Glass, Conductive Paste, and Solar Cell

Abstract

To provide a glass whereby: when forming an electrode on a semiconductor substrate such as a solar cell through an insulation film, it is possible to suppress contact resistance between the electrode and the semiconductor substrate, and also suppress excessive fire-through, resulting in improved conversion efficiency of the solar cell.SOLUTION: A glass contains, by mol% in terms of oxide, PbO of 40% or more and 70% or less, SiO2 of 4% or more and 20% or less, Al2O3 of 1% or more and 10% or less, B2O3 of 0% or more and 30% or less, and Ga2O3 of 1% or more and 20% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to glass, conductive paste, and solar cells, and particularly to glass suitable for forming electrodes of solar cells, a conductive paste using glass powder made of the glass, and a solar cell having electrodes formed by the conductive paste.

Background Art

[0002] Conventionally, electronic devices in which a conductive layer serving as an electrode is formed on a semiconductor substrate such as silicon (Si) have been used for various applications. This conductive layer serving as an electrode is formed by applying a conductive paste in which conductive metal powders such as aluminum (Al), silver (Ag), and copper (Cu) and glass powder are dispersed in an organic vehicle onto the semiconductor substrate and firing at a temperature required for electrode formation.

[0003] When forming an electrode on a semiconductor substrate in this way, an insulating film may be formed over the entire surface of the semiconductor substrate on which the electrode is to be formed, and a patterned electrode may be formed so as to partially penetrate the insulating film and contact the semiconductor substrate.

[0004] For example, in a solar cell, an antireflection film is provided on a semiconductor substrate serving as a light-receiving surface, and electrodes are provided thereon in a pattern. The antireflection film is for reducing the surface reflectance while maintaining a sufficient visible light transmittance to enhance the light-receiving efficiency, and is usually composed of an insulating material such as silicon nitride, titanium dioxide, silicon dioxide, or aluminum oxide.

[0005] Also, in a double-sided light-receiving solar cell capable of receiving light on the back side as well, a passivation film made of the same insulating material as the antireflection film is provided over the entire back surface, and electrodes are formed thereon in a form that partially contacts the semiconductor substrate.

[0006] Since the electrode needs to be formed in contact with the semiconductor substrate, when forming the electrode, the insulating film is removed according to the pattern of the electrode to be formed, and the electrode is formed on the portion where the insulating film is removed. As a method of removing the insulating film, there is a method of physically removing it with a laser or the like, but this method involves an increase in the manufacturing process and an increase in the equipment introduction cost. Therefore, in recent years, a method called fire-through, in which a conductive paste containing conductive metal powder and glass powder, that is, a paste-like electrode material, is applied on the insulating film and heat-treated to penetrate the insulating film into the conductive paste, has been adopted.

[0007] The above technique for forming an electrode on a semiconductor substrate is also applied to the formation of an electrode on a pn junction type semiconductor substrate in a solar cell.

[0008] Patent Document 1 discloses a specific glass composition in the glass used for the conductive paste, which contains, by mass%, 60 to 95% of PbO, 0 to 10% of B2O3, and 1 to 30% of SiO2 + Al2O3.

[0009] Patent Document 2 describes a lead glass used for a sealing material that can be sealed at a relatively low temperature. As a specific glass composition, it contains, by mass%, 5 to 40% of B2O3, 0 to 5% of Al2O3, 0 to 60% of PbO, 2 to 20% of ZnO, 5 to 50% of BaO, 0 to 30% of V2O5, 0 to 25% of Sb2O3, 0 to 20% of SiO2, and 0 to 40% of Bi2O3.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0011] As disclosed in Patent Documents 1 and 2, for the glass used in forming the electrodes of a solar cell, technologies have been developed to improve the formability of the electrodes and reduce the contact resistance between the electrodes and the semiconductor substrate.

[0012] However, the glass described in Patent Document 1 has a problem that the fluidity of the glass during firing is high, erosion of the antireflection film progresses due to excessive fire-through, and the open-circuit voltage (Voc) deteriorates. In addition, there is a problem that the conversion efficiency with the solar cell is low due to high contact resistance. Further, the glass described in Patent Document 2 has a problem that the reactivity with silicon nitride (SiN) used for the passivation film is high and the fire-through progresses excessively, resulting in deterioration of Voc.

[0013] Particularly in solar cells such as TOPCon (Tunnel Oxide Passivated Contact), at present, technologies for improving the conversion efficiency of solar cells by reducing the contact resistance between the electrodes and the semiconductor substrate and suppressing excessive fire-through are under development.

[0014] The present invention aims to provide a glass used for forming an electrode, which can improve the conversion efficiency of a solar cell by suppressing the contact resistance between the electrode and the semiconductor substrate and suppressing excessive fire-through when forming the electrode on a semiconductor substrate such as a solar cell via an insulating film. The present invention further aims to provide a conductive paste containing glass powder made of the glass and a solar cell with improved conversion efficiency by using the conductive paste.

Means for Solving the Problems

[0015] The inventors of the present invention have found that the above problems can be solved by setting the glass composition within a specific range, and have completed the present invention. The present invention provides a glass, a conductive paste, and a solar cell having the following configurations.

