JP5134722B1 - Solar cell and paste material used therefor - Google Patents

Abstract

Disclosed is a paste material for forming an aluminum electrode which has water resistance and can maintain good electrode characteristics.
A solar cell including a semiconductor substrate, a light-receiving surface electrode formed on one surface of the semiconductor substrate, and an aluminum electrode formed on the other surface side of the semiconductor substrate, The solar cell contains phosphorus (P) at a content of 30 ppm to 700 ppm in a form different from the glass component. The formation of the aluminum electrode of the solar cell includes an aluminum powder, a glass binder, an organic vehicle, and a phosphorus supplier containing phosphorus (P) in the chemical composition. A paste material containing phosphorus (P) at a content of 30 ppm to 700 ppm is used in an aluminum electrode obtained by firing the paste material.
[Selection] Figure 2

Description

  The present invention relates to a solar cell (cell) and a paste material used therefor. Specifically, the present invention relates to a paste material containing aluminum for forming an aluminum electrode (as a back electrode) on the back surface side of a light receiving surface of a solar cell (typically a silicon-based solar cell).

As a typical example of a solar cell that converts solar light energy into electric power, a solar cell that uses crystalline silicon (single crystal or polycrystal) as a semiconductor substrate, a so-called crystalline silicon solar cell is known. As such a crystalline silicon solar cell, for example, a single-sided light receiving solar cell 10 as shown in FIG. 1 is known.
This solar cell 10 includes an n-Si layer 16 formed by pn junction formation on the light-receiving surface side of a p-type silicon substrate (Si wafer: p-Si layer made of p-type crystalline silicon) 11. An antireflection film 14 made of titanium oxide or silicon nitride formed by CVD or the like, and a surface electrode (light receiving surface electrode) 12 made of Ag, typically formed by screen printing and baking a silver paste, are provided. . On the other hand, on the back side of the p-type silicon substrate (p-Si layer) 11, a back side external connection electrode 22 made of Ag formed by screen printing and baking a silver paste in the same manner as the front side electrode 12, And an aluminum electrode 20 having a so-called back surface field (BSF) effect.

  The aluminum electrode 20 is formed on substantially the entire back surface by printing and baking a paste material mainly composed of aluminum powder. During this firing, an Al—Si alloy layer (not shown) is formed, and aluminum diffuses into the p-type silicon substrate (p-Si layer) 11 to form a p + layer 24. By forming the p + layer 24, that is, the BSF layer, the photogenerated carriers are prevented from recombining in the vicinity of the back electrode, and, for example, an open circuit voltage (Voc) and a short circuit current (Isc) are improved. The

JP 2011-066353 A JP 2007-115738 A JP 2011-103301 A

  By the way, since such a solar cell is assumed to be used outdoors for a long time, it is required to have excellent long-term reliability such as weather resistance and durability. For example, in a solar cell module in which a plurality of solar cells (cells) are connected, there is a concern that the opening can be a moisture intrusion route when the power extraction part to the outside of the solar cell module opens due to some influence. Is done. In the unlikely event that such a situation occurs, the surface electrode 12 made of Ag has water resistance, but the aluminum electrode 20 generates hydrogen by reacting with moisture, so that the solar cell (cell ) May expand. This is a problem to be improved because it leads to the deterioration of the durability of the solar cell module. Therefore, for solar cells (cells), it is desired to improve the water resistance of the aluminum electrode, more preferably the hot water resistance.

In order to satisfy such a requirement for the aluminum electrode 20, for example, Patent Document 1 discloses a paste material suitable for a back electrode of a silicon-based solar cell, and includes Al powder, glass frit, vehicle, and Al powder. A paste material containing 0.3 to 5.0 parts by mass of Sn powder with respect to 100 parts by mass has been proposed. Thus, it is disclosed that the hot water resistance can be sufficiently enhanced without impairing various characteristics of the back electrode made of the aluminum electrode 20.
In addition to the cited reference 1, it is desired that paste materials for aluminum electrodes that realize the aluminum electrode 20 with improved water resistance be provided in various forms.

  This invention is made | formed in view of this point, and it aims at providing the paste material for forming the aluminum electrode which is excellent in water resistance and can maintain a favorable electrode characteristic. Another object of the present invention is to provide a solar cell including a back electrode formed using such paste material.

  In order to achieve the above object, a solar cell (typically a cell) provided by the present invention includes a semiconductor substrate (typically a silicon substrate) and a light receiving surface formed on one surface of the semiconductor substrate. An electrode and an aluminum electrode formed on the other surface side of the semiconductor substrate are provided. The aluminum electrode contains phosphorus (P) at a content of 30 ppm to 700 ppm (that is, 30 μg to 700 μg / Al electrode 1 g, the same applies hereinafter) in a form different from the glass component. Here, phosphorus (P) can exist in the form of a simple substance or a compound (may be in the form of an ion).

  In such a solar cell, phosphorus (P) is contained as a substance that improves the water resistance of the aluminum electrode. Therefore, in the solar cell using such an aluminum electrode as the back electrode, even if moisture enters the cell, the reaction between the moisture and aluminum is suppressed. And when phosphorus (P) is contained by the content rate of 30 ppm-700 ppm on a mass basis in an aluminum electrode, water resistance can be provided, maintaining an electrode characteristic in a favorable range. Although the cause of this improvement in water resistance is not clear, the water resistance (for example, hot water resistance) of the aluminum electrode is immersed in, for example, hot water at 80 ° C. by including such a small amount of phosphorus (P). Even under such severe conditions, resistance of 5 minutes or more can be realized. Thereby, corrosion of the aluminum electrode and cell expansion can be suppressed, and the water resistance and durability of the solar cell can be maintained well.

  In addition, the said patent document 2 and 3 are mentioned as a prior art of the solar cell which contains some phosphorus (P) in an aluminum electrode. For example, Patent Document 2 relates to a paste composition for forming an electrode on a silicon semiconductor substrate. In an aluminum paste composition containing aluminum powder, an organic vehicle, and a plasticizer, phosphoric acid is used as a plasticizer. The use of ester plasticizers is disclosed. However, according to the study by the present inventors, in this type of aluminum paste composition, the phosphoric ester plasticizer is easily scattered from the aluminum electrode by firing, and the above concentration in the aluminum electrode is as described above. It is difficult to leave this phosphorus (P). In addition, in order to achieve the above water resistance, it is necessary to contain a large amount of plasticizer in the aluminum paste composition, and adding a large amount of such a plasticizer increases the warpage of the silicon semiconductor substrate, The peel strength of the obtained aluminum electrode is remarkably lowered, and the aluminum electrode cannot be practically used as an electrode.

  Further, Patent Document 3 discloses a conductive paste containing phosphorus (P) as a glass component (metal glass). However, metallic glass generally contains three or more kinds of elements, and these atoms are strongly attracted. Further, from the study by the present inventors, even when phosphorus (P) is contained in the aluminum electrode, when this is a glass component, the phosphorus (P) improves the water resistance of the aluminum electrode. It has been confirmed that the function to improve cannot be demonstrated. In this respect, the solar cell disclosed herein is required to contain phosphorus (P) at the above concentration in a form different from the glass component in the aluminum electrode.

  In a preferred embodiment of the solar cell disclosed herein, the aluminum electrode contains a glass component together with an aluminum component, and the content of the glass component is 2.6% by mass of the aluminum component. % Or less. The glass component is usually a binder for sintering in a paste material used when forming an aluminum electrode, and serves to bind aluminum particles together and firmly fix the aluminum electrode film to a semiconductor substrate. It also has a function of imparting water resistance to the aluminum electrode. However, in the solar cell disclosed here, the water resistance is improved by the inclusion of phosphorus, so the content of the glass component in the aluminum electrode can be reduced. Moreover, the curvature amount of the semiconductor substrate resulting from a glass component can be reduced by reducing content of a glass component. In addition, the conductivity of the aluminum electrode itself can be improved, and the degree of freedom in the composition of the aluminum electrode is further expanded. Particularly preferably, a glass-free solar cell can be provided.

