JP5144857B2 - Conductive paste composition for solar cell - Google Patents

Description

  The present invention relates to a conductive paste composition suitable for a solar cell electrode formed by a fire-through method.

For example, a general silicon-based solar cell is provided with an antireflection film and a light-receiving surface electrode on an upper surface of a silicon substrate which is a p-type polycrystalline semiconductor via an n ⁺ layer, and on the lower surface via a p ⁺ layer. It has a structure provided with electrodes (hereinafter simply referred to as “electrodes” when they are not distinguished from each other), and power generated at the pn junction of the semiconductor by light reception is taken out through the electrodes. The antireflection film is for reducing the surface reflectance and increasing the light receiving efficiency while maintaining sufficient visible light transmittance, and is made of a thin film such as Si ₃ N ₄ , TiO ₂ , or SiO ₂ .

Since the above-described antireflection film has a high electric resistance value, it prevents an electric power generated at the pn junction of the semiconductor from being efficiently extracted. Therefore, the light-receiving surface electrode of the solar cell is formed by a method called fire-through, for example. In this electrode forming method, for example, after the antireflection film is provided on the entire surface of the n ⁺ layer, a conductive paste is applied on the antireflection film in an appropriate shape by using, for example, a screen printing method, and is fired. Apply. The conductive paste is mainly composed of, for example, silver powder, glass frit (a piece of flaky or powdered glass that is crushed as necessary after melting and quenching the glass raw material), an organic vehicle, and an organic solvent. In the baking process, the glass component in the conductive paste breaks the antireflection film, so that an ohmic contact is formed by the conductive component in the conductive paste and the n ⁺ layer (for example, Patent Documents). See 1). According to the above electrode forming method, the process is simplified as compared with the case where the antireflection film is partially removed and an electrode is formed on the removed portion, and there is a problem of misalignment between the removed portion and the electrode forming position. Does not occur.

  In the formation of the light-receiving surface electrode of such a solar cell, various proposals have been made for the purpose of improving the fire-through property and improving the ohmic contact, and thus increasing the fill factor (FF) and energy conversion efficiency. It has been done. For example, by adding a Group 5 element such as P, V, Bi, etc. to the conductive paste, there is one that promotes the redox action on the antireflection film of glass and silver and improves the fire-through property (for example, (See Patent Document 1). In addition, by adding chloride, bromide, or fluoride to the conductive paste, these additives help the glass and silver break the antireflection film, thereby improving ohmic contact (for example, patents). See reference 2.)

In addition, an electrode material containing Ag powder, organic vehicle, glass frit, and at least one of Ti, Bi, Co, Zn, Zr, Fe, and Cr on an antireflection film made of Si ₃ N ₄ or SiO ₂ or the like. There is one that attempts to obtain stable ohmic contact and solder adhesive strength by baking (see, for example, Patent Document 3). Ti, Bi and the like are preferably 0.05 to 5 parts by weight with respect to 100 parts by weight of Ag. The reason why the above effect is obtained is not shown, but if Ti, Bi, etc. are contained, they dissolve in the glass during the firing process of the electrode material, and then the electrode material acts on the antireflection film. Therefore, the reaction with the antireflection film is stabilized as compared with the case where Bi or the like is not contained.

Also proposed is a thick film conductive composition in which Ag powder, ZnO with a particle size of 7-100 (nm), and glass frit with a softening point in the range of 300-600 (° C) are dispersed in an organic solvent. (See, for example, Patent Document 4). This thick film conductive composition is for forming a light-receiving surface electrode of a solar cell, and conductivity and solder adhesion are improved by adding Zn. For the same purpose, it has been proposed to use MnO ₂ instead of ZnO (see, for example, Patent Document 5).

Japanese Examined Patent Publication No. 03-046985 Japanese Patent No. 3707715 JP 2001-313400 A JP 2006-302890 A JP 2006-302891 A JP 58-096777 A JP 2008-192921 A JP 2005-347276 A JP 2008-226816 A Japanese Patent Laid-Open No. 08-097417 JP 2004-323910 A

  By the way, when manufacturing a solar cell module, there is a step of bonding a cell and solder, and it is important in terms of module reliability to secure this bonding strength immediately after bonding or after aging. . The conductive pastes described in Patent Documents 1 and 2 still have low solder adhesive strength, and it has been desired to further increase the adhesive strength. On the other hand, in the conductive pastes described in Patent Documents 3 to 5, although the adhesive strength is improved, there is a problem that the conversion efficiency is lowered because the contact resistance is increased.

  In addition, since the light receiving surface electrode blocks incident sunlight in the solar battery, the amount of energy entering the solar battery cell is reduced according to the area occupied by the light receiving surface electrode. On the other hand, it has been proposed to increase the light receiving area by narrowing the line width of the light receiving surface electrode. For example, it is desired to reduce the conventional line width of about 130 (μm) to 110 (μm) or less. ing. However, as the line width is narrowed, it becomes difficult to obtain a good ohmic contact, the contact resistance increases, and the current density decreases as a result. Therefore, the conversion efficiency decreases. In particular, the above-described conductive paste with improved solder bond strength has a high contact resistance, making it more difficult to meet the demand for thinning.

  Further, since the n-layer of the solar cell has a high recombination rate of minority carriers, the generated electrons and holes are recombined, and a phenomenon (heat loss) occurs that does not contribute to power generation and changes to heat. In particular, short-wavelength light is easily absorbed by the n layer because of the large light absorption coefficient of the semiconductor, and recombination is likely to occur. On the other hand, by increasing the sheet resistance of the solar cell substrate to make the n layer thin, and by reducing the impurity concentration and increasing the diffusion distance of minority carriers, heat loss due to recombination is suppressed and the short wavelength is reduced. There is one that contributes to the generation of sunlight (referred to as a shallow emitter or shallow junction cell). Thereby, current increases and conversion efficiency is increased. However, if the impurity concentration in the vicinity of the surface is lowered, the ohmic contact is deteriorated, so that the contact resistance is increased, the current density is lowered, and the conversion efficiency is lowered. Therefore, the conductive paste with improved solder bond strength has a problem of high contact resistance from the beginning, and it is more difficult to apply to the above-described shallow emitter.

