JP6178621B2 - SOLAR CELL, MANUFACTURING METHOD THEREOF, AND SOLAR CELL MODULE - Google Patents

Description

  The present invention relates to a solar cell and a manufacturing method thereof. Furthermore, the present invention relates to a solar cell module.

  As energy problems and global environmental problems become more serious, solar cells are attracting attention as alternative energy alternatives to fossil fuels. In a solar cell, electric power is generated by taking out carriers (electrons and holes) generated by light irradiation to a photoelectric conversion unit made of a semiconductor junction or the like to an external circuit. In order to efficiently extract carriers generated in the photoelectric conversion unit to an external circuit, a collector electrode is provided on the photoelectric conversion unit of the solar cell.

  For example, in a crystalline silicon solar cell using a single crystal silicon substrate or a polycrystalline silicon substrate, a collector electrode made of a thin metal is provided on the light receiving surface. Further, even in a heterojunction solar cell having an amorphous silicon layer and a transparent electrode layer on a crystalline silicon substrate, a collector electrode is provided on the transparent electrode layer.

  The collector electrode of a solar cell is generally formed by pattern printing of a silver paste by a screen printing method. Although this method is simple in itself, there are problems that the material cost of silver is large and the silver paste material containing a resin is used, so that the resistivity of the collector electrode is increased. In order to reduce the resistivity of the collector electrode formed using the silver paste, it is necessary to print the silver paste thickly. However, when the printed thickness is increased, the line width of the electrode also increases, so that it is difficult to make the electrode thin, and the light shielding loss due to the collecting electrode increases.

  As a technique for solving these problems, a method of forming a collecting electrode by a plating method that is excellent in terms of material cost and process cost is known. For example, Patent Documents 1 to 3 disclose a solar cell method in which a metal layer made of copper or the like is formed on a transparent electrode constituting a photoelectric conversion unit by a plating method. In this method, first, a resist material layer (insulating layer) having an opening corresponding to the shape of the collector electrode is formed on the transparent electrode layer of the photoelectric conversion portion, and electroplating is performed on the resist opening of the transparent electrode layer. As a result, a metal layer is formed. Thereafter, the resist is removed to form a collector electrode having a predetermined shape. Patent Document 3 discloses that the line width of the plating electrode is made equal to or smaller than the base electrode layer by forming the plating electrode layer using a mask after the base electrode layer is formed.

  In Patent Document 4, an insulating layer such as SiO2 is provided on the transparent electrode, and then a groove penetrating the insulating layer is provided to expose the surface or side surface of the transparent electrode layer so as to be electrically connected to the exposed portion of the transparent electrode. Discloses a method of forming a metal collector electrode. Specifically, a method has been proposed in which a metal seed is formed on the exposed portion of the transparent electrode layer by a photoplating method or the like, and a metal electrode is formed by electroplating using the metal seed as a starting point. Such a method is more advantageous in terms of material cost and process cost because it is not necessary to use a resist as in Patent Documents 1 and 2. Moreover, by providing a low-resistance metal seed, the contact resistance between the transparent electrode layer and the collector electrode can be reduced.

  In Patent Document 5, by increasing the unevenness of the conductive seed, the entire surface of the photoelectric conversion portion other than the conductive seed is covered and a discontinuous opening is formed on the conductive seed when forming the insulating layer. In addition, it is described that a plating layer is formed through the opening.

  Patent Document 6 proposes a method in which a passivation layer (insulating layer) made of a polymer resin is formed on a transparent electrode layer, and a collector electrode made of a base electrode made of a conductive paste and a metal layer is formed thereon. The metal layer is formed on the base electrode by electroplating. In this method, the passivation layer is partially dissolved by the solvent or monomer component contained in the paste when the base electrode is formed, and electrical contact between the transparent electrode and the collector electrode is obtained. Further, since the base electrode is formed on the insulating layer, shunts and short circuits due to contact between the defective portion of the semiconductor layer and the base electrode are prevented.

  On the other hand, the collector electrode has been studied in detail from the viewpoint of environmental resistance. In Patent Document 7, Ag paste or the like is used as the collector electrode, and the concentration and molecular weight of the resin constituting the collector electrode are appropriately adjusted. As a result, Ag ions that can be generated by the penetration of moisture in the air from the gaps of the collector electrode diffuse into the semiconductor layer through pinholes existing on the surface of the photoelectric conversion part, and the reliability of the solar cell is reduced. It is described that can be suppressed. Patent Document 7 describes that it is preferable to have 70% by weight or more of a conductive substrate from the viewpoint of conductivity.

JP 60-66426 A JP 2000-58885 A JP 2010-98232 A JP 2011-199045 A Special table 2013-507781 gazette JP-A-5-63218 JP-A-6-196737

According to the method of the above-mentioned patent document 4, it is possible to form a collector electrode with a fine line pattern by plating without using an expensive resist material. In Patent Document 4, the side surface of the transparent electrode layer and the metal collecting electrode are in contact with each other in the groove penetrating the insulating layer and the transparent electrode layer. However, since the thickness of the transparent electrode layer is generally about 100 nm, Small contact area. Therefore, there is a problem that the resistance between the transparent electrode and the collector electrode is increased, and the function as the collector electrode cannot be sufficiently exhibited.

  As in Patent Document 6, the method of dissolving a part of the passivation layer has a problem that it is difficult to sufficiently reduce the contact resistance between the transparent electrode layer and the collector electrode. That is, since an insulating passivation layer is interposed between the transparent electrode layer and the collector electrode, the contact resistance is increased, and there is a problem from the viewpoint of reducing the resistance.

  Further, according to the study by the present inventors, when the second conductive layer is formed on the base layer by plating or the like, there is a place where the second conductive layer penetrates the base layer and contacts the outermost surface layer of the photoelectric conversion unit. If the amount is too large, the collector electrode peels off at the interface between the photoelectric conversion portion and the underlayer, but the second conductive layer penetrates the underlayer and partially contacts the outermost surface layer of the photoelectric conversion portion, thereby peeling off the collector electrode. It became clear that the resistance between the photoelectric conversion part and the second conductive layer can be reduced while being suppressed.

  However, in Patent Document 6, no consideration has been given to the effect of forming the collector electrode by plating or the like, or the peeling between the collector electrode and the photoelectric conversion portion. In Patent Document 5, the plating layer contacts the conductive seed deposited on the conductive surface (transparent electrode layer) through the opening of the electrically insulating layer that discontinuously covers the conductive seed. Although the structure has been studied, no investigation has been made on the reduction in resistance caused by the plating layer penetrating and contacting the conductive surface through the conductive seed.

  Patent Document 7 discusses the problem of long-term reliability degradation of solar cells that may occur due to diffusion of metal ions contained in the collector electrode due to the penetration of moisture in the air from the gap of the collector electrode. No investigation has been made on the influence of the plating solution on the collector electrode when the collector electrode is formed by plating, or on the formation of a plating layer embedded in the void portion of the collector electrode formed by printing or the like.

  An object of the present invention is to solve the problems of the prior art relating to the formation of a collector electrode of a solar cell as described above, to improve the conversion efficiency of the solar cell, and to reduce the manufacturing cost of the solar cell.

  As a result of intensive studies in view of the above problems, the present inventors have found that the conversion efficiency of the crystalline silicon solar cell can be improved by using a predetermined collector electrode, and that the collector electrode can be formed at low cost. And found the present invention.

  That is, the present invention relates to the following.

A solar cell having a photoelectric conversion part and a patterned collector electrode on one main surface of the photoelectric conversion part, the collector electrode in order from the photoelectric conversion part side, a patterned first conductive layer, A first conductive layer forming region in which the first conductive layer is formed on the first main surface of the photoelectric conversion unit, including an insulating layer in which an opening is formed and a patterned second conductive layer; A first conductive layer non-formation region where the first conductive layer is not formed, and in the first conductive layer formation region, the first conductive layer is covered with the insulating layer , and the second conductive layer is formed. some of the layers are electrically connected to the first conductive layer through the opening in the insulating layer provided between the second conductive layer and the first conductive layer, of the first Nishirube conductive layer Pass through the opening so that at least one region of the first electrode reaches the outermost surface layer on one main surface side of the photoelectric conversion unit. And that solar cells are embedded in the first conductive layer Te.

  Among the second conductive layers, when the second conductive layer A is the one embedded in the first conductive layer through the opening so as to reach the outermost surface layer on the one main surface side of the photoelectric conversion portion, It is preferable that the second conductive layer A is formed in the vicinity of the end portion of the first conductive layer.

  The second conductive layer A is preferably localized in the vicinity of the end portion of the first conductive layer.

  In the cross section parallel to the line width direction of the first conductive layer, when the width of the region where the second conductive layer A is in contact with the outermost surface layer on the one main surface side of the photoelectric conversion unit is A0, the first conductive layer It is preferable that the ratio of A0 to the layer width satisfies 0.1% to 10%.

  When the first conductive layer has a conductive material and a binder resin, and the volume ratio of the conductive material and the binder resin to the first conductive layer is V1 and V2, respectively, V1 = 70 to 90 parts, And it is preferable to satisfy | fill V2 = 10-30 parts.

  The conductive material preferably includes a low-melting-point material having a heat flow start temperature T1 lower than the heat-resistant temperature of the photoelectric conversion unit.

  The second conductive layer is preferably a plating layer.

  It is preferable that the outermost surface layer on the one principal surface side of the photoelectric conversion portion is a transparent electrode layer.

  The photoelectric conversion part has a silicon-based thin film and a transparent electrode layer as an outermost surface layer in this order on one main surface of a one-conductivity-type crystalline silicon substrate, and has the collector electrode on the transparent electrode layer. preferable.

    It is preferable that the insulating layer is also formed on the first conductive layer non-formation region of the photoelectric conversion portion.

    It is preferable to produce a solar cell module including the solar cell.

    The method for producing a solar cell of the present invention includes a first conductive layer forming step in which a first conductive layer is formed on the photoelectric conversion portion; an insulating layer forming step in which an insulating layer is formed on the first conductive layer; It is preferable to have a plating process in which a second conductive layer that is electrically connected to the first conductive layer is formed in this order by a plating method through an opening provided in the insulating layer.

    In the first conductive layer forming step, it is preferably formed by applying a coating material containing a conductive material and a binder resin on one main surface of the photoelectric conversion unit, and then curing the coating material.

    The first conductive layer includes a low melting point material having a heat flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion unit, and is higher than the heat flow start temperature T1 of the low melting point material after the insulating layer forming step. It is preferable that the opening is formed by performing a heat treatment at an annealing temperature Ta.

    The first conductive layer includes a low melting point material having a heat flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion unit, and is higher than the heat flow start temperature T1 of the low melting point material in the insulating layer forming step. It is preferable that the opening is formed simultaneously with the formation of the insulating layer by forming the insulating layer at the substrate temperature Tb.

  In the insulating layer forming step, an insulating layer is preferably formed also on the first conductive layer non-formation region of the photoelectric conversion portion.

  It is preferable that the photoelectric conversion unit includes a silicon-based thin film and a transparent electrode layer in this order on one main surface of a one-conductivity-type crystalline silicon substrate, and the collector electrode is formed on the transparent electrode layer.

  According to the present invention, since the collector electrode can be formed by a plating method, the resistance of the collector electrode is reduced, and the conversion efficiency of the solar cell can be improved. In addition, the conventional method for forming a collector electrode by a plating method requires a patterning process of an insulating layer using a resist or the like, but according to the present invention, a plating method can be used without using an expensive resist for pattern formation. It is possible to form a pattern electrode. Further, by setting the second conductive layer that penetrates the first conductive layer and reaches the outermost surface layer of the photoelectric conversion portion within a predetermined range, it is possible to reduce the resistance while suppressing the peeling of the collector electrode. Therefore, a highly efficient solar cell can be provided at low cost.

It is typical sectional drawing which shows the solar cell of this invention. It is typical sectional drawing which shows the heterojunction solar cell concerning one Embodiment. It is a conceptual diagram of the manufacturing process of the solar cell by one Embodiment of this invention. It is a conceptual diagram which shows an example of the shape change at the time of the heating of a low melting-point material. It is a conceptual diagram for demonstrating the shape change at the time of the heating of low melting-point material powder, and necking. It is a SEM photograph of metal fine particles in which sintering necking has occurred. It is a structure schematic diagram of a plating apparatus. It is typical sectional drawing which shows the collector electrode concerning one Embodiment. It is a schematic diagram of the cross section parallel to the line width direction of the collector electrode for demonstrating the edge part vicinity of a 1st conductive layer. It is a figure which shows the optical characteristic of the insulating layer in an Example.