  In a preferred embodiment of the solar cell disclosed herein, the aluminum electrode contains the phosphorus (P) at a content of 80 ppm to 400 ppm. According to such a configuration, in addition to improving the water resistance of the aluminum electrode, these electrode characteristics can be maintained in a good range with a better balance, and a higher quality solar cell can be realized. In particular, the above hot water resistance can have a resistance of 10 minutes or more even under severe conditions such as immersion in hot water at 80 ° C.

Moreover, this invention provides the paste material as another side surface which implement | achieves the said objective. That is, the paste material disclosed here is a paste material for forming an aluminum electrode of a solar cell, and includes an aluminum powder, a resin, a solvent, and a phosphorus supplier containing phosphorus in chemical composition. It is out. And content of this phosphorus supply agent is prescribed | regulated so that phosphorus (P) may be contained in the aluminum electrode obtained by baking the said paste material by the form different from a glass component with the content rate of 30 ppm-700 ppm. It is characterized by that. In the paste material disclosed herein, the resin is preferably a cellulosic polymer.
In the present specification, the “paste material” refers to a composition (mixture) prepared in a paste form (or a slurry form or an ink form).

  In the paste material disclosed herein, the phosphorus supply agent is contained for the purpose of improving the water resistance of an aluminum electrode obtained by firing the paste material. When phosphorus (P) is added to the paste material in the form of an additive, it is usually burned off together with the resin, solvent, and the like by firing. Therefore, in the present invention, it is required that phosphorus (P) be contained (residual) in a ratio of 30 ppm to 700 ppm in the fired aluminum electrode. With such a configuration, a paste material capable of realizing an aluminum electrode excellent in water resistance is provided.

  In a preferred embodiment of the paste material disclosed herein, it further contains a glass binder, and the glass binder is contained in a proportion of 2.6 parts by mass or less when the aluminum powder is 100 parts by mass. . This is understood as an amount corresponding to approximately 2.0% by mass or less when the entire aluminum electrode after firing is 100% by mass. This glass binder is a binder for sintering in a paste material used when forming an aluminum electrode, and serves to bind aluminum particles together and firmly fix the aluminum electrode film to a semiconductor substrate. Have It also has a function of imparting water resistance to the aluminum electrode. In such a paste material, the water resistance is improved by the inclusion of phosphorus, so that the content of the glass binder can be reduced. Further, by reducing the content of the glass binder, as a result, a paste material that can reduce the amount of warpage of the semiconductor substrate caused by the glass binder can be obtained. In addition, the conductivity of the resulting aluminum electrode itself can be improved, and the degree of freedom of the composition of the paste material can be expanded.

In a preferred embodiment of the paste material disclosed herein, the phosphorus supply agent is a phosphorus material contained in the paste material used to form a predetermined amount of aluminum electrode. It contains a phosphorus-containing compound that realizes that the residual ratio of phosphorus remaining on the aluminum electrode is 20% or more.
Even when phosphorus is contained in the paste material, the residual rate of phosphorus remaining in the aluminum electrode after firing is relatively low. That is, even when a phosphorus supply agent containing phosphorus is used, the amount of phosphorus remaining on the aluminum electrode obtained by firing can be extremely small. For example, if a phosphorus-containing compound having a phosphorus residual ratio of less than 20% is used as a phosphorus supply agent and phosphorus (P) is allowed to remain at a ratio of 30 ppm to 700 ppm on an aluminum electrode obtained by firing, the phosphorus-containing compound The amount of added becomes extremely large, which may lead to, for example, deterioration of other electrode characteristics of the aluminum electrode. However, according to such a configuration, since the phosphorus supply agent capable of remaining 20% or more of phosphorus is selected and used, both the water resistance and the electrode characteristics are obtained without deteriorating the electrode characteristics of the aluminum electrode. A paste material capable of forming an excellent aluminum electrode is provided.

  In a preferred embodiment of the paste material disclosed herein, the phosphorus supply agent includes alkyl ether phosphoric acid and / or phosphorus resinate. According to this configuration, for example, when alkyl ether phosphoric acid is used as the phosphorus supply agent, phosphorus (P) contained in the paste material remains in the aluminum electrode at a rate of about 50% or more on a mass basis. obtain. For example, when phosphorus resinate is used as the phosphorus supply agent, phosphorus (P) contained in the paste material can remain in the aluminum electrode at a rate of about 20% or more on a mass basis. Therefore, a paste material capable of improving the water resistance can be realized with a small addition amount without impairing other characteristics of the aluminum electrode more than necessary.

  In a preferred embodiment of the paste material disclosed herein, the phosphorus supply agent is included in a proportion of 0.13 parts by mass to 2.6 parts by mass when the aluminum powder is 100 parts by mass. According to this configuration, the water resistance of the aluminum electrode obtained by firing is improved by the inclusion of phosphorus. Therefore, in addition to the role as a binder, it is possible to reduce the content of the glass binder that functions to increase water resistance. As a result, a paste material that can reduce the amount of warpage of the semiconductor substrate due to the glass binder and improve the conductivity of the aluminum electrode itself is realized. In addition, the degree of freedom in designing the composition of the paste material can be expanded.

  In a preferred embodiment of the paste material disclosed herein, the glass binder is 0.13 parts by mass to 2.6 parts by mass, and the phosphorus supply agent is 0.13 parts by mass with respect to 100 parts by mass of the aluminum powder. Part to 2.6 parts by mass. By making the balance of the compounding amount of the glass binder and the phosphorus supplier compounded in the paste material the above ratio with respect to the aluminum powder, the improvement of the water resistance of the aluminum electrode obtained by firing and the semiconductor substrate The effect of reducing the amount of warpage can be suitably exerted in a balanced manner, and a higher quality solar cell can be provided.

It is sectional drawing which shows an example of the structure of a solar cell typically. It is a figure which shows the relationship between phosphorus (P) content in the aluminum electrode which concerns on one Embodiment, and the time which shows hot water resistance. It is a figure which shows the relationship between the phosphorus supply amount in the paste material which concerns on one Embodiment, and the curvature amount of the solar cell obtained by this. It is a figure which shows the relationship between the phosphorus supply agent amount in the paste material which concerns on one Embodiment, and the open circuit voltage Voc of the solar cell obtained by this. It is a figure which shows the relationship between the phosphorus supply agent amount in the paste material which concerns on one Embodiment, and the conversion efficiency Eff of the solar cell obtained by this. It is a figure which shows the relationship between the amount of phosphorus supply agents in the paste material which concerns on one Embodiment, and BSF resistance of the solar cell obtained by this. It is a figure which shows the relationship between the phosphorus supply amount in the paste material which concerns on one Embodiment, and the water resistant time of the solar cell obtained by this.

Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than those specifically mentioned in the present specification and matters necessary for carrying out the present invention (for example, a method of mixing a raw material powder (solid content) of a paste material, a resin and a solvent, The method of applying to the substrate, etc.) can be grasped as a design matter of those skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.
In addition, the structure itself of the solar cell according to an embodiment manufactured using the paste material disclosed herein can be considered in the same manner as the conventional solar cell 10 as shown in FIG. It does not include a configuration that makes it particularly different. That is, the solar cell according to this embodiment typically has a pn junction on the light receiving surface side of a p-type silicon substrate (Si wafer: p-Si layer made of p-type crystalline silicon) 11 as illustrated in FIG. The n-Si layer 16 formed by the formation is provided, and an antireflection film 14 and a surface electrode (light receiving surface electrode) 12 made of Ag are provided on the surface thereof. On the other hand, on the back side of the p-type silicon substrate (p-Si layer) 11, a back side external connection electrode 22 made of Ag and an aluminum electrode 20 having a so-called BSF effect can be provided.

