Preparation method of low-cost electroplating copper grid line heterojunction solar cell

Abstract

The invention belongs to the technical field of semiconductors and solar cells, and discloses a preparation method of a low-cost electroplating copper grid line heterojunction solar cell. In order to solve the problems that the cost is high when a metal electrode in a heterojunction solar cell is prepared through traditional screen printing low-temperature paste, and efficiency improvement is limited due to the fact that a narrower grid line is difficult to prepare in a screen printing mode, in-situ pretreatment is carried out on an N face to carry out surface roughening; then, copper electroplating is carried out in a light-induced horizontal electroplating mode to prepare an N-face copper electrode; and then turning over the battery piece to carry out P-surface copper electroplating so as to prepare the heterojunction solar battery with double-surface copper-electroplated grid lines.

Description

Technical Field

[0001] This invention relates to the field of semiconductor and solar cell technology, specifically to a method for preparing a low-cost electroplated copper grid heterojunction solar cell. Background Technology

[0002] Among various silicon-based solar cell technologies, silicon-based heterojunction solar cells possess higher conversion efficiencies due to the use of amorphous silicon thin-film passivation on both the front and back sides, making them the most likely next-generation mass-produced photovoltaic technology. However, the biggest limitation to the large-scale mass production of heterojunction solar cells lies in the high cost of the low-temperature silver paste material used. Extensive research has been conducted to address this issue, with electroplated copper electrodes replacing screen-printed low-temperature silver paste electrodes to reduce material costs. However, traditional copper electroplating techniques require PVD to sputter a metal seed layer onto the ITO surface to improve the adhesion between the subsequent copper plating and the substrate. Without a seed layer, the direct adhesion between the electroplated copper and ITO is too weak, easily leading to grid line detachment and reduced cell efficiency and stability. Research has found that the PVD sputtering process causes significant sputtering damage to the ITO surface, resulting in an efficiency decrease of over 0.3%. Furthermore, it requires an additional PVD unit, increasing equipment investment, making it not a very suitable technology for mass production. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the above-mentioned background technology and provide a low-cost and simple method for preparing electroplated copper grid heterojunction solar cells that is more suitable for large-scale mass production. The method of this invention can realize a PVD-free sputtering metal seed layer process, and the bonding force between the subsequent electroplated copper and TCO can meet the requirements for use.

