Electrically conductive paste, solar cell, and method for manufacturing solar cell

Abstract

Provided is an electrically conductive paste that is for manufacturing a crystalline silicon solar cell and that is suitable for formation of an electrode by a laser process. This electrically conductive paste is for formation of an electrode for a solar cell, the paste comprising: (A) electrically conductive particles; (B) organic vehicles; and (C) a glass frit, wherein the product BGF·G of the basicity BGF of the glass frit (C) and the contained amount G of the glass frit (C), where the unit of the contained amount G of the glass frit (C) is expressed as parts by weight in the electrically conductive paste with respect to 100 parts by weight of the electrically conductive particles (A) contained in the electrically conductive paste, is in the range of 0.25-1.45.

Description

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 primarily silver particles (silver powder).

[0004] Patent Document 1 describes a method for improving the ohmic contact behavior between a contact grid and an emitter layer of a silicon solar cell. Specifically, the method described in Patent Document 1 includes the following steps: a silicon solar cell (1) is first provided with the emitter layer, the contact grid (5), and a back contact (3). The contact grid (5) is electrically connected to one pole of a voltage source. A contact device electrically connected to the other pole of the voltage source is connected to the back contact (3). The voltage source applies a voltage in the opposite direction to the forward direction of the silicon solar cell (1) that is lower than the breakdown voltage. During this voltage application, a point light source (13) is induced across the sun-facing side of the silicon solar cell (1). A small section of the sun-facing side is point-illuminated, inducing a current flowing in a partial area. This current acts on the small section for 1 ms to 100 ms. The current is equivalent to a reduction of 10 to 30 times the magnitude of the short-circuit current of the silicon solar cell (1) measured under standard test conditions, based on the ratio of the area of ​​the subsection to the area of ​​the silicon solar cell (1).

[0005] 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.

[0006] Patent Document 3 describes a method for manufacturing a crystalline silicon solar cell using a conductive paste for forming an electrode of a crystalline silicon solar cell that contains an inorganic material. Patent Document 3 describes a conductive paste that contains conductive particles and glass frit as the inorganic material. The glass frit contained in the conductive paste of Patent Document 3 contains 70 to 90 wt % of PbO relative to 100 wt % of the glass frit, and Al ２ O ３ It is stated that it does not contain

[0007] Patent Document 4 describes a conductive composition containing silver powder, a glass powder containing PbO, and a vehicle made of an organic substance. Patent Document 4 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 4 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] JP 2019-525471 A JP 2021-513218 A JP 2011-86754 A JP 2009-231826 A

[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 therefore sometimes referred to as the passivation film 2. 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 establishing electrical continuity between the light-incident surface electrode 20 and the impurity diffusion layer 4. In the example shown in FIG. 5 , when the electrode pattern is fired, the electrode pattern fires through the anti-reflection film 2, causing the anti-reflection film 2 to disappear and bring the light-incident surface electrode 20 and the impurity diffusion layer 4 into 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 further collected by the light-incident side busbar electrode 20a. A metal ribbon for interconnection, which is surrounded by solder, is soldered to the light-incident side busbar electrode 20a, and current is extracted to the outside via this metal ribbon.

[0011] In order to obtain a crystalline 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] 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 Documents 1 and 2 describe specific examples 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 to the crystalline silicon solar cell so that a current flows in the reverse direction to the forward direction, and irradiating the light-incident surface of the solar cell with light from a point light source. Generally, the laser treatment process can improve the fill factor (FF) without reducing the open circuit voltage (Voc), which is a solar cell characteristic. 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 , when the laser treatment process is used, the anti-reflection film 2 is present in most of the area between the light-incident surface electrode 20 and the impurity diffusion layer 4. In the laser treatment process, the above-mentioned predetermined voltage is applied to the pn junction so that a current flows in the reverse direction to the forward direction. Light is irradiated from a point light source to generate carriers (electrons and holes), causing a current to flow in a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4, resulting in localized heating. This localized heating creates a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4 where the impurity diffusion layer 4 is not present. As a result, as shown in FIG. 13 , it is believed that a small electrically conductive area (locally conductive portion) of an AgSi alloy 30 (an alloy of silver and silicon) is formed locally in the impurity diffusion layer 4 in contact with the light-incident surface electrode 20. Because the AgSi alloy 30 is formed locally in a limited area, it is not shown in FIG. 1 . 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, except for the area where the local conduction 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).

[0013] Furthermore, in the case of conventional crystalline silicon solar cells, when the light-incident side surface electrode 20 is formed, the electrode pattern of conductive paste is fired, causing the anti-reflection film 2 to fire through and come into contact with the impurity diffusion layer 4. This fire-through process 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 anti-reflection film 2 is not generally fired through when the light-incident side surface electrode 20 is formed. Therefore, by using the laser treatment process, damage to the impurity diffusion layer 4 can be suppressed.

[0014] Therefore, an object of the present invention is to provide a conductive paste suitable for forming electrodes by a laser treatment process for the production of crystalline silicon solar cells.

[0015] Another object of the present invention is to provide a method for manufacturing a high-performance crystalline silicon solar cell using a conductive paste suitable for forming electrodes by a laser treatment process. Another object of the present invention is to provide a high-performance crystalline silicon solar cell manufactured by a manufacturing method that includes forming electrodes by a laser treatment process.

[0016] In order to solve the above problems, the present invention has the following configuration.

[0017] (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 has a basicity B ＧＦand 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.

[0018] (Configuration 2) Configuration 2 is the conductive paste of configuration 1, in which the (A) conductive particles include silver particles.

[0019] (Configuration 3) Configuration 3 is the conductive paste of configuration 1 or 2, in which the organic vehicle (B) contains at least one selected from ethyl cellulose, rosin ester, acrylic, and an organic solvent.

[0020] (Configuration 4) Configuration 4 is the (C) content C of PbO in mol% in the glass frit ＰｂＯ and the content G of the glass frit (C), ＰｂＯ The conductive paste of any one of configurations 1 to 3, wherein G is in the range of 20 to 139.

[0021] (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 0.3 to 4.0 parts by weight.

[0022] (Configuration 6) Configuration 6 is the conductive paste of any one of configurations 1 to 4, in which the content G of the glass frit (C) is 0.5 to 1.5 parts by weight.

[0023] (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 300 to 600°C.

[0024] (Configuration 8) In Configuration 8, the glass frit (C) is a glass frit containing ZnO, V ２ O ５ , W.O. ３ and Nb ２ O ３ 8. The conductive paste of any one of configurations 1 to 7, comprising at least one selected from the following:

[0025] (Configuration 9) Configuration 9 is the conductive paste of any one of Configurations 1 to 8, further comprising (D) aluminum particles.

