JP6139261B2 - 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 take out carriers generated in the photoelectric conversion unit to an external circuit, a solar cell provided with a collector electrode on the photoelectric conversion unit is used. For example, 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.

  In thin film solar cells using amorphous silicon thin films, crystalline silicon thin films, etc., compound solar cells such as CIGS and CIS, organic thin film solar cells, dye-sensitized solar cells, etc. In order to reduce the surface resistance, a transparent electrode layer is provided on the light receiving surface side surface of the photoelectric conversion unit.

  In such a configuration, since the transparent electrode layer can function as a collector electrode, it is not necessary to provide a separate collector electrode in principle. However, conductive oxides such as indium tin oxide (ITO) and zinc oxide constituting the transparent electrode layer have a problem that the internal resistance of the solar battery cell is increased because the resistivity is higher than that of metal. Although the resistivity of the transparent electrode layer can be reduced by crystallization, the light absorption increases accordingly, and the resistivity of the transparent electrode layer and the light absorption are in a trade-off relationship.

  The increase in internal resistance due to the transparent electrode layer can be solved by separately forming a collector electrode (metal electrode as an auxiliary electrode) on the surface of the transparent electrode layer. A solar cell in which a collecting electrode is formed on a transparent electrode layer is used. For example, Patent Document 1 discloses a technique for improving electrical characteristics while suppressing light absorption by forming two amorphous transparent electrode layers having different numbers of carriers.

  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.

  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 2 to 5 disclose a solar cell method in which a metal layer made of copper or the like is formed on a transparent electrode by a plating method.

  In forming a collecting electrode by plating, there is a step of immersing solar cells in a non-neutral liquid material. In that case, since a transparent electrode layer has electroconductivity, it will plate. Therefore, a part of the transparent electrode layer is protected with an insulating material (insulating layer) in order to prevent the transparent electrode layer from coming into direct contact with the liquid material and to form a collecting electrode anywhere on the transparent electrode layer. There is a need to.

  In the methods of Patent Documents 2 and 3, 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. A metal layer is formed by plating. Thereafter, the resist is removed to form a collector electrode having a predetermined shape.

  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 2 and 3. Moreover, by providing a low-resistance metal seed, the contact resistance between the transparent electrode layer and the collector electrode can be reduced.

  Further, in Patent Document 5, a conductive seed having appropriate roughness and porosity is used, and by increasing the unevenness of the conductive paste, the entire surface of the photoelectric conversion portion other than the conductive seed is formed during the formation of the insulating layer. And discontinuous openings on the conductive seed, and a plating layer is formed starting from the openings.

  Here, it is generally known that an amorphous transparent electrode layer is poor in acid resistance and alkali resistance, and is easily dissolved by contact with these substances, resulting in a decrease in solar cell performance. In order to solve this, for example, in Patent Document 6, by forming a crystalline layer thicker than the seed crystal layer on the seed crystal layer (a mixture of amorphous and crystalline) as a transparent electrode layer, It is disclosed that the plating resistance can be improved at the time of plating using an acid solution or an alkaline solution.

  Further, Patent Document 7 relates to a solar cell having a substrate / electrode / amorphous transparent electrode layer / photoelectric conversion part, to the transparent electrode layer by an acid component of a buffer layer formed between the transparent electrode layer and the photoelectric conversion part. To suppress damage, crystallize the portion of the amorphous transparent electrode layer in contact with the buffer layer, and leave the transparent electrode layer on the electrode side amorphous so that it has low resistance and excellent flexibility. It is described that a flexible solar cell can be manufactured.

WO2012 / 020682 International Publication Pamphlet Japanese Patent Publication No. 60-66426 JP 2000-58885 A JP 2011-199045 A Special table 2013-507781 gazette JP 2012-219301 A JP 2011-181904 A

  According to the study by the present inventors, even when the amorphous transparent electrode layer is protected by an insulating layer, the plating solution penetrates onto the transparent electrode layer, the transparent electrode layer is eluted, and the solar cell characteristics are deteriorated. A problem occurred.

  In Patent Document 6, it is considered that the plating resistance can be improved by using a thick crystalline layer. However, since the crystalline ratio of the entire film is large and the light absorption is increased, the performance of the solar battery cell is deteriorated. it is conceivable that.

Patent Document 7 describes that an electrode (collecting electrode) is formed on the amorphous transparent electrode layer side (light incident side) by plating, screen printing, or the like, and the transparent electrode layer is exposed to the plating solution. Is amorphous, it is thought that corrosion by the plating solution occurs and the photoelectric conversion characteristics deteriorate.
As described above, nothing has been studied so far from the viewpoint of protection from the plating solution and light absorption.

  The present invention solves the problems of the prior art relating to the formation of the light-receiving surface side transparent electrode layer of the photoelectric conversion part of the solar cell as described above, improves the conversion efficiency of the solar cell, and reduces the manufacturing cost of the solar cell. The purpose is to reduce.

  As a result of intensive studies in view of the above problems, the present inventors can improve the conversion efficiency of solar cells by using a predetermined transparent electrode layer and collector electrode, and further improve the yield in the manufacturing process and long-term use. Therefore, the present inventors have found that a solar cell can be manufactured at a low cost and have reached the present invention.

  That is, the present invention relates to the following.

  A solar cell having a photoelectric conversion unit and a transparent electrode layer and a collector electrode in this order on one main surface of the photoelectric conversion unit, wherein the collector electrode is a first collector electrode and a second collector electrode from the transparent electrode layer side. The transparent electrode layer has an amorphous first transparent electrode layer and a crystalline second transparent electrode layer in this order from the photoelectric conversion part side, and the film thickness of the first transparent electrode layer A solar cell in which d1 and the film thickness d2 of the second transparent electrode layer satisfy d1> d2.

  The second transparent electrode layer is preferably completely crystallized.

  It is preferable that the film thickness of the first transparent electrode layer and the second transparent electrode layer satisfy d1 + d2 = 10 to 140 nm, and the film thickness d2 of the second transparent electrode layer satisfies d2 = 5 to 50 nm.

It is preferable that the film thicknesses of the first transparent electrode layer and the second transparent electrode layer satisfy 0.5 > d2 / (d1 + d2) ≧ 0.036.

  The first transparent electrode layer and the second transparent electrode layer each contain a metal dopant, and when the metal dopant amounts of the first transparent electrode layer and the second electrode layer are W1 and W2, respectively, W1 = 5 to 15 weights %, W2 = 0.1 to 10% by weight, and preferably satisfies W2 <W1.

  It is preferable that the photoelectric conversion part has a silicon-based thin film on one main surface of the one-conductivity-type crystalline silicon substrate.

