JP2014078594A - Paste composition and solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a paste composition for forming a silver electrode, by which an excellent electric property (e.g. curve factor) and an excellent solder-bonding strength can be obtained even in the case of forming, by a fire-through method, a light-receiving surface electrode (silver electrode) of a solar battery of a shallow emitter structure with a thin n-layer.SOLUTION: A paste composition for forming a silver electrode 12 provided on a light-receiving surface of a crystalline silicon-based solar battery 10 comprises silver powder, glass powder, tellurium-containing powder, and transition metal-containing powder which are dispersed in an organic medium. To a total of 100 mass% of the paste composition, 0.1-1.5 mass% of the tellurium-containing powder and 0.001-0.5 mass% of the transition metal-containing powder are blended.

Description

  The present invention relates to a paste composition and a solar cell. More specifically, the present invention relates to a silver electrode forming paste composition used when a silver electrode is formed on the light-receiving surface side of a crystalline silicon solar cell by a fire-through method.

As a typical example of a solar cell that converts solar light energy into electric power, a solar cell that uses crystalline silicon (single crystal or polycrystal) as a semiconductor substrate, a so-called crystalline silicon solar cell is known. As such a crystalline silicon solar cell, for example, a single-sided light receiving type solar cell 110 as shown in FIG. 3 is known.
This solar cell 110 includes an n-Si layer 116 formed by a pn junction on the light-receiving surface (upper surface in FIG. 3) side of a p-type silicon substrate (Si wafer: p-Si layer made of p-type crystalline silicon) 111. The n-Si layer 116 includes an antireflection film 114 made of silicon nitride or titanium oxide, and a surface electrode (light-receiving surface electrode) 112 made of silver (Ag). On the other hand, on the back surface (bottom surface in FIG. 3) side of the p-type silicon substrate (p-Si layer) 111, a back surface side external connection electrode 122 made of silver (Ag) as well as the light receiving surface electrode 112 and a so-called back surface electric field. And an aluminum electrode 120 exhibiting a (BSF; Back Surface Field) effect.

  One of the methods for forming the light receiving surface electrode 112 is a method called a fire through method. In this fire-through method, for example, an antireflection film 114 is formed on almost the entire surface of the silicon substrate 111, and a paste composition for forming a light receiving surface electrode (hereinafter simply referred to as an electrode) is formed on the antireflection film 114 in any form. The paste is sometimes applied directly and fired. The electrode paste used here is mainly composed of, for example, conductive particles, glass powder, and an organic medium. Then, the glass powder in the electrode paste oxidizes the antireflection film 114 during firing and incorporates it into the glass, so that the electrical connection between the conductive particles in the paste and the n-Si layer 116 (specifically, Achieves ohmic contact). According to this method, the number of processes can be reduced as compared with an electrode formation method that involves partial removal of the antireflection film 114, and the distance between the removal portion of the antireflection film 114 and the formation position of the light receiving surface electrode 112 is reduced. There is no worry about gaps or overlaps. Therefore, such a fire-through method is preferably employed for forming the light receiving surface electrode 112.

When forming the light receiving surface electrode 112 of such a solar cell, the solar obtained by suppressing the contact resistance between the light receiving surface electrode 112 and the n-Si layer 116 by realizing good electrical connection. Efforts are being made to increase the fill factor (FF) and energy conversion efficiency of the battery (single cell).
For example, in Cited Document 1, a low contact resistance is obtained by adding 0.5% by weight of glass powder not containing Cd and particles of transition metal source oxides such as Fe, Mn, Co, and Cu to the paste. A silver conductor composition having a well and having high adhesive strength is disclosed. In Cited Document 2, substantially 3.6 to 6.5% by weight of Zn and Ti alone and oxides are added to the paste, and a Mn-containing additive is further added to 1% by weight. A thick film conductive composition that improves both electrical performance and solder adhesion by adding 4.5 wt% is disclosed.
Reference 3 discloses a conductive paste for electrode formation that does not cause an increase in contact resistance by adding 0.01 wt% to 10 wt% of TeO ₂ in the paste.

Patent No. 4291146 JP 2006-302891 A Japanese Patent No. 4754655 JP 2011-181680 A

  By the way, in the general configuration of the solar cell as described above, short-wavelength light has low transmittance, so it reaches the pn junction and is absorbed by the n-Si layer without contributing to power generation and changed to heat. It was. For this reason, the thickness (depth) of the n-Si layer is reduced in order to deliver light having a shorter wavelength to the pn junction portion with the highest possible intensity and to extract more current, that is, to increase the photoelectric conversion efficiency. Attempts have been made to make it thin. For example, it has been proposed that the n-Si layer, which has conventionally been about 300 nm to 500 nm thick, be made shallow emitters with a thickness of about 250 nm or less. According to such a configuration, high energy light in the vicinity of near-ultraviolet light (for example, a wavelength of 200 nm to 380 nm) can be used for power generation. Therefore, improvement in power generation efficiency of the solar cell is expected.

  However, when the thickness of the n-Si layer is reduced in this way, the n-Si layer itself increases in resistance and increases in sheet resistance, and it is necessary to reduce the dopant concentration in order to suppress surface recombination. From the above, there is a problem that it is difficult to obtain a good ohmic contact between the light-receiving surface electrode and the n-Si layer, and the contact resistance increases. In addition, when the above-described fire-through method is applied to the formation of the light-receiving surface electrode, the electrode paste may not only reach the n-Si layer but also erode to the vicinity of the pn junction interface beyond the n-Si layer. Strict conditions were required, and there was a risk of adverse effects on the fill factor (FF) and energy conversion efficiency of solar cells.

Note that the electrode paste suitable for the fire-through method disclosed in the above-mentioned Patent Documents 1 and 2 has a problem that the contact resistance is relatively high while the solder adhesive strength is improved. Therefore, it is difficult to use the solar cell having a shallow emitter structure because it further increases the contact resistance.
In addition, the electrode paste suitable for the fire-through method disclosed in Patent Document 3 does not sufficiently exhibit its characteristics when applied to a solar cell having a shallow emitter structure, and solder wettability is extremely impaired. Therefore, it was difficult to use.