  The present invention has been made against the background of the above circumstances, and an object of the present invention is to provide a conductive paste composition for a solar cell that can increase solder adhesive strength without impairing electrical characteristics.

In order to achieve such an object, the gist of the present invention is that a conductive material for a solar cell, which is used to form an electrode of a silicon-based solar cell, which includes a conductive silver powder, a glass frit, and an organic medium. The paste composition contains at least one of Ni and NiO having an average particle size in the range of 10 to 91 (nm) in a ratio of 0.5 (wt%) or less with respect to the entire paste composition .

Thus, the conductive paste composition, fines Ni or NiO (hereinafter, referred to as Ni, etc.) to include, providing a conductive film such as the light-receiving surface electrode in sheet silicon substrate of the n layer, the The conductor film has a high solder bond strength. Moreover, since a conductive path is suitably formed between the silicon substrate and the silver in the conductor film, the contact resistance between the silicon substrate and the conductor film is reduced, and the electrical characteristics are equal to or better than when Ni or the like is not added. Therefore, thinning is facilitated, and when applied to the light receiving surface electrode, the light receiving area can be increased, so that a photoelectric conversion efficiency equal to or higher than that can be obtained. Therefore, according to the present invention, a conductive paste composition for a solar cell that can increase the solder adhesive strength without impairing electrical characteristics is obtained, and such a conductive paste composition is suitable for a light-receiving surface electrode of a solar cell. It is. In particular, since the conductive paste composition of the present invention can reduce the contact resistance as described above, it is highly effective when applied to a shallow emitter in which ohmic contact tends to be deteriorated because the impurity concentration is low. can get.

In addition, when soldering to an electrode formed from a conductive paste not containing Ni, Ag in the electrode melts with Sn in the solder to generate a metal compound, thereby generating stress at the interface between the electrode and the solder. This is thought to cause a decrease in solder bond strength. When using a conductive paste containing Ni, the reaction rate of Ag and Sn is suppressed because Ni has a low reaction rate with Sn, so the formation of metal compounds is suppressed, and the adhesive strength is improved. It is done. Further, when the average particle diameters of Ni and NiO are less than 10 (nm), they are easy to aggregate, and thus the dispersibility is poor and handling is difficult. On the other hand, when the average particle size exceeds 91 (nm), the photoelectric conversion efficiency is lowered as compared with the case where the average particle size is not added. These average particle diameters are calculated from, for example, values measured visually using an SEM image. Note that the conductive paste may contain other Ni compounds such as NiC.
In addition, the adhesion strength increases as the addition amount of Ni and NiO increases, but if added excessively, the resistance value of the conductor film itself generated from the conductive paste increases and the contact resistance between the conductor film and the cell increases. Conversion efficiency tends to decrease. Therefore, in order to obtain a sufficiently high conversion efficiency by sufficiently reducing the resistance value, it is necessary to keep the addition amount of Ni or the like to 0.5 (wt%) or less. According to the present invention, since extremely fine Ni is used, the effect of improving the solder bond strength can be sufficiently obtained even if the addition amount is very small.

  Incidentally, adding Ni to the conductive paste for solar cells has been conventionally performed. For example, in Patent Document 6, the electrode on the light-receiving surface side of the solar cell is about 5 to 10 (wt%). It is shown that an Ag paste containing Ni is used. In this patent document 6, the particle size of Ni is not described at all, and the use of fine powder and the effect thereof are not suggested. Although the amount of Ni added is remarkably increased, the solar cell described in Patent Document 6 has a low output, so it is considered that efficiency reduction due to the addition of a large amount of Ni did not become a problem. Therefore, even if such an Ag paste is used, since there is too much Ni, the contact resistance increases on the contrary, the conductivity decreases and the FF value decreases.

  In Patent Document 7, when forming the back Al electrode of the solar cell, Ag powder, Ni powder having an average particle size of 0.1 to 1.0 (μm), glass frit and vehicle are used, and Ni powder is used for Ag powder. It is described that a conductive paste having a composition of 0.5 to 2.0 (wt%) is used. Even in this technology, the Ni content is remarkably increased, but Ni becomes an acceptor when entering Si. Therefore, if such a paste is used on the front surface side consisting of the n layer, the efficiency will be reduced, but there may be a large amount of Ni on the back surface side. When Ni having a relatively large particle size is used, the content necessary for enjoying the desired effect is increased, but this is not a problem when used on the back side.

  Patent Document 8 describes a conductive paste suitable for forming a light-receiving surface electrode of a solar cell to which a fluoride such as Ni or a bromide is added by a fire-through method. In this technique, by adding Ni fluoride or the like, the fire-through property is increased and the contact resistance is lowered. In order to prevent Ni from being easily oxidized from forming an oxide film, it is added by fluoride or bromide. No consideration is given to improving the adhesive strength, and the particle size and addition amount of the Ni compound are not particularly considered.

  In addition, although applied to solar cell electrodes having different structures, in Patent Document 9, metal nanoparticles are dispersed in a dispersion medium and selected from PVP, PVP copolymer, PVA, and cellulose ether. An electrode-forming composition containing an organic polymer is described. The metal nanoparticles include silver nanoparticles 75 (wt%) or more having a primary particle diameter of 10 to 50 (nm) and a total of 0.02 to 25 (wt%) of gold, nickel, and the like. This is used for the back electrode of a super straight solar cell with a transparent conductive film, and is intended to prevent the formation of a space such as an air layer at the bonding interface between the transparent conductive film and the back electrode. No consideration is given to increasing the strength.

  Further, as a method of reducing contact resistance by adding nickel, when manufacturing a MIS field effect transistor as shown in Patent Document 10, a Ni film or the like is formed on a wafer and heat treatment is performed. There is something. By utilizing the fact that only the refractory metal film deposited in the region in contact with the silicon exposed surface changes into a refractory metal silicide film in a self-aligned manner, the surfaces of the impurity diffusion region and the gate electrode are simultaneously self-aligned. Silicide is used to reduce the resistance. Nickel is thought to prevent impurity diffusion into silicon and contribute to good ohmic contact. As described above, the techniques described in Patent Documents 6 to 10 increase the conductivity or decrease the contact resistance by using Ni or a Ni compound, respectively, and the surface of the solar cell formed by the fire-through method. There is no indication that improvement in the adhesive strength of the side electrode has been desired or that this can be improved by adding Ni or the like.

