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

Abstract

The present application relates to the technical field of solar cells, in particular to a low-warp flexible thin-film solar cell and a preparation method thereof, the solar cell is composed of a supporting substrate, an MXene-PI composite substrate, an epitaxial layer, a P electrode, an N electrode and an anti-reflection film layer; the MXene-PI composite substrate is a composite substrate formed by covering an MXene layer on the front surface of a PI film substrate. On the basis of the traditional process, a layer of two-dimensional transition metal carbide MXene is deposited in advance on the front surface of the PI film substrate through a spin coating process, which not only can reduce the thermal expansion coefficient of the composite substrate, but also can enhance the mechanical properties of the battery, improve the deformation resistance, and effectively solve the problem of excessive warping of the flexible battery.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, specifically to a low-warpage flexible thin-film solar cell and its preparation method. Background Technology

[0002] Space solar cell technology is upgrading towards higher efficiency and greater flexibility. Historically, rigid silicon-based cells, due to their weak light absorption and poor flexibility, were unable to meet the demands and were replaced by gallium arsenide (GaAs) cells. Gallium arsenide is a direct bandgap semiconductor with a high light absorption coefficient, a theoretical efficiency exceeding 33%, and is also resistant to high temperatures and radiation. Subsequently, flexible GaAs cells with inverted structures solved the problem of light shading by electrodes in upright structures and, with their bendability and lightweight characteristics, have been widely used in the aerospace field.

[0003] Polyimide (PI) films, with their comprehensive advantages such as high temperature resistance, radiation resistance, high mechanical strength, and lightweight, have been widely used as substrate materials for flexible GaAs solar cells. In traditional flexible solar cell fabrication processes, metals need to be deposited on both sides of the substrate to maintain the bond between the substrate and the epitaxial layer. However, the significant difference in the coefficients of thermal expansion between the PI film and the metal leads to the accumulation of thermal stress, which is difficult to eliminate and causes excessive warping on both sides of the cell. This not only affects the uniformity of light absorption but also leads to a decrease in energy conversion efficiency. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a low-warpage flexible thin-film solar cell and its preparation method. This solar cell has a low coefficient of thermal expansion, good mechanical properties, and can improve warpage, making it suitable for the aerospace field.

[0005] The purpose of this invention is to provide a low-warpage flexible thin-film solar cell, which is composed of a supporting substrate, an MXene-PI composite substrate, an epitaxial layer, a P electrode, an N electrode, and an anti-reflection coating layer (ARC layer);

[0006] The MXene-PI composite substrate is formed by coating a MXene layer onto the front side of a PI film substrate. Specifically, the thickness of the MXene layer is 0.54 μm.

[0007] The inventors discovered through preliminary research that when MXene (two-dimensional transition metal carbide) is deposited on the back side of the PI film, stress concentrates at the PI-GaAs interface. Since MXene is far from the stress source, it cannot effectively alleviate stress concentration, and the PI film layer, being the primary stress-bearing layer, amplifies thermal deformation due to its viscoelastic properties. However, depositing MXene on the front side of the PI film forms a "PI-MXene-GaAs" sandwich structure, allowing the thermal expansion difference to transition layer by layer, reducing stress concentration at the interface. Simultaneously, the MXene layer directly supports GaAs, improving interfacial bonding strength and suppressing relative deformation between GaAs and the PI film. Therefore, this invention, based on traditional processes, pre-deposits a layer of MXene on the front side of the PI film substrate via spin coating, forming a composite substrate that, together with the GaAs epitaxial layer, forms a "PI-MXene-GaAs" sandwich structure. This reduces the thermal expansion coefficient of the composite substrate, making it closer to that of GaAs, thus reducing stress concentration at the interface. Furthermore, the direct support of MXene for GaAs improves interfacial bonding strength, suppresses relative deformation between GaAs and the PI film, enhances the mechanical properties of the battery, and increases its resistance to deformation. This effectively solves the problem of excessive warping in flexible batteries, broadening its application range.

[0008] Furthermore, the epitaxial layer is formed by stacking multiple GaAs-based thin films with different doping and composition (single junction), or by stacking multiple III-V compound semiconductor sub-cells with different band gaps in series (multi-junction).

[0009] Specifically, multilayer GaAs-based thin films with different doping and compositions include Al 0.4 Ga 0.6 As, In 0.5 Ga 0.5 As; multiple III-V compound semiconductor sub-cells with different band gaps are the top cell GaInP, the middle cell GaAs, and the bottom cell InGaAs.

