Electrically conductive paste, use of electrically conductive paste, solar cell, and method for producing solar cell

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 (BGF·G) of the basicity BGF 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

[0001] The present invention relates to a conductive paste used for forming electrodes of semiconductor devices, etc. In particular, the present invention relates to a conductive paste for forming electrodes of solar cells. The present invention also relates to a solar cell manufactured using the conductive paste for forming electrodes, and a method for manufacturing a solar cell.

[0002] Semiconductor devices such as crystalline silicon solar cells, which use crystalline silicon substrates made by processing single-crystal silicon or polycrystalline silicon into flat plates, typically have electrodes formed on the silicon substrate surface using a conductive paste for electrode formation to establish electrical contact with the outside of the device. Among semiconductor devices with electrodes formed in this manner, 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 power generated by the crystalline silicon solar cell to be extracted to the outside.

[0003] Conventional electrodes for crystalline silicon solar cells are formed using a conductive paste containing conductive powder, glass frit, an organic binder, a solvent, and other additives. The conductive powder is mainly silver particles (silver powder).

[0004] The glass frit contained in the conductive paste generally contains lead (Pb). However, lead (Pb) may have a negative impact on the environment. Therefore, lead-free glass frit that does not contain lead has been proposed.

[0005] 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 a derivative thereof 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 a derivative thereof and (b) the lead-free glass frit.

[0006] Patent Literature 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 Literature 2 involves applying a predetermined voltage in a forward direction and a reverse direction to the silicon solar cell, and 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.

[0007] Patent Document 3 describes a conductive composition containing silver powder, glass powder containing PbO, and a vehicle made of an organic substance. Patent Document 3 also describes that the conductive composition is a conductive composition for forming an electrode that penetrates a silicon nitride layer and is conductive with an n-type semiconductor layer formed below the silicon nitride layer. Patent Document 3 also describes that the glass powder contained in the conductive composition has a basicity of 0.6 to 0.8 and a glass transition point of 300°C to 450°C.

[0008] Japanese Patent No. 5934411 Special Publication No. 2021-513218 Japanese Patent Laid-Open No. 2009-231826

[0009] FIG. 5 shows an example of a cross-sectional schematic diagram of a typical crystalline silicon solar cell. As shown in FIG. 5, a crystalline silicon solar cell generally includes an impurity diffusion layer 4 (e.g., a p-type impurity diffusion layer in which p-type impurities are diffused) 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 referred to as a passivation film. Furthermore, an electrode pattern for a 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, followed by drying and firing at a predetermined temperature to form the light-incident surface electrode 20. In typical crystalline silicon solar cells, 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. Note that fire-through refers to etching the anti-reflection film 2, which is an insulating film, with glass frit or the like contained in a conductive paste, thereby electrically connecting the light-incident surface electrode 20 and the impurity diffusion layer 4. In the example shown in FIG. 5 , the electrode pattern fires through the anti-reflection film 2 during firing, causing the anti-reflection film 2 to disappear. As a result, the light-incident surface electrode 20 and the impurity diffusion layer 4 are in contact. A p-n 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 entering 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. During this process, light is absorbed in the n-type crystalline silicon substrate 1, generating electron-hole pairs. These electron-hole pairs are separated by the electric field created by the p-n junction: electrons from the n-type crystalline silicon substrate 1 to the back electrode 15, and holes 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 a current via these electrodes.

[0010] FIG. 2 shows an example of a schematic diagram of the light-incident side surface of a crystalline silicon solar cell. As shown in FIG. 2, a busbar electrode (light-incident side busbar electrode 20a) and a light-incident side finger electrode 20b (sometimes simply referred to as "finger electrode 20b") are arranged on the light-incident side surface of the crystalline silicon solar cell as a light-incident side surface electrode 20. In the example shown in FIGS. 5 and 2, electrons of electron-hole pairs generated by incident light entering the crystalline silicon solar cell are collected by the finger electrode 20b and then further collected by the light-incident side busbar electrode 20a. A metal ribbon for interconnection, the periphery of which is covered with solder, is soldered to the light-incident side busbar electrode 20a. This metal ribbon extracts current to the outside.

[0011] In order to obtain a crystalline silicon solar cell with high conversion efficiency, the contact resistance between the light-incident side surface electrode 20 and the impurity diffusion layer 4 is required to be low.

[0012] 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 of 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. When products are manufactured using materials containing lead, there is a risk that the lead will pollute the environment when the products are disposed of. Therefore, it is desirable to use lead-free materials when manufacturing products. It is preferable to use lead-free glass frit in the solar cell manufacturing process.

[0013] Meanwhile, a solar cell manufacturing method 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 technique for obtaining low contact resistance by forming a light-incident surface electrode 20, applying a predetermined voltage so that a current flows in the reverse direction of the forward direction of the p-n 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, performing a laser treatment process can improve the fill factor (FF) of solar cell characteristics without reducing the open circuit voltage (Voc). In the laser treatment process, it is preferable that when the electrode pattern of the conductive paste is fired at a predetermined temperature, 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. Figure 1 shows an example of a cross-sectional schematic diagram illustrating a structure in which a light-incident surface electrode 20 is formed on the light-incident surface of a crystalline silicon solar cell 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-side surface electrode 20 and the impurity diffusion layer 4. In the laser treatment process, the above-mentioned predetermined voltage is applied to the pn junction so that a current flows in the reverse direction to the forward direction, 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-side surface electrode 20 and the impurity diffusion layer 4, resulting in localized heating. This localized heating creates a small area between the light-incident-side surface electrode 20 and the impurity diffusion layer 4 where the impurity diffusion layer 4 is not present. As a result, as shown in FIGS. 6 and 7 , it is believed that an AgSi alloy 30 (an alloy of silver and silicon) is locally formed as a small electrically conductive portion (local conductive portion) in the impurity diffusion layer 4 in contact with the light-incident-side surface electrode 20. Note that the AgSi alloy 30 is formed locally in a limited area, and is therefore not shown in FIG. 1 . 6 and 7 show the approximate position of the AgSi alloy 30, and do not precisely show the boundary of the AgSi alloy 30.It is believed that these locally formed minute electrically conductive portions enable good electrical conduction between the light-incident side surface electrode 20 and the impurity diffusion layer 4. Furthermore, 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, excluding the area where the local conductive portion is formed. As a result, the fill factor (FF) can be improved without reducing the open-circuit voltage (Voc) as a performance of the solar cell. Therefore, the conductive paste used to form the light-incident side surface electrode 20 by the laser treatment process needs to have properties different from conventional conductive pastes (conductive pastes that can fire through the anti-reflection film 2).

[0014] Furthermore, in the case of conventional crystalline silicon solar cells, when the light-incident side 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. This firing causes damage to the impurity diffusion layer 4, resulting in 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 during firing to form the light-incident side surface electrode 20. Therefore, by using the laser treatment process, damage to the impurity diffusion layer 4 can be suppressed.

[0015] Therefore, an object of the present invention is to provide a conductive paste suitable for forming electrodes by a laser treatment process for manufacturing crystalline silicon solar cells, the conductive paste containing lead-free glass frit.

[0016] Another object of the present invention is 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. Another object of the present invention is 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.

[0017] In order to solve the above problems, the present invention has the following configuration.

[0018] (Configuration 1) Configuration 1 is a conductive paste for forming an electrode of a solar cell, comprising: (A) conductive particles; (B) an organic vehicle; and (C) a glass frit, wherein the glass frit (C) is substantially free of PbO, and the basicity B of the glass frit (C) is ＧＦ and the product B of 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 taken as 100 parts by weight, ＧＦ A conductive paste having G in the range of 0.25 to 1.45.

[0019] (Configuration 2) In Configuration 2, the glass frit (C) is Bi ２ O ３ 2. The conductive paste of embodiment 1, comprising:

[0020] (Configuration 3) Configuration 3 is the (C) Bi in the glass frit in units of mol%. ２ O ３ The content of (C Ｂｉ２Ｏ３ ) and the content G of the glass frit (C), Ｂｉ２Ｏ３ The conductive paste of configuration 1 or 2, wherein G is in the range of 10 to 200.

[0021] (Configuration 4) Configuration 4 is the conductive paste of any one of Configurations 1 to 3, in which the (A) conductive particles include silver particles.

[0022] (Configuration 5) Configuration 5 is the conductive paste of any one of configurations 1 to 4, in which the content G of the glass frit (C) is 0.1 to 5.0 parts by weight.

[0023] (Configuration 6) Configuration 6 is the conductive paste of any one of configurations 1 to 5, in which the content G of the glass frit (C) is 0.3 to 3.0 parts by weight.

[0024] (Configuration 7) Configuration 7 is the conductive paste of any one of configurations 1 to 6, wherein the glass transition temperature of the glass frit (C) is 250 to 600°C.

[0025] (Configuration 8) In Configuration 8, the glass frit (C) is SiO ２ , B ２ O ３ , V ２ O ５ , Bi ２ O ３ , TeO2, BaO, CuO, Li ２ 8. The conductive paste of any one of configurations 1 to 7, further comprising at least one selected from O and ZnO.

[0026] (Configuration 9) 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.

[0027] (Configuration 10) Configuration 10 is a conductive paste for forming an electrode of a solar cell, wherein the solar cell 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 back electrode disposed so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type; a passivation film disposed in contact with the surface of the semiconductor layer of the second conductivity type; and a light-incident-side surface electrode disposed on at least a part of the surface of the passivation film, wherein the light-incident-side surface electrode is a light-incident-side surface electrode that has been treated by applying light from a point light source to 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 the forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type, and the conductive paste is a conductive paste for forming the light-incident-side surface electrode.

[0028] (Configuration 11) Configuration 11 is a solar cell including: 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 electrode disposed so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type; a passivation film disposed in contact with the surface of the semiconductor layer of the second conductivity type; and a light-incident-side surface electrode disposed on at least a part of the surface of the passivation film, wherein the light-incident-side surface electrode is the light-incident-side surface electrode that has been treated to irradiate 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 between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type in a direction opposite to the forward current; and the light-incident-side surface electrode is a fired body of the conductive paste of any of Configurations 1 to 10.

