JP5756447B2 - 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 a sufficient visible light transmittance, and is made of a thin film such as silicon nitride, titanium dioxide, or silicon dioxide.

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 formation method, for example, after the antireflection film is provided on the entire surface of the n ⁺ layer, a conductive paste, that is, a paste-like electrode material is appropriately formed on the antireflection film by using, for example, a screen printing method. And apply a baking process. As a result, the electrode material is heated and melted, and at the same time, the antireflection film in contact with the electrode material is melted, and the light receiving surface electrode and the semiconductor are brought into contact with each other. 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 a 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). In this conductive paste, conductivity is obtained by blending various trace components made of a metal or a compound such as phosphorus, vanadium, bismuth, and tungsten. 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. There is an advantage that 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 value) and energy conversion efficiency. Has been made. For example, by adding a Group 5 element such as phosphorus, vanadium, bismuth or the like to the conductive paste, there is one that promotes the redox action on the glass and silver antireflection film 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.) Examples of the fluoride include lithium fluoride, nickel fluoride, and aluminum fluoride. It is also shown that a Group 5 element is added in addition to the above additives. The glass is, for example, borosilicate glass.

  In addition, it is proposed that the conductive paste contains 0.5 to 5 parts by weight of silver phosphate with respect to 100 parts by weight of silver powder to assist the action of breaking the antireflection film and ensure ohmic contact (for example, , See Patent Document 3). In addition, by using a glass containing zinc oxide as a main component and containing no lead, and using a paste containing silver, gold, and antimony, there is no intrusion of the electrode, so that the junction does not break down and low contact resistance is obtained. There are some (see, for example, Patent Document 4).

In a silver-containing paste containing 85 to 99 (wt%) silver and 1 to 15 (wt%) glass, the glass was mixed with 15 to 75 (mol%) PbO and 5 to 50 (mol%) SiO. comprises _2, it is a composition that does not contain B ₂ O ₃ has been proposed (e.g., see Patent Document 5.). This silver-containing paste is used for forming an electrode of a solar cell, and the ohmic contact is improved by using the glass having the above composition. The above glass, a _{P 2 O 5 0.1~8.0 (mol%} ), or Sb ₂ O ₅ to be able to contain 0.1 to 10.0 (mol%), further, an alkali metal oxide of 0.1 to 15.0 (mol%) (Na ₂ O, K ₂ O, Li ₂ O).

Further, the present applicant has a glass frit PbO 46~57 (mol%), the _{B 2 O 3 1~7 (mol%} ), a SiO ₂ glass containing in the range of 38-53 (mol%) A solar cell electrode paste composition was proposed previously (see Patent Document 6). In this paste composition, the optimum firing temperature range at the time of forming the electrode of the solar cell is widened by selecting the composition range of PbO, B ₂ O ₃ and SiO ₂ as described above. The optimum firing temperature range for individual substrates may vary due to variations in the manufacturing process, but the wider the optimum firing temperature range, the greater the possibility that the firing temperature will fall within that range. Average output is improved.

Further, the present applicant has a glass frit _{Li 2 O 0.6~18 (mol%)} , a PbO 20~65 (mol%), the _{B 2 O 3 1~18 (mol%} ), 20~ the SiO ₂ A paste composition for a solar cell electrode made of glass containing 65 (mol%) was previously proposed (see Patent Document 7). This paste composition enables thinning of the light-receiving surface electrode without deteriorating ohmic contact or line resistance, and the softening point is sufficiently lowered by including Li ₂ O in an amount of 0.6 to 18 (mol%). It has been shown that moderate erodibility can be obtained. Li is generally desired to be avoided in semiconductor applications, and tends to give excessive erosion, especially in glass with a large amount of Pb, but in solar cell applications, the appropriate amount is included to improve fire-through. It has been found that. Moreover, since Li is a donor element, it also has an action of reducing contact resistance.

Japanese Patent Laid-Open No. 62-049676 JP 11-213754 A Japanese Patent Laid-Open No. 08-148446 JP-A-55-103775 Special table 2008-520094 JP 2010-199334 A JP 2011-066354 A JP 2012-142422 A International Publication No. 2010/016186

  By the way, in the solar cell described above, an attempt has been made to reduce the surface recombination speed by thinning the n layer located on the light receiving surface side so that more current can be taken out, that is, to form a shallow emitter. Yes. When a shallow emitter is used, the short wavelength side near 400 (nm) also contributes to power generation, so it is considered an ideal solution in terms of improving the efficiency of solar cells. The shallow emitter has an n-layer thickness on the light-receiving surface side of 70 to 100 (nm), which is thinner than the conventional silicon solar cell 100 to 200 (nm). Since the portion that cannot be effectively used by changing to heat before reaching the pn junction is reduced, there is an advantage that the short-circuit current increases and the power generation efficiency is increased.

  However, in the shallow emitter, since it is necessary to make the cell have a high sheet resistance, the concentration of the donor element (for example, phosphorus) in the vicinity of the surface is lowered or the pn junction is shallow. When the donor element concentration near the surface decreases, the barrier barrier between Ag and Si increases, and it becomes difficult to ensure ohmic contact of the light-receiving surface electrode. Further, when the pn junction becomes shallow, it becomes very difficult to control the penetration depth so that the antireflection film is sufficiently broken by fire-through and the electrode does not penetrate the pn junction.