As schematically shown in FIG. 1, the solar cell 100 of the present invention includes a collector electrode 7 on one main surface of a photoelectric conversion unit 50. The collector electrode 70 includes a first conductive layer 71 and a second conductive layer 72 in order from the photoelectric conversion unit 50 side. An insulating layer 9 having an opening is formed between the first conductive layer 71 and the second conductive layer 72. A part of the second conductive layer 72 is electrically connected to the first conductive layer 71 through the opening 9 h of the insulating layer 9.

  Hereinafter, the present invention will be described in more detail by taking, as an example, a heterojunction crystalline silicon solar cell (hereinafter sometimes referred to as a “heterojunction solar cell”) that is an embodiment of the present invention. A heterojunction solar cell is a crystalline silicon solar cell in which a diffusion potential is formed by having a silicon thin film having a band gap different from that of single crystal silicon on the surface of a single crystal silicon substrate of one conductivity type. The silicon-based thin film is preferably amorphous. Among them, a thin intrinsic amorphous silicon layer interposed between a conductive amorphous silicon thin film for forming a diffusion potential and a crystalline silicon substrate is a crystalline silicon solar cell having the highest conversion efficiency. It is known as one of the forms.

FIG. 2 is a schematic cross-sectional view of a crystalline silicon solar cell according to an embodiment of the present invention. The crystalline silicon solar cell 101 includes, as the photoelectric conversion unit 50, the conductive silicon thin film 3 a and the light incident side transparent electrode layer 6 a on one surface (light incident side surface) of the one conductivity type single crystal silicon substrate 1. In this order. It is preferable that the other surface (opposite surface on the light incident side) of the one conductivity type single crystal silicon substrate 1 has the conductivity type silicon thin film 3b and the back surface side transparent electrode layer 6b in this order. A collector electrode 7 including a first conductive layer 71 and a second conductive layer 72 is formed on the light incident side transparent electrode layer 6 a on the surface of the photoelectric conversion unit 50. An insulating layer 9 having an opening is formed between the first conductive layer 71 and the second conductive layer 72.

  It is preferable to have intrinsic silicon-based thin films 2a and 2b between the one-conductivity-type single crystal silicon substrate 1 and the conductive silicon-based thin films 3a and 3b. It is preferable to have the back metal electrode 8 on the back side transparent electrode layer 6b.

  First, the one conductivity type single crystal silicon substrate 1 in the crystalline silicon solar cell of the present invention will be described. In general, a single crystal silicon substrate contains an impurity that supplies electric charge to silicon in order to provide conductivity. Single crystal silicon substrates include an n-type in which atoms (for example, phosphorus) for introducing electrons into silicon atoms and a p-type in which atoms (for example, boron) for introducing holes into silicon atoms are contained. That is, “one conductivity type” in the present invention means either n-type or p-type.

  In heterojunction solar cells, electron / hole pairs are efficiently separated and recovered by providing a strong electric field with the heterojunction on the incident side where the most incident light is absorbed as the reverse junction. Can do. Therefore, the heterojunction on the light incident side is preferably a reverse junction. On the other hand, when holes and electrons are compared, electrons having smaller effective mass and scattering cross section generally have higher mobility. From the above viewpoint, the single crystal silicon substrate 1 used for the heterojunction solar cell is preferably an n-type single crystal silicon substrate. The single crystal silicon substrate 1 preferably has a texture structure on the surface from the viewpoint of light confinement.

A silicon-based thin film is formed on the surface of the one conductivity type single crystal silicon substrate 1 on which the texture is formed. As a method for forming a silicon-based thin film, a plasma CVD method is preferable. As conditions for forming a silicon-based thin film by plasma CVD, a substrate temperature of 100 to 300 ° C., a pressure of 20 to 2600 Pa, and a high frequency power density of 0.004 to 0.8 W / cm ² are preferably used. As a source gas used for forming a silicon-based thin film, a silicon-containing gas such as SiH ₄ or Si ₂ H ₆ or a mixed gas of a silicon-based gas and H ₂ is preferably used.

The conductive silicon thin film 3 is a one-conductivity type or reverse conductivity type silicon thin film. For example, when n-type is used as the one-conductivity-type single crystal silicon substrate 1, the one-conductivity-type silicon-based thin film and the reverse-conductivity-type silicon-based thin film are n-type and p-type, respectively. B ₂ H ₆ or PH ₃ is preferably used as the dopant gas for forming the p-type or n-type silicon-based thin film. Moreover, since the addition amount of impurities such as P and B may be small, it is preferable to use a mixed gas diluted with SiH ₄ or H ₂ in advance. When forming a conductive silicon thin film, a gas containing a different element such as CH ₄ , CO ₂ , NH ₃ , GeH ₄ is added to alloy the silicon thin film, thereby reducing the energy gap of the silicon thin film. It can also be changed.

  Examples of silicon-based thin films include amorphous silicon thin films, microcrystalline silicon (thin films containing amorphous silicon and crystalline silicon), and the like. Among these, it is preferable to use an amorphous silicon thin film. For example, as a preferable configuration of the photoelectric conversion unit 50 when an n-type single crystal silicon substrate is used as the one-conductivity-type single crystal silicon substrate 1, the transparent electrode layer 6a / p-type amorphous silicon thin film 3a / i type is used. Examples include a laminated structure in the order of amorphous silicon thin film 2a / n type single crystal silicon substrate 1 / i type amorphous silicon thin film 2b / n type amorphous silicon thin film 3b / transparent electrode layer 6b. In this case, for the reason described above, it is preferable that the p-layer side be the light incident surface.

  The intrinsic silicon thin films 2a and 2b are preferably i-type hydrogenated amorphous silicon composed of silicon and hydrogen. When i-type hydrogenated amorphous silicon is deposited on a single crystal silicon substrate by CVD, surface passivation can be effectively performed while suppressing impurity diffusion into the single crystal silicon substrate. Further, by changing the amount of hydrogen in the film, it is possible to give an effective profile to the carrier recovery in the energy gap.

  The p-type silicon thin film is preferably a p-type hydrogenated amorphous silicon layer, a p-type amorphous silicon carbide layer, or a p-type amorphous silicon oxide layer. A p-type hydrogenated amorphous silicon layer is preferable from the viewpoint of suppressing impurity diffusion and reducing the series resistance. On the other hand, the p-type amorphous silicon carbide layer and the p-type amorphous silicon oxide layer are wide gap low-refractive index layers, which are preferable in terms of reducing optical loss.

  The photoelectric conversion unit 50 of the heterojunction solar cell 101 preferably includes the transparent electrode layers 6a and 6b on the conductive silicon thin films 3a and 3b. The transparent electrode layer is formed by a transparent electrode layer forming step. The transparent electrode layers 6a and 6b are mainly composed of a conductive oxide. As the conductive oxide, for example, zinc oxide, indium oxide, or tin oxide can be used alone or in combination. From the viewpoints of conductivity, optical characteristics, and long-term reliability, an indium oxide containing indium oxide is preferable, and an indium tin oxide (ITO) as a main component is more preferably used. Here, “main component” means that the content is more than 50% by weight, preferably 70% by weight or more, and more preferably 90% by weight or more. The transparent electrode layer may be a single layer or a laminated structure composed of a plurality of layers.

  A doping agent can be added to the transparent electrode layer. For example, when zinc oxide is used as the transparent electrode layer, examples of the doping agent include aluminum, gallium, boron, silicon, and carbon. When indium oxide is used as the transparent electrode layer, examples of the doping agent include zinc, tin, titanium, tungsten, molybdenum, and silicon. When tin oxide is used as the transparent electrode layer, examples of the doping agent include fluorine.

  The doping agent can be added to one or both of the light incident side transparent electrode layer 6a and the back surface side transparent electrode layer 6b. In particular, it is preferable to add a doping agent to the light incident side transparent electrode layer 6a. By adding a doping agent to the light incident side transparent electrode layer 6a, the resistance of the transparent electrode layer itself can be reduced and resistance loss between the transparent electrode layer 6a and the collector electrode 7 can be suppressed.

  The film thickness of the light incident side transparent electrode layer 6a is preferably 10 nm or more and 140 nm or less from the viewpoints of transparency, conductivity, and light reflection reduction. The role of the transparent electrode layer 6a is to transport carriers to the collector electrode 7, and it is only necessary to have conductivity necessary for that purpose, and the film thickness is preferably 10 nm or more. By setting the film thickness to 140 nm or less, absorption loss in the transparent electrode layer 6a is small, and a decrease in photoelectric conversion efficiency accompanying a decrease in transmittance can be suppressed. Moreover, if the film thickness of the transparent electrode layer 6a is within the above range, an increase in carrier concentration in the transparent electrode layer can also be prevented, so that a decrease in photoelectric conversion efficiency due to a decrease in transmittance in the infrared region is also suppressed.

  The method for forming the transparent electrode layer is not particularly limited, but a physical vapor deposition method such as a sputtering method, a chemical vapor deposition (MOCVD) method using a reaction between an organometallic compound and oxygen or water is preferable. In any film forming method, energy by heat or plasma discharge can be used.

  The substrate temperature at the time of producing the transparent electrode layer is appropriately set. For example, when an amorphous silicon thin film is used as the silicon thin film, the temperature is preferably 200 ° C. or lower. By setting the substrate temperature to 200 ° C. or lower, desorption of hydrogen from the amorphous silicon layer and accompanying dangling bonds to silicon atoms can be suppressed, and as a result, conversion efficiency can be improved.

  It is preferable that the back surface metal electrode 8 is formed on the back surface side transparent electrode layer 6b. As the back surface metal electrode 8, it is desirable to use a material having high reflectivity from the near infrared to the infrared region and high conductivity and chemical stability. Examples of the material satisfying such characteristics include silver and aluminum. The method for forming the back surface metal electrode layer is not particularly limited, but a physical vapor deposition method such as a sputtering method or a vacuum evaporation method, a printing method such as screen printing, or the like is applicable.

A collecting electrode 7 is formed on the transparent electrode layer 6a. The collector electrode 7 includes a first conductive layer 71 and a second conductive layer 72. The first conductive layer 71 includes a conductive material. The conductive material than the heat resistant temperature of the photoelectric conversion unit has a thermal flow temperature T ₁ of the low temperature, it is preferable to contain a low melting point material.

  In the present embodiment, the insulating layer 9 having an opening is formed between the first conductive layer 71 and the second conductive layer 72. In the collector electrode 7 of the present invention, a part of the second conductive layer 72 is electrically connected to the first conductive layer 71. Here, “partially conducting” means a state in which the insulating layer is typically formed with an opening and the opening is filled with the material of the second conductive layer. In addition, when the thickness of a part of the insulating layer becomes as thin as about several nm (that is, a region having a thin film thickness is locally formed), the second conductive layer 72 becomes the first conductive layer. The thing which is conducting to 71 is also included. For example, when the low-melting-point material of the first conductive layer 71 is a metal material such as aluminum, the first conductive layer 71 and the second conductive layer are interposed via an oxide film (corresponding to an insulating layer) formed on the surface thereof. A state in which the gap is conducted is exemplified.

  A method for forming an opening for electrically connecting the first conductive layer and the second conductive layer in the insulating layer 9 is not particularly limited, and methods such as laser irradiation, mechanical drilling, and chemical etching can be employed. Moreover, the method of forming the opening part at the time of forming an insulating layer by making the surface uneven structure of the first conductive layer larger than that of the photoelectric conversion part can be used. In one embodiment, a method of forming an opening in an insulating layer formed thereon by using a low-melting-point material as the conductive material in the first conductive layer and causing the low-melting-point material to heat flow. .

  As a method of forming the opening by thermal flow of the low melting point material in the first conductive layer, after forming the insulating layer 9 on the first conductive layer 71 containing the low melting point material, the thermal flow start temperature T1 of the low melting point material. A method in which the surface shape of the first conductive layer is changed by heating (annealing) and an opening (crack) is formed in the insulating layer 9 formed thereon; or a low-melting-point material is contained. There is a method in which when the insulating layer 9 is formed on the first conductive layer 71, the low melting point material is heated to flow by heating to T1 or more, and the opening is formed simultaneously with the formation of the insulating layer.

  Hereinafter, an example of a method for forming an opening in the insulating layer using the thermal flow of the low melting point material in the first conductive layer will be described with reference to the drawings. Note that the present invention is not limited to the following embodiment.

