JP6014847B2 - Solar cell and method for manufacturing the same - Google Patents

Description

  The present invention relates to a solar cell and a manufacturing method thereof, and more particularly to a solar cell using a semiconductor substrate such as a silicon substrate and a manufacturing method thereof.

  In recent years, development of clean energy has been desired due to the problem of depletion of energy resources and global environmental problems such as an increase in CO2 in the atmosphere. In view of effective use of resources and prevention of environmental pollution, solar cells that directly convert sunlight into electric energy are widely used, and development is progressing with the aim of further enhancement of functionality. Therefore, solar power generation using solar cells has been developed and put into practical use as a new energy source, and is on the path of development.

  Solar cells (solar cells) are well-known devices for converting solar radiation into electrical energy. Conventionally, a solar cell forms a pn junction by diffusing an impurity having a conductivity type opposite to that of a silicon substrate, for example, on a light receiving surface of a monocrystalline or polycrystalline silicon substrate, Products manufactured by forming electrodes on the back side on the opposite side are mainly used. In addition, in order to reduce raw material costs, silicon substrates are becoming thinner.

  In recent years, with the miniaturization, thinning, and high integration of the sun, it is necessary to insulate and protect a small area, and formation of a dense patterned protective layer and the like is required. In other words, it has become necessary to form a layer that protects against α-rays that have become more precise and have become the enemy of semiconductors, and external stresses such as pressure applied during resin molding.

  In a conventional manufacturing technique, as a method for forming a protective layer on a solar cell, it is practical to obtain a thin film by spin-coating a polyamic acid or a polyimide resin varnish for a protective film on a wafer. However, since there is a problem that a thin film cannot be formed only on a necessary portion of the wafer, a process for forming a desired pattern, for example, a photolithography technique or the like is necessary, which is complicated.

  In addition, the polyimide resin used for the polyimide layer is often a polyamic acid, and in order to process it into a polyimide, a step of heating and ring closure (imidization) (350 to 500 ° C.) is necessary. It was. Therefore, there is a problem of workability such as a large shrinkage of the resin during the imidization reaction, and it has been difficult to form the resin protective layer as a dense pattern on a thin wafer or the like.

  In addition, a method of forming a resin pattern by exposure using a polyimide resin imparted with photosensitivity has also been proposed, but the photosensitivity imparting material is limited and is expensive and may not be applied in a wet manner. There was a problem such as. Further, in the component, the generated outgas component adheres to the metal layer bonded to the electrode, which may reduce the reliability of the product, and there is often a restriction that the photosensitizing material cannot be used.

  Further, high efficiency and low price of single crystal silicon solar cells and polycrystalline silicon solar cells are important issues. In order to increase the efficiency of the cell, a back surface point contact cell having a structure in which an electrode is formed on a part of the back surface and the other part is covered with a surface passivation film such as a silicon oxide film or a silicon nitride film is promising. In order to realize this structure at low cost, a solar cell formed by using a screen printing method on a polyimide layer in a region where no electrode is provided on the back side has been proposed (Patent Document 1). On the other hand, since the polyimide layer is an organic material, there is a problem that the characteristics deteriorate when a high temperature above a certain level (about 500 ° C. or higher) is applied for a long time, and it is required to form the back electrode at a lower temperature. ing.

  In order to form an electrode, a method of depositing nickel or copper by electrolytic plating after forming a silver seed layer using a screen printing method using a silver paste is used. Since it must produce by screen printing, there exists a problem that manufacturing cost becomes high (patent document 2).

  Further, a flexible printed wiring board is also known in which a thermoplastic polyimide layer is provided on a non-thermoplastic polyimide, copper is sputtered on the surface, and then a metal layer is formed by electrolytic plating (for example, Patent Document 5). There was a problem that the adhesion strength in the fine wiring was lowered.

JP2012-38915 JP2010-108994A JP-A-4-146690 JP 2000-167980 A JP 2003-251773 A International Publication No. W099 / 19771

  In view of the above problems, the problem to be solved by the present invention is to use a lower cost manufacturing process as a crystalline silicon solar cell in which an electrode is formed on the surface opposite to the light receiving surface of the crystalline silicon substrate. It is to present a structure and manufacturing method of a crystalline silicon solar cell that can be manufactured.

The solar cell of the present invention employs the following configuration in order to solve the above problems.
(1) A crystalline silicon substrate made of single crystal silicon or polycrystalline silicon, and an insulating layer made of polyimide having a plurality of openings on the surface opposite to the light receiving surface of the crystalline silicon substrate, or an insulating layer made of an inorganic material A step of preparing a structure in which an insulating layer having a plurality of openings laminated with an insulating layer made of polyimide is formed, the insulating layer made of polyimide having a tetracarboxylic dianhydride and a carboxyl group Applying and drying a polyimide obtained by reacting diamine, and a nickel layer that becomes an electrode by an electroless plating method simultaneously on both the opening and the insulating layer made of polyimide. And a step of forming a nickel-containing metal layer.
(2) The method according to (1) , further comprising a method of forming a copper layer or a copper-containing metal layer on the nickel layer or the nickel-containing metal layer by electrolytic plating.
(3) The method according to (2) , further comprising a step of heat-treating at 100 ° C. to 300 ° C. after forming the copper layer or the copper-containing metal layer .
(4) The insulating film made of an inorganic material includes one or more of a silicon nitride film, a silicon oxide film, an aluminum oxide film, a silicon oxynitride film, and a titanium oxide film. The method described .
(5) The method according to any one of (1) to (4), wherein the carboxyl group-containing diamine is 3,3′-dicarboxy-4,4′-diaminodiphenylmethane .

