JP2010010495A - Silicon type solar cell method of manufacturing and aluminum paste used for the method of manufacturing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solar cell manufacturing method that can prevent deformation occurrences such as a conspicuous warp in a silicon semiconductor substrate, and that guarantees a high BSF effect. <P>SOLUTION: The solar cell manufacturing method includes: use of aluminum paste, as a material to form aluminum electrodes, which includes aluminum powder whose main particle diameter (D50) in a particle size distribution based on laser diffraction method is in the range of 2 μm to 8.5 μm, a bismuth type glass frit containing bismuth oxide, as an indispensable constituent, having a glass softening point of no more than 580°C, and an organic vehicle; formation of aluminum electrodes by applying the aluminum paste to the substrate and baking it; and removal of a low-density aluminum layer, which easily collapses, consisting of a material containing aluminum that is part of the baked aluminum electrode. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a solar cell (cell) and a method for producing the same, and more particularly to a method for producing a silicon-based solar cell and an aluminum paste suitably used for the method.

Conventionally, as a typical example of a solar cell mainly composed of silicon (semiconductor substrate) such as crystalline silicon and amorphous silicon (hereinafter collectively referred to as “silicon-based solar cell”), a single-sided light receiving type solar as shown in FIG. A battery 10 is known.
This solar cell 10 includes an n-Si layer 16 formed by pn junction formation on the light-receiving surface side of a p-Si layer (p-type crystalline silicon) 18 of a silicon semiconductor substrate (Si wafer) 11, and on its surface. An antireflection film 14 made of titanium oxide or silicon nitride formed by CVD or the like, and a surface electrode (light-receiving surface electrode) 12 made of Ag typically formed by screen printing and baking a silver paste are provided. . On the other hand, on the back surface side of the p-Si layer 18, similarly to the front surface electrode 12, a back surface side external connection electrode 22 made of Ag formed by screen printing and baking a silver paste and a so-called back surface electric field (BSF; And an aluminum electrode 20 having a back surface field effect.
The aluminum electrode 20 is formed on substantially the entire back surface by printing and baking an aluminum paste mainly composed of aluminum powder. During this firing, an Al—Si alloy layer (not shown) is formed, and aluminum diffuses into the p-Si layer 18 to form the p ⁺ layer 24. By forming the p ⁺ layer 24, that is, the BSF layer, the photogenerated carriers are prevented from recombining in the vicinity of the back electrode, and for example, an improvement in short circuit current and open circuit voltage (Voc) is realized.

Incidentally, in order to effectively realize the BSF effect, it is necessary to form the aluminum electrode 20 with a certain thickness (for example, 50 to 60 μm). On the other hand, for reasons such as reduction in manufacturing cost of solar cells (solar cells) and downsizing of solar cell modules, it is required to further reduce the thickness of the silicon semiconductor substrate (Si wafer) 11, that is, the solar cell element itself. It has been.
However, the thinning of the silicon semiconductor substrate (Si wafer) 11 is caused by the difference between the thermal expansion coefficient of the substrate 11 itself and the thermal expansion coefficient of the aluminum electrode 20 during firing for forming the aluminum electrode 20. (Wafer) It helps to cause deformation such as warping and bending. For this reason, conventionally, various devices for preventing the occurrence of deformation such as warping have been made.
For example, Patent Document 1 discloses an aluminum paste for forming an aluminum electrode, wherein an aluminum-containing organic compound is added. Patent Document 2 discloses an inorganic compound powder that is an aluminum paste for forming an aluminum electrode and has a thermal expansion coefficient smaller than that of aluminum and any one of a melting temperature, a softening temperature, and a decomposition temperature higher than the melting point of aluminum. An aluminum paste characterized by containing is disclosed.
In Patent Document 3, an aluminum electrode is formed by firing as a method for suppressing cell cracking in the subsequent process due to warpage of the silicon semiconductor substrate (cell) generated after firing due to the difference in thermal shrinkage between aluminum and silicon. A method is described in which the surface of a substrate (cell) is subjected to ultrasonic etching so that the reflectance with respect to light having a wavelength of 600 to 1000 nm is 30% or more.