[0029] Configuration 12 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-side surface electrode containing silver disposed on at least a portion of the surface of the passivation film, wherein the silicon emitter layer of the second conductivity type 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 containing an alloy of silver and silicon, and the light-incident-side surface electrode is a fired body of the conductive paste of any of Configurations 1 to 10.

[0030] (Configuration 13) Configuration 13 is a method for manufacturing a solar cell, comprising: preparing 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 electrode so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type; forming a passivation film so as to be in contact with the surface of the semiconductor layer of the second conductivity type; forming a light-incident-side surface electrode on at least a part of the surface of the passivation film; and irradiating the light-incident-side surface of the solar cell with light from a point light source 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 the forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type, wherein the light-incident-side surface electrode is a fired body of the conductive paste of any of Configurations 1 to 10.

[0031] A fourteenth aspect of the present invention is the use of the conductive paste according to any one of claims 1 to 10 for forming an electrode of a solar cell.

[0032] In configuration 15, the back electrode is a fired body of a conductive paste for a back electrode, the conductive paste for a back electrode includes: second conductive particles; a second organic vehicle; and a second glass frit, the second glass frit being substantially free of PbO, and the second glass frit being 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 13. The solar cell according to claim 11, further comprising at least one selected from the group consisting of ZnO and ZnO.

[0033] Aspect 16 is the second glass frit, ２ 16. The solar cell of claim 15, comprising:

[0034] Aspect 17 is a method for manufacturing a back electrode, wherein the back electrode is a fired body of a conductive paste for a back electrode, the conductive paste for a back electrode includes: second conductive particles; a second organic vehicle; and a second glass frit, the second glass frit being substantially free of PbO, and the second glass frit being 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.

[0035] 18. The method of claim 18, wherein the second glass frit is TeO ２ 18. The method for producing a solar cell according to claim 17, comprising:

[0036] According to the present invention, it is possible to provide a conductive paste suitable for forming an electrode by a laser treatment process for the production of a crystalline silicon solar cell, the conductive paste including a lead-free glass frit.

[0037] Another object of the present invention is 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 provides a high-performance lead-free crystalline silicon solar cell manufactured by a manufacturing method that includes forming electrodes by a laser treatment process.

[0038] 1 is an example of a cross-sectional schematic diagram showing a structure in which a light-incident-side surface electrode is formed on the light-incident-side surface of a crystalline silicon solar cell by a laser treatment process using the conductive paste of this embodiment. FIG. 2 is an example of a cross-sectional schematic diagram of the light-incident-side surface of a crystalline silicon solar cell. FIG. 3 is an example of a cross-sectional schematic diagram of a bifacial crystalline silicon solar cell formed using the conductive paste of this embodiment. FIG. 4 is an example of a cross-sectional schematic diagram of the vicinity of the light-incident-side surface electrode (finger electrode) of a general crystalline silicon solar cell, showing a state in which the anti-reflection film (passivation film) between the electrode and the impurity diffusion layer has disappeared due to fire-through. FIG. 5 is a cross-sectional SEM (scanning electron microscope) photograph (magnification: 20,000 times) of the vicinity of the passivation film on the light-incident-side surface of a solar cell in which a light-incident-side surface electrode is formed using the conductive paste of Reference Example 1. FIG. 6 is a cross-sectional SEM photograph (magnification: 20,000 times) of the vicinity of the passivation film on the light-incident-side surface of a solar cell in which a light-incident-side surface electrode is formed using the conductive paste of Example 1. 1 is a cross-sectional SEM photograph (magnification: 20,000 times) of the vicinity of the passivation film on the light-incident surface of a solar cell in which a light-incident surface electrode is formed using the conductive paste of Comparative Example 1. 2 is a cross-sectional SEM (scanning electron microscope) photograph (magnification: 20,000 times) of the vicinity of the passivation film on the light-incident surface of a crystalline silicon solar cell of Reference Example 1, which is a diagram for explaining the depth d of the AgSi alloy region. 3 is a cross-sectional SEM (scanning electron microscope) photograph (magnification: 20,000 times) of the vicinity of the passivation film on the light-incident surface of a crystalline silicon solar cell of Reference Example 1, which is a diagram for explaining the passivation film residual ratio Lp / (Lp+Le).

[0039] Hereinafter, embodiments of the present invention will be described in detail. Note that the following embodiments are forms for realizing the present invention, and are not intended to limit the scope of the present invention.

[0040] In this specification, "crystalline silicon" includes both single crystal and polycrystalline silicon. Furthermore, "crystalline silicon substrate" refers to a material obtained by forming crystalline silicon into a shape suitable for element formation, such as a flat plate, for the formation of 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 heterogeneous substrates such as glass, can also be used as crystalline silicon substrates. Furthermore, "crystalline silicon solar cell" refers to a solar cell manufactured using a crystalline silicon substrate.

[0041] In this specification, the glass frit is made mainly of a plurality of types of oxides, for example, metal oxides, and is generally used in the form of glassy particles.

[0042] In this specification, lead-free glass frit refers to glass frit that is substantially free of lead (Pb). Since glass frit is manufactured using metal oxides as raw materials, lead-free glass frit refers to glass frit that is substantially free of lead oxide (PbO). When manufacturing lead-free glass frit, lead-containing materials (PbO) are not 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 the glass frit.

[0043] As shown in Fig. 1, finger electrodes 20b are arranged on the light-incident surface of a crystalline silicon solar cell as light-incident surface electrodes 20. In the example shown in Fig. 1, the holes of electron-hole pairs generated by incident light entering the crystalline silicon solar cell are collected in the finger electrodes 20b via the impurity diffusion layer 4 (e.g., the p-type impurity diffusion layer 4). Therefore, low contact resistance is required between the finger electrodes 20b and the impurity diffusion layer 4. The conductive paste of this embodiment can be preferably used to form the finger electrodes 20b.

[0044] In this specification, the light-incident side 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".

[0045] This embodiment is a conductive paste for forming an electrode of a solar cell. The conductive paste of this embodiment includes (A) conductive particles, (B) an organic vehicle, and (C) glass frit. The (C) glass frit included in the conductive paste of this embodiment is substantially free of PbO. That is, the (C) glass frit included in the conductive paste of this embodiment is 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.

[0046] The glass frit (C) contained in the conductive paste of this embodiment is substantially free of lead. Furthermore, materials other than the glass frit (C) contained in the conductive paste of this embodiment are also substantially free of 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.

[0047] The photoelectric conversion efficiency of a solar cell (sometimes simply referred to as "conversion efficiency") is expressed as the product of the fill factor (FF), open circuit voltage (Voc), and short circuit current (Jsc). Basically, FF and Voc have a trade-off relationship. Therefore, it is difficult to simultaneously increase both FF and Voc. Meanwhile, Patent Document 2 describes that employing a laser treatment process during the manufacture of a crystalline silicon solar cell can improve the ohmic contact behavior between the grid-shaped electrode serving as the light-incident surface electrode 20 and the impurity diffusion layer 4 (emitter layer). It also describes that the contact resistance between the light-incident surface electrode 20 and the impurity diffusion layer 4 can be significantly reduced. Therefore, by performing the laser treatment process, FF can be improved without reducing Voc.

[0048] The present inventors have found that applying a laser treatment process to a solar cell having a light-incident surface electrode 20 formed using a conventional conductive paste (e.g., the conductive paste described in Patent Document 3) 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 this is due to the conventional conductive paste having excessive fire-through (reactivity) with the anti-reflection film 2 (passivation film). Furthermore, the present inventors have found that by adjusting the basicity and content of the lead-free glass frit within an appropriate range, the reactivity of the glass frit with the anti-reflection film 2 (passivation film) can be made appropriate. Using a lead-free glass frit as the glass frit can prevent lead pollution due to lead emissions into the environment. 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 a level comparable to that of a lead-containing glass frit. Having obtained the above findings, the present inventors have discovered a conductive paste that can be preferably used in the production of crystalline silicon using a laser treatment process, and have arrived at the present invention.

[0049] By forming an electrode of a crystalline silicon solar cell using the conductive paste of this embodiment and performing a laser treatment process, low contact resistance can be obtained 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.

[0050] In the laser treatment process using the conductive paste of this embodiment, the antireflection film 2 (passivation film) is not essentially fired through when the light-incident side surface electrode 20 is formed. Furthermore, even when the laser treatment process is performed on the light-incident side surface electrode 20, most of the antireflection film 2 (passivation film) in contact with the light-incident side surface electrode 20 is not lost. That is, the antireflection film 2 (passivation film) is present over most of the area between the light-incident side surface electrode 20 and the impurity diffusion layer 4 (e.g., 90% or more of the interface area, preferably 95% or more, and more preferably 99% or more) except for the area where a small localized electrical conduction portion (local conduction portion) is formed. Therefore, by using the laser treatment process when forming the light-incident side surface electrode 20, damage to the impurity diffusion layer 4 can be suppressed.

[0051] 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.

[0052] The conductive paste of this embodiment can be preferably used to form a light-incident-side surface electrode 20 formed on the surface (light-incident surface) of an anti-reflection film 2 (passivation film) formed on an 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 described above, the conductive paste of this embodiment can be used to form electrical contact between the back surface electrode 15 and the crystalline silicon substrate 1 of the solar cell via the back surface passivation film.

[0053] The conductive paste of the present invention will be described below using an example in which it is used to form a light-incident 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 surface is a p-type impurity diffusion layer 4. In this specification, the impurity diffusion layer 4 in the case of a solar cell using a crystalline silicon substrate 1 may also 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.

[0054] 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 it can effectively passivate the surface of the silicon substrate. When the passivation film is a multiple layer, it is preferable that the passivation film is a laminated film (SiN / SiO ｘ It should be noted that SiN / SiO ｘ When the film is a passivation film, the surface of the silicon substrate can be passivated more effectively. ｘ The SiO ｘ A film was formed, and SiO ｘIt is preferable to form a SiN film on the film. ｘ The film can be a native oxide film of a silicon substrate.

