JP2009246277A - Paste composition for solar cell electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conductive paste composition for a solar cell electrode, which preferably forms a large cross section printing pattern with a thin width diameter without damaging the printing characteristics. <P>SOLUTION: Spreading of paste on a paste applying surface is preferably suppressed while keeping appropriate mobility at the time of squeezing since the paste thixotropy is improved in view of the fact that an ethylcellulose with a degree of ethoxylation as low as 45-47% is contained with a proportion of 60% or more of the entire resin. This forms the large cross section printing pattern with the narrow width diameter without damaging the printing characteristics. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a conductive paste composition suitable for a solar cell electrode.

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. The antireflection film is for reducing the surface reflectance while maintaining sufficient visible light transmittance, and is made of a thin film of silicon nitride, titanium dioxide, silicon dioxide or the like.

The light-receiving surface electrode of the solar cell is formed by a method called fire-through, for example. In this electrode forming method, for example, after the antireflection film is provided on the entire surface of the n ⁺ layer, a conductive paste is applied on the antireflection film in an appropriate shape by using, for example, a screen printing method, and is fired. Apply. The conductive paste is composed mainly of, for example, silver powder, glass powder, an organic vehicle, and an organic solvent, and the glass component in the conductive paste breaks the antireflection film during the firing process. An ohmic contact is formed by the conductor component in the conductive paste and the n ⁺ layer (see, for example, Patent Document 3).
Japanese Patent Laid-Open No. 10-247418 JP 2003-059336 A JP 2007-019106 A

  By the way, the opaque light receiving surface electrode containing silver as a conductor component as described above blocks sunlight incident on the light receiving surface. As the area occupied by the light receiving surface electrode increases, the light receiving area capable of absorbing solar energy is reduced, so that the conversion efficiency of the solar cell is lowered. In order to suppress the reduction of the light receiving area, it is conceivable to reduce the electrode width dimension of the light receiving surface electrode (that is, the width dimension of the print pattern). However, in order to reduce the electrode width dimension, it is necessary to reduce the coating thickness to suppress the spread (sagging) of the conductive paste on the coating surface. For this reason, if the electrode width dimension is reduced, the electrode cross-sectional area is reduced, so that the electric resistance value is increased and the current density is lowered, so that it is difficult to improve the conversion efficiency.

  In contrast, by using a conductive paste with a viscosity of 200 to 500 (Pas) and a thixotropy index of 1.5 to 4.5, the electrode width is reduced while keeping the coating thickness thick. It has been proposed to form a light-receiving surface electrode that suppresses the reduction of the area and has a narrow width and a low electric resistance (see, for example, Patent Document 3). In Patent Document 3, the viscosity is a value measured at 25 (° C.) and 0.5 (rpm) using a 3 ° cone with an E-type viscometer. The thixotropy index has various definitions. In the field of screen printing, the thixotropy index generally represents the shear rate dependency of the viscosity. In Patent Document 3, the viscosity measured at 0.5 (rpm) is 2.5. It is a value divided by the viscosity measured in (rpm).

  Since the conductive paste described in Patent Document 3 has a relatively high viscosity and low fluidity, it is difficult to spread on the coating surface, so the width of the printed pattern is reduced while keeping the coating thickness relatively thick. be able to. It is desired that the conductive paste has fluidity suitable for screen printing, that is, has a sufficiently low viscosity so that it can easily pass through the mesh. If the thixotropy index is sufficiently large, the viscosity is lowered by the shear stress acting from the squeegee during squeezing, so that the flow on the coated surface can be suppressed without impairing the printability. If the thixotropy index is too large, the viscosity rapidly increases after passing through the mesh and the fluidity is lost, so that it becomes difficult to obtain a smooth print pattern.

