Solar cell and manufacture thereof

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a polycrystalline or single crystal solar cell and a method for producing the same, and more particularly to an improvement in a metal electrode of the solar cell and a method for forming the same.

【0002】

[Conventional technology]

8 and 9 are schematic cross-sectional views for explaining a manufacturing process of a conventional solar cell.

【0003】

First, in FIG. 8A, a p-type silicon substrate 1 having a main surface parallel to the crystal plane of the Miller index (100) and having a thickness of about 0.4 mm is prepared. This silicon substrate 1 is etched in an aqueous solution of about 80 ° C containing several% sodium hydroxide and isopropyl alcohol for about 30 minutes to remove distortion on the surface of the substrate, and the main surface of the substrate 1 is shown in FIG. 8 ( A) It is made into a pyramid-shaped uneven shape as shown by being enlarged in the circle 1A inside. This etching is called texture etching, and the pyramid-shaped uneven shape 1A reduces the light reflection on the main surface of the substrate 1 and improves the efficiency of taking light into the substrate 1.

【0004】

In FIG. 8 (B), n-type impurities (generally phosphorus) are diffused from the substrate 1 by heat treatment at about 1000 ° C for about 30 minutes.⁺Layer 2 is formed, thereby forming p-type substrates 1 and n⁺A pn junction is formed with layer 2.

【0005】

In FIG. 8C, an antireflection film 3 made of a metal oxide is formed on the front surface of the substrate 1 as a light receiving surface by using a known spraying method.

【0006】

In FIG. 8 (D), the antireflection film 3 is covered with an acid resistant tape (not shown) and then etched with a mixed solution of hydrofluoric acid and nitric acid for about 30 seconds.⁺Unnecessary parts formed on the back surface and the side surface of the substrate 1 in the diffusion layer 2 are removed.

【0007】

In FIG. 9A, the back silver electrode 4 and the back aluminum electrode 5 are formed by screen-printing the silver paste and the aluminum paste on the back surface of the substrate 1 and firing at 700 ° C to 800 ° C. At the same time, p⁺Layer 6 is formed, which serves to efficiently collect carriers.

【0008】

In FIG. 9B, the silver paste is screen-printed on the antireflection film 3 and fired at 700 ° C to 750 ° C to form the front silver electrode 7. During this firing, the silver paste reacts with the antireflection film 3 and penetrates, n⁺Join with layer 2.

【0009】

Finally, in FIG. 9 (C), the flux-coated substrate 1 is immersed in silver-containing solder (QQS571Sn62, etc.) heated to about 190 ° C for about 30 seconds, and the front electrode solder on the front silver electrode 7 is obtained. A back electrode solder layer 9 is formed on the layer 8 and the back silver electrode 4. Then, the residual flux is washed and removed with an organic solvent such as xylene, toluene, and acetone, whereby the solar cell is completed.

【0010】

Figures 10 (A) and 10 (B) show the front and back of the thus completed solar cell, respectively. On the front surface of the solar cell, the main electrode portion solder layer 8a and the grid electrode portion solder layer 8b covering each of the main electrode portion and the grid electrode portion of the front silver electrode 7 are observed. Further, on the back surface of the solar cell, a back electrode solder layer 9 covering the back silver electrode 4 provided in the opening of the back aluminum electrode 5 is observed.

【0011】

[Problems to be Solved by the Invention]

In the conventional solar cell obtained by the manufacturing method as shown in FIGS. 8 and 9, the silver electrode portion, particularly the front silver electrode 7, is coated with solder, and thus has excellent electrical characteristics. , It is difficult to control the solder temperature for uniformly forming the front electrode solder layer 8 on the front silver electrode 7 and the speed at which the substrate 1 is pulled out from the solder layer. That is, in some cases, the front electrode solder layer 8 is not uniformly formed on the front silver electrode 7, and it is necessary to observe the coating state of the front electrode solder layer 8 for each solar cell and repair it in case of a defect. is there. Further, in the step of coating the solder layer on the silver electrode, the substrate 1 is rapidly heated and cooled, so that the substrate 1 is easily cracked by the thermal stress, which causes a decrease in the yield of the solar cell. Furthermore, if the substrate 1 is immersed in commonly used 6-4 solder, the silver electrode 7 gradually dissolves in the solder and may disappear if the silver electrode is thin, so it is an expensive solder containing a few percent of silver. (QQS571Sn62 etc.) is mainly used.

