JP5833298B2 - Method for manufacturing solar cell and electrode forming paste material used in the method - Google Patents

Description

  The present invention relates to a technique for forming fine linear electrodes on a light receiving surface of a solar cell (solar cell). Moreover, it is related with the manufacturing method of the solar cell to which this technique is applied, and the paste material for electrode formation used for this method.

In recent years, solar cells (solar cells) have been rapidly spreading from the viewpoint of environmental awareness and energy saving, and accordingly, a cell structure with higher performance than before, that is, a cell structure with good photoelectric conversion efficiency and high output. There is a need for solar cells.
One measure for realizing this requirement is to increase the light receiving area per unit cell area of the solar cell. For example, as one means for expanding the light receiving area, there is a thinning (fine line) of a linear electrode formed on the light receiving surface.

On the cell light-receiving surface of the so-called silicon type solar cell, which is currently in the mainstream, a grid composed of linear bus bar (connection) electrodes formed of a conductor such as silver, and streaky thin lines connected to the bus bar (For current collection) electrodes are provided. Hereinafter, these electrodes are also collectively referred to as light receiving surface electrodes.
The portion where such a light receiving surface electrode is formed is a non-light receiving portion (light shielding portion) on the light receiving surface of the solar cell. For this reason, if the light receiving surface electrode is made thinner (fine line) than before, the non-light receiving part (light shielding part) is reduced accordingly, the light receiving area per cell unit area is expanded, and the output per cell unit area is improved. Can be made. At this time, if the electrode is not thickened by the thinned line, the line resistance of the electrode increases, and the output characteristics of the solar cell are reduced by that much. Therefore, a fine line of the light-receiving surface electrode and an improvement in the electrode thickness, that is, a high aspect ratio (ratio of electrode thickness to line width: increase thickness / line width; the same shall apply hereinafter) are simultaneously required. The following patent document 1 is mentioned as a prior art of this light-receiving surface electrode.

  As described in Patent Document 1, a conventional light-receiving surface electrode includes a metal powder such as silver as a conductor component and an organic vehicle component made of a binder or a solvent, and is in a paste form (slurry or ink form). ) Is formed on the light receiving surface of the solar cell (cell) with a predetermined wiring pattern by screen printing using the material prepared in the above (hereinafter also referred to as “electrode forming paste”). The following Patent Documents 2 to 6 are listed as conventional techniques for forming such a linear electrode in a predetermined pattern on a semiconductor substrate using a technique such as screen printing.

JP 2006-341547 A JP 2004-355862 A JP 2004-234900 A JP 2006-302962 A JP 2006-324590 A JP 2009-283362 A

  In screen printing performed using a conventional paste material, it is difficult to form a fine current collecting (grid) electrode having a predetermined pattern, for example, having a line width (line width) of 60 μm or less. For example, when screen printing is performed using a general screen mask (screen mesh), there is a possibility of disconnection at the intersection of meshes, which is not preferable. In order to avoid the risk of such disconnection, it is possible to adopt a metal mask and screen-print the light receiving electrode for wiring. Since the metal mask does not have a mesh in the opening of the mask, the resistance when the electrode forming paste material passes is small and clogging is small, so that the occurrence of disconnection can be prevented. Also, by using a metal mask rather than using a general screen mask (screen mesh), the discharge amount of the electrode forming paste material increases, so the electrode thickness (film thickness) also increases, and a higher aspect ratio. It is expected that a fine linear electrode can be formed.

However, when a metal mask is used, the discharge amount of the electrode forming paste increases. Therefore, if the conventional electrode forming paste is used as it is, an electrode that greatly extends beyond the desired line width is formed. This is not preferable.
Therefore, the present invention has been made in view of such conventional problems, and the main purpose thereof is a method suitable for realizing a fine line and a high aspect ratio of a light-receiving surface electrode of a solar cell and the method. It is to provide the material used. Another object of the present invention is to provide a solar cell (solar cell) in which a fine light-receiving surface electrode is formed on a semiconductor substrate by using the method and material, and a method for manufacturing the solar cell.

In order to achieve the above object, the present invention provides a suitable paste material for forming a linear electrode (light receiving surface electrode) on the light receiving surface of a solar cell.
That is, the paste for forming the linear electrode on the light receiving surface of the solar cell provided by the present invention is:
Comprising a conductive powder and an organic vehicle component in which the powder is dispersed;
Rotational speed: the viscosity at 20 rpm is 200 to 400 Pa · s, and
Rotational speed: Viscosity at ₅ rpm: η ₅ and Viscosity at rotational speed: 50 rpm: η ₅₀ Viscosity ratio: η ₅ / η ₅₀ is in the range of 5.5 to 7.5 This is an electrode forming paste.
Here, the viscosity is a viscosity that can be measured by a commercially available rotational viscometer. For example, by using a rotational viscometer (Brookfield rotational viscometer) manufactured by Brookfield, which is standard in the field, the above conditions of the rotational speed range (for example, a No. 4 spindle is used) are easy. The viscosity ratio can be calculated by measuring the viscosity.

