Boron diffusion method, TOPCon solar cell and preparation method of TOPCon solar cell

Abstract

The invention relates to a boron diffusion method, a TOPCon solar cell and a preparation method of the TOPCon solar cell. The boron diffusion method comprises the steps that the temperatures from a diffusion furnace opening to a furnace tail are set to be T1, T2, T3, T4, T5 and T6 in sequence; in the deposition step, T1 to T6 are independently selected from 780 DEG C to 900 DEG C, the difference between any two adjacent temperatures is smaller than or equal to 20 DEG C, when a diffusion source is introduced from the furnace tail to the furnace opening, T1 is larger than T2, T1 is larger than T3, T6 is larger than T5, and T6 is larger than T4, or when the diffusion source is introduced from the furnace opening to the furnace tail, T1 is larger than T2, T3 is larger than T4, T5 is larger than T6; in the step of heating and knot pushing, T1 to T6 are independently selected from 850-1000 DEG C, the difference between any two adjacent temperatures is smaller than or equal to 20 DEG C, when gas is introduced from the furnace tail to the furnace opening, T1 is larger than T2, T1 is larger than T3, T6 is larger than T5, and T6 is larger than T4, or when gas is introduced from the furnace opening to the furnace tail, T1 is larger than T2, T3 is larger than T4, T5 is larger than T6. The boron-rich layer obtained through the method has excellent sheet resistance uniformity after SE laser doping, and the yield and conversion efficiency can be improved when the boron-rich layer is applied to a solar cell.

Description

Technical Field

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

[0002] In recent years, the research and development and industrialization of photovoltaic technology have advanced by leaps and bounds. With the joint efforts of the industry, the conversion efficiency of solar cell production has been continuously innovated. The mass production conversion efficiency of passivated emitter rear cell (PERC cell) is gradually approaching its theoretical limit. Therefore, the technical focus of the industry has shifted from P-type to N-type. Among them, the tunnel oxide passivated contact (TOPCon) cell has been widely concerned by the industry because of its advantages such as low attenuation, large potential for efficiency improvement, and low cost of production line upgrade, and has become the battery technology with the highest market share in the current N-type field. The current actual production efficiency of TOPCon cells has been increased from the initial 23% to more than 25%, and the process route has been relatively mature. Therefore, "efficiency improvement and cost reduction" have become the core content of the current TOPCon technology development.

[0003] In the TOPCon cell production process, superimposing laser doping selective emitter (SE laser doping) is currently the preferred method for TOPCon cell efficiency improvement. However, problems such as poor uniformity of sheet resistance behind SE laser doping, unstable efficiency improvement, abnormal appearance after oxidation, and the appearance of EL black edges exist in the production process, reducing the yield and conversion efficiency of solar cells. Summary of the Invention

[0004] Based on this, in view of the above problems, it is necessary to provide a boron diffusion method, a TOPCon solar cell and a preparation method thereof. The boron diffusion method enables the boron-rich layer on the surface of the silicon wafer to have excellent sheet resistance uniformity after SE laser doping, and improves the yield and conversion efficiency of the battery when applied to the TOPCon solar cell.

[0005] The present invention provides a boron diffusion method, which sequentially includes boat loading, vacuum pumping and heating, constant temperature leak detection, backpressure, pre-oxidation, deposition, temperature rising and driving the junction, and temperature dropping and boat unloading.

[0006] The temperatures from the furnace mouth to the furnace tail in the diffusion furnace are set as T1, T2, T3, T4, T5, and T6 in sequence.

[0007] In the deposition step, T1 to T6 are each independently selected from 780°C - 900°C, and the difference between any two adjacent temperatures is less than or equal to 20°C. When the diffusion source is introduced from the furnace tail to the furnace mouth, T1 > T2, T1 > T3, T6 > T5, T6 > T4; or when the diffusion source is introduced from the furnace mouth to the furnace tail, T1 > T2 > T3 > T4 > T5 > T6.

[0008] In the step of temperature increase and diffusion push, T1 to T6 are each independently selected from 850°C to 1100°C, the difference between any two adjacent temperatures is less than or equal to 20°C, and when the gas is introduced from the furnace tail to the furnace mouth, T1 > T2, T1 > T3, T6 > T5, T6 > T4, or when the gas is introduced from the furnace mouth to the furnace tail, T1 > T2 > T3 > T4 > T5 > T6.

[0009] In one embodiment, in the deposition step, T1 to T6 are each independently selected from 790°C to 840°C.

[0010] In one embodiment, in the step of temperature increase and diffusion push, T1 to T6 are each independently selected from 900°C to 950°C.

[0011] In one embodiment, in the deposition step, the diffusion source includes nitrogen, a boron source, and oxygen. Among them, the flow rate of nitrogen is 1000 sccm - 4000 sccm, the flow rate of the boron source is 100 sccm - 400 sccm, the flow rate of oxygen is 300 sccm - 1500 sccm, the deposition pressure is 90 mbar - 300 mbar, and the deposition time is 300 s - 1200 s.