[0026] (Configuration 10) Configuration 10 is the conductive paste of Configuration 9, in which the content of the (D) aluminum particles in the conductive paste is 0.1 to 2.0 parts by weight, based on 100 parts by weight of the (A) conductive particles in the conductive paste.

[0027] (Configuration 11) Configuration 11 is a conductive paste for forming an electrode of a solar cell, the 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 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 the conductive paste for forming the light-incident-side surface electrode.

[0028] (Configuration 12) Configuration 12 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 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 the conductive paste according to any one of Configurations 1 to 10.

[0029] (Configuration 13) Configuration 13 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 part 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 14) Configuration 14 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 according to any one of Configurations 1 to 10.

[0031] (Configuration 15) Configuration 14 is the use of the conductive paste of any one of Configurations 1 to 11 to form an electrode of a solar cell.

[0032] According to the present invention, a conductive paste suitable for forming an electrode by a laser treatment process can be provided for the production of a crystalline silicon solar cell.

[0033] The present invention also provides a method for manufacturing a high-performance crystalline silicon solar cell using a conductive paste suitable for forming electrodes by a laser treatment process.The present invention also provides a high-performance crystalline silicon solar cell manufactured by a manufacturing method including forming electrodes by a laser treatment process.

[0034] 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 schematic diagram of the light-incident side surface of a crystalline silicon solar cell. FIG. 3 is an example of a schematic diagram of the back surface of a crystalline silicon solar cell. FIG. 4 is an example of a cross-sectional schematic diagram of a bifacial crystalline silicon solar cell using the conductive paste of this embodiment. FIG. 5 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. 6 is a plan view schematic diagram showing a resistivity measurement pattern for an electrode formed using the conductive paste. FIG. 7 is a plan view schematic diagram showing a contact resistance measurement pattern for an electrode formed using the conductive paste. FIG. 8 is a plan view schematic diagram showing a photoluminescence imaging (PL) measurement pattern for an electrode formed using the conductive paste. 11 is an image of the photoluminescence intensity measured by the photoluminescence imaging method (PL method) for the sample of Example 4. 12 is an image of the photoluminescence intensity measured by the photoluminescence imaging method (PL method) for Comparative Example 1. 13 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 sample in which the light-incident side surface electrode 20 is formed using the same conductive paste as the sample shown in FIG. 11. 14 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 sample in which the light-incident side surface electrode 20 is formed using the same conductive paste as the sample shown in FIG. 12. 15 is a cross-sectional SEM photograph (magnification: 20,000 times) showing that an AgSi alloy, which is a locally minute electrically conductive portion (locally conductive portion), is formed in the impurity diffusion layer in contact with the light-incident side surface electrode of a sample prepared under the same conditions as in Example 5.

[0035] 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.

[0036] 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.

[0037] In this specification, glass frit is a material containing multiple types of oxides, for example, metal oxides as main materials, and is generally used in the form of glassy particles.

[0038] 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 conductive paste of this embodiment is characterized by appropriately controlling the basicity of the (C) glass frit and the content of the (C) glass frit in the conductive paste.

[0039] 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 are in a trade-off relationship, and it is difficult to simultaneously increase both FF and Voc. For example, Patent Documents 1 and 2 disclose that employing a laser treatment process during the manufacture of a crystalline silicon solar cell can improve the ohmic contact behavior between a grid-shaped electrode, which is the light-incident surface electrode, and an impurity diffusion layer (emitter layer), thereby significantly reducing the contact resistance between the light-incident surface electrode and the impurity diffusion layer. Therefore, by performing a laser treatment process, FF can be improved without reducing Voc.

[0040] The present inventors have discovered that when a laser treatment process is applied to a solar cell having a light-incident surface electrode formed using a conventional conductive paste (e.g., the conductive paste described in Patent Document 3), it adversely affects the anti-reflection film (passivation film) and the impurity diffusion layer (and substrate), resulting in a decrease in the conversion efficiency of the solar cell. The present inventors have also discovered that this is due to the conventional conductive paste having excessively strong fire-through (reactivity) with the anti-reflection film (passivation film). Furthermore, the present inventors have found that the reactivity of the glass frit with the anti-reflection film (passivation film) can be made appropriate by adjusting the basicity and content of the glass frit within an appropriate range. Based on the above findings, the present inventors have discovered a conductive paste that can be preferably used when producing crystalline silicon using a laser treatment process, and have arrived at the present invention.

[0041] 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 of the solar cell without impairing the function of the anti-reflection film 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 a light-incident surface electrode by a laser treatment process when manufacturing a crystalline silicon solar cell.

[0042] 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.

[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] 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.

[0046] 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, 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.

[0047] The conductive paste of the present invention will be described below using as an example the case of forming a light-incident side 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.

[0048] 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 preferably 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 preferably 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 SiN / SiO ｘ It is preferable to form a film of SiO ｘ The film can be a native oxide film of a silicon substrate.

[0049] 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.

[0050] In the laser treatment process, the conductive paste of this embodiment is subjected to the application of the above-described predetermined voltage and irradiation with light from a point light source, 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. 13 , a localized electrically conductive portion (local conduction portion) of AgSi alloy 30 is formed in the impurity diffusion layer 4 (silicon emitter layer) in contact with the light-incident side surface electrode 20. This locally formed electrically conductive portion is believed to enable good electrical conduction between the light-incident side surface electrode 20 and the impurity diffusion layer 4 (silicon emitter layer). Therefore, the conductive paste used to form the light-incident side surface electrode by the laser treatment process must have properties different from conventional conductive pastes (conductive pastes that can fire through the anti-reflection coating 2).

[0051] The conductive paste of this embodiment will be specifically described.

[0052] <(A) Conductive Particles> The conductive paste of this embodiment contains (A) conductive particles.

[0053] 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.

[0054] 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).

[0055] 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.

[0056] 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).

[0057] <(B) Organic Vehicle> The conductive paste of this embodiment contains (B) an organic vehicle.

[0058] The organic vehicle may contain an organic binder and a solvent. The organic binder and the solvent serve to adjust the viscosity of the conductive paste, and are not particularly limited. The organic binder may be dissolved in a solvent before use.

[0059] 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.

[0060] 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.

[0061] 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, diethylene glycol monobutyl ether acetate (butyl carbitol acetate), etc.). The amount of the solvent added is usually 0.5 to 30 parts by weight, and preferably 2 to 25 parts by weight, per 100 parts by weight of the silver particles. A specific example of the organic solvent is diethylene glycol monobutyl ether acetate (butyl carbitol acetate).