  The paste material disclosed here is a back electrode forming paste material used for forming an aluminum electrode as a back electrode in a solar cell. Such a paste material is a mixture mainly composed of aluminum powder. And typically, a glass binder is included. A liquid solvent so that these aluminum powder and glass binder can be put together in a paste form (or can be in the form of a slurry or an ink, the same shall apply hereinafter) and evenly spread on the back surface of the semiconductor substrate. (Typically, a vehicle composed of a resin and a solvent.) And the paste material disclosed here contains the phosphorus supply agent which consists of a compound which contains phosphorus further in chemical composition in addition to the said material. This phosphorus supply agent is defined so that phosphorus (P) is contained in an aluminum electrode obtained by firing the paste material at a content of 30 ppm to 700 ppm (that is, a ratio of 30 μg to 700 μg per 1 g of Al electrode). It is characterized by having.

  The aluminum powder contained as the main solid content in the paste material disclosed here is an aggregate of particles mainly composed of aluminum (Al), and typically an aggregate of particles made of Al alone. However, even if such an aluminum powder contains a trace amount of impurities other than Al and Al-based alloys (particles), if it is an aggregate of Al-based particles as a whole, it will be referred to as “aluminum powder” here. Can be included. The aluminum powder may be produced by a conventionally known production method and does not require special production means.

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

  Here, in the case where an aluminum electrode as a back electrode is formed on one surface (typically, the back surface side of the light receiving surface) of a semiconductor substrate (for example, Si substrate) constituting the solar cell, a dried coating film before firing. The film thickness is preferably 100 μm or less, more preferably 80 μm or less, still more preferably 60 μm or less, for example, 30 μm ± 10 μm. By forming the aluminum electrode with such a film thickness, the BSF effect in the back electrode can be effectively expressed.

As an aluminum powder suitable for forming a dry coating film having such a film thickness, it is appropriate that the average particle diameter of particles (primary particles) constituting the powder is 20 μm or less, preferably 1 μm or more. It is 10 μm, more preferably 2 μm or more and 8 μm or less, for example, 5 μm ± 1 μm. The average particle diameter here refers to the particle diameter when the cumulative volume is 50% in the particle size distribution of the powder, that is, D50 (median diameter). Such D50 can be easily measured by a particle size distribution measuring apparatus based on a laser diffraction scattering method (that is, a particle size distribution is determined by a scattering pattern when a laser beam is irradiated and scattered on a measurement sample).
For example, a plurality of aluminum powders (typically two types) having different average particle diameter differences (for example, the difference is in the range of 3 μm to 7 μm) are mixed, and the average particle diameter of the mixed powder is You may use the aluminum (mixed) powder which exists in the said range. By using the aluminum powder having an average particle diameter as described above, a dense aluminum electrode suitable as a back electrode can be formed.

  Although there is no restriction | limiting in particular as content of the said aluminum powder in the paste material disclosed here, The said paste material is 100 mass% as a whole, 60 mass% or more and 90 mass% or less, More preferably, 65 mass It is preferable to adjust the content rate so that the aluminum powder is not less than 85% and not more than 85% by mass, for example, not less than 70% and not more than 80% by mass. When the content of the aluminum powder in the manufactured paste material is within the above range, an aluminum electrode (film) with improved denseness and electrical conductivity can be formed.

  The paste material disclosed here can contain a glass binder (typically glass frit) as its constituent material. Such a glass binder is not an essential component of the paste material disclosed herein, but can be mainly included as a binder for sintering the aluminum powder, and enhances the binding between the aluminum powders by firing, It has the role of improving the adhesive strength between the aluminum electrode as the back electrode of the battery and the substrate. And as above-mentioned, it also plays the role which reduces the reactivity with the water | moisture content of the aluminum electrode which consists of fine powder-form aluminum and is comparatively high.

Such a glass binder preferably has a relatively high thermal expansion coefficient so as to approach the thermal expansion coefficients of aluminum and the semiconductor substrate. By using a glass binder having a high thermal expansion coefficient, for example, it is possible to obtain an effect of suppressing deformation of the semiconductor substrate (wafer) itself, such as warping or bending, during firing for forming an aluminum electrode. Examples of such glass include borosilicate glass, zinc glass, alkali glass, glass containing barium oxide, bismuth oxide, and the like, or a combination of two or more thereof. As specific examples, glass mainly composed of the following oxides, that is, B ₂ O ₃ —SiO ₂ —ZnO-based glass, R ₂ O—ZnO—SiO ₂ —B ₂ O ₃ -based glass (here, R ₂ O is an alkali metal oxide), RO—ZnO—SiO ₂ —B ₂ O ₃ glass (where RO is an alkaline earth metal oxide), Bi ₂ O ₃ —B ₂ O ₃ —ZnO glass, B ₂ A glass powder made of O ₃ —SiO ₂ —Bi ₂ O ₃ glass or the like is preferable.

Among these, the glass binder is preferably a zinc-based glass based on a B ₂ O ₃ —SiO ₂ —ZnO system. As this type of glass binder, lead borosilicate glass having a low softening temperature has been conventionally used. However, from the viewpoint of lead regulation and environmental considerations, the above zinc-based glass is preferably used. it can. This zinc-based glass (zinc borosilicate glass) has a thermal expansion coefficient close to that of silicon compared to other types of glass, and the amount of warpage is relatively small. Moreover, it is preferable also in that it has a chemically stable property and durability such as high thermal shock temperature and resistance to water.

  In addition, glass (for example, phosphoric acid glass, metallic glass containing phosphorus, etc.) containing phosphorus (P) as a glass constituent component (for example, network former or the like) exists in glass. However, according to the study by the present inventors, phosphorus (P), which is a glass component, is contained in the glass as a glass skeleton, so that phosphorus (P) derived from the phosphorus supplier disclosed here is used as the glass skeleton. It has been confirmed that the function (function to improve water resistance) cannot be achieved. That is, phosphorus (P) contained in the glass as a glass component such as a network former does not have a function of increasing the water resistance of the aluminum electrode. In this respect, since the paste material disclosed here contains phosphorus (P) in a form different from that of the glass component, it is clearly distinguished from the paste material containing phosphorus (P) in the glass binder. Can be done. In addition, even when phosphorus (P) is contained in the metallic glass, the atoms constituting the metallic glass attract each other strongly, and thus the function as a phosphorus supplying agent (function to improve water resistance) cannot be achieved. .

  Therefore, from such a viewpoint, as such a glass binder, glass containing phosphorus (P) as a glass component may be used, or glass containing no phosphorus (P) in its chemical composition (phosphorus-free glass). ) May be used. In the invention disclosed herein, these glass components function in the same manner regardless of whether phosphorus (P) is contained or not. No difference is recognized. From this, even if phosphorus (P) is contained in the glass binder, this phosphorus (P) does not have a function of improving water resistance, so that phosphorus (P ) Is clearly distinguished. Hereinafter, what is described as phosphorus (P) in the present specification indicates phosphorus (P) derived from a phosphorus supply unless otherwise specified.

Such a glass binder has a specific surface area of about 0.degree. In order to stably fire and fix (stick) the paste material (coating film) supplied on the substrate (for example, silicon substrate). It is preferably about 5 m ² / g or more and 50 m ² / g or less, and those having an average particle diameter of 3 μm or less are suitable.

  Moreover, although the mixing | blending containing this glass binder is not specifically limited, When said aluminum powder is 100 mass parts, it can contain in the ratio of about 2.6 mass parts-6.4 mass parts, for example. . When the entire paste material is 100% by mass, for example, it can be about 2% by mass to 5% by mass, preferably 3% by mass to 4% by mass.

