JP7499073B2 - Conductive connecting material and solar cell - Google Patents

Description

The present invention relates to a conductive connecting material and a solar cell.

Conductive pastes containing conductive particles such as silver paste and a binder made of a thermosetting resin are used for mounting electronic components on printed circuit boards. Conductive pastes can also be used to form desired conductive patterns. For example, there are solar cells in which electrode patterns are formed using silver paste. However, conductive pastes have the disadvantages of being relatively high in electrical resistance and expensive.

Solder paste containing solder particles and flux is also used to mount electronic components. Solder paste is relatively inexpensive and has low electrical resistance. Solder paste cannot be used to form conductive patterns because it cannot be layered on the surface of materials with low wettability, such as insulating substrates. Patent Document 1 describes a technique for improving the adhesiveness of solder paste to materials with relatively low wettability by adding a thermosetting resin having a curing temperature lower than the melting point of the solder particles to the solder paste. However, although the solder paste described in Patent Document 1 can improve adhesion to electrodes, it cannot be layered on materials that repel solder, such as solder resist.

JP 2019-141878 A

In a heterojunction back-contact solar cell, electrodes of different polarities may be arranged in an intricate manner, and therefore an insulating material such as solder resist may be arranged between the electrodes to prevent short circuits. In this case, it is necessary to form a conductive pattern on the insulating material as well. Therefore, the objective of the present invention is to provide an inexpensive conductive connection material that can form a low-resistance conductive pattern on the surface of an insulating material, and an inexpensive solar cell.

The conductive connecting material according to one embodiment of the present invention includes solder particles, conductive particles covered with a metal that does not melt in the operating temperature range, and a thermosetting resin having a curing temperature lower than the melting point of the solder particles.

In the conductive connection material, the content of the conductive particles per 100 parts by mass of the solder particles may be 5 parts by mass or more and 1500 parts by mass or less.

In the conductive connecting material, the content of the thermosetting resin per 100 parts by mass of the solder particles may be 0.8 parts by mass or more and 300 parts by mass or less.

In the conductive connection material, the content of the thermosetting resin per 100 parts by mass of the conductive particles may be 5 parts by mass or more and 50 parts by mass or less.

A solar cell according to one aspect of the present invention includes a semiconductor substrate, a plurality of strip-shaped first semiconductor layers and a plurality of strip-shaped second semiconductor layers alternately stacked on the back surface of the semiconductor substrate, a plurality of first finger electrodes stacked on the first semiconductor layers, and a plurality of second finger electrodes stacked on the second semiconductor layers, and an insulating layer covering the back surfaces of the first semiconductor layer, the second semiconductor layer, the first finger electrodes, and the second finger electrodes and having a plurality of first connection openings formed to partially expose the first finger electrodes and a plurality of second connection openings formed to partially expose the second finger electrodes, a first bus bar stacked on the back surface of the insulating layer and connecting the plurality of first finger electrodes exposed from the first connection openings, and a second bus bar stacked on the back surface of the insulating layer and connecting the plurality of second finger electrodes exposed from the second connection openings, and the first bus bar and the second bus bar include solder, conductive particles having a metal having a melting point higher than the melting point of the solder at least on the surface, and a thermosetting resin that disperses and holds the conductive particles.

The present invention provides an inexpensive conductive connection material that can form a conductive pattern on the surface of an insulating material, and an inexpensive solar cell.

1 is a schematic plan view showing a configuration of a solar cell according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of the solar cell of FIG. 1 .

Specific embodiments of the present invention are described in detail below. The present invention is not limited to the following embodiments, and can be modified as appropriate within the scope of the object of the present invention.

The conductive connecting material according to one embodiment of the present invention includes solder particles, conductive particles having a metal on at least the surface thereof that has a melting point higher than that of the solder particles, and a thermosetting resin having a curing temperature lower than that of the solder particles. The "melting point" refers to the solidus temperature as defined in JIS-Z3198-1 (2014). The "curing temperature" refers to the temperature at which the differential heat quantity peaks in differential scanning calorimetry as defined in JIS-K7121 (1987).

This conductive connecting material is applied to the surface of a substrate in a desired pattern and heated to form a conductor while maintaining the pattern shape. Specifically, when the conductive connecting material is heated, the thermosetting resin first hardens, maintaining the solder particles and conductive particles in a dispersed state. When further heated, the solder particles reflow and wet the conductive particles, forming a continuous layer that connects the conductive particles. Therefore, even when this conductive connecting material is applied to the surface of a substrate that is not wettable by solder, the reflowed solder is not repelled and broken, and a continuous conductor can be formed.

