Selectively doped TOPCon solar cell and preparation method thereof

Abstract

The invention discloses a selectively doped TOPCon solar cell and a preparation method thereof, and belongs to the technical field of photovoltaics. In order to solve the problems of'contact-recombination 'contradiction and difficult process adjustment existing in the uniform doping of the front surface of the existing TOPCon battery, the invention proposes to divide an emitter region on the front surface of the battery into a metal contact region and a non-metal contact region. According to the preparation method, after conventional boron diffusion, high doping of a metal contact area and non-doping of a non-metal contact area are realized through three procedures of laser ablation, selective texturing and thermal oxidation in sequence. According to the structure, the metal contact resistance is effectively reduced, meanwhile, doping induced recombination of carriers in an illumination area is thoroughly eliminated, and the conversion efficiency of the cell is improved by more than 0.2%. In addition, the structure is better in compatibility with low-silver-content paste, the silver paste cost can be greatly reduced, the production process is simplified, and the process stability is improved.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and more specifically to a TOPCon solar cell based on selective doping and its fabrication method. Background Technology

[0002] TOPCon (Tunnel Oxide Passivated Contact) is a high-efficiency crystalline silicon solar cell based on tunneling oxide layer and polycrystalline silicon passivated contact technology. Compared with traditional PERC (Passivated Emitter and Rear Cell) cells and other high-efficiency technologies such as HJT (Heterojunction Technology), TOPCon cells have the following core advantages: First, they have higher conversion efficiency, with mass production efficiencies generally reaching over 25%, and laboratory efficiencies exceeding 26%, significantly higher than PERC cells (approximately 23.5%); second, they have a better temperature coefficient, with less power decay in high-temperature environments, resulting in actual power generation 3%-5% higher than PERC cells under most climatic conditions; finally, their manufacturing process is highly compatible with existing PERC production lines, allowing for upgrades and modifications to existing lines, significantly reducing the technology iteration costs for companies.

[0003] The existing typical process flow for TOPCon cells includes texturing, boron doping, alkaline polishing, deposition of tunneling oxide and polycrystalline silicon layers (crystalline silicon), annealing, RCA cleaning, atomic layer deposition (ALD), front and back passivation film deposition, screen printing, sintering, and testing and sorting. Among these, boron atom doping (boron doping) on ​​the front side of the cell is a key step in forming the P-type emitter and constructing the PN junction. The purpose of this doping process is to form good conductive contacts; increasing the doping concentration effectively improves the ohmic contact performance between the metal electrode and the silicon wafer. The cell structure fabricated using existing processes is as follows: Figure 1 As shown.

[0004] However, this existing technology's uniform doping inherently presents a "contact-recombination" contradiction: to improve the contact between the metal electrode and silicon, a high doping concentration is required; but a high concentration of dopant atoms introduces recombination centers into the silicon bandgap. These recombination centers act like "traps," capturing electrons and holes, causing charge carriers to recombine and disappear, thereby reducing the battery's open-circuit voltage and conversion efficiency. In actual production, to achieve a balance between contact and recombination, the doping concentration is controlled within a compromise range, preventing the battery efficiency from reaching its theoretical maximum.

[0005] Furthermore, setting this equilibrium point also brings significant flexibility issues and capacity losses in actual production. With changes in upstream materials such as metal pastes, the optimal doping concentration needs to be adjusted accordingly to match the new contact characteristics. Each process adjustment has an adjustment window (usually exceeding 24 hours), during which the produced cells have lower efficiency, leading to direct production and economic losses.

[0006] Therefore, existing front-side doping technologies for TOPCon batteries struggle to simultaneously achieve optimal metal contact and minimal carrier recombination, hindering breakthroughs in battery efficiency and impacting production stability and economics due to frequent process adjustments during mass production. Consequently, there is an urgent need in this field for a battery and fabrication method capable of overcoming these bottlenecks. Summary of the Invention

