JP2010284853A - Squeegee for screen printing, and manufacturing method of photovoltaic cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a squeegee allowing uniform screen printing within a surface, and allowing screen printing causing no line breaking at a central part in the longitudinal direction nor printing blurring, with simple structure. <P>SOLUTION: Both ends in the longitudinal direction are cut off to form notch parts in a contact face of the squeegee 11 with a screen plate. The center in the longitudinal direction of the squeegee 11 is protruded in relation to the screen plate from both ends. Pressing pressure to the screen plate by the squeegee 11 is more easily applied to the central part than to both ends in the longitudinal direction, allowing uniform printing in the longitudinal direction. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a screen printing squeegee used for forming a light receiving surface electrode of a solar battery cell.

  Screen printing is widely used as a technology for forming thick films, thin films, and patterns in the manufacture of various products in many fields related to electronic products such as ceramic parts and flat panel displays.

  Conventionally, in screen printing, a squeegee that can be attached to a squeegee holder of a screen printing apparatus is used. Place the desired ink or paste on a mesh-like screen plate that is tensioned tightly on a metal frame such as aluminum, and move on the screen plate while applying pressure with the squeegee installed in the squeegee holder. Let By printing a desired ink or paste on the surface of the printing material previously set on the printing stage movable under the screen plate, printing on the printing material is performed.

  The printed pattern shape transferred at this time is a mask pattern prepared in advance using a photosensitive emulsion or the like on the printing material side of the mesh screen plate. The portion where the ink or paste is to be transferred must be opened as an opening. When extruding ink or paste using a squeegee in the actual printing process, the ink or paste is transferred to the printing material through the opening of the mask. By creating a desired print pattern shape in advance on a screen mask, the pattern shape can be easily printed and transferred onto a substrate.

  The material and shape of the squeegee used in screen printing can be selected depending on the application. Examples of the main squeegee material include metals, rubbers, silicone rubbers, polyurethane resins, and the like, and generally those having high solvent resistance are used. The main squeegee shapes include a flat shape, a square shape, a sword tip shape, a micro squeegee, and the like, and it is necessary to select a tool that is considered to be optimal depending on the shape of the squeegee holder used and the desired finish.

  Here, in a photovoltaic cell, an electrode is formed using screen printing. The structure of a general solar cell is shown in FIG. 1, and the manufacturing process is shown in FIG.

In the case of manufacturing a crystalline silicon solar battery cell, the p-type silicon substrate 1 is etched, and then the n-type diffusion layer 2 is diffused on one side of the light-receiving surface by n-type dopant, and the surface reflectance is reduced thereon. The antireflection film 3 is formed. A part of the back surface is printed with silver paste by a screen printing method and dried at about 150 to 200 ° C., and then an aluminum paste is printed on almost the entire uncoated portion by a screen printing method and about 150 to 200 ° C. Dry with. At this time, the aluminum paste is printed so as to overlap a part of the dried silver paste. Further, a silver paste is printed in a pattern on the antireflection film 3 on the light receiving surface side by a screen printing method, dried at about 150 to 200 ° C., and then fired at about 700 to 750 ° C. A silver electrode 4 and an aluminum electrode 5 are formed on the back surface of the cell, and a light receiving surface electrode (silver electrode) 6 is formed on the light receiving surface. In addition, when an aluminum paste acts as a p-type dopant during firing, a p ⁺ -type diffusion layer 7 is also formed on the back side of the substrate, which greatly contributes to improvement of electrical characteristics.

  In the above-described electrode forming step of the solar battery cell, a screen printing method having high versatility and excellent productivity is often employed. Electrodes are formed on the light receiving surface and the back surface by printing a silver paste or an aluminum paste on a silicon substrate through a screen mask produced based on the printed pattern shape. In this electrode formation process, it is important that the printed pattern shape is basically transferred to the silicon substrate with high accuracy and uniformity, but this is necessary due to differences in the printed pattern shape such as fine line printing and solid printing. Paste characteristics such as viscoelasticity and points to be noted change. For example, in the printing process of an aluminum paste used on the back surface of a solar battery cell, solid printing is performed on a large area. Therefore, the in-plane film thickness uniformity of printing (reduction of printing unevenness) becomes important. On the other hand, in the printing process of the silver paste used on the light receiving surface, in order to sufficiently perform the function as a collecting electrode, the wire breakage occurred during printing with the sub-electrode having a fine line shape in the entire electrode pattern. It is important that electrode formation defects such as print fading do not occur.

