JP2007026934A - Conductive paste and solar cell element produced using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conductive paste in which bleeding and plugging of a mesh portion are suppressed, having excellent mold releasing performance and easiness in making printing paste thick, and in which a printed wiring of high precision can be obtained, and to provide a solar cell element using the conductive paste. <P>SOLUTION: The conductive paste contains a conductive metal powder and an organic vehicle, has a viscosity of 200 Pa s or more and 350 Pa s or less at a shear rate of 10 s<SP>-1</SP>and a viscosity of 80 Pa s or more and 120 Pa s or less at a shear rate of 40 s<SP>-1</SP>at a temperature of 25°C, and has a gelling point in which, when the value of storage elastic modulus G' and loss elastic modulus G" are measured on the strain applied to the conductive paste in the range of 0% to 20% at a frequency of 1 Hz, the amount of measured values of the storage elastic modulus G' and the loss elastic modulus G" in the range is reversed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for forming a conductive circuit printed board represented by a solar cell, and more particularly to a method for manufacturing a solar cell element using a conductive paste for printing.

  Conventional methods for forming a conductor circuit on the surface of a substrate with a conductive paste include a screen printing method, a drawing method for drawing a circuit with a pen, and an etching method in which a circuit is formed by applying a predetermined circuit forming material on the entire surface of the substrate and then etching. There are laws. However, since it is superior in terms of productivity and reliability, a screen printing method with excellent reproducibility of fine lines is used in most cases.

As a typical conductive paste, there is one in which conductive metal powder is mixed and dispersed in an organic vehicle in which a resin is dissolved in a solvent. Such a typical conductive paste is printed by a screen printing method using a printing pattern and a resin squeegee and then baked to form a conductor wiring having a line width of about 100 μm on the substrate. Such a conductor wiring is used as a surface collector electrode of a solar cell or a thick film circuit on a ceramic substrate.
JP 2003-124052 A

However, in the above conventional technique, when a film is formed by screen printing on a substrate using a general printing pattern and a resin squeegee as described above, a general conductive paste is formed into a figure after printing. There are problems such as blurring and clogging in the mesh portion.

  These are particularly serious problems when trying to produce fine conductor circuit patterns, causing wiring defects due to circuit disconnection and the like, resulting in variations in characteristics of solar cells and generation of characteristic defects.

  Moreover, it is considered that the printing blur is caused by the filling amount of the paste into the pattern and the viscosity at the time of leveling after printing. The viscosity at the time of leveling is considered to be attributed to the elastic component in the paste when the viscosity is changed from a viscosity under a high shear rate during printing to a low shear rate during the leveling process. Therefore, it is considered that the clogging of the mesh portion is caused by the detachability of the paste filled in the pattern, that is, the viscous component under the shear rate applied when the paste is transferred to the substrate. When the paste viscosity is measured while changing the shear rate, the logarithm of each of the shear rate and the viscosity value is graphed to show a substantially straight line. Therefore, by controlling the inclination of this straight line, the viscosity at the time of paste filling, at the time of transfer, at the time of releasing the plate, and at the time of leveling is controlled. In addition, at low shear rates, such as during leveling, the change in the structure when the paste shear rate is applied is included in the linear response region, so the effect of the ratio of the viscous and elastic components that make up the paste becomes significant. . Therefore, in order to solve the problems at the time of printing as shown above, logarithm and logarithm of the measured value of the shear rate and the viscosity when the paste viscosity is measured by continuously changing the shear rate respectively. It is necessary to control the inclination of the straight line and the ratio of the viscous component and the elastic component in the paste linear region.

