Preparation method of flexible multi-junction solar cell

Abstract

The invention discloses a preparation method of a flexible multi-junction solar cell, which is used for preparing a GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer in the same reactor and improving the quality of a Ge bottom cell of the multi-junction solar cell. The method comprises the following steps: 1, introducing high-temperature flushing gas into a reactor cavity for cleaning; 2, epitaxially growing a buffer layer and a sacrificial layer on the GaAs substrate in sequence; step 3, growing a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer on the sacrificial layer in an inverted manner; 4, manufacturing a back electrode on the epitaxial layer of the multi-junction solar cell; 5, preparing a flexible thin film substrate on the epitaxial layer of the multi-junction solar cell; 6, removing the sacrificial layer, and separating the GaAs substrate from the epitaxial layer of the multi-junction solar cell; and 7, preparing an antireflection film and a front electrode on one side, far away from the flexible thin film substrate, of the epitaxial layer of the multi-junction solar cell to obtain the flexible multi-junction solar cell.

Description

Technical Field

[0001] This invention belongs to the field of solar cell technology, specifically relating to a method for preparing a flexible multi-junction solar cell. Background Technology

[0002] High-efficiency GaAs-based solar cells have been widely used in spacecraft. To further improve the conversion efficiency and power-to-weight ratio of solar cells, the industry has developed GaInP / GaAs / Ge triple-junction solar cells based on GaAs single-junction solar cells. However, in this triple-junction structure, GaAs, as the intermediate cell, has a low output current density, becoming a bottleneck for current matching and affecting overall performance. Replacing the intermediate cell material from GaAs with GaInAs, which has a lower In content, not only increases the output current density of the intermediate cell while minimizing lattice mismatch, but also allows for the fabrication of intermediate cells with varying In content to match solar spectra under different conditions, thereby effectively improving the overall conversion efficiency of the device. Therefore, GaInP / GaInAs / Ge is a more ideal triple-junction solar cell structure than GaInP / GaAs / Ge.

[0003] Currently, the most widely used space multi-junction solar cells are rigid GaInP / GaInAs / Ge triple-junction solar cells fabricated on Ge substrates. Due to the heavy weight of Ge substrates and the complex mechanical structures required for the deployment and retraction of the panels, rigid GaInP / GaInAs / Ge triple-junction solar cells have a relatively low power-to-weight ratio. Furthermore, lightweight flexible solar cells are bendable and can be fabricated into flexible solar panels, simplifying the associated mechanical structures and helping to reduce satellite launch costs. Therefore, flexibility is an important development direction for current space solar cells.

[0004] Existing methods for realizing flexible GaInP / GaInAs / Ge multi-junction solar cells typically involve grinding and etching the Ge substrate to thin it after device fabrication, aiming for lightweight and flexibility. This process not only consumes expensive Ge substrates but is also complex, has limited thinning thickness, and generates excess Ge cell current due to the thicker Ge sub-cells (i.e., the bottom cell). This excess current cannot be utilized and is converted into heat, causing the device to heat up. In practical applications, prolonged heat accumulation reduces the solar cell's conversion efficiency.

[0005] In addition, existing epitaxial layer lift-off techniques typically involve bonding or hot-pressing commercial flexible substrates onto the epitaxial layer, which can easily lead to gaps and stress imbalances between the epitaxial layer and the flexible substrate, reducing the thermal conductivity and mechanical reliability of flexible solar cells.

[0006] Epitaxial growth of Ge-based bottom cells is considered an ideal method for realizing flexible GaInP / GaInAs / Ge multi-junction solar cells. However, epitaxial growth of Ge-based bottom cells faces the challenge of the "memory effect." If III-V and IV materials are epitaxially grown in the same reactor chamber, some reactive gases and deposited materials remain in the chamber, causing cross-contamination between the III-V and IV materials and altering the doping characteristics of the epitaxial layer. For example, residual As and P elements can contaminate the Ge bottom cell during epitaxial growth, turning a p-type doped Ge epitaxial layer into an n-type doped layer. Similarly, residual Ge elements can contaminate the GaInP top cell and GaInAs middle cell in the next epitaxial growth cycle, causing additional n-type doping. Using multi-chamber growth techniques increases device fabrication costs and significantly reduces production efficiency.

[0007] To reduce the cost of space-use solar cells while improving the power-to-weight ratio and reliability of devices, a novel fabrication process for lightweight and flexible GaInP / GaInAs / Ge multi-junction solar cells is urgently needed. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for preparing flexible multi-junction solar cells, preparing GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafers in the same reactor, improving the quality of Ge base cells in multi-junction solar cells, and producing lightweight flexible GaInP / GaInAs / Ge solar cells.