  However, when the inventors of the present application tried to reproduce the conductive paste described in Patent Document 3, the viscosity of the conductive powder and the resin was determined within a certain range as described in the same document. In addition, it was difficult to control the thixotropy index, and it was impossible to obtain a print pattern having a narrow cross-sectional area with a large cross-sectional area while maintaining printability. And, as a result of reviewing the paste composition in response to this result, it is not possible to obtain desired characteristics simply by setting the composition ratio of the conductive powder and the resin within a certain range, and the resin constituting the paste has a certain range. It has been found that preferable results can be obtained by using ethyl cellulose having a degree of ethoxylation.

  The present invention has been made against the background described above, and its purpose is to provide a conductive paste composition for solar cell electrodes that can suitably form a print pattern having a narrow width and a large cross-sectional area without impairing printability. To provide things.

  In order to achieve such an object, the gist of the present invention is a paste composition for a solar cell electrode comprising a conductor powder, a glass powder, and a vehicle, wherein (a) the vehicle is 45 to 47 ( %) Of ethyl cellulose having a degree of ethoxylation of 60% (% by weight) or more and ethyl cellulose having a degree of ethoxylation of 48% (%) or more comprising a resin and a solvent.

  In this way, the solar cell electrode paste composition uses ethyl cellulose as the resin, and contains 45 to 47 (%) and ethyl cellulose with a low degree of ethoxylation in a proportion of 60 (%) or more of the entire resin. Therefore, the thixotropy of the paste is enhanced, so that the spread on the coated surface of the paste can be suitably suppressed while ensuring appropriate fluidity during squeezing. Therefore, the conductive paste composition for solar cell electrodes capable of forming a print pattern having a narrow width and a large cross-sectional area without impairing printability is obtained.

Incidentally, ethylcellulose is partially etherified with ethyl chloride (C ₂ H ₅ Cl) to partially hydroxylate the hydroxyl group (-OH) of cellulose ((C ₆ H ₁₀ O ₅ ) _n ), that is, ethoxy group (C ₂ H ₅ O The ratio of substitution with respect to all hydroxyl groups before etherification is referred to as ethoxylation degree or ethoxylation ratio. As a resin for a printing paste such as a conductive paste, ethylcellulose having a degree of ethoxylation in the range of 48 to 49.5 (%), for example, 49 (%) is generally used (see, for example, Patent Document 1). ). Such ethyl cellulose is suitable for screen printing because it does not have stringing or the like, and is suitable for an electrode paste because it easily burns off during firing.

  As a result of preparing and evaluating a conductive paste by variously changing the formulation composition using ethyl cellulose in the above-mentioned range of ethoxylation degree, the present inventors have come to the conclusion that it is difficult to obtain an appropriate thixotropy. . And, as a result of conducting a test by replacing a part of ethyl cellulose with ethyl cellulose having a low ethoxylation degree that is not usually used for electrode paste, when this is included at a ratio of 60% or more with respect to the whole resin Furthermore, it has been found that an appropriate thixotropy can be obtained while maintaining printability. The present invention has been made based on such knowledge. The ethyl cellulose having an ethoxylation degree of 48 (%) or more that occupies the rest of the resin may have an ethoxylation degree of 48 to 49.5 (%) that is generally used for printing applications, for example. It can be expensive.

  In addition, a conductive paste containing ethyl cellulose having an ethoxylation degree of 45 to 53 (%) wider than the above range has also been proposed (see, for example, Patent Document 2). This conductive paste is used for forming a conductor pattern of an electric circuit, but the range of the degree of ethoxylation is merely a slightly larger numerical value range for commercially available ethyl cellulose. That is, the above-mentioned Patent Document 2 aims to improve transferability by using a cellulose resin and an acrylic resin in combination. Not shown.

  Here, preferably, in the paste composition, the conductor powder is a spherical silver powder. In this way, since the filling rate of the conductor powder in the coating film is increased, in combination with the use of highly conductive silver, it is compared with the case where conductor powder of other shapes such as scales is used. And the electrical conductivity of the conductor produced | generated from the coating film becomes high. For this reason, the cross-sectional area required for obtaining a certain level of conductivity is reduced, so that the line width can be further reduced while ensuring the necessary conductivity. Therefore, if the paste composition is applied to the light-receiving surface electrode to reduce the line width, the light-receiving area capable of absorbing solar energy can be further increased, so that a solar cell with higher conversion efficiency can be obtained.

