A stepped temperature cycle promoted boron diffusion process and application thereof

Abstract

The present application belongs to the technical field of solar cell manufacturing, and provides a boron diffusion process with step temperature circulation promotion and application thereof. The boron diffusion process comprises: placing a silicon wafer into a boron diffusion device, and sequentially performing pre-oxidation, pre-deposition and step temperature circulation boron diffusion promotion on the silicon wafer. The step temperature circulation boron diffusion promotion process comprises sequentially performed initial stage, middle stage and final stage, each stage adopts the circulation mode of temperature rising oxygen-free boron diffusion promotion and then temperature falling oxygen-containing boron diffusion promotion for multiple cycles, and the temperature rising temperature and temperature falling temperature of each stage are optimized to form a boron diffusion doped layer on the surface of the silicon wafer. The boron diffusion process can reduce the oxidation temperature, eliminate the concentric circle defect, increase the junction depth and surface doping concentration of boron diffusion doping, improve the uniformity of boron diffusion doping, further improve the FF, Jsc, Uoc and Eta of the cell, and also can reduce the thermal budget and improve the service life of the device.

Description

Technical Field

[0001] This invention relates to the field of solar cell manufacturing technology, and in particular to a stepped temperature cycle-driven boron diffusion process and its application. Background Technology

[0002] Currently, solar cells are mainly classified into three categories: crystalline silicon cells (such as PERC cells, TOPCon cells, HJT cells, IBC cells, etc.), thin-film cells (such as CIGS cells, CdTe cells), and novel cells (such as perovskite cells, organic cells). Among them, crystalline silicon cells dominate the market due to their high efficiency and mature technology. Thin-film cells have advantages in flexibility and lightweight, but their photoelectric conversion efficiency (referred to as cell efficiency) is relatively low. While novel cells such as perovskite cells have great potential for efficiency (theoretical cell efficiency exceeding 33%), their stability still needs improvement. Currently, N-type crystalline silicon cells such as TOPCon (tunneling oxide passivated contact cells) are becoming the mainstream development direction due to their high efficiency (mass-produced cell efficiency greater than or equal to 25%) and low degradation.

[0003] Boron diffusion in solar cells (such as TOPCon cells) is a key technology that incorporates boron atoms into silicon wafers through high-temperature thermal diffusion, primarily used to form P-type semiconductor regions (such as P+ emitters or back passivation layers). The core principle of boron diffusion is to decompose a boron source (such as BBr3 or BCl3) at high temperatures, forming borosilicate glass (BSG) on the silicon wafer surface. Boron atoms then diffuse into the silicon lattice, replacing silicon atom positions and forming acceptor levels, thus achieving P-type doping of the silicon wafer surface. However, during silicon wafer production, the high oxygen content (13-15 ppm) at the beginning and end of the silicon ingot makes it prone to oxygen precipitation during high-temperature boron diffusion. This leads to concentric circle defects in the cell's EL (electroluminescence) detection, reducing cell yield and increasing costs. These high-risk silicon wafers typically require price reductions or remelting, further increasing cell production costs.

[0004] To address the aforementioned shortcomings, existing solutions, such as CN117352595A, provide a boron diffusion process based on low-temperature cyclic push-bonding. After pre-oxidation and boron source deposition, cyclic boron diffusion push-bonding is performed at a constant push-bonding temperature. Each cycle includes an oxygen-free stage and an oxygen-filled stage, with at least five cycles performed. The push-bonding temperature during both the oxygen-free and oxygen-filled stages is kept constant (controlled at 950-1000℃). This reduces the oxygen-containing push-bonding time and temperature for boron diffusion, achieving lower energy consumption in boron diffusion production. It also helps improve concentric circle defects detected by battery EL (electrochemical detection) and achieves a higher surface boron doping concentration, thereby optimizing the battery's contact resistance. Furthermore, although this boron diffusion process can increase the boron doping concentration on the silicon wafer surface, the boron-doped junction depth is relatively shallow (the sheet resistance of Examples 1-4 in CN117352595A is 90-180 Ω / □, while the sheet resistance of the comparative example is 130-150 Ω / □), indicating that this cyclic boron diffusion process sacrifices some junction depth (the lower the sheet resistance, the shallower the junction depth). A shallower boron-doped junction depth increases the emitter resistance, leading to an increase in series resistance, which in turn reduces the fill factor of the cell and consequently lowers the cell efficiency.