  An insulating layer having an opening formed between the first collector electrode and the second collector electrode, wherein the first collector electrode is covered with the insulating layer, and the opening provided in the insulating layer It is preferable that the first collector electrode and the second collector electrode are electrically connected via each other.

  It is preferable to have the insulating layer also on the second transparent electrode layer.

  The first collector electrode preferably includes a low melting point material, and the heat flow start temperature T1 of the low melting point material is 250 ° C. or less.

  It is preferable that the low melting point material includes a metal material.

  The second collector electrode is preferably composed mainly of copper.

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

  The manufacturing method of the solar cell includes a transparent electrode layer forming step in which the first transparent electrode layer and the second transparent electrode layer are formed in this order on one main surface of the photoelectric conversion unit; a first collector electrode on the transparent electrode layer A first collecting electrode forming step in which a second collecting electrode is formed by plating on the first collecting electrode in this order, after the transparent electrode layer forming step and before the plating step It is preferable to have an insulating layer forming step in which an insulating layer is formed on the second transparent electrode layer.

  The insulating layer forming step is included after the first collector electrode forming step, the insulating layer is formed on the first collector electrode in the insulating layer forming step, and the opening is provided in the insulating layer in the plating step. It is preferable that a second collector electrode that is electrically connected to the first collector electrode is formed by plating.

  The first collector electrode includes a low melting point material having a thermal flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion unit, and is higher than the thermal 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 collector electrode includes a low melting point material having a heat flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion portion, 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.

  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 of forming a collecting electrode by plating requires a patterning process of the insulating layer. According to the present invention, the pattern electrode can be formed by plating without using a resist or the like for pattern formation. Is possible. 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 surface observation image of a transparent electrode layer. It is a conceptual diagram of the manufacturing process of the solar cell by one Embodiment of this invention. FIG. 4C is a conceptual diagram in the case where an insulating layer is not formed over the first collector electrode by using a mask or the like in the step of FIG. 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 a figure which shows the optical characteristic of the insulating layer in an Example.

  As schematically shown in FIG. 1, the solar cell 110 of the present invention includes a transparent electrode layer 100 and a collector electrode 70 on one main surface of a photoelectric conversion unit 50. The transparent electrode layer 100 includes an amorphous first transparent electrode layer 101 and a crystalline second transparent electrode layer 102 in order from the photoelectric conversion unit 50 side. The collector electrode 70 includes a first collector electrode 71 and a second collector electrode 72 in order from the second transparent electrode layer 102 side. An insulating layer 9 is formed on the second transparent electrode layer 102. In the present invention, by using the transparent electrode layer 100, the photoelectric conversion part can be protected from the plating solution while suppressing light absorption.

  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 111 has, as the photoelectric conversion unit 50, a conductive silicon thin film 3a in this order on one surface (surface on the light incident side) of the one conductive single crystal silicon substrate 1. On one main surface side of the photoelectric conversion unit 50, the transparent electrode layer 100 including the first transparent electrode layer 101 and the second transparent electrode layer 102 in this order is provided. It is preferable that the other surface (surface opposite to the light incident side) of the one conductivity type single crystal silicon substrate 1 has the conductivity type silicon-based thin film 3b and the back surface side transparent electrode layer 103 in this order. A collecting electrode 70 including a first collecting electrode 71 and a second collecting electrode 72 is formed on the transparent electrode layer 100 on the one main surface side of the photoelectric conversion unit 50. An insulating layer 9 is formed on the second transparent electrode layer 102.

  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 a back metal electrode 8 on the back side transparent electrode layer 103.

  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, when a n-type single crystal silicon substrate is used as the one-conductivity-type single crystal silicon substrate 1, a suitable configuration of the photoelectric conversion unit 50 is a p-type amorphous silicon thin film 3a / i-type amorphous silicon system. A laminated structure in the order of a thin film 2a / n-type single crystal silicon substrate 1 / i-type amorphous silicon-based thin film 2b / n-type amorphous silicon-based thin film 3b can be given. 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 111 includes a transparent electrode layer 100 on the conductive silicon thin film 3a. Moreover, it is preferable to provide the transparent electrode layer 103 on the conductive silicon-based thin film 3b. The transparent electrode layer is formed by a transparent electrode layer forming step.

The transparent electrode layer 100 in the present invention includes an amorphous first transparent electrode layer 101 and a crystalline second transparent electrode layer 102 in order from the photoelectric conversion portion side.
In the present invention, the first transparent electrode layer mainly functions for electrical connection with the photoelectric conversion portion and light capture, and the second transparent electrode layer mainly protects the first transparent electrode layer from the external environment. To work.

  In the present invention, “crystalline” means (1) having a clear peak structure by X-ray diffraction method, or (2) using a transparent electrode layer as shown in FIG. After being immersed in 10% hydrochloric acid for 20 seconds, washed with pure water and air-dried with a blower, when the surface shape was observed with a scanning electron microscope (SEM), as shown in FIG. It means that there is no clear difference in the surface condition before and after. Moreover, the thing without a clear difference after 40 second immersion is more suitable, and the thing without a clear difference after 60 second immersion is still more suitable.

  For example, when the film thickness of the transparent electrode layer is as thin as about 10 to 20 nm, (1) a clear peak may not be observed by the X-ray diffraction method, but when hydrochloric acid treatment is performed under the conditions of (2) If there is no clear difference between the two, it can be defined as crystalline.

  (1) Measurement by the X-ray diffraction method can be performed by the 2θ / θ method, and the measurement range of 2θ can be set to 20 to 80 °. Among these, (1) those having a clear peak structure by the X-ray diffraction method and having no clear difference in the surface state before and after the hydrochloric acid treatment under the condition (2) are more preferable.

  “Amorphous” means a material other than the above “crystalline”, that is, no peak is observed by X-ray diffraction method, and it is immersed in 10% hydrochloric acid at 25 ° C. for 20 seconds and then washed with pure water. When the surface shape is observed with an SEM, the surface shape is clearly different between before and after the treatment as shown in FIG. 3 (b1) or FIG. 3 (b2). When the entire surface is covered with amorphous, the transparent electrode layer on the entire surface is damaged by the acid treatment as shown in FIG. When small crystal grains are present in the amorphous material, the crystal grains are not dissolved by the acid treatment, but the amorphous layer portion is dissolved while leaving the crystal grain portion as shown in FIG. 3 (b2). As described above, a substance that has a clear difference due to hydrochloric acid treatment can be defined as amorphous.

  More preferably, the crystalline material in the present invention is completely crystallized. Complete crystallization means that the entire observation site is covered with crystal grains when the surface is observed with a transmission electron microscope.

