Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
  • H01B1/20—Conductive material dispersed in non-conductive organic material
  • H01B1/22—Conductive material dispersed in non-conductive organic material the conductive material comprising metals or alloys
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/20—Electrodes

Definitions

• the present invention relates to a conductive paste used to form electrodes for semiconductor devices and the like.
• the present invention relates to a conductive paste for forming electrodes for solar cells.
• the present invention also relates to solar cells manufactured using the conductive paste for forming electrodes, and a method for manufacturing solar cells.
• Semiconductor devices such as crystalline silicon solar cells that use crystalline silicon, which is made by processing single crystal silicon or polycrystalline silicon into a flat plate, as a substrate generally have electrodes formed on the surface of the silicon substrate using a conductive paste for electrode formation to allow electrical contact with the outside of the device.
• electrodes are formed in this way, the production volume of crystalline silicon solar cells has increased significantly in recent years.
• These solar cells have an impurity diffusion layer, an anti-reflective 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 back electrode allow the electricity generated by the crystalline silicon solar cell to be extracted to the outside.
• Electrodes for crystalline silicon solar cells are formed using a conductive paste that contains conductive particles, glass frit, organic binders, solvents and other additives. Silver particles are mainly used as the conductive particles.
• Patent document 1 describes a conductive paste containing (i) 100 parts by weight of a conductive powder containing a metal selected from the group consisting of silver, nickel, copper, and mixtures thereof, (ii) 0.3 to 8 parts by weight of aluminum powder having a particle size of 3 to 11 ⁇ m, (iii) 3 to 22 parts by weight of glass frit, and (iv) an organic medium.
• Patent document 1 also describes a method for manufacturing a p-type electrode of an N-type base solar cell, including the steps of: preparing an N-type base semiconductor substrate including an n-base layer, a p-type emitter on the n-base layer, a first passivation layer on the p-type emitter, and a second passivation layer on the n-base layer; applying the conductive paste described above on the first passivation layer; and firing the conductive paste.
• Patent document 2 describes a method for improving the ohmic contact behavior between the contact grid and the emitter layer of a silicon solar cell. Specifically, patent document 2 describes the following: A silicon solar cell (1) is first provided with the emitter layer, the contact grid (5) and the 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 the application of this voltage, a point 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 to induce 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 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).
• Patent document 3 describes a process for improving the ohmic contact behavior between a contact grid and an emitter layer in a silicon solar cell. Specifically, the process described in patent document 3 involves applying a predetermined voltage in the forward and reverse directions of the silicon solar cell, guiding a point light source to the solar surface side of the silicon solar cell, thereby irradiating a cross section of a subsection on the solar surface side.
• Patent Document 4 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 4 describes a conductive paste that contains conductive particles and glass frit as the inorganic material. It describes that the glass frit contained in the conductive paste of Patent Document 4 contains 70 to 90% by weight of PbO and does not contain Al 2 O 3 in 100% by weight of the glass frit.
• Patent Document 5 describes a conductive composition that contains silver powder, glass powder containing PbO, and a vehicle made of an organic substance.
• the conductive composition is a conductive composition for forming an electrode that penetrates a silicon nitride layer and conducts with an n-type semiconductor layer formed below the silicon nitride layer.
• Patent Document 5 also describes that the basicity of the glass powder contained in the conductive composition is 0.6 to 0.8, and that the glass transition point is 300°C to 450°C.
• Figure 5 shows an example of a schematic cross-sectional view of a typical crystalline silicon solar cell.
• an impurity diffusion layer 4 e.g., a p-type impurity diffusion layer 4 in which p-type impurities are diffused
• a crystalline silicon substrate 1 e.g., an n-type crystalline silicon substrate 1.
• An anti-reflection film 2 is formed on the impurity diffusion layer 4.
• the anti-reflection film 2 also functions as a passivation film, and is sometimes called a passivation film.
• the electrode pattern of the light-incident surface electrode 20 (surface electrode) is printed on the anti-reflection film 2 using a conductive paste by screen printing or the like, and the conductive paste is dried and fired at a predetermined temperature to form the light-incident surface electrode 20.
• the conductive paste fires through the anti-reflection film 2 during firing at this predetermined temperature. This fire-through allows the light-incident surface electrode 20 to be formed so as to contact the impurity diffusion layer 4.
• the fire-through is to etch the anti-reflection film 2, which is an insulating film, with glass frit or the like contained in the conductive paste, and to electrically connect the light-incident surface electrode 20 and the impurity diffusion layer 4.
• the anti-reflection film 2 disappears due to the electrode pattern being fired through during firing of the electrode pattern. Therefore, the light-incident surface electrode 20 and the impurity diffusion layer 4 are in contact with each other. A pn junction is formed at the interface between the n-type crystalline silicon substrate 1 and the impurity diffusion layer 4. Most of the incident light that is incident on the crystalline silicon solar cell is transmitted through the anti-reflection film 2 and the impurity diffusion layer 4. The transmitted incident light is incident on the n-type crystalline silicon substrate 1, where the light is absorbed, and electron-hole pairs are generated. The electron-hole pairs are separated by the electric field due to the pn junction.
• the electrons move from the n-type crystalline silicon substrate 1 to the back electrode 15, and the holes move from the p-type impurity diffusion layer 4 to the light-incident surface electrode 20.
• the electrons and holes (carriers) are extracted to the outside as electric current through these electrodes.
• FIG 2 shows an example of a schematic diagram of the light incident surface of a crystalline silicon solar cell.
• a busbar electrode (light incident busbar electrode 20a) and a light incident finger electrode 20b (sometimes simply referred to as “finger electrode 20b") are arranged on the light incident surface of the crystalline silicon solar cell as the light incident surface electrode 20.
• the electrons of the electron-hole pairs generated by the incident light entering the crystalline silicon solar cell are collected by the finger electrode 20b and are further collected by the light incident busbar electrode 20a.
• a metal ribbon for interconnection, surrounded by solder, is soldered to the light incident busbar electrode 20a. This metal ribbon extracts the current to the outside.
• the contact resistance between the light-incident surface electrode 20 and the impurity diffusion layer 4 must be low.
• the laser treatment process refers to a technology for obtaining a low contact resistance by forming the light-incident surface electrode 20, applying a predetermined voltage so that a current flows in the opposite direction to the forward direction of the crystalline silicon solar cell, and irradiating the light-incident surface of the solar cell with light from a point light source.
• the fill factor (FF) can be improved without decreasing the open circuit voltage (Voc), which is one of the solar cell characteristics.
• FIG. 1 shows an example of a schematic cross-sectional view showing a structure in which the 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.
• a voltage is applied so that a current flows in the opposite direction to the forward direction at the pn junction, and light is irradiated from a point light source to generate carriers (electrons and holes), so that a current flows in a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4, and the area is locally heated. Due to the local heating, a small area where the impurity diffusion layer 4 does not exist is locally formed between the light-incident surface electrode 20 and the impurity diffusion layer 4. As a result, as shown in FIG. 12, it is considered that an AgSi alloy 30, which is a locally minute electrically conductive part, is formed between the light-incident surface electrode 20 and the impurity diffusion layer 4.
• the AgSi alloy 30 is locally formed in a limited area, so it is omitted in FIG. 1. It is considered that the locally formed small electrically conductive part enables good electrical conduction between the light-incident surface electrode 20 and the impurity diffusion layer 4. Therefore, it is considered that the AgSi alloy 30 is formed in an area of, for example, 1% or less (preferably 0.1% or less) of the area of the region where the light-incident surface electrode 20 and the impurity diffusion layer 4 contact each other. As a result, the fill factor (FF) can be improved without decreasing the open-circuit voltage (Voc) of the solar cell.
• FF fill factor
• the conductive paste used to form the light-incident surface electrode 20 by the laser processing process must have properties different from those of conventional conductive pastes (conductive pastes that can fire through the anti-reflection film 2).
• the size of the AgSi alloy 30 is a minute electrically conductive portion of 200 to 1800 nm or less, and the AgSi alloy 30 can be confirmed in a SEM photograph of a cross section of the AgSi region 30 as shown in FIG. 12.
• the electrode pattern of the conductive paste is fired, so that the conductive paste fires through the anti-reflection film 2 and the electrode pattern comes into contact with the impurity diffusion layer 4.
• the impurity diffusion layer 4 is damaged, resulting in a problem of a decrease in the performance of the crystalline silicon solar cell.
• the anti-reflection film 2 is not basically fired through when the light-incident surface electrode 20 is formed. Therefore, by using the laser treatment process, damage to the impurity diffusion layer 4 can be suppressed.
• the anti-reflection film 2 which functions as a passivation film, exists in most of the area between the light-incident surface electrode 20 and the impurity diffusion layer 4. Therefore, recombination of carriers on the surface of the impurity diffusion layer 4 in the area where the light-incident surface electrode 20 exists can be suppressed.
• a conductive paste for forming an electrode on a p-type impurity diffusion layer usually contains 1 part by weight or more of aluminum particles per 100 parts by weight of conductive particles such as silver particles.
• the present inventors have found that the conductive paste containing aluminum particles reduces the adhesion of the electrode to the p-type impurity diffusion layer, causing a problem that the electrode is easily peeled off from the p-type impurity diffusion layer of the solar cell. In this specification, this electrode problem may be referred to as a problem of "electrode reliability.” In other words, the conductive paste containing aluminum particles significantly impairs the reliability of the electrode to the p-type impurity diffusion layer.
• metal ribbons for interconnection are soldered to the electrodes of the solar cells. If the conductive paste for forming the electrodes contains aluminum particles, there is a problem in that the soldering strength of the metal ribbon to the electrodes decreases.