FIG. 3 is a process conceptual diagram showing an embodiment of a method for forming the collector electrode 7 on the photoelectric conversion unit 50 of the solar cell. In this embodiment, first, the photoelectric conversion unit 50 is prepared (photoelectric conversion unit preparation step, FIG. 3A). For example, in the case of a heterojunction solar cell, as described above, a photoelectric conversion unit including a silicon-based thin film and a transparent electrode layer is prepared on one conductivity type silicon substrate.

  A first conductive layer 71 including a conductive material 710 is formed on one main surface of the photoelectric conversion portion (first conductive layer forming step, FIG. 3B). In the present embodiment, the conductive material 710 includes a low melting point material 711 and a high melting point material 713.

  As the first conductive layer, a material containing a binder resin 712 in addition to the conductive material is preferably used. In particular, the first conductive layer preferably contains the binder resin in a range of 3 to 30% by volume ratio. When the first conductive layer contains the binder resin in a suitable range, at least one region of the second conductive layer 72 is the outermost surface layer on the one main surface side of the photoelectric conversion unit 50 in the opening 9 h of the insulating layer 9. As a result, the structure embedded in the first conductive layer 71 (FIG. 3E) can be easily formed, and as a result, the solar cell characteristics can be improved (details will be described later).

An insulating layer 9 is formed on the first conductive layer 71 (insulating layer forming step, FIG. 3C). The insulating layer 9 may be formed only on the first conductive layer 71, and is also formed on a region where the first conductive layer 71 of the photoelectric conversion unit 50 is not formed (first conductive layer non-formation region). It may be. In particular, when a transparent electrode layer is formed on the surface of the photoelectric conversion unit 50 as in a heterojunction solar cell, the insulating layer 9 is preferably formed also on the first conductive layer non-formation region.
After the insulating layer is formed, an annealing process by heating is performed (annealing process, FIG. 3D). Due to the annealing treatment, the first conductive layer 71 is heated to the annealing temperature Ta, and the low melting point material is heat-fluidized to change the surface shape, and accordingly, the insulating layer 9 formed on the first conductive layer 71 is deformed. Occurs. The deformation of the insulating layer 9 is typically the formation of an opening 9h in the insulating layer. The opening 9h is formed in a crack shape, for example.

  After forming an opening in the insulating layer by annealing, a second conductive layer 72 is formed by a plating method (plating step, FIG. 3E). Although the first conductive layer 71 is covered with the insulating layer 9, the first conductive layer 71 is exposed at a portion where the opening 9 h is formed in the insulating layer 9. Therefore, the first conductive layer is exposed to the plating solution, and metal can be deposited starting from the opening 9h. According to such a method, the second conductive layer corresponding to the shape of the collector electrode can be formed by plating without providing a resist material layer having an opening corresponding to the shape of the collector electrode.

  As described above, a part of the second conductive layer 72 is electrically connected to the first conductive layer through the opening 9h of the insulating layer on the first conductive layer. In the present invention, at least one region of the second conductive layer that is conducted to the first conductive layer through the opening 9h is at least one region on the main surface side of the photoelectric conversion unit 50 as shown in FIG. It is embedded in the first conductive layer 71 so as to reach the surface layer. Among the second conductive layers, the one embedded in the first conductive layer through the opening so as to reach the outermost surface layer on the one main surface side of the photoelectric conversion portion is referred to as “second conductive layer A” (in the drawing) The code is 72a).

  Further, in the opening 9 h of the insulating layer 9, the opening in which the second conductive layer A is formed (that is, the second embedded in the first conductive layer and in contact with the outermost surface layer on the one main surface side of the photoelectric conversion unit). The opening in which the conductive layer A is formed is defined as an opening 9ha (outermost surface layer connection opening 9ha).

  By adopting a structure in which the second conductive layer is embedded in the first conductive layer, the adhesion strength at the interface between the first conductive layer and the second conductive layer is improved, and peeling of the second conductive layer can be prevented. . In addition, the second conductive layer embedded in the first conductive layer reaches the outermost surface layer on the one main surface side of the photoelectric conversion unit, so that the solar cell is in contact with the photoelectric conversion unit during operation (during power generation). Since the current flows directly between the photoelectric conversion portion and the second conductive layer without going through the first conductive layer where the resistance tends to be high, the solar cell characteristics can be further improved.

  In particular, in the cross section parallel to the line width direction of the first conductive layer, when the width of the region where the second conductive layer A is in contact with the outermost surface layer on the one principal surface side of the photoelectric conversion unit is A0, The ratio of A0 to the line width (L1) of the conductive layer is preferably included in 0.1 to 10%. By forming the second conductive layer A in the above range, it is possible to suppress the peeling of the collector electrode as compared with the case where the second conductive layer A is formed in the majority of the line width of the first conductive layer. .

  In addition, since the formation region of the second conductive layer A is localized in the vicinity of the end of the first conductive layer, the peeling of the second conductive layer can be further suppressed and the peeling between the photoelectric conversion unit and the second conductive layer can be prevented while preventing the peeling. Resistance can be reduced. From the viewpoint of efficient carrier extraction, the region where the second conductive layer A is formed is more preferably near the end of the first conductive layer.

(First conductive layer)
The first conductive layer 71 is a layer that functions as a conductive underlayer when the second conductive layer is formed by a plating method. Therefore, the first conductive layer only needs to have conductivity that can function as a base layer for electrolytic plating. In the present specification, it is defined as being conductive if the volume resistivity is 10 ⁻² Ω · cm or less. Further, if the volume resistivity is 10 ² Ω · cm or more, it is defined as insulating.

  As described above, the first conductive layer is preferably formed of the conductive material 710 and the binder resin 712. In order to sufficiently improve the conductivity of the first conductive layer formed by the screen printing method, it is desirable to cure the first conductive layer by heat treatment or exposure treatment. Therefore, when heat treatment is performed, it is preferable to use a material that can be cured at the above drying temperature as the binder resin, and an epoxy resin, a phenol resin, an acrylic resin, or the like is applicable. When such a binder resin 712 and a conductive material 710 having a low-melting-point material 711 are used, the shape of the low-melting-point material changes with curing, and as shown in FIG. This is because an opening (crack) is likely to occur in the insulating layer near the material. Note that the ratio between the binder resin and the conductive material may be set to be equal to or higher than a so-called percolation threshold (a critical value of a ratio corresponding to the content of the conductive material at which the conductivity develops).

  Here, as will be described later, according to the study by the present inventors, the second conductive layer is embedded in the first conductive layer, and the region in contact with the outermost surface layer of the photoelectric conversion unit is appropriately set within a predetermined range. Thus, it has been clarified that the solar cell characteristics can be improved while the peeling between the collector electrode and the photoelectric conversion portion is suppressed. That is, when the plating solution reaches the inside of the first conductive layer and the photoelectric conversion part below the first conductive layer without limitation, problems such as peeling of the collecting electrode from the photoelectric conversion part and deterioration of reliability occur. However, as in the present invention, the second conductive layer is formed so as to penetrate the first conductive layer and reach the surface of the photoelectric conversion part only within a predetermined range, and the peeling is suppressed while being suppressed. It became clear that resistance was possible.

  The formation method and formation range of the second conductive layer A in contact with the outermost surface layer of the photoelectric conversion part are not particularly limited, but it is preferable to appropriately set the material or conditions for forming the first conductive layer.

  As the first conductive layer, a layer containing a conductive material and a binder resin is preferably used. The volume ratio of the binder resin to the conductive paste forming the first conductive layer is preferably in the range of 3 to 30%, and the volume ratio of the conductive material to the entire conductive paste is preferably in the range of 70 to 97%. .

  By setting the volume ratio of the binder resin to 3% or more, peeling from the photoelectric conversion portion of the collector electrode can be further suppressed. That is, by setting the volume ratio of the binder resin contained in the first conductive layer within the above range, voids are less likely to occur in the first conductive layer, and when the first conductive layer comes into contact with the plating solution, the first conductive layer It is considered that the plating solution can be prevented from reaching the inside of the first conductive layer and the photoelectric conversion portion below the first conductive layer through the gap. In this case, it is considered that the second conductive layer A reaching the outermost surface layer on the one main surface side of the photoelectric conversion part can be easily set within a predetermined range.

  Moreover, it is thought that the increase in resistance of the first conductive layer can be suppressed by setting the volume ratio of the binder resin to 30% or less. In addition, when the second conductive layer is formed by the plating method, it is considered that defects during plating such as non-uniform plating deposition due to a voltage drop in the first conductive layer can be suppressed. Among these, from the viewpoint of more reliably preventing the peeling of the collector electrode and ensuring high conductivity, more preferably, the volume ratio of the binder resin to the entire conductive paste is in the range of 5 to 20%. The volume ratio with respect to the whole conductive paste is in the range of 80 to 95%.

  The first conductive layer can be easily formed by curing with heat or light using the binder resin and conductive material (first conductive layer material) having a volume ratio as described above. In general, by setting the volume ratio of the first conductive layer material within the above range, the volume ratio between the conductive material and the binder resin after forming the first conductive layer can be easily set within the above range.

  The film thickness of the first conductive layer 71 is preferably 20 μm or less from the viewpoint of cost, and more preferably 10 μm or less. On the other hand, from the viewpoint of setting the line resistance of the first conductive layer 71 in a desired range, the film thickness is preferably 0.5 μm or more, and more preferably 1 μm or more.

  Here, when a conductive paste or the like is usually used as the first conductive layer, the resistance increases due to the paste material. Therefore, it is necessary to increase the thickness from the viewpoint of reducing resistance. On the other hand, in the present invention, as described above, the second conductive layer is formed in addition to the first conductive layer, and the second conductive layer A reaching the outermost surface layer of the photoelectric conversion portion is formed, thereby reducing the low resistance. Therefore, the thickness of the first conductive layer can be reduced. Therefore, it is preferable from the viewpoint of further reducing the production cost.

The first conductive layer 71 includes a conductive material. It does not specifically limit as a conductive material, For example, silver, copper, aluminum, etc. can be used. Etc. can be used. Conductive material preferably comprises a low melting point material of the heat flow temperature T _1. The heat flow start temperature is a temperature at which the material causes heat flow by heating and the surface shape of the layer containing the low melting point material changes, and is typically the melting point. In the case of a polymer material or glass, the material may soften at a temperature lower than the melting point to cause heat flow. In such a material, it can be defined that heat flow start temperature = softening point. The softening point is a temperature at which the viscosity becomes 4.5 × 10 ⁶ Pa · s (the same as the definition of the softening point of glass).

The low melting point material is preferably a material that causes heat flow in the annealing process and changes the surface shape of the first conductive layer 71. Therefore, the thermal flow temperature T ₁ of the low-melting material is preferred over the annealing temperature Ta is low. In the present invention, the annealing process is preferably performed at an annealing temperature Ta lower than the heat resistant temperature of the photoelectric conversion unit 50. Therefore, the heat flow temperature T ₁ of the low melting point material, it is preferred to heat the temperature of the photoelectric conversion unit is cold.

The heat-resistant temperature of the photoelectric conversion unit is irreversibly reduced in the characteristics of a solar cell including the photoelectric conversion unit (also referred to as “solar battery cell” or “cell”) or a solar battery module manufactured using the solar battery cell. Temperature. For example, in the heterojunction solar cell 101 shown in FIG. 2, the single crystal silicon substrate 1 constituting the photoelectric conversion unit 50 hardly changes its characteristics even when heated to a high temperature of 500 ° C. or higher. When the amorphous silicon-based thin films 2 and 3 are heated to about 250 ° C., thermal deterioration or diffusion of doped impurities may occur, resulting in irreversible deterioration of solar cell characteristics. Therefore, in the heterojunction solar cell, the first conductive layer 71 is preferably heat flow temperature T ₁ is comprises a low melting point material 250 ° C. or less.

The lower limit of the thermal flow temperature T ₁ of the low melting point material is not particularly limited. From the viewpoint of easily forming the opening 9h in the insulating layer 9 by increasing the amount of change in the surface shape of the first conductive layer during the annealing treatment, the low melting point material is thermally flowable in the first conductive layer forming step. It is preferable not to produce. For example, when the first conductive layer is formed by coating or printing, heating may be performed for drying. In this case, the heat flow temperature T ₁ of the low-melting material is preferred over the heating temperature for the drying of the first conductive layer is a high temperature. From this viewpoint, the heat flow temperature T ₁ of the low melting point materials is preferably at least 80 ° C., more preferably at least 100 ° C..

