WO2023157935A1

WO2023157935A1 - Solar cell

Info

Publication number: WO2023157935A1
Application number: PCT/JP2023/005543
Authority: WO
Inventors: 淳一小池
Original assignee: 株式会社マテリアル・コンセプト
Priority date: 2022-02-16
Filing date: 2023-02-16
Publication date: 2023-08-24

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

solar cell

The present invention relates to electrode wiring of a solar cell and its peripheral structure.

The need for solar cells as a renewable energy source is increasing. Conventionally, silver paste containing expensive silver has been used as a material for bus electrodes and finger electrodes, which are current collecting electrodes, making it difficult to reduce the cost of the electrodes. Therefore, a proposal has been made to convert the silver (Ag) electrode to a copper (Cu) electrode to reduce the cost. Hereinafter, silver may be referred to as "Ag", copper as "Cu", and silicon as "Si".

As an example, Patent Document 1 discloses a solar cell device 10 having a silicon semiconductor substrate 1, a Cu-containing metal layer 4, an Ag-containing finger wiring 4, and an interface layer 3 containing an oxide or an organic compound. The Ag-containing finger wiring 4 is laminated on the light receiving surface side of the silicon semiconductor substrate 1, the interface layer 3 is laminated on the light receiving surface side of the silicon semiconductor substrate 1, and the Cu-containing metal layer 4 is laminated on the interface layer 3. , and spaced apart from the Ag-containing finger wires 2 are proposed. However, since there is an insulating interfacial layer between the Cu-containing metal layer and the Ag-containing finger wiring, the electrical resistance of the wiring as a whole increases. As a result, the series resistance of the photovoltaic cell increases, deteriorating the fill factor (FF) and conversion efficiency.

Further, in order to increase the conductivity between the copper electrode and the silicon substrate, when the copper electrode is arranged so as to be in direct contact with the silicon substrate, interdiffusion between Cu in the copper electrode and Si in the silicon substrate occurs. In addition to this, there is a problem that the diffusion rate of Cu in Si is very high (see Non-Patent Document 1). Cu that has entered the silicon substrate forms an acceptor level at a deep energy position in the silicon bandgap and shortens the carrier life in the diode. becomes.

WO2016/152481

As described above, in order to reduce the manufacturing cost of the solar cell device, when expensive silver is replaced with inexpensive copper as the material of the bus electrodes and finger electrodes, the electrode material copper easily diffuses into the silicon substrate. As a result, there is a problem that deterioration of solar cell characteristics is caused. Furthermore, if 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 will be higher, increasing the series resistance (Rs) of the cell, so the curve There is a problem that the factor (FF) and conversion efficiency deteriorate. In addition, the copper electrode is said to peel off from the cell surface because the adhesion of the copper electrode to the silicon substrate and the materials placed under the copper electrode such as anti-reflection coatings (SiN, _SiO2, etc.) is insufficient. There are also challenges.

An object of the present invention is to provide a solar cell that has good solar cell characteristics and adhesion even if the silver electrode in the solar cell is replaced with a copper electrode.

The inventors of the present invention have studied the above problems and have found that by adjusting the composition of the Cu-containing bus electrode, the adhesion between the Cu electrode and its underlying layer can be improved without sacrificing the solderability of the TAB wire. I found out what I can do. Furthermore, the inventors have found that Cu in the Cu-containing bus electrode can be prevented from diffusing into the silicon substrate through the Ag electrode and the Al electrode, and have completed the present invention. Specifically, the present invention includes the following embodiments (1) to (5).

(1) An embodiment according to 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 arranged on the light receiving surface side of the silicon substrate,
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, covers a part of the silver finger electrode, and is in contact with the insulating layer in a part other than the silver finger electrode,
Aluminum finger electrodes, aluminum bus electrodes and second copper bus electrodes are arranged on the back 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,
A part of the aluminum bus electrode is covered with the second copper bus electrode, and a distance of 5 μm or more and 170 μm or less from the interface end between the aluminum bus electrode and the second copper bus electrode is provided. A solar cell wherein a region of the sintered body comprises an alloy containing aluminum and copper.

(2) In an embodiment according to the present invention, the first copper bus electrode or the second copper bus electrode contains at least one Te oxide or Se oxide having a softening point of 450° C. or less,
In the cross section perpendicular to the length direction of the copper bus electrode, the percentage of the area where Te or Se, which is a constituent element of the Te oxide or the Se oxide, is 4% or more and 45% or less. (1) is a solar cell.

(3) In an embodiment according to 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 more and 450° C. or less,
(1) or ( 2) is the solar cell described in .

