Boron diffusion method, TOPCon battery and preparation method of TOPCon battery

Abstract

The invention relates to a boron diffusion method, a TOPCon battery and a preparation method thereof, and the boron diffusion method comprises the following steps: placing a pretreated N-type silicon wafer in a diffusion furnace, and carrying out the pre-oxidation at a first temperature; performing constant source deposition at the first temperature; raising the temperature in the diffusion furnace from the first temperature to a second temperature, and performing limited source knot pushing at the second temperature; increasing the temperature in the diffusion furnace from the second temperature and then reducing the temperature, and carrying out gradient cooling oxidation. In the boron diffusion process, the oxide layer is prepared through gradient cooling oxidation, oxide layer phase change of different doping concentrations can be formed, the boron diffusion rate is regulated and controlled, the problem of surface boron depletion is solved, longitudinal P- / P + high-low junctions are further improved, surface energy band bending is reduced, and therefore the capacity of migration of carriers to an N-type region is improved.

Description

Technical Field

[0001] The invention relates to the technical field of solar cells, and in particular to a boron diffusion method, a TOPCon cell and a preparation method thereof. Background Art

[0002] Solar cells are divided into N-type and P-type according to the doping type of the original silicon wafer. In 2013, the Fraunhofer-ISE Institute in Germany proposed the tunnel oxide passivation contact (TOPCon) solar cell technology based on N-type silicon. TOPCon cells mainly use boron diffusion doping to manufacture PN junctions. Compared with phosphorus diffusion, the solid solubility of boron in silicon is lower, which makes the diffusion of boron more challenging. In the boron diffusion process, it first diffuses through the boron source to form a shallow junction rich in boron atoms. A boron-rich layer will be formed on the surface. The boron-rich layer (BRL) is rich in SiB. 4 and SiB 6 , which will cause trouble for the subsequent process to clean, so borosilicate glass (BSG) is formed through an oxidation process, which is easy to remove using HF in the subsequent process. Therefore, constant source diffusion and limited source diffusion (push junction / oxidation) technology are usually used in combination.

[0003] In the boron diffusion process, constant temperature oxidation is a key step, which includes the following functions: (1) high temperature oxidation forms an oxide layer, repairs lattice defects, and converts the boron interstitial doping method into a boron substitutional doping method; (2) the surface boron-rich layer is oxidized to form a BSG layer, which makes it easier to clean with HF in the subsequent process and reduces the recombination loss; (3) the promotion of high temperature can redistribute impurities to obtain the appropriate concentration and depth, forming an ideal PN junction; (4) the BSG layer formed by oxidation plays a role in protecting the PN junction in the subsequent process.

[0004] However, during the isothermal oxidation process, the boron doping concentration of the silicon wafer is related to the junction depth: Figure 1 As shown, Figure 1 This is the ECV curve of the constant temperature oxidation process of the boron diffusion process in the prior art. The surface concentration ratio of the ECV curve reflects the degree of boron depletion. The starting point of the doping concentration of the ECV test curve is defined as Cs, and the peak point is defined as Cmax. The ratio of Cs / Cmax reflects the degree of boron depletion. The lower the value, the more serious the depletion. As the junction depth increases, the concentration initially (Cs) rises to the peak value (Cmax), and the boron doping concentration of the silicon wafer decreases with increasing depth. This phenomenon is mainly due to the segregation effect of oxidation and the redistribution of boron atoms. The redistribution of boron is related to the segregation coefficient, which is defined as the ratio of the equilibrium concentration of impurities in the silicon wafer to the concentration of the impurity in the silicon dioxide layer. The segregation coefficient is approximately 3 / 10. This makes the surface boron tend to enter the silicon dioxide layer. During the oxidation process, oxygen molecules pass through the oxide layer to reach the Si / SiO 2The surface reacts with silicon to form SiO 2 , and then Si / SiO 2 The interface is pushed into the interior of the silicon wafer. Therefore, after the boron-doped N-type silicon wafer is oxidized, the boron concentration near the surface of the silicon wafer is depleted. Due to the boron depletion, the boron concentration on the surface of the silicon wafer decreases, and together with the increased boron concentration inside the silicon wafer, a P - / P + High-low junctions cause longitudinal band bending, such as Figure 2 As shown, at the same time, under the action of concentration gradient, the relatively high oxidation temperature will cause the boron inside the silicon wafer to diffuse deeper. The rate at which the silicon dioxide thickness increases per unit area per unit time is defined as the oxidation rate, and the rate at which boron diffuses into the silicon wafer is defined as the diffusion rate. The ratio of the diffusion rate to the oxidation rate will also affect the degree of boron depletion, resulting in efficiency loss. Summary of the invention

[0005] Based on this, the purpose of the present invention is to provide a boron diffusion method, a TOPCon battery and a preparation method thereof to improve Si / SiO 2 Interfacial boron depletion problem.