Low melting point material, if the heat flow temperature T ₁ is the above-mentioned range, be organic, it may be inorganic. The low melting point material may be electrically conductive or insulating, but is preferably a metal material having conductivity. If the low-melting-point material is a metal material, the resistance value of the first conductive layer can be reduced. Therefore, when the second conductive layer is formed by electroplating, the uniformity of the film thickness of the second conductive layer can be improved. it can. If the low melting point material is a metal material, the contact resistance between the photoelectric conversion unit 50 and the collector electrode 7 can be reduced.

  As the low melting point material, a simple substance or an alloy of a low melting point metal material or a mixture of a plurality of low melting point metal materials can be suitably used. Examples of the low melting point metal material include indium, bismuth, and gallium.

The first conductive layer 71, as a conductive material, in addition to the low melting point material preferably contains a refractory material having a thermal flow temperature T ₂ of the relatively high temperature than the low-melting-point material. Since the first conductive layer 71 includes the high melting point material, the first conductive layer and the second conductive layer can be efficiently conducted, and the conversion efficiency of the solar cell can be improved. For example, when a material having a large surface energy is used as the low melting point material, when the first conductive layer 71 is exposed to a high temperature by the annealing process and the low melting point material enters a liquid phase state, as conceptually shown in FIG. In some cases, the particles of the low-melting-point material are aggregated to become coarse particles, and the first conductive layer 71 may be disconnected. On the other hand, since the high melting point material does not enter a liquid phase state even when heated during the annealing process, the low melting point material as shown in FIG. 4 can be obtained by including the high melting point material in the first conductive layer forming material. Disconnection of the first conductive layer due to coarsening can be suppressed.

Heat flow temperature T ₂ of the high-melting material is preferably higher than the annealing temperature Ta. That is, when the first conductive layer 71 contains a low melting point material and a high melting point material, the heat flow starting temperature T _{1 of} the low melting point material, the heat flow starting temperature T _{2 of} the high melting point material, and the annealing temperature Ta in the annealing process are: , T ₁ <Ta <T ₂ is preferably satisfied. The high melting point material may be an insulating material or a conductive material, but a conductive material is preferable from the viewpoint of reducing the resistance of the first conductive layer. When the low melting point material has low conductivity, the resistance of the first conductive layer as a whole can be reduced by using a material having high conductivity as the high melting point material. As the conductive high melting point material, for example, a single metal material such as silver, aluminum, copper, or a plurality of metal materials can be preferably used.

  When the first conductive layer 71 contains a low-melting-point material and a high-melting-point material, the content ratio is to suppress disconnection due to the coarsening of the low-melting-point material as described above, to the conductivity of the first conductive layer, to the insulating layer. From the standpoint of easiness of forming the opening (increase in the number of starting points of metal deposition of the second conductive layer) and the like, it is appropriately adjusted. The optimum value varies depending on the material used and the combination of particle sizes. For example, the weight ratio of the low melting point material to the high melting point material (low melting point material: high melting point material) is 5:95 to 67:33. It is a range. The weight ratio of the low melting point material: the high melting point material is more preferably 10:90 to 50:50, and further preferably 15:85 to 35:65.

For example, when a particulate low-melting-point material such as metal particles is used as the material of the first conductive layer 71, the particle diameter D _{L of the} low-melting-point material is used from the viewpoint of facilitating formation of an opening in the insulating layer by annealing. Is preferably 1/20 or more of the film thickness d of the first conductive layer, more preferably 1/10 or more. Particle size _{D L} of the low-melting material, more preferably 0.25 [mu] m, more preferably not less than 0.5 [mu] m. When the first conductive layer 71 is formed by a printing method such as screen printing, the particle size of the particles can be set as appropriate according to the mesh size of the screen plate. For example, the particle size is preferably smaller than the mesh size, and more preferably ½ or less of the mesh size. When the particles are non-spherical, the particle size is defined by the diameter of a circle having the same area as the projected area of the particles (projected area circle equivalent diameter, Heywood diameter).

The shape of the particles of the low melting point material is not particularly limited, but a non-spherical shape such as a flat shape is preferable. In addition, non-spherical particles obtained by combining spherical particles by a technique such as sintering are also preferably used. Generally, when the metal particles are in a liquid phase, the surface shape tends to be spherical in order to reduce the surface energy. If low melting point material of the first conductive layer before annealing is non-spherical, the annealing is heated in heat flow starting temperature above T _1, since the particles approaches the spherical shape, the surface shape of the first conductive layer The amount of change is greater. Therefore, it is easy to form an opening in the insulating layer 9 on the first conductive layer 71.

As described above, the first conductive layer 71 is conductive, and the volume resistivity may be 10 ⁻² Ω · cm or less. The volume resistivity of the first conductive layer 71 is preferably 10 ⁻⁴ Ω · cm or less. When the first conductive layer has only the low melting point material, the low melting point material only needs to have conductivity. In the case where the first conductive layer contains a low melting point material and a high melting point material, at least one of the low melting point material and the high melting point material may be conductive. For example, the combination of low melting point material / high melting point material includes insulation / conductivity, conductivity / insulation, conductivity / conductivity. In order to make the first conductive layer have a lower resistance, Both the low melting point material and the high melting point material are preferably conductive materials.

In addition to the combination of the low-melting-point material and the high-melting-point material as described above as the material for the first conductive layer 71, by adjusting the size (for example, particle size) of the material, the first conductive layer 71 is heated by the annealing process. It is also possible to suppress disconnection of the conductive layer and improve conversion efficiency. For example, if a material having a high melting point such as silver, copper, gold or the like is fine particles having a particle size of 1 μm or less, sintering necking (particulate particles) at a temperature T ₁ ′ of about 200 ° C. or lower than the melting point Therefore, it can be used as the “low melting point material” of the present invention. When the material that causes such sintering necking is heated to the sintering necking start temperature T ₁ ′ or higher, deformation occurs in the vicinity of the outer periphery of the fine particles, so that the surface shape of the first conductive layer is changed, and the insulating layer An opening can be formed in 9. Further, even when the fine particles are heated to a temperature higher than the sintering necking start temperature, the fine particles maintain the solid state if the temperature is lower than the melting point T ₂ ′. Disconnection due to is difficult to occur. That is, it can be said that a material that causes sintering necking such as metal fine particles is a “low melting point material” in the present invention, but also has a side surface as a “high melting point material”.

In a material that causes such sintering necking, it can be defined that sintering necking start temperature T ₁ ′ = heat flow start temperature T ₁ . FIG. 5 is a diagram for explaining the sintering necking start temperature. FIG. 5A is a plan view schematically showing the particles before sintering. Since they are not sintered, the particles are in point contact with each other. FIG. 5B and FIG. 5C are cross-sectional views schematically showing a state where the particles after sintering are cut along a cross section passing through the center of each particle. FIG. 5 (B) shows the state after the start of sintering (sintering initial stage), and FIG. 5 (C) shows the state where the sintering has progressed from (B). In FIG. 5B, the grain boundary between the particle A (radius r _A ) and the particle B (radius r _B ) is indicated by a dotted line having a length a _AB .

Sintering necking onset temperature _{T 1} ', the ratio of _{r A} and _r larger value max _(r A, _{r B)} of the _B and the grain boundary between the length _{a AB, a AB / max (} r A, r _B ) is defined as the temperature at which it becomes 0.1 or more. That is, the temperature at which a _AB / max (r _A , r _B ) of at least a pair of particles is 0.1 or more is referred to as a sintering necking start temperature. In FIG. 6, for the sake of simplicity, the particles are shown as spherical, but when the particles are not spherical, the radius of curvature of the particles near the grain boundary is regarded as the radius of the particles. When the radius of curvature of the particle near the grain boundary varies depending on the location, the largest radius of curvature among the measurement points is regarded as the radius of the particle. For example, as shown in FIG. _6A, a grain boundary having a length of _AB is formed between a pair of fine particles A and B that have been sintered. In this case, the shape of the particle A in the vicinity of the grain boundary is approximated by an arc of a virtual circle A indicated by a dotted line. On the other hand, the grain boundaries near the particle B, one is approximated by an arc of a virtual circle B ₁ indicated by broken lines, and the other is approximated by an arc of a virtual circle B ₂ indicated by a solid line. As shown in FIG. 6B, since r _B2 > r _B1 , r _B2 is regarded as the radius r _{B of the} particle B. Note that the above virtual circle is determined by a method in which the boundary is defined by black and white binarization processing of the observation image of the cross section or surface, and the center coordinates and radius are calculated by the least square method based on the coordinates of the boundary in the vicinity of the grain boundary. it can. If it is difficult to strictly measure the sintering necking start temperature according to the above definition, the first conductive layer containing fine particles is formed, and the temperature at which an opening (crack) is generated in the insulating layer by heating is set. It can be regarded as the sintering necking start temperature. As will be described later, when heating is performed during the formation of the insulating layer, the temperature at which an opening (crack) is generated by heating the substrate during the formation of the insulating layer can be regarded as the firing necking start temperature.

  As described above, as the conductive material of the first conductive layer, in addition to the material having a low melting point material, for example, a material having no low melting point material (for example, only the above high melting point material) may be used. it can. Even if it does not have a low melting point material, as described above, after forming an insulating layer so as to cover the first conductive layer, a method of separately forming an opening in the insulating layer, or the first conductive layer The opening is formed in the insulating layer on the first conductive layer by, for example, forming the opening when forming the insulating layer by making the surface uneven structure of the electrode larger than the surface uneven structure of the photoelectric conversion portion. be able to.

  The material for forming the first conductive layer preferably includes an insulating material in addition to the above-described conductive material (for example, a low melting point material and / or a high melting point material). As the insulating material, a paste containing a binder resin or the like can be preferably used. In order to sufficiently improve the conductivity of the first conductive layer formed by the screen printing method, it is desirable to cure the first conductive layer by heat treatment. Therefore, as the binder resin contained in the paste, it is preferable to use a material that can be cured at the drying temperature, and an epoxy resin, a phenol resin, an acrylic resin, or the like is applicable.

  When a conductive material containing a low-melting-point material is used, the shape of the low-melting-point material changes with the curing of the binder resin, and as shown in FIG. Openings (cracks) are likely to occur. Note that the ratio between the binder resin and the conductive material may be set to be equal to or higher than a so-called percolation threshold (a critical value of a ratio corresponding to the content of the conductive material at which the conductivity develops).

The first conductive layer 71 can be produced by a known technique such as an inkjet method, a screen printing method, a conductive wire bonding method, a spray method, a vacuum deposition method, or a sputtering method. The first conductive layer 71 that is patterned in a predetermined shape of the comb or the like. A screen printing method is suitable for forming the patterned first conductive layer from the viewpoint of productivity. In the screen printing method, a method of printing a collecting electrode pattern using a printing paste containing a conductive material and a screen plate having an opening pattern corresponding to the pattern shape of the collecting electrode is preferably used.

On the other hand, when a material containing a solvent is used as the printing paste, a drying step for removing the solvent is required. As described above, the drying temperature in this case is preferably lower than the heat resistant temperature of the photoelectric conversion part. For example, when the photoelectric conversion part has a transparent electrode layer or an amorphous silicon thin film, the drying temperature is preferably 250 ° C. or lower, more preferably 200 ° C. or lower, and 180 ° C. or lower. Further preferred. It is preferable than the heat flow temperature T ₁ of the low melting point material is a low temperature. The drying time can be appropriately set, for example, from about 5 minutes to 1 hour.

  As mentioned above, although demonstrated centering on the case where a 1st conductive layer is formed by the printing method, the formation method of a 1st conductive layer is not limited to a printing method. For example, the first conductive layer may be formed by vapor deposition or sputtering using a mask corresponding to the pattern shape. Further, the pattern may be formed by an ink jet method or the like.

Insulation layer)
An insulating layer 9 is formed on the first conductive layer 71 . Since the first one conductivity layer 71 is formed in a predetermined pattern (e.g. comb), on the surface of the photoelectric conversion unit 50 includes a first conductive layer forming region where the first conductive layer is formed, the first conductive There is a first conductive layer non-formation region in which no layer is formed. The insulating layer 9 is formed at least in the first conductive layer formation region. In the present invention, the insulating layer 9 is preferably formed also on the first conductive layer non-formation region, and particularly preferably formed on the entire surface of the first conductive layer non-formation region. When the insulating layer is also formed in the region where the first conductive layer is not formed, the photoelectric conversion part can be protected chemically and electrically from the plating solution when the second conductive layer is formed by plating. It becomes. For example, when a transparent electrode layer is formed on the surface of the photoelectric conversion unit 50 like a heterojunction solar cell, an insulating layer is formed on the surface of the transparent electrode layer, so that the transparent electrode layer and the plating solution Contact is suppressed and precipitation of the metal layer (second conductive layer) on the transparent electrode layer can be prevented. Also, from the viewpoint of productivity, it is more preferable that the insulating layer is formed in the entire first conductive layer formation region and the first conductive layer non-formation region.