[0007] This invention aims to solve the problems of efficiency limitations and difficulties in adjusting the manufacturing process caused by uniform front-side doping in existing TOPCon batteries. To achieve the above objectives, this invention adopts the following technical solution: In a first aspect, the present invention provides a method for fabricating a selectively doped TOPCon solar cell, comprising the following steps: S1. Texturing and Boron Diffusion: Texturing and boron diffusion are performed on the silicon wafer to form a heavily doped layer and a borosilicate glass layer. S2. Laser ablation: Laser ablation is performed on the silicon wafer after boron diffusion is completed to selectively remove the borosilicate glass layer in some areas in order to define the metal contact area and the non-metal contact area. S3. Selective texturing: The laser-treated silicon wafer is immersed in an alkaline etching solution to remove the silicon material and the underlying boron doped layer in the non-metallic contact area, and to form a new textured surface in the non-metallic contact area while retaining the boron doped layer in the metallic contact area. S4. Thermal oxidation: The silicon wafer that has been selectively texturized is subjected to thermal oxidation treatment to grow a silicon dioxide protective layer in the non-metallic contact area. S5. Perform subsequent processes for conventional TOPCon cells, including at least the deposition of the back tunneling oxide layer and polycrystalline silicon layer, the deposition of the front passivation film, and the fabrication of the electrodes.

[0008] Preferably, the subsequent processes of the conventional TOPCon cell described in step S5 include alkaline polishing, crystalline silicon, annealing, RCA cleaning, ALD alumina, front and back films, printing, sintering, laser-enhanced contact optimization, and testing and sorting.

[0009] Among them, alkaline polishing: using an alkaline solution to polish the silicon wafer to remove the diffusion layer on the back and edges of the silicon wafer, so as to avoid the formation of leakage channels.

[0010] Crystalline silicon: A dense silicon oxide tunneling layer, 1-2 nanometers thick, is formed on the back of the silicon wafer using thermal oxidation or plasma oxidation techniques. This film allows electrons to pass through rapidly through the "tunneling effect" while preventing holes and reducing carrier recombination. A doped polycrystalline silicon layer, or polysilicon layer, is then deposited on the silicon oxide tunneling layer using chemical vapor deposition.

[0011] Annealing: High temperature activates the doped phosphorus atoms in the poly silicon layer, enabling them to conduct electricity and repairing defects generated during silicon wafer processing, thus optimizing passivation quality.

[0012] RCA cleaning: By chemical etching, the borosilicate glass layer and the silicon dioxide protective layer generated in step (4) in the metal contact area on the front of the battery are removed, and the polycrystalline silicon or doped layer that may have diffused or deposited on the front and edge of the silicon wafer on the back of the battery in step (6) is removed to avoid the formation of leakage channels. ALD (Aluminum Oxide): A dense aluminum oxide film is prepared on the front and back of the battery using atomic layer deposition technology.

[0013] Front and back films: Silicon nitride films are redeposited on aluminum oxide films by plasma-enhanced chemical vapor deposition (PECVD).

[0014] Printing: Silver paste is printed on the metal contact area on the front of the battery using screen printing technology to form the front electrode, and silver paste is printed on the grooved area on the back to form the back electrode.

[0015] Sintering: The printed electrode cell is placed in a high-temperature sintering furnace, so that the silver paste electrode penetrates the silicon nitride / alumina film on the front side and forms a good ohmic contact with the heavily boron-doped layer below, ensuring effective current conduction.

[0016] Laser Enhanced Contact Optimization (LECO): A large number of charge carriers are generated by laser irradiation. The charge carriers conduct at local points between the electrode and the silicon wafer to form a high current. The resulting high-temperature molten silver silicon forms an ohmic contact, achieving better ohmic contact and reducing surface damage.

[0017] Testing and sorting: Photovoltaic performance tests are conducted on the sintered solar cells, and they are graded and sorted according to parameters such as conversion efficiency and open-circuit voltage.

[0018] The core of this invention lies in introducing three key processes sequentially after the conventional boron diffusion step: laser ablation, selective texturing, and thermal oxidation.

[0019] Laser ablation: This method uses a laser to selectively remove borosilicate glass layers from specific areas, thereby defining the pattern that needs to be processed subsequently, namely the metal contact area and the non-metal contact area.

[0020] Selective texturing: Utilizing the difference in etching rates between silicon and borosilicate glass using a specially formulated alkaline etching solution, doped silicon in the non-metallic contact area is precisely removed, restoring it to an undoped state and forming a textured surface structure, while the doped layer in the metallic contact area remains unaffected.

[0021] Thermal oxidation: A thin layer of silicon dioxide is grown in the exposed undoped silicon region to protect the silicon in the region from corrosion during the subsequent alkaline polishing process.

[0022] Preferably, in step S1, the boron doping concentration of the heavily doped layer is 0.8 × 10⁻⁶. 21 -1.2×10 21 atoms / cm³, contact resistance is 0.6×10 -5 -1.5×10 -5 Ω cm 2 .