Here, preferably, the Ni and NiO are included in a ratio of 0.2 (wt%) or less with respect to the entire paste composition. As described above, the adhesion strength increases as the amount of Ni added increases, but if added excessively, the resistance value of the conductor film itself generated from the conductive paste increases and the contact resistance between the conductor film and the cell decreases. Since the conversion efficiency tends to increase and the conversion efficiency tends to decrease , the addition amount of Ni or the like is preferably as small as possible within the range where an improvement in adhesive strength is observed, and is preferably limited to 0.2 (wt%) or less.

  In addition, the manufacturing method and physical property of Ni powder and NiO powder are not specifically limited, For example, an appropriate thing can be used from various commercial items. In addition, as a manufacturing method, for example, as shown in Patent Document 11, a nickel salt powder or a nickel hydroxide powder is suspended by a polyol method, and this is heated to be reduced to nickel powder. A method of crushing the agglomerated nickel powder thus obtained can be mentioned. According to this method, fine nickel powder having excellent dispersibility can be obtained.

  Preferably, in the conductive paste composition for a solar cell, the glass frit having a softening point in the range of 300 to 600 (° C.) is used. If it does in this way, the conductive paste composition for solar cells which can be used suitably when a solar cell is broken by an anti-reflective film by a fire through method and a light-receiving surface electrode is formed is obtained. That is, by using a glass frit having a softening point in the above temperature range, the conductive paste composition for a solar cell has a good fire-through property and a pn by glass even in a shallow emitter having a thin n layer. Bonding is less likely to break. If the softening point is less than 300 (° C), the erosion of the paste composition becomes too strong and the pn junction is likely to be broken. Contact cannot be obtained.

  Preferably, the glass frit has an average particle size (D50) in the range of 0.3 to 3.0 (μm) and is contained in a proportion in the range of 1 to 20 (vol%) with respect to the entire paste. It is. If the average particle size of the glass frit is too small, melting will be too early when the electrode is fired, resulting in a decrease in electrical characteristics, but if it is 0.3 (μm) or more, moderate melting properties can be obtained, so that the electrical characteristics are further enhanced. . In addition, since agglomeration is unlikely to occur, better dispersibility can be obtained during paste preparation. Also, when the average particle size of the glass frit is significantly larger than the average particle size of the conductive silver powder, the dispersibility of the whole powder is lowered, but if it is 3.0 (μm) or less, better dispersibility can be obtained. . Moreover, a further melting property of the glass can be obtained. In addition, if the glass amount is 1 (vol%) or more, the melting property of the antireflection film is further improved, so that a better ohmic contact can be obtained. Further, if the amount of glass is 20 (vol%) or less, it is difficult to form a single insulating layer, so that higher conductivity can be obtained. Therefore, in order to obtain a better ohmic contact, it is preferable to satisfy both the average particle diameter and the ratio in the paste. The ratio in the paste is particularly preferably 2 to 10 (vol%).

  The average particle size of the glass frit is a value obtained by the air permeation method. The air permeation method is a method for measuring the specific surface area of a powder from the permeability of a fluid (for example, air) to the powder layer. The basis of this measurement method is the Kozeny-Carmann equation, which shows the relationship between the wetted surface area of all particles constituting the powder layer and the flow velocity and pressure drop of the fluid passing therethrough. The specific surface area of the sample is obtained by measuring the flow velocity and pressure drop with respect to the powder layer filled under the conditions determined by the above. In this method, the gap between the filled powder particles is regarded as pores, and the wetted surface area of the particles that resists the flow of air is determined. Usually, the value is smaller than the specific surface area determined by the gas adsorption method. Show. An average particle diameter assuming spherical particles can be calculated from the obtained specific surface area and particle density.

  Preferably, the conductive silver powder has an average particle size (D50) in the range of 0.3 to 3.0 (μm). Further, if the average particle size of the silver powder is 3.0 (μm) or less, better dispersibility can be obtained, and thus higher conductivity can be obtained. Moreover, if it is 0.3 (μm) or more, aggregation is suppressed and better dispersibility can be obtained. Since silver powder of less than 0.3 (μm) is extremely expensive, 0.3 (μm) or more is preferable from the viewpoint of manufacturing cost. Further, if the average particle diameter of both the conductive silver powder and the glass frit is 3.0 (μm) or less, there is an advantage that clogging hardly occurs even when the electrode is printed and formed with a fine line pattern.

  The silver powder is not particularly limited, and may be any shape such as a spherical shape or a scale shape. However, when the spherical powder is used, the printability is excellent and the filling rate of the silver powder in the coating film is increased. Therefore, compared to the case where other shapes of silver powder such as scales are used, the coating film The electrical conductivity of the electrode generated from is increased. Therefore, it is particularly preferable because the line width can be further reduced while ensuring the necessary conductivity.

  Preferably, the solar cell conductive paste composition has a viscosity ratio (that is, [10 (rpm)) within a range of a viscosity of 150 to 250 (Pa · s) at 25 (° C.)-20 (rpm). [Viscosity] / [viscosity at 100 (rpm)]) is 3-8. By using a paste having such a viscosity characteristic, the viscosity is suitably reduced during squeezing and transmitted through the screen mesh. After the transmission, the viscosity returns to a high viscosity and the expansion of the printing width is suppressed. Thus, a fine line pattern can be easily obtained while maintaining the printability such that clogging does not easily occur and clogging does not occur. The viscosity of the paste composition is more preferably in the range of 160 to 200 (Pa · s), and the viscosity ratio is more preferably in the range of 3.2 to 6.0. In addition, a viscosity ratio of 4 to 6 is desirable for thinning a design line width of 100 (μm) or less.

  Note that increasing the film thickness so that the cross-sectional area can be maintained even if the line width is reduced includes, for example, increasing the emulsion thickness of the printing plate, increasing the tension, and reducing the line diameter. It is also possible to widen the aperture. However, when the emulsion thickness is increased, the separation of the plate is deteriorated, so that the stability of the printed pattern shape cannot be obtained. In addition, when the tension is increased or the wire diameter is reduced, the screen mesh is easily stretched, so that it is difficult to maintain the dimensional and shape accuracy and the durability of the printing plate making is lowered. In addition, since it is provided with a large width, a bus bar that is unnecessary to increase the film thickness is also increased, resulting in a problem of waste of material.

