JP2014199876A - Crystal silicon-based solar cell and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide a highly efficient and highly reliable crystal silicon-based solar cell by forming a predetermined back electrode to suppress a reduction in fill factor.SOLUTION: The crystal silicon-based solar cell has a light incident side silicon-based thin-film layer, a light incident side transparent electrode layer, and a front electrode in this order on a first main surface side of a single-crystal silicon substrate of one conductivity type and has a back side silicon-based thin-film layer, a back side transparent electrode layer, and a back electrode in this order on a second main surface side of the substrate. The back electrode has a first conductive layer and a second conductive layer in this order from the back side transparent electrode layer side. Preferably, the second conductive layer contains copper as a main component and is formed so as to cover at least the first conductive layer and to be in contact with the back side transparent electrode layer.

Description

  The present invention relates to a crystalline silicon solar cell and a method for manufacturing the same.

  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 take out the carrier generated in the photoelectric conversion unit to an external circuit, a back electrode is provided on the back surface side (the side opposite to the light incident surface) of the solar cell.

  For example, in a heterojunction solar cell having an amorphous silicon layer and a transparent electrode layer on a crystalline silicon substrate, a back electrode obtained by patterning Ag paste by a screen printing method is generally used. In this method, the construction method itself is simple and low-cost, but the material cost of silver is large, and since the back electrode is formed using an Ag paste having a resin other than silver, there is a problem that the resistivity is relatively high. is there.

  Also, in solar cells such as thin-film silicon solar cells and crystalline silicon solar cells, back electrodes, etc., in which a metal such as silver, aluminum, or copper having a relatively high reflectance is formed on the entire surface by sputtering or the like, are also used. ing. For example, Patent Document 1 and Patent Document 2 describe a thin-film silicon solar cell in which a metal material such as silver, aluminum, or copper having a high reflectance is formed on the entire surface by sputtering or the like and used as a back electrode. In Patent Document 1, aluminum-containing zinc oxide is used as the back surface side transparent electrode layer, and the back electrode is formed thereon. In Patent Document 2, one or more thin films are used to form the back electrode. Is formed.

  However, when the back electrode made of the Ag paste is used as the back electrode, or the silver thick film formed on the entire surface by sputtering is used, there is a problem that the amount of silver used is large, leading to high cost. As a result, studies have been made to use low-cost copper. Among them, in the heterojunction solar cell, a back electrode having a thickness larger than that of the thin-film silicon solar cell is preferably used from the viewpoint of reducing resistance, and thus studies using Cu are particularly vigorously performed.

  In order to solve this, Patent Document 3 describes a metal film having a structure of Ag / Cu (or Al) / Ag in which copper or aluminum is formed between two silver electrodes as a back electrode of a solar cell. It describes that low-cost Cu is used.

WO2008 / 62685 International Publication Pamphlet JP 2009-246031 A JP 2012-142452 A

  However, according to the study by the present inventors, when Cu is used as the back electrode, the contact resistance between the transparent electrode layer and copper is increased by a long-term environmental test, which leads to a decrease in the curvature factor. It became clear. Therefore, when Cu is used instead of Ag as the back electrode as in Patent Document 2, it is considered that the contact resistance after the environmental test increases. In Patent Document 3, Ag having a film thickness of about 0.1 to 1 μm is used between Cu and the transparent electrode layer used as the back electrode, and it is considered that the increase in the contact resistance can be prevented. Issues remain in terms of cost.

  As described above, in the heterojunction solar cell, it has not been sufficiently studied from the viewpoint of forming a low-cost back electrode while maintaining the solar cell characteristics. An object of the present invention is to solve the above-described problems by using a predetermined electrode and to improve the conversion efficiency of a solar cell.

  The crystalline silicon solar cell of the present invention relates to the following.

  On the first main surface side of the single conductivity type single crystal silicon substrate, the light incident side silicon-based thin film layer, the light incident side transparent electrode layer, and the surface electrode are provided in this order, and on the second main surface side of the substrate, A crystalline silicon solar cell having a back-side silicon-based thin film layer, a back-side transparent electrode layer, and a back-side electrode in this order, wherein the back-side electrode includes a first conductive layer and a second conductive layer from the transparent electrode layer side. The second conductive layer includes copper as a main component, covers at least the first conductive layer, and is in contact with the back-side transparent electrode layer.

  The first conductive layer is preferably island-shaped.

  The first conductive layer is preferably made of only a dense conductive material, and substantially the entire surface of the dense conductive material on the back side transparent electrode layer side is in contact with the back side transparent electrode layer.

  The first conductive layer has a maximum distance d1 between the interface between the back surface-side transparent electrode layer and the interface with the second conductive layer in a direction perpendicular to the substrate surface satisfying 0 nm <d1 <9 nm. Is preferred.

  The first conductive layer preferably contains as a main component one selected from Ag, Al, Au, Ni, Ti, or Sn.

  The surface electrode preferably has a third conductive layer mainly composed of copper.

  The second conductive layer and the third conductive layer are preferably plating layers.

It is preferable to use a solar cell module using the crystalline silicon solar cell.
The method for producing a crystalline silicon solar cell according to the present invention includes a back electrode forming step of forming a back electrode on a second main surface opposite to the substrate of the back side transparent electrode layer, The back electrode forming step includes a step of forming a first conductive layer on the back side transparent electrode layer and a second conductive layer that covers the first conductive layer and is in contact with the back side transparent electrode layer It is preferable to have these steps in this order.

  The first conductive layer is preferably formed by a sputtering method.

  A surface electrode forming step of forming a surface electrode including a third conductive layer on the first main surface opposite to the substrate of the light incident side transparent electrode layer, wherein the surface electrode forming step includes: It is preferable to have a step of forming the third conductive layer on the first main surface side of the light incident side transparent electrode layer by plating.

  It is preferable that the second conductive layer in the back electrode and the third conductive layer in the front electrode are simultaneously formed by a plating method.

  According to the present invention, even when copper is used as the back electrode, it can be used with higher reliability than the conventional one, and a high-efficiency and low-cost solar cell can be provided. .