[0023] Preferably, in step S1, the diffusion source for boron diffusion is boron trichloride or boron tribromide, the diffusion temperature is 950-1100℃, and the diffusion time is 60-180 minutes.

[0024] Preferably, in step S2, the laser is a picosecond laser with an energy density ≥4 mJ / mm², a laser wavelength of 532nm, a pulse width of 100kHz, and a spot diameter of 190 micrometers.

[0025] Preferably, in step S3, the alkaline corrosive solution is a KOH solution with a mass fraction of 4%-6%, the temperature is 55-85℃, and the soaking time is 60-440 seconds.

[0026] Preferably, in step S3, the depth to which the silicon material in the non-metallic contact area is removed during the selective texturing step is 1-4 μm.

[0027] Preferably, in step S4, the thermal oxidation temperature is 700-900℃, and the oxidation is carried out for 15-60 minutes in a mixed atmosphere with a nitrogen to oxygen volume ratio of 10:(0.5-1), and the thickness of the silicon dioxide protective layer is 5-15nm.

[0028] Secondly, the present invention provides a selectively doped TOPCon solar cell prepared by the above method, comprising: a patterned metal contact region and a non-metal contact region on the front side of an N-type silicon substrate; the metal contact region being a heavily boron-doped layer, on which a metal electrode is covered; the non-metal contact region being an intrinsic silicon region formed after removing the doped layer, wherein an aluminum oxide film and a silicon nitride film are sequentially deposited on the surface of the non-metal contact region; and a silicon oxide tunneling layer, a doped polycrystalline silicon layer, an aluminum oxide film, and a silicon nitride film are sequentially disposed on the back side of the N-type silicon substrate. The structure is as follows. Figure 2As shown.

[0029] Preferably, the boron doping concentration in the metal contact region is not less than 0.8 × 10⁻⁶. 21 atoms / cm 3 The boron doping concentration in the non-metallic contact region is less than 1×10⁻⁶. 19 atoms / cm 3 .

[0030] The front emitter of this battery is no longer uniform, but is patterned into a metal contact region and a non-metal contact region. The metal contact region is kept with a high concentration of doping to ensure excellent ohmic contact; the non-metal contact region is undoped, which fundamentally eliminates doping recombination loss in this area, and its surface is covered with a silicon dioxide protective layer.

[0031] Compared with the prior art, the beneficial technical effects of this invention are reflected in: Significantly improved efficiency: By eliminating doping and recombination in the illuminated area (non-metallic contact area) and optimizing the conductivity of the metallic contact area, the battery's conversion efficiency breaks through the original bottleneck. Experimental data shows that compared with conventional processes, this invention can improve battery efficiency by more than 0.20% (e.g., from 26.024% to 26.261%).

[0032] Significantly reduced costs: This structure has more relaxed requirements for the contact area and can be well compatible with low silver content paste (reduced from 90% silver content to 82% silver content), significantly reducing the cost of silver paste on the front side of the battery.

[0033] High process stability: Due to the high doping design of the metal contact area, it is more adaptable to the slurry. Therefore, when changing the slurry, there is no need to frequently adjust the overall boron diffusion process parameters, which simplifies process control, shortens the process window period, and improves production yield and stability. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of a conventional TOPCon battery with uniform doping on the front side in the prior art. Figure 2 This is a schematic diagram of the front structure of the selectively doped TOPCon battery described in this invention; Figure 3 This is a schematic diagram of the structure after the texturing and boron diffusion steps of the present invention are completed; Figure 4 This is a schematic diagram of the structure after the selective texturing step of the present invention is completed; Figure 5 This is a schematic diagram of the structure after the thermal oxidation step described in this invention is completed. Detailed Implementation

[0035] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0036] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0037] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0038] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0039] Example 1 A method for fabricating a selectively doped TOPCon solar cell includes the following steps: (1) Texturing and Boron Diffusion: An N-type silicon substrate is provided, and the silicon wafer is subjected to conventional texturing to form a pyramidal textured structure; subsequently, boron diffusion is performed to form a heavily doped layer and a borosilicate glass layer. The boron diffusion employs a high-doping process, using boron trichloride as the diffusion source, with a diffusion temperature of 1050℃ and a diffusion time of 120 minutes, ultimately resulting in a boron doping concentration of 1.1 × 10⁻⁶ in the front emitter region. 21 atoms / cm 3 The contact resistance is 1×10 -5 Ω cm 2 The structure obtained in this step is as follows: Figure 3 As shown.