The glass frit is not particularly limited, and either leaded glass or lead-free glass can be used. For example, PbO 46~57 (mol%) in terms of _{oxide, B 2 O 3 1~7 (mol} %), leaded glass in a proportion in the range of SiO ₂ 38~53 (mol%), as oxide containing containing in _{Li 2 O 0.6~18 (mol%)} , PbO 20~65 (mol%), B 2 O 3 1~18 (mol%), in a proportion in the range of SiO ₂ 20~65 (mol%) Li leaded glass, Bi ₂ O ₃ 10~29 in terms of oxide (mol%), ZnO 15~30 ( mol%), SiO 2 0~20 (mol%), B 2 O 3 20~33 (mol% ), Li ₂ O, Na ₂ O, and K ₂ O can be used as lead-free glass containing a total amount of 8 to 21 (mol%).

  In the leaded glass, PbO is a component that lowers the softening point of the glass and is essential to enable low-temperature firing. In order to obtain good fire-through properties, it is preferable that PbO is 46 (mol%) or more and 57 (mol%) or less. The amount of PbO is more preferably 49 (mol%) or more, and further preferably 54 (mol%) or less. That is, the range of 49 to 54 (mol%) is more preferable.

In the leaded glass, B ₂ O ₃ is a glass-forming oxide (that is, a component that forms a glass skeleton), and is an essential component for lowering the softening point of glass. In order to obtain good fire-through properties, B ₂ O ₃ is preferably 1 (mol%) or more and 7 (mol%) or less. The amount of B ₂ O ₃ is more preferably 3 (mol%) or more, and further preferably 5 (mol%) or less. That is, the range of 3 to 5 (mol%) is more preferable.

In the leaded glass, SiO ₂ is a glass-forming oxide and is an essential component for increasing the chemical resistance of the glass. In order to obtain good fire-through properties, it is preferable that SiO ₂ is 38 (mol%) or more and 53 (mol%) or less. The amount of SiO ₂ is more preferably 43 (mol%) or more, and more preferably 48 (mol%) or less. That is, the range of 43 to 48 (mol%) is more preferable.

  Moreover, the said leaded glass may contain another various glass component and additive in the range which does not impair the characteristic. For example, Al, Zr, Na, Li, Ca, Zn, Mg, K, Ti, Ba, Sr, etc. may be contained. These may be included in a total range of 10 (mol%) or less, for example.

In the Li-containing lead-containing glass, Li ₂ O is an essential component in addition to PbO, B ₂ O ₃ and SiO ₂ . Li ₂ O is a component that lowers the softening point of the glass. In order to obtain good fire-through properties, Li ₂ O is preferably 0.6 (mol%) or more and 18 (mol%) or less. If Li ₂ O is less than 0.6 (mol%), the softening point becomes too high and the erosion property to the antireflection film tends to be insufficient. On the other hand, if it exceeds 18 (mol%), the erodibility becomes too strong, and the electrical characteristics tend to deteriorate. Incidentally, Li is generally an impurity for semiconductors because it promotes diffusion, and Li tends to deteriorate the characteristics, so it is desirable to avoid it in semiconductor applications. In particular, when the amount of Pb is usually large and Li is contained, the erodibility tends to be too strong and control tends to be difficult. However, in the solar cell application as described above, no deterioration in characteristics was observed using glass containing Li, and on the other hand, an appropriate amount was included, thereby improving the fire-through property and improving the characteristics. Li is a donor element and can reduce contact resistance. In addition, it was recognized that the composition range of the glass capable of obtaining good fire-through properties was increased by adopting a composition containing Li. However, even in solar cell applications, if it is excessively contained, the erodibility becomes too strong, and the electrical characteristics tend to deteriorate. The amount of Li ₂ O is more preferably 6 (mol%) or more, and further preferably 12 (mol%) or less. That is, the range of 6 to 12 (mol%) is more preferable. Further, about 6 (mol%) is particularly preferable.

  In addition, in the Li-containing lead-containing glass, the amount of PbO is preferably 20 (mol%) or more and 65 (mol%) or less in order to obtain good fire-through properties. If the amount of PbO is less than 20 (mol%), the softening point becomes too high, so that it is difficult to erode the antireflection film, and it becomes difficult to obtain a good ohmic contact. On the other hand, when it exceeds 65 (mol%), the softening point becomes too low, so that erosion becomes too strong and the pn junction is likely to be broken. The amount of PbO is more preferably 22.4 (mol%) or more, and further preferably 50.8 (mol%) or less. That is, the range of 22.4 to 50.8 (mol%) is more preferable. Moreover, about 30-40 (mol%) is especially preferable.

In the Li-containing lead-containing glass, the amount of B ₂ O ₃ is preferably 1 (mol%) or more and 18 (mol%) or less in order to obtain good fire-through properties. If the amount of B ₂ O ₃ is less than 1 (mol%), the softening point becomes too high, so that it is difficult to erode the antireflection film, and it becomes difficult to obtain a good ohmic contact, and the moisture resistance tends to decrease. is there. In particular, in an embodiment in which Li is contained in the glass, it is extremely difficult to melt if B ₂ O ₃ is not contained in an amount of 1 (mol%) or more. On the other hand, if it exceeds 18 (mol%), the softening point becomes too low, so that the erosion becomes too strong and the pn junction is broken. The amount of B ₂ O ₃ is more preferably 2.8 (mol%) or more, and further preferably 12 (mol%) or less. That is, the range of 2.8 to 12 (mol%) is more preferable. Moreover, about 6-12 (mol%) is especially preferable.

In the Li-containing lead-containing glass, the SiO ₂ content is preferably 20 (mol%) or more and 65 (mol%) or less in order to obtain good fire-through properties. If the amount of SiO ₂ is less than 20 (mol%), chemical resistance is insufficient and glass formation tends to be difficult.On the other hand, if it exceeds 65 (mol%), the softening point becomes too high and the antireflection film is eroded. It tends to be difficult to obtain a good ohmic contact. The amount of SiO ₂ is more preferably 27.0 (mol%) or more, and further preferably 48.5 (mol%) or less. That is, the range of 27.0 to 48.5 (mol%) is more preferable. Moreover, about 30-35 (mol%) is especially preferable.