【0012】

In view of the above prior art, the present invention eliminates the need to coat the metal electrode with solder by chemically treating the metal electrode, and has high performance and reliability even if the metal electrode is not included. It is intended to provide expensive solar cells.

【0013】

[Means for solving problems]

According to one aspect of the present invention, the solar cell has a crystalline semiconductor substrate that produces a photoelectric conversion action and an electrode formed by screen-printing a metal paste on the surface of the semiconductor substrate and firing it. The electrode contains a surface chemically treated with a chemical solution containing at least one selected from hydrobromic acid, hydrofluoric acid, DL-apple acid, stearic acid, adipic acid, salicylic acid, citric acid, and lactic acid. It is characterized by having.

【0014】

In this solar cell, since the electrodes have a surface chemically treated with an appropriate chemical solution, the electrodes do not need to be coated with solder, and yet they exhibit high-performance and highly reliable output characteristics. be able to.

【0015】

According to another aspect of the present invention, the method for manufacturing a solar cell is to form a crystalline semiconductor substrate that produces a photoelectric conversion action, screen-print a metal paste on the surface of the semiconductor substrate, and fire it. The electrode is formed and the surface of the electrode is chemically treated with a chemical solution containing at least one selected from hydrobromic acid, hydrofluoric acid, DL-apple acid, stearic acid, adipic acid, salicylic acid, citric acid, and lactic acid. It is characterized by including a step of performing.

【0016】

In this method of manufacturing a solar cell, since the electrode is chemically treated with an appropriate chemical solution, the step of coating the electrode with solder can be eliminated, and the solar cell is damaged or soldered due to thermal stress in the solder coating step. There is no need to worry about coating defects.

【0017】

BEST MODE FOR CARRYING OUT THE INVENTION

Conventionally, there are the following three reasons (A1) to (A3) as reasons for coating the silver electrode of a solar cell with solder.

【0018】

(A1) By covering the silver electrode with solder, discoloration of the silver electrode can be reduced. That is, the unsoldered silver electrode may be discolored by the permeated moisture even after the solar cell is assembled into a module containing a plurality of cells.

【0019】

(A2) When assembling a module containing multiple solar cells, it is necessary to connect the interconnector for connecting the cells with a soldering iron. At this time, if the silver electrode is coated with solder, soldering can be performed without using flux, which simplifies the connection work of the interconnector.

【0020】

(A3) By covering the silver electrode with solder, its series resistance is reduced and the efficiency of the solar cell is increased.

【0021】

By the way, recently, forming a fine front electrode has become an indispensable condition for improving the efficiency of solar cells, and due to advances in screen plate making and electrode printing technology, a narrow width of 50 to 100 microns has been adopted. It has become possible to easily form a fine electrode having the electrode. Even if the color of such a fine electrode is slightly discolored, the appearance of the solar cell is not impaired, and the effect of the discoloration of the electrode on the long-term stability of the electrical characteristics of the solar cell is small. Regarding the above reason (A1), it is no longer a problem even if the solder is not coated. Further, the main electrode portion of the front electrode on the light receiving surface side of the solar cell is invisible from the outside because the interconnector is connected when the module is assembled.

【0022】

Furthermore, when connecting the interconnector to the solar cell with a soldering iron, the amount of non-chlorine flux used is limited to the required part of the interconnector (see Fig. 2 (A)). Since it is possible to perform simple cleaning and non-cleaning by reducing the amount of the above-mentioned reason (A2), the above-mentioned reason (A2) is no longer a problem.

