A thin-film solar cell flexible substrate and a method of manufacturing the same

Abstract

The application discloses a flexible substrate of a thin-film solar cell and a preparation method thereof. The preparation method comprises the following steps: growing an inverted epitaxial layer on a GaAs substrate, evaporating a metal thin film on the epitaxial layer; spin-coating photoresist on the surface of the metal thin film to perform a photoetching patterning process, and exposing a region without photoresist covering as a patterned region; growing a metal thick film on the surface of the metal thin film, the metal thick film filling the patterned region and covering the top of the photoresist, and the metal thick film parts between the photoresist located on the patterned region forming metal support columns, and other parts of the metal thick film forming a metal cover plate; removing the photoresist between the metal thick film and the metal thin film, forming hollow isolation channels between adjacent metal support columns, and then forming a hollow composite metal structure, the hollow composite metal structure serving as a flexible substrate of a solar cell, and solving the problems of a mismatch of a thermal expansion coefficient, flexibility and strength in the prior art under the premise of guaranteeing mechanical support performance of the substrate.

Description

Technical Field

[0001] This invention belongs to the field of flexible thin-film solar cell manufacturing technology, and specifically relates to a flexible substrate for thin-film solar cells and its preparation method. Background Technology

[0002] Flexible thin-film solar cells, with their core advantages such as extreme lightweight, foldability, and high power-to-weight ratio, have broad application prospects in aerospace, curved power supply, and other scenarios, and can effectively overcome the bottlenecks of traditional rigid solar panels, which are heavy and have limited storage volume.

[0003] Currently, flexible metal substrates are one of the mainstream technologies for thin-film solar cells, with two representative approaches: One type uses pure copper electroplated flexible substrates, such as the scheme disclosed in Chinese patent CN201810904936.5, which uses electroplated copper to prepare flexible substrates, featuring high preparation efficiency and low cost. However, copper's coefficient of thermal expansion is much higher than that of semiconductor epitaxial materials such as GaAs. During the high-temperature cooling process, the difference in shrinkage between the copper substrate and the epitaxial layer will generate huge tensile stress, which can easily lead to warping, dark cracks, or even failure of the epitaxial layer. At the same time, copper is a "fast-diffusion" element in semiconductors, which can easily migrate to the active region of the battery to form recombination centers, causing a decrease in photoelectric conversion efficiency. In addition, copper is easily oxidized in air to form non-conductive oxides, which will increase the series resistance of the battery, reduce long-term reliability, and impose stringent requirements on the packaging process.

[0004] Another approach uses a periodic Cu / Ni composite substrate, such as the scheme disclosed in Chinese patent CN202310980698.7, which improves the mechanical strength of thin-film batteries by periodically electroplating a Cu / Ni composite layer. However, nickel itself has high hardness and high deposition tensile stress, which may cause epitaxial layer cracking during the initial nickel layer deposition; the multi-layer periodic structure introduces multiple stress abrupt interfaces (for example, three stress abrupt interfaces are introduced in three cycles), and the interface stress concentration is prone to material delamination; at the same time, the bending stiffness of a 30μm thick Cu / Ni composite layer is much higher than that of pure copper of the same thickness, resulting in a significant decrease in flexibility, making it impossible to achieve small radius bending, and it can only be used as a lightweight rigid sheet, which is difficult to adapt to curved surface bonding scenarios. In addition, Cu and Ni are prone to forming galvanic cells in humid environments, which accelerates corrosion, and the interfacial diffusion under high temperature conditions also poses a risk of interface embrittlement.

[0005] In summary, existing flexible metal substrate solutions cannot simultaneously achieve low thermal stress matching, excellent bending flexibility, and sufficient mechanical support performance. Stress failure caused by thermal expansion coefficient mismatch and the contradiction between flexibility and strength have become key technical problems restricting the improvement of yield and reliability of flexible thin-film solar cells. Summary of the Invention

[0006] The purpose of this invention is to provide a flexible substrate for thin-film solar cells and a method for preparing the same. The invention creates a hollow metal composite structure as a flexible substrate for solar cells, thereby solving the problems of thermal expansion coefficient mismatch and the contradiction between flexibility and strength in the prior art while ensuring the mechanical support performance of the substrate.