  In order to reliably obtain ohmic contact by the above fire-through, it is necessary to lower the viscosity of the glass at the firing temperature so that the glass is supplied promptly and uniformly to the electrode-silicon interface. As a method of reducing the viscosity, the amount of alkali or the like is adjusted to lower the softening point, or the composition, that is, the composition ratio of Pb, Si, B, Zn, which is a component that forms the glass skeleton, is changed (hereinafter referred to as the composition). , “Composition change”). Since the composition change has a large effect on the erosion amount control, the alkali amount is generally increased, but the erosion rate at the time of fire-through is increased, so it is more difficult to control the firing conditions such as temperature. Become. That is, in any case, it is difficult to achieve both ohmic contact and erosion amount control.

In contrast, the present applicant has a glass frit PbO 50~70 (mol%), B 2 O 3 and 1 to 8 (mol%), wherein the SiO ₂ 20 to 40 a (mol%), Pb A conductive paste composition for solar cells made of glass having a / Si (mol ratio) in the range of 1.4 to 2.5 and not containing Li ₂ O has been previously proposed (see Patent Document 8). According to this paste composition, a sufficiently low contact resistance can be obtained even for a substrate with a high sheet resistance, and a good fire-through property can be obtained without particularly increasing the amount of alkali. It can be suitably applied. The Pb / Si ratio is a necessary condition for achieving both fire-through performance and leakage current suppression. In addition, as described above, fire-through properties may be improved by appropriately including Li, but it is preferably not included from the viewpoint of leakage current suppression. In this paste composition, the composition is adjusted appropriately. By doing so, a composition containing no Li is realized.

  In addition, although not particularly intended for shallow emitters, a conductive paste composition for solar cells containing alkaline earth metals such as Ca and Mg or low melting point metals such as Zn, Pb, Sn, Bi, Te and Se The thing is proposed (refer patent document 9). This paste composition is a paste composition containing ultrafine metal or a metal compound thereof, and in the prior art which tried to obtain stable high conductivity and excellent adhesive force, the coating film contracted during firing and contacted. It is intended to suppress the increase in resistance and the generation of microcracks in the substrate.

  Among the above-mentioned low melting point metals, Te or Te compounds are remarkably effective in improving the electrical characteristics of the light-receiving surface electrode, and in recent years, various attempts have been made to add them to conductive pastes. In order to obtain ohmic contact in the formation of the light-receiving surface electrode by fire-through, it is necessary to increase the amount of Ag dissolved in the glass layer formed at the interface between the light-receiving surface electrode and the silicon substrate. When it coexists, the amount of Ag dissolution increases. In addition, since the change in the amount of dissolved Ag with respect to the temperature change is small, Ag dissolved in the glass gradually precipitates in the temperature-decreasing process of the firing process, so the optimum firing temperature range (that is, the firing margin) is widened. These are considered to bring about improvement of characteristics.

  On the other hand, however, Te has a strong anti-erosion effect, so if the amount of addition increases, the fire-through becomes insufficient, and on the contrary, the electrical characteristics are lowered and the optimum firing temperature range is narrowed. Therefore, it cannot be said that the effect of adding Te to the conductive paste composition has been sufficiently obtained, and further improvement in characteristics has been desired.

  The present invention has been made in the background of the above circumstances, and an object thereof is to provide a conductive paste composition for solar cells that further improves the FF value and output characteristics.

In order to achieve such an object, the gist of the present invention is a conductive paste composition for a solar cell comprising a conductive powder, a glass frit, a Te compound powder, and a vehicle, the glass frit includes a Li ₂ O of 3 to 30 (mol%) with an acid halide terms, and PbO of 28 to 60 (mol%), and B ₂ O ₃ of 3~18 (mol%), 10~40 ( mol% ) Ri formed of glass and a SiO ₂ of, and, and PbO and the Te compound powder of the glass, Pb / Te (mol ratio) of Ru near be in the range of 0.4 to 5.5.

In this way, the glass frit in the conductive paste composition for a solar cell, and PbO of 28 to 60 (mol%), and B ₂ O ₃ of 3~18 (mol%), 10~40 ( mol%) SiO ₂ and the PbO and the Te compound powder contained in the paste composition separately from the glass frit are within a range of Pb / Te (mol ratio) of 0.4 to 5.5. Therefore, it is possible to sufficiently obtain the effect of increasing the amount of Ag dissolved by the coexistence of Pb and Te while ensuring sufficient erosion. In addition, a sufficient donor compensation effect can be obtained when Li ₂ O is contained in a sufficiently large amount of 3 to 30 (mol%). When Li ₂ O is excessive, the erosion becomes too strong and the electrical characteristics tend to deteriorate. However, since the erosion is alleviated by Te, the amount of Li can be increased sufficiently. Therefore, when the light-receiving surface electrode is formed by fire-through using this paste composition, the FF value and output characteristics of the solar cell are further enhanced.

  In the glass frit composition, PbO is a component for lowering the softening point of glass and enabling low-temperature firing. In order to obtain good fire-through properties, the Pb amount needs to be 28 (mol%) or more. If it is less than this, since the softening point becomes too high, vitrification becomes difficult and it becomes difficult to erode the antireflection film, and it becomes difficult to obtain good ohmic contact. On the other hand, if it exceeds 60 (mol%), the softening point becomes too low and the erosion becomes so strong that the pn junction is liable to be broken, resulting in a problem that the FF value becomes small. The amount of PbO is more preferably 30 (mol%) or more, and still more preferably 58 (mol%) or less.

