Flexible solar cell capable of preventing copper diffusion and preparation method thereof

Abstract

The invention provides a flexible solar cell capable of preventing copper diffusion and a preparation method thereof, and the preparation method of the solar cell comprises the steps: growing an inverted epitaxial layer on a semiconductor substrate, then growing a metal layer on the epitaxial layer, the structure of the metal layer is Cr / Ti / Pt / Pd / Ni / Au, the Cr / Ti / Pt / Pd / Ni is a metal gradient buffer layer, and the Au is a seed layer; electroplating a Cu layer on the seed layer, wherein the Cu layer and the metal layer form a back electrode; and then temporarily bonding the Cu layer and the rigid substrate together, removing the semiconductor substrate, manufacturing a front electrode and an anti-reflection film, finally removing the temporary rigid substrate, and performing annealing, scribing and end face corrosion to complete the preparation of the flexible solar cell. The metal layers are progressively grown layer by layer between the epitaxial layer and the Cu layer according to the mode that the thermal expansion coefficient is slowly increased, copper diffusion is effectively prevented, thermal stress can be relieved, the reliability is high, and the cost is low.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of semiconductor devices, and particularly relates to a flexible solar cell for preventing copper diffusion and a preparation method thereof. BACKGROUND

[0002] The flexible multi-junction gallium arsenide solar cell has good photoelectric conversion efficiency and high mass specific power. Due to the bendable characteristic, it can be attached to different curved surfaces, so that it has broad application prospects in the fields of aerospace, civil market and the like. In the traditional preparation method of the flexible solar cell, electroplated copper is often used as the flexible substrate, as described in Chinese patent CN201810904936.5. The method improves the production efficiency and reduces the production cost by one bonding and debonding process.

[0003] However, the use of copper as the flexible substrate of the solar cell mainly has the following defects: 1. Mismatch of thermal expansion coefficients of copper and epitaxy The thermal expansion coefficient of copper (about 17 ppm / ℃) is much higher than that of GaAs (about 6 ppm / ℃), which means that when cooling from high temperature, the shrinkage amount of copper is much larger than that of GaAs. When the temperature change is larger, the tensile stress on GaAs from copper is also larger. In addition, GaAs is a brittle material with poor ductility. When the tensile stress generated by thermal stress exceeds the breaking strength of GaAs itself, cracks will be generated in the epitaxy with GaAs as the substrate, thereby reducing the device yield.

[0004] 2. Copper is extremely easy to diffuse into the epitaxy If no barrier layer is arranged between copper and GaAs, copper and GaAs are in direct contact, the concentration of copper at the interface is extremely high, and copper is extremely easy to diffuse into the GaAs with a lower concentration. The diffusion coefficient of copper is several orders of magnitude higher than that of common dopants (such as Si and Zn) in GaAs. Once entering the GaAs lattice, copper will introduce deep level defect centers, become effective recombination centers and trap centers, cause the resistivity of the solar cell to increase, the photoelectric conversion efficiency to decrease, and the device performance to deteriorate seriously. In the prior art, although a Pt / Au structure is used as the barrier layer, Pt is used as the barrier layer, and Au provides ohmic contact and solderability, the amount of Pt is not mentioned, and the barrier effect of the single barrier layer is limited. In addition, the amount of Au is large, and the cost is high, which is not suitable for mass production.

[0005] 3. Copper surface is easy to oxidize If the copper surface is not protected, it is easy to oxidize and discolor after long-term exposure to air. On the one hand, the conductivity of the generated oxide is much lower than that of metallic copper, the contact resistance increases significantly, the short-circuit current decreases, and thus the photoelectric conversion efficiency decreases. On the other hand, when soldering, the molten solder cannot wet the oxide layer, which will cause false soldering or false soldering. SUMMARY

[0006] The present application aims to provide a flexible solar cell preventing copper diffusion and a preparation method thereof, which effectively prevents copper diffusion, relieves thermal stress, has high reliability and low cost.