It is typical sectional drawing which shows the crystalline silicon type solar cell which concerns on the comparative example of this invention. It is a typical sectional view showing a crystalline silicon system solar cell concerning one embodiment of the present invention. It is typical sectional drawing which shows a back surface side transparent conductive layer and a back surface electrode (a 1st conductive layer and a 2nd conductive layer).

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In each drawing of the present application, dimensional relationships such as thickness and length are appropriately changed for clarity and simplification of the drawings, and do not represent actual dimensional relationships.

  The crystalline silicon solar cell of the present invention has a light incident side silicon thin film layer and a light incident side transparent electrode layer in this order on the first main surface side of the one-conductivity type single crystal silicon substrate. On the second main surface side, a back side silicon-based thin film layer, a back side transparent electrode layer, and a back electrode are provided in this order. The back electrode includes a first conductive layer and a second conductive layer in this order from the back transparent electrode layer side. The second conductive layer is composed mainly of copper, covers at least the first conductive layer, and is in contact with the transparent electrode layer.

  Hereinafter, the present invention will be described in more detail using a crystalline silicon solar cell which is an embodiment of the present invention as an example. However, the present invention is not limited to the crystalline silicon solar cell shown in FIG. In the present invention, the film thickness unless otherwise specified means the film thickness in the direction perpendicular to the textured slope on the silicon substrate.

(First embodiment)
The crystalline silicon-based solar cell of the present invention has a one-conductivity-type or reverse-conductivity-type silicon-based thin film on the first main surface side of a one-conductivity-type single-crystal silicon substrate (hereinafter also referred to as “substrate” or “silicon substrate”) (Light incident side silicon-based thin film) and light incident side transparent electrode layer. Moreover, it is preferable to have a surface electrode further on the first main surface side of the light incident side transparent electrode layer. Also, on the second main surface side of the substrate, a back electrode having a one-conductivity-type or reverse-conductivity-type silicon thin film (back-side silicon thin film), a back-side transparent electrode layer, a first conductive layer and a second conductive layer is provided. Have in order. Further, it is preferable that an intrinsic silicon-based thin film is further provided between the substrate 1 and the conductive silicon-based thin film (light incident side silicon-based thin film or back surface side silicon-based thin film).

  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 according to the present invention includes a single conductive single crystal silicon substrate 1 and a reverse conductive silicon thin film 3, and a single conductive single crystal silicon substrate 1 and a single conductive silicon thin film 5. It is preferable to have a first intrinsic silicon-based thin film 2 and a second intrinsic silicon-based thin film 4 respectively. Moreover, it is preferable that the surface electrode 8 is formed in this order on the light incident side transparent electrode layer 6. A first conductive layer 9-1 and a second conductive layer 9-2, which are back electrodes 9, are formed on the back-side transparent electrode layer 7.

  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 impurities for supplying electric charge to silicon in order to have conductivity. Single crystal silicon substrates are classified into an n-type supplied with phosphorus atoms for introducing electrons into Si atoms and a p-type supplied with boron atoms for introducing holes (also called holes). That is, “one conductivity type” in the present invention means either n-type or p-type. When used in solar cells, electron-hole pairs can be efficiently separated and recovered by providing a strong electric field with the incident-side heterojunction that absorbs the most light incident on the single crystal silicon substrate as the reverse junction. it can. Therefore, the heterojunction on the incident side is preferably a reverse junction. On the other hand, when holes and electrons are compared, the mobility is generally higher for electrons having a smaller effective mass and scattering cross section. From the above viewpoint, the single crystal silicon substrate 1 used in the present invention is preferably an n-type single crystal silicon substrate. Moreover, it is preferable that the one conductivity type single crystal silicon substrate 1 has a texture structure on the surface from the viewpoint of light confinement.

After the texture is formed, a silicon-based thin film is formed on the surface of the single conductivity type single crystal silicon substrate. As a method for forming a silicon-based thin film, a plasma CVD method is preferable. As conditions for forming the silicon-based thin film, 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 the silicon-based thin film, a silicon-containing gas such as SiH ₄ or Si ₂ H ₆ or a mixture of these gases and H ₂ is used.

The silicon-based thin film in the present invention means a one-conductivity-type or reverse-conductivity-type conductivity-type silicon thin film. For example, when n-type is used as the one-conductivity-type single crystal silicon substrate 1, one-conductivity-type silicon-based thin film, The reverse conductivity type silicon thin films are n-type and p-type, respectively. B ₂ H ₆ or PH ₃ is preferably used as a dopant gas for forming the p-type or n-type silicon thin film in the present invention. 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. Further, by adding a gas containing a different element such as CH ₄ , CO ₂ , NH ₃ , GeH _4, etc., it is possible to alloy and change the energy gap.

  Examples of the silicon-based thin film in the present invention include an amorphous silicon thin film and microcrystalline silicon (thin film containing amorphous silicon and crystalline silicon). Among these, an amorphous silicon-based thin film is preferably used. . As the silicon-based thin film, a conductive silicon-based thin film can be used. At this time, as shown in FIG. 2, it is preferable to use a reverse conductivity type silicon thin film on the light incident side of the one conductivity type single crystal silicon substrate 1. This is because electrons are excited most by incident light on the light incident surface side of the one-conductivity-type single crystal silicon substrate 1, and a recombination loss can be suppressed by having a strong electric field gradient on the light incident surface side. . In order to suppress recombination on the back side, it is preferable to use a one-conductivity-type silicon thin film on the back side.

  For example, when an n-type single crystal silicon substrate is used as the single conductivity type single crystal silicon substrate 1, a preferred configuration of the present invention is transparent electrode layer / p-type amorphous silicon thin film / i-type amorphous silicon. Thin film / n-type single crystal silicon substrate / i-type amorphous silicon thin film / n-type amorphous silicon thin film / transparent electrode layer, etc. In this case, the back surface is preferably n-layer for the above reasons. .