• the present invention aims to provide a conductive paste to solve the above problems. That is, the present invention aims to provide a conductive paste suitable for forming electrodes by a laser treatment process for the manufacture of crystalline silicon solar cells. The present invention also aims to provide a conductive paste for forming electrodes for solar cells that can suppress the occurrence of electrode reliability problems such as the electrodes being easily peeled off from the p-type impurity diffusion layer of the solar cell.
• the present invention also aims to provide a method for manufacturing high-performance crystalline silicon solar cells that uses a conductive paste suitable for forming electrodes by a laser treatment process and that can prevent electrode reliability problems from occurring.
• the present invention also aims to provide a high-performance crystalline silicon solar cell that can prevent electrode reliability problems from occurring and that is manufactured by a manufacturing method that includes forming electrodes by a laser treatment process.
• the present invention has the following configuration.
• a first aspect of the present invention relates to a conductive paste for forming an electrode on a passivation film disposed on a surface of a p-type semiconductor layer disposed on a surface of an n-type semiconductor substrate, the conductive paste comprising: (A) silver particles; (B) an organic vehicle; and (C) a glass frit,
• the conductive paste further contains 0.5 parts by weight or less of (D) aluminum particles relative to 100 parts by weight of the (A) silver particles, or does not contain the (D) aluminum particles.
• a configuration 2 is the conductive paste of configuration 1, wherein the (C) glass frit includes at least one selected from ZnO, V2O5 , WO3 , and Nb2O3 .
• Configuration 3 is the conductive paste of configuration 1 or 2, wherein the conductive paste contains 0.3 parts by weight or less of the (D) aluminum particles per 100 parts by weight of the (A) silver particles, or contains no (D) aluminum particles.
• Configuration 4 is the conductive paste of any one of configurations 1 to 3, wherein the conductive paste contains 0.5 to 3.0 parts by weight of the (C) glass frit per 100 parts by weight of the (A) silver particles.
• Configuration 5 is the conductive paste of any one of configurations 1 to 4, in which the glass transition point of the glass frit (C) is 350 to 450°C.
• a configuration 6 is the conductive paste of any one of configurations 1 to 5, in which a product B GF ⁇ G of a basicity B GF of the (C) glass frit and a content G of the (C) glass frit in parts by weight in the conductive paste when a content of the (A) silver particles in the conductive paste is taken as 100 parts by weight is in the range of 0.3 to 2.
• a configuration 7 is the conductive paste of any one of configurations 1 to 6, in which the product C PbO G of the content C PbO of PbO in the (C) glass frit in units of mol % and the content G of the (C) glass frit is in the range of 26 to 105.
• Configuration 8 is the conductive paste of any of Configurations 1 to 7, in which the ratio D/G of the content G of the (C) glass frit in parts by weight in the conductive paste to the content D of the (D) aluminum particles in parts by weight in the conductive paste, when the content of the (A) silver particles in the conductive paste is 100 parts by weight, is 0.4 or less.
• the solar cell is The n-type semiconductor substrate; the p-type semiconductor layer disposed on one surface of the n-type semiconductor substrate; a second electrode disposed so as to be electrically connected to the other surface of the n-type semiconductor substrate; a passivation film disposed in contact with a surface of the p-type semiconductor layer; a first electrode disposed on at least a portion of a surface of the passivation film; the first electrode is a first electrode that has been subjected to a process of irradiating light from a point light source onto a surface of the solar cell on which the first electrode is formed, while applying a voltage between the second electrode and the first electrode so that a current flows between the p-type semiconductor layer and the n-type semiconductor substrate in a direction opposite to a forward direction;
• the conductive paste of any one of configurations 1 to 8, wherein the conductive paste is a conductive paste for forming the first electrode of the solar cell.
• Configuration 10 includes an n-type semiconductor substrate; a p-type semiconductor layer disposed on one surface of the n-type semiconductor substrate; a second electrode disposed so as to be electrically connected to the other surface of the n-type semiconductor substrate; a passivation film disposed in contact with a surface of the p-type semiconductor layer; a first electrode disposed on at least a portion of a surface of the passivation film, the first electrode is a first electrode that has been subjected to a process of irradiating light from a point light source onto a surface of the solar cell on which the first electrode is formed, while applying a voltage between the second electrode and the first electrode so that a current flows between the p-type semiconductor layer and the n-type semiconductor substrate in a direction opposite to a forward direction;
• the solar cell has the first electrode formed by firing the conductive paste according to any one of configurations 1 to 9 of the solar cell.
• Aspect 11 is the solar cell of aspect 10, further comprising an AgSi alloy disposed in at least a portion between the first electrode and the p-type semiconductor layer and in contact with the first electrode and the p-type semiconductor layer.
• Aspect 12 is a method for manufacturing a solar cell, comprising the steps of: Providing an n-type semiconductor substrate; forming a p-type semiconductor layer on one surface of the n-type semiconductor substrate; forming a second electrode electrically connected to the other surface of the n-type semiconductor substrate; forming a passivation film in contact with a surface of the p-type semiconductor layer; forming a first electrode on at least a portion of a surface of the passivation film; applying a voltage between the second electrode and the first electrode so that a current flows between the p-type semiconductor layer and the n-type semiconductor substrate in a direction opposite to a forward direction, and irradiating the surface of the solar cell on which the first electrode is formed with light from a point light source;
• a method for producing a solar cell wherein the first electrode is a sintered body of the conductive paste according to any one of configurations 1 to 9.
• Configuration 13 is the solar cell of configuration 12, further including forming an AgSi alloy in at least a portion between the first electrode and the p-type semiconductor layer so as to be in contact with the first electrode and the p-type semiconductor layer by irradiating the light from the point light source onto the surface of the solar cell on which the first electrode is formed, while applying the voltage between the second electrode and the first electrode.
• a conductive paste suitable for forming electrodes by a laser processing process for the manufacture of crystalline silicon solar cells it is possible to provide a conductive paste for forming electrodes for solar cells that can suppress the occurrence of electrode reliability problems such as the electrodes being easily peeled off from the p-type impurity diffusion layer of the solar cell.
• the present invention it is possible to provide a manufacturing method for high-performance crystalline silicon solar cells that uses a conductive paste suitable for forming electrodes by a laser processing process, and can prevent problems with electrode reliability.
• a manufacturing method for high-performance crystalline silicon solar cells that uses a conductive paste suitable for forming electrodes by a laser processing process, and can prevent problems with electrode reliability.
• FIG. 1 is an example of a schematic cross-sectional view showing a structure in which a light-incident surface electrode is formed on the light-incident surface of a crystalline silicon solar cell by a laser treatment process using the conductive paste of this embodiment.
• 1 is a schematic diagram of an example of the light incident surface of a crystalline silicon solar cell.
• 1 is an example of a schematic diagram of the back surface of a crystalline silicon solar cell.
• 1 is an example of a schematic cross-sectional view of a bifacial crystalline silicon solar cell using the conductive paste of this embodiment.
• FIG. 1 is an example of a schematic cross-sectional view showing a structure in which a light-incident surface electrode is formed on the light-incident surface of a crystalline silicon solar cell by a laser treatment process using the conductive paste of this embodiment.
• 1 is a schematic diagram of an example of the light incident surface of a crystalline silicon solar cell.
• 1 is an example of a schematic diagram of the back surface
• FIG. 1 is an example of a schematic cross-sectional view of a typical crystalline silicon solar cell near the light-incident surface electrode (finger electrode), showing that the anti-reflection film (passivation film) between the electrode and the impurity diffusion layer has disappeared due to fire-through.
• FIG. 2 is a schematic plan view showing a resistivity measurement pattern for an electrode formed using a conductive paste.
• FIG. 2 is a schematic plan view showing a contact resistance measurement pattern and a tape peeling test pattern for an electrode formed using a conductive paste.
• FIG. 2 is a schematic plan view showing a pattern for measuring a photoluminescence imaging method (PL method) for an electrode formed using a conductive paste.
• PL method photoluminescence imaging method
• 1 is an image of the photoluminescence emission intensity of the sample of Example 1 measured by a photoluminescence imaging method (PL method).
• 1 is an image of the photoluminescence emission intensity of the sample of Comparative Example 2 measured by a photoluminescence imaging method (PL method).
• 10 is a cross-sectional SEM photograph (magnification: 20,000 times) of a sample in which a light-incident surface electrode is formed using the same conductive paste as the sample shown in FIG. 9, near the passivation film on the light-incident surface.
• Example 1 is a cross-sectional SEM photograph (magnification 2x) showing that an AgSi alloy, which is a minute electrically conductive portion, is formed locally between the light-incident side surface electrode and the impurity diffusion layer of a sample prepared under the same conditions as in Example 1.
• crystalline silicon includes single crystal and polycrystalline silicon.
• crystalline silicon substrate refers to a material in which crystalline silicon is formed into a shape suitable for forming elements, such as a flat plate, in order to form semiconductor devices such as electric or electronic elements. Any method may be used to manufacture crystalline silicon. For example, the Czochralski method can be used for single crystal silicon, and the casting method can be used for polycrystalline silicon. Other manufacturing methods, such as polycrystalline silicon ribbons manufactured by the ribbon pulling method, and polycrystalline silicon formed on a heterogeneous substrate such as glass, can also be used as the crystalline silicon substrate.
• crystalline silicon solar cell refers to a solar cell manufactured using a crystalline silicon substrate.
• glass frit refers to a material that is primarily made of multiple types of oxides, such as metal oxides, and is generally used in the form of glass-like particles.
• This embodiment is a conductive paste for forming electrodes for solar cells.
• the conductive paste of this embodiment contains (A) silver particles, (B) an organic vehicle, and (C) glass frit.
• the conductive paste of this embodiment further contains 0.5 parts by weight or less of (D) aluminum particles per 100 parts by weight of (A) silver particles, or does not contain (D) aluminum particles.
• the light-to-electricity conversion efficiency of a solar cell (sometimes simply referred to as “conversion efficiency") is expressed as the product of the fill factor (FF), open circuit voltage (Voc), and short circuit current (Jsc).