[0010] Furthermore, the P electrode is composed of Ti, Pt, and Au layers of different thicknesses. Specifically, it can be stacked in a Ti / Pt / Au configuration with a film thickness of 3μm-5μm.

[0011] Furthermore, the N-electrode is composed of Ag, Ge, and Au layers of different thicknesses. Specifically, it can be stacked in an Au / Ge / Ag / Au configuration with a film thickness of 4 μm-6 μm.

[0012] Furthermore, the antireflective coating has a Ti3O5 / Al2O3 structure, and the optical thickness of the film is 1 / 4 of the target wavelength.

[0013] This invention also provides a method for preparing a low-warpage flexible thin-film solar cell, comprising the following steps:

[0014] (1) An epitaxial layer with an inverted structure was grown on a GaAs substrate using MOCVD (metal-organic chemical vapor deposition);

[0015] (2) Preparation of MXene dispersion;

[0016] (3) Using spin coating, a layer of MXene dispersion is coated on the front side of the PI film substrate to form an MXene-PI composite substrate;

[0017] (4) Different metal films are deposited on the front and back sides of the MXene-PI composite substrate and on the surface of the epitaxial layer, and the front side of the MXene-PI composite substrate is bonded to the metal of the epitaxial wafer.

[0018] (5) Spin-coat a layer of bonding adhesive onto a temporary sapphire substrate, and then temporarily bond it to the back gold of the MXene-PI composite substrate;

[0019] (6) Use chemical etching solution to remove the cut-off layer in the GaAs substrate and epitaxial layer, exposing the underlying contact layer GaAs.

[0020] (7) Photolithography, development, evaporation and etching are performed on the epitaxial layer to form a fine gate, a main gate and an N electrode;

[0021] (8) An anti-reflection film is deposited between the N electrodes by optical coating, overlay and etching;

[0022] (9) Fabricate the P electrode by overlay and etching to remove the epitaxial layer in the P electrode region and expose the P electrode. Finally, remove the temporary substrate sapphire by pyrolysis.

[0023] (10) Spin-coating protective adhesive onto the surface of the battery semi-finished product, then cutting, removing the adhesive, cleaning, and baking to obtain a complete flexible solar cell.

[0024] Furthermore, in step (2), the preparation method of the MXene dispersion is as follows: add intercalating agent and vitamin C to the MAX phase for dispersion, then ultrasonically peel off, centrifuge to remove MAX phase impurities, and finally disperse in an organic solvent to obtain the MXene dispersion.

[0025] Because MXene sheets are tightly stacked together by strong van der Waals forces, hydrogen bonds, and electrostatic interactions, attempting direct separation using methods such as ultrasound is extremely inefficient. In contrast to the traditional intercalating agent hexadecyltrimethylammonium bromide (CTAB), using 8-phenylaminonaphthalene-1-sulfonate ammonium salt (ANS-NH4) as an intercalating agent provides strong steric hindrance by inserting the naphthalene and benzene rings between the MXene sheets in a planar parallel manner. The sulfonate anions can electrostatically attract the positively charged metal sites on the MXene surface, more effectively widening the interlayer spacing and separating multilayer MXene into single-layer MXene. In this technical solution, the MXene dispersion serves as the dispersion medium for the MXene sheets. Through spin coating, MXene nanosheets are uniformly dispersed on the PI film surface, forming a dense, continuous, and strongly adherent MXene layer, ensuring the structural uniformity of the composite substrate. Furthermore, the thickness and morphology of the MXene layer can be precisely controlled by adjusting parameters such as the dispersion concentration and viscosity, meeting the process requirements of flexible battery substrates for ultra-thin and uniform coatings.

[0026] Furthermore, the intercalating agent is ANS-NH4; the solvent is NMP (N-methylpyrrolidone).

[0027] Furthermore, in step (3), the spin coating speed is 2500±50 rpm and the time is 15±2 s.

[0028] Furthermore, in step (4), the metal source for evaporation is a combination of several of Ni, Cr, Ti, Pt, Au, Ag, Al, and Pd. Specifically, the front side of the PI film is TiPtAu, and the back side is TiAlPdTi; TiAu is evaporated onto the front side of the epitaxial wafer.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] This invention, based on traditional processes, pre-deposits a layer of MXene on the front side of a PI film substrate via spin coating to form a composite substrate, creating a "PI-MXene-GaAs" sandwich structure with the GaAs epitaxial layer. This structure reduces the thermal expansion coefficient of the composite substrate, making it closer to that of GaAs; furthermore, MXene directly supports GaAs, improving interfacial bonding strength and enhancing the battery's mechanical properties. Through this dual mechanism of thermal expansion coefficient step buffering and mechanical property enhancement, it effectively reduces battery warpage and improves the battery's resistance to deformation, making it widely applicable in the aerospace field. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the low-warpage flexible thin-film solar cell of the present invention;