(4) In an embodiment according to the present invention, part or all of the silver finger electrode has a laminated structure consisting of a first layer electrode and a second layer electrode,
the electrode of the first layer contains silver, penetrates the insulating layer and is in contact with the surface of the silicon substrate;
the second layer electrode comprises 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 line width of the electrode of the second layer is 20 μm or more and 100 μm or less, is wider than the line width of the electrode of the first layer, and the end in the width direction is in contact with the insulating layer, or (2) is the solar cell.

(5) In an embodiment according to the present invention, part of the silver finger electrode has a laminated structure consisting of a first layer electrode and a second layer electrode,
The electrodes of the first layer comprise silver, extend through the insulating layer and contact the surface of the silicon substrate, are spaced longitudinally, and are spaced apart lengthwise of the individual electrodes of the first layer. the distance between the electrodes of the first layer is 1 mm or more and 7 mm or less in the length direction and 0.8 mm or more and 3 mm or less in the width direction;
(1) or (2) above, wherein the electrodes of the second layer contain copper, are arranged in contact with the electrodes of the first layer, and electrically connect the electrodes of the first layer that are spaced apart; The solar cell described in .

According to the present invention, an inexpensive copper bus electrode can be applied as the bus electrode of the solar cell, and a solar cell in which the adhesion between the Cu bus electrode and its underlying layer is good can be provided. Furthermore, the baking temperature for forming the electrodes can be lowered, and the Cu of the copper bus electrodes can be prevented from diffusing into the silicon substrate through the Ag finger electrodes and the Al finger electrodes. ADVANTAGE OF THE INVENTION According to this invention, the manufacturing cost of a solar cell can be reduced significantly, without impairing performance and reliability.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically an example of the wiring structure arrange|positioned at the light-receiving surface side of the solar cell which concerns on this embodiment, (a) is a figure which shows the structure where a silver finger electrode continues, (b) is a figure. 4A and 4B show a structure in which the silver finger electrodes are discontinuous; FIG. 4 is a diagram showing a cross-sectional SEM image of an aluminum bus electrode and a second copper bus electrode arranged on the back surface side of a solar cell according to the present embodiment; (a) is a diagram showing the entire cross section; (b) is an enlarged view of region X in (a) above, and (c) is an enlarged view of region Y in (a) above. 3 is a diagram schematically showing a cross-sectional structure along line AA' of FIG. 2; FIG. FIG. 2 is a diagram showing a SEM image and a composition distribution image by SEM-EDX 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 the present embodiment, and (a) is a cross section; (b) is a diagram showing a distribution image of Ag, (c) is a diagram showing a distribution image of Cu, and (d) is a diagram showing a distribution image of Te. It is a diagram.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The present invention is not limited by this embodiment. In this specification, the notation "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less".

(Basic structure of solar cell)
The solar cell according to this embodiment has a silicon substrate, and the silicon substrate has an insulating layer on a part of its surface. Silver finger electrodes and first copper bus electrodes are arranged on the light receiving surface side of the silicon substrate, and aluminum finger electrodes, aluminum bus electrodes and second copper bus electrodes are arranged on the back surface side of the silicon substrate. . The insulating layer can be formed of _a material such as SiN, _SiO2 , _Al2O3 . Of the surfaces of the silicon substrate, the "light-receiving surface" refers to the surface that is exposed to sunlight, and the "rear surface" refers to the surface opposite to the light-receiving surface. In this specification, the "silicon substrate" may be simply referred to as "substrate".

(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 electrode is formed of a sintered body of fine particles containing aluminum. In this specification, the "copper bus electrode" is referred to as "Cu bus electrode", the "silver finger electrode" is referred to as "Ag finger electrode", and the "aluminum finger electrode" is referred to as "Al finger electrode". and the "aluminum bus electrode" may also be referred to as "Al bus electrode".

The first copper bus electrodes arranged on the light-receiving surface side cover part of the silver finger electrodes, and the parts other than the silver finger electrodes are in contact with the insulating layer. 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. With such an electrode structure, among the conductive electrodes, the copper bus electrode is not in direct contact with the silicon substrate, so carrier disappearance occurring at the interface between the electrode and the silicon substrate can be suppressed. . Therefore, the fill factor (FF) and the open circuit voltage (Voc) can be increased.

FIGS. 1(a) and 1(b) schematically show an electrode structure in which the silver finger electrodes 2 arranged on the light receiving surface side are covered with the first copper bus electrodes 1. FIG. The silver finger electrodes 2 may be formed continuously from one end of the substrate to the other, as shown in FIG. You may form discontinuously in the part covered with 1. FIG.