  The Li-containing lead-containing glass may contain other various glass components and additives as long as the characteristics are not impaired. For example, Al, Zr, Na, Ca, Zn, Mg, K, Ti, Ba, Sr, etc. may be contained. Since Al is an effective component for obtaining the stability of the glass, it hardly affects the properties but is preferably contained. These may be included in a total range of 30 (mol%) or less, for example. For example, Al and Ti are each preferably 6 (mol%) or less, and more preferably 3 (mol%) or less. Zn is preferably 30 (mol%) or less, and more preferably 15 (mol%) or less. By using a composition containing appropriate amounts of these Al, Ti, and Zn, the parallel resistance Rsh is improved, and the open-circuit voltage Voc and the short-circuit current Isc are improved, so that higher electrical characteristics can be obtained.

In the lead-free glass, the amount of B ₂ O ₃ is preferably 20 (mol%) or more and 33 (mol%) or less in order to obtain good fire-through properties. If it is less than 20 (mol%), the softening point tends to be too high, and if it exceeds 33 (mol%), the electric characteristics of the solar cell tend to be insufficient. While the softening point increases with decreasing B ₂ O _{3, the} electrical characteristics decrease with increasing B ₂ O ₃ (for example, in silicon-based solar cells, this is considered to be due to the higher reactivity with Si, the substrate material). Therefore, the ratio is preferably determined in consideration of a desired softening point and electrical characteristics, for example, 30 (mol%) or less is preferable.

In the lead-free glass, Bi ₂ O ₃ is a component that lowers the softening point of the glass and is preferably included to enable low-temperature firing. If it is less than 10 (mol%), the softening point tends to be too high, and if it exceeds 29 (mol%), the electric characteristics of the solar cell tend to be insufficient. In order to obtain as high electrical characteristics as possible, the amount of Bi ₂ O ₃ is preferably as small as possible, and more preferably 20 (mol%) or less. Further, in order to sufficiently lower the softening point, a larger amount of Bi ₂ O ₃ is preferable, and 15 (mol%) or more is preferable. That is, the range of 15 to 20 (mol%) is particularly preferable.

  Further, in the lead-free glass, ZnO is a component that lowers the softening point of the glass and increases durability (that is, long-term reliability), and if it is less than 15 (mol%), the softening point is too high and the durability is low. The nature is also insufficient. On the other hand, if it exceeds 30 (mol%), the balance with other components is affected, but the glass is easily crystallized. The smaller the amount of ZnO, the higher the softening point and the lower the durability. On the other hand, the larger the amount, the easier it is to crystallize, so 20 (mol%) or more is more preferred, and 30 (mol%) or less is more preferred. That is, the range of 20 to 30 (mol%) is particularly preferable.

In the lead-free glass, the alkaline components Li ₂ O, Na ₂ O, K ₂ O are components that lower the softening point of the glass, and if the total amount is less than 8 (mol%), the softening point becomes too high. If it exceeds 21 (mol%), the electrical characteristics of the solar cell become insufficient. The softening point increases as the amount of the alkali component decreases, while the electrical characteristics decrease as the amount of the alkali component increases. Therefore, it is more preferably 10 (mol%) or more, and more preferably 20 (mol%) or less. That is, the range of 10 to 20 (mol%) is particularly preferable.

In the lead-free glass, SiO ₂ is a glass-forming oxide, and lead-free glass also has an effect of improving the stability of the glass. However, since the softening point increases as the number increases, it is necessary to keep it at 20 (mol%) or less. In order to obtain sufficient stability, 4 (mol%) or more is more preferable, and in order to keep the softening point at a sufficiently low value, 11 (mol%) or less is more preferable. That is, 4 to 11 (mol%) is particularly preferable.

Further, the glass frit can be synthesized from various raw materials that can be vitrified within the composition range, and examples thereof include oxides, carbonates, nitrates, etc. Examples of the Si source include silicon dioxide SiO _2. a boric acid B ₂ O ₃ is a B source, red lead Pb ₃ O ₄ as a Pb source, a bismuth oxide as a Bi source, a zinc oxide as a Zn source, the Li source lithium carbonate, Na Sodium carbonate can be used as the source, and potassium carbonate can be used as the K source.

  Moreover, in any of leaded glass and lead-free glass, when other components are contained in addition to the main components, their oxides, hydroxides, carbonates, nitrates, etc. may be used.

  In addition, since the conductive paste composition of the present invention improves the adhesive strength while maintaining the electrical characteristics as described above, it is easy to make a thin line, so that an ohmic contact is obtained by the fire-through method. It can be suitably used for a light-receiving surface electrode of a silicon-based solar cell.

It is a schematic diagram which shows the cross-sectional structure of the solar cell with which the paste for electrodes of one Example of this invention was applied for formation of a light-receiving surface electrode. It is a figure which shows an example of the light-receiving surface electrode pattern of the solar cell of FIG.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are appropriately simplified or modified, and the dimensional ratios, shapes, and the like of the respective parts are not necessarily drawn accurately.

FIG. 1 is a diagram schematically showing a cross-sectional structure of a silicon-based solar cell 10 in which an electrode paste according to an embodiment of the conductive paste composition of the present invention is used to form a light-receiving surface electrode 20. In FIG. 1, a solar cell 10 includes a silicon substrate 12 which is, for example, a p-type polycrystalline semiconductor, an n layer 14 and a p ⁺ layer 16 respectively formed on the upper and lower surfaces thereof, and a reflection formed on the n layer 14. A prevention film 18 and a light-receiving surface electrode 20 and a back electrode 22 formed on the p ⁺ layer 16 are provided. The thickness dimension of the silicon substrate 12 is, for example, about 100 to 200 (μm).

The n layer 14 and the p ⁺ layer 16 are provided by forming layers having a high impurity concentration on the upper and lower surfaces of the silicon substrate 12, and the thickness dimension of the high concentration layer is, for example, 70% for the n layer 14. ˜100 (nm), and the p ⁺ layer 16 is about 500 (nm), for example. The n layer 14 is about 100 to 200 (nm) in a general silicon solar cell, but is thinner than that in this embodiment, and has a structure called a shallow emitter. The impurity contained in the n layer 14 is an n-type dopant such as phosphorus (P), and the impurity contained in the p ⁺ layer 16 is a p-type dopant such as aluminum (Al) or boron (B). .