[0016] [1] In terms of mol% representation in terms of oxides, Containing 40% or more and 70% or less of PbO, 4% or more and 20% or less of SiO2, 1% or more and 10% or less of Al2O3, 0% or more and 30% or less of B2O3, and 1% or more and 20% or less of Ga2O3 Glass. [2] The glass according to [1], wherein the total content (PbO + Bi2O3 + SiO2 + B2O3) of PbO, Bi2O3, SiO2, and B2O3 is 70% or more in terms of mol% in terms of oxide conversion. [3] The glass according to [1] or [2], wherein the ratio of the total content (Ga2O3 + B2O3) of Ga2O3 and B2O3 to the total content (SiO2 + Ga2O3 + B2O3) of SiO2, Ga2O3, and B2O3 is 40% or more and 90% or less in terms of mol% in terms of oxide conversion. [4] The glass according to any one of claims [1] to [3], wherein the glass transition temperature is 280°C or more and 430°C or less. [5] A conductive paste containing a glass powder, a conductive metal powder, and an organic vehicle, the glass being the glass according to any one of [1] to [4]. [6] A solar cell including an electrode formed using the conductive paste according to [5]. [7] A solar cell comprising a silicon substrate having a sunlight receiving surface, a first insulating film provided on the sunlight receiving surface of the silicon substrate, a second insulating film provided on the surface of the silicon substrate opposite to the sunlight receiving surface, a first electrode that penetrates a part of the first insulating film and contacts the silicon substrate, a second electrode that penetrates a part of the second insulating film and contacts the silicon substrate, wherein the first electrode includes at least one metal selected from the group consisting of Al, Ag, Cu, Au, Pd, and Pt, and a glass containing 40% or more and 70% or less of PbO, 4% or more and 20% or less of SiO2, 1% or more and 10% or less of Al2O3, 0% or more and 30% or less of B2O3, and 1% or more and 20% or less of Ga2O3 in terms of mol% in terms of oxide conversion. ​

Advantages of the Invention

[0017] The glass of the present invention has a specific composition range. In particular, by containing a specific amount of Ga2O3 together with PbO, which has a high effect of promoting fire-through, the excessive reaction between silicon nitride used for the passivation film and the glass can be suppressed, and excessive fire-through can be suppressed. Further, by containing Ga2O3, Ga diffuses into the Si substrate, and the contact resistance can be reduced.

[0018] Therefore, when the glass of the present invention is used in a conductive paste together with a conductive component, excellent fire-through properties can be obtained, the contact resistance between the electrode and the semiconductor substrate can be reduced, and the conversion efficiency of the solar cell can be improved.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present invention will be described. <Glass> The glass according to the present embodiment contains, in terms of mol% in terms of oxide, 40% or more and 70% or less of PbO, 4% or more and 20% or less of SiO2, 1% or more and 10% or less of Al2O3, 0% or more and 30% or less of B2O3, and 1% or more and 20% or less of Ga2O3. In the following description, unless otherwise specified, the “%” indication of the content of each component of the glass is in terms of mol% in terms of oxide. In this specification, the “~” representing a numerical range includes the upper and lower limits. Further, in this specification, “containing 0%” means not containing.

[0021] The content of each component in the glass according to this embodiment is determined from the results of inductively coupled plasma (ICP-AES: Inductively Coupled Plasma-Atomic Emission Spectroscopy) analysis or electron probe micro analyzer (EPMA: Electron Probe Micro Analyzer) analysis of the obtained glass.

[0022] Hereinafter, in the description of the glass components, "conductive paste" means "conductive paste containing the glass of the present invention". Also, "electrode" means "electrode obtained using the conductive paste containing the glass of the present invention".

[0023] PbO has reactivity with an insulating film and a silicon substrate, and has a function of improving the softening fluidity of the glass. Thereby, for example, when an electrode is formed on a semiconductor substrate or the like using a conductive paste containing the glass according to this embodiment, the contact resistance between the electrode and the substrate or the like can be reduced, and the bonding strength can be improved.

[0024] The glass according to this embodiment contains PbO in a proportion of 40% or more and 70% or less. By setting the content of PbO to 40% or more, fire-through progresses easily, and sufficient contact between the electrode and the insulating film and the semiconductor substrate can be ensured. The content of PbO is preferably 45% or more, more preferably 48% or more, and still more preferably 55% or more. Also, by setting the content of PbO to 70% or less, excessive fire-through can be prevented, and erosion of the insulating film can be suppressed. The content of PbO is preferably 67% or less, more preferably 63% or less, and still more preferably 59% or less.

[0025] SiO2 is a component that improves the stability and weather resistance of glass and also adjusts the reactivity with an insulating film or a silicon substrate. The glass of this embodiment contains SiO2 at a ratio of 4% or more and 20% or less. By setting the content of SiO2 to 4% or more, it becomes easier to vitrify and electrode formation becomes easier. The content of SiO2 is preferably 6% or more, more preferably 8% or more, and still more preferably 10% or more. Also, when the content of SiO2 is 20% or less, an increase in the glass transition point is suppressed, the glass exhibits excellent fluidity during firing, and the reactivity with an insulating film or a silicon substrate is improved. The content of SiO2 is preferably 18% or less, more preferably 16% or less, and still more preferably 14% or less.

[0026] Al2O3 is a component that improves the weather resistance of glass. The glass according to this embodiment contains Al2O3 at a ratio of 1% or more and 10% or less. By setting the content of Al2O3 to 1% or more, the weather resistance is improved and the glass can be stabilized. The content of Al2O3 is preferably 2% or more, more preferably 3% or more. Also, when the content of Al2O3 is 10% or less, an increase in the glass transition point is suppressed and excellent fluidity is exhibited during softening. The content of Al2O3 is preferably 8% or less, more preferably 6% or less, and still more preferably 5% or less.