[0007] To achieve the above objectives, this application provides a method for fabricating a flexible substrate for thin-film solar cells, comprising: A GaAs substrate is provided, and an epitaxial layer of a flip-chip multi-junction solar cell is grown on the GaAs substrate to obtain an epitaxial wafer; Ti, Mo, and Ag are sequentially vapor-deposited on the epitaxial layer to form a metal thin film; Photolithography patterning is performed by spin-coating photoresist onto the surface of a metal thin film. After spin coating, exposure, and development, the area exposed without photoresist coverage becomes the arrayed pattern area. A thick metal film is grown on the surface of a patterned metal thin film. The thick metal film is made of any one of Al, Cu, N, Ag, Au, and Pt, and the thickness of the thick metal film is greater than the thickness of the photoresist. The thick metal film fills the patterned area and covers the top of the photoresist. The thick metal film portion located on the patterned area between the photoresist forms a metal support pillar. The metal support pillar and the thick metal film portion above the photoresist form a metal cover plate. The photoresist between the thick metal film and the thin metal film is removed, and a hollow isolation channel is formed between adjacent metal support pillars, thereby forming a hollow composite metal structure consisting of a bottom metal thin film, an array of metal support pillars and an upper metal cover plate. The hollow composite metal structure serves as a flexible substrate for solar cells.

[0008] Furthermore, the flexible substrate also includes a stress balancing layer. After removing the photoresist, a stress balancing layer is deposited on the surface of a thick metal film using a magnetron sputtering method. The stress balancing layer is a TiW thin film with a thickness of 100nm-300nm.

[0009] Furthermore, before preparing the stress balancing layer, the epitaxial wafer after removing the photoresist was immersed in a mixed solution of phosphoric acid and water with a volume ratio of 1:16 for 2 minutes to remove the oxide layer on the surface of the metal thick film. Then it was rinsed with water, ultrasonically soaked in isopropanol for 5 minutes, and finally baked in a vacuum oven at 100°C for 10 minutes.

[0010] Furthermore, the thicknesses of Ti, Mo, and Ag are 50 Å, 1000 Å, and 30000 Å, respectively, wherein Ti serves as an adhesion layer, Mo as a barrier layer, and Ag as a back reflective layer or a welding layer.

[0011] Furthermore, the photoresist has a thickness of 10 μm, and the metal thick film is deposited and grown using evaporation, magnetron sputtering, or electroplating processes, with a thickness of less than 30 μm.

[0012] Furthermore, the metal thick film is made of Al and has a thickness of 20 μm. After the metal thick film is prepared, a blue film is attached to the other side of the GaAs substrate opposite to the epitaxial layer. Then, the epitaxial wafer is placed on a grinding machine with the metal thick film facing upward to grind and thin it, eliminating the uneven steps on the surface of the metal thick film and making the surface of the metal thick film smooth. The thickness of the thinned metal thick film is 15 μm, and the thickness of the metal cover plate is 5 μm.

[0013] Furthermore, the shape of the patterned region is one of regular hexagon, square or circle, arranged in a periodic array, and the total area of ​​the patterned region accounts for 70% to 80% of the wafer area.

[0014] Furthermore, the patterned region is a regular hexagonal array, with the distance between opposite sides of the regular hexagons being 250 μm and the distance between the sides of adjacent hexagons being 30 μm. The total area of ​​the patterned region accounts for 75% of the wafer area.

[0015] Furthermore, after obtaining the flexible substrate, the process also includes the following steps: temporarily bonding the flexible substrate to the rigid substrate, etching away the GaAs substrate, preparing the front electrode and antireflection film, and obtaining the flexible thin-film solar cell chip after debonding, annealing, dicing, and end-face etching.

[0016] This application also provides a flexible substrate for thin-film solar cells, which is prepared by any of the above-described preparation methods.

[0017] After adopting the above solution, the beneficial effects of the present invention are as follows: 1. Significantly improves thermal stress matching and reduces the risk of epitaxial failure: This application uses hollow isolation channels to divide the continuous thick metal film into array-type independent metal support pillars, decomposes the overall stress at the wafer level into the independent deformation of multiple local metal support pillars, and restricts the deformation of each metal support pillar to a local range, blocking the long-distance accumulation path of in-plane stress and avoiding the linear superposition of stress from the center to the edge.