  According to the present invention, in a crystalline silicon solar cell having an insulating layer made of a soluble polyimide having a carboxyl group or an insulating layer made of an inorganic material and an insulating layer made of a soluble polyimide having a carboxyl group. By using soluble polyimide to which electroless plating can be applied, it is possible to use copper, which is a lower cost material than silver, for the electrode on the non-light-receiving surface side. Thus, since it can be simultaneously formed on both the contact portion and the insulating layer made of polyimide, a highly efficient crystalline silicon solar cell can be realized while simplifying the manufacturing process.

It is drawing which shows the cross-section of the 1st form of the solar cell of this invention. It is an enlarged drawing of an electrode periphery among the cross-sectional structures of the 1st form of the solar cell of this invention. It is drawing which shows another cross-sectional structure of the 1st form of the solar cell of this invention. It is an enlarged drawing of an electrode periphery among another cross-sectional structures of the 1st form of the solar cell of this invention. It is drawing which shows the cross-section of the 2nd form of the solar cell of this invention. It is drawing which shows the cross-section of the 3rd form of the solar cell of this invention. It is drawing which shows arrangement | positioning of the back surface electrode of the 3rd form of the solar cell of this invention.

In the method for producing a solar cell of the present invention, the polyimide used for the insulating layer is directly imidized in the presence of an acid catalyst by dissolving tetracarboxylic dianhydride and a diamine having a carboxyl group in an organic solvent (that is, , Without going through a polyamic acid). It is also possible to produce an imide oligomer in which a triple bond such as an ethynyl group is introduced into the molecular chain or / and at the molecular chain end, and to form a good organic insulating layer by a heat treatment reaction during solar cell production. (The manufacturing method will be described later).

    In the present invention, since a metal layer can be formed on a polyimide insulating layer by a plating method, it is essential that the polyimide structure contains a carboxyl group. That is, using 3,5-diaminobenzoic acid and / or 3,3′-dicarboxy-4,4′-diaminodiphenylmethane as a diamine raw material for producing the polyimide resin composition has various characteristics and costs. In terms of surface.

  In addition, aromatic diamines such as 3,3'-dicarboxybenzidine and 3,5-bis (4-aminophenoxy) benzoic acid, which are not available as commercial products, are also preferable in terms of various characteristics. Moreover, although it is possible to synthesize a polyimide containing a carboxyl group by using an aliphatic diamine compound such as diaminocyclohexanecarboxylic acid or 2,3-diaminopropanoic acid, the heat resistance tends to decrease. These compounds are used alone or in combination of two or more.

  300-3000 g / mol is suitable for the quantity of the carboxyl group contained in a polyimide, ie, the carboxylic acid equivalent of a polyimide resin, and 500-2000 g / mol is more suitable. When the amount is 300 g / mol or less, the amount of carboxyl groups becomes too large, and the weight reduction due to the decarboxylation reaction of the carboxyl groups during high-temperature treatment cannot be ignored. There is a tendency that a decrease in the adhesiveness occurs. In addition, since the carboxylic acid equivalent of this case is described by the weight (unit: g / mol) of the polyimide resin per 1 mol of carboxylic acid, the smaller the number, the more carboxyl groups are included.

  If the molecular weight of the polyimide cannot be increased due to restrictions such as viscosity or solid content, heat curing can be achieved even if the initial molecular weight is low by introducing a reactive group in the polymer chain and / or at the end of the polymer chain. It is also possible to obtain various favorable physical properties.

  As the reactive group, an ethynyl group that does not remove water (condensation reaction) by thermosetting is most preferable. The ethynyl group reacts with ethynyl bonds by heating at 200 ° C. or higher, preferably 300 ° C. or higher, resulting in a double bond structure with excellent heat resistance, and a benzene ring structure (three ethynyl groups react) at high concentrations. Can be formed. Specific examples of the polyimide raw material containing an ethynyl group include 4,4 ′-(1,2-ethynyl) bisphthalic dianhydride in the case of a polymer chain, and 3-ethynylaniline in the case of a polymer end. Examples include amino compounds such as -ethynylaniline, and acid anhydrides such as 4-ethynylphthalic anhydride, 4- (phenylethynyl) phthalic anhydride, and (phenylethynyl) trimellitic anhydride. These compounds are used alone or in combination of two or more.

  In addition to thermosetting reaction by triple bond such as ethynyl bond, reaction using double bond by maleic anhydride, reaction using 1,3-dioxazolinebenzene that reacts with hydroxyl group or carboxyl group It is also possible to cause a thermosetting reaction. In particular, in the present case, since the carboxyl group is already included in the imide structure, it can be easily used. However, if a large amount of carboxyl groups are consumed by the thermosetting reaction, there is a tendency that poor formation of the metal layer at the time of plating or a decrease in adhesion with the metal layer occurs.

  On the other hand, as the tetracarboxylic dianhydride and diamine constituting the polyimide used in the present invention, other diamines and / or tetracarboxylic dianhydrides are usually used together with the diamine having the carboxyl group. Thereby, it can adjust to an appropriate carboxylic acid equivalent, and in addition, various functions, such as heat resistance, an electrical property, film | membrane physical property, and adhesiveness, can be provided.