• FF and Voc are in a trade-off relationship, and it is difficult to simultaneously increase both FF and Voc.
• Patent Documents 2 and 3 describe that by adopting a laser treatment process during the manufacture of crystalline silicon solar cells, the ohmic contact behavior between the grid-shaped electrode, which is the light-incident surface electrode, and the impurity diffusion layer (emitter layer) can be improved, and the contact resistance between the light-incident surface electrode and the impurity diffusion layer can be significantly reduced. Therefore, by performing a laser treatment process, FF can be improved without decreasing Voc.
• the conductive paste containing aluminum particles reduces the adhesion of the electrode to the p-type semiconductor layer (p-type impurity diffusion layer 4), causing the electrode to easily peel off from the p-type semiconductor layer (p-type impurity diffusion layer 4) of the solar cell (problem of "electrode reliability").
• the conductive paste containing aluminum particles significantly reduces the reliability of the electrode to the p-type semiconductor layer (p-type impurity diffusion layer 4).
• the conductive paste of this embodiment further contains 0.5 parts by weight or less of (D) aluminum particles per 100 parts by weight of (A) silver particles, or does not contain (D) aluminum particles. Therefore, by using the conductive paste of this embodiment, a highly reliable electrode can be obtained.
• the present inventors have found that when a laser treatment process is applied to a solar cell in which a light-incident surface electrode is formed using a conventional conductive paste (for example, the conductive paste described in Patent Document 4), 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.
• a conventional conductive paste for example, the conductive paste described in Patent Document 4
• 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 found that this is because the fire-through property (reactivity) of the conventional conductive paste to the anti-reflection film (passivation film) is too strong.
• the present inventors have found that the reactivity of the glass frit to the anti-reflection film (passivation film) can be made appropriate by setting the basicity and content of the glass frit within an appropriate range.
• the conductive paste of this embodiment it can be preferably used when manufacturing crystalline silicon using a laser treatment process.
• FIG. 1 shows a schematic cross-sectional view of an example of a crystalline silicon solar cell in which a light-incident surface electrode 20 is formed using the conductive paste of this embodiment.
• the electrode formed using the conductive paste of this embodiment is referred to as the "first electrode.”
• the light-incident surface electrode 20 is the first electrode.
• the conductive paste of this embodiment can be preferably used to form the first electrode (light-incident surface electrode 20) by a laser treatment process when manufacturing a crystalline silicon solar cell.
• the anti-reflection film 2 (passivation film) is not essentially fired through when the first electrode (light-incident surface electrode 20) is formed. Furthermore, when the laser treatment process is performed on the first electrode (light-incident surface electrode 20), most of the anti-reflection film 2 (passivation film) in contact with the light-incident surface electrode 20 does not disappear. Therefore, by using the laser treatment process when forming the first electrode (light-incident surface electrode 20), damage to the impurity diffusion layer 4 can be suppressed.
• finger electrodes 20b are arranged on the light incident surface of the crystalline silicon solar cell as light incident surface electrodes 20.
• the holes of the electron-hole pairs generated by the incident light entering the crystalline silicon solar cell are collected in the finger electrodes 20b via the impurity diffusion layer 4 (e.g., p-type impurity diffusion layer 4). Therefore, the contact resistance between the finger electrodes 20b and the impurity diffusion layer 4 is required to be low.
• the conductive paste of this embodiment can be preferably used to form the finger electrodes 20b.
• the light incident side surface electrode 20 and the back 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".
• the electrode formed using the conductive paste of this embodiment is the "first electrode”.
• the other electrode different from the first electrode may be referred to as the "second electrode”.
• the back electrode 15 is the second electrode.
• the light incident side surface electrode 20 is the second electrode.
• the surface on the side where the first electrode of the solar cell is formed may be referred to as the light incident side surface.
• One type of crystalline silicon solar cell is a bifacial crystalline silicon solar cell that generates electricity by receiving light from two surfaces (first and second light-incident surfaces) (see Figure 4).
• the conductive paste of this embodiment can be used to form an electrode (first electrode) that is formed on the light-incident surface on which a p-type impurity diffusion layer (p-type semiconductor layer) is formed.
• the conductive paste of this embodiment can be preferably used to form a light-incident surface electrode 20 formed on the surface (light-incident surface) of the anti-reflection film 2 (passivation film) formed on the impurity diffusion layer 4, but is not limited thereto.
• 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.
• the conductive paste of this embodiment can be used to form an electrical contact between the back surface electrode 15 and the crystalline silicon substrate 1 of the solar cell through the back surface passivation film.
• the back surface electrode 15 is the first electrode.
• the conductive paste of this embodiment will be described taking as an example the case of forming a light incident side surface electrode 20 (surface electrode) of a crystalline silicon solar cell using an n-type crystalline silicon substrate 1.
• the light incident side surface electrode 20 is the first electrode.
• the impurity diffusion layer 4 formed on the light incident side surface is a p-type impurity diffusion layer 4.
• an anti-reflection film 2 is formed on the surface of the p-type impurity diffusion layer 4.
• the passivation film can be a film consisting of a single layer or multiple layers.
• the passivation film is a single layer, it is preferably a thin film (SiN film) made of silicon nitride (SiN) from the viewpoint of effectively passivating the surface of the silicon substrate.
• the passivation film is a multiple layer, it can be a laminated film (SiN/SiO x film) of a thin film made of silicon nitride and a thin film made of silicon oxide.
• the SiN/SiO x film is the passivation film
• the SiO x film can be a natural oxide film of the silicon substrate.
• the x of the SiO x film can be in the range of 1 to 2.
• the crystalline silicon solar cell can have a light incident busbar electrode 20a and/or a back TAB electrode 15a.
• the light incident busbar electrode 20a has a function of electrically connecting the finger electrode 20b for collecting the current generated by the solar cell and the metal ribbon for interconnection.
• the back TAB electrode 15a has a function of electrically connecting the back surface electrode 15b for collecting the current generated by the solar cell and the metal ribbon for interconnection. If the finger electrode 20b comes into contact with the crystalline silicon substrate 1, the surface defect density of the surface (interface) of the crystalline silicon substrate 1 where the finger electrode 20b comes into contact increases, and the solar cell performance decreases.
• the conductive paste of this embodiment has low fire-through property (reactivity) with respect to the anti-reflective film 2, so it does not completely fire through the anti-reflective film 2. Therefore, when the finger electrode 20b is formed using the conductive paste of this embodiment, the passivation film in the part in contact with the crystalline silicon substrate 1 can be kept in its original state, and an increase in the surface defect density that causes carrier recombination can be prevented. Therefore, the conductive paste of the present embodiment described above can be suitably used as a conductive paste for forming the finger electrodes 20b of a crystalline silicon solar cell. The entire electrode 20 can be formed using the conductive paste of the present embodiment.
• the light incident side busbar electrode 20a can be formed using a conductive paste different from that for the finger electrodes 20b, which are the first electrodes. In this case, of the light incident side surface electrode 20, only the finger electrodes 20b are the first electrodes. The same applies to the back electrode 15.
• a laser treatment process can be performed on the electrode (light incident surface electrode 20) formed using the conductive paste of this embodiment.
• a voltage is applied and light from a point light source is irradiated, causing a current to flow in a small area between the light incident surface electrode 20 and the impurity diffusion layer 4, resulting in local heating.
• an AgSi alloy 30, which is a local electrically conductive portion, is formed between the light incident surface electrode 20 and the impurity diffusion layer 4.
• the AgSi alloy 30 is formed locally in a limited area, it is not shown in FIG. 1. It is believed that this locally formed electrically conductive portion enables good electrical conduction between the light incident surface electrode 20 and the impurity diffusion layer 4. Therefore, the conductive paste used to form the light incident surface electrode 20 by the laser treatment process must have properties different from those of conventional conductive pastes (conductive pastes that can fire through the anti-reflection film 2).
• the conductive paste of the present embodiment contains (A) silver particles.
• the silver particles may be silver particles or silver alloy particles, or silver-coated particles (a metal other than silver is used as the core material, and this core material is coated with silver).
• the conductive paste of this embodiment may contain metals other than silver, such as gold, copper, nickel, zinc, and tin.
• the silver particles have a silver content of 90% by weight or more.
• a large number of silver particles (Ag particles) may be referred to as silver powder (Ag powder). The same applies to other particles.
• the particle shape and particle size (also called particle diameter) of the silver particles are not particularly limited. For example, spherical and scaly particle shapes can be used.
• the particle size of the silver particles can be determined by the particle size (D50) of 50% of the total particle size. In this specification, D50 is also called the average particle size.
• the average particle size (D50) can be determined from the results of particle size distribution measurement performed by the Microtrack method (laser diffraction scattering method).
• the average particle size (D50) of the silver particles is preferably 0.5 to 2.5 ⁇ m, and more preferably 0.8 to 2.2 ⁇ m.
• the average particle size (D50) of the silver particles within a specified range, the reactivity of the conductive paste with the passivation film during firing of the conductive paste can be suppressed. Note that if the average particle size (D50) is larger than the above range, problems such as clogging may occur during screen printing.
• the size of silver particles can also be expressed as the BET specific surface area (also simply referred to as "specific surface area").
• the BET specific surface area of silver particles is preferably 0.1 to 1.5 m 2 /g, and more preferably 0.2 to 1.2 m 2 /g.
• the BET specific surface area can be measured, for example, using a fully automatic specific surface area measuring device Macsoeb (manufactured by MOUNTEC Corporation).
• the conductive paste of the present embodiment contains (B) an organic vehicle.
• the organic vehicle may contain an organic binder and a solvent.
• the organic binder and the solvent serve to adjust the viscosity of the conductive paste, and are not particularly limited.