As a result of having the viscosity characteristics as described above, the electrode forming paste disclosed here has a mask for screen printing on the light-receiving surface of the semiconductor substrate constituting the cells of the solar cell, particularly by adopting the screen printing method. When being applied (supplied), the paste exhibits good fluidity at a relatively high shear rate, and the “slipping” of the paste from the mask (screen mask or metal mask made of various materials) is good. For this reason, it is possible to prevent the occurrence of disconnection (that is, supply failure site). In addition, after the screen printing method is performed and a predetermined pattern is printed through various masks (that is, the shear rate after application to the light receiving surface is extremely low), the viscosity is good (maintaining the shape). Performance) and an undesired expansion of the line width can be prevented. That is, since the shape of the wiring pattern applied to the light receiving surface before baking is difficult to spread, an electrode pattern with good shape accuracy can be formed.
Therefore, according to the paste material having the above-described configuration, it is possible to form a thin light-receiving surface electrode having a desired aspect ratio. In other words, it is possible to achieve further fine line formation of the linear electrodes formed on the light receiving surface, and to improve the output per unit area of the cell light receiving surface.

  In the present invention, “screen printing” refers to the formation of an electrode in a target pattern through the opening of the mask in which the opening is formed in a pattern corresponding to the target pattern (that is, a wiring pattern to be formed on the light receiving surface). It is a term that refers to a printing technique for applying (supplying) a paste to the light receiving surface, and is not limited to the mask material used. Masks such as screen masks (mesh) made of various synthetic resins (for example, polyester), metal screen meshes (for example, stainless steel mesh), metal masks in which openings are formed by processing metal materials by etching or laser processing, etc. The printing (patterning) using is a wiring pattern forming technique included in “screen printing” in the present specification.

In a preferred embodiment of the electrode forming paste disclosed herein, the viscosity ratio: η ₅ / η ₅₀ is in the range of 6-7.
According to the paste material having such characteristics, viscosity characteristics suitable for forming the light receiving surface electrode on the light receiving surface of the semiconductor substrate by screen printing are exhibited. Therefore, it is possible to more preferably form a thin line-shaped light-receiving surface electrode (particularly a grid electrode for current collection) having a desired aspect ratio with a predetermined wiring pattern.

In another preferred embodiment of the electrode forming paste disclosed herein, a metal powder (typically silver powder) having an average particle size of 1 to 3 μm as the conductive powder is 80 to 90% by mass of the entire paste. The organic vehicle component is contained in a proportion of 10 to 20% by mass of the whole paste.
According to the paste material having such characteristics, it is possible to form an electrode having good viscosity characteristics as described above and having good conductivity (in other words, a high-density metal electrode). For this reason, a low resistance and high output solar cell can be manufactured.
Particularly preferably, the glass powder is contained in a proportion of 5% by mass or less of the entire paste. According to such a structure, the adhesiveness with a board | substrate improves and the electrode which is more excellent in mechanical strength can be formed in a light-receiving surface.

Further, the present invention provides a method of forming an electrode on a light receiving surface of a cell (substrate) of a solar cell using any of the electrode forming pastes disclosed herein, and forming the electrode as one step. A method of manufacturing a solar cell is provided.
That is, the manufacturing method disclosed here is a method of manufacturing a solar cell including a semiconductor substrate and a linear light receiving surface electrode of a predetermined pattern formed on the light receiving surface of the substrate,
Comprising an electrically conductive powder and an organic vehicle component in which the powder is dispersed;
Rotational speed: the viscosity at 20 rpm is 200 to 400 Pa · s, and
Rotational speed: Viscosity at ₅ rpm: η ₅ and Viscosity at rotational speed: 50 rpm: η ₅₀ Viscosity ratio: η ₅ / η ₅₀ is in the range of 5.5 to 7.5 Preparing an electrode forming paste; and
Applying the paste in a predetermined pattern to the light receiving surface by screen printing and baking to form a light receiving surface electrode of the predetermined pattern;
Is included.

  According to this manufacturing method, by performing screen printing using the paste material having the above-described configuration, it is possible to accurately form a thin light-receiving surface electrode having a desired aspect ratio on a semiconductor substrate. That is, it is possible to realize a fine line of the light receiving surface electrode to expand the light receiving area per unit area of the solar cell (light receiving surface), and to improve the output per unit area of the cell light receiving surface. Therefore, according to this manufacturing method, a high output solar cell can be provided.

In a preferred embodiment of the production method disclosed herein, the viscosity ratio: η ₅ / η _{50 of the} paste is in the range of 6-7.
By forming the light-receiving surface electrode using a paste material having such characteristics, it is more preferable to provide a thin-line light-receiving surface electrode (especially a grid electrode for current collection) having a desired aspect ratio with a predetermined wiring pattern. A high output solar cell can be manufactured.

In another preferred embodiment of the production method disclosed herein, the paste comprises a metal powder (typically silver powder) having an average particle size of 1 to 3 μm as the conductive powder, and 80 to 90 mass of the entire paste. %, And the organic vehicle component is contained at a ratio of 10 to 20% by mass of the entire paste.
By forming the light-receiving surface electrode using a paste material with such characteristics, a low-resistance, high-output solar cell is manufactured that has an electrode with good conductivity (in other words, a high-density metal electrode). can do.
Preferably, the paste used contains glass powder in a proportion of, for example, 5% by mass or less of the total paste. According to such a structure, the solar cell which improved the adhesiveness with a board | substrate and was equipped with the electrode which is more excellent in mechanical strength in the light-receiving surface can be manufactured.