  Such diamines include 2,4-diaminotoluene, 4,4′-diamino-2,2′-dimethyl-1,1′-biphenyl, 4,4′-diamino-2,2′-ditrifluoromethyl. -1,1'-biphenyl, 4,4'-diamino-3,3'-ditrifluoromethyl-1,1'-biphenyl, m-phenylenediamine, p-phenylenediamine, 4,4'-diamino-3, 3′-dihydroxy-1,1′-biphenyl, 4,4′-diamino-3,3′-dimethyl-1,1′-biphenyl, 9,9′-bis (3-methyl-4-aminophenyl) fluorene 3,7-diamino-dimethyldibenzothiophene 5,5-dioxide, 2,2-bis (3-hydroxy-4-aminophenyl) propane, 2,2-bis (4-aminophenyl) propa 2,2-bis (3-methyl-4-aminophenyl) propane, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl Sulfone, 4,4′-diaminodiphenyl sulfide, 2,6-diaminopyridine, 4,4 ′-(hexafluoroisopropylidene) dianiline, 2,2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane, 2,2-bis (3-amino-4-methylphenyl) hexafluoropropane, 4,4 ′-(9-fluorenylidene) dianiline, 1,3-bis (3-aminophenoxy) benzene, 1,3-bis ( 3-hydroxy-4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) Benzene, 1,4-bis (4-aminophenoxy) benzene, 2,2-bis [4- (4-aminophenoxy) phenyl] propane, 2,2-bis [4- (4-aminophenoxy) phenyl] hexa Fluoropropane, α, α-bis (4-aminophenyl) -1,3-diisopropylbenzene, α, α-bis (4-aminophenyl) -1,3-dihexafluoroisopropylidenebenzene, α, α-bis (4-aminophenyl) -1,4-diisopropylbenzene, α, α-bis (4-aminophenyl) -1,4-dihexafluoroisopropylidenebenzene, α, α-bis [4- (4-aminophenoxy) ) Phenyl] -1,3-diisopropylbenzene, α, α-bis [4- (4-aminophenoxy) phenyl] -1,4-diisopropylbenzene, etc. Can. These diamines are used alone or in combination of two or more.

  Examples of tetracarboxylic dianhydrides include pyromellitic dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, bis (3,4-dicarboxyphenyl) ether dianhydride, 3 , 3 ′, 4,4′-benzophenonetetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenylsulfonetetracarboxylic dianhydride, 2,2-bis (3,4-dicarboxyphenyl) Propane dianhydride and 4,4 ′-(4,4′-isopropylidenediphenoxy) bisphthalic dianhydride, 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride, 4, 4 ′-(1,2-ethynyl) bisphthalic dianhydride, 1,4-bis (3,4-dicarboxytrifluorophenoxy) tetrafluorobenzene dianhydride, bicyclo [2.2.2 Oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, etc., and bis (3,4-dicarboxyphenyl) ether dianhydride, ', 4,4'-biphenylsulfonetetracarboxylic dianhydride can be suitably used. These tetracarboxylic dianhydrides may be used alone or in combination of two or more.

  The polyimide contained in the polyimide resin composition of the present invention can be produced by a known synthesis method in which tetracarboxylic dianhydride and diamine are dissolved in an organic solvent and imidized directly in the presence of an acid catalyst. . Various raw materials may be added all at once or sequentially. It can be suitably used depending on the raw materials used and various characteristics. The mixing ratio of tetracarboxylic dianhydride and diamine is preferably 0.9 to 1.1 mol% of the total amount of diamine with respect to 1 mol% of the total amount of acid dianhydride. Here, as the acid catalyst, chemical imidation using a catalyst such as acetic anhydride / triethylamine or valerolactone / pyridine can be suitably used. The reaction temperature is preferably 80 to 250 ° C., and the reaction time can be appropriately selected depending on the scale of the batch, the reaction conditions employed, and the like.

  The number average molecular weight of the polyimide resin thus obtained is preferably 6000 to 60000, and more preferably 7000 to 40000. If the number average molecular weight is less than 6000, film properties such as breaking strength tend to decrease. If the number average molecular weight exceeds 60000, the viscosity increases, causing stringing problems, and a varnish suitable for printing and coating can be obtained. I want to. Here, the number average molecular weight is a polystyrene equivalent value based on a calibration curve prepared using standard polystyrene by a gel permeation chromatography (GPC) apparatus.

  Moreover, it is preferable that it is 1000-10000, and, as for the number average molecular weight in the case of the imide oligomer which has a reactive group, it is more preferable that it is 2000-8000. If the number average molecular weight is less than 1000, the curing conditions tend to be problematic, for example, the heat curing time must be lengthened. If it exceeds 10,000, the number of polymer ends decreases and the curing reaction itself is less likely to occur. There is a tendency to end up.

Since the solvent contained in the composition of the present invention is based on a screen printing method capable of forming a pattern directly at low cost, it is preferable to use a solvent having a vapor pressure of 1 mmHg or less at room temperature. Further, it is desirable that a part or more of a hydrophobic solvent is included so as not to be affected by environmental humidity during printing. Incidentally, the evaporation rate of the solvent in the ink can be measured by using a commercially available differential thermal / thermogravimetric simultaneous measuring device and observing the reduced weight. In the following examples, TG-DTA 2000S manufactured by MAC. Science Co., Ltd. was used, and measurement was performed under the condition that an N ₂ flow rate of 150 ml / min, a temperature of 40 degrees, and a sample amount of 20 μl were dropped onto a cup having an opening of 5 mmφ. Is doing.

  Specific examples of the solvent that can be used in the present invention include benzoic acid esters such as methyl benzoate and ethyl benzoate, acetates such as benzyl acetate and butyl carbitol acetate, and ethers such as diethylene glycol dibutyl ether. . By using a solvent that is hardly soluble in water, whitening (polyimide precipitation phenomenon) and viscosity change are less likely to occur due to moisture absorption particularly in screen printing. Further, when the vapor pressure at room temperature becomes 1 mmHg or more, plate thirst tends to occur in screen printing, and continuous printability tends to be inferior.

