Back structure of solar cell and method for manufacturing the same

Abstract

The back structure of a solar cell and a preparation method thereof, the back structure of the solar cell comprising a silicon substrate, a silicon dioxide layer, a transition layer and a boron-silicon conversion layer which are sequentially stacked; the transition layer is doped with boron oxide, and the content of the boron oxide at the interface between the transition layer and the silicon dioxide layer is less than the content of the boron oxide at the interface between the transition layer and the boron-silicon conversion layer. The back structure of the solar cell can be applied to the second texturing process of the solar cell to protect the back of the silicon substrate.

Description

Technical Field

[0001] This application relates to the field of solar cell technology, specifically to a back structure of a solar cell and its fabrication method. Background Technology

[0002] TOPCon cells (tunneling oxide passivated contact cells) have achieved mass production efficiencies exceeding 25.5%. To further improve efficiency, the industry has introduced front-side Polyfinger technology: laser-grooving is performed on the front side of the boron-diffused silicon wafer to deposit boron-doped polycrystalline silicon, forming localized passivated contacts to reduce metal recombination losses. This process involves two texturing processes. Before the second alkaline texturing, the front side needs to be cleaned with weak HF to form a uniform textured surface in the grooved area. During this process, the back side must be effectively protected to avoid corrosion from alkaline solutions and HF.

[0003] Conventional back-side protection solutions mainly fall into three categories: First, a silica sacrificial layer is used. Silica is grown on the back side via thermal oxidation after boron diffusion. However, this method suffers from weak interfacial bonding between the silica and the underlying borosilicate glass, making it prone to microcracks during thermal cycling. Second, PECVD is used to deposit a mask. However, this process is susceptible to pinhole defects due to prolonged alkaline texturing and requires additional equipment. Third, single-sided cleaning / texturing equipment is used, but the investment for a single unit is 2-5 million RMB, resulting in high modification costs and hindering widespread adoption. Therefore, a novel back-side protection technology that balances protective effectiveness, process window, and low cost is urgently needed. Summary of the Invention

[0004] The purpose of this application is to provide a back-side structure of a solar cell and its fabrication method, thereby solving the problems of poor protection effect and high cost of existing back-side protection technologies for solar cells.

[0005] To achieve the objectives of this application, the following technical solution is provided: In a first aspect, the present invention provides a back structure of a solar cell, comprising a silicon substrate, a silicon dioxide layer, a transition layer and a borosilicate conversion layer stacked sequentially; wherein the transition layer is doped with boron oxide, and the boron oxide content at the interface between the transition layer and the silicon dioxide layer is less than the boron oxide content at the interface between the transition layer and the borosilicate conversion layer.

[0006] In some embodiments, the boron oxide content in the transition layer increases in a gradient from the side facing the silicon dioxide layer to the side facing the borosilicate conversion layer along the thickness direction.

[0007] In some embodiments, the boron oxide content at the interface between the transition layer and the silicon dioxide layer is less than or equal to 2 wt%.

[0008] In some embodiments, the boron oxide content at the interface between the transition layer and the borosilicate conversion layer is 3wt% to 10wt%.

[0009] In some embodiments, both the silicon dioxide layer and the borosilicate conversion layer are doped with boron oxide, and the boron oxide content in the silicon dioxide layer is less than the boron oxide content in the borosilicate conversion layer.

[0010] In some embodiments, the boron oxide content in the transition layer near the silicon dioxide layer is greater than the boron oxide content in the silicon dioxide layer, and the boron oxide content in the transition layer near the borosilicate conversion layer is less than the boron oxide content in the borosilicate conversion layer.

[0011] In some embodiments, the boron oxide content in the silicon dioxide layer is less than or equal to 2 wt%.

[0012] In some embodiments, the boron oxide content in the transition layer is 0 wt% to 10 wt%.

[0013] In some embodiments, the boron oxide content in the borosilicate conversion layer is 3wt% to 10wt%.

[0014] In some embodiments, the thickness of the silicon dioxide layer is 6 nm to 15 nm.

[0015] In some embodiments, the thickness of the transition layer is 2nm to 5nm.

[0016] In some embodiments, the thickness of the borosilicate conversion layer is 4 nm to 10 nm.

[0017] In a second aspect, the present invention provides a method for fabricating a back structure of a solar cell, the method being used to fabricate a back structure of a solar cell as described in the first aspect, the method comprising: placing a silicon substrate in a boron diffusion furnace for boron diffusion to form an initial borosilicate conversion layer on the back side of the silicon substrate; and transferring the silicon substrate having the initial borosilicate conversion layer to an oxidation furnace for oxidation conversion to obtain a solar cell.

[0018] In some embodiments, the temperature for boron diffusion is 900°C to 1050°C.

[0019] In some embodiments, the thickness of the initial borosilicate conversion layer is 20 nm to 60 nm.

[0020] In some embodiments, the boron oxide content in the initial borosilicate conversion layer is 15wt% to 30wt%.

[0021] In some embodiments, the oxidation conversion temperature is 800°C to 900°C.

[0022] In some embodiments, the duration of the oxidative conversion is 10 min to 25 min.

[0023] In some embodiments, the volume ratio of oxygen to nitrogen in the oxidative conversion is 1:1 to 1:3.

[0024] The back structure of the solar cell provided by the present invention is formed on the back of a silicon substrate and includes a silicon dioxide layer, a transition layer and a borosilicate conversion layer. The transition layer can guide the gradual transition of boron oxide content between the silicon dioxide layer and the borosilicate conversion layer, without any abrupt interface of composition, and the coefficient of thermal expansion is continuously transitioned. This makes the solar cell exhibit excellent corrosion resistance during hydrofluoric acid cleaning and secondary texturing, and the corrosion is also removed in the subsequent alkaline polishing step without affecting the performance of the solar cell. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the back structure of a solar cell according to one embodiment; Figure 2 This is a flowchart illustrating the fabrication process of the back structure of a solar cell according to one embodiment. Figure 3 This is a flowchart illustrating the fabrication process of a solar cell according to one implementation method. Figure 4 This is a schematic diagram illustrating the fabrication process of the back structure of a solar cell according to one embodiment. Figure 5 This is a schematic diagram illustrating the corrosion resistance mechanism of the back structure of a solar cell in one embodiment. Figure 6 This is a schematic diagram of the back structure cross-section of the solar cell in Example 1 and an EDS line scan result image; Figure 7 These are mechanical differences in the back structure of the solar cells provided in the embodiments and comparative examples; Figure 8 This is a comparison of the morphology of the back structure of the solar cells provided in the embodiments and comparative examples, as well as the cell performance. Detailed Implementation

[0027] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0029] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used in this application includes any and all combinations of one or more of the associated listed items.