[0012] In one embodiment, in the step of temperature increase and diffusion push, the gas is selected from nitrogen, the flow rate of nitrogen is 2000 sccm - 4500 sccm, the pressure is 200 mbar - 600 mbar, and the time is 600 s - 2000 s.

[0013] In one embodiment, in the steps of vacuum pumping and temperature increase, constant temperature leak detection, backpressure, pre - oxygen, and deposition, the temperature from the furnace mouth to the furnace tail in the diffusion furnace is the same.

[0014] In one embodiment, in the pre - oxygen step, the diffusion source includes nitrogen and oxygen. Among them, the flow rate of nitrogen is 2000 sccm - 5000 sccm, the flow rate of oxygen is 800 sccm - 2000 sccm, the time is 100 s - 400 s, and the thickness of the formed silicon oxide layer is 1.5 nm - 3.5 nm.

[0015] In one embodiment, in the step of temperature decrease and boat removal, the temperature is 650°C - 850°C, the pressure is 600 mbar - 800 mbar, the flow rate of nitrogen is 5000 sccm - 30000 sccm, and the temperature decrease time is 900 s - 3000 s.

[0016] A method for preparing a TOPCon solar cell includes the boron diffusion method as described above.

[0017] A TOPCon solar cell is prepared by the method for preparing a TOPCon solar cell as described above.

[0018] In the boron diffusion process provided by the present invention, a one-step deposition is performed by setting a specific gradient temperature in the deposition stage, so that the boron source in the diffusion furnace is evenly distributed and evenly deposited on the surface of the silicon wafer to form a boron-rich layer. Then, a specific gradient temperature is set in the temperature rise and junction pushing stage to realize the redistribution of the boron source. Through the above settings, on the one hand, the problems of abnormal appearance and EL black edges caused by excessive deposition of unactivated boron elements in the boron-rich layer on the surface of the silicon wafer, such as a large drop in square resistance and a fast deposition rate of the oxide layer, are improved, and the yield of the battery cell is improved. On the other hand, the square resistance extreme difference and square resistance uniformity of the boron-rich layer after SE laser doping are improved, so that the electrical properties of the solar cell, such as the open circuit voltage, short circuit current, and fill factor, are improved and the efficiency is stabilized, thereby improving the conversion efficiency of the solar cell.

[0019] At the same time, compared with the boron diffusion method in the traditional process, since the deposition in the boron diffusion method of the present invention only requires one step, the boron diffusion time is short, which can increase production and reduce costs in large-scale production in factories. DETAILED DESCRIPTION

[0020] For ease of understanding of the present invention, the present invention will be described more fully below with reference to relevant 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 understanding of the disclosure of the present invention more thorough and comprehensive.

[0021] 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 herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0022] The boron diffusion method provided by the present invention comprises the following steps: entering a boat, vacuuming and heating, constant temperature leak detection, back pressure, pre-oxygenation, deposition, heating and knot pushing, and cooling and exiting the boat.

[0023] During the entire boron diffusion process, the temperatures from the furnace mouth to the furnace tail in the diffusion furnace are set to T1, T2, T3, T4, T5, and T6 respectively.

[0024] In the step of entering the boat, the textured silicon wafer is inserted into the diffusion furnace tube in a horizontal or vertical manner, heated and continuously purged with nitrogen to prevent impurities from entering the diffusion furnace. Preferably, T1 to T6 are independently selected from 780°C-900°C, and T1=T2=T3=T4=T5=T6.

[0025] In order to better prevent impurities from entering the diffusion furnace, during the boat loading step, the nitrogen flow rate is 1000 sccm - 5000 sccm, the boat loading time is 400 s - 1000 s, and the pressure is atmospheric pressure.

[0026] During the vacuum pumping and heating step, after closing the diffusion furnace, vacuum pumping and heating are carried out, and no gas is introduced during the whole step. Preferably, during the vacuum pumping step, the pressure of the diffusion furnace is pumped down to 90 mbar - 150 mbar, and T1 to T6 are each independently selected from 780 °C - 900 °C, with the temperature difference between any two adjacent temperatures being less than or equal to 20 °C. And when the diffusion source is introduced from the furnace tail to the furnace mouth during the whole diffusion step, T1 > T2, T1 > T3, T6 > T5, T6 > T4; or when the diffusion source is introduced from the furnace mouth to the furnace tail during the whole diffusion step, T1 > T2 > T3 > T4 > T5 > T6. The whole process lasts for 600 s - 1200 s.

[0027] During the constant temperature leak detection step, in order to detect whether there is a leak in the diffusion furnace, the pressure is maintained constant, and no gas is introduced during the whole process, lasting for 60 s - 120 s.

[0028] During the backpressure step, the pressure is raised and nitrogen is introduced into the diffusion furnace. Preferably, the pressure is raised to 100 mbar - 200 mbar, and the nitrogen flow rate is 5000 mbar - 10000 mbar. The whole process lasts for 200 s - 600 s.