【0023】

Under these circumstances, if a highly efficient solar cell can be obtained without coating the electrodes with solder, the solder coating process can be omitted, the yield can be improved, and an inexpensive solar cell can be provided. become.

【0024】

However, simply omitting the solder coating of the silver electrode raises the problem for the above reason (A3).

【0025】

In this regard, FIG. 11 shows the output characteristics of a conventional solar cell in the manufacturing stage of FIG. 9 (B). In FIG. 11, the horizontal axis represents the output voltage (V) and the vertical axis represents the output current (A). The output shown in FIG. 11 is about 48% lower than the output of the cell of FIG. 9 (C) in which the silver electrode is coated with solder. This is because the solder is not covered with a thin grid-shaped front electrode (width 50 to 100 μm), so the series resistance of the grid electrodes is large, and the contact resistance between the grid electrodes and the surface of the silicon substrate is large. Presumed. However, the resistivity of the silver electrode is 3 × 10.^-8Whereas Ωm, the resistivity of solder is larger than that, 15 × 10^-8Since it is Ωm, it is presumed that the large contact resistance between the electrode portion and the surface of the silicon substrate is the main cause of the decrease in the output of the solar cell having the electrode not coated with solder.

【0026】

Therefore, if the contact resistance between the electrode and the surface of the silicon substrate can be reduced by some method instead of the solder coating treatment, it is expected that the conventional solder coating is not always necessary.

【0027】

As a method of reducing the contact resistance between the electrode and the surface of the substrate, the composition ratio of silver in the silver paste used is increased, the type of glass frit in the silver paste is changed, and the firing conditions (transporting the semiconductor substrate in the furnace). It is possible to consider optimizing the speed of the paste, the firing temperature, etc.), but it is very difficult to obtain the same electrical characteristics and tensile strength of the electrodes as the solder-coated solar cell.

【0028】

Therefore, in the present invention, the output characteristics are improved without the need for solder coating by reducing the contact resistance between the electrode and the surface of the silicon substrate by chemically treating the solar cell after firing the electrode and before solder coating. Attempts to provide a soldered solar cell.

【0029】

In FIG. 1, the solar cell in the state shown in FIG. 9 (B) before being solder-coated was immersed in one of various chemical solutions having a concentration of 0.5%, washed with pure water, and dried. The electrical characteristics of the solar cell measured later are shown. In FIG. 1, the horizontal axis and the vertical axis represent the processing time (seconds) and the maximum output Pm (W), respectively. As the chemical solution, an aqueous solution containing any one of hydrogen bromide (HBr), DL-apple acid, hydrofluoric acid (HF), hydrochloric acid (HCl), ammonia, and acetic acid at a concentration of 0.5% was used. .. As can be seen from FIG. 1, the maximum output Pm of the solar cell can be significantly improved by selecting an appropriate treatment time regardless of which chemical solution is used.

【0030】

However, among various chemicals, for ammonia, a slight discoloration is observed on the electrode surface after chemical treatment and the solderability deteriorates, but for hydrobromide acid and DL-malic acid, there is no change in the appearance of the electrode. Excellent electrical characteristics can be obtained.

【0031】

From the results shown in Fig. 1, the surface of the silver powder contained in the silver paste is oxidized during the firing of the silver electrode, and this oxide film increases the contact resistance between the silicon surface and the silver electrode. It is estimated that the maximum output of the cell is low. Therefore, in order to improve the electrical characteristics of the solar cell after firing, it is considered that the oxide film in the electrode may be dissolved and treated with a chemical solution that restores the surface of the metal particles. However, among the inorganic acids, for example, nitric acid and sulfuric acid are not preferable because they oxidize the metal or change it into sulfide. In addition, reducing acids such as hydrochloric acid and phosphoric acid are not preferable because they are highly corrosive.

