WO2024257439A1 - Electrically conductive paste, use of electrically conductive paste, solar cell, and method for producing solar cell - Google Patents

Abstract

Provided is an electrically conductive paste which is suitable for forming an electrode by means of a laser treatment process in order to produce a crystalline silicon solar cell. The electrically conductive paste contains a lead-free glass frit.　The electrically conductive paste is used for forming an electrode of a solar cell, and contains (A) electrically conductive particles, (B) an organic vehicle and (C) a glass frit. The glass frit (C) contains substantially no PbO. The product (B_GF·G) of the basicity B_GF of the glass frit (C) and the content G of the glass frit (C) in terms of parts by weight in the electrically conductive paste, if the content of the electrically conductive particles (A) in the electrically conductive paste is taken to be 100 parts by weight, falls within the range 0.25-1.45.

Description

Conductive paste, use of conductive paste, solar cell, and method for manufacturing solar cell

The present invention relates to a conductive paste used to form electrodes for semiconductor devices and the like. In particular, the present invention relates to a conductive paste for forming electrodes for solar cells. The present invention also relates to solar cells manufactured using the conductive paste for forming electrodes, and a method for manufacturing solar cells.

Semiconductor devices such as crystalline silicon solar cells that use crystalline silicon, which is made by processing single crystal silicon or polycrystalline silicon into a flat plate, as a substrate generally have electrodes formed on the surface of the silicon substrate using a conductive paste for electrode formation in order to make electrical contact with the outside of the device. Among semiconductor devices in which electrodes are formed in this way, the production volume of crystalline silicon solar cells has increased significantly in recent years. These solar cells have an impurity diffusion layer, an anti-reflection film, and a light-incident surface electrode on one surface of the crystalline silicon substrate, and a back electrode on the other surface. The light-incident surface electrode and the back electrode allow the electricity generated by the crystalline silicon solar cell to be extracted to the outside.

　Conventional electrodes for crystalline silicon solar cells are formed using a conductive paste that contains conductive powder, glass frit, organic binder, solvent and other additives. Silver particles (silver powder) are mainly used as the conductive powder.

The glass frit contained in conductive paste generally contains lead (Pb). However, lead (Pb) may have adverse effects on the environment. For this reason, lead-free glass frit that does not contain lead has been proposed.

As an example of a conductive paste containing lead-free glass frit, Patent Document 1 describes a conductive paste containing (a) about 85 wt % to about 99.5 wt % of a conductive metal or its derivative based on the solid content, (b) about 0.5 wt % to 15 wt % of a lead-free glass frit containing tellurium-bismuth-selenium-lithium-oxide based on the solid content, and (c) an organic carrier. Note that in Patent Document 1, the weight of the solid content is the total weight of (a) the conductive metal or its derivative and (b) the lead-free glass frit.

Patent document 2 describes a process for improving the ohmic contact behavior between a contact grid and an emitter layer in a silicon solar cell. Specifically, the process described in patent document 2 involves applying a predetermined voltage in the forward and reverse directions of the silicon solar cell, guiding a point light source to the solar surface side of the silicon solar cell, thereby irradiating a cross section of a subsection on the solar surface side.

Patent Document 3 describes a conductive composition that contains silver powder, glass powder containing PbO, and a vehicle made of an organic substance. Patent Document 3 describes that the conductive composition is a conductive composition for forming an electrode that penetrates a silicon nitride layer and conducts with an n-type semiconductor layer formed below the silicon nitride layer. Patent Document 3 also describes that the basicity of the glass powder contained in the conductive composition is 0.6 to 0.8, and that the glass transition point is 300°C to 450°C.

Patent No. 5934411 Special Publication No. 2021-513218 JP 2009-231826 A

Figure 5 shows an example of a schematic cross-sectional view of a typical crystalline silicon solar cell. As shown in Figure 5, in a typical crystalline silicon solar cell, an impurity diffusion layer 4 (e.g., a p-type impurity diffusion layer in which p-type impurities are diffused) is formed on the light-incident surface (light-incident surface) of a crystalline silicon substrate 1 (e.g., an n-type crystalline silicon substrate 1). An anti-reflection film 2 is formed on the impurity diffusion layer 4. The anti-reflection film 2 also functions as a passivation film, and is sometimes called a passivation film. Furthermore, the electrode pattern of the light-incident surface electrode 20 (surface electrode) is printed on the anti-reflection film 2 using a conductive paste by screen printing or the like, and the conductive paste is dried and fired at a predetermined temperature to form the light-incident surface electrode 20. In a typical crystalline silicon solar cell, the conductive paste fires through the anti-reflection film 2 during firing at this predetermined temperature. This fire-through allows the light-incident surface electrode 20 to be formed so as to contact the impurity diffusion layer 4. The fire-through is to etch the anti-reflection film 2, which is an insulating film, with glass frit or the like contained in the conductive paste, and electrically connect the light-incident surface electrode 20 and the impurity diffusion layer 4. In the example shown in FIG. 5, the anti-reflection film 2 disappears when the electrode pattern is baked, because the electrode pattern fires through the anti-reflection film 2. As a result, the light-incident surface electrode 20 and the impurity diffusion layer 4 are in contact with each other. A pn junction is formed at the interface between the n-type crystalline silicon substrate 1 and the impurity diffusion layer 4. Most of the incident light incident on the crystalline silicon solar cell passes through the anti-reflection film 2 and the impurity diffusion layer 4 and enters the n-type crystalline silicon substrate 1. In this process, the light is absorbed in the n-type crystalline silicon substrate 1, and electron-hole pairs are generated. These electron-hole pairs are separated by the electric field of the pn junction, with the electrons being separated from the n-type crystalline silicon substrate 1 to the back electrode 15, and the holes being separated from the p-type impurity diffusion layer 4 to the light-incident surface electrode 20. The electrons and holes (carriers) are extracted to the outside as electric current through these electrodes.

Figure 2 shows an example of a schematic diagram of the light incident surface of a crystalline silicon solar cell. As shown in Figure 2, a busbar electrode (light incident busbar electrode 20a) and a light incident finger electrode 20b (sometimes simply referred to as "finger electrode 20b") are arranged on the light incident surface of the crystalline silicon solar cell as the light incident surface electrode 20. In the example shown in Figures 5 and 2, the electrons of the electron-hole pairs generated by the incident light entering the crystalline silicon solar cell are collected by the finger electrode 20b and further collected by the light incident busbar electrode 20a. A metal ribbon for interconnection, surrounded by solder, is soldered to the light incident busbar electrode 20a. This metal ribbon extracts the current to the outside.

In order to obtain a crystalline silicon solar cell with high conversion efficiency, the contact resistance between the light-incident surface electrode 20 and the impurity diffusion layer 4 must be low.

The glass frit contained in the conductive paste is generally a glass frit containing lead oxide (PbO) (lead-containing glass frit). This is because the conductive paste for forming electrodes in crystalline silicon solar cells contains lead-containing glass frit, which can reduce the contact resistance between the light-incident surface electrode 20 and the impurity diffusion layer 4. However, lead has adverse effects on the human body. If products are manufactured using materials containing lead, there is a risk that the lead will pollute the environment when the products are disposed of. For this reason, it is desirable to use lead-free materials that do not contain lead when manufacturing products. It is preferable to use lead-free glass frit in the solar cell manufacturing process.

On the other hand, a method for manufacturing a solar cell using a laser treatment process has been proposed to obtain low contact resistance between the light-incident surface electrode 20 and the impurity diffusion layer 4. Patent Document 2 describes a specific example of the laser treatment process. In this specification, the laser treatment process refers to a technology for obtaining low contact resistance by applying a predetermined voltage to the light-incident surface electrode 20 after forming the light-incident surface electrode 20 so that a current flows in the opposite direction to the forward direction of the pn junction of the crystalline silicon solar cell, and irradiating the light-incident surface of the solar cell with light (e.g., wavelength 400 nm to 1500 nm) from a point light source. Generally, by performing the laser treatment process, the fill factor (FF) of the solar cell characteristics can be improved without decreasing the open circuit voltage (Voc). In the laser treatment process, when the electrode pattern of the conductive paste is baked at a predetermined temperature, it is preferable that the conductive paste does not fire through the anti-reflection film 2 in most of the anti-reflection film 2 in contact with the electrode pattern. FIG. 1 shows an example of a cross-sectional schematic diagram showing a structure in which a light-incident surface electrode 20 is formed on the light-incident surface of a crystalline silicon solar cell by using a laser treatment process. As shown in FIG. 1, an anti-reflection film 2 is present in most of the area between the light-incident surface electrode 20 and the impurity diffusion layer 4. In the laser treatment process, the above-mentioned predetermined voltage is applied so that a current flows in the opposite direction to the forward direction in the pn junction, and light from a point light source is irradiated to generate carriers (electrons and holes). This laser treatment process causes a current to flow in a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4, causing local heating. Due to the local heating, a small area where the impurity diffusion layer 4 does not exist is locally formed between the light-incident surface electrode 20 and the impurity diffusion layer 4. As a result, as shown in FIGS. 6 and 7, it is considered that an AgSi alloy 30 (an alloy of silver and silicon) which is a small electrically conductive part (local conductive part) is locally formed in the impurity diffusion layer 4 in contact with the light-incident surface electrode 20. Since the AgSi alloy 30 is formed locally in a limited area, it is not shown in FIG. 1. The dotted ellipse in FIGS. 6 and 7 indicates the approximate position of the AgSi alloy 30, and does not strictly indicate the boundary of the AgSi alloy 30. It is considered that the locally formed minute electrically conductive portion enables good electrical conduction between the light incident side surface electrode 20 and the impurity diffusion layer 4. In addition, the anti-reflection film 2 (passivation film) is present in most of the area between the light incident side surface electrode 20 and the impurity diffusion layer 4 other than the area where the local conductive portion is formed. As a result, the fill factor (FF) can be improved without decreasing the open circuit voltage (Voc) as the performance of the solar cell. Therefore, the conductive paste used to form the light incident side surface electrode 20 by the laser processing process needs to have properties different from those of conventional conductive pastes (conductive pastes that can fire through the anti-reflection film 2).

In addition, in the case of conventional crystalline silicon solar cells, when the light incident surface electrode 20 is formed, the electrode pattern of the conductive paste is fired, causing it to fire through the anti-reflection film 2 and come into contact with the impurity diffusion layer 4. When this fire-through occurs, the impurity diffusion layer 4 is damaged, causing a problem of reduced performance of the crystalline silicon solar cell. In contrast, in the laser treatment process, the electrode pattern of the conductive paste does not essentially fire through the anti-reflection film 2 when fired to form the light incident surface electrode 20. Therefore, by using the laser treatment process, damage to the impurity diffusion layer 4 can be suppressed.

The present invention aims to provide a conductive paste that is suitable for forming electrodes by a laser treatment process for the manufacture of crystalline silicon solar cells, the conductive paste including a lead-free glass frit.

The present invention also aims to provide a method for manufacturing a high-performance crystalline silicon solar cell using a conductive paste that is suitable for forming electrodes by a laser treatment process and that contains a lead-free glass frit. The present invention also aims to provide a high-performance lead-free crystalline silicon solar cell manufactured by a manufacturing method that includes forming electrodes by a laser treatment process.

To solve the above problems, the present invention has the following configuration.

(Configuration 1)
Configuration 1 is a conductive paste for forming an electrode of a solar cell,
(A) conductive particles;
(B) an organic vehicle; and
(C) a glass frit,
The (C) glass frit is substantially free of PbO,
The conductive paste is such that a product BGF _{·G of a basicity BGF} of the (C) glass _frit and a content G of the (C) glass frit in the conductive paste expressed in parts by weight when a content of the (A) conductive particles in the conductive paste is taken as 100 parts by weight is a range of 0.25 to 1.45.

(Configuration 2)
Configuration 2 is the conductive paste of configuration ₁ , wherein the (C) glass frit includes _Bi2O3 .

(Configuration 3)
Structure 3 is the conductive paste of structure 1 or 2, in which the product C _Bi2O3 ·G of the content of Bi ₂ O ₃ in the (C) glass frit in mol % (C _Bi2O3 ) and the content G of the (C) glass frit is in the range of 10 to 200.

(Configuration 4)
Configuration 4 is the conductive paste of any one of configurations 1 to 3, wherein the (A) conductive particles include silver particles.

(Configuration 5)
A configuration 5 is the conductive paste of any one of configurations 1 to 4, wherein the content G of the glass frit (C) is 0.1 to 5.0 parts by weight.

(Configuration 6)
A configuration 6 is the conductive paste of any one of configurations 1 to 5, wherein the content G of the glass frit (C) is 0.3 to 3.0 parts by weight.

(Configuration 7)
A seventh aspect of the present invention is the conductive paste according to any one of the first to sixth aspects, wherein the glass frit (C) has a glass transition point of 250 to 600°C.

(Configuration 8)
A configuration 8 is the conductive paste of any one of configurations 1 to 7, wherein the glass frit (C) includes at least one selected from SiO ₂ , B ₂ O ₃ , V ₂ O ₅ , Bi ₂ O ₃ , TeO2, BaO, CuO, Li ₂ O, and ZnO.

(Configuration 9)
A configuration 9 is the conductive paste of any one of configurations 1 to 8, wherein (B) the organic vehicle includes at least one selected from ethyl cellulose, rosin ester, acrylic, and an organic solvent.

(Configuration 10)
The present invention relates to a conductive paste for forming an electrode of a solar cell,
Solar cells,
a semiconductor substrate of a first conductivity type;
a semiconductor layer of a second conductivity type disposed on one surface of the semiconductor substrate of the first conductivity type;
a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate;
a passivation film disposed in contact with a surface of the second conductive type semiconductor layer;
a light incident side surface electrode disposed on at least a part of a surface of the passivation film;
the light-incident-side surface electrode is a surface electrode on the light-incident side of the solar cell that has been subjected to a process of irradiating light from a point light source onto the light-incident-side surface of the solar cell while applying a voltage between the back electrode and the light-incident-side surface electrode so that a current flows in a direction opposite to a forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type;
The conductive paste according to any one of configurations 1 to 9, wherein the conductive paste is a conductive paste for forming the light-incident side surface electrode.

(Configuration 11)
The present invention relates to a semiconductor substrate having a first conductivity type;
a semiconductor layer of a second conductivity type disposed on one surface of the semiconductor substrate of the first conductivity type;
a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate;
a passivation film disposed in contact with a surface of the second conductive type semiconductor layer;
a light-incident side surface electrode disposed on at least a portion of a surface of the passivation film,
the light-incident side surface electrode is a surface electrode on which light from a point light source is irradiated onto the light-incident side surface of the solar cell while a voltage is applied between the back electrode and the light-incident side surface electrode so that a current flows between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type in a direction opposite to a forward direction;
The solar cell has a surface electrode on the light-incident side, the surface electrode being a fired body of the conductive paste according to any one of configurations 1 to 10.