[0062] <(C) Glass Frit> The conductive paste of the present embodiment contains (C) glass frit.

[0063] 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 0.25 to 1.45, preferably in the range of 0.3 to 1.4, and more preferably in the range of 0.4 to 1.2. 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.

[0064] The basicity of the glass frit can be calculated by the method described in Patent Document 4 (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.

[0065] 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:

[0066] A ｉ =Z ｉ ・Z ０２－ / (r ｉ +r ０２－ ) ２ = Zi 2 / (r ｉ +1.40) ２ Z ｉ : valence of cation, oxygen ion is 2r ｉ : Ionic radius of cation (Å)

[0067] 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

[0068] This B ｉBCaO = 1, BSiO ２ When normalized as = 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

[0069] 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.

[0070] 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 BCaO = 1, BSiO ２ = 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 frit ＧＦ and the product B of the content G ＧＦ ・G is also a dimensionless number.

[0071] The basicity (B ＧＦ The basicity (B) is preferably 0.30 to less than 0.80, more preferably 0.35 to 0.75, and even more preferably 0.40 to 0.70. 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.

[0072] The content G of the glass frit in the conductive paste of this embodiment is preferably 0.3 to 4.0 parts by weight, more preferably 0.4 to 3.0 parts by weight, even more preferably 0.5 to 2.0 parts by weight, and particularly preferably 0.5 to 1.5 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 temperature, the reactivity of the glass frit with the passivation film can be made appropriate.

[0073] The glass frit contained in the conductive paste of this embodiment is PbO, SiO ２ , Al ２ O ３ , B ２ O ３ , ZnO, V ２ O ５ , W.O. ３ and Nb ２ O ３ The glass frit contained in the conductive paste of the present embodiment preferably contains one or more selected from PbO, SiO ２ , Al ２ O ３ , B ２ O ３ and ZnO.

[0074] In the conductive paste of this embodiment, (C) the glass frit is ZnO, V ２ O ５ , W.O. ３ and Nb ２ O ３ By including at least one of these oxides in the glass frit, the basicity of the glass frit can be adjusted to an appropriate range.

[0075] The glass frit preferably contains PbO. The content of PbO in the glass frit (100 mol%) is preferably 25 to 60 mol%, more preferably 30 to 55 mol%, and even more preferably 40 to 55 mol%. By including PbO in the glass frit, reactivity with the passivation film can be suppressed and contact resistance can be reduced.

[0076] The glass frit is SiO ２ The glass frit (100 mol%) preferably contains SiO ２ The content of the glass frit is preferably 20 to 65 mol %, more preferably 25 to 60 mol %. ２ By including the compound, the reactivity with the passivation film can be suppressed.

[0077] The glass frit is Al ２ O ３ The glass frit (100 mol%) preferably contains Al. ２ O ３ The content of Al is preferably 3.0 to 6.8 mol %, more preferably 3.5 to 6 mol %. ２ O ３ By including the compound, the reactivity with the passivation film can be suppressed.

[0078] The glass frit is B ２ O ３ It is preferable that the glass frit (100 mol%) contains B. ２ O ３ The content is preferably 3.0 to 15 mol %, more preferably 3.5 to 12 mol %.

[0079] The glass frit preferably contains ZnO. The content of ZnO in the glass frit (100 mol %) is preferably 5 to 20 mol %, and more preferably 8 to 15 mol %. By including ZnO in the glass frit, the basicity of the glass frit can be adjusted to an appropriate range.

[0080] The conductive paste of this embodiment has (C) a PbO content C in the glass frit in units of mol%. ＰｂＯ and (C) the product C of the glass frit content G ＰｂＯ G is preferably in the range of 20 to 139, more preferably in the range of 22 to 130, and even more preferably in the range of 26 to 105. ＰｂＯ If G exceeds 139, the reactivity between the glass frit and the passivation film becomes too high. ＰｂＯ If G is less than 20, the contact resistance between the resulting electrode and the impurity diffusion layer becomes too high.

[0081] In the conductive paste of this embodiment, the glass transition point (Tg) of the (C) glass frit is preferably 300 to 600°C, more preferably 320 to 500°C, and even more preferably 350 to 450°C. By setting the glass transition point (Tg) of the (C) glass frit to 300°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 and the impurity diffusion layer.

[0082] The glass transition point (Tg) can be measured as follows. That is, 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. The temperature is raised from room temperature to 900°C at a temperature rise rate of 10°C / min as the measurement conditions, 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 thus obtained can be determined as the glass transition point Tg.

[0083] 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.

[0084] The glass frit particles can be one type of particles containing a predetermined amount of each of the required oxides. Alternatively, particles made of a single oxide can be used as different particles for each of the required oxides. Alternatively, a combination of multiple types of particles with different compositions of the required oxides can be used.

[0085] <Aluminum Particles> The conductive paste of this embodiment may further contain (D) aluminum particles. The (D) aluminum particles may be contained as particles separate from the (A) conductive particles.

[0086] In a crystalline silicon substrate, aluminum has properties as a p-type impurity. When a conductive paste printed on crystalline silicon is fired, the aluminum in the conductive paste diffuses into the crystalline silicon and becomes a p-type impurity. Therefore, when an electrode is formed on the surface of a p-type impurity diffusion layer of a crystalline silicon substrate, a conductive paste containing aluminum particles can provide low contact resistance between the electrode and the p-type impurity diffusion layer. Therefore, when an electrode is formed on the surface of a p-type impurity diffusion layer of a crystalline silicon substrate, it is preferable that the conductive paste contain aluminum particles.

[0087] When the conductive paste of this embodiment contains (D) aluminum particles, the content of (D) aluminum particles in the conductive paste is preferably 0.1 to 2.0 parts by weight, and more preferably 0.5 to 2.0 parts by weight, based on 100 parts by weight of the content of (A) conductive particles in the conductive paste. By containing a predetermined amount of aluminum particles in the conductive paste, low contact resistance can be obtained between the electrode and the p-type impurity diffusion layer.

[0088] On the other hand, when an electrode is formed on an n-type impurity diffusion layer of a crystalline silicon substrate or on the surface of an n-type crystalline silicon substrate, aluminum in the conductive paste will diffuse into the n-type impurity diffusion layer, etc., adversely affecting the solar cell characteristics. Therefore, when an electrode is formed on an n-type impurity diffusion layer of a crystalline silicon substrate or on the surface of an n-type crystalline silicon substrate, it is preferable that the conductive paste of this embodiment does not contain (D) aluminum particles (the content of (D) aluminum particles is zero).

