A thin-film solar cell based on in-situ evaporation of a flexible substrate and a method of manufacturing the same

Abstract

This invention discloses a thin-film solar cell based on an in-situ evaporation-deposited flexible substrate and its fabrication method. The fabrication method includes growing an epitaxial layer on a GaAs substrate, and sequentially evaporating a P-side electrode and a composite flexible substrate on the epitaxial layer. The composite flexible substrate includes a first metal support layer, a first adhesion transition layer, an insulating stress regulation layer, a second adhesion transition layer, and a second metal support layer stacked sequentially along the growth direction. The first and second metal support layers have the same thickness and are both made of Ni; the first and second adhesion transition layers have the same thickness and are both made of Ti; the insulating stress regulation layer is made of SiO2. Subsequent fabrication methods include temporary bonding, fabrication of the N-side electrode and antireflection film, lead-out of the P-side electrode on the same side, removal of the temporary substrate, and annealing. This application uses an in-situ evaporation method to prepare a multilayer symmetrical composite flexible substrate to actively balance thermal stress and avoids additional bonding interface defects.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor device technology, and specifically relates to a thin-film solar cell based on in-situ evaporation of a flexible substrate and its preparation method. Background Technology

[0002] As spacecraft power systems face increasingly stringent requirements for power-to-weight ratio (typically >1000W / kg), traditional rigid triple-junction gallium arsenide solar cells, due to their heavy and non-power-generating native Ge or GaAs substrates, have become a bottleneck restricting performance improvement. To overcome this limitation, the industry commonly employs "substrate transfer" technology, which involves peeling off the functional epitaxial layer from the native substrate and transferring it to a lightweight, flexible substrate. Existing typical technical solutions include the following two categories.

[0003] Option 1 (Patent Application CN201510291455.8): First, the epitaxial substrate is thinned to approximately 100 μm and polished. Then, a Ti / Ag / Au bonding layer is deposited on the P-side. Next, the bonding layer is thermo-bonded to a copper-molybdenum-copper flexible substrate with a matching coefficient of thermal expansion (CTE). Finally, N-side electrodes and an anti-reflection film are fabricated. This option uses a CMC flexible substrate with a CTE matching GaAs to reduce thermal mismatch stress and partially replaces Au with Ag to reduce costs.

[0004] Option 2 (Patent Application CN202011604325.2): A release layer and a multi-junction cell epitaxial layer are sequentially grown on a GaAs substrate. A Pd / Zn / Cu layer is deposited as a seed layer on the surface of the bottom cell contact layer, followed by electroplating of a 30μm~50μm thick copper support layer, and then further electrodepositing a carbon nanotube thin film (approximately 10μm). Subsequently, the GaAs substrate is removed using the release layer to fabricate the N-side electrode and antireflection coating. This option utilizes the high elastic modulus and corrosion resistance of carbon nanotubes to improve the stress mismatch between the copper substrate and the epitaxial layer, and enhances oxidation resistance.

[0005] However, both of the above-mentioned solutions still have fundamental technical shortcomings in terms of practical engineering and mass production, as follows: (1) Insufficient interface adhesion reliability In Scheme 1, thermocompression bonding is a solid-state connection, which is extremely sensitive to surface flatness and cleanliness. In actual processes, nanoscale voids are easily formed at the bonding interface. These voids repeatedly expand and contract during vacuum thermal cycling, leading to a decrease in effective contact area and local current accumulation. In Scheme 2, the support layer formed by electroplating differs significantly from the vapor-deposited layer in grain size, internal stress, and impurity content. Furthermore, hydrogen embrittlement and residual organic additives generated during electroplating form a weak bonding layer at the atomic scale at the vapor / electroplating interface, which is prone to delamination failure under thermal stress and mechanical bending.

[0006] (2) The process is complex and has poor compatibility. In Option 1, the hot-press bonding process is time-consuming (60-120 minutes) and has low capacity. Furthermore, it still requires a certain thickness of Au as an anti-oxidation layer, limiting cost control. In Option 2, the copper and nickel ions introduced by the electroplating process are deep-level impurities in III-V compound semiconductors; even trace amounts of contamination can severely degrade the photoelectric conversion efficiency of the battery. Simultaneously, the uniformity of the electroplating thickness is affected by various factors, making it difficult to precisely control the thickness tolerance of the 30μm-50μm copper layer during mass production, which will affect the uniform distribution of stress.

