JP2794141B2 - Method for manufacturing photoelectric conversion device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the formation of a back electrode of a photoelectric conversion device, and is particularly useful when a solar cell is connected to form a module.

[0002]

2. Description of the Related Art A silicon solar cell widely used as a photoelectric conversion device will be described as an example. FIGS. 2A and 2B are a schematic sectional view and a plan view of an example of a conventional solar cell. It is manufactured as follows.

For example, an N-type diffusion layer 1 is provided on the surface of a P-type silicon substrate 10 by thermal diffusion in an atmosphere containing POCl ₃ or the like to form a PN junction. Thereafter, unnecessary N-type diffusion layers on the back and side surfaces are removed, and Al
P / P ⁺ for BSF effect by baking paste etc.
In general, there is provided a structure in which a BSF layer 7 as a high-low barrier layer and an aluminum electrode 6-1 of a fired aluminum layer are provided. In addition, a SiO ₂ film 2 for passivation and a reflection reduction are provided on the light receiving surface side. An anti-reflection film 3 is formed.
Finally, a fishbone-shaped light-receiving surface collecting electrode 8 is provided on the light-receiving surface side to complete the cell. In recent years of mass production, instead of the conventional vacuum deposition method, printing and baking of a metal paste containing Ag as a main component (hereinafter referred to as silver paste) penetrates the antireflection film and forms an N-type diffusion layer 1 on the surface. Is used.

As a single cell, a solar cell is completed by such a process, but in order to be used in an actual solar cell system, a plurality of cells are connected in series to form a structure called a module. Need to be In this case, as shown in FIG. 4 (b), the light-receiving surface collecting electrode 8 of one cell 20 and the aluminum electrode 6-1 on the back surface of the next cell 20 are connected by a thin lead frame 11 coated with copper wire. A connection method is used in which connections are made sequentially.

However, in the case of a high-efficiency solar cell having the above-mentioned BSF structure, since the back surface is made of aluminum, it cannot be connected without general soldering, so that the following FIGS. ) And FIGS. 3 (a) and (b)
The structure shown in FIG.

First, as shown in FIG. 2B, the aluminum paste is not printed on the part 10-1 on the back side, but the fishbone type is formed on the entire back surface of the silicon substrate 10 including the unprinted part. The silver paste (pad silver electrode) 5-1 to which the solder is attached is formed by printing and baking a silver paste in the pattern described above. In the portion 10-1 where the aluminum paste is not printed, the silver electrode 5-1 is directly connected to the silicon substrate 10.

FIGS. 3A and 3B are plan views showing the structure of another back electrode, and the aluminum electrode 6-1 is removed from the structure shown in FIGS. It will be. In the figure, a silver electrode 5-2 is BSF
Connected directly to layer 7. It is manufactured as follows. The process up to the process on the back surface is the same as that in FIG.

First, an aluminum paste is printed on the entire back surface and baked to form the BSF layer 7. Then, unnecessary aluminum layers and baked layers are removed by dipping in an acid (for example, aqua regia), and exposed. On the surface of the BSF layer 7 thus formed, a silver electrode 5-2 having a fishbone-like pattern and good adhesion is formed.

[0009]

In the case of the manufacturing method shown in FIGS. 2A and 2B, when the aluminum paste is fired,
First, a eutectic reaction between aluminum and silicon on the substrate occurs,
In the process of raising the processing temperature, diffusion of impurities from the aluminum eutectic layer to silicon occurs to form a P ⁺ layer,
Excess aluminum solidifies during the cooling process. At this time, glass, resin components and the like contained in the paste remain on the surface of the sintered aluminum layer. This layer is called a combustion layer (or a dust layer), and although there is electrical conduction, it is brittle, and the silver electrode 5-1 has strong adhesiveness at the pad portion 10-1 at the contact portion with the silicon substrate 10. However, the adhesive force at the interface in contact with the combustion layer is 1 kg / cm ² or less, which is extremely weak and often peels off, increasing the contact resistance, decreasing the fill factor of the cell, and deteriorating the characteristics. It is a factor that makes things worse. Regarding the resistance component on the back surface, considering the current flow in the silver electrode on the back surface, when a light-generating current flows on the back surface, the resistance loss flowing through the fired aluminum layer is better than that of the fired aluminum layer 6-1. The resistance loss is at least one order of magnitude smaller than the resistance loss flowing through the upper silver electrode 5-1. The current does not flow through the thin silver electrode 5-1 above the aluminum electrode 6-1 but flows through the fired aluminum layer having a low resistance and overlaps. After flowing at the interface with the silver electrode 5-1, it is collected by the lead frame 11 connected to the silver electrode 5-1 and flows into the next cell. The silver other than the peripheral portion does not contribute to the reduction of the resistance portion in the interconnection of the cells at all, which means that unnecessary and excessive silver was used.

