JPWO2012165167A1 - Solar cell and paste composition for forming aluminum electrode of solar cell - Google Patents

Abstract

Provided are a solar cell including an aluminum electrode with improved adhesive strength and a paste composition for forming the aluminum electrode. The paste composition, as a glass frit, has the following conditions: (1) The glass softening point is 400 ° C. or higher and 600 ° C. or lower; (2) The thermal expansion coefficient is 60 × 10 ⁻⁷ / ° C. or higher and 80 × 10 ^−7. / ° C or less;
(3) A glass frit comprising: SiO ₂ , B ₂ O ₃ , ZnO and / or PbO, Al ₂ O ₃ , and at least one alkali metal oxide as essential components.

Description

The present invention relates to a solar battery (cell) and a manufacturing method thereof, and to an aluminum electrode forming paste composition used in the manufacturing method.
Note that this application claims priority based on Japanese Patent Application No. 2011-125062 filed on June 3, 2011, the entire contents of which are incorporated herein by reference. .

Conventionally, as a typical example of a solar cell mainly composed of silicon (semiconductor substrate) such as crystalline silicon and amorphous silicon (hereinafter also referred to as “silicon-based solar cell”), a single-sided light receiving type solar cell as shown in FIG. 1000 is known (see, for example, Patent Documents 1 to 4).
This solar cell 1000 includes an n-Si layer 116 formed by pn junction formation on the light-receiving surface side of a p-Si layer (p-type crystalline silicon) 118 of a silicon semiconductor substrate (Si wafer) 111, and on the surface thereof. An antireflective film 114 made of titanium oxide or silicon nitride formed by CVD or the like and a paste composition (hereinafter also referred to as “silver paste”) typically composed mainly of silver (Ag) powder is screen-printed and fired. A surface electrode (light-receiving surface electrode) 112 made of Ag.
On the other hand, on the back surface of the p-Si layer 118 (referred to as the surface opposite to the light receiving surface; hereinafter the same), it is made of Ag formed by screen printing and baking silver paste in the same manner as the surface electrode 112. A back surface side external connection electrode 122 and an aluminum electrode 120 having a so-called back surface field (BSF) effect are provided.

The aluminum electrode 120 is formed on substantially the entire back surface by printing and baking a paste composition (hereinafter also referred to as “aluminum paste”) mainly composed of aluminum (Al) powder. During this firing, an Al—Si alloy layer (not shown) is formed, and aluminum diffuses into the p-Si layer 118 to form the p ⁺ layer 124. By forming the p ⁺ layer 124, that is, the BSF layer, it is possible to prevent the photogenerated carriers from recombining in the vicinity of the back electrode, and to realize, for example, improvement in short circuit current and open circuit voltage (Voc). it can.

Japanese Patent Application Publication No. 2010-10495 Japanese Patent Application Publication No. 2011-23598 Japanese Patent Application Publication No. 2010-192480 Japanese Patent Application Publication No. 2007-59380

As shown in FIG. 7, in the conventional silicon-based solar cell 1000, an Ag surface electrode (light-receiving surface electrode) 112 for taking out current is formed on the light-receiving surface side, and recombination prevention of electrons is performed on the back-surface side. A BSF layer 124 is formed. The Ag surface electrode 112 and the aluminum electrode 120 for forming the BSF layer 124 are typically formed by a screen printing method, and are formed by simultaneously sintering both surfaces.
Moreover, a lead wire (lead frame: not shown) for current extraction is soldered to the solar cell wafer 1000 having the electrodes (112, 120, 122) formed on both surfaces in this way. Then, a plurality of solar cell wafers 1000 are connected in series using the lead wires to be modularized, and predetermined power can be supplied in such a modularized state.

  Here, in the structure shown in FIG. 7, since the solder cannot be used for the aluminum electrode 120 used on the back surface, the Ag electrode 122 is formed on the soldered portion. As a result, the formation of the Ag electrode 122 hinders the uniformization of the BSF layer 124, and the formation of the Ag electrode 122 uses silver, which is a noble metal more expensive than aluminum, as the conductive component. It becomes a factor of.

In an attempt to cover the entire back surface with an aluminum electrode, the present inventor uses a conductive adhesive film (that is, a film containing an adhesive and conductive particles) without using solder, and a method of thermocompression bonding of the lead wire Was considering. Specifically, the connection is executed without using solder by thermally bonding the lead wire to the position of the external connection electrode 122 made of Al via a conductive adhesive film.
However, it has been found that this technique using a conductive adhesive film cannot provide sufficient adhesive strength on the electrode 122 made of aluminum. That is, as a result of examination by the present inventors, when the adhesive portion of the lead wire (conductive ribbon wire) through the conductive adhesive film is peeled off, peeling occurs in the aluminum film in the electrode 122 made of aluminum, which is sufficient. It was thought that this was the reason why it was not possible to obtain a sufficient adhesive strength. More specifically, it was found that the aluminum film itself in the electrode 122 made of aluminum has no strength, and the adhesive strength after firing between the aluminum particles forming the aluminum film is weak.

  On the other hand, if the lead wire can be bonded on the aluminum electrode of the solar cell wafer, the advantage that the BSF layer 124 can be formed on the entire surface is obtained, and the use of silver is not necessary. A significant cost reduction corresponding to the price difference can be achieved.

  The present invention has been created in view of the above points, and one object of the present invention is to provide a paste composition for forming an aluminum electrode (particularly a backside external connection electrode) with improved adhesive strength. To do. Another object of the present invention is to provide a solar cell including an aluminum electrode (particularly a backside external connection electrode) formed using such a paste composition and a method for producing the solar cell.