[0005] In addition, CN115083893A discloses a low-temperature boron diffusion process, which, through three stages of variable-temperature boron diffusion deposition (785-890℃), low-temperature advancement (900-960℃), and oxidation after source activation (850-950℃), controls the peak temperature of boron diffusion below 1000℃, thus achieving low-temperature boron diffusion and helping to improve the concentric circle defects in battery EL detection. However, this low-temperature boron diffusion process has the following defects: (1) It has extremely high requirements for temperature difference control of 5-20℃ and precise time control of 1-5 minutes in each boron diffusion deposition stage. Even small process fluctuations may lead to deterioration of sheet resistance uniformity, so its process window is narrow. This low-temperature boron diffusion process, while reducing the temperature, puts forward higher requirements for process stability. (2) Moreover, although this low-temperature boron diffusion process can optimize sheet resistance uniformity, from its test structure, its boron doping effect is not good and the junction depth is still shallow, which is not conducive to improving the battery's fill factor, battery efficiency, and other electrical performance. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a stepped-temperature cyclic boron diffusion process and its application in the fabrication of solar cells.

[0007] Based on this, the present invention discloses a boron diffusion process with stepped temperature cycling, comprising the following process steps:

[0008] Step 1: Place the silicon wafer in a boron diffusion apparatus, heat it, and introduce oxygen to pre-oxidize the silicon wafer;

[0009] Step 2: Heat up and introduce a boron doping source to pre-deposit a borosilicate glass layer on the silicon wafer surface;

[0010] Step 3: Perform step-temperature cyclic boron diffusion on the pre-deposited silicon wafer, which specifically includes:

[0011] Step 31, Initial stage: First, perform oxygen-free boron expansion and push-junction at 1000-1100℃, then perform oxygen-boron expansion and push-junction at 920-980℃. Repeat this process multiple times to complete the push-junction in the initial stage.

[0012] Step 32, Intermediate Stage: First, perform oxygen-free boron expansion and push-bonding at 950-1010℃, then perform oxygen-boron expansion and push-bonding at 920-980℃. Repeat this process multiple times to complete the push-bonding in the intermediate stage.

[0013] Step 33, Final Stage: First, perform oxygen-free boron diffusion and push junction at 955-1015℃, then perform oxygen-filled boron diffusion and push junction at 870-930℃. Repeat this process multiple times to complete the final stage of push junction formation, thereby forming a boron-doped layer on the silicon wafer surface.

[0014] Preferably, in step 3, the number of cycles in each stage is 3-7 times, for a total of 9-21 cycles, and the oxidation flow rate for oxygen-boron propagation in each stage is 15000-25000 sccm; the oxygen introduction time in each cycle of each stage accounts for 20% of the current cycle time, and the time of each cycle in each stage is the same; the total process time of step 3 is 4400-4600s.

[0015] More preferably, in step 3, the number of cycles in each stage is 5, and the oxidation flow rate for oxygen-boron diffusion junction in each stage is 20000 sccm; when entering the oxygen-free boron diffusion junction, the N2 atmosphere is maintained; the oxygen introduction time in each cycle of each stage accounts for 20% of the total time of the current cycle, and the time of each cycle in each stage is the same; the entire process time of step 3 is 4500s.

[0016] More preferably, in the initial stage of step 31, the temperature of the oxygen-free boron diffusion junction is 1000°C, while the temperature of the oxygen-boron diffusion junction is 950°C.

[0017] More preferably, in the intermediate stage of step 32, the temperature of the oxygen-free boron extension junction is 980°C, while the temperature of the oxygen-boron extension junction is 950°C.

[0018] More preferably, in the final stage of step 33, the temperature of the oxygen-free boron diffusion junction is 985°C, while the temperature of the oxygen-boron diffusion junction is 900°C.

[0019] Preferably, in step 3, the temperature difference during the entire stepped temperature cycle boron diffusion process is less than or equal to 100°C.