[0007] To achieve the above-mentioned purpose, the present application provides a preparation method of a flexible solar cell preventing copper diffusion, comprising: providing a semiconductor substrate, growing an inverted epitaxial layer of a multi-junction cell on the semiconductor substrate; growing a metal layer on the epitaxial layer, the metal layer having a structure of Cr / Ti / Pt / Pd / Ni / Au, wherein Cr / Ti / Pt / Pd / Ni is a metal gradual buffer layer and Au is a seed layer; electroplating a Cu layer on the seed layer, the Cu layer and the metal layer constituting a back electrode; providing a rigid substrate, temporarily bonding the Cu layer and the rigid substrate together; removing the semiconductor substrate; making a front electrode and an anti-reflection film on the exposed epitaxial surface of the epitaxial wafer after removing the semiconductor substrate; removing the rigid substrate by debonding to obtain a flexible cell wafer; annealing, dicing and end surface etching the flexible cell wafer to complete the preparation of the flexible solar cell.

[0008] Optionally, in the metal layer, the thicknesses of Cr, Ti, Pt, Pd, Ni and Au are 500-1000 Å, 500-1000 Å, 300-600 Å, 300-600 Å, 800-1200 Å and 200-600 Å, respectively.

[0009] Optionally, after electroplating the Cu layer, Ni and Sn are electroplated on the Cu layer in sequence as a protective layer of Cu, the protective layer, the Cu layer and the metal layer constituting the back electrode.

[0010] Optionally, the thickness of the Cu layer is 20-40 μm, and in the protective layer, the thickness of Ni is 500-1500 Å and the thickness of Sn is 300-800 Å.

[0011] Optionally, the epitaxial layer comprises a buffer layer, a cutoff layer, an N-type contact layer, a double-junction or triple-junction epitaxial layer and a P-type contact layer grown in sequence from bottom to top.

[0012] Optionally, removing the semiconductor substrate comprises: The semiconductor substrate is a GaAs substrate, after temporary bonding, the semiconductor substrate is ground by a grinding wheel to a thickness of 200 microns, then the epitaxial wafer is immersed in a mixed solution of ammonia, hydrogen peroxide and water, and soaked for 30 minutes until the GaAs substrate and the buffer layer are completely removed, the cutoff layer is exposed, then the cutoff layer is etched and removed by a hydrochloric acid solution.

[0013] Optionally, the fabrication of the front electrode comprises: By photolithography, a pattern is made on the surface exposed by removing the semiconductor substrate from the epitaxial layer, then Au / AuGe / Ag / Au is evaporated on the surface by electron beam evaporation to form the front electrode, the thickness of Au, AuGe, Ag and Au is 1500 angstrom, 4000 angstrom, 40000 angstrom and 2000 angstrom respectively.

[0014] Optionally, the fabrication of the anti-reflection film comprises: The epitaxial wafer on which the front electrode is prepared is immersed in a selective etching solution to remove the N-type contact layer except the front electrode, then TiO2 / Al2O3 is evaporated on the surface of the epitaxial wafer after selective etching by electron beam evaporation to form the anti-reflection film, the thickness of TiO2 and Al2O3 is 30 nm and 70 nm respectively.

[0015] Optionally, a metal layer is evaporated on the epitaxial layer by electron beam evaporation.

[0016] The application also provides a flexible solar cell for preventing copper diffusion, which is prepared by the preparation method of the flexible solar cell according to any one of the above.

[0017] After the above scheme, the application has the following advantages: 1. Effectively relieving thermal stress: the metal layers of Cr / Ti / Pt / Pd / Ni / Au are grown layer by layer between the epitaxial layer and the Cu layer according to the way of gradually increasing the thermal expansion coefficient, the thermal expansion coefficient of the metal layers increases layer by layer, which fully prepares for electroplating copper, significantly reduces the thermal mismatch stress between Cu and GaAs epitaxial, avoids the dark cracking of the epitaxial layer, and improves the device yield.