Aspect 12 includes a first conductivity type crystalline silicon substrate;
a silicon emitter layer of a second conductivity type disposed on one surface of the crystalline silicon substrate of the first conductivity type;
a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type crystalline silicon substrate;
a passivation film disposed in contact with a surface of the second conductivity type silicon emitter layer;
a light-incident surface electrode including silver disposed on at least a portion of a surface of the passivation film,
the second conductive type silicon emitter layer has a local conductive portion that is in direct contact with the light incident side surface electrode without a passivation film therebetween,
the local conductive portion includes an alloy of silver and silicon;
The solar cell has a surface electrode on the light-incident side, the surface electrode being a fired body of the conductive paste according to any one of configurations 1 to 10.

(Configuration 13)
A thirteenth aspect of the present invention is a method for manufacturing a solar cell, comprising the steps of:
Providing a semiconductor substrate of a first conductivity type;
forming a semiconductor layer of a second conductivity type on one surface of the semiconductor substrate of the first conductivity type;
forming a back surface electrode so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate;
forming a passivation film in contact with a surface of the second conductive type semiconductor layer;
forming a light incident side surface electrode on at least a part of a surface of the passivation film;
applying a voltage between the back electrode and the light-incident surface electrode so that a current flows in a direction opposite to a forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type; and irradiating the light from a point light source onto the light-incident surface of the solar cell.
The method for producing a solar cell, wherein the light-incident side surface electrode is a fired body of the conductive paste according to any one of configurations 1 to 10.

Configuration 14 is the use of a conductive paste according to any one of claims 1 to 10 to form an electrode for a solar cell.

In configuration 15, the back electrode is a fired body of a conductive paste for a back electrode,
The conductive paste for the back electrode is
Second conductive particles;
A second organic vehicle; and
a second glass frit;
the second glass frit is substantially free of PbO;
_13. The solar cell of embodiment ₁₁ or _{12, wherein the second glass frit comprises at least one selected from SiO2, B2O3, Bi2O3, P2O5 , Li2O , Na2O , Al2O3 , TeO2 , TiO2} , _ZrO2 , and ZnO.

16. The solar cell of embodiment 15, wherein the second glass frit comprises _TeO2 .

Aspect 17 is a method for manufacturing a back electrode, comprising the steps of:
The conductive paste for the back electrode is
Second conductive particles;
A second organic vehicle; and
a second glass frit;
the second glass frit is substantially free of PbO;
14. The _{method for} producing _{a solar cell according to claim 13, wherein the second glass frit comprises at least one selected from SiO2, B2O3, Bi2O3, P2O5 , Li2O , Na2O , Al2O3 , TeO2 , TiO2} , _ZrO2 , and ZnO.

18. The method of claim 17, wherein the second glass frit comprises _TeO2 .

The present invention provides a conductive paste suitable for forming electrodes by a laser processing process for the manufacture of crystalline silicon solar cells, the conductive paste including a lead-free glass frit.

The present invention also aims to provide a method for manufacturing a high-performance crystalline silicon solar cell using a conductive paste that is suitable for forming electrodes by a laser processing process and that contains a lead-free glass frit. The present invention can also provide a high-performance lead-free crystalline silicon solar cell manufactured by a manufacturing method that includes forming electrodes by a laser processing process.

FIG. 1 is an example of a schematic cross-sectional view showing a structure in which a light-incident surface electrode is formed on the light-incident surface of a crystalline silicon solar cell by a laser treatment process using the conductive paste of this embodiment. 1 is a schematic diagram of an example of the light incident surface of a crystalline silicon solar cell. 1 is an example of a schematic diagram of the back surface of a crystalline silicon solar cell. 1 is an example of a schematic cross-sectional view of a bifacial crystalline silicon solar cell using the conductive paste of this embodiment. FIG. 1 is an example of a schematic cross-sectional view of a typical crystalline silicon solar cell near the light-incident surface electrode (finger electrode), showing that the anti-reflection film (passivation film) between the electrode and the impurity diffusion layer has disappeared due to fire-through. 1 is a cross-sectional SEM (scanning electron microscope) photograph (magnification: 20,000 times) of a solar cell in which a light-incident surface electrode is formed using the conductive paste of Reference Example 1, near the passivation film on the light-incident surface. 1 is a cross-sectional SEM photograph (magnification: 20,000 times) of a solar cell in which a light-incident surface electrode is formed using the conductive paste of Example 1, near a passivation film on the light-incident surface. 1 is a cross-sectional SEM photograph (magnification: 20,000 times) of a solar cell in which a light-incident surface electrode is formed using the conductive paste of Comparative Example 1, near a passivation film on the light-incident surface. This is a cross-sectional SEM (scanning electron microscope) photograph (magnification: 20,000 times) of the crystalline silicon solar cell of Reference Example 1 near the passivation film on the light-incident surface, and is a figure for explaining the depth d of the AgSi alloy region. This is a cross-sectional SEM (scanning electron microscope) photograph (magnification: 20,000 times) of the crystalline silicon solar cell of Reference Example 1 near the passivation film on the light-incident surface, and is a figure for explaining the passivation film residual rate Lp/(Lp+Le).

The following is a detailed description of an embodiment of the present invention. Note that the following embodiment is a form for realizing the present invention, and does not limit the scope of the present invention.

In this specification, "crystalline silicon" includes single crystal and polycrystalline silicon. Furthermore, "crystalline silicon substrate" refers to a material in which crystalline silicon is formed into a shape suitable for forming elements, such as a flat plate, in order to form semiconductor devices such as electric or electronic elements. Any method may be used to manufacture crystalline silicon. For example, the Czochralski method can be used for single crystal silicon, and the casting method can be used for polycrystalline silicon. Other manufacturing methods, such as polycrystalline silicon ribbons manufactured by the ribbon pulling method, and polycrystalline silicon formed on a heterogeneous substrate such as glass, can also be used as the crystalline silicon substrate. Furthermore, "crystalline silicon solar cell" refers to a solar cell manufactured using a crystalline silicon substrate.

In this specification, glass frit refers to a material that is primarily made of multiple types of oxides, such as metal oxides, and is generally used in the form of glass-like particles.

In this specification, lead-free glass frit means glass frit that does not substantially contain lead (Pb). Since glass frit is manufactured using metal oxides as raw materials, lead-free glass frit means glass frit that does not substantially contain lead oxide (PbO). When manufacturing lead-free glass frit, no lead-containing material (PbO) is intentionally used. However, lead-free glass frit may contain trace amounts of lead that are inevitably mixed in as an impurity. Specifically, the lead-free glass frit of this embodiment may contain 0.1 wt % or less of lead as an impurity relative to 100 wt % of glass frit.

As shown in FIG. 1, finger electrodes 20b are arranged on the light incident surface of the crystalline silicon solar cell as light incident surface electrodes 20. In the example shown in FIG. 1, the holes of the electron-hole pairs generated by the incident light entering the crystalline silicon solar cell are collected in the finger electrodes 20b via the impurity diffusion layer 4 (e.g., p-type impurity diffusion layer 4). Therefore, the contact resistance between the finger electrodes 20b and the impurity diffusion layer 4 is required to be low. The conductive paste of this embodiment can be preferably used to form the finger electrodes 20b.

In this specification, the light-incident surface electrode 20 and the back surface electrode 15, which are electrodes for extracting current from the crystalline silicon solar cell to the outside, may be collectively referred to simply as "electrodes".

This embodiment is a conductive paste for forming electrodes for solar cells. The conductive paste of this embodiment contains (A) conductive particles, (B) an organic vehicle, and (C) glass frit. The (C) glass frit contained in the conductive paste of this embodiment does not substantially contain PbO. In other words, the (C) glass frit contained in the conductive paste of this embodiment is a lead-free glass frit. Furthermore, in the conductive paste of this embodiment, the basicity of the (C) glass frit and the content of the (C) glass frit in the conductive paste are appropriately controlled.

The glass frit (C) contained in the conductive paste of this embodiment does not substantially contain lead. Furthermore, materials other than the glass frit (C) contained in the conductive paste of this embodiment also do not substantially contain lead. Therefore, the conductive paste of this embodiment is a lead-free conductive paste. Therefore, lead pollution of the environment can be prevented when solar cells manufactured using the conductive paste of this embodiment are discarded.

The light-to-electricity conversion efficiency of a solar cell (sometimes simply referred to as "conversion efficiency") is expressed as the product of the fill factor (FF), the open circuit voltage (Voc), and the short circuit current (Jsc). Basically, FF and Voc are in a trade-off relationship. Therefore, it is difficult to simultaneously increase both FF and Voc. On the other hand, Patent Document 2 describes that by adopting a laser treatment process during the manufacture of a crystalline silicon solar cell, it is possible to improve the ohmic contact behavior between the grid-shaped electrode that is the light incident side surface electrode 20 and the impurity diffusion layer 4 (emitter layer), and further describes that it is possible to significantly reduce the contact resistance between the light incident side surface electrode 20 and the impurity diffusion layer 4. Therefore, by performing a laser treatment process, it is possible to improve FF without decreasing Voc.

The present inventors have found that when a laser treatment process is applied to a solar cell in which a light-incident surface electrode 20 is formed using a conventional conductive paste (for example, the conductive paste described in Patent Document 3), it adversely affects the anti-reflection film 2 (passivation film) and the impurity diffusion layer 4 (and the substrate 1), resulting in a decrease in the conversion efficiency of the solar cell. The present inventors have also found that the cause is that the fire-through property (reactivity) of the conventional conductive paste to the anti-reflection film 2 (passivation film) is too strong. Furthermore, the present inventors have found that the reactivity of the glass frit to the anti-reflection film 2 (passivation film) can be made appropriate by setting the basicity and content of the lead-free glass frit within an appropriate range. By using lead-free glass frit as the glass frit, lead pollution due to the discharge of lead into the environment can be prevented. Furthermore, when the conductive paste (lead-free glass frit) of this embodiment is used, the contact resistance of the resulting electrode can be reduced to the same extent as that of a lead-containing glass frit. With the above knowledge, the inventors discovered a conductive paste that can be preferably used when manufacturing crystalline silicon using a laser processing process, leading to the invention.

By forming an electrode of a crystalline silicon solar cell using the conductive paste of this embodiment and performing a laser treatment process, it is possible to obtain low contact resistance between the electrode and the impurity diffusion layer 4 of the solar cell without impairing the function of the anti-reflection film 2 as a passivation film. Therefore, by performing a laser treatment process using the conductive paste of this embodiment, a crystalline silicon solar cell with high conversion efficiency can be obtained. The conductive paste of this embodiment can be preferably used to form the light incident surface electrode 20 by a laser treatment process when manufacturing a crystalline silicon solar cell.

In the laser treatment process using the conductive paste of this embodiment, the anti-reflection film 2 (passivation film) is not basically fired through when the light-incident surface electrode 20 is formed. Furthermore, even if the laser treatment process is performed on the light-incident surface electrode 20, most of the anti-reflection film 2 (passivation film) in contact with the light-incident surface electrode 20 does not disappear. In other words, the anti-reflection film 2 (passivation film) is present in most of the area between the light-incident surface electrode 20 and the impurity diffusion layer 4 (e.g., 90% or more of the area of the interface, preferably 95% or more, and more preferably 99% or more) except for the area where a small localized electrical conductive portion (local conductive portion) is formed. Therefore, by using the laser treatment process when forming the light-incident surface electrode 20, damage to the impurity diffusion layer 4 can be suppressed.

One type of crystalline silicon solar cell is a bifacial power generation crystalline silicon solar cell that generates electricity by receiving light from two surfaces (first and second light incident surfaces). In this case, the conductive paste of this embodiment can be preferably used to form electrodes on the first and second light incident surfaces.

The conductive paste of this embodiment can be preferably used to form a light-incident surface electrode 20 formed on the surface (light-incident surface) of the anti-reflection film 2 (passivation film) formed on the impurity diffusion layer 4, but is not limited thereto. For example, the conductive paste of this embodiment may be used to form a back surface electrode 15 on the surface (back surface) opposite the light-incident surface. A passivation film may be formed on the back surface of a crystalline silicon solar cell, and the back surface electrode 15 may be formed on the passivation film. In the case of a solar cell with this structure, as explained above, the conductive paste of this embodiment can be used to form an electrical contact between the back surface electrode 15 and the crystalline silicon substrate 1 of the solar cell through the back surface passivation film.

The conductive paste of the present invention will be described below by taking as an example the case of forming a light incident side surface electrode 20 (surface electrode) of a crystalline silicon solar cell using an n-type crystalline silicon substrate 1. In the case of this crystalline silicon solar cell, the impurity diffusion layer 4 formed on the light incident side surface is a p-type impurity diffusion layer 4. Note that in this specification, the impurity diffusion layer 4 in the case of a solar cell using a crystalline silicon substrate 1 may be referred to as a "silicon emitter layer." In addition, an anti-reflection film 2 is formed on the surface of the p-type impurity diffusion layer 4.

The passivation film (anti-reflection film 2) can be a film consisting of a single layer or multiple layers. When the passivation film is a single layer, it is preferable that the passivation film is a thin film (SiN film) made of silicon nitride (SiN) because the surface of the silicon substrate can be effectively passivated. When the passivation film is a multiple layer, the passivation film can be a laminated film (SiN/SiO _x film) of a thin film made of silicon nitride and a thin film made of silicon oxide. When the passivation film is a SiN/SiO _x film, it is preferable to form the SiO _x film so that the SiO _x film is in contact with the silicon substrate 1, and to form a SiN film on the SiO _x film, because the surface of the silicon substrate can be more effectively passivated. The SiO _x film can be a natural oxide film of the silicon substrate.

The crystalline silicon solar cell can have a light incident side busbar electrode 20a and/or a backside TAB electrode 15a. The light incident side busbar electrode 20a has the function of electrically connecting the finger electrode 20b for collecting the current generated by the solar cell and the metal ribbon for interconnection. Similarly, the backside TAB electrode 15a has the function of electrically connecting the entire backside electrode 15b for collecting the current generated by the solar cell and the metal ribbon for interconnection. If the finger electrode 20b comes into contact with the crystalline silicon substrate 1, the surface defect density of the surface (interface) of the crystalline silicon substrate 1 where the finger electrode 20b comes into contact increases, and the solar cell performance decreases. The conductive paste of the present invention, particularly as a conductive paste for the finger electrode 20b, has low fire-through (reactivity) to the anti-reflective film 2, so it does not completely fire through the anti-reflective film 2. Therefore, when the finger electrode 20b is formed using the conductive paste of the present invention, the passivation film in the portion in contact with the crystalline silicon substrate 1 can be kept intact, and an increase in the surface defect density that causes carrier recombination can be prevented. Therefore, the conductive paste of the present embodiment described above can be suitably used as a conductive paste for forming the finger electrode 20b of a crystalline silicon solar cell. The conductive paste of the present embodiment can also be suitably used as the back electrode 15 (back finger electrode 15c) of a bifacial crystalline silicon solar cell, as shown in FIG. 4. The entire electrode 20 can also be formed using the conductive paste of the present embodiment.

In the laser treatment process, the above-mentioned predetermined voltage is applied and light from a point light source is irradiated, causing a current to flow in a small area between the light incident surface electrode 20 and the impurity diffusion layer 4 (silicon emitter layer), resulting in localized heating. As a result, as shown in FIG. 7, a AgSi alloy 30, which is a localized electrically conductive portion (locally conductive portion), is formed in the impurity diffusion layer 4 (silicon emitter layer) in contact with the light incident surface electrode 20. It is believed that this locally formed electrically conductive portion enables good electrical conduction between the light incident surface electrode 20 and the impurity diffusion layer 4 (silicon emitter layer). Therefore, the conductive paste of this embodiment used to form the light incident surface electrode 20 by the laser treatment process has properties different from conventional conductive pastes (conductive pastes that can fire through the anti-reflection film 2).

