JP2011138922A - Solar cell and screen printing plate for manufacturing solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell which has sufficient adhesive strength in connecting a lead wire to a back silver electrode of the solar cell, characteristics of which do not degrade, and which can be produced in an inexpensive manner; and to provide a screen printing plate used to manufacture the solar cell. <P>SOLUTION: The solar cell includes a bus bar electrode 106 at least on its rear surface, wherein: a line width of the bus bar electrode is 2.0 mm or more but less than 2.5 mm, the maximum thickness is 9.0 μm or larger, and a proportion of the minimum thickness to the maximum thickness of the electrode is 0-80%; the line width of the bus bar electrode is 2.5 mm ore more but less than 3.0 mm, the maximum thickness is 6.6 μm or larger, and a proportion of the minimum thickness to the maximum thickness of the electrode is 0-80%; or the line width of the bus bar electrode is 3.0-5.0 mm, the maximum thickness is 4.0 μm or larger, and a proportion of the minimum thickness to the maximum thickness of the electrode is 30-80%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a low-cost and high-efficiency solar cell and a screen plate making used for the production.

As shown in FIGS. 1 to 3, a general solar battery cell has a PN junction 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. Is forming. On the N-type diffusion layer 101, an antireflection film 102 such as a SiN _x film is formed. The BSF layer 103 and the aluminum electrode 104 are formed on the back side of the P-type semiconductor substrate 100 by applying an aluminum paste and baking it. Further, a finger electrode 107 for collecting current on the light receiving surface side, a thick electrode called a bus bar electrode 105 formed to collect current therefrom, and a bus bar electrode 106 on the back surface side are electrically conductive including silver. It is formed by applying a functional paste and baking.

  Moreover, in order to obtain a predetermined voltage and current, the solar battery cell is configured and used as a solar battery module by connecting a plurality of solar battery cells in series or in parallel. As a connection method in this case, it is common to solder the bus bar electrode 105 on the front surface and the bus bar electrode 106 formed on the back surface of the solar battery cell using lead wires. As the lead wire, a copper wire coated with solder is mainly used.

  In recent years, the demand for solar cells is expanding as one of clean energy against the background of environmental problems, but the high cost of energy compared to general commercial power is an obstacle to its spread. Reduction of solar cell manufacturing costs is an important issue, but at the same time, the performance of solar cells must be maintained to a minimum.

  So far, as one of the cost-reduction measures for solar cell manufacturing, we have considered thinning the backside silver electrode. However, when the electrode is thinned, the adhesive strength when soldering the lead wire is lowered, and sufficient reliability cannot be obtained.

  As a method for improving the electrode adhesive strength that has been reported so far, a copper foil is adhered thickly on the electrode bus bar to increase the adhesive strength (Patent Document 1: Japanese Patent Laid-Open No. 2000-340812), or a hole in the electrode. The adhesive strength is increased by emptying and soldering (Patent Document 2: Japanese Patent Laid-Open No. 2005-243790), or the paste material is examined to increase the adhesive strength (Patent Document 3: Japanese Patent Laid-Open No. 2007-281033). Some suggestions have been made. However, these require man-hours and cost for materials.

  On the other hand, in JP-A-2006-339342 (Patent Document 4), in order to improve the adhesive strength, an attempt was made to reduce costs by forming irregularities on the bus bar using two plates having different patterns. However, when the electrode width is reduced, the lead wire must also be reduced, and it has been found that sufficient adhesion strength cannot be obtained due to the reduction of the adhesion area. In addition, since printing is performed twice as separate versions, the number of steps is large and the cost is increased.

JP 2000-340812 A JP 2005-243790 A JP 2007-281023 A JP 2006-339342 A

  The present invention has been made in view of the above problems, and has a sufficient adhesive strength when connecting a lead wire to a back surface silver electrode of a solar battery cell, and does not deteriorate in characteristics and can be manufactured at low cost. An object of the present invention is to provide a battery and a screen plate for use in the production thereof.