[0032] Figure 2This is a schematic diagram of the MXene-PI composite substrate structure formed after spin-coating the MXene dispersion according to the present invention;

[0033] Figure 3 This is a schematic diagram of the structure of the MXene-PI composite substrate and the epitaxial layer after metal film deposition according to the present invention;

[0034] Figure 4 This is a schematic diagram of the structure of the MXene-PI composite substrate and the epitaxial layer after bonding according to the present invention;

[0035] Figure 5 This is a schematic diagram of the structure after the N electrode of this invention has been fabricated;

[0036] Figure 6 This is a schematic diagram of the structure after the antireflection coating layer of this invention has been fabricated;

[0037] Figure 7 This is a schematic diagram of the structure after the P electrode of this invention has been fabricated;

[0038] Figure 8 This is a schematic diagram of the structure of the solar cell of the present invention after it has been manufactured and cut.

[0039] Explanation of the labels in the diagram:

[0040] 1. GaAs contact layer; 2. Epitaxial layer; 3. PI film substrate; 4. MXene layer; 5. First bonding layer; 6. Supporting substrate; 7. Second bonding layer; 8. N electrode; 9. Antireflective film layer; 10. P electrode. Detailed Implementation

[0041] 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. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0042] In the description of this application, it should be understood that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this application.

[0043] In the description of this application, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is usually based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this application and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this application; the directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0044] Please see Figures 1 to 8 It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the illustrations only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the shape, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0045] One embodiment of the present invention provides a low-warpage flexible thin-film solar cell, the structural schematic of which is shown below. Figure 1 As shown, from bottom to top, the layers are: a supporting substrate 6, a PI film substrate 3, an MXene layer 4, a second bonding layer 7, a first bonding layer 5, an epitaxial layer 2, a contact layer GaAs 1, a P-electrode 10, an N-electrode 8, and an anti-reflection film layer 9. The supporting substrate is a PI film substrate with a gold backing; the PI film substrate and the MXene layer form an MXene-PI composite substrate; the metal deposited below the epitaxial layer is the P-electrode (lower electrode); the N-electrode and the anti-reflection film layer are fabricated above the epitaxial layer, with the N-electrode and the anti-reflection film layer alternating.

[0046] In some embodiments, the battery P electrode is composed of Ti, Pt, and Au layers of different thicknesses, which can be stacked in the form of Ti / Pt / Au, with a film thickness of 3μm-5μm; the N electrode is composed of Au, Ge, Ag, and Au layers of different thicknesses, which can be stacked in the form of Au / Ge / Ag / Au, with a film thickness of 4μm-6μm.

[0047] In some embodiments, the antireflective film has a Ti3O5 / Al2O3 structure, and the optical thickness of the film is 1 / 4 of the target wavelength, which can minimize reflection.

[0048] In some embodiments, the epitaxial layer is formed by stacking multiple GaAs-based thin films with different doping and different compositions (single junction), or by stacking multiple III-V compound semiconductor sub-cells with different band gaps in series (multi-junction).

[0049] Another embodiment of the present invention provides a method for fabricating a low-warpage flexible thin-film solar cell, the schematic diagram of which is shown below.Figures 2 to 8 As shown, it includes the following steps:

[0050] (1) An epitaxial layer with an inverted structure was grown on a GaAs substrate using MOCVD;

[0051] Specifically, MOCVD is used to sequentially grow epitaxial layers on a GaAs substrate, including a GaAs buffer layer, a GaInP stop layer, a GaAs contact layer, a cap layer, a window layer, an emitter region, a base region, a buffer layer, and a tunnel junction.

[0052] (2) Preparation of MXene dispersion: The preparation method is as follows: intercalating agent and vitamin C are added to the MAX phase for dispersion, then ultrasonically exfoliated, centrifuged to remove MAX phase impurities, and finally dispersed in an organic solvent to obtain the final product. This invention utilizes the excellent two-dimensional layered structure and high modulus of the MXene layer itself to not only significantly reduce the thermal expansion coefficient of the composite substrate and suppress warping deformation of the PI film caused by thermal expansion and contraction during bonding, alloying, and other processes; but also to enhance the tensile strength and bending stiffness of the composite substrate, improve the overall mechanical properties and deformation resistance of the battery, thereby effectively solving the problems of folding and damage that occur during the cutting, testing, welding, and transportation of flexible batteries, and ensuring the stability of the battery structure.