The paste composition provided by the present invention (that is, a composition prepared in a paste form) is a paste composition for forming an aluminum electrode of a solar cell.
And the paste composition for aluminum electrode formation of the solar cell disclosed here contains aluminum powder, glass frit, and an organic vehicle. And the glass frit has the following conditions:
(1) The glass softening point is 400 ° C. or higher and 600 ° C. or lower;
(2) The thermal expansion coefficient is 60 × 10 ⁻⁷ / ° C. or more and 80 × 10 ⁻⁷ / ° C. or less;
(3) Including SiO ₂ , B ₂ O ₃ , ZnO and / or PbO, Al ₂ O ₃ , and at least one alkali metal oxide as essential components;
It is characterized by comprising.

  In the paste composition (aluminum paste) having the above-described configuration, the paste composition is applied to the silicon semiconductor substrate by having the glass frit (glass powder) having the properties shown in (1) to (3) above. Thus, the adhesive strength of the formed aluminum electrode can be improved. Therefore, for example, it is possible to stably join an aluminum electrode (for example, a backside external connection electrode) formed from the paste composition and another connection member (for example, a conductive adhesive film including an adhesive and conductive particles). Can be maintained. As a result, it is possible to replace the conductive component of the conventional electrode, for example, the external connection electrode, from a noble metal such as silver with inexpensive aluminum, and to reduce the cost corresponding to the material price difference between silver and aluminum. Can be realized. If the Ag electrode for external connection on the back side can be replaced with an aluminum electrode, the BSF layer can be formed uniformly on the entire back side of the silicon semiconductor substrate.

In a preferred embodiment of the paste composition (aluminum paste) disclosed herein, the glass frit includes 100 mol% of the following components as a whole, and the molar content of each component is
SiO ₂ 20~35mol%,
B ₂ O ₃ 5~30mol%,
ZnO and / or PbO 25-45 mol%,
Al ₂ O ₃ 2~10mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 5 to 15 mol%,
At least one of CaO, SrO and BaO 0-10 mol%,
Bi ₂ O ₃ 0-5 mol%,
And the total of these components is 95 mol% or more (for example, 100 mol%) of the entire glass frit.
By employing the glass frit having such a composition, the adhesive strength (peeling strength) of an aluminum electrode (for example, a backside external connection electrode) formed from the paste composition can be further improved.

In a further preferred embodiment of the paste composition (aluminum paste) disclosed herein, the glass frit has 100 mol% of the following components as a whole, and the molar content of each component is
SiO ₂ 20~30mol%,
B ₂ O ₃ 20~30mol%,
ZnO 25-35 mol%,
Al ₂ O ₃ 3~7mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 10 to 15 mol%,
At least one of CaO, SrO and BaO, 2 to 10 mol%,
And the total of these components is 95 mol% or more (for example, 100 mol%) of the entire glass frit.
By employing the glass frit having such a composition, the adhesive strength (peeling strength) of an aluminum electrode such as a back-side external connection electrode formed from the paste composition can be further improved. Moreover, such high adhesive strength can be realized without Pb.

  Another preferred embodiment of the paste composition (aluminum paste) disclosed herein is characterized in that the glass softening point of the glass frit is 500 ° C. or higher and 600 ° C. or lower. By using a glass frit having a glass softening point in such a temperature range, an aluminum electrode having high adhesive strength (peel strength) can be formed.

  Moreover, in another preferable aspect of the paste composition (aluminum paste) disclosed herein, the total paste composition is 100% by mass, the content of the aluminum powder is 60 to 80% by mass, and the above The glass frit content is 2 to 10% by mass. When the solid content of the paste composition is the above content, it is easy to uniformly apply the paste composition containing the solid content to the silicon semiconductor substrate (typically in a film shape), and the paste composition is applied. By baking the substrate, an aluminum electrode having a good appearance and strong adhesive strength can be formed.

As described above, by using any of the paste compositions (aluminum paste) disclosed herein, an aluminum electrode having high adhesive strength can be formed on the silicon semiconductor substrate.
Therefore, according to the present invention, a silicon semiconductor substrate, a light receiving surface electrode formed on the light receiving surface side which is one surface of the substrate, and an aluminum electrode formed on the back surface side which is the other surface of the substrate are provided. A solar cell comprising:
At least a part of the aluminum electrode as a glass composition has the following conditions:
(1) The glass softening point is 400 ° C. or higher and 600 ° C. or lower;
(2) The thermal expansion coefficient is 60 × 10 ⁻⁷ / ° C. or more and 80 × 10 ⁻⁷ / ° C. or less;
(3) Including SiO ₂ , B ₂ O ₃ , ZnO and / or PbO, Al ₂ O ₃ , and at least one alkali metal oxide as essential components;
The solar cell characterized by containing the glass composition which comprises can be provided.
The solar cell having such a configuration can achieve high adhesive strength (peel strength) in an aluminum electrode containing the glass composition (glass component) having the above properties. For this reason, in the solar cell (silicon-based solar cell) disclosed here, the conductive component of the portion where the Ag electrode has been used in the past, for example, the external connection electrode, can be replaced with inexpensive aluminum from noble metal such as silver. This makes it possible to reduce the manufacturing cost of the electrode (substitution from the Ag electrode to the aluminum electrode).
In addition, according to the present invention, it is possible to provide a solar cell in which a BSF layer is formed on the entire back surface side of a silicon semiconductor substrate in which an Ag electrode for back side external connection is replaced with an aluminum electrode.

In a preferred embodiment of the solar cell disclosed herein, the glass composition is composed of 100 mol% of the following components as a whole, and the molar content of each component is
SiO ₂ 20~35mol%,
B ₂ O ₃ 5~30mol%,
ZnO and / or PbO 25-45 mol%,
Al ₂ O ₃ 2~10mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 5 to 15 mol%,
At least one of CaO, SrO and BaO 0-10 mol%,
Bi ₂ O ₃ 0-5 mol%,
And the total of these components is 95 mol% or more (typically 100 mol%) of the entire glass composition.