  The present invention has been made in the background of the above circumstances, and its purpose is, for example, the case where the light-receiving surface electrode (silver electrode) of a solar cell having a thin shallow emitter structure with n layers is formed by the fire-through method. However, it is an object of the present invention to provide a paste composition for forming a silver electrode, which has excellent electrical characteristics (for example, fill factor) and solder adhesive strength. Another object of the present invention is to provide a solar cell including a light-receiving surface electrode formed using such a paste composition.

  In order to achieve the above object, the paste composition provided by the present invention is a paste composition for forming a silver electrode disposed on the light receiving surface of a crystalline silicon solar cell. In such paste composition, silver powder, glass powder, tellurium-containing powder, and transition metal-containing powder are dispersed in an organic medium, and when the total paste composition is 100% by mass, the above It is characterized in that the tellurium-containing powder is blended at a ratio of 0.1 mass% to 1.5 mass%, and the transition metal-containing powder is blended at a ratio of 0.001 mass% to 0.5 mass%.

  According to the above configuration, the paste composition contains the transition metal-containing powder together with the tellurium-containing powder. Therefore, when the silver electrode disposed on the light-receiving surface of the crystalline silicon-based solar cell is formed by the fire-through method, this is required. A good conductive path is formed between the silver electrode formed by the paste composition and the n-Si phase, and a strong bond is realized. That is, a good electrical and physical connection can be achieved. Thereby, the electrical characteristics (for example, conversion efficiency, a fill factor, etc.) of the obtained solar cell are improved, and a strong bond between the silver electrode and the substrate and high solder bonding strength are realized.

  The applicants have also proposed a paste composition capable of forming a light-receiving surface electrode by a fire-through method in a solar cell having a shallow emitter structure (see, for example, Patent Document 4). According to this paste composition, although relatively excellent solder adhesive strength and low contact resistance can be realized, it cannot be said that the effect of improving electric characteristics such as power generation efficiency based on the shallow emitter structure is sufficiently exhibited. There was room for further improvement.

Therefore, in a preferred embodiment of the paste composition disclosed herein, the crystalline silicon solar cell is characterized in that the sheet resistance of the silicon substrate is 60Ω / □ or more.
For example, this paste composition can form a good electrical bond with the n-Si layer having a high sheet resistance by the fire-through method. Further, it is considered that excessive erosion and diffusivity are not exhibited during fire-through. Therefore, the sheet resistance of the silicon substrate is 60 Ω / □ or more, and it can be used for forming a light receiving surface electrode of a solar cell having a shallow emitter structure, and the solar cell can be improved due to the shallow emitter structure. Without impairing the electrical characteristics, it is possible to achieve a strong bond between the silver electrode and the substrate and a high solder bonding strength. From this point of view, the paste composition disclosed herein can exhibit its features more clearly when used in the manufacture of a solar cell having a shallow emitter structure.

In a preferred embodiment of the paste composition disclosed herein, the transition metal-containing powder is at least one metal selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Or it is the powder of the oxide.
According to this configuration, the fill factor of the light-receiving surface electrode obtained from this paste composition can be effectively increased.

In a preferred embodiment of the paste composition disclosed herein, when the entire paste composition is 100% by mass,
Silver powder: 40% by mass to 95% by mass,
The glass powder: 0.5% by mass or more and 5% by mass or less,
The tellurium-containing powder: 0.1% by mass or more and 1.5% by mass or less,
The transition metal-containing powder: 0.001% by mass to 0.5% by mass,
It is characterized by being blended at a ratio of
According to such a configuration, a paste composition in which properties such as electrical conductivity, applicability, and sinterability are well-balanced is provided.

In a preferred embodiment of the paste composition disclosed herein, the glass powder is an oxide equivalent composition, the following composition:
SiO ₂ : 20 mol% or more and 65 mol% or less,
B ₂ O ₃ : 1 mol% or more and 18 mol% or less,
PbO: 20 mol% or more and 65 mol% or less,
Li ₂ O: 0.6 mol% or more and 18 mol% or less,
It is characterized by having.
According to such a configuration, a paste composition excellent in adhesive strength and electrical connection between the formed silver electrode and the n-Si layer, solder strength, and the like is provided.

In a preferred embodiment of the paste composition disclosed herein, the glass powder has a softening point in a range of 300 ° C. to 600 ° C.
According to such a configuration, a paste composition excellent in fire-through property can be realized in which the antireflection film can be suitably oxidized and incorporated, and excessive erosion of the n-Si layer is suppressed.

The solar cell provided by the present invention is characterized in that a light-receiving surface is provided with a silver electrode formed using any of the paste compositions described above.
According to such a configuration, a good conductive path is formed between the silver electrode and the n-Si phase on the light receiving surface of the crystalline silicon solar cell, and a strong bond is realized. That is, a good electrical and physical connection is realized. Thereby, the solar cell excellent in electrical characteristics (for example, a curve factor and adhesive strength), solder adhesive strength, etc. is implement | achieved.

In a preferred aspect of the solar cell disclosed herein, the crystalline silicon-based solar cell has an n-Si layer formed on the light-receiving surface side of a p-type silicon substrate, and antireflection on the n-Si layer. A film and the silver electrode,
The sheet resistance of the silicon substrate is 60Ω / □ or more.
The paste composition as described above can form a good electrical bond with an n-Si layer having a high sheet resistance, for example. Further, it is considered that the n-Si layer does not exhibit excessive erosion and diffusibility by firing. Therefore, according to such a configuration, even if the sheet resistance of the silicon substrate is 60Ω / □ or more and the shallow emitter structure is formed, the electrical characteristics such as the power generation efficiency improved due to the shallow emitter structure are impaired. Therefore, a solar cell having a strong bond between the silver electrode and the substrate and having a high solder bonding strength is realized.

In a preferred aspect of the solar cell disclosed herein, the silver electrode is formed by penetration firing of a coating film of the paste composition disposed on the antireflection film (hereinafter simply referred to as “fire”). It may be expressed as “It is formed by the through method.”).
According to such a configuration, since the shallow emitter structure as described above is simply formed using the fire-through method, there is no gap or overlap in the formation position of the antireflection film and the light receiving surface electrode at low cost. A solar cell excellent in electrical characteristics such as conversion efficiency can be provided.

