Back electrode of thin solar cell for space and preparation method of back electrode

Abstract

The invention discloses a back electrode of a thin solar cell for space and a preparation method thereof, and the back electrode comprises a back electrode main body which is disposed on the back surface of a solar cell substrate. The back electrode main body is provided with a honeycomb structure, the honeycomb structure is composed of a plurality of honeycomb-shaped pore units, the side walls of the honeycomb-shaped pore units form a regular hexagon, and the adjacent honeycomb-shaped pore units are connected through the side walls. The back electrode of the solar cell comprises the honeycomb structure, so that the strength of the back electrode is improved, effective support is provided for the solar cell, and the risk of warping of the solar cell is reduced.

Description

Technical Field

[0001] The invention relates to the field of solar cells, and in particular to a back electrode of a thin solar cell for space use and a preparation method thereof. Background Art

[0002] With the development of space technology, future spacecraft are moving towards large-scale and ultra-miniaturization. This trend puts higher requirements on the area, weight-to-power ratio and other aspects of space solar cells. In order to obtain high-weight-to-power solar cells for space, thinning the germanium (Ge) substrate of solar cells is an effective solution.

[0003] However, the moment of inertia of the thinned solar cell section is reduced, making it more prone to warping and deformation when subjected to external forces. In addition, due to the high thermal expansion coefficient mismatch between the materials of the solar cell substrate and the back electrode, large internal stress is easily generated. Therefore, after the preparation of the thin back electrode of the space solar cell is completed, the solar cell will be severely warped. Severely warped solar cells are very likely to break during processing and service, which directly affects the yield and service life of the solar cell. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of serious warping of thin solar cells for space use due to external forces or large internal stress. By designing a back electrode containing a honeycomb structure, the external force can be effectively dispersed and the internal stress of existing thin batteries for space use during production and service can be reduced, thereby reducing the risk of warping and deformation of thin batteries for space use.

[0005] In order to achieve the above-mentioned object, the present invention provides a back electrode of a thin solar cell for space use, comprising a back electrode body, wherein the back electrode body is arranged on the back side of a solar cell substrate;

[0006] The back electrode body has a honeycomb structure, which is composed of a plurality of honeycomb pore units. The side walls of the honeycomb pore units form a regular hexagon, and adjacent honeycomb pore units are connected through the side walls.

[0007] Furthermore, the Young's modulus of the material of the back electrode body is greater than 120 GPa, the Poisson's ratio is 0.3-0.4, and the thermal expansion coefficient is 4.5×10 -6 m / K~20×10 -6 m / K.

[0008] Furthermore, the material of the back electrode body is a conductive metal material, and the conductive metal material at least includes any one of Cu and Mo.

[0009] Furthermore, the inner regular hexagon formed by the inner wall of the honeycomb pore unit side wall has a side length of 0.175mm to 0.875mm; the outer regular hexagon formed by the outer wall of the honeycomb pore unit side wall has a side length of 0.376mm to 1.732mm; and the thickness of the honeycomb pore unit side wall is 0.02mm to 1.12mm.

[0010] Furthermore, the thickness of the back electrode body is 0.002 mm to 0.02 mm.

[0011] Furthermore, a transition layer is provided between the back electrode body and the solar cell substrate for forming an ohmic contact with the solar cell substrate; the transition layer has the same honeycomb structure as the back electrode body.

[0012] Furthermore, a welding layer is provided on the bottom surface of the back electrode body for providing a welding interface for the solar cell so as to be connected with different solar cells to form a solar cell array; the welding layer has the same honeycomb structure as the back electrode body.

[0013] Furthermore, the transition layer includes a first metal layer and a second metal layer; the material of the first metal layer is Pd, and the thickness of the first metal layer is 0.02 μm to 1 μm; the material of the second metal layer is Ag, and the thickness of the second metal layer is 0.1 μm to 2 μm.

[0014] Furthermore, the material of the welding layer is Ag, and the thickness of the welding layer is 0.1 μm to 3 μm.

[0015] The present invention also provides a method for preparing the back electrode of the above-mentioned thin solar cell for space use, characterized by comprising:

[0016] providing a substrate;

[0017] forming a photoresist pattern on the back side of the substrate to expose a honeycomb structure pattern on the back side of the substrate;

[0018] A transition layer, a back electrode body and a welding layer are sequentially evaporated on the honeycomb structure pattern to form a back electrode with a honeycomb structure.