[0009] To solve the above-mentioned technical problems, the embodiments of the present invention adopt the following technical solutions: A method for fabricating a flexible multi-junction solar cell includes the following steps: Step 1: Introduce high-temperature flushing gas into the reactor cavity to clean it; Step 2: In the reactor cavity, a buffer layer and a sacrificial layer are epitaxially grown sequentially on a GaAs substrate using an epitaxial growth process; Step 3: In the reactor cavity, an epitaxial growth process is used to grow a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer on the sacrificial layer inverted, forming a GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer. Step 4: Remove the GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer from the reactor cavity and fabricate the back electrode on the multi-junction solar cell epitaxial layer; Step 5: Fabricate a flexible thin film substrate on the epitaxial layer of the multi-junction solar cell with the back electrode, so that the flexible thin film substrate and the epitaxial layer of the multi-junction solar cell form an integral structure. Step 6: Use a wet etching process to remove the sacrificial layer and separate the GaAs substrate from the epitaxial layer of the multi-junction solar cell; Step 7: Prepare an antireflection film and a front electrode on the side of the epitaxial layer of the multi-junction solar cell away from the flexible thin film substrate to obtain a flexible multi-junction solar cell.

[0010] As a preferred example, in step 1, the temperature of the rinsing gas is 400–650°C; the rinsing gas is hydrogen or hydrogen chloride gas.

[0011] As a preferred example, in step 2, the buffer layer material is GaAs or GaInAs, and the sacrificial layer material is AlAs, AlInP, or AlGaAs.

[0012] As a preferred example, step 3 specifically includes: epitaxially growing a front contact layer, a top cell, a first tunnel junction, a middle cell, a second tunnel junction, and a bottom cell sequentially on the surface of the sacrificial layer, wherein the top cell is a GaInP sub-cell, the middle cell is a GaInAs sub-cell, and the bottom cell is a Ge sub-cell.

[0013] As a preferred example, in step 3, the epitaxial growth of the bottom cell is performed using a low-temperature process, wherein the low-temperature range is 550–620°C; and intermittent doping technology is used to adjust the doping concentration of the bottom cell to achieve a concentration of 10. 23 Orders of magnitude of p-type doping in the substrate of the bottom cell.

[0014] As a preferred example, the method for fabricating the flexible multi-junction solar cell further includes: step 8: polishing the GaAs substrate separated in step 5, and using it as a new GaAs substrate to repeatedly fabricate other flexible multi-junction solar cells.

[0015] As a preferred example, in step 8, the same GaAs substrate is reused more than 20 times.

[0016] As a preferred example, in step 5, a flexible thin film substrate is prepared on the epitaxial layer of a multi-junction solar cell using a spin coating method; the flexible thin film substrate material is an organic polymer.

[0017] As a preferred example, in step 5, the thickness of the flexible thin film substrate is 10 to 50 micrometers, and it is prepared by a layered spin coating and segmented curing process. An adhesion promoter is coated on the surface of the epitaxial layer before spin coating.

[0018] As a preferred example, the thickness of the bottom battery is 5 micrometers.

[0019] Compared with existing technologies, the method for preparing flexible multi-junction solar cells of the present invention can prepare GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafers in the same reactor, improve the quality of Ge substrate cells in multi-junction solar cells, and produce lightweight flexible GaInP / GaInAs / Ge solar cells. An embodiment of the present invention provides a method for preparing flexible multi-junction solar cells, comprising the following steps: Step 1: Introducing high-temperature flushing gas into the reactor cavity to clean the reactor cavity; Step 2: In the reactor cavity, using an epitaxial growth process, sequentially growing a buffer layer and a sacrificial layer on a GaAs substrate; Step 3: In the reactor cavity, using an epitaxial growth process, invertingly growing a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer on the sacrificial layer to form a GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer; Step 4: From... The GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer is removed from the reactor chamber, and a back electrode is fabricated on the multi-junction solar cell epitaxial layer. Step 5: A flexible thin film substrate is fabricated on the multi-junction solar cell epitaxial layer with the back electrode, so that the flexible thin film substrate and the multi-junction solar cell epitaxial layer form an integral structure. Step 6: A wet etching process is used to remove the sacrificial layer and separate the GaAs substrate from the multi-junction solar cell epitaxial layer. Step 7: An anti-reflection film and a front electrode are fabricated on the side of the multi-junction solar cell epitaxial layer away from the flexible thin film substrate to obtain a flexible multi-junction solar cell. In the above method, the introduction of high-temperature rinsing gas into the reactor chamber can accelerate the evaporation of residual materials, which facilitates the subsequent fabrication of GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafers in the same reactor and improves the quality of the bottom cell. Attached Figure Description

[0020] To more clearly illustrate the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only a part of the embodiments of the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0021] Figure 1 A schematic diagram of the structure of a flexible multi-junction solar cell prepared by the method of an embodiment of the present invention; Figure 2 A schematic diagram of the structure of a GaInP / GaInAs / Ge multijunction solar cell epitaxial wafer prepared by the method of an embodiment of the present invention; Figure 3 This is a flowchart of a method according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure corresponding to step 2 in the method of this embodiment of the invention; Figure 5 This is a schematic diagram of the structure corresponding to step 3 in the method of this embodiment of the invention; Figure 6This is a schematic diagram of the structure corresponding to step 4 in the method of this embodiment of the invention; Figure 7 This is a schematic diagram of the structure corresponding to step 5 in the method of this embodiment of the invention; Figure 8 This is a schematic diagram of the structure corresponding to step 6 in the method of this embodiment of the invention; Figure 9 This is a schematic diagram of the structure corresponding to step 7 in the method of this embodiment of the invention; Figure 10 This is a schematic diagram of the GaAs substrate structure reused in step 8 of the method of this embodiment of the invention.