In order to increase the adhesive force at the interface with the silver electrode 5-1 in contact with the combustion layer, the structure shown in FIGS. 3A and 3B in which the aluminum fired layer 6-1 is removed is used. Indicates that while the sheet resistance of the aluminum fired layer is several mΩ, the BSF layer 7
When the area of the current extracting portion is small, the resistance loss due to the current flowing through the BSF layer 7 is large, and therefore the occupancy of the back surface silver electrode must be increased. Therefore, as shown in FIGS. 3A and 3B, a fish-bone or grid-like silver electrode 5 similar to the light-receiving surface electrode is used.
-2 must be formed. This means that the process becomes complicated and the amount of silver material used increases, which leads to an increase in cost.

As described above, in order to form a module, in a method of forming pad silver electrodes for connecting cells by printing and baking after forming a BSF layer, the amount of silver used is reduced and the bonding strength is increased. It is difficult to improve the fill factor in the module by reducing the resistance loss between them. An object of the present invention is to solve the above-mentioned problems and to provide a cell and a module having a high adhesion and a high fill factor by forming a BSF layer and a pad silver electrode on the back surface by a simple process.

[0012]

According to the present invention, a silver paste is applied to a connection area of a lead frame on a peripheral portion of a back surface of a silicon substrate and dried, so as to overlap a part of the silver paste in the connection area of the lead frame. The BSF layer and the pad silver electrode were formed at the same time by applying an aluminum paste on the back surface and drying and then firing.

[0013]

According to the present invention, the BSF layer and the pad silver electrode can be formed simultaneously by firing. In addition, the bonding strength of the pad silver electrode is strong and the occupancy of the electrode can be reduced.

[0014]

1 (a) and 1 (b) are a schematic sectional view of an embodiment of the present invention and a schematic diagram of a back electrode thereof, respectively. It is manufactured as follows.

After the cleaning of the P-type silicon substrate 10, chemical etching for removing a crushed layer is performed. Subsequently, the P-type silicon substrate 10 is placed in POCl ₃ gas for 850-90.
Thermal diffusion is performed at 0 ° C. to form an N-type diffusion layer 1 having a sheet resistance of about 50Ω. Subsequently, the P formed on the surface at this time
After removing the SG film (Phospho-silicate glass) with hydrofluoric acid, SiO ₂ 2 serving as a passivation layer is formed at 800 ° C. on the surface of the silicon substrate 10 in an oxidizing atmosphere.

Thereafter, an anti-reflection film 3 of, for example, a TiO ₂ film is deposited and formed on the light-receiving surface by CVD, and unnecessary N-type diffusion layers 1 on the back and side surfaces are chemically removed, and the process proceeds to electrode formation.

First, a silver paste to be the pad silver electrode 5 is applied to a part of the peripheral portion of the back surface of the silicon substrate 10 at a predetermined position of a lead frame led from the light receiving surface side of an adjacent cell to a width of 4 to 6 mm. Is applied by printing in a size of 15 to 20 mm in length and dried. Subsequently, aluminum paste 6 is printed and dried on the entire back surface so that the periphery of the pad silver electrode 5 except for one side on the lead frame 11 side is overlapped by about 2 mm to leave the connection portion of the lead frame. And then fired in an oxidizing atmosphere at about 750 ° C.
The F layer 7 and the pad silver electrode 5 are formed simultaneously. For this reason, the adhesive force, which was a conventional problem, between the aluminum fired layer and the silver electrode formed thereon was significantly improved.

The cell characteristics vary depending on the printing area of the aluminum paste 6, but in this case, the portion where the aluminum paste is not printed can be made to be 2% or less of the entire rear surface area, and hardly affects the cell characteristics.

