CN118507554A - A main grid-free heterojunction battery and its preparation method, and battery assembly - Google Patents

Abstract

The present invention provides a busbar-free heterojunction battery and a preparation method thereof, and a battery assembly, comprising a heterojunction battery cell, a plurality of fine grid lines arranged on both sides of the heterojunction battery cell, and two thick grid lines arranged on both sides of the heterojunction battery cell; the fine grid lines form a grid-like structure; the two thick grid lines are arranged symmetrically with respect to the center of the heterojunction battery cell, and are respectively connected to the fine grid lines on the corresponding sides; the edges on both sides of the heterojunction battery cell provided with the thick grid lines are respectively provided with insulating grooves; the above-mentioned busbar-free heterojunction batteries are connected in series to obtain a battery assembly, and adjacent busbar-free heterojunction batteries are fixed one front and one back along the edge of one side provided with the thick grid lines, and adjacent thick grid lines are electrically connected to each other. The above-mentioned busbar-free heterojunction battery achieves power improvement under the premise of reducing costs and avoiding short circuit of battery cells, and the battery assembly achieves gapless arrangement, greatly simplifies the process flow, improves the utilization rate per unit area, and achieves total power improvement.

Description

A main grid-free heterojunction battery and its preparation method, and battery assembly

Technical Field

The present invention relates to the technical field of solar cell modules, and in particular to a main grid-free heterojunction cell and a preparation method thereof, and a cell module.

Background Art

At present, the competition in the photovoltaic industry is becoming increasingly fierce. Photovoltaic products are beginning to pursue the ultimate cost reduction and efficiency improvement. In terms of reducing battery costs, the busbar-free technology developed in recent years has been highly anticipated. The main feature of the busbar-free technology is to remove the busbar that originally needs to be printed on the battery surface, and only retain the secondary grid design. The original busbar is used to collect the secondary grid current, and the busbar-free technology uses a welding strip to replace it; due to the large amount of silver used in the busbar, after adopting the busbar-free technology, the consumption of silver paste printed on the battery is reduced by about half, and the cost of silver paste accounts for about 40% of the entire battery manufacturing cost, so the busbar-free technology can achieve considerable cost reduction effects.

While the busbar-free technology brings considerable benefits to the cost reduction of battery products, it also requires the support of supporting processes and equipment on the downstream component side. At present, the welding process of busbar-free battery cell components mainly adopts the method of positioning glue brushing, supplemented by low-temperature heating to fix the welding ribbon on the surface of the battery cell. In order to make the welding ribbon better play the role of replacing the busbar, the diameter of the welding ribbon needs to be further reduced, and the number of welding ribbons on the battery surface needs to be further increased. The overall process difficulty is greatly increased: first, it is necessary to add a dispensing machine to be responsible for the positioning and dispensing of glue on the surface of the battery cell, and at the same time, it is necessary to ensure that the glue traces are small and the bonding is firm to meet the tensile requirements; secondly, after the diameter of the welding ribbon is reduced, the current collection effect is improved by increasing the number of welding ribbons. If there is a welding problem, rework is extremely difficult; finally, after the diameter of the welding ribbon is reduced, it is much softer than the original texture. During the high-temperature lamination process, the liquid glue squeezes outward and easily pulls the welding ribbon off, causing appearance problems of the battery component.

Summary of the invention

Purpose of the invention: The purpose of the present invention is to provide a busbar-free heterojunction battery and its preparation method and battery assembly that can reduce costs and effectively prevent battery short circuits.

Technical solution: A busbar-free heterojunction battery, comprising a heterojunction battery cell, a plurality of fine grid lines arranged on the front and back sides of the heterojunction battery cell, a first thick grid line arranged on the front side of the heterojunction battery cell, and a second thick grid line arranged on the back side of the heterojunction battery cell; the fine grid lines include a plurality of mutually parallel first fine grid lines and a plurality of mutually parallel second fine grid lines, and the first fine grid lines and the second fine grid lines are not parallel to each other, each of the second fine grid lines is connected to at least two of the first fine grid lines, and at least one of the second fine grid lines is connected between two adjacent first fine grid lines; the first and second thick grid lines are symmetrically arranged with respect to the center of the heterojunction battery cell, the first thick grid line connects the plurality of fine grid lines on the front side of the heterojunction battery cell, and the second thick grid line connects the plurality of fine grid lines on the back side of the heterojunction battery cell; the heterojunction battery cell is provided with insulating grooves parallel to the edges of the first and second thick grid lines on both sides thereof.

