JP2012054442A - Method of manufacturing solar cell and screen plate making process for use therein - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a screen plate making process in which the joint of a finger electrode and a bus bar electrode can be made linear like the finger electrode without increasing the manufacturing cost of a solar cell or the shadow loss, and thereby disconnection of the joint of the bus bar electrode and the finger electrode can be prevented without losing good appearance of the solar cell.SOLUTION: In the screen plate making process used for manufacturing a solar cell where a bus bar electrode and a finger electrode perpendicular thereto are formed by printing conductive paste, an opening for the bus bar electrode and an opening for the finger electrode are connected at a plurality of parts, the opening width of one half or less of the connections is larger than the opening width for the finger electrode, and the opening width of remaining connections is equal to the width for the finger electrode.

Description

  The present invention relates to a method for manufacturing a highly efficient solar cell with good productivity and high reliability, and more particularly, a connection between a bus bar electrode and a finger electrode printed using a screen printing method is disconnected. The present invention relates to a method for manufacturing a solar cell capable of forming a high aspect ratio electrode without any problem. Moreover, this invention relates to the screen platemaking used for manufacture of this solar cell.

A cross-sectional view (FIG. 1), a front surface structure (FIG. 2), and a back surface structure (FIG. 3) of a solar cell manufactured using a conventional technique will be described. In a general solar battery cell, a pn junction is formed by diffusing an n-type dopant into a p-type semiconductor substrate 100 such as silicon to form an n-type diffusion layer 101. On the n-type diffusion layer 101, an antireflection film 102 such as a SiN _x film is formed. On the back surface side of the p-type semiconductor substrate 100, an aluminum paste is applied to almost the entire surface, and the BSF layer 103 and the aluminum electrode 104 are formed by sintering. On the back surface, thick electrodes 106 and 306 called bus bars for current collection are formed by applying and baking a conductive paste containing silver or the like. On the other hand, finger electrodes 207 for collecting current and thick electrodes called bus bar electrodes 105 and 205 formed to collect current from the finger electrodes are arranged in a comb shape on the light receiving surface side so as to intersect at substantially right angles. The

  And when manufacturing this kind of solar cell, as a method of electrode formation, there are a vapor deposition method, a plating method, a printing method, a drawing method, etc., but the surface finger electrode 207 is easy to form and low cost. For reasons such as these, generally, it is formed by a printing / firing method as shown below. That is, as the surface electrode material, generally a conductive paste containing silver powder, glass frit, organic vehicle, and organic solvent as main components is used, and after applying this conductive paste by a screen printing method or the like. The surface electrode is formed by high-temperature sintering in a firing furnace.

Screen printing is the following method.
First, the screen plate making used in the screen printing method has a substantially rectangular shape by covering a mesh material in which warp yarns and weft yarns orthogonal to each other are knitted with a photosensitive emulsion and removing the emulsion partly by exposure. These pattern holes are formed. This screen plate making is placed on the substrate, and a printing paste (ink) placed on the screen plate making is spread on the pattern, and a flexible spatula called a printing squeegee is applied to an appropriate squeegee hardness (60 to 80 degrees), By moving at a squeegee angle (60 to 80 degrees), a pressure (printing pressure) (0.2 to 0.5 MPa), and a printing speed (20 to 100 mm / sec), it is attached to the substrate through the pattern holes, Further, the printing paste adhered to the printing material is dried to form a printing pattern.

  At this time, the printing paste does not adhere to the portion corresponding to the warp and weft in the pattern hole immediately after the printing paste falls through the opening where the mesh material in the pattern hole does not exist and adheres to the substrate. However, since the flow of the printing paste adhering to the portion corresponding to the opening portion thereafter occurs, a continuous printing pattern with a uniform thickness is obtained.

  Thus, the screen printing method uses the same pattern as the pattern holes formed in the screen plate making by transferring the printing paste filled into the pattern openings on the screen plate making to the printed material by the movement of the printing squeegee. Is formed on the substrate.

  The contact resistance (contact resistance) between the surface finger electrode 207 and the Si substrate 100 formed by such a method and the wiring resistance of the electrode have a great influence on the conversion efficiency of the solar cell, and high efficiency (low cell series resistance, In order to obtain a high fill factor FF (curve factor)), the contact resistance and the wiring resistance value of the surface finger electrode 207 are required to be sufficiently low.