  Further, not only the hydrophobic solvent but also a hydrophilic solvent (that is, a solvent miscible with water) can be used in combination, and the evaporation rate of the ink can be controlled. Specific examples include acetic esters such as diethylene glycol monoethyl ether acetate, glymes such as triglyme and tetraglyme, ethers such as tripropylene glycol dimethyl ether and diethylene glycol diethyl ether ether, and sulfolane. In addition, the description that it is miscible with water is to clearly indicate that solvents having different vapor pressures and properties are used in combination, and is not necessarily miscible with water. However, the good solvent differs depending on the various raw materials used and the synthesized polyimide composition, and therefore, combining with an organic solvent that is hardly soluble in water is preferable because a solvent that is miscible with water has a wider range of options. Moreover, the reason why the vapor pressure at room temperature is 1 mmHg or less is the same reason as in the case of a hydrophobic solvent. By using the solubility of polyimide in each solvent and the difference in the evaporation rate of each solvent, pattern sagging during drying, etc. It is possible to avoid printing pattern defects.

  It is useful to mix two or more kinds of solvents as described above, and the mixing ratio is preferably 30% by weight to 80% by weight of the hydrophobic solvent with respect to the whole mixed solvent. If the proportion of the hydrophobic solvent is less than 30% by weight, the hydrophobicity of the solvent is not sufficiently exhibited, and this tends to cause whitening and a change in viscosity during screen printing.

  In addition, as a diluent for adjusting the evaporation rate or adjusting the viscosity at the time of preparing the resin composition, a lactone solvent such as γ-butyrolactone, a ketone solvent such as cyclohexanone, ethylene carbonate, propylene carbonate, etc. A carbonate solvent can also be used. In particular, when the pattern to be formed is sufficiently large or when continuous printability is not so much required, this is an effective method in that the solubility of polyimide is increased and the storage stability is improved. The most recommended solvent is γ-butyrolactone, which can also be used during polyimide synthesis.

  The proportion of the solid content of the polyimide resin in the composition of the present invention is preferably 15 to 60% by weight, and more preferably 25 to 50% by weight. If it is less than 15% by weight, the film thickness that can be generated by one printing and coating tends to be thin, and multiple printings and coatings tend to be required. If it exceeds 60% by weight, the viscosity of the resin composition is too high. There is a tendency to end up.

  The resin composition of the present invention has thixotropic properties as described later. Since the thixotropic property can be imparted by adding an inorganic filler, it is also an effective means to contain the inorganic filler in the resin composition of the present invention. Examples of the inorganic filler for imparting thixotropy include inorganic fillers composed of at least one of silica, alumina, and titania. Specific examples include amorphous silica of 0.01 to 0.03 μm and / or spherical silica, alumina, or titania having a particle size of 0.1 to 0.3 μm. Moreover, it is more preferable to use an inorganic filler surface-treated with a trimethylsilylating agent or the like for the purpose of enhancing storage stability. The content of the inorganic filler in the composition is usually 0 to 50% by weight, preferably 2 to 30% by weight. When the content of the inorganic filler is within this range, appropriate thixotropy is imparted.

  Moreover, if there is no influence on a product, additives, such as a coloring agent, an antifoamer, a leveling agent, an adhesiveness imparting agent, can be added to the polyimide resin composition of this invention as needed. Examples of the colorant include phthalocyanine blue, phthalocyanine green, iodine green, disazo yellow, crystal violet, titanium oxide, carbon black, naphthalene black and the like. The antifoaming agent is used to eliminate bubbles generated during printing, coating, and curing, and an acrylic or silicone surfactant is appropriately used. Specifically, BYK-A501 from BYK Chemi, DC-1400 from Dow Corning, SAG-30, FZ-328, FZ-2191, FZ-5609 from Nihon Unicar, etc. are listed as silicone-based defoamers. It is done. The leveling agent is used to lose unevenness on the surface of the film that occurs during printing and coating. Specifically, it is preferable to contain about 100 ppm to about 2% by weight of a surfactant component, and an acrylic or silicone leveling agent can suppress foaming and make the coating film smooth. Preferably, it is non-ionic without ionic impurities. Examples of suitable surfactants include FC-430 from 3M, BYK-051 from BYK Chemi, Y-5187, A-1310, and SS-2801 to 2805 from Nippon Unicar. Examples of the adhesion-imparting agent include imidazole compounds, thiazole compounds, triazole compounds, organoaluminum compounds, organotitanium compounds, and silane coupling agents. These additives are preferably blended in an amount of 10 parts by weight or less with respect to 100 parts by weight of the polyimide resin component. When the compounding amount of the additive exceeds 10 parts by weight, the physical properties of the obtained coating film tend to be lowered and the problem of contamination by volatile components also occurs. For this reason, it is most preferable not to add said additive.

  The viscosity at 25 ° C. of the polyimide resin composition of the present invention is preferably 3500 to 30000 mPa · s, more preferably 4000 mPa · s to 20000 mPa · s, and particularly preferably 6000 to 18000 mPa · s. If it is less than 3500 mPa · s, sagging or the like is likely to occur, and a sufficient film thickness and resolution cannot be obtained, and if it exceeds 40000 mPa · s, transferability and printing workability tend to be inferior. The numerical value of the viscosity of the present invention is expressed as an apparent viscosity obtained using a rheometer under the condition of a rotational speed of 333 rad / s.

  This viscosity value is an important factor not only for the shape retention immediately after coating, but also for the fluidity that easily deforms and flows with a squeegee during screen printing. In screen printing, when the viscosity is high, rolling of the resin composition is deteriorated, so that coating with a scraper becomes insufficient, and coating unevenness or scratches tend to occur.