Further, preferably, the glass composition has 100% by mole of the following components as a whole, and the molar content of each component is
SiO ₂ 20~30mol%,
B ₂ O ₃ 20~30mol%,
ZnO 25-35 mol%,
Al ₂ O ₃ 3~7mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 10 to 15 mol%,
At least one of CaO, SrO and BaO, 2 to 10 mol%,
And the total of these components is 95 mol% or more (typically 100 mol%) of the entire glass composition.
Preferably, the glass softening point of the glass composition is 500 ° C. or higher and 600 ° C. or lower.

According to the present invention, an aluminum electrode having an adhesive strength (peel strength) equal to or higher than that of a conventional Ag electrode can be formed on the silicon semiconductor substrate as the external connection electrode.
Accordingly, one particularly preferable aspect of the solar cell disclosed herein is that an external connection electrode is formed on the back surface side of the silicon semiconductor substrate, and the external connection electrode is formed by an aluminum electrode containing the glass composition. It is configured.
In a preferred embodiment, a conductive adhesive film is affixed on an aluminum electrode containing the glass composition that constitutes the external connection electrode.

Further, the present invention provides, as another aspect for realizing the above object, a silicon semiconductor substrate, a light-receiving surface electrode formed on the light-receiving surface side that is one surface of the substrate, and the other surface of the substrate A method for producing a solar cell comprising an aluminum electrode formed on a back surface side is provided.
That is, the solar cell manufacturing method disclosed herein is characterized in that at least a part of the aluminum electrode formed on the back surface side is formed using any paste composition provided by the present invention. .

FIG. 1 is a cross-sectional view schematically showing an example of the structure of a solar cell 100 according to an embodiment of the present invention. 2A and 2B are a top view and a cross-sectional view schematically showing the configuration of the back surface side of the substrate 11 in the solar cell 100, respectively. FIG. 3 is a perspective view schematically showing a configuration in which the conductive adhesive film 30 is disposed on the external connection aluminum electrode 22 on the back surface side of the solar cell 100. 4A and 4B are cross-sectional views showing a structure in which a lead wire (tab wire) 35 is disposed on the external connection aluminum electrode 22 with a conductive adhesive film 30 interposed therebetween. FIG. 5 is a cross-sectional view schematically showing the configuration of the strength measuring apparatus 300 that performs the adhesive strength evaluation. FIG. 6 is a graph showing the relationship between the adhesive strength and the softening point of the glass frit. FIG. 7 is a cross-sectional view schematically showing an example of the structure of a conventional solar cell 1000.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, matters other than matters particularly 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.

First, the aluminum electrode forming paste composition provided by the present invention will be described in detail.
The aluminum electrode forming paste composition disclosed herein is an aluminum paste used for forming an aluminum electrode in a solar cell, and includes a paste form (expressed as ink form) containing aluminum powder, glass frit, and an organic vehicle. In other words, other constituent components are not particularly limited as long as they can realize the object of the present invention.
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. 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.

The aluminum powder used 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).
The aluminum powder contained in the aluminum electrode forming paste composition disclosed herein suitably has an average particle size of 20 μm or less. For example, an aluminum powder having an average particle size of about 1 μm to 10 μm can be preferably used.
In this specification, the average particle diameter refers to a particle diameter 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 a particle size distribution measuring apparatus based on a laser diffraction method.
Although there is no particular limitation, the content of the aluminum powder is suitably about 55 to 85% by mass, more preferably 60 to 80% by mass, based on 100% by mass of the entire paste composition. When the aluminum powder content is as described above, an aluminum electrode with improved density (for example, an aluminum electrode for external connection having a film thickness of 100 μm or less, for example, a film thickness of 10 μm to 100 μm) is suitable on a silicon semiconductor substrate. Can be formed.

Of the solid content in the aluminum electrode forming paste composition disclosed herein, glass frit (glass powder) is an inorganic additive that improves the adhesive strength of the aluminum electrode. In particular, in the paste composition (aluminum paste) provided by the present invention, the glass frit has the above-described conditions (1) to (3), so that the formed aluminum electrode has high adhesive strength (peel strength). Can be granted.
That is, as the glass frit (glass composition) contained in the aluminum electrode forming paste composition disclosed herein, for example, the thermal expansion coefficient (linear thermal expansion coefficient) is 60 × 10 ⁻⁷ / ° C. or more and 80 × 10 8. Those having ⁻⁷ / ° C. or lower are suitable, and those having a thermal expansion coefficient (linear thermal expansion coefficient) of 65 × 10 ⁻⁷ / ° C. or higher and 75 × 10 ⁻⁷ / ° C. or lower are more preferable.
In this specification, the “thermal expansion coefficient” is a temperature measured by a thermomechanical analyzer (TMA) based on a general differential expansion method (TMC) to a temperature below the glass softening point (for example, 400 ° C. or 500 ° C. Calculated as an average value during (° C).

  Further, the glass frit contained in the aluminum electrode forming paste composition disclosed herein is calculated by a glass softening point (measured by a general thermomechanical analyzer (TMA) in the same manner as the thermal expansion coefficient). A glass softening point) of 400 ° C. or higher and 600 ° C. or lower is suitable. Further, those having a glass softening point of 500 ° C. or more and 600 ° C. or less are particularly preferable.