The conductive paste of this embodiment will now be described in detail.

<(A) Conductive particles>
The conductive paste of the present embodiment contains (A) conductive particles.

In the conductive paste of this embodiment, metal particles or alloy particles can be used as the conductive particles. Examples of metals contained in the metal particles or alloy particles include silver, gold, copper, nickel, zinc, and tin. Silver particles (Ag particles) can be used as the metal particles. The conductive paste of this embodiment can contain metals other than silver, such as gold, copper, nickel, zinc, and tin. In order to obtain low electrical resistance and high reliability, it is preferable that the conductive particles are silver particles made of silver. Note that silver particles made of silver can contain other metal elements as unavoidable impurities. Also, a large number of silver particles (Ag particles) may be called silver powder (Ag powder). The same applies to other particles.

The particle shape and particle size (also called particle diameter) of the conductive particles are not particularly limited. For example, spherical and scaly particle shapes can be used. The particle size of the conductive particles can be determined by the particle size (D50) of 50% of the total particle size. In this specification, D50 is also called the average particle size. The average particle size (D50) can be determined from the results of particle size distribution measurement performed by the Microtrack method (laser diffraction scattering method).

The average particle diameter (D50) of the conductive particles is preferably 0.5 to 2.5 μm, and more preferably 0.8 to 2.2 μm. By having the average particle diameter (D50) of the conductive particles within a specified range, the reactivity of the conductive paste with the passivation film during firing of the conductive paste can be suppressed. Note that if the average particle diameter (D50) is larger than the above range, problems such as clogging may occur during screen printing.

The size of silver particles can be expressed as the BET specific surface area (also simply referred to as "specific surface area"). The BET specific surface area of silver particles is preferably 0.1 to 1.5 m ² /g, and more preferably 0.2 to 1.2 m ² /g. The BET specific surface area can be measured, for example, using a fully automatic specific surface area measuring device Macsoeb (manufactured by MOUNTEC Corporation).

<(B) Organic Vehicle>
The conductive paste of the present embodiment contains (B) an organic vehicle.

The organic vehicle may contain an organic binder and a solvent. The organic binder and the solvent serve to adjust the viscosity of the conductive paste, and are not particularly limited. The organic binder may also be dissolved in a solvent before use.

In the conductive paste of this embodiment, it is preferable that the (B) organic vehicle contains at least one selected from ethyl cellulose, rosin ester, acrylic, and an organic solvent. By containing at least one selected from ethyl cellulose, rosin ester, acrylic, and an organic solvent, the (B) organic vehicle can be screen printed favorably, and the shape of the printed pattern can be made appropriate.

The organic binder can be selected from cellulose-based resins (e.g., ethyl cellulose, nitrocellulose, etc.) and (meth)acrylic resins (e.g., polymethyl acrylate, polymethyl methacrylate, etc.). The organic vehicle contained in the conductive paste of this embodiment preferably contains at least one selected from ethyl cellulose, rosin ester, butyral, acrylic, and an organic solvent. The amount of organic binder added is usually 0.1 to 30 parts by weight, and preferably 0.2 to 5 parts by weight, per 100 parts by weight of silver particles.

As the organic solvent, one or more selected from alcohols (e.g., terpineol, α-terpineol, and β-terpineol, etc.) and esters (e.g., hydroxyl group-containing esters, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, and diethylene glycol monobutyl ether acetate (butyl carbitol acetate), etc.) can be used. The amount of the solvent added is usually 0.5 to 30 parts by weight, and preferably 2 to 25 parts by weight, per 100 parts by weight of silver particles. A specific example of the organic solvent is diethylene glycol monobutyl ether acetate (butyl carbitol acetate).

<(C) Glass Frit>
The conductive paste of the present embodiment contains (C) glass frit.

The glass frit contained in the conductive paste of this embodiment is a lead-free glass frit. Therefore, the glass frit contained in the conductive paste of this embodiment does not substantially contain lead (Pb). However, the glass frit used in this embodiment may contain a small amount of lead that is inevitably mixed in as an impurity. Specifically, the glass frit used in this embodiment may contain 0.1% by weight or less of lead as an impurity relative to 100% by weight of the glass frit.

In the conductive paste of this embodiment, the product BGF _{·G of the basicity BGF} of the (C) glass frit and the content G of the (C) glass _frit in parts by weight in the conductive paste when the content of the (A) conductive particles in the conductive paste is 100 parts by weight is in the range of 0.05 to 1.5, preferably in the range of 0.1 to 1.4, and more preferably in the range of 0.15 to 1.35. By setting the product _BGF ·G of the basicity _BGF of the glass frit and the content G in an appropriate range, the reactivity of the glass frit with respect to the anti-reflection film 2 (passivation film) can be made appropriate. Therefore, the conductive paste of the embodiment can be preferably used when manufacturing crystalline silicon using a laser processing process.

The basicity of the glass frit can be calculated by the method described in Patent Document 3 (JP Patent Publication No. 2009-231826). In other words, the basicity of the glass powder can be defined using the formula shown in K. Morinaga, H. Yoshida and H. Takebe: J. Am Cerm. Soc., 77, 3113 (1994). Specifically, it is as follows.

The bonding force between M _i -O of the oxide M _i O is given by the following formula as the attractive force Ai between a cation and an oxygen ion.

A _i =Z _i・Z ₀₂₋ /(r _i +r ₀₂₋ ) ² =Zi・2/(r _i +1.40) ²
Z _i : valence of cation, oxygen ion is 2
r _i : ionic radius of cation (Å)

The ionic radius r _i of the oxygen ion is 1.40 nm. The reciprocal B _i (=1/A _i ) of A _i in the above formula is defined as the oxygen donating ability of the single component oxide M _i O.
B _i ≡ 1/A _i

_The B _i -index of each single-component oxide is obtained by normalizing B _CaO = 1 and B _SiO2 = 0. If the B _i -index of each component is expanded to a multi-component system using the cation fraction, the basicity (=B _GF ) of a melt of a glass oxide (glass frit) of any composition can be calculated.
B _GF =Σn _i・B _i
n _i : cation fraction

The basicity ( _BGF ) defined in this way represents the oxygen donating ability as described above, and the larger the value, the easier it is to donate oxygen and the easier it is to exchange oxygen with other metal oxides. In other words, it can be said that "basicity" represents the degree of dissolution in a glass melt.

The content G of the glass frit (C) is a dimensionless number because it is a ratio to the content G of the conductive particles (A). Also, as described above, B _i is a standardized value with B _CaO = 1 and B _SiO2 = 0, so the basicity B _GF (= Σn _i · B _i ) of the glass frit (C) is a dimensionless number. Therefore, the product B _GF · G of the basicity B _GF of the glass frit (C) and the content G is also a dimensionless number.

The basicity ( _BGF ) of the glass frit of this embodiment is preferably 0.10 to 1.5, more preferably 0.15 to 1.3, and even more preferably 0.20 to 1.1. When the basicity ( _BGF ) is within such a range, the reactivity of the glass frit with respect to the passivation film can be made appropriate by adjusting the amount of the glass frit added to the conductive paste.

The content G of the glass frit in the conductive paste of this embodiment is preferably 0.1 to 5.0 parts by weight, more preferably 0.2 to 4.0 parts by weight, even more preferably 0.3 to 3.0 parts by weight, and particularly preferably 0.4 to 2.7 parts by weight, relative to 100 parts by weight of the conductive particles. By appropriately adjusting the content G of the glass frit in the conductive paste together with the basicity ( _BGF ), the reactivity of the glass frit with respect to the passivation film can be made appropriate. More specifically, in order to obtain a conductive paste suitable for forming an electrode by a laser treatment process, the content of the glass frit is reduced from that of the conventional method, and the basicity of the glass frit is set to an appropriate range, thereby suppressing the reactivity with the passivation film and improving Voc.

The glass frit contained in the conductive paste of the present embodiment preferably contains one or more selected from SiO ₂ , B ₂ O ₃ , V ₂ O ₅ , Bi ₂ O ₃ , TeO ₂ , BaO, CuO, Li ₂ O, and ZnO. When the glass frit contains at least one of these oxides, the basicity of the glass frit can be adjusted to an appropriate range.

The glass frit preferably contains Bi ₂ O _3. The content of Bi ₂ O ₃ in the glass frit (100 mol%) is preferably 10 to 80 mol%, more preferably 15 to 75 mol%, and even more preferably 20 to 70 mol%. By including Bi ₂ O ₃ in the glass frit, the reactivity with the passivation film can be adjusted to an appropriate range while being lead-free, and the contact resistance can be reduced.

In the conductive paste of this embodiment, the _{product CBi2O3} · _{G of the content of Bi2O3} in the glass frit (C) in mol% ( _CBi2O3 ) and the content G of the glass frit (C) is preferably in the range of 10 to 200, more preferably in the range of 13 to 170, and even more preferably in the range of 15 to 150. By having the product _CBi2O3 ·G with the content G of the glass _frit (C) in the above range, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while being lead-free.

The glass frit may contain SiO ₂ to the extent that it does not adversely affect the conductive paste of the present embodiment. When the glass frit contains SiO ₂ , the content of SiO ₂ in the glass frit (100 mol%) is preferably 10 to 60 mol%, and more preferably 15 to 40 mol%. By containing an appropriate amount of SiO ₂ in the glass frit, the reactivity with the passivation film can be controlled.

The glass frit may contain B ₂ O ₃ to the extent that it does not adversely affect the conductive paste of the present embodiment. When the glass frit contains B ₂ O ₃ , the content of B ₂ O ₃ in the glass frit (100 mol%) is preferably 3 to 60 mol%, and more preferably 4 to 50 mol%. By containing an appropriate content of B ₂ O ₃ in the glass frit, the reactivity with the passivation film can be controlled.

The glass frit may contain _V2O5 to the extent that it does not adversely affect the conductive paste of the present embodiment. When the glass _frit contains _V2O5 , the content of _V2O5 in the glass frit (100 mol% ₎ is preferably less than 8 _{mol%, more preferably 5 mol% or less. When the glass frit contains V2O5 , the} basicity of _the glass frit can be reduced. Therefore, when the basicity of the glass frit is high, the basicity of the glass frit can be adjusted to an appropriate range by containing an appropriate content of _V2O5 .

The glass frit may contain _TeO2 to the extent that it does not adversely affect the conductive paste of the present embodiment. When the glass frit contains _TeO2 , the content of _TeO2 in the glass frit (100 mol%) is preferably less than 80 mol%, more preferably 50 mol% or less. When the glass frit contains _TeO2 , the basicity of the glass frit can be reduced. Therefore, when the basicity of the glass frit is high, the basicity of the glass frit can be adjusted to an appropriate range by containing an appropriate content of _TeO2 .

The glass frit may contain BaO to the extent that it does not adversely affect the conductive paste of this embodiment. When the glass frit contains BaO, the content of BaO in the glass frit (100 mol%) is preferably 3 to 20 mol%, and more preferably 5 to 10 mol%. By containing an appropriate content of BaO in the glass frit, the reactivity with the passivation film can be adjusted to an appropriate range.

The glass frit may contain CuO to the extent that it does not adversely affect the conductive paste of this embodiment. When the glass frit contains CuO, the content of CuO in the glass frit (100 mol%) is preferably 10 to 40 mol%, and more preferably 20 to 30 mol%. By containing an appropriate amount of CuO in the glass frit, the reactivity with the passivation film can be adjusted to an appropriate range.

The glass frit may contain Li ₂ O to the extent that it does not adversely affect the conductive paste of the present embodiment. When the glass frit contains Li ₂ O, the content of Li ₂ O in the glass frit (100 mol%) is preferably 3 to 40 mol%, and more preferably 5 to 30 mol%. When the glass frit contains an appropriate content of Li ₂ O, the reactivity with the passivation film can be adjusted to an appropriate range.

The glass frit may contain ZnO to the extent that it does not adversely affect the conductive paste of this embodiment. When the glass frit contains ZnO, the content of ZnO in the glass frit (100 mol%) is preferably 5 to 70 mol%, and more preferably 15 to 60 mol%. By including ZnO in the glass frit, the basicity of the glass frit can be adjusted to an appropriate range.

In the conductive paste of this embodiment, the glass transition point (Tg) of the glass frit (C) is preferably 250 to 600°C, more preferably 270 to 500°C, and even more preferably 300 to 470°C. By making the glass transition point (Tg) of the glass frit (C) 250°C or higher, it is possible to suppress reactivity with the passivation film. In addition, by making the glass transition point (Tg) 600°C or lower, it is possible to reduce the contact resistance between the resulting electrode (e.g., the light-incident surface electrode 20) and the impurity diffusion layer 4.

The glass transition point (Tg) can be measured as follows. First, a differential thermobalance (TG-DTA2000S, manufactured by Mac Science Co., Ltd.) is used, and the sample glass powder and reference material are set on the differential thermobalance. Next, the temperature is raised from room temperature to 900°C at a heating rate of 10°C/min as the measurement conditions, and a curve (DTA curve) is obtained in which the temperature difference between the sample glass powder and the reference material is plotted against temperature. The first inflection point of the DTA curve obtained in this way can be determined as the glass transition point Tg.

The shape of the glass frit particles is not particularly limited, and for example, spherical or amorphous shapes can be used. The particle size is also not particularly limited. From the viewpoint of workability, etc., the average particle size (D50) of the particles is preferably in the range of 0.1 to 10 μm, and more preferably in the range of 0.5 to 5 μm.

The glass frit particles can be one type of particle containing a predetermined amount of each of the required oxides. Also, particles made of a single oxide can be used as different particles for each of the required oxides. Also, multiple types of particles with different compositions of the required oxides can be used in combination. In order to obtain the synergistic effects of different types of oxides, it is preferable that the glass frit particles be one type of particle containing a predetermined amount of each of the required oxides.

<Other ingredients>
The conductive paste of the present embodiment may contain additives and other substances in addition to those mentioned above, provided that they do not adversely affect the solar cell characteristics of the resulting solar cell.

The conductive paste of this embodiment may further contain additives selected from plasticizers, defoamers, dispersants, leveling agents, stabilizers, and adhesion promoters, as necessary. Of these, the plasticizer may be at least one selected from phthalates, glycolates, phosphates, sebacates, adipic acids, and citrates.

The conductive paste of this embodiment may contain additives other than those described above, provided that they do not adversely affect the solar cell characteristics of the resulting solar cell. For example, the conductive paste of this embodiment may further contain at least one additive selected from titanium resinate, titanium oxide, cobalt oxide, cerium oxide, silicon nitride, copper manganese tin, aluminosilicate, and aluminum silicate. By containing these additives, the adhesive strength of the electrode to the passivation film can be improved. These additives may be in the form of particles (additive particles). The amount of additive added per 100 parts by weight of silver particles is preferably 0.01 to 5 parts by weight, more preferably 0.05 to 2 parts by weight. In order to obtain a higher adhesive strength, the additive is preferably copper manganese tin, aluminosilicate, or aluminum silicate. The additive may contain both aluminosilicate and aluminum silicate.