In another preferred embodiment of the manufacturing method disclosed herein, the paste is applied to the light receiving surface by screen printing using a metal mask having openings corresponding to a predetermined pattern.
By using a metal mask having no intersection such as a mesh, an electrode with a high aspect ratio can be more suitably formed. Moreover, even if a metal mask that has no resistance to paste flow such as the intersection of meshes and the paste movement from the mask opening to the light receiving surface tends to be excessive, the electrode forming paste disclosed here has the above-mentioned viscosity characteristics. Therefore, excessive paste movement from the opening of the metal mask to the light receiving surface can be suppressed (as a result, excessive extension of the line width can be suppressed), and the desired line width can be maintained.

In a more preferable aspect of the manufacturing method, the screen printing is performed as follows.
Disposing the metal mask on the light receiving surface of the substrate;
Supplying a paste on the surface of the metal mask arranged above,
Filling the opening in the metal mask with the paste by moving a squeegee at a moving speed of 100 to 300 mm / sec on the surface of the metal mask supplied with the paste;
Is included.
According to the manufacturing method having such a configuration, a light receiving surface electrode having an accurate size (line width) can be efficiently formed by smooth movement of the squeegee, and a high-performance solar cell can be efficiently manufactured.

  By performing screen printing employing the electrode forming paste disclosed herein, for example, the line width is 70 μm or less (more preferably 65 μm or less, particularly 60 μm or less) and the thickness is at least 20 μm (more preferably at least It is possible to form a current collecting electrode having a predetermined pattern such as 25 μm on the light receiving surface of the substrate. Therefore, the manufacturing method disclosed here can be preferably employed to manufacture a solar cell that achieves a fine line and a high aspect ratio of the light-receiving surface electrode.

It is sectional drawing which shows an example of the structure of a solar cell typically. It is a top view which shows typically the pattern of the electrode formed in the light-receiving surface of a solar cell. It is a figure which illustrates typically the suitable one aspect | mode of the screen printing to the light-receiving surface of the semiconductor substrate performed using the paste for electrode formation, (a) shows filling of the paste material to the mask opening part by a squeegee. FIG. 4B is a diagram showing that the paste coating material is supplied to the light receiving surface of the substrate through the mask opening. It is a figure which shows typically the formation state of the light-receiving surface electrode formed using each paste produced in the Example. It is a graph which shows the relationship between the viscosity of each paste produced in the Example, and the line | wire width (micrometer) of the light-receiving surface electrode (grid electrode) formed using this paste. It is a graph which shows the relationship between the viscosity of each paste produced in the Example, and the film thickness (micrometer) of the light-receiving surface electrode (grid electrode) formed using this paste.

Hereinafter, preferred embodiments of the present invention will be described. It should be noted that technical matters other than the contents particularly mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters for those skilled in the art based on the prior art. The present invention can be carried out based on the technical contents disclosed in the present specification and the common general technical knowledge in the field.
In addition, the viscosity of the paste material mentioned in this specification has shown the value of the viscosity measured at normal temperature. Here, “normal temperature” refers to a temperature range of 15 to 35 ° C., and typically refers to a temperature range of 20 to 30 ° C. (for example, 25 ° C.).

The electrode forming paste disclosed here is a material mainly composed of conductive powder and an organic vehicle component for dispersing the powder, as in the conventional paste material of this type (conductor paste). is there.
As the “conductive powder” that is the main component of the solid content of the paste, a simple substance of a noble metal such as silver (Ag), platinum (Pt), palladium (Pd), gold (Au) or an alloy thereof (Ag—Pd alloy) , Pt—Pd alloys, etc.), and alloys of the above precious metals and other metals. From the viewpoint of low cost and low electrical resistance, typically, a powder made of silver or a silver-based alloy (hereinafter referred to as “Ag powder”) is particularly preferably used.

As the Ag powder and other conductive powders, those having an average particle diameter of 5 μm or less based on a laser diffraction method (light scattering method) are suitable, and the average particle diameter is 3 μm or less (typically 1 to 3 μm, for example, 1 ˜2 μm) is preferably used.
The shape of the particles constituting the conductive powder is not particularly limited, but typically, a spherical shape, a scaly shape, a conical shape, a rod shape, or the like can be preferably used. Spherical or scaly particles are preferably used for reasons such as easy formation of a dense light-receiving surface electrode with good fillability. As the conductive powder to be used, those having a sharp (narrow) particle size distribution are preferable. For example, a conductive powder having a sharp particle size distribution that does not substantially contain particles having a particle diameter of 10 μm or more is preferably used. As this index, the ratio (D10 / D90) of the particle size (D10) when the cumulative volume is 10% and the particle size (D90) when the cumulative volume is 90% in the particle size distribution based on the laser diffraction method can be adopted. When all the particle sizes constituting the powder are equal, the value of D10 / D90 is 1, and conversely, the value of D10 / D90 approaches 0 as the particle size distribution becomes wider. It is preferable to use a powder having a relatively narrow particle size distribution such that the value of D10 / D90 is 0.2 or more (for example, 0.2 to 0.5).
The paste using the conductive powder having such an average particle diameter and particle shape has a good filling property of the conductive powder and can form a dense electrode. This is advantageous in forming a fine wiring pattern on the light receiving surface with high shape accuracy.