[0022] In the figure: GaAs substrate 100, buffer layer 200, sacrificial layer 300, GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer 400, front contact layer 410, top cell 420, first tunnel junction 430, middle cell 440, second tunnel junction 450, bottom cell 460, back electrode 500, flexible thin film substrate 600, anti-reflection film 700, front electrode 800. Detailed Implementation

[0023] To make the content of this invention clearer, the following description, in conjunction with the accompanying drawings, further illustrates the invention. This invention is not limited to this specific embodiment. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0024] like Figure 3 As shown, a method for fabricating a flexible multi-junction solar cell according to an embodiment of the present invention includes the following steps: Step 1: Introduce high-temperature flushing gas into the reactor chamber to clean it.

[0025] Step 2: In the reactor cavity, a buffer layer 200 and a sacrificial layer 300 are epitaxially grown sequentially on a GaAs substrate 100 using an epitaxial growth process.

[0026] Step 3: In the reactor cavity, an epitaxial growth process is used to grow a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer 400 on the sacrificial layer 300 inverted, forming a GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer.

[0027] Step 4: Remove the GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer from the reactor cavity, and fabricate a back electrode 500 on the multi-junction solar cell epitaxial layer 400. Preferably, the back electrode 500 is prepared using PVD (physical vapor deposition) and electroplating processes.

[0028] Step 5: A flexible thin film substrate 600 is fabricated on the multi-junction solar cell epitaxial layer 400 on which the back electrode 500 is fabricated, so that the flexible thin film substrate 600 and the multi-junction solar cell epitaxial layer 400 form an integral structure.

[0029] Step 6: Use a wet etching process to remove the sacrificial layer 300 and separate the GaAs substrate 100 from the multi-junction solar cell epitaxial layer 400.

[0030] Step 7: Prepare an antireflective film 700 and a front electrode 800 on the side of the epitaxial layer 400 of the multi-junction solar cell away from the flexible thin film substrate 600 to obtain a flexible multi-junction solar cell. Preferably, the antireflective film 700 is prepared using PVD (physical vapor deposition), chemical vapor deposition, or atomic layer deposition. The preparation process of the front electrode includes PVD, electroplating, screen printing, or inkjet printing.

[0031] In step 1, a high-temperature flushing gas is introduced into the reactor chamber. The temperature of the flushing gas is 400–650°C, and the gas flow rate is 500–3500 sccm. The flushing gas is either hydrogen or hydrogen chloride. Introducing the high-temperature flushing gas into the reactor chamber accelerates the evaporation of residual materials (especially As). The introduced gas physically removes the materials remaining in the reactor chamber after the previous reaction. These residual materials specifically include As, P, and Ge. Without flushing, the residual substances from the previous reaction will evaporate during cell epitaxy, altering the doping characteristics of the materials and reducing device performance. In extreme cases, if there is excessive As residue, the epitaxially grown p-type Ge layer will transform into an n-type layer, causing the Ge-based cell to fail.

[0032] Preferably, step 1 further includes: after cleaning the reactor cavity, coating the reactor cavity with a Ge film. Specifically, after cleaning the reactor cavity, isobutylgermanium precursor is introduced, and Ge material is grown without an epitaxial substrate to prepare a 2-5 micrometer Ge coating, i.e., a Ge film, within the reactor cavity. The Ge film coating on the reactor cavity helps mitigate the memory effect of As and P elements. Within the process window of this invention, Ge material evaporation is minimal. At atmospheric pressure, As sublimates at 615 degrees Celsius, while Ge does not directly sublimate at atmospheric pressure, with a melting point of approximately 938°C and a boiling point of approximately 2833°C. Under the low-pressure environment (between 40 and 110 mbar) and temperature window of this invention, no significant Ge material evaporation is observed.

[0033] Preferably, in step 2, the buffer layer 200 is made of GaAs or GaInAs, and the sacrificial layer 300 is made of AlAs, AlInP, or AlGaAs.

[0034] The epitaxial growth process is one of metal-organic vapor deposition (MOCVD), molecular beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE). In this embodiment, the epitaxial layer of the device is grown in the same reactor chamber. Using different growth equipment (i.e., different reactors) in different process stages can effectively solve the memory effect problem of As material, but it also brings high equipment costs. If multiple reactors are used to complete different process stages, at least two machines are required, resulting in additional equipment costs. In addition, the yield of multi-reactor fabrication processes is lower than that of single-reactor fabrication processes. Before device growth, time is required to evacuate the reactor chamber, followed by heating to the process temperature before device growth. After growth, time is required to restore from a vacuum (or low pressure) state to an atmospheric pressure state. The method of this embodiment grows the device in the same reactor, which is significantly less expensive than multi-reactor fabrication processes.