  As the material of the insulating layer 9, a material that exhibits electrical insulation is used. The insulating layer 9 is preferably a material having chemical stability with respect to the plating solution. By using a material having high chemical stability with respect to the plating solution, the insulating layer is hardly dissolved during the plating step when forming the second conductive layer, and damage to the surface of the photoelectric conversion portion is less likely to occur. Moreover, when the insulating layer 9 is formed also on the 1st conductive layer non-formation area | region, it is preferable that an insulating layer has a large adhesion strength with the photoelectric conversion part 50. FIG. For example, in the heterojunction solar cell, the insulating layer 9 preferably has a high adhesion strength with the transparent electrode layer 6a on the surface of the photoelectric conversion unit 50. By increasing the adhesion strength between the transparent electrode layer and the insulating layer, it becomes difficult for the insulating layer to be peeled off during the plating step, and metal deposition on the transparent electrode layer can be prevented.

  For the insulating layer 9, it is preferable to use a material with little light absorption. Since the insulating layer 9 is formed on the light incident surface side of the photoelectric conversion unit 50, more light can be taken into the photoelectric conversion unit if light absorption by the insulating layer is small. For example, when the insulating layer 9 has sufficient transparency with a transmittance of 90% or more, the optical loss due to light absorption in the insulating layer is small, and without removing the insulating layer after forming the second conductive layer, the solar Can be used as a battery. Therefore, the manufacturing process of a solar cell can be simplified and productivity can be further improved. When the insulating layer 9 is used as it is as a solar cell without being removed, the insulating layer 9 is more preferably made of a material having sufficient weather resistance and stability against heat and humidity in addition to transparency. .

The material of the insulating layer may be an inorganic insulating material or an organic insulating material. As the inorganic insulating material, for example, materials such as silicon oxide, silicon nitride, titanium oxide, aluminum oxide, magnesium oxide, and zinc oxide can be used. As the organic insulating material, for example, materials such as polyester, ethylene vinyl acetate copolymer, acrylic, epoxy, and polyurethane can be used. From the viewpoint of facilitating the formation of an opening in the insulating layer due to interface stress caused by the change in the surface shape of the first conductive layer in the annealing treatment, the material of the insulating layer is an inorganic material having a small breaking elongation. It is preferable. Among these inorganic materials, from the viewpoint of plating solution resistance and transparency, silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, sialon (SiAlON), yttrium oxide, magnesium oxide, barium titanate, samarium oxide, Barium tantalate _, tantalum oxide, magnesium fluoride, titanium oxide, strontium titanate and the like are preferably used. Among these, from the viewpoint of electrical properties and adhesion to the transparent electrode layer, etc., silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, sialon (SiAlON), yttrium oxide, magnesium oxide, barium titanate, samarium oxide, Barium tantalate _, tantalum oxide, magnesium fluoride, and the like are preferable, and silicon oxide, silicon nitride, and the like are particularly preferably used from the viewpoint that the refractive index can be appropriately adjusted. These inorganic materials are not limited to those having a stoichiometric composition, and may include oxygen deficiency or the like.

  The film thickness of the insulating layer 9 is appropriately set according to the material and forming method of the insulating layer. When a material containing a low-melting-point material is used as the first conductive layer, the thickness of the insulating layer 9 is such that the insulating layer 9 opens to the insulating layer due to interface stress or the like caused by a change in the surface shape of the first conductive layer in the annealing process It is preferable that the portion is thin enough to be formed. From this viewpoint, the thickness of the insulating layer 9 is preferably 1000 nm or less, and more preferably 500 nm or less. In addition, by appropriately setting the optical characteristics and film thickness of the insulating layer 9 in the first conductive layer non-forming portion, the light reflection characteristics are improved, the amount of light introduced into the solar cell is increased, and the conversion efficiency is further improved. It becomes possible to improve. In order to obtain such an effect, the refractive index of the insulating layer 9 is preferably lower than the refractive index of the surface of the photoelectric conversion unit 50. Further, from the viewpoint of imparting suitable antireflection properties to the insulating layer 9, the film thickness is preferably set within a range of 30 nm to 250 nm, and more preferably within a range of 50 nm to 250 nm. The film thickness of the insulating layer on the first conductive layer forming region and the film thickness of the insulating layer on the first conductive layer non-forming region may be different. For example, in the first conductive layer formation region, the thickness of the insulating layer is set from the viewpoint of facilitating the formation of the opening by annealing, and in the first conductive layer non-formation region, an optical film having appropriate antireflection characteristics The film thickness of the insulating layer may be set to be thick.

  When a transparent electrode layer (generally having a refractive index of about 1.9 to 2.1) is provided on the surface of the photoelectric conversion unit 50 as in a heterojunction solar cell, the effect of preventing light reflection at the interface is enhanced and the solar cell. In order to increase the amount of light introduced into the inside, the refractive index of the insulating layer is preferably an intermediate value between air (refractive index = 1.0) and the transparent electrode layer. Moreover, when a photovoltaic cell is sealed and modularized, it is preferable that the refractive index of an insulating layer is an intermediate value of a sealing agent and a transparent electrode layer. From this viewpoint, the refractive index of the insulating layer 9 is preferably, for example, 1.4 to 1.9, more preferably 1.5 to 1.8, and further preferably 1.55 to 1.75. The refractive index of the insulating layer can be adjusted to a desired range depending on the material, composition, etc. of the insulating layer. For example, in the case of silicon oxide, the refractive index is increased by reducing the oxygen content. In addition, unless otherwise indicated, the refractive index in this specification is a refractive index with respect to the light of wavelength 550nm, and is a value measured by spectroscopic ellipsometry. Further, it is preferable that the optical film thickness (refractive index × film thickness) of the insulating layer is set so as to improve the antireflection characteristics according to the refractive index of the insulating layer.

  The insulating layer can be formed using a known method. For example, in the case of an inorganic insulating material such as silicon oxide or silicon nitride, a dry method such as a plasma CVD method or a sputtering method is preferably used. In the case of an organic insulating material, a wet method such as a spin coating method or a screen printing method is preferably used. According to these methods, it is possible to form a dense film with few defects such as pinholes.

  Among these, from the viewpoint of forming a film having a denser structure, the insulating layer 9 is preferably formed by a plasma CVD method. By this method, not only a thick film with a thickness of about 200 nm but also a thin insulating film with a thickness of about 30 to 100 nm can be formed.

  For example, in the case of having a texture structure (uneven structure) on the surface of the photoelectric conversion portion 50 as in the crystalline silicon solar cell shown in FIG. The layer is preferably formed by a plasma CVD method. By using a highly dense insulating layer, it is possible to reduce damage to the transparent electrode layer during the plating process and to prevent metal deposition on the transparent electrode layer. Such a highly dense insulating film functions as a barrier layer for water, oxygen, and the like for the layer inside the photoelectric conversion unit 50 as in the silicon thin film 3 in the crystalline silicon solar cell of FIG. Therefore, the effect of improving the long-term reliability of the solar cell can be expected.

  Note that the shape of the insulating layer 9 between the first conductive layer 71 and the second conductive layer 72, that is, the insulating layer 9 on the first conductive layer forming region, is not necessarily a continuous layer shape, and is an island shape. It may be. Note that the term “island” in this specification means a state in which a part of the surface has a non-formation region where the insulating layer 9 is not formed.

  In the present invention, the insulating layer 9 can also contribute to improving the adhesion between the first conductive layer 71 and the second conductive layer 72. For example, when a Cu layer is formed by plating on the Ag layer that is the base electrode layer, the adhesion between the Ag layer and the Cu layer is small, but the Cu layer is formed on an insulating layer such as silicon oxide. Therefore, it is expected that the adhesion of the second conductive layer is enhanced and the reliability of the solar cell is improved.

As described above, when the first conductive layer has, for example, a low melting point material, an annealing process is performed after the insulating layer is formed on the first conductive layer 71 and before the second conductive layer 72 is formed. During the annealing process, the first conductive layer 71 is heated to a temperature higher than the thermal flow temperature T ₁ of the low melting point material, for the low-melting-point material is fluidized state, the surface shape of the first conductive layer is changed. Along with this change, an opening 9h is formed in the insulating layer 9 formed thereon. Therefore, in the subsequent plating process, a part of the surface of the first conductive layer 71 is exposed to the plating solution and becomes conductive, and as shown in FIG. 3E, metal is deposited starting from this conductive portion. It becomes possible.

  In this case, the opening is mainly formed on the low melting point material 711 of the first conductive layer 71. When the low melting point material is an insulating material, it is insulative immediately below the opening, but since the plating solution penetrates into the conductive high melting point material existing around the low melting point material, the first conductive layer and It is possible to conduct the plating solution.

The annealing temperature (heating temperature) Ta during the annealing treatment is preferably higher than the thermal flow start temperature T ₁ of the low melting point material, that is, T ₁ <Ta. The annealing temperature Ta preferably satisfies T ₁ + 1 ° C. ≦ Ta ≦ T ₁ + 100 ° C., and more preferably satisfies T ₁ + 5 ° C. ≦ Ta ≦ T ₁ + 60 ° C. The annealing temperature can be appropriately set according to the composition and content of the material of the first conductive layer.

Further, as described above, the annealing temperature Ta is preferably lower than the heat resistant temperature of the photoelectric conversion unit 50. The heat-resistant temperature of the photoelectric conversion unit varies depending on the configuration of the photoelectric conversion unit. For example, the heat resistant temperature in the case of having a transparent electrode layer or an amorphous silicon-based thin film, such as a heterojunction solar cell or a silicon-based thin film solar cell, is about 250 ° C. Therefore, in the case of a heterojunction solar cell in which the photoelectric conversion portion includes an amorphous silicon thin film or a silicon thin film solar cell, the annealing temperature is 250 from the viewpoint of suppressing thermal damage at the amorphous silicon thin film and its interface. It is preferable that the temperature is set to be equal to or lower. In order to realize a higher performance solar cell, the annealing temperature is more preferably 200 ° C. or less, and further preferably 180 ° C. or less. Accordingly, the heat flow temperature T ₁ of the low melting point material of the first conductive layer 71 is preferably less than 250 ° C., more preferably less than 200 ° C., more preferably less than 180 ° C..

  On the other hand, a crystalline silicon solar cell having a reverse conductivity type diffusion layer on one main surface of a one conductivity type crystalline silicon substrate does not have an amorphous silicon thin film or a transparent electrode layer, and therefore has a heat resistance temperature of 800 ° C. to It is about 900 ° C. Therefore, the annealing process may be performed at an annealing temperature Ta higher than 250 ° C.

  Note that the method for forming the opening in the insulating layer is not limited to the method in which the annealing treatment is performed after the insulating layer is formed as described above. For example, as shown in FIG. 3 (broken arrows), the opening 9h can be formed simultaneously with the formation of the insulating layer 90.

  For example, the opening is formed substantially simultaneously with the formation of the insulating layer by forming the insulating layer while heating the substrate. Here, “substantially simultaneously with the formation of the insulating layer” means that a separate process such as annealing is not performed in addition to the insulating layer forming process, that is, during or immediately after the formation of the insulating layer. Means the state. The term “immediately after film formation” includes the period from the end of film formation of the insulating layer (after the stop of heating) to the time when the substrate is cooled and returned to room temperature. In addition, when an opening is formed in the insulating layer on the low-melting-point material, even after the insulating layer on the low-melting-point material has been formed, the insulating layer is formed around the periphery. Thus, the case where the insulating layer around the low melting point material is deformed and an opening is formed is included.