  By making the second transparent electrode layer that may come into contact with the plating solution in the plating step or the like into a crystalline state, it is possible to further prevent a decrease in solar cell performance and a decrease in reliability. In the transparent electrode layer 100 according to the present invention, the film thickness d1 of the first transparent electrode layer and the film thickness d2 of the second transparent electrode layer satisfy d1> d2. By forming a laminated structure of an amorphous first transparent electrode layer and a crystalline second transparent electrode layer and satisfying the above film thickness, a transparent electrode layer with high resistance to plating solution is formed without losing the light capturing effect can do.

  The total film thickness d1 + d2 of the first transparent electrode layer 101 and the second transparent electrode layer 102 is preferably 10 nm or more and 140 nm or less from the viewpoint of transparency, conductivity, and light reflection reduction. Among these, 30 nm or more is preferable from the viewpoint of conductivity, and 80 nm or more is more preferable. Moreover, 120 nm or less is preferable from a viewpoint of light reflection reduction, and 100 nm or less is more preferable.

  Here, when a collector electrode is formed by a plating method on a solar cell work product in which a transparent electrode layer is formed on one main surface side of the photoelectric conversion part, as described later, it is generally not neutral as a plating solution. A neutral solution (for example, PH ≦ 6.0, 8.0 ≦ PH, etc.) is used. In this case, since the transparent electrode layer having conductivity is also plated, it is generally necessary to protect the transparent electrode layer with an insulating layer or the like. However, even when covered with an insulating layer, the plating solution may penetrate to the transparent electrode layer when exposed to the plating solution.

  In the present invention, by using the second transparent electrode layer as the transparent electrode layer, even if a defect (for example, a pinhole) occurs in the insulating layer 9, the transparent electrode layer or photoelectric layer that exists under the defective part is used. It is possible to further suppress damage in the plating process to the conversion part.

  For example, when there is a minute defect such as a gap between the insulating layer and the first collector electrode or a pinhole formed in the insulating layer 9, the plating solution entering from the defect may corrode the transparent electrode layer. In connection with this, a transparent electrode layer may peel from a photoelectric conversion part, or the damage to a photoelectric conversion part may arise. In addition, as described later, when a non-volatile non-neutral solution such as sulfuric acid is used as the plating solution, the non-neutral components are less likely to volatilize, so long-term reliability such as when modularized may be lost. .

  On the other hand, in the present invention, by using the second transparent electrode layer, even when a defect or the like exists in the insulating layer, damage in the plating process to the transparent electrode layer is suppressed, and from the photoelectric conversion portion of the transparent electrode layer Can be prevented. That is, the effect of protecting the surface of the photoelectric conversion portion from the plating solution can be expected by the second transparent electrode layer.

  From the viewpoint of further improving the resistance from the plating solution, the film thickness d2 of the second transparent electrode layer is preferably 5 nm or more, more preferably 10 nm or more, and particularly preferably 30 nm or more.

Further, from the viewpoint of reducing loss due to light absorption, d2 is preferably 60 nm or less, more preferably 50 nm or less, and particularly preferably 40 nm or less. In the transparent electrode layer, from the viewpoint of suppressing light absorption and improving plating resistance, d1 + d2 is preferably 0.5 > d2 / (d1 + d2) ≧ 0.036, and d1 + d2 is 0.35 ≧ d2 / (d1 + d2). ≧ 0.07 is more preferable.

  The film thickness of the transparent electrode layer can be determined by cross-sectional observation with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The transparent electrode layer 100 in the present invention contains a conductive oxide as a main component. 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.

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

  The metal dopant can be added to one or both of the one main surface side transparent electrode layer 100 and the back surface side transparent electrode layer 103. In particular, it is preferable to add a metal dopant to the transparent electrode layer 100. By adding a metal dopant to the transparent electrode layer 100, the resistance of the transparent electrode layer itself can be reduced and resistance loss between the transparent electrode layer 100 and the collector electrode 7 can be suppressed.

In the present invention, when the metal dopant amounts of the first transparent electrode layer and the second electrode layer are W1 and W2, respectively, W2 <W1 is satisfied, that is, the dopant concentration is more transparent than the first transparent electrode layer 101. The electrode layer 102 is preferably lower.
Thereby, the first transparent electrode layer is likely to be amorphous, and the crystallinity of the second transparent electrode layer can be further improved. From the viewpoint of reducing light absorption, the metal dopant amount is preferably 5 to 15% by weight, and more preferably 7 to 12% by weight, of the first transparent electrode layer 101.

  From the viewpoint of crystallization, the dopant concentration of the second transparent electrode layer 102 is preferably W2 = 0.1 to 10% by weight, and more preferably 1 to 6% by weight.

  In the present invention, the transparent electrode layer 100 is preferably composed mainly of indium oxide. Here, “main component” means that the content is more than 50% by weight, preferably 70% by weight or more, and more preferably 85% by weight or more. Moreover, it is preferable that a transparent electrode layer contains at least 1 type of metals, such as Sn, W, As, Zn, Ge, Ca, Si, and C, as a metal dopant according to a utilization condition. Among them, indium tin oxide (ITO) using Sn as a metal dopant is more preferably used.

  The transparent electrode layer 100 in the present invention preferably has a transmittance of 65% or more at a wavelength of 400 to 900 nm, more preferably 70% or more, and particularly preferably 75% or more. By setting it as this range, light absorption can be suppressed and the light confinement effect can be improved more.

  Although the transparent electrode layer 100 in this invention has an amorphous 1st transparent electrode layer and a crystalline 2nd transparent electrode layer in order from the one main surface side of a photoelectric conversion part, as long as the function of this invention is not impaired. Furthermore, you may have another transparent electrode layer. For example, a third transparent electrode layer having different crystallinity can be formed between the first transparent electrode layer and the second transparent electrode layer. The transparent electrode layer 103 on the back side in the present invention is not particularly limited and can be formed in the same manner as the first transparent electrode layer and the second transparent electrode layer.

  A method for forming the transparent electrode layer 100 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 100 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. The crystallinity of the transparent electrode layer can be obtained by changing the material and composition of the conductive oxide, the film forming conditions (film forming method, substrate temperature, type and amount of introduced gas, film forming pressure, power density, etc.), etc. Can be adjusted accordingly.

  In particular, the smaller (larger) the amount of oxygen introduced into the film forming chamber and the larger (smaller) the power density at the time of film forming, the more likely it becomes crystalline (amorphous).