  As the intrinsic silicon-based thin film in the present invention, a substantially intrinsic i-type silicon-based thin film is preferably used. In this case, it is preferable to use i-type hydrogenated amorphous silicon composed of silicon and hydrogen. When film formation is performed by CVD using i-type hydrogenated amorphous silicon, surface passivation can be effectively performed while suppressing impurity diffusion into the single crystal silicon substrate. Another reason is that by changing the amount of hydrogen in the film, the energy gap can have an effective profile for carrier recovery.

  The p-type silicon thin film layer is preferably a p-type hydrogenated amorphous silicon layer, a p-type amorphous silicon carbide layer, or a p-type oxidized amorphous silicon layer. From the viewpoint of impurity diffusion and series resistance, a p-type hydrogenated amorphous silicon layer is preferable. On the other hand, a p-type amorphous silicon carbide layer or a p-type oxidized amorphous silicon layer is preferable as a wide-gap low-refractive index layer in terms of reducing optical loss.

  When a one-conductivity-type silicon-based thin film is used as the back-side silicon-based thin film in the present invention as shown in FIG. 2, the one-conductivity-type silicon-based thin film 5 is recombined on the back surface mainly due to the BSF (Back Surface Field) effect. It helps to suppress. The one conductivity type silicon-based thin film 5 preferably includes at least an amorphous silicon-based thin film. Further, a crystalline silicon thin film may be provided between the amorphous silicon thin film and the back side transparent electrode layer in order to improve electrical contact with the back transparent electrode layer. Note that the term “crystalline” includes those partially including an amorphous state as commonly used in the technical field of thin film photoelectric conversion devices.

  In order to obtain a sufficient BSF effect, the one-conductivity-type silicon thin film 5 needs to have a certain thickness. Thereby, minority carriers (holes in the case of using an n-type single crystal silicon substrate) can be prevented from diffusing to the back electrode side, and recombination on the back surface can be suppressed.

  As described above, from the viewpoint of sufficiently exhibiting the BSF effect, it is preferable to increase the thickness of the one-conductivity-type silicon thin film 5 to some extent. In this case, the thickness of the one conductivity type silicon-based thin film 5 is preferably 5 nm or more, more preferably 10 nm or more, and particularly preferably 30 nm or more. On the other hand, the upper limit of the thickness of the one-conductivity-type silicon-based thin film 5 is not particularly limited, but is preferably 100 nm or less, more preferably 70 nm or less, and particularly preferably 50 nm or less from the viewpoint of reducing manufacturing costs.

  The crystalline silicon solar cell in the present invention has a back surface side transparent electrode layer 7 on the one-conductivity type silicon thin film 5. A light incident side transparent electrode layer 6 is provided on the reverse conductivity type silicon-based thin film 3.

  The transparent electrode layer in the present invention contains a conductive oxide as a main component. As the conductive oxide, for example, zinc oxide, indium oxide, and tin oxide can be used alone or in combination. From the viewpoint of conductivity, optical characteristics, and long-term reliability, an indium oxide containing indium oxide is used. An oxide is preferable, and in particular, an oxide containing indium tin oxide (ITO) as a main component is more preferably used. Here, in the present invention, “main component” means containing more than 50% of the material, preferably 70% or more, and more preferably 90% or more.

  The transparent electrode layer in the present invention may be used as a single layer or may be a laminated structure composed of a plurality of layers. Further, a doping agent can be added to the transparent electrode layer. For example, when zinc oxide is used as the transparent electrode layer, examples thereof include aluminum, gallium, boron, silicon, and carbon. When indium oxide is used, zinc, tin, titanium, tungsten, molybdenum, silicon, and the like can be given. In the case of using tin oxide, fluorine and the like can be mentioned.

  In the transparent electrode layer of the present invention, a doping agent can be added to one or both of the transparent electrode layer 6 and the transparent electrode layer 7 on the light incident side shown in FIG. It is preferable to add to the transparent electrode layer 6. This is because the collector electrode formed on the light incident side is generally comb-shaped, so that the resistance loss that can occur in the transparent electrode layer can be suppressed.

  The film thickness of the light incident side transparent electrode layer 6 of the present invention is preferably 10 nm or more and 140 nm or less from the viewpoint of transparency, conductivity, and reduction of light reflection of the cell. The role of the transparent electrode layer is to transport carriers to the surface electrode, and it is only necessary to have conductivity necessary for that purpose. On the other hand, from the viewpoint of transparency, by setting the thickness to 140 nm or less, the absorption loss of the transparent electrode layer itself is small, so that a decrease in photoelectric conversion efficiency due to a decrease in transmittance can be suppressed. Moreover, if the carrier concentration in a transparent electrode layer is made low, the photoelectric conversion efficiency accompanying the increase in the light absorption of an infrared region can also be suppressed. Furthermore, since the light incident side transparent electrode layer 6 also serves as an antireflection film, a light confinement effect can be expected by setting the film thickness to an appropriate thickness.

  On the other hand, the film thickness of the back-side transparent electrode layer 7 is preferably 5 nm or more and 180 nm or less from the viewpoint of increasing back-surface reflection. Further, from the viewpoint of suppressing light absorption, that is, suppressing reduction in current (Jsc) due to absorption of long wavelength light, 150 nm or less is preferable, and 80 nm or less is more preferable.

  The method for forming the transparent electrode layer is not particularly limited, but includes a physical vapor deposition method such as a sputtering method and a chemical vapor deposition (MOCVD) method using a reaction between an organometallic compound and oxygen or water. preferable. In any film forming method, energy by heat or plasma discharge can be used. The substrate temperature during the production of the transparent electrode layer may be set as appropriate, but is preferably 200 ° C. or lower when an amorphous silicon thin film is used as the silicon thin film. This is because by producing at 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.

  A back electrode 9 is formed on the back side transparent electrode layer 7. As the back electrode 9, a first conductive layer 9-1 and a second conductive layer 9-2 are formed in this order from the back transparent electrode layer 7 side.

  Here, in the thin film silicon-based solar cell, since integration is generally performed and the area for each unit cell is small, and the current density for power generation is small, the film thickness is usually 100 to 100. A back electrode formed on the entire surface of about 300 nm is used. At this time, it is necessary to select a material to be used from the viewpoint of increasing the reflectance and reducing the series resistance. However, since the film of the back electrode is thin, generally high-cost silver is used.