(Fabrication of solar cell)
The solar cell according to this embodiment can be produced, for example, by the following method. As the silicon substrate, a silicon wafer having a texture structure on the surface and forming a p/n junction can be used. On the back side of the silicon substrate opposite to the light-receiving side, a layer of Al ₂ O ₃ is deposited on the back side by atomic layer deposition (ALD), and a plasma-assisted chemical vapor deposition is performed on the Al ₂ O ₃ layer. A SiN layer is formed by a growth method (PECVD). Next, a partial opening is formed in the arranged SiN layer/Al ₂ O ₃ layer by laser light. After that, an aluminum paste for forming aluminum bus electrodes and aluminum finger electrodes is printed by a screen printer.

The SiN layer has an antireflection role. The Al ₂ O ₃ layer has a role of passivation. Since both the SiN layer and the Al ₂ O ₃ layer have insulating properties, they also function as insulating layers on the substrate.

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 for forming finger electrodes is printed.

Then, the silicon substrate on which each of the above pastes is printed is subjected to heat treatment (fire-through) in which the silicon substrate is heated in the atmosphere at a temperature of 600° C. to 900° C. for several minutes. By this heat treatment, connection between the silver finger electrodes and the silicon substrate is obtained on the light-receiving surface of the silicon substrate, and connection between the aluminum bus electrodes and the aluminum finger electrodes and the silicon substrate is obtained on the back surface.

Next, in the substrate obtained by the above heat treatment, a copper paste for forming the second copper bus electrodes is partially printed on the aluminum bus electrodes on the back side. On the other hand, on the light-receiving surface side of the substrate, a copper paste for forming the first copper bus electrodes is printed so as to cover the silver finger electrodes. The substrate on which these copper pastes are printed is dried in the air at 100° C. for 3 minutes, and then subjected to an oxidizing heat treatment by heating in the air, so that the copper paste is changed to copper oxide and sintered. After that, the substrate obtained above is subjected to a reduction heat treatment in which it is heated in a reducing gas atmosphere, and the copper oxide is changed to copper to obtain the first copper bus electrode and the second copper bus electrode. As for the heating conditions for the oxidation heat treatment or reduction heat treatment, 300 to 420° C. and 1 to 30 minutes can be used. A mixed gas containing hydrogen (eg, 3% by volume) and nitrogen can be used as the reducing gas atmosphere.

(Interface region between aluminum bus electrode and second copper bus electrode)
The aluminum bus electrode arranged on the back surface side of the substrate in the solar cell according to this embodiment is formed of a sintered body of fine particles containing aluminum, and a second copper bus electrode is formed on part of the aluminum bus electrode. is formed. The sintered body of the aluminum bus electrode has a region having an alloy containing aluminum and copper near the end 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 and the copper of the second copper bus electrode react to form an alloy containing aluminum and copper in the sintered body. A region is formed. The alloy region can enhance the adhesion between the copper bus electrode and the contact electrode.

FIGS. 2(a) to 2(c) are scanning electron microscope (SEM) images of a region including the aluminum bus electrode 4 arranged on the back side and the second copper bus electrode 3 partially overlapping thereon. ) is an SEM image observed in . The respective portions of the second copper bus electrodes 3 and aluminum bus electrodes 4 formed using copper paste and aluminum paste can be distinguished based on the size and shape of the particles, as shown in FIG. 2(a). Therefore, the structure near the interface between the aluminum bus electrode 4 and the second copper bus electrode 3 can be specified.

FIG. 3 is a diagram schematically showing the cross-sectional structure along the line A-A' marked in FIG. 2(a). As shown in FIG. 3, the second copper bus electrodes 3 are arranged to partially cover the aluminum bus electrodes 4, so that the second copper bus electrodes 3 and the aluminum bus electrodes 4 overlap each other. A portion 8 is formed. The aluminum bus electrode 4 is in contact with the Al finger electrodes 10 on the side opposite to the overlapping portion 8 . The interface 7 where the second copper bus electrode 3 and the aluminum bus electrode 4 are in contact continues to the end of the overlapping portion 8 . The edge of the interface located at the edge of the overlapping portion 8 is at the position indicated by reference numeral 9 in FIG. This specification refers to the edge of the above interface as the "interface edge". The interface edge 9 can be identified at the position indicated by the white dotted line in the SEM image of FIG. 2(a).

Here, two regions (indicated by reference numerals 5 and 6) are selected in the aluminum bus electrode 4 shown in FIG. (c). FIG. 2(b) shows region X remote from the interface edge and FIG. 2(c) shows region Y adjacent to the interface edge. The image contrast of region Y shows patchy tissue compared to the image contrast of region X. Analysis of region Y by X-ray energy dispersive spectroscopy (SEM-EDX) revealed that an alloy containing aluminum and copper was formed. By changing the temperatures of the oxidation heat treatment and the reduction heat treatment, it is possible to know how far the region in which the alloy containing aluminum and copper is formed extends from the edge of the interface.