In order to achieve the above object, the following solar cell and screen plate making are provided.
Claim 1:
A solar cell having a bus bar electrode on at least the back surface,
The bus bar electrode line width is 2.0 mm or more and less than 2.5 mm, the maximum thickness is 9.0 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 0 to 80%, or
The bus bar electrode line width is 2.5 mm or more and less than 3.0 mm, the maximum thickness is 6.6 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 0 to 80%, or
The bus bar electrode line width is 3.0 mm or more and 5.0 mm or less, the maximum thickness is 4.0 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 30 to 80%.
A solar cell characterized by being
Claim 2:
A screen engraving for a back surface of a solar cell, comprising at least a closed portion of 50% or less in a bus bar opening.
Claim 3:
The screen plate making according to claim 2, wherein the distance between the closest openings is 100 µm or less.
Claim 4:
The solar cell which has the bus-bar electrode manufactured by the screen platemaking of any one of Claim 2 or 3 in a back surface.

  According to the present invention, the surface area of the electrode is increased by forming irregularities on the surface of the bus bar, and the adhesion strength at the time of bonding the lead wire is improved and it is difficult to peel off. At this time, no characteristic deterioration is observed. Further, since the amount of electrode paste used can be saved, it is extremely effective for cost reduction. Furthermore, the present invention can be realized by a slight design change of screen plate making.

It is sectional drawing of the electrode of a common solar cell. It is a top view which shows the surface shape of a common solar cell. It is a back view which shows the back surface shape of a common solar cell. It is sectional drawing of the bus-bar electrode after printing with the pattern of FIG. It is a front view which shows an example of the shape of the printing pattern based on this invention. It is sectional drawing of the bus-bar electrode after printing with the pattern of FIG. It is a top view which shows the other example of the shape of a printing pattern based on this invention. It is sectional drawing of the bus-bar electrode after printing with the pattern of FIG. It is a top view which shows another example of the shape of a printing pattern based on this invention. (A) is a top view which shows the shape of the printing pattern based on this invention, (B) is an enlarged view of A. FIG. It is a graph which shows the relationship between bus bar thickness and tensile strength. It is a graph which shows the relationship between a bus-bar width and tensile strength. It is a graph which shows the relationship between a bus-bar width and thickness.

  An example of a method for manufacturing 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% by mass of hydrogen peroxide may be mixed in a hydrochloric acid solution and heated to 60 to 90 ° C. for washing.

On this substrate, an emitter layer is formed by vapor phase diffusion using phosphorus oxychloride. Common silicon solar cell, it is necessary to form only on the light receiving surface of the PN junction, or diffused in a state superimposed two substrates to each other in order to achieve this, SiO ₂ film Ya on the back surface before spreading A SiN _x film or the like is formed as a diffusion mask, and it is necessary to devise such that a PN junction cannot be formed on the back surface. After diffusion, the glass on the surface is removed with hydrofluoric acid.

Next, an antireflection film is formed on the light receiving surface. For film formation, the above-described plasma CVD apparatus is used to form a SiN _x film of about 100 nm. 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, the back electrode and the light receiving electrode are formed by screen printing. 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.

  When the back bus bar is printed with the pattern shown in FIG. 3, the surface of the back bus bar 106 becomes flat (FIG. 4). In order to have sufficient adhesive strength (= 2N or more) in the experiments so far, at least the bus bar thickness is 6.2 μm (FIG. 11 solid line) or the bus bar width is 2.3 mm (FIG. 12 solid line). It was found that the adhesive strength sharply decreased below these values. The isointensity lines of tensile strength as shown in FIGS. From this figure, it can be seen that the tensile strength when the bus bar width is 2.0 to 2.5 mm needs to have a certain thickness. If the discharge rate changes depending on the conductive paste viscosity, the tensile strength may not be obtained. It is essential to stably produce solar cells that can obtain sufficient tensile strength even if the bus bar thickness is somewhat small.