  In addition, since the spherical conductor powder as described above has a smooth surface shape, it facilitates the flow of the paste, and thus easily escapes from the plate. There is also an advantage that a reduction in the cross-sectional area is preferably suppressed. That is, since the cross-sectional shape is prevented from collapsing from the rectangular cross-section as the film thickness is increased, the cross-sectional area is increased. In addition, there is an advantage that continuous printing is easy because clogging of the printing screen hardly occurs. If the paste is easy to flow, the paste tends to spread on the coated surface. However, according to the paste of the present invention, the thixotropy is increased by including ethylcellulose having a low degree of ethoxylation as described above. Therefore, even if the above spherical powder is used, the spread of the paste on the coated surface is suitably suppressed.

  Preferably, the conductor powder has an average particle size of 2 (μm) or less. In this way, since the conductor powder having a sufficiently small average particle diameter is used, the filling rate of the conductor powder in the coating film is increased, so that the conductivity of the conductor generated from the coating film is increased. .

  Moreover, the said conductor powder is not specifically limited, According to required electroconductivity and other physical properties, an appropriate thing can be used. However, since the paste composition for solar cell electrodes is required to have high conductivity and solderability, silver is most preferable from such a viewpoint.

  Further, the glass powder is not particularly limited, and an appropriate one can be used depending on the processing temperature, conductivity, and other physical properties. For example, lead borosilicate glass is preferable.

  Preferably, the conductive paste composition contains 70 to 84 parts by weight of the conductor powder, 3 to 8 parts by weight of the glass powder, and 10 to 20 parts by weight of the vehicle. It is a waste. In this way, a paste composition that can produce an electrode having good printability, high conductivity, and good solder wettability can be obtained. If the conductor powder is too small, high conductivity cannot be obtained, and if it is excessive, the fluidity is lowered and the printability is deteriorated. If the glass powder is too small, the adhesion to the substrate is insufficient, and if it is excessive, the glass floats on the electrode surface after firing and solder wettability is deteriorated.

  Moreover, since the paste composition of the present invention can form a conductor pattern having a narrow width and a large cross-sectional area as described above, it can be suitably used for a light receiving surface electrode among solar cell electrodes. However, it is not limited to the light receiving surface electrode, and can be used as a back surface electrode. For example, the back electrode is composed of an aluminum film covering the entire surface and a strip-like electrode overlapping therewith, but is also suitable as a constituent material of the strip-like electrode.

  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 a paste composition 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 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 dimension of the high concentration layer, that is, the layers 14 and 16 are formed. The thickness dimension is, for example, about 0.5 (μm). The impurity contained in the n ⁺ layer 14 is, for example, phosphorus (P) that is an n-type dopant, and the impurity contained in the p ⁺ layer 16 is, for example, boron (B) that is a p-type dopant.

  The antireflection film 18 is a thin film made of, for example, silicon nitride, and is provided with an optical thickness of about 1/4 of the visible light wavelength, for example, 10% or less, for example, 2 (%). It has a very low reflectivity.

The light receiving surface electrode 20 is made of, for example, a thick film conductor having a uniform thickness, and as shown in FIG. 2, the light receiving surface has a planar shape that forms a comb shape having a plurality of thin wire portions. 24 is provided on substantially the entire surface. The above thick film conductor is made of thick film silver containing about 93 (% by weight) Ag and about 7 (% by weight) lead borosilicate glass. Further, the thickness dimension of the conductor layer is, for example, in the range of 15 to 20 (μm), for example, about 17 (μm), and the width dimension of each thin line portion is in the range of, for example, 80 to 130 (μm), for example, About 100 (μm), the cross-sectional area is in the range of 1400 to 1900 (μm ² ), for example, about 1500 (μm ² ). For this reason, the light-receiving surface electrode 20 has a low line resistance within a range of, for example, 500 to 530 (Ω), for example, about 515 (Ω), and has high conductivity.