Therefore, Patent Document 1 includes a conductive metal powder and an organic vehicle, and has a viscosity of 1.0 Pa · s to 10 Pa · s and a slip rate of 10 s ^{−1 at} a temperature of 25 ° C. and a shear rate of 500 s ⁻¹ . Storage elastic modulus G ′ and loss elasticity measured by changing the strain applied to the conductive paste within a linear region of the conductive paste at a frequency of 1 Hz and a viscosity of 5 Pa · s to 20 Pa · s or less. A conductive paste having a ratio (tan δ (G ″ / G ′)) to a rate G ″ of 2.0 or more and 8.0 or less is disclosed. However, for example, when the conductive paste is printed to form an electrode of a solar cell element, the viscosity is low and the viscosity component is considerably larger than the elastic component. Therefore, an electrode having a large thickness and a sufficient thickness cannot be formed.

  In order to extract the electric power obtained by the incidence of sunlight on the solar cell element without loss, it is necessary to increase the cross-sectional area of the electrode provided on the solar cell element and suppress the resistance loss of the electrode. In the electrode provided on the light receiving surface side of the battery element, in order to secure a sufficient light receiving area, it is important to reduce the line width of the electrode and increase the thickness of the electrode. Therefore, when the above conductive paste is used, the problem is that the light receiving area is hindered due to the large blurring, and the resistance loss of the electrode is increased due to the lack of thickness, thereby producing a highly efficient solar cell element. Can not do it.

  Therefore, an object of the present invention is to provide a conductive paste that suppresses bleeding and clogging of the mesh portion, has good release properties, is easy to thicken the printing paste, and can obtain a highly accurate printed wiring. In addition, another object is to provide a solar cell element using the conductive paste.

  In order to achieve the above object, the conductive paste according to the present invention is configured as follows.

The conductive paste according to the present invention is a conductive paste containing a conductive metal powder and an organic vehicle, and has a viscosity of 200 Pa · s to 350 Pa · s at a slip rate of 10 s ^{−1 at} a temperature of 25 ° C. The storage elastic modulus G ′ and the loss elastic modulus in the range of 0% to 20% of the strain applied to the conductive paste at a speed of 40 s ⁻¹ with a viscosity of 80 Pa · s to 120 Pa · s and a frequency of 1 Hz. The value of G ″ is measured, and it has a point that the measured values of the storage elastic modulus G ′ and the loss elastic modulus G ″ are reversed within the above range, that is, a gelation point.

  In addition, the conductive metal powder contains 70 wt% or more and 90 wt% or less with respect to the paste weight.

  In addition, the conductive paste contains glass powder.

  Further, the total of the conductive metal powder and the glass powder includes 70 wt% or more and 90 wt% or less with respect to the paste weight, and the glass powder is included 2 wt% or more and 8 wt% or less. To do.

  The organic binder is added in an amount of 1% by weight to 6% by weight with respect to the paste weight, and 0.5% by weight to 1.7% of an additive for improving the dispersibility of the conductive metal powder and maintaining the dispersion stability. It is characterized by having a weight percent or less.

  The organic binder contains at least one resin selected from cellulose and acrylic.

  And the solar cell element of this invention has the conductive layer formed by baking the said electrically conductive paste.

  Furthermore, the solar cell element of the present invention is obtained by screen-printing and baking the conductive paste on a silicon substrate.

As described above, the conductive paste in the present invention is a conductive paste containing a conductive metal powder and an organic vehicle, and has a viscosity of 200 Pa · s to 350 Pa · s at a slip rate of 10 s ^{−1 at} a temperature of 25 ° C. Hereinafter, when the shear rate is 40 s ⁻¹ , the viscosity is 80 Pa · s or more and 120 Pa · s or less, and the strain applied to the conductive paste at a frequency of 1 Hz is a storage elastic modulus G ′ within a range of 0% to 20%. The value of the loss modulus G ″ is measured, and the measured value of the storage modulus G ′ and the loss modulus G ″ is reversed within the above range, that is, has a gelation point. As a result, it is possible to suppress bleeding and clogging of the mesh part, to improve the plate release property, to easily increase the thickness of the printing paste, and to improve problems in printing.