[0020] Preferably, in step 1, the heating temperature for pre-oxidation is 780-820℃, the pressure is 120-180 mbr, the oxygen flow rate is 300-700 sccm, and the pre-oxidation process time is 220-260 s.

[0021] Preferably, in step 2, the pre-deposition temperature is 830-870℃, the boron doping source is BCl3, the flow rate of BCl3 is 100-140 sccm, the oxygen flow rate is 200-800 sccm, and the pre-deposition process time is 430-470 s.

[0022] This invention discloses the application of a stepped temperature cyclic boron diffusion process, that is, the application of a stepped temperature cyclic boron diffusion process to the fabrication of solar cells.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects:

[0024] The boron diffusion process of this invention, based on pre-oxidation and pre-deposition, incorporates the stepped temperature cyclic boron diffusion process described in steps 31-33 (this stepped temperature cyclic boron diffusion process includes an initial stage, a middle stage, and a final stage performed sequentially; each stage employs a cyclical method of heating up oxygen-free boron diffusion junctions and then cooling down oxygen-purifying boron diffusion junctions for multiple cycles, and the heating and cooling temperatures of each stage are optimized), to replace the high-temperature post-oxidation process in the traditional boron diffusion process. This reduces the oxidation temperature, effectively avoiding concentric circle defects in battery EL detection caused by oxygen atom precipitation in high-oxygen silicon wafers, while also reducing thermal budget and extending the lifespan of boron diffusion equipment such as quartz tubes.

[0025] Furthermore, in the stepped temperature cyclic boron diffusion process of the present invention, in the stepped temperature cyclic boron diffusion process of step 3, each stage of each cycle first raises the temperature to rapidly advance the junction depth without oxygen, then lowers the temperature and introduces oxygen to lock the surface boron doping concentration and repair lattice damage. While eliminating concentric defects, it can also increase the junction depth and surface doping concentration of the boron-doped layer, improve the uniformity of boron doping, and thus reduce the sheet resistance fluctuation of boron doping. Therefore, when this stepped temperature cyclic boron diffusion process is applied to the preparation of solar cells, it can further improve the fill factor, short-circuit current density, and open-circuit voltage of the cell, and further improve the cell efficiency.

[0026] Furthermore, compared to existing boron diffusion processes such as CN115083893A, the requirements for temperature difference control and time regulation at each stage are relatively relaxed, thus increasing the process window. Detailed Implementation

[0027] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to specific embodiments.

[0028] A stepped-temperature cyclic-progressive boron diffusion process according to the present invention includes the following process steps:

[0029] Step 1, Pre-oxidation: The silicon wafer is fed into a boron diffusion device, and O2 with a flow rate of 300-700 sccm is introduced at a temperature of 780-820℃ and a pressure of 120-180 mbr for a process time of 220-260 s to pre-oxidize the silicon wafer.

[0030] Step 2, Pre-deposition: At a temperature of 830-870℃, BCl3 with a flow rate of 100-140 sccm is introduced as a boron doping source, and O2 with a flow rate of 200-800 sccm is introduced. The process time is 430-470s to pre-deposit a borosilicate glass layer on the silicon wafer surface.

[0031] Step 3: Perform a stepped temperature cyclic boron diffusion process, which includes:

[0032] Step 31, Initial stage: First, perform oxygen-free boron expansion and push-bonding at 1000-1100℃, then perform oxygen-boron expansion and push-bonding at 920-980℃; repeat this cycle 3-7 times to complete the initial stage of push-bonding.

[0033] In step 31, during the oxygen-free boron diffusion process at 1000-1100℃ (preferably 1000℃), the high-temperature environment significantly increases the interstitial diffusion rate of boron, thus rapidly advancing the junction depth. Simultaneously, the oxygen-free conditions at this high temperature suppress boron oxidation and volatilization, preventing premature decrease in surface boron concentration. Subsequently, during the oxygen-bearing boron diffusion process at 920-980℃ (preferably 950℃), moderate cooling reduces thermal stress, and oxygen repairs lattice damage (such as silicon vacancies) caused by the high-temperature oxygen-free environment. Therefore, compared to existing isothermal diffusion processes (i.e., in the existing boron diffusion process such as CN117352595A, where the diffusion temperature remains constant between the oxygen-free and oxygen-bearing stages), the stepped-temperature cyclic boron diffusion process of this invention increases the junction depth while maintaining a stable high level of boron doping concentration on the silicon wafer surface, achieving efficient utilization of the initially deposited boron.