[0053] Specifically, Ti3AlC2 was first etched with a lithium fluoride-hydrochloric acid (10 mol / mL aqueous solution) mixture at 40±3℃ for 9-10 h to obtain multilayer MXene. The multilayer MXene precipitate was then repeatedly centrifuged and washed with deionized water until the supernatant was nearly neutral. Next, at 30±3℃, an intercalating agent ANS-NH4 (0.25 mol / mL) and vitamin C were added to the obtained multilayer MXene. The mixture was then ultrasonically exfoliated to separate the multilayer MXene into single-layer nanosheets. The precipitate was collected by centrifugation and washed repeatedly with acetonitrile to thoroughly remove moisture. Finally, the completely dried MXene precipitate was redispersed in N-methylpyrrolidone (NMP) to obtain an MXene dispersion. This dispersion can improve the interfacial bonding with the PI film through hydrogen bonding; the added vitamin C can inhibit the oxidation of MXene during dispersion and recombination.

[0054] (3) Using a spin-coating process, a layer of MXene dispersion is coated on the front side of the PI film substrate to form an MXene-PI composite substrate, such as... Figure 2 As shown;

[0055] Specifically, MXene dispersion was coated onto the PI surface using a spin coating method at a speed of 2500±50 rpm for 15±2 s. The thickness of the MXene layer was 0.54 μm, and then dried to form an MXene-PI composite substrate.

[0056] (4) Different metal films are deposited on the front and back sides of the MXene-PI composite substrate and on the surface of the epitaxial layer, and the front side of the MXene-PI composite substrate is bonded to the metal of the epitaxial wafer, such as... Figures 3 to 4 As shown;

[0057] Specifically, under high vacuum conditions, different metal films are deposited on the front and back sides of the MXene-PI composite substrate and the epitaxial layer using electron beam or thermal resistance evaporation. The metal source uses several combinations of Ni, Cr, Ti, Pt, Au, Ag, Al, Pd, and Ge, with the gold backing of the MXene-PI composite substrate serving as the battery's support substrate. Then, using a metal bonding process, under set temperature and pressure conditions, the front side of the MXene-PI composite substrate is bonded to the metal layer beneath the epitaxial layer, thus forming a bonding layer. More specifically, the front side of the MXene-PI composite substrate is TiPtAu, and the back side is TiAlPdTi. TiAu is deposited on the front side of the epitaxial wafer.

[0058] (5) Spin-coat a layer of bonding adhesive onto the temporary sapphire substrate, and then temporarily bond it to the back gold of the MXene-PI composite substrate.

[0059] (6) Use a chemical etching solution to remove the cut-off layer in the GaAs substrate and epitaxial layer, exposing the underlying contact layer GaAs; specifically, use ammonia, hydrogen peroxide and deionized water in a certain chemical ratio to remove the GaAs substrate, and then use HCl to etch away the GaInP cut-off layer in the epitaxial layer, exposing the underlying GaAs, which is the contact layer GaAs.

[0060] (7) Photolithography, development, evaporation, and etching are performed above the epitaxial layer to form the fine gate, main gate, and N-electrode, such as... Figure 5 As shown; the electrode fabrication process here adopts conventional processes, and any feasible method can be used. Specifically, the N electrode can be composed of Au / Ge / Ag / Au layers with different film thicknesses, ranging from 4μm to 6μm.

[0061] (8) An anti-reflection film is deposited between the N electrodes by optical coating, overlaying, and etching, such as... Figure 6 As shown; specifically, the process is carried out under high vacuum conditions, the structure of the antireflective film is Ti3O5 / Al2O3, and the optical thickness of the film is 1 / 4 of the target wavelength, which can minimize reflection.

[0062] (9) Fabricate the P-electrode. Through overlay and etching, remove the epitaxial layer in the P-electrode region to expose the P-electrode. Finally, remove the temporary sapphire substrate using a pyrolysis process. Figure 7As shown; specifically, using the lower electrode overlay and lower electrode etching processes, P electrodes composed of Ti / Pt / Au layers with different film thicknesses of 3μm-5μm are fabricated; using a pyrolysis process, the temporary sapphire substrate is peeled off from the battery. By realizing the recycling of the temporary substrate, not only are the high cost and high pollution emission problems caused by using GaAs as a temporary substrate effectively avoided, but the difficulty and related costs of removing the sapphire temporary substrate are further reduced.

[0063] (10) Spin-coating a protective adhesive onto the surface of the semi-finished battery, then cutting, removing the adhesive, cleaning, and baking to obtain a complete flexible solar cell, such as... Figure 8 As shown.