【0032】

FIG. 2 is a schematic side view for explaining the connection of the interconnector to the solar cell. FIG. 2 (A) shows the connection of the interconnector to the solar cell according to the present invention, and FIG. 2 (B) shows the connection of the interconnector to the conventional solar cell. In FIG. 2, reference numeral 1S represents a component of the solar cell excluding the front electrode, and the interconnector 10 is coated with solder. In FIG. 2A, the flux 11 is applied onto the interconnector 10 corresponding to the front electrode 7a treated with the chemical solution. In this state, the interconnector 10 is joined to the front electrode 7a treated with a chemical solution using a soldering iron. On the other hand, in the conventional solar cell shown in FIG. 2 (B), the front electrode 7 is covered with the solder layer 8. In this state, the solder-coated interconnector 10 is joined to the front electrode 7 using a soldering iron.

【0033】

FIG. 3 shows the bonding strength (electrode strength) of the front electrode to the silicon substrate by applying a tensile force to the 3 mm wide interconnector soldered to the front electrode of the solar cell by the method shown in FIG. ) Is shown. As shown in FIG. 3 (A), the interconnector 10 was subjected to a tensile force at an angle of 45 ° with respect to the surface of the solar cell 1S. The graph of FIG. 3 (B) shows the electrode tensile strength (g) of the front electrode treated with various chemicals.

【0034】

As is clear from FIG. 3 (B), the front electrode 7a treated with hydrobromide (HBr) or malic acid has excellent electrical properties as shown in FIG. It can be seen that it also has the tensile strength of the electrode. Solar cells treated with hydrofluoric acid (HF) have excellent electrical properties as shown in Fig. 1, but if they are treated with a chemical solution for a long time, they are shown in Fig. 3 (B). There is a problem that the electrode strength is lowered as described above. This is because the glass frit in the silver paste is partially dissolved by hydrofluoric acid to reduce the contact resistance between the electrode and the silicon surface, resulting in good electrical properties, but the silver powder is applied to the silicon substrate. It is considered that this is because the electrode strength is lowered because the glass frit that acts to fix the glass frit is melted more than necessary. Therefore, when hydrofluoric acid is used as a chemical solution, it is preferable to reduce the concentration of hydrofluoric acid or shorten the treatment time.

【0035】

From the above, as the treatment chemicals, hydrogen bromide acid, hydrofluoric acid, DL-malic acid, stearic acid, adipic acid, salicylic acid, citric acid, lactic acid, etc., which do not contain chlorine, cause a problem of electrode corrosion. It is preferable because the electrical characteristics can be improved without any problem. In particular, hydrobromic acid and DL-malic acid are preferable in terms of reliability because they can be washed with water and have high electrode strength.

【0036】

As can be seen from the above, a solar cell having a high output without solder coating on the metal electrode by chemically treating the fired metal electrode of the solar cell according to the present invention to reduce the contact resistance between the electrode and the silicon surface. You can get a cell. Chemical agents that can be used at this time are organic acids such as stearic acid, adipic acid, salicylic acid, citric acid, lactic acid, DL-apple acid, hydrofluoric acid and hydrofluoric acid, which are generally used as activators for flux for soldering. There are inorganic acids such as hydrofluoric acid. The reason why a high output solar cell can be obtained without solder coating the electrodes is that these agents dissolve or reduce the oxide film on the surface of the metal powder contained in the electrodes to restore the surface of the metal powder. It is presumed that this is because the contact resistance between the electrode and the silicon surface is lowered. It should be noted that the solar cell, whose contact resistance has been lowered and a high output can be obtained, does not cause a change in electrical characteristics over time in a constant temperature and humidity test (temperature 45 ° C, humidity 95%, test time 720 hours). ) Has been confirmed. In addition, since hydrobromic acid and DL-malic acid are well dissolved in water, they hardly remain in the solar cell by washing with running water for 5 to 10 minutes, and there is no problem in reliability.

【0037】

[Example]

4 and 5 are schematic cross-sectional views for explaining a manufacturing process of a solar cell according to an embodiment of the present invention.