[0018] Meanwhile, the hollow isolation channel is equivalent to introducing stress relief grooves into the metal film layer, which can cut the large continuous metal film layer into independent "islands" or "frames" to form a porous or grid-like structure. When the temperature rises, each independent part can expand outward freely without transferring the huge accumulated stress to the entire wafer, thus forming a decoupling effect. From a macroscopic mechanical perspective, this porous or grid-like structure greatly reduces the equivalent Young's modulus of the metal film layer. According to the bimetallic strip effect formula, stress is proportional to modulus. When the modulus is reduced, the generated thermal stress is naturally reduced, and the interfacial shear stress caused by thermal expansion mismatch is also reduced. This greatly alleviates the problem of epitaxial layer warping and dark cracking after cooling from high-temperature processes, and can significantly improve the device fabrication yield.

[0019] 2. Balancing excellent flexibility and mechanical support strength: Unlike existing Cu / Ni composite substrates that sacrifice flexibility for strength by using a high-hardness nickel layer, this application adopts a sandwich hollow structure of "bottom metal thin film + arrayed metal support pillars + upper metal cover plate". By utilizing mechanical principles similar to I-beams, the moment of inertia of the cross section is increased, which provides good bending stiffness and anti-collapse ability in the vertical direction, and can provide stable mechanical support for the fragile semiconductor epitaxial layer. At the same time, the hollow isolation channel retains the space for in-plane bending deformation, and the flexible substrate as a whole still has excellent bending flexibility, which can be adapted to applications such as curved surface bonding and folding storage.

[0020] 3. Optimize thermal performance and improve thermal shock reliability: The thermal conductivity of air or vacuum medium in the hollow isolation channel is much lower than that of the metal body, which can form a natural thermal barrier layer, slowing down the direct transfer of heat from the metal film to the GaAs epitaxial layer, or protecting the internal device when the external temperature fluctuates drastically; under the thermal shock conditions of rapid heating and cooling, the hollow isolation channel provides deformation buffer space for the thermal expansion and contraction of the metal film, which can effectively reduce the interfacial shear force caused by instantaneous thermal deformation. Combined with the slight bending and torsional deformation of the metal support column (structural compliance), it can absorb some of the strain energy generated by thermal mismatch, reduce the risk of interface delamination and cracking, and improve the long-term reliability of the device under temperature cycling.

[0021] 4. Strong process compatibility, combining lightweight and integrated scalability: The fabrication process of this invention is based on conventional semiconductor photolithography, coating, and polishing processes, which is highly compatible with existing thin-film solar cell fabrication processes and requires no additional special equipment; moreover, the hollow structure of the flexible substrate removes some metal materials, directly reducing the overall weight of the substrate, which meets the ultra-lightweight requirements of aerospace scenarios; at the same time, the flat surface formed by the upper metal cover plate can provide a good operating surface for subsequent processes, and the hollow isolation channel can be expanded to integrate vertical electrical interconnects, fluid channels and other functions, adapting to heterogeneous integration scenarios.

[0022] 5. Potential acoustic, vibration damping, and diffusion and corrosion suppression properties: The hollow structure extends the sound propagation path, providing vibration damping and sound insulation effects. This allows flexible solar cells to better withstand mechanical vibrations and reduces vibration damage to the GaAs lattice. The flexible substrate can be fabricated using a single metal material, avoiding the corrosion problems of Cu / Ni composite structures and eliminating the risk of interface embrittlement caused by interdiffusion at multi-layer metal interfaces, thus improving the long-term reliability of the device. Attached Figure Description

[0023] Figure 1 This is a structural diagram of the metal thin film layer after deposition according to the present invention; Figure 2 This is a structural diagram of the present invention after photolithography patterning process; Figure 3 This is a structural diagram of the metal film deposited according to the present invention; Figure 4 This is a structural diagram of the metal film after grinding and thinning according to the present invention; Figure 5 This is a structural diagram of the present invention after the photoresist has been removed; Figure 6 This is a structural diagram of the present invention after the deposition of the stress-balanced layer; Figure 7 This is a structural diagram of the solar cell chip prepared according to the present invention; Figure 8 This is a process flow diagram of the present invention.