Further, the back electrode 22 is formed by applying a full-surface electrode 26 formed by applying a thick film material containing aluminum as a conductor component on the p ⁺ layer 16 over almost the entire surface, and a strip-like application on the full-surface electrode 26. The band-shaped electrode 28 made of thick film silver is formed. The belt-like electrode 28 is provided in order to make it possible to solder a conducting wire or the like to the back electrode 22.

  Since the solar cell 10 configured as described above has high conductivity because the line width of the light-receiving surface electrode 20 is narrow and the cross-sectional area is large as described above, the short-circuit current density Jsc, filter The factor FF is large compared to the conventional case, and the open-circuit voltage Voc is kept at the same level as the conventional case. As a result, the conversion efficiency is high.

  The light receiving surface electrode 20 as described above is formed by, for example, a well-known fire-through method. An example of the manufacturing method of the solar cell 10 including the formation of the light receiving surface electrode will be described below.

First, a vehicle is prepared with the composition shown in Table 1 below. For example, the following constituent materials are mixed using a stirrer or the like, and heated at 90 (° C.) for about 6 to 8 hours to dissolve ethyl cellulose.

  Of the above constituent materials, the two types of ethyl cellulose are both commercially available products (for example, Etocel (registered trademark) manufactured by Dow Chemical Co., Ltd.), but ethyl cellulose having an ethoxylation degree of 48 to 49.5 (%) has been conventionally used as an electrode. Used for conductive pastes (for example, Etocel (registered trademark) STD type). Further, ethylcellulose having an ethoxylation degree of 45 to 47 (%) is not used as an electrode paste, but is used as an explosive material or the like (for example, Etocel (registered trademark) MED type). These two types of ethyl cellulose are used in a ratio of STD type of 40 (wt%) and MED type of 60 (wt%). The vehicle thus prepared is hereinafter referred to as an improved vehicle.

Next, using the vehicle, a conductive paste is prepared with the composition shown in Table 2 below. This preparation process is performed by mixing the following constituent materials and dispersing them with, for example, a three-roll mill.

  Among the above constituent materials, the Ag powder is a spherical powder, and the average particle size is about 2 (μm). Separately prepared glass frit was used for lead borosilicate glass.

While preparing the conductive paste as described above, the n ⁺ layer 14 and the p ⁺ are diffused or implanted into an appropriate silicon substrate by a well-known method such as thermal diffusion or ion plantation. By forming the layer 16, the silicon substrate 12 is produced. Next, a silicon nitride thin film is formed thereon by an appropriate method such as spin coating, and the antireflection film 18 is provided.

Next, the conductive paste is screen-printed on the antireflection film 18 with the pattern shown in FIG. The printing line width was 100 (μm). This is dried at, for example, 120 (° C.), and further subjected to a baking treatment at 820 (° C.) in a near infrared furnace. As a result, the glass component in the conductive paste dissolves the antireflection film 18 in the firing process, and the conductive paste breaks the antireflection film 18, so that the electrical component between the conductive component in the conductive paste, that is, silver and the n ⁺ layer 14. As shown in FIG. 1, ohmic contact between the silicon substrate 12 and the light-receiving surface electrode 20 is obtained. The light receiving surface electrode 20 is formed in this way.

  Next, for example, an aluminum paste is applied to the entire back surface of the silicon substrate 12 by a screen printing method or the like, and a baking process is performed to form the entire surface electrode 26 made of a thick aluminum film. Further, the band-like electrode 28 is formed by applying the conductive paste on the surface of the entire surface electrode 26 in a band-like form using a screen printing method or the like and performing a baking treatment. Thereby, the back electrode 22 which consists of the full surface electrode 26 which covers the whole back surface, and the strip | belt-shaped electrode 28 provided in strip shape on a part of the surface is formed, and the said solar cell 10 is obtained.