  And when the electrode of a solar cell element is screen-printed using the conductive paste which improved the printability in this way, the dispersion | variation in a printed figure pattern area is suppressed and the film thickness of a figure pattern becomes thick. In the solar cell element formed by baking after forming the conductive paste film, there is an effect of improving the characteristics.

  Until now, the viscosity of the conductive paste for screen printing has often been evaluated only by a rotational viscometer. However, the viscosity measured at a certain number of revolutions is the viscosity at a certain shear rate, and it is extremely difficult to estimate the paste's viscoelastic behavior during screen printing, where the shear rate applied to the paste changes every moment. is there.

  In addition, since the conductive paste is a viscoelastic body, it is estimated that the ratio of the viscous component and the elastic component in each paste has a significant effect on printability, but it is impossible to evaluate viscoelasticity with a rotational viscometer. It is.

  Therefore, in the present invention, the conductive paste of the present invention is designed by controlling the paste viscosity when the slip rate applied to the conductive paste changes every moment, and the viscous and elastic components in the conductive paste using a rheometer. .

  In the conductive paste of the present invention, the paste viscosity change measurement and dynamic viscoelasticity measurement when the shear rate is continuously changed are measured using a rheometer between the two disks or cones of the measurement unit. Evaluation was made by placing and applying a shear rate or strain to the paste and measuring the stress generated in the paste.

In the present invention, at 25 ° C., until 0～40S ^-1, the time of constantly changing the shear rate, the measurement of the viscosity change in the paste, upon changing the strain to 0-20% at a frequency 1Hz The storage elastic modulus and loss elastic modulus were measured, and the storage elastic modulus and loss elastic modulus within the range were determined.

Regarding the viscosity of the paste, when the viscosity is extremely high, there is a case where printing cannot be performed due to the problem of the ability to remove the paste from the plate making. On the other hand, when the viscosity is extremely low, there is a case where a thick film cannot be formed due to a problem of blurring after printing. Based on these matters, it is considered that the paste viscosity needs to be within a certain range. This viscosity range, the viscosity at a shear rate ^{10s -1} is not higher than 200 Pa · s or more 350 Pa · s, the viscosity at a shear rate ^{40 s -1} is less 80 Pa · s or more 120 Pa · s.

  However, just keeping the viscosity within this range cannot improve the problem of bleeding after printing and clogging of the mesh portion of the plate making.

  Therefore, the storage elastic modulus G ′ and the loss elastic modulus G ″ when the strain is changed from 0 to 20% at a frequency of 1 Hz are measured, and the measured storage elastic modulus G ′ and loss elastic modulus G ″ are the above values. It was confirmed that a thick film can be formed without causing blurring after printing while suppressing clogging of the mesh portion of the plate making by having a point that reverses within the range, that is, a gelation point.

  When a sinusoidal wave is applied as a strain to such a conductive paste sample, the elasticity can be measured separately using the same phase and the viscosity being 90 degrees out of phase. The frequency may be set within the range in which the conductive paste can linearly respond, and it has been confirmed that 0 to 5 Hz is within the linear response range in the plurality of pastes used in this evaluation. Therefore, the dynamic viscoelasticity measurement was evaluated by paying attention to 1.0 Hz which is within the range where the paste can linearly respond. In addition, the strain measurement range was set to 0% to 20%. If measurement was performed within this range, most printing conductive pastes were measured from the linear viscoelastic range to the range where changes in elasticity and viscosity occurred. Because it was confirmed that it can be performed, it was set in this range.

  When the storage elastic modulus G ′ is larger than the loss elastic modulus G ″ in the linear viscoelastic region, the conductive paste becomes an elastic body and thus does not flow easily. Although it is extremely inferior, it does not cause blurring.