[0024] Label Explanation: 1. GaAs substrate; 2. Epitaxial layer; 3. Metal thin film; 4. Photoresist; 5. Patterned area; 6. Metal thick film; 61. Metal support pillar; 62. Metal cover plate; 7. Hollow isolation channel; 8. Stress balancing layer; 9. Front electrode; 10. Antireflection coating. Detailed Implementation

[0025] 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.

[0026] like Figures 1-8 As shown, the present invention also provides a method for preparing a flexible substrate for a thin-film solar cell, comprising the following steps: S1, GaAs thin-film battery flip-chip epitaxial growth A GaAs substrate 1 is provided, specifically a 4-inch GaAs substrate. Using an MOCVD device, an epitaxial layer 2 for flip-chip multi-junction solar cells is grown on the GaAs substrate 1 to obtain an epitaxial wafer. All semi-finished structures containing an epitaxial layer 2 described below can be referred to as epitaxial wafers.

[0027] S2, Fabrication of a thin metal film 3 Take the above-mentioned epitaxial wafer, and ultrasonically clean it sequentially in acetone and isopropanol for 10 minutes each, then rinse it with deionized water and spin dry at high speed. For example... Figure 1 As shown, Ti, Mo and Ag are then sequentially vapor-deposited on the epitaxial layer 2 to form a metal thin film 3, wherein the thicknesses of Ti, Mo and Ag are 50 Å, 1000 Å and 30000 Å, respectively.

[0028] Among them, Ti is the adhesion layer, which plays an adhesion role; Mo is the barrier layer, which can effectively block the diffusion and migration of metal atoms to the active region in the epitaxial layer 2, avoid the photoelectric conversion efficiency decay caused by the formation of recombination centers by fast-diffusion elements, and has high hardness, which also has a stress buffering effect; Ag is the back reflection layer, which can also be used as a welding layer.

[0029] S3, Photolithography to create patterns Photoresist 4 is spin-coated onto the surface of the metal thin film 3 for photolithography patterning. The thickness of the photoresist 4 is 10 μm. After spin coating, exposure, and development, the exposed areas without photoresist 4 form an array of patterned regions 5. The shape of the patterned regions 5 can be one of square, circle, or regular hexagon, arranged in a periodic array, and the total area of ​​the patterned regions 5 accounts for 70% to 80% of the wafer area (i.e., the surface area of ​​the metal thin film 3). The structure after photolithography is as follows. Figure 2 As shown.

[0030] Preferably, the shape of the patterned region 5 is a regular hexagonal array, with the distance between opposite sides of the hexagons being 250 μm, the distance between the sides of adjacent hexagons being 30 μm, and the total area of ​​the patterned region 5 accounting for about 75% of the wafer area.

[0031] S4, Fabrication of Thick Metal Film 6 A thick metal film 6 is deposited on the patterned metal thin film 3 by means of evaporation, magnetron sputtering or electroplating. The material of the thick metal film 6 is any one of Al, Cu, N, Ag, Au and Pt, and the thickness of the thick metal film 6 is greater than the thickness of the photoresist 4. The thick metal film 6 fills the patterned area 5 and covers the top of the photoresist 4. The portion of the thick metal film 6 located on the patterned area 5 between the photoresist 4 forms a metal support pillar 61. The metal support pillar 61 and the portion of the thick metal film 6 above the photoresist 4 form a metal cover plate 62.

[0032] Specifically, the thickness of the metal thick film 6 is less than 30 μm, which meets the requirements of thin and light flexible substrates, and the thickness needs to be greater than 10 μm to form the upper metal cover plate 62 structure.

[0033] Preferably, the metal thick film 6 is made of Al, deposited and grown by vapor deposition, and has a thickness of 20 μm. Its surface morphology is as follows: Figure 3 As shown.

[0034] S5, Grinding to thin thick metal film 6 like Figure 3 As shown, the surface of the newly deposited thick metal film 6 is uneven and stepped, and it needs to be ground and thinned to make the surface of the metal cover plate 62 flat, so as to provide a good operating surface for subsequent processes.

