solar cells

Abstract

The present invention provides a solar cell which employs copper as an electrode material, while having good solar cell characteristics and adhesion.　A solar cell according to the present invention has a configuration in which: a silver finger electrode 2 and a first copper bus electrode 1 are arranged on the light-receiving surface side of a silicon substrate; the first copper bus electrode 1 covers a part of the silver finger electrode 2; an Al finger electrode, an Al bus electrode 4 and a second copper bus electrode 3 are arranged on the back surface side of the silicon substrate; the Al finger electrode and the Al bus electrode 4 are formed of a sintered body of fine particles that contain aluminum; the second copper bus electrode 3 covers a part of the Al bus electrode 4; and the Al bus electrode 4 contains an alloy, which contains aluminum and copper, in a region of the sintered body extending from a position that is 5 µm away from an interfacial edge 9 between the Al bus electrode 4 and the second copper bus electrode 3 to a position that is 170 µm away from the interfacial edge 9.

Description

[Technical Field]

[0001] The present invention relates to electrode wiring of a solar cell and its peripheral structure. [Background technology]

[0002] The need for solar cells as a renewable energy source is increasing. Traditionally, expensive silver paste has been used as the material for the bus electrodes and finger electrodes, which are the current collecting electrodes, making it difficult to reduce the cost of these electrodes. For this reason, proposals have been made to convert silver (Ag) electrodes to copper (Cu) electrodes to reduce costs. Hereinafter, silver will sometimes be referred to as "Ag," copper as "Cu," and silicon as "Si."

[0003] As an example, Patent Document 1 proposes a solar cell device 10 having a silicon semiconductor substrate 1, a Cu-containing metal layer 4, Ag-containing finger wires 4, and an interface layer 3 containing an oxide or an organic compound, in which the Ag-containing finger wires 4 are stacked on the light-receiving surface side of the silicon semiconductor substrate 1, the interface layer 3 is stacked on the light-receiving surface side of the silicon semiconductor substrate 1, and the Cu-containing metal layer 4 is stacked on the interface layer 3 and spaced apart from the Ag-containing finger wires 2. However, because an insulating interface layer exists between the Cu-containing metal layer and the Ag-containing finger wires, the electrical resistance of the entire wiring increases. As a result, the series resistance of the solar cell increases, resulting in a deterioration in fill factor (FF) and conversion efficiency.

[0004] Furthermore, when a copper electrode is placed in direct contact with a silicon substrate to increase the conductivity between the copper electrode and the silicon substrate, not only does interdiffusion occur between the Cu in the copper electrode and the Si in the silicon substrate, but the Cu diffusion rate in the Si is also very fast (see Non-Patent Document 1). Cu that penetrates into the silicon substrate forms an acceptor level at a deep energy position in the silicon band gap, shortening the carrier lifetime in the diode and reducing the open circuit voltage (Voc), which causes degradation of solar cell characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2016 / 152481 [Non-patent literature]

[0006] [Non-Patent Document 1] ASGrove, Physics and Technology of Semiconductor Devices, p40(1967) Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, when expensive silver is replaced with inexpensive copper as the material for the bus electrodes and finger electrodes to reduce the manufacturing cost of solar cell devices, the copper electrode material easily diffuses into the silicon substrate, resulting in degradation of solar cell performance. Furthermore, when an interfacial layer is placed between the copper electrode and the silicon substrate to prevent this copper diffusion, the electrical resistance of the entire wiring increases, increasing the cell's series resistance (Rs), resulting in degradation of the fill factor (FF) and conversion efficiency. Another issue is that copper electrodes have poor adhesion to the silicon substrate and materials placed underneath them, such as anti-reflective coatings (e.g., SiN, SiO2), which can cause the copper electrode to peel off from the cell surface.

[0008] An object of the present invention is to provide a solar cell that has good solar cell characteristics and adhesion even when silver electrodes are replaced with copper electrodes. [Means for solving the problem]

[0009] The inventors have investigated the above-mentioned problems and found that by adjusting the composition of the Cu-containing bus electrode, it is possible to improve the adhesion between the Cu electrode and its underlying material without sacrificing the soldering properties of the TAB wire. Furthermore, they have found that it is possible to prevent Cu from the Cu-containing bus electrode from diffusing into the silicon substrate via the Ag electrode and Al electrode, which led to the completion of the present invention. Specifically, the present invention includes the following embodiments (1) to (5).

[0010] (1) An embodiment of the present invention is a solar cell having a silicon substrate, the silicon substrate has an insulating layer on a part of its surface, a silver finger electrode and a first copper bus electrode are disposed on a light-receiving surface side of the silicon substrate; the silver finger electrode comprises silver and is in contact with a surface of the silicon substrate; the first copper bus electrode contains at least copper, covers a portion of the silver finger electrodes, and is in contact with the insulating layer at a portion other than the silver finger electrodes; aluminum finger electrodes, an aluminum bus electrode, and a second copper bus electrode are disposed on a back surface side of the silicon substrate; the aluminum finger electrodes and the aluminum bus electrodes are formed of a sintered body of fine particles containing aluminum, In the solar cell, a portion of the aluminum bus electrode is covered with the second copper bus electrode, and a region of the sintered body located at a distance of 5 μm or more and 170 μm or less from the edge of the interface between the aluminum bus electrode and the second copper bus electrode contains an alloy containing aluminum and copper.