  As a method of forming the opening substantially simultaneously with the formation of the insulating layer, for example, in the insulating layer forming step, the substrate is heated to a temperature Tb higher than the thermal flow start temperature T1 of the low melting point material 711 of the first conductive layer 71. However, a method of forming the insulating layer 9 on the first conductive layer 71 is used. Since the insulating layer 9 is formed on the first conductive layer in which the low melting point material is in a fluid state, stress is generated at the film forming interface at the same time as the film formation, for example, a crack-shaped opening is formed in the insulating layer. The

  The substrate temperature Tb at the time of forming the insulating layer (hereinafter referred to as “insulating layer forming temperature”) represents the substrate surface temperature (also referred to as “substrate heating temperature”) at the time of starting the formation of the insulating layer. In general, the average value of the substrate surface temperature during the formation of the insulating layer is usually equal to or higher than the substrate surface temperature at the start of film formation. Therefore, if the insulating layer forming temperature Tb is higher than the heat flow starting temperature T1 of the low melting point material, deformation of the opening or the like can be formed in the insulating layer.

  For example, when the insulating layer 9 is formed by a dry method such as a CVD method or a sputtering method, the substrate surface temperature in the insulating layer formation is set higher than the thermal flow start temperature T1 of the low melting point material, thereby opening the opening. The part can be formed. When the insulating layer 9 is formed by a wet method such as coating, the opening is formed by setting the substrate surface temperature when drying the solvent to be higher than the thermal flow start temperature T1 of the low melting point material. be able to. Note that the “film formation start point” when the insulating layer is formed by a wet method refers to the time point when the solvent starts drying. The preferable range of the insulating layer formation temperature Tb is the same as the preferable range of the annealing temperature Ta.

  The substrate surface temperature can be measured, for example, by attaching a temperature display material (also called a thermo label or a thermo seal) or a thermocouple to the substrate surface. In addition, the temperature of the heating unit (such as a heater) can be appropriately adjusted so that the surface temperature of the substrate falls within a predetermined range.

  When annealing treatment is performed in the insulating layer forming step, the material and composition of the insulating layer, and the film forming conditions (film forming method, substrate temperature, type and amount of introduced gas, film forming pressure, power density, etc.) are adjusted as appropriate. Thus, an opening can be formed in the insulating layer.

  When the insulating layer 9 is formed by the plasma CVD method, from the viewpoint of forming a dense film, the insulating layer forming temperature Tb is preferably 130 ° C. or higher, more preferably 140 ° C. or higher, and further preferably 150 ° C. or higher. Moreover, it is preferable that the highest temperature reached on the substrate surface during the formation of the insulating layer is lower than the heat-resistant temperature of the photoelectric conversion part.

  The film deposition rate by plasma CVD is preferably 1 nm / second or less, more preferably 0.5 nm / second or less, and further preferably 0.25 nm / second or less from the viewpoint of forming a denser film. As film forming conditions when silicon oxide is formed by plasma CVD, a substrate temperature of 145 ° C. to 250 ° C., a pressure of 30 Pa to 300 Pa, and a power density of 0.01 W / cm 2 to 0.16 W / cm 2 are preferable. After the opening is formed substantially simultaneously with the formation of the insulating layer, when there is a portion where the opening is not sufficiently formed, the above-described annealing step may be further performed.

  In addition, the method for forming the opening in the insulating layer is not limited to the above. As described above, the surface uneven structure of the first conductive layer is formed on the first main surface of the photoelectric conversion unit, and the first conversion of the photoelectric conversion unit is performed. A relatively thin insulating layer is formed on one main surface of the photoelectric conversion part on the first conductive layer formation region and the first conductive layer non-formation region, using a larger surface uneven structure in the conductive layer non-formation region. In this case, the opening can be easily formed in the insulating layer on the first conductive layer simultaneously with the formation of the insulating layer.

(Second conductive layer)
As described above, after the insulating layer 9 having the opening 9h is formed, the second conductive layer 72 is formed on the insulating layer 9 in the first conductive layer forming region by plating. At this time, the metal deposited as the second conductive layer is not particularly limited as long as it is a material that can be formed by a plating method. For example, copper, nickel, tin, aluminum, chromium, silver, gold, zinc, lead, palladium, etc. Alternatively, a mixture of these can be used.

  During operation of the solar cell (power generation), current flows mainly through the second conductive layer. Therefore, from the viewpoint of suppressing resistance loss in the second conductive layer, it is preferable that the line resistance of the second conductive layer is as small as possible. Specifically, the line resistance of the second conductive layer is preferably 1 Ω / cm or less, and more preferably 0.5 Ω / cm or less. On the other hand, the line resistance of the first conductive layer only needs to be small enough to function as a base layer during electroplating, for example, 5 Ω / cm or less.

  The second conductive layer can be formed by either an electroless plating method or an electrolytic plating method, but the electrolytic plating method is preferably used from the viewpoint of productivity. In the electroplating method, since the metal deposition rate can be increased, the second conductive layer can be formed in a short time.

  Taking the acidic copper plating as an example, a method of forming the second conductive layer by the electrolytic plating method will be described. FIG. 7 is a conceptual diagram of the plating apparatus 10 used for forming the second conductive layer. A substrate 12 on which an insulating layer having a first conductive layer and an opening is formed on a photoelectric conversion portion, and an anode 13 are immersed in a plating solution 16 in the plating tank 11. The first conductive layer 71 on the substrate 12 is connected to the power source 15 via the substrate holder 14. By applying a voltage between the anode 13 and the substrate 12, copper is selectively formed on the first conductive layer not covered with the insulating layer 9, that is, with an opening formed in the insulating layer by annealing treatment as a starting point. Can be deposited.

The plating solution 16 used for acidic copper plating contains copper ions. For example, a known composition mainly composed of copper sulfate, sulfuric acid, and water can be used, and a metal that is the second conductive layer is deposited by passing a current of 0.1 to 10 A / dm ² through this. be able to. An appropriate plating time is appropriately set according to the area of the collecting electrode, current density, cathode current efficiency, set film thickness, and the like.

  The second conductive layer may be composed of a plurality of layers. For example, after a first plating layer made of a material having high conductivity such as Cu is formed on the first conductive layer through the opening of the insulating layer, the second plating layer having excellent chemical stability is formed on the first plating layer. By forming on the surface of one plating layer, a collector electrode having low resistance and excellent chemical stability can be formed.

  It is preferable to provide a plating solution removing step after the plating step to remove the plating solution remaining on the surface of the substrate 12. By providing the plating solution removing step, it is possible to remove the metal that can be deposited starting from the opening 9h other than the opening 9h of the insulating layer 9 formed by annealing. Examples of the metal that is deposited starting from other than the opening 9h include those starting from a pinhole of the insulating layer 9 or the like. By removing such a metal by the plating solution removing step, the light-shielding loss is reduced, and the solar cell characteristics can be further improved.

  As described above, a part of the second conductive layer is electrically connected to the first conductive layer through the opening 9h of the insulating layer. At this time, at least one region of the second conductive layer conducted through the opening 9h is, as shown in FIG. 8, the outermost surface layer on the one main surface side of the photoelectric conversion unit 50 in the opening 9h of the insulating layer 9. Embedded in the first conductive layer 71 so as to reach. Among the second conductive layers, those embedded in the first conductive layer so as to reach the outermost surface layer on the one main surface side of the photoelectric conversion unit through the opening 9h of the insulating layer are described as “second conductive layer A”. (Symbol 72a in the figure).

  Here, “embedded in the first conductive layer” means not only the one in contact with the outermost surface layer of the photoelectric conversion unit but also the one embedded in the first conductive layer as indicated by reference numeral 72b in FIG. Including. By adopting a structure in which the second conductive layer is embedded in the first conductive layer, the adhesion strength between the first conductive layer and the second conductive layer is improved, and peeling of the second conductive layer can be prevented. Further, when the second conductive layer embedded in the first conductive layer reaches the outermost surface layer on the one main surface side of the photoelectric conversion unit (that is, the second conductive layer A is formed), the solar cell is operated. In (power generation), part of the current flows directly between the photoelectric conversion unit and the second conductive layer without going through the first conductive layer having a relatively high resistance, so that the solar cell characteristics are further improved. It becomes possible.

  Here, “at least one region of the second conductive layer conducted through the opening 9h” may be one region of the second conductive layer conducted through the opening 9h. All regions (all reach the outermost surface layer of the photoelectric conversion unit) may be used.

  Here, according to the study by the present inventors, when the second conductive layer A is too much, in particular, a material having a transparent electrode layer as the outermost surface layer of the photoelectric conversion part, such as a heterojunction solar cell, was used. In this case, the photoelectric conversion part and the collector electrode may be peeled off, but by making the ratio of the second conductive layer A within a predetermined range, the carrier can be efficiently taken out while suppressing the peeling. It became clear.

  In the cross section parallel to the line width direction of the first conductive layer, when the width of the region where the second conductive layer A is in contact with the outermost surface layer on the one main surface side of the photoelectric conversion portion is A0, the line width of the first conductive layer The ratio of A0 to (L1) preferably satisfies 0.1% to 10%. By setting it as said range, peeling with a photoelectric conversion part and a collector electrode can be prevented more. Note that the ratio of A0 to the width of the first conductive layer can be obtained, for example, by observing a cross section parallel to the line width direction of the collector electrode with an SEM (scanning electron microscope) or the like.

  The second conductive layer A is preferably formed near the end of the first conductive layer. Here, “the vicinity of the end portion of the first conductive layer” means that the first conductive layer (line width L1) when the cross section parallel to the line width direction of the first conductive layer is observed as shown in FIG. It means a region (reference numeral 71B in the figure) from the end on both sides to the position 20% inside, that is, a region where the distance from the end of the first conductive layer is 0 or more and 0.2L1 or less. By forming the second conductive layer A in the vicinity of the end portion 71B of the first conductive layer, the second conductive layer A and the light receiving surface on which carriers are generated and the second conductive layer A are formed compared to the case where the second conductive layer A is formed near the central portion of the collector electrode. Since the distance to the two conductive layers A is short, carriers can be taken out to the external circuit more efficiently.

  The second conductive layer A is preferably localized in the vicinity 71B of the first conductive layer. In the present invention, the phrase “the second conductive layer A is localized in the vicinity of the end portion” means that the density of the second conductive layer A in the vicinity of the end portion of the first conductive layer is higher than that in the central portion. That is, the second conductive layer A being localized in the vicinity of the end portion 71B of the first conductive layer means that the density of the second conductive layer A is higher in the vicinity of the region end portion than in the central portion.

  Among these, from the viewpoint of further suppressing peeling of the photoelectric conversion portion and the collector electrode and efficiently extracting carriers, it is preferable that the second conductive layer A in the central portion is less, and the second conductive layer A is the first conductive layer. It is more preferable that it is formed only in the vicinity of the end portion and not in the central portion. Further, from the viewpoint of further improving the carrier extraction effect while further suppressing peeling in the vicinity of the end portion, the end portion of the first conductive layer among the second conductive layers A existing in the vicinity of the end portion of the first conductive layer. It is more preferable to localize in a region where the distance from is greater than 0 and equal to or less than 0.1L1.

  In general, a transparent electrode layer such as ITO or an insulating layer such as silicon oxide is hydrophilic, and a contact angle with water on the surface of the substrate 12 or the surface of the insulating layer 9 is about 10 ° or less. There are many cases. On the other hand, from the viewpoint of facilitating removal of the plating solution by air blow or the like, the contact angle with the water on the surface of the substrate 12 is preferably set to 20 ° or more. In order to increase the contact angle of the substrate surface, the surface of the substrate 12 may be subjected to water repellent treatment. The water repellent treatment is performed, for example, by forming a water repellent layer on the surface. By the water repellent treatment, the wettability of the substrate surface to the plating solution can be reduced.

  Instead of the water repellent treatment on the surface of the insulating layer 9, an insulating layer 9 having water repellency may be formed. That is, by forming the insulating layer 9 having a large contact angle θ with water (for example, 20 ° or more), a separate water-repellent treatment step can be omitted, so that the productivity of the solar cell can be further improved. As a method for imparting water repellency to the insulating layer, for example, the insulating layer is formed by a plasma CVD method in which the film forming conditions of the insulating layer (for example, the flow rate ratio of silicon source gas and oxygen source gas introduced into the film forming chamber) are changed. And a method of forming a silicon oxide layer as the above.

  In the present invention, the insulating layer removing step may be performed after the collector electrode is formed (after the plating step). In particular, when a material having a large light absorption is used as the insulating layer, it is preferable to perform an insulating layer removing step in order to suppress a decrease in solar cell characteristics due to the light absorption of the insulating layer. The method for removing the insulating layer is appropriately selected according to the characteristics of the insulating layer material. For example, the insulating layer can be removed by chemical etching or mechanical polishing. An ashing method can also be applied depending on the material. At this time, from the viewpoint of further improving the light capturing effect, it is more preferable that all of the insulating layer on the first conductive layer non-forming region is removed. When the water repellent layer 91 is formed on the insulating layer 9, it is preferable that the water repellent layer 91 is also removed together with the insulating layer 9. Note that in the case where a material with low light absorption is used for the insulating layer, the insulating layer removing step does not need to be performed.

