JP2011066353A - Aluminum paste for solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide aluminum paste for a solar cell capable of suppressing warpage of a back electrode while keeping an electrical characteristic, film strength and film appearance. <P>SOLUTION: Since a small amount of Sn powder is included in this aluminum paste, when an overall electrode 26 is formed on the back of a silicon substrate 12 by printing, drying and baking by using the paste, warpage of the silicon substrate 12 is reduced. In addition, when Sn is added, waterproofness of the overall electrode 26 formed of aluminum is improved and, since the additive amount is 0.3-5.0 (pts.mass) in 100 (pts.mass) of Al, that is, an extremely small amount such as 0.21-3.5 (pts.mass) in 100 (pts.mass) of paste, the conductivity, film strength and appearance of the overall electrode 26 are not influenced at all. Since the warpage can be suppressed without reducing the film thickness, a BSF effect can be sufficiently exerted. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an aluminum paste suitable for a back electrode of a solar cell.

For example, a general silicon-based solar cell is provided with an antireflection film and a light-receiving surface electrode on an upper surface of a silicon substrate which is a p-type polycrystalline semiconductor via an n ⁺ layer, and on the lower surface via a p ⁺ layer. It has a structure with electrodes. By receiving light, electric power is generated in the pn junction of the semiconductor and is taken out through the electrode.

The n ⁺ layer is formed by diffusing an n-type dopant such as P (phosphorus) in a silicon substrate. For example, in a silicon substrate having a thickness of 160 to 300 (μm), the thickness is about 0.2 to 0.6 (μm). Is provided. The antireflection film is made of a thin film such as silicon nitride, titanium dioxide, or silicon dioxide, and is provided to reduce the surface reflectance and increase the light receiving efficiency while maintaining a sufficient visible light transmittance. The light-receiving surface electrode is made of a thick film material containing silver or the like as a conductor component, and is formed on the antireflection film by a so-called fire-through method or the like.

  On the other hand, the back electrode is composed of, for example, two strip electrodes provided in a strip shape, and a full-surface electrode provided on substantially the entire surface excluding the strip electrode. The entire surface electrode is made of a thick film material containing aluminum as a conductor component, and the strip electrode is made of a thick film material containing silver as a conductor component. This strip electrode is provided for the purpose of enabling soldering of a lead wire or the like to the back electrode.

In forming the full-surface electrode, an aluminum paste is applied to the back surface by screen printing or the like, dried, and then fired at a temperature corresponding to the paste composition. At this time, the aluminum in the paste diffuses into the silicon substrate to form the p ⁺ layer, an electric field is generated on the back side due to the difference in Fermi level between the pp ⁺ layers, and the collection efficiency of the generated carriers is improved. Surface Field) effect is obtained. In the aluminum paste, for example, an organic vehicle and glass frit for improving the strength of the aluminum electrode are added to aluminum powder.

JP 2000-090734 A Japanese Patent Laid-Open No. 2003-223813 JP 2001-313402 A JP 2008-166344 A JP 2007-234625 A Japanese Examined Patent Publication No. 06-105792 JP 2007-273760 A

  Incidentally, in the solar cell described above, in recent years, it has been studied to reduce the thickness of the silicon substrate to 200 (μm) or less for the purpose of reducing the cost. However, if the silicon substrate is thinned, stress is generated in the substrate due to the difference in thermal expansion coefficient between silicon and aluminum during firing for electrode formation, so that the silicon substrate warps and eventually breaks in the manufacturing process. There was a problem.

  On the other hand, various techniques for reducing warpage when the substrate thickness is reduced have been proposed. For example, there is one that reduces the thickness dimension of an aluminum electrode (see, for example, Patent Document 1). According to this technology, warping is suppressed because the stress during firing is reduced, but the amount of aluminum diffused on the back surface is reduced and the BSF effect is reduced. There is a problem that the appearance of the film tends to be reduced and the appearance of the film is also lowered.