<Method of manufacturing conductive paste>
Next, a method for producing the conductive paste of the present embodiment will be described. The conductive paste of the present embodiment can be produced by adding silver particles, glass frit, and other additives and/or additives as necessary to an organic binder and a solvent, mixing them, and dispersing them.

Mixing can be performed, for example, with a planetary mixer. Dispersion can be performed with a three-roll mill. Mixing and dispersion are not limited to these methods, and various known methods can be used.

<Solar Cell>
Next, the solar cell of this embodiment will be described. The conductive paste of this embodiment described above is preferably used to form an electrode of the solar cell. That is, the conductive paste of this embodiment described above is preferably used to form a predetermined electrode of a crystalline silicon solar cell whose manufacturing process includes performing a laser treatment process on the predetermined electrode.

In this embodiment, a solar cell has at least a portion of the electrode formed using the conductive paste described above. Figures 1 and 4 show schematic cross-sectional views of a crystalline silicon solar cell. The conductive paste of this embodiment is substantially free of lead. Furthermore, in the crystalline silicon solar cell of this embodiment, materials other than the conductive paste can also be made substantially free of lead. Therefore, the crystalline silicon solar cell of this embodiment can be a lead-free solar cell.

In the solar cell of this embodiment, crystalline silicon, silicon carbide, germanium, gallium arsenide, and the like can be used as the material for the semiconductor substrate. From the standpoint of safety and cost as a solar cell, it is preferable that the material for the semiconductor substrate is crystalline silicon (single crystal silicon, polycrystalline silicon, etc.).

The solar cell of this embodiment includes a semiconductor substrate of a first conductivity type, a semiconductor layer of a second conductivity type disposed on one surface of the semiconductor substrate of the first conductivity type, a passivation film (anti-reflection film 2) disposed in contact with the surface of the semiconductor layer of the second conductivity type, and a light-incident surface electrode 20 disposed on at least a portion of the surface of the passivation film. The solar cell of this embodiment may also include a back electrode 15 disposed so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type. In the example of FIG. 1, the semiconductor substrate of the first conductivity type is a crystalline silicon substrate 1, the semiconductor layer of the second conductivity type is an impurity diffusion layer 4, and the passivation film is an anti-reflection film 2.

The semiconductor substrate of the first conductivity type is an n-type semiconductor substrate or a p-type semiconductor substrate. The semiconductor layer of the second conductivity type is a p-type semiconductor layer or an n-type semiconductor layer. When the semiconductor substrate is an n-type semiconductor substrate, a p-type semiconductor layer (p-type impurity diffusion layer 4) is disposed on one surface of the semiconductor substrate. When the semiconductor substrate is a p-type semiconductor substrate, an n-type semiconductor layer (n-type impurity diffusion layer 4) is disposed on one surface of the semiconductor substrate. The interface between the semiconductor substrate of the first conductivity type and the semiconductor layer of the second conductivity type corresponds to a pn junction. The material of the semiconductor substrate is preferably silicon. Therefore, the semiconductor substrate is preferably a crystalline silicon substrate.

The passivation film can be an anti-reflective film 2. The passivation film is preferably a thin film made of silicon nitride.

The light incident surface electrode 20 of the solar cell of this embodiment can be a sintered body of the conductive paste of this embodiment. The conductive paste of this embodiment can be used to manufacture a solar cell with this structure.

The conductive paste of this embodiment can be preferably used to form the light-incident surface electrode 20 of a crystalline silicon solar cell using a laser treatment process. The laser treatment process refers to a process in which light from a point light source is irradiated onto the light-incident surface of the solar cell while applying a voltage to the back electrode 15 and the light-incident surface electrode 20 so that a current flows in the opposite direction to the forward direction at the pn junction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type. Carriers (electron-hole pairs) are generated inside the semiconductor substrate by the light from the point light source, and the application of a voltage makes it possible to move the carriers, that is, to flow a current. The voltage is applied so that the direction of current flow at the pn junction is opposite to the forward direction. Therefore, when the semiconductor substrate is an n-type semiconductor substrate and the semiconductor layer is a p-type semiconductor layer, a voltage is applied to the back electrode 15 and the light-incident surface electrode 20 so that a current flows from the n-type semiconductor substrate to the p-type semiconductor layer. Furthermore, if the semiconductor substrate is a p-type semiconductor substrate and the semiconductor layer is an n-type semiconductor layer, a voltage is applied to the back electrode 15 and the light-incident side surface electrode 20 so that a current flows from the n-type semiconductor layer to the p-type semiconductor substrate.

When the crystalline silicon solar cell is a bifacial solar cell as shown in FIG. 4, light can be incident from two surfaces (the light incident surface and the back surface). Therefore, by irradiating at least one of the light incident surface or the back surface of the bifacial solar cell with light from a point light source, an AgSi alloy, which is a local conductive portion, can be formed in the impurity diffusion layer in contact with at least one of the electrodes (the light incident surface electrode 20 or the back surface electrode 15). In addition, by irradiating at least one surface of the bifacial solar cell with light from a point light source, an AgSi alloy can be formed near the electrodes on the two surfaces of the bifacial solar cell (the light incident surface electrode 20 and the back surface electrode 15).

The first conductivity type semiconductor substrate of the solar cell of this embodiment is preferably an n-type semiconductor substrate, and more preferably an n-type crystalline silicon substrate 1. The second conductivity type semiconductor layer of the solar cell of this embodiment is preferably a p-type semiconductor layer, and more preferably a p-type impurity diffusion layer 4 made of crystalline silicon. In general, the mobility of electrons, which are carriers in the n-type crystalline silicon substrate 1, is higher than the mobility of holes, which are carriers in the p-type crystalline silicon substrate 1. Therefore, in order to obtain a solar cell with high conversion efficiency, it is advantageous to use an n-type crystalline silicon substrate 1.

In the following explanation, a solar cell will be used as an example in which the first conductivity type semiconductor substrate is an n-type crystalline silicon substrate 1, and the second conductivity type semiconductor layer is a p-type impurity diffusion layer 4 (sometimes simply referred to as "impurity diffusion layer 4").

As shown in FIG. 1, when the laser treatment process is used, the anti-reflection film 2 (passivation film) is present in most of the area between the light-incident surface electrode 20 and the impurity diffusion layer 4. In the laser treatment process, the above-mentioned predetermined voltage is applied so that a current flows in the opposite direction to the forward direction in the pn junction, and light (e.g., laser light) from a point light source is irradiated, so that a current flows in a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4, causing local heating. As a result, as shown in FIGS. 6 and 7, an AgSi alloy 30 (an alloy of silver and silicon) is formed as a local electrically conductive portion (local conductive portion) between the light-incident surface electrode 20 and the impurity diffusion layer 4. That is, the local conductive portion contains an alloy of silver and silicon. In addition, in the local conductive portion, the impurity diffusion layer 4 (a silicon emitter layer of the second conductivity type) is directly in contact with the light-incident surface electrode 20 without the anti-reflection film 2 (passivation film). This locally formed electrically conductive portion (locally conductive portion) enables good electrical conduction between the light incident side surface electrode 20 and the impurity diffusion layer 4. The conductive paste of this embodiment has a lower reactivity with the anti-reflection film 2 than conventional conductive pastes, and has a reactivity with the anti-reflection film 2 (passivation film) appropriate for the laser treatment process. Therefore, the conductive paste of this embodiment can be preferably used to form the light incident side surface electrode 20 of a crystalline silicon solar cell using a laser treatment process.

The crystalline silicon solar cell shown in FIG. 1 can have a back electrode 15 with the structure shown in FIG. 3. The back electrode 15 is arranged so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type. As shown in FIG. 3, the back electrode 15 can generally include a full back electrode 15b and a back TAB electrode 15a electrically connected to the full back electrode 15b.

FIG. 4 shows an example of a cross-sectional schematic diagram of a bifacial crystalline silicon solar cell. The bifacial crystalline silicon solar cell shown in FIG. 4 has an impurity diffusion layer 4 and an anti-reflection film 2 (passivation film and back surface passivation film). In a bifacial crystalline silicon solar cell, the conductive paste of this embodiment can be used to form the light incident surface electrode 20 (particularly, finger electrode 20b) on the light incident surface and the back surface electrode 15 (back surface finger electrode 15c). This allows a laser processing process to be used to form an electrically conductive portion (locally conductive portion) in the passivation film (anti-reflection film 2) on the light incident surface and the back surface passivation film (anti-reflection film 2).

Therefore, the conductive paste of the present embodiment described above can be suitably used as a conductive paste for forming the finger electrodes 20b of a crystalline silicon solar cell. The conductive paste of the present embodiment can also be used as a conductive paste for forming the back electrode 15 of a bifacial crystalline silicon solar cell.

The busbar electrodes of the crystalline silicon solar cell shown in FIG. 1 include the light incident side busbar electrode 20a shown in FIG. 2 and the backside TAB electrode 15a as shown in FIG. 3. A metal ribbon for interconnection, the periphery of which is covered with solder, is soldered to the light incident side busbar electrode 20a and the backside TAB electrode 15a. This metal ribbon allows the current generated by the solar cell to be taken out of the crystalline silicon solar cell. The bifacial crystalline silicon solar cell shown in FIG. 4 can also have the light incident side busbar electrode 20a and the backside TAB electrode 15a having the same shape as the light incident side busbar electrode 20a.

The width of the busbar electrodes (light incident side busbar electrode 20a and backside TAB electrode 15a) can be approximately the same as that of the metal ribbon for interconnection. In order for the busbar electrodes to have low electrical resistance, the wider the width, the better. On the other hand, in order to increase the area of incidence of light on the light incident side surface, the narrower the width of the light incident side busbar electrode 20a is. Therefore, the busbar electrode width can be 0.05 to 5 mm, preferably 0.08 to 3 mm, more preferably 0.1 to 2 mm, and even more preferably 0.15 to 1 mm. In addition, the number of busbar electrodes can be determined according to the size of the crystalline silicon solar cell. The number of busbar electrodes is arbitrary. Specifically, the number of busbar electrodes can be three or four, or more. The optimal number of busbar electrodes can be determined so as to maximize the conversion efficiency of the crystalline silicon solar cell by simulating the operation of the solar cell. Since the crystalline silicon solar cells are connected in series to each other by metal ribbons for interconnection, it is preferable that the number of light-incident side busbar electrodes 20a and the back TAB electrodes 15a are the same. For the same reason, it is preferable that the widths of the light-incident side busbar electrodes 20a and the back TAB electrodes 15a are the same.

In order to increase the area of incidence of light on the crystalline silicon solar cell, it is better that the area occupied by the light incident surface electrode 20 on the light incident surface is as small as possible. Therefore, it is preferable that the finger electrodes 20b on the light incident surface are as narrow as possible and that there are as few of them as possible. On the other hand, in terms of reducing electrical loss (ohmic loss), it is preferable that the finger electrodes 20b are wide and there are many of them. In addition, in terms of reducing the contact resistance between the finger electrodes 20b and the crystalline silicon substrate 1 (impurity diffusion layer 4), it is preferable that the finger electrodes 20b are wide. From the above, the number of busbar electrodes can be determined according to the size of the crystalline silicon solar cell and the width of the busbar electrodes. The optimal width and number of finger electrodes 20b (the spacing between the finger electrodes 20b) can be determined by simulating the operation of the solar cell so as to maximize the conversion efficiency of the crystalline silicon solar cell. The width and number of back finger electrodes 15c of the back electrode 15 of the bifacial crystalline silicon solar cell shown in FIG. 4 can also be determined in a similar manner.

<Method of manufacturing solar cell>
Next, a method for manufacturing the solar cell of this embodiment will be described. The solar cell can be a crystalline silicon solar cell. In the following description, an example in which the solar cell is a crystalline silicon solar cell will be described.

The method for manufacturing a solar cell of this embodiment includes the steps of printing the above-mentioned conductive paste on the surface of the anti-reflection film 2 on the semiconductor layer of the second conductivity type (impurity diffusion layer 4), drying, and firing to form an electrode (light-incident surface electrode 20). The method for manufacturing a solar cell of this embodiment will be described in more detail below.

The method for manufacturing a solar cell according to this embodiment includes a step of preparing a semiconductor substrate (e.g., crystalline silicon substrate 1) of a first conductivity type (p-type or n-type). As the semiconductor substrate of the first conductivity type, it is preferable to use an n-type crystalline silicon substrate 1. The following describes an example in which a crystalline silicon solar cell is manufactured using an n-type crystalline silicon substrate 1.

In order to obtain high conversion efficiency, it is preferable that the surface of the crystalline silicon substrate 1 on the light incident side has a pyramidal texture structure.

Next, the method for manufacturing a solar cell of this embodiment includes a step of forming a semiconductor layer of a second conductivity type on one surface of the semiconductor substrate of the first conductivity type.

The manufacturing method of the crystalline silicon solar cell of this embodiment includes a step of forming a second conductive type semiconductor layer (impurity diffusion layer 4) on one surface of the crystalline silicon substrate 1 prepared in the above-mentioned step. When an n-type crystalline silicon substrate 1 is used as the crystalline silicon substrate 1, a p-type impurity diffusion layer 4 can be formed by diffusing a p-type impurity such as B (boron) as the impurity diffusion layer 4. It is also possible to manufacture a crystalline silicon solar cell using a p-type crystalline silicon substrate 1. In that case, an n-type impurity diffusion layer 4 is formed by diffusing an n-type impurity such as P (phosphorus) as the impurity diffusion layer 4.

When forming the impurity diffusion layer 4, it can be formed so that the sheet resistance of the impurity diffusion layer 4 is 40 to 150 Ω/□ (square), preferably 45 to 120 Ω/□.

In addition, in the manufacturing method of the crystalline silicon solar cell of this embodiment, the depth to which the impurity diffusion layer 4 is formed can be 0.3 μm to 1.0 μm. The depth of the impurity diffusion layer 4 refers to the depth from the surface of the impurity diffusion layer 4 to the pn junction. The depth of the pn junction can be the depth from the surface of the impurity diffusion layer 4 to the point where the impurity concentration in the impurity diffusion layer 4 becomes the impurity concentration of the substrate.

The method for manufacturing a solar cell of this embodiment includes a step of forming a back electrode 15 so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate (n-type crystalline silicon substrate 1). The back electrode 15 can be formed either before or after the light-incident surface electrode 20 is formed. Furthermore, the firing for forming the back electrode 15 can be performed simultaneously with or separately from the firing for forming the light-incident surface electrode 20.

Specifically, the manufacturing method of the crystalline silicon solar cell of this embodiment forms the back electrode 15 by printing and firing a conductive paste on the other surface (back surface) of the crystalline silicon substrate 1.

When manufacturing a bifacial crystalline solar cell as shown in FIG. 4, a second impurity diffusion layer 16 can be formed. On the other hand, by forming a back electrode 15 using the conductive paste (conductive composition) of this embodiment and performing a laser treatment process, a low-resistance electrically conductive portion (local conductive portion) can be formed between the back electrode 15 and the crystalline silicon substrate 1. Therefore, in the case of a bifacial crystalline solar cell, it is preferable to form the back electrode 15 using the conductive paste of this embodiment. In this case, the back electrode 15 is a fired body of the conductive paste of this embodiment.