JP 2000-90734 A Japanese Patent Laid-Open No. 2003-223813 JP 2004-235272 A

  The present invention was created to solve the above-mentioned conventional problems by an approach different from that of each of the above-mentioned patent documents, and the silicon semiconductor substrate (wafer) undergoes significant warping deformation while ensuring a high BSF effect. It aims at providing the manufacturing method of the solar cell (cell) which can prevent doing. It is another object of the present invention to provide a solar cell (cell) that is manufactured by such a manufacturing method and has no deformation such as the warp. Another object of the present invention is to provide an aluminum paste for forming an aluminum electrode that is suitably used in such a manufacturing method.

  The inventor applied an aluminum paste satisfying a predetermined condition on a silicon semiconductor substrate (that is, a semiconductor substrate made of silicon) and then baked, and a part of the aluminum electrode was easily baked after baking. I found it to collapse. In other words, in an aluminum electrode formed on a silicon semiconductor substrate using an aluminum paste that satisfies a predetermined condition, a part of the aluminum electrode collapses as a low-density layer after firing, resulting in high-density warping. It has been found that a difficult thin aluminum electrode can be easily formed on a silicon substrate, and the present invention has been completed.

  That is, one method provided by the present invention includes a silicon semiconductor substrate (hereinafter sometimes abbreviated as “Si substrate”), a light-receiving surface electrode formed on one surface side of the substrate, and the substrate. It is a method of manufacturing a silicon-type solar cell provided with the aluminum electrode formed in the other surface side. In the silicon-based solar cell manufacturing method disclosed herein, as the material for forming the aluminum electrode, aluminum powder having a particle size distribution based on the laser diffraction method (D50) of 10 μm or less and bismuth oxide are essential. An aluminum electrode is formed by using an aluminum paste containing bismuth glass frit having a glass softening point of 580 ° C. or less and an organic vehicle, and applying the aluminum paste on the substrate and baking it. And removing a readily disintegrating low-density aluminum layer made of a material containing aluminum, which is a part of the fired aluminum electrode.

In the manufacturing method having such a configuration, (1) aluminum powder having a particle size distribution based on laser diffraction method (D50) of 10 μm or less, (2) bismuth oxide as an essential component and glass softening point of 580 ° C. or less. An aluminum electrode is formed on the Si substrate using an aluminum paste containing the bismuth-based glass frit and (3). An organic vehicle that is a dispersion medium for these components. And in the manufacturing method of this invention, when the aluminum paste coating material (aluminum layer) obtained by apply | coating this paste on Si substrate is baked, the one part that consists of a substance containing aluminum is easy to disintegrate the low density. An aluminum layer (hereinafter also simply referred to as “low density layer” or “collapsed aluminum layer”) is formed.
After the firing, the collapsed aluminum layer can be physically easily removed, and after the collapsed aluminum layer is removed, an aluminum electrode having a high density and a reduced thickness is formed. Such a high-density and thin-film aluminum electrode has a property of being difficult to warp.
Therefore, according to the manufacturing method of the present invention, it is possible to manufacture a highly reliable silicon-based solar cell (cell) that guarantees a high BSF effect and has no deformation such as warpage or a low degree of the silicon semiconductor substrate.

In a preferred aspect of the method for producing a silicon-based solar battery (cell) disclosed herein, the low-density layer is removed by blowing a gas to the fired aluminum electrode.
In the manufacturing method of this aspect, the collapsing aluminum layer is removed by a simple operation such as blowing a gas (typically an inert gas such as air or nitrogen gas) or rolling the substrate surface with an adhesive roller. can do.

In another preferred embodiment of the method for producing a silicon-based solar battery (cell) disclosed herein, the glass frit has a specific surface area based on the BET method of at least 2 m ² / g.
In another preferred embodiment of the method for producing a silicon-based solar battery (cell) disclosed herein, the content of aluminum powder in the aluminum paste is 70 to 80% by mass, and the glass frit is Content rate is 2-10 mass%, It is characterized by the above-mentioned. In the production methods of these embodiments, the collapsed aluminum layer can be formed particularly easily.

Moreover, this invention provides the paste-form aluminum electrode formation material used suitably for the said manufacturing method.
That is, an aluminum paste for forming an aluminum electrode of a silicon-based solar cell provided by the present invention comprises an aluminum powder having a center particle size (D50) of particle size distribution based on laser diffraction method of 10 μm or less, and bismuth oxide. A bismuth-based glass frit having an essential component and a glass softening point of 580 ° C. or lower and an organic vehicle are included.
The aluminum paste coated product (aluminum layer) obtained on the Si substrate by using the aluminum paste of the present invention is such that a part of the aluminum layer forms a low-density collapsed aluminum layer after firing. By removing with an appropriate physical means, as described above, a high-density and thin-film aluminum electrode with less warpage can be formed on the Si substrate.
Therefore, according to the present invention, by using the aluminum paste disclosed herein, a highly reliable silicon-based solar cell that guarantees a high BSF effect and has no deformation such as warpage or the like, or a low degree thereof. Cell).