  In addition, if the ink does not have shape retention ability to maintain the printed shape immediately after being applied to the desired pattern shape by screen printing or the like, bleeding and sagging occur in the outer periphery of the pattern, so a thick film with high resolution can be obtained. Can't get. If the viscosity is simply increased, sagging and the like can be suppressed, but a problem of plate separation and a problem of flatness of the coating film occur in screen printing. Accordingly, the thixotropy coefficient is important in order to prevent bleeding and sagging. Usually, rheometer measurement can be quantified and evaluated from the area obtained by hysteresis curve (measurement of viscosity rotation speed dependence), but it is a method of evaluating with a TI value using a more general viscometer. Is the simplest. In the present invention, the thixotropy coefficient is expressed as the apparent viscosity of the resin composition at shear rates of 33 (rad / s) and 333 (rad / s), and the ratio η33 / η333 between η33 and η333.

  The complex viscosity of the resin varnish measured at a frequency of 33 rad / s is preferably 14,000 to 120,000 mPa · s. If it exceeds 120,000 mPa · s, the paste remains in the mesh portion of the plate when screen printing is performed, and the separation of the plate tends to deteriorate.

  Therefore, the polyimide resin composition of the present invention preferably has a thixotropy coefficient (η33 / η333) at 25 ° C. in the range of 1.5 to 4.0, more preferably 1.8 to 3.5. 2.5 to 3.2 is particularly preferable. If the thixotropy coefficient is 1.5 or more, sufficient resolution can be easily obtained by screen printing. On the other hand, if the thixotropy coefficient is 4.0 or less, workability during printing is improved.

The polyimide resin composition of the present invention preferably has high wettability with silicon substrates, ceramic substrates, glass substrates, glass epoxy substrates, metal substrates typified by Ni, Cu, and Al substrates, and PI coating substrates. That is, the contact angle at room temperature is preferably 20 to 90 ° in any of silicon, SiO ₂ film, polyimide resin, ceramic, and metal surface. If it is 90 ° or less, a uniform coating film can be obtained without flares, repellencies and pinholes. If it exceeds 90 °, the resin paste will bounce on the substrate, and pinholes, patterning defects, etc. will occur. Conversely, if it is 20 ° or less, sagging occurs during leveling after coating, and the pattern accuracy tends to decrease, which is not preferable. The contact angle is such that when a droplet of the heat-resistant resin paste is dropped on various substrates, a tangent is drawn from the contact point between the droplet and the substrate, and the angle between the tangent and the substrate is defined as the contact angle. The “room temperature” mainly refers to a temperature around 25 ° C. In addition, the contact angle of a composition can be adjusted with a polyimide resin composition, a solvent, surfactant, an antifoamer, and a leveling agent.

  The insulating film in the solar cell can be formed by applying and drying the polyimide composition of the present invention on the base layer in the solar cell. As a method for applying the polyimide resin composition of the present invention, a screen printing method, a dispensing method, and an ink jet method are preferable. Particularly, the screen printing method is optimal in that a large area can be applied in a short time. With a single application, it is possible to stably form a film having a thickness after drying of 1 μm or more, preferably 2 μm or more. Considering the insulation reliability, it is desirable to obtain a thickness of at least 5 μm by one application. Therefore, in the screen printing method, a mesh plate having a wire diameter of 50 μm or less and 420 mesh or more and a resin having a rubber hardness of 70 to 90 degrees are used. It is desirable to screen print using a squeegee. The specifications of the screen plate such as the mesh diameter and the number of meshes can be appropriately selected depending on the desired film thickness and pattern size. In addition, fine lines can be drawn by the dispensing method, and it is possible to achieve that the line width of the wet coating film is within + 20% even when left at room temperature for one day as compared with the line width immediately after application. . Furthermore, it is possible to draw fine lines by the ink jet method, and it is possible to achieve that the line width of the wet coating film is within + 100% even when left at room temperature for one day as compared with the line width immediately after coating. .

  The polyimide resin composition can obtain an insulating film and a protective film having excellent electrical characteristics, heat resistance, and chemical resistance by performing leveling, vacuum drying, and final curing process after printing. The leveling is preferably performed for 10 minutes or more. The vacuum drying is preferably performed because the finish of the coating film is improved, but may not always be necessary when a leveling agent or an antifoaming agent is added. The final curing temperature and time can be appropriately selected depending on the solvent of the polyimide resin composition and the applied film thickness.

As described above, an insulating layer having a plurality of openings made of polyimide or a plurality of openings in which an insulating layer made of an inorganic material and an insulating layer made of polyimide are laminated on the surface opposite to the light receiving surface of the crystalline silicon substrate. After forming the insulating layer, an electrode having a contact with the silicon substrate is formed by plating through the plurality of openings . In particular , the electrode preferably includes a laminated structure of a seed layer made of nickel or a nickel-containing metal and copper or a copper-containing metal, and an electroless plating method is simultaneously applied to both the contact portion and the insulating layer made of polyimide. A nickel layer or a nickel-containing metal layer is formed using the same . In this case, it is preferable to form the seed layer by an electroless plating method and copper or a copper-containing metal by an electrolytic plating method. The electroless plating method and the method itself for forming a metal layer by an electrolytic plating method are well known, and the electroless plating method can be performed by a conventional method using a commercially available electroless plating solution. The method is specifically described in the following examples. In addition, it is preferable to heat-treat the entire structure at 100 ° C. to 300 ° C. for about 10 minutes to 60 minutes after electrode formation. By this heat treatment, the contact resistance between the silicon substrate and the electrode is reduced, and the adhesion between the polyimide layer and the electrode is improved.

  Hereinafter, preferred specific examples of the present invention will be described in more detail with reference to the drawings.

  The crystalline silicon substrate 1 used in the present invention may be either single crystal silicon or polycrystalline silicon. The crystalline silicon substrate 1 used in the present invention may use either p-type crystalline silicon or n-type crystalline silicon. The single crystal silicon or polycrystalline silicon used for the crystalline silicon substrate 1 may be any material, but single crystal silicon or polycrystalline silicon having a resistivity of 0.5-10 Ω · cm is desirable.