  On the other hand, the crystalline silicon solar cell in the present invention needs to have a thickness of the back electrode to some extent, and generally has a thickness of 300 nm or more. This is because the size of a crystalline silicon substrate used as a crystalline silicon-based solar cell (also referred to as a cell) is usually 5 to 6 inches square, and the current density generated is increased to about 5 A to 9 A per cell. Because. For this reason, from the viewpoint of further reducing loss due to series resistance, the film thickness of the back electrode formed on the entire cell surface is generally thicker than that of the thin-film silicon-based solar cell.

  In the crystalline silicon solar cell, the film thickness of the back electrode depends on the form of the back electrode, but is preferably 300 nm or more, more preferably 500 nm or more, and particularly preferably 700 nm or more from the viewpoint of reducing resistance. Moreover, from a viewpoint of suppression of material cost and reduction of the curvature in a cell, 1500 nm or less is preferable, 1200 nm or less is more preferable, and 1000 nm or less is especially preferable.

  Thus, in the crystalline silicon solar cell, the amount of the metal material used as the back electrode is larger than that of the thin film silicon solar cell. Use low-cost copper.

  However, according to the study by the present inventors, when copper was used as the back electrode as described above, the solar cell characteristics were deteriorated after a long-term environmental test. Specifically, the series resistance before and after the long-term environmental test of the solar cell module in the case of using Cu as the back electrode increased by 2 to 3% compared to the case of using Ag as the back electrode. This is considered due to the fact that the contact resistance between the Cu electrode and the transparent electrode layer was increased after a long-term environmental test.

  In the present invention, a predetermined first conductive layer 9-1 is provided between the second conductive layer 9-2 containing copper as a main component as the back electrode and the back-side transparent electrode layer 7. By forming the predetermined first conductive layer, it is possible to suppress a decrease in solar cell characteristics after a long-time environmental test. At this time, the second conductive layer is formed to cover the first conductive layer and to be in contact with the back-side transparent electrode layer. That is, a first conductive layer non-formation region not having the first conductive layer is formed on the back side transparent electrode layer.

  The first conductive layer is not particularly limited as long as it has a lower contact resistance with the back side transparent electrode layer than the second conductive layer mainly composed of copper after a long-term environmental test. It is preferable that one of Ag, Al, Au, Ni, Ti or Sn showing better contact resistance with the layer is a main component, and Ag, Al, Au, Ni, Ti or Sn is used as the first conductive layer. It is more preferable to use Thereby, even when the contact resistance between the second conductive layer and the back-side transparent electrode layer increases after the environmental test, the series resistance component in the solar cell can be reduced through the first conductive layer. Note that “after a long-term environmental test” usually means a change in resistance before and after standing at a temperature of 85 ° C. and a humidity of 85% for 1000 hours.

  Here, when light reflection at the back electrode is considered, in a normal crystalline silicon solar cell, the light reaching the back electrode is considered to be mostly on the longer wavelength side than about 700 nm. The reflectance of Cu exceeds 95% in the range of about 700 to 1500 nm in the case of normal incidence, whereas Al, Ni, Ti, Sn, etc. are inferior to Cu in the above wavelength range.

  Therefore, when the above-mentioned material having a large film thickness is used as the first conductive layer, the contact resistance is considered to increase, but the reflectance is lowered. Therefore, from the viewpoint of further suppressing the decrease in reflectivity, the first conductive layer is formed in an island shape by reducing the contact resistance after a long-term environmental test while suppressing the decrease in reflectivity. It is preferable.

  On the other hand, when Au or Ag is used as the first conductive layer, Au or Ag has a higher reflectance in the above-mentioned wavelength range than Cu, but Au and Ag are very expensive, so use as little as possible. It is preferable to do. Moreover, when island-like Ag etc. are used as a 1st conductive layer, it will behave like a nanoparticle and it will be estimated that a merit is obtained in the light scattering etc. in a back surface side.

  From the above viewpoint, the first conductive layer in the present invention is formed on the back surface side transparent electrode layer so as to have a region that is not partially formed (first conductive layer non-formation region), and the second conductive layer. Is formed so as to cover the first conductive layer and to contact the transparent electrode layer on the back surface side.

  The first conductive layer is preferably formed in an island shape. In the first conductive layer in the present invention, the maximum value d1 of the distance between the interface with the backside transparent electrode layer and the interface with the second conductive layer in the direction perpendicular to the substrate surface is the material used. However, it is not particularly limited as long as there is a portion (typically an island-shaped portion) that does not have the first conductive layer on the transparent electrode layer, but 0 nm <d1 <9 nm is preferable. Among these, d1> 2 nm is more preferable. Further, from the viewpoint of further improving the conversion efficiency, d1 <8.5 nm or less is preferable, and d1 <8 nm or less is more preferable.

  Here, d1 means a distance between the interface between the back surface side transparent electrode layer of the first conductive layer and the interface with the second conductive layer in the direction perpendicular to the substrate. For example, in a cross section perpendicular to the substrate surface, when the first conductive layer is a hemisphere, d1 is the radius of the sphere, and when the first conductive layer is a sphere, d1 is the diameter of the sphere. The “substrate surface” means a textured slope when the substrate has a texture structure. That is, the distance in the direction perpendicular to the texture slope can be obtained as d1.

  By making the first conductive layer into an island shape, the second conductive layer is in direct contact with the back side transparent electrode layer, but the contact resistance between the second conductive layer and the back side transparent electrode layer increases after the environmental test. Even if it did, the series resistance between a back surface transparent electrode layer and a 2nd conductive layer can be reduced by interposing the 1st conductive layer which exists in an island form, and the fall of a solar cell characteristic can be suppressed.

  Further, by having the island-shaped first conductive layer, compared with the case where the first electrode layer is formed on the entire surface as in Patent Document 3, the adhesiveness with the second conductive layer formed thereon or the second It is estimated that the adhesion between the conductive layer and the transparent electrode layer is improved.