Preferably, the specific surface area based on the BET method of the glass frit contained in the aluminum paste of the present invention is at least 2 m ² / g. Moreover, it is preferable that the content rate of the aluminum powder in this paste is 70-80 mass%. Moreover, it is preferable that the content rate of the glass frit in this paste is 2-10 mass%.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, matters other than matters specifically mentioned in the present specification (for example, the form, composition, mixing ratio, etc. of aluminum powder and glass frit) and matters necessary for the implementation of the present invention (for example, a paste preparation method, The general manufacturing process of solar cells (cells) that does not characterize the present invention can be understood as a design matter for those skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

The aluminum paste disclosed here is an aluminum paste used for forming an aluminum electrode in a silicon-based solar cell (also referred to as a solar cell or a solar cell element), and includes aluminum powder, glass frit, and an organic vehicle. As long as the object of the present invention can be realized, there are no particular restrictions on the other components.
In this specification, the aluminum powder refers to an aggregate of particles mainly composed of aluminum (Al), and is typically an aggregate of particles composed of Al alone. However, an impurity other than Al or an alloy mainly composed of Al is used. Even a trace amount may be included in the “aluminum powder” as long as it is an aggregate of particles mainly composed of aluminum as a whole. The aluminum powder itself may be produced by a conventionally known production method and does not require special production means.

Further, regarding the particle size distribution of the aluminum powder, the “center particle size” refers to a particle size at a cumulative volume of 50% in the particle size distribution of the powder, that is, D50 (median diameter). Such D50 can be easily measured by various particle size distribution measuring devices based on a laser diffraction method (that is, a particle size distribution is determined by a scattering pattern when a laser beam is irradiated and scattered on a measurement sample).
As for the aluminum powder contained in the aluminum paste disclosed here, the center particle size (D50) of the particle size distribution based on the laser diffraction method is suitably 10 μm or less, and the center particle size is 2 μm to 8.5 μm. preferable. Those having a center particle diameter of 3 to 6 μm are particularly preferred for the above-mentioned use. By including an aluminum powder having a center particle diameter in such a range, a collapsed aluminum layer can be formed more easily.

The particles constituting the aluminum powder to be used are typically spherical, but are not limited to the so-called spherical shape. It may contain flake shaped or irregular shaped particles.
As the aluminum powder having a center particle diameter D50 of 10 μm or less, for example, a center particle diameter D50 of 2 μm to 8.5 μm (particularly preferably 3 μm to 6 μm), 70% by mass or more of particles (primary particles) constituting the powder are spherical. Or it is preferable to have a shape similar to it. For example, it is preferable that 70% by mass or more of the particles constituting the aluminum powder have an aspect ratio (ie, major axis / minor axis ratio) of 1 to 1.3.
The aluminum powder is preferably a powder having a relatively narrow particle size distribution (in other words, a uniform particle size). As this index, the ratio (D10 / D90) of the particle size (D10) when the cumulative volume is 10% and the particle size (D90) when the cumulative volume is 90% in the particle size distribution based on the laser diffraction method can be adopted. When all the particle sizes constituting the powder are equal, the value of D10 / D90 is 1, and conversely, the value of D10 / D90 approaches 0 as the particle size distribution becomes wider. It is preferable to use a powder having a relatively narrow particle size distribution such that the value of D10 / D90 is 0.2 or more (for example, 0.2 to 0.5).
Although it does not specifically limit, the quantity used as the content of aluminum powder of about 65-85 mass% of the whole paste is suitable, and the quantity which is about 70-80 mass% is still more preferable. When the aluminum powder content is as described above, an aluminum electrode film with improved denseness can be formed on the Si substrate.