[0007] In summary, existing technologies have failed to perfectly solve the problem of actively balancing thermal stress between the thick metal support layer and the thin film epitaxial layer, and each has inherent shortcomings such as poor interface reliability or low process compatibility. Therefore, there is an urgent need to develop a novel flexible substrate integration technology that is entirely dry, vacuum in-situ fabricated, employs a multilayer symmetrical structure to actively offset thermal stress, and requires no additional bonding interfaces. Summary of the Invention

[0008] The purpose of this invention is to provide a thin-film solar cell based on in-situ evaporation of a flexible substrate and its preparation method. The method uses in-situ evaporation to prepare a multilayer symmetrical composite flexible substrate to actively balance thermal stress and avoids additional bonding interface defects.

[0009] To achieve the above objectives, this application provides a method for fabricating a thin-film solar cell based on in-situ evaporation of a flexible substrate, comprising: Provide a GaAs substrate, and grow an epitaxial layer for a flip-chip multi-junction solar cell on the GaAs substrate; A P-side electrode and a composite flexible substrate are sequentially deposited on the epitaxial layer. The composite flexible substrate includes a first metal support layer, a first adhesion transition layer, an insulating stress regulation layer, a second adhesion transition layer, and a second metal support layer, which are sequentially stacked along the growth direction. The first metal support layer and the second metal support layer have the same thickness and are both made of Ni. The first adhesion transition layer and the second adhesion transition layer have the same thickness and are both made of Ti. The insulating stress regulation layer is made of SiO2.

[0010] A rigid substrate is provided to temporarily bond a composite flexible substrate to the rigid substrate; Remove the GaAs substrate and fabricate an N-side electrode and an antireflection film on the front side of the epitaxial layer; A P-side electrode window is etched on the anti-reflection film, and the P-side electrode is exposed by wet deep etching, thus enabling the P-side electrode to be led out on the same side. The temporary substrate is removed and alloying annealing is performed to complete the fabrication of the thin-film solar cell.

[0011] Furthermore, the thickness of the first metal support layer and the second metal support layer is 5μm~30μm, the thickness of the first adhesion transition layer and the second adhesion transition layer is 30nm~100nm, and the thickness of the insulating stress regulation layer is 1μm~5μm.

[0012] Furthermore, the insulating stress-regulating layer is prepared by deposition using ion-assisted electron beam evaporation technology at a deposition temperature of 25℃~50℃, an evaporation rate of 1Å / s-10 Å / s, and an oxygen partial pressure of 2×10⁻⁶ within the electron beam evaporation stage. -2 Pa, with Ar as the ion source + The ion energy was set to 100 eV, and the current density was 0.1 mA / cm². 2 -0.5mA / cm 2 .

[0013] Furthermore, the first metal support layer and the second metal support layer are prepared by electron beam evaporation, and the plating rate is 10 Å / s to 30 Å / s; the first adhesion transition layer and the second adhesion transition layer are also prepared by electron beam evaporation, and the plating rate is 1 Å / s to 10 Å / s.

[0014] Furthermore, the P-side electrode consists of an Au layer and an Ag layer, with thicknesses of 10nm~100nm and 3μm~5μm, respectively, and the Au layer is located close to the epitaxial layer.

[0015] Furthermore, the P-side electrode window is etched on the antireflective film, and the P-side electrode is exposed by wet deep etching, which includes: spin-coating photoresist on the light-receiving surface, photolithographically etching the P-side electrode window area, and then using a mixed solution of hydrochloric acid, phosphoric acid and hydrogen peroxide alternately to perform selective wet deep etching, vertically removing the antireflective film and epitaxial layer corresponding to the P-side electrode window area until the Au layer in the P-side electrode is exposed.

[0016] Furthermore, the epitaxial layer includes an N-type GaAs contact layer, a GaInP top cell, a first tunnel junction, a GaAs middle cell, a second tunnel junction, a buffer layer, an InGaAs bottom cell, and a P-type InGaAs contact layer stacked sequentially along the growth direction. A P-side electrode and a composite flexible substrate are then deposited sequentially on the P-type InGaAs contact layer.