  In order to solve this, it is effective to increase the adhesive strength by forming irregularities on the back bus bar to increase the surface area.

  As a method for forming the unevenness on the electrode, for example, a method of pressing the unevenness after printing to form a groove or scraping the electrode can be considered. However, when the substrate is pressure-bonded, cracking occurs or the substrate is damaged. In addition, a method for forming irregularities on a bus bar by printing twice using two plates having different screen printing patterns has been reported (Patent Document 4: Japanese Patent Application Laid-Open No. 2006-339342). This also has the problem that man-hours increase. A better technique is realized by having a closed portion in the bus bar printing pattern of screen making. The simplest pattern is a pattern having one closing portion 208b at the center of the opening 208a as shown in FIG. A cross section of the backside bus bar 106 printed using this plate is shown in FIG. The adhesion area can be improved by increasing the adhesion area with the lead wire. Further, if printing is performed with a pattern having continuous openings and closed portions as shown in FIG. 7, a cross-sectional view as shown in FIG. 8 is obtained, and the adhesive area is further increased and the adhesive force is improved. Moreover, compared with the conventional box-type bus bar electrode, the amount of paste used is small, and the adhesive strength can be maintained. Further, a rectangular or rhombus pattern as shown in FIG. 9 may be used.

  The tensile strength when unevenness is formed using the pattern shown in FIG. 7 is shown by broken lines in FIGS. When the bus bar thickness was fixed at 10 mm, the tensile strength was improved with the concave and convex electrodes when the bus bar width was 3 to 4 mm compared to the flat electrode. On the other hand, when the bus bar width was constant at 4 mm, the tensile strength was maintained and the required thickness could be reduced by about 2 μm. The result is shown by a broken line in FIG.

  The shape of the concave-convex shape of the backside bus bar according to the present invention is not particularly limited, but as described above, for example, a concave shape as shown in FIG. 6 printed with a plate-making pattern shown in FIG. Or what has a continuously uneven | corrugated shape like FIG. 8 printed by the platemaking pattern shown by FIG. Alternatively, it is printed with the plate-making pattern shown in FIG. 9, and the bus bar vertical and horizontal cross-sectional views are as shown in FIG.

Therefore, in the present invention, the bus bar electrode on the back surface is
The bus bar electrode line width is 2.0 mm or more and less than 2.5 mm, the maximum thickness is 9.0 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 0 to 80%, or
The bus bar electrode line width is 2.5 mm or more and less than 3.0 mm, the maximum thickness is 6.6 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 0 to 80%, or
It is necessary that the bus bar electrode line width is 3.0 mm or more and 5.0 mm or less, the maximum thickness is 4.0 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 30 to 80%.
The ratio of the maximum electrode thickness X to the minimum thickness Y is (Y / X) × 100.
It is represented by

  Moreover, when forming the said unevenness | corrugation by screen plate-making, it is preferable that screen plate-making has a 50% or less closed part in the opening part for bus bars, and it is preferable that the distance between the adjacent opening parts is 100 micrometers or less.

  The unevenness of the bus bar of the present invention uses screen plate making having an opening and a closing portion and bleeding of paste. If the opening is not more than 50%, printing using paste bleeding according to the present invention cannot be performed. When the distance between the openings exceeds 100 μm, the paste is not sufficiently blotted, the bus bar is split, and the substrate is exposed. At this time, since it is difficult to bond the substrate and the lead wire, the module strength is insufficient. On the other hand, when the number of closed portions is too small, the paste fills between the openings, but since the unevenness is difficult to be formed, the surface area cannot be gained and the adhesive strength becomes the same as before. In addition, the effect of reducing the amount of silver used is small.