Taking ITO film formation as an example, for example, using a target containing 1 to 5% by weight of tin, the amount of oxygen introduced is 0.3% by volume to 10% by volume with respect to the total amount of gas introduced. it is preferred that film formation is performed under the conditions of a density ^{0.2mW / cm 2 ~1.2mW / cm 2} . By setting it as the said range, a crystalline transparent electrode layer can be formed more easily. A target containing 5 to 10% by weight of tin was used. The amount of oxygen introduced was 0.3% by volume to 10% by volume with respect to the total amount of gas introduced, and the power density was 0.2 mW / cm ^{2 to} 1.2 mW / It is preferable that film formation is performed under conditions of cm ² . By setting it as the above range, an amorphous transparent electrode layer is more easily formed.

  On the back side transparent electrode layer 103, the back side metal electrode 8 is preferably formed. 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 100. The collector electrode 7 includes a first collector electrode 71 and a second collector electrode 72.

In the present embodiment, the insulating layer 9 is formed between the first collector electrode 71 and the second collector electrode 72. In the collector electrode 7 of the present invention, a part of the second collector electrode 72 is electrically connected to the first collector electrode 71. Here, “partially conducting” means that the insulating layer typically has an opening, and the opening is filled with the material of the second collector electrode, thereby conducting. In addition, since the film thickness of a part of the insulating layer 9 is very thin (ie, a region having a thin film thickness is locally formed), the second collector electrode 72 is The thing connected to the electrode 71 is also included. For example, when the low melting point material of the first collector electrode 71 is a metal material such as aluminum, the first collector electrode 71 and the second collector electrode 72 are connected via an oxide film (corresponding to an insulating layer) formed on the surface thereof. There is a state in which the gap is electrically connected. In the present invention, in addition to the above, as shown in FIG. 5, the entire surface of the first collector side of the first collector electrode may be electrically connected to the second collector electrode.

  The method for forming the opening for conducting the first collector electrode 71 and the second collector electrode 72 in the insulating layer 9 is not particularly limited, and methods such as laser irradiation, mechanical drilling, and chemical etching are adopted. it can. In one embodiment, a method of forming an opening in an insulating layer formed thereon by causing the low melting point material in the first collector electrode 71 to heat flow can be mentioned.

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

  Hereinafter, a method of forming an opening in the insulating layer using the thermal flow of the low melting point material in the first collector electrode 71 will be described with reference to the drawings.

  FIG. 4 is a process conceptual diagram showing an embodiment of a method for forming the collector electrode 70 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. 4A). For example, in the case of a heterojunction solar cell, a photoelectric conversion unit including a silicon-based thin film is prepared on a one-conductivity type silicon substrate as described above.

  The transparent electrode layer 100 is formed on one main surface of the photoelectric conversion part (transparent electrode layer forming step, FIG. 4A). In the transparent electrode layer forming step, the first transparent electrode layer 101 and the second transparent electrode layer 102 are formed from the photoelectric conversion portion side. A first collector electrode 71 including a low melting point material 711 is formed (first collector electrode forming step, FIG. 4B). An insulating layer 9 is formed on the first collector electrode 71 (insulating layer forming step, FIG. 4C). The insulating layer 9 may be formed only on the first collector electrode 71, and is also formed on a region of the transparent electrode layer 100 where the first collector electrode 71 is not formed (first collector electrode non-formation region). It may be. In particular, in the present invention, since the transparent electrode layer is formed on the surface of the photoelectric conversion unit 50, it is preferable that the insulating layer 9 is also formed on the first collector electrode non-formation region.

  After the insulating layer is formed, an annealing process by heating is performed (annealing process, FIG. 4D). Due to the annealing process, the first collector electrode 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 collector electrode 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 the opening in the insulating layer by annealing, the second collector electrode 72 is formed by plating (plating step, FIG. 4E). The first collector electrode 71 is covered with the insulating layer 9, but the first collector electrode 71 is exposed at the portion where the opening 9 h is formed in the insulating layer 9. Therefore, the first collector electrode is exposed to the plating solution, and metal can be deposited starting from the opening 9h. According to such a method, the second collector electrode 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.

(First collector electrode)
The first collector electrode 71 is a layer that functions as a conductive underlayer when the second collector electrode is formed by a plating method. Therefore, the 1st collector electrode should just have the electroconductivity of the grade which can function as a base layer of electroplating. 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.

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

The first collector electrode 71 includes 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 collector electrode 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, it is preferable that the annealing process is performed at an annealing temperature Ta lower than the heat resistant temperature of the photoelectric conversion unit 50 or the transparent electrode layer. 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 or the transparent electrode layer is low.

The heat-resistant temperature of the photoelectric conversion part or the transparent electrode layer is a solar battery (also referred to as “solar battery cell” or “cell”) or a solar battery module manufactured using the solar battery cell including the photoelectric conversion part and the transparent electrode layer. Is a temperature at which the characteristics of irreversibly decrease. 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 thin films 2 and 3 and the transparent electrode layer 6 are heated to about 250 ° C., thermal degradation may occur or diffusion of doped impurities may occur, resulting in an irreversible decrease in solar cell characteristics. Therefore, in the heterojunction solar cell, the first collector electrode 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 collector electrode during the annealing process, the low melting point material is thermally flowable in the first collector electrode forming step. It is preferable not to produce. For example, when the first collector electrode 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 collector electrode at 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 collector electrode can be reduced. Therefore, when the second collector electrode is formed by electroplating, the uniformity of the film thickness of the second collector electrode can be improved. it can. Moreover, if the low melting point material is a metal material, the contact resistance between the transparent electrode layer 100 on the photoelectric conversion unit 50 and the collector electrode 70 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 collector electrode 71, in addition to the above 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 collector electrode 71 has a high melting point material, the first collector electrode and the second collector electrode 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 collector electrode 71 is exposed to a high temperature by the annealing process and the low melting point material is in a liquid phase state, as conceptually shown in FIG. In some cases, the particles of the low-melting-point material are aggregated into a coarse granular shape, and the first collector electrode 71 is 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. Disconnection of the first collector electrode 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 collector electrode 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 further reducing the resistance of the first collector electrode. When the low melting point material has low conductivity, the resistance of the first collector electrode 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 collector electrode 71 contains a low-melting-point material and a high-melting-point material, the content ratio thereof is to suppress disconnection due to the coarsening of the low-melting-point material as described above, to the conductivity of the first collector electrode, 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 collector electrode) 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.

When a particulate low melting point material such as metal particles is used as the material of the first collector electrode 71, from the viewpoint of facilitating the formation of an opening in the insulating layer by annealing, the particle size D _L of the low melting point material is: It is preferably 1/20 or more of the film thickness d of the first collector electrode, and 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. Moreover, when the 1st collector electrode 71 is formed by printing methods, such as screen printing, the particle size of particle | grains can be set suitably according to the mesh size etc. of a 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 is a non-spherical first collector electrode of the pre-annealing, when heated to the heat flow temperature above T ₁ by the annealing treatment, the particles approaches the spherical shape, the surface shape of the first collector electrode The amount of change is greater. Therefore, it is easy to form an opening in the insulating layer 9 on the first collector electrode 71.