[0017] Furthermore, the preparation of the N-side electrode and antireflection film includes: spin-coating a negative photoresist onto the surface of the N-type GaAs contact layer, forming an electrode grid pattern after exposure and development, and then sequentially depositing Pd, Ge, Au, Ag, and Au using electron beam evaporation, with the thicknesses of Pd, Ge, Au, Ag, and Au being 10nm~50nm, 10nm~50nm, 10nm~50nm, 3μm~8μm, and 10nm~50nm, respectively, followed by organic lift-off to form the N-side electrode; Then, using a mixed solution of citric acid, hydrogen peroxide, and water, and with the N-side electrode as a mask, the N-type GaAs contact layer outside the N-side electrode is selectively etched away to expose the GaInP top cell. TiO2 and Al2O3 are then sequentially deposited on the exposed top cell surface as antireflection films, with the thicknesses of TiO2 and Al2O3 being 1 nm to 20 nm and 5 nm to 100 nm, respectively.

[0018] Furthermore, after the composite flexible substrate is prepared, a protective layer is deposited on the surface of the second metal support layer. The material of the protective layer is Au, and the thickness is 10nm~100nm.

[0019] This application also provides a thin-film solar cell based on in-situ evaporation of a flexible substrate, which is prepared by any of the above preparation methods.

[0020] After adopting the above solution, the beneficial effects of the present invention are as follows: 1. Complete elimination of thermal stress deformation, resulting in high battery flatness: This application symmetrically sets metal support layers on both sides of the insulating stress control layer. When the temperature changes, the two Ni metal support layers of equal thickness will generate thermal expansion forces of equal magnitude and symmetrical direction relative to the central plane. These two forces cancel each other out at the central SiO2 layer (insulating stress control layer), and the macroscopic net stress approaches zero. Therefore, the battery cell always remains flat regardless of the annealing temperature. At the same time, the SiO2 layer can be preset to have a specific compressive stress, actively compensating for residual tensile stress and offsetting the tensile stress generated when the two thick Ni metal layers expand due to heat, so that the residual stress can be controlled within a range close to zero. Compared with existing solutions that rely on CTE matching or single-layer buffering, this application solves the problems of epitaxial layer cracking and battery warping from a mechanistic perspective.

[0021] 2. High interfacial bonding strength, significantly improved resistance to delamination and bending: This application introduces a Ti-based adhesive transition layer between the metal support layer (Ni) and the SiO2 layer. Ti and Ni can form a continuous solid solution or intermetallic compound, achieving metallic bonding. Ti can also react with oxygen in SiO2 to form Ti-O-Si covalent bonds, acting as a chemical bond "converter," transforming the weakly bonded Ni / SiO2 interface into two strongly bonded Ni / Ti and Ti / SiO2 interfaces, significantly improving the overall adhesion of the composite substrate. Compared to the nanoscale voids at the bonding interface or the hydrogen embrittlement and organic residues at the electroplated interface in existing technologies, this application achieves impurity-free and void-free atomic-level chemical bonding, resulting in a significant increase in peel strength, making it suitable for flexible battery applications subject to repeated bending.

[0022] 3. Dry vacuum in-situ preparation, simple process and good compatibility: This application uses a fully vacuum evaporation method to prepare the composite flexible substrate, eliminating the need for high-temperature and high-pressure bonding equipment and wet electroplating baths. This avoids the mechanical damage to the epitaxial layer caused by hot-press bonding, the low throughput problem caused by long processing times, and completely eliminates the risk of contamination of III-V semiconductors by deep-level impurities such as copper and nickel during electroplating. This method is highly compatible with existing III-V standard semiconductor production lines, which is conducive to large-scale mass production.

[0023] 4. Precise and controllable support layer thickness, with good batch consistency: Electron beam evaporation offers high thickness control precision (rate stability within ±5%) and can be monitored online in real time; while electroplating processes are affected by multiple factors such as current distribution, plating solution flow rate, and temperature, resulting in large thickness tolerances. The metal support layer prepared in this application has uniform thickness and consistent stress distribution, effectively ensuring batch-to-batch performance stability and making it suitable for mass production.