As a glass frit (that is, a powdery or flaky glass material) which is also a component for stably baking (fixing) (pasting) the paste component (coating film) adhered on the Si substrate (wafer). What is contained in the aluminum paste is preferably a so-called bismuth-based glass frit having a low melting point containing bismuth oxide (Bi ₂ O ₃ ) as one of main components. A glass frit made of several kinds of oxide glasses characterized in that the content of bismuth oxide is 40% by mass or more (for example, 40 to 75% by mass) of the whole glass frit is preferable.
Suitable bismuth-based glass frit includes those containing, in addition to bismuth oxide, barium oxide (BaO), zinc oxide (ZnO), silicon oxide (SiO ₂ ), boron oxide (B ₂ O ₃ ), and the like. For example, a glass frit mainly composed of bismuth oxide, boron oxide, and zinc oxide (Bi ₂ O ₃ —B ₂ O ₃ —ZnO-based glass), a glass frit mainly composed of bismuth oxide, boron oxide, and silicon oxide. (Bi ₂ O ₃ —B ₂ O ₃ —SiO ₂ glass), glass frit (Bi ₂ O ₃ —SiO ₂ —PbO glass) mainly composed of bismuth oxide, silicon oxide and lead oxide, and the like. It is done.
As a particularly preferred bismuth-based glass frit, 80% by mass or more (preferably 90% by mass or more, particularly preferably 95% by mass or more, eg 100% by mass) of the entire glass frit is bismuth oxide, barium oxide, zinc oxide, silicon oxide and The thing comprised with the boron oxide is mentioned. In particular, the content of each component when the total amount of these five components is 100% by mass,
Bi ₂ O ₃ 40 to 75 wt%,
3-20% by mass of BaO,
ZnO 5-20% by mass,
B ₂ O ₃ 5 to 20 wt%,
_{3 to} 15% by mass of SiO2,
Are preferred.
As other trace components of these five components (typically, the total amount of trace components is 10% by mass or less of the entire glass frit), for example, aluminum oxide (Al ₂ O ₃ ), lead oxide (PbO), tin oxide (SnO) Metal oxides such as zirconium oxide (ZrO ₂ ) and chromium oxide (Cr ₂ O ₃ ), alkaline earth metal oxides such as magnesium oxide (MgO), calcium oxide (CaO) and strontium oxide (SrO), and / or Alternatively, one or more of alkali metal oxides such as lithium oxide (Li ₂ O), sodium oxide (Na ₂ O), and potassium oxide (K ₂ O) may be included. A lead-free glass frit is preferable from the viewpoint of the global environment.

The glass frit having the above composition suitably has a specific surface area based on the BET method of approximately 0.5 to ¹⁰ m ² / g, preferably at least 2 m ² / g. For example, it is particularly preferably ² to 10 m ² / g (specifically about ² to 6 m ² / g). With this specific surface area, the collapsed aluminum layer can be formed quickly.
The center particle size (D50) of the glass frit is particularly preferably 2 μm or less, particularly about 1 μm or less.
Although it does not specifically limit, The quantity used as about 1-10 mass% of the whole paste for glass frit content is suitable, and the quantity which is about 2-10 mass% is still more preferable. An amount of approximately 3-8% by weight is particularly preferred. By setting the content to this level, it is possible to realize both the rapid formation of the collapsed aluminum layer and the formation of a dense aluminum electrode at a high level.

One of the auxiliary components of the aluminum paste is an organic medium (vehicle) in which aluminum powder or glass frit is dispersed. The organic solvent constituting the vehicle is not particularly limited as long as it can disperse aluminum powder and glass frit satisfactorily, and those conventionally used for this type of paste can be used without any particular limitation. For example, as a solvent constituting a vehicle, a high-boiling organic solvent such as ethylene glycol and a diethylene glycol derivative (glycol ether solvent), toluene, xylene, butyl carbitol (BC), terpineol or the like is used alone or in combination. Can do.
In addition, various resin components can be included as an organic binder constituting the vehicle. Any resin component may be used as long as it can impart good viscosity and coating film forming ability (adhesiveness to a silicon substrate) to the aluminum paste, and those used in this type of conventional paste are used without particular limitation. be able to. Examples thereof include those mainly composed of acrylic resin, epoxy resin, phenol resin, alkyd resin, cellulosic polymer, polyvinyl alcohol, rosin resin and the like. Among these, cellulosic polymers such as ethyl cellulose are particularly preferable.
Although it does not specifically limit, the quantity used as about 10-30 mass% of the whole paste of organic vehicle is suitable, and the quantity which is about 15-25 mass% is more preferable.