The composition of the solder particles is not particularly limited, but medium-temperature or medium-low-temperature solder particles are preferably used. The lower limit of the melting point of the solder particles is preferably 150°C, more preferably 160°C. Meanwhile, the upper limit of the melting point of the solder particles is preferably 217°C, more preferably 200°C. By setting the melting point of the solder particles to the lower limit or higher, it becomes easier to select the thermosetting resin described below. In addition, by setting the melting point of the solder particles to the upper limit or lower, the range of use of the conductive connection material is less likely to be restricted.

The lower limit of the average particle size of the solder particles (circle equivalent diameter when observed under a microscope) is preferably 1 μm, and more preferably 3 μm. On the other hand, the upper limit of the average particle size of the solder particles is preferably 50 μm, and more preferably 30 μm. By setting the average particle size of the solder particles to the lower limit or more, the cost of the solder particles can be reduced. In addition, by setting the average particle size of the solder particles to the upper limit or less, it becomes possible to selectively apply the conductive connection material by printing or the like.

The lower limit of the solder particle content in the conductive connection material is preferably 5% by mass, more preferably 30% by mass, and even more preferably 50% by mass. On the other hand, the upper limit of the solder particle content is preferably 95% by mass, and more preferably 80% by mass. By making the solder particle content equal to or greater than the lower limit, the electrical resistance of the conductor formed can be reduced and manufacturing costs can be suppressed. Furthermore, by making the solder particle content equal to or less than the upper limit, sufficient conductive particles and thermosetting resin can be contained.

At least the surface of the conductive particles is formed of a metal that has wettability to solder and does not melt when the solder particles are reflowed. In other words, the conductive particles may be coated with a metal, or may be entirely formed of the same metal. The lower limit of the difference between the melting point of the metal on the surface of the conductive particles and the melting point of the solder particles is preferably 50°C, more preferably 100°C, in order to allow some leeway in the reflow temperature of the solder particles. Examples of the metal material on the surface of the conductive particles include silver, gold, copper, nickel, and aluminum, and among these, silver, which has excellent conductivity, is particularly preferred.

The lower limit of the average particle size of the conductive particles is preferably 1 μm, and more preferably 3 μm. On the other hand, the upper limit of the average particle size of the conductive particles is preferably 30 μm, and more preferably 20 μm. By setting the average particle size of the conductive particles to the lower limit or more, the cost of the conductive particles can be reduced. In addition, by setting the average particle size of the conductive particles to the upper limit or less, the surface area of the conductive particles can be increased, and the solder can be effectively prevented from breaking.

The lower limit of the conductive particle content per 100 parts by mass of solder particles is preferably 5 parts by mass, and more preferably 7 parts by mass. On the other hand, the upper limit of the conductive particle content per 100 parts by mass of solder particles is preferably 1500 parts by mass, more preferably 100 parts by mass, and even more preferably 50 parts by mass. By setting the conductive particle content to the lower limit or more, it is possible to effectively prevent the solder from breaking. In addition, by setting the conductive particle content to the upper limit or less, it is possible to reduce the amount of relatively expensive conductive particles used and thus keep costs down.

The thermosetting resin has a curing temperature lower than the melting point of the solder particles so that it can cure before the solder particles reflow and hold the conductive particles so that they do not move. The lower limit of the difference between the curing temperature of the thermosetting resin and the melting point of the solder particles is preferably 25°C, and more preferably 30°C, so that the conductive particles can be reliably held before the solder particles melt. As the thermosetting resin, epoxy resin, urethane resin, acrylic resin, etc. can be used.

The lower limit of the thermosetting resin content per 100 parts by mass of solder particles is preferably 0.8 parts by mass, and more preferably 1.0 parts by mass. On the other hand, the upper limit of the thermosetting resin content per 100 parts by mass of solder particles is preferably 300 parts by mass, and more preferably 30 parts by mass. By making the content of the thermosetting resin equal to or greater than the lower limit, the conductive particles can be properly maintained. In addition, by making the content of the thermosetting resin equal to or less than the upper limit, an increase in the electrical resistance of the conductor formed can be suppressed.

The lower limit of the content of the thermosetting resin per 100 parts by mass of the conductive particles is preferably 5 parts by mass, and more preferably 10 parts by mass. On the other hand, the upper limit of the content of the thermosetting resin per 100 parts by mass of the conductive particles is preferably 50 parts by mass, and more preferably 30 parts by mass. By making the content of the thermosetting resin equal to or greater than the lower limit, the conductive particles can be held in place. Also, by making the content of the thermosetting resin equal to or less than the upper limit, the molten solder can reliably wet the conductive particles.

The conductive connecting material may also contain flux. Flux can remove oxide films and foreign matter from the surfaces of the joining metals, enhancing the adhesive effect. There are no particular limitations on the type of flux as long as it does not reduce solderability. The conductive connecting material may further contain a solvent, etc.