[0030] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0031] This invention provides a back-side structure for a solar cell; please refer to [reference needed]. Figure 1 It includes a silicon substrate 100, a silicon dioxide layer 200, a transition layer 300 and a borosilicate conversion layer 400 stacked sequentially; the transition layer 300 is doped with boron oxide, and the boron oxide content at the interface between the transition layer 300 and the silicon dioxide layer 200 is less than the boron oxide content at the interface between the transition layer 300 and the borosilicate conversion layer 400.

[0032] In a specific embodiment, the solar cell is preferably an N-type TOPCon solar cell. Unlike the traditional BSG application structure with boron diffusion junction on the front side, the silicon dioxide layer 200, transition layer 300 and borosilicate conversion layer 400 described in this invention are all arranged on the back side of the silicon substrate 100. The three work together to form the back side anti-corrosion and anti-oxidation back side structure of the solar cell of this invention.

[0033] In a specific embodiment, the silicon substrate 100 can be n-type monocrystalline silicon, which has... <100> The crystal orientation facilitates the formation of a uniform and regular back-side film, effectively ensuring the deposition uniformity and adhesion of each film layer and reducing interfacial recombination problems caused by crystal orientation defects. The resistivity of the silicon substrate 100 is 0.4 Ω·cm to 1.5 Ω·cm, and the thickness of the silicon substrate 100 is 130 μm to 170 μm. Optionally, its resistivity can be 0.6 Ω·cm, 0.8 Ω·cm, 1.0 Ω·cm, or 1.2 Ω·cm; and its thickness can be 130 μm, 140 μm, 150 μm, 160 μm, or 170 μm.

[0034] In a specific embodiment, the silicon dioxide layer 200 is disposed close to the silicon substrate 100. This layer uses dense silicon dioxide generated by thermal oxidation as the matrix material, and is uniformly doped with trace amounts of boron oxide. It should be noted that the boron oxide content in this layer is extremely low, forming a dense structure close to pure silicon dioxide, with Si-O-Si network bond angles approaching 148°. This achieves basic passivation of the back side of the silicon substrate 100, effectively reducing the carrier recombination rate on the silicon wafer surface.

[0035] In a specific embodiment, the borosilicate conversion layer 400 (BSG conversion layer) is positioned away from the substrate. This layer also uses thermally oxidized silicon dioxide as the substrate and is highly doped with boron oxide to form a stable borosilicate glass functional layer. It should be noted that some Si-O-Si bonds in the borosilicate conversion layer 400 are replaced by Si-OB bonds, resulting in a relatively loose yet continuous network structure. This effectively isolates the back of the battery from external moisture, acid and alkali corrosive media, and process impurities.

[0036] In a specific embodiment, a transition layer 300 is located between the silicon dioxide layer 200 and the borosilicate conversion layer 400, and includes thermally oxidized silicon dioxide and boron oxide doped therein. The transition layer 300 includes a first interface facing the silicon dioxide and a second interface facing the borosilicate conversion layer 400. The boron oxide content at the first interface is less than the boron oxide content at the second interface. Preferably, along the thickness direction, the boron oxide content in the transition layer 300 increases in a gradient from the side facing the silicon dioxide layer 200 to the side facing the borosilicate conversion layer 400.

[0037] In a specific embodiment, the boron oxide content in the transition layer 300 increases in a gradient, meaning there are no abrupt changes in composition or obvious interfacial delamination throughout, forming a uniform and controllable gradual transition of composition. The transition layer 300 can effectively alleviate the thermal stress generated by the stacking of multiple film layers, significantly optimize the interfacial bonding state, and significantly improve the overall interface quality and structural stability of the battery back side.

[0038] The back structure of the solar cell provided by the present invention is formed on the back of a silicon substrate 100, and includes a silicon dioxide layer 200, a transition layer 300, and a borosilicate conversion layer 400. The transition layer 300 can guide the gradual transition of boron oxide content between the silicon dioxide layer 200 and the borosilicate conversion layer 400, and there is no abrupt interface between the two. This allows the thermal expansion coefficient of the entire stack to achieve a smooth and continuous dynamic transition from the inside to the outside. This enables the solar cell to exhibit excellent corrosion resistance during hydrofluoric acid cleaning and secondary texturing, and the corrosion is synergistically removed in the subsequent alkaline polishing step without affecting the performance of the solar cell.

[0039] Meanwhile, since there are no abrupt cross sections in the back structure of the solar cell, the B2O3 content changes continuously from 100 on the silicon substrate to the surface, and the coefficient of thermal expansion can also transition continuously. In the subsequent thermal cycling of tunneling oxide layer preparation and sintering, the thermal stress of the back structure of the solar cell can be reduced by more than 50% compared with the combination of conventional silicon dioxide and borosilicate glass, effectively preventing the generation of interface microcracks and peeling defects.

[0040] In some embodiments, the boron oxide content at the interface between the transition layer 300 and the silicon dioxide layer 200 is less than or equal to 2 wt%. Optionally, the boron oxide content at the interface between the transition layer 300 and the silicon dioxide layer 200 can be 0 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, or 2 wt%. The extremely low boron oxide doping at this interface of the transition layer 300 can maximize the preservation of the dense passivation characteristics of the thermally oxidized silicon dioxide layer 200 and avoid the enrichment of high-concentration boron components at the silicon wafer passivation interface.

[0041] In some embodiments, the boron oxide content at the interface between the transition layer 300 and the borosilicate conversion layer 400 is 3wt% to 10wt%. Optionally, the boron oxide content at the interface between the transition layer 300 and the borosilicate conversion layer 400 can be 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, or 10wt%. This range can eliminate abrupt differences in composition between layers and, in conjunction with the overall gradient distribution of the transition layer 300, achieve a continuous and smooth transition in film composition and thermal expansion coefficient.

[0042] In some embodiments, the boron oxide content in the silicon dioxide layer 200 is lower than that in the borosilicate conversion layer 400. Through gradient doping control, the overall boron oxide doping content within the bottom silicon dioxide layer 200 is significantly lower than that in the top borosilicate conversion layer 400, resulting in an overall boron content distribution trend of low boron at the bottom and high boron at the top. This structural design avoids the problem of interface state elevation and passivation failure caused by direct contact between high boron components and the passivation interface of the silicon substrate 100, while maximizing the outer protective performance by utilizing the stable characteristics of the top high borosilicate glass.