[0024] 5. Simplified, low-cost, and non-destructive process for leading out P-side electrodes on the same side: This application adopts a wet deep etching process to lead out P-side electrodes on the same side, and uses the Au layer in the P-side electrode as a natural corrosion stop layer. By utilizing the corrosion resistance of Au, the corrosion can be precisely stopped at the Au surface, which greatly simplifies the same-side electrode process, significantly improves the yield, and eliminates the need for expensive ICP dry etching equipment, thus avoiding damage to the active area of ​​the battery by plasma. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the flip-chip double-junction solar cell of the present invention; Figure 2 This is a schematic diagram of the composite flexible substrate of the present invention; Figure 3 This is a flowchart of the preparation method of the present invention.

[0026] Label Explanation: 1. Protective layer; 2. Composite flexible substrate; 21. First metal support layer; 22. First adhesion transition layer; 23. Insulation stress regulation layer; 24. Second adhesion transition layer; 25. Second metal support layer; 3. P-side electrode; 4. Epitaxial layer; 5. N-side electrode; 6. Antireflection coating. Detailed Implementation

[0027] 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.

[0028] Key references Figures 1-2 The present invention provides a thin-film solar cell based on in-situ evaporation of a flexible substrate, comprising, from bottom to top, a protective layer 1, a composite flexible substrate 2, a P-side electrode 3, an epitaxial layer 4, an N-side electrode 5, and an antireflection film 6.

[0029] The epitaxial layer 4 is an epitaxial layer for a multi-junction solar cell. Its specific structure includes, from bottom to top, a P-type InGaAs contact layer, an InGaAs bottom cell, a buffer layer, a second tunnel junction, a GaAs middle cell, a first tunnel junction, a GaInP top cell, and an N-type GaAs contact layer, which is an epitaxial layer for a triple-junction solar cell. The antireflection film 6 and the N-surface electrode 5 are both located on the N-type GaAs contact layer of the epitaxial layer 4.

[0030] Key references Figure 2 The composite flexible substrate 2 comprises, from top to bottom, a first metal support layer 21, a first adhesion transition layer 22, an insulating stress regulating layer 23, a second adhesion transition layer 24, and a second metal support layer 25. The first metal support layer 21 and the second metal support layer 25 are both made of Ni and have the same thickness of 5 μm to 30 μm. The first adhesion transition layer 22 and the second adhesion transition layer 24 are both made of Ti and have the same thickness of 30 nm to 100 nm. The insulating stress regulating layer 23 is made of SiO2 and has a thickness of 1 μm to 5 μm. This application provides the following advantages for the five-layer symmetrical composite flexible substrate 2: Two metal support layers of equal thickness (i.e., Ni layer, CTE≈13×10) are used. -6 The first metal support layer 21 and the second metal support layer 25 are respectively placed on both sides of the SiO2 layer (i.e., the insulating stress control layer 23). When the temperature changes, the first metal support layer 21 and the second metal support layer 25 will generate thermal expansion forces of equal magnitude and symmetrical direction relative to the central plane. These two forces cancel each other out at the central insulating stress control layer 23, and macroscopically, the net stress approaches zero. Therefore, the cell remains flat regardless of the annealing temperature. Existing technologies rely solely on CTE matching or single-layer buffering, which cannot completely eliminate thermal stress. The five-layer symmetrical composite flexible substrate 2 structure set in this application solves the problems of epitaxial layer cracking and cell warping from a mechanistic perspective.

[0031] Furthermore, if a thick metal (such as Ni) is directly deposited onto an oxide (such as SiO2), the interfacial bonding is extremely weak due to the difference in chemical bond types (metallic vs. covalent) and the significant difference in lattice constants, making it prone to delamination and peeling during subsequent processes or bending. Therefore, this application embeds an ultrathin Ti layer (i.e., an adhesion transition layer) between the Ni and SiO2 layers. Ti is a highly reactive transition metal; its work function and electronic structure enable it to form a continuous solid solution or intermetallic compound with Ni, achieving metallic bonding. Moreover, Ti can react with oxygen atoms in the SiO2 layer to form Ti-O-Si bond bridges, achieving covalent bonding. Thus, the Ti layer acts as a chemical bond "converter," transforming the weakly bonded Ni / SiO2 interface into two strongly bonded Ni / Ti and Ti / SiO2 interfaces, significantly improving the overall adhesion of the composite substrate. Compared to the nanoscale voids at the bonding interface or the hydrogen embrittlement and organic residues at the electroplating interface in existing technologies, this application achieves impurity-free and void-free atomic-level chemical bonding, which significantly improves peel strength and is suitable for flexible battery applications subject to repeated bending.