  The first conductive layer in the present invention has a dense conductive material. Especially, it is preferable that a 1st conductive layer consists only of a dense conductive material. Here, the “dense conductive material” means that in the first conductive layer forming portion, there is almost no spatial gap in the material constituting the first conductive layer.

  For example, as shown in FIG. 3B, when a paste material such as an Ag paste is used as the first conductive layer, it usually has metal fine particles and a resin. In this case, each of the fine particles becomes a “dense conductive material”. However, since a resin or the like exists between the fine particles and a spatial gap exists between the fine metal particles, such a first conductive layer is provided. Does not fall under the category of “dense conductive material only”. In the form of FIG. 3B, the contact area between the back surface side transparent electrode layer 7 and the conductive material is close to a point.

  On the other hand, as shown in FIG. 3A, when an island-like metal film or the like is formed by a sputtering method or a vapor deposition method, a spatial space is formed in the metal film forming each island in the first conductive layer. It is understood that there is almost no gap and is composed of “only a dense conductive material”. In the case of the form as shown in FIG. 3A, almost the entire surface of the dense conductive material in the first conductive layer on the back surface side transparent conductive electrode layer side is in contact with the back surface side transparent electrode layer.

  Here, “substantially the entire surface” means that 90% or more of the surface of the dense conductive material on the back surface side transparent electrode layer side is in contact with the back surface side transparent electrode layer. Among these, 95% or more is preferable, and 100%, that is, the entire surface is more preferable. For example, the dense conductive material includes a concave portion in a part of the surface on the back surface side transparent conductive layer side, and a portion not in contact with the concave portion of the concave portion. The “dense conductive material only” may be any material that is substantially made of only a dense material, and may contain some impurities.

  Here, the contact resistance is considered to be influenced by the contact area with the conductive material contained in the first conductive layer, but as shown in FIG. 3A, only the dense conductive material as the first conductive layer is used. By using this, the contact area with the back surface side transparent electrode layer is widened, and accordingly, a contact resistance reduction effect can be expected more than the paste material of FIG.

  For example, when a material having higher reflectance than Cu such as Ag is used as the first conductive layer, the entire surface of the dense conductive material is in contact with the back side transparent conductive electrode layer as shown in FIG. There is an advantage that the area to be reflected increases, leading to an increase in back surface reflection (reflectance). Accordingly, it is possible to further suppress the decrease in reflectance caused by the escape of light from the gap between the fine particles and the increase in light absorption due to the multiple reflection in the gap, which may occur when using silver paste or the like. It becomes possible.

  The shape of the back electrode in the present invention is not particularly limited as long as the second conductive layer covers the first conductive layer and is in contact with the back side transparent electrode layer. That is, the back electrode may be (1) formed on almost the entire surface of the back side transparent electrode layer, or (2) part of the back side transparent electrode layer (typically in a pattern by screen printing or the like). It may be formed. In addition, the 2nd conductive layer is not adhering on a part of 1st conductive layer, and the 1st conductive layer is partially exposed. Here, “substantially the entire surface” means that the back electrode is formed on 90% or more of the back side transparent electrode layer, and the component constituting the back electrode (first conductive layer or second conductive layer). It is only necessary that substantially the entire surface of the back-side transparent electrode layer is covered with any of the above.

  In the case of (1), from the viewpoint of sufficiently reducing the series resistance, the back electrode is preferably formed at 95% or more of the back side transparent electrode layer, more preferably 100%, that is, the entire surface. preferable. Here, that the back electrode is formed on the entire surface means a state in which the second conductive layer is formed over the entire region where the first conductive layer is not formed.

As described above, each of the contact area S ₁ of the back side transparent electrode first conductive layer to the layer and a second conductive _layer, S ₂ is the like reflectance and resistivity of the material used as the first conductive layer The contact area ratio (S ₁ / S ₂ ) is preferably about 0.1 to 2.0. By setting it as the said range, it is estimated that the adhesiveness of a 2nd conductive layer and a 1st conductive layer improves, and the adhesiveness of a 2nd conductive layer and a back surface side transparent electrode layer also improves in connection with it.

The ratio of the contact area of dense conductive material contained in each of the first conductive layer and the second conductive layer _{(S 1A /} S _2A) is preferably from 0.2 to 1.5. It is estimated that the fall of a reflectance or the raise of a resistivity can be suppressed more by setting it as the said range.

  When the film thickness is d2, the second conductive layer in the present invention is preferably 300 nm or more, more preferably 500 nm or more, and particularly preferably 700 nm or more from the viewpoint of reducing electric resistance. Moreover, from a viewpoint of suppression of material cost and reduction of the curvature in a cell, 1500 nm or less is preferable, 1200 nm or less is more preferable, and 1000 nm or less is especially preferable.

  On the other hand, the thickness of the back electrode greatly depends on the number of interconnectors such as tab wires for connecting solar cells or solar cells and external circuits when modularization is performed. For example, comparing the case where two tab wires are connected to the case where three tab wires are connected, the distance between tab wires is larger in the case of two wires than in the case of three, and carriers are present in the back electrode layer. Must travel a long distance. For this reason, in the case of using cells having back electrodes of the same thickness, the series resistance is higher in the case of two tab wires than in the case of three tab wires, and the curvature factor is lowered. Therefore, in order to obtain the same curvature factor, it is necessary to increase the thickness of the back electrode when there are two tab wires. For example, when two tab wires are used and a thin film made of only a dense conductive material is used as the second conductive layer, d2 is preferably in the above range from the viewpoint of reducing resistance.

  Here, since silver exhibits the same electrical conductivity as copper, for example, when silver is used as the second conductive layer, a film thickness comparable to that when copper is used is required, resulting in high costs. . On the other hand, in the present invention, since the main component of copper is used as the second conductive layer, the back electrode can be formed at low cost even when the film thickness is large as described above.

  In addition, when a paste or the like is used as the second conductive layer, the paste material is generally formed by adding metal fine particles to the resin paste or the like. Therefore, the electrical conductivity is higher than that of a dense conductive material alone. Lower. For this reason, for example, when Ag paste is formed on the entire surface, d2 needs to be about 1 μm to 20 μm, which causes an increase in cost.