B ₂ O ₃ is a glass-forming oxide (that is, a component that forms a glass skeleton), and is a component for lowering the softening point of glass.In order to obtain good fire-through properties, 3 (mol %) Or more. If it is less than this, since the softening point becomes too high, it becomes difficult to erode the antireflection film, and it becomes difficult to obtain a good ohmic contact, and the moisture resistance also decreases. There is also a problem that the leakage current tends to increase as Voc decreases. 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 easily broken. Even if the amount is too large, Voc tends to decrease and leakage current increases. The amount of B ₂ O ₃ is more preferably 16 (mol%) or less.

SiO ₂ is a glass-forming oxide and is a component for increasing the chemical resistance of glass. If it is less than 10 (mol%), chemical resistance is insufficient and glass formation becomes difficult. On the other hand, if it exceeds 40 (mol%), the softening point becomes too high and it is difficult to vitrify and it is difficult to erode the antireflection film, and it becomes difficult to obtain a good ohmic contact. The amount of SiO ₂ is more preferably 12 (mol%) or more, and more preferably 38 (mol%) or less.

Li ₂ O is a component that lowers the softening point of the glass. If it is less than 3 (mol%), the softening point becomes too high and the erosion property to the antireflection film becomes insufficient. On the other hand, if it exceeds 30 (mol%), the erodibility becomes too strong, and the electrical characteristics 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 solar cell applications such as those described above, no deterioration in properties was observed even when glass containing Li was used, but the proper amount was included on the contrary, thereby improving the fire-through property and enhancing the properties. This is thought to be due to the erosion-inhibiting effect of Te. As described above, the present invention has been made on the basis of such a finding that the paste composition containing Te can increase the amount of Li in the glass. 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 4 (mol%) or more, and more preferably 28 (mol%) or less.

Te in the paste increases the amount of Ag dissolved as described above, and enables an increase in the amount of Li due to the erosion-inhibiting action, but when the Pb / Te (mol ratio) is less than 0.4, The amount of Ag dissolved cannot be increased sufficiently, and the effect of addition is not recognized. On the other hand, if it exceeds 5.5, the erosion control action becomes too strong, making fire-through difficult. Te is preferably contained in the range of 0.1 to 1.4 in terms of TeO ₂ / glass weight ratio. In order to obtain a sufficient addition effect, it is preferably contained in a weight ratio of 0.1 or more, and in order not to make the erosion inhibiting action excessive, it is preferable to keep the weight ratio to 1.4 or less. The Te amount is more preferably 0.2 or more and more preferably 0.6 or less in terms of TeO ₂ / glass weight ratio.

  In addition, although it is difficult to specify the above-described components and each component described later in any form in the glass, as described above, these ratios are all values converted to oxides.

Here, preferably, the glass has Pb / (Si + Al) (mol ratio) in the range of 1.0 to 3.2. It is preferable that PbO and SiO ₂ in the glass are not only within the above range, but also Pb / (Si + Al) is within the above range. Note that Al is an optional component, and when Al is not included, the above ratio is Pb / Si. If it is less than 1.0, the fire-through property is lowered, and the contact resistance between the light-receiving surface electrode and the n layer is increased. On the other hand, if it exceeds 3.2, the leakage current (diode current) Id becomes remarkably large, so that in any case, the FF value decreases and sufficient output characteristics cannot be obtained.

Preferably, the glass contains 5.0 (mol%) or less of SO ₂ in terms of oxide. In this way, without increasing the amount of alkali or changing the composition, it is possible to reduce the viscosity when the glass is softened while maintaining the same erosion as when SO ₂ is not included. Therefore, since the surface tension at the time of softening is lowered, the glass component is quickly supplied to the electrode-substrate interface, so that a uniform thin glass layer is formed at the interface and the erosion uniformity is improved. Thus, better electrical characteristics can be obtained. Therefore, the amount of penetration of the electrode material during the fire-through can be easily controlled, and an ohmic contact can be obtained more easily. For example, when an electrode is formed by a fire-through method on a solar cell having a thin n-layer shallow emitter structure in which a high sheet resistance substrate of about 80 to 120 (Ω / □) is used, the composition should include SO _2. Is preferred.

The SO ₂ is well known as a component that lowers the viscosity of the glass, but the conductive paste containing Ag has not been studied because there is a concern about the reaction between Ag and S. However, if the amount is at least as small as about 5 (mol%) in the glass, no reaction with Ag is observed, and the effect of lowering the viscosity can be suitably enjoyed. Moreover, when glass is supplied quickly to the electrode-substrate interface as described above, it is difficult for the glass to remain in the electrode, so that solder erosion is likely to occur during soldering, and sufficient adhesive strength can be obtained. However, if SO ₂ is contained in the glass, solder erosion is less likely to occur even if the amount of glass remaining in the electrode is reduced. For this reason, there exists an advantage which can make an output characteristic and a solder characteristic compatible. In addition, since S is a similar element to Te, S has a good compatibility, so there is an advantage that a synergistic effect can be obtained.