As described above, the first collector electrode 71 may be conductive and have a volume resistivity of 10 ⁻² Ω · cm or less. The volume resistivity of the first collector electrode 71 is preferably 10 ⁻⁴ Ω · cm or less. When the first collector electrode has only the low melting point material, the low melting point material only needs to have conductivity. When the first collector electrode contains a low melting point material and a high melting point material, it is sufficient that at least one of the low melting point material and the high melting point material has conductivity. For example, the combination of the low melting point material / the high melting point material includes insulation / conductivity, conductivity / insulation, conductivity / conductivity. In order to make the first collector electrode 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 collecting electrode 71, the first current by heating at the annealing treatment is adjusted by adjusting the size (for example, particle size) of the material. It is also possible to suppress the disconnection of the collector electrode and improve the 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 a material that causes such sintering necking is heated to a sintering necking start temperature T ₁ ′ or higher, deformation occurs in the vicinity of the outer periphery of the fine particles. 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 a solid phase 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. 7 is a diagram for explaining the sintering necking start temperature. FIG. 7A 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. 7B and FIG. 7C are cross-sectional views schematically showing a state in which the particles after the sintering is started are cut along a cross section passing through the center of each particle. FIG. 7 (B) shows a state after the start of sintering (sintering initial stage), and FIG. 7 (C) shows a state where the sintering has progressed from (B). In FIG. 7B, 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. 7, for simplification, the particles are shown as spherical. However, 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. 8A, 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. 8B, 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 temperature at which the first collector electrode containing fine particles is formed and the 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.

  In addition to the low melting point material (and high melting point material) described above, a paste containing a binder resin or the like can be preferably used as the material for forming the first collector electrode. In order to sufficiently improve the conductivity of the first collector electrode formed by the screen printing method, it is desirable to cure the first collector electrode 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. In this case, the shape of the low melting point material changes with hardening, and as shown in FIG. 4D, an opening (crack) is likely to occur in the insulating layer near the low melting point material during the annealing process. Note that the ratio between the binder resin and the conductive low melting point material may be set to be equal to or higher than a so-called percolation threshold (a critical value of the ratio corresponding to the low melting point material content at which conductivity is manifested).

  The first collector electrode 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 collecting electrode 71 is preferably patterned into a predetermined shape such as a comb shape. A screen printing method is suitable for forming the patterned first collector electrode from the viewpoint of productivity. In the screen printing method, a method of printing a collector electrode pattern using a printing paste containing a low melting point material made of metal particles and a screen plate having an opening pattern corresponding to the pattern shape of the collector 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 earlier, the drying temperature in this case, it is preferred to 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.

  The first collector electrode may be composed of a plurality of layers. For example, a laminated structure including a lower layer having a low contact resistance with the transparent electrode layer on the surface of the photoelectric conversion portion and an upper layer containing a low melting point material may be used. According to such a structure, an improvement in the curve factor of the solar cell can be expected with a decrease in contact resistance with the transparent electrode layer. Moreover, the resistance of the first collector electrode can be further reduced by adopting a laminated structure of the low-melting-point material-containing layer and the high-melting-point material-containing layer.

  As described above, the case where the first collector electrode is formed by the printing method has been mainly described, but the method of forming the first collector electrode is not limited to the printing method. For example, the first collector electrode may be formed by vapor deposition or sputtering using a mask corresponding to the pattern shape.

(Insulating layer)
An insulating layer 9 is formed on one main surface side of the photoelectric conversion unit (that is, on the transparent electrode layer 100).
Here, when the first collector electrode 71 is formed in a predetermined pattern (for example, a comb shape), the first collector electrode forming region in which the first collector electrode is formed on the surface of the transparent electrode layer 100, There is a first collector electrode non-formation region in which one collector electrode is not formed.

  The insulating layer 9 is formed at least in the first collector electrode non-forming region as shown in FIG. 4C in the insulating layer forming step. In the present invention, the insulating layer 9 is preferably formed on the entire surface of the first collector electrode non-formation region. When the insulating layer is formed on the entire surface of the first collector electrode non-forming region, when the second collector electrode is formed by plating, the photoelectric conversion portion and the transparent electrode layer 100 are chemically and electrically removed from the plating solution. It is possible to protect. 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 deposition of the metal layer (second collector electrode) on the transparent electrode layer can be prevented.

  The insulating layer in the present invention may be formed before the plating step, and the insulating layer forming step may be either after the transparent electrode layer forming step and before the first collector electrode forming step, or after the first collector electrode forming step and before the plating step. It may be. For example, as shown in FIG. 5, when it is formed before the first collector electrode forming step, a portion corresponding to the first collector electrode is protected by a mask, and an insulating layer is formed on a portion other than the first collector electrode. The method of forming etc. are mentioned. Moreover, when forming after a 1st collector electrode formation process, an insulating layer is formed at least on the 1st collector electrode non-formation area | region.

  In this case, it is preferable that the insulating layer 9 is also formed on the first collector electrode formation region. That is, by forming so as to cover the first collector electrode, the photoelectric conversion part can be further protected from the plating solution as described later. Also, from the viewpoint of productivity, it is more preferable that an insulating layer is formed in the entire first collector electrode formation region and the first collector electrode 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 collector electrode, and damage to the surface of the transparent electrode layer is less likely to occur.

  In addition, since the insulating layer 9 is formed on the first collector electrode formation region, the insulating layer preferably has a high adhesion strength with the transparent electrode layer. 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 collector electrode, 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 the interface stress caused by the change in the surface shape of the first collector electrode in the annealing treatment, the material of the insulating layer is an inorganic material having a small elongation at break. 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. The film thickness of the insulating layer 9 is preferably thin enough that an opening can be formed in the insulating layer due to, for example, stress at the interface caused by a change in the surface shape of the first collector electrode in the annealing process. 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 collector electrode non-formation part, 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 transparent electrode layer 100. 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. When forming an insulating layer in both the first collector electrode formation region and the first collector electrode non-formation region, the film thickness of the insulation layer on the first collector electrode formation region and the insulation on the first collector electrode non-formation region The layer thicknesses may be different. For example, in the first collector electrode 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 collector electrode non-formation region, an optical film having appropriate antireflection characteristics The film thickness of the insulating layer may be set to be thick.