【0038】

First, in FIG. 4A, a p-type silicon substrate 1 having a main surface parallel to the crystal plane of the Miller index (100) and having a thickness of about 0.4 mm is prepared. This silicon substrate 1 is etched in an aqueous solution of 80 ° C containing several% sodium hydroxide and isopropyl alcohol for about 30 minutes to remove distortion on the surface of the substrate, and the main surface of the substrate 1 is shown in FIG. 4 (A). ) Is made into a pyramid-shaped uneven shape as shown by being enlarged in the circle 1A inside. This etching is called texture etching, and the pyramid-shaped uneven shape 1A reduces the light reflection on the main surface of the substrate 1 and improves the efficiency of taking light into the substrate 1.

【0039】

In FIG. 4 (B), n-type impurities (generally phosphorus) are diffused from the substrate 1 by heat treatment at about 1000 ° C for about 30 minutes.⁺Layer 2 is formed, thereby forming p-type substrates 1 and n⁺A pn junction is formed with layer 2.

【0040】

In FIG. 4C, an antireflection film 3 made of a metal oxide is formed on the front surface of the substrate 1 as a light receiving surface by using a known spraying method.

【0041】

In FIG. 4 (D), the antireflection film 3 is covered with an acid resistant tape (not shown) and then etched with a mixed solution of hydrofluoric acid and nitric acid for about 30 seconds.⁺Unnecessary parts formed on the back surface and the side surface of the substrate 1 in the diffusion layer 2 are removed.

【0042】

In FIG. 5A, the back silver electrode 4 and the back aluminum electrode 5 are formed by screen-printing the silver paste and the aluminum paste on the back surface of the substrate 1 and firing at 700 ° C to 800 ° C. At the same time, p⁺Layer 6 is formed, which serves to efficiently collect carriers.

【0043】

In FIG. 5B, the silver paste is screen-printed on the antireflection film 3 and fired at 700 ° C to 750 ° C to form the front silver electrode 7. During this firing, the silver paste reacts with the antireflection film 3 and penetrates, n⁺Join with layer 2. Then, the substrate 1 is immersed in a 0.5% concentration normal temperature aqueous solution in which hydrobromic acid is mixed with pure water for about 20 seconds, and washed with pure water (5 MΩ or more). The washed substrate 1 is dried with warm air, thereby completing the solar cell.

【0044】

FIG. 6 shows the front and back of the solar cells manufactured by the process shown in FIGS. 4 and 5. FIG. 6 (A) shows the front surface of the solar cell, and both the main electrode portion 7a and the grid electrode portion 7b (width 50 to 100 μm) of the front electrode are chemically treated and not solder-coated. .. FIG. 6B shows the back surface of the solar cell, and the back silver electrode 4 in contact with the silicon substrate 1 through the opening formed in the aluminum electrode layer 5 is also treated with a chemical solution, and the solder coating is applied. It has not been.

【0045】

FIG. 7 shows the output characteristics of the solar cells manufactured by the process shown in FIGS. 4 and 5. In this graph, the horizontal axis represents the output voltage (V) and the vertical axis represents the output current (A). As can be seen from the comparison between the graph of FIG. 7 and the graph of FIG. 11, this book is compared with the output characteristics of a conventional solar cell having a fired electrode in the state shown in FIG. 9 (B). It will be clear that the output characteristics of the solar cell having the electrodes chemically treated after firing according to the invention are significantly improved.

【0046】

Although hydrobromic acid was used as a chemical solution in the above examples, it goes without saying that an organic acid such as DL-malic acid or citric acid can be used instead of hydrobromic acid. A flux containing an acid or hydrobromic acid as an activator can also be used. However, when flux is used, it is necessary to select treatment conditions that increase both the electrical characteristics and the electrode strength by lengthening the immersion time or increasing the drug concentration. Moreover, in the embodiment, n⁺/ p / p⁺Although the type of solar cell has been described, the present invention is n.⁺Needless to say, it can also be applied to / p-type solar cells.