[0032] Preferably, the composite flexible substrate 2 is prepared by electron beam evaporation. Compared with the existing preparation methods that use hot-press bonding and electroplating, this application uses full vacuum evaporation, which is completed entirely in an electron beam evaporation stage. It is fully compatible with III-V semiconductor processes, eliminating the need for high-temperature and high-pressure bonding equipment and wet electroplating baths. This avoids the mechanical damage to the epitaxial layer caused by hot-press bonding and the low throughput problem caused by long processing times. It also completely eliminates the risk of contamination of III-V semiconductors by deep-level impurities such as copper and nickel during electroplating. Moreover, electron beam evaporation has high thickness control precision (rate stability within ±5%) and can be monitored online in real time; while the electroplating process is affected by many factors such as current distribution, plating solution flow rate, and temperature, resulting in large thickness tolerances. The metal support layer prepared in this application has uniform thickness and consistent stress distribution, effectively ensuring the stability of performance between batches and making it suitable for mass production.

[0033] Furthermore, the stress (compressive or tensile stress) of the SiO2 layer evaporated by electron beam depends on deposition parameters such as substrate temperature, evaporation rate, oxygen partial pressure, and ion source auxiliary energy. This application, by actively adjusting these parameters during the evaporation process, can precisely control the compressive stress of the SiO2 layer to a specific magnitude, achieving a residual stress controllable within a near-zero range. In one embodiment, the specific parameters can be controlled as follows: deposition temperature of 25℃~50℃, evaporation rate of 1 Å / s-10 Å / s, and oxygen partial pressure within the electron beam evaporation stage (vacuum coating equipment) of 2×10⁻⁶. -2 Pa, with Ar as the ion source + The ion energy was set to 100 eV, and the current density was 0.1 mA / cm². 2 -0.5mA / cm 2Through the coordinated control of the above parameters, the prepared SiO2 layer has a preset compressive stress of 1 GPa, which can be used to offset the thermally induced tensile stress generated in the first metal support layer 21 and the second metal support layer 25 (both with a thickness of 10 μm) during the annealing alloying process.

[0034] Specifically, the P-side electrode 3 consists of an Au layer and an Ag layer, with thicknesses of 10nm~100nm and 3μm~5μm, respectively. The Au layer is close to the epitaxial layer 4, and the P-side electrode 3 can form a low-resistance ohmic contact with the epitaxial layer 4. At the same time, Au has a very high reflectivity, which can effectively reflect photons back to the absorption layer, increasing the probability of light absorption, which is crucial for improving the short-circuit current density of the battery.

[0035] Optionally, regarding the P-side electrode 3, this application adopts a structure with P-side electrodes led out on the same side. That is, the P-side electrode window is etched on the anti-reflection film 6 using photolithography, and then the anti-reflection film 6 and epitaxial layer 4 corresponding to the P-side electrode window area are removed by wet deep etching to expose the Au layer of the P-side electrode 3, thus realizing the lead-out of the P-side electrode on the same side. In this process, this application utilizes the corrosion resistance of Au, which allows the etching to stop precisely on the Au surface, greatly simplifying the same-side electrode process, significantly improving the yield, and eliminating the need for expensive ICP dry etching equipment, thus avoiding damage to the active area of ​​the battery by plasma.

[0036] Optionally, the material of the protective layer 1 is Au, and the thickness is 10nm~100nm. The protective layer 1 covers the surface of the second metal support layer 25, which can prevent Ni from oxidizing during long-term storage and ensure its reliability.

[0037] Key references Figure 3 The present invention also provides a method for preparing a thin-film solar cell based on in-situ evaporation of a flexible substrate, which includes the following steps: S1, Flip-chip epitaxial growth of multi-junction cells A GaAs substrate is provided, specifically an N-type GaAs substrate. Using an MOCVD device, an epitaxial layer 4 of a flip-chip multi-junction solar cell is grown on the GaAs substrate to obtain an epitaxial wafer.