  The present invention has a transparent electrode layer (generally a refractive index of about 1.9 to 2.1) on the surface of the photoelectric conversion unit 50, and enhances the light reflection preventing effect at the interface, thereby increasing the inside of the solar battery cell. In order to increase the amount of light introduced into the insulating layer, 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 where the surface of the transparent electrode layer 100 has a texture structure (uneven structure), such as 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.

  In the case where the insulating layer 9 is formed between the first collector electrode 71 and the second collector electrode 72 as in the embodiment shown in FIG. 4, that is, the first collector electrode is formed so as to cover the first collector electrode. When the insulating layer is formed on the region, the shape of the insulating layer 9 is not necessarily a continuous layer shape and may be an island shape. 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 embodiment, the insulating layer 9 can also contribute to improving the adhesion between the first collector electrode 71 and the second collector electrode 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 collector electrode is enhanced and the reliability of the solar cell is improved.

In the present embodiment, after the insulating layer is formed on the first collector electrode 71 and before the second collector electrode 72 is formed, an annealing process is performed. During annealing, the first collector electrode 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 collector electrode is changed. Along with this change, an opening 9h is formed in the insulating layer 9 formed thereon. Therefore, in the subsequent plating step, a part of the surface of the first collector electrode 71 is exposed to the plating solution and becomes conductive, so that the metal is deposited starting from this conductive portion as shown in FIG. It becomes possible.

  The opening is mainly formed on the low melting point material 711 of the first collector electrode 71. When the low melting point material is an insulating material, it is insulative immediately below the opening, but the plating solution penetrates into the conductive high melting point material existing around the low melting point material. 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 collector electrode.

As described above, the annealing temperature Ta is preferably lower than the heat resistant temperature of the photoelectric conversion unit 50 or the transparent electrode layer. 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. The heat resistance temperature of the transparent electrode layer 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 collector electrode 71 is preferably less than 250 ° C., more preferably less than 200 ° C., more preferably less than 180 ° 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. 3C ′, the opening can be formed simultaneously with the formation of the insulating layer 9.

  For example, by forming the insulating layer while heating the substrate, the deformed portion is formed almost simultaneously with the formation of the insulating layer. “Substantially simultaneously with the formation of the insulating layer” means a state during or immediately after the formation of the insulating layer, which does not have a new step other than the step of forming the insulating layer. For example, it includes a case where a deformed portion is generated after the film formation of the insulating layer is finished (after the heating is stopped) until the substrate surface temperature returns to room temperature or the like. Further, when an opening is formed in an insulating layer on a certain low melting point material, the insulating layer around the low melting point material is formed even after the formation of the insulating layer on the low melting point material is finished. This includes the case where the insulating layer on the low melting point material is deformed and an opening is formed.

  As a method of forming the opening almost simultaneously with the formation of the insulating layer, for example, while heating the substrate to a temperature Tb higher than the thermal flow start temperature T1 of the low melting point material 711 of the first collector electrode 71 in the insulating layer forming step. A method of forming the insulating layer 9 on the first collector electrode 71 is used. Since the insulating layer 9 is formed on the first collector electrode 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, and for example, an opening on the crack is formed in the insulating layer. The

  The substrate temperature Tb (hereinafter referred to as “insulating layer forming temperature”) when forming the insulating layer means 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 formation 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, the opening can be formed by setting the substrate surface temperature during the formation of the insulating layer to be higher than the thermal flow start temperature T1 of the low melting point material. it can. Further, 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 the solvent is dried to form a film to a temperature higher than the thermal flow start temperature T1. It can be carried out. The “film formation start point” in the case where 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 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 ultimate temperature of the substrate surface at the time of film formation of an insulating layer is lower than the heat-resistant temperature of a photoelectric conversion part or a transparent electrode layer.

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, the opening can be easily formed in the insulating layer. As an example of film formation when silicon oxide is used as the insulating layer, it is preferable to use plasma CVD. The film forming conditions, a substrate temperature of 145 ° C. to 250 DEG ° C., a pressure 30Pa～300Pa, it is preferable that the film formation is performed under the conditions of the power density ^{0.01W / cm 2 ~0.160W / cm 2} .

  After the opening is formed almost simultaneously with the formation of the insulating layer, when there is a portion where the opening is not sufficiently formed, the above-described annealing treatment may be further performed.

(Secondary electrode)
After the insulating layer is formed on the transparent electrode layer 100 as described above, the second collector electrode 72 is formed on the first collector electrode by a plating method. When the insulating layer having the opening 9 is formed on the first collector electrode, the second collector electrode is formed on the insulating layer in the first collector electrode formation region. At this time, the metal deposited as the second collector electrode 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 (during power generation), current flows mainly through the second collector electrode. Therefore, from the viewpoint of suppressing resistance loss at the second collector electrode, the line resistance of the second collector electrode is preferably as small as possible. Specifically, the line resistance of the second collector electrode 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 collector electrode only needs to be small enough to function as a base layer during electroplating, and may be, for example, 5 Ω / cm or less.

The second collector electrode can be formed by either an electroless plating method or an electrolytic plating method, but from the viewpoint of productivity, it is preferable to use the electrolytic plating method. In the electrolytic plating method, the deposition rate of the metal can be increased, so that the second collector electrode can be formed in a short time.
In the electrolytic plating method, a plating solution (non-neutral solution) containing a metal to be plated is used as the second collector electrode. As the non-neutral solution, an acidic solution is generally used as described later.

  The method for forming the second collector electrode by the electrolytic plating method will be described taking acid copper plating as an example. Here, a case where an insulating layer having an opening is formed on the first collector electrode will be described. FIG. 9 is a conceptual diagram of the plating apparatus 10 used for forming the second collector electrode. A substrate 12 having an insulating layer in which a first collector electrode and an opening are formed on a transparent electrode layer, and an anode 13 are immersed in a plating solution 16 in a plating tank 11. The first collector electrode 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 deposited on the first collector electrode that is not covered with the insulating layer 9, that is, with an opening formed in the insulating layer as a starting point. be able to.

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 collector electrode is deposited by applying a current of 0.1 to 10 A / dm ² to 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 collector electrode 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 collector electrode via an insulating layer, the second plating layer having excellent chemical stability is formed on the first plating layer. By forming on the surface of the 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. 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.

  The plating solution is removed by, for example, a method in which the plating solution remaining on the surface of the substrate 12 taken out from the plating tank is removed by air blow type air washing, followed by washing with water and further blowing off the washing solution by air blowing. Can do. By reducing the amount of the plating solution remaining on the surface of the substrate 12 by performing air cleaning before rinsing, the amount of the plating solution brought in at the time of rinsing can be reduced. As a result, the amount of cleaning liquid required for water washing can be reduced, and the labor of waste liquid treatment that accompanies water washing can be reduced, reducing the environmental burden and cost of washing and improving the productivity of solar cells. Can be made.