[0027] B2O3 has a function of reducing the contact resistance by diffusion of B into the Si substrate, further improves the fluidity during softening of the glass, and improves the bonding strength with the semiconductor substrate. Also, B2O3 is a component for forming the network structure of the glass and contributes to the stabilization of the glass. The glass according to this embodiment contains B2O3 at a ratio of 0% or more and 30% or less. When B2O3 is contained, its content is preferably 4% or more, more preferably 8% or more, and still more preferably 10% or more. Also, by setting the content of B2O3 to 30% or less, the weather resistance can be improved. The content of B2O3 is preferably 28% or less, more preferably 26% or less, and still more preferably 24% or less.

[0028] Ga2O3 is a component that suppresses the excessive reaction of the glass with silicon nitride used for the passivation film, thereby suppressing excessive firing through. Further, Ga has a function of reducing the contact resistance by diffusing into the Si substrate. The glass according to this embodiment contains Ga2O3 at a ratio of 1% or more and 20% or less. By setting the content of Ga2O3 to 1% or more, excessive firing through can be suppressed, the contact resistance can be reduced, and the conversion efficiency of the solar cell can be improved. Also, the weather resistance can be improved and the glass can be stabilized. The content of Ga2O3 is preferably 1.5% or more, more preferably 2.5% or more. Also, from the viewpoint of stabilizing the glass, the content of Ga2O3 is 20% or less, preferably 18% or less, more preferably 15% or less, and even more preferably 10% or less.

[0029] Bi2O3 has a function of improving the softening fluidity of the glass similar to PbO, but has a weaker force of firing through than PbO. Therefore, by replacing a part of PbO with Bi2O3 in the glass composition, excessive firing through can be suppressed and the contact property between the electrode and the insulating film and the semiconductor substrate can be improved. Therefore, the glass according to this embodiment may contain Bi2O3. When containing Bi2O3, the content of Bi2O3 is preferably 2% or more, more preferably 4% or more. Also, from the viewpoint of ensuring the firing through property, the content of Bi2O3 is preferably 15% or less, more preferably 10% or less.

[0030] TiO2 is a component that promotes crystallization during firing and suppresses the excessive progress of firing through, and may be contained. When containing TiO2, the content of TiO2 is preferably 1% or more, more preferably 2% or more, from the viewpoint of suppressing the excessive progress of firing through. Also, from the viewpoint of ensuring the fluidity of the glass during softening, the content of TiO2 is preferably 12% or less, more preferably 10% or less.

[0031] La2O3 is a component that promotes crystallization during firing and suppresses excessive progress of fire-through, and it may be contained. When La2O3 is contained, from the viewpoint of suppressing excessive progress of fire-through, the content of La2O3 is preferably 0.5% or more, more preferably 1.0% or more, and still more preferably 1.5% or more. Further, from the viewpoint of ensuring fire-through properties, the content of La2O3 is preferably 10% or less, more preferably 8% or less, and still more preferably 6% or less.

[0032] Nb2O5 is a component that improves the fluidity and reactivity of the glass and also improves the weather resistance, and it may be contained. When Nb2O5 is contained, its content is preferably 0.5% or more, more preferably 1.0% or more. Further, in order to enhance the reactivity and improve the electrical characteristics of the solar cell, the content of Nb2O5 is preferably 10% or less, more preferably 8% or less, and still more preferably 6% or less.

[0033] For the glass according to this embodiment, it is preferable that the total content of PbO, Bi2O3, SiO2, and B2O3 (PbO + Bi2O3 + SiO2 + B2O3) is 70% or more. When (PbO + Bi2O3 + SiO2 + B2O3) is within the above range, the glass can be stabilized. (PbO + Bi2O3 + SiO2 + B2O3) is more preferably 75% or more, and still more preferably 80% or more. Further, from the viewpoint of suppressing excessive fire-through, (PbO + Bi2O3 + SiO2 + B2O3) is preferably 98% or less, more preferably 95% or less, and still more preferably 93% or less.

[0034] For the glass according to this embodiment, it is preferable that the ratio of the total content of Ga2O3 and B2O3 (Ga2O3 + B2O3) to the total content of SiO2, Ga2O3, and B2O3 (SiO2 + Ga2O3 + B2O3) is 40% or more and 90% or less. When the ratio is 40% or more, the p of the semiconductor wafer +Diffusion of B and Ga, which can function as acceptors in the layer, into the Si substrate is promoted, so that contact resistance can be reduced. This ratio is more preferably 45% or more, still more preferably 50% or more. Also, when the ratio is 90% or less, the glass can be stabilized. It is more preferably 85% or less, still more preferably 80% or less. The ratio of the total content of Ga2O3 and B2O3 to the total content of SiO2, Ga2O3 and B2O3 (SiO2 + Ga2O3 + B2O3) refers to the ratio of the total content of Ga2O3 and B2O3 (Ga2O3 + B2O3) when the total content of SiO2, Ga2O3 and B2O3 (SiO2 + Ga2O3 + B2O3) is taken as 100%.