As a glass frit (glass composition) having the preferred thermal expansion coefficient and glass softening point, SiO ₂ , B ₂ O ₃ , ZnO and / or PbO, Al ₂ O ₃ , and at least one kind It is preferable to include an alkali metal oxide (for example, selected from Li ₂ O, Na ₂ O, and K ₂ O) as an essential component.
As a suitable example of the glass frit (glass composition) contained in the aluminum electrode forming paste composition disclosed herein, the following components as a whole are assumed to be 100 mol%, and the molar content of each component is:
SiO ₂ 20~35mol%,
B ₂ O ₃ 5~30mol%,
ZnO and / or PbO 25-45 mol%,
Al ₂ O ₃ 2~10mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 5 to 15 mol%,
At least one of CaO, SrO and BaO 0-10 mol%,
Bi ₂ O ₃ 0-5 mol%,
The total of these components is 95 mol% or more (typically 100 mol%) of the entire glass frit (glass composition).
Furthermore, as a glass frit having a particularly preferable composition, 100 mol of all the following components are contained.
%, The molar content of each component is
SiO ₂ 20~30mol%,
B ₂ O ₃ 20~30mol%,
ZnO 25-35 mol%,
Al ₂ O ₃ 3~7mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 10 to 15 mol%,
At least one of CaO, SrO and BaO, 2 to 10 mol%,
The total of these components is 95 mol% or more (typically 100 mol%) of the entire glass frit (glass composition).

By containing the glass frit having the above composition, the adhesive strength (peel strength) of the aluminum electrode to be formed can be further improved.
For example, SiO ₂ which is an essential component is a main component constituting a glass skeleton. If the SiO ₂ content is too high, the glass softening point becomes too high, which is not preferable. On the other hand, if the SiO ₂ content is too low, the chemical resistance and water resistance decrease, which is not preferable.
B ₂ O ₃ is a component having a high effect of lowering the softening point and the melting temperature of the glass frit. If the B ₂ O ₃ content is too low, the effect of lowering the softening point and melting temperature of the glass cannot be obtained. On the other hand, if the B ₂ O ₃ content is too high, the water resistance may be lowered, which is not preferable.
ZnO or PbO is a component that can lower the softening point of the glass frit (glass composition) or adjust the thermal expansion coefficient, and any one or both of ZnO and PbO can be used. It is preferable to contain in the range of the said content rate. If the content of these components is too high, the glass softening point is too low, which is not preferable. A PbO free material is preferred.

Al ₂ O ₃ is a component that controls the fluidity of the glass frit during melting and is involved in the adhesion stability during the formation of the aluminum electrode. If the Al ₂ O ₃ content is too low, the adhesion stability is lowered, which is not preferred. If the Al ₂ O ₃ content is too high, the chemical resistance of the glass may be lowered, which is not preferred.
_Further, alkali metal oxide components such as _{Li 2 O, Na 2 O,} K 2 O is a component to increase the thermal expansion coefficient. If the content of these alkali metal oxide components is too low, the thermal expansion coefficient may be too low. On the other hand, if the content is too high, the thermal expansion coefficient becomes excessively high, which is not preferable.

The glass frit can contain an arbitrary component in addition to the essential components described above.
For example, it is preferable to contain an alkaline earth metal oxide component such as CaO, SrO, or BaO at a content of 10 mol% or less. By containing at least one alkaline earth metal oxide, the thermal expansion coefficient can be adjusted more easily, and the chemical resistance, etc. can be improved by diversifying the glass composition (multiple types of constituent metal elements). Is preferable because it can improve the stability of the glass. It is preferable to contain an alkaline earth metal oxide component such as CaO, SrO, or BaO at a content of, for example, 1 mol% to 10 mol% (for example, 2 to 10 mol%, particularly 5 to 10 mol%).
Further, Bi ₂ O ₃ may be contained in an appropriate amount, for example, in a proportion of 10 mol% or less (preferably 5 mol% or less) for the purpose of improving the stability of the fired glass (and thus the fired aluminum electrode). Good.
In addition, other oxide components such as Zr, Ti, V, Nb, La, Ce, Sn, and P are appropriately included at a ratio of 5 mol% or less (for example, about 0.1 to 5 mol%) of the entire glass composition. Also good.

In order to stably bake and fix (bake) the paste composition (coating film) applied on the silicon semiconductor substrate, the glass frit contained in the paste composition is a ratio based on the BET method. Those having a surface area of about 0.5 m ² / g or more and 50 m ² / g or less are preferred, and those having an average particle diameter (D50) of 2 μm or less (particularly about 1 μm or less) are preferred.
In addition, the content of the glass powder in the paste composition is not particularly limited, but about 1 to 15% by mass is appropriate when the entire paste composition is 100% by mass, and about 2 to 10% by mass is appropriate. preferable. According to the paste composition containing the glass frit having the above elemental composition with such a content, an aluminum electrode having high adhesive strength (for example, an aluminum electrode for external connection on the back surface side) can be suitably formed.

The paste composition disclosed here includes the above-described aluminum powder and glass frit (glass powder) as a solid content, and a liquid medium (organic vehicle) for dispersing the solid content.
The organic solvent constituting such a vehicle is not particularly limited as long as it can disperse aluminum powder and glass frit satisfactorily, and those used in conventional pastes of this type can be used without particular limitation. For example, as the organic solvent constituting the vehicle, high boiling point organic solvents such as ethylene glycol and diethylene glycol derivatives (glycol ether solvents), toluene, xylene, butyl carbitol (BC), butyl diglycol acetate (BDGA), terpineol and the like are used. It can be used in combination of multiple types. 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 provide the paste composition disclosed herein with good viscosity and coating film forming ability (adhesion to a silicon substrate), and is used in conventional pastes of this type. Things can be used without particular limitation. 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 paste composition disclosed herein, like conventional aluminum pastes for solar cells, is typically easily prepared by mixing aluminum powder, glass frit (glass powder), and a suitable organic vehicle. Can do. For example, a three-roll mill or other kneader may be used to mix and stir a predetermined mixing ratio of aluminum powder and glass frit together with an organic vehicle at a predetermined mixing ratio.