  As a result of evaluating the characteristics of the light-receiving surface electrode 20 for the solar cell 10 thus obtained, a vehicle having a composition different from that shown in Table 1 above or a composition different from that shown in Table 2 above was obtained. This will be described together with evaluation results of other examples and comparative examples using paste.

  Table 3 summarizes the preparation compositions and evaluation results of Examples and Comparative Examples. In Table 3, Example 1 is a paste prepared using the improved vehicle shown in Table 1 with the specifications in Table 2. Examples 2 to 5 are pastes using the same improved vehicle but having different blending compositions. Of these, Examples 2 and 3 were obtained by increasing or decreasing the amount of silver powder as compared with Example 1 while maintaining the total amount of silver powder and glass frit as solids. In Examples 4 and 5, the solid content was reduced or increased compared to Example 1. In Examples 6 and 7, the ratio of MED type in ethyl cellulose was changed when preparing the vehicle. Example 6 was obtained by changing the MED type to 75 (%), and Example 7 was all made MED type. The preparation specifications of the paste are the same as in Example 1. Those used in Examples 6 and 7 are also called improved vehicles unless there is a particular need to distinguish them.

  Further, the pastes of Comparative Examples 1 to 4 were prepared with the same paste preparation composition as in Example 1 except that the vehicle and silver powder were different as follows. In Comparative Example 1, a conventional vehicle shown in Table 4 was used in place of the improved vehicle. In this conventional vehicle, the total amount of ethyl cellulose is STD type, and the paste of Comparative Example 1 is a conventional paste that does not contain MED type ethyl cellulose. In Comparative Example 2, instead of the improved vehicle, a vehicle in which the proportion of MED-type ethyl cellulose remained at 50 (wt%), which is smaller than that, was used. In Comparative Example 3, the above-described conventional vehicle was used, and a scaly silver powder was used in place of the spherical silver powder. In Comparative Example 4, scaly silver powder was used instead of spherical silver powder.

  As shown in the evaluation results of Table 3 above, when the improved vehicle and spherical Ag powder were used as in Examples 1 to 7, the resistivity was 3.1 to 3.3 (Ω · cm), which is the same as the conventional (Comparative Example 1). The film thickness is increased while the line width is reduced. That is, the fired film thickness was 10 to 12 (μm) in the past, but 14 to 19 (μm) in Examples 1 and 3, 15 to 20 (μm) in Example 2, and It increased to 14 to 18 (μm), in Example 5 to 16 to 20 (μm), and in Example 6 to 16 to 21 (μm). Further, the line width is 126 to 136 (μm) in the prior art, 98 to 105 (μm) in Example 1, 99 to 104 (μm) in Example 2, and 101 to 107 in Example 3. (μm), 102 to 108 (μm) in Example 4, 97 to 104 (μm) in Example 5, 98 to 105 (μm) in Example 6, 98 to 103 (μm) in Example 7, respectively Reduced.

  In addition, the resistivity is prepared for a silicon substrate separately from the solar cell manufacturing, the paste is applied to the silicon substrate in a sufficiently large area, dried at 120 (° C.), and then in a near infrared furnace 820 ( The conductive film for evaluation was formed by baking at [° C.] and measured. The resistivity of the conductor film for evaluation was measured by a four-terminal method using a resistivity meter, and converted by a fired film thickness for comparison.