  The method for producing such conductive powder such as Ag powder is not particularly limited. For example, conductive powder (typically Ag powder) produced by a known wet reduction method, gas phase reaction method, gas reduction method or the like can be classified and used as necessary. Such classification can be performed using, for example, a classification device using a centrifugal separation method.

On the other hand, as the “organic vehicle component” in which the conductive powder is dispersed, those conventionally used in this type of paste material can be used without any particular limitation. Typically, the vehicle is composed of organic binders and organic solvents of various compositions.
Examples of the organic binder include cellulose polymers such as ethyl cellulose and hydroxyethyl cellulose, acrylic resins such as polybutyl methacrylate, polymethyl methacrylate, and polyethyl methacrylate, epoxy resins, phenol resins, alkyd resins, polyvinyl alcohol, and polyvinyl butyral. An organic binder based on is preferably used. In particular, a cellulosic polymer (for example, ethyl cellulose) is preferable, and a viscosity characteristic capable of performing particularly good screen printing can be realized.

Organic solvents include ester solvents such as butyl cellosolve acetate and butyl carbitol acetate (BCA: diethylene glycol monobutyl ether acetate), ether solvents such as butyl carbitol (BC: diethylene glycol monobutyl ether), ethylene glycol and diethylene glycol derivatives Organic solvents such as toluene, xylene, mineral spirit, terpineol and mentanol are preferably used.
A preferable solvent constituting the organic vehicle is an organic solvent having a boiling point of about 200 ° C. or higher (typically about 200 to 260 ° C.). An organic solvent having a boiling point of about 230 ° C. or higher (typically about 230 to 260 ° C.) is more preferably used. Particularly preferred solvent components include butyl carbitol (BC), butyl carbitol acetate (BCA), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and the like.

The content of the conductive powder in the entire paste is suitably about 80% by mass or more (typically 80 to 90% by mass) when the entire paste is 100% by mass, for example, 85% by mass. It is preferable to set the degree. Increasing the content of the conductive powder is preferable from the viewpoint of forming a dense electrode pattern with good shape accuracy. On the other hand, if the content is too high, the handleability of the paste, the suitability for screen printing, and the like may decrease, which is not preferable.
Further, the content of the organic vehicle is suitably 5 to 20% by mass when the total paste is 100% by mass, and 10 to 20% by mass (particularly 10 to 15% by mass). preferable.
Moreover, it is preferable that an organic binder is contained in the ratio of 15 mass parts or less (typically 1-10 mass parts) with respect to 100 mass parts of electroconductive powder among organic vehicle components. Particularly preferably, it is contained at a ratio of 5 to 10 parts by mass with respect to 100 parts by mass of the conductive powder.
In addition, the said numerical range regarding the content rate of each component should not be interpreted exactly | strictly, As long as the objective of this invention can be achieved, slight deviation from this range is accept | permitted.

In addition to the main conductive powder and organic vehicle component, various inorganic and / or organic additives can be included as long as the object of the present invention can be achieved. Preferable examples of the inorganic additive, glass powder and other ceramic powder _{(ZnO 2, Al 2 O 3} , SiO 2 , etc.), other various fillers and the like. Among these, addition of ceramic powder, particularly glass powder (glass frit) is preferable. Typically, the average particle size of this type of powder (filler) is adjusted to be equal to or less than that of the conductive powder. For example, glass powder or other powder (filler) having an average particle size of 3 μm or less, preferably 2 μm or less, typically about 0.1 to 2 μm based on a laser diffraction method (light scattering method) is used. Can do.

The glass component to be added may be a normal amorphous or crystalline glass. Specific examples include those containing oxides such as lead, boron and silicon, such as lead silicate glass, lead aluminosilicate glass, lead borosilicate glass and lead aluminoborosilicate glass. Although a glass softening point is not specifically limited, It is preferable that it is about 300-600 degreeC (for example, 400-500 degreeC). When such a glass powder is added, an electrode having excellent adhesion to the electrode can be formed by welding the glass component to a substrate (typically a silicon substrate) when the pace coating material is baked. .
The content ratio of the glass powder and other ceramic powder is suitably 10% by mass or less when the entire paste is 100% by mass, and typically 5% by mass or less (eg, about 1 to 5% by mass). is there.

  In addition to the above components, various additive components can be added to the electrode forming paste disclosed herein as necessary. Examples thereof include additives such as surfactants, antifoaming agents, antioxidants, dispersants, polymerization inhibitors.

The electrode forming paste of the present invention suitably used for the solar cell manufacturing method disclosed herein has a viscosity (hereinafter “η ₂₀ ”) at a rotational speed of 20 rpm of 200 to 400 Pa · s, and Rotational speed: Viscosity at ₅ rpm: η ₅ and Viscosity at rotational speed: 50 rpm: η ₅₀ Viscosity ratio: η ₅ / η ₅₀ is in the range of 5.5 to 7.5. Preferably, η ₅ / η ₅₀ is in the range of 6-7. Typically, the viscosity is a viscosity measured by a Brookfield rotational viscometer by rotating the paste material under test at any one of the above rotational speed conditions using an appropriate spindle (for example, No. 4 spindle). .
The electrode forming paste used in the solar cell manufacturing method and the light-receiving surface electrode forming method disclosed herein has a viscosity such that η ₂₀ is 200 to 400 Pa · s, so that when performing normal screen printing. Shows a suitable viscosity. Details will be described below.