[0006] The present invention provides a boron diffusion method, which comprises the following steps:

[0007] S1. The pretreated N-type silicon wafer is placed in a diffusion furnace and pre-oxidized at a first temperature;

[0008] S2. performing constant source deposition at a first temperature;

[0009] S3. Raising the temperature within the diffusion furnace from a first temperature to a second temperature, and performing finite source push at the second temperature;

[0010] S4. The temperature in the diffusion furnace is first raised from the second temperature and then lowered to perform gradient temperature reduction oxidation.

[0011] The present invention prepares the oxide layer by gradient temperature reduction oxidation during the boron diffusion process, which can form oxide layer phase changes with different doping concentrations, regulate the boron diffusion rate, and thus improve the surface boron depletion problem, thereby improving the longitudinal P - / P + The high-low junction reduces the surface energy band bending, thereby improving the ability of carriers to migrate to the N-type region.

[0012] As a further improvement of the above solution of the present invention, the gradient temperature reduction oxidation comprises:

[0013] The temperature in the diffusion furnace is raised from the second temperature to the third temperature, primary oxidation is performed to form a primary oxide layer; and the temperature in the diffusion furnace is lowered from the third temperature to the fourth temperature, secondary oxidation is performed to form a secondary oxide layer.

[0014] In the first oxidation stage of the present invention, due to the high growth temperature, the generated oxide layer contains more boron, and the high temperature leads to low density of the oxide layer, which increases the boron concentration in the first oxidation layer. At the same time, boron diffuses into the silicon wafer due to the concentration difference in the silicon wafer at high temperature. In the second oxidation stage, since the oxidation temperature is lower than the previous oxidation temperature, as the oxide layer grows, Si / SiO 2 The interface migrates to the silicon substrate. Due to the lowering of temperature, the higher concentration of boron in the primary oxide layer changes its solid solubility. The higher concentration of boron in the primary oxide layer migrates to the newly generated oxide layer in the secondary oxidation stage, effectively slowing down the Si / SiO 2 Boron penetrates into SiO at the interface 2 The rate in the medium helps to suppress the reduction of surface boron concentration, and the square resistance range is controlled in 400-450Ω.

[0015] As a further improvement of the above solution of the present invention, the primary oxidation comprises: at a third temperature, introducing oxygen into the diffusion furnace, the oxygen flow rate is 15000-25000 sccm, the pressure in the diffusion furnace is 600-900 mbar, and the primary oxidation time is 1000-1500 s;

[0016] And / or, the third temperature is 1000°C-1050°C.

[0017] As a further improvement of the above solution of the present invention, the secondary oxidation comprises: at a fourth temperature, oxygen is introduced into the diffusion furnace, the oxygen flow rate is 8000-20000 sccm, the pressure in the diffusion furnace is 600-900 mbar, and the secondary oxidation time is 3000-4500 s;

[0018] And / or, the fourth temperature is 990°C-1040°C.

[0019] As a further improvement of the above solution of the present invention, the pre-oxidation comprises: at a first temperature, introducing oxygen and nitrogen into the diffusion furnace, the oxygen flow rate is 300-1500sccm, the nitrogen flow rate is 500-3000sccm, the pressure in the diffusion furnace is 140-220bar, and the pre-oxidation time is 120-240s;

[0020] And / or, the first temperature is 780°C-860°C.

[0021] The present invention forms a thinner pre-oxidation layer through a pre-oxidation step, which is helpful for the growth of a subsequent boron-rich layer (BRL) and helps to provide sufficient boron atoms for the subsequent formation of an oxidation layer in a primary oxidation stage.