  In recent years, in the solar cell industry that has attracted particular attention, there is a demand for a technique for improving its electrical characteristics without sacrificing long-term reliability. In addition, with the recent increase in solar cell production, intensifying sales competition has emerged in a very prominent form, and it is desired to provide products with excellent cost performance as well as electrical characteristics to the market. Yes.

  As described above, a screen printing method having high versatility and excellent productivity is generally used as a method for forming an electrode of a solar battery cell. By introducing the electrode forming step by the screen printing method, it is possible to produce solar cells in a large amount and at a low cost only by a simple process.

  However, the formation of the light receiving surface electrode of the solar battery cell causes a loss of the light receiving area (shadow loss), and the miniaturization of the light receiving surface electrode is progressing year by year for the purpose of reducing this loss. Furthermore, since the silicon wafer is made thinner to reduce the material cost, the screen printing conditions in the solar cell electrode forming process are inevitably adjusted within a limited range. In particular, regarding the in-plane uniformity of the light receiving surface electrode on the silicon substrate, the effect of the miniaturization of the light receiving surface electrode on the finished state of the electrode after printing is very large, and the electrode paste cannot sufficiently pass through the printing pattern of the screen plate. Electrode formation defects such as print fading, electrode defects, and disconnection due to the above are easily caused by thinning. The inventor found from various examination results that the electrode formation failure tends to occur particularly in a form corresponding to a portion where the central portion in the longitudinal direction of the squeegee is difficult to be pressed.

  As shown in FIG. 3, on the light receiving surface 8 of the solar battery cell, a main grid 9 of light receiving surface electrodes is formed in a direction orthogonal to the traveling direction of the squeegee. Note that the sub-electrode of the light-receiving surface electrode is formed in parallel with the traveling direction of the squeegee. Here, the specific region 10 in which defective electrode formation is likely to occur is located in the center of the main grid 9.

  In addition, since the light-receiving surface electrode mainly functions as a collector electrode, the influence of poor electrode formation on the light-receiving surface due to printing fading, electrode defects, disconnection, or the like has a great influence on the electrical characteristics of the solar cells. In particular, the fill factor (FF), which is a factor that is influenced by the resistance component of the electrode, is greatly reduced, leading to a large loss of electrical characteristics.

  In order to prevent print fading, electrode defects, wire breakage, etc. in the fine line portion of such a miniaturized light receiving surface electrode, the paste characteristics such as viscoelasticity possessed by the electrode paste itself are improved. It is considered that the best way is to make the printed pattern of the mask more smoothly transmitted. However, it is not easy to improve the composition of the electrode paste used to a desired state, and there is a high possibility that the development period will be prolonged. Although it is conceivable that the electrode forming process is a method other than screen printing, an electrode forming technique that is more versatile and more productive than screen printing has not yet been established. As other methods, changing the printing conditions during screen printing, for example, slowing the squeegee moving speed during printing is one of the workarounds for poor electrode formation, but when the squeegee moving speed becomes slow, Inevitably, the time required for the printing process becomes longer, resulting in a reduction in production tact.

  For the above reasons, manufacturing solar cells that overcome the problems of print defects, electrode defects and wire breakage that tend to occur due to thinning, and provide stable output with little extra loss. In order to do so, it was necessary to establish some kind of improvement method.

  For example, Patent Document 1 describes that a cutout portion is provided along the longitudinal direction of a squeegee as a screen printing squeegee that enables printing with a uniform ink thickness without fading.