[0089] The aluminum particles mainly contain aluminum elements. The purity of the aluminum in the aluminum particles is, for example, preferably 99.7% or more, more preferably 99.9% or more. The aluminum particles may contain impurities other than aluminum, for example, other metal elements that are inevitably contained. The aluminum particles may also contain alloys of aluminum and other metal elements, oxides of aluminum, etc.

[0090] The shape of the aluminum particles is exemplified by, but not limited to, a spherical shape or an oval spherical shape. From the viewpoints of good printability and good reactivity with a semiconductor substrate, the shape of the aluminum particles is preferably spherical.

[0091] The average particle size (D50) of the aluminum particles is not particularly limited. It is preferable for the average particle size (D50) of the aluminum particles to be 1 μm or more and 20 μm or less in terms of improving the printability of the paste composition and improving the reactivity with the semiconductor substrate. A more preferable average particle size (D50) of the aluminum particles is 2 to 4 μm.

[0092] <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.

[0093] 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.

[0094] 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.

[0095] <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.

[0096] 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.

[0097] <Solar Cell> Next, a solar cell of the present embodiment will be described. This embodiment is a solar cell in which at least a portion of an electrode is formed using the conductive paste described above. Figures 1 and 4 show schematic cross-sectional views of a crystalline silicon solar cell.

[0098] 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 viewpoints of safety and cost as a solar cell, the material of the semiconductor substrate is preferably crystalline silicon (single crystal silicon, polycrystalline silicon, etc.).

[0099] 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.

[0100] The semiconductor substrate of the first conductivity type is an n-type semiconductor substrate or a p-type semiconductor substrate. The semiconductor layer of the second conductivity type is a p-type semiconductor layer or an n-type semiconductor layer. When the semiconductor substrate is an n-type semiconductor substrate, a p-type semiconductor layer (p-type impurity diffusion layer 4) is disposed on one surface of the semiconductor substrate. When the semiconductor substrate is a p-type semiconductor substrate, an n-type semiconductor layer (n-type impurity diffusion layer 4) is disposed on one surface of the semiconductor substrate. The material of the semiconductor substrate is preferably silicon. Therefore, the semiconductor substrate is preferably a crystalline silicon substrate.

[0101] The passivation film can be an anti-reflection film 2. The passivation film is preferably a thin film made of silicon nitride.

[0102] 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.

[0103] 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.

[0104] The first conductivity type semiconductor substrate of the solar cell of this embodiment is preferably an n-type semiconductor substrate, more preferably an n-type crystalline silicon substrate. The second conductivity type semiconductor layer of the solar cell of this embodiment is preferably a p-type semiconductor layer, more preferably a p-type impurity diffusion layer 4 made of crystalline silicon. Generally, the mobility of electrons, which are carriers in an n-type crystalline silicon substrate, is higher than the mobility of holes, which are carriers in a p-type crystalline silicon substrate. Therefore, in order to obtain a solar cell with high conversion efficiency, it is advantageous to use an n-type crystalline silicon substrate.

[0105] In the following explanation, a solar cell will be described as an example in which the semiconductor substrate of the first conductivity type is an n-type crystalline silicon substrate 1 and the semiconductor layer of the second conductivity type is a p-type impurity diffusion layer 4 (sometimes simply referred to as the "impurity diffusion layer 4").

[0106] As shown in FIG. 1 , when a 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 FIG. 13 , 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 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.

[0107] 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.

[0108] An example of a cross-sectional schematic diagram of a bifacial crystalline silicon solar cell is shown in Figure 4. The bifacial crystalline silicon solar cell shown in Figure 4 has an impurity diffusion layer 4, an anti-reflection film 2, and a back surface passivation film 14. In the bifacial crystalline silicon solar cell, the conductive paste of this embodiment is used to form the light-incident-side surface electrode 20 (particularly, the finger electrode 20b) on the light-incident surface and the back surface electrode 15 (back surface finger electrode 15c), and a laser treatment process can be used 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 14.

[0109] 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 solar cells.

[0110] The busbar electrodes of the crystalline silicon solar cell shown in Fig. 1 include a light-incident side busbar electrode 20a shown in Fig. 2 and a backside TAB electrode 15a as shown in Fig. 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. Current generated by the solar cell is extracted to the outside of the crystalline silicon solar cell through these metal ribbons. The bifacial crystalline solar cell shown in Fig. 4 can also have a light-incident side busbar electrode 20a and a backside TAB electrode having a shape similar to that of the light-incident side busbar electrode 20a.

[0111] 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.

[0112] 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.

[0113] <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.

[0114] The method for manufacturing a solar cell of this embodiment includes the steps of printing the above-mentioned conductive paste on the surface of the anti-reflection film 2 on the semiconductor layer of the second conductivity type (impurity diffusion layer 4), drying, and firing to form an electrode (light-incident surface electrode). The method for manufacturing a solar cell of this embodiment will be described in more detail below.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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. In this case, an n-type impurity diffusion layer 4 is formed by diffusing an n-type impurity such as P (phosphorus).

[0119] 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 / □.

[0120] 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.

[0121] 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 a first conductivity type semiconductor substrate (n-type crystalline silicon substrate). 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.

[0122] 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 .

[0123] 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 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.

[0124] 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.

[0125] 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).

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] In the manner described above, the crystalline silicon solar cell of this embodiment can be manufactured.

[0134] 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 and lead-free solders, can be used as the solder.

[0135] 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.

[0136] The present embodiment will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0137] In the examples and comparative examples, the performance of the conductive pastes of the examples and comparative examples of this embodiment was evaluated by using a measurement substrate simulating a single-crystal silicon solar cell to evaluate the degree of deterioration of the passivation film using the photoluminescence imaging method (PL method), as well as the contact resistance and specific resistance of the formed electrodes.

[0138] <Materials and Preparation Proportions of Conductive Paste> Tables 1 to 3 show the compositions of the conductive pastes of Examples 1 to 14 and Comparative Examples 1 to 4. The compositions shown in Tables 1 to 3 and the compositions of each component below are shown in parts by weight of each component when the (A) silver particles are taken as 100 parts by weight. The components contained in the conductive paste are as follows.

[0139] (A) Silver Particles Table 4 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 the Examples and Comparative Examples. Tables 1 to 3 show the blending amounts of silver particles A1 and A2 in the conductive pastes of the Examples and Comparative Examples. 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 analyzer, Macsoeb (manufactured by MOUNTEC). The BET specific surface area was measured by the single-point BET method using nitrogen gas adsorption after pre-drying at 100°C and flowing nitrogen gas through it for 10 minutes.