Preferably, the first thin grid lines are cross-connected with the second thin grid lines to form a grid structure.

Preferably, the first thin gate lines and the second thin gate lines are perpendicular to each other.

The present invention also provides a method for preparing the above-mentioned busbar-free heterojunction battery, comprising the following steps:

(1) Using a special stencil, the grid line circuit is printed on the front of the heterojunction cell;

(2) Curing in a high-temperature curing furnace;

(3) Using special stencils, the gate line circuit is printed on the back of the heterojunction cell;

(4) Sintering the heterojunction cell at high temperature;

(5) Using ultraviolet laser, an insulating groove is opened at the edge of the heterojunction cell with a thick grid line to obtain a main grid-free heterojunction cell.

Specifically, the insulating groove has a depth of 50-100 nm and a width of 40-80 μm; two ends of the insulating groove are aligned with two ends of the thick gate line.

The present invention also provides a battery assembly, comprising at least two of the above-mentioned busbarless heterojunction batteries, and the adjacent busbarless heterojunction batteries are affixed and fixed one front and one back along the edge of one side where a thick grid line is provided; the thick grid line provided on one side where two busbarless heterojunction batteries are affixed is electrically connected.

Preferably, the separation tension between two adjacent busbar-free heterojunction batteries is greater than or equal to 20N.

Preferably, after the two thick grid lines of the busbar-free heterojunction battery are attached to each other, the thick grid lines are fixedly connected by a conductive tape.

Preferably, after the two thick grid lines of the busbar-free heterojunction solar cell are attached to each other, the thick grid lines are fixedly connected by a welding strip.

Preferably, after the two thick grid lines of the busbar-free heterojunction battery are attached to each other, the thick grid lines are fixedly connected by a conductive adhesive.

Beneficial effects: Compared with the prior art, the present invention has the following significant effects:

1. Compared with the traditional main-grid-free heterojunction battery design in which only one row of parallel auxiliary grids is printed, the present invention provides a design of two groups of fine grid lines that are staggered and connected to each other in two directions, and a pair of thick grid lines are symmetrically arranged at the center of the edges of the front and back sides of the battery to collect the current of the fine grid lines. This design greatly improves the fault tolerance rate in the case of circuit disconnection without increasing the consumption of silver paste, ensuring that the current has dozens of paths to flow to the collection point.

2. Traditional busbar-less heterojunction cells usually reserve blank areas near the edge of the cell to prevent the front and back circuits from being connected and causing a short circuit. The present invention uses a design in which an insulating groove is opened by laser at the edge of the cell to ensure that the front and back circuits of the cell will not be connected and cause a short circuit.

3. In the process of battery assembly packaging and welding, traditional main-grid-free heterojunction batteries need to use a section of welding ribbon to pass through the gap between two batteries to connect the positive and negative electrodes of the two batteries to complete the battery series connection. This method always causes a gap between the battery cells, and hidden cracks are prone to appear at the edge of the gap. However, since the battery edge of the present invention adopts special insulation treatment, the batteries are tightly connected without short circuit. The two batteries are arranged alternately, one front and one back, so that the thick grid lines at both ends of the battery cell can be tightly connected. The two batteries can be simply and quickly connected by using conductive tape, which greatly reduces the equipment requirements, simplifies the series connection process of the battery assembly, and provides more process options.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the front structure of a busbar-free heterojunction battery according to Example 1 of the present invention.

FIG2 is a schematic cross-sectional view of a busbar-free heterojunction battery according to Example 1 of the present invention.

FIG3 is a side view of a busbar-less heterojunction battery according to Example 1 of the present invention.

FIG. 4 is a schematic diagram of the structure of a busbar-free heterojunction battery according to Embodiment 2 of the present invention.

FIG5 is a schematic diagram of the structure of a conventional busbar-free heterojunction battery in the art provided by the present invention.

FIG. 6 is a schematic diagram of the structure of a battery assembly according to Embodiment 3 of the present invention.

FIG. 7 is a schematic diagram of the structure of a battery assembly according to Embodiment 4 of the present invention.