[0018] 2. Excellent copper diffusion barrier performance: In the metal layer, the combination of Pt / Pd / Ni is a multi-layer barrier structure, wherein Pt is the first diffusion barrier layer, which is used to protect the Ti layer and the underlying GaAs epitaxy, and can effectively prevent the upward diffusion of Au and Cu; Pd is the second diffusion barrier layer and wetting layer, which can enhance the barrier effect and improve the quality of the subsequent plated layer, further ensure that Cu and Au cannot reach the sensitive area of the lower layer, greatly improve the long-term reliability, and provide a smooth surface for subsequent Au plating; Ni as the third diffusion barrier layer can effectively inhibit the lateral migration of Au atoms at high temperature, and provide a good substrate for Au plating. The multi-layer metal combination prevents Cu diffusion, makes up for the shortcomings of the single-layer diffusion barrier layer in the prior art, fully protects the GaAs epitaxy, and enhances the reliability of the performance of the flexible solar cell.

[0019] 3. Preventing copper oxidation and enhancing weldability: The application plating Ni, Sn as a protective layer on the surface of Cu, Ni can not only protect the surface of Cu, but also prevent the mutual diffusion between Cu and Sn, avoid the formation of brittle intermetallic compounds, and greatly improve the long-term reliability of the solder joint; Sn enhances the weldability of the back electrode, and the process is mature, compared with surface plating Ag or Au, which greatly saves the material cost and has high cost performance. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 It is the initial structure diagram of the epitaxial wafer of the application; Figure 2 It is the three-junction flip-chip epitaxial structure diagram of the epitaxial wafer of the application; Figure 3 It is the double-junction flip-chip epitaxial structure diagram of the epitaxial wafer of the application; Figure 4 It is the structure diagram of the application of evaporating metal gradient buffer layer; Figure 5 It is the structure diagram of the application of evaporating metal layer, Cu layer and protective layer; Figure 6 It is the structure diagram of the back electrode of the application; Figure 7 It is the structure diagram of the temporary key rigid substrate of the application; Figure 8 It is the structure diagram of removing the semiconductor substrate of the application; Figure 9 It is the structure diagram of evaporating the front electrode and the anti-reflection film of the application; Figure 10 It is the structure diagram of removing the rigid substrate of the application; Figure 11 It is the flow chart of the preparation method of the application.

[0021] Label explanation: 1, semiconductor substrate; 2, epitaxial layer; 3, metal layer; 31, metal gradual buffer layer; 32, seed layer; 4, Cu layer; 5, protective layer; 6, back electrode; 7, rigid substrate; 8, front electrode; 9, anti-reflective film; 10, temporary bonding glue. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application, and the range of the present application includes two end values.

[0023] As shown in Figure 11 , the present application provides a preparation method of a flexible solar cell for preventing copper diffusion, comprising the following steps: S1, growing an epitaxial layer 2: providing a semiconductor substrate 1, growing an inverted epitaxial layer 2 of a multi-junction cell on the semiconductor substrate 1 by a MOCVD device, and the structure of the epitaxial wafer after growing the epitaxial layer 2 is as shown in Figure 1 .

[0024] Optionally, the semiconductor substrate 1 is a GaAs substrate, the epitaxial layer 2 can be an epitaxial layer structure of an inverted three-junction or two-junction GaAs thin-film cell, or even an epitaxial layer structure of more than three junctions, which is not limited here.

[0025] Specifically, as shown in Figures 2-3 , the epitaxial layer 2 includes a buffer layer, a cutoff layer, an N-type contact layer, a two-junction or three-junction epitaxial layer, and a P-type contact layer, which are sequentially grown from bottom to top. In addition, a GaAs cap layer (not shown in the figure) can be grown on the P-type contact layer to protect the epitaxial structure and achieve high-quality ohmic contact.