In this way, the cross-sectional area, whereas was 1248~1453 (μm ²⁾ In the conventional, in Example 1 1423~1890 (μm ^2), in Example ² 1503~1804 (μm 2), carried out In Example 3, 1405-1857 (μm ² ), in Example 4, from 1432 to 1869 (μm ² ), in Example 5, from 1521 to 1908 (μm ² ), in Example 6, from 1435 to 1925 (μm ² ), and from Example 7 As a result, the line resistance was reduced to 508 to 538 (Ω) in Examples 1 to 7 as compared with the conventional line resistance of 654 (Ω) as a result of being increased to 1310 to 1960 (μm ² ). . Therefore, even when the light receiving surface electrode 20 is formed using any of the conductive pastes of Examples 1 to 7, since the effective light receiving area can be increased while reducing the line resistance, the conversion efficiency Eff is conventionally 15.28. For example, in Example 1, it was dramatically increased to 16.45 (%).

  In addition, the evaluation of solar cell characteristics uses a solar simulator to measure the current value and voltage value while changing the output current value by changing the load, and the short-circuit photocurrent density Jsc, open circuit voltage Voc, fill factor This was done by obtaining FF and conversion efficiency Eff. According to the embodiment, since the effective light receiving area increases because the line width dimension is reduced, the short circuit current density Jsc increases. In addition, since the cross-sectional area increases and the line resistance decreases, the fill factor FF increases. On the other hand, the open-circuit voltage Voc was maintained at about 0.61 (V), which is the same level as before, regardless of the conductive paste. Thus, according to the present embodiment, Jsc and FF increase and Voc is maintained, so that the conversion efficiency Eff is improved.

  The paste of Comparative Example 1 uses a vehicle consisting only of ethyl cellulose having a degree of ethoxylation of 48 to 49.5 (%) and BDGA. Therefore, when the viscosity is adjusted to be suitable for printing, it easily flows on the coated surface. Become. Therefore, when the light-receiving surface electrode is formed using this paste, a thin conductor film 30 having a wide width dimension is formed as schematically shown in FIG. On the other hand, the paste of the example uses a vehicle containing ethyl cellulose having an ethoxylation degree of 45 to 47 (%) in a ratio of 60 (%) or more of the whole ethyl cellulose, so that thixotropy is improved. It becomes difficult to flow on the coated surface while maintaining good printability. For this reason, the cross section of the light-receiving surface electrode 20 formed using this is narrower and higher in height than the comparative example 1 as schematically shown in FIG. The width can be reduced and the line resistance can be lowered.

  In addition, the vehicle used in the paste of Comparative Example 2 contains MED type ethyl cellulose, but the proportion of the total resin is only 50 (wt%), so both the fired film thickness and the line width are comparative examples. This is equivalent to 1 or slightly improved. That is, although a tendency to improve thixotropy is recognized, it is insufficient, and the line resistance is 659 (Ω), which is equivalent to that of Comparative Example 1.

  In Examples 6 and 7, an improved vehicle whose resin is MED type 75 (wt%) or 100 (wt%) is used. Even in such a specification, both the fired film thickness and the line width are examples. It is equivalent to 1, and the line resistance is also equivalent as described above. Therefore, according to the evaluation results of Comparative Examples 1 and 2, Examples 1, 6, and 7 and the like, it is essential that MED type ethyl cellulose is contained in a proportion of 60 (wt%) or more of the entire resin. It is clear that is not particularly defined.

Further, in Comparative Example 3 in which the flaky Ag powder was used in the conventional vehicle, since the flaky Ag powder reduces the flowability of the paste, the fired film thickness increases to 15 to 20 (μm), and the line The width is also slightly narrower, 120-124 (μm). However, in the paste of Comparative Example 3, the scale-like Ag powder prevents the paste from smoothly passing through the screen plate making and also reduces the plate separation property. Therefore, as schematically shown in FIG. The cross section of 32 becomes a shape close to a triangle. As a result, although the fired film thickness is increased as described above, the cross-sectional area is reduced to 900 to 1879 (μm ² ), and the line resistance is increased to 728 (Ω). In addition, the scaly powder has a disadvantage that the contact area between the powders is small and the resistivity is as high as 4.4 (Ω · cm). Therefore, the fact that the effective light receiving area is increased by reducing the line width dimension is not effectively utilized, and the light emission efficiency is 15.70 (%), which is a slight improvement over Comparative Example 1 or remains at an equivalent level. On the other hand, in Examples 1-7, since spherical powder is used, the filling rate in the coating film is increased, and therefore, when scaly silver powder is used as in Comparative Example 3, In comparison, the conductivity of the conductor generated from the coating film is increased.