[0019] Compared with the prior art, the beneficial effects of the present invention at least include:

[0020] (1) The back electrode of the thin solar cell for space provided by the present invention includes a back electrode body, and the back electrode body has a honeycomb structure, which optimizes the high warping problem of existing solar cells. The honeycomb structure is composed of a number of regular hexagonal honeycomb pore units. On the one hand, when the honeycomb structure is subjected to external force or internal stress, it can evenly disperse the stress through the side walls of a number of regular hexagonal honeycomb pore units, thereby improving the strength (structural stability) of the back electrode, thereby providing effective support for the solar cell and reducing the risk of warping of the solar cell; on the other hand, because the honeycomb structure has a high porosity, it can greatly reduce the density and weight of the required materials while ensuring the overall strength of the back electrode structure. Therefore, the back electrode of the present invention has the advantages of high strength and light weight at the same time.

[0021] (2) Furthermore, the back electrode main material of the present invention is selected from a material that matches the thermal expansion coefficient of the solar cell substrate, which can reduce the difference in expansion or contraction of the back electrode and the solar cell substrate at different rates and amounts due to temperature changes, thereby reducing internal stress, enhancing the structural stability of the entire solar cell, and improving the solar cell's anti-bending ability.

[0022] (3) Furthermore, in the back electrode of the present invention, the transition layer, the back electrode body, and the welding layer all adopt the same honeycomb structure, further achieving uniform stress distribution of the entire back electrode structure, thereby further improving the bending resistance of the overall solar cell and reducing the risk of warping or fracture of the solar cell during production, processing, and service. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The figure is a schematic structural diagram of a back electrode of a thin solar cell for space use according to the present invention.

[0024] Figure 2 It is a schematic diagram of an enlarged cross section of a part of the honeycomb structure of the present invention (including three honeycomb pore units).

[0025] Description of the accompanying drawings:

[0026] Substrate 10,

[0027] Back electrode 20 , transition layer 21 , first metal layer 211 , second metal layer 212 , back electrode body 22 , honeycomb structure 221 , welding layer 23 . DETAILED DESCRIPTION

[0028] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] In the description of the present invention, it should be noted that the terms "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0030] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0031] The “thin solar cell” described herein refers to a micrometer-sized solar cell, for example, a solar cell having a thickness of 30 μm to 90 μm.

[0032] The "internal stress" described herein refers to stress caused by internal factors of the material, including but not limited to temperature changes, phase changes, processing processes, etc.

[0033] As described in the background art, existing thin solar cells for space use (hereinafter referred to as "solar cells") have serious warping or deformation problems due to thinning and large internal stress, which affects the product yield and service life of the solar cells.

[0034] In order to solve the above problems, the present invention proposes to optimize the structure and materials of the back electrode of the solar cell. In the fields of construction, aerospace, etc., honeycomb structures are used to make various building panels, spacecraft sidings, etc. due to their advantages such as light weight and high strength. The honeycomb structure is composed of regular hexagonal units with periodic topological distribution. This geometric shape enables the material to effectively disperse stress through each regular hexagonal unit when subjected to external force, thereby improving the strength of the material; at the same time, the honeycomb structure has a high porosity, thereby greatly reducing the density and weight of the required material without affecting the overall strength of the material structure. In the present invention, the inventor innovatively proposed for the first time to apply the honeycomb structure to the back electrode of the solar cell, thereby strengthening the back electrode structure of the solar cell, and then enhancing the bending resistance of the solar cell. In addition, by selecting a material that matches the thermal expansion coefficient of the solar cell substrate, the internal stress caused by temperature changes in the material can be significantly reduced, thereby further improving the bending resistance of the solar cell.

[0035] The present invention provides a back electrode 20 of a thin solar cell for space use, comprising a back electrode body 22, see Figure 1 The back electrode body 22 is arranged on the back side of the solar cell substrate 10.

[0036] In the back electrode 20 provided by the present invention, at least the back electrode body 22 has a honeycomb structure 221, which is composed of a plurality of honeycomb pore units, wherein the honeycomb pore unit comprises six edges (side walls) and six vertices; the side walls of the honeycomb pore units form a regular hexagon, and adjacent honeycomb pore units are connected by side walls, see Figure 2 When the honeycomb pore unit is subjected to internal stress or external force, the internal stress or external force can be uniformly diffused into the entire honeycomb structure 221 along the regular hexagonal side walls of the honeycomb pore unit, so that the honeycomb structure 221 effectively disperses the stress, thereby providing effective support for the solar cell and improving the bending resistance of the solar cell.