  A first embodiment of the solar cell of the present invention will be described. However, the present invention is not limited to the solar cell manufactured by the method described below.

  First, a texture structure is formed on the surface of the crystalline silicon substrate 1 (p-type single crystal silicon substrate). The texture structure may be formed on both surfaces of the substrate 1 or on one surface (light receiving surface side). In order to form the texture structure, first, the as-sliced substrate is immersed in a heated potassium hydroxide or sodium hydroxide solution to remove the damaged layer of the substrate. Then, a texture structure is formed on the surface of the substrate by dipping in a heated potassium hydroxide / isopropyl alcohol-based solution. Alternatively, the damage layer of the substrate may be removed and the surface texture may be simultaneously performed by immersing the as-sliced substrate directly in a heated potassium hydroxide / isopropyl alcohol-based solution. Alternatively, the surface texture may be formed by immersing the as-sliced substrate in a solution containing hydrofluoric acid and nitric acid.

Subsequently, after washing the substrate 1 with a solution such as hydrochloric acid or hydrofluoric acid, a phosphorus diffusion layer (n ⁺ layer) 2 is formed on the crystalline silicon substrate 1 by thermal diffusion of POCl ₃ or the like. The phosphorus diffusion layer 2 can also be formed by applying a solution containing phosphorus and performing heat treatment. The depth of the phosphorus diffusion layer 2 is preferably 0.2-1 μm, and the sheet resistance is preferably 50-100 Ω / □. Thereafter, a silicon nitride film is formed as the antireflection film 3 on the diffusion layer 2 in a thickness range of 60-100 nm. The antireflection film 3 is not limited to a silicon nitride film, and one or two types selected from silicon oxide, aluminum oxide, silicon oxynitride film, and titanium oxide are used. The silicon nitride film can be formed by a plasma CVD method, a thermal CVD method, a sputtering method, or the like, but a plasma CVD method that can be formed in a temperature range of 350 to 500 ° C. is desirable.

  On the other hand, on the non-light-receiving surface side of the crystalline silicon substrate 1, a BSF (back surface) which is a layer doped with a group III element such as aluminum or boron having the same conductivity type as the crystalline silicon substrate 1 at a higher concentration than the crystalline silicon substrate 1. field) layer 4 is formed. The BSF layer 4 is formed by applying aluminum or a paste containing boron as a main component in a predetermined shape on the back side, performing a heat treatment, and then removing the aluminum metal layer or the boron glass layer on the surface. The For the application of the aluminum paste or the paste containing boron as a main component, methods such as screen printing, ink jetting, and spin coating can be used. The electrode 5 on the light receiving surface side is formed by forming a paste mainly composed of silver on the antireflection film 3 by screen printing and performing a heat treatment (fire through). The heat treatment for producing the BSF layer 4 and the surface electrode 5 may be performed simultaneously. The BSF layer 4 can be formed on the entire back surface, or in the form of dots or lines. In FIG. 2, the BSF layer 4 is formed on a part of the silicon substrate 1 on the non-light receiving surface side, but may be formed on the entire surface of the non-light receiving surface side.

Further, a contact between the back electrode 6 (non-light receiving surface side electrode) and the BSF layer 4 is formed through a plurality of openings of the polyimide layer 7. The polyimide layer 7 can be formed by printing in a predetermined pattern having openings by screen printing or the like, but the openings may be in the form of dots or lines. The back electrode 6 is simultaneously formed on both the contact portion on the non-light-receiving surface side and the polyimide layer 7.

Here, FIG. 2 shows an enlarged view around the contact portion of the back electrode (part A in FIG. 1). It has a laminated structure of a seed layer 61 forming an ohmic contact with the silicon substrate 1 and copper or a metal layer 62 containing copper formed thereon. The seed layer also functions as a barrier layer for preventing copper from diffusing into the silicon substrate. The back electrode 6 is formed as a seed layer 61 by simultaneously forming nickel or a metal containing nickel on the silicon substrate 1 (BSF layer 4) on both the contact portion and the insulating layer made of polyimide by electroless plating. Further, the copper or a metal containing copper is simultaneously laminated on the seed layer by using an electrolytic plating method or an electroless plating method. Thereafter, heat treatment is performed to obtain ohmic contact between the seed layer made of nickel or a metal containing nickel and the silicon substrate 1 (BSF layer 4). In this case, it is desirable that nickel or a part of the metal layer 61 containing nickel forms silicide with the BSF layer 4.

  In this backside electrode configuration, nickel or nickel-containing metal layer 61 and copper or copper-containing metal layer 62 are both produced by plating, and after deposition, a low-resistance electrode at a heat treatment temperature of 500 ° C. or less. Can be produced. For this reason, the back electrode 6 can be formed without deteriorating the polyimide layer 7.

  Further, an insulating layer 8 in which one or two of a silicon nitride film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, and a titanium oxide film is laminated between the polyimide layer 7 and the silicon substrate 1 is provided. It may be formed (FIGS. 3 and 4). In this case, after the insulating layer 8 is formed, a polyimide layer 7 having a predetermined pattern is formed thereon, and the insulating layer 8 is etched with hydrofluoric acid or the like using the polyimide layer 7 as a mask to form a contact portion pattern. be able to. In this structure, it can be expected that the surface passivation effect at the interface between the silicon substrate 1 and the insulating layer 8 is further enhanced. Moreover, in this structure, although the insulating layer 8 is formed between the polyimide layer 7 and the silicon substrate 1, except for this, as illustrated in the solar cell having only the polyimide layer 7, the back electrode 6 and the structure of the BSF layer 4 may be changed.