  In addition, the electrode area must be reduced so that as much light as possible can be captured on the light receiving surface. In order to improve the short circuit current (Jsc) while maintaining the FF, the finger electrode must be thin and have a large cross-sectional area in the longitudinal section, that is, a high aspect ratio finger electrode must be formed.

  Among the methods for forming the electrodes of the solar cell, as a method for forming a high aspect ratio, ultrafine wire, a method of forming a groove in a cell and filling a paste (Patent Document 1: JP-A-2006-54374), A printing method using an inkjet method is disclosed. However, the former is not preferable because it includes a step of forming a groove in the substrate and may damage the substrate. The latter ink-jet method is a method suitable for forming a thin line because it applies a pressure to eject droplets from a thin nozzle, but it is difficult to increase the height.

  On the other hand, the screen printing method is easy to create a printing pattern, can minimize damage to the substrate by adjusting the printing pressure, has a high working speed per cell, is low cost, and has excellent productivity. It is. By using a conductive paste having high thixotropy, it is possible to form an electrode having a high aspect ratio while maintaining the shape after being transferred.

  As described above, screen printing is a method that is cheaper than other printing methods and suitable for forming electrodes with a high aspect ratio.

However, in the case of screen printing, the printing direction (traveling direction of the printing squeegee) is an element that promotes disconnection.
FIG. 4 shows an opening of a conventional screen plate making, 401 is a bus bar electrode opening and its width is Wb, 402 is a finger electrode opening and its width is Wf. Reference numeral 403 denotes a connecting portion between the bus bar electrode opening and the finger electrode opening, and the opening width Wc of the opening connecting portion 403 of this conventional screen plate making is formed to be the same as the finger electrode opening width Wf. Generally, in order to prevent disconnection of the finger electrode, the printing direction and the finger electrode are substantially parallel, while the printing direction and the bus bar electrode are substantially vertical. At this time, the width of the connection portion between the bus bar electrode and the finger electrode on the printing start side of the electrode after printing becomes very narrow (508 in FIG. 5). This is particularly noticeable in the case of fine line printing. This is because, at the connection portion between the finger electrode and the bus bar electrode, the printing squeegee falls in the bus bar electrode opening, and the amount of paste applied to the connection portion is reduced. On the other hand, the width of the connection portion between the bus bar electrode and the finger electrode on the printing end side tends to increase due to the large amount of paste applied (509 in FIG. 5).

  In order to solve the above problem, a method of widening the width of the connection portion between the bus bar electrode and the finger electrode is disclosed (Patent Document 2: JP 2009-272405 A). However, when this method is used, the connection portion between the bus bar electrode and the finger electrode is excessively thick, so that it is blurred or fooled. For this reason, there is a problem that the shadow loss increases and the characteristics are lowered. Naturally, a solar cell is a device used under sunlight, and unlike other semiconductor devices, it has many opportunities to be exposed to the public. Therefore, not only the characteristics but also the appearance of solar cells are very important elements. The proposal has a problem in that the connection between the bus bar electrode and the finger electrode becomes thick, so that the thickness of the finger electrode becomes discontinuous and the aesthetic appearance is impaired.

  Conventionally, JP 2005-150540 A (Patent Document 3) describes a screen plate making with an opening width larger than the entire surface on one side, but the screen plate making of Patent Document 3 has a width of 100 μm or more. This is intended for thick electrodes, and if the opening width of the entire surface on one side is increased with such a thick line, a large amount of expensive paste is used, which increases the cost.

JP 2006-54374 A JP 2009-272405 A JP-A-2005-150540

  The present invention has been made in view of the above problems, and an object of the present invention is to produce a solar cell with high conversion efficiency at low cost by forming an electrode having a high aspect ratio and low resistance. And a screen plate making used in the method.