When the rotational speed: viscosity at ₅ rpm: η ₅ is constant, the viscosity ratio: η ₅ / η ₅₀ increases as the viscosity η _{50 at the} rotational speed: 50 rpm decreases. Such a rotational speed: When the viscosity η _{50 at 50} rpm (that is, when the shearing speed is larger than that when the rotational speed is 5 rpm) is low, the shearing speed is increased when the paste is screen-printed. Indicates that the paste "spills out" or flows out from various screen printing masks (typically metal masks). In particular, when applying paste material to the light-receiving surface of a semiconductor substrate that constitutes a cell of a solar cell by using a screen printing method via a screen printing mask (specifically, using a squeegee to open the mask opening) Viscosity characteristics that are preferred when the paste material is filled in). This is because the paste exhibits good fluidity and “pull” of the paste from the mask is good.

On the other hand, the rotational speed: viscosity at 50 rpm: If eta ₅₀ is constant, the rotational speed: The value of the viscosity ratio as the viscosity eta ₅ increases (η ₅ / η ₅₀₎ at 5rpm increases. Such a rotational speed: The viscosity η5 at ₅ rpm (that is, when the shearing speed is smaller than that when the rotational speed is 50 rpm) is high, which means that the paste printed on the light-receiving surface of the substrate is difficult to flow (the paste dripping is small). ) From the viewpoint. As a result, after the screen printing method is executed and printed on the light receiving surface of the substrate in a predetermined pattern through various masks (that is, the shear rate after being applied to the light receiving surface is extremely low). , Good viscosity (shape maintaining performance) can be exhibited, and undesired expansion of the line width can be prevented.
That is, since the shape of the wiring pattern applied to the light receiving surface before baking is difficult to spread, an electrode pattern with high shape accuracy can be formed. In addition, when the value of the viscosity ratio (η5 / η50) is too large, the leveling property of the paste generally tends to decrease, which is not preferable.

Such viscosity (η ₅ , η ₂₀ , η ₅₀ ) and / or viscosity ratio (η ₅ / η ₅₀ ) are, for example, the content ratio of the conductive powder contained in the paste, the average particle diameter or the conductivity of the conductive powder. Although it can be adjusted by the particle size distribution of the powder, etc., preferably the kind of the solvent constituting the organic vehicle while maintaining the above-described suitable shape (outer shape and average particle size and particle size distribution) of the conductive powder, It can be set by adjusting the type of organic binder and the content ratio thereof.

  The electrode forming paste disclosed here can be prepared typically by mixing the conductive powder and the organic vehicle component as in the case of this type of conventional conductor paste. At this time, an additive as described above (for example, an inorganic filler such as glass powder or an auxiliary component such as a viscosity modifier) may be added and mixed as necessary. For example, by using a three-roll mill or other kneader, the conductive powder and various additives are mixed together with the organic vehicle component at a predetermined blending ratio and kneaded together to obtain pastes for forming electrodes having various compositions. Can be prepared.

  Next, a solar cell manufacturing method including forming linear electrodes in a predetermined pattern on the light receiving surface of a semiconductor substrate for solar cells using the electrode forming paste disclosed herein will be described. The solar cell manufacturing method is the same as that for manufacturing a conventional solar cell (solar cell) except for the material that characterizes the present invention and the process using the material. The portions related to the use of the same materials as those described above do not characterize the present invention, and thus will not be described in detail.

1 and 2 schematically show an example of a solar cell (solar cell) 10 that can be suitably manufactured by implementing the present invention, and is made of single crystal, polycrystalline, or amorphous silicon (Si). This is a so-called silicon type solar cell that uses the wafer as the semiconductor substrate 11. A cell 10 shown in FIG. 1 is a general single-sided light receiving solar cell 10.
Specifically, this type of solar cell 10 includes an n-Si layer 16 formed by pn junction formation on the light-receiving surface side of a p-Si layer (p-type crystalline silicon) 18 of a silicon substrate (Si wafer) 11. And an antireflection film 14 made of titanium oxide or silicon nitride formed by CVD or the like, and light receiving surface electrodes 12 and 13 made of an electrode forming paste containing Ag powder or the like.
On the other hand, on the back side of the p-Si layer 18, a back side external connection electrode formed of a predetermined paste material (typically a conductive paste in which the conductive powder is Ag powder) in the same manner as the light receiving surface electrode 12. 22 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 an aluminum paste mainly composed of aluminum powder. During this firing, an Al—Si alloy layer (not shown) is formed, and aluminum diffuses into the p-Si layer 18 to form the 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 improvement in short circuit current and open circuit voltage (Voc) is realized.

As shown in FIG. 2, on the light receiving surface 11A side of the silicon substrate 11 of the solar cell 10, as the light receiving surface electrodes 12, 13, several mutually parallel linear bus bar (connecting) electrodes 12, A grid-like grid (collecting current) electrode (also referred to as a finger electrode) 13 parallel to each other connected so as to intersect the bus bar electrode 12 is formed.
A large number of grid electrodes 13 are formed to collect photogenerated carriers (holes and electrons) generated by light reception. The bus bar electrode 12 is a connection electrode for collecting carriers collected by the grid electrode 13. Therefore, as described above, the bus bar electrode 12 and the grid electrode 13 (particularly the large number of grid electrodes 13) provided on the light receiving surface 11A side are made as fine lines as possible, thereby increasing the photoelectric conversion efficiency of the solar cell 10 and increasing the high efficiency. Output can be achieved.

