Silicon wafer, preparation method thereof and solar cell

Abstract

The invention discloses a silicon wafer and a preparation method thereof, and a solar cell, the silicon wafer comprises a silicon substrate, the side edge of the silicon substrate is coated with a protective coating, and the protective coating integrally extends to the front edge and / or the back edge of the silicon substrate; according to the invention, the edge of the silicon wafer is prevented from being chemically corroded and mechanically damaged by arranging the protective coating, so that the silicon wafer maintains the original bendable performance in the process of manufacturing the solar cell, and the reliability of the bendable silicon wafer is powerfully guaranteed; and therefore, the production qualification rate of the manufactured bendable solar cell (namely a flexible cell piece) is ensured.

Description

Technical Field

[0001] The present invention relates to the field of solar cells, and in particular to a silicon wafer and a preparation method thereof. The present invention also relates to a solar cell made from the silicon wafer. Background Art

[0002] Monocrystalline silicon solar cells account for a large proportion of the current photovoltaic market (approximately more than 95%), and have many advantages such as high conversion efficiency, good stability, and low cost. Since monocrystalline silicon is a brittle material and is very fragile, it is usually encapsulated with rigid glass to make photovoltaic modules, which do not have the characteristics of bendability and flexibility. With the development of photovoltaic application technology, more and more scenarios require lightweight and flexible photovoltaic modules, such as roofs, car roofs, portable power supplies, wearable electronic devices, etc. Therefore, there is an urgent practical application demand to improve the bendability and flexibility of monocrystalline silicon solar cells.

[0003] Relevant research has confirmed that the fragility of monocrystalline silicon solar cells is closely related to the line marks left on the edge when the silicon wafer is sliced ​​and the velvet structure on the surface. The velvet structure is prepared by the anisotropic reaction of the monocrystalline silicon wafer in an alkaline solution. It is a pyramid-shaped light-trapping structure that captures more sunlight. However, the line marks and pyramid structures left on the edge will reduce the mechanical strength of the silicon wafer, because the concave valleys of the line marks and the valleys of the adjacent pyramids form a relatively sharp angle, which is similar to forming many microscopic "notches" on the silicon wafer. When the silicon wafer is bent, vibrated, or subjected to thermal shock (dramatic temperature changes), stress concentration will occur at the notch position, causing the silicon wafer to easily break. Silicon wafer breakage may occur during the production of cells, the production of battery modules, and the use of photovoltaic products.

[0004] Further, the invention patent application with application number CN202211090758X discloses a method and structure for rounding the edge of a silicon wafer to make the silicon wafer soft; another invention patent application with application number CN2023101757573 discloses a method for edge processing using plasma etching; another invention patent application with application number CN2023101363247 discloses an edge-polished single-crystal texturing silicon wafer and a preparation method, which prepares the edge area of ​​the silicon wafer with a width of 0.1~5mm into a smooth surface without a pyramid structure, which can effectively reduce the stress concentration generated at the pyramid angle and improve the bendability and flexibility of the silicon wafer.

[0005] However, as the applicant further implemented and developed, it was found that the above method can significantly improve the bendability of silicon wafers. However, when the silicon wafers are made into solar cells, the silicon wafers need to go through a series of solar cell manufacturing processes, and the bendability of the silicon wafers will gradually decrease, seriously affecting the production qualification rate of flexible cells.

[0006] Please refer to Figure 1 and Figure 2 As shown, Figure 1 As can be seen, the edge of the silicon wafer is very smooth after polishing. The smooth edge can improve the bendability of the silicon wafer. Figure 2 for Figure 1 The laser micrograph of the edge of a single-crystal textured silicon wafer after undergoing a series of solar cell manufacturing processes shows that many defects have been generated at the edge of the silicon wafer, resulting in a decrease in the bendability of the silicon wafer.

[0007] To this end, the applicant hopes to seek technical solutions to solve the above technical problems. Summary of the invention

[0008] In view of this, the purpose of the present invention is to provide a silicon wafer and a preparation method thereof, and a solar cell. By setting a protective coating, the edges of the silicon wafer are protected from chemical corrosion and mechanical damage, so that the silicon wafer maintains its original bendability during the process of manufacturing solar cells, effectively ensuring the reliability of the use of bendable silicon wafers, and further ensuring the production qualification rate of bendable solar cells (i.e., flexible solar cells).