  In addition, it has also been proposed to add inorganic compound powders such as silica and alumina having a smaller thermal expansion coefficient than aluminum and a higher melting point than aluminum to the aluminum paste (see, for example, Patent Document 2). According to this technique, since the thermal expansion coefficient of the aluminum electrode is reduced by the added inorganic compound powder, the stress at the time of firing is reduced and the warpage is suppressed. Characteristics are degraded. In addition, since the sintering of aluminum is inhibited by the inorganic compound powder, there is a problem that the film strength is also lowered.

  In addition, it has been proposed to add silicon powder to aluminum paste (see, for example, Patent Document 3). Silicon powder is added for the purpose of decreasing the thermal expansion coefficient of the aluminum electrode and suppressing warping, but the Al-Si eutectic point is 577 (° C), which is lower than the melting point of aluminum 660 (° C). The amount of liquid phase in the film increases, and the thickness of the alloy layer formed on the back surface also increases. For this reason, contrary to the said objective, curvature will increase on the contrary. Although addition of Al-Mg alloy powder or Mg powder has also been proposed (see, for example, Patent Document 4), since the Al-Mg eutectic point is as low as 450 (° C) when the Al ratio is high, In the same manner as when silicon powder is added, warpage increases.

  In addition, there is one in which an aluminum-containing alloy powder having a melting point higher than that of aluminum is added to an aluminum paste (see, for example, Patent Document 5). Titanium, vanadium, etc. are mentioned as a constituent element of an alloy. According to this technique, since the precipitation temperature of the solid phase increases due to the addition of the alloy powder, the amount of the liquid phase decreases and the warpage is suppressed. However, there is a problem that the conductivity of the aluminum electrode is lowered by the high melting point alloy powder. In the techniques described in these Patent Documents 1 to 5, the warpage cannot be suppressed, or other characteristics such as electrical characteristics, tensile strength, and film appearance are deteriorated, so that the solar cell characteristics cannot be obtained.

  The present invention has been made against the background of the above circumstances, and an object thereof is to provide an aluminum paste for a solar cell that can suppress warpage of a back electrode while maintaining electrical characteristics, film strength, and film appearance. .

  In order to achieve such an object, the gist of the present invention is an aluminum paste for solar cells containing Al powder, glass frit, and a vehicle, and Sn powder is added to 100 parts by mass of the Al powder. It is to be included at a ratio in the range of 0.3 to 5.0 parts by mass.

  In this way, since the aluminum paste contains a small amount of Sn powder, if the back electrode is formed on the silicon substrate by printing, drying and firing using this paste, the warpage of the silicon substrate is reduced. The Although the Al-Sn eutectic point is 228 (° C) and it is significantly lower than the melting point of Al, it is not certain what effect it is, but unlike the case of adding Si powder or Mg powder, Warpage is suppressed. In addition, the addition of Sn improves the water resistance of the aluminum electrode, and the addition amount is very small, 0.3 to 5.0 (parts by mass), so it has no effect on the conductivity, film strength and appearance of the aluminum electrode. Absent. In addition, since the warpage can be suppressed without reducing the film thickness, the BSF effect can be fully enjoyed. Therefore, when used for the back electrode, warpage is suppressed, and an aluminum paste for solar cells excellent in electrical characteristics, film strength, water resistance, and appearance can be obtained.

  Note that if the amount of Sn added is less than 0.3 (parts by mass), the effect of suppressing warpage of the silicon substrate cannot be sufficiently obtained. On the other hand, even if it exceeds 5.0 (parts by mass), the warp reduction effect is hardly changed, and a large amount of blisters and balls are generated and the film appearance is remarkably deteriorated. Therefore, it is essential that the Sn powder amount is in the range of 0.3 to 5.0 (parts by mass) with respect to 100 parts (parts by mass) of the Al powder.