  On the contrary, when the storage elastic modulus G ′ is smaller than the loss elastic modulus G ″ in the linear viscoelastic range, the viscosity of the paste is too strong compared to the elasticity, so that it is very easy to flow. Causes sagging and blurring, but the plate-making ability is improved and clogging of the mesh portion of the plate-making can be suppressed.

  Accordingly, when the strain is changed from 0 to 20%, the values of the storage elastic modulus G ′ and the loss elastic modulus G ″ are reversed. Therefore, when this conductive paste is screen-printed, it suppresses clogging of printed blemishes and the mesh part of the plate making, improves plate separation, and increases the thickness of the printing paste. It was confirmed that this can be done easily.

  Further, the ratio (tan δ (G ″ / G ′)) between the storage elastic modulus G ′ and the loss elastic modulus G ″ in the linear viscoelastic region is preferably 0.4 or more and 1.2 or less. If it is less than, the plate-making ability is extremely inferior, and clogging to the mesh part of the plate-making cannot be suppressed. This is not preferable because thickening cannot be performed.

  The solar cell element using the conductive paste of the present invention can be produced by a known manufacturing method using the conductive paste of the present invention for an electrode.

  Hereinafter, the solar cell element of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a cross-sectional view showing the structure of the solar cell element of the present invention. In FIG. 1, 1 is a semiconductor substrate, 2 is a diffusion layer, 3 is an antireflection film, 4 is a surface electrode, 5 is a back electrode, and 6 is a back surface electric field region.

  First, the semiconductor substrate 1 is prepared. This semiconductor substrate 1 is a substrate having a specific resistance of about 0.2 to 2.0 Ω · cm containing one conductivity type semiconductor impurity such as boron (B). A single crystal silicon substrate is formed by a pulling method or the like, and a polycrystalline silicon substrate is formed by a casting method or the like. The polycrystalline silicon substrate can be mass-produced and is more advantageous than the single crystal silicon substrate in terms of manufacturing cost. An ingot formed by a pulling method or a casting method is cut into an appropriate size, such as 10 cm × 10 cm or 15 cm × 15 cm, and sliced to a thickness of 500 μm or less, more preferably 300 μm or less to obtain a semiconductor substrate 1.

  Thereafter, a very small amount of the surface is etched with NaOH, KOH, hydrofluoric acid, or hydrofluoric acid in order to clean the cut surface of the substrate.

  Further, it is desirable to form minute protrusions on the surface on the light receiving surface side of the semiconductor substrate by using a dry etching method or a wet etching method in order to effectively take light into the substrate.

  Next, the semiconductor substrate 1 is placed in a diffusion furnace, and heat treatment is performed in a gas containing an impurity element such as phosphorus oxychloride, thereby diffusing phosphorus atoms in the surface portion of the semiconductor substrate 1 to reduce the sheet resistance to 30 to A diffusion layer 2 having an n-type conductivity type of about 300Ω / □ and a thickness of about 0.2 to 0.5 μm is formed.

  Then, the n-type diffusion layer 2 is left only on the surface side of the semiconductor substrate 1 and other portions are removed, followed by washing with pure water. The removal of the n-type diffusion layer 2 other than the surface side of the semiconductor substrate 1 is performed by, for example, applying a resist film on the surface side of the semiconductor substrate 1 and etching and removing it using a mixed solution of hydrofluoric acid and nitric acid. This is done by removing it.

  Further, an antireflection film 3 is formed on the surface side of the semiconductor substrate 1. The antireflection film 3 is made of, for example, a silicon nitride film, and is formed by, for example, a plasma CVD method in which a mixed gas of silane and ammonia is plasmatized by glow discharge decomposition and deposited. The antireflection film 3 is formed so as to have a refractive index of about 1.8 to 2.3 in consideration of a refractive index difference with the semiconductor substrate 1 and is formed to a thickness of about 500 to 1000 mm. . This silicon nitride film has a passivation effect when formed, and has an effect of improving the electrical characteristics of the solar cell together with an antireflection function.