Preferably, the glass contains 18 (mol%) or less of Al ₂ O ₃ in terms of oxide. Al ₂ O ₃ is an effective component for obtaining the stability of glass, and if it is sufficiently contained, it can suppress deterioration of electrical characteristics in reliability evaluation (for example, 85 ° C, 85RH, 1000hr). . Further, when Al ₂ O ₃ is contained, the viscosity of the glass is lowered, and further, the series resistance Rs is lowered to increase the FF value and the firing temperature range tends to be widened. However, if it becomes excessive, it has the effect of increasing the leakage current and lowering Voc instead, so it is preferable to keep it at 18 (mol%) or less.

Preferably, the glass contains 6.0 (mol%) or less of P ₂ O ₅ in terms of oxide. In this way, P contained in the glass diffuses to the electrode-substrate interface and the donor concentration at the interface is increased, so that the lack of donor element concentration in the shallow emitter is compensated, and the gap between the electrode and the substrate is compensated. There is an advantage that an ohmic contact can be easily obtained. In the shallow emitter, in order to obtain a sufficient donor compensation effect, the impurity solubility in Si is 1 × 10 ⁻¹⁹ (atom / cm ³ ) or more in the vicinity of the fire-through firing temperature of 760 to 800 (° C.). It is desirable to include a plurality of types of donor elements. Since P is a donor element like Li, for example, a composition containing both Li and P is preferable. In addition to these, Sb, As, etc. can also be used.

By the way, in the high sheet resistance cell constituting the shallow emitter, the thickness of the antireflection film made of, for example, Si ₃ N _{4 is} about 80 (nm), and the amount of erosion by the electrode is in the range of 80 to 90 (nm). It is desirable to control, that is, control with an accuracy of 10 (nm). When the donor element concentration is compensated as described above, even if the erosion is slightly excessive for ensuring conduction, the output decrease due to the excessive erosion is suppressed, and therefore an ohmic contact is easily obtained.

The glass may have a composition containing other components such as TiO ₂ and ZnO. By setting the composition containing appropriate amounts of 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. That is, the FF value is higher and the leakage current is further reduced. There is also an advantage that the amount of PbO can be reduced. These contents are TiO ₂ of 18 (mol%) or less and ZnO of 30 (mol%) or less in terms of oxide. Since TiO ₂ and ZnO tend to increase the leakage current when they are excessive, it is preferable to set the above amount as the upper limit.

In addition, TiO ₂ tends to increase the FF value, but when added excessively, the softening point rises and tends to increase the contact resistance, and also has the effect of increasing the leakage current as described above. It is preferable to keep it below 18 (mol%).

  Further, since the open circuit voltage Voc decreases when the ZnO content becomes excessive, it is preferably kept at 30 (mol%) or less.

  The glass frit has an average particle size (D50) in the range of 0.3 to 3.0 (μm). If the average particle size of the glass frit is too small, melting will be too early when the electrode is baked, resulting in a decrease in electrical characteristics. . In addition, since agglomeration is unlikely to occur, better dispersibility can be obtained during paste preparation. Also, the dispersibility of the entire powder is lowered when the average particle size of the glass frit is significantly larger than the average particle size of the conductive powder, but better dispersibility can be obtained when it is 3.0 (μm) or less. Moreover, a further meltability of the glass can be obtained. Therefore, in order to obtain a better ohmic contact, the average particle diameter is preferable.

  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 a 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 making up 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 powder particles can be calculated from the obtained specific surface area and particle density.

  Preferably, the conductive powder is a silver powder having an average particle diameter (D50) in the range of 0.3 to 3.0 (μm). Although copper powder, nickel powder, etc. can be used as the conductive powder, silver powder is most preferable in order to obtain high conductivity. 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 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 by a fine line pattern.

  The silver powder is not particularly limited, and thinning can be performed while maintaining conductivity even when a powder of any shape such as a spherical shape or a scale shape is used. 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, so that, together with the use of highly conductive silver, other shapes such as scales are used. Compared with the case where the silver powder of this is used, the electrical conductivity of the electrode produced | generated from the coating film becomes high. Therefore, it is particularly preferable because the line width can be further reduced while ensuring the necessary conductivity.

  Preferably, the conductive paste composition for solar cells has a viscosity ratio (that is, [10 (rpm) within a range of 150 to 250 (Pa · s) at 25 (° C.) to 20 (rpm). )] / [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 240 (Pa · s), and the viscosity ratio is more preferably in the range of 3.2 to 6.5. 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.

  Preferably, the conductive paste composition for a solar cell includes the conductive powder in a proportion in the range of 64 to 90 parts by weight and the vehicle in a range of 3 to 20 parts by weight. In this way, a paste composition can be obtained that can easily form an electrode having good printability, thin line width, and high conductivity.

  Preferably, the conductive paste composition contains the glass frit in a range of 1 to 10 parts by weight with respect to 100 parts by weight of the conductive powder. If it is contained in an amount of 1 part by weight or more, sufficient erosion property (fire-through property) can be obtained, so that a good ohmic contact can be obtained. Further, if it is kept at 10 parts by weight or less, it is difficult to form an insulating layer, and sufficient conductivity can be obtained. The amount of glass based on 100 parts by weight of the conductive powder is more preferably 1 to 8 parts by weight, and still more preferably 1 to 7 parts by weight.

  In addition, the conductive composition of the present invention can be suitably used for the light-receiving surface electrode because it can suitably control silver deposition during the electrode formation by fire-through as described above.