[0011] (2) In an embodiment of the present invention, the first copper bus electrode or the second copper bus electrode contains at least one of Te oxide or Se oxide, which has a softening point of 450°C or less, In the solar cell according to (1) above, in a cross section perpendicular to the longitudinal direction of the copper bus electrode, the area ratio of Te or Se, which is a constituent element of the Te oxide or the Se oxide, distributed is 4% or more and 45% or less.

[0012] (3) In an embodiment of the present invention, the first copper bus electrode or the second copper bus electrode contains a thermoplastic organic polymer having a glass transition temperature of 300°C or higher and 450°C or lower, The solar cell according to (1) or (2) above, wherein the area ratio of carbon, a constituent element of the thermoplastic organic polymer, distributed in a cross section perpendicular to the longitudinal direction of the copper bus electrode is 9% or more and 40% or less.

[0013] (4) In an embodiment of the present invention, a part or all of the silver finger electrodes have a laminated structure consisting of a first layer electrode and a second layer electrode, the first electrode contains silver, penetrates the insulating layer, and contacts the surface of the silicon substrate; the second layer electrode includes copper and is disposed on the first layer electrode; The line width of the electrode of the first layer is 15 μm or more and 60 μm or less, The solar cell according to (1) or (2) above, wherein the line width of the second layer electrode is 20 μm or more and 100 μm or less, which is wider than the line width of the first layer electrode, and the ends in the width direction are in contact with the insulating layer.

[0014] (5) In an embodiment of the present invention, a part of the silver finger electrode has a laminated structure consisting of a first layer electrode and a second layer electrode, the first-layer electrodes contain silver, penetrate the insulating layer, and contact the surface of the silicon substrate; the first-layer electrodes are spaced apart in the longitudinal direction; the length of each first-layer electrode is 1 mm or more and 15 mm or less; the spacing between the first-layer electrodes is 1 mm or more and 7 mm or less in the longitudinal direction and 0.8 mm or more and 3 mm or less in the width direction; The solar cell according to (1) or (2) above, wherein the second layer electrode contains copper, is disposed in contact with the first layer electrode, and electrically connects the separated first layer electrodes to each other. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a solar cell that can use inexpensive copper bus electrodes as bus electrodes and that has good adhesion between the Cu bus electrode and its underlying substrate. Furthermore, it is possible to lower the firing temperature for forming the electrodes, and it is possible to prevent Cu in the copper bus electrode from diffusing into the silicon substrate via the Ag finger electrodes and Al finger electrodes. According to the present invention, it is possible to significantly reduce the manufacturing cost of solar cells without sacrificing performance and reliability. [Brief explanation of the drawings]

[0016] [Figure 1] 1A and 1B are diagrams showing an example of a wiring structure arranged on the light-receiving surface side of a solar cell according to this embodiment, in which (a) shows a structure in which silver finger electrodes are continuous, and (b) shows a structure in which the silver finger electrodes are discontinuous. [Figure 2] 1A and 1B are SEM images of cross sections of an aluminum bus electrode and a second copper bus electrode arranged on the back side of a solar cell according to an embodiment of the present invention, where (a) is a view showing the entire cross section, (b) is an enlarged view of region X in (a), and (c) is an enlarged view of region Y in (a). [Figure 3] FIG. 3 is a diagram schematically showing a cross-sectional structure taken along line AA′ in FIG. 2. [Figure 4] 1A and 1B are diagrams showing SEM images and SEM-EDX composition distribution images of a cross section of a first copper bus electrode and a silver finger electrode arranged on the light-receiving surface side of a solar cell according to this embodiment, where (a) is a diagram showing an SEM image of the cross section, (b) is a diagram showing an Ag distribution image, (c) is a diagram showing a Cu distribution image, and (d) is a diagram showing a Te distribution image. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described. The present invention is not limited to this embodiment. In this specification, the expression "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less." means.

[0018] (Basic structure of solar cells) The solar cell according to this embodiment has a silicon substrate, which has an insulating layer on a portion of its surface. Silver finger electrodes and a first copper bus electrode are arranged on the light-receiving surface side of the silicon substrate, and aluminum finger electrodes, an aluminum bus electrode, and a second copper bus electrode are arranged on the back surface side of the silicon substrate. The insulating layer can be made of a material such as SiN, SiO2, or Al2O3. Of the surfaces of the silicon substrate, the "light-receiving surface" refers to the surface that is exposed to sunlight, and the "back surface" refers to the surface located opposite the light-receiving surface. In this specification, the "silicon substrate" may also be simply referred to as a "substrate."

[0019] (Electrode structure) The silver finger electrodes contain silver and are in contact with the surface of the silicon substrate. The first copper bus electrode contains at least copper. The aluminum finger electrodes contain aluminum and are in contact with the surface of the silicon substrate. The aluminum bus electrodes are formed of a sintered body of fine particles containing aluminum. In this specification, the "copper bus electrodes" may be referred to as "Cu bus electrodes," the "silver finger electrodes" as "Ag finger electrodes," the "aluminum finger electrodes" as "Al finger electrodes," and the "aluminum bus electrodes" as "Al bus electrodes."

[0020] The first copper bus electrode, located on the light-receiving surface side, covers a portion of the silver finger electrode and is in contact with the insulating layer except for the silver finger electrode. The second copper bus electrode covers a portion of the aluminum bus electrode and is in contact with the insulating layer except for the aluminum bus electrode. With this electrode structure, the copper bus electrode, which is one of the conductive electrodes, does not directly contact the silicon substrate, thereby suppressing carrier annihilation at the interface between the electrode and the silicon substrate. This increases the fill factor (FF) and open-circuit voltage (Voc).