In order to solve the above-mentioned problems, the present invention provides a screen plate for use in manufacturing a solar cell in which a conductive paste is printed to form bus bar electrodes and finger electrodes orthogonal to the bus bar electrodes. The opening and the finger electrode opening are connected at a plurality of locations, the opening width of half or less on one side of the connecting portion is larger than the finger electrode opening width, and the remaining connecting portion has the same opening width as the finger electrode width. A screen plate making is provided (claim 1).
In this case, all of the bus bar electrode openings on one side of the printing start side have finger connection portions having an opening width larger than the finger electrode opening width, and the remaining connection portions have the same opening width as the finger electrode width. It is preferable that it is present (claim 2).
In this case, it is preferable that the opening width of the finger electrode of the screen plate making is 80 μm or less. If the opening width is generally over 80 to 100 μm, the disconnection as described above rarely occurs. This technique is effective for fine lines with an opening width of 80 μm or less. If the opening width is 50 to 80 μm, particularly 60 to 70 μm, this method is very effective.
Further, if the width of the finger electrode after printing is not equal to the width of the finger electrode after printing, the aesthetic appearance of the solar cell is impaired, the appearance is deteriorated, and the yield is lowered. Therefore, it is preferable that the opening width of the bus bar electrode and the finger electrode connecting portion of the screen making is larger than 1.0 times and 1.5 times or less of the finger electrode opening width.

The present invention further relates to a method for manufacturing a solar cell in which a conductive paste is printed using the screen plate making to form bus bar electrodes and finger electrodes, and the printing is performed along the longitudinal direction of the finger electrodes. A method for manufacturing a solar cell is provided (claim 5).
In the method for manufacturing a solar cell that is printed using the screen plate making having the above-described features, it is desirable that the printing direction is substantially parallel to the finger electrodes in order to prevent disconnection of the finger electrodes.
When the printing direction is substantially parallel to the finger electrode, the width of the connection portion between the bus bar electrode on the printing start side and the finger electrode is often narrower with respect to the finger electrode after printing. Therefore, it is desirable that at least the opening width of the connection portion between the bus bar electrode and the finger electrode on the printing start side is larger than the finger electrode.

  If the screen plate-making of this invention is used, it will not increase a solar cell manufacturing cost or a shadow loss, and also can make the connection part of a finger electrode and a bus-bar electrode into the same straight line as a finger electrode. Therefore, disconnection of the connection part of a bus-bar electrode and a finger electrode can be prevented, without impairing the beauty | look of a solar cell.

It is sectional drawing of the electrode of a common solar cell. It is a figure which shows the surface shape of a common solar cell. It is a figure which shows the back surface shape of a common solar cell. It is an opening part enlarged view of the conventional screen plate making. It is an enlarged view after printing by the conventional screen plate making. It is an enlarged view of an opening of a comparative screen plate making. It is an enlarged view after printing by comparative screen plate making. It is an opening part enlarged view of the screen platemaking of this invention. It is an enlarged view after printing by screen plate making of the present invention.

  The screen plate making according to the present invention is used for the production of a solar cell, and an example of the method for producing the solar cell of the present invention will be described below. 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 mass% hydrogen peroxide solution may be mixed in the 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. In general silicon solar cells, it is necessary to form a pn junction only on the light-receiving surface, and in order to achieve this, diffusion is performed in a state where two substrates are overlapped, or a SiO ₂ film or the like is formed on the back surface before diffusion. It is necessary to devise such that a pn junction cannot be formed on the back surface by forming a SiN _x film or the like as a diffusion mask. After diffusion, the glass formed on the surface is removed with hydrofluoric acid or the like.

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. 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.

  Next, a back electrode is formed by a screen printing method. A paste in which silver powder and glass frit are mixed with an organic binder is screen-printed on the back surface of the substrate in a bus bar shape, and then a paste in which aluminum powder is mixed with an organic binder is screen-printed in a region other than the bus bar. After printing, the back electrode is formed by baking at a temperature of 700 to 800 ° C. for 5 to 30 minutes. The back electrode is preferably formed by a printing method, but can also be formed by a vapor deposition method, a sputtering method, or the like.

  Next, the surface electrode is formed by a screen printing method. Screen engraving having a comb-shaped printing pattern designed on the surface of the substrate with a paste prepared by mixing silver powder, glass frit and an organic binder with a finger electrode width of 30 to 80 μm and a finger electrode interval of 0.5 to 4.0 mm. Use to print. After printing, the printed paste is dried and baked at a temperature of 700 to 800 ° C. for 5 to 30 minutes to form a surface electrode.