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. Boric acid H ₃ BO ₃ or boron oxide B ₂ O ₃ as the B source, lead Pb ₃ O ₄ as the Pb source, lithium carbonate Li ₂ CO ₃ as the Li source, and ammonium sulfate ( NH ₄ ) ₂ SO ₄ , aluminum oxide Al ₂ O ₃ can be used as the Al source, and ammonium dihydrogen phosphate NH ₄ H ₂ PO ₄ can be used as the P source.

  Moreover, what is necessary is just to use those oxides, hydroxides, carbonates, nitrates, etc. also about the component added besides each component mentioned above.

  In addition, the glass constituting the conductive paste of the present invention may contain other various glass components and additives as long as the properties are not impaired. For example, Na, Ca, Mg, K, Ba, Sr, etc. may be contained. These may be included in a total range of 30 (mol%) or less, for example.

It is a schematic diagram which shows the cross-sectional structure of the solar cell with which the paste composition 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 solar cell module 12 including a silicon-based solar cell 10 to which a conductive composition according to an embodiment of the present invention is applied. In FIG. 1, the solar cell module 12 includes the solar cell 10, a sealing material 14 for sealing the solar cell 10, a surface glass 16 provided on the sealing material 14 on the light receiving surface side, and a solar cell from the back surface side. 10 and a protective film (that is, a back sheet) 18 provided to protect the sealing material 14. The sealing material 14 is made of, for example, EVA, and is appropriately blended with a crosslinking agent, an ultraviolet absorber, an adhesion protective agent and the like so as to have sufficient weather resistance. The protective film 18 is made of, for example, fluorine resin, polyethylene terephthalate (PET) resin, or a laminate of a plurality of resin films made of PET, EVA, etc., and has high weather resistance, water vapor barrier properties, etc. I have.

The solar cell 10 includes, for example, a silicon substrate 20 which is a p-type polycrystalline semiconductor, an n layer 22 and a p ⁺ layer 24 respectively formed on the upper and lower surfaces thereof, and a reflection formed on the n layer 22. A prevention film 26 and a light receiving surface electrode 28, and a back electrode 30 formed on the p ⁺ layer 24 are provided. The thickness dimension of the silicon substrate 20 is, for example, about 100 to 200 (μm).

The n layer 22 and the p ⁺ layer 24 are provided by forming layers having a high impurity concentration on the upper and lower surfaces of the silicon substrate 20, and the thickness dimension of the high concentration layer is, for example, 70 n. ˜100 (nm), and the p ⁺ layer 24 is about 500 (nm), for example. The n layer 22 is about 100 to 200 (nm) in a general silicon-based 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 22 is an n-type dopant such as phosphorus (P), and the impurity contained in the p ⁺ layer 24 is a p-type dopant such as aluminum (Al) or boron (B). .

The antireflection film 26 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 1/4 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 28 is made of, for example, a thick film conductor having a uniform thickness. As shown in FIG. 2, the light receiving surface electrode 28 is a comb having a large number of thin line portions substantially on the entire surface of the light receiving surface 32. Are provided in a planar shape.

The above thick film conductor is composed of thick film silver containing 1 to 10 parts by weight of glass (eg, 6.0 parts by weight) and 0.3 to 3.0 parts by weight of TeO ₂ with respect to 100 parts by weight of Ag. but, at the value that the glass in terms oxides, in the range of 28 to 60 a PbO (mol%), the range of the _{B 2 O 3 3~18 (mol%} ), SiO 2 and 10 to 40 (mol %), Li ₂ O in the range of 3 to 30 (mol%), SO ₂ 5 (mol%) or less, Al ₂ O ₃ 18 (mol%) or less, P ₂ O ₅ 6 ( mol glass), TiO ₂ is 5 (mol%) or less, ZnO is 25 (mol%) or less, and Na ₂ O is 1 (mol%) or less. In the lead glass, PbO, SiO ₂ , and Al ₂ O ₃ are included so that the Pb / (Si + Al) molar ratio is in the range of 1.0 to 3.2. TeO ₂ is contained so that the TeO ₂ / glass weight ratio is in the range of 0.1 to 1.4 and Pb / Te (mol ratio) is in the range of 0.4 to 5.5. TeO ₂ is more preferably in the range of 0.6 to 2.4 parts by weight with respect to 100 parts by weight of Ag.

  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 wire portion is, for example, in the range of 80-130 (μm), for example, It is about 100 (μm) and has sufficiently high conductivity.

The back electrode 30 is formed by applying a full-surface electrode 34 formed by applying a thick film material containing aluminum as a conductor component on the p ⁺ layer 24 over substantially the entire surface, and a strip-like application on the full-surface electrode 34. The band-shaped electrode 36 made of thick silver is formed. The strip electrode 36 is provided in order to make it possible to solder a solder ribbon 38 or a conductive wire to the back electrode 30. A solder ribbon 38 is welded to the light receiving surface electrode 28 in the same manner as the back surface side.

The light receiving surface electrode 28 as described above is formed by, for example, a well-known fire-through method using an electrode paste composed of conductor powder, TeO ₂ powder, glass frit, vehicle, and solvent. It is. 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, the glass frit is produced. Lithium carbonate Li ₂ CO ₃ as Li source, aluminum oxide Al ₂ O ₃ as Al source, ammonium dihydrogen phosphate NH ₄ H ₂ PO ₄ as P source, silicon dioxide SiO ₂ as Si source, B source Prepare boric acid H ₃ BO ₃ , red lead Pb ₃ O ₄ as the Pb source, and ammonium sulfate (NH ₄ ) ₂ SO ₄ as the S source, respectively, and weigh them to have an appropriate composition within the above-mentioned range. Mix. 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 addition, the average particle diameter of the said glass powder is computed using the air permeation method.