[0022] As a further improvement of the above scheme of the present invention, the constant source deposition is divided into three steps, and each deposition step includes: introducing a boron source, oxygen, nitrogen, and furnace door nitrogen into the diffusion furnace, the boron source flow rate is 100-150sccm, the oxygen flow rate is 300-500sccm, the nitrogen flow rate is 2000-4000sccm, the furnace door nitrogen flow rate is 300-1500sccm, the pressure in the diffusion furnace is 140-220bar, and the deposition time is 120-200s; the temperature of the next deposition step is 10℃-20℃ higher than the deposition temperature of the previous step.

[0023] The constant source deposition of the present invention is carried out in three steps. The temperature of the next deposition step is 10°C-20°C higher than that of the previous deposition step. The purpose is to optimize the intra-wafer / inter-wafer uniformity of the silicon wafer. After deposition, a boron-rich layer is formed on the surface of the silicon wafer.

[0024] As a further improvement of the above solution of the present invention, the finite source push-in includes: at the second temperature, nitrogen is introduced into the diffusion furnace, the nitrogen flow rate is 2000-5000sccm, the furnace door nitrogen flow rate is 300-1500sccm, the pressure in the diffusion furnace is 220-450bar, and the finite source push-in time is 500-900s;

[0025] And / or, the second temperature is 860°C-950°C.

[0026] As a further improvement of the above solution of the present invention, the boron diffusion method further comprises the following steps:

[0027] S5. Cooling: Cooling the temperature in the diffusion furnace from the fourth temperature to the fifth temperature, introducing oxygen, the oxygen flow rate is 8000-20000sccm, the pressure in the diffusion furnace is 600-900mbar, the cooling time is 1800-3000s, and then closing the diffusion furnace; the fifth temperature is 700℃-800℃.

[0028] The present invention uses high temperature to preliminarily push boron impurities to form a shallow junction, at which time the on-chip square resistance is controlled to be 170-210Ω.

[0029] The present invention provides a method for preparing a TOPCon battery, which includes a boron diffusion step, wherein the boron diffusion step adopts the boron diffusion method as described above.

[0030] The present invention provides a TOPCon battery, which is prepared by the preparation method as described above.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The present invention prepares the oxide layer by gradient temperature reduction oxidation during the boron diffusion process, which can form oxide layer phase changes with different doping concentrations, regulate the boron diffusion rate, increase the boron depletion from 0.65 to 0.86, improve the surface boron depletion problem, and thus improve the longitudinal P - / P + The high-low junction reduces the surface energy band bending, thereby improving the ability of carriers to migrate to the N-type region.

[0033] 2. In the process of gradient cooling, in the primary oxidation stage, due to the high growth temperature, the generated oxide layer contains more boron, and the high temperature leads to low density of the oxide layer, which increases the boron concentration in the primary oxidation layer. At the same time, boron diffuses into the silicon wafer due to the concentration difference in the silicon wafer at high temperature; in the secondary oxidation stage, due to the lower oxidation temperature than the primary oxidation temperature, as the oxide layer grows, Si / SiO 2 The higher concentration of boron in the oxide layer formed in the first oxidation stage will change its solid solubility due to the lowering temperature, and will migrate to the newly formed oxide layer in the second oxidation stage, effectively slowing down the Si / SiO 2 Boron penetrates into SiO at the interface 2 The rate in the reaction mixture helps to inhibit the decrease of surface boron concentration.

[0034] 2. The present invention effectively increases the boron doping concentration of the surface P-type emitter through temperature reduction oxidation, thereby improving the battery conversion efficiency by 0.08%; at the same time, it reduces the overall energy consumption loss, and provides reliable theoretical support for reducing the overall oxidation temperature; the use of a gradient cooling method provides an effective improvement idea for quartz consumables; it also improves the square resistance uniformity of the silicon wafer from 5.29% to 3.65%; and under the condition of ensuring that the oxygen flow rate and oxidation time are sufficient, the gradient cooling oxidation method provides a guarantee for good efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is an ECV curve diagram of the constant temperature oxidation process of the boron diffusion process in the prior art;

[0036] Figure 2 It is a schematic diagram of the longitudinal energy band bending on the surface of a silicon wafer during the constant temperature oxidation process of the boron diffusion process in the prior art;

[0037] Figure 3 It is an ECV curve diagram of the gradient temperature reduction oxidation of the embodiment of the present invention and the constant temperature oxidation of the comparative example. DETAILED DESCRIPTION

[0038] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively below in conjunction with specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thoroughly understood.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0040] This embodiment provides a method for preparing a TOPCon battery, which includes the following steps:

[0041] S1. Form a pyramid velvet surface on the original silicon wafer through a NaOH alkaline solution.