[0034] It should be noted that the oxygen-free boron diffusion process in step 31 has high requirements for temperature control (preferably 1050℃±50℃). If the temperature is too high, the boron will diffuse too quickly, resulting in an excessively deep junction and PN junction penetration. If the temperature is too low, the diffusion will be insufficient, resulting in an excessively shallow junction.

[0035] Step 32, Intermediate Stage: First, perform oxygen-free boron expansion and push-bonding at 950-1010℃, then perform oxygen-boron expansion and push-bonding at 920-980℃; repeat this cycle 3-7 times to complete the push-bonding in the intermediate stage.

[0036] In step 32, during the oxygen-free stage, appropriately lowering the temperature (slightly lower than the initial stage) can slow down the diffusion rate of boron, making the boron diffusion distribution more uniform and thus reducing abrupt changes in the boron diffusion concentration gradient. During the oxygen-introducing stage, the lattice repair effect of oxygen is maintained, and at the same time, through the synergistic regulation of temperature and oxygen, the activation efficiency of boron within the silicon wafer is optimized, making its concentration distribution more uniform, ultimately reducing the fluctuation of sheet resistance.

[0037] Step 33, Final Stage: First, perform oxygen-free boron expansion and push-bonding at 955-1015℃, then perform oxygen-boron expansion and push-bonding at 870-930℃; repeat this cycle 3-7 times to complete the final stage of push-bonding.

[0038] In step 33, during the oxygen-free stage, the temperature is lowered to lock in the depth of the formed boron-doped junction, effectively suppressing excessive boron diffusion. The oxygen-introducing stage employs a low-temperature process, significantly reducing boron volatilization while simultaneously repairing surface defects and passivating interface states.

[0039] In step 3, the delay in switching from oxygen-free to oxygen-filled will cause fluctuations in the surface boron doping concentration, so a rapid switch is required.

[0040] In step 3, the oxygen introduction time accounts for 20% of the total cycle time, meaning the oxygen-to-oxygen switching ratio is 1:4 (i.e., the ratio of oxygen-to-oxygen time is 1:4). Each of the three stages, 31, 32, and 33, is repeated 3-7 times, for a total of 9-21 cycles. The duration of each cycle is the same, and the oxygen introduction time is also the same (therefore, the oxygen-free time is also the same in each cycle). The entire process time for step 3 is 4400-4600 seconds.

[0041] In step 3, the oxygen flow rate is controlled at 15000-25000 sccm during each oxygenation process, and an N2 atmosphere is maintained when oxygenation stops (i.e., an N2 atmosphere is maintained during the oxygen-free process). By intermittent oxygenation, the high-temperature post-oxidation of the traditional boron diffusion process is avoided, the oxidation temperature is reduced, and concentric circle defects in battery EL detection can be effectively avoided.

[0042] After step 33, a boron-doped layer is formed on the silicon wafer surface. Compared to existing isothermal push-junction processes such as CN117352595A (i.e., in existing boron diffusion processes, the push-junction temperature remains constant during the oxygen-free and oxygen-filled stages), the junction depth of the boron-doped layer in this invention is increased by 30-50%, and the boron doping concentration is stably maintained at 2×10⁻⁶. 19 -4×10 19atoms / cm 3 (e.g., 3.5×10) 19 atoms / cm 3 This process is superior to existing isothermal push-junction processes such as CN117352595A; at the same time, the surface recombination rate is significantly reduced, which improves the minority carrier lifetime by 10-15%.

[0043] The present invention discloses a method for preparing a solar cell (taking a TOPCon cell as an example), comprising the following preparation steps:

[0044] Step 1: Texturing the pre-selected silicon wafer to form a textured surface on the wafer.

[0045] In step one, alkaline texturing is used, and the texturing time is controlled at 400-440 seconds to form a pyramid-shaped textured surface on the silicon wafer, which has a reflectivity of 8-10%.