  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), for example, an average particle diameter of about 1.6 (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.

The TeO ₂ powder is, for example, a commercially available powder having an average particle size of about 10 (μm) (for example, manufactured by Rare Metal Co., Ltd.) using an appropriate pulverizer such as a planetary mill or a ball mill. pulverized to an average particle size of about μm).

Prepared above paste material, respectively, for example in the range of the conductor powder 77 to 90 (wt%), the range of the glass frit 1 to 10 (wt%), a TeO ₂ powder 0.3 to 3.0 (wt%) In this range, the vehicle is weighed in the ratio of 5 to 14 (wt%), the solvent is weighed in the range of 2 to 5 (wt%), mixed using a stirrer, etc., and then dispersed by, for example, a three roll mill Process. Thereby, the electrode paste is obtained.

While preparing the electrode paste as described above, the n layer 22 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 24, the silicon substrate 20 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 26 is provided.

  Next, the electrode paste is screen-printed on the antireflection film 26 with the pattern shown in FIG. This is dried at, for example, 150 (° C.), and further subjected to a baking treatment at a temperature in the range of 700 to 900 (° C.) in a near infrared furnace. As a result, the glass component in the electrode paste melts the antireflection film 26 in the firing process, and the electrode paste breaks the antireflection film 26. Therefore, the conductor component in the electrode paste, that is, silver and the n layer 22 Electrical connection is obtained, and an ohmic contact between the silicon substrate 20 and the light-receiving surface electrode 28 is obtained as shown in FIG. The light receiving surface electrode 28 is formed in this way.

  The back electrode 30 may be formed after the above process, but may be formed by firing at the same time as the light receiving surface electrode 28. When the back electrode 30 is formed, the full surface electrode 34 made of a thick aluminum film is formed on the entire back surface of the silicon substrate 20 by, for example, applying an aluminum paste by screen printing or the like and performing a baking process. Further, the strip electrode 36 is formed by applying the electrode paste on the surface of the entire surface electrode 34 in a strip shape by screen printing or the like and performing a baking treatment. Thereby, the back electrode 30 which consists of the full surface electrode 34 which covers the whole back surface, and the strip | belt-shaped electrode 36 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 28.

In the solar cell 10 of the present embodiment, the light receiving surface electrode 28 is provided by the fire-through method as described above. As described above, the light receiving surface electrode 28 has a PbO content in the range of 28 to 60 (mol%). A glass containing B ₂ O ₃ within a range of ₃ to 18 (mol%), SiO ₂ within a range of 10 to 40 (mol%), and Li ₂ O within a range of 3 to 30 (mol%). Since it is composed of thick film silver containing TeO _{2 in} the range of 1 to 10 parts by weight and in the range of 0.3 to 3.0 parts by weight with respect to 100 parts by weight of silver, the thick film silver paste has an amount of Li In spite of the fact that it is extremely increased, it has moderate erodibility during fire-through and Ag precipitates gently. Therefore, an ohmic contact is obtained and the solar cell 10 having excellent electrical characteristics is obtained.

  Next, the result of having manufactured and evaluated the solar cell 10 according to said manufacturing process by changing glass composition variously is demonstrated. About the solar cell characteristic, the output was measured using the commercially available solar simulator, and the curve factor FF value was calculated | required. The evaluation results are shown in Tables 1 and 2 together with the glass composition. In Tables 1 and 2, the “output characteristics” indicate the result of determining suitability based on the FF value, with an FF value of 75 or more being “◯” (that is, an example), and a value less than 75 being “x” (that is, Comparative example). The FF value is a determination as to whether or not a good ohmic contact is obtained. In general, a solar cell can be used if the FF value is 70 or more. In the examples, those having an FF value greater than 75 were considered acceptable.

Each sample was prepared using spherical Ag powder having an average particle size of 1.6 (μm) and glass frit having an average particle size of 1.5 (μm). The mixing ratio is basically based on Ag powder 83 (wt%), glass frit 3 (wt%), TeO ₂ powder 1 (wt%), vehicle 8 (wt%), and solvent 5 (wt%), and the printability is equivalent. Therefore, the amount of the vehicle and the amount of the solvent were appropriately adjusted so that the viscosity at 25 (° C.)-20 (rpm) was 220 to 240 (Pa · s). The printing plate making for forming the light-receiving surface electrode 28 was made by providing a 20 (μm) thick emulsion on a SUS325 screen mesh having a wire diameter of 23 (μm). The printing conditions were set so that the width of the grid line would be 80 (μm). The sheet resistance of the substrate was evaluated using 90 ± 10 (Ω / □).