The aluminum paste provided by the present invention can be handled in the same manner as an aluminum paste conventionally used for forming a back surface aluminum electrode (and thus a p ⁺ layer or BSF layer) on a Si substrate. This method can be employed without any particular limitation. Typically, an aluminum paste is applied to a Si substrate (wafer) by a screen printing method, a dispenser coating method, a dip coating method, or the like so as to obtain a desired thickness or coating film pattern. Next, the coated material is dried at an appropriate temperature (room temperature to about 100 ° C.). After drying, the dried coating film is baked by heating for a predetermined time in an appropriate baking furnace (for example, a high-speed baking furnace) under appropriate heating conditions (for example, 700 to 800 ° C.). Thereby, the coated material is baked on the Si substrate, and the aluminum electrode 20 as shown in FIG. 1 is formed. In general, the aluminum electrode 20 is fired, and the P ⁺ layer (BSF layer) 24 can also be formed as described above.

According to the aluminum paste provided by the present invention, a low-density layer (that is, a collapsed aluminum layer) that is a part of the aluminum electrode and is made of a material containing aluminum is formed after the dried coating film is fired.
Such a collapsed aluminum layer can be easily removed by various physical means. A dense aluminum electrode layer excluding the collapsed aluminum layer and a physical removal method (means) that does not adversely affect the Si substrate such as scratches are preferred. For example, it is preferable to spray (inject) fluid (for example, gas or water) from a supply port such as a suitable nozzle on the fired aluminum electrode. It is particularly preferable to blow a gas (typically an inert gas such as air or nitrogen gas). Alternatively, the collapsed aluminum layer may be removed by washing (for example, washing with water or washing with an organic solvent such as alcohol) after the baking. Alternatively, the collapsing aluminum layer may be removed by physical means such as roller application of the substrate surface with an adhesive roller.

According to the present invention, by removing the collapsed aluminum layer, thinner and dense aluminum with a BSF layer (p ⁺ layer) equivalent to the case of forming (firing) a conventional thick aluminum electrode (for example, a film thickness of 30 to 40 μm). An electrode (for example, a film thickness of 10 μm or less, typically 5 to 10 μm) can be formed. Further, by removing the collapsed aluminum layer after the firing, it is possible to prevent the Si substrate from warping or other deformations or cracks due to the difference between the thermal expansion coefficient of the Si substrate and the thermal expansion coefficient of the aluminum film. Can do.

The materials and processes used for solar cell production may be exactly the same as in the prior art except that the aluminum paste of the present invention is used and the step of removing the collapsed aluminum layer is added, and is formed by the aluminum paste of the present invention. A silicon-based solar cell such as a crystalline silicon solar cell (see FIG. 1 described above as an example) provided with an aluminum electrode can be manufactured. For example, by performing screen printing or the like using a conventional silver paste, Ag electrodes having a predetermined pattern can be formed on the light receiving surface side and the back surface side. In addition, an n ⁺ layer and an antireflection film can be formed on the light receiving surface side by performing the same process as in the prior art.
Since the manufacturing process itself of such a solar cell (element) may remain the conventional technique and does not particularly characterize the present invention, detailed description thereof will be omitted.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples. The numerical values related to the particle size distribution (specifically, the center particle size D50) described below use ion-exchanged water as a dispersion medium, and a laser diffraction / scattering particle size distribution measuring device (Horiba, Ltd. product: LA). -920).

<Production Example: Preparation of aluminum paste>
Table 1 shows the center particle diameters of a total of seven types (A1 to 7) of aluminum powder used in this production example. Tables 2 and 3 show the composition, glass softening point, and specific surface area based on the BET method of a total of 13 types of glass frit used in this production example. Bi-1 to 7 are bismuth glass frit having a composition as shown in the table, Pb-1 to Pb-3 are lead glass frit, and Zn-1 to 2 are zinc glass frit.
Thus, these powder raw materials were appropriately selected and used to prepare aluminum pastes having the contents (blending ratios) shown in Tables 4-7. Specifically, any aluminum powder and glass frit listed in Tables 1 to 3 and an organic vehicle (here 5% by weight ethyl cellulose and 95% by weight) so as to have the composition (blending ratio) as shown in the table. % Terpineol) and mixed well using a three-roll mill. As a result, 27 types of aluminum pastes of Samples 1 to 27 were prepared.