[0004] To achieve the objectives of this invention, the method for preparing a low-cost electroplated copper grid heterojunction solar cell includes the following steps: (1) Texturing and cleaning: First, place the N-type silicon wafer in KOH solution for double-sided texturing, and then clean the impurities on the surface of the silicon wafer; (2) Amorphous silicon thin film deposition: Intrinsic amorphous silicon passivation layer, phosphorus-doped silicon thin film, and boron-doped silicon thin film are prepared on the front and back sides of silicon wafer by chemical vapor deposition. (3) Transparent conductive thin film deposition: TCO (transparent conductive oxide) thin films were prepared on the front and back sides of the silicon wafer by ion sputtering to obtain samples with TCO prepared on both sides; After completing step (3), continue with the following steps: (Scheme ①-4) A layer of silver paste (2-5μm) is screen-printed on the P side of the sample as a seed layer for subsequent electroplating, and then annealed and cured. (Scheme ①-5) Trench patterning preparation: A layer of photosensitive ink is printed on the N side of the silicon wafer using screen printing. Then, gate trenches of different widths are formed on the silicon wafer surface by exposure and development. The bottom of the trench exposes the TCO layer, while the TCO layer in the non-trench area is covered by ink. Similarly, the TCO in the non-gate area is protected by patterning on the P side. (Scheme ①-6) N-side photoinduced copper plating: First, place the patterned silicon wafer in the equipment. At this time, only the N-side of the cell is in contact with the solution. The silver paste seed layer on the P-side is connected to the cathode of the external circuit, and there is an irradiation light source at the bottom of the solution. Then, the silicon wafer is transferred to the electroplating tank with the rollers to prepare for copper plating. (Scheme ①-7) After the N-side copper plating is completed, the battery cell is dried and flipped over. At this time, the P-side is facing down and in contact with the copper plating solution, and the N-side is connected to the negative terminal of the external power supply to provide current. (Scheme ①-8) Film removal: Place the solar cell in an alkaline solution, spray at a pressure of 0.8kg-3.0kg, and react for 10s-100s to remove the photosensitive ink from the surface of the silicon wafer. Finally, wash and dry the wafer to form a complete electroplated copper grid heterojunction solar cell. Alternatively, after completing step (3), continue with the following steps: (Scheme ②-4) First, place the sample prepared with TCO on both sides in an annealing furnace for annealing, and then place the sample in a PVD device to prepare a metal seed layer on the P side; (Scheme ②-5) A layer of photosensitive ink is printed on the N side of the silicon wafer using screen printing. Then, the grid trenches of different widths are formed on the silicon wafer surface by exposure and development. The TCO layer is exposed at the bottom of the trenches, and the TCO layer in the non-trench area is covered by ink. Similarly, the same method is used to expose and develop the P side, and the metal seed layer is partially exposed to form the trenches to be electroplated. (Scheme ②-6) N-side photoinduced copper plating: First, place the patterned silicon wafer in the equipment. At this time, only the N-side of the cell is in contact with the solution. The metal seed layer on the P-side is connected to the cathode of the external circuit, and there is an irradiation light source at the bottom of the solution. Then, the silicon wafer is transported to the electroplating tank with rollers to prepare for copper plating. (Scheme ②-7) After the N-side copper plating is completed, the battery cell is dried and flipped over. At this time, the P-side is facing down and in contact with the copper plating solution, and the N-side is connected to the negative terminal of the external power supply to provide current. (Scheme ②-8) Film removal: Place the solar cell in an alkaline solution, spray at a pressure of 0.8kg-3.0kg, and react for 10s-100s to remove the photosensitive ink from the surface of the silicon wafer. Finally, wash and dry the wafer to form a complete electroplated copper grid heterojunction solar cell.

[0005] Furthermore, in some embodiments of the present invention, the TCO in step (3) is a stacked structure of one or more of ITO, FTO, SnOx, and AZO.

[0006] Furthermore, in some embodiments of the present invention, the graphicization method in the steps (Schemes ①-5) is to directly inkjet print the insulating material, or to screen print ink and to deposit the insulating dielectric film layer using PVD or CVD.

[0007] Furthermore, in some embodiments of the present invention, the insulating dielectric film layer in step (scheme ①-5) is one or more of silicon nitride, silicon oxide, and aluminum oxide, the purpose of which is to protect the TCO area of ​​the P side while not covering the silver paste seed layer printed on the P side.

[0008] Furthermore, in some embodiments of the present invention, the solution in steps (Scheme ①-6) and (Scheme ②-6) is a sodium-containing salt solution. The main purpose of this step is to generate some metal particles on the N-side TCO surface by means of photoinduced assisted electrolysis, which serve as contact points to enhance the adhesion of subsequent copper electroplating.

[0009] Furthermore, in some embodiments of the present invention, the solution in the electroplating tank in steps (Scheme ①-6) and (Scheme ②-6) is a salt solution containing copper ions, and the electroplating principle is photo-induced enhanced electroplating. The reason for utilizing photoinduction in this step is that: under illumination, the built-in electric field of the PN junction of the battery cell decreases on the N-side, allowing the external current on the P-side to conduct to the N-side. Copper ions in the solution receive electrons generated on the N-side, thereby producing copper atoms and forming copper grid lines. Simultaneously, since the TCO surface has poor conductivity, photoinduction can generate a uniform potential on the battery surface, which also helps improve the uniformity of copper plating. This step requires the previous steps to screen-print a silver paste seed layer or prepare a metal seed layer as a conductive electrode; therefore, the seed layer area cannot be covered during the P-side patterning step, otherwise, conductivity will be impossible.