[0021] 1(a) and (b) are schematic diagrams showing an electrode structure in which silver finger electrodes 2 arranged on the light-receiving surface side are covered with a first copper bus electrode 1. The silver finger electrodes 2 may be formed continuously from one end of the substrate to the other end, as shown in Fig. 1(a), or may be formed discontinuously in the portion covered by the first copper bus electrode 1, as shown in Fig. 1(b).

[0022] (Fabrication of solar cells) The solar cell according to this embodiment can be fabricated, for example, by the following method. A silicon wafer having a textured surface and a p / n junction formed thereon can be used as the silicon substrate. On the back surface of the silicon substrate, opposite the light-receiving surface, an Al2O3 layer is formed on the back surface by atomic layer deposition (ALD), and a SiN layer is formed on the Al2O3 layer by plasma-enhanced chemical vapor deposition (PECVD). Next, partial openings are formed in the SiN / Al2O3 layer using a laser beam. After that, an aluminum paste is printed using a screen printer to form aluminum bus electrodes and aluminum finger electrodes.

[0023] The SiN layer serves as an anti-reflection layer, the Al2O3 layer serves as a passivation layer, and since both the SiN layer and the Al2O3 layer have insulating properties, they also function as an insulating layer on the substrate.

[0024] On the other hand, on the light-receiving surface side of the silicon substrate, a SiN layer is formed on the light-receiving surface, and then silver paste is printed to form finger electrodes.

[0025] The silicon substrate on which each of the above pastes has been printed is then subjected to a heat treatment (fire-through) in which it is heated in the atmosphere for several minutes at a temperature of 600°C to 900°C. This heat treatment establishes connections between the silver finger electrodes and the silicon substrate on the light-receiving surface of the silicon substrate, and between the aluminum bus electrodes and the aluminum finger electrodes and the silicon substrate on the back surface.

[0026] Next, on the substrate obtained by the heat treatment, copper paste for forming a second copper bus electrode is printed partially on the aluminum bus electrode on the back side. Meanwhile, copper paste for forming a first copper bus electrode is printed on the light-receiving side of the substrate so as to cover the silver finger electrodes. The substrate with these copper pastes printed thereon is dried in air at 100°C for 3 minutes, and then subjected to an oxidation heat treatment in which the substrate is heated in air to convert the copper paste into copper oxide and sintered. The obtained substrate is then subjected to a reduction heat treatment in which the substrate is heated in a reducing gas atmosphere to convert the copper oxide into copper, thereby obtaining the first copper bus electrode and the second copper bus electrode. The heating conditions for the oxidation heat treatment or reduction heat treatment can be 300 to 420°C and 1 to 30 minutes. The reducing gas atmosphere can be a mixed gas consisting of hydrogen (e.g., 3% by volume) and nitrogen.

[0027] (Interface area between aluminum bus electrode and second copper bus electrode) In the solar cell according to this embodiment, the aluminum bus electrode disposed on the back side of the substrate is formed from a sintered body of fine particles containing aluminum, and a second copper bus electrode is formed on a portion of the aluminum bus electrode. The sintered body of the aluminum bus electrode has a region containing an alloy containing aluminum and copper near the edge of the interface between the aluminum bus electrode and the second copper bus electrode. Near the edge of the interface between the aluminum bus electrode and the second copper bus electrode, the aluminum of the aluminum bus electrode reacts with the copper of the second copper bus electrode, forming a region containing an alloy containing aluminum and copper within the sintered body. This alloy region can improve adhesion between the copper bus electrode and the contact electrode.

[0028] 2(a) to 2(c) show SEM images, taken with a scanning electron microscope (SEM), of a region including the aluminum bus electrode 4 disposed on the back side and the second copper bus electrode 3 formed partially overlapping it. As shown in FIG. 2(a), the second copper bus electrode 3 and the aluminum bus electrode 4 formed using the copper paste and the aluminum paste can be distinguished based on the particle size and shape, and therefore the structure near the interface between the aluminum bus electrode 4 and the second copper bus electrode 3 can be identified.

[0029] FIG. 3 is a schematic diagram showing the cross-sectional structure taken along line A-A' in FIG. 2(a). As shown in FIG. 3, the second copper bus electrode 3 is disposed so as to cover a portion of the aluminum bus electrode 4, forming an overlapping portion 8 between the second copper bus electrode 3 and the aluminum bus electrode 4. The aluminum bus electrode 4 contacts an Al finger electrode 10 on the side opposite the overlapping portion 8. An interface 7 between the second copper bus electrode 3 and the aluminum bus electrode 4 continues to the edge of the overlapping portion 8. The edge of the interface located at the edge of the overlapping portion 8 is located at the position indicated by the reference numeral 9 in FIG. 3. In this specification, the edge of the interface is referred to as the "interface edge." The interface edge 9 can be identified as being located at the position indicated by the white dotted line in the SEM image in FIG. 2(a).

[0030] Two regions (indicated by reference numerals 5 and 6) were selected in the aluminum bus electrode 4 shown in FIG. 2(a), and enlarged SEM images of these regions are shown in FIGS. 2(b) and 2(c). FIG. 2(b) shows region X, which is distant from the interface edge, and FIG. 2(c) shows region Y, which is adjacent to the interface edge. The image contrast of region Y exhibits a patchy structure compared to the image contrast of region X. Analysis of region Y using an X-ray energy dispersive spectroscopy (SEM-EDX) revealed that an alloy containing aluminum and copper had formed. By changing the temperatures of the oxidation heat treatment and reduction heat treatment, it was possible to determine the distance from the interface edge to which the region in which the alloy containing aluminum and copper had formed had progressed.