[0035] Preferably, step 3 specifically includes: epitaxially growing a front contact layer 410, a top cell 420, a first tunnel junction 430, a middle cell 440, a second tunnel junction 450, and a bottom cell 460 sequentially on the surface of the sacrificial layer 300. The top cell 420 is a GaInP sub-cell, the middle cell 440 is a GaInAs sub-cell, and the bottom cell 460 is a Ge sub-cell. When epitaxially growing the bottom cell 460, a low-temperature process is used, with the low temperature range being 550–620°C; and intermittent doping technology is used to adjust the doping concentration of the bottom cell 460 to achieve a concentration of 10. 23 Orders of magnitude of p-type doping in the substrate of the bottom cell.

[0036] Specifically, under the conditions of a reactor chamber pressure of 40–110 mbar and a temperature of 550–720 °C, GaAs material buffer layer 200, AlAs material sacrificial layer 300, GaAs front contact layer 410, n-type doped AlInP window layer, n-type doped GaInP top cell emitter layer, p-type doped GaInP top cell base layer, p-type doped AlGaInP back field layer, p++ type doped AlGaAs first tunnel junction layer, n++ type doped GaInP first tunnel junction layer, n-type doped GaInP window layer, n-type doped GaAs emitter layer, p-type doped GaInAs base layer, p-type doped AlGaAs back field layer, p++ type doped GaAs second tunnel junction layer, n++ type doped GaAs second tunnel junction layer, and n-type doped GaInP window layer are epitaxially grown sequentially on the surface of GaAs substrate 100. After epitaxy of the III-V group materials, the reactor chamber is cooled to 550–620°C to reduce the volatilization rate of residual As and P elements in the reactor chamber. Isobutylgermanium gas is then introduced to grow an n-type doped Ge emitter layer without introducing a dopant gas. Because the memory effect of As and P elements causes n-type doping of Ge, an n-type doped Ge emitter layer can be obtained without introducing an n-type dopant gas. Subsequently, a p-type doped Ge base layer for the Ge sub-cell (i.e., the bottom cell) is grown at high speed to dilute the contamination of the Ge epitaxial layer by As and P elements. P-type dopant gas is intermittently introduced to achieve low-concentration p-type doping of the Ge sub-cell (i.e., the bottom cell) base layer.

[0037] The epitaxial growth temperature of conventional Ge materials is above 650℃. In the above method, by appropriately lowering the growth temperature of the Ge subcell (i.e., the bottom cell) to 550–620℃, the sublimation phenomenon of As material is suppressed while ensuring the crystal quality of the Ge epitaxial layer, thus alleviating the memory effect of As element during the growth of Ge subcell.

[0038] Existing high-efficiency solar cells rely on a structure of a highly doped n-type emitter layer and a low-doped p-type substrate. However, due to the precision limitations of existing epitaxial equipment, p-type doped Ge epitaxial layers grown using conventional techniques such as reducing dopant gas flow rates still have relatively high doping concentrations, thus limiting the efficiency of epitaxially grown Ge solar cells. Typically, the doping concentration of the p-layer (base layer) should be an order of magnitude lower than that of the n-layer (emitter layer) to ensure high efficiency. However, currently, Ge cells fabricated using epitaxial processes can only achieve a lower doping concentration in the p-layer than in the n-layer; the concentrations are relatively close, resulting in limited efficiency. This invention addresses this by intermittently introducing a low-flow-rate p-type doping gas during the growth of the Ge sub-cell (i.e., the base cell) substrate and performing annealing after growth to allow dopant diffusion, thereby achieving a p-type Ge epitaxial layer with a lower doping concentration than conventional processes.

[0039] To avoid the equipment and time costs associated with using multiple reactors for epitaxial growth, this invention uses a single reactor chamber to grow III-V and IV group epitaxial layers. Regarding the memory effect problem that occurs when epitaxially growing GaInP / GaInAs / Ge multi-junction solar cells in the same reactor, this invention reduces the As and P element memory effect during the epitaxial growth of Ge sub-cells (i.e., bottom cells) through various process methods. In this invention, the reactor is pre-washed at high temperature before epitaxial growth to reduce residual material from the previous reaction. During the epitaxial growth of Ge sub-cells (i.e., bottom cells), the volatilization of As and P elements is reduced through low-temperature epitaxy, while the contamination of the Ge epitaxial layer by As and P elements is suppressed by increasing the epitaxial growth rate of Ge material. This invention also utilizes an intermittent doping process to achieve a concentration of 10... 23 The order-of-magnitude p-type doping of the Ge sub-cell substrate overcomes the drawback of the difficulty in controlling the doping concentration in traditional epitaxial Ge processes, and achieves a Ge sub-cell epitaxial layer with highly controllable quality.

[0040] Preferably, the method further includes: Step 8: Polishing the GaAs substrate 100 separated in Step 5, and using it as a new GaAs substrate for repeated fabrication of other flexible multi-junction solar cells. This preferred method reuses the GaAs substrate 100 separated in Step 5, improving device utilization. Preferably, the same GaAs substrate 100 is reused more than 20 times, for example, 20 times, 28 times, 35 times, etc.