  Incidentally, although the said patent document 6 describes the aluminum paste containing 5-50 (mass part) Sn powder with respect to Al powder 100 (mass part), as above-mentioned, 5.0 (mass part) is described. If a large amount of Sn powder is added, blisters and balls are generated in large quantities. In the above-mentioned Patent Document 6, Sn powder is added for the purpose of enabling plating of an aluminum electrode and thus soldering of a lead wire, and no consideration is given to warpage suppression or film appearance.

Patent Document 7 describes an aluminum paste using an aluminum powder coated with a low melting point solder alloy (ie, Sn alloy). This aluminum paste suppresses the warpage of the substrate as low-temperature firing to such an extent that aluminum does not melt. The aluminum powder is fixed to each other with a solder alloy, and the back electrode is fixed to the substrate. Therefore, even if the back electrode is formed with this aluminum paste, since the p ⁺ layer is not formed because aluminum does not diffuse into the silicon substrate, the BSF effect cannot be obtained. The BSF effect and warpage suppression can be achieved by providing a two-layer structure in which a first electrode layer using aluminum powder not coated with a solder alloy is provided on the substrate side and a second electrode layer using coated aluminum powder is provided thereon. Although it has been explained that it is possible, such a two-layer structure is complicated in order to increase the manufacturing cost and to suppress warping when forming the first electrode layer Since it is necessary to make this thin, it is difficult to obtain a sufficient BSF effect while suppressing warpage.

  Here, preferably, the Sn powder has an average particle diameter in the range of 2 to 10 (μm). If the particle size of the Sn powder is 2 (μm) or more, agglomeration is unlikely to occur, so that better dispersibility can be obtained during paste preparation. If the particle size is 10 (μm) or less, good dispersibility can be obtained with a smaller addition amount. The average particle size is more preferably 2.5 to 5.0 (μm). The addition amount of Sn powder is more preferably in the range of 1.0 to 5.0 (parts by mass).

  Preferably, the aluminum paste is in a range of 60 (parts by mass) to 80 (parts by mass) of aluminum powder and 5 (parts by mass) or less of glass powder in the entire paste 100 (parts by mass). Including each. Sn powder is contained in the paste 100 (parts by mass) in the range of 0.21 to 3.5 (parts by mass). When the aluminum powder is 60 (parts by mass) or more, the conductivity of the electrode is further increased and the amount of Al diffusion into the substrate is further increased. Further, if the aluminum powder is kept at 80 (parts by mass) or less, the amount of vehicle can be sufficiently increased, so that a paste with more excellent printability can be obtained. The glass powder is essential to be added in order to increase the adhesion strength of the electrode to the substrate, but in order to obtain sufficiently high conductivity, it is desirable to keep it at 5 (parts by mass) or less. The organic vehicle occupies the balance of the other pastes of the aluminum powder, glass powder, and Sn powder, for example.

  Preferably, the aluminum powder has an average particle size in the range of 2 to 9 (μm). When the particle size is increased, the dispersibility tends to be inferior, and even when the particle size is significantly decreased, the dispersibility tends to be inferior due to aggregation or the like. From the above viewpoint, the average particle size of the aluminum powder is more preferably in the range of 5 to 7 (μm).

  The organic vehicle is not particularly limited. For example, what melt | dissolved appropriate resin, such as a cellulose polymer and an acrylic resin, with terpineol, butyl carbitol, etc. can be used.

  Further, the glass powder is not particularly limited. Various materials such as lead-based, bismuth-based, and borosilicate-based materials can be used. However, in consideration of environment, non-lead type is preferable. The average particle size of the glass powder is preferably in the range of 1 to 10 (μm), for example.

  Moreover, the said aluminum paste can contain suitably other additives in the range which does not disturb the characteristic other than said each component. Examples of the additive include plasticizers such as phthalic acid esters and adipic acid esters generally used in pastes, and dispersants typified by vinyl polymers and polycarboxylic acids.