The antireflection film 18 is a thin film made of, for example, silicon nitride Si ₃ N ₄ , and is provided with an optical thickness of, for example, about ¼ of the visible light wavelength, for example, about 80 (nm). It is configured to have an extremely low reflectance of 10 (%) or less, for example, 2 (%).

The light-receiving surface electrode 20 is made of, for example, a thick film conductor having a uniform thickness. As shown in FIG. 2, the light-receiving surface electrode 20 is a comb having a large number of thin line portions substantially on the entire surface of the light-receiving surface 24. Are provided in a planar shape. The thick film conductor is composed of thick film silver containing Ag in the range of about 78 to 99 (wt%) and containing Ni in the range of 0.5 (wt%) or less as a conductor component. glass component, in terms of oxide values, the range of the PbO 20~65 (mol%), for example 22.4 (mol%) or so, the range of the _{B 2 O 3 1~18 (mol%} ), for example 9.0 (mol%), SiO ₂ in the range of ₂₀ to 65 (mol%), for example, about 35.6 (mol%), Al ₂ O ₃ in the range of 0 to 6 (mol%), for example, 3.0 (mol%) the extent, in the range of Li ₂ O and 0.6-18 (mol%), for example 12.0 (mol%) or so, the range of the TiO ₂ to 0~6 (mol%), for example, 3.0 (mol%) or so, the ZnO 0 Li-containing lead-containing glass each contained in a range of ˜30 (mol%), for example, about 15.0 (mol%). In addition, the thickness dimension of the conductor layer is, for example, in the range of 20-30 (μm), for example, about 25 (μm), and the width dimension of each thin line portion is in the range of, for example, 80-130 (μm) It is about 100 (μm) and has sufficiently high conductivity.

  In addition, the Ni contains a slight amount of NiC, but as shown in the manufacturing process described later, since the heat treatment when forming the conductor layer is performed in an oxidizing atmosphere, Ni in the conductor film is changed to NiO. It is considered that there is a high possibility that

The back electrode 22 is formed by applying a full-surface electrode 26 formed by applying a thick film material containing aluminum as a conductor component on the p ⁺ layer 16 over almost the entire surface, and a strip-like application on the full-surface electrode 26. The band-shaped electrode 28 made of thick film silver is formed. The belt-like electrode 28 is provided in order to be able to solder a lead wire or the like to the back electrode 22.

  In the solar cell 10 of this example, since the light-receiving surface electrode 20 is composed of thick film silver containing conductive zinc oxide, which is a low-resistance n-type semiconductor, as described above, the line width is 100 (μm). Despite being thinned to a certain extent, a good ohmic contact with the n layer 14 is obtained, and the contact resistance is low.

  The light-receiving surface electrode 20 as described above is formed by, for example, a well-known fire-through method using an electrode paste composed of conductive powder, glass frit, conductive zinc oxide, vehicle, and solvent. It is a thing. An example of the manufacturing method of the solar cell 10 including the formation of the light receiving surface electrode will be described below.

First, a glass frit is produced. When using a glass frit made of Li-containing lead-containing glass as described above, lithium carbonate Li ₂ CO ₃ as the Li source, silicon dioxide SiO ₂ as the Si source, boric acid B ₂ O ₃ as the B source, Pb Pb ₃ O ₄ as the source, aluminum oxide Al ₂ O ₃ as the Al source, titanium oxide TiO ₂ as the Ti source, and zinc oxide ZnO as the Zn source, respectively. Weigh and mix so that This is put into a crucible, melted at a temperature in the range of 900 to 1200 (° C.) according to the composition for about 30 minutes to 1 hour, and rapidly cooled to be vitrified. This glass is pulverized using an appropriate pulverizing apparatus such as a planetary mill or a ball mill. The average particle size (D50) after pulverization is, for example, about 0.3 to 3.0 (μm).

In place of the glass frit, PbO 46~57 (mol%) in terms of _{oxide, B 2 O 3 1~7 (mol} %), leaded in a proportion in the range of SiO ₂ 38~53 (mol%) glass or, Bi ₂ O ₃ in terms of oxides 10~29 (mol%), ZnO 15~30 (mol%), SiO 2 0~20 (mol%), B 2 O 3 20~33 (mol%), Lead-free glass containing a total amount of Li ₂ O, Na ₂ O, and K ₂ O in the range of 8 to 21 (mol%) can also be used. In the case of using the leaded glass, for example, sodium oxide Na ₂ O as a Na source, lithium oxide Li ₂ O as a Li source, and potassium carbonate as a K source are produced. Vitrification is carried out in the same manner as in the case of. When lead-free glass is used, the above Li-containing glass is used except that bismuth oxide is used as the Bi source, NH ₄ H ₂ PO ₄ is used as the P source, calcium oxide CaO is used as the Ca source, and BaCO ₃ is used as the Ba source. Glass frit is produced in the same manner as leaded glass and leaded glass. Table 1 shows the glass composition used in this example. In Table 1, Nos. 1, 3 and 4 are Li-containing glass, No. 2 is leaded glass, and No. 5 is lead-free glass. In the present embodiment, any of these can be used.

  On the other hand, as the conductor powder, for example, a commercially available spherical silver powder having an average particle diameter (D50) in the range of 0.3 to 3.0 (μm) is prepared. By using such silver powder having a sufficiently small average particle diameter, the filling rate of the silver powder in the coating film can be increased and the electrical conductivity of the conductor can be increased. The vehicle is prepared by dissolving an organic binder in an organic solvent. For example, butyl carbitol acetate is used as the organic solvent, and ethyl cellulose is used as the organic binder. The ratio of ethyl cellulose in the vehicle is, for example, about 15 (wt%). A solvent added separately from the vehicle is, for example, butyl carbitol acetate. That is, although it is not limited to this, the same solvent as that used for the vehicle may be used. This solvent is added for the purpose of adjusting the viscosity of the paste.

  Separately, fine powder Ni is prepared. Fine powder Ni can be produced using the polyol method or the like as described in Patent Document 11, but in this example, for example, Mitsui Mining Co., Ltd. (Mitsui Mining Co., Ltd.) having an average particle size of 10 to 91 (nm). Fine nickel powder manufactured by Co., Ltd. was used.