FIG8 is a schematic structural diagram of a conventional battery assembly in the art provided by the present invention.

Figure numerals: 1, heterojunction cell; 2, first thin grid line; 3, second thin grid line; 4, first thick grid line; 5, second thick grid line; 6, insulating groove; 7, auxiliary grid; 8, conductive tape; 9, welding tape.

DETAILED DESCRIPTION

The present invention is further explained below in conjunction with the accompanying drawings and specific implementation methods.

The present invention provides a busbar-free heterojunction battery, comprising a heterojunction battery cell, a plurality of fine grid lines arranged on the front and back sides of the heterojunction battery cell, a first thick grid line arranged on the front side of the heterojunction battery cell, and a second thick grid line arranged on the back side of the heterojunction battery cell; the fine grid lines comprise a plurality of mutually parallel first fine grid lines and a plurality of mutually parallel second fine grid lines, and the first fine grid lines and the second fine grid lines are not parallel to each other, each of the second fine grid lines is connected to at least two of the first fine grid lines, and at least one of the second fine grid lines is connected between two adjacent first fine grid lines; the first and second thick grid lines are symmetrically arranged with respect to the center of the heterojunction battery cell, the first thick grid line connects the plurality of fine grid lines on the front side of the heterojunction battery cell, and the second thick grid line connects the plurality of fine grid lines on the back side of the heterojunction battery cell; the heterojunction battery cell is provided with insulating grooves parallel to the edges of the first and second thick grid lines on both sides thereof.

The present invention also provides a method for preparing the above-mentioned busbar-free heterojunction battery, comprising the following steps:

(1) Printing the gate line circuit on the front side of the heterojunction cell using a special stencil corresponding to the above-mentioned fine gate line and thick gate line designs;

(2) Using a high-temperature curing furnace to cure the gate line circuit;

(3) Using special stencils, the gate line circuit is printed on the back of the heterojunction cell;

(4) Sintering the heterojunction cell at high temperature;

(5) Using ultraviolet laser, an insulating groove is opened at the edge of the heterojunction cell with a thick grid line to obtain a main grid-free heterojunction cell.

The insulating groove has a depth of 50-100 nm and a width of 40-80 μm. Both ends of the insulating groove are aligned with both ends of the thick grid line. In this embodiment, both ends are about 2 mm away from the edge of the heterojunction cell.

The present invention also provides a battery assembly, comprising at least two of the above-mentioned busbarless heterojunction batteries, and the adjacent busbarless heterojunction batteries are affixed and fixed one front and one back along the edge of one side where a thick grid line is provided; the thick grid line provided on one side where two busbarless heterojunction batteries are affixed is electrically connected.

After the thick grid lines of two pieces of the busbar-free heterojunction batteries are attached together, the thick grid lines can be fixed by conductive tape, welding tape, conductive glue, etc. In this embodiment, the separation tension between two adjacent busbar-free heterojunction batteries must be greater than or equal to 20N.

The technical solution of the present invention is further illustrated below in conjunction with specific embodiments and comparative examples.

In the following embodiments and comparative examples, the heterojunction cells selected are all heterojunction cells with a length of 182 mm, a width of 91 mm, and a thickness of 120 μm. Unless otherwise specified, the process flow and manufacturing parameters used are the same.

Example 1

Please refer to Figures 1 and 2. This embodiment provides a busbar-free heterojunction battery, including a heterojunction battery cell 1, a plurality of fine grid lines arranged on the front and back sides of the heterojunction battery cell 1, a first thick grid line 4 arranged on the front side of the heterojunction battery cell 1, and a second thick grid line 5 arranged on the back side of the heterojunction battery cell 1; in this embodiment, the fine grid lines include a plurality of mutually parallel first fine grid lines 2 and a plurality of mutually parallel second fine grid lines 3, the first fine grid lines 2 and the second fine grid lines 3 are perpendicular to each other, and both form an angle of 45° with the edge of the heterojunction battery cell 1, the spacing between adjacent first fine grid lines 2 is _d1 , _{1mm≤d1≤3mm} , the spacing between adjacent second fine grid lines 3 is _d2 , and _d2 = _d1 , that is, the first fine grid lines 2 and the second fine grid lines 3 are cross-connected to each other to form a square grid structure. The first thick grid line 4 is arranged at the edge of the front side of the heterojunction battery cell 1, and is connected to all the thin grid lines on the front side of the heterojunction battery cell 1. The second thick grid line 5 is connected to all the thin grid lines on the back side of the heterojunction battery cell 1 based on the same arrangement. In the present embodiment, the first and second thick grid lines are arranged on the long sides of the heterojunction battery cell 1.