[0026] Optionally, the buffer layer is an N-type GaAs buffer layer, the cutoff layer is a GaInP cutoff layer, the N-type contact layer is an N-type GaAs contact layer, and the P-type contact layer is a P-type InGaAs contact layer.

[0027] Optionally, as shown in Figure 2 , the three-junction epitaxial layer is composed of a GaInP top cell, a GaInP tunnel junction, a GaAs middle cell, a GaInP tunnel junction, and an InGaAs bottom cell, which are sequentially stacked from bottom to top. As shown in Figure 3 , the two-junction epitaxial layer is composed of a GaInP sub-cell, a GaInP tunnel junction, and a GaAs sub-cell, which are sequentially stacked from bottom to top.

[0028] S2, Depositing metal layer 3: The epitaxial wafer prepared in step S1 is organically cleaned, and then a metal layer 3 is grown on the epitaxial layer 2. Specifically, the metal layer 3 is deposited by electron beam evaporation. The structure of the metal layer 3 is Cr / Ti / Pt / Pd / Ni / Au, where Cr / Ti / Pt / Pd / Ni is a metal gradient buffer layer 31 and Au is a seed layer 32.

[0029] Optionally, the thicknesses of Cr, Ti, Pt, Pd, Ni, and Au are 500Å-1000Å, 500Å-1000Å, 300Å-600Å, 300Å-600Å, 800Å-1200Å, and 200Å-600Å, respectively.

[0030] Preferably, the thicknesses of Cr, Ti, Pt, Pd, Ni, and Au are 1000 Å, 1000 Å, 500 Å, 500 Å, 1000 Å, and 500 Å, respectively.

[0031] Traditional methods use electroplated copper as a flexible substrate. The coefficient of thermal expansion of copper is about 17 ppm / ℃, while that of GaAs is about 6 ppm / ℃. The difference in the coefficients of thermal expansion between the two is an order of magnitude. During temperature changes, this can easily lead to dark cracks in the epitaxial layer, reducing device yield.

[0032] In existing technologies, Ti / Pt / Au is deposited under copper as a buffer metal layer. However, the coefficient of thermal expansion of Ti is 8.6ppm / ℃-9.0ppm / ℃, that of Pt is 8.8ppm / ℃-9.0ppm / ℃, and that of Au is about 14.2ppm / ℃. The change in the coefficient of thermal expansion from Pt to Au is large, and the thermal stress caused by thermal mismatch is not effectively relieved.

[0033] In this application, a metal layer 3 of Cr / Ti / Pt / Pd / Ni / Au structure is deposited below the Cu layer 4. Cr is close to the epitaxial layer 2, and Au is close to the subsequent Cu layer 4. The coefficient of thermal expansion increases layer by layer from Cr to Au, which provides sufficient preparation for copper electroplating, significantly reduces the thermal mismatch stress between Cu and GaAs epitaxy, avoids dark cracks in the epitaxial layer 2, and improves device yield.

[0034] And, as Figure 4As shown, the coefficient of thermal expansion of Cr is approximately 6.2 ppm / ℃, which is similar to that of GaAs epitaxy. This allows it to combine with Ti on GaAs to form better adhesion and also provides a gradient buffer from GaAs to Ti. Cr and Ti then act as the first and second adhesion layers, ensuring a strong bond between the epitaxial layer 2 and the subsequent metal structure layers. Pd has a coefficient of thermal expansion of 11.8 ppm / ℃, while Ni has 12.5-13.5 ppm / ℃. The Pd / Ni combination also forms a gradient buffer between Pt and Au. This multi-level buffered gradient metal layer design significantly solves the thermal stress problem caused by thermal mismatch from GaAs to Cu, eliminates dark cracks inside the solar cell epitaxy, and greatly improves the yield of Cu-based flexible solar cells.