It is a schematic diagram which shows the cross-sectional structure of the solar cell with which the aluminum paste of one Example of this invention was applied for formation of a back surface electrode. It is a figure which shows the back surface electrode of the solar cell of FIG. It is the evaluation result of the relationship between Sn addition amount and the curvature of a silicon substrate.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are appropriately simplified or modified, and the dimensional ratios, shapes, and the like of the respective parts are not necessarily drawn accurately.

FIG. 1 is a diagram schematically showing a cross-sectional structure of a silicon-based solar cell 10 to which an aluminum paste for solar cell according to an embodiment of the present invention is applied. In FIG. 1, a solar cell 10 is formed on a silicon substrate 12 which is, for example, a p-type polycrystalline semiconductor, an n ⁺ layer 14 and a p ⁺ layer 16 respectively formed on the upper and lower surfaces thereof, and the n ⁺ layer 14. The antireflection film 18 and the light receiving surface electrode 20, and the back electrode 22 formed on the p ⁺ layer 16 are provided. The thickness dimension of the silicon substrate 12 is in the range of, for example, 100 to 200 (μm), for example, about 180 (μm), and is a rectangular thin plate of about 125 × 125 (mm) or a disk shape of about 125 (mm) in diameter. It is what constitutes.

The n ⁺ layer 14 and the p ⁺ layer 16 are provided by forming layers having a high impurity concentration on the upper and lower surfaces of the silicon substrate 12, and the thickness of the high concentration layer is the same as that of the n ⁺ layer 14. For example, about 70 to 100 (nm), and the p ⁺ layer 16 is about 500 (nm), for example. The n ⁺ layer 14 is about 100 to 200 (nm) in a general silicon solar cell, but is thinner than that in the present embodiment, and has a structure called a shallow emitter. The impurity contained in the n ⁺ layer 14 is an n-type dopant such as phosphorus (P), and the impurity contained in the p ⁺ layer 16 is a p-type dopant such as aluminum (Al).

The antireflection film 18 is a thin film made of, for example, silicon nitride Si ₃ N ₄ , and is provided with an optical thickness of, for example, about ¼ of the visible light wavelength, for example, about 80 (nm). It is configured to have an extremely low reflectance of 10 (%) or less, for example, 2 (%).

  Further, the light receiving surface electrode 20 is made of, for example, a thick film conductor having a uniform thickness, and is provided in a planar shape that forms a comb shape having a large number of thin wire portions on substantially the entire surface of the light receiving surface 24. Yes. The above thick film conductor has a Ag in the range of 67 to 98 (wt%), for example, about 94.3 (wt%), and a glass in the range of 2 to 33 (wt%), for example, about 5.7 (wt%). It consists of thick film silver. The glass is, for example, borosilicate glass or lead glass.

Further, as schematically shown in FIG. 2, the back electrode 22 has a strip electrode 28 made of thick film silver provided on the p ⁺ layer 16 in a strip shape, and substantially the entire surface excluding the strip electrode 28. A full-surface electrode 26 made of a thick film having aluminum as a conductor component provided so as to partially overlap the belt-like electrode 28. The thick film constituting the entire surface electrode 26 is made of aluminum, a small amount of Sn, and glass. For example, Sn is about 0.3 to 5 (parts by mass) and glass is 2.9 (parts by mass) with respect to 100 parts (parts by mass) of aluminum. Part) is included. The strip electrode 28 is provided in order to make it possible to solder a conductive wire or the like to the back electrode 22.

Solar cell 10 of the embodiment, since it is a n ⁺ pp ⁺ structure from the light receiving surface 24 side is n ⁺ layer 14, p-type silicon substrate 12, p ⁺ layer 16 provided as described above, The BSF effect is suitably obtained. Therefore, there is an advantage of having high current collection efficiency.