[0046] Step 2: Perform boron diffusion treatment on the silicon wafer to prepare a boron-diffused emitter (i.e., a p+ emitter, which is a boron-diffused doped layer) on the front side of the silicon wafer, and cover the front side of the boron-diffused emitter with a borosilicate glass layer; while the back side and edges of the silicon wafer form a swirling diffusion layer, accompanied by a borosilicate glass layer.

[0047] In step two, the boron diffusion treatment refers to steps 1-3 of the stepped temperature cyclic boron diffusion process described in the above-described specific embodiments of the present invention. Of course, the stepped temperature cyclic boron diffusion process described in the above-described specific embodiments of the present invention can also be applied to the fabrication of other types of solar cells.

[0048] Step 3: Perform single-sided acid washing on the silicon wafer to remove the borosilicate glass layer on the back and edges of the silicon wafer.

[0049] Step 4: Perform alkaline polishing on the back of the silicon wafer to form a polished surface with a reflectivity of over 30% on the back of the silicon wafer; after polishing, remove the borosilicate glass layer on the front of the silicon wafer.

[0050] Step 5: Fabricate a TOPCon structure on the back side of the silicon wafer (which includes a tunneling oxide layer and an n-type doped amorphous silicon layer deposited sequentially on the back side of the silicon wafer, with the thickness of the tunneling oxide layer being 1-2 nm).

[0051] Step 6: Anneal the silicon wafer to transform the n-type doped amorphous silicon layer into an n-type doped polycrystalline silicon layer (i.e., an n+poly layer).

[0052] Step 7: Clean the front and back oxides generated during the silicon wafer annealing process using HF solution or HCl solution.

[0053] Step 8: Deposit a 2-10 nm thick aluminum oxide layer on the front side of the p+ emitter.

[0054] Step 9: Prepare a silicon nitride antireflection film on both the front side of the alumina layer and the back side of the n+poly layer. The thickness of the silicon nitride antireflection film on the front side is 79-85 nm, and the thickness of the silicon nitride antireflection film on the back side is 85-95 nm.

[0055] Step 10: Apply metal paste to both the front and back sides of the silicon wafer using a screen printing process, and then sinter to form metal electrodes. The lower end of the front metal electrode passes through the front silicon nitride antireflective film and the aluminum oxide layer in sequence, making ohmic contact with the p+ emitter; while the upper end of the back metal electrode passes through the back silicon nitride antireflective film, making ohmic contact with the n+ poly layer. After completing Step 10, the TOPCon battery of this invention is obtained.

[0056] The following are specific embodiments of a stepped temperature cyclic boron diffusion process and a method for preparing TOPCon batteries according to the present invention.

[0057] Example 1

[0058] This embodiment of a stepped-temperature cyclic-progressive boron diffusion process includes the following process steps:

[0059] Step 1, Pre-oxidation: The silicon wafer is fed into a boron diffusion device, and O2 with a flow rate of 500 sccm is introduced at a temperature of 800℃ and a pressure of 150 mbr for 240 s to pre-oxidize the silicon wafer.

[0060] Step 2, Pre-deposition: At a temperature of 850℃, BCl3 with a flow rate of 120 sccm and O2 with a flow rate of 600 sccm are introduced for a process time of 450s to pre-deposit a borosilicate glass layer on the silicon wafer surface.

[0061] Step 3: Perform a stepped temperature cyclic boron diffusion process, which includes:

[0062] Step 31, Initial stage: First, perform oxygen-free boron expansion and push-bonding at 1000℃, then perform oxygen-boron expansion and push-bonding at 950℃ (i.e., oxygen-free at 1000℃ + oxygen-bearing at 950℃); repeat this cycle 5 times to complete the initial stage of push-bonding.

[0063] Step 32, Intermediate Stage: First, perform oxygen-free boron expansion and push-bonding at 980℃, then perform oxygen-boron expansion and push-bonding at 950℃ (i.e., oxygen-free at 980℃ + oxygen-bearing at 950℃); repeat this cycle 5 times to complete the push-bonding in the intermediate stage.

[0064] Step 33, Final Stage: First, perform oxygen-free boron expansion and pushing at 985℃, then perform oxygen-boron expansion and pushing at 900℃ (i.e., oxygen-free at 985℃ + oxygen-bearing at 900℃); repeat this cycle 5 times to complete the final stage of pushing and pushing.