[0009] The technical solution adopted by the present invention is as follows: A silicon wafer comprises a silicon substrate, wherein the side of the silicon substrate is coated with a protective coating, and the protective coating extends integrally to the front edge and / or the back edge of the silicon substrate.

[0010] Preferably, the material of the coating includes polytetrafluoroethylene PTFE and / or modified polytetrafluoroethylene material (including soluble polytetrafluoroethylene PFA, polyperfluoroethylene propylene FEP, etc.); wherein the weight proportion of the polytetrafluoroethylene and / or modified polytetrafluoroethylene material in the coating is 90-100wt%, and it not only has good acid and alkali resistance and oxidation resistance, and can withstand high temperatures up to 300°C, but also has a strong bonding force with the silicon substrate, so that the edge of the silicon substrate can obtain a reliable coating protection effect.

[0011] Preferably, the thickness of the protective coating is in the range of 1-100 μm; and / or the coverage width of the protective coating on the front edge or the back edge of the silicon substrate is in the range of 0.1-5 mm.

[0012] Preferably, the silicon substrate is a polished single crystal silicon wafer obtained by chemically polishing an original silicon wafer.

[0013] Preferably, a method for preparing the silicon wafer according to the above comprises the following steps: S1). The original silicon wafer is chemically polished, then cleaned and dried to obtain a polished silicon wafer as a silicon substrate; S2). In the above step S1), a protective adhesive is applied to the side, front edge and / or back edge of the polished silicon wafer to obtain a polished silicon wafer coated with a protective adhesive; S3). Sintering the protective adhesive in the polished silicon wafer obtained in the above step S2) to obtain the silicon wafer.

[0014] Preferably, in the step S1), the chemical polishing is performed by polishing with an alkaline solution, or the chemical polishing is performed by two-step chemical polishing, wherein the first step of chemical polishing is performed by polishing with an alkaline solution, and the second step of chemical polishing is performed by polishing with an acidic solution.

[0015] Preferably, the alkaline solution comprises at least one of NaOH and KOH, wherein the mass fraction of the alkali in the alkaline solution is 1%-25%, and / or the temperature of the alkaline solution is 65-95°C, and / or the treatment time of the alkaline corrosion polishing is 1-10min; the acidic solution comprises an oxidant and HF, the mass fraction of the oxidant in the acidic solution is 30%-60%, the mass fraction of HF in the acidic solution is 0.5%-15%, and / or the temperature of the acidic solution is 15-40°C, and / or the treatment time of the acidic solution polishing is 3-30min; the oxidant is preferably HNO3 or ozone, wherein the mass fraction of HNO3 in the acidic solution is 30%-60%; the mass fraction of ozone in the acidic solution is 20ppm-200ppm.

[0016] Preferably, in the step S3), before sintering, the protective adhesive is dried (specifically, the solvent of the protective adhesive is dried) to obtain a preformed protective coating; wherein the drying temperature is 100-200°C, and the drying time is 1-10 min; the sintering temperature is 250-400°C, and the sintering time is 10-60 min; preferably, after drying and before sintering, pressure is applied to the preformed protective coating to reduce the thickness of the preformed protective coating, and further preferably, the thickness of the preformed protective coating is reduced to 1-50 μm to improve the uniformity of the thickness of the preformed protective coating, and then it is sintered.

[0017] Preferably, a solar cell is made of a silicon wafer, and the silicon wafer is made according to the silicon wafer described above or by the method for making the silicon wafer described above.

[0018] Preferably, the solar cell is a silicon heterojunction solar cell; wherein, the process of making the silicon heterojunction solar cell using silicon wafers includes texturing and cleaning the silicon wafers, and / or depositing an amorphous silicon thin film on the silicon wafers, and / or depositing a transparent conductive thin film on the silicon wafers, and / or making electrodes on the silicon wafers by screen printing or electroplating.