[0043] In some embodiments, the boron oxide content of the transition layer 300 near the silicon dioxide layer 200 is greater than the boron oxide content in the silicon dioxide layer 200, while the boron oxide content of the transition layer 300 near the borosilicate conversion layer 400 is less than the boron oxide content in the borosilicate conversion layer 400. By gradually increasing the boron oxide content in each layer, the boron oxide content in each layer can be continuously increased step by step, eliminating the problem of abrupt changes in composition between adjacent layers. This allows the thermal expansion coefficient, physical and mechanical properties, and chemical properties of each layer to transition gradually, strengthening the interlayer stress buffering effect and improving the adhesion tightness and high-temperature process tolerance of the back layer.

[0044] In some embodiments, the boron oxide content in the silicon dioxide layer 200 is less than or equal to 2 wt%. Optionally, the boron oxide content in the silicon dioxide layer 200 can be 0 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, or 2 wt%. It should be noted that the boron oxide inside the silicon dioxide layer 200 is uniformly distributed throughout, and the overall boron oxide content of the silicon dioxide layer 200 is always less than or equal to the boron oxide content at the first interface of the transition layer 300.

[0045] In some embodiments, the boron oxide content in the transition layer 300 is 0 wt% to 10 wt%. Optionally, the boron oxide content in the transition layer 300 can be 1 wt% to 3 wt%, 1 wt% to 5 wt%, 1 wt% to 7 wt%, 1 wt% to 10 wt%, 2 wt% to 3 wt%, 2 wt% to 5 wt%, 2 wt% to 7 wt%, or 2 wt% to 10 wt%. Utilizing this wide and controllable range of boron content, and in conjunction with the gradient distribution characteristic that continuously increases from the inside out, the transition layer 300 can connect the low-boron silicon dioxide layer 200 to the high-boron borosilicate conversion layer 400.

[0046] In some embodiments, the boron oxide content in the borosilicate conversion layer 400 is 3wt% to 10wt%. The boron oxide content in the borosilicate conversion layer 400 can be 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, or 10wt%. It should be noted that the boron oxide inside the borosilicate conversion layer 400 is uniformly distributed, and the overall boron oxide content of the borosilicate conversion layer 400 is greater than or equal to the boron oxide content at the second interface of the transition layer 300, ensuring that the outermost protective layer maintains a high-boron stable composition and fully utilizes the excellent properties of borosilicate glass.

[0047] In some embodiments, the thickness of the silicon dioxide layer 200 is 6 nm to 15 nm. Optionally, the thickness of the silicon dioxide layer 200 can be 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, or 15 nm. Meeting the above range can maintain the interface passivation capability of the silicon substrate 100, effectively reduce the back-side interface recombination rate, ensure smooth carrier tunneling transport, and provide a flat and stable substrate interface for the transition layer 300, thus balancing passivation performance and electrical output performance.

[0048] In some embodiments, the thickness of the transition layer 300 is 2nm to 5nm. Optionally, the thickness of the transition layer 300 can be 2nm, 2.5nm, 3nm, 3.5nm, 4nm, 4.5nm, or 5nm. Meeting the above range allows for a continuous and stable increase in boron oxide content within a very small layer thickness range, achieving a smooth transition in composition and thermal expansion coefficient between the silicon dioxide layer 200 and the borosilicate conversion layer 400, releasing interlayer thermal stress, and thoroughly optimizing the interface bonding state without affecting the battery's electrical performance, thus eliminating interface delamination and microcrack defects.

[0049] In some embodiments, the thickness of the borosilicate conversion layer 400 is 4 nm to 10 nm. Optionally, the thickness of the borosilicate conversion layer 400 can be 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, or 10 nm. Meeting the above range can form a uniform and dense outer protective film, effectively blocking external corrosive media and process damage, stabilizing the overall structure of the back film layer, and at the same time not interfering with back carrier transport and electrode contact, thus adapting to mass production process requirements.

[0050] In some embodiments, the silicon dioxide layer 200 accounts for 60% to 75% of the thickness of the back structure (the combination of silicon dioxide layer 200, transition layer 300, and borosilicate conversion layer 400), and the borosilicate conversion layer 400 accounts for 25% to 40% of the thickness of the back structure. Optionally, the silicon dioxide layer 200 accounts for 60%, 65%, 70%, and 75% of the total thickness; and the borosilicate conversion layer 400 accounts for 25%, 30%, 35%, and 40% of the total thickness.

[0051] In some embodiments, the density of the silicon dioxide layer 200 is 2.18 g / cm³. 3 ~2.25g / cm 3 Optionally, the density of the silicon dioxide layer 200 can be 2.18 g / cm³. 3 2.19 g / cm 3 2.2g / cm 3 2.21 g / cm 3 2.22 g / cm 3 2.23 g / cm 3 2.24 g / cm 3 2.25g / cm 3 .

[0052] In some embodiments, the density of the borosilicate conversion layer 400 is 2.10 g / cm³. 3 ~2.17g / cm 3 Optionally, the density of the borosilicate conversion layer 400 can be 2.10 g / cm³. 3 2.11 g / cm 3 2.12 g / cm 3 2.13 g / cm 3 2.14 g / cm 3 2.15g / cm 3 2.16 g / cm 3 2.17 g / cm 3 .

[0053] In some embodiments, the refractive index of the silicon dioxide layer 200 is 1.46 to 1.47. The refractive index of the borosilicate conversion layer 400 is 1.47 to 1.49.

[0054] This invention provides a method for fabricating the back structure of a solar cell. Please refer to [link / reference]. Figure 2 The preparation method is used to fabricate the back structure of the solar cell provided in the above embodiments, and the preparation method includes: Step S100: The silicon substrate is placed in a boron diffusion furnace for boron diffusion to form an initial boron-silicon conversion layer on the back side of the silicon substrate. In step S200, the silicon substrate with the initial borosilicate conversion layer is transferred to an oxidation furnace for oxidation conversion to obtain the back structure of the solar cell.