[0029] During the pre - oxygen step, the pressure and temperature are maintained constant, and nitrogen and oxygen are introduced into the diffusion furnace. The introduced oxygen can form a thin and dense silicon oxide layer on the surface of the silicon wafer, which can prevent the derivatives generated during the deposition of boron source from corroding the silicon wafer and damaging the silicon surface structure, resulting in an increase in load.

[0030] In one embodiment, the nitrogen flow rate is 2000 sccm - 5000 sccm, the oxygen flow rate is 800 sccm - 2000 sccm, the time is 100 s - 400 s, and the thickness of the generated silicon oxide layer is 1.5 nm - 3.5 nm.

[0031] In the deposition step, a diffusion source is introduced into a diffusion furnace to perform one-step deposition on a silicon wafer, thereby forming a boron-rich layer on the surface of the silicon wafer. Among them, T1 to T6 are each independently selected from 780°C to 900°C, and the temperature difference between any two adjacent temperatures is less than or equal to 20°C. And when the diffusion source is introduced from the furnace tail to the furnace mouth, T1 > T2, T1 > T3, T6 > T5, T6 > T4, or when the diffusion source is introduced from the furnace mouth to the furnace tail, T1 > T2 > T3 > T4 > T5 > T6, so that the sheet resistance from the boron-rich layer on the surface of the silicon wafer at the furnace mouth of the diffusion furnace to the boron-rich layer on the surface of the silicon wafer at the furnace tail reaches substantially the same, and at the same time, the amount of boron in the boron-rich layer deposited on the surface of the silicon wafer is substantially the same. Preferably, T1 to T6 are each independently selected from 790°C to 840°C.

[0032] In order to better deposit the silicon wafer, in the deposition step, the diffusion source includes nitrogen, a boron source, and oxygen. Among them, the boron source is preferably boron trichloride. The flow rate of nitrogen is 1000 sccm - 4000 sccm, the flow rate of the boron source is 100 sccm - 400 sccm, the flow rate of oxygen is 300 sccm - 1500 sccm, the deposition pressure is 90 mbar - 300 mbar, and the deposition time is 300 s - 1200 s.

[0033] In order to better improve the uniformity of boron diffusion, in one embodiment, in the steps of vacuum pumping and heating, constant temperature leak detection, backpressure, pre-oxygen, and deposition, the temperature from the furnace mouth to the furnace tail in the diffusion furnace is the same.

[0034] In the step of heating and pushing for annealing, the temperature and pressure rise. T1 to T6 are each independently selected from 850°C to 1100°C, and the temperature difference between any two adjacent temperatures is less than or equal to 20°C. And when the gas is introduced from the furnace tail to the furnace mouth, T1 > T2, T1 > T3, T6 > T5, T6 > T4, or when the gas is introduced from the furnace mouth to the furnace tail, T1 > T2 > T3 > T4 > T5 > T6, so as to realize the redistribution of the boron source.

[0035] Preferably, T1 to T6 are each independently selected from 900°C to 950°C.

[0036] In order to better realize the redistribution of the boron source, in the step of heating and pushing for annealing, the gas is selected from nitrogen. The flow rate of nitrogen is 2000 sccm - 4500 sccm, the pressure is 200 mbar - 600 mbar, and the time is 600 s - 2000 s.

[0037] In the step of cooling and taking out the boat, the temperature is lowered and nitrogen is continuously introduced. After the cooling is completed, the pressure is raised back to atmospheric pressure, and the silicon wafer after boron diffusion is taken out and cooled.

[0038] Preferably, in the step of cooling and taking out the boat, T1 to T6 are each independently selected from 650°C to 850°C, and T1 = T2 = T3 = T4 = T5 = T6, the pressure is 600 mbar to 800 mbar, the flow rate of nitrogen is 5000 sccm to 30000 sccm, and the cooling time is 900 s to 5000 s.

[0039] Through the above boron diffusion method, on the one hand, it improves the problems of large decrease in sheet resistance due to excessive deposition of unactivated boron in the boron-rich layer on the silicon wafer surface, appearance abnormalities caused by fast deposition rate of the oxide layer, and EL black edges, and improves the yield of the cell. On the other hand, it also improves the sheet resistance difference and sheet resistance uniformity after SE laser doping of the boron-rich layer, thereby reducing the damage of the laser to the pyramids on the silicon wafer surface, improving the electrical properties such as open circuit voltage, short circuit current, and fill factor of the prepared solar cell and stabilizing the efficiency improvement, and further improving the conversion efficiency of the solar cell. Compared with the boron diffusion method in the traditional process, since only one step of deposition is required in the boron diffusion method of the present invention, the boron diffusion time is short, which can be as low as 90 min, thus achieving the effect of increasing production and reducing costs in large-scale factory production.

[0040] The present invention also provides a method for preparing a TOPCon solar cell, including the boron diffusion method as described above. The TOPCon solar cell obtained by this preparation method has excellent electrical properties such as open circuit voltage, short circuit current, and fill factor, and high conversion efficiency.