• the organic binder may also be dissolved in a solvent before use.
• the (B) organic vehicle contains at least one selected from ethyl cellulose, rosin ester, acrylic, and an organic solvent.
• the (B) organic vehicle can be screen printed favorably, and the shape of the printed pattern can be made appropriate.
• the organic binder can be selected from cellulose-based resins (e.g., ethyl cellulose, nitrocellulose, etc.) and (meth)acrylic resins (e.g., polymethyl acrylate, polymethyl methacrylate, etc.).
• the organic binder contained in the conductive paste of this embodiment preferably contains at least one selected from ethyl cellulose, rosin ester, butyral, and acrylic.
• the amount of organic binder added is usually 0.1 to 30 parts by weight, and preferably 0.2 to 5 parts by weight, per 100 parts by weight of silver particles.
• the organic solvent may be at least one selected from alcohols (e.g., terpineol, ⁇ -terpineol, ⁇ -terpineol, etc.) and esters (e.g., hydroxyl 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 silver particles.
• a specific example of the organic solvent is diethylene glycol monobutyl ether acetate (butyl carbitol acetate).
• the conductive paste of the present embodiment contains (C) glass frit.
• the glass frit contained in the conductive paste of the present embodiment may include at least one selected from PbO, SiO2, Al2O3, B2O3 , ZnO , V2O5 , WO3 , and Nb2O3 .
• the basicity of the glass frit which will be described later, can be adjusted to an appropriate range.
• the glass frit preferably contains PbO.
• the content of PbO in the glass frit (100 mol%) is preferably 25-60 mol%, more preferably 30-55 mol%, and even more preferably 40-55 mol%.
• PbO it is possible to suppress reactivity with the passivation film and reduce contact resistance.
• the glass frit preferably contains SiO 2.
• the content of SiO 2 in the glass frit (100 mol%) is preferably 20 to 65 mol%, and more preferably 25 to 60 mol%. By containing SiO 2 , reactivity with the passivation film can be suppressed.
• the glass frit preferably contains Al 2 O 3.
• the content of Al 2 O 3 in the glass frit (100 mol%) is preferably 3.0 to 6.8 mol%, and more preferably 3.5 to 6 mol%.
• the glass frit preferably contains B 2 O 3.
• the content of B 2 O 3 in the glass frit (100 mol%) is preferably 3.0 to 15 mol%, and more preferably 3.5 to 12 mol%.
• the glass frit (C) contained in the conductive paste of the present embodiment preferably further contains at least one selected from ZnO, V 2 O 5 , WO 3 and Nb 2 O 3 in addition to the above-mentioned components.
• 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%.
• ZnO the basicity of the glass frit can be adjusted to an appropriate range and also contributes to reliability.
• the glass frit preferably contains at least one selected from V 2 O 5 , WO 3 and Nb 2 O 3.
• the content of each of these oxides in the glass frit (100 mol%) is preferably 0.2 to 5 mol%, and more preferably 0.5 to 2 mol%.
• the glass transition point (Tg) of the glass frit (C) is preferably 300 to 600°C, more preferably 320 to 500°C, and even more preferably 350 to 450°C.
• the glass transition point (Tg) of the glass frit (C) is preferably 300 to 600°C, more preferably 320 to 500°C, and even more preferably 350 to 450°C.
• 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 the sample glass powder and reference material are set on the differential thermobalance. The measurement conditions are a temperature rise rate of 10°C/min from room temperature to 900°C, and a curve (DTA curve) is obtained in which the temperature difference between the sample glass powder and the reference material is plotted against temperature. The first inflection point of the DTA curve obtained in this way can be determined as the glass transition point Tg.
• TG-DTA2000S manufactured by Mac Science Co., Ltd.
• the average particle size (D50) of the particles is preferably in the range of 0.1 to 10 ⁇ m, and more preferably in the range of 0.5 to 5 ⁇ m.
• the glass frit particles can be one type of particle containing a predetermined amount of each of the required oxides. Also, particles made of a single oxide can be used as different particles for each of the required oxides. Also, a combination of multiple types of particles with different compositions of the required oxides can be used.
• 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 in parts by weight when the content of the (A) silver particles in the conductive paste is 100 parts by weight is preferably in the range of 0.3 to 2, more preferably in the range of 0.4 to 1.6, and even more preferably in the range of 0.5 to 1.4.
• the product BGF ⁇ G of the basicity BGF of the glass frit and the content G in an appropriate range, the reactivity of the glass frit with respect to the anti-reflection film 2 (passivation film) can be made appropriate. Therefore, the conductive paste of the embodiment can be preferably used when producing crystalline silicon using a laser treatment process.
• the basicity of the glass frit can be calculated by the method described in Patent Document 5 (JP Patent Publication No. 2009-231826).
• the basicity of the glass powder can be determined using the formula shown in "K. Morinaga, H. Yoshida and H. Takebe: J. Am Cerm. Soc., 77, 3113 (1994)". Specifically, it is as follows.
• the bonding force between M i -O of the oxide M i O is expressed as the cation-oxygen ion attractive force Ai by the following formula.
• Z i valence of cation, oxygen ion is 2
• r i ionic radius of cation ( ⁇ )
• the ionic radius r i of the oxygen ion is 1.40 nm.
• B GF basicity of a melt of a glass oxide (glass frit) of any composition
• the basicity BGF thus defined 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.
• the basicity BGF can be said to represent the degree of dissolution in a glass melt.
• the content G of the glass frit (C) is a dimensionless number because it is a ratio to the content G of the silver particles (A).
• the basicity BGF of the glass frit of this embodiment is preferably 0.30 to less than 0.80, more preferably 0.32 to 0.75, and even more preferably 0.35 to 0.70.
• the reactivity of the glass frit with respect to the passivation film can be made appropriate by adjusting the amount of the glass frit added to the conductive paste.
• the product C PbO ⁇ G of the content C PbO of PbO in the glass frit (C) in mol% and the content G of the glass frit (C) is preferably in the range of 26 to 105, more preferably in the range of 35 to 104, and even more preferably in the range of 45 to 103. If the product C PbO ⁇ G exceeds 105, the reactivity between the glass frit and the passivation film becomes too high. Also, if the product C PbO ⁇ G is less than 26, the contact resistance between the obtained electrode and the p-type semiconductor layer becomes too high.
• the conductive paste of this embodiment preferably contains 0.5 to 3.0 parts by weight of (C) glass frit per 100 parts by weight of (A) silver particles.
• the conductive paste of this embodiment more preferably contains 0.5 parts by weight or more and less than 3 parts by weight of (C) glass frit per 100 parts by weight of (A) silver particles, even more preferably contains 0.7 parts by weight or more and 2.5 parts by weight or less, and particularly preferably contains 0.9 parts by weight or more and 2.2 parts by weight or less.
• the conductive paste of the present embodiment may further include (D) aluminum particles.
• the (D) aluminum particles may be included as particles separate from the (A) silver particles.
• aluminum has the properties of a p-type impurity.
• the conductive paste printed on the crystalline silicon is fired, the aluminum in the conductive paste diffuses into the crystalline silicon and becomes a p-type impurity. Therefore, when forming an electrode on the surface of the p-type semiconductor layer of the crystalline silicon substrate 1, the conductive paste can contain aluminum particles, thereby making it possible to obtain low contact resistance between the electrode and the p-type semiconductor layer. Therefore, when forming an electrode on the surface of the p-type semiconductor layer of the crystalline silicon substrate 1, the conductive paste can contain aluminum particles.
• the inventors have found that when the conductive paste contains aluminum particles, the adhesion of the electrode to the p-type semiconductor layer decreases, causing the electrode to easily peel off from the p-type semiconductor layer of the solar cell (a problem of "electrode reliability"). In other words, when the conductive paste contains aluminum particles, the reliability of the electrode with respect to the p-type semiconductor layer is significantly impaired.
• metal ribbons for interconnection are soldered to the electrodes of the solar cells. If the conductive paste for forming the electrodes contains aluminum particles, there is a problem in that the soldering strength of the metal ribbon to the electrodes decreases.
• the conductive paste of this embodiment contains (D) aluminum particles in a predetermined amount or less, or does not contain (D) aluminum particles.
• the (D) aluminum particles are as follows:
• the conductive paste of this embodiment further contains 0.5 parts by weight or less of (D) aluminum particles per 100 parts by weight of (A) silver particles, or does not contain (D) aluminum particles.
• the upper limit of the content of (D) aluminum particles in the conductive paste of this embodiment is preferably 0.3 parts by weight or less per 100 parts by weight of (A) silver particles, more preferably less than 0.3 parts by weight, and even more preferably 0.25 parts by weight or less.
• the conductive paste of this embodiment can be a conductive paste that does not contain (D) aluminum particles. Note that "not containing (D) aluminum particles” means that "(D) aluminum particles" are not intentionally added, and does not exclude the inclusion of aluminum components as unavoidable impurities.
• the lower limit of the content of the aluminum particles (D) in the conductive paste of this embodiment can be 0.01 parts by weight, preferably 0.1 parts by weight, and more preferably 0.2 parts by weight, per 100 parts by weight of the silver particles (A).
• the upper limit of the content of the aluminum particles (D) in the conductive paste of this embodiment is as described above.
• the ratio D/G of the content G of (C) glass frit in the conductive paste in parts by weight to the content D of (D) aluminum particles in the conductive paste in parts by weight is preferably 0.4 or less, more preferably 0.35 or less, and even more preferably 0.32 or less.
• the aluminum in the conductive paste diffuses into the n-type semiconductor layer or the n-type crystalline silicon substrate, adversely affecting the solar cell characteristics. Therefore, when an electrode is formed on the surface of an n-type semiconductor layer or 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).
• Aluminum particles mainly contain the element aluminum.
• the purity of aluminum in the aluminum particles is, for example, preferably 99.7% or more, and more preferably 99.9% or more.