[0035] Furthermore, the combination of Pt, Pd, and Ni serves as a multilayer barrier structure. Pt acts as the first diffusion barrier layer, protecting the Ti layer and the underlying GaAs epitaxial layer, effectively preventing the downward diffusion of Au and Cu from the upper layer. Pd acts as the second diffusion barrier layer and wetting layer, enhancing the barrier effect and improving the quality of subsequent coatings, further ensuring that Cu and Au cannot reach the sensitive areas of the lower layers, greatly improving long-term reliability, and providing a smooth surface for subsequent Au plating. Ni acts as the third diffusion barrier layer, effectively suppressing the lateral migration of Au atoms at high temperatures and providing a good substrate for Au plating. This multilayer metal combination barrier structure prevents Cu diffusion, overcoming the shortcomings of existing single-layer diffusion barrier layers, fully protecting the GaAs epitaxial layer, and enhancing the reliability of flexible solar cells.

[0036] Furthermore, this application selects Au as the seed layer 32. Au possesses excellent oxidation and corrosion resistance, ensuring that the surface remains clean and highly conductive throughout the entire process from deposition completion to entry into the electroplating bath, thereby obtaining a high-quality, defect-free copper plating layer. Moreover, Au has a very low resistivity, far superior to other commonly used seed layer materials (such as titanium, titanium nitride, and tantalum), ensuring uniform current distribution during electroplating and achieving uniform copper deposition. Gold and copper can also form a solid solution, exhibiting good interdiffusion and adhesion. With proper process control, the Cu layer 4 can firmly adhere to the gold layer.

[0037] Optionally, organic cleaning of epitaxial wafers includes immersing the epitaxial wafers in acetone and isopropanol for 10 minutes each, then rinsing with water and swirl-drying.

[0038] S3, Electroplated copper: such as Figure 5 As shown, a Cu layer 4 is electroplated on the seed layer 32, and Ni and Sn are sequentially electroplated on the Cu layer 4 to serve as a protective layer 5 for the Cu layer 4. All the plated metals constitute the back electrode 6 of the flexible solar cell, as shown. Figure 6As shown, metal layer 3, Cu layer 4 and protective layer 5 constitute the back electrode 6, while metal layer 3 and Cu layer 4 serve as a flexible substrate.

[0039] In protective layer 5, Ni can protect the Cu surface and prevent the interdiffusion between Cu and Sn, avoiding the formation of brittle intermetallic compounds and greatly improving the long-term reliability of the solder joint; Sn enhances the solderability of the back electrode, the process is mature, and compared with surface plating of Ag or Au, it greatly saves material costs and has a high cost performance.

[0040] Optionally, the thickness of the Cu layer 4 is 20μm-40μm; in the protective layer 5, the thickness of the Ni is 500Å-1500Å and the thickness of the Sn is 300Å-800Å.

[0041] Preferably, the thickness of Ni is 1000 Å and the thickness of Sn is 500 Å. S4. Temporary Bonding: A rigid substrate 7 is provided, which can be glass or sapphire. The rigid substrate 7 is then organically cleaned by immersing it in acetone and isopropanol for 10 minutes each, followed by rinsing with water and spin-drying. Next, temporary bonding adhesive 10 is spin-coated onto the Cu layer 4 to temporarily bond the Cu layer 4 and the rigid substrate 7 together. The bonded structure is as follows. Figure 7 As shown.

[0042] S5. Removal of Semiconductor Substrate 1: Thin the temporarily bonded epitaxial wafer by grinding away 200 μm of the back semiconductor substrate with a grinding wheel. Then immerse it in a mixed solution of ammonia, hydrogen peroxide, and water (volume ratio 1:2:1) for 30 minutes until the GaAs substrate and buffer layer of the epitaxial wafer are completely removed, exposing the stop layer. Next, immerse it in hydrochloric acid solution to etch and remove the stop layer for 1 minute, making the GaAs surface of epitaxial layer 2 bright, with the structure as shown. Figure 8 As shown.