  Then, the front electrode 4 and the back electrode 5 are simultaneously formed by applying and baking a silver paste on the surface of the semiconductor substrate 1 and applying an aluminum paste and a silver paste on the back surface.

  The conductive paste of the present invention is added for conductive metal powder having conductivity such as silver, an organic vehicle in which an organic binder is dissolved in an organic solvent, and for improving dispersibility and maintaining dispersion stability of the conductive metal powder. Contains agents. Moreover, it has a gelation point in the range of 1 Hz, which is a linear response region, and a strain of 0 to 20%.

  In the conductive paste of the present invention, the conductive metal powder uses a flake powder of about 3 to 10 μm and a spherical powder of about 0.1 to 1.0 μm, and is 70 wt% or more and 90 wt% with respect to the paste weight. It is preferable to contain within the following ranges. The paste may contain glass powder, and the particle size distribution of the solid component composed of the conductive metal powder and the glass powder is controlled within a range of about 0.1 to 10 μm, and the conductive metal powder and the glass powder are pasted. It is preferable to contain 70 to 90% by weight with respect to the weight, of which glass powder is contained within the range of 2 to 8% by weight with respect to the important paste. If a solid component is 70 weight% or more, the extreme viscosity tendency of a paste can be reduced and the state where printing blurring tends to occur can be suppressed. On the contrary, if it is 90 weight% or less, the extreme elasticity tendency of a paste can be reduced and the state which a paste tends to clog in a printing pattern can be suppressed. Therefore, it becomes possible to produce a conductive paste having a gelation point by setting the particle size distribution and the content within the above range.

  However, if the particle size distribution range of the solid component is 0.01 to 1.0 μm, it becomes extremely elastic due to an increase in specific surface area, and the paste tends to clog in the printed pattern. When the thickness is 20 μm, the viscosity tends to be extremely high due to a decrease in the specific surface area, and printing blemishes are likely to occur.

In the aforementioned organic vehicle, the organic binder is preferably contained in an amount of 1% by weight to 6% by weight based on the paste weight. When the organic binder is 1% by weight or more and when the organic binder is 6% by weight or less, the viscosity of the organic vehicle in which the organic binder is dissolved in the organic solvent becomes appropriate, and the viscosity of the paste has a slip rate of 10 s ^−1. The viscosity at the time of 200 Pa · s to 350 Pa · s, and the viscosity at a shear rate of 40 s ⁻¹ can satisfy the range of 80 Pa · s to 120 Pa · s.

  Moreover, it is preferable that an additive contains 0.5 to 1.7 weight% with respect to the paste weight. The additive contributes to improvement in dispersibility and dispersion stability of the conductive metal powder in the paste. Therefore, if the amount is less than 0.5% by weight, the effect of dispersing is insufficient, so that a structure that easily causes aggregation of metal powders is formed, resulting in a paste having a very strong elastic tendency. On the other hand, when the amount is more than 1.7% by weight, the interfacial energy in the paste system is lowered, and the paste is very strong in viscosity.

  Examples of the organic solvent used in the conductive paste of the present invention include butyl carbitol, butyl carbitol acetate, butyl cellosolve, butyl cellosolve acetate, terpineol, hydrogenated terpineol, hydrogenated terpineol acetate, methyl ethyl ketone, isobornyl acetate, and nopylacetate. It is done. Examples of the organic binder include cellulose resins such as methyl cellulose, ethyl cellulose, and nitrocellulose, acrylic resins such as methyl methacrylate, and butyral resins. Examples of the additive include anionic and nonionic dispersants and surfactants.

  Since the organic vehicle used at this time decomposes and volatilizes at about 400 ° C., the components do not remain in the electrodes 4 and 5 after firing. The glass frit is used to give strength to the baked electrodes 4 and 5. Some glass frit contains oxides such as lead, boron, and silicon, and has various softening points of about 300 to 600 ° C., but some remain in the electrodes 4 and 5 after firing, and some silicon. In order to weld the electrodes 4 and 5 and the silicon substrate 1.