[0041] Preferably, in step 5, a flexible thin film substrate 600 is prepared on the epitaxial layer 400 of the multi-junction solar cell using a spin-coating method; the flexible thin film substrate 600 is made of an organic polymer. Preferably, the organic polymer is polyimide.

[0042] Preferably, in step 5, the flexible thin film substrate 600 has a thickness of 10–50 micrometers, is prepared using a layered spin-coating and segmented curing process, and an adhesion promoter is coated onto the surface of the epitaxial layer before spin-coating. The flexible thin film substrate is prepared on the epitaxial layer of a multi-junction solar cell with a back electrode, forming an integrated structure with the multi-junction solar cell epitaxial layer. Specifically, an adhesion promoter is coated onto a clean and clear multi-junction solar cell epitaxial layer. A polyimide solution is spin-coated at 2000 rpm for 60 seconds each time, for three spin-coatings. Then, it is dried at 90–120°C for 2–5 minutes, then heated to 150–180°C and dried for another 5–10 minutes. Finally, it is slowly heated to 260°C under nitrogen and held for 1 hour to obtain a flexible thin film substrate integrated with the multi-junction solar cell epitaxial layer. Polyimide undergoes significant volume shrinkage during curing. If a thick coating is applied in one step, the surface layer will restrict the shrinkage of the underlying layer after curing, generating huge internal stress, leading to cracks or warping of the film. Layered spin coating allows each layer to shrink freely at a smaller thickness, resulting in uniform stress distribution. A common method for curing polyimide films is to directly raise the ambient temperature to the temperature required for imidization. This can lead to incomplete evaporation of moisture and solvents within the polyimide film, causing blistering and peeling. Segmented curing allows for effective evaporation of moisture and solvents from the polyimide film, improving yield.

[0043] This method directly fabricates a flexible thin-film substrate on the epitaxial layer of a multi-junction solar cell, improving the integration between the flexible thin-film substrate and the epitaxial layer. Compared to traditional bonding flexible substrate processes, it reduces the gaps between the epitaxial layer and the flexible thin-film substrate, thus improving the device reliability and heat dissipation capacity of the multi-junction solar cell. Traditional bonding methods involve attaching pre-existing flexible thin-film tape to the epitaxial wafer or pressing it using hot pressing. Compared to spin coating, both adhesive and hot pressing methods struggle to achieve the same uniform film thickness, are more prone to creating gaps between the film and the device, thereby reducing reliability and thermal conductivity. Furthermore, the pressure applied during hot pressing can cause the epitaxial wafer of the multi-junction solar cell to break. Compared to traditional adhesive bonding flexible substrate processes, this invention directly fabricates a flexible substrate on the epitaxial layer of the multi-junction solar cell, improving the integration between the flexible thin-film substrate and the epitaxial layer, reducing the gaps between the epitaxial layer and the flexible thin-film substrate, and thus improving the device reliability and heat dissipation capacity of the multi-junction solar cell.

[0044] Preferably, in step 6, the wet etching process can use a hydrofluoric acid solution with a mass concentration of 5-20%. Applying external stress to the flexible thin film substrate 600 accelerates the etching and peeling speed of the sacrificial layer 300. There are various methods for applying external stress in the prior art, such as applying stress by a robotic arm gripping the flexible thin film substrate.

[0045] In step 7, a front electrode is fabricated on the top cell side of the epitaxial layer of a multi-junction solar cell using photolithography, physical vapor deposition, or other processes. Subsequently, an anti-reflection film is prepared using physical vapor deposition, chemical vapor deposition, or atomic layer deposition.

[0046] The method described in the above embodiments grows a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer on a GaAs substrate, and separates the multi-junction solar cell epitaxial layer from the GaAs substrate by setting a sacrificial layer between the GaAs substrate and the multi-junction solar cell epitaxial layer, transferring the multi-junction solar cell epitaxial layer to a flexible thin film substrate. While obtaining a flexible multi-junction solar cell device, the GaAs substrate can be recycled, which greatly reduces the fabrication cost of the flexible GaInP / GaInAs / Ge multi-junction solar cell device.

[0047] like Figure 1 As shown, the flexible multi-junction solar cell prepared by the above preparation method includes, from bottom to top, a flexible thin film substrate 600, a back electrode 500, a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer 400, an anti-reflection film 700, and a front electrode 800.

[0048] like Figure 2 The diagram shows the structure of the GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer during the fabrication of flexible multi-junction solar cells using the above method. In this method, the GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer adopts a triple-junction GaInP / GaInAs / Ge structure, obtained through inverted epitaxial growth. Specifically, a buffer layer 200, a sacrificial layer 300, a front contact layer 410, a top cell 420, a first tunnel junction 430, a middle cell 440, a second tunnel junction 450, and a bottom cell 460 are sequentially epitaxially grown on the surface of a GaAs substrate 100.