Please refer to FIG. 2 , the first thick grid line 4 and the second thick grid line 5 are symmetrically arranged around the center of the heterojunction cell, and the heterojunction cell 1 is provided with insulating grooves 6 parallel to the edges of the thick grid lines on both sides thereof.

As shown in FIG. 3 , the insulating groove 6 is symmetrically opened at one side edge of the heterojunction cell 1 , and two ends of the insulating groove 6 are respectively aligned with two ends of the first thick grid line 4 and the second thick grid line 5 to ensure the insulation effect.

The method for preparing the above-mentioned busbar-free heterojunction battery comprises the following specific steps:

(1) Using a special stencil corresponding to the above-mentioned gate line design, the gate line circuit is printed on the front side of the heterojunction cell, and the amount of silver paste used on the front side is recorded as 22 mg;

(2) Use a high temperature curing furnace for curing, the curing temperature is 150-180℃, and the time is 10-15s;

(3) Use a special stencil to print the gate line circuit on the back of the heterojunction cell, and record the amount of silver paste used on the back as 29 mg;

(4) Sintering the heterojunction cell at a high temperature of 180-220°C for 15-30 seconds;

(5) Using a 300-400nm ultraviolet laser, an insulating groove is opened on both sides of the long edge of the heterojunction cell. The insulating groove has a depth of 50-100nm, a width of 55-65μm, and a length of 178-179mm;

(6) A busbar-free heterojunction cell was obtained through steps (1) to (5). The conversion efficiency of the cell was tested and the average conversion efficiency of the single cell was 25.5%.

Example 2

Please refer to FIG. 4 , in which a main-grid-free heterojunction battery is provided in this embodiment, including a heterojunction battery cell 1, a plurality of fine grid lines arranged on the front and back sides of the heterojunction battery cell 1, and a first thick grid line 4 and a second thick grid line 5 arranged on the front side of the heterojunction battery cell 1; in this embodiment, the fine grid lines include a plurality of mutually parallel first fine grid lines 2 and a plurality of mutually parallel second fine grid lines 3, the spacings between adjacent first fine grid lines 2 are all the same value, and the second fine grid lines 3 can be further divided into 4 groups, each group is on a straight line, the 4 straight lines are parallel to each other, and each group of the first fine grid lines is on a straight line. Among the two fine grid lines 3, each second fine grid line 3 is staggered with a pair of first fine grid lines 2, each second fine grid line 3 connects two adjacent first fine grid lines 2, and two second fine grid lines 3 are arranged between two adjacent first fine grid lines 2; the first thick grid line 4 is arranged at the edge of the front side of the heterojunction battery cell 1, and is connected to all the first thin grid lines 2 on the front side of the heterojunction battery cell 1, and the second thick grid line 5 is connected to all the second thin grid lines 3 on the back side based on the same arrangement; in the present embodiment, the first and second thick grid lines are arranged on the long sides of the heterojunction battery cell 1.

Please refer to FIG. 2 , based on the same arrangement as in Example 1, the first thick grid line 4 and the second thick grid line 5 are arranged symmetrically about the center of the heterojunction cell, and the heterojunction cell 1 is provided with insulating grooves 6 parallel to the edges of the two sides of the thick grid lines.

Please refer to FIG. 3 , based on the same configuration as in Example 1, the insulating groove 6 is symmetrically opened at one side edge of the heterojunction cell 1 , and both ends of the insulating groove 6 are respectively aligned with both ends of the first thick grid line 4 and the second thick grid line 5 to ensure the insulation effect.

This embodiment provides a gate line design method different from that of Embodiment 1, which can further reduce the amount of silver paste used while achieving the same basic functions.

Comparative Example 1

Please refer to Figure 5. This comparative example provides a conventional main-grid-free heterojunction battery, including a heterojunction battery cell 1 and a secondary grid 7. The secondary grid 7 is a plurality of parallel grid lines. The secondary grid 7 maintains a certain distance from the edge of the heterojunction battery cell 1 to avoid short circuit at the edge. No insulating groove is provided at the edge of the heterojunction battery cell 1.