[0031] In the solar cell according to this embodiment, a region of the sintered body located at a distance of 5 μm or more and 170 μm or less from the edge of the interface between the aluminum bus electrode and the second copper bus electrode contains an alloy containing aluminum and copper. Hereinafter, this specification may refer to the region containing the alloy containing aluminum and copper as the "alloy layer." The above-specified distance for the alloy layer will be explained below.

[0032] The range of the region where the alloy layer is formed can be determined, for example, as follows. The surface structure of the Al bus electrode is observed at 4000x magnification using an SEM. At this time, the observation region is moved from the interface edge toward the Al finger electrode, and multiple SEM images are taken. In these SEM images, alloy particles exhibiting a patchy structure and Al particles without a patchy structure are clearly distinguishable, as shown in FIGS. 2(b) and 2(c). The captured multiple images are stitched together, and multiple rectangular frames are drawn within the image. The first rectangle is positioned so that one long side overlaps the interface edge 9 and the other long side is included in the Al bus electrode. The structure within this rectangular frame is then observed, and the number of alloy particles and Al particles contained within the frame is determined. If 10% or more of the obtained total number of particles exhibit a patchy structure, the first rectangular region is determined to be an alloy layer. The alloy layer containing aluminum and copper in this embodiment is identified by this method.

[0033] If the structure of the first rectangle is not an alloy layer, the distance from the interface edge is determined to be zero. If the structure of the first rectangle is an alloy layer, a second rectangle is placed farther from the interface edge than the first rectangle, with one long side overlapping the long side of the first rectangle on the Al bus electrode side. Then, the structure within the frame is observed in the same manner as for the first rectangle, and the proportion of particles exhibiting a mottled structure is measured. If 10% or more of the particles contained within the frame of the second rectangle exhibit a mottled structure, the region of the second rectangle is determined to be an alloy layer. For example, if the size of the rectangular frame is 20 μm × 5 μm so that it is 5 μm away from the interface edge, if the second rectangle is not an alloy layer, the distance from the interface edge is determined to be 5 μm. By repeating this procedure, it is possible to determine the distance to which the alloy layer has progressed from the interface edge.

[0034] The alloy layer containing the aluminum and copper alloy is preferably present within a range of 5 μm to 170 μm from the edge of the interface between the aluminum bus electrode and the copper bus electrode. If the alloy layer extends beyond 170 μm, copper may diffuse into the silicon substrate in contact with the aluminum bus electrode, significantly deteriorating battery performance. The lower limit of the distance from the edge of the interface to the edge of the alloy layer is preferably 5 μm or more, more preferably 10 μm or more, even more preferably 15 μm or more, or even more preferably 47 μm or more. On the other hand, the upper limit of this distance is preferably 170 μm or less, more preferably 100 μm or less, and particularly preferably 91 μm or less. Even when the heat treatment temperature and heat treatment time are changed, it is sufficient to select conditions that allow the distance from the edge of the interface to the edge of the alloy layer to be within a range of 5 μm to 170 μm.

[0035] (Te oxide, Se oxide) At least one of the first copper bus electrode and the second copper bus electrode of the solar cell according to this embodiment preferably contains copper and at least one of Te oxide (tellurium oxide) and Se oxide (selenium oxide), both of which have a softening point of 450° C. or less. Hereinafter, the first copper bus electrode and the second copper bus electrode may be collectively referred to as the "copper bus electrodes."

[0036] The copper bus electrode is produced by firing a copper paste printed on an insulating layer. If the copper paste contains an oxide that softens or melts at the firing temperature, the oxide softens or melts and then cools and distributes during the firing process. This results in improved adhesion between the oxide and the insulating layer, preventing the bus electrode from peeling off from the insulating layer. From this perspective, the copper bus electrode preferably contains at least one of Te oxide and Se oxide, both of which have a softening point of 450°C or lower.

[0037] Furthermore, copper paste containing oxides with a softening point of 450°C or less can be fired at low temperatures, preventing Cu from diffusing into the silicon substrate via the silver finger electrodes or Al bus electrodes during firing. Mixing Te oxide and Se oxide is preferable, as it reduces the melting point to 350°C or less. In addition to tellurium (Te) and selenium (Se) oxides, other oxides include bismuth (Bi) and vanadium (V) oxides.

[0038] Here, the softening point of the oxide according to this embodiment is defined as the temperature at the fourth inflection point obtained by differential thermal analysis.

[0039] (The percentage of the area where Te or Se is distributed (area ratio)) As described below, the distribution of Te oxide or Se oxide contained in the copper bus electrode corresponds to the distribution of Te or Se. In this embodiment, the copper bus electrode was specified based on the area ratio where Te or Se is distributed. That is, in a cross section perpendicular to the longitudinal direction of the copper bus electrode, the area ratio where Te (tellurium) or Se (selenium), a constituent element of Te oxide or Se oxide, is distributed is preferably 4% or more and 45% or less. Hereinafter, this "area ratio" will be referred to as "area ratio." If the area ratio of Te or Se is less than 4%, insufficient adhesion may occur. On the other hand, if the area ratio is too high, the electrical resistance of the copper bus electrode increases. Furthermore, the increase in oxides present on the surface of the copper bus electrode may cause a problem in which solder does not sufficiently adhere to the copper bus electrode during the process of soldering a TAB wire to the copper bus electrode. Therefore, the lower limit of the area ratio of Te or Se is preferably 4% or more, more preferably 10% or more, and even more preferably 13% or more. The upper limit of the area ratio is preferably 45% or less, and more preferably 40% or less.