JP 2004-130735 A

  It takes time and effort to produce a squeegee with the cutouts. Therefore, the present invention enables the formation of an electrode in a solar battery cell that enables uniform screen printing in a plane with a simpler structure and enables screen printing without causing disconnection or blurring in a specific region. The purpose is to provide a squeegee for screen printing suitable for the above.

  In the squeegee for screen printing according to the present invention, the center in the longitudinal direction of the squeegee protrudes toward the screen plate from both ends at the contact surface contacting the screen plate on which the printing pattern is formed.

  In this squeegee, the center in the longitudinal direction is more closely attached to the screen plate than both ends, and it is easier to press in the center part in the longitudinal direction than in the conventional squeegee. The extrusion amount of the ink or paste that was non-uniform becomes uniform in the longitudinal direction. For this reason, it is possible to eliminate electrode fading and electrode formation defects such as electrode defects due to printing non-uniformity occurring in the central portion in the longitudinal direction.

  Both ends in the longitudinal direction of the contact surface of the squeegee are removed to form a notch, and the notch is formed at a position facing a region outside the print pattern located near the center of the screen plate.

  By removing both ends in the longitudinal direction of the squeegee, the center in the longitudinal direction protrudes toward the screen plate from both ends. When the squeegee is pressed against the screen plate, the center portion of the contact surface is in close contact with the screen plate, but a gap is formed between the screen plate and the cutout portion. Since a printing pattern is not formed on the screen plate facing this notch, printing is not affected even if there is a gap. Further, due to the presence of the notch, when the squeegee moves, the friction between both ends of the squeegee and the screen plate is reduced, and the screen plate is hardly damaged.

  The notch is formed by chamfering the end in the longitudinal direction. The shape of the notch is a curved surface such as an inclined surface, an arc, an ellipse, or a sector. The corners at both ends of the squeegee are eliminated and the screen plate can be prevented from being damaged.

  The electrode of the solar battery cell is formed by applying a conductive paste to the semiconductor substrate using the squeegee. In particular, in the light receiving surface electrode of the solar battery cell, an electrode parallel to the traveling direction of the squeegee and an electrode parallel to the longitudinal direction are formed. When such a light receiving surface electrode is formed, an electrode parallel to the traveling direction can be uniformly formed by screen printing using the squeegee.

  According to the present invention, a squeegee with a simple structure in which both ends in the longitudinal direction are removed enables uniform ink and paste extrusion in the longitudinal direction, thereby preventing printing defects such as print fading, electrode defects, and disconnection. it can. In addition, friction between the squeegee at both ends and the screen plate during movement of the squeegee can be reduced, the screen plate can be prevented from being damaged, and the life of the screen plate can be extended.

Cross-sectional view of solar cell of the present invention Diagram showing the manufacturing process of solar cells The figure which shows the generation | occurrence | production area | region of the electrode formation defect in a photovoltaic cell Cross-sectional view of solar cells for each manufacturing process Perspective view of squeegee Figure showing a squeegee with a conventional structure The figure which shows the squeegee of this invention The figure which shows a mode when the conventional squeegee contacts the screen plate The figure which shows a mode when the squeegee of this invention contacts the screen plate. The figure which shows evaluation of the screen printing using this invention and the conventional squeegee The figure which shows the squeegee of other forms

  The manufacturing process of the photovoltaic cell of this embodiment is shown in FIG. First, a surface resistance value of about 50Ω / □ is obtained by thermal diffusion (about 800 to 900 ° C.) using phosphorus as a dopant on one side surface of an acid-etched p-type polycrystalline silicon substrate 1 having a thickness of 180 μm and a 156 mm square size. An n-type diffusion layer 2 having