It is sectional drawing which showed typically an example of the structure of the solar cell of this invention. It is the side view which showed typically the mode of the measurement of adhesive strength. It is sectional drawing which showed typically an example of the structure of the conventional solar cell.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, a method of applying a paste composition to a substrate, a baking method, a structure of a solar cell, etc.) It can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

[Paste composition]
The paste composition disclosed here is a paste composition for forming a silver electrode disposed on the light receiving surface of a crystalline silicon solar cell. This paste composition is essentially constituted by dispersing silver powder, glass powder, tellurium-containing powder, and transition metal-containing powder in an organic medium. In such a paste composition, when the total paste composition is 100% by mass, the tellurium-containing powder is 0.1% by mass to 1.5% by mass, and the transition metal-containing powder is 0.001% by mass to 0.00%. It is characterized by being blended at a ratio of 5% by mass or less. In addition, as long as the object of the present invention is realized, there is no strict limitation on other components and their blending ratio (amount), for example, addition of a dispersant or the like that can be generally used for this type of paste composition. An agent may be included.

[Silver powder]
The paste composition disclosed here contains silver (Ag) powder as the main solid content. Such silver powder may be an aggregate of particles mainly composed of silver (Ag), and is preferably an aggregate of particles made of Ag alone. However, even if such silver powder contains a trace amount of impurities other than Ag and Ag-based alloys (particles), it is included in the “silver powder” as long as it is an aggregate of Ag-based particles as a whole. Can be done. The silver powder may be produced by a conventionally known production method and does not require special production means.

  The shape of the particles constituting such silver powder is not particularly limited. Although it is typically spherical, it is not limited to a so-called true spherical shape. In addition to the spherical shape, for example, a flake shape or an irregular shape can be mentioned, and the silver powder may be composed of particles having such various shapes. When the silver powder is composed of particles having a small average particle size (for example, several μm size), it is preferable that 70% by mass or more of the particles (primary particles) have a spherical shape or a similar shape. For example, a silver powder having an aspect ratio (that is, a ratio of a major axis to a minor axis of the particle) of 70% by mass or more of particles constituting the silver powder is preferably 1 to 1.5.

When forming an Ag electrode as a light-receiving surface electrode on one surface (typically a light-receiving surface, but it may be the back surface) of a substrate (for example, a Si substrate) constituting a solar cell, a desired The application amount and application form of the paste composition can be taken into consideration so that the dimensions (line width, film thickness, etc.) and shape can be realized. Here, the silver powder suitable for forming the light-receiving surface electrode of such a solar cell is not particularly limited, but those having an average particle diameter of 20 μm or less are suitable. Yes, preferably from 0.01 μm to 10 μm, more preferably from 0.3 μm to 5 μm, for example, 2 μm ± 1 μm. Here, the average particle diameter means a particle diameter at a cumulative volume of 50% in a particle size distribution measured by a laser diffraction / scattering method, that is, D50 (median diameter).
For example, it is also possible to use a silver (mixed) powder in which a plurality of silver powders (typically two types) having different average particle diameters are mixed and the average particle diameter of the mixed powder is within the above range. it can. By using silver powder having an average particle diameter as described above, a dense Ag electrode suitable as a light-receiving surface electrode can be formed.

  The content of the silver powder in the paste composition disclosed herein is not particularly limited, but when the total paste composition is 100% by mass, the content is 40% by mass or more and 95% by mass or less. Preferably, the content is adjusted so that the silver powder is 60% by mass or more and 90% by mass or less, for example, 70% by mass or more and 80% by mass or less. In the case where the silver powder content in the manufactured paste composition is within the above range, an Ag electrode (film) having high conductivity and further improved denseness can be formed.

[Glass powder]
Of the solid content in the paste composition disclosed here, the glass powder contained as an accessory component is essential for forming a silver electrode as a light-receiving surface electrode of a solar cell from the antireflection film by the fire-through method. It may also be an inorganic additive that improves the adhesion strength to the substrate.
As such a glass powder, a glass frit can be typically used. Examples of the glass constituting such a glass frit include, for example, lead-based, zinc-based, borosilicate-based, alkali-based glass, and glass containing barium oxide, bismuth oxide, or the like, or two or more of them. Combinations are mentioned. Specific examples include glasses mainly composed of various oxides, that is, PbO—SiO ₂ —Li ₂ O glass, PbO—SiO ₂ —B ₂ O ₃ glass, and SiO ₂ —B ₂ O ₃ —. Lead-containing glass typified by PbO—Li ₂ O glass, B ₂ O ₃ —SiO ₂ —ZnO glass, R ₂ O—ZnO—SiO ₂ —B ₂ O ₃ glass (where R ₂ O is Alkali metal oxide), RO—ZnO—SiO ₂ —B ₂ O ₃ glass (where RO is an alkaline earth metal oxide), Bi ₂ O ₃ —B ₂ O ₃ —ZnO glass and B ₂ O _3. A glass powder made of lead-free glass (so-called lead-free glass) represented by —SiO ₂ —Bi ₂ O ₃ -based glass or the like is preferable.

More specifically, for example, a glass powder having a representative composition as shown below (oxide conversion composition; the whole glass frit is 100 mol%) is shown as a preferred example.
[Lead glass]
_{46~57mol% PbO-1~7mol% B 2} O 3 -38~53mol% SiO 2
[Li-containing lead-based glass]
0.6-18 mol% Li ₂ O-20-65 mol% PbO-1-18 mol% B ₂ O ₃ -20-65 mol% SiO ₂ ;
For _{example, 0.6~18mol% Li 2 O-20~65mol} % PbO-3~18mol% B 2 O 3 -20~65mol% SiO 2
[Lead-free glass]
10 to 29 mol% Bi ₂ O ₃ -15 to ₃₀ mol% ZnO-0 to 20 mol% SiO ₂ -20 to 33 mol% B ₂ O ₃ -8 to 21 mol% (Li ₂ O, Na ₂ O, K ₂ O)

  For example, the above glass powder having a softening point in the range of 300 ° C. or more and 600 ° C. or less is suitable when the silver electrode is formed by breaking the antireflection film in the fire-through method. If the softening point is less than 300 ° C., the erosion of the paste composition during firing becomes too strong and the pn junction is easily eroded. On the other hand, if the softening point exceeds 600 ° C., the action of eroding the antireflection film is insufficient. This is not preferable because it is difficult to obtain electrical bonding between the electrode and the substrate.