[0042] S2. Boron source is used in a low pressure quartz tube diffusion furnace to carry out P in a high temperature boron diffusion furnace + Preparation of the emitter, and then preparation of a thick BSG layer by high temperature oxidation, and condensation and redistribution of impurity boron; specifically:

[0043] S21. Heating the boat: Place the original silicon wafer after texturing in the diffusion furnace, set the temperature of the diffusion furnace to 700-820℃, stabilize the temperature to ensure the uniform deposition of the subsequent oxide layer and square resistance;

[0044] S22. Pumping and leak detection: Conduct constant temperature leak detection at 700-820℃ and pressure 110-140mbar to ensure the sealing of the furnace tube and guarantee the stability of the subsequent process;

[0045] S23. Pre-oxidation: The temperature of the diffusion furnace is raised to 780-860°C, and 1500sccm of oxygen and 2000sccm of nitrogen are introduced at 780-860°C for pre-oxidation. The pressure in the furnace is 140-220bar, and the pre-oxidation process time is 120-240s to form a pre-oxidation layer; a thin pre-oxidation layer is conducive to the growth of a subsequent boron-rich layer (BRL), which helps to provide sufficient boron atoms for the subsequent oxidation layer;

[0046] S24. Constant source deposition: Deposition is divided into three steps. The temperature of the diffusion furnace is raised to 780-860°C, and 100-150sccm of boron source, 300-500sccm of oxygen, 2000-4000sccm of nitrogen, and 300-1500sccm of furnace door nitrogen are introduced at 780-860°C. The mixed gas is introduced according to the flow rate of boron source: oxygen: nitrogen: furnace door nitrogen = 1:3:27:12, the pressure is 140-220bar, and the time for each step is 120-200s. The temperature of the next deposition is higher than that of the previous deposition. The deposition temperature is 10-20°C higher than that of the previous step, in order to optimize the uniformity of the silicon wafer within and between wafers, so that a boron-rich layer is formed on the surface of the silicon wafer after deposition; a purge step is set before deposition and after each deposition step: nitrogen is introduced at 3500sccm for purge at 780-860°C, the pressure is 140-220bar, the purge time is 120-300s, and the purge temperature is 10-20°C higher than that of the previous step, in order to blow the boron oxide generated during the source deposition process out of the furnace tube, reduce the furnace tube, and provide a heating time buffer for the next deposition temperature;

[0047] S25. Limited source push junction: Raise the temperature of the diffusion furnace to 860-950℃, and introduce nitrogen at 2000-5000sccm at 860-950℃, with 300-1500sccm of furnace door nitrogen, for 500-900s, and pressure of 220-450bar. Use high temperature to initially push the boron impurities to form a shallow junction. At this time, the internal square resistance of the chip is controlled to 170-210Ω.

[0048] S26. Primary oxidation stage: the temperature of the diffusion furnace is raised to 1000-1050°C, and oxygen is introduced at 15000-25000sccm at 1000-1050°C, the pressure is 600-900mbar, and the time is 1000-1500s to form a primary oxide layer; due to the high growth temperature, the primary oxide layer contains more boron from the boron rich layer (BRL) and the borosilicate glass layer (BSG); the high temperature leads to a low density of the oxide layer, which further increases the boron concentration in the primary oxide layer, so its boron concentration is higher than that of SiO formed at a lower temperature. 2 Boron concentration; on the other hand, at high temperatures, boron diffuses into the silicon wafer due to the concentration difference in the silicon wafer;

[0049] S27. Secondary oxidation stage: the temperature of the diffusion furnace is lowered to 990-1040°C, and oxygen is introduced at 8000-20000sccm at 990-1040°C, the pressure is 600-900mbar, and the time is 3000-4500s to form a secondary oxidation layer; the key to this step is that the oxidation temperature is lowered by 10-15°C compared with the primary oxidation temperature. As the oxide layer grows, Si / SiO 2The interface migrates to the silicon substrate. Due to the lowering of temperature, the higher concentration of boron in the primary oxide layer changes its solid solubility. The higher concentration of boron in the primary oxide layer migrates to the newly formed secondary oxide layer, effectively slowing down the Si / SiO 2 Boron penetrates into SiO at the interface 2 The rate in the medium helps to inhibit the reduction of surface boron concentration; the square resistance range is controlled in 400-450Ω;

[0050] S28. Cooling: Cool the diffusion furnace to 700-800°C, introduce oxygen at 8000-20000sccm, pressure at 600-900mbar, time at 1800-3000s, then turn off the heating. At this time, the furnace tube uses residual heat for oxidation, and cooling is done to prepare for boat discharge.