[0035] The glass according to this embodiment adds Ga2O3 that can suppress excessive reaction with silicon nitride used for the passivation film together with PbO having high fire-through property. Thereby, excessive fire-through can be suppressed, electrodes can be formed while suppressing erosion of insulating films such as antireflection films, and Voc can be improved. Further, the glass according to this embodiment can promote diffusion of impurities from the glass into the p + layer and reduce contact resistance. For this reason, during semiconductor wafer manufacturing, Ga, which is a Group 13 element used for forming the p + layer, acts as an acceptor, and it is considered that contact resistance can be reduced by diffusion of Ga as an impurity into the Si substrate. Also, since B is also a Group 13 element like Ga, it is considered to act as an acceptor and reduce contact resistance. Therefore, it is preferable to contain B2O3 together with Ga2O3.

[0036] The glass according to this embodiment may contain other optional components. Specific examples of other optional components include various oxide components commonly used in glass, such as P2O3, As2O5, Sb2O5, Na2O, K2O, Fe2O3, CuO, Sb2O3, SnO2, MnO, MnO2, CeO2.

[0037] Other components are used in combination of one or more according to the purpose. The content of other optional components is preferably 20% or less, more preferably 15% or less, still more preferably 10% or less, and even more preferably 5% or less for each component. Further, the total content of other components is preferably 20% or less, more preferably 10% or less.

[0038] The glass according to this embodiment preferably has a glass transition temperature (Tg) of 280°C or higher and 430°C or lower. When the glass transition temperature is 280°C or higher, excessive fire-through can be suppressed and Voc is improved. The glass transition temperature is more preferably 290°C or higher, still more preferably 300°C or higher. Further, when the glass transition temperature is 430°C or lower, excellent fluidity is exhibited during softening, so that the reaction between the electrode and the semiconductor substrate proceeds, the contact resistance is reduced, and the contact property between the electrode and the insulating film can be improved. The glass transition temperature is more preferably 420°C or lower, still more preferably 410°C or lower.

[0039] In this specification, the glass transition temperature (Tg) is obtained by determining the first inflection point of the DTA chart measured at a heating rate of 10°C / min using a differential thermal analysis (DTA) apparatus TG8110 manufactured by Rigaku Corporation as Tg.

[0040] The manufacturing method of the glass according to this embodiment is not particularly limited. Specifically, for example, it can be manufactured by the method shown below.

[0041] First, a raw material mixture is prepared. The raw materials are not particularly limited as long as they are raw materials used for manufacturing ordinary oxide-based glass, and oxides, carbonates, etc. can be used. In the obtained glass, the types and ratios of the raw materials are appropriately adjusted so as to fall within the above composition range to obtain a raw material mixture.

[0042] Next, the raw material mixture is heated by a known method to obtain a melt. The temperature for heating and melting (melting temperature) is preferably 800 to 1400°C, more preferably 900 to 1300°C. The time for heating and melting is preferably 30 to 300 minutes.

[0043] Thereafter, the glass according to the present embodiment is obtained by cooling and solidifying the melt. The cooling method is not particularly limited. For example, methods such as rapid cooling by a roll-out machine, a press machine, dropping into a cooling liquid, etc. can be mentioned. The obtained glass is preferably completely amorphous, that is, the crystallinity is 0%. However, as long as the effects of the present invention are not impaired, it may contain crystallized portions.

[0044] The shape of the glass obtained above is not particularly limited, and it may be, for example, a block shape, a plate shape, a thin plate shape (flake shape), a powder shape, etc.

[0045] The glass according to the present embodiment has a function as a binder and has conductivity, and is preferably used for a conductive paste. The conductive paste containing the glass according to the present embodiment has high conductivity and is preferably used, for example, for forming electrodes of a solar cell. When the glass according to the present embodiment is contained in the conductive paste, the glass is preferably contained as glass powder.

[0046] <Glass powder> The glass powder according to the present embodiment is composed of the glass according to the present embodiment, and D 50 is preferably 0.3 μm or more and 3.0 μm or less. D 50 being 0.3 μm or more improves the dispersibility when made into a conductive paste. Also, D 50 being 3.0 μm or less makes it difficult for locations where there is no glass powder to exist around the conductive metal powder, so the adhesiveness between the electrode and a semiconductor substrate, etc. is further improved. D 50 is more preferably 0.5 μm or more. Also, it is more preferably 2.7 μm or less, and even more preferably 2.0 μm or less.

[0047] In addition, in this specification, "D 50"D50" represents the volume-based median particle size in the cumulative particle size distribution. Specifically, in the cumulative particle size curve of the particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer, it represents the particle size when the cumulative amount occupies 50% based on volume.

[0048] The glass powder according to this embodiment can be obtained by pulverizing the glass produced as described above to have the above-specified particle size distribution by, for example, a dry pulverization method or a wet pulverization method.

[0049] As a method for pulverizing glass to obtain the glass powder according to this embodiment, for example, a method of dry pulverizing a glass of an appropriate shape and then wet pulverizing it is preferable. Dry pulverization and wet pulverization can be carried out using a pulverizer such as a roll mill, a ball mill, a jet mill, etc. The particle size distribution can be adjusted, for example, by adjusting the pulverizer such as the pulverization time in each pulverization or the size of the balls in the ball mill. In the case of the wet pulverization method, it is preferable to use water as the solvent. After wet pulverization, moisture is removed by drying or the like to obtain the glass powder. In order to adjust the particle size of the glass powder, in addition to pulverizing the glass, classification may be performed as necessary.

[0050] <Conductive paste> The glass according to this embodiment can be applied to a conductive paste as glass powder. The conductive paste using the glass according to this embodiment contains the glass powder, conductive metal powder, and organic vehicle according to the above embodiment.