[0055] The crystalline silicon solar cell can have a light-incident busbar electrode 20a and / or a backside TAB electrode 15a. The light-incident busbar electrode 20a functions to electrically connect the finger electrodes 20b for collecting current generated by the solar cell to the metal ribbon for interconnection. Similarly, the backside TAB electrode 15a functions to electrically connect the full-surface backside electrode 15b for collecting current generated by the solar cell to the metal ribbon for interconnection. If the finger electrodes 20b come into contact with the crystalline silicon substrate 1, the surface defect density on the surface (interface) of the crystalline silicon substrate 1 where the finger electrodes 20b come into contact increases, resulting in a decrease in solar cell performance. The conductive paste of the present invention, particularly as a conductive paste for the finger electrodes 20b, has low fire-through (reactivity) with respect to the anti-reflection film 2 and therefore does not completely fire-through the anti-reflection film 2. Therefore, when finger electrodes 20b are formed using the conductive paste of the present invention, the portion of the passivation film in contact with crystalline silicon substrate 1 can be maintained intact, preventing an increase in the surface defect density that causes carrier recombination. Therefore, the conductive paste of the present embodiment described above can be suitably used as a conductive paste for forming finger electrodes 20b of crystalline silicon solar cells. The conductive paste of the present embodiment can also be suitably used as a back electrode 15 (back finger electrode 15c) of a bifacial crystalline silicon solar cell, as shown in FIG. 4 . Furthermore, the entire electrode 20 can be formed using the conductive paste of the present embodiment.

[0056] 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 region between the light-incident side surface electrode 20 and the impurity diffusion layer 4 (silicon emitter layer), resulting in localized heating. As a result, as shown in FIG. 7 , an AgSi alloy 30, which is a localized electrically conductive portion (local conduction portion), is formed in the impurity diffusion layer 4 (silicon emitter layer) in contact with the light-incident side surface electrode 20. It is believed that this locally formed electrically conductive portion enables good electrical conduction between the light-incident side 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 side surface electrode 20 by the laser treatment process has properties different from conventional conductive pastes (conductive pastes that can fire through the anti-reflection coating 2).

[0057] The conductive paste of this embodiment will be specifically described.

[0058] <(A) Conductive Particles> The conductive paste of this embodiment contains (A) conductive particles.

[0059] In the conductive paste of this embodiment, metal particles or alloy particles can be used as the conductive particles. 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 also contain metals other than silver, such as gold, copper, nickel, zinc, and tin. From the viewpoint of achieving low electrical resistance and high reliability, the conductive particles are preferably silver particles made of silver. Silver particles made of silver can contain other metal elements as unavoidable impurities. A large number of silver particles (Ag particles) may also be referred to as silver powder (Ag powder). The same applies to other particles.

[0060] The particle shape and particle size (also referred to as particle diameter) of the conductive particles are not particularly limited. Examples of particle shapes that can be used include spherical and scale-like particles. The particle size of the conductive particles can be defined as the particle size (D50) of 50% of the total particle size. In this specification, D50 is also referred to as the average particle size. The average particle size (D50) can be determined from the results of particle size distribution measurement performed using a microtrack method (laser diffraction scattering method).

[0061] 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 ensuring that the average particle diameter (D50) of the conductive particles is within the 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.

[0062] The size of silver particles can also be expressed as a 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, more preferably 0.2 to 1.2 m ２ The BET specific surface area can be measured using, for example, a fully automatic specific surface area measuring device Macsoeb (manufactured by MOUNTEC).

[0063] <(B) Organic Vehicle> The conductive paste of this embodiment contains (B) an organic vehicle.

[0064] The organic vehicle may contain an organic binder and a solvent. The organic binder and the solvent play a role in adjusting the viscosity of the conductive paste, and are not particularly limited. The organic binder may be dissolved in a solvent before use.

[0065] In the conductive paste of this embodiment, the (B) organic vehicle preferably 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 suitably screen-printed, and the shape of the printed pattern can be made appropriate.

[0066] 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 typically 0.1 to 30 parts by weight, and preferably 0.2 to 5 parts by weight, per 100 parts by weight of silver particles.

[0067] The organic solvent can be one or more selected from alcohols (e.g., terpineol, α-terpineol, β-terpineol, etc.) and esters (e.g., hydroxy group-containing esters, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, and diethylene glycol monobutyl ether acetate (butyl carbitol acetate)). The amount of the solvent added is typically 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).

[0068] <(C) Glass Frit> The conductive paste of the present embodiment contains (C) glass frit.

[0069] 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 trace amount of lead that is inevitably mixed in as an impurity. Specifically, the glass frit used in this embodiment may contain 0.1 wt % or less of lead as an impurity relative to 100 wt % of the glass frit.

[0070] The conductive paste of this embodiment has (C) a basicity B of the glass frit ＧＦ and the product B of the content G of (C) glass frit in parts by weight in the conductive paste when the content of (A) conductive particles in the conductive paste is taken as 100 parts by weight. ＧＦ G 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. ＧＦ and the product B of the content G ＧＦ By setting G in an appropriate range, the reactivity of the glass frit with 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 treatment process.

[0071] The basicity of the glass frit can be calculated by the method described in Patent Document 3 (JP 2009-231826 A). That is, 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.

[0072] Oxide M ｉ O's M ｉ The bonding force between -O is given by the following equation as the attractive force Ai between a cation and an oxygen ion:

[0073] A ｉ =Z ｉ ・Z ０２－ / (r ｉ +r０２－ ) ２ = Zi 2 / (r ｉ +1.40) ２ Z ｉ : valence of cation, oxygen ion is 2r ｉ : Ionic radius of cation (Å)

[0074] Ionic radius r of oxygen ion ｉ is 1.40 nm. ｉ The reciprocal B of ｉ (= 1 / A ｉ ) as a single component oxide M ｉ The oxygen donating capacity of O is ｉ ≡1 / A ｉ

[0075] This B ｉ B ＣａＯ = 1, B ＳｉＯ２ = 0, the B of each single component oxide ｉ -index is given. B of each component ｉ - When the index is expanded to a multi-component system using the cation fraction, the basicity (= B ＧＦ ) can be calculated. ＧＦ =Σn ｉ ・B ｉ n ｉ : cation fraction

[0076] The basicity (B ＧＦ ) 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, "basicity" can be said to represent the degree of dissolution in a glass melt.

[0077] The content G of (C) glass frit is a ratio to the content A of conductive particles, and is therefore a dimensionless number. ｉ Is B ＣａＯ = 1, B ＳｉＯ２ = 0, so (C) the basicity B of the glass frit ＧＦ (=Σn ｉ ・B ｉ ) is a dimensionless number. Therefore, (C) the basicity B of the glass fritＧＦ and the product B of the content G ＧＦ ・G is also a dimensionless number.

[0078] The basicity (B ＧＦ The basicity (B) is preferably 0.10 to 1.5, more preferably 0.15 to 1.3, and even more preferably 0.20 to 1.1. ＧＦ When the temperature (Tc) of the glass frit is in this range, the reactivity of the glass frit with the passivation film can be made appropriate by adjusting the amount of glass frit added to the conductive paste.

[0079] 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. The content G of the glass frit in the conductive paste is expressed in terms of basicity (B ＧＦ ) and by appropriately adjusting the content of glass frit, the reactivity of the glass frit with 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 glass frit is reduced more than before and the basicity of the glass frit is set within an appropriate range, thereby suppressing the reactivity with the passivation film and improving Voc.

[0080] The glass frit contained in the conductive paste of this embodiment is SiO ２ , B ２ O ３ , V ２ O ５ , Bi ２ O ３ , TeO ２ , BaO, CuO, Li ２ It is preferable that the glass frit contains one or more selected from 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.

[0081] The glass frit is Bi２ O ３ Preferably, Bi in the glass frit (100 mol%) ２ O ３ The content of Bi is preferably 10 to 80 mol %, more preferably 15 to 75 mol %, and further preferably 20 to 70 mol %. ２ O ３ By including the compound, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while being lead-free.

[0082] The conductive paste of this embodiment includes (C) Bi in terms of mol% in the glass frit. ２ O ３ The content of (C Ｂｉ２Ｏ３ ) and (C) the glass frit content G, Ｂｉ２Ｏ３ G 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. (C) The product C of the glass frit content G Ｂｉ２Ｏ３ By setting G within the above range, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while still being lead-free.

[0083] The glass frit may contain SiO in an amount that does not adversely affect the conductive paste of this embodiment. ２ The glass frit may contain SiO ２ In the case where the glass frit contains SiO ２ The content of the glass frit is preferably 10 to 60 mol %, more preferably 15 to 40 mol %. ２ By including the compound, the reactivity with the passivation film can be controlled.

[0084] The glass frit is B within a range that does not adversely affect the conductive paste of this embodiment. ２ O ３ The glass frit may contain B. ２ O ３When the glass frit contains B, the B in the glass frit (100 mol%) ２ O ３ The content of B is preferably 3 to 60 mol %, more preferably 4 to 50 mol %. ２ O ３ By including the compound, the reactivity with the passivation film can be controlled.

[0085] The glass frit is V within a range that does not adversely affect the conductive paste of this embodiment. ２ O ５ The glass frit may contain V. ２ O ５ When the glass frit contains V, the V in the glass frit (100 mol%) ２ O ５ The content of is preferably less than 8 mol %, more preferably 5 mol % or less. ２ O ５ By including V, the basicity of the glass frit can be reduced. Therefore, when the basicity of the glass frit is high, an appropriate content of V ２ O ５ By including the above, the basicity of the glass frit can be adjusted to an appropriate range.

[0086] The glass frit contains TeO in an amount that does not adversely affect the conductive paste of this embodiment. ２ The glass frit may contain TeO ２ When the glass frit contains TeO ２ The content of TeO is preferably less than 80 mol %, more preferably 50 mol % or less. ２ Therefore, when the basicity of the glass frit is high, an appropriate content of TeO ２ By including the above, the basicity of the glass frit can be adjusted to an appropriate range.

[0087] 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 including an appropriate amount of BaO in the glass frit, the reactivity with the passivation film can be adjusted to an appropriate range.

[0088] 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 including an appropriate amount of CuO in the glass frit, the reactivity with the passivation film can be adjusted to an appropriate range.

[0089] The glass frit contains Li in an amount that does not adversely affect the conductive paste of this embodiment. ２ The glass frit may contain Li. ２ When O is contained, Li in the glass frit (100 mol%) ２ The O content is preferably 3 to 40 mol %, more preferably 5 to 30 mol %. ２ By including O, the reactivity with the passivation film can be adjusted to an appropriate range.

[0090] 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.