After cleaning the substrate surface, a silicon nitride film having a thickness of about 70 to 100 nm is formed as an antireflection film 3 on the surface on which the n-type diffusion layer 2 is formed by plasma CVD. Next, a silver electrode 4 is formed by printing a silver paste for a back electrode as a back electrode on a part of the back surface of the substrate by a screen printing method and drying at about 150 ° C. Further, the aluminum electrode 5 is formed by printing the substantially entire surface by the screen printing method while making the aluminum paste overlap the silver electrode 4 only on a part of the same surface, and drying again at about 150 ° C. Thereafter, the silver paste for the light receiving surface electrode is printed on a predetermined portion of the light receiving surface by a screen printing method and dried at about 150 ° C., thereby forming the light receiving surface electrode 6. Subsequently, the p ⁺ diffusion layer 7 is formed by firing at about 740 ° C. in the air, and the electrodes formed on the front and back surfaces of the solar battery cell are fired. In this way, the solar battery cell shown in FIG. 1 is manufactured.

  In the light-receiving surface electrode 6, two parallel main grids and sub-electrodes made up of a number of thin lines orthogonal to the main grid are formed. A printing pattern is formed on the screen plate in accordance with such a light receiving surface electrode. The traveling direction of the squeegee is a direction parallel to the sub-electrode.

  In the screen printing for forming the light receiving surface electrode 6 and the back surface electrode, a screen plate on which a predetermined printing pattern is formed according to the shape of each electrode is placed above the silicon substrate 1 and moved while pressing the squeegee. The metal paste is printed on the silicon substrate 1 according to the print pattern.

  The micro squeegee used for this screen printing is shown in FIGS. The squeegee 11 is attached to a squeegee holder 14. And in the contact surface 12 which contacts the screen plate of the squeegee 11 so that the press with respect to the screen plate in the center part of the longitudinal direction of the squeegee 11 goes up, the edge part 13 of the longitudinal direction (long side direction) of the squeegee 11 is left and right. Is removed to form a notch A. That is, by polishing the broken-line circle portion 15 shown in FIG. 6 with a commercially available squeegee grinder, a part of the squeegee 11 is scraped off, and a notch A that is recessed with respect to the screen plate from the center in the longitudinal direction is formed. The As shown in FIG. 7, in the inclined surface 12 of the squeegee 11, the center in the longitudinal direction of the squeegee 11 protrudes toward the screen plate from both ends, and the notch A becomes an inclined surface from the center toward the end. The height difference between the vertex 16 at the center of the squeegee 11 in the longitudinal direction and the deepest removed portion 17 at the end is 1.5 mm. In addition, it is good to chamfer the corner | angular part of the both ends of the contact surface 12 of the squeegee 11. FIG.

  Thus, the squeegee 11 has a simple structure in which both end portions in the longitudinal direction are thinner than the center portion. And since the process of the squeegee 11 only scrapes off the both ends of the contact surface 12, it can be performed easily.

  Here, screen printing was carried out using the squeegee 11 of the present invention as shown in FIG. 7 and the conventional squeegee 11 as shown in FIG. 6 having a flat contact surface and a right end. FIG. 9 shows the contact state between the screen plate and the squeegee during printing when a conventional micro squeegee is used, and FIG. 10 shows the contact state between the screen plate and the squeegee during printing when the squeegee of the present invention is used. Show. In the figure, 19 is a screen plate mesh portion, and 20 is a metal frame portion.

  And the finish of the electrode when using the same printing conditions and the same electrode paste, especially the sub-electrode on the light-receiving surface side, which is the fine line printing location, print fading or electrode loss in a specific area (longitudinal center) Then, it was compared whether or not disconnection occurred. As the degree of print fading, relative evaluation was carried out by sequentially indicating from the state where disconnection or print fading did not occur as ◯ → Δ → ×. The result is shown in FIG.

  When the conventional squeegee 11 is used, print fading or disconnection occurs in a specific area near the location where the central portion of the squeegee 11 passes during movement. When the squeegee 11 of the present invention was used, print fading or disconnection did not occur in a specific area. Further, compared to the case where the conventional squeegee 11 is used, the use of the squeegee 11 of the present invention makes it easier to press at the center portion of the squeegee. As a result, the amount of electrode paste pushed out at a location where the electrode paste was not sufficiently permeated was larger than when the conventional squeegee 11 was used, so the amount of paste printed onto the silicon substrate 1 as the printing object increased. .