[0140] (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.

[0141] (C) Glass Frit Table 5 shows the composition, basicity, and glass transition point of glass frits A to G used in the conductive pastes of the Examples and Comparative Examples. The average particle size (D50) of glass frits A to G was 2 μm. Tables 1 to 3 show the type (any of A to G) and content G (parts by weight) of (C) glass frit in the conductive pastes of the Examples and Comparative Examples.

[0142] The glass transition points of glass frits A to G were measured. Table 5 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 platinum cells as samples, and alumina powder was used as a standard sample. DTA curves were 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 point (extrapolated point) of the first endotherm in the DTA curve was taken as the glass transition point.

[0143] Glass frits A to G 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 yielded a glass frit with an average particle size (D50) of 2 μm.

[0144] (D) Aluminum particles Table 6 shows the product number, manufacturer, shape, and average particle diameter (D50) of aluminum particles D1 and D2 used in the conductive pastes of Examples and Comparative Examples. Tables 1 to 3 show the blending amounts (parts by weight) of aluminum particles D1 and D2 in the conductive pastes of Examples and Comparative Examples.

[0145] Next, materials in the weight proportions shown in Tables 1 to 3 were mixed in a planetary mixer, and then dispersed in a three-roll mill to form a paste, thereby preparing conductive pastes for the examples and comparative examples.

[0146] <Evaluation of the Reactivity of Conductive Paste with Passivation Films by Photoluminescence Imaging (PL)> The reactivity of conductive pastes with passivation films was evaluated using the photoluminescence imaging method (referred to as the "PL method"). PL is a non-destructive, non-contact method that allows for rapid evaluation of the reactivity of conductive pastes with passivation films. Specifically, PL is a method in which a sample is irradiated with light with energy greater than the forbidden band width to cause light emission, and the state of defects within the crystal and surface / interface defects is evaluated based on the state of the light emission. When a sample contains defects within a single-crystal silicon substrate and surface / interface defects, the defects act as recombination centers for electron-hole pairs generated by light irradiation, correspondingly reducing the band-edge emission intensity of photoluminescence. In other words, when the passivation film is eroded by a printed / fired electrode, forming surface defects at the interface between the passivation film and the single-crystal silicon substrate (i.e., the surface of the single-crystal silicon substrate), the photoluminescence intensity of the area where the surface defects are formed (i.e., the area of ​​the electrode formed on the sample) decreases. The reactivity of the prototype conductive paste with the passivation can be evaluated based on the intensity of this photoluminescence.

[0147] The method for fabricating a prototype substrate for evaluation by the PL method is as follows.

[0148] The substrate used was an n-type single crystal silicon substrate (substrate thickness: 200 μm).

[0149] 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.

[0150] Next, textured structures (concave and convex shapes) were formed on both surfaces of the substrate by wet etching. Specifically, pyramidal textured structures were formed on both surfaces (the light-incident surface and the back surface) by wet etching (sodium hydroxide aqueous solution). The substrate was then washed with an aqueous solution containing hydrochloric acid and hydrogen peroxide. Next, boron was implanted into one surface (the light-incident surface) of the substrate having the textured structure to form a p-type diffusion layer to a depth of approximately 0.5 μm. The sheet resistance of the p-type diffusion layer was 60 Ω / □. Furthermore, phosphorus was implanted into the other surface (the 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 implantations were simultaneously performed by thermal diffusion.

[0151] 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, and then a silicon nitride film was formed to a thickness of about 60 nm by plasma CVD using silane gas and ammonia gas. 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).

[0152] The substrate thus obtained was cut into a square of 25 mm x 25 mm to prepare substrate 1.

[0153] Next, as shown in FIG. 8, a square electrode pattern 22 of 13 mm×13 mm was printed on the surface of the substrate 1 using a conductive paste for forming electrodes, and then dried.

[0154] The substrate 1, on whose surface the electrode pattern 22 was printed using the conductive paste as described above, was fired using a belt furnace (firing furnace) CDF7210 manufactured by Despatch Industries, Inc., at a peak temperature of 720°C and an in-out time of the firing furnace of 50 seconds.

[0155] In this manner, a substrate (sample) for PL measurement was prepared.

[0156] Measurements using the PL method were carried out using a Photoluminescence Imaging System (model number LIS-R2) manufactured by BT Imaging Co., Ltd. Light from an excitation light source (wavelength 650 nm, output 3 mW) was irradiated onto the back surface of the substrate (the surface on which the electrode pattern of the light-incident surface electrode 20 was not formed), and an image of the photoluminescence emission intensity was obtained.

[0157] 9 and 10 show images of the photoluminescence intensity measured by the PL method.

[0158] FIG. 9 shows an image of the photoluminescence intensity measured by the PL method for a sample in which an electrode pattern 22 was formed using the conductive paste of Example 4. As is clear from FIG. 9 , the image of the portion in which the electrode pattern 22 was formed is brighter than that shown in FIG. 10 (described later). This indicates that the decrease in photoluminescence intensity in the portion in which the electrode pattern of the light-incident side surface electrode 20 was formed was suppressed. Therefore, in the case of the sample shown in FIG. 9 , the passivation function of the passivation film was maintained by forming the electrode pattern of the light-incident side surface electrode 20. Therefore, in the case of the sample in FIG. 9 in which the conductive paste of Example 4 was used, it can be said that the surface defect density on the surface of the single-crystal silicon substrate 1 did not increase.

[0159] On the other hand, the sample shown in Fig. 10 was fabricated using the conductive paste of Comparative Example 1 to form the light-incident side surface electrode 20. As is clear from Fig. 10, the image of the portion where the electrode pattern of the light-incident side surface electrode 20 was formed is darker than that of the sample shown in Fig. 9. This indicates that the photoluminescence intensity of the portion where the electrode pattern of the light-incident side surface electrode 20 was formed was reduced. Therefore, in the case of the sample shown in Fig. 10 using the conductive paste of Comparative Example 1, the formation of the electrode pattern of the light-incident side surface electrode 20 impaired the passivation function of the passivation film, and the surface defect density on the surface of the single-crystal silicon substrate 1 increased.

[0160] Tables 1 to 3 show the measured photoluminescence intensity (PL value) values ​​for the examples and comparative examples. The PL value is the average value of the photoluminescence intensity near the electrode. The PL value is a numerical value that varies depending on the spectrum and intensity of the irradiated light from the excitation light source, the optical system used for measurement, and other factors, and is expressed in arbitrary units. When the conditions for measuring the PL value, such as the spectrum and intensity of the irradiated light and the optical system used for measurement, are the same, the degree of carrier recombination (the degree of degradation of the passivation function) in each sample can be evaluated by comparing the magnitude of the PL value of each sample. The higher the PL value, the better the passivation function provided by the passivation film.