[0091] In the conductive paste of this embodiment, the glass transition point (Tg) of the (C) glass frit is preferably 250 to 600°C, more preferably 270 to 500°C, and even more preferably 300 to 470°C. By setting the glass transition point (Tg) of the (C) glass frit to 250°C or higher, it is possible to suppress reactivity with the passivation film. Furthermore, by setting the glass transition point (Tg) to 600°C or lower, it is possible to reduce the contact resistance between the resulting electrode (e.g., the light-incident side surface electrode 20) and the impurity diffusion layer 4.

[0092] 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 a glass powder sample and a reference material are set on the differential thermobalance. Next, the temperature is increased from room temperature to 900°C at a temperature increase rate of 10°C / min as a measurement condition, and a curve (DTA curve) is obtained by plotting the temperature difference between the glass powder sample and the reference material against the temperature. The first inflection point of the DTA curve obtained in this way can be determined as the glass transition point Tg.

[0093] The shape of the glass frit particles is not particularly limited, and for example, spherical, irregular, etc. may be used. The particle size is also not particularly limited. From the viewpoint of workability, the average particle diameter (D50) of the particles is preferably in the range of 0.1 to 10 μm, more preferably in the range of 0.5 to 5 μm.

[0094] The glass frit particles can be one type of particles containing predetermined amounts of the required oxides. Alternatively, particles made of a single oxide can be used as different particles for each of the required oxides. Alternatively, multiple types of particles with different compositions of the required oxides can be used in combination. To synergistically obtain the effects of different types of oxides, it is preferable that the glass frit particles be one type of particles containing predetermined amounts of the required oxides.

[0095] <Other Components> The conductive paste of this embodiment may contain additives other than those described above, as long as they do not adversely affect the solar cell characteristics of the resulting solar cell.

[0096] The conductive paste of this embodiment may further contain additives selected from plasticizers, antifoaming agents, dispersants, leveling agents, stabilizers, adhesion promoters, etc. Among these, the plasticizer may be at least one selected from phthalates, glycolates, phosphates, sebacates, adipic acids, citrates, etc.

[0097] 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. The inclusion of these additives can improve the adhesive strength of the electrode to the passivation film. 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. To achieve higher adhesive strength, the additive is preferably copper manganese tin, aluminosilicate, or aluminum silicate. The additive may include both aluminosilicate and aluminum silicate.

[0098] <Method for manufacturing conductive paste> Next, a method for manufacturing the conductive paste of this embodiment will be described. The conductive paste of this embodiment can be manufactured by adding silver particles, glass frit, and other additives and / or additives as needed to an organic binder and a solvent, mixing them, and dispersing them.

[0099] Mixing can be performed, for example, using a planetary mixer, and dispersion can be performed using a three-roll mill. The mixing and dispersion methods are not limited to these, and various known methods can be used.

[0100] <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 subjecting the predetermined electrode to a laser treatment process.

[0101] This embodiment is a solar cell in which at least a portion of an electrode is formed using the above-described conductive paste. Cross-sectional schematic diagrams of a crystalline silicon solar cell are shown in Figures 1 and 4. The conductive paste of this embodiment is substantially free of lead. Furthermore, the crystalline silicon solar cell of this embodiment can be configured so that materials other than the conductive paste also contain substantially no lead. Therefore, the crystalline silicon solar cell of this embodiment can be a lead-free solar cell.

[0102] In the solar cell of this embodiment, the material of the semiconductor substrate can be crystalline silicon, silicon carbide, germanium, gallium arsenide, etc. From the viewpoint of safety and cost as a solar cell, the material of the semiconductor substrate is preferably crystalline silicon (single crystal silicon, polycrystalline silicon, etc.).

[0103] 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-side 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 Figure 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 the anti-reflection film 2.

[0104] The first conductivity type semiconductor substrate is an n-type semiconductor substrate or a p-type semiconductor substrate. The second conductivity type semiconductor layer 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 first conductivity type semiconductor substrate and the second conductivity type semiconductor layer corresponds to a pn junction. The material of the semiconductor substrate is preferably silicon. Therefore, the semiconductor substrate is preferably a crystalline silicon substrate.

[0105] The passivation film can be an anti-reflection film 2. The passivation film is preferably a thin film made of silicon nitride.

[0106] The light-incident side surface electrode 20 of the solar cell of this embodiment can be a fired 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.

[0107] 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 p-n junction between the second conductivity type semiconductor layer and the first conductivity type semiconductor substrate. The light from the point light source generates carriers (electron-hole pairs) inside the semiconductor substrate, and application of a voltage enables the carriers to move, i.e., current to flow. The voltage is applied so that the direction of current flow at the p-n 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, when 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.

[0108] In addition, when the crystalline silicon solar cell is a bifacial solar cell as shown in Figure 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 serves as a local conduction 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).

[0109] 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. Generally, 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.

[0110] In the following explanation, a solar cell 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") will be used as an example.

[0111] As shown in FIG. 1 , when the laser treatment process is used, an 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. In the laser treatment process, the above-mentioned predetermined voltage is applied so that a current flows in the reverse direction to the forward direction at the pn junction. Light (e.g., laser light) is irradiated from a point light source, causing a current to flow in a small area between the light-incident-side surface electrode 20 and the impurity diffusion layer 4, resulting in 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 conduction portion) between the light-incident-side surface electrode 20 and the impurity diffusion layer 4. That is, the local conduction portion contains an alloy of silver and silicon. Furthermore, in the local conduction portion, the impurity diffusion layer 4 (a silicon emitter layer of the second conductivity type) is in direct contact with the light-incident-side 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 lower reactivity with the anti-reflection film 2 than conventional conductive pastes, and has 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 for forming the light-incident side surface electrode 20 of a crystalline silicon solar cell using the laser treatment process.

[0112] The crystalline silicon solar cell shown in Fig. 1 can have a back electrode 15 having a 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 an entire back electrode 15b and a back TAB electrode 15a electrically connected to the entire back electrode 15b.

[0113] 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 a light-incident surface electrode 20 (particularly, finger electrode 20b) on the light-incident surface and a back surface electrode 15 (back surface finger electrode 15c). This allows a laser treatment process to form electrically conductive portions (locally conductive portions) in the passivation film (anti-reflection film 2) on the light-incident surface and the back surface passivation film (anti-reflection film 2).

[0114] Therefore, the conductive paste of this embodiment can be suitably used as a conductive paste for forming finger electrodes 20b of crystalline silicon solar cells. The conductive paste of this embodiment can also be used as a conductive paste for forming back electrode 15 of bifacial crystalline silicon solar cells.

[0115] The busbar electrodes of the crystalline silicon solar cell shown in Figure 1 include a light-incident side busbar electrode 20a shown in Figure 2 and a backside TAB electrode 15a as shown in Figure 3. Metal ribbons for interconnection, the periphery of which is covered with solder, are soldered to the light-incident side busbar electrode 20a and the backside TAB electrode 15a. This metal ribbon allows current generated by the solar cell to be extracted to the outside of the crystalline silicon solar cell. The bifacial crystalline silicon solar cell shown in Figure 4 can also have a light-incident side busbar electrode 20a and a backside TAB electrode 15a having a shape similar to that of the light-incident side busbar electrode 20a.

[0116] The width of the busbar electrodes (light-incident side busbar electrode 20a and backside TAB electrode 15a) can be approximately the same as the width of the metal ribbon used for interconnection. A wider width is preferable for the busbar electrodes to have low electrical resistance. On the other hand, a narrower width for the light-incident side busbar electrode 20a is preferable to increase the area of ​​light incident on the light-incident side surface. 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. The number of busbar electrodes can be determined depending on the size of the crystalline silicon solar cell. The number of busbar electrodes is optional. Specifically, the number of busbar electrodes can be three, four, or more. The optimal number of busbar electrodes can be determined by simulating solar cell operation so as to maximize the conversion efficiency of the crystalline silicon solar cell. Since the crystalline silicon solar cells are connected in series with each other by the interconnect metal ribbons, it is preferable that the number of light-incident side busbar electrodes 20a and the backside 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 backside TAB electrodes 15a are the same.

[0117] To increase the area of ​​light incident on the crystalline silicon solar cell, it is preferable that the area occupied by the light-incident surface electrode 20 on the light-incident surface be as small as possible. Therefore, it is preferable that the finger electrodes 20b on the light-incident surface be as narrow as possible and as few as possible in number. On the other hand, from the viewpoint of reducing electrical loss (ohmic loss), it is preferable that the finger electrodes 20b be wide and numerous. Furthermore, from the viewpoint of reducing contact resistance between the finger electrodes 20b and the crystalline silicon substrate 1 (impurity diffusion layer 4), it is also preferable that the finger electrodes 20b be wide. From the above, the number of busbar electrodes can be determined depending on 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 solar cell operation 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.

[0118] <Method for manufacturing solar cell> Next, a method for manufacturing a solar cell according to 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.

[0119] 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 side surface electrode 20). The method for manufacturing a solar cell of this embodiment will be described in more detail below.

[0120] The method for manufacturing a solar cell according to this embodiment includes the step of preparing a semiconductor substrate (e.g., a crystalline silicon substrate 1) of a first conductivity type (p-type or n-type). As the semiconductor substrate of the first conductivity type, an n-type crystalline silicon substrate 1 is preferably used. The following description will be given taking as an example a case where a crystalline silicon solar cell is manufactured using an n-type crystalline silicon substrate 1.

[0121] From the viewpoint of obtaining 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.

[0122] 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 a first conductivity type.

[0123] The method for manufacturing a crystalline silicon solar cell according to this embodiment includes the step of forming a second conductivity type semiconductor layer (impurity diffusion layer 4) on one surface of the crystalline silicon substrate 1 prepared in the above-described 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). It is also possible to manufacture a crystalline silicon solar cell using a p-type crystalline silicon substrate 1. In this case, an n-type impurity diffusion layer 4 is formed by diffusing an n-type impurity such as P (phosphorus).

[0124] When the impurity diffusion layer 4 is formed, 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 / □.

[0125] Furthermore, in the method for manufacturing a 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.

[0126] The method for manufacturing a solar cell according to 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 side surface electrode 20 is formed. The firing for forming the back electrode 15 can be performed simultaneously with or separately from the firing for forming the light-incident side surface electrode 20.