The conductive connection material having the above composition contains solder particles, conductive particles, and a thermosetting resin that hardens at a temperature lower than the melting point of the solder particles. Therefore, the thermosetting resin hardens and holds the conductive particles before the solder particles melt, and the molten solder then forms a continuous layer with the conductive particles as nuclei, so that a continuous conductor can be formed on the surface of a substrate that does not have wettability with solder. In addition, the conductive connection material is relatively inexpensive because it uses a small amount of relatively expensive conductive particles. Furthermore, the conductive connection material can form a conductive pattern with low resistance because electrical connection can be made by solder, which has low electrical resistance.

Next, an embodiment of a solar cell of the present invention will be described with reference to the attached drawings. Note that the same or equivalent parts in each drawing will be given the same reference numerals. For simplification, illustrations and reference numerals of members may be omitted, in which case other drawings should be referred to. Figure 1 is a schematic plan view showing the configuration of a solar cell 1 according to one embodiment of the present invention using the conductive connecting material described above. Figure 2 is a schematic cross-sectional view of the solar cell 1.

The solar cell 1 is a so-called heterojunction back contact type solar cell. The solar cell 1 comprises a semiconductor substrate 11, a plurality of first semiconductor layers 21 and a plurality of second semiconductor layers 22 formed in strips extending in a first direction on the back side (opposite the light incident surface) of the semiconductor substrate 11 and arranged alternately in a second direction intersecting the first direction, a plurality of first finger electrodes 31 and a plurality of second finger electrodes 32 stacked in strips extending in the first direction at the center in the second direction on the back side of the first semiconductor layer 21 and the second semiconductor layer 22, and a plurality of first and second finger electrodes 31 and 32 stacked in strips extending in the first direction at the center in the second direction on the back side of the first semiconductor layer 21 and the second semiconductor layer 22. The device includes an insulating layer 41 that covers the back side of the finger electrode 32 and has a plurality of first connection openings 411 formed to partially expose the first finger electrode 31 and a plurality of second connection openings 412 formed to partially expose the second finger electrode 32, a first bus bar 51 that is laminated on the back side of the insulating layer 41 and connects the plurality of first finger electrodes 31 exposed from the first connection openings 411, and a second bus bar 52 that is laminated on the back side of the insulating layer 41 and connects the plurality of second finger electrodes 32 exposed from the second connection openings 412.

The semiconductor substrate 11 is formed of a crystalline silicon material such as single crystal silicon or polycrystalline silicon. The semiconductor substrate 11 is, for example, an n-type semiconductor substrate in which a crystalline silicon material is doped with an n-type dopant. An example of an n-type dopant is phosphorus (P). The semiconductor substrate 11 functions as a photoelectric conversion substrate that absorbs incident light from the light receiving surface side and generates photocarriers (electrons and holes). By using crystalline silicon as the material for the semiconductor substrate 11, the dark current is relatively small, and a relatively high output (stable output regardless of illuminance) can be obtained even when the intensity of the incident light is low.

The first semiconductor layer 21 and the second semiconductor layer 22 have different conductivity types. For example, the first semiconductor layer 21 is formed of a p-type semiconductor, and the second semiconductor layer 22 is formed of an n-type semiconductor. The first semiconductor layer 21 and the second semiconductor layer 22 can be formed of, for example, an amorphous silicon material containing a dopant that imparts the desired conductivity type. An example of a p-type dopant is boron (B), and an example of an n-type dopant is phosphorus (P) as described above.

The first semiconductor layer 21 and the second semiconductor layer 22 are each formed in a strip shape extending in a first direction. In the solar cell 1, a plurality of first semiconductor layers 21 and a plurality of second semiconductor layers 22 are alternately provided in a second direction intersecting the first direction. The first semiconductor layer 21 and the second semiconductor layer 22 are preferably arranged so as to cover substantially the entire surface of the semiconductor substrate 11. The first semiconductor layer 21 and the second semiconductor layer 22 attract carriers generated in the semiconductor substrate 11 and collect charges.

The first finger electrodes 31 are laminated on the back surface of the first semiconductor layer 21 so as to extend in a first direction, and the second finger electrodes 32 are laminated on the back surface of the second semiconductor layer 22 so as to extend in the first direction. The first finger electrodes 31 and the second finger electrodes 32 extract electric charges from the first semiconductor layer 21 and the second semiconductor layer 22. The first finger electrodes 31 and the second finger electrodes 32 are formed from a metal such as copper.

The insulating layer 41 is formed from an insulating material whose main component is, for example, epoxy resin. The first connection openings 411 are formed in one or more rows aligned in the second direction. The second connection openings 412 are formed in one or more rows aligned in the second direction, offset from the rows of the first connection openings 411.

The first bus bar 51 and the second bus bar 52 connect the first finger electrode 31 and the second finger electrode 32, respectively, and serve as an electrical path for extracting power from the solar cell 1 to the outside.