  Here, in the screen plate making of the present invention, as shown in FIG. 8, the width Wc of the opening 808 on the printing start side connecting portion is set to the bus bar electrode at the connecting portion between the bus bar electrode opening 401 and the finger electrode opening 402. The width is narrower than the opening width Wb and wider than the finger electrode opening width Wf, and the width of the opening 809 of the print end side connecting portion is made the same as the finger electrode opening width (FIG. 8). In this case, it is preferable that 1 to 50%, particularly 15 to 30%, of the print start side connection portion is formed wider than the finger electrode opening width Wf.

  In this case, the width Wc of the opening 608 of the printing start side connection portion and the width of the opening 609 of the printing end side connection portion shown in FIG. 6 are each made narrower than the bus bar electrode opening width Wb and wider than the finger electrode opening width Wf. When screen plate making is used, as shown in FIG. 7, the width of the finger electrode 708 existing in the print start side connection area is substantially equal to the original width Wf of the finger electrode 207, and the finger electrode on the print start side is the bus bar electrode. On the other hand, the finger electrode 709 existing in the printing end side connection region is printed and formed in a state wider than the original width Wf of the finger electrode 207. Further, when the screen plate making shown in FIG. 8 is used, as shown in FIG. 9, the finger electrode 207 has the width of the finger electrodes 908 and 909 existing in the connection area on the printing start side and the printing end side. A linear finger electrode 207 having the same width as that of the width Wf is printed.

  In the case of printing finger electrodes that are generally used, the opening width of the screen plate making is more than 80 to 100 μm. In this case, since the finger electrode is sufficiently thick and can be printed thickly, the disconnection as described above rarely occurs. However, when the opening width is 80 μm or less, the difference in film thickness between the bus bar electrode and the finger electrode becomes large, and disconnection occurs due to the difference in heat shrinkage.

  Therefore, in the present invention, when printing a fine line pattern having an opening width of 80 μm or less, preferably 50 to 80 μm, particularly 60 to 70 μm, the opening width of the connection portion between the bus bar electrode and the finger electrode of the screen making is determined as the bus bar electrode. Printing is performed using a screen plate making that is smaller and larger than the finger electrodes.

  If the opening width of the connection between the bus bar electrode and finger electrode at the start and end of printing is made extremely wide, the width after printing becomes thick, and the finger thickness becomes discontinuous, which impairs the aesthetics of the cell. Therefore, there is a risk that the appearance will be poor. In addition, the increase in the electrode area increases the shadow loss and degrades the cell characteristics. That is, the width of the finger electrode after printing must be equal to the width of the finger electrode after printing. Therefore, the opening width of the connection part is preferably larger than the opening width of the finger electrode and not more than 1.5 times.

Regarding the printing direction of screen printing, in order to prevent disconnection of the finger electrode, the printing direction is preferably substantially parallel to the finger electrode.
In the case of the printing direction, since the printing direction is perpendicular to the bus bar electrode, the printing squeegee falls on the bus bar electrode, and the amount of application at the connection portion between the bus bar electrode on the printing start side and the finger electrode is reduced. , Make it easier to invite disconnection. For this reason, it is preferable to perform printing using a screen plate-making method in which at least the opening width of the connection portion between the bus bar electrode and the finger electrode on the printing start side is smaller than the bus bar electrode and larger than the finger electrode.

  After forming an electrode using the above methods, it is sintered by heat treatment at 700 to 800 ° C. for 5 to 30 minutes in the atmosphere. Thereby, a bus-bar electrode and a finger electrode are formed. The back electrode and the light-receiving surface electrode can be baked at the same time.

  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.

In order to confirm the effectiveness of the present invention, the following steps were performed on 30 semiconductor substrates to produce solar cells.
The printing pattern is 80 μm (example), 100 μm (comparison) with respect to the conventional pattern A (finger electrode opening (Wf) 60 μm: comparative example), with the opening (Wc) of the bus bar electrode and the finger electrode on the printing start side connecting portion. Example) Screen plate making was prepared. The opening at the printing end side connection portion was the same as the finger electrode opening (Wf). All the opening widths (Wb) of the bus bar electrodes were unified to 2 mm.