[0127] Specifically, in the method for manufacturing a crystalline silicon solar cell of this embodiment, a conductive paste is printed on the other surface (back surface) of crystalline silicon substrate 1 and then fired to form back surface electrode 15 .

[0128] 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.

[0129] 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 conductivity type semiconductor layer (impurity diffusion layer 4). The passivation film can be an anti-reflection film 2.

[0130] Specifically, in the method for manufacturing a 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-described 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 on 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. Furthermore, 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).

[0131] The method for manufacturing a solar cell according to this embodiment includes a step of forming a light-incident-side surface electrode 20 on at least a portion of the surface of the passivation film (anti-reflection film 2). In the manufacturing method according to this embodiment, the above-described conductive paste is used to form the light-incident-side surface electrode 20. Therefore, the light-incident-side surface electrode 20 is a fired body of the above-described conductive paste.

[0132] In the method for manufacturing a 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 side surface electrode 20. Note that firing to form the back electrode 15 can be carried out simultaneously with firing to form the light-incident side surface electrode 20.

[0133] 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 electrodes 20 a and the light-incident side finger electrodes 20 b of the light-incident side surface electrode 20 can be formed using the conductive paste of this embodiment.

[0134] Following the printing and drying of the pattern of the light-incident side surface electrode 20, a conductive paste for forming the back surface electrode 15 is printed and dried. The conductive paste of this embodiment can be preferably used to form electrodes (light-incident side surface electrode 20 and, in some cases, back surface electrode 15) of solar cells such as crystalline silicon solar cells.

[0135] The printed, dried conductive paste is then fired in air under specified firing conditions using a firing furnace such as a tubular furnace. The firing conditions are as follows: the firing atmosphere is air, and the firing temperature is 500 to 1000°C, more preferably 600 to 1000°C, even more preferably 500 to 900°C, and particularly preferably 700 to 900°C. The firing is preferably carried out for a short period of time, and the temperature profile (temperature-time curve) during firing preferably has a peak. For example, the peak temperature is preferably the above-mentioned temperature, and 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.

[0136] During firing, it is preferable to simultaneously fire the conductive pastes for forming the light-incident-side surface electrode 20 and the back surface electrode 15, thereby simultaneously forming both electrodes. By printing a predetermined conductive paste on the light-incident-side surface and the back surface and firing them simultaneously, firing to form the electrodes can be performed only once. This allows crystalline silicon solar cells to be manufactured at lower cost.

[0137] The method for manufacturing a solar cell of this embodiment includes performing the above-mentioned laser treatment process. 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 a direction opposite 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.

[0138] In the manner described above, the crystalline silicon solar cell of this embodiment can be manufactured.

[0139] The crystalline silicon solar cells of this embodiment obtained as described above can be electrically connected with metal ribbons for interconnection and laminated with glass plates, sealing materials, protective sheets, etc. to obtain a solar cell module. Metal ribbons covered with solder (e.g., copper ribbons) can be used as the metal ribbons for interconnection. Commercially available solders, such as those containing tin as a main component, specifically lead-containing solders or lead-free solders, can be used as the solder. To obtain lead-free solar cells, it is preferable to use lead-free solder.

[0140] In the crystalline silicon solar cell of this embodiment, a high-performance crystalline silicon solar cell can be obtained by forming a predetermined electrode of the solar cell using the conductive paste of this embodiment and performing a laser treatment process.

[0141] The conductive paste of this embodiment contains lead-free glass frit. Therefore, the electrode formed on the surface of the solar cell is also lead-free. Therefore, when the 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, lead-free solar cells can be manufactured.

[0142] <Depth d of the Region of AgSi Alloy 30> In this specification, the depth d of the region of AgSi alloy 30 (sometimes simply referred to as "AgSi alloy 30") refers to the length of the longest line segment (the length d of the line segment connecting B1 and B2 in FIG. 9) among the line segments connecting any one point (B1 in FIG. 9) at the interface between the electrode and AgSi alloy 30 to any one point (B2 in FIG. 9) at the interface between the substrate and AgSi alloy 30 in an SEM photograph obtained by SEM observation of a cross section of AgSi alloy 30 as shown in FIG. 9. Specifically, the depth d of AgSi alloy 30 can be obtained by superimposing the region of AgSi alloy 30 determined for EDX measurement on an SEM photograph obtained by SEM observation of a 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.

[0143] 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. When the depth d of the AgSi alloy 30 is within this range, contact resistance is reduced, and a highly efficient crystalline silicon solar cell having a high fill factor (FF) can be obtained.

[0144] <Remaining rate of passivation film 2> In this specification, the degree 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 expressed 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 region near the AgSi alloy 30 where the AgSi alloy 30 is not formed.

[0145] A method for measuring the passivation film 2 residual ratio will be described using an example SEM photograph of a cross section of a solar cell shown in Figure 10. To obtain the passivation film 2 residual ratio, a cross section including the passivation film 2 and the AgSi alloy 30 is first observed with an SEM at a magnification of 20,000 times to obtain an SEM photograph. 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 sum 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 at the portion where the AgSi alloy 30 is formed is measured in this SEM photograph. Length Le corresponds to the length of the passivation film 2 lost during the solar cell manufacturing process. In the example shown in Figure 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 sum of Le1 and Le2. The remaining rate of the passivation film 2 can be obtained as Lp / (Lp+Le). Note that the portion of the passivation film 2 that has disappeared during the solar cell manufacturing process can be identified by EDX measurement. Furthermore, the lengths Le1 and the like can be measured by approximating the passivation film 2 and the like as being straight lines.

[0146] In the crystalline silicon solar cell of this embodiment, the residual rate of the passivation film 2 is 10 to 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%. When the residual rate of the passivation film 2 is in an appropriate range, a highly efficient crystalline silicon solar cell having a high open circuit voltage (Voc) and fill factor (FF) can be obtained.

[0147] <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 forming electrodes of the passivation film 2 to the film thickness Db after firing for forming electrodes (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)."

[0148] 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 predetermined range, an increase in the surface defect density that causes carrier recombination when the solar cell of this embodiment generates power can be prevented.

[0149] In this specification, the pre-firing film thickness Da of the passivation film 2 refers to the film thickness of the passivation film 2 when the passivation film 2 is formed on a predetermined substrate. The film thickness Da immediately after film formation can be measured by SEM observation of a cross section near the passivation film 2 before forming an electrode.

[0150] In this specification, the film thickness Db after the solar cell is completed refers to 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 an electrode on the surface of the solar cell by firing.

[0151] In this specification, the thickness Db 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 refers to the thickness of the passivation film 2 near the AgSi alloy 30 of a solar cell completed by forming an electrode pattern using a predetermined conductive paste on the passivation film 2 formed on a predetermined substrate, and then performing predetermined processing such as predetermined firing to form the electrodes and AgSi alloy 30. The thickness Db is sometimes referred to as the "thickness Db of the solar cell after completion." The thickness Db of the solar cell after completion can be measured by SEM observation of a 5.7 μm × 3.9 μm image area of ​​the cross section including the passivation film 2 and AgSi alloy 30 of a solar cell completed by forming the electrodes and AgSi alloy 30. In other words, 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 including the AgSi alloy 30 of the completed solar cell. Specifically, the film thickness Db of the passivation film 2 after the solar cell is completed can be obtained by observing a 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 area: 5.7 μm × 3.9 μm), dividing the SEM photograph into six equal parts vertically, measuring the film thickness (five locations) of the passivation film 2 at five boundaries of the six-part image, and calculating the average film thickness of the five locations.

[0152] <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.

[0153] 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. In this specification, the conductive paste for forming the rear electrode is particularly 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.

[0154] An example of a bifacial solar cell as shown in FIG. 4 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 surface on which a p-type diffusion layer is formed, and the back electrode 15 formed using a conductive back electrode paste is an electrode on the front surface on which an n-type diffusion layer is formed. The conductive back electrode paste 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 surface on which an n-type diffusion layer is formed, and the back electrode 15 is an electrode on the front surface on which a p-type diffusion layer is formed (an electrode formed using the conductive paste of the above-described embodiment).

[0155] The conductive paste for the back electrode can be preferably used to form 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 back electrode 15 of the crystalline silicon solar cell shown in Fig. 1. In the crystalline silicon solar cell of this embodiment, a light-incident side 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.

[0156] The conductive paste for the back electrode will be described below. The conductive paste for the back electrode contains (A2) conductive particles, (B2) an organic vehicle, and (C2) glass frit, which will be described below.

[0157] <(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 may be the same as the (A) conductive particles contained in the conductive paste of the present embodiment described above. In this specification, the conductive particles contained in the conductive paste for the back electrode may also be referred to as "second conductive particles."

[0158] <(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."

[0159] <(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 it adjusts its reactivity with the passivation film to an appropriate range, reduces 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 prevents lead contamination of the environment. Note that, from the viewpoint of reducing 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.

[0160] In this specification, the glass frit contained in the conductive paste for the back electrode may be referred to as the "second glass frit."

[0161] 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 is substantially free of lead (Pb). However, the (C2) glass frit used in the conductive paste for the back electrode may contain a trace 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 wt % or less of lead as an impurity relative to 100 wt % of the (C2) glass frit.

[0162] The conductive paste for the back electrode has a glass frit basicity B ＧＦ and the product B of the content G of the glass frit (C2) in the conductive paste for the back electrode in parts by weight when the content of the conductive particles (A2) in the conductive paste for the back electrode is taken as 100 parts by weight, ＧＦG 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. (C2) Basicity B of the Glass Frit ＧＦ and the product B of the content G ＧＦ By setting G within 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.

[0163] The basicity (B ＧＦ The basicity (B) is preferably 0.10 to 1.5, more preferably 0.15 to 1.3, and even more preferably 0.20 to 1.1. ＧＦ When the surface roughness (C2) of the glass frit is within this range, the reactivity of the glass frit (C2) with the passivation film can be made appropriate by adjusting the amount of the glass frit (C2) added to the conductive paste for the back electrode.

[0164] 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). The content G2 of the glass frit (C2) in the conductive paste for the back electrode is determined based on the basicity (B ＧＦ ) and by appropriately adjusting the temperature, (C2) the reactivity of the glass frit with respect to the passivation film can be made appropriate.

[0165] The glass frit (C2) contained in the conductive paste for the back electrode is 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 including 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.