Such illustrated solar cell 10 is roughly manufactured through the following process.
That is, an appropriate silicon wafer is prepared, and the p-Si layer 18 and the n-Si layer 16 are formed by doping predetermined impurities by a general technique such as a thermal diffusion method or ion plantation. A substrate (semiconductor substrate) 11 is produced. Next, an antireflection film 14 made of silicon nitride or the like is formed by a technique such as spin coating.
Further, an electrode forming paste is printed on the formed antireflection film 14 with a wiring pattern as shown in FIG. 2 based on a screen printing method. Although the line width to be printed is not particularly limited, a grid (collecting current) electrode having a line width of about 70 μm or less (preferably 65 μm or less, particularly 60 μm or less) by adopting the electrode forming paste of the present invention. A coating film is formed. Next, the substrate is dried in an appropriate temperature range (typically 100 to 200 ° C., for example, about 130 to 150 ° C.). The contents of a suitable screen printing method will be described later.

On the other hand, on the back side of the silicon substrate 11, first, as in the case of the light-receiving surface electrodes (typically Ag electrodes) 12, 13, a predetermined paste material (typically a conductor whose conductive powder is Ag powder). The paste was used to screen-print in a predetermined pattern and dried to form a back-side conductor paste coating that would become the back-side external connection electrode 22 (see FIG. 1) after firing. Next, a paste containing aluminum powder as a conductor component is applied to the entire back surface by a screen printing method or the like, and dried to form an aluminum film.
Next, the silicon substrate 11 to which the paste coating (dried film-like coating) is applied on both surfaces is appropriately fired in an air atmosphere using a firing furnace such as a near-infrared fast firing furnace (for example, 700 to 700). 900 ° C.).
By such firing, a fired aluminum electrode 20 is formed together with the light-receiving surface electrodes (typically Ag electrodes) 12 and 13 and the backside external connection electrode (typically Ag electrode) 22, and at the same time, Al—Si (not shown). The alloy layer is formed and aluminum diffuses into the p-Si layer 18 to form the p + layer (BSF layer) 24 described above, and the solar cell 10 is manufactured.
Instead of simultaneous firing as described above, for example, firing for forming the light-receiving surface electrodes (typically Ag electrodes) 12 and 13 on the light-receiving surface side, and rear surface side aluminum electrodes and external connection electrodes are used. You may implement separately the baking for forming.

In the light-receiving surface electrode forming process performed in the solar cell manufacturing method disclosed herein, when screen printing is performed using an electrode forming paste having a desired composition, preferably, (a) and (b) in FIG. As shown in FIG. 5, it is preferable to perform general screen printing using a metal mask 50 in which openings 51 corresponding to the wiring pattern are formed in an accurate shape by etching or laser processing.
Specifically, a predetermined metal mask 50 is arranged on the light receiving surface 11A of the semiconductor substrate (silicon substrate) 11, and an electrode forming paste 70 having a desired composition is supplied onto the surface of the arranged metal mask 50. . Then, by moving the squeegee 60 having an appropriate shape at a high speed on the surface of the metal mask 50, preferably at a movement speed of 100 to 300 mm / second (for example, 150 to 250 mm / second), an appropriate displacement speed (shear speed). Can be applied to the electrode forming paste 70. As a result, the fluidity of the paste is increased, and the opening 70 of the metal mask 50 can be efficiently filled with the paste 70.
Preferably, the printing pressure of the squeegee (pressure applied to the squeegee) is not particularly limited because it can be appropriately changed depending on the material and structure of the squeegee itself, but typically about 0.05 to 0.3 MPa is appropriate. Typically, it is about 0.1 to 0.2 MPa. In addition, the angle between the metal mask surface and the squeegee (attack angle) is preferably slightly higher than the conventional angle, and an attack angle of 70 to 85 degrees (more preferably 75 to 80 degrees) is particularly preferable.

The opening width of the opening 51 of the metal mask 50 to be used is preferably a size that can realize a fine line of the light receiving surface electrode. For example, when a grid electrode having a line width of 70 μm or less is formed, a metal mask having an opening width corresponding to the size, typically 60 μm or less (for example, 30 to 60 μm), or 50 μm or less (for example, 30 to 50 μm) is used. Can be used. Although not particularly limited, for example, a mask having an opening width that is narrower by about 5 to 20% than the line width of the target grid electrode (for example, a mask having an opening width of about 48 to 57 μm if an electrode having a line width of about 60 μm is intended). Should be used. The metal mask to be used may be one in which both an opening for forming a grid electrode line and an opening for forming a bus bar electrode line are formed, or only an opening for a grid electrode line is formed. It may be a thing. When using a metal mask in which only an opening for forming a grid electrode line is used, the bus bar electrode can be formed using another screen printing plate.
According to the solar cell manufacturing method disclosed here (that is, the electrode forming method by screen printing included in the method), for example, the grid electrode 13 having a line width of 70 μm or less (preferably 65 μm or less, particularly 60 μm or less) is formed. can do. Moreover, there is no restriction | limiting in particular about a bus-bar electrode, For example, a bus-bar electrode with a line | wire width of about 1000-3000 micrometers can be formed.
The shape of the squeegee 70 is not particularly limited as long as it does not hinder smooth movement and filling of the paste 70 into the mask opening 51. Typically, the squeegee 70 is made of a synthetic resin having a thickness of about 1 to 10 mm (for example, made of polyurethane or A flat plate (made of fluororesin) (typically hardness of 60 to 80 degrees, particularly hardness of 80 degrees) can be suitably used.