  When thin-line thick film printing is performed at high speed and continuously, it is presumed that the ratio between the storage elastic modulus G ′ and the loss elastic modulus G ″ has an influence on the shape defect due to blurring or clogging of the mesh portion. In particular, since printing is performed at high speed and continuously, blurring after printing becomes noticeable.

  Therefore, a paste having a gelation point where the storage elastic modulus G ′ and the loss elastic modulus G ″ intersect within a strain range of 1 Hz, which is a linear response region, and 0-20%, which is a low strain region, is produced to suppress blurring. be able to.

  As shown in FIG. 2A, the surface electrode 4 is composed of a surface bus bar electrode 4a for taking out an output from the surface and a surface finger electrode 4b for current collection provided so as to be orthogonal thereto. Further, as shown in FIG. 2B, the back electrode 5 includes a back bus bar electrode 5a and a back current collecting electrode 5b for extracting output from the back surface.

  The back surface collecting electrode 5b is formed by applying an aluminum paste made of a conductive metal powder made of aluminum powder, an organic vehicle and glass powder to form a paste, for example, by a screen printing method. It is baked by baking for about 30 minutes. At this time, the back surface electric field region 6 is formed which prevents aluminum from diffusing into the semiconductor substrate 1 and recombination of carriers generated on the back surface. The back surface electric field region 6 is also called a BSF (Back Surface Field) region, and prevents a decrease in efficiency due to recombination of photogenerated carriers near the back surface of the semiconductor substrate 1.

  Further, the front bus bar electrode 4a, the front finger electrode 4b, and the rear bus bar electrode 5a are applied by applying a silver paste made of a conductive metal powder, an organic vehicle, and a glass frit, for example, by a screen printing method. After drying, baking is performed by baking at a maximum temperature of 600 to 800 ° C. for about 1 to 30 minutes. As a coating method, a known method other than the screen printing method may be used. The surface electrode 4 may be formed by etching away a portion corresponding to the electrode of the antireflection film 3 or may be directly formed on the antireflection film 3 by a technique called fire-through.

  After the back bus bar electrode 5a for output extraction is formed, the back current collecting electrode 5b is formed so as not to cover a part of the back bus bar electrode 5a. Note that the order of forming the back surface bus bar electrode 5a and the back surface collecting electrode 5b may be reversed. Further, the back electrode 5 does not have the above-described structure, and may have a structure composed of a bus bar electrode mainly composed of silver and finger electrodes similar to the front electrode 4.

  As described above, in the electrode of the solar cell element produced using the conductive paste of the present invention, clogging of the mesh portion of the plate making is suppressed, so that productivity is not lowered, Since it is possible to increase the thickness of the printing paste, the surface finger electrode 4b of the solar cell element can be formed with a narrow and thick electrode, thereby reducing the light receiving area. In addition, as a result of suppressing the resistance loss of the surface finger electrode, a highly efficient solar cell element can be formed.

[Production of paste]
First, butyl carbitol acetate as an organic solvent, butyral resin as an organic binder, and a nonionic dispersant as an additive are mixed and dissolved to prepare an organic vehicle, and the prepared organic vehicle, glass powder, A conductive paste was prepared by mixing with silver powder and uniformly dispersing with three rolls.

(Paste viscosity measurement)
The paste viscosity of the paste of this example when the shear rate was continuously changed was measured. A rheometer (manufactured by Physica, type: MCR-300) was used for the measurement. The measurement conditions were a constant temperature of 25 ° C., the shear rate was applied for 60 seconds at 10 s ⁻¹ and 40 s ⁻¹ time points, and the viscosity was measured after 60 seconds.