The paste composition disclosed here can be handled in the same manner as an aluminum paste conventionally used to form an aluminum electrode (and thus a p ⁺ layer or BSF layer) as a back electrode on a substrate, A conventionally known method can be employed without any particular limitation. Typically, the paste composition is applied (applied) to a silicon semiconductor substrate by a screen printing method, a dispenser coating method, a dip coating method, or the like so as to obtain a desired film thickness or coating film pattern. The thickness of the substrate can be set in consideration of the desired solar cell size, the thickness of the aluminum electrode formed on the substrate, the strength (for example, the breaking strength) of the substrate, and the like. Although it does not specifically limit, about 5 micrometers-300 micrometers are suitable, and about 5 micrometers-200 micrometers (especially about 10 micrometers-100 micrometers) are suitable.
Next, the paste coating product is dried at an appropriate temperature (for example, room temperature or higher, typically about 100 ° C.). After drying, the dried coating film is baked by heating in an appropriate baking furnace (for example, a high-speed baking furnace) under appropriate heating conditions (for example, 600 ° C. to 900 ° C., preferably 700 ° C. to 800 ° C.) for a predetermined time. I do. Thereby, the paste application product is baked on the substrate, and an external connection aluminum electrode 22 as shown in FIG. 1 to be described later can be formed.

  FIG. 1 is a cross-sectional view showing a configuration of a solar cell 100 according to an embodiment of the present invention. The solar cell 100 of this embodiment includes a silicon semiconductor substrate (Si wafer) 11, a light receiving surface electrode 12 formed on one surface side (front surface side) of the substrate 11, and the other surface side (back surface side) of the substrate 11. ) Formed on the aluminum electrode (20, 22).

  The silicon semiconductor substrate 11 of the present embodiment is made of silicon such as crystalline silicon or amorphous silicon. In the configuration of the present embodiment, the n-Si layer 16 formed by pn junction formation is located on the light receiving surface side of the p-Si layer (p-type crystalline silicon) 18 of the substrate 11. An antireflection film 14 made of titanium oxide or silicon nitride formed by general chemical vapor deposition (CVD) or the like is located on the surface of the n-Si layer 16. In addition, the surface of the antireflection film 14 is provided with a surface electrode (light receiving surface electrode) 12 made of Ag, typically formed by screen printing and baking a silver paste.

In the configuration of the present embodiment, the aluminum electrode forming paste composition disclosed herein (hereinafter sometimes referred to as “first aluminum paste” for convenience) is formed on the back side of the p-Si layer 18 as a material. An aluminum electrode 22 is provided. The aluminum electrode 22 corresponds to the external connection electrode 22 on the back surface side that is electrically connected to another external connection member such as a lead wire (conductive ribbon line).
The external connection aluminum electrode 22 of this embodiment is a first aluminum having a predetermined composition as in the case of forming this type of conventional electrode (for example, forming an external connection Ag electrode using a silver paste as a material). It is formed by screen printing and baking the paste.

  On the back surface side of the p-Si layer 18, an aluminum electrode 20 made of another aluminum paste (hereinafter sometimes referred to as “second aluminum paste” for convenience) is formed together with the external connection aluminum electrode 22. The aluminum electrode 20 is formed on substantially the entire back surface of the p-Si layer 18 and is an aluminum electrode (hereinafter referred to as “back surface wide area aluminum electrode”) that exhibits a back surface field (BSF) effect. . In the configuration of this embodiment, the external connection aluminum electrode 22 is linearly formed on the back surface side of the p-Si layer 18, and the back surface wide aluminum electrode 20 is formed on the back surface side of the p-Si layer 18. The external connection aluminum electrode 22 is formed on substantially the entire surface other than the region where the external connection aluminum electrode 22 is formed.

Further, in the configuration of the present embodiment, the back surface wide aluminum electrode 20 covers a part of the external connection aluminum electrode 22 (specifically, both edges of the linear structure) and the external connection aluminum electrode 22 It is formed to have an opening 23 that is exposed. The back surface wide aluminum electrode 20 covers both edges of the external connection aluminum electrode 22, and the both sides are formed so that the side surface of the back surface wide aluminum electrode 20 and the side surface of the external connection aluminum electrode 22 are in contact with each other. It is also possible.
In this embodiment, the back surface wide aluminum electrode 20 may be a general aluminum paste used for forming an aluminum electrode of a conventional solar cell, including aluminum powder, glass frit, and an organic vehicle. The content is not particularly limited. Therefore, the detailed description is omitted because it does not characterize the present invention.

In the present embodiment, typically, the first aluminum paste is applied to the silicon semiconductor substrate (wafer) 11 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. To do. Next, the coated material is dried at an appropriate temperature (room temperature to about 100 ° C.). Further, a second aluminum paste is applied to the silicon semiconductor substrate 11 so that the opening 23 exposing the external connection aluminum electrode 22 is formed. Next, the coated material is dried at an appropriate temperature (room temperature to about 100 ° C.). Thereafter, 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 substrate, and the external connection aluminum electrode 22 and the back surface wide aluminum electrode 20 as shown in FIG. 1 are formed. In the present embodiment, the external connection aluminum electrode 22 and the back surface wide aluminum electrode 20 are fired, and the p ⁺ layer (BSF layer) 24 can be formed.