[0010] Furthermore, in some embodiments of the present invention, the alkaline solution in steps (Scheme ①-8) and (Scheme ②-8) is a sodium hydroxide or potassium hydroxide solution; preferably, the (mass) concentration of the alkaline solution is 3%-20%; preferably, the temperature of the alkaline solution is 30-70°C.

[0011] Furthermore, in some embodiments of the present invention, the material of the metal seed layer in (Scheme ②-4) is a common metal material such as copper or aluminum, or its alloy, in order to enhance the conductivity of the TCO surface on the P side and serve as a seed layer for subsequent electroplating.

[0012] Compared with the prior art, the advantages of the present invention are as follows: (1) In order to solve the problems of high cost when preparing metal electrodes in heterojunction solar cells by traditional screen printing low temperature paste, and the difficulty in preparing narrower grid lines by screen printing, which limits the efficiency improvement, this invention uses in-situ pretreatment of TCO surface on N side to roughen surface, and then uses photo-induced horizontal electroplating to electroplat copper to prepare N side copper electrode; then the cell is flipped over and copper is electroplated on P side to prepare heterojunction solar cell with double-sided copper grid lines.

[0013] (2) This invention proposes a method for preparing copper grid heterojunction solar cells by two-stage cross-horizontal electroplating. The N-side is prepared by photo-induced enhanced horizontal electroplating to directly prepare copper grid lines with high bonding force on the TCO, which effectively avoids sputtering damage to the N-side caused by the traditional preparation of copper seed layer, and can bring higher conversion efficiency. The P-side is also prepared by horizontal electroplating. This double-sided cross-horizontal electroplating method can effectively avoid the problem of mutual interference between the N-side and P-side currents in the traditional surface electroplating method. Moreover, the cathode electrodes used in the cross-horizontal electroplating method proposed in this invention do not directly contact the electrolyte, which effectively avoids the cumbersome steps of copper plating on the electrodes and subsequent deplating. Attached Figure Description

[0014] Figure 1 This is a flowchart of the method in Embodiment 1 of the present invention; Figure 2 This is a flowchart of the method in Embodiment 2 of the present invention; Figure 3 This is a schematic diagram of the N-plane light-induced seedless electroplating device in the method of the present invention; Figure 4 This is a schematic diagram of horizontal copper electroplating on the P-side in the method of this invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Additional aspects and advantages of this invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It should be understood that the following description is merely illustrative and not intended to limit the invention.

[0016] The terms “comprising,” “including,” “having,” “containing,” or any other variations thereof, as used herein, are intended to cover a non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0017] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1 to 5” is disclosed, the described range should be interpreted as including the ranges “1 to 4”, “1 to 3”, “1 to 2”, “1 to 2 and 4 to 5”, “1 to 3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0018] The indefinite articles “a” and “an” preceding an element or component of this invention do not impose any limitation on the quantity (i.e., number of times) of the element or component. Therefore, “an” or “a” should be interpreted as including one or at least one, and the singular form of an element or component also includes the plural form, unless the quantity clearly refers only to the singular form.