[0019] The present invention arranges a protective coating on the side, front edge and / or back edge of the silicon substrate, so that the edge of the silicon substrate is reliably covered and protected. The protective coating can prevent the edge of the silicon wafer from chemical corrosion and mechanical damage, so that the silicon wafer maintains its original bendability during the process of manufacturing solar cells (including harsh environments such as strong alkaline chemical liquid, strong acidic chemical liquid, strong oxidizing chemical liquid, high temperature, vacuum, and high-energy particle bombardment), effectively guarantees the reliability of the use of bendable silicon wafers, and further ensures the production qualification rate of the bendable solar cells (i.e., flexible battery cells) manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a laser micrograph of the edge of a single crystal textured silicon wafer with an edge polished structure; Figure 2 yes Figure 1 Laser micrograph of the edge of a single crystal textured silicon wafer after undergoing a series of solar cell manufacturing steps; Figure 3 is a schematic diagram of a flexible single crystal silicon wafer with an edge protection coating provided in Example 1 of the present invention; Figure 4 is a schematic diagram of an edge section of a flexible single crystal silicon wafer with an edge protection coating provided in Example 1 of the present invention; Figure 5 is a laser microscopic photograph of the edge of a flexible single crystal silicon wafer with an edge protection coating provided in Example 1 of the present invention; Figure 6 is a laser micrograph of the edge section of the flexible single crystal silicon wafer with a preformed PTFE protective coating in Example 1 of the present invention; Figure 7 is a laser micrograph of the edge section of the flexible single crystal silicon wafer after the preformed PTFE protective coating is flattened in Example 1 of the present invention; Figure 8 It is the amount of downward compression deformation when the textured silicon wafer and the solar cell are broken in the three-point bending test in Example 1 of the present invention and Comparative Example 1 respectively. DETAILED DESCRIPTION

[0021] This embodiment provides a silicon wafer, including a silicon substrate, wherein the side of the silicon substrate is coated with a protective coating, and the protective coating extends integrally to the front edge and / or the back edge of the silicon substrate.

[0022] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0023] In order to further illustrate the implementation process and implementation effect of the present application, based on the above implementation scheme, the present application further proposes the following examples and comparative examples: Example 1: Prepare a silicon wafer according to the following steps: (1) Cleaning the original single crystal silicon wafer in an alkaline solution containing KOH and H2O2 to remove pollutants on the surface of the original single crystal silicon wafer, wherein the mass fraction of KOH in the alkaline solution is 1%, the mass fraction of H2O2 is 1%, the temperature of the alkaline solution is 65°C, and the cleaning time of the alkaline solution is 3 minutes; after the alkaline solution cleaning is completed, it is then cleaned in pure water for 2 minutes; (2) performing alkaline etching polishing on the original single crystal silicon wafer cleaned in the above step (1) in a KOH solution with a mass fraction of 10%, wherein the temperature of the KOH solution is 80° C. and the alkaline etching polishing time is 3 min; after the alkaline etching polishing is completed, the wafer is rinsed in pure water for 2 min; (3) performing acid etching polishing on the original single crystal silicon wafer obtained after alkaline etching polishing in the above step (2) in an acid mixed solution of HNO3 and HF; the mass fraction of HNO3 in the acid mixed solution is 50%, the mass fraction of HF is 2%, the temperature of the acid mixed solution is 20°C, and the acid etching polishing is 10 minutes; after the acid etching polishing is completed, it is rinsed in pure water for 2 minutes, and then dried in hot air at a temperature of 80°C for 10 minutes; and a polished silicon wafer 01 as a silicon substrate is obtained; (4) Applying polytetrafluoroethylene (PTFE) protective adhesive (polytetrafluoroethylene (PTFE) content is 88%, and the rest is volatile solvent) to the side, front edge and back edge of the polished silicon wafer 01 obtained in the above step (3), so that the front edge, back edge and side of the polished silicon wafer 01 are all covered by the PTFE protective adhesive, wherein the width of the covered area of ​​the front edge and back edge of the polished silicon wafer 01 is controlled within the range of 0.5-1mm, and the side of the polished silicon wafer 01 is completely covered, and then using hot air at a temperature of 150°C to dry the PTFE protective adhesive for 2 minutes; obtaining a preformed PTFE protective coating (see Figure 6 shown); (5) Using a flat press, apply pressure to the preformed PTFE protective coating so that the preformed PTFE protective coating is flattened (see Figure 7 As shown in the figure, the thickness of the flattened PTFE protective coating is 20-30 μm, and then high-temperature sintering is performed, the sintering temperature is 350°C, and the sintering time is 30 minutes. After sintering, a flexible single crystal silicon wafer with an edge protective coating 02 is obtained (please refer to Figure 3 , Figure 4 and Figure 5 ), which is the silicon wafer obtained in Example 1.