【0047】

[Effect of the invention]

As described above, according to the present invention, it is possible to provide a solar cell capable of obtaining a high output without coating the fired electrode with solder. More specifically, the following effects can be obtained.

【0048】

(B1) Since expensive solder (QQS571Sn62, etc.), flux for solder coating of electrodes, and organic solvents such as xylene, toluene, and acetone used for cleaning the flux after solder coating are not required, solar cells are not required. It is possible to significantly reduce the cost in manufacturing the cell.

【0049】

(B2) Instead of expensive manufacturing equipment such as soldering equipment and explosion-proof type flux cleaning equipment that can control the temperature with high precision, it is possible to manufacture solar cells using cheaper chemical treatment equipment and cleaning equipment. Therefore, it is possible to reduce the cost of the solar cell by reducing the amount of capital investment.

【0050】

(B3) In the process of solder coating the electrodes, the substrate is rapidly heated and cooled and easily cracked, and the substrate also cracks due to thermal stress caused by the difference in the coefficient of thermal expansion between the solder, the substrate, and the electrode material. The facilitation causes a decrease in the yield of the solar cell, but in the present invention, the chemical solution treatment at room temperature is possible and no stress is applied to the substrate, so that the yield of the solar cell is greatly improved. It will be.

【0051】

(B4) In the process of solder coating the electrodes, silver contained in the electrodes of the solar cell is gradually dissolved in the solder, so expensive solder containing several% of silver is used, but the cell is still immersed in the solder layer. Sometimes the silver contained in the electrode is slightly melted and the electrode layer becomes thin, and the adhesive strength of the electrode to the substrate decreases due to thermal stress. On the other hand, in the present invention, the silver contained in the electrode does not melt and is not subjected to heat stress, so that the adhesive strength of the electrode to the substrate can be higher than that of the conventional one. Is.

【0052】

(B5) In the present invention, since the electrodes are not solder-coated, it is not necessary to observe and repair the solder-coated defects for each solar cell, and the inspection process of the solar cells can be reduced.

【0053】

(B6) In the solar cell manufactured by the present invention, since the electrodes are not coated with solder, the surface of the cell is less uneven, and it is possible to reduce cell cracking in the process of laminating a plurality of cells and assembling them into a module. it can.

[Simple explanation of drawings]

[Figure 1]

It is a graph which shows the maximum output of the solar cell which was chemically treated with various chemicals.

[Figure 2]

It is a side view for demonstrating the connection method of the interconnector to a solar cell.

[Fig. 3]

It is a figure which shows the method of the tensile strength test of the electrode of a solar cell, and the result.

[Fig. 4]

It is the schematic sectional drawing for demonstrating the manufacturing process of the solar cell according to one Example of this invention.

[Fig. 5]

It is sectional drawing which shows the process which follows FIG.

[Fig. 6]

It is a figure which shows the front surface and the back surface of the solar cell manufactured by the process of FIG. 4 and FIG.

[Fig. 7]

It is a graph which shows the output characteristic of the solar cell manufactured by the process of FIG. 4 and FIG.

[Fig. 8]

It is sectional drawing for demonstrating an example of the manufacturing process of the conventional solar cell.

[Fig. 9]

It is sectional drawing which shows the process which follows FIG.

[Fig. 10]

It is a figure which shows the front surface and the back surface of the solar cell manufactured by the process of FIG. 8 and FIG.

[Fig. 11]

It is a graph which shows the output characteristic of the conventional solar cell in the state of FIG. 9B.

[Explanation of symbols]

1 p-type silicon semiconductor substrate 1A Pyramid-shaped uneven surface 2 n⁺Diffusion layer 3 Anti-reflective coating 4 Back silver electrode 5 Back aluminum electrode 7 Front silver electrode 7a, 7b Chemically treated front silver electrode 8,8a, 8b Solder layer coated on the front silver electrode 9 Solder layer coated on the back silver electrode 10 interconnector 11 Flux for soldering