[0041] The present invention also provides a TOPCon solar cell prepared by the method for preparing a TOPCon solar cell as described above.

[0042] Hereinafter, the boron diffusion method, the TOPCon solar cell and its preparation method will be further described through the following specific examples.

[0043] Example 1

[0044] Loading the boat: The textured silicon wafers are inserted horizontally into the diffusion furnace tube. The temperatures T1 to T6 from the furnace mouth to the furnace tail are set to 800°C, 800°C, 800°C, 800°C, 800°C, 800°C in sequence. During the loading process, nitrogen with a flow rate of 2000 sccm is introduced from the furnace tail to the furnace mouth for purging, the pressure is normal pressure 1013 mbr, and the loading time lasts for 400 s.

[0045] Vacuum pumping and heating up: After closing the diffusion furnace door, the pressure is pumped down to 95 mbar, and T1 to T6 are set to 830°C, 820°C, 810°C, 822°C, 824°C, 828°C in sequence. The whole process lasts for 660 s.

[0046] Constant temperature leak detection: The pressure is maintained at 95 mbar, and T1 to T6 are maintained at 830 °C, 820 °C, 810 °C, 822 °C, 824 °C, and 828 °C in sequence. The whole process lasts for 60 s.

[0047] Backpressure: The pressure rises to 200 mbar, and nitrogen with a flow rate of 3500 sccm is introduced. T1 to T6 are maintained at 830 °C, 820 °C, 810 °C, 822 °C, 824 °C, and 828 °C in sequence. The whole process lasts for 300 s.

[0048] Pre-oxygen: The pressure is maintained at 200 mbar, and nitrogen with a flow rate of 3360 sccm and oxygen with a flow rate of 1000 sccm are introduced from the furnace tail to the furnace mouth. T1 to T6 are maintained at 830 °C, 820 °C, 810 °C, 822 °C, 824 °C, and 828 °C in sequence. The whole process lasts for 200 s, and the thickness of the silicon oxide layer formed on the silicon wafer surface is 1.95 nm.

[0049] Deposition: The pressure is maintained at 200 mbar, and nitrogen with a flow rate of 2700 sccm, boron trichloride with a flow rate of 210 sccm, and oxygen with a flow rate of 540 sccm are introduced from the furnace tail to the furnace mouth. T1 to T6 are maintained at 830 °C, 820 °C, 810 °C, 822 °C, 824 °C, and 828 °C in sequence. The whole process lasts for 410 s.

[0050] Heating and diffusion: The pressure rises and is maintained at 400 mbar, and nitrogen with a flow rate of 3300 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are heated to 935 °C, 922 °C, 915 °C, 926 °C, 924 °C, and 929 °C in sequence. The whole process lasts for 1200 s.

[0051] Cooling and wafer unloading: The pressure rises and is maintained at 800 mbar, and nitrogen with a flow rate of 16000 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are cooled to 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, and 750 °C in sequence. The cooling lasts for 1200 s. After cooling, nitrogen backpressure and wafer unloading are carried out to obtain the boron-diffused silicon wafer.

[0052] Example 2

[0053] Wafer loading: The textured silicon wafer is inserted vertically into the diffusion furnace tube. The temperatures from the furnace mouth to the furnace tail, T1 to T6, are set to 800 °C, 800 °C, 800 °C, 800 °C, 800 °C, and 800 °C in sequence. During the wafer loading process, nitrogen with a flow rate of 2000 sccm is introduced from the furnace tail to the furnace mouth for purging. The pressure is normal pressure 1013 mbr, and the wafer loading time lasts for 400 s.

[0054] Vacuum pumping and heating: After closing the diffusion furnace door, pump the pressure down to 95 mbar, and set T1 to T6 to 830 °C, 820 °C, 815 °C, 815 °C, 820 °C, and 825 °C in sequence. The whole process lasts for 900 s.

[0055] Constant temperature leak detection: Maintain the pressure at 95 mbar, and keep T1 to T6 at 830 °C, 820 °C, 815 °C, 815 °C, 820 °C, and 825 °C in sequence. The whole process lasts for 60 s.

[0056] Backpressure: Raise the pressure back to 140 mbar, and introduce nitrogen with a flow rate of 3500 sccm from the furnace tail to the furnace mouth. Keep T1 to T6 at 830 °C, 820 °C, 815 °C, 815 °C, 820 °C, and 825 °C in sequence. The whole process lasts for 300 s.

[0057] Pre-oxygen: Maintain the pressure at 140 mbar, introduce nitrogen with a flow rate of 3360 sccm and oxygen with a flow rate of 1000 sccm from the furnace tail to the furnace mouth. Keep T1 to T6 at 830 °C, 820 °C, 815 °C, 815 °C, 820 °C, and 825 °C in sequence. The whole process lasts for 200 s, and the thickness of the silicon oxide layer formed on the silicon wafer surface is 1.95 nm.