  As described above, the case where the collector electrode 7 is provided on the light incident side of the heterojunction solar cell has been mainly described, but a similar collector electrode may be formed on the back surface side. Since a solar cell using a crystalline silicon substrate, such as a heterojunction solar cell, has a large amount of current, in general, power generation loss due to loss of contact resistance between the transparent electrode layer / collector electrode tends to be significant. On the other hand, in the present invention, since the collector electrode having the first conductive layer and the second conductive layer has a low contact resistance with the transparent electrode layer, it is possible to reduce power generation loss due to the contact resistance. .

  The present invention also relates to a crystalline silicon solar cell other than a heterojunction solar cell, a solar cell using a semiconductor substrate other than silicon such as GaAs, a pin junction or a pn junction of an amorphous silicon thin film or a crystalline silicon thin film. Various types of organic thin film solar cells such as silicon-based thin film solar cells having a transparent electrode layer formed thereon, compound semiconductor solar cells such as CIS and CIGS, dye-sensitized solar cells and organic thin films (conductive polymers) Applicable to solar cells.

A crystalline silicon solar cell has a structure in which a diffusion layer of reverse conductivity type (for example, n-type) is provided on one main surface of a single conductivity type (for example, p-type) crystalline silicon substrate, and the collector electrode is provided on the diffusion layer. Can be mentioned. Such a crystalline silicon solar cell is generally provided with a conductive layer such as a p ⁺ layer on the back side of one conductive layer. Thus, if the photoelectric conversion unit does not include an amorphous silicon layer and the transparent electrode layer, heat flow temperature T ₁ and the annealing temperature Ta of the low melting point material may be higher than 250 ° C..

Examples of the silicon thin film solar cell include an amorphous silicon thin film solar cell having an amorphous intrinsic (i type) silicon thin film between a p type thin film and an n type thin film, and a p type thin film and an n type thin film. Examples thereof include a crystalline silicon-based semiconductor solar cell having a crystalline intrinsic silicon thin film between the thin film. A tandem thin film solar cell in which a plurality of pin junctions are stacked is also suitable. In such a silicon-based thin-film solar cell, it in consideration of the heat resistance of the transparent electrode layer or an amorphous silicon-based thin film, the thermal flow temperature T ₁ and the annealing temperature Ta of the low melting point material is 250 ° C. or less Preferably, it is 200 degrees C or less, More preferably, it is 180 degrees C or less.

  The solar cell of the present invention is preferably modularized for practical use. The modularization of the solar cell is performed by an appropriate method. For example, a bus bar is connected to a collector electrode via an interconnector such as a tab, so that a plurality of solar cells are connected in series or in parallel, and sealed with a sealant and a glass plate to be modularized. Done.

  Hereinafter, the present invention will be specifically described with reference to the examples of the heterojunction solar cell shown in FIG. 2, but the present invention is not limited to the following examples.

(Adhesion strength evaluation)
As a method for evaluating the adhesion strength of the collector electrode of the solar cell, the solar cell after the collector electrode was formed was subjected to a peeling experiment with a tape (mending tape: manufactured by 3M), and then an optical microscope (OLS3000: manufactured by Olympus). ) Was observed. A 20 × objective lens was used for observation. In the peeling experiment, a tape was attached to a position of 2 to 3 cm from the end of the finger electrode toward the center so as to cover the end of the finger electrode. Thereafter, the tape attached from the end of the tape (the end side of the collector electrode) was peeled off by hand, and then the presence or absence of the portion where the second conductive layer at the finger end was peeled off from the first conductive layer was observed. At this time, the adhesion evaluation was evaluated with 1 to 5 points, with 5 points being the highest and 1 point being the lowest.

(Cross section observation)
In order to measure the presence / absence of the formation of the second conductive layer A and the ratio of A0 to the width of the first conductive layer, a cross section parallel to the line width direction of the collector electrode was taken for the collector electrode where peeling did not occur in the adhesion strength observation. 20 points were observed at random. When observing the cross section of the collector electrode, a laser beam was applied from the back side of the cell in the direction perpendicular to the collector electrode to form a groove on the back side of the cell, and the cell was cut along the groove. The obtained cross section was observed by SEM. For each cross section, after observing the entire cross section at an observation magnification of 1000 times and measuring the width of the first conductive layer, the presence or absence of the second conductive layer A was observed at an observation magnification of 5000 times, and there was a second conductive layer A. In this case, the ratio of A0 to the width of the first conductive layer is measured by measuring the width (A0) of the region where the two conductive layers A are in contact with the outermost surface layer on the one main surface side of the photoelectric conversion portion at an observation magnification of 5000 times. (Formation area ratio of the second conductive layer A) was calculated.

Example 1
The heterojunction solar cell of Example 1 was manufactured as follows.

  As a single conductivity type single crystal silicon substrate, an n-type single crystal silicon wafer having an incident plane of (100) and a thickness of 200 μm was used, and this silicon wafer was immersed in a 2 wt% HF aqueous solution for 3 minutes. After the silicon oxide film was removed, rinsing with ultrapure water was performed twice. This silicon substrate was immersed in a 5/15 wt% KOH / isopropyl alcohol aqueous solution maintained at 70 ° C. for 15 minutes, and the texture was formed by etching the surface of the wafer. Thereafter, rinsing with ultrapure water was performed twice. When the surface of the wafer was observed with an atomic force microscope (manufactured by AFM Pacific Nanotechnology), the surface of the wafer was most etched and a pyramidal texture with an exposed (111) surface was formed. It was.

The etched wafer was introduced into a CVD apparatus, and an i-type amorphous silicon film having a thickness of 5 nm was formed on the light incident side as an intrinsic silicon-based thin film 2a. The film formation conditions for the i-type amorphous silicon were: substrate temperature: 150 ° C., pressure: 120 Pa, SiH ₄ / H ₂ flow rate ratio: 3/10, and input power density: 0.011 W / cm ² . In addition, the film thickness of the thin film in a present Example measures the film thickness of the thin film formed on the glass substrate on the same conditions by the spectroscopic ellipsometry (brand name M2000, JA Woollam Co., Ltd. product). It is a value calculated from the film forming speed obtained by this.

On the i-type amorphous silicon layer 2a, a p-type amorphous silicon film having a thickness of 7 nm was formed as the reverse conductivity type silicon-based thin film 3a. The film forming conditions for the p-type amorphous silicon layer 3a were as follows: the substrate temperature was 150 ° C., the pressure was 60 Pa, the SiH ₄ / B ₂ H ₆ flow rate ratio was 1/3, and the input power density was 0.01 W / cm ² . . The B ₂ H ₆ gas flow rate mentioned above is the flow rate of the diluted gas diluted with H ₂ to a B ₂ H ₆ concentration of 5000 ppm.

Next, an i-type amorphous silicon layer having a thickness of 6 nm was formed as an intrinsic silicon-based thin film 2b on the back side of the wafer. The film formation conditions for the i-type amorphous silicon layer 2b were the same as those for the i-type amorphous silicon layer 2a. On the i-type amorphous silicon layer 2b, an n-type amorphous silicon layer having a thickness of 4 nm was formed as a one-conductivity-type silicon-based thin film 3b. The film forming conditions for the n-type amorphous silicon layer 3b were: substrate temperature: 150 ° C., pressure: 60 Pa, SiH ₄ / PH ₃ flow rate ratio: 1/2, input power density: 0.01 W / cm ² . The PH ₃ gas flow rate mentioned above is the flow rate of the diluted gas diluted with H ₂ to a PH ₃ concentration of 5000 ppm.

On this, as transparent electrode layers 6a and 6b, indium tin oxide (ITO, refractive index: 1.9) was formed to a thickness of 100 nm. Using indium oxide as a target, a transparent electrode layer was formed by applying a power density of 0.5 W / cm ² in an argon atmosphere at a substrate temperature of room temperature and a pressure of 0.2 Pa. On the back surface side transparent electrode layer 6b, silver was formed as a back surface metal electrode 8 with a film thickness of 500 nm by sputtering. On the light incident side transparent electrode layer 6a, the collector electrode 7 having the first conductive layer 71 and the second conductive layer 72 was formed as follows.

For the formation of the first conductive layer 71, SnBi metal powder (particle diameter D _L = 25 to 35 μm, melting point T ₁ = 141 ° C.) as a low melting point material and silver powder (particle diameter D _H = 2 to 3 μm, melting point T ₂ = 971 ° C.) at a weight ratio of 20:80, and a printing paste containing an epoxy resin as a binder resin was used. The binder resin, the SnBi metal powder, and the silver powder were mixed so that the amount of the binder resin contained in the printing paste was 7% of the entire printing paste by volume ratio. The printed paste was screen printed using a # 230 mesh (opening width: l = 85 μm) screen plate having an opening width (L = 80 μm) corresponding to the collector electrode pattern, and dried at 90 ° C. .

  The wafer on which the first conductive layer 71 is formed is put into a CVD apparatus, and a silicon oxide layer (refractive index: 1.5) is formed as an insulating layer 9 with a thickness of 80 nm on the light incident surface side by the plasma CVD method. It was.

The film forming conditions of the insulating layer 9 were: substrate temperature: 135 ° C., pressure 133 Pa, SiH ₄ / CO ₂ flow rate ratio: 1/20, input power density: 0.05 W / cm ² (frequency 13.56 MHz). The refractive index (n) and extinction coefficient (k) of the insulating layer formed on the light incident surface side under these conditions are as shown in FIG. Thereafter, the wafer after forming the insulating layer was introduced into a hot-air circulating oven, and an annealing process was performed at 180 ° C. for 20 minutes in an air atmosphere.

The substrate 12 that has been subjected to the annealing process as described above was put into the plating tank 11 as shown in FIG. In the plating solution 16, copper sulfate pentahydrate, sulfuric acid, and sodium chloride were added to a solution prepared so as to have a concentration of 120 g / l, 150 g / l, and 70 mg / l, respectively. (Product: ESY-2B, ESY-H, ESY-1A) added were used. Using this plating solution, plating is performed under conditions of a temperature of 40 ° C. and a current of 3 A / dm ² , and copper is uniformly formed as the second conductive layer 72 with a thickness of about 10 μm on the insulating layer on the first conductive layer 71. Precipitated in Almost no copper was deposited in the region where the first conductive layer was not formed.

  Thereafter, the silicon wafer on the outer periphery of the cell was removed with a width of 0.5 mm by a laser processing machine, and the heterojunction solar cell of the present invention was produced.

  As a result of observing the cross section of the collector electrode of Example 1 along the above procedure, it was confirmed that the second conductive layer A was formed, and the formation region ratio was 0.8%.

(Example 2)
A heterojunction solar cell was produced in the same manner as in Example 1 except that the volume ratio of the binder resin contained in the printing paste for forming the first conductive layer 71 was changed to 4% as shown in Table 1. It was done. Moreover, the measurement of the solar cell characteristics, the observation of the adhesion strength of the collector electrode, and the cross-sectional observation of the collector electrode were performed in the same manner as in Example 1. As a result of observing the cross section of the collector electrode of Example 2, it was confirmed that the second conductive layer A was formed, and the formation region ratio thereof was 2.1%.

(Example 3)
A heterojunction solar cell was produced in the same manner as in Example 1 except that the volume ratio of the binder resin contained in the printing paste for forming the first conductive layer 71 was changed to 20% as shown in Table 1. It was done. Measurement of solar cell characteristics, observation of the adhesion strength of the collector electrode, and observation of the cross section of the collector electrode were also performed in the same manner as in Example 1. As a result of observing the cross section of the collector electrode of Example 3, it was confirmed that the second conductive layer A was formed, and the formation region ratio thereof was 0.3%.

(Comparative Example 1)
Example 1 except that a silver paste that does not contain a low-melting-point material (that is, a ratio of metal material powder to silver powder of 0: 100) was used as the printing paste for forming the first conductive layer. The first conductive layer (silver electrode) 71 was formed in the same manner as described above. Thereafter, none of the insulating layer forming step, the annealing step, and the second metal layer forming step was performed, and a heterojunction solar cell using this silver electrode as a collecting electrode was fabricated. Although the measurement of the solar cell characteristics and the observation of the adhesion strength of the collecting electrode were performed in the same manner as in Example 1, the cross-sectional observation of the collecting electrode was not performed.