The parameters of the busbar-free heterojunction battery in this comparative example and the busbar-free heterojunction battery provided in Example 1 are shown in Table 1 below:

Table 1

It can be seen from the data in Table 1 that the busbarless heterojunction battery provided in Example 1 of the present invention reduces the use of 25 mg of silver paste per piece compared to the existing conventional busbarless heterojunction battery, while achieving a 0.1% improvement in conversion efficiency. Calculated at a silver paste price of 7,500 yuan/kg, each printed busbarless heterojunction battery can save 0.1875 yuan in cost, equivalent to 0.0044 yuan/W.

Example 3

Please refer to Figure 6. This embodiment provides a battery assembly, including multiple busbarless heterojunction batteries as described in Example 1, and adjacent busbarless heterojunction batteries are tightly fitted and fixed one front and one back along the edge of one side where the thick grid lines are provided, that is, the first thick grid line 4 of a busbarless heterojunction battery is electrically connected to the second thick grid line 5 of the adjacent busbarless heterojunction battery.

In this embodiment, after the thick grid lines of the two pieces of the busbar-free heterojunction batteries are attached to each other, they are fixedly connected by a conductive tape 8. The conductive tape 8 can be set continuously or intermittently according to needs, but it is necessary to ensure that the separation tension between the two adjacent pieces of the busbar-free heterojunction batteries is greater than or equal to 20N. In this embodiment, the width of the conductive tape 8 is greater than or equal to 4mm.

After the busbar-free heterojunction battery is fixedly connected, due to the special insulation treatment on the edge of the heterojunction battery cell 1 and the design of the insulation groove 6, the heterojunction battery cell 1 can be directly and tightly connected without any short circuit problem. Moreover, since the colors of the front and back sides of the heterojunction battery cell 1 are highly similar, the thick grid lines on the front and back sides are designed with central symmetry. Therefore, a one-front-one-back arrangement method is adopted, so that the busbar-free heterojunction batteries can be tightly fitted between each other.

Taking a 182/144 half-cell module as an example, the preparation method of the battery module includes the following steps:

(1) The suction cup sucks up a single heterojunction battery without a main grid, and lays 12 batteries alternately in the order of one front and one back, with the thick grid lines on the front and back of adjacent batteries closely connected;

(2) The conveyor belt is started to transport the laid battery string to the tape attachment work area;

(3) The robot cuts a transparent conductive tape with a length of 178-180 mm and a width greater than or equal to 4 mm, and attaches the tape above the thick grid lines of the adjacent cells on the front side of the battery string;

(4) After the front tape is attached, the suction cup group is evenly attached to the front surface of the battery string, driving the battery string to reverse so that the back of the battery string faces upward;

(5) The robot cuts a transparent conductive tape with a length of 178-180 mm and a width greater than or equal to 4 mm, and attaches the tape above the thick grid line of the adjacent battery on the back of the battery string;

(6) The robot flips the battery string and places it back on the platform, completing the series connection of the busbar-less heterojunction batteries.

Example 4

Please refer to Figure 7. This embodiment provides a battery assembly, including multiple busbarless heterojunction batteries as described in Example 1, and adjacent busbarless heterojunction batteries are tightly fitted and fixed one front and one back along the edge of one side where the thick grid lines are provided, that is, the first thick grid line 4 of a busbarless heterojunction battery is electrically connected to the second thick grid line 5 of the adjacent busbarless heterojunction battery.

Different from Example 3, in this embodiment, after the thick grid lines 4 of the two pieces of the busbar-free heterojunction batteries are attached to each other, they are fixedly connected by welding strips 9. Since the adjacent busbar-free heterojunction batteries are directly and tightly attached, only a small section of welding strips 9 is needed to fix the thick grid lines 4 to complete the series connection of the battery string. Compared with the traditional welding process, it is simpler and consumes less materials such as welding strips and solders. In some other foreseeable implementations, heating and curing of the conductive adhesive can also be used as a connection process for connecting the thick grid lines 4.