The above composition is representative, and various kinds of materials are used for the purpose of obtaining good adhesion to the substrate, electrode film formability, erosion to the reaction antireflection film, and good ohmic contact. It goes without saying that the ingredients may be adjusted or additional glass modifying ingredients may be added.
However, in the paste composition disclosed here, the tellurium (Te) component contained in the tellurium-containing powder described later does not need to be contained in the glass powder. In order to clarify this point, for example, a powder made of tellurium glass in which Te forms a glass network structure may be distinguished from the glass powder here.

As a suitable glass frit contained in this paste composition, the paste composition (coating film) applied on a substrate (for example, Si substrate) is stably baked and fixed (baked). The specific surface area is preferably about 0.1 m ² / g to 10 m ² / g. The average particle size is preferably 0.01 μm or more and 10 μm or less, and more preferably 0.1 μm or more and 5 μm or less.
Further, the content of the glass powder in the paste composition is not particularly limited, but is approximately 0.5% by mass or more and 5% by mass or less, preferably 0.5% by mass or more and 3% by mass of the entire paste composition. % Or less, more preferably 1% by mass or more and 3% by mass or less.

Next, the tellurium-containing powder characterizing the paste composition disclosed herein and the transition metal-containing powder will be described.
[Tellurium-containing powder]
Tellurium-containing powder is an essential component contained as a solid content of the paste composition. Such tellurium-containing powder may be an aggregate of particles mainly composed of tellurium (Te) and a compound thereof. Preferably, in addition to metal tellurium alone, a compound of tellurium and other metal, oxide, oxo Inorganic compounds such as acids, hydroxides, halides, sulfates, phosphates, nitrates, carbonates, acetates, metal complexes (coordination compounds), organic compounds such as telluride, telluroxide, telluron, and these It may be an aggregate of particles composed of a mixture or a composite. Typically, particles of telluric acid represented by the general formula, Te (OH) ₆ , and tellurium oxide represented by TeO ₂ , Te ₂ O ₃ , Te ₂ O ₅ , TeO _3, etc. are exemplified. The These may be used alone or in combination of two or more.

  There is no restriction | limiting in particular also about the shape etc. of this tellurium containing powder, You may be arbitrary in the range which can be disperse | distributed uniformly in a paste composition. The average particle size of the particles constituting these powders is suitably 0.01 μm or more and 10 μm or less, preferably 0.5 μm or more and 10 μm or less.

  The above tellurium-containing powder is contained at a ratio of approximately 0.1% by mass or more and 1.5% by mass or less, for example, when the total paste composition is 100% by mass. When the content of the tellurium-containing powder is less than 0.1% by mass, for example, the effect of improving electric characteristics such as a fill factor (FF) of a solar cell including a silver electrode formed by this paste composition is sufficiently exerted. Not. The tellurium-containing powder content is preferably 0.3% by mass or more, and more preferably 0.5% by mass or more. In addition, when the content of the tellurium-containing powder exceeds 1.5% by mass, for example, the effect of improving electrical characteristics such as FF of the solar cell is reduced. The tellurium-containing powder content is preferably 1.3% by mass or less, more preferably 1.2% by mass or less.

[Transition metal-containing powder]
In addition, the paste composition disclosed herein must contain a transition metal-containing powder as a solid content together with the tellurium-containing powder. The transition metal-containing powder exhibits the effect of improving the solder bond strength by being contained in the paste composition. In the paste composition disclosed herein, the tellurium-containing powder and the transition metal-containing powder are contained at the same time. For example, in addition to the solder adhesive strength of the silver electrode formed by this paste composition, the silver The effect of enhancing the electrical characteristics such as FF of a solar cell including an electrode is synergistically enhanced. In addition, the combined use of the tellurium-containing powder and the transition metal-containing powder suppresses excessive erosion of the n-Si layer or the like when the paste composition is fired, for example, a low dopant concentration such as a shallow emitter structure. Even when it is bonded to the n-Si layer, good electrical connection is realized.

  The transition metal-containing powder may be a transition metal simple substance or a powder of various compounds thereof. Preferably, it is at least one of powder of transition metal or transition metal oxide. In addition, as a transition metal in this case, elements belonging to Group 3 to Group 11 of the periodic table can be considered, and more specifically, transition metals included in the fourth period of the periodic table. It is preferable because the effect of increasing the adhesive strength is great. Among them, titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn). preferable. These transition metal-containing powders may be used alone or in combination of two or more.

The average particle diameter of the particles constituting these powders is suitably 1 nm or more and 200 nm or less, preferably 5 nm or more and 200 nm or less, more preferably 15 nm or more and 200 nm or less.
Moreover, there is no restriction | limiting in particular about the shape etc. of this transition metal containing powder, You may be arbitrary in the range which can be disperse | distributed uniformly in a paste composition. Moreover, such a transition metal-containing powder may be supplied to the paste composition in the form of a paste in which the transition metal-containing powder is dispersed in an organic medium, for example.

  The above transition metal-containing powder is contained at a ratio of approximately 0.001 mass% to 0.5 mass% when the total paste composition is 100 mass%, for example. When the content of the transition metal-containing powder is less than 0.001% by mass, for example, the effect of improving electrical characteristics such as FF of a solar cell provided with a silver electrode formed by this paste composition is not sufficiently exhibited. Moreover, also when content of a transition metal containing powder exceeds 0.5 mass%, the effect which improves electrical characteristics, such as FF of a solar cell on the contrary, reduces. The content of the transition metal-containing powder is preferably 0.001% by mass or more and 0.1% by mass or less.

[Organic medium]
The paste composition disclosed herein contains the above-described silver powder, glass powder, tellurium-containing powder and transition metal-containing powder as a solid content, and an organic medium for dispersing these solid content as a balance (typically Vehicle). Any organic medium may be used as long as it can disperse the above-described solid content, particularly silver powder, and any of those used in conventional pastes of this type can be used without particular limitation. For example, as a solvent constituting an organic medium, a high boiling point organic solvent such as ethylene glycol and a diethylene glycol derivative (glycol ether solvent), toluene, xylene, butyl carbitol (BC), terpineol or the like is used alone or in combination. be able to.
In addition, the vehicle can contain various resin components as an organic binder. Any resin component may be used as long as it can give the paste composition good viscosity and coating film forming ability (adhesiveness to the substrate), and those used in conventional pastes of this type are not particularly limited. be able to. Examples thereof include those mainly composed of acrylic resin, epoxy resin, phenol resin, alkyd resin, cellulosic polymer, polyvinyl alcohol, rosin resin and the like. Among these, cellulosic polymers such as ethyl cellulose are particularly preferable.