• the aluminum particles may contain impurities other than aluminum, for example, other metal elements that are inevitably included.
• the aluminum particles may also contain alloys of aluminum and other metal elements, and oxides of aluminum.
• the aluminum particles may contain components other than aluminum as unavoidably mixed impurities.
• the shape of the aluminum particles is, for example, spherical or elliptical, but is not limited to these. From the viewpoints of good printability and good reaction with semiconductor substrates, it is preferable that the shape of the aluminum particles is spherical.
• the average particle diameter (D50) of the aluminum particles is not particularly limited. If the average particle diameter (D50) of the aluminum particles is 1 ⁇ m or more and 20 ⁇ m or less, it is preferable in that the printability of the paste composition is improved and the reactivity with the semiconductor substrate is also improved. A more preferable average particle diameter (D50) of the aluminum particles is 2 to 4 ⁇ m.
• the conductive paste of the present embodiment may contain additives and other substances in addition to those mentioned above, provided that they do not adversely affect the solar cell characteristics of the resulting solar cell.
• the conductive paste of this embodiment may further contain, as necessary, one or more additives selected from plasticizers, defoamers, dispersants, leveling agents, stabilizers, and adhesion promoters.
• plasticizers may be selected from phthalates, glycolates, phosphates, sebacates, adipic acids, and citrates.
• 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.
• 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.
• 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.
• the additive is preferably copper manganese tin, aluminosilicate, or aluminum silicate.
• the additive may contain both aluminosilicate and aluminum silicate.
• the conductive paste of this embodiment does not contain inorganic carbon powder such as graphite, graphene, or carbon nanotubes. If inorganic carbon powder is contained in the electrode, the resistivity of the electrode may increase, which may reduce the efficiency of the solar cell.
• the conductive paste of the present embodiment can be produced by adding silver particles, glass frit, and other additives and/or additives as necessary to an organic binder and a solvent, mixing them, and dispersing them.
• Mixing can be performed, for example, with a planetary mixer.
• Dispersion can be performed with a three-roll mill. Mixing and dispersion are not limited to these methods, and various known methods can be used.
• FIG. 1 and 4 show schematic cross-sectional views of a crystalline silicon solar cell.
• crystalline silicon, silicon carbide, germanium, gallium arsenide, etc. can be used as the material of the semiconductor substrate. From the standpoint of safety and cost as a solar cell, it is preferable that the material of the semiconductor substrate is crystalline silicon (single crystal silicon, polycrystalline silicon, etc.).
• the solar cell of this embodiment shown in FIG. 1 includes an n-type semiconductor substrate, a p-type semiconductor layer disposed on one surface of the n-type semiconductor substrate, a passivation film (anti-reflection film 2) disposed in contact with the surface of the p-type semiconductor layer, and a light-incident surface electrode 20 (first electrode) that is an electrode 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 (second electrode) disposed so as to be electrically connected to the other surface of the n-type semiconductor substrate.
• the n-type semiconductor substrate is a crystalline silicon substrate 1 containing n-type impurities
• the p-type semiconductor layer is an impurity diffusion layer 4
• the passivation film is an anti-reflection film 2.
• the material of the semiconductor substrate is preferably silicon. Therefore, the semiconductor substrate is preferably a crystalline silicon substrate 1.
• the passivation film can be an anti-reflection film 2.
• the passivation film is preferably a thin film (SiN film) made of silicon nitride (SiN).
• the light incident surface electrode 20 (first electrode) of the solar cell of this embodiment can be a sintered body of the conductive paste of this embodiment.
• the conductive paste of this embodiment can be used to manufacture a solar cell with this structure.
• the conductive paste of this embodiment can be preferably used to form the light-incident surface electrode 20 of a crystalline silicon solar cell using a laser treatment process.
• the laser treatment process refers to a process in which light from a point light source is irradiated onto the surface (light-incident surface) on the side where the light-incident surface electrode 20 (first electrode) of the solar cell is formed, while a voltage is applied to the back electrode 15 and the light-incident surface electrode 20 so that a current flows in the opposite direction to the forward direction at the pn junction between the p-type semiconductor layer (impurity diffusion layer 4) and the n-type semiconductor substrate (substrate 1).
• the light from the point light source generates carriers (electron-hole pairs) inside the semiconductor substrate, and the application of a voltage makes it possible to move the carriers, that is, to flow a current.
• the voltage is applied so that the direction of current flow at the pn junction is opposite to the forward direction. Therefore, when the semiconductor substrate is an n-type semiconductor substrate and the semiconductor layer is a p-type semiconductor layer, a voltage is applied to the back electrode 15 and the light-incident surface electrode 20 so that a current flows from the n-type semiconductor substrate to the p-type semiconductor layer.
• the n-type semiconductor substrate of the solar cell of this embodiment is preferably an n-type crystalline silicon substrate 1.
• the p-type semiconductor layer of the solar cell of this embodiment is preferably a p-type impurity diffusion layer 4 in which the crystalline silicon contains p-type impurities.
• the mobility of electrons, which are carriers in the n-type crystalline silicon substrate 1 is higher than the mobility of holes, which are carriers in the p-type crystalline silicon substrate 1. Therefore, in order to obtain a solar cell with high conversion efficiency, it is advantageous to use an n-type crystalline silicon substrate 1.
• the conductive paste of this embodiment is preferably a conductive paste for forming the light incident surface electrode 20 (first electrode) of the solar cell described above.
• the n-type semiconductor substrate is an n-type crystalline silicon substrate 1 and the p-type semiconductor layer is a p-type impurity diffusion layer 4 (sometimes simply referred to as "impurity diffusion layer 4").
• an anti-reflection film 2 (passivation film) is present in most of the area between the light-incident surface electrode 20 (first electrode) and the impurity diffusion layer 4.
• a laser treatment process is performed on the light-incident surface electrode 20 (first electrode).
• a voltage is applied so that a current flows in the opposite direction to the forward direction at the pn junction, and light (e.g., laser light) is irradiated from a point light source, so that a current flows in a small area between the light-incident surface electrode 20 and the impurity diffusion layer 4, causing local heating.
• light e.g., laser light
• an AgSi alloy 30, which is a locally electrically conductive portion, is formed between the light-incident surface electrode 20 and the impurity diffusion layer 4. Note that since the AgSi alloy 30 is locally formed in a limited area, it is not shown in FIG. 1. It is believed that this locally formed electrically conductive portion enables good electrical conduction between the light-incident surface electrode 20 and the impurity diffusion layer 4.
• the conductive paste of this embodiment has a lower reactivity with the anti-reflection film 2 than conventional conductive pastes, and has a reactivity with the anti-reflection film 2 (passivation film) appropriate for the laser treatment process. Therefore, the conductive paste of this embodiment can be preferably used to form the light incident surface electrode 20 of a crystalline silicon solar cell using a laser treatment process.
• the crystalline silicon solar cell shown in FIG. 1 can have a back electrode 15 with the structure shown in FIG. 3.
• the back electrode 15 is arranged so as to be electrically connected to the other surface of the n-type semiconductor substrate.
• the back electrode 15 can generally include a full back electrode 15b and a back TAB electrode 15a electrically connected to the full back electrode 15b.
• FIG. 4 shows an example of a cross-sectional schematic diagram of a bifacial crystalline silicon solar cell.
• the bifacial crystalline silicon solar cell shown in FIG. 4 has an impurity diffusion layer 4, an anti-reflection film 2, and a back surface passivation film (back surface anti-reflection film 14).
• the front surface impurity diffusion layer 4 is a p-type impurity diffusion layer (p-type semiconductor layer)
• the light incident side front surface electrode 20 (first electrode) can be formed using the conductive paste of this embodiment, as in the crystalline silicon solar cell shown in FIG. 1 described above.
• the back surface electrode 15 when the back surface impurity diffusion layer (second impurity diffusion layer 16) is a p-type impurity diffusion layer (p-type semiconductor layer), the back surface electrode 15 (first electrode) can be formed using the conductive paste of this embodiment. In this case, as in the solar cell shown in FIG. 1 described above, a portion electrically connected to the back surface passivation film (back surface anti-reflection film 14) can be formed using a laser processing process.
• the impurity diffusion layer on the back surface is a p-type impurity diffusion layer (p-type semiconductor layer)
• the back surface electrode 15 of the bifacial crystalline silicon solar cell can be said to be an electrode (first electrode) similar to the light incident surface electrode 20 of the solar cell shown in FIG. 1.
• the light incident surface electrode 20 shown in FIG. 4 can be said to be a second electrode.
• the conductive paste of the present embodiment described above can be suitably used as a conductive paste for forming the finger electrode 20b (first electrode) of a crystalline silicon solar cell.
• the conductive paste of the present embodiment can also be suitably used as a conductive paste for forming the back electrode 15 (back finger electrode 15c, first electrode) of a bifacial crystalline solar cell.
• the busbar electrodes of the crystalline silicon solar cell shown in FIG. 1 include the light incident side busbar electrode 20a shown in FIG. 2 and the backside TAB electrode 15a as shown in FIG. 3.
• a metal ribbon for interconnection is soldered to the light incident side busbar electrode 20a and the backside TAB electrode 15a. This metal ribbon allows the current generated by the solar cell to be extracted to the outside of the crystalline silicon solar cell.
• the bifacial crystalline solar cell shown in FIG. 4 can also have the light incident side busbar electrode 20a and the backside TAB electrode 15a having the same shape as the light incident side busbar electrode 20a.
• the width of the busbar electrodes (light incident side busbar electrode 20a and backside TAB electrode 15a) can be approximately the same as that of the metal ribbon for interconnection. In order for the busbar electrodes to have low electrical resistance, the wider the width, the better. On the other hand, in order to increase the area of incidence of light on the light incident side surface, the narrower the width of the light incident side busbar electrode 20a is. Therefore, the busbar electrode width can be 0.05 to 5 mm, preferably 0.08 to 3 mm, more preferably 0.1 to 2 mm, and even more preferably 0.15 to 1 mm.