[0051] As the conductive metal powder contained in the conductive paste according to this embodiment, powders of metals usually used for electrodes formed on circuit boards (including multilayer electronic components) such as semiconductor substrates and insulating substrates are used without particular limitation. Specifically, powders such as Ag, Al, Cu, Au, Pd, Pt, etc. can be mentioned as the conductive metal powder. Among these, from the viewpoint of productivity, Ag powder and Al powder are preferable. From the viewpoint of suppressing aggregation and obtaining uniform dispersibility, the particle diameter of the conductive metal powder is D 50 is preferably 0.3 μm or more and 10 μm or less.

[0052] The content of the conductive metal powder in the conductive paste is preferably 63.0% by mass or more and 97.9% by mass or less based on the total mass of the conductive paste. When the content of the conductive metal powder is 63.0% by mass or more, further sintering of the conductive metal powder can be suppressed, and the occurrence of glass floating and the like can be suppressed. Further, by setting the content of the conductive metal powder to 97.9% by mass or less, it is easy to sufficiently cover the periphery of the conductive metal powder with glass precipitates. In addition, the adhesiveness between the electrode and a circuit board such as a semiconductor substrate or an insulating substrate can be improved. The content of the conductive metal powder based on the total mass of the conductive paste is more preferably 95.0% by mass or less.

[0053] The content of the glass in the conductive paste is preferably, for example, 0.1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the conductive metal powder. By setting the content of the glass to 0.1 part by mass or more, it is easy to sufficiently cover the periphery of the conductive metal powder with glass precipitates. In addition, the adhesiveness between the electrode and a circuit board such as a semiconductor substrate or an insulating substrate can be improved. Further, by setting the content of the glass powder to 10 parts by mass or less, further sintering of the conductive metal powder can be suppressed, and the occurrence of glass floating and the like can be suppressed. The content of the glass powder with respect to 100 parts by mass of the conductive metal powder is more preferably 0.5 part by mass or more and 8 parts by mass or less.

[0054] Examples of the organic resin binder used for the organic vehicle include organic resins such as cellulose-based resins such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, oxyethyl cellulose, benzyl cellulose, propyl cellulose, and nitrocellulose, and acrylic resins obtained by polymerizing one or more of acrylic monomers such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-hydroxyethyl methacrylate, butyl acrylate, and 2-hydroxyethyl acrylate.

[0055] Examples of solvents used for organic vehicles include, for example, in the case of cellulose-based resins, solvents such as terpineol, butyl diglycol acetate, ethyl diglycol acetate, and propylene glycol diacetate are preferably used. Further, for example, in the case of acrylic resins, solvents such as methyl ethyl ketone, terpineol, butyl diglycol acetate, ethyl diglycol acetate, and propylene glycol diacetate are preferably used.

[0056] The ratio of the organic resin binder to the solvent in the organic vehicle is not particularly limited, but is selected so that the resulting organic resin binder solution has a viscosity capable of adjusting the viscosity of the conductive paste. Specifically, the mass ratio represented by the organic resin binder: solvent is preferably about 3:97 to 15:85.

[0057] The content of the organic vehicle in the conductive paste is preferably 2% by mass or more and 30% by mass or less based on the total amount of the conductive paste. By setting the content of the organic vehicle to 2% by mass or more, an increase in the viscosity of the conductive paste is suppressed, the coatability such as printing of the conductive paste is improved, and a good conductive layer (electrode) is easily formed. Further, by setting the content of the organic vehicle to 30% by mass or less, a decrease in the content ratio of the solid content of the conductive paste is prevented, and a sufficient coating film thickness can be obtained.

[0058] As one aspect of the conductive paste according to the present embodiment, a conductive paste containing 63.0 to 97.9% by mass of a metal selected from the group consisting of Ag, Al, Cu, Au, Pd, and Pt based on the total mass of the conductive paste, and in terms of mol% of the oxide, containing 40% or more and 70% or less of PbO, 4% or more and 20% or less of SiO2, 1% or more and 10% or less of Al2O3, 0% or more and 30% or less of B2O3, and 1% or more and 20% or less of Ga2O3, and containing 0.1 to 10 parts by mass of the glass based on 100 parts by mass of the metal, and containing 2 to 30% by mass of the organic vehicle based on the total mass of the conductive paste can be mentioned.

[0059] In addition to the glass, conductive metal powder, and organic vehicle described above, known additives can be blended into the conductive paste according to the present embodiment as needed and within the limits that do not conflict with the objectives of the present embodiment.

[0060] Examples of such additives include various inorganic oxides. Specific examples of inorganic oxides include B2O3, ZnO, SiO2, Al2O 3、 TiO2, MgO, ZrO2, and Sb2O3, as well as composite oxides thereof. These inorganic oxides have the effect of moderating the sintering of the conductive metal powder during the firing of the conductive paste, and thereby have the function of adjusting the bonding strength after firing. The size of the additive composed of these inorganic oxides is not particularly limited, but for example, those with D 50 of 10 μm or less can be preferably used.

[0061] The content of the inorganic oxide in the conductive paste is appropriately set according to the purpose, but is preferably 10% by mass or less, more preferably 7% by mass or less, based on the glass powder. When the content of the inorganic oxide relative to the glass powder is 10% by mass or less, a decrease in the fluidity of the conductive paste during electrode formation is suppressed, and sufficient bonding strength between the electrode and a circuit board such as a semiconductor substrate or an insulating substrate can be ensured. Also, in order to obtain a practical compounding effect (adjustment of the bonding strength after firing), the lower limit value of the above content is preferably 0.5% by mass or more, more preferably 1.0% by mass or more.