  Further, in Comparative Example 4 using the improved vehicle but using the flaky Ag powder, since the vehicle and the Ag powder each decrease the flowability of the paste, the fired film thickness is further 15 to 25 (μm). As the thickness increases, the line width also decreases to 98-106 (μm). As a result, the line resistance is sufficiently low as 542 (Ω). However, since the screen plate making ability and the plate releasability are further lowered, as shown schematically in FIG. 6, the cross-section of the conductor film 34 is closer to a triangle, and the mesh marks are larger and formed. Variations in the film thickness of the conductor film are large, and the lineability is also poor. Therefore, a temporary characteristic can be obtained, and the conversion efficiency Eff is 16.43 (%) equivalent to that in Example 1. However, it is difficult to form a stable film, which is not suitable as a conductive paste for a solar cell electrode.

  In addition, although the comparative example 4 using the improved vehicle and the scale-like Ag powder has a problem in printability as described above, the line width, the line resistance, and the like are excellent, and as a result, the same as the example. Conversion efficiency is obtained. Therefore, in Table 3 above, this was taken as a comparative example in view of the evaluation results, but it can be used as a paste specification that provides the same characteristics as the examples by devising the printing method. That is, it is preferable to use a spherical Ag powder, but it is not essential, and a paste using a scaly powder is also included in the scope of the present invention.

  As described above, according to this example, the MED type, that is, 45 to 47 (%) and ethyl cellulose having a low degree of ethoxylation are contained in a proportion of 60 (%) or more of the entire resin, so that the thixotropy of the paste is increased. Therefore, the spread on the coated surface of the paste can be suitably suppressed while ensuring appropriate fluidity during squeezing. Therefore, a print pattern having a narrow width and a large cross-sectional area can be formed without impairing printability.

  In addition, according to this example, since the spherical silver powder is used as the conductor powder, the filling rate in the coating film is increased, so compared with the case where powder of other shapes such as scales is used. Thus, the conductivity can be increased, and the line width can be further reduced to increase the conversion efficiency.

  In addition, according to the present embodiment, the spherical powder having a smooth surface shape facilitates the flow of the paste, and thus easily escapes from the plate. For this reason, it is possible to suppress the deterioration of the plate separation and the reduction of the cross-sectional area of the printed pattern with the suppression of the spread on the coated surface. There is an advantage that can be formed.

  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.

It is a schematic diagram which shows the cross-section of the solar cell to which the paste composition of one Example of this invention was applied. It is a figure which shows an example of the light-receiving surface electrode pattern of the solar cell of FIG. 6 is a view showing a cross section of a conductor film of Comparative Example 1. FIG. 3 is a view showing a cross section of a conductor film of Example 1. FIG. 10 is a view showing a cross section of a conductor film of Comparative Example 3. FIG. It is a figure which shows the cross section of the conductor film of the comparative example 4.

Explanation of symbols

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 (3)

A solar cell electrode paste composition comprising a conductor powder, a glass powder, and a vehicle,
The vehicle includes a resin and a solvent in which ethyl cellulose having an ethoxylation degree of 45 to 47 (%) occupies 60 (% by weight) or more and ethyl cellulose having an ethoxylation degree of 48 (%) or more occupies the balance. A solar cell electrode paste composition.   The solar cell electrode paste composition according to claim 1, wherein the conductor powder is a spherical silver powder.   3. The solar cell according to claim 1, comprising 70 to 84 parts by weight of the conductor powder, 3 to 8 parts by weight of the glass powder, and 10 to 20 parts by weight of the vehicle. Paste composition.

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