[0055] In a specific embodiment, step S100 specifically includes: placing the silicon substrate in a boron diffusion furnace, using a boron source to perform boron diffusion at a preset temperature, and forming boron-doped P atoms on the front and back sides of the silicon substrate. + Type-type emitter; after diffusion, an initial borosilicate conversion layer is formed on both the front and back surfaces of the silicon substrate, and the thickness and compositional uniformity (1σ) of the initial borosilicate conversion layer are <3%.

[0056] In a specific embodiment, in step S100, the boron source can be BBr3 or BCl3. Formation of P + The silicon substrate of the emitter is controlled to have a sheet resistance of 120Ω / sq to 250Ω / sq; optionally, the sheet resistance can be 120Ω / sq, 130Ω / sq, 140Ω / sq, 150Ω / sq, 160Ω / sq, 170Ω / sq, 180Ω / sq, 190Ω / sq, 200Ω / sq, 210Ω / sq, 220Ω / sq, 230Ω / sq, 240Ω / sq, or 250Ω / sq.

[0057] In some embodiments, the temperature for boron diffusion in step S100 is 900℃~1050℃. Optionally, the temperature for boron diffusion can be 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, or 1050℃.

[0058] In some embodiments, in step S100, the thickness of the initial borosilicate conversion layer is 20 nm to 60 nm. Optionally, the thickness of the initial borosilicate conversion layer can be 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, or 60 nm. Preferably, the thickness of the initial borosilicate conversion layer is 30 nm to 50 nm.

[0059] In some embodiments, in step S100, the boron oxide content in the initial borosilicate conversion layer is 15 wt% to 30 wt%. Optionally, the boron oxide content in the initial borosilicate conversion layer can be 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 22 wt%, 24 wt%, 26 wt%, 28 wt%, or 30 wt%. Optionally, the boron oxide content in the initial borosilicate conversion layer is 20 wt% to 25 wt%.

[0060] In a specific embodiment, step S200 specifically includes: directly transferring the silicon substrate that has completed boron diffusion to an oxidation furnace and performing thermal oxidation conversion in an oxygen-containing atmosphere; wherein, the oxygen-containing atmosphere can be dry oxygen (O2) or a dry oxygen / nitrogen mixture; after the thermal oxidation conversion is completed, the initial borosilicate conversion layer is converted into a three-layer structure, namely a silicon dioxide layer, a transition layer and a borosilicate conversion layer.

[0061] In some embodiments, the oxidation conversion temperature in step S200 is 800℃~900℃. Optionally, the oxidation conversion temperature can be 800℃, 810℃, 820℃, 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, 890℃, or 900℃. Preferably, the oxidation conversion temperature is 830℃~870℃.

[0062] In some embodiments, the oxidation conversion time in step S200 is 10 min to 25 min. Optionally, the oxidation conversion time can be 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, 16 min, 17 min, 18 min, 19 min, 20 min, 21 min, 22 min, 23 min, 24 min, or 25 min. Preferably, the oxidation conversion time is 15 min to 20 min.

[0063] In some embodiments, in step S200, the volume ratio of oxygen to nitrogen in the oxidation conversion is 1:1 to 1:3.

[0064] In a specific embodiment, in step S200, please refer to... Figure 4 During the thermal oxidation conversion process, the initial borosilicate conversion layer undergoes three processes: densification of SiO2 components, migration and volatilization of B2O3, and growth of new SiO2 at the interface. After the reaction is complete, the initial borosilicate conversion layer transforms into a back-side structure with a total thickness of 10 nm to 25 nm. The thickness reduction is due to the partial volatilization of B2O3 reducing the total mass, and the volume shrinkage caused by the densification of the SiO2 network. The specific physicochemical reactions involved include: Figure 4 (a) in the figure represents the densification of the SiO2 component in the initial borosilicate conversion layer: the original amorphous SiO2 in the initial borosilicate conversion layer undergoes network recombination at high temperature, and the Si-O-Si bond angle converges from the dispersed distribution in the initial borosilicate conversion layer (average 140°~145°) to the characteristic value (148°) of thermally oxidized SiO2, thus increasing the network density. Figure 4 (b) illustrates the migration and volatilization of B2O3: In the initial borosilicate conversion layer, B2O3 migrates from the interface region near the silicon substrate towards the surface. The driving forces for this migration are the B2O3 concentration gradient (Fick diffusion) and the displacement effect of newly formed SiO2 on B2O3 during oxidation. The B2O3 reaching the surface volatilizes in gaseous form at temperatures above 800°C and under oxygen flow (the boiling point of B2O3 is 1860°C, but under airflow purging, the partial pressure of surface B2O3 remains at a low level, promoting continuous volatilization). Figure 4(c) represents the growth of newly formed SiO2 at the interface: silicon atoms in the silicon substrate diffuse to the interface through the formed oxide layer (diffusion coefficient D). Si Approximately 10 at 850°C -18 cm 2 / s), reacts with oxygen at the Si / SiO2 interface to generate new SiO2, following the Deal-Grove equation: t_ox = (X_ox / s), 2 ) / B + X_ox / (B / A), where B is the parabolic rate constant and B / A is the linear rate constant. Under dry oxygen conditions at 850°C, B≈0.02 μm 2 / h, B / A≈0.07 μm / h.

[0065] In a specific embodiment, in step S200, the initial borosilicate conversion layer is a precursor for the thermal oxidation reaction. Under preset reaction temperature and atmosphere conditions, the SiO2 component in the initial borosilicate conversion layer undergoes structural reorganization and densification at high temperature, while the B2O3 component migrates to the surface and partially volatilizes under the action of high temperature and oxygen flow. By precisely controlling the oxidation temperature, oxygen partial pressure and time, the initial borosilicate conversion layer is transformed into a back structure with a gradient structure.

[0066] In a specific embodiment, in step S200, the silicon dioxide layer corresponds to the portion of the initial borosilicate conversion layer near the interface with the silicon substrate. During the high-temperature oxidation process, silicon atoms in the silicon substrate diffuse to the interface and react with oxygen to generate new SiO2 (following the parabolic oxidation stage of the Deal-Grove model). At the same time, the original B2O3 in the initial borosilicate conversion layer is displaced and migrates towards the surface.

[0067] In a specific embodiment, in step S200, the borosilicate conversion layer corresponds to the portion of the initial borosilicate conversion layer near the interface with the silicon substrate. During the high-temperature oxidation process, although some B2O3 volatilizes (B2O3+1.5O2→B2O3↑, the gaseous products are carried away by the airflow), some B2O3 remains, thus forming the borosilicate conversion layer.