[0043] S6. Fabrication of the front electrode 8: A pattern is fabricated on the exposed bright GaAs surface after removing the semiconductor substrate 1 from the epitaxial wafer using photolithography. Then, Au / AuGe / Ag / Au is deposited on this epitaxial surface as the front electrode 8 using electron beam evaporation, as shown in the structure. Figure 9 As shown.

[0044] Specifically, the thicknesses of Au, AuGe, Ag, and Au are 1500Å, 4000Å, 40000Å, and 2000Å, respectively.

[0045] S7. Selective etching: Immerse the epitaxial wafer prepared in step S6 into a selective etching solution, which is a mixture of citric acid, hydrogen peroxide and water (with a volume ratio of 2:2:1). The immersion time is 2 minutes to remove the N-type GaAs contact layer other than the front electrode 8.

[0046] S8. Fabrication of antireflection film 9 and etching: On the surface etched in step S7, TiO2 / Al2O3 is deposited as antireflection film 9 by electron beam evaporation, with the structure as follows... Figure 9 As shown; then, a pattern is made on the antireflective film 9 by photolithography, exposing the front electrode 8, and photoresist covers the area except for the front electrode 8; then the epitaxial wafer that has undergone photolithography is immersed in an acidic solution for 1 minute. This acidic solution can be a mixture of hydrofluoric acid and water with a volume ratio of 1:10 to etch away the antireflective film 9 on the front electrode 8. Finally, it is rinsed with water and dried by spin drying.

[0047] Optionally, in the antireflective coating 9, the thickness of TiO2 is 30 nm and the thickness of Al2O3 is 70 nm.

[0048] S9. Removal of rigid substrate 7: The solar cell fabricated in step S8 is placed on a heating platform and baked at 150°C for 3 minutes. The bonding is then debonded via thermal sliding, and the solar cell is peeled off from the rigid substrate 7 to obtain a flexible solar cell with the structure shown below. Figure 10 As shown.

[0049] After being peeled off, the flexible solar cells need to be soaked in two stripping solutions for 15 minutes each, then rinsed with water for 10 minutes, then soaked in isopropyl alcohol for 1 minute, and then baked in an oven at 110°C for 15 minutes.

[0050] Optionally, the stripped rigid substrate 7 can be immersed in two stripping solutions for 15 minutes each, rinsed with water, and spun dry before being reused for the next temporary bonding.

[0051] S10, Annealing, Scribing, and End-face Etching: The flexible solar cell prepared in step S9 is placed in a tubular annealing furnace and annealed at 200°C for 20 minutes. Then, adhesive is applied to the front side of the flexible solar cell, and it is cut with a laser or diamond blade, retaining the effective area in the cell pattern. The cut flexible solar cell is then immersed in an etching solution to remove the cutting residue particles on the end face. The etching solution is a mixture of citric acid, hydrogen peroxide, and water with a volume ratio of 2:2:1. Finally, the cell is immersed in acetone for 15 minutes each, rinsed with water for 10 minutes, and immersed in isopropanol for 1 minute. It is then baked in an oven at 110°C for 15 minutes to complete the preparation of the flexible solar cell.

[0052] Accordingly, this application also provides a flexible solar cell that prevents copper diffusion, which is prepared by the above-described method for preparing flexible solar cells.

[0053] It is worth noting that the thicknesses of the semiconductor substrate 1, epitaxial layer 2, metal layer 3, Cu layer 4, protective layer 5, back electrode 6, rigid substrate 7, front electrode 8, antireflective film 9, and temporary bonding adhesive 10 shown in the accompanying drawings are merely examples and do not represent their actual thicknesses. Furthermore, the actual proportions between the semiconductor substrate 1, epitaxial layer 2, metal layer 3, Cu layer 4, protective layer 5, back electrode 6, rigid substrate 7, front electrode 8, antireflective film 9, and temporary bonding adhesive 10 are not as shown in the drawings and are for reference only.