And in the paste material disclosed here, the water resistance of the aluminum electrode obtained by baking is improved by containing a phosphorus supply agent. Then, it becomes possible to reduce the content of the glass binder that has also played the role of improving the water resistance of the aluminum electrode. Therefore, in the paste material disclosed herein, the content of the glass binder is preferably 2.6 parts by mass or less, more preferably 2.3 parts by mass or less, preferably 100 parts by mass of aluminum powder. It is desirable that the amount be 2.0 parts by mass or less. Preferably, a glass-free blend that does not contain a glass binder can also be used.
In addition, when the content of the glass binder is reduced in this way, the amount of warpage of the semiconductor substrate due to this can be reduced. Moreover, since a glass binder shows insulation, the electrical conductivity of aluminum electrode itself can be improved by reducing this. In addition, the degree of freedom in designing the composition of the paste material can be expanded.

  The paste material disclosed here includes the aluminum powder as described above and, if necessary, a glass binder, and also includes a liquid medium for dispersing them. In such a paste material, the liquid medium in which the solid material (aluminum powder and glass binder) in the paste material is dispersed is typically a vehicle (can include a vehicle whose concentration is adjusted with a solvent). .

  Here, the vehicle is composed of a resin and a solvent (typically an organic solvent) that dissolves or disperses the resin. Such a solvent is not particularly limited as long as it can disperse the above-mentioned solid material, particularly aluminum powder, well, and those used in this type of conventional paste can be used without any particular limitation. For example, examples of the organic solvent constituting the vehicle include high-boiling organic solvents such as ethylene glycol and diethylene glycol derivatives (glycol ether solvents), toluene, xylene, butyl carbitol (BC), and terpineol, One kind or a combination of plural kinds can be used.

  Various resins can be included as an organic binder constituting the vehicle. Any resin component may be used as long as it can give the paste material good viscosity and coating film forming ability (adhesiveness to the substrate), and those used in conventional pastes of this type should be used without any particular limitation. Can do. Examples thereof include those mainly composed of acrylic resin, epoxy resin, phenol resin, alkyd resin, cellulosic polymer, polyvinyl alcohol, rosin resin and the like. Among these, cellulosic polymers such as ethyl cellulose are particularly preferable.

The ratio of the resin and the solvent as the liquid medium can be arbitrarily set in consideration of the viscosity and applicability of the target paste material. As a rough guide, when the total amount of the solid material (aluminum powder and glass binder) in the paste material is 100% by mass, the resin is 1% by mass to 5% by mass (more preferably 2% by mass to 4%). For example, setting to be included at a ratio of (mass% or less) is exemplified. More specifically, for example, the ratio of the resin to the total of the resin and the solvent can be appropriately adjusted within a range of about 3% by mass to 15% by mass.
Further, although not particularly limited, the ratio of the liquid medium in the paste material disclosed herein is, for example, that the liquid medium (total of resin and solvent) is 1. It is appropriate that the amount is 0 to 40% by mass, preferably 1.5 to 35% by mass, more preferably 18 to 32% by mass. When the aluminum powder is 100 parts by mass, approximately 13 parts by mass to 51 parts by mass is appropriate, preferably 19 parts by mass to 45 parts by mass, and more preferably 23 parts by mass to 41 parts by mass. Can be about.

Next, the phosphorus supply agent (phosphorus-containing compound) that characterizes the paste material disclosed herein will be described.
This phosphorus supply agent may be included in the paste material in the form of a solid material or in the form of a liquid material. Such a phosphorus supply agent contains phosphorus (P) in the paste material so that phosphorus (P) is contained at a content of 30 ppm to 700 ppm in a form different from the glass component in the aluminum electrode obtained by firing the paste material. P) is supplied.
Phosphorus (P) remains in the aluminum electrode after firing, thereby improving the water resistance of the aluminum electrode. Although the cause is not clear, it is considered that the activity of aluminum is lowered by some action to suppress the reaction with moisture. Note that the form of phosphorus (P) supplied in the paste may be in the form of a simple substance or a compound (which may be in the form of ions) as long as it is different from the glass component.

  Here, the content rate and the effect of phosphorus (P) contained in the aluminum electrode will be described. That is, when phosphorus (P) is contained in the aluminum electrode, the water resistance of the aluminum electrode is improved, but when the content is less than 30 ppm, the effect of improving the water resistance can be sufficiently obtained. difficult. When the content of phosphorus (P) is 30 ppm or more, as an index, for example, in the evaluation of hot water resistance soaked in hot water at 80 ° C., water resistance capable of realizing resistance of 5 minutes or more is provided. Can do. Further, as the content of phosphorus (P) in the aluminum electrode increases, the water resistance is improved, but the electrode characteristics tend to decrease the open circuit voltage Voc and increase the resistance value of the BSF layer. The amount of warpage of the substrate also increases. Therefore, the upper limit of the phosphorus (P) content is set to 700 ppm in consideration of these actions comprehensively. Exceeding 700 ppm is not preferable because the amount of warpage of the semiconductor substrate is increased, among others. Thus, the content rate of phosphorus (P) contained in the aluminum electrode after firing is preferably 30 ppm to 700 ppm, more preferably 60 ppm to 560 ppm, and still more preferably 80 ppm to 400 ppm. For example, by setting the phosphorus (P) content to 80 ppm or more, it is possible to achieve a resistance of 10 minutes or more in the evaluation of the hot water resistance. In addition, by setting the phosphorus (P) content to 400 ppm or less, for example, electrode characteristics such as conversion efficiency (Eff), open-circuit voltage (Voc), and resistance of the BSF layer of the aluminum electrode, and the warpage amount of the semiconductor substrate Can be in a good range with a good balance.

  In order to achieve the phosphorus (P) content in such an aluminum electrode, the amount of phosphorus contained in the paste material used to form a predetermined amount of aluminum electrode as the phosphorus supply agent is as described above. It is preferable that the paste material contains a phosphorus supply agent that realizes that the residual rate of phosphorus remaining on the aluminum electrode after a fixed amount of firing is 20% or more.

  That is, for example, even when phosphorus (P) is contained in an additive such as a dispersant or a plasticizer used in this type of paste material, generally, phosphorus ( The residual amount of P) is extremely small. This is because this type of additive is used on the premise that the component is scattered together with the vehicle and the like by firing, and the addition method for leaving the component is not particularly considered. Furthermore, it is because phosphorus (P) contained in these additives is almost easily scattered by firing. As described above, even when an additive containing phosphorus is used, the amount of phosphorus remaining on the aluminum electrode obtained by firing can be extremely small. In the unlikely event that phosphorus (P) is allowed to remain at a ratio of 30 ppm to 700 ppm on the fired aluminum electrode, the amount of the additive added becomes extremely large, for example, reducing other electrode characteristics of the aluminum electrode, etc. Can lead to That is, it can be expected that some components other than phosphorus (P) in the additive remain in the aluminum electrode, and thus adversely affect various characteristics of the aluminum electrode. Therefore, in order to include the above-mentioned concentration of phosphorus in the fired aluminum electrode without unnecessarily degrading the electrode characteristics of the aluminum electrode, the phosphorus residual ratio (mass basis) in the phosphorus supply agent is 20% or more. Preferably there is. Such a residual ratio of phosphorus is more preferably 30% or more, and more specifically 50% or more, for example, 60% or more.