[0062] Additives known in the conductive paste, such as defoamers and dispersants, may be added to the conductive paste. Note that the above organic vehicle and these additives are usually components that disappear during the electrode formation process. For the preparation of the conductive paste, known methods using a rotary mixer equipped with stirring blades, a kneader, a roll mill, a ball mill, etc. can be applied.

[0063] The application and firing of the conductive paste onto a circuit board such as a semiconductor substrate or an insulating substrate can be carried out by the same method as the application and firing in the conventional electrode formation. Examples of the application method include screen printing and the dispensing method. The firing temperature depends on the type and surface state of the conductive metal powder contained, but generally a temperature in the range of 500 to 1000 °C can be exemplified. The firing time is appropriately adjusted according to the shape, thickness, etc. of the electrode to be formed. Also, a drying treatment at about 80 to 200 °C may be provided between the application and firing of the conductive paste.

[0064] <Solar cell> The solar cell according to this embodiment includes an electrode formed using the conductive paste described in the above <Conductive paste>, specifically, an electrode baked on a semiconductor substrate. In the solar cell according to this embodiment, it is preferable that at least one of the electrodes is an electrode provided in such a manner that it partially penetrates an insulating film by fire-through and contacts the semiconductor substrate using the above conductive paste.

[0065] Examples of such an electrode that penetrates the insulating film of the solar cell include an electrode provided in such a manner that it partially penetrates an insulating film, which is an antireflection film, as an electrode on the light-receiving surface of a solar cell using a pn junction type semiconductor substrate and contacts the semiconductor substrate. Examples of the insulating material constituting the insulating film, which is an antireflection film, include silicon nitride, titanium dioxide, silicon dioxide, aluminum oxide, etc., and it is preferable that the insulating material consists of two layers. In this case, the light-receiving surface may be one side or both sides of the semiconductor substrate, and the semiconductor substrate may be either n-type or p-type, but in order to increase the efficiency of the solar cell more, it is preferable that it is both sides, and it is preferable that the semiconductor substrate is n-type. The electrode provided on the light-receiving surface of such a solar cell can be formed by fire-through using the above conductive paste.

[0066] A configuration example of the solar cell according to the present embodiment will be described below, but the configuration of the solar cell according to the present embodiment is not limited to the configuration example. The solar cell according to the configuration example includes a silicon substrate having a sunlight receiving surface (hereinafter simply referred to as the "receiving surface" or "surface"), a first insulating film provided on the receiving surface of the silicon substrate, and a second insulating film provided on the surface opposite to the receiving surface of the silicon substrate (hereinafter simply referred to as the "back surface"), a first electrode that penetrates a part of the first insulating film and contacts the silicon substrate, and a second electrode that partially contacts the silicon substrate through the opening of the second insulating film.

[0067] More specifically, for example, as shown in FIG. 1, the solar cell 10 has a p + layer 1b provided on the light receiving surface S1 of the n-type Si semiconductor substrate 1. An antireflection film (silicon nitride / aluminum oxide) 2b is further formed on its surface. However, in some regions, an Ag-Al electrode 3b is formed in contact with the p + layer 1b through the antireflection film (silicon nitride / aluminum oxide) 2b.

[0068] Similarly, on the non-light receiving surface S2 of the n-type Si semiconductor substrate 1, an n + layer 1a is provided, and an antireflection film (silicon nitride / aluminum oxide) 2a is further formed on its surface. However, in some regions, an Ag electrode 3a is formed in contact with the n + layer 1a through the antireflection film (silicon nitride / aluminum oxide) 2a.

[0069] The Ag-Al electrode 3b and the Ag electrode 3a can be formed by applying and firing the Ag-Al electrode 3b or the Ag electrode 3a through the conductive paste in a part of the region on the surfaces of the antireflection films (silicon nitride / aluminum oxide) 2b and 2a.

[0070] In the configuration example, the first electrode is an electrode formed by fire-through using the conductive paste according to the present embodiment, and preferably contains at least one selected from the group consisting of metals including Al, Ag, Cu, Au, Pd, and Pt, and the glass according to the present embodiment.

[0071] The first electrode preferably contains 90% by mass or more and 99.9% by mass or less of the metal and 0.1% by mass or more and 10% by mass or less of the glass according to this embodiment. Further, the first electrode more preferably contains at least Ag.

[0072] In addition, in this configuration example, it is more preferable that the first electrode and the second insulating film include a metal oxide film in contact with both surfaces of the silicon substrate and a silicon nitride film further on the metal oxide film. The metal oxide film more preferably consists of aluminum oxide or silicon dioxide.

Examples

[0073] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples. Examples 1 to 11 are examples, and Examples 12 to 21 are comparative examples.

[0074] (Examples 1 to 20) Glass was manufactured as thin plate glass by the following method, and glass powder was manufactured from the thin plate glass. The particle size distribution of the glass powder was measured, and the glass transition temperature of the glass was measured using the glass powder.

[0075] <Manufacture of glass (thin plate glass)> The raw material powders were blended and mixed so as to have the compositions shown in Tables 1 and 2 in terms of mol% based on oxides, and melted in an electric furnace at 1000 to 1600 ° C for 30 minutes to 1 hour using a crucible to form thin plate glass made of the glass having the compositions shown in Tables 1 and 2.