[0035] The grinding and thinning steps include: attaching a blue film to the other side of the GaAs substrate 1 opposite to the epitaxial layer 2, and then placing the epitaxial wafer with the blue film facing down and the metal thick film 6 facing up on the grinding machine to perform grinding and thinning, eliminating the uneven steps on the surface of the metal thick film 6, so that the surface of the metal thick film 6 is flat.

[0036] Preferably, after thinning, the thickness of the metal film 6 is reduced from the original 20 μm to 15 μm, then the thickness of the metal cover plate 62 is 5 μm, and the structure after thinning is as follows: Figure 4 As shown.

[0037] S6, Adhesive Removal Cleaning After peeling off the blue film, the thinned epitaxial wafer is sequentially immersed in three stripping solutions (acetone solution) for 10 minutes each, followed by ultrasonic soaking in isopropanol solution for 10 minutes. This removes the photoresist 4 inside the metal film layer (i.e., the area between the metal thin film 3 and the metal thick film 6) and the residual photoresist after peeling off the blue film, creating a hollow isolation channel 7 between adjacent metal support pillars 61. This forms a hollow composite metal structure consisting of the bottom metal thin film 3, the array of metal support pillars 61, and the upper metal cover plate 62. This hollow composite metal structure serves as a flexible support substrate for the thin-film solar cell. The specific structure is as follows: Figure 5 As shown.

[0038] Using hollow composite metal structures as flexible substrates has the following advantages: Firstly, it significantly improves thermal stress matching. The hollow isolation channel 7 structure blocks the stress accumulation path. In a planar structure, stress accumulates linearly from the center to the edge (with the highest stress at the edge). The hollow isolation channel 7 cuts off the stress transmission path. The deformation of each metal support pillar 61 is limited to a local range, avoiding long-distance stress superposition. At the same time, the setting of the hollow isolation channel 7 is equivalent to introducing stress relief grooves in the metal film layer, which can cut the large area of ​​continuous metal film layer into independent "islands" or "frames" to form a porous or grid-like structure. When the temperature rises, each independent part can expand outward freely without transmitting the huge accumulated stress to the entire wafer, forming a decoupling effect. From a macroscopic mechanical perspective, this porous or grid-like structure greatly reduces the equivalent Young's modulus of the metal film layer. According to the bimetallic strip effect formula, stress is proportional to modulus. The reduction in modulus naturally reduces the thermal stress, and thus the interfacial shear stress caused by thermal expansion mismatch is also reduced. This greatly alleviates the problem of tensile warping and dark cracking of the epitaxial layer 2 after cooling from high-temperature processes, and can significantly improve the device fabrication yield.

[0039] Secondly, it enhances mechanical strength and resistance to deformation. The metal cover plate 62 provides planar support for the patterned metal, and the planar design facilitates subsequent processes. In sheet metal mechanics, a well-designed cavity structure enhances mechanical properties. The hollow structure of the flexible substrate increases the interfacial moment of inertia, similar to the principle of an I-beam in construction, increasing the structural thickness and improving bending stiffness. This means the metal cover plate 62 is less prone to collapse or bending in the vertical direction. Furthermore, it prevents overall instability. Compared to a simple thin metal foil, this "sandwich structure" with supporting pillars better maintains its shape and provides sufficient mechanical support to protect the fragile GaAs epitaxial layer.

[0040] Furthermore, it optimizes thermal performance, providing both insulation and buffering. The thermal conductivity of the air or vacuum medium inside the hollow isolation channel 7 is much lower than that of metal, forming a thermal barrier layer that slows down the direct transfer of heat from the metal film to the GaAs epitaxial layer, or protects internal devices during drastic external temperature fluctuations. It also adapts to thermal shock; during rapid temperature increases and decreases (thermal shock), the "buffer space" provided by the hollow structure allows the metal room to expand and contract, reducing the risk of delamination or cracking caused by instantaneous thermal expansion and contraction, creating an energy-absorbing effect similar to a "lattice" or "honeycomb." This structure aligns with the design concept of zero-thermal-expansion lattice composite materials. Although the metal itself expands significantly, the hollow structure allows the metal skeleton to undergo slight bending or torsional deformation (i.e., structural compliance) during thermal expansion, thus "absorbing" some of the strain energy generated by the mismatch in thermal expansion coefficients, rather than converting it all into destructive interfacial shear stress.