[0065] In step 3, the oxygen introduction time accounts for 20% of the total cycle time, meaning the oxygen-to-oxygen switching ratio is 1:4. Each of the three stages, 31, 32, and 33, is repeated 5 times, for a total of 15 cycles. The duration of each cycle is the same, and the oxygen introduction time is also the same in each cycle (therefore, the oxygen-free time is also the same in each cycle). The entire process time for step 3 is 4500 seconds.

[0066] In step 3, the oxygen flow rate is 20,000 sccm during each oxygenation process, and an N2 atmosphere is maintained when oxygenation stops (i.e., an N2 atmosphere is maintained during the oxygen-free process). By intermittent oxygenation, the high-temperature post-oxidation of the traditional boron diffusion process is avoided, the oxidation temperature is reduced, and concentric circle defects in battery EL detection are avoided.

[0067] This embodiment of a TOPCon battery preparation method includes the following preparation steps:

[0068] Step 1: Texturing the pre-selected silicon wafer to form a textured surface on the wafer.

[0069] In step one, the resistivity of the silicon wafer is 1.5Ω / □; texturing is performed using an alkaline solution formed by mixing KOH and H2O2 in a volume ratio of 1:5, at a texturing temperature of 70℃ and a texturing time of 400s, to form a 1.1µm high pyramid-shaped textured surface on the silicon wafer, with a reflectivity of 10%.

[0070] Step 2: Perform boron diffusion treatment on the silicon wafer to prepare a boron-diffused emitter (i.e., a p+ emitter) on the front side of the silicon wafer, and cover the front side of the boron-diffused emitter with a borosilicate glass layer; while the back side and edges of the silicon wafer form a swirling diffusion, accompanied by a borosilicate glass layer.

[0071] In step two, the boron diffusion treatment refers to steps 1-3 of the boron diffusion process with stepped temperature cyclic propagation described above in this embodiment.

[0072] Step 3: Perform single-sided acid washing on the silicon wafer to remove the borosilicate glass layer on its back and edges.

[0073] In step three, the pickling solution used is a 30% HF solution, the pickling speed is 3.5 m / min, and the actual pickling and etching duration is 1 min.

[0074] Step 4: Perform alkaline polishing on the back side of the silicon wafer to form a polished surface with a reflectivity of over 30% on the back side of the silicon wafer; after polishing, remove the borosilicate glass layer on the front side of the silicon wafer with HF solution.

[0075] Step 5: Prepare a TOPCon structure on the back side of a silicon wafer using PVD (Physical Vapor Deposition) technology (which includes a tunneling oxide layer and an n-type doped amorphous silicon layer deposited sequentially on the back side of the silicon wafer): First, oxygen ions are decomposed by plasma to deposit a 2nm thick tunneling oxide layer on the back side of the silicon wafer; then, an amorphous silicon layer is deposited on the back side of the tunneling oxide layer by magnetron sputtering of a silicon target. At the same time, PH3 gas is introduced to achieve in-situ phosphorus doping of the amorphous silicon layer, resulting in an n-type doped amorphous silicon layer.

[0076] Step 6: The silicon wafer processed in Step 5 is sent into an annealing furnace for annealing to transform the n-type doped amorphous silicon layer into an n-type doped polycrystalline silicon layer (i.e., an n+poly layer).

[0077] Step 7: Clean the front and back oxides generated during the silicon wafer annealing process using HCl solution.

[0078] Step 8: Deposit a 7nm thick aluminum oxide layer on the front side of the p+ emitter using ALD (atomic layer deposition) technology.

[0079] Step 9: Using PECVD (Plasma Enhanced Chemical Vapor Deposition) technology, a silicon nitride antireflection film is prepared on both the front side of the alumina layer and the back side of the n+poly layer. The thickness of the silicon nitride antireflection film on the front side is 83 nm, and the thickness of the silicon nitride antireflection film on the back side is 90 nm.