Next, the method for manufacturing a solar cell of this embodiment includes forming a passivation film so as to be in contact with the surface of the second conductive type semiconductor layer (impurity diffusion layer 4). The passivation film can be an anti-reflection film 2.

Specifically, in the manufacturing method of the crystalline silicon solar cell of this embodiment, an anti-reflection film 2 that also functions as a passivation film is formed on the surface of the impurity diffusion layer 4 formed in the above-mentioned process. A silicon nitride film (SiN film) can be formed as the anti-reflection film 2. When a silicon nitride film is used as the anti-reflection film 2, the silicon nitride film layer also functions as a passivation film for the light incident surface. Therefore, when a silicon nitride film is used as the anti-reflection film 2, a high-performance crystalline silicon solar cell can be obtained. In addition, since the anti-reflection film 2 is a silicon nitride film, it can exhibit an anti-reflection function against incident light. The silicon nitride film can be formed by a method such as PECVD (Plasma Enhanced Chemical Vapor Deposition).

The manufacturing method of the solar cell of this embodiment includes a step of forming a light incident surface electrode 20 on at least a portion of the surface of the passivation film (anti-reflection film 2). In the manufacturing method of this embodiment, the above-mentioned conductive paste is used to form the light incident surface electrode 20. Therefore, the light incident surface electrode 20 is a sintered body of the above-mentioned conductive paste.

In the manufacturing method of the crystalline silicon solar cell of this embodiment, a conductive paste is printed on the surface of the anti-reflection film 2 and then fired to form the light incident surface electrode 20. Note that firing to form the back electrode 15 can be performed simultaneously with firing to form the light incident surface electrode 20.

Specifically, first, the pattern of the light incident side surface electrode 20 printed using the conductive paste of this embodiment is dried for several minutes (e.g., 0.5 to 5 minutes) at a temperature of about 100 to 150°C. At this time, the light incident side busbar electrode 20a and the light incident side finger electrode 20b of the light incident side surface electrode 20 can be formed using the conductive paste of this embodiment.

Following the printing and drying of the pattern for the light-incident surface electrode 20, a conductive paste for forming the back electrode 15 is printed and dried. The conductive paste of this embodiment can be preferably used to form electrodes (light-incident surface electrode 20, and in some cases back electrode 15) for solar cells such as crystalline silicon solar cells.

Then, the printed conductive paste is dried and fired in air under specified firing conditions using a firing furnace such as a tubular furnace. Firing conditions include a firing atmosphere of air and a firing temperature of 500 to 1000°C, more preferably 600 to 1000°C, even more preferably 500 to 900°C, and particularly preferably 700 to 900°C. Firing is preferably performed for a short period of time, and the temperature profile (temperature-time curve) during firing is preferably peak-shaped. For example, with the above temperature as the peak temperature, the in-out time of the firing furnace is preferably 10 to 100 seconds, more preferably 20 to 80 seconds, and even more preferably 40 to 60 seconds.

When firing, it is preferable to fire the conductive pastes for forming the light-incident surface electrode 20 and the back surface electrode 15 at the same time, forming both electrodes at the same time. In this way, by printing a specific conductive paste on the light-incident surface and back surface and firing them at the same time, it is possible to fire the electrodes only once. This allows crystalline silicon solar cells to be manufactured at a lower cost.

The method for manufacturing a solar cell of this embodiment includes carrying out the laser treatment process described above. That is, the method for manufacturing a solar cell of this embodiment includes irradiating the light incident surface of the solar cell with light (e.g., laser light) from a point light source while applying a voltage between the back electrode 15 and the light incident surface electrode 20 so that a current flows in the opposite direction to the forward direction between the second conductivity type semiconductor layer (p-type impurity diffusion layer 4) and the first conductivity type semiconductor substrate (n-type crystalline silicon substrate 1). The laser treatment process enables good electrical conduction between the light incident surface electrode 20 and the impurity diffusion layer 4.

In the manner described above, the crystalline silicon solar cell of this embodiment can be manufactured.

The crystalline silicon solar cell of this embodiment obtained as described above can be electrically connected by a metal ribbon for interconnection, and laminated with a glass plate, a sealing material, a protective sheet, etc. to obtain a solar cell module. As the metal ribbon for interconnection, a metal ribbon (e.g., a ribbon made of copper) covered with solder can be used. As the solder, a solder that is available on the market, such as one that contains tin as a main component, specifically a lead-containing leaded solder or a lead-free solder, can be used. To obtain a lead-free solar cell, it is preferable to use a lead-free solder as the solder.

In the crystalline silicon solar cell of this embodiment, a high-performance crystalline silicon solar cell can be obtained by forming the required electrodes of the solar cell using the conductive paste of this embodiment and performing a laser treatment process.

The conductive paste of this embodiment contains lead-free glass frit. Therefore, the electrode formed on the surface of the solar cell is also a lead-free electrode. Therefore, when a solar cell manufactured using the conductive paste of this embodiment is disposed of, lead pollution of the environment can be prevented. In other words, by using the conductive paste of this embodiment, a lead-free solar cell can be manufactured.

<Depth d of the region of AgSi alloy 30>
In this specification, the depth d of the region of the AgSi alloy 30 (sometimes simply referred to as "AgSi alloy 30") refers to the length of the line segment that is the maximum length (the length d of the line segment that connects B1 and B2 in FIG. 9) among the line segments that connect an arbitrary point (B1 in FIG. 9) at the interface between the electrode and the AgSi alloy 30 to an arbitrary point (B2 in FIG. 9) at the interface between the substrate and the AgSi alloy 30 in the SEM photograph obtained by SEM observation of the cross section of the AgSi alloy 30 as shown in FIG. 9. Specifically, the depth d of the AgSi alloy 30 can be obtained by superimposing the region of the AgSi alloy 30 determined in the EDX measurement on the SEM photograph obtained by SEM observation of the cross section near the passivation film 2 at a magnification of 20,000 times, determining the above-mentioned predetermined line segment, and measuring the length of the predetermined line segment.

The depth d of the AgSi alloy 30 is preferably 100 to 4000 nm, more preferably 120 to 3000 nm, even more preferably 130 to 2500 nm, and particularly preferably 150 to 2000 nm. By setting the depth d of the AgSi alloy 30 within this range, the contact resistance is reduced, and a highly efficient crystalline silicon solar cell with a high fill factor (FF) can be obtained.

<Residual rate of passivation film 2>
In this specification, the extent to which the passivation film 2 (anti-reflection film 2) exists between the electrode and the impurity diffusion layer 4 of the crystalline silicon substrate 1 after firing to form the electrode of the solar cell of this embodiment can be indicated as the remaining rate of the passivation film 2. Note that the passivation film 2 disappears in the portion where the AgSi alloy 30 is formed. Since the AgSi alloy 30 is not formed in the portion where the passivation film 2 exists, the remaining rate of the passivation film 2 is considered to be the proportion of the area in the vicinity of the AgSi alloy 30 where the AgSi alloy 30 is not formed.

The method for measuring the residual rate of the passivation film 2 will be described using an example of an SEM photograph of a cross section of a solar cell shown in Figure 10. First, in order to obtain the residual rate of the passivation film 2, a SEM photograph is obtained by observing the cross section including the passivation film 2 and the AgSi alloy 30 with an SEM at a magnification of 20,000 times. The horizontal length (horizontal to the substrate surface) of this SEM photograph is 5.7 μm, and the vertical length (perpendicular to the substrate surface) is 3.9 μm. Next, the total length Lp of the cross section of the passivation film 2 in this SEM photograph is measured. In the example shown in Figure 10, the total length Lp of the cross section of the passivation film 2 in the SEM photograph is the total length of Lp1, Lp2, Lp3, and Lp4. Next, the total length Le of the cross section of the interface between the AgSi alloy 30 and the electrode in the part where the AgSi alloy 30 is generated in this SEM photograph is measured. The length Le corresponds to the length of the passivation film 2 that has disappeared during the manufacturing process of the solar cell. In the example shown in FIG. 10, the total length Le of the cross section of the interface between the AgSi alloy 30 and the electrode in the portion where the AgSi alloy 30 is formed is the total length of Le1 and Le2. The remaining rate of the passivation film 2 can be obtained as Lp/(Lp+Le). The portion where the passivation film 2 has disappeared during the manufacturing process of the solar cell can be identified by measurement using EDX. The length of Le1, etc. can be measured by approximating the passivation film 2, etc. as a straight line.

In the crystalline silicon solar cell of this embodiment, the remaining rate of the passivation film 2 is 10-90%, preferably 30% or more and less than 90%, more preferably 50% or more and less than 90%, and even more preferably 70% to 89%. By keeping the remaining rate of the passivation film 2 within an appropriate range, a highly efficient crystalline silicon solar cell with a high open circuit voltage (Voc) and fill factor (FF) can be obtained.

<Film thickness ratio before and after firing of passivation film 2>
In this specification, the film thickness ratio before and after firing of the passivation film 2 is the ratio (Db/Da) of the film thickness Da before firing for electrode formation of the passivation film 2 to the film thickness Db after firing for electrode formation (after the solar cell is completed). In this specification, the film thickness ratio before and after firing may be simply referred to as the "film thickness ratio (Db/Da)."

In the solar cell of this embodiment, the film thickness ratio (Db/Da) is preferably 15% to 85%, more preferably 20% to 70%, and even more preferably 30% to 60%. By keeping the film thickness ratio (Db/Da) of the passivation film 2 within a specified range, it is possible to prevent an increase in the surface defect density that causes carrier recombination when the solar cell of this embodiment generates electricity.

In this specification, the pre-firing thickness Da of the passivation film 2 refers to the thickness of the passivation film 2 when the passivation film 2 is formed on a specified substrate. The thickness Da immediately after film formation can be measured by SEM observation of the cross section near the passivation film 2 before forming the electrodes.

In this specification, the film thickness Db after the solar cell is completed is the film thickness of the passivation film 2 in a scanning electron microscope photograph of a 5.7 μm × 3.9 μm cross section including the AgSi alloy 30 of a solar cell completed by forming electrodes on the surface of the solar cell by firing.

In this specification, the film thickness Db of the passivation film 2 in a scanning electron microscope photograph of a 5.7 μm×3.9 μm cross section including AgSi alloy 30 of a solar cell completed with electrodes formed on the surface of the solar cell refers to the film thickness of the passivation film 2 near the AgSi alloy 30 of a solar cell completed by forming an electrode pattern using a specified conductive paste on the passivation film 2 formed on a specified substrate, and forming the electrodes and AgSi alloy 30 by performing a specified treatment such as a specified firing. The film thickness Db is sometimes referred to as the "film thickness Db after the solar cell is completed." The film thickness Db after the solar cell is completed can be measured by SEM observation of an image range of 5.7 μm×3.9 μm of a cross section including the passivation film 2 and AgSi alloy 30 of a solar cell completed with the electrodes and AgSi alloy 30 formed. That is, the thickness Db of the solar cell after completion is the thickness Db of the passivation film 2 in a scanning electron microscope photograph of a 5.7 μm × 3.9 μm cross section of the completed solar cell including the AgSi alloy 30. Specifically, the thickness Db of the passivation film 2 after the solar cell is completed can be obtained by observing the cross section including the passivation film 2 and the AgSi alloy 30 with an SEM at a magnification of 20,000 times to obtain an SEM photograph (SEM image range: 5.7 μm × 3.9 μm), dividing the SEM photograph vertically into six equal parts, measuring the thickness (five places) of the passivation film 2 at the five boundaries of the six equal parts, and obtaining the average thickness of the five places.

<Conductive paste for rear electrode>
A conductive paste (conductive paste for rear electrode) that can be used to form the rear electrode of the solar cell of this embodiment will be described.

In this specification, the term "rear electrode" refers to the surface opposite to the surface on which an electrode is formed using the conductive paste of this embodiment described above. In this specification, the conductive paste for forming the rear electrode is specifically referred to as the "conductive paste for rear electrode." Note that the conductive paste for rear electrode is a lead-free conductive paste, similar to the conductive paste of this embodiment described above.

Note that, as an example of a bifacial solar cell as shown in FIG. 4, there is a bifacial solar cell using an n-type Si substrate. In the case of a bifacial solar cell using an n-type Si substrate, the front electrode (light incident side front electrode 20) is an electrode on the front side on which a p-type diffusion layer is formed, and the back electrode 15 formed using a conductive paste for the back electrode is an electrode on the front side on which an n-type diffusion layer is formed. The conductive paste for the back electrode may also be used to form the front electrode (light incident side front electrode 20) of a bifacial solar cell using a p-type Si substrate. In this case, the front electrode (light incident side front electrode 20) is an electrode on the front side on which an n-type diffusion layer is formed, and the back electrode 15 is an electrode on the front side on which a p-type diffusion layer is formed (an electrode formed using the conductive paste of the above-mentioned embodiment).

The conductive paste for the back electrode can be preferably used to form the back electrode 15 (back finger electrode 15c) of the bifacial crystalline silicon solar cell shown in FIG. 4. The conductive paste for the back electrode can also be preferably used to form the back electrode 15 of the crystalline silicon solar cell shown in FIG. 1. In the crystalline silicon solar cell of this embodiment, the light incident surface electrode 20 formed using the conductive paste of this embodiment described above is disposed on the surface opposite to the back electrode formed using the conductive paste for the back electrode.

The conductive paste for the back electrode is described below. The conductive paste for the back electrode contains (A2) conductive particles, (B2) an organic vehicle, and (C2) glass frit, which are described below.

<(A2) Conductive particles>
The conductive paste for the back electrode contains (A2) conductive particles. The (A2) conductive particles contained in the conductive paste for the back electrode are the (A) The conductive particles may be the same as the conductive particles. In this specification, the conductive particles contained in the conductive paste for the back electrode may be referred to as "second conductive particles".

<(B2) Organic Vehicle>
The conductive paste for the back electrode contains an organic vehicle (B2). As the organic vehicle (B2), an organic vehicle similar to the organic vehicle (B) described above can be used. In this specification, the organic vehicle contained in the conductive paste for the back electrode may be referred to as a "second organic vehicle."

<(C2) Glass Frit>
The conductive paste for the back electrode contains (C2) glass frit. The (C2) glass frit contained in the conductive paste for the back electrode preferably contains Te. By containing Te, the (C2) glass frit is a lead-free glass frit that does not contain lead (Pb), but can adjust the reactivity to the passivation film to an appropriate range, reduce the contact resistance between the electrode and the impurity diffusion layer 4 (or the second impurity diffusion layer 16) of the crystalline silicon substrate 1, and prevent lead contamination of the environment. Note that, from the viewpoint of lowering the contact resistance, the glass frit containing Te is preferably contained in the conductive paste used for the electrode on the surface on which the n-type diffusion layer is formed.

In this specification, the glass frit contained in the conductive paste for the back electrode may be referred to as the "second glass frit."