[0040] (2) Laser ablation: The silicon wafer that has completed boron diffusion is sent to a laser machine. A picosecond laser with a wavelength of 532nm, a repetition frequency of 100kHz, and a spot diameter of 190μm is used to ablate the borosilicate glass layer in the non-metallic contact area at an energy density of 5 mJ / mm² to form a preset pattern.

[0041] (3) Selective texturing: The laser-treated silicon wafer is immersed in a 5% KOH solution (containing TS52 additive from the Shichuang brand, concentration 0.7wt%) and treated at 70°C for 200 seconds. During this process, approximately 3.3 μm of silicon in the non-metallic contact areas without borosilicate glass protection is etched away, forming a new textured surface and completely removing the doped layer in that area; while the metallic contact areas protected by borosilicate glass remain unaffected. The structure obtained from this step is as follows: Figure 4 As shown.

[0042] (4) Thermal oxidation: The selectively texturized silicon wafer is placed in a tube furnace and treated at 760°C with nitrogen gas at a flow rate of 10 L / min and oxygen gas at a flow rate of 1 L / min for 35 minutes. This grows a 10 nm thick silicon dioxide protective layer in the non-metallic contact area, protecting the silicon from reacting with the alkaline solution in subsequent processes. The structure obtained in this step is as follows: Figure 5 As shown.

[0043] (5) Alkaline polishing: Alkaline solution is used to polish the silicon wafer to remove the diffusion layer on the back and edge of the silicon wafer and avoid the formation of leakage channels.

[0044] (6) Crystalline silicon: A dense silicon oxide tunneling layer with a thickness of 1-2 nanometers is generated on the back side of the silicon wafer using thermal oxidation or plasma oxidation technology. This film allows electrons to pass through quickly through the "tunneling effect" while preventing holes and reducing carrier recombination. A doped polycrystalline silicon layer, i.e., a poly silicon layer, is deposited on the silicon oxide tunneling layer using chemical vapor deposition technology.

[0045] (7) Annealing: High temperature activates the doped phosphorus atoms in the poly silicon layer, enabling them to conduct electricity and repairing defects generated during the processing of the silicon wafer, thus optimizing the passivation quality.

[0046] (8) RCA cleaning: Remove the borosilicate glass layer and the silicon dioxide protective layer generated in step (4) from the metal contact area on the front of the battery by chemical etching, and remove the polycrystalline silicon or doped layer that may have diffused or deposited on the front and edge of the silicon wafer on the back of the battery in step (6) to avoid the formation of leakage channels. (9) ALD aluminum oxide: A dense aluminum oxide film is prepared on the front and back of the battery using atomic layer deposition technology.

[0047] (10) Front and back films: Silicon nitride films are redeposited on aluminum oxide films by plasma enhanced chemical vapor deposition (PECVD).

[0048] (11) Printing: Silver paste is printed on the metal contact area on the front of the battery to form the front electrode by screen printing technology, and silver paste is printed on the grooved area on the back to form the back electrode.

[0049] (12) Sintering: The printed electrode cell is placed in a high-temperature sintering furnace so that the silver paste electrode penetrates the silicon nitride / alumina film on the front side and forms a good ohmic contact with the heavily boron-doped layer below, ensuring that the current is effectively discharged.

[0050] (13) Laser Enhanced Contact Optimization (LECO): A large number of charge carriers are generated by laser irradiation. The charge carriers conduct at local points between the electrode and the silicon wafer to form a high current. The high temperature molten silver silicon generated forms an ohmic contact, achieving better ohmic contact and reducing surface damage.

[0051] (14) Testing and sorting: Photovoltaic performance tests are conducted on the sintered solar cells, and they are sorted according to parameters such as conversion efficiency and open circuit voltage.

[0052] Comparative Example 1 The same N-type silicon substrate and subsequent TOPCon process steps as in Example 1 were used, except that after texturing in step (1), the cell was manufactured using a conventional boron diffusion process, i.e., a doping concentration of 0.2 × 10⁻⁶ was selected. 19 For batteries with atoms / cm³, steps (2)-(4) of this invention are skipped, and steps (5)-(14) are performed directly, that is, the uniform doped structure on the front side is maintained.