[0037] In some embodiments, the inner regular hexagon formed by the inner wall of the honeycomb pore unit side wall has a side length of 0.175 mm to 0.875 mm, the outer regular hexagon formed by the outer wall of the honeycomb pore unit side wall has a side length of 0.376 mm to 1.732 mm, and the thickness of the honeycomb pore unit side wall is 0.02 mm to 1.12 mm.

[0038] In some embodiments, the thickness of the back electrode body 22 is 0.002 mm to 0.02 mm.

[0039] In some embodiments, in order to reduce the internal stress generated by the solar cell during the processing and forming process or under temperature changes, the back electrode body 22 is preferably made of a material with a larger elastic modulus (Young's modulus), a smaller Poisson's ratio, and a thermal expansion coefficient that is more matched with the solar cell substrate 10 (usually a Ge substrate). The elastic modulus reflects the rigidity of the material. The larger the elastic modulus, the greater the rigidity of the material, which means that the deformation of the material is smaller under the same external force. The Poisson's ratio reflects the lateral deformation ability of the material. The smaller the Poisson's ratio, the smaller the ratio of the lateral strain to the axial strain of the material when it is axially stretched or compressed, that is, the lateral deformation of the material when it is stressed is smaller. When the thermal expansion coefficient of the back electrode 20 material matches that of the solar cell substrate 10, the difference in expansion or contraction of the back electrode 20 and the solar cell substrate 10 at different rates and amounts due to temperature changes can be reduced, thereby reducing internal stress. In some embodiments, the Young's modulus of the material of the back electrode body 22 is greater than 120GPa, the Poisson's ratio is 0.3-0.4, and the thermal expansion coefficient is 4.5×10 -6 m / K~20×10 -6 In some embodiments, the material of the back electrode body 22 is a conductive metal material, the conductive metal material at least includes any one of Cu and Mo, and the purity of the metal is not less than 99.99%.

[0040] In the back electrode 20 provided by the present invention, a transition layer 21 is further provided between the back electrode body 22 and the solar cell substrate 10. The transition layer 21 includes a first metal layer 211 and a second metal layer 212, which are used to form an ohmic contact with the solar cell substrate 10 and ensure a reliable connection between the solar cell substrate 10 and the back electrode 20. The first metal layer 211 and the second metal layer 212 may be the same or different. The first metal layer 211 is connected to the solar cell substrate 10, and the second metal layer 212 is connected to the back electrode body 22. In some embodiments, the material of the first metal layer 211 is Pd, and the thickness of the first metal layer 212 is 0.02μm to 1μm; the material of the second metal layer is Ag, and the thickness of the second metal layer is 0.1μm to 2μm, and the purity of the metal used in the first metal layer and the second metal layer is not less than 99.99%.

[0041] In the back electrode 20 provided by the present invention, a welding layer 23 is further provided on the bottom surface of the back electrode body 22, and the welding layer 23 is used to provide a welding interface for the solar cell to connect with different solar cells to form a solar cell array. The material of the welding layer 23 is Ag, and the purity of Ag is not less than 99.99%. The thickness of the welding layer is 0.1μm to 3μm.

[0042] Furthermore, in some preferred embodiments, the transition layer 21 and / or the welding layer 23 also have the same honeycomb structure 221 as the back electrode body 22, so as to achieve uniform stress distribution of the entire back electrode 20 structure, thereby further improving the bending resistance of the overall solar cell and reducing the risk of warping or fracture of the solar cell during production, processing and service.

[0043] The present invention also provides a method for preparing the back electrode of the above-mentioned thin solar cell for space use, comprising the following steps:

[0044] Step 1, providing a substrate. In this embodiment, the substrate is a Ge substrate.

[0045] Step 2, forming a photoresist pattern on the back side of the substrate to expose a honeycomb structure pattern on the back side of the substrate.

[0046] The back side of the substrate is subjected to a photolithography process, and after being coated with a photoresist and developed, a honeycomb structure pattern is formed on the back side of the substrate, so that in the subsequent step S3, different materials can be evaporated on the honeycomb structure pattern to form a back electrode with a honeycomb structure.