[0019] Furthermore, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., described below refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms are not necessarily directed at the same embodiment or example. Moreover, the technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0020] Example 1 A method for fabricating a low-cost electroplated copper grid heterojunction solar cell includes the following steps: (1) Texturing and cleaning: The initial process is the same as that of conventional silicon-based heterojunction solar cells. First, the N-type silicon wafer is placed in KOH solution for double-sided texturing, and then the impurities on the surface of the silicon wafer are cleaned. (2) Amorphous silicon thin film deposition: Intrinsic amorphous silicon passivation layer, phosphorus-doped silicon thin film, and boron-doped silicon thin film are prepared on the front and back sides of silicon wafer by chemical vapor deposition. (3) Deposition of transparent conductive thin film: A transparent conductive oxide thin film is prepared on the front and back sides of the silicon wafer by ion sputtering. The transparent conductive oxide thin film is ITO. (4) A 5μm layer of silver paste was screen-printed on the P side of the sample as a seed layer for subsequent electroplating, and then annealed and cured. (5) Trench patterning preparation: A layer of photosensitive ink is printed on the N side of the silicon wafer by screen printing, and then the gate trenches of different widths are formed on the surface of the silicon wafer by exposure and development. The bottom of the trench is the exposed TCO layer, and the TCO layer in the non-trench area is covered by ink. Similarly, a graphic method is also used on the P side to protect the TCO in the non-grid area. At this time, the purpose of the graphic screen printing insulating ink is to protect the TCO area on the P side while not covering the silver paste seed layer printed on the P side. (6) N-side photoinduced copper plating: First, place the patterned silicon wafer in the equipment. At this time, only the N-side of the cell is in contact with the solution. The silver paste seed layer on the P-side is connected to the cathode of the external circuit, and there is 50kw / m at the bottom of the solution. 2 The light source has high irradiance, and the main component of the solution is a sodium-containing salt solution. The main purpose of this step is to generate some metal particles on the N-side TCO surface using photo-induced assisted electrolysis, which serve as contact points to enhance the adhesion of subsequent copper electroplating. After surface treatment, the silicon wafer is transported to the electroplating tank by rollers to prepare for copper electroplating. The solution in the electroplating tank is mainly a salt solution containing copper ions, and the electroplating principle is also photo-induced enhanced electroplating. The reason for using photoinduction in this step is that, under illumination, the built-in electric field of the PN junction on the N-side of the solar cell decreases. At this time, the external current on the P-side can be transmitted to the N-side. Copper ions in the solution receive electrons generated on the N-side, thus producing copper atoms and forming copper grid lines. Simultaneously, since the TCO surface has poor conductivity, photoinduction can generate a uniform potential on the cell surface, which also helps improve the uniformity of copper plating. This step requires a silver paste seed layer printed in the previous step as a conductive electrode; therefore, the seed layer area must not be covered during the P-side patterning step, otherwise conductivity will be impossible.

[0021] (7) After the N-side copper plating is completed, the battery cell is dried and flipped over. At this time, the P-side is facing down and in contact with the copper plating solution. The N-side is connected to the negative terminal of the external power supply to provide current. The equipment structure is similar to step (6), but at this time, light-induced electroplating is no longer used because the holes generated on the P-side of the battery under the action of light are not conducive to the occurrence of copper plating. Moreover, the P-side already has a thin silver paste seed layer as a conductive layer, which has good conductivity. Therefore, only ordinary horizontal electroplating is needed.

[0022] (8) Removal of film: Place the sample in a 5% sodium hydroxide or potassium hydroxide solution at a solution temperature of 60°C, a spray pressure of 3.0 kg, and a reaction time of 100 s to remove the photosensitive ink on the silicon wafer surface; finally, wash and dry the sample to form a complete electroplated copper grid heterojunction solar cell.