  Further, at the same time when the back electrode 6 is formed, the nickel or nickel-containing metal and the copper or copper-containing metal are laminated on the silver electrode of the front electrode 5 to form a three-layer laminate. An electrode structure may be formed. Furthermore, in order to prevent copper oxidation, a metal containing tin may be laminated on the metal containing copper or copper by a plating method.

  Next, a second embodiment of this embodiment will be described. FIG. 5 is a drawing showing an example of a cross-sectional structure of the solar cell of the present invention. Only the structure on the light receiving surface side is different, and the same structure as that of the first embodiment may be formed on the back surface side of the crystalline silicon substrate 1.

  An example in which n-type single crystal silicon is used as the crystalline silicon substrate 1 in this embodiment will be described. First, in order to reduce reflection of incident light on the light receiving surface, a texture structure is formed on the surface of the crystalline silicon substrate 1 (only on the light receiving surface side). On the non-light-receiving surface side of the crystalline silicon substrate 1, an n-type BSF layer 4 is formed by diffusing Group V impurities such as phosphorus. On the contrary, an amorphous silicon layer is formed on the surface on the light receiving surface side by chemical vapor deposition (CVD). A heterojunction is formed between the amorphous silicon layer 9 and the crystalline silicon substrate 1. The amorphous silicon layer 9 is preferably formed on the silicon substrate 1 by forming an amorphous layer in which an undoped layer (i layer) 91 and a boron doped layer (p layer) 92 are laminated. Alternatively, the layer (p layer) 92 doped with may be formed. A transparent conductive film 10 such as indium oxide (ITO) or zinc oxide (ZnO) is formed on the amorphous silicon layer 92 by sputtering or vapor deposition. Furthermore, the surface electrode 5 (light-receiving surface side electrode) is formed on the transparent conductive film. The surface electrode is formed by printing or plating a copper or a metal containing copper or silver or a metal containing silver. As the back electrode 6 (electrode on the non-light-receiving surface side), one of the structures shown in the first embodiment can be selected. This embodiment has a solar cell structure using a heterojunction between the silicon substrate 1 and the amorphous silicon layers 91 and 91 instead of the pn junction between the silicon substrate 1 and the diffusion layer 2 of the first embodiment. .

  Furthermore, a third embodiment of the solar cell according to the present invention will be described. FIG. 6 is a drawing showing an example of a cross-sectional structure of the solar cell of the present invention.

  An example in which n-type single crystal silicon is used as the crystalline silicon substrate 1 in this embodiment will be described. A texture structure is formed only on the light receiving surface side of the crystalline silicon substrate 1. An n-type front surface field (FSF) layer 11 doped with a group V element such as phosphorus is formed on the light receiving surface side of the crystalline silicon substrate 1. On the surface of the SFS layer 11, an antireflection film 3 such as SiN (which may also serve as a surface passivation film) is formed. Note that only the antireflection film 3 may be formed except for the diffusion layer 11.

On the other hand, a p-type diffusion layer (p ⁺ layer) 2 in which boron or aluminum is diffused in a predetermined portion is formed on the non-light-receiving surface side. A pn junction is formed between the p-type diffusion layer 2 and the n-type single crystal silicon substrate 1. Further, an n-type BSF layer (n ⁺ layer) 4 is formed in a predetermined portion by diffusion of phosphorus. Here, the diffusion layer 2 and the first back electrode 63 are connected by a contact portion, and the BSF layer 4 and the second back electrode 64 are connected by a contact portion. In the place excluding the contact portion, the polyimide layer 7, or the insulating film 8 made of one or two of the silicon nitride film, silicon oxide film, aluminum oxide film, silicon oxynitride film, and titanium oxide film, and the polyimide layer 7 Is formed. The first back electrode 63 and the second back electrode 64 have a laminated structure of a seed layer 61 forming a contact with the silicon substrate 1 and copper or a metal 62 containing copper formed thereon. The seed layer 61 can be formed by plating nickel or a metal containing nickel. As shown in FIG. 7, the first electrode 63 and the second electrode 64 are formed on part of the contact portion and the insulating layer 7 made of polyimide so as not to be connected to each other. In the configuration of the third embodiment, a solar cell structure that can be manufactured at low cost can be realized by using the configuration of the present embodiment even in a back electrode type crystalline silicon solar cell.

  Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the following examples.

Example 1
1. Synthesis and synthesis of polyimide Example 1
A 2-liter separable three-necked flask equipped with a stainless steel vertical stirrer was equipped with a ball condenser equipped with a water separation trap. 148.91 g (480 mmol) of bis- (3,4-dicarboxyphenyl) ether dianhydride (ODPA), 23.86 g of 1,3-bis (3-aminopropyl) tetramethyldisiloxane (PAM-E) 96 mmol), 3,3′-dicarboxy-4,4′-diaminodiphenylmethane (MBAA) 41.22 g (144 mmol), 2,2-bis [4- (4-aminophenoxy) phenyl] propane (BAPP) 98.52 g (240 mmol), γ-valerolactone 4.8 g, pyridine 7.6 g, methyl benzoate (BAME) 385 g, γ-butyrolactone 385 g, and toluene 100 g were charged. After stirring for 30 minutes at 180 rpm in a nitrogen atmosphere at room temperature, the mixture was heated to 180 ° C. and stirred for 5 hours. During the reaction, toluene-water azeotrope was removed. By removing the reflux from the system, a 28% concentration polyimide solution was obtained. In addition, the theoretical carboxylic acid equivalent of this polyimide resin is 1025 g / mol.