The (C2) glass frit contained in the conductive paste for the back electrode is a lead-free glass frit. Therefore, the (C2) glass frit contained in the conductive paste for the back electrode does not substantially contain lead (Pb). However, the (C2) glass frit used in the conductive paste for the back electrode may contain a small amount of lead that is inevitably mixed in as an impurity. Specifically, the (C2) glass frit used in the conductive paste for the back electrode may contain 0.1% by weight or less of lead as an impurity per 100% by weight of the (C2) glass frit.

The conductive paste for the back electrode has a product BGF _·G of the basicity BGF of the (C2) glass frit and the content G of the (C2) glass frit in the conductive paste for the back electrode in parts by weight when the content of the (A2) conductive particles in the conductive paste for the back electrode is taken as 100 parts by weight, and the product _BGF ·G is preferably in the range of 1 to 3, more preferably in the range of 1.2 to 2.5, and even more preferably in the range of 1.5 to 2.3. By setting the product _BGF ·G of the basicity _BGF of the (C2) glass frit and the content G in an appropriate range, a crystalline silicon solar cell with appropriate performance can be obtained in combination with the light incident side surface electrode 20 formed using the conductive paste of this embodiment.

The basicity ( _BGF ) of the (C2) glass frit of this embodiment is preferably 0.10 to 1.5, more preferably 0.15 to 1.3, and even more preferably 0.20 to 1.1. When the basicity ( _BGF ) is within such a range, the reactivity of the (C2) glass frit with respect to the passivation film can be made appropriate by adjusting the amount of the (C2) glass frit added in the conductive paste for the back electrode.

The content G2 of the glass frit (C2) in the conductive paste for the back electrode is preferably 0.1 to 5.0 parts by weight, more preferably 0.5 to 4.0 parts by weight, even more preferably 0.3 to 3.5 parts by weight, and particularly preferably 1.0 to 3.0 parts by weight, relative to 100 parts by weight of the conductive particles (A2). By appropriately adjusting the content G2 of the glass frit (C2) in the conductive paste for the back electrode together with the basicity (B _GF ), the reactivity of the glass frit (C2) with respect to the passivation film can be made appropriate.

The (C2) glass frit contained in the conductive paste for the back electrode preferably contains at least one selected from SiO ₂ , B ₂ O ₃ , Bi ₂ O ₃ , P ₂ O ₅ , Li ₂ O, Na ₂ O, Al ₂ O ₃ , TeO ₂ , TiO ₂ , ZrO ₂ and ZnO. By containing at least one of these oxides in the (C2) glass frit, the basicity of the (C2) glass frit can be adjusted to an appropriate range.

The (C2) glass frit preferably contains _TeO2 . When the (C2) glass frit contains _TeO2 , the content of _TeO2 in the (C2) glass frit (100 mol%) is preferably less than 80 mol%, and more preferably 60 mol% or less. The content of _TeO2 in the (C2) glass frit (100 mol%) is preferably 30 mol% or more, and more preferably 40 mol% or more. By containing _TeO2 in the (C2) glass frit, the reactivity to the passivation film can be adjusted to an appropriate range while being lead-free, and the contact resistance can be reduced.

In the conductive paste for the back electrode, the product C _TeO2 ·G2 of the content (C _TeO2 ) of _TeO2 in the (C2) glass frit in mol % and the content G2 of the (C2) glass frit is preferably in the range of 10 to 200, more preferably in the range of 50 to 170, and even more preferably in the range of 80 to 150. By having the product C _TeO2 ·G2 of the content G2 of the (C2) glass frit in the above range, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while being lead-free.

The (C2) glass frit preferably contains Bi ₂ O ₃ in a range that does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains Bi ₂ O ₃ , the content of Bi ₂ O ₃ in the (C2) glass frit (100 mol%) is preferably 10 to 80 mol%, more preferably 15 to 75 mol%, and even more preferably 20 to 70 mol%. By containing Bi ₂ O ₃ in the (C2) glass frit, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while being lead-free.

The (C2) glass frit may contain SiO ₂ to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains SiO ₂ , the content of SiO _{2 in the (C2) glass frit (100 mol%) is preferably 10 to 60 mol%, and more preferably 15 to 40 mol%. By containing an appropriate content of SiO 2 in} the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain B ₂ O ₃ to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains B ₂ O ₃ , the content of B ₂ O ₃ in the (C2) glass frit (100 mol%) is preferably 3 to 60 mol%, and more preferably 4 to 50 mol%. By containing an appropriate content of B ₂ O ₃ in the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain P ₂ O ₅ to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains P ₂ O ₅ , the content of P ₂ O ₅ in the (C2) glass frit (100 mol%) is preferably 1 to 10 mol%, and more preferably 2 to 5 mol%. By containing an appropriate content of P ₂ O ₅ in the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain Li ₂ O to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains Li ₂ O, the content of Li ₂ O in the (C2) glass frit (100 mol%) is preferably 3 to 40 mol%, and more preferably 5 to 30 mol%. By containing an appropriate content of Li ₂ O in the (C2) glass frit, the reactivity with the passivation film can be adjusted to an appropriate range.

The (C2) glass frit may contain Na ₂ O ₃ to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains Na ₂ O ₃ , the content of Na ₂ O ₃ in the (C2) glass frit (100 mol%) is preferably 5 to 15 mol%, and more preferably 7 to 13 mol%. By containing an appropriate content of Na ₂ O ₃ in the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain Al ₂ O ₃ to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains Al ₂ O ₃ , the content of Al ₂ O ₃ in the (C2) glass frit (100 mol%) is preferably 1 to 10 mol%, and more preferably 3 to 8 mol%. By containing an appropriate content of Al ₂ O ₃ in the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain _TiO2 to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains _TiO2 , the content of _TiO2 in the (C2) glass frit (100 mol%) is preferably 0.5 to 8 mol%, and more preferably 1 to 4 mol%. By containing an appropriate content of _TiO2 in the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain _ZrO2 to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains _ZrO2 , the content of _ZrO2 in the (C2) glass frit (100 mol%) is preferably 0.5 to 8 mol%, and more preferably 1 to 4 mol%. By containing an appropriate content of _ZrO2 in the (C2) glass frit, the reactivity with the passivation film can be controlled.

The (C2) glass frit may contain ZnO to the extent that it does not adversely affect the conductive paste for the back electrode. When the (C2) glass frit contains ZnO, the content of ZnO in the (C2) glass frit (100 mol%) is preferably 2 to 20 mol%, and more preferably 5 to 15 mol%. By including ZnO in the (C2) glass frit, the basicity of the (C2) glass frit can be adjusted to an appropriate range.

The (C2) glass frit preferably contains Li ₂ O, TeO ₂ , and ZnO. The (C2) glass frit preferably contains SiO ₂ , B ₂ O ₃ , Bi ₂ O ₃ , P ₂ O ₅ , Na ₂ O, Al ₂ O ₃ , TiO _2, and ZrO _2. When the (C2) glass frit in the conductive paste for the back electrode contains a predetermined component, a solar cell with high performance can be obtained.

The conductive paste for the back electrode preferably has a glass transition point (Tg) of 250 to 600°C, more preferably 270 to 500°C, and even more preferably 300 to 470°C. By making the glass transition point (Tg) of the (C2) glass frit 250°C or higher, it is possible to suppress reactivity with the passivation film. Furthermore, by making the glass transition point (Tg) 600°C or lower, it is possible to reduce the contact resistance between the resulting electrode (e.g., the light-incident surface electrode 20) and the second impurity diffusion layer 16.

(C2) The shape of the glass frit particles is not particularly limited, and for example, spherical or amorphous shapes can be used. The particle size is also not particularly limited. From the viewpoint of workability, etc., the average particle size (D50) of the particles is preferably in the range of 0.1 to 10 μm, and more preferably in the range of 0.5 to 5 μm.

(C2) The glass frit particles can be one type of particle containing a predetermined amount of each of the required oxides. Also, particles made of a single oxide can be used as different particles for each of the required oxides. Also, multiple types of particles with different compositions of the required oxides can be used in combination. In order to obtain the synergistic effects of different types of oxides, it is preferable that the glass frit particles (C2) are one type of particle containing a predetermined amount of each of the required oxides.

<Other ingredients>
The conductive paste for a back electrode of this embodiment can contain additives and additives other than those described above, similar to the conductive paste described above, to the extent that they do not adversely affect the solar cell characteristics of the obtained solar cell.

<Method of manufacturing conductive paste for rear electrode>
The conductive paste for the back electrode can be produced by the same method as the conductive paste of this embodiment described above.

<Lead-free solar cells>
The lead-free crystalline silicon solar cell of this embodiment has a lead-free electrode and a local conductive portion. The lead-free electrode is an electrode formed using a lead-free conductive paste. In general, the portions of a crystalline silicon solar cell other than the electrodes can be formed using materials that do not contain lead. Therefore, in this specification, a crystalline silicon solar cell having a lead-free electrode is referred to as a lead-free crystalline silicon solar cell.

Specifically, the lead-free crystalline silicon solar cell of this embodiment is a solar cell including a crystalline silicon substrate of a first conductivity type, a silicon emitter layer of a second conductivity type disposed on one surface of the crystalline silicon substrate of the first conductivity type, a back electrode disposed so as to be electrically connected to the other surface of the crystalline silicon substrate of the first conductivity type, a passivation film disposed in contact with the surface of the silicon emitter layer of the second conductivity type, and a light-incident surface electrode containing silver disposed on at least a portion of the surface of the passivation film. The silicon emitter layer of the second conductivity type has a local conductive portion that is in direct contact with the light-incident surface electrode without a passivation film. The local conductive portion contains an alloy of silver and silicon. The light-incident surface electrode is a sintered body of the conductive paste of this embodiment described above. It is preferable that the back electrode is a sintered body of the conductive paste for the back electrode described above. The crystalline silicon substrate, silicon emitter layer, passivation film, and local conductive portion of the lead-free solar cell are the same as those of the solar cell of this embodiment described above.

The light incident surface electrode and back electrode of a lead-free solar cell are lead-free electrodes that do not contain lead. Therefore, the light incident surface electrode and back electrode are electrodes formed using a lead-free conductive paste.

In the lead-free crystalline silicon solar cell of this embodiment, a specific electrode of the solar cell is formed using the conductive paste of this embodiment described above, and a laser processing process is performed to form a local conductive portion (AgSi alloy). As the lead-free crystalline silicon solar cell of this embodiment has a local conductive portion (AgSi alloy), a high-performance crystalline silicon solar cell can be obtained.

The conductive paste of this embodiment and the conductive paste for the back electrode described above contain lead-free glass frit. Therefore, the electrode formed on the surface of the solar cell is also a lead-free electrode. Therefore, when a solar cell manufactured using the conductive paste of this embodiment is disposed of, lead pollution of the environment can be prevented.

The present embodiment will be explained in detail below using examples, but the present invention is not limited to these.

<Examples 1 to 8 and Comparative Examples 1 and 2>
In Examples 1 to 8 and Comparative Examples 1 and 2, single crystal silicon solar cells were fabricated and the electrical characteristics of the single crystal silicon solar cells were measured to evaluate the performance of the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2 of this embodiment.

<<Conductive paste materials and preparation ratio>>
Table 1 shows the compositions of the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. The compositions shown in Table 1 and the compositions of each component below are shown in parts by weight of each component when the (A) conductive particles are taken as 100 parts by weight. The components contained in the conductive paste are as follows.

(A) Silver Particles Table 2 shows the product number, manufacturer, shape, average particle size (D50), TAP density, and BET specific surface area of silver particles A1 and A2 used in the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. Table 1 shows the blending amounts of silver particles A1 and A2 in the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. The average particle size (D50) was determined by measuring the particle size distribution using the microtrack method (laser diffraction scattering method) and obtaining the median diameter (D50) from the results of the particle size distribution measurement. The same applies to the average particle sizes (D50) of the other components. The BET specific surface area was measured using a fully automatic specific surface area measuring device Macsoeb (manufactured by MOUNTEC). The BET specific surface area was measured by the BET one-point method using nitrogen gas adsorption after preliminary drying at 100°C and flowing nitrogen gas for 10 minutes.

(B) Organic Vehicle An organic binder and a solvent were used as the organic vehicle. Ethyl cellulose (0.4 parts by weight) with an ethoxy content of 48 to 49.5% by weight was used as the organic binder. Diethylene glycol monobutyl ether acetate (butyl carbitol acetate) (3 parts by weight) was used as the solvent.

(C) Glass Frit Table 3 shows the composition, basicity and glass transition point of glass frits GF1 to GF6 used in the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. The average particle size (D50) of glass frits GF1 to GF6 was set to 2 μm. Table 1 shows the type and content G (parts by weight) of glass frit (C) in the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. Glass frits GF1 to GF6 are lead-free glass frits.

The glass transition points of glass frits GF1 to GF6 were measured. Table 3 shows the measured glass transition points of glass frits A to G. The glass transition points of the glass frits were measured as follows. That is, about 50 mg of glass frits A to G were placed in a platinum cell as samples, and alumina powder was used as a standard sample. A DTA curve was obtained in an air atmosphere using a differential thermal analyzer (TG-8120, manufactured by Rigaku Corporation) at a heating rate of 20°C/min from room temperature to 800°C. The starting point (extrapolated point) of the first endotherm in the DTA curve was determined as the glass transition point. Note that the starting point of the first endotherm in the DTA curve of glass frit GF4 could not be clearly identified. Therefore, the "glass transition point" column for glass frit GF4 in Table 3 is marked "unclear."

Glass frits GF1 to GF6 were manufactured as follows. First, the oxide powders used as raw materials were weighed, mixed, and placed in a crucible. The crucible was placed in a heated oven, and the contents of the crucible were heated to the melting temperature, and maintained at the melting temperature until the raw materials were sufficiently melted. Next, the crucible was removed from the oven, and the molten contents were stirred uniformly. Next, the contents of the crucible were quenched at room temperature using two stainless steel rolls to obtain a plate-shaped glass. Finally, the plate-shaped glass was crushed in a mortar while being uniformly dispersed, and sieved through a mesh sieve to obtain glass frit with the desired particle size. By sieving through a 100-mesh sieve and remaining on a 200-mesh sieve, a glass frit with an average particle size (D50) of 149 μm was obtained. By further crushing this glass frit, a glass frit with an average particle size (D50) of 2 μm was obtained.

Next, the above-mentioned predetermined types and amounts of (A) conductive particles, (B) organic vehicle, and (C) glass frit were mixed in a planetary mixer, and then dispersed in a triple roll mill to form a paste, thereby producing the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2.

<<Manufacturing of monocrystalline silicon solar cells>>
A bifacial single crystal silicon solar cell was manufactured as shown in Fig. 4. A P (phosphorus) doped n-type single crystal silicon substrate (substrate thickness: 200 µm) was used as the substrate.

First, a silicon oxide layer of approximately 20 μm was formed on the substrate by dry oxidation, and then the substrate was etched with a mixed solution of hydrogen fluoride, pure water, and ammonium fluoride to remove damage to the substrate surface. In addition, heavy metals were cleaned with an aqueous solution containing hydrochloric acid and hydrogen peroxide.