[0064] In summary, this invention forms an MXene-PI composite substrate by pre-depositing a layer of MXene on the front side of a PI film substrate, and simultaneously forms a "PI-MXene-GaAs" sandwich structure with a GaAs epitaxial layer. This structure not only reduces the thermal expansion coefficient of the composite substrate, but also improves the interfacial bonding strength. It fully utilizes the dual mechanisms of thermal expansion coefficient step buffering and mechanical property enhancement to effectively reduce battery warpage and improve battery deformation resistance, making it widely applicable in the aerospace field.

[0065] Finally, it should be emphasized that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-warpage flexible thin-film solar cell, characterized in that, The solar cell consists of a supporting substrate, an MXene-PI composite substrate, an epitaxial layer, a P electrode, an N electrode, and an anti-reflection film layer; The MXene-PI composite substrate is a composite substrate formed by covering the front side of a PI film substrate with an MXene layer. The MXene layer is obtained by spin-coating an MXene dispersion onto the surface of a PI membrane. The MXene dispersion is prepared by adding an intercalating agent and vitamin C to the MAX phase for dispersion, followed by ultrasonic exfoliation, centrifugation to remove MAX phase impurities, and finally dispersion in an organic solvent. During fabrication, different metal films are deposited on the front and back sides of the MXene-PI composite substrate and on the surface of the epitaxial layer, and the front side of the MXene-PI composite substrate is bonded to the metal of the epitaxial wafer.

2. The low-warpage flexible thin-film solar cell according to claim 1, characterized in that, The epitaxial layer is formed by stacking multiple GaAs-based thin films with different doping and composition, or by stacking multiple III-V compound semiconductor sub-cells with different band gaps in series.

3. The low-warpage flexible thin-film solar cell according to claim 1, characterized in that, The P electrode is composed of Ti, Pt, and Au layers of different thicknesses.

4. A low-warpage flexible thin-film solar cell according to claim 1, characterized in that, The N electrode is composed of Au, Ge, Ag and Au layers of different thicknesses.

5. A low-warpage flexible thin-film solar cell according to claim 1, characterized in that, The antireflective coating has a Ti3O5 / Al2O3 structure, and the optical thickness of the film is 1 / 4 of the target wavelength.

6. A method for preparing a low-warpage flexible thin-film solar cell according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: (1) An epitaxial layer with an inverted structure was grown on a GaAs substrate using MOCVD; (2) Preparation of MXene dispersion; (3) Using spin coating, a layer of MXene dispersion is coated on the front side of the PI film substrate to form an MXene-PI composite substrate; (4) Different metal films are deposited on the front and back sides of the MXene-PI composite substrate and on the surface of the epitaxial layer, and the front side of the MXene-PI composite substrate is bonded to the metal of the epitaxial wafer. (5) Spin-coat a layer of bonding adhesive onto a temporary sapphire substrate, and then temporarily bond it to the back gold of the MXene-PI composite substrate; (6) Use chemical etching solution to remove the cut-off layer in the GaAs substrate and epitaxial layer, exposing the underlying contact layer GaAs. (7) Photolithography, development, evaporation and etching are performed on the epitaxial layer to form a fine gate, a main gate and an N electrode; (8) An anti-reflection film is deposited between the N electrodes by optical coating, overlay and etching; (9) Fabricate the P electrode by overlay and etching to remove the epitaxial layer in the P electrode region and expose the P electrode. Finally, remove the temporary substrate sapphire by pyrolysis. (10) Spin-coating protective adhesive onto the surface of the battery semi-finished product, then cutting, removing the adhesive, cleaning, and baking to obtain a complete flexible solar cell.

7. The method for preparing a low-warpage flexible thin-film solar cell according to claim 6, characterized in that, In step (2), the preparation method of the MXene dispersion is as follows: add intercalating agent and vitamin C to the MAX phase for dispersion, then ultrasonically peel off, centrifuge to remove MAX phase impurities, and finally disperse in an organic solvent to obtain the dispersion.

8. The method for preparing a low-warpage flexible thin-film solar cell according to claim 7, characterized in that, The intercalating agent is ANS-NH4; the organic solvent is NMP.

9. The method for preparing a low-warpage flexible thin-film solar cell according to claim 6, characterized in that, In step (3), the spin coating speed is 2500±50 rpm and the time is 15±2 s.

10. The method for preparing a low-warpage flexible thin-film solar cell according to claim 6, characterized in that, In step (4), the metal source for vapor deposition is a combination of several of Ni, Cr, Ti, Pt, Au, Ag, Al, and Pd.

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