Method for simultaneously obtaining rigid solar cells and flexible solar cells using a single epitaxial process

Abstract

The application discloses a method for simultaneously obtaining rigid solar cells and flexible solar cells by one epitaxial process, and can obtain a high-performance rigid three-junction gallium arsenide solar cell and a high-performance flexible gallium arsenide three-junction / double-junction solar cell from a single germanium wafer through one epitaxial process. The single expensive epitaxial growth process can simultaneously produce two complete high-performance multi-junction cells, greatly reduces the epitaxial cost, and improves the material utilization efficiency and production efficiency. Since the substrate reuse mode is not used, the epitaxial substrate quality is excellent and controllable, and the quality and epitaxial cost are double optimized.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor manufacturing, and in particular to a method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process. Background Technology

[0002] Space solar cells typically employ III-V group multi-junction gallium arsenide (GaAs) solar cell structures, such as germanium-based GaInP / GaAs / Ge triple-junction cells, to achieve high conversion efficiency. On the other hand, flexible GaAs solar cells, due to their advantages of being bendable, lightweight, and easy to integrate, have shown great application potential in wearable devices, drones, portable power supplies, and other fields.

[0003] Currently, rigid and flexible solar cells are typically manufactured using separate process lines. The stripping process for flexible gallium arsenide (GaAs) solar cells is currently the most promising low-cost technology. The main method involves epitaxially forming a flip-chip triple-junction GaAs epitaxial wafer with a GaInAs / GaAs / GaInP triple-junction structure on the surface of a GaAs wafer, followed by device fabrication. The device fabrication process involves first preparing a flexible lower electrode substrate on the epitaxial surface, then exposing the edges of the epitaxial layer through etching, and then gradually separating the epitaxial layer with the lower electrode substrate using an immersion method to form a flexible wafer. The flexible wafer is then fixed onto a rigid temporary substrate for subsequent device fabrication. Theoretically, after the flexible wafer is separated from the rigid GaAs substrate, the GaAs substrate can be reused through polishing, thus achieving low cost. However, current technology requires etching the wafer edges before each separation of the flexible wafer, which can easily damage the GaAs edges, resulting in poor epitaxial quality at the edges after reuse. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a novel epitaxial structure and device fabrication method that can obtain a high-performance rigid triple-junction gallium arsenide solar cell and a high-performance flexible gallium arsenide triple / double-junction solar cell from a single germanium wafer through a single epitaxial process.

[0005] This invention is implemented as follows: a method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process includes the following steps: (1) Clean the germanium substrate to ensure that the germanium surface is ready for epitaxy; (2) On a germanium substrate, first epitaxial GaAs subcells are formed, and then epitaxial GaInP subcells are formed to form a Ge / GaAs / GaInP forward cell structure. (4) Continue to extend AlAs as a sacrificial layer; (4) Continue to epitaxially extend GaInP subcells, then extend GaAs subcells, and finally extend GaInAs subcells to form a GaInAs / GaAs / GaInP reverse cell structure. (5) The GaInAs epitaxial surface is cleaned, and then a flexible battery support substrate is prepared. (6) The surface of the Ge substrate is cleaned, and then a metal under electrode layer is prepared on its surface; (7) Apply a protective film to the Ge substrate or perform temporary bonding protection; (8) Place the wafer in HF solution and immerse it to etch the AlAs sacrificial layer to achieve separation of the flexible wafer from the rigid wafer; (9) The rigid wafer is removed from the protective film or temporarily bonded, and then rigid processing is carried out to finally realize the production of rigid battery; (10) Flexible wafers are temporarily bonded and then processed using flexible technology to finally produce flexible batteries.

[0006] Furthermore, in step (5), the substrate is a metal substrate or a PI substrate with a thickness of more than 10 μm.

[0007] Preferably, if the substrate is metal, a protective film or adhesive is applied to the surface.

[0008] Furthermore, in step (6), the thickness of the lower metal electrode layer is 2μm~8μm.

[0009] Furthermore, in step (9), concentrated hydrochloric acid is used to remove the GaInP barrier layer on the surface of the newly peeled forward battery structure, followed by processing of the upper electrode grid lines, anti-reflection film, and dicing process to form a rigid battery.

[0010] Furthermore, in step (10), concentrated hydrochloric acid is used to remove the GaInP blocking layer on the surface of the newly peeled reverse battery structure, and then the upper electrode grid line, anti-reflection film, and dicing process are continued to form a flexible battery. The beneficial effects of this invention are: (1) A single expensive epitaxial growth process can produce two complete, high-performance multi-junction cells at the same time, which greatly reduces the cost of epitaxy and improves material utilization efficiency and production efficiency.