Synthesis Comparative Example 1
The same apparatus as in Synthesis Example 1 was used. ODPA 148.91 g (480 mmol), PAM-E 23.86 g (96 mmol), 4,4 ′-(1,3-phenylenediisopropylidene) bisaniline (Bisaniline-M) 70.28 g (204 mmol), BAPP 73.89 g (180 mmol), 4.8 g of γ-valerolactone, 7.6 g of pyridine, 385 g of BAME, 385 g of tetraglyme, and 100 g of toluene were charged. After stirring for 30 minutes at 180 rpm in a nitrogen atmosphere at room temperature, the mixture was heated to 180 ° C. and stirred for 5 hours. During the reaction, toluene-water azeotrope was removed. By removing the reflux from the system, a 28% concentration polyimide solution was obtained.

  In this time, the electrode forming of the crystalline Si solar cell and the pretreatment method of the electroless Ni + Cu plating of the metal formed on the BSF layer, the electroless Ni + Cu plating and the plating composition are as follows.

Pretreatment method UV treatment 30mm, 2 minutes
↓
Alkaline degreasing (45 ° C, 2 minutes)
NaOH 8 g / dm ³ , sodium citrate 10 g / dm ³ , surfactant 2 g / dm ³
↓
Etching (20 ° C, 1 minute)
Phosphoric acid 5vol%, nitric acid 5vol%
↓
Conditioning (45 ° C, 1 minute)
↓

Electroless plating and electrolytic plating method Catalytic treatment (45 ° C, 3 minutes)
PdCl ₂ 0.3 g / dm ³ , HCl 0.3%
↓
Reduction-treated hypophosphorous acid 30 g / dm ³
↓
Electroless Ni plating 45 ℃ for 10 minutes (0.7μm)
↓
Electric Cu plating room temperature 3A / dm ²
3 μm
↓
Furnace 120 ℃ 1 hour

Example 2
A single crystal silicon solar cell having the structure shown in FIG. 2 was fabricated using a p-type single crystal silicon substrate (crystalline silicon substrate 1) using boron as a dopant.

After the texture of the surface of the single crystal silicon substrate 1, a phosphorus diffusion layer (diffusion layer 2) using POCl ₃ was formed. Next, as the antireflection film 3, an SiN film having a thickness of 80 nm was formed by plasma CVD. Next, a pattern made of silver paste was printed on the entire surface of the SiN film and an aluminum paste was printed on the entire back surface by screen printing, and baked at 750 ° C. As a result, a contact between the surface electrode 5 and the silicon substrate 1 (diffusion layer 2) by the silver paste on the surface was formed. At the same time, the BSF layer 4 and the back surface aluminum layer were formed from the back surface side aluminum paste. Next, the aluminum metal layer on the back side was removed with hydrochloric acid, leaving only the BSF layer 4. Thereafter, the polyimide ink capable of plating according to the present invention (Example Synthesis Example 1) was printed in a predetermined pattern by screen printing to form an insulating film layer 8 on the back surface. Next, nickel was continuously produced by pre-treatment and electroless plating described in the paragraphs 0067 and 0068, and copper was produced by electrolytic plating, and a heat treatment at 200 ° C. was applied to form the back electrode 6. At the same time, nickel and copper were formed on the silver electrode formed on the light receiving surface side, and the surface electrode 5 made of a silver / nickel / copper laminate was formed.

As a result of fabricating a single crystal silicon solar cell by the above process, a short-circuit current of 34.3 mA / cm ² , an open-circuit voltage of 0.612 V, a fill factor of 0.723, and a conversion efficiency of 15.2% were obtained. As a result, the effect of the present invention became clear.

Comparative Example 1
In the same process as described above, polyimide ink (Comparative Synthesis Example 1) was printed in a predetermined pattern by screen printing to form an insulating film layer 8 on the back surface. Next, although nickel plating was performed by the pretreatment and electroless plating, plating could not be performed.

DESCRIPTION OF SYMBOLS 1 1st conductivity type silicon substrate 2 2nd conductivity type impurity diffusion layer 3 Surface reflection preventing film 4 BSF layer 5 Front surface electrode 6 Back surface electrode 7 Insulating layer made of polyimide 8 Insulating layer made of inorganic material 9 Amorphous silicon layer 10 Transparent Conductive film 11 FSF layer 61 Seed layer of first back electrode or second back electrode 62 Metal of copper or copper of first back electrode or second back electrode 63 First back electrode 64 Second back electrode 91 Non-doped Amorphous silicon layer (i layer)
92 Second conductivity type amorphous silicon layer

Claims (5)

A crystalline silicon substrate made of single crystal silicon or polycrystalline silicon, and an insulating layer made of polyimide having a plurality of openings on the surface opposite to the light receiving surface of the crystalline silicon substrate, or made of an insulating layer made of an inorganic material and polyimide A step of preparing a structure in which an insulating layer having a plurality of openings laminated with an insulating layer is formed, and the insulating layer made of polyimide reacts with tetracarboxylic dianhydride and a diamine having a carboxyl group A nickel layer or a nickel-containing metal that becomes an electrode by an electroless plating method simultaneously on both of the step formed by applying and drying the polyimide obtained by drying, and on the opening and the insulating layer made of polyimide And a step of forming a layer. The nickel layer or nickel-containing metal layer, The method of claim 1, further comprising a method of forming a copper layer or a copper-containing metal layer by electrolytic plating. The method of Claim 2 which further includes the process of heat-processing at 100 to 300 degreeC after forming the said copper layer or a copper containing metal layer . The method according to claim 1, wherein the insulating film made of the inorganic material includes one or more of a silicon nitride film, a silicon oxide film, an aluminum oxide film, a silicon oxynitride film, and a titanium oxide film. . The method according to claim 1, wherein the carboxyl group-containing diamine is 3,3′-dicarboxy-4,4′-diaminodiphenylmethane .