In the solar cell according to the present embodiment, the region of the sintered body at a distance of 5 μm or more and 170 μm or less from the end of the interface between the aluminum bus electrode and the second copper bus electrode contains an alloy containing aluminum and copper. ing. In this specification, a region having an alloy containing aluminum and copper may be hereinafter referred to as an "alloy layer." The above specified distances for the alloy layers are explained below.

　The range of the area where the alloy layer is formed can be determined, for example, as follows. Observe the surface texture of the Al bus electrode at a magnification of 4000 times using SEM. At this time, the observation area is moved from the interface edge toward the Al finger electrode, and a plurality of SEM images are taken. In these SEM images, as shown in FIGS. 2(b) and 2(c), alloy particles exhibiting a mottled structure and Al particles not exhibiting a mottled structure can be clearly distinguished. It connects multiple images and draws multiple rectangular frames in them. The first rectangle is arranged so that one long side overlaps with the interface end portion 9 and the other long side is included in the Al bus electrode side. Then, the structure within the rectangular frame is observed, and the number of alloy particles and Al particles contained within the frame is determined. When 10% or more of the obtained grains exhibit a mottled structure, the first rectangular region is determined to be an alloy layer. The alloy layer containing aluminum and copper in this embodiment is specified by such a method.

If the structure of the first rectangle is not an alloy layer, the distance from the edge of the interface is determined to be zero. When the structure of the first rectangle is an alloy layer, the second rectangle is arranged at a position farther from the interface edge than the first rectangle, and one of the long sides of the second rectangle is the Al bus electrode of the first rectangle. Place it so that it overlaps with the long side on the side. Then, the texture within the frame was observed in the same manner as for the first rectangle, and the ratio of the number of particles exhibiting a patchy texture was measured. If the particles exhibit a patchy texture, the second rectangular area is determined to be the alloy layer. For example, when the size of the rectangular frame is 20 μm×5 μm so that it is separated from the edge of the interface by 5 μm, if the second rectangle is not an alloy layer, it is determined that the distance from the edge of the interface is 5 μm. do. By repeating such operations, it is possible to determine how far the alloy layer extends from the edge of the interface.

The alloy layer containing the alloy containing aluminum and copper is preferably present in a range at a distance of 5 μm or more and 170 μm or less from the end of the interface between the aluminum bus electrode and the copper bus electrode. If the alloy layer expands beyond 170 μm, copper may diffuse into the silicon substrate in contact with the aluminum bus electrode, greatly deteriorating battery characteristics. 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, and even more preferably 15 μm or more and 47 μm or more. On the other hand, the upper limit of the distance is preferably 170 μm or less, more preferably 100 μm or less, and particularly preferably 91 μm or less. Even if the heat treatment temperature and heat treatment time are changed, the conditions may be selected so that the distance from the interface edge to the alloy layer edge is in the range of 5 μm or more and 170 μm or less.

(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 the present embodiment contains copper and has a softening point of 450° C. or less Te oxide (tellurium oxide) or Se It preferably contains at least one kind of oxide (selenium oxide). Hereinafter, the first copper bus electrode and the second copper bus electrode may be collectively referred to as "copper bus electrode".

The copper bus electrodes are made by firing the copper paste printed on the insulating layer. If an oxide that softens or melts at the firing temperature is contained in the copper paste, the oxide softens or melts and then cools and distributes due to the firing process. As a result, the adhesion between the oxide and the insulating layer is enhanced, and the bus electrode can be prevented from peeling off from the insulating layer. From that point of view, the copper bus electrode preferably contains at least one of Te oxide and Se oxide having a softening point of 450° C. or lower.

In addition, since the copper paste containing an oxide having a softening point of 450° C. or lower can be fired at a low temperature, Cu diffuses into the silicon substrate through the silver finger electrodes or the Al bus electrodes during the firing process. can prevent you from doing it. Mixing Te oxide and Se oxide is preferable because the melting point can be lowered to 350° C. or lower. Other than oxides of tellurium (Te) and selenium (Se), there are oxides of bismuth (Bi), vanadium (V), and the like.

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.