[0041] Furthermore, the main body of the flexible substrate in this application can be made of a single metal material, which avoids the corrosion problem of the Cu / Ni composite structure battery and eliminates the risk of interface embrittlement caused by interdiffusion at the multilayer metal interface, thereby improving the long-term operational reliability of the device.

[0042] S7, Create a stress balancing layer 8 The flexible substrate also includes a stress balancing layer 8. After removing the photoresist 4, the stress balancing layer 8 is deposited on the surface of the metal thick film 6 by magnetron sputtering. The stress balancing layer 8 is a TiW thin film with a thickness of 100nm-300nm, preferably 200nm. The structure after preparation is shown in Figure 6.

[0043] In composite structures of metals (Al or Ag) and GaAs, the shrinkage rate of the metal is much greater than that of GaAs during subsequent cooling processes, resulting in enormous tensile stress at the interface, which can lead to wafer warping or delamination. Therefore, this application utilizes the compressive stress of the TiW thin film to counteract the tensile stress of the metal film, while simultaneously leveraging the high hardness of the TiW thin film to protect the fragile hollow array, thus acting as both a "stress balancer" and a "structural reinforcement."

[0044] Furthermore, before preparing the stress balancing layer 8, the epitaxial wafer after removing the photoresist 4 needs to be immersed in a mixed solution of phosphoric acid and water with a volume ratio of 1:16 for 2 minutes to remove the oxide layer on the surface of the metal thick film 6. Then, it is rinsed with water, ultrasonically soaked in isopropanol for 5 minutes, and finally baked in a vacuum oven at 100°C for 10 minutes.

[0045] After obtaining the flexible substrate, the fabrication steps of the finished battery also include: spin-coating a temporary bonding adhesive onto the surface of the flexible substrate to temporarily bond it to a rigid substrate, which can be glass, sapphire, silicon wafer, etc.; then etching away the GaAs substrate 1 to prepare the front electrode 9 and the antireflection film 10; and finally, after debonding, annealing, dicing, and end-face etching, obtaining the flexible thin-film solar cell chip, with the structure as shown in the figure. Figure 7 As shown.

[0046] like Figure 7As shown, this application also provides a flexible substrate for thin-film solar cells, which is prepared by the preparation method described in any one of the above claims. It includes a metal thin film 3, a metal thick film 6, and a stress balancing layer 8 sequentially stacked along the back side of the epitaxial layer 2. The metal thin film 3 and the metal thick film 6 form a metal film layer. The portion of the metal thick film 6 near the metal thin film 3 is provided with an array of multiple hollow isolation channels 7. The hollow isolation channels 7 are formed after the photoresist 4 is removed. The portions of the metal thick film 6 between the hollow isolation channels 7 form metal support pillars 61. The thickness of the metal thick film 6 is greater than the thickness of the hollow isolation channels 7. The portions of the metal thick film 6 covering the metal support pillars 61 and the hollow isolation channels 7, excluding the metal support pillars 61, are covered by a metal cover plate 62. The metal thin film 3, the array of metal support pillars 61, and the metal cover plate 62 constitute a hollow composite metal structure.

[0047] The metal thin film 3 includes Ti, Mo and Ag stacked sequentially along the back side of the epitaxial layer 2, with the thicknesses of Ti, Mo and Ag being 50 Å, 1000 Å and 30000 Å respectively. Ti serves as an adhesion layer, Mo as a barrier layer, and Ag as a back reflective layer or a welding layer.

[0048] The metal thick film 6 is made of any one of Al, Cu, N, Ag, Au, and Pt, and its thickness is less than 30 μm. The hollow isolation channel 7 has a thickness of 10 μm.

[0049] The stress balancing layer is an 8TiW thin film, and the thickness of the TiW thin film is 100nm-300nm.

[0050] It is worth noting that the thicknesses of the GaAs substrate 1, epitaxial layer 2, metal thin film 3, photoresist 4, patterned region 5, thick metal film 6, metal support pillar 61, metal cover plate 62, hollow isolation channel 7, stress balancing layer 8, front electrode 9, and antireflection film 10 shown in the accompanying drawings are merely examples and do not represent their actual thicknesses. Furthermore, the actual proportions between the GaAs substrate 1, epitaxial layer 2, metal thin film 3, photoresist 4, patterned region 5, thick metal film 6, metal support pillar 61, metal cover plate 62, hollow isolation channel 7, stress balancing layer 8, front electrode 9, and antireflection film 10 are not as shown in the drawings and are for reference only.