2A and 2B are a top view and a cross-sectional view schematically showing the configuration of the back surface side of the substrate 11 of the silicon-based solar cell 100 of the present embodiment. 2 (a) and 2 (b), the back side of the substrate 11 is shown as being positioned upward for convenience.
In the configuration shown in FIG. 7, the external connection aluminum electrode 22 is formed at a position where the back side external connection electrode (Ag electrode) 122 is located. In the example shown in FIGS. 2A and 2B, the external connection aluminum electrode 22 can function as an electrode having a width of 2 to 6 mm, for example. In general, it is difficult to join aluminum and solder. Therefore, in the configuration of the present embodiment, the conductive adhesive film 30 is attached so as to be crimped to the external connection aluminum electrode 22 (see FIG. 3 described later). The conductive adhesive film 30 (FIG. 3) is typically an anisotropic conductive adhesive film. For example, as an adhesive component, epoxy resin, phenoxy resin, acrylic resin, polyimide resin, polyamide resin, polycarbonate resin are used. Other thermosetting resins and thermoplastic resins can be used. Thus, various conductive particles (typically metal particles such as nickel, copper, and noble metals) are dispersed in the resin (adhesive) component, and the conductive adhesive film 30 is By heating and pressurizing, the conductive adhesive film 30 can be affixed to a predetermined site, and conduction can be ensured by the conductive particles. In addition, there is no restriction | limiting in particular in the electroconductive adhesive film to be used, The electroconductive adhesive film of this kind of application marketed can be employ | adopted without a restriction | limiting in particular.

  FIG. 3 is a perspective view schematically showing a configuration in which the conductive adhesive film 30 as described above is arranged on the external connection aluminum electrode 22 on the back surface side of the solar cell 100. As shown in FIG. 3, the conductive adhesive film 30 is disposed on the surface of the external connection aluminum electrode 22, and the lead wire, that is, the tab wire (conductive ribbon wire) 35 is disposed on the surface of the conductive adhesive film 30. It will be.

4A and 4B show a cross-sectional structure in which the tab wire 35 is disposed on the external connection aluminum electrode (bus bar electrode) 22 with the conductive adhesive film 30 interposed therebetween.
First, as shown in FIG. 4A, a conductive adhesive film 30 is laminated on the external connection aluminum electrode 22 formed on the substrate 11, and a tab wire 35 is formed on the conductive adhesive film 30. Laminate. The conductive adhesive film 30 has a configuration in which conductive particles 31 (for example, nickel particles plated with gold) are uniformly dispersed in an adhesive component 32 made of, for example, an epoxy thermosetting resin.
Next, as shown in FIG. 4B, the laminate of the external connection aluminum electrode 22, the conductive adhesive film 30, and the tab wire 35 is pressed and heated using the conductive adhesive film 30 as a tab wire bonding material. Thus (see arrow 50), the conductive particles 31 in the conductive adhesive film 30 conduct the external connection aluminum electrode 22 and the tab wire 35 (see arrow 55). At the same time, the adhesive component (here, thermosetting resin) 32 in the conductive adhesive film 30 is thermally cured, whereby reliable conduction equivalent to solder bonding can be realized. In addition, when soldering the tab wire 35, it is necessary to apply a high temperature of 200 ° C. or higher, but in the bonding using the conductive adhesive film 30, low-temperature bonding at about 180 ° C. can be performed. Therefore, the influence (for example, generation | occurrence | production of distortion by a heat | fever) to the solar cell (cell) 100 by heating can be reduced or prevented beforehand.

In the embodiment described above, the external connection aluminum electrode 22 composed of the first aluminum paste and the back surface wide aluminum electrode 20 composed of the second aluminum paste are used as the back surface aluminum electrodes, but the present invention is not limited thereto. .
For example, all the back surface aluminum electrodes including the back surface wide aluminum electrode 20 can be formed by the first aluminum paste.

  Hereinafter, some test examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in the test examples.

In the following tests, the difference in adhesive strength was evaluated when the properties of the glass frit which is a solid component of the aluminum electrode forming paste composition (aluminum paste) were made different from each other.
In this test, a total of 8 types of glass samples (sample 1 to sample 8) showing the glass composition (mol%), the glass softening point (° C.) and the thermal expansion coefficient (thermal expansion coefficient) were used in Table 1 below. .

Thus, a total of eight types of aluminum paste (Test Examples 1 to 8) each including the glass frit (Samples 1 to 8) shown in Table 1 were prepared.
The aluminum paste according to each test example was different only in the properties of the glass frit described above, and other components (aluminum powder, organic vehicle) and mixing ratio were the same.
That is, the contents of the aluminum paste of each test example are as follows.
(1) Aluminum powder:
Aluminum powder having an average particle size of 6 μm was used in an amount of 66% by mass of the entire paste.
(2) Organic vehicle:
As an organic solvent, terpineol was used in an amount of 26% by mass based on the entire paste.
Further, as an organic binder, ethyl cellulose was used in an amount of about 2% by mass of the entire paste.
(3) Glass frit:
The glass frit having the property of any of Samples 1 to 8 was used in an amount of 6% by mass of the entire paste.