No.1~23 is a TeO ₂ / glass weight ratio from .12 to .50, Pb / Te to (mol ratio) as .8-4.8, Pb, Si, and each element of Al, Pb / (Si + Al ) ratio Are considered. Among these, Nos. 2 to 4, 6 to 10, 12, 14, 15, 18 to 22 are examples, and Nos. 1, 5, 11, 13, 16, 17, and 23 are comparative examples. In the examples, when Pb is 28 to 60 (mol%), Si is 10 to 40 (mol%), and Pb / (Si + Al) is all in the range of 1.0 to 3.2, the FF value is 75 to 78. Results were obtained. In particular, in the range of Pb / (Si + Al) of 1.1 to 2.7, a more preferable result with an FF value of 76 or more was obtained. On the other hand, in the comparative example, Pb / (Si + Al) is as small as 0.9, or as large as 3.3 or 3.4, or Pb is as small as 26 (mol%) or as large as 62 (mol%). Alternatively, since the Si content is as low as 8 (mol%) or as high as 42 (mol%), the FF value stays at 74 in all cases. From these evaluation results, excellent output characteristics can be obtained if Pb is in the range of 28 to 60 (mol%), Si is in the range of 10 to 40 (mol%), and Pb / (Si + Al) is in the range of 1.0 to 3.2. It is thought that

  In the above examples, Nos. 2 and 3 confirmed the lower limit value of Pb / (Si + Al), No. 6 confirmed the lower limit value of Pb amount, and No. 10 represents the Pb amount. The upper limit was confirmed, No.14 was the lower limit of Si and the upper limit of Pb / (Si + Al), No.22 was the upper limit of Si and the lower limit of Pb / (Si + Al) It was confirmed.

In Nos. 24-31, the amount of B was examined by setting the TeO ₂ / glass weight ratio to 0.20 to 0.29 and Pb / Te (mol ratio) to 1.7 to 2.7. In the examples, when the amount of B is in the range of 3 to 18 (mol%), a high FF value of 75 to 78 was obtained. In particular, when the B content was in the range of 3 to 16 (mol%), a more preferable result was obtained in which the FF value was 76 or more, and in the range of 6 to 12 (mol%), the FF value was 77 or more. On the other hand, in the comparative example, since No. 24 has a small B amount of 1 (mol%), the FF value stays at 73, and No. 31 has a high 20 (mol%), so the FF value is Stayed at 74. In the examples, No. 25 is a confirmation of the lower limit of the B amount, and No. 30 is a confirmation of the upper limit of the B amount.

In Nos. 32-43, the amount of Li was examined by setting the TeO ₂ / glass weight ratio to 0.20 to 0.40 and Pb / Te (mol ratio) to 1.2 to 3.0. In the examples, when the amount of Li is in the range of 3 to 30 (mol%), a high FF value of 75 to 78 was obtained. In particular, when the Li content was in the range of 10 to 28 (mol%), a more preferable result was obtained in which the FF value was 76 or more, and in the range of 10 to 15 (mol%), the FF value was 78. On the other hand, in the comparative example, No. 32 lacked Li and No. 33 had a low Li of 1 (mol%). Further, No. 42 had a high Li content of 32 (mol%) and No. 43 had a high Li content of 35 (mol%), so the FF value remained at 73 to 74.

In Nos. 44 to 46, the amount of Al was examined by setting the TeO ₂ / glass weight ratio to 0.37 to 0.42 and Pb / Te (mol ratio) to 1.0 to 1.3. Al is an arbitrary component as seen in No. 30 and No. 69 to 73 described later, and as shown in No. 44 and 45, by setting it to 15 to 18 (mol%), FF Satisfactory results with a value of 75 were obtained. On the other hand, in No. 46 of the comparative example, since the Al content is as large as 20 (mol%), the FF value slightly decreased and remained at 74. Al, which is an optional component, may have a glass composition containing it, but it is preferable to keep it at 18 (mol%) or less.

Nos. 47 to 49 are those in which the amount of P was examined with a TeO ₂ / glass weight ratio of 0.39 and Pb / Te (mol ratio) of 1.3. P is an optional component as seen in No. 2, 19, etc., and as shown in No. 47, 48, by setting the content to 6 (mol%), a sufficient FF value of 75 is obtained. Results were obtained. On the other hand, in the comparative example No. 49, P was as high as 8 (mol%), so the FF value remained at 74.

Nos. 50 and 51 are those in which the amount of S was examined by setting the TeO ₂ / glass weight ratio to 0.39 to 0.42 and Pb / Te (mol ratio) to 1.2 to 1.3. S is an optional component as seen in No. 2, 8, etc., and by setting the content to 5 (mol%) as shown in No. 50 of the example, a sufficient FF value of 75 is obtained. Results were obtained. On the other hand, as shown in Comparative Example No. 51, when the S content increased to 7 (mol%), the FF value remained at 74.

Nos. 52 to 75 were examined for Pb / Te (mol ratio). Of these, Nos. 60 to 67 were glass systems lacking Na, and Nos. 68 to 75 were made of Pb-B-Si-Li. Each glass was evaluated. In Examples Nos. 53 to 57, TeO ₂ was added in the range of TeO ₂ / glass weight ratio in the range of 0.10 to 1.33, and in the range of Pb / Te in the range of 0.4 to 5.4, the high FF value of 75 to 78 was obtained was gotten. In particular, good results were obtained with an FF value of 76 or more when Pb / Te was 0.9 to 2.6, and an FF value of 78 when Pb / Te was 1.8. On the other hand, in No. 59 that does not contain Te and No. 52 in which Pb / Te is as small as 0.2, the FF value stays at 73, and in No. 58 where Pb / Te is as large as 5.8, the FF value stays at 74. As a result.