[0058] Deposition: Maintain the pressure at 140 mbar, introduce nitrogen with a flow rate of 2600 sccm, boron trichloride with a flow rate of 250 sccm, and oxygen with a flow rate of 640 sccm from the furnace tail to the furnace mouth. Keep T1 to T6 at 830 °C, 820 °C, 815 °C, 815 °C, 820 °C, and 825 °C in sequence. The whole process lasts for 410 s.

[0059] Heating and diffusion: Raise the pressure back and maintain it at 140 mbar, introduce nitrogen with a flow rate of 3800 sccm from the furnace tail to the furnace mouth. Raise T1 to T6 to 938 °C, 934 °C, 932 °C, 932 °C, 937 °C, and 931 °C in sequence. The whole process lasts for 1240 s.

[0060] Cooling and wafer unloading: Raise the pressure back and maintain it at 800 mbar, introduce nitrogen with a flow rate of 16000 sccm from the furnace tail to the furnace mouth. Cool T1 to T6 to 800 °C, 800 °C, 800 °C, 800 °C, 800 °C, and 800 °C in sequence. The cooling lasts for 1000 s. After cooling, perform nitrogen filling and backpressure to unload the wafer, and obtain the silicon wafer after boron diffusion.

[0061] Example 3

[0062] Loading the boat: Insert the textured silicon wafers horizontally into the diffusion furnace tube. Set the temperatures T1 to T6 from the furnace mouth to the furnace tail to be 900 °C, 890 °C, 880 °C, 892 °C, 894 °C, and 898 °C in sequence. During the loading process, purge with nitrogen at a flow rate of 2000 sccm from the furnace tail to the furnace mouth, with the pressure at atmospheric pressure of 1013 mbar, and the loading time lasts for 600 s.

[0063] Vacuum pumping and heating up: After closing the diffusion furnace door, pump the pressure down to 95 mbar. Set T1 to T6 to be 900 °C, 890 °C, 880 °C, 892 °C, 894 °C, and 898 °C in sequence, and the whole process lasts for 900 s.

[0064] Constant temperature leak detection: Maintain the pressure at 95 mbar, and maintain T1 to T6 at 900 °C, 890 °C, 880 °C, 892 °C, 894 °C, and 898 °C in sequence. The whole process lasts for 60 s.

[0065] Backpressure: Raise the pressure back to 300 mbar, and purge with nitrogen at a flow rate of 140 sccm from the furnace tail to the furnace mouth. Maintain T1 to T6 at 900 °C, 890 °C, 880 °C, 892 °C, 894 °C, and 898 °C in sequence, and the whole process lasts for 500 s.

[0066] Pre-oxygen: Maintain the pressure at 300 mbar, and purge with nitrogen at a flow rate of 3360 sccm and oxygen at a flow rate of 1000 sccm from the furnace tail to the furnace mouth. Maintain T1 to T6 at 900 °C, 890 °C, 880 °C, 892 °C, 894 °C, and 898 °C in sequence. The whole process lasts for 200 s, and the thickness of the silicon oxide layer formed on the silicon wafer surface is 2.80 nm.

[0067] Deposition: Maintain the pressure at 300 mbar, and purge with nitrogen at a flow rate of 4000 sccm, boron trichloride at a flow rate of 400 sccm, and oxygen at a flow rate of 1500 sccm from the furnace tail to the furnace mouth. Maintain T1 to T6 at 900 °C, 890 °C, 880 °C, 892 °C, 894 °C, and 898 °C in sequence. The whole process lasts for 1200 s.

[0068] Heating up and driving the junction: Raise the pressure and maintain it at 600 mbar, and purge with nitrogen at a flow rate of 4500 sccm from the furnace tail to the furnace mouth. Raise T1 to T6 to 905 °C, 892 °C, 885 °C, 897 °C, 899 °C, and 8903 °C in sequence, and the whole process lasts for 2000 s.

[0069] Cooling and taking out the wafer: The pressure rises and is maintained at 800 mbar. Nitrogen with a flow rate of 30000 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are cooled to 800 °C, 800 °C, 800 °C, 800 °C, 800 °C, and 800 °C in sequence. The cooling lasts for 5000 s. After the cooling is completed, nitrogen is filled to increase the pressure and the wafer is taken out to obtain the boron-diffused silicon wafer.

[0070] Example 4

[0071] Loading the wafer: The textured silicon wafer is inserted horizontally into the diffusion furnace tube. The temperatures from the furnace mouth to the furnace tail, T1 to T6, are set to 800 °C, 790 °C, 780 °C, 792 °C, 794 °C, and 798 °C in sequence. During the loading process, nitrogen with a flow rate of 2000 sccm is introduced from the furnace tail to the furnace mouth for purging. The pressure is normal pressure, 1013 mbr, and the loading time lasts for 400 s.

[0072] Vacuum pumping and heating up: After the diffusion furnace door is closed, the pressure is pumped down to 95 mbar. T1 to T6 are set to 800 °C, 790 °C, 780 °C, 792 °C, 794 °C, and 798 °C in sequence. The whole process lasts for 660 s.