[0068] In a specific embodiment, in step S200, the transition layer corresponds to the middle position of the initial borosilicate conversion layer, wherein the B2O3 content is positively correlated with the thickness, that is, the content is lower near the silicon dioxide layer and higher near the borosilicate conversion layer. Its formation mechanism is that during the high-temperature oxidation process, the diffusion coefficient of B2O3 in the SiO2 matrix is ​​10. -16 ~10 -14 cm 2 During the migration of B2O3 molecules from the initial borosilicate conversion layer to the surface, they form a dynamic equilibrium with the newly formed SiO2 at the interface, resulting in a continuous gradient rather than a step change in the B2O3 concentration distribution.

[0069] For some implementation methods, please refer to Figure 2 The preparation method further includes the following steps before step S100: Step S000: The silicon substrate is subjected to a first texturing process to form a uniform pyramidal textured surface structure on the front and back sides of the silicon substrate.

[0070] In a specific embodiment, in step S000, an n-type monocrystalline silicon wafer is used ( <100> The silicon substrate (with a crystal orientation and resistivity of 0.4 Ω·cm to 1.5 Ω·cm) is cleaned and then subjected to anisotropic alkaline texturing in an alkaline texturing solution. The resulting pyramid size is controlled to be 1 μm to 3 μm. Furthermore, the silicon substrate after the first texturing is subjected to boron diffusion treatment. Optionally, the alkaline texturing solution can be a 1% to 3% KOH or IPA solution, and the texturing temperature can be 75℃ to 80℃.

[0071] This invention also provides a method for fabricating a solar cell, which is based on the above-described method for fabricating the back structure of a solar cell. Please refer to [reference needed]. Figure 3 It also includes the following steps: Step S300: A laser is used to scan a silicon substrate with a backside structure so that a grooved area is formed on the front side of the silicon substrate according to a preset pattern. Step S400: Immerse the silicon substrate with the grooved area in the pickling solution to remove the residue in the laser-grooved area; Step S500: The silicon substrate with the grooved area is subjected to a second texturing process to form a uniform pyramid textured surface in the laser-grooved textured area on the front side. Step S600: The silicon substrate with a pyramid textured surface is placed in an oxidation furnace for thermal oxidation to form a tunnel oxide layer in the laser-grooved textured area on the front side. Step S700: The silicon substrate with the tunneling oxide layer is acid-washed and polished to remove the back structure on the back side of the silicon substrate. In step S800, the silicon substrate is subjected to LPCVD deposition, phosphorus diffusion, passivation, screen printing, and sintering to obtain a solar cell.

[0072] In a specific embodiment, step S300 specifically includes: using a nanosecond or picosecond pulsed laser (wavelength 532nm~1064nm, preferably 532nm green light) to perform laser scanning and grooving on the front side of the silicon substrate according to a preset Polyfinger pattern. Optionally, the laser energy density is controlled at 0.5J / cm². 2 ~3J / cm 2 The groove width is 10μm~30μm.

[0073] In a specific embodiment, step S400 specifically includes: immersing the silicon substrate with the grooved area in a weak HF acid solution to remove borosilicate glass residue in the laser-grooved area. Optionally, the concentration of HF acid is 1%~5%, preferably 2%~3%; the immersion temperature of HF acid is 20℃~25℃, and the immersion time is 60s~120s.

[0074] In a specific embodiment, step S500 specifically includes: placing the pickled silicon substrate with the grooved area into an alkaline texturing solution. Since the laser-grouted area on the front side is not protected, a uniform pyramidal textured surface will be formed in the laser-grouted area on the front side. Optionally, the alkaline texturing solution can be a 1%~3% KOH or NaOH solution, the texturing temperature can be 70℃~80℃, and the texturing time can be 5min~10min.

[0075] In a specific embodiment, in step S500, since the back side of the silicon substrate is protected by a back structure, the acidic oxides on the borosilicate conversion layer will consume some of the OH groups. - This reduces the corrosive effect of the alkaline texturing solution on the modified surface; and the low permeability of the silica layer further prevents OH... - Diffusion into the silicon substrate. The overall etching rate of the back structure is less than or equal to 0.5 nm / min. After the second texturing process at 5 min to 10 min, the total loss of the back structure is 2.5 nm to 5 nm, and the back silicon substrate is not etched at all.

[0076] In a specific embodiment, step S600 specifically includes: placing a silicon substrate with a pyramidal textured surface in an oxidation furnace and performing thermal oxidation in an oxygen atmosphere or a mixed oxygen atmosphere, so as to form a tunneling oxide layer (SiO2) in the laser-grooved textured surface area on the front side. x (Thickness is 1.0nm~2.0nm).

[0077] In a specific embodiment, in step S600, the thermal oxidation temperature is 600℃~750℃. Optionally, the thermal oxidation temperature can be 600℃, 620℃, 640℃, 660℃, 680℃, 700℃, 720℃, or 750℃. Preferably, the thermal oxidation temperature is 650℃~700℃, and the thermal oxidation temperature is lower than the thermal oxidation conversion temperature in step S200 above, so that the back-side structure will not change significantly.

[0078] In a specific embodiment, in step S700, the silicon substrate with the tunneling oxide layer is subjected to HF acid cleaning to completely remove the back-side structure of the silicon substrate; then, an alkaline solution is used for polishing to make the back-side of the silicon substrate smooth. Optionally, the concentration of the HF acid cleaning solution is 5%~10%, and the cleaning time is 30s~60s; the alkaline solution used for polishing is KOH or TMAH with a concentration of 5%~20%, and the polishing temperature is 60℃~75℃.

[0079] In a specific embodiment, step S800 is a standard process for fabricating solar cells, including: depositing an intrinsic polycrystalline silicon layer using LPCVD technology, the thickness of which is 100nm~200nm; and performing phosphorus diffusion treatment to form n + -poly-Si, sheet resistance of 30Ω / sq~60Ω / sq; RCA cleaning; Al2O3 deposition on the front side with a thickness of 3nm~8nm; and SiN deposition using PECVD technology. x SiN x The thickness is 70nm~85nm; SiN is deposited on the back side using PECVD technology. x SiN x The thickness is 75nm~85nm; then silver paste is screen-printed on the front and back sides and sintered.