[0051] S3. Use HF to remove the BSG layer on the back side, then use NaOH to etch away the PN junction on the back side and the edge, and polish the back side at the same time.

[0052] S4. Use plasma enhanced chemical vapor deposition (PECVD) to first deposit a tunnel oxide layer (1-2nm), then deposit a layer of phosphorus-doped amorphous silicon layer of about 40±5nm. Then deposit a tunnel oxide layer (1-2nm), then deposit a layer of phosphorus-doped amorphous silicon layer of about 60±5nm, and then form a layer of SiO x layer (10-20nm).

[0053] S5. The amorphous silicon on the back side is converted into polycrystalline silicon by annealing at 900°C in a tube furnace, and the square resistance on the back side is about 45±5Ω.

[0054] S6. Remove the BSG on the front side and the PSG on the back side using HF solution.

[0055] S7. Deposit an aluminum oxide film (AlOx) with a thickness of about 4.0±0.5 nm on the front side of the silicon wafer by atomic layer deposition (ALD).

[0056] S8. Deposit a silicon nitride (SiNx) film on both sides of the silicon wafer by PECVD.

[0057] S9. Screen printing to make metal electrodes.

[0058] S10. Laser induced sintering to obtain TOPCon battery.

[0059] Next, taking an N-type original silicon wafer with a size of 183*183 mm, a thickness of 130 um, and a resistivity of 0.4-1.6 Ω·cm as an example, this embodiment is described in detail.

[0060] Example

[0061] S1. Form a pyramid velvet surface on an N-type original silicon wafer (size 183*183mm, thickness 130um, resistivity 0.4-1.6Ω·cm) in a NaOH alkaline solution.

[0062] S21. Heating the boat: Place the N-type original silicon wafer after texturing in the diffusion furnace, set the temperature of the diffusion furnace to 820℃, stabilize the temperature, and ensure the uniform deposition of the subsequent oxide layer and square resistance;

[0063] S22. Pumping and leak detection: Conduct constant temperature leak detection at 820℃ and 140mbar to ensure the sealing of the furnace tube and guarantee the stability of the subsequent process;

[0064] S23 pre-oxidation: at 825 ° C, oxygen 1500sccm, nitrogen 2000sccm, furnace door nitrogen 500sccm pre-oxidation, furnace pressure of 220bar, pre-oxidation process time 240s, forming a pre-oxidation layer;

[0065] S24. Constant source deposition:

[0066] One purge: At 825°C, nitrogen was introduced at 3500 sccm for purge, the furnace pressure was 220 bar, and the purge time was 120 s;

[0067] Primary deposition: 110 sccm of boron trichloride, 370 sccm of oxygen, 3500 sccm of nitrogen, and 1500 sccm of furnace door nitrogen were introduced at 825°C, the furnace pressure was 220 bar, and the deposition time was 120 s.

[0068] Secondary purge: Raise the temperature of the diffusion furnace to 835°C, introduce 3500sccm of nitrogen and 1500sccm of furnace door nitrogen at 835°C for purge, the pressure in the furnace is 220bar, and the purge time is 180s;

[0069] Secondary deposition: 130 sccm of boron trichloride, 400 sccm of oxygen, 3500 sccm of nitrogen, and 1500 sccm of furnace door nitrogen were introduced at 835°C, the pressure in the furnace was 220 bar, and the deposition time was 180 s.

[0070] Three times of purging: the temperature of the diffusion furnace was raised to 845°C, and nitrogen was introduced at 3000 sccm and furnace door nitrogen at 500 sccm for purging at 845°C. The pressure in the furnace was 220 bar and the purging time was 180 s.