  For example, 77 to 88 (wt%) of spherical Ag powder having an average particle diameter of about 1.6 (μm) is prepared, and 1 to 10 glass frit selected from those shown in Table 1 is prepared. (wt%), 0.01 to 0.5 (wt%) of the fine powder Ni, 4 to 14 (wt%) of the vehicle, 2 to 8 (wt%) of the solvent, and mixed using a stirrer or the like For example, dispersion processing is performed with a three-roll mill. Thereby, an electrode paste is obtained. In this example, in order to make the printability between samples equal, the viscosity of 20 (rpm) -25 (° C.) was adjusted to 160 to 180 (Pa · s), and the printing plate making was SUS325. A mesh having a wire diameter of 23 (μm) and an emulsion thickness of 20 (μm) was used.

While preparing the electrode paste as described above, the n layer 14 and the p ⁺ layer are diffused or implanted into an appropriate silicon substrate by a well-known method such as a thermal diffusion method or ion plantation. By forming 16, the silicon substrate 12 is produced. Next, a silicon nitride thin film is formed thereon by an appropriate method such as PE-CVD (plasma CVD), and the antireflection film 18 is provided.

  Next, the electrode paste is screen-printed on the antireflection film 18 with the pattern shown in FIG. In printing, the printing conditions were set so that the width of the grid line after firing was 100 (μm). This is dried at, for example, 150 (° C.), and further subjected to a baking treatment at a temperature in the range of 740 to 900 (° C.) in a near infrared furnace. As a result, the glass component in the electrode paste melts the antireflection film 18 in the firing process, and the electrode paste breaks the antireflection film 18, so that the conductor component in the electrode paste, that is, silver and the n layer 14 Electrical connection is obtained, and an ohmic contact between the silicon substrate 12 and the light-receiving surface electrode 20 is obtained as shown in FIG. The light receiving surface electrode 20 is formed in this way.

  In addition, although the said back surface electrode 22 may be formed after the said process, it can also be formed by baking simultaneously with the light-receiving surface electrode 20. FIG. When the back electrode 22 is formed, the entire surface electrode 26 made of a thick aluminum film is formed by applying, for example, an aluminum paste to the entire back surface of the silicon substrate 12 by screen printing or the like and performing a baking process. Further, the strip electrode 28 is formed by applying the electrode paste on the surface of the entire surface electrode 26 in a strip shape using a screen printing method or the like and performing a baking treatment. Thereby, the back electrode 22 which consists of the full surface electrode 26 which covers the whole back surface, and the strip | belt-shaped electrode 28 provided in strip shape on a part of the surface is formed, and the said solar cell 10 is obtained. In the above process, when manufacturing by simultaneous firing, a printing process is performed before firing the light-receiving surface electrode 20.

  The composition of the electrode paste is variously changed, the solar cell 10 is manufactured according to the above manufacturing process, the output is measured using a commercially available solar simulator, the curve factor FF value is evaluated, and the light receiving surface electrode 20 Tables 2 to 5 show the results (Nos. 1 to 23) of soldering terminals and measuring the peel strength with a tensile tester together with comparative examples (Nos. 24 to 28) together with the preparation specifications of each sample. Show. In each table, the additive column indicates the particle size of Ni or NiO added, that is, the type, that is, the amount of Ni and NiO, and the amount added to 100 parts by weight of the paste. In the glass No. column, the type of glass used is shown using the frit numbers shown in Table 1 above. All glass additions are 3 parts by weight. Further, although not shown in the table, the Ag powder was added in an amount of 84 parts by weight of spherical powder having an average particle diameter of 1.6 (μm). Further, the amount of the organic component, that is, the vehicle was all 13 parts by weight. That is, the total amount of Ag powder, glass frit, and vehicle was 100 parts by weight. Tables 2 to 4 summarize examples for each element of interest, and experimental data that overlap each other are shown again with the same example numbers.

  Moreover, in the said Tables 2-5, the measured FF value (%) and the quality determination were described in the FF determination column. In the pass / fail judgment, “less than” 75% or less was “impossible”, 75% or more “good”, and particularly FF value 77 (%) “very good”. This pass / fail judgment is in line with the actual situation where an FF value of 75% or more is required in the market. In addition, in the solder adhesive strength column, the adhesive strength measured immediately after bonding and after aging is shown, the strength after aging is less than 1 (N) is `` impossible '', 1-3 (N) is `` good '', 3 Those exceeding (N) were considered “very good”. The adhesive strength after the lapse of time was measured after storage in a high-temperature bath at 85 (° C) for 1 week after soldering. The solar cell is required to have durability capable of maintaining the quality for a long period of, for example, 20 years or more, and the evaluation of the adhesive strength is an accelerated test for judging the presence or absence of the durability. Strength criteria were in line with the market demands.

Table 2 above evaluates the influence on the FF value and the adhesive strength by variously changing the Ni particle diameter between 10 and 91 (nm). In Example Nos. 1 to 12 where the Ni particle size is 10 to 91 (nm), a good or very good FF value of 75 to 77 (%) is obtained, whereas the Ni particle size is 195 (nm) In comparative examples No. 27 and 28 (see Table 5), which were large, the FF value was 73% or less, which was insufficient. Solder adhesive strength was good at 2.2 (N) or more after aging. In Comparative Examples No. 24 and 25 (see Table 5) to which Ni was not added, a good FF value of 76 (%) was obtained, but the adhesive strength after aging was extremely low at 0.9 (N). . Comparing these, it can be seen that if Ni is added, the effect of improving the adhesive strength can be obtained regardless of the particle size, but if the particle size exceeds 91 (nm), the FF value decreases and becomes insufficient. In addition, according to the evaluation results of Examples No. 1 to 6 (excluding No. 2) in which Ni powder of 22 nm or less was added in a relatively small amount of 0.1 parts by weight or less, a small amount of fine Ni should be added. It can be seen that the effect of improving the FF value can be obtained. The effect of improving the FF value is obtained by forming nickel silicide between the added Ni and the Si of the cell, thereby improving the parallel resistance Rsh, reducing the leakage current, and reducing the contact resistance. It is thought that it is.