[0161] For confirmation, cross sections of the samples shown in Figures 9 and 10 were observed using a scanning electron microscope (SEM). Figure 11 shows a cross-sectional SEM photograph (magnification: 20,000 times) of the vicinity of the passivation film of a sample (corresponding to Example 4) in which the light-incident-side surface electrode 20 was formed using the same conductive paste under the same conditions as the sample shown in Figure 9. Figure 12 shows a cross-sectional SEM photograph (magnification: 20,000 times) of the vicinity of the passivation film of a sample (corresponding to Comparative Example 1) in which the light-incident-side surface electrode 20 was formed using the same conductive paste under the same conditions as the sample shown in Figure 10. As is clear from Figure 11, in the case of samples with high PL values, the antireflection film 2 (passivation film) maintained its shape almost unchanged even after the light-incident-side surface electrode 20 was formed, and the antireflection film 2 (passivation film) was not eroded by the glass frit. On the other hand, as is clear from FIG. 12, in the case of samples with low PL values, the anti-reflection film 2 (passivation film) is eroded by the glass frit, and most of the anti-reflection film 2 (passivation film) is lost. In other words, the conductive paste of Comparative Example 1 can be said to be a conductive paste that is capable of firing through the passivation film. Note that the portion marked "Glass" in FIG. 12 is a glass component resulting from the glass frit contained in the conductive paste. From the above, it is clear that the reactivity of a conductive paste with the anti-reflection film 2 (passivation film) can be evaluated by measuring the PL value using the above-mentioned PL method.

[0162] As is clear from Tables 1 to 3, the PL values ​​of the samples obtained using the conductive pastes (conductive compositions) of Examples 1 to 14 of this embodiment were 5522 (Example 2) or higher. In contrast, the PL value of Comparative Example 1 was 4900, and the PL value of Comparative Example 2 was a low value of 4800. Therefore, it can be said that the passivation function of the passivation film is lower in Comparative Examples 1 and 2 compared to the examples of this embodiment, resulting in reduced solar cell performance. This also suggests that when electrodes are formed using the conductive pastes of Comparative Examples 1 and 2, the open circuit voltage (Voc), one of the solar cell characteristics, is reduced.

[0163] In addition, when a cross section of the sample of Comparative Example 2 having a PL value of 4800 near the interface between the electrode and the silicon substrate was observed by SEM, it was confirmed that, similar to Comparative Example 1 having a PL value of 4900 (see FIG. 12 ), no anti-reflection film 2 (passivation film) remained in the sample of Comparative Example 2. On the other hand, when a cross section of the sample of Example 2 having a PL value of 5522 near the interface between the electrode and the silicon substrate was observed by SEM, it was confirmed that, similar to Example 4 (see FIG. 11 ), the anti-reflection film 2 (passivation film) remained between the electrode and the silicon substrate in the sample of Example 2. Therefore, from the evaluation results by the photoluminescence imaging method (PL method) used in the evaluation of Examples 1 to 14 and Comparative Examples 1 to 4, it was revealed that when the PL value is 5000 or more, preferably when the PL value is 5500 or more, the anti-reflection film 2 (passivation film) remains between the electrode and the silicon substrate, and a solar cell with high performance (particularly a high Voc) can be expected to be obtained.

[0164] It was confirmed that the PL value did not change significantly before and after the laser treatment process, which is thought to be because the laser treatment process is a process for forming a small localized electrically conductive portion and does not affect the majority of the anti-reflection film 2 (passivation film).

[0165] <Measurement of Contact Resistance Before Laser Treatment Process> In the same manner as the sample for measurement by the PL method, a p-type impurity diffusion layer was formed on one surface of an n-type crystalline silicon substrate (substrate thickness: 200 μm), and further, a silicon nitride film (anti-reflection film 2 serving as a passivation film) having a thickness of approximately 60 nm was formed on the p-type impurity diffusion layer, thereby obtaining a substrate for measuring contact resistance.

[0166] The conductive pastes used to form electrodes on the surface (light incident surface) of the substrate on which the p-type diffusion layer was formed in the single crystal silicon solar cells of the Examples and Comparative Examples were those shown in Tables 1 to 3.

[0167] The conductive paste was printed by screen printing onto the anti-reflection film 2 of the substrate in a 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, and then dried at 150° C. for approximately 1 minute.

[0168] A commercially available Ag paste was printed by screen printing as the back electrode 15 (the electrode on the surface where 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 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 15 was approximately 20 μm. It was then simultaneously fired on both sides using a belt furnace (firing furnace) CDF7210 manufactured by Despatch Industries, Inc., at a peak temperature of 720°C with an in-out time of the furnace of 50 seconds. In this manner, a single-crystal silicon solar cell was fabricated.

[0169] The solar cell thus obtained was cut into a 15 mm × 15 mm square as shown in Figure 7 to obtain a contact resistance measurement sample. As shown in Figure 7, light incident side finger electrodes 20b, each 60 μm wide and 15.0 mm long, were arranged at 1.5 mm intervals on the light incident side surface of the cut solar cell (contact resistance measurement sample). These light incident side finger electrodes 20b were used as contact resistance measurement patterns. The contact resistance of the contact resistance measurement patterns of the examples and comparative examples before the laser treatment process was determined by the TLM (Transfer Length Method) method using a GP 4 TEST Pro manufactured by GP Solar.

[0170] Three samples were prepared under the same conditions for measuring the contact resistance, and the measured value was calculated as the average value of the three samples.

[0171] <Measurement of Contact Resistance After Laser Treatment Process> Similar to the measurement of contact resistance before the laser treatment process described above, a single-crystal silicon solar cell was fabricated, and a laser treatment process was performed on the light-incident surface of this solar cell. That is, laser light was irradiated onto the light-incident surface of the solar cell while applying a negative voltage to the back electrode 15 and a positive voltage to each of the light-incident 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 irradiated laser light intensity was 100 W / cm. ２ The voltage application and laser light irradiation time were set to 2 seconds.

[0172] The solar cell thus obtained was cut into a square of 15 mm x 15 mm as shown in Figure 7, and the contact resistance after the laser treatment process was measured in the same manner as in the contact resistance measurement before the laser treatment process.

[0173] First, the contact resistance before the laser treatment process was 450 mΩ cm ２ If the contact resistance is 300 Ω cm or less, it can be used as an electrode for a solar cell by subjecting it to a laser treatment process. ２ If the thickness is below 1000 nm, the material can be more preferably used as an electrode for a solar cell by subjecting it to a laser treatment process.