[0080] It should be noted that the back structure of the solar cell provided by this invention can be applied to the secondary texturing process of solar cells (step S500). Please refer to [link / reference]. Figure 5 Its differentiated corrosion resistance mechanisms include: Figure 5 (a) illustrates the corrosion process under weak hydrofluoric acid (HF): the corrosion reaction of SiO2 by HF is SiO2 + 6HF → H2SiF6 + 2H2O. For the silica layer, due to the integrity and high density of the Si-O-Si network, the penetration and reaction of HF molecules require overcoming a relatively high activation energy, resulting in a corrosion rate of 1 nm / min to 2 nm / min. For the borosilicate conversion layer, the presence of B2O3 disrupts part of the Si-O-Si network (the bond energy of the Si-OB bond is about 15% lower than that of the Si-O-Si bond), and this layer has a slightly lower density, resulting in a slightly higher HF corrosion rate (2 nm / min to 3 nm / min). However, since the silica layer accounts for more than 60% of the total thickness and is located in a critical position close to the silicon substrate, even if the borosilicate conversion layer is consumed relatively quickly in HF, the silica layer can still maintain effective protection for the silicon substrate. For weak HF cleaning, the total thickness loss of the back structure is 2 nm to 6 nm.

[0081] Figure 5(b) shows the corrosion process under alkaline conditions: the corrosion reaction of SiO2 by alkaline solution is SiO2 + 2OH-. - →SiO3 2- + H2O. In the borosilicate conversion layer, since B2O3 is an acidic oxide, it preferentially reacts with OH- in an alkaline environment. - The reaction forms a boron-rich buffer region on the surface of the borosilicate conversion layer, consuming some of the OH groups. - OH - The driving force for inward diffusion. Therefore, the effective corrosion rate of the back structure in alkaline solution does not exceed 0.5 nm / min, and the total loss of the back structure is 2.5 nm to 5 nm.

[0082] The technical solution of the present invention will be described in detail below through specific embodiments.

[0083] Example 1 This embodiment provides a solar cell and its fabrication method. This embodiment uses an n-type monocrystalline silicon wafer as the silicon substrate, and the wafer has the following specifications: <100> The crystal orientation is such that the resistivity is 0.8 Ω·cm to 1.2 Ω·cm, and the thickness is 150 μm. The specific steps of this preparation method include the following: (1) Fabrication of the back structure of the solar cell: After the first texturing, the silicon substrate was placed in a diffusion furnace containing BBr3 and diffused at 960°C to obtain a silicon substrate with a sheet resistance of 180Ω / sq and an initial borosilicate conversion layer thickness of 40nm; the B2O3 content in the initial borosilicate conversion layer was about 22wt%. Then, the silicon substrate was transferred to an oxidation furnace and thermally oxidized for 18min at 850°C under dry oxygen (O2 flow rate 12slm, N2 flow rate 8slm) to obtain a silicon substrate with a back structure, which includes a silicon dioxide layer, a transition layer and a borosilicate conversion layer.

[0084] (2) Fabrication of solar cells: The silicon substrate with the back structure is laser-grooved on the front side (wavelength 532nm); then it is cleaned with weak HF acid (3% HF, 90s); the cleaned silicon substrate is texturized a second time (2% KOH + texturizing additive, 75°C, 8min); the silicon substrate after the second texturization is thermally oxidized to form a 1.5nm thick tunneling oxide layer (680°C, O2); then it is polished with alkaline solution (10% KOH, 70°C, removal amount is 3μm) to obtain a semi-finished material; then the semi-finished material is processed into solar cells.

[0085] The back structure obtained in step (1) was detected by ellipsometer (wavelength 632.8 nm) and X-ray photoelectron spectroscopy (XPS). The total thickness of the back structure was measured to be 18 nm. Among them, the thickness of the silicon dioxide layer was 12 nm, the B2O3 content inside was less than 2 wt%, and the refractive index was 1.465; the thickness of the transition layer was 2 nm; the thickness of the borosilicate conversion layer was 4 nm, the B2O3 content was 6 wt%, and the refractive index was 1.480.

[0086] from Figure 6 The diagram shows a cross-sectional view of the back structure of the solar cell and the EDS line scan results. Figure 6 (a) in the diagram illustrates the relative positions of the three-layer structure; Figure 6 (b) in the figure represents the atomic-level flatness of the Si / SiO2 interface. Figure 6 (c) shows the gradient distribution of boron content from the silicon substrate to the surface as displayed by EDS line scan (the B signal gradually rises from near zero to a peak and then remains on a plateau).

[0087] The semi-finished material obtained in step (2) was inspected by transmission electron microscopy (TEM). No white circle defects were found on the back side, and the standard deviation of the back side reflectivity uniformity was <0.3%, confirming that there were no microcracks or peeling at the interface between the back side gradient structure and the silicon substrate. The test results of the obtained finished solar cell were Voc=746mV, FF=84.2%, and efficiency=25.6%.

[0088] Example 2 This embodiment provides a solar cell and its fabrication method. This embodiment uses an n-type monocrystalline silicon wafer as the silicon substrate, and the wafer has the following specifications: <100> The crystal orientation is such that the resistivity is 0.8 Ω·cm to 1.2 Ω·cm, and the thickness is 150 μm. The specific steps of this preparation method include the following: (1) Fabrication of the back structure of the solar cell: After the first texturing, the silicon substrate was placed in a diffusion furnace containing BBr3 and diffused at 960°C to obtain a silicon substrate with a sheet resistance of 180Ω / sq and an initial borosilicate conversion layer thickness of 40nm; the B2O3 content in the initial borosilicate conversion layer was about 22wt%. Then, the silicon substrate was transferred to an oxidation furnace and thermally oxidized for 15min at 880°C and dry oxygen (O2 flow rate 12slm, N2 flow rate 8slm) to obtain a silicon substrate with a back structure, which includes a silicon dioxide layer, a transition layer and a borosilicate conversion layer.

[0089] (2) Fabrication of solar cells: The silicon substrate with the back structure is laser-grooved on the front side (wavelength 532nm); then it is cleaned with weak HF acid (3% HF, 90s); the cleaned silicon substrate is texturized a second time (2% KOH + texturizing additive, 75°C, 8min); the silicon substrate after the second texturization is thermally oxidized to form a 1.5nm thick tunneling oxide layer (680°C, O2); then it is polished with alkaline solution (10% KOH, 70°C, removal amount is 3μm) to obtain a semi-finished material; then the semi-finished material is processed into solar cells.