Table 3 shows the results of evaluating the influence on the FF value and the adhesive strength by changing the Ni addition amount in the range of 0.01 to 0.5 parts by weight in the case of using two types of glass frits 1 and 2, respectively. It is. All the Ni powders used had an average particle size of 22 (nm). When the glass frit 1 was used, a very good result was obtained with a high FF value of 77 (%) when the Ni addition amount was 0.1 parts by weight or less. When 0.5 part by weight was added and when the glass frit 2 was used, good results with an FF value of 75 (%) or more were obtained. In addition, when glass frit 1 is used, the solder bond strength is very good, exceeding 3 (N) when the amount of Ni added is 0.06 parts by weight or more, and 2 (N) in other cases. The above good results were obtained. On the other hand, in Comparative Example No. 26 (see Table 5) in which the Ni addition amount was 1.0 part by weight, the solder bond strength was as high as 3.1 (N), and a very good result was obtained, but the FF value was 71 (%). It was extremely low and insufficient. According to these evaluation results, the adhesive strength increases as the Ni addition amount increases, but the tendency that the FF value decreases to a peak of about 0.05 parts by weight is observed. The value drops to 75 (%), and is significantly lower at 1.0 part by weight. Therefore, Ni amount is rather preferably be kept below 0.5 part by weight, not more preferably be kept below 0.2 parts by weight. Although not shown in Table 3, when the addition amount was 2 parts by weight or more, a decrease in adhesive strength was observed. This is thought to be because the excessive addition of Ni hinders the sintering of Ag.

  Note that there is no particular lower limit for the amount of Ni added, and an effect of improving the adhesive strength can be obtained even with a very small amount added, but it becomes difficult to ensure the homogeneity of the paste as the amount added is reduced. In the above evaluation, the case where the amount is less than 0.01 parts by weight is not evaluated.

  In the evaluation results shown in Table 3 above, in Nos. 5 and 6 in which the glass frit 1 was used and Ni fine powder having a particle size of 22 (nm) was added in the range of 0.06 to 0.1 parts by weight, the FF value was 77 (% ) And a very good result with an adhesive strength of 3.4 to 3.5 (N). According to these results, it is preferable to use the glass frit 1 for the glass, the most preferable particle diameter of Ni fine powder is 22 (nm), and the addition amount is most preferably in the range of 0.06 to 0.1 parts by weight.

  Table 4 summarizes the results of evaluation using glass frits 1 to 5 in a system in which 0.1 parts by weight of Ni fine powder having a particle size of 22 (nm) with the most preferable results obtained as described above was added. Therefore, Nos. 22 and 23 using NiO instead of Ni as an additive were also evaluated. According to this evaluation result, when the glass frits 1 and 5 were used, a very high FF value of 77 (%) was obtained and an extremely high adhesive strength of 3.5 (N) was obtained. Is considered to be preferable to glass frits 2 to 4. In particular, in Example No. 21 using the glass frit 5, an extremely high adhesive strength of 4.9 (N) was obtained after aging, and this was the best in the range evaluated this time. For NiO, only the evaluation using glass frit 1 was carried out. When 0.02 part by weight was added, the FF value was 77 (%), the adhesive strength was 2.9 (N), and when 0.1 part by weight was added, the FF value was The result was 77 (%) and the adhesive strength was 3.9 (N). Also in the case of NiO, it is considered preferable to add 0.1 parts by weight.

  In addition, when Examples No. 1 to No. 23 and Comparative Examples No. 26 to 28 in Tables 2 to 5 are compared with Comparative Examples No. 24 and 25, the strength immediately after bonding is a relatively high value. However, in the former case where Ni is added, the adhesive strength is high even after the lapse of time, whereas in the latter case where Ni is not added, the strength after the lapse of time is remarkably lowered. Although any tendency to decrease the strength due to aging is observed, it can be seen that the addition of Ni remarkably alleviates the degree of decrease.

According to the evaluation results described above, by the addition of fine Ni or NiO and Ag powder electrode paste for the conductor components are provided as the light-receiving surface electrode 20 on the n-layer 14 of the sheet silicon substrate 12, the light-receiving The surface electrode 20 has high solder adhesive strength. In addition, since a conductive path is suitably formed between the silicon substrate 12 and Ag in the light receiving surface electrode 20, the contact resistance between the silicon substrate 12 and the light receiving surface electrode 20 is reduced, and Ni or the like is not added. Since it has equivalent or better electrical characteristics, thinning becomes easy and the light receiving area can be increased, so that equivalent or higher photoelectric conversion efficiency can be obtained. Therefore, an electrode paste suitable for the light-receiving surface electrode 20 of the solar cell 10 that can increase the solder adhesive strength without impairing the electrical characteristics can be obtained.

  As mentioned above, although this invention was demonstrated in detail with reference to drawings, this invention can be implemented also in another aspect, A various change can be added in the range which does not deviate from the main point.

For example, in the above embodiment, the antireflection film 18 is made of a silicon nitride film. However, the constituent material is not particularly limited, and various other materials such as titanium dioxide TiO ₂ generally used for solar cells. Those consisting of can be used as well.

  In the examples, the case where the present invention is applied to the silicon-based solar cell 10 has been described. However, the present invention is similar to the present example in that the energy level of the conductive zinc oxide in the electrodes is It is applicable to any solar cell that can form a light-receiving surface electrode by the fire-through method, as long as it is in the middle magnitude relation between the conductor component in the substrate and the energy level of the substrate. If it exists, the board | substrate material of application object will not be specifically limited.

10: solar cell, 12: silicon substrate, 14: n layer, 16: p ⁺ layer, 18: antireflection film, 20: light receiving surface electrode, 22: back electrode, 24: light receiving surface, 26: full surface electrode, 28: Strip electrode

Claims (2)

A conductive paste composition for solar cells, comprising conductive silver powder, glass frit, and an organic medium, and used for forming an electrode of a silicon-based solar cell,
A conductive paste composition for solar cells , comprising at least one of Ni and NiO having an average particle size of 10 to 91 (nm) in a proportion of 0.5 (wt%) or less with respect to the entire paste composition object. The conductive paste composition for a solar cell according to claim 1, wherein the Ni and NiO are contained in a proportion of 0.2 (wt%) or less with respect to the entire paste composition.

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