First, as shown in FIG. 1, a boron-doped {100} p-type as-cut silicon substrate 100 having a 15 cm square, a thickness of 250 μm, and a specific resistance of 2.0 Ω · cm is prepared, and a damage layer is formed with a concentrated potassium hydroxide aqueous solution. Removal, texture formation, emitter layer 101 heat-treated at 850 ° C. in a phosphorus oxychloride atmosphere was formed, phosphorus glass was removed with hydrofluoric acid, washed and dried. Next, a SiN _x film 102 is formed using a plasma CVD apparatus, and a paste obtained by mixing silver powder and glass frit with an organic binder is screen-printed in a bus bar shape on the back surface (106), and then aluminum powder is coated with an organic binder. The mixed paste was screen printed in an area other than the bus bar (104). A semiconductor substrate having a back electrode formed by drying the organic solvent was produced.

  Next, on this semiconductor substrate, a conductive paste containing silver powder, glass frit, an organic vehicle, and an organic solvent as main components and a metal oxide as an additive is applied to a screen plate having the above printing pattern. Was applied on the antireflection film formed on the semiconductor substrate at a squeegee hardness of 70 degrees, a squeegee angle of 70 degrees, a printing pressure of 0.3 MPa, and a printing speed of 50 mm / sec. After printing, the organic solvent was dried in a clean oven at 150 ° C. and then fired in an air atmosphere at 800 ° C.

The 30 solar cells thus fabricated were evaluated by electrode observation with an optical microscope and a solar simulator (in a 25 ° C. atmosphere, irradiation intensity: 1 kW / m ² , spectrum: AM1.5 global). In the appearance inspection, the finger width after printing and the width of the connection portion were observed with an optical microscope, and when the difference was ± 20% or more of the finger electrode width, the appearance was regarded as defective. Moreover, the presence or absence of the disconnection was visually inspected with a microscope. The average results of Example 1 are shown in Table 1.

  The width of the connection portion between the bus bar electrode and the finger electrode was the same as that of the finger electrode in B compared with the standard condition A, but was thicker than the finger electrode in C, resulting in poor appearance. Moreover, when the disconnection of the connection part of a bus-bar electrode and a finger electrode was confirmed with the microscope, it was not confirmed by BC.

  The short-circuit current decreased at C where the opening of the connection portion between the bus bar electrode and the finger electrode was large. This is due to the shadow loss due to the increase in width. On the other hand, the curve factor showed 75.2%, which is about 2% higher than the level A where disconnection occurred.

  Disconnection could be avoided by making the opening width of the connection portion between the bus bar electrode and the finger electrode larger than the finger electrode. However, when the opening width is 1.6 times or more, the width is wide and the appearance is poor, or the short-circuit current is lowered and the conversion efficiency is lowered.

  In the conventional method, disconnection of the connection portion between the bus bar electrode and the finger electrode occurred. However, if the screen plate making of the present invention is used, an electrode having a high aspect ratio can be formed without disconnection without increasing the number of steps.

100 p-type semiconductor substrate 101 n-type diffusion layer 102 antireflection film (SiN _x film)
103 BSF layer 104 Aluminum electrode 105, 205 Front surface bus bar 106, 306 Back surface bus bar 207 Finger electrode 403, 608, 808 Opening of printing start side connection portion 609, 809 in screen plate making Opening 708, 908 of connecting end portion in screen plate making Finger electrodes 709 and 909 existing in the print start side connection area A finger electrode existing in the print end side connection area

Claims (5)

  A screen plate for use in the production of a solar cell in which a conductive paste is printed to form a bus bar electrode and a finger electrode perpendicular to the bus bar electrode. The screen plate is connected to the bus bar electrode opening and the finger electrode opening at a plurality of locations. The screen plate making is characterized in that the opening width of half or less of the connecting portions is larger than the finger electrode opening width, and the remaining connecting portions have the same opening width as the finger electrode width.   It has a finger connection part having an opening width larger than the finger electrode opening width on one side of the printing start side of the bus bar electrode opening, and the opening widths of the remaining connection parts are the same as the finger electrode width. The screen plate making according to claim 1, wherein   3. The screen plate making according to claim 1, wherein the opening width of the finger electrode of the screen plate making is 80 [mu] m or less.   The opening width of the bus bar electrode and the finger electrode connecting portion of the screen plate making is larger than 1.0 times and 1.5 times or less of the finger electrode opening width. Screen plate making as described.   It is a manufacturing method of the solar cell which prints an electrically conductive paste using the screen platemaking of any one of Claim 1 thru | or 4, and forms a bus-bar electrode and a finger electrode, Comprising: Along the longitudinal direction of a finger electrode A method for producing a solar cell, wherein the printing is performed.

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