(Proportion of area where Te or Se is distributed (area ratio))
As will be described later, the distribution of Te oxide or Se oxide contained in the copper bus electrode corresponds to the distribution of Te or Se. This embodiment specified the copper bus electrode based on the percentage of the area where Te or Se is distributed. That is, in the cross section perpendicular to the length direction of the copper bus electrode, the ratio of the area where Te (tellurium) or Se (selenium), which is a constituent element of Te oxide or Se oxide, is distributed is 4% or more and 45%. The following are preferable. In this specification, the above-mentioned "ratio of area" is hereinafter referred to as "area ratio". If the area ratio of Te or Se is less than 4%, the adhesion may be insufficient. On the other hand, if the area ratio is too large, the electrical resistance of the copper bus electrode increases. Furthermore, since the amount of oxide present on the surface of the copper bus electrodes increases, there is a possibility that the solder will not adhere sufficiently to the copper bus electrodes in the process of soldering the TAB wires to the copper bus electrodes. Therefore, the lower limit of the area ratio of Te or Se is preferably 4% or more, more preferably 10% or more, and still more preferably 13% or more. The upper limit of the area ratio is preferably 45% or less, more preferably 40% or less.

Here, the method for measuring the area ratio of Te or Se contained in the copper bus electrode will be described below using Te as an example. A sample obtained by a predetermined manufacturing method has Cu bus electrodes and Ag finger electrodes formed on the light receiving surface side. After exposing a cross section of the sample perpendicular to the longitudinal direction of the Cu bus electrode, the composition distribution of the cross section was examined using a scanning electron microscope (SEM) and an X-ray energy dispersive spectrometer (SEM-EDX). Ta. Samples subjected to cross-sectional observation were produced from arbitrary five points on the Cu bus electrode. FIG. 4(a) shows an SEM image of the cross-sectional structure. FIGS. 4(b), (c), and (d) respectively show composition distribution images of Cu, Ag, and Te elements obtained by SEM-EDX, and Cu is present in FIG. 4(b). Regions and regions where Te exists in FIG. 4(d) are shown in white. The area of Te distribution and the area of Cu distribution were measured at five arbitrarily selected points in the cross section, and the area ratio of Te was obtained by dividing the area of Te by the total area of Cu and Te. Their average value was taken as the area ratio of Te. In FIG. 4D, 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.

(thermoplastic organic polymer)
In order 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. The ratio of the area where carbon, which is a constituent element of the thermoplastic organic polymer, is distributed is preferably 9% or more and 40% or less.

The bus electrode containing a thermoplastic organic polymer having a glass transition temperature of 300°C or higher and 450°C or lower, which constitutes the organic layer, can enhance adhesion with the insulating layer. When the bus electrodes are produced by the firing process, they are not thermally decomposed, but are softened or melted, and then cooled to form bus electrodes 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 material layer decreases during firing of the bus electrodes, and the amount of the organic material layer that penetrates into the voids of the bus electrodes increases. Since the amount of the organic layer present at the interface between the two is reduced, the adhesion strength may be reduced. On the other hand, when the glass transition temperature is higher than 450°C, there is a possibility that sufficient adhesion strength cannot be obtained.

The size of the organic layer formed near the interface can be evaluated by the ratio of the area where the carbon contained in the thermoplastic organic polymer forming the organic layer is distributed. The "area ratio" is hereinafter referred to as "area ratio". The area ratio of carbon can be obtained by SEM-EDX analysis of the cross section of the bus electrode in the same manner as the measurement method described for the area ratio of Te. The area ratio of carbon in the cross section perpendicular to the length direction of the bus electrode is preferably 9% or more and 40% or less. If the area ratio of carbon is less than 9%, the adhesion strength between the bus electrode and the insulating layer is reduced. On the other hand, if the area ratio of carbon exceeds 40%, a problem may occur that the solder does not sufficiently adhere to the bus electrodes in the process of soldering the TAB wires to the bus electrodes. 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.

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.

(first layer and second layer of finger electrodes)
The solar cell according to this embodiment has a laminated structure in which part or all of the finger electrodes are composed of first-layer electrodes and second-layer electrodes.

In one embodiment, the electrode of the first layer contains silver, penetrates through the insulating layer and is in contact with the surface of the silicon substrate. A second layer electrode comprises 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, the line width of the second layer electrode is 20 μm or more and 100 μm or less, and is wider than the line width of the first layer electrode, It is preferable that the end in the width direction is in contact with the insulating layer.

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

In another embodiment, the electrodes of the first layer contain silver, penetrate through the insulating layer and are in contact with the surface of the silicon substrate, and are spaced apart in the length direction. The length of each electrode of the first layer is 1 mm or more and 10 mm or less, and the distance between the electrodes of the first layer is 1 mm or more and 5 mm or less in the length direction and 0.8 mm in the width direction. 2 mm or less. The electrodes of the second layer contain copper, are arranged in contact with the electrodes of the first layer, and electrically connect the spaced electrodes of the first layer.