[0011] (2) Since the substrate reuse method is not used, the quality of the epitaxial substrate is excellent and controllable, achieving dual optimization of quality and epitaxial cost. Attached Figure Description

[0012] Figure 1 This is the epitaxial structure of the gallium arsenide solar cell of the present invention.

[0013] In the figure, 101 is the Ge wafer substrate, 102 is the nucleation layer and buffer layer, 103 is the first tunnel junction, 104 is the forward intermediate cell GaAs, 105 is the second tunnel junction, 106 is the forward top cell GaInP, 107 is the AlAs sacrificial layer, 108 is the reverse top cell GaInP, 109 is the third tunnel junction, 110 is the reverse intermediate cell GaAs, 111 is the fourth tunnel junction, and 112 is the reverse bottom cell GaInAs. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0015] This invention proposes an epitaxial structure of "forward epitaxy-sacrificial layer-reverse epitaxy".

[0016] The extension process is as follows: (1) Clean the germanium substrate to ensure that the germanium surface is ready for epitaxy; (2) On a germanium substrate, first epitaxial GaAs subcells are formed, and then epitaxial GaInP subcells are formed to form a Ge / GaAs / GaInP forward cell structure. (3) Continue to extend AlAs as a sacrificial layer; (4) Continue to epitaxially extend GaInP subcells, then extend GaAs subcells, and finally extend GaInAs subcells to form GaInAs / GaAs / GaInP reverse cell structure.

[0017] After epitaxy, the reverse structure is stripped away using device fabrication processes, ultimately forming a high-performance rigid triple-junction gallium arsenide solar cell and a high-performance flexible gallium arsenide triple-junction solar cell.

[0018] The device manufacturing process is as follows: (1) Clean the surface of GaInAs epitaxial layer and then prepare a flexible battery support substrate. The substrate can be a metal substrate or a PI substrate with a thickness of more than 10 μm. If it is a metal substrate, the surface is protected by film or adhesive coating. (2) The surface of the Ge substrate is cleaned, and then a metal lower electrode layer with a thickness of 2μm~8μm is prepared on its surface; (3) Apply a protective film to the Ge substrate, or perform temporary bonding protection; (4) Place the wafer in HF solution and immerse it to etch the AlAs sacrificial layer to achieve separation of the flexible wafer from the rigid wafer; (5) Remove the protective film or temporarily bond the rigid wafer, and then process it according to the normal rigid process to finally realize the production of rigid battery; (6) The flexible wafer is temporarily bonded and then processed according to the normal flexible process to finally produce a flexible battery.

[0019] Example 1. Substrate preparation A 4-inch diameter P-type germanium single-crystal substrate offset 2°–6° to the (100) plane was provided. First, the substrate underwent ultrasonic cleaning with acetone-isopropanol-deionized water to remove surface particles and organic matter. Subsequently, the substrate was immersed in dilute hydrofluoric acid (HF:H2O = 1:10) for 1 minute to remove the native oxide layer, and then dried with high-purity nitrogen. The treated germanium substrate was then quickly loaded into a metal-organic chemical vapor deposition (MOCVD) reaction chamber.

[0020] 2. High-temperature heat treatment of germanium substrate In a hydrogen atmosphere, the reaction chamber temperature is raised to 650°C - 750°C and maintained for 5-15 minutes. This process aims to further clean the substrate surface and reconstruct surface atoms to obtain an atomically flat epitaxial surface, laying the foundation for subsequent high-quality epitaxial growth.

[0021] 3. Epitaxial growth of a "forward" triple-junction solar cell structure Growth is carried out in a temperature range of 650°C - 750°C.

[0022] 3.1 Nucleation Layer and Buffer Layer First, a 50-100 nm thick GaAs nucleation layer is grown on a germanium substrate. This step employs a low growth rate (<2 μm / h) and an appropriate V / III ratio (50-100) to ensure the formation of a single-orientation, low-dislocation-density GaAs crystal structure on the germanium substrate. This layer is a crucial transition layer for achieving high-quality epitaxy of all subsequent III-V materials.

[0023] 3.2 Formation of germanium-based solar cells (PN junctions) During growth, gallium atoms from group III sources diffuse into the germanium substrate, forming a PN junction in situ, thus creating a germanium sub-cell. By controlling the temperature and growth pauses at this point, the junction depth and electrical performance can be optimized.

[0024] 3.3 First Tunnel Connection Grow an AlGaAs / GaAs superlattice tunnel junction.

[0025] First, grow a heavily doped N++ type GaAs layer with a thickness of 20-30 nm and a doping concentration ≥1×10⁻⁶. 19 cm -3 .