[0073] Constant temperature leak detection: The pressure is maintained at 95 mbar. T1 to T6 are maintained at 800 °C, 790 °C, 780 °C, 792 °C, 794 °C, and 798 °C in sequence. The whole process lasts for 60 s.

[0074] Pressure increasing: The pressure rises to 90 mbar. Nitrogen with a flow rate of 3500 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are maintained at 800 °C, 790 °C, 780 °C, 792 °C, 794 °C, and 798 °C in sequence. The whole process lasts for 300 s.

[0075] Pre-oxidation: The pressure is maintained at 90 mbar. Nitrogen with a flow rate of 3360 sccm and oxygen with a flow rate of 1000 sccm are introduced from the furnace tail to the furnace mouth. T1 to T6 are maintained at 800 °C, 790 °C, 780 °C, 792 °C, 794 °C, and 798 °C in sequence. The whole process lasts for 200 s. The thickness of the silicon oxide layer formed on the silicon wafer surface is 1.60 nm.

[0076] Deposition: The pressure is maintained at 90 mbar. Nitrogen with a flow rate of 1000 sccm, boron trichloride with a flow rate of 100 sccm, and oxygen with a flow rate of 300 sccm are introduced from the furnace tail to the furnace mouth. T1 to T6 are maintained at 800 °C, 790 °C, 780 °C, 792 °C, 794 °C, and 798 °C in sequence. The whole process lasts for 300 s.

[0077] Temperature increase and pushing for diffusion: The pressure rises and is maintained at 200 mbar. Nitrogen with a flow rate of 2000 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are heated to 995 °C, 982 °C, 975 °C, 986 °C, 984 °C, and 989 °C in sequence, and the whole process lasts for 600 s.

[0078] Temperature decrease and wafer unloading: The pressure rises and is maintained at 600 mbar. Nitrogen with a flow rate of 5000 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are cooled to 800 °C, 800 °C, 800 °C, 800 °C, 800 °C, and 800 °C in sequence. The cooling lasts for 900 s. After the cooling is completed, nitrogen is filled to increase the pressure and the wafer is unloaded to obtain the silicon wafer after boron diffusion.

[0079] Example 5

[0080] Example 5 is carried out with reference to Example 1, and the differences are as follows: In the steps of vacuum pumping and heating, constant temperature leak detection, pressure backfilling, pre-oxygen introduction, and deposition, T1 to T6 are 810 °C, 800 °C, 790 °C, 802 °C, 804 °C, and 808 °C in sequence.

[0081] In the step of temperature increase and pushing for diffusion, T1 to T6 are 950 °C, 937 °C, 930 °C, 941 °C, 939 °C, and 944 °C in sequence.

[0082] Example 6

[0083] Example 6 is carried out with reference to Example 1, and the differences are as follows: In the steps of vacuum pumping and heating, constant temperature leak detection, pressure backfilling, pre-oxygen introduction, and deposition, T1 to T6 are 840 °C, 830 °C, 820 °C, 832 °C, 834 °C, and 838 °C in sequence.

[0084] In the step of temperature increase and pushing for diffusion, T1 to T6 are 915 °C, 902 °C, 895 °C, 906 °C, 904 °C, and 909 °C in sequence.

[0085] Example 7

[0086] Example 7 is carried out with reference to Example 1, and the differences are as follows: During the whole boron diffusion process, the gas is introduced from the furnace mouth to the furnace tail. In the steps of vacuum pumping and heating, constant temperature leak detection, pressure backfilling, pre-oxygen introduction, and deposition, T1 to T6 are 820 °C, 822 °C, 825 °C, 826 °C, 830 °C, and 840 °C in sequence.

[0087] In the step of temperature increase and pushing for diffusion, T1 to T6 are 915 °C, 917 °C, 918 °C, 919 °C, 920 °C, and 930 °C in sequence.

[0088] Comparative Example 1

[0089] Loading the boat: Insert the textured silicon wafers horizontally into the diffusion furnace tube. Set the temperatures T1 to T6 from the furnace mouth to the furnace tail to be 800°C, 800°C, 800°C, 800°C, 800°C, 800°C in sequence. During the loading process, purge with nitrogen at a flow rate of 2000 sccm from the furnace tail to the furnace mouth, with the pressure at atmospheric pressure of 1013 mbar, and the loading time lasts for 400 s.

[0090] Vacuum pumping and heating up: After closing the diffusion furnace door, pump the pressure down to 95 mbar. Set T1 to T6 to be 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence, and the whole process lasts for 560 s.

[0091] Constant temperature leak detection: Maintain the pressure at 95 mbar, and maintain T1 to T6 at 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence. The whole process lasts for 60 s.

[0092] Backpressure: Pump the pressure down to 90 mbar, and purge with nitrogen at a flow rate of 1500 sccm from the furnace tail to the furnace mouth. Maintain T1 to T6 at 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence, and the whole process lasts for 300 s.