[0024] The flexible single-crystal silicon wafer with edge protection coating provided in the first embodiment is further used to manufacture solar cells, and the manufacturing method of the solar cells adopts any known solar cell manufacturing process; preferably, in the present embodiment, the flexible single-crystal silicon wafer with edge protection coating provided in the first embodiment is further used to manufacture silicon heterojunction solar cells, and the manufacturing process includes sequentially performing texturing and cleaning, amorphous silicon thin film deposition, transparent conductive thin film deposition, and screen printing of grid line electrodes on the flexible single-crystal silicon wafer with edge protection coating (in other embodiments, electrodes can also be manufactured by electroplating) to obtain a flexible heterojunction solar cell with edge protection coating.

[0025] In order to further verify the effect achieved by this embodiment, the flexible single crystal silicon wafer with edge protection coating provided in this embodiment 1 is used. The present application samples the flexible single crystal silicon wafer after the texturing and cleaning process and the battery cell after the screen printing process for three-point bending test. The test results are as follows: Figure 8 As shown: Since the PTFE protective coating can protect the edge of the silicon wafer from corrosion by chemical solutions, the edge of the silicon wafer remains in a polished state after texturing and cleaning, confirming that the flexible single-crystal silicon wafer provided in Example 1 has good bendability, and the average downward deformation during fragmentation (referred to as "fragmentation deformation") is 18.3 mm; in the process after texturing and cleaning, the PTFE protective coating can also protect the edge of the silicon wafer from mechanical damage by loading equipment and transmission mechanisms, maintaining the original bendability, and ultimately making the average fragmentation deformation of the battery cell after screen printing 18.1 mm, without significant changes.

[0026] Comparative Example 1:

[0027] The difference between Comparative Example 1 and Example 1 is that in this Comparative Example 1, a mask adhesive is used to replace the PTFE protective adhesive in step (4) of Example 1, and the drying condition is 150°C for 20 minutes, and step (5) is cancelled; wherein, the composition of the mask adhesive is a mixture of polyvinyl butyral and tetrahydrofuran, wherein the mass fraction of polyvinyl butyral is 25%. In the subsequent production process, tetrahydrofuran is used to clean the silicon wafer after the texturing step to dissolve and remove the polyvinyl butyral mask at the edge of the silicon wafer to obtain a single crystal texturized silicon wafer with an edge polishing structure. The difference from the texturized silicon wafer in Example 1 is that the texturized silicon wafer in this Comparative Example 1 does not have an edge protection coating. The other structures of the texturized silicon wafer are the same as those in Example 1, and the subsequent production process is also the same as that in Example 1. The flexible single crystal silicon wafer after the texturing cleaning process and the battery cell after the screen printing process were sampled and subjected to a three-point bending test. The test results are as follows: Figure 8 As shown: The average crushing deformation of the flexible monocrystalline silicon wafer provided in Comparative Example 1 after texturing and cleaning is 18.2 mm, which is equivalent to the average crushing deformation of the flexible monocrystalline silicon wafer with edge protection coating provided in Example 1 after texturing and cleaning; however, in the subsequent production process, since the flexible monocrystalline silicon wafer provided in Comparative Example 1 is not protected by the edge protection coating, the edges are damaged after collision and friction with the loading device and the transmission mechanism, and finally the average crushing deformation of the corresponding battery cell after screen printing is reduced to 13.9 mm, and the bendability is significantly reduced. Since the location and degree of damage to the silicon wafer are somewhat random, the discreteness of the crushing deformation distribution of the battery cell in Comparative Example 1 is relatively large, and the crushing deformation of the severely damaged battery cell is reduced by more than half.