  The proportion of the organic medium in the entire paste composition is suitably 5% by mass or more and 60% by mass or less, preferably 7% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less. It is as follows. Further, the organic binder contained in the vehicle is a ratio of about 1 to 15% by mass, preferably about 1 to 10% by mass, more preferably about 1 to 7% by mass of the entire paste composition. Should be included. By adopting such a configuration, it is easy to form (apply) a coating film having a uniform thickness on the substrate as a silver electrode (film), it is easy to handle, and it takes a long time to dry before firing the silver electrode film. It is preferable because it can be suitably dried without the need for.

As described above, the preferable combination of the silver powder, glass powder, tellurium-containing powder, transition metal-containing powder and organic medium in the paste composition disclosed herein is when the total paste composition is 100% by mass. For example, it can be suitably prepared by using the following formulation as a guide.
Silver powder: 40% to 95% by weight Glass powder: 0.5% to 5% by weight Tellurium-containing powder: 0.1% to 1.5% by weight Transition metal-containing powder: 0.001% by weight 0.5 mass% or less Organic medium: 5 mass% or more and 60 mass% or less Here, what is characteristic is that the ratio of tellurium-containing powder and transition metal-containing powder in the solid content (typically with respect to silver powder) is The presence of such a component, although in a very small amount, reduces the contact resistance between the n-Si layer having a low dopant concentration and the silver electrode, improves the solder bond strength, and thus the conversion efficiency of the solar cell. It is indispensable for improving the characteristics of The presence of such a component is caused by excessive paste composition components even under extremely disadvantageous manufacturing conditions in which a light-receiving surface electrode of a solar cell having a high sheet resistance of 60 Ω / □ or more is formed by a fire-through method. For example, it is indispensable for realizing a good electrical junction while suppressing erosion (which reaches a pn junction, for example).

The paste composition disclosed herein can be easily prepared typically by mixing the above-described constituent materials, similarly to the conventional paste for forming an electrode of a solar cell. For example, using a three-roll mill or other kneader, silver powder, glass powder, tellurium-containing powder and transition metal-containing powder having a predetermined mixing ratio may be mixed and stirred together with an organic medium at a predetermined mixing ratio.
When mixing the tellurium-containing powder or transition metal-containing powder with other constituent materials (containing components), a dispersion or slurry composition in which the powder is dispersed in a liquid medium such as an aqueous solvent or alcohol in advance. It may be provided in the form of a product (hereinafter also simply referred to as “slurry”).

[Production of silver electrode]
The paste composition obtained as described above can be handled in the same manner as, for example, a silver paste conventionally used for forming a silver electrode as a light-receiving surface electrode on a substrate. That is, a conventionally known method can be employed without particular limitation for the formation of the silver electrode using the paste composition disclosed herein. For example, when the silver electrode (light receiving surface electrode 12) in the solar cell 10 shown in FIG. 1 is formed by the fire-through method, thermal diffusion of phosphorus (P) or the like on the light receiving surface of the substrate 11 as in the prior art. Then, an n layer (n-Si layer) 16 is formed, and an antireflection film 14 is further formed thereon by CVD or the like. Thereafter, the paste composition disclosed herein is supplied (applied) on the antireflection film 14 so as to have a desired film thickness (for example, about 20 μm) and a desired coating film pattern. The paste composition can be typically supplied by a screen printing method, a dispenser coating method, a dip coating method, or the like. As such a substrate, a silicon (Si) substrate 11 is suitable, and typically a Si wafer. The thickness of the substrate 11 includes a desired solar cell size, film thicknesses of the Ag electrode 12, the back electrode 20, the antireflection film 14 and the like formed on the substrate 11, and the strength (for example, destruction) of the substrate 11. (Strength) and the like. Generally, the thickness is 100 μm or more and 300 μm or less, preferably 150 μm or more and 250 μm or less, for example, 160 μm or more and 200 μm. The thickness of the n layer 16 is about 300 to 500 nm in conventional silicon solar cells, but the paste composition has a shallow emitter with a thinner n layer 16 and a lower dopant concentration. The present invention can also be applied to the substrate 11 having a structure. The thickness of the n layer 16 can be, for example, 500 nm or less, more limited to 250 nm or less, and can be, for example, about 200 nm or less.

When the fire-through method is not adopted, after forming the n layer 16 and the antireflection film 14 on the light receiving surface of the substrate 11, the antireflection film 14 is peeled off with a desired Ag electrode pattern, and the peeled portion is removed. Supplying the paste composition for forming an Ag electrode at a desired film thickness can be mentioned.
Next, the paste coating product is dried at an appropriate temperature (for example, room temperature or higher, typically about 100 ° C.). After drying, the dried coating film is baked by heating in an appropriate baking furnace (for example, a high-speed baking furnace) under appropriate heating conditions (for example, 600 ° C. to 900 ° C., preferably 700 ° C. to 800 ° C.) for a predetermined time. I do. As a result, the paste application product is baked onto the substrate 11 to form a silver electrode 12 as shown in FIG.

  As described above, the paste composition disclosed herein includes tellurium-containing powder and transition metal-containing powder as solids. This paste composition has a lower contact resistance of the silver electrode film obtained than the paste composition containing only the tellurium-containing powder out of the two kinds of powders. As a result, it is possible to manufacture a solar cell with high energy conversion efficiency. And In addition, since the paste composition has a higher adhesive strength of the silver electrode film obtained than the paste composition containing only the transition metal-containing powder among the two kinds of powders, the electrical properties, durability and reliability are consequently obtained. High solar cell can be produced. Therefore, according to such a paste composition, a solar cell having excellent solar cell characteristics (for example, FF of 78% or more and silver electrode adhesive strength of 6N or more) can be realized.