[0049] Preferably, the front contact layer 410 is made of GaAs. The GaInP top cell 420 comprises an n-type doped AlInP window layer, an n-type doped GaInP top cell emitter layer, a p-type doped GaInP top cell base layer, and a p-type doped AlGaInP back field layer. The AlInP window layer has a thickness of 25 nm and a doping concentration of 5 × 10⁻⁶. 18 cm - ³; The GaInP emitter layer has a thickness of 70 nm and a doping concentration of 3 × 10⁻⁶. 18 cm - ³; The GaInP substrate thickness is 1300 nm, and the doping concentration is 1×10⁻⁶. 17 cm - ³; The AlGaInP back field layer is 100 nm thick and has a doping concentration of 4 × 10⁻⁶.18 cm - ³.

[0050] Preferably, the first tunneling junction 430 comprises a p++ type doped AlGaAs first tunneling junction layer and an n++ type doped GaInP first tunneling junction layer. The p++ AlGaAs layer has a thickness of 25 nm and a doping concentration of 6 × 10⁻⁶. 19 cm - ³; n++ GaInP layer thickness is 20 nm, doping concentration is 1×10 19 cm - ³.

[0051] Preferably, the GaInAs solar cell 440 comprises an n-type doped GaInP window layer, an n-type doped GaInAs emitter layer, a p-type doped GaInAs substrate, and a p-type doped AlGaAs back surface layer. The GaInP window layer has a thickness of 20 nm and a doping concentration of 1 × 10⁻⁶. 18 cm - ³; The GaInAs emitter layer has a thickness of 140 nm and a doping concentration of 1×10⁻⁶. 18 cm - ³; The GaInAs substrate thickness is 3000 nm and the doping concentration is 1×10⁻⁶. 17 cm - ³; The AlGaAs backfield layer has a thickness of 150 nm and a doping concentration of 1×10⁻⁶. 18 cm - ³.

[0052] Preferably, the second tunneling junction 450 comprises a p++ type doped GaAs second tunneling junction layer and an n++ type doped GaAs second tunneling junction layer. The p++ GaAs layer has a thickness of 15 nm and a doping concentration of 8 × 10⁻⁶. 19 cm - ³; n++ GaAs layer thickness is 15nm, doping concentration is 1×10 19 cm - ³.

[0053] Preferably, the bottom cell 460 comprises an n-type doped GaInP window layer, an n-type doped Ge emitter layer, and a p-type doped Ge substrate. The GaInP window layer has a thickness of 20 nm and a doping concentration of 1 × 10⁻⁶. 18 cm - ³; The Ge emitter layer has a thickness of 200 nm and a doping concentration of 2 × 10⁻⁶. 18 cm - ³; Ge substrate thickness is 1000 nm, doping concentration is 5 × 10⁻⁶ 17 cm - ³.

[0054] The multi-junction solar cells with the above structure can achieve more perfect lattice matching. GaInAs material with added In has a smaller lattice constant compared to GaAs, resulting in better lattice matching with Ge. Smaller lattice mismatch can introduce less lattice mismatch stress during the epitaxial growth of Ge subcells, leading to devices with higher crystal structure quality, reduced carrier recombination within the device (i.e., reduced internal losses), and improved efficiency.

[0055] The lattice mismatch between the individual sub-cells of the multi-junction solar cell with the above structure is less than 0.5%. The thickness of the Ge sub-cell (i.e., the bottom cell) is less than 5 micrometers. The method in this embodiment uses epitaxial growth of the Ge sub-cell, which allows for precise control of the Ge sub-cell thickness, thereby appropriately reducing the excess current of the Ge sub-cell, effectively controlling device heating, mitigating the reduction in solar cell conversion efficiency caused by overheating, and improving the stability of the multi-junction solar cell in space applications. This embodiment of the invention obtains the Ge sub-cell through epitaxial growth without using a Ge substrate, avoiding the waste of Ge substrate in existing flexible multi-junction solar cell processes based on thinning the Ge substrate, improving cost-effectiveness while also having ecological benefits.

[0056] Here is an example.

[0057] The present invention discloses a method for fabricating a flexible multi-junction solar cell, comprising the following steps: Step 1: Set the MOCVD reactor chamber pressure to 100 mbar and the temperature to 650℃. Introduce hydrogen gas into the reactor at a flow rate of 500-3500 sccm and flush the reactor chamber with hydrogen gas for 4 hours to clean it.

[0058] Step 2, as follows Figure 4 As shown, in the reactor chamber after cleaning in step 1, a GaAs substrate is provided. A GaAs buffer layer and an AlAs sacrificial layer are epitaxially grown on the GaAs substrate using an MOCVD process. Specifically, a 200 nm thick GaAs buffer layer is first epitaxially grown on the GaAs substrate using an MOCVD process, followed by a 25 nm thick AlAs sacrificial layer. In the MOCVD epitaxial process, the Group III MOCVD precursor is TMGa or TMAl, and the Group V MOCVD precursor is AsH3. The GaAs substrate can be a brand new substrate suitable for epitaxy or a reused substrate after polishing.