  Moreover, when using what was formed in a part (for example, grid shape) of a back surface side transparent electrode layer as a back surface electrode, a 2nd conductive layer is also formed in a grid shape. In this case, the grid usually has finger electrodes. Since the finger electrode has a narrow line width and a high resistance, it is necessary to increase the line width of the finger electrode or to increase the film thickness from the viewpoint of further reducing the resistance. If the grid also has bus bar electrodes, the total amount of necessary materials varies depending on the number, height, line width, etc. of the bus bar electrodes and finger electrodes. For example, a thickness of about several μm to several tens of μm is required.

  On the other hand, the second electrode layer can be formed at a low cost even when it is necessary to increase the thickness of the second conductive layer, for example, copper paste, by using the second conductive layer as in the present invention. It becomes possible to do. In addition, when the film thickness of the 1st conductive layer used as a back electrode and a 2nd conductive layer has a certain amount of thickness, cross-sectional observation by a step meter, SEM, or resistance value at the time of film-forming on a flat board | substrate It can be obtained by converting from On the other hand, when the thickness is small, such as an island-shaped first conductive layer, it can be obtained by observation with SEM or TEM.

In the present invention, it is preferable to use a thin film made of only a dense conductive material from the viewpoint of reducing resistance and suppressing material costs as the second conductive layer, and a back electrode having such a second conductive layer is used. (1) It is preferable to use one formed on almost the entire back surface side transparent electrode layer, and more preferable to use one formed on the entire surface. Moreover, it is preferable to use what consists only of a dense conductive material also as a 1st conductive layer, and as a back surface electrode, both the 1st conductive layer and the 2nd conductive layer consist only of a dense conductive material, It is particularly preferred to form.
In the present invention, the second conductive layer is composed mainly of copper. Among these, it is preferable to use copper from the viewpoint that it can be more easily produced at low cost.

  The back electrode may have another conductive layer in addition to the first conductive layer and the second conductive layer as long as the function of the present invention is not impaired. For example, in order to suppress oxidation of the second conductive layer containing copper as a main component, it is preferable to form a conductive layer on the second conductive layer. In this case, for example, Patent Document 3 describes that Ag / Cu / Ag forms silver on copper or aluminum to suppress modification, but silver is also relatively susceptible to sulfidation and oxidation. It can happen. Therefore, it is more preferable to use Ti, Sn, Cr, or the like as the conductive layer from the viewpoint of being more difficult to be modified and being manufactured at a lower cost.

  In the present invention, the method for producing the first conductive layer is preferably a sputtering method or a vapor deposition method. Thereby, the 1st conductive layer which consists only of a dense conductive material can be formed. In particular, when Ag, Au, or the like is used as the first conductive layer, it is preferably formed by a sputtering method from the viewpoint of reducing the manufacturing cost and from the viewpoint of good coating.

  A method for forming the second conductive layer is not particularly limited, and a physical vapor deposition method such as a sputtering method or a vacuum evaporation method, a screen printing method, a plating method, or the like is applicable. In particular, when the back electrode is (1) formed on almost the entire surface, it is preferable to form it by a sputtering method or a plating method. In this case, it is possible to easily form the second conductive layer made of only a dense conductive material.

  In addition, when the back side transparent electrode layer and the first conductive layer are formed by sputtering, it is possible to continuously form the back electrode in the same apparatus, preventing the back electrode from being oxidized, and the film interface (transparent electrode layer and back electrode) Improvement in adhesion at the interface) can be expected. Also, from the viewpoint of productivity, the sputtering method is preferable, and it is more preferable to form both the first conductive layer and the second conductive layer by the sputtering method. In this case, for example, by forming the light incident side transparent electrode layer, the back side transparent electrode layer, the first conductive layer, and the second conductive layer in this order by sputtering, the number of steps can be reduced, so that productivity is improved. From the viewpoint of Moreover, since it is expected that the second conductive layer is thick to some extent, the plating method is preferable from the viewpoint of cost reduction.

  A surface electrode 8 is preferably formed on the light incident side transparent electrode layer 6 as shown in FIG. The surface electrode 8 has at least a third conductive layer. In this case, it is also possible to use an underlayer and a third conductive layer formed in this order as surface electrodes on the light incident side transparent electrode layer. As the surface electrode, it can be produced by a known technique such as an ink jet method, a screen printing method, a wire bonding method, a spray method, a vacuum deposition method, a sputtering method, etc., but from the viewpoint of productivity, a screen printing method using an Ag paste or a copper A plating method using, etc. is preferred.

  Moreover, as a 3rd conductive layer, it is preferable to use what has copper as a main component, and it is preferable to form a plating layer by the plating method. In particular, when the second conductive layer used as the back electrode is formed by plating, it can be formed simultaneously with the third conductive layer used as the front electrode, and the number of steps can be reduced, which is preferable from the viewpoint of productivity.

  In addition, in order to prevent an electrical short circuit in the outer peripheral portion of the one-conductivity-type single crystal silicon substrate, it is preferable to perform an insulating treatment step after forming the solar battery cell. Insulation treatment process includes forming a groove in the silicon substrate with laser light, cleaving the silicon substrate according to the groove, mechanically cutting the crystalline silicon substrate using a dicing saw, and removing by etching, etc. You can also It is also possible to insulate by forming a film using a mask when forming the front and back transparent electrode layers and the back electrode layer. In addition, as long as it can exclude an electrical short circuit part, the method of only forming a groove | channel with a laser beam may be sufficient.

  As described above, by using the crystalline silicon solar cell of the present invention, it is possible to produce a crystalline silicon solar cell with high efficiency and high reliability. In addition, it is possible to produce a crystalline silicon solar cell that is low in cost and excellent in productivity.

  The crystalline silicon 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.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to a following example. FIG. 2 is a schematic cross-sectional view showing a crystalline silicon solar cell according to one embodiment of the present invention.