The solar cell 10 is manufactured, for example, by preparing an aluminum paste as described below and forming a full-surface electrode 26 on the back surface using the aluminum paste. First, aluminum powder 100 (mass part), organic vehicle 32-40 (mass part), glass powder 2.9 (mass part), and Sn powder 0.3-5.0 (mass part) are mixed. Assuming that the entire paste is 100 (wt%), aluminum powder is 70 (wt%), glass powder is 2.0 (wt%), Sn powder is 0.21 to 3.5 (wt%), vehicle is 24.5 to 27.79 (wt%) Become. The aluminum powder is, for example, a spherical or nearly spherical powder having an average particle size of about 6 (μm). The glass powder is, for example, SiO ₂ —B ₂ O ₃ —ZnO glass, and the average particle size is, for example, about 5 (μm). Sn powder was spherical particles produced by a water-atomization method, and those having an average particle diameter of 2.5 to 10 (μm) as shown in Table 1. A sample prepared with such a composition was kneaded by, for example, a three-roll mill to obtain an aluminum paste.

In Table 1, Examples 1 to 9 and Comparative Examples 13 and 14 use Sn powder, but Examples 10 and 11 use Sn / Ag3 / Cu0.5 alloy instead of Sn powder. The different examples used and Comparative Example 12 are those to which no metal powder or the like is added, Comparative Example 15 is the one to which Si powder is added, and Comparative Example 16 is the one to which SiO ₂ powder is added. The particle diameters and addition amounts of the metal powders and the like of each Example and Comparative Example are as shown in Table 1. In Comparative Examples 13 and 14, the amount of Sn powder added was 0.1 (parts by mass) and 8.0 (parts by mass) with respect to 100 parts (parts by mass) of Al powder.

Next, for example, a rectangular thin plate-shaped p-type silicon substrate 12 having a thickness of about 180 (μm) and a size of about 125 × 125 (mm) is prepared. A silver electrode, that is, a belt-like electrode 28 is formed by printing, for example, by a thick film screen printing method, and a drying process is performed. Next, the above-described aluminum paste is printed on this using, for example, a thick film screen printing method. For printing plate making, for example, stainless steel 200 mesh was used, and the amount of paste applied was 7 (mg / cm ² ). After printing, it was dried at 80 (° C.), and baked at a baking temperature of 700 to 800 (° C.) using a near-infrared high-speed baking furnace in the air atmosphere. That is, the silver electrode and the aluminum electrode were collectively fired. Thereby, the said light-receiving surface electrode 20 and the back surface electrode 22 are formed, and the said solar cell 10 is obtained.

  The solar cell 10 manufactured as described above was evaluated for warpage, film strength, water resistance, and appearance. The results are shown in the right column of Table 1. The warpage was placed on a horizontal surface with the light-receiving surface 24 side up, and the height of the lower surface was taken as the amount of warpage. Further, the film strength was evaluated by determining whether or not the electrode film was peeled off by sticking the adhesive tape to the entire surface electrode 26 and peeling it off. The case where it did not exfoliate was set as (circle) and the case where it peeled off was set as x. Further, the water resistance was evaluated by immersing the entire surface electrode 26 in warm water of 80 (° C.) and generating bubbles on the film surface. The case where bubbles were generated in 2 minutes or less was evaluated as x, the case where bubbles were generated in 2 to 10 minutes was evaluated as Δ, and the time until bubbles were generated was determined as 10 minutes. Further, the appearance was evaluated as x when blisters or balls were observed by observing the film surface, and ◯ when there was no blister.

  As shown in the above evaluation results, with respect to Al 100 (parts by mass), Sn is contained in the range of 0.3 to 5.0 (parts by mass), that is, Sn in paste 100 (parts by mass) is 0.21 to 3.5 (parts by mass). In Examples 1 to 9 included in the range of), the warpage was 1.0 to 1.8 (mm). Further, even in Examples 10 and 11 containing 2.0 (parts by mass) of Sn / Ag3 / Cu0.5, the warpage remained at 1.0 to 1.2 (mm). Further, in all the examples, the film strength and the appearance were both good, and the water resistance was all good except that Example 1 was slightly inferior.