[0090] The back structure obtained in step (1) was detected by ellipsometer (wavelength 632.8 nm) and X-ray photoelectron spectroscopy (XPS). The total thickness of the back structure was measured to be 16 nm. Among them, the thickness of the silicon dioxide layer was 11 nm; the thickness of the transition layer was 2 nm; the thickness of the borosilicate conversion layer was 3 nm; and the B2O3 content was 4 wt%.

[0091] The semi-finished material obtained in step (2) was inspected by transmission electron microscopy (TEM). No white circle defects were found on the back side, and the standard deviation of the back side reflectivity uniformity was <0.3%, confirming that there were no microcracks or peeling at the interface between the back side gradient structure and the silicon substrate. The test results of the obtained finished solar cell were Voc=747mV, FF=84.3%, and efficiency=25.7%.

[0092] Example 3 This embodiment provides a solar cell and its fabrication method. This embodiment uses an n-type monocrystalline silicon wafer as the silicon substrate, and the wafer has the following specifications: <100> The crystal orientation is such that the resistivity is 0.8 Ω·cm to 1.2 Ω·cm, and the thickness is 150 μm. The specific steps of this preparation method include the following: (1) Fabrication of the back structure of the solar cell: After the first texturing, the silicon substrate was placed in a diffusion furnace containing BBr3 and diffused at 960°C to obtain a silicon substrate with a sheet resistance of 180Ω / sq and an initial borosilicate conversion layer thickness of 40nm; the B2O3 content in the initial borosilicate conversion layer was about 22wt%. Then, the silicon substrate was transferred to an oxidation furnace and thermally oxidized for 22min at 820°C under dry oxygen (O2 flow rate 12slm, N2 flow rate 8slm) to obtain a silicon substrate with a back structure, which includes a silicon dioxide layer, a transition layer and a borosilicate conversion layer.

[0093] (2) Fabrication of solar cells: The silicon substrate with the back structure is laser-grooved on the front side (wavelength 532nm); then it is cleaned with weak HF acid (3% HF, 90s); the cleaned silicon substrate is texturized a second time (2% KOH + texturizing additive, 75°C, 8min); the silicon substrate after the second texturization is thermally oxidized to form a 1.5nm thick tunneling oxide layer (680°C, O2); then it is polished with alkaline solution (10% KOH, 70°C, removal amount is 3μm) to obtain a semi-finished material; then the semi-finished material is processed into solar cells.

[0094] The back structure obtained in step (1) was detected by ellipsometer (wavelength 632.8 nm) and X-ray photoelectron spectroscopy (XPS). The total thickness of the back structure was measured to be 20 nm. Among them, the thickness of the silicon dioxide layer was 12 nm; the thickness of the transition layer was 2 nm; the thickness of the borosilicate conversion layer was 6 nm; and the B2O3 content was 8 wt%.

[0095] The semi-finished material obtained in step (2) was inspected by transmission electron microscopy (TEM). No white circle defects were found on the back side, and the standard deviation of the back side reflectivity uniformity was <0.3%, confirming that there were no microcracks or peeling at the interface between the back side gradient structure and the silicon substrate. The test results of the obtained finished solar cell were Voc=745mV, FF=84.1%, and efficiency=25.5%.

[0096] Comparative Example 1 This comparative example provides a solar cell and its fabrication method. This comparative example uses an n-type monocrystalline silicon wafer as the silicon substrate, and the wafer has the following specifications: <100> The crystal orientation is such that the resistivity is 0.8 Ω·cm to 1.2 Ω·cm, and the thickness is 150 μm. The specific steps of this preparation method include the following: (1) Fabrication of the back structure of the solar cell: After the first texturing, the silicon substrate was placed in a diffusion furnace containing BBr3 and diffused at 960°C to obtain a silicon substrate with a sheet resistance of 180Ω / sq and an initial borosilicate conversion layer thickness of 40nm. Then, the silicon substrate was immersed in 10% HF for 5min to remove the initial borosilicate conversion layer. The silicon substrate was then transferred to an oxidation furnace and thermally oxidized at 800°C under dry oxygen (O2 flow rate 12slm, N2 flow rate 8slm) for 15min to obtain a pure silicon dioxide layer.

[0097] (2) Fabrication of solar cells: The silicon substrate with a silicon dioxide layer is laser-grooved on the front side (wavelength 532nm); then it is cleaned with weak HF acid (3% HF, 90s); the cleaned silicon substrate is texturized a second time (2% KOH + texturizing additive, 75°C, 8min); the silicon substrate after the second texturization is thermally oxidized to form a 1.5nm thick tunneling oxide layer (680°C, O2); then it is polished with alkaline solution (10% KOH, 70°C, removal amount is 3μm) to obtain a semi-finished material; then the semi-finished material is processed into solar cells.

[0098] The silicon substrate obtained in step (1) was examined by transmission electron microscopy (TEM). It was found that there was an obvious interface step between the silicon dioxide layer and the borosilicate glass covering area (because the silicon substrate was slightly corroded after the initial borosilicate conversion layer was removed by HF).

[0099] Transmission electron microscopy (TEM) revealed localized cracks (crack density approximately 5 cracks / cm) at the interface between the silicon dioxide layer and the formed borosilicate glass, resulting in localized degradation of the back-side passivation quality. The final solar cell tested showed a Voc of 742mV, an FF of 83.6%, and an efficiency of 25.2%.

[0100] from Figure 7 The mechanical differences between the back surface structures of the two different solar cells provided in the embodiments and comparative examples can be seen in the figures. Figure 7 (a) shows the stress distribution of the back structure of Example 1 in the finite element simulation during thermal cycling from 800°C to room temperature (maximum stress <50 MPa). Figure 7 (b) shows the stress distribution of the back structure of Comparative Example 1 under the same thermal cycle (interfacial stress > 200 MPa, stress concentration occurs). Figure 7 (c) shows the comparison of the interface integrity of the two structures after three thermal cycles (Example 1 has no cracks, while Comparative Example 1 has microcracks).

[0101] from Figure 8 The examples and comparative examples show a comparison of the morphology of the back structure of two different solar cells and their cell performance; among them, Figure 8 (a) shows optical photographs of the back of four samples (none should be without white circles); among them, Figure 8 (b) shows a comparison of TEM cross-sectional views (the interface in the example has no cracks, while the interface in the comparative example has microcracks); where, Figure 8 (c) is a bar chart comparing battery electrical performance parameters (Voc, FF, efficiency).