[0076] <Manufacture of glass powder> In each example, the obtained thin plate glass was pulverized by combining dry pulverization and wet pulverization as follows to adjust the particle size distribution. The particle size distribution of the obtained glass powder was measured, and the glass transition temperature was measured using the glass powder.

[0077] Dry pulverization was performed with a ball mill for 6 hours, and coarse particles were removed with a 150-mesh sieve. Next, after the dry pulverization obtained above, the glass powder from which the coarse particles were removed was D50 Using a ball mill, wet grinding was carried out with water so that 50 was within a predetermined range, thereby producing glass powder with a desired particle size distribution. During this wet grinding, alumina balls with a diameter of 5 mm were used to obtain a predetermined D 50 After that, the slurry obtained by wet grinding was filtered and dried at 130 °C using a dryer to remove moisture, thereby producing glass powder.

[0078] <Evaluation> For the glass of each example, the D 50 and glass transition temperature of the glass powder were evaluated in the following manner. The results are shown in Tables 1 and 2 together with the composition.

[0079] (D 50 ) 0.02 g of glass powder was mixed with 60 cc of isopropyl alcohol (IPA) and dispersed for 1 minute by ultrasonic dispersion. The sample was put into a Microtrac measuring machine (laser diffraction / scattering type particle size distribution measuring device) to obtain the value of D 50 .

[0080] (Glass transition temperature) The obtained glass powder was filled into an aluminum pan, and the heating rate was measured at 10 °C / min using a differential thermal analyzer TG8110 manufactured by Rigaku Corporation. The first inflection point of the DTA chart obtained by the measurement was taken as the glass transition temperature (indicated as "Tg" in Tables 1 and 2).

[0081] <Production of Conductive Paste> Conductive pastes for forming Ag - Al electrodes each containing the glass powder of Examples 1 to 20 were prepared by the following method.

[0082] First, 85 parts by mass of butyl diglycol acetate was mixed with 15 parts by mass of ethyl cellulose and stirred at 85 °C for 2 hours to prepare an organic vehicle. Next, 15 parts by mass of the obtained organic vehicle was mixed with 84.5 parts by mass of Ag powder (spherical silver powder: AG-4-8F, manufactured by DOWA Electronics Co., Ltd.) and 0.5 parts by mass of Al powder (atomized aluminum powder #600F, manufactured by Minako Co., Ltd.), and then kneaded by a kneader for 10 minutes. Then, the glass powders of Examples 1 to 21 were blended at a ratio of 2 parts by mass per 100 parts by mass of the metal powders (Ag powder and Al powder), and further kneaded by a kneader for 90 minutes to obtain a conductive paste for forming an Ag-Al electrode.

[0083] <Evaluation> (Measurement of contact resistance Rc) Using the conductive paste for forming an Ag-Al electrode prepared above, an Ag-Al electrode was formed on a semiconductor substrate through an insulating film (a two-layer film composed of a silicon nitride layer and an aluminum oxide layer) as follows, and the contact resistance at that time was evaluated.

[0084] The method for measuring the contact resistance will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram schematically showing a cross section of an example of a double-sided light-receiving type solar cell on an n-type Si substrate on which an electrode is formed using the conductive paste according to the present embodiment. FIG. 2 is a diagram showing an electrode pattern formed on a Si substrate used when evaluating the contact resistance Rc [Ω].

[0085] Using an n-type crystalline Si semiconductor substrate sliced to a thickness of 160 μm, first, in order to clean the sliced surface of the Si semiconductor substrate, the front and back surfaces were etched with hydrofluoric acid in a very small amount. Then, an uneven structure for reducing the light reflectance was formed on the light-receiving surface of the Si semiconductor substrate using a wet etching method.

[0086] Next, a p + layer was formed by diffusion on the light-receiving surface of the Si semiconductor substrate. B (boron) was used as the doping element for p-type conversion. In this way, an n-type Si semiconductor substrate having a p + layer was obtained. Next, on the light-receiving surface (p +An antireflection film was formed on the surface of the layer. As the material of the antireflection film, silicon nitride and aluminum oxide were mainly used. After forming an aluminum oxide layer with a thickness of 10 nm by ALD (Atomic Layer Deposition), a silicon nitride layer with a thickness of 80 nm was formed on the upper layer by plasma CVD. Next, on the p + An insulating film was formed on the back surface (the back of the n-type Si substrate) of the layer with respect to the p-layer of the semiconductor substrate. As the material of the antireflection film, silicon nitride was mainly used and it was formed with a thickness of 80 nm by plasma CVD.

[0087] Next, the conductive paste for forming an Ag-Al electrode obtained by using the glass powder of Examples 1 to 21 was applied in a line shape by screen printing on the surface on the light-receiving surface side of the obtained Si semiconductor substrate with an antireflection film, and dried at 120 °C.

[0088] Then, firing was performed at a peak temperature of 740 °C for 100 seconds using an infrared light heating furnace to form a surface Ag-Al electrode, and a single-sided cell for contact resistance measurement was completed. Note that the Ag-Al electrode was formed in a shape that penetrates the antireflection film and contacts the p + layer of the Si semiconductor substrate.