Next, a texture (bumpy shape) was formed on both sides of the substrate by wet etching. Specifically, a pyramidal texture structure was formed on both sides (the main light-incident surface and the back surface) by wet etching (sodium hydroxide solution). The substrate was then washed with an aqueous solution containing hydrochloric acid and hydrogen peroxide.

Next, boron was injected into one of the textured surfaces (the light-incident surface) of the substrate to form a p-type diffusion layer to a depth of approximately 0.5 μm. The sheet resistance of the p-type diffusion layer was 60 Ω/□.

Furthermore, phosphorus was injected into the other surface (back surface) of the substrate having the textured structure to form an n-type diffusion layer to a depth of approximately 0.5 μm. The sheet resistance of the n-type diffusion layer was 20 Ω/□. Boron and phosphorus were simultaneously injected by thermal diffusion.

Next, a thin oxide film layer of 1 to 2 nm was formed on the surface (light incident surface) of the substrate on which the p-type diffusion layer was formed, and on the surface (rear surface) of the substrate on which the n-type diffusion layer was formed. After that, a silicon nitride film was formed to a thickness of about 60 nm by plasma CVD using silane gas and ammonia gas. Specifically, a silicon nitride film (anti-reflective film 2) with a film thickness of about 70 nm was formed by plasma CVD by glow discharge decomposition of a mixed gas of _NH3 / _SiH4 = 0.5 at 1 Torr (133 Pa).

The conductive paste used to form the electrodes on the surface (light incident surface) of the substrate on which the p-type diffusion layer was formed for the single crystal silicon solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 was that shown in Table 1.

The conductive paste was printed by screen printing. An electrode pattern consisting of a 1.5 mm wide light incident side busbar electrode 20a and a 60 μm wide light incident side finger electrode 20b was printed on the anti-reflection film 2 of the above-mentioned substrate so that the film thickness was approximately 20 μm, and then dried at 150°C for approximately 1 minute.

A commercially available Ag paste was printed by screen printing to form the back electrode 15 (the electrode on the surface on which the n-type diffusion layer is formed). The electrode pattern of the back electrode 15 has the same electrode pattern shape as the light-incident side surface electrode 20. It was then dried at 150°C for approximately 60 seconds. After drying, the conductive paste for the back electrode had a film thickness of approximately 20 μm. It was then fired simultaneously on both sides using a belt furnace (firing furnace) CDF7210 manufactured by Despatch Industries, Inc., with a peak temperature of 720°C and an in-out time of the furnace of 50 seconds. In this manner, a single crystal silicon solar cell was produced.

<<Measurement of the electrical characteristics of solar cells before the laser treatment process>>
The electrical characteristics of the single crystal silicon solar cell were measured as follows. That is, the current-voltage characteristics of the prototype solar cell were measured using a solar simulator SS-150XIL manufactured by Eiko Seiki Co., Ltd. under irradiation with solar simulator light (energy density 100 mW/cm ² ) at 25°C and AM1.5, and the fill factor (FF), open circuit voltage (Voc) and conversion efficiency (%) were calculated from the measurement results. Two single crystal silicon solar cells were produced under the same manufacturing conditions, and the measured values were calculated as the average of the two.

<<Laser treatment process>>
A laser treatment process was performed on the light incident surface of the single crystal silicon solar cells of the above-mentioned Examples 1 to 8 and Comparative Examples 1 and 2. That is, a laser light was irradiated onto the light incident surface of the solar cell while applying a negative voltage to the back electrode 15 and a positive voltage to each of the light incident surface electrodes 20 formed on the light incident surface in the pattern shown in Figure 2 so that a current flows in the opposite direction to the forward direction between the p-type impurity diffusion layer 4 of the solar cell and the n-type crystalline silicon substrate 1. The applied voltage during the laser treatment process was 20 V, the intensity of the irradiated laser light was 100 W/ ^cm2 , and the voltage application and laser light irradiation time were 2 seconds.

<<Measurement of electrical properties of solar cells after laser treatment process>>
The electrical properties of the solar cell after the laser treatment process were measured as well as the electrical properties of the solar cell before the laser treatment process.

As is clear from Table 1, the electrical properties of the solar cells produced using the conductive pastes of Examples 1 to 8 of this embodiment before the laser treatment process were low, for example the conversion efficiency was in the range of 0.5 to 1.6%. In contrast, the electrical properties of the solar cells produced using the conductive pastes of Comparative Examples 1 and 2 before the laser treatment process were similarly low, for example the conversion efficiency was in the range of 0.8 to 1.0.

As is clear from Table 1, the electrical characteristics of the solar cells produced using the conductive pastes of Examples 1 to 8 of this embodiment (wherein the product _BGF ·G of the basicity _BGF of the (C) glass frit and the content G of the (C) glass frit when the content of the (A) conductive particles is taken as 100 parts by weight) are in the range of 0.17 to 1.35) after the laser treatment process are much higher than the electrical characteristics before the laser treatment process. Specifically, the fill factor (FF) of the examples was in the range of 77.8 to 82.4%, the open circuit voltage (Voc) was in the range of 0.630 to 0.717 V, and the conversion efficiency was in the range of 21.3 to 24.2%. In contrast, the electrical characteristics of the solar cells produced using the conductive pastes of Comparative Examples 1 and 2 after the laser treatment process were lower than the electrical characteristics of Examples 1 to 8. Specifically, the fill factor (FF) of Comparative Example 1 (the product BGF _·G of the basicity BGF of the (C) glass _frit and the content G of the (C) glass frit when the content of the (A) conductive particles is 100 parts by weight is 0.11) was 31.9%, and the conversion efficiency was 6.4%. The open circuit voltage (Voc) of the solar cell of Comparative Example 1 was 0.700 V, but the fill factor (FF) was low, so the conversion efficiency was low. In addition, the fill factor (FF) of Comparative Example 2 (the product _BGF ·G of the basicity _BGF of the (C) glass frit and the content G of the (C) glass frit when the content of the (A) conductive particles is 100 parts by weight is 2.04) was in the range of 77.1%, the open circuit voltage (Voc) was in the range of 0.620 V, and the conversion efficiency was in the range of 19.7%. Therefore, the electrical characteristics of the solar cell of Comparative Example 2 were lower than the electrical characteristics of the solar cells of Examples 1 to 8. Therefore, it is clear that the solar cells fabricated using the conductive pastes of Examples 1 to 8 of this embodiment have superior electrical characteristics after the laser treatment process compared to the solar cells fabricated using the conductive pastes of Comparative Examples 1 and 2.

<Reference Examples 1 to 4>
In Examples 1 to 8 and Comparative Examples 1 and 2, single crystal silicon solar cells were fabricated using a conductive paste containing a lead-containing glass frit, and the electrical characteristics of the single crystal silicon solar cells were measured.

Table 4 shows the compositions of the conductive pastes of Reference Examples 1 to 4. The compositions shown in Table 4 and the compositions of each component below are shown as parts by weight of each component when (A) the conductive particles are taken as 100 parts by weight. The components contained in the conductive paste are as follows:

(A) Conductive Particles As the conductive particles, silver particles A1 shown in Table 2 were used. Table 4 shows the blending amount of the conductive particles.

(B) Organic Vehicle As the organic vehicle, the same organic binders and solvents as in Examples 1 to 8 and Comparative Examples 1 and 2 were used in the same amounts.

(C) Glass Frit Table 5 shows the composition, basicity and glass transition point of the glass frits GF11 and GF12 used in the conductive pastes of Reference Examples 1 to 4. All of the glass frits GF11 and GF12 contain PbO. The average particle size (D50) of the glass frits GF11 and GF12 was set to 2 μm. Table 4 shows the type and content G (parts by weight) of the glass frit (C) of the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. The glass transition points of the glass frits GF11 and GF12 were measured in the same manner as in the case of the above-mentioned glass frits GF1 to GF6. The glass frits GF11 and GF12 were manufactured in the same manner as in the case of the above-mentioned glass frits GF1 to GF6.

As shown in Table 4, aluminum (Al) particles were added as component (D) to the conductive pastes of Reference Examples 2 to 4. As the (D) Al particles, Al particles manufactured by Toyo Aluminum (product number: TFH-A02P, spherical, average particle size (D50): 2 μm) were used. Table 4 shows the amount (parts by weight) of the (D) Al particles in the conductive pastes of Reference Examples 2 to 4. Note that no (D) Al particles were added to the conductive paste of Reference Example 1.

Next, similar to the above-mentioned Examples 1 to 8 and Comparative Examples 1 and 2, predetermined types and amounts of (A) conductive particles, (B) organic vehicle, (C) glass frit, and (D) Al particles, if necessary, were mixed in a planetary mixer, and further dispersed in a triple roll mill to form a paste, thereby producing the conductive pastes of Reference Examples 1 to 4.

<Manufacturing of single crystal silicon solar cells>
Solar cells of Reference Examples 1 to 4 were manufactured in the same manner as the solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above, and the electrical characteristics of the solar cells were measured before and after the laser treatment process. Table 4 shows the measurement results of the electrical characteristics of Reference Examples 1 to 4.

As is clear from Tables 1 and 4, the electrical properties of the solar cells produced using the conductive pastes of Examples 1 to 8 of this embodiment after the laser treatment process were comparable to the electrical properties of the solar cells produced using the conductive pastes of Reference Examples 1 to 4 after the laser treatment process. Therefore, it has become clear that when a laser treatment process is performed on solar cells produced using the conductive pastes with lead-free glass frit of Examples 1 to 8 of this embodiment, solar cells with similar performance to those produced using conductive pastes with lead-containing glass frit can be manufactured.

<SEM photo>
The cross sections near the anti-reflection film 2 (passivation film) of the solar cells of Reference Example 1, Example 1, and Comparative Example 1, which had been subjected to a laser treatment process, were observed by a scanning electron microscope (SEM). Fig. 6 is a cross-sectional SEM photograph of a solar cell in which a light-incident side surface electrode 20 was formed using the conductive paste of Reference Example 1. Fig. 7 is a cross-sectional SEM photograph of a solar cell in which a light-incident side surface electrode 20 was formed using the conductive paste of Example 1. Fig. 8 is a cross-sectional SEM photograph of a solar cell in which a light-incident side surface electrode 20 was formed using the conductive paste of Comparative Example 1.

When the laser treatment process is used, the anti-reflection film 2 (passivation film) is present in most of the area between the light-incident surface electrode 20 and the impurity diffusion layer 4. In the laser treatment process, the above-mentioned predetermined voltage is applied so that a current flows in the opposite direction to the forward direction in the pn junction, and light (e.g., laser light) from a point light source is irradiated, so that a current flows in a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4, causing local heating. As a result, as shown in Figures 6 and 7, an AgSi alloy 30 (an alloy of silver and silicon) is formed as a local electrically conductive portion (local conductive portion) between the light-incident surface electrode 20 and the impurity diffusion layer 4. That is, the local conductive portion contains an alloy of silver and silicon. In addition, in the local conductive portion, the impurity diffusion layer 4 (silicon emitter layer of the second conductivity type) is directly in contact with the light-incident surface electrode 20 without the anti-reflection film 2 (passivation film). This locally formed electrically conductive portion (locally conductive portion) enables good electrical conduction between the light incident surface electrode 20 and the impurity diffusion layer 4. The conductive paste of this embodiment containing lead-free glass frit has low reactivity with the anti-reflection film 2, similar to the conductive paste containing a specific lead-containing glass frit, and has reactivity with the anti-reflection film 2 (passivation film) suitable for the laser treatment process. Therefore, the conductive paste of this embodiment can be preferably used to form the light incident surface electrode 20 of a crystalline silicon solar cell using a laser treatment process. In addition, since the conductive paste of this embodiment is a conductive paste containing lead-free glass frit, lead pollution of the environment can be prevented when the solar cell is discarded.

On the other hand, as shown in FIG. 8, in the case of a solar cell in which the light-incident surface electrode 20 was formed using the conductive paste of Comparative Example 1, after the laser treatment process was performed, the AgSi alloy 30 (an alloy of silver and silicon), which is a local electrically conductive portion (local conductive portion), was not formed between the light-incident surface electrode 20 and the impurity diffusion layer 4. Therefore, it was made clear that the conductive paste of Comparative Example 1 cannot be said to be a conductive paste suitable for forming an electrode by a laser treatment process.

Furthermore, the fill factor (FF) of the solar cell in which the light-incident surface electrode 20 was formed using the conductive paste of Comparative Example 2 before the laser treatment process was 62.4%. The fill factor (FF) of the solar cell in Comparative Example 2 before the laser treatment process was higher than the fill factor (FF) of the solar cell in the Example before the laser treatment process. This suggests that in the solar cell in which the light-incident surface electrode 20 was formed using the conductive paste of Comparative Example 2, the electrode pattern fired through the passivation film when the conductive paste (electrode pattern) was fired. As a result, in the solar cell using the conductive paste of Comparative Example 2, the passivation film disappeared, so the open circuit voltage (Voc) after the laser treatment process was low. Therefore, it was revealed that the conductive paste of Comparative Example 2 cannot be said to be a conductive paste suitable for forming electrodes by a laser treatment process.

<Examples 9 to 14 and Reference Example 5>
In Examples 9 to 14 and Reference Example 5, a bifacial single crystal silicon solar cell was fabricated, and the electrical characteristics of the single crystal silicon solar cell were measured to evaluate the performance of the conductive paste of Examples 9 to 14 and Reference Example 5 of this embodiment.

In Examples 9 to 14 and Reference Example 5, the electrodes of a bifacial solar cell using an n-type Si substrate are formed with the conductive paste of this embodiment. That is, the surface electrode (light incident surface electrode 20) in Examples 9 to 14 and Reference Example 5 is the surface electrode on which the p-type impurity diffusion layer 4 is formed, and the back electrode 15 is the surface electrode on which the n-type second impurity diffusion layer 16 is formed.

Table 6 shows the compositions of the conductive pastes for forming the front electrodes and the conductive pastes for forming the back electrodes in Examples 9 to 14 and Reference Example 5. The compositions shown in Table 6 and the compositions of each component below are shown as parts by weight of each component when the (A) conductive particles are taken as 100 parts by weight.

<<Conductive paste for forming surface electrodes>>
The conductive paste for forming the surface electrode in Examples 9 to 11 and Reference Example 5 is the same as the conductive paste for forming the surface electrode in Example 1. The conductive paste for forming the surface electrode in Example 12 is the same as the conductive paste for forming the surface electrode in Example 7. The conductive paste for forming the surface electrode in Example 13 is the same as the conductive paste for forming the surface electrode in Example 2. The conductive paste for forming the surface electrode in Example 14 is the same as the conductive paste for forming the surface electrode in Example 8. Note that, here, the electrode formed with the conductive paste for forming the surface electrode is referred to as "surface electrode 20".

<<Materials and preparation ratios of conductive paste for forming rear electrode>>
The conductive pastes for forming the rear electrodes in Examples 9 to 14 and Reference Example 5 are as follows. Note that, here, the electrode formed from the conductive paste for forming the rear electrode is referred to as "rear electrode 15".