[0080] Step 10: Apply metal paste to both the front and back sides of the silicon wafer using screen printing, and then sinter to form metal electrodes. The lower end of the front metal electrode passes through the front silicon nitride antireflective film and the aluminum oxide layer in sequence, making ohmic contact with the p+ emitter; while the upper end of the back metal electrode passes through the back silicon nitride antireflective film, making ohmic contact with the n+ poly layer. After completing Step 10, the TOPCon battery of this embodiment is obtained.

[0081] Comparative Example 1

[0082] The preparation method of the TOPCon battery in this comparative example is the same as that in Example 1, except that:

[0083] The boron diffusion treatment in step two of this comparative example employs a conventional boron diffusion process, which includes sequential pre-oxidation, pre-deposition, propulsion, and post-oxidation steps. The process conditions for pre-oxidation and pre-deposition are the same as steps 1-2 of Example 1. The propulsion step's process conditions are: continuous N2 flow at a flow rate of 3000 sccm, a propulsion temperature of 890°C, and a propulsion time of 600 s. The post-oxidation step's process conditions are: continuous O2 flow at a flow rate of 17000 sccm, a post-oxidation temperature of 1050°C, and a post-oxidation time of 4200 s. Thus, the TOPCon battery of this comparative example is obtained.

[0084] Comparative Example 2

[0085] The preparation method of the TOPCon battery in this comparative example is the same as that in Example 1, except that:

[0086] In step two of this comparative example, the boron diffusion treatment employs an existing isothermal cyclic boron diffusion process, as described in CN117352595A. Specifically, during cyclic boron diffusion, the temperatures of both the oxygen-free and oxygen-filled boron diffusion junctions are maintained at 980°C, the oxygen-to-oxygen on / off ratio is 1:4, the oxygen flow rate during oxygen-filled boron diffusion is 20000 sccm, and a total of 20 cycles are performed. The entire cyclic boron diffusion process takes 4200 seconds. This yields the TOPCon battery of this comparative example.

[0087] Performance testing

[0088] The performance of the TOPCon batteries prepared in Example 1 and Comparative Examples 1-2 was tested, and the test results are shown in Table 1 below:

[0089] Table 1

[0090]

[0091] In Table 1, Eta is the battery efficiency, Uoc is the open-circuit voltage, Jsc is the short-circuit current density, and FF is the fill factor; EL detection is electroluminescence detection, and the presence of concentric circle defects in the battery can be determined based on the EL detection results.

[0092] As can be seen from Table 1:

[0093] Compared to Comparative Example 1 (traditional boron diffusion process, BSL) and Comparative Example 2 (which uses an existing isothermal cyclic boron diffusion process such as CN117352595A), in Embodiment 1 of the present invention, when performing boron diffusion on the silicon wafer surface, based on the pre-oxidation and pre-deposition in steps 1 and 2 above, the step-temperature cyclic boron diffusion process described in steps 31-33 above not only allows this step-temperature cyclic boron diffusion process to replace the high-temperature post-oxidation process in the traditional boron diffusion process (as shown in Comparative Example 1), but also reduces the oxidation temperature (for example, step 33 oxygen-boron diffusion process). The oxidation temperature required for the process can be as low as 900℃, which further reduces the concentric circle defects detected by the battery EL test. It can also increase the junction depth and surface doping concentration of the boron diffused emitter (i.e., p+ emitter), improve the uniformity of boron diffusion doping, and thus reduce the sheet resistance fluctuation of boron diffusion doping (the sheet resistance fluctuation of boron diffusion doping can be <5% when combined with the stepped temperature cycle boron diffusion process in steps 31-33; the sheet resistance fluctuation of the traditional boron diffusion process in Comparative Example 1 can reach 10-15%). Therefore, it can further improve the battery's FF, Jsc, and Uoc electrical performance, and further improve the battery efficiency.

[0094] Furthermore, the lifespan of quartz tubes used in existing high-temperature boron diffusion processes is 12-18 months (due to high temperature and boron corrosion), while the lifespan of quartz tubes used in existing phosphorus diffusion processes is 36 months (the phosphorus diffusion process temperature is lower, similar to the boron diffusion process temperature in Example 1 of this invention). Therefore, even without considering production losses caused by downtime, the boron diffusion process with stepped temperature cycling as described in Example 1 of this invention can reduce costs related to quartz tube wear by nearly 70% and reduce energy consumption per tube by 40-50%, lowering the thermal budget and helping to extend the lifespan of boron diffusion equipment such as quartz tubes. Compared to the conventional boron diffusion process, such as in Comparative Example 1, the boron diffusion process of this invention also helps to shorten process time and improve production efficiency.