<Test Example 1: Calculation of warpage immediately after firing and after air blow>
Using the obtained aluminum paste of Samples 1 to 27 for forming an aluminum electrode, solar cells corresponding to each sample were manufactured.
Specifically, a commercially available 125 mm square (□) solar cell p-type single crystal silicon substrate (plate thickness 200 μm) was prepared, and the surface thereof was subjected to an alkali etching treatment using an aqueous NaOH solution. Next, an n-Si layer having a thickness of about 0.5 μm is formed on the light-receiving surface of the Si substrate by applying a phosphorus-containing solution to the light-receiving surface of the Si substrate on which the texture structure is formed by the etching process and performing heat treatment (N ⁺ layer) was formed (see FIG. 1)
Next, an antireflection film (titanium oxide film) having a thickness of about 50 to 100 nm was formed on the n-Si layer by plasma CVD (PECVD). Furthermore, a coating film (thickness 20 to 50 μm) to be a surface electrode (Ag electrode) was formed on the antireflection film by a screen printing method using a predetermined surface electrode (Ag electrode) forming silver paste (see FIG. 1). ).
On the back side of the Si substrate, any aluminum paste was printed (applied) by screen printing (using stainless steel screen mesh SUS # 165, the same applies hereinafter) to form a dry aluminum film having a thickness of 30 μm. . Next, this Si substrate was baked to form a baked aluminum electrode. Specifically, firing was performed at a firing temperature of 700 to 800 ° C. using a near-infrared high-speed firing furnace in an air atmosphere.
Next, the warpage amount of the Si substrate immediately after firing was examined. That is, the Si substrate after firing was placed on a horizontal test stand so that the surface on which the aluminum electrode was formed was directed upward, and the dimension between the lowest part and the uppermost part in the thickness direction of the Si substrate was measured. . The measured value was defined as the amount of warpage (mm) after firing in this test example. The results are shown in the corresponding columns of Tables 4-7.

  Subsequently, an air blowing process for injecting air from a gas injection nozzle was performed on the aluminum electrode of each Si substrate. The processing conditions were such that the wind speed at the nozzle tip was 15 to 50 m / second, and this processing was performed for about 3 to 10 seconds per one Si substrate. After the air blow treatment, the warpage amount of the Si substrate was measured by the same method as described above. The measured value was defined as the amount of warpage (mm) after air blowing in this test example. The results are shown in the corresponding columns of Tables 4-7.

  As is apparent from the results in each table, the glass frit is a frit made of bismuth glass having a glass softening point of 580 ° C. or lower (that is, other bismuth glass frit except Bi-4 having a glass softening point of 590 ° C.). When using, form a low-density layer (collapsed aluminum layer) that is easy to collapse after firing, and then remove the collapsed aluminum layer by air blow to form a dense and thin aluminum electrode We were able to. That is, it was realized that the amount of warpage was reduced to 1 mm or less (specifically about 0.8 mm) in the Si substrate after removing the collapsed aluminum layer, compared to the amount of warpage after firing. In addition, the formation of such a collapsed aluminum layer was not recognized at all in the aluminum paste containing lead-based glass frit and the aluminum paste containing zinc-based glass frit.

In particular, when the aluminum pastes of Samples 8, 17 and 19 were used, part of the collapsed aluminum layer collapsed immediately after firing, and the amount of warpage of the Si substrate was reduced even immediately after firing. Even in such a case, it is necessary to remove the debris of the collapsed aluminum layer from the Si substrate, and the air blowing process is useful for this purpose.
Further, by comparison between aluminum pastes produced by varying the center particle size (average particle size) of the aluminum powder (see Table 4), the center particle size (average particle size) is particularly 3 to 6 μm (here 3. An aluminum powder in the range of 2 to 5.5 μm) has been found to be preferred.
In addition, it was confirmed that the specific surface area of the bismuth-based glass frit is desirably relatively large, for example, 2 m ² / g or more (for example, 2 to ⁶ m ² / g).
Further, with respect to the content of the bismuth-based glass frit, it was confirmed that the collapsed aluminum layer can be easily formed as the content increases within the range confirmed in this test example. However, in order to maintain the performance as an aluminum electrode at a desired level, the glass frit content is suitably about 1 to 10% by mass of the entire paste, and is about 2 to 10% by mass. Further preferred. An amount of approximately 3-8% by weight is particularly preferred.
Further, there is a correlation between the glass frit content and the specific surface area, and it was confirmed that the glass frit content can be relatively reduced by using a bismuth glass frit having a large specific surface area.