Finally, a light-receiving surface collecting electrode 8 is formed by printing silver paste and baking at about 700 ° C., and is provided with a pad for a module, in other words, a pad silver electrode 5 for cell connection on the back surface. The battery 20 is completed. At this time, the adhesive strength at the aluminum / silver interface is 15 kg / cm ^2.
As described above, the reliability related to the connection of the lead frame has been greatly improved.

Further, a pad silver electrode having a low contact resistance and a high adhesive strength is formed between the BSF layer and the pad silver electrode.
In addition, as described above, the pad silver for cell connection formed on the back surface has high adhesion and high reliability, and has low contact resistance at the silver-aluminum interface. And the characteristics are good.

Further, by forming the above-mentioned light receiving surface collecting electrode 8 simultaneously with the pad silver electrode 5 on the back surface, the process can be further simplified. After printing and drying the silver and aluminum paste on the back surface, printing and drying the silver paste on the light receiving surface side, and firing in an oxidizing atmosphere at about 800 ° C., the electrodes on both front and back surfaces are fired only once. Can also be formed.

FIG. 5 shows the process according to the invention and FIG.
FIG. 3 shows the process used to manufacture (a) and (b)
5 is a table comparing the processes used for manufacturing (a) and (b). It can be seen that the process of the present invention has been greatly simplified as compared to the prior art.

Table 1 below shows a comparison of the characteristics between the module connected with the cell according to the present invention shown in FIG. 1 and the module using the cell having the conventional structure shown in FIGS. 2 (a) and 2 (b). .

[0024]

[Table 1]

As shown in Table 1, for example, 100 m
In a module in which 36 m-square cells are connected in series,
The fill factor of the module was greatly improved, and the efficiency could be improved by 1% or more compared with the conventional case. Also, when connecting cells to form a module, FIG.
As shown in (a), the length of the lead frame 11 required for connection between the adjacent cells 20 and 20 was shortened, and workability was able to be improved.

[0026]

According to the present invention, it is possible to provide a method for forming a pad silver electrode having a low contact resistance and a high adhesive force despite the small contact portion. In addition, since electrodes on the back surface or both surfaces can be formed by one firing, the process can be greatly simplified. In addition, the pad silver for cell connection formed on the back surface has strong adhesion and high reliability, and also has low contact resistance between the silver and aluminum interfaces, so the module with serially connected cells has a high fill factor and good characteristics. become.

Further, since the occupation ratio of the pad silver electrode can be reduced, the material cost can be reduced and the cost can be reduced.

[Brief description of the drawings]

1 (a) and 1 (b) are a schematic sectional view and a schematic view of a back electrode of a solar cell according to the present invention, respectively.

FIGS. 2A and 2B are a schematic sectional view of a conventional solar cell and a schematic view of a back electrode thereof, respectively.

3 (a) and 3 (b) are a schematic cross-sectional view of another example of a conventional solar cell and a schematic view of a back electrode thereof, respectively.

FIGS. 4A and 4B are side views showing connection of cells by a lead frame according to the present invention and a conventional example, respectively.

FIG. 5 is a comparison table between the present invention and a conventional example.

[Explanation of symbols]

1 N-type diffusion layer 2 SiO ₂ film 3 antireflection film 5 pad silver electrode 6 aluminum paste (aluminum paste fired layer) 7 BSF layer 8 light-receiving surface collecting electrode 10 silicon substrate

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yuji Yokozawa 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Kazutaka Nakajima 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (56) References JP-A-62-9680 (JP, A) JP-A-62-108579 (JP, A) JP-A-3-263877 (JP, A) JP-A-6-500670 (JP, A) (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 31/04

Claims (2)

(57) [Claims] 1. A step of applying and drying a silver paste on a connection area of a lead frame at a peripheral portion of a back surface of a silicon substrate, and applying an aluminum paste on the back surface so as to overlap a part of the silver paste in an area of the lead frame formation area. A step of drying, a step of forming a BSF layer and a pad silver electrode by firing,
A method for manufacturing a photoelectric conversion device having: 2. A method for manufacturing a photoelectric conversion device according to claim 1, further comprising a step of applying and drying a silver paste on a region of the light-receiving surface of the silicon substrate where a collector is to be formed, and a step of baking the collector simultaneously with baking the back surface. .