[0026] Regrow a layer of heavily doped P++ type Al 0.3Ga 0.7 As layer, 20-30 nm thick, doping concentration ≥ 1×10⁻⁶ 19 cm -3 .

[0027] The tunnel junction should exhibit excellent ohmic properties and a high tunneling probability after annealing.

[0028] 3.4 Growth of GaAs mesocells Back field layer: A layer of P-type Al is grown 0.3 Ga 0.7 As, thickness 50-100 nm, doping concentration approximately 1×10⁻⁶ 18 cm -3 It is used to reflect minority carriers.

[0029] Base region: A layer of p-type GaAs is grown, with a thickness of 2.0-3.0 μm and a doping concentration of approximately 1 × 10⁻⁶. 17 cm -3 This is the main light-absorbing area of ​​the battery.

[0030] Emitter region: A layer of N-type GaAs is grown, with a thickness of 0.2-0.5 μm and a doping concentration of approximately 1 × 10⁻⁶. 18 cm -3 .

[0031] Window layer: Grow an N-type AlInP layer with a thickness of 30-50 nm and a doping concentration ≥ 5 × 10⁻⁶. 17 cm -3 This is used to reduce the surface recombination rate.

[0032] 3.5 Second Tunnel Connection A GaInP / GaAs superlattice tunnel junction is grown.

[0033] First, grow a heavily doped N++ type GaInP layer with a thickness of 20-30 nm and a doping concentration ≥1×10⁻⁶. 19 cm -3 .

[0034] A second heavily doped P++ type GaAs layer, 20-30 nm thick, with a doping concentration of 1 × 10⁻⁶, is grown. 19 cm -3 .

[0035] 3.6 Growth of GaInP Top Cells Back surface layer: A layer of P-type AlInP is grown, with a thickness of 50-100 nm and a doping concentration of 1×10⁻⁶. 18 cm -3 It is used to reflect minority carriers.

[0036] Base region: A layer of P-type Ga is grown0.51 In 0.49 P, thickness 0.8-1.5 μm, doping concentration approximately 1×10⁻⁶ 17 cm -3 This is the main light-absorbing area of ​​the battery.

[0037] Emitter region: A layer of N-type Ga is grown 0.51 In 0.49 P, thickness 0.1-0.3 μm, doping concentration approximately 1×10⁻⁶ 18 cm -3 .

[0038] Window layer: An N-type AlInP layer with a thickness of 30-50 nm is grown to reduce the surface recombination rate.

[0039] Cap layer: An N-type GaAs layer is grown with a thickness of 300-500 nm.

[0040] Isolation layer: Grow an N-type Ga layer 0.51 In 0.49 P, thickness 0.1-0.3μm.

[0041] 4. Growth of AlAs sacrificial layers The growth temperature was adjusted to 600°C - 700°C. An undoped or lightly N-type doped AlAs layer was grown, with a thickness controlled between 100-300 nm. After growth, a brief in-situ annealing was performed at this temperature to improve crystal quality.

[0042] 5. Epitaxial growth of a "reverse" triple-junction solar cell structure 5.1 Growth of GaInP Top Cells All structures are similar to those in Section 3.6, but their growth order is reversed.

[0043] 5.2 Third Tunnel Crossing The structure is similar to that in Section 3.5, but its growth order is reversed.

[0044] 5.3 Growth of GaAs mesocells The structure is similar to that in Section 3.4, but its growth order is reversed.

[0045] 5.4 Fourth Tunnel Crossing The structure is similar to that in Section 3.3, but its growth order is reversed.

[0046] 5.5 Growth of GaInAs Substrates for Solar Cells This cell is used to absorb photons with longer wavelengths. Ga is grown under strain equilibrium. 0.7 In 0.3 As layer.

[0047] Window layer: N-type InGaP or AlGaAs.

[0048] Launch region: N-type Ga 0.7 In 0.3 As, thickness 0.3-0.6μm.

[0049] Base region: P-type Ga0 .7 In 0.3 As, thickness 2.5-4.0μm.

[0050] Back field layer: P-type InGaAsP or AlGaAs.

[0051] Cap layer: P-type GaInAs, thickness 300-500nm.

[0052] 6. Post-epitaxy process and peeling 6.1 Fabrication of the back electrode of flexible battery Ti / Pt / Au-Cu electrodes were fabricated on a P-type GaInAs contact layer using evaporation and electroplating processes.

[0053] 6.2 Bonding Flexible Substrates A 10-20 μm thick bonding adhesive is spin-coated onto the surface of the Cu electrode and then bonded to the PET film.

[0054] Curing is carried out at 150°C - 200°C to form a strong bond.