[0038] Optionally, the epitaxial layer 4 may be the epitaxial layer of a three-junction solar cell, comprising an N-type GaAs contact layer, a GaInP top cell, a first tunnel junction, a GaAs middle cell, a second tunnel junction, a buffer layer, an InGaAs bottom cell, and a P-type InGaAs contact layer stacked sequentially along the growth direction.

[0039] Optionally, an etching stop layer is also grown between the GaAs substrate and the epitaxial layer 4 to protect the epitaxial layer 4 from corrosion during subsequent removal of the GaAs substrate.

[0040] S2, Epitaxial Cleaning Take the above epitaxial wafer, and ultrasonically clean it in acetone and isopropanol for 10 minutes each, then rinse it with deionized water and spin dry at high speed.

[0041] S3, in-situ vacuum-deposited P-side electrode 3, and composite flexible substrate 2 The cleaned epitaxial wafer is placed in an electron beam evaporation stage, and the following layers are deposited sequentially without breaking the vacuum: The P-side electrode 3 is formed by sequentially depositing an Au layer and an Ag layer on a P-type InGaAs contact layer, with the growth thicknesses of the Au layer and Ag layer being 10nm~100nm and 3μm~5μm, respectively.

[0042] The composite flexible substrate 2 includes the following five-layer structure (structure as follows) Figure 2 (as shown) The first metal support layer 21 is formed by depositing 5μm~30μm Ni on the P-side electrode 3 using a high deposition rate (i.e., deposition rate) of 10Å / s~30Å / s.

[0043] The first adhesive transition layer 22 is formed by depositing 30nm~100nm of Ti on the first metal support layer 21 at a deposition rate of 1Å / s~10Å / s.

[0044] The insulating stress control layer 23 is prepared by deposition using ion-assisted electron beam evaporation technology, forming a 1μm~5μm thick SiO2 layer deposited on the first adhesion transition layer 22. By adjusting the electron beam evaporation process parameters, a preset compressive stress is achieved to counteract the tensile stress between the upper and lower thick Ni metal layers (i.e., the first metal support layer 21 and the second metal support layer 25). The parameters are controlled as follows: deposition temperature of 25℃~50℃, evaporation rate of 1Å / s-10 Å / s, and high-purity O2 is introduced into the cavity of the electron beam evaporation stage to maintain an oxygen partial pressure of 2×10⁻⁶ within the cavity. -2 Pa, using Ar+ as the ion source, with ion energy set at 100 eV and current density at 0.1 mA / cm². 2 -0.5mA / cm 2 .

[0045] Through the synergistic control of the above parameters, the prepared SiO2 layer (i.e., the insulating stress control layer 23) has a preset compressive stress of 1 GPa. This compressive stress can be used to offset the thermally induced tensile stress generated by the first metal support layer 21 and the second metal support layer 25 (both with a thickness of 10 μm) during the alloy annealing process. It is calculated that the total tensile stress per unit width generated by the two Ni layers during annealing at 400°C is approximately 5500 N / m. The 1 μm-5 μm thick SiO2 layer with a compressive stress of 1 GPa can provide a reverse compensation force of the same order of magnitude, so that the composite flexible substrate 2 can still maintain macroscopic flatness after high-temperature annealing, and the net stress applied to the epitaxial layer is close to zero.

[0046] The second adhesive transition layer 24 is formed by vapor deposition on the insulating stress regulation layer 23, and the preparation process is the same as that of the first adhesive transition layer 22.

[0047] The second metal support layer 25 is formed by vapor deposition on the second adhesive transition layer 24, and the preparation process is the same as that of the first metal support layer 21.

[0048] Optionally, after the second metal support layer 25 is prepared, an Au layer with a thickness of 10nm~100nm is deposited on the second metal support layer 25 as a protective layer 1 to prevent Ni from oxidizing during long-term storage and ensure its reliability.

[0049] S4, Temporary Bonding and GaAs Substrate Debonding A rigid substrate, such as sapphire crystal, with a thickness of 500 μm, is provided. A temporary bonding adhesive is uniformly spin-coated onto the surface of the composite flexible substrate 2. The adhesive-coated surface is then aligned and bonded to the rigid substrate at a temperature above 120°C and a vacuum degree of not less than 1.0 × 10⁻⁶. -1 Temporary bonding is completed under the condition of mbar.