Several 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 examples.
The average particle size described below was measured using a laser diffraction / scattering particle size distribution measuring device (Horiba, Ltd. product: LA-920).
In addition, the viscosity was measured using a No. 4 spindle (spindle “SC-4-14”) with a rotational viscometer (HBT type DV III +) manufactured by Brookfield, and each rotational speed (5, 20, 50 rpm) at 25 ° C. ).

<Preparation of electrode forming paste>
A total of 17 types of pastes shown in Tables 1 and 2 below were prepared.
As the conductive powder, Ag powder having an average particle diameter of 2 μm based on the laser diffraction method was used. Moreover, ethyl cellulose and BCA (butyl carbitol acetate) were used as the organic vehicle component. Further, as the glass frit, a general lead borosilicate glass powder (average particle diameter based on laser diffraction formula: 0.5 to 1.6 μm) often used in the preparation of a conductor paste in the electronic material field was used. Specifically, these materials were mixed so as to have a composition (blending ratio) as shown in Tables 1 and 2, and were well kneaded using a three-roll mill. Thus, a total of 17 types of electrode forming pastes shown as samples 1 to 17 in the table were prepared.
Subsequently, the viscosity: η ₅ , η ₂₀ and η ₅₀ at the rotational speeds of ₅ rpm, 20 rpm and 50 rpm of each of the obtained pastes were measured, and the viscosity ratio η ₅ / η ₅₀ was further determined. Each measured value is shown in the corresponding column of the table.
As is apparent from the values shown in the table, pastes having various viscosities and viscosity ratios were obtained by adjusting the composition of the organic vehicle component.

<Production of test solar cell (light-receiving surface electrode)>
Using the obtained pastes of Samples 1 to 17 for forming the light-receiving surface electrodes (that is, the grid electrodes and the bus bar electrodes), solar cells corresponding to the respective samples were manufactured.
That is, a commercially available 156 mm square (□) solar cell p-type single crystal silicon substrate (thickness: 200 μm) was prepared, and the surface thereof was subjected to an alkali etching treatment using an aqueous NaOH solution. Next, an n-Si layer having a thickness of about 0.5 μm is formed on the light-receiving surface of the Si substrate by applying a phosphorus-containing solution to the light-receiving surface of the Si substrate on which the texture structure is formed by the etching process and performing heat treatment. (N + layer) was formed. Next, an antireflection film (titanium oxide film) having a thickness of about 50 to 100 nm was formed on the n-Si layer by plasma CVD (PECVD).
Thereafter, a coating film to be a light-receiving surface electrode (Ag electrode) was formed on the antireflection film by a screen printing method in the air at room temperature using the paste of any sample.

Specifically, it includes two linear bus bar electrodes parallel to each other as shown in FIG. 2 and FIG. 4 described later, and a plurality of grid electrodes parallel to each other so as to be orthogonal to the bus bar electrodes. A wiring pattern was formed on the light receiving surface by screen printing. The line width of the target grid electrode was set to 60 to 65 μm. The line width of the bus bar electrode was set to 2000 μm.
Thus, a metal mask having openings corresponding to the wiring pattern and the line width size of each electrode is disposed on the light receiving surface of the substrate, and after supplying paste onto the mask, silicon rubber (or urethane) Rubber may also be used.) A squeegee made of 80 (hardness) is brought into contact with the metal mask at an attack angle of 80 degrees and a printing pressure of 0.1 MPa, the squeegee is moved at a high speed of 200 mm / second, and the paste is applied to the opening of the mask. Filled and printed.
After the screen printing, the substrate was dried at 150 ° C., and then the substrate was baked to form a light receiving surface electrode. Specifically, firing was performed in a firing temperature range of 700 to 800 ° C. using a near-infrared high-speed firing furnace in an air atmosphere.

<Evaluation of electrode formation (1)>
The shape accuracy was evaluated by observing the pattern of the light receiving surface electrode (grid electrode, bus bar electrode) formed on the light receiving surface of the test substrate as described above with an optical microscope. Pattern observation is mainly performed by (1). Presence or absence and unintentional irregularities (swells) on the surface of the line (electrode) and / or the outer edge, and (2). The presence or absence of protrusions on the line and the extent thereof, (3). Presence or absence of line bleeding (line crushing) and its extent, (4). It was performed from the viewpoint of the presence or absence and the degree of disconnection of the line.
The observation results are schematically shown in FIG. The screen printing (that is, the squeegee moving direction) is performed from the bottom to the top of the illustrated substrate. A bold circle or ellipse in the figure indicates a part where a line break or defect is recognized by microscopic observation.
As is apparent from this figure, when the paste having a viscosity ratio (η ₅ / η ₅₀ ) of 3 or 8 is used compared to the paste having a viscosity ratio of 5 to 7 The line defects were significantly reduced. In particular, the lines (particularly grid electrodes) formed using the paste having a viscosity ratio of 6 or 7 were not disconnected, and only slight protrusions and bleeding were observed.