[Measurement of ratio tan δ between storage elastic modulus G ′ and loss elastic modulus G ″ of paste]
The dynamic viscoelasticity of the paste of this example was measured. The same rheometer as described above was used for the measurement. The measurement conditions were that the storage elastic modulus G ′ and the loss elastic modulus G ″ were measured by changing the strain applied to the conductive paste from 0 to 20% at a frequency of 1 Hz at a constant temperature of 25 ° C.

[Manufacture of solar cell elements]
The damage layer on the surface of the p-type semiconductor substrate 1 of polycrystalline silicon having a thickness of 250 μm, an outer shape of 15 cm × 15 cm, and a resistance of 1.5 Ω · cm was etched and washed with NaOH. Next, the semiconductor substrate 1 is placed in a diffusion furnace and heated in phosphorus oxychloride (POCl ₃ ) to diffuse phosphorus atoms on the surface of the semiconductor substrate 1, thereby forming an n-type diffusion layer 2. Formed. A silicon nitride film having a thickness of 850 mm to be the antireflection film 3 was formed thereon by plasma CVD.

  And the aluminum paste was apply | coated to the whole back surface side, it heated, and the back surface electric field area | region 6 was formed. The aluminum paste was removed by ultrasonic cleaning.

  The conductive paste produced on the front side and the back side of the semiconductor substrate 1 is 230 mesh, a printing pattern with an emulsion part thickness of 20 μm, and a resin squeegee with a rubber hardness of 70 degrees by screen printing, as shown in FIG. It was applied to the shape shown in a) and dried. In addition, it formed in the grid | lattice form similarly to the surface side also on the back side.

[Investigation of blemishes and missing parts in printed graphic patterns]
The width of the surface finger electrode having a design line width of 100 μm of the graphic pattern printed using the printed pattern shown in FIG. 2A was observed with an optical microscope and measured. In addition, the surface finger electrode was observed with an optical microscope to determine whether a defect portion was generated.

[Investigation of clogging of printed pattern after printing]
In order to investigate whether the mesh part clogging had arisen in the printed pattern after printing, the printed pattern after printing was observed with the optical microscope from the back surface.

[Investigation of thickness of graphic pattern after printing and film thickness difference between center and edge]
The film thickness of the surface finger electrode after printing was measured. The measurement was carried out using a stylus type surface roughness meter manufactured by Tokyo Seimitsu Co., Ltd.

In the samples 1 to 11 shown in Table 1, the result of the viscosity, the point where the storage elastic modulus G ′ and the loss elastic modulus G ″ intersect, that is, the presence of gelation points, blemishes, clogging and defects, and the film thickness of the surface finger electrode The results are shown in Table 2. Among these results, the amount of blurring is 15 μm or less, the figure pattern after printing is free of defects, and the film thickness is 15 μm or more. In Table 1, the sample numbers marked with * are outside the scope of the present invention, and all others are within the scope of the present invention.

Here, attention is focused on these viscosities and gelation points at 10 s ⁻¹ and 40 s ⁻¹ . No. Nos. ¹ , 5, and 6 satisfy the gelation point at a shear rate of 10 s ^-1 at a viscosity of 200 Pa · s to 350 Pa · s, at a shear rate of 40 s ^-1 at 80 Pa · s to 120 Pa · s. It was a conductive paste and had good printability with no clogging or defects.

However, the viscosity at a shear rate of 10 s ⁻¹ is greater than 350 Pa · s, the viscosity at a shear rate of 40 s ⁻¹ is greater than 120 Pa · s, In the conductive pastes 2, 7, 9, and 10, clogging and defective portions were observed.

Further, the viscosity at a shear rate of 10 s ⁻¹ is less than 200 Pa · s, the viscosity at a shear rate of 40 s ⁻¹ is less than 120 Pa · s, or has no gelation point. In 3, 4, 8, and 11, the result was that the amount of fringes on the finger electrode was larger than 15 μm and the film thickness of the finger electrode was smaller than 15 μm.