[0174] Next, the value of the contact resistance after the laser treatment process is 20 mΩ cm ２ Since the contact resistance is 9 mΩ cm or less, it can be preferably used as the electrode 20 of a solar cell. ２ The following conditions can be more preferably met for use as an electrode for a solar cell:

[0175] As is clear from Tables 1 to 3, the contact resistance before the laser treatment process of the samples obtained using the conductive pastes (conductive compositions) of Examples 1 to 14 of this embodiment was 440 mΩ cm ２ In contrast, the contact resistance of Comparative Example 3 was 640 mΩ cm ２The contact resistance of Comparative Example 4 was 804 mΩ cm ２ Furthermore, the contact resistance after the laser treatment process of the samples obtained using the conductive pastes (conductive compositions) of Examples 1 to 14 of this embodiment was 18 mΩ cm ２ In contrast, the contact resistance of Comparative Example 3 was 21 mΩ cm ２ The contact resistance of Comparative Example 4 was 32 mΩ cm ２ Therefore, it can be said that the contact resistance in Comparative Examples 3 and 4 is higher than that in the Examples of this embodiment, and therefore the performance of the solar cell is degraded. In other words, it can be said that Comparative Examples 3 and 4 cannot be preferably used as electrodes for solar cells.

[0176] [Measurement of Resistivity] The resistivity of the conductive film patterns for resistivity measurement obtained by firing the conductive pastes of the Examples and Comparative Examples was measured.

[0177] The resistivity of the examples and comparative examples was measured by the following procedure. A silicon substrate measuring 15 mm in width, 15 mm in length, and 180 μm in thickness was prepared. A pattern made of conductive paste as shown in FIG. 6 was printed on the substrate using a 325-mesh stainless steel screen.

[0178] Next, the silicon substrates on whose surfaces the patterns of the examples and comparative examples were printed were simultaneously fired on both sides using a belt furnace (firing furnace) CDF7210 manufactured by Despatch Industries, Inc., at a peak temperature of 720°C and an in-out time of the furnace of 50 seconds. In this manner, samples for resistivity measurement were prepared.

[0179] The resistivity of the conductive film patterns of the resistivity measurement samples obtained by firing the conductive pastes of the Examples and Comparative Examples was measured. First, the resistance value was measured using a four-terminal method using a Toyo Corporation Model 2001 multimeter. The cross-sectional area of ​​the conductive film pattern was measured using a Lasertec Corporation OPTELICS H1200 confocal microscope and a Lasertec Corporation 1500SD2 surface roughness and shape measuring instrument. Measurements were taken at 50 locations over a 1.6 mm range, and the average value was calculated. The resistivity was calculated using the cross-sectional area and the measured resistance value.

[0180] Four samples were prepared under the same conditions for measuring the resistivity, and the measured value was calculated as the average of the four values. The measurement results are shown in Tables 1 to 3.

[0181] As is clear from Tables 1 to 3, the resistivity of the conductive films obtained using the conductive pastes (conductive compositions) of Examples 1 to 14 of this embodiment and Comparative Examples 1 to 4 was 8 μΩ cm or less (Examples 11 and 18). Generally, a resistivity of 15 μΩ cm or less can be said to be suitable for use as an electrode. Therefore, it can be said that an electrode with a desirable resistivity can be obtained by using the conductive paste of this embodiment.

[0182] As mentioned above, when the conductive pastes of Comparative Examples 1 and 2 are used, there is a problem of low PL values. Furthermore, when the conductive pastes of Comparative Examples 3 and 4 are used, there is a problem of high contact resistance between the electrode and the impurity diffusion layer. Therefore, it is clear that high-performance solar cells cannot be manufactured using the conductive pastes of Comparative Examples 1 to 4.

[0183] In contrast, the electrodes formed using the conductive paste of this embodiment have a sufficiently high PL value, low contact resistance, and low specific resistance. Therefore, it can be said that by using the conductive paste of this embodiment, a high-performance crystalline silicon solar cell can be obtained.

[0184] As described above, it was confirmed that the contact resistance of electrodes formed using the conductive paste of this embodiment was reduced by performing a laser treatment process. When the contact resistance was low, the fill factor (FF) increased. Furthermore, the results of the PL method evaluation described above indicated that when the conductive paste of this embodiment was used, the residual rate of the anti-reflection film 2 (passivation film) between the electrode 20 and the impurity diffusion layer 4 was high, thereby suppressing carrier recombination. Therefore, when the conductive paste of this embodiment was used, it could be said that there was little risk of a decrease in open-circuit voltage (Voc). From the above, it can be said that when an electrode was formed using the conductive paste of this embodiment, a solar cell having the characteristic of being able to improve the fill factor (FF) without decreasing the open-circuit voltage (Voc) could be obtained.

[0185] FIG. 13 shows a high-magnification scanning electron microscope (SEM) photograph (magnification: 20,000 times) of a cross section of a solar cell fabricated under the same conditions as in Example 5 after a laser treatment process. As shown in FIG. 13 , it can be seen that AgSi alloy 30 (local conductive portion), which is a small localized electrically conductive portion (local conductive portion), is formed in the impurity diffusion layer 4 (silicon emitter layer) in contact with the light-incident side surface electrode of the solar cell fabricated under the same conditions as in Example 5. In the solar cell of this embodiment, the conductive paste of this embodiment was used to form the electrode (light-incident side surface electrode 20), so it can be said that a local conductive portion was formed in the impurity diffusion layer 4. Therefore, it can be said that low contact resistance was achieved between the electrode and the impurity diffusion layer 4 of the solar cell by forming the electrode (light-incident side surface electrode 20) using the conductive paste of this embodiment.

[0186]

[0187]

[0188]

[0189]

[0190]

[0191]

[0192] REFERENCE SIGNS LIST 1 crystalline silicon substrate 2 anti-reflection film (passivation film) 4 impurity diffusion layer 14 rear surface passivation film 15 rear surface electrode 15a rear surface TAB electrode (rear surface bus bar electrode) 15b rear surface electrode (full rear surface electrode) 15c rear 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, (A) conductive particles; (B) an organic vehicle; and (C) containing a glass frit, The basicity B of the (C) glass frit 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.

2. The conductive paste according to claim 1 , wherein the (A) conductive particles include silver particles.

3. The conductive paste according to claim 1 or 2, wherein the organic vehicle (B) comprises at least one selected from the group consisting of ethyl cellulose, rosin ester, acrylic, and an organic solvent.