[0051] 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.

[0052] 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 preparing a flexible substrate for a thin-film solar cell, characterized in that, include: A GaAs substrate is provided, and an epitaxial layer of a flip-chip multi-junction solar cell is grown on the GaAs substrate to obtain an epitaxial wafer; Ti, Mo, and Ag are sequentially vapor-deposited on the epitaxial layer to form a metal thin film; Photolithography patterning is performed by spin-coating photoresist onto the surface of a metal thin film. After spin coating, exposure, and development, the area exposed without photoresist coverage becomes the arrayed pattern area. A thick metal film is grown on the surface of a patterned metal thin film. The thick metal film is made of any one of Al, Cu, N, Ag, Au, and Pt, and the thickness of the thick metal film is greater than the thickness of the photoresist. The thick metal film fills the patterned area and covers the top of the photoresist. The thick metal film portion located on the patterned area between the photoresist forms a metal support pillar. The metal support pillar and the thick metal film portion above the photoresist form a metal cover plate. The photoresist between the thick metal film and the thin metal film is removed, and a hollow isolation channel is formed between adjacent metal support pillars, thereby forming a hollow composite metal structure consisting of a bottom metal thin film, an array of metal support pillars and an upper metal cover plate. The hollow composite metal structure serves as a flexible substrate for solar cells.

2. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 1, characterized in that: The flexible substrate also includes a stress balancing layer. After removing the photoresist, a stress balancing layer is deposited on the surface of a thick metal film using a magnetron sputtering method. The stress balancing layer is a TiW thin film with a thickness of 100nm-300nm.

3. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 2, characterized in that: Before preparing the stress balance layer, the epitaxial wafer after removing the photoresist was immersed in a mixed solution of phosphoric acid and water with a volume ratio of 1:16 for 2 minutes to remove the oxide layer on the surface of the metal thick film. Then it was rinsed with water, ultrasonically soaked in isopropanol for 5 minutes, and finally baked in a vacuum oven at 100°C for 10 minutes.

4. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 1, characterized in that: The thicknesses of Ti, Mo, and Ag are 50 Å, 1000 Å, and 30000 Å, respectively, wherein Ti serves as an adhesion layer, Mo as a barrier layer, and Ag as a back reflective layer or a welding layer.

5. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 1, characterized in that: The photoresist has a thickness of 10 μm, and the metal thick film is deposited and grown using evaporation, magnetron sputtering, or electroplating processes, with a thickness of less than 30 μm.

6. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 5, characterized in that: The metal thick film is made of Al and has a thickness of 20 μm. After the metal thick film is prepared, a blue film is attached to the other side of the GaAs substrate opposite to the epitaxial layer. Then, the epitaxial wafer is placed on a grinding machine with the metal thick film facing up to grind and thin it, eliminating the uneven steps on the surface of the metal thick film and making the surface of the metal thick film smooth. The thickness of the thinned metal thick film is 15 μm, and the thickness of the metal cover plate is 5 μm.

7. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 1, characterized in that: The shape of the patterned region is one of regular hexagon, square or circle, arranged in a periodic array, and the total area of ​​the patterned region accounts for 70% to 80% of the wafer area.

8. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 7, characterized in that: The patterned area is a regular hexagonal array with a side-to-side distance of 250 μm between the hexagons and a side-to-side distance of 30 μm between adjacent hexagons. The total area of ​​the patterned area accounts for 75% of the wafer area.

9. The method for preparing a flexible substrate for a thin-film solar cell as described in claim 1, characterized in that: After obtaining the flexible substrate, the process also includes the preparation of the finished battery: temporarily bonding the flexible substrate to the rigid substrate, etching away the GaAs substrate, preparing the front electrode and antireflection film, and obtaining the flexible thin-film solar cell chip after debonding, annealing, dicing, and end-face etching.

10. A flexible substrate for thin-film solar cells, characterized in that: It is prepared by the preparation method according to any one of claims 1-9.

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