From the above, the viscosity when the shear rate is 10 s ⁻¹ is 200 Pa · s or more and 350 Pa · s or less, and when the shear rate is 40 s ⁻¹ , the viscosity is 80 Pa · s or more and 120 Pa · s or less. The clogged portion does not cause a defect in the formed film. Further, the storage elastic modulus G ′ and the loss elastic modulus G ″ are measured within a range of 0% to 20% of the strain applied to the conductive paste at a frequency of 1 Hz. By setting the conductive paste so that the measurement value of the rate G ″ is reversed, that is, has a gelation point, the printed pattern pattern is not blurred. Moreover, a thick film as designed can be formed by using a conductive paste that satisfies the above conditions.

[Printing the paste above and firing to investigate characteristics]
Sample No. on which the conductive paste was printed. 1 to 11 were fired at a maximum temperature of 750 ° C. for about 5 minutes to produce solar cell elements.

The measurement results of various characteristics of the solar cell element are shown in the following table.

As is apparent from Table 3, the viscosity when the shear rate is 10 s ⁻¹ is 200 Pa · s to 350 Pa · s, the viscosity when the shear rate is 40 s ⁻¹ is 80 Pa · s to 120 Pa · s, and the frequency. The storage elastic modulus G ′ and the loss elastic modulus G ″ are measured in the range of 0% to 20% of the strain applied to the conductive paste at 1 Hz, and the storage elastic modulus G ′ and the loss elastic modulus G ″ are within the above ranges. Of sample No. having a gelation point at which the magnitude of the measured value is reversed. The solar cell elements produced using the conductive pastes 1, 5, and 6 have a conversion efficiency exceeding 16%, and the conversion efficiency can be improved by suppressing the amount of blurring and ensuring the film thickness. I understood.

It is a figure which shows the structure of the cross section of a common solar cell element. It is a figure which shows an example of the electrode shape of a common solar cell element, (a) is a light-receiving surface side (front surface), (b) is a non-light-receiving surface side (back surface).

Explanation of symbols

1: Semiconductor substrate 2: Diffusion layer 3: Antireflection film 4: Front electrode 4a: Front bus bar electrode 4b: Front finger electrode 5: Back electrode 5a: Back bus bar electrode 5b: Back current collecting electrode 6: Back surface electric field region

Claims (8)

A conductive paste containing conductive metal powder and organic vehicle,
At a temperature of 25 ° C., the viscosity at a shear rate of 10 s ⁻¹ is 200 Pa · s to 350 Pa · s, the viscosity at a shear rate of 40 s ⁻¹ is 80 Pa · s to 120 Pa · s, and the frequency is 1 Hz. A conductive paste having a gelation point at which the measured values of the storage elastic modulus G ′ and the loss elastic modulus G ″ are reversed when the strain applied to the conductive paste is in the range of 0% to 20%. 2. The conductive paste according to claim 1, wherein the conductive metal powder contains 70 wt% or more and 90 wt% or less with respect to the paste weight. The conductive paste according to claim 1, wherein glass powder is contained. The total of the conductive metal powder and the glass powder includes 70% by weight or more and 90% by weight or less with respect to the paste weight, and the glass powder is included by 2% by weight or more and 8% by weight or less. Item 4. The conductive paste according to Item 3. The organic binder is added in an amount of 1% by weight to 6% by weight with respect to the paste weight, and 0.5% by weight to 1.7% by weight of additives for improving the dispersibility of the conductive metal powder and maintaining the dispersion stability. The conductive paste according to claim 1, further comprising: The conductive paste according to any one of claims 1 to 5, wherein the organic binder contains at least one resin selected from cellulose and acrylic. The solar cell element which has a conductive layer formed by baking the electrically conductive paste in any one of Claims 1 thru | or 6. The solar cell element formed by screen-printing and baking the electrically conductive paste in any one of Claims 1 thru | or 7 on a silicon substrate.

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