[0059] Step 3, as follows Figure 5As shown, in the reactor cavity, GaInP / GaInAs / Ge multi-junction solar cell epitaxial layers are grown on an AlAs sacrificial layer. Specifically, in the reactor cavity, a GaAs material front contact layer is first epitaxially grown using MOCVD, followed by the sequential epitaxial growth of a GaInP sub-cell (i.e., top cell), a p++ AlGaAs / n++ GaInP first tunnel junction, a GaInAs sub-cell (i.e., middle cell), a p++ GaAs / n++ GaAs second tunnel junction, and a window layer for a Ge sub-cell (i.e., bottom cell). Each sub-cell adopts an n-on-p structure. The top and middle cells are configured with a back surface field layer to improve carrier collection and a window layer to reduce surface recombination. The Group III MOCVD precursors are TMGa, TMAl, and TMIn, while the Group V MOCVD precursors are PH3 and AsH3. The n-type doping source is Si2H6, and the p-type doping source is DEZn. The growth temperature for the front contact layer and top cell was 720℃, the growth temperature for the p++ AlGaAs / n++ GaInP first tunnel junction was 600℃, the growth temperature for the middle cell was 650℃, and the growth temperature for the p++ GaAs / n++ GaAs second tunnel junction was 550℃. After the epitaxial growth of the III-V group materials was completed, the reactor chamber was cooled to 580℃ for the epitaxial growth of the emitter layer and substrate of the Ge bottom cell. Isobutylgermanium was used as the Ge precursor, and TMGa was used as the p-type doping source for the Ge bottom cell substrate. Due to the memory effect of As and P elements, n-type doping of the emitter layer of the Ge bottom cell could be achieved without an n-type doping source. During the epitaxial growth of the p-type substrate of the Ge bottom cell, intermittent doping was performed in a cycle of 15 seconds, with TMGa (trimethylgallium) introduced for 2 seconds and no TMGa introduced for the rest of the time. After epitaxy, annealing was performed at 620℃ to diffuse the doped Ga element and form a relatively uniform p-type doped profile.

[0060] Step 4, as follows Figure 6 As shown, a back electrode 500 is fabricated on the epitaxial layer 400 of a multi-junction solar cell.

[0061] Step 5, as follows Figure 7 As shown, an organic polymer flexible thin film substrate is prepared on the epitaxial layer of a multi-junction solar cell with a back electrode, so that the organic polymer flexible thin film substrate and the epitaxial layer form an integrated structure. Specifically, an adhesion promoter is coated on the epitaxial layer of the multi-junction solar cell, and a polyimide solution is spin-coated at a speed of 2000 rpm for 60 seconds each time, and the spin-coating is performed three times. Then, it is dried at 120°C for 2 minutes, then heated to 160°C and dried for another 10 minutes. Finally, it is slowly heated to 260°C under nitrogen and held at that temperature for 1 hour to obtain a flexible polyimide thin film substrate integrated with the epitaxial layer of the multi-junction solar cell.

[0062] Step 6, as follows Figure 8 As shown, a wet etching process is used to remove the sacrificial layer and separate the GaAs substrate from the epitaxial layer of the multi-junction cell. Specifically, the main active component of the wet etching solution is hydrofluoric acid, which has a high etching rate for the AlAs sacrificial layer but a low etching rate for the device layer, thus achieving selective etching of the sacrificial layer.

[0063] Step 7, as follows Figure 9 As shown, an antireflection film and a front electrode are fabricated on the side of the epitaxial layer of a multi-junction solar cell away from the flexible thin film substrate to obtain a flexible multi-junction solar cell.

[0064] Step 8, as follows Figure 10 As shown, the GaAs substrate stripped in step 6 is polished and reused for epitaxial growth in the flexible multi-junction solar cell fabrication process. Specifically, the novel GaAs substrate for epitaxy can be reused at least 20 times in the flexible multi-junction solar cell fabrication process.

[0065] This invention discloses a method for fabricating a flexible multi-junction solar cell. It grows Ge sub-cells via epitaxial growth without using a Ge substrate, and combines this with a wet etching epitaxial lift-off process to obtain a lightweight and flexible device. This avoids the waste of Ge substrate associated with existing flexible GaInP / GaInAs / Ge multi-junction solar cell processes, significantly reducing device fabrication costs while conserving valuable Ge resources. Based on the structure of a GaInP / GaInAs / Ge triple-junction solar cell, this invention precisely controls the thickness of the Ge sub-cell (i.e., the bottom cell) through epitaxial growth, reducing heat generation caused by excessive current in the Ge sub-cell and improving the long-term stability of the device in practical applications. This invention achieves a low-doped Ge sub-cell (i.e., bottom cell) substrate through intermittent doping, thereby improving the performance of the Ge sub-cell and increasing the device conversion efficiency. The main factor limiting the performance of epitaxially grown Ge sub-cells (i.e., bottom cells) is the difficulty in achieving a low-concentration p-type Ge substrate. This is primarily because the gas flow rate is low during Ge material epitaxy, and the flow rate of the doping gas is even lower than that of the Ge material gas. Existing equipment has limited precision in controlling gas flow, with a lower limit to the gas flow rate, making it difficult to achieve the ideal p-type Ge doping concentration. Intermittent doping involves intermittently introducing dopant gas, followed by annealing to allow the dopant to diffuse, forming a relatively uniform low-concentration doping layer. Intermittent doping improves the conversion efficiency of Ge sub-cells by achieving a lower concentration of p-doped layers.