[0093] Pre-oxygen: Maintain the pressure at 90 mbar, and purge with nitrogen at a flow rate of 1100 sccm and oxygen at a flow rate of 1600 sccm from the furnace tail to the furnace mouth. Maintain T1 to T6 at 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence. The whole process lasts for 200 s, and the thickness of the silicon oxide layer formed on the silicon wafer surface is 2.3 nm.

[0094] Deposition: Maintain the pressure at 90 mbar, and maintain T1 to T6 at 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence. Conduct a primary deposition by introducing nitrogen at a flow rate of 2000 sccm, boron trichloride at a flow rate of 150 sccm, and oxygen at a flow rate of 600 sccm. The primary deposition lasts for 600 s; then perform backpressure purge, raise the pressure to 130 mbar, and introduce nitrogen at a flow rate of 4300 sccm. Maintain T1 to T6 at 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence, lasting for 150 s; maintain the pressure at 130 mbar, and maintain T1 to T6 at 870°C, 860°C, 860°C, 860°C, 860°C, 860°C in sequence. Conduct a secondary deposition by introducing nitrogen at a flow rate of 1300 sccm, boron trichloride at a flow rate of 150 sccm, and oxygen at a flow rate of 600 sccm. The secondary deposition lasts for 200 s.

[0095] Temperature rising and pushing to form junctions: The pressure rises and is maintained at 400 mbar. Nitrogen with a flow rate of 3300 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are heated to 960 °C, 960 °C, 960 °C, 960 °C, 960 °C, and 960 °C in sequence, and the whole process lasts for 1080 s.

[0096] Temperature decreasing and taking out the boat: The pressure rises and is maintained at 800 mbar. Nitrogen with a flow rate of 16000 sccm is introduced from the furnace tail to the furnace mouth. T1 to T6 are cooled to 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, and 750 °C in sequence. The temperature decrease lasts for 2520 s. After the temperature decrease is completed, nitrogen is filled to increase the pressure and the boat is taken out to obtain the boron-diffused silicon wafers.

[0097] Comparative Example 2

[0098] Comparative Example 2 is carried out with reference to Example 1, the difference being that: in the steps of vacuum pumping and heating, constant temperature leak detection, pressure backfilling, pre-oxidation, and deposition, T1 to T6 are 850 °C, 780 °C, 810 °C, 840 °C, 820 °C, and 900 °C in sequence.

[0099] Comparative Example 3

[0100] Comparative Example 3 is carried out with reference to Example 1, the difference being that: in the steps of vacuum pumping and heating, constant temperature leak detection, pressure backfilling, pre-oxidation, and deposition, T1 to T6 are 800 °C, 820 °C, 810 °C, 810 °C, 820 °C, and 800 °C in sequence.

[0101] Comparative Example 4

[0102] Comparative Example 4 is carried out with reference to Example 1, the difference being that: in the step of temperature rising and pushing to form junctions, T1 to T6 are 1100 °C, 1060 °C, 1010 °C, 1010 °C, 1070 °C, and 1100 °C in sequence.

[0103] Comparative Example 5

[0104] Comparative Example 5 is carried out with reference to Example 1, the difference being that: in the step of temperature rising and pushing to form junctions, T1 to T6 are 1000 °C, 1020 °C, 1040 °C, 1040 °C, 1020 °C, and 1000 °C in sequence.

[0105] Test Example 1

[0106] The boron-diffused silicon wafers obtained in Examples 1 - 7 and Comparative Examples 1 - 5 are subjected to laser SE laser doping, and the change in sheet resistance before and after doping is tested. The specific test method is as follows. The test results of Examples 1 - 7 and Comparative Examples 1 - 5 are shown in Tables 1 - 12.

[0107] SE Laser Doping: After boron diffusion on the silicon wafer, laser doping is carried out using a laser square pattern for measuring sheet resistance. A 1064nm infrared nanosecond continuous laser is used with a laser power of 78W, a spot width of 100μm, and a scanning speed of 25m / s.

[0108] Sheet Resistance Test: An offline NAPSON RG - 200PV sheet resistance tester is used to measure the sheet resistance at the center point and the four sides of the silicon wafer after boron diffusion and the silicon wafer after SE laser doping, and the average sheet resistance and the in - wafer sheet resistance uniformity are calculated.

[0109] Table 1

[0110]

[0111] Table 2

[0112]

[0113] Table 3

[0114]

[0115]

[0116] Table 4

[0117]

[0118] Table 5

[0119]

[0120] Table 6

[0121]

[0122] Table 7

[0123]

[0124]

[0125] Table 8

[0126]

[0127] Table 9

[0128]

[0129] Table 10

[0130]

[0131]

[0132] Table 11

[0133]

[0134] Table 12

[0135]

[0136] It can be seen from Tables 1 - 12 that the sheet resistance difference and sheet resistance uniformity after SE laser doping of the boron-rich layer on the surface of the boron-diffused silicon wafers obtained in the examples are better.