[0095] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present invention.

[0096] The technical solution provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A boron diffusion process with stepped temperature cyclic propagation, characterized in that, The process includes the following steps: Step 1: Place the silicon wafer in a boron diffusion apparatus, heat it, and introduce oxygen to pre-oxidize the silicon wafer; Step 2: Heat up and introduce a boron doping source to pre-deposit a borosilicate glass layer on the silicon wafer surface; Step 3: Perform step-temperature cyclic boron diffusion on the pre-deposited silicon wafer, which specifically includes: Step 31, Initial stage: First, perform oxygen-free boron expansion and push-bonding at 1000-1100℃, then perform oxygen-boron expansion and push-bonding at 920-980℃. Repeat this process multiple times to complete the push-bonding in the initial stage. Step 32, Intermediate Stage: First, perform oxygen-free boron expansion and push-bonding at 950-1010℃, then perform oxygen-boron expansion and push-bonding at 920-980℃. Repeat this cycle multiple times to complete the push-bonding in the intermediate stage. Step 33, Final Stage: First, perform oxygen-free boron diffusion and push junction at 955-1015℃, then perform oxygen-filled boron diffusion and push junction at 870-930℃. Repeat this process multiple times to complete the final stage of push junction formation, thereby forming a boron-doped layer on the silicon wafer surface.

2. The boron diffusion process with stepped temperature cyclic propagation according to claim 1, characterized in that, In step 3, each stage is repeated 3-7 times, for a total of 9-21 cycles. The oxidation flow rate for oxygen-boron propagation in each stage is 15000-25000 sccm. The oxygen introduction time for each cycle in each stage accounts for 20% of the current cycle time, and the cycle time is the same for each stage. The total process time for step 3 is 4400-4600 s.

3. The boron diffusion process with stepped temperature cyclic propagation according to claim 2, characterized in that, In step 3, the number of cycles for each stage is 5, and the oxidation flow rate for oxygen-boron propagation junction in each stage is 20000 sccm; when entering the oxygen-free boron propagation junction, the N2 atmosphere is maintained; the entire process time for step 3 is 4500s.

4. A stepped temperature cyclically driven boron diffusion process according to any one of claims 1-3, characterized in that, In the initial stage of step 31, the temperature of the oxygen-free boron diffusion junction is 1000°C, while the temperature of the oxygen-boron diffusion junction is 950°C.

5. A stepped temperature cyclically driven boron diffusion process according to any one of claims 1-3, characterized in that, In the intermediate stage of step 32, the temperature of the oxygen-free boron extension junction is 980°C, while the temperature of the oxygen-boron extension junction is 950°C.

6. A stepped temperature cyclically driven boron diffusion process according to any one of claims 1-3, characterized in that, In the final stage of step 33, the temperature of the oxygen-free boron diffusion junction is 985°C, while the temperature of the oxygen-boron diffusion junction is 900°C.

7. The boron diffusion process with stepped temperature cyclic propagation according to claim 1, characterized in that, In step 3, the temperature difference during the entire stepped temperature cycle boron diffusion process is less than or equal to 100°C.

8. The boron diffusion process with stepped temperature cyclic propagation according to claim 1, characterized in that, In step 1, the heating temperature for pre-oxidation is 780-820℃, the pressure is 120-180mbar, the oxygen flow rate is 300-700sccm, and the pre-oxidation process time is 220-260s.

9. The boron diffusion process with stepped temperature cyclic propagation according to claim 1, characterized in that, In step 2, the pre-deposition temperature is 830-870℃, the boron doping source is BCl3, the flow rate of BCl3 is 100-140 sccm, the oxygen flow rate is 200-800 sccm, and the pre-deposition process time is 430-470 s.

10. The application of the step-temperature cyclically driven boron diffusion process according to any one of claims 1-9, characterized in that, A boron diffusion process with stepped temperature cycling is applied to the fabrication of solar cells.

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