[0050] Next, the GaAs substrate was thinned to about 50 μm using a mechanical polishing machine. Then, a mixed solution of sulfuric acid, hydrogen peroxide and water (volume ratio of 1:5:1) was used as the etching solution to completely etch away the remaining GaAs substrate. Then, hydrochloric acid was used to continue etching for about 1 minute to remove the etching stop layer. Finally, it was rinsed with deionized water and dried.

[0051] S5. Fabrication of N-face electrode A negative photoresist is spin-coated onto the surface of the N-type GaAs contact layer. After exposure and development, an electrode grid pattern is formed. Then, Pd, Ge, Au, Ag, and Au are sequentially deposited by electron beam evaporation. The thicknesses of Pd, Ge, Au, Ag, and Au are 10nm~50nm, 10nm~50nm, 10nm~50nm, 3μm~8μm, and 10nm~50nm, respectively. Finally, the N-face electrode 5 is formed by organic lift-off.

[0052] S6. Preparation of antireflective coating A selective etching solution of citric acid, hydrogen peroxide, and water was used as the substrate. Using the N-side electrode 5 as a mask, the N-type GaAs contact layer outside the N-side electrode 5 was selectively etched away at 40°C, exposing the GaInP top cell. Then, TiO2 and Al2O3 were sequentially deposited as anti-reflection films 6 on the exposed top cell surface. The deposition thicknesses of TiO2 and Al2O3 were 1 nm to 20 nm and 5 nm to 100 nm, respectively. The anti-reflection film 6 on the main grid lines of the N-side electrode was removed by overlay etching to facilitate subsequent soldering and testing.

[0053] S7, same side P-side electrode 3 leads out Photoresist is spin-coated onto the light-receiving surface of the antireflective film 6, and the P-side electrode window area is etched using photolithography. Selective wet deep etching is performed using alternating solutions of hydrochloric acid, phosphoric acid, and hydrogen peroxide to vertically remove the antireflective film 6 and epitaxial layer 4 corresponding to the P-side electrode window area until the Au layer in the P-side electrode 3 is exposed. This application utilizes the corrosion resistance of Au, allowing etching to precisely stop at the Au surface, significantly simplifying the same-side electrode process, significantly improving yield, and eliminating the need for expensive ICP dry etching equipment, thus avoiding damage to the active area of ​​the battery from plasma.

[0054] S8: Debonding and Alloying Annealing The epitaxial wafer obtained in step S7 is heated until the temporary bonding adhesive fails at a temperature of 150℃~250℃ to remove the rigid substrate and residual adhesive from the wafer source, resulting in a flexible thin-film solar cell. The obtained flexible thin-film solar cell is placed in an alloy furnace tube and annealed at 200℃~300℃ for 10min~60min. This step simultaneously achieves: low-resistance ohmic contact is formed on the N-side electrode 5 and low-resistance ohmic contact is formed on the P-side electrode 3, and due to the symmetrical stacked design of the composite flexible substrate 2, the solar cell exhibits no warping or cracking after annealing.

[0055] S9: Cutting The solar cell is separated into individual solar cells by blade cutting. Since the epitaxial layer 4 has been removed in advance by wet deep etching at the cutting path, the blade only cuts the composite flexible substrate 2 during cutting without exposing any pn junction. Therefore, there is no need to perform end face passivation treatment to complete the device fabrication.

[0056] It is worth noting that the thicknesses of the protective layer 1, composite flexible substrate 2, P-side electrode 3, epitaxial layer 4, N-side electrode 5, and antireflection film 6 shown in the accompanying drawings are merely examples and do not represent their actual thicknesses. Furthermore, the actual proportions between the protective layer 1, composite flexible substrate 2, P-side electrode 3, epitaxial layer 4, N-side electrode 5, and antireflection film 6 are not as shown in the drawings and are for reference only.

[0057] 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.