[0089] The contact resistance of the single-sided cell manufactured using the conductive paste for forming an Ag-Al electrode containing the glass powder of each of the above examples was measured by the TLM method (Transfer length Method). The contact resistance Rc [Ω] between the Ag-Al electrode formed via an insulating film (a two-layer film composed of a silicon nitride layer and an aluminum oxide layer) on the p + layer side and the Ag-Al electrode of the n-type Si semiconductor substrate was evaluated. For the contact resistance Rc [Ω], the anode side of the tester was fixed to the pattern P1 in FIG. 2, and the cathode side of the tester was applied to each position of the patterns P2, P3, P4, and P5 to measure the electrical resistance, and the contact resistance Rc [Ω] was obtained. The results are shown in Tables 1 and 2. Note that N / D in Tables 1 and 2 indicates that the paste thickened and printing could not be performed, or the contact resistance Rc [Ω] could not be measured because the resistance value was large.

[0090] (Residual ratio of silicon nitride layer (SiN residual ratio)) After measuring the contact resistance Rc, the residual ratio of the silicon nitride layer, which is one of the insulating films, was measured using an n-type Si semiconductor substrate to evaluate the reactivity between each paste and the silicon nitride layer. To remove the Ag-Al electrodes from the n-type Si semiconductor substrate, the substrate was immersed in a 50% aqueous nitric acid solution for 2 hours, then the substrate and a 1% aqueous hydrofluoric acid solution were placed in a vial and sealed, and immersed for 5 minutes while applying ultrasonic waves. Next, the substrate was washed with ion-exchanged water. By energy dispersive X-ray spectroscopy (SEM-EDS), nitrogen on the electrode (N1) and nitrogen outside the electrode (N2) were quantified, and the ratio of nitrogen on the electrode to nitrogen outside the electrode (N1 / N2) was calculated as the residual ratio of the silicon nitride layer. Note that N / D in Table 2 indicates that the paste thickened and printing was not possible.

[0091]

Table 1

[0092]

Table 2

[0093] As shown in Tables 1 and 2, Examples 1 to 11 had a higher SiN residual ratio compared to Comparative Examples 12 to 21. It was found that they promoted crystallization, suppressed the flow of glass during high-temperature firing, and could suppress excessive fire-through. Examples 12 to 17 were examples that did not contain Ga2O3, Example 18 did not contain Al2O3, SiO2, and Ga2O3, and the content of B2O3 was more than 30%, Example 19 did not contain Al2O3 and Ga2O3, and the content of PbO was more than 70%, Example 20 did not contain Ga2O3, and the content of Al2O3 was more than 10%, and Example 21 did not contain Ga2O3, the content of PbO was less than 40% and the content of SiO2 was more than 30%.

[0094] In addition, Examples 1 to 11, which are examples, had a lower contact resistance Rc and excellent insulating film penetration, that is, fire-through property, compared with the comparative examples. From this result, it was found that by using the glass of the present invention in the formation of a solar cell, excessive fire-through can be suppressed to increase Voc, and further, the contact resistance between the electrode and the semiconductor substrate can be reduced to improve the conversion efficiency.

Explanation of Signs

[0095] 10…Solar cell, 1…n-type Si semiconductor substrate, 1a…n + layer, 1b…p + layer, 2a…Anti-reflection film (silicon nitride / aluminum oxide), 2b…Anti-reflection film (silicon nitride / aluminum oxide), 3a…Ag electrode, 3b…Ag-Al electrode, S1…Light-receiving surface, S2…Non-light-receiving surface.

Claims

1. In terms of mol% in terms of oxide, PbO is 40% or more and 70% or less, SiO ２ at 4% or more and 20% or less, Al ２ O ３ is 1% or more and 10% or less, B ２ O ３ is 0% or more and 30% or less, and Ga ２ O ３ is 1% or more and 20% or less A glass containing the same.

2. In terms of molar percentage based on oxides, PbO, Bi ２ O ３ , SiO ２ , B ２ O ３ The total content of (PbO + Bi ２ O ３ + SiO ２ + B ２ O ３ ) is 70% or more. The glass according to claim 1.

3. In terms of molar percentage in terms of oxide, SiO ２ , Ga ２ O ３ and B ２ O ３ The total content of (SiO ２ + Ga ２ O ３ + B ２ O ３ ), the ratio of the total content of Ga ２ O ３ and B ２ O ３ (Ga ２ O ３ + B ２ O ３ ) is 40% or more and 90% or less. The glass according to claim 1 or 2.

4. The glass according to any one of Claims 1 to 3, having a glass transition temperature of 280°C or more and 430°C or less.

5. A conductive paste containing a glass powder, a conductive metal powder, and an organic vehicle, made of the glass according to any one of Claims 1 to 4.

6. A solar cell including an electrode formed using the conductive paste according to Claim 5.

7. A silicon substrate having a sunlight receiving surface, A first insulating film provided on the sunlight receiving surface of the silicon substrate, A second insulating film provided on the surface of the silicon substrate opposite to the sunlight receiving surface, A first electrode that penetrates a part of the first insulating film and contacts the silicon substrate, A second electrode that penetrates a part of the second insulating film and contacts the silicon substrate, A solar cell comprising: The first electrode includes at least one metal selected from the group consisting of Al, Ag, Cu, Au, Pd, and Pt, and a glass containing, in terms of mol% of oxide conversion, 40% or more and 70% or less of PbO, 4% or more and 20% or less of SiO ２ , 1% or more and 10% or less of Al ２ O ３ , 0% or more and 30% or less of B ２ O ３ , and 1% or more and 20% or less of Ga ２ O ３ , and a solar cell containing the same.

Images