• the number of busbar electrodes can be determined according to the size of the crystalline silicon solar cell.
• the number of busbar electrodes is arbitrary, but specifically, it can be three or four or more.
• the optimal number of busbar electrodes can be determined by simulating the operation of the solar cell so as to maximize the conversion efficiency of the crystalline silicon solar cell. Since the crystalline silicon solar cells are connected to each other in series by the metal ribbon for interconnection, it is preferable that the number of light incident side busbar electrodes 20a and backside TAB electrodes 15a is the same. For the same reason, it is preferable that the widths of the light incident side busbar electrode 20a and the back surface TAB electrode 15a are the same.
• the finger electrodes 20b on the light incident surface are as narrow as possible and that there are as few of them as possible.
• the finger electrodes 20b are wide and there are many of them.
• the finger electrodes 20b are wide.
• the number of busbar electrodes can be determined according to the size of the crystalline silicon solar cell and the width of the busbar electrodes.
• the optimal width and number of finger electrodes 20b (the spacing between the finger electrodes 20b) can be determined by simulating the operation of the solar cell so as to maximize the conversion efficiency of the crystalline silicon solar cell.
• the width and number of back finger electrodes 15c of the back electrode 15 of the bifacial crystalline silicon solar cell shown in FIG. 4 can also be determined in a similar manner.
• the solar cell can be a crystalline silicon solar cell.
• the solar cell is a crystalline silicon solar cell.
• the method for manufacturing a solar cell in this embodiment includes preparing an n-type semiconductor substrate.
• the n-type semiconductor substrate it is preferable to use an n-type crystalline silicon substrate 1.
• the following describes an example in which a crystalline silicon solar cell is manufactured using an n-type crystalline silicon substrate 1.
• the surface of the crystalline silicon substrate 1 on the light incident side has a pyramidal texture structure.
• the method for manufacturing a solar cell includes a step of forming a p-type semiconductor layer (impurity diffusion layer 4) on one surface of the crystalline silicon substrate 1 (n-type semiconductor substrate) prepared in the above-mentioned step.
• the impurity diffusion layer 4 can be formed by diffusing p-type impurities (group 13 elements) such as B (boron), Al (aluminum) and/or Ga (gallium).
• the impurity diffusion layer 4 When forming the impurity diffusion layer 4, it can be formed so that the sheet resistance of the impurity diffusion layer 4 is 40 to 150 ⁇ / ⁇ (square), preferably 45 to 120 ⁇ / ⁇ .
• the depth to which the impurity diffusion layer 4 is formed can be 0.3 ⁇ m to 1.0 ⁇ m.
• the depth of the impurity diffusion layer 4 refers to the depth from the surface of the impurity diffusion layer 4 to the pn junction.
• the depth of the pn junction can be the depth from the surface of the impurity diffusion layer 4 to the point where the impurity concentration in the impurity diffusion layer 4 becomes the impurity concentration of the substrate.
• the method for manufacturing a solar cell of this embodiment includes forming a second electrode so as to be electrically connected to the other surface of the n-type semiconductor substrate.
• the second electrode is the back electrode 15.
• a conductive paste is printed on the other surface (back surface) of the crystalline silicon substrate 1, and then fired to form a second electrode (back electrode 15).
• the second electrode can be formed either before or after the light-incident surface electrode 20 is formed.
• the firing to form the second electrode can be performed simultaneously with or separately from the firing to form the light-incident surface electrode 20.
• 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 p-type semiconductor layer (impurity diffusion layer 4).
• the passivation film can be an anti-reflection film 2.
• an anti-reflection film 2 that also functions as a passivation film is formed on the surface of the impurity diffusion layer 4 formed in the above-mentioned process.
• a silicon nitride film SiN film
• the silicon nitride film layer also functions as a passivation film for the light incident surface. Therefore, when a silicon nitride film is used as the anti-reflection film 2, a high-performance crystalline silicon solar cell can be obtained.
• the anti-reflection film 2 is a silicon nitride film, it can exhibit an anti-reflection function against incident light.
• the silicon nitride film can be formed by a method such as PECVD (Plasma Enhanced Chemical Vapor Deposition).
• the manufacturing method of the solar cell of this embodiment includes forming a light incident surface electrode 20 (first electrode) on at least a portion of the surface of the passivation film (anti-reflection film 2).
• the above-mentioned conductive paste is used to form the light incident surface electrode 20. Therefore, the light incident surface electrode 20 is a sintered body of the above-mentioned conductive paste.
• the manufacturing method of the crystalline silicon solar cell of this embodiment includes a step of forming the light-incident surface electrode 20 by printing and firing a conductive paste on the surface of the anti-reflection film 2. Note that firing to form the back electrode 15 can be performed simultaneously with firing to form the light-incident surface electrode 20.
• 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.
• the light incident side busbar electrode 20a and the light incident side finger electrode 20b of the light incident side surface electrode 20 can be formed using the conductive paste of this embodiment.
• a conductive paste for forming the back electrode 15 can be printed and dried after this.
• Firing conditions include a firing atmosphere in air and a firing temperature of 500 to 1000°C, more preferably 600 to 1000°C, even more preferably 500 to 900°C, and particularly preferably 700 to 900°C. Firing is preferably performed for a short period of time, and the temperature profile (temperature-time curve) during firing is preferably peak-shaped.
• 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.
• the method for manufacturing a solar cell of this embodiment includes carrying out the laser treatment process described above. That is, the method for manufacturing a solar cell of this embodiment includes irradiating the surface (light incident surface) on the side where the first electrode of the solar cell is formed with light (e.g., laser light) from a point light source while applying a voltage between the second electrode (rear electrode 15) and the first electrode (light incident surface electrode 20) so that a current flows in the opposite direction to the forward direction between the p-type semiconductor layer (p-type impurity diffusion layer 4) and the n-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.
• a second impurity diffusion layer 16 can be formed.
• a back electrode 15 (first electrode) using the conductive paste of this embodiment and performing a laser treatment process, a low-resistance conductive portion can be formed between the back electrode 15, which is the first electrode, and the crystalline silicon substrate 1. Therefore, in the case of a bifacial solar cell, the back electrode 15 can be formed using the conductive paste of this embodiment. In this case, the back electrode 15 (first electrode) is a fired body of the conductive paste of this embodiment.
• the crystalline silicon solar cell of this embodiment can be manufactured.
• the crystalline silicon solar cell of this embodiment obtained as described above can be electrically connected with a metal ribbon for interconnection and laminated with a glass plate, a sealing material, a protective sheet, etc. to obtain a solar cell module.
• a metal ribbon for interconnection a metal ribbon covered with solder (e.g., a ribbon made of copper) can be used.
• solder a solder mainly composed of tin, specifically a lead-containing solder and a lead-free solder, or any other solder available on the market can be used.
• a high-performance crystalline silicon solar cell can be obtained by forming the required electrodes of the solar cell using the conductive paste of this embodiment and performing a laser treatment process.
• a measurement substrate simulating a single crystal silicon solar cell was used to evaluate the degree of degradation of the passivation film using the photoluminescence imaging method (PL method), as well as the contact resistance and resistivity of the formed electrodes, to evaluate the performance of the conductive paste in the examples and comparative examples of this embodiment.
• PL method photoluminescence imaging method
• Tables 1 to 3 show the compositions of the conductive pastes of Examples 1 to 12 and Comparative Examples 1 and 2.
• 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 pastes are as follows.
• Silver particles Table 4 shows the product number, manufacturer, shape, average particle size (D50), TAP density, and BET specific surface area of silver particles A1 and A2 used in the conductive pastes of 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 size (D50) was determined by measuring the particle size distribution using the microtrack method (laser diffraction scattering method) and obtaining the median diameter (D50) from the results of the particle size distribution measurement. The same applies to the average particle sizes (D50) of the other components.
• a fully automatic specific surface area measuring device Macsoeb manufactured by MOUNTEC was used to measure the BET specific surface area.
• the BET specific surface area was measured by the BET one-point method using nitrogen gas adsorption after pre-drying at 100°C and flowing nitrogen gas for 10 minutes.
• (B) Organic Vehicle An organic binder and a solvent were used as the organic vehicle. Ethyl cellulose (0.4 parts by weight) with an ethoxy content of 48 to 49.5% by weight was used as the organic binder. Diethylene glycol monobutyl ether acetate (butyl carbitol acetate) (3 parts by weight) was used as the solvent.
• (C) Glass Frit Table 5 shows the composition, basicity and glass transition point of the glass frits GF1 to GF4 used in the conductive pastes of the Examples and Comparative Examples.
• the average particle size (D50) of the glass frits GF1 to GF4 is 2 ⁇ m.
• Tables 1 to 3 show the type (any of GF1 to GF4) and the content G (parts by weight) of the (C) glass frit in the conductive pastes of the Examples and Comparative Examples.
• the glass transition points of glass frits GF1 to GF4 were measured. Table 5 shows the measured glass transition points of glass frits GF1 to GF4.
• the glass transition points of the glass frits were measured as follows. That is, approximately 50 mg of glass frits GF1 to GF4 were placed in a platinum cell as samples. Alumina powder was used as the standard sample. A DTA curve of the sample was obtained in an air atmosphere at a heating rate of 20°C/min from room temperature to 800°C using a differential thermal analyzer (TG-8120, manufactured by Rigaku Corporation). The starting point (extrapolated point) of the first endotherm in the DTA curve was taken as the glass transition point.