[0040] Here, a method for measuring the area fraction of Te or Se contained in a copper bus electrode will be described below, using Te as an example. A sample obtained by a specific fabrication method had a Cu bus electrode and Ag finger electrodes formed on the light-receiving surface side. A cross section perpendicular to the longitudinal direction of the Cu bus electrode was exposed for the sample, and the composition distribution of the cross section was examined using a scanning electron microscope (SEM) and an X-ray energy dispersive spectroscopy (SEM-EDX). The samples used for cross-sectional observation were prepared from five randomly selected locations on the Cu bus electrode. Figure 4(a) shows an SEM image of the cross-sectional structure. Figures 4(b), (c), and (d) show composition distribution images of Cu, Ag, and Te, respectively, obtained by SEM-EDX. The areas containing Cu in Figure 4(b) and the areas containing Te in Figure 4(d) are indicated in white. The area of ​​Te distribution and the area of ​​Cu distribution were measured at five randomly selected points on the cross section, and the area of ​​Te was divided by the total area of ​​Cu and Te to obtain the area ratio of Te. The average value was taken as the area ratio of Te. In Figure 4(d), the area ratio of Te was calculated to be 6.4%. Since the distribution of Te corresponds to the distribution of Te oxide, the effect of Te oxide can be evaluated based on the area ratio of Te. The same applies to the area ratio of Se.

[0041] (thermoplastic organic polymer) To ensure adhesion between the copper bus electrode and the insulating layer, an organic layer may be provided near the interface between the copper bus electrode and the insulating layer. The copper bus electrode of the solar cell according to this embodiment preferably contains a thermoplastic organic polymer having a glass transition temperature of 300°C or higher and 450°C or lower, and the proportion of the area where carbon, a constituent element of the thermoplastic organic polymer, is distributed in a cross section perpendicular to the longitudinal direction of the bus electrode is preferably 9% or higher and 40% or lower.

[0042] A bus electrode containing a thermoplastic organic polymer constituting the organic layer and having a glass transition temperature of 300°C or higher and 450°C or lower can enhance adhesion to the insulating layer. When the bus electrode is produced by baking, it does not thermally decompose but softens or melts, and then cools to form a bus electrode in close contact with the insulating layer. If the glass transition temperature of the thermoplastic organic polymer is lower than 300°C, the viscosity of the organic layer decreases during baking, increasing the amount of the polymer penetrating into the voids in the bus electrode. This reduces the amount of organic layer present at the interface between the bus electrode and the insulating layer, potentially reducing adhesion strength. On the other hand, if the glass transition temperature is higher than 450°C, sufficient adhesion strength may not be obtained.

[0043] The size of the organic layer formed near the interface can be evaluated by the area ratio of carbon contained in the thermoplastic organic polymer forming the organic layer. Hereinafter, this "area ratio" will be referred to as "area ratio." The carbon area ratio can be obtained by SEM-EDX analysis of a cross section of the bus electrode, similar to the measurement method described above for the area ratio of Te. In a cross section perpendicular to the longitudinal direction of the bus electrode, the carbon area ratio is preferably 9% or more and 40% or less. If the carbon area ratio is less than 9%, the adhesion strength between the bus electrode and the insulating layer decreases. On the other hand, if the carbon area ratio exceeds 40%, there is a risk of a problem in which the solder does not sufficiently adhere to the bus electrode during the process of soldering the TAB wire to the bus electrode. The lower limit of the carbon area ratio is preferably 9% or more, more preferably 25% or more. The upper limit of the carbon area ratio is preferably 40% or less.

[0044] The glass transition temperature of the thermoplastic organic polymer constituting the organic layer is defined as the temperature at which the loss tangent measured by dynamic viscoelasticity measurement becomes maximum.

[0045] (First and second layers of finger electrodes) In the solar cell according to this embodiment, some or all of the finger electrodes have a laminated structure made up of a first layer electrode and a second layer electrode.

[0046] In one embodiment, the first layer electrode contains silver and penetrates the insulating layer to contact the surface of the silicon substrate. The second layer electrode contains copper and is disposed on the first layer. The line width of the first layer electrode is 15 μm or more and 60 μm or less, and the line width of the second layer electrode is 20 μm or more and 100 μm or less, and is preferably wider than the line width of the first layer electrode, with the widthwise ends contacting the insulating layer.

[0047] If the line widths of the first and second layers are too small, a sufficient amount of paste will not be extruded from the openings in the screen printing plate during paste printing, making it impossible to form continuous wiring. On the other hand, if the line widths are too large, the light-receiving area will be reduced, resulting in a deterioration in conversion efficiency. The second layer electrode must be wider than the first layer electrode in order to transmit electricity from the first layer electrode to the outside. Furthermore, since the second layer electrode contains copper, it is preferable that the second layer electrode be in contact with an insulating layer except for the portion in contact with the first layer electrode in order to suppress Cu diffusion into the substrate.

[0048] In another embodiment, the first layer electrodes contain silver, penetrate the insulating layer, and contact the surface of the silicon substrate, and are spaced apart in the longitudinal direction. Each of the first layer electrodes has a length of 1 mm to 10 mm, and the spacing between the first layer electrodes is 1 mm to 5 mm in the longitudinal direction and 0.8 mm to 2 mm in the width direction. The second layer electrodes contain copper, and are arranged in contact with the first layer electrodes, electrically connecting the spaced-apart first layer electrodes.