<Adhesive strength test (1)>
Next, the adhesive strength test was done about the aluminum electrode produced on the silicon semiconductor substrate using each aluminum paste of Test Examples 1-8 manufactured as mentioned above. Here, the procedure for forming the aluminum electrode on one surface (back surface) of the silicon semiconductor substrate 11 using the aluminum pastes of Test Examples 1 to 8 is as follows.
That is, a commercially available 125 mm square p-type single crystal silicon substrate (plate thickness 200 μm) for solar cells was prepared, and the surface thereof was subjected to alkali etching treatment using an aqueous sodium hydroxide solution. Next, an n-Si layer having a thickness of about 0.5 μm is formed on the light-receiving surface of the silicon substrate by applying a phosphorus-containing solution to the light-receiving surface of the silicon substrate on which the texture structure is formed by the etching process and performing heat treatment. (N ⁺ layer) was formed.
Next, an antireflection film (titanium oxide film) having a thickness of about 50 nm to about 100 nm was formed on the n-Si layer by plasma CVD (PECVD). Further, a coating film (thickness of 20 μm or more and 50 μm or less) to be a surface electrode (Ag electrode) was formed on the antireflection film by a screen printing method using a predetermined silver paste for forming a surface electrode (Ag electrode).
On the other hand, on the back side of the silicon semiconductor substrate, the paste composition according to any one of the above Test Examples 1 to 8 was printed (applied) by screen printing (using stainless steel screen mesh SUS # 165). A coating film having a thickness of about 30 μm was formed in a line shape with a width of about 5 mm. Next, this substrate was baked to form a linear aluminum electrode 28 for test evaluation. Specifically, firing was performed at a firing temperature of about 700 ° C. or more and 800 ° C. or less using a near-infrared high-speed firing furnace in an air atmosphere.
The adhesion strength of the aluminum electrode formed from the paste composition for forming an aluminum electrode according to each test example was measured by performing an adhesion strength evaluation test using the test evaluation cell (solar cell) thus obtained. .

Evaluation of the adhesive strength (peel strength) of the formed aluminum electrode (that is, peel strength evaluation) was performed using a strength measuring device 300 as shown in FIG.
Specifically, the strength measuring apparatus 300 shown in FIG. 5 fixes a glass substrate 41 on a fixing jig 40 via a fixing screw 43 and a locking plate 44, and an epoxy adhesive on the glass substrate 41. The light-receiving surface side of the test silicon semiconductor substrate 11 obtained above was fixed by 42.
A conductive adhesive film 30, which is a commercial product, is attached to the formed aluminum electrode 28 on the exposed surface side of the silicon semiconductor substrate 11 fixed on the glass substrate 41 in this way by thermocompression bonding, and on the conductive adhesive film 30. Further, a tab line 35 was pasted.
Then, as shown in FIG. 5, the strength measuring device 300 is inclined so that the bottom surface of the fixing jig 40 is 135 °, and the extension portion 35e formed in advance on the tab wire 35 is pulled upward in the vertical direction. (See arrow 45), the adhesive strength of tab wire 35 / conductive adhesive film 30 / electrode 28 was measured.

The results are shown in the corresponding column of Table 2. The adhesive strength evaluation test (peel strength test) is performed on a plurality of (two) aluminum electrodes formed from the aluminum paste according to each test example, and the average of the test results (measured values) for the two aluminum electrodes. The value (that is, the number of measurements n = 2) was evaluated as the adhesive strength.
In addition, the graph of FIG. 6 has shown the relationship between the adhesive strength obtained by this test, and the softening point of the glass frit in an aluminum paste.

As shown in Table 2 and FIG. 6, it was confirmed that the glass softening point in the aluminum paste exhibited high adhesive strength in the range of 400 ° C. to 600 ° C. (particularly 500 ° C. to 600 ° C.). In particular, when the aluminum paste of Test Example 6 having a glass softening point around 550 ° C. was used, extremely high adhesive strength was observed.
Further, the glass frit having a thermal expansion coefficient of 60 × 10 ⁻⁷ / ° C. or higher and 80 × 10 ⁻⁷ / ° C. or lower (particularly 65 × 10 ⁻⁷ / ° C. or higher and 75 × 10 ⁻⁷ / ° C. or lower) is high. It was observed to show adhesive strength.

<Adhesive strength test (2)>
Further, a linear aluminum electrode having a film thickness of about 30 μm and a width of about 2 mm is formed on the silicon semiconductor substrate 11 by using the aluminum paste according to Test Example 6 according to the same procedure as the above-described adhesive strength test (1). When the same peel strength evaluation test was conducted, an average value (n = 2) of an adhesive strength of 3.25 N / 2 mm was observed as shown in the Example column of Table 3.
As a comparative control, a linear Ag electrode having a film thickness of about 30 μm and a width of about 2 mm is formed on the silicon semiconductor substrate 11 using a normal silver paste instead of the aluminum paste according to Test Example 6, and solder is formed on the surface thereof. When the same peel strength evaluation test was performed by attaching the tab wire 35 via the adhesive, as shown in the column of Comparative Example A in Table 3, the average value (n = 2) was 3.5 N / 2 mm adhesion. Strength was observed.
In addition, Comparative Example B in Table 3 is a result when the aluminum paste according to Test Example 1 is used instead of the aluminum paste according to Test Example 6.

  As can be understood from Table 3, it was confirmed that the aluminum electrode formed from the aluminum paste according to Test Example 6 can obtain the same adhesive strength as the soldered Ag electrode in Comparative Example A. Moreover, when compared with the adhesive strength of the aluminum electrode in Comparative Example B, it is confirmed that the adhesive strength of the aluminum electrode formed from the aluminum paste according to Test Example 6 is remarkably improved.

As is clear from the description of the embodiment and test examples described above, in the solar cell provided by the present invention, an aluminum electrode can be employed as the backside external connection electrode. As a result, it is possible to perform bonding using a conductive adhesive film, which cannot be used with ordinary aluminum electrodes because of low adhesive strength. Therefore, the BSF layer can be uniformly formed on the entire back surface of the silicon semiconductor substrate (wafer) as compared with the conventional configuration in which the Ag electrode is used as the back surface side external connection electrode. In addition, cost reduction corresponding to the material price difference between silver and aluminum can be achieved.
Moreover, since the bonding using the conductive adhesive film can be used by the aluminum electrode for external connection with improved adhesion strength (peeling strength) of the aluminum film, a pattern not including the electrode portion for solder bonding can be used. Can be built. As a result, the aluminization of the entire back surface of the substrate (wafer) improves the power generation efficiency of the solar cell due to the uniform formation of the BSF layer, and improves the power generation efficiency of the solar cell due to omitting the bus bar electrode pattern. Can be realized.