[0166] (C2) The glass frit is TeO ２ (C2) The glass frit preferably contains TeO ２ (C2) TeO in the glass frit (100 mol%) ２ The content of TeO in the (C2) glass frit (100 mol%) is preferably less than 80 mol%, and more preferably 60 mol% or less. ２ The content of (C2) the glass frit is TeO is preferably 30 mol % or more, and more preferably 40 mol % or more. ２ By including the compound, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while being lead-free.

[0167] The conductive paste for the back electrode is (C2) TeO in terms of mol% in the glass frit. ２ The content of (C ＴｅＯ２ ) and (C2) the glass frit content G2, ＴｅＯ２ G2 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. (C2) The product C of the glass frit content G2 ＴｅＯ２ By setting G2 within the above range, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while still being lead-free.

[0168] (C2) The glass frit is Bi within a range that does not adversely affect the conductive paste for the back electrode. ２ O ３ (C2) The glass frit preferably contains Bi. ２ O ３ When the glass frit (C2) contains Bi, the Bi content in the glass frit (100 mol%) is ２ O ３The content of (C2) the glass frit is preferably 10 to 80 mol %, more preferably 15 to 75 mol %, and even more preferably 20 to 70 mol %. ２ O ３ By including the compound, it is possible to adjust the reactivity with the passivation film to an appropriate range and reduce the contact resistance while being lead-free.

[0169] (C2) The glass frit is made of SiO to the extent that it does not adversely affect the conductive paste for the back electrode. ２ (C2) The glass frit may contain SiO ２ When the glass frit (C2) contains SiO ２ The content of (C2) glass frit is preferably 10 to 60 mol %, more preferably 15 to 40 mol %. ２ By including the compound, the reactivity with the passivation film can be controlled.

[0170] (C2) The glass frit is B within a range that does not adversely affect the conductive paste for the back electrode. ２ O ３ (C2) The glass frit may contain B ２ O ３ When the glass frit (C2) contains B, the glass frit (100 mol%) ２ O ３ The content of (C2) glass frit is preferably 3 to 60 mol %, more preferably 4 to 50 mol %. ２ O ３ By including the compound, the reactivity with the passivation film can be controlled.

[0171] (C2) The glass frit is used in an amount not adversely affecting the conductive paste for the back electrode. ２ O ５ (C2) The glass frit may contain P ２ O ５ In the case where the glass frit (C2) contains P ２ O ５The content of (C2) glass frit is preferably 1 to 10 mol %, and more preferably 2 to 5 mol %. ２ O ５ By including the compound, the reactivity with the passivation film can be controlled.

[0172] (C2) The glass frit contains Li within a range that does not adversely affect the conductive paste for the back electrode. ２ (C2) The glass frit may contain Li. ２ When O is contained, (C2) Li in the glass frit (100 mol%) ２ The O content is preferably 3 to 40 mol %, more preferably 5 to 30 mol %. (C2) The glass frit has an appropriate Li content. ２ By including O, the reactivity with the passivation film can be adjusted to an appropriate range.

[0173] (C2) The glass frit contains Na in an amount that does not adversely affect the conductive paste for the back electrode. ２ O ３ (C2) The glass frit may contain Na ２ O ３ When the glass frit (100 mol%) contains Na, the glass frit (C2) contains Na. ２ O ３ The content of (C2) glass frit is preferably 5 to 15 mol %, more preferably 7 to 13 mol %. ２ O ３ By including the compound, the reactivity with the passivation film can be controlled.

[0174] (C2) The glass frit contains Al in an amount that does not adversely affect the conductive paste for the back electrode. ２ O ３ (C2) The glass frit may contain Al. ２ O ３ In the case where the glass frit contains (C2), Al in the glass frit (100 mol%) ２ O ３The content of (C2) glass frit is preferably 1 to 10 mol %, more preferably 3 to 8 mol %. ２ O ３ By including the compound, the reactivity with the passivation film can be controlled.

[0175] (C2) The glass frit contains TiO within a range that does not adversely affect the conductive paste for the back electrode. ２ (C2) The glass frit may contain TiO ２ In the case where the glass frit (C2) contains TiO ２ The content of (C2) glass frit is preferably 0.5 to 8 mol %, more preferably 1 to 4 mol %. ２ By including the compound, the reactivity with the passivation film can be controlled.

[0176] (C2) The glass frit is ZrO in an amount that does not adversely affect the conductive paste for the back electrode. ２ (C2) The glass frit may contain ZrO ２ When the glass frit (C2) contains ZrO ２ The content of (C2) glass frit is preferably 0.5 to 8 mol %, more preferably 1 to 4 mol %. ２ By including the compound, the reactivity with the passivation film can be controlled.

[0177] 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.

[0178] (C2) The glass frit is Li ２ O, TeO ２and ZnO. The glass frit (C2) 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 conductive paste for the back electrode contains the glass frit (C2) containing the predetermined component, a solar cell with high performance can be obtained.

[0179] In the conductive paste for the back electrode, the glass transition temperature (Tg) of the (C2) glass frit is preferably 250 to 600°C, more preferably 270 to 500°C, and even more preferably 300 to 470°C. By setting the glass transition temperature (Tg) of the (C2) glass frit to 250°C or higher, it is possible to suppress reactivity with the passivation film. Furthermore, by setting the glass transition temperature (Tg) to 600°C or lower, it is possible to reduce the contact resistance between the resulting electrode (e.g., the light-incident side surface electrode 20) and the second impurity diffusion layer 16.

[0180] The shape of the particles of (C2) glass frit is not particularly limited, and for example, spherical or irregular shapes can be used. The particle size is also not particularly limited. From the viewpoint of workability, the average particle diameter (D50) of the particles is preferably in the range of 0.1 to 10 μm, more preferably in the range of 0.5 to 5 μm.

[0181] The particles of the (C2) glass frit can be one type of particles containing predetermined amounts 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, a combination of multiple types of particles with different compositions of the required oxides can be used. To synergistically obtain the effects of different types of oxides, the particles of the (C2) glass frit are preferably one type of particles containing predetermined amounts of each of the required oxides.

[0182] <Other Components> The conductive paste for a back electrode of the present embodiment can contain 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 resulting solar cell.

[0183] <Method for Manufacturing Conductive Paste for Back Electrode> The conductive paste for the back electrode can be manufactured by the same method as the conductive paste of this embodiment described above.

[0184] <Lead-free solar cell> 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. Generally, the portions of a crystalline silicon solar cell other than the electrode can be formed using lead-free materials. 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.

[0185] 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 directly contacts 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.

[0186] The light-incident side surface electrode and the back surface electrode of the lead-free solar cell are lead-free electrodes that do not contain lead, and therefore, the light-incident side surface electrode and the back surface electrode are electrodes formed using a lead-free conductive paste.

[0187] In the lead-free crystalline silicon solar cell of this embodiment, a predetermined electrode of the solar cell is formed using the conductive paste of this embodiment described above, and a laser treatment process is performed to form a local conductive portion (AgSi alloy). By having the lead-free crystalline silicon solar cell of this embodiment have the local conductive portion (AgSi alloy), a high-performance crystalline silicon solar cell can be obtained.

[0188] The conductive paste of this embodiment and the above-described conductive paste for the back electrode contain lead-free glass frit. Therefore, the electrode formed on the surface of the solar cell is also lead-free. Therefore, when the solar cell manufactured using the conductive paste of this embodiment is disposed of, lead pollution of the environment can be prevented.

[0189] The present embodiment will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0190] <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 properties of the single crystal silicon solar cells were measured to evaluate the performance of the conductive pastes of Examples 1 to 8 of the present embodiment and Comparative Examples 1 and 2.

[0191] <<Materials and Preparation Ratios of Conductive Paste>> 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 as 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.

[0192] (A) Silver Particles Table 2 shows the product number, manufacturer, shape, average particle diameter (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 diameter (D50) was determined by measuring particle size distribution using the Microtrac method (laser diffraction scattering method) and obtaining the median diameter (D50) from the particle size distribution measurement results. The same applies to the average particle diameters (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 single-point method using nitrogen gas adsorption after pre-drying at 100°C and flowing nitrogen gas for 10 minutes.

[0193] (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.

[0194] (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 (C) glass frit 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.

[0195] 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, approximately 50 mg of glass frits A to G were placed in a platinum cell as a sample, 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 onset (extrapolated point) of the first endotherm in the DTA curve was taken as the glass transition point. Note that the onset 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."

[0196] Glass frits GF1 to GF6 were produced as follows. First, the raw oxide powders 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 that temperature until the raw materials were fully melted. Next, the crucible was removed from the oven, and the molten contents were uniformly stirred. Next, the contents of the crucible were quenched at room temperature using two stainless steel rollers to obtain a plate-shaped glass. Finally, the plate-shaped glass was crushed and uniformly dispersed in a mortar and sieved through a mesh sieve to obtain glass frit with the desired particle size. By sieving the glass that passed through a 100-mesh sieve and remained on a 200-mesh sieve, a glass frit with an average particle size (D50) of 149 μm was obtained. Further crushing of this glass frit resulted in a glass frit with an average particle size (D50) of 2 μm.

[0197] 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 three-roll mill to form a paste, thereby producing the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2.

[0198] <<Manufacturing of Single Crystal Silicon Solar Cell>> 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.

[0199] First, a silicon oxide layer of approximately 20 μm was formed on the substrate by dry oxidation, and then the substrate surface was etched with a solution containing hydrogen fluoride, pure water, and ammonium fluoride to remove damage.Furthermore, heavy metals were removed using an aqueous solution containing hydrochloric acid and hydrogen peroxide.

[0200] Next, textures (concave and convex shapes) were formed on both sides of the substrate by wet etching. Specifically, pyramidal texture structures were 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.

[0201] Next, boron was implanted into one surface (light incident surface) of the substrate having the textured structure to form a p-type diffusion layer to a depth of about 0.5 μm, with a sheet resistance of 60 Ω / □.

[0202] Furthermore, phosphorus was implanted 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 / □. The boron and phosphorus were simultaneously implanted by thermal diffusion.

[0203] 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 (back 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, NH ３ / SiH ４ A silicon nitride film (anti-reflection film 2) having a thickness of about 70 nm was formed by plasma CVD using glow discharge decomposition of a mixed gas of SiO 2 =0.5 at 1 Torr (133 Pa).