(Comparative Example 2)
Using a mask corresponding to a region where the first conductive layer is formed, after forming a silicon oxide layer as an insulating layer in a portion other than the region where the first conductive layer is formed, a portion where the silicon oxide layer is not formed ( A heterojunction solar cell is produced in the same manner as in Example 1 except that the first conductive layer is formed on the insulating layer non-formed portion and the second conductive layer is formed directly on the first conductive layer by plating. It was done. Measurement of solar cell characteristics, observation of the adhesion strength of the collector electrode, and observation of the cross section of the collector electrode were also performed in the same manner as in Example 1.

(Comparative Example 3)
A heterojunction solar cell was produced in the same manner as in Comparative Example 2 except that the volume ratio of the binder resin contained in the printing paste for forming the first conductive layer 71 was changed to 35% as shown in Table 1. It was done. Measurement of solar cell characteristics, observation of the adhesion strength of the collector electrode, and observation of the cross section of the collector electrode were also performed in the same manner as in Example 1.

  The production conditions and solar cell characteristics (open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and conversion efficiency (Eff)) of the heterojunction solar cells of the above Examples and Comparative Examples are shown in Table 1. Table 1 also shows the adhesion strength, the presence / absence of formation of the second conductive layer A, and the formation region ratio of each of the examples and comparative examples.

  From the comparison between each Example and Comparative Example 1, the solar cell of the present invention has improved conversion efficiency (Eff) as compared with the conventional solar cell having a collecting electrode made of a silver paste electrode. This is probably because the solar cell of the example has a low-resistance plating layer as the second conductive layer, so that the resistance of the collector electrode is reduced and the fill factor (FF) is improved.

  Moreover, in each Example, the short circuit current (Jsc) is also improved as compared with Comparative Example 1. This is presumably because the reflectance at the outermost surface (air interface of the solar cell) was lowered because the insulating layer 9 having a low refractive index was provided on the transparent electrode layer 6a having a high refractive index. This can be estimated from the fact that in FIG. 10, the insulating layer (silicon oxide) has a lower refractive index than the transparent electrode layer (ITO) in the wavelength range that the solar cell can use for photoelectric conversion, and has almost no light absorption. . Thus, when an insulating layer having transparency and an appropriate refractive index is formed, it is understood that a solar cell having high conversion characteristics can be obtained even if the insulating layer is not removed after the formation of the second conductive layer.

  In Comparative Example 3, since the volume ratio of the binder resin was higher than in each Example and the second conductive layer A was not formed, the resistance was not reduced due to the contact between the second conductive layer and the transparent electrode layer. Therefore, it is considered that the curve factor (FF) is low and the conversion efficiency (Eff) is low as compared with the respective examples having the second conductive layer A.

  Comparing the formation region ratio of the second conductive layer A of the example, as the resin volume ratio increases, that is, Example 2 (resin volume ratio 4%), Example 1 (resin volume ratio 7%), Example 3 There were many formation places of the 2nd conductive layer A in order of (resin volume ratio 20%). This is because when the binder resin content is low, voids are likely to occur in the first conductive layer, the second conductive layer is likely to reach the outermost surface of the photoelectric conversion part, and conversely when the binder resin content is high. This is presumably because the second conductive layer is less likely to reach the outermost surface of the photoelectric conversion part.

  Comparing the solar cell characteristics of each Example, Example 2 in which a large amount of the second conductive layer A was formed had the highest characteristics, and Example 3 in which the second conductive layer A was relatively small had the lowest characteristics. . This is thought to be because the effect of reducing resistance due to the second conductive layer being in contact with the transparent electrode layer is increased and the fill factor (FF) is improved due to the formation of a large amount of the second conductive layer A.

  Regarding the adhesion strength of the collector electrode of each example and comparative example, since no peeled part was observed in any of Example 3, Comparative Example 1, and Comparative Example 3, the relative adhesion evaluation was 5 out of 5 points. Points. In Example 1, peeling occurred in two of the 96 end portions of the finger electrode, both of which were mild peeling of about 10 μm. The relative adhesion evaluation of Example 1 was evaluated as 4 out of 5 points. In Example 3, peeling occurred in 5 of the 96 end portions of the finger electrode, and all were mild peeling of about 10 μm. The relative adhesion evaluation of Example 3 was evaluated as 3 out of 5 points. In the case of mild peeling as in Example 1 and Example 3, it is considered that peeling is in a range that does not affect the photoelectric conversion region, and thus there is little influence on the solar cell characteristics. In Comparative Example 2, peeling occurred at 19 points out of the 96 end portions of the finger electrode, and peeling was about 10 to 100 μm. The relative adhesion evaluation of Comparative Example 2 was evaluated as 1 out of 5 points.

  In Comparative Example 1 and Comparative Example 3, the second conductive layer A was not formed, and it is considered that the first conductive layer and the transparent electrode layer were strongly attached in the first conductive layer formation region. In Example 3, the second conductive layer A is formed at a ratio of 0.3% of the width of the first conductive layer. Compared with Examples 1 and 2, the second conductive layer A was formed in the region where the second conductive layer A was formed. Since the ratio is small, it is considered that the adhesion strength of the collector electrode is improved as compared with Examples 1 and 2.

  In Comparative Example 2, the adhesion strength was significantly reduced as compared with each Example. In Comparative Example 2, although the volume ratio of the resin contained in the first conductive layer was the same as in Example 1, the adhesion strength was reduced. This is because in Comparative Example 2, the second conductive layer A was excessively formed as compared with each Example due to the absence of the insulating layer on the first conductive layer, and the collector electrode was peeled off. Conceivable. That is, in Example 1, an insulating layer is provided on the first conductive layer, and the second conductive layer is embedded in the first conductive layer only from the opening of the insulating layer to the outermost surface of the photoelectric conversion unit. The second conductive layer A is formed, but Comparative Example 2 does not have an insulating layer on the first conductive layer, and the second conductive layer enters a void existing on the surface of the first conductive layer. It is considered that the second conductive layer A was formed excessively and the adhesion strength of the collector electrode was lowered.

  In the solar cell characteristics of Comparative Example 2, the fill factor (FF) is lower than that of the example. From the height of the area ratio in which the second conductive layer A was formed, it was presumed that Comparative Example 2 showed a high fill factor (FF). However, in actuality, the second conductive layer A was excessively formed. It is considered that the adhesion strength of the electrode was weak and the continuity between the collecting electrode and the transparent electrode layer was poor, and thus the solar cell characteristics lower than that of Example 1 were exhibited.

  As described above, in the present invention, by forming the second conductive layer A in the above range, it is possible to improve the solar cell characteristics while suppressing a decrease in the adhesion strength between the collector electrode and the photoelectric conversion portion. I understood it.

  As described above with reference to the examples, according to the present invention, the collector electrode of the solar cell can be produced without performing the patterning of the insulating layer, so that a high-power solar cell is provided at a low cost. It becomes possible to do. In the present invention, an insulating layer is provided between the first conductive layer and the second conductive layer forming the collector electrode, and the second conductive layer in a predetermined range is the outermost surface on the one main surface side of the photoelectric conversion unit. By being embedded in the first conductive layer through the opening so as to reach the surface layer, the resistance of the solar battery cell is reduced, and a solar battery having high characteristics can be provided.

1. 1. One conductivity type single crystal silicon substrate 2. Intrinsic silicon-based thin film 5. Conductive silicon thin film 6. Transparent electrode layer Collector electrode 71. First conductive layer 71B. Near the end of the first conductive layer 710. Conductive material 711. Low melting point material 712. Binder resin 713. High melting point material 72. Second conductive layer 72a. Second conductive layer A
72b. Second conductive layer embedded in the first conductive layer 8. Back metal electrode 9. Insulating layer 9h. Opening 9ha. Uppermost surface connection opening 50. Photoelectric conversion unit 100. Solar cell 101. Heterojunction solar cell 10. Plating apparatus 11. Plating tank 12. Substrate 13. Anode 14. Substrate holder 15. Power supply 16. Plating solution

Claims (17)

A solar cell having a photoelectric conversion part and a patterned collector electrode on one main surface of the photoelectric conversion part,
The collector electrode, in order from the photoelectric conversion portion side, includes a patterned first conductive layer, an insulating layer in which an opening is formed, and a patterned second conductive layer ,
On the first main surface of the photoelectric conversion portion, there are a first conductive layer forming region in which the first conductive layer is formed and a first conductive layer non-forming region in which the first conductive layer is not formed. And
In the first conductive layer forming region, the first conductive layer is covered with the insulating layer , and a part of the second conductive layer is provided between the first conductive layer and the second conductive layer. It said being electrically connected to the first conductive layer through the opening of the insulating layer, so that at least one region of said first Nishirube conductive layer reaches the outermost layer of the one main surface side of the photoelectric conversion unit, embedded in the first conductive layer through the opening, the solar cell. When the second conductive layer A is the second conductive layer A that is embedded in the first conductive layer through the opening so as to reach the outermost surface layer on the one main surface side of the photoelectric conversion unit,
The solar cell according to claim 1, wherein the second conductive layer A is formed in the vicinity of an end portion of the first conductive layer.   The solar cell according to claim 2, wherein the second conductive layer A is localized near the end of the first conductive layer.   In the cross section parallel to the line width direction of the first conductive layer, when the width of the region where the second conductive layer A is in contact with the outermost surface layer on the one main surface side of the photoelectric conversion unit is A0, the first conductive layer The solar cell according to claim 2 or 3, wherein a ratio of A0 to a layer width satisfies 0.1% or more and 10% or less.   When the first conductive layer has a conductive material and a binder resin, and the volume ratio of the conductive material and the binder resin to the first conductive layer is V1 and V2, respectively, V1 = 70 to 90 parts, And the solar cell of any one of Claims 1-4 satisfy | filling V2 = 10-30 parts.   The solar cell according to claim 5, wherein the conductive material includes a low-melting-point material whose thermal flow start temperature T1 is lower than a heat resistant temperature of the photoelectric conversion unit.   The solar cell according to claim 1, wherein the second conductive layer is a plating layer.   The solar cell of any one of Claims 1-7 whose outermost surface layer of the one main surface side of the said photoelectric conversion part is a transparent electrode layer.   The photoelectric conversion unit has a silicon-based thin film and a transparent electrode layer as an outermost surface layer in this order on one main surface of one conductivity type crystalline silicon substrate, and the collector electrode on the transparent electrode layer, Item 9. The solar cell according to Item 8. The solar cell according to claim 1, wherein the insulating layer is also formed on the first conductive layer non-formation region of the photoelectric conversion unit.   A solar cell module provided with the solar cell of any one of Claims 1-10. It is a method of manufacturing the solar cell of any one of Claims 1-10,
A first conductive layer forming step in which a patterned first conductive layer is formed on the photoelectric conversion portion;
An insulating layer forming step in which an insulating layer is formed on the first conductive layer; and a second that is electrically connected to the first conductive layer by a plating method through an opening provided in the insulating layer on the first conductive layer. The manufacturing method of a solar cell which has the plating process in which a conductive layer is formed in this order.   In the first conductive layer formation step, the first conductive layer is formed by applying a coating material containing a conductive material and a binder resin on one main surface of the photoelectric conversion unit, and then curing the coating material. The method for manufacturing a solar cell according to claim 12. The first conductive layer includes a low-melting-point material having a heat flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion unit,
The solar cell according to claim 12 or 13, wherein after the insulating layer forming step, the opening is formed by performing a heat treatment at an annealing temperature Ta higher than a thermal flow start temperature T1 of the low melting point material. Manufacturing method. The first conductive layer includes a low-melting-point material having a heat flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion unit,
In the insulating layer forming step, the opening is formed simultaneously with the formation of the insulating layer by forming the insulating layer at a substrate temperature Tb higher than the thermal flow start temperature T1 of the low melting point material. Item 14. A method for producing a solar cell according to Item 12 or 13.   The method for manufacturing a solar cell according to claim 12, wherein an insulating layer is also formed on the first conductive layer non-formation region of the photoelectric conversion unit in the insulating layer forming step.   The photoelectric conversion unit has a silicon-based thin film and a transparent electrode layer in this order on one main surface of a one-conductivity-type crystalline silicon substrate, and the collector electrode is formed on the transparent electrode layer. 16. The method for producing a solar cell according to any one of 16 above.