Further, in Examples Nos. 60 to 67, TeO ₂ was added in a range of TeO ₂ / glass weight ratio of 0.10 to 1.00, and in the range of Pb / Te of 0.5 to 5.5, the FF value was high as 75 to 78. FF values were obtained. In particular, good results were obtained with an FF value of 76 or more when Pb / Te was 0.9 to 2.7 and an FF value of 78 when Pb / Te was 1.8. On the other hand, in No. 67 that does not include Te, the FF value stays at 73, and in No. 60 where Pb / Te is as small as 0.3 and No. 66 where Pb / Te is as large as 5.7, the FF value remains at 74. As a result.

Further, in Examples Nos. 69 to 73, TeO ₂ was added in a range of 0.11 to 1.40 in the TeO ₂ / glass weight ratio. When Pb / Te was in the range of 0.4 to 5.5, the FF value was high as 75 to 78. FF values were obtained. In particular, good results were obtained with an FF value of 76 or more when Pb / Te was 1.0 to 3.0, and an FF value of 78 when Pb / Te was 2.0. On the other hand, in the case of No. 75 containing no Te, No. 68 having a small Pb / Te of 0.2 and No. 74 having a large Pb / Te of 5.7, the FF value remained at 74.

Moreover, according to the paste composition of a present Example from all the Examples shown in the said Table 1 and Table 2, all are Pb 28-59 (mol%) and B 3-18 (mol) in conversion of an oxide. %), Si 10-40 (mol%), Al 0-18 (mol%), Li 3-30 (mol%), Ti 0-5 (mol%), Zn 0-22 (mol) %), P is 0~6 (mol%), Na is 0~1 (mol%), S is 0~5 (mol%), Pb / (Si + Al) is 1.0 to 3.2, TeO ₂ / glass In the range of 0.10 to 1.40 and Pb / Te in the range of 0.4 to 5.5, good results with an FF value of 75 or more are obtained. Also, Pb is 30 to 58 (mol%), B is 3 to 16 (mol%), Si is 12 to 39 (mol%), Al is 0 to 10 (mol%), Li is 4 to 28 (mol%) ), Ti is 0 to 5 (mol%), Zn is 0 to 17 (mol%), P is 0 to 2 (mol%), Na is 0 to 1 (mol%), S is 0 to 1 (mol%) ), Pb / (Si + Al) in the range of 1.1 to 2.7, TeO ₂ / glass in the range of 0.20 to 0.60, and Pb / Te in the range of 0.9 to 3.3, a better result with an FF value of 76 or more is obtained. Also, Pb is 30 to 56 (mol%), B is 4 to 12 (mol%), Si is 15 to 32 (mol%), Al is 0 to 10 (mol%), Li is 5 to 15 (mol%) ), Ti is 0 to 5 (mol%), Zn is 0 to 17 (mol%), P is 0 to 2 (mol%), Na is 0 to 0.5 (mol%), S is 0 to 0.5 (mol%) ), Pb / (Si + Al) is in the range of 1.1 to 2.2, TeO ₂ / glass is in the range of 0.22 to 0.40, and Pb / Te is in the range of 1.3 to 2.5. Also, Pb is 30 to 56 (mol%), B is 4 to 12 (mol%), Si is 15 to 32 (mol%), Al is 0 to 4.5 (mol%), Li is 5 to 15 (mol%) ), Ti is 0 to 5 (mol%), Zn is 0 to 17 (mol%), P is 0 to 2 (mol%), Na is 0 to 0.5 (mol%), S is 0 to 0.5 (mol%) ), Pb (Si + Al) is 1.1 to 2.0, TeO ₂ / glass is 0.22 to 0.40, Pb / Te is 1.3 to 2.3, and particularly good results with an FF value of 78 are 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.

10: solar cell, 12: solar cell module, 14: sealing material, 16: surface glass, 18: protective film, 20: silicon substrate, 22: n layer, 24: p ⁺ layer, 26: antireflection film, 28 : Light receiving surface electrode, 30: Back electrode, 32: Light receiving surface, 34: Full surface electrode, 36: Strip electrode, 38: Solder ribbon

Claims (5)

A conductive paste composition for solar cells, comprising conductive powder, glass frit, Te compound powder , and vehicle,
The glass frit, and Li ₂ O 3 and 30 in oxides terms (mol%), and PbO of 28 to 60 (mol%), and B ₂ O ₃ of 3~18 (mol%), 10~40 Ri formed of glass and a SiO ₂ of (mol%), and, the PbO and the Te compound powder of the glass, Pb / Te (mol ratio), wherein near Rukoto the range of 0.4 to 5.5 A conductive paste composition for solar cells.   The conductive paste composition for a solar cell according to claim 1, wherein the glass has a Pb / (Si + Al) (mol ratio) in the range of 1.0 to 3.2. The conductive paste composition for a solar cell according to claim 1 or ₂ , wherein the glass contains SO ₂ of 5.0 (mol%) or less in terms of oxide. 4. The conductive paste composition for a solar cell according to claim 1, wherein the glass contains 18 (mol%) or less of Al ₂ O ₃ in terms of oxide. 5. The conductive paste composition for a solar cell according to any one of claims 1 to 4, wherein the glass contains 6.0 (mol%) or less of P ₂ O ₅ in terms of oxide.

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