As a phosphorus supply agent that can achieve the phosphorus residual rate, for example, alkyl ether phosphoric acid and / or phosphorus resinate can be exemplified as one specific example.
Examples of the alkyl ether phosphoric acid include polyoxyethylene oleyl ether phosphoric acid, polyoxyethylene alkyl ether phosphoric acid, polyoxyethylene alkyl phenyl ether phosphoric acid TEA, polyoxyethylene ether phosphoric acid Na, polyoxyethylene octyl ether phosphoric acid. And polyoxyethylene lauryl ether phosphate TEA, polyoxypropylene glyceryl ether phosphate and the like.
Here, for example, polyoxyethylene oleyl ether phosphoric acid is a component used as an anionic surfactant or a cationic surfactant, and is represented by the following structural formula as a general formula.

In formula (1), R represents an alkyl group. Typically, R is an alkyl group in which C is 1-50, typically C can be an alkyl group having 5-25, for example, C can be an alkyl group having 10-20. In addition, n represents a natural number, typically n is any one of 1 to 100 or a natural number in a range between two, typically n is 1 to 10, for example, n is It can take values 1, 2, 3, 4 or 5.
Specific examples of such polyoxyethylene oleyl ether phosphoric acid include oleth-3-phosphoric acid and oleth-5-phosphoric acid.
In addition, as the phosphorus resinate, various solutions containing an organic phosphorus compound in which phosphorus (P) is dissolved in an organic solvent (which may be in the form of paste, slurry, or ink) can be used.

  For example, when the above paste material is formed using alkyl ether phosphoric acid as a phosphorus supply agent, phosphorus (P) in a proportion of 20% or more, typically about 50% or more by mass, remains in the aluminum electrode after firing. Can do. For example, as a specific example, when oleth-3-phosphate is used, phosphorus (P) can remain in the aluminum electrode after firing at a residual rate of about 50%. As for oleth-5-phosphate, phosphorus (P) can remain in the aluminum electrode after firing at a residual rate of about 60%. For example, when the paste material is formed using phosphorus resinate as a phosphorus supply agent, phosphorus (P) in a proportion of about 20% or more on a mass basis can remain in the fired aluminum electrode. For reference, for plasticizers and dispersants containing phosphorus that are widely used, the residual rate of phosphorus is less than 20%, typically about 10% or less, typically 5% or less. It can be.

  Here, it is not preferable to use a large amount of a material having a low phosphorus residual rate as a phosphorus supply agent because it leads to deterioration of the quality of the aluminum electrode and solar cell obtained after firing. Therefore, the phosphorus residual ratio in the phosphorus supply agent is preferably 20% or more, and more preferably 30% or more. For example, 40% or more is more desirable.

  According to such a configuration, a paste material capable of forming an aluminum electrode with water resistance more efficiently with a smaller amount of phosphorus supply agent is realized. The content of such a phosphorus supply agent in the paste material cannot be generally described because it depends on the phosphorus residual ratio after firing of the phosphorus supply agent used as described above. For example, for 100 parts by mass of aluminum powder In addition, it is preferable that the phosphorus supply agent is contained at a ratio of 0.13 parts by mass to 2.6 parts by mass. More preferably, they are 0.3 mass part-2.0 mass parts, More specifically, they are 0.5 mass part-1.6 mass parts. In addition, when the paste material is 100% by mass, the ratio of the phosphorus supply agent can be approximately 0.1% by mass to 2.0% by mass. The content of the phosphorus supply agent is 0.13 parts by mass or more with respect to 100 parts by mass of the aluminum powder in order to leave at least 30 ppm of phosphorus (P) in the aluminum electrode after firing. This is because it is considered that the lower limit of the content of the phosphorus supplier is required to be about this level. Further, the content of the phosphorus supply agent is 2.6 parts by mass or less with respect to 100 parts by mass of the aluminum powder because the amount of warpage of the semiconductor substrate (that is, solar cell (cell)) obtained after firing is excessively large. This is for preventing (for example, exceeding about 2 mm). According to the paste material disclosed here, the above-described effects can be obtained by adding a very small amount of phosphorus supply agent as described above. Moreover, even when a component other than phosphorus (P) in the phosphorus supply agent remains on the aluminum electrode, the adverse effect of the component on the solar battery cell can be minimized.

  In consideration of the blending ratio of the glass binder and the phosphorus supplier, the glass binder is 0.13 parts by mass to 2.6 parts by mass and the phosphorus supplier is 0 with respect to 100 parts by mass of the aluminum powder. It is preferable to make it contain in the ratio of .13 mass parts-2.6 mass parts. As a result, the effects of improving the water resistance of the aluminum electrode obtained after firing and reducing the amount of warpage of the semiconductor substrate can be obtained in a balanced and efficient manner, and a higher quality aluminum electrode and solar cell can be realized. it can.

The paste material disclosed here can be easily prepared by mixing the above-mentioned materials in the same manner as the conventional aluminum paste for solar cells. For example, typically, a three-roll mill or other kneader is used to mix and stir a predetermined mixing ratio of aluminum powder, a glass binder, and a phosphorus supply agent together with a vehicle at a predetermined mixing ratio.
In the above mixing step, the phosphorus supply agent may be added in the form of a liquid or in the form of a powder as it is while other materials are being mixed. You may mix and disperse | distribute with the material of a part. Moreover, after mixing or disperse | distributing with liquids, such as aqueous solvents and organic solvents, such as alcohol, to make a dispersion liquid or a slurry-like composition, it can also mix with another material.

The paste material disclosed here can be handled in the same manner as the aluminum paste conventionally used to form an aluminum electrode (and thus a p ⁺ layer, ie, a BSF layer) as a back electrode on a semiconductor substrate. Any conventionally known method can be employed without any particular limitation. Typically, a paste material is applied (applied) to a semiconductor substrate by a screen printing method, a dispenser coating method, a dip coating method, or the like so as to obtain a desired film thickness (for example, 30 μm or less) or a coating film pattern. Such a semiconductor substrate is preferably a silicon (Si) substrate, typically a Si wafer. The thickness of the substrate can be set in consideration of the desired solar cell size, the thickness of the aluminum electrode formed on the substrate, the strength of the substrate (for example, the breaking strength), etc. 100 μm or more and 300 μm or less are appropriate, 150 μm or more and 250 μm or less are preferable, for example, 160 μm or more and 200 μm.

Next, the paste material application is dried at an appropriate temperature (for example, above room temperature, typically about 100 ° C.). The means for drying is not particularly limited. In addition to drying by heating, for example, various means such as suction and air blowing may be employed alone or in combination of two or more. After drying, the dried coating film is baked by heating in an appropriate baking furnace (for example, a high-speed baking furnace) under appropriate heating conditions (for example, 600 ° C to 900 ° C, preferably 700 ° C to 800 ° C) for a predetermined time. I do. Thereby, the paste material applied product is baked onto the semiconductor substrate, and the aluminum electrode 20 as shown in FIG. 1 is formed. In general, the aluminum electrode 20 is fired, and the P ⁺ layer (BSF layer) 24 can also be formed as described above. That is, the aluminum electrode 20 to be the back electrode is formed on the p-type silicon substrate 11 by firing, and the aluminum atoms are diffused into the substrate 11 to form the p ⁺ layer 24 containing aluminum as an impurity. It will be.

  As described above, the paste material disclosed herein includes a predetermined amount of phosphorus (P) in an aluminum electrode formed by firing. As a result, the water resistance of the aluminum electrode is improved, so that the content of the glass binder contained not only as a binder but also for the purpose of enhancing the durability of the aluminum electrode in the paste coating is reduced. Can do. As a result, in a solar cell obtained using this paste material, it is possible to effectively suppress and prevent deformation such as warpage of the semiconductor substrate resulting from the difference in thermal expansion coefficient between the glass component and the semiconductor substrate. Moreover, the aluminum electrode obtained using this paste material can reduce the content of the glass component and maintain good electrical conduction between the aluminum particles.