[0049] As described above, the arrangement of the first-layer silver finger electrodes may be continuous from one end of the substrate to the other, or may be interrupted along the way. Selecting the length of each electrode in the first layer and the spacing between the electrodes in the first layer within the specified ranges is preferable because carriers inside the silicon substrate can efficiently migrate from the emitter section to the first-layer electrodes. Furthermore, in the case of interrupted silver finger electrodes, the copper finger electrodes in the second layer are preferable because they can connect the individual silver finger electrodes to each other and extract carriers inside the substrate to the outside. Furthermore, if the length and width of the first-layer electrodes are greater than the specified ranges, the light-receiving area becomes smaller, resulting in a deterioration in conversion efficiency. On the other hand, if the length and width of the first-layer electrodes are smaller than the specified ranges, the proportion of photoinduced carriers generated in the substrate that recombine and disappear before reaching the first-layer electrodes increases, which is undesirable because it reduces the fill factor (FF) and open-circuit voltage (Voc). [Example]

[0050] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0051] (Example 1) Distance from the interface edge to the alloy layer A silicon substrate with a textured surface and a p / n junction was used. An Al2O3 layer was formed on the backside of the silicon substrate by atomic layer deposition (ALD), and a SiN layer was formed on top of the Al2O3 layer by plasma-enhanced chemical vapor deposition (PECVD). Next, partial openings were formed in the SiN / Al2O3 layer using laser light. Next, aluminum paste was printed by screen printing to form Al bus electrodes and Al finger electrodes. A SiN layer was formed on the light-receiving surface of the silicon substrate, and silver paste was printed on top of that. The silicon substrate was then subjected to a heat treatment in air at 600°C for 3 minutes (this heat treatment is also called "fire-through") to form the silver finger electrodes and Al bus electrodes.

[0052] Next, copper paste was printed partially on the Al bus electrode on the back surface of the obtained substrate. The copper paste for forming a first copper bus electrode was printed on the light-receiving surface of the substrate so as to cover the silver finger electrodes. After printing, the substrate was air-dried at 100°C for 3 minutes and then subjected to an oxidation heat treatment in air at a predetermined temperature for 3 minutes to convert the copper paste into copper oxide. The obtained substrate was then subjected to a reduction heat treatment in a mixed gas atmosphere consisting of 3% by volume of hydrogen and the remainder nitrogen at a predetermined temperature for 3 minutes to produce a sample in which a first copper bus electrode was formed from copper oxide. The same heating temperature was used for the oxidation heat treatment and the reduction heat treatment.

[0053] As shown in Table 1, samples were produced by changing the oxidation heat treatment temperature and reduction heat treatment temperature (T) within the range of 280°C to 450°C. For the obtained samples, a cross section near the edge of the interface between the Al bus electrode and the second copper bus electrode on the back side was observed using an SEM to confirm the existence of an alloy layer containing aluminum and copper formed on the Al bus electrode. The distance (L) from the edge of the interface between the Al bus electrode and the second copper bus electrode to the edge of the alloy layer was then measured. The results are shown in Table 1.

[0054] Furthermore, the voltage-current curves of each sample were measured under light irradiation with an intensity of AM (Air Mass) = 1.5, and the series resistance Rs (T) and parallel resistance Rsh (T) were obtained for samples heat-treated at the oxidation heat treatment temperature and reduction heat treatment temperature (T) (°C). Using these values, the rate of change in Rs (ΔRs) and the rate of change in Rsh (ΔRsh) were calculated using the Rs (380°C) and Rsh (380°C) of the sample heat-treated at 380°C as references, according to the following equations (1) and (2). The results are shown in Table 1.

[0055] ΔRs={Rs(T)-Rs(380℃)} / Rs(380℃)...Equation (1) ΔRsh={Rsh(T)-Rsh(380℃)} / Rsh(380℃)...Equation (2)

[0056] The overall ratings A to D shown in Table 1 are classified according to the following criteria. Criteria A: The absolute values ​​of both ΔRs and ΔRsh are less than 0.2. Criterion B: The absolute values ​​of both ΔRs and ΔRsh are less than 0.3. Criterion C: The absolute values ​​of both ΔRs and ΔRsh are less than 1.0. Criterion D: The absolute value of either ΔRs or ΔRsh is 1.0 or more. The cases with ratings A, B and C were evaluated as good, and the case with rating D was evaluated as unsuitable.

[0057] In Table 1, samples (Test Examples 1-2 to 1-7) whose distance from the interface to the end of the alloy layer was within the range of the present invention (5 μm or more and 170 μm or less) were rated A, B, or C, indicating small ΔRs and ΔRsh and a negligible effect on the conversion efficiency (FF). On the other hand, samples (Test Examples 1-1 and 1-8) whose distance was outside the range of the present invention were rated D, indicating that when the distance was less than 5 μm, ΔRs was large, and when the distance exceeded 170 μm, the absolute values ​​of both ΔRs and ΔRsh were large, resulting in a decrease in conversion efficiency in both cases. Table 1 also indicates that when the oxidation and reduction heat treatment times were each 3 minutes, the preferred heat treatment temperature range was 320°C or more and 420°C or less.