[0204] For the single crystal silicon solar cells of Examples 1 to 8 and Comparative Examples 1 and 2, the conductive pastes used for forming electrodes on the surface (light incident surface) of the substrate on which the p-type diffusion layer was formed were those shown in Table 1.

[0205] The conductive paste was printed by screen printing on the anti-reflection film 2 of the substrate described above in an electrode pattern consisting of 1.5 mm wide light-incident side busbar electrodes 20a and 60 μm wide light-incident side finger electrodes 20b, so as to have a film thickness of approximately 20 μm. The printed pattern was then dried at 150° C. for approximately 1 minute.

[0206] A commercially available Ag paste was printed by screen printing as the back electrode 15 (the electrode on the surface on which the n-type diffusion layer was formed). The electrode pattern of the back electrode 15 was the same as that of the light-incident side surface electrode 20. It was then dried at 150°C for approximately 60 seconds. After drying, the film thickness of the conductive paste for the back electrode was 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 fabricated.

[0207] <<Measurement of Electrical Characteristics of Solar Cell Before Laser Treatment Process>> The electrical characteristics of the single crystal silicon solar cell were measured as follows: The current-voltage characteristics of the prototype solar cell were measured using a solar simulator SS-150XIL manufactured by Eiko Seiki Co., Ltd. under the conditions of 25°C, AM1.5, solar simulator light (energy density 100 mW / cm ２ The fill factor (FF), open circuit voltage (Voc), and conversion efficiency (%) were calculated from the measurement results. Two single-crystal silicon solar cells were fabricated under the same manufacturing conditions, and the measured values ​​were calculated as the average of the two.

[0208] <<Laser Treatment Process>> A laser treatment process was performed on the light-incident surface of the single-crystalline silicon solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above. That is, a laser beam 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 flowed 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, and the intensity of the irradiated laser beam was 100 W / cm. ２ The voltage application and laser light irradiation time were set to 2 seconds.

[0209] <<Measurement of Electrical Properties of Solar Cell After Laser Treatment Process>> The electrical properties of the solar cell after the laser treatment process were measured in the same manner as the electrical properties of the solar cell before the laser treatment process.

[0210] 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, with conversion efficiencies ranging from 0.5 to 1.6%, for example. 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, with conversion efficiencies ranging from 0.8 to 1.0%, for example.

[0211] As is clear from Table 1, the conductive pastes of Examples 1 to 8 of this embodiment ((C) glass frit basicity B ＧＦ and the product B of the content G of the glass frit (C) when the content of the conductive particles (A) is 100 parts by weight. ＧＦThe electrical properties of the solar cells fabricated using the conductive pastes of Comparative Examples 1 and 2 after the laser treatment process were significantly higher than those 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 properties of the solar cells fabricated using the conductive pastes of Comparative Examples 1 and 2 after the laser treatment process were lower than those of Examples 1 to 8. Specifically, in Comparative Example 1 ((C) Basicity B of the glass frit ＧＦ and the product B of the content G of the glass frit (C) when the content of the conductive particles (A) is 100 parts by weight. ＧＦ The fill factor (FF) of the solar cell of Comparative Example 1 (G=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 conversion efficiency was low because the fill factor (FF) was low. ＧＦ and the product B of the content G of the glass frit (C) when the content of the conductive particles (A) is 100 parts by weight. ＧＦ The fill factor (FF) (G=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.

[0212] Reference Examples 1 to 4 In Examples 1 to 8 and Comparative Examples 1 and 2, single crystal silicon solar cells were fabricated using conductive pastes containing lead-containing glass frit, and the electrical characteristics of the single crystal silicon solar cells were measured.

[0213] 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 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.

[0214] (A) Conductive particles Silver particles A1 shown in Table 2 were used as conductive particles. Table 4 shows the blending amounts of the conductive particles.

[0215] (B) Organic Vehicle As the organic vehicle, the same organic binder and solvent as in Examples 1 to 8 and Comparative Examples 1 and 2 were used in the same amounts.

[0216] (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 glass frits GF11 and GF12 contain PbO. The average particle size (D50) of glass frits GF11 and GF12 was 2 μm. Table 4 shows the type and content G (parts by weight) of (C) glass frit in the conductive pastes of Examples 1 to 8 and Comparative Examples 1 and 2. The glass transition points of glass frits GF11 and GF12 were measured in the same manner as for glass frits GF1 to GF6 described above. Glass frits GF11 and GF12 were manufactured in the same manner as for glass frits GF1 to GF6 described above.

[0217] As shown in Table 4, aluminum (Al) particles were added as component (D) to the conductive pastes of Reference Examples 2 to 4. Al particles manufactured by Toyo Aluminum (product number: TFH-A02P, spherical, average particle size (D50): 2 μm) were used as the (D) Al particles. Table 4 shows the blending amounts (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.

[0218] Next, similarly 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, if necessary, (D) Al particles were mixed in a planetary mixer, and further dispersed in a three-roll mill to form a paste, thereby producing the conductive pastes of Reference Examples 1 to 4.

[0219] <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 properties of the solar cells were measured before and after the laser treatment process. Table 4 shows the measurement results of the electrical properties of Reference Examples 1 to 4.

[0220] As is clear from Tables 1 and 4, the electrical properties after the laser treatment process of the solar cells produced using the conductive pastes of Examples 1 to 8 of this embodiment were comparable to the electrical properties after the laser treatment process of the solar cells produced using the conductive pastes of Reference Examples 1 to 4. Therefore, it was revealed that when the laser treatment process was performed on solar cells produced using the conductive pastes having lead-free glass frit of Examples 1 to 8 of this embodiment, solar cells with performance similar to that produced when a conductive paste having lead-containing glass frit was used can be manufactured.

[0221] <SEM Photographs> Cross sections near the antireflection film 2 (passivation film) of the solar cells of Reference Example 1, Example 1, and Comparative Example 1 that had been subjected to a laser treatment process were observed with 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.

[0222] 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-side 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 reverse direction to the forward direction at the pn junction, and light (e.g., laser light) is irradiated from a point light source. This causes a current to flow in a small area between the light-incident-side surface electrode 20 and the impurity diffusion layer 4, resulting in 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 conduction portion) between the light-incident-side surface electrode 20 and the impurity diffusion layer 4. That is, the local conduction portion contains an alloy of silver and silicon. In addition, in the local conduction portion, the impurity diffusion layer 4 (a silicon emitter layer of the second conductivity type) is in direct contact with the light-incident-side 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 containing lead-free glass frit, like a conductive paste containing a predetermined lead-containing glass frit, has low reactivity with the anti-reflection film 2 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 side surface electrode 20 of a crystalline silicon solar cell using a laser treatment process. Furthermore, because the conductive paste of this embodiment contains lead-free glass frit, lead contamination of the environment can be prevented when the solar cell is disposed of.

[0223] 8, in the case of a solar cell in which the light-incident side surface electrode 20 was formed using the conductive paste of Comparative Example 1, after the laser treatment process, no AgSi alloy 30 (an alloy of silver and silicon), which is a local electrically conductive portion (local conductive portion), was formed between the light-incident side surface electrode 20 and the impurity diffusion layer 4. Therefore, it was revealed that the conductive paste of Comparative Example 1 cannot be said to be a conductive paste suitable for forming an electrode by the laser treatment process.

[0224] 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 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 during firing of the conductive paste (electrode pattern). As a result, the passivation film disappeared in the solar cell using the conductive paste of Comparative Example 2, and therefore the open-circuit voltage (Voc) after the laser treatment process was low. Therefore, it was revealed that the conductive paste of Comparative Example 2 is not a conductive paste suitable for forming electrodes using a laser treatment process.

[0225] <Examples 9 to 14 and Reference Example 5> In Examples 9 to 14 and Reference Example 5, bifacial 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 9 to 14 and Reference Example 5 of this embodiment.

[0226] In Examples 9 to 14 and Reference Example 5, the electrodes of a bifacial solar cell using an n-type Si substrate are formed using the conductive paste of this embodiment. That is, the front surface electrode (light-incident side front surface electrode 20) in Examples 9 to 14 and Reference Example 5 is the electrode on the front surface on which the p-type impurity diffusion layer 4 is formed, and the back surface electrode 15 is the electrode on the front surface on which the n-type second impurity diffusion layer 16 is formed.

[0227] Table 6 shows the compositions of the conductive pastes for forming the front electrode and the conductive pastes for forming the back electrode in Examples 9 to 14 and Reference Example 5. The compositions shown in Table 6 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.

[0228] <<Conductive Paste for Forming Surface Electrode>> 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. Here, the electrode formed with the conductive paste for forming the surface electrode is referred to as "surface electrode 20."

[0229] <<Materials and Preparation Ratios of Conductive Paste for Forming Rear Electrode>> The conductive pastes for forming 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 rear electrode is referred to as “rear electrode 15.”

[0230] (A) Silver Particles Table 6 shows the blending amounts of silver particles A1 and A2 in the conductive pastes for forming back electrodes in Examples 9 to 14 and Reference Example 5. Note that silver particles A1 and A2 are the same as silver particles A1 and A2 used in the conductive pastes for forming light-incident side surface electrodes in Examples 1 to 8. Table 2 shows the product number, manufacturer, shape, average particle diameter (D50), tap density, and BET specific surface area of ​​silver particles A1 and A2 used in the conductive pastes for forming back electrodes in Examples 9 to 14 and Reference Example 5.

[0231] (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 surface electrode in Examples 1 to 8 and Comparative Examples 1 and 2.

[0232] (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 pastes for forming rear electrodes in Examples 9 to 14 and Reference Example 5. The average particle size (D50) of the glass frits GFA, GFB, and GFC was 2 μm. Table 6 shows the type and content G2 (parts by weight) of (C) glass frit in the conductive pastes for forming rear electrodes in 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.

[0233] The method for measuring the glass transition points of the glass frits GFA, GFB, and GFC was the same as the method for measuring the glass transition points of the above-mentioned glass frits A to G. Table 7 shows the measured values ​​of the glass transition points of the glass frits GFA, GFB, and GFC.

[0234] The manufacturing methods of the glass frits GFA, GFB, and GFC are the same as the manufacturing methods of the glass frits A to G described above.