Comparative Example 2

Please refer to Figure 8. This comparative example provides a conventional battery assembly of busbarless heterojunction batteries connected in series, including multiple busbarless heterojunction batteries described in Comparative Example 1 and multiple welding ribbons 9. Two busbarless heterojunction batteries pass through the gap between the heterojunction battery cells 1 through a section of welding ribbon 9 to connect the positive and negative electrodes of the two busbarless heterojunction batteries. This welding method ensures that there is always a gap between the two busbarless heterojunction batteries, and the edge of the gap is prone to hidden cracks.

Taking the 182/144 half-cell module as an example, the power test is performed on the module. The battery module provided in Example 3 is tested under the same conditions. The power data and process comparison are shown in Table 2 below:

Table 2

From the data and process comparison in Table 2, it can be seen that the battery assembly provided in Example 3 has obvious advantages. It does not require heating in the process flow, which reduces energy consumption, has a simple process, is easy to rework, can significantly improve production capacity, and can effectively avoid 0.5-0.7% battery cell efficiency attenuation. Since the single busbar-free heterojunction battery cells are tightly fitted without gaps, it not only eliminates the problem of hidden cracks that may be caused by the welding ribbon squeezing the battery cell, but also further improves the utilization rate per unit area of the battery assembly, so that the single-block power is increased by 7W, achieving a 1.15% performance improvement.

Claims (10)

1. A busbar-free heterojunction battery, characterized in that it comprises a heterojunction battery cell, a plurality of fine grid lines arranged on the front and back sides of the heterojunction battery cell, a first thick grid line arranged on the front side of the heterojunction battery cell, and a second thick grid line arranged on the back side of the heterojunction battery cell; the fine grid lines comprise a plurality of mutually parallel first fine grid lines and a plurality of mutually parallel second fine grid lines, and the first fine grid lines and the second fine grid lines are not parallel to each other, each of the second fine grid lines is connected to at least two of the first fine grid lines, and at least one of the second fine grid lines is connected between two adjacent first fine grid lines; the first thick grid line and the second thick grid line are symmetrically arranged around the center of the heterojunction battery cell, the first thick grid line connects the plurality of fine grid lines on the front side of the heterojunction battery cell, and the second thick grid line connects the plurality of fine grid lines on the back side of the heterojunction battery cell; the heterojunction battery cell is provided with insulating grooves parallel to the edges of the first and second thick grid lines on both sides thereof. 2 . The busbar-free heterojunction battery according to claim 1 , wherein the first fine grid lines are cross-connected with the second fine grid lines to form a grid structure. 3 . 3 . The busbar-less heterojunction battery according to claim 1 , wherein the first thin grid line and the second thin grid line are perpendicular to each other. 4. A method for preparing a busbar-free heterojunction battery according to any one of claims 1 to 3, characterized in that it comprises the following steps: (1) Using a special stencil, the grid line circuit is printed on the front of the heterojunction cell; (2) Curing in a high-temperature curing furnace; (3) Using special stencils, the gate line circuit is printed on the back of the heterojunction cell; (4) Sintering the heterojunction cell at high temperature; (5) Using ultraviolet laser, an insulating groove is opened at the edge of the heterojunction cell with a thick grid line to obtain a main grid-free heterojunction cell. 5 . The method for preparing a busbar-free heterojunction battery according to claim 4 , wherein the insulating groove has a depth of 50-100 nm and a width of 40-80 μm; and two ends of the insulating groove are aligned with two ends of the thick grid line. 6. A battery assembly, characterized in that it comprises at least two busbar-free heterojunction batteries as described in claim 1, and adjacent busbar-free heterojunction batteries are affixed and fixed one front and one back along the edge of one side where thick grid lines are provided; and the thick grid lines provided on one side where adjacent busbar-free heterojunction batteries are affixed are electrically connected. 7. The battery assembly according to claim 6 is characterized in that the separation tension between two adjacent busbar-free heterojunction batteries is greater than or equal to 20N. 8 . The battery assembly according to claim 6 , wherein after the two thick grid lines of the busbar-free heterojunction battery are attached to each other, the thick grid lines are fixedly connected by a conductive tape. 9 . The battery assembly according to claim 6 , wherein after the thick grid lines of two pieces of the busbar-free heterojunction battery are attached to each other, the thick grid lines are fixedly connected by a welding strip. 10 . The battery assembly according to claim 6 , wherein after the two thick grid lines of the busbar-free heterojunction battery are attached to each other, the thick grid lines are fixedly connected by conductive adhesive.