[0071] Three depositions: 130 sccm of boron trichloride, 400 sccm of oxygen, 2800 sccm of nitrogen, and 400 sccm of furnace door nitrogen were introduced at 845°C, the pressure in the furnace was 220 bar, and the deposition time was 180 s;

[0072] Four purges: the temperature of the diffusion furnace was raised to 850°C. At 850°C, 40 sccm of boron trichloride, 5000 sccm of nitrogen and 500 sccm of furnace door nitrogen were introduced for purge. The pressure in the furnace was 220 bar and the purge time was 180 s.

[0073] S25. Finite source inference:

[0074] One push: raise the temperature of the diffusion furnace to 875°C, and introduce nitrogen at 4000 sccm and furnace door nitrogen at 500 sccm at 875°C for 360 seconds at a pressure of 600 bar.

[0075] Secondary push: 10,000 sccm of nitrogen was introduced at 875°C, 500 sccm of nitrogen at the furnace door, time was 120 s, and pressure was 600 bar.

[0076] S26. Primary oxidation stage: the diffusion furnace temperature was raised to 1045°C, and oxygen was introduced at 1045°C at 20,000 sccm, a pressure of 800 mbar, and a time of 1500 s to form a primary oxide layer;

[0077] S27. Secondary oxidation stage: the diffusion furnace temperature was cooled to 1035°C, and oxygen was introduced at 1035°C at 15000sccm, a pressure of 800mbar, and a time of 3300s to form a secondary oxidation layer;

[0078] S28. Cooling: Cool the diffusion furnace to 800°C, introduce 10,000 sccm of oxygen, 800 mbar of pressure, and 2000 s; cool the diffusion furnace to 750°C, introduce 15,000 sccm of nitrogen and 2000 sccm of furnace door nitrogen, 1060 mbar of pressure, and 3000 s; then turn off the heating, at which time the furnace tube uses residual heat for oxidation, and cools down in preparation for boat discharge.

[0079] S3. Use HF to remove the BSG layer on the back side, then use NaOH to etch away the PN junction on the back side and the edge, and polish the back side at the same time.

[0080] S4. Use plasma enhanced chemical vapor deposition (PECVD) to first deposit a tunnel oxide layer (1-2nm), then deposit a layer of phosphorus-doped amorphous silicon layer of about 40±5nm. Then deposit a tunnel oxide layer (1-2nm), then deposit a layer of phosphorus-doped amorphous silicon layer of about 60±5nm, and then form a layer of SiO x layer (10-20nm).

[0081] S5. The amorphous silicon on the back side is converted into polycrystalline silicon by annealing at 900°C in a tube furnace, and the square resistance on the back side is about 45±5Ω.

[0082] S6. Remove the BSG on the front side and the PSG on the back side using HF solution.

[0083] S7. Deposit an aluminum oxide film (AlOx) with a thickness of about 4.0±0.5 nm on the front side of the silicon wafer by atomic layer deposition (ALD).

[0084] S8. Deposit a silicon nitride (SiNx) film on both sides of the silicon wafer by PECVD.

[0085] S9. Screen printing to make metal electrodes.

[0086] S10. Laser induced sintering to obtain TOPCon battery.

[0087] Comparative Example

[0088] The difference between this comparative example and the embodiment is that the number of N-type original silicon wafers in the comparative example is 1600, and the following steps are used in this comparative example to replace steps S26 and S27 of the embodiment:

[0089] S26. Constant temperature oxidation stage: the temperature of the diffusion furnace is raised to 1045°C, and oxygen is introduced at 1045°C at a flow rate of 20,000 sccm and a pressure of 800 mbar for 4,800 seconds to form an oxide layer.

[0090] Figure 3 The ECV test curves of the gradient temperature reduction oxidation in the boron diffusion of the embodiment and the constant temperature oxidation in the comparative example are shown in FIG. Figure 3 It can be seen that the ratio of Cs / Cmax of the gradient cooling boron oxide depletion in the embodiment is 0.86, while the ratio of Cs / Cmax of the constant temperature boron oxide depletion in the comparative example is 0.65, which indicates that the gradient cooling oxidation method of this embodiment effectively improves the problem of surface boron depletion.

[0091] The TOPCon cells prepared in the examples and comparative examples were screen printed in a single line and tested by an IV tester. The test conditions were as follows: light intensity: the sunlight intensity under AM1.5G conditions should be 1000 W / m 2 , that is, the light intensity is 100MW / cm; Temperature: the temperature of the photovoltaic cell is 25°C. The test results are shown in Table 1.

