Flexible solar cell capable of preventing copper diffusion
By using a Cr/Ti/Pt/Pd/Ni/Au multilayer metal structure and a Ni/Sn protective layer in flexible solar cells, the thermal mismatch and diffusion problems between copper and gallium arsenide epitaxial layers were solved, improving device yield and reliability while reducing costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-30
- Publication Date
- 2026-04-10
AI Technical Summary
In existing flexible solar cells, the thermal expansion coefficient of copper does not match that of the gallium arsenide epitaxial layer, resulting in significant thermal stress. Copper atoms diffuse into the epitaxial layer, forming deep-level defects, and the copper surface is prone to oxidation, affecting the performance and reliability of the cells.
A multilayer metal structure of Cr/Ti/Pt/Pd/Ni/Au is used as a buffer layer and a barrier layer, combined with a Ni/Sn protective layer, to form a gradual buffer of thermal expansion gradient, which prevents copper diffusion and oxidation.
It effectively alleviates thermal stress, prevents copper diffusion, improves device yield and reliability, reduces costs, and enhances battery performance and welding reliability.
Smart Images
Figure CN121843282A_ABST
Abstract
Description
[0001] This case is a divisional application of a Chinese patent, with the parent application number being 202511415051.5 and the application date being 2025-09-30. Technical Field
[0002] This invention belongs to the field of semiconductor device technology, and specifically relates to a flexible solar cell that prevents copper diffusion. Background Technology
[0003] Flexible multi-junction gallium arsenide solar cells have broad application prospects in aerospace, portable electronic devices, and wearable energy systems due to their high photoelectric conversion efficiency, high specific power, and bendability. Traditional flexible solar cells often use electroplated copper as a supporting substrate to provide good conductivity and mechanical support.
[0004] However, the existing technology of using electroplated copper as the supporting substrate for flexible solar cells still has the following prominent problems: Severe mismatch in thermal expansion coefficients: The thermal expansion coefficient of copper (approximately 17 ppm / ℃) is much higher than that of gallium arsenide (GaAs) epitaxial layers (approximately 6 ppm / ℃). This means that when cooling from high temperature, the shrinkage of copper is much greater than that of gallium arsenide. During temperature changes, significant thermal stress will be generated between the copper layer and the epitaxial layer, which can easily lead to cracks in the brittle gallium arsenide epitaxial layer, reducing device yield and reliability.
[0005] Copper atoms diffuse into the epitaxial layer very easily: Copper has an extremely high diffusion coefficient in the epitaxial layer. If no effective blocking structure is set, copper atoms diffuse into the low-concentration gallium arsenide interior, forming deep-level defects, which become recombination centers and trap centers, leading to increased battery resistance and decreased photoelectric conversion efficiency.
[0006] Copper surfaces are prone to oxidation: Copper is easily oxidized in air to form a high-resistivity oxide layer, which not only increases contact resistance and reduces short-circuit current, but also affects the reliability of subsequent welding processes, easily leading to incomplete or false soldering.
[0007] While existing technologies have attempted to use a Pt / Au structure as a barrier layer to alleviate the above problems, with Pt as the barrier layer and Au providing ohmic contact and solderability, the barrier effect of Pt is limited, failing to systematically solve the problems of thermal mismatch and diffusion. Furthermore, the amount of Au used is large, resulting in high costs and making it unsuitable for mass production. Summary of the Invention
[0008] The purpose of this invention is to provide a flexible solar cell that prevents copper diffusion. By designing a metal gradient buffer layer and a composite barrier structure, the problems of thermal mismatch stress between copper and the epitaxial layer and copper atom diffusion are solved.
[0009] To achieve the above objectives, the present invention provides a flexible solar cell that prevents copper diffusion, comprising: Epitaxial layer, which is the epitaxial layer of a multi-junction cell; A metal layer located on the epitaxial layer, the structure of the metal layer being Cr / Ti / Pt / Pd / Ni / Au, wherein Cr / Ti / Pt / Pd / Ni is a metal gradient buffer layer and Au is a seed layer; The Cu layer located on the seed layer, along with the metal layer, forms the back electrode on the back side of the epitaxial layer.
[0010] Furthermore, in the metal layer, 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.
[0011] Furthermore, a protective layer is provided on the Cu layer, the protective layer comprising Ni and Sn grown sequentially, and the protective layer, Cu layer and metal layer constitute the back electrode.