The first bus bar 51 and the second bus bar 52 contain solder, conductive particles having a metal on at least the surface thereof that has a melting point higher than that of the solder, and a thermosetting resin that disperses and holds the conductive particles. Such first bus bar 51 and second bus bar 52 can be formed from the conductive connection material described above.

The solar cell 1 having the above-described configuration can be formed by a simple process of applying and heating the first bus bar 51 and the second bus bar 52 using a relatively inexpensive conductive connecting material, and is therefore relatively inexpensive yet has low current collection resistance.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-mentioned embodiments, and various modifications and variations are possible. For example, the conductive connecting material according to the present invention may contain additives other than those described above. Furthermore, the solar cell according to the present invention may include further components, such as a transparent electrode interposed between the semiconductor layer and the finger electrode.

The present invention will be described in more detail below based on examples. Note that the present invention is not limited to these examples.

By adding other materials to commercially available solder paste, we produced prototype conductive connection materials with prototype numbers 1 to 15, each with a different composition, as shown in Table 1 below. The content is shown as the mass content per 100 mass parts of solder particles. The base solder paste contains 13 mass parts of flux per 100 mass parts of solder particles. The particle diameter of the solder particles was 25 to 55 μm. Note that "-" in the table indicates that no entry exists because the substance is not contained.

A test specimen was created by forming electrodes on a substrate in stripes of 550 μm wide, 25 μm thick, and 210 μm apart using silver paste, and then laminating a stripe-shaped insulating layer of 300 μm wide, 35 μm thick, and 460 μm apart using insulating paste so that the center of the electrode was exposed over a width of 300 μm. A prototype of the conductive connection material was applied to the surface of this test specimen in a line shape of 550 μm wide, 30 to 100 μm high, and 156 mm long using a dispenser in a direction perpendicular to the electrode, and then heated on a hot plate. After cooling, the resistivity of the conductor formed by the conductive connection material was measured. Note that the "∞" resistivity in the table means that a continuous conductor could not be formed, and therefore no conductivity was obtained.

As described above, the conductive connection materials of prototype numbers 2 to 4 and 6 to 8, which contain sufficient conductive particles covered with a metal that does not melt in the operating temperature range compared to the solder particles, and a thermosetting resin with a curing temperature lower than the melting point of the solder particles, were able to form a continuous conductor, but the other conductive connection materials were unable to form a continuous conductor. The amount of conductive particles required to form a continuous conductor is thought to be about 5 parts by mass per 100 parts by mass of solder particles, although this depends on the manufacturing conditions, etc.

The resistivity of a conductor formed from a conductive connecting material increases as the conductive particle content increases. Furthermore, the unit price of conductive particles in a conductive connecting material is particularly high. For this reason, from the standpoint of electrical resistance and cost, it is desirable not to make the conductive particle content in the conductive connecting material higher than necessary.

REFERENCE SIGNS LIST 1 Solar cell 11 Semiconductor substrate 21 First semiconductor layer 22 Second semiconductor layer 31 First finger electrode 32 Second finger electrode 41 Insulating layer 411 First connection opening 412 Second connection opening 51 First bus bar 52 Second bus bar

Claims (4)

Solder particles and
Conductive particles having a metal on at least a surface thereof, the metal having a melting point higher than the melting point of the solder particles;
a thermosetting resin having a curing temperature lower than the melting point of the solder particles;
a first bus bar and a second bus bar formed from a conductive connecting material,
A semiconductor substrate;
a plurality of strip-shaped first semiconductor layers and a plurality of strip-shaped second semiconductor layers alternately stacked on a rear surface of the semiconductor substrate;
a plurality of first finger electrodes respectively stacked on the first semiconductor layer and a plurality of second finger electrodes respectively stacked on the second semiconductor layer;
an insulating layer covering rear surfaces of the first semiconductor layer, the second semiconductor layer, the first finger electrodes, and the second finger electrodes, the insulating layer having a plurality of first connection openings formed to partially expose the first finger electrodes and a plurality of second connection openings formed to partially expose the second finger electrodes;
Equipped with
the first bus bar is laminated on a rear surface of the insulating layer and connects the first finger electrodes exposed from the first connection openings;
the second bus bar is laminated on a rear surface of the insulating layer and connects the second finger electrodes exposed from the second connection openings . 2 . The solar cell according to claim 1 , wherein a content of the conductive particles relative to 100 parts by mass of the solder particles is 5 parts by mass or more and 1500 parts by mass or less. 3. The solar cell according to claim 1, wherein a content of the thermosetting resin relative to 100 parts by mass of the solder particles is 0.8 parts by mass or more and 300 parts by mass or less. 4. The solar cell according to claim 1, wherein a content of the thermosetting resin relative to 100 parts by mass of the conductive particles is 5 parts by mass or more and 50 parts by mass or less.

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