• TG-8120 differential thermal analyzer
• Glass frits GF1 to GF4 were manufactured as follows. First, the oxide powders were weighed, mixed, and placed in a crucible. The crucible was placed in a heated oven. The contents of the crucible were heated to the melting temperature. The melting temperature was maintained until the raw materials were sufficiently melted. Next, the crucible was removed from the oven, the molten contents were stirred uniformly, and the contents of the crucible were quenched at room temperature using two stainless steel rolls to obtain a plate-shaped glass. Finally, the plate-shaped glass was crushed in a mortar while being uniformly dispersed, and sieved through a mesh sieve to obtain glass frit with the desired particle size.
• Table 6 shows the product number, manufacturer, shape and average particle size (D50) of aluminum particles D1 and D2 used in the conductive pastes of the 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 the Examples and Comparative Examples.
• the reactivity of the conductive paste with the passivation film was evaluated by the photoluminescence imaging method (referred to as the "PL method").
• the PL method is capable of evaluating the reactivity of the conductive paste with the passivation film in a non-destructive, non-contact, and short time.
• the PL method is a method in which a sample is irradiated with light having an energy larger than the forbidden band width to cause it to emit light, and the state of defects in the crystal and surface/interface defects is evaluated from the state of the light emission.
• the defects act as recombination centers of electron-hole pairs generated by the light irradiation, and the band edge emission intensity by photoluminescence decreases correspondingly.
• the passivation film is eroded by the printed/fired electrode and a surface defect is formed 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 emission intensity of the part where the surface defect is formed i.e., the part of the electrode formed on the sample
• the reactivity of the conductive paste with the passivation film can be evaluated based on the intensity of this photoluminescence.
• the method for preparing the substrate for evaluation using the PL method is as follows.
• the substrate used was an n-type single crystal silicon substrate (substrate thickness 200 ⁇ m).
• a silicon oxide layer of approximately 20 ⁇ m was formed on the substrate by dry oxidation, and then the substrate was etched with a mixed solution of hydrogen fluoride, pure water, and ammonium fluoride to remove damage to the substrate surface.
• heavy metals were cleaned with an aqueous solution containing hydrochloric acid and hydrogen peroxide.
• a textured structure (uneven shape) was formed on both sides of the substrate by wet etching. Specifically, a pyramidal textured structure was formed on both sides (main light-incident surface and back surface) by wet etching (sodium hydroxide solution). After that, it was washed with an aqueous solution containing hydrochloric acid and hydrogen peroxide. Next, boron was injected into one surface (light-incident surface) of the substrate having the textured structure, forming a p-type impurity diffusion layer 4 to a depth of about 0.5 ⁇ m. The sheet resistance of the p-type impurity diffusion layer 4 was 60 ⁇ / ⁇ .
• phosphorus was injected into the other surface (back surface) of the substrate having the textured structure, forming an n-type impurity diffusion layer to a depth of about 0.5 ⁇ m.
• the sheet resistance of the n-type impurity diffusion layer was 20 ⁇ / ⁇ . Boron and phosphorus were injected simultaneously by thermal diffusion.
• a thin oxide film of 1 to 2 nm was formed on the surface (light incident surface) of the substrate on which the p-type impurity diffusion layer 4 was formed, and on the surface (rear surface) of the substrate on which the n-type impurity diffusion layer was formed.
• a silicon nitride film of about 60 nm in thickness was formed by plasma CVD using silane gas and ammonia gas.
• the substrate thus obtained was cut into a square of 25 mm x 25 mm to prepare substrate 1.
• a square electrode pattern 22 of 13 mm x 13 mm was printed on the surface of the substrate 1 using a conductive paste for forming electrodes, and then dried.
• the measurements using the PL method were carried out using a Photoluminescence Imaging System (model number LIS-R2) manufactured by BT Imaging.
• 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 22 of the light-incident surface electrode 20 is not formed) to obtain an image of the photoluminescence emission intensity.
• Figures 9 and 10 show images of the photoluminescence intensity measured using the PL method.
• Figure 9 shows an image of the photoluminescence intensity measured by the PL method for a sample in which the electrode pattern 22 was formed using the conductive paste of Example 1.
• the image of the part in which the electrode pattern 22 was formed is brighter than that of Figure 10 described later. This shows that the decrease in the photoluminescence intensity in the part in which the electrode pattern 22 of the light-incident side surface electrode 20 was formed was suppressed. Therefore, in the case of the sample shown in Figure 9, it can be said that the passivation function of the passivation film was maintained by forming the electrode pattern 22 of the light-incident side surface electrode 20. Therefore, in the case of the sample using the conductive paste of Example 1 shown in Figure 9, it can be said that the surface defect density of the surface of the single crystal silicon substrate did not increase.
• the conductive paste of Comparative Example 2 was used to form the light incident side surface electrode 20.
• the image of the portion of the light incident side surface electrode 20 where the electrode pattern 22 is formed is darker than that of the sample shown in FIG. 9. This indicates that the photoluminescence emission intensity of the portion of the light incident side surface electrode 20 where the electrode pattern 22 is formed has decreased. Therefore, in the case of the sample using the conductive paste of Comparative Example 2 shown in FIG. 10, the formation of the electrode pattern 22 of the light incident side surface electrode 20 impairs the passivation function of the passivation film, and the surface defect density of the surface of the single crystal silicon substrate has increased.
• Tables 1 to 3 show the measured values of photoluminescence intensity (PL value) 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, as well as the optical system used for measurement, and is a value in any unit.
• the degree of carrier recombination degree of deterioration of the passivation function
• FIG. 11 shows a cross-sectional SEM photograph (magnification: 20,000 times) of the vicinity of the passivation film of a sample (corresponding to Example 1) in which the light-incident surface electrode 20 was formed under the same conditions using the same conductive paste as the sample shown in FIG. 9.
• the anti-reflection film 2 in the case of a sample with a high PL value, even after the light-incident surface electrode 20 was formed, the anti-reflection film 2 (passivation film) maintained almost the same shape, and the anti-reflection film 2 (passivation film) was not eroded by the glass frit.
• FIG. 11 shows a cross-sectional SEM photograph (magnification: 20,000 times) of the vicinity of the passivation film of a sample (corresponding to Example 1) in which the light-incident surface electrode 20 was formed under the same conditions using the same conductive paste as the sample shown in FIG. 9.
• FIG. 12 shows an SEM photograph (magnification: 20,000 times) of the cross section of a sample prepared under the same conditions as Example 1, observed by a high-magnification scanning electron microscope (SEM).
• SEM scanning electron microscope
• Such a conductive paste is believed to be a conductive paste that can fire through the passivation film. From the above, it is clear that the reactivity of the conductive paste with the anti-reflective film 2 (passivation film) can be evaluated by measuring the PL value using the above-mentioned PL method.
• the PL values of the samples obtained using the conductive pastes of Examples 1 to 12 of this embodiment were 5113 (Example 5) or more.
• the PL value of Comparative Example 1 was 9868, and the PL value of Comparative Example 2 was 5520.
• the PL value is 5000 or more, it can be said that there is no problem with the reactivity of the conductive paste with the anti-reflective film 2 (passivation film). Therefore, when the conductive pastes of Examples 1 to 12 and Comparative Examples 1 and 2 are used, it can be said that there is no problem with the reactivity with the anti-reflective film 2 (passivation film).
• the PL value did not change significantly before and after the laser treatment process. This is thought to be because the laser treatment process is a process for forming tiny localized electrically conductive parts, and does not affect most of the anti-reflection film 2 (passivation film).
• a p-type impurity diffusion layer 4 was formed on one surface of an n-type crystalline silicon substrate 1 (substrate thickness: 200 ⁇ m), and further, a silicon nitride film (anti-reflection film 2 serving as a passivation film) having a thickness of about 60 nm was formed on the p-type impurity diffusion layer 4 to obtain a substrate for measuring contact resistance.
• the conductive paste (conductive paste for forming the first electrode) used for forming the electrode on the surface (light incident surface) of the substrate on which the p-type impurity diffusion layer 4 is formed in the single crystal silicon solar cells of the examples and comparative examples was the one shown in Tables 1 to 3.
• the conductive paste was printed by screen printing.
• a pattern consisting of a 1.5 mm wide light incident side busbar electrode 20a and a 60 ⁇ m wide light incident side finger electrode 20b was printed on the anti-reflection film 2 of the above-mentioned substrate so that the film thickness was approximately 20 ⁇ m, and then it was dried at 150°C for approximately 1 minute.
• a commercially available Ag paste was printed by screen printing to form the back electrode 15 (the electrode on the surface on which the n-type diffusion layer is formed).
• the electrode pattern of the back electrode 15 is the same as that of the light-incident side surface electrode 20. It was then dried at 150°C for approximately 60 seconds. After drying, the conductive paste for the back electrode 15 had a film thickness of approximately 20 ⁇ m. Then, using a belt furnace (firing furnace) CDF7210 manufactured by Despatch Industries, Inc., both sides were simultaneously fired at a peak temperature of 720°C with an in-out time of the firing furnace of 50 seconds. In this manner, a single crystal silicon solar cell was produced.
• the solar cell thus obtained was cut into a 15 mm x 15 mm square as shown in Figure 7 to obtain a sample for contact resistance measurement.
• light incident side finger electrodes 20b with a width of 60 ⁇ m and a length of 15.0 mm were arranged at intervals of 1.5 mm on the light incident side surface of this cut solar cell (sample for contact resistance measurement).
• These light incident side finger electrodes 20b (first electrodes) were used as the pattern for contact resistance measurement.
• 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 method (Transfer length Method) using a GP 4 TEST Pro manufactured by GP Solar.
• the material can be used as an electrode for a solar cell by performing a laser treatment process.
• the contact resistance value before the laser treatment process is 300 ⁇ cm2 or less, the material can be more preferably used as an electrode for a solar cell by performing a laser treatment process.