[0058] 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 thin-film solar cell based on in-situ evaporation of a flexible substrate, characterized in that, include: Provide a GaAs substrate, and grow an epitaxial layer for a flip-chip multi-junction solar cell on the GaAs substrate; A P-side electrode and a composite flexible substrate are sequentially deposited on the epitaxial layer. The composite flexible substrate includes a first metal support layer, a first adhesion transition layer, an insulating stress regulation layer, a second adhesion transition layer, and a second metal support layer, which are sequentially stacked along the growth direction. The first metal support layer and the second metal support layer have the same thickness and are both made of Ni. The first adhesion transition layer and the second adhesion transition layer have the same thickness and are both made of Ti. The insulating stress regulation layer is made of SiO2. A rigid substrate is provided to temporarily bond a composite flexible substrate to the rigid substrate; Remove the GaAs substrate and fabricate an N-side electrode and an antireflection film on the front side of the epitaxial layer; A P-side electrode window is etched on the anti-reflection film, and the P-side electrode is exposed by wet deep etching, thus enabling the P-side electrode to be led out on the same side. The temporary substrate is removed and alloying annealing is performed to complete the fabrication of the thin-film solar cell.

2. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 1, characterized in that: The thickness of the first metal support layer and the second metal support layer is 5μm~30μm, the thickness of the first adhesive transition layer and the second adhesive transition layer is 30nm~100nm, and the thickness of the insulating stress regulating layer is 1μm~5μm.

3. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 2, characterized in that: The insulating stress-regulating layer was prepared by ion-assisted electron beam evaporation (IBE) deposition at a temperature of 25°C to 50°C and an evaporation rate of 1 Å / s to 10 Å / s. The oxygen partial pressure within the electron beam evaporation stage was 2 × 10⁻⁶. -2 Pa, with Ar as the ion source + The ion energy was set to 100 eV, and the current density was 0.1 mA / cm². 2 -0.5mA / cm 2 .

4. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 2, characterized in that: The first and second metal support layers are prepared by electron beam evaporation with a plating rate of 10 Å / s to 30 Å / s; the first and second adhesion transition layers are also prepared by electron beam evaporation with a plating rate of 1 Å / s to 10 Å / s.

5. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 1, characterized in that: The P-side electrode consists of an Au layer and an Ag layer, with thicknesses of 10nm~100nm and 3μm~5μm, respectively. The Au layer is located close to the epitaxial layer.

6. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 1, characterized in that: The process of etching a P-side electrode window on the antireflective film and exposing the P-side electrode by wet deep etching includes: spin-coating photoresist on the light-receiving surface, photolithographically etching the P-side electrode window area, and then using a mixture of hydrochloric acid solution, phosphoric acid and hydrogen peroxide solution alternately for selective wet deep etching to vertically remove the antireflective film and epitaxial layer corresponding to the P-side electrode window area until the Au layer in the P-side electrode is exposed.

7. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 1, characterized in that: The epitaxial layer includes an N-type GaAs contact layer, a GaInP top cell, a first tunnel junction, a GaAs middle cell, a second tunnel junction, a buffer layer, an InGaAs bottom cell, and a P-type InGaAs contact layer stacked sequentially along the growth direction. A P-side electrode and a composite flexible substrate are then deposited sequentially on the P-type InGaAs contact layer.

8. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 7, characterized in that: The preparation of the N-side electrode and antireflection film includes: spin-coating negative photoresist on the surface of the N-type GaAs contact layer, forming an electrode grid pattern after exposure and development, and then sequentially depositing Pd, Ge, Au, Ag, and Au using electron beam evaporation, with the thicknesses of Pd, Ge, Au, Ag, and Au being 10nm~50nm, 10nm~50nm, 10nm~50nm, 3μm~8μm, and 10nm~50nm, respectively, followed by organic lift-off to form the N-side electrode; Then, using a mixed solution of citric acid, hydrogen peroxide, and water, and with the N-side electrode as a mask, the N-type GaAs contact layer outside the N-side electrode is selectively etched away to expose the GaInP top cell. TiO2 and Al2O3 are then sequentially deposited on the exposed top cell surface as antireflection films, with the thicknesses of TiO2 and Al2O3 being 1 nm to 20 nm and 5 nm to 100 nm, respectively.

9. The method for fabricating a thin-film solar cell based on an in-situ evaporated flexible substrate as described in claim 1, characterized in that: After the composite flexible substrate is prepared, a protective layer is deposited on the surface of the second metal support layer. The material of the protective layer is Au, and the thickness is 10nm~100nm.

10. This application also provides a thin-film solar cell based on in-situ evaporation of a flexible substrate, characterized in that: It is prepared by the preparation method according to any one of claims 1-9.

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