(A) Silver Particles Table 6 shows the amounts of silver particles A1 and A2 blended in the conductive pastes for forming the back electrodes of Examples 9 to 14 and Reference Example 5. The silver particles A1 and A2 are the same as the silver particles A1 and A2 used in the conductive pastes for forming the light-incident side surface electrodes of Examples 1 to 8. Table 2 shows the part number, manufacturer, shape, average particle size (D50), TAP density, and BET specific surface area of the silver particles A1 and A2 used in the conductive pastes for forming the back electrodes of Examples 9 to 14 and Reference Example 5.

(B) Organic Vehicle The type and amount of the organic vehicle used in the conductive paste for forming the back electrode in Examples 9 to 14 and Reference Example 5 were the same as the type and amount of the organic vehicle (B) used in the conductive paste for forming the light-incident side front electrode in Examples 1 to 8 and Comparative Examples 1 and 2.

(C) Glass Frit Table 7 shows the composition, basicity and glass transition point of the glass frits GFA, GFB and GFC used in the conductive paste for forming the back electrode of Examples 9 to 14 and Reference Example 5. The average particle size (D50) of the glass frits GFA, GFB and GFC was set to 2 μm. Table 6 shows the type and content G2 (parts by weight) of the glass frit (C) in the conductive paste for forming the back electrode of Examples 9 to 14 and Reference Example 5. The glass frits GFA and GFB are lead-free glass frits. The glass frit GFC is a lead-containing glass frit.

The method for measuring the glass transition points of glass frits GFA, GFB, and GFC is the same as the method for measuring the glass transition points of glass frits A to G described above. Table 7 shows the measured glass transition points of glass frits GFA, GFB, and GFC.

The manufacturing methods for glass frits GFA, GFB, and GFC are the same as the manufacturing methods for glass frits A to G described above.

Next, the above-mentioned predetermined types and amounts of (A) conductive particles, (B) organic vehicle, and (C) glass frit were mixed in a planetary mixer, and then dispersed in a triple roll mill to form a paste, thereby producing the conductive pastes for forming the back electrodes of Examples 9 to 14 and Reference Example 5.

<<Manufacturing of monocrystalline silicon solar cells>>
As in Examples 1 to 8 and Comparative Examples 1 and 2, bifacial single crystal silicon solar cells as illustrated in Fig. 4 were manufactured as the solar cells of Examples 9 to 14 and Reference Example 5. However, for the conductive paste for forming the electrode on the front surface (light incident surface) of the substrate on which the p-type diffusion layer was formed and the conductive paste for forming the back electrode 15 (the electrode on the front surface on which the n-type diffusion layer was formed) of the single crystal silicon solar cells of Examples 9 to 14 and Reference Example 5, the conductive paste for forming the front surface electrode and the conductive paste for forming the back electrode shown in Table 6 above were used.

<<Measurement of the electrical characteristics of solar cells before the laser treatment process>>
The electrical characteristics of the solar cells of Examples 9 to 14 and Reference Example 5 before the laser treatment process were measured in the same manner as the solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above.

<<Laser treatment process>>
The laser treatment process of Examples 9 to 14 and Reference Example 5 was carried out in the same manner as the solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above.

<<Measurement of the electrical properties of solar cells after laser treatment process>>
The electrical characteristics of the solar cells after the laser treatment process of Examples 9-14 and Reference Example 5 were measured in the same manner as the solar cells of Examples 1-8 and Comparative Examples 1 and 2 described above.

<<Depth d of AgSi alloy 30>>
The depth d of the AgSi alloy 30 of the front electrode 20 in each of Examples 9 to 14 and Reference Example 5 was measured as follows. Table 6 shows the measurement results.

Figure 9 illustrates the depth d of the AgSi alloy 30 of the surface electrode 20. The depth d of the AgSi alloy 30 was measured as the maximum length of the line segment (length d of the line segment connecting B1 and B2 in Figure 9) from any point (B1 in Figure 9) at the interface between the electrode and the AgSi alloy 30 to any point (B2 in Figure 9) at the interface between the substrate and the AgSi alloy 30 in an SEM photograph obtained by SEM observation of the cross section of the AgSi alloy 30. The SEM photograph shown in Figure 9 was obtained by SEM observation of the cross section including the passivation film 2 and the AgSi alloy 30 of the completed solar cell at a magnification of 20,000 times.

<<Residual rate of passivation film 2>>
The remaining rate of the passivation film 2 near the surface electrode 20 in Examples 9 to 14 and Reference Example 5 was measured as follows. The measurement results of the remaining rate of the passivation film 2 near the surface electrode 20 are shown in the "Remaining rate" column of "Evaluation of surface electrode" in Table 6.

Specifically, first, a cross section of the completed solar cell including the passivation film 2 and the AgSi alloy 30 near the front electrode 20 was observed with an SEM at a magnification of 20,000 times to obtain an SEM photograph (SEM image area: 5.7 μm × 3.9 μm). The horizontal length (horizontal to the substrate surface) of this SEM photograph is 5.7 μm, and the vertical length (perpendicular to the substrate surface) is 3.9 μm. Next, as illustrated in FIG. 10, the total length Lp of the cross section of the passivation film 2 in this SEM photograph was measured. In the example shown in FIG. 10, the total length Lp of the cross section of the passivation film 2 in the SEM photograph is the total length of Lp1, Lp2, Lp3, and Lp4. Next, the total length Le of the cross section of the interface between the AgSi alloy 30 and the electrode in the part where the AgSi alloy 30 was generated in this SEM photograph was measured. The length Le corresponds to the length of the passivation film 2 that disappeared during the manufacturing process of the solar cell. In the example shown in FIG. 10, the total length Le of the cross section of the interface between the AgSi alloy 30 and the electrode in the portion where the passivation film 2 has disappeared is the total length of Le1 and Le2. The remaining rate of the passivation film 2 can be obtained as Lp/(Lp+Le). The lengths of Lp1, etc. were measured by approximating the passivation film 2, etc. as a straight line.

<Film thickness and film thickness ratio of passivation film 2 before and after firing>
The pre-firing film thickness Da of the passivation film 2 near the front electrode 20 and the film thickness Db after the solar cell was completed were measured for the solar cells of Examples 9 to 14 and Reference Example 5. The "Pre-firing film thickness (Da)" column of "Evaluation of front electrode" in Table 6 shows the measured pre-firing film thickness Da of the passivation film 2 near the front electrode 20. The "Post-firing/treatment film thickness (Db)" column of "Evaluation of front electrode" in Table 6 shows the measured film thickness Db of the passivation film 2 near the front electrode 20 after the solar cell was completed. The "Film thickness ratio (Db/Da)" column of "Evaluation of front electrode" in Table 6 shows the film thickness ratio (Db/Da) of the passivation film 2 before and after firing.

The pre-firing thickness Da of the passivation film 2 was measured by SEM observation of the cross section of the passivation film 2 near the surface electrode 20 immediately after deposition. That is, the passivation film 2 was formed on the surface of a specified crystalline silicon substrate 1 under the same conditions as in Examples 9 to 14 and Reference Example 5, and the cross section of the passivation film 2 was observed by SEM to obtain the pre-firing thickness Da of the passivation film 2 in Examples 9 to 14 and Reference Example 5.

The thickness Db of the completed solar cell is the passivation film 2 of the completed solar cell after electrodes are formed on the surface of the solar cell and a specified laser processing process is performed as necessary. Specifically, first, a SEM photograph (SEM image area: 5.7 μm × 3.9 μm) was obtained by observing the cross section of the completed solar cell including the passivation film 2 and AgSi alloy 30 with an SEM at a magnification of 20,000 times. Next, the SEM photograph was divided into six equal parts vertically, and the thickness (five locations) of the passivation film 2 was measured at five boundaries of the six equal parts. The thickness Db of the completed solar cell was taken as the average thickness of the five locations of the passivation film 2.

The film thickness ratio before and after firing of the passivation film 2 is the ratio (Db/Da) of the film thickness Da of the passivation film 2 before firing measured as described above to the film thickness Db after the solar cell is completed.

The results shown in Table 6 reveal that by forming the front electrode 20 using the conductive paste of this embodiment and forming the back electrode 15 using a specified conductive paste for forming the back electrode, a crystalline silicon solar cell with appropriate performance can be obtained.

A conductive paste containing lead-containing glass frit was used to form the back electrode 15 of the crystalline silicon solar cell of Reference Example 5. A conductive paste containing lead-free glass frit was used to form the back electrode 15 of the crystalline silicon solar cells of Examples 9 to 14. The crystalline silicon solar cells of Examples 9 to 14 have performance comparable to that of the crystalline silicon solar cell of Reference Example 5, and it has become clear that by forming the front electrode 20 using the conductive paste of this embodiment and forming the back electrode 15 using a specified conductive paste for forming the back electrode, a lead-free crystalline silicon solar cell with good performance can be manufactured.

In particular, by forming the front electrode 20 using the conductive paste of this embodiment, forming the back electrode 15 using glass frit containing Te as the conductive paste for forming the back electrode, and forming a local conductive portion using a laser treatment process, it has become clear that even when the electrodes on the two front surfaces of the solar cell are formed using a conductive paste made of a material that does not contain lead, it is possible to obtain an environmentally friendly bifacial crystalline silicon solar cell that has high solar cell characteristics comparable to those of a solar cell that contains lead-containing glass frit.

1 Crystalline silicon substrate 2 Anti-reflection film (passivation film)
4 impurity diffusion layer 15 rear electrode 15a rear TAB electrode (rear bus bar electrode)
15b Back electrode (full back electrode)
15c Back surface finger electrode 16 Second impurity diffusion layer 20 Light incident side surface electrode (surface electrode)
20a: light incident side bus bar electrode 20b: light incident side finger electrode 22: electrode pattern 30: AgSi alloy (local conductive portion)

Claims (18)

A conductive paste for forming an electrode of a solar cell,
(A) conductive particles;
(B) an organic vehicle; and
(C) a glass frit,
The (C) glass frit is substantially free of PbO,
A conductive paste, wherein a product BGF _{·G of a basicity BGF} of the (C) glass _frit and a content G of the (C) glass frit in the conductive paste in parts by weight when a content of the (A) conductive particles in the conductive paste is taken as 100 parts by weight, is in a range of 0.25 to 1.45. The conductive paste according to claim 1 , wherein the (C) glass frit contains Bi ₂ O ₃ . The conductive paste according to claim 1 or 2, wherein the product C _Bi2O3 ·G of the content (C _Bi2O3 ) of Bi ₂ O ₃ in the glass frit (C) in mol% and the content G of the glass frit (C) is in the range of 10 to 200. The conductive paste according to any one of claims 1 to 3, wherein the (A) conductive particles include silver particles. The conductive paste according to any one of claims 1 to 4, wherein the content G of the glass frit (C) is 0.1 to 5.0 parts by weight. The conductive paste according to any one of claims 1 to 5, wherein the content G of the glass frit (C) is 0.3 to 3.0 parts by weight. The conductive paste according to any one of claims 1 to 6, wherein the glass transition point of the glass frit (C) is 250 to 600°C. The conductive paste _according to any one of claims _{1 to} 7, wherein the glass frit (C) contains at least one selected from the group consisting of _SiO2 , _B2O3 , _V2O5 , _Bi2O3 , _TeO2 , BaO, CuO, _Li2O and ZnO. The conductive paste according to any one of claims 1 to 8, wherein (B) the organic vehicle contains at least one selected from ethyl cellulose, rosin ester, acrylic, and an organic solvent. A conductive paste for forming an electrode of a solar cell,
Solar cells,
a semiconductor substrate of a first conductivity type;
a semiconductor layer of a second conductivity type disposed on one surface of the semiconductor substrate of the first conductivity type;
a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate;
a passivation film disposed in contact with a surface of the second conductive type semiconductor layer;
a light-incident side surface electrode disposed on at least a portion of a surface of the passivation film;
the light-incident-side surface electrode is a surface electrode on the light-incident side of the solar cell that has been subjected to a process of irradiating light from a point light source onto the light-incident-side surface of the solar cell while applying a voltage between the back electrode and the light-incident-side surface electrode so that a current flows in a direction opposite to a forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type;
The conductive paste according to any one of claims 1 to 9, which is a conductive paste for forming the light-incident side surface electrode. a semiconductor substrate of a first conductivity type;
a semiconductor layer of a second conductivity type disposed on one surface of the semiconductor substrate of the first conductivity type;
a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate;
a passivation film disposed in contact with a surface of the second conductive type semiconductor layer;
a light-incident side surface electrode disposed on at least a portion of a surface of the passivation film,
the light-incident side surface electrode is a surface electrode on which light from a point light source is irradiated onto the light-incident side surface of the solar cell while applying a voltage between the back electrode and the light-incident side surface electrode so that a current flows between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type in a direction opposite to a forward direction;
A solar cell, wherein the light incident side surface electrode is a fired body of the conductive paste according to any one of claims 1 to 10. a first conductivity type crystalline silicon substrate;
a silicon emitter layer of a second conductivity type disposed on one surface of the crystalline silicon substrate of the first conductivity type;
a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type crystalline silicon substrate;
a passivation film disposed in contact with a surface of the second conductivity type silicon emitter layer;
a light-incident surface electrode including silver disposed on at least a portion of a surface of the passivation film,
the second conductive type silicon emitter layer has a local conductive portion that is in direct contact with the light incident side surface electrode without a passivation film therebetween,
the local conductive portion includes an alloy of silver and silicon;
A solar cell, wherein the light incident side surface electrode is a fired body of the conductive paste according to any one of claims 1 to 10. A method for manufacturing a solar cell, comprising the steps of:
Providing a semiconductor substrate of a first conductivity type;
forming a semiconductor layer of a second conductivity type on one surface of the semiconductor substrate of the first conductivity type;
forming a back surface electrode so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate;
forming a passivation film in contact with a surface of the second conductive type semiconductor layer;
forming a light incident side surface electrode on at least a part of a surface of the passivation film;
applying a voltage between the back electrode and the light-incident surface electrode so that a current flows in a direction opposite to a forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type; and irradiating the light from a point light source onto the light-incident surface of the solar cell.
The method for producing a solar cell, wherein the light incident side surface electrode is a fired body of the conductive paste according to any one of claims 1 to 10. 　Use of the conductive paste according to any one of claims 1 to 10 for forming an electrode for a solar cell. The back electrode is a fired body of a conductive paste for a back electrode,
The conductive paste for the back electrode is
Second conductive particles;
A second organic vehicle; and
a second glass frit;
the second glass frit is substantially free of PbO;
_13. The solar cell of claim ₁₁ or _{12, wherein the second glass frit comprises at least one selected from SiO2, B2O3, Bi2O3, P2O5 , Li2O , Na2O , Al2O3 , TeO2 , TiO2} , _ZrO2 , and ZnO. The solar cell of claim 15 , wherein the second glass frit comprises TeO ₂ . The back electrode is a fired body of a conductive paste for a back electrode,
The conductive paste for the back electrode is
Second conductive particles;
A second organic vehicle; and
a second glass frit;
the second glass frit is substantially free of PbO;
14. _{The method of claim 13, wherein the second glass frit comprises at least one selected from SiO2, B2O3, Bi2O3, P2O5 , Li2O , Na2O , Al2O3 , TeO2 , TiO2 , ZrO2} , _and ZnO. 20. The method of claim 17, wherein the second glass frit comprises _TeO2 .