[0028] Embodiment 2:

[0029] The only difference between Example 2 and Example 1 is that in Example 2, step (3) is eliminated, and the original single crystal silicon wafer obtained after alkaline etching and polishing in step (2) is directly used as the polished silicon wafer 01 of the silicon substrate, and step (4) is implemented.

[0030] Using the flexible monocrystalline silicon wafer provided in Example 2, the present application takes samples of the flexible monocrystalline silicon wafer after the texturing and cleaning process and the battery cell after the screen printing process for three-point bending test; among them, the average fragmentation deformation of the flexible monocrystalline silicon wafer provided in Example 2 after texturing and cleaning is 15.8mm; the average fragmentation deformation of the battery cell after screen printing is still 15.8mm. Embodiment 3:

[0031] The only difference between Example 3 and Example 1 is that in Example 3, a soluble polytetrafluoroethylene PFA adhesive (polytetrafluoroethylene PFA content is 90%, and the rest is volatile solvent) is used to replace the PTFE protective adhesive in step (4) of Example 1.

[0032] Using the flexible monocrystalline silicon wafer provided in Example 3, the present application takes samples of the flexible monocrystalline silicon wafer after the texturing and cleaning process and the battery cell after the screen printing process for three-point bending test; among them, the average fragmentation deformation of the flexible monocrystalline silicon wafer provided in Example 3 after texturing and cleaning is 18.3mm; the average fragmentation deformation of the battery cell after screen printing is 18.2mm. Embodiment 4:

[0033] The only difference between Example 4 and Example 1 is that in Example 4, a fluoroethylene propylene (FEP) adhesive (the FEP content is 86%, and the rest is a volatile solvent) is used to replace the PTFE protective adhesive in step (4) of Example 1.

[0034] Using the flexible monocrystalline silicon wafer provided in Example 4, the present application takes samples of the flexible monocrystalline silicon wafer after the texturing and cleaning process and the battery cell after the screen printing process for three-point bending test; among them, the average fragmentation deformation of the flexible monocrystalline silicon wafer provided in Example 4 after texturing and cleaning is 18.1mm; the average fragmentation deformation of the battery cell after screen printing is still 17.9mm.

[0035] Comparative Example 2:

[0036] The difference between Comparative Example 2 and Example 1 is that in this Comparative Example 2, a mask adhesive is used to replace the PTFE protective adhesive in step (4) of Example 1, and the drying condition is 150°C for 20 minutes, and step (5) is eliminated; wherein the mask adhesive is a mixture of polyvinyl butyral and tetrahydrofuran, wherein the mass fraction of polyvinyl butyral is 25%. That is, the difference between the silicon wafer in Example 1 and the silicon wafer in this Comparative Example 2 is that the edge protective coating material of the silicon wafer is changed to polyvinyl butyral, and the subsequent production process is the same as that of Example 1.

[0037] Using the flexible monocrystalline silicon wafer provided in Comparative Example 2, the present application samples the flexible monocrystalline silicon wafer after the texturing and cleaning process and the battery cell after the screen printing process for three-point bending test; wherein, the average fragmentation deformation of the flexible monocrystalline silicon wafer provided in Comparative Example 2 after texturing and cleaning is 18.3 mm; the average fragmentation deformation of the battery cell after screen printing is reduced to 13.2 mm; and since the polyvinyl butyral mask will melt and volatilize at the production process temperature of heterojunction solar cells (about 200°C), it will cause pollution to the silicon wafer and production equipment.

[0038] It should be noted that the three-point bending test method involved in the entire application refers to the national standard GB / T34171-2017 (Three-point bending method for testing the bending performance of thin and ultra-thin glass). The thickness of the single crystal textured silicon wafer in the embodiment and the comparative example is 100±5μm, the length×width is 156.8mm×156.8mm, and the span of the lower pressure rod in the three-point bending test is 30mm.