[0023] Example 2 A method for fabricating a low-cost electroplated copper grid heterojunction solar cell includes the following steps: (1) Texturing and cleaning: The initial process is the same as that of conventional silicon-based heterojunction solar cells. First, the N-type silicon wafer is placed in KOH solution for double-sided texturing, and then the impurities on the surface of the silicon wafer are cleaned. (2) Amorphous silicon thin film deposition: Intrinsic amorphous silicon passivation layer, phosphorus-doped silicon thin film, and boron-doped silicon thin film are prepared on the front and back sides of silicon wafer by chemical vapor deposition. (3) Deposition of transparent conductive thin film: A transparent conductive oxide thin film is prepared on the front and back sides of the silicon wafer by ion sputtering. The transparent conductive oxide thin film is ITO. (4) First, place the sample prepared with TCO on both sides in an annealing furnace for annealing, and then place the sample in a PVD device to prepare a copper metal seed layer on the P side. The purpose is to enhance the conductivity of the TCO surface on the P side and to serve as a seed layer for subsequent electroplating. (5) Trench patterning preparation: A layer of photosensitive ink is printed on the N side of the silicon wafer by screen printing, and then the gate trenches of different widths are formed on the silicon wafer surface by exposure and development. The bottom of the trench is exposed TCO layer, and the TCO layer in the non-trench area is covered by ink. Similarly, the same method is used on the P side for exposure and development, and the metal seed layer is partially exposed to the trench to be electroplated. (6) N-side photoinduced copper plating: First, place the patterned silicon wafer in the equipment. At this time, only the N-side of the cell is in contact with the solution. The metal seed layer on the P-side is connected to the cathode of the external circuit, and there is 50kw / m at the bottom of the solution. 2 The light source has high irradiance, and the main component of the solution is a sodium-containing salt solution. The main purpose of this step is to generate some metal particles on the N-side TCO surface using photo-induced assisted electrolysis, which serve as contact points to enhance the adhesion of subsequent copper electroplating. After surface treatment, the silicon wafer is transported to the electroplating tank by rollers to prepare for copper electroplating. The solution in the electroplating tank is mainly a salt solution containing copper ions, and the electroplating principle is also photo-induced enhanced electroplating. The reason for using photoinduction in this step is that, under illumination, the built-in electric field of the PN junction on the N-side of the solar cell decreases. At this time, the external current on the P-side can be transmitted to the N-side. Copper ions in the solution receive electrons generated on the N-side, thus producing copper atoms and forming copper grid lines. Simultaneously, since the TCO surface has poor conductivity, photoinduction can generate a uniform potential on the cell surface, which also helps improve the uniformity of copper plating. This step requires a metal seed layer prepared in the previous step as a conductive electrode; therefore, the seed layer area must not be covered during the P-side patterning step, otherwise, conductivity will be impossible.

[0024] (7) After the N-side copper plating is completed, the battery cell is dried and flipped over. At this time, the P-side is facing down and in contact with the copper plating solution. The N-side is connected to the negative terminal of the external power supply to provide current. The equipment structure is similar to step (6), but at this time, light-induced electroplating is no longer used because the holes generated on the P-side of the battery under the action of light are not conducive to the occurrence of copper plating. Moreover, the P-side already has a metal seed layer as a conductive layer, which has good conductivity. Therefore, only ordinary horizontal electroplating is needed.

[0025] (8) Removal of film: Place the sample in a 5% sodium hydroxide or potassium hydroxide solution at a solution temperature of 60°C, a spray pressure of 3.0 kg, and a reaction time of 100 s to remove the photosensitive ink on the silicon wafer surface; finally, wash and dry the sample to form a complete electroplated copper grid heterojunction solar cell.