In addition, the material and process for solar cell manufacture other than forming an aluminum electrode (back surface electrode) using the paste material disclosed here may be completely the same as those in the past. And a solar cell (typically crystalline silicon type solar cell) provided with the back electrode formed with the said paste material can be manufactured, without performing a special process. A typical example of the structure of such a crystalline silicon solar cell is the structure shown in FIG. As a process after forming the aluminum electrode, for example, after forming the n ⁺ layer 16 and the antireflection film 14 as in the conventional case, a conventional silver paste mainly composed of silver is used to print on a desired area on the back surface with a screen.・ Dry and print and dry silver paste in a pattern on the light receiving side. Thereafter, the paste material is printed and dried so as to overlap a part of the silver paste formation region on the back surface, and firing is performed. In this way, the solar battery (cell) 10 is formed.

  Therefore, according to such paste material, a solar cell having good water resistance can be realized. In such a solar cell, since the aluminum electrode as the back electrode contains a predetermined amount of phosphorus, even if moisture enters the cell for some reason, the aluminum electrode does not react with moisture and the cell expands. Is prevented. Therefore, it can be a solar cell having water resistance and durability. In addition, the water resistance of the aluminum electrode is improved, and a solar cell (cell) having favorable solar cell characteristics such as conversion efficiency (Eff), open circuit voltage (Voc), and BSF resistance can be realized. For example, in such a solar cell, it has a resistance of about 10 minutes in hot water (80 ° C.) immersed in hot water at 80 ° C., and the hot water is about 5 times or more than when no phosphorus is contained in an aluminum electrode. Can be improved.

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

[Preparation of paste material]
(1) An aluminum powder having an average particle diameter (D50) of 5 μm was prepared as the aluminum powder.
(2) Compounding ratio of glass frit (B ₂ O ₃ : 25 mol%, SiO ₂ : 12 mol%, ZnO: 63 mol%) made of zinc-based (B ₂ O ₃ —SiO ₂ —ZnO-based) glass as a glass binder Glass frit composed of
(3) The following four commercially available phosphorus-containing compounds were prepared as phosphorus supply agents. That is, (A) oleth-3-phosphate, (B) oleth-5-phosphate, (C) phosphorus resinate, (D) tolyl phosphate, and (E) tributyl phosphate were prepared.

The aluminum powder, the glass binder, and the phosphorus supply agent prepared as described above were kneaded together with a resin (ethyl cellulose) as a binder as a vehicle component and an organic solvent (terpineol) to obtain a paste material.
Here, the mass ratio of the aluminum powder and the glass binder contained in the paste material was adjusted so that aluminum powder: glass powder was 78: 0.2.
The mass ratio of the aluminum powder to the resin was adjusted so that the ratio of aluminum powder: resin was 78: 1.2.

And about the compounding ratio of a phosphorus supply agent, first, without adding a phosphorus supply agent, a solvent is 78: 20.6 by mass ratio of an aluminum powder and a solvent (namely, the sum total of all the components of a paste material is 100). The paste material thus prepared was designated as paste material 1. For example, the blending (mass%) of each material in the paste material 1 is as follows.
(Paste material 1)
Aluminum powder: 78
Glass binder: 0.2
Phosphorus supply agent: 0
Resin: 1.2
Solvent: 20.6

Then, while maintaining the proportions of the aluminum powder, the glass binder, and the resin in the entire paste material, the phosphorus supply agent (A) is 0.1% by mass, 0.1% by mass when the total paste is 100% by mass. The paste materials 2 to 4 were added at a ratio of 2% by mass and 0.4% by mass, and the balance was the solvent. For example, the blending (mass%) of each material in the paste material 4 is as follows.
(Paste material 4)
Aluminum powder: 78
Glass binder: 0.2
Phosphorus supply agent: 0.4
Resin: 1.2
Solvent: 20.2

  In addition, the ratio of phosphorus (P) with respect to aluminum (Al) in the paste material 4 is about 0.0333 mass%. Therefore, the phosphorus supply agents (B) to (B) so that the paste material 4 and the phosphorus content in the paste material are the same (that is, the blending ratio of phosphorus (P) to aluminum (Al) is 0.0333 mass%). The paste materials 5-8 which adjusted the compounding quantity of E) were prepared. That is, in paste materials 5 to 8, the proportions of aluminum powder, glass binder, and resin in the entire paste material are the same as in paste 1, and the amounts of phosphorus supply agents (B) to (E) are set respectively. Paste material 5: 0.50% by mass, paste material 6: 0.14% by mass, paste material 7: 0.31% by mass, paste material 8: 0.22% by mass, and the remainder as a solvent. Table 1 shows the relationship between the phosphorus supply agents of these paste materials 1 to 8 and the blending ratio thereof. In addition, the ratio with respect to 100 mass parts of aluminum powders of a phosphorus supply agent (A) is "Al reference | standard compounding quantity (mass part)", and the quantity of phosphorus (P) contained in paste material is "P quantity (mass%)" Are also shown in Table 1.

[Production of solar cells]
Using the paste materials 1-8 obtained as described above for forming an aluminum electrode, a solar battery (cell) was produced.
Specifically, a commercially available 125 mm (5 inch) square p-type single crystal silicon substrate (plate thickness: 180 μm) for solar cells is prepared, and the surface thereof is mixed acid containing hydrofluoric acid and nitric acid. Acid etching treatment.
Next, a phosphorous-containing solution is applied to the light-receiving surface of the silicon substrate on which a fine uneven structure is formed by the above-described etching process, and heat treatment is performed, so that the thickness of the silicon substrate is about 0.5 μm. A Si layer (n ⁺ layer) was formed (see FIG. 1).
Next, an antireflection film (silicon nitride film) having a thickness of about 80 nm was formed on the n-Si layer by plasma CVD (PECVD). Furthermore, after forming a coating film (thickness of 20 μm or more and 40 μm or less) to be a surface electrode (Ag electrode) by a screen printing method on the antireflection film using a predetermined silver paste for surface electrode (Ag electrode) formation, the same A coating film to be a back electrode (Ag electrode) was formed in a pattern and dried (see FIG. 1).
Each paste material of paste materials 1 to 8 is printed (applied) by screen printing (using stainless steel screen mesh SUS # 200, the same applies hereinafter) so as to overlap a part of the Ag electrode on the back side of the silicon substrate. Then, a coating film having a film thickness of about 40 μm was formed. Next, the silicon substrate was baked to form an aluminum electrode (back surface electrode). Specifically, firing was performed for 1 minute at a set temperature of 900 ° C. using a near-infrared high-speed firing furnace in an air atmosphere. Here, firing was performed using a belt-type firing furnace with the aluminum electrode surface of the silicon substrate facing down. This obtained the solar cell (samples 1-8) as an evaluation cell.

[Measurement of phosphorus (P) content]
Next, the amount of phosphorus (P) contained in the aluminum electrodes of the produced solar cells of Samples 1 to 8 was examined by the following procedure.
First, the weight of the aluminum electrode is calculated from the difference between the weight of only the silicon substrate measured in advance in the manufacture of the evaluation cell and the weight of the cell obtained after firing.
Next, the obtained cell for evaluation was pulverized, dissolved in hydrofluoric acid to obtain a sample solution, and the P concentration in the sample solution was measured using an ICP emission spectroscopic analyzer (manufactured by SII, SPS3000). The measurement was performed at n = 2.
In addition, since the silicon substrate originally contains phosphorus (P) doped when the back surface of the silicon substrate is n-type processed, the amount of P corresponding to this is obtained from the value of Sample 1 that does not contain the phosphorus supply agent. The phosphorus concentration contained only in the aluminum electrode was calculated by subtracting from the measured value. The results are shown in Table 1 as “P amount (ppm)”. Further, the residual ratio of phosphorus was calculated from the ratio of the amount of phosphorus (P) added to the paste material and the amount of phosphorus (P) remaining in the fired aluminum electrode, and the results are shown in Table 1.