[0058] [Table 1]

[0059] (Example 2) Area ratio of Te Te oxide particles were added to a paste containing copper particles at the weight ratio (%) shown in Table 2. The weight ratio is the ratio (%) of the weight of the oxide particles to the total weight of the copper particles and the oxide particles. Using the copper paste prepared in this manner, a sample having a copper bus electrode was produced using the same procedure as in Example 1, with oxidation heat treatment and reduction heat treatment temperatures of 380°C selected. A cross section perpendicular to the longitudinal direction of the copper bus electrode located on the light-receiving surface side of the obtained sample was formed and exposed, and the distribution of Te in the cross section was measured using an X-ray energy dispersive spectroscopy (SEM-EDX) device to obtain the area ratio (%) of Te. The results are shown in Table 2.

[0060] Next, a 1.4 mm wide TAB wire was soldered to each sample, and the soldered TAB wire was lifted by hand to evaluate the acceptability of the solder bond. When the TAB wire was lifted by hand, if it adhered to the sample it was judged as "pass", and if it peeled off from the sample it was judged as "fail". Next, using samples with "passable" soldering, the TAB wire was pulled perpendicular to the sample surface, and the tensile strength (N) of the TAB wire when it peeled off from the sample was measured. The measurement results are shown in Table 2.

[0061] In the overall evaluation, ratings A and B were evaluated as good, and rating D was evaluated as unsuitable. As shown in Table 2, Test Examples 2-2 to 2-6, in which the area ratio of Te falls within the range of the present invention (4% or more and 45% or less), were evaluated as rating A or B, confirming that the solderability was good and solar cells with high tensile strength of the TAB wire were obtained. On the other hand, Test Examples 2-1, 2-7, and 2-8, which are outside the range of the present invention, were evaluated as rating D, indicating that the solderability and adhesive strength with the TAB wire were unsuitable.

[0062] [Table 2]

[0063] (Example 3) Area ratio of carbon contained in thermoplastic organic polymer Polyimide powder with a glass transition temperature of 190°C and a melting point of 320°C was added to a paste containing copper particles at the weight ratio (%) shown in Table 3. The weight ratio is the ratio (%) of the weight of the polyimide powder to the combined weight of the copper particles and the powder. Using the copper paste prepared in this manner, samples with copper bus electrodes were fabricated using the same procedure as in Example 1, with oxidation heat treatment and reduction heat treatment temperatures of 380°C selected. A cross section perpendicular to the longitudinal direction of the copper bus electrode located on the light-receiving surface side of the obtained sample was formed and exposed. The carbon distribution in the cross section was then measured using an X-ray energy dispersive spectroscopy (SEM-EDX) to obtain the carbon area ratio (%). Furthermore, the "solderability" and "TAB wire tensile strength" were measured using the same procedure as in Example 2. The results are shown in Table 3. The criteria for the "overall evaluation" shown in Table 3 were the same as in Example 2.

[0064] As shown in Table 3, Test Examples 3-2 to 3-4, in which the area ratio of carbon was within the range of the present invention (9% or more and 40% or less), were rated A, meaning that solar cells with good solderability and high TAB wire tensile strength were obtained. On the other hand, Test Examples 3-1, 3-5, and 3-6, which were outside the range of the present invention, were rated D, meaning that the TAB wire solderability and adhesive strength were inadequate.

[0065] [Table 3]

[0066] (Example 4) Line width of first layer and line width of second layer A predetermined electrode was fabricated on the back side of the substrate using the same procedure as in Example 1. Next, a silver paste was printed on the light-receiving surface of the substrate using the same procedure as in Example 1, followed by a drying process and a fire-through heat treatment to form a finger electrode (hereinafter referred to as the "first-layer electrode") with a thickness of 5 to 8 μm. Furthermore, a copper paste was printed so as to overlap the first-layer electrode, and the electrode was dried in the air at 100°C for 3 minutes. This was then subjected to an oxidation heat treatment in the air at 380°C for 3 minutes, followed by a reduction heat treatment in a reducing atmosphere (a mixed gas atmosphere containing argon gas and 3% by volume of hydrogen gas) at 380°C for 3 minutes to form a second-layer electrode containing copper. The second-layer electrode was formed so that its widthwise edge was in contact with the insulating layer. The combined thickness of the first-layer electrode and the second-layer electrode was 20 to 40 μm. Various samples of the first-layer electrode and the second-layer electrode, each with the line widths shown in Table 4, were fabricated.

[0067] The open-circuit voltage (Voc) (unit: V) and series resistance (Rs) (unit: mΩ) were measured using the obtained samples. Furthermore, a tape test was performed to check whether the electrode peeled off from the substrate to evaluate the adhesion between the electrode and the substrate. In the overall evaluation, the criteria for Grade A were Voc of 0.74 or more, Rs of 500 or less, and no peeling (adhesion) in the tape test. The criteria for Grade B were Voc of less than 0.74, Rs of 650 or less, and no peeling in the tape test. The criteria for Grade D were peeling in the tape test. Solar cells with Grade A or B were evaluated as having good conversion characteristics and adhesion, while solar cells with Grade C were evaluated as unsuitable. As shown in Table 4, Test Examples 4-2 to 4-6, in which the line widths of the first layer electrode and the second layer electrode were within the range of the present invention (line width of the first layer electrode: 15 μm or more and 60 μm or less, and line width of the second layer electrode: 20 μm or more and 100 μm or less), were rated A and B, respectively, and showed good conversion characteristics and adhesion. On the other hand, Test Example 4-1, which was outside the range of the present invention, was rated C, and showed inadequate conversion characteristics and adhesion.