On the other hand, in Comparative Example 12 in which Sn was not added, the warpage was 2.4 (mm), and in Comparative Example 13 in which 0.1 (mass part) was added, the warpage was 2.2 (mm), both of which were 2 (mm) or more. It was a warp. In Comparative Example 15 to which Si was added, the warpage was as large as 2.6 (mm). Moreover, the comparative example 12 which does not add Sn was inferior in both water resistance and appearance. In Comparative Example 13 in which Sn is 0.1 (mass part) with respect to Al 100 (mass part), that is, 0.07 (mass part) in paste 100 (mass part) and Comparative Example 15 in which Si is added, the warpage is large. In addition, the water resistance was slightly inferior. In Comparative Example 14 where Sn is 8.0 (parts by mass) with respect to Al 100 (parts by mass), that is, 5.6 (parts by mass) in paste 100 (parts by mass), the warpage was as small as 1.3 (mm). Many balls were seen, resulting in poor appearance. In Comparative Example 16 to which SiO ₂ was added, although the warpage was as small as 1.2 (mm), the film strength was weak and the water resistance was insufficient.

  According to the above evaluation results, when Sn or Sn alloy is contained 0.3 to 5.0 (parts by mass) with respect to Al 100 (parts by mass), that is, 0.21 to 3.5 (parts by mass) in paste 100 (parts by mass), warpage occurs. It is apparent that can be made a small value of less than 2.0 (mm), the film strength is high, and a good appearance can be obtained. In particular, if the addition amount is 1.0 (parts by mass) with respect to Al 100 (parts by mass), that is, 0.7 (parts by mass) or more in the paste 100 (parts by mass), the warpage remains at 1.4 (mm) or less and water resistance Sex is also sufficiently enhanced.

  FIG. 3 is a graph showing the above evaluation results, with the horizontal axis representing the amount of Sn or Sn alloy added to Al 100 (parts by mass) and the vertical axis representing the amount of warpage. The tendency for the warpage to decrease sharply from where the addition amount exceeds 0.31 (parts by mass) with respect to Al 100 (parts by mass), that is, 0.21 (parts by mass) in paste 100 (parts by mass), is confirmed. It can be seen that the change in warping becomes almost horizontal when the mass is 1.0 (mass part) or more with respect to (mass part), that is, 0.7 (mass part) or more in the paste 100 (mass part). Further, although the difference was relatively small, it was recognized that the warp tends to be smaller as the particle size is larger.

  As described above, according to this embodiment, since a small amount of Sn powder is contained in the aluminum paste, the entire surface electrode 26 is formed on the back surface of the silicon substrate 12 by printing, drying, and baking using this paste. Then, the warp of the silicon substrate 12 is reduced. Moreover, when Sn is added, the water resistance of the entire surface electrode 26 made of aluminum is improved, and the addition amount is 0.3 to 5.0 (parts by mass) with respect to Al 100 (parts by mass), that is, 0.21 in the paste 100 (parts by mass). Since it is a very small amount of ~ 3.5 (parts by mass), it does not affect the conductivity, film strength and appearance of the entire surface electrode 26 at all. In addition, since the warpage can be suppressed without reducing the film thickness, the BSF effect can be fully enjoyed.

  As mentioned above, although this invention was demonstrated in detail with reference to drawings, this invention can be implemented also in another aspect, A various change can be added in the range which does not deviate from the main point.

10: solar cell, 12: silicon substrate, 14: n ⁺ layer, 16: p ⁺ layer, 18: antireflection film, 20: light receiving surface electrode, 22: back electrode, 24: light receiving surface, 26: full surface electrode, 28 : Strip electrode

Claims (2)

An aluminum paste for solar cells containing Al powder, glass frit, and vehicle,
An aluminum paste for solar cells, comprising Sn powder in a proportion of 0.3 to 5.0 parts by mass with respect to 100 parts by mass of the Al powder.   The aluminum paste for solar cells according to claim 1, wherein the Sn powder has an average particle size in the range of 2 to 10 (µm).

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