[0137] Test Example 2

[0138] According to the Baseline production line process, the boron-diffused silicon wafers obtained in Examples 1 - 7 and Comparative Examples 1 - 5 were subjected to SE laser doping and then made into TOPCon solar cells. The open circuit voltage (Uoc), short circuit current (Isc), ohmic internal resistance (Rser), parallel resistance (Rshunt), fill factor (FF) and conversion efficiency (Eta) of the TOPCon solar cells were measured using a halm tester. The test results are shown in Table 13.

[0139] Table 13

[0140]

[0141] It can be seen from Table 13 that the TOPCon solar cell made from the boron-diffused silicon wafer obtained in Example 1 after SE laser doping has a conversion efficiency 0.07% higher, a Uoc 0.4 mV higher, an Isc 1 mA higher, and an FF 0.21 higher than the TOPCon solar cell made from the boron-diffused silicon wafer obtained in Comparative Example 1 after SE laser doping. The TOPCon solar cell made from the boron-diffused silicon wafer obtained in Example 2 after SE laser doping has a conversion efficiency 0.09% higher, a Uoc 0.8 mV higher, and an FF 0.15 higher than the TOPCon solar cell made from the boron-diffused silicon wafer obtained in Comparative Example 1 after SE laser doping. By comparing Examples 1 - 7 and Comparative Examples 1 - 5, it can be seen that the TOPCon solar cells prepared by SE laser doping of the boron-diffused silicon wafers obtained in the present invention have excellent electrical properties.

[0142] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0143] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A boron diffusion method, successively including boat loading, vacuum pumping and heating, constant temperature leak detection, backpressure, pre-oxidation, deposition, heating and pushing junction, and cooling and boat unloading. It is characterized in that the temperatures from the furnace mouth to the furnace tail in the diffusion furnace are set as T1, T2, T3, T4, T5, and T6 in sequence; in the deposition step, T1 to T6 are respectively and independently selected from 780°C - 900°C, the difference between any two adjacent temperatures is less than or equal to 20°C, and when the diffusion source is introduced from the furnace tail to the furnace mouth, T1 > T2, T1 > T3, T6 > T5, T6 > T4, or when the diffusion source is introduced from the furnace mouth to the furnace tail, T1 > T2 > T3 > T4 > T5 > T6; in the heating and pushing junction step, T1 to T6 are respectively and independently selected from 850°C - 1100°C, the difference between any two adjacent temperatures is less than or equal to 20°C, and when the gas is introduced from the furnace tail to the furnace mouth, T1 > T2, T1 > T3, T6 > T5, T6 > T4, or when the gas is introduced from the furnace mouth to the furnace tail, T1 > T2 > T3 > T4 > T5 > T6.

2. The boron diffusion method according to claim 1, It is characterized in that in the deposition step, T1 to T6 are respectively and independently selected from 790°C - 840°C.

3. The boron diffusion method according to claim 1, It is characterized in that in the heating and pushing junction step, T1 to T6 are respectively and independently selected from 900°C - 950°C.

4. The boron diffusion method according to any one of claims 1 - 3, It is characterized in that in the deposition step, the diffusion source includes nitrogen, boron source, and oxygen, wherein the flow rate of nitrogen is 1000 sccm - 4000 sccm, the flow rate of the boron source is 100 sccm - 400 sccm, the flow rate of oxygen is 300 sccm - 1500 sccm, the deposition pressure is 90 mbar - 300 mbar, and the deposition time is 300 s - 1200 s.

5. The boron diffusion method according to any one of claims 1 - 3, It is characterized in that in the heating and pushing junction step, the gas is selected from nitrogen, the flow rate of nitrogen is 2000 sccm - 4500 sccm, the pressure is 200 mbar - 600 mbar, and the time is 600 s - 2000 s.

6. The boron diffusion method according to any one of claims 1 - 3, It is characterized in that in the steps of vacuum pumping and heating, constant temperature leak detection, backpressure, pre-oxidation, and deposition, the temperatures from the furnace mouth to the furnace tail in the diffusion furnace are the same.

7. The boron diffusion method according to any one of claims 1 - 3, It is characterized in that in the pre-oxidation step, the diffusion source includes nitrogen and oxygen, wherein the flow rate of nitrogen is 2000 sccm - 5000 sccm, the flow rate of oxygen is 800 sccm - 2000 sccm, the time is 100 s - 400 s, and the thickness of the formed silicon oxide layer is 1.5 nm - 3.5 nm.

8. The boron diffusion method according to claim 1, It is characterized in that In the step of cooling down and taking out the boat, the temperature is 650°C - 850°C, the pressure is 600 mbar - 800 mbar, the flow rate of nitrogen is 5000 sccm - 30000 sccm, and the cooling time is 900 s - 3000 s.

9. A method for preparing a TOPCon solar cell, characterized in that, it includes the boron diffusion method according to any one of claims 1 - 8.

10. A TOPCon solar cell, characterized in that, it is prepared by the method for preparing a TOPCon solar cell according to claim 9.

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