[0047] In this embodiment, step 2 comprises:

[0048] Step 2.1, placing the substrate on a coating machine, coating a negative photoresist on the back of the substrate, placing the substrate in an oven at a temperature higher than 80° C. for more than 20 minutes to remove the solvent and form a negative photoresist layer.

[0049] Step 2.2, a mask is covered on the negative photoresist layer, the mask has a honeycomb structure pattern, the substrate is placed on an exposure table for exposure for 10s to 30s, and then the substrate is placed in an oven at a temperature higher than 80°C for more than 20min to increase the hardening degree of the exposed negative photoresist. During the exposure process, the negative photoresist area not covered by the honeycomb structure pattern of the mask will be hardened due to exposure, and the area will not be dissolved by the developer during subsequent development.

[0050] Step 2.3, placing the substrate in a developing tank containing a developer, and allowing it to stand for 250s to 500s to develop, so as to remove the unexposed negative photoresist (ie, the negative photoresist area covered by the honeycomb structure pattern of the mask).

[0051] Step 2.4: placing the substrate in clean water for cleaning for 80s to 300s to remove the developer remaining on the back side of the substrate.

[0052] The step 2.3 and the step 2.4 are cycled two to three times to form a desired photoresist pattern to expose a honeycomb structure pattern on the back side of the substrate.

[0053] Step 2.5, placing the substrate in an oven at a temperature higher than 80° C. for more than 6 minutes to make the photoresist pattern more stable.

[0054] Step 3, sequentially vapor-depositing the transition layer, the back electrode body, and the welding layer on the honeycomb structure pattern to form a back electrode with a honeycomb structure.

[0055] The substrate is placed in a reaction chamber of an evaporation device, and material vapor of a transition layer is introduced into the reaction chamber through the evaporation device to form a transition layer with a honeycomb structure; then, material vapor of a back electrode body is introduced into the reaction chamber through the evaporation device to form a back electrode body with the same honeycomb structure; finally, material vapor of a welding layer is introduced into the reaction chamber through the evaporation device to form a welding layer with the same honeycomb structure, thereby forming a back electrode with a honeycomb structure.

[0056] In this embodiment, step 3 includes:

[0057] Step 3.1, fixing the substrate formed with the honeycomb structure pattern on a support sheet inside the evaporation equipment.

[0058] Step 3.2, adding Pd into the crucible of the evaporation device, setting the evaporation rate of Pd to 2 angstroms / second to 15 angstroms / second, and after Pd is heated and evaporated inside the evaporation device, it is deposited on the honeycomb structure pattern to form a Pd layer.

[0059] Step 3.3, add Ag into the crucible of the evaporation device, set the evaporation rate of Ag to 4 angstroms / second to 20 angstroms / second, and after Ag is heated and evaporated inside the evaporation device, it is deposited on the Pd layer to form an Ag layer. At this point, a transition layer with a honeycomb structure is formed on the back of the substrate.

[0060] Step 3.4, adding Cu into the crucible of the evaporation equipment, setting the evaporation rate of Cu to 8 Å / s to 25 Å / s, after Cu is heated and evaporated inside the evaporation equipment, it is deposited on the Ag layer to form a Cu layer, that is, forming a back electrode body with a honeycomb structure.

[0061] Step 3.5, adding Ag into the crucible of the evaporation equipment, setting the evaporation rate of Ag to 4 Å / s to 20 Å / s, after Ag is heated and evaporated inside the evaporation equipment, it is deposited on the Cu layer to form an Ag layer, that is, a welding layer with a honeycomb structure is formed.

[0062] Step 4: soaking the substrate in an organic solution.

[0063] In step 3, when the substrate is placed in the reaction chamber of the evaporation equipment for evaporation, since the front and side of the substrate will be partially exposed in the reaction chamber without shielding, a small amount of metal material will be evaporated to these areas. Therefore, it is necessary to soak the substrate in an organic solution to remove impurities. The organic solution can be any one of ethanol or acetone. The impurities include metal materials and other residues evaporated in the undesirable areas of the substrate surface. In this embodiment, the substrate is soaked in acetone for more than 60 minutes.

[0064] Furthermore, the substrate is placed in anhydrous ethanol at a temperature higher than 45° C. for more than 30 seconds to completely remove the organic solution and moisture.