<Test Example 2: Measurement of open circuit voltage>
In this test example, first, on the back side of each Si substrate on which the surface electrode (Ag electrode) was formed in Test Example 1, a back electrode (Ag electrode) similar to the silver paste for forming the surface electrode (Ag electrode) was formed. Screen-printed in a predetermined pattern using a forming silver paste and dried to form a back-side Ag coating material having a thickness of 20 to 50 μm (that is, a back-side external connection electrode made of Ag after firing: see FIG. 1) ) Was formed.
Subsequently, each aluminum paste of Samples 1 to 27 was printed (applied) by screen printing to form a dry aluminum film having a thickness of 30 μm. Next, this Si substrate was baked at a baking temperature of 700 to 800 ° C. in a near-infrared high-speed baking furnace in the air atmosphere. By this firing, a fired aluminum electrode was formed together with the front electrode (Ag electrode) and the back-side external connection Ag electrode. Further, the air blow treatment was performed to physically remove the collapsed aluminum layer formed on the fired aluminum electrode.
Next, for the solar cell thus obtained (see FIG. 1), a voltmeter is connected between the front and back Ag electrodes, and the maximum voltage, that is, the open voltage (Voc) when sunlight is irradiated on the light receiving surface is measured. did. The results are shown in the corresponding columns of Tables 4-7.

  As shown in each table, an open circuit voltage of about 600 mV was obtained for any of the solar cells (cells) used in this test example. Therefore, by performing the process of removing the collapsible aluminum layer, it is possible to prevent the Si substrate from being deformed or cracked in the subsequent manufacturing process (or use process), and the electrical characteristics of the solar cell. It was confirmed that there is no risk of adversely affecting the characteristics (output characteristics). Further, by removing the collapsed aluminum layer, it is possible to form a dense aluminum electrode thinner than the conventional one (typically 30 μm or less, preferably 20 μm or less), and equivalent to the conventional thick film aluminum electrode formation. It was suggested that an effect (for example, BSF effect) can be exhibited.

It is sectional drawing which shows an example of the structure of a solar cell typically.

Explanation of symbols

10 Solar cell 11 Silicon substrate (Si wafer)
12 Surface electrode (light-receiving surface electrode)
14 Antireflection film 16 n-Si layer 18 p-Si layer 20 Back surface aluminum electrode 22 Back side external connection electrode 24 p ⁺ layer

Claims (7)

A method for producing a silicon-based solar cell comprising a silicon semiconductor substrate, a light-receiving surface electrode formed on one surface side of the substrate, and an aluminum electrode formed on the other surface side of the substrate,
As a material for forming an aluminum electrode, an aluminum powder having a particle size distribution based on a laser diffraction method (D50) of 10 μm or less, and a bismuth glass having bismuth oxide as an essential component and a glass softening point of 580 ° C. or less. Using an aluminum paste containing a frit and an organic vehicle;
Applying the aluminum paste onto the substrate and firing to form an aluminum electrode; and
Removing a low-density aluminum layer that is part of the fired aluminum electrode and is made of a substance containing aluminum, which is easily disintegrated;
A method for producing a silicon-based solar cell, comprising:   The method according to claim 1, wherein the low-density layer is removed by blowing a gas to the fired aluminum electrode. The production method according to claim 1 or 2, wherein the specific surface area of the glass frit based on the BET method is at least 2 m ² / g.   The content rate of the aluminum powder in the said aluminum paste is 70-80 mass%, and the content rate of the said glass frit is 2-10 mass%, The any one of Claims 1-3 characterized by the above-mentioned. The manufacturing method as described. An aluminum paste for forming an aluminum electrode of a silicon-based solar cell,
An aluminum powder having a center particle size (D50) of particle size distribution based on laser diffraction method of 10 μm or less;
Bismuth-based glass frit having bismuth oxide as an essential component and having a glass softening point of 580 ° C. or lower;
An organic vehicle,
Including aluminum paste. The aluminum paste according to claim 5, wherein the glass frit has a specific surface area based on the BET method of at least 2 m ² / g.   The aluminum paste of Claim 5 or 6 whose content rate of the said aluminum powder in a paste is 70-80 mass%, and whose content rate of the said glass frit is 2-10 mass%.

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