In this way, the arrangement form of the silver finger electrodes of the first layer may be continuous from one end of the substrate to the other end, or may be divided in the middle. By selecting the length of each electrode of the first layer and the distance between the electrodes of the first layer within the above-specified range, carriers inside the silicon substrate can be efficiently discharged from the emitter. It is preferable in terms of shifting to the electrodes of the first layer. Furthermore, in the case of the silver finger electrodes arranged separately, the copper finger electrodes of the second layer are preferable in that the carriers in the substrate can be extracted to the outside by connecting the individual silver finger electrodes. Further, if the length and width of the electrodes of the first layer are larger than the above-specified ranges, the light-receiving area becomes small, resulting in degradation of conversion efficiency. On the other hand, when the length and width of the electrodes of the first layer are smaller than the ranges specified above, the rate at which the photoinduced carriers generated in the substrate recombine and disappear before reaching the electrodes of the first layer is increased, and the fill factor (FF) and open-circuit voltage (Voc) are decreased.

Examples of the present invention will be described below. The invention is not limited to these descriptions.

(Example 1) Distance from Interface Edge to Alloy Layer A silicon substrate having a textured structure on the surface and having a p/n junction was used. An Al ₂ O ₃ layer was formed on the back surface of the silicon substrate by atomic layer deposition (ALD), and a SiN layer was formed on the Al ₂ O ₃ layer by plasma enhanced chemical vapor deposition (PECVD). A laser beam was then used to form a partial opening in the SiN layer/Al ₂ O ₃ layer. Next, an aluminum paste for forming Al bus electrodes and Al finger electrodes was printed by screen printing. A SiN layer was formed on the light-receiving surface of the silicon substrate, and silver paste was printed thereon. Thereafter, the silicon substrate was subjected to heat treatment at 600° C. for 3 minutes in the atmosphere (this heat treatment is also called “fire-through”) to form silver finger electrodes and Al bus electrodes.

Next, a copper paste was partially printed on the Al bus electrode on the back surface of the obtained substrate. A copper paste for forming the first copper bus electrodes was printed on the light receiving surface side of the substrate so as to cover the silver finger electrodes. The substrate after printing was dried in the atmosphere at 100° C. for 3 minutes, and then subjected to oxidation heat treatment in the atmosphere at a predetermined temperature for 3 minutes to change the copper paste into copper oxide. Next, the obtained substrate was subjected to reduction heat treatment for 3 minutes at a predetermined temperature in a mixed gas atmosphere of 3% by volume of hydrogen and the balance of nitrogen to form the first copper bus electrodes from copper oxide. A sample was made. The same heating temperature was applied to the above oxidation heat treatment and reduction heat treatment.

As shown in Table 1, samples were prepared by changing the oxidizing heat treatment temperature and the reduction heat treatment temperature (T) in the range of 280°C to 450°C. In the obtained sample, the cross section near the end of the interface between the Al bus electrode on the back side and the second copper bus electrode was observed with an SEM to confirm the alloy layer containing aluminum and copper formed on the Al bus electrode. . Then, 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 measured. Table 1 shows the results.

Furthermore, each sample was used to measure the voltage-current curve under light irradiation with an intensity of AM (Air Mass) = 1.5, and heat treated at the temperature (° C.) of the oxidation heat treatment temperature and the reduction heat treatment temperature (T). Rs (380 ° C.) and Rsh (380 ° C.) of the sample heat-treated at 380 ° C. As a reference, the rate of change of Rs (ΔRs) and the rate of change of Rsh (ΔRsh) were calculated by the following formulas (1) and (2). Table 1 shows the results.

ΔRs={Rs(T)−Rs(380° C.)}/Rs(380° C.) Equation (1)
ΔRsh={Rsh(T)−Rsh(380° C.)}/Rsh(380° C.) Equation (2)

Judgments A to D shown in the overall judgment in Table 1 are classified according to the following criteria.
Judgment A: The absolute values of both ΔRs and ΔRsh are less than 0.2.
Judgment B: The absolute values of both ΔRs and ΔRsh are less than 0.3.
Judgment C: The absolute values of both ΔRs and ΔRsh are less than 1.0.
Determination D: The absolute value of either ΔRs or ΔRsh is 1.0 or more.
Judgments A, B, and C were evaluated as good, and judgment D was evaluated as unsuitable.

In Table 1, the samples (Test Examples 1-2 to 1-7) in which the distance from the interface to the end of the alloy layer is within the range of the present invention (5 μm or more and 170 μm or less) are judged A, B or C. It can be said that ΔRs and ΔRsh are small and the effect on the conversion efficiency (FF) is minor. On the other hand, the samples (Test Examples 1-1 and 1-8) in which the above distance is outside the scope of the present invention are judged D, and if it is less than 5 μm, ΔRs is large, and if it is more than 170 μm, ΔRs and Both absolute values of ΔRsh were large, indicating that both cases resulted in a decrease in conversion efficiency. Further, according to Table 1, when each heat treatment time of oxidation and reduction is 3 minutes, the preferable range of heat treatment temperature is 320° C. or more and 420° C. or less.