[0065] Step 5, attaching a mask having the honeycomb structure pattern on the back electrode on the back side of the substrate, stripping off the negative photoresist outside the honeycomb structure pattern of the mask (ie, the negative photoresist exposed in step S2.2), and then removing the mask.

[0066] Step 6: performing high temperature annealing treatment on the substrate.

[0067] During the high-temperature annealing process, an alloying process occurs inside the back electrode to increase the bonding strength between the substrate and the transition layer, between the transition layer and the back electrode body, and between the back electrode body and the welding layer, thereby preventing detachment (stratification) between different layers inside the back electrode, helping to reduce internal stress and improving the structural stability of the back electrode.

[0068] In this embodiment, the substrate is placed in an alloy furnace and annealed at a temperature above 300° C. for at least 3 minutes. After the annealing is completed, a back electrode of a thin solar cell for space use with low mass, high strength and strong bending resistance is obtained.

[0069] In summary, the present invention provides a thin solar cell back electrode for space use, which adopts a honeycomb structure and effectively solves the high warping problem of existing solar cells. The honeycomb structure not only improves the overall strength and structural stability of the back electrode by uniformly dispersing stress, provides effective support for the solar cell, and reduces the risk of warping, but also, due to its high porosity, significantly reduces the weight of the material while maintaining strength. In addition, the thermal expansion coefficient of the back electrode material matches that of the solar cell substrate, reducing the internal stress caused by temperature changes, further enhancing the structural stability and bending resistance of the solar cell, and making the back electrode have the advantages of both high strength and light weight.

[0070] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A back electrode of a thin solar cell for space use, characterized in that: It includes a back electrode body, which is arranged on the back side of the solar cell substrate; The back electrode body has a honeycomb structure, which is composed of a plurality of honeycomb pore units. The side walls of the honeycomb pore units form a regular hexagon, and adjacent honeycomb pore units are connected through the side walls.

2. The back electrode of the thin solar cell for space use according to claim 1, characterized in that: The Young's modulus of the material of the back electrode body is greater than 120 GPa, the Poisson's ratio is 0.3-0.4, and the thermal expansion coefficient is 4.5×10 -6 m / K~20×10 -6 m / K.

3. The back electrode of the thin solar cell for space use according to claim 2, characterized in that: The material of the back electrode body is a conductive metal material, and the conductive metal material at least includes any one of Cu and Mo.

4. The back electrode of the thin solar cell for space use according to claim 1, characterized in that: The inner regular hexagon formed by the inner wall of the honeycomb pore unit side wall has a side length of 0.175mm to 0.875mm; the outer regular hexagon formed by the outer wall of the honeycomb pore unit side wall has a side length of 0.376mm to 1.732mm; the thickness of the honeycomb pore unit side wall is 0.02mm to 1.12mm.

5. The back electrode of the thin solar cell for space use according to claim 1, characterized in that: The thickness of the back electrode body is 0.002 mm to 0.02 mm.

6. The back electrode of the thin solar cell for space use according to claim 1, characterized in that: A transition layer is provided between the back electrode body and the solar cell substrate for forming an ohmic contact with the solar cell substrate; the transition layer has the same honeycomb structure as the back electrode body.

7. The back electrode of the thin solar cell for space use according to claim 1, characterized in that: A welding layer is provided on the bottom surface of the back electrode body for providing a welding interface for the solar cell so as to be connected with different solar cells to form a solar cell array; the welding layer has the same honeycomb structure as the back electrode body.

8. The back electrode of the thin solar cell for space use according to claim 6, characterized in that: The transition layer includes a first metal layer and a second metal layer; the material of the first metal layer is Pd, and the thickness of the first metal layer is 0.02 μm to 1 μm; the material of the second metal layer is Ag, and the thickness of the second metal layer is 0.1 μm to 2 μm.

9. The back electrode of the thin solar cell for space use according to claim 7, characterized in that: The material of the welding layer is Ag, and the thickness of the welding layer is 0.1 μm to 3 μm.

10. A method for preparing a back electrode of a thin solar cell for space use as claimed in any one of claims 1 to 9, characterized in that: include: providing a substrate; forming a photoresist pattern on the back side of the substrate to expose a honeycomb structure pattern on the back side of the substrate; A transition layer, a back electrode body and a welding layer are sequentially evaporated on the honeycomb structure pattern to form a back electrode with a honeycomb structure.

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