[0102] The test results from Examples 1-3 and Comparative Example 1 show that fabricating a backside structure on the back of the silicon substrate can effectively alleviate thermal stress and improve interface quality. Based on the transition layer connecting the silicon dioxide layer and the borosilicate conversion layer, there is no abrupt change in composition at the interface, and the coefficient of thermal expansion exhibits a continuous transition. During the subsequent thermal cycling of tunneling oxide layer preparation at 600-750°C and sintering at 700-850°C, the thermal stress inside the gradient structure was reduced by more than 50% compared to Comparative Example 1. TEM observation showed that the interface microcrack density decreased from 5 cracks / cm in the comparative example to no more than 0.5 cracks / cm, significantly improving the consistency of backside passivation quality.

[0103] Meanwhile, the alkaline environment provided in the preparation process can reduce the corrosion rate, and the B2O3 in the initial borosilicate conversion layer preferentially reacts with OH in the alkaline environment. - The reaction forms a transition layer, consuming some OH-. - OH - The driving force for inward diffusion. The effective etching rate of the entire back structure in 1%~3% KOH (70-80°C) is ≤0.5nm / min, which is more than 60% lower than that of Comparative Example 1 (0.5nm / min~1.5nm / min). For production lines with long texturing times (8 min~10 min), the total loss of the gradient structure is 2.5nm~5nm, while the loss of Comparative Example 1 is 4nm~15nm, expanding the process window by more than 3 times.

[0104] Furthermore, the fabrication process provided by this invention directly uses the initial borosilicate conversion layer, which is naturally formed after boron diffusion, as a thermal oxidation precursor, eliminating the need for additional deposition processes or material inputs. Compared to the PECVD approach, this invention consumes only oxygen and nitrogen, reducing the cost per wafer by 0.5 to 1.0 yuan. During the thermal oxidation conversion of the initial borosilicate conversion layer, the oxidation rate significantly decreases after the B2O3 is depleted, resulting in batch-to-batch uniformity (1σ) of the total thickness of the back-side structure being <5%, superior to Comparative Example 1. Therefore, for large-scale production (monthly capacity > 1GW), the improved thickness consistency directly translates into improved stability of the back-side passivation quality, reducing yield fluctuations by 30%.

[0105] Furthermore, the back-side structure achieves a balance between resistance to HF acid and alkali solutions. The silica layer primarily provides resistance to HF acid corrosion (corrosion rate 1 nm / min ~ 2 nm / min), while the borosilicate conversion layer provides the main alkali corrosion resistance through an acidic oxide buffering mechanism (effective corrosion rate ≤ 0.5 nm / min). The protective effect of the back-side structure in different chemical environments is superior to that of Comparative Example 1. Moreover, the total thickness of the back-side structure is 10 nm ~ 25 nm, and the removal time during the 5% ~ 10% HF pre-cleaning before alkali polishing does not exceed 15 seconds, without increasing production line time. The residual B2O3 in the borosilicate conversion layer dissolves as H3BO3 during HF cleaning, leaving no solid residue and avoiding contamination of subsequent production.

[0106] Furthermore, the embodiments provided by this invention can be completed in a conventional boron diffusion furnace or oxidation furnace without the need for additional equipment. The oxidation temperature (800-900°C) is lower than the boron diffusion temperature (900-1050°C), thus not causing boron diffusion redistribution.

[0107] In the description of the embodiments of this application, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and are not intended to 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 of this application.

[0108] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims

1. A back-side structure of a solar cell, characterized in that, It includes a silicon substrate, a silicon dioxide layer, a transition layer, and a borosilicate conversion layer stacked sequentially; the transition layer is doped with boron oxide, and the boron oxide content at the interface between the transition layer and the silicon dioxide layer is less than the boron oxide content at the interface between the transition layer and the borosilicate conversion layer.

2. The back structure of the solar cell according to claim 1, characterized in that, Along the thickness direction, the boron oxide content in the transition layer increases in a gradient from the side facing the silicon dioxide layer to the side facing the borosilicate conversion layer.

3. The back structure of the solar cell according to claim 1, characterized in that, The boron oxide content at the interface between the transition layer and the silicon dioxide layer is less than or equal to 2 wt%; The boron oxide content at the interface between the transition layer and the borosilicate conversion layer is 3wt%~10wt%.

4. The back structure of the solar cell according to claim 1, characterized in that, Both the silicon dioxide layer and the borosilicate conversion layer are doped with boron oxide, and the boron oxide content in the silicon dioxide layer is less than that in the borosilicate conversion layer.

5. The back structure of the solar cell according to claim 4, characterized in that, The boron oxide content in the transition layer near the silicon dioxide layer is greater than the boron oxide content in the silicon dioxide layer, while the boron oxide content in the transition layer near the borosilicate conversion layer is less than the boron oxide content in the borosilicate conversion layer.

6. The back structure of the solar cell according to claim 5, characterized in that, The boron oxide content in the silica layer is less than or equal to 2 wt%; and / or The boron oxide content in the transition layer is 0 wt% to 10 wt%; and / or The boron oxide content in the borosilicate conversion layer is 3wt%~10wt%.

7. The back structure of the solar cell according to claim 1, characterized in that, The thickness of the silicon dioxide layer is 6 nm to 15 nm; and / or The thickness of the transition layer is 2nm~5nm; and / or The thickness of the borosilicate conversion layer is 4 nm to 10 nm.

8. A method for fabricating the back structure of a solar cell, characterized in that, The preparation method is used to fabricate the back structure of a solar cell as described in any one of claims 1-7, and the preparation method includes: A silicon substrate is placed in a boron diffusion furnace for boron diffusion, forming an initial boron-silicon conversion layer on the back side of the silicon substrate; The silicon substrate having the initial borosilicate conversion layer is transferred to an oxidation furnace for oxidation conversion to obtain the back structure of a solar cell.

9. The preparation method according to claim 8, characterized in that, The temperature for boron diffusion is 900℃~1050℃; and / or The initial borosilicate conversion layer has a thickness of 20 nm to 60 nm; and / or The boron oxide content in the initial borosilicate conversion layer is 15wt%~30wt%.

10. The preparation method according to claim 8, characterized in that, The oxidation conversion temperature is 800℃~900℃; and / or The duration of the oxidation conversion is 10 min to 25 min; and / or The volume ratio of oxygen to nitrogen in the oxidation conversion is 1:1 to 1:3.

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