Example 1
As shown in FIG. 2, the crystalline silicon solar cell of Example 1 was manufactured as follows.
An n-type single crystal silicon substrate 1 having an incident plane of (100) and a thickness of 200 μm is used as a single conductivity type single crystal silicon substrate 1 and the n-type single crystal silicon substrate 1 is made into a 2 wt% HF aqueous solution. It was immersed for 3 minutes, the silicon oxide film on the surface was removed, and rinse with ultrapure water was performed twice. Next, the substrate was dipped in a 5/15 wt% KOH / isopropyl alcohol aqueous solution maintained at 70 ° C. for 15 minutes, and a texture was formed by etching the substrate surface. Thereafter, rinsing with ultrapure water was performed twice. When the surface of the single crystal silicon substrate 1 was observed with an atomic force microscope (manufactured by AFM Pacific Nanotechnology), etching was most advanced on the substrate surface, and a pyramidal texture with an exposed (111) plane was formed. It had been.

  The single crystal silicon substrate 1 after the etching was introduced into a CVD apparatus, and an i-type amorphous silicon layer 2 was formed as a first intrinsic silicon thin film 2 on the light incident side by 5 nm. The film thickness of the thin film formed in this example was measured by spectroscopic ellipsometry (trade name M2000, manufactured by JA Woollam Co., Ltd.) when the film was formed on a glass substrate under the same conditions. The film forming speed was obtained and calculated on the assumption that the film was formed at the same film forming speed.

The film forming conditions for the i-type amorphous silicon layer 2 were a substrate temperature of 170 ° C., a pressure of 120 Pa, a SiH ₄ / H ₂ flow rate ratio of 3/10, and an input power density of 0.011 W / cm ² . A p-type amorphous silicon layer 3 having a thickness of 7 nm was formed on the i-type amorphous silicon layer 2 as a reverse conductive silicon thin film 3. The film formation conditions for the p-type amorphous silicon layer 3 were a substrate temperature of 150 ° C., a pressure of 60 Pa, a SiH ₄ / B ₂ H ₆ flow rate ratio of 1/3, and an input power density of 0.01 W / cm ² . . The B ₂ H ₆ gas used above was a gas diluted with H ₂ to a B ₂ H ₆ concentration of 5000 ppm.

Next, 6 nm of i-type amorphous silicon layers 4 were formed on the back surface side as the second intrinsic silicon-based thin film 4. The film forming conditions for the i-type amorphous silicon layer 4 were a substrate temperature of 150 ° C., a pressure of 120 Pa, a SiH ₄ / H ₂ flow rate ratio of 3/10, and an input power density of 0.011 W / cm ² . An n-type amorphous silicon layer 5 having a thickness of 8 nm was formed on the i-type amorphous silicon layer 4 as a one-conductivity-type silicon-based thin film 5. The film forming conditions for the n-type amorphous silicon layer 5 were a substrate temperature of 150 ° C., a pressure of 60 Pa, a SiH ₄ / PH ₃ flow rate ratio of 1/2, and an input power density of 0.01 W / cm ² . As the PH ₃ gas mentioned above, a gas diluted with H ₂ to a PH ₃ concentration of 5000 ppm was used.

  All of these silicon-based thin films were formed without using a mask, and were formed on the entire surface of the silicon substrate 1 on the film forming surface side (surface exposed to plasma).

On this, indium tin oxide (ITO, refractive index: 1.9) was deposited as a light incident side transparent electrode layer 6 to a thickness of 100 nm. Film forming conditions were such that the substrate temperature was room temperature, and a sintered body of indium oxide and tin oxide was applied as a target by applying a power density of 0.5 W / cm ² in an argon atmosphere of 0.2 Pa. Thereafter, the back-side transparent electrode layer 7 was formed to a thickness of 50 nm under the same film-forming conditions as the light incident-side transparent electrode layer 6. After forming the transparent electrode layer 7, 8 nm of Ni was formed by sputtering as the first conductive layer 9 of the back electrode, and then 500 nm of copper was formed as the second conductive layer 10 by using the sputtering method. The transparent electrode layer, the first conductive layer, and the second conductive layer were all formed without using a mask.

  In addition, the thickness of the back surface electrode in this invention was measured using SEM (Field emission type scanning electron microscope S4800, Hitachi High-Technologies company make). At this time, the back electrode was formed on the entire surface of the transparent electrode layer. The first conductive layer was formed in an island shape, and the second conductive layer was formed on a region where the first conductive layer was formed so as to cover the first conductive layer and to be in contact with the transparent electrode layer. . On the light incident side transparent electrode layer 6, a surface electrode 8 was formed with Ag paste using a screen printing method.

  Then, it moved to the laser processing apparatus and the groove | channel was formed over the perimeter of the outer peripheral part of the light-incidence side of a crystalline silicon substrate with the laser beam. The position of the groove was 0.5 mm from the edge of the crystalline silicon substrate. As the laser light, the third harmonic (wavelength 355 nm) of a YAG laser was used, and the depth of the groove was set to about one third of the thickness of the crystalline silicon substrate. Then, it bent and fractured along the groove | channel, and it was set as the insulation process process by removing the crystalline silicon substrate outer peripheral part. Thereafter, annealing was performed at 190 degrees for 1 hour.

A crystalline silicon solar cell was produced as described above. Using a solar simulator having a spectral distribution of AM1.5, the solar cell characteristics were measured by irradiating simulated sunlight with an energy density of 100 mW / cm ² at 25 ° C.

  Furthermore, a mini module including a single crystalline silicon-based solar cell was manufactured, and an environmental test was performed in which the mini module was left in an environment of a temperature of 85 degrees and a humidity of 85% for 1000 hours. The structure of the mini-module is a backsheet / sealing material / wiring member-connected crystalline silicon solar cell / sealing material / glass, and is connected to an external measuring instrument via a wiring member attached to the crystalline silicon solar cell. It connected and measured the solar cell characteristic using the said solar simulator. Before and after the environmental test, the solar cell outputs were compared, and if the retention rate = (output after the environmental test) ÷ (output before the environmental test) × 100 ≧ 95.0 (%), it was determined to be acceptable.