  And since electrode formation defects, such as a disconnection and printing fading, did not arise, the resistance of the electrode itself was reduced and the average maximum output (especially FF) of a photovoltaic cell improved. Furthermore, the uniformity in the printing surface of the paste filling amount in the gaps of the screen mesh stretched on the screen plate was increased, and as a result, the uniformity in the printing surface of the electrode paste printed on the substrate was improved. Therefore, the variation in the electrical characteristics of the solar cells was suppressed, and the non-defective product rate indicating the probability of satisfying the electrical characteristics exceeding a certain standard was also improved.

  Further, by removing both ends in the longitudinal direction of the squeegee 11, it becomes difficult for local pressing to the screen plate at both ends 18 in the longitudinal direction of the squeegee 11, and the mesh stretched on the screen plate during movement of the squeegee. The load on the mesh portion 19 due to friction with the portion 19 is reduced.

  In the case of the present invention, the thickness of both end portions 18 in the longitudinal direction of the squeegee 11 in contact with the screen plate is thinner than the conventional one, and the folding angle of the mesh portion 19 of the screen plate at both end portions 18 is as follows. The present invention is smaller than the conventional one. Therefore, in the present invention, the tension at the center portion of the screen plate is larger than that of the conventional one. In the central portion in the longitudinal direction, the squeegee 11 is in close contact with the screen plate, and the substantial pressure applied to the substrate is increased.

  In addition, when the squeegee 11 of the present invention is used, local pressure is less likely to be applied to both ends 18 in the longitudinal direction of the squeegee 11, thereby reducing the load on the mesh portion 19 due to friction and breaking the mesh. Is less likely to occur than before. Thereby, the lifetime of the screen plate can be extended.

  By the way, in the squeegee 11 shown in FIG. 7, the notch A has reached the vicinity of the center in the longitudinal direction, but as shown in FIG. 11, the notch A is formed so as not to cover the printing pattern formed on the screen plate. May be. In the longitudinal direction, the print pattern is formed corresponding to the main grid of the light receiving surface electrode 6. The position of the notch A is at a position facing a region outside the print pattern located near the center of the screen plate. Of the contact surface 12 of the squeegee 11, the region facing the print pattern is a flat surface, and the notch A is an inclined surface.

  By adopting such a squeegee structure, the flat contact surface 12 of the squeegee 11 comes into contact with the printing pattern of the screen plate and presses the screen plate evenly. Therefore, the amount of paste extruded onto the silicon substrate 1 is larger and more uniform than before, and the in-plane uniformity of the printed paste is improved. Further, at both ends in the longitudinal direction of the squeegee 11, a gap is formed between the squeegee 11 and the screen plate, and the load on the screen plate can be reduced. Since the print pattern is not formed in the area where the gap is formed, the print is not affected even if there is a gap.

  In addition, this invention is not limited to the said embodiment, Of course, many corrections and changes can be added to the said embodiment within the scope of the present invention. The shape of the notch is not limited to a flat surface, but may be an arc, an ellipse, a sector, or a trapezoid.

1 Silicon substrate 4 Silver electrode 5 Aluminum electrode 6 Light receiving surface electrode 11 Squeegee 12 Contact surface A Notch

Claims (4)

A squeegee for screen printing, characterized in that the center in the longitudinal direction of the squeegee protrudes toward the screen plate from both ends on the contact surface contacting the screen plate on which the printing pattern is formed. The both ends in the longitudinal direction of the contact surface are removed to form a notch, and the notch is formed at a position facing a region outside the print pattern located near the center of the screen plate. Item 2. A squeegee for screen printing according to item 1. 3. The squeegee for screen printing according to claim 2, wherein the cutout portion is formed by chamfering an end portion in a longitudinal direction. The manufacturing method of the photovoltaic cell characterized by apply | coating an electrically conductive paste to a semiconductor substrate using the squeegee in any one of Claims 1-3.

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