[Evaluation of hot water resistance]
Next, the hot water resistance of the solar cells of Samples 1 to 8 prepared in the same manner was examined.
That is, warm water maintained at a temperature of 80 ° C. was prepared, and the cells were immersed in the warm water so that the aluminum electrode of the obtained solar cell was in sufficient contact with the warm water. Thereafter, the time until bubbles were generated from the aluminum electrode was measured. In addition, time until foam | bubble generation | occurrence | production was measured making 10 minutes an upper limit.
These results are shown in Table 1 as water resistance time. Further, FIG. 2 shows the relationship between the content of phosphorus (P) contained in the aluminum film and the time indicating hot water resistance. In FIG. 2, a white marker indicates that the measurement of the water resistance time is rounded up in 10 minutes.

From FIG. 2, it was confirmed that a strong correlation was found between the concentration of phosphorus (P) in the aluminum electrode and the hot water resistance. That is, it was found that the hot water resistance is improved as the concentration of phosphorus (P) contained in the aluminum electrode is higher. Moreover, even if the amount of phosphorus (P) supplied in the paste material is the same from the results of the hot water resistance of samples 5 to 8, it is contained in the aluminum electrode obtained after firing due to the difference in the type of phosphorus supply agent. It was also confirmed that there was a large difference in the amount of phosphorus (P) (that is, the residual ratio).
Further, for example, when it is desired to secure the hot water resistance of the aluminum electrode for 5 minutes or longer, it has been confirmed that it is effective to set the amount of phosphorus (P) contained in the aluminum to 30 ppm or more. Moreover, when it was desired to ensure the hot water resistance of the aluminum electrode for 10 minutes or more, it was found that it is effective to set the amount of phosphorus (P) contained in the aluminum to 80 ppm or more.

[Evaluation of solar cell characteristics]
According to the above paste material preparation method, the proportion of the aluminum powder, the glass binder, and the resin in the entire paste material is the same as in paste 1, and the proportion of the phosphorus supply agent (A) in the entire paste material is Material 9: 0.3% by mass, Paste material 10: 0.6% by mass, Paste material 11: 0.9% by mass, Paste material 12: 1.2% by mass, Paste material 13: 1.5% by mass, Paste The material 14: 2.0 mass% and the paste material 15: 2.5 mass% were used, the amount of solvent was adjusted according to this, and the paste materials 9-15 were prepared.
And using these paste materials 9-15, the solar cell (samples 9-15) as a cell for evaluation was produced according to preparation procedures of said solar cell. In addition, the ratio with respect to 100 mass parts of aluminum powders of a phosphorus supply agent (A) is made into "Al reference | standard compounding quantity (mass part)", and the phosphorus amount contained in the amount of paste materials is made into "P quantity (mass%)" These are also shown in Table 2.

For these samples 9 to 15, the amount of phosphorus (P) contained in the aluminum electrode was measured by the same method as described above. From the result, the amount of phosphorus (P) derived from the phosphorus supply agent and the residual ratio were determined and shown in Table 2.
Moreover, with respect to samples 9 to 15, evaluation of hot water resistance and (1) measurement of the warpage amount of the silicon substrate were performed by the same method as described above. For samples 9 to 13, measurements of (2) open circuit voltage Voc, (3) conversion efficiency Eff, and (4) BSF resistance were further performed.

(1) Evaluation of warpage amount of silicon substrate The warpage amount of the silicon substrate of the obtained solar cells (samples 9 to 15) was examined. That is, the fired silicon substrate was placed on a horizontal test stand so that the surface on which the aluminum electrode was formed faced down, and the dimension between the lowest part and the uppermost part in the thickness direction of the silicon substrate was measured. . The measured value was defined as the amount of warpage (mm) in this test example. Moreover, the target of the amount of curvature here was 2.0 mm or less.

(2)-(3) Evaluation of open circuit voltage Voc and conversion efficiency Eff The open circuit voltage Voc and conversion efficiency Eff of the obtained solar cell (samples 9-13) were investigated. These measurements are based on JIS C
This was carried out according to the method described in 8913.
(4) Evaluation of BSF resistance The BSF resistance of the obtained solar cells (samples 9 to 13) was examined by the following procedure. That is, the evaluation cell was immersed in a 5% hydrochloric acid solution to dissolve the aluminum electrode in the hydrochloric acid solution. After all the aluminum electrodes were dissolved, the evaluation cell was taken out of the hydrochloric acid solution, washed with water and dried, and the resistance value of the BSF layer formed on the surface of the silicon substrate from which the aluminum electrode was removed was measured. Note that the resistance was measured at 9 points in one evaluation cell.

  The relationship between the blending amount of the phosphorus supply and the measurement results of the warpage amount of the silicon substrate, the open circuit voltage Voc, the conversion efficiency Eff, and the BSF resistance is shown in FIGS. In the graph, an average value of a plurality of measured values is shown.

FIG. 3 shows that the amount of warpage of the silicon substrate tends to increase as the amount of phosphorus supply increases. For example, when the goal is to suppress the warpage of a silicon substrate having a thickness of 125 mm × 125 mm and a thickness of 180 μm to less than 2.0 mm (1.6% of the length of one side of the substrate), phosphorus (P ) It was confirmed that it was effective to adjust the amount to 700 ppm or less.
From FIG. 4, the open circuit voltage of the solar cell has no particular effect in the range where the amount of the phosphorus supply agent A is up to 0.9 mass%, but tends to decrease when it exceeds 0.9 mass%. Was confirmed.
From FIG. 5, it was confirmed that the conversion efficiency of the solar cell is not affected by the amount of the phosphorus supply agent in the paste material.

From FIG. 6, it can be seen that the BSF resistance of the solar cell tends to increase as the blending amount increases until the blending amount of the phosphorus supply agent reaches 1.2 mass%.
The amount of phosphorus (P) in the aluminum electrode is more preferably 10 to 50 ppm from the amount of these phosphorus supply agents and the accompanying warpage of the silicon substrate, open circuit voltage Voc, conversion efficiency Eff, and BSF resistance. It was confirmed that it was suitable.

[Evaluation of the amount of glass binder]
According to the above paste material preparation method, the ratio of the aluminum powder and the resin in the entire paste material remains the same as in paste 1, and the ratio of the glass binder and phosphorus supply agent (A) in the entire paste material is changed. The paste materials 16-20 were adjusted. That is, in the paste material 16, the proportions of the glass binder and the phosphorus supply agent (A) in the entire paste material are 3.0% by mass and 0% by mass, respectively, and the paste material 17 is 1.2% by mass, 0.00%. 4 mass%, paste material 18 is 1.5 mass% and 0.4 mass%, paste material 19 is 2.0 mass% and 0.4 mass%, and paste material 20 is 2.5 mass% and 0 mass%. 4% by mass, and the balance was the solvent.
And using these paste materials 16-20, the solar cell (samples 16-20) as a cell for evaluation was produced according to said solar cell preparation procedures.

  These samples 16 to 20 were evaluated for hot water resistance and the amount of warpage of the silicon substrate in the same manner as described above, and the results are shown in Table 3.

  From Table 3, it was confirmed that better hot water resistance (water resistance) can be obtained even if the blending amount of the glass binder is greatly reduced by blending the phosphorus supply agent into the paste material. Furthermore, since the amount of warping of the silicon substrate can be reduced by reducing the amount of the glass binder, the balance of the semiconductor substrate can be improved while improving the hot water resistance by properly balancing the phosphorus supplier and the glass binder. It was found that the amount of warpage could be reduced. For example, in the conventional paste material, the ratio of the glass binder is about 4.5 parts by mass with respect to 100 parts by mass of Al. However, according to the paste material disclosed herein, for example, the glass binder is 2.6 parts by mass or less, for example 2.0 parts by mass or less, further 0.13 parts by mass to 100 parts by mass of aluminum powder. Even if it is significantly reduced to about 1.5 parts by mass, it can be obtained by including about 0.13 parts by mass to 0.51 parts by mass of phosphorus (P) with respect to 100 parts by mass of the aluminum powder by the phosphorus supply agent. It was confirmed that the water resistance of the solar cell can be sufficiently improved and the amount of warpage can be sufficiently reduced.