[Production of solar cells]
In addition, the materials and processes for manufacturing the solar cell other than forming the silver electrode (typically, the light receiving surface electrode) of the solar cell using the paste composition disclosed here are exactly the same as the conventional ones. It's okay. And a solar cell (typically crystalline silicon type solar cell) provided with the silver electrode formed with the said paste composition can be manufactured, without performing a special process. A typical example of the structure of such a crystalline silicon solar cell is the structure shown in FIG.
As a process other than the silver electrode formation, formation of the aluminum electrode 20 as the back electrode 20 can be mentioned. The procedure for forming the aluminum electrode 20 is as follows. For example, first, the paste composition disclosed here is printed to form the light-receiving surface electrode 12 on the light-receiving surface as described above, and the silver paste (which may be the paste composition disclosed herein) is also formed on the back surface. Is printed in the desired area and dried. Thereafter, the aluminum electrode paste material is printed and dried so as to overlap a part of the silver paste forming region on the back surface, and all the coating films are fired. Usually, the aluminum electrode 20 is baked, and a p ⁺ layer (BSF layer) 24 can also be formed. That is, the aluminum electrode 20 to be the back electrode is formed on the p-type silicon substrate 11 by firing, and the aluminum atoms are diffused into the substrate 11 to form the p ⁺ layer 24 containing aluminum as an impurity. It will be. In this manner, a solar cell (cell) 10 including a silver electrode formed using the paste composition disclosed herein on the light receiving surface is manufactured.

  EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the following examples.

(Embodiment 1)
[Preparation of silver electrode forming paste composition]
(1) Silver particles (AG48F, manufactured by DOWA Electronics Co., Ltd.) having an average particle diameter (D50) of 2 μm were prepared as silver powder.
(2) As the glass powder, a glass frit having an average particle diameter (D50) of 1 μm and represented by the following composition, PbO: 38 mol%, SiO ₂ : 32 mol%, Li ₂ O: 12 mol% was used. In the composition of the glass powder, each component is represented by a value converted to an oxide.
(3) TeO ₂ particles having an average particle diameter (D50) of 0.1 μm (manufactured by Rare Metal Co., Ltd., purity 99% or more) were prepared as tellurium-containing components.
(4) As transition metal powders, Ni, NiO, Ti, Fe ₂ O ₃ , CoO, ZnO, Cu, CuO, MnO ₂ , Mn ₂ O ₃ , and Mn ₃ O ₄ were prepared.
(5) An organic vehicle composed of a binder (ethyl cellulose) and an organic solvent (terpineol) was prepared as an organic medium.

(Sample 1)
Paste composition prepared by mixing (1) silver powder, (2) glass powder, (3) tellurium-containing powder and (4) Ni powder as transition metal powder prepared above, and (5) kneading with an organic medium. A product (sample 1) was obtained.
Here, Ni particles (Ni slurry: 117X manufactured by JFE Mineral Co., Ltd.) having an average particle diameter (D50) of 0.15 μm were used as the Ni powder.
The blending ratio of each material is 85% by weight of silver powder, 2% by weight of glass powder, 1% by weight of tellurium-containing powder, and 0.06% by weight of Ni powder when the total paste composition is 100% by weight. %. Further, the organic medium was blended so that the binder occupies 1% by mass of the entire paste composition, and the remainder was prepared with an organic solvent so as to have a concentration suitable for printing.

(Sample 2)
In the paste composition of Sample 1, (4) A paste composition (Sample 2) was obtained in the same manner as Sample 1 except that Ni powder was not added.
(Sample 3)
In the paste composition of Sample 1, (3) A paste composition (Sample 3) was obtained in the same manner as Sample 1 except that no tellurium-containing powder was added.
(Sample 4)
In the paste composition of Sample 1, a paste composition (Sample 4) was obtained in the same manner as Sample 1 except that (3) both the tellurium-containing powder and (4) Ni powder were not added.

[Production A of Solar Cell for Evaluation]
Using the paste compositions of Samples 1 to 4 prepared above as the silver electrode forming paste, solar cells for evaluation (see FIG. 1) were prepared according to the following procedure.
That is, first, a commercially available p-type single crystal silicon substrate (plate thickness 180 μm) for a solar cell having a size of 156 mm square was prepared, and its surface was acid-etched using a mixed acid obtained by mixing hydrofluoric acid and nitric acid. . Next, a phosphorus-containing solution is applied to the light-receiving surface of the silicon substrate on which a fine uneven structure (not shown in FIG. 1) is formed by the above-described etching process, and heat treatment is performed to thicken the light-receiving surface of the silicon substrate. An n-Si layer (n ⁺ layer) having a thickness of about 0.5 μm was formed. An antireflection film (silicon nitride film) having a thickness of about 80 nm was formed on the n-Si layer by plasma CVD (PECVD).

  Next, using the paste compositions of Samples 1 to 4 prepared above, a coating film that becomes a silver electrode by screen printing (using a stainless steel screen mesh SUS # 165. The same applies hereinafter) on the antireflection film ( A thickness of 10 μm to 30 μm). Similarly, a coating film to be a back electrode (silver electrode) was formed in a pattern. These coating films were dried at 85 ° C. and subjected to the next step.

Thereafter, a predetermined aluminum paste for back electrode was printed (applied) by screen printing so as to overlap a part of the silver electrode pattern on the back side of the silicon substrate, thereby forming a coating film having a film thickness of about 55 μm. Next, by firing this silicon substrate, a silver electrode (light-receiving surface electrode) was formed by a fire-through method. Firing was performed at a firing temperature of approximately 700 ° C. or higher and 800 ° C. or lower in an air atmosphere using a near infrared high-speed baking furnace. Thereby, the photovoltaic cell for evaluation was obtained.
Hereinafter, the solar cells produced using the paste compositions of Samples 1 to 4 will be referred to as the solar cells of Samples 1 to 4, respectively.

[Curve factor (FF)]
Using a solar simulator (Beger, PSS10), the IV characteristics of the solar cells of Samples 1 to 4 were measured, and a fill factor (FF) was obtained from the obtained IV curves. The fill factor (FF) value is expressed as a ratio of maximum generated power and short-circuit current (short-circuit current density) × open-circuit voltage.
It calculated based on the "crystal type photovoltaic cell output measuring method" prescribed | regulated to C-8913. The calculation results of the FF values are shown in percentage form and shown in Table 1.