4. The content C of PbO in the glass frit (C) in mol% ＰｂＯ and the content G of the glass frit (C), ＰｂＯ The conductive paste according to claim 1 or 2, wherein G is in the range of 20 to 139.

5. 3. The conductive paste according to claim 1, wherein the content G of the glass frit (C) is 0.3 to 4.0 parts by weight.

6. 3. The conductive paste according to claim 1, wherein the content G of the glass frit (C) is 0.5 to 1.5 parts by weight.

7. 3. The conductive paste according to claim 1, wherein the glass transition temperature of the glass frit (C) is 300 to 600°C.

8. The glass frit (C) contains ZnO, V ２ O ５ , W.O. ３ and Nb ２ O ３ The conductive paste according to claim 1 or 2, comprising at least one selected from the group consisting of:

9. The conductive paste according to claim 1, wherein the SiO 2 content of the glass frit (C) is 20 to 65 mol %.

10. The PL value of an n-type single crystal silicon substrate having an electrode pattern formed using the conductive paste is 5522 or more, The thickness of the n-type single crystal silicon substrate is 200 μm, the n-type single crystal silicon substrate has a textured structure; the n-type single crystal silicon substrate has a p-type diffusion layer having a sheet resistance of 60 Ω / □ formed on one surface of the n-type single crystal silicon substrate, a silicon nitride film having a thickness of 60 nm formed on the p-type diffusion layer, and an n-type diffusion layer having a sheet resistance of 20 Ω / □ formed on the other surface of the n-type single crystal silicon substrate; the electrode pattern is formed by printing the electrode pattern on the surface of the silicon nitride film using the conductive paste, and firing the electrode pattern at a peak temperature of 720°C with an in-out time of a firing furnace of 50 seconds; the PL value is the average value of the photoluminescence intensity when the surface of the n-type single-crystal silicon substrate on which the electrode pattern is not formed is irradiated with light from an excitation light source having a wavelength of 650 nm and an output of 3 mW, and the photoluminescence intensity is measured; The conductive paste according to claim 1 or 2, wherein the photoluminescence intensity is measured using a Photoluminescence Imaging System manufactured by BT Imaging, model number LIS-R2.

11. A conductive paste for forming an electrode of a solar cell, Solar cells, a semiconductor substrate of a first conductivity type; a second conductivity type semiconductor layer disposed on one surface of the first conductivity type semiconductor substrate; a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate; a passivation film disposed in contact with a surface of the second conductivity type semiconductor layer; a light-incident side surface electrode disposed on at least a part of the surface of the passivation film; Including, the light-incident-side surface electrode is a surface electrode on which light from a point light source is irradiated onto the light-incident-side surface of the solar cell while applying a voltage between the back electrode and the light-incident-side surface electrode so that a current flows between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type in a direction opposite to a forward direction, 3. The conductive paste according to claim 1, wherein the conductive paste is a conductive paste for forming the light-incident surface electrode.

12. a semiconductor substrate of a first conductivity type; a second conductivity type semiconductor layer disposed on one surface of the first conductivity type semiconductor substrate; a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate; a passivation film disposed in contact with a surface of the second conductivity type semiconductor layer; a light-incident side surface electrode disposed on at least a part of the surface of the passivation film; A solar cell comprising: the light-incident-side surface electrode is a surface electrode on which light from a point light source is irradiated onto the light-incident-side surface of the solar cell while applying a voltage between the back electrode and the light-incident-side surface electrode so that a current flows between the semiconductor layer of the second conductivity type and the semiconductor substrate of the first conductivity type in a direction opposite to a forward direction, A solar cell, wherein the light-incident surface electrode is a fired body of the conductive paste according to claim 1 or 2.

13. a first conductivity type crystalline silicon substrate; a second conductivity type silicon emitter layer disposed on one surface of the first conductivity type crystalline silicon substrate; a back surface electrode disposed so as to be electrically connected to the other surface of the first conductivity type crystalline silicon substrate; a passivation film disposed in contact with a surface of the second conductivity type silicon emitter layer; a light-incident surface electrode containing silver arranged on at least a part of the surface of the passivation film; A solar cell comprising: the second conductivity type silicon emitter layer has a local conduction portion that is in direct contact with the light-incident side surface electrode without a passivation film therebetween, the local conductive portion includes an alloy of silver and silicon; the light-incident side surface electrode is a fired body of a conductive paste, The conductive paste is (A) conductive particles; (B) an organic vehicle; and (C) containing a glass frit, A solar cell, wherein the product BGF·G of the basicity BGF of the (C) glass frit and the content G of the (C) glass frit in the conductive paste expressed in parts by weight when the content of the (A) conductive particles in the conductive paste is taken as 100 parts by weight is a range of 0.25 to 1.

45.

14. A solar cell as described in claim 13, wherein the (A) conductive particles include silver particles.

15. A solar cell as described in claim 13 or 14, wherein (B) the organic vehicle comprises at least one selected from ethyl cellulose, rosin ester, acrylic and organic solvent.

16. The solar cell according to claim 13 or 14, wherein the product C PbO ·G of the content C PbO of PbO in the (C) glass frit in mol % and the content G of the (C) glass frit is in the range of 20 to 139.

17. The solar cell according to claim 13, wherein the content G of the glass frit (C) is 0.3 to 4.0 parts by weight.

18. The solar cell according to claim 13, wherein the content G of the glass frit (C) is 0.5 to 1.5 parts by weight.

19. The solar cell according to claim 13 or 14, wherein the glass transition point of the glass frit (C) is 300 to 600°C.

20. The solar cell according to claim 13 or 14, wherein the glass frit (C) contains at least one selected from ZnO, V 2 O 5 , WO 3 and Nb 2 O 3 .

21. The solar cell according to claim 13 or 14, wherein the SiO 2 content of the glass frit (C) is 20 to 65 mol %.

22. A method for manufacturing a solar cell, comprising: providing a semiconductor substrate of a first conductivity type; forming a semiconductor layer of a second conductivity type on one surface of the semiconductor substrate of the first conductivity type; forming a back surface electrode so as to be electrically connected to the other surface of the first conductivity type semiconductor substrate; forming a passivation film in contact with a surface of the second conductivity type semiconductor layer; forming a light incident side surface electrode on at least a part of the surface of the passivation film; 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 between the second conductivity type semiconductor layer and the first conductivity type semiconductor substrate in a direction opposite to a forward current direction, A method for manufacturing a solar cell, wherein the light-incident side surface electrode is a fired body of the conductive paste according to claim 1 or 2.

23. 3. Use of the conductive paste according to claim 1 or 2 for forming an electrode of a solar cell.