Finally, a silver paste in which silver powder and glass frit are mixed with an organic binder is screen-printed as a light-receiving surface electrode, and then the silver powder is passed through the SiN _x film by heat treatment (fire-through) to make the electrode and silicon conductive. The back electrode and the light-receiving surface electrode can be baked at the same time.

Examples of the present invention will be described below, but the present invention is not limited thereto.
In order to confirm the effectiveness of the present invention, a solar cell was produced by the following method.
The bus bar electrode has a line width of 2.5 mm, an electrode maximum thickness of 12 μm, and a ratio of the minimum thickness to the electrode maximum thickness of 30%. A conventional print pattern A was printed with the following pattern in the same manner with the line width and the maximum electrode thickness set to a ratio of the minimum thickness to the maximum electrode thickness of 81%.
There are two types of printing patterns, the conventional pattern A (opening ratio 100%: FIG. 3) and the pattern B of the present invention (opening ratio 71%: FIG. 10: patterns in which 100 μm openings and 40 μm openings are alternately continuous). Was prepared, and each pattern was made with three types of meshes ST150, ST250, and ST350.

  After removing a damaged layer with a hot concentrated potassium hydroxide aqueous solution on 60 boron-doped {100} p-type ascut silicon substrates having a diffusion thickness of 250 μm and a specific resistance of 1 Ω · cm, in a potassium hydroxide / 2-propanol aqueous solution The film was dipped in to form a texture, followed by washing in a hydrochloric acid / hydrogen peroxide mixed solution. Next, it heat-processed in the phosphorus oxychloride atmosphere in the state which accumulated the back surfaces at 870 degreeC, and formed the emitter layer. After diffusion, the glass was removed with hydrofluoric acid, washed and dried.

After the above treatment, a SiN _x film was formed as a light-receiving surface antireflection film on all samples using a plasma CVD apparatus. Next, 10 sheets of silver paste were printed on the back plate A, B, and 3 types of meshes as the back electrode and dried. Next, an aluminum paste was screen-printed as a back electrode and a silver paste was screen-printed on the light-receiving surface and dried. When the levels A and B were baked in an air atmosphere at 780 ° C. and IV measurement was performed, the characteristics of the patterns A and B were equivalent. Next, a tensile strength test was performed. The adhesive tensile strength is defined as a peak of force when a lead wire having a width of 2 mm is soldered to a bus bar, the bonded lead wire is pulled in a vertical direction, and the lead wire is peeled from the substrate. A measurement result of 2N or more was considered acceptable. As a result, only ST150 mesh was passed in pattern A. No peeling occurred in pattern B.

  In the conventional printed pattern, when the back surface silver electrode was made thinner, even if the electrode thickness was confirmed to be peeled, no peeling was observed using the printed pattern according to the present invention.

100 P-type semiconductor substrate 101 N-type diffusion layer 102 Antireflection film 103 BSF layer 104 Aluminum electrode 105 Front bus bar electrode 106 Back bus bar electrode 107 Finger electrode 208a Bus bar opening 208b Bus bar closing part

Claims (4)

A solar cell having a bus bar electrode on at least the back surface,
The bus bar electrode line width is 2.0 mm or more and less than 2.5 mm, the maximum thickness is 9.0 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 0 to 80%, or
The bus bar electrode line width is 2.5 mm or more and less than 3.0 mm, the maximum thickness is 6.6 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 0 to 80%, or
The bus bar electrode line width is 3.0 mm or more and 5.0 mm or less, the maximum thickness is 4.0 μm or more, and the ratio of the minimum thickness to the electrode maximum thickness is 30 to 80%.
A solar cell characterized by being   A screen engraving for a back surface of a solar cell, comprising at least a closed portion of 50% or less in a bus bar opening.   The screen plate making according to claim 2, wherein the distance between the closest openings is 100 µm or less.   The solar cell which has the bus-bar electrode manufactured by the screen platemaking of any one of Claim 2 or 3 in a back surface.

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