(Example 2)
A crystalline silicon solar cell was fabricated and evaluated in the same manner as in Example 1 except that 3 nm of Ni was formed as the first conductive layer 9 described in Example 1. In Example 2, the first conductive layer is formed in an island shape, and the second conductive layer covers the first conductive layer, and the region where the first conductive layer is not formed so as to be in contact with the transparent electrode layer Was formed on top.

(Comparative Example 1)
A crystalline silicon solar cell was fabricated and evaluated in the same manner as in Example 1 except that 20 nm of Ni was formed as the first conductive layer 9 described in Example 1. In Comparative Example 1, as shown in FIG. 1, the first conductive layer was formed in a layer shape, and was formed on the entire surface of the transparent electrode layer. The second conductive layer was formed so as to cover the first conductive layer, but was not in contact with the transparent electrode layer.

(Comparative Example 2)
A crystalline silicon solar cell was fabricated and evaluated in the same manner as in Example 1 except that Ni was not formed as the first conductive layer 9 described in Example 1. In Comparative Example 2, the second conductive layer was formed on the entire surface of the transparent electrode layer.

  A crystalline silicon solar cell was produced as described above.

  Table 1 shows the relationship between the film thickness of the formed Ni and the characteristics of the minimodule.

  In Comparative Example 2 in which Ni was not formed as the first conductive layer 9 and Cu used as the second conductive layer was in contact with the straight back-side transparent electrode layer, Jsc was the highest. This is not because Ni has a reflectance of only about 65% to 75% at wavelengths of 700 nm to 1200 nm, but Cu that is about 95% or more functions as a back reflecting layer on the entire surface, so Jsc is another example. It became higher than that. On the other hand, in Comparative Example 1 where Ni was formed on the entire surface, Jsc was the lowest. In Examples 1 and 2 according to the present invention, Jsc was improved over Comparative Example 1. This is probably because Cu is in contact with the transparent electrode layer in the island-shaped Ni gap, so that the reflectance at the portion is improved.

  Next, Table 2 shows the characteristics and retention rates of each mini-module after the 1000-hour environmental test.

  In Comparative Example 2, a decrease in the curvature factor, which is considered to be caused by an increase in contact resistance between ITO and Cu, was observed. On the other hand, from the viewpoint of the retention rate, by forming Ni in an island shape (Examples 1 and 2) or the entire surface (Comparative Example 1), it is possible to suppress a decrease in the curvature factor, which is 95% or more. A retention rate could be obtained. This is presumably because the contact resistance of Ni to ITO after a long environmental test is lower than that of Cu. From the above, it is considered that the first conductive layer 9 is preferably formed in an island shape in order to maintain a high reflectance (ie, Jsc) and a high retention after the environmental test.

  As described above, by using the crystalline silicon-based solar battery of the present invention, it is possible to provide a highly reliable high-output solar battery cell at a low cost.

1. 1. One conductivity type single crystal silicon substrate 1. First intrinsic silicon-based thin film layer 3. Reverse conductivity type silicon-based thin film layer 4. Second intrinsic silicon-based thin film layer 5. One conductivity type silicon-based thin film layer 6. Light incident side transparent electrode layer 7. Back side transparent electrode layer Collector electrode 8-1. First conductive layer 8-2. Second conductive layer 9. Back electrode 9-1. First conductive layer 9-2. Second conductive layer

Claims (12)

On the first main surface side of the single conductivity type single crystal silicon substrate, the light incident side silicon-based thin film layer, the light incident side transparent electrode layer, and the surface electrode are provided in this order, and on the second main surface side of the substrate, A crystalline silicon solar cell having a back side silicon thin film layer, a back side transparent electrode layer, and a back electrode in this order,
The back electrode has a first conductive layer and a second conductive layer in this order from the transparent electrode layer side,
The second conductive layer is a crystalline silicon solar cell comprising copper as a main component, covering at least the first conductive layer, and being in contact with the back-side transparent electrode layer.   The crystalline silicon solar cell according to claim 1, wherein the first conductive layer has an island shape.   The first conductive layer is made of only a dense conductive material, and substantially the entire surface of the dense conductive material on the back surface side transparent electrode layer side is in contact with the back surface side transparent electrode layer. A crystalline silicon-based solar cell according to 1.   The first conductive layer has a maximum distance d1 between the interface with the backside transparent electrode layer and the interface with the second conductive layer in a direction perpendicular to the substrate surface, satisfying 0 nm <d1 <9 nm. The crystalline silicon solar cell according to any one of claims 1 to 3.   5. The crystalline silicon solar cell according to claim 1, wherein the first conductive layer is mainly composed of one selected from Ag, Al, Au, Ni, Ti, or Sn.   The said surface electrode is a crystalline silicon type solar cell of any one of Claims 1-5 which has a 3rd conductive layer which has copper as a main component.   The crystalline silicon solar cell according to claim 6, wherein the second conductive layer and the third conductive layer are plating layers.   The solar cell module using the crystalline silicon type solar cell of any one of Claims 1-7. It is a manufacturing method of a crystalline silicon system solar cell given in any 1 paragraph of Claims 1-7,
A back electrode forming step of forming a back electrode on the second main surface opposite to the substrate of the back side transparent electrode layer;
The back electrode forming step includes forming a first conductive layer on the back side transparent electrode layer;
And a step of forming a second conductive layer so as to cover the first conductive layer and to be in contact with the transparent electrode layer on the back surface side in this order.   The method for manufacturing a crystalline silicon solar cell according to claim 9, wherein the first conductive layer is formed by a sputtering method. A surface electrode forming step of forming a surface electrode including a third conductive layer on the first main surface opposite to the substrate of the light incident side transparent electrode layer;
The crystalline silicon-based solar according to claim 9 or 10, wherein the surface electrode forming step includes a step of forming the third conductive layer on the first main surface side of the light incident side transparent electrode layer by a plating method. Battery manufacturing method. The manufacturing method of the crystalline silicon solar cell of Claim 11 with which the 2nd conductive layer in the said back surface electrode and the 3rd conductive layer in the said surface electrode are formed simultaneously by the plating method.

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