode and the back surface electrode are formed by sequentially patterning and then simultaneously sintering, but sequential printing and sintering may be repeated.

Further, the first layer electrode paste on the lower layer side is not limited to a paste containing silver particles as a main component, but a metal oxide having a melting point higher than 800 ° C. and a eutectic temperature with silicon. It is desirable to use the object as a glass frit. For example, zirconia ZrO ₂ , alumina, vanadium oxide V ₂ O ₅ or the like is desirable. This is because it has the property that it does not dissolve at the thermal decomposition temperature of the organic component, and the amount of warpage of the polycrystalline silicon substrate 11 after firing can be reduced.

<Example 1>
As Example 1, in the step of forming the light-receiving surface side electrode 12 (FIGS. 4-6), the first layer electrode paste and the second layer electrode paste were adjusted as follows.
First layer electrode paste (without silver particle powder or glass frit)
Metal oxide 70-95wt%
Resin components such as ethyl cellulose 1 to 10 wt%, effective solvent 0.5 to 10 wt%
(Silver particle powder and glass frit may also be mixed)
Second layer electrode paste Silver particle powder (particle size 0.5 μm) 70-88 wt%, glass frit 0.1-5 wt%
Resin components such as ethyl cellulose 1 to 5 wt%, effective solvent 0.5 to 5 wt%

  Then, using the coating apparatus shown in FIG. 5, the first electrode nozzle 100a is filled with the first layer electrode paste 102a, and the second electrode nozzle 100b is filled with the second layer electrode paste 102b. 100 is scanned in the direction of the arrow in the figure to form a desired electrode pattern (FIGS. 4-6). And it sintered at 800 degreeC for 2 seconds.

  Other processes were formed in the same manner as the manufacturing process of the solar cell of the first embodiment. The solar cell thus obtained is a light-receiving surface side electrode having good adhesion and low resistance.

<Example 2>
As Example 2, in the step of forming the light-receiving surface side electrode 12 (FIGS. 4-6), the first layer electrode paste and the second layer electrode paste were adjusted as follows.
First layer electrode paste Silver particle powder (particle size 1 μm) 70-88 wt%, glass frit 0.1-5 wt%
Resin components such as ethyl cellulose 1 to 5 wt%, effective solvent 0.5 to 5 wt%
Second layer electrode paste (same as in Example 1)
Silver particle powder (particle diameter 0.5 μm) 70 to 88 wt%, glass frit 0.1 to 5 wt%
Resin components such as ethyl cellulose 1 to 5 wt%, effective solvent 0.5 to 5 wt%

  Also in this embodiment, the coating apparatus shown in FIG. 5 is used to fill the first electrode nozzle 100a with the first layer electrode paste 102a and the second electrode nozzle 100b with the second layer electrode paste 102b. And the discharge nozzle 100 is scanned in the direction of the arrow in the figure to form a desired electrode pattern (FIGS. 4-6). And it sintered at 800 degreeC for 2 seconds.

  The solar cell thus obtained is also a light-receiving surface side electrode with good adhesion and low resistance.

  In the step of applying heat at a high temperature for a semiconductor device such as a solar battery cell, when there is a possibility of contamination with heavy metals or the like, the time for applying the temperature is shortened or lowered slightly. Recent electrode materials have been heated at a low temperature of 800 ° C. for 2 seconds for a short time. Conventionally, when the conditions of such electrode materials are slightly different from the optimum conditions, the reaction may suddenly proceed too much or the reaction may be insufficient. If the reaction suddenly progresses too much, unnecessary residual stress concentration occurs at the Si / electrode interface, and if the reaction is insufficient, the fire-through of the antireflection film, which is an insulator, becomes insufficient, resulting in a high resistance electrode. Sometimes formed.

On the other hand, according to the method of the present embodiment, the electrode cross-sectional area is kept constant and light is received by the solar cell while keeping the firing conditions of the conductive paste for electrodes (assuming the above range). When printing thickly and sintering, electrode adhesion area becomes small, stress concentration becomes large, and it is avoided that it becomes easy to generate | occur | produce electrode peeling. In the present embodiment, the metal reaction rate of the material such as zirconia ZrO ₂ , alumina, and vanadium oxide V ₂ O ₅ is controlled by controlling the sintering reaction rate.

  In addition, it is desirable to adjust the amount of the organic solvent as appropriate depending on whether the silver paste material is applied for screen printing or is applied from a nozzle.

Embodiment 2. FIG.
FIG. 7: is a figure which shows the flowchart of Embodiment 2 of the manufacturing method of the solar cell concerning this invention. 8A to 8B are diagrams illustrating the light receiving surface side electrode forming step.