[0054] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0055] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for fabricating a flexible solar cell that prevents copper diffusion, characterized in that, include: A semiconductor substrate is provided, on which a flip-chip epitaxial layer for a multi-junction cell is grown; A metal layer is grown on the epitaxial layer. The structure of the metal layer is Cr / Ti / Pt / Pd / Ni / Au, where Cr / Ti / Pt / Pd / Ni is a metal gradient buffer layer and Au is a seed layer. A Cu layer is electroplated on the seed layer, and the Cu layer and the metal layer constitute the back electrode. A rigid substrate is provided to temporarily bond the Cu layer to the rigid substrate; Remove the semiconductor substrate; A front electrode and an antireflection film are fabricated on the epitaxial surface exposed after the semiconductor substrate is removed from the epitaxial wafer. Debonding removes the rigid substrate, resulting in a flexible solar cell; The flexible solar cells are prepared by annealing, dicing, and end-face etching.

2. The method for fabricating a flexible solar cell with copper diffusion prevention as described in claim 1, characterized in that: The thicknesses of Cr, Ti, Pt, Pd, Ni, and Au in the metal layer are 500Å-1000Å, 500Å-1000Å, 300Å-600Å, 300Å-600Å, 800Å-1200Å, and 200Å-600Å, respectively.

3. The method for fabricating a flexible solar cell with copper diffusion prevention as described in claim 1, characterized in that: After electroplating a Cu layer, Ni and Sn are sequentially electroplated on the Cu layer as protective layers for Cu. The protective layer, Cu layer, and metal layer constitute the back electrode.

4. The method for fabricating a flexible solar cell with copper diffusion prevention as described in claim 3, characterized in that: The thickness of the Cu layer is 20μm-40μm; In the protective layer, the thickness of Ni is 500Å-1500Å, and the thickness of Sn is 300Å-800Å.

5. The method for fabricating a flexible solar cell that prevents copper diffusion as described in claim 1, characterized in that: The epitaxial layer includes, from bottom to top, a buffer layer, a stop layer, an N-type contact layer, a double-junction or triple-junction epitaxial layer, and a P-type contact layer.

6. The method for fabricating a flexible solar cell with copper diffusion prevention as described in claim 5, characterized in that, Removing the semiconductor substrate includes: The semiconductor substrate is a GaAs substrate. After temporary bonding, the semiconductor substrate is ground away by 200 μm of thickness with a grinding wheel. Then, the epitaxial wafer is immersed in a mixed solution of ammonia, hydrogen peroxide and water for 30 minutes until the GaAs substrate and buffer layer are completely removed, exposing the stop layer. The stop layer is then removed by etching with hydrochloric acid solution.

7. The method for fabricating a flexible solar cell with copper diffusion prevention as described in claim 6, characterized in that, Fabrication of the front electrode includes: Patterns are created on the surface exposed by removing the semiconductor substrate from the epitaxial layer using photolithography. Then, Au / AuGe / Ag / Au are deposited on this surface as front electrodes by electron beam evaporation, with thicknesses of Au, AuGe, Ag, and Au of 1500 Å, 4000 Å, 40000 Å, and 2000 Å, respectively.

8. The method for fabricating a flexible solar cell with copper diffusion prevention as described in claim 7, characterized in that, The production of antireflective coatings includes: The epitaxial wafer for preparing the front electrode is immersed in a selective etching solution to remove the N-type contact layer other than the front electrode; then TiO2 / Al2O3 is deposited as an antireflection film on the surface of the selectively etched epitaxial wafer by electron beam evaporation, with the thicknesses of TiO2 and Al2O3 being 30 nm and 70 nm, respectively.

9. The method for fabricating a flexible solar cell that prevents copper diffusion as described in claim 1, characterized in that: A metal layer is deposited on an epitaxial layer by electron beam evaporation.

10. A flexible solar cell that prevents copper diffusion, characterized in that: It is prepared by the method for preparing a flexible solar cell as described in any one of claims 1-9.

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