[0235] 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 three-roll mill to form a paste, thereby producing the conductive pastes for forming back electrodes of Examples 9 to 14 and Reference Example 5.

[0236] <<Manufacturing of Single Crystal Silicon Solar Cells>> As in Examples 1 to 8 and Comparative Examples 1 and 2, bifacial single crystal silicon solar cells such as the one 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 back surface 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 surface electrode shown in Table 6 above were used.

[0237] <<Measurement of Electrical Properties of Solar Cells Before Laser Treatment Process>> The electrical properties 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 for the solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above.

[0238] <<Laser Treatment Process>> The laser treatment process in Examples 9 to 14 and Reference Example 5 was carried out in the same manner as in the solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above.

[0239] <<Measurement of Electrical Properties of Solar Cells After Laser Treatment Process>> The electrical properties of the solar cells after the laser treatment process of Examples 9 to 14 and Reference Example 5 were measured in the same manner as for the solar cells of Examples 1 to 8 and Comparative Examples 1 and 2 described above.

[0240] <<Depth d of AgSi Alloy 30>> The depth d of the AgSi alloy 30 of the front surface electrode 20 in Examples 9 to 14 and Reference Example 5 was measured as follows. Table 6 shows the measurement results.

[0241] 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 longest line segment (the length d of the line segment connecting B1 and B2 in FIG. 9 ) among the line segments connecting any one point (B1 in FIG. 9 ) at the interface between the electrode and the AgSi alloy 30 to any one point (B2 in FIG. 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 FIG. 9 was obtained by SEM observation of a cross section including the passivation film 2 and the AgSi alloy 30 of a completed solar cell at a magnification of 20,000 times.

[0242] <<Residual Rate of Passivation Film 2>> The residual 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 "Residual Rate" column under "Evaluation of Surface Electrode" in Table 6 shows the measurement results of the residual rate of the passivation film 2 near the surface electrode 20.

[0243] Specifically, a cross section of a completed solar cell, including the passivation film 2 and the AgSi alloy 30 near the front electrode 20, was first observed using an SEM at 20,000x magnification to obtain an SEM photograph (SEM image area: 5.7 μm × 3.9 μm). The horizontal (horizontal to the substrate surface) length of this SEM photograph was 5.7 μm, and the vertical (perpendicular to the substrate surface) length was 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 sum 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 at the portion where the AgSi alloy 30 was formed was measured in this SEM photograph. The length Le corresponds to the length of the passivation film 2 lost during the solar cell manufacturing process. 10, the total length Le of the cross section of the interface between the AgSi alloy 30 and the electrode at the portion where the passivation film 2 has disappeared is the sum of Le1 and Le2. The remaining rate of the passivation film 2 can be obtained as Lp / (Lp+Le). The lengths Lp1 and others were measured by approximating the passivation film 2 and others as being straight lines.

[0244] <Film Thickness and Film Thickness Ratio Before and After Firing of Passivation Film 2> For the solar cells of Examples 9 to 14 and Reference Example 5, the pre-firing film thickness Da of the passivation film 2 near the front surface electrode 20 and the film thickness Db after the solar cell was completed were measured. The "Film Thickness (Da) before Firing" column under "Evaluation of Front Surface Electrode" in Table 6 shows the measured film thickness Da of the passivation film 2 near the front surface electrode 20 before Firing. The "Film Thickness (Db) after Firing / Treatment" column under "Evaluation of Front Surface Electrode" in Table 6 shows the measured film thickness Db of the passivation film 2 near the front surface electrode 20 after the solar cell was completed. The "Film Thickness Ratio (Db / Da)" column under "Evaluation of Front Surface Electrode" in Table 6 shows the film thickness ratio (Db / Da) of the passivation film 2 before and after Firing.

[0245] The pre-firing film 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 film formation. That is, the passivation film 2 was formed on the surface of a predetermined 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 film thickness Da of the passivation film 2 in Examples 9 to 14 and Reference Example 5.

[0246] 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, if necessary, a predetermined laser treatment process is performed. Specifically, a cross section of the completed solar cell, including the passivation film 2 and the AgSi alloy 30, was first observed using an SEM at a magnification of 20,000 times to obtain an SEM photograph (SEM image area: 5.7 μm × 3.9 μm). Next, the SEM photograph was divided into six equal parts vertically, and the thicknesses of the passivation film 2 (five locations) at the boundaries of the six equal parts were measured. The thickness Db of the completed solar cell was the average value of the thicknesses of the passivation film 2 at the five locations.

[0247] The film thickness ratio of the passivation film 2 before and after firing 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.

[0248] The results shown in Table 6 reveal that a crystalline silicon solar cell with appropriate performance can be obtained by forming the front surface electrode 20 using the conductive paste of this embodiment and forming the back surface electrode 15 using a predetermined conductive paste for forming the back surface electrode.

[0249] A conductive paste containing lead-containing glass frit was used to form back electrode 15 of the crystalline silicon solar cell of Reference Example 5. Also, a conductive paste containing lead-free glass frit was used to form back electrode 15 of the crystalline silicon solar cells of Examples 9 to 14. The crystalline silicon solar cells of Examples 9 to 14 had performance comparable to that of the crystalline silicon solar cell of Reference Example 5, and therefore it was revealed that lead-free crystalline silicon solar cells with good performance could be manufactured by forming front electrode 20 using the conductive paste of this embodiment and forming back electrode 15 using a predetermined conductive paste for forming a back electrode.

[0250] In particular, it has been revealed that by forming the front surface electrode 20 using the conductive paste of this embodiment, forming the back surface electrode 15 using a glass frit containing Te as the conductive paste for forming the back surface electrode, and forming a local conductive portion using a laser treatment process, 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, 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.

[0251]

[0252]

[0253]

[0254]

[0255]

[0256]

[0257]

[0258] DESCRIPTION OF SYMBOLS 1 Crystalline silicon substrate 2 Anti-reflection film (passivation film) 4 Impurity diffusion layer 15 Back surface electrode 15a Back surface TAB electrode (back surface bus bar electrode) 15b Back surface electrode (full back surface 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 (locally conductive portion)

Claims

1. A conductive paste for forming an electrode of a solar cell, comprising: (A) conductive particles; (B) an organic vehicle; and (C) a glass frit, wherein the glass frit is substantially free of PbO, and the basicity B of the glass frit is 0.01 to 0.

01. ＧＦ and the content G of the glass frit (C) in the conductive paste in parts by weight when the content of the conductive particles (A) in the conductive paste is taken as 100 parts by weight, ＧＦ A conductive paste having G in the range of 0.25 to 1.

45.

2. The (C) glass frit is Bi ２ The conductive paste of claim 1 , further comprising O3.

3. The Bi content in the glass frit (C) in units of mol% ２ Content of O3 (C Ｂｉ２Ｏ３ ) and the content G of the glass frit (C) Ｂｉ２Ｏ３ The conductive paste according to claim 1 or 2, wherein G is in the range of 10 to 200.

4. The conductive paste according to any one of claims 1 to 3, wherein the (A) conductive particles include silver particles.

5. 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.

6. 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.

7. 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.

8. The (C) glass frit is SiO ２ , B ２ O ３ , V ２ O ５ , Bi ２ O ３ , TeO ２ , BaO, CuO, Li ２ The conductive paste according to any one of claims 1 to 7, comprising at least one selected from O and ZnO.

9. A conductive paste according to any one of claims 1 to 8, wherein (B) the organic vehicle contains at least one selected from the group consisting of ethyl cellulose, rosin ester, acrylic, and an organic solvent.

10. A conductive paste for forming an electrode of a solar cell, comprising: 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 electrode disposed so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type; a passivation film disposed in contact with the surface of the semiconductor layer of the second conductivity type; and a light incident side surface electrode disposed on at least a portion of the surface of the passivation film, wherein the light incident side surface electrode is a light incident side surface electrode that has been treated by irradiating the light incident side surface of the solar cell with light from a point light source 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 the forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type, and the conductive paste according to any one of claims 1 to 9 is a conductive paste for forming the light incident side surface electrode.

11. A solar cell comprising: 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 electrode disposed so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type; a passivation film disposed in contact with the surface of the semiconductor layer of the second conductivity type; and a light incident side surface electrode disposed on at least a portion of the surface of the passivation film, wherein the light incident side surface electrode is a light incident side surface electrode that has been treated to irradiate the light incident side surface of the solar cell with light from a point light source 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 the forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type, and the light incident side surface electrode is a fired body of a conductive paste according to any one of claims 1 to 10.

12. A solar cell comprising: 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, wherein the silicon emitter layer of the second conductivity type has a local conductive portion in direct contact with the light-incident surface electrode without a passivation film, the local conductive portion containing an alloy of silver and silicon, and the light-incident surface electrode is a fired body of the conductive paste according to any one of claims 1 to 10.

13. A method for manufacturing a solar cell, comprising: preparing 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 electrode so as to be electrically connected to the other surface of the semiconductor substrate of the first conductivity type; forming a passivation film so as to be in contact with the surface of the semiconductor layer of the second conductivity type; forming a light incident surface electrode on at least a portion of the surface of the passivation film; and irradiating the light incident surface of the solar cell with light from a point light source while applying a voltage between the back electrode and the light incident surface electrode so that a current flows in a direction opposite to the forward direction between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type, wherein the light incident surface electrode is a fired body of a conductive paste according to any one of claims 1 to 10.

14. Use of the conductive paste according to any one of claims 1 to 10 for forming an electrode of a solar cell.

15. The rear electrode is a fired body of a conductive paste for a rear electrode, the conductive paste for a rear electrode includes second conductive particles, a second organic vehicle, and a second glass frit, the second glass frit is substantially free of PbO, and the second glass frit includes 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 The solar cell according to claim 11 or 12, comprising at least one selected from the group consisting of ZnO and ZnO.

16. The second glass frit is TeO ２ The solar cell of claim 15 , comprising:

17. The rear electrode is a fired body of a conductive paste for a rear electrode, the conductive paste for a rear electrode includes second conductive particles, a second organic vehicle, and a second glass frit, the second glass frit is substantially free of PbO, and the second glass frit includes 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.

18. The second glass frit is TeO ２ The method for producing a solar cell according to claim 17 , comprising:

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