<Electrode formation evaluation (2)>
Next, the line width and film thickness of the light receiving surface electrode (grid electrode) formed on the light receiving surface of the test substrate are examined by microscopic observation (micrometer), and the average line width (μm) of the formed grid electrode and The average thickness (film thickness: μm) is shown in the corresponding column of Tables 1 and 2, and the graphs of FIGS. 5 and 6 show the viscosity of each paste and the average line width of the grid electrode formed using the paste ( FIG. 5) or the relationship with the average film thickness (FIG. 6).

As is apparent from these tables and graphs, when a paste having a viscosity ratio (η ₅ / η ₅₀ ) of 6 to 7 is used, a grid electrode having a target line width of 60 to 65 μm is preferably formed. I was able to confirm it. Further, the film thickness is 21 μm or more (typically 23 μm or more), which indicates that a highly accurate grid electrode with a high aspect ratio is formed.
On the other hand, when the paste having the viscosity ratio of 3 was used, an electrode having a remarkably high line width (85 μm or more) was formed. This indicates that it is difficult to maintain the shape of the screen-printed coating on the light receiving surface. Also, when a paste having a viscosity ratio of 5 is used, the line width of the electrode exceeds 65 μm, but not as marked as when a paste having a viscosity ratio of 3 is used. It was shown that the shape maintenance performance on the surface was slightly inferior. On the contrary, when a paste having a viscosity ratio of 8 was used, it was recognized that the viscosity at the time of screen printing was too high and a sufficient amount of paste was not filled in the opening of the metal mask. For this reason, the film thickness tended to be relatively low.

<Electrode performance evaluation>
The performance of the light receiving surface electrode (grid electrode) formed on the light receiving surface of the test substrate as an electrode (conductor) was evaluated by a line resistance value (mΩ).
Specifically, using a digital high tester manufactured by Hioki Electric Co., Ltd. (HIOKI), the probe of the tester was brought into contact with the grid electrode of the light receiving surface electrode at intervals of 38 mm, and the value was measured. Measurement was performed at a total of 10 positions different from each other, and the average value thereof was taken as the line resistance value (mΩ).
The average value (mΩ) of the line resistance values thus measured is shown in the corresponding columns of Tables 1 and 2. As apparent from the line resistance values in the table, all the electrodes showed sufficiently low resistance values, but the resistance values of the electrodes using the paste having the viscosity ratio of 8 showed generally high values. This reflects that a defect such as disconnection has occurred in the line as a result of the viscosity at the time of screen printing being too high to fill the opening of the metal mask with a sufficient amount of paste.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

10 Solar cell
11 Semiconductor substrate (Si substrate)
11A Light-receiving surface 12 Bus bar electrode (light-receiving surface electrode)
13 Grid electrode (light-receiving surface electrode)
14 Antireflection film 16 n-Si layer 18 p-Si layer 20 Back surface aluminum electrode 22 Back side external connection electrode 24 p + layer 50 Metal mask 51 Opening 60 Squeegee 70 Electrode forming paste

Claims (4)

A method of manufacturing a solar cell comprising a semiconductor substrate and a linear light-receiving surface electrode of a predetermined pattern having a line width of 70 μm or less and a thickness of at least 21 μm formed on the light-receiving surface of the substrate,
It contains a conductive powder and an organic vehicle component in which the powder is dispersed. The viscosity at a rotational speed of 20 rpm is 200 to 400 Pa · s. The viscosity at a rotational speed of ₅ rpm: η ₅ and the rotational speed: Viscosity at 50 rpm: Viscosity ratio with η ₅₀ : Preparing an electrode forming paste characterized in that η ₅ / η ₅₀ is in the range of 5.5 to 7.5, and
The paste is applied to the light receiving surface by one screen printing so that the line width is 70 μm or less and the thickness is at least 21 μm, and the light receiving surface electrode of the predetermined pattern is baked. Forming,
Including
Application of the paste to the light receiving surface is performed by screen printing using a metal mask having an opening having an opening width of 60 μm or less corresponding to the predetermined pattern ,
The screen printing is
Disposing the metal mask on the light receiving surface of the substrate;
Supplying the paste on the surface of the arranged metal mask;
Filling the opening in the metal mask with the paste by moving a squeegee at a moving speed of 100 to 300 mm / sec on the surface of the metal mask supplied with the paste;
It includes, the production method. The said viscosity ratio: (eta) ₅ / (eta) _{50 of the} said paste exists in the range of 6-7, The manufacturing method of Claim 1 characterized by the above-mentioned.   The paste contains silver powder having an average particle size of 1 to 3 μm as the conductive powder in a proportion of 80 to 90% by mass of the whole paste, and an organic vehicle component in a proportion of 10 to 20% by mass of the whole paste. The manufacturing method according to claim 1, wherein:   The said paste contains glass powder in the ratio of 5 mass% or less of the whole paste, The manufacturing method of Claim 3 characterized by the above-mentioned.

Images