[0039] The key technical means of this embodiment include coating the edge of the polished silicon wafer with a protective coating that is resistant to acid and alkali corrosion, oxidation and high temperature, to prevent the edge of the silicon wafer from being corroded by chemical liquids and mechanical damage by loading equipment and transmission mechanisms, thereby maintaining the bendability of the silicon wafer.

[0040] The flexible single crystal silicon wafer with edge protection coating provided in this embodiment is suitable for manufacturing thin silicon heterojunction solar cells with a thickness of less than 120 μm, and further for manufacturing bendable and windable flexible solar cell modules. It can also be applied to other potential single crystal silicon solar cells with a production process temperature not higher than 300°C, and has broad application prospects and practical value.

[0041] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0042] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A silicon wafer, characterized in that: It comprises a silicon substrate, wherein the side of the silicon substrate is covered with a protective coating, and the protective coating is integrally extended to the front edge and / or the back edge of the silicon substrate.

2. The silicon wafer according to claim 1, characterized in that: The material of the coating includes polytetrafluoroethylene and / or modified polytetrafluoroethylene material; wherein the weight proportion of the polytetrafluoroethylene and / or modified polytetrafluoroethylene material in the coating is 90-100wt%.

3. The silicon wafer according to claim 1, characterized in that: The thickness of the protective coating is in the range of 1-100 μm; and / or the coverage width of the protective coating on the front edge or the back edge of the silicon substrate is in the range of 0.1-5 mm.

4. The silicon wafer according to claim 1, characterized in that: The silicon substrate is a polished single crystal silicon wafer obtained by chemically polishing an original silicon wafer.

5. A method for preparing a silicon wafer according to any one of claims 1 to 4, characterized in that: The steps are as follows: S1). The original silicon wafer is chemically polished, then cleaned and dried to obtain a polished silicon wafer as a silicon substrate; S2). In the above step S1), a protective adhesive is applied to the side, front edge and / or back edge of the polished silicon wafer to obtain a polished silicon wafer coated with a protective adhesive; S3). Sintering the protective adhesive in the polished silicon wafer obtained in the above step S2) to obtain the silicon wafer.

6. The method for preparing a silicon wafer according to claim 5, characterized in that: In the step S1), the chemical polishing adopts alkaline solution polishing, or the chemical polishing adopts two-step chemical polishing, wherein the first step of chemical polishing adopts alkaline solution polishing and the second step of chemical polishing adopts acidic solution polishing.

7. The method for preparing a silicon wafer according to claim 6, characterized in that: The alkaline solution includes at least one of NaOH and KOH, wherein the mass fraction of the alkali in the alkaline solution is 1%-25%, and / or the temperature of the alkaline solution is 65-95°C, and / or the treatment time of the alkaline corrosion polishing is 1-10min; the acidic solution includes an oxidant and HF, wherein the mass fraction of the oxidant in the acidic solution is 30%-60%, the mass fraction of HF in the acidic solution is 0.5%-15%, and / or the temperature of the acidic solution is 15-40°C, and / or the treatment time of the acidic solution polishing is 3-30min.

8. The method for preparing a silicon wafer according to claim 5, characterized in that: In the step S3), before sintering, the protective adhesive is dried to obtain a preformed protective coating; wherein the drying temperature is 100-200°C and the drying time is 1-10 min; the sintering temperature is 250-400°C and the sintering time is 10-60 min; preferably, after drying and before sintering, pressure is applied to the preformed protective coating so that the thickness of the preformed protective coating is reduced.

9. A solar cell made of silicon wafer, characterized in that: The silicon wafer is prepared by using the silicon wafer according to any one of claims 1-4 or by using the method for preparing the silicon wafer according to any one of claims 5-8.

10. The solar cell according to claim 9, characterized in that: The solar cell is a silicon heterojunction solar cell; wherein the process of making the silicon heterojunction solar cell using silicon wafers includes texturing and cleaning the silicon wafers, and / or depositing an amorphous silicon film on the silicon wafers, and / or depositing a transparent conductive film on the silicon wafers, and / or making electrodes on the silicon wafers by screen printing or electroplating.

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