[Adhesive strength]
Next, the adhesive strength of the silver electrode in the solar cells of Samples 1 to 4 produced as described above was evaluated. Evaluation of the adhesive strength (peel strength) of the silver electrode was performed using a strength measuring device 300 as shown in FIG.
Specifically, the glass substrate 41 is fixed on the fixing jig 40 of the strength measuring device 300 shown in FIG. 2 via the fixing screw 43 and the locking plate 44, and the epoxy adhesive 42 is placed on the glass substrate 41. Then, the solar cell 10 for evaluation was placed with the light receiving surface side facing up and the back surface side facing down, and fixed.
The tab wire 35 was soldered via the solder layer 30 on the silver electrode 12 positioned on the upper surface side of the solar cell for evaluation fixed on the glass substrate 41 in this way.
Then, as shown in FIG. 2, the strength measuring device 300 is tilted so that the bottom surface of the fixing jig 40 is 135 °, and the extension 35e formed in advance on the tab wire 35 is pulled upward in the vertical direction. (See arrow 45), the adhesive strength of tab wire 35 / solder layer 30 / silver electrode 12 was measured. The measurement results of the adhesive strength are shown in Table 1.

[Evaluation]
The fill factor (FF value) is a factor that directly affects the leakage current and the series resistance Rs, and is an important numerical value for evaluating the performance of the solar cell, such as being used in the management of the solar cell manufacturing process. . The fill factor of a crystalline silicon solar cell is typically in the range of 70% to 80%. In the latter half of the 70% FF value, it is evaluated that the performance as a solar cell is greatly improved by increasing the FF value even by 0.1%.

From the results of Table 1, the solar cell of Sample 1 formed using the paste composition of Sample 1 containing both tellurium-containing powder and Ni powder has a very good FF value of about 79%, It was confirmed that the adhesive strength with the n-Si layer was extremely high.
On the other hand, the solar cells of Samples 2 to 4 formed using the paste composition not containing either or both of tellurium-containing powder and Ni powder have an FF value of 75% and a relatively good value. Although it was, it was a low value compared with Sample 1.

From the comparison of samples 2 to 4, the paste composition of sample 2 in which tellurium-containing powder is added to the paste composition (sample 4) that does not contain both tellurium-containing powder and Ni powder has an improved FF value. However, it can be seen that by including the tellurium-containing powder, the solder wettability is poor and the adhesive strength remains low. Moreover, although the paste composition of the sample 3 containing Ni powder was inferior compared with the case where the improvement degree of FF value contains a tellurium containing powder, it was confirmed that solder wettability is improved and adhesive strength improves.
From the above considerations, it is expected that the adhesive strength of the sample 1 paste composition containing both tellurium-containing powder and Ni powder is lower than the adhesive strength of the sample 3 paste composition containing only Ni powder. . However, contrary to this expectation, it is noted that in this embodiment, the adhesive strength of the paste composition (sample 1) containing both tellurium-containing powder and Ni powder is greatly improved.

  From the above, for the paste composition of Sample 1 containing both tellurium-containing powder and Ni powder, good electrical connection is possible by the fire-through method, and the formed silver electrode also has a high FF value, It was confirmed that it had both good performance and high adhesive strength.

(Embodiment 2)
[Production B of Solar Cell for Evaluation]
Using the paste compositions of Sample 1 and Sample 2 prepared above as the silver electrode forming paste, an n-Si layer (n layer) was formed on the light receiving surface of the silicon substrate so that the sheet resistance was 60 Ω / □ or more. Except for this, a solar cell for evaluation was produced in the same manner as in the production A of the solar cell described above. The n-Si layer is about 300 to 500 nm in a general crystalline silicon-based solar cell, but the n-Si layer formed here is thinner (thickness is about 70 nm) and is called a shallow emitter. Make up structure.
The fill factor (FF) of the solar cell thus produced was measured by the same method as in the first embodiment, and is shown in Table 2 below. For reference, the fill factor of a normal solar battery cell having no shallow emitter structure, which was prepared using the paste compositions of Sample 1 and Sample 2 in Embodiment 1, was also shown.

  As shown in Table 2, the paste composition of Sample 1 containing both tellurium-containing powder and Ni powder has an extremely high FF of about 79% even when a shallow emitter structure is formed by increasing the sheet resistance of the substrate. Good characteristics were obtained. That is, it is considered that the paste composition contains both the tellurium-containing powder and the Ni powder, so that a good shallow emitter structure is formed without the paste composition eroding the pn junction interface when the substrate is fired. Even when the sheet resistance is increased and the dopant concentration of the n-Si layer is lowered, the contact resistance can be kept low because the electrical connection between the formed silver electrode and the n-Si layer is good. It is considered that the increase in series resistance Rs due to the increase in sheet resistance was sufficiently compensated.

  On the other hand, the solar cell formed using the paste composition of Sample 2 had a FF value of 73%, which was lower than that of a normal configuration solar cell. This is because, in addition to the increase in sheet resistance, the n-Si layer and the pn junction interface are eroded by diffusion of paste composition components during firing, and the contact resistance between the silver electrode and the n-Si layer increases. For example, it is considered that the series resistance Rs is increased and the FF value cannot be maintained.

(Embodiment 3)
Varying the amount of tellurium-containing powder and Ni powder in the paste composition of sample 1 prepared above as shown in Table 3 below, preparing paste compositions of various compositions, and preparing these paste compositions A solar cell for evaluation was produced in the same manner as in Production A (Embodiment 1) of the above solar cell. Then, the fill factor (FF) of the produced solar battery cell was measured by the same method as in the first embodiment and shown in Table 3 below.

  As shown in Table 3, it can be seen that the blending amount of the tellurium-containing powder is preferably about 0.1% by mass or more and 1.5% by mass or less in terms of FF characteristics. As for the blending amount of the Ni powder, a high value of FF of 78% or more is obtained in the range of 0.001% by mass or more and 0.6% by mass or less. It can be seen that since the tendency to decrease FF is observed, it is preferable to set the ratio to about 0.001% by mass or more and 0.5% by mass or less.