• the contact resistance before the laser treatment process of the samples obtained using the conductive pastes of Examples 1 to 12 of this embodiment was 447 m ⁇ cm 2 (Example 9) or less. Therefore, when the conductive pastes of the examples of this embodiment are used to form electrodes of solar cells, it can be said that they can be used as solar cell electrodes by performing a laser treatment process.
• the contact resistance before the laser treatment process of the samples obtained using the conductive pastes of Examples 5, 6 and 8 of this embodiment was 251 m ⁇ cm 2 (Example 8) or less.
• the contact resistance before the laser treatment process of the samples obtained using the conductive pastes of Comparative Examples 1 and 2 was lower than 300 m ⁇ cm 2 .
• the light incident surface of the solar cell was irradiated with laser light while applying a negative voltage to the back electrode 15 (second electrode) and a positive voltage to each of the light incident surface electrodes 20 (first electrodes) of the pattern shown in FIG. 2.
• the applied voltage during the laser treatment process was 20 V
• the irradiated laser light intensity was 100 W/cm 2
• the voltage application and laser light irradiation time were 2 seconds.
• the solar cell thus obtained was cut into a 15 mm x 15 mm square as shown in Figure 7, and the contact resistance after the laser treatment process was measured using the same method as that used to measure the contact resistance before the laser treatment process.
• the contact resistance value after the laser treatment process is 20 m ⁇ cm2 or less, it can be preferably used as the electrode 20 of a solar cell. Furthermore, if the contact resistance value after the laser treatment process is 9 m ⁇ cm2 or less, it can be more preferably used as the electrode of a solar cell.
• the contact resistance after the laser treatment process of the samples obtained using the conductive pastes of Examples 1 to 12 of this embodiment was 16 m ⁇ cm 2 (Example 7) or less. Therefore, when the conductive pastes of the Examples of this embodiment are used to form an electrode of a solar cell, it can be preferably used as the electrode 20 of the solar cell.
• the contact resistance after the laser treatment process of the samples obtained using the conductive pastes of Examples 3 to 6 and 12 of this embodiment was 9 m ⁇ cm 2 or less. Therefore, when the conductive pastes of Examples 3 to 6 and 12 of this embodiment are used to form an electrode of a solar cell, it can be more preferably used as an electrode of a solar cell.
• the contact resistance after the laser treatment process of the samples obtained using the conductive pastes of Comparative Examples 1 and 2 was lower than 9 m ⁇ cm 2 .
• the resistivity of the examples and comparative examples was measured by the following procedure. That is, a silicon substrate with a width of 15 mm, a length of 15 mm, and a thickness of 180 ⁇ m was prepared. A pattern made of conductive paste as shown in Figure 6 was printed on this substrate using a 325 mesh stainless steel screen.
• the silicon substrates on which the patterns of the examples and comparative examples were printed were simultaneously fired on both sides in 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, samples for resistivity measurement were prepared.
• the resistivity of the conductive film patterns of the samples for resistivity measurement obtained by firing the conductive pastes of the examples and comparative examples was measured.
• the resistance value was measured by the four-terminal method using a Toyo Corporation multimeter Model 2001.
• the cross-sectional area of the conductive film pattern was measured using a Lasertec Corporation confocal microscope OPTELICS H1200 and a surface roughness and shape measuring instrument 1500SD2. Measurements were taken at 50 points over a range of 1.6 mm, and the average value was calculated.
• the resistivity was calculated using the cross-sectional area and the measured resistance value.
• the resistivity of the samples obtained using the conductive pastes of Examples 1 to 12 of this embodiment was 5 to 6 ⁇ cm. Therefore, when the conductive pastes of the Examples of this embodiment are used to form electrodes for solar cells, it can be said that there is no problem with the resistivity of the electrodes.
• the resistivity of the samples obtained using the conductive pastes of Comparative Examples 1 and 2 was 6 ⁇ cm.
• a p-type impurity diffusion layer 4 was formed on one surface of an n-type crystalline silicon substrate 1 (substrate thickness 200 ⁇ m), and a silicon nitride film (anti-reflection film 2, which is a passivation film) with a thickness of approximately 60 nm was further formed on the p-type impurity diffusion layer 4 to obtain a substrate for tape peeling tests.
• the conductive pastes used to form electrodes on the surface (light incident surface) of the substrate on which the p-type impurity diffusion layer 4 is formed for the single crystal silicon solar cells of the examples and comparative examples were those shown in Tables 1 to 3.
• the conductive paste was printed by screen printing. As with the sample for measuring contact resistance, a pattern consisting of a 1.5 mm wide light incident side busbar electrode 20a and a 60 ⁇ m wide light incident side finger electrode 20b was printed on the anti-reflection film 2 of the above-mentioned substrate to a film thickness of approximately 20 ⁇ m, and then dried at 150°C for approximately 60 seconds.
• the silicon substrate with the patterns of the examples and comparative examples printed on its surface was baked in a belt furnace (baking furnace) CDF7210 manufactured by Despatch Industries, Inc. at a peak temperature of 720°C with an in-out time of the baking furnace of 50 seconds.
• a belt furnace baking furnace
• CDF7210 manufactured by Despatch Industries, Inc.
• the electrode-attached substrate thus obtained was cut into a 15 mm x 15 mm square as shown in Figure 7 to obtain a sample for the tape peeling test.
• the sample for the tape peeling test was immersed in a 1% aqueous solution of acetic acid at room temperature for 1 hour. After immersion, it was removed from the acetic acid aqueous solution, washed with pure water, and dried.
• Six electrodes finger electrodes
• Scotch (registered trademark) Mending Tape 810 width 12 mm was attached to the six electrodes and peeled off. The number of electrodes that peeled off when peeled off was confirmed.
• Tables 1 to 3 show the results of the tape peeling test. "Good” indicates that none of the six electrodes peeled off. "Usable” indicates that 1 to 5 electrodes peeled off. "Poor” indicates that all six electrodes peeled off.
• a substrate for measuring soldering adhesion strength simulating a solar cell was fabricated using the prepared conductive paste, and the soldering adhesion strength was measured.
• a light incident side bus bar electrode 20a was formed on the substrate for measuring soldering adhesion strength.
• the measurement substrate was prepared as follows:
• the substrate used was a p-type Si single crystal silicon substrate (substrate thickness 200 ⁇ m).
• a silicon oxide layer of approximately 20 ⁇ m was formed on the substrate by dry oxidation, and then the substrate was etched with a mixed solution of hydrogen fluoride, pure water, and ammonium fluoride to remove damage to the substrate surface.
• heavy metals were cleaned with an aqueous solution containing hydrochloric acid and hydrogen peroxide.
• a textured structure was formed on the light-incident surface. Specifically, a pyramidal textured structure was formed on the light-incident surface by wet etching (sodium hydroxide solution).
• a silicon nitride film was formed as a back surface passivation film to a thickness of about 60 nm on the entire surface of the substrate by plasma CVD using silane gas and ammonia gas.
• the solar cell substrate obtained in this manner was cut into a 15 mm x 15 mm square for use.
• the printing of the conductive paste for forming the light incident side busbar electrode 20a was performed by screen printing. Using the conductive paste of the examples and comparative examples containing glass frit and silver particles shown in Tables 1 to 3, a pattern of the light incident side busbar electrode 20a with a length of 1.3 mm and a width of 2 mm was printed on the passivation film of the above-mentioned substrate so that the film thickness was approximately 20 ⁇ m, and then it was dried at 150°C for approximately 1 minute.
• the back electrode 15 is not necessary when measuring the adhesive strength of the light-incident side busbar electrode 20a. Therefore, the back electrode 15 was not formed.
• the substrate with the conductive paste printed on its surface as described above was baked under specified conditions in air using a near-infrared baking furnace (NGK Insulators, Ltd., high-speed baking test furnace for solar cells) that uses a halogen lamp as the heating source.
• the baking conditions were a peak temperature of 775°C, and baking in air with an in-out time of 30 seconds. In this way, a substrate for measuring soldering adhesion strength was prepared.
• the soldered metal ribbon adhesive strength measurement specimen was prepared and measured as follows.
• the ring-shaped part on one end of the ribbon was pulled at 90 degrees to the substrate surface with a digital tensile gauge (A & D Co., Ltd., Digital Force Gauge AD-4932-50N) and the fracture strength of the adhesive was measured to measure the solder adhesive strength. Ten specimens were prepared, and the measured value was calculated as the average value of the 10 specimens.
• the adhesive strength of the metal ribbon When the adhesive strength of the metal ribbon is 1.5 N/mm or more, it can be said that the adhesive strength is sufficient for use. When the adhesive strength of the metal ribbon is 0.5 N/mm or more, it can be said that the adhesive strength is sufficient for use in specific applications.
• One example of a specific application is when the light incident side finger electrode 20b is formed using the conductive paste of this embodiment, and the light incident side busbar electrode 20a is formed using another type of conductive paste. In this case, the adhesive strength of the metal ribbon to the light incident side finger electrode 20b is not particularly required, so even if the adhesive strength of the metal ribbon is 0.5 N/mm, no particular problem will arise.
• the adhesive strength of the metal ribbon of the electrode obtained using the conductive paste of Example 4 of this embodiment was 0.5 N/mm. Therefore, it can be said that the electrode formed using the conductive paste of Example 4 has an adhesive strength that can be used for specific applications.
• the adhesive strength of the metal ribbon of the electrode obtained using the conductive paste of Examples 1 to 3 and 5 to 12 of this embodiment was 1.5 N/mm or more. Therefore, it can be said that the electrodes obtained using the conductive paste of Examples 1 to 3 and 5 to 12 of this embodiment have a good adhesive strength that can withstand sufficient use.
• the adhesive strength of the metal ribbon of the electrode obtained using the conductive paste of Comparative Examples 1 and 2 was 0.6 to 1.3 N/mm, which was an adhesive strength that can be used for specific applications.

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