  Here, in general, a transparent electrode layer such as ITO or an insulating layer such as silicon oxide is hydrophilic, and the contact angle between the surface of the substrate 12, that is, the surface of the transparent electrode layer 100 or the surface of the insulating layer 9, and water is In many cases, it is about 10 ° or less. 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 collector electrode non-forming region is removed. 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 collector electrode and the second collector electrode has a low contact resistance with the transparent electrode layer, it is possible to reduce power generation loss due to the contact resistance. .

  Further, the present invention provides a solar cell using a semiconductor substrate other than silicon such as GaAs other than a heterojunction solar cell, a transparent electrode layer on a pin junction or a pn junction of an amorphous silicon thin film or a crystalline silicon thin film. Applicable to various solar cells such as formed silicon-based thin-film solar cells, compound semiconductor solar cells such as CIS and CIGS, dye-sensitized solar cells and organic thin-film solar cells such as organic thin films (conductive polymers) is there.

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.
In each example, measurement was performed by the following method.

(Evaluation method)
[Film thickness measurement]
The film thickness of the thin film in this example was determined by observing the cross section with SEM (Field Emission Scanning Electron Microscope S4800, manufactured by Hitachi High-Technologies Corporation). Note that it is difficult to identify the interface between the first transparent electrode layer 101 and the second transparent electrode layer 102 by SEM observation. Therefore, the film thickness of these layers is individually formed as a film thickness estimation sample (a transparent conductive layer is formed on a silicon substrate), and the correlation between the film thickness and the film formation time is obtained. The film thickness was calculated based on the film formation time. The film thickness was defined by measuring the vertical direction as an axis.

[Crystallinity of transparent electrode layer]
The crystallinity of the transparent electrode layer was evaluated according to the following (1) and (2), and it was judged that one satisfying either one was crystalline, and one not satisfying both was amorphous.
(1) X-ray diffraction method Using a sample in which a transparent electrode layer is formed on an alkali-free glass having a thickness of 1.1 mm, the presence or absence of a peak is identified by the X-ray diffraction method. Sex was evaluated. X-ray diffraction measurement was performed by the 2θ / θ method, and the 2θ measurement range was 20 to 80 °.
(2) SEM surface observation after the acid treatment and before the formation of the insulating layer (with a transparent electrode layer as the outermost surface layer of the photoelectric conversion part) for 20 seconds and 60 seconds in 10% hydrochloric acid at 25 ° C. Thereafter, the surface was washed with pure water and air-dried with a blower, and the surface shape was observed with a scanning electron microscope (SEM) at a magnification of 50,000 times, and the surface state before and after the treatment was confirmed. △ if a clear difference can be observed in the 20-second treatment, ◯ if no clear difference is seen in the 20-second treatment, and a clear difference appears in the 60-second treatment, and no clear difference appears even in the 60-second treatment. ◎.

[Photoelectric conversion characteristics]
The light of AM1.5 was irradiated with the light quantity of 100 mW / cm < ² > with the solar simulator, and the open end voltage (Voc), the short circuit current density (Jsc), the fill factor (FF), and the conversion efficiency (Eff) were measured.

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 .

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 top of this, a first transparent electrode layer 101 and a second transparent electrode layer 102 were formed with a film thickness of 70 nm of amorphous 10% indium tin oxide (ITO) and 30 nm of crystalline 5% ITO, respectively. 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. Subsequently, as the back surface side transparent electrode layer 103, amorphous 10% ITO was formed into a film with a thickness of 80 nm under the same conditions, and silver was formed on the back surface electrode layer 103 as a back surface metal electrode 8 with a film thickness of 500 nm by sputtering. . On the light incident side transparent electrode layer 100, the collector electrode 7 having the first collector electrode 71 and the second collector electrode 72 was formed as follows.

For the formation of the first collector electrode 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 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 collector electrode 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 on the light incident surface side with a thickness of 80 nm by plasma CVD. 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 a second collector electrode 72 with a thickness of about 10 μm on the insulating layer on the first collector electrode 71. Precipitated in Almost no copper was deposited in the region where the first collector electrode 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.

(Example 2)
A collector electrode is formed by a plating process in the same manner as in Example 1 except that the first transparent electrode layer and the second transparent electrode layer are each made of 90 nm amorphous 10% ITO and 10 nm crystalline 5% ITO. Was done.

(Reference Example 1)
The first transparent electrode layer and the second transparent electrode layer were formed in the same manner as in Example 1 except that the amorphous 10% ITO was only 100 nm, and the collector electrodes were formed by the plating process.

(Reference Example 2)
A collector electrode is formed by a plating process in the same manner as in Example 1 except that the first transparent electrode layer and the second transparent electrode layer are each 10 nm of amorphous 10% ITO and 90 nm of crystalline 5% ITO. Was done.

(Reference Example 3)
A collector electrode is formed by a plating process in the same manner as in Example 1 except that the first transparent electrode layer and the second transparent electrode layer are each made of 30% crystalline 5% ITO and 70 nm amorphous 10% ITO. Was done.

(Comparative Example 1)
The first transparent electrode layer and the second transparent electrode layer are made of a single layer of amorphous 10% ITO having a thickness of only 100 nm, and a silver paste (that is, a metal material powder) that does not contain a low melting point material as a printing paste for forming the first collector electrode. The first collector electrode (silver electrode) 71 was formed in the same manner as in Example 1, except that the ratio of silver powder to silver powder was 0: 100. 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.

  Moreover, in order to make it easier to compare the plating resistance due to the difference in crystallinity of the transparent electrode layer, the overplating of Example 1, Example 2 and Reference Example 1 was performed over 5 times the normal plating time. At that time, it was examined whether electrode peeling occurred. Δ indicates poor adhesion and peeling of the electrode occurs, ○ indicates that electrode peeling does not occur, and ◎ indicates that the adhesiveness is more suitable.

  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, Comparative Examples and Reference Examples Is shown in Table 1.

  From the comparison of each example with Reference Example 1 and Comparative Example 1, the solar cell of the present invention has improved conversion efficiency (Eff) compared to a conventional solar cell having a collector electrode made of a silver paste electrode. Yes. This is presumably because the resistance of the collector electrode was lowered and the fill factor (FF) was improved in the solar cell of the example.

  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 100 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.