[0092] Table 1

[0093] Experimental Group Number of batteries Eta Uoc Isc FF R Rs IRev2 Gradient temperature reduction oxidation 2100 26.68％ 0.735 14.320 86.53 2540.0 0.00056 0.057 Constant temperature oxidation 1600 26.60％ 0.734 14.343 86.14 2232.1 0.00053 0.066

[0094] It can be seen from Table 1 that the efficiency of the gradient temperature reduction oxidation method of the embodiment is 0.08% higher than that of the constant temperature oxidation method of the comparative example.

[0095] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0096] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A boron diffusion method, characterized in that: It includes the following steps: S1. The pretreated N-type silicon wafer is placed in a diffusion furnace and pre-oxidized at a first temperature; S2. performing constant source deposition at a first temperature; S3. Raising the temperature within the diffusion furnace from a first temperature to a second temperature, and performing finite source push at the second temperature; S4. The temperature in the diffusion furnace is raised from the second temperature and then lowered to perform gradient temperature reduction oxidation.

2. The boron diffusion method according to claim 1, characterized in that: The gradient temperature reduction oxidation comprises: The temperature in the diffusion furnace is raised from the second temperature to the third temperature, primary oxidation is performed to form a primary oxide layer; and the temperature in the diffusion furnace is lowered from the third temperature to the fourth temperature, secondary oxidation is performed to form a secondary oxide layer.

3. The boron diffusion method according to claim 2, characterized in that: The primary oxidation comprises: introducing oxygen into the diffusion furnace at a third temperature, the oxygen flow rate is 15000-25000 sccm, the pressure in the diffusion furnace is 600-900 mbar, and the primary oxidation time is 1000-1500 s; And / or, the third temperature is 1000°C-1050°C.

4. The boron diffusion method according to claim 2, characterized in that: The secondary oxidation comprises: at a fourth temperature, introducing oxygen into the diffusion furnace, the oxygen flow rate is 8000-20000 sccm, the pressure in the diffusion furnace is 600-900 mbar, and the secondary oxidation time is 3000-4500 s; And / or, the fourth temperature is 990°C-1040°C.

5. The boron diffusion method according to claim 1, characterized in that: The pre-oxidation comprises: introducing oxygen and nitrogen into the diffusion furnace at a first temperature, the oxygen flow rate is 300-1500sccm, the nitrogen flow rate is 500-3000sccm, the pressure in the diffusion furnace is 140-220bar, and the pre-oxidation time is 120-240s; And / or, the first temperature is 780°C-860°C.

6. The boron diffusion method according to claim 1, characterized in that: The constant source deposition is divided into three steps, and each deposition step includes: introducing a boron source, oxygen, nitrogen, and furnace door nitrogen into the diffusion furnace, the boron source flow rate is 100-150sccm, the oxygen flow rate is 300-500sccm, the nitrogen flow rate is 2000-4000sccm, the furnace door nitrogen flow rate is 300-1500sccm, the pressure in the diffusion furnace is 140-220bar, and the deposition time is 120-200s; the temperature of the next deposition step is 10℃-20℃ higher than the deposition temperature of the previous step.

7. The boron diffusion method according to claim 1, characterized in that: The finite source push-in includes: at the second temperature, introducing nitrogen into the diffusion furnace, the nitrogen flow rate is 2000-5000sccm, the furnace door nitrogen flow rate is 300-1500sccm, the pressure in the diffusion furnace is 220-450bar, and the finite source push-in time is 500-900s; And / or, the second temperature is 860°C-950°C.

8. The boron diffusion method according to claim 2, characterized in that: The boron diffusion method further comprises the following steps: S5. Cooling: The temperature in the diffusion furnace is cooled from the fourth temperature to the fifth temperature, oxygen is introduced, the oxygen flow rate is 8000-20000 sccm, the pressure in the diffusion furnace is 600-900 mbar, the cooling time is 1800-3000 s, and then the diffusion furnace is closed; the sixth temperature is 700℃-800℃.

9. A method for preparing a TOPCon battery, comprising a boron diffusion step, characterized in that: The boron diffusion step adopts the boron diffusion method according to any one of claims 1 to 8.

10. A TOPCon battery, characterized in that: It is prepared by the preparation method as claimed in claim 9.

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