[0055] 6.3 Fabrication of the back electrode of a rigid battery The surface of the Ge substrate is cleaned with HF, and then a Ti / Pt / Ag / Au metal under electrode layer with a thickness of 2μm~8μm is prepared on its surface.

[0056] 6.4 Rigid Battery Back Protection A blue film was attached to the surface electrode of the Ge substrate, and then baked at 90°C for 20 minutes for film protection.

[0057] 6.5 Selective Wet Etching and Stripping The sample was immersed in a 10% hydrofluoric acid aqueous solution. The HF acid penetrated from the sample edge to the AlAs sacrificial layer through capillary action. At room temperature, the HF acid etched AlAs at an extremely high rate, while the etch rate on the GaInP layers on both sides was extremely low (negligible). After 2-4 hours, the AlAs layer was completely etched, and the forward epitaxial structure separated from the reverse epitaxial structure.

[0058] 6.6 Final Device Fabrication Flexible battery: On the surface of the newly peeled "reverse" battery structure, the GaInP blocking layer is removed using concentrated hydrochloric acid, and then processes such as upper electrode grid lines, anti-reflection film, and dicing are continued to form a flexible battery.

[0059] Rigid cell: On the surface of the newly peeled "forward" cell structure, the GaInP blocking layer is removed using concentrated hydrochloric acid, and then further processing such as upper electrode grid lines, anti-reflection film, and dicing is carried out to form a rigid cell.

[0060] 7. Ultimately, two independent, high-efficiency multijunction solar cells were successfully obtained from the same germanium wafer: Product A (rigid): Germanium-based GaInP / GaAs / Ge triple junction solar cell.

[0061] Product B (Flexible): GaInP / GaAs / GaInAs triple-junction flexible thin-film solar cell.

[0062] like Figure 1 The figure shows the epitaxial structure of the gallium arsenide solar cell prepared by the present invention. In the figure, 101 is the Ge wafer substrate, 102 is the nucleation layer and buffer layer, 103 is the first tunnel junction, 104 is the forward intermediate cell GaAs, 105 is the second tunnel junction, 106 is the forward top cell GaInP, 107 is the AlAs sacrificial layer, 108 is the reverse top cell GaInP, 109 is the third tunnel junction, 110 is the reverse intermediate cell GaAs, 111 is the fourth tunnel junction, and 112 is the reverse bottom cell GaInAs.

[0063] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the scope of the claims.

Claims

1. A method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process, characterized in that, Includes the following steps: (1) Clean the germanium substrate to ensure that the germanium surface is ready for epitaxy; (2) On a germanium substrate, first epitaxial GaAs subcells are formed, and then epitaxial GaInP subcells are formed to form a Ge / GaAs / GaInP forward cell structure. (3) Continue to extend AlAs as a sacrificial layer; (4) Continue to epitaxially extend GaInP subcells, then extend GaAs subcells, and finally extend GaInAs subcells to form a GaInAs / GaAs / GaInP reverse cell structure. (5) The GaInAs epitaxial surface is cleaned, and then a flexible battery support substrate is prepared. (6) The surface of the Ge substrate is cleaned, and then a metal under electrode layer is prepared on its surface; (7) Apply a protective film to the Ge substrate or perform temporary bonding protection; (8) Place the wafer in HF solution and immerse it to etch the AlAs sacrificial layer to achieve separation of the flexible wafer from the rigid wafer; (9) The rigid wafer is removed from the protective film or temporarily bonded, and then rigid processing is carried out to finally realize the production of rigid battery; (10) Flexible wafers are temporarily bonded and then processed using flexible technology to finally produce flexible batteries.

2. The method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process according to claim 1, characterized in that, In step (5), the substrate is a metal substrate or a PI substrate with a thickness of more than 10 μm.

3. The method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process according to claim 2, characterized in that, If the substrate is metal, a protective film or adhesive is applied to the surface.

4. The method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process according to claim 1, characterized in that, In step (6), the thickness of the lower metal electrode layer is 2μm~8μm.

5. The method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process according to claim 1, characterized in that, In step (9), concentrated hydrochloric acid is used to remove the GaInP barrier layer on the surface of the newly peeled forward battery structure, followed by the processing of the upper electrode grid lines, anti-reflection film, and dicing process to form a rigid battery.

6. The method for simultaneously obtaining rigid and flexible solar cells using a single epitaxial process according to claim 1, characterized in that, In step (10), concentrated hydrochloric acid is used to remove the GaInP barrier layer on the surface of the newly peeled reverse battery structure, and then the upper electrode grid line, anti-reflection film and dicing process are continued to form a flexible battery.

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