  According to the method for manufacturing the solar cell of the present embodiment, the integrated discharge nozzle shown in FIG. 5 may be used instead of the integrated discharge nozzle. That is, the present embodiment is characterized in that the pattern formation of the second layer electrode (step S1006b) is performed after the pattern formation of the first layer electrode (step S1006a) by the screen printing method. The other steps are exactly the same as those shown in FIG. 6 in the first embodiment, and thus description thereof is omitted here.

  That is, as shown in FIG. 8A, after the antireflection film 13 is formed (step S1004) and the back surface n layer is removed ((back surface n layer removal) step S1005), the first layer electrode is formed by screen printing. 12a is formed ((first layer electrode printing) step S1006a).

  Thereafter, as shown in FIG. 8B, a second layer electrode (pattern) 12b is formed by a screen printing method ((second layer electrode printing) step S1006b). And a solar cell as shown in FIGS. 1-3 is formed through a sintering process.

  Also by this method, it is possible to form a highly reliable solar cell in which electrode peeling does not easily occur.

  Also in this case, the conductive powder is a component for ensuring electrical continuity after the conductive paste is baked, and is not limited to silver powder but has conductivity such as other metal powders. However, silver powder having high conductivity is preferable for the light-receiving surface side electrode of the solar cell. The particle size of the powder is not particularly limited, but it is desirable that the 50% average particle size is 1/5 or less with respect to the width of the electrode. This is because if the 50% average particle diameter exceeds 1/5 of the width of the electrode, it becomes difficult to fill the intaglio groove with the conductive paste, and the frequency of occurrence of disconnection of the electrode after printing increases. . On the other hand, if the particle size is too small, the viscosity of the conductive paste containing the conductive powder with a desired content ratio is increased, and a problem occurs in the printability. The particle size is desirable.

  As described above, the solar cell and the method for manufacturing the solar cell according to the present invention are useful for forming an electrode pattern having a high aspect ratio, and in particular, a high-aspect-ratio collector electrode having a width of 40 μm or less and a film thickness of 80 μm or more. It is suitable for forming a grid electrode.

DESCRIPTION OF SYMBOLS 11 Polycrystalline silicon substrate 11a p-type polycrystalline silicon substrate 11b N-type impurity diffusion layer 12 Light-receiving surface side electrode 12a First layer electrode 12b Second layer electrode 13 Antireflection film 14 Back surface side electrode 100 Discharge nozzle 100a For first layer electrode Nozzle 100b Second layer electrode nozzle 102a First layer electrode paste 102b Second layer electrode paste

Claims (8)

A semiconductor substrate comprising a first conductivity type semiconductor region and a second conductivity type semiconductor region, and having a pn junction;
A solar cell comprising a light receiving surface side electrode provided in a grid shape on the light receiving surface side of the semiconductor substrate, and a back surface side electrode provided on the back surface side of the semiconductor substrate,
The light receiving surface side electrode,
A first layer electrode configured in a dot pattern;
A solar cell having a two-layer structure with a second layer electrode formed so as to cover an upper layer of the first layer electrode.   2. The solar cell according to claim 1, wherein the first layer electrode includes a metal oxide having a eutectic temperature with silicon and a melting point higher than a sintering temperature of the light-receiving surface side electrode. The solar cell according to claim 2, wherein the metal oxide includes at least one of zirconia ZrO ₂ , alumina, and vanadium oxide V ₂ O ₅ .   The first layer electrode and the second layer electrode are configured in a pattern using a paste containing silver particles, and the average particle diameter of the silver particles of the second layer electrode before sintering is the first layer. 2. The solar cell according to claim 1, wherein the solar cell is one half or less of an average particle diameter of silver particles of the electrode. The average particle diameter of the silver particles of the second layer electrode is 0.5 μm or less,
The solar cell according to claim 4, wherein an average particle diameter of silver particles of the first layer electrode is 1 μm or more. Forming a semiconductor substrate having a pn junction, the semiconductor substrate having a first conductivity type semiconductor region and a second conductivity type semiconductor region;
Forming grid-shaped light receiving surface side electrodes arranged in parallel on the first main surface of the semiconductor substrate;
Applying a conductive paste on the second main surface of the semiconductor substrate to form a back electrode forming layer to be a back electrode,
Forming the light receiving surface side electrode,
Forming a first layer electrode so as to have a dot pattern;
Forming a second layer electrode formed to cover the upper layer of the first layer electrode,
A method for manufacturing a solar cell, which is a step of forming an electrode having a two-layer structure. Forming the light receiving surface side electrode,
The nozzle for the first layer electrode and the nozzle for the second layer electrode are installed close to each other, the nozzle for the second layer electrode continuously discharges when scanning between both ends of the pattern, and the nozzle for the first layer electrode is disposed at both ends of the pattern. The method for manufacturing a solar cell according to claim 6, wherein the solar cell is configured to discharge intermittently when scanning the space. Forming the first layer electrode and the second layer electrode;
All are screen printing processes, The manufacturing method of the solar cell of Claim 6 characterized by the above-mentioned.

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