[0066] This invention uses high-temperature gas to flush the reactor chamber before epitaxy (after each round of preparation, a certain amount of material remains in the reactor chamber, including various materials. The residual As material has a significant impact on device performance and needs to be flushed away). Then, Ge sub-cells are grown at high speed using a low-temperature process to suppress the memory effect, effectively alleviating the memory effect problem of growing GaInP / GaInAs / Ge multi-junction cells in the same reactor chamber.

[0067] This invention improves the surface quality of the GaAs substrate by incorporating a buffer layer between the sacrificial layer and the GaAs substrate, reducing the amount of polishing required before reuse and minimizing single-use losses. This increases the number of GaAs substrate reuses to over 20 times, further reducing device fabrication costs. Furthermore, this invention directly fabricates flexible substrates on the epitaxial layer of multi-junction solar cells, improving the integration between the flexible substrate and the epitaxial layer. Compared to traditional bonding flexible substrate processes, this reduces the gap between the epitaxial layer and the flexible substrate, enhancing the device reliability and heat dissipation capabilities of multi-junction solar cells.

[0068] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for fabricating a flexible multi-junction solar cell, characterized in that, Includes the following steps: Step 1: Introduce high-temperature flushing gas into the reactor cavity to clean it; Step 2: In the reactor cavity, an epitaxial growth process is used to sequentially grow a buffer layer (200) and a sacrificial layer (300) on a GaAs substrate (100). Step 3: In the reactor cavity, an epitaxial growth process is used to grow a GaInP / GaInAs / Ge multi-junction solar cell epitaxial layer (400) on the sacrificial layer (300) inverted, forming a GaInP / GaInAs / Ge multi-junction solar cell epitaxial wafer. Step 4: Remove the GaInP / GaInAs / Ge multijunction solar cell epitaxial wafer from the reactor cavity and fabricate a back electrode (500) on the multijunction solar cell epitaxial layer (400). Step 5: A flexible thin film substrate (600) is fabricated on the epitaxial layer (400) of the multi-junction solar cell with the back electrode (500), so that the flexible thin film substrate (600) and the epitaxial layer (400) of the multi-junction solar cell form an integral structure; Step 6: Use wet etching process to remove the sacrificial layer (300) and separate the GaAs substrate (100) from the multi-junction solar cell epitaxial layer (400); Step 7: Prepare an antireflection film (700) and a front electrode (800) on the side of the epitaxial layer (400) of the multi-junction solar cell away from the flexible thin film substrate (600) to obtain a flexible multi-junction solar cell.

2. The method for fabricating a flexible multi-junction solar cell according to claim 1, characterized in that, In step 1, the temperature of the rinsing gas is 400–650°C; the rinsing gas is hydrogen or hydrogen chloride gas.

3. The method for fabricating a flexible multi-junction solar cell according to claim 1, characterized in that, In step 2, the buffer layer (200) is made of GaAs or GaInAs, and the sacrificial layer (300) is made of AlAs, AlInP, or AlGaAs.

4. The method for fabricating a flexible multi-junction solar cell according to claim 1, characterized in that, Step 3 specifically includes: epitaxially growing a front contact layer (410), a top cell (420), a first tunnel junction (430), a middle cell (440), a second tunnel junction (450), and a bottom cell (460) sequentially on the surface of the sacrificial layer (300). The top cell (420) is a GaInP sub-cell, the middle cell (440) is a GaInAs sub-cell, and the bottom cell (460) is a Ge sub-cell.

5. The method for fabricating a flexible multi-junction solar cell according to claim 4, characterized in that, In step 3, when epitaxially growing the bottom cell (460), a low-temperature process is used to grow the bottom cell, and the low-temperature range is 550-620℃; and the doping concentration of the bottom cell (460) is adjusted using intermittent doping technology to achieve a concentration of 10. 23 Order-of-magnitude p-type doping in the substrate of the bottom cell.

6. The method for fabricating a flexible multi-junction solar cell according to claim 1, characterized in that, Also includes: Step 8: Polish the GaAs substrate (100) separated in Step 5 to serve as a new GaAs substrate for repeated fabrication of other flexible multi-junction solar cells.

7. The method for fabricating a flexible multi-junction solar cell according to claim 6, characterized in that, In step 8, the same GaAs substrate (100) is reused more than 20 times.

8. The method for fabricating a flexible multi-junction solar cell according to claim 1, characterized in that, In step 5, a flexible thin film substrate (600) is prepared on the epitaxial layer (400) of a multi-junction solar cell using a spin coating method; the flexible thin film substrate (600) is made of an organic polymer.

9. The method for fabricating a flexible multi-junction solar cell according to claim 8, characterized in that, In step 5, the thickness of the flexible thin film substrate (600) is 10-50 micrometers. It is prepared by layered spin coating and segmented curing process, and an adhesion promoter is coated on the surface of the epitaxial layer before spin coating.

10. The method for fabricating a flexible multi-junction solar cell according to claim 4, characterized in that, The thickness of the bottom battery (460) is 5 micrometers.

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