  Although the present invention has been described in detail above, these are merely examples, and the present invention can be implemented in other modes, and various modifications can be made without departing from the spirit of the present invention. For example, you may form at least one part of a light-receiving surface electrode (For example, the bus-bar electrode which comprises a light-receiving surface electrode, or a grid line) using the paste composition for aluminum electrode formation disclosed here.

11 Silicon semiconductor substrate (Si wafer)
DESCRIPTION OF SYMBOLS 12 Light-receiving surface electrode 14 Antireflection film 16 n-Si layer 18 p-Si layer 20 Back surface wide area aluminum electrode 22 Aluminum electrode 23 for external connection Opening 24 BSF layer 30 Conductive adhesive film 31 Conductive particle 32 Adhesive material component 35 Tab Wire (lead wire)
35e Tab extension 40 Fixing jig 41 Glass substrate 100, 200, 1000 Solar cell 300 Strength measuring device

Claims (12)

A paste composition for forming an aluminum electrode of a solar cell,
Aluminum powder,
Glass frit with the following conditions:
(1) The glass softening point is 400 ° C. or higher and 600 ° C. or lower;
(2) The thermal expansion coefficient is 60 × 10 ⁻⁷ / ° C. or more and 80 × 10 ⁻⁷ / ° C. or less;
(3) Including SiO ₂ , B ₂ O ₃ , ZnO and / or PbO, Al ₂ O ₃ , and at least one alkali metal oxide as essential components;
A glass frit comprising:
An organic vehicle,
A paste composition for forming an aluminum electrode, comprising: The glass frit is 100 mol% of the following components as a whole, and the molar content of each component is
SiO ₂ 20~35mol%,
B ₂ O ₃ 5~30mol%,
ZnO and / or PbO 25-45 mol%,
Al ₂ O ₃ 2~10mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 5 to 15 mol%,
At least one of CaO, SrO and BaO 0-10 mol%,
Bi ₂ O ₃ 0-5 mol%,
The paste composition according to claim 1, wherein the total of these components is 95 mol% or more of the entire glass frit. The glass frit is 100 mol% of the following components as a whole, and the molar content of each component is
SiO ₂ 20~30mol%,
B ₂ O ₃ 20~30mol%,
ZnO 25-35 mol%,
Al ₂ O ₃ 3~7mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 10 to 15 mol%,
At least one of CaO, SrO and BaO, 2 to 10 mol%,
The paste composition according to claim 2, wherein the total of these components is 95 mol% or more of the entire glass frit.   The paste composition as described in any one of Claims 1-3 whose glass softening point of the said glass frit is 500 degreeC or more and 600 degrees C or less.   5. The content of the aluminum powder is 60 to 80% by mass, and the content of the glass frit is 2 to 10% by mass, based on 100% by mass of the entire paste composition. The paste composition according to one item. A solar cell comprising a silicon semiconductor substrate, a light-receiving surface electrode formed on the light-receiving surface side which is one surface of the substrate, and an aluminum electrode formed on the back surface side which is the other surface of the substrate,
At least a part of the aluminum electrode, as a glass composition, has the following conditions:
(1) The glass softening point is 400 ° C. or higher and 600 ° C. or lower;
(2) The thermal expansion coefficient is 60 × 10 ⁻⁷ / ° C. or more and 80 × 10 ⁻⁷ / ° C. or less;
(3) Including SiO ₂ , B ₂ O ₃ , ZnO and / or PbO, Al ₂ O ₃ , and at least one alkali metal oxide as essential components;
A solar cell comprising a glass composition comprising: The glass composition has the following components as a whole at 100 mol%, and the molar content of each component is
SiO ₂ 20~35mol%,
B ₂ O ₃ 5~30mol%,
ZnO and / or PbO 25-45 mol%,
Al ₂ O ₃ 2~10mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 5 to 15 mol%,
At least one of CaO, SrO and BaO 0-10 mol%,
Bi ₂ O ₃ 0-5 mol%,
The solar cell according to claim 6, wherein the total of these components is 95 mol% or more of the entire glass composition. The glass composition has the following components as a whole at 100 mol%, and the molar content of each component is
SiO ₂ 20~30mol%,
B ₂ O ₃ 20~30mol%,
ZnO 25-35 mol%,
Al ₂ O ₃ 3~7mol%,
At least one of Li ₂ O, Na ₂ O and K ₂ O, 10 to 15 mol%,
At least one of CaO, SrO and BaO, 2 to 10 mol%,
The solar cell according to claim 7, wherein the total of these components is 95 mol% or more of the entire glass composition.   The solar cell as described in any one of Claims 6-8 whose glass softening point of the said glass composition is 500 degreeC or more and 600 degrees C or less. On the back side, an external connection electrode is formed,
The solar cell according to any one of claims 6 to 9, wherein the external connection electrode is constituted by an aluminum electrode containing the glass composition.   The solar cell of Claim 10 with which the electroconductive adhesive film is affixed on the aluminum electrode containing the said glass composition which comprises the said electrode for external connection. A method of manufacturing a solar cell comprising a silicon semiconductor substrate, a light receiving surface electrode formed on the light receiving surface which is one surface of the substrate, and an aluminum electrode formed on the back surface which is the other surface of the substrate Because
A method for producing a solar cell, wherein at least a part of an aluminum electrode formed on the back surface side is formed using the paste composition according to any one of claims 1 to 5.

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