[0012] Furthermore, the thickness of the Cu layer is 20μm-40μm; in the protective layer, the thickness of the Ni is 500Å-1500Å, and the thickness of the Sn is 300Å-800Å.
[0013] Furthermore, the epitaxial layer has a front electrode and an antireflection film on its front side.
[0014] Furthermore, the front electrode is made of Au / AuGe / Ag / Au material, and the thicknesses of Au, AuGe, Ag, and Au are 1500Å, 4000Å, 40000Å, and 2000Å, respectively; the antireflection film is made of TiO2 / Al2O3 material, and the thicknesses of TiO2 and Al2O3 are 30nm and 70nm, respectively.
[0015] Furthermore, the epitaxial layer includes 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 stacked sequentially, with the metal layer located on the P-type contact layer.
[0016] Furthermore, the dual-junction epitaxial layer is composed of GaInP sub-cells, GaInP tunnel junctions, and GaAs sub-cells stacked sequentially; the triple-junction epitaxial layer is composed of GaInP top cell, GaInP tunnel junction, GaAs middle cell, GaInP tunnel junction, and InGaAs bottom cell stacked sequentially.
[0017] Furthermore, the buffer layer is an N-type GaAs buffer layer, the stop layer is a GaInP stop 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.
[0018] Furthermore, a GaAs capping layer for protecting the epitaxial structure is provided between the P-type contact layer and the metal layer.
[0019] After adopting the above solution, the beneficial effects of the present invention are as follows: 1. Excellent thermal stress relief capability: This application sets a metal layer between the epitaxial layer and the Cu layer. The metal layer adopts a Cr / Ti / Pt / Pd / Ni / Au multilayer structure. Its thermal expansion coefficient increases gradually from the Cr layer (about 6.2 ppm / ℃) to the Au layer (about 14.2 ppm / ℃), forming a gradual buffer of thermal expansion gradient. This significantly reduces the thermal mismatch stress between the copper layer and the epitaxial layer, effectively prevents epitaxial layer cracking, and improves device yield and mechanical reliability.
[0020] 2. Excellent Copper Diffusion Barrier Performance: In the metal layer, the Pt / Pd / Ni combination forms a multilayer barrier structure. Pt serves as the first diffusion barrier layer, protecting the Ti layer and the underlying GaAs epitaxial structure, 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 plating layers. This further ensures that Cu and Au cannot reach the sensitive areas of the lower layer, greatly improving long-term reliability and providing a smooth surface for subsequent Au plating. Ni, as the third diffusion barrier layer, effectively suppresses the lateral migration of Au atoms at high temperatures and provides 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 structure, and enhancing the reliability of flexible solar cells.
[0021] 3. Improved copper surface oxidation resistance and solderability: This application sets Ni and Sn as protective layers on the Cu surface. Ni can protect the Cu surface and prevent Cu oxidation, and also prevent the interdiffusion between Cu and Sn, avoiding the formation of brittle intermetallic compounds, which greatly improves 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. Attached Figure Description
[0022] Figure 1 This is an initial structural diagram of the epitaxial wafer of the present invention; Figure 2 This is a diagram of the three-junction flip-chip epitaxial structure of the epitaxial wafer of the present invention; Figure 3 This is a diagram of the double-junction flip-chip epitaxial structure of the epitaxial wafer of the present invention; Figure 4 This is a structural diagram of the vapor-deposited metal gradient buffer layer of the present invention; Figure 5This is a structural diagram of the vapor-deposited metal layer, Cu layer, and protective layer of the present invention; Figure 6 This is a structural diagram of the back electrode of the present invention; Figure 7 This is a structural diagram of the temporary bonded rigid substrate of the present invention; Figure 8 This is a structural diagram of the present invention with the semiconductor substrate removed; Figure 9 This is a structural diagram of the vapor-deposited front electrode and antireflection film of the present invention; Figure 10 This is a structural diagram of the present invention with the rigid substrate removed; Figure 11 This is a flowchart of the preparation method of the present invention.
[0023] Label Explanation: 1. Semiconductor substrate; 2. Epitaxial layer; 3. Metal layer; 31. Metal gradient buffer layer; 32. Seed layer; 4. Cu layer; 5. Protective layer; 6. Back electrode; 7. Rigid substrate; 8. Front electrode; 9. Antireflective coating; 10. Temporary bonding adhesive. Detailed Implementation
[0024] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application, and the range values mentioned in this application all include endpoint values.