(Embodiment 4)
(Samples 5-17)
Instead of the nickel particles (Ni particles) in Sample 1, the transition metal powder shown in Table 2 was used with a partly changed blending amount, and the other conditions were the same as in Sample 1, and the paste composition (Sample 5 to Sample 5) was used. 17) was obtained.
That is, the paste composition of Sample 5 was replaced with the Ni particles of Sample 1 using the same amount of NiO particles.
In the paste composition of sample 6, instead of the Ni particles of sample 1, the same amount of Ti particles was used.
In the paste composition of sample 7, instead of the Ni particles of sample 1, the same amount of Fe ₂ O ₃ particles was used.
In the paste composition of sample 8, instead of the Ni particles of sample 1, the same amount of CoO particles was used.
The paste composition of Sample 9 was obtained by using the same amount of ZnO particles instead of the Ni particles of Sample 1.

In the paste composition of sample 10, instead of the Ni particles of sample 1, Cu particles were used at a ratio of 0.03% by weight with respect to the entire paste composition.
The paste composition of Sample 11 was replaced with the same amount of CuO particles instead of the Cu particles of Sample 9.
The paste composition of sample 12 was replaced with the same amount of MnO ₂ particles instead of the Cu particles of sample 9.
In the paste composition of Sample 13, the same amount of Mn ₂ O ₃ particles was used instead of the Cu particles of Sample 9.
In the paste composition of sample 14, instead of the Cu particles of sample 9, the same amount of Mn ₃ O ₄ particles was used.

In the paste composition of sample 15, the same amount of SiO particles was used instead of the Ni particles of sample 1.
The paste composition of Sample 16 was replaced with the same amount of boron (B) instead of the Ni particles of Sample 1.
The paste composition of Sample 17 was obtained by using the same amount of carbon (C) particles instead of the Ni particles of Sample 1.

  Using the paste compositions of Samples 5 to 17 obtained above as the light-receiving surface electrode forming paste, solar cells for evaluation were manufactured according to Preparation A for solar cells for evaluation described above. For the sake of convenience, solar cells for evaluation produced using the paste compositions of Samples 5 to 17 will be referred to as the solar cells of Samples 5 to 17, respectively.

[Evaluation]
For the solar cells of Samples 5 to 17, the fill factor (FF) was calculated in the same procedure as in Samples 1 to 4, and the adhesive strength of the silver electrode on the light receiving surface was measured. These results are shown in Table 4. In addition, the result about the solar cell of Sample 1 is also shown for reference.

From Table 4, for solar cells of Samples 5 to 14, a paste composition containing a tellurium-containing powder and a transition metal element of the fourth period in the periodic table of elements or an oxide thereof in an extremely small amount was used. It was confirmed that both the adhesive strengths were improved. The transition metal element alone and its oxide are both excellent in FF and high adhesive strength, and a good silver electrode film can be formed. In particular, Ni, NiO, Mn ₂ O ₃ and Mn ₃ O ₄ were added. In this case, it was found that a good silver electrode was obtained with an FF of 79% or more and an adhesive strength of 7N or more.
In addition, about the solar cell of samples 15-17, it replaced with transition metal containing powder, for example, although the paste containing semimetals, such as boron (B), carbon (C), and silicon (Si), was used, Such an effect was not obtained. In addition, according to further studies by the present inventors, the above effect can be obtained even when a powder containing a base metal such as aluminum (Al) and tin (Sn) is used instead of the transition metal-containing powder. It was difficult to get.

  The present invention provides a paste composition for forming a silver electrode, which contains, in addition to silver particles, a tellurium-containing powder and a transition metal-containing powder. By using such a paste composition, even when a silver electrode of a solar cell having a thin n-Si layer is formed by the fire-through method, the adhesive strength is high and the electrode characteristics are good (for example, a high fill factor is realized). Yes) a silver electrode can be formed, and a high-quality solar cell can be realized.

10 Solar cell 11 Substrate 12 Light-receiving surface electrode (silver electrode)
14 Antireflection film 16 n-Si layer (n layer)
20 Back electrode (aluminum electrode)
22 Back side external connection electrode 24 p ⁺ layer 110 solar cell 111 substrate 112 surface electrode (light-receiving surface electrode)
114 Antireflection film 116 n-Si layer (n layer)
120 Aluminum electrode (back electrode)
122 Back side external connection electrode 124 p ⁺ layer 300 Strength measuring device

Claims (9)

A paste composition for forming a silver electrode disposed on a light receiving surface of a crystalline silicon solar cell,
Silver powder, glass powder, tellurium-containing powder, and transition metal-containing powder are dispersed in an organic medium,
When the total paste composition is 100% by mass, the tellurium-containing powder is 0.1% by mass to 1.5% by mass, and the transition metal-containing powder is 0.001% by mass to 0.5% by mass. The paste composition is blended at a ratio of   The paste composition according to claim 1, wherein the crystalline silicon-based solar cell has a sheet resistance of a silicon substrate of 60Ω / □ or more. The transition metal-containing powder is
The paste composition according to claim 1 or 2, which is a powder of at least one metal selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn or an oxide thereof. When the total paste composition is 100% by mass,
Said silver powder: 40 mass% or more and 95 mass% or less,
The glass powder: 0.5% by mass or more and 5% by mass or less,
The tellurium-containing powder: 0.1% by mass or more and 1.5% by mass or less,
The transition metal-containing powder: 0.001% by mass to 0.5% by mass,
The paste composition of any one of Claims 1-3 currently mix | blended in the ratio. The glass powder is an oxide conversion composition, the following composition,
SiO ₂ : 20 mol% or more and 65 mol% or less,
B ₂ O ₃ : 1 mol% or more and 18 mol% or less,
PbO: 20 mol% or more and 65 mol% or less,
Li ₂ O: 0.6 mol% or more and 18 mol% or less,
The paste composition according to claim 1, comprising:   The paste composition of Claims 1-4 in which the said glass powder has a softening point in the range of 300 to 600 degreeC.   A solar cell which equips a light-receiving surface with the silver electrode formed using the paste composition of any one of Claims 1-6. The crystalline silicon-based solar cell includes an n-Si layer formed on the light-receiving surface side of a p-type silicon substrate, and includes an antireflection film and the silver electrode on the n-Si layer.
The solar cell according to claim 7, wherein the silicon substrate has a sheet resistance of 60Ω / □ or more.   The solar cell according to claim 7 or 8, wherein the silver electrode has a coating film of the paste composition disposed on the antireflection film formed by through firing.

Images