  When Example 1, Example 2, and Reference Example 2 are compared, in an example where the thicknesses of the first transparent electrode layer (amorphous layer) and the second transparent electrode layer (crystalline layer) satisfy d1> d2, d1 The characteristics were improved as compared with Reference Example 2 where <d2. In particular, the current value was improved as the thickness of the crystalline layer was reduced from Reference Example 2 (90 nm) → Example 1 (30 nm) → Example 2 (10 nm). This is probably because the amorphous transparent electrode layer has a smaller light absorption than the crystalline transparent electrode layer, and the total amount of light that can be taken into the solar cell is relatively large.

  For Example 1, Example 2, and Reference Example 1, the crystallinity of the transparent conductive layer was examined. In Reference Example 1 having only the amorphous layer, the characteristics of the amorphous layer were shown both in (1) X-ray diffraction method and (2) SEM surface observation after acid treatment. In Reference Example 1, a clear difference was observed after treatment with hydrochloric acid as shown in FIG. Example 1 shows the characteristics of the crystalline layer in (1) and (2), respectively, and (2) a clear difference is seen as shown in FIG. There wasn't.

  In Example 2, since the thickness of the crystalline layer was as thin as 10 nm, (1) no peak was observed by X-ray diffraction method, and (2) the surface was slightly dissolved after 40 seconds of acid treatment, but (2) acid After 20 seconds from the treatment, no clear difference was observed as in Example 1, indicating the characteristics of the crystalline layer. When Examples 1 and 2 were compared, Example 1 with a thick crystalline layer was confirmed to be in a more suitable state. This is presumably because the thicker the crystalline layer, the more the crystal grows and the denser crystal grains are formed.

The overplating test results showed that the plating resistance was higher in Examples 1 and 2 having a crystalline layer than in Reference Example 1 in which only the amorphous layer (the outermost surface layer was an amorphous layer). Further, in Example 1 (30 nm) where the crystalline layer is thick as the outermost surface layer, the plating resistance was improved as compared with Example 2 (10 nm). This is because even when overplating is performed for a longer time than the normal plating time, the use of a crystalline layer as the outermost surface layer prevents penetration of the plating solution into the interface between the insulating layer, the first collector electrode and the transparent conductive layer. This is considered to be because the damage to the transparent conductive layer could be further suppressed. This result also corresponds to the crystallinity of the transparent conductive layer, and the higher the crystallinity, the higher the plating resistance.
Therefore, it is considered that the reliability after standing for a long time is further improved by using the solar cell having the transparent electrode layer of the present invention.

  As described above with reference to the examples, according to the present invention, by using the two-layer transparent electrode layer, the chemical resistance is maintained without dropping the current value as compared with the solar cell having a single amorphous layer. It is possible to produce a solar battery cell with

1. 1. One conductivity type single crystal silicon substrate 2. Intrinsic silicon-based thin film Conductive silicon thin film 70. Collector electrode 71. First collecting electrode 711. Low melting point material 72. Second collector electrode 8. Back metal electrode 9. Insulating layer 9h. Opening 50. Photoelectric conversion unit 100. Transparent electrode layer 101. First transparent electrode layer 102. Second transparent electrode layer 103. Back side transparent electrode layer 110. Solar cell 111. Heterojunction solar cell 10. Plating apparatus 11. Plating tank 12. Substrate 13. Anode 14. Substrate holder 15. Power supply 16. Plating solution

Claims (16)

A solar cell having a photoelectric conversion part and a transparent electrode layer and a collecting electrode in this order on one main surface of the photoelectric conversion part,
The collector electrode has a first collector electrode and a second collector electrode in this order from the transparent electrode layer side,
The transparent electrode layer has an amorphous first transparent electrode layer and a crystalline second transparent electrode layer in this order from the photoelectric conversion portion side,
The solar cell in which the film thickness d1 of the first transparent electrode layer and the film thickness d2 of the second transparent electrode layer satisfy d1> d2.   The solar cell according to claim 1, wherein the second transparent electrode layer is completely crystallized.   3. The film thickness of the first transparent electrode layer and the second transparent electrode layer satisfies d1 + d2 = 10 to 140 nm, and the film thickness d2 of the second transparent electrode layer satisfies d2 = 5 to 50 nm. Solar cells. 4. The solar cell according to claim 1, wherein the film thickness of the first transparent electrode layer and the second transparent electrode layer satisfies 0.5 > d2 / (d1 + d2) ≧ 0.036.   The first transparent electrode layer and the second transparent electrode layer each contain a metal dopant, and when the metal dopant amounts of the first transparent electrode layer and the second electrode layer are W1 and W2, respectively, W1 = 5 to 15 weights %, W2 = 0.1 to 10% by weight, and the solar cell according to claim 1 satisfying W2 <W1.   The solar cell of any one of Claims 1-5 in which the said photoelectric conversion part has a silicon-type thin film on one main surface of one conductivity type crystalline silicon substrate. Including an insulating layer having an opening formed between the first collector electrode and the second collector electrode;
The first collector electrode is covered with the insulating layer;
The solar cell according to claim 1, wherein the first collector electrode and the second collector electrode are electrically connected through the opening provided in the insulating layer.   The solar cell of Claim 7 which has the said insulating layer also on a said 2nd transparent electrode layer.   The solar cell according to claim 7 or 8, wherein the first collector electrode includes a low melting point material, and a thermal flow start temperature T1 of the low melting point material is 250 ° C or lower.   The solar cell according to claim 9, wherein the low melting point material includes a metal material.   The solar cell according to claim 1, wherein the second collector electrode contains copper as a main component.   A solar cell module provided with the solar cell of any one of Claims 1-11. It is a manufacturing method of the solar cell of any one of Claims 1-11,
A transparent electrode layer forming step of forming the first transparent electrode layer and the second transparent electrode layer in this order on one main surface of the photoelectric conversion part;
A first collector electrode forming step in which a first collector electrode is formed on the transparent electrode layer;
A plating step in which a second collector electrode is formed on the first collector electrode by a plating method, in this order,
The manufacturing method of a solar cell which has an insulating layer formation process in which an insulating layer is formed on the second transparent electrode layer after the transparent electrode layer formation process and before the plating process. Having the insulating layer forming step after the first collector electrode forming step,
In the insulating layer forming step, the insulating layer is formed on the first collector electrode,
The method for manufacturing a solar cell according to claim 13, wherein a second collector electrode that is electrically connected to the first collector electrode is formed by plating through an opening provided in the insulating layer in the plating step. The first collector electrode includes a low melting point material having a thermal flow start temperature T1 lower than a heat resistant temperature of the photoelectric conversion unit,
The solar cell manufacturing according to claim 14 , wherein the opening is formed by performing a heat treatment at an annealing temperature Ta higher than a thermal flow start temperature T <b> 1 of the low melting point material after the insulating layer forming step. Method. The first collector electrode includes a low melting point material having a thermal 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 15. A method for producing a solar cell according to Item 14 .