(Example 2) Area ratio of Te Te oxide particles were added to the paste containing the 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 value of the weight of the copper particles and the weight of the oxide particles. Using the copper paste prepared in this way, a sample having a copper bus electrode was produced by selecting an oxidation heat treatment temperature and a reduction heat treatment temperature of 380° C. in the same procedure as in Example 1. For the obtained sample, after forming a cross section perpendicular to the longitudinal direction of the copper bus electrode arranged on the light receiving surface side and exposing it, using an X-ray energy dispersive spectrometer (SEM-EDX), Te in the cross section was measured. The distribution was measured to obtain the area ratio (%) of Te. Table 2 shows the results.

Next, a TAB wire with a width of 1.4 mm was soldered to each sample, and the solderability was evaluated by lifting the soldered TAB wire by hand. When the TAB wire was lifted by hand, the case where the TAB wire adhered to the sample was judged as "acceptable", and the case where the TAB wire was separated from the sample was judged as "impossible". Next, using a sample that was "acceptable" for soldering, the TAB wire was pulled perpendicularly to the sample surface, and the tensile strength (N) of the TAB wire when the TAB wire was peeled off from the sample was measured. Table 2 shows the measurement results thereof.

In the overall judgment, judgment A and judgment B were evaluated as good, and judgment D was evaluated as unsuitable. As shown in Table 2, Test Examples 2-2 to 2-6 in which the area ratio of Te is within the range of the present invention (4% or more and 45% or less) are judged A or B, and solderability was good, and it was confirmed that a solar cell having a TAB wire with high tensile strength could be obtained. On the other hand, Test Examples 2-1, 2-7, and 2-8, which are outside the scope of the present invention, were judged as D, and were unsuitable for solderability to TAB wires and adhesion strength.

(Example 3) Area ratio of carbon contained in thermoplastic organic polymer In a paste containing copper particles, a polyimide powder having a glass transition temperature of 190 ° C. and a melting point of 320 ° C. is added to the weight ratio (%) shown in Table 3. was added with The weight ratio is the ratio (%) of the weight of the polyimide powder to the sum of the weight of the copper particles and the weight of the powder. Using the copper paste prepared in this way, a sample having a copper bus electrode was produced by selecting an oxidation heat treatment temperature and a reduction heat treatment temperature of 380° C. in the same procedure as in Example 1. After forming and exposing a cross-section perpendicular to the longitudinal direction of the copper bus electrode arranged on the light-receiving surface side of the obtained sample, carbon concentration in the cross-section was measured using an X-ray energy dispersive spectrometer (SEM-EDX). The distribution was measured to obtain the carbon area ratio (%). Furthermore, the same procedures as in Example 2 were used to measure "solderability" and "tensile strength of TAB wire". Those results are shown in Table 3. The criteria for the “comprehensive evaluation” shown in Table 3 are the same as in Example 2.

As shown in Table 3, Test Examples 3-2 to 3-4, in which the area ratio of carbon is within the range of the present invention (9% or more and 40% or less), were evaluated as A, and the solderability was good. As a result, a solar cell having a TAB wire with high tensile strength was obtained. On the other hand, Test Examples 3-1, 3-5, and 3-6, which are outside the scope of the present invention, were judged as D, and the solderability and adhesive strength of the TAB wire were unsuitable.

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

Using the obtained sample, the open circuit voltage (Voc) (unit: V) and series resistance (Rs) (unit: mΩ) were measured. Furthermore, in order to evaluate the adhesion between the electrodes and the substrate, a tape test was performed to check whether the electrodes were separated from the substrate. In the comprehensive judgment, the criteria for judgment A are Voc of 0.74 or more, Rs of 500 or less, and peeling in the tape test of "no (adhesion)". Criteria for judgment B are Voc of less than 0.74, Rs of 650 or less, and no delamination in the tape test. The criterion for judgment D is "yes" in the tape test. A solar cell with a judgment of A or B is evaluated as having good conversion characteristics and adhesiveness, and a solar cell with a judgment of C is evaluated as unsuitable. As shown in Table 4, the line width of the first layer electrode and the line width of the second layer electrode are within the range of the present invention (the line width of the first layer electrode is 15 μm or more and 60 μm or less, the second layer electrode Test Examples 4-2 to 4-6 included in the line width of 20 μm or more and 100 μm or less) were judged A and B, and showed good conversion characteristics and adhesion. On the other hand, Test Example 4-1, which is outside the scope of the present invention, was judged C, and was unsuitable for conversion characteristics and adhesiveness.