[0026] Those skilled in the art will readily understand that the above description is merely a partial example of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating a low-cost electroplated copper grid heterojunction solar cell, characterized in that, The method for fabricating the low-cost electroplated copper grid heterojunction solar cell includes the following steps: (1) Texturing and cleaning: First, place the N-type silicon wafer in KOH solution for double-sided texturing, and then clean the impurities on the surface of the silicon wafer; (2) Amorphous silicon thin film deposition: Intrinsic amorphous silicon passivation layer, phosphorus-doped silicon thin film, and boron-doped silicon thin film are prepared on the front and back sides of silicon wafer by chemical vapor deposition. (3) Deposition of transparent conductive thin film: TCO thin films were prepared on the front and back sides of the silicon wafer by ion sputtering to obtain samples with TCO prepared on both sides; After completing step (3), continue with the following steps: (Scheme ①-4) A layer of silver paste is screen-printed on the P side of the sample as a seed layer for subsequent electroplating, and then annealed and cured. (Scheme ①-5) Trench patterning preparation: A layer of photosensitive ink is printed on the N side of the silicon wafer using screen printing. Then, gate trenches of different widths are formed on the silicon wafer surface by exposure and development. The bottom of the trench exposes the TCO layer, while the TCO layer in the non-trench area is covered by ink. Similarly, the TCO in the non-gate area is protected by patterning on the P side. (Scheme ①-6) N-side photoinduced copper plating: First, place the patterned silicon wafer in the equipment. At this time, only the N-side of the cell is in contact with the solution. The silver paste seed layer on the P-side is connected to the cathode of the external circuit, and there is an irradiation light source at the bottom of the solution. Then, the silicon wafer is transferred to the electroplating tank with the rollers to prepare for copper plating. (Scheme ①-7) After the N-side copper plating is completed, the battery cell is dried and flipped over. At this time, the P-side is facing down and in contact with the copper plating solution, and the N-side is connected to the negative terminal of the external power supply to provide current. (Scheme ①-8) Film removal: Place the solar cell in an alkaline solution, spray at a pressure of 0.8kg-3.0kg, and react for 10s-100s to remove the photosensitive ink from the surface of the silicon wafer. Finally, wash and dry the wafer to form a complete electroplated copper grid heterojunction solar cell. Alternatively, after completing step (3), continue with the following steps: (Scheme ②-4) First, place the sample prepared with TCO on both sides in an annealing furnace for annealing, and then place the sample in a PVD device to prepare a metal seed layer on the P side; (Scheme ②-5) A layer of photosensitive ink is printed on the N side of the silicon wafer using screen printing. Then, the grid trenches of different widths are formed on the silicon wafer surface by exposure and development. The TCO layer is exposed at the bottom of the trenches, and the TCO layer in the non-trench area is covered by ink. Similarly, the same method is used to expose and develop the P side, and the metal seed layer is partially exposed to form the trenches to be electroplated. (Scheme ②-6) N-side photoinduced copper plating: First, place the patterned silicon wafer in the equipment. At this time, only the N-side of the cell is in contact with the solution. The metal seed layer on the P-side is connected to the cathode of the external circuit, and there is an irradiation light source at the bottom of the solution. Then, the silicon wafer is transported to the electroplating tank with rollers to prepare for copper plating. (Scheme ②-7) After the N-side copper plating is completed, the battery cell is dried and flipped over. At this time, the P-side is facing down and in contact with the copper plating solution, and the N-side is connected to the negative terminal of the external power supply to provide current. (Scheme ②-8) Film removal: Place the solar cell in an alkaline solution, spray at a pressure of 0.8kg-3.0kg, and react for 10s-100s to remove the photosensitive ink from the surface of the silicon wafer. Finally, wash and dry the wafer to form a complete electroplated copper grid heterojunction solar cell.

2. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, In the steps (Scheme ①-4), a layer of 2-5μm silver paste is printed.

3. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, In step (3), TCO is a stacked structure of one or more of ITO, FTO, SnOx, and AZO.

4. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, The graphic representation method in the steps (Scheme ①-5) is to directly inkjet print the insulating material, or screen print ink, or deposit the insulating dielectric film layer using PVD or CVD.

5. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, In the steps (Scheme ①-5), the insulating dielectric film is one or more of silicon nitride, silicon oxide, and aluminum oxide.

6. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, The medicinal solution in steps (Scheme ①-6) and (Scheme ②-6) is a sodium-containing salt solution.

7. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, The solution in the electroplating tank in steps (Scheme ①-6) and (Scheme ②-6) is a salt solution containing copper ions.

8. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, In steps (Scheme ①-8) and (Scheme ②-8), the alkaline solution is a sodium hydroxide or potassium hydroxide solution.

9. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 8, characterized in that, The concentration of the alkaline solution is 3%-20%; preferably, the temperature of the alkaline solution is 30-70℃.

10. The method for preparing a low-cost electroplated copper grid heterojunction solar cell according to claim 1, characterized in that, The material of the metal seed layer in (Scheme ②-4) is copper or aluminum.

Images