[0068] [Table 4]

[0069] (Example 5) Line width of the first layer Samples having first-layer electrodes with the line widths shown in Table 5 were prepared in the same manner as in Example 4, except that a second-layer electrode was not formed. Using the obtained samples, the same items as in Example 4 were measured and an overall evaluation was made. The results are shown in Table 5.

[0070] [Table 5]

[0071] (Example 6) Electrode length, longitudinal spacing, and width spacing of the first layer In accordance with the sample of judgment A shown in Example 4, conditions were selected in which the line width of the first layer electrodes was 30 μm and the line width of the second layer electrodes was 50 μm. When forming the first layer electrodes, silver paste was printed so that they were spaced apart at regular intervals in the longitudinal direction, and drying and fire-through treatments were performed using the same procedures as in Example 4 to produce first layer silver finger electrodes. Copper paste was then printed to form continuous second layer electrodes that overlapped the top of the first layer and connected the spaced first layer electrodes. Then, using the same procedures as in Example 4, the sample was dried in the air at 100°C for 3 minutes, followed by oxidation heat treatment and reduction heat treatment at 380°C for 3 minutes to produce a sample with second layer electrodes.

[0072] Using the obtained samples, the length, longitudinal spacing, and widthwise spacing of the individual electrodes spaced apart in the first layer of electrodes were measured. Furthermore, the same items as in Example 4 were measured and an overall evaluation was made. The measurement results are shown in Table 6. Test Examples 6-2 to 6-5, which fall within the scope of the present invention (first layer electrode length of 1 mm or more and 15 mm or less, longitudinal spacing of 1 mm or more and 7 mm or less, and widthwise spacing of 0.8 mm or more and 2 mm or less), were evaluated as A, showing good conversion characteristics and adhesiveness. On the other hand, Test Example 6-1, which falls outside the scope of the present invention, was evaluated as C, showing inadequate conversion characteristics and adhesiveness.

[0073] [Table 6] [Explanation of symbols]

[0074] 1. First copper bus electrode 2 silver finger electrodes 3 Second copper bus electrode 4. Aluminum bus electrode 5 Area X 6 Area Y 7 Interface between aluminum bus electrode and second copper bus electrode 8. Portion where the aluminum bus electrode and the second copper bus electrode overlap 9 Interface edge 10 Aluminum finger electrodes

Claims

1. In a solar cell having a silicon substrate, the silicon substrate has an insulating layer on a part of its surface, a silver finger electrode and a first copper bus electrode are disposed on a light-receiving surface side of the silicon substrate; the silver finger electrode comprises silver and is in contact with a surface of the silicon substrate; the first copper bus electrode includes at least copper, covers a portion of the silver finger electrode, and is in contact with the insulating layer at a portion other than the silver finger electrode; aluminum finger electrodes, an aluminum bus electrode, and a second copper bus electrode are disposed on a back surface side of the silicon substrate; the second copper bus electrode covers a portion of the aluminum bus electrode and is in contact with the insulating layer at a portion other than the aluminum bus electrode; the aluminum finger electrodes and the aluminum bus electrodes are formed of a sintered body of fine particles containing aluminum, a second copper bus electrode that is electrically connected to the aluminum bus electrode and the second copper bus electrode; an aluminum bus electrode that is electrically connected to the second copper bus electrode and the aluminum bus electrode; an aluminum-copper alloy layer that is electrically connected to the second copper bus electrode and the aluminum bus electrode; and a distance from the interface edge to the alloy layer edge that is in the range of 5 μm to 170 μm.

2. the first copper bus electrode or the second copper bus electrode contains at least one of Te oxide or Se oxide, which has a softening point of 450°C or less; 2. The solar cell according to claim 1, wherein a ratio of an area where Te or Se, a constituent element of the Te oxide or the Se oxide, is distributed in a cross section perpendicular to the longitudinal direction of the copper bus electrode is 4% or more and 45% or less.

3. the first copper bus electrode or the second copper bus electrode comprises a thermoplastic organic polymer having a glass transition temperature of 300° C. or higher and 450° C. or lower; 3. The solar cell according to claim 1, wherein a ratio of an area where carbon, a constituent element of the thermoplastic organic polymer, is distributed in a cross section perpendicular to the longitudinal direction of the copper bus electrode is 9% or more and 40% or less.

4. a part or all of the silver finger electrodes have a laminated structure consisting of a first layer electrode and a second layer electrode, the first electrode contains silver, penetrates the insulating layer, and contacts the surface of the silicon substrate; the second layer electrode includes copper and is disposed on the first layer electrode; the line width of the first electrode is 15 μm or more and 60 μm or less; 3. The solar cell according to claim 1, wherein the line width of the second layer electrode is 20 μm or more and 100 μm or less, which is wider than the line width of the first layer electrode, and the ends in the width direction are in contact with the insulating layer.

5. a part of the silver finger electrode has a laminated structure consisting of a first layer electrode and a second layer electrode, the first-layer electrodes contain silver, penetrate the insulating layer, and contact the surface of the silicon substrate; the first-layer electrodes are arranged at intervals in the longitudinal direction; the length of each first-layer electrode is 1 mm or more and 15 mm or less; the intervals between the first-layer electrodes are 1 mm or more and 7 mm or less in the longitudinal direction and 0.8 mm or more and 3 mm or less in the width direction; 3. The solar cell according to claim 1, wherein the second layer electrode contains copper, is disposed in contact with the first layer electrode, and electrically connects the first layer electrodes that are spaced apart from each other.

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