(Example 5) Line width of the first layer By the same procedure as in Example 4, except that the electrodes of the second layer were not formed, the line widths shown in Table 5 were applied to the electrodes of the first layer. A sample with electrodes was prepared. Using the obtained sample, the same items as in Example 4 were measured and overall judgment was made. Those results are shown in Table 5.

(Example 6) First layer electrode length, lengthwise spacing, and widthwise spacing A condition was selected in which the line width of the second layer electrode was 50 μm. Then, when forming the electrodes of the first layer, the silver paste is printed so as to be arranged at regular intervals in the length direction, and the drying treatment and the fire-through treatment are performed by the same procedure as in Example 4. to prepare a first layer silver finger electrode. Copper paste was then printed to form a continuous second layer electrode overlying the top of the first layer and connecting the spaced first layer electrodes. After that, according to the same procedure as in Example 4, air drying was performed at 100 ° C. for 3 minutes, and then oxidation heat treatment and reduction heat treatment were performed at 380 ° C. for 3 minutes to obtain a sample having a second layer electrode. made.

Using the obtained sample, the length, lengthwise interval, and widthwise interval of the individual electrodes arranged apart from each other in the electrodes of the first layer were measured. moreover. The same items as in Example 4 were measured to make a comprehensive judgment. Table 6 shows the measurement results thereof. Test Example 6 included in the scope of the present invention (the electrode length of the first layer is 1 mm or more and 15 mm or less, the distance in the length direction is 1 mm or more and 7 mm or less, the distance in the width direction is 0.8 mm or more and 2 mm or less) -2 to 6-5 were rated A and exhibited good conversion properties and adhesion. On the other hand, Test Example 6-1, which is outside the scope of the present invention, was judged C, and was unsuitable for conversion characteristics and adhesiveness.

REFERENCE SIGNS LIST 1 first copper bus electrode 2 silver finger electrode 3 second copper bus electrode 4 aluminum bus electrode 5 region X
6 Area Y
7 interface between aluminum bus electrode and second copper bus electrode 8 portion where aluminum bus electrode and second copper bus electrode overlap 9 interface edge 10 aluminum finger electrode

Claims

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 arranged on the light receiving surface side of the silicon substrate,
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, covers a part of the silver finger electrode, and is in contact with the insulating layer in a part other than the silver finger electrode,
Aluminum finger electrodes, aluminum bus electrodes and second copper bus electrodes are arranged on the back 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,
A part of the aluminum bus electrode is covered with the second copper bus electrode, and a distance of 5 μm or more and 170 μm or less from the interface end between the aluminum bus electrode and the second copper bus electrode is provided. A solar cell, wherein a region of the sintered body comprises an alloy containing aluminum and copper.
The first copper bus electrode or the second copper bus electrode contains at least one Te oxide or Se oxide having a softening point of 450° C. or less,
In a cross section perpendicular to the length direction of the copper bus electrode, a ratio of an area in which Te or Se, which is a constituent element of the Te oxide or the Se oxide, is distributed is 4% or more and 45% or less. Item 1. The solar cell according to item 1.
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,
3. The method according to claim 1 or 2, wherein in a cross section perpendicular to the length direction of the copper bus electrode, the ratio of the area where carbon, which is a constituent element of the thermoplastic organic polymer, is distributed is 9% or more and 40% or less. A solar cell as described.
Some or all of the silver finger electrodes have a laminated structure consisting of a first layer electrode and a second layer electrode,
the electrode of the first layer contains silver, penetrates the insulating layer and is in contact with the surface of the silicon substrate;
the second layer electrode comprises 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,
3. The line width of the electrodes of the second layer is 20 μm or more and 100 μm or less, is wider than the line width of the electrodes of the first layer, and the ends in the width direction are in contact with the insulating layer. The solar cell described in .
A part of the silver finger electrode has a laminated structure consisting of a first layer electrode and a second layer electrode,
The electrodes of the first layer comprise silver, extend through the insulating layer and contact the surface of the silicon substrate, are spaced longitudinally, and are spaced apart lengthwise of the individual electrodes of the first layer. the distance between the electrodes of the first layer is 1 mm or more and 7 mm or less in the length direction and 0.8 mm or more and 3 mm or less in the width direction;
3. The electrode of the second layer according to claim 1 or 2, wherein the electrode of the second layer contains copper, is arranged in contact with the electrode of the first layer, and electrically connects the electrodes of the first layer that are spaced apart. solar cell.