[Effect of residual rate of phosphorus supply agent]
Using the phosphorus supply agent (D) whose phosphorus residual ratio in the aluminum electrode was relatively low, the blending amount of the phosphorus supply agent (D) was adjusted so that the phosphorus content remaining in the aluminum electrode was 30 ppm or more. Thus, the characteristics of the obtained solar cell were evaluated.
First, in accordance with the above paste material preparation method, the aluminum powder, the glass binder, and the resin amount are the same as those of the paste 1, and the amount of the phosphorus supply agent (D) is the paste material 21: 0% by mass, the paste material 22 : 1.8% by mass, paste material 23: 3.6% by mass, paste material 24: 10% by mass, paste materials 21 to 24 were prepared using the remainder as a solvent.
And using this paste material 21-24, the solar cell (samples 21-24) as a cell for evaluation was produced according to said solar cell preparation procedures.

With respect to these samples 21 to 24, the amount of phosphorus (P) contained in the aluminum electrode was measured by the same method as described above. From the results, the amount of phosphorus (P) derived from the phosphorus supply agent and the residual rate were determined and shown in Table 4.
Moreover, (1) hot water resistance and (2) tape pull strength were evaluated with respect to these samples 21-24.
(1) Evaluation of hot water resistance The hot water resistance of the obtained solar cells (samples 21 to 24) was examined according to the above method. The results are shown in the corresponding column of Table 4.
(2) Evaluation of tape pull strength The tape pull strength of the obtained solar cells (samples 21 to 24) was evaluated. That is, after pressing the cellophane tape (CT-15153P) made from Nichiban with a finger to the surface of the aluminum electrodes of the above samples 21 to 24, the tape was peeled off, and the state of the electrodes attached to the peeled tape surface was observed visually. The observed results were evaluated in three stages. Here, the above three-step evaluation was performed with the goal of being equivalent to the current product. That is, “○” indicates that the electrode is not attached to almost the entire surface of the pressed tape, and “△” indicates that the electrode is partially attached to the pressed tape. The state where the electrode is attached to the entire surface is indicated by “x”. The results are shown in the corresponding column of Table 4.

As can be seen from the results in Table 4, since the aluminum electrodes of Samples 21 to 24 contain phosphorus (P) at a content rate of 30 ppm or more, a phosphorus supply agent (D ), It was confirmed that the hot water resistance was improved according to the phosphorus content.
However, in the evaluation of the tape pull strength, for the sample 21 to which no phosphorus supply agent (D) was added, the aluminum electrode was hardly peeled off and good electrode film adhesion was obtained. As for samples 22 to 24 using a large amount of phosphorus supply agent (D) in order to achieve the amount, all the aluminum electrode films were peeled cleanly, resulting in extremely low electrode film adhesion. It was. That is, since such a solar cell is difficult to put into practical use, it was confirmed that it is desirable to use a phosphorus supply agent having a high phosphorus residual ratio in the aluminum electrode after firing.

[Evaluation of paste material using glass binder containing phosphorus (P)]
Using phosphorous glass as the glass binder, paste material 25 was prepared in accordance with the paste material preparation method described above.
The proportion (mass%) of aluminum powder, glass binder (phosphorus glass), phosphorus supply agent, resin and solvent in the entire paste material was as follows.

(Paste material 25)
Aluminum powder: 78
Glass binder: 1.0
Phosphorus supply agent: 0
Resin: 1.2
Solvent: 19.8

As the phosphorus _{glass, P 2 O 5: 1.4mol%} , SiO 2: 11.5mol%, B 2 O 3: 23.5mol%, ZnO: glass frit consists of 63.6Mol% mixing ratio Was used. Moreover, the same thing as the paste material 1 was used about aluminum powder, resin, and a solvent.
And using this paste material 25, the solar cell (sample 25) as a cell for evaluation was produced according to the production procedure of the solar cell.

The sample 25 was evaluated for hot water resistance by the same method as described above. As a result, the time until water bubbles are generated from the surface of the aluminum electrode immersed in hot water at 80 ° C. (water resistance time) is 1.1 minutes, which is slightly more water resistant than sample 1 in which no phosphorus supply agent is blended. The result was inferior.
The amount of phosphorus (P) contained in the aluminum electrode was 137 ppm.
From this, even if phosphorus (P) is contained in the paste material, for example, when the phosphorus (P) is contained in the form of a glass component, the function of improving the water resistance of the aluminum electrode is achieved. Not confirmed.

  The present invention provides a paste material for forming a back electrode comprising aluminum powder, glass powder, a vehicle and a phosphorus supply agent. An aluminum electrode of a solar cell obtained by using such a paste material contains a solar cell having excellent water resistance by containing phosphorus (P) at a content of 30 ppm to 700 ppm in a form different from the glass component. Can be realized.

10 Solar cell 11 P-type silicon substrate (Si wafer)
12 Surface electrode (light-receiving surface electrode)
14 Antireflection film 16 n-Si layer (n ⁺ layer)
20 Aluminum electrode (back electrode)
22 Back side external connection electrode 24 p ⁺ layer

Claims (10)

A solar cell comprising a semiconductor substrate, a light receiving surface electrode formed on one surface of the semiconductor substrate, and an aluminum electrode formed on the other surface side of the semiconductor substrate,
The solar cell in which the aluminum electrode contains phosphorus (P) at a content of 30 ppm to 700 ppm in a form different from the glass component. The aluminum electrode contains a glass component together with an aluminum component,
The content rate of this glass component is a solar cell of Claim 1 which is 2.6 mass% or less by making an aluminum component into 100 mass%.   The solar cell according to claim 1 or 2, wherein the aluminum electrode contains the phosphorus (P) at a content of 80 ppm to 400 ppm. A paste material for forming an aluminum electrode of a solar cell,
Aluminum powder,
Resin,
Solvent,
A phosphorus supplier containing phosphorus (P) in the chemical composition;
The content of the phosphorus supply agent is defined such that the aluminum electrode obtained by firing the paste material contains phosphorus (P) in a form different from the glass component at a content of 30 ppm to 700 ppm. material. In addition, it contains a glass binder,
The paste material according to claim 4, wherein the glass binder is contained in a proportion of 2.6 parts by mass or less when the aluminum powder is 100 parts by mass.   As the phosphorus supply agent, the residual rate of phosphorus remaining in the predetermined amount of the sintered aluminum electrode is 20% or more with respect to the amount of phosphorus contained in the paste material used to form the predetermined amount of aluminum electrode. The paste material of Claim 4 or 5 containing the phosphorus containing compound which implement | achieves becoming.   The paste material according to any one of claims 4 to 6, comprising alkyl ether phosphoric acid and / or phosphorus resinate as the phosphorus supply agent.   The said phosphorus supply agent is a paste material of any one of Claims 4-7 contained in the ratio of 0.13 mass part-2.6 mass parts, when the said aluminum powder is 100 mass parts.   The glass binder is contained in a proportion of 0.13 parts by mass to 2.6 parts by mass, and the phosphorus supply agent is contained in a ratio of 0.13 parts by mass to 2.6 parts by mass with respect to 100 parts by mass of the aluminum powder. The paste material according to any one of claims 5 to 8.   The paste material according to any one of claims 4 to 9, wherein the resin is a cellulosic polymer.

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