[0053] Performance testing The photoelectric performance of the batteries prepared in Example 1 and Comparative Example 1 was tested, and the test results are shown in Table 1.

[0054] Table 1

[0055] Test results show that the selective doping structure and method provided by this invention can significantly improve the conversion efficiency and other key performance parameters of TOPCon cells.

[0056] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

[0057] It should be particularly noted that the various embodiments listed in this specification and accompanying drawings are intended to illustrate the technical solutions and advantages of the present invention, and not to limit the scope of protection of the present invention. Without departing from the core ideas and technical effects of the present invention, those skilled in the art can make any form of improvement, substitution, combination, or modification to the structural arrangement, process parameters, material selection, control logic, etc., of the described embodiments; any obvious changes based on the same concept should be considered equivalent solutions of the present invention and should be included within the scope of protection defined by the claims of the present invention. The actual scope of protection of the present invention is determined by the appended claims and should be correctly understood in conjunction with the specification and accompanying drawings.

Claims

1. A method for fabricating a selectively doped TOPCon solar cell, characterized in that, Includes the following steps: S1. Texturing and Boron Diffusion: Texturing and boron diffusion are performed on the silicon wafer to form a heavily doped layer and a borosilicate glass layer. S2. Laser ablation: Laser ablation is performed on the silicon wafer after boron diffusion is completed to selectively remove the borosilicate glass layer in some areas in order to define the metal contact area and the non-metal contact area. S3. Selective texturing: The laser-treated silicon wafer is immersed in an alkaline etching solution to remove the silicon material and the underlying boron doped layer in the non-metallic contact area, and to form a new textured surface in the non-metallic contact area while retaining the boron doped layer in the metallic contact area. S4. Thermal oxidation: The silicon wafer that has been selectively texturized is subjected to thermal oxidation treatment to grow a silicon dioxide protective layer in the non-metallic contact area. S5. Perform subsequent processes for conventional TOPCon cells, including at least the deposition of the back tunneling oxide layer and polycrystalline silicon layer, the deposition of the front passivation film, and the fabrication of the electrodes.

2. The preparation method according to claim 1, characterized in that, In step S1, the boron doping concentration of the heavily doped layer is 0.8 × 10⁻⁶. 21 -1.2×10 21 atoms / cm³, contact resistance 0.6×10 -5 -1.5×10 -5 Ω cm 2 .

3. The preparation method according to claim 2, characterized in that, In step S1, the diffusion source used for boron diffusion is boron trichloride or boron tribromide, the diffusion temperature is 950-1100℃, and the diffusion time is 60-180 minutes.

4. The preparation method according to claim 1, characterized in that, In step S2, the laser is a picosecond laser with an energy density ≥4 mJ / mm², a wavelength of 532nm, a pulse width of 100kHz, and a spot diameter of 190 micrometers.

5. The preparation method according to claim 1, characterized in that, In step S3, the alkaline corrosive solution is a KOH solution with a mass fraction of 4%-6%, the temperature is 55-85℃, and the soaking time is 60-440 seconds.

6. The preparation method according to claim 1, characterized in that, In step S3, the depth to which the silicon material in the non-metallic contact area is removed is 1-4 μm.

7. The preparation method according to claim 1, characterized in that, In step S4, the thermal oxidation temperature is 700-900℃, and the oxidation is carried out for 15-60 minutes in a mixed atmosphere with a nitrogen to oxygen volume ratio of 10:(0.5-1). The thickness of the silicon dioxide protective layer is 5-15nm.

8. A selectively doped TOPCon solar cell prepared by the method according to any one of claims 1-7, characterized in that, include: The front side of the N-type silicon substrate has patterned metal contact areas and non-metal contact areas; The metal contact area is a heavily boron-doped layer, and a metal electrode is covered on the heavily boron-doped layer; The non-metallic contact region is an intrinsic silicon region formed after removing the doped layer, and an aluminum oxide film and a silicon nitride film are sequentially deposited on the surface of the non-metallic contact region. The back side of the N-type silicon substrate is sequentially provided with a silicon oxide tunneling layer, a doped polycrystalline silicon layer, an aluminum oxide film, and a silicon nitride film.

9. The TOPCon solar cell according to claim 8, characterized in that, The boron doping concentration in the metal contact area is not less than 0.8 × 10⁻⁶. 21 atoms / cm 3 The boron doping concentration in the non-metallic contact region is less than 1×10⁻⁶. 19 atoms / cm 3 .

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