[0025] like Figure 11 As shown, this application provides a flexible solar cell that prevents copper diffusion, comprising the following steps: S1. Growing Epitaxial Layer 2: A semiconductor substrate 1 is provided. A flip-chip epitaxial layer 2 for a multi-junction solar cell is grown on the semiconductor substrate 1 using an MOCVD device. The epitaxial wafer structure after growing epitaxial layer 2 is as follows: Figure 1 As shown.
[0026] Optionally, the semiconductor substrate 1 is a GaAs substrate, and the epitaxial layer 2 can be an epitaxial layer structure of a flip-chip triple-junction or double-junction GaAs thin-film battery, or even an epitaxial layer structure with more than three junctions, without limitation.
[0027] Specifically, such as Figure 2-3As shown, the epitaxial layer 2 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. Furthermore, a GaAs capping 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 contacts.
[0028] Optionally, the buffer layer is an N-type GaAs buffer layer, the stop layer is a GaInP stop 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.
[0029] Optional, such as Figure 2 As shown, the triple-junction epitaxial layer consists of GaInP top cell, GaInP tunnel junction, GaAs middle cell, GaInP tunnel junction, and InGaAs bottom cell, stacked sequentially from bottom to top. Figure 3 As shown, the double-junction epitaxial layer consists of GaInP subcells, GaInP tunnel junctions, and GaAs subcells stacked sequentially from bottom to top.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] And, as Figure 4 As 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 6 As 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] Specifically, the thicknesses of Au, AuGe, Ag, and Au are 1500Å, 4000Å, 40000Å, and 2000Å, respectively.
[0047] 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.
[0048] 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.
[0049] Optionally, in the antireflective coating 9, the thickness of TiO2 is 30 nm and the thickness of Al2O3 is 70 nm.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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 flexible solar cell that prevents copper diffusion, characterized in that, include: Epitaxial layer, which is the epitaxial layer of a multi-junction cell; A metal layer located on the epitaxial layer, the structure of the metal layer being Cr / Ti / Pt / Pd / Ni / Au, wherein Cr / Ti / Pt / Pd / Ni is a metal gradient buffer layer and Au is a seed layer; The Cu layer located on the seed layer, along with the metal layer, forms the back electrode on the back side of the epitaxial layer.
2. The flexible solar cell for preventing copper diffusion 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. A flexible solar cell for preventing copper diffusion as described in claim 1, characterized in that: A protective layer is provided on the Cu layer, the protective layer comprising Ni and Sn grown sequentially, and the protective layer, Cu layer and metal layer constitute the back electrode.
4. A flexible solar cell for preventing copper diffusion as described in claim 3, characterized in that: The thickness of the Cu layer is 20μm-40μm; in the protective layer, the thickness of the Ni is 500Å-1500Å and the thickness of the Sn is 300Å-800Å.
5. A flexible solar cell for preventing copper diffusion as described in claim 1, characterized in that: The epitaxial layer has a front electrode and an antireflection film on its front side.
6. A flexible solar cell for preventing copper diffusion as described in claim 5, characterized in that: The front electrode is made of Au / AuGe / Ag / Au material, and the thicknesses of Au, AuGe, Ag and Au are 1500Å, 4000Å, 40000Å and 2000Å, respectively; the antireflection film is made of TiO2 / Al2O3 material, and the thicknesses of TiO2 and Al2O3 are 30nm and 70nm, respectively.
7. A flexible solar cell for preventing copper diffusion as described in claim 1, characterized in that: The epitaxial layer includes 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 stacked sequentially, with the metal layer located on the P-type contact layer.
8. A flexible solar cell for preventing copper diffusion as described in claim 7, characterized in that: The double-junction epitaxial layer is composed of GaInP sub-cells, GaInP tunnel junctions, and GaAs sub-cells stacked sequentially; the triple-junction epitaxial layer is composed of GaInP top cell, GaInP tunnel junction, GaAs middle cell, GaInP tunnel junction, and InGaAs bottom cell stacked sequentially.
9. A flexible solar cell for preventing copper diffusion as described in claim 7, characterized in that: The buffer layer is an N-type GaAs buffer layer, the stop layer is a GaInP stop 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.
10. A flexible solar cell for preventing copper diffusion as described in claim 7, characterized in that